Influence of fluid shear on primary nucleation of low molecular organic compounds in solution

Abstract: In this project, two experimental systems have been set up to study the influence of fluid shear on primary nucleation. In the first set of experiments, in total about 3000 nucleation experiments have been done with butyl paraben/ethanol solutions, curcumin/ethanol solutions and m-hydroxybenzoic acid/1-propanol solutions in capped vials agitated with magnetic stir bars. The induction time has been measured at different supersaturation levels, temperatures and agitation rates. There is an overall tendency in the experiments that the induction time is initially reduced with increasing agitation but then increases after passing a minimum value. To extend and widen the information in the low agitation rate region, levels of agitation below the minimum stirring rate of the motor were investigated by adopting an operation strategy of intermittent agitation. By calculating the average energy input for discontinuous stirring in the crystallizing vial with different intervals, the induction time was found to decrease with increasing agitation power input raised to approximately 0.2 in the low agitation region. .The induction time in the second set of experiments, carried out using a Taylor-Couette flow system, is found to be inversely related to the flow shear rate. By fitting the parameters of the classical nucleation theory to experimental data, it is shown that the results can be explained as an influence on the pre-exponential factor in the nucleation rate equation. Two different approaches to modify the expression for the pre-exponential factor to account for the convective mass transfer effect, has been investigated.This work includes a comprehensive review of previous published work on the influence of fluid shear on primary nucleation in solution. Based on the different mechanisms proposed, including the agitation-enhanced mass transfer, agitation suppressed nucleation and shear induced molecular alignment, the experimental results are discussed.