Chemical absorption or reactive absorption involves a chemical reaction between the substance being absorbed and the absorbing medium. In some cases, it occurs in combination with physical absorption. Chemical absorption depends upon the stoichiometry of the reaction and the concentration of the reactants.
An example of chemical absorption is the purification of natural gas by passing the natural gas through an aqueous solution of an ethanolamine in which any acid gases, such as hydrogen sulfide (H2S) and carbon dioxide] (CO2), are removed from the natural gas by reacting with the ethanolamine (see the Amine gas treating and Natural gas processing articles). Another example is the removal of any hydrogen sulfide gas from the feedstock to an ammonia production plant by contacting the hydrogen sulfide with a bed of solid zinc oxide (ZnO) with which it reacts to form solid zinc sulfide (ZnS) (see the Ammonia production article).
Either type of absorption may be reversible or irreversible. The physical absorption of small amounts of oxygen in water can be reversed by heating the water. The reactive absorption of acid gases by an aqueous solution of ethanolamine can be reversed by distillation of the ethanolamine solution. However, the reactive absorption of hydrogen sulfide by zinc oxide cannot be reversed. The reactive absorption of carbon dioxide by an aqueous solution of sodium hydroxide (NaOH) is also irreversible and there not as economically acceptable as the reversible reactive absorption of carbon dioxide by ethanolamine solutions.