Absorption and stripping

In short

Absorption is the transfer of one or more solutes from a gas mixture into a liquid solvent; stripping (desorption) is the reverse, driving dissolved solute out of a liquid into a gas or steam. The pair almost always runs as a loop — absorber plus stripper — so the solvent is regenerated and recycled, as in amine treating of natural gas, CO₂ removal in urea plants and ammonia or SO₂ scrubbing. It is a guaranteed scoring topic in GATE-CH (equilibrium and operating lines, NTU/HTU, Kremser) and in university Mass Transfer-II papers, and it is standard interview ground for IOCL, BPCL, HPCL, GAIL, ONGC, RIL, Tata Chemicals, Coromandel, UPL, Linde, Air Liquide, Thermax and L&T Hydrocarbon. On the job it decides solvent circulation rate, column height and diameter, steam consumption in the regenerator and whether a stack meets CPCB limits.

Explained

What absorption and stripping are

• Absorption: solute A moves from the gas phase to the liquid phase because its partial pressure in the gas is above its equilibrium partial pressure over the liquid. Driving force (y − y*) is positive.
• Stripping: the same equipment and equations with the driving force reversed — the liquid is supersaturated relative to the gas, so (x − x*) is positive. A stripper is run hot and/or at low pressure; an absorber cold and/or at high pressure.
• Physical absorption: solute dissolves without reacting (CO₂ in water, benzene vapour in oil, acetone in water). Capacity follows Henry's law; regeneration is easy (flash or stripping).
• Chemical absorption (reactive): solute reacts in the liquid — CO₂/H₂S in MEA, MDEA or K₂CO₃, SO₂ in lime slurry or NaOH, NOx in water/alkali. Reaction keeps the interfacial solute concentration near zero, so the effective driving force and the absorption rate are much larger; the penalty is heat of reaction and reboiler steam.
• Selective vs bulk removal: a selective solvent (e.g. MDEA for H₂S over CO₂) picks one component; physical solvents (Selexol, Rectisol, water) remove more or less everything with solubility.
• Mode of contact: counter-current (standard, best driving force), co-current (used when only one transfer unit is needed, or for very fast reactions), cross-flow (spray scrubbers).
• Equipment: packed columns (random — Pall, Raschig, Intalox; structured — gauze/sheet metal), tray columns (sieve, valve, bubble-cap), spray towers, venturi scrubbers (dust + gas together), bubble columns and stirred tanks (slow chemical reaction, liquid-film controlled), falling-film absorbers (HCl, highly exothermic).

Equilibrium and solubility

• Solubility data are given as p*–x or y*–x curves at the operating temperature; the equilibrium curve is the backbone of every graphical design.
• Henry's law (dilute solutions): p* = H·x, or y* = m·x with m = H/P. H has units of pressure (bar, kPa, atm) per unit mole fraction; m is dimensionless.
• Effect of temperature: solubility of a gas falls as temperature rises (H and m increase) — hence absorb cold, strip hot.
• Effect of pressure: at constant temperature, raising total pressure lowers m = H/P, so high pressure favours absorption; a pressure let-down favours stripping.
• Solvent selection criteria: high solubility (low m) for the solute, high selectivity, low volatility (to avoid solvent loss), low viscosity, non-corrosive, non-foaming, thermally and chemically stable, cheap, non-toxic, and easily regenerable. Water first; then oils, amines, alkali solutions.
• A gas-film-controlled system has a very soluble gas (NH₃ or HCl in water, small m); a liquid-film-controlled system has a sparingly soluble gas (O₂, CO₂ in water, large m).

