(433f) Radiation Effects on Heat Transfer in Slender Packed Bed Reactors

Radial heat transfer is one of the most challenging aspects of a safe, stable, and economic operation of packed bed reactors.However, the comprehensive description of heat transfer is far from trivial. During the last two decades, particle resolved computational fluid dynamics (PRCFD) has enabled researchers to gain insights into transport phenomena in packed beds that are nearly impossible with experiments. With PRCFD, we showed in previous studies that thermal radiation significantly contributes to the overall heat transfer mechanism¹ in slender packed beds and can therefore also influence the local reaction rate². Since PRCFD is still too computationally demanding for daily engineering problems, simplified engineering models are widely applied. In this contribution, we compare heat transfer in packed beds calculated from PRCFD and from a classical 2D plug flow model with and without thermal radiation conditions. In order to compare the classical 2D plug flow heat transfer model, the wall Nusselt number and the effective thermal conductivity of the bed were governed from the axial temperature profile of the CFD simulations.

Figure (A) and (B) show impressively the intensified surface temperature of the spheres in the near-wall region. This effect is reflected by the wall Nusselt number. However, the correlations from Martin & Nilles underpredict this near-wall phenomenon, see Figure (C). Contrarily, the effective thermal conductivity of the bed is well predicted by the ZBS correlation, which indicates minor thermal radiation effects in the bulk region of the bed. We also present the comparison of radial temperature profiles at different axial positions between the CFD simulations and the 2D model without and with thermal radiation in the two beds, see Figure (D).

[1] Wehinger, G. D. (2019). Chem. Ing. Tech., 91(5), 583-591.

[2] Wehinger, G. D., & Flaischlen, S. (2019). Ind. Eng. Chem. Res., 58(31), 14410-14423.