Numerical simulations of transfer and transport properties inside packed beds of spherical particles

Performance analysis, statistical modeling, and multiple response optimization of a novel fixed-bed quartz reactor packed with Ba-Pt@γ-AL2O3 using response surface methodology

In the present study, a novel fixed-bed continuous reactor with a preheating chamber was designed to be utilized for the practical application of removal studies of dangerous pollutants, especially NOX removal by NOX Storage Reduction (NSR) catalysts on a laboratory scale. The reactor's design and operational parameters, including outer wall temperature (50-600 °C), volumetric flow rate (0.3-3 L/min), wall temperature time (0.16-10 min), and granule surface area inside the preheating chamber (0-270 cm2), were statistically modeled and optimized using Response Surface Methodology (RSM). For more logical and effective parameter optimization, the ratio of gas and catalyst temperatures and pressure drop to the reactor outer wall temperature (GT/ROWT, CT/ROWT, and PD/ROWT) were also included in the optimization process. Experimental results showed that gas temperature, catalyst temperature, and pressure drop ranged from 31 to 177 °C, 51-585 °C, and 7-153 Pa, respectively. Optimal conditions were determined to be an outer wall temperature of 230 °C, a volumetric flow rate of 3 L/min, a wall temperature time of 0.16 min, and a granule surface area of 67.3 cm2. The results demonstrated that outer wall temperature, flow rate, time, and surface area of granules have significant and interaction effects on the responses and should be considered when researchers assess the removal efficiency of thermal catalysts.

Modeling CO2 Adsorption in a Thin Discrete Packing

Local dynamics of CO2 adsorption in a discrete packing contained in a thin tube was assessed by 3D modeling. Thin tube packed bed adsorbers are currently used over tube structures in thermochemical energy storage systems and atmospheric revitalization of confined spaces. Driven by the interplay between key factors such as the exothermicity and the fluid flow, the advective transport was found less effective than the diffusive one on the breakthrough trends of CO2 which displayed significant concentration gradients at both inter- and intraparticle scales. The lack of angular symmetry inside the particles by the reduction in resistance to mass transfer in the area of solid particles exposed to high velocities led to greater convective transports from the bulk of the gaseous phase to the pores. The result of the modeling agreed with the experimental data obtained at the exit of the adsorber, helping reduction in reliance on the empirical dispersion models used in the one-dimensional modeling.

Theoretical and experimental investigation of the influence of intraphase diffusion on the oxidation of toluene over manganese-based oxide

The influence of intraphase diffusion on the oxidation of toluene over manganese-based oxide, MnFeOx, under isothermal conditions was investigated. The one-dimensional (1D) heterogeneous model was developed to explain the experimental results. The model can be applied under certain operating conditions, i.e. conditions with negligible resistance to external mass transfer. The article describes the numerical solution of the equations of the proposed model. The relevance of the 1D heterogeneous model compared to the 1D pseudohomogeneous model was also emphasized. Validation of the proposed models was carried out by comparing with experimentally measured results. Both models were found to describe the experimental system studied quite well, with the 1D heterogeneous model providing better insight into the relationship between the rate of chemical reaction and the rate of intraphase diffusion.

Analysis of Particle-Resolved CFD Results for Dispersion in Packed Beds

Dispersion is the spreading of a solute while it is moved by a flowing medium. The study of dispersion in catalytic chemical reactors is fundamental to their design, since dispersion influences the reactant and product transport within the bed. In this paper, longitudinal and transverse dispersion of an inert tracer in slender packed beds of spheres and spherocylinders is studied using Computational Fluid Dynamics simulations. The focus is on the analysis of dispersion from full field data. The purpose is to develop a methodology that can later also be used to characterize dispersion from full field experimental data such as MRI measurements. Results obtained by means of particle-resolved CFD simulations are discussed. Spatial distributions and residence times are analyzed and the results are interpreted by comparison to results obtained from 1D and 2D convection-diffusion equations.

Computational Fluid Dynamics for Fixed Bed Reactor Design

Flow, heat, and mass transfer in fixed beds of catalyst particles are complex phenomena and, when combined with catalytic reactions, are multiscale in both time and space; therefore, advanced computational techniques are being applied to fixed bed modeling to an ever-greater extent. The fast-growing literature on the use of computational fluid dynamics (CFD) in fixed bed design reflects the rapid development of this subfield of reactor modeling. We identify recent trends and research directions in which successful methodology has been established, for example, in computer generation of packings of complex particles, and where more work is needed, for example, in the meshing of nonsphere packings and the simulation of industrial-size packed tubes. Development of fixed bed reactor models, by either using CFD directly or obtaining insight, closures, and parameters for engineering models from simulations, will increase confidence in using these methods for design along with, or instead of, expensive pilot-scale experiments.

