Geometry Of Packed Bed Research Articles

CFD modeling of natural convection in pebble bed geometry with finite volume method

Abstract In this study, we used the finite volume method to computationally model natural convective flow in packed bed geometry. Using the OpenFOAM® v2112 code, we performed the computational analysis. We successfully meshed the intricate packed bed flow geometry, which consists of several spheres positioned at random. The spheres have sizes of 0.006 and 0.01 m, and the associated Rayleigh numbers are 1.83 × 107 and 8.48 × 107 respectively. We used the packed bed heights of H/d = 5, 10, and 20 in the simulations. By comparing the results of the OpenFOAM® v2112 simulations of the natural convection flow for all self-heating sphere in a packed bed, we demonstrated that the velocity distributions and Nusselt values are in good agreement with the experimental data. Additionally, it was evident from the velocity and temperature distributions in a packed bed core that there was a major temperature rise at nearby low velocity fields and a minor velocity rise in the intermediate and upper elevations. We showed that increasing the height of the pebble-bed core and correspondingly increasing the quantity of spheres inside it makes the flow more difficult and also generates local hot spots. This study is notable for using the finite volume method to evaluate natural convection flow in all self-heating packed beds and for simulating packed bed flow using a significant number of spheres. These two factors contribute to the originality of this work.

Numerical simulation of convective heat transfer in packed bed using open-source computational fluid dynamics

The shape and stacking method of the packing elements will affect the overall performance of the packed bed structure. In this study, the convection heat transfer in packed beds was simulated using the customized OpenFOAM “porousFoam_pimple” solver, which could provide the direction for the actual design and optimization of packed bed. Blender was used to generate the packed beds of spheres and fibers. Moreover, the influence of packed bed geometry on the convective heat transfer performance was investigated. The average pressure gradient and comprehensive heat transfer efficiency vary greatly with the parameters, such as fiber diameter, fiber axial, inlet velocity angle, and fiber interleaving degree. The customized “porousFoam_pimple” solver can be used for the numerical simulation of the convective heat transfer in packed beds with particles of different shapes under different operating conditions, providing a reliable and practical tool for the design and optimization of packed beds.

Investigation of natural convection heat transfer of self-heating packed beds

This study investigated heat transfer characteristics of natural convective flow in the packed bed geometry experimentally and numerically. The natural convection experiments for self-heating condition were performed using the copper sulfate electroplating system based on the analogy between heat and mass transfers. Also, the same conditions were simulated with Lattice-Boltzmann Method (LBM) and the results were compared. The spheres diameters were d = 4, 6 and 10 mm and the corresponding Rayleigh numbers were 5.43 × 106, 1.83 × 107 and 8.48 × 107 respectively. The packed bed height to sphere diameter ratios, H/d were 5, 10 and 20. The Nusselt numbers (Nud), velocity and shear stress distributions of both analyses were in good agreement. Both results showed the same order (10−2 m/s) of irregular velocity distributions resulting from the random arrangement of the spheres. In spite of the laminar flow condition, wake and vortex generated by the packed bed geometry increased the turbulent kinetic energies to one- or two-order higher values than those at the entrance. The temperature profiles of flow inside packed bed were approximately uniform for d = 4 mm cases but there were temperature gradients in case of d = 10 mm. This revealed that the use of smaller fuel sphere can prevent from unwanted hot spots inside the packed bed. The Nud values from LBM results and newly developed empirical correlation for natural convective flow in packed bed were consistent.

An improved contact modification routine for a computationally efficient CFD simulation of packed beds

