Composite inorganic membranes for hydrogen reaction, separation and purification.
El-Zarouk, Khaled Mohamed
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Silica-alumina composite membranes for hydrogen separation and high temperature chemical reactions were prepared using both conventional and modified dip-coating techniques. These were deposited on commercially available a.-alumina macroporous support of 10 millimetre (mm) outer diameter, 7 mm inner diameter and average pore size of 6000 nanometre (nm) wash coated with Titania. The reactants of the coating technique were silicone elastomer and isopentane promoted by a catalyst. The catalyst (silicone curing agent) was added as a templating agent to control the eventual adhesion and densification of the elastomer sol. In particular, the microporous membranes were prepared by creating suction in the bore side of the membrane and involved continuous stirring of the coating mixture during the process, and their pore characteristics were analysed. Then, the effects of thermal treatment on the gas transport and micro pore structure of the resulting membranes were investigated. The pore size of the silica membrane prepared by conventional technique was in the range of approximately 8 to 11 nm while that prepared by modified dip-coating was in the range of about 3 to 4 nm. In addition, the membranes were segmented into five categories; silica membrane for hydrogen reaction, silica membrane for separation, silica membrane for purification, palladium (Pd)-impregnated membrane and silica on gamma -alumina (y-alumina). The hydrogen permeation of the silica membrane prepared for hydrogen reaction was of the order of 10-7 mol/m2.s.pa, while the nitrogen permeance was of the order of 10- mollm2.s.pa. at pressure differential of 0.5-2.0 bar and temperature range of 323-473 Kelvin (K). The maximum hydrogen I nitrogen (H2 I N2) selectivity, determined from single-component permeances to H2 and N2 was approximately 3.58. These permeances were decreased for the silica membrane prepared for hydrogen separation when the dip coating, drying and calcination was applied 7 times instead of 3 times as in the case of the hydrogen reaction membrane. The silica membrane for H2 separation provides permeances of about 5.8 x 10-9 mole.meter-2.second-l.pascarl (mol!m2.s.pa) for H2 and 9.4 x 10-10 mol/m2.s.pa for N2, with higher H21N2 selectivity of about 8. Higher mixed gas separation factors of H2:N2 > 400 and H2 permeance of 4.1 x 10-9 ol!m2.s.pa were achieved with silica membrane for H2 purification prepared with the modified dip-coating using suction technique with silicone elastomer as precursor. This technique was especially effective in plugging the macroporous support which possessed a wide pore size distribution. The membrane permeated gases except propane (C3Hg) by the activated diffusion mechanism at permeation temperature range of 298 -573 K, and the activation energies are in the order of 10.6 - 13 kilojoules I mole (kJ/mol) and 26.1-28.7 kJ/mol for H2 and N2 respectively. The tests have demonstrated that this composite membrane has the capability to separate hydrogen from gas mixtures with almost complete H2 selectivity and to produce high purity H2 (up to 99.0 %) from a 50 I 50 % H21N2 mixture stream. A theoretical model for a propane dehydrogenation reaction scheme in tubular and annular membrane reactors is developed. This model is applied to three different membranes namely: a silica-alumina membrane, a silica-y-alumina membrane and a Pd impregnated membrane. Results indicate that the Pd impregnated membrane provided very high theoretical conversions (82 % at 600°C) compared with the other two composite membranes.