Investigation Of Gas-solids Flow In A Pharmaceutical Fluidized Bed Drying Process By CFD Simulation And ECT Measurement

Bottom-spray fluidized bed is one of the most widely used reactors in pharmaceutical industry for pellet coating and granulation.In this type of fluidized bed,a Wurster tube is usually put inside the fluidized bed chamber to increase the heat and mass transfer efficiency between the coating droplets and particles,to improve the particle movement and to avoid the generation of particle clusters efficiently.However,it is difficult to measure the hydrodynamic characteristics of the gas-particles flow in a Wurster type fluidized bed and optimize the geometrical structure of the bed and the process operation parameters due to its complex structure and high flow velocity inside the Wurster tube.Therefore,finding a proper method to monitor the gas-particles flow and the particle drying process is very important to improve the process efficiency and the end-point product quality.In this research,computational fluid dynamics(CFD)simulation based on Eulerian-Eulerian two-phase flow method(TFM),electrical capacitance tomography(ECT)experimental measurement method and a mathematical modeling based on three-phase theory,are combined to investigate the complex gas-particles flow and the particle drying process in a Wurster type fluidized bed.The main objective of this research is to understand the flow field and the particle drying process,to optimize the geometrical structure of the bed and the process operation parameters.To understand the gas-particles flow in the bed and find the most proper numerical simulation model,CFD simulation and ECT measurement are used in a lab-scale Wurster fluidized bed.Three different drag models,i.e.Wen-Yu,Gidaspow and EMMS,are applied to investigate their effects on the CFD simulation results.The simulation results indicate that the particles mainly accumulate at the bottom of the bed,and go through the coating,drying,annulus and transportation zone in sequence.The pressure signal in the bed has obvious periodicity.The results of Wen-Yu and Gidaspow models have a good agreement with the ECT measurement result.But the EMMS model based on particle cluster is not suitable for this lab-scale Wurster type fluidized bed.To optimize the geometrical structure of the bed and the operation conditions,different structure dimensions and operation conditions,including the gap between tube and air distributor,tube height,tube diameter and superficial gas velocity,are adopted in CFD simulation and ECT measurement to find the most proper value for each parameter.The sensitivity analysis of these parameters indicates that tube diameter has the biggest effect on the flow field.Considering the bubble diameter,all three parameters,the gap,tube diameter and tube height,have bigger effect on the flow field.To understand the particle drying process further,a mathematical modeling based on three-phase theory and CFD simulation are combined.The results of these two methods are compared to investigate the feasibility of their combination.The simulation results indicate that the particle drying process has three stages,i.e.pre-warming,constant-rate and falling-rate.The heat and mass transfer between gas and particles mainly occur in the interface of bubble phase and particle phase.The mathematical modeling results indicate that different tube dimensions,superficial gas velocities and particle parameters have great effect on the particle drying process,and the particle temperature has a one-to-one correspondence with the particle moisture.However,the comparison of CFD simulation and mathematical modeling results shows that the combination of two-fluid model(TEM)and Gunn's relationship is not suitable for the investigation of the drying process in a pharmaceutical fluidized bed.In summary,this research indicates the gas-particles flow and the particle drying process in a pharmaceutical fluidized bed,gives the most proper values for the drag model,tube dimension and superficial gas velocity,and gets the effect degree of each parameter on the flow field.The results provide valuable information for the CFD simulation as well as measurement for Wurster type fluidized beds.The results might be used to optimize the particle coating and drying process in pharmaceutical industry.