Local structure effects on pressure drop in slender fixed beds of spheres

For slender fixed beds, the void fraction and flow properties are complex topics. Different factors can influence the local bed structure. For statistical analysis, 2800 fixed beds have been generated and the impact of friction factor and reactor-to-particle diameter ratio on the distribution has been shown. With particle-resolved computational fluid dynamics, all local structure effects are taken into account for the flow simulations. Pressure drop measurements and simulations showed that these effects can lead to areas with low flow resistance, leading to overestimated pressure drop by typical correlations up to 85 %

Phosphorus dosing during catalytic n‑butane oxidation in a μ-reactor: a proof of concept

The selective oxidation of n-butane to maleic anhydride over vanadium–phosphorus oxide catalysts is subject to a dynamic change in the catalyst activity. This phenomenon is called phosphorus dynamics and plays a vital role in the prediction of catalytic reaction rates, but to date, no models measured under transport limitation free conditions have been published. This study presents the first investigation of the phosphorus dynamics over extended periods of time (multiple days on stream) under transport limitation free conditions in a μ-fixed-bed reactor. Initially, temperature variation experiments are conducted to investigate whether phosphorus dynamics takes place in a μ-reactor and to determine the onset of phosphorus loss. Then, a setup for dosing of liquid organophosphorous species on the scale of nL min–1 is proposed, and functionality is demonstrated via step test experiments. Results of the temperature variation showed that phosphorus loss occurs in the μ-reactor but starts at temperatures exceeding those of industrial scale reactors by 30–80 K. It was further observed that addition of steam to the feed increases the intensity of the phosphorus dynamics and lowers the onset temperature. Step test results demonstrated the functionality of the dosing setup if a suitable inert material is chosen and the metal surfaces downstream the dosing are treated according to a passivation procedure proposed in this study. The addition of steam appears to be required for appropriate distribution of the dosed organophosphorous species over the catalyst bed

A conjugated heat and mass transfer model to implement reaction in particle-resolved CFD simulations of catalytic fixed bed reactors

Modeling catalytic fixed bed reactors with a small tube-to-particle diameter ratio requires a detailed description of the interactions between fluid flow, intra-particle transport, and the chemical reaction(s) within the catalyst. Particle-resolved computational fluid dynamics (PRCFD) simulations are the most promising approach to accurately predict the behavior of these reactors, since they take explicitly into account the local packed bed structure. In this work, a conjugated heat and mass transfer model for use in PRCFD simulations is presented to couple the fluid flow through the fixed bed with transport and reaction in the porous catalyst, while guaranteeing the no-slip boundary condition at the fluid-solid interface. For this purpose, the solutions of the solid and fluid domain are computed separately and are coupled by calculation and updating the boundary condition at the particle surface. Due to the consideration of secondary gradients, the developed transfer model is also valid for unstructured calculation meshes containing non-orthogonal cells at the fluid-solid interface. Such meshes are often used to resolve complex geometries, such as a packed bed, in a computationally efficient manner. The coupling approach is validated using cases for which an analytical solution or literature correlations derived from experimental data are available. The simulation results of a short catalytic packed bed with rings catalyzing the partial oxidation of n-butane to maleic anhydride exemplify the potential of PRCFD involving reactions to analyze the catalyst performance in great detail

How Temperature Measurement Impacts Pressure Drop and Heat Transport in Slender Fixed Beds of Raschig Rings

The axial temperature profile of a packed bed is often measured by thermocouples placed either directly in the bed or inside a thermowell centered in the reactor tube. Quantifying the impact of the thermocouple well on fluid flow, heat transport, and consequently on the measured temperatures is still an unresolved challenge for lab-scale reactors but especially, and even more so for multitubular reactors in industry. Particle-resolved computational fluid dynamics (PRCFD) simulations are a suitable approach to investigate the changes in transport phenomena exerted by inserting thermocouple wells into packed beds because they take into account the local packed bed structures. In this study, PRCFD simulations are performed based on design of simulation experiments (DoSE). The effect of the thermowell diameter and its thermal conductivity on the deviations between packed beds with and without thermowells is statistically quantified for characteristic integral quantities like pressure drop and tube wall-bed Nusselt number. The axial temperature profiles inside the thermowells can be computed efficiently with reasonably accuracy applying the Nusselt number correction as derived in this study from the DoSE in a one-dimensional pseudo-homogeneous energy balance

Quo vadis multiscale modeling in reaction engineering? A perspective

This work reports the results of a perspective workshop held in summer 2021 discussing the current status and future needs for multiscale modeling in reaction engineering. This research topic is one of the most challenging and likewise most interdisciplinary in the chemical engineering community, today. Although it is progressing fast in terms of methods development, it is only slowly applied by most reaction engineers. Therefore, this perspective is aimed to promote this field and facilitate research and a common understanding. It involves the following areas: (1) reactors and cells with surface changes focusing on Density Functional Theory and Monte-Carlo simulations; (2) hierarchically-based microkinetic analysis of heterogeneous catalytic processes including structure sensitivity, microkinetic mechanism development, and parameter estimation; (3) coupling first-principles kinetic models and CFD simulations of catalytic reactors covering chemistry acceleration strategies and surrogate models; and finally (4) catalyst-reactor-plant systems with details on linking CFD with plant simulations, respectively. It therefore highlights recent achievements, challenges, and future needs for fueling this urgent research topic in reaction engineering