Che 10402

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Distillation Distillation Hardware

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Pr Process File Reference: CHE10402

For additional information on this subject, contact R.A. Al-Husseini on 874-2792

Engineering Encyclopedia

Process Distillation Distillation Hardware

CO NT ENT

P AGE

DISTILLATION HARDWARE .............................................................................. 1 NOMENCLAT NOME NCLATURE URE ............ ...... ............ ............ ............ ............. ............. ............ ............ ............ ............. ............ ........... ............ ............ ............ ...... 1 SELECTING TOWER CONTACTING DEVICES................................................. DEVICES ................................................. 3 Sieve Trays ....................................................................................... 5 Valve Trays....................................................................................... Trays ....................................................................................... 5 Bubble-Cap Trays ............................................................................. 6 Packing ........................................................................................... 10 Figure 10 ......................................................................................... 12 Grids ............................................................................................... 13 Baffle Sections ................................................................................ 14 FACTORS AFFECTING AFFECTING TRAY PERFORMANCE PERFORMANCE ............. ................... ............ ............ ........... ........... ......... ... 17 Maximum Vapor Rate Considerations ............................................ 27 Minimum Vapor Rate Considerations Considerations ............. ................... ............ ............ ........... ........... ........ .. 27 Maximum Liquid Rate Considerations Considerations ............. ................... ............ ............ ........... ........... ........ 27 Minimum Liquid Rate Considerations ............................................ 27 MAIN TRAY DESIGN PARAMETERS............................................................... 28 Hardware Definitions ................................................................................. 29 Tower Diameter and Tray Spacing ............ .................. ............ ............ ........... ........... ............. ......... 29 Downcomer Area ............................................................................ 30 Downcomer Clearance .................................................................... 30 Outlet Weir Height and Weir Length .............................................. 30 Multipass Trays............................................................................... 31 Contacting Area Definitions ........................................................... 31 Tray Pressure Balance ................................................................................ 32 Valve Tray Design Options ........................................................................ 37 Tower Internals........................................................................................... 39 Tray Transitions .............................................................................. 39 Saudi Aramco DeskTop Standards



CO NT ENT

P AGE

DISTILLATION HARDWARE .............................................................................. 1 NOMENCLAT NOME NCLATURE URE ............ ...... ............ ............ ............ ............. ............. ............ ............ ............ ............. ............ ........... ............ ............ ............ ...... 1 SELECTING TOWER CONTACTING DEVICES................................................. DEVICES ................................................. 3 Sieve Trays ....................................................................................... 5 Valve Trays....................................................................................... Trays ....................................................................................... 5 Bubble-Cap Trays ............................................................................. 6 Packing ........................................................................................... 10 Figure 10 ......................................................................................... 12 Grids ............................................................................................... 13 Baffle Sections ................................................................................ 14 FACTORS AFFECTING AFFECTING TRAY PERFORMANCE PERFORMANCE ............. ................... ............ ............ ........... ........... ......... ... 17 Maximum Vapor Rate Considerations ............................................ 27 Minimum Vapor Rate Considerations Considerations ............. ................... ............ ............ ........... ........... ........ .. 27 Maximum Liquid Rate Considerations Considerations ............. ................... ............ ............ ........... ........... ........ 27 Minimum Liquid Rate Considerations ............................................ 27 MAIN TRAY DESIGN PARAMETERS............................................................... 28 Hardware Definitions ................................................................................. 29 Tower Diameter and Tray Spacing ............ .................. ............ ............ ........... ........... ............. ......... 29 Downcomer Area ............................................................................ 30 Downcomer Clearance .................................................................... 30 Outlet Weir Height and Weir Length .............................................. 30 Multipass Trays............................................................................... 31 Contacting Area Definitions ........................................................... 31 Tray Pressure Balance ................................................................................ 32 Valve Tray Design Options ........................................................................ 37 Tower Internals........................................................................................... 39 Tray Transitions .............................................................................. 39 Saudi Aramco DeskTop Standards



Downcomer Downcome r Seal ............................................................................. 40 Seal Pan .......................................................................................... 41 Antijump Baffle .............................................................................. 42 Wire Mesh Entrainment Screens (Demisters)................................. (Demisters) ................................. 43

Saudi Aramco DeskTop Standards



Nomenclature Ad A1 Af Ap

Clearance area between downcomer and tray below, in.2, cm 2 Area of one hole, ft2, m 2 Tray free area, ft2, m 2 Hole area, ft2, m 2

Aw

Active area (available for holes, valves, caps), ft2, m 2 Waste area, ft2, m 2

b

Notch width, width , in., mm

Co

Orifice discharge coefficient

CSB

Tray capacity parameter, ft/s, m/s

c

Downcomer clearance, in., mm

D

Nozzle diameter, in., mm

D

Vapor apor--free li liqui quid in in dow downc nco omer mer hei height ght, in. in.,, mm mm

DT

Tower diameter, ft, m

d

Depth of V notch, in., mm

Fh

Factor in calculation of effective liquid head

f

Aeration factor

FP

Flow parameter, (L/VL)(r V/r L)0.5

H

Tray spacing, in., mm

hd

Downcomer contraction pressure loss, in. liquid, mm liquid

he

Effective liquid head over weir, in.

ho

Head of liquid over weir, in., mm

hon

Liquid height on notch, in., mm

hr

Residual pressure drop, in. liquid, mm liquid

ht

Total tray pressure drop, in. liquid, mm liquid

hv

Hydrostatic head of vapor, in. liquid, mm liquid

hw

Weir height, in., mm

hwi

Inlet weir height, in., mm

hwo

Outlet weir height, in., mm

K 1 L

Viscosity/pressure correction factor of Va Liquid volumetric rate, ft3/s, m 3/h

La

Average static liquid head, in. liquid, mm liquid

At


1



lfp

Flow path length, in., mm

lud

Length of clearance under downcomer, in., mm

lw

Effective weir length, in., mm

lwi

Inlet weir length, in., mm

LLH LLH

High High Liqu Liquiid Lev Level el

LLL

Low Low Li Liquid uid Lev Level

N p n

Number Num ber of o f tray passes passe s

Pd

Dry-tray vapor pressure drop, in. liquid, mm liquid

Pw

Wet tray pressure drop, in. liquid, mm liquid

Q

Liquid flow rate, gpm, L/s

r

Downcomer rise, in., mm Downcomer area, ft2, m 2

Sd

Number of notches in weir

Sdo

Downcomer inlet area, ft2, m 2 Downcomer outlet area, ft2, m 2

St

Column cross sectional area, ft2, m 2

TT

For towers, towers, refers refers to the height height of the tower tower measur measured ed Tange Tangentnt-toto-Tange Tangent nt

