Computational and Experimental Study of the Flow in Evaporative Crystallizers

Various evaporative-crystallization systems rely on the natural circulation generated by boiling as the only driving force for the fluid flow. The circulation resulting from the balance between the buoyancy forces of the vapor bubbles and the frictional resistance plays an important role in the convective-boiling heat transfer, and it is desired that this circulation be as high as practically possible to maximize the capacity of the equipment and to lead to high-quality product yield. Although the basic mechanisms that govern the individual processes of boiling, buoyancy, and two-phase interactions have been extensively studied in simpler geometries, their combined behavior in the complex geometry of evaporative-crystallizers and the interaction of numerous physical and chemical variables make it difficult to understand and optimize the key parameters leading to improved product yield. In the present study measurements and computations have been reported both in a lab-scale test rig and in a full-scale crystallizer in order to obtain a better understanding of the physical processes. It is observed that one of the key physical parameters that influence the circulation rate is the drag coefficient, and that, existing correlations have to be corrected for flow contamination and high void fractions to obtain reasonable agreement with measurements.