Stable multi-membrane module prepared by counter diffusion CVD for high temperature hydrogen separation

Porous inorganic membranes are widely considered over polymeric membranes for their resistance to physical and chemical degradation. Last decade showed much interest towards the development of hydrogen selective inorganic membranes with considerable attention focused towards silica membranes. Silica membranes are usually in the form of silica layers placed over rigid and porous ceramic supports such as Vycor glass, alumina, etc., using sol-gel and chemical vapor deposition (CVD). The hydrogen permeance values of these membranes are reported over a wide range (10^-7 to 10^-9 mol m^-2 s ^-1 Pa^-1) and the selectivity for nitrogen ranges up to 3000. In the CVD process, suitable precursors were allowed to react near the supporting mesoporous/microporous substrate to produce a solid deposit inside/over these substrates resulting in complete or partial pore plugging. Two types of configurations are possible here; one is the one-sided geometry in which the reactants are made to react in the gas phase or near the surface of the substrate, producing an even deposit on the surface of the porous substrate. The second type is the opposing reactant geometry or counter diffusion CVD in which the reactants are fed through opposite sides of a porous support, which reacts inside the pores to form a deposit on the pore wall. This reduces the pore size and finally the reaction stops automatically when the reactants could no longer pass through these narrow pores. Thus a homogeneous micropore size distribution could be achieved using this method. Few authors have already reported the preparation and properties of singular membranes using the above method. In this presentation, we will explain the fabrication and permeation properties of a multi-membrane module prepared by counter diffusion CVD method using tetramethylorthosilicate (TMOS) and O₂ as precursors. In this process, the reaction takes place simultaneously inside the supports arranged parallel to each other. Single component gas permeation studies were performed to evaluate this multi-membrane module. Permeation of larger molecules did not vary much within the temperature range of 373-873K. Activated transport displayed by the hydrogen molecules in the above range (Ea=22.9 kJ/mol) points to the narrow nature of the membrane pores. H2 permeance through this module was 5.02 × 10^-8 mol m^-2 s^-1 Pa^-1 yielding very high selectivity ratio with N₂ (>1000). This module also maintained a steady permeance ratio of 500 at 76 kPa of steam at 773K.