FRI - Internal design vol 1

FRI VOLUME 5: FRACTIONATION DESIGN HANDBOOK

TRAYED TOWERS – GENERAL NOTES

Issued:

03/01/1978

1.00

Revised:

TRAYED TOWERS – GENERAL NOTES LIQUID FEEDS - Make sure the design does not produce a non-uniform flow pattern on the

tray. In particular, look for for ways of absorbing the momentum of the the incoming liquid and of distributing it over the tray. INTERMEDIATE FEEDS - Not only is it necessary to absorb liquid momentum and get a

good distribution, it is also necessary to consider ways of achieving good mixing between the feed liquid and the liquid coming from the tray above. FLASHING INTERMEDIATE FEEDS - Care must be taken that downcomer action is not

spoiled by the flashing feed overloading the downcomer with vapor or by causing boiling in the downcomer through heat transfer from a hotter ho tter feed. ACCESS - All technical solutions which are acceptable from a process design point of view

must also be assessed in terms of mechanical design and ease of access.

Page 1 of 1


INLETS – TOP

Issued:

03/01/1978

1.01

Revised:

INLETS - TOP

INLETS - TOP .............................................. .................................................... ............................................................................. ......................... 1 1.

Reflux or Non-Flashing Feed to Top Tray .................................................... ......................................................................... ..................... 2

2.


3.

Flashing Feed to Top Tray...................................................... .................................................................................................. ............................................ 4

4.

Flasing Feed to Top Tray............................................. ...................................................... ........................................................5

5.

Flashing Feed Top Tray ........................................................ ...................................................................................................... .............................................. 6

Page 1 of 6


INLETS – TOP

Issued:

03/01/1978

1.01

Revised:

INLETS - TOP

INLETS - TOP .............................................. .................................................... ............................................................................. ......................... 1 1.


2.


3.

Flashing Feed to Top Tray...................................................... .................................................................................................. ............................................ 4

4.

Flasing Feed to Top Tray............................................. ...................................................... ........................................................5

5.

Flashing Feed Top Tray ........................................................ ...................................................................................................... .............................................. 6

Page 1 of 6

Issued: 03/01/1978 Revised:

1

INLETS - TOP

Reflux or Non-Flashing Feed to Top Tray

DESIGN CRITERIA – A. False Downcomer 1. Clearance above tray floor set to give approximately 1” head loss under baffle but not less than 1/2” clearance. B. Inlet Weir 1. Hiw shall be such that it functions as a weir and not as a submerged weir. 2. For low liquid rates, elbow can be omitted and nozzle located at tray floor level.

Page 2 of 6

1.01

Issued: 03/01/1978

INLETS - TOP

Revised:

2

Reflux or Non-Flashing Feed to Top Tray

DESIGN CRITERIA -

A.

B.

Feed to center downcomer seal area 1.

Details same as single pass trays.

2.

Feed distributor may not be required for small diameters.

Feed to side downcomer seal areas 1.

Same principle as single pass tray. Balanced flow is essential.

Page 3 of 6

1.01

Issued: 03/01/1978

INLETS - TOP

Revised:

3

1.01

Flashing Feed to Top Tray

NOTES -

1.

Strength of baffle must be sufficient to withstand forces exerted.

2.

Use plate at top of false downcomer for open-end nozzle and/or reinforcing.

3.

Alternate "a" can be used when vertical escape velocity of vapor behind false downcomer does not exceed vapor velocity in perforated section.

.

Page 4 of 6

Issued: 03/01/1978

INLETS - TOP

Revised:

4

1.01

Flashing Feed to Top Tray

NOTES -

1.


2.

Use plate at top of false downcomer for open-end nozzle and for reinforcing.

3.

Design a can be used when vertical escape velocity of vapor behind false downcomer does not exceed vapor velocity in perforated section.

Page 5 of 6

Issued: 03/01/1978

INLETS - TOP

Revised:

5

1.01

Flashing Feed Top Tray

NOTES -

1.


2.

Design a can be used when vertical escape velocity of vapor behind false downcomer does not exceed vapor velocity in perforated section.

3.

If design b is used, liquid penetration on the inlet side of a sieve tray may be encountered.

4.

Inlet pipe should be centered accurately over distribution device.

Page 6 of 6


INLETS – INTERMEDIATE

Issued:

03/01/1978

1.02

Revised:

INLETS - INTERMEDIATE

INLETS - INTERMEDIATE .................................................... ................................................ 1 1.

Intermediate Liquid Feed ............................................. .....................................................2

2.

Intermediate Liquid Feed ............................................. .....................................................3

3.

Intermediate Vapor Feed ................................................. ................................................. 4

4.

Intermediate Vapor Feed ................................................. ................................................. 5

5.

Intermediate Vapor & Liquid Feed ................................................... ................................ 6

6.

Intermediate Vapor & Liquid Feed ................................................... ................................ 7

Page 1 of 7


1


1.02

Intermediate Liquid Feed

NOTES -

1. Do not feed into downcomer if the resultant mixture can generate vapor (nozzle "a"), if the system has a foaming tendency, or in high pressure systems. 2. Install insulating plate with nozzles "b" or "c" if feed is hot, if required for additional strength, or if wear plate is advisable. 3. Use distributor if the feed quantity is a high percentage of the liquid on the tray, if the area fed is large, or for special applications where immediate good mixing is required such as solvent extractive distillation.

Page 2 of 7


2


1.02

Intermediate Liquid Feed

NOTES -

1. For nozzles "a" and "c" - do not feed into downcomer if the resultant mixture can generate vapor, if the system has a foaming tendency, or in high pressure systems. 2. Install insulating plate with nozzles "b" and "d" if feed is hot, if required for extra strength, or if wear plate is advisable. 3. Balanced flow is essential.

Page 3 of 7


3


1.02

Intermediate Vapor Feed

NOTES -

1. Orient at 90° to 270° range depending on elevation above tray and volume of feed vapor. 2. Normal escape area at each end of feed device is 1.0 to 1.50 times nozzle area unless a larger area is required to reduce the vapor velocity. 3. Open end nozzles are acceptable for very low vapor feed rates. 4. Design of the feed device must not interfere with flow of liquid into the downcomer or entrained vapor leaving the downcomer.

Page 4 of 7


4


1.02

Intermediate Vapor Feed

NOTES -

1. Nozzle "a" should be located at 90° or 270°. 2. Normal escape area at each end of feed device is 1.0 to 1.50 times nozzle area unless a larger area is required to reduce the vapor velocity. 3. Open end nozzles are acceptable for very low vapor feed rates.

Page 5 of 7


5


1.02

Intermediate Vapor & Liquid Feed

DESIGN CRITERIA -

A. Locate nozzle "a" in the vicinity of 90° or 270° if fluid fall into the downcomer could interfere with its operation. B. See general note regarding obstruction under tray above. C. Design "b" is preferred where the liquid component of the feed is a large fraction of the total liquid load on the tray.

Page 6 of 7


6


1.02

Intermediate Vapor & Liquid Feed

A. Nozzle "b" and "d" favored over nozzle "a" and "c" where the liquid component of the feed is a large fraction of the total liquid load on the tray. B. Orient nozzles in locations shown.

Page 7 of 7


INLETS – BOTTOM

Issued:

03/01/1978

1.03

Revised:

INLETS - BOTTOM

INLETS - BOTTOM ............................................................. .................................................... 1 1.

Inlets Below Bottom Tray ................................................................................................. 2

2.


3.


Page 1 of 4


1

INLETS – BOTTOM

1.03

Inlets Below Bottom Tray

DESIGN CRITERIA -

A.

Orientation at 90° or 270° is required for open-ended inlets.

B.

Other orientations require the use of deflector devices to prevent impingement on the downcomer or seal pan.

C.

Large diameter inlets (ie., diameter greater than tray spacing) not provided with deflector devices may require a greater clearance between the top of the inlet and the tray above.

.

Page 2 of 4

Issued: 03/01/1978

INLETS – BOTTOM

Revised:

2

1.03


DESIGN CRITERIA -

A.


B.


C.


D.

If design "a" is used, interruption of liquid curtain from seal pan is recommended if inlet device is located below seal pan.

E.

Side downcomers are normally preferred.

Page 3 of 4

Issued: 03/01/1978

INLETS – BOTTOM

Revised:

3

1.03


DESIGN CRITERIA -

A.


B.


C.


Page 4 of 4


OUTLETS – TOP

Issued:

03/01/1978

1.04

Revised:

OUTLETS - TOP

OUTLETS - TOP....................................................................................................................... 1 1.

Vapor Outlets Above Top Tray Without Disentrainment Device ...................................... 2

2.

Vapor outlets Above Top Tray Without Disentrainment Device ....................................... 3

3.

Vapor Outlets Above Top Tray Without Disentrainment Device ...................................... 4

Page 1 of 4


1

OUTLETS - TOP

1.04

Vapor Outlets Above Top Tray Without Disentrainment Device

NOTES -

1.

The above criteria are recommended to minimize entrainment and reduce flow disturbance on the top tray.

2.

Design b is suitable only for smaller diameter columns.

Page 2 of 4


2

OUTLETS - TOP

1.04

Vapor outlets Above Top Tray Without Disentrainment Device

NOTES -

1. .

Available information does not allow the specification of dimensions P, Q, R, S, V, W, X, and α.

Page 3 of 4


3

OUTLETS - TOP

Vapor Outlets Above Top Tray Without Disentrainment Device

NOTES -

1.

Available information does not allow the specification of dimensions M, N & Ø.

Page 4 of 4

1.04


OUTLETS – INTERMEDIATE

Issued:

03/01/1978

1.05

Revised:

OUTLETS -INTERMEDIATE

OUTLETS -INTERMEDIATE ................................................................................................. 1 1.

Total Liquid Drawoffs .................................................. ..................................................... 2

2.

Partial Liquid Drawoffs ..................................................................................................... 3

3.

Vapor Drawoff Intermediate Tray ................................................ ..................................... 4

4.

Chimney Trays ................................................. ................................................................. 5

5.

Chimney Trays ................................................. ................................................................. 6

Page 1 of 6


1


1.05

Total Liquid Drawoffs

DESIGN COMMENTS -

A.

Designs “a” and “b” are preferred.

B.

Design “c” needs a stagnant liquid pool to provide the downcomer seal. This may lead to fouling or solid deposition in some cases.

C.

Design “d” does not have an overflow provision. Damage can occur when (1) the draw-off rate is inadequate, or (2) a liquid-filled column is drained from the column base.

Page 2 of 6

Issued: 03/01/1978


Revised:

2

1.05

Partial Liquid Drawoffs

NOTES -

1.

Design “a” is preferred over designs “b” and “c” because their increased complexity and cost are not justified.

2.

If design “d” is used, liquid penetration on the inlet side of a sieve tray may be encountered.

3.

If downcomer build-up is not limiting, normal tray spacing may be adequate.

4.

See 1.05-1 for definition of e.

Page 3 of 6


3


1.05

Vapor Drawoff Intermediate Tray

NOTES -

1.

Nozzle “A” is usually sufficient for small diameter towers.

2.

Nozzle “B” should be used for larger columns whenever vapor withdrawn is a significant portion of the total vapor flow and/or there is concern that horizontal vapor flow could interfere with tray action.

3.

Nozzle “C” is likely to contain less liquid than nozzle “A” at the same height.

4.

A shield above nozzle “B” may be desirable to protect from weepage from the tray above.

Page 4 of 6


4


Chimney Trays

NOTES -

1.

Design “A” is the preferred design.

2.

Design notes and alternate designs are shown on the following page.

Page 5 of 6

1.05

Issued: 03/01/1978


Revised:

5

1.05

Chimney Trays

NOTES -

1.

The number of chimneys should be selected on the basis of providing proper vapor distribution at a reasonable cost. Chimneys may be round or rectangular.

2.

Downcomer flooding can occur if the liquid in the downcomer is substantially more aerated than the liquid sealing the downcomer.

3.

Sumps provide additional liquid head without increasing the weight of liquid on the tray.

4.

Seal welding should be considered if leakage must be minimized.

5.

The total chimney area is normally 15 to 25% of the tower area.

6.

The annular area between the top of the chimney and the hat should be equal to, or greater than 1.25 times the chimney area.

7.

The overflow weir height is set by the residence time required.

Page 6 of 6

FRI VOLUME 5: FRA CTIONATION DESIGN HANDBOOK

INTERMEDIATE LIQUID DRAWOFF

Issued:

1/15/1994

Revised:

1.05.1


Intermediate Liquid Drawoff.....................................................................................................1 Introduction ......................... ...................................................................................................... 2 Downcomer Trapout ................................................. ................................................................. 2 Chimney Tray ...................................................................................... ...................................... 4 Draw Nozzle Setting ................................................. ................................................... .............. 5

Page 1 of 14

Issued: 1/15/1994 Revised: 10/1/2006

1


1.05.1

Introduction

An intermediate liquid drawoff may be a partial draw where only part of the liquid in the column is withdrawn while the remainder flows down the column as internal reflux. It may also be a total draw where all of the liquid is withdrawn. In this case, a portion of the liquid withdrawn (or in some cases all of that liquid) may be returned to the tower below the draw point as external reflux. The external reflux may be cooled, heated, stripped, etc. before it is returned to the column. A partial draw gives only indirect control of the internal reflux which may not be acceptable in some cases, e.g. when the internal reflux rate is small compared to the draw rate and a small change in draw rate could upset the tower. The common means for withdrawing liquid are downcomer trapouts and chimney trays. A downcomer trapout is essentially an extended downcomer and as such it normally provides little residence time for vapor disengagement; the liquid withdrawn may therefore be aerated. This can cause problems unless the design provides for it. A downcomer trapout may not achieve the required draw rate if the draw tray leaks or weeps. (As shown in Figure 1, the "draw tray" is defined by the last tray where the liquid withdrawn contacts vapor; not by the location of the draw nozzle. Some designs have a draw sump located on the tray below the draw tray). Startup, in particular, can be a problem if vapor rates are not high enough to keep liquid on the draw tray. Use of a leak-resistant type draw tray, e.g. a bubble cap tray, may be preferred for a downcomer trapout for a total draw or a partial draw where most of the liquid is removed. As shown in Section 1.06, downcomer trapouts are commonly used to withdraw liquid to a reboiler. A chimney tray can provide residence time for vapor disengagement so that a relatively clear liquid is withdrawn. It also provides surge volume for smoother column control. By means of gasketing or seal welding, a chimney tray can be constructed for no leakage. Disadvantages of a chimney tray are that it increases tower height and cost. Some designers install vortex breakers on intermediate liquid drawoffs. Vortex breakers are discussed in Section 4.04.

2

Downcomer Trapout Partial Draw - Figure 1 shows several designs for downcomer trapouts with a partial draw from a side downcomer. Design "A" is generally preferred. This design relies on an overflow from the draw sump to provide a downcomer seal. Normally the draw sump is a full segmental pan the same width as the downcomer. By installing a draw box under the draw sump as in Design "A1" it is possible to gain additional head over the nozzle with only a small loss in tower free area. This draw box is normally a cube slightly larger than the diameter of the nozzle. A draw sump smaller than a full segmental is sometimes used when only a small fraction of the liquid is withdrawn. A disadvantage of Designs "A" and "A1" is that the downcomer may lose its seal if its liquid height is too low. This could occur as the result of a draw rate which is too high, leakage through the tray or draw sump, an upset condition, etc.

Page 2 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Design "D" is similar to Design "B". Increased tray spacing may be required because of a large downcomer backup caused by the inlet weir. With sieve trays, this design also requires a larger calming zone at the tray inlet in order to eliminate precipitant weeping at the inlet row of perforations.

A double pan arrangement as in Designs "E" and "F" provides a positive downcomer seal. These designs may also improve degassing of the liquid withdrawn. Loss of tray bubbling area is a potential disadvantage. Figure 2 shows a downcomer trapout design for a partial drawoff from an intermediate downcomer. This design is similar to Design "A" for a tray with a side downcomer. Intermediate downcomer designs similar to side downcomer Designs "B" - "F" are possible.

On two-pass trays, drawoff from a center downcomer is preferred to drawoff from the two side downcomers as this results in a simpler design for the draw piping. Partial drawoff from a three-pass tray is not recommended because it is very difficult to design a draw system which will provide the same ratio of overflow to drawoff at both downcomers. With four-pass trays, drawoff from a tray with two off-center downcomers is preferred as the design difficulties on the alternate tray are similar to those on a three-pass tray. Total Draw - It should be noted that it is unlikely that a downcomer trapout will give an absolutely total draw. There is likely to be some of weeping through even a leak-resistant tray. Some designs have drainholes in the draw sump so that it will drain completely on shutdown. Nonetheless, the amount of weeping may be small enough that such a draw could be considered essentially "total" for many services. If an absolutely total draw is required, a seal welded chimney tray or an internal head should be used. An example is a multi-service tower where the liquid introduced below the draw tray is a different material than the liquid taken from the draw tray. Figure 3 shows several designs for downcomer trapouts with a total draw from a side downcomer. It is good practice to provide the trapout with a provision for overflow to permit operation if the draw becomes restricted. The overflow provision must be made in such a way that the downcomer is liquid sealed. Designs "A" and "B" are preferred. Design "C" is similar to partial draw Design "C". This design needs a stagnant liquid pool to provide the downcomer seal. This may lead to fouling or solids deposition in some cases. Design "D" is the simplest and it does not require as much tower height as the other designs do. However, it does not have an overflow provision. Tray damage can occur if liquid builds up on the draw tray because the draw is restricted or if a liquid-filled column is drained from the base. Figure 4 shows two designs for a downcomer trapout with an intermediate downcomer. Design "A" is preferred as it provides an overflow. Design "B" is analogous to Design "D" for a side downcomer.

On two-pass trays, drawoff from a center downcomer is preferred to drawoff from the two side downcomers as thisresults in a simpler design for the draw piping. Similarly, with four-pass trays, drawoff from a tray with two off-center downcomers is preferred. It is possible to pump a portion of the draw from a total draw trapout directly if the piping design assures that flow preferentially goes to the pump. It is usually not practical to pump the entire flow directly from a total draw downcomer trapout because the residence time is small and there are exposures to poor control pulling the sump dry and damaging the pump. However, some designers have successfully used trapout Design "A" to pump a total draw by enlarging the draw sump and adding a level controller.

Page 3 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Summary of Design Consideration for Downcomer Trapouts

1. Partial or total drawoff of liquid may be withdrawn as a side stream product or as a pumparound stream. 2. Drawoffs from downcomers are the most common and require lower investment than chimney tray drawoffs. 3. Choice between partial or total drawoff with pump-back depends on tower control and flow stability of the internal reflux. 4. Partial drawoff rate should not exceed 70 % of the sum of the internal reflux rate plus product rate at the drawoff tray (pumparound stream rates not included). This is important because control and stability of the tray section below is affected more by changes in the drawoff rate; a total drawoff with a pump-back metered internal reflux rate eliminates this concern. 5. Drawoff arrangements without a seal pan may not provide a downcomer seal and may cause premature downcomer flooding. These arrangements are not as effective in disengaging vapor from the liquid. They are generally acceptable for drawoffs to side-stream strippers and disengaging drums. They are generally not acceptable for drawoffs that are pumped. This type of arrangement may be acceptable for pumparound streams drawn from low pressure (less than 50 psia (350 kPa)) towers where vapor disengaging in the downcomer is rapid. 6. Drawoff arrangements with a seal pan generally occupy more of the tray cross-sectional area, therefore reducing the tray active area and capacity. The effect of reduced active area may be critical with high vapor rates and relatively low liquid rates and should be evaluated. The use of sloped downcomers minimizes the reduction in tray active area. 7. Vapor disengaging and drawoff box design are especially important in high pressure light ends towers and foaming services, where the surface tension is low, the liquid rate is usually high, small bubbles are formed and vapor/liquid separation is difficult. 8. Avoid use of drawoffs with internal piping. Heat transfer effects on the surface of the piping need to be evaluated to avoid flashing the drawoff liquid. Insulation of the internal piping may reduce or prevent flashing. When this type of drawoff must be used, the drawoff internal piping and nozzle should be sized for self-venting flow. (See Section - Total Draw, page 1.05.1-5) 9. Minimize the liquid holdup time for thermally sensitive or unstable materials.

3.

Chimney Tray Figure 5 shows several chimney tray designs. With a partial draw, overflow into a downcomer or downpipe conveys liquid from the chimney tray to the tray below. With a total draw tray, provision for overflow is not required. An overflow provision may be justified, however, if it allows the tower to continue to operate in case of a draw restriction. Liquid overflow into the vapor risers can lead to entrainment and premature tower flooding. The overflow downcomer should be liquid sealed to prevent vapor rise through it.

A chimney tray should be used when: 1. 2. 3. 4.

Appreciable liquid holdup is required, as for product surge or for water settling. Leakage cannot be tolerated for process reasons. A drawoff box would occupy excessive tray cross-sectional areas. A partial drawoff rate is greater than 70% of the total liquid rate on the drawoff tray (Pumparound stream rates not included). 5. A liquid drawoff is required in a tray section with some proprietary tray designs. 6. A liquid drawoff feeds a mid-column thermosiphon reboiler. The downcomer must be designed for the total liquid flow during startup before the side boiler is in operation.

Page 4 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Use of a draw sump is preferred to a nozzle flush with the chimney tray floor as this provides additional head above the outlet nozzle without increasing the riser height. Side draw sumps are shown in Figure 5. A center sump is often used in larger diameter towers. Care must be taken in the design of the downcomer from the tray above the chimney tray. If that downcomer is sealed with a seal pan as in Designs "A" and "B", fall of liquid from the pan may cause splashing and frothing on the chimney tray. Minimizing the liquid fall will minimize these effects. Extending the downcomer to the tray deck as in Design "C" can result in downcomer flooding due to excessive backup as the fluid in the downcomer is aerated to a greater degree than the liquid on the tray and requires a greater depth to produce the same hydrostatic pressure. The number and arrangement of the vapor risers should be selected on the basis of providing proper vapor distribution at a reasonable cost. Risers may be round or rectangular. Rectangular risers are less expensive to fabricate. In trayed towers, the total riser area is typically 15 to 25% of the tower area. The annular area between the top of the riser and the hat should be equal to or greater than 1.25 times the riser area. A minimum spacing of 18 inches (450mm) between the hat and the tray above is recommended.

4.

Draw Nozzle Setting Partial Draw - With a partial drawoff from a downcomer trapout or a chimney tray, a liquid level is maintained above the draw nozzle and the draw piping is liquid full. The liquid withdrawn will generally be a bubble-point liquid and a conservative design approach is to design the draw piping such that the pressure at all points is greater than the source pressure to avoid flashing/degassing at any point in the draw piping. Then the point where the draw piping turns downward (point P1 of Figure 6) becomes controlling for nozzle sizing.

An energy balance requires that:

P 0 − P 1 ρ

+

V 02

2

− V 1

2 g c

+

( Z 0 − Z 1 ) g g c

− E =

0

The losses (E) include an abrupt contraction at the nozzle and friction losses in the draw piping. The contraction loss is calculated as half a velocity head. For P 1 = P0 and V0 assumed to be essentially zero, the minimum required head above the centerline of the nozzle is:

h=

1.5(V 1 ) 2 2 g

+ Δ P f

The available head above the centerline of the draw nozzle must be greater than the required head. The available and required heads are compared in terms of height of clear liquid. With downcomer trapout Designs "1A" - "1C" , the head available is the downcomer backup plus the liquid head in the draw sump. Downcomer backup is usually calculated in height of clear liquid. Since the liquid withdrawn is aerated, the height in the sump is multiplied by an aeration factor to convert to a clear liquid head. An aeration factor of 0.4-0.5 is commonly used. Calculation of the required head is based on an aerated liquid rate (clear liquid flow rate divided by the aeration factor) and this calculated head is multiplied by the aeration factor to convert to a clear liquid head.

Page 5 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

With a chimney tray, the head available will be set by height of the overflow weir. A chimney tray generally provides adequate residence time for deaeration and there may be a significant density gradient across the height of liquid with an essentially clear liquid at the draw nozzle and a frothy liquid spilling over the overflow weir. This variation needs to be taken into account when comparing the available and required heads. Total Draw - Under some conditions, a total draw from a downcomer trapout can result in unstable flow as the liquid falling in the vertical draw piping siphons the draw box level down, trapping slugs of vapor which results in surging flow. Surging flow should generally be avoided and it can be avoided by designing the piping for free-fall self-venting flow. Experimental work has shown that free-fall self-venting flow occurs when (74):

( N Fr ) l =

V l gD

ρ l ρ l − ρ g

≤

0.31

If the liquid density is much greater than the vapor density, the density term can be dropped from the equation giving the following equation for the minimum pipe diameter for free-fall self-venting flow:

D = 0.0765(Q ) 0.4 D = 1.115(Q ) 0.4

US Eng. Units SI Units

If the outlet nozzle is sized for free-fall self-venting flow, the nozzle entrance loss and friction losses will be very small. When designing drawoff boxes and sizing drawoff nozzles one must consider the dimensions of the drawoff box (depth and length) the number of nozzles from the center downcomers and drawoff nozzle sizes. Nozzle sizing procedure for a total draw from a chimney tray will depend on whether a controlled level is held on the tray. If the level is controlled by an external valve and level controller so that the draw box is not pulled dry, the draw nozzle and piping are sized such that the head available at the lowest controlled liquid level is sufficient to overcome the nozzle entrance loss and friction losses as with a partial draw. The allowance for aeration need to reflect the reduced residence time at that level. If the liquid level on the chimney tray is not controlled, the draw may be subject to unstable flow as described above for a total draw from a downcomer trapout and the outlet nozzle should generally be sized for free-fall self-venting flow. Other Designs Considerations 1.

