Distillation Column Tray Selection & Sizing – 1 - Separation Technologies

► Tower Packing

► Packing Trays

► Liquid Packing

► Pump Selection

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D istil lati on Column Tr ay Sel ection & Si zi ng – 1 - Separ ati on Technolog ies

The factors discussed below influence the choice between trays & packings. As these are guidelines for selection of trays or packings for a particular translated into service, it is recommended to analyze each design case on its own merit for selection. Sr. No.System Favouring Tray Column

Sys te m Favouring Packe d Column

1 Solid handling

Vaccum system energy gain or separation improvement.

High liquid rate

Low pressure drop application

2 composition Feed and and variable

3

4

temperature reduction an can Rev Revampsam beps- The The pressu pressure re drop gain, capacity

foundation fou atend suppo L arndations ge di diamseand r co csupports olumnrts s

Small diameter co columns< 90 900 mm mm

Perf Perfor orm mance ance pre predi dict ctiion is easy easy

polym pol ymeri erisati and and degr degradati adation. on. Corr Corros osi ivsation e syst syon stem em

5

6 Less

weight saving

in cost

of Foaming system

7 colils, Inte Interb rboi oil &lers, er side s, in idraw nterc tercon onde den nsers sers,, cool cooliingLow 8High turn down requirements

reducing liqui quid

holdu oldup p

for

Batch Distillation

9 Chemical reactions The industry, based on its experience, has standardised the type to be used in certain services. If this reference is not available the guideline as per Appendix 1 are to be used Types of Tray

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The factors discussed below influence the choice between trays & packings. As these are guidelines for selection of trays or packings for a particular translated into service, it is recommended to analyze each design case on its own merit for selection. Sr. No.System Favouring Tray Column

Sys te m Favouring Packe d Column

1 Solid handling

Vaccum system energy gain or separation improvement.

High liquid rate

Low pressure drop application

2 composition Feed and and variable

3

4

temperature reduction an can Rev Revampsam beps- The The pressu pressure re drop gain, capacity

foundation fou atend suppo L arndations ge di diamseand r co csupports olumnrts s

Small diameter co columns< 90 900 mm mm

Perf Perfor orm mance ance pre predi dict ctiion is easy easy

polym pol ymeri erisati and and degr degradati adation. on. Corr Corros osi ivsation e syst syon stem em

5

6 Less

weight saving

in cost

of Foaming system

7 colils, Inte Interb rboi oil &lers, er side s, in idraw nterc tercon onde den nsers sers,, cool cooliingLow 8High turn down requirements

reducing liqui quid

holdu oldup p

for

Batch Distillation

9 Chemical reactions The industry, based on its experience, has standardised the type to be used in certain services. If this reference is not available the guideline as per Appendix 1 are to be used Types of Tray

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The particular tray selection and its design can materially affect the performance of a given distillation, absorption, or stripping system. Each tray should be design designed ed so as to gi give as effi effici cien entt a contact contact between between the the vapou vapourr and and li liqui quid as possi possibl ble, e, with withiin reason reasonabl ablee econom economiic li limits. Valve tray:

Valve trays are perforated sheet metal decks on which round, liftable valves are mounted. The vapour flows through valves which are installed parallel to the outlet weir. Valve trays combine high capacity and excellent efficiency with a wide operating range.

Advantages: Excellent Excellent liquid/ liquid/ vapour contacting contac ting.. Higher Higher capacity. cap acity. Higher flexibility flexibility than sieve trays. tra ys. Can Ca n handle higher higher loadings. Low-pressure Low-p ressure drop than bubble cap.

Sieve tray: tray:

Sieve trays are flat perforated plate in which vapour rises through small holes in tray floor, & bubbles through liquid in fairly uniform manner. They have comparable capacity cap acity as valve valve trays. tra ys.

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Distillation Column Tray Selection & Sizing – 1 - Separation Technologies

Advantages:

Simple construction Low entrainment, low cost Low maintenance cost Low fouling tendency

Disadvantages:

Less- flexible to varying loads than other two types Bubble cap tray:

Vapour rises through risers or uptakes into bubble cap, out through slots as bubbles into surrounding liquid on tray. It is mainly used in special applications.

Advantages:

Moderate capacity Most flexible (high & low vap. & liquid rates) Can provide excellent turndown. Disadvantages:

High entrainment,

High fouling tendency High cost, High pressure drop

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Dual flow trays:

A dual flow tray is a sieve tray with no downcomers. This tray operates with liquid continuously weeping through the holes. Due to the absence of downcomers, dual flow tray gives more tray area hence a greater capacity than any of the common tray types. They are ideal for revamp where if some efficiency can be sacrificed for more capacity. They are least expensive to make and easiest to install and maintain.

