Distillation Column Tray Selection & Sizing – 1
Once the process design stage ends, the equipment design begins. This stage of design converts process requirements into actual hardware.
One of the most prominent hardwares used for mass transfer is tray. Tray columns are widely used in various types of mass transfer operations. All the simulation results, which predict a certain number of theoretical stages, can be converted to actual trays depending upon tray efficiency for a particular service.
In any conventional tray vapour rises through the liquid pool on the tray deck and then disengages from the liquid in the space above the deck. Liquid enters the tray from a downcomer 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.
- Vapour space above the active area: This is the zone in which liquid is separated from vapour.
- Downcomer between trays. This zone has two functions, first moving liquid from one contacting tray to another and second disengaging vapour from liquid.
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 selection of trays or packings for a particular service, it is recommended to analyze each design case on its own merit for selection.
|System Favouring Tray Column||System Favouring Packed Column|
|Solid handling||Vaccum system|
|High liquid rate||Low pressure drop application|
|can be||translated into capacity||gain,||
|energy gain or separation improvement.|
|Large diameter columns||Small diameter columns< 900 mm|
|Performance prediction is easy||Corrosive system|
|foundations and supports|
|Interboilers, intercondensers, cooling||Low||liquid||holdup||for||
|colils, & side draw||polymerisation and degradation.|
|High turn down requirements||Batch Distillation|
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
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 the vapour and liquid as possible, within reasonable economic limits.
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.
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 as valve trays.
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.
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
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:
Comparison between Common Conventional Trays.
|Sr.||Factors||Sieve Tray||Valve Tray||Bubble-Cap Tray||Dual-Flow Tray|
|Low to moderate|
|Low to Moderate|
~ 2-3 times of sieve
|8||Fouling||Low||Low to||High: Tends to collect||Extremely Low|
|9||Effects of||Low||Low to||High||Very Low|
|10||Well Known||but readily||Well Known|
|information||can occur in large|
|dia. (>8 feet)|
|Often used||Where high||Extremely low Liquid||Capacity revamps,|
|Main||flow & Where|
|11||when turndown||turndown is||Highly fouling and|
|Application||leakage must be|
|is not critical||required||corrosive services|
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 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 & Recessed 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
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):
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 thickness (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) 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 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.
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.
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.
The pressure exerted by the vapour is insufficient to hold up the liquid on the tray. Therefore, liquid starts to leak through perforations.
c) Pressure 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.
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
- Nomograph Method
- FRI Tray design handbook
However in this technical guideline we are describing method using 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 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:
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.
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: Recommended VCL values for different foaming tendencies
VCL in downcomer, ft/s
|Low||Low pressure (<100-psia) light hydrocarbon,||0.4-0.5||0.5-0.6||
|stabilizers, air-water simulators|
|Medium||Oil systems, crude oil distillation, absorbers,||0.3-0.4||0.4-0.5||
|med. pressure (100-300 psia) hydrocarbon|
|High||Amine, glycerine, glycols, high-pressure||0.2-0.25||0.2-||
|(>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.
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 in no 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
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 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*(R2 – h2)0.5
?/2 = ?/2 - ?
Sector area = ASECT = ? R2 * ? / (2 * ?)
Area of triangle (ABC) = ATRI. = w*h/2
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.
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-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*(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 location from centerline. This needs to be done on a case-by-case basis.
Liquid Flow Path Length (FPL):
FPL= (tray diameter) minus (side DC width of the tray) minus (bottom width of DC of tray above)
|Downcomer width (Centre downcomer, Bottom of Downcomer)|
|Downcomer width (Side downcomer, Top of Downcomer)|
|Downcomer width (Centre downcomer, Top of Downcomer)|
|Downcomer width (Side downcomer, Bottom of Downcomer)|
C) Detailed Design
The flooding check is performed using following Correlations:
- 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:
|1||Flooding Mechanism||Entrainment (Jet) flood only|
|2||Tray Type||Sieve or Valve trays only|
|4||Gas Velocity||1.5-13 ft/s|
|5||Liquid Load||0.5-12 gpm/in of outlet weir|
|6||Gas Density||0.03-10 lb/ft3|
|7||Liquid Density||20-75 lb/ft3|
|8||Surface Tension||5-80 dyne/cm|
|9||Liquid Viscosity||0.05-2.0 cP|
|10||Tray Spacing||14-36 in|
|11||Hole Diameter||1/8-1 in|
|12||Fractional Hole Area||0.06-0.20|
|13||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.
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 tends to be conservative, especially at high pressure and liquid rate.
This correlation possess following restriction:
|Flooding Mechanism||Entrainment (Jet) flood only|
|Tray Type||Sieve Tray, Valve and Bubble-cap Tray|
|Hole size||Hole£ ½ in (sieve tray)|
|Weir height||< 15% Tray Spacing|
Steps to calculate % Flood using Fair’s correlation:
i. Calculate flow parameter
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:
4. Hydraulic checks
Hydraulic check involves checking following parameters:
- Flow Regime
- Downcomer residence time
- Pressure Drop
- Downcomer backup
ii.Determination of Flow 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 presents oscillations. This is a liquid continuous flow 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.
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:
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.
Lw – weir length in inches, AB – Active area ft2
p – pitch in inches
hc – clear liquid height, inches
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.
This method is used for Spray Regime; Es is entrainment lb of liquid / lb of vapour.