Separation of Hydrocarbon Isomer Vapors with Silicalite Zeolite Membranes

Abstract

The permeation behavior of n-octane, isooctane, and n-hexane vapors through continuous, silicalite zeolite membranes on porous alumina supports was investigated between 383 and 523 K. For binary and ternary mixtures, n-octane permeated through the membrane faster than either n-hexane or isooctane, and a selectivity for n-octane over isooctane of 40 was obtained in a ternary mixture at 413 K. Selectivity was a function of temperature, however, and lower selectivities were obtained above and below 413 K. The n-octane/n-hexane selectivity was 9 at 413 K and decreased with increasing temperature. Permeances were a strong function of other organics present in the feed. The permeance of n-octane increased a factor of 16 in the presence of isooctane and n-hexane. The pure component permeances thus could not be used to predict separations for mixtures. Whereas n-octane was always the faster permeating compound in mixtures, pure isooctane permeated up to 5 times faster than pure n-octane, and pure n-hexane permeated faster than pure isooctane. That is, the largest molecule did not have the lowest permeance for pure components, and the smallest molecule did not have the highest permeance in mixtures. Thus relative permeances could not be predicted on the basis of size or shape alone. The permeances of the pure compounds were activated with activation energies between 18 and 45 kJ/mol. The mixed feed permeances also increased with temperature but did not follow an exponential relationship. Transport limitations caused by membrane saturation, diffusion through boundary layers on the feed side, and diffusion through the alumina support were not important.