A vortex breaker may be needed at the entrance of the drawoff nozzle see Section 4.04 in the FRI Design Practices Manual.

2.

Vent lines for drawoff lines (when to provide vent lines; where to locate vents; can be problematic; provide block valves to commission or decommission as required).

3.

Water drawoffs may be needed for processes such as absorbers, deethanizers, and oil/water separation inside or outside the tower.

4.

The length of horizontal drawoff piping immediately following the drawoff nozzle should be minimized between the nozzle and the first vertical downturn. The pipe diameter of the first Page 6 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

vertical downturn should be sized to be self-venting and be at least 10 ft (3 m) in length or 10 pipe diameters (whichever is larger) before it can be swaged or contain a horizontal run. 5.

The corners on drawoff boxes may be tapered to maximize free area below.

6.

Support beam interference with the tray/downcomer below should be minimized or eliminated.

7.

Seal pan weir horizontal stiffener interference should be minimized at entrance to drawoff boxes. See the comments under Partial Drawoff section in the paragraphs discussing Figures 2 & 4.

8.

Be sure to consider foaming characteristics and aeration factor when sizing the drawoff nozzle and sump box. The drawoff nozzle diameter should be increased in proportion to the foaming factor for foaming services.

9.

Seal welding drawoff boxes and sumps is recommended to prevent leakage.

10. Keep in mind the effect of drawoff internals on reduced tray bubble area and free area. 11. It can be difficult to design drawoffs from multiple flow pass trays to obtain the correct liquid flow rate. See the comments in the Downcomer Trapout section discussing Figure 2. 12. The drawoff box should drain completely at shutdown. 13. Avoiding splashing/carryover of pumparound return liquid into side stream drawoff boxes. 14. The orientation of drawoff nozzle and box should be such that it minimizes the exterior piping in the case of multiple drawoff nozzles. On the interior of the tower they should be arranged so as not to interfere with the downcomers or vapor passage and should not require interior piping. 15. Avoid dumping liquid into drawoff boxes as this may cause aeration of the liquid in the drawoff box which can cause choking of the drawoff nozzle. 16. The preferred orientation for the long side of rectangular chimneys is parallel to the liquid flow to the sump.

Page 7 of 14

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1.05.1

Nomenclature

Units D DCW E g g c h (N fr )l Q P TS V V' Z Δ p ρ g ρl

Inside diameter of draw pipe Downcomer width Friction or head loss Gravitational constant Coversion factor Head relative to centerline of draw nozzle Liquide Froude number, dimensionless Liquid rate (hot, nonaerated) Pressure Tray spacing Superficial velocity Clear liquid superficial velocity Elevation Friction loss Gas density Liquid density

US Engr. ft ft ft-lb f lb 32.17 ft/s 2 32.17 ft-lb/lb f -s2 ft

SI m m m-N/kg 9.8066 m/s2 1kg-m/N-s2 m

US gpm lbf /ft2

m 3/s Pa

ft/s ft /s ft ft lb/ft 3 lb/ft 3

m/s m/s m m kg/m3 kg/m3

References

1. Andersen, A. E., and Jubin, J. C., "Case Histories of the Distillation Practitioner", Chemical Engineering Progress, Vol. 60, No. 10, October 1964, pages 60 - 63. 2. Fleming, B., and Sloley, A. W., "Feeding and Drawing Products: The Forgotten Part of Distillation", Paper presented at the 95 Chem Show & 46th CPI Exposition, New York, December 4-7, 1995. 3. FRI Fractionation Tray Design Manual, Volume 5 - Design Practices, Section 1.05, January 15, 1994. 4. Kister, H. Z., Distillation Operation, New York: McGraw-Hill, Inc., 1990, p. 103-114. 5. Kister, H. Z., "Outlets and Internal Devices for Distillation Columns", Chemical Engineering , July 28, 1980, pages 79 - 83. 6. Simpson, L. L., "Sizing Piping for Process Plants", Chemical Engineering , June 17, 1968, p. 204. 7. Sloley, A. W., "Don't Get Drawn Into Distillation Difficulties", Chemical Engineering Progress , June 1998, p.63-78

Page 8 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Note 1, Seal Pan Drain Hole A 1/2in (13mm) drain hole is needed to assure that the seal pan completely empties upon shutdown. Note 2, General Dimension Guideline (See Ref. 4) d - Inside nozzle diameter. DCW - Downcomer width. C - Clearance under downcomer. S - Horizontal distance between the downcomer panel and seal-pan weir. S = C + 1/2in (13mm). TS - Tray spacing. W - Seal-pan weir height. W = 2 in (50mm)(minimum). X - Height between sump box bottom and seal- pan bottom. X = 1.5d to 2d or d + 2in (50mm) which ever is larger. Y - Height between seal-pan bottom or tray deck and the tray inlet weir or drawoff box overflow weir. 3C + 1 in (25mm). Z - Horizontal distance between the seal-pan weir and the sump wall should be ≥ 4 in (100mm). Figure 1. DOWNCOMER TRAPOUT - PARTIAL LIQUID DRAWOFF FROM SIDE DOWNCOMER

Page 9 of 14

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Note 1, Seal Pan Drain Hole A 1/2in (13mm) drain hole is needed to assure that the seal pan completely empties upon shutdown. Note 2, General Dimension Guideline (See Ref. 4) d - Inside nozzle diameter. C - Clearance under downcomer S - Horizontal distance between the downcomer panel and seal-pan weir. S = C + 1/2in (13mm). W - Seal-pan weir height. W = 2 in (50mm)(minimum) X - Height between sump box bottom and seal- pan bottom. X = 1.5d to 2d or d + 2in (50mm) which ever is larger. Y - Height between seal-pan bottom or tray deck and the tray inlet weir or drawoff box overflow weir.. 3C + 1 in (25mm). Z - Horizontal distance between the seal-pan weir and the sump wall should be ≥ 4 in (100mm). Figure 2. TRAPOUT – PARTIAL LIQUID DRAWOFF FROM A CENTER DOWNCOMER

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1.05.1

Figure 3. DOWNCOMER TRAPOUT - TOTAL LIQUID DRAWOFF FROM SIDE DOWNCOMER

Figure 4. DOWNCOMER TRAPOUT – TOTAL LIQUID DRAWOFF FROM CENTER DOWNCOMER

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1.05.1

Notes:

1. The downcomer is sealed by a seal pan. 2. The downcomer is sealed by the overflow weir which may result in excessive downcomer backup. 3. The backup in the downcomer to the chimney tray should be checked considering the difference in aeration and the head balance. 4. For wide risers, the distance between the bottom of the tray and the top of the hat should be at least ½ of the hat width.

Figure 5. LIQUID DRAWOFF FROM CHIMNEY TRAY

Page 12 of 14

Issued: 1/15/1994 Revised: 10/1/2006


(1)

Design D Calming Section (Multi-downcomer) Tray Partial Drawoff Notes: 1. Design D provided by courtesy of Shell Global Solutions (US) I nc.

Figure 5 (Cont.). LIQUID DRAWOFF FROM CHIMNEY TRAY

Page 13 of 14

1.05.1

Issued: 1/15/1994 Revised: 10/1/2006


1.05.1

Design A(1) Note: 1. Design A picture provided courtesy of Sulzer Chemtech USA, Inc. Figure 6. TYPICAL GAS RISER WITH COVER

Note: 1. A vortex breaker (one tier of grating) should be installed in the drawoff box above the drawoff nozzle. Refer to FRI DPM Section 4.0.4 Figure 7. AVAILABLE HEAD FOR PARTIAL LIQUID DRAWOFF

Page 14 of 14


REBOILER CIRCUITS FOR TRAYED COLUMNS

Issued:

10/01/2006

1.06

Revised:

REBOILER CIRCUITS FOR TRAYED COLUMNS

Reboiler Circuits for Trayed Columns .....................................................................................1 1. Introduction .......................................................................................................................... 2 2. Preliminary Work By Designer ................................................... ......................................... 2 3. Selection of Reboiler Type ................................................................................................... 3 4. General Considerations For Reboiler Selection ................................................................... 3 5. Reboiler Selection by Process Issue ........................................... .......................................... 5 6. Tower Bottom Arrangements ................................................ ............................................... 8 7. Reboilers and Tower Elevation ................................................ .......................................... 14 8. Bottom Section Design Notes .............................................. ............................................... 15 9. Things to Avoid ................................................. ................................................................. 19

Page 1 of 52


1

REBOIL ER CIRCUITS FOR TRAYED COLUMNS

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Introduction

The reboiler generally supplies most of the energy required to effect component separation. If too much heat is supplied, the tower will flood; conversely, if too little heat is available, separation performance decreases via poor reflux ratio (pinching), excessive weeping or poor tray action. Proper design of reboiling systems involves coordinating aspects both outside and inside the tower. This Design Practice focuses on both of these aspects to ensure proper operation of the overall reboiler system. Important literature discussing these topics is also cited. The design of tower reboiler circuits and bottom sections can be broken into several parts: fluid flow systems, exchanger types, liquid sumps and associated baffling, drawoff arrangements and return arrangements. Although these are not generally studied as much as the mass transfer equipment above, tower bottom sections should be considered key tower internals. A number of fractionator problems can be attributed to either improper bottom section design or poor reboiler circuit layout; taken together, they are thought to be the second-most common cause of tower problems. To better illustrate this, consider the objectives a properly designed tower bottom and reboiler system must accomplish: • • • •

• • • •

• • • • •

Provide adequate hydraulics for the reboiler circuit. Separate and distribute the incoming vapor and liquid phases properly. Absorb fluid momentum and prevent mechanical damage to surrounding internals. Prevent entrainment of bottom tray overflow liquid or bottom pool liquid by reboiler return fluids. Provide adequate liquid inventory for startup and step changes in reboiler duty. Provide sufficient liquid residence and degassing time for downstream equipment. Provide adequate net positive suction head (NPSH) for any bottoms pumps. Maximize mass transfer capabilities of the reboiler (nearly one theoretical stage can be obtained via use of a staging baffle). Maximize available temperature driving force in the reboiler. Accommodate transients in the concentration of heavy components in the feed. Allow removal of fouling material from the tower bottom. Allow safe column shutdown in the event of a process upset. Minimize overall column height.

Thus many factors come into play in designing a successful reboiler and tower bottom arrangement. The available literature does not organize the process well and, especially, does not contain much information about multipass trayed towers. This Design Practice section is intended to clarify the design process and extend coverage to multipass towers.

2

Preliminary Work By Designer

The design process begins by definition of the design basis and selection of the reboiler type that most suits the particular application. ‘ Reboiler type’ refers to exchanger and circulation type, such as vertical thermosyphon, kettle, internal, horizontal forced circulation, etc. The designer proceeds through the following steps: • •

•

Run tower simulation(s). Determine reboiler type and method of process liquid circulation, using criteria from Selection of Reboiler Type and Tables 1 and 2. Select tower bottom configuration (such as once-through, constant head recirculating, etc.) using

Page 2 of 52


•


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criteria from the Tower Bottom Arrangements section and Table 3. Choose limiting case(s) for duty and bottoms product rate to be used in detailed design.

After this, the designer would be ready to begin developing details of the tower bottom and reboiler circuit.

3

Selection of Reboiler Type

As noted above, selecting a reboiler type means determining the method of fluid circulation (thermosyphon, forced or none) and selecting an exchanger type (vertical, horizontal, kettle or internal). These decisions need to be made before any tower bottom internals can be designed, since internals vary substantially for different reboiler types. To aid in the selection process, considerations are discussed below starting with general rules of thumb and progressing to more specific issues. Simple examples of common reboiler types are shown in Figure 1, and a comparative summary of reboiler types is given in Table 1. Note that the figures are mainly conceptual, so a Fractionation Specialist should be consulted to review the detailed design of any particular system.

4.

General Considerations For Reboiler Selection

It is typical to consider thermosyphon systems first to see if one can meet the process requirements. Thermosyphon reboilers are the most widely used type in distillation systems. The name ‘thermosyphon’ stems from the fact that they utilize the density difference between liquid in the tower bottom and mixedphase fluid in the reboiler and return line to drive reboiler process flow; in other words, they are gravityflow systems. To summarize their advantages, they are relatively compact and economical, require no pumps, and offer relatively high heat transfer rates (small exchanger size) with relatively low residence times in the heated zone. Of course, thermosyphon systems are not applicable in all circumstances. They are not recommended for the following conditions: • • • • •

High liquid viscosity (viscous loss dampens f luid circulation). Fouling systems (pumped systems achieve higher velocities to help mitigate fouling). Adequate driving head cannot be attained economically (consider a kettle system). Large operating load variations or turndown ratios are required (consider a pumped system). High reliability is a key factor (kettle or forced circulation systems are preferred for this).

Sometimes thermosyphon reboilers can be troublesome with vacuum systems because the large volume of vapor can sweep liquid from tube walls and reduce heat transfer in the exchanger. This is especially true for services where the liquid boiling range or circulation driving head varies routinely. However, if the driving head is kept steady, a reliable vaporization curve is available, and care is taken in modeling the hydraulics, thermosyphon reboilers can be successfully used in vacuum service. Most existing vacuum thermosyphon systems tend to use vertical exchangers. Some discussion about what constitutes high liquid viscosity is warranted here. One source lists 25 cP (1) (25 mPas) as the cutoff point for thermosyphon flow while another lists 0.5 cP (2) (0.5 mPas) as the maximum for tubeside flow in a vertical exchanger. All other known sources avoid the issue by simply listing “high liquid viscosity” as a limitation of thermosyphon systems – which is not very useful. Examination of some assumed hydraulic parameters for a typical hydrocarbon thermosyphon system (Appendix 1) indicates appreciable flow resistance begins to occur at about 3 to 4 cP (3 to 4 mPa s), so this is the recommended viscosity range where forced circulation should begin to take precedence over thermosyphon circulation. Page 3 of 52



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TABLE 1 Comparison of Reboiler Types (Adapted from Ref. 3) Vertical Thermosyphon (Figure 1a)

Horizontal Thermosyphon (Figure 1b)

Kettle (Figures 1e & 1f)

Forced Circulation (Figure 1c)

Internal Reboiler (Figures 1g & 1b)

Boiling side

Typically tube

Typically shell

Shell

Typically tube

Shell

Heat transfer rate

High

Moderately high

Low to moderate

High

Low to moderate

Plot space requirement

Small

Large

Large

Vertical: small Horizontal: large

Minimal to small

Process piping

Small quantity and simple to design

Standard quantity, 2-phase return

Standard quantity, single phases only

Extra piping w/2 phase & controls

None

Pump required

No

No

No

Yes

No

Extra column skirt height requirement (if bottoms product is not pumped)

Yes – to accommodate vertical exchanger

Yes – to drive thermosyphon No flow (but less than vertical t’syphon)

Yes – to provide reboiler circulation pump NPSH

No

ΔT requirement

High – inside tube walls exposed to vapor at top

Moderate

Low

High

Moderate to high

Low

Low

High

Low

High

Low

Moderate

High

Very Low

Moderate

Poor

Poort (but better than vertical t’syphon)

Poor

Good

Poor

Good, if multiple large shells used

Very Good: Large areas handled in a single shell

Good, if multiple large shells used

Relatively easy

If vertical, can be difficult depending on congestion (easy if horizontal)

Poor, unless tower bottom swaged out for larger bundles Next to impossible while on-stream, but easy during shutdowns

Residence time in heated zone Process side fouling tendency Performance with high viscosity liquids Ability to handle large surface area

Modest: approx 4 medium shells max

Maintenance and cleaning

Can be difficult, depending on congestion

Susceptibility to instability

High, but moderate for constant head

Design data Capital cost Operating cost, excluding heating medium

Safety issues

Relatively easy

Low

Low

Low

Readily available

High, but moderate for constant head Some available

Readily available

Readily available

Low

Moderate

High

Moderate

Readily Available Very low, unless tower swage out for larger bundles

None

None

Pumping cost, and occasional pump None maintenance cost

Normal

Normal; kettle exchanger can hold liquid inventory to help in emergency shutdown

Pump seal leakage is important for flammables/toxics

None

Normal

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Flange leakage is a major concern, especially for flammables/toics



1.06

If one of the above thermosyphon limitations applies, a kettle or forced circulation system is generally chosen. For towers in clean services, an internal reboiler might be considered. The chief advantage of this type is low capital cost due to savings on an exchanger shell, external piping and internal tower baffling. However internal reboilers have a number of limitations, which are listed later. A special arrangement applicable to high vacuum conditions in clean services is the falling film reboiler (Figure 2). These can be considered a hybrid between forced and gravity circulation. Falling film reboilers are vertical units where the process fluid is pumped onto the top tube sheet and flows downward as a thin film on the inside walls of the tubes. Boiling essentially starts immediately and the generated vapor flows downward in the inner space of the tubes. The two phases collect in the lower channel and are conveyed to the column via carefully designed means, generally a specially designed bottom channel. Falling film reboilers are often used with very heat sensitive materials, in viscous services, vacuum services, and with wide boiling mixtures. The only suppression of vaporization is the slight pressure increase from the column draw to the top tube sheet. There is minimal liquid head to overcome. The bulk temperature of the liquid in the reboiler essentially never exceeds the reboiler outlet temperature. Falling film reboiler systems are therefore extremely flexible, but they are also expensive, requiring a specially designed heat exchanger as well as the equipment any forced circulation system would have. One critical item needing special attention is the method for assuring that each tube is properly fed. If a thermosyphon system were applicable, the next decisions would be to determine the flow and exchanger type. The choices for flow type are once-through and recirculating; the choices for exchanger type are vertical and horizontal. Once-through flow is useful for strippers and other low-boilup services where the mass flowrate of vapor in the reboiler return is less than about 40% of the bottoms product mass flowrate (4). Recirculating flow is required in services where reflux rates are high compared to product rates, such as splitters. More information about once-through vs. recirculating reboilers is given in the Tower Bottom Arrangements section below.

Selection of a vertical or horizontal exchanger can be made based on each option’s advantages and disadvantages as given in Table 2. To supplement this, additional information about preferred exchanger types is given in the next section, ‘Reboiler Selection by Process Issue,” below. Note that published literature gives conflicting accounts about vertical vs. horizontal exchanger usage in the process industries (5, 6 and 7) , leading to some confusion about selection practices. A particularly clear discussion of factors affecting this decision is given by Sloley (8), who goes on to explain that vertical exchangers predominate in chemical applications, while horizontal exchangers are prevalent in refining applications.

5

Reboiler Selection by Process Issue Fouling Service

Fouling service, as used herein, refers to a fouling tendency of process fluid in the tower bottom rather than the heat transfer medium. The preferred bottoms arrangement for fouling service is forced circulation, which falls under the classification “Preferential Baffle” below. Forced circulation systems (using a pump) can achieve much greater reboiler circuit velocities than thermosyphon systems, which aids in keeping exchanger tubes clean. The forced circulation system can include vertical or horizontal exchangers, so long as the fouling fluid is allocated to the tubeside (this is more typical of vertical exchangers). If a forced system is not suitable, the next best alternative is considered to be a vertical thermosyphon system. Kettle and internal reboilers should be avoided due to long residence times in the heated zone and high vaporization rates.

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Revised:

1.06

TABLE 2 Reboiler Exchanger Comparison: Horizontal vs. Vertical Advantages Vertical

Horizontal

Disadvantages

Minimal plot space requirements

Exchanger area is limited

Return piping typically short to very short

Greater skirt height requirement

Relatively small capital cost

Requires relatively high ΔT driving force

Good for large exchanger area requirement

Occupies moderate-to-large plot space

Requires moderate ΔT driving force

Higher capital cost than vertical

Better access for maintenance

Returns piping design must avoid slug flow

Often requires less tower or skirt heigh

Vacuum Systems

These can be a problem for thermosyphon systems because the large volume occupied by vapor canreduce liquid contact area in the reboiler exchanger, leading to poor heat transfer. Thermosyphon driven reboilers can be successfully used in vacuum service, but the hydraulics must be carefully studied by a competent designer. Small errors in predicted friction losses or hydrostatic head above the exchanger can lead to large errors in the vaporization percentage and return line fluid density, which can render these systems inoperable. Forced circulation systems are easier to design for low-pressure services. One particular forced circulation setup for vacuum service is the suppressed-vaporization system ( Figure 1d), where the flow control valve is placed downstream from the reboiler exchanger (9). No vaporization occurs in the exchanger itself (sensible heat transfer only), so the heated liquid flashes as it traverses the downstream valve. Another way to suppress vaporization in forced circulation systems is to use an orifice at the column return nozzle. Note these valves or orifices can experience erosion as the liquid flashes across them under vacuum conditions, producing high exit velocities. For this reason, some practitioners recommend the use of control valves having contoured plugs, and some completely avoid suppressed vaporization systems. Also, the high fluid velocities produced by a valve or orifice at the tower inlet can cause fluid distribution problems or mechanical damage inside the tower unless specific provisions are made to handle these. With clean process fluids, the designer of a low-pressure system may wish to consider a falling film reboiler as described previously. Safety

Forced circulation systems involve pumps and often pump seals. The hazards of a seal leak should be considered, especially for flammable or toxic fluids. Thermosyphon systems eliminate pump seal leakage problems. Internal reboilers have large flange connections that may have substantial moment arms applied by the heavy tube bundles, if they are not supported properly. The flanges are prone to leak and have been known to cause fires. Ease of Maintenance

During shutdowns, access space (rather than reboiler type) is generally the most important factor for ease

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of maintenance. However, selection of the reboiler TEMA type also greatly affects maintenance of reboilers. For instance, the designer can specify exchanger inlet and outlet heads that allow the tubes to be inspected and cleaned without requiring removal of external piping. The reboiler can also be designed for easy tube bundle removal to facilitate inspection and mechanical cleaning or hydroblasting (10). Selection of the correct shell type is also very important to ensure proper fluid circulation, minimize fouling potential and maximize on-stream time. In services where on-line cleaning is necessary, internal reboilers should be avoided. In fouling services, a spare exchanger is often provided, but this is not practical with internal reboilers. If the process side is dirtier than the heating medium, a design that allocates process fluid to the tube side should be considered. Conversely, if the heating medium is dirtier, it should be allocated to the tube side. Typically, vertical exchangers have the process fluid on the tubeside, and horizontal exchangers have the process fluid on the shell side – although these are not absolute rules. For kettle and internal reboilers, however, process fluid is always on the shell side. Reliability

From a process standpoint, kettle reboilers are considered the most reliable, although vertical thermosyphon systems are also considered quite good. Forced circulation systems can be robust, but this really depends on the reliability of the pump. Horizontal thermosyphon systems and internal reboilers are considered average in terms of reliability. Stability When Perturbed

Forced circulation systems with flow control upstream of the exchanger are the most stable when subjected to tower swings, followed by kettle and internal reboiler systems. Vertical and horizontal thermosyphon systems are more sensitive to operating perturbations – however, use of a constant head baffle in the tower bottom design greatly improves their stability. Approach Temperature

For a given heating medium, once-through systems give the greatest cool end ‘approach temperature,’ or thermal driving force, in the reboiler exchanger; this is because the process side feed to the exchanger is comprised entirely of liquid from the bottom tray, which is the coolest possible reboiler feed. Conversely, recirculating systems with high tower reflux ratios provide the smallest driving force because a large percentage of the reboiler feed is material from the reboiler effluent. In cases where a high reflux ratio system is limited by reboiler approach temperature, a preferential baffle can help even if one would not be included for mass transfer purposes. As for exchanger types, vertical exchangers require the greatest driving forces, while kettle types require the least. Forced circulation systems allow for the greatest driving forces without concern for process side fouling because they can be designed with high process fluid velocities. Required Heat Transfer Area

Vertical reboilers are limited in tube length (see “Vertical Thermosyphon Systems” under “Reboilers and Tower Elevation,” below), and are also limited to about four shells per tower (11). Thus they cannot provide very large heat exchange areas. Horizontal and kettle reboilers are greatly preferred when large area is required. Internal reboilers can also limit available heat transfer area unless the tower has been increased in height or swaged out to accommodate more or larger bundles. Capital Cost

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Internal reboilers are typically the least expensive because they eliminate external process piping and reboiler exchanger shell(s), although in some cases this advantage is negated by bottom section height or diameter increases to accommodate larger heat transfer bundles. Vertical thermosyphon systems generally rank second in cost because the return piping is usually very short. Horizontal thermosyphon and forced circulation systems are considered moderately expensive. Kettle systems are typically the most capital intensive due to exchanger shell size and foundation requirements. Operating Cost

Ignoring the cost of the heating utility, thermosyphon systems have no operating costs due to the use of gravity acting on density differences to drive reboiler fluid flow. Forced circulation systems are more expensive to operate due to pumping and associated pump maintenance costs. Plot Space Requirement

Internal reboilers occupy very little plot space, followed by vertical exchangers, which generally require small plot spaces. Horizontal exchangers and kettle systems require relatively large plot spaces, especially if removable bundles are desired. Proper exchanger head selection can help minimize plot space requirements. It is beyond the scope of this practice to cover actual design of the reboiler circuit piping and exchanger(s). For thermosyphon and kettle systems, the flow through the reboiler must be calculated from a pressure balance. It is essential that accurate assessments be made of fluid densities, extent of vaporization and friction losses so that the correct flow driving force and resistances are used in the pressure balance. An article by Kern (12) describes the pressure balance particularly well and gives criteria for piping design. A comprehensive review of design correlations for vertical, horizontal and kettle exchangers is given by Fair (13) . Additional information about horizontal and vertical reboiler systems is contained in articles by Collins (14) and Orrell(15).

