(487z) Effect of Reactant and Product Gases On Membrane Flux During Methanol Steam Reforming in a Packed Bed Membrane Reactor: Surface Coverage Effects

Methanol steam reforming has been widely studied in packed bed membrane reactors for its utility in the generation of high purity hydrogen for fuel cells[1, 2]. The reactants and products of this reaction (methanol, steam, carbon dioxide, and carbon monoxide) may reduce the H2 flux through the membrane. The objective of the current study is to understand the causes of this decrease in flux due to these membrane contaminants. A 3.4 micron thick Pd-Ag (23 wt% Ag) nanopore membrane was used in a membrane separator apparatus [3, 4]. Initially pure H2 was introduced into the shell side of a membrane permeation apparatus containing the Pd-Ag membrane and the flux through the membrane was measured for temperatures between 225 ? 300 oC and retentate pressures of 3 and 5 bars with the permeate pressure at 1 bar. Then various concentrations of methanol, water, carbon dioxide and carbon monoxide were introduced along with the H2. The decrease in H2 flux was noted for each case. The three main causes for this decrease are attributed to the lowering of the retentate side hydrogen partial pressure, concentration polarization [4] and surface coverage effects (i.e. decrease in effective surface area of the membrane). A Langmuir adsorption model describing the coverage of the contaminant on the Pd membrane was incorporated into a 2-dimensional membrane permeator model. This model along with the experimental results was used to estimate the coefficients of adsorption of the various gases on the membrane surface. These coefficients were verified using independent experimental data. The coefficients were then used in a model of a packed bed membrane reactor carrying out methanol steam reforming. The findings of this study are used to evaluate the potential of a methanol membrane reformer for coupled hydrogen generation and purification.

References

1. Harold, M.P., B. Nair, and G. Kolios, ?Hydrogen Generation in a Pd Membrane Fuel Processor: Assessment of Methanol-Based Reaction Systems,? Chemical Engineering Science, 58, 2551-2571 (2003).

2. Lattner, J.R., and M.P. Harold, ?Comparison of Methanol Based Fuel Processors for PEM Fuel Cell Systems,? Appl. Catalysis B. Environmental, 56, 149-169 (2005).

3. Nair, B., and M.P. Harold, ?Pd Encapsulated and Nanopore Hollow Fiber Membranes: Synthesis and Permeation Studies,? J. Membrane Sci., 290, 182-195 (2007).

4. Nair, B., and M.P. Harold, ?Experiments and Modeling of Transport in Composite Pd and Pd/Ag Coated Alumina Hollow Fibers,? J. Membrane Sci , 311, 53-67 (2008).