(414y) Hexane Isomer Permeation and Separation Properties of Silicalite-1 Membrane Under Pressurized Conditions

Membrane separation has been studied extensively for gas separation and water purification as energy-saving processes in the recent decades. Some of them have already been put to practical use, for example, desalination of seawater and dehydration of ethanol. Petroleum refinery and petroleum chemistry can be the next target of membrane separation because separation and purification occupy the largest part of energy consumption in petroleum industry.

Zeolites draw attention as membrane material for hydrocarbon separation because of their high thermal and chemical resistances. Flux improvement is one of the issues of hydrocarbon separation with zeolite membrane. In general, the difference of partial pressures between feed and permeate sides strongly affects permeation flux. In this study, we investigated hydrocarbon separation property of silicalite-1 membrane which has high separation ability based on molecular sieving effect. In addition, we discuss the dependency of flux on the difference of partial pressures in the feed and permeation sides.

Silicalite-1 membrane was prepared on a tubular a-alumina support (i.d. = 7.0 mm, o.d. = 10 mm, average pore size = 150 nm) by secondary-growth method. Silicalite-1 seed crystals were supported on the outer surface of support by dip coating method. The seeded support was immersed in a synthesis gel and hydrothermal treatment was carried out. Vapor permeation property of silicalite-1 membrane was investigated under pressurized conditions. Hexane isomers (n-hexane, 2-methylpentane and 2,2-dimethylbutane) were used for model substances of hydrocarbons. Hydrocarbon vapor was fed to the outer surface of tubular membrane and the permeate side was swept with inert gas. The membrane temperature and pressure of feed side were adjusted from 373 - 673 K and 1 - 4 atm, respectively.

n-Hexane permeation flux was investigated under pressurized conditions in the unary system. The n-hexane flux increased with increasing difference of partial pressures between feed and permeate sides over the whole temperature range tested. Besides, the flux increment was larger at higher temperature.

Permeation flux is determined by the adsorption amount and diffusion rate of permeation molecule. Consequently, diffusion rate at lower temperature and adsorption amount at higher temperature mainly dominate the permeation flux. Therefore, it is deduced that the n-hexane flux increment under pressurized conditions is elucidated in increasing of adsorption amount. In particular, the effect was greater in higher temperature range where the adsorption value was sufficiently low. In this experiment, n-hexane flux reached 8.2 kg m^-2 h^-1 at 673 K at 4 atm of partial pressure difference.

In a similar way, n-hexane permselectivity from hexane isomers was investigated. The n-hexane flux under pressurized conditions increased with pressure and reached a plateau above 2 atm at 573 K. On the other hand, n-hexane flux increased linearly with increasing partial pressure up to 4 atm at 673 K. This trend was able to be explained with adsorption inhibition by other components. Since other components also adsorbed easily at lower temperature, the adsorption amount of n-hexane was less increased even under pressurized conditions. Accordingly, the n-hexane flux had a maximum at relatively low pressures at lower temperature. The flux and the separation factor of n-hexane and 2,2-dimethylbutane at 673 K and 4 atm were 2.9 kg m^-2 h^-1 and 279, respectively.