Hydrogen is an excellent cover gas for furnace brazing and electronic manufacture; it’s used as a carrier gas for gas chromatography or as a flame-detector gas, and it’s a necessity for ammonia production and most fuel cells. If you are working in one of these fields you can buy bottled hydrogen (cylinders) or a hydrogen generator . The main difference is cost. Cylinder hydrogen is typically the choice for small demand applications. A palladium membrane hydrogen purifier is added ( we make these) if high purity is important. Hydrogen generators are more generally used for larger -demand applications. They are more expensive at the start, but provide convenience and long-term savings. The essay below goes through the benefits and drawbacks of each as applies to gas chromatography.
Point of use Cylinder Hydrogen Is Simple and Allows Easy Monitoring and Control. At the smallest laboratories, those with one or two gas chromatographs, you’ll generally find you are best served by a single hydrogen cylinder for each GC, aided by a hydrogen purifier of some sort. This is called “point of use” hydrogen. Each cylinder is typically belted to a wall and used until the cylinder is empty. At that point, the application is stopped (the purifier is often stopped too) and a new cylinder switched in. There is usually a short break- in period where GC results are unreliable, but after one or two runs, everything is as before. The biggest advantage here is simplicity including ease of pressure control and monitoring. You can always check the pressure right by the GC and adjust it as needed. Long term cost is usually higher, though, and you have to stop whenever a cylinder needs switching.
Multi-cylinder Systems or Generators Provide Fewer interruptions. Larger laboratories tend to use multiple hydrogen cylinders with complex switchover systems, or hydrogen generators. Multiple cylinders are racked together and connected to a manifold and a single, larger purifier (we make these too). Tanks are emptied in series so that there is no disruption. When each tank empties, it is switched out in a way that maintains the flow. One problem is that the pressure and flow does not typically stay constant as the cylinders switch and as additional GCs or other processes are brought on line or taken off.
Purity can suffer too, as there is more tubing and more connections in the system. There is thus more room for leaks and degassing. This can be solved by replacing the single large purifier by point-of-use purifiers, installed just prior to the GC or other application.
Cylinder packs come with a safety disadvantage: with so many cylinders, there is a potential for disastrous leaks or mistakes that empty many cylinders at once — too fast to disperse the large amount of hydrogen released. Maintenance becomes an issue too since the manifolds and automatic switches become complicated quickly. Complex systems can require a trained technician to trouble-shoot and maintain; I sometimes do that, and customers don’t seem to mind, but it’s an issue.
Hydrogen generators can be cheaper and you avoid cylinder changes; Hydrogen generators are fed by tap-water or a very large tank of methanol -water. Running out is less of a problem, and adding more water or methanol to the tank does not affect the hydrogen output.
Safety is improved by limiting the output of the generator to the amount the room will vent. A room with 100 ft3 of air and some circulation can generally host a hydrogen generator 2-3 slpm output with no fear of reaching explosive limits. It’s also worthwhile to fit the hydrogen generator with an alarm or safety that shuts down if a leak is detected (we provide these for purifiers too).
Generator Options: Methanol-based hydrogen generators or electrolysis. Both options are are available in outputs from 250 ccm to 50 slpm. For larger-yet output, you’ll probably want an electrolyzer. In general, either generator will pay for itself in the first year if you use the gas, continuously, or nearly so.
In 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 H2O –> 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. Desiccants are cheaper, but the gas is at best 99.9% pure, good enough to feed FIDs, but not good enough to be used as a carrier gas, or for chemical production. Over time desiccants wear out; they require constant monitoring and changing as they become filled with water vapor. Often electrolytic hydrogen generators also require the addition of a caustic electrolyte solution as caustic can leak out, or leave by corrosion mechanisms.
In Reformer-based hydrogen generators a methanol-water mix is pumped to about 300 psi and heated to about 350 °C. It is then sent over a catalyst where it is converted to a hydrogen-containing gas-mix by the reaction CH3OH + H2O –> 3H2 + CO2. Pure hydrogen is extracted from the gas mix by passing it through a membrane, either within the reactor (a membrane reactor), or by use of a membrane purifier external to the reactor.
Cost comparisons. Hydrogen in cylinders is fairly expensive if you use gas continuously. In Detroit, where we are, hydrogen costs about $70 each cylinder low low-purity gas, or $200 for high purity gas. Each cylinder contains 135 scf of gas. If you use 1/10 cylinder per day, you will find you’re spending about $7,300 per year on hydrogen gas, with another $1000 spent on cylinder rental and delivery. This is about the cost of a comparable hydrogen generator plus the water or methanol and electricity run it. If you use significantly less hydrogen you save money with cylinders, if you use more there is significant savings with a generator.
Most hydrogen generators have delivery pressure limitations compared to cylinders. Cylinders have no problem supplying hydrogen at 200 psi or greater pressures. By contrast, generators are limited to only the 60-150 psig range only. This pressure limitation is not likely to be a problem, even for GCs that need higher pressure gas or when the generator must be located far from the instruments, but you have to be aware of the issue when buying the generator. Electrolysis systems that use caustic provide the highest pressures, but they tend to be the most expensive, and least safe as the operate hot and caustic can drip out. Fuel cell generators and reformers provide lower pressure gas (90 psi maximum, typically), but they are safer. In general generators should be located close to the instruments to minimize supply line pressure drop. If necessary it can pay to use cylinders and generators or several generators to provide a range of delivery pressures and a shorter distance between the hydrogen generator and the application.
Click here for the prices of REB Research hydrogen generators. By comparison, I’ve attached prices for electrolysis-based hydrogen generators here (it’s 2007 data; please check the company yourself for current prices). Finally, the price of membrane purifiers is listed here.
Maintenance required for optimal performance. Often electrolytic hydrogen generators require the addition of a caustic electrolyte solution; desiccant purified gas will require the monitoring and changing of desiccant cartridges to remove residual moisture from the hydrogen. Palladium membrane purifiers systems, and reformer systems need replacement thermocouples and heaters every few years. Understanding the required operating and maintenance procedures is an important part of making an informed decision.
Conclusion:
Cylinder hydrogen supplies are the simplest sources for labs but present a safety, cost, and handling concerns, particularly associated with cylinder change-outs. Generators tend to be more up-front expensive than cylinders but offer safety benefits as well as benefits of continuous supply and consistent purity. They are particularly attractive alternative for larger labs where large hydrogen supply can present larger safety risks, and larger operating costs.
R. E. Buxbaum, January 30, 2013, partially updated Apr. 2022.