Tag Archives: membrane

A simpler way to recycle the waste fuel of a SOFC.

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My favorite fuel cells burn hydrogen-rich hydrocarbon fuels, like methane (natural gas) instead of pure hydrogen. Methane is far more energy dense, and costs far less than hydrogen per energy content. The US has plenty of methane and has pipelines that distribute it to every city and town. It’s a low CO2 fuel, and we can lower the CO2 impact further by mixing in hydrogen to get hythane. Elon Musk has called hydrogen- powered fuel cells “fool cells”, methane-powered fuel cells look a lot less foolish. They easily compete with his batteries and with gasoline. Besides, Musk has chosen methane as the fuel for his proposed starship to Mars.

Solid oxide fuel cells, SOFCs, can use methane directly without any pre-reformer. They operate at 800°C or so. At these temperatures, methane reacts with water (steam) within the fuel cell to form hydrogen by the reaction, CH4 + H2O –> 3H2 + CO. The hydrogen, and to a lesser extent the CO is oxidized in the fuel cell to create electricity,, but the methane is not 100% consumed, generally. Unused methane, CO, and some hydrogen exits a solid oxide fuel cell along with the products of combustion, CO2 and water.

Several researchers have looked for ways to recycle this waste fuel to capture the energy value. Six years ago, I patented a membrane method to extract the waste fuel and recycle it, see a description here. I now see this method as too complex, and have applied for a patent on a simpler version, shown below as Figure 1. As before the main work is done by a membrane but here I dispense with the water gas shift reactor, and many of the heat exchangers of the previous approach.

Simple way to improve fuel use in a high temperature fuel cell, using just a membrane.

The fuel cell system of Fig. 1 operates at somewhat elevated pressure, 2 atm or more. It is expected that the majority of the exhaust going to the membrane will be CO2 and water. Most of this will pass through the membrane and will exhaust to the air. The rest is mixed with fresh methane and recycles to the fuel cell. Despite the pressure of the fuel cell, very a little energy is needed for recirculation since the methane does not go through the membrane. The result is a light, simple, and energy efficient process. If you are interested, please contact me at REB Research. Or you can purchase the silicone membrane module here. Alternately, see here for flux information and other applications.

Robert Buxbaum, December 8, 2022.

A more accurate permeation tester

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There are two ASTM-approved methods for measuring the gas permeability of a material. The equipment is very similar, and REB Research makes equipment for either. In one of these methods (described in detail here) you measure the rate of pressure rise in a small volume.This method is ideal for high permeation rate materials. It’s fast, reliable, and as a bonus, allows you to infer diffusivity and solubility as well, based on the permeation and breakthrough time.

Exploded view of the permeation cell.

For slower permeation materials, I’ve found you are better off with the other method: using a flow of sampling gas (helium typically, though argon can be used as well) and a gas-sampling gas chromatograph. We sell the cells for this, though not the gas chromatograph. For my own work, I use helium as the carrier gas and sampling gas, along with a GC with a 1 cc sampling loop (a coil of stainless steel tube), and an automatic, gas-operated valve, called a sampling valve. I use a VECO ionization detector since it provides the greatest sensitivity differentiating hydrogen from helium.

When doing an experiment, the permeate gas is put into the upper chamber. That’s typically hydrogen for my experiments. The sampling gas (helium in my setup) is made to flow past the lower chamber at a fixed, flow rate, 20 sccm or less. The sampling gas then flows to the sampling loop of the GC, and from there up the hood. Every 20 minutes or so, the sampling valve switches, sending the sampling gas directly out the hood. When the valve switches, the carrier gas (helium) now passes through the sampling loop on its way to the column. This sends the 1 cc of sample directly to the GC column as a single “injection”. The GC column separates the various gases in the sample and determines the components and the concentration of each. From the helium flow rate, and the argon concentration in it, I determine the permeation rate and, from that, the permeability of the material.