Dual Flow Tray

Baffle Tray

Baffle trays:

For a baffle tray column the gas flows upwards through the baffle openings and in doing so contacts the liquid showering down from one baffle to the next. Baffle tray columns have almost same flooding capacity as cross flow trays. Types of baffles used are disc & donut and segmental baffles for various column diameters. Dual flow and baffle trays are used for fouling applications, solid / slurry handling services, corrosive services. Proprietary types of trays:

MD Trays – Linde / UOP, Ripple Trays – Stone & Webster Engg. Corp. Rectangular Valve (BDH), ValveGrid (MVG/SVG), SHELL HIFI, ConSep Trays – SulzerBallast Tray,

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Flexitray, Bi-FRAC, SUPERFRAC and ULTRAFRAC Trays – Koch-Glitsch Engg.Co., Tunnel Trays- Montz, Nye trays- Nye Engg Co, Comparison between Common Conventional Trays . Sr. Factors No.

Sie ve Tray

Valve Tray

1

Capacity

High

High

2

Efficiency

High

High

3

Turndown

~50%

~25-30%

4

Entrainment

Moderate

Moderate

5

Pressure Drop Moderate

Moderate

6

Cost

Low

~1.2 sieve times trays

7 8 9 10 11

Maintenance Fouling Tendency Effects Corrosion of Design information Main Application

Low Low Low Well Known when Often is not turndown critical used

Low Moderate to Moderate Low to moderate Low to Proprietary, available but readily Where turndown requiredhigh is

Bubble -Cap Tray

Dual-Flow Tray

Moderately High

Very High

Moderately High

Least

10%

Least

High Low to moderate High Low to Moderate ~ 2-3 times traysof sieve

Least

Relatively High

Low

Solids High: Tends to collect Extremely Low High Very Low Well Known Available.Instability dia. can Some occur (>8 information feet) in large Extremely minimized leakage flow & Where must lowbeLiquid Highly Capacity corrosive fouling revamps, services and

Tray Parameters a)

No. of passes (Np):

The numbers of flowpaths of liquid on tray are 1, 2, 3 or 4 as per liquid capacity requirement of column. From a capacity viewpoint, a liquid rate greater than 6 gpm / inch of weir (weir loading), is the rate at which a higher number of flow paths should be considered. The maximum allowable weir

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loading is 13 gpm/in of weir length. If the weir loading exceeds this the tray needs redesign with higher number of passes. b)

Tray Spacing (S):

Tray spacing is the distance between two trays. Generally tray spacing ranges from 8 to 36 inches (200 mm to 900 mm). Prime factor in setting tray spacing is the economic trade-off between column height and column diameter. Most columns have 600 mm tray spacing. Cryogenic columns have tray spacing of 200-300 mm. c)

Outlet Weirs (hw):

An outlet weir maintains a desired liquid level on the tray. As the liquid leaves the contacting area of the tray, it flows over the tray weir to enter into the downcomer.

d)

Downcomer Clearance (hcl ):

This is the vertical distance between the tray floor and the bottom edge of the downcomer apron. The Normalpractice is to use a downcomer clearance of 1/2 inch less than the overflow weir height to provide a static liquid seal e)

Inlet Weirs & Recess ed Seal Pans:

Inlet weirs and recessed seal pans are primarily used for achieving a downcomer seal in cases where a potential positive sealing problem exists and clearance under downcomer is limited f)

Downcomers:

Passage of liquid from the top tray to the bottom of tray occurs via downcomers. Downcomers are conduits having circular, segmental, or rectangular cross sections that convey liquid from upper tray to a lower tray in a distillation column.

g)

Downcomer width (Chord height, WDC):

It is maximum horizontal distance between tower wall and weir. h)

Flow path length (FPL):

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Flow path length is the distance between the inlet downcomer & outlet downcomer. The minimum limit for flow path length is 400 mm in order to provide good contacting between vapour and liquid. This is also necessary for the mechanical reason of providing tray manway.

i)

Tray deck thicknes s (t):

Trays normally used in commercial service need a minimum material thickness to provide structural strength (personnel walk on them during installation) and corrosion allowance. A thickness of 10 to 12 gauge (2.5 to 3.5 mm) is customary for carbon steel, while 12 to 14 gauge (1.9 to 2.5 mm) is used for stainless steel trays (in general no C.A. for SS)

j)

