A specialized cell designed for vapor liquid equilibrium (VLE) measurements at cryogenic temperatures and high pressures was used to measure new (p,T,x,y) data for binary mixtures of methane + ethane, + propane, + 2-methylpropane (isobutane), and + butane from (203 to 273) K at pressures up to 9 MPa. A literature review of VLE data for these binary mixtures indicates that a significant number have large uncertainties; however, because estimates of uncertainties in measured phase compositions are often not quantified sufficiently, it can be difficult for equation of state (EOS) developers to identify which data sets are of poor quality. Robust quantitative uncertainties were estimated for the VLE data acquired in this work, which allowed the identification of literature data sets that should not be included in future EOS development. The new data were compared with the predictions of the Peng−Robinson (PR) EOS and the Groupe European de Recherche Gaziere (GERG-2008) multiparameter EOS. The former describes the new data measured at low pressures within experimental uncertainty but deviates systematically from the data as the bubble point pressure is increased; for the binary mixtures containing either of the butanes, the maximum relative deviation of the data from the PR EOS amounted to nearly 10 % of the methane liquid mole fraction. The GERG-2008 EOS was better able to describe the new high-pressure data for the CH 4 + C 3 H 8 system than the PR EOS. However, for both the CH 4 + C 4 H 10 mixtures, the GERG EOS deviated from these data by an amount twice as large as the PR EOS because of ambiguity about which VLE literature data sets should be used in model development. The new data resolve these ambiguities and should facilitate the development of improved EOS as needed, for example, in simulations of low temperature natural gas separation processes.

New pTxy data are reported for methane + pentane and methane + hexane at pressures up to 14 MPa over the temperature range (173 to 333) K using a custom-built vapor−liquid equilibria apparatus. For methane (1) + pentane (2), a mixture with overall mole fraction z 2 ≈ 0.02 was prepared gravimetrically, and measurements were performed along an isochoric pathway. For the methane (1) + hexane (3) mixture, liquid hexane was pumped into the evacuated cell using an HPLC pump, and then after the addition of methane, isothermal measurements were made at 11 temperatures. Two liquid phases were observed close to the bubble point in the methane + hexane mixture at (183.15 and 233.15) K at pressures of (3.31 and 12.99) MPa, respectively. Our data are compared with previous literature data and with the predictions of the Groupe European de Recherche Gaziere (GERG-2004 XT08) multiparameter equation of state (EOS) and the Peng−Robinson and Advanced Peng−Robinson cubic equations of state implemented in commercial process simulation software. The differences from the GERG-2004 EOS in the liquid phase mole fraction x 1 were up to 0.1 for methane + pentane and up to 0.3 for methane + hexane. The systematic increase in the deviations with pressure, at constant temperature, is clear evidence of the need for tuning of the EOS parameters, especially at high pressure. The differences are smaller for the Peng−Robinson and the Advanced Peng−Robinson EOS; however, all three EOS failed to predict the second liquid phase in methane + hexane. Our data agree with the x 1 values reported by Chen et al. (J. Chem. Eng. Data 1976, 21, 213−219) for the appearance of a second liquid phase.

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