Testing of Multiple-States Method for Inferential Determination of Nitrogen Content in Natural Gas, 18-9382Printer Friendly Version
Inclusive Dates: 04/01/03 - 07/31/03
Background - Since 1998, Southwest Research Institute (SwRI) has been developing an innovative technology for determining the energy content of natural gas. The technology uses a patented computer algorithm created at SwRI to inferentially determine, in nearly real time, various natural gas properties, including heating value, molecular weight, and density. Relatively simple measurements of the gas temperature, pressure, and sound speed, as well as the diluent concentrations for nitrogen and carbon dioxide are all that are required as inputs for the computational algorithm. Traditionally, the natural gas industry has relied on gas chromatography to analyze the energy content or heating value of natural gas. The cost of installing, operating, and maintaining gas chromatographs is at least twice as much as the projected cost of implementing the new SwRI technology. Thus, this new SwRI technology will provide substantial cost savings, with no appreciable loss in measurement precision, when used in place of conventional gas chromatographs. Also, because of its expected cost effectiveness, this technology can be deployed at many field measurement sites where gas chromatography is not economically viable.
In 20002001, SwRI engineers created the first energy meter prototype module that measured all of the required algorithm inputs, except the nitrogen concentration. A recent review of existing technology, conducted by SwRI, concluded that no simple, accurate and inexpensive method for determining nitrogen concentration in natural gas currently exists. As a follow-up to that review, SwRI demonstrated the potential of inferentially determining nitrogen concentration in natural gas blends by using the previously-developed algorithm correlations applied at two different thermodynamic states. The technique, called the multiple-states method, demonstrated positive results for a selected gas composition, with less than ±0.10 mol percent error in the nitrogen content of the gas blend.
Approach - The project objective was to determine the viability of using the multiple-states method in the energy meter algorithm to inferentially determine nitrogen concentration for a broad range of gas compositions, temperatures, and pressures. In addition, the project studied the possibility of refining the algorithm to increase the method accuracy by selecting a new set of reference gas blends.
The multiple-states method was tested analytically over a broad temperature and pressure range for three gas compositions with varying nitrogen content. A commercial software package was used to apply equation-of-state calculations to determine the speed of sound of the test gas at the different thermodynamic states. Experimental results were simulated analytically by altering the input measurements for the gas speed of sound and carbon dioxide concentration by allowable error amounts. The algorithm output was compared to the known nitrogen content of the selected gas compositions.
Accomplishments - The multiple-states method appears to be a viable method for determining nitrogen concentration accuratelyto within ±0.10 to 0.30 mol percent, depending on the accuracy of the input measurements and the gas composition. The accuracy of the method relies heavily on the formulation of the algorithm and its associated reference gases. Because of the success of the project and the viability of the method, the energy meter project was able to secure client funding for 20032004. In addition, the quick-look project generated additional tasks for the energy meter algorithm research in 2003 that may aid in the improvement of the accuracy of the multiple-states method.