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.
- 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 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.
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