Separation of close-boiling hydrocarbons with silicalite zeolite membranes

The permeation behaviour of n-heptane, isooctane and n-hexane vapours through continuous silicalite zeolite membranes on porous alumina supports has been investigated between 359 and 523 K. Pure isooctane, n-heptane and n-hexane have similar permeances. In the presence of n-heptane and n-hexane, in both binary and ternary mixtures, however, the isooctane permeance decreases by a factor of up to ca. 50, whereas the n-heptane and n-hexane permeances are almost the same as the pure component permeances. Thus, separation selectivities for n-heptane-isooctane as high as 138 were obtained at 373 K in ternary mixtures with n-hexane, but such separations cannot be predicted from the pure gas permeances. Selectivity is a strong function of temperature, and lower selectivities were obtained above and below 373 K for the ternary mixture. The highest selectivity of 57 for a binary n-heptane-isooctane mixture was obtained at 359 K, and selectivity decreased with increasing temperature. The permeances of all compounds in the mixtures increase with temperature.