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Hydrocarbon dew point

The hydrocarbon dew point is the temperature (at a given pressure) at which the hydrocarbon components of any hydrocarbon-rich gas mixture, such as natural gas, will start to condense out of the gaseous phase. It is often also referred to as the HDP or the HCDP. The maximum temperature and the pressure at which such condensation takes place is called the cricondentherm.^[1] The hydrocarbon dew point is a function of the gas composition as well as the pressure.

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The hydrocarbon dew point is universally used in the natural gas industry as an important quality parameter, stipulated in contractual specifications and enforced throughout the natural gas supply train, from producers through processing, transmission and distribution companies to final end users.

The hydrocarbon dew point of a gas is a different concept from the water dewpoint, the latter being the temperature (at a given pressure) at which water vapor present in a gas mixture will condense out of the gas.

Relation of the term GPM to the hydrocarbon dew point

In the United States, the hydrocarbon dewpoint of processed, pipelined natural gas is related to and characterized by the term GPM which is the gallons of liquifiable hydrocarbons contained in 1,000,000 cubic feet of natural gas at a stated temperature and pressure. When the liquifiable hydrocarbons are characterized as being hexane or higher molecular weight components, they are reported as GPM (C6+).^[2]^[3]

However, it should be noted that the quality of raw produced natural gas is also often characterized by the term GPM meaning the gallons of liquifiable hydrocarbons contained in 1,000 cubic feet of the raw natural gas. In such cases, when the liquifiable hydrocarbons in the raw natural gas are characterized as being ethane or higher molecular weight components, they are reported as GPM (C2+). Similarly, when characterized as being propane or higher molecular weight components, they are reported as GPM (C3+).^[4]

Care must be taken not to confuse the two different definitions of the term GPM.

References

^ Hydrocarbon Dew Point
^ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, October 15, 2004)
^ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, September 28, 2005)
^ A.J. Kidnay and William Parish (2006). Fundamentals of Natural Gas Processing, 1st Edition, CRC Press. ISBN 0-8493-3406-3. (See page 110)