Monitoring the quality of fluidization using the short-term predictability of pressure fluctuations

In various industrial applications of bubbling fluidized beds, defluidizing parts of the bed or even of the complete bed can occur as a result of agglomeration and sintering of particles due to unintentional maloperation of the bed, changes in operating conditions, or variations in gas or solids feed. Defluidization malt be prevented by increasing the gas velocity or changing the solids feed, if the change in the "quality" of the fluidized state of particles is detected early enough. An analysis method is proposed that uses the short-term predictability of time series of (local) pressure fluctuations in the fluidized bed to detect changes early in the hydrodynamic state of the bed. It is based on the comparison of an original time series of pressure fluctuations with successive time series measured during operation of a fluidized bed. The comparison is based on a discriminating statistic computed for the original time series as well as for each successive series. The null hypothesis that the original and successive time series are similar is rejected if the discriminating statistics of both time series significantly differ. Experimental application of the method is illustrated for fluidization at elevated temperatures (ca. 120 degrees C) of agglomerating plastic particles in a 5-cm-ID laboratory fluidized bed. The method recognizes the change in the hydrodynamics due to the incipient agglomeration of the particles. In this particular case the time period between the moment of detection of a significant change in the hydrodynamics and the end of the experiment when the bed becomes defluidized seems sufficiently large to take preventive measures. The average bed-pressure drop is not a sensitive quantity to detect changes early in the fluidization behavior.