What is the difference between absorption and stripping?

Absorption transfers a component from a gas phase into a liquid solvent — the gas is the feed and the liquid enters lean and leaves rich. Stripping is the reverse: transferring a component out of a liquid into a gas (or steam), regenerating the liquid so it can be reused. The two operations use the same equipment and the same graphical and HTU-NTU design methods, just with the driving force reversed — stripping needs the operating line to sit below the equilibrium line where absorption needs it above.

What sets the minimum liquid rate in an absorption column?

The minimum liquid rate is the lowest solvent flow that can still achieve the required removal, corresponding to the operating line just touching the equilibrium curve (an infinite number of transfer units would be needed). It is found where the operating line pivots to be tangent to, or intersects, the equilibrium curve at the gas inlet condition. Real columns are designed at 1.2 to 1.5 times the minimum liquid rate, the same capital-versus-driving-force trade-off used for reflux ratio in distillation.

What is the stripping factor and why does it matter?

The stripping factor S = mG/L (equilibrium slope m times gas rate G, divided by liquid rate L) compares how strongly a component favors the vapor phase against how much liquid is available to hold it. S greater than 1 favors stripping (the component wants to leave the liquid); S less than 1 favors absorption (the component wants to stay dissolved). Choosing an operating gas-to-liquid ratio that gives a favorable stripping or absorption factor (typically 1.4-2 for absorbers) keeps the required number of stages or transfer units reasonable.

What is HTU and NTU in packed tower design?

NTU (Number of Transfer Units) is a dimensionless measure of how difficult the separation is — how many "units" of mass-transfer driving force must be traversed — found by integrating across the concentration change relative to the equilibrium driving force. HTU (Height of a Transfer Unit) is a packing-specific property (meters per transfer unit) reflecting how efficiently that packing achieves mass transfer. Required packed height is simply Z = HTU × NTU, directly analogous to HETP × theoretical stages.

What are the most common industrial applications of absorption and stripping?

Amine absorption (gas treating) removes acid gases — CO2 and H2S — from natural gas or refinery gas streams using an amine solvent, which is then regenerated by stripping (typically with steam) in a companion column. Other major applications include VOC stripping from contaminated groundwater or wastewater using air, ammonia or SO2 scrubbing from flue gas, and glycol dehydration of natural gas, which combines absorption (water removal) with stripping (glycol regeneration).

Engineering·13 min read·July 12, 2026

💧 Absorption & Stripping Column Design

A practical guide to absorption and stripping column design — gas absorption fundamentals, the graphical operating-line method, HTU-NTU packed tower sizing, and common applications like acid gas removal and VOC stripping.

📖Chemical & Process Engineering Handbook (Full Access)

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Two Sides of the Same Mass-Transfer Coin

Not every separation relies on boiling point differences the way distillation does. Absorption and stripping separate based on differences in solubility: a component's tendency to dissolve in (or escape from) a liquid solvent. In absorption, a gas stream contacts a liquid solvent and one or more components transfer from the gas into the liquid. In stripping, the reverse happens — a liquid stream contacts a gas (often steam or air) and a dissolved component transfers back out into the gas phase, regenerating the liquid. The two operations are mirror images, share the same equipment (packed or trayed columns), and are designed with the same graphical and rate-based methods, which is why they are almost always taught and applied together.

Topics covered

gas absorptionstripping column designpacked tower designoperating lineminimum liquid rateHTU NTUheight of transfer unitnumber of transfer unitsstripping factoracid gas removalamine treatingVOC strippingHenry lawmass transfer