Platinum Metals Rev., 1962, 6, (2), 47
Palladium Alloy Diffusion Process for Hydrogen Purification
First Commercial-Scale Plant in Operation
The development of a silver-palladium alloy having a hydrogen diffusion rate more than twice that of pure palladium, as well as a high degree of stability on heating and cooling in hydrogen, was described in Platinum Metals Review, 1960, 4, 130-131, by Dr J. B. Hunter, of J. Bishop – Co Platinum Works, Malvern, Pennsylvania. In the course of this article the author also outlined the design of a range of diffusion cells based upon this alloy suitable for laboratory, pilot plant and full-scale operation in the separation and purification of hydrogen. Since that time further design and engineering work have been carried out, and a number of relatively small-scale units have been put into service. Now the first commercial-scale plant for volume production of high purity hydrogen has been put on stream. This is at the Los Angeles plant of the National Cylinder Gas Division of the Chemetron Corporation, where the installation is producing 115,000 cubic feet a day of hydrogen with an impurity level as low as 0. 1 part per million.
The basic unit of the system consists of a bundle of thin-walled small diameter silver-palladium alloy tubes, manifolded together into a header, and assembled into an outer envelope. This construction provides a thin membrane for diffusion, coupled with high surface area in a small volume and adequate mechanical strength. Multiples of this unit can of course be assembled to provide the throughput required.
In the National Cylinder Gas plant the feed hydrogen, more than 99.7 per cent pure, is preheated to around 3I5°C, while the diffusion cells are maintained at the same temperature. The diffusion rate depends upon the partial pressure of hydrogen in the gas stream and the rate of circulation of the feed gas. The temperature of around 315°C is the minimum at which the rate of diffusion through the alloy becomes a practical proposition, but the hydrogen pressure differential across the alloy tube walls and the rate of bleed-off from the cells are factors which influence the economics of operating a diffusion unit.
The National Cylinder Gas installation employs electrolytic hydrogen, the 0.3 per cent of impurities consisting primarily of oxygen, with some nitrogen, methane and water. Hydrogen produced from hydrocarbon reforming processes would of course contain also carbon monoxide and carbon dioxide, but these would not affect performance. The only impurities that do in fact interfere with diffusion are unsaturated hydrocarbons and sulphur, but the former can be removed by a hot air purge of the cells, while the latter can readily be removed by normal methods before being allowed to reach the diffusion unit.
The factors that led National Cylinder Gas to install their diffusion plant were mainly the safety, simplicity and ease of operation of the Bishop system. The unit has no moving parts, and the capital cost represents almost the total expense. Control of the process is simple, consisting only of analysis of the output stream.