💧 Absorption & Stripping Column Design

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.

Gas Absorption Fundamentals

Absorption is governed by vapor-liquid equilibrium, just like distillation — but because absorption solutes are often dilute, the equilibrium relationship is frequently approximated with Henry's law rather than Raoult's law:

y* = m·x

where y* is the gas-phase mole fraction in equilibrium with liquid-phase mole fraction x, and m (the Henry's law constant, or equilibrium slope) captures how strongly the component prefers the gas phase over the liquid. A small m means the solute is highly soluble (favors the liquid, easy to absorb); a large m means it resists absorption and needs more liquid or more stages to capture. Unlike distillation, absorption columns rarely need a reboiler or condenser — the liquid solvent is simply fed at the top, the gas at the bottom, and they contact countercurrently as the liquid falls and the gas rises.

The Operating Line and Countercurrent Contact

Just as in McCabe-Thiele distillation, an absorption column's performance can be visualized on an x-y diagram carrying the equilibrium curve and an operating line derived from a mass balance around the top (or bottom) of the column down to any intermediate point:

y = (L/G)x + [y_out − (L/G)x_in]

where L and G are the liquid and gas molar flow rates. Because the gas enters at the bottom rich in solute and leaves at the top lean, and the liquid enters at the top lean and leaves at the bottom rich, the operating line for absorption sits above the equilibrium curve (the driving force pushes solute from gas into liquid). For stripping, the operating line sits below the equilibrium curve, reversing the driving force. The vertical or horizontal distance between the operating line and the equilibrium curve at any point is the local driving force for mass transfer — the larger the gap, the faster the transfer and the fewer stages or shorter the packed bed needed.

Minimum Liquid Rate and the L/G Ratio

Exactly as minimum reflux governs distillation economics, a minimum liquid-to-gas ratio (L/G)_min governs absorber economics. It is the lowest solvent rate that can still achieve the specified removal, found where the operating line becomes tangent to (or intersects) the equilibrium curve — at that point the driving force approaches zero somewhere in the column, and an infinite contacting area (infinite stages or infinite packed height) would be required. Real absorbers are designed with a liquid rate of roughly 1.2 to 1.5 × (L/G)_min, the same capital-versus-driving-force trade-off distillation makes with reflux ratio: more liquid means a shorter, cheaper column but higher solvent circulation, pumping, and (if regenerated) stripping/reboiler cost.

The Stripping Factor

A single dimensionless group, the stripping factor, summarizes whether conditions favor absorption or stripping:

S = mG / L

When S < 1, the liquid phase dominates and the component tends to stay absorbed (favorable for absorption); when S > 1, the gas phase dominates and the component tends to leave the liquid (favorable for stripping). Absorber designs typically target an absorption factor A = L/(mG) (the reciprocal of S) in the range of 1.4-2, balancing a reasonable number of theoretical stages against solvent circulation cost. Stripping columns aim for the mirror-image target on S.

Packed Tower Design: The HTU-NTU Method

While trayed absorbers can be designed stage-by-stage exactly like a distillation column (with an efficiency correction), most industrial absorbers and strippers use packed columns, sized with the rate-based HTU-NTU method rather than counting discrete stages:

Z = HTU × NTU

• NTU (Number of Transfer Units) is a dimensionless measure of separation difficulty, found by integrating the inverse driving force (y − y*) across the required composition change. A larger required composition change, or a smaller average driving force (equilibrium curve close to the operating line), both increase NTU. For dilute systems with a straight equilibrium line, NTU has a convenient closed-form solution in terms of the absorption or stripping factor and the terminal compositions — analogous to the Kremser equation used for stage-wise absorbers.
• HTU (Height of a Transfer Unit) is a packing- and system-specific property (units of length, e.g., meters) reflecting how effectively a given packing achieves mass transfer per unit of driving force — essentially the packed-column analog of HETP. It depends on the mass-transfer coefficient, the interfacial area the packing provides, and the molar gas velocity, and is obtained from correlations or vendor/pilot data for the specific packing and system.

Multiplying the two gives the required packed bed height directly — a shorter HTU (more efficient packing) or fewer transfer units (an easier separation, larger driving force) both reduce the height needed.

Column Diameter and Flooding

Just as in distillation, the column diameter is set by the maximum allowable gas velocity before flooding — the point where liquid can no longer drain against the rising gas and the bed floods with liquid, spiking pressure drop and destroying contact. Generalized pressure-drop correlations (GPDC charts, essentially the same tool used for distillation packing) give the flooding velocity as a function of gas and liquid loading and physical properties; design velocity is typically set at 60-80% of flooding to leave adequate margin. Structured or high-capacity random packing is common in absorbers handling large gas volumes (like flue-gas scrubbers) precisely because it pushes the flooding limit higher for a given diameter.

Common Industrial Applications

• Acid gas removal (amine treating): natural gas or refinery gas is contacted with an aqueous amine solution (MEA, DEA, MDEA) in an absorber to remove CO₂ and H₂S down to pipeline or process specifications. The rich amine is then regenerated in a companion stripper (typically reboiled with steam), releasing the acid gases overhead for disposal or further processing (e.g., a Claus sulfur recovery unit) and returning lean amine to the absorber — a closed-loop absorption/stripping pair that is one of the most common unit operations in gas processing.
• VOC stripping: volatile organic compounds are stripped from contaminated groundwater or wastewater using air or steam, exploiting the compounds' high volatility (large Henry's constant) to transfer them efficiently out of the water into a gas phase for subsequent treatment (e.g., activated carbon or thermal oxidation).
• Glycol dehydration: natural gas is contacted with lean triethylene glycol (TEG) in an absorber to remove water vapor and prevent hydrate formation downstream; the wet glycol is regenerated in a stripping/reboiler system, closely paralleling the amine treating loop.
• Flue gas scrubbing: SO₂ or other acid gases are absorbed from combustion flue gas into an alkaline slurry or solution (e.g., limestone slurry in wet FGD systems), a large-scale application of the same absorption principles at high gas volumes and low pressure drop tolerance.

Design Workflow Summary

A typical absorber/stripper design proceeds: define the required removal (inlet and target outlet gas composition), select a solvent and obtain its equilibrium (Henry's constant) data, choose an operating liquid rate at roughly 1.2-1.5× the minimum L/G, compute the required NTU from the terminal compositions and the absorption/stripping factor, obtain HTU from packing vendor data or correlations for the chosen packing and physical properties, multiply to get packed height, then size the diameter from a flooding correlation at the design gas and liquid rates. The parallels to distillation are deliberate — absorption and stripping are, at their core, the same graphical and mass-transfer logic applied to a solubility-driven separation instead of a volatility-driven one.