6

Tower Bottom Arrangements

This section discusses the relative merits and weaknesses of various tower bottom arrangements that feed the reboiler and provide residence time. The descriptions here pertain to internal features, such as baffles or drawoff configurations, which comprise the tower bottom design. A summary of this information is given in Table 3. Flow Classifications: Before describing the tower bottom arrangements in detail, it is useful to discuss the two primary flow classifications into which all bottom arrangements fit. Once-through systems are arrangements where liquid from the bottom tray traverses the reboiler only once; the liquid portion of the reboiler effluent is collected as net product and is kept separate from bottom tray liquid. Recirculating systems allow a portion of the reboiler effluent liquid to remix into the reboiler feed, thus permitting some of the liquid to traverse the reboiler two or more times. There are two main differences in these flow schemes: once-through systems give a full stage of mass transfer in the reboiler (the maximum available), but their boilup ratios are limited by the maximum vaporization rate available in the reboiler exchanger. Conversely, recirculating systems provide only a partial stage of mass transfer in the reboiler, but allow unlimited boilup ratios. Because liquid leaving the bottom tray is the coolest stream possible for reboiler feed, once-through arrangements also give the greatest cold-end approach temperature in the reboiler exchanger. They are also good for thermally polymerizing or fouling materials where it is desirable to avoid repeated contact with hot reboiler tubes.

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Unbaffled tower bottoms are the most common type of tower bottom arrangement. Note that there are several different reboiler types that lack baffle(s) in the tower bottom:

1. Once-through trapouts (sump liquid is reboiler return material, unmixed with bottom tray liquid) 2. Kettle systems (sump holds liquid for residentce time, but pre and post reboiler liquids do not mix) 3. Recirculating systems (bottom tray overflow mixes with reboiler return liquid) Further descriptions of the first two cases can be found in the respective sections below. The third item, unbaffled recirculating systems, is the primary subject of this subsection. Unbaffled recirculating systems are simple and inexpensive, which are the main reasons they are so widely employed. They are compatible with both thermosyphon and forced circulation reboilers. Figures 1a through 1d show simplified examples of unbaffled recirculating systems. Advantages: Simple design requires no baffle inspection or maintenance. Reboiler and product draws may be combined in a single draw nozzle. Like all recirculating systems, allows unlimited boilup ratios in the tower TABLE 3 Comparison of Tower Bottom Arrangements Type

Once-Through Trapout

Once-Through Collector

Preferential Baffle

Primary Advantages

Full theoretical stage. No bottoms recontact with hot reboiler tubes. Full theoretical stage. No bottoms recontact with hot reboiler tubes. Compatible with forced circulation. Preferred for forced circulation. Does not limit boilup ratio, although baffle unnecessary at high boilup ratios.

Constant Head

Thermosyphon flow stability during upsets. Does not limit boilup ratios.

Standard Kettle

Simple bottom configuration with vapor-only returns. Full theoretical stage.

Trapout Kettle

More product residence time available than standard kettle.

Unbaffled

Simple, low cost. Good for high boilup ratios.

Internal Pool

Low cost.

Internal Bath

Low cost. Nearly full theoretical stage.

Page 9 of 52

Primary Disadvantages

Boilup ratio limited to 40%. Gives highest reboiler outlet temperature. Incompatible with forced circulation. Chimney tray: fewer active tray(s). Partition baffle: reduction of bottoms product residence time. Partial theoretical stage only. For thermosyphon systems, operating perturbations can affect reboiler flow, prolonging upsets. Partial theoretical stage only. Constant head compartment(s) must be leak tight. Increased likelihood of reboiler fouling. High cost. Long residence time of bottoms material in heated zone. Precise exchanger elevation required. Product spends more time at max temperature than standard kettle. May require more height than standard kettle. For recirculating systes, gives lowest separation efficiency. For thermosyphon systems, operating perturbations can affect reboiler flow, prolonging upsets. On-stream cleaning nearly impossible. Bottom liquid level difficult to assess. Long residence time of bottoms material in heated zone. On-stream cleaning nearly impossible. Boilup ratios limited similar to once-through. Long residence time of bottoms material in heated zone.



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Weaknesses: Affords no mass transfer benefit; the reboiler simply becomes an enthalpy addition point. Thus it is the least efficient arrangement in terms of component separation. In thermosyphon applications, swings in the tower bottom liquid level can affect the reboiler circulation rate and duty, prolonging tower upsets. To counteract this, these systems may require a more responsive heating fluid control system than other types of bottom arrangements. These duty and control considerations apply only to exchangers with bare or low-fin tubes; exchangers with nucleate-boiling enhanced tubes provide much more stable heat transfer rates as circulation varies. Once-Through Trapout arrangements involve a total draw downcomer trapout (see Sect 1.05.1) to capture essentially all of the liquid leaving the bottom tray and feed it directly to the reboiler. Liquid in the reboiler return is directed to an unbaffled tower bottom, and is drawn solely as bottoms product. None of the bottoms liquid is recycled back to the reboiler, hence the name once-through. Figure 3 shows trapouttype configurations for single pass and two pass trayed towers. Trapout arrangements are generally limited to simple, single draw configurations; multi-draw configurations such as dual draws from two or four pass towers are better handled with chimney trays (see “Once-Through Collector” section, below). Trapouts are limited to use with thermosyphon flow systems because they do not provide sufficient residence time for a pump. Advantages: Can achieve one full theoretical stage of separation, if the trapout draw does not leak. The high elevation of the trapout draw generally provides good driving force for thermosyphon flow. Limitations: With a thermosyphon system, reboil vapor is limited to about 40% of the bottoms product rate, due to the normal limitation of 30% maximum vaporization (by weight) in thermosyphon exchangers (16). This makes once-through thermosyphon systems appropriate only for low-boilup systems such as strippers. Although use of forced flow could increase the boilup rate to about parity with the bottoms product rate, trapout draw systems suffer from a lack of liquid inventory to prevent pump cavitation. Thus a once-through collector system (see below) would be used for forced flow. Finally, in cases where the desired vapor boilup rate exceeds the bottoms product rate, a recirculating reboiler such as a preferential baffle arrangement (see below) must be used instead. Weaknesses: The trapout draw box must be carefully constructed to avoid leakage. Thermosyphon flow is not compatible with high viscosity liquids. Once-Through Collector systems remove the limitation of low liquid inventory inherent in oncethrough trapout systems (see above). They are essentially once-through systems with a means of collecting liquid to smooth out variations in flow to the reboiler caused by perturbations in tower operation. Once-through collector arrangements are compatible with both thermosyphon and forced flow systems. In forced flow applications, a liquid level control scheme must be added. As with once-through trapout systems, reboil vapor is limited to about 40% of the bottoms rate for thermosyphon driven flow and equal to bottoms rate for pump driven flow. For vapor boilup ratios greater than these, a recirculating reboiler such as a preferential baffle arrangement (see below) must be used. Figure 4 shows two variations of the oncethrough collector system: • •

Chimney tray collector ( Figures 4a, 4c and 4d) Partitioned bottom ( Figure 4b)

Both of these options have advantages and weaknesses. Chimney tray arrangements can provide plenty of residence time, but they take up height, so fewer active trays can be installed in a fixed tower height. Partitioned bottom arrangements can increase reboiler feed inventory without reducing tray counts as long as sufficient bottoms residence time is available on the product side of the baffle. But they provide less

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liquid flow driving head than trapout or chimney tray arrangements, and care must be exercised to ensure the baffle is leak tight. Preferential Baffle arrangements are recirculating systems that utilize a baffle in the tower bottom to segregate bottom tray overflow liquid from reboiler return liquid. An opening in the baffle allows some reboiler return liquid to flow into and mix with bottom tray liquid. Thus the reboiler draw preferentially contains bottom tray liquid, but also contains recirculated liquid to make up the additional reboiler flow demand. The liquid level on each side of the baffle is equal, except for a small differential from liquid flowing through the hole. Preferential baffles are also known in the literature as baffles with a large hole or baffles with underflow . They may be used with thermosyphon or forced circulation systems. Figure 5 shows various configurations for single pass and multipass trayed towers. In the multipass versions of Figures 5c and 5d, extra spacing should be provided between the bottom tray and seal pans to accommodate liquid backup caused by the short overflow notches on the seal pan weirs. Note that some companies do not believe the added complexity and expense of preferential baffles are justified by their performance benefits, and omit these baffles entirely (see also Limitations below). Advantages: The internal baffle does not need to be liquid tight. Gives more separation than an unbaffled arrangement, but less than a full mass transfer stage. Like all recirculating systems, allows unlimited reflux ratios in the tower. Limitations: Preferential baffle systems do not develop a full equilibrium stage for the reboiler. As the tower reflux ratio increases, the ratio of recirculated material to bottom tray liquid in the reboiler feed also increases, and the usefulness of the reboiler as a separation stage steadily drops. When the ratio of tower bottoms product to reboiler draw rate falls below 20% (e.g. splitting close-boiling components), a preferential baffle is considered no longer useful, and it should be omitted to provide an unbaffled bottom arrangement. For towers in such splitting services, it is generally better to add a tray to the tower than install a bottom baffle to utilize the reboiler for separation. An additional limitation for thermosyphon driven systems is that they cannot handle high viscosity liquids. Weaknesses: The weaknesses of preferential baffle systems are similar to those for unbaffled towers. For thermosyphon driven flow, a change in the bottoms liquid level will affect the reboiler circulation rate, and thus the reboiler duty, causing the tower profiles and separation performance to swing. Even conditions or events downstream of the tower that cause product inventory changes can affect tower operation with a preferential thermosyphon reboiler. Some preferential thermosyphon systems have been known to work well only at one particular liquid level. It can be seen that all of these issues are related to duty control, and preferential baffle arrangements may therefore require responsive control schemes on the heating medium. As mentioned previously for unbaffled tower arrangements, use of nucleate-boiling enhanced reboiler tubes can mitigate these control issues. Constant Head arrangements are recirculating systems that maintain a constant depth liquid pool above the reboiler draw. The most common configuration has a partition baffle which separates the tower bottom into product and reboiler draw compartments ( Figures 6a, 6b, 6d and 6g). Liquid from the bottom tray is directed into the reboiler draw side, as is liquid from the reboiler return. Then, return liquid in excess of the reboiler draw requirement spills over a weir to the product side, where the level can be varied to provide rate control to downstream equipment. Constant head partition baffles are also referred to in the literature as baffles with overflow. Other constant head configurations include chimney tray and collector box configurations (Figures 6c, 6e and 6f ), where an inventory of liquid is kept inside the tower above the bottom liquid pool, using a tray or box with an overspill weir. These alternative arrangements generally provide less liquid holdup than the partition baffle, although they may be less expensive to build.

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The tower bottom should be designed to make the bottom tray liquid pass through the reboiler at least once before proceeding to the product compartment. Constant head arrangements are used only with thermosyphon circulating systems. Extra height should be provided between the bottom tray and the seal pans in certain multipass versions ( Figures 6d and 6g) to accommodate liquid backup caused by short overflow notches on the seal pan weirs. Advantages: Changes in product rate or level do not affect the reboiler circulation rate and duty, thus uncoupling the tower from minor downstream events. Like all recirculating systems, unlimited reflux ratios are allowed in the tower. Limitations: Baffle, tray or box leakage must be less than bottoms product rate, thus construction quality becomes more important as the tower reflux ratio increases. Thermosyphon circulation is not compatible with high viscosity liquids. Weaknesses: Constant head systems generally require more internal pieces and better workmanship than other bottom arrangements. They also often require more tower height than other options. Because the bottom tray and reboiler return liquids are both directed to the reboiler feed compartment(s), constant head systems can collect fouling products or nonvolatile components in the reboiler loop. The reboiler feed piping should have means to drain these materials at low points. Kettle arrangements appear deceptively simple from a process standpoint. Liquid from the bottom tray of the tower is drawn and directed to a kettle reboiler exchanger. The kettle exchanger is a special type of heat exchanger which has a tube bundle immersed in a liquid bath, with substantial vapor disengaging space above the bundle. Vapor and liquid are separated in the exchanger’s disengaging space, so the return line carries vapor only. Kettle arrangements are once-through systems; reboiler effluent liquid does not recirculate or back-mix with bottom tray liquid.

Kettle reboilers are typically designed with an overflow weir, which creates a separate product compartment within the exchanger shell. Kettle designs with overflow weirs must have removable tube bundles (U-tube bundles or TEMA “S” or “T” type return heads). Some alternative kettle designs do not have overflow weirs; in this case the liquid bath is maintained via level control. Fixed tubesheets (nonremovable tube bundles) may be used in this type of exchanger. There are three types of kettle arrangements. The first or standard arrangement is most prevalent (Figure 1e). It collects bottom tray liquid in the tower bottom and feeds a kettle exchanger having an internal weir. No level control scheme is necessary on the tower bottom because the liquid level in the tower is governed by the weir elevation in the kettle exchanger. However level control is required on the bottoms product compartment of the exchanger. The second type of kettle arrangement utilizes a trapout draw from the bottom tray, or a chimney collector tray, to feed a kettle exchanger with an internal weir ( Figure 1f ). Product overflowing the exchanger bath weir is then routed back to the tower bottom where it is collected for residence time purposes. In this case, level control is placed on the tower bottom rather than the exchanger product compartment. The trapout version typically requires more tower height in the bottom section because liquid must flow back from the exchanger to the tower sump. The kettle reboiler elevation also tends to be higher for these systems. Figure 1f also shows two options for returning vapor from the kettle exchanger: above or below the collector tray. Note that the chimney riser area and riser vapor velocity are very different for these two options. In the case where the return vapor is introduced above the chimney tray, the risers act basically as vents, and very little riser area is required. When the return vapor is introduced below the chimney tray, the riser area must be substantially greater to handle the full process vapor rate. The third type of kettle arrangement is basically a variation of the first arrangement. The overspill weir

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inside the kettle exchanger is eliminated, and the entire liquid inventory of the exchanger is placed on level control. Not only does this reduce the buildup of fouling material in the exchanger, it also permits manipulation of the fluid level to affect liquid entrainment into the return line. However, sensing the liquid level in a boiling liquid pool can be difficult, as mentioned below for internal reboilers. No multipass versions versions of the kettle arrangements are shown shown herein because they are logical logical extensions of the single pass standard kettle ( Figure 1e), two-pass trapout ( Figure 3b), or multipass collector ( Figure 4c and 4d) arrangements. Advantages: A full theoretical stage of separation can be achieved by kettle arrangements. The tower bottom configuration requires no baffles. The tower internals do not need to separate mixed phase fluids nor absorb large fluid forces. Kettle reboilers with removable tube bundles are relatively easy to inspect and clean. Limitations: The elevation of the kettle exchanger is limited to a modest range dictated by the desired liquid level in the tower bottom. Since kettle reboilers are once-through devices, they technically are limited in terms of achievable tower reflux ratios (see discussion under OnceThrough arrangements above). But this limitation comes into play only in extreme cases, since kettle exchangers typically allow vaporization rates up to 80% (17). This means they can achieve vapor boilup to bottoms product ratios of about 4:1, and even greater vaporization rates are possible if the service service is very clean. Weaknesses: Kettle reboiler exchangers are relatively expensive. They have a long residence time at maximum temperature in the exchanger, and perform poorly with thermally or chemically fouling materials. In addition, they are improperly designed more often than other types of reboilers because they appear so simple. Attention must be paid to the kettle pressure balance (described at the end of the “Selection of Reboiler Type” section) , which gives the liquid head required to drive flow from the tower to the exchanger and back through the vapor return piping; also sufficient disengaging space must be allotted in the kettle exchanger. As system pressure increases, detailed kettle entrainment calculations become more important. This is necessitated by the decreasing rate of vapor/liquid phase separation at higher operating pressures, due to lower surface tension and smaller phase density differences. Internal reboilers, also known as stab-in reboilers or stab-in bundles, are reboiler exchanger bundles, which are inserted directly into the tower shell below the bottom tray. The bundle is submerged either in the tower bottom liquid pool ( Figure 1g) or in a bath of liquid formed by damming the bottom tray overflow liquid ( Figure 1h). With a bath arrangement, lighter materials boil off from the bath and the remaining liquid overflows to the sump as bottoms product, where it is collected for residence time purposes. Note that the Design Practice Committee generally recommends against using internal reboilers because they are known to have caused numerous operating and capacity problems in previous applications. Advantages: A properly designed bath-type internal reboiler can achieve nearly a full theoretical stage of separation (similar to kettle types). Internal reboilers can be inexpensive in cases where they eliminate exchanger shells and associated process piping without substantially increasing the tower shell cost. Limitations: Unless multiple exchanger bundles are used, internal reboilers are limited to small diameter towers because tube bundle heat transfer area cannot grow as fast as tower crosssectional area with increasing tower diameter. Multiple bundles may increase tower height, offsetting any cost advantage. The bath type arrangement is similar to a once-through reboiler and may limit the boilup ratio.

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Weaknesses: On-stream cleaning is nearly impossible; the tower must be shut down for exchanger maintenance. Similar to kettle reboilers, performance is poor with fouling materials. Internal reboilers require extra tower shell height and incorporate large flange connections, which can leak, especially if the bundle is not supported properly inside the vessel. For the bottom pool arrangement, the tower bottom liquid level can be difficult to assess because of froth generated by the internal exchanger (18). The lower liquid level tap must also be located well below the tube bundle to ensure that two-phase material cannot reach it and cause a false low-level reading. False level readings can mislead operators about the true froth height in the tower, and result in flooding by entrainment of froth to the tray above the reboiler bundle. For the bath arrangement, excessive frothing and hydraulic restrictions, caused by improper design of the bath basin, often bottleneck towers.

7

Reboilers and Tower Elevation

To minimize tower and foundation capital costs, it is generally desired to minimize the overall tower height. Typically this means designing the tower (including the reboiler type and bottom section) first, based on process requirements, then selecting the minimum tower skirt height which provides adequate head for all of the following purposes: • • •

Reboiler circulation (thermosyphon driving force or pump NPSH) Bottoms product pump NPSH Tower or reboiler drainage to downstream equipment, if required

The sections below discuss head considerations for various reboiler types in more detail to allow an assessment of their contribution to required tower skirt height. Note that when a preferential baffle or unbaffled bottoms arrangement is specified, the liquid head used in the reboiler flow calculations should be based on the lowest operating liquid level allowed (typically designated LLL). But the thermal and hydraulic design of the reboiler circuit should comprehend both HLL and LLL process limits, and the reboiler inlet and outlet lines should be sized to handle circulation rates at HLL operating conditions. If a constant head baffle arrangement is used for a thermosyphon system, there will be different liquid levels to consider on the reboiler and product sides of the baffle, and the designer should use LLL on the product side for all product hydraulic calculations. Vertical Thermosyphon Systems

Generally this type of exchanger is hung off the tower itself, and the height of the system is determined by the selected length of the exchanger tubes. Common tube lengths are from 6 to 20 feet (2 to 6 meters), with the longer lengths applicable to designs which require large heat transfer areas (19). Note that reboiler tube length should decrease with decreasing column process pressure to minimize liquid hydrostatic head (shorter tubes in a vertical exchanger reduce hydrostatic head), which maximizes LMTD because vaporization can start at a lower temperature. This becomes an important design consideration in applications operating near atmospheric pressure. If the reboiler feed piping enters the exchanger channel from below, additional skirt height may be required for this as well. Horizontal Thermosyphon Systems

In this case, the reboiler exchanger is typically located at a minimum practical distance above grade to allow for piping clearances, ease of maintenance, or condensate drainage if necessary (the reboiler tubeside outlet nozzle is usually located above the top of the condensate drum for this purpose). Then a pressure balance calculation is performed for the reboiler circuit (including return piping), which gives the required liquid height above the exchanger necessary to drive the desired reboiler flow. Page 14 of 52



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Kettle Systems

Properly designed kettle systems do not usually require significant liquid head for reboiler flow, so the skirt height is typically governed by another factor such as bottoms pump NPSH requirement. In the case of a tower whose pressure is sufficiently above downstream equipment to drive bottoms product onward without a pump, the skirt height may be quite low. Conversely, if bottoms evacuation or drainage requirements dictate a significant skirt height, the kettle exchanger itself may need to be situated on a high foundation because of the elevation relationship between kettle overspill weir elevation and tower bottom liquid level. This elevation difference is given by the kettle pressure balance as described previously. Forced Circulation Systems

The liquid head necessary for a forced circulation system is based on the NPSH requirement of the reboiler circulation pump. Typically, the tower bottom tangent line is elevated about 15 feet (4.5 meters)(20) to provide sufficient NPSH. If a separate product pump is used, its NPSH requirement may govern. Internal Reboilers

These add shell height to the tower itself, but they do not affect the tower skirt height at all.

8

Bottom Section Design Notes

Although it is beyond the scope of this Practice to give step-by-step design methods for tower bottom arrangements, this section section includes information information to help the tower tower designer avoid pitfalls. pitfalls. Bottom Tray Design

When using once-through designs with an active tray at the bottom, this tray should be designed to minimize liquid bypassing, such as weeping, even at turndown conditions (e.g. avoid excessive hole area, or use a valve tray). An alternative to this would be to provide a chimney tray above the reboiler return to catch liquid weeping from above. Chimney trays are particularly useful to collect and properly route liquid from 3 or 4-pass tray sections. For once-through designs employing a trapout sump and draw nozzle, the draw sump is typically sized for 1 ft/s (0.3 m/s) downward superficial velocity, while the nozzle is sized for 3 ft/s (0.9 m/s)(21). Some practitioners specify a self-venting size nozzle in this service to ensure that vapor is not entrained into the trapout draw line. Regardless of reboiler type, the bottom tray design should also provide more vertical spacing to the seal pan than is used in the tray section above. In addition, the downcomer clearance in the seal pan should be more generous than the trays above to allow for accumulation of solids. Because of the possibility of fluid slugs or flow instabilities in two-phase reboiler return lines, the seal pan weir should provide more static seal than typical trays to prevent loss of the seal during transients. Some practitioners specify a static static seal of up to 2 inches (50mm) here. If the trays in the bottom section of the tower are proprietary designs such as UOP MD trays, special bottom tray and tower bottom designs will be required. Reboiler Return Piping

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Issued: 10/01/2006


Revised:

For mixed-phase returns, only vertical and horizontal pipe runs should be used–no sloped piping should be included since this can contribute to formation of liquid slugs. The vertical portions of any piping returning from the reboiler exchanger(s) should be checked for slug flow. The Lockhart-Martinelli flow map has been used successfully for this purpose (22). Sometimes elimination of slug flow in the vertical leg(s) will dictate a smaller pipe size than recommended for the tower inlet nozzle (see Inlet Nozzles and Distributor Pipes below). In such cases, the return piping should swage up to the inlet nozzle size just upstream of the elbow where the final vertical leg meets the final horizontal section connecting to the tower. A reducing elbow can be used here instead of separate reducer and elbow fittings. For a vapor return from a kettle exchanger, the return line is typically sized for a pressure drop of about 0.3 psi per 100 ft (0.07 bar per 100 meters) of equivalent straight pipe. Inlet Nozzles and Distributor Pipes

These are generally sized in according to a maximum velocity criterion. For flush nozzles or nozzleswith distributor pipes in mixed phase service, it is common to size the reboiler return nozzle as follows (23):

V max

−

C ρ m

, ft/s or m/s

Where: V max = maximum superficial fluid velocity, ft/s or m/s C = 3000 for design or 4000 max. for revamp, English units = 4500 for design or 6000 max. for revamp, SI units

ρ m

=

100 % wt _ vapor % wt _ liquid +

ρ v

ρ l

ρ v = vapor density, lb/ft 3 or kg/m3 ρ l  liquid density, lb/ft 3 or kg/m3

Higher velocities are permissible if a phase separation device such as a vane pack is used at the fluid inlet, but the value of C in the above equation should be limited to 10,000 (15,000 for SI units) to mitigate erosion and vibration problems. Note that some practitioners limit the value of C in grassroots design to 2000 (or 3000 for SI units). For a vapor return from a kettle reboiler, the tower inlet nozzle generally matches the line size (see Reboiler Return Piping above). When four-pass trays are used above the reboiler return, it is considered important that two return nozzles be provided to properly distribute vapor to the bottom tray, and that attention be paid to the nozzle orientations relative to the downcomers. Some practitioners allow a single return nozzle as long as sufficient vertical space is provided. If proper measures are not taken, the bottom tray could flood prematurely due to uneven splitting of vapor to each tray pass. In addition, external Page 16 of 52

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equipment and piping should be symmetrical. Three-pass trays in the bottom section present a special concern because simple single or dual return nozzle designs do not split the reboiler return vapor flow appropriately, and they risk causing entrainment of bottom seal pan overflow liquid. This is one reason why 3-pass trays have not been included in the arrangement figures. The preferred method of distributing vapor to (and collecting liquid from) a 3-pass bottom tray section is to use a chimney tray with perforated plates in the risers to force the desired vapor splits. This increases pressure drop in the tower but not in the reboiler loop. Figure 7 shows current recommendations for reboiler return inlet arrangements, along with some configurations that are not recommended. One common reboiler return arrangement in the process industries is a flush nozzle oriented parallel to the bottom tray seal pan weir(s) ( Figures 6a and 7, arrangement A). But inlet nozzles are often fitted with a slotted or perforated internal distributor pipe, especially for mixed-phase service or multipass trays. Criteria for laying out perforated pipe distributors are given in FRI Design Practices Section 1.11.5-1. Note that various FRI practitioners are split about evenly between requiring pipe distributors and avoiding any type of internal pipe distributor with thermosyphon or kettle reboilers due to the added pressure drop and its effect on reboiler process flow (24).

