(582r) Membrane Reformers: Optimization of Catalysts and Membranes for Production of Ultra-Pure Hydrogen through Steam Reforming of Methanol

Membrane reformers couple reaction and separation in a single unit which makes them a promising tool to increase efficiency of many chemical processes. Selective removal of a reaction product through membrane in these devices enhances the per-pass conversion for equilibrium limited reactions such as dehydrogenation compared to conventional fixed bed reactors. Therefore, in order to generate and separate high purity hydrogen, on-board membrane reformer is considered a promising technology. Further, as the name suggests, reforming catalysts and membranes are the two intertwined constituents which control the performance of this system. A catalyst with low CO selectivity will enable higher hydrogen partial pressures near membrane. With higher hydrogen partial pressure difference across the membrane, permeation is also enhanced. Further, permeation of hydrogen through the membrane also maximizes the mass transfer rate of hydrogen from the catalyst by maintaining a concentration gradient. This work is therefore focused on the primary optimization of varying catalysts and membrane performance prior to their integration in a single unit.

A light weight and high energy density liquid fuel such as methanol is easy to store and can be reformed to produce hydrogen rich gas at temperatures as low as 250°C. Catalysts for stable methanol reforming activity were synthesized with Cu as an essential element supported on various mixed oxides. Catalyst testing was performed with varying operational parameters such as weight hour space velocity (WHSV, kg_cat/mol_methanol.s)), temperature and steam-to-methanol (S/M) ratio. Further, active metal% and bi-metal compositions were varied to study the product selectivity for the optimal reaction. The obtained data was also fitted using different rate equations to identify the most probable reaction mechanism.

In addition to catalysts, separation studies were performed with membranes synthesized with a series of modified electroless deposition-hydrogen heat treatment steps. This was continued until a non-porous palladium-based film was achieved on asymmetric tubular support. A non-porous Pd-film morphology allows only hydrogen to permeate through it according to solution-diffusion mechanism. With support total surface area of 28.3 cm², membrane deposition was carried out for three combinations a) pure Pd, b) 90%Pd-10%Ag and c) 90%Pd-8%Ag-2%Au. Surface characterizations were performed with FESEM-EDX and AFM. Further, the prepared membranes were tested with simulated compositions of reformate to determine membrane perm-selectivity.

The studies illustrated in this work will thereby present a complete insight to individual membrane and catalyst performances. Lastly, with reference to the prior art, this work will also present the feasibility of the optimal catalyst and membrane integration using a dimensionless DaPe number. DaPe is the product of Damkohler and Peclet number which provides the ratio of maximum reaction rate per unit volume over maximum permeation rate per unit volume, a defining feature of a membrane reactor.