Adsorption and ion exchange

In short

Adsorption is the selective concentration of a solute (adsorbate) onto the surface of a porous solid (adsorbent); ion exchange is a special stoichiometric adsorption in which mobile counter-ions on a resin swap with ions of like charge in solution. Both are equilibrium-plus-rate separations used where distillation or absorption fails — trace impurities, close-boiling gases, heat-sensitive liquids and dissolved salts. They are examined in GATE (CH) mass transfer every few years through isotherms, bed capacity and breakthrough calculations, and in university Mass Transfer II / Separation Processes papers. In Indian practice they appear as activated-carbon filters in water and effluent plants, PSA oxygen and nitrogen plants, molecular-sieve ethanol dehydration for E20 blending, and DM plants feeding boilers in every refinery, fertiliser plant and thermal station.

Explained

Fundamentals, adsorbents and classification

• Adsorption is a surface phenomenon (solute accumulates at the interface); absorption is bulk dissolution into a phase. Sorption covers both; desorption/elution is the reverse.
• Physical adsorption (physisorption): van der Waals forces, heat of adsorption roughly 5–40 kJ/mol (of the order of the latent heat), reversible, multilayer possible, fast, non-specific, decreases with temperature.
• Chemical adsorption (chemisorption): chemical bond formation, 40–400 kJ/mol, monolayer only, often irreversible, specific, needs activation energy so it may rise with temperature over a range.
• Adsorbent requirements: large internal area (300–1,500 m²/g), controlled pore size, mechanical strength, low pressure drop, regenerability, low cost.
• Pore classification (IUPAC): micropores < 2 nm (most of the capacity), mesopores 2–50 nm (transport), macropores > 50 nm (access channels).
• Common adsorbents:
  • *Activated carbon* — from coconut shell (India is a major producer), coal or wood; non-polar, removes organics, colour, chlorine, VOCs; 600–1,500 m²/g.
  • *Silica gel* — polar, for drying gases to moderate dew points, regenerated ~150 °C.
  • *Activated alumina* — polar, deep gas drying, also fluoride and arsenic removal from drinking water.
  • *Zeolite molecular sieves (3A, 4A, 5A, 13X)* — crystalline aluminosilicates with uniform pores, separate by size and polarity; 3A for ethanol dehydration, 5A/13X for O₂/N₂ PSA.
  • *Polymeric resins and carbon molecular sieves*; *bone char*, *bagasse/rice-husk carbons* studied widely in Indian effluent work.
• Mechanism of separation: (i) equilibrium (capacity difference), (ii) kinetics (diffusion-rate difference — carbon molecular sieve N₂ PSA), (iii) steric/molecular-sieving, (iv) ion exchange (stoichiometric).

Adsorption equilibria — isotherms

• An isotherm plots equilibrium loading q (kg adsorbate/kg adsorbent) against fluid concentration C or partial pressure p at constant T. Isobar: q vs T; isostere: ln p vs 1/T (slope gives heat of adsorption).
• Linear (Henry) isotherm: q = K·C, valid at low coverage.
• Langmuir: monolayer, identical sites, no lateral interaction — gives a saturation limit q_m. Linearised as C/q = 1/(q_m K_L) + C/q_m.
• Freundlich: empirical, q = K_F C^1/n; straight line on log–log plot; 1/n < 1 means favourable.
• BET: extends Langmuir to multilayers; the standard route to specific surface area by N₂ adsorption at 77 K.
• Favourability (Brunauer types): Type I favourable/concave (good for adsorption, hard to regenerate), linear, Type III unfavourable/convex (poor loading, easy regeneration), and irreversible (horizontal).
• Temperature effect: adsorption is exothermic, so loading falls as T rises — the basis of thermal swing regeneration; loading rises with pressure — the basis of pressure swing.

Rates, mass transfer and fixed-bed behaviour

• Resistances in series: external film diffusion → macropore (pore/Knudsen) diffusion → micropore/surface diffusion → adsorption step (usually fast for physisorption).
• Contacting modes: (i) batch/stirred tank, (ii) fixed bed (most common), (iii) moving bed/simulated moving bed (SMB, used for p-xylene and sugars), (iv) fluidised bed.
• Fixed-bed adsorption is unsteady: a mass-transfer zone (MTZ) of finite length travels down the bed. Ahead of it the bed is fresh; behind it the bed is saturated.
• Breakthrough curve: effluent concentration versus time or throughput. Breakpoint is the time when the outlet reaches the permitted limit C_b (often 5% of feed); exhaustion is near-feed concentration.
• The bed is split into LUB (length of unused bed) and the saturated length. LUB ≈ half the MTZ length and is nearly independent of bed length, so a longer bed is used more efficiently — the basis of scale-up from a laboratory column.
• A favourable isotherm with good mass transfer gives a sharp, self-sharpening front; unfavourable isotherms and slow diffusion give a spread front and large LUB.
• Design parameters: superficial velocity (gases 0.15–0.5 m/s), bed depth, L/D ≈ 1–4 for gases and 1–3 for carbon filters, EBCT (empty-bed contact time, 5–30 min for water treatment), pressure drop by Ergun equation.