When thermosyphon loop pressure drop is critical, certain other arrangements can be used to distribute vapor from mixed-phase reboiler returns without imposing as much pressure drop as pipe distributors give. Notable examples are multiple vane separators (a.k.a. “hooded inlets”) and tangential inlets (a.k.a. “vapor horns”). These devices also reduce the liquid content of the vapor feeding the bottom tray by coalescing small droplets from the reboiler return into larger droplets which fall out of the vapor phase. Another option is to introduce the mixed-phase return below a chimney tray to promote phase separation and distribute vapor to the trays above. Vane devices are often used beneath low-pressuredrop mass transfer sections that are sensitive to vapor distribution, such as dualflow trays, packed beds and shed decks. Vane device design and performance are beyond the scope of this Practice. Typically, vane devices are designed and supplied by the tray vendor. For a vapor return from a kettle exchanger, a flush nozzle (lacking any internal distributor pipe) may be used if the velocity head of the entering vapor is equal to or less than the pressure drop of the bottom tray. The velocity head is defined as V²/2g, where V is the superficial fluid velocity and g is the gravity acceleration constant. Some practitioners do not allow the use of flush vapor return nozzles below active trays unless the inlet F factor is below a certain value, such as 15 or 20 PSF ½ (18 or 24 Pa ½). As noted above, certain practitioners recommend against the use of perforated distributor pipes with kettle reboilers due to the effect of added pressure drop on reboiler hydraulics (24). If a flush nozzle is selected for the reboiler return, in certain cases it may be necessary to include a wear plate on the opposite tower wall to protect the shell metal from corrosion or erosion. Circumstances that may warrant the use of a wear plate include: • • • •

High fluid inlet velocity Corrosive services, such as sour water stripping Forced circulation Possibility of solids in the return fluid

The reboiler return nozzle should not be located too close to the bottom tray deck. Recommended distances from the top of the reboiler nozzle to the tray are as follows (25): Page 17 of 52

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Flush nozzles: One tray spacing or 18 inches (450mm) min. Nozzles with a pipe distributor: One tray spacing or 12 inches (300mm) min. Multiple vane devices: 45° to point on opposite wall Some practitioners require more space than this, such as 2/3 of tray spacing from the top of the nozzle to the tray. Computational fluid dynamics can be used to optimize the reboiler return configuration. The bottom of the reboiler return nozzle should be one nozzle diameter or 12 inches (300mm) minimum from the maximum sump liquid level. A summary of these key dimensions is given in Figure 8. Note that additional spacing should be provided for foaming systems. Baffle Placement and Details

In towers employing baffles to segregate bottom tray liquid from reboiler return liquid, the baffle should be placed so that the appropriate volume is provided on each side while minimizing the overall height of the tower bottom. As before, key dimensions from this discussion are summarized in Figure 8. Designs for towers operating at high pressures (>400 psia / >2750 kPa for hydrocarbons) should avoid baffle-type arrangements in favor of unbaffled tower bottoms to maximize residence time for vapor/liquid disengagement. Gamma scans taken during troubleshooting of demethanizer towers with preferential baffles have shown that the reboiler inlet can actually be a two phase mixture rather than clear liquid, giving a much lower driving head than expected. High-pressure services should provide extra residence time in the reboiler sump. For constant head arrangements, the reboiler compartment inside the tower should contain at least as much volume as the reboiler draw piping plus the process side of the reboiler exchanger. This provides some surge capacity for instances when a step change is made in reboiler duty (via increased heatingmedium flow). The product side should provide adequate residence time between high and low levels to accommodate process requirements downstream of the tower. An access hatchway should be provided near the bottom of a constant head baffle to allow inspection access to both sides of the tower bottom, but any such hatchway should be mechanically sound and well gasketed to prevent leakage. As mentioned previously, the reboiler draw line(s) should include a drain connection at the low point to allow removal of fouling materials and non-volatile components which can build up in constant head reboiler circuits. For preferential baffle arrangements, the entire tower bottom area may be used in calculating liquid residence time of the bottoms product; the volume between high and low liquid levels should provide adequate residence time for downstream process requirements. If single-pass trays are used above, the baffle is typically placed on the same chord as the bottom tray seal pan. In the case of multipass trays above, the baffle should be located to provide about 1 ft/s (0.3 m/s) downcomer velocity based on the reboiler liquid draw rate. The hole which admits product side liquid to the reboiler side should be located near the bottom and should be 18” (450mm) in diameter or larger to permit inspection access to the other side. In large towers, the baffle hole size should be increased to provide as much area as the reboiler draw nozzle(s). Often a kick (or sloped section) is added to constant head or preferential baffles to provide more space for liquid in the reboiler return to be collected and routed to the desired compartment. In the case of a constant head arrangement, the kick should not cause the net bottoms overspill to exceed 0.33 ft/s (0.1 m/s) velocity through the opening to the product side. For a preferential baffle, the kick normally needs to extend only out to the tower centerline to allow overspill from the notched weir(s) of the bottom seal pan(s) to fall into the reboiler side. The slope of any kick should be at least 15° from horizontal to promote quick drainage of large liquid volumes such as those Page 18 of 52

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encountered in close-boiling systems (26). However, the slope should not exceed 20° so that installers and inspectors can comfortably stand on it as necessary (27). In cases where the sump baffle extends above the reboiler return nozzle (e.g. Figure 4b), its top edge should be at least one tray spacing below the bottom tray. In cases where there is no sump baffle (e.g. Figures 1a and 1b), or where the baffle ends below the reboiler return nozzle ( Figure 5c), the top of the seal pan weir or trapout box should be at least one tray spacing from the bottom tray. Sump baffles are normally made of plate material and welded to their tower attachments, so they are usually considered part of the pressure vessel. In sufficiently large towers, a removable hatchway is sometimes provided in the baffle kick to afford inspection access. Any such hatchway should be gasketed as appropriate to meet the leakage tolerance of the baffle. Outlet Nozzles and Vortex Breakers

New reboiler and product draw nozzles in the bottom head should be sized to avoid friction losses sufficient to hamper thermosyphon circulation or reduce available pump NPSH. Typically this means sizing bottom head nozzles for 3 to 4 ft/s (0.9 to 1.2 m/s) average velocity. In revamp cases, velocities of up to 5 or even 6 ft/s (1.5 to 1.8 m/s) may be permissible for bottom nozzles, but only if deemed acceptable after performing a detailed hydraulic analysis of the draw circuit in question. Vortex breakers should be provided on all bottom product draw nozzles. They may also be used on dedicated reboiler draw nozzles in cases where the reboiler compartment liquid level can vary, but consideration must be made about the probability of the vortex breaker plugging and causing a capacity limitation. This is especially true for thermosyphon systems and for tower services prone to fouling or plugging. Vortex breakers are discussed in Sections 4.04-1 through 4.04-4. If the selected draw nozzle size is larger than the draw piping size, the reducer between these sizes should be located at least 10 nozzle diameters downstream of the nozzle entrance. This allows for turbulent flow profiles to develop fully and provides maximum opportunity for deaeration before the line size decreases. As a general rule, drawoff piping should drop 10 feet (3 meters) vertically from the nozzle before swaging down or switching to a horizontal run.

9

Things to Avoid Figure 9 shows some bottom section design mistakes, which should be avoided. In Figure 9a, the reboiler return fluid is directed toward the bottom tray downcomer and/or seal pan. This design can fail in a number of ways, including (1) backup of the bottom tray downcomer, (2) entrainment of seal pan overflow liquid by the returning vapor, (3) mechanical failure of the bottom tray downcomer from fluid impingement, or (4) heat transfer from reboiler return fluid to the liquid in the downcomer, causing vaporization and choking inside the downcomer.

In Figure 9b, the reboiler return pipe has been routed through the downcomer. Again, this can failby vaporizing liquid in the downcomer and choking it. Also, if the bottom section trays are heavily liquid loaded, this design might block enough downcomer area to cause backup flooding. In Figure 9c, hot liquid from the reboiler return is kept separate from cooler bottom tray liquid by anextended baffle. But heat transfer through the baffle can vaporize light components in the bottom tray liquid, hindering or choking downflow. This problem can be especially serious if the bottom tray downcomer itself extends into the liquid pool, affording no opportunity for vapor to escape.

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1.06

References

1. Kister, H. Z., Distillation Operation, McGraw Hill, Inc., New York, 1990, p. 436. 2. Collins, G. K., “Horizontal-thermosiphon-reboiler design,” Chemical Engineering, July 19, 1976, p. 149. 3. Kister, Op Cit, pp. 436-437 (adapted from Table 15.1). 4. Jacobs, J. K., “Reboiler Selection Simplified,” Hydrocarbon Processing andPetroleum REFINER 40:70 (1961), p. 190. 5. Kister, Op Cit, p. 434. 6. Collins, G. K., Op Cit, p. 149. 7. Fair, J. R., “REBOILERS / A General Review of Predictive Models and Design Practices,” December 7, 1992, p. 1 (prepared for The Encyclopedia of Chemical Processing and Design, J. J. McKetta, Editor). 8. Sloley, A. W., “Properly Design Thermosyphon Reboilers,” CEP, March 1997, pp. 54-55. 9. Shah, G. C., "Troubleshooting reboiler systems," CEP, July 1979. 10. Mukherjee, R., “Effectively Design Shell-and-Tube Heat Exchangers”, Chemical Engineering Progress, February 1998. 11. Design guideline reported by an FRI Design Practices Committee member. 12. Kern, R., “How to design piping for reboiler systems,” Chemical Engineering, August 4, 1975, pp. 107-113. 13. Fair, Op Cit, entire document (pp. 1-34). 14. Collins, Op Cit, entire article (pp. 149-152). 15. Orrell, W. H., “Physical Considerations in Designing Vertical Thermosyphon Reboilers,” Chemical Engineering, September 17, 1973, entire article (pp. 120-122). 16. Jacobs, Op Cit, p. 190. 17. Ibid, p. 194. 18. Hepp, P. S., “Internal column reboilers – liquid level measurement,” Chemical Engineering Progress 59:2, February 1963, pp. 66-69. 19. Design guidelines reported by several FRI Design Practices Committee members. 20. Design guideline reported by an FRI Design Practices Committee member. 21. Design guideline reported by an FRI Design Practices Committee member. 22. Collins, Op Cit, p. 152. 23. Ibid, p. 150. 24. Kister, Op Cit, p. 89. 25. Design guideline reported by an FRI Design Practices Committee member. 26. Kister Op Cit, p. 100. 27. Design guideline reported by an FRI Design Practices Committee member.

Page 20 of 52



Figure 1a Vertical Thermosyphon

Page 21 of 52

1.06



Figure 1b Horizontal Thermosyphon

Page 22 of 52

1.06



Figure 1c Forced Circulation

Page 23 of 52

1.06



Figure 1d Suppressed Vaporization

Page 24 of 52

1.06



Figure 1e Standard Kettle

Page 25 of 52

1.06



Figure 1f Trapout Kettle

Page 26 of 52

1.06



Figure 1g Internal (Stab-in) – Pool Style (NOT RECOMMENDED)

Page 27 of 52

1.06



Figure 1h Internal – Bath Style (NOT RECOMMENDED)

Page 28 of 52

1.06



Figure 2 Falling Film Reboiler

Page 29 of 52

1.06



Figure 3a Once-Through Trapout Single Pass

Page 30 of 52

1.06



Figure 3b Once-Through Trapout Two Pass

Page 31 of 52

1.06



Figure 4a Once-Through Collector Chimney Tray Version

Page 32 of 52

1.06



Figure 4b Once-Through Collector Tower Bottom Partition Version

Page 33 of 52

1.06



Figure 4c Once-Through Collector Two Pass Single or Dual Circuit

Page 34 of 52

1.06



Figure 4d Once-Through Collector Four Pass

Page 35 of 52

1.06



Figure 5a Preferential Single Pass

Page 36 of 52

1.06



Figure 5b Preferential Two Pass Center (NOT RECOMMENDED)

Page 37 of 52

1.06



Figure 5c Preferential Two Pass Side

Page 38 of 52

1.06



Figure 5d Preferential Four Pass

Page 39 of 52

1.06



Figure 6a Constant Head Single Pass w/Flush Nozzle & Boot Partition

Page 40 of 52

1.06



Figure 6b Constant Head Single Pass w/Distributor Pipe & Boot Partition

Page 41 of 52

1.06



Figure 6c Constant Head Single Pass w/Collector Box

Page 42 of 52

1.06



Figure 6d Constant Head Two Pass

Page 43 of 52

1.06



Figure 6e Constant Head Two Pass w/Collector Tray

Page 44 of 52

1.06



Figure 6f Constant Head Two Pass w/Collector Boxes

Page 45 of 52

1.06



Figure 6g Constant Head Four Pass

Page 46 of 52

1.06



Figure 7 Return Nozzle Arrangments

Page 47 of 52

1.06



Figure 8 Typical Dimensions in Tower Bottom Designs

Page 48 of 52

1.06



Figure 9a Things to Avoid Reboiler Return Impinges on Seal Pan

Page 49 of 52

1.06



Figure 9b Things to Avoid Reboiler Return Through Downcomer

Page 50 of 52

1.06



Figure 9c Things to Avoid Submerged Baffle between Hot and Cold Liquids

Page 51 of 52

1.06



1.06

APPENDIX 1 APPROXIMATE HYDRAULIC CALCULATION FOR A HYDROCARBON T HERMSYPHON SYSTEM (For Use in Setting Viscosity Guidelines)

Noting that viscosities of water and typical hydrocarbon streams near their saturation points are between 0.2 and 0.5cP (0.2 and 0.5 mPa ⋅s), we can use 0.5cP (0.5 mPa ⋅s) as a starting point. Pressure drop in single-phase turbulent pipe flow varies with viscosity to the 0.2 power. Assuming that the typical bottoms draw line is 40 to 50 equivalent feet (12 to 15 equiv. meters) with an average velocity of 4 ft/s (1.2 m/s), we get a viscous line loss of maybe 5” (125mm) liquid. For an exchanger pressure drop of 1 psi (0.069 bar) and a liquid gravity of 0.77, we have a loss of about 36” (915mm) liquid in the exchanger. For a return line loss of 0.3 psi (0.021 bar) we get about 11” (280mm) liquid loss. Thus a representative total loss around the reboiler loop might be 52” (1300mm) clear liquid. Now assuming that the pressure drop in the two phase regions follows the 0.2 power behavior with viscosity (conservative for vapor-rich regimes such as annular or mist flow), a 5” (125mm) or 10% increase in loop pressure drop would result at a viscosity of 0.8cP (0.8 mPa ⋅s). At 1.4cP (1.4 mPa⋅s), the loop pressure drop would increase by 12” (300mm) or 23%; at 3.3cP (3.3 mPa⋅s) the drop would increase by 24” (600 mm) or 46% and at 7.5cP (7.5 mPa ⋅s) the drop would increase by 37.5” (940mm) or 72%. Although these estimates are probably conservative, it appears that liquid viscosities in the range of 3 to 4cP (3 to 4 mPa ⋅s) begin to have an appreciable effect on resistance to thermosyphon flow.

Page 52 of 52


TRAY TRANSITION

Issued: Revised:

TRAY TRANSITION

DESIGN CRITERIA -

A.

Trays above transition rotated 90 ° to trays below.

B.

Refer to appropriate pages for feed arrangements.

Page 1 of 1

03/01/1978

1.07


TRAY BLANKING

Issued:

09/01/1981

1.11

Revised:

Trays are blanked to reduce the minimum operating point as required by a variety of situations. Trays that must operate at rates well below those for which they were designed might require blanking; trays which must have additional inherent capacity for some future expansion requirement might require blanking prior to the expansion; trays whose diameters are established by considerations other than the loads they must handle might require blanking; and there may be other reasons. Four types of blanking patterns, as shown in the attached sketches, are common: - inlet and/or outlet blanking Sketch 2 - blanking the sides Sketch 3 - blanking with strips parallel to flow Sketch 4 - blanking with strips perpendicular to flow Sketch 1

The selection of a blanking pattern should depend on a number of considerations, not the least of which would be cost. The ideal way to blank a tray would be as shown in Sketch 2, that is, to blank segmental areas on both sides of the tray such that the active (perforated) area is, or approaches a rectangle. In such an arrangement, suitable hardware must be provided to assure that all liquid passes over the active area only. This method is likely to be more expensive than any of the others and should only be used if the benefits are expected to justify the additional cost. Since the readily available information on this subject is relatively skimpy, engineering judgment will be required. It would appear, however, that the key variable is (effective) flow path length, and that if other methods will significantly reduce flow path length, especially if they would reduce it below 30 inches, the method in Sketch 2 should be considered. The Sketch 2 method of blanking trays is frequently used when hole areas must be reduced by half or more, as other methods will generally either reduce the flow path significantly or will reduce the percent hole area in the active area beyond the limits for which reliable meaningful data exist. As shown on the sketch, baffles are provided to keep liquid away from the blanked areas. These baffles should be as high as possible, subject to interference with the support members of the tray above. The blanked areas, which will, never-the-less, receive some liquid from leakage and/or splashing, should drain easily and selectively into the downcomer to the tray below. Inlet and/or outlet blanking ( Sketch 1) should be considered when the area to be blanked is relatively small and/or the flow path length is long. Inlet and outlet blanking is preferred to one or the other, particularly when much blanking is required, because it is desired that the vapor flow between trays not have a general side to side component. The effect of such, however, is unknown. The need for inlet weirs in these kinds of arrangements, and the preferred location of outlet weirs, are questions which have not been addressed. The blanking methods shown in Sketches 3 and 4 are expected to be acceptable as long as the effective flow path length is adequate to maintain the desired efficiency and the strips are not too wide. The only meaningful F.R.I. test of blanking with strips is covered in the September 1969 Progress Report which describes the use of 5.37 inch blanking strips installed parallel to flow (as in Sketch 3) on an 8 foot diameter tray having a flow path length of 60 inches. The tray was blanked from 14 percent hole area to 8 percent hole area and was compared to a normal 8 percent hole area tray. The report concluded that there were no significant differences, but anyone planning to use blanking strips parallel to flow is advised to read the September 1969 Progress Report. Also, users are cautioned that the use of strips wider than 5-1/2” inches, and/or blanking with strips parallel to liquid flow on trays with FPL's lower than 60 inches may adversely affect performance, particularly tray efficiency. In general, the selection of the Sketch 3 vs. the Sketch 4 method depends on the orientation of the tray panels. It is simpler and more economic to install blanking strips parallel to the long dimension of the tray panels. Neither Sketch 3 nor Sketch 4 methods are generally used when more than half the hole area is to be blanked or if the hole area will be reduced below five percent of the active area. Other F.R.I. tests on blanking are reported in the Progress Report for January 1958 and October 1959, and the 1960 Annual Report. Page 1 of 2


TRAY BLA NKING

SIEVE TRAY BLANKING

Page 2 of 2

1.11


TRAY LEVELNESS

Issued:

11/01/2001

1.12

Revised:

Distillation trays that are significantly out of level do not function properly. In difficult separations, they may not function at all. There is a growing list of cases in which releveled trays have corrected problems of poor tray efficiency, unstable turndown performance and troubled separations. Yet, because of the broad dispersal of responsibility for tray levelness - from the tray and tower designers and manufacturers, the tower foundation designer, the tower erector, the tray installer - and the numerous procedures and checks required to insure levelness, there often is a tendency to overlook this important factor. Listed below are some practical aspects on the subject of tray levelness: 1.

The sun shining on an uninsulated tower can temporarily affect tray and tray ring levelness. On tall, large diameter towers, solar distortion of the tower has been observed to cause tray tilting in excess of one half inch high point to low point. If solar influences are causing tower distortion during tray installation or inspection, the hours before sunrise are the best time to take critical levelness measurements. When the tower is operating, the sun effect is negated by the tower internal temperatures and tower insulation. High winds causing tower sway and lean can also cause problems when taking levelness measurements.

2.

Tray levelness specifications should be clear to everyone involved. Does "tray levelness: plus/minus one quarter inch" mean that the high point on the tray can be vertically one half inch from the low point? The expression "levelness to be (within) one quarter inch" also causes considerable debate among installation personnel. One good method is to specify the maximum allowable vertical distance from the highest point to the lowest point on the active portion of the tray. Also, it is a good idea to state whether the measurements are to be made on the tray deck or on the deck supporting structure, such as the top surfaces of the tower rings and support beams. Minor deck protuberances caused by fastener loads may not be functionally objectionable, but if included in levelness measurements, they can make meeting tight levelness tolerances very costly or entirely unfeasible.

3.

As several entities are usually involved in the final result - tray manufacturers, tower fabricators, erectors - each should be constrained so that the finished installed assembly is within limits. Consideration should be given to limiting the levelness of the tower rings and the trueness of tray parts. One practice is to allow the tray rings to be out of level one half of the maximum installed tray levelness allowance.

4.

The technique of measuring ring and tray levelness is not a task to be taken lightly. For erected towers of larger diameter, an excellent method of measuring is with the use of a temporarily fixed optical level and a length calibrated vertical rod held on the surface to be measured. The differences between such readings taken across the tray will accurately determine levelness. With small diameter erected towers, the optical level cannot be used because of focusing restrictions. The manometer type liquid tube is a common measuring device employing two people, each reading the liquid level at the ends of the tube against calibrated vertical scales held on the surfaces to be compared. Unless fastidious effort is applied, this technique is subject to serious error. Among the many factors that affect accuracy are air bubbles in the liquid, dirt and oil in the tube affecting the meniscus, density differences in the liquid, oscillations, parallax, each person reading a different elevation on the meniscus, and, background noise affecting communication. An improvement in the method is to connect one end of the tube to the bottom of a temporarily fixed large diameter reservoir in which the liquid level remains essentially constant. Then all that is required is for readings to be taken by a single person at the other end of Page 1 of 3


TRAY LEVELNESS

1.12

the tube at various points about the surface to be measured; the differences in readings reflect levelness. Care must be taken that the weight of inspecting personnel does not create erroneous readings by temporarily causing the tray to sag. 5.

Proving levelness of tower rings or installed trays in a vessel resting horizontally in a vessel fabricator's shop is a great deal more difficult than when the tower is in operating position fixed to its foundation. Only by careful measurements can it be determined that the rings are parallel to each other and the baseplate, and perpendicular to the towers longitudinal centerline. Typically, vessels trayed in the horizontal will have a greater potential for out of level trays because of the difficulties in measuring "level" in this position. In addition, there is also a good probability that there will be an erection error when the vessel is sited. A final levelness inspection, after the vessel is erected, is advisable.

6.

Despite the science that has been brought to foundation design, tower foundations have been known to settle, causing once level trays to go out of level. This tipping creates a nonrandom out of levelness that is considerably more serious than random out of level trays which are more common with installation and manufacturing deficiencies. When all trays are tilted to one side, the vapor preferentially rises up one side of the tower through the high side because of the smaller hydraulic head of froth on this side.

7.