U

Mini Minimu mum m vap vapor or vel veloci ocity trou trough gh hol hole, ft/ ft/s, m/s m/s

Ui

Liquid velocity at downcomer entrance, ft/s, m/s

Uo

Linear vapor velocity trough holes, ft/s, m/s

V

Sup Superficial cial vapor apor vel velocit city, ft/s, m/ m/s

Va VL

Allowable superficial vapor velocity, ft/s, m/s Total tray vapor load, ft3/s, m 3/s

V N

Vapor velocity based on the net tray area available for liquid disengagement, ft/s,

Sdi

m/s b

Ratio of hole ho le area are a to tray t ray area a rea available av ailable for holes ho les

mL

Liquid viscosity, cP, cP

r L r V

Liquid density at operating conditions, lb/ft3, kg/m3 Vapor density at operating conditions, lb/ft3, kg/m3

s

Surface tension, dynes/cm, dynes/cm

*

Dist Distin ingu guis ishe hess val value uess ass assoc ocia iate ted d wit with h inb inboa oard rd down downco come merr tra trays ys


2


Process Distillation Hardware

Selecting Tower Contacting Devices A contacting device must have good liquid and vapor handling capacities, good contacting efficiency, reasonable pressure drop, and predictable turndown characteristics, and it must also be economical. The devices available fall into two broad categories: cross-flow and countercurrent. They are shown conceptually in Figure 1.

CROSS-FLOW VERSUS COUNTERCURRENT DEVICES

FIGURE 1

With cross-flow devices, the liquid flows horizontally across a flat plate, called a tray, that contains a contacting device that thoroughly disperses the vapor into the liquid. The dispersion process must produce sufficient interfacial area and maintain the phases in contact with each other long enough to promote adequate mass transfer between the phases. As the liquid flows across the tray, it is contacted by the rising vapor. At the far side of the tray, the liquid enters a downcomer, which carries it to the tray below where the contacting process is repeated. The contacting area must be large enough to handle the required liquid and vapor rates while promoting the desired mass transfer. Likewise, the downcomer must be large enough to handle the liquid being processed.


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With countercurrent devices, the liquid flow is truly countercurrent to the vapor flow. The efficiency of contact depends on the area available for mass transfer. In trays, this is provided by bubbling vapor through the liquid, thereby producing sufficient interfacial area for mass transfer. With packing, the interfacial area for mass transfer is provided by the surface area of the packing. With baffle trays, the interfacial area is created by forcing the vapor to flow through descending curtains of liquid, which breaks the liquid curtains into droplets. Generally, as the surface area of a device increases, the efficiency increases. However, as the surface area increases, capacity decreases while cost rises. Thus, the final design involves optimizing the capacity, efficiency, cost, and other process considerations for the variety of possible internal designs. Most Saudi Aramco units use valve trays; however, sieve trays are very common in the petroleum industry. There are very few Saudi Aramco towers with packing, although potential applications exist, especially in vacuum crude distillation and in debottlenecking existing units.

Cross-Flow Devices Figure 2 illustrates a typical arrangement and key components for a one-pass tray and a two pass tray.

CROSS-FLOW CONTACTING DEVICES

FIGURE 2 The most common types of trays in use today are sieve, valve, and bubble cap trays. Following is a review of these types of trays. Figure 6, after the section on bubble caps, summarizes their characteristics. Saudi Aramco DeskTop Standards

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Sieve Trays The contacting area consists of flat plates containing perforations, usually 1/2 in. (13 mm) in diameter (Figure 3). They are the simplest trays to fabricate and are therefore the cheapest. They also exhibit good capacity, excellent efficiency, and good turndown characteristics (about 3/1). Their flat surface facilitates maintenance. Thus, they may be used in fouling services, provided the hole size is large enough.

SIEVE TRAY DECK

FIGURE 3

Valve Trays For most Saudi Aramco fractionation services, valve trays are the first choice. Valve trays contain proprietary devices manufactured by Glitsch Inc., Koch Engineering, and Nutter Engineering. The valve size, shape, weight, and other parameters vary from vendor to vendor. Turndown is excellent, reaching 5/1. The valve tray capacity and efficiency are about equal to those of a sieve tray, but cost is roughly 10% higher. Valve trays are not recommended for severely fouling service, because deposits may interfere with the valve movement. Valves specified with a dimple have a lower probability to stick to the tray deck in the closed position. Valves can also be specified with an anti-rotation device that will prevent rotation of the valve and wear of the valve legs. Figure 4 illustrates valves from the three main vendors.


5



VALVE TRAYS

FIGURE 4

Bubble-Cap Trays Figure 5 illustrates bubble cap trays, which were the first type of tray developed for continuous distillation. Although they provide excellent vapor-liquid contact over a wide range of throughputs, they are relatively expensive to fabricate, install, and maintain. As distillation hardware evolved, bubble-cap trays were largely displaced first by sieve trays and later by valve trays. Despite their expense, bubble cap trays are sometimes specified in fouling, low pressure drop, and high turndown services.


6



BUBBLE-CAP TRAYS

FIGURE 5

TRAYS -- A SUMMARY OF CHARACTERISTICS

Tray Type

Cost per Unit Area

Capacity

Efficiency

Flexibility

Remarks

Sieve

Medium to high.

High. Equal to or better than other tray types.

Lowest of all trays with downcomers.

Medium. 3/1 can usually be achieved.

Alternative to valve trays when high turndown is not required.

Valve

Medium to high; as good as sieve trays.

High. As good as sieve trays.

Medium. About 10% greater than Sieve Trays.

High. Possibly up to 5/1.

First choice for most applications. Not recommended for moderate to severe fouling services.

Bubble Cap

Medium to high, except low to medium at high liquid rate.

Medium to high.

High. At least twice the cost of sieve trays.

High. 5/1 or slightly higher.

Use for high flexibility where fouling of valve trays may be a problem.

