Forced circulation evaporative crystallizers exhibit exceptional versatility, catering to a broad spectrum of applications by manipulating the crystallizer’s geometry and accompanying components. These systems operate based on a fundamental set of principles, where supersaturation is induced by circulating a process slurry through the heat exchanger’s tubes.
This controlled circulation mitigates the temperature differential between the heating medium and process slurry, elevating the fluid’s temperature. The liquid phase undergoes evaporation as the fluid reaches an area within the crystallizer body, characterized by a sufficiently low pressure known as the boiler surface. The resulting crystals are recovered and processed in the dewatering system, which involves washing and dewatering procedures to maintain the desired purity levels. The residual mother liquor is collected in a dedicated tank and reintroduced into the crystallizer, closing the loop.
Fine-tuning of crystal purity is attainable through the strategic use of a purge stream, with increased purging yielding higher purity levels. Furthermore, purity enhancement can be achieved by incorporating an elutriation and wash leg to eliminate impurities from the solution before separation.
Adding a slurry return draft tube inlet results in a larger average crystal size distribution and augments the washing efficiency, leading to higher product quality. This is achieved by reducing supersaturation at the boiling surface by creating an upward flow, facilitating the transport of larger crystals to the boiling surface, thereby increasing the available surface area for crystal growth. This, in turn, reduces supersaturation and limits the nucleation of new crystals, commonly referred to as fines. The draft tube inlet also diverts supersaturation from the vessel walls toward the boiling surface, resulting in extended washout cycles.
Incorporating an elutriation leg refines the crystal size distribution (CSD), narrowing its range by redirecting fine crystals back into the crystallizer for continued growth while providing an exit pathway for larger crystals to prevent excessive growth.