As an example, let’s assume that the sample gas flow is 20 sccm, as in the diagram above, and that the GC determines the H2 concentration to be 1 ppm. The permeation rate is thus 20 x 10-6 std cc/minute, or 3.33 x 10-7 std cc/s. The permeability is now calculated from the permeation area (12.56 cm2 for the cells I make), from the material thickness, and from the upstream pressure. Typically, one measures the thickness in cm, and the pressure in cm of Hg so that 1 atm is 76cm Hg. The result is that permeability is determined in a unit called barrer. Continuing the example above, if the upstream hydrogen is 15 psig, that’s 2 atmospheres absolute or or 152 cm Hg. Lets say that the material is a polymer of thickness is 0.3 cm; we thus conclude that the permeability is 0.524 x 10-10 scc/cm/s/cm2/cmHg = 0.524 barrer.

This method is capable of measuring permeabilities lower than the previous method, easily lower than 1 barrer, because the results are not fogged by small air leaks or degassing from the membrane material. Leaks of oxygen, and nitrogen show up on the GC output as peaks that are distinct from the permeate peak, hydrogen or whatever you’re studying as a permeate gas. Another plus of this method is that you can measure the permeability of multiple gas species simultaneously, a useful feature when evaluating gas separation polymers. If this type of approach seems attractive, you can build a cell like this yourself, or buy one from us. Send us an email to reb@rebresearch.com, or give us a call at 248-545-0155.

Robert Buxbaum, April 27, 2022.

Upgrading landfill and digester gas for sale, methanol

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We live in a throw-away society, and the majority of it, eventually makes its way to a landfill. Books, food, grass clippings, tree-products, consumer electronics; unless it gets burnt or buried at sea, it goes to a landfill and is left to rot underground. The product of this rot is a gas, landfill gas, and it has a fairly high energy content if it could be tapped. The composition of landfill gas changes, but after the first year or so, the composition settles down to a nearly 50-50 mix of CO2 and methane. There is a fair amount of water vapor too, plus some nitrogen and hydrogen, but the basic process is shown below for wood decomposition, and the products are CO2  and methane.

System for sewage gas upgrading, uses REB membranes.

C6 H12 O6  –> 3 CO2  + 3 CH4 

This mix can not be put in the normal pipeline: there is too much CO2  and there are too many other smelly or condensible compounds (water, methanol, H2S…). This gas is sometimes used for heat on site, but there is a limited need for heat near a landfill. For the most part it is just vented or flared off. The waste of a potential energy source is an embarrassment. Besides, we are beginning to notice that methane causes global-warming with about 50 times the effect of CO2, so there is a strong incentive to capture and burn this gas, even if you have no use for the heat. I’d like to suggest a way to use the gas.

We sell small membrane modules too.

The landfill gas can be upgraded by removing the CO2. This can be done via a membrane, and REB Research sells a membranes that can do this. Other companies have other membranes that can do this too, but ours are smaller, and more suitable to small operations in my opinion. Our membrane are silicone-based. They retain CH4 and CO and hydrogen, while extracting water, CO2 and H2S, see schematic. The remainder is suited for local use in power generation, or in methanol production. It can also be used to run trucks. Also the gas can be upgraded further and added to a pipeline for shipping elsewhere. The useless parts can be separated for burial. Find these membranes on the REB web-site under silicone membranes.

Garbage trucks in New York powered by natural gas. They could use landfill gas.

There is another gas source whose composition is nearly identical to that of landfill gas; it’s digester gas, the output of sewage digesters. I’ve written about sewage treatment mostly in terms of aerobic bio treatment, for example here, but sewage can be treated anaerobically too, and the product is virtually identical to landfill gas. I think it would be great to power garbage trucks and buses with this. Gas. In New York, currently, some garbage trucks are powered by natural gas.

