Phase equilibria of carbon dioxide and methane gas-hydrates predicted with the modified analytical S-L-V equation of state (original) (raw)
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Fluid Phase Equilibria, 2013
Formation of gas hydrates is an important feature of the water-carbon dioxide system. An accurate description of thermodynamic properties of this system requires a consistent description of both fluid (liquid, vapor, and supercritical fluid) and solid states (ice, dry ice, and hydrates) and of their respective phase equilibria. In this study, we slightly modified and refitted the gas hydrate model by A.L. Ballard and E.D. Sloan [Fluid Phase Equil. 194 (2002) 371-383] to combine it with highly accurate equations of state (EoS) in form of the Helmholtz energy and Gibbs energy for other phases formed in the water-carbon dioxide system. The mixture model describing the fluid phases is based on the IAPWS-95 formulation for thermodynamic properties of water by W. Wagner and A. Pruß [J. Phys. Chem. Ref. Data 31 (2002) 387-535] and on the reference EoS for CO 2 by R. Span and W. Wagner [J. Phys. Chem. Ref. Data 25 (1996) 1509-1596]. Both pure-fluid equations are combined using newly developed mixing rules and an excess function explicit in the Helmholtz energy. Pure-component solid phases were modeled with the IAPWS formulation for water ice Ih by R. Feistel and W. Wagner [J. Phys. Chem. Ref. Data 35 (2006) 1021-1047] and with the dry ice EoS by A. Jäger and R. Span [J. Chem. Eng. Data 57 (2012) 590-597]. Alternatively, the hydrate model was combined with the GERG-2004 EoS [O. Kunz, R. * Corresponding author, email: vins.vaclav@seznam.cz, telephone: +420 266 053 152, fax: + 420 286 584 695 2 Klimeck, W. Wagner, M. Jaeschke, GERG Technical Monograph 15, VDI Verlag GmbH, Düsseldorf, 2007]. Since the gas hydrate model uses the fugacity of the gas component in the coexisting phase as an input variable, the accuracy of the predicted phase equilibria was significantly improved by using highly accurate EoSs for coexisting phases. The new hydrate model can be used in a temperature range of 150 ÷ 295 K and at pressures up to 500 MPa. Together with the models describing the fluid and pure solid phases it allows for the desired accurate and consistent description of all phases and phase equilibria including, e.g., flash calculations into two and three phase regions.
Fluid Phase Equilibria, 2010
In this work, experimental dissociation data for clathrate hydrates of methyl cyclopentane, methyl cyclohexane, cyclopentane or cyclohexane+carbon dioxide are reported at different temperatures. The experimental data were generated using an isochoric pressure-search method. The reliability of this method is examined by generating new dissociation data for clathrate hydrates of methyl cyclopentane +methane and comparing them with the experimental data reported in the literature. The acceptable agreement demonstrates the reliability of the experimental method used in this work. The experimental data for all measured systems are finally compared with the corresponding literature data in the absence of the above mentioned cyclic compounds to identify their promotion effects.
A thermodynamic model to predict three phase (L-H-V and I-H-V) equilibria of gas hydrates is presented. In this model we have employed a fugacity based approach where the hydrate phase is modeled using van der Waals-Platteeuw solid solution theory and the liquid phase activity coefficients are determined from the modified UNIFAC method. For the vapour phase fugacity calculations we have investigated three equations of state (EOS): Peng-Robinson-Stryjek-Vera (PRSV), Patel-Teja (PT) and Soave-Redlich-Kwong (SRK). This model employs only parameters reported in the literature. The coexistence pressures predicted by our model for the sI hydrates of methane, carbon dioxide and ethane are in reasonable agreement with experiments, whereas our model overestimates the coexistence pressures for the sII clathrates of nitrogen and propane. The predicted cage occupancies are found to increase with increasing temperature in the L-H-V equilibria. For I-H-V equilibria the cage occupancy is observed to decrease with temperature. We have also estimated the solubility of each guest in the liquid phase (for L-H-V equilibria) using the Henry's law. The solubilities predicted using all three EOS are in good agreement for all guest molecules, with the exception of nitrogen where at relatively higher temperatures the estimates from the PRSV EOS are noticeably lower than the corresponding predictions from the PT and SRK EOS.
Thermodynamic and Kinetic Modeling of CH4/CO2 Hydrates Phase transitions
International Journal of Energy and Environment, 2013
Natural gas hydrates in reservoirs are thermodynamically unstable due to exposure to mineral surfaces and possibly undersaturated phases of water and hydrate formers. Changes in global temperatures also alter the stability regions of the accumulations of gas hydrates worldwide. The fact that hydrates in porous media never can reach equilibrium, and formation can occur from different phases, as well as dissociate according to different thermodynamic driving forces imposes very complex phase transition dynamics. These phase transitions dynamics are solutions to coupled differential equations of mass transport, heat transport and phase transition kinetics. The availability of free energy as functions of temperature, pressure and the composition of all components in all phases in states outside of equilibrium is therefore necessary in kinetic theories based on minimisation of free energy. For this purpose we have applied an extended adsorption theory for hydrate, SRK equation of state for methane/CO2 gas and solubilities of these components in water for the limit of water thermodynamics. The thermodynamic model is developed for calculation of free energy of super saturated phase along all different gradients (mole fractions, pressure and temperature) of super saturation.
