Effect of Equation of States on High-Pressure Volumetric Measurements of Methane-Coal Sorption Isotherms-Part 1: Volumes of Free Space and Methane Adsorption Isotherms

Abstract

High-pressure volumetric analysis (HPVA) experiments were carried out using Particulate System's HPVA-II 200 at temperatures of 25 and 40 °C and pressures up to 7 MPa to quantify methane adsorption capacities of a coal sample. Six well-known equations of state (EOSs), namely, Peng-Robinson (PR), Soave-Redlich-Kwonk (SRK) and their volume translated forms (PR-Peneloux and SRK-Peneloux), Soave modified Benedict-Webb-Rubin (SBWR), and Lee-Kesler (LK) were used for data interpretation, and the results were compared with those calculated by the equipment using Z-factors from NIST-refprop software implementation of McCarty and Arp EOS for helium and Setzmann and Wagner EOS for methane. Because of variations in the Z-factors of helium and supercritical methane obtained from the different EOSs, large variations were observed in calculated isotherms and Langmuir parameters with the PR-Peneloux and PR-EOS family showing the largest relative deviations, while SRK-EOS and SRK-Peneloux gave more negative adsorption for each temperature. All the EOSs gave significantly lower density for the adsorbed phase at 40 °C compared to the value at 25 °C. This is consistent with the negative effect of temperature on adsorption.