Separation of hydrogen from a H₂ H₂ OHI gaseous mixture using a silica membrane

Abstract Hydrogen separation characteristics of silica membranes prepared by chemical‐vapor deposition (CVD)‐in an H 2 H 2 O HI gaseous mixture were evaluated for the application to hydrogen iodide decomposition in the thermochemical iodine–sulfer (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°C for S1, S2, S3, L1, and L2 membranes, respectively. Separation experiments of the H 2 H 2 OHI gaseous mixture (a molar composition of 0.09:0.78:0.13) in the modified membranes were carried out at 300–600°C. Hydrogen permeance was almost the same as the single‐component permeance. The separation factors of H 2 H 2 O 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°C).