Separation of supercritical carbon dioxide and caffeine with mesoporous silica and microporous silicalite membranes

摘要

The regeneration of supercritical carbon dioxide from a mixture containing caffeine by the prepared mesoporous silica and microporous silicalite membrane filters was studied. The mesoporous silica layer, with an average pore opening of 2.5 nm, was deposited on a tubular alpha-alumina filter. The experimental data showed that a caffeine rejection as high as 0.98 in the first 6 h and a supercritical carbon dioxide permeation flux of 0.074 mol/m~2/s could be obtained at 35 deg C and 13.79 MPa. The mass transfer resistance in the coated meosporous silica layer was found to be negligible and the adsorption in the silica layer to be the major mechanism in the separation. Because of the occurrence of adsorption, a steady state operation was hard to achieve and caffeine rejection would drop when the adsorption equilibrium was approached. For the prepared microporous silicalite membrane with pore operatings of 0.5-0.6 nm, a caffeine rejection of 1.0 was observed at a temperature of 35 deg C and a pressure equal to or lower than 12 MPa. The separation under these pressures was based on the molecular sieving mechanism. Due to the presence of some defects, mainly resulting from inter-grain porosity in the coated layer, a high caffeine rejection but not 1.0 could be achieved when the pressure was larger than 12 MPa. At these pressure, adsorption also occurred in the microporous silicalite layer. The mass transfer resistance in a thin silicalite layer was not significant, resulting in a CO_2 permeation flux sufficiently close to that of the parent filter.