Absorption (chemistry)

In chemistry, absorption is a process by which a substance incorporated in one state is transferred into another substance of a different state (e.g., gases being absorbed by a liquid or liquids being absorbed by a solid). As an industrial process, the most commonly encountered use of absorption is for the separation and/or purification of a gas mixture by the absorption of part of the mixture in a solvent.

Types of absorption

Absorption may be either a physical or a chemical process:

Physical absorption of a gas or part of a gas mixture in a liquid solvent involves the mass transfer that occurs at the interface between the gas and the liquid and the rate at which the gas diffuses into the liquid. Physical absorption of gases in a liquid solvent depends on the following parameters: solubility of the gases and the pressure and temperature conditions.

An example of physical absorption of a gas into a liquid is the absorption of ammonia (NH₃) into water (H₂). Another example is the separation of low molecular weight gases such as propane (C₃H₈) and butane (C₄H₁₀) from a hydrocarbon gas mixture of methane (CH₄), ethane (C₂H₆), propane and butane by absorbing the propane and butane in a solvent that is a mixture of much higher molecular weight hydrocarbon liquids.

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 ethanolammine solutions.