Particle-resolved computational fluid dynamics (PRCFD) is the most detailed modeling approach for fixed bed reactors. A major issue of PRCFD is to ensure a high quality of the underlying 3D calculation mesh around the particle-particle and particle-wall contacts, which otherwise results in convergence problems. A still unresolved challenge, especially for packed beds of non-spherical particles, is to avoid this poor-quality mesh at contacts by modification of the packed bed geometry, while providing a reasonable number of mesh cells and physical accuracy. Based on the so called local caps method, a novel mesh-type independent routine was developed to modify all occurring contacts of a packed bed of rings and to generate a computationally efficient high-quality mesh. Thereby, one of the two particles involved in a point or line contact was inflated and used as cap geometry, whereas area contacts were combined via the contact surface. The impact of the cap size on mesh quality, packed bed structure, fluid flow and heat transport was investigated for a technically relevant slender packed bed under various conditions. For a small ratio between solid and fluid thermal conductivity (λPλF≤23.1), and ReP=50−600, a cap size smaller than 0.8% of the outer particle diameter did not have significant impact on the simulation results. To apply the approach to other packing structures and particle shapes, it is recommended to investigate at least three different cap sizes to determine at which cap size the independence from contact modification is achieved.

Lead adsorption in a serpentine millichannel‐based packed‐bed device: Effect of hydrodynamics and mixing characteristics

AbstractThe present study depicts the hydrodynamics and the mixing characteristics in a millichannel‐based serpentine fixed‐bed device to attain the particular demands of the miniature adsorption devices' fabrication. Residence time distribution analyses were accomplished to analyze the velocity distribution inside the packed bed geometry. The effects of operating variables on the system hydrodynamics and mixing and their impact on the lead adsorption characteristics were enunciated. New correlations were proposed for the frictional resistance and axial dispersion of the fluid. The parametric effects on the lead ions [Pb(II)] adsorption were studied in the same millichannel geometry packed with the graphene oxide coated glass beads. The kinetics of the adsorptive removal process is assessed by the Thomas model and the Yoon–Nelson model. The regeneration study of the said millichannel‐based fixed‐bed device was also executed.

Three-dimensional CFD simulation of anaerobic reactions in a continuous packed-bed bioreactor

In this study, a lab-scale continuous packed-bed bioreactor with multi-sized distributed particles has been simulated in a structured layout using computational fluid dynamics. The goal was to achieve a well-estimated simple model which can describe the hydrodynamic regime of flow as well as mass transfer of species within the bioreactor. A MATLAB program was coded to create the packed-bed geometry by generating non-overlapping spherical particles. The flow hydrodynamics was assumed to be single-phase creeping flow regime (Stokes flow). The kinetic data of batch experiments at similar operating conditions were used to formulate the rate of anaerobic reactions. The anaerobic methanogenesis reactions were assumed to occur at the particle surfaces (on the biofilm). The bioreactor temperature was fixed at 37°C. The commercial software COMSOL Multiphysics 5.0 was employed to solve the governing equations. Velocity profiles and concentration distribution of volatile fatty acids (VFAs) are shown inside the packed-bed reactor. Simulation results were found to be in good agreement with the experimental data at various influent flow rates (23.8 – 43.5 mL/h) and various influent concentrations of VFAs (0.001 – 0.042 M). In all cases, the relative error was less than 15% which is acceptable. This proposed model is applicable for scale-up and optimization purposes as well as evaluation of other kinetic models.

Systematically Improving Espresso: Insights from Mathematical Modeling and Experiment

Summary Espresso is a beverage brewed using hot, high-pressure water forced through a bed of roasted coffee. Despite being one of the most widely consumed coffee formats, it is also the most susceptible to variation. We report a novel model, complimented by experiment, that is able to isolate the contributions of several brewing variables, thereby disentangling some of the sources of variation in espresso extraction. Under the key assumption of homogeneous flow through the coffee bed, a monotonic decrease in extraction yield with increasingly coarse grind settings is predicted. However, experimental measurements show a peak in the extraction yield versus grind setting relationship, with lower extraction yields at both very coarse and fine settings. This result strongly suggests that inhomogeneous flow is operative at fine grind settings, resulting in poor reproducibility and wasted raw material. With instruction from our model, we outline a procedure to eliminate these shortcomings.

