Borax Crystallization Kinetics in a Pitched‐Blade Turbine/Straight‐Blade Turbine Dual‐Impeller Crystallizer

Abstract

The influence of hydrodynamic conditions on crystallization kinetics and properties of borax crystals obtained in a dual-impeller batch cooling crystallizer was investigated. The two impellers used, i.e., pitched-and straight-blade turbines, were mounted on the same shaft. Hydrodynamics was analyzed by means of the mixing time values and specific fluid flow patterns generated. Results indicate that wider metastable zones were generally observed at impeller positions characterized by longer mixing times. In those cases, the growth rate constants were lower, resulting in a formation of smaller but more regularly shaped crystals. These findings imply that the dual-impeller position should be taken into account in order to produce crystals of desired characteristics.

Influence of Impeller Diameter on Crystal Growth Kinetics of Borax in a Mixed Dual‐Impeller Batch‐Cooling Crystallizer

“…To evaluate its effect, a number of samples was taken during crystallization. Crystals were seized by a method described in a previous paper . Results on longest linear crystal size vs process time are presented in Fig.…”

“…To clarify the relation between k g and hydrodynamic conditions in the crystallizer, the dimensionless mixing time was experimentally determined by a method described in a previously published paper and the results are listed in Tab. .…”

“…Experiments in this work have been conducted in a 15‐dm 3 flat‐bottom crystallizer (Fig. ) which was described in detail in a previously published paper . The liquid height‐to‐crystallizer diameter ratio of 1.3 required mixing to be performed by two impellers.…”

“…At the end of the process, crystal size distribution was determined by sieving. All experimental procedures mentioned above have been described in detail in a previously published paper .…”

“…In comparison to single-impeller systems, hydrodynamics in multiple impeller systems is more complicated [10][11][12][13]. Flows generated by each impeller may or may not interact, depending on impeller characteristics [10][11][12][13][14][15]. A possible interaction will most definitely reflect on the fluid flow pattern and, consequently, the convective flow in the crystallizer [16][17][18][19].…”