Models bubble columns

The proneness of the primarily produced minute gas bubbles to coalescence depends on three parameters a) on the size of the primarily produced gas bubbles b) on the material system c) on the state of flow in the bubble column [50]. Therefore, fixing the minimum diameter of a bubble column is surely not a sufficient criterion. One would have to carry out preliminary tests in differently scaled columns to find out the necessary minimum size of the model bubble column, see also Example 34. 

Hagesaether et al [29, 30] adopted this approach modeling bubble column dispersions and found that with the choice of parameter values used in their... 

Bertola F, Grundseth J, Hagesaether L, Dorao C, Luo H, Hjarbo KW, Svend-sen HF, Vanni M, Baldi G, Jakobsen HA (2005) Numerical Analysis and Experimental Validation of Bubble Size Distribution in Two-Phase Bubble Column Reactors. Multiphase Science Technology 17(1-2) 123-145 Breim G, Braeske H, Durst F (2002) Investigation of the unsteady two-phase flow with small bubbles in a model bubble column using phase-Doppler anemometry. Chem Eng Sci 57(24) 5143-5159... 

Adopting the approach used by Bhavaraju et al. [69] in modeling bubble columns, Sano and Mori [70] andMurty et al. [71] assumed the rate of energy dissipation to be equal to the sum of the rate of energy dissipation associated with liquid circulation and that due to bubble sUp. 

Jakobsen HA, Lindborg H, Dorao CA (2005) Modeling bubble column reactors progress and limitations. Ind Eng Chem Res 48 5107-5151... 

Brenn G, Braeske H, Durst F (2002) Investigation of the unsteady two-phase flow with small bubbles in a model bubble column using phase-Doppler anemometry. Chem Eng Sci 57 5143-5159... 

The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model was then applied to the precipitation of CaC03 via CO2 absorption into Ca(OH)2aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution. 

Rigopoulos, Stelios and Alan G. Jones, 2001. Dynamic Modelling of a Bubble Column for Particle Fonuation via a Gas-Liquid Reaction. Chemical Engineering Science (in press). 

Tadaki and Maeda (Tl) examined the desorption of carbon dioxide from water in a bubble-column and analyzed the experimental results under the assumption that while the gas phase moves in piston flow, the liquid undergoes axial mixing that can be characterized by the diffusion model. (Shulman and Molstad, in contrast, assumed piston flow for both phases.) Only poor agreement was obtained between the theoretical model and the experimental... 

Owing to the high computational load, it is tempting to assume rotational symmetry to reduce to 2D simulations. However, the symmetrical axis is a wall in the simulations that allows slip but no transport across it. The flow in bubble columns or bubbling fluidized beds is never steady, but instead oscillates everywhere, including across the center of the reactor. Consequently, a 2D rotational symmetry representation is never accurate for these reactors. A second problem with axis symmetry is that the bubbles formed in a bubbling fluidized bed are simulated as toroids and the mass balance for the bubble will be problematic when the bubble moves in a radial direction. It is also problematic to calculate the void fraction with these models. 

Ekambra etal. [21] compared the results from ID, 2D, and 3D simulations of a bubble column with experimental results. They obtained similar results for holdup and axial velocity, while eddy viscosity, Reynolds stresses, and energy dissipation were very different in the three simulations as shown in Figure 15.7. This example also illustrates the importance of selecting the right variables for model vahdation. A 2D model will yield good results for velocity but will predict all variables based on turbulent characteristics poorly. 

The main part of the report describes the results of systematic investigations into the hydrodynamic stress on particles in stirred tanks, reactors with dominating boundary-layer flow, shake flasks, viscosimeters, bubble columns and gas-operated loop reactors. These results for model and biological particle systems permit fundamental conclusions on particle stress and the dimensions and selection of suitable bioreactors according to the criterion of particle stress. 

