Flaming ice

burning hydrateI am doing a bit of reading up on a substance that looks like ice, but, as you can see, burns. The substance is called gas hydrate or methane hydrate. It is a crystal and it looks like ice because it is quite like ice. It is a crystalline arrangement of water molecules, like ice. But the arrangement is a bit different. The water molecules form an array of cages. A cage of water molecules is shown at the top left — H20 molecules are shown in red (oxygen atom) and white (hydrogen). In these cages sit methane molecules. A methane, CH4, molecule in the cage is also shown. It is the methane that allows these crystals to burn.

I think gas hydrates are cool, but the main interest is in stopping them forming. As I guess you should expect they tend to form at cold or sub-zero temperatures, when you have water and methane together. They are also stabilised by high pressure — typically you need about ten atmospheres or more of pressure to form them.

The problem is that in oil or gas pipelines in cold places such as the arctic or on the ocean seabed (in deep water the temperature is always low) you have the perfect conditions for forming these gas hydrates. The temperature is low, there is loads of methane around*, there is at least some water around as it is very hard to get rid of, and the pipelines have to be at high pressure to move the oil or gas along.

So gas hydrates tend to form, and if enough form then the pipeline becomes completely blocked by their ice-like crystals.Of course this is extremely bad news for the company trying to pump oil or gas through the pipeline. Worse, the obvious way to unblock a pipeline is to drop the pressure, but this can turn a blockage into a missile. As these gas hydrates are only stable at high pressure if the pressure is lowered they will tend to melt. However, as they melt from the outside this causes them to unstick from the inside of the pipe. This sounds good but if the pressure has only been reduced in one side you then have many kilograms of flammable solid being accelerated along a pipe by a hefty pressure differential. Not what you need in a rather expensive pipeline.

* In oil there will be a fair bit of dissolved methane.

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