Measurement of Residence Time Behavior in a Continuously Operated Spouted Bed

Continuous gas-solid processes are applied in many industries with particulate products as e.g. for agglomeration, granulation, coating or drying. An important criterion for these apparatuses, which often combine several unit operations in one plant, is a narrow residence time distribution and low back-mixing between the stages in order to obtain a constant product quality. A novel tracking method with magnetizable particles was established in order to measure the residence time distribution in continuously operated fluidized or spouted beds. In this work, a continuously operated horizontal prismatic spouted bed with four unseparated chambers (ProCell 25, Glatt) was investigated. The tracer was produced by coating the bed material, namely Cellets^®500 particles, with a magnetizable paint. The applicability of the particles as tracer material was evaluated regarding the requirements and confirmed. The residence time distribution was determined by applying an impulse of tracer material during continuous process operation at the inlet and taking samples of the outflow in defined time intervals. The tracer material was separated by a magnetic rod and the measurement of the weight concentration in every sample allowed the fast calculation of the exit age distribution.

Experiments were performed with different bed masses and gas volume flow rates. Besides the original chamber configuration, plates with different opening geometries, as e.g. a mousehole, were inserted to physically separate the four chambers. It was found that the sealing of the plates is an important step in order to force the particles to flow through the opening geometry. In addition, experiments with closed in- and outlet boundary conditions were performed to characterize the back-mixing and the flow behavior of the particles dependent on the separation plates. Therefore, the tracer particles were inserted in one chamber and the distribution after a defined process duration was used for quantification of back-mixing. It was found that the back-mixing can be reduced by decreasing the size of the mousehole in the separation plates. Nevertheless, the size reduction is limited as a minimum hole diameter has to be provided for reaching the desired mass flow rate through the apparatus. The experimental data are used for validation of simulations (e.g. CFD-DEM, rCFD) of continuous spouted bed operation.