Abstract
This paper addresses design and control issues related to a continuous crystallization process. Design addresses the configuration of actuators for this process. Results from both model studies and from experimental analysis show how different operation and design choices, related to the actuators, influence the process behaviour. The main conclusion is that classified removal of small crystals (fines) and large product 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 removal rate can be used to stabilize these cycles effectively. Fine particles are classified with an annular zone. The size of the annular zone can be decreased without affecting the dynamic controllability, significantly. Classified removal of product crystals from the crystallizer is achieved with a vibrating screen. The aperture size of the screen and the flow feeding the screen are considered important design parameters. Good product quality is obtained with a low feed flow and a large aperture size. Controllability analysis further reveals that for both 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.
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