AZEOTROPIC MIXTURES MAY BE BROKEN BY MEMBRANE DISTILLATION

The feasibility of removing water from azeotropic mixtures is explored. Although the selectivity of membrane distillation processes is largely determined by vapor liquid equilibria (VLE), differences in diffusion rates across the membrane and an eventual additional gas gap may shift the selectivity away from unity, the value expected for azeotropes. Experiments in a plate-and-frame membrane distillation module, with azeotropic mixtures of HCl/H2O and of propionic acid/H2O, yielded retention selectivities of the solute between 0.6 and 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 distillation was shifted to higher acid strength, whereas it disappeared altogether for the propionic acid/water system. Preliminary design considerations indicate that membrane distilation might be utilized analogously to pervaporation in order to break azeotropes, and that membrane distillation might complement pervaporation for mixtures with higher water contents.