Assessment of clathrate hydrate phase equilibrium data for CO2+CH4/N2+water system
Fluid Phase Equilibria, 2013
Outlier diagnostic in phase equilibrium data of binary clathrate hydrates containing CO 2 is the main aim of the present work. The treated experimental data are concerning the clathrate hydrates of CO 2 + CH 4 /N 2 in the presence of water. The utilized algorithm applies the basis of a mathematical approach, in which the statistical Hat matrix, Williams plot, and the residuals of two models results bring about the probable outliers detection. The range of applicability of the applied models and quality of the existing experimental data are also investigated. The van der Waals and Platteeuw (vdW-P) solid solution theory is used to model the hydrate phase, and the Valderrama-Patel-Teja equation of state (VPT-EoS) along with the non-density dependent (NDD) mixing rules is applied to deal with the fluid phases in the first model. The compositions of the vapor phase in equilibrium with gas hydrate and liquid water as well as the equilibrium pressures are predicted through the mentioned model. The second model includes a correlation proposed by Adisasmito et al., which is utilized to represent the hydrate dissociation pressures for three-phase equilibrium conditions (liquid water-vapor-hydrate). It is interpreted from the obtained results that the applied models for calculation/estimation of the phase behavior of the investigated binary clathrate hydrate systems have wide ranges of applicability. Consequently, we may, with high confidence level, say that among all data treated, one experimental equilibrium pressure value and four experimental hydrate dissociation values are probable doubtful ones.
Thermodynamic Modeling of CH4/CO2 hydrate Phase Transition
International Journal of Energy, Environment and Economics
Natural gas hydrates in reservoirs are thermodynamically unstable due to exposure to mineral surfaces and possibly undersaturated phases of water and hydrate formers. Changes in global temperatures also alter the stability regions of the accumulations of gas hydrates worldwide. The fact that hydrates in porous media never can reach equilibrium, and formation can occur from different phases, as well as dissociate according to different thermodynamic driving forces imposes very complex phase transition dynamics. These phase transitions dynamics are solutions to coupled differential equations of mass transport, heat transport and phase transition kinetics. The availability of free energy as functions of temperature, pressure and the composition of all components in all phases in states outside of equilibrium is therefore necessary in kinetic theories based on minimisation of free energy. For this purpose we have applied an extended adsorption theory for hydrate, SRK equation of state for methane/CO2 gas and solubilities of these components in water for the limit of water thermodynamics. The thermodynamic model is developed for calculation of free energy of super saturated phase along all different gradients (mole fractions, pressure and temperature) of super saturation.
Hydrate phase equilibria of gaseous mixtures of methane+ carbon dioxide+ hydrogen sulfide
In this communication, experimental dissociation conditions for clathrate hydrates of methane + carbon dioxide + hydrogen sulfide in liquid water -hydrate-vapor equilibrium are reported. The dissociation temperatures and pressures are in the ranges of (273.9 to 288.25) K and (0.207 to 1.94) MPa, respectively. Concentrations of methane, carbon dioxide and hydrogen sulfide in the feed gas are also varied. To perform the measurements, an isochoric pressure-search method was used. The dissociation data obtained in the present work are compared with the predictions of a thermodynamic model (HWHYD version 1.
Fluid Phase Equilibria, 2011
In this paper, a set of experimental data on the phase equilibrium of gas hydrates in the presence of binary gas mixtures comprising CO 2 is presented. The procedure established allows for the determination of both the composition of the gas phase as well as the hydrate phase without the need to sample the hydrate. The experimental results obtained in these measurements have been described by means of the classical model of van der Waals and Platteeuw. The values of internal parameters of the reference state and the Kihara parameters have been re-discussed and their interdependency is pointed. Finally the new set of parameters is validated against experimental data from other sources available in the literature, or invalidated against other sources. Finally, we conclude on the difference of experimental data between laboratories. The differences are not on the classical (Pressure, Temperature, gas composition) data which appear equivalent between laboratories. The difference stands on the measurement composition of the hydrate phase.
Chemical engineering transactions, 2020
Gas hydrates are crystalline structures formed by water molecules and compounds of low molecular weights, being formed under suitable conditions of pressure and temperature. Although initially considered as inconveniences to the natural gas industries, they are currently considered as promising alternatives for solving some important global issues, such as contributing to the reduction of effects caused by the greenhouse gases. This concern related to the control of emissions of polluting gases has mobilized hundreds of countries that, at the United Nations Climate Change Conference (COP), agreed to reduce emissions of carbon dioxide and other gases by 2100. However, despite several strategies in the reduction of carbon dioxide emissions have been proposed, many rely on political incentives and substantial investments to convert pre-existing technologies to clean technologies, making such applicability and adaptability problematic. Thus, innovative Carbon Capture and Storage (CCS) t...