DESIGN ISSUES RELATED TO THE CONTROL OF CONTINUOUS CRYSTALLIZERS

This paper addresses design and control issues related to a continuous crystallization process. Design addresses the configuration of actuat ors for this process. Results from both model studies and from experim ental analysis show how different operation and design choices, relate d to the actuators, influence the process behaviour. The main conclusi on is that classified removal of small crystals (fines) and large prod uct crystals improves the shape of the crystal size distribution (CSD) at steady-state, however, at the expense of open-loop cycling of the CSD. A simple SISO feedback control strategy that acts on the fines re moval rate can be used to stabilize these cycles effectively. Fine par ticles are classified with an annular zone. The size of the annular zo ne can be decreased without affecting the dynamic controllability, sig nificantly. Classified removal of product crystals from the crystalliz er is achieved with a vibrating screen. The aperture size of the scree n and the flow feeding the screen are considered important design para meters. Good product quality is obtained with a low feed flow and a la rge aperture size. Controllability analysis further reveals that for b oth designs only two degrees of freedom exist, i.e. a quantity related to the small particles in the CSD and the total mass production rate per unit of time can be controlled independently.