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  • Yohannis Tobo

Computational fluid dynamics modeling of chemical reactors: Impact of model selection

For internal flow processes like reactors, measuring the detailed processes applying the experimental technique is challenging because of the occurrence of complex and simultaneous flow of different phases mixtures, chemical reactions and heat and mass transfer, in addition to the challenges in operating conditions, mixtures properties, safety and scale of the reactor. In such circumstances, Computational Fluid Dynamics (CFD) modeling and simulation is a powerful tool for detailed analysis and design optimization in a short time.

The CFD model helps to get a detailed insight into the reactors' design optimization and reaction product characterization. The CFD model is straightforward for processes involving only single-phase hydrodynamics modeling. Such models are usually preferred because of their simplicity in implementing the model and the model solution is obtained quickly. In contrast, modeling flows a mixture of different phases, including chemical reactions, interphase heat and mass transfer, is challenging because of the complexity of model implementation, computational resource requirement and computational time. Even though the model is complex, it mimics the actual processes and provides extended results.

Simplifying models to hydrodynamics models in a flow involving different phases of mixtures, chemical reactions and mass transfer could reduce the accuracy of the model predictions because chemical reactions and interphase mass transfer affect the composition of the mixture phases and hydrodynamics behavior. Also, the impact of thermal and molecular diffusion transport cannot be modeled in the hydrodynamics model, which affects the transport of chemical compositions. So, characterizing and concluding the reactor based on only hydrodynamics analysis needs attention as it may mislead the conclusion.

For chemical reactors, accurate and complete information from the CFD model is obtained when chemical reactions, hydrodynamics, heat and mass transfer and diffusion transport models are solved. Such models allow the characterization of the reactants and products and compute the reactor conversion efficiency on top of hydrodynamics analysis.

Including chemical reactions in the CFD model gives the opportunity to study the behavior of reactions. Since there are several intermediate reactions at different reaction rates, the slowest reactions delay the subsequent reactions and affect the residence time of the solid biomass inside the reactor.

In addition, the inclusion of chemical reaction and heat and mass transfer allows us to analyze the time required by the reactant to reach the minimum temperature to initiate the reaction and the distance the reactants travel from the initial to the final point before being wholly consumed. The model also helps study reaction product distribution due to the combined hydrodynamics, diffusion transport and residence time distribution.

In general, modeling and simulation of internal flow processes involving the flow of different phases of mixtures involving chemical reactions, interphase heat and mass transfer are challenging, but it is the most accurate model. The model represents the actual process in the virtual environment and provides detailed results which are challenging or impossible to measure in real experiments. Ignoring the implementation of the process inside the reactor reduces the model's accuracy and design optimization based on hydrodynamics alone might not hit the intended goals.



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