Factors Influencing Chemical Absorption of CO2 and H2S in Biogas Purification: A Review

Absorption is a process in which a substance (solute) is brought into contact with a solvent normally for a purpose of separation. pH and concentration of the solvent, temperature of both the gas and the solvent, and absorbing surface area influence the absorption process. Any deviation of pH from the optimal level by adding acid or base into the system can lower the absorption rate. Concentration of carbon dioxide in the gas stream and the loading rate influence the rate of absorption. Monoethanolamine (MEA) has an optimum absorption rate at liquid temperature of 35 degrees C. Ammonia has high removal efficiency at ambient temperature and there exists a direct relationship between absorption rate and temperature. Absorption of CO2 with sodium hydroxide is normally done at a temperature range of 1-16 degrees C (30-60 degrees F). Calcium oxide requires high temperature for absorption ranging from 300-390 degrees C, while 22-80 degrees C is used for H2S absorption with K2CO3. A linear increase results from heating the CO2 gas stream prior to absorption. The rate of absorption is influenced by the contact area between the absorbing fluid and the gas.