distillation column tray selection & sizing – 1 - separation technologies.pdf

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Distillation Column Tray Selection & Sizing – 1Introduction:

Once the process design stage ends, the equipment designbegins. This stage of design converts process requirementsinto actual hardware.

One of the most prominent hardwares used for masstransfer is tray. Tray columns are widely used in varioustypes of mass transfer operations. All the simulationresults, which predict a certain number of theoreticalstages, can be converted to actual trays depending upontray efficiency for a particular service.

In any conventional tray vapour rises through the liquidpool on the tray deck and then disengages from the liquidin the space above the deck. Liquid enters the tray from adowncomer above and leaves via a downcomer below.

Conventional Tray has three functional zones:

Active area for mixing vapour and liquid: This is the zone where mass transfer occurs.1. Vapour space above the active area: This is the zone in which liquid is separated from vapour.2. Downcomer between trays. This zone has two functions, first moving liquid from one contacting tray toanother and second disengaging vapour from liquid.

3.

Each of these zones takes up vertical and horizontal space in the tower.

Selection Guide for Tray Column:

The factors discussed below influence the choice between trays & packings. As these are guidelines for selectionof trays or packings for a particular service, it is recommended to analyze each design case on its own merit forselection.

Sr. No.System Favouring TrayColumn

System Favouring PackedColumn

Distillation Column Tray Selection & Sizing – 1 - Separation Technologies http://seperationtechnology.com/distillation-column-tray-selection-1/

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1Solid handling Vaccum system

2High liquidrate Low pressure drop application

Feedcomposition and

temperatureRevamps- The pressure drop

reduction

3can be

translated into capacity gain,

anvariable

energy gain or separationimprovement.

4Large diametercolumns Small diameter columns< 900 mm

5Performance prediction is easy Corrosive system

6Less weight saving in cost

ofFoamingsystem

foundations andsupports

7Interboilers, intercondensers, cooling Low liquid holdup for

reducing

colils, & side drawpolymerisation anddegradation.

8High turn downrequirements

BatchDistillation

9Chemical reactions

The industry, based on its experience, has standardised the type to be used in certain services. If this reference isnot available the guideline as per Appendix 1 are to be used

Types of Tray

The particular tray selection and its design can materially affect the performance of a given distillation,absorption, or stripping system. Each tray should be designed so as to give as efficient a contact between thevapour and liquid as possible, within reasonable economic limits.

Valve tray:

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


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Advantages: Excellent liquid/ vapour contacting. Higher capacity. Higher flexibility than sieve trays. Can handle higher loadings. Low-pressure drop than bubble cap.

Sieve tray:

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

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 ontray. It is mainly used in special applications.


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Advantages:

Moderate capacityMost flexible (high & low vap. & liquid rates)Can provide excellent turndown.

Disadvantages:

High entrainment, High fouling tendency High cost, High pressure drop

Dual flow trays:

A dual flow tray is a sieve tray with no downcomers. This tray operates with liquid continuously weepingthrough the holes. Due to the absence of downcomers, dual flow tray gives more tray area hence a greatercapacity than any of the common tray types. They are ideal for revamp where if some efficiency can besacrificed 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 liquidshowering down from one baffle to the next. Baffle tray columns have almost same flooding capacity as crossflow 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:


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MD Trays – Linde / UOP,Ripple Trays – Stone & Webster Engg. Corp.Rectangular Valve (BDH),ValveGrid (MVG/SVG),SHELL HIFI, ConSep Trays – SulzerBallast Tray,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 Sieve Tray Valve Tray Bubble-Cap Tray Dual-Flow TrayNo.

