kilomentor

Kilomentor has already written about steam distillation. Steam distillation is however just a special case of the technique of co-distillation. What makes the steam case unique is that the substance that is being co-distilled with the components of interest is water, which is practically free. Therefore, if one needs to distil a large amount of water making a lot of waste water, in order to recover a small amount of product the cost of materials is still not high. On scale any co-distillate liquid can readily be recovered and does not need to be purified before it is recycled.

The other essential criterion for a liquid used in co-distillation is that it must be easily separated from the compounds you are interested in purifying. Usually this means that the substance you are trying to purify must have a low solubility in the co-distillation liquid. Another usual requirement is that the liquid have a high, but not inaccessible boiling point, which at the same time not higher than the desired material. Kerosene and nujol (paraffin) fractions would qualify because many compounds are essentially insoluble in hydrocarbons. Silicone oil is also a possibility. This particular possibility came to my attention once when I had to recover a compound that had been mixed into a heating bath when a flask accidentally broke. I realized how easy it was to recover the lost product by extraction from the silicone oil. At the other end of the polarity range solvents like ethylene glycol, propylene glycol and glycerine can be used as codistillation liquids for high boiling non-polar constituents.

With these codistillations the liquid is not typically totally vaporized and injected into the still pot as is done in steam distillation but more simply a mixture of the liquids is heated to reflux in the normal way used in regular distillation. As the distillate is collected fresh (or recycled)co-distillation liquid is added into the still pot by some addition device.

What is necessary for efficient co-distillation is to wrap the vapour path to the condenser well and to try to supply supplemental heating. Fractionation is not possible in co-distillation and for energy conservation reasons one does not want to vaporize the distillate more than once in its journey to the intended condensing surface.

If all that is needed is a higher temperature than is provided by the typical steam distillation, then you may be better served by superheated steam distillation where the water vapor is kept at a temperature distinctly different from 100 C. Vacuum steam distillation will also serve to purify very high boiling compounds.