Accurate Prediction of Compressibility Factors by the GERG Virial Equation(includes associated paper 23568 )

Abstract

Summary. The European Gas Research Group (GERG, Groupe European de Researches Gaziroes) 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 amid 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 averace 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%. Introduction Metering of large-volume flows of natural gas at high pressure requires accurate knowledge of the compressibility factor. A number of gas companies use modified versions of the American Gas Assn. (AGA) NX-19 equation. These methods are satisfactory when the pressure is not too much and the temperature range is limited. In certain cases. however, the difference between measured and predicted compressibility factors may be as great as 1 %. British Gas used an extended corresponding-states method, and Gaz de France used a modified version of the Redlich-Kwong EOS. These two equations show RMS errors of about 0.25% between measured and predicted data from an early (1982) version of the GERG data bank. 3 None of these methods mentioned is fully satisfactory. Therefore. several years ago, GERG decided to develop an EOS to predict the compressibility factor of natural gases with an uncertainty of less than 0. I %. The initial results of this work were presented in 1986.4 This paper presents details of the final development of the GERG virial equation. The accuracy of the com-pressibility factor calculation obtained is discussed. Specifications of GERG Virial Equation The input data for calculation of the compressibility factor of natural gas by the GERG virial equation are temperature, pressure, and composition of the mixture. For the composition of the mixture, the mole fraction of each of the 13 groups of components listed in Table 1 is required. Table I also gives the specifications for the GERG virial EOS.