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Purifying Natural Gas
A significant source of natural gas is "associated-dissolved" gas pulled up as a byproduct from crude oil wells, which must be separated out from the oil. In some cases, the relief of pressure by tapping the well will cause the fuels to separate on their own when deposited and allowed to settle in a large tank. Gravity pulls the heavier oil downward and the natural gas is siphoned off from the top. Most often, this is done right away on the site of the oil well itself.
The next impurities removed from natural gas are usually natural gas liquids that have condensed thanks to the lower pressure above ground. These liquids, along with any free water molecules, are usually caught in a sort of filter called an inlet slug catcher. It catches the liquids while letting the gaseous molecules through. This step also usually occurs right on the sight of the wellhead.
The condensate separator does not remove all of the water present in the "wet" natural gas, so it must be dehydrated in facilities specifically designed for the purpose. Some of the water is absorbed by introducing ethylene glycol, while still more is "adsorbed" in desiccant-laden dehydration towers. Inside the towers, this "adsorption" process uses silica gel and activated alumina to bind the water molecules to their surfaces.
After the water from the gas, a group of other contaminants, consisting mainly of hydrogen sulfide, carbon dioxide, water vapor, helium and oxygen, will be removed by routing the gas through an amine solution --- a reusable chemical that absorbs sulfur compounds from the natural gas. The gas is then pushed through a series of filtering tubes which, by virtue of slowing the flow down, cause the molecules to redistribute by weight with the heavier, unwanted molecules flowing out through lower filters. At the same time, the centrifugal force of the tubes removes any remaining water vapor or solid particulates.
The natural gas runs through the filters until the hydrogen sulfide and carbon dioxide reach acceptably low levels, after which they are directed to a Nitrogen Rejection Unit. In one kind of NRU the temperature of the gas stream is drastically lowered and the nitrogen is vented off. Another method involves running the gas through a special absorbent oil meant to absorb hydrocarbons. The oil is then depressurized multiple times, allowing the nitrogen and any helium present in the mix to be extracted.
Possibly at the same time the stream is run through the NRU, or after, the methane will finally be extracted from the stream. As in the first NRU method, the methane may be extracted cryogenically by swiftly lowering the temperature to minus-120 degrees Fahrenheit. This causes the other hydrocarbons to crystallize but leaves methane as a gas. Another method involves routing the gas through an absorption oil that separates the methane and leaves an oil enriched by the other hydrocarbons. By this time the methane produced is considered "pipeline-quality" or "dry" natural gas, and is about 98 percent pure when pumped away.
After pumping off the methane, the hydrocarbon-enriched oil is boiled to separate out the oil for reuse. The hydrocarbons are then cooled to reform as liquids, only to be reheated again in steps to separate off each component chemical by its particular boiling point. The hydrocarbon fuels produced this way are usually ethane, butane, propane, and various pentanes that are often trucked away for distribution and sale as well.