Recovery of vinyl chloride from by‐streams of polyvinyl chloride production by TPSA in a multitubular adsorber

AbstractThis work aims at developing an efficient and feasible adsorption‐based separation process for the separation of vinyl chloride and nitrogen, on activated carbon, by employing a multitubular packed bed geometry, with adsorbent material inside the tubes. Using this geometry, a 2‐dimensional mathematical model of a temperature pressure swing adsorption process was used to developed a 6‐step three multitubular adsorbers system capable of separating and purifying an industrial scale gas stream of a 40:60% (v/v) vinyl chloride/nitrogen mixture into a 95% (v/v) vinyl chloride stream and a nitrogen stream with a vinyl chloride limit concentration of 8 ppm (w/w). The process reported energy consumption of 4.88 × 106 J/kgVCM and recovery capacity of 24.35 kgVCM/(m3unit h). The multitubular geometry enabled the use of lower adsorbent loads, shorter cycle times, and lower regeneration temperatures. An equivalent 1‐dimensional model has also shown to satisfactorily estimate the performance of the current equipment.

Synergistic sorbent separation for one-step ethylene purification from a four-component mixture.

Purification of ethylene (C2H4), the largest-volume product of the chemical industry, currently involves energy-intensive processes such as chemisorption (CO2 removal), catalytic hydrogenation (C2H2 conversion), and cryogenic distillation (C2H6 separation). Although advanced physisorbent or membrane separation could lower the energy input, one-step removal of multiple impurities, especially trace impurities, has not been feasible. We introduce a synergistic sorbent separation method for the one-step production of polymer-grade C2H4 from ternary (C2H2/C2H6/C2H4) or quaternary (CO2/C2H2/C2H6/C2H4) gas mixtures with a series of physisorbents in a packed-bed geometry. We synthesized ultraselective microporous metal-organic materials that were readily regenerated, including one that was selective for C2H6 over CO2, C2H2, and C2H4.

Selecting for ethylene

Gas Separations Purification of ethylene from other gases produced during its synthesis, such as acetylene, ethane, and carbon dioxide, is an energy-intensive process. Chen et al. use a mixture of microporous metal-organic framework physisorbents that are selective for one of these four gases. A series of sorbents in a packed-bed geometry produced ethylene pure enough for making polymers. Science , this issue p. [241][1] [1]: /lookup/doi/10.1126/science.aax8666

A data driven artificial neural network model for predicting radiative properties of metallic packed beds

In this work, we demonstrate that machine learning methods can be reliably used to predict radiative properties of dispersed media, i.e. packed beds, as a function of packed bed geometry and material properties. The computationally expensive Monte Carlo ray tracing (MCRT) method, which is widely used in this context, is replaced by Neural Networks (NN). We demonstrate that the data-driven surrogate prediction works accurately and generally. The results of both MCRT and NN models agree well with each other and with previously measured literature results. We also measure the uncertainty of the NN results using statistical methods. It is recommended that the developed model be used for efficient inverse problems and optimizations in relevant future work.

Adsorption of zinc ions on bone char using helical coil-packed bed columns and its mass transfer modeling

AbstractThis study reports the assessment of helical coil-packed bed columns for Zn2+ adsorption on bone char. Zn2+ adsorption breakthrough curves have been obtained using helical coil columns with different characteristics and a comparison has been conducted with respect to the results of straight fixed-bed columns. Results showed that the helical coil adsorption columns may offer an equivalent removal performance than that obtained for the traditional packed bed columns but using a compact structure. However, the coil diameter, number of turns, and feed flow appear to be crucial parameters for obtaining the best performance in this packed-bed geometry. A mass transfer model for a mobile fluid flowing through a porous media was used for fitting and predicting the Zn2+ breakthrough curves in helical coil bed columns. Results of adsorbent physicochemical characterization showed that Zn2+ adsorption on bone char can be attributed to an ion-exchange mechanism. In summary, helical coil columns appear to be a ...

