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In contrast to high-pressure gas cylinders, in-house gas generators use filters to extract the target gas from lab air or water. Whether the application is gas chromatography, liquid chromatography, mass spectroscopy, or thermal analysis, lab gas generators are a safer, more convenient, and less expensive surrogate to gas cylinders. Selecting the most appropriate lab gas generator amongst the many manufacturers and models can be difficult (Table 1). This article describes the benefits of using in-house gas generation (i.e., easier, safer, less expensive), illustrates the process by which several gases are extracted (i.e., nitrogen, hydrogen, ultra dry air, purge gas, zero air, and exhaust air), and discusses key purchasing considerations for matching the best lab gas generator to the end-user’s needs.
Table 1 - Lab gas generator manufacturers
- Air Liquide (www.airliquide.com)
- Dionex Corp. (www.dionex.com)
- LABREPCO (www.labrepco.com)
- Matheson Gas (www.mathesongas.com)
- Neu-Tec Group, Inc. (www.neutecgroup.com)
- Parker Balston (www.labgasgenerators.com)
- Parker Domnick Hunter (www.parker.com)
- Peak Scientific (www.peakscientific.com)
- Proton (www.protononsite.com)
- Restek (www.restek.com)
Benefits of lab gas generators
The three primary benefits of producing in-house gas using a lab gas generator are safety, convenience, and cost. Depending on the application and the lab environment, another benefit may include better use of limited lab space.
Safety
Transporting, handling, and securing high-pressure gas cylinders can be dangerous if safety precautions are not observed. Should the cylinder be mishandled or dropped, it is possible that extensive property damage or worse, serious personal injury, may occur. Many laboratories and research institutions have strict regulations for the storage of high-pressure gas cylinders such as not storing cylinders near exits or exit routes, securing cylinders with a chain or belt, and not storing cylinders near flammable or combustible substances.
Unlike gas from a cylinder, a lab gas generator operates at a much lower pressure and volume. A typical gas cylinder contains a large quantity of gas at high pressure (0.44 L to 126.3 L at about 2000 psi), creating a number of potentially dangerous scenarios. If a leak occurs, a large volume of toxic gas could be released into the laboratory, displacing the air, and potentially asphyxiating those working within. Lab gas generators generally have a much lower maximum output (600 to 1200 mL/min at 100 psi) and therefore would present a much lower hazard in the event of a leak in the system.
Convenience
Invariably, the use of high-pressure gas cylinders will create unplanned and potentially costly down time due to an empty cylinder. Dependence on gas vendors can lead to other inconveniences such as long-term contracts, price fluctuations, delayed delivery or incompatible delivery scheduling, or valve and regulator rental fees. Using an in-house gas generator can eliminate these inconveniences by providing the necessary gas at the pressure and flow required on a 24 hr a day/7 days a week basis with little to no operator interaction necessary. By extracting the necessary gas from the lab air or water supply, gas generators provide automatic, reliable, and relatively inexpensive gas, thereby eliminating the reliance on an external delivery service.
Cost
The cost of purchasing and operating a gas generator can be offset in less than one year after purchase, depending on the application and workload. The upfront, operation, and maintenance cost of in-house gas generation is generally much less than the cost of ordering, storing, and changing high-pressure gas cylinders.
Lab gas on demand
In-house gas generators are available for individual gases, or for multiple gases, as needed to support the desired system. For example, a tri-gas generator, such as the SOURCE LCMS-5000NA TriGas Generator (Parker Balston, Haverhill, MA), can generate pure nitrogen, zero air, and source exhaust air from compressed air.
Atmospheric air contains approximately 78% nitrogen, 21% oxygen, and 1% trace elements and molecules.1 Before extracting the target gas, dust and particulate matter are prefiltered from compressed air. Nitrogen can then be selectively filtered from the air using a series of hollow membrane fibers. Pressure, flow rate, and membrane size and quality are the primary variables determining the nitrogen production. Controlling the flow rate of compressed air through the filter will alter the nitrogen purity, with a faster flow rate, increasing the oxygen content of the filtered product. If high-flow, high-purity nitrogen is needed (e.g., to supply total organic carbon analyzers), pressure swing adsorption (PSA) systems can be used. PSA systems use molecular sieves of activated carbon to absorb oxygen leaving nitrogen gas approaching 99.9% pure.2
The in-house generation of hydrogen gas is accomplished by the electrolysis of water using either a metallic electrode or an ionomeric proton exchange membrane (PEM). The resulting hydrogen gas then passes through a series of membranes and desiccants to further purify and dry the gas. When metallic electrodes are used, a strong water soluble electrolyte and a bundle of palladium tubes (cathode) are used. Oxygen and other impurities are collected on the anode, leaving hydrogen gas with purity in excess of 99.99999%.3 PEMs conduct hydrogen ions (protons) and are impermeable to hydrogen and oxygen. In the presence of water and the correct electrical potential, PEMs dissociate water to provide hydrogen and oxygen.
