Separation Factor as a Criterion

The second consideration for the selection of a separation method is the separation factor, SF, that can be achieved by the particular separation method for the separation between two key components of the feed. This factor, for the separation of key component 1 from key component 2 between phases I and II, for a single stage of contacting, is defined as 

When a multicomponent mixture forms nearly ideal liquid and vapor solutions, and the ideal gas law holds, the K values and relative volatility can be readily estimated from vapor pressure data. Such K values are referred to as ideal or Raoult s law K values. Then, the SF for vapor-liquid separation operations employing an ESA (partial evaporation, partial condensation, or distillation) is given by 

For vapor-liquid separation operations (e.g., azeotropic and extractive distillation) that use an MSA that causes the formation of a nonideal liquid solution, but operate at near-ambient pressure, expressions for the K values of the key components are based on a modified Raoult s law that incorporates liquid-phase activity coefficients. Thus, the separation factor is given by 

If an MSA is used to create two liquid phases, such as in liquid-liquid extraction, the SF is referred to as the relative selectivity, P  

Unless values of SF are about 10 or above, absorption and stripping operations cannot achieve sharp separation between two components. Nevertheless, these operations are used widely for preliminary or partial separations where the separation of one key component is sharp, but only a partial separation of the other key component is adequate. The degree of sharpness of separation is given by the recovery factor RF, 

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