PVT measurements of carbon dioxide + nitrogen and an equation of state development for natural and hydrocarbon gases

Abstract

Densities of five mixtures of carbon dioxide + nitrogen (x[CO2] = 0.10560, 0.25147, 0.50365, 0.71105, 0.90921) and pure nitrogen were measured at 300 and 320 K at pressures to 10 MPa using the Burnett technique. Second and third virial coefficients and densities were derived for each mixture from the measurements. Cross second and third virial coefficients were determined at each temperature. The second virial coefficients for the five mixtures have an absolute accuracy of 0.2 cm^3/mol and the third virial coefficients have an estimated accuracy of 50 cm^6/mol^2$. The estimated accuracy for the cross second virial coefficients coefficients is 0.5 cm^3/mol. Several methods for calculating cross third virial coefficients are compared with the experimental values. The methods considered are: ideal solution, geometric mean, the Orbey and Vera correlation, Lennard-Jones, the MHEMHS model, and an algebraic determination using two mixture third virial coefficients. A continuously weighed pycnometer has been used to measure densities of the mixtures in the dense gas and liquid regions at temperatures from 225 to 450 K at pressures to 70 MPa. A procedure has been developed which uses this apparatus to detect phase transitions. Saturation densities and pressures are reported for those isotherms where phase transitions occur. The accuracy of the densities obtained from the pycnometer is 0.1 kg/m^3 or 0.1 per cent whichever is greater. A flexible foil gravity mixer is described which has been developed to assist in the preparation of gas mixtures of accurately known composition without contaminating the mixtures. A new automated temperature measurement and control scheme is described which has been implemented on both the Burnett apparatus and the pycnometer. The new controller is capable of temperature control to [plus or minus] mK and interfaces with a personal computer via a D/A converter. A new equation of state has been developed for hydrocarbon gases based upon the truncated virial equation. The second and third virial coefficients are calculated from the specific gravity at temperatures between 220 and 360 K at pressures to 10 MPa yielding an average uncertainty in the compressibility factor of about 0.1 per cent...