Vapor-Liquid Equilibria Measurements of the Methane plus Pentane and Methane plus Hexane Systems at Temperatures from (173 to 330) K and Pressures to 14 MPa

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) approximate to 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.