As a bonus, here’s how to make methanol from partially upgraded landfill or digester gas. As a first step 2/3 of the the CO2 removed. The remained will convert to methanol. by the following overall chemistry:

3 CH4 + CO2 + 2 H2O –> 4 CH3OH. 

When you removed the CO2., likely most of the water will leave with it. You add back the water as steam and heat to 800°C over Ni catalyst to make CO and H2. That’s done at about 800°C and 200 psi. Next, at lower temperature, with an appropriate catalyst you recombine the CO and H2 into methanol; with other catalysts you can make gasoline. These are not trivial processes, but they are doable on a smallish scale, and make economic sense where the methane is essentially free and there is no CNG customer. Methanol sells for $1.65/gal when sold by the tanker full, but $5 to $10/gal at the hardware store. That’s far higher than the price of methane, and methanol is far easier to ship and sell in truckload quantities.

Robert Buxbaum, June 8, 2021

Brown’s gas for small scale oxygen production.

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Some years ago I wrote a largely negative review of Brown’s gas, but the COVID crisis in India makes me want to reconsider. Browns gas can provide a simple source of oxygen for those who are in need. First, an explanation, Browns gas is a two-to-one mix of hydrogen and oxygen; it’s what you get when you do electrolysis of water without any internal separator. Any source of DC electricity will do, e.g. the alternator of a car or a trickle charger of the sort folks buy for their car batteries, and almost any electrode will do too (I’d suggest stainless steel). You can generate pressure just by restricting flow from the electrolysis vessel, and it can be a reasonable source of small-scale oxygen or hydrogen. The reaction is:

H2O –> H2 + 1/2 O2.

The problem with Brown’s gas is that it is explosive, more explosive than hydrogen itself, so you have to handle it with care; avoid sparks until you separate the H2 from the O2. Even the unseparated mix has found some uses, e.g. as a welding gas, or for putting in cars to avoid misfires, increase milage, and decrease pollution. I think that methanol reforming is a better source of automotive hydrogen: hydrogen is a lot safer than this hydrogen-oxygen mix.

Browns gas to oxygen for those who need it.

The mix is a lot less dangerous if you separate the oxygen from the hydrogen with a membrane, as I show in the figure. at right. If you do this it’s a reasonable wy to make oxygen for patients who need oxygen. The electrolysis cell can be a sealed bottle with water and the electrodes; add a flow restriction as shown to create the hydrogen pressure that drives the separation. The power can be an automotive trickle charger. You can get this sort of membranes from REB Research, here and many other suppliers. REB provide consulting services if you like.

In a pinch, you don’t even need the membrane, by the way. You can rely on your lungs to make the separation. A warning, though, the mix is dangerous. Avoid all sparks. Also, don’t put salt into the water. You can can put in some baking soda or lye to speed the electrolysis, but If you put salt in, you’ll find you don’t make oxygen, but will instead make chlorine. And chlorine is deadly. If you’re not sure, smell the gas. If it smells acrid, don’t use it. This is the chlorine-forming reaction.

2NaCl + 2 H2O –> H2 + Cl2 + 2NaOH

Ideally you should vent the hydrogen stream out the window, but for short term, emergency use, the hydrogen can be vented into your home. Don’t do this if anyone smokes (not that anyone should smoke about someone on oxygen). This is a semi-patentable design, but I’m giving it away; not everything that can be patented should be.

Robert Buxbaum, May 13, 2021.

Getter purifiers versus Membrane purifiers

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There are two main types of purifiers used for gases: getters and membranes. Both can work for you in almost any application, and we make both types at REB Research – for hydrogen purification mostly, but sometimes for other applications. The point of this essay is which one makes more sense for which application. I’ll mostly talk about hydrogen purification, but many of the principles apply generally. The way both methods work is by separating the fast gas from the slower gas. With most getters and most membranes, hydrogen is the fast gas. That is to say, hydrogen usually is the component that goes through the membrane preferentially, and hydrogen is the gas that goes through most getters preferentially. It’s not always the case, but generally.

Scematic of our getter beds for use with inert gasses. There are two chambers; one at high temperature to remove water, nitrogen, methane, CO2, and one at lower temperature the remove H2. The lower temperature bed can be regenerated.

