CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part I Vessel with One and Two Impellers

A simulation of flow field and tracer homogenization was performed using the commercial CFD software FLUENT 6.1. The aim is to investigate the potential of CFD software to predict concentration distribution of added tracer in cylindrical vessels. The calculated results – dimensionless velocity profiles, power and pumping numbers, dimensionless concentration curves, and mixing times – were compared with experiments in stirred vessels. In Part I, the study was performed for vessels agitated by one or two impellers on a centric shaft. Two different impellers were used – a 6‐bladed 45° pitched blade turbine and a standard Rushton turbine. The standard k‐? turbulence model and multiple reference frames method were used for the simulations. The influence of the grid type was also investigated; three types of grid – a structured, unstructured and a special user‐defined grid – were studied.     

IMPELLER CLEARANCE EFFECT ON OFF BOTTOM PARTICLE SUSPENSION IN AGITATED VESSELS

The effect of impeller off bottom clearance on the power input requirement for off bottom solid suspension was examined for 45° pitched blade impellers in flat and round bottom fully baffled agitated vessels. Results showed a similar dependence as obtained for radial flow impellers when similar flow patterns were observed. The dependence appears to be independent of the impeller diameter to tank diameter ratio and vessel shape.     

Numerical Investigation of the Effect of Impeller Design Parameters on the Performance of a Multiphase Baffle‐Stirred Reactor

The turbulent gas‐liquid flow field in an industrial 100‐m³ stirred tank was calculated by using computational fluid dynamics based on the finite‐volume method. Turbulent effects were modeled with the shear stress transport model, and gas‐liquid bubbly flow was modeled with the Eulerian‐Eulerian approach using the Grace correlation for the drag force interphase momentum transfer. The relative motion between the rotating impeller and the stationary baffled tank was considered by using a multiple frames of reference algorithm. The effects of Rushton and pitched‐blade impeller design parameters such as blade geometry, location, and pumping direction on the mixing performance were investigated. It was found that a combination of Rushton turbines with up‐pumping pitched‐blade turbines provides the best mixing performance in terms of gas holdup and interfacial area density. The approach outlined in this work is useful for performance optimization of biotechnology reactors, as typically found in fermentation processes.     

Determination of Dissipation Rate in Stirred Vessels Through Direct Measurement of Fluctuating Velocity Gradients

2‐D and 3‐D PIV and 4‐channel LDA techniques were employed to measure the turbulence energy dissipation rate in vessels stirred by Rushton impellers. The measurements were made in vessels of diameter T = 100 mm and 294 mm and stirred by impellers of diameter D = T/3. The impellers were rotated at speeds corresponding to Reynolds number of 40,000 to ensure fully‐turbulent flow in the vessels. ? was determined directly from measurements of the Reynolds stress gradients by analyzing the PIV images over interrogation areas down to 0.1 mm. Similar LDA data were obtained in the larger 294 mm vessel, with a resolution of around 50 μm. The fluctuating gradient results obtained with the two techniques compare well and show that direct measurement of the ? distribution is feasible with both PIV and LDA methods.     

Effect of Dual Impeller‐Sparger Geometry on the Hydrodynamics and Mass Transfer in Stirred Vessels

The understanding of the effect of impeller‐sparger configurations on gas dispersion and mass transfer is very important to improve the performance of gas/liquid contactor systems. The influence of the impeller positions, the upper turbine diameter, the sparger ring diameter and its location in regard to the lower impeller on the power consumption, the volumetric mass‐transfer coefficient and the overall oxygen transfer efficiency were studied in a nonstandard curved bottomed reactor with an agitated system with dual disk style turbines. In the range of the gas flow rates studied, the most efficient impeller‐sparger arrangement for the oxygen transfer is the impeller system with turbines of different diameters located at C = 0.25 and IC = 0.5, and with the sparger of smaller diameter than the lower impeller settled below the impeller. A new model to estimate the k_La with an average relative error of 8 %, which takes the reactor operation conditions and the influence of the impeller‐sparger geometry into account, was also proposed.     

Mixing of Oil in Water Through Electrical Resistance Tomography and Response Surface Methodology

The mixing performance of the oil‐in‐water dispersion system was evaluated. Using an electrical resistance tomography system composed of two measuring planes, the effect of parameters such as impeller type, impeller speed, oil type, and oil volume fraction on the mixing performance through axial mixing indices were explored. The oil type and the oil volume fraction were identified as the most influential factors on the mixing index. Castor oil, with the highest viscosity of the tested oils, was found as the most difficult oil to disperse. The Scaba impeller was the most efficient impeller in dispersing oil in water. The interactions between oil type and impeller type as well as between impeller speed and oil type, had the greatest impact on the mixing index.     