FIGURE 6


7



Downcomer Configurations The standard type of downcomer is the straight, or chordal, downcomer shown in Figures 7 and 8. For a given tower diameter, a certain amount of the available tray cross-sectional area, the downcomer area, is needed for liquid handling with the remainder, the bubble area, available for vapor flow. Therefore, any changes that reduce the tray area used by downcomers, increases the area available for vapor flow. Such a goal can be achieved by using stepped or sloped downcomers as shown in Figure 7. The process performance characteristics of sloped and stepped downcomers containing the same inlet and outlet areas are identical. They can therefore be used interchangeably. The required downcomer cross-sectional area is greater at the top of the downcomer where most of the vapor disengagement takes place. Sloped or stepped downcomers provide the required area at the top of the downcomer, and at the same time, they reduce the tray area taken by the downcomers at the bottom. As a result, the tray area available for vapor-liquid contact and vapor disengagement with stepped or sloped downcomers is higher than for straight downcomers. Sloped or stepped downcomers are most effective when used in trays with moderate-to-high liquid rates to increase their vapor capacity (existing units) or to reduce the required tray diameter (new units).

STEPPED VERSUS SLOPED DOWNCOMERS

FIGURE 7


8



Downcomer Configurations (Cont'd) For uniform liquid flow distribution onto a tray, the chord at the bottom of the downcomer must have certain minimum length, often expressed as a percentage of the tower diameter. In some services where very low liquid rates must be handled, this minimum chord length provides a downcomer whose area is too large for the liquid flow rate being handled (that is, the chord is about 6.8% of the tower cross-sectional area). A modified arc (also known as segmental) downcomer can be specified (Figure 8) to overcome this limitation while still meeting the minimum requirement. The modified arc downcomer has an area less than the 6.8% provided by the minimum (65% of tower diameter) chordal downcomer, but has a projected weir length at least equal to the minimum. Some older towers may contain a full arc-type downcomer. This style of downcomer functions in the same manner as a modified arc but is more expensive to build and thus is no longer used in new towers.

STRAIGHT, MODIFIED ARC, AND ARC-TYPE DOWNCOMERS

FIGURE 8


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Countercurrent Devices Packing, grids, and baffle sections are the three types of countercurrent devices reviewed in this section. Figure 13 at the end of the section, summarizes their characteristics.

Packing Although a packed tower design may result in a smaller tower diameter, the total cost of the installation with packing, packing supports, distributors, and redistributors is generally higher than that of a trayed tower. The most common uses of packing in distillation services are: •

Applications where pressure drop across the internals is critical, such as in vacuum distillation.

•

Revamps, especially where downcomers consume a large percentage of the tower's cross-sectional area or where downcomer filling is high; examples are, heavily liquid loaded towers such as debutanizers and depropanizers.

•

Corrosive services where ceramic packings are more economical than alloy trays.

•

In towers less than 2 ft in diameter.

Saudi Aramco units using packing are the crude vacuum distillation columns, ADIP extractors (2-in. polypropylene Intalox Saddles), and Merox oxidizers (1.5-in. carbon Raschig rings).

Random Packings (Also Called Dumped Packings) Random packings are the most frequently used countercurrent devices (Figure 9). Their name derives from the fact that they are dumped into the column and orient randomly. The most widely used packing today is the Pall ring. It comes in a number of sizes and materials of construction. As the ring size increases, the capacity increases while the pressure drop, cost, and efficiency decrease. Thus, for a given design, there is an optimum economic combination of ring size, tower diameter, and tower height.


10



COUNTERCURRENT DEVICES - RANDOM PACKING

FIGURE 9

Since 1978, several other packings have come on the market that provide improved performance characteristics. These include Norton Company's Intalox Metal Tower Packing (IMTP) also known as Metal Intalox, and Nutter Engineering's Nutter Ring. Raschig rings are used infrequently, while Intalox saddles are generally preferred for applications requiring ceramic packing.


11



Structured (Ordered) Packings Structured packing devices are fabricated in bundles from crimped sheet metal and installed in the tower in layers having a fixed orientation. Since they provide more surface area per unit volume than random packings, they are more efficient. However, they cost two-to-four times as much. Of the contacting devices available, structured packings provide the lowest pressure drop per theoretical stage of contacting as well as the best capacity/efficiency combination. This feature makes them especially attractive in vacuum towers. There are several brands and suppliers, including Flexipac by Koch Engineering, Gempak by Glitsch, Intalox Structured by Norton, Montz by Nutter Engineering, and Mellapak by Sulzer. One of these devices, by Koch Engineering, is shown in Figure 10.

STRUCTURED PACKING BY KOCH ENGINEERING

FRONT VIEW

ISOMETRIC VIEW

PASTEUP

NEED TO BRING IN SCAN

SCAN REMOVED BECAUSE OF DIFFICULTY PUTTING ON DISC

Figure 10


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Grids Grids are similar to structured packing in that they are fabricated in panels and installed in an ordered manner. However, their efficiency characteristics are much poorer due to their high open area and low surface area per unit of volume. The first grid to appear on the market, circa 1961, was the Glitsch grid. It was intended for use in services where entrainment removal was critical but where fouling was too severe to use crinkled wire mesh screens. In recent years, several new grids have come on the market. They are Flexigrid by Koch Engineering and Snapgrid by Nutter Engineering. Pictures of these major grids are shown in Figure 11.

VARIOUS TYPES OF GRIDS

FIGURE 11

Because of their high capacity and low pressure drop, grids have also been used in heat transfer sections (pumparounds) of vacuum crude distillation and other heavy hydrocarbon fractionators. The liquid is introduced on the top layer of grid via spray nozzles.


13



Baffle Sections There are two basic types of baffle sections. The first type is sheds; the second type is disc and donuts (Figure 12). These devices operate differently from grids or packing. In baffle sections, the liquid cascades from baffle to baffle in the form of liquid curtains. As the vapor flows through these curtains, the liquid is broken up into droplets and mass transfer occurs. However, this is a very inefficient liquid/vapor contacting mechanism. For severe fouling services, baffle sections are about the only internal available if long run lengths are required. Because of their high open area, they have high capacity but very poor efficiency. Thus, baffle sections require a disproportionate amount of tower height for the functions they perform.

SHEDS/DISC AND DONUTS

FIGURE 12


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COUNTERCURRENT DEVICES -- A SUMMARY OF CHARACTERISTICS

Device

Capacity

Efficiency

Cost per Unit Area

Flexibilit y

Remarks

Packing (Pall Medium. Rings, Metal Intalox, Nutter Rings, etc.)

Medium to High.

Medium to > 3/1. low, depending on material of construction.

Good for DP service. Mainly used in vacuum pipestills and in various high liquid rate absorbers.

Structured Packing Flexipac; Montz Gempak; Mellapak IntaloxStructured

Medium to very high depending on size used.