Hole pitch (P):

Centre to centre distance between holes is called pitch. Normal practice is to use a hole pitch to hole diameter ratio between 2.2 to 3.8.

k)

Syste m (Derating) factors:

Derating factors are often closely related to the foaming tendency of the system. Higher the foaming tendency, the lower is the Derating factor. System factors are used in three of the rating correlations (jet flood, down comer backup flood, down comer choke) to account for system effects on hydraulic capacity limits. It includes both foaming effects and high vapour density.

l)

Bubbling (Active) Area (AB):

Bubbling area is the column area, which is actually available for vapour bubbling through liquid. It can be defined as column area minus downcomer areas, downcomer seal & large calming zones. m) % Hole Area:

This is the ratio of hole area to bubbling area. The default practice is to target a hole area of 8 to 10 % of bubbling area for pressure services. The acceptable range for percentage hole area is 5 % to 15 %. However for some critical services, we can go % hole area up to 17-17.5% provided that weeping is under control. Hole areas below 5 % are not used.

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n)


Anti jump baffles:

Anti jump baffles plates suspended vertically above centre or off centre downcomers, which stops liquid jumping from one deck onto the opposite deck, flow path Tray Hydraulic Parameters

Following are the some important output parameters of tray hydraulics.

a)

Flood:

Jet Flood: In spray regime operation flooding is brought about by excessive vapour flow, causing excessive liquid to be entrained in the vapour up the column. In froth and emulsion flows regimes operation excessive froth entrainment in the vapour up the column causes jet flooding. Down-comer Back-up Flood: Occurs when the pressure available for a given height of liquid and froth in the downcomer cannot overcome the total pressure drop across the tray This pressure imbalance causes the froth in the downcomer to start backing-up until it reaches the tray above, causing an increased accumulation of liquid on it. It requires high liquid and vapour loads. Downcomer Choke Flood: The mechanism by which this type of flooding occurs is one related to frictional pressure losses in the downcomer becoming excessive. In addition, the vapour carried into the downcomer must separate from the liquid and then flow counter-current to the liquid entering the downcomer. When the combination of vapour exiting and the liquid entering becomes excessive, the downcomer entrance is choked causing the liquid to backup on the tray. It requires relatively high liquid rates, surpassing a velocity limitation on the downcomer. b) Weeping/Dumping

The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations.

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c)


Pres sure Drop:

Pressure drop is an important consideration while designing a tray. It becomes more critical for the vacuum systems than the high-pressure systems. The tray pressure drop is viewed as the sum of the pressure drop through the valves or sieves and pressure drop through the aerated liquid on the tray deck. d)

Turndown ratio:

Turndown ratio defines the range of vapour load between which the column can operate without substantially affecting its’ primary separation objective (i.e. fractionation efficiency) or over which acceptable tray performance is achieved. The tray efficiency stays at or above the design value throughout the turndown range. Tray Sizing

The sizing procedure is an iterative calculation. A preliminary design is set, and then refined by checking against the performance correlations until an adequate design is achieved. The sizing calculations are performed at the point where column loading is expected to be highest and lowest for each section, i.e., i) The top tray ii) Above every feed, product drawoff, or point of heat addition or removal. iii) Below every feed, product drawoff, or point of heat addition or removal. iv) The bottom tray. v) At any point in the column where the calculated vapour or liquid loading peaks The sizing is done at all above load points and also detailed sizing is checked at all above load points. All design parameters given in the design procedure below are calculated at all above load points at turndown and turn-up loads so that the feasibility of design for varied loads is tested. a) Preliminary determination of tower area:

The methods used for determining tower diameter are: “C” Factor Method

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Nomograph Method FRI Tray design handbook However in this technical guideline we are describing method using C-Factor Method. C-Factor Method:

The following calculations are done at all the loading points mentioned above and diameters are found separately. If the difference in calculated diameter at different sections exceeds 20 percent, different diameters for the sections are likely to be economical. The section having different diameter should be at least 20ft in length else same diameter can be maintained. i. Tray Area

Assume appropriate values for following parameters (based on system requirements) for preliminary diameter calculation. dH = Hole diameter, inches (¼ to ½ inch) S = Tray spacing, inches (18 – 24″) hct = Clear Liquid height at the transition from the froth to spray regime, in of liquid. Assumption: The starting values for these can be d H=1/4″, S=24″, h ct =2″ Calculate C- Factor (C SB) using following Kister and Haas Correlation:

ii. Flood Velocity Calculation

This is the velocity of upward vapour at which liquid droplets are suspended. Calculate Flood Velocity (u N ) using following equation:

