Fluctuation–dissipation analysis of heat and mass flow in energy transport at different CO2 hydrate dissociation interfaces

Abstract

Understanding the mechanism of kinetic behaviors is necessary for gas hydrate application. In this paper, the system of three-phase and two-interface was established, and the kinetic dissociation characteristics in different CO2 hydrate interfacial areas were compared using molecular dynamic simulations. It is found that the dissociation rate in the right interfacial region with relatively more carbon dioxide molecules is faster than that with relatively more water molecules. The mechanism of different kinetic behaviors at different dissociation interfaces was analyzed from the perspective of mass transfer, heat transfer and energy transport. The results show that the fluctuation–dissipation theory holds well for the number of hydrate-like CO2 molecules in the whole and part of the bulk hydrate at the near-equilibrium state, the interfacial latent heat flow in both interfacial areas, the interfacial average atomic potential energy difference, and the interfacial sensible heat flow in the left interfacial area at the non-equilibrium state. In which, the potential energy transport from the adjacent liquid to hydrate-like water molecules has an inherent correlation with the interfacial latent heat flow, and both have the similar fluctuation period, 900 ps and 1000 ps. The interfacial thermal resistance (ITR) was used to represent the dissipation of the net heat flow fluctuation amplitude, the values for the left and right interfacial regions are 5.57 × 10-10 m2·K/W and 5.46 × 10-10 m2·K/W, which reveals the faster heat transfer (including latent and sensible heat) in the right interfacial regions with relatively more carbon dioxide molecules. In addition, it was inferred that the mass transfer plays a leading role when there is no initial temperature gradient between the hydrate and the water (gas) phase, and in contrary, the heat transfer dominants.