Medium to very high depending on size used.

High - at least two times dumped packing cost.

> 3/1.

Best efficiency per unit of DP.

Glitsch Grid Flexigrid Snapgrid

Very high. Poor as Medium to fractionation high. device. Good for entrainment removal and heat transfer.

Low: less than 2/1.

Good for high vapor-low liquid service to minimize effect of entrainment. Used in wash zones of heavy hydrocarbon fractionators where moderate coking occurs.


15



Sheds and Disc Very high. Poor as and Donuts fractionation device.

Medium.

Low. < 1.5/1

Used in severe fouling service; e.g., slurry pumparound in cat fractionator.

FIGURE 13


16



Factors Affecting Tray Performance Flow Regimes - Spray and Froth Movies taken during operations in various towers have indicated that different flow regimes can exist on a tray. The first is the froth regime. In this regime, vapor passes through the liquid on the tray as discrete bubbles of irregular shape. As the vapor rate increases, jets and bubbles of rapidly changing shape are observed. If the vapor rate is raised still further, a gas jet issues from the orifice and some of the liquid is shattered into droplets in a regime called the spray regime. In the spray regime, the vapor phase is continuous, whereas in the froth regime, the liquid phase is continuous (Figure 14). Spray regime operation occurs primarily at high vapor velocities and low liquid rates. The froth regime in high pressure systems is also referred to as the emulsion regime.

FROTH REGIME VERSUS SPRAY REGIME OPERATION

FIGURE 14


17



Flow Regimes - Spray and Froth (Cont'd) Operation in the spray regime can be very detrimental to good tower performance, causing tray efficiency to drop sharply because the liquid and vapor residence times are reduced. While spray regime operations have been observed on all the widely used trays discussed earlier, the spray regime has been investigated primarily with sieve trays. Under spray regime conditions, the vapor rate is sufficient to "blow through" the liquid, thereby making the vapor phase continuous. In fact, the term blowing is often used to describe the spray regime. Because the liquid rate is usually set by the process and cannot be increased, the most effective way to suppress the spray regime is to dissipate the jet leaving the orifice as quickly as possible. The most obvious way to dissipate the jet is to increase the open area on the tray, thereby reducing jet velocity. A second way is to use smaller orifices; for example 1/8-in. holes versus 1/2- or 1/4-in. holes used on sieve trays. Because the distance to dissipate a jet is a function of the orifice diameter, the smaller the orifice the faster the jet will dissipate. A third way is to use valve trays. Because the vapor leaves the valve element almost horizontally, its vertical velocity component is greatly reduced and its jet is more quickly dissipated.

Entrainment Entrainment is defined as the liquid carried by the vapor from a given tray to the tray above. As the vapor rate in the contacting area is increased, the amount of energy being dissipated also increases. This energy creates the interfacial area needed for good contacting between the liquid and the vapor. It also expands the froth or spray height on the tray, thereby decreasing the distance between the top of the spray and the tray above. As this disengaging distance decreases further, some of the liquid is carried, or entrained, to the tray above as droplets (Figure 15). The smallest drops will be entrained to the tray above while the largest drops will fall back to the entrainment generation tray. As the quantity of entrainment increases, the tray above becomes overloaded and floods, and the tray efficiency drops sharply. Flooding must be avoided to maintain good tower control and design fractionation. The quantity of entrainment generated is dependent on vapor rate, liquid rate, system physical properties, and certain hardware parameters.


18



GENERATING ENTRAINMENT

FIGURE 15

Jet Flooding Jet flooding sets the vapor-handling capacity of almost all cross-flow trays. In jet flooding, the liquid is projected or jetted to the tray above by the vapor leaving the tray's orifice. If sufficient liquid is entrained to the tray above, the liquid will overload the downcomers, and the tray will flood. When flooding occurs, the liquid begins to back up on the tray until the inter-tray space is filled with a dense froth (Figure 16). This causes the next higher tray to flood, and flooding moves up the tower until the liquid is carried out the top of the tower. When flooded, the tower fractionates poorly and is very difficult to control. The approach to flooding conditions is quantified as % jet flood or % flood. This is the ratio, expressed as percent, of the vapor velocity between the trays, V, divided by the maximum vapor velocity that will not cause flooding. The maximum velocity is called allowable vapor velocity, Va.


19



Jet Flooding (Cont'd) Because jet flooding sets the maximum capacity of the tower, it must not be exceeded. Furthermore, as the percent of the jet flood velocity moves from 90% to 100%, the entrainment rate increases exponentially and the tray efficiency falls off sharply. Perry's Chemical Engineers' Handbook and vendor literature provide correlations for determining jet flooding.

PERCENT JET FLOOD VERSUS EFFICIENCY

FIGURE 16


20



Downcomer Inlet Velocity As the liquid leaves the contacting area on a tray, it enters the downcomer. Since it enters as a froth (20-50% liquid by volume), it must be disengaged before it flows to the tray below. The downcomer provides residence time for disengaging and acts as a conduit for liquid flow to the tray below. If the entrance area is too small and the froth cannot readily enter the downcomer, the froth height will increase in the contacting area. This height will continue to increase until there is sufficient head to force the froth into the downcomer or until the froth reaches the tray above, causing flooding.

Downcomer Residence Time The difference between the liquid and vapor densities, ρL-ρV, is one measure of the difficulty of separation in the downcomer. Thus, based on buoyancy considerations, as the difference of ρL-ρV gets smaller, disengaging becomes more difficult. For this reason, the downcomer sizing criteria allow lower velocities (higher residence time) for high-pressure systems, where ρL-ρV is low.

Downcomer Filling The liquid height in the downcomer is called downcomer filling, expressed in inches of clear liquid or as a percent of the tray spacing. Since the liquid enters the downcomer as a froth, the actual fluid level in the downcomer will be higher than the filling calculated as clear liquid (Figure 17). The exact height depends on the average froth density in the downcomer. As the liquid travels downward in the downcomer, the vapor disengages and escapes from the top of the downcomer. If the downcomer is sized properly, the liquid leaving should be essentially clear liquid. Thus, there is a froth density gradient down the downcomer that ranges from the froth density on the tray (at the top) to clear liquid at the bottom.


