Sources of Laboratory Hydrogen
Hydrogen Cylinders versus Hydrogen Generators
Hydrogen is gaining popularity for a variety of uses in the laboratory. It is an excellent cover gas for furnace brazing, and, when used as a carrier gas for gas chromatography, it increases speed, resolution, and sensitivity over helium, especially when used with FID. Options for point-of-use hydrogen sources for GC-FID are limited to hydrogen cylinders and hydrogen generators, with cylinder hydrogen as the most commonly used laboratory supply source, often aided by palladium membrane hydrogen purifiers. Hydrogen generators are gaining in popularity providing some safety and long-term cost savings at the expense of shorter-term capital costs. Understanding the benefits and drawbacks of each option helps in choosing the best hydrogen supply source for a laboratory system.
Cylinder Hydrogen Supply Options:
Cylinder hydrogen is the most commonly used supply source in laboratories for single GCs, and other low volume hydrogen applications. There are two different approaches in using hydrogen cylinders:
? Single, Point-of-use hydrogen cylinders. An individual hydrogen cylinder is used to supply the carrier/fuel gas to the application. It is generally belted to a wall, located next to each GC or other hydrogen application. This approach is most applicable to smaller laboratories with one or two hydrogen applications.
? Multi-cylinder configurations. Multiple hydrogen cylinders, often racked together, are connected to manifolds with a switchover system. This can be used to provide larger volumes of hydrogen gas for several GC's in larger laboratories, or for large braze furnaces, and larger-scale fuel cell tests. Gas Hydrogen supply is initiated from one side of the switchover system, and is switched automatically as the primary hydrogen source is depleted.
Cylinder Hydrogen Supply Benefits:
Easy pressure monitoring and control. Hydrogen cylinders provide the user with easy control of the gas delivery pressure. Good GC performance requires that the hydrogen delivery pressure must be constant. Similarly, furnace applications require the hydrogen pressure to be maintained in a small range. Cylinder hydrogen provides this control reliably through the use of cheap, mechanical pressure regulators. Point-of-use cylinders are particularly good in this regard: the cylinder can be located quite near to the GC or other application, and this minimizes the pressure drop in the supply line. Further, the proximity of the gas source to the application, delivery pressure is easy monitored and adjusted.
Fewer interruptions in gas supply. Multi-cylinder configurations used with switchover systems provide a more continuous supply of gas, avoiding the most damaging problems that typically occur during cylinder changeout. GC operation is not interrupted during cylinder changes, as changeout of the primary cylinder is performed when while the secondary side of the switchover system is delivering gas. This more continuous hydrogen supply enhances reliability. It reduces the potential for running out of carrier or fuel gas and, avoids contamination of the system. As the hydrogen cylinder packs are located further from the GC or application, there is a somewhat greater tendency for the pressure to vary as the flow varies.
Cylinder Hydrogen Supply Concerns and Considerations:
Cylinder handling and storage. With point-of-use cylinder supply, the frequent changing of cylinders can be expected to interrupt the GC, furnace, or experiment. The act of changing cylinders can be difficult and time consuming, and the hydrogen bottles must be monitored continuously to ensure that gas does not run out. Multi-cylinder configurations with switchover systems are less disruptive to the laboratory operation, but are also more expensive in terms of capital cost, cylinder rental, and manpower. Generally, they require a trained technician to trouble-shoot the switchover and handle multiple cylinder changes. For both the point-of-use and multi-cylinder configurations, cylinder changeouts increase the risk of fire or the introduction air contaminants.
Safety. The flammable nature of hydrogen and the high pressure of cylinders (2400 psig) present concerns for cylinder storage and handling in laboratories, particularly when multi-cylinder packs are used. A single leak on a multi-cylinder line can easily raise the hydrogen content in a laboratory to the explosive level (about 4.5%). In addition, the size and weight of hydrogen cylinders present hazards to personnel. Care must be taken in while handling cylinders during changeouts, and cylinders should be secured to the wall or bench top using appropriate cylinder holders and restraints. Because of these concerns, many labs are seeking ways to reduce or eliminate the use of hydrogen cylinders.
Product Quality Variations. Very high purity hydrogen is required for many laboratory applications. And even where lower purity (e.g. zero-grade) is acceptable, e.g. as a GC carrier gas, the contaminant level should be kept constant, and not allowed to vary as is typical from on hydrogen cylinder to another. Cylinder-to-cylinder variation may preclude sensitive GC analyses, as it introduces another variable into the operation. Similarly, high dew-point hydrogen cannot be used when brazing high-chrome refractory alloys.
Selection of Appropriate Gas Delivery Equipment to Ensure Hydrogen Purity. Oxygen and moisture cannot be prevented from entering the system during cylinder changes. To minimize the impact of these contaminants, high purity gas handling equipment should be used. Regulators should be of brass or stainless steel with stainless steel diaphragms and metal-to-metal seals. Stainless steel diaphragms do not adsorb and release as much oxygen or moisture into the system, and a metal-to-metal seal will minimize ambient air leakage into the regulator. To minimize diffusion of ambient contaminants into the system, stainless steel or copper tubing should be used from the cylinder to the GC. To further protect the system from oxygen and moisture, point-of-use purifiers should be installed in the hydrogen lines just prior to the GC or furnace to remove residual contaminants. These components add cost to the hydrogen supply system, but are essential for system performance.
Hydrogen Generator Supply Options:
High purity hydrogen generators are becoming more popular as hydrogen supply sources in laboratories. Generators are available in different hydrogen production capacities typically ranging from 150 ccm to 5 slpm: outputs that can supply single or multiple GC's, modest-sized braze furnaces, IC tool production, and laboratory-scale fuel cell testing. Hydrogen generators offer a number of safety, reliability, and convenience benefits to the user at the expense of higher initial capital cost. There are two main options for laboratory hydrogen generators.
