A vibrating-wire viscometer of very high precision was used to measure the viscosity of methane and of two natural gases. The experimental data were, in general, taken at temperatures of 260, 280, 300, and 320 K and at pressures up to 20 MPa, and additionally in the case of methane at temperatures of 340 and 360 K and at pressures up to 29 MPa. The estimated uncertainty is ±0.3 and ±0.5% for methane and the natural gases, respectively. The new experimental data for methane were used together with zero-density or low-density viscosity values from this study and from the literature to develop a viscosity equation for natural gas composed of two contributions. The mixing rule of Wilke [J. Chem. Phys. 18: 517 1950] was applied for the zero-density viscosity part which is based on zero-density correlations for twelve components (methane, nitrogen, carbon dioxide, ethane, propane, n-and isobutane, n-and isopentane, n-hexane, n-heptane, and n-octane) and agrees with the values derived from experiment within ±0.3%. The density dependence of the residual viscosity part was correlated with methane data only, neglecting any temperature dependence, whereas the composition dependence is characterized by a pseudo-critical viscosity value. For methane the agreement between the correlated and experimental data is within ±0.5%. The values predicted with the correlation and the experimental data agree within ±1% for both the high calorific, H, natural gas and the low calorific, L, natural gas.

A GERG (Groupe EurQpeen de Recherches Gazieres) equation of state (EOS) is presented to calculate the compressibility factor of natural gases. The equation, which does not require detailed gas analysis, can predict the compressibility factor when three of the four following gas properties are known: the gross calorific value, the relative density, and the mole fractions of N 2 and CO 2 , The new equation, known as the SGERG-88 vi rial equation, is based on ideas presented in an earlier study. The new equation, however, applies to wider ranges oftemperature and pressure and can be applied to gases containing hydrogen if the mole fraction of the hydrogen is known. The equation was tested on natural gases purchased, transported, and marketed by the European gas companies. Excellent agreement between computed and experimental compressibility factors was found. This difference is, in most cases (94 % of the data points), less than 0.1 % in the temperature range of 265 to 335 K and for pressures up to 12 MPa.

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The European Gas Research Group (GERG, Groupe Europeen de Recherches Gazieres) performed an extensive research project involving the measurement of compressibility factors of pure gases and binary mixtures and of natural gases in the temperature range of 265 to 335 K and at pressures up to 12 MPa. These pure gas and binary mixture data, together with high-quality data from the literature, were used to develop the GERG virial equation for the accurate prediction of the compressibility factor of natural gas (multicomponent) mixtures. Pressure, temperature, and a 13-component composition are used as input data. Eighty-four sets of experimental natural gas data made up of more than 4,000 data points were used to validate the GERG virial equation of state (EOS). The target accuracy of the GERG virial equation is ±0.1 %. A comparison of experimental and predicted compressibility factors shows that this target was achieved. The average root-mean-square (RMS) error for the differences between the experimental and the predicted compressibility factors is 0.06% for pressures up to 12 MPa. Even in the high-pressure range between 8 and 12 MPa, the RMS error is only 0.07%.

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