CO2 Hydrate Particles Research Articles

Stochastic Cellular Automata Modeling of CO2 Hydrate Growth and Morphology.

Carbon dioxide (CO2) hydrates are important in a diverse range of applications and technologies in the environmental and energy fields. The development of such technologies relies on fundamental understanding, which necessitates not only experimental but also computational studies of the growth behavior of CO2 hydrates and the factors affecting their crystal morphology. As experimental observations show that the morphology of CO2 hydrate particles differs depending on growth conditions, a detailed understanding of the relation between the hydrate structure and growth conditions would be helpful. To this end, this work adopts a modeling approach based on hybrid probabilistic cellular automata to investigate variations in CO2 hydrate crystal morphology during hydrate growth from stagnant liquid water presaturated with CO2. The model, which uses free energy density profiles as inputs, correlates the variations in growth morphology to the system subcooling ΔT, i.e., the temperature deficiency from the triple CO2-hydrate-water equilibrium temperature under a given pressure, and properties of the growing hydrate-water interface, such as surface tension and curvature. The model predicts that when ΔT is large, parabolic needle-like or dendrite crystals emerge from planar fronts that deform and lose stability. In agreement with chemical diffusion-limited growth, the position of such planar fronts versus time follows a power law. In contrast, the tips of the emerging parabolic crystals steadily grow in proportion to time. The modeling framework is computationally fast and produces complex growth morphology phenomena under diffusion-controlled growth from simple, easy-to-implement rules, opening the way for employing it in multiscale modeling of gas hydrates.

Heat Transfer and Pressure Drop Simulation of CO2-Hydrate Mixture in Tube

The formation of CO2 hydrate during CO2 transportation presents a complex two-phase flow within tube. A two-dimensional CFD model for CO2 hydrate mixture flow in tube is derived based on the Eulerian multiphase flow modeling approach in which the two phases consist of CO2 gas and CO2 hydrate particles. A coupled Eulerian multiphase and nonisothermal flow model without phase-change is developed based on COMSOL Multiphysics built-in application modes. The model couples the mass, momentum, and energy equations for the two phases to solve the temperature and flow characteristics of the CO2 hydrate mixture flow in tube. CO2 hydrate particles are found to settle down during flow even under high velocity operation. The pressure drop increased linearly with inlet volume fraction from 1.29[Formula: see text]kPa for 0.1–5.2[Formula: see text]kPa for 0.5, and the related overall heat transfer coefficients of the CO2 hydrate mixture computed from the model ranged from 980 to 4000[Formula: see text]W/m2K with variation of CO2 hydrate volume fraction.

Scale‐up of a continuous‐jet hydrate reactor for CO2 ocean sequestration

AbstractIn previous work in our laboratories and in the ocean, we have investigated the formation and dissociation of composite CO2 particles made of CO2 hydrate, liquid CO2, and water. The composite is formed by partially converting liquid CO2 into hydrate at mid‐ocean depth (1000–1200 m) conditions. Partial conversion of CO2 into hydrate enables injection of CO2 in seawater in the form of a dense composite material that will drive CO2 to ocean depths well below the injection point. Thermodynamic conditions allowing the formation of negatively buoyant composite particles have been established for a laboratory‐scale, continuous‐jet hydrate reactor (CJHR). An investigation has been performed to explore the issues related to the scale‐up of the CJHR. Hydrate was formed using two CJHR geometries; the first sprayed water in CO2, and the second sprayed CO2 in water. Using a plot of Ohnesorge vs. Reynolds numbers allowed flow rates to be selected that would yield a spray regime and maximize hydrate formation. The effect of varying pressure and liquid flow rates on hydrate behavior was observed. Depending on these parameters, hydrate particles were observed to sink, float, or be neutrally buoyant. A two‐order‐of‐magnitude scale‐up in the flow rates of the two fluids has been achieved with the larger CJHR geometries without losing the important characteristics of the hydrate particles (i.e., density and cohesiveness). Temperature changes as a result of hydrate formation were also monitored. A mathematical model has been developed to predict the fate of sinking CO2 hydrate particles after release in the seawater. These results can guide further field and laboratory investigations related to the scale‐up of ocean CO2 sequestration. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Modeling descending carbon dioxide injections in the ocean

An integral double plume model is used to explore the fate of solid CO2 hydrate particles released continuously into a quiescent ocean for the purposes of CO2 sequestration. Such a release is desirable because hydrate particles are negatively buoyant and dissolution of CO2 enhances this negative buoyancy through the solute density effect. Plume depth and thickness are shown to increase with CO2 mass loading and initial hydrate particle diameter, and exceed the equivalent rise height and thickness associated with positively buoyant droplet releases. The depths also greatly exceed those associated with a single particle release, highlighting the importance of the “plume” effect. Plumes initially composed of multiple particle sizes produce greater maximum plume depth, but similar average plume depth and dilution compared with plumes composed of homogeneous particles, suggesting that adequate simulations can be made using a single particle size. Although the model is valid only for discharge to quiescent receiving water, analysis shows the effects of an ambient current decrease with increasing mass flow rate or decreasing particle size, making results relevant for most likely release scenarios.

Ocean Disposal of CO2: Conditions for Producing Sinking CO2 Hydrate

Carbon dioxide sequestration at intermediate ocean depths in the form of a sinking CO2 stream has been shown to be technically feasible. Buoyancy characteristics of CO2 hydrate particles obtained from a coflow reactor operating under conditions that produce negatively buoyant CO2 hydrate composite particles were studied. Using a simplified process model, the limiting ratios of CO2 to water to produce sinking hydrate composite streams were established for pressure and temperature conditions of intermediate ocean depths. A kinetic model involving a mass transfer step and a reaction step was proposed to explain experimental data using a seafloor process simulator. Results suggest that the reactor operates under mass‐transfer controlled conditions. Although the density of freshwater is only a few percentage units less than that of seawater, operating conditions derived from freshwater‐based studies are significantly different for ambient seawater. Owing to the higher density of seawater, greater conversion of CO2 to hydrate is needed to obtain a sinking CO2 composite stream than at the same depth of freshwater.

Numerical study on effects of hydrate formation on deep sea CO2 storage.

When liquid CO2 is disposed into the deep sea or stored on the sea floor in order to prevent anthropogenic CO2 from entering the atmosphere, the CO2 reacts immediately with the seawater and forms solid CO2 hydrate. In this study, a growth model is proposed for CO2 hydrate layers. In addition, an estimation is shown for the CO2 concentration profile, the growth rate of the layer, and its thickness in the quasi-steady state. The simulation results indicate that the hydrate layer diminishes CO2 dissolution from the storage pool into the upper sea. This growth process is extremely sensitive to the density difference between the CO2 hydrate particles and its environmental liquid.