Ioannis N. Tsimpanogiannis

On the upscaling of reaction-transport processes in porous media with fast or finite kinetics

Abstract We show that for reaction-transport processes with fast kinetics (in the limit of thermodynamic equilibrium), conventional volume averaging for determining effective kinetic parameters applies only when the macroscopic variable approaches its equilibrium value. Even under such conditions, computing the effective mass transfer coefficient requires solving an eigenvalue problem, which couples the local microstructure with the global. Two examples, one involving a simple advection–dissolution problem and another a drying problem in a pore network, illustrate the theoretical predictions. Similar considerations apply for the case of finite kinetics, when the macroscale concentration approaches an equilibrium value. In that case, the effective kinetic parameter is not equal to the local, as typically assumed, but it becomes a function of the local Thiele modulus.

Prediction of the phase equilibria of methane hydrates using the direct phase coexistence methodology

The direct phase coexistence method is used for the determination of the three-phase coexistence line of sI methane hydrates. Molecular dynamics (MD) simulations are carried out in the isothermal-isobaric ensemble in order to determine the coexistence temperature (T3) at four different pressures, namely, 40, 100, 400, and 600 bar. Methane bubble formation that results in supersaturation of water with methane is generally avoided. The observed stochasticity of the hydrate growth and dissociation processes, which can be misleading in the determination of T3, is treated with long simulations in the range of 1000-4000 ns and a relatively large number of independent runs. Statistical averaging of 25 runs per pressure results in T3 predictions that are found to deviate systematically by approximately 3.5 K from the experimental values. This is in good agreement with the deviation of 3.15 K between the prediction of TIP4P/Ice water force field used and the experimental melting temperature of ice Ih. The current results offer the most consistent and accurate predictions from MD simulation for the determination of T3 of methane hydrates. Methane solubility values are also calculated at the predicted equilibrium conditions and are found in good agreement with continuum-scale models.

Pore-Network Modeling of Isothermal Drying in Porous Media

In this paper we present numerical results obtained with a pore-network model for the drying of porous media that accounts for various processes at the pore scale. These include mass transfer by advection and diffusion in the gas phase, viscous flow in the liquid and gas phases and capillary effects at the liquid-gas interface. We extend our work by studying the effect of capillarity-induced flow in macroscopic liquid films that form at the pore walls as the liquid-gas interface recedes. A mathematical model that accounts for the effect of films on the drying rates and phase distribution patterns is presented. It is shown that film flow is a major transport mechanism in the drying of porous materials, its effect being dominant when capillarity controls the process, which is the case in typical applications.

Hydrogen storage in sH hydrates: a Monte Carlo study.

Grand canonical Monte Carlo simulations are performed to evaluate the hydrogen-storage capacity of the recently discovered hydrogen hydrates of the sH type, at 274 K and up to 500 MPa. First, the pure H2 hydrate is investigated in order to determine the upper limit of H 2 content in sH hydrates. It is found that the storage capacity of the hypothetical pure H2 hydrate could reach 3.6 wt % at 500 MPa. Depending on pressure, the large cavity of this hydrate can accommodate up to eight H2 molecules, while the small and medium ones are singly occupied even at pressures as high as 500 MPa. Next, the binary H2-methylcyclohexane sH hydrate is examined. In this case, the small and medium cavities are again singly occupied, resulting in a maximum H2 uptake of 1.4 wt %. Finally, the results from simulations on pure H2 and binary hydrates are utilized to investigate the potential of H2 storage in sH hydrates where the promoter molecules occupy the medium instead of the large cavities.

Immiscible CO2‐H2O fluids in the shallow crust

The significance of a single CO2-H2O fluid phase is well known for metamorphic systems, and CO2-H2O immiscibility is explicit in fluid inclusion literature, especially regarding hydrothermal ores. Complex multiphase CO2-H2O behavior exists over wide temperature and pressure ranges overlapping other important geochemical processes. The character and physical-chemical properties of multiple phases possible for CO2 and H2O, and the potential impact of these coexisting phases on geochemical processes in the crust, are not broadly appreciated. We propose that immiscible supercritical CO2 fluid and a liquid rich in H2O coexist in the shallow crust, to 400°C and 300 MPa, and that interactions among the two fluids and host rock are significant processes that produce recognizable geochemical and textural evidence. Supercritical CO2 fluids bring potential complexity to fluid-rock systems by influencing aqueous reactions via carbonic acid equilibria, penetrating complex geometries inaccessible to aqueous fluid, and dissolving and redistributing metals as organometallic compounds. The distal margin of a contact metamorphic aureole is one example we discuss in which interaction between two disparate CO2-H2O fluids controls H2O activity and the progress and distribution of metamorphic hydration reactions. In another example, supercritical CO2 produces acidity, carbonate saturation, and silica supersaturation in brine. Separation and emplacement of this brine into a rock-dominated system buffered to neutral pH enhances precipitation of carbonates and quartz, chalcedony, or amorphous silica in veins. Other possible examples of CO2-H2O fluid immiscibility coupled with multiphase fluid-rock interactions are clay desiccation, diagenetic and postdiagenetic silicate reactions, origin and distribution of carbonate cements in sedimentary basin sandstones, fluid-mass transfer, and anthropogenic CO2 sequestration.