Photocatalysis in a packed bed: Degradation of organic dyes by immobilized silver nanoparticles

Complete photocatalytic degradation of methyl orange, methylene blue and rhodamine chloride was accomplished using immobilized silver nanoparticles as catalysts and tethered calix[7]hydroquinone molecules as nano-reactors. The nanoparticles were immobilized on quartz beads that were assembled in a packed bed configuration. The large interfacial area afforded by the packed bed geometry, presentation of the nanoparticles as a single, tethered layer, and the optically transparent packing material facilitated intimate interaction between the catalyst, dye solution and radiation. The degradation was accomplished using either UV or visible radiation and the nanoparticles displayed self-cleaning and anti-poisoning properties thereby demonstrating excellent durability and catalytic efficiency. The single-layer coverage of Ag nanoparticles facilitates use of very small amounts of the photocatalyst for degradation of relatively large amounts of dye. The method addresses multiple issues that constrain scale-up of photocatalytic systems that deploy nanoparticles as catalysts.

Dynamic simulation of integrated rock-bed thermocline storage for concentrated solar power

In contrast to wind and photovoltaic, concentrated solar power plants can be equipped with thermal energy storage in order to decouple intermittent energy supply and grid feed-in. The focus of this study is the technical evaluation of a cost-efficient storage concept for solar tower power plants. Consisting of a quartzite-rock bed that is charged with a hot air flow and discharged by cold air counter-flow, the storage essentially operates like a regenerator. For such systems, the discharge temperature typically declines with time. Furthermore, the use of a randomly packed bed results in considerable pressure loss. In order to describe the relevant flow and heat transfer mechanisms in rock beds used for thermal storage, a mathematical model written in the modelling language Modelica is developed and validated. Good agreement with experimental data from literature is obtained. With the aid of the validated model, a rock-bed thermal storage for application in a semi-industrial scale solar power plant (1.5MWel) is designed and optimised with respect to electrical efficiency of the plant during the charge and discharge cycle. The storage capacity is equivalent to four hours of full-load operation. Results show that compressor work should be considered directly in the selection of packed-bed geometry in order to minimise the efficiency penalty of storage integration in the solar plant.

Wetting dynamics and contact angles of powders studied through capillary rise experiments

Wettability is an important property involved in the industrial use of granular solids and powders. It is commonly described with the contact angle and an experimental method for its determination in dynamic conditions is proposed in this work. The method is based on the capillary rise of the wetting liquid into a packed bed of the material under analysis. Differently from the classical Washburn method, the packed bed is closed to the atmosphere and the air pressure increase is measured allowing to evaluate the powder contact angle through a dynamic balance of the pressure forces. In the expression of such forces a new equivalent capillary radius for the powder bed is used based on an alternative definition of the particle equivalent diameter. This diameter is closely related to the length of the three phase line which divide the wet portion of the bed from the dry one and mirrors the physics of wetting process better than the classical Sauter diameter. A way to determine it with optical microscopy is given. Also the measure of the packed bed porosity (entering in the equivalent capillary radius definition) has been improved by using the effective porosity concept [Hapgood et al., J. Coll. Interface Sci. 253 (2002) 353–366] and by modifying the way of estimating it. The proposed experimental technique, coupled to the theoretical model for the packed bed, can describe accurately the packed bed geometry and the wetting dynamics by following the changes of the contact angle from its initial maximum value up to the final equilibrium one.

Plasma Heating and Temperature Difference Between Gas Pellets in Packed Bed With Dielectric Barrier Discharge Under Natural Convection Condition

A particular phenomenon was found in packed beds with dielectric barrier discharge (DBD): The temperature of the gaseous phase is significantly higher than that of the pellet phase. This is completely different from the conventional packed bed where the phases have essentially the same temperature. A simplified model was developed to estimate this phenomenon in the packed bed with DBD under natural convection. The power consumption mechanism was investigated. The mechanisms for the electrical energy of inlet plasma transformed into heat energy in the two phases, gaseous and pellets of the packed bed reactor, are different. The energy transformation in the gaseous phase includes electron absorption as well as ion absorption. The energy absorbed by electrons, including ohmic heating and stochastic heating, maintains plasma condition. The energy absorbed by ions transfers to other heavy particles with frequent collisions in plasma sheath and, finally, raises the macro temperature of the gas. The energy consumption in pellet phase is dielectric polarization loss and depends on packed bed geometry and DBD. These mechanisms of energy consumption are introduced into the packed bed heat transfer model to develop the simplified model. Some experiments are performed and the results agree well with the numerical simulation of this model.