Regeneration and industrial cycles

• Thermal swing adsorption (TSA): heat with hot gas or steam to desorb; long cycles (hours to days), used for drying and solvent recovery (steam regeneration of carbon, then decanting/distilling the solvent).
• Pressure swing adsorption (PSA): adsorb at high pressure, desorb by blowdown to near-atmospheric; short cycles (seconds to minutes). VSA/VPSA desorbs under vacuum. Used for O₂ (13X/LiX), N₂ (CMS), H₂ purification and biogas upgrading; medical-oxygen PSA plants were installed across Indian district hospitals after 2021.
• Inert purge, displacement purge and chemical/steam regeneration are the other routes; carbon that is fouled is sent for off-site thermal reactivation.
• Skarstrom PSA cycle: pressurise → adsorb (product) → countercurrent blowdown → countercurrent purge, with two or more beds so flow is continuous.
• Two-bed lead–lag arrangement is standard: one adsorbing while the other regenerates; a guard bed protects the main bed.
• Practical limits: capacity loss over cycles (heel build-up), attrition and dust, exothermic temperature rise, fire risk in carbon beds handling ketones and spontaneous heating of wet carbon.

Ion exchange — resins, equilibrium and operation

• An ion-exchange resin is a crosslinked polymer (styrene–divinylbenzene, or acrylic) carrying fixed ionic groups with mobile counter-ions. Gel (microporous) and macroporous types; DVB content sets crosslinking, swelling and strength.
• Types: strong acid cation (SAC, –SO₃H), weak acid cation (WAC, –COOH), strong base anion (SBA, quaternary ammonium — Type I and Type II), weak base anion (WBA, amine); also chelating resins (heavy metals) and inorganic exchangers (zeolite, greensand).
• Capacity: total capacity in eq/L of wet resin (SAC ≈ 1.8–2.2 eq/L, SBA ≈ 1.0–1.4 eq/L); operating capacity is lower and depends on regenerant dose, leakage limit and flow.
• Exchange is stoichiometric and reversible: 2R–Na + Ca²⁺ ⇌ R₂–Ca + 2Na⁺.
• Selectivity sequences (dilute solution): cations — Fe³⁺ > Al³⁺ > Ca²⁺ > Mg²⁺ > K⁺ > Na⁺ > H⁺ (SAC); anions — SO₄²⁻ > NO₃⁻ > Cl⁻ > HCO₃⁻ > OH⁻ (SBA). Higher valence and lower hydrated radius are held more strongly; high regenerant concentration reverses the preference (electroselectivity), which is why concentrated brine or acid regenerates the bed.
• Service cycle: service (downflow) → backwash (declassify, remove fines) → regeneration (co-current or counter-current) → slow rinse (displacement) → fast rinse → return to service.
• Processes:
  • *Softening:* SAC in Na form with NaCl regeneration — removes hardness, leaves TDS unchanged.
  • *Dealkalisation:* WAC in H form removes hardness associated with bicarbonate, CO₂ stripped in a degasser.
  • *Demineralisation (DM):* SAC(H) → degasser → SBA(OH); two-bed gives ~1–10 µS/cm; mixed bed (MB) polisher gives < 0.1 µS/cm and silica in ppb for high-pressure boilers.
  • *Condensate polishing, nitrate and fluoride removal, sugar decolourisation, recovery of chromium and nickel from plating rinses, uranium and rare-earth recovery.*
• Problems: organic fouling of SBA, iron fouling, silica leakage at end of run, oxidative attack by chlorine (dechlorinate feed on carbon), osmotic shock, resin attrition; throw-away capacity falls ~1–3%/year.
• Comparison with RO: ion exchange is best for low TDS (< ~500 mg/L) and for final polishing; RO is cheaper above that; modern plants use RO followed by MB or electrodeionisation (EDI), which needs no chemical regeneration.
• Indian context and codes: boiler feed-water quality follows IS 10496 (water for low- and medium-pressure boilers) and the boiler maker's/IBR requirements; drinking water must meet IS 10500: 2012 (with amendments) — TDS 500 mg/L desirable (2,000 permissible), total hardness 200 mg/L desirable, fluoride 1.0 mg/L, nitrate 45 mg/L, iron 0.3 mg/L (no relaxation); arsenic 0.01 mg/L. Activated carbon for water is specified in IS 877 and granular/powdered carbon grades under related IS specifications — check the current edition before quoting. Regeneration effluent (acid/alkali/brine) is a trade effluent under the Water (Prevention and Control of Pollution) Act, 1974, consented by the State Pollution Control Board, with CPCB inlet norms for pH 5.5–9.0; spent resin and spent carbon handling follows the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 as amended. Zero liquid discharge is mandated for several highly polluting categories, which makes brine disposal from softeners a design constraint.