? Electrolytic Hydrogen generators: Purified water, either purchased separately, or purified on-site is mixed with an electrolyte, generally KOH, and converted to hydrogen and oxygen by the electrolytic reaction H2Og H2 + ½ O2. As the hydrogen produced is generally "wet", containing water vapor, the hydrogen is then purified by use of a desiccant, or by passage through a metal membrane purifier.
? Reformer-based hydrogen generators: A methanol-water mix or ammonia is converted to a hydrogen-containing gas-mix at temperatures around 350 C and pressures about 300 psi. Pure hydrogen is generally produced from the mix by passing it through a membrane, either within the reactor (a membrane reactor), or a membrane purifier external to the reactor.
Hydrogen Generator Benefits:
Safety enhancements. Hydrogen gas generators offer significant safety benefits over gas cylinders. A generator output is typically low enough that the laboratory air can never build up to explosive levels. Further, the pressure of hydrogen in the generator is generally much lower than in cylinders, and the instantaneous volume is typically low. The use of hydrogen generators thus eliminates the need for personnel to handle large quantities of flammable, high-pressure gas. Further, unlike cylinders, most hydrogen generators can be fitted with alarm features to alert the user to operating problems, and most have automatic shut down capabilities that trigger if the unit malfunctions. All of these factors contribute greatly to the overall safety of in the lab.
Continuous gas supply. Once installed and operating properly, hydrogen generators provide a continuous supply of high purity gas. The need to change and store cylinders is eliminated, saving time and cost. One adds water or methanol-water as needed, and hydrogen is produced as long as there is electricity in the lab. Eliminating cylinder changeouts also reduces downtime due to interruptions in carrier or fuel gas supply, and minimizes the potential for ambient air contamination.
Consistent gas purity. Hydrogen generators often contain metal membranes, and thus will supply hydrogen at purity levels of 99.9999%. This purity level remains constant over time, eliminating gas purity as an operating variable in the analysis. This consistent purity provides reliability for the GC system. Electrolysis systems with only a desiccant to remove water vapor from the hydrogen should be used only where high hydrogen purity less important than high hydrogen pressure. Even with a fresh cartridge, this gas is quite impure. The purity decreases with time as the desiccant wears out, but this can be the best option if the delivery pressure must be above 100 psi.
Space savings. The relatively small size of hydrogen generators allows them to be conveniently located on the lab bench, without consuming a lot of valuable bench space. Eliminating the hydrogen cylinders can also free up a lot of valuable storage space in the lab.
Operating cost savings. The cost savings of on-site hydrogen generation accrue as the delivery volumes get larger. Hydrogen in cylinders cost, in Detroit where we are, about $60 for each cylinder low low-purity hydrogen gas: 135 scf of 99.98% pure. If you use this much every day, for example, you will find that you've spent about $20,000 every year on hydrogen gas, on top of the cost of cylinder rental and delivery. This is more than the cost of a comparable hydrogen generator plus the water or methanol and electricity run it. You also won't have to pay people to switch out cylinders as often, and you won't have the safety and zoning hazard associated with large quantities of explosive hydrogen on site. If you use less hydrogen per day, the cost savings accrue more-slowly; if you use more, they accrue more rapidly.
Hydrogen Generator Concerns and Considerations:
Delivery pressure limitations. It is important to be aware that most laboratory hydrogen generators are capable of supplying hydrogen at only 60-100 psig. This pressure limitation may be a problem for labs that use higher pressure gas, or when the generator must be located remotely from the equipment it supplies, or where multiple instruments are fed from one generator. The generator should be located as close as possible to the instrument that it supplies to minimize the effects of supply line pressure drop. Depending on the complexity of the lab and the number of GC's in use, it may pay to select several generators to provide with a broad delivery pressure range and a short distance between the hydrogen generator and the application.
High capital investment. Hydrogen generators typically cost from $5000-$15000 depending on the capacity. This relatively high front-end capital investment may be a consideration for some laboratories. However, it is important to remember that hydrogen purifiers are not much cheaper, and it is important to compare this one-time capital cost to the safety benefits and ongoing operating costs of cylinder hydrogen, cylinder rental, and cylinder handling. The prices for REB Research hydrogen generators can be found here; the prices for electrolysis-based hydrogen generators, here.
Maintenance required for optimal performance. Often electrolytic hydrogen generators require the addition of a caustic electrolyte solution or will require the monitoring and changing of desiccant cartridges to remove residual moisture from the hydrogen. Palladium membrane purifiers systems, and reformer systems similarly require that thermocouples and heaters be replaced every few years. If this maintenance is not performed, the generator will not operate properly, and reliability will suffer. Understanding the required operating and maintenance procedures is an important part of making an informed decision on generator type (or on buying a generator at all).
While cylinders are the most common hydrogen supply source for labs, they present a number of safety and handling concerns. The flammability of hydrogen and the high pressure of the cylinders pose storage and handling problems. The need to inventory and change cylinders regularly may also be inconvenient and costly. Many users are seeking ways to eliminate the hydrogen cylinders from their labs to improve safety and productivity.
To combat these concerns, many users are converting to hydrogen generators for their hydrogen supply. Generators tend to be more up-front expensive than cylinders while offering safety benefits as well as benefits of continuous supply and consistent purity. The safety, reliability, and convenience of hydrogen generators make them particularly attractive alternative for larger labs where large hydrogen supply can present larger safety risks, and larger operating costs.
Copyright (c) 2011 REB Research & Consulting.