Model-based operational constraints for fluidised bed crystallisation

Fluidised bed reactors are used for water softening in water treatment plants. Recent research shows that under current operation of reactors the crystallisation of calcium carbonate can be hampered. Until now the operational constraints on the fluidised bed have not been exactly known. Experiments were carried out to investigate the fluidisation behaviour of calcium carbonate pellets in water. The results of the fluidisation experiments are compared to two commonly used modelling approaches of Ergun and Richardon-Zaki. Using the experimental data the models are calibrated. The calibrated Richardson-Zaki model is used to determine operational constraints on pellet size at the bottom of the reactor and water flow through the reactor. The model-based constraints are compared to operational data of the Weesperkarspel full-scale treatment plant of Waternet (The Netherlands). It can be concluded that the current operation of the treatment plant violates the calculated constraints with consequences for effluent quality and corrective maintenance. By using models for determining the operation of the fluidised bed, the softening process can thus be improved.