Prediction of Gas-hydrate Equilibrium, Stability and Kinetic Nucleation in Porous Media

Natural gas-hydrates are crystalline inclusion compounds with gas molecules (guest compounds) trapped within a host lattice formed by water molecules in an ice-like hydrogen-bonded framework. Natural gas-hydrates have the potential to become an important carbon-based resource addressing the increasing energy demand, and they pose a risk in terms of climate change. Accurate estimates of gas-hydrates global inventory, understanding of formation and dissociation processes of gas-hydrates, and evaluation of their environmental impact require models that accurately describe gas-hydrate stability in sediments and predict gas-hydrate kinetic nucleation processes. The hypothesis driving this work is that incorporation of selected sediment properties, i.e., surface energies and pore diameter, can lead to more accurate predictions of hydrate equilibrium, stability and nucleation in porous media. In this work, a model for gas-hydrate equilibrium in porous media was developed from basic thermodynamic principles and tested against available experimental data published in the scientific literature. The proposed model predicts reported experimental data with high accuracy for the range of pore sizes (3.4 ~ 24.75 nm) of different materials reported in the literature. It was found that the wettability of the pore surface affects the shape of the hydrate phase inside the pore and consequently influences the equilibrium pressures of gas-hydrates formed in porous media. A predictive macroscopic mathematical model describing the kinetic nucleation of gas-hydrates was developed based on Classical Nucleation Theory (CNT) in order to formulate correction factors for three types of interfaces mostly encountered in natural sediments (gas-liquid interface, liquid-solid interface and three-phase boundary lines). This approach, which incorporates the interfacial properties of sediments, can efficiently provide a fundamental understanding on the dependence of the formation mechanism of gas hydrates on a wide range of interfacial properties (wettability, substrate size, interfacial tension). The model predicts that hydrate nucleation is energetically favorable on confined surfaces with smaller contact-angle values, i.e., hydrophilic surfaces. Comparison between different types of interfaces leads to the conclusion that the nucleation of gas hydrates preferentially occurs in larger sediment pores. At the beginning of methane hydrate formation, for example, hydrate will preferentially nucleate at the gas-liquid interface. With the increase of hydrate volume or growth of the hydrate phase, the center of crystal growth moves towards the liquid-solid interface. In natural systems, gas hydrates form first on the concave liquid/solid interface and gas/liquid interface in sandstone sediments, gas/liquid interface and gas/liquid/solid triple boundary line in clay sediments and gas/liquid interface in pipeline with oil droplets. The inclusion of sediment properties in the model for gas-hydrate equilibrium in sediments predict experimental data within a margin of %AAD lower than 2%, a significant improvement upon previous modeling attempts. Additionally, the inclusion of sediment properties in the models for kinetic nucleation of gas hydrates result in mathematical models that capture the qualitative information obtained from examination of gas-hydrate core samples. Therefore, the hypothesis of the present work was proven

Fabrication and Mechanical Properties of Micro-Architectured 3D Scaffolds

Freeze casting is a physical process for the fabrication of porous anisotropic materials. In this method, an aqueous slurry of ceramic particles is frozen directionally, creating lamellar columns of ice that push particles between the growing crystals. Then, the frozen material is lyophilized to remove the ice and sintered to densify the ceramic. Scaffolds made by freeze casting often have significant strength in the solidification direction, while they lack sufficient strength in the transverse direction. To enhance strength in the transverse direction, magnetite particles are added to a slurry of paramagnetic particles, and an external magnetic field is applied during solidification. Interactions between the magnetite and paramagnetic particles compete with thermal and viscous forces, resulting in different colloidal behaviors. Under relatively weak magnetic fields, the particles are attracted to one another, forming aligned chains that are trapped by the ice front and result in bridges spanning the lamellar walls. When interactions between magnetite and paramagnetic particles are strong, the alignment of magnetite also results in alignment of the paramagnetic particles. Under stronger magnetic fields, however, a gradient magnetic force attracts particles toward the field’s poles, creating biphasic regions of iron-rich and iron-poor microstructures. To further investigate the relationship between microstructure and mechanical properties observed, 3D printed scaffolds mimicking patterns observed in magnetic freeze casting were designed and fabricated for comparison. The 3D printed scaffolds were tested in compression in three orthogonal directions. To compare their performance, permutated radar charts were used to simultaneously analyze the strength, toughness, resilience, elastic modulus and strain to failure across each orthogonal direction

Particles, Drops, and Bubbles in Gradient Fields

Particles, drops, and bubbles submerged in a host liquid are omnipresent in nature and industry. Often, they are subjected to gradients in concentration or temperature (or both). These gradients might change locally or evolve over time, placing the system far from its equilibrium. This gives rise to extraordinary rich physics at the intersection of fluid dynamics, chemical engineering, and colloid and interface science. In this thesis, we investigate the behaviour of particles, drops, and bubbles in applied gradient fields. We focus on small-scale, idealized table-top experiments closely combined with theoretical and numerical modelling to study these objects under conditions that are far from equilibrium. For the latter, we consider particles, drops, and bubbles at a water-ice interface during unidirectional solidification (Part I) and immiscible drops in density stratified ethanol-water mixtures (Part II).In Part I we deal with the freezing of suspensions and oil-in-water emulsions in order to study the interaction between different objects and an approaching water-ice solidification front. To do so in a controlled manner, we apply a thermal gradient over our sample and ensure slow, uni-directional freezing. We then change the type of object near the front to add more and more complexity to the system, starting with solid particles (chapter 1) before moving on to drops (chapter 1-4) and eventually bubbles (chapter 5).In Part II we study the dynamics of immiscible drops in a density stratified ethanol-water mixture. These studies further investigate the peculiar observation that these drops can show continuous bouncing, against gravity, caused by an oscillatory solutal Marangoni flow around the drop. In chapter 6 we look in depth into the onset of the bouncing instability and extend the experimental parameter space by changing the viscosity of the oil, in order to determine the different mechanisms that trigger it. Finally, in chapter 7, we dive further into the characteristics of the bouncing cycle through well-performed experiments and numerical simulations, aiming for a one-to-one comparison.<br/

Frost Heave in Colloidal Soils.

We develop a mathematical model of frost heave in colloidal soils. The theory accountsfor heave and consolidation while not requiring a frozen fringe assumption. Two solidificationregimes occur: a compaction regime in which the soil consolidates to accommodate the ice lenses, and a heave regime during which liquid is sucked into the consolidated soil from an external reservoir, and the added volume causes the soil to heave. The ice fraction is found to vary inversely with thefreezing velocity V , while the rate of heave is independent of V , consistent with field and laboratoryobservations. © 2011 Society for Industrial and Applied Mathematics