Solids mixing and wall-to-bed heat transfer in cross-flow bubbling fluidized bed reactors with different immersed tube bundles

In this thesis a cold flow model was used to investigate the solids mixing effects and wall-to-bed heat transfer with respect to the influence of the lateral crossflow in a bubbling fluidized bed under continuous particle exchange. The superficial gas velocity, the solids circulation rate, and the tube bundle heat exchanger geometry were varied. Geldart group B solids were used. The impulse response tracer measurement was utilized to obtain the particle residence time distribution and a mathematical routine was applied to calculate the mean residence time and characteristic values to analyze solids mixing phenomena. The wall-to-bed heat transfer was determined by using an electrically heated heat transfer probe. Results of the tracer experiments show that, depending on the geometric tube bundle configuration, the mixing characteristics of the cross flow fluidized bed resemble those of an ideally mixed stirred tank reactor, but are more or less superimposed by the characteristics of dispersed plug flow. Results of the heat transfer measurements show that the heat transfer coefficient increases with increasing gas velocity and decreases when the tubes are arranged more densely in the bed. Regarding the variation of crossflow in the defined operating range for solids circulation, there is no significant effect on the wall-bed heat transfer. In addition the operability, i.e., solids circulation rate as a function of superficial gas velocity, was investigated using a multistage fluidized bed cold flow model connected via internal downcomers. The operability was investigated with and without external downcomer aeration. Three flow regimes for the internal downcomers were investigated in terms of their pressure variations as a possibility for a detection system. The use of external aeration in the internal downcomers result in a significant increase in solids circulation by 20% and the pressure signals can be used to determine the flow regimes in the downcomers. (author)