Robert P. Warzinski

Molecular dynamics simulations of methane hydrate decomposition

Molecular dynamics simulations have been carried out to study decomposition of methane hydrate at different cage occupancies. The decomposition rate is found to depend sensitively on the hydration number. The rate of the destruction of the cages displays Arrhenius behavior, consistent with an activated mechanism. During the simulations, reversible formation of partial water cages around methane molecules in the liquid was observed at the interface at temperatures above the computed hydrate decomposition temperature.

Molecular Dynamics Simulations of the Thermal Conductivity of Methane Hydrate

Nonequilibrium molecular dynamics simulations with the nonpolarizable SPC/E (Berendsen et al., J. Phys. Chem. 1987, 91, 6269) and the polarizable COS/G2 (Yu and van Gunsteren, J. Chem. Phys. 2004, 121, 9549) force fields have been employed to calculate the thermal conductivity and other associated properties of methane hydrate over a temperature range from 30 to 260 K. The calculated results are compared to experimental data over this same range. The values of the thermal conductivity calculated with the COS/G2 model are closer to the experimental values than are those calculated with the nonpolarizable SPC/E model. The calculations match the temperature trend in the experimental data at temperatures below 50 K; however, they exhibit a slight decrease in thermal conductivity at higher temperatures in comparison to an opposite trend in the experimental data. The calculated thermal conductivity values are found to be relatively insensitive to the occupancy of the cages except at low (T<or=50 K) temperatures, which indicates that the differences between the two lattice structures may have a more dominant role than generally thought in explaining the low thermal conductivity of methane hydrate compared to ice Ih. The introduction of defects into the water lattice is found to cause a reduction in the thermal conductivity but to have a negligible impact on its temperature dependence.

Modeling clathrate hydrate formation during carbon dioxide injection into the ocean

Because of carbon dioxides potential role in global warming, there is considerable interest in methods of long-term sequestering of anthropogenic emissions of CO[sub 2] outside of the atmosphere. The work reported here predicts the effect of hydrate formation on the fate of CO[sub 2] droplets discharged into the ocean under hydrate-forming conditions. New information on hydrate growth rates recently determined by one of the authors is incorporated into the model. It is seen that hydrate film formation on CO[sub 2] droplets into the deep ocean will increase estimates of required injection depths and decrease the maximum allowable droplet size suitable for effective sequestration to occur. 13 refs., 2 figs.

Dynamic morphology of gas hydrate on a methane bubble in water: Observations and new insights for hydrate film models

Predicting the fate of subsea hydrocarbon gases escaping into seawater is complicated by potential formation of hydrate on rising bubbles that can enhance their survival in the water column, allowing gas to reach shallower depths and the atmosphere. The precise nature and influence of hydrate coatings on bubble hydrodynamics and dissolution is largely unknown. Here we present high-definition, experimental observations of complex surficial mechanisms governing methane bubble hydrate formation and dissociation during transit of a simulated oceanic water column that reveal a temporal progression of deep-sea controlling mechanisms. Synergistic feedbacks between bubble hydrodynamics, hydrate morphology, and coverage characteristics were discovered. Morphological changes on the bubble surface appear analogous to macroscale, sea ice processes, presenting new mechanistic insights. An inverse linear relationship between hydrate coverage and bubble dissolution rate is indicated. Understanding and incorporating these phenomena into bubble and bubble plume models will be necessary to accurately predict global greenhouse gas budgets for warming ocean scenarios and hydrocarbon transport from anthropogenic or natural deep-sea eruptions.

Formation of gas hydrates from single-phase aqueous solutions

Abstract Experiments and theory both demonstrate that crystalline hydrates can form from a single-phase system consisting of liquid water with dissolved hydrate former under appropriate conditions of temperature, pressure, and dissolved hydrate former content. At these conditions, the pressure required for hydrate formation will be equal to or greater than that required when a separate gas or liquid phase of hydrate former is present. It is possible to form hydrates from a single phase when the mole fraction of dissolved hydrate former is greater than the mole fraction which would exist in the water-rich liquid phase at the three-phase (vapor/water-rich liquid/hydrate or VLH) hydrate equilibrium pressure. However, this would represent non-equilibrium or super-saturated conditions with respect to hydrate formation. It is also possible to form hydrates from a single water-rich phase when the mole fraction of the dissolved hydrate former is less than that which would exist in the presence of a gas phase at the three-phase VLH hydrate equilibrium pressure. The pressure requirement for the formation of hydrate increases as the mole fraction of hydrate former decreases under these conditions. The possibility of forming hydrates from dissolved hydrate formers may have application to potential commercial processes; for example, in the sequestration of CO 2 in the deep ocean and in the recovery of hydrates from suboceanic and permafrost regions.

