Azeotropic mixtures may be broken by membrane distillation

The feasibility of removing water from azeotropic mixts. is explored. Although the selectivity of membrane distn. processes is largely detd. by vapor liq. equil. (VLE), differences in diffusion rates across the membrane and an eventual addnl. gas gap may shift the selectivity away from unity, the value expected for azeotropes. Expts. in a plate-and-frame membrane distn. module, with azeotropic mixts. of HCl/H2O and of propionic acid/H2O, yielded retention selectivities of the solute 0.6-0.8. They could be decreased by increasing the length of the diffusion path, but the permeation flux decreased also. For the HCl/H2O system, the apparent azeotropic point in membrane distn. was shifted to higher acid strength, whereas it disappeared altogether for the propionic acid/water system. Preliminary design considerations indicate that membrane distn. might be utilized analogously to pervaporation in order to break azeotropes, and that membrane distn. might complement pervaporation for mixts. with higher water contents. [on SciFinder (R)]