CONICET | Buscador de Institutos y Recursos Humanos

The degree of supersaturation of the mother liquor is a key factor in improving the monitoring andcontrol of the final stage of industrial sugar crystallization. However, the difficulty of obtaining onlinesupersaturation measurements is one of the challenges associated with monitoring and controllingsugar crystallization. There is no direct method or single instrument for measuring supersaturation.It can only be calculated or inferred from other measurements. In the literature, estimators ofmother liquor supersaturation are reported, typically focused on the first stage of crystallization. TheSeedMaster series transmitters are the sole industrial instruments that provide online supersaturationinformation by calculating it from external measurements. The purpose of this study is to designa first-principles model-based soft-sensor as a practical alternative to obtain real-time informationabout supersaturation in the last stage of sugar crystallization. The proposed estimator relies on twomodels: a supersaturation model and a second simplified model of the last stage of crystallization. Theparameters of both models were estimated based on real industrial data. The estimation is performedin three steps: 1. An Unscented Kalman Filter estimates the states of the crystallization model andtheir variance. 2. The estimated supersaturation value is obtained by substituting the estimated statesinto the supersaturation model. 3. The estimators bias, and variance are calculated to establisherror bounds. The main characteristics of the obtained estimator are: practical unbiasedness, nearlyminimum variance and robustness. The performance and behavior of the supersaturation estimatorare contrasted using real data from an industrial crystallization plant (Urbano Noris factory, Holguín,Cuba). Regardless of its initial conditions, the estimator converges to the three standard deviation errorband in less than three minutes. The exact time may vary depending on how much the estimatorsinitial conditions deviate from those of the process. After this time (Reach Time), the estimates remainwithin the calculated error limits of three standard deviations. The maximum absolute errors obtainedwere less than 0.019 units, corresponding to a maximum relative error of less than 1.5%. These valuesare favorable since they are well below critical values (0.125 units of absolute error). Moreover, theerror bands are much smaller than the operating zone width (approximately 0.25 units), which is anecessary condition for any supersaturation estimator to be useful. Finally, it should be noted that theerrors have been reduced compared to the values reported in previous research focused on the sugarindustry using other techniques.

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