Economics dictate that the out of levelness allowance be related to the diameter of the vessel. A brief analysis of data submitted by eight member companies on the subject indicates that the high point to low point allowance of installed trays falls essentially in a band limited by the following straight lines: Maximum allowance, inches = 0.19 + 0.015D AND Minimum allowance, inches = 0.06 + 0.009D where D is the inside diameter of the column in feet. Allowances over 7/16 inch are outside of the scope of the data available. Some systems require a higher degree of tray levelness than others. Some companies recognize this in their specifications by having two classes of out of level allowances.

8.

All trays sag somewhat due to their own dead weight. Trays over twenty feet in diameter will normally have a dead weight sag that will be significant with respect to the levelness requirement. Past practice has often allowed the tray support elements be manufactured with an upward camber so that the tray will be level when installed. It is, however, desirable that the tray be level during operation when in addition to the gravitational force, process forces are acting on the tray deck and structure. For this reason, the practice of cambering tray parts to eliminate dead weight sag needs reexamination. The net process force, due to the action of liquid and vapor in the column is normally upward. It counters the dead weight sag and is generally of a magnitude equivalent to the dead weight gravitational force. A tray built with upward camber may, therefore, hump up in the center when the process force, acting up, opposes the gravitational force acting down.

Page 2 of 3


TRAY LEVELNESS

1.12

The fact that the net process force acts upward is not difficult to perceive. The pressure drop each tray, which of course is an upward acting force, is mainly composed of two additive elements, the hydraulic head of liquid on the tray and the dry plate pressure drop across the tray deck. The weight of liquid on the tray is balanced by part of the tray pressure drop, leaving the force attributable to the dry plate drop to oppose the force of the dead weight. Analysis of the many variables encountered in normal designs shows that either force may be larger depending on the particular case. The weight of downcomer liquid and the location of the downcomers on the tray should be included in any particular analysis. To summarize on this point, on large trays, a structural analysis of all of the forces acting on an operating tray will provide insight into tray levelness in a meaningful way. It may be prudent to allow for the sag during installation knowing that the tray will come into level when the tower goes into operation.

Page 3 of 3


TRAY GASKETING

Issued:

07/15/1982

Rev i s ed :

1.13

04/15/1985

TRAY GASKETING

Trays are generally assembled from components of such size that they can be removed through tower manways which may be as small as 18 inch nominal nominal diameter. Therefore, except when cartridge trays are supplied, even the the smallest of trayed towers towers contain trays assembled from from panels. Where these panels join each other, and where they join the tower shell, regions of possible leakage exist. If such leakage is likely, and intolerable or importantly undesirable, some method of eliminating or at least limiting that leakage must be provided. To control leakage, joints may be seal welded or gasketed. gasketed. Where continued removability removability is required, or other reasons make seal welding undesirable, gasketing is the more appropriate approach. Gasketing of tray joints joints should not be done done where it is not truly required. required. Gasketing is costly, both both to purchase and to install. Gasketing increases maintenance requirements. Also, an improperly gasketed tray, or one whose gaskets have deteriorated, will often leak much more severely than one designed to have metal to metal seals only. A number of Gasketing Gasketing applications applications are common. Total drawoff trays, trays, generally chimney type, type, from which minimal leakage can be tolerated, tolerated, are frequently gasketed. gasketed. Bubble cap trays, which which have been selected for their minimal leak characteristic, characteristic, are very often gasketed. Gasketing is sometimes suggested suggested for joints in downcomer receiving areas, particularly in low liquid rate towers, but detailed analyses of the forces involved generally show joint leakage in these areas to be unlikely. There is some controversy about the use of gaskets in the active areas of sieve or valve trays. Although it is sometimes recommended and indeed, installed, most users having input to this document have difficulty supporting efforts to eliminate minute openings in trays that have five to fifteen percent hole areas by design. The above comments relate to the leakage of liquid through through gaps in the tray structure. There is one area where the leakage leakage of vapor may may lead to column flooding: flooding: the downcomer. Most designs utilize at least two panels to form the downcomer. Each is bolted to both downcomer bolting bolting bars and to the other panel along a horizontal horizontal joint. joint. Vapor leakage at this joint may seriously affect downcomer performance. For example, a continuous gap 1/16" wide will give a superficial upward vapor velocity in the downcomer of the order of 0.1 m/s. This could seriously seriously interfere with with the vapor/liquid vapor/liquid separation and may cause premature flooding by reducing the froth density in the downcomer. Good metal to metal seals or effective gasketing should should be regarded as essential. essential. Also, difficulties in sealing the downcomer downcomer at the bolting bars are common, particularly with field-installed bolting bars which are frequently poorly orientated. This is not so serious serious since the vapor leakage is confined to the corners of the downcomer. Nevertheless, reasonable reasonable measures should be taken to eliminate eliminate any gaps. For a rough estimate of leakage, use the following

V

ρ

=

ν

V ρV

D K v

= = = =

κ

ν

D

superficial upward vapor velocity in the downcomer from leakage (ft/sec, m/s) vapor density (lb/ft 3, kg/m3) column diameter (ft, m) 1 (British units), 0.4 (SI units)

Page 1 of 1

FRI VOLUME 5: FRACTIONATION FRACTIONATION DESIGN HANDBOOK

TRAY DESIGN, CONSTRUCTION AND ASSEMBLY

Issued: Rev i s ed :

07/15/1983 01/15/1997

1.14.1

Tray Design, Construction and Assembly

Modern distillation trays are generally of light gauge metal and of sectionalized design in which the tray parts are sized to pass through tower manholes and are assembled inside the vessel. The tower fabricator builds and installs tower attachments to support the trays according to specifications supplied by the tray supplier. Although there are several proprietary tray designs designs that employ employ novel support systems and assembly techniques, conventional single or multi-pass cross-flow tray decks are usually supported by an understructure of major major and minor beams supplied with the trays. The minor beams (deck beams) support the tray deck panels longitudinally and are either of the integral beam (integral truss) design or of the loose beam (loose truss) truss) design. When required, major beams carry the loads loads developed in the minor beams, to the tower wall. Not all trays have major beams. The concept of integral integral beam construction construction was developed developed within the the tray industry. industry. It enjoys the reputation of being economical economical and easy to install. In integral beam construction, the minor minor beams are formed by bending down one or both longitudinal edges of the tray deck panels to form the support beams integrally with the decks. Often, the lower edge of the beam web is stiffened with an additional narrow width integrally integrally bent flange. The integral beams are not usually usually connected directly to the main beam(s) or tower wall; the loads developed in the beams are transmitted to the other supports through the overlapping of deck panels panels and through adjacent adjacent deck panel fasteners. Shear clips and special beam hangers may be used if a more positive means of transferring loads is required. Loose beam construction usually usually follows the principles employed employed in traditional structural structural design. The independent minor beams are connected directly to the major beams and tower walls with bolt through connector brackets. Minor beams may alternatively be connected connected by hangers welded to the minor minor beam and overlapping the major major beam or support ring. Retainers are necessary with hangers hangers to prevent uplift. After the supporting beam structure is installed, the flat deck plates are laid on the top flanges of the minor beams and restrained by large diameter, frictional hold-down washers. The tray parts are secured to the tower by clips clips and to each other by means of threaded threaded fasteners. Tray clips are used to clamp the peripheral tray sections to the top surfaces of flat horizontal rings welded to the tower wall for field installed installed trays. These arrangements allow for ease of tower tower fit up; they provide for differences in thermal expansion between the trays and the tower, and for tower out-of-roundness. The tray decks are coupled together with nuts and bolts by either bolting through holes in the mating parts or by a large diameter frictional hold-down washer arrangement. The nuts and bolts can be of the welded-on variety or they they can be loose. One manufacturer uses a system whereby whereby adjacent deck panels interlock together using a system of slots and tabs instead of bolts and washers. Shop installed trays are installed with the vessel in the horizontal position before shipment to the field. This may result in labor cost savings savings compared to field installed installed trays. However, this can be a special problem for large diameter vessels. For vessels greater than six foot or 1800 mm diameter, the "uppermost" portion of a tray deck may be out of reach and require the use of a ladder or platform to finish installing installing the tray. Tray decks can work themselves themselves loose during shipment shipment or during vessel vessel erection. In some cases tray damage has occurred. It is necessary to inspect every tray after the tower has been erected. With vessels larger than five five to six feet (1600 to 1800 mm) in diameter, repair and readjustment of the tray decks in the field may offset much of the potential cost savings. Welded-on fasteners, where either the nut or the bolt is shop welded to the blind side of the tray (the side opposite the one where the installation activity is taking place, usually underneath) and its mating half is threaded on from the other side during assembly assembly in the tower, increase initial tray tray cost. Installation costs are reduced, however, since only one man is needed needed to work from the most accessible side. side. Loose fasteners require two men, one on either side of the the tray, to to assemble assemble each fastener. Some trays, by Page 1 of 2

Issu ed: 7/15/19 7/15/1983 83 Revised: 01/15/1997

TRAY DESIGN, CONSTRUCTION CONSTRUCTION AND ASSEMBL Y

1.14.1

virtue of their constrictive their constrictive design or tray spacing, spacing, cannot use loose fasteners. fasteners. To achieve the same effect and reduce fastener costs, swaged-and-threaded holes have been used as substitutes for welded-on nuts. This method, method, however, however, has limited limited user acceptance. acceptance. The method method of attachment referred to as "caged nuts" is similar to the welded-on fasteners, except that a nut is attached to a tray deck with the use of a small bracket that holds the nut in place and prevents it from rotating until the bolt placed through it at the time of the installation of the tray is tightened down. Trays may be designed to be either top installed or bottom installed (See Definitions, Definitions, Section 0.03-1). In a top installed tray, traying in a tower proceeds upward as each tray is installed above the tray previously installed. In a bottom installed tray, traying traying progresses progresses downward in the tower. Bottom installed installed trays are normally used only for special situations, situations, such as when traying is is to proceed rapidly in in both directions from tower manholes located at the top and bottom of the column, or in situations where trays above a process fluid entrance nozzle are expected to sustain damage, and repair or replacement is more easily done from below. below. Some trays, by virtue of their their design and large large tray spacing, can be installed installed from either the top or the bottom. The tray purchaser is advised to specify whether trays are to be top or bottom installed, whether or not the tray fasteners are to be welded to the tray parts so that installation can be accomplished with a workman on the top (or bottom) side of the tray only, and whether manways are to be operable from one or from both sides. Using the term "top installability" in a tray specification does not assure that a tray installer can install all parts and fasten them from the top side of the tray only. To be sure, you must include the term "only", as in "trays must be installable from the top side only". To accomplish this it is necessary to weld either a nut or a bolt to the underside underside of the tray part. Welding a nut or bolt refers to tack welding welding in most cases and not to seal welding as the latter method can deform the fastener being welded and leave weld splatter on the threads. Caged nuts are another another alternative to to parts with welded welded designs. It is also necessary to provide a method to assure that the tray clamps on the blind side of the tray are oriented properly. This can be carried out using using marks on the hardware (such as a slot slot for a screwdriver). Manways in the tray tray floor allow passage through the trays for inspection inspection and maintenance. maintenance. The removable manway panel, generally a portion of the active tray floor, is usually specified to be either removable from above the tray (top) (top) only or from above above and below (top and bottom). bottom). The top operable manway is more economical and trouble free. Top and bottom operability operability is often provided by by means of rotatable clips which are secured with threaded nuts. nuts. Proprietary manways designed to enable quick and easy operation are also available.

Page 2 of 2


TRAY MECHANICAL STRENGTH

Issued:

01/15/1997

1.14.2

Revised:


Tray Mechanical Strength.........................................................................................................1 1.

Standard Designs ............................................................................................................... .2

2.

Heavy Duty Designs ...........................................................................................................2

3.

Mechanical Strength Design Factors ............................................................ ...................... 3

4.

Shear Clips ............................................... ........................................................................... 4

5.

Major Beams.............................................. ................................................... ...................... 4

Page 1 of 5


1


1.14.2

Standard Designs

Typical user standards set 20 pounds per square foot (psf) to 30 (98-147 kg/m 2) as a minimum downward, static, uniform loading applied to the active area (or bubbling area) of the tray. The tray would be designed to withstand this loading without exceeding a specified amount of deflection. The inlet area of the tray would be designed for a greater loading, such as 60 psf to 65 psf (294-319 kg/m 2) because of the higher liquid level encountered due to downcomer backup above the inlet panel. The total allowable deflection must be specified. Deflection is normally expressed as a function of the span length. One typical limitation is: Deflection Allowed = Span Length 900 Examples: Span Length

Deflection

4' 8' 12' 16' 20'

0.053" 0.107" 0.160" 0.213" 0.267"

(1.2 m) (2.4 m) (3.7 m) (4.9 m) (6.1 m)

(1.35 mm) (2.71 mm) (4.06 mm) (5.42 mm) (6.77 mm)

These deflections may be verified during tray installation. The actual dynamic load deflection during normal operation is usually not considered because of the difficulty of verification. In most cases, it would be less than the static load. Trays are also designed to withstand a 250 to 300 pound (113 to 136 kg) concentrated downward load applied at any point on the tray. This allows for loadings produced by workers moving about on the tray decks during installation or inspections. In this case, the design limitation is to withstand the concentrated loading without permanently deforming the tray and not the deflection criteria given above. Most tray manufacturers use the ASME Code for allowable stress and yield values for the specified material, but designing to ASME Code is not mandatory unless specified by the user. The uniform loading values relate to a normal liquid level on the tray: 4" water depth = 21 psf 102 mm = 102 kg/m 2 6" water depth = 31 psf 152 mm = 152 kg/m 2 Owner/users sometimes specify greater loadings for severe services. Upthrust force (uplift) must be specifically defined, if required. Collector/chimney trays are usually designed for a liquid (water) depth of riser height plus 2" (50 mm).

2

Heavy Duty Designs

A full tray spacing of liquid load will collapse trays designed for standard loads. Consider the forces produced by these liquid depths, some of which correspond to typical tray spacings:

Page 2 of 5

Issued: 01/15/1997


Revised:

12" water depth = 62 psf 18" water depth = 94 psf 24" water depth = 125 psf

1.14.2

305 mm = 305 kg/m 2 457 mm = 457 kg/m 2 610 mm = 610 kg/m 2

When trays are repeatedly damaged due to vibrations, pulsations, high liquid levels, or severe uplift forces such as steam releases, the tray design is usually strengthened to 60, 90, or 144 psf (294, 441, or 706 kg/m 2) (1 psi or 703 kg/m 2) loading. Some severe services require 288 psf (2 psi or 1406 kg/m 2) or greater. A thorough vessel design review is necessary when heavy-duty designs are used to ensure that the excessive force loading causes no damage to be done to the vessel. This is also prudent when using through-bolted designs (for whatever reason) and trays that are seal-welded in place since these designs fail when metal tears and not when frictionally clamped panels slip apart. The failure sequence should always be: trays fail first; then major beams; then the vessel. Designs are generally considered to be "heavy-duty" when the trays are designed for one psi or greater. For most designs, there is a 25% to 50% step-charge increase in the cost of trays that is encountered somewhere in the 90 to 144 loading range. The added costs are a function of column diameter, number of passes, and material of construction. .

3

Mechanical Strength Design Factors

The most important variables in tray mechanical design are: · · · · · · ·

design loading deck and beam material thickness material of construction truss depth, panel width, and span bolt/clamp spacing major beams shear clips

The most cost effective strengthening is by using shorter beam spans, panel widths, and deeper channel trusses with shear clips. Thicker decks may be used, but may not in itself be adequate. Often closer bolt/clamp spacings are used; and in some cases thicker trusses and decks may be required.

Page 3 of 5


4.


1.14.2

Shear Clips

Figure 1A. Tray Ring Attachment

Figure 1 B. Downcomer Attachment

A shear clip is a vertical plate that is attached to the end of an integral truss to aid in transmitting the mechanical loading to the support structure (tray support ring or downcomer truss, for example). A shear clip may be bolted or welded at each end. Figure 1 illustrates two typical shear clip attachments and two methods of connecting it to the support structure. thermosyphon circulation.

5

Major Beams

The lowest cost major beam is often a solid I-beam. Beam depth should be checked versus the tray spacing and flow direction. Deep beams may adversely affect both vapor and liquid flow and, therefore, the capacity and efficiency of the tray. Maintenance access across the tray may also be restricted. A beam may be considered "deep" if the bottom of the beam extends substantially into the spray height of the tray below. The depth of a beam that is normal to liquid flow or that is located above the downcomer of the tray below should not exceed 25% of the tray spacing (30% at the very maximum). Do not neglect the possible consequence of a beam located below the bottom tray (or bottom packed bed) in a column. The location of vapor inlets or reboiler return nozzles should be reviewed relative to the positions of beams and trusses that project below the lowest fractionating device in a column for any potentially adverse effects. Alternatively, lattice type beams allow some degree of vapor equalization through the struts. More than one tray may be supported on a lattice type beam if the beam depth is made equal to some multiple of the tray spacing. Windows may be cut into l-Beams for vapor equalization. Major beams should normally be oriented in the direction parallel to the liquid flow for minimum interference. When this approach is used, then the minor beams (trusses) may become an interference with process flow and access. Also, when major beams are oriented parallel to the liquid flow, they must often cross through the downcomers, which can reduce the downcomer capacity. For cost, process, and access reasons, major beams are used in very large towers only as a last resort. Because an increase in the number of flow paths reduces span length significantly, large diameter one-pass trays are more likely to have beam interference problems than are multi-pass trays. However, all designs

Page 4 of 5



having major beams should be carefully reviewed for vapor and liquid flow effects.

Page 5 of 5

1.14.2


PERFORATED PIPE DISTRIBUTORS

Issued:

7/15/1983

1.15

Revised:


Ensuring proper feed distribution during column operations is necessary to achieve adequate column efficiencies. Many columns use perforated pipe distributors for the introduction of their feed streams. A good pipe distributor design requires a balance among three flow variables: 1.

Kinetic energy of the fluid.

2.

Lost work due to friction.

3.

Pressure drops across distributor perforations.

The external piping connected to pipe distributors can also affect their operation. A sufficient length of straight pipe should precede the first outlet port in order to minimize entrance effects. Symmetry, when branching to multiple pipe distributors, will help reduce flow splitting problems. For single phase feeds, the most common pipe distributor design consists of a constant diameter pipe with equally spaced holes of equal diameter. The usual design criteria are listed below. 1.

Length to diameter ratio below 150.

2.

Total hole area equal to pipe flow area.

3.

Inlet pipe velocity of normal magnitude (5 to 10 ft/sec for liquid).

4.

Orifices large enough to avoid blockage (diameters less than 1/4 to 1/2-inch are not recommended).

A design based on the criteria offered will give a maldistribution of about ± 10%, which is fully satisfactory for most purposes. Maldistribution is here defined as the difference between the flows through the first and last holes expressed as percentage of the flow through the first hole.

100(Qn − Q1 ) Q1 If required, a better distribution may be obtained with a pipe distributor of constant diameter that has circular holes of equal diameter and variable spacing or equally spaced holes of variable diameter. The economics of manufacturing a pipe distributor may dictate the geometry of the design. A good design incorporating various flow situations and pipe geometries requires a computerized finite difference solution to the problem (Ref. 5). However, specific design methods have been published and utilize one or more of the following assumptions (Ref. 2, 3, 4, 6). 1.

One flow regime, either turbulent or laminar.

2.

Constant friction factor.

3.

Constant orifice coefficient.

4.

Perfect momentum transfer.

Page 1 of 2



1.15

For two phase or flashing feeds, good distribution is difficult if not impossible to achieve (Ref. 4). No design methods have been found in the literature, and there appears to be a wide diversity of practice in the industry. A few general points are listed below. 1. Usually, it is not possible to separate vapor from liquid in a pipe distributor - all holes will discharge a two phase mixture, though liquid will usually tend to travel to the far end of the distributor. 2. In the feed line and the distributor, the plug flow regime should be avoided wherever there is a change of direction; otherwise vibration and damage may result. 3. Where two phase flow divides into two or more distributor pipes, the pipework configuration must be as nearly symmetrical as possible if an equal split of vapor and liquid is to be achieved. 4. Particularly with flashing feeds, if the discharge orifices have been designed for a high pressure drop to suppress vaporization in the feed line, discharge velocities will be high. Downcomer panels and tray decks in the vicinity of such a distributor will probably require reinforcement and/or sacrificial wear plates. Also, care must be taken to ensure that tray action is not adversely affected. The forces generated by the "jetting" of a fluid from a pipe distributor can damage the pipe, its connections, and surrounding assemblies. Long pipe distributors should be supported at both ends from the column wall. Depending on an analysis of the forces involved, some distributors may require intermediate supports. The supports should not prevent longitudinal thermal expansion. Downcomer panels and trays in the vicinity of a feed distributor pipe may require reinforcement, especially in two phase services. 1.

Usually, it is not possible to separate vapor from liquid in a pipe distributor - all holes will discharge a two phase mixture, though liquid will usually tend to travel to the far end of the distributor.

2.

In the feed line and the distributor, the plug flow regime should be avoided wherever there is a change of direction; otherwise vibration and damage may result.

3.

Where two phase flow divides into two or more distributor pipes, the pipework configuration must be as nearly symmetrical as possible if an equal split of vapor and liquid is to be achieved.

4.

Particularly with flashing feeds, if the discharge orifices have been designed for a high pressure drop to suppress vaporization in the feed line, discharge velocities will be high. Downcomer panels and tray decks in the vicinity of such a distributor will probably require reinforcement and/or sacrificial wear plates. Also, care must be taken to ensure that tray action is not adversely affected.

The forces generated by the "jetting" of a fluid from a pipe distributor can damage the pipe, its connections, and surrounding assemblies. Long pipe distributors should be supported at both ends from the column wall. Depending on an analysis of the forces involved, some distributors may require intermediate supports. The supports should not prevent longitudinal thermal expansion. Downcomer panels and trays in the vicinity of a feed distributor pipe may require reinforcement, especially in two phase services.

Page 2 of 2


LOW LIQUID RATE DESIGNS

Issued:

10/30/1985

Revised:

1.16


Low Liquid Rate Desigsn..........................................................................................................1 1. Introduction .............................................................................................. ............................ 2 2. Tray Capacity ..................................................................................................... .................. 2 3. Entrainment..................................................................... ......................................................2 4. Efficiency.................................................. ............................................................................ 3 5. Number of Flow Paths ..........................................................................................................3 6. Operating Seal ................................................... ................................................................... 3 7. Weirs................................................ ..................................................................................... 3 8. Splash Baffles ....................................................................................................................... 4 9. Recessed Inlet Sumps .......................................................................................................... .4 10. Pipe Downcomers ............................................. .................................................................. 4 11. Gasketing ................................................ ............................................................................ 4 12. Turndown ................................................ ............................................................................ 4 13. Vacuum Distillation Trays ................................................... ............................................... 5

Page 1 of 5


1


1.16

Introduction

The one overriding fact about low liquid rate trays is the common report from operating units stating poor or deteriorated performance. A low liquid rate tray is considered as one having a net liquid rate of less than 0.5 gpm/sq ft (1.2 m 3/hr/m2) of tower area. Trays may be operated as low as 0.05 gpm/sq ft (0.12 m3/hr/m2), but would suffer most of the problems mentioned in this note. F.R.I. tests (13, 14) have shown stable operation at 0.08 gpm/ft sq (0.2 m 3/hr/m2), but generally with high entrainment - 40% to 1000% of net liquid flow. Efficiencies were not measured. Note that: a.

Low liquid rate implies small downcomer area. For simplicity, liquid rate is referenced to tower area rather than bubbling area as the result is essentially the same.

b. Many low liquid rate trays exhibit significant rates of entrainment. In this discussion the term "net liquid rate" refers to the net liquid flow down the column from tray to tray, ignoring the re-circulation of liquid through entrainment.

2

Tray Capacity

The capacity of sieve trays is reduced at liquid rates below 3 gpm/inch (27 m 3/hr/m) of outlet weir length (15). Recent tests (16) have shown that valve trays do not suffer as great a reduction. Although bubble cap trays are frequently used in low liquid rate service, their capacity is generally lower than both sieve and valve trays, due to higher entrainment generation..

3

Entrainment

Nearly all high vapor, low liquid rate trays operate in the spray regime. Under these circumstances blowing can occur whereupon the tray loses the liquid level and appears dry. Consider a tray operating in the spray regime with a normal liquid level on the tray. Potentially, liquid leaves the tray by three processes: a. entrainment to the tray above, b. liquid overflowing the outlet weir into the downcomer, and c. carryover of spray into the downcomer as vapor expands to fill the available free area above it. When the sum of the two entrainment components (a + c) equals the liquid feed to the tray, there is no liquid left to maintain a crest over the weir. Any further increase in vapor rate will blow the tray dry there will be little or no liquid on the tray floor. In conditions of high entrainment each tray receives entrainment as well as giving up an equivalent amount. If all the top tray entrainment is returned as reflux, a good operating seal may be established - as was the case in all the F.R.I. test work on both sieve and bubble cap trays. However, the bottom tray receives no entrainment from below and may well operate dry.

