Fluid Dynamics of a Wurster-type Fluidized Bed - A Numerical and Experimental Study

Doctoral thesis, 2004

The fluid dynamics of a Wurster bed, a fluidized bed used in the pharmaceutical industry for the coating of small particles or pellets, is investigated numerically and experimentally. The main objective of the thesis is to obtain greater knowledge of the flow properties in the Wurster bed. The long term goal is to simulate the entire process enabling the use of experimentally validated computational fluid dynamics as an industrial design tool. An optical probe was developed to be able to measure the local instantaneous solids volume fraction. A theory was derived for the response signal of the probe and experimentally tested in a circulating fluidized bed where the solids volume fraction registered by the optical probe was compared to pressure drop measurements for two different particle sizes. The calibration theory is based on a model of the blockage of light by the particles together with measured characteristics of the probe. The probe gave qualitatively good agreement with the measured pressure drop. Measurements using the optical probe as well as a capacitance probe technique to obtain bubble measurements were compared to numerical calculations of the fluid dynamics in the Wurster bed. The numerical model was an Eulerian two-fluid model. Two different closure models for the rheology of the solids phase were compared, one based on the concept of elasticity and the other based on kinetic theory of granular flow. The measured and calculated solids volume fractions were in good agreement. The capacitance probe results indicated a change in the behaviour of the Wurster bed when the fluidization velocity was increased, where the bed went from a bubbling to a channeling behaviour. Both frictional and kinetic contributions to the solids phase stress must be considered in the simulation of a fluidized bed. Different models for the frictional stress were investigated numerically by studying particle flow in a hopper, a flow case dominated by enduring particle-particle contacts. The simulations were compared to experimental results from the literature.