Adsorption

Adsorption captures molecules on the surface of porous solids. Learn adsorbents (activated carbon, zeolites, silica gel, MOFs), isotherms (Langmuir, Freundlich), fixed-bed operation and breakthrough curves, mass-transfer zones, bed sizing, and regeneration (TSA, PSA).

A gram of activated carbon can have over 1000 m² of internal surface area. Molecules stick to that surface, so adsorbents remove trace contaminants from water and air, dry gases, and separate air into oxygen and nitrogen.

Beds eventually saturate and must be regenerated or replaced.

An analogy

An adsorbent is like a car park with thousands of spaces inside a small building: molecules drive in and park on the surface until it's full.

Adsorbents

Activated carbon (organics, odours), zeolites/molecular sieves (drying, size-selective), silica gel and activated alumina (drying), polymeric resins, and new MOFs. Physical adsorption is weak and reversible; chemisorption forms chemical bonds.

Isotherms

At equilibrium, loading depends on concentration and temperature. Langmuir assumes a monolayer with finite sites; Freundlich is empirical for heterogeneous surfaces. Lower temperature and higher concentration increase loading.

Fixed beds and breakthrough

Fluid flows through a packed bed; a mass-transfer zone moves through it. Outlet concentration stays near zero until the zone reaches the end (breakthrough), then rises to the inlet value. Beds are switched at a breakthrough limit.

Regeneration cycles

Temperature-swing adsorption (TSA) heats the bed to desorb (gas drying, solvent recovery). Pressure-swing adsorption (PSA) lowers pressure (hydrogen purification, oxygen concentrators). Moving and simulated-moving beds give continuous separation (sugar, xylene isomers).

Formulas

• Langmuir isotherm: q = q_m K_L C / (1 + K_L C)
• Freundlich isotherm: q = K_F C^(1/n)
• Ideal bed life: t = m_ads q / (Q C_in)
• Usable capacity: t_bt ≈ t_ideal × (1 − ½ MTZ/L)

Worked example: Carbon loading

Given: Langmuir q_m = 0.3 g/g, K = 0.05 L/mg; C = 20 mg/L

Find: Equilibrium loading.

• q = 0.3 × 0.05 × 20/(1 + 1) = 0.15 g/g

Answer: 0.15 g/g

Worked example: Bed life

Given: 500 kg carbon at q = 0.15; Water 10 m³/h at 20 mg/L; 70 % of capacity usable before breakthrough

Find: Service time.

• Capacity = 500 × 0.15 = 75 kg
• Load = 10 × 20 = 200 g/h
• Ideal t = 375 h; with 70 % → ≈ 262 h (11 days)

Answer: ≈ 262 h

Common mistakes

• Wrong: Using full equilibrium capacity for bed life. — Right: Account for the mass-transfer zone and breakthrough limit.
• Wrong: Ignoring competitive adsorption. — Right: Other components occupy sites and cut capacity.

Applications

• Water & air purification: Activated carbon removes organics, odours and micropollutants.
• Gas drying: Molecular sieves dry natural gas and instrument air.
• Gas separation: PSA for hydrogen, oxygen and nitrogen production.

Key takeaways

• Huge internal surface area captures molecules.
• Langmuir: q = q_m K C/(1 + K C).
• Beds saturate — design for breakthrough and MTZ.
• Regenerate by TSA or PSA.

Going further

• Breakthrough modelling: Linear driving force models predict curve shape and MTZ length.
• Simulated moving bed: Continuous chromatographic separation by switching ports.
• MOFs and COFs: Tailorable pores for CO₂ capture and hydrogen storage.