Hydrogen is going to be part of energy transition. Hydrogen liquefaction is going to be part of hydrogen value chain.
Are you ready to liquefy hydrogen? Before you start, you might want to avoid four (4) mistakes in your process design.
Mistake #1: Peng-Robinson is a popular physical property package for oil and gas industry. But please don't use it for hydrogen liquefaction. Instead, RefProp is perceived to be most accurate when it comes to hydrogen liquefaction. As you can see in this Hysys calculation, to cool 1 kg/h hydrogen from 298.15 K (or 25 C) to 18 K, Peng-Robinson requires 4375 kJ/h, while RefProp only requires 3953 kJ/h
Mistake #2: Hydrogen has two (2) isomers, one is para-hydrogen and the other one is ortho-hydrogen. At cryogenic conditions, ortho-hydrogen will slowly convert to para hydrogen and release heat at the same time. If you model hydrogen as 25% para and 75% ortho, surprisingly, the duty is the same. Both of them are 3953 kJ/h. Why? Because Refprop assumes 25% para and 75% ortho if you simply specify hydrogen. So it does not sound like a mistake if you simply specify hydrogen?
Let us continue to look at mistake #3, then you'll understand you need to model para and ortho hydrogen, instead of just normal hydrogen. At a temperature of 18 K, 25% para and 75% ortho is in liquid state. However, if you wait for a couple of days and let all ortho is converted into para hydrogen which releases heat to vaporize hydrogen, you will be shocked that there is not even one single droplet of hydrogen in your tank. To prevent this from happening, you need to use catalyst to convert ortho to para hydrogen and further cool it. In this example, the further cooling duty is 475 kJ/h, or 12% extra cooling duty. However, the extra compression horsepower will be about 20% more after doing an exergy or Gibbs free energy analysis. The same analysis also shows the minimum liquefaction power is about 4.1 kWh/kg hydrogen.
Mistake #4: it looks like we have modeled hydrogen properties correctly so far. But let us do some common sense check. As we know, at ambient temperature, the thermal equilibrium between para and ortho is about 25% and 75%. If we do not make any adjustment to RefProp, RefProp calculates only about 19% para hydrogen, instead of 25%. So we need to make some adjustment to RefProp so that it matches experimental data.
Hydrogen liquefaction is still relatively new. We can probably make a million mistakes, but I hope you can at least avoid these four (4) mistakes after watching this video.
Thanks for watching this video. It is brought to you by Guofu Chen. More interesting topics can be found at showcase.guofuchen.com
