Process intensification of cooling crystallization of cholesterol from acetone solution using CO2 gas bubbles: Experiments and modeling

Abstract

The present work investigates dependence of heterogeneous nucleation of solids on the gas bubble-solution interface in a process termed as "Precipitation by Gas-Bubbling" (PGB), through experiments and modeling. This process involves bubbling of CO2 gas at ambient condition through a solution of cholesterol in acetone, which is simultaneously cooled by an external coolant. The process parameters selected are coolant temperature, initial solution concentration, and CO2 flow rate. The flowing CO2 gas bubbles facilitate reduction in metastable zone width and induction time for cholesterol precipitation, rendering more efficient surface-mediated crystallization, thereby enabling process intensification of cooling crystallization. Faster precipitation of cholesterol at higher CO2 flow rate, i.e., higher CO2 bubble-solution interfacial area, demonstrates gas bubble-solution interface acting as a substrate for heterogeneous nucleation. Accordingly a mechanism for heterogeneous nucleation of cholesterol is proposed and validated by developing a model for prediction of final average particle size. The kinetic model parameters, namely, the pre-exponential factor as per the classical nucleation theory, and the crystal growth coefficient are regressed by comparing the predicted values of average particle size with the corresponding experimental data, and are found to be proportional to the surface area of gas bubbles and in turn on CO2 flow rate.