Kinetics and heat transfer during crystallization of gas hydrates and ice

Crystallization refers to a liquid-to-solid phase transition and has been extensively studied due to its relevance in wide-ranging engineering systems. Although the thermodynamics of crystallization is well understood, the kinetics of liquid-solid phase transitions remains an area of active research. Unlike the reverse process of melting, crystallization consists of two different stages: nucleation of a seed crystal, followed by growth into the liquid medium. The kinetics of nucleation primarily depends on interfacial energies and temperature, whereas growth is influenced by heat and mass transfer characteristics of the system. Understanding the overall kinetics of liquid-solid phase transitions is therefore quite complex, and is the focus of this dissertation. This dissertation focuses on the crystallization of gas hydrates and ice. Hydrates are ice-like crystalline solids which form under high pressure and low temperature conditions from water (cage of host molecules) and another liquid or gas (guest molecule). Hydrates can enable novel applications in areas such as carbon capture and sequestration (CCS), flow assurance, natural gas transportation/storage and desalination. Understanding ice formation/melting is critical in the fields of atmospheric sciences, global warming, cryopreservation, infrastructure protection and desalination. Specific topics of this dissertation center around the nucleation and growth kinetics of hydrates and ice. The first task aims at improving the significantly sluggish nucleation kinetics of carbon dioxide (CO₂) hydrates. CO₂ hydrates offer promising new options for carbon dioxide capture and sequestration. However, nucleation times of CO₂ hydrates can range from hours to days, which is too slow and stochastic for any practical application. Building up on previous research, it is discovered that magnesium and magnesium alloy surfaces can nucleate CO₂ hydrates in a few minutes. This represents a 20X improvement in nucleation time over previous research and makes possible on-demand nucleation, which is critical for industrial applications. XPS and FTIR characterization of these surfaces reveal interesting mechanistic insights into the phenomena underlying this ultrafast nucleation. The second task studies the influence of three-phase contact lines and interfacial chemistry on the nucleation kinetics of ice. From classical understanding of nucleation, it is well-known that homogenous nucleation is much slower than heterogeneous nucleation, with the latter occurring on a metal surface with high surface energy. Heterogeneous nucleation is therefore analytically modeled at the conjunction of two phases. It is shown that the conjunction of three-phases at contact lines can further accelerate nucleation. Experimentally, this is often observed in bulk water where ice nucleation originates at the three-phase contact lines present in the system, and not inside the bulk water. Controlled experimentation investigating ice nucleation at fluid-fluid-solid interfaces and modeling using classical nucleation theory show the importance of considering three-phase line contacts during ice nucleation. The third task models the film growth kinetics of hydrates on a stagnant gas-water interface. Film growth of hydrates is the stage of hydrate growth after nucleation where a layer of hydrate grows at the interface of gas and water. It has been conventionally understood that the kinetics of film growth is controlled by the rate of heat transfer in the system. Using scaling arguments, this dissertation shows that heat transfer is not a significant factor, as previously believed. It is shown that hydrate film growth can be effectively modelled using gas diffusion-limited kinetics. The new model shows excellent agreement with multiple experimental datasets (from literature) on hydrate formation from a single as well as mixture of gases. This work enables a novel understanding of the kinetics underlying film growth and provides a fundamentals-based equation for describing the reaction model of hydrate formation. The fourth task explores the coupling between heat and mass transfer during hydrate growth. Hydrate growth kinetics has been modeled in studies spanning several decades; however, most models tend to focus on heat and mass transfer separately; there is inadequate understanding on how these effects are coupled. The focus of this task is to develop a mathematical formulation to capture such effects. The developed mathematical formulation is incorporated in a framework to simulate hydrate growth in bubble column reactors. Bubble column reactors exhibit one of the highest growth rates and the process can be significantly heat transfer-limited if not designed appropriately. The simulation captures the evolution of bubble velocities, bubble radius, temperature, conversion rate etc. for various flow rates of gas and operating conditions. This work also identifies methods to enhance heat dissipation and captures the influence of various operating parameters on hydrate growth. In summary, this dissertation significantly advances the current understanding of crystallization of hydrates and ice. Via experimentation and modeling (analytical, numerical), this dissertation reports novel approaches to speed up hydrate formation, while also providing a deeper understanding of the mechanisms underlying the nucleation of hydrates and ice.Mechanical Engineerin