What follows is a shorter, easier version of my old essay on how membrane reactors work to get you past the normal limits of thermodynamics. Also, I’m happy to say, our membrane reactors designs have gotten simpler, and that deserves an update.

At left, is our current, high pressure membrane reactor design, available in one-tube to  72 tube reactor assembly; high pressure, or larger, I suppose. Typically, the area around the shell is used for heat transfer. One needs to add heat to promote endothermic reactions like methanol reforming CH3OH + H2O –> 3H2 + CO2, or ammonia cracking 2NH3 –> N2 + 3H2. You need to take away heat from exothermic reactions, like the water gas shift reaction, CO + H2O –> CO2 + H2. Generally you want to have some heat transfer to help drive the reaction.

The reactor is a shell containing metallic tubes that filter gas. Normally the idea is for hydrogen to be formed in the shell area, and leave by diffusion through the tube walls and down the tubes, leaving as pure hydrogen (only hydrogen can go through metals). We typically suggest to have the reactor sit this way, with the tubes pointed up, and the body half-filled or more with catalyst. According to normal thermodynamics, the extent of a reaction like ammonia cracking will be negatively affected by overall pressure, and the extent of the WGS reaction is only affected by operating temperature. The membrane reactor allows you to remove hydrogen while the reaction progresses, and allows you to get good conversion at higher pressures too. That’s because hydrogen removal shifts the equilibrium so the reaction goes further. The effect is particularly significant at higher pressures. By combining the steps of reaction with separation, we can operate a higher pressures, delivering ultra high purity, avoiding the normal limitations of thermodynamics.

The water-gas reaction, CO + H2O –> CO2 + H2, deserves special mention since it’s common and exothermic. In a normal reactor, your only option to drive the reaction to near completion is by operating at low temperature where the catalyst barely works, 200 °C, or lower. You also have to remove the heat of reaction. In a membrane reactor, you can operate at a much higher temperature, especially if you work at higher pressure. At higher temperature the catalysts work better, and you don’t have to work so hard to remove heat.

At our company, REB Research, we sell membrane reactors; and catalysts, membrane tubes, and consulting.