Theoretical Study of Gas Hydrate Decomposition Kinetics—Model Development

Mesomorphology of clathrate hydrates from molecular ordering

Clathrate hydrates are crystals formed by guest molecules that stabilize cages of hydrogen-bonded water molecules. Whereas thermodynamic equilibrium is well described via the van der Waals and Platteeuw approach, the increasing concerns with global warming and energy transition require extending the knowledge to non-equilibrium conditions in multiphase, sheared systems, in a multiscale framework. Potential macro-applications concern the storage of carbon dioxide in the form of clathrates, and the reduction of hydrate inhibition additives currently required in hydrocarbon production. We evidence porous mesomorphologies as key to bridging the molecular scales to macro-applications of low solubility guests. We discuss the coupling of molecular ordering with the mesoscales, including (i) the emergence of porous patterns as a combined factor from the walk over the free energy landscape and 3D competitive nucleation and growth and (ii) the role of molecular attachment rates in crystallization-diffusion models that allow predicting the timescale of pore sealing. This is a perspective study that discusses the use of discrete models (molecular dynamics) to build continuum models (phase field models, crystallization laws, and transport phenomena) to predict multiscale manifestations at a feasible computational cost. Several advances in correlated fields (ice, polymers, alloys, and nanoparticles) are discussed in the scenario of clathrate hydrates, as well as the challenges and necessary developments to push the field forward.

Dynamic Dissociation Behaviors of sII Hydrates in Liquid Water by Heating: A Molecular Dynamics Simulation Approach

An understanding of the dynamic behavior of subtle hydrate dissociation in the liquid water phase is fundamental for gas production from marine hydrate reservoirs. Molecular dynamics simulations are performed in this study to investigate the dissociation kinetics of pure propane and binary propane + methane sII hydrates in a liquid water environment. The results show that faster hydrate dissociation rates are observed at higher initial temperatures. The hydrate phase dissociates from the cluster surface to the inside in a layer-by-layer manner under the simulation temperature conditions, which is similar to the behavior of sI hydrates and is independent of the hydrate crystal type. Compared to the binary sII hydrate, the pure sII hydrate dissociates more easily under the same initial temperature conditions, which can be attributed to the stabilizing effect of guest molecules in the hydrate cages. The empty cages collapse in one step, in contrast to the two-step pathway induced by the guest-host interaction. In addition, a hydrocarbon phase forms in the binary hydrate dissociation system instead of nanobubbles. These results can provide molecular-level insights into the dynamic mechanism of hydrate dissociation and theoretical guidance for gas recovery by thermal injection from marine hydrate reservoirs.

Study on the decomposition mechanism and kinetic model of natural gas hydrate slurry in water-in-oil emulsion flowing systems

Hydrate slurry decomposition in flow systems is a significant subject that involves flow assurance and development of marine natural gas hydrates. Firstly, the decomposition mechanism of hydrate slurry is studied in this work, and it is proposed that desorption of the gas from the surface of the decomposed hydrate particles might be the main reason for the coalescence of particles and water droplets during the hydrate slurry decomposition. Secondly, a hydrate slurry decomposition kinetic model comprehensively considering the influencing factors (i.e., the intrinsic kinetics, heat and mass transfer) is proposed in this work, based on the classic intrinsic kinetic model and the hydrate slurry dissociation experiments conducted in a flow loop system. The fugacity difference is used as the driving force for the hydrate decomposition, and the influence of particle coalescence, and heat and mass transfer is also considered. The effect of the heat and mass transfer is coupled with the apparent decomposition reaction rate constant. Meanwhile, the time-dependent interfacial parameters would significantly impact on the hydrate dissociation rate, which are considered to enhance the predictive precision of the decomposition kinetic model. Further, the integrated decomposition kinetics model proposed in this paper could well describe the trends of the amount of released gas and the dissociation rate of the experimental flow systems. Through combining the experimental results of the hydrate slurry decomposition, the decomposition parameters under actual flowing conditions were obtained.

Hydrate slurry decomposition in flow systems is a significant subject that involves flow assurance and development of marine natural gas hydrates.

Origin of Self-preservation Effect for Hydrate Decomposition: Coupling of Mass and Heat Transfer Resistances

Gas hydrates could show an unexpected high stability at conditions out of thermodynamic equilibrium, which is called the self-preservation effect. The mechanism of the effect for methane hydrates is here investigated via molecular dynamics simulations, in which an NVT/E method is introduced to represent different levels of heat transfer resistance. Our simulations suggest a coupling between the mass transfer resistance and heat transfer resistance as the driving mechanism for self-preservation effect. We found that the hydrate is initially melted from the interface, and then a solid-like water layer with temperature-dependent structures is formed next to the hydrate interface that exhibits fractal feature, followed by an increase of mass transfer resistance for the diffusion of methane from hydrate region. Furthermore, our results indicate that heat transfer resistance is a more fundamental factor, since it facilitates the formation of the solid-like layer and hence inhibits the further dissociation of the hydrates. The self-preservation effect is found to be enhanced with the increase of pressure and particularly the decrease of temperature. Kinetic equations based on heat balance calculations is also developed to describe the self-preservation effect, which reproduces our simulation results well and provides an association between microscopic and macroscopic properties.

Theoretical study of gas hydrate decomposition kinetics: model predictions

In order to provide an estimate of intrinsic gas hydrate dissolution and dissociation kinetics, the Consecutive Desorption and Melting Model (CDM) was developed in a previous publication (Windmeier, C.; Oellrich, L. R. J. Phys. Chem. A 2013, 117, 10151-10161). In this work, an extensive summary of required model data is given. Obtained model predictions are discussed with respect to their temperature dependence as well as their significance for technically relevant areas of gas hydrate decomposition. As a result, an expression for determination of the intrinsic gas hydrate decomposition kinetics for various hydrate formers is given together with an estimate for the maximum possible rates of gas hydrate decomposition.