(80c) Density Collection Technique for Gas Mixtures Using an Automated Isochoric Apparatus and a Magnetic Suspension Densitometer

Accurate knowledge of the PVT behavior of gases is essential for the natural gas industry. The gas densities at reservoir and pipeline conditions are necessary for production, transportation and utilization of the resource. Density plays a key role in determining economic value of natural gas for custody transfer, and it is important to have accurate gas density data covering the broad range of pipeline conditions. It is also essential to have such data to validate or develop equations of state (EOS) that can calculate phase equilibrium properties and provide accurate density predictions. For pipeline conditions, accurate cricondentherm and cricondenbar values are necessary to avoid two-phase flow (liquid + gas). In our laboratory, we have constructed an automated isochoric apparatus to measure densities and phase envelopes, and we have commissioned a magnetic suspension densitometer (MSD) for density measurements. Using the isochoric apparatus, we collect isomolar pressure vs temperature data. The isochoric apparatus can operate at temperatures ranging from 100 K to 500 K and at pressures up to 200 MPa. The temperature gradients across the cell are less than ±0.002 K. We measure pressure with 0.01% accuracy and 0.0001% resolution. The MSD can measure isothermal densities up to 200 MPa between 200 and 500 K. Temperature stability is ±5 mK and pressures have an uncertainty of ±0.002%. The MSD can provide densities accurate to ±0.06%. By crossing the isomolar PT lines obtained from isochoric apparatus with isothermal density lines obtained from the MSD, we can determine the densities of the isomolar lines at intersections with the isothermal lines. Because the volume of the isochoric cell changes with temperature and pressure, the density of an isomolar line is not quite constant. The actual densities result from applying the volume distortion equation. It is necessary to have accurate material property coefficients to correct the volumes accurately. By using our density collection technique, we determine our isochoric cell material parameters experimentally. The procedure is to use the known isothermal densities at thee intersection points with isomolar lines to determine the coefficients in the volume distortion expression. We illustrate the technique with experimental data for a pseudo-natural gas mixture.