Mar-2011

Practical hydrocarbon dew point specification for natural gas transmission lines

Hydrocarbon liquid dropout can cause a number of problems in gas transmission lines, including increased pressure drop, reduced line capacity and equipment problems such as compressor damage.

Jerry A Bullin and Carl Fitz, Bryan Research & Engineering, Inc
Todd Dustman, Questar Pipeline Company

Article Summary

To avoid liquid dropout, most current operating specifications for gas transmission lines require that the lines be operated above the hydrocarbon dew point (HDP) or cricondentherm hydrocarbon dew point (CHDP). The HDP may be determined either by direct measurement such as the Bureau of Mines chilled mirror method or by calculation using an equation of state (EOS) with a measured composition. This project (GPA Project No. 081) was undertaken to determine a practical HDP specification, allowing small amounts of liquids that have no significant impact on operations. Results from the project show that 0.002 gallons of liquid per thousand standard cubic feet of gas (GPM) has a negligible effect on pressure drop and should not disrupt pipeline operations. Calculation of an accurate HDP from a GC analysis such as typically available at a custody transfer point may be useful but is highly dependent on the characterisation of the heavy fraction. An extended analysis of the heavy fraction is best. However, an empirical method has been developed to predict the C6, C7, C8, C9 and heavier composition when only a lumped C6+ fraction characterisation is available.

Introduction
Gas transmission lines are one of the core assets of the energy infrastructure in the United States. As a result, the operation of these lines must be as trouble-free as possible. A major operational consideration for gas pipelines is hydrocarbon liquid condensation from the natural gas. Hydrocarbon liquid in gas pipelines can cause operational issues, including increased pressure drop, reduced line capacity and equipment problems such as compressor damage. In order to avoid hydrocarbon condensation or “liquid dropout” in gas pipelines, several different control parameters have historically been monitored and assigned limits, including C6+ GPM (gallons of liquid per thousand standard cubic feet of gas), mole fraction C6+, HDP and CHDP.

The HDP is defined as the point at which the first droplet of hydrocarbon liquid condenses from the vapour. It can also be thought of as the minimum temperature above which no condensation of hydrocarbons occurs at a specified pressure. The CHDP, illustrated in Figure 1, defines the maximum temperature at which this condensation can occur regardless of pressure. The CHDP is heavily influenced by the C6+ GPM as shown in Figure 2 for 40 natural gas mixtures from Dustman et al1 and Brown et al.2 However, the relationship between CHDP and C6+ GPM is not exact due to differences in composition of the lumped C6+ fraction. The CHDP of a gas with C6+ GPM of 0.07 ranges from about 28 to 55°F (-2 to 13°C) as shown in Figure 2. This is a 27°F (15°C) variability in the CHDP. This variability is overcome by specifying the acceptable CHDP directly.

Most current operating specifications for gas transmission lines require that the lines be operated above the HDP or CHDP. The HDP may be determined by direct measurement using manual or automated dew point analysers. In the field, HDP is commonly measured using the Bureau of Mines chilled mirror method, where the natural gas sample flows continually across the surface of a small mirror, which is cooled by the flow of a low temperature gas on the other side. As the temperature is slowly reduced, the operator watches through an eyepiece for hydrocarbon condensation on the mirror surface. When condensation is detected, the dew point temperature and pressure are manually recorded (Starling3, George and Burkey4).

When the gas composition is known, a convenient method of determining the HDP is by calculation using a validated EOS. When the pressure and composition are specified, an EOS such as Peng-Robinson (PR) or Soave-Redlich-Kwong (SRK) can be used to accurately calculate the HDP. It must be noted that many variations of the generic PR and SRK EOS exist, and are not all equal. The most accurate contain modifications based on pure component properties and binary interactions. Therefore, it is necessary to validate an EOS by comparing to many sets of vapour-liquid equilibria (VLE) and dew point data.

While the dew point identifies the condition at which vapour first begins to condense to liquid, it provides no information about the quantity of condensation resulting from a small degree of cooling. The condensation rate of liquids in gas transmission lines may vary widely depending on the composition, temperature and pressure of the system. Condensation rates resulting from cooling were studied by the National Physical Laboratory in the United Kingdom2 for several different natural gases. The calculated condensation rate varied from practically nil at 9°F (5°C) below the dew point for a very lean natural gas to 500 mg/m3 (0.006 actual GPM) only 1°F (0.5°C) below the dew point for another natural gas. A pipeline containing the lean natural gas could be operated quite satisfactorily 9°F (5°C) below the dew point with little liquid dropout. On the other hand, a large amount of liquid dropout would occur if a pipeline with the second natural gas were to operate 9°F (5°C) below the dew point. Clearly the dew point alone does not provide enough information to completely identify conditions at which a pipeline can be operated without liquids problems. More information is needed about the degree of condensation that takes place below the dew point.

The objective of the present work is to develop a “practical” HDP that considers both the HDP curve and the degree of condensation that takes place below the dew point. The practical HDP should use the gas composition and an EOS to identify acceptable operating conditions for natural gas transmission lines. The current project is an extension to GPA Project 063 “Measuring Hydrocarbon Dew Points in Natural Gas”, which produced Research Reports RR-1964 and RR-199.5

Review of equations of state to calculate dew point
Equations of state that have been appropriately modified and validated can be used to accurately calculate the dew point of natural gas mixtures based on the composition. Two of the most popular generic EOS are the PR EOS6 and SRK EOS.7 These equations use critical temperature, critical pressure and acentric factor to describe the pure fluid. Mixtures require an additional one or two binary interaction parameters, which may be temperature dependent and can be obtained by fitting binary VLE data. Adding to the complexity, different mixing rules have been developed to improve phase equilibria predictions8 and numerous enhancements have been proposed such as Graboski and Daubert’s modifications contained in the API version of the SRK.9 Due to these possible variations and modifications, different computer programs that use the PR or SRK EOS will not necessarily produce the same answer.