Thermodiffusion of CO₂ in Water by Nonequilibrium Molecular Dynamics Simulations

Abstract

The components of a fluid mixture may segregate due to the Soret effect, a coupling phenomenon in which mass flux can be induced by a thermal gradient. In this work, we evaluate systematically the thermodiffusion of the CO 2 −H 2 O mixture, and the influence of the geothermal gradient on CO 2 segregation in deep saline aquifers in CO 2 storage. The eHeX method, a nonequilibrium molecular dynamics simulation approach, is judiciously selected to simulate the phenomenon. At 350 K, 400 bar, and CO 2 mole fraction of 0.02 (aquifer conditions), CO 2 accumulates on the cold side, and the thermal diffusion factor is close to 1 in a number of force fields. The lower the temperature, the higher is the separation and the thermal diffusion factor. In colder regions, water self-association is stronger, whereas the CO 2 −H 2 O cross-association and the CO 2 −CO 2 interactions enhance at higher temperatures. Thermodiffusion and gravitational segregation have opposite effects on CO 2 segregation. At typical subsurface conditions, the Soret effect is more pronounced than gravity segregation, and CO 2 concentrates in the top (colder region). Our work sets the stage to model the effect of electrolytes on CO 2 segregation in subsurface aquifers and other areas of interest.

Effect of Salinity on CO2 Thermodiffusion in Aqueous Mixtures by Molecular Dynamics Simulations

“…In our simulations, the nonbonded interactions between particles accounted for the van der Waals and electrostatics interactions, which are represented by the Lennard–Jones (LJ) and Coulombic potential, respectively. For the Soret effect in the CO 2 –H 2 O mixture, a set of force field combinations gave similar results . In our work, the force fields for CO 2 and H 2 O molecules were EPM2 and SPCE, both semiflexible (fixed bond length but a harmonic potential for the angles).…”

“…The temperature of inversion is around 350 K since there is almost no detectable CO 2 segregation under this condition. The CO 2 migration trend from thermophobic to thermophilic is also observed by increasing temperature in the binary CO 2 –H 2 O mixture . Without salt in the mixture, inversion occurs at a higher temperature (around 400 K).…”

“…Omrani et al assumed that the thermodynamic factor is one, not accounting for nonideality. From MD simulations and the CPA EoS, however, we showed that the thermodynamic factor is lower than 1 . Therefore, improved predictions on the CO 2 diffusivity were obtained by accounting for the mixture nonideality.…”

“…A thermostat can be used to represent thermodiffusion in NEMD simulations. To minimize the disturbance that the Langevin thermostat causes in the mixture dynamics, a larger damp factor may be required, which slows down the method convergence . Therefore, we select the eHeX method, an enhanced version of the HeX method, to represent the Soret effect.…”

“…In recent work, we have evaluated the thermodiffusion of the CO 2 –H 2 O under subsurface conditions . At lower temperatures, CO 2 tends to concentrate in the cold region, where the CO 2 –CO 2 interactions are weaker and the hydrogen bonds are more organized.…”