(539c) The Effect of Axial Dispersion on Crystal Size Distribution in a Meso-Scale Continuous Oscillatory Baffled Crystallizer

Su, Q., Purdue University
Nagy, Z. K., Purdue University
Rielly, C., Loughborough University
The effect of axial dispersion on crystal size distribution in a meso-scale continuous oscillatory baffled crystallizer

Iyke Onyemelukwe1, Qinglin Su2, Zoltan K. Nagy1,2, Chris D. Rielly1

1 Department of Chemical Engineering, Loughborough University,
Loughborough, LE11 3TU, UK

2 Davidson School of Chemical Engineering, Purdue University,
West Lafayette, IN 47906, USA


Tubular crystallizers, specifically the continuous oscillatory baffled crystallizer (COBC), for continuous manufacturing have been an active research area in recent years, e.g. investigating how axial dispersion is affected by flow regime, operating conditions and the geometry of the periodic constrictions [1] and how the resulting residence time distribution (RTD) influences the supersaturation distribution and the resulting product crystal size distribution [2]. In this study, an experimental setup of a meso-scale COBC was developed to study the effects of axial dispersion on a cooling crystallization process of the glycine in water system. First, the axial dispersion effect, characterized by the Peclet number, Pe, was measured experimentally by using an imperfect pulse method, tracking the concentration-time history of a tracer at two axial locations after the tracer input for both the liquid and particle phases. Dispersion models for the tracer concentration in each phase were then fitted to calculate the dispersion parameters. The method used a fast Fourier transformation (FFT) to convolute the measured input concentration profile with the parameterized model RTD to almost perfectly match the measured output profile; The effects of the oscillation frequency and amplitude on the dispersion coefficients were studied.

A seeded cooling crystallization of glycine and water system was run in the meso-COBC under the same range of through-flow and oscillatory operating conditions. Broad crystal size distributions of final crystals were observed. Furthermore, a steady-state mathematical model of COBC, consisting of population balance model of crystals, mass balance equation of solute, and energy balance equation for heat transfer, was developed to provide an in-depth understanding of the dispersion effect on crystal size distribution. It was found that the axial dispersion had a less significant effect on the crystal size distribution under the current operating conditions, whereas the fast cooling rate along the tube had a far greater impact, which results in extensive primary nucleation. An optimal trajectory of temperature along the tube was important in controlling the supersaturation and the crystal size distribution [3]. However, the nature of the counter-current heat transfer, which leads to a series of natural cooling curves, one for each change in jacket temperature, makes difficult the control of temperature trajectory in a limited number of tube segments. Perspectives on using the meso-scale COBC for crystallization kinetics screening for continuous manufacturing and for scaling up to larger COBC facilities will be given.



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