Why store propane but not natural gas?

17 May 2009

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 sort of room temperature, so you can have really quite weak-like 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 that temperatures you have to have tanks that are strong enough to survive 200 to 220 atmospheres of pressure and from the experiment that we are 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.

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