21



DOWNCOMER FILLING

FIGURE 17

Downcomer Sizing Criteria Downcomer inlet velocity, based on vapor-free liquid, normally should not exceed 0.4 ft/s to assure an adequate area for vapor disengaging. For foamy liquids, the inlet velocity is limited to 0.2 ft/s. However, based on Saudi Aramco experience with crude stabilizer columns, downcomer inlet velocity can be higher without downcomer flooding limitation. Allowable downcomer filling is 50% for normal systems and 40% for foaming systems. Further, the downcomer shall be sized to allow 5 seconds minimum residence time for low pressure columns and 7 to 8 seconds for high-pressure columns (greater than 400 psi) and systems with high foam stability.


22



Weeping At low vapor velocities, the dry-tray pressure drop of the tray is insufficient to support the liquid head on the tray; as a result, some liquid begins to flow intermittently through the vapor openings. This liquid bypassing begins at the "weep point." As the vapor rate decreases further, more liquid pours through the holes and weeping becomes continuous. Although the total quantity of liquid that weeps is constant at a given vapor rate, the weep rate per hole fluctuates. That is, some holes are weeping while others are in the vapor bubbling mode. At any instant, a given hole may be bubbling, weeping, or doing neither, in a random distribution across the contacting area of the tray. Although weeping can occur on all tray types, it is less of a problem in valve trays, which are the most widely used tray in Saudi Aramco plants. Since weeping occurs only at reduced rates, it is the major factor in determining tray turndown, the range of vapor loadings over which acceptable fractionation is achieved. (See Tray Turndown discussion.) For sieve trays, turndown ratio is usually between 2/1 to 3/1; for valve trays, 3/1 to 5/1. As Figure 18 shows, when vapor rate decreases, weeping increases very rapidly and tray efficiency begins to decrease sharply. Weepage up to 20% of the liquid rate has small effect on efficiency and is acceptable.

EFFECT OF WEEPING ON EFFICIENCY

FIGURE 18


23



Dumping When all the liquid flows through the holes on a tray, that is, no liquid flows over the weir, dumping is said to occur. When dumping takes place, tray efficiency is extremely poor and the products will be offspec. Trays should not be operated in the dumping region.

Tray Turndown Turndown is a measure of the hydraulic flexibility of the tray. It is defined as the ratio of maximum to minimum loadings in a range over which acceptable tray performance is achieved. This usually is the range over which the tray efficiency stays at or above the design value (Figure 18). As Figure 18 shows, there is a relatively flat portion of the efficiency curve where design (or better) efficiency is obtained. At low vapor rates, however, excessive weeping decreases efficiency; at high vapor rates (above 90% of flood), excessive entrainment decreases efficiency.

Entrainment-Weeping -- Tray Operating Window Figure 19a shows the fractional weepage and entrainment curves for a typical sieve tray with a moderate to high liquid rate. Using Figures 19a and b, note the difference in operating ranges for 20% fractional weepage and entrainment. The moderate to high liquid rate provides a good turndown ratio. The low liquid rate provides a poor turndown ratio. Sieve trays can usually be designed to provide a turndown ratio of 2/1 to 3/1; valve trays, up to 5/1. If the liquid rate on a tray is low (say below 1.5 gpm/in. of weir per pass), the operating window on the tray is extremely small or nonexistent. This is shown in Figure 19b.


24



EFFECT OF LIQUID RATE ON SIEVE TRAY TURNDOWN

(a)

(b)

FIGURE 19

Tray Efficiency The vapor and liquid phases must be dispersed thoroughly and remain in contact long enough for mass transfer to occur and to achieve good efficiency. The vapor residence time is the time for the vapor to flow through the volume of froth on the tray. Likewise, the liquid residence time is the time for the liquid to flow through the volume of froth on the tray. Both of these variables depend on liquid and gas rates as well as the weir height and bubble area on the tray. The efficiency is also affected by the vapor and liquid diffusivities. Since these values are fixed for a given system, there is no way to change them through tray hardware changes. To achieve good efficiency, a designer must optimize the weir height, open area, bubble area, number of liquid passes, and other variables. Excessive weeping, entrainment, and operation in the spray regime must be avoided. Figure 20 illustrates the effect of tower loading on valve and sieve tray efficiencies. The operating range for the valve tray is wider, reaching to very low turndown. Sieve tray efficiency may be somewhat higher near design loadings.


25



EFFECT OF TOWER LOADING ON TRAY EFFICIENCY VALVE TRAY VERSUS SIEVE TRAY

FIGURE 20

Tray Performance Diagram A fractionating tray must be operated within a certain range of vapor and liquid rates to give optimum performance and an economical design. Outside this range, efficiency drops off or the tower becomes inoperable. The effects of vapor and liquid rates on tray performance are depicted schematically on Figure 21 and are summarized in the text that follows. TYPICAL TRAY PERFORMANCE DIAGRAM


26



Maximum Vapor Rate Considerations A very high vapor rate may cause: •

Jet flooding, excessive entrainment, or spray regime operation.

•

High pressure drop across the tray, resulting in excessive downcomer filling and subsequent tray flooding.

Minimum Vapor Rate Considerations A very low vapor rate may cause: •

Weeping or dumping.

•

Poor contacting and tray efficiency because of inadequate vapor and liquid mixing.

These conditions can result from insufficient vapor loading or from excessive open area on the tray, both of which produce a low vapor velocity through the tray openings.

Maximum Liquid Rate Considerations High liquid rates may cause: •

Tray flooding because of insufficient disengaging in the downcomers, excessive tray pressure drop, and excessive downcomer filling.

•

Tray flooding because of excessive downcomer entrance or exit velocity and downcomer bridging.

Minimum Liquid Rate Considerations Low liquid rates may cause: •

Spray regime operation at high vapor rates.

•

Vapor bypassing up the downcomers, if the downcomer is not sealed.

•

Poor contacting and low tray efficiency, because of inadequate liquid residence time on the tray due to operation in the spray regime.


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Main Tray Design Parameters The development of a design for a new tray generally follows the steps listed below: 1.

Define the design vapor and liquid loadings.

2.

Determine tray spacing, diameter, and layout.

3.

Calculate hydraulics, pressure drop, and downcomer (DC) filling.

4.

Evaluate flexibility.

5.

Produce a balanced tray design.

The tray design procedure is iterative and involves repeating several of these steps, even when a tray design computer program is used. In this section we will focus on: •

Hardware definitions.

•

Tray pressure balance.

•

Main tray design variables that the engineer needs to determine performance and key parameters that the tray design must satisfy.

•

Valve tray design options

The Saudi Aramco Design Practice, ADP-C-001, contains information and criteria for tray design. A Glitsch valve tray design manual is provided as a separate class handout.


