Tag Archives: safety

Hydrogen Cylinders versus Hydrogen Generators for Gas Chromatography

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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.

What is the best hydrogen storage medium?

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Answering best questions is always tricky since best depends on situation, but I’ll cover some hydrogen storage options here, and I’ll try to explain where our product options (cylinder gas purifiers and methanol-water reformers) fit in.

The most common laboratory option for hydrogen storage is inside a tank; typically this tank is made of steel, but it can be made of aluminum, fiberglass or carbon fiber. Tanks are the most convenient source for small volume users since they are instantly ready for delivery at any pressure up to the storage pressure; typically that’s 2000 psi (135 atm) though 10,000 (1350 atm) is available by special order. The maximum practical density for this storage is about 50 g/liter, but this density ignores the weight of the tank. The tank adds a factor of 20 or to the weight, making tanks a less-favored option for mobile users. Tanks also add significantly to the cost. They also tend to add impurities to the gas, and there’s a safety issue too: tanks sometimes fall over, and compressed gas can explode. For small-volume, non-mobile users, one can address safety by chaining up ones tank and adding a metal membrane hydrogen purifier; This is one of our main products.

Another approach is liquid hydrogen; The density of liquid hydrogen is higher than of gas, about 68 g/liter, and you don’t need as a tank that’s a big or heavy. One problem is that you have to keep the liquid quite cold, about 25 K. There are evaporative losses too, and if the vent should freeze shut you will get a massive explosion. This is the storage method preferred by large users, like NASA.

Moving on to metal hydrides. These are heavy and rather expensive but they are safer than the two previous options. To extract hydrogen from a metal hydride bed the entire hydride bed has to be heated, and this adds complexity. To refill the bed, it generally has to be cooled, and this too adds complexity. Generally, you need a source of moderately high pressure, clean, dry hydrogen to recharge a bed. You can get this from either an electrolysis generator, with a metal membrane hydrogen purifier, or by generating the hydrogen from methanol using one of our membrane reactor hydrogen generators.

Borohydrides are similar to metal hydrides, but they can flow. Sorry to say, they are more expensive than normal metal hydrides and they can not be regenerated.They are ideal for some military use

And now finally, chemical materials: water, methanol, and ammonia. Chemical compounds are a lot cheaper than metal hydrides or metal borohydrides, and tend to be far more readily available and transportable being much lighter in weight. Water and/or methanol contains 110 gm of H2/liter;  ammonia contains 120 gms/liter, and the tanks are far lighter and cheaper too. Polyethylene jugs weighing a few ounces suffices to transport gallon quantities of water or methanol and, while not quite as light, relatively cheap metallic containers suffice to hold and transport ammonia.

The optimum choice of chemical storage varies with application and customer need. Water is the safest option, but it can freeze in the cold, and it does not contain its own chemical energy. The energy to split the water has to come externally, typically from electricity via electrolysis. This makes water impractical for mobile applications. Also, the hydrogen generated from water electrolysis tends to be impure, a problem for hydrogen that is intended for storage or chemical manufacture. Still, there is a big advantage to forming hydrogen from something that is completely non-toxic, non-flammable, and readily available, and water definitely has a place among the production options.

Methanol contains its own chemical energy, so hydrogen can be generated by heating alone (with a catalyst), but it is toxic to drink and it is flammable. I’ve found a  my unique way of making hydrogen from methanol-water using  a membrane reactor. Go to my site for sales and other essays.

Finally, ammonia provides it’s own chemical energy like methanol, and is flammable, like methanol; we can convert it to hydrogen with our membrane reactors like we can methanol, but ammonia is far more toxic than methanol, possessing the power to kill with both its vapors and in liquid form. We’ve made ammonia reformers, but prefer methanol.