Development of Pd Alloy Composite Membranes for Small Scale Hydrogen Generation

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 increase the hydrogen yield and reduce the capital and operating costs of producing hydrogen, and consequently, reduce the price of hydrogen 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. 

Pall Corporation is actively involved in commercial development of Pd-alloy composite membrane technology for hydrogen separation and production.  Central to Pall’s Pd-alloy composite membrane technology is the development of an appropriate substrate material consisting of a porous metal tube coated with a homogeneous fine pore size ceramic diffusion barrier coating that enables deposition of thin yet defect-free Pd-alloy films.  Welding non-porous metal tubes to the porous metal substrate tubes addresses the membrane sealing issues and allows fabrication of large-scale modules using conventional tube-sheet and shell and tube vessel manufacturing technology. 

Pall Corporation has different techniques available for preparing Pd-alloy composite membranes and has the ability to vary alloy composition as well as membrane thickness as desired.  We are currently producing the Pd-alloy composite membrane elements of up to 12” in length which have undergone extensive in-house testing and are also undergoing trials/tests at customer sites.  For a Pd-alloy composite membrane of ~ 3 micron thickness, the typical pure hydrogen flux rate at 20 psi hydrogen partial pressure differential and 400 ^oC membrane temperature is about 150 scfh/ft².  The typical H₂/Ar binary gas selectivity for the same membrane is about 10,000.  The Pd-alloy composite membranes have successfully been demonstrated in 500 hour duration exposure tests 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. 

This paper will specifically address recent experience with development of small multi-tubular membrane modules with 12” long membrane elements for hydrocarbon reformer-based small high-purity hydrogen sources needed for portable/backup power generation of up to 10 kW capacity.  Issues related to integration of the hydrogen purifier module in an overall reformer system will be discussed and membrane module performance test results observed in various test configurations with single and multi-tube modules will be presented.  This presentation will describe the current status of commercialization of these membrane modules.

Scale-up of the composite membranes to 1 m length would be needed for supporting large scale hydrogen production applications.  Pall Corporation currently produces the base support porous metal tubes in lengths up to 8’ and has the capability and future plans of large scale production of the Pd-alloy composite membrane elements and modules.  The composite membrane technology will reduce the capital cost of equipment required for large scale hydrogen production by combining the hydrogen generation and purification steps.  This paper will also present the techno-economic evaluation of the membrane separator or integrated membrane reactor process for large scale hydrogen production and approaches for meeting the hydrogen production cost targets.