EVALUATION OF SECOND VIRIAL COEFFICIENTS AND ISOCHORIC INFLECTION LOCI FROM BURNETT - ISOCHORIC PVT MEASUREMENTS ON THE HYDROGEN - METHANE SYSTEM
MAGEE, JOE WILTON, JR.
Doctor of Philosophy
Measurement of the PVT relations of a 20.05 mol % hydrogen - 79.95 mol % methane mixture was conducted between 140 and 600 K with pressures to 72 MPa. The primary measurements were conducted along lines of constant density (isochores) with each isochore anchored to a series of Burnett isotherms at 273.150 K. The density of each isochore was calculated from the optimal virial expansion truncated after six terms and is shown to have an accuracy approaching 0.01% at high pressures. An investigation of the deviation of the compressibility factor from linearity along isochores showed that deviations are present at both high and low reduced temperatures. The high temperature deviations are negative but the sign of the low temperature deviations depends on density. Any deviations from linearity carry significant messages concerning the behavior of the second temperature derivative of the equation of state at constant density ((PAR-DIFF)('2)Z/(PAR-DIFF)(1/T)('2))(,p). Precise PVT data for pure hydrogen, pure methane, and a 20.05 mol % hydrogen-methane mixture were used to evaluate the isochoric inflection loci defined by ((PAR-DIFF)('2)Z/(PAR-DIFF)(1/T)('2))(,p) = 0. Hydrogen's locus has been shown to be "open" in contrast to methane's "closed" locus. The presence of hydrogen dominates the behavior of the binary mixture, as shown by its "open" isochoric inflection locus. Second and third virial coefficients of the 20.05 mol % hydrogen-methane mixture were evaluated for temperatures from 190 to 500 K. Second cross virial coefficients were calculated for temperatures from 273.15 K to 420 K for the hydrogen-methane system. A new empirical equation of state was derived and tested. The equation incorporates the behavior of second virial coefficients and deviations from linearity along isochores. It consists of a power series in both density and reciprocal temperature and permits analytical calculation of thermodynamic properties.