Science Questions

Why store propane but not natural gas?

Sun, 17th May 2009

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Question

Nick Brown asked:

Why store propane but not natural gas?

Answer

Dave - Itís all about boiling points really. If you look ó the bigger the molecule, the higher temperature it will boil at.  So butane will boil at sort of -0.1 degrees centigrade, so you donít 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 thatís not difficult to make with a small, light steel tank; and itís 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 thatís going to involve about 2,000 tonnes of pressure on every square metre of that tank so itís going to have to be made up incredibly strong steel, itís going to be very heavy.  It just makes it expensive and impractical to carry around ó and very dangerous to carry around in vehicles.

Chris Smith -   So thatís why we have it coming down the pipeline but we donít store it locally.  We use propane, much easier to compress and get lots of it into a small space in a tank. 

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

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Nic Browne asked the Naked Scientists: Hi Gang, My name is Nic Browne and I live in Victoria, Australia. I download your podcasts on iTunes every week and I absolutely love your show. Could you please answer a question for me. What is the difference between LPG gas used in heaters, and natural gases that do the same thing? It's been bothering me for a while so an answer would be nice. Cheers, Nic What do you think? Nic Browne , Wed, 4th Feb 2009

The main difference is that LPG (Liquid Petroleum Gas), as its name suggests, can be liquified (by compression) at room temperature and so you don't need extremely big containers to store a useful amount of it. lightarrow, Wed, 4th Feb 2009

People do store natural gas.
http://en.wikipedia.org/wiki/LNG_storage_tank
But with propane you need a relatively low pressuure to make it a liquid (at normal temperatures). With natural gas you have to keep it very cold or it boils. Bored chemist, Wed, 4th Feb 2009

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