1,721,112 research outputs found
Fixed bed reactors of non-spherical pellets: Importance of heterogeneities and inadequacy of azimuthal averaging
No full textDespite the substantial simplicities inherent in pseudo-continuum models of fixed bed reactors, there is a continued interest in the use of such models for predicting fluid flow and transport scalars. In this paper, we aim to quantitatively address the inadequacy of 2D pseudo-continuum models for narrow-tube fixed beds. We show this by comparing with spatially resolved 3D results obtained by a robust and integrated numerical workflow, consisting of a sequential Rigid Body Dynamics and Computational Fluid Dynamics (RBD-CFD) approach. The RBD is founded on a physics-based hard-body packing algorithm, recently proposed by the authors (Moghaddam, E.M., Foumeny, E.A., Stankiewicz, A.I., Padding, J.T., 2018. A Rigid Body Dynamics Algorithm for Modelling Random Packing Structures of Non-Spherical and Non-Convex Pellets. Ind. Eng. Chem. Res. 57, 14988–15007), which offers a rigorous method to handle resting contacts between particles. The methodology is benchmarked for simulations of flow fields in all flow regimes, for 5 ≤ Rep ≤ 3,000, in random packings of spheres and cylinders with tube-to-pellet diameter ratios, N, between 2.29 and 6.1. The CFD results reveal a remarkable influence of local structure on the velocity distribution at the pellet scale, particularly in low-N packings, where the spatial heterogeneity of the structure is very strong along the bed axis. It is also demonstrated that azimuthal averaging of the 3D velocity field over the bed volume, which has been considered as an advancement over plug flow idealization in classical pseudo-continuum models, cannot reflect the role of vortex regions emerging in the wake of the pellets, and leads to underestimation of the local velocity values by more than 400% of the inlet velocity.Large Scale Energy StorageIntensified Reaction and Separation SystemsComplex Fluid Processin
Spherical versus elongated particles – Numerical investigation of mixing characteristics in a gas fluidized bed
No full textThe possibility to offer good intermixing between particles is one of the main properties that make fluidized beds such an important industrial appliance. In this work, we use CFD-DEM simulations to compare mixing characteristics of spherical (AR-1) to elongated spherocylindrical particles (AR-4) of aspect ratio In simulation of AR-4 particles, single-particle and multi-particle correlations for hydrodynamic forces are tested. The results show that elongated particles have more vigorous intermixing and lower mixing times compared to spherical particles. Multi-particle correlations have a slight effect on particle mixing, and they increase the difference between AR-1 and AR-4 particles at higher gas velocities. Including hydrodynamic lift force and torque in the case of AR-4 particles leads to more vigorous mixing and lower mixing times.Complex Fluid Processin
Hydraulic modelling of liquid-solid fluidisation in drinking water treatment processes
No full textIn drinking water treatment plants, multiphase flows are a frequent phenomenon. Examples of such flows are pellet-softening and filter backwashing where liquid-solid fluidisation is applied. A better grasp of these fluidisation processes is needed to be able to determine optimal hydraulic states. In this research, models were developed, and experiments performed to gain such hydraulic knowledge. As a result, treatment processes can be made more flexible. In a rapidly changing environment, drinking water production must be flexible to ensure robustness and to tackle challenges related to sustainability and long-term changes. In the hydraulic models, the voidage in the fluidised bed and the particle size of the suspended granules are crucial variables. Voidage prediction is challenging as the fluidised bed is a dynamic environment showing highly heterogeneous behaviour that is hard to describe with an effective model. And particle size causes a conundrum due to the irregular shapes of the applied granules. Through the combination of hydraulic dimensionless Reynolds and Froude numbers, an accurate voidage prediction model has now been developed. With a straightforward pseudo-3D image analysis for non-spherical particles measuring particle mass and density, the dimensioned shapes of, for instance, ellipsoids can be determined. Particle shape factors included in models are not constant as is commonly believed, but dynamic. Applying advanced computational fluid dynamics simulations confirmed significant heterogeneous particle-fluid patterns in fluidised beds. Comprehensive sedimentation experiments showed that the average drag coefficient and terminal setting velocity of individual grains can be estimated reasonably well, but with a significant degree of data spread around the mean values. For engineering purposes, this is relevant information which should be taken into consideration. A new soft-sensor was designed to determine the voidage gradient and particle size profile in a fluidised bed. The expansion degree of highly erratic, polydisperse and porous granular activated carbon grains can be predicted with a model, but in full-scale processes the grains are subject to change, and therefore it is most likely that the prediction accuracy will deteriorate rapidly. For reliable drinking water quality, smart models provide solutions to complex challenges, but they are only effective when they are calibrated and validated in advanced pilot plants and are applied in full-scale processes with diligence and commitment on the part of multidisciplinary teams.Complex Fluid ProcessingSanitary Engineerin
Spheres vs. rods in fluidized beds: Numerical and experimental investigations
