Pervaporation, an energy saving separation process, can be useful in pharmaceutical processing. However, the organic solvents involved in pharmaceutical product synthesis are chemically demanding; very few polymers are able to withstand them. An ideal membrane would be polymeric having a high thermal, chemical and mechanical stability. Such a membrane is made of a copolymer of polydimethyldioxole and tetrafluoroethylene known as PDD-TFE of the CMS-3 variety with a very high free volume. This novel membrane is used to separate a variety of organic-organic and aqueous-organic mixtures. An earlier study based on water-ethanol-isopropanol has shown evidence that the membrane selectivity may be based on size exclusion. Thus, solvents with larger molecular dimensions may not able to penetrate the membrane and remain in the feed; the permeate is enriched in the molecularly smaller solvent. Separation systems of methanol-toluene, ethyl acetate-toluene, tetrahydrofuran (THF)- toluene, N,N-dimethylformamide (DMF)-water, N,N-dimethylacetamide (DMAc)-water and N,N-dimethylsulfoxide (DMSO)-water are explored using a 25 µm thick PDD-TFE membrane at various temperatures and feed compositions. Depending on the system, a wide range of separation factors (αij) are achieved.
The highest water-organic solvent separation factors are obtained in the dehydration of aprotic solvents. A feed containing of 99 wt% DMAc and 1 wt% water would yield an αij of 12,373 for water at 50°C. For mixtures of DMSO and water, similarperformance has been observed resulting in an αij of 8,834. For systems of DMF and water, the highest αij of 12,514 is achieved at 50°C with a feed containing 90 wt% DMF and 10 wt% water. Separation of a 95 wt% THF and 5 wt% water feed at 50°C results in a separation factor of 497. Compared to the results for aprotic solvent systems with water, separation of the organic-organic mixtures yielded limited performance. The PDD-TFE membrane is selective for methanol over toluene with an αij of 7.8 at 30°C for a 72.6 wt%-27.4 wt% toluene-methanol feed. Very poor separation is observed for THF- toluene mixtures. The maximum αij is 1.6 at 50°C using a 25 wt%-75 wt% toluene-THF feed. For mixtures of ethyl acetate and toluene, an almost constant αij of 6 is found at all temperatures and compositions. For water for systems of DMAc and DMSO, water fluxes range from 4.0 to 9.8 g/(m2-h). In DMF-water mixtures, water exhibits significantly higher flux at 77 g/(m2-h). For most other solvents, permeation through the membrane is relatively small, 5 g/(m2-h) at maximum.
Overall permeability coefficients for solvents studied correlate a relationship with the longest molecular solvent size. Such a correlation describes permeation of highly polar solvents such as methanol only when methanol dimerization in the highly hydrophobic membrane is postulated. Analysis of the permeability coefficient shows a decreasing trend with temperature, unlike that of traditional glassy polymers. This is affected by Langmuir sorption of all solvents onto the membrane including water.
Dehydration of this novel membrane has also been explored for other solvent mixtures such as water-ethylene glycol, and acetone-butanol-ethanol-water. Very high aij values have been determined; 12,800 for water-ethylene glycol, 7,180 for water-butanol, 900 for water-ethanol, and 235 for water-acetone have been observed at 30°C.
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