(620d) Development of Pd-Alloy Composite Membranes for Hydrogen Separation

Economical generation of pure hydrogen represents a critical technology component for power generation by PEM fuel cells in a variety of mobile and stationary power applications. Hydrogen is conventionally produced by steam reforming of hydrocarbon fuels followed by a two-step water gas shift (WGS) reaction and hydrogen separation and purification by pressure swing adsorption (PSA). Combining the WGS reactor and hydrogen purification steps into a single membrane reactor, has the potential to significantly reduce the capital and operating costs of producing hydrogen, and consequently, reduce the price of hydrogen to the consumer. Due to simultaneous reaction and product separation, it is possible to increase the efficiency of the overall process by shifting the equilibrium and thereby producing more hydrogen than would be possible using the conventional approach, providing additional benefit to the consumer. High performance, high-temperature hydrogen separation membranes thus represent a key enabling technology for efficient hydrogen production using synthesis gas derived from a variety of feedstocks.

Palladium-alloy foils and extruded tubes are known to be completely selective for hydrogen separation, however, they are a relatively expensive option for large scale industrial applications. These product formats exhibit low hydrogen flux rates due to the thickness necessary for structural stability. By depositing thin palladium-alloy film on a porous substrate, hydrogen flux and structural stability of the composite Pd-alloy membrane is increased while reducing the membrane costs. Both the thin metal film deposition process and the porous substrate characteristics influence development of successful composite membranes for hydrogen separation application. In a U.S. DOE sponsored program, Pall Corporation has successfully developed high flux and selectivity hydrogen separation membranes by depositing thin Pd-alloy films on tubular ceramic/porous stainless steel composite (AccuSep® Inorganic media) substrates. Both the plating parameters as well as AccuSep® Inorganic media substrate characteristics have been optimized for preparing thin, defect-free Pd-alloy membranes with high hydrogen selectivity and membrane flux. These composite membranes have demonstrated performance exceeding U.S. DOE's year 2010 hydrogen flux target with structural stability against thermal cycling necessary for their commercial application.

The composite membrane synthesis process is currently being scaled-up to 30? long tubular elements. The tubular membranes are fabricated with non-porous tube sections welded at each end for ease of using and sealing with conventional metal fittings. In addition, the non-porous tube sections may directly be welded to a tubesheet for fabricating multi-tubular membrane modules.

This paper will present experimentally observed membrane test results in various test configurations and will describe the current status of commercialization of these membranes. The Pd-alloy composite membranes have successfully been demonstrated in a 500 hour duration exposure test in synthesis gas environment as well as in multiple rapid thermal cycle tests with stable performance with respect to high hydrogen flux and selectivity over time. The effect of syngas mixtures on the durability of a palladium alloy membrane will be presented. A simulated syngas composition typical of that from a ethanol reformer with a WGS reactor is used in these tests. The specific conditions include a gas mixture of 50% H2, 1% CO, 30% CO2, 19% H2O at 400 oC and pressures from 20 to 75 psig. Results of experiments designed to measure the effects of concentration polarization will also be reported.

The composite membrane technology will reduce the capital cost of equipment required for hydrogen production by combining the hydrogen generation and purification steps. The U.S. DOE has set year 2010 targets of 250 scfh/ft2 for pure hydrogen flux (400 °C, 20 psig pressure difference) and $1000/ft2 for membrane module cost. Combining these goals into a single goal of $4/scfh of hydrogen capacity sets a relevant, challenging goal for commercialization. This paper will present the techno-economic evaluation of the membrane reactor process for hydrogen production and approaches for meeting the hydrogen production cost target.