Phase-coexistence Simulations of Fluid Mixtures by the Markov Chain Monte Carlo Method Using Single-Particle Models

TL;DR

This paper introduces a simplified single-particle Lennard-Jones model for CO2 and N2, validated through Markov Chain Monte Carlo simulations, achieving comparable accuracy to complex models while significantly improving computational efficiency.

Contribution

The study develops and validates a simplified single-particle Lennard-Jones model for fluid mixtures, demonstrating its effectiveness and efficiency in phase-coexistence simulations.

Findings

Single-particle models agree well with experimental phase diagrams.

Models show similar accuracy in gas-phase predictions as complex models.

Computational efficiency is significantly improved, especially at high liquid densities.

Abstract

We present a single-particle Lennard-Jones (L-J) model for CO2 and N2. Simplified L-J models for other small polyatomic molecules can be obtained following the methodology described herein. The phase-coexistence diagrams of single-component systems computed using the proposed single-particle models for CO2 and N2 agree well with experimental data over a wide range of temperatures. These diagrams are computed using the Markov Chain Monte Carlo (MC) method based on the Gibbs-NVT ensemble. This good agreement validates the proposed simplified models. That is, with properly selected parameters, the single-particle models have similar accuracy in predicting gas-phase properties as more complex, state-of-the-art molecular models. To further test these single-particle models, three binary mixtures of CH4, CO2 and N2 are studied using a Gibbs-NPT ensemble. These results are compared against experimental data over a wide range of pressures. The single-particle model has similar accuracy in the gas phase as traditional models although its deviation in the liquid phase is greater. The simplified model improves the computational efficiency significantly, particularly in the case of high liquid density where the acceptance rate of the particle-swap trial move increases. The MC method based on Gibbs-NVT ensemble is a viable alternative to simulate phase-coexistence of fluid mixtures. We compare, at constant temperature and pressure, the Gibbs-NPT and Gibbs-NVT ensembles to analyze their performance differences and results consistency. As theoretically predicted, the agreement between the simulations implies that Gibbs-NVT can be used to validate Gibbs-NPT predictions when experimental data is not available.