Absorption

Gas absorption removes components from a gas by dissolving them into a liquid. Learn Henry's law, minimum liquid rate, absorption factor, Kremser equation for stages, packed columns (HTU/NTU), stripping, and chemical absorption such as amine CO₂ capture.

Scrubbers clean flue gases, amine units remove CO₂ and H₂S from natural gas, and absorbers recover valuable solvents. Gas and liquid flow counter-currently through trays or packing so the solute moves from gas into liquid.

The liquid is often regenerated in a stripper so it can be reused.

An analogy

Absorption is like a sponge soaking up a spill: the cleaner the sponge (lean solvent) and the more you use, the more you capture.

Physical absorption

Solute gas dissolves according to Henry's law. Higher pressure and lower temperature increase solubility. Counter-current flow maximises driving force throughout the column.

Liquid rate and absorption factor

Too little liquid saturates; the minimum L/G makes the operating line touch equilibrium. Practical L/G ≈ 1.2–1.5 × minimum. Absorption factor A > 1 is needed for high recovery; stages then follow from Kremser.

Packed vs tray columns

Random or structured packing gives low pressure drop and good efficiency for corrosive or foaming systems; height = HTU × NTU. Trays suit large liquid flows and are easier to inspect. Columns are sized for flooding limits (≈ 70–80 % of flood).

Chemical absorption and CO₂ capture

Reactive solvents (MEA, MDEA, carbonates) greatly increase capacity and selectivity. Regeneration in a stripper with a reboiler consumes most of the energy (≈ 2.5–4 GJ/t CO₂ for amines). Mass transfer with reaction uses enhancement factors; solvent degradation and corrosion are practical issues.

Formulas

• Henry's law: p_A = H x_A or y = m x
• Minimum liquid (dilute): (L/G)_min ≈ m × fraction absorbed (lean solvent)
• Kremser: N = ln[((y_in − m x_in)/(y_out − m x_in))(1 − 1/A) + 1/A] / ln A
• Packed height: Z = HTU × NTU

Worked example: Scrubber design

Given: Gas 100 kmol/h with 5 % solute; remove 95 %; Henry slope m = 1.2, lean solvent; L/G = 1.5 × minimum

Find: L/G, A and theoretical stages.

• (L/G)_min ≈ 1.2 × 0.95 = 1.14 → L/G = 1.71 → L ≈ 171 kmol/h
• A = 1.71/1.2 = 1.425
• N = ln[(0.05/0.0025)(1 − 0.702) + 0.702]/ln 1.425 = 5.4

Answer: L ≈ 171 kmol/h, ≈ 5.4 stages

Worked example: Packed height

Given: NTU = 6.2; HTU = 0.6 m

Find: Packed height.

• Z = 0.6 × 6.2 = 3.7 m (+ distributors, margin)

Answer: ≈ 3.7 m of packing

Common mistakes

• Wrong: Operating with A < 1 and expecting high recovery. — Right: A < 1 limits maximum recovery regardless of stages.
• Wrong: Ignoring heat of absorption. — Right: Temperature rise reduces solubility — cool the solvent or use intercoolers.

Applications

• Gas treating: Amine units remove H₂S/CO₂ from natural gas and refinery streams.
• Air pollution control: Wet scrubbers for SO₂, HCl and NH₃.
• Carbon capture: Post-combustion CO₂ capture from power and cement plants.

Key takeaways

• Henry's law equilibrium; counter-current flow.
• L/G ≈ 1.2–1.5 × minimum; A = L/mG > 1.
• Kremser gives stages; Z = HTU × NTU for packing.
• Chemical solvents need energy-intensive regeneration.

Going further

• Enhancement factor: Chemical reaction in the liquid film speeds absorption.
• Flooding correlations: Eckert/Sherwood charts size column diameter.
• Solvent selection: Capacity, selectivity, volatility, degradation and regeneration energy.