Abstract
The secondary nucleation process of 2,2-azobisisobutyronitrile (AIBN) seeded crystallisation in methanol in a stirred tank reactor was studied at varying initial supersaturation levels, temperatures, crystal seed numbers, and stirrer speeds. The average secondary nucleation rate, induction time, and agglomeration ratio were measured using on-line microscopic imaging. The initial supersaturation level, temperature, and stirrer speed were found to be positively correlated with the secondary nucleation rate. A small change in the crystal seed number, i.e., 1-20, did not substantially affect the secondary nucleation rate throughout the secondary nucleation process. An increase in the initial supersaturation level and crystal seed number decreased the induction time, and an increase in the strength of agitation promoted the initiation of secondary nucleation at a stirring rate greater than 250 revolutions per minute (rpm). Temperature exerted a complex effect on the induction time. Regarding the agglomeration ratio, the initial supersaturation level positively correlated with the agglomeration ratio, while the stirrer speed negatively correlated with this parameter. Finally, based on the measured data, the average secondary nucleation rate, induction time, and final crystal suspension density were correlated. This study provides guidance for the control of supersaturation, induction time, stirring, and other factors in the crystal seed addition process in AIBN crystallisation.
Highlights
Nucleation is the process in which nuclei with sufficient stability to exist are produced in a solution or a slurry [1]
At a stirrer speed less than 250 rpm, a slight increase in the induction time was observed as the stirrer speed increased, which was postulated to be due to the error between experiments in this study
The predicted induction time has an error of less than 15% when it is greater than 1000 seconds, effect of initial supersaturation on induction time at higher temperature may be caused by that the but when the induction time is less than 1000 seconds, the deviation of the fit becomes larger, high molecular kinetic energy enhances the frequency and intensity of atoms at high temperature
Summary
Nucleation is the process in which nuclei with sufficient stability to exist are produced in a solution or a slurry [1]. In early experimental studies on secondary nucleation, samples were frequently and intermittently collected manually out of the crystalliser to analyse the CSD off-line, and the secondary nucleation process was examined through numerical regression analysis [8,9]. FBRM may have difficulty in recognising whether a particle is a single crystal or an agglomerate [14], which causes measurement errors. On modelling the secondary nucleation, an empirical power formula is often used [27,28,29] It assumes that the secondary nucleation rate has power relationships with supersaturation, the stirring rate and particle suspension density. Models were built to correlate the average secondary nucleation rate, induction time, agglomeration, and final crystal suspension density, with crystallisation conditions including initial supersaturation, temperature, crystal seed number, and stirrer speed
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