Key points

• Adsorption - surface accumulation of solute on a porous solid; selective, used for trace and close-boiling separations.
• Physisorption vs chemisorption - weak van der Waals, multilayer, reversible, ~5–40 kJ/mol versus chemical bonds, monolayer, often irreversible, 40–400 kJ/mol.
• Isotherm - q versus C at constant T; Langmuir (monolayer, saturates), Freundlich (empirical power law), BET (multilayer, used for surface area).
• Favourable isotherm - concave to the concentration axis (Type I); gives high loading and a sharp self-sharpening front but is harder to regenerate.
• Adsorbent choice - activated carbon for non-polar organics, silica gel/alumina for drying, molecular sieves for size- and polarity-based gas separation.
• Micropores carry capacity, macropores carry transport; BET N₂ area at 77 K is the standard measure.
• Fixed bed is unsteady-state - a mass-transfer zone travels through the bed; the bed is sized on breakpoint, not on equilibrium capacity alone.
• Breakthrough curve - outlet C vs time; breakpoint at the allowed limit, exhaustion near feed concentration.
• LUB - length of unused bed ≈ half the MTZ; constant with bed length, so scale-up adds saturated length to a measured LUB.
• Regeneration - TSA (heat, long cycles), PSA/VSA (pressure, seconds to minutes), inert or displacement purge, steam.
• Skarstrom cycle - pressurise, adsorb, blowdown, purge; two beds give continuous product.
• Ion exchange - stoichiometric, reversible swap of counter-ions; capacity in eq/L of resin, not kg/kg.
• Resin types - SAC, WAC, SBA (Type I/II), WBA; DVB crosslinking sets swelling and strength.
• Selectivity - rises with valence and falls with hydrated size; reversed by concentrated regenerant (electroselectivity).
• Softening removes hardness only; demineralisation removes all ions; mixed bed polishes to < 0.1 µS/cm.
• Counter-current regeneration gives lower leakage and 25–40% less chemical than co-current.
• Ion exchange suits low TDS; RO is cheaper for high TDS, with MB or EDI as the polisher.
• Water standards - IS 10500: 2012 for drinking water; regeneration effluent is a consented trade effluent under the Water Act, 1974.

Formulas and examples

Example 1 — Freundlich constants from batch data. Activated carbon is contacted with phenol solution. Equilibrium data: C = 20 mg/L, q = 60 mg/g; C = 200 mg/L, q = 190 mg/g.

• Take logs: ln q₂/q₁ = (1/n)·ln C₂/C₁ → ln(190/60) = 1.1526; ln(200/20) = 2.3026.
• 1/n = 1.1526/2.3026 = 0.5005 ≈ 0.50.
• K_F = q/C^0.5 = 60/√20 = 60/4.472 = 13.4 (mg/g)(L/mg)^0.5. Check: at C = 200, q = 13.4 × 14.14 = 189.5 mg/g ✔.
• Interpretation: 1/n = 0.5 < 1, a favourable isotherm.

Example 2 — Batch adsorbent requirement. 10 m³ of effluent at C₀ = 150 mg/L phenol must be brought to 10 mg/L using fresh carbon obeying the isotherm above (q in mg/g, C in mg/L).

• Equilibrium loading at C = 10: q = 13.4 × √10 = 13.4 × 3.162 = 42.4 mg/g.
• Phenol removed = 10 m³ × (150 − 10) g/m³ = 1,400 g.
• Carbon = 1,400 g / 0.0424 g per g = 33,000 g ≈ 33 kg (single stage).
• Two-stage countercurrent would cut this sharply: with an intermediate 40 mg/L, stage 2 carbon = (40−10)/0.0424... — the point for exams is that staging reduces adsorbent because each stage leaves the carbon in equilibrium with a higher concentration.
• At ₹120/kg for coconut-shell carbon, one-shot cost ≈ ₹3,960 per batch.

