Natural gas processing


Natural gas processing

Natural gas processing plants, or fractionators, are used to purify the raw natural gas extracted from underground gas fields and brought up to the surface by gas wells. The processed natural gas, used as fuel by residential, commercial and industrial consumers, is almost pure methane and is very much different from the raw natural gas.

Raw natural gas typically consists primarily of methane (CH4), the shortest and lightest hydrocarbon molecule. It also contains varying amounts of:

*Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons. When processed and purified into finished by-products, all of these are collectively referred to NGL (Natural Gas Liquids).
*Acid gases: carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).
*Other gases: nitrogen (N2) and helium (He).
*Water: water vapor and liquid water.
*Liquid hydrocarbons: perhaps some natural gas condensate (also referred to as "casinghead gasoline" or "natural gasoline") and/or crude oil.
*Mercury: very small amounts of mercury primarily in elemental form, but chlorides and other species are possibly present. [http://www.uop.com/objects/87MercuryRemoval.pdf "Mercury Removal from Natural Gas and Liquids"] UOP website page]

The raw natural gas must be purified to meet the quality standards specified by the major pipeline transmission and distribution companies. Those quality standards vary from pipeline to pipeline and are usually a function of a pipeline system’s design and the markets that it serves. In general, the standards specify that the natural gas:

*Be within a specific range of heating value (caloric value). For example, in the United States, it should be about 1,035 ± 5% Btu per cubic foot of gas at 1 atmosphere and 60 °F (41 MJ ± 5% per cubic metre of gas at 1 atmosphere and 0 °C).
*Be delivered at or above a specified hydrocarbon dew point temperature (below which some of the hydrocarbons in the gas might condense at pipeline pressure forming liquid slugs which could damage the pipeline).
*Be free of particulate solids and liquid water to prevent erosion, corrosion or other damage to the pipeline.
*Be dehydrated of water vapor sufficiently to prevent the formation of methane hydrates within the gas processing plant or subsequently within the sales gas transmission pipeline. [ [http://www.ipt.ntnu.no/~jsg/undervisning/naturgass/parlaktuna/Chap7.pdf Dehydration of Natural Gas] by Prof. Jon Steiner Gudmundsson, Norwegian University of Science and Technology] [http://www.processgroup.com.au/Technologies/GasProcessing/GlycolDehyrdation/tabid/84/Default.aspx Glycol Dehydration] (includes a flow diagram}]
*Contain no more than trace amounts of components such as hydrogen sulfide, carbon dioxide, mercaptans, nitrogen, and water vapor.
*Maintain mercury at less than detectable limits (approximately 0.001 ppb by volume) primarily to avoid damaging equipment in the gas processing plant or the pipeline transmission system from mercury amalgamation and embrittlement of aluminum and other metals. [ [http://www.tigg.com/ACTIVATED-CARBON/desulfurization.html "Desulfurization of and Mercury Removal From Natural Gas"] by Bourke, M.J. and Mazzoni, A.F., Laurance Reid Gas Conditioning Conference, Norman, Oklahoma, March 1989.] [ [http://www.gaschem.com/mercur.html Using Gas Geochemistry to Assess Mercury Risk] , OilTracers, 2006]

Types of raw natural gas wells

Raw natural gas comes primarily from any one of three types of wells: crude oil wells, gas wells, and condensate wells.

Natural gas that comes from crude oil wells is typically termed "associated gas". This gas can exist separate from the crude oil in the underground formation, or dissolved in the crude oil.

Natural gas from gas wells and from condensate wells, in which there is little or no crude oil, is termed "non-associated gas". Gas wells typically produce only raw natural gas, while condensate wells produce raw natural gas along with a very low density liquid hydrocarbon called "natural gas condensate" (sometimes also called "natural gasoline" or simply "condensate").

Raw natural gas can also come from methane deposits in the pores of coal seams. Such gas is referred to as "coalbed gas" and it is also called "sweet gas" because it is relatively free of hydrogen sulfide.

