Control of pellets coating in a Wurster type fluidized bed based on electrical capacitance tomography measurement

Wurster type fluidized bed is commonly used for pellets coating in many industrial sectors. However, there are few reports on the optimization and automatic control of these processes in literature. The main reason is that it is difficult to apply reliable in-line measurement. In this paper, electrical capacitance tomography (ECT) is applied to reconstruct the solids distribution in a lab-scale Wurster type fluidized bed and the results were used in a control loop of pellets coating process. The features of different flow regimes exist in fluidized bed are extracted from the ECT images using a simple but efficient approach. The objective of control is to keep a stable minimum fluidization state of the solids flow and avoid the occurrence of undesirable flow regimes in the annular zone in the bed including intermittent, plug and defluidization. A PID controller was applied to keep a low volume fraction of solids in the annulus zone by manipulating the air flow rate. The peristaltic pump was also manipulated by a simple control logic based on the detected flow regimes. It was found that the control strategy based on ECT measurement was useful for keeping a stable flow regime, and avoiding the occurrence of defluidization.