Ultra dry air, purge gas, zero air, and exhaust air generation
A coalescing compressed air filter can remove water, hydrocarbon lubricants, and synthetic lubricants from compressed air. Typically these filters are composed of borosilicate glass hollow fibers in a fluorocarbon resin binder. There are many techniques used to remove moisture, particulate matter, and hydrocarbons from air. Typically, a two-stage filtration removes 99.99% of particulate matter and droplets, whereas a three-stage filter can remove compressor oil vapor.4-6
Key purchasing considerations
Before purchasing an in-house gas generator, it is important to consider the intended application and the corresponding system requirements. Table 2 outlines several factors to consider when selecting a lab gas generator.
Table 2 - Lab gas generator specifications and considerations
- Cost
- Ease of use
- Filtration detection method
- Flow rate
- Inlet pressure
- Purity
- Reliability/ruggedness/customer service
- Size, weight, and footprint
- Standards and certifications
- Temperature range
Defining requirements
- Flow rate: Determining the optimum flow rate will be application specific. Always consult with the gas-supplied equipment manufacturer for the ideal operating flow rate. Adjusting the flow rate may affect the purity of the generated gas, as described in the nitrogen generation section.
- Minimum and maximum inlet pressure: Depending on the facilities available, or the application, operators may prefer a lab gas generator with or without a built-in compressed air supply. If using an in-house compressed gas supply, ensure the pressure is within the minimum and maximum inlet pressure of the lab gas generator.
- Dimensions: Floor or bench space is often at a premium in the laboratory. Minimizing equipment footprint in the lab is critical to efficient use of limited laboratory space. Lab gas generators come in multiple shapes and sizes, including compact designs such as the Domnick Hunter Scientific (Haverhill, MA) or the Peak Scientific (Billerica, MA) models.
- Maintenance: Although operational maintenance is typically minimal, lab gas generation equipment must be regularly maintained to ensure that it is delivering the required quality and operational costs are kept to a minimum. Therefore it is essential that the manufacturer’s recommended maintenance instructions are followed. A comprehensive maintenance package from the manufacturer is highly recommended.
Validation
On-site testing is often difficult due to the complexity of the test and the availability of test equipment. For this reason, each lab gas generator should be tested against traceable standards, either by the manufactureror by an independent third party. Validation documentation should accompany the lab gas generator upon delivery or request.
Conclusion
The in-house production of gases is a safer, more convenient, and more cost-effective method of providing required gases to the laboratory. Using the constant supply of laboratory air and water, lab gas generation provides gases on a 24 hr a day/7 days a week basis, eliminating down-time lags associated with the dependence on cylinder-based gas sourcing. In addition to matching the correct specifications to the lab gas needs, buyers should consider the reputation of the manufacturer, customer service, and availability of replacement parts or consumables.
References
- CRC Handbook of Chemistry and Physics, 92nd ed. CRC Press: Boca Raton, FL, 2011.
- Nitrogen Generators with Research Grade Purity; available at: www.labgasgenerators.com/Docs/Products/Balston%20_AGS_H_Catalog_Nitrogen_Generators_Section.pdf; accessed April 18, 2012.
- Hydrogen Generators for Fuel Gas; available at: www.parker.com/literature/Balston%20Filter/AGS/AGS%20Catalogs/PDFs/Balston_An alytical_Gas_Systems_Catalog_Hydrogen_Generators_Section.pdf; accessed April 18, 2012.
- Ultra Dry Gas Generator; available at: www.labgas.com.au/linked/parker%20balston%20nmr%20dry%20air%20generator.pdf; accessed April 18, 2012.
- FT-IR Purge Gas Generators; available at: www.labgasgenerators.com/Docs/Products /Balston_AGS_H_Catalog_FTIR_Gas_Generators_Section.pdf; accessed April 18, 2012.
- Zero Air Gas Generators ZA015-ZA300; available at: www.peakscientific.com/media/za_leaflet_us_copy1.pdf; accessed April 18, 2012.
T. Keith Brock, BS, is a Contributing Writer; e-mail: [email protected]