The permselectivity of ion-exchange membranes for non-electrolyte liquid mixtures : I. Separation of alcohol/water mixtures with nafion hollow fibers

Abstract

The separation of alcohol/water mixtures by Nafion hollow fiber membranes is reported; water is the preferred permeate. The permeation rates and separations were shown to be highly dependent on the type of counter-ions. For the alkali cation series, flux decreases in the following sequence: H > Li > Na > K > Cs. This sequence is reversed for the separation factor. Thus, for the azeotropic feed composition of isopropanol/water, Cs + displays a separation factor of 56 and Li + of 10 when coupled with relatively high permeation rates of 100—2000 g/m 2-hr (for membranes of 90μm thickness). These flux and separation factors are conspicuously high compared to other polymer membrane systems. Compared to mono- and divalent ions, trivalent ions display uncommonly low separations even at low permeation rates. The apparent energy of activation of transport, as measured in pervaporation, is less than 6 kcal/mol for both feed components, indicative of transport through discrete paths. The absorption studies indicate strong synergisms associated with an extensive swelling of the membrane, displaying a maximum when exposed to ∼ 21 mole% alcohol solution. This study reaffirms that ion-exchange membranes can be used to efficiently separate liquid mixtures and that without changing the membrane in the permeator, mass transport characteristics can be altered at will merely by loading the membrane with the proper counter-ion(s).