The dehydration of nitric acid using pervaporation and a nafion perfluorosulfonate/perfluorocarboxylate bilayer membrane

In this paper, the dehydration of nitric acid using pervaporation is investigated. Experiments were conducted using the commercially available, Nafion 90209 bilayer membrane. It was shown that this approach is able to separate water from nitric acid feed concentrations up to 80 wt.%, including breaking the azeotrope at 68 wt.%. It was observed that the separation factor decreased from 105 to 48 as the concentration of the feed was increased and that the water flux decreased from 2.1 to 0.1 kg m−2 h−1. The effect of varying the permeate side pressure from < 1 to 70 mmHg was also investigated. A maximum in the permeate flux of 0.2 kg m−2 h−1 was observed as the permeate pressure was reduced below 8 mmHg with a feed concentration of 50 wt.% nitric acid. The peculiarity is attributed to a dehydration effect within the bilayer membrane because of significantly different diffusion rates through the two ionomer layers of the membrane. A simple resistance-in-series model was also used to illustrate the contribution of the membrane to the separation process and to compare the membrane contribution with the vapor–liquid equilibria (VLE) contribution.