1 Capacity High HighModerately High Very High

2 Efficiency High HighModerately High Least

3 Turndown ~50% ~25-30%10% Least

4 Entrainment Moderate ModerateHigh

Low to moderate

5 Pressure Drop Moderate ModerateHigh

Low to Moderate

6 Cost Low ~1.2 times~ 2-3 times of sieve Least

sieve traystrays

7 Maintenance Low Low toRelatively High Low

Moderate

8 Fouling Low Low toHigh: Tends tocollect Extremely Low

Tendency Moderate Solids9 Effects of Low Low to High Very Low

Corrosion moderateProprietary, Some information

Design Available.Instability10 Well Known but readily Well Knowninformation can occur in largeavailable

dia. (>8 feet)

Often used Where highExtremely lowLiquid Capacity revamps,

Main flow & Where11 when turndownturndown is Highly fouling andApplication leakage must beis not critical required corrosive services

minimized

Tray Parameters


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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 acapacity viewpoint, a liquid rate greater than 6 gpm / inch of weir (weir loading), is the rate at which a highernumber of flow paths should be considered. The maximum allowable weir 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 to900 mm). Prime factor in setting tray spacing is the economic trade-off between column height and columndiameter. 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, itflows 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. TheNormalpractice is to use a downcomer clearance of 1/2 inch less than the overflow weir height to provide astatic liquid seal

e) Inlet Weirs & Recessed Seal Pans:

Inlet weirs and recessed seal pans are primarily used for achieving a downcomer seal in cases where a potentialpositive 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 conduitshaving circular, segmental, or rectangular cross sections that convey liquid from upper tray to a lower tray in adistillation column.

g) Downcomer width (Chord height, WDC):

It is maximum horizontal distance between tower wall and weir.

h) Flow path length (FPL):

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

i) Tray deck thickness (t):


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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 3.5mm) is customary for carbon steel, while 12 to 14 gauge (1.9 to 2.5 mm) is used for stainless steel trays (ingeneral 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 diameterratio between 2.2 to 3.8.

k) System (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 comerbackup flood, down comer choke) to account for system effects on hydraulic capacity limits. It includes bothfoaming 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 bedefined 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 % ofbubbling area for pressure services. The acceptable range for percentage hole area is 5 % to 15 %. However forsome critical services, we can go % hole area up to 17-17.5% provided that weeping is under control. Hole areasbelow 5 % are not used.

n) Anti jump baffles:

Anti jump baffles plates suspended vertically above centre or off centre downcomers, which stops liquidjumping 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


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entrained in the vapour up the column. In froth and emulsion flows regimes operation excessive frothentrainment 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 thetotal pressure drop across the tray This pressure imbalance causes the froth in the downcomer to start backing-upuntil it reaches the tray above, causing an increased accumulation of liquid on it. It requires high liquid andvapour loads.

Downcomer Choke Flood:

The mechanism by which this type of flooding occurs is one related to frictional pressure losses in thedowncomer becoming excessive. In addition, the vapour carried into the downcomer must separate from theliquid and then flow counter-current to the liquid entering the downcomer. When the combination of vapourexiting and the liquid entering becomes excessive, the downcomer entrance is choked causing the liquid tobackup 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 leakthrough perforations.

c) Pressure Drop:

Pressure drop is an important consideration while designing a tray. It becomes more critical for the vacuumsystems than the high-pressure systems. The tray pressure drop is viewed as the sum of the pressure dropthrough 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 substantiallyaffecting its’ primary separation objective (i.e. fractionation efficiency) or over which acceptable trayperformance 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 checkingagainst the performance correlations until an adequate design is achieved. The sizing calculations are performedat 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


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parameters given in the design procedure below are calculated at all above load points at turndown and turn-uploads 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

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 foundseparately. If the difference in calculated diameter at different sections exceeds 20 percent, different diametersfor the sections are likely to be economical. The section having different diameter should be at least 20ft inlength else same diameter can be maintained.

i. Tray Area

Assume appropriate values for following parameters (based on system requirements) for preliminary diametercalculation.

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 dH=1/4″, S=24″, h ct=2″

Calculate C-Factor (CSB) 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 (uN)using following equation:


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iii. Net Area Calculation

The net area represents smallest area available for vapour flow in the inter-tray spacing. Calculate Net Area (AN)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 havetaken care of foaming

Table: Recommended VCL values for different foaming tendencies

Foaming

Example

VCL in downcomer, ft/s

18-in 24-in 30-inTendency

Spacing Spacing Spacing

LowLow pressure (<100-psia) lighthydrocarbon, 0.4-0.5 0.5-0.6

0.5-0.6

stabilizers, air-water simulators

MediumOil systems, crude oil distillation,absorbers, 0.3-0.4 0.4-0.5

0.4-0.5

med. pressure (100-300 psia)hydrocarbon


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HighAmine, glycerine, glycols,high-pressure 0.2-0.25 0.2-

0.2-0.3

(>300-psi) light hydrocarbons 0.25

v. Tower Diameter Calculation

TotalTowerArea (AT) = AD + AN

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:

The Fractional area should around 10% but avoid less than 8% in normal circumstances. Note that AD should inno circumstance be less that 5% of AT

Net Area (AN):

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.

