Fixed bed reactors are widely applicable in a range of chemical process industries. Their ease of use and simplified operation make them an attractive and preferred option in reactor selection, however the geometric complexities within the bed as a result of the unstructured packing has made the design of such beds historically based on pseudo-homogenous models together with correlation-based transport parameters. Low tube-to-particle diameter ratio (N) beds, in particular, are selected for highly exothermic or endothermic reactions, such as in methane steam reforming or alkane dehydrogenation. Due to the large fraction of tube to catalyst particle contact in these low N beds, wall effects induce a mass transfer boundary layer at the wall, and in the case of thermal beds, a simultaneous resistance to heat transfer. Computational Fluid Dynamics (CFD) has been shown to be an accurate tool for experimental validation and predictive analysis of packed beds, and may be used to derive more accurate design parameters for fixed bed reactors. More specifically, the elucidation of dispersion, or the transport of reactant and product within the bed due to molecular diffusion and convective flow is of fundamental interest to the design of fixed beds. Computational Fluid Dynamics was used in this research to study solute dispersion in eight beds of varying N at a range of particle Reynolds numbers in the laminar flow regime. In the first stage of research, flow development was simulated in three-dimensional packed beds of spheres. Then, the reactor wall was sectioned to include a boundary condition of pure methane, from which the solute could laterally disperse into the bed. In the second stage, a two-dimensional representation of the bed was created using the commercial Finite Element Analysis software COMSOL Multiphysics. In these models, axial velocity profiles and radial methane concentration profiles taken from the 3-D models were supplied, and a fitting procedure by use of the Levenberg-Marquardt Least-Squares optimization algorithm was completed to fit radial dispersion coefficients and near-wall mass transfer coefficients to the CFD data. These optimization runs were conducted for all N at a number of bed depths in each case. Two sub-studies were conducted in which a constant velocity profile and a local velocity profile were supplied to the 2-D model, and the optimization re-run. It was found that this two parameter model did not fully account for various mechanisms of dispersion in the bed, namely the increasing rate of dispersion from the tube wall boundary layer up to the bed center, but only accounted for a diffusive-dispersion at the wall and a constant-rate, convective-dispersion everywhere else in the bed. Length dependency of dispersion coefficients were also noted, particularly in the developing sections of the bed. Nevertheless, the combined CFD and optimization procedure proved to be an accurate and time-efficient procedure for the derivation of dispersion coefficients, which may then lend themselves to the standard design of packed bed reactors.

Fixed bed reactors are widely applicable in a range of chemical process industries. Their ease of use and simplified operation make them an attractive and preferred option in reactor selection, however the geometric complexities within the bed as a result of the unstructured packing has made the design of such beds historically based on pseudo-homogenous models together with correlation-based transport parameters. Low tube-to-particle diameter ratio (N) beds, in particular, are selected for highly exothermic or endothermic reactions, such as in methane steam reforming or alkane dehydrogenation. Due to the large fraction of tube to catalyst particle contact in these low N beds, wall effects induce a mass transfer boundary layer at the wall, and in the case of thermal beds, a simultaneous resistance to heat transfer. Computational Fluid Dynamics (CFD) has been shown to be an accurate tool for experimental validation and predictive analysis of packed beds, and may be used to derive more accurate design parameters for fixed bed reactors. More specifically, the elucidation of dispersion, or the transport of reactant and product within the bed due to molecular diffusion and convective flow is of fundamental interest to the design of fixed beds. Computational Fluid Dynamics was used in this research to study solute dispersion in eight beds of varying N at a range of particle Reynolds numbers in the laminar flow regime. In the first stage of research, flow development was simulated in three-dimensional packed beds of spheres. Then, the reactor wall was sectioned to include a boundary condition of pure methane, from which the solute could laterally disperse into the bed. In the second stage, a two-dimensional representation of the bed was created using the commercial Finite Element Analysis software COMSOL Multiphysics. In these models, axial velocity profiles and radial methane concentration profiles taken from the 3-D models were supplied, and a fitting procedure by use of the Levenberg-Marquardt Least-Squares optimization algorithm was completed to fit radial dispersion coefficients and near-wall mass transfer coefficients to the CFD data. These optimization runs were conducted for all N at a number of bed depths in each case. Two sub-studies were conducted in which a constant velocity profile and a local velocity profile were supplied to the 2-D model, and the optimization re-run. It was found that this two parameter model did not fully account for various mechanisms of dispersion in the bed, namely the increasing rate of dispersion from the tube wall boundary layer up to the bed center, but only accounted for a diffusive-dispersion at the wall and a constant-rate, convective-dispersion everywhere else in the bed. Length dependency of dispersion coefficients were also noted, particularly in the developing sections of the bed. Nevertheless, the combined CFD and optimization procedure proved to be an accurate and time-efficient procedure for the derivation of dispersion coefficients, which may then lend themselves to the standard design of packed bed reactors.

