Hydrocarbon dew point is the temperature at which hydrocarbon-rich gases such as natural gas begin to condense. It is the function of gas composition and pressure. The determination of hydrocarbon dew point for natural gas has become critical for the natural gas industry as hydrocarbon liquids in the gas stream can lead to an increase in compression costs, issues with pressure regulator freezing, hydrate formation and damage of equipments such as gas turbines.
One of the simplest and commonly used methods of hydrocarbon dew point measurement is the manually operated cooled or chilled mirror dew point meters that have a metallic mirror surface with a pressurized sample chamber.
The chamber in turn includes a glass viewing port through which the operator can observe the mirror surface. These instruments are used for periodic spot check measurements.
Hydrocarbon liquid dropout in gas transmission lines can cause several problems including reduced line capacity, compressor damage and pressure drop.
In 2011, Bullin et al from the Bryan Research & Engineering, Inc. proposed a method to determine hydrocarbon dew point specification that allows small quantity of liquid without affecting the operations of gas transmission lines. Experimental results showed that 0.002 GPM of liquid has a negligible effect on pressure drop and does not have any significant impact on pipeline operations.
Hydrocarbon dew point in natural gas becomes a cause of concern for Saudi Aramco and Saudi Electricity Company that operate more than 150 aeroderivative and industrial combustion gas turbines.
Wide variations have been observed in the hydrocarbon dew points of gases supplied to these turbines during the automated on-line hydrocarbon dew point monitoring conducted by Newbound et al of Saudi Arabian Oil Company in 2003.
Variations in dew point temperatures were found to be high during the winter months. Moreover, dramatic operational changes on two major gas plants have been observed.
However, the sensitivity of the turbines to liquids in the fuel can be improved by increasing the firing temperatures. Hence it was concluded that hydrocarbon dew point monitoring can recommend fuel preheating temperatures and the desired fuel conditioning equipment.
In 2009, Brown et al from the National Physical Laboratory, Teddington, U.K presented a detailed comparison on the performance of direct and indirect methods of measuring the hydrocarbon dew point for real and synthetic natural gas mixtures.
The relative order of the dew points determined by direct and indirect methods for gas mixtures was found to be reversed. In addition, the utilization of automatic chilled mirror instrument for measuring the dew point of gases having low condensation rates resulted in a discrepancy of up to 2 K when compared to the gases having high condensation rates.
Some of the recent applications of hydrocarbon dew point monitoring device include the following:
- Pipeline quality and custody transfer
- Gas turbine feed gas monitoring
- Control of contractual blending operations
- Optimizing of switch times for pressure swing
- Monitoring inlet and outlet of storage facilities.
Current industry trends have increased the profile of hydrocarbon dew point as a critical gas quality parameter.
Work is still being carried out to advance the industrial methods of determining natural gas hydrocarbon dew point to accurately predict the required changes in the natural gas supply.
Current methods of determining dew points, particularly software packages, are likely to under-predict the hydrocarbon dew points of natural gas as the heating value and line pressure of the gas stream increases.
However, the accuracy of dew points can be improved with the proper characterization of heavy ends of hydrocarbons.
Research has found that the best method of characterizing hydrocarbons would depend on the actual composition of the gas. In addition, a reliable and accurate hydrocarbon dew point monitoring device can assist in controlling the gas processing operations.
- AMETEK Model 241CE II Hydrocarbon Dew Point Analyzer - Tectra
- Newbound TD and Wagiealla WS. On-line hydrocarbon dew point monitoring in fuel gas. ASME Turbo Expo 2003, collocated with the 2003 International Joint Power Generation Conference. 2003;4:113-117.
- Bullin JA and Fitz C. Practical hydrocarbon dew point specification from natural gas transmission lines. Bryan Research and Engineering, Inc.
- Brown AS, et al. Measurement of the hydrocarbon dew point of real synthetic natural gas mixtures by direct and indirect methods. Energy Fuels. 2009;23(3):1640-1650.
- Hydrocarbon dew point is a critical consideration for pipeline operations - Webb Kentrol/Sevco.