Synthetic chemistry labs use tons of solvents. Preparative column chromatography consumes anywhere from half a liter to one liter per half gram of purified compound. Other solvent-intensive operations include extraction, HPLC, and chemical reactions. Rotary evaporation is the most common laboratory operation taking place immediately after flash column chromatography. Elution typically involves a blend of polar and apolar solvents, ethyl acetate and hexane, for example. A Google search for prices for these solvents turns up approximately $65 and $75 per liter, respectively. Reagent acetonitrile will set you back about $60 per liter, but upgrading to HPLC grade will cost $30 more per liter. Add to these costs the price of disposing spent solvent, and the argument for recycling becomes compelling.
Solvent recycling usually begins (and sometimes ends) with rotary evaporation. Depending on your needs, regenerating chromatography-grade solvents may be as simple as adding a drying agent like magnesium sulfate and filtering.
Testing for condensate composition (in the case of two or more cosolvents) is always a good idea, even if you use a low-temperature trap. Remember, compounds exhibiting good retention/separation characteristics at 4 percent ethyl acetate in hexane will fly through the column at 6 percent. Refractive index will provide a quick reading, provided you’ve created a standard composition curve for your concentration ranges. Here, the chemist’s old standby formulae for dilution and addition will serve you well.
In some instances, distillation makes sense, particularly in the case of three or more cosolvents. To stick with our example, ethyl acetate boils at 170.8° C and hexane at 154.4° C, so fractional distillation should be a cinch.
Choice of distillation equipment depends on the level of purity you’re looking for. For chromatography-grade recycling, a simple distillation column will do. Recovering solvents to HPLC or analytical grade requires serious fractional distillation, which may not be worth the bother or operating costs. Spent HPLC- or analytical-grade solvents may be recycled for lesscritical operations, however.
If your lab already has solvent stills set up, it’s easy enough to add another heating mantle, a large round-bottom flask, a condenser, and a collection vessel. Most glassware vendors sell everything you will need. If you don’t already recycle, consider dedicating the corner of a relatively unused hood for this purpose.
This is all that most labs interested in do-it-yourself solvent recycling will need. Multi-lab organizations with high solvent usage have likely already installed dedicated recycling units from such vendors as Unique Equipment, Parts Cleaning Technologies, Baron Blakeslee, CB Mills, Finish Thompson, and others.
Midsize to large labs wishing to upgrade solvent recycling beyond simple stills should look into products like the Hei-Vap benchtop or Hei-Vap Industrial rotary evaporators from Heidolph North America (Elk Grove Village, IL). Both devices function similarly, but the Industrial model accommodates a 20-liter flask.
Hei-Vap uses temperature sensors within the condenser coils, which detect phase transitions typical of boiling solvents and adjust the vacuum to achieve the best separation. For the liquid phase described earlier, Hei-Vap collects the lower-boiling component, hexane, and separates it from the higher-boiling ethyl acetate.
“For solvent recovery, Hei-Vap replaces a dedicated still,” says Jason Paloskey, business development director. An add-on unit, Distimatic allows unattended 24/7 operation, refills the evaporation flask, and discharges condensate automatically. “Many of our customers run Hei-Vap all day from 55-gallon drums of used solvent.”
Some customers use Hei-Vap as a primary still to upgrade reagent-grade solvents to HPLC grade, which alone could shorten the return on investment.
Whatever your decision, make sure the operation of in-house stills is in good hands. Post rules regarding heating to dryness, which will ruin hundreds of dollars’ worth of equipment and could also cause an explosion. Make sure the pot is emptied and cleaned periodically, and perhaps install a level sensor or timer that turns off the heating element at an appropriate endpoint.
Distilling waste solvents is high-tech stuff, but it does entail hazards. Do it right and your lab will save thousands of dollars per year.
For additional resources on evaporators, including useful articles and a list of manufacturers, visit www.labmanager.com/evaporators