The
study of the reactivity of catalysts requires one to assess
chemical kinetics and mechanisms in the absence of mass transport
artifacts. 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 protocol for the
calculations above. Particularly, we present protocols for estimating
the effectiveness factor for external and the Weisz-Prater number
for the internal mass transfer limitations. Data previously published
on the oxidation of propane over mixed vanadium–aluminum (hydr)oxides
was taken as a case study. Based on these protocols, we did a sensitivity
study of the models used for calculations. 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%.
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%.
The study of the reactivity of catalysts requires assessing chemical
kinetics and mechanism in the absence of mass transport artifacts.
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 protocol for the
calculations above. 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. 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%.
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