Chapter-9 - Chapter 9 Vapor-Liquid Equilibrium Distillation...

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Chapter 9 Vapor-Liquid Equilibrium Distillation is one of the most commonly used separation processes in the chemical process industries. It is based on the di f erences in boiling points of components in the mixture. A typical distillation column is shown in Figure 9.1. The feed enters the distillation column in any state (from subcooled liquid to superheated vapor). The parts of the distillation column above and below the feed point are called the rectifying and stripping sections , respectively. The vapor stream from the top of the distillation column is sent to a condenser. Part of the liquid is taken as the overhead product, while the rest is returned to the column as "re f ux". The overhead product is rich in more volatile component(s). The ratio of the re f ux f ow rate to the overhead product f ow rate is called the re f ux ratio . Part of the liquid stream from the bottom of the distillation column is taken as the bottoms product, which is rich in less volatile component(s). The rest is vaporized in a reboiler and returned to the column. Condenser Feed Reboiler Bottoms product Liquid Overhead product Vapor Rectifying section Stripping section Equilibrium Stage Liquid Vapor Figure 9.1 A distillation column. The trays placed in the column are used to bring the vapor and liquid streams, which f ow in countercurrent direction, into contact with each other. The trays may be of various types, i.e., sieve, bubble-cap, valve, etc. In a sieve tray column, for example, liquid f owsacrossthe tray through channels separated by ba F es and vapor f ows from the bottom, up through holes in the tray. When the two streams mix with each other, if the component fugacities are not the same in the vapor and liquid phases, the resulting driving force causes mass transfer to take place. While the more volatile component transfers from the liquid to the vapor phase, the less volatile component transfers from the vapor to the liquid phase. The system reaches 279
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equilibrium, or, in other words, mass transfer between the phases stops, when component fugacities in the vapor and liquid phases are equal to each other. Once the system reaches equilibrium, the two phases leave the tray as two separate streams. The combination of these stages, i.e., mixing, reaching equilibrium, and separation, is called an equilibrium stage .T h e purpose of this chapter is to provide the necessary tools to determine the compositions of the liquid and vapor streams leaving each equilibrium stage. 9.1 VAPOR-LIQUID EQUILIBRIUM CALCULATIONS When vapor is in equilibrium with liquid, besides temperature and pressure of the vapor and liquid phases, fugacities of each component in the vapor and liquid phases must be equal to each other, i.e., b f V i ( T,P,y i )= b f L i ( T,P,x i ) i =1 , 2 ,...,k (9.1-1) There are mainly two approaches for the modeling of vapor-liquid equilibrium: In the f rst approach, also known as the γ - φ model, fugacity of component i in the vapor phase is expressed in terms of the fugacity coe cient, and the fugacity of component i in the liquid phase is
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Chapter-9 - Chapter 9 Vapor-Liquid Equilibrium Distillation...

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