Mass balance, operating line and minimum solvent rate

• Rigorous balance on a solute-free basis (carrier gas Gs, solvent Ls, mole ratios Y = y/(1−y), X = x/(1−x)) gives a straight operating line on Y–X coordinates. In mole fractions the line is straight only if the solution is dilute (roughly y < 0.05–0.10).
• Counter-current balance: Gs(Y₁ − Y₂) = Ls(X₁ − X₂); subscript 1 = bottom (rich gas in, rich liquid out), 2 = top (lean gas out, lean solvent in).
• For an absorber the operating line lies above the equilibrium curve; for a stripper it lies below. If they touch, the driving force is zero — a pinch — and infinite height is needed.
• Minimum liquid rate (absorption): assume exit liquid leaves in equilibrium with the entering gas, X₁* = Y₁/m-equivalent, then (Ls/Gs)min = (Y₁ − Y₂)/(X₁* − X₂). Design rate is typically 1.2 to 2.0 times the minimum (commonly 1.5).
• Minimum gas rate (stripping): (Gs/Ls)min = (X₁ − X₂)/(Y₁* − Y₂) with Y₁* in equilibrium with the entering rich liquid.
• Economic trade-off: more solvent → shorter column but higher pumping, cooling and regeneration-steam cost; less solvent → taller column. Optimum is usually near 1.3–1.5 × minimum.
• Non-isothermal absorbers: heat of solution raises the liquid temperature down the column, bending the equilibrium curve upward; large-duty units (amine absorbers) need a stage-wise enthalpy balance, not an isothermal design.

Rate of mass transfer — two-film theory and HTU–NTU

• Two-film (Whitman) theory: all resistance sits in thin stagnant films on either side of the interface; equilibrium holds at the interface (y_i = m x_i).
• Individual rates: N_A = k_y(y − y_i) = k_x(x_i − x). Overall: N_A = K_y(y − y*) = K_x(x* − x).
• Resistances add: 1/K_y = 1/k_y + m/k_x and 1/K_x = 1/k_x + 1/(m k_y). Small m (very soluble gas) → gas film controls; large m → liquid film controls.
• In packed beds the interfacial area a (m²/m³) is unknown separately, so the volumetric coefficients K_y·a, k_y·a are used, with units kmol/(m³·s·unit mole fraction).
• Design equation for a packed absorber, dilute and constant flows: Z = HOG × NOG, where HOG = G′/(K_y a) is the height of a transfer unit (m) and NOG = ∫dy/(y − y*) is the number of transfer units (dimensionless).
• HTU is largely a property of the packing, liquid and gas loads and physical properties; NTU is set purely by the separation required and the equilibrium relation. Keeping them separate is the whole point of the HTU–NTU method.
• For trays, the equivalent quantities are number of theoretical stages N and tray efficiency; for packing, HETP = Z/N (typically 0.3–0.6 m for random packing, 0.2–0.5 m for structured packing — vendor data decide).
• Graphical NOG: plot operating and equilibrium lines, integrate 1/(y − y*) against y, or use the straight-line shortcut (log-mean driving force) or the Colburn/Kremser equation.

Sizing a column: diameter, flooding, pressure drop

• Diameter is fixed by the allowable gas velocity. In packing, liquid holdup rises with gas rate until flooding — liquid is held up, pressure drop shoots up, efficiency collapses. Design at 60–75% of flooding velocity.
• Pressure drop and flooding are read from the generalised pressure drop correlation (GPDC/Eckert chart) using the flow parameter (L/G)√(ρ_G/ρ_L) and a capacity parameter containing the packing factor F_p; typical design drop is 20–40 mm H₂O per metre of packing (about 200–400 Pa/m).
• Random packing: cheap, tolerant of fouling, good for corrosive duty (ceramic, plastic); needs liquid redistributors every 5–10 column diameters (or 3–6 m) and a minimum wetting rate.
• Structured packing: very low pressure drop and low HETP, high capacity — used for vacuum and large flue-gas duties; sensitive to fouling and costly.
• Trays: preferred for high liquid loads, where intercoolers or side-draws are needed, for fouling/solids service, and at large diameters; higher pressure drop. Sieve trays are cheapest; valve trays give better turndown; bubble-cap trays give the widest turndown.
• Standard hydraulic limits on trays: weeping and dumping at low vapour rate, entrainment/jet flooding and downcomer flooding at high rates.
• Internals: liquid distributor (quality decides HETP as much as the packing), hold-down plate, support plate, mist eliminator, demister and sump.