Our getter beds for use with inert gasses have two chambers; one is high temperature to remove water, nitrogen, etc. and one at lower temperature the remove H2. The lower temperature bed can be regenerated.

Consider the problem of removing water and similar impurities from a low-flow stream of helium for a gas chromatograph. You probably want to use a getter because there are not really good membranes that differentiate helium from impurities. And even with hydrogen, at low flow rates the getter system will probably be cheaper. Besides, the purified gas from a getter leaves at the same pressure as it entered. With membranes, the fas gas (hydrogen) leaves at a lower pressure. The pressure difference is what drives membrane extraction. For inert gas drying our getters use vanadium-titanium to absorb most of the impurities, and we offer a second, lower temperature bed to remove hydrogen. For hydrogen purification with a bed, we use vanadium and skip the second bed. Other popular companies use other getters, e.g. drierite or sodium-lead. Whatever the getter, the gas will leave purified until the getter is used up. The advantage of sodium lead is that it gets more of the impurity (Purifies to higher purity). Vanadium-titanium removes not only water, but also oxygen, nitrogen, H2S, chlorine, etc. The problem is that it is more expensive, and it must operate at warm (or hot) temperatures. Also, it does not removed inert gases, like helium or argon from hydrogen; no getter does.

To see why getters can be cheaper than membranes if you don’t purify much gas, or if the gas starts out quite pure, consider a getter bed that contains 50 grams of vanadium-titanium (one mol). This amount of getter will purify 100 mols of fast gas (hydrogen or argon, say) if the fast gas contains 1% water. The same purifier will purify 1000 mols of fast gas with 0.1% impurity. Lets say you plan to use 1 liter per minute of gas at one atmosphere and room temperature, and you start with gas containing 0.1% impurity (3N = 99.9% gas). Since the volume of 100 mols of most gases a these conditions is 2400 liters. Thus, you can expect our purifier to last for 400 hours (two weeks) at this flow rate, or for four years if you start with 99.999% gas (5N). People who use a single gas chromatograph or two, generally find that getter-based purifiers make sense; they typically use only about 0.1 liters/minute, and can thus get 4+ years’ operation even with 4N gas. If you have high flows, e.g. many chromatographs or your gas is less-pure, you’re probably better off with a membrane-based purifier, shown below. That what I’ll discuss next.

Our membrane reactors and most of our hydrogen purifiers operate with pallium-membranes and pressure-outside. Only hydrogen permeates through the palladium membrane.

Our membrane reactors and most of our hydrogen purifiers operate with pallium-membranes and pressure-outside. Only hydrogen permeates through the palladium membrane.

The majority of membrane-based purifiers produced by our company use metallic membranes, usually palladium alloys, and very often (not always) with pressure on the outside. Only hydrogen passes through the membranes. Even with very impure feed gases, these purifiers will output 99.99999+% pure H2 and since the membrane is not used up, they will typically operate forever so long as there is no other issue — power outages can cause problems (we provide solutions to this). The main customers for our metallic membrane purifiers are small laboratories use and light manufacturers. We also manufacture devices that combine a reformer that makes 50% pure hydrogen from methanol + steam where the membranes are incorporated with the reactor — a membrane reformer, and this has significant advantages. There is no equivalent getter-based device, to my knowledge because it would take too much getter to deal with such impure gas.

Metal membranes are impermeable to inert gases like helium and argon too, and this is an advantage for some customers, those who don’t want anything but hydrogen. For other customers, those who want a cheaper solution, or are trying to purify large amounts of helium, we provide polymeric membranes, a lower cost, lower temperature option. Metal membranes are also used with deuterium or tritium, the higher isotopes of hydrogen. The lighter isotopes of hydrogen permeate these membranes faster than the heavier ones for reasons I discuss here.

