Gerg Project: Wide-Range Reference Equation of State for Natural Gases

摘要

A group of European gas companies, GERG, supported the development of a new equation of state for the thermodynamic properties of natural gases covering the gas and liquid region including the vapour-liquid phase equilibrium. The new equation, GERG02, was developed on the basis of a multi-fluid approximation using pure substance equations for each component and experimental data for binary mixtures only. Therefore, the representation of multicomponent mixture data is predictive. The results calculated with the new equation for thermal and caloric properties of natural gas mixtures show substantial improvements in comparison to the AGA8-DC92 and the GERG88 equation, which are known to be the current internationally accepted standard for density or compression factor calculations at pipeline conditions (see ISO 12213-part 2 and 3, 1997). The new reference equation allows high accuracy calculations for thermal and caloric properties in the homogeneous region (gas, liquid and supercritical) and also enables calculations for the vapour-liquid equilibrium. The reference equation can be used as a database or reference for technical applications and processes with natural gases, LNG, LPG, natural gas vehicles and hythane mixtures. The uncertainty in density and in speed of sound at pipeline conditions is less than +-0.1%. As a result of the poor data situation, the description of the liquid phase and the vapour- liquid phase equilibrium is less accurate but still as good as possible compared to the uncertainty of the available experimental data. Gas densities measured for rich natural gases containing high fractions of ethane (up to 20%), propane (up to 14%) and butane (up to 6%). are calculated by the new reference equation within less than +-0.1% to +-0.2%. The AGA8-DC92 equation does not accurately describe the densities of these gas mixtures. For LNG like mixtures the new equation reproduces experimental saturated liquid densities to within +-0.2%. For comparison, the still widely used Peng-Robinson cubic equation deviate by more than 10%.