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Hardware Definitions Figures 22 and 23 are typical layouts of single-pass trays, illustrating the tray characteristics discussed in the following sections.

Tower Diameter and Tray Spacing These two parameters set the capacity of the tower. As the distance between trays (the tray spacing, H) increases, tower capacity increases. For most services, the most economical spacing falls between 18-24 in. Spacings above 36 in. provide little capacity advantage and are not usually recommended. Likewise, tray spacings as low as 12 in. can be used, but this increases the tower diameter (DT) required to handle a given set of vapor and liquid loadings. In addition, low spacings make maintenance much more difficult. The Saudi Aramco Engineering Standards specify minimum tray spacing requirement at various tower diameters.

TRAY LAYOUT DEFINITIONS

FIGURE 22


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DOWNCOMER ARRANGEMENTS

FIGURE 23

Downcomer Area This is the area in Figures 22 and 23 (S di, Sdo ) that handles liquid as it flows from a given tray to the tray below. The edge of the downcomer is usually chordal in shape, and its maximum width is called the downcomer rise (r).

Downcomer Clearance This is the vertical clearance (c) between the tray floor and the bottom edge of the downcomer apron (Figure 22).

Outlet Weir Height and Weir Length As the liquid leaves the contacting area and enters the downcomer, it flows over the outlet weir (Figure 22). The height of the weir (hw) is set by the designer to provide liquid holdup on the tray and promote efficient liquid/vapor contacting. The weir length (lw) is the same as the downcomer chord length. Inlet weirs are discussed later in the downcomer seal section.


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Multipass Trays As the liquid rate on a tray increases, the capacity of the tower can usually be increased if the liquid flow is split into more than one path (Figure 24). Such split-flow trays are called multipass trays. On multipass trays, the downcomers nearest the tower centerline are inboard downcomers, while those farthest away are called outboard downcomers.

TRAY PASS ARRANGEMENTS

FIGURE 24

Contacting Area Definitions During tray design, such terms as active area , hole area , waste area , and free area are used. They are explained below (see also Figures 22 and 23).

Active or Bubble Area (A t ) This is the area between the downcomers where vapor/liquid contacting occurs. It is used in calculating tray efficiency, but does not set the tray's capacity.

Hole/valve/cap Area (A p ) This is the open area or hole area provided within the bubble area to permit vapor to enter, contact, and pass through the liquid on the tray. For a sieve tray, it equals the total area of all the holes on a given tray. The hole area is usually expressed as a fraction of the active area (A p/At) and is determined by various correlations for each tray type.


31



Waste Area (A w ) Waste area is any part of the active area that is farther than 3 in. from the edge of a contacting device. Since vapor does not contact the liquid in this area, it is not included in the active area. Waste area frequently occurs when tray blanking, inlet weirs, or recessed inlet boxes have been specified.

Free Area (A f ) Test data have shown that as the vapor flows through and leaves the active area (At) it expands over the downcomer(s) and its velocity drops. Thus, an area greater than the bubble area is available for vapor flow. This larger area is known as the free area (Af ). The free area is what determines the tray's capacity. When multipass trays are designed, the tray (either inboard or outboard) that has the smallest free area is used to set the tray's capacity. For trays with sloped or stepped downcomers, the average free area is used.

Flow Path Length (l fp ) The length of the contacting area in the direction of the liquid flow (see Figure 23). Tray Pressure Balance A tray pressure balance illustrates the factors that determine downcomer filling. Figure 25 illustrates the pressure balance for a two-pass sieve tray; the pressure balance for a one-pass tray and for a valve tray is similar. The components of the pressure balance are described below. Their values are generally expressed in terms of clear liquid height at the tray conditions, for example, inches of clear liquid. •

Dry tray pressure drop (Pd). It is the pressure drop through the tray openings, sieve holes in this case. It does not take into account any effects from the presence of the liquid (dry tray).

•

Average liquid static head (La).

•

The sum of Pd and La is the pressure drop between the trays.

•

Pressure loss under the downcomer or downcomer contraction pressure loss (hd) results from the flow of the liquid through the downcomer clearance.

•

Downcomer liquid filling (D).

A pressure balance between the trays through two paths, through the tray openings and through the downcomer, results in the following equations.


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Tray Pressure Balance (Cont'd) Pressure balance for inboard downcomer filling:

D* = h*d + P *d + L*a + La

* Pressure balance for outboard downcomer filling: D = hd + P d + La +La

The * distinguishes inboard from outboard downcomer trays (Figure 25). It was assumed that the vapor density, r V, is significantly lower than the liquid de nsity, r L. Pressure balance for single pass trays:

D = hd + P d + 2La


33



For light ends columns operating under high pressures, it is necessary to consider the effect of vapor density in design calculations. The liquid height in the downcomer in such cases should be determined by the equations available in ADP-C-001, 3.4.1.

PRESSURE BALANCE FOR A TWO-PASS SIEVE TRAY

Inboard Downcomer: D* = h*d + P* d + L* a + L a for r V << r L Outboard Downcomer: D = hd + P d + L a + L* a for r V << r L FIGURE 25


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Main Tray Design Variables and Performance Parameters Below is a list of the main variables determined during tray design and a list of key performance parameters affected by the tray design. A discussion relating the items in the two lists follows.

Main Tray Design Variables • • • • • • • • •

Tray diameter. Tray spacing. Number of tray passes. Downcomer area. Downcomer type. Active or bubble area. Open or hole area. Weir height. Downcomer clearance.

Key Performance Parameters • • • • • • •

Jet flooding. Downcomer filling (or downcomer flooding). Downcomer inlet velocity. Dry and total tray pressure drop. Pressure drop under the downcomer. gpm/in. of weir. Weeping and tray flexibility.

Tray diameter and tray spacing are the two most important variables in tray design because they determine the diameter and height of the tower. They affect two important performance parameters: jet flooding and downcomer filling (downcomer flooding). The tray diameter is the main variable in determining the velocity of vapor between trays and, as a result, jet flooding. Tray spacing also affects jet flooding. Higher tray spacing between trays allows more liquid droplets to settle before they reach the tray above; thus it helps reduce entrainment and the percent jet flood. Improvement for tray spacings above 36 in. is marginal. Because of the trade-off between tower diameter (tray diameter) and height (tray spacing), finding the most economical design may require the evaluation of alternative tray spacings. Tray diameter and tray spacing also affect downcomer filling. Vapor velocities determined by the tray diameter affect the tray pressure drop of a tray and as a result the downcomer filling. Tray spacing sets the height available for vapor disengagement in the downcomers. Liquid height is the numerator and tray spacing the denominator in determining the percent downcomer filling.


