SEPARATION OF HYDROCARBON ISOMER VAPORS WITH SILICALITE ZEOLITE MEMBRANES

The permeation behavior of n-octane, isooctane, and n-hexane vapors th rough continuous, silicalite zeolite membranes on porous alumina suppo rts was investigated between 383 and 523 K. For binary and ternary mix tures, n-octane permeated through the membrane faster than either n-he xane or isooctane, and a selectivity for n-octane over isooctane of 40 was obtained in a ternary mixture at 413 K. Selectivity was a functio n of temperature, however, and lower selectivities were obtained above and below 413 K. The 12-octane/n-hexane selectivity was 9 at 413 K an d decreased with increasing temperature. Permeances were a strong func tion 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. Th e pure component permeances thus could not be used to predict separati ons for mixtures. Whereas n-octane was always the faster permeating co mpound 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 pe rmeance in mixtures. Thus relative permeances could not be predicted o n the basis of size or shape alone. The permeances of the pure compoun ds were activated with activation energies between 18 and 45 kJ/mol. T he mixed feed permeances also increased with temperature but did not f ollow an exponential relationship. Transport limitations caused by mem brane saturation, diffusion through boundary layers on the feed side, and diffusion through the alumina support were not important.