CONTROL OF THE CRYSTAL MEAN SIZE IN A PILOT-PLANT POTASH CRYSTALLIZER

On-line measurements of the mean crystal size, the weight percent soli ds in the product and fines steams, and an estimate of the supersatura tion in a 1 m(3) forced-circulation pilot plant crystallizer are perfo rmed in this study. From the open loop responses to step changes in th e feed, product, recirculation, and fines dissolution flow rates, it i s determined that the on-line sensors can provide acceptable measureme nts ofprocess variables. It is found that the sensors used to measure the weight percent of fines and the supersaturation are sensitive to c hanges in the brine composition. The open loop responses are also used to design a PI-controller for the control of the mean crystal size. B ased on a first order process model of the measured mean size, a casca de PI-controller is designed using the fines dissolution flow rate as the slave controlled variable. At a temperature difference of 15 K bet ween the feed tank and the crystallizer, the controller results in som e improvement in the measured mean size of crystals. The controller do es not have adequate set point tracking abilities and causes some osci llatory behaviour as a result of disturbances. The controller performa nce and especially its set point tracking capability was improved when the temperature difference between the feed tank and the crystallizer was reduced to 5 K and further modifications to the crystallizer circ uit were incorporated.