PERFORMANCE EVALUATION OF A FIXED ELEMENT CRYSTALLISER

DURING THE 2012 and 2013 non-crush seasons, two horizontal crystallisers were replaced at Victoria Mill. Victoria Mill currently has six operational horizontal crystallisers, of which four are fixed element design (FED). Two of these were installed leading up to the 2013 crushing season and were changed from the traditional double-helical rotating coils. This new design comprised cooling water flowing through 18 non-rotating elements equally spaced along the length of the crystalliser with stirring arms rotating between each element. This report compares this recent design with documented industry performance data for current crystalliser installations. Three main aspects were evaluated including heat transfer coefficient, residence time distribution and exhaustion. The Heat Transfer Coefficient (HTC) of the new FED crystallisers was determined and compared to the historical performance of more traditional crystalliser designs. The results from this project showed the FED crystallisers predominantly produced HTC values in excess of 30 W/m2.K, which compared favourably with typical reciprocating or rotating coil values which have been reported in ranges from 14 to 30 W/m2.K. Lithium chloride (LiCl) tracer tests were conducted to determine massecuite residence time in the crystallisers. These data helped identify some stagnation at the ends of each vessel, little evidence of short circuiting, and mean residence times generally close to predicted levels. A massecuite pressure filter was used to evaluate massecuite exhaustion. This was completed by observing the purity drop of mother molasses across the crystalliser. Both FED crystallisers showed effective exhaustion performance and although not measured directly, this was associated with commensurate crystallisation rates. Using the data collected and the analyses performed, recommendations have been made with respect to future crystalliser design and selection.