Description of a natural gas processing plant

There are a great many ways in which to configure the various unit processes used in the processing of raw natural gas. The block flow diagram below is a generalized, typical configuration for the processing of raw natural gas from non-associated gas wells. It shows how raw natural gas is processed into sales gas pipelined to the end user markets. [ [http://www.eia.doe.gov/pub/oil_gas/natural_gas/feature_articles/2006/ngprocess/ngprocess.pdf "Natural Gas Processing: The Crucial Link Between Natural Gas Production and Its Transportation to Market"] ] [ [http://www.uop.com/gasprocessing/6070.html "Example Gas Plant"] ] [ [http://www.axens.net/upload/presentations/fichier/axens_gpagcc_2004v2.pdf "From Purification to Liquefaction Gas Processing"] ] [ [http://www.spe.org/specma/binary/files/5804785Syn10682.pdf "Feed-Gas Treatment Design for the Pearl GTL Project"] ] [ [http://lnglicensing.conocophillips.com/NR/rdonlyres/B78B6727-E5F4-4505-B9C3-96CC94D7B30D/7357/AICHELNGNGLIntegrationPaper.pdf "Benefits of integrating NGL extraction and LNG liquefaction"] ] It also shows how processing of the raw natural gas yields these byproducts:

*Natural gas condensate
*Sulfur
*Ethane
*Natural gas liquids (NGL): propane, butanes and C5+ (which is the commonly used term for pentanes plus higher molecular weight hydrocarbons)

Raw natural gas is commonly collected from a group of adjacent wells and is first processed at that collection point for removal of free liquid water and natural gas condensate. The condensate is usually then transported to an oil refinery and the water is disposed of as wastewater.

The raw gas is then pipelined to a gas processing plant where the initial purification is usually the removal of acid gases (hydrogen sulfide and carbon dioxide). There a many processes that are available for that purpose as shown in the flow diagram, but amine treating is the most widely used process. In the last ten years, a new process based on the use of polymeric membranes to dehydrate and separate the carbon dioxide and hydrogen sulfide from the natural gas stream is gaining acceptance.

The acid gases removed by amine treating are then routed into a sulfur recovery unit which converts the hydrogen sulfide in the acid gas into elemental sulfur. There are a number of processes available for that conversion, but the Claus process is by far the one usually selected. The residual gas from the Claus process is commonly called "tail gas" and that gas is then processed in a tail gas treating unit (TGTU) to recover and recycle residual sulfur-containing compounds back into the Claus unit. Again, as shown in the flow diagram, there are a number of processes available for treating the Claus unit tail gas. The final residual gas from the TGTU is incinerated. Thus, the carbon dioxide in the raw natural gas ends up in the incinerator flue gas stack.

The next step in the gas processing plant is to remove water vapor from the gas using either the regenerable absorption in liquid triethylene glycol (TEG), commonly referred to as glycol dehydration, or a Pressure Swing Adsorption (PSA) unit which is regenerable adsorption using a solid adsorbent. [ [http://www.uop.com/objects/96%20MolecularSieves.pdf Molecular Sieves] (includes a flow diagram of a PSA unit)] Other newer processes requiring a higher pressure drop like membranes or dehydration at supersonic velocity using, for example, the Twister Supersonic Separator may also be considered.

Mercury is then removed by using adsorption processes (as shown in the flow diagram) such as activated carbon or regenerable molecular sieves.

Nitrogen is next removed and rejected using one of the three processes indicated on the flow diagram:

*Cryogenic process ["Gas Processes 2002", Hydrocarbon Processing, pages 84-86, May 2002 (schematic flow diagrams and descriptions of the Nitrogen Rejection and Nitrogen Removal processes)] using low temperature distillation. This process can be modified to also recover helium, if desired.
*Absorption process [http://www.aet.com/gtip1.htm "Market-Driven Evolution of Gas Processing Technologies for NGLs "] Advanced Extraction Technology Inc. website page] using lean oil or a special solvent [ [http://www.aet.com/aetnrubig.pdf AET Process Nitrogen Rejection Unit] Advanced Extraction Technology Inc. website page] as the absorbent.
*Adsorption process using activated carbon or molecular sieves as the adsorbent. This process may have limited applicability because it is said to incur the loss of butanes and heaver hydrocarbons.