<p> An Experimental investigation has been conducted to determine the effects on degree of reduction and efficiency of utilization of gases of changes in the levels of several variables when reducing fixed beds of commercial hematite pellets. The effects on reduction behaviour of changes in the levels of reducing gas composition and flow rate, and reaction temperature, were determined by graphical and numerical techniques.</p> / Thesis / Master of Engineering (MEngr)

This thesis presents an investigation on multiphase reactor hydrodynamics using magnetic resonance imaging (MRI). The study demonstrates experimental techniques by which computational and quasi-analytical fluid models may be validated. Three types of industrially-important multiphase reaction vessels are considered: a co-current upflow gas-liquid-solid bed, a co-current downward trickle bed (gas, liquid, solid), and a gas-solid fluidised bed. These reactors were selected as they commonly demonstrate local hydrodynamic anisotropy which affects the global performance of industrial units. MRI was used to obtain 2D velocity images of the gas and liquid phases in the packed beds, and of the gas and the solid phases in the fluidised bed. This study reports the first spatially resolved velocity measurements of both the gas and liquid phases in a co-current upflow bed, and the gas and solid phases of an isolated bubble in a fluidised bed. The experimental vessels were: 52 mm in diameter using 5 mm glass spheres in the upflow bed at 8 bara, 27 mm with 5 mm glass spheres in the trickle bed at 6.75 bara, and 52 mm using 1.2 mm poppy seeds as the fluidised particles at 8.5 bara. The experiments were conducted at a laboratory temperature of 25.0 ± 3.0 °C. In the upflow bed, time-averaged velocity images were acquired over a 2.5 h experimental time. This was done to capture the steady state behaviour of the vessel operating in the pulsing flow regime. The temporally-stable trickle flow state in the trickle bed was imaged over 15-100 minutes. In both packed beds, severe spatial anisotropy in the distribution of flow between pores was revealed. Furthermore, the data were used to determine classical design features such as catalyst wetting and liquid holdup which compared well with literature models. The trickle bed data were further analysed using a morphological algorithm which unambiguously identified the gas-liquid and liquid-solid interfaces. The interfacial flow fields were found to be similar to the bulk flow, with most voxels exhibiting static behaviour. The amount of interaction between the phases was found to be minimal, which is typical of the low interaction regime. A single bubble injection system was employed in the fluidised bed which allowed the injection of isolated bubbles into the incipiently fluidised bed. It also enabled the triggered acquisition of NMR data at precise time intervals. The bubble was found to be an indented ellipsoidal shape, which rose with atypical behaviour which caused it to collapse. Rise velocity was found to be consistent with theory, and the injected bubbles were sufficiently spatially reproducible to acquire 2D velocity images using single-point imaging. These velocity images showed flow behaviour characteristic of a 'fast' rising bubble, with a gas recirculation cloud 37 mm in diameter. The particle field was shown to have very high flow in the bubble wake, revealing the mechanism of bubble collapse. The flow data were compared to classical two-phase fluidisation theory, which revealed noteworthy differences in the division of flow between the particulate and bubbling regions.

Ces travaux de recherche s'intéressent à la caractérisation des transferts thermiques dans les milieux fluide-particules, et en particulier, les lits fluidisés au sein desquels un solide divisé est mis en suspension par un fluide. La grande diversité d'échelles spatiales et temporelles dans ces procédés nécessite d'étudier les interactions hydrodynamiques, thermiques et/ou chimiques entre les particules et le fluide à l'aide d'une approche multi-échelles. Une étude des transferts thermiques dans des lits fixes puis fluidisés, est réalisée à deux échelles : locale (Particle Resolved Simulation) et moyennée (Discrete Element Method-Computional Fluids Dynamics). L'étude PRS permet de caractériser les couplages locaux des transferts thermiques entre particules ainsi que la dynamique de ces transferts dans les configurations fluidisées. Une étude comparative entre les échelles met en évidence les limites du modèle DEM-CFD à capter les fluctuations des transferts thermiques observées dans les simulations PRS. Dans un dernier temps, les fermetures du modèle DEM-CFD sont améliorées de manière à réintroduire les fluctuations perdues par le changement d'échelles. / This work aims at characterizing heat transfer into fluid-solid flows, and more particularly fluidized beds, into which a solid phase is suspended by a flowing fluid. The wide range of spatial and temporal scales present in such processes encourage to study hydrodynamic, thermal and/or chemical interactions between the particles and the fluid through a multi-scale strategy. The analysis of thermal interactions was first carried out for fixed bed configurations and then, fluidized beds at two overlapping scales: local (PRS; Particle Resolved Simulation) and mesoscopic (DEMCFD; Discrete Element Method-Computional Fluids Dynamics). The PRS approach accounts for the local coupling of heat transfer between the particles and its dynamics into fluidized beds. A comparative study of the two scales indicated the limits of the DEM-CFD model to capture the heat transfer fluctuations observed into PRS. In a last step, the closure laws for DEM-CFD were improved to reintroduce the fluctuations lost at this scale.