Chapter-10 - Chapter 10 Solubility of Gases in Liquids In...

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Chapter 10 Solubility of Gases in Liquids In some separation processes, we are mainly interested in the transfer of a component from the gas phase to the liquid phase. For example, stringent environmental protection laws in many countries require the concentrations of pollutants in the industrial emission gases (CO 2 , H 2 S, SO 2 , NO x ) to be kept below speci fi ed limits. For this purpose gas absorbers, scrubbers, and wetted wall columns are used to bring the gas stream into contact with liquid solvent so as to transfer the undesired component from the gas to the liquid. A typical gas absorber is shown in Figure 10.1. Within the column, the contact between the gas and liquid streams is achieved either on trays, as in the case of distillation, or in a packed bed. The purpose of using a packed bed is to enhance the mass transfer between the phases by increasing the interfacial area. While the gas stream to be cleaned enters from the bottom of the column, the liquid solvent is fed from the top of the column and is distributed over the packing by a distribution plate (or a spray nozzle). Absorption in the liquid phase takes place either by physical absorption or by a chemical reaction. The e ffl uent gas leaves the column from the top. The liquid stream leaving the absorber is fi rst sent to a stripper to recover the dissolved solute and then recycled back to the absorption column. Liquid in Gas out Gas in Liquid out Figure 10.1 A packed gas absorber. In recent years, hollow fi ber membrane contactors have found wide application in the bio- logical treatment of gas streams containing volatile organic compounds (VOC’s), and also in the removal of acid gas 1 from waste gas streams. These devices, in a way, resemble shell-and- tube heat exchangers. The membrane wall not only keeps liquid and gas phases separated from each other but also provides an interfacial area for the two phases. The gas stream may fl ow either in the tube or shell side, in cocurrent or countercurrent fl ow with the liquid stream. For more detail about membrane contactors, see Gabelman and Hwang (1999) and Sirkar (1997). 1 An acid gas is a gas containing signi fi cant amounts of acidic gases, such as CO 2 and H 2 S. 357
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10.1 HENRY’S LAW The solubility of a gas in a liquid is determined by the equations of phase equilibrium. Consider the gaseous and liquid phases of a binary system that are in equilibrium. The condition of equilibrium states that b f V i ( T, P, y i ) = b f L i ( T, P, x i ) i = 1 , 2 (10.1-1) Expressing the left- and right-hand sides of Eq. (10.1-1) in terms of fugacity coe cient and activity coe cient, respectively, yields y i P b φ V i ( T, P, y i ) = γ i ( T, P, x i ) x i f L i ( T, P ) (10.1-2) The di culty in using Eq. (10.1-2) comes from the fact that if the temperature of the mixture, T , is greater than the critical temperature of component i , then the pure component i exists only as a gas and it is impossible to calculate f L i ( T, P ) . For example, consider a rigid cylinder containing water (1) and methane (2) in equilibrium at 298 K and 1 bar . Since the critical
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