Databases for Identification of Chemicals by Gas Chromatography: Natural Gas and Alternative Refrigerant Applications

T.J. Bruno

Objective: To provide an efficient, fast, and reliable method for identifying a wide variety of chemical compounds in both the laboratory and the field. Initial applications include the heavier components of natural gas (the C₆+ fraction), natural gas treatment materials, and alternative refrigerant fluids.

Problem: The design and operation of many processes involving fluids rely on an accurate chemical analysis of the fluid stream composition. For example, calorific value of natural gas is calculated from a chromatographic analysis of each individual gas stream. Custody transfer of natural gas is therefore based upon a detailed compositional analysis. The most common analysis of natural gas at present considers only the lighter components. This approach introduces significant uncertainty to subsequent calculations based on the gas composition. The inclusion of the heavier fraction into the analysis is a complex problem, because the gas consists of upwards of 400 organic and inorganic constituents. Moreover, the composition varies with season, with source-well long-term history, and with short-term usage and storage history. Thus, a fast, low-cost, and reliable method is required for the efficient commerce and use of this vital natural resource. In the refrigeration industry, the thermal properties of a mixed working fluid is strongly composition dependent. Proper refrigeration machine design and operation therefore depends upon an accurate composition measurement. Not only must the initial filling be at precisely controlled mixture compositions, but also maintenance fillings must be so as well. Because the constituents of mixed working fluids leak at different rates, replenishment maintenance requires an accurate fluid chemical analysis.

Approach: Gas chromatography offers an economical and accurate solution to the problem of natural gas analysis and mixed refrigerant fluid analysis. In addition to being one of the most well understood and economical analytical methods available, it is very amenable to field applications. What is needed is an interactive database that can be incorporated into the control and analysis software of both laboratory and field gas chromatographic instrumentation. We have approached the problem of standardizing and automating these analyses through the measurement of standard chromatographic retention parameters on the most useful stationary phases available, and also on some novel developmental phases. The standard retention parameters that we have measured include net retention volumes, relative retentions, and Kovats retention indices. These parameters are corrected for instrumental variation and are, therefore, reproducible from instrument to instrument. The measurements are performed on a specially modified commercial gas chromatograph that provides highly accurate retention information. The column temperature dependence of each retention parameter is modeled with appropriate equations to allow predictions at all relevant temperatures. These models then form the heart of an interactive database that allows off-line identification of peaks and also the optimization of more complex analyses.

Results and Future Plans: The measurement of all standard retention parameters (at several column temperatures) has been completed for 90 natural gas hydrocarbons, 23 natural gas odorization compounds, and 120 alternative refrigerant fluids on ten useful stationary phases. These stationary phases include the methyl silicones and derivatives, porous polymer and solid adsorbents, and some novel stationary phases that include sol/gel phases and clay phases. Several versions of the databases have been released, and are being used in the gas industry and by regulatory agencies. Next year, we will complete the final versions of the databases, suitable for several computer platforms.

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