AN = AT - AD

Bubbling (Active) area (AB):

The total cross-section area AT less the area at the inlet & outlet downcomer is called as bubbling area.

AB = AT - ADT - ADB


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Below figure shows the Typical Tray Layout.

Weir Length and Downcomer Width: SinglePass Tray:

The calculation of Weir Length and Downcomer Width involves geometrical relationship between downcomerarea, downcomer width, and downcomer length.

Following Figure shows downcomer geometry:

Calculate downcomer width and weir length using following method

? = sin-1(h/R)


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w = 2*R COS (?) or w = 2*(R2 – h2)0.5

?/2 = ?/2 - ?

Sector area = ASECT = ? R2 * ? / (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 andfence type of weir. Blocked (Picket fence) weirs are used for handling low liquid loading.

Down-comer area

AD = ASECT- ATRI

Two Pass Tray:

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

The side downcomer area can be calculated as that for single pass tray. It should be noted that side down-comersare on both sides.

Center downcomer calculations can be done as follows in similar manner as side down-comer:


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? = sin-1 (h/R)

w = 2*R COS (?1) or w = 2*(R2 – h2)0.5

? = 2*(?/2- ?)

Sector area = ASECT = ? R2 * ? / (2 * ?)

Area of center downcomer = Area of circle -2*area of sector + 2*Area of Triangle Area of downcomer = ?*R2 –2* ASECT + h1*w1

In case of more than two pass trays we have to define one more parameter, i.e. off-center downcomer locationfrom centerline. This needs to be done on a case-by-case basis.

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)

w4dc=Downcomer width (Side downcomer, Bottom of

Downcomer)


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C) Detailed Design

Flooding Check:

The flooding check is performed using following Correlations:

Kister and Haas correlation.1. Downcomer choke-Koch correlation2. Fair’s correlation3. Smith et al. correlation4.

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 geometryon 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. Factors Applicability1 Flooding Mechanism Entrainment (Jet) flood only2 Tray Type Sieve or Valve trays only3 Pressure 1.5-500 psia4 Gas Velocity 1.5-13 ft/s5 Liquid Load 0.5-12 gpm/in of outlet weir6 Gas Density 0.03-10 lb/ft37 Liquid Density 20-75 lb/ft38 Surface Tension 5-80 dyne/cm9 Liquid Viscosity 0.05-2.0 cP10 Tray Spacing 14-36 in11 Hole Diameter 1/8-1 in12 Fractional Hole Area 0.06-0.2013 Weir Height 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.


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

2. Jet Flood: Fair’s correlation

The Fair correlation has been standard of the industry for entrainment flood prediction. Fair’s correlation tendsto be conservative, especially at high pressure and liquid rate.

This correlation possess following restriction:


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Sr.NoFactors Applicability

1Flooding Mechanism Entrainment (Jet) flood only

2Tray Type Sieve Tray, Valve and Bubble-cap Tray

3Hole size Hole£ ½ in (sieve tray)Weir height < 15% Tray Spacing

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 % LoadUtilization using Kister and Haas correlation:

4. Hydraulic checks


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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 regimeis described as one where the size and shape of bubbles is non-uniform and with rather large size distribution, aswell as travelling at varying velocities. The liquid surface is either wavy or it presents oscillations. This is aliquid 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 iscompletely disrupted and is no longer a continuous phase on top of the tray; liquid is a dispersed phase presentonly 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 shearingaction of the high velocity liquid “tears off” the vapour bubbles leaving the orifices on the tray. Most of the gasis 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:

If entrainment is excessive, column diameter or tray spacing are usually increased. As recommended value, theentrainment 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 traysoperating at a high liquid to vapour ratio, 0.1 lb of liquid entrained per pound of liquid is an excessive quantityof 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

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