GAS ABSORPTION

GAS ABSORPTIONGas AbsorptionThe removal of one or more selected components from a mixture of gases by absorption into a suitable liquid is the second major operation of chemical engineering that is based on interphase mass transfer controlled largely by rates of diffusionExamples:acetone can be recovered from an acetoneair mixture by passing the gas stream into water in which the acetone dissolves while the air passes outammonia may be removed from an ammoniaair mixture by absorption in water.oxides of nitrogen are absorbed in water to give nitric acidcarbon dioxide is absorbed in a solution of sodium hydroxide, a chemical reaction occursAbsorption processes are therefore conveniently divided into two groups, those in which the process is solely physical and those where a chemical reaction is occurring.In considering the design of equipment to achieve gas absorption, the main requirement is that the gas should be brought into intimate contact with the liquid, and the effectiveness of the equipment will largely be determined by the success with which it promotes contact between the two phasesIn absorption, the feed is a gas introduced at the bottom of the column, and the solvent is fed to the top, as a liquid; the absorbed gas and solvent leave at the bottom, and the unabsorbed components leave as gas from the topDifference between distillation and gas absorptionDistillationGas AbsorptionVapor is produced by partial vaporization of the liquid there fore at its boiling pointThe liquid is below its boiling pointDiffusion of molecules is in both directionDiffusion is from gas to liquid onlyThe L/V for distillation is less than in gas absorptionConditions of equilibrium between liquid and gasWhen two phases are brought into contact they eventually reach equilibrium. Thus, water in contact with air evaporates until the air is saturated with water vapour, and the air is absorbed by the water until it becomes saturated with the individual gases. In any mixture of gases, the degree to which each gas is absorbed is determined by its partial pressure. At a given temperature and concentration, each dissolved gas exerts a definite partial pressure. Three types of gases may be considered from this aspecta very soluble one, such as ammonia, a moderately soluble one, such as sulfur dioxide, and a slightly soluble one, such as oxygen.it is important to note that the solubility falls with a rise of temperature.In many instances the absorption is accompanied by the evolution of heat, and it is therefore necessary to fit coolers to the equipment to keep the temperature sufficiently low for an adequate degree of absorption to be obtained.