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iii. Ne t Area Calculation

The net area represents smallest area available for vapour flow in the inter-tray spacing. Calculate N et Area (A N) from the flood velocity using following equation: Assume the column is to be designed for 80% of flood.

iv. Downcomer Area Calculation Calculate downcomer area (AD) from clear liquid velocity in downcomer using following formula:

Where, QL

= Liquid Flow Rate, ft3/s

VCL

= Clear Liquid Velocity in Downcomer

Value of VCL obtained from table below. No derating factor is required for this calculation, as VCL values have taken care of foaming Table: Re commended VCL values for different foaming tendencies

VCL inSpacing downcomer, ft/s Spacing 18-in 24-in Spacing 30-in Foaming Example Tendency med. pressure (100- 300 psia) Low pressure (<100-psia) light

0.5-0.6

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Low

hydrocarbon, stabilizers, air-water simulators Oil systems, crude oil distillation, absorbers,

Medium

0.4-0.5 0.5-0.6

0.3-0.4 0.4-0.5

0.4-0.5

hydrocarbon 0.2-0.3 High

(>300-psi) Amine, glycerine, light hydrocarbons glycols, high-pressure

0.2-0.25 0.20.25

v. Tower Diameter Calculation

TotalTowerArea (AT) = AD + A N

b)

Preliminary tray layout:

A Preliminary layout is needed as layout influences the column size.

Downcomer Layout: Check the % of Downcomer area with respect to tower area:

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The Fractional area should around 10% but avoid less than 8% in normal circumstances. Note that A D should in no circumstance be less that 5% of AT Net Area (A N): The total tower cross-section area AT less the area at the top of the downcomer (sometime refer to as free area, the term free area.) The net area represents the smallest area available for vapour flow in the inter-tray spacing. A N = AT - A D Bubbling (Active) area (AB): The total cross-section area AT less the area at the inlet & outlet downcomer is called as bubbling area. A B

= AT - A DT - A DB

Below figure shows the Typical Tray Layout.

Weir Length and Downcomer Width: SinglePass Tray:

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The calculation of Weir Length and Downcomer Width involves geometrical relationship between downcomer area, downcomer width, and downcomer length. Following Figure shows downcomer geometry:

Calculate downcomer width and weir length using following method

? = sin-1(h/R)

w = 2*R COS (?) or w = 2*(R 2 – h2)0.5 ?/2 = ?/2 - ?

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Sector area = ASECT = ? R 2 * ? / (2 * ?) Area of triangle (ABC) = ATRI. = w*h/2 Where, Lw = Weir Length = w* (1-fractional weir blockage) wdc = Downcomer Width = R -h AD = Adc = Downcomer Area Fractional weir blockage is the fraction of total weir length that is available for liquid flow by using picket and fence type of weir. Blocked (Picket fence) weirs are used for handling low liquid loading. Down-comer area A

D = ASECT- ATRI

Two Pass Tray: Two pass trays have alternating arrangements of one center-downcomer and two side-downcomers.

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The side downcomer area can be calculated as that for single pass tray. It should be noted that side down-comers are on both sides. Center downcomer calculations can be done as follows in similar manner as side down-comer: ? = sin-1 (h/R)

w = 2*R COS (?1) or w = 2*(R 2 – h2)0.5 ? = 2*(?/2- ?)

Sector area = ASECT = ? R 2 * ? / (2 * ?) Area of center downcomer = Area of circle -2*area of sector + 2*Area of Triangle Area of downcomer = ?*R 2 – 2* ASECT + h1*w1 In case of more than two pass trays we have to define one more parameter, i.e. off-center downcomer location from centerline. This needs to be done on a case-by-case basis.

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Liquid Flow Path Length (FPL): ForSinglePassTray: FPL= (tray diameter) minus (side DC width of the tray) minus (bottom width of DC of tray above)

Where,

w1dc

=Downcomer width (Centre downcomer, Bottom of Downcomer)

w2dc

= Downcomer width (Side downcomer, Top of Downcomer)

w3dc

=Downcomer width (Centre downcomer, Top of Downcomer) =Downcomer width (Side downcomer, Bottom of

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w4dc


Downcomer)

C) Detailed Design Flooding Check:

The flooding check is performed using following Correlations: 1. 2. 3. 4.

Kister and Haas correlation. Downcomer choke-Koch correlation Fair’s correlation Smith et al. correlation

1. Jet Flood: Kister and Haas correlation

This correlation possess following advantage: -

It gives a close approximation to the effects of physical properties, operating variable, and tray geometry on the flood point.