The next step is to recover of the natural gas liquids (NGL) for which most large, modern gas processing plants use another cryogenic low temperature distillation process involving expansion of the gas through a turbo-expander followed by distillation in a demethanizing fractionating column. [ [http://www.aet.com/turbo.htm "Cryogenic Turbo-Expander Process"] Advanced Extraction Technology Inc. website page] ["Gas Processes 2002", Hydrocarbon Processing, pages 83-84, May 2002 (schematic flow diagrams and descriptions of the NGL-Pro and NGL Recovery processes)] Some gas processing plants use lean oil absorption process rather than the cryogenic turbo-expander process.

The residue gas from the NGL recovery section is the final, purified sales gas which is pipelined to the end-user markets.

The recovered NGL stream is processes through a fractionation train consisting of three distillation towers in series: a dethanizer, a depropanizer and a debutanizer. The overhead product from the deethanizer is ethane and the bottoms are fed to the depropanizer. The overhead product from the depropanizer is propane and the bottoms are fed to the debutanizer. The overhead product from the debutanizer is a mixture of normal and iso-butane, and the bottoms product is a C5+ mixture. The recovered streams of propane, butanes and C5+ are each "sweetened" in a Merox process unit to convert undesirable mercaptans into disulfides and, along with the recovered ethane, are the final NGL by-products from the gas processing plant.

References

External links

* [http://www.ucalgary.ca/ENCH/class_notes/ench607/mainmenu.pdf Natural Gas Processing Principles and Technology] (an extensive and detailed course text by Dr. A.H. Younger, University of Calgary, Alberta, Canada).

* [http://www.naturalgas.org/naturalgas/processing_ng.asp Processing Natural Gas] a website maintained by the Natural Gas Supply Association.

* [http://www.epa.gov/ttn/chief/ap42/ch05/final/c05s03.pdf Natural Gas Processing] (part of the US EPA's AP-42 publication)

* [http://primis.phmsa.dot.gov/comm/FactSheets/FSNaturalGasProcessingPlants.htm Natural Gas Processing Plants] (a US Department of Transportation website)


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • Natural-gas processing — is a complex industrial process designed to clean raw natural gas by separating impurities and various non methane hydrocarbons and fluids to produce what is known as pipeline quality dry natural gas.[1] Contents 1 Background 2 Types of raw… …   Wikipedia

  • Natural gas processing plant —   Facilities designed to recover natural gas liquids from a stream of natural gas that may or may not have passed through lease separators and/or field separation facilities. These facilities control the quality of the natural gas to be marketed …   Energy terms

  • 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… …   Wikipedia

  • 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… …   Wikipedia

  • Natural gas storage — Natural gas, like many other commodities, can be stored for an indefinite period of time in natural gas storage facilities for later consumption. Contents 1 Usage 2 Measures and definitions 3 Types 3.1 …   Wikipedia

  • Natural gas — For other uses, see Natural gas (disambiguation). Natural gas extraction by countries in cubic meters per year. Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons[1] …   Wikipedia

  • Gas processing unit —   A facility designed to recover natural gas liquids from a stream of natural gas that may or may not have passed through lease separators and/or field separation facilities. Another function of natural gas processing plants is to control the… …   Energy terms

  • Natural gas plant liquids —   Those hydrocarbons in natural gas that are separated as liquids at natural gas processing plants, fractionating and cycling plants, and in some instances, field facilities. Lease condensate is excluded. Products obtained include liquefied… …   Energy terms

  • Natural gas prices — at the Henry Hub in US Dollars per MBtu for the 2000 2010 decade. Natural gas prices, as with other commodity prices, are mainly driven by supply and demand fundamentals. However, natural gas prices may also be linked to the price of crude oil… …   Wikipedia

  • natural gas — a combustible mixture of gaseous hydrocarbons that accumulates in porous sedimentary rocks, esp. those yielding petroleum, consisting usually of over 80 percent methane together with minor amounts of ethane, propane, butane, nitrogen, and,… …   Universalium


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.