GAS LQUID CONTACTORS
INTERPHASE MASS TRANSFER
G-L/V-L
(Absorption/Stripping/Distillation/Humidification/Dehumidification
L-L (Solvent Extraction)
G-S (Drying/Adsorption)
L-S (Leaching/Crystallization and Ion exchange)
Generation of Interfacial
area in Tray column
Objective:
Intimate contact between the phases &
high mass transfer rate
High degree of turbulent mixing
Dispersion of one phase into another
Equipment & devices for Interphase mass transfer
Gas dispersed in Liquid : Tray/Bubble/Agitated vessels
Liquid dispersed in Gas: Spray towers, Packed towers, Venturi scrubbers
Both phases are Continuous: Falling film contactor
Bubble cap /Sieve tray/Valve tray
Tray or Plate Column:
(i) Vertical cylindrical shell &
(ii) Set of tower internals
(a) Trays
(b) Arrangements for flow of liquid from one tray to the lower one through the
downcomer
(c) inlet/outlet nozzles
SHELL: metal/an alloy, Piped shells/Plate shells
MoC: corrosiveness of the fluids, T, P and cost.
If Column dia is small, several flanged sections in order that the trays may be fitted
Dc is less than 1 m - rarely used, Dc > 10 m
Tray: Tray support rings, Metal angles rounded & welded,
Big diameter trays need support beams
Skirt support - column weight, liquid load and wind stress, Seismic factor
Ladder on the shell wall, Small platforms with railings, Insulation, Man hole
Tray: ( Heart of the a column)
Trays functions:
1. Gas to flow through the holes or passages – Gas-Liquid dispersion
2. Mass transfer between the phases
Bubble cap tray:
Oldest (low carbon steel/stainless steel)
Major components: a bell shaped cap and a riser (chimney)
Slots on its wall
Shape: Rectangular, triangular, trapezoidal or saw tooth type
Equilateral triangular pitch, Bubble cap: 1” to 6”
Rectangular slot: 0.5”- 1.5”, Width: 0.125” – 0.375”
Number of slots: 12-70
Shroud ring, hsr
(6 mm)
Skirt clearance, hsc
12.5 to 38 mm
Dia of cap (dc): 5 – 15 cm
Pc : 1.25 – 1.5 dc
Sieve /Perforated Tray:
Simplest form
Holes relatively small diameter (0.125” – 0.5”)
Clean fluid services : 0.1875”
Fouling : 0.5”
Vacuum services : 0.125”
Punching than drilling
Pitch: 2.5dh – 5dh (3.8dh)
Free area: 6 - 10%
(Hole area/tower C/S, less DC area)
Thick. of plate to dia of hole: 0.4 – 0.7
Valve Tray: New class of tray
Variable area for the gas/vapor
Movable disk, normally circular
Guides – can slide vertically up/down
Self cleaning
Mechanical wear & corrosion
Sticking of the tray
Alternate rows of heavy & light valves are placed
Heavy : 12/14 Gauge sheets
Light: 16 Gauge sheets
No. of valves: 12-16 vales/ft2 of tray area
Orifice/Disk Dia : 1.5 to 2 in.
Raise of Disk: 3/16 – 7/16 in.
Flow area of fully open valves : 10-15% of the active area (Aa = Ac-Ad)
Downcomers (DC):
• DC: Passage of liquid flows down from one tray to the next
• Downcomer clearance – lower edge of the downcomer plate
should be dipped in the liquid on the lower tray with clearance
0.5 – 1 inch
• Upper portion of the DC : V-L disengagement
• Lower portion of the DC: Clear liquid to next tray
• Residence time for G-L disengagement: (Downcomer Volume/Vol
flow rate of clear liquid)
• Clear liquid velocity : 0.3 – 0.5 ft/s
• Straight/inclined plate – Segmental downcomer
Types of Downcomers
Flow arrangement
Weirs
Vertical plate - To maintain desired depth of G-L dispersion
Weir length (lw) : 60-85% of the tower diameter
DC area: 5-15% of the tray area
Weir Height: 1 – 2 inches
lw /Dc = 0.77 for 12% C/S area
Height of the weir (hw): 25-50 mm for sieve plates
For Bubble Cap: hw = hsc + hsr + hs+ hss
Feed inlet/outlet nozzle
Selection of Types of Tower
Factors to be considered:
1. Cost BCT:ST:VT is 3 : 1.5 : 2 for mild steel as MoC
2. Capacity: Dc is more for given flow rate, then capacity of tray is low. [ST:VT:BCT]
3. Operating range of liquid & vapor flow rates
For Satisfactory operation BCT > ST > VT
VT is not suitable for low vapor flow rates
Turndown ratio: Flow rateMax/Flow rateMin
Bubble cap give good for wider range of TDR
4. Efficiency: Generally not considered. h is almost same for all trays.
5. Pressure drop: ST < VT < BCT
For Vacuum Distillation, Packed tower is best choice
Low Vac. Operation, Tray column is suitable.
