The occurrence of liquid hydrocarbons in natural gas transmission lines has increased in recent years as a result of the shrinking price spread between natural gas and natural gas liquids (NGL’s). Consequently, there is increasing interest among many pipeline companies in monitoring hydrocarbon dew point (HCDP) and liquids in the transmission lines to ensure the safety and reliability of the system. This paper examines the methods available for determining the HCDP of natural gases and their implementation in transmission systems. A case study is presented on Questar Pipeline Company’s management and control of HCDP issues in their interstate gas transmission system in Utah, Wyoming and Colorado.

Process simulators have been used for years to design and model actual operation of all types of different plant processes. The majority of process simulators provide a “steady-state” picture of plant operations and do not account for changes in inlet or ambient conditions. Steady state simulators are very useful when first designing a plant under a certain set of conditions, or when developing a baseline for plant operation. These simulators are also much more affordable than the dynamic simulators that are available in today’s market. Unfortunately, plant operating conditions very seldom match design conditions and it is difficult for the Operator to discern what effect the changing conditions have on his process without performing numerous simulations using trial and error and manual manipulation. Even then, these results are often times suspect. Crosstex Energy Services, L.P. and Bryan Research and Engineering, Inc. undertook a project to model one of the Crosstex gas processing facilities using the ProMax simulation software. Using the program’s capabilities to rate the performance of various plant equipment, as it executes the simulation, and by utilizing available parametric study features that allow numerous runs to be made consecutively, without interruption, the ProMax simulator was able to provide a series of “snapshots” that provided a realistic and accurate prediction of how the plant will respond to changes in conditions. While this is still a prediction of steady state operation, the simulator has approached the dynamic threshold and only lacks the time derivative to cross over into that next dimension. This paper will show the steps that were taken to reach this point, the benefits it provided and how it might be used at other plant locations.

This paper focuses on the modeling of solid phase behavior in systems that are frequently encountered in natural gas processing. The ability to perform accurate calculation of freezing or solids formation conditions in processes from dry ice, hydrates, and water ice is quite important. Although the primary focus in this work is on dry ice formation from carbon dioxide, analogies with hydrate formation are presented. A description of the phase equilibria at different conditions of temperature and pressure is included. The paper compares the predicted results from simulation with selected experimental data sets, and illustrates that accurate results are obtained over a wide variety of conditions. However, due to the complicated phase behavior of these systems, improper interpretation of results, or incorrect use of the tools within the simulator is possible due to the multiplicity of incipient formation points. One fact that is not well known is that lowering the temperature may cause a solid that has formed to melt under certain conditions of pressure and composition. While recent work has been done to mitigate the incorrect application of these tools, knowledge of some of the different types of phase behavior is generally desirable to understand and exploit the results. Phase diagrams are presented to aid in understanding the solid formation behavior.

The use of process simulators to model plant operations can provide a plant hundreds of thousands of dollars each year in increased production and lower energy costs. This paper looks at several example plants where process simulators were utilized to optimize their operation and measurable results were obtained. Each of these plants were able to improve their bottom line profit because a process simulator was available and plant personnel were dedicated to using it to improve plant performance and efficiency. In today’s economic roller coaster, where product margins can be positive one day and negative the next, a plant must be designed and operated with the utmost operating flexibility, while maintaining high energy efficiency. The process simulator allows both the designer and the operator to maximize this flexibility and determine the best way to operate the plant at both ends of the operating spectrum. Today, a plant that does not use a simulator to monitor its operation is simply throwing money away.

Most engineers have little familiarity with the software technologies that provide the framework for the variety of applications they employ, from word processors to process simulators. While adequate for casual use of an application, a more thorough understanding of these technologies is required in order to extend an application and provide custom behavior. Usually the effort required to perform small customizations is not significant provided the framework is understood. This paper introduces Object-Oriented Programming (OOP), OLE Automation, and Extensible Markup Language (XML), three common technologies used in programs. A conceptual discussion of OOP is presented along with examples where the paradigm may be encountered. Automation is introduced with illustrations of how this feature can extend an application. Finally, XML is summarized along with a discussion of some of the tools and supporting technologies used with XML. The aim of this paper is to give a basic understanding of how and when these technologies can be exploited based on specific applications and tasks.

Due to the increase in natural gas prices in the past few years, the benefits of optimizing natural gas gathering and processing systems have become substantially greater. These benefits can be observed from an analysis of the operating conditions, updating gas contracts, and adding gas to existing systems when excellent opportunities exist. A new technique has been developed to accurately model gas processing systems to incorporate an economic simulation with the process simulation. This new technique utilizes an Excel interface with process simulation software to include economic factors with the simulation results. As a result, an extended analysis of the operating conditions of the facility as well as the economic conditions can be simultaneously combined to provide a complete model of the system. This methodology can be extended to include a parametric study of the available process and economic variables. Excel solvers may also be used to generate the economic optimum set operating conditions.

Hydrate inhibition with methanol continues to play a critical role in many operations. Opportunities exist at many facilities for optimizing the amount of methanol required based on the operating conditions. To properly predict these requirements, the distribution of the methanol between the gas and liquid phases is of key importance. Significant contributions by the GPA research program both in past years and current or future research projects make it possible to better predict methanol requirements for hydrate inhibition from commercial simulators. However, a proper understanding of experimental methods and actual sample and overall compositions is very important to an accurate interpretation of the results.

Hydrate inhibition with methanol continues to play a critical role in many operations. Numerous opportunities exist for optimizing methanol usage based on the operating conditions, seasonal variations in temperature, and accurate prediction of the hydrate formation temperature. To properly predict the requirements, the distribution of methanol between the gas and liquid phases is of key importance. These opportunities for optimization have been made possible primarily through research data from the GPA.

Over the years, the Gas Processors Association (GPA) has appropriated funding toward research that has served the Gas Processing Industry in many ways. Perhaps the most significant manner in which the benefits of this research have been realized is through more accurate measurements of phase equilibria, enthalpy, density, and other physical properties leading to more efficient engineering analysis and design. In particular, the accuracy of process simulators has been dramatically improved since these basic properties are involved in virtually every calculation. This article will review many of the projects undertaken by GPA. The article will provide examples where accurate predictions were not possible for engineering calculations due to lack of data, but today are performed routinely due to data collected under GPA research. Finally, the article will suggest some areas of possible research where current data are limited.

Vapor-liquid equilibrium is predicted by using the Soave modification of the Redlich-Kwong equation of state (EOS). The concept of equal fugacities is used to calculate the equilibrium constant, Ki=yi/zi, then, it is shown how the Murphree tray efficiency can be applied on the liquid or the vapor phases to modify that constant. The derivatives needed are calculated numerically, and it is shown that for absorbers and distillation columns, Murphree tray efficiency applied this way can be used to simulate the actual number of stages. Murphree tray efficiency values can be specified for one, several or all of the components on any stages of a column. A dehydration example is shown, the dew point depression values of a mixture of water-gas, using a triethylene glycol solution for dehydration purposes, are calculated by incorporating the method into a process simulator program called PROSIM® and compared with the values reported in the literature.