9.3 Absorption, Stripping, and Extraction

9.3 Absorption, Stripping, and Extraction

Quick Answer: Absorption moves solute gas → liquid (lean in, rich out). Stripping is the reverse (liquid → gas) for regeneration or cleanup. Extraction moves solute between immiscible liquids (raffinate depleted, extract enriched). Choose by solubility, selectivity, heat sensitivity, and dilute vs concentrated feeds.

Not every separation is a distillation column. Acid-gas removal from natural gas, amine systems, sour-water strippers, wastewater VOC treatment, and aromatic/specialty separations all use absorption, stripping, or liquid–liquid extraction. These unit operations sit squarely in Domain C with the mass-transfer fundamentals of Section 9.1 and the equilibrium ideas of Domain B.

Gas Absorption and Scrubbing

Gas absorption (gas scrubbing when the goal is purification/emission control) contacts a gas mixture with a liquid solvent that preferentially dissolves one or more solutes (e.g., H₂S, CO₂, NH₃, SO₂, light organics).

Goals

Countercurrent tower picture

Typical packed or tray absorber:

• Gas enters bottom (rich in solute), leaves top (lean gas / treated gas).
• Liquid solvent enters top lean, leaves bottom rich.
• Countercurrent flow maximizes average driving force: richest gas meets richest liquid; leanest gas meets leanest liquid.

Equilibrium: Henry’s law or solubility curves give (y^) (or p) in equilibrium with liquid x. Operating line from material balance must stay on the correct side of equilibrium (driving force (y - y^* > 0) for absorption of solute into liquid).

Minimum liquid rate: as L decreases, the operating line pivots toward the equilibrium curve and pinches—infinite height required. Practical absorbers use L above (L_{\min}).

Worked L/G sketch (dilute)

Gas in: G = 100 mol/s, (y_{\mathrm{in}} = 0.02) mole fraction solute (dilute). Target (y_{\mathrm{out}} = 0.002). Inlet lean liquid (x_{\mathrm{in}} = 0). Suppose equilibrium is (y^* = 1.5 x) (mole fraction units).

Solute removed from gas ≈ (G(y_{\mathrm{in}}-y_{\mathrm{out}}) = 100(0.018) = 1.8,\mathrm{mol/s}).

If outlet liquid is at equilibrium with inlet gas (limiting pinch idea for minimum L with rich end pinch): (x_{\mathrm{out,max}} = y_{\mathrm{in}}/1.5 = 0.0133).

Then (L_{\min} x_{\mathrm{out,max}} \approx 1.8) ⇒ (L_{\min} \approx 1.8/0.0133 \approx 135,\mathrm{mol/s}) (order-of-magnitude dilute material balance). Real designs take L > (L_{\min}) and account for non-dilute systems, heat effects, and kinetics (reactive amines).

Physical vs chemical absorption: physical solvents (e.g., cold methanol-type processes conceptually) rely on solubility; chemical solvents (amines for CO₂/H₂S) react with the solute, steepening effective capacity—still limited by rates, heats of reaction, and regeneration energy.

Stripping (Desorption)

Stripping removes a dissolved solute from a liquid by contact with a gas that has low solute partial pressure (steam, air, nitrogen, or fuel gas).

Amine plant loop (conceptual): absorber takes acid gas into lean amine → rich amine → stripper (steam/reboiler) releases acid gas → lean amine recycles. The stripper is desorption driven by high T (lower solubility / reverse reaction) and stripping vapor.

Sour water strippers and wastewater air strippers use the same logic: reduce liquid concentration of NH₃, H₂S, or VOCs. Raising T, lowering P, or increasing strip-gas rate improves removal when equilibrium and rates allow.

Exam trap: stripping is not distillation, though a stripper column may look similar. Distillation uses vapor generated from the same mixture (boilup) and reflux of condensate; stripping emphasizes an external or generated strip gas to lower solute partial pressure above the liquid.

Liquid–Liquid Extraction (LLE)

Liquid–liquid extraction transfers a solute from one liquid phase (the feed, often aqueous or organic) into a second immiscible liquid solvent.

Single-stage mixer-settler: mix → settle → separate extract and raffinate. Multistage countercurrent trains or columns improve recovery—same staging logic as absorption (operating line vs equilibrium).

When partition helps

If (K_D) is large, solute prefers the extract phase; fewer stages and less solvent achieve a target recovery. If phases partially miscible, solvent recovery and phase splitting become design constraints (tie lines on ternary diagrams—recognize existence, not draw full diagrams on this exam).

Worked partition sketch

Feed: 100 kg with 5 kg solute (5 wt%). Equal mass solvent, single equilibrium contact, (K_D = 4) on mass-fraction basis in each phase (simplified), immiscible solvents of constant mass 100 kg each after split.

Let raffinate solute mass fraction = x, extract = 4x (if K_D = y/x = 4 and phases equal mass… careful: with equal phase masses, solute balance:

[ 5 = 100x + 100(4x) = 500x \Rightarrow x = 0.01,\quad y = 0.04 ]

Solute in raffinate = 1 kg; in extract = 4 kg → 80% recovery in one stage. A second countercurrent stage would recover more of the remaining 1 kg.

When Each Operation Is Preferred Industrially

Solvent selection checklist (absorption or extraction):

1. High capacity / favorable equilibrium for the solute
2. Selectivity against unwanted components
3. Easy regeneration (strip, distill, or crystallize)
4. Low toxicity, corrosion, vapor pressure, cost
5. Immiscibility (for LLE) and density difference for settling

Connecting Rate and Equilibrium Across Separators

Material balances still rule: overall and component balances close every absorber, stripper, and extractor—just as in Domain A. Never compute a rate coefficient when the stem only needs a balance or an equilibrium loading.

Common UPDA Traps

1. Calling lean solvent “rich” after the absorber bottom—rich leaves the bottom.
2. Claiming stripping changes the thermodynamic K of a reaction the way a catalyst does—different concepts.
3. Confusing raffinate (depleted feed phase) with extract (solvent phase).
4. Designing absorption with L < L_min (pinch—impossible for target recovery at finite height).
5. Using distillation intuition (reflux ratio) without mapping to L/G or solvent rate analogies.
6. Ignoring that chemical solvents need regeneration energy—absorption is not free separation.

Domain C Mass-Transfer Wrap (Chapter 9)

Together with fluid mechanics and heat transfer (Chapters 7–8), this completes the transport-phenomena third of Domain C for the UPDA/MMUP Chemical exam. Domain D next turns from physical separations to chemical reaction engineering—rates that create or destroy species rather than only redistribute them between phases.