Separation of hydrogen from a H2-H2O-HI gaseous mixture using a silica membrane

Hydrogen separation characteristics of silica membranes prepared by chemical-vapor deposition (CVD)-in an H-2-H2O-HI gaseous mixture were evaluated for the application to hydrogen iodide decomposition in the thermochemical iodine-sulfur (IS) process. Porous alumina tubes with a pore size of 10 nm and different gas-permeating-portion lengths [20 mm (S membranes) and 100 mm (L membranes)] were modified by CVD using tetraethoxysilane as the Si source. Pore closure was controlled by CVD to maintain high hydrogen permeance while lowering only HI permeance. At a certain point, CVD was stopped using He/N-2 selectivity as the indicator of pore closure. H-2/N-2 selectivities of the modified membranes, which were measured by a single-component permeation experiment, showed 53, 9.2, 4.1, 135, and 6.6 at 600 degrees C for S1, S2, S3, L1, and L2 membranes, respectively. Separation experiments of the H-2-H2O-HI gaseous mixture (a molar composition of 0.09:0.78:0.13) in the modified membranes were carried out at 300-600 degrees C. Hydrogen permeance was almost the same as the single-component permeance. The separation factors of H-2-H2O and H-2-HI were over 3 and 150, respectively. The S3 membrane showed the highest hydrogen permeance (on the order of 10(-7) mol.Pa-1.m(-2).s(-1)) and the highest separation factor of H-2-HI (650 at 450 degrees C).