(535j) Effect of Flow Mixing Patterns on Carbon Dioxide - Calcium Hydroxide Interfacial Interaction during Calcium Carbonate Precipitation in Taylor Couette Crystallizers

The objective of this study is to investigate the variety of flow patterns in a three-phase system, reacting gaseous carbon dioxide (CO₂) with aqueous calcium hydroxide (Ca(OH)₂) to precipitate solid calcium carbonate crystals (CaCO₃) in a Taylor-Couette reactor (TCR), and to examine the influence of the corresponding flow dynamics, CO₂ gas bubble size and interfacial area on the properties of calcium carbonate crystals. It is known that the interfacial area in a gas–liquid system in a Taylor-Couette reactor varies substantially depending on the flow regime, since a variety of flow states can be observed due to the interaction between centrifugal and buoyancy forces. While previous studies only focused on the influence of dissipated energy, in the current study we extend our understanding of changes in two phase interfacial transport with variation in flow regime behavior, by assessing vortex dynamics using flow visualization tools. The reactor geometry, reactant concentration, rotating speed of the inner cylinder, reactant flow rate and fluid properties were all varied. In particular, we attempted to differentiate between the effect of rotational speed (dissipated energy) and the additional effect of introducing axial flowrates, which was reported to have a more significant effect on flow patterns and interfacial mass-transfer. Additionally, a unique factor of this study was the variation of fluid properties, such as viscosity and surface tension, by precipitating calcium carbonate crystals in the medium of glycerol. Although glycerol is used mainly to control the hydrodynamics of the system, it actually exhibits a better solubility of calcium hydroxide (Ca(OH)₂) than water, providing a greater flexibility in varying aqueous reactant concentration than in conventional precipitation. In fact, aqueous solutions of alcohols have been previously adopted in synthesizing calcium-carbonate particles with nanoscales, particular morphologies, and novel functions. The first part of this study included an experimental characterization of flow patterns by assessing gas bubble dispersion and bubble size, identifying flow-patterns with highest interfacial area. Another set of experiments were used to estimate mass-transfer rates and interfacial area. Finally, the ideal mixing flow regimes identified from these experiments were used for calcium carbonate precipitation. It was found that flow-patterns, particularly “ring flow” plays a critical role in fine-tuning crystal properties such as internal crystal structure, particle microstructure and morphology.