Choice of Separator—Summary

For a heterogeneous or multiphase mixture, separation usually can be achieved by phase separation. Such phase separation should be carried out before any homogeneous separation. Phase separation tends to be easier and should be done first. 

Distillation is by far the most commonly used method for the separation of homogeneous fluid mixtures. The cost of distillation varies with operating pressure, which, in turn, is mainly determined by the molecular weight of the materials being separated. Its widespread use can be attributed to its ability to 

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

If an azeotropic mixture is to be separated by distillation, then use of pressure change to alter the azeotropic composition should be considered before use of an extraneous mass-separating agent. Avoiding the use of extraneous materials often can prevent environmental problems later in the design. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flow rate, temperature, and pressure are important variables to be set, but no attempts should be made to carry out any optimization at this stage. 

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