35



Main Tray Design Variables and Performance Parameters (Cont'd) The Saudi Aramco Engineering Standards, AES-C-001, 5.1.3, specify minimum tray spacing requirements for tower access and service. Towers with high liquid rates use more than one tray pass . The upper acceptable limit of liquid rate per tray pass is about 15 gpm/in. of weir or according to the ADP criteria, 5000 gph/ft of diameter. Tray hydraulics at higher liquid rates become unpredictable. The downcomer area determines the inlet velocity in the downcomer and along with the tray spacing, the residence time of liquid in the downcomer. ADP specify maximum inlet velocity and minimum residence time requirements. Vendors (see Glitsch valve tray design manual) have similar criteria. When the downcomer inlet velocity sets the size of the downcomer (usually at high liquid rates), it may be possible to increase the cross sectional space available for vapor flow by using sloped downcomers . The liquid leaving the downcomer is relatively clear of vapor; thus, higher velocities (on clear liquid basis) at the bottom of the downcomer are acceptable. The active or bubble area of a tray normally is the area left after the downcomer area is determined. Very small residence time and small flow path lengths along the bubble area may result in low tray efficiencies. The open or hole area of a tray affects tray performance parameters such as dry tray pressure drop, weeping and tray flexibility, and the transition between the froth and spray regimes on the tray. Lowering the open area increases the vapor velocity through the holes, the dry tray pressure drop, and as a result, the downcomer filling. High vapor velocities through the holes, especially when the liquid rates are low, may result in a spray rather than froth vaporliquid contact on the tray. High open area reduces the flexibility of the tray and may result in weeping and dumping at turndown conditions. The weir height is a key factor in determining the liquid height on the tray. As such, it affects tray performance parameters such as pressure drop, weeping, and tray flexibility. It also affects the spray-froth transition and the tray efficiency. Along with the downcomer clearance, it determines the downcomer sealing . The downcomer clearance also affects the pressure loss under the downcomer (or downcomer contraction pressure loss hd) and therefore, downcomer filling. For trays with high liquid rates, shaped lip downcomers help reduce pressure drop.


36



Valve Tray Design Options Valve trays are normally specified for new column designs. They are relatively inexpensive and provide good vapor/liquid contact over a wide throughput range. Valve tray designs are normally developed by the various tray fabricators that submit quotations. Precise design methods vary, but all fabricators follow the same general procedures. Most major tray fabricators issue design manuals for general use which illustrate these procedures. Sample design manuals include the following: (a)

Glitsch, Inc. (Dallas, Texas) Ballast Tray Design Manual (Bulletin No. 4900) 5th Edition (December 1989)

(b)

Koch Engineering Company, Inc. (Wichita, Kansas) Flexitray (R) Design Manual (Bulletin 960-1, 1982)

(c)

Nutter Engineering Company (Tulsa, Oklahoma) Float Valve Design Manual April 1976

Such manuals can be used to: •

Determine the preliminary size of new columns and tray components.

•

Check tray fabricators' designs for new columns and trays.

•

Evaluate existing columns under operating conditions differing from original designs.

Tray fabricators' design manuals are updated periodically, so care must be taken to utilize the latest manuals. Tray designs developed by tray vendors or via vendor manuals should be checked using the Saudi Aramco design criteria (ADP-C-001). For final design, the tray should be rated by the vendor. Figure 26 is typical of tray design results from valve tray vendors. PROCESS/PRO II can be used to develop new tray designs and rate existing ones. PROCESS/PRO II uses the Glitsch valve tray design method. For sieve and bubble cap trays, it derates the valve tray results by 5 and 15% respectively. It is necessary to simulate the tower to rate or design a tray with PROCESS or PRO II.


37



BALLAST TRAYS DESIGNED BY GLITSCH INC.

FIGURE 26


38



Tower Internals The following is a list of common tower internals. The design of most of these internals is covered in ADP-C-001. In addition to ADP, vendors can provide designs and design criteria. • • • • • • • •

Tray support. Tray pass transitions. Downcomer seal. Antijump baffle. Wire mesh entrainment screens. Tower inlets. Tower drawoffs. Reboiler drawoffs.

An overview of tray pass transitions, downcomer seal, anti-jump baffle, and wire mesh entrainment screens is below.

Tray Transitions Figure 27 illustrates the tray transition arrangemen t for one-pass trays to two-pass trays. TRAY TRANSITIONS

FIGURE 27


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Downcomer Seal The downcomer must be sealed by liquid to prevent some of the vapor from bypassing the contacting region by flowing upward through the liquid in the downcomer. In most designs, the liquid holdup (or clear liquid height) will sufficiently seal the downcomer clearance without additional hardware devices. When this is not possible, however, ways to provide a seal via hardware techniques are shown in Figure 28.

DOWNCOMER SEALING TECHNIQUES

FIGURE 28


40



Seal Pan Seal pans provide seal for the bottom tray. A typical seal pan arrangement is shown in Figure 29.

SEAL PAN

FIGURE 29


41



Antijump Baffle Figure 30 illustrates a typical antijump baffle. Antijump baffles are used to prevent liquid collision in the center of the inboard downcomer. The specific need for this baffle varies with liquid rate and the type of tray used.

ANTIJUMP BAFFLE

FIGURE 30


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Wire Mesh Entrainment Screens (Demisters) In some towers, entrainment of liquid can cause serious product contamination and degradation. Crinkled wire mesh screens are installed to provide a surface upon which the entrained liquid can coalesce to prevent this problem. These screens must be carefully designed. If the velocity through the screen is too low, maximum coalescence will not occur. If the velocity is too high, coalesced liquid will be re-entrained from the screen. Screen coking may also be a problem, depending on temperature, type of screen, and feedstock quality. Each tower must be considered individually and past or similar experience relied upon. An example of wire mesh screen efficiency is provided in Figure 31. For optimum performance, the kinetic energy factor F = V [r V/(r L-r V)]0.5 for the vapor entering the screen should fall within the design range of the screen. If it falls below this range, the crosssectional area of the screen should be reduced somewhat by addition of a donut-shaped baffle around the screen.

