Doctoral Defense by Ronja Heming
Abstract:
The development of energy-efficient and environmentally sustainable purification processes is gaining importance in response to increasing environmental challenges and resource scarcity. Suspension melt crystallization (freeze concentration) represents a promising alternative for dewatering and purification processes, as it combines high separation efficiency with relatively low energy consumption, relying solely on electricity. This work focuses on the experimental investigation and process understanding of suspension melt crystallization, particularly the interaction between the crystallization unit and the continuously operated wash column for solid-liquid separation and crystal purification. Using a binary aqueous system with sodium chloride, the process parameters of a scraped cooling crystallizer with forced circulation were systematically analyzed regarding their impact on process stability and particle size distribution. A vibration-based monitoring method was developed and validated to detect early signs of process disturbances such as layer formation. In addition, the mechanical piston-type wash column was characterized in terms of purification efficiency under different operating conditions. It was shown that high purification efficiencies above 99.9 % are achievable with optimized wash front height and melt temperature. Furthermore, a mechanistic process model was developed, incorporating hydrodynamics and energy balances to predict nucleation, crystal growth, and particle size distribution. Model validation with binary aqueous sodium chloride and magnesium sulfate systems, respectively, demonstrated good agreement with experimental data and provided insights into process optimization. Finally, the integration of a concentrate recycling loop was explored, showing its potential to significantly increase water yield without compromising product quality. In doing so, technical limitations were highlighted that can be addressed through targeted process modifications.