Page 2 of 5



1.16

In low liquid rate situations, particularly where no reflux is returned to the column, a demister should always be installed above the top tray. The distance between the top tray and the demister should preferably be 1.5 times the tray spacing; it should not be less than one tray spacing.

4

Efficiency

High levels of entrainment on low liquid trays can seriously affect tray efficiency. This is because the liquid entrainment constitutes a form of back-mixing and is best considered in relation to the net liquid flow. Some of the F.R.I. tests recorded entrainment levels up to 1000% of the net liquid flow and, although not measured, the overall efficiency must have been very low.

5

Number of Flow Paths

Low liquid rate trays should be designed for the fewest number of flow paths and particular effort is usually made to reduce the net weir length. Special orbit flow or maze flow path designs are often used to increase the liquid residence time and provide more liquid depth on the tray. There is no diameter limit for single pass trays, as there are a number of 50 ft (15 m) diameter wash trays in service.

6

Operating Seal

It is imperative that an operating seal be established on each tray. Start-up time is dependent upon the available liquid volume. Extra liquid flow during start-up is advisable to wet all components and establish the operating seal. Under certain conditions there is not enough liquid to establish the seal, especially for trays having only ½ inch (13 mm) of downcomer seal. For normal designs consider increasing the delta seal height between weir height and downcomer clearance. Maximize the weir height while minimizing the downcomer clearance. This approach requires a relatively large volume of liquid to completely fill the tray.

7

Weirs

Inlet weirs are frequently used to seal the downcomer using a smaller volume of liquid than normal designs. One shortcoming is that all the seal liquid must flow over the outlet weir into the downcomer, as leakage through a sieve tray cannot flow back over the inlet weir. Notched weirs (that is, weirs with triangular notches) are not required for liquid distribution as the tray normally operates in the spray regime with violent mixing action. One purpose for notched weirs is to increase the liquid depth on the tray. This may also be accomplished by increasing the weir height. Picket fence weirs (also referred to as castellated weirs) shorten the net overflow weir length as a means of increasing the height over the weir and liquid depth. Another feature is that a picket height of 1/3 to 1/2 the tray spacing will act as a splash baffle to prevent spray carryover into the downcomer. Consider the spray height when selecting the picket height.

Page 3 of 5


8


1.16

Splash Baffles

Vertical baffles placed along the outlet weir may be used to deflect spray and prevent blowing a tray dry. At liquid rates above 3.0 gpm per inch (27 m 3/hr/m) of weir, these may tend to increase the tray pressure drop. Special hoods placed over the downcomer area are sometimes used. However, these are complicated and expensive to install.

9

Recessed Inlet Sumps

The use of an inlet sump permits a downcomer to seal with minimum liquid volume. If the downcomer extends below the deck into the sump, then an operating seal is established at virtually any liquid rate. One advantage is that any leakage through a sieve tray at turndown conditions will preferentially flow into the sump. On the other hand a gasketed/bolted sump construction is very difficult to install properly and will likely leak more than the total liquid feed to the column. It is recommended that all low liquid rate sumps be seal welded in place to be leak free. This is the recommended design for low liquid rate trays.

10

Pipe Downcomers

Pipe downcomers are used to minimize both the downcomer seal area and the top area available for spray carryover. In most cases the best design also includes hats above the pipes and seal cups below. It is important to note that, in cases where the full vapor rate is flowing before the liquid rate can be established, vapor flow up in the unsealed downpipes may prevent liquid flow from becoming established and all liquid may then be blown overhead. It is therefore recommended that the liquid rate is established before the vapor flow commences. If, for any reason, this is not possible, the use of pipe downcomers is not recommended.

11

Gasketing

Gasketing should be considered for all low liquid rate trays. Judgment is needed to justify exotic or expensive gasketing. It should be noted that the extra tray installation time and labor to install gasketing adds significant cost. Large diameter towers are especially subject to having sizable gaps at panel overlap junctions, support rings and major beam intersections. (See Section 1.13 of this manual.)

12

Turndown

The turndown ratio for low liquid sieve trays is limited, as the dump point approaches or equals the weep point. Any weepage therefore will cause loss of operating liquid seal and allow vapor by-pass. Bubble cap trays are the better choice when low vapor rates are required in conjunction with low liquid rates.

Page 4 of 5


13


1.16

Vacuum Distillation Trays

This application pushes trays to their limit, especially large diameter columns. Sieve and valve trays have a very narrow range of acceptable performance when designed for minimum pressure drop. In many cases the specified pressure drop requires that design rates operate very near the weep point. Consideration should be given to using packing when the expected performance of trays is questionable.

Page 5 of 5


TRAY DATA SHEETS

Issued:

12/15/1990

1.17

Revised:

TRAY DATA SHEETS

The FRI "Standard" Tray Data Sheets were developed to enhance and improve communications between purchasers and suppliers of tower internals. Purchasers will benefit by receiving quotations that are more uniform and more likely to be fully responsive to their needs. This can reduce the bid evaluation effort and recycle with suppliers. The standard sheets will also help ensure that suppliers receive the information they require to quote each project to the customer's wishes. The FRI Standard Data Sheets are for the most part self-explanatory. In general, the more complete the data, the more uniform and "correct" will be the designs offered. Some of the process data defining vapor and liquid rates are redundant. This was intentionally done to provide flexibility, particularly since tabulated data from the various process simulators are not standardized. Redundant data need not be supplied but are useful to suppliers to catch inconsistencies or mis-copied numbers. Process data, whether from simulations or otherwise, must be the internal loads. Care must be taken where there are subcooled or superheated streams entering the tower. Pumparounds are a common case of a subcooled stream which can dramatically influence tower loadings. Alternate feed locations must be clearly indicated. The data sheets should define more than one set of conditions to show the design range for trays with alternate services. The "Derating Factor" is used to increase the design margin to ensure that the desired performance is met. This factor is applied to the capacity calculations to accommodate foaming, high pressure operation, or other characteristics which are expected to produce less capacity than predicted by the standard correlations. The correct design practice is to apply a single derating factor - usually, but not always, the most conservative value of those which apply. Derating factors typically range from 0.6 to 0.9 and can be different for trayed and packed towers. When specified these factors will be included in the calculations used to predict flooding values. Therefore, purchasers must be careful to avoid "double derating” (e.g. specifying a reduced maximum allowable % jet flood in addition to a derating factor). Similarly, if multiple derating factors apply, the values should not be multiplied. Because some derating is often included in a suppliers proprietary correlations to account for common problems, such as high pressure operation, entering a specific purpose for derating helps those suppliers avoid "double derating". Please refer to Section 0.03 of this manual for additional discussion of aerating. These data sheets are not intended to be stand-alone documents. The purchaser should include with these data sheets any additional information which could impact the design of the tower internals. It is good engineering practice to include sketches and/or drawings of the tower and any special features, such as existing internal supports, preferred or existing feed arrangements, drawoffs, etc. The locations of all inlets (including alternate feeds) and outlets should be clearly shown, including both elevations and orientations. The locations of vessel manholes should be shown and care must be taken that the manhole size noted on the data sheet is the inside diameter, not the nominal nozzle size. In addition to sketches and/or drawings, more pages of text may be required. This would almost always be true when the purchaser is making a " hardware purchase" and has chosen to define details of tray or packed tower geometry that are normally established by the supplier. In a less extreme case, the purchaser may have only a few specific requirements, such as a higher than typical outlet weir on a tray where a liquid residence time need exists. The presence of solids or a two-phase (vapor/liquid) feed will impact the selection and/or design of tower internals. Either occurrence should be accompanied by further explanation. Pertinent information on solids includes the amount and size distribution of the solids, their source, density, and their sticking tendency. Two phase (vapor/liquid) feed conditions should be described on the Feed Data Sheet. The key design information is the mass flow and density of the vapor and liquid phases which are normally obtained from simulation program stream summary printouts.

Page 1 of 8


TRAY DATA SHEETS

1.17

Also, included on the data sheet are a number of mechanical considerations often of specific interest to the process engineer preparing the data sheet. Some of these may also be included in the purchaser's mechanical specification(s) covering tower internals, but some may not. Each point should be considered. It should also be understood that completely filling out this data sheet does not negate the need for including the purchaser's general mechanical specification(s) or otherwise defining what general mechanical specification(s) apply. The data sheet was developed as a service to the membership and all other users, designers, and suppliers of fractionation equipment. It can be used as is, it can be used with any modifications the user desires; or it can be used as input to the user's existing form. The intent is to improve communications, and different users have different approaches as to how that should be done.

Page 2 of 8

Issued: 12/15/1990

TRAY DATA SHEETS

Revised:

1.17

Sheet 1 of 2 TRAY DATA SHEET

Client _______________________

Plant Location __________________

Engineer ________________

Job No. _____________________

Inquiry No. ____________________

Date ___________________

Item No. __________________________________

Service _________________________________

Tray No. 1 = Top/Btm

Section (Name/Description) Tray Numbers Included Loading at Actual Tray No. Number of Trays Required NORMAL VAPOR TO:

Rate, lbs/h Density, lbs/ft3 3

Rate, actual ft /s Molecular Weight, lb/lb mole Viscosity, cP Pressure, psia Temperature, °F Design Range, % of Normal NORMAL LIQUID FROM:

Rate, lbs/h Density, lbs/ft

3

Rate, US GPM (hot) Molecular Weight, lb/lb mole Surface Tension, dynes/cm Viscosity, cP Temperature, °F Design Range, % of Normal

Page 3 of 8

Issued: 12/15/1990

TRAY DATA SHEETS

Revised:

1.17

Sheet 2 of 2 TRAY DATA SHEET Item No. ____________________________________ Service ___________________________________

Section (Name/Description) Tray Numbers Included PERFORMANCE REQUIREMENTS:

Max. ΔP per Tray, mmHg Max. Allowable Jet Flood, % Max. % DC Liq. Velocity, % Max. DC Backup Clear Liq, in. Derating Factor, fraction Purpose for Derating (Foaming, System, Safety) MECHANICAL REQUIREMENTS:

Tower Diameter, inches Number of Passes Tray Spacing, inches Type of Tray Hole/B Cap Diameter, inches Deck Material/Thickness Valve/B Cap Material Hardware Material Support Material/Thickness Total Corrosion Allowance Vessel Manhole I.D., inches MISCELLANEOUS:

Solids Present: Anti-Jump Baffles: Recessed Seal Pans: Specify Equal Design Load: or

Yes / No Yes / No / Vendor Preference Yes / No / Vendor Preference Bubbling Areas / Flow Path Lengths per pass ____ PSF with ______ inch deflection at ___________ F. ____ Standard: 30 PSF with 1/8" at 300 F.

Page 4 of 8

Issued: 12/15/1990

TRAY DATA SHEETS

Revised:

1.17

Sheet 1 of 2 TRAY DATA SHEET

Client

EXAMPLE

Job No.

Plant Location

PN-979

Item No.

PASADENA

Inquiry No.

89-12345

C-1701

Engineer Date

Service

10/16/89 Splitter


Section (Name/Description)

Top

Btm - Max

Btm - Min

21-36

1-20

1-20

Loading at Actual Tray No.

21

1

1

Number of Trays Required

16

20

20

Rate, lbs/h

17280

27460

8630

Density, lbs/ft3

1.126

1.284

1.116

Pressure, psia

85

88

88

Temperature, °F

222

322

228

20-100

100

100

7510

38960

26360

33.75

35.46

34.82

Surface Tension, dynes/cm

7.75

7.39

8.34

Viscosity, cP

0.118

0.120

0.126

27-100

100

100

Tray Numbers Included

NORMAL VAPOR TO:

3

Rate, actual ft /s Viscosity, cP Molecular Weight, lb/lb mole

Design Range, % of Normal NORMAL LIQUID FROM:

Rate, lbs/h Density, lbs/ft

3

Rate, US GPM (hot) Molecular Weight, lb/lb mole

Temperature, °F Design Range, % of Normal

Page 5 of 8

DP

Issued: 12/15/1990

TRAY DATA SHEETS

Revised:

1.17

Sheet 2 of 2 Item No.

C-1701

TRAY DATA SHEET Service

Section (Name/Description) Tray Numbers Included

Splitter

Top

Btm - Max

Btm - Min

21-36

1-20

1-20

PERFORMANCE REQUIREMENTS:

Max. ΔP per Tray, mmHg

None

None

None

Max. Allowable Jet Flood, %

None

None

None

Max. % DC Liq. Velocity, %

None

None

None

Max. DC Backup Clear Liq, in.

None

None

None

Derating Factor, fraction

0.95

0.95

0.95

Safety

Safety

Safety

Tower Diameter, inches

30

30

Number of Passes

1

1

Tray Spacing, inches

18

24

Valve

Valve

--

--

410/14GA

410/14GA

Valve/B Cap Material

410

410

Hardware Material

410

410

Support Material/Thickness

CS/0.25

CS/0.25

Total Corrosion Allowance

None

None

Vessel Manhole I.D., inches

18

18

Purpose for Derating (Foaming, System, Safety) MECHANICAL REQUIREMENTS:

Type of Tray Hole/B Cap Diameter, inches Deck Material/Thickness

MISCELLANEOUS:


Yes X No ________ Yes X No X Vendor Preference Yes X No X Vendor Preference Bubbling Areas / Flow Path Lengths per pass ____ PSF with ______ inch deflection at ___________ F. ✓ Standard: 30 PSF with 1/8" at 300 F.

Page 6 of 8

Issued: 12/15/1990

TRAY DATA SHEETS

Revised:

1.17

Sheet 1 of 2 TRAY DATA SHEET METRIC

Client ______________________

Plant Location __________________

Engineer _______________

Job No. _____________________

Inquiry No. ____________________

Date __________________

Item No. __________________________________

Service ________________________________


Section (Name/Description) Tray Numbers Included Loading at Actual Tray No. Number of Trays Required NORMAL VAPOR TO:

Rate, kg/h Density, kg/m3 3

Rate, Actual m /s Molecular Weight, kg/kg mole Viscosity, mPa s Pressure, kPa (bar a) Temperature, C Design Range, % of Normal NORMAL LIQUID FROM:

Rate, kg/in Density, kg/m3 Rate, Actual m3/h Molecular Weight, kg/kg mole Surface Tension, mN/m(dynes/cm) Viscosity, mPa s Temperature, C Design Range, % of Normal

Page 7 of 8


TRAY DATA SHEETS

1.17

Sheet 2 of 2 TRAY DATA SHEET METRIC Item No. ___________________________________ Service ________________________________

Section (Name/Description) Tray Numbers Included PERFORMANCE REQUIREMENTS:

Max. ΔP per Tray, mmHg(mbar) Max. Allowable Jet Flood, % Max. % DC Liq. Velocity, % Max. DC Backup Clear Liq, mm Derating Factor, fraction Purpose for Derating (Foaming, System, Safety) MECHANICAL REQUIREMENTS:

Tower Diameter, mm Number of Passes Tray Spacing, mm Type of Tray Hole/B Cap Diameter, mm Deck Material/Thickness, mm Valve/B Cap Material Hardware Material Support Material/Thickness, mm Total Corrosion Allowance, mm Vessel Manhole I.D., mm MISCELLANEOUS:


Yes / No Yes / No / Vendor Preference Yes / No / Vendor Preference Bubbling Areas / Flow Path Lengths per pass ____ kPa (mbar) with ______ mm deflection at ___________ C. ____ Standard: 1.4 kPa with 3 mm at 150 C. Page 8 of 8


LEAK TIGHTNESS

Issued:

05/01/1989

1.18

Revised:

LEAK TIGHTNESS

Leak Tightness..........................................................................................................................1 1.

Effect of Leakage on Tower Performance.................................................. ........................ 2

2.

Leakage in the Tray ................................................... .........................................................2

3.

Tray Specification for Leakage Abatement .............................................. .......................... 2

4.

Leakage Test ....................................................................................................................... 3

Page 1 of 3


1

LEA K TIGHTNESS

1.18

Effect of Leakage on Tower Performance

Leakage of liquid from fractionation trays, internal pans, and packed tower distributors is always undesirable (24) but its importance varies dramatically with application. In most situations, some leakage can be tolerated. However, there are cases in which there are important incentives to minimize leakage and special provisions for leakage reduction are warranted. Some examples are: •

•

Low liquid rate applications where excessive leakage may cause channeling or drying up to take place on the tray; Packed column distributors, where excessive leakage may cause Maldistribution;

•

Total drawoffs, where liquid from the section above would contaminate the section below; and

•

Partial drawoffs, where excessive leakage may starve the draw off sump.

Liquid leakage in tray towers will lower tray efficiency, because it permits a portion of the liquid to leave the tray without proper contact with the vapor. The extent of this problem depends on the amount and location of the leakage, its fraction of the total liquid on the tray, the system involved, and the process operating conditions (24). In refinery operations, where much of the leak tightness tradition evolved, especially with respect to drawoff hardware, product degradation is a prime consideration.

2

Leakage in the Tray

Since a tray is assembled in panels and fitted around a tray ring, there are places where the metal - to -metal fit leaves gaps. Some common locations of liquid leakage are around the periphery of the tray where it lays on the ring, at Joints between tray plates, where chimneys join the tray deck, at manway joints, at joints between tray plates and outlet weirs, and between the weir and weir clamping bars. Severe, or even moderate, column out-of-roundness can result in badly fitted trays either at the periphery or at support beams.

3

Tray Specification for Leakage Abatement

In most sieve and valve trays, the amount of leakage would be small compared to tray weeping, and little leakage abatement is required other than ensuring all bolts are properly tightened or gaps between metal joints are properly covered by seal plates. Unique leakage abatement specifications are often made for chimney trays, distributors, draw pans, bubble cap trays, and some special valve trays in situations where leakage must be minimized. In these situations, gasketing (see Section 1.16 of this manual) is often required and a leak test frequently specified. When absolute minimum leakage is desired, the tray panels should be seal-welded, or special leakage-resistant designs should be used. When gasketing is used particular care needs to be taken at multi-panel joints and that gasket material is carefully placed. Sealing compound should be used with caution because of the possibility of attack by the process fluids with which it will be in contact. In these special situations, a leakage rate should be specified. A leak test measures the time it takes for liquid level on the tray or internal pan to drop a prescribed liquid height. In selecting the time specification (tolerance of the leakage), consideration must be given that in normal operation the liquid leakage will be less than that observed in the test because there is a pressure differential acting against the leakage.

Page 2 of 3

Issued: 05/01/1989

LEA K TIGHTNESS

Revised:

1.18

However, in some situations, the converse may occur, because thermal expansion at normal operating conditions may widen the gaps through which liquid leaks. This is very important because an unnecessarily stringent requirement will not only add cost to that tray but will increase the installation time and effort. A severe test should be imposed only when the liquid loading is very small or loss of product will result in a significant economic penalty. The following table was developed for testing bubble cap trays and drawoff pans for leakage tightness in refinery service. Leakage Class

SERVICE

1

All towers except those covered below in Class 2 or Class 3.

2

GPH/FT

3

2

M /H/M

3.0

0.12

2

Vacuum towers, except as in Class 3.

1.5

0.06

3

Trays immediately above the flash zones or the wash sections of vacuum towers.

0.5

0.02

Leakage rate is observed, and, hence, specified as loss of level on the tray (i.e. one inch (25 mm) of liquid level drop per given time). The most common time specification is a drop of one inch (25 mm) in 10 or 20 minutes roughly corresponding to Classes 1 and 2, respectively.

4.

Leakage Test

A leakage test can only be conducted inside the tower after the tray is properly installed. Before a leakage test is conducted, the weep holes should be plugged and the tray cleaned. The assembled tray should be visually inspected first to uncover any levelness problems, poor fit-up or other poor workmanship, or anything else causing an unsealed area. Then the tray should be filled with water up to the weir for a contacting tray or to a prescribed height for chimney trays. The drop in liquid level for a given time is then recorded. The leakage pattern can be observed under the test tray. Where gasketed joints are used, the engineer responsible for accepting the tray installation should be aware that it frequently requires several reassemblies before the required leak rate can be achieved

Page 3 of 3


CARTRIDGE TRAYS

Issued:

07/25/2008

1.19

Revised:

CARTRIDGE TRAYS

Cartridge Trays..........................................................................................................................1 1.

General ................................................................................................................................ 2

2.

Advantages and Disadvantages .......................................................................................... 2

3.

Performance Compared to Conventional Trays ................................................... .............. 5

4.

Installation Method Summary ................................................. ........................................... 5

5.

Tray Design Considerations ............................................................................................... 6

6.

Gasket Design Considerations ................................................ ............................................ 6

7.

Tower Design Considerations ............................................................................................ 7

8.

Tray/Tower Interfaces ................................................. .......................................................7

9.

Bundle Removal Consideration ............................................... ........................................... 8

10. Retrofits .................................................. ............................................................................ 8 11. Inspections during Fabrication ........................................................................................... 8 12. Installation Preparation ....................................................................................................... 8 13. Installation .......................................................................................................................... 9

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1

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1.19

General

Cartridge trays, also known as package trays or post supported trays, are widely used in columns too small for normal sectional tray installation and maintenance. This is generally considered to be 3 feet (0.91 m) in diameter or less. However, sectional trays have been provided to the industry in diameters as small as 2.4 feet (0.74 m). The decision of whether or not a tower is too small to install sectional trays will vary with the region of the world and the preference of the designer. Packing is usually the preferred choice for these small diameter columns, but there are several reasons why trays would be advantageous. Examples include fouling systems, certain foaming systems, high pressure applications, and systems that require high efficiency and a large turndown. Cartridge trays can be specified for columns up to 7 foot (2.1 m) in diameter if special circumstances warrant the extra design costs. Cartridge trays consist of a series of trays pre-assembled in a bundle. The number of trays contained in a bundle varies with tower diameter. Larger diameter bundles may only have 4-5 trays, whereas smaller diameter bundles may have 8-10 trays. The reason for the difference is the increased frictional resistance generated during installation between the tower shell and the bundle for larger diameter bundles. The trays themselves can be of any type: sieve, valve, bubble cap, dual-flow, etc. The tray decks are connected together by tie-rods that run the length of the bundle. Spacer rods are inserted over the tie-rods to ensure the correct tray spacing. The bundle is bolted at the ends to make a complete unit that is inserted through a body flange into the column. The bundles can be bolted together as they are installed so that all the trays in a section of column are connected together. The bundles rest on support clips welded to the shell at the bottom of a column section. The downcomers are envelope-style and are integral to the tray deck. The trays are sealed to the column wall by circumferential gaskets. Figure 1 shows bundles that are ready for inspection. Figure 2 shows a larger diameter application – 4.5’ ID (1.37 m). Figure 3 shows the detail of the connection between the bundles. The tie-rods have threaded connections on both ends. A bracket and double nuts are used to join the tie-rods together. The overall height of the bracket and nuts is set to give the proper tray spacing for the trays at the interface between the bundles.

Cartridge tray bundles are supplied fully assembled by the manufacturer. In rare cases, assembly may be quoted as an option to significantly reduce the purchase price. However, considerable time and effort will be needed to field assemble the bundles. Resolution of fit-up problems in the field is difficult and inefficient. In addition, field crews generally have little experience with cartridge trays. For these reasons, bundle assembly by the manufacturer is strongly recommended.

2

Advantages and Disadvantages

Cartridge trays are generally three to five times as expensive as conventional trays, but the column shell is less expensive because individual tray support rings are not required and commercially available pipe can often be used as the vessel shell. Cartridge tray columns may require more body flanges and manways to facilitate installation compared to a conventionally trayed column, but this is highly dependent on the system and on the tower design. Cartridge trays can be installed much quicker than conventional trays. A bundle of 8-10 trays can be installed in 1-2 hours whereas ring supported trays generally require 1-2 hours per tray. Maintenance of cartridge trays usually involves removing several or all bundles from a column, whereas ring supported trays can be accessed via column and tray manways. A study done by a member company in the 1980’s indicated that for towers less than 42” (1.07 m) diameter the total installed cost for a column and its internals is less for cartridge trays than ring supported

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1.19

trays. A rough comparison by a different member company in 2006 also found that the total installed cost for a 42” (1.07 m) diameter tower would be approximately the same for cartridge trays and for ring supported trays. Conventional wisdom indicates that ring supported trays can be installed in towers that are at least 36” (0.91 m) diameter, but it is difficult to work within such a tight space. Table 1 shows the advantages and disadvantages of each type of installation and may help facilitate the decision between the two types of installations for a 36” (0.91 m) tower.

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Table 1: Advantages and Disadvantages of Cartridge Trays and Ring Supported Trays For 36” Diameter Towers Cartridge Trays Ring Supported Trays Work is done outside of column.

Do not have to work in a tight space. Installation

Crane work always required.

Each tray is individually attached to a tray support ring by a clamping and bolting system.

