GAS AND SURFACE DIFFUSION IN MODIFIED y-ALUMINA SYSTEMS

The transport of pure gases through a microporous membrane is described. The alumina-based membrane (pores 2.5-4 nm) is suitable for Knudsen diffusion separation . To improve the separation factor, interaction with and mobility on the pore wall of one of the gases of a mixture is necessary. To introduce surface diffusion of oxygen and hydrogen, a y-alumina membrane was impregnated with silver . If temperature and atmosphere are controlled carefully, finely dispersed silver up to 17% by weight can be introduced. At higher loads and under oxidizing conditions, particle growth occurs. In adsorption experiments, little oxygen adsorption on the silver-modified y-alumina could be detected . This is due to a decrease in accessible surface area of the silver because of particle growth of silver under oxygen . The mobility of hydrogen on the surface was tested by counterdiffusion experiments, of which the theory is given. Hydrogen shows a considerable mobility on the surface at 293 K . At low pressures the flux ratio of hydrogen to nitrogen improved from 3 .8 to 8.8. Magnesia was introduced into the y-alumina membrane to enhance the adsorption and mobility of CO2. It is known that 30% of the CO 2 transport on non-modified yalumina is surface diffusion . The highest achievable magnesia load was 2 .2% by weight . Introduction of magnesia into the y-alumina surface gives more strong base sites and fewer weak base sites . This results in stronger bonding of C0 2 on the surface, but the amount adsorbed is comparable with the amount of C02 adsorbed on non-modified y-alumina . The contribution of surface diffusion to the total transport decreases with the introduction of magnesia, as is shown by counterdiffusion. The more strongly bonded CO 2 is less mobile, resulting in a smaller surface flux .