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The '''mole fraction''' is a measure of the concentration of a component substance in a mixture of substances. It is defined as the number of [[mole]]s of a component substance in a mixture divided by the total number of moles of the mixture.<ref>{{cite book|author=N.A. Gokcen and R.G. Reddy|title=Thermodynamics|edition=2nd Edition|publisher=Plenum Press|year=1996|id=ISBN 0-306-45380-0}}</ref> It may be expressed as:
'''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 field]]s. It [[Condensation|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:<ref>[http://www.eia.doe.gov/emeu/iea/glossary.html#L International Energy Glossary] (a page from the website of the [[Energy Information Administration]])</ref><ref>[http://www.eia.doe.gov/pub/oil_gas/natural_gas/feature_articles/2006/ngprocess/ngprocess.pdf Natural gas processing] (a page from the website of the [[Energy Information Administration]])</ref>  


:<math>x_\mathrm{a} = \frac{n_\mathrm{a}}{n_\mathrm{a} + n_\mathrm{b} + n_\mathrm{c} \ldots} = \frac{n_\mathrm{a}}{n_\mathrm{total}}</math>
*Crude [[oil well]]s – 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.


{|border="0" cellpadding="2"
*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.
|-
|align=right|where:
|-
|align=right|<font style="vertical-align:-20%;"><math>x_\mathrm{a}</math></font>
|align=left|= the mole fraction of component <font style="vertical-align:-5%;"><math>\mathrm{a}</math></font> in the mixture
|-
|align=right|<font style="vertical-align:-20%;"><math>n_\mathrm{a}</math></font>
|align=left|= the number of moles of component <font style="vertical-align:-5%;"><math>\mathrm{a}</math></font> in the mixture
|-
|align=right|<font style="vertical-align:-20%;"><math>n_\mathrm{b}</math></font>
|align=left|= the number of moles of component <font style="vertical-align:0%;"><math>\mathrm{b}</math></font> in the mixture
|-
|align=right|<font style="vertical-align:-20%;"><math>n_\mathrm{c}</math></font>
|align=left|= the number of moles of component <font style="vertical-align:-5%;"><math>\mathrm{c}</math></font> in the mixture
|-
|align=right|<font style="vertical-align:-20%;"><math>n_\mathrm{total}</math></font>
|align=left|= the total number of moles in the mixture
|}


Mole fractions are dimensional numbers and they are often referred to as ''mol fractions''.
*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''.  


==Applications==


Mole fractions are very commonly used in [[chemistry]], [[thermodynamics]], [[chemical engineering]] and other disciplines to express concentrations of specific substances in [[gas]] mixtures and [[liquid]] [[solutions]].
 
==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:<ref>[http://www.marathonpetroleum.com/msds/0245MAR001.pdf Natural Gas Condensate] [[Marathon Oil Company]] [[MSDS]]</ref><ref>[http://seweb1.phillips66.com/hes/msds.nsf/5bbae21d3705607e86256bd5006a8bc9/bdb184e665400a87862565e5004b85a2?OpenDocument Natural Gas Condensate] [[Phillips Petroleum Company]] MSDS</ref><ref>[http://seweb1.phillips66.com/hes%5Cmsds.nsf/MSDSID/US741060/$file/30017244.pdf Condensate (Alaska)] [[ConocoPhillips]] of Alaska MSDS</ref><ref>[http://www.hess.com/ehs/msds/Nat_Gas_Cond_Sweet_15017_clr.pdf  Natural Gas Condensate] [[Amerada Hess Corporation]] MSDS</ref>
 
*[[Hydrogen sulfide]] (H<sub>2</sub>S}
*[[Thiol]]s traditionally also called [[mercaptan]]s (denoted as RSH, where R is an organic group such as methyl, ethyl, etc.)
*[[Carbon dioxide]] (CO<sub>2</sub>) 
*[[Alkane|Straight-chain alkanes]] having from 2 to 12 [[carbon]] atoms (denoted as C<sub>2</sub> to C<sub>12</sub>)
*[[Cyclohexane]] and perhaps other [[naphthene]]s
*[[Aromatics]] ([[benzene]], [[toluene]], [[xylenes]] and [[ethylbenzene]])
 
==Separating the condensate from the raw natural gas==
 
[[Image:NaturalGasCondensate.png|frame|right|Schematic flow diagram of the separation of condensate from raw natural gas]]
There are quite literally hundreds of different equipment configurations for the processing required to separate natural gas condensate from a raw natural gas. The [[Process flow diagram|schematic flow diagram]] to the right depicts just one of the possible configurations.<ref>[http://www.mse.co.uk/pdf/papers/tp01.pdf Simplified Process Flow Diagram]</ref>
 
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 evaporation|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 transport|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 [[dehydration|dehydrated]] and [[acid gas]]es and other impurities will be removed from the gas. Then the  [[ethane]] (C<sub>2</sub>), propane (C<sub>3</sub>), butanes (C<sub>4</sub>) and C<sub>5</sub>  plus higher [[molecular weight]] hydrocarbons (referred to as C<sub>5</sub>+) 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 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.


==References==
==References==
{{reflist}}
{{reflist}}
==External links==
*[http://www.naturalgas.org/naturalgas/processing_ng.asp Processing raw natural gas]
*[http://www.petrostrategies.org/Learning%20Center/Gas_Processing.htm Preparing raw natural gas for sales]
*[http://www.epa.gov/ttn/chief/ap42/ch05/final/c05s03.pdf Natural Gas Processing] (part of the [[AP 42 Compilation of Air Pollutant Emission Factors|US EPA's AP-42 publication]] and includes a schematic diagram)

Revision as of 15:18, 3 September 2008

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.


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]

Separating the condensate from the raw natural gas

Schematic flow diagram of the separation of condensate from raw natural gas

There are quite 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 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 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.

References

  1. ↑ International Energy Glossary (a page from the website of the Energy Information Administration)
  2. ↑ Natural gas processing (a page from the website of the Energy Information Administration)
  3. ↑ Natural Gas Condensate Marathon Oil Company MSDS
  4. ↑ Natural Gas Condensate Phillips Petroleum Company MSDS
  5. ↑ Condensate (Alaska) ConocoPhillips of Alaska MSDS
  6. ↑ Natural Gas Condensate Amerada Hess Corporation MSDS
  7. ↑ Simplified Process Flow Diagram

External links

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