Vapor Recycles and Purges

When a mixture contains components with a broad range of volatilities, either a partial condensation from the vapor phase or a partial vaporization from the liquid phase followed by a simple phase split often can produce an effective separation. This is in essence a single-stage distillation process. However, by its very nature, a single-stage separation does not produce pure products hence further separation of both liquid and vapor streams is often required. 

Consider the single-stage phase split shown in Fig. 3.1a. Overall material balances and component material balances can be written as 

V = vapor flow rate from the separator L = liquid flow rate from the separator Zi = mole fraction of component i in the feed y = mole fraction of component i in the vapor Xi = mole fraction of component i in the liquid 

The vapor-liquid equilibrium relationship can be deflned in terms of K values by 

Equations (4.5) to (4.7) can now be solved to give expressions for the vapor- and liquid-phase compositions leaving the separator  

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