Fluid dynamic modelling of bubble column reactors

Numerical simulations of rectangular shape bubble column reactors (BCR) are validated starting from preliminary simulations aimed at identifying proper simulation parameters for a given system and resulting up to the numerical simulation with mass transfer and chemical reactions. The transient, three dimensional simulations are carried out using FLUENT software and the results obtained for a system with low gas flow rate (48 L/h) indicated that we need enough fine mesh grid and appropriate closure of interfacial forces to predict reliably plume oscillation period, liquid axial velocity and gas holdup profiles. In case of high flow rate (260 L/h), we compared the results for the effect of different interfacial closure forces and change in inlet boundary condition for gas volume fraction. There is no change in hydrodynamic results when there is change in gas volume fraction at inlet boundary condition. The effect of virtual mass interfacial force on the simulation results is also negligible. However, the major effects of applying lift force on results of plume oscillation period, liquid axial velocity and gas holdup is predicted. For comparable simulation results to experimental data, it is suggested that requirement of enough fine grids and appropriate correlations for interfacial forces, especially the combination of drag and lift forces is necessary. To study the bubble size distribution in BCR the numerical simulations are carried out with QMOM population balance technique for air-water fluid system. After finalization of the generic moment boundary conditions with simulations with PBM using QMOM without breakage and coalescence phenomena, then we simulated the system with breakage and coalescence and eventually, the simulation results are compared with experimental and simulation data taken from the scientific literature. For better hydrodynamics results of BCR as compared to experimental results, the interfacial lift force with combination of drag force is predicted for QMOM. The discretization scheme for gas volume fraction and moments of first order upwind provided the expected results of bubble size distribution. The simulation result of QMOM with breakage and coalescence models were also in good agreement with hydrodynamics experimental results and simulation results of class methods and DQMOM for bubble size distribution results. The modelling of chemical absorption of pure CO2 gas in caustic solution is carried out in a rectangular BCR with identical simulation parameters settings of previous work. For applicability of available kinetic and physical data we developed concentration differential equations to estimate the species molar concentration with respect to time in MATLAB code. The obtained profiles of evaluation of concentration and pH were in similar fashion as compared to available CFD simulated concentration and pH profiles at a point in the bubble column with respect to time. CFD simulation taking into account the mass transfer and chemical reaction, the E-E approach is used with assumption of uniform bubble size for modelling of chemisorption of the CO2 gas bubbles into NaOH aqueous solution. The adopted models successfully predicted the hydrodynamics results and are in good agreement with experimental and simulation results, however, reaction processes results are not as per expectation and further improvement in adopted simulation methods is required for better result

Numerical Investigation of Particles Breakage and Growth in Gas-Solid Processes: Spiral Jet Milling and Polyolefin Polymerization in Fluidized Bed Reactors

L'abstract è presente nell'allegato / the abstract is in the attachmen

Computational fluid dynamic modeling of fluidized bed polymerization reactors

Polyethylene is one of the most widely used plastics, and over 60 million tons are produced worldwide every year. Polyethylene is obtained by the catalytic polymerization of ethylene in gas and liquid phase reactors. The gas phase processes are more advantageous, and use fluidized bed reactors for production of polyethylene. Since they operate so close to the melting point of the polymer, agglomeration is an operational concern in all slurry and gas polymerization processes. Electrostatics and hot spot formation are the main factors that contribute to agglomeration in gas-phase processes. Electrostatic charges in gas phase polymerization fluidized bed reactors are known to influence the bed hydrodynamics, particle elutriation, bubble size, bubble shape etc. Accumulation of electrostatic charges in the fluidized-bed can lead to operational issues. In this work a first-principles electrostatic model is developed and coupled with a multifluid computational fluid dynamic (CFD) model to understand the effect of electrostatics on the dynamics of a fluidized-bed. The multifluid CFD model for gas-particle flow is based on the kinetic theory of granular flow closures. The electrostatic model is developed based on a fixed, size-dependent charge for each type of particle (catalyst, polymer, polymer fines) phase. The combined CFD model is first verified using simple test cases, validated with experiments and applied to a pilot-scale polymerization fluidized bed reactor. The CFD model reproduced qualitative trends in particle segregation and entrainment due to electrostatic charges observed in experiments. For the scale up of fluidized bed reactor, filtered models are developed and implemented on pilot scale reactor