Chemical absorption, industrial systems and Indian practice

• Acid-gas (amine) treating: absorber at high pressure and 40–50 °C with MEA/DEA/MDEA/aMDEA; rich amine is flashed, heated in a lean/rich exchanger and steam-stripped in a regenerator at 1.5–2 bar, 110–125 °C. Standard in refineries (IOCL, BPCL, HPCL, RIL), gas processing (GAIL, ONGC) and ammonia-urea plants, where CO₂ recovered from syngas is feed to the urea reactor.
• Enhancement factor and the Hatta number classify reactive absorption: slow reaction → bulk-liquid-volume controlled (use bubble columns/stirred tanks), fast reaction → film controlled (use packing or spray, maximise area a).
• Flue gas desulphurisation (FGD): wet limestone scrubbers (SO₂ + CaCO₃ → gypsum) dominate Indian coal power retrofits; also seawater and dry/semi-dry sorbent injection. The 2015 MoEFCC notification set SO₂ limits for coal plants, but compliance deadlines and applicability have been revised several times, most recently narrowing mandatory FGD largely to plants near large/critically polluted areas — always check the current notification before quoting a limit.
• Other Indian duties: HCl absorption in graphite falling-film units (chlor-alkali, UPL, Aarti), NH₃ scrubbing in fertiliser and refrigeration plants, benzene/solvent recovery from vent gas by oil absorption followed by steam stripping, air-stripping of ammonia and VOCs in effluent treatment, and sour-water strippers in every refinery.
• Codes and law: column shells are designed to ASME Section VIII Div. 1 or the Indian unfired-pressure-vessel code (IS 2825, an old but still-referenced standard) with SMPV(U) Rules where applicable; refinery practice follows OISD standards; emissions and effluents fall under the Air (Prevention and Control of Pollution) Act 1981, the Water Act 1974 and the Environment (Protection) Act 1986 with CPCB/SPCB consent conditions; hazardous-chemical storage follows the MSIHC Rules under EPA; worker safety in these plants is now governed by the Occupational Safety, Health and Working Conditions Code, which replaced the Factories Act from 21 November 2025.

Key points

• Absorption - solute moves gas → liquid, driven by (y − y*); favoured by low temperature and high pressure.
• Stripping - solute moves liquid → gas, driven by (x − x*); favoured by high temperature and low pressure.
• Henry's law - p* = Hx, y* = mx; valid for dilute, non-reacting, sparingly soluble systems.
• Operating line - straight on solute-free (Y–X) coordinates; above the equilibrium curve for absorption, below it for stripping.
• Pinch point - operating and equilibrium lines touch; driving force zero, infinite stages or height.
• Minimum L/G - from exit liquid in equilibrium with inlet gas; design at 1.2–2.0 × minimum.
• Absorption factor A = L/(mG) - A > 1 is needed for deep absorption; stripping factor S = mG/L = 1/A.
• Two-film theory - interface at equilibrium; resistances in series, 1/K_y = 1/k_y + m/k_x.
• Gas-film control - very soluble gas, small m (NH₃, HCl in water); liquid-film control - sparingly soluble gas, large m (O₂, CO₂ in water).
• HTU (HOG) - packing/hydraulics property, G′/(K_y a), metres; NTU (NOG) - separation difficulty, dimensionless; Z = HOG × NOG.
• HETP - packed height per theoretical stage, 0.2–0.6 m typically; links packed and tray design.
• Kremser–Colburn - closed-form stage or NTU count when both lines are straight.
• Flooding - capacity limit of packing; design at 60–75% of flooding velocity, 200–400 Pa/m drop.
• Trays vs packing - trays for high liquid load, fouling, side-draws and big diameters; packing for low pressure drop, corrosive and vacuum duty.
• Chemical absorption - reaction cuts interfacial concentration to near zero, raising rate; cost is regeneration heat.
• Absorber-stripper loop - the solvent is the recycle; stripper steam, not the absorber, usually sets operating cost.
• Solvent choice - high solubility and selectivity, low volatility and viscosity, non-corrosive, regenerable, cheap.

Formulas and examples

Example 1 — solvent rate and operating line. 100 kmol/h of air containing 5 mol% NH₃ is scrubbed with ammonia-free water at 1 atm, 30 °C; 95% of the NH₃ must be removed. Equilibrium: y* = 1.2x (dilute, treat L and G as constant).

• NH₃ in = 5 kmol/h; out = 0.25 kmol/h → y₂ = 0.0025 (so y₁ − y₂ = 0.0475).
• Exit liquid in equilibrium with inlet gas: x₁* = y₁/m = 0.05/1.2 = 0.04167.
• (L/G)min = 0.0475/0.04167 = 1.14 → L_min = 114 kmol/h.
• Design at 1.5 × minimum: L = 171 kmol/h (≈ 3,080 kg/h water).
• Actual exit liquid: x₁ = G(y₁ − y₂)/L = 100 × 0.0475/171 = 0.02778; check m·x₁ = 0.0333 < y₁ ✓ (no pinch).
• Absorption factor A = L/(mG) = 171/(1.2 × 100) = 1.425.

