Absorption

Abstract

Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scrubbing a gas mixture with a suitable liquid. One or more of the constituents of the gas mixture dissolves or is absorbed in the liquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the liquid or the solvent, and in other cases, it reacts with the liquid chemically. The purpose of such scrubbing operations may be gas purification, eg, removal of air pollutants from exhaust gases; product recovery; or production of solutions of gases for various purposes. Gas absorption is usually carried out in vertical countercurrent columns. The solvent is fed in at the top of the absorber, the gas mixture from the bottom. The absorbed substance is washed out by the solvent, which is often recovered in a subsequent stripping or desorption operation. The absorber may be a packed column, plate tower, or simple spray column, or a bubble column. The fundamental physical principles underlying the process of gas absorption are the solubility of the absorbed gas and the rate of mass transfer. Information on both must be available when sizing equipment for a given application. In standard packed absorption columns, a gas mixture travels up through a gas absorption tower and the solute is transferred to the liquid phase and thus gradually removed from the gas. The liquid accumulates solute on its way down through the column. Multicomponent mass‐transfer effects, nonisothermal gas absorption, and axial dispersion effects result in considerable departures from ideal flow assumptions. Neglect of axial dispersion results in an overestimation of the driving forces and may lead to an unsafe design. Bubble tray absorption columns may be designed graphically based on a so‐called McCabe‐Thiele diagram. The required number of actual plates, however, is larger than the number of theoretical plates. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved. A computer is normally required to establish the correct concentration and temperature profiles through the tower.