Doctor of Philosophy

dissertationFroth flotation is a highly complex, multiphase, and multiscale process that is usually performed in large tanks called mechanical flotation cells. The aim of this research is to investigate the single and multiphase flow hydrodynamics in lab scale flotation cells by decoupling the hydrodynamics from physicochemical effects. Both experimental and numerical approaches are used to study the behavior of flows in lab and pilot scale flotation cells. Nonintrusive experimental techniques such as particle image velocity (PIV) and electrical resistance tomography (ERT) techniques are used to measure flow velocities, solids holdup, mixing efficiency, and to interpret flow pattern. Eulerian-Eulerian computational fluid dynamics (CFD) models are developed and tested for solid-liquid (slurry) and gas-liquid flows in stirred tanks and flotation cells. Using single phase CFD simulations, the effect of flotation specific impeller blade shape and impeller size on mean flow and pumping behavior is tested in lab scale flotation cells for the first time. In the absence of a stator, the mean flow is found to transition from radial to axial type flow when the off-bottom clearance is below the critical value. This prediction is experimentally verified using time averaged PIV data. Based on the analysis of pumping and power number data, the rectangular shaped blade design is found to be the most efficient. The impeller blade shape is found to critically affect the flow in the vicinity of the impeller and a design with the largest surface area is needed to create an intense turbulence zone, needed for mixing and dispersion of incoming air. Eulerian-Eulerian CFD model is used to study the solid phase suspension and mixing characteristics for monosized silica particles. Experimental comparison with the results from the literature for stirred tanks and in-house ERT measurements suggest that the model performs reasonably well. Population balance equation model (PBM) is coupled with CFD to study gas dispersion, mixing, and local bubble size distribution in the stirred tank and flotation cell using quadrature method of moments (QMOM) approach in ANSYS Fluent solver. The default QMOM model in Fluent is found to be inaccurate due to independent solution of moment transport equations and therefore is supplied with a moment correction algorithm from the literature to successfully identify and correct the invalid moment sequence during the CFD simulation. The new model is found to be superior to the current models in its ability to satisfactorily predict the overall gas holdup and local bubble size distribution for stirred tanks under moderate aeration and agitation rates. This model is extended to study the development of flow regimes based on the gas dispersion pattern in a generic flotation cell. Though highly useful, the coupled CFD-PBM approach is computationally intensive and requires considerable effort to achieve an accurate solution. This motivated us to develop a PBM based on the high-order moment conserving method of classes (HMMC) approach for a pilot scale XCELL flotation cell for frother concentration over critical coalescence concentration, thus, only considering breakage of bubbles. Nonlinear optimization solvers in Matlab are used to calculate the point estimates of adjustable parameters in breakage models. The 95% bootstrap calculated using empirical bootstrap indicates very high confidence in estimated parameters. The HMMC model provides an accurate description of steady state bubble size distribution and the mean number diameters only using overall gas holdup and specific energy as inputs

Process Simulation of Technical Precipitation Processes - The Influence of Mixing

This work develops and shows up methods to tackle multi-scale challenges in particle formation during precipitation crystallization. Firstly, molecular, micro- and meso-scale interactions in confined impinging jet mixers are investigated and simulatively predicted. Secondly, to build up on developed methods, macroscale as present for instance in stirred tank reactors is added to the considerations

Computational Engineering for Nuclear Solvent Extraction Equipment

The ultimate objective of this work is to leverage modern computational tools to provide a unique and contemporary approach to pulse sieve-plate extraction column (PSEC) design and optimisation. Particular attention is given to providing novel analysis on: the functionality of operation, methods of performance analysis, determination of flooding, and development of simulation approaches that faithfully represent PSEC hydrodynamic behaviour. A detailed assessment is undertaken of the dispersive mixing and turbulence characterisation of an industrially representative PSEC. This is achieved with computational fluid dynamics (CFD) running turbulence resolving large eddy simulation (LES), coupled with the volume of fluid (VOF) multiphase approach. Found was the dependency of PSEC functionality on turbulence production, and not on the viscous plate-induced stresses, generated therein. Consequently, the standard round-hole sieve-plate design is found to perform poorly at producing and distributing the types of flow and turbulence beneficial to droplet size reduction. This milestone discovery marks the first explicit contribution to knowledge of PSEC operation in decades. Subsequently, a number of typical unsteady Reynolds averaged Navier-Stokes (URANS) turbulence modelling methods were compared against the benchmark LES. The URANS models, highly representative of the available PSEC CFD literature, were not able to produce agreeable solutions in the important hydrodynamic characteristics of the flows. Therefore the standard has been set for turbulence characterisation in PSEC simulation with LES. The appropriate LES VOF method was carried forward to a campaign of 25 unique case runs that resulted in synergistically rich data set. Novel means of flooding identification was developed and tested. From this a number statistical analysis methodologies were employed to develop tools which successfully resolve the operational envelope and diagnose the likelihood of flooding during operation based on easily measurable variables. Lastly, a state-of-the-art two-fluid hybrid VOF/Eulerian-Eulerian multiphase CFD model, with population balance, was implemented and interrogated. The model was successful in capturing all scales of the multiphase behaviour to further improve the faithful description of the complete fluid interactions. The population balance produced predictions for the droplet size distributions inline with available examples from literature and therefore provides exciting opportunities for accurate mass transfer predictions in pulse column simulations