36” (0.91 m) ID column is a very If column is out round, then installation tight space to work within can be hard or impossible, depending on the severity of t the out-of-roundness. Time for Installation

Approximately 1-2 hours per bundle

Approximately 1-2 hours per tray

Shell must be built to special roundness tolerance to facilitate insertion of the bundle. Have empty shell with support clips at bottom of each section. Nozzles need to be welded to the outside of the shell so there is no interference with the cartridge insertion. Shell Design

All welds must be ground flush with the ID of the shell. Where nozzles are welded to the shell care must be taken to minimize distortion and flattening of the shell.

Each tray has a tray support ring welded into the vessel shell. Adds 10% to the cost of vessel needed for cartridge trays due to additional labor and materials for the rings. Possibly fewer or even no body flanges needed.

May need more body flanges, which are expensive and must be level and square. May want more vessel manways to gain access for inspection & repair. 4X Tray Cost Requires all connected bundles to be removed from column. Gaskets usually Future Maintenance must be replaced before bundles are reinserted. Numerous body flanges increase the Operation potential for leaks. Page 4 of 16

1X Tray manways allow access to individual trays. Vessel manholes are easier to seal.


CARTRIDGE TRAYS

1.19

There is another factor that might influence the decision between using cartridge trays and ring supported trays for an application. The designer must pay extra attention to detail during the design, fabrication, and installation phases of a cartridge tray project to make it successful. For this reason, many designers will only specify cartridge trays when the job can be done no other way. Other designers choose to increase the tower diameter to 36” (0.91 m) to be able to specify ring supported trays, thereby avoiding the extra engineering time required for cartridge trays.

3

Performance Compared to Conventional Trays

While most design considerations and operating characteristics for package trays are unchanged fromthose of sectional trays (see FRI Handbook, Volume I), there are several exceptions. Some recommend derating the capacity of all small columns (diameters less than 36" (0.91 m)) by 10-20%. (1) To maintain mechanical integrity, a cross-flow cartridge tray must utilize envelope type downcomers. Although the area behind the downcomers is available for vapor-liquid disengaging, the correlations developed for segmental downcomer inlet velocity may not apply. Due to the envelope downcomer, seal ring, and tray support rods, a cartridge tray will have significant wasted area. See Figure 4. Therefore, the calculations of effective bubbling area, downcomer area, and outlet weir length are not as straight forward as for conventional trays. For these reasons final cartridge tray performance determinations are often best left to the manufacturer. Finally, small diameter columns will have short flow paths, which may reduce the tray efficiency by decreasing the efficiency boost caused by the cross-flow effect.

4.

Installation Method Summary

The installation method for cartridge trays drives the need for attention to detail during the design, fabrication, and installation phases of a project. During installation, each bundle is inserted into the column through a body flange. Often times, all trays are inserted through the top body flange, so the area above the column must be available to work in. First, the bundle that contains the bottom trays is lifted with a crane above the body flange. The proper orientation is established, and then the bundle is lowered into the column. Figure 5 shows a bundle being installed. Just before the last tray of the bundle is inserted into the column, two wood beams that can support the weight of the bundle are inserted under the tray. The bundle is disconnected from the crane, and the partially inserted bundle rests on the wood beams. The next bundle is lifted, its orientation established, and then bolted to the partially inserted bundle. The wood beams are removed, and the joined bundles are then lowered into the column to just before the last tray where the wooden blocks are again inserted. This process is repeated for as many bundles as necessary. As the trays are lowered into the column, the circumferential gaskets ride down the column, creating the seal with the wall. The trays eventually rest on support clips at the bottom of the column. Figure 6 shows one type of gasket during installation. If everything goes well, the bundles will be inserted into the column under the power of their own weight, the gaskets will seal properly, the trays will rest properly on the support clips, and all equipment will mate up at designated places. Cartridge trays should always be installed vertically on site for many reasons: •

to prevent damage to the trays and to the gaskets during installation and shipment Page 5 of 16


• • • •

5

CARTRIDGE TRAYS

1.19

to prevent rotation of the bundles during transport to ensure the bundles are resting evenly on the tray support clips to ensure proper mating of bundles between tower sections to ensure proper mating of tower internals

Tray Design Considerations

Think of cartridge trays as being akin to a pre-fabricated house. They must endure shipping and installation without losing their structural integrity. The tray spacing and overall bundle length should not change even when they are shipped over bumpy roads and lifted by cranes in their shipping crates. Specifying heavier duty hardware and tray decks helps to make the bundles more rigid and less prone to coming loose during shipment and installation. Specify stronger tie-rods and spacer pipe than the standard vendor design: ½” (13 mm) diameter tie-rods with ½” (13 mm) SCH80 spacer pipe (3). This results in a very tight fit between the tie-rod and spacer pipe and gives the bundle more rigidity. Specify 10 ga tray decks for the same reason (3).

6

Gasket Design Considerations

The gasket design is very important and extra thought should be given to it during the design phase of a project. Tray performance will suffer greatly if the gasket seal loosens over time or fails completely due to incompatible materials of construction. Figure 7 shows several different gasket designs. One type of flexible gasket seal is shown in Figure 7a. The gasket material can be made of any flexible material, including flexible metal strips. This design makes the bundles the easiest to install and remove because the gaskets flip and slide along the tower wall. See Figure 6. Request a heavy-duty clamp ring and check the design to ensure plenty of bolting. (3) The gaskets and clamp ring are usually cut into 6 or 8 pieces for cost effectiveness. It is very important that the pieces of gasket and clamp ring do not end at the same place, making a spot that is likely to leak. This is also illustrated in Figure 6. Straight gasket pieces forced to the circumference of the tower are discouraged. Gasket material should be cut to size from sheets. An alternative flexible gasket seal, the “C” channel double gasket, is shown in Figure 7b. The rope gasket seal ( Figure 7c) normally uses a standard gasket material and makes the bundles fairly easy to install. Keep in mind that some gasket materials tend to "uncoil" after they are rolled into the ring. Tie wires and special crating are used to keep the gasket in place if they are installed at the tray fabricator’ s shop. The compression ring, also called the piston ring, type ( Figure 7d) offers a reliable liquid seal and can be reused when the bundles are removed. (2) The bundles are more difficult to install/remove than the other designs due to metal-to-metal friction between the gasket and the tower wall and due to the rust that often develops at this interface. This design is commonly used in the natural gas industry, in high temperature applications where finding reliable gasket materials can be problematic, and in instances where the trays are not intended to be removed. In practice, most end-user companies have developed preferences for one design or the other based on their particular needs and experience. It is recommended that gasket types 7a, 7b, and 7c be installed after the trays have arrived on site to avoid damaging them during shipment.

Page 6 of 16


7

CARTRIDGE TRAYS

1.19

Tower Design Considerations

The roundness of the tower is the most important part of a cartridge tray installation. The trays must seal to the tower with the gasket, or they won’ t work! But, if the tower shell is too tight, the trays get stuck! There are anecdotes of installations where pressure had to be applied to force the trays past a tight spot. There is a risk of seriously damaging the trays when this happens. Therefore, the roundness specification for the tower shell and how well the vessel fabricator can deliver on this specification will determine how easily the trays are installed. Communicating the importance of the roundness specification to the vessel fabricator early and often is recommended. The problem that usually occurs is that any place where there is significant welding on the tower causes the metal to pull in, resulting in a tight spot. Large nozzles, lifting lugs, and vessel supports are the usual culprits. The ASME specification for out-of-roundness is 1% of the OD for pressure vessels and roughly 0.5% of the OD for vessels rated for full vacuum. (There is a formula for vessels rated for full vacuum.) If the vessel is rated for full vacuum, make sure to call attention to this while reviewing fabrication drawings. Thicker shells make it easier to meet the roundness specification, so pay closer attention if there is no corrosion allowance. The tray fabricator should supply a template to the vessel fabricator to pass through the column to ensure that the bundles can pass. However, this is done after the column is fabricated. Repairs can be made to tight spots by applying pressure to the shell with a hydraulic jack. The repair must be done in a way to ensure that an inelastic force has been applied that will not relax during shipment of the vessel. Clever vessel fabricators will apply a counterforce while welding these problem areas and avoid the need for repairs all together. It is important to have access to all sets of tray support clips so that visual verification of the bottom tray of each section resting properly on its support clips can be made. If the trays are not resting evenly on their support clips, this situation must be remedied to ensure individual tray levelness and proper operation of the trays.

8

Tray/Tower Interfaces

During the design phase of a project, consider how other tower hardware will mate up with the trays. The orientation and elevation of all hardware that is installed after the tray bundles (thermowells, feed pipes, etc.) must be considered so that it does not hit an obstruction. It is also important to make allowances for slight imperfections in the elevation and orientation of the bundles. For instance, a reflux pipe, a feed pipe, or a thermowell should not have to mate up to other tower elements to a very close tolerance. The bundles may rotate slightly during installation, or the height of the bundle may be off by 1/8” -1/4” (3-6 mm). The column must have a smooth wall to permit bundle installation, so pay close attention to feed and draw-off arrangements. Reflux is usually introduced behind a false downcomer panel on the top tray. Vapor or liquid feed pipes can be designed as “stab-in" piping. Seal pans for draw-offs cannot utilize the column wall. Sometimes there is a pressing need to weld internals to the column shell. Examples include a flashing feed device or a chimney tray with a liquid draw-off. The bundles cannot be installed past such obstructions, so the column must be divided into more than one section. Tray support clips are welded to the shell at the bottom of each section. The downcomer from the bottom tray in the upper section is required to mate up with the trays in the lower section of the column. Ensuring that there is adequate downcomer clearance at this interface is very important to prevent premature downcomer flooding. A tower manway should be incorporated into the shell design at these points to permit visual verification and/or field repairs. See Figure 8. Have a method prepared ahead of time for a field repair and incorporated into the tray design, just in case it is needed.

Page 7 of 16


9

CARTRIDGE TRAYS

1.19

Bundle Removal Consideration

Removing cartridge tray bundles from a tower can be difficult in itself, but removing bundles from a tower in a fouling service is guaranteed to be challenging. The need for removing the bundles for maintenance or future capacity expansions should be considered during the design phase of a project. Modifications can be made to the bundle and column design to facilitate their removal. Using a gasket design like the one shown in Figure 7b is recommended for fouling services. Using a reinforced plastic gasket such as Teflon® steam valve packing gives good sealing and strength, yet it is more likely to yield sufficiently when removing the bundle. Pay special attention to the strength of the bundle and consider specifying stronger tie-rods and spacer pipe so the bundles can better withstand the forces required for removal. A support structure, a method for pulling the bundle out, and sufficient clearance around the tower to accommodate the bundle as it is removed are all worthy of consideration. One member company with a tower in an extreme fouling service devised special mechanical devices to push the cartridges out the bottom of the tower for periodic cleaning. cleaning. In other cases, column shells have been removed from the plant, plant, laid out horizontally, and the bundles pulled out using a bull-dozer.

10

Retrofits

A word of caution about retrofitting an existing column shell for cartridge trays. Ideally there would be some way to measure the column’ s roundness before proceeding with the design. Knowing if there are tight spots or places where the column exceeds its maximum diameter is key to understanding how to modify the tray design to make up for these deficiencies. If an inspection cannot be done before the design proceeds, specify wider gaskets and smaller tray diameters than the standard design. Be prepared to reuse the old internals if the new cartridge trays cannot be installed due to unexpected tight spots in the column. In addition, all existing tower attachments (clips, rings, internal feed attachments, etc.) must be ground flush to the vessel wall. Care must be taken not to compromise the integrity of the vessel during this process.

11

Inspections During Fabrication

Cartridge tray projects are not the place for skimping on inspections. Inspect the column and witness the passing of the template at the vessel fabricator’s shop to verify the roundness. The other items that should be inspected carefully at the vessel fabricator shop are the location and levelness of the tray support clips and the squareness and levelness of the body flanges. These items ensure that the trays will be level and will mate up properly, which is critical for good operation. Inspect the trays at the tray fabricator shop and ask to see the shipping crates. Cheap shipping crates could result in damage to the bundles and unnecessary repairs at the site. The cartridge itself must be checked for roundness and squareness of the complete bundle unit. If the trays are out of round, the cartridge will not fit in the shell.

12

Installation Preparation

There are many things that the project team can do several days before installation to ensure a smooth installation day. It is a good idea to pass the tray template through the column after it arrives on site to make sure that any repairs to the column have not relaxed, causing the vessel to pull in again during shipment. Deliver the bundles close to the installation site, remove them from their shipping crates, and reinspect them. Recheck the bundle length, tray spacing, and make sure all bolts are tight. Make sure nothing is loose or has been damaged during shipment. Pay careful attention to the tray gaskets. It is better Page 8 of 16


CARTRIDGE TRAYS

1.19

to install the gaskets after the trays have arrived on site because they will likely be damaged during shipment. Make sure there are spare gaskets available in case they are damaged during installation. When using the flexible gasket design, ensure no metal pieces like washers extend beyond the metal tray deck. If a metal gasket ring is used, a compression device will be needed to compress the ring for insertion in the shell.

13

Installation

If the bundles have been prepared ahead of time and the tray template has been passed through the column onsite as suggested in the above section (Installation Preparation), then the two main issues to deal with on installation day are: 1. Maintaining the orientation of the bundles during insertion, and 2. Ensuring the bottom tray decks rest on the support clips. Maintaining the orientation of the bundles as they are lowered in the column is important im portant to make sure that other tower attachments will mate up properly. Choose the orientation that will be used for matching. Make match marks on each of the trays or gaskets and on the column body flange. Once the trays are installed, it is important to make sure that the bottom tray of each section is resting on its support clips. Visual verification through a manway is the best way to do this. The trays may rotate slightly as they are pushed in the column, and a bolt may end up hitting the support clips instead of the tray deck. If a bolt is hitting the support clips, then the tray must be rotated so that the deck will rest on it instead. Otherwise the trays will not be level and performance will suffer. Be prepared ahead of time by having a confined space entry permit ready. If manway access is not possible, then measure from the top tray deck to the top of the flange in several places to check for the correct elevation and for levelness. Another option is to use a camera that could be snaked through a nozzle to provide the visual verification that the deck is r esting on the support clips.

References

1. Glitsch Inc., "Ballast Tray Design Manual", Bulletin 4900, 6th Edition, 1993. 2. Nutter, I.E., "Self-Sealing Pre-Assembled Fluid Fluid Contact Tray Unit", US Patent No. 3,179,389, 3,179,389, April 20, 1965. 3. Sands, Ruth R., “Distillation: How to Specify & Install Cartridge Trays” , Chem. Eng., Vol. 113, No. 4, pp.86-92 4. Sulzer Chemtech, Inc., “Package Trays for Pipe-Size Process Vessels” , Bulletin PT-1.

Page 9 of 16


CARTRIDGE TRAYS

Figure 1: Cartridge Tray Bundles Ready for Inspection Upper photo courtesy of DuPont. Lower photo courtesy of Sulzer Chemtech Page 10 of 16

1.19


CARTRIDGE TRAYS

Figure 2: Larger diameter application – 4.5’ ID (1.37 m). Photo courtesy of Sulzer Chemtech.

Page 11 of 16

1.19


CARTRIDGE TRAYS

Figure 3: Schematic showing detail of the connection between bundles.

Page 12 of 16

1.19


CARTRIDGE TRAYS

Figure 4: Schematic showing wasted area due to seal ring.

Page 13 of 16

1.19


CARTRIDGE TRAYS

Figure 5: Cartridge Tray Bundle Being Installed

Figure 6: Circumferential Gasket Detail Page 14 of 16

1.19


CARTRIDGE TRAYS

7a. CLAMP RING FLEXIBLE GASKET

7b. “C” CHANNEL DOUBLE FLEXIBLE GASKET

7c. ROPE GASKET

7d. PISTON RING GASKET

Figure 7: Cartridge Tray Seal Mechanisms Page 15 of 16

1.19


CARTRIDGE TRAYS

Figure 8: Schematic Showing the Importance of Mating Two Sections of Column Together

Page 16 of 16

1.19


DE-RATING FACTORS – TRAYED COLUMNS

Issued:

01/15/1994

1.20

Revised:


De-Rating Factors – Trayed Columns.......................................................................................1 1. Introduction .............................................................................................. ............................ 2 2. De-Rating Staging Calculations ........................................................................................... 2 3. De-Rating Tray Hydraulics Calculations............................................... ............................... 5

Page 1 of 10


1


1.20

Introduction

Many facets of distillation column design involve some degree of uncertainty. Inaccurate, or possibly estimated, system properties (physical properties and vapor/liquid equilibrium, or VLE, data) and errors associated with design correlations for hydraulics and efficiency are among the more obvious sources of uncertainty; other sources of error are listed in the FRI Annual Report from 1973. To account for these uncertainties, the column design is often de-rated by applying correction factors, known as de-rating factors, to the design calculations. Three de-rating factors have been defined by the Design Practices Committee: Foaming Factor, System Factor, and Safety Factor. Extensive definitions of these de-rating factors are given in Section 0.03 of Volume 5 of the FRI Design Handbook. Brief definitions are provided below for convenience: Foaming Factor - allows for the tendency of the system to foam; System Factor - provides a margin of safety when designing outside the range of the physical property database of a design correlation; Safety Factor - allows for uncertainty in the design correlations.

De-rating factors can be applied at many points in the design procedure, both in the staging and hydraulic calculations. Often, a single de-rating factor is used to account for uncertainties in more than one area. Sometimes de-rating factors are applied implicitly (when rules-of thumb are used, for example), so the designer may not even be aware that the design has been de-rated. Occasionally, a design is de-rated in several places. For designs involving great uncertainty, particularly new designs, it is very tempting to derate every step of the design. Such indiscriminate application of de-rating factors will result in fractionation equipment that is oversized, and probably will provide much less turndown than is actually desired. This is a problem of particular concern when several individuals are assigned various responsibilities in the design procedure. For example, a thermodynamicist may add safety factor to the physical properties and VLE by predicting the separation to be more difficult than it really is. The process engineer may perform the staging calculations using a reflux ratio greatly in excess of minimum reflux and product specifications much more stringent than actually required. The engineer performing the hydraulics calculations could then set the tower diameter, tray spacing, and downcomer dimensions such that the column operates at, say, 70% of its predicted jet flooding velocity, downcomer backup does not exceed, say, 50% of the tray spacing, and the velocity in the downcomer is less than, say, 50% of the choke velocity. Foaming and system factors could be applied as appropriate to further de-rate the hydraulic design. While de-rating a design is prudent to ensure successful operation, it is important to realize when de-rating factors have been applied and to avoid excessive de-rating such as that illustrated above. Excessive de-rating of a design can result in a greatly oversized tower, which will in turn lead to oversized auxiliaries. The application of de-rating factors to staging calculations is discussed briefly below, and the de-rating of hydraulics calculations is discussed in the following section.

2

De-Rating Staging Calculations

The number of trays required to make a given separation can be determined by a variety of procedures, generically referred to here as staging calculations. There are at least three places in the staging calculations where the design can be de-rated: the System Properties used in the calculation, the Internal Traffic determined by the calculation, and the Tray Efficiency used to convert the number of ideal stages to a number of real trays. De-rating the design to account for uncertainty in one area may be equivalent to de-rating in another area. Further comments on de-rating these areas of the staging calculations are provided below. As always, the designer should realize when de-rating factors have been applied and be

Page 2 of 10



1.20

wary of double de-rating. System Properties - The number of stages is found by some staging calculation, usually performed by computer. The accuracy of the simulation is highly dependent upon the accuracy of the system properties. For situations where the VLE and physical properties are known, the results of the staging calculation are often used directly, without modification. In some situations, however, the staging results may be de-rated to account for the inability of binary interaction models to predict the true behavior of the fluid mixture. This may be done by assuming worst-case system properties such that the most difficult separation, within reason, is predicted.

Often, preliminary designs (and sometimes final designs) must be based on estimated system properties. In these situations, one way of de-rating the staging calculations is to use more stringent product purity specifications than are actually required. This effectively increases the number of stages and the reflux rate required to make the separation. When the design is de-rated in this manner, additional de-rating factors are usually not applied. An alternative method of de-rating the staging calculations is to perform the calculations with the proper specifications and/or non-conservative predictions of system properties, then arbitrarily increase the number of trays. The potential effects of this de-rating on tower performance can be assessed by resimulating the tower with the additional trays. If the resimulation is performed with the reflux held constant, then adding trays is equivalent to using a more stringent purity specification - how much more stringent can be determined from the results of the resimulation. The resimulation could also be performed with reduced reflux (within the constraints of minimum reflux, of course) in order to hold the separation constant. If the original system properties (and estimate of tray efficiency) were correct, then the actual tower may very well be operated in this way. It is important that the designer recognize the implications of the additional trays and ensure that the internals can operate properly at the reduced reflux rates if the system properties and tray efficiency allow. Refer to the discussion of efficiency de-rating for additional comments and warnings on de-rating designs in this way. Internal Traffic - The liquid and vapor rates used to perform the hydraulic calculations are obtained from the column staging calculations and are influenced by the number of stages used in the design. For a given separation, reflux (and, hence, the liquid and vapor rates in the tower) and the number of ideal stages are related; a typical reflux versus stages curve is shown in Figure 1. The asymptotes on the curve represent the minimum reflux and the minimum number of stages needed to make the separation. Minimum reflux and stages can be determined by using rigorous computer simulations to construct a diagram similar to Figure 1, or by using Chien's method. Determination of these limiting values by shortcut methods is discussed by Henley and Seader (53),(54). The actual reflux is usually selected to be 1.1 to 1.5 times the minimum; in the face of uncertainty, one may increase the selected ratio. For instance, if 1.15 times the minimum is the economic optimum, the presence of uncertainty may favor the selection of a higher ratio such as 1.2. Often, the number of stages is set according to the optimum ratio (1.15 in this example), while reflux and reboil according to the higher ratio (1.2 in this example). This practice leads to vapor and liquid rates, reboiler and condenser duties and pump duties which are all de-rated. It is important to remember that additional de-rating factor will probably be applied to the hydraulic calculations (the tower might be designed at, say, 80% of the jet flooding velocity). Double de-rating the design in this way will result in a column much larger than actually necessary, and could affect hydraulic flexibility and thus the type of tray selected.

If the reflux ratio is selected to be some multiple, say C, of minimum reflux, then the required number of stages can be determined directly from the reflux versus stages curve, as shown in Figure 2. The number of stages used for design may then be increased such that the reflux requirements are fairly insensitive to the number of stages that are actually developed in the tower (N1 on Figure 2). If the design reflux ratio remains at C*R MIN, as shown, then the design point has effectively, but arbitrarily, been moved to a curve

Page 3 of 10



1.20

based on a higher purity product specification. De-rating the design by using higher product purities than required was described in the discussion discussion on de-rating system properties. properties. The designer should be cognizant cognizant of this de-rating if the the tower is designed designed by selecting N and R in this way. As mentioned previously, previously, additional de-rating is probably not justified. A similar situation may arise arise when designing internals internals for tower revamps. If the tower is being revamped revamped with higher capacity internals, then there there may be some uncertainty as to how many stages will be lost. In this event, it may be necessary to design on the portion of the reflux versus stages curve where the number of stages developed is insensitive to reflux ratio (N2 on Figure 2). If the internals are designed at a higher reflux ratio than necessary (R2 on Figure 2), then the design has effectively been de-rated by designing to a higher purity than is required. The designer should be aware of the effect of such a de-rating, and should ensure that the internals selected can operate properly at the most optimistic, as well as the most pessimistic, prediction prediction of reflux rate. Efficiency - Tray efficiency has been described as the last great frontier to be conquered by distillation researchers. Accurate prediction methods that can be applied applied with confidence are not yet available, and so it is not possible to give any quantitative advice regarding efficiency prediction that can be applied to the entire spectrum of feasible separations. Some designers report excellent results when estimating tray efficiency for hydrocarbon systems using methods methods based on the two-film theory (such as the FRI or AIChE models). Other designers have reported very poor results when applying these same models, particularly to highly non-ideal chemical systems. Strictly empirical methods are sometimes used because the methods are simple to use and the designer feels that the correlation does a reasonable job of predicting the tray efficiency of the system of interest. Two such correlations are commonly used: the Drickamer and Bradford correlation is used to predict tray efficiencies in systems comprising homologous hydrocarbons only, and the O'Connell correlation is applicable to all distillation systems.

