Modeling approach to packed bed process with solid particle reactions

Abstract

In the present study, a mathematical simulation has been conducted for iron ore pellet indurator as an industrial packed bed process. As one of the crucial submodels, gas-solid particle reaction models need to be considered in relation with the reaction and the particle characteristics. In this study, the application of reaction models has been discussed with coke combustion in a pellet. The shrinking core model has frequently been used in the previous studies. However, since the model basically assumes a single species nonporous particle, it may need to be questioned for a pellet, a porous agglomerate of multi-component fine grains, in terms of its validity over reaction regimes and arbitrariness in the parameters. The application of the grain model is expected to reduce the limitations of the shrinking core model as it incorporates the pore structure of a pellet. First of all, the two models are compared at the single pellet scale in a simplified condition as the preliminary step to construct the entire indurator simulation. The shrinking core model is prepared with 5 cases in which the modeling parameters are considered differently based on the expressions from the previous studies. The results are discussed in terms of the overall conversion fraction profiles, the reaction modulus, and the diffusivity coefficients by varying temperature $400 ^\circ C$ - $1200 ^\circ C$ . The results show that the reaction progression using the shrinking core model depends largely on the effective diffusion coefficients, and the shrinking core model needs a fitting to make the more realistic approximation. 3 among the 5 cases of the shrinking core model which showed close results to the grain model are selected for further comparison in the overall indurator simulation. In the next step of the entire indurator simulation, a calculation is firstly conducted for the induration case without coke to validate the overall bed model by comparing the bed temperature results to the data from the previous study. Afterward, a simulation is conducted for the induration case with coke using the grain model and validated to the previous data as the base case. Then, the 3 cases of the shrinking core model from the preliminary step are compared to the grain model case in terms of bed temperature and conversion fraction profiles. Moreover, the result of the grain model reaction modulus shows that coke combustion takes place mostly in the diffusion controlled condition. Considering that the shrinking core model for a porous particle is theoretically valid limitedly for the diffusion controlled condition, as well as the low coke content, the results may imply that the previous simulations which used the shrinking core model could possibly present the plausible bed temperature results. However, the process conditions such as gas condition, particle size and porosity, and solid composition can be modified. In those cases, the validity of the process simulation using the shrinking core model may need to be re-evaluated. The grain model is expected to be more advantageous as it incorporates the pore structure in relation to pellet porosity and enables the quantitative analyses for the reaction progression. In this regard, the additional simulations are made for the changes in the pellet diameter, porosity, the mean grain diameter, and the coke content are made, which are limited with the shrinking core model.