Robert Buxbaum, August 26, 2018

My latest invention: improved fuel cell reformer

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Last week, I submitted a provisional patent application for an improved fuel reformer system to allow a fuel cell to operate on ordinary, liquid fuels, e.g. alcohol, gasoline, and JP-8 (diesel). I’m attaching the complete text of the description, below, but since it is not particularly user-friendly, I’d like to add a small, explanatory preface. What I’m proposing is shown in the diagram, following. I send a hydrogen-rich stream plus ordinary fuel and steam to the fuel cell, perhaps with a pre-reformer. My expectation that the fuel cell will not completely convert this material to CO2 and water vapor, even with the pre-reformer. Following the fuel cell, I then use a water-gas shift reactor to convert product CO and H2O to H2 and CO2 to increase the hydrogen content of the stream. I then use a semi-permeable membrane to extract the waste CO2 and water. I recirculate the hydrogen and the rest of the water back to the fuel cell to generate extra power, prevent coking, and promote steam reforming. I calculate the design should be able to operate at, perhaps 0.9 Volt per cell, and should nearly double the energy per gallon of fuel compared to ordinary diesel. Though use of pure hydrogen fuel would give better mileage, this design seems better for some applications. Please find the text following.

Use of a Water-Gas shift reactor and a CO2 extraction membrane to improve fuel utilization in a solid oxide fuel cell system.

Inventor: Dr. Robert E. Buxbaum, REB Research, 12851 Capital St, Oak Park, MI 48237; Patent Pending.

Solid oxide fuel cells (SOFCs) have improved over the last 10 years to the point that they are attractive options for electric power generation in automobiles, airplanes, and auxiliary power supplies. These cells operate at high temperatures and tolerate high concentrations of CO, hydrocarbons and limited concentrations of sulfur (H2S). SOFCs can operate on reformate gas and can perform limited degrees of hydrocarbon reforming too – something that is advantageous from the stand-point of fuel logistics: it’s far easier to transport a small volume of liquid fuel that it is a large volume of H2 gas. The main problem with in-situ reforming is the danger of coking the fuel cell, a problem that gets worse when reforming is attempted with the more–desirable, heavier fuels like gasoline and JP-8. To avoid coking the fuel cell, heavier fuels are typically reforming before hand in a separate reactor, typically by partial oxidation at auto-thermal conditions, a process that typically adds nitrogen and results in the inability to use the natural heat given off by the fuel cell. Steam reforming has been suggested as an option (Chick, 2011) but there is not enough heat released by the fuel cell alone to do it with the normal fuel cycles.

Another source of inefficiency in reformate-powered SOFC systems is basic to the use of carbon-containing fuels: the carbon tends to leave the fuel cell as CO instead of CO2. CO in the exhaust is undesirable from two perspectives: CO is toxic, and quite a bit of energy is wasted when the carbon leaves in this form. Normally, carbon can not leave as CO2 though, since CO is the more stable form at the high temperatures typical of SOFC operation. This patent provides solutions to all these problems through the use of a water-gas shift reactor and a CO2-extraction membrane. Find a drawing of a version of the process following.

RE. Buxbaum invention: A suggested fuel cycle to allow improved fuel reforming with a solid oxide fuel cell

RE. Buxbaum invention: A suggested fuel cycle to allow improved fuel reforming with a solid oxide fuel cell

As depicted in Figure 1, above, the fuel enters, is mixed with steam or partially boiled water, and heated in the rectifying heat exchanger. The hot steam + fuel mix then enters a steam reformer and perhaps a sulfur removal stage. This would be typical steam reforming except for a key difference: the heat for reforming comes (at least in part) from waste heat of the SOFC. Normally speaking there would not be enough heat, but in this system we add a recycle stream of H2-rich gas to the fuel cell. This stream, produced from waste CO in a water-gas shift reactor (the WGS) shown in Figure 1. This additional H2 adds to the heat generated by the SOFC and also adds to the amount of water in the SOFC. The net effect should be to reduce coking in the fuel cell while increasing the output voltage and providing enough heat for steam reforming. At least, that is the thought.

