Investigation of the complex gas-solids flow characteristics in a fluidized bed with a Wurster tube by process tomography and CFD simulation

A Wurster fluidized bed, widely used in the coating process of the pharmaceutical industry, is modeled to investigate the characteristics of gas-solids flow, concerning the effect of Wurster tube and operating parameters. Two main methods used in this research are computational fluid dynamic (CFD) simulation and electrical capacitance tomography (ECT) measurement. The CFD model is based on the kinetic theory of granular flow (KTGF). The drag model based on the energy-minimization multi-scale (EMMS) method is applied and compared with Wen-Yu and Gidaspow models. Key process parameters, including the solids volume fraction, solids velocity, bubble diameter and pressure drop, are analyzed and verified by the measurement results. Both the CFD simulation and ECT measurement results provide valuable information revealing the hydrodynamic characteristics in the dense-phase zone, and the combination of these two techniques provides a good method to investigate the flow field in Wurster fluidized beds. For the lab-scale Wurster fluidized bed, the results of Wen-Yu and Gidaspow drag models have a good agreement with the ECT measurement. The design of the Wurster tube has significant effects on the flow characteristics. To guarantee the solids circulation in the bed while avoiding the non-uniform coating or undesired agglomeration, optimized parameters of the tube geometric dimensions and fluidization conditions are chosen to improve the solids circulation efficiency and the product quality based on the CFD simulation and ECT measurement results. The sensitivity analysis gives the guiding direction for further optimization of a lab-scale Wurster fluidized bed.