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The removal of 5 specific active pharmaceutical ingredients (API's) with molecular weight of 189, 313, 435, 531, and 721, respectively, from toluene, methylene chloride, and methanol was studied by using solvent resistant nanofiltration. Three membranes of the StarMem series (120, 122, and 228), with cut-off values of 200, 220, and 280 respectively, were used in the experiments. Although the rejections expected from the size difference between solutes and membrane pores are high, the results largely depended on the solvent used. For toluene, rejections were rather small, due to the low molecular weight of the solutes of interest (all API's except for the largest compound). Modelling of the rejection curve showed that the minimum molecular weight of a solute to obtain a rejection of 90% in toluene with the membranes used, is ca. 600. The application in methylene chloride was unsuccessful due to partial dissolution of the membrane top layer; other polymeric membranes such as the Solsep series might be more successful. The rejections in methanol were sufficiently high (.90%) to allow implementation: the rejection can be significantly increased by using a module design with double membrane passage and recirculation of the retentate, as was calculated from mass balances. A comparison of a (single pass) nanofiltration system with a throughput distillation unit, currently in use, showed that the energy consumption is 200 times lower in the nanofiltration system.

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