Pervaporation Recovers Organic Solvents from Pharmaceutical, Chemical Process Streams
Cleaning up solvent waste streams from pharmaceutical processes typically requires several steps, including energy-intensive distillation or flashing under vacuum. A group at Rowan University (Glassboro, N.J.) believes the time is right for drug firms to switch from these older methods to more process-friendly, environmentally benign membranes, particularly those that facilitate pervaporation ("permeation/evaporation"). Liquids permeate these membranes, diffuse through, and change to a vapor on the downstream side. The net effect is to concentrate materials present in very low concentration waste or process streams without regard to boiling point or azeotrope formation.
Convincing companies to switch to pervaporation membranes has not been easy for C. Steward Slater, chemical engineering professor and department chairman at Rowan. Neighboring chemical and pharmaceutical companies have been eager to investigate the membrane research and development capabilities that Slater has assembled in his new department (see figure), but not always so eager to implement them.
C. Stewart Slater explains operation of a hollow-fiber membrane at an industrial seminar.
One of Slater's research projects uses modified silicone membranes for separating trace organics from primarily aqueous waste streams. There are two reasons to do this: solvent recovery, and the ability to discharge water as water and not as chemical waste. Silicone polymers, which are permeable to organics, work reasonably well as crude pervaporation membrane materials. But, as Slater points out, for many mixtures the separation isn't 100 percent effective. "The recovered organic solvent is reasonably pure, but some water also comes through the membrane in quantities large enough that the organic material requires further processing."
To enhance membrane performance, Slater uses composite membranes made primarily of silicone impregnated with silicalite, a zeolite with a high ratio of silicon to aluminum. The result is a 60-fold improvement in organic/water selectivity. For example, an unmodified silicone membrane gives ethyl acetate selectivities of about 20 (materials ranging from 92.5% to 96.4% pure). Silicalite-doped silicone's selectivity for ethyl acetate is as high as 1281. The ethyl acetate results are particularly impressive in light of similar membranes' organic selectivity of around 50 in ethanol/water or methanol/water systems.
The only tradeoff for the higher selectivity is flux: The greater the selectivity, the lower the throughput for a particular membrane.
By Angelo DePalma
For more information, contact:
C. Stewart Slater, Professor and Chairman,
Dept. of Chemical Engineering, Rowan University,
312 Harry M. Rowan Hall, Glassboro, NJ 08028.
Tel: 609- 256-4631.