Three-phase equilibria of hydrates from computer simulation. I. Finite-size effects in the methane hydrate

TL;DR

This study uses computer simulations to analyze how finite system sizes affect the accuracy of determining the three-phase equilibrium temperature of methane hydrates, highlighting the importance of system size in hydrate stability predictions.

Contribution

It provides a detailed analysis of finite-size effects on hydrate phase equilibrium calculations using two water models, clarifying discrepancies in previous research.

Findings

Finite-size effects significantly influence $T_3$ calculations.

Smaller systems can produce inaccurate $T_3$ due to bubble formation.

Larger, more realistic systems show better convergence of $T_3$.

Abstract

Clathrate hydrates are vital in energy research and environmental applications. Understanding their stability is crucial for harnessing their potential. In this work, we employ direct coexistence simulations to study finite-size effects in the determination of the three-phase equilibrium temperature ($T_3$) for methane hydrates. Two popular water models, TIP4P/Ice and TIP4P/2005, are employed, exploring various system sizes by varying the number of molecules in the hydrate, liquid, and gas phases. The results reveal that finite-size effects play a crucial role in determining $T_3$. The study includes nine configurations with varying system sizes, demonstrating that smaller systems, particularly those leading to stoichiometric conditions and bubble formation, may yield inaccurate $T_3$ values. The emergence of methane bubbles within the liquid phase, observed in smaller configurations, significantly influences the behavior of the system and can lead to erroneous temperature estimations. Our findings reveal finite size effects on the calculation of the $T_3$ by direct coexistence simulations and clarify the system size convergence for both models, shedding light on discrepancies found in the literature. The results contribute to a deeper understanding of the phase equilibrium of gas hydrates and offer valuable information for future research in this field.