Calculation of mass transfer limitations of a gas-phase reaction in an isothermal fixed bed reactor: tutorial and sensitivity analysis

The study of the reactivity of solid catalysts requires assessing chemical kinetics and mechanism in the absence of mass transport artifacts. These artifacts consist of the formation of concentration gradients either on the external or internal (inside nanopores) surface of the solid. Despite the existence of models and criteria for assessing the presence of mass transfer limitations during catalytic tests for gas-phase reactions in isothermal fixed bed reactors, the literature does not present straightforward protocols for performing the latter calculations. In this work, we present a systematic and complete protocol for the calculations above. The developed protocol serves as a tutorial for students and researchers. Particularly, the effectiveness factor for external and the Weisz-Prater number for the internal mass transfer limitations were developed. The oxidation of propane over mixed vanadium-aluminum (hydr)oxides was taken as a case study. Based on these protocols we perform a sensitivity study of the models for the following modifications: (i) the equation of state for modeling the thermodynamic properties of the gas phase, (ii) the particle size, (iii) the conversion of propane at two different temperatures and, (iv) the reactant used as a basis of the calculations; i.e., switching from propane to oxygen. Results showed that the model for calculating the effectiveness factor was poorly sensitive to all the above modifications. Meanwhile, the Weisz-Prater number was much more sensitive to the studied modifications, even reaching deviations up to ~200%.