SOFCs differ from proton conducting FCS, e.g. PEM FCs, in that the ion that moves is oxygen, not hydrogen. As a result, water produced in the fuel cell ends up in the hydrogen-rich stream and not in the oxygen stream. Having this additional water in the fuel stream of the SOFC can promote fuel reforming within the FC. This presents a difficulty in exhausting the waste water vapor in that a means must be found to separate it from un-combusted fuel. This is unlike the case with PEM FCs, where the waste water leaves with the exhaust air. Our main solution to exhausting the water is the use of a membrane and perhaps a knockout drum to extract it from un-combusted fuel gases.

Our solution to the problem of carbon leaving the SOFC as CO is to react this CO with waste H2O to convert it to CO2 and additional H2. This is done in a water gas shift reactor, the WGS above. We then extract the CO2 and remaining, unused water through a CO2- specific membrane and we recycle the H2 and unconverted CO back to the SOFC using a low temperature recycle blower. The design above was modified from one in a paper by PNNL; that paper had neither a WGS reactor nor a membrane. As a result it got much worse fuel conversion, and required a high temperature recycle blower.

Heat must be removed from the SOFC output to cool it to a temperature suitable for the WGS reactor. In the design shown, the heat is used to heat the fuel before feeding it to the SOFC – this is done in the Rectifying HX. More heat must be removed before the gas can go to the CO2 extractor membrane; this heat is used to boil water for the steam reforming reaction. Additional heat inputs and exhausts will be needed for startup and load tracking. A solution to temporary heat imbalances is to adjust the voltage at the SOFC. The lower the voltage the more heat will be available to radiate to the steam reformer. At steady state operation, a heat balance suggests we will be able to provide sufficient heat to the steam reformer if we produce electricity at between 0.9 and 1.0 Volts per cell. The WGS reactor allows us to convert virtually all the fuel to water and CO2, with hardly any CO output. This was not possible for any design in the PNNL study cited above.

The drawing above shows water recycle. This is not a necessary part of the cycle. What is necessary is some degree of cooling of the WGS output. Boiling recycle water is shown because it can be a logistic benefit in certain situations, e.g. where you can not remove the necessary CO2 without removing too much of the water in the membrane module, and in mobile military situations, where it’s a benefit to reduce the amount of material that must be carried. If water or fuel must be boiled, it is worthwhile to do so by cooling the output from the WGS reactor. Using this heat saves energy and helps protect the high-selectivity membranes. Cooling also extends the life of the recycle blower and allows the lower-temperature recycle blowers. Ideally the temperature is not lowered so much that water begins to condense. Condensed water tends to disturb gas flow through a membrane module. The gas temperatures necessary to keep water from condensing in the module is about 180°C given typical, expected operating pressures of about 10 atm. The alternative is the use of a water knockout and a pressure reducer to prevent water condensation in membranes operated at lower temperatures, about 50°C.

Extracting the water in a knockout drum separate from the CO2 extraction has the secondary advantage of making it easier to adjust the water content in the fuel-gas stream. The temperature of condensation can then be used to control the water content; alternately, a separate membrane can extract water ahead of the CO2, with water content controlled by adjusting the pressure of the liquid water in the exit stream.

Some description of the membrane is worthwhile at this point since a key aspect of this patent – perhaps the key aspect — is the use of a CO2-extraction membrane. It is this addition to the fuel cycle that allows us to use the WGS reactor effectively to reduce coking and increase efficiency. The first reasonably effective CO2 extraction membranes appeared only about 5 years ago. These are made of silicone polymers like dimethylsiloxane, e.g. the Polaris membrane from MTR Inc. We can hope that better membranes will be developed in the following years, but the Polaris membrane is a reasonably acceptable option and available today, its only major shortcoming being its low operating temperature, about 50°C. Current Polaris membranes show H2-CO2 selectivity about 30 and a CO2 permeance about 1000 Barrers; these permeances suggest that high operating pressures would be desirable, and the preferred operation pressure could be 300 psi (20 atm) or higher. To operate the membrane with a humid gas stream at high pressure and 50°C will require the removal of most of the water upstream of the membrane module. For this, I’ve included a water knockout, or steam trap, shown in Figure 1. I also include a pressure reduction valve before the membrane (shown as an X in Figure 1). The pressure reduction helps prevent water condensation in the membrane modules. Better membranes may be able to operate at higher temperatures where this type of water knockout is not needed.

