Monitoring and Control of the Fines Content in the Crystal Product from a DTB Crystallizer

Size distribution is one of the key features of a crystal product to be controlled to ensure good technological characteristics. Controlling the entire size distribution is often unnecessary: in order to obtain a stable crystal size distribution (CSD) and suppress the disturbances, the manipulation of the fines removal rate can be effectively used, as suggested by Rohani and Lee (1987). However, an on-line measurement device must be available. The target of such a measuring instrument may be measuring either the magma density of fines or the entire product size distribution. As far as the magma density of fines is concerned, turbidimetry is one of the most suitable techniques for its determination, whereas light reflectance or diffraction-based instruments are usually adopted to measure the entire size distribution. The former technique, currently adopted in the Lasentec FBRM, only gives a qualitative indication of the size distribution; light diffraction, on the other hand, requires a troublesome dilution of the suspension. The present work deals with a control scheme of a continuous cooling crystallizer based on the use of a specifically developed on-line measuring device which is cheap and easy to apply and operate. The control system was checked on a continuous pilot-scale crystallizer of potassium sulphate from aqueous solutions.