Wurster type fluidized beds are widely used in pharmaceutical industry for pellets coating. They are considered to be the best choice for pellets coating as they can produce a superior film compared with other coating strategies. For a Wurster type fluidized bed, there exists a concentric cylindrical tube which is called Wurster tube inside a conical chamber, and the flow region is mainly divided into two parts, i.e. the coating zone inside the Wurster tube and annulus zone outside the Wurster tube. A dual-fluid nozzle is placed in the bottom of the Wurster tube for spraying the coating solutions. In this research, process tomography sensors including electrical capacitance tomography (ECT) and microwave tomography (MWT) are designed and used to investigate the complex gas-solids flow characteristics in a Wurster type fluidized bed for coating and drying. One set of 12-4-8 combined electrical capacitance tomography (ECT) sensor was designed and used to monitor the solid concentration inside the chamber. The ECT measurement was conducted synchronously both inside and outside of the Wurster tube, i.e. coating zone and drying zone, to achieve a complete monitor of fluidised bed process with Wurster tube. The MWT sensor has 16 antennas and the measurement frequency is in the range of 1.0 GHz and 2.5 GHz. A series of tests were carried out by varying the operational parameters including the Wurster tube gap, air flow velocity and particle mass load. Different flow patterns as well as flow stability were evaluated based on the tomography measurement results. To evaluate the flow characteristics inside the bed, fast flourier transform (FFT) was applied to investigate the fluctuation in the coating zone and drying zone. The hydrodynamics of gas-solids flow with different moisture content were revealed by the tomography images and spectrum structure based on FFT analysis.
To verify and optimize the measurement result, computational fluid dynamic (CFD) based on the kinetic theory of granular flow (KTGF) is used to investigate the flow characteristics in the same bed. Different drag models, i.e. the energy-minimization multi-scale (EMMS) model, Wen-Yu and Gidaspow model, are used to investigate the gas-particle interface effect on the flow characteristics. Key process parameters, including the solids concentration, solids velocity and pressure drop, are analyzed and verified by the measured results. Both the CFD simulation and tomography measurement results provide valuable information to reveal the hydrodynamic characteristics in the Wurster type fluidized bed. With the lab-scale fluidized bed, the results of Wen-Yu and Gidaspow drag models have a good agreement with the ECT measurement. The results also indicate that the design of the Wurster tube has significant effects on the flow. Based on the CFD simulation and ECT measurement results, optimized parameters are chosen to improve the solids circulation efficiency and coating quality.