Comment by MBCook

One of the reasons we use cryogenic liquidfied gases is so the density can be in the same ball-park of more-easily liquified gases which do not need low temperature to keep from expanding until the tank ruptures.

>One of the reasons we use cryogenic liquidfied gases is so the density can be in the same ball-park of more-easily liquified gases which do not need low temperature to keep from expanding until the tank ruptures.

Propane, butane, LPG are all gases but the pressure which needs to be contained as the gas is turned into a liquid using pressure, is not too high for the typical welded BBQ tank. Designed to hold about 350 psi.

The two lighter hydrocarbons, methane & ethane can be compressed way beyond what a high-pressure spun cylinder (like the typical 3000 psi rated heavy oxygen tank welders use) can handle, and still not liquefy.

So similar to oxygen, nitrogen, argon, hydrogen and other "fixed" gases, methane needs to be liquefied cryogenically or any reasonable size tank will still not hold enough to last but a very small fraction of the time compared to the same capacity cryogenic storage.

But "storage" is doing a lot of work here.

Interestingly, with cryogenics you're going to need to handle even less pressure than the BBQ tanks, and the same size container ends up holding way more than the high-pressure cylinder at 3000 psi.

A typical liquid nitrogen cylinder runs at about 50 psi, the tank will be rated quite a bit higher than that but not considered "high-pressure" by anybody. Thinner and non-curved shapes can be fine which can be lighter in weight than higher-pressure ratings would require, but you really have to have plenty of good thermal insulation to boot.

The thing is, once you refill your cryogenic tank with cold liquid gas, you can never actually shut the tank completely. There is no additional cooling. The only thing keeping it cold is the low temperature of the liquid itself, no matter how good the insulation is, heat will gradually soak in and given enough time the whole thing would eventually end up at ambient temperature. Not cold enough to remain as a liquid any more.

That would be eventually explosive whether it was a flammable gas or not.

Instead, the tank is continuously venting a constant stream of gas from top.

IOW the rate of heat absorbtion is compensated for under equilibrium as it boils the liquid a little bit constantly and there has to be a way for that gas to escape. The remaining liquid maintains the low temperature because the boiling point of the gas (at that low pressure) is still in the cryogenic range.

The liquid self-refrigerates by evaporation to the (negative) boiling point of the substance. Which is why liquid helium is so much colder than liquid nitrogen in an identical cryo tank.

That means if you fill a tank with one of these cryo gases, depending on your usage rate the losses to evaporation may be more than the amount you are utilizing.

Or if you fill the cryo tank and don't use any at all for a while, it will empty itself by evaporation anyway and it could be before you got to use any of it.

Is the shape round? I bet it's round.