(789a) Kinetic Studies On Binary Clathrate Hydrates

While early clathrate hydrate studies aimed at the suppression of hydrate formation in oil pipelines, several applications have emerged where the formation of clathrate hydrates could be useful, including natural gas storage, carbon dioxide sequestration and desalination. In these settings, it is valuable to explore ways to produce hydrate at lower pressures, higher temperatures and increased formation rates than are presently available.

One innovation to facilitate hydrate formation is to employ a binary hydrate, which can boost the thermodynamic driving force significantly. In hydrates, the ice-like structures of hydrogen-bonded water networks form cavities of different sizes. While the occupation of each type of cavity by guest molecules promotes hydrate formation, no single guest species effectively fits all cage sizes at the same time. By introducing two guest species simultaneously -- one liquid primary former (such as cyclopentane) fitting the large cavities and one helper gas (such as methane or CO₂) fitting the small cavities – the cage stabilization effect is strengthened, allowing hydrate to form and grow under conditions unavailable to corresponding single-component hydrates.

Though thermodynamically favorable, the formation kinetics of binary clathrate hydrates are not well understood. Prior kinetic studies mainly deal with single component hydrates, but the growth of binary hydrates is more complicated. It entails the co-presence of hydrate, water, liquid former and  gas helper, comprising a complex coexistence of four-phases. The hydrate growth process may take place primarily at the water - liquid hydrate former interface, consuming the helper gas dissolved in both liquid phases. The solubility of the helper gas in the liquid primary former is usually far below the hydrate stoichiometric level. For example, cyclopentane-methane hydrate has the sII structure with a 2:1 ratio of small:large cavities, and its formation consumes methane and cyclopentane roughly in the same ratio, yet a liquid phase composed of 67% methane and 33% cyclopentane does not exist in the temperature/pressure range of interest. As a result, transport issues arise additional to the factors involved in single-component hydrate growth since the hydrate growth is potentially limited by the transfer of the helper gas from the vapor phase to and between the two liquid phases.

This experimental study investigates the kinetics of the binary hydrate growth, with emphasis on the interphase transport of the hydrate components. The hydrates of the former pairs, cyclopentane-methane and cyclopentane-CO₂, are generated in a batch reactor as a function of temperature, pressure, stirring rate and level of saturation. The growth rate is tracked by the pressure and temperature trajectories, and comparative studies are performed to identify the roles of various factors involved in the hydrate growth, such as the degree of subcooling, agitation intensity, interfacial areas of the liquid phases and solubility of the helper gas in the primary hydrate former. These findings extend our fundamental understanding of the formation and growth kinetics of binary clathrate hydrates. They also guide the selection of promising binary hydrate formers, as well as the optimal operating conditions to realize economic clathrate hydrate production.