Efficiency prediction methods are based on limited data however, and may or may not include data that reasonably approximate approximate the system of interest. interest. The best efficiency estimates estimates can be obtained from carefully collected and analyzed pilot plant data, research data (such as that produced by FRI), or high quality plant data on a system system similar to the one of interest. interest. Plant efficiency data should be of of sufficient quality if they satisfy the guidelines guidelines set forth by AIChE in their their column testing procedure: material balances should close to within 3% and energy balances to within 5% (refer to the AIChE Equipment Testing Procedure for Tray Distillation Columns for additional details) (28). Information of this quality is not always available, however, forcing the designer or troubleshooter to rely on some estimation method. Because of the uncertainty associated with the prediction methods, it seems reasonable to apply some derating factor to the estimate. estimate. Unfortunately, the open literature literature offers little guidance guidance on how to do this. FRI recommends subtracting 10 efficiency points from the efficiency predicted for dualflow trays by the methods described in Section 3.4 of Volume 1 of the FRI Handbook (55) ; extending this correction to other tray types, however, is not appropriate. The inherent uncertainty in the efficiency prediction methods and the scarcity of recommended means for de-rating efficiency estimates should serve as a warning to the designer or troubleshooter troubleshooter to exercise caution. The uncertainty associated with with efficiency prediction also underscores the need for pilot testing when working with new or unusual systems. In some cases, the efficiency estimate estimate may be implicitly de-rated. de-rated. For example, tower designs designs in which the number of trays is set arbitrarily are often greatly over designed. designed. The "required" number of trays may have been determined many years ago, and not reduced in newer installations despite subsequent advances in distillation technology. technology. The overdesign of such towers is evidenced by the fact that they often often continue to perform acceptably even when several trays are dislodged during upsets. For designs involving a fair degree of uncertainty, it is tempting to add a few trays to the final design in order to increase the chances of the column making the desired separation. separation. The incremental cost of a few trays and some additional shell height is not great, and should certainly be considered along with other alternatives for ensuring successful successful operation of the column. column. However, the designer must be aware that that Page 4 of 10

Issued: 01/15/1994


Revised:

1.20

this practice is, in effect, an arbitrary arbitrary de-rating of the column column efficiency. The design could have have similarly been de-rated by arbitrarily increasing the diameter to allow additional reflux; this may limit turndown, however, particularly if sieve trays are to be used. used. If either method of arbitrarily arbitrarily de-rating efficiency efficiency (either extra trays or diameter) is used, the tower performance should be assessed, by simulation, at turndown conditions using the maximum expected efficiency to ensure that the desired range of operation can be obtained in the event the trays do perform at the efficiency predicted predicted without de-rating. de-rating. The results may lead to the use of valve trays where they might not have been considered otherwise.

3

De-Rating Tray Hydraulics Calculations

FRI has identified three capacity limits for trayed columns, columns, and has included correlations correlations in the FRI Design Handbook for their prediction: Jet (Entrainment) Flooding - Massive entrainment of liquid to the tray floor above fills the tray space with liquid and floods the tower. Downcomer Backup Flooding - Liquid cannot exit the downcomer at an acceptable rate; first the downcomer, then the tray space, fills with froth or foam, flooding the column. System Limitation - An ultimate capacity limit, related to system properties but not related to the type or configuration of the tower internals.

A fourth flooding flooding criterion, downcomer downcomer choke, is commonly applied applied by designers. designers. Downcomer choke flooding occurs when froth cannot enter the downcomer at an acceptable rate; the tray spacing then becomes filled with liquid and the tower floods. FRI has developed a correlation for predicting downcomers maximum inlet velocity, velocity, but this prediction is not not an FRI flood criterion. Other downcomer choke prediction prediction methods methods are available available elsewhere which are used used by many many designers. However, designers often use the FRI correlation as a conservative downcomer choke prediction method. A fifth capacity limit, Blowing Flood, has also been observed, and is considered by some vendors when performing tray ratings. This phenomenon is of concern at low liquid to vapor rates: excessive vapor velocity though the the active area blows the liquid liquid clear of the tray deck. FRI does not include a separate separate correlation for blowing flood in the Design Handbook. FRI defines a safety factor for each of the three capacity limits described in their Design Handbook as well as the downcomer inlet velocity (downcomer choke) criterion:

⎡ V load @ F lood ⎤ ⎥ ⎣V load @ Design ⎦

Safety Factor ≥ ⎢ Where:

V load = V

ρ V ρ L

V ρV ρ L

= = =

− ρ V

vapor rate, ft 3 /s V m3 /s vapor density, lb/ft 3 V kg/m3 liquid density, lb/ft 3 V kg/m3 Page 5 of 10



1.20

Because of the error associated with fitting a correlation to observed data, errors associated with the observations themselves, and error that may be incurred when extrapolating capacity correlations to untested systems, the safety factor for each of the capacity limits is typically selected to be greater than 1.0 For sieve and bubble bubble cap trays, FRI provides provides charts of probability probability of flooding flooding versus safety (56),(57) factor ; safety factor may be set to make the probability of flooding as low as the designer wishes (see Section 5.2 of Volume 1 of the Design Handbook for details). Many procedures have been published published for predicting the velocity velocity at which jet flooding flooding will occur. The design procedures developed by internals vendors are among the methods most commonly used for designing sieve and valve trays. Vendor recommendations, recommendations, along with other published guidelines, guidelines, for the maximum percentage of the flooding velocity at which a column should be designed are listed in Table I. Note that the de-rating factors used in the vendor procedures are all less than one, unlike the safety factor defined by FRI, which is greater than one (the reciprocal of the F RI de-rating factor can be used for direct comparison). The de-rating factors recommended by vendors appear in the denominator denominator of the downcomer or active area calculation and have the effect of increasing the required area. Some designers prefer to design to a given percentage of the flooding velocity, say 85%, in order to have the trays operate at maximum maximum efficiency. Such a design has not not been de-rated, per se, since the the velocity was not selected to compensate for uncertainty in the design correlation, but to obtain a specific point on the known efficiency versus vapor vapor rate curve. For towers designed in this way, additional additional de-rating of the active area should be undertaken with caution if at all. Most vendor design procedures also involve a factor to account for the tendency of the system to foam. This foam factor is applied to the jet flooding correlation and to the downcomer calculations and results in more active area and downcomer area than would necessary to achieve the same throughout if the system did not not foam. A consolidated listing of published foam factors is provided in Table II. Because FRI has not tested very many foaming systems, their design procedures do not include foam factors, although the bubble cap and sieve tray procedures do recommend that greater safety factors be used for "mineral oil absorbers and strippers, and for systems which foam badly." Weeping does not constitute a capacity limit, but does represent the lower operating region of the column (a significant amount amount of weeping can be tolerated without severely degrading efficiency). The FRI method for calculating weeping from sieve trays allows a weeping safety factor, analogous to the capacity safety factors, to be be calculated. No recommendations for acceptable limits of such such a weeping safety factor factor are supplied, however, since the amount of weeping weeping that that can be tolerated depends upon the turndown desired for the column. If the desired turndown cannot cannot be achieved with sieve trays, it may may be necessary to use valve trays. In Section 5.5 of Volume 1 of the Design Handbook, FRI reports a standard deviation of 0.095 in the correlation of the weep-point data (58). Ruff recommends a safety factor of about 40% on the predicted vapor velocity required to ensure no weeping, but this recommendation is based on a purely theoretical analysis analysis (59).

Page 6 of 10



1.20

References

28. AIChE Equipment Testing Procedure. Tray Distillation Columns. A Guide to Performance Evaluation. 2nd Edition, 1987. 53. Chien, H.Y., "Rigorous Method for Calculating Minimum Reflux Rates in Distillation," AIChE Journal, 24(4), 606, 1978. 54. Henley, E.J., and Seader, J.D., "Equilibrium- Stage Separation Operations in Chemical Engineering", John Wiley & Sons, New York, 1981. 55. FRI Handbook, Vol. 1, Section 3.4, p.10. (Dualflow Safety Factor). 56. FRI Handbook, Vol. 1, Section 4.2, p.2. (Bubble Cap Safety Factor). 57. FRI Handbook, Vol. 1, Section 5.2, pp.1-4, 12, 14 (Sieve Tray Safety Factor). 58. FRI Handbook, Vol. 1, Section 5.5, p.5. (Sieve Tray Weeping - Reliability). 59. Ruff, K., Pilhofer, T., and Mersmann, A., "Ensuring Flow Through All the Openings of Perforated Plates for Fluid Dispersion," Int. Chem. Eng., 18 (3), p. 395, 1978. 60. Glitsch Ballast Tray Design Manual, Bulletin No. 4900, 6th Edition, 1993. 61. "Flexitray Design Manual", Koch Engineering Company Bulletin 960-1, 1982. 62. Norton Tray Design Manual, to be published. 63. Nutter Float Valve Design Manual, Rev.1, August 1981.

Page 7 of 10



TABLE I TRAYED COLUMNS - JET FLOOD SAFETY FACTOR (SIEVE AND VALVE TRAYS)

Note: % Flood reported is based on vendor's flooding correlation; direct comparison of values between vendors is not recommended

GLITSCH

Vacuum: Design Rate < 77% of Flood Other: Design Rate < 82% of Flood Small Diameter (< 36"): Design Rate < 65 to 75% of Flood

KOCH

Design Rate < 85% of Flood

NORTON

Design Rate < 80% of Flood for new towers Design Rate < 85% of Flood for revamped towers This value is corrected for high pressure, foaming systems, and tray spacing.

NUTTER

Design Rate < 80% of Flood for new towers Design Rate < 90% of Flood for revamped towers This value is corrected for predicted entrainment.

FRI

Bubble Cap Trays: Jet Flood Safety Factor > 1.3 to 1.4 Dualflow Trays: 2.0 > Jet Flood Safety Factor > 1.2 to 1.3

Page 8 of 10

1.20



1.20

TABLE II TRAYED COLUMNS - FOAM FACTOR

SYSTEM

FACTOR

Oil Absorbers (below 0° F) Oil Absorbers (above 0° F)

0.8-0.95 0.85

Amine Absorbers Amine Regenerators

0.73-0.8 0.85

Glycol Absorbers Glycol Regenerators

0.5-0.75 0.65-0.85

CO2 Absorbers CO2 Regenerators

0.85 0.8

Hot Carbonate Absorbers Hot Carbonate Regenerators

0.85 0.9

Dethanizers Demethanizers Depropanizers

0.85-1.0 0.8-1.0 0.9

H2S Strippers Vacuum Towers Crude Towers

0.85-0.9 0.85-1.0 0.85-1.0

Caustic Wash Caustic Regenerators

0.65 0.3-0.6

Sour Water Strippers Alcohol Synthesis Absorbers Fluorine Systems (BF3, Freon) MEK Units Sulfolane Systems Furfural Refining Towers

0.5-0.7 0.35 0.9 0.6 0.85-1.0 0.8-0.85

REFERENCES:

Page 9 of 10

(60),(61),(62),(63)

Issued: 01/15/1994


Revised:

FIGURE 1 REFLUX VS. STAGES CURVE

) R ( O I T A R X U L F E R

R MIN N MIN

NUMBER OF STAGES (N)

FIGURE 2 REFLUX VS. STAGES CURVES FOR DIFFERENT PURITY SEPARATIONS ) R ( O I T A R X U L F E R

R vs. N for Required Separation

R2

R vs. N for Separation Greater than Required (Minimum Reflux and Minimum Stages asypmtotes are not shown for clarity)

R @ N2

R = C*R MIN R MIN

N MIN

N2

N @ C*R MIN

N1

NUMBER OF STAGES (N)

Page 10 of 10

1.20


TRAY DAMAGE CAUSED BY HARMONIC VIBRATIONS

Issued:

02/01/2006

1.21

Revised:


Tray Damage Caused by Harmonic Vibrations...........................................................................1 1.

Introduction ........................................................................................................................ 2

2.

Background ............................................... .......................................................................... 2

3.

Observations ................................................... .................................................................... 3

4.

Caculations and Recommendations .................................................. .................................. 3

5.

Conclusions ........................................................................................................................ 5

6.

Special Notes ................................................. ..................................................................... 6

Page 1 of 11


1


1.21

Introduction

For many years it has been observed that under certain operating conditions, distillations trays can tear themselves apart. These conditions are believed to be associated with the natural frequency of the tray structure. This phenomenon generally occurs at low vapor loadings and/or low dry tray pressure drops. A correlation is provided to help the tray designer to understand and identify the potential for vibrational damage and help keep this phenomenon from occurring in the future. It is the intent of this paper to address the process issues surrounding the initiation of vibrations. Structural issues are not addressed here but have been addressed by Winter (6).

2

Background

One of the earliest recorded articles on the subject of harmonic vibrations on distillation trays was authored by Waddington in 1973 (3). He pointed out at that time that for laboratory scale sieve tray columns there was a flow regime that could potentially have “synchronous bubbling” that could enhance vibration on trays. This regime is where the operating tray can become unstable with respect to vapor distribution. In other words, there is insufficient vapor side pressure drop (dry tray pressure drop) to overcome the liquid head on the tray. Normally most trays operate with sufficient dry tray pressure drop to enable the vapor to spread across the entire bubbling area in a uniform nature. Under normal conditions, the tray is operating with a net upwards force. This means that any opening on the tray deck (including unintended openings at tray seams and panel splices) will have vapor passing upward through it. There should be NO liquid leakage through any opening on the tray deck. Only at minimum operating conditions (or turndown) does a tray normally experience sufficiently low dry tray pressure drop that weeping through the tray deck may start to occur. When this happens, the tray can be said to be operating at or near force neutral conditions. Finally, when the tray has so little vapor that it can no longer maintain any liquid on the tray deck and practically no liquid makes it to the outlet, it can be said that ”dumping” conditions prevail and the tray is operating with a net downwards force. When trays operate with either a net upwards or net downwards force, the tray deck is physically deflected in that direction by that force. Unless an external mechanical pulsation is applied to the flowing vapor or liquid to the trays, there is little opportunity for trays to harmonically vibrate under either of these two conditions. It is however, the force neutral condition that allows a set of trays to potentially (but not always) find their natural frequency and commence vibration. As explained by Brierley (2), if that natural tray frequency corresponds to the bubble formation frequency, then a condition exists such that the tray structure may vibrate so much as to fatigue the support mechanism and ultimately allow the tray to fail. This is harmonic vibration. Please note that simply having the right conditions to enable harmonic vibrations does not ensure that such vibrations will occur, only that the potential is present. Pulsation is a form of harmonic vibration that distillation towers may be subjected to. This phenomenon is not covered in this topic. This effect is different than harmonic vibrations even though it looks similar. Many times pulsation is the result of standing waves on a tray set that is exposed to the atmosphere. In other words the tower has no pressure control. Pulsation typically does not match the natural frequency of the tray structure. Typical pulsation frequencies range between 3 and 10 Hertz. Trays of most any diameter have been known to show severe physical damage under pulsation conditions and these conditions should be avoided.

Page 2 of 11

Issued: 02/01/2006


Revised:

3

1.21

Observations

There have been several recorded cases of trays “self destructing” due to vibrations (9, 4, 2). When economic conditions force process units to operate at capacities significantly less than design there tends to be an increase in the occurrence of this phenomenon. Many distillation towers that operate at reduced internal traffic may be at or near the force neutral region. Table 1 below shows a list of several towers that have experienced destructive harmonic vibrations. All are operating at loads significantly below their maximum capacity. Table 1 Recent Observed Harmonic Vibration Damaged Trays

Diameter

Passes

162" 1 144" 2 90" 1 120" 1 96" 1 118.1” 1 177.2” 2 212.6" 2 *RV = Round Valve

ρV,

lb/ft3

0.0183 0.09 0.10 0.11 0.01 0.955 0.062 0.175

Type Tray Sieve MVG Sieve Sieve RV* RV* Sieve Sieve

TS, in.

Thickness

30 24 24 24 21 17.7 17.7 19.7

10 GA 14 GA 14 GA 14 GA 12 GA 3 mm 3 mm 2 mm

Dry Drop, in. H2O 0.17

0.66 1.1 0.91

% Open Area

% Flood

11.30% 14.6% 12% 13% 11.80% 11.2% 14.7% 11.7%

14% 14.5% 16% 45% 38% 49% 62% 57%

As can be seen from Table 1, both one and two-pass trays are affected, as well as both fixed opening devices (i.e. Sieve and MVG) and movable ones (i.e. as the round valve, RV). It is interesting to note that all have tower diameters greater than 7 feet (2.13 m). Another thing to note is that all have vapor densities less than 1.0 lb/ft 3 (16.0 kg/m3). Both of these observations were made by previous authors (4, 2).

4

Caculations and Recommendations

Brierley’s paper provided a prediction for when harmonic vibrations can potentially occur. He agreed with Waddington (3) that the “vibration mechanism is due to pressure pulsations generated in association with synchronous bubble formation across a large part of the tray area.” Priestman(9) noted that these pressure pulsations caused tray damage when they were in the range of 20 to 40 cycles per second (Hz). Practice has shown that vibration frequencies as low as 10 Hertz can also result in tray damage. Brierley also expanded on Waddington’s theory that negative resistance to vapor flow will promote harmonic vibrations. Negative resistance is synonymous with force neutral. As Brierley did, if one plots the total tray pressure drop as a function of vapor loading at constant liquid rate (Figure 1) the pressure drop goes through a minimum value. The minimum is real of course, assuming there is little or no weeping on the tray. Brierley contended that any point on this curve to the left of the minimum value was subject to harmonic vibrations provided the natural frequency of the tray equaled the bubble formation frequency. He also derived an equation that helps predict where this minimum pressure drop occurs. By differentiating the total tray pressure drop with respect to vapor hole velocity, he arrived at Equation 1 below.

Page 3 of 11



V c = 15.5 ( ρ L − ρ V ) H CL F P / (C ( ρ V

0 .5

))

1.21

(Eq. 1)

Where, Vc = Critical vapor hole velocity, ft/s 3 ρL = Liquid Density, lb/ft 3 ρV = Vapor Density, lb/ft HCL = Clear Liquid Hgt @ operational Velocity V, ft. of liquid V = Hole Velocity, ft/s FP = Fractional open area based on bubbling area C = Velocity Head of a Dry Tray

C = 2.0 C H g C ρ W / 12.0

(Eq. 2)

Where, CH = Dry Tray Drop Hole Coefficient, from Summers and van Sinderen (1) gC = gravity constant = 32.174 ft/s 2 3 ρW = Liquid Density of Water = 62.428 lb/ft The minimum total pressure drop will occur when V/V C is equal to 1.0. It is recommended however, that to avoid the force neutral region, that V/V C be considerably different than 1.0. It is recommended to maintain values of V/V C above 1.6. Lockett (8), gave a similar argument as Brierley for determining a critical minimum vapor rate that allows harmonic vibrations. He derived a slightly different correlation because he used a different clear liquid height correlation than Brierley. Today, the most reliable correlation for predicting clear liquid height on a tray is Colwell’s equation (7). FRI uses this equation as its basis for determining sieve tray froth and clear liquid height. However, Colwell’s work to determine the clear liquid height is an iterative calculation. As a result, it would be extremely difficult to re-derive Brierley’s work with Colwell’s clear liquid height equation. An easier way to look at the clear liquid height is to plot the minimum pressure drop predicted from Colwell’s equation and then observe how well the above Equation 1 matches the results. This was done by Summers (10) who arrived at Equation 3 below. Equation 3 can safely be used with FRI’s determination of HCL. 0 .5

V c = 23 ( ρ L − ρ V ) H CL F P / (C ( ρ V ))

(Eq. 3)

Again it is recommended, however, that to avoid the force neutral region, that V/V C be considerably different than 1.0. It is recommended to maintain values of V/V C above 1.6 based on equation 3. A good practice to employ, if the predicted value of V/V C is less than 1.6, is to increase the dry tray pressure drop by reducing the tray open area. A high dry tray pressure drop ensures the user that the tray is operating with a net force upwards. If you force the V/V C calculation to be greater than 1.6 you can rearrange Equations 2 & 3 to be: 0 .5

V / V C = 1.6 < 2 V C H g C ρ W ( ρ V ) / 23 ( ρ L − ρ V ) H CL F P

Page 4 of 11

(Eq. 4)



1.21

Dry Tray pressure drop is defined as: ΔΡ

''

DRY

= C H V

2

(Eq. 5)

ρ V

Where,

ΔΡ ' = Dry Tray Drop, feet of water F-Factor is defined as: 0 .5

F = CFS V ( ρ V ) / Α Α

(Eq. 6)

Where, CFSV = cubic feet per second of vapor AA = Tray Active Area, ft 2 F = F-factor, ft/sec (lb/ft 3)0.5 F-Factor can also be written as:

F = V F P ( ρ v

0.5

)

(Eq. 7)

Substituting Equations 5 and 7 into Equation 4 results in:

1.6 < 2 ΔΡ ' DRY g C ρ W / 23 F ( ρ L − ρ V ) H CL

(Eq. 8)

If you convert dry drop from inches water to inches of liquid and assume low pressure Equation 8 looks like this:

1.6 < 2 ΔΡ DRY gC / 23 F H CL

(Eq. 9)

Where,

ΔΡ DRY = Dry Tray Drop, feet of hot liquid Rearranging Equation 9 results in the profound and simpler conclusion for low pressure, that to keep the tray from having harmonic vibration one must make sure that the ratio of the dry tray pressure drop over the hydrostatic head of liquid on the tray must be greater than:

ΔΡ DRY / H CL < 0.572 F

(Eq. 10)

The attached Figures 2 to 6 show damage from several towers that exhibited harmonic vibration. Figure 7 shows a photograph through a sight glass of a low loaded tray while in harmonic vibration.

5

Conclusions

The practice of calculating the harmonic vibration potential can be applied to several tray types including Page 5 of 11



1.21

sieve, fixed valve, and movable valve. Again, this is not meant to indicate that harmonic vibrations will most definitely occur, only to indicate that conditions are ideal. The natural frequency of the tray structure still has to match the bubble generation frequency for harmonic vibrations to actually occur. If the beam length, for example, is short enough (i.e. small diameter towers or short flow path lengths) then the tray’s natural frequency will never match the bubble formation frequency. A good Design Practice would be to avoid a combination of the following parameters in a tray design: 1. Diameter between 84” and 180” (2.1 and 4.5m) or integral truss length between 60" and 120" (1.5 and 3.0m) 2. Vapor density between 0.01 and 0.1 lb/ft 3 (0.16 and 1.6 Kg/m 3) 3. Number of tray passes less than 3 4. Vibration Factor (V/V C) less than 1.6 For those applications where the first three factors are present, special operating procedures should be established to minimize the time where the tower would operate under the fourth condition (i.e. minimize turndown time or startup). Another approach would be to look at the structural aspects of the trays and make adjustments (i.e. change the beam length or depth). Please note that simply changing tray panels out for a thicker material will not be a sufficient change to affect the natural frequency of the integral tray truss. Harmonic vibrations can tear trays apart very quickly and one must be careful, when starting up a unit, not to create or maintain this situation for an extended period of time.

6

Special Notes

Please note that Dualflow trays always operate under force neutral conditions. Therefore one would expect these trays to be highly susceptible to harmonic vibrations. This has been observed at low pressure and in tower diameters that have a natural frequency that allows this phenomenon to occur. Dualflow trays may need extra support structure to keep beam lengths short and help avoid vibration damage. Larger diameter towers tend to not exhibit harmonic vibrations even though their individual beam lengths may be ideal for this behavior. It is felt that when there is a multiplicity of beams and/or trusses, that the beams interact with each other defeating the ability for them to become harmonic. When troubleshooting damaged towers, a telltale sign that harmonic vibrations has occurred is that none of the damaged tray panels will show signs of being deformed. The trays will appear torn yet not bent.

Page 6 of 11



1.21

References

1.)

Summers, D.R., van Sinderen, A.H., “Dry Tray Pressure Drop of Rectangular Float Valves and VGrid Trays”, AIChE Spring National Meeting, Houston, TX on April 25, 2001, unpublished.

2.)

Brierley, R.J.P., Whyman, P.J.M., Erskine, J.B., I. Chem. E. Symposium Series No. 56 (1979) 2.4 / 45-63

3.)

Waddington, W. “Vibration Excitation of Sieve Tray Columns by Bubbling,” Masters Thesis, Sheffield University

4.)

Winter, J.R., “Avoid Vibrational damage to Distillation Trays”, Chemical Engineering Progress, May 1993 pp 42-47

5.)

Fujita, K., Tanaka, M., Shiraki, K., and Yamazaki, T., “Study of Flow-Induced Vibrations of a Sieve Tray Column, JSME Vol. 54, No. 502 (1988) pp 1194-1203

6.)

Winter, J.R., “Distillation Tray Structural Parameter Study, Phase I”, 19 th NASTRAN User’s Colloquium, Williamsburg, VA, April 17, 1991.

7.)

Colwell, C.J., “Clear Liquid Height and Froth Density on Sieve Trays”, Industrial Engineering Chemistry, Process Design and Development, Vol. 20, No. 2 (1981) pp 298-307

8.)

Lockett, M.J., Distillation Tray Fundamentals, Cambridge University Press (1986), pp 116-117

9.)

Priestman, G.H., Brown, D.J., Kohler, H.K., “Pressure Pulsations in Sieve-Tray Column”, I. Chem. E. Symposium Series No. 56 (1979) 2.4 / 1-16

10.) Summers, D.R., “Harmonic Vibrations Cause Tray Damage”, AIChE Annual Meeting, San Francisco, CA on November 18, 2003. 11.) Summers, D.R., “Bad Vibrations”, Hydrocarbon Engineering, March 2005, pp 51-54.

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FRI - Internal design vol 1

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