Absorption Tower Design Theory
Gas absorption is a mass transfer operation where a gaseous solute is dissolved into a liquid solvent in a packed or tray column.
What is Absorption?
Absorption (or scrubbing) is a separation process where a gas mixture contacts a liquid solvent, and one or more components transfer from gas to liquid phase.
Common Applications: CO2 capture, H2S removal, SO2 scrubbing, VOC recovery, ammonia absorption.
Material Balance
For countercurrent flow (gas up, liquid down) on a solute-free basis:
G'(Y1 - Y2) = L'(X1 - X2)
Where G' and L' are solute-free flow rates, Y = y/(1-y) and X = x/(1-x) are mole ratios. For dilute systems, Y ≈ y and X ≈ x.
Equilibrium Relationship
For dilute systems, Henry's Law applies:
y* = m × x (or pA = H × xA)
Where m is the dimensionless Henry's constant (slope of equilibrium line). Low m means high solubility → easier absorption.
Absorption Factor
The absorption factor A determines the ease of separation:
A = L / (m × G)
- A > 1: Operating line above equilibrium - absorption favored
- A = 1: Lines parallel - infinite stages needed
- A < 1: Operating line below equilibrium - incomplete absorption
Kremser Equation
For theoretical stages with linear equilibrium and operating lines:
N = ln[(1 - 1/A)(y1 - mx2)/(y2 - mx2) + 1/A] / ln(A)
Where y1 = inlet gas, y2 = outlet gas, x2 = inlet liquid.
NTU-HTU Method (Packed Columns)
For packed columns, height is calculated using transfer units:
Z = NOG × HOG
Number of Transfer Units (NOG)
NOG = ∫ dy / (y - y*) = (y1 - y2) / ΔyLM
Height of Transfer Unit (HOG)
HOG depends on packing type, flow rates, and physical properties. Typical values: 0.3-1.5 m for random packing, 0.15-0.5 m for structured packing.
Minimum Liquid Rate
At minimum liquid rate, operating line passes through the pinch point:
Lmin = G × (y1 - y2) / (y1/m - x2)
Actual liquid rate is typically 1.2-1.5 times Lmin.
Operating Diagram
The y-x diagram shows:
Operating Line: Passes through (x2, y2) with slope L/G
Equilibrium Line: y* = mx (passes through origin)
Stage Steps: Horizontal to equilibrium, vertical to operating line
Column Types
| Type | Advantages | Disadvantages |
|---|---|---|
| Packed Column | Low pressure drop, good for corrosive/foaming | Poor liquid distribution, scale-up issues |
| Tray Column | Easy scale-up, handles varying loads | Higher pressure drop, not for foaming |
| Spray Tower | Very low ΔP, handles solids | Poor efficiency, backmixing |
References
- Kremser, A. (1930). "Theoretical Analysis of Absorption Process". Natl. Petroleum News.
- Treybal, R.E. (1980). Mass-Transfer Operations. McGraw-Hill.
- Seader, J.D. and Henley, E.J. (2006). Separation Process Principles. Wiley.
- Perry, R.H. and Green, D.W. (2008). Perry's Chemical Engineers' Handbook. McGraw-Hill.