Many industrial processes involve one or more streams of electrolyte solutions. Melt crystallization is well known to be used for liquid purification been the limit of application eutectic conditions. In the other hand Eutectic freeze crystallization (EFC) has been reported to be economically more beneficial than the conventional techniques for the recovery of pure salt or acids and clean water (Van der Ham et al. 1999, Van der Ham et al. 2003, Vaessen et al. 2003). Van der Ham et al. showed that compared to triple-stage evaporative crystallization savings up to 70% of the energy consumption could be achieved depending on the type of salt. In an EFC process salt and ice are crystallized simultaneously at conditions just below the eutectic point of the aqueous solution. The selectivity of crystal growth leads to very pure products even when the solution contains many impurities as is often the case for industrial streams. Impurities can however, shift the eutectic point of the pure system. During eutectic freeze crystallization separation of the ice and the salt crystals can be perform gravitationally within the crystallizer or by centrifugal forces in or out of the crystallizer due to the difference in density between either the ice or the salt, and the mother liquor. In previous studies several prototypes of EFC crystallizers have been developed. The most important design characteristics of these crystallizers are the heat transfer rate that dictates the production rate, the residence time, the solid suspension and the separation efficiency, all them governed by the fluid dynamics in the crystallizer. Several Melt and EFC crystallizer designs and implementations have been reported in literature. As an example Vaessen et al. compared the performance of the Cooled Disk Column Crystallizer (CDCC-1) and the Scraped Cooled Wall Crystallizer (SCWC-1) (Vaessen et al. 2003). The heat transfer was higher in the CDCC-1 up to 7.2 kW m-2 compared with the 3.8 kW m-2 of the SCWC-1 due to the more effective removal of the developing ice scale layer from the heat exchanger surface. The gravitational separation on the other hand, was shown to proceed more effectively in the SCWC-1, because settling of the salt crystals and floatation of the ice crystals was not hampered by the horizontally positioned perforated cooling disks. However, separation difficulties still occurred at high solid percentages, and ice scale formation at low scraping rates (Genceli 2008). In this paper we present a novel Stirred Scraped Cooled Wall Crystallizer with an optimized fluid dynamics design for higher heat transfer, crystal suspension and for efficient gravitational separation to increase the throughput. Heat transfer and fluid dynamics were simultaneously investigated by using thermocromic liquid crystal slurry and crystallization kinetics were investigated in situ in a continuous crystallizer were crystal size distribution, morphology and purity of the crystals were determined.

References:

Genceli, F. E. Scaling-Up Eutectic Freeze Crystallization, PhD dissertation, Delft, 2008

Vaessen, R.J.C., Janse, B.J.H., Seckler, M.M., Witkamp, G.J. Evaluation of the Performance of a Newly Developed Eutectic Freeze Crystallizer Scraped Cooled Wall Crystallizer, Chemical Engineering Research and Design 81(2003), A10, 1363-1372

Van der Ham, F., Witkamp, G.J., De Graauw, J., Van Rosmalen, G.M. Eutectic Freeze Crystallization Simultaneous Formation and Separation of Two Solid Phases, J. Cryst. Growth 198-199(1999), 744-748

Van der Ham, F., Seckler, M.M., Witkamp, G.J. Eutectic Freeze Crystallization in a New Apparatus: The Cooled Disk Column Crystallizer, Chem. Eng. Proc. 43(2003), 161

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