SOS: save our solvent!
facility, one of the major expenses will be solvents. And just like
the price of petrol and diesel it just keeps going up and up.
Huw Kidwell examines how a new recycling approach could
keep costs down and help protect the environment to boot.
Solvents are absolutely essential in laboratories for chromatography, spectroscopic measurements, analyses, filtration, extraction and crystallisation. But the situation is more complicated than that - not only is solvent expensive, when it is used and goes to waste there is a massive disposal problem, which again costs money. In these times of more and more recycling of useful materials the same principle needs to be applied to waste laboratory solvent and not a throw away policy. From methanol and acetonitrile used in reverse phase HPLC to the litres and litres of solvent taken off a reaction mixture during a pilot scale process, all of this could be recycled and reused indefinitely. Recycling means less crude oil is required to make solvent and also that less carbon is produced through incineration - so it's a win-win solution all around. This problem is as pertinent for commercial laboratories as it is for manufacturing plants. A group in Rowan University College of Engineering in Glassboro, New Jersey, US seem to have come up with a partial 'green chemistry' solution to the problem. Funded with grants of over $200,000 from the US Environmental Protection Agency (EPA) the project involves the use of pervaporation membrane systems in the separation of solvent water mixtures. Pervaporation is a combination of membrane permeation and evaporation that can be used to separate a miscible solvent system. The process is conducted at low temperature and pressure and has been used in the past as an inexpensive way of separating constant-boiling azeotropes such as 95:5 ethanol:water to give pure ethanol, which would otherwise need to use a tenary solvent distillation with benzene (a carcinogen) or strong drying agents. Pervaporation is also used for the dehydration of organic solvents, the removal of organics from aqueous streams and the purification of waste waters. The process of pervaporation involves the separation of two or possibly more miscible solvents across a thin polymer membrane by their differing rates of diffusion and also an evaporative phase change. A concentrate and vapour pressure gradient is used to allow one component to preferentially permeate across the membrane. Application of vacuum to the permeate side is a necessary part of the process together with the immediate condensation of the permeated vapours. Pervaporation is usually used to separate a minor component of a liquid mixture, thus high selectivity through the membrane is essential. Green engineering, green chemistry The director of the project is Dr C. Stewart Slater, a chemical engineering professor in Rowan University. Dr Slater and his group have been investigating the use of solvent pervaporation for industrial scale processes. However when questioned he saw no reason that the process could not be used to treat solvent on a smaller laboratory scale. Dr Slater commented: "In pharmaceutical manufacturing a process might well use 100kg of solvent to produce 1kg of drug material and this solvent is marked as waste and then incinerated... What we aim to do is to apply some green engineering principles to this and use technology in a feasible and economical way but without creating pollution... green engineering refers to things we know we should be doing." He went on to say: "Membrane pervaporation is an enormously useful and versatile process which can be designed into a system for the recycling of the majority of commonly used solvents... all that is required is a life cycle assessment of the solvent to understand exactly what purification will be needed." The Slater group is now working with several large pharmaceutical companies in the US, such as Bristol-Myers Squibb and Novartis, developing green engineering designs to test within their manufacturing processes. Applying the process on a commercial scale is some way off yet but the principles have been established. It now remains to prove that membrane pervaporation can be used to recycle solvent on a commercial scale (even in conjunction with other technologies if required) and get a useable economically viable system in the marketplace for laboratory and pharmaceutical use.
