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Natural Gas Processing


Topics

  1. How is Natural Gas Treated to Prepare It for Sales?

  2. Natural Gas Liquids

  3. Natural Gas Hydrates

  4. References


How is Natural Gas Treated to Prepare It for Sales? 

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.

 

Natural Gas Processing Schematic

[Click on image to view full-size]

[Click on image to view full-size]

 

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.

Recovery with Different Gas Processing Technologies

[Click on image to view full-size]

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

See Natural Gas Liquids - Current Market Trends

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Natural Gas Hydrates

State-owned Japan Oil, Gas and Metals National Corporation (JOGMEC) announced on March 12, 2013 that it had successfully conducted the world’s first tests to extract natural gas from methane hydrate deposits.

Methane hydrate, sometimes referred to as "methane ice," is a crystalline substance which forms when methane and water molecules combine under low temperature, high pressure conditions.

A team aboard the research drilling ship Chikyu began extraction of gas from a layer of methane hydrates about 1,000 feet below the seabed Tuesday morning by applying a depressurization method. JOGMEC will begin analyzing data while it continues the flow test through the end of this month.

JOGMEC highlighted that some 40 trillion cubic feet of methane is held in the methane hydrate deposits under the sea in the eastern Nankai Trough, an amount equivalent to approximately 11 years of liquefied natural gas (LNG).

The success of this test could signal a dramatic leap in the area of unconventional gas resources, opening up an energy revival for Japan much like the shale revolution in the US. Japan plans to begin commercial production from methane hydrate deposits by 2018.

Source: Davis, Dorothy. "Japan first to extract gas from 'methane ice'," March 12, 2013, PennEnergy.

Please see Methane clathrate for more information about natural gas hydrates.

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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

Natural Gas is America's New Energy Frontier - American Petroleum Institute

Natural Gas Processing Principles and Technology (an extensive and detailed course text by Dr. A.H. Younger, University of Calgary, Alberta, Canada).

Natural Gas Processing (part of the US EPA's AP-42 publication)

Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation to Market 

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