Interrelation among Liquid Velocity,Impeller Speed and Gas Flow Rate in the Stirred Airlift Reactor

We investigate the liquid circulation velocity in the draft‐tube airlift reactor, with mechanical agitation in the internal column as a riser, at the impeller speed of up to 40 s^–1. An influence of the gas flow rate and the stirrer speed on the riser and downcomer gas hold‐up difference and on the liquid circulation velocity is also investigated. The character of the liquid circulation velocity changes depends on the relation between the gas flow rate and the impeller speed. A monotonic increase of the liquid circulation velocity with an increase of the gas velocity is observed at the impeller speed of lower than 15 s^–1. A distinct decrease of the liquid velocity is found at higher impeller speeds and at low gas velocities. The decrease is larger for higher impeller speeds. We observe the minimum on the curve of the liquid velocity dependence on the gas velocity followed by the monotonic increase of the liquid velocity with an increase of the gas flow velocity. The minimum of the liquid circulation velocity appears if the ratio of the gas flow number to the impeller speed is about 0.0006. The minima are shifted towards the higher gas velocities at higher impeller speeds. An experimental equation for the prediction of the liquid circulation velocity in the stirred airlift reactor is presented.     

The Influence of Macroscopic Elongational Flow on Dispersion Processes in Agitated Tanks

The influence of elongational and shear gradients in the macroscopic flow field in agitated tanks on dispersion processes is investigated. Measurements of droplet size distribution for a liquid‐liquid dispersion process using phase‐Doppler anemometry (PDA) reveal that axial‐flow impellers, such as the 24°‐pitched‐blade turbine and propeller, produce smaller droplets than the Rushton turbine at the same average specific power and energy input. These results stand in contradiction to the usual assumption that only the maximum turbulent shear stress determines the breakup process and the Rushton turbine is well known to produce higher turbulent shear stresses. Particle image velocimetry (PIV) measurements of the macroscopic flow field indicate that the 24°‐pitched‐blade turbine and propeller produce larger areas with higher elongational gradients. Therefore, the proposed consideration of particle breakup due to macroscopic elongational flow in addition to turbulent stresses improves the understanding of dispersion processes in agitated tanks.     

Flow Patterns in Rheologically Evolving Model Fluids Produced by Hybrid Dual Mixing Systems

The flow patterns produced by two dual mixing systems composed of independently driven impellers were studied. The dual impellers included a turbine rotating at high speed (Rushton or Smith) and a slowly rotating helical ribbon agitator (HR). Visualizations and power input were used to evaluate mixing performance. The influence of the rotational speed ratio on the flow patterns was evaluated. For high shear‐thinning fluids, N_T/N_HR modifies the flow patterns considerably. Three typical behaviors were found with shear thinning fluids: segregation of two principal flow patterns (N_T/N_HR < 10), turbine dominance (N_T/N_HR > 10), and a well‐distributed flow pattern throughout the tank (N_T/N_HR = 10). For low‐viscosity fluids, the motionless HR reduced the vortex length and the T‐HR systems eliminated vortex when the impellers rotated in opposite directions at N_T/N_HR = 10. Finally, a relationship between the dimensionless vortex length and the Froude number is proposed for individual turbines as well as for the turbine‐motionless HR systems.     

Liquid Homogenization in Aerated Multi‐Impeller Stirred Vessel

The macroflow of fluid in a tall cylindrical vessel stirred with multiple stirrers was studied in the case of aeration of a liquid charge. The time of homogenization of the charge (mixing time) was calculated from the time dependency of the tracer concentration measured at various locations. Two types of stirrer were used in the experiments: six‐bladed Rushton turbines and/or pitched‐blade turbines with inclined blades pumping the liquid down or up. Four stirrers of the same type were located on the shaft. Other variables during the experiments were the stirrer frequency and the gas flow rate. It was found that the liquid macroflow in the vessel could be interpreted by the cell model or by the axial dispersion model for unaerated as well as for aerated systems. The influence of the aeration on the macroflow and mixing time was explained by the interaction of buoyancy and radial forces, and equations for the model parameters were proposed containing gas flow numbers and Froude numbers.