Guang-Jin Chen

A new approach to gas hydrate modelling

A two-step hydrate formation mechanism is proposed for gas hydrate formation: (1) a quasi-chemical reaction process to form basic hydrate and (2) an adsorption process of smaller gas molecules in the linked cavities of basic hydrate. Based on the new concepts introduced in a previous article and the two kinds of equilibrium: the quasi-chemical reaction equilibrium of step 1 and the physical adsorption equilibrium of step 2, a simpler hydrate model has been developed. Extensive test results indicate that the new model is adequate for predicting the hydrate formation conditions for pure gases and gas mixtures.

Thermodynamic modeling of hydrate formation based on new concepts

A new hydrate model distinguished from the well known van der Waals-Platteeuw-type model has been developed on the basis of a more reasonable description of the mechanism of hydrate formation. The new concepts proposed include: local stability, linked cavity, basic hydrate, and basic hydrate component (for mixtures). The corresponding thermodynamic model derived has been extensively tested by prediction of hydrate formation from pure gases and gas mixtures. Comparison with experimental data reported in the literature and predictions based on a typical VDW-P-type model indicate that the new model is capable not only of improving prediction accuracy, but also of explaining some intriguing experimental observations in hydrate formation.

Replacement mechanism of methane hydrate with carbon dioxide from microsecond molecular dynamics simulations

Replacement of CH4 in hydrate form with CO2 is a candidate for recovering CH4 gas from its hydrates and storing CO2. In this work, microsecond molecular dynamics simulations were performed to study the replacement mechanism of CH4 hydrate by CO2 molecules. The replacement process is found to be controlled cooperatively by the chemical potentials of guest molecules, “memory effect”, and mass transfer. The replacement pathway includes the melting of CH4 hydrate near the hydrate surface and the subsequent formation of an amorphous CO2 hydrate layer. A large number of hydrate residual rings left after the melting of CH4 hydrate facilitate the nucleation of CO2 hydrate and enhance the dynamic process, indicating the existence of so-called “memory effect”. In the dynamic aspect, the replacement process takes place near the surface of CH4 hydrate rather easily. However, as the replacement process proceeds, the formation of the amorphous layer of the CO2 hydrate provides a significant barrier to the mass transfer of the guest CH4 and CO2 molecules, which prevents the CH4 hydrate from further dissociation and slows down the replacement rate.

Microsecond Molecular Dynamics Simulations of the Kinetic Pathways of Gas Hydrate Formation from Solid Surfaces

In this paper, we report microsecond molecular dynamics simulations of the kinetic pathway of CO(2) hydrate formation triggered by hydroxylated silica surfaces. Our simulation results show that the nucleation of the CO(2) hydrate is a three-stage process. First, an icelike layer is formed closest to the substrates on the nanosecond scale. Then, on the submicrosecond timescale, a thin layer with intermediate structure is induced to compensate for the structure mismatch between the icelike layer and the final stable CO(2) hydrate. Finally, on the microsecond timescale, the nucleation of the first CO(2) hydrate motif layer is generated from the intermediate structure that acts as nucleation seeds. We also address the effects of the distance between two surfaces.

A study on the application of scaling equation for asphaltene precipitation

Abstract The scaling equation proposed by Rassamdana et al. [H. Rassamdana, B. Dabir, M. Nematy, M. Farhani, M. Sahimi, AIChE J. 42 (1996) 10–22; H. Rassamdana, M. Sahimi, AIChE J. 42 (1996) 3318–3332.] is an attractive tool for modeling asphaltene precipitation, as it is simple and the properties of complex asphaltenes are not involved. In this work, the universality of exponents Z and Z ′ in the scaling equation has been examined. The results show that Z ′ can be taken as a universal constant ( Z ′=−2), while Z depends on oil composition but is independent of the specific precipitant ( n -alkane) used. For the oils studied, the optimum value of exponent Z is in the range of 0.10≤ Z ≤0.50. The predictive capability of the scaling equation has been checked by comparison with literature precipitation data, and further extended to the prediction of the onset of asphaltene deposition and asphaltene precipitation in the gas-injected crude oils.