-

It describes spray regime entrainment.

-

It was derived from a much wider database of commercial and pilot-scale column data.

-

It can predict sieve and valve tray entrainment flooding within ± 15 and ± 20 percent respectively.

This correlation possess following restriction: Sr.no. 1 2 3 4 5 6 7 8

Factors Flooding Mechanism Tray Type Pressure Gas Velocity Liquid Load Gas Density Liquid Density Surface Tension

Applicability Entrainment (Jet) flood only Sieve or Valve trays only 1.5-500 psia 1.5-13 ft/s 0.5-12 gpm/in of outlet weir 0.03-10 lb/ft3 20-75 lb/ft3 5-80 dyne/cm

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9 10

Liquid Viscosity Tray Spacing

0.05-2.0 cP 14-36 in

11 12 13

Hole Diameter Fractional Hole Area Weir Height

1/8-1 in 0.06-0.20 0-3 in

Steps to calculate % Flooding using Kister and Haas correlation: i. Calculate Weir Load (QL): Liquid Load describes the flux of liquid across the tray.

ii. Clear Liquid height at the transition from the froth to spray ((hct )

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2. Jet Flood: Fair’s correlation

The Fair correlation has been standard of the industry for entrainment flood prediction. Fair’s correlation tends to be conservative, especially at high pressure and liquid rate. This correlation possess following restriction:

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Sr.No Factors

Applicability

1 Flooding Mechanism

Entrainment (Jet) flood only

Tray Type

Sieve Tray, Valve and Bubble-cap Tray

Hole size Weir height

Hole£ ½ in (sieve tray) < 15% Tray Spacing

2

3

Steps to calculate % Flood using Fair’s correlation: i. Calculate flow parameter

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3. Down-comer choke-Koch correlation:

This is the more conservative correlation for checking Down-comer Design. Steps to calculate % Load Utilization using Kister and Haas correlation:

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4. Hydraulic checks

Hydraulic check involves checking following parameters: -

Flow Regime

-

Entrainment

-

Downcomer residence time

-

Pressure Drop

-

Downcomer backup

ii.Determination of Flow Regime Froth Regime This is the most commonly encountered flow regime in operating columns. The froth formed under this regime is described as one where the size and shape of bubbles is non-uniform and with rather large size distribution, as well as travelling at varying velocities. The liquid surface is either wavy or it

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presents oscillations. This is a liquid continuous flow regime. Spray Regime This regimes occurs at relatively high vapour velocities (i.e. large vapour flow rates) and low liquid loads, characteristics which are typical of vacuum systems. The vapour velocity is so large, that the liquid phase is completely disrupted and is no longer a continuous phase on top of the tray; liquid is a dispersed phase present only in the form of drops, and therefore the continuous phase is the vapour. Emulsion Regime This flow regime is typically encountered in high-pressure systems and relatively high liquid loads. The shearing action of the high velocity liquid “tears off” the vapour bubbles leaving the orifices on the tray. Most of the gas is emulsified in small bubbles within the liquid, with the mixture behaving as a uniform two-phase fluid, obeying the Francis weir formula. This is a liquid continuous flow regime. The determination of regime on tray given below is only for information and has no use in sizing. ii. Froth-Emulsion Transition Check This correlation is applicable for Sieve trays only. The value of actual flow parameter is calculated as below:

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If the value of actual flow parameter exceeds 0.0208 then the regime of operation is emulsion. iii. Froth-Spray Transition Check: Porter and Jenkins correlation for the froth to spray transition.

Where, Lw – weir length in inches, AB – Active area ft2 p – pitch in inches hc – clear liquid height, inches 5. Entrainment:

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If entrainment is excessive, column diameter or tray spacing are usually increased. As recommended value, the entrainment from the tray should not exceed about 0.10 lb liquid entrained per pound of liquid flow. Methods to determine Entrainment: Fair’s entrainment correlation This method holds good for froth and emulsion regime. However it is less accurate for spray regime. For a trays operating at a high liquid to vapour ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantity of entrained liquid.

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Kister and Haas Correlation This method is used for Spray Regime; Es is entrainment lb of liquid / lb of vapour.

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May 7th, 2012 in Design Distillation System Categories Adsorption Crystallization Process Design Distillation System Distillation Technology Experimental Setups Membrane Separation Operations of Distillation System Other Unit Operations Process Simulation

Distillation Column Tray Selection & Sizing – 1 - Separation Technologies

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