The influence of dissociative adsorption on effectiveness factors of porous catalysts

Abstract

The actual concentration profile of a reacting fluid inside a porous catalyst is something of a mystery. However, there are powerful models to predict the concentration distribution, and effectiveness factor as a function of the Thiele modulus. This work provides a model for the porous catalyst particle considering heterogeneous kinetics for steady‐state Langmuir‐Hinshelwood rate expressions, including the cases when adsorption stoichiometry is other than 1:1, as for dissociative adsorption (1:2), which have not been covered by the preceding literature. The isothermal and the non‐isothermal particles are considered. Numerical solutions for the concentration profile of a limiting reactant were calculated by three methods: finite differences with Newton‐Raphson recursion, Chebyshev orthogonal collocation, and integration by Adams method as done by Weisz and Hicks. The Weisz‐Hicks method outperformed the other two. Plots show the results for specific cases, which are briefly discussed. Adsorption stoichiometry and pellet geometry are of paramount importance for the assessment of effectiveness factors. The results demonstrate that the effectiveness of a catalyst pellet with prevalent dissociative adsorption of a reactant is always inferior to the effectiveness with prevalent simple adsorption.

How to Achieve Flexible Green Ammonia Production: Insights via Three-Dimensional Computational Fluid Dynamics Simulation

“…Based on empirical correlations to calculate effective factors η η = b 0 + b 1 T + b 2 X + b 3 T 2 + b 4 X 2 + b 5 T 3 + b 6 X 3 X is N 2 conversion rate, and b 0 – b 6 is a function of pressure. The heat transfer calculations could be obtained by eq according to Soares et al: normalΔ H r = 4.184 { − true[ 0.54526 + 846.609 T + 459.734 × 10 6 T 3 true] p − 5.34685 T − 0.2525 × 10 − 3 T 2 + 1069197 × 10 − 6 T − 3 − 9157.09 } The mixture transport can be modeled using the Euler Equation. Since the fluid flow is highly turbulent with relatively high velocity, the CFD solver solves the Reynolds Number formula for the transport equation.…”

“…Based on empirical correlations to calculate effective factors η X is N 2 conversion rate, and b 0 – b 6 is a function of pressure. The heat transfer calculations could be obtained by eq according to Soares et al: The mixture transport can be modeled using the Euler Equation. Since the fluid flow is highly turbulent with relatively high velocity, the CFD solver solves the Reynolds Number formula for the transport equation.…”

Solving boundary value problems in heterogeneous catalysis with orthogonal collocation and arc‐length continuation

“…Indeed, realistic models have many aspects and can include complex reaction mechanisms as well as nonisothermal particles. Particular cases include consecutive and parallel reactions [23,29], reaction orders different than unity [18,19,24], absorption [14], geometrical aspects [16,35], isothermal and nonisothermal Langmuir-Hinshelwood kinetics [20,26,34,38], possibly with dissociative adsorption [31]. One class of distinct important BVPs arises when one accounts for mass and heat transfer by convection at the surface of the catalyst particle.…”