Long term performance cobalt oxide silica membrane module for high temperature H2 separation

Abstract

Here we show the long term performance at high temperatures of a multi-tube module containing 8 membranes in 4 parallel lines with a total of 545 cm² area. The membranes were prepared via thin film dip coating of cobalt oxide silica (CoO_xSi) sol–gel on tubular alumina supports. A preliminary study found that the sol–gel containing 20 mol% cobalt oxide formed the best microporous structure with the highest surface area and pore volume. All resulting membranes delivered permeances of ∼1 × 10⁻⁷ mol m⁻² s⁻¹Pa⁻¹ at 500 °C, indicating a high repeatability for the membrane fabrication process. The permselectivities of helium (He) and hydrogen (H₂) over carbon dioxide (CO₂) and nitrogen (N₂) increased from 10–20 at 100 °C to values close to 1000 at 500 °C. Additionally, the apparent energies of activation (E_act) for the smaller kinetic diameter gases He and H₂ at 12.2 and 19.5 kJ mol⁻¹ were high and contrary to the negative values for larger gases N₂ and CO₂ at −1.8 and −7.4 kJ mol⁻¹. These remarkable results were attributed to the molecular sieving mechanism of the microporous silica which was enhanced by the embedding of cobalt oxide into the matrix, delivering structural control with an average pore size of 3 Å. The E_act for H₂ permeance was higher than that of He, indicating that the cobalt oxide played an important role in H₂transport. Two membrane lines performed exceptionally well for binary gas mixture processing with H₂ purity reaching values close to 100% in the permeate stream for argon (Ar) concentrations of up to 80% in the retentate stream. A major finding here is that the binary gas selectivity was independent of temperature, contrary to the permselectivity observed for single gas permeance. Further, the H₂ flow rate was greatly affected by the concentration of Ar in the mixture, while the temperature dependency played only a marginal role. In particular, competitive adsorption in the percolative pathways containing pore constrictions or bottlenecks of the anisotropic CoO_xSi matrix allowed Ar to impede H₂ diffusion. Finally, the CoO_xSi membranes proved thermally stable and robust for 2000 h of testing for various thermal cycles up to 500 °C.