It seems likely that, no matter what improvements in membrane technology, the membrane will have to operate at pressures above about 6 atm, and likely above about 10 atm (upstream pressure) exhausting CO2 and water vapor to atmosphere. These high pressures are needed because the CO2 partial pressure in the fuel gas leaving the membrane module will have to be significantly higher than the CO2 exhaust pressure. Assuming a CO2 exhaust pressure of 0.7 atm or above and a desired 15% CO2 mol fraction in the fuel gas recycle, we can expect to need a minimum operating pressure of 4.7 atm at the membrane. Higher pressures, like 10 or 20 atm could be even more attractive.

In order to reform a carbon-based fuel, I expect the fuel cell to have to operate at 800°C or higher (Chick, 2011). Most fuels require high temperatures like this for reforming –methanol being a notable exception requiring only modest temperatures. If methanol is the fuel we will still want a rectifying heat exchanger, but it will be possible to put it after the Water-Gas Shift reactor, and it may be desirable for the reformer of this fuel to follow the fuel cell. When reforming sulfur-containing fuels, it is likely that a sulfur removal reactor will be needed. Several designs are available for this; I provide references to two below.

The overall system design I suggest should produce significantly more power per gm of carbon-based feed than the PNNL system (Chick, 2011). The combination of a rectifying heat exchange, a water gas reactor and CO2 extraction membrane recovers chemical energy that would otherwise be lost with the CO and H2 bleed steam. Further, the cooling stage allows the use of a lower temperature recycle pump with a fairly low compression ratio, likely 2 or less. The net result is to lower the pump cost and power drain. The fuel stream, shown in orange, is reheated without the use of a combustion pre-heater, another big advantage. While PNNL (Chick, 2011) has suggested an alternative route to recover most of the chemical energy through the use of a turbine power generator following the fuel cell, this design should have several advantages including greater reliability, and less noise.

Claims:

1.   A power-producing, fuel cell system including a solid oxide fuel cell (SOFC) where a fuel-containing output stream from the fuel cell goes to a regenerative heat exchanger followed by a water gas shift reactor followed by a membrane means to extract waste gases including carbon dioxide (CO2) formed in said reactor. Said reactor operating a temperatures between 200 and 450°C and the extracted carbon dioxide leaving at near ambient pressure; the non-extracted gases being recycled to the fuel cell.

Main References:

The most relevant reference here is “Solid Oxide Fuel Cell and Power System Development at PNNL” by Larry Chick, Pacific Northwest National Laboratory March 29, 2011: http://www.energy.gov/sites/prod/files/2014/03/f10/apu2011_9_chick.pdf. Also see US patent  8394544. it’s from the same authors and somewhat similar, though not as good and only for methane, a high-hydrogen fuel.

Robert E. Buxbaum, REB Research, May 11, 2015.

Simple electroplating of noble metals

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Electro-coating gold onto a Pd tube by dissolving an iron wire.

Electro-coating gold onto at Pd-coated tube by dissolving an iron wire at REB Research.

Here’s a simple trick for electroplating noble metals: gold, silver, copper, platinum. I learned this trick at Brooklyn Technical High School some years ago, and I still use it at REB Research as part of our process to make hydrogen permeation barriers, and sulfur tolerant permeation membranes.  It’s best used to coat reasonably inactive, small objects,  e.g. to coat copper on a nickel or silver on a penny for a science fair.

As a first step, you make a dilute acidic solution of the desired noble metal. Dissolve a gram or so of copper sulphate, silver nitrate, or gold chloride per 250 ml of water. Make sure the solution is acidic using pH paper, add acid if needed aiming for a pH of 3 to 4. Place some solution into a test tube or beaker of a size that will hold the object you want to coat. As a next step, attach an iron or steel wire to the object, I typically use bailing wire from the hardware store wrapped several times about the top of the object, and run the length of the object; see figure. Place the object into your solution and wait for 5 to 30 minutes. Coating works without the need for any other electric source or any current control.

