Organic Solvents and Various Means for their Removal

 

Co-distillation

If two liquids are essentially immiscible, distillate comes over when the sum of their vapor pressures equals the pressure inside the distillation apparatus. The effect is that a small amount of a high boiling material will co-distill along with a larger amount of a lower boiling material. So long as the low boiling material is inexpensive and the higher boiling material is easily separable from this large amount of low boiler, co-distillation can physically separate the high boiler from less volatile or non volatile  components mixed with it. Aside from using water (which is classified below as steam distillation), the inexpensive material most frequently used in a co-distillation is kerosene/paraffin/lamp oil/coal oil. Much less frequently silicone oil (dimethicone KF-96L-2cs) has been used as the high boiling component.

Steam Distillation

Steam distillation represents the particular case of co-distillation using water. Many higher boiling solvents can be chased by steam distillation. Nitrobenzene and 1,1,2,2-tetrachloroethane are frequently removed this way after Friedel-Craft reaction. The steam can be preheated to temperatures above 100 C thereby co-distilling a larger portion of the lower boiling material while minimizing the volume of water. Codistillation including steam distillation can also be done under vacuum so that the material of interest is not exposed to as much heat.

Reversible Degradation

Some special compounds can be used as solvents that are reversibly cracked into smaller more volatile fragments by heating. Piperylene sulfone has solvent characteristics very similar to tetramethylene sulfone. The essential difference for our consideration here is that the former is fragmented by heat into  1,3-pentadiene and sulfur dioxide. These can be trapped together as a distillate whereupon they reform piperylene sulfone.

  Likewise, dicyclopentadiene upon heating can disaggregate into the monomeric cyclopentadiene and be distilled out of a reactor and away from less-volatile products.

Hydroxymethanesulfonic acid is a strong acid solvent that upon heating breaks apart into formaldehyde, water, and sulfur dioxide before reforming when they are recondensed together. 

Degradation

Some solvents can be hydrolyzed into water-soluble fragments.

 

Acetic anhydride can be hydrolyzed into acetic acid.

 

Dimethylformamide can be hydrolyzed by acid into dimethylamine and formic acid.

 

Dimethylacetamide can be hydrolyzed into acetic acid and dimethylamine.

 

Propylene carbonate can be hydrolyzed into 1,2-propanediol and carbon dioxide.