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GAS ABSORPTION
● also known as scrubbing
● an operation in which a gas mixture is contacted with a liquid for the purpose
of preferentially dissolving one or more components of the gas mixture and to provide a
solution of them in the liquid.
● there is a mass transfer of the component of the gas from the gas phase to the
liquid phase.
● the operation of absorption is applied in industry to purify process streams or
recover valuable components of the stream. It is used extensively to remove toxic or
noxious components (pollutants) from effluent gas streams.
The absorption process requires the following steps:
1. diffusion of the solute gas molecules through the host gas to the liquid boundary
layer based on a concentration gradient
2. solvation of the solute gas in the host liquid based on gas-liquid solubility
3. diffusion of the solute gas based on concentration gradient, thus depleting the liquid
boundary layer and permitting further solvation.
Method of Operation:
A. Counter-current Operation
● it was widely used in the industry.
● the gas enters the column or tower from below as leaves at the top, while liquid
enters from the top and flows in opposite direction and exits from the bottom.
Figure for Dilute System:
Notations : In terms of mole fraction and total flowrates
y : mole fraction of solute A in the gas phase
x : mole fraction of solute A in the liquid phase
G : total molar flowrate of the gas stream (gas flux), kg-moles/m2.s
L : total molar flowrate of the liquid stream, kg-moles/m2.s
Inside the column, mass transfer takes place as the solute (component A) is
absorbed by the liquid. The quantities of L and x (for the liquid side) and G and y (for the
gas side) varies continuously: as we gradually move up the column, component A is
continuously being transferred from the gas phase to the liquid phase. Thus, in going up
the column, there is a decrease in the total gas flowrate, and a decrease in the
concentration of A in the gas phase. At the same time, in going down the column, there
is an increase in the total liquid flowrate, and an increase in the concentration of A in
the liquid phase. Thus,
G1 > G > G2 L1 > L > L2
y1 > y > y2 x1 > x > x2
The relationship between these variables L, x, G and y is the operating line
equation. The operating line equation is obtained by material balance around the
column.
At steady-state: IN = OUT
Thus, G y + L 1 x1 = L x + G1 y1
Using the dilute system assumptions, we simply the equation and obtain:
G y = L x + G y1 - L x1
Re-arranging:
Y =[ ] x +[
Since L and G are assumed to be approximately constant, the operating line is a straight
line of the form y = mx + c, with the gradient of L / G, the liquid-to-gas ratio.
The operating line connects the 2 end points - point 1 (x1 , y1) that represents
conditions at the bottom of the column, and point 2 (x2 , y2) that represents conditions
at the top of the column.
B. Co-current Gas Absorption
● This mode of operation is seldom used in practice.
● It is less efficient than counter-current operation.
The main points to note about this operation are as follow:
● The operating line has negative slope.
● The is no minimum liquid-to-gas ratio.
● To produce an exit liquid and gas streams at equilibrium (xe, ye) on the
equilibrium curve, an infinitely tall column must be used.
ABSORPTION EQUIPMENT
● Tray (or plate or stage) Columns - The types of trays used in absorption include: sieve
tray, valve tray and bubble-cap trays. These internals are the same as those covered in
"Distillation".
● Packed Column - Both random and structured packings had been used.
● Spray Column - The gas flows upward continuously through an open chamber in
which scrubbing liquid droplets falls from spray nozzles through the gas. The gas
pressure drop is small, but separation is not as good as the bubble column. This column
is widely used for its simplicity, low pressure drop, and resistance to scale deposition
and plugging.
● Bubble Column - The gas is forced under pressure through perforated pipes
submerged in the scrubbing liquid. As such the gas phase is dispersed and the liquid
phase is continuous. As the bubbles rise through the liquid, absorption of the gas occurs.
This type of device suffers from the high pressure drop due to the liquid hydrostatic
head.
Types of Absorption Processes:
Physical Absorption
● can be used as an alternative to distillation for the separation of light gases.
Chemical Absorption
● is important for removal of acid gases and for chemicals production.
Application of Gas Absorption
1. Coking
2. Coal-Fired power plant
3. Natural Gas Production
4. Microelectronics Manufacturing
5. Ground Water Remediation
GAS DESORPTION
● also known as Stripping
● The process of removing gas dissolved in a liquid and where the liberated gas
is taken up in a gas or vapor in contact with the liquid.
● The mass transfer is in the opposite direction, i.e. from the liquid phase to the
gas phase.