The iron wire creates the electricity used in electroplating the noble metal. Iron has a higher electro-motive potential than hydrogen and hydrogen has a higher potential than the noble metals. In acid solution, the iron wire dissolves but (it’s hoped) the substrate does not. Each iron atom gives up two electrons, becoming Fe++. Some of these electrons go on to reduce hydrogen ions making H2 (2H+ + 2e– –> H2), but most should go to reduce the noble metal ions in the solution to form a coat of metallic gold, silver, or copper on both the wire and the object. See an example of how I do calculations regarding voltage, electron number, and Gibbs free energy.

Transferring electrons requires you have good electrical contact between the wire and the object. Most of the noble metal coats the object, not the wire since the object is bigger, typically. Thanks to my teachers at Brooklyn Technical High School for teaching me. For a uniform coat, it helps to run the wire down parallel to the entire length of tube; I think this is a capacitance, field effect. For a larger object, you may want several wires if you are plating a larger object. For a thicker coat, I found you are best off making many thin coats and heating them. This reduces tension forces in the coat, I think.

The picture shows a step in the process we use making our sulfur-resistant hydrogen permeation membranes (buy them here), used, e.g. to concentrate impurities in a hydrogen stream for improved gas chromatography. The next step is to dissolve the gold or copper into the palladium.

Go here for a great periodic table cup from REB Research, or for the rest of our REB Research products. I occasionally make silver-coated pennies for schoolchildren, but otherwise use this technology only for in-house production.

R.E. Buxbaum, July 20, 2013.

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.

How and why membrane reactors work

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Here is a link to a 3 year old essay of mine about how membrane reactors work and how you can use them to get past the normal limits of thermodynamics. The words are good, as is the example application, but I think I can write a shorter version now. Also, sorry to say, when I wrote the essay I was just beginning to make membrane reactors; my designs have gotten simpler since.

At left, for example, is a more modern, high pressure membrane reactor design. A common size is  72 tube reactor assembly; high pressure. The area around the shell is used for heat transfer. Normally the reactor would sit with this end up, and the tube area filled or half-filled with catalyst, e.g. for the water gas shift reaction, CO + H2O –> CO2 + H2 or for the methanol reforming CH3OH + H2O –> 3H2 + CO2, or ammonia cracking 2NH3 –> N2 + 3H2. According to normal thermodynamics, the extent of reaction for these reactions will be negatively affected by pressure (WGS is unaffected). Separation of the hydrogen generally requires high pressure and a separate step or two. This setup combines the steps of reaction with separation, give you ultra high purity, and avoids the normal limitations of thermodynamics.

Once equilibrium is reached in a normal reactor, your only option to drive the reaction isby adjusting the temperature. For the WGS, you have to operate at low temperatures, 250- 300 °C, if you want high conversion, and you have to cool externally to remove the heat of reaction. In a membrane reactor, you can operate in your preferred temperature ranges and you don’t have to work so hard to remove, or add heat. Typically with a MR, you want to operate at high reactor pressures, and you want to extract hydrogen at a lower pressure. The pressure difference between the reacting gas and the extracted hydrogen allows you to achieve high reaction extents (high conversions) at any temperature. The extent is higher because you are continuously removing product – H2 in this case.

Here’s where we sell membrane reactors; we also sell catalyst and tubes.

Big new hydrogen purifier ships

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We shipped out our largest hydrogen purifier to date on Thursday, one designed for use in hydrogen-powered airplanes. I’m pretty happy; lots of throughput, light weight, low pressure drop, quite durable. We had a pizza party Friday to celebrate(if we didn’t invite you, sorry). I’m already working on design improvements (lessons learned) in case we get another order, or another, similar customer. I think we could do even better in our next version.