MODIFIED REDLICH-KWONG EQUATIONS OF STATE FOR CH4 AND CH4-H2O FLUID MIXTURES

Modified Redlich-Kwong (MRK) equations of state are proposed for CH4 and CH4 - H2O fluid mixtures, expressing the parameters a and b as functions of temperature and pressure. For CH4 the new MRK was obtained by fitting 2,831 P - V - T-data for temperatures 150 K less-than-or-equal-to T less-than-or-equal-to 623 K and pressures up to 10.3 kbar. In the P - T range of investigation, calculated volumes relative to experimentally determined volumes have an average percent deviation of 0.2 (for T > 210 K the average percent deviation is 0.08). Extrapolation to high pressures and temperatures has been tested with shockwave data so that the MRK should yield good results to 2000 K and at least 200 kbar and is therefore really useful for geochemical conditions. The MRK for the binary CH4-H2O is obtained using the new MRK for CH4 and a similar MRK for H2O, proposed by GREVEL & CHATTERJEE (1992), assuming conventional mixing rules for the parameters a(mix) and b(mix) and introducing adjustable corrective factors epsilon and tau for the cross coefficients. These factors were fitted to three existing sets of P - V - T - X(CH4)-data (WELSCH, 1973; BAZAEV et al., 1974; SRETENSKAYA et al., 1986). As an independent proof of the validity of the MRK for the binary the accuracy of reproducing H(m)ex measurements of WORMALD & COLLING (1984) was investigated and showed a good agreement between measured and calculated data.