An assessment of fluidized bed dynamics with CPFD simulations

The computational particle fluid dynamic (CPFD) method has been used to simulate a laboratory-scale fluidized bed, which has been designed for plastic pyrolysis. The simulations have been performed under cold-mode condition, where only the fluidization of sand particles is considered. The objective of the work is to gain an in-depth understanding of the hydrodynamic behavior of the fluidized bed, which is of particular importance with regard to an efficient mixing and heating of the bed materials as well as the final product yield. The focus of the work is assessing the dynamic behavior of the fluidized bed in terms of the total kinetic energy of all sand particles KS and the bubble frequency fB. For validation of the numerical approach, the calculated pressure drop Δp shows good agreement with measured data. In accordance with measurement and theoretical analysis, Δp increases with the bed inventory mS and remains nearly constant with the bulk gas flow velocity uG. It has been shown that KS increases with uG, which is due to the increased gas flow momentum flux with uG, leading to a reinforced gas-to-solid momentum exchange. The same behavior has been found for the influence of the sand particle mass mS on KS, where KS increases with mS. uG has been found to have a subordinate effect on fB, whereas fB decreases with mS. An increase in the gas temperature TG has led to a decreased KS, while the bed height hB and Δp remain nearly constant. This is due to the decreased density or momentum flux of the gas flow at higher TG. While up-scaling the fluidized bed, KS and fB have found to be strongly increased, whereas uG, Δp and hB were kept constant. The results reveal that it is not sufficient to use solely the general “static” parameters, i.e., hB and Δp, for characterizing hydrodynamic properties of a fluidized bed. In this case, KS and fB represent measures for the available kinetic energy and its fluctuation frequency of the whole fluidized bed system, which are more suitable for assessing the hydrodynamic behavior of the fluidized bed under up-scaled and elevated temperature conditions