Preparation of a multi-membrane module for high-temperature hydrogen separation

Hydrogen permselective multi-membrane modules were prepared by simultaneously depositing silica using a counter-diffusion chemical vapor deposition (CVD) procedure. Mesoporous gamma-alumina layer prepared over alpha-alumina substrates were arranged inside a tubular casing and attached to the casing at either ends using glass sealing. Tetramethylorthosilicate and oxygen were allowed through either sides of the gamma-alumina layer at 873 K, which produced a deposit inside this layer thereby reducing the pore diameter. This module displayed an activated transport for hydrogen molecules while effectively filtered larger molecules like N₂, CH₄ and CO₂. Hydrogen permeation values increased from 1.57×10^-10 mol m^-2 s^-1 Pa^-1 at 373 K to 4.38×10^-8 mol m^-2 s^-1 Pa^-1 at 873 K, with activation energy (E_a) of 22.92 kJ mol^-1. The permeation values for N₂, CH₄ and CO₂ molecules remained below 10^-11 mol m^-2 s^-1 Pa^-1, yielding a selectivity ratio with H₂ of >2000, which indicates the superior quality of this module. The permeance values were also comparable with that obtained for single membranes prepared using a similar method. Hydrothermal stability of this module, when analyzed under membrane reactor conditions (H₂O/N₂ = 3; 76 kPa), was found to be stable for more than 15 h.