Assessment of Shear Sensitivity of Breakage and Attrition In Crystal Suspensions
Jan Sefcik1, Katarzyna Sypek1, and Miroslav Soos2. (1) Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow, United Kingdom, (2) Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Wolfgang-Pauli-Str. 10, Zurich, 8093, Switzerland
Flow conditions during crystallization and post-crystallization processing are well known to influence agglomeration and breakage/attrition processes in crystal suspensions. The objective of this work has been to determine a range of flow conditions which can be used for a given crystal suspension in order to avoid significant crystal breakage or attrition. This has been achieved by exposing small volumes of the crystal suspension to a well controlled flow through a small diameter contraction, where the flow rate as well as the contraction geometry can be varied in order to cover a desired range of flow conditions ranging from laminar to turbulent and covering a range of shear rates relevant for industrial operating conditions. Flow fields and corresponding shear rates in contractions were computed with a Computational Fluid Dynamics package Fluent. Experimental diagnostics of crystal breakage has been based on standard in situ analytical technology using Focused Beam Reflectance measurements with Lasentec. We have used model crystal systems such as platelet-like aspirin crystals recrystallized from ethanol. Dry powders were dispersed in saturated solvent solutions and the resulting slurries were exposed to a certain number of passes through specific contraction geometries at a range of flow rates. Effects of flow conditions on breakage of crystals were determined for various initial crystal size distributions, solvents and solid loadings. This allowed us to estimate upper critical shear rate values, below which no significant breakage of crystals was observed in given crystal slurries.