No full textFor the past century, fluidized beds have been standard equipment in many branches of industry. In most applications they are used to manipulate granular and powder-like materials, whose particles can roughly be approximated as spheres. Therefore, numerical models and investigations have focused mainly on fluidized beds with spherical particles. Recent decades witnessed an increase in the use of fluidized beds in biomass processing. Unlike other materials typically used in fluidized beds, biomass is characterized by relatively large and elongated particles. For the sake of simplicity, numerical models for simulating fluidization of elongated particles have so far neglected a lot of specifics that can occur during this process and even applied the same models and conclusions that were developed for fluidization of spherical particles. The goal of this thesis is to define what is necessary for performing physically correct Computational Fluid Dynamics - Discrete Element Model (CFD-DEM) simulations of elongated particles fluidization. This thesis emphasizes the difference in fluidization between spherical and elongated particles and looks into ways to include specific particle and fluid interactions related to elongated particles into numerical (CFD-DEM) model. Results fromCFD-DEMsimulationswere validated using two experimental techniques, magnetic particle tracking (MPT) and X-ray tomography (XRT). This thesis is part of larger project of multi-scale modeling of fluidized beds with elongated particles and is focusing on the middle scale, bridging fully resolved, direct numerical simulations (DNS) with large scale, two fluid model (TFM) or multi-phase - particle in cell (MP-PIC) models, capable of simulating industrial sized fluidized beds. This thesis first looks in to the effect of including shape induced lift force and hydrodynamic torque, which were so far neglected in CFD-DEM simulations of elongated particles. It is shown that including lift force and hydrodynamic torque leads to considerable changes in the particle vertical velocity and particle preferred orientation in the fluidized bed. Looking into the mixing characteristics, as one of the most important parameters of fluidized beds, also considerable differenceswere found. Further differences in fluidization behaviour of spherical and elongated particles, as well as the effect of increasing particle aspect ratio, were shown experimentally, using MPT. Clear differences between spherical and elongated particles were found concerning the particle velocity and rotational velocity distributions. The effect of increasing particle aspect ratio and gas inlet velocity on fluidization of elongated particles was shown. Using XRT, the difference in bubbling and slugging fluidization between spherical and elongated particles was shown. In the end, the effect of newly developed multi-particle correlations for hydrodynamic forces and torque was tested, and it is concluded that they can improve the accuracy of simulations of dense fluidized beds containing elongated particles. The findings of this thesis clearly show that the models and assumptions developed for fluidization of spherical particles cannot simply be transferred to the fluidization of elongated particles. The results presented here give a new insight in the fluidization of elongated particles. They are also valuable for validation and development of larger scale models capable of simulating industrial size fluidized bed with elongated particles.Complex Fluid Processin
Elongated particles in fluidized beds: From lab-scale experiments to constitutive models
No full textGas-solid fluidized beds are widely used in various industries due to their favourable mixing, and mass and heat transfer characteristics. Fluid catalytic cracking, polymerization, drying, and granulation are a few examples of their applications. In recent years, there has been increased application of fluidized beds in biomass gasification and clean energy production. Fluidization has been extensively studied, experimentally, theoretically and numerically, in the past. However, most of these studies focused on spherical particles while in practice granules are rarely spherical. Particle shape can have a significant effect on fluidization characteristics. It is therefore important to study the effect of particle shape on fluidization behavior in detail. One of the main reasons we still do not completely understand the fluidization phenomenon is because of complex hydrodynamic interactions and its large separation of scales. Industrial fluidized bed reactors of tens of meters in diameter can have hydrodynamic scales varying from micrometers to meters. Experimental setups of such large size are extremely expensive and therefore not practical. On the other hand, theoretical and empirical correlations are not accurate for scale-up and are rarely available for non-spherical particle shapes. Because of this, we need a different approach. One that takes advantage of experimental measurements and numerical simulations. The tasks are divided into three parts based on scales, each focusing on a particular aspect : DNS (direct numerical simulation), CFD-DEM (computational fluid dynamics - discrete element model) and TFM (two fluid model) or MP-PIC (multi-phase - particle in cell). In this thesis, the focus is on CFD-DEM modelling, a ’bridge’ that connects the DNS and TFM/MP-PIC models.Intensified Reaction and Separation System
Adsorption and Electrokinetics at Silica-Electrolyte Interfaces: A Molecular Simulation Study
No full textExperimentally investigating the nanoscale behavior at oxide-electrolyte interfaces has proven to be extremely challenging. Molecular Dynamics (MD) simulations have arisen as a potential computational alternative to gain atomic level insights at these interfaces. But how accurately do these simulations represent the physics and chemistry at the interface? In many situations we do in fact not know. Validation at the interface remains challenging. The force fields used in MD simulations, that describe the inter-particle interactions, are generally optimized for purposes deviating considerably from interfaces. Yet, these same force fields are blindly used to model surface-fluid interactions, yielding wildly varying results of for example ion adsorption. This dissertation tackles the problem of simulating interfaces by critically looking at MD simulations and proposing novel solutions, both for MD simulations in general and specifically targeting their validity and limitations with regards to modeling interfaces…Complex Fluid Processin
