6.2 CO2 Bulk Removal
Carbon dioxide can be removed from syngas by chemical and physical adsorption with a washing liquid or by adsorption with solid adsorbentia. The choice for chemical or physical absorption (or a combination of both) depends on the partial pressure in the gas. For chemical absorption in commercial processes substituted amines are used, while solvents like methanol or polyethylene glycol are used for physical absorption. The CO2 concentration can be removed to values lower than 0.1 vol% by these processes. When the syngas contains significant concentrations of other gases besides H2 and CO2, adsorption on solid absorbentia is preferred. Examples of these materials comprise silica gel, active carbon, zeolites, and molecular sieves. The adsorption takes place at low temperatures and high pressures and the regeneration takes place either by higher temperatures (TSA; Thermal Swing Adsorption) or lower pressure (PSA; Pressure Swing Adsorption). In these processes also H2S can be removed.