Hydrocarbon Dew Point

The hydrocarbon dew point is the temperature (at a state 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. The maximum

temperature and the pressure at which this condensation takes place is called the

cricondentherm.

It is often also referred to as the HDP or the HCDP. 

Importance of the hydrocarbon dew point
Therefore, 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 the final end users.

If the hydrocarbon dew point of pipelined natural gas is too high, some liquids may condense out in the gas pipeline. This not only degrades the heating value of the remaining gas, it increases the potential for problems in the pipeline transmission systems and causes problems for the end users of the gas such as industrial combustion equipment and household gas appliances.

Consequences

If natural gas is allowed to cool below its dew point, in storage or use, then hydrocarbon liquids are formed. This

can have deleterious consequences depending upon what the natural gas is being used for

• sampling – unrepresentative sample

• electricity generation – damage to turbines

• transportation – liquid in low points of system Measurement

There are three commonly used ways of determining hydrocarbon dew point, the cold mirror method (de facto

standard), the gravimetric method (based on ISO 6570) and calculation from composition data provided by

chromatography.

Cold mirror (dewscope)

This method tends to be the benchmark against which other techniques are compared.

The principle is based on observation of the formation of a film of hydrocarbon condensate on the surface of an

illuminated cooled mirror. Natural gas is passed over the mirror surface, at 27 bar, which is cooled by a coolant such as carbon dioxide.

Experience indicates that a dewscope measures a slightly lower temperature than the actual true dew point. This is due to the need to build up a finite amount of liquid on the mirror before it can be seen. During this time, the mirror is still cooling, and the temperature is read, not as the first molecule condenses, but a little later when an observable amount has accumulated. The nature of the liquid formation is unknown.

Gravimetric method

This method is designed to measure the mass of condensate that is separated from natural gas at a defined temperature and pressure. It can be a fully automated method, and is described in ISO 6570: Measurement of potential hydrocarbon liquid.

The gas pressure, the amount of gas flowed, the cooling bath and gas temperatures and the mass of condensate are logged. From this record, it is easy to plot the amount of condensate collected against time, or the amount of

condensate per cubic metre of gas as a function of time.

The points to note about this method are • it does not give a direct correlation to hydrocarbon dew point – although the amount of liquid produced at a given temperature is of more use for pipeline operations

• amounts of liquid being collected are usually very small so are averaged out over a longer period of time

• low rates of condensate collection can be measured, but only with hindsight, considering data averaged

over an hour or longer

• slow response time for monitoring increases in hydrocarbon content.

Calculation from analysis

This relies on analysing natural gas by chromatography then using a computer program (see below inbrief) to calculate the dew point by solving an equation of state.

Points to note about this approach 

• ideally need to measure every component present

(this is not possible due to the complexity of natural gas)

• analysis to n-pentane with a C6+ backflush is not good enough

• some assumptions about grouping of hydrocarbons have to be made regarding the make up and

treatment of carbon number six and above

• depending upon the assumptions made above the calculated dew point temperature can vary

• the quality of the chromatographic data is of vital importance in this method.

Which method is the correct one? This depends upon accuracy required, frequency of results, degree of

automation and costs of operation.

In brief it provides

• dew temperature at any given pressure

• maximum dew temperature (cricondentherm)

• amount of liquid formed in two phase region

• choice of equation of state

• single phase EOS solutions

• 2-phase vapour-liquid equilibrium EOS solutions

• choice of pressure and temperature units

• 250+ component thermodynamic properties database