Natural Gas Processing

Natural gas produced from underground reservoirs must be processed to remove water, impurities, and heavier hydrocarbons. The impurities are usually hydrogen sulfide and carbon dioxide. The heavier hydrocarbons or natural gas liquids (NGLs) are ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), and natural gasoline.

Natural gas liquids are removed from the natural gas feed to recover the heavier hydrocarbons and to allow the treated gas to move through the pipeline network. The liquids are removed from the stream because they are more valuable (have higher prices) as raw materials to produce chemicals and gasoline than they are as fuel. In addition the heavier liquids can condense out of the natural gas stream and cause problems in transporting the gas through pipelines.

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Natural gas liquids are recovered using four different technologies: refrigeration, cryogenic recovery, oil absorption, and dry-bed adsorption. Natural gas liquids are recovered by cooling or refrigerating the natural gas until the liquids are condensed out. The plants use Freon or propane to cool the gas.

Cryogenic recovery processes are done at temperatures lower than -150 °F. The low temperatures allow the plant to recover over 90% of the ethane in the natural gas. Most new gas processing plants use cryogenic recovery technology. Oil absorption is a process used by older gas processing plants and in many refinery gas plants.

This process is not as efficient as cryogenic processing and only 70% propane and all of the butane and natural gasoline are recovered. Dry-bed adsorption is used to remove water and some of the natural gas liquids from the natural gas. The liquids are adsorbed on the surface of the desiccant such as silica gel. Desiccants are added to many products, such as medicines, to keep them dry. The adsorption process recovers 10 - 15% of the butane and 50 - 90% of the natural gasoline.

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Natural Gas Liquids

After removal from the natural gas stream, the natural gas liquids are separated in a series of distillation towers into their primary components: ethane, propane, butane, and natural gasoline.

Natural Gas Liquids Boiling Points

Chemical	Boiling Point
Methane	-259 ^oF.
Ethane	-128 ^oF.
Propane	- 44 ^oF.
Isobutane	10.9 ^oF.
Normal Butane	31.1 ^oF.
Isopentane	82.2 ^oF.
Normal Pentane	96.9 ^oF.

The following table indicates the distribution of natural gas liquids in the marketplace. The major product uses are for petrochemical and refinery feed stocks to produce products like ethylene, propylene, butylene, and gasoline. The main use of propane is in fuel applications such as home heating in places that are not served by natural gas pipelines and fuel for outdoor cooking.

Natural Gas Liquids Products and Markets

(Percent of Consumption)

Product	Petrochemical Feed Stock	Domestic and Industrial Fuel	Refinery Feed Stock	Other
Ethane	97.0	1.5	0	1.5
Propane	42.0	52.0	0	6.0
n-Butane	21.0	0	66.0	13.0
i-Butane	43.0*	0	57.0	0
Natural Gasoline	28.0	0	44.0	28.0

* Includes MTBE

Source: J. Richard Moore

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References

For more information about natural gas processing, check out the following references:

	NaturalGas.org - an educational website covering a variety of topics related to the natural gas industry. The purpose of this website is to provide visitors with a comprehensive information source for topics related to natural gas, and present an unbiased learning tool for students, teachers, industry, media, and government. This site has been developed and is maintained by the Natural Gas Supply Association.
	Gas Processors Association
	American Gas Association
	Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation to Market, Energy Information Administration

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