Mixing and crystallization kinetics in gas-liquid reactive crystallization

A study of gas-liquid reactive crystallization for CO2-BaCl2-H2O system was performed in a continuous flow crystallizer. The influences of mixing on the crystallization kinetics of barium carbonate crystals were investigated. The mixing parameters are stirrer speed, feed concentration, gas-flow rate, pH of solution, addition rate of NaOH solution, and mean residence time. Under pH-stat operation, the crystallization mechanism can be assessed by the addition rate of NaOH solution, which acts as an indicator for the absorption rate of carbon dioxide. Assuming a size-independent agglomeration mechanism, the nucleation rate, growth rate and agglomeration kernel can be obtained, simultaneously, at steady state, by the method of moments. Evidence shows that feed concentration, feed rate, gas-flow rate, and stirrer speed have a significant influence on the nucleation rates and mean particle sizes. This shows the effect of micromixing. The crystallization mechanism tends to be reaction limited when the feed concentration of barium chloride solution is higher than 5 mM, while at lower stirrer speeds and feed concentrations, the mechanism tends to be both mixing and reaction controlled. The growth rate depends on the mean supersaturation value and the pH of the solution and the mass-transfer resistance cannot be completely eliminated in this work. For a monodispersal collision model, in the viscous sub-range of turbulence, the agglomeration kernel can be expressed as β ∝ d3–1/4, showing a low efficiency of collision. The result is also demonstrated by the agglomeration kernel expression. Comparison with a liquid-liquid-mixing reactive crystallization system is also discussed.