Example 2 — NOG and ideal stages for the same duty.

• Δy₁ = y₁ − m x₁ = 0.05 − 0.0333 = 0.01667; Δy₂ = 0.0025 − 0 = 0.0025.
• Δy_lm = (0.01667 − 0.0025)/ln(6.667) = 0.014167/1.8971 = 0.007468.
• NOG = 0.0475/0.007468 = 6.36 transfer units.
• Colburn check: ln[20 × 0.29825 + 0.70175]/0.29825 = 1.8971/0.29825 = 6.36 ✓.
• Ideal trays, Kremser: N = 1.8971/ln 1.425 = 1.8971/0.35417 = 5.36; at 60% overall efficiency, actual trays = 5.36/0.6 = 8.9 → 9 trays.

Example 3 — diameter and packed height. Same gas: 100 kmol/h at 1 atm, 303 K; design superficial velocity 1.0 m/s (about 70% of flooding for 38 mm Pall rings); K_y a = 0.10 kmol/(m³·s·Δy).

• Volumetric gas flow = 100 × 22.414 × (303/273) = 2,488 m³/h = 0.691 m³/s.
• Area = 0.691/1.0 = 0.691 m² → D = √(4 × 0.691/π) = 0.94 m; take D = 0.95 m, S = 0.709 m².
• G′ = (100/3600)/0.709 = 0.0392 kmol/(m²·s); HOG = 0.0392/0.10 = 0.39 m.
• Z = HOG × NOG = 0.39 × 6.36 = 2.49 m → specify 2.7 m of packing (10% margin), in one bed with a redistributor at the top.
• Implied HETP = Z/N = 2.49/5.36 = 0.46 m, a normal value for this packing.

Example 4 — which film controls. For the NH₃–water system k_y a = 0.08 and k_x a = 0.5 kmol/(m³·s), m = 1.2.

• 1/K_y a = 1/0.08 + 1.2/0.5 = 12.5 + 2.4 = 14.9 → K_y a = 0.0671 kmol/(m³·s).
• Gas-film share = 12.5/14.9 = 83.9% → gas-film controlled; increasing gas turbulence (higher G, better packing) helps, more water circulation does little.

Example 5 — steam stripping. 100 kmol/h of water containing 2 mol% of a volatile solute must be reduced to 0.2 mol%; open steam enters solute-free; at stripper conditions y* = 20x.

• G_min = L(x₁ − x₂)/(m x₁ − y₂) = 100 × 0.018/0.40 = 4.5 kmol/h.
• Design at 1.5 × minimum: G = 6.75 kmol/h ≈ 122 kg/h steam; S = mG/L = 20 × 6.75/100 = 1.35.
• NOL = ln[(0.02/0.002)(1 − 0.7407) + 0.7407]/(1 − 0.7407) = ln(3.3333)/0.25926 = 1.20397/0.25926 = 4.64 transfer units.
• Ideal plates: N = 1.20397/ln 1.35 = 1.20397/0.30010 = 4.01 → 4 theoretical stages (say 7 actual trays at ~60% efficiency).

Where it is used

• Refineries and gas processing: amine absorbers and regenerators for H₂S/CO₂, sour-water strippers, LPG amine treating and lean-oil absorption of C₃+ — IOCL, BPCL, HPCL, RIL Jamnagar, GAIL and ONGC gas terminals.
• Fertiliser and ammonia plants: CO₂ removal from syngas by aMDEA/hot potassium carbonate, with the recovered CO₂ fed to urea synthesis; ammonia scrubbers on vents — RCF, IFFCO, Coromandel, Chambal.
• Power and metallurgy: wet limestone FGD on coal units (gypsum by-product) and acid-gas scrubbing on smelter/sulphuric-acid plant tail gas — NTPC, Tata Power, Hindustan Zinc, Hindalco.
• Chlor-alkali, agrochemicals and pharma: HCl and Cl₂ absorption in graphite falling-film and packed units, NOx and SO₂ alkali scrubbers, solvent-vapour recovery from dryer and reactor vents — UPL, Aarti Industries, Gujarat Alkalies, Divi's.
• Environment and utilities: VOC and ammonia air-stripping in ETPs/CETPs, odour scrubbers in STPs, CO₂ capture pilots and biogas upgrading to CBG under the SATAT programme; Thermax, Praj and Linde supply such packages.
• Job roles: process design engineer (column sizing, solvent circulation, steam balance), plant/operations engineer (flooding, foaming, amine losses), EHS and environment engineer (CPCB consent and stack compliance), technical sales and commissioning of scrubbers.

