Controllability of particulate processes in relation to the sensor characteristics

Some considerations are given that should be regarded when selecting a sens or for the control of the particle size distribution (PSD) in particulate p rocesses, such as crystallizers, yet-mills and granulators. As an example, the control of the crystal size distribution (CSD) dynamics in a continuous crystallization process is taken. Different methods are discussed to deriv e process information from a typical PSD sensor, comprising an array detect or. It is shown that generally a limited number of degrees of freedom are p resent in the process output signals. This is due to the principle of measu rement and the limited number of degrees of freedom in the process itself. Factors limiting the controllability of processes are discussed. It is show n that proper placement of the sensor in the particle size domain is import ant for feedback control, because the sensor characteristics will determine the location of the zeros in the process transfer function. For an industr ial crystallization process, it is shown that measurement of the CSD below 30 mu m limits the achievable closed-loop speed of response, due to the pre sence of a right halve plane zero. Effective control of the CSD is achieved , using a simple feedback controller, that measures only the fine crystals in the range of 40-200 mu m. Experimental results from the UNIAK pilot crys tallizer are depicted. (C) 2000 Elsevier Science S.A. All rights reserved.