# How much energy needed to liquify H?

I want to know how much pounds (or the correct measure) would an air compressor need to liquify Hydrogen

As its boiling point is somewhat close to -250°C i want to know how to calculate the pressure needed to liquify it, and how much "men" pushing a 20 meters lever compressing a serial of one-directioned-flux valves, would be required to achieve such mechanical power. Or how much "Gasoline regular sedan car engines" would be required for it

Note: Im not refering to H2 because im refering to hydrogen itself, i say that because my question has been edited to saying "$H_2$" at the title which i believe is kind of deuterium or so.. and im not meaning this

• I assume you mean -250°C. – BowlOfRed Aug 15 '17 at 19:07
• To cool a gas you need to start with a gas that will cool when it expands. Most gases at room temperature, except helium and hydrogen will cool when they expand (this is why storing hydrogen gas under pressure at room temperature is so dangerous, if there is a leak the gas streaming out heats up a lot and will then react with oxygen). This means that you can just compress air, let it cool down to room temperature and then let that expand though a valve to make liquefied air. You can use that to cool hydrogen to below -80 C. At these temperatures hydrogen will cool upon expansion. – Count Iblis Aug 15 '17 at 19:22
• @CountIblis will -80°C compressed hydrogen will storage more hydrogen at a cubic meter than a 20°C compressed hydrogen at the same cubic meter? Thank you very much – LuxBellum Aug 15 '17 at 19:29
• Yes, at the same pressure you can store more at a lower temperature. Using the Joule–Thomson effect you'll then get liquefied hydrogen provided the temperature is below the so-called inversion temperature. – Count Iblis Aug 15 '17 at 19:32
• So if i understood well, below -80C pressurized hydrogen will cool when expanded through a valve, but when reaching critical fluid point which someone told me to be 250°C on hydrogen, its opposite, it will heat upon expansion... – LuxBellum Aug 15 '17 at 19:36

You'll probably be wanting to use the equation $$pV = nRT$$ where $p$ is the pressure, $V$ is the volume, $n$ is the number of moles $R$ is the molar gas constant $(8.31)$ and $T$ is the temperature in Kelvin. You can easily rearrange this equation to get $$p = \frac{nRT}{V}$$ I'm a bit confused as to why you've brought in fusion however hydrogen has a boiling point of around 20.25 Kelvin. If you were to use $1$ mole of hydrogen and pressurise say a volume of 1 $m^3$ then the pressure would need to be approximately $168$Pa or about $0.00166$ atm (quite small)!

• Corrected, sorry i meant boiling, and thank you very much for the formula! – LuxBellum Aug 15 '17 at 18:32
• No problem, if this answered your question it's helpful to both me and the site the 'mark as answer' :) – CooperCape Aug 15 '17 at 18:42
• Why do you think that the ideal gas equation has much to say about the process of liquification? – Jon Custer Aug 15 '17 at 18:45
• He asked for the pressure required to do such a thing. In my opinion this provides it. I'm not quite sure how far into non-ideal gases it's 'ideal' (ha..ha..) to get into? – CooperCape Aug 15 '17 at 18:46
• Dont downvote him, his answer helped me a lot – LuxBellum Aug 15 '17 at 23:13

Hydrogen has a critical temperature of 33K. So you can't get a true liquid simply by pressurizing room temperature hydrogen. It would be a supercritical fluid. You can squeeze more into a container with higher pressure, but it won't be a liquid.

Liquification can make transporting some materials easier (like propane, LNG). Once liquified, the materials can remain in this state with sufficient pressure. They can be poured, drain through valves, etc. There is some vapor present, but you can often move the liquid and ignore the amount in vapor.

With materials where the critital temperature is low, you can't do this step at room temperature. Instead of liquifying, the material just gets denser and denser, but still acts in many ways like a gas. There's no fluid portion that you can pour. The fluid will expand to fill a container like a gas.

This doesn't put a hard limit on the density you can reach. You can theoretically make a container of supercritical hydrogen as dense as cryogenic liquid hydrogen. The problem is that the pressures needed to do so are huge. You would need massively strong tanks and valves. Any leak or failure would be very dangerous. You might be able to compress a few grams of hydrogen to the density you want. But at room temperature, the device to hold it at that pressure might have to be several hundred kg or more to not fail. So the engineering side usually means that this is never done. You lose more in the cost and complexity of the necessary equipment than you gain from the extra quantity that can be squeezed into the tank.

• Does that mean that if i manage to liquify it... lets say with laser cooling... (random example) and once liquified i transfer it onto a strong container, it will re-expand? The reason im thinking this is because, liquified a container holds more mass than a gas of the same element onto the same volume container, am i right? – LuxBellum Aug 15 '17 at 19:16
• If you have a sufficiently strong container, it won't expand, but it will no longer be a liquid, but a supercritical fluid. For normal objects, liquids are more dense than gases. But at high pressures, that's not always the case. – BowlOfRed Aug 15 '17 at 19:18
• Ok should i be more specific, 1 cubic meter of supercritical fluid hydrogen, will carry more hydrogen mass than a 1 cubic meter of regular gas state hydrogen? How do i know which elements or molecules are more likely to be "more dense" as supercriticalized and which aren't when liquified? – LuxBellum Aug 15 '17 at 19:21