- Natural-gas condensate
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Natural-gas condensate is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields.
It condenses out of the raw gas if the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas.
The natural gas condensate is also referred to as simply condensate, or gas condensate, or sometimes natural gasoline because it contains hydrocarbons within the gasoline boiling range. Raw natural gas may come from any one of three types of gas wells:[1][2]
- Crude oil wells—Raw natural gas that comes from crude oil wells is called associated gas. This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil.
- Dry gas wells—These wells typically produce only raw natural gas that does not contain any hydrocarbon liquids. Such gas is called non-associated gas.
- Condensate wells—These wells produce raw natural gas along with natural gas liquid. Such gas is also non-associated gas and often referred to as wet gas.
Contents
Composition of natural-gas condensate
There are hundreds of wet gas fields worldwide and each has its own unique gas condensate composition. However, in general, gas condensate has a specific gravity ranging from 0.5 to 0.8 and may contain:[3][4][5][6]
- Hydrogen sulfide (H2S)
- Thiols traditionally also called mercaptans (denoted as RSH, where R is an organic group such as methyl, ethyl, etc.)
- Carbon dioxide (CO2)
- Straight-chain alkanes having from 2 to 12 carbon atoms (denoted as C2 to C12)
- Cyclohexane and perhaps other naphthenes
- Aromatics (benzene, toluene, xylenes, and ethylbenzene)
Separating the condensate from the raw natural gas
Main article: Natural-gas processing
Schematic flow diagram of the separation of condensate from raw natural gas
There are literally hundreds of different equipment configurations for the processing required to separate natural gas condensate from a raw natural gas. The schematic flow diagram to the right depicts just one of the possible configurations.[7]
The raw natural gas feedstock from a gas well or a group of wells is cooled to lower the gas temperature to below its hydrocarbon dew point at the feedstock pressure and that condenses a good part of the gas condensate hydrocarbons. The feedstock mixture of gas, liquid condensate and water is then routed to a high pressure separator vessel where the water and the raw natural gas are separated and removed. The raw natural gas from the high pressure separator is sent to the main gas compressor.
The gas condensate from the high pressure separator flows through a throttling control valve to a low pressure separator. The reduction in pressure across the control valve causes the condensate to undergo a partial vaporization referred to as a flash vaporization. The raw natural gas from the low pressure separator is sent to a "booster" compressor which raises the gas pressure and sends it through a cooler and on to the main gas compressor. The main gas compressor raises the pressure of the gases from the high and low pressure separators to whatever pressure is required for the pipeline transportation of the gas to the raw natural gas processing plant. The main gas compressor discharge pressure will depend upon the distance to the raw natural gas processing plant and it may require that a multi-stage compressor be used.
At the raw natural gas processing plant, the gas will be dehydrated and acid gases and other impurities will be removed from the gas. Then, the ethane (C2), propane (C3), butanes (C4), and pentanes (C5)—plus higher molecular weight hydrocarbons referred to as C5+—will also be removed and recovered as byproducts.
The water removed from both the high and low pressure separators will probably need to be processed to remove hydrogen sulfide (H2S) before the water can be disposed of or reused in some fashion.
Some of the raw natural gas may be re-injected into the gas wells to help maintain the gas reservoir pressures.
Drip gas
Drip gas is natural gas condensate, a naturally occurring form of gasoline found near many oil and natural gas wells, in natural gas pipelines, and as a byproduct of natural gas extraction. It is also known as "condensate," "natural gasoline," "casing head gas," "raw gas," "white gas," and "liquid gold."[8][9] Drip gas has industrial uses as a cleaner and solvent, as a lantern and stove fuel, and as a denaturing additive for fuel alcohol.
Historical use in vehicles
Some very early internal combustion engines—such as the first types made by Karl Benz, and early Wright brothers aircraft engines—used natural gasoline, which could be either drip gas or a similar range of hydrocarbons distilled from crude oil. Natural gasoline has an octane rating of about 30 to 50, sufficient for the low-compression engines of the early 20th century. By 1930, improved engines and higher compression ratios required higher-octane, refined gasolines to produce power without knocking or detonation.
Beginning in the Great Depression, drip gas was used as a replacement for commercial gasoline by people in oil-producing areas. "In the days of simple engines in automobiles and farm tractors it was not uncommon for anyone having access to a condensate well to fill his tank with 'drip,'" according to the Oklahoma Historical Society. Sometimes it worked fine. "At other times it might cause thundering backfires and clouds of foul-smelling smoke."[10]
Woody Guthrie's autobiographical novel Seeds of Man begins with Woody and his uncle Jeff tapping a natural gas pipeline for drip gas. The gas also has a mention in Badlands, the Terrence Malick movie.[11]
It was sold commercially at gas stations and hardware stores in North America until the early 1950's. The White gas sold today is a similar product but is produced at refineries with the benzene removed.[12]
In 1975, the New Mexico State Police's drip gas detail – three men in pickup trucks – began patrolling oil and gas fields, catching thieves and recovering barrels of stolen gas. The detail stopped its work in 1987.[13]
The use of drip gas in cars and trucks is now illegal in many states. It is also harmful to modern engines due to its low octane rating, high heat of combustion and lack of additives. It has a distinctive smell when used as a fuel, which allowed police to catch people using drip gas illegally.[14][15]
Composition
Under the United States Code of Federal Regulations, drip gas consists of butane, pentane, and hexane hydrocarbons. Within set ranges of distillation, drip gas may be extracted and used to denature fuel alcohol.[16]
References
- ^ International Energy Glossary (a page from the website of the Energy Information Administration)
- ^ Natural gas processing (a page from the website of the Energy Information Administration)
- ^ Natural Gas Condensate Marathon Oil Company MSDS
- ^ Natural Gas Condensate Phillips Petroleum Company MSDS
- ^ Condensate (Alaska) ConocoPhillips of Alaska MSDS
- ^ Natural Gas Condensate Amerada Hess Corporation MSDS
- ^ Simplified Process Flow Diagram
- ^ Mamdouh R. Gadallah and Ray L. Fisher, Applied Seismology: A Comprehensive Guide to Seismic Theory and Application, ISBN 1593700229.
- ^ New Mexico State Police Association, New Mexico State Police, 1933-2000, ISBN 1563115875.
- ^ Oklahoma Historical Society, Encyclopedia of Oklahoma History and Culture.
- ^ Badlands script.
- ^ http://fuel.papo-art.com/
- ^ New Mexico State Police, 1933-2000.
- ^ "Drip Gas Was A Real Gas for Me As A Kid" by Jack Cawthon, June 9 2004.
- ^ Burning Drip Gas in Horntown, Oklahoma, by Clayton Adair.
- ^ "Authorized Materials for Fuel Alcohol". Alcohol and Tobacco Tax and Trade Bureau. http://www.ttb.gov/pdf/authorized_denaturants_fuel_alcohol.pdf. Retrieved 2008-03-06.
External links
- Processing raw natural gas
- Preparing raw natural gas for sales
- Natural Gas Processing (part of the US EPA's AP-42 publication and includes a schematic diagram)
Categories:- Chemical engineering
- Chemical processes
- Natural gas
- Unit processes
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