Example 3 — Langmuir from linearisation. Data for a dye on bagasse carbon: (C, q) = (5, 20), (50, 100), (200, 150) with C in mg/L and q in mg/g.

• C/q = 0.250, 0.500, 1.333 (L/g × ... i.e. g·L/(mg·g)).
• Slope from the end points = (1.333 − 0.250)/(200 − 5) = 1.083/195 = 0.005556 → q_m = 1/0.005556 = 180 mg/g.
• Intercept = 0.250 − 0.005556×5 = 0.2222 = 1/(q_m·K_L) → K_L = 1/(180 × 0.2222) = 0.025 L/mg.
• Check at C = 50: q = 180 × 0.025 × 50/(1 + 0.025×50) = 225/2.25 = 100 mg/g ✔.

Example 4 — Fixed bed: saturation capacity, breakpoint and LUB. Air at 1 atm, 25 °C carrying 2,000 ppm (v) of toluene flows at 0.5 m³/s through a carbon bed, D = 1.0 m, L = 1.0 m, bulk density 450 kg/m³, equilibrium loading at feed concentration 0.25 kg toluene/kg carbon. The measured breakpoint is at 85% of the ideal (stoichiometric) time.

• Toluene concentration: 2,000 ppm = 0.002 × (1/22.4 × 298/273)... use ideal gas: moles/m³ = 101,325/(8.314×298) = 40.9 mol/m³ → toluene = 0.002 × 40.9 = 0.0818 mol/m³ × 92 g/mol = 7.53 g/m³.
• Feed rate of toluene = 0.5 × 7.53 = 3.76 g/s = 13.5 kg/h.
• Carbon mass = 450 × (π/4 × 1²) × 1.0 = 353 kg; saturation capacity = 353 × 0.25 = 88.3 kg.
• Ideal breakthrough time t_s = 88.3/13.5 = 6.5 h.
• Breakpoint t_b = 0.85 × 6.5 = 5.6 h → carbon actually used = 0.85 × 88.3 = 75 kg.
• LUB = L(t_s − t_b)/t_s = 1.0 × (6.5 − 5.6)/6.5 = 0.15 m.
• To scale to a 12-hour cycle: used length needed = 12/6.5 × 1.0 × 0.85... simpler — required saturated length = (13.5 × 12 kg)/(450 × 0.785 × 0.25) = 162/88.3 = 1.83 m; total bed = 1.83 + 0.15 = 1.98 ≈ 2.0 m.

Example 5 — Softener run length. Raw water with total hardness 300 mg/L as CaCO₃ is softened at 20 m³/h by a SAC bed holding 1.5 m³ of resin with an operating capacity of 1.4 eq/L (= 1,400 eq/m³).

• Hardness = 300/50 = 6 meq/L = 6 eq/m³.
• Bed capacity = 1.5 × 1,400 = 2,100 eq.
• Water treated per run = 2,100/6 = 350 m³ → run length = 350/20 = 17.5 h.
• Salt needed at 100 g NaCl per eq (a typical 1.7× stoichiometric dose; 1 eq NaCl = 58.5 g) = 2,100 × 100 g = 210 kg NaCl per regeneration ≈ 0.6 kg salt per m³ of soft water; at ₹8/kg, ≈ ₹1,680 per regeneration, i.e. ₹4.8 per m³ of water.
• Brine volume at 10% w/v = 210/0.10 = 2,100 L of 10% brine.

Where it is used

• Water and wastewater: activated-carbon filters and DM plants in refineries (Jamnagar, Panipat), fertiliser plants, thermal stations (NTPC) and pharma units; carbon polishing of ETP outlets to meet CPCB/SPCB consent limits; defluoridation (activated alumina) in Rajasthan and Gujarat, arsenic removal in West Bengal and Bihar.
• Industrial gases: PSA/VPSA oxygen and nitrogen generators (Linde, Air Liquide, Inox Air Products and many Indian PSA skid builders), hospital medical-oxygen PSA plants, H₂ purification in refineries, biogas-to-CBG upgrading under the SATAT programme.
• Ethanol and food: molecular-sieve 3A dehydration to fuel-grade ethanol for the E20 programme in sugar/distillery units; carbon and resin decolourisation of sugar syrups, juices and edible oils.
• Pharma, specialty chemicals and electronics: resin-based purification and chromatographic/SMB separations; ultrapure water from MB + EDI for API and semiconductor/solar cell fabs.
• Job roles: process/design engineer at water-treatment OEMs (Ion Exchange India, Thermax, VA Tech Wabag), utility and offsite engineer in refineries and power plants, applications engineer with resin and carbon suppliers, R&D on low-cost biosorbents in IIT/NIT and CSIR labs.
• Environment and catalysis: VOC solvent-recovery beds in paint and pharma plants, mercury guard beds, Hg/dioxin control in incinerators, and adsorption-based CO₂ capture pilots.

