Adsorption process involves the transfer of solute in the gas stream to the surface of a
solid material, where they concentrate mainly as a result of physical or Vander wall forces.
Commercial adsorbents are generally granular solids with a large surface area per unit
S.S. Kapdi et al. / Renewable Energy 30 (2005) 1195–1202 1197
volume. By a proper choice of adsorbent, the process can remove CO2, H2S, moisture and
other impurities either selectively or simultaneously from biogas.
Gas purification can also be carried out using some form of silica, alumina, activated
carbon or silicates, which are also known as molecular sieves [12].
Adsorption is generally accomplished at high temperature and pressure. It has good
moisture removal capacities, simple in design and easy to operate. But it is a costly process
with high pressure drops and high heat requirements.
Schomaker et al. [13] reported that CO2 could be removed from biogas by pressure
swing adsorption which consists of at least three active carbon beds. One of the beds is
fed with biogas under pressure (6 bar) CO2 is adsorbed. When there is saturation of CO2
in the adsorption bed, the process is shifted to the second bed. The saturated bed is
depressurized to ambient pressure. The efficiency of this process is up to 98%.
Continuous monitoring of a small-scale installation (26 m3/h) in Sweden using pressure
swing adsorption technique through carbon molecular sieves have given excellent results
in terms of clean gas, energy efficiency and cost [9].
Pandey and Fabian [14] used naturally occurring zeolite-Neopoliton Yellow Tuff
(NYT) for adsorption. They found that the active component for CO2 adsorption is
chabazite, which has adsorption capacity of 0.4 kg CO2 per kg of chabazite at 1.50 bar and
22 8C. During the adsorption process the H2S content is also reduced.