Is Glycerin a General Green Solution to Solvent Recovery and Replacement for Process Chemistry?
kilomentor | 13 June, 2011 07:15
At laboratory scale, when one solvent needs to be replaced with another, the solution contents are placed in a r.b. flask, set spinning on the vacuum rotary evaporator with appropriate heating and condensing facilities and when the first solvent has been completely evaporated then the required new replacement solvent is added and the solutes brought back into solution by swirling, scrapping and heating. There is no equivalent procedure for solvent exchanges at scale. In the plant situation, because stirring and heat transfer become ineffective below the minimum stirrable volume, the solvent and substrates combined volumes can never be taken below the minimum stirrable volume. Thus, solvent is never completely removed so no essentially solvent free state arises.When the solvent replacement is of a more volatile one with a less volatile one, the latter solvent can be used to chase the former in some form of distillation; but when the replacement solvent is of more volatile one, tricks, such as using azeotropes, come into play. Even where an azeotrope can work there are still fractions of mixed solvents that become environmentally troublesome waste.It is proposed that a general solution to this problem is to use the minimum stirrable volume of glycerin as a chaser for the first solvent. Glycerin is cheap, biodegradable and has a bp of 182C @ 10 mm Hg. It would be expected to remain behind in a standard distillation when combined with any of the common organic reaction solvents. Even DMSO (bp 189), DMF (bp153), NMP (bp 81-82 10mm hg) would be expected to be chased by glycerin. All the first solvent could be distilled because the volume in the reactor would not go below the minimum stirrable volume. The less volatile solutes from the starting solvent would be retained in the glycerol although not necessarily kept in solution. Advantageously glycerol is immiscible with many common organic solvents such as hexane, methylene chloride, acetone, chloroform, benzene, probably toluene. Thus the solutes can be extracted from the glycerin phase into the new lower boiling solvent. If the transfer is inhibited by partition coefficients, the glycerin can be diluted with some water to perhaps improve the transfer. Whatever glycerin is taken into the new lower boiling solvent can be removed using a drying agent that strongly binds it such as calcium chloride.The distilled first solvent contaminated with traces of glycerin upon simple treatment might be ready for reuse. Thus the first solvent is no longer a waste and there are no mixed fractions of solvents to dispose of. The waste glycerin is a biodegradable material and the quantity used is no more than the minimum stirrable volume of the reactor.