EXAMPLE OF WIRE MESH EFFICIENCY

Kinetic Energy Factor, F = V

ρV

0.5

ρ L – ρ V

FIGURE 31


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GLOSSARY active area

The tray deck area where the liquid-vapor contacts take place.

antijump baffle

Tower internal device placed over the inlet of an inboard downcomer in order to prevent liquid from one side from jumping to the other side. See figure in the text.

arc downcomer

A type of downcomer. See figure in Downcomer Configuration section.

baffle sections

Horizontal or low-angle contacting devices creating cascades of liquid for contact with rising vapor. There are two basic types of baffle sections: sheds, and disks and donuts. See the figures in the text.

blank tray

Tray used to collect liquid from higher trays or packing. Blank trays do not provide vapor-liquid contact. A synonymous term is chimney tray.

bubble cap tray

A type of tray. The vapor goes through risers and inverted caps making contact with the liquid when leaving the caps. See the figures in the text.

cartridge tray

Prefabricated tray and downcomer assembly. See figure in text.

chimney tray

Tray used to collect liquid from higher trays or packing. Chimney trays do not provide vapor-liquid contact. A synonymous term is blank tray .

choking

Accumulation of froth bridged over the inlet of a downcomer, slowing down the transfer of liquid to the trays below.

chordal downcomer

Vertical straight downcomer across a chord of the tower cross section. Synonymous with straight downcomer . See Figure 7, Downcomer Configuration section.

column

A vertical vessel containing contacting devices such as trays or packing, used to perform separations such as distillation or extraction. A synonymous term is tower.

countercurrent devices

Devices in which the liquid flow is truly countercurrent to the vapor flow.


44



cross-flow devices

Devices in which liquid flows horizontally across a flat plate.

debottlenecking

Removal of a process or equipment constraint.

demisting

Elimination of entrained liquid droplets at the top of a packed bed or a trayed tower.

disc and donuts

A type of baffle section. See the figures in the text.

downcomer area

The cross-sectional area of downcomers.

downcomer clearance

The vertical distance between the bottom of the downcomer and the tray deck.

downcomer contraction pressure drop

Pressure drop of the liquid passing under the downcomer.

downcomer filling

Height of liquid in the downcomer. It is often expressed in inches of clear liquid or a percent (clear liquid) of the tray spacing.

downcomer flooding

Overloading of the tray interspace with liquid, caused by high downcomer filling.

downcomer rise

The horizontal radial distance between the center of the chord of a straight outboard downcomer and the vessel wall.

downcomer seal

Hydraulic seal of the downcomer outlet. See figures in the text.

downcomers

Tower internals that allow the tray liquid to pass to the tray below.

dry-tray pressure drop

Part of the pressure drop that is not related to the presence of the liquid on the tray, that is, the pressure of the vapor through the contacting device.

dumped packing

Packing type, consisting of small (2-in. is typical) devices with large open space, placed in the tower (dumped) in random orientation. A synonymous term is random packing .

dumping

Weeping of all the liquid, so that no liquid flows over the weir.


45



entrainment


Liquid carryover by the vapor to the tray above.

46



flexibility

Refers to capacity related flexibility. See Turndown.

flooding

Overloading of the tray interspace with liquid. Frequently, the term refers to jet flooding.

flow regimes

The movement of liquid and vapor on a tray.

free area

The tray cross-sectional area available for vapor flow.

froth

A flow regime in which vapor passes through a liquid on the tray as discrete bubbles of irregular shape.

grids

Countercurrent contacting devices fabricated in panels and installed in an ordered manner. In contrast to structured packing, grids provide wide clearances. See the figures in the text.

hole area

The open area provided within the bubble area to permit vapor to enter, contact, and pass through the liquid on the tray.

inboard downcomer

Downcomer positioned by the vessel wall.

jet flooding

Overloading of the tray interspace with liquid, caused by excessive entrainment.

modified arc downcomer

A type of downcomer. See Figure 8 in Downcomer Configuration section.

multiple downcomer tray

Proprietary type of tray. See figure in Downcomer Configuration section.

outboard downcomer

Downcomer positioned by the vessel wall.

packing

Devices that provide countercurrent vapor-liquid contact in distillation columns.

percent jet flood flood)

(% The ratio, expressed as a percent, of the vapor velocity between the trays, V, divided by the maximum vapor velocity that will not cause flooding.


47



plates

Contact points of all the vapor and liquid in a column, such as it occurs on column trays. The term theoretical plates is used to indicate that equilibrium is reached at the contact point between all the vapor and all the liquid. The actual plates reflect the obtained tray efficiency. A synonymous term is stages.

pumparound

Heat removal from a stream pumped from a tray to a higher tray.

random packing

Packing type, consisting of small (2-in. is typical) devices with large open space, placed in the tower (dumped) in random orientation. A synonymous term is dumped packing .

seal pan

Tower internal device placed over the inlet of an inboard downcomer in order to prevent liquid from one side from jumping to the other side. See figure in the text.

sheds

A type of baffle section. See Figure 7 in the text.

sieve tray

A perforated plate type of tray.

sloped downcomer


spray

A flow regime in which a gas jet issuing from the orifice shatters some liquid into droplets.

stages

Contact points of all the vapor and liquid in a column, such as occurs on column trays. The term theoretical stages is used to indicate that equilibrium is reached at the contact point between. The actual stages reflect the obtained tray efficiency. A synonymous term is plates .

stepped downcomer


straight downcomer

Vertical straight downcomer across a chord of the tower cross section. Synonymous with chordal downcomer . See Figure 8 in Downcomer Configuration section.

structured packing

Countercurrent contacting devices fabricated from thin crimped sheets of metal and installed in layers having a fixed orientation. See the figures in the text.


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superficial velocity

Velocity based on the tower diameter rather than the crosssectional area available for flow.

support ring

Horizontal ring welded to the tower walls that are used to support a tray.

tower

See column.

tray loadings

Tray vapor and liquid rates.

tray pass number

The number of individual paths of liquid on a tray.

tray spacing

The vertical distance between two trays.

tray turndown

The ratio of maximum to minimum tray loadings in a range over which acceptable performance is achieved.

truss

Tray support beam.

turndown

Operation at reduced capacity.

ultimate capacity

The largest vapor load a tower can handle, as predicted by the Stokes law on droplet entrainment.

valve tray

A type of tray with contacting devices that can be opened and closed. See the figures in text.

waste area

Any area in the active area that is farther than 3 in. from the edge of a contacting device.

weeping

Liquid flow through the tray openings.

weir

A vertical strip at the inlet or outlet of a tray used to maintain liquid height on the tray or a liquid seal at the outlet of the downcomer. See figure in text.


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Che 10402

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