Natural gas - distribution

This is a very short article about natural gas distribution. I will describe how the natural gas is distributed in my country.

Simply speaking, the natural gas comes form a drill in the ground. Sure, there can be many drills in one single natural gas field and, of course, there are many gas fields around the world. The important thing is that the composition of the natural gas can be different even from two drills in the same gas field, and it can be very different if you compare natural gas from two separate fields.

The most important components of the natural gas are methane and ethane. These two always come in the largest proportion and thus they represent the largest contribution as energy source. Somewhat lesser amount of the propane and butane is also present. Of course there are also others, higher order hydrocarbons but the portion of them is usually too small to represent any important energy source (although, please note, there is more energy in a single higher order hydrocarbon molecule than in a lower order one). In addition, as always, you can find some impurities that do not contribute to energy – like carbon dioxide, nitrogen and other gases (some of them are very unfriendly to environment and if present in too high concentrations, the natural gas my require cleanup).

It is not handy to handle the raw natural gas having all its fractions mixed up because all these fractions behave somewhat differently (for example have different boiling points). Usually the propane and butane (and pentane, hexane…) are separated and bottled under pressure. The propane and butane are very handy for bottling because when you put them under reasonable pressure they become liquid and so you can put a lot of it into a bottle. In addition, the propane and butane are much more saturated energy sources than the methane and ethane thus it still makes a sense to distribute them in bottles (a more expensive way).

It would be too expensive and clumsy to put methane and ethane into bottles so pipes carry these into our homes or wherever it is needed.

The law requires that the methane-ethane gas that is distributed into our homes have some minimal energy density per volume. Usually the required energy density is somewhere in between of pure methane and pure ethane. Therefore the gas distributor must supply some mixture where some minimal proportion of ethane is present.

The gas distributor, of course, is not willing to distribute any richer mixture of the gas than required by law because the excessive ethane can be used for other, more profitable purposes. The excessive ethane is separated and used for production of ethylene and acetylene. The ethylene is further used to produce some sorts of plastics (like polyethylene) and the acetylene is bottled and used as a highly concentrated heat source for welding.

But what if our natural gas drills don’t produce enough ethane even to satisfy the minimal requirements of the law? In this case the distributor must produce the ethane from methane to enrich the mixture. Sure, the profitability of such drills is questionable. But don’t worry – this rarely happens in the real life because the gas producer and distributor companies are very important ones and the government will simply change the law to accommodate for it. Dot.

As I said the methane and ethane are distributed through pipes. To make this more economical the gas is pressurized to a very high pressure (say 60 bar) before entering a main pipeline – thus making that more of it can flow through limited cross-section of the pipe (this is equivalent to having a high voltage in main electric power lines). Hundreds of kilometers later the gas comes into distribution and reduction station located close to a populated area (a city).

This is where the gas is depressurized to a lower pressure that is suitable and safe for distribution in our homes. As you know, when a gas is depressurized its volume grows and its temperature drops down. As the pressure was very high, the temperature after depressurization drops down to a very low value. The distributor company doesn’t want to send this cold gas to our homes because our homes are very close and there wont be enough time for the gas to reheat again as it travels through this short pipeline.

That is why the distributor actively heats the gas in the distribution and reduction station – the distributors burns some small amount of gas to reheat the cold gas back to normal temperature. Then it sends it to our homes. Why bothering with this reheating?

Because a cold gas is more dense and thus it has more energy per volume than a warm gas. As we pay the gas per volume, the distributor is not interested in selling the cold gas. The warmed gas is more voluminous and the distributor will receive more money for it. Sure. Why then doesn’t the distributor sell us a very hot gas? Again, because the law requires that the distributed gas has some minimal energy per volume unit.

Of course, as the natural gas has no odor, and due to safety issues, it is requested for the distributor to odorize the gas using some smelly substance (usually ethyl-mercaptane) prior its sending to our homes. I can’t say nothing much about this stinky stuff except that a small bottle of it lasts for weeks for a whole town. (I was always wondering how do they produce this stuff – I mean, how do they find workers?) Very large consumers of natural gas get their gas non-odorized to cut odorization expenses.

Danijel Gorupec, 2006

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