Plate Design Procedure
1. Calculate the Maximum & Minimum Vapor flow rates
2. Collect/Estimate system physical properties
3. Select trial plate spacing
4. Estimation of Colum Diameter, based on flooding
5. Decide liquid flow arrangement
6. Make trial plate layout – Downcomer area, Active area, Hole area, Hole size, weir
height
7. Check Weeping rate
8. Check plate pressure drop
9. Check downcomer backup
10. Decide plate layout details – Calming zone, unperforated areas, Hole pitch
11. Recalculate % flooding based on chosen diameter of the column
12. Check entrainment
13. Optimize design
14. Draw up the plate and sketch the layout
Plate Design Procedure
Flooding fixes the upper limit of vapour velocity
Vactual = 70-90% of Vflooding
The entrainment of liquid droplets in
vapour upto 10% is normally accepted.
Jet Flooding : Froth of V-L mixture
touches the above tray
Downcomer Flooding: Liquid elevation in
the DC above the weir plate
Limitations:
System is not foaming
Tray spacing : 0.15 – 1 m, Weir height is less than 15% of plate spacing
Sieve plate perforations: <= 13 mm
CSB is valid only if Ah/Aa >= 0.1
Ah/Aa < 0.1, then CSB must be multiplied by 0.9 and 0.8 for A h/Aa is 0.08 & 0.06
For Single pass tray : An: Ac – Ad, Aa: Ac – 2Ad
Ah: Total area of all active holes, Aa : Active area/bubbling area of tray
An : Net Area available for V-L disengagement
For Single pass tray : An: Ac – Ad,
Aa: Ac – 2Ad
Increasing the lt, decreases Dc, increases tray h,
No. of trays decreases, Height of tray increases
For Dc less than 1 m, lt : 0.2 – 0.3 m,
Dc above 1 m, lt : 0.3 – 0.6 m,
Larger lt is required in some plates to accommodate
nozzles, side streams, manholes
Checking for Liquid Entrainment:
kg of liquid droplets entrained per kg of gross liquid flow
Height of the Weir decides the liquid over the tray
It is a Straight and Rectangular plates.
BCT: To minimize the fluctuations, V Notch Weir plates used
Weir length (lw) : 60-85% of Dc,
DC area: 5-15% of the Ac/s
lw /Dc = 0.77 for 12% C/S area
Weir Height: 1 – 2 inches for sieve tray,
For BCT: hw = hsc + hsr + hs+ hss
Increasing the hw, increases tray h, also increases the DP
For hw : 40 mm – 90 mm, for above atm.
Pressure (40-50 mm more common)
hw : 6 mm – 12 mm, for vacuum operations
In newer designs, inlet weir is avoided to avoid hydraulic jump
Decreases the DC area, increases the DP
Increases the lw, increases the Dc
Weep Point:
Lower limit of operating range, Liquid leakage through the plate
Minimum point of Vapor flow for stable operation
Eduljee’s Equation:
Height of liquid crest over weir plate : Francis Formula
Hole size: 2.5 mm – 12 mm (5 mm is preferred)
Plate thickness :
Pitch: 2.5dh – 5dh (3.8dh)
Thick. of plate to dia of hole: 0.4 – 0.7
Tray Pressure drop:
Dptotal, tray = DPdry + DPliquid height + DPresidual
= hd + hliquid height + hresidual
= hd + (hw + how) + hresidual
DPdry: Vapor rises through perforations of dry plate
Dry Plate Pressure drop:
uh : Velocity through the holes (m/s) = Qv/Aholes Ah : 10-20% Aactive
DPliquid height: Vapor rises through pool of clear liquid
Dpresidual : Vapor Liquid disengagement space
Downcomer Backup:
Downcomer Residence time: (minimum 3 sec)
