Transport and related (gas and vapour) separation in ceramic membranes

The main transport mechanisms in pores with a size of 3 to 100 nm are Knudsen and surface diffusion. According to Knudsen diffusion the transport of gases is inversely proportional to the square root of the molar mass. Surface diffusion is an additional mechanism to Knudsen diffusion and decreases, in most cases, the separation possibilities. Only surface diffusion of hydrogen (on metals) can increase the separation factors. Multilayer adsorption and capillary condensation can be very selective mechanisms for the separation of condensable gases from gas mixtures in pores of 2 to 10 nm. The application is limited because of the temperature/pressure regime which is needed for condensation and which may lead to technical inconveniences. In pores smaller than 2 nm and in zeolites micropore diffusion occurs. Transport of gases is activated with activation energies smaller than 25 kJ/mol for amorphous microporous systems. In silica-type of microporous toplayers H₂,He and CO₂ are transported much faster than any other type of gas. At 200°C a separation factor of over 200 is found for the mixture H₂/C₃H₆. The transport mechanism in amorphous microporous materials is still under discussion. In this contribution the mentioned transport mechanisms will be discussed and their possibilities in gas and vapour separation will be illustrated.