Interview questions

• Q: What is the difference between adsorption and absorption? A: Adsorption concentrates solute on the surface of a solid; absorption dissolves it throughout the bulk of a liquid phase.
• Q: How do you tell physisorption from chemisorption experimentally? A: By the heat of adsorption (5–40 against 40–400 kJ/mol), reversibility, and whether loading falls monotonically with temperature; physisorption is multilayer, chemisorption is monolayer and specific.
• Q: What does a favourable isotherm mean for a fixed bed? A: It is concave to the concentration axis, gives high loading at low concentration and a self-sharpening mass-transfer front, so the breakthrough is sharp — but desorption is harder.
• Q: Why is a fixed-bed adsorber always unsteady-state? A: The solid's loading keeps changing with time and position as the mass-transfer zone moves through the bed; there is no steady profile until the whole bed is saturated.
• Q: What is LUB and why is it useful? A: Length of unused bed, roughly half the mass-transfer zone; it is almost independent of bed height, so a lab column gives the LUB and the plant bed is that plus the saturated length needed for the design cycle.
• Q: Why does PSA use pressure and TSA use temperature? A: Loading rises with pressure and falls with temperature; pressure can be changed in seconds for small, cheap units, while heating and cooling a bed takes hours but gives deeper regeneration.
• Q: Which molecular sieve dries ethanol and why 3A? A: 3A has ~0.3 nm pores, which admit water (0.28 nm) but exclude the ethanol molecule (~0.44 nm), so it breaks the ethanol–water azeotrope without a solvent.
• Q: Does softening reduce TDS? A: No — it swaps Ca²⁺ and Mg²⁺ for Na⁺, so hardness falls but total dissolved solids stay about the same (slightly changed by equivalent weights).
• Q: Why is counter-current regeneration preferred in DM plants? A: The most exhausted resin layer meets fresh regenerant last-in/first-out, leaving the outlet end in the pure form, which gives lower ionic leakage and 25–40% less acid or alkali.
• Q: Why must chlorine be removed before an ion-exchange bed? A: Free chlorine oxidises the DVB crosslinks, causing resin swelling, capacity loss and leaching; a carbon bed or bisulphite dosing is placed upstream.
• Q: When would you choose RO over ion exchange? A: Above roughly 500 mg/L TDS, RO costs less in chemicals and effluent; ion exchange is then kept only as a mixed-bed or EDI polisher.
• Q: What does rising conductivity and silica at the end of a DM run indicate? A: The anion bed is exhausting — silica, the weakest-held anion, leaks first, which is why silica is used as the end-point signal for the SBA unit.

Common mistakes

• Treating a fixed bed as steady-state and sizing it on equilibrium capacity, forgetting the unused bed and the breakpoint limit.
• Mixing up the Langmuir and Freundlich linear plots (C/q vs C against log q vs log C) and so reporting q_m from a Freundlich fit, which has no saturation limit.
• Unit chaos in isotherms: mg/g with kg/kg, mg/L with kg/m³, or forgetting that K_F's units depend on 1/n.
• Quoting ion-exchange capacity in kg/kg instead of eq/L of resin, or forgetting the 50 mg/L = 1 meq/L CaCO₃ conversion.
• Assuming a softener reduces TDS or removes silica, or that a DM plant removes organics and colour (that needs carbon).
• Ignoring that a strongly favourable isotherm, while good for loading, makes thermal or chemical regeneration expensive.
• In the lab, not allowing enough contact time or not filtering before analysis, then blaming the isotherm for scatter; and in the plant, skipping backwash so the bed channels and breaks through early.

Related topics

• Mass transfer: diffusion · Absorption and stripping · Membrane separations · Extraction and leaching · Design of trays and packed columns · Chemical reaction engineering: catalysis and porous solids