Interview questions

• Q: Why is an absorber run cold and at high pressure while the stripper is hot and at low pressure? A: Gas solubility rises with pressure and falls with temperature, so m = H/P is small in the absorber (good capacity) and large in the stripper (easy release of the solute).
• Q: Why is the operating line straight on mole-ratio coordinates but curved on mole-fraction coordinates? A: Because the solute-free carrier flows Gs and Ls are genuinely constant, whereas total G and L change as solute transfers; mole fractions give a straight line only when the solution is dilute.
• Q: What is the minimum liquid rate and why can it never be used? A: It is the rate at which the exit liquid reaches equilibrium with the entering gas; the driving force becomes zero at that point, so infinite column height would be needed.
• Q: What does the absorption factor A tell you? A: A = L/(mG) is the ratio of the operating-line slope to the equilibrium-line slope; A > 1 means the lines diverge and deep absorption is possible, A ≤ 1 means a pinch limits recovery regardless of height.
• Q: Difference between HTU and NTU? A: NTU is the difficulty of the separation, fixed by the inlet/outlet compositions and equilibrium; HTU is the height of packing needed per transfer unit, fixed by packing, flows and physical properties. Height is their product.
• Q: Which film controls for ammonia-water and for oxygen-water, and why? A: NH₃ is very soluble (small m) so gas-film resistance dominates; O₂ is sparingly soluble (large m) so liquid-film resistance dominates, from 1/K_y = 1/k_y + m/k_x.
• Q: How does a chemical reaction in the liquid increase the absorption rate? A: It consumes solute at or near the interface, keeping the liquid-side concentration near zero and the driving force near maximum, and can shift the resistance entirely to the gas film.
• Q: What is flooding in a packed column and how is it avoided? A: Rising gas rate holds up liquid until the bed fills, pressure drop rises steeply and mass transfer collapses; design at 60–75% of flooding velocity using the GPDC/Eckert chart.
• Q: What is HETP and why is it used for packing? A: Height of packing equivalent to one theoretical plate; it lets a stage-wise calculation be converted into packed height, and it is obtained from vendor or pilot data.
• Q: Why are liquid distributors and redistributors so important? A: Maldistribution and wall flow leave packing dry, cutting effective interfacial area; a poor distributor can double the apparent HETP, so redistribution is provided every 5–10 diameters.
• Q: In an amine unit, why does the stripper, not the absorber, dominate operating cost? A: Regeneration needs reboiler steam to reverse the heat of absorption and generate stripping vapour; this energy, typically the largest utility in the loop, scales with solvent circulation rate.
• Q: Your absorber outlet gas suddenly goes off-spec with no change in feed. What would you check? A: Lean solvent quality and temperature, circulation rate, degradation or fouling of the solvent, foaming, tray/packing damage or maldistribution, and whether gas rate has pushed the column towards flooding or entrainment.

Common mistakes

• Using mole fractions in the overall balance for a concentrated gas; the operating line is straight only in mole-ratio (solute-free) units unless the stream is dilute.
• Confusing the two Kremser forms: the denominator is ln A for ideal stages and (1 − 1/A) for transfer units — they give different numbers for the same duty.
• Mixing up m and 1/m, or quoting H in pressure units and then using it as if dimensionless; always divide by total pressure to get m.
• Taking the design liquid rate as the minimum rate, or forgetting the 1.2–2.0 factor, and reporting an impossibly short column.
• Treating K_y and K_y·a as interchangeable — one is per unit area, the other per unit packed volume; the units differ by m²/m³.
• Assuming an isothermal absorber in a high-duty or reactive system, where heat of solution raises the liquid temperature and shrinks the real driving force.
• Sizing the diameter at or near the flooding velocity, and ignoring minimum wetting rate at the turndown end.

Related topics

Mass transfer: diffusion · Distillation · Adsorption and ion exchange · Design of trays and packed columns · Extraction and leaching · Membrane separations