A Nonequilibrium Chemical Affinity-Based Hydrate Growth Model: Applied to the CH4 Hydrate

While hydrate formation conditions are commonly present in natural gas offshore production, where hydrates can cause undesirable blockages, the natural reserves of CH4 hydrate on the seafloor have been studied as a potential energy source. Different applications for these crystalline structures are also being developed due to their gas storage capacity. Hence, understanding the hydrate growth phenomenon may lead to alternative operational procedures to avoid blockages, explore natural reserves, and develop new gas storage technologies. However, despite the literature advances to describe hydrate equilibrium states, the formation dynamics of these solids are still obscure. A generic model capable of including the thermodynamic factor in the growth is fundamental to better describe the phenomenon in all these scenarios. Therefore, we propose a new model for hydrate growth kinetics using reaction chemical affinity as a driving force. This model is based on nonequilibrium thermodynamics, clarifying conceptually relevant but rarely investigated problems. DeDonder's affinity allows the inclusion of nonideal thermodynamic factors for all hydrate-forming components in the growth model, explicitly relating the thermodynamic behavior of hydrate formation to the kinetics. The definition of a coupling factor between reaction and diffusion makes it necessary to estimate only the reaction constant of the model. We show that the model accurately describes the experimental data by evaluating the reaction-diffusion limiting conditions on the growth rate of CH4 hydrate in freshwater. Depending on the pressure, the chemical affinity-based model confirms that reaction and diffusion mechanisms govern methane hydrate growth. Lastly, we verify that including water activity in the driving force alters the growth dynamics even for systems close to ideality.