Separation of mixtures of benzene and n-alkanes using an FAU-type zeolite membrane

The vapor permeation properties of an FAU-type zeolite membrane, which was formed on the outer surface of a porous alpha-Al2O3 support tube, were determined in the temperature range of 358-413 K. The measurements were carried out using single-component benzene (Bz) and n-alkanes (C-4-C-7), as well as mixtures of benzene and n-alkanes. The hydrocarbons were diluted with nitrogen, and the partial pressures of hydrocarbons on the feed side were determined to be 7-8 kPa for the single-component systems and 4-5 kPa for each component of the binary systems. The permeate side was swept with a now of nitrogen. The permeances were dependent on the permeation temperature and, to a greater extent, on the presence of benzene. For the single-component systems, the permeances were dependent on the diffusivities of the permeants, and the diffusivity of benzene through the membrane was the smallest of all the hydrocarbons tested. For the binary systems, however, benzene permeated at faster rates than any of the n-alkanes, and the permeances to n-alkanes were not directly dependent on their chain lengths. This benzene-selective permeation was realized using the FAU-type zeolite membrane with pore openings larger than the molecular size of benzene. The separation factors were affected by the partial pressures of permeants on the permeate side. When the partial pressures on the permeate side were in the range of 100-140 Pa for benzene and 1-4 Pa for n-alkanes, the separation factors at 373 K were 57 for Bz/n-C-4, 70 for Bz/n-C-5, 63 for Bz/n-C-6, and 27 for Bz/n-C-7. The adsorption selectivities for the binary systems on the feed-side surface of the membrane were 24 for Bz/n-C-4 and 15 for Bz/n-C-7, and were much higher than those for the single-component systems. This suggests that the adsorption of the n-alkanes on the FAU-type zeolites was strongly retarded by the selective adsorption of benzene.