Glycerol: A Possibly General Method for Changing Solvents in Process Chemistry Reactors

In the laboratory completely changing the solvent that has dissolved within it any mixture of solutes to a different solvent (so long as the solute mixture is stable under the conditions of concentration ) is to completely remove the first solvent on a rotary evaporator, add the second solvent and stir to redissolve any solutes that have precipitated.

This is not even conceivable in industrial reactors because, in almost every case,  they have an appreciable non-stirrable volume. That is to say, when the solvent has not yet been completely removed further evaporation becomes impossible because the impeller blade no longer contacts the liquid. A further problem is that precipitating solutes would accumulate on the walls of the reactor making heat transfer impossible without solute degradation.

 

A possible rather general procedural solution for this difficulty may exist. 

1.    The first reaction solvent is completely distilled (chased) out of the reactor and away from the mixture of less volatile solutes by adding the minimal stirrable volume of glycerol. Glycerol (bp. 290 ℃ with decomposition).

2.    Since neither acetone nor methylene chloride are miscible with glycerol and they do not form any binary azeotrope

 (i) A mixture of some useful proportion of acetone and methylene chloride is added to the glycerol solution or slurry to deliver an upper phase that upon stirring extracts the required non-volatile substrates and the lower glycerol phase is cut away.

or (ii) the desired second solvent, if it is less dense and also immiscible with glycerin, is used to extract the non-volatile substrate, and the lower glycerol phase is cut away.

Then

If (i) above is used, distill the methylene chloride away from the acetone leaving an acetone solution.

If (ii ) above is used, then wash the desired second solvent solution with water or brine to remove traces of glycerin.

Then

If (i)  above has been followed, add the new second solvent and distill away the lower boiling acetone (almost all solvents have higher boiling points than acetone). If this second solvent is to be both protic and miscible with water, residual glycerol needs to have been removed at the stage of the acetone-methylene chloride extract by passing it through a plug of silica or alumina.

Note: For this to work, the mixture of less volatile solutes (usually the entire contents of a reaction mixture ) must be stable in glycerol under the conditions required to evaporate the first solvent and at least the desired components extractable back out of the glcerol.