Hydrocarbon dew point

  1. Pages with script errors
  2. Chemical engineering
  3. Natural gas
  4. Hydrocarbons


For the meteorological dew point, see 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 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.

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 chain, 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 (28,000 m3) 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, 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 (28 m3) 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.

Although GPM is an additional parameter of some value, most pipeline operators and others who process, transport, distribute or use natural gas are primarily interested in the actual HCDP, rather than GPM. Furthermore, GPM and HCDP are not interchangeable and one should be careful not to confuse what each one exactly means.

Methods of HCDP determination

There are primarily two categories of HCDP determination. One category involves "theoretical" methods, and the other involves "experimental" methods.

Theoretical methods of HCDP determination

The theoretical methods use the component analysis of the gas mixture (usually via gas chromatography, GC) and then use an equation of state (EOS) to calculate what the dew point of the mixture should be at a given pressure. The Peng-Robinson and Kwong-Redlich-Soave equations of state are the most commonly used for determining the HCDP in the natural gas industry.

The theoretical methods using GC analysis suffer from four sources of error:

  • The first source of error is the sampling error. Pipelines operate at high pressure. To do a field GC analysis, the pressure has to be regulated down to close to atmospheric pressure. In the process of reducing pressure, some of the heavier components may drop out, particularly if the pressure reduction is done in the retrograde region. Therefore the gas reaching the GC is fundamentally different (usually leaner in the heavy components) than the actual gas in the pipeline. If a sample bottle is collected for delivery to a laboratory, significant care must be taken not to introduce any contaminants to the sample and make sure that the sample bottle represents the actual gas in the pipeline.
  • The second source is the error on the analysis of the gas mix components. A typical field GC will have at best (under ideal conditions and frequent calibration) ~2% (of range) error in the quantity of each gas analyzed. Since the range for most field-GCs for C6 components is 0-1 mol%, there will be about 0.02 mol% uncertainty in the quantity of C6+ components. While this error does not change the BTU value by much, it will introduce a significant error in the HC dew point determination. Furthermore, since the exact distribution of C6+ components is an unknown (the amount of C6, C7, C8, ...), this further introduces additional errors in any HC dew point calculations. When using a C6+ GC these errors can be as high as 100 °F or more, depending on the gas mixture and the assumptions made regarding the composition of the C6+ fraction. For "pipeline quality" natural gas, a C9+ GC analysis will reduce the uncertainty to approximately 25-30 °F. Using a laboratory C12+ GC analysis can reduce the error further to less than 1 °C. However, using a C12 laboratory system can introduce additional errors, namely sampling error. If the gas has to be collected in a sample bottle and shipped to a laboratory for C12 analysis, sampling errors can be significant. Obviously there is also a lag time error between the time the sample was collected and the time it was analyzed.[5]
  • The third source of errors is calibration errors. All GCs have to be calibrated routinely with a calibration gas representative of the gas under analysis. If the calibration gas is not representative, or calibrations are not routinely performed, there will be errors introduced.
  • The fourth source of error relates to the errors embedded in the equation of state model used to calculate the dew point. Different models are prone to varying amounts error at different pressure regimes and gas mixes. There is sometimes a significant divergence of calculated dew point based solely on the choice of equation of state used.

The significant advantage of using the theoretical models is that the HCDP at several pressures (as well as the cricondentherm) can be determined from a single analysis. This provides for operational uses such as determining the phase of the stream flowing through the flow-meter, determining if the sample has been affected by ambient temperature in the sample system, and avoiding [[Amine gas treating|amine] foaming from liquid hydrocarbons in the amine contactor.

GC vendors with a product targeting the HCDP analysis include ABB, Thermo-fisher, Emerson, as well as other companies.

Experimental methods of HCDP determination

In the "experimental" methods, one actually cools a surface on which gas condenses and then measures the temperature at which the condensation takes place. The experimental methods can be divided into manual and automated systems. Manual systems, such as the Bureau of Mines dewpoint meter, depend on an operator to manually cool the chilled mirror slowly and to visually detect the onset of condensation. The automated methods use automatic mirror chilling controls and sensors to detect the amount of light reflected by the mirror and detect when condensation occurs through a drop in measured light. The chilled mirror technique is a first principle measurement which is not subject to "calculation" or "choice of model" errors. However, some liquids must condense before the detection of condensation on the mirror can occur, and therefore results in values lower than the theoretical methods.[5]

The experimental method also suffers from two sources of error. The first error is in the detection of condensation. A manual chilled mirror device relies on the operator to determine when a mist has formed on the mirror, and is highly subjective. In most cases, the operator under-reports the dew point because by the time condensation has accumulated enough to be visible to an operator, the dewpoint has already been reached and passed. The automated chilled mirror devices are significantly more repeatable but can be affected by contaminants that condense on the mirror before the hydrocarbons. The second source of error is the speed of the cooling of the mirror and the measurement of the temperature of the mirror when the condensation is detected. This error can be minimized by controlling the cooling speed, or having a fast condensation detection system.

The experimental methods only provide a HCDP at the pressure at which the measurement is taken, and cannot provide the cricondentherm or the HCDP at other pressures. However, in any event, most pipeline operators like to know the HCDP at their current line pressure. Given that the input pressure of the experimental systems can be adjusted by a regulator, one can also measure the HCDP at other pressures by adjusting the input pressure.

Companies who offer an automated chilled mirror system include, Ametek, Michell Instruments, and SpectraSensors[6]

See also

References

  1. ↑ Hydrocarbon Dew Point
  2. ↑ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, October 15, 2004)
  3. ↑ White Paper on Liquid Hydrocarbon Drop Out in Natural Gas Infrastructure (NGC+ Liquid Hydrocarbon Dropout Task Group, September 28, 2005)
  4. ↑ Script error (See page 110)
  5. ↑ 5.0 5.1 Script error
  6. ↑ HCD4000(TM) Hydrocarbon Dewpoint Analyzer

External links

de:Kohlenwasserstoff-Taupunkt


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