Gas permeability through graphite foil: The influence of physical density, membrane orientation and temperature

Gas permeability parameters of microporous foils based on exfoliated graphite prepared by controlled pressing as membrane material for gas separation are reported. Permeability of H2, N2, CH4, CO2 and C1-C4 lower hydrocarbons for pressed graphite foil (GF) samples with various densities (240-1500 kg/m3) within the temperature range 20-90 °C was studied. It was found that GF samples with density around 1000 kg/m3 demonstrate high H2/CO2 ideal selectivity (α = 14-22) which significantly exceeds Knudsen selectivity. The influence of anisotropy structure features of GF membranes, physical foil density, transport pore distribution and gas flow direction on gas transport and separation properties was investigated as well. The evaluation of the surface flow contribution to the overall gas flow was carried out by approximation method using temperature dependences of gas permeance. It is concluded that the presence of noticeable surface flow can lead as to a decrease of gas permeance in comparison with Knudsen flow (for example, for CO2) as to an increase of gas permeance (for example, for lower hydrocarbons). Development of separation process for particular gas mixture compositions needs to take into account obtained tendency. Future potential applications of GF membranes are discussed as well.