Why store propane but not natural gas?

17 May 2009

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Question

Why store propane but not natural gas?

Answer

Dave - It's all about boiling points really.

If you look, the bigger the molecule, the higher temperature it will boil at. So butane will boil at sort of -0.1 degrees Centigrade, so you don't need very much pressure to keep that as a liquid at room temperature so you can have really quite weak tanks.

Propane boils at about -42 degrees Centigrade, which means you have to have 10 or 12 atmospheres of pressure to keep that a liquid at 40 degrees Centigrade. But that's not difficult to make with a small, light steel tank; and it's quite practical to carry around, that would be sensible.

Natural gas is methane, which boils at -161 degrees Centigrade, roughly. To keep that a liquid at all you have to have tanks that are strong enough to survive 200 to 220 atmospheres of pressure, and from the experiment that we were doing earlier that's going to involve about 2,000 tonnes of pressure on every square metre of that tank, so it's going to have to be made up incredibly strong steel, it's going to be very heavy. It just makes it expensive and impractical to carry around - and very dangerous to carry around in vehicles.

Chris Smith - So that's why we have it coming down the pipeline but we don't store it locally. We use propane, much easier to compress and get lots of it into a small space in a tank.

Dave - That's right. In a pipeline it never has to be at a very high pressure. You never have to liquefy it...

Comments

If hydrogen has a BP=-252C and it can be compressed in a cylinder why can't Natural gas (methane) w/BP=-161C ? Thanks in advance

The reason we put hydrogen into cylinders is because there is no other practical and safe way to transport it for the given volumes we use and the applications for which it is employed. So the cylinders are hugely heavy, but that's what it takes. Natural gas can be safely and sensibly transmitted by pipeline because there is a high demand for it. That being the case, cylinders are not practical and no one uses them, although you could if so minded. But because propane or butane are perfectly viable alternatives, what would be the point?

When propane is burning, the fumes are much more toxic to the human body than that of natural gas, so that would be one reason to develop a way to store it in tanks and sell it.

Burning Methane: CH4 + 2O2 -> CO2 + 2H2O (1 molecule of carbon dioxide and 2 of water)

Burning Propane: C3H8 + 5O2 - > 3CO2 + 4H2O (3 molecules of carbon dioxide and 4 of water)

In other words, the products of burning natural gas (CH4) and propane (C3H8) are identical and differ only in number. So you are quite mistaken that the products are different.

So if its on constant flow and can be turned on or off, can we use it with portable generators? Since we use it with stationary ones all ready. all we would need todo is tap into the existing line. Instead of standing in line during power outages at gas stations til they run empty or lose power themselves.

Sorry, Marshall, I don't understand your comment.

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