Permeation mechanism through zeolite NaA and T-type membranes for practical dehydration of organic solvents

Commercially available zeolite NaA and T-type membranes, which were grown on the surface of porous cylindrical mullite tubes by hydrothermal treatment, showed excellent water permselective performance in pervaporation (PV) and vapor permeation (VP) for water/organic mixtures. Both of zeolite membranes were composed of inter-grown zeolite crystal layer on the outer surface of the tube, each of which is ca. 10 μm for NaA and ca. 20 μm for T-type membrane in thickness. Their zeolite crystal layer was composed of two layers, namely the top surface layer and the intermediate layer under the top surface. Average Kelvin diameters were 1.4 nm for the NaA membrane and 0.8 nm for the T-type membrane, respectively. Zeolite NaA membranes showed extremely high PV and VP performance even for water/methanol mixture. T-type membranes also showed extremely high PV and VP performance for water/alcohol mixtures except for water/methanol mixture. The experimental data were discussed from the new theoretical consideration. The consideration consists of the following: a very fine and narrow non-zeolitic pore penetrates from the zeolite crystal layer to the mullite support. The majority of zeolitic pores in top surface layer are assembling in the non-zeolitic pore. The water molecules in the feed are selectively adsorbed in zeolitic pores in the top surface layer, and then are transported to the non-zeolitic pore through the zeolitic pores. At narrower space in the non-zeolitic pore, the capillary condensation occurs, and then the condensate evaporates and diffuses into the permeation side.