Progress in Research of Gas Hydrate

It is of great significance to study gas hydrate because of following reasons. (1) Most organic carbon in the earth reserves in the form of natural gas hydrate, which is considered as a potential energy resource for the survival of human being in the future. (2) A series of novel technologies are based on gas hydrate. (3) Gas hydrate may lead to many hazards including plugging of oil/gas pipelines, accelerating global warming up, etc. In this paper, the latest progresses in exploration and exploitation of natural gas hydrate, the development of hydrate-based technologies including gas separation, gas storage, CO2 sequestration via forming hydrate, as well as the prevention of hydrate hazards are reviewed. Additionally, the progresses in the fundamental study of gas hydrate, including the thermodynamics and kinetics are also reviewed. A prospect to the future of gas hydrate research and application is given.

Equation of state analog correlations for the viscosity and thermal conductivity of hydrocarbons and reservoir fluids

Abstract Based on the geometric similarity of P–V–T, T–μ–P and T–λ–P diagrams, viscosity (μ) and thermal conductivity (λ) data of hydrocarbons and their mixtures have been successfully correlated using a cubic equation of state (EOS) type equation. The advantages of using EOS-analog expressions are simple in form, applicable to both gas/liquid, high-pressure/low-pressure, and smooth phase transition of μ and λ in the near-critical region could be achieved. First, a modified viscosity correlation based on PR (Peng–Robinson) EOS type expression is presented, which improves the prediction of the viscosity of reservoir fluids (including CO2-injected enhanced oil recovery systems). Then, the development of EOS type expression for fluid thermal conductivity is described. The results of extensive tests with data indicate the superiority of the proposed EOS-analog viscosity and thermal conductivity correlations over typical empirical and semi-empirical correlations used in the petroleum industry.

A hybrid absorption–adsorption method to efficiently capture carbon

Removal of carbon dioxide is an essential step in many energy-related processes. Here we report a novel slurry concept that combines specific advantages of metal-organic frameworks, ion liquids, amines and membranes by suspending zeolitic imidazolate framework-8 in glycol-2-methylimidazole solution. We show that this approach may give a more efficient technology to capture carbon dioxide compared to conventional technologies. The carbon dioxide sorption capacity of our slurry reaches 1.25 mol l−1 at 1 bar and the selectivity of carbon dioxide/hydrogen, carbon dioxide/nitrogen and carbon dioxide/methane achieves 951, 394 and 144, respectively. We demonstrate that the slurry can efficiently remove carbon dioxide from gas mixtures at normal pressure/temperature through breakthrough experiments. Most importantly, the sorption enthalpy is only −29 kJ mol−1, indicating that significantly less energy is required for sorbent regeneration. In addition, from a technological point of view, unlike solid adsorbents slurries can flow and be pumped. This allows us to use a continuous separation process with heat integration.

Nucleation of the CO2 hydrate from three-phase contact lines.

Using molecular dynamics simulations on the microsecond time scale, we investigate the nucleation and growth mechanisms of CO(2) hydrates in a water/CO(2)/silica three-phase system. Our simulation results indicate that the CO(2) hydrate nucleates near the three-phase contact line rather than at the two-phase interfaces and then grows along the contact line to form an amorphous crystal. In the nucleation stage, the hydroxylated silica surface can be understand as a stabilizer to prolong the lifetime of adsorbed hydrate cages that interact with the silica surface by hydrogen bonding, and the adsorbed cages behave as the nucleation sites for the formation of an amorphous CO(2) hydrate. After nucleation, the nucleus grows along the three-phase contact line and prefers to develop toward the CO(2) phase as a result of the hydrophilic nature of the modified solid surface and the easy availability of CO(2) molecules. During the growth process, the population of sI cages in the formed amorphous crystal is found to increase much faster than that of sII cages, being in agreement with the fact that only the sI hydrate can be formed in nature for CO(2) molecules.

A review on the gas hydrate research in China

This paper is aimed at giving a brief overview on the gas hydrate-related research activities carried out by Chinese workers in the past decade. The contents are divided into three sections: (1) Basic research, (2) Status of the exploration of natural gas hydrate resources in the South China Sea region, and (3) The development of hydrate-based new technologies. A comprehensive list of references contributed by Chinese workers (most are published in international journals in English) is attached for those who are interested in knowing more details about our work.