chapter10 - 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 to keep the concentrations of pollutants in the industrial emission gases (CO 2 , H 2 S, SO 2 ,NO x ) below speci f ed limits. For this purpose gas absorbers, scrubbers, and wetted wall columns are used to contact gas stream 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 gas and liquid streams is achieved either on trays, as in the case of distillation, or in a packed bed. The purpose of using packed bed is to enhance the mass transfer between the phases by increasing 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 f uent gas leaves the column from the top. The liquid stream leaving the absorber is f rstsen ttoa stripper to recover the dissolved solute and then recycled back to the absorption column. Gas in Gas out Liquid in Liquid out Figure 10.1 A packed gas absorber. In recent years, the use of hollow f ber membrane contactors f nds wide application in biological 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 to shell- and-tube heat exchangers. The membrane wall not only keeps liquid and gas phases separated from each other but also provides interfacial area for the two phases. The gas stream may F ow either in the tube or shell side, in cocurrent or countercurrent F ow with the liquid stream. For more detail in membrane contactors, see Gabelman and Hwang (1999) and Sirkar (1997). 1 An acid gas is a gas containing signi f cant amounts of acidic gases, such as CO 2 and H 2 S. 321
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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 ( i γ i ( 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 ( ) . For example, consider a rigid cylinder containing water (1) and methane (2) in equilibrium at 298 K and 1bar . Since the critical temperature of methane is 190 .
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chapter10 - Chapter 10 Solubility of Gases in Liquids In...

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