CO2 permeation through thermoplastic materials under cryo-compressed conditions for ship-based CO2 transport

Transporting CO2 via ships under cryo-compressed conditions is one of the main scenarios expected for the deployment of the CO2 transport chain in the coming years. Nevertheless, the performance of non-metallic materials, such polymers that can be used as liners, protective layers, or even as sealing gaskets, under these conditions is scarcely reported in literature, generating knowledge gaps limiting the possibility to take educated decisions during the design of new or the repurposing of existing infrastructure. In this work, CO₂ sorption and permeation behaviour in four thermoplastic polymers (PVDF, PTFE, PEEK, and UHMWPE) were systematically investigated at temperatures down to −46 °C and pressures above saturation. CO₂ solubility in both the gaseous and liquid phases was modelled by means of a thermodynamic framework based on the well-established lattice fluid (LF) and non-equilibrium lattice fluid (NELF) equations of state, enabling the prediction of sorption isotherms beyond the experimental conditions. Despite the different polymer nature, three materials showed an expected behaviour, with permeability dropping at lower temperatures, due to the dominant effect of diffusion over solubility. Nevertheless, a quite peculiar behaviour was observed by PTFE, which displays a transient non-linear trend with local maxima dependent on the operating pressure. Such complex experimental permeability trends observed for the four polymeric materials are accurately described using the Standard Transport Model, aiming to improve the understanding of the experimental results under subcritical and cryo-compressed conditions. The model validation conducted in this study is anticipated to provide a foundation for future modelling efforts, enabling the exploration of system performance beyond current experimental constraints.