Aerosol & Particulate Research Lab

Gas Absorption

Definition: transfer of a gaseous component (absorbate) from the gas phase to a liquid (absorbent) phase through a gas-liquid interface.
Q: What are the key parameters that affect the effectiveness?
Q: How can we improve absorption efficiency?
Mass transfer rate:
gas phase controlled absorption
liquid phase controlled absorption
Reading: Chap 13
Q: Does it matter if it’s gas phase or liquid phase controlled?

Gas Absorption Equipment
Mist
Eliminator
Liquid
Spray
Packing
Liquid outlet
Dirty gas in
Spray
nozzle
Clean gas out
Countercurrent packed tower
Spray tower
Mycock et al., 1995
Redistributor
Q: Limitations of a spray tower?
Q: Why redistributor?
Clean gas out
Dirty gas in

Berl
Saddle
Intalox
Saddle
Raschig
Ring
Lessing
Ring
Pall
Ring
Tellerette
Three-bed cross flow packed tower
Liquid spray
Dry Cell
Packing
Mycock et al., 1995
Q: Criteria for good packing materials?

Mass Balance
In = Out
Slope of Operating
Line = Lm/Gm
Lm: molar liquid flow rate
Gm: molar gas flow rate
x: mole fraction of solute in pure liquid
y: mole fraction of solute in inert gas
Known: ??

Unknown: ??
(for a dilute system)
Gas in
Liquid out
Gas out
Liquid in

Generally, actual liquid flow rates are specified at 25 to 100% greater than the required minimum.
G = 84.9 m3/min (= 3538 mole/min). Pure water is used to remove SO2 gas. The inlet gas contains 3% SO2 by volume. Henry’s law constant is 42.7 (mole fraction of SO2 in air/mole fraction of SO2 in water). Determine the minimum water flow rate (in kg/min) to achieve 90% removal efficiency.
Q: How much is X2 if fresh water is used? What if a fraction of water is recycled?

Channeling: the gas or liquid flow is much greater at some points than at others
Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the void space between packing
Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow
Problems with high gas flow
Gas flow rate is 3538 mole/min and the minimum liquid flow rate is 2448 kg/min to remove SO2 gas. The operating liquid rate is 50% more than the minimum. The packing material selected is 2” ceramic Intalox Saddles. Find the tower diameter and pressure drop based on 75% of flooding velocity for the gas velocity. Properties of air:: molecular weight: 29 g/mole; density: 1.17×10-3 g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp.

L: mass flow rate
of liquid
G: mass flow rate
of gas
G’: mass flux of gas
per cross sectional
area of column
F: Packing factor
: specific gravity
of the scrubbing
liquid
L: liquid viscosity
(in cP; 0.8 for water)
(dimensionless)

Mass Transfer
Two-Film Theory (microscopic view)
Cussler, “Diffusion”, Cambridge U. Press, 1991.
pG
CI
pI
CL
J: flux
k: mass transfer coefficient
(gas phase flux)
(liquid phase flux)
(overall flux)

Macroscopic analysis of a packed tower
Mole balance on the solute over the
differential volume of tower
(equivalent concentration
to the bulk gas pressure)
(equivalent pressure to the
bulk concentration in liquid)
(overall liquid phase MT coefficient)
(overall gas phase MT coefficient)

Mole balance on the solute in the gas only
NTU?
HTU?
a: packing area per volume
(tower height)

Mass balance
Equilibrium
x1, y1*
x1, y1
xZ, yZ*
xZ, yZ
Alternative solution:
Assumptions for dilute/soluble systems?

Pure amine
Lm = 0.46 gmole/s
0.04% CO2
1.27% CO2
Gm = 2.31
gmole/s
C* = 7.3% CO2 in amine
Q: A Packed tower using organic amine at 14 oC to absorb CO2. The entering gas contains 1.27% CO2 and is in equilibrium with a solution of amine containing 7.3% mole CO2. The gas leaves containing 0.04% CO2. The amine, flowing counter-currently, enters pure. Gas flow rate is 2.31 gmole/s and liquid flow rate is 0.46 gmole/s. The tower’s cross-sectional area is 0.84 m2. KOGa = 9.34×10-6 s-1atm-1cm-3. The pressure is 1 atm. Determine the tower height that can achieve this goal.

Absorption of concentrated vapor
Mole balance on the controlled volume
Gas flux
Liquid flux
x1, y1
x1, y1*
xZ, yZ*
xZ, yZ

Mole balance on the gas in a differential tower volume

HTU (ft)
HTU
For a given packing material and pollutant, HTU does not change much.

Summary
Transfer from gas phase to liquid phase; Gas phase or liquid phase controlled mass transfer.
Equipment: spray tower and packed tower.
Equilibrium line (Henry’s law) and operating line (mass balance).
Design: (a) liquid flow rate by mass balance; (b) tower diameter by flooding condition; (c) tower height by mass transfer rate
Dilute and concentrated system

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