TECHNOLOGY
NCG Purifies Hydrogen by Diffusion National Cylinder Gas is now operating palladium alloy diffusion process commercially at its Los Angeles plant Volume purification of hydrogen has been achieved using the palladium alloy diffusion process. This process, one of the simplest and safest known methods of reducing impurities in hydrogen to below V 1 0 p.p.m., is now putting out more than 115,000 cu. ft. per day at Chemetron's National Cylinder Gas plant in Los Angeles. The palladium alloy diffusion process is not new. Many companies—Milton Roy and Englehard Industries, among them—supply lab and semicommercial units from stock. And many have announced capability to install commercial units. Chemetron's Los Angeles unit, however, is the first to go on a commercial scale. The process works because hydrogen diffuses through drawn palladium metal, unlike other gases. Theory as to just how and why this happens is still incomplete. Nobody really knows, for instance, whether the process is ionic or molecular.
The basic unit of the system is a patented tube made of an alloy of palladium and silver by J. Bishop & Co. Platinum Works, of Malvern, Pa. Bundles of these tubes, baffled and with one end closed are manifolded into a header with the closed ends inside. Hydrogen, more than 99.7% pure, is preheated to 600° F., and a heat transfer medium keeps the bundle at 600° F. or above. The diffusion rate through the platinum depends on the hydrogen partial pressure in the gas stream. Thus diffusion depends on the system pressure, percentage of hydrogen, and rate of circulation of the electrolytic hydrogen. Temperature has to be kept above 600° F.—the temperature at which diffusion rate becomes practical. The other factors—pressure, differential across the membrane, and circulation rate—are matters of economics, according to G. G. Pinney of NCG.
PURE GAS. Chemetron uses palladium alloy diffusion cells, such as the one shown here, to purify more than 115,000 cu. ft. of hydrogen per day 48
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Since the NCG installation uses electrolytic hydrogen, the 0.3% impurities are primarily oxygen and some nitrogen, methane, and water. Probably a greater volume of hydrogen is produced from refinery streams, so more-typical impurities would include methane, carbon monoxide, and carbon dioxide. In small scale operations, NCG has reduced impurities in such gases below detectable limits, Mr. Pinney says. Unsaturated hydrocarbons and sulfur are the only constituents of a gas stream that can interfere with diffusion. These can blanket the palladium alloy surface, apparently reducing its porosity to hydrogen. They aren't permanent poisons, however, and can be swept away by an air wash. The nature of a high-hydrogen stream makes olefins unlikely. Besides, both olefins and sulfur can be removed by standard methods before gas reaches the diffusion process. The deciding factors that led NCG to the diffusion process are its safety, simplicity, and ease of operation. The unit itself has no moving parts which could wear. And capital costs represent almost its total expense. Control, which is by analysis of the vented stream, is simple. Thus operation requires only periodic attention and there is almost no added operational expense. Safety is inherent in the process. Since palladium catalyzes water formation, oxygen can't build up to explosive limits, but combines with hydrogen. NCG has other commercial installations under way, but hasn't said where, as yet. It also has not decided whether purification equipment will become a standard part of its line. This depends on how well the equipment fits with its present line of manifolds, regulators, and other items supplied to gas users. Also, Bishop has licensed several companies to act as re-sellers of its basic silverpalladium tube, so the emerging competitive situation is not clear as yet.
