ENCH 445: Lecture 15 --  Selection of Separation Processes

Basic Principle: 

Examine physical properties and exploit differences in physical properties between components being separated.

General Rules:

1.  Consider processes that exploit vapor-liquid equilibrium first (e.g., distillation) since these processes tend to be energy efficient and simple to design, build, and operate.  Difference in boiling points between components being separated usually gives a good indictation of the feasibility:

       a.  A boiling point difference (delta_T_boiling) of greater than 100C usually indicates a single stage process is feasible.

       b.  A boiling point difference between 10 and 100C usually indicates that a multistage distillation process is feasible.

       c.  A boiling point difference less than 10C usually means distillation is unlikely to work since a very large number of stages is required.

In general, a boiling point difference of 10C corresponds to a relative volatility of 1.2 between components.  In close cases, estimate the relative volatility and use it as a criterion since a relative volatility greater than 1.2 is usually needed for multistage distillation to be successful.

2.  Don't use distillation if:

     a.  Boiling points are high (e.g., greater than 200C)

     b.  Components decompose when heated

     c.  Boiling point difference is less than 10C (see above)

3.  If vapor-liquid equilibrium processes are not promising, consider liquid-liquid extraction next.  Solubilities in various solvents are the key physical property.

    a.   For physically interacting solvents (e.g., no hydrogen bonding) the "cohensive energy density" (which is also related to the solubility parameter in regular solution theory) is the key property.  Try to select a solvent so that the solubility parameter of the solvent is within 3 (cal/cm3)2 of the solubility parameter of one of the components to be separated and more than 3 units from the other component.

   b.  For chemically interacting solvents, try to exploit hydrogen bonding, acid-base interactions, or other "specific" chemical interactions.

4.  Consider membrane processes next.  A key consideration is whether an appropriate membrane material exists.

5.  Consider adsorption processes next. 

Other considerations:

1.  Favor processes that remove a minor component from a major component.

2.  Adsorption processes become favorable as the solute concentration becomes very small since the fact these processes are batch process becomes less of a disadvantage under these conditions.  This is because an adsorption bed can be left on stream for long time between regeneration cycles when the solute concentation is dilute.