Bubbling process in stirred tank reactors I: Agitator effect on bubble size, formation and rising

The study of the hydrodynamics generated by impellers and its effect on the generation of bubbles and on their rising and dispersion is of key importance to improve the knowledge about the contact between phases and the mass transfer rates, particularly in cases where it is the limiting step. CFD simulations and high-speed video techniques are used to study the hydrodynamics developed by five different impellers, each located at three different positions above the dispersion device. Furthermore, two dispersion devices with one and two holes, respectively, are also used. The effect of the impellers on the characteristics of the bubbles and of the dispersions generated has been analysed. Bubbles generated under stirring are smaller than those generated in stagnant fluids. It is also shown that the initial bubble size at the orifice determines the contribution of the impeller and the perforated plate to the Sauter mean diameter. Although bubble formation is chaotic, the formation period is predictable based on three variables: the location of the impeller, its rotational speed and the gas flow rate. Bubble mean diameter was correlated to classical equations based on Kolmogorov's theory. Only when impellers are capable of breaking the bubbles, Kolmogorov's theory is completely verified.     

Large Eddy Simulation of Flow in Stirred Vessels

Large eddy simulations (LES) of the flow in stirred tanks were performed. One of the advantages with LES is that it can provide details of the flow field that cannot be obtained with so‐called Reynolds averaged equations and the corresponding models. Simulations were done on both single and multi‐impeller systems. Both Rushton and curved‐blade radial impellers were studied in both a fixed and a rotating frame of reference. The results show that the periodicity is much stronger and present in a larger part of the vessel for the Rushton turbine than for a curved‐blade turbine.     

Investigation of turbulence modulation in solid–liquid suspensions using parallel competing reactions as probes for micro-mixing efficiency

The Bourne and the Villermaux competitive reaction chemistries were applied to study the effects of suspended particles on the yield of an undesired product and hence to infer their effects on local dissipation rates. Two-phase micro-mixing experiments were carried out in a 1 l stirred vessel, agitated by a pitched-blade turbine, using four particle size ranges: 70–100, 250–300, 700–750 and 1000 μm. Experiments were carried out with up to 1.75 vol% particles in the Bourne scheme and 3 vol% in the Villermaux scheme. Both reaction schemes gave qualitatively similar results, although stronger effects of added particles were obtained with the Bourne chemistry. The effect of 700–750 μm particles could not be distinguished from experimental error, but the other size ranges gave increased by-product yields and suppressed the dissipation rates. These results confirmed earlier two-phase PIV observations: smaller particles (70–100 and 250–300 μm) gave maximum suppression at ∼1 vol%. Above this volume fraction, the level of suppression decreased and in some cases turbulence augmentation occurred, indicating that particle concentration, as well as size, is an important factor.     

CFD simulations of gas-liquid-solid stirred reactor: Prediction of critical impeller speed for solid suspension

In this work, simulations have been performed for three phase stirred dispersions using computational fluid dynamics model (CFD). The effects of tank diameter, impeller diameter, impeller design, impeller location, impeller speed, particle size, solid loading and superficial gas velocity have been investigated over a wide range. The Eulerian multi-fluid model has been employed along with the standard k-ε turbulence model to simulate the gas-liquid, solid-liquid and gas-liquid-solid flows in a stirred tank. A multiple reference frame (MRF) approach was used to model the impeller rotation and for this purpose a commercial CFD code, FLUENT 6.2. Prior to the simulation of three phase dispersions, simulations were performed for the two extreme cases of gas-liquid and solid-liquid dispersions and the predictions have been compared with the experimental velocity and hold-up profiles. The three phase CFD predictions have been compared with the experimental data of Chapman et al. [1983. Particle-gas-liquid mixing in stirred vessels, part III: three phase mixing. Chemical Engineering Research and Design 60, 167-181], Rewatkar et al. [1991. Critical impeller speed for solid suspension in mechanical agitated three-phase reactors. 1. Experimental part. Industrial and Engineering Chemistry Research 30, 1770-1784] and Zhu and Wu [2002. Critical impeller speed for suspending solids in aerated agitation tanks. The Canadian Journal of Chemical Engineering 80, 1-6] to understand the distribution of solids over a wide range of solid loading (0.34-15 wt%), for different impeller designs (Rushton turbine (RT), pitched blade down and upflow turbines (PBT45)), solid particle sizes (120-) and for various superficial gas velocities (0-10 mm/s). It has been observed that the CFD model could well predict the critical impeller speed over these design and operating conditions.     

Investigations on hydrodynamics and mass transfer in gas–liquid stirred reactor using computational fluid dynamics

In this work, the hydrodynamics and mass transfer in a gas–liquid dual turbine stirred tank reactor are investigated using multiphase computational fluid dynamics coupled with population balance method (CFD–PBM). A steady state method of multiple frame of reference (MFR) approach is used to model the impeller and tank regions. The population balance for bubbles is considered using both homogeneous and inhomogeneous polydispersed flow (MUSIG) equations to account for bubble size distribution due to breakup and coalescence of bubbles. The gas–liquid mass transfer is implemented simultaneously along with the hydrodynamic simulation and the mass transfer coefficient is obtained theoretically using the equation based on the various approaches like penetration theory, slip velocity, eddy cell model and rigid based model. The CFD model predictions of local hydrodynamic parameters such as gas holdup, Sauter mean bubble diameter and interfacial area as well as averaged quantities of hydrodynamic and mass transfer parameters for different mass transfer theoretical models are compared with the reported experimental data of [Alves et al., 2002a] and [Alves et al., 2002b] . The predicted hydrodynamic and mass transfer parameters are in reasonable agreement with the experimental data.     

Modelling of a RDC using a combined CFD-population balance approach

In the present study we propose an extension of the Euler/Lagrangian approach for liquid-liquid two phase flows when the volume fraction of the dispersed phase is not small. The continuous phase velocity is obtained by solving the Reynolds-averaged Navier-Stokes equations augmented with the k-ε turbulence model. The motion of the dispersed phase is calculated by solving the equations of motion taking into account inertia, drag and buoyancy forces. The coupling between the phases is described by momentum source terms and the terms that account for turbulence generation by the droplets’ motion. Collision and breakage of the droplets are treated by a single particle Monte-Carlo stochastic simulation method. This method is based on a mass flow formulation and operator splitting technique. For validation of the numerical procedure the droplet size distribution and flow fields in a rotating disc contactor are calculated and compared with the existing experimental results.     

LIQUID-LIQUID MIXING IN STIRRED VESSELS: A REVIEW

Liquid-liquid mixing is a key process in industries that is commonly accomplished in mechanical agitation systems. Liquid-liquid mixing performance in a stirred tank can be evaluated by various parameters, namely minimum agitation speed, mixing time, circulation time, power consumption, drop size distribution, breakup and coalescence, interfacial area, and phase inversion. The importance of these liquid-liquid mixing parameters, the measurement method, and the results are discussed briefly. Input parameters such as impeller type, power number, flow pattern, number of impellers, and dispersed phase volume fraction, in addition to physical properties of phases such as viscosity and density, are reviewed. Scale-up aspects are also included.