Simulation of isoenthalps and Joule-Thomson inversion curves of pure fluids and mixtures

This paper examines the molecular simulation of expansion and compression processes of fluids under common non-isothermal conditions. It is shown that accurate isoenthalps can be traced if simulated configurational properties obtained from the particular molecular force-field adopted are complemented by experimentally-based data for the ideal-gas contributions to the heat capacity. The simulation of inversion curves is also analyzed and found to be particularly challenging as conventional extrapolation techniques have limited applicability. Several approaches were investigated to overcome such difficulties including (1) simulation of several isobars and isoenthalps, (2) thermodynamic integration, and (3) histogram reweighting techniques. Contrary to the results of a recent study, our calculations show that the Johnson et al., equation of state does provide a good description of the inversion curve for the Lennard-Jones fluid. Isoenthalps and the inversion curve for a model:gas condensate mixture were also simulated; calculations of this sort could be used to identify conditions at which isoenthalpic decompression leads to heating of hydrocarbon reservoir fluids.