Sunday, January 13, 2008

Processing Natural Gas to be Clean and Pure

Before natural gas goes into the pipelines, it needs to be removed of all impurities. Processing natural gas to pipeline dry gas quality involves four main removal processes: oil and condensate removal, water removal, separation of natural gas liquids, and lastly, sulfur and carbon dioxide removal. In addition to these, heaters and scrubbers are installed, usually at or near the wellhead. The scrubbers serve primarily to remove sand and other large-particle impurities. With natural gas that contains even low quantities of water, natural gas hydrates have a tendency to form when temperatures drop. To reduce the occurrence of hydrates (resembling ice like crystals), small natural gas-fired heating units are typically installed along the gathering pipe wherever it is likely that these hydrates may form.

First the natural gas needs to be separated from the oil in which it is dissolved. This separation of natural gas from oil is most often done using equipment installed at or near the wellhead. Although dry pipeline quality natural gas is virtually identical across different geographic areas, raw natural gas from different regions may have different compositions and separation requirements. Natural gas is dissolved in oil underground primarily due to the pressure that the formation is under. The most basic type of separator is known as a conventional separator. Simply, it consists of a closed tank where the force of gravity serves to separate the heavier liquids like oil, and the lighter gases, like natural gas. Sometimes specialized equipment is necessary to separate oil and natural gas. An example of this type of equipment is the Low-Temperature Separator (LTX). This is most often used for wells producing high pressure gas along with light crude oil or condensate. Using pressure differentials to cool the wet natural gas and separate the oil and condensate, wet gas enters the separator, being cooled slightly by a heat exchanger. The process allows the gas to expand lowering of the temperature in the separator. By varying the pressure of the gas in various sections of the separator, it is possible to vary the temperature, which causes the oil and some water to be condensed out of the wet gas stream. This basic pressure-temperature relationship can work in reverse as well, to extract gas from a liquid oil stream.

Next, it is necessary to remove most of the associated water from the wet gas stream. Most of the liquid, free water associated with extracted natural gas is removed by simple separation methods at or near the wellhead. The treatment for 'dehydrating' the natural gas involves one of two processes: either absorption, or adsorption.
Absorption occurs when the water vapor is taken out by a dehydrating agent. An example of absorption dehydration is known as Glycol Dehydration. In this process, a liquid desiccant dehydrator serves to absorb water vapor from the gas stream. Glycol, the principal agent in this process, has a chemical affinity for water.
The primary form of dehydrating natural gas using adsorption is solid-desiccant dehydration. It usually consists of two or more adsorption towers, which are filled with a solid desiccant. Solid-desiccant dehydrators are more effective than glycol dehydrators, and are usually installed as a type of straddle system along natural gas pipelines.

The third step in the process of removing the impurities is the separation of the natural gas liquids (NGLs). In most instances, natural gas liquids have a higher value as separate products, and it is thus economical to remove them from the gas stream. The removal of natural gas liquids usually takes place in a relatively centralized processing plant, and uses techniques similar to those used to dehydrate natural gas.

In addition to water, oil, and NGL removal, one of the most important parts of gas processing involves the removal of sulfur and carbon dioxide. Natural gas from some wells contains significant amounts of sulfur and carbon dioxide. Because of the rotten smell provided by its sulfur content, this natural gas is commonly called 'sour gas'. Sour gas is extremely harmful, even lethal, to breathe. Sour gas can also be extremely corrosive. The sulfur that exists in the natural gas stream is extracted and marketed on its own.

Gas processing ensures that the natural gas intended for use is as clean and pure as possible, making it the clean burning and environmentally sound energy choice. Companies like Triple Diamond Energy Corporation make sure that processing removes all impurities so only clean and pure natural gas goes in the pipeline network across the nation.

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