Wall-to-Bed Heat Transfer in Bubbling Fluidized Bed Reactors with an Immersed Heat Exchanger and Continuous Particle Exchange

Measurements were carried out to investigate the wall-to-bed heat transfer in a cross-flow bubbling fluidized bed under continuous solids exchange. The experiments were performed with different tube bundle heat exchangers immersed in a lab-scale gas-solid fluidized bed cold flow model 0.4 m x 0.2 m in cross section. Geldart group B particles were used, and the gas velocity, the solids circulation rate, and the tube bundle heat exchanger geometry were varied. Tracer experiments that were previously performed with this setup showed that the mixing characteristics of the cold flow model mostly resembled mixed flow but superimposed by dispersed plug flow caused by the cross-flow of particles. Because of this knowledge, it was assumed that the influence of the imposed cross-flow may have a positive effect on the heat transfer by intensifying particle convection due to the lateral movement of solids. Due to the limited studies that focus on the effect of cross-flow on the heat transfer coefficient, a well-established technique comprising an electrically heated heat transfer measurement probe was used to determine this effect. The present results show that more heat is transported from the probe to the particles when the gas velocity is increased and less heat is transported when tubes are arranged more densely in the bed. These findings are in good agreement with the findings reported in the literature. However, and with the exception of one test series, no significant increase in the heat transfer coefficient, which was expected to intensify by the cross-flow, was observed. Based on the novelty that the study focuses on the influence of cross-flow on heat transfer in bubbling fluidized beds, the findings represent a missing link and are part of a series of previously conducted studies. Furthermore, they can be helpful for understanding the mixing behavior and accompanying heat transfer in cross-flow bubbling fluidized bed reactors.

Automated summary

The study investigated wall-to-bed heat transfer in a cross-flow bubbling fluidized bed with continuous solids exchange. Results showed that increasing gas velocity leads to more heat being transported from the probe to the particles, while arranging the tubes more densely in the bed results in less heat transfer.