Multiphase Flow Modelling of Electrochemical Systems: an analytical approach
No full textThe primary objective of this work is to provide new analytical models to support the theoretical understanding of multiphase flows in various electrochemical systems. Most of the previous works in this field use either experiments or numerical simulations to understand the hydrodynamics of the multiphase flows. In this thesis, various new analytical models are derived for different cell configurations such as PEM diffusion layer, flow-through electrolysers, parallel plate electrolyzers and zero-gap electrolysers. New design equations are provided that can be readily used as a first estimate for a new electrochemical cell. The novelty of this work lies in providing new analytical approaches to develop a theoretical understanding of electrochemical cells
Kidney stone in a chip: Understanding calcium oxalate kidney stone formation
No full textKidney stone formation is a global health problem with increasing prevalence. Stone formation is a physiochemical process involving crystallization of inorganic salts in the presence of biological constituents in the urinary system. To inhibit kidney stone formation, a better understanding of the underlying physicochemical mechanism of stone formation in the kidney is required. In this thesis, the solubility, nucleation and growth of calcium oxalate (CaOx), the most common inorganic constituent of kidney stones, were studied under different conditions such as ion concentration, pH value, and also the role of inhibitors in water or artificial urine was investigated. The first step towards this work was obtaining the solubility curve of calcium oxalate monohydrate (COM) in the solvent, such as ultrapure water and different buffers, to elucidate the physicochemical conditions which can cause the kidney stone formation (Chapter 2). Beside the solubility study, advanced technology to observe crystal formation in small scale and a very short time was needed. The volume, structure and flow properties inside the kidney inspired us to use microfluidic technology with comparable volume and flow rate. The developed microfluidic devices that mimic pathways in the human kidney were used to study the nucleation and growth of calcium oxalate crystals. The developed devices rendered an alternate perspective to the study of kidney stone formation and showed that microfluidics can provide precise, simple and fast detection of stone formation under various experimental conditions. Initially, the designed microfluidic device allowed us to build a testing platform for the study of nucleation kinetics of CaOx inside isolated environments provided by droplets. Preliminary experiments were performed by dissolving calcium chloride and sodium oxalate in ultrapure water. The aqueous solution, containing the ions, forms the droplet phase and oil were used as the continuous phase. Altering the pH values, as well as increasing the concentration of additives such as magnesium and osteopontin (OPN), were shown to slow down the nucleation kinetics, or even inhibit nucleation (Chapter 3). Complex Fluid Processin
Computational modelling and parameter estimation in fixed beds of non-spherical pellets
No full textFixed bed arrangements find wide applications, particularly in reaction engineering, where they are employed as multi-tubular catalytic reactors for the transformation of reactants into desired products. The importance of such complicated reactors can be realized by their extensive applications as the process workhorse in various industries, e.g. chemical, pharmaceutical and petrochemical. The design of such systems is predominantly rooted in macroscopic models, e.g. pseudo-continuum approaches, with effective parameters extracted from averaged semi-empirical correlations. However, such simplistic design procedures are inadequate for design of tubular fixed beds with low tube-to-particle diameter ratios, say dt/dp<10, where lateral heterogeneities of the tortuous structure lead to dominance of localised phenomena. These local or “pellet-scale“ effects cannot be captured nor explained by pseudocontinuum models, and call for 3D spatially-resolved simulations of flow and transport scalars. However, majority of the prevailing efforts within the context of “particle-revolved CFD simulation”, have dealt with fixed beds of spheres, because generating random packing of nonspherical pellets necessitates a cumbersome and complicated strategy to account for the orientation freedom of such pellets, specifically when collisions occur...Large Scale Energy Storag
Direct numerical simulations of flow around non-spherical particles
No full textThis work focuses on creating a recipe for parametrizing flow around assemblies of non-spherical particles. A multi-relaxation time lattice Boltzmann method (MRT-LBM) is used to simulate the flow. The research focuses on 3 different developments. First, different boundary conditions available in the literature for LBM are tested to identify the best for the flow problem. The second part of the thesis focuses on developing more widely applicable scaling laws for drag and lift of various isolated non-spherical particles. In the third part, a recipe to describe hydrodynamic forces on assemblies of axisymmetric, non-spherical particles is proposed. With the described parameters, drag, lift and torque correlations are proposed accordingly. This research is funded by the European Research Council under its consolidator grant scheme, contract no. 615096 (NonSphereFlow).Complex Fluid Processin
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