Machine Design

It's a gas, gas, gas

Gas generators create hydrogen and nitrogen from water and compressed air.

By Brian Keith
Global Product Manager
Parker Hannifin Corp.
Filtration and Separation Div.
Tewksbury, Mass.

Edited by Victoria Reitz

Nitrogen generators replace bulky and costly nitrogen cylinders.

Hydrogen generators from Parker Hannifin eliminate the need for dangerous cylinders.

While fast gases like oxygen, carbon dioxide, and water vapor quickly permeate the membrane, most of the nitrogen flows along the membrane fiber in a separate stream.

Pressure-swing adsorption gas separation adsorbs oxygen over nitrogen using a carbon-molecular sieve (CMS).

Gas cylinders and dewars are becoming relics thanks to the advent of gas-generation systems. These generators turn compressed gas into nitrogen, and deionized water into hydrogen gas. Over the last several years, advances in technology have significantly increased use of these on-site gas generators. Membranes, specialized adsorbents and catalysts, improved air compressors, and enhanced electronic controls have fueled this wave of progress.

Gas generators provide consistent, reliable, on-site supplies of gases that are instantly ready for use. And since gas generators eliminate the need to store gases, they also do away with injuries caused by lifting and dropping heavy metal cylinders. Doing away with cylinders also eliminates the costs of maintenance, shipping, returning, and storing them. There is also no need to keep hydrogen generators away from oxidizing gases or to locate generators in specially constructed areas.

Nitrogen generators use either membrane or pressure-swing-adsorption (PSA) technologies. Membrane generators made by Balston, a division of Parker, produce up to 99.5% pure, commercially sterile nitrogen at dewpoints to –58°F from compressed air. They use proprietary membrane-separation technology to divide air into two streams: one 95 to 99.5% pure nitrogen, the other oxygen-rich with carbon dioxide and other trace gases.

Compressed air flows through bundles of individual, hollow fibers made of semipermeable membranes. Each fiber has a perfectly circular cross section and a uniform bore through its center. Because the fibers are so small, a great many can be packed into a limited space, providing enough surface area to produce a relatively high-volume flow of nitrogen.

Oxygen, water vapor, and other trace gases permeate the membrane fiber and are discharged through a permeate port. Nitrogen stays within the hollow fibers and flows through an outlet port.

Balston PSA nitrogen generators produce up to 99.95% pure, compressed nitrogen at dewpoints to –40°F from nearly any compressed-air supply. The generators continually transform air into nitrogen at safe, regulated pressures without operator attention.

PSA generators use a combination of filtration and PSA technologies to create nitrogen. Compressed air is prefiltered to remove all contaminants down to 0.1 micron. The gas separation process preferentially adsorbs oxygen over nitrogen using carbon-molecular sieve (CMS). At high pressures, the CMS has a greater affinity for oxygen, carbon dioxide, and water vapor than it does at low pressures. By raising and lowering pressure within the

CMS bed, all contaminants are captured and released, leaving the CMS unchanged. This process lets nitrogen pass through. Depressurizing the CMS releases adsorbed oxygen and other contaminant gases to the atmosphere.

Generators can also make hydrogen using one of two basic technologies: proton-exchange membranes (PEM), and palladium membranes. Hydrogen gas generators are most often used in gas chromatographs and in labs for hydrogenation reactions and engine-emission analysis.

PEM systems from Parker produce flows from 90 to 1200 cc/min of 99.99999% pure hydrogen at pressures from 0 to 100 psig. The Balston H2-500 hydrogen generator uses deionized water and electricity. It produces hydrogen gas through electrolytic dissociation of water via the PEM. The resultant hydrogen stream then passes through a palladium membrane for purification. Because of the membrane's small pore size, only hydrogen and its isotopes penetrate it. A membrane thus keeps the hydrogen gas at a purity level of two orders of magnitude greater than desiccant or silica gel-based systems.

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