Airlift loop reactor (ALR), basically a specially structured bubble column, has been widely used in chemical industry, biotechnology and environmental protection, due to its high efficiency in mixing, mass transfer, heat transfer etc [1]. In these processes, multiple reactions are commonly involved, in addition to their complicated aspects of mixing, mass transfer, and heat transfer. The interaction of all these obviously affects selectivity of the desired products [2]. It is, therefore, essential to develop efficient computational flow models to reveal more about such a complicated process and to facilitate design and scale up tasks of the reactor. However, in the past decades, most involved studies were usually carried out in air-water system and the assumed reactor constructions were oversimplified which kept itself far away from the real industrial conditions [3] [4]. 

Reactor model of micro bubble column performance... 

GL 26] [R 3] [P 28] See the discussion of results in the section Reactor model of micro bubble column performance, above [10]. 

Saxena, S. C., Rosen, M., Smith, D. N., and Ruether, J. A., Mathematical Modeling of Fischer-Tropsch Slurry Bubble Column Reactors, Chem. Eng. Comm., 40 97 (1986)... 

The computation performed in this study is based on the model equations developed in this study as presented in Sections II.A, III.A, III.B, and III.C These equations are incorporated into a 3-D hydrodynamic solver, CFDLIB, developed by the Los Alamos National Laboratory (Kashiwa et al., 1994). In what follows, simple cases including a single air bubble rising in water, and bubble formation from a single nozzle in bubble columns are first simulated. To verify the accuracy of the model, experiments are also conducted for these cases and the experimental results are compared with the simulation results. Simulations are performed to account for the bubble-rise phenomena in liquid solid suspensions with single nozzles. Finally, the interactive behavior between bubbles and solid particles is examined. The bubble formation and rise from multiple nozzles is simulated, and the limitation of the applicability of the models is discussed. 

In system 1, the 3-D dynamic bubbling phenomena in a gas liquid bubble column and a gas liquid solid fluidized bed are simulated using the level-set method coupled with an SGS model for liquid turbulence. The computational scheme in this study captures the complex topological changes related to the bubble deformation, coalescence, and breakup in bubbling flows. In system 2, the hydrodynamics and heat-transfer phenomena of liquid droplets impacting upon a hot flat surface and particle are analyzed based on 3-D level-set method and IBM with consideration of the film-boiling behavior. The heat transfers in... 

In a bubble-column reactor for a gas-liquid reaction, Figure 24.1(e), gas enters the bottom of the vessel, is dispersed as bubbles, and flows upward, countercurrent to the flow of liquid. We assume the gas bubbles are in PF and the liquid is in BMF, although nonideal flow models (Chapter 19) may be used as required. The fluids are not mechanically agitated. The design of the reactor for a specified performance requires, among other things, determination of the height and diameter. 

In most cases the frictional component of the pressure gradient is negligible in bubble columns but if necessary it can be calculated using the homogeneous model discussed in Section 7.5. 

This approach is proven for design and prediction of the performance of multiple-bed down-flow reactors. The complication, and a critical difference between this and a bubble column, is that the gas bubbles are formed in situ. The gas flux, and thus gas hold-up, will vary over the bed height. For the down-flow beds, a simplified linear gas hold-up profile was inherent in the design models, but with no apparent penalty in design accuracy. 

Wolf-Dieter Deckwer was working along this line and performed experimental investigations to characterise bubble column reactors without and with suspended catalysts, modelled transport processes and chemical reactions in them. 

In 1976 he was appointed to Associate Professor for Technical Chemistry at the University Hannover. His research group experimentally investigated the interrelation of adsorption, transfer processes and chemical reaction in bubble columns by means of various model reactions a) the formation of tertiary-butanol from isobutene in the presence of sulphuric acid as a catalyst b) the absorption and interphase mass transfer of CO2 in the presence and absence of the enzyme carboanhydrase c) chlorination of toluene d) Fischer-Tropsch synthesis. Based on these data, the processes were mathematically modelled Fluid dynamic properties in Fischer-Tropsch Slurry Reactors were evaluated and mass transfer limitation of the process was proved. In addition, the solubiHties of oxygen and CO2 in various aqueous solutions and those of chlorine in benzene and toluene were determined. Within the framework of development of a process for reconditioning of nuclear fuel wastes the kinetics of the denitration of efQuents with formic acid was investigated. 

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