Solid-fluid equilibria of mixtures of interest in LNG production: Measurement and modelling of methane + carbon dioxide + neo-pentane systems

A precise understanding of the solubility limits of solids in methane-rich mixtures is essential for assessing crystallization risks in the production of liquefied natural gas (LNG). While recent studies have provided new experimental data and modelling approaches dealing with the solubility of heavy components (primarily aromatic compounds) in binary mixtures with methane, the phase equilibrium behavior of multi-component mixtures at cryogenic temperatures remains insufficiently understood. Given its high solubility in methane, the presence of neo-pentane in the feed mixture may potentially reduce the crystallization risk of other heavy components (i.e., chemicals having a triple-point temperature above the LNG temperature like carbon dioxide), either by enhancing their solubility at specific temperature and pressure conditions or by extending the pressure range over which the fluid phase remains stable. This study presents new insights into the thermodynamic behavior of systems comprised of methane, carbon dioxide, and neo-pentane, with the aim of investigating the effect of neo-pentane on the solubility of carbon dioxide in methane. New experimental data were determined for: (1) the binary carbon dioxide + neo-pentane system, including both liquid-vapor equilibrium (VLE) and solid-fluid equilibrium (SFE) measurements down to 170 K, and (2) the ternary methane + carbon dioxide + neo-pentane system, with SFE measurements down to 120 K. Both systems were studied using two different static-analytic apparatuses. The experimental results were compared with modelling results obtained by coupling the Peng-Robinson Equation of State (PR78 EoS) for the fluid phases with the Classical Approach for the solid phases.