Understanding Retrograde Gas Condensation: A Molecular Perspective

Abstract Retrograde gas condensation is an intriguing and seemingly counterintuitive phase behavior phenomenon in which gas condenses as pressure decreases at certain conditions and fluid compositions. This work aims to provide an explanation that bridges the gap between phenomenological observations and microscopic mechanisms using thermodynamic modeling and molecular dynamics (MD) simulation. Phase envelopes for a selection of binary hydrocarbon mixtures have been generated and compared with experimental results using the Peng Robinson (PR) equation of state (EoS). After capturing key features of retrograde condensation through EoS modeling, MD simulation with an initial energy minimization step using the steepest descent algorithm followed by equilibration and production runs in the NVT ensemble has been used to simulate the constant composition expansion of a ternary C1-nC4-nC10 mixture; the intermolecular energies and other thermodynamic and transport quantities were computed from the MD simulations to elucidate the retrograde condensation behavior. The OPLS-AA force field was used along with the leap-frog algorithm to integrate Newton’s equations of motion with the velocity rescaling thermostat to maintain the temperature. EoS modeling was in good agreement with experimental data for the studied binary mixtures. Additionally, EoS modeling was used to showcase the differences between the critical temperature and cricondentherm of the various binary mixtures; this showcases the extent of the negative slope region in the phase envelope, which is associated with retrograde condensation. MD simulations revealed that the intermolecular interaction energies among heavier hydrocarbon components with low vapor pressures increase as pressure decreases at certain regions above the critical temperature and below the cricondentherm. Gas condensate systems typically exhibit methane mole fractions of 0.6–0.8; however, the liquid dropouts were predominantly composed of heavier hydrocarbon compounds. In the MD simulation, pressures corresponding to aggregation of nC10 also showed a slow decay in the radial distribution function between terminal and secondary carbon atoms in the nC10 with nC4, suggesting the assemblage of the heavier fractions in liquid-like molecular clusters. Furthermore, variation in the ratio of self-diffusion of the heavier components compared to the lighter components was observed, which can be attributed to the increased intermolecular interactions of the heavier components at the liquid dropout region. Despite their extensive application in studying complex fluids, reports of MD simulations in retrograde condensation are almost missing from the literature; this study aims to utilize molecular modeling to elucidate the molecular origin of retrograde condensation.