Coupling Hydrodynamic and Biokinetic Growth Models in Aerated Wastewater Treatment Processes

In this thesis, a coupled hydrodynamic and wastewater biokinetic finite volume based CFD model for an aeration tank in OpenFOAM has been created to understand the effect of the hydrodynamics on the biological processes. A pilot-scale aeration tank that is aerated using fine membrane diffusers along the base has been designed and manufactured. A procedure for conducting lab experiments using an acoustic Doppler velocimeter to record velocity measurements was outlined. A series of aeration tank experiments with flow rates ranging from 18 – 108 L/min through membrane diffuser setups that involved 1 or 3 diffusers were conducted in which ADV velocity measurements were taken and have been used to validate a CFD model. Additionally, it was found that certain diffuser configurations showed pseudo - 2D behaviour such that the recorded data could be used to validate 2D simulations of the aeration tank. A CFD model using the Eulerian-Eulerian multiphase formulation in OpenFOAM was created to replicate the bubble driven fluid flow and free surface effects in the pilot-scale aeration tank. The influence of the inlet conditions, bubble diameter size and bubble dynamic models on the generated results were investigated and compared with the experimental data to validate the modelling choices. As a result, a 2D and 3D CFD model of the aeration tank was defined and validated against the experimental ADV data. Using the results, a procedure for coupling the biokinetics into the hydrodynamics was described in OpenFOAM. The difficulties that arose from transferring a two-phase solution with a free surface to a single-phase solver was outlined and solutions to the issues were defined and assessed. The mass transfer of oxygen into the fluid was modelled and compared with experimental results from the membrane diffuser manufactures to confirm the accuracy of the model. The oxygen mass transfer model was used to assess how the membrane diffuser setup and flow rate impacts the oxygenation of the reactor. It was found that increasing the number of aerating diffusers while keeping the total air flow rate the same significantly increased the oxygenation of the tank in comparison to just increasing the air flow rate which was found to only slightly increase the oxygenation. Additionally, a curve fitting procedure was described to derive a global oxygen transfer rate coefficient and saturation value from the CFD simulation for specific aeration tank setups and assessment of the values found they could give insight to the hydrodynamic behaviour in the reactor. The simulations were further extended to include the biokinetics to describe the biological interactions. A simple biokinetic aeration model was proposed to assess the impact of the hydrodynamics, inlet and outlet locations, and flow rate on the biological processes in tank. It was found that inadequate mixing in the 2D simulation resulted in twice the required amount of time to reach the maximum biomass concentrations compared with the equivalent perfectly mixed reactor. It was shown that the location of the inlet and outlet with the same hydrodynamic flow fields could influence the biological processes. It was found that there was no difference in the biological performance of the 3D reactor with an aerating flow rate of 0.3 and 0.6 L/s such that it would be inefficient to aerate the tank at 0.6 L/s. Finally, the full ASM1 was implemented into the coupled model and compared with the conventional ASM1 model to assess the performance of the aeration tank at producing and removing nitrates and ammonium. It was found that inadequate mixing resulted in reduced efficiency of the reactor at producing and removing nitrates and ammonium, respectively, which would further impact the performance of the sequential rectors

Second Microgravity Fluid Physics Conference

The conference's purpose was to inform the fluid physics community of research opportunities in reduced-gravity fluid physics, present the status of the existing and planned reduced gravity fluid physics research programs, and inform participants of the upcoming NASA Research Announcement in this area. The plenary sessions provided an overview of the Microgravity Fluid Physics Program information on NASA's ground-based and space-based flight research facilities. An international forum offered participants an opportunity to hear from French, German, and Russian speakers about the microgravity research programs in their respective countries. Two keynote speakers provided broad technical overviews on multiphase flow and complex fluids research. Presenters briefed their peers on the scientific results of their ground-based and flight research. Fifty-eight of the sixty-two technical papers are included here