New Mass-Transfer Model for Predicting Hydrate Film Thickness at the Gas-Liquid Interface under Different Thermodynamics-Hydrodynamics-Saturation Conditions

The accurate prediction of hydrate film thickness is critical for addressing the hydrate-related issues in the fields of environment and energy resource, including the disposal of greenhouse gases into the ocean, evaluating the rising lifetime of bubble plumes in deepwater, and flow assurance problems in subsea pipelines. However, the microscopic mass-transfer mechanism in the existing hydrate film growth models has not been thoroughly studied, especially in the pore updating inside the hydrate film. In this work, an integrated mechanistic model of hydrate film growth at the gas-liquid interface is developed. Multiple mass-transfer behaviors that potentially control the thickening growth of hydrate film are considered: (i) gas diffusion and water permeation through the hydrate film; (ii) substance dissipation along the hydrate pore channels; and (iii) gas transfer between the water/hydrate interface and the surrounding fluid. In the new model, a characteristic parameter is introduced to evaluate the pore updating efficiency, which is determined by correlating with the published experimental data. Based on the numerical solution of unsteady convection diffusion equation, the gas concentration distribution in the flowing water around the hydrate film is obtained, and the mass transfer coefficients at different flow velocities are estimated correspondingly. Using the proposed model, the dynamic evolution rules of hydrate film growth at two specific conditions are investigated. The model predictions for hydrate film steady-state thickness show a good agreement with the measured results from the literature. Further, a sensitivity analysis on the growing thickness of hydrate film is performed with various influencing factors, which involves system subcooling, water flow velocity, and dissolved gas concentration in liquid water. Eventually, a case study for the scenario of the spatial and temporal changes in film thickness on a rising methane bubble in Monterey Bay Canyon is carried out. This work provides new insights into the interphase mass-transfer characteristics for hydrate dynamic growth and can serve as a useful reference for predicting the hydrate film thickness.