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Abstract:
Crystallization processes involve different kinetic processes such as nucleation of crystals, their subsequent growth and breakage. The rate equations of these kinetic processes are typically nonlinear and may give rise to complex nonlinear behavior including self sustained oscillations. These instabilities in turn lead to poor product quality and should therefore be avoided. A reliable prediction and thorough understanding of potential sources for instability is not only of scientific interest but also an important issue for better design and control of crystallization processes. In this contribution focus is on ammonium sulfate crystallization. This system has been studied intensively in the past and can be viewed as a standard test system for crystallization processes. Focus is on crystallizers with fines dissolution and classified product removal. Using methods from numerical bifurcation analysis we investigate, how stability in such a system depends on the various operational parameters, like cut sizes and recycle rates of fines dissolution and product classification. Theoretical predictions of three different population models are compared with each other: (i) a model with simple kinetics and ideal fines classification as introduced in [1], (ii) a model with complex kinetics involving breakage due to attrition and ideal fines classification as introduced in [2], and (iii) a model with simple kinetics and nonideal fines classification as introduced in [3]. It is found that predictions of the instability regions with models (i) and (ii) are rather close. In contrast to this the nonideal fines classification can change stability rather drastically. Hence, at least for the stability analysis, a detailed model of the classification seems to be more important than a detailed model of the population kinetics. References: [1] P. Pathath and A. Kienle. Chem. Engng. Sci. 57 (2002), 4391-4399. [2] A. Gerstlauer, S. Motz, A. Mitrovic and E.D. Gilles. Chem. Engng. Sci. 57, 4311-4328. [3] A. Mitrovic. PhD Thesis, University Stuttgart, VDI Fortschrittsberichte Nr. 3/749, VDI-Verlag, Düsseldorf.
