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What is Hydrocarbon Dew Point?Hydrocarbon Dew Point (HCDP) is not an easy parameter to measure, due to numerousfactors including gas composition, contaminants and additives, high pressures, and thepresence of corrosive compounds.

HCDP indicates the temperature at which heavy hydrocarbon components begin to condenseout of the gaseous phase, when the gas is cooled at constant pressure: Hence theexpression “hydrocarbon liquid drop out”. Therefore, a higher HCDP normally indicates ahigher proportion of heavy hydrocarbon components.

The HCDP is defined as the series of matching pressure and temperature points at whichhydrocarbons condense into liquid from a natural gas mixture. It is typically displayed on aphase diagram (see below) as a function of gas pressure and temperature, for natural gaswith a given composition. The dew point line divides the two-phase gas-liquid region andthe single-phase gas region. Two dew point temperatures are possible at a given pressureand two dew point pressures are possible at a given temperature.

H 2 O & Hydrocarbon Dew Point Curves for a Typical Natural Gas Mixture

This phase envelope phenomenon provides for behavior known as retrograde condensation.The word “retrograde” means moving backward and this phenomenon was given the name

because it is contradictory to the phase behavior of pure components, which condense withincreasing pressure and/or decreasing temperature. The maximum pressure at which liquidscan form is called the cricondenbar, and the maximum temperature at which liquids canform is called the cricondentherm.

Note that given the shape of the phase envelope, the measurement of hydrocarbon dewpoint and potential hydrocarbon liquid is usually carried out at a pressure between 25 and30 bar (ideally 27 bar) where liquid drop out occurs at the highest temperatures.

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When condensate forms from a gas mixture, the distribution of hydrocarbons changes sothat the liquid phase becomes enriched in the heavier components while the gas phasebecomes depleted of these heavier components. As the gas is cooled below its original dew

point temperature, the entire dew point curve shifts cooler for the remaining gas phase thatis now depleted in heavier components. The chilled gas temperature becomes the newHCDP of the gas stream.

Effect of C6+ Distribution on Hydrocarbon Dew Point Curves 1086 Btu/scf, 0.1 mol C6+

Natural gas contains a wide range of hydrocarbons, from light short chain aliphatics toheavy, long chain molecules making it difficult to measure the dew point. Each hydrocarbonhas a slightly different dew point temperature for a given pressure, thought the importanttemperature is that at which the first molecules start to condense out, as other moleculeswill condensate soon after.

Hence, the HCDP is very sensitive to the specific components of the gas stream and isstrongly influenced by the concentration of the heavier hydrocarbons, especially C6+. Thepresence of heavier hydrocarbons will increase the HCDP and failure to include them in aHCDP calculation will under predict the HCDP. Therefore, an accurate determination of theHCDP requires evaluation of distribution of the individual components in the C6+ fraction (at

least C9 but possibly higher).

It is also critical to note that processing a gas stream primarily removes or extracts heavyhydrocarbons and thus reduces the HCDP of a given mixture. The level of hydrocarbonremoval directly impacts the HCDP.

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