Microkinetics-based reactor models

We highlight in this section some of these more recent developments of reactor models for Fe-based catalysts based on global kinetic descriptions. Our intent is to describe the state of the art, pointing out the main features and Umitations of the two recent models. There remains a need to build on the emerging understanding of the mechanistic features of SCR through the use of reactors based on microkinetic models. That will undoubtedly be an area of activity in the coming years [15]. 

The role of kinetic and reactor modeling is crucial in the continued advancement of these catalysts as they are optimized for specific applications. We have described different mechanisms for SCR for feed compositions spanning the standard to fast to NO2 types. Convergence to the correct mechanisms is essential if predictive mechanistic-based kinetic models are to be developed. To date the kinetic models have been of the global variety. While these are usefiil for reactor optimization, microkinetic models are needed to guide rational catalyst design and the discovery of new catalyst formulations. 

A model for the riser reactor of commercial fluid catalytic cracking units (FCCU) and pilot plants is developed This model is for real reactors and feedstocks and for commercial FCC catalysts. It is based on hydrodynamic considerations and on the kinetics of cracking and deactivation. The microkinetic model used has five lumps with eight kinetic constants for cracking and two for the catalyst deactivation. These 10 kinetic constants have to be previously determined in laboratory tests for the feedstock-catalyst considered. The model predicts quite well the product distribution at the riser exit. It allows the study of the effect of several operational parameters and of riser revampings. 

Commonly, the design of a reactor requires the prediction of the rate of reaction. Two different approaches have been used to develop suitable kinetic models for the WGS reaction. The first is based on microkinetics by taking into account the elementary steps from the adsorption of the chemical species to the reaction and the product desorption the second is based on the macrokinetics that are empirical models in which the rate of reaction depends proportionally on the concentration of reactants and products and exponentially on temperature (typically expressed using the Arrhenius equation). The microkinetics approach is more complex, in particular from a mathematical and computational point of view, but it offers the possibility to better model the surface coverage and the enthalpy of the reaction (i.e., the temperature increase on the catalyst surface). Two different mechanisms for the WGS reaction are proposed in the scientific literamre the redox mechanism and the associative mechanism. 

The microkinetic modehng provides us with only numerical results and is not convenient for further use in reactor design and optimization. But with the information obtained from the microkinetic analysis, a macrokinetic-based L-H model can be easily derived. In this situation, there is no need to make any assumption. The values of kinetic parameters can also be retrieved from microkinetic study. As an example, an L-H model using Eq. (2.24) as the RDS was proposed ... 

---