Adsorption

Adsorption is a surface phenomenon in which molecules from a gas or liquid stream — called the adsorbate — adhere to the surface of a solid material called the adsorbent. Unlike absorption, the adsorbate does not penetrate into the bulk of the solid; it binds only to the outer surface and to the inner walls of pores. The driving force can be physical (van der Waals attraction, called physisorption) or chemical (covalent or ionic bonding, called chemisorption). Capacity depends on surface area, pore size distribution, and the affinity between adsorbate and adsorbent.

Adsorbents used in waste-processing and biogas plants include activated carbon (surface area 800–1,500 m²/g, used for H₂S, VOC, and siloxane removal), molecular sieves and zeolites (selective by pore size, used for water and CO₂ removal in PSA), silica gel (for moisture), and iron oxide impregnated media (for H₂S chemisorption). A 5 TPD CBG plant typically uses 200–400 kg of activated carbon for H₂S polishing, replaced every 3–6 months depending on loading.

Adsorbents have finite capacity and must be either replaced or regenerated. Pressure Swing Adsorption (PSA) regenerates by depressurising — used widely for biogas upgrading and oxygen generation. Temperature Swing Adsorption (TSA) regenerates with hot gas — used for solvent and moisture recovery. Throwaway beds are simpler but generate spent-media disposal cost; impregnated iron oxide spent media is classified as hazardous waste under the Hazardous and Other Wastes (Management and Transboundary Movement) Rules, 2016 if H₂S loading exceeds threshold. Key trade-offs are selectivity versus capacity, pressure drop versus contact time, and capex (regenerative systems) versus opex (throwaway systems).