MONITORING CRYSTALLIZATION .2. ON LINE MONITORING OF WHITE SUGAR CRYSTALLIZATION THROUGH SOFTWARE SENSORS

In this paper we present the theoretical bases and experimental implem entation of a methodology which allows, on-line and in real-time, the characterization of the semi-batch evaporative crystallization process in cane sugar refining. This methodology does not require the use or installation of 'sensitive' sensors, such as consistency and total den sity meters, or the theoretical knowledge of 'less accurately known' q uantities such as crystal growth rates and evaporation rates. The algo rithm proposed is based on a theoretical model of the crystallization process which includes growth rate dispersion mechanisms and the chara cterization of crystal size distribution (CSD) by its first six linear moments. A transformation is performed which eliminates the growth ra te and leaves the state estimation dependent only on measures availabl e on-line. The latter are readings of the 'input' variables to the sys tem and of three internal variables characteristic of the crystallizat ion state. The 'input' readings are those related to - (i) liquors/syr up feed: flowrate, brix, temperature and purity; (ii) steam; flowrate, temperature and pressure; (iii) stirrer power: current. The 'internal variables' required are the vacuum pressure, the brix of solution in the massecuite and its temperature. These are all known to be reliable measurements. The time trajectories of the following properties and i ndices of performance are obtained, viz.-(i) supersaturation; (ii) tot al mass and volume; (iii) mass fraction of crystals; (iv) CSD characte rised by an average crystal size and a coefficient of variation; and ( v) energy consumption indices. As subsidiary results from the practica l point of view, but most relevant for theoretical studies are the est imates simultaneously obtained of the growth rate and evaporation rate throughout the operation. The concept is now implemented at industria l scale at RAR, undergoing experimental tests for robustness to noises and sensor failure. Based upon this data-bank on the process operatio n, which is now available, studies are being carried out both on alter native, more fundamental, descriptions of the process behaviour and on alternative forms of controlled operation.