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OS5A-3:METHANE HYDRATE DISSOCIATION KINETICS USING NONEQUILIBRIUM MOLECULAR DYNAMICS
发布时间:2014-07-28
Gabriel G. S. FERREIRA1，Frederico W. TAVARES2, Charlles R. A. ABREU2 , Paulo L. C. LAGE1
1. Programa de Engenharia Química COPPE, Universidade Federal do Rio de Janeiro, BRAZI; 2. Escola de Química (EQ), Universidade Federal do Rio de Janeiro, BRAZIL
The methane hydrate formation and dissociation is very often predicted using empirical intrinsic kinetics models, where the hydrate dissociation rate is proportional to a driving force. One of the most popular model is the one formulated by Kaschiev and Firoozabadi (J. Crystal Growth 241, 220, 2002). They derived a generalized driving force model for gas-water systems, which depends on the difference of chemical potential in the system due to the hydrate phase formation. One of the main issues of this formulation is the uncertainties of the experimental procedure used on the data regression. Although experimental data of hydrate dissociation reported in the literature are obtained under controlled environment with high stirring speeds, it is common to find laboratory set-up dependent results. This can be related to the assumption of no thermal or mass transfer resistances, which is a very strong hypothesis (even for reactors with high stirring speeds), which renders quite difficult to get reliable intrinsic kinetics experimental data.
Seeking for a better understanding on the dynamics of hydrate dissociation, several molecular dynamics simulations studies were carried out. Although useful for a qualitative analysis, most of these works used an isothermal approach, applying thermostat methods to overheat the hydrate. In this approach, thermal gradients in the hydrate-fluid interface cannot exist and the dissociation rate results have no connection to the actual physical phenomena. Recently, Bagherzedah et. al. (J. Chem. Thermodynamics 44, 13, 2012) performed constant energy non-equilibrium hydrate dissociation simulations, developing a methodology to estimate the hydrate dissociation rate at 273K in contact with water at temperatures above the hydrate stability. Although this method is promising, very few results were published and no comparison to other results were performed so far. We performed similar molecular dynamics simulations using a different water model (TIP3P). Although the results were qualitatively satisfactory, the hydrate dissociation rates were much higher than those reported in their work. This was expected because TIP3P model predicts a much lower transition temperature for water/ice, but this result shows that further analysis on the methodology is necessary. The current work also analyzed a convergence analysis for the number of molecules in the system to give size-independent results.
1. Programa de Engenharia Química COPPE, Universidade Federal do Rio de Janeiro, BRAZI; 2. Escola de Química (EQ), Universidade Federal do Rio de Janeiro, BRAZIL
The methane hydrate formation and dissociation is very often predicted using empirical intrinsic kinetics models, where the hydrate dissociation rate is proportional to a driving force. One of the most popular model is the one formulated by Kaschiev and Firoozabadi (J. Crystal Growth 241, 220, 2002). They derived a generalized driving force model for gas-water systems, which depends on the difference of chemical potential in the system due to the hydrate phase formation. One of the main issues of this formulation is the uncertainties of the experimental procedure used on the data regression. Although experimental data of hydrate dissociation reported in the literature are obtained under controlled environment with high stirring speeds, it is common to find laboratory set-up dependent results. This can be related to the assumption of no thermal or mass transfer resistances, which is a very strong hypothesis (even for reactors with high stirring speeds), which renders quite difficult to get reliable intrinsic kinetics experimental data.
Seeking for a better understanding on the dynamics of hydrate dissociation, several molecular dynamics simulations studies were carried out. Although useful for a qualitative analysis, most of these works used an isothermal approach, applying thermostat methods to overheat the hydrate. In this approach, thermal gradients in the hydrate-fluid interface cannot exist and the dissociation rate results have no connection to the actual physical phenomena. Recently, Bagherzedah et. al. (J. Chem. Thermodynamics 44, 13, 2012) performed constant energy non-equilibrium hydrate dissociation simulations, developing a methodology to estimate the hydrate dissociation rate at 273K in contact with water at temperatures above the hydrate stability. Although this method is promising, very few results were published and no comparison to other results were performed so far. We performed similar molecular dynamics simulations using a different water model (TIP3P). Although the results were qualitatively satisfactory, the hydrate dissociation rates were much higher than those reported in their work. This was expected because TIP3P model predicts a much lower transition temperature for water/ice, but this result shows that further analysis on the methodology is necessary. The current work also analyzed a convergence analysis for the number of molecules in the system to give size-independent results.
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