Separation of hydrogen from an H-2-H2O-HBr system with an SiO2 membrane formed in macropores of an alpha-alumina support tube

A porous alpha-alumina tube of 2.5 mm o.d. and 1.9 mm i.d. was used as the support of a silica membrane used for hydrogen separation at high temperature. The pore structures of the tube were as follows:size distribution, 110-180 nm; average size 150 nm; and porosity, 0.4-0.55. Macropores of the tube were plugged with silica formed by thermal decomposition of tetraethylorthosilicate at 600 degrees C. To improve the step coverage of the deposition in residual pinholes, the reactant was continuously evacuated through the porous wall of the support. The hydrogen permeation of the membrane formed was of the order of 10(-8) mol m(-2) s(-1) Pa-1 at 600 degrees C, while the nitrogen permeance was below 10(-11) mol m(-2) s(-1) Pa-1. The membrane was applied to separate hydrogen in the presence of HBr and abundant steam at 200-400 degrees C in a thermochemical water decomposition process (UT-3 process). The permeance of H-2 from the H-2-H2O-HBr mixture was nearly the same as obtained with pure H-2 and that of H2O was smaller than the detection limit in the present study, 10(-10) mol m(-2) s(-1) Pa-1 . This means that the permselectivity of hydrogen to water was at least 100. HBr molecules could not permeate the membrane because of their large size and the permeance was below 10(-12) mol m(-2) s(-1) Pa-1. The membrane was durable in the H-2-H2O-HBr atmosphere at 400 degrees C.