Advances in fixed-bed reactor modeling using particle-resolved computational fluid dynamics (CFD)

In 2006, Dixon et al. published the comprehensive review article entitled “Packed tubular reactor modeling and catalyst design using computational fluid dynamics.” More than one decade later, many researchers have contributed to novel insights, as well as a deeper understanding of the topic. Likewise, complexity has grown and new issues have arisen, for example, by coupling microkinetics with computational fluid dynamics (CFD). In this review article, the latest advances are summarized in the field of modeling fixed-bed reactors with particle-resolved CFD, i.e. a geometric resolution of every pellet in the bed. The current challenges of the detailed modeling are described, i.e. packing generation, meshing, and solving with an emphasis on coupling microkinetics with CFD. Applications of this detailed approach are discussed, i.e. fluid dynamics and pressure drop, dispersion, heat and mass transfer, as well as heterogeneous catalytic systems. Finally, conclusions and future prospects are presented.

CFD Analysis of BED Textural Characteristics on TBR Behavior: Hydrodynamics and Scaling-up

In recent years, CFD has played an important role in the understanding and design of TBR’s. In this work, through CFD with Eulerian approach, a three-phase heterogeneous reactor model was developed, were the accuracy of Interfacial Momentum Exchange Model (IMEM) for the gas-solid interaction, the effect of a more detailed catalytic bed geometry description, and the pellet shape over TBR hydrodynamics of two fluid phases interacting with the solid phase was studied. Then, a second model was developed, where the validated hydrodynamic model was coupled with mass transport for an HDS process of light gasoil. Additionally, in order to insight into the scaling up process of a TBRs, the proposed columns behaviors were compared against literature columns using four different ways, and it was found that the best predictions were obtained when the models’ holdup were equaled to those evaluated in literature columns. Since in reliable literature deviations in pressure drop predictions of around 30% can be found, the model results show significant improvement against literature, achieving 5 times better accuracy in predicting pressure drops, and 50% improvement in holdup prediction; the coupled model reproduces the same conversion values compared with literature data, and predicts conversions with 95% accuracy

Recirculation zones induce non-Fickian transport in three-dimensional periodic porous media

In this work, the influence of pore space geometry on solute transport in porous media is investigated performing computational fluid dynamics pore-scale simulations of fluid flow and solute transport. The three-dimensional periodic domains are obtained from three different pore structure configurations, namely, face-centered-cubic (fcc), body-centered-cubic (bcc), and sphere-in-cube (sic) arrangements of spherical grains. Although transport simulations are performed with media having the same grain size and the same porosity (in fcc and bcc configurations), the resulting breakthrough curves present noteworthy differences, such as enhanced tailing. The cause of such differences is ascribed to the presence of recirculation zones, even at low Reynolds numbers. Various methods to readily identify recirculation zones and quantify their magnitude using pore-scale data are proposed. The information gained from this analysis is then used to define macroscale models able to provide an appropriate description of the observed anomalous transport. A mass transfer model is applied to estimate relevant macroscale parameters (hydrodynamic dispersion above all) and their spatial variation in the medium; a functional relation describing the spatial variation of such macroscale parameters is then proposed.

Computational Fluid Dynamics-Based Hydrodynamics Studies in Packed Bed Columns: Current Status and Future Directions

A careful review of the literature reveals that extensive research has been done on the hydrodynamics in packed bed columns using turbulence models. It can be noted that the choice of turbulence model is influenced by the number of phases, type of fluid, Reynolds number range and the type of packing. Thus, comparison of turbulence models for the selection of a suitable model assumes great importance for the better prediction of flow pattern. This is due to the fact that poor prediction of the flow pattern can lead to a limited heat and mass transfer model as the rate of transfer processes in packed bed is governed by the hydrodynamics of the packed bed. The aim of this paper is to give a review of the computational fluid dynamics (CFD)-based hydrodynamics studies of packed bed columns with the primary interest of studying pressure drop and drag coefficient in packed beds. From the literature survey in Science Direct database, more than 48,000 papers related to packed bed columns have been published with more than 3,000 papers focused on the hydrodynamic studies of the bed to date. Unfortunately, there are only a few studies reported on the hydrodynamics of packed columns under supercritical fluid condition. Therefore, it is imperative that the future work has to focus on the hydrodynamics of supercritical packed column and particularly on the selection of suitable turbulence model.

Predictive model of solute transport with reversible adsorption in spatially periodic hierarchical porous media

Solute transport in hierarchical porous media with reversible adsorption is predicted by the volume averaging method. A transient macroscopic advection-diffusion equation is derived to describe the multiscale solute transport problem. The theoretical expression of the dispersion tensor is obtained which is the function of pore-scale velocity profile. Steady closure equations are derived to calculate the dispersion tensor. With the aid of pore-scale simulation in unit cells of the hierarchical porous media, the dispersion tensor can be calculated. The model is verified by comparing its predictions and obtaining favorable agreement with results of direct numerical simulations and with experimental data for columns comprised of ordered, porous pillars. It is straightforward to use the model to predict the solute transport behavior in fixed beds packed with particles.