Supercritical-fluid solubilization of catalyst precursors: The solubility and phase behavior of molybdenum hexacarbonyl in supercritical carbon dioxide and application to the direct liquefaction of coal☆

Abstract A method utilizing supercritical carbon dioxide for impregnating coal with catalytic amounts of molybdenum hexacarbonyl useful for promoting coal dissolution during direct liquefaction is described. Results are presented that describe the solubility and phase behavior of molybdenum hexacarbonyl in supercritical carbon dioxide at 40, 50, and 60 °C. A model based upon the Peng-Robinson equation-of-state was used both to initially predict and subsequently model the observed solubility behavior. Preliminary liquefaction results of Illinois No. 6 coal impregnated with molybdenum hexacarbonyl from supercritical carbon dioxide are also presented.

Control of a fluid particle under simulated deep-ocean conditions in a high-pressure water tunnel

An apparatus that permits the observation of liquid CO2 particles in a simulated deep-ocean environment was designed, modeled, constructed, and tested. Analysis concerning the vertical stability and control of the fluid particle in a countercurrent flow loop is presented. The vertical position of the particle was found to vary due to the gradual dissolution of the CO2 into the water and various other random effects. Using linearized equations for spherical particle motion in the fluid flow, a second-order dynamic equation is derived and analyzed. The servocontrol system consists of a video system that digitizes particle position in a viewing window and a centrifugal pump that can control the speed of the countercurrent flow in order to maintain the particle within the observation window of a charge-coupled device camera. Currently, the system is being used to obtain information on the dissolution behavior of CO2 in seawater at various conditions of pressure and temperature that simulate ocean depths down ...

THERMAL PROPERTIES OF METHANE HYDRATE BY EXPERIMENT AND MODELING AND IMPACTS UPON TECHNOLOGY

Thermal properties of pure methane hydrate, under conditions similar to naturally occurring hydrate-bearing sediments being considered for potential production, have been determined both by a new experimental technique and by advanced molecular dynamics simulation (MDS). A novel single-sided, Transient Plane Source (TPS) technique has been developed and used to measure thermal conductivity and thermal diffusivity values of low-porosity methane hydrate formed in the laboratory. The experimental thermal conductivity data are closely matched by results from an equilibrium MDS method using in-plane polarization of the water molecules. MDS was also performed using a non-equilibrium model with a fully polarizable force field for water. The calculated thermal conductivity values from this latter approach were similar to the experimental data. The impact of thermal conductivity on gas production from a hydrate-bearing reservoir was also evaluated using the Tough+/Hydrate reservoir simulator.

The Impact of CO2 Clathrate Hydrate on Deep Ocean Sequestration of CO2: Experimental Observations and Modeling Results

Abstract: CO2 clathrate hydrate is a crystalline compound that can form under temperature and pressure conditions associated with the injection and storage of CO2 in the deep ocean (below 500 m). At depths being considered for injection of CO2 (between 1,000 and 1,500 m), in the absence of hydrate formation, the buoyant CO2 would simply rise as it dissolved in the seawater. If, however, the hydrate phase forms, it will affect this process. The impact could be positive or negative, depending on how hydrate forms and whether it is associated with undissolved CO2. This paper summarizes experimental and theoretical information relating to formation conditions for the hydrate, the relative density of the hydrate, the formation of a hydrate shell on drops of liquid CO2, and the impact that a hydrate shell has on dissolution of CO2. The future direction of the work is also briefly described.

Observations of CO2 clathrate hydrate formation and dissolution under deep-ocean disposal conditions

Publisher Summary This chapter discusses the observations and implications for the effective disposal of carbon dioxide (CO 2 ) in the ocean. Simply discharging the CO 2 at great depths may be insufficient if hydrate coatings form on liquid droplets of CO 2 . Observations of single droplets indicate that the coating may impede the dissolution and permit the CO 2 droplet to rise to unacceptably shallow depths. The possibility of growth of the hydrate shell cannot be ruled out based on the observations of single droplets. If CO 2 is not dissolved, then the growth of the hydrate coatings becomes more likely, especially as droplets collide and fresh CO 2 is released. Density observations indicate that pure hydrates formed in the first case are negatively buoyant and sink. Problems with plugging in the event of a flow interruption in such systems may be avoided by operating at slightly under saturated conditions with respect to CO 2 . Observations indicate that these conditions favor the formation of a semi-solid mass rather than a solid hydrate plug.