Experimental study on temperature characteristics and energy conversion in packed bed reactor with dielectric barrier discharge

The temperature characteristics and energy conversion in packed bed reactor combined with a dielectric barrier discharge (DBD) plasma was investigated experimentally. The pellet temperatures of two types packed bed reactor, cylindrical reactor and parallel-plate reactor, was measured in conditions of various inlet voltage of DBD plasma. The relationship between pellet temperature of the packed bed and applied voltage of DBD plasma was discovered. The experimental result indicates a tendency that the pellet temperature of packed bed increases as the applied voltage of inlet plasma increases. When the voltage of inlet plasma is high enough, the pellet temperature increment decreases. Simultaneously,the packed bed temperature is sensitive to the inlet plasma energy and there is a potential application to heat exchanger. Moreover the proportion of energy consumption of plasma inputting into packed bed reactor was analyzed and calculated. The mechanisms that electrical energy of inlet plasma is transformed into heat energy in the two phases, gaseous and pellets of the packed bed reactor are different. The energy consumption in pellet phase is dielectric polarization loss and depends on packed bed geometry and DBD plasma etc. The energy consumption in gaseous phase is plasma sheath procedure. The important factors effecting on gas discharge are gaseous component and voltage, frequency of power.

Effects of porosity, pumping power, and L/ D ratio on the thermal characteristics of an N 2O catalytic igniter with packed bed geometry

In this study thermal characteristics of an N 2O catalytic igniter with packed bed geometry which is recently introduced as a hybrid ignition system for small satellites are theoretically considered. For the purpose a so-called porous medium approach has been opted for modeling the N 2O catalytic igniter with packed bed geometry. Using the Brinkman-extended Darcy equation with Ergun term for fluid flow and the one-equation model for heat transfer, both velocity and temperature distributions are presented. To ensure the validity of the approach, the calculated wall temperature solutions have been compared with previous experimental data. Based on the results, parameters of engineering importance such as the porosity, the pumping power and the ratio of length to diameter of the catalytic igniter are identified. Their effects on thermal performance of the catalytic igniter are systematically presented. Finally an effective volume flow rate has been defined to determine the optimum values of the parameters capable of delivering maximum catalyst performance.

Numerical treatment of pebble contact in the flow and heat transfer analysis of a pebble bed reactor core

This paper studies the numerical treatment of the inter-pebble regions in the modeling of a packed bed geometry for the computational fluid dynamics (CFD) analysis of a pebble bed reactor (PBR) core, where the pebbles are physically in contact with each other. In some studies, the inter-pebble regions have been approximated with gaps, in consideration of the problems on mesh quality or economy of the CFD calculation. To examine such a methodology, a sensitivity analysis for the gap size was conducted with two spherical pebbles, where the inter-pebble region was modeled by means of two kinds of inter-pebble gap and two kinds of direct contact. The cases of direct contact showed numerous differences in the results of the flow regime around the pebbles as well as in the wake, compared to the cases of the inter-pebble gap. No large differences were found between the two cases of direct contact. Based on the result of the sensitivity analysis, the two cases of inter-pebble modeling, i.e., the 1-mm gap and area-contact, were applied to the PBR simulation. It was concluded that the flow regimes and their relevant flow-induced local heat transfer were significantly dependent on the modeling of the inter-pebble region.

팩 베드 형상을 가지는 N2O 촉매 점화기의 열적현상

In this paper, thermal characteristics of a nitrous oxide (N₂O) catalytic reactor with packed-bed geometry are theoretically and numerically investigated. Several researchers experimentally presented that catalytic decomposition of N₂O in a packed bed generates about 82kJ/㏖e in the exothermic reaction. Based on the results they have studied the catalytic decomposition of N₂O in a packed bed to use it not only as a monopropellant thrust for small satellites but also as an igniter system for hybrid rockets. So we aim to identify important parameters existing in an N₂O packed-bed geometry, and to clarify its critical effect on thermal characteristics of the catalytic igniter using a porous medium approach.