Diffusion Coefficients in CO₂/n-Alkane Binary Liquid Mixtures by Molecular Simulation

Abstract

The objective of this work was to determine Fick diffusion coefficients in CO2/n-alkane binary mixtures without experimental test. For doing so, Maxwell-Stefan (MS) diffusivity was calculated by molecular simulation. Simultaneously, a thermodynamic factor was estimated using the PC-SAFT (perturbed chain statistical associating fluid theory) equation of state (eos). The binary Fick diffusivities are calculated as the product of both quantities. The binary mixtures investigated contain CO2 and various n-alkanes (nC10, nC16, nC22, nC28, nC44), at their bubble pressure at varying temperatures between 298 and 373 K. The calculated values of Fick diffusivities were compared against the experimental ones for the systems where literature data exist. An average deviation of 26% was found for the CO2/n-decane and 15% for CO2/n-hexadecane mixtures. These results support that molecular simulation can be employed as a tool for the determination of Fick diffusivities in high pressure systems, like in oil reservoirs, without the need to construct a complicated and expensive experimental setup. This method only requires the phase behavior of the desired system, and it can be used for multicomponent mixtures. As an example, predictions of Fick diffusivities were done for CO2 binary mixtures with heavy n-alkanes (nC22, nC28, nC44).

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