The role of intermolecular interactions in the prediction of the phase equilibria of carbon dioxide hydrates

The direct phase coexistence methodology was used to predict the three-phase equilibrium conditions of carbon dioxide hydrates. Molecular dynamics simulations were performed in the isobaric-isothermal ensemble for the determination of the three-phase coexistence temperature (T3) of the carbon dioxide-water system, at pressures in the range of 200-5000 bar. The relative importance of the water-water and water-guest interactions in the prediction of T3 is investigated. The water-water interactions were modeled through the use of TIP4P/Ice and TIP4P/2005 force fields. The TraPPE force field was used for carbon dioxide, and the water-guest interactions were probed through the modification of the cross-interaction Lennard-Jones energy parameter between the oxygens of the unlike molecules. It was found that when using the classic Lorentz-Berthelot combining rules, both models fail to predict T3 accurately. In order to rectify this problem, the water-guest interaction parameters were optimized, based on the solubility of carbon dioxide in water. In this case, it is shown that the prediction of T3 is limited only by the accuracy of the water model in predicting the melting temperature of ice.

Influence of combining rules on the cavity occupancy of clathrate hydrates by Monte Carlo simulations

Assessing the exact amount of gas stored in clathrate-hydrate structures can be addressed by either molecular-level simulations (e.g. Monte Carlo) or continuum-level modelling (e.g. van der Waals–Platteeuw-theory-based models). In either case, the Lorentz–Berthelot (LB) combining rules are by far the most common approach for the evaluation of the parameters between the different types of atoms that form the hydrate structure. The effect of combining rules on the calculations has not been addressed adequately in the hydrate-related literature. Only recently the use of the LB combining rules in hydrate studies has been questioned. In the current study, we report an extensive series of Grand Canonical Monte Carlo simulations along the three-phase (H–Lw–V) equilibrium curve. The exact geometry of hydrate crystals is known from diffraction experiments and, therefore, the formation of hydrates can be simulated as a process of gas adsorption in a solid porous material. We examine the effect of deviations from the LB combining rules on the cavity occupancy of argon hydrates and work towards quantifying it. The specific system is selected as a result of the characteristic behaviour of argon to form hydrates of different structures depending on the prevailing pressure. In particular, an sII hydrate is formed at lower pressures, while an sI hydrate is formed at intermediate pressures, and finally an sH hydrate is formed at higher pressures.

Atomistic Molecular Dynamics Simulations of CO2 Diffusivity in H2O for a Wide Range of Temperatures and Pressures

Molecular dynamics simulations were employed for the calculation of diffusion coefficients of CO2 in H2O. Various combinations of existing force fields for H2O (SPC, SPC/E, and TIP4P/2005) and CO2 (EPM2 and TraPPE) were tested over a wide range of temperatures (283.15 K < T < 623.15 K) and pressures (0.1 MPa < P < 100.0 MPa). All force-field combinations qualitatively reproduce the trends of the experimental data; however, two specific combinations were found to be more accurate. In particular, at atmospheric pressure, the TIP4P/2005-EPM2 combination was found to perform better for temperatures lower than 323.15 K, while the SPC/E-TraPPE combination was found to perform better at higher temperatures. The pressure dependence of the diffusion coefficient of CO2 in H2O at constant temperature is shown to be negligible at temperatures lower than 473.15 K, in good agreement with experiments. As temperature increases, the pressure effect becomes substantial. The phenomenon is driven primarily by the higher compressibility of liquid H2O at near-critical conditions. Finally, a simple power-law-type phenomenological equation is proposed to correlate the simulation values; the proposed correlation should be useful for engineering calculations.

Experimental and Computational Investigation of the sII Binary He−THF Hydrate

The objective of this work is to study the binary He-THF hydrate with both experimental and theoretical approaches. Experimental data for the hydrate equilibrium at pressures up to 12.6 MPa are reported for the binary He-THF hydrate with stoichiometric THF composition (i.e., 5.56 mol % THF). These data are used to calibrate a thermodynamic model [J. Phys. Chem. C2009, 113, 422] for the prediction of hydrate equilibrium that is based on the van der Waals-Platteeuw statistical thermodynamic theory. Then this model is used to extrapolate the obtained experimental data to much higher pressures, and good agreement is observed with other available experimental data at pressures up to 150 MPa. This model is also capable of estimating the cavity occupancies for He and THF. The results show that the large cavities are completely occupied by THF molecules, whereas the small ones are partially occupied by He atoms. The He occupancy of the small cavities is less than 60%, even at high pressures (100 MPa). The occupancies predicted from this model are in close agreement with similar results from molecular simulations and a previously reported thermodynamic approach.

Monte Carlo study of sII and sH argon hydrates with multiple occupancy of cages

A series of grand canonical Monte Carlo simulations are performed to investigate the argon uptake inside sII and sH argon hydrates at 274 K and pressures up to 700 MPa. Our results indicate that the small cavities of the sII argon hydrate, as well as the small and medium cavities of the sH hydrate are singly occupied without the observation of any multiple occupancy. On the other hand, the large cavities of the sII hydrate are found to be doubly occupied, and the large cavities of the sH hydrate are found to be filled with up to five argon molecules. The results from our simulations are in very good agreement with the available experimental data concerning cage occupancies and with similar results from other types of molecular simulations.