Gas–Liquid–Solid Multiphase Flow and Phase-Transition Mechanisms of Cryogenic CH4–CO2 Systems in Pipelines

Abstract

CH₄–CO₂ mixtures are widely encountered in LNG liquefaction, CCUS transportation, cryogenic biogas upgrading, cryogenic carbon capture, and the reinjection of CO₂-rich associated gas. Low temperatures or rapid depressurization can bring the system close to the solid-CO₂ formation boundary, thereby triggering dry-ice crystallization and desublimation, particle agglomeration, and pipeline blockage. Based on the original review, this paper condenses the literature discussion while retaining five central themes: thermodynamic phase equilibrium, solid-phase nucleation, in-pipe multiphase flow, coupled heat and mass transfer, and numerical modeling. The results show that phase boundaries shift significantly with pressure, composition, and impurities; heterogeneous nucleation is more likely to dominate wall-frost formation; deposition and re-entrainment of dry-ice particles govern pressure-drop and heat-transfer risks; and engineering models require a coordinated choice between EOS thermophysical-property accuracy and CFD capability for resolving phase interfaces.