Absorption is a basic chemical engineering operation and is probably the most well-established gas control technique. It is used extensively in the separation of corrosive, hazardous, or noxious pollutants from waste gases. The major advantage of absorption is its flexibility; an absorber can handle a range of feed rates.

Absorption, also called scrubbing, involves transferring pollutants from a gas phase to a contacting solvent. The transfer occurs when the pollutant partial pressure in the gas phase is higher than its vapor pressure in equilibrium with the solvent. To maximize the mass-transfer driving force (i.e., the difference in pollutant concentration between the gas and liquid phase), the absorber generally operates in a countercurrent fashion.

Absorption systems can be classified as physical absorption and absorption with a chemical reaction. In physical absorption, the pollutants are dissolved in a solvent and can be desorbed for recovery. The absorption of ammonia by water or the absorption of hydrocarbon by oil are typical examples.

If a solvent to absorb a significant quantity of the pollutant cannot be found, a reactant mixed with the solvent can be used. The pollutants must first be absorbed into the liquid phase for the reaction to occur. In this case, the pollutant concentration in the liquid is reduced to a low level. As a result, high absorption capacity is achieved.

The reaction can be reversible or irreversible. Typical reversible reactions are H2S/ethanolamines, CO2/alkali, carbonates, and some flue gas desulfurization (FGD) systems. The reversible reactions allow the pollutants to be recovered in a concentrated form and the solvent to be recycled to the absorber. If the reactions are irreversible, the reaction products must be disposed or marketed (e.g. ammonium sulfate). A few FGD systems are irreversible (vide infra).

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