Effect Of Impeller Type Research Articles

Investigation of gas-liquid mass transfer in slurry systems driven by the coaxial mixer

The coaxial mixer was applied to the gas-liquid mass transfer process in slurry systems containing up to 8 vol% solids. The effects of impeller type, impeller speed, solid content, and gas flow rate were examined, and a correlation for the volumetric mass transfer coefficient was established. The findings indicate that at identical impeller speeds, the combination of an anchor and Rushton turbine exhibited the highest volumetric mass transfer coefficient and gas hold-up among coaxial mixers. However, when the pitched blade turbine with a down-pumping direction was utilized as the inner impeller, the coaxial mixer demonstrated superior performance under the same power consumption conditions. Additionally, the lower anchor speed was found to improve gas-liquid mass transfer, whereas a higher speed would lead to poor dispersion of gas phase, thereby deteriorating the performance of coaxial mixers. Increasing solid content caused a continuous decline in both the volumetric mass transfer coefficient and gas hold-up, while a rise in gas flow rate had the positive effect. Finally, the correlation developed for the volumetric mass transfer coefficient in slurry systems showed a deviation of less than 20% between predicted and measured values.

Investigation of effects of impeller geometry on the spatial distribution of turbulent field and micro-mixing performance in a stirred reactor equipped with multiple impellers

There are few studies on the effects of spatial distribution of hydrodynamics on the micro-mixing in stirred reactors. In this work, a statistical approach is introduced to quantitatively assess the spatial distributions of hydrodynamic parameters, using the turbulent dissipation rate as an example. According to the results, the effects of impeller geometry and angular velocity on the turbulent dissipation rate can be well described in terms of the volume-averaged values and spatial distribution functions. In the present study, a more uniform distribution of the turbulent dissipation rate is achieved by the multiple impellers with an inclinational angle of 15° at the same rotational speed. However, increasing the inclinational angle can decrease the volume-averaged value. The cumulative percentage of area-averaged turbulent dissipation rate is well modeled by an exponential function. The effects of impeller types on the meso- and micro-mixing processes are also investigated. It is found that the micro-mixing process has more significant influences than the meso-mixing process in the bottom region and near the free surface. The effects of impeller types on the micro-mixing performance are further investigated using the Villermaux–Dushman reaction system, both experimentally and numerically. An empirical correlation that considers the spatial distribution of turbulent parameters for the reaction rate of I2 synthesis is proposed. The results show that the predictions of segregation index, which represents micro-mixing performance, are improved by a numerical model coupled with the new reaction rate model.

CFD-PBM Investigation on Droplet Size Distribution in a Liquid–Liquid Stirred Tank: Effect of Impeller Type

The influence of impeller type on the liquid–liquid dispersion system was investigated by computational fluid dynamics coupled with the population balance model (PBM). Rushton turbine (RT), pitched-blade turbine (PBT), and parabolic blade turbine (PB) produce different flow fields and turbulence distribution. The droplet size distribution (DSD), breakage frequency, and coalescence rate were predicted with the Alopaeus breakage model and the CT coalescence model. The simulated velocity field and DSD were validated with particle image velocimetry and image analysis, respectively. The analysis of the interaction between eddy and droplet shows that the turbulent inertial regime was the dominant mechanism of droplet breakage in the impeller region for radial-type impellers (RT and PB), while the turbulent viscous regime for an axial impeller (PBT). RT and PB can produce smaller droplets than PBT, about 16.4 and 10.7%, respectively. The average turbulent dissipation rates of RT and PB are about 7 and 10 times that of PBT in the impeller region, which explains the better performance of these two radial impellers in terms of droplet breakup.

Solid-liquid suspension in a stirred tank driven by an eccentric-shaft: Electrical resistance tomography measurement

Aiming at improving particle suspension performance in the stirred tank, the eccentric-shaft agitation and two-half-pitched-blade (THPB) impeller were proposed. The power consumption and suspension characteristics of THPB impellers under the eccentric-shaft, concentric and eccentric agitation schemes were experimentally studied. Effects of solid loadings and impeller rotation speeds were discussed. Results were compared with their counterparts of the two-flat-blade (TFB) impeller. It was found that, when running at the same rotation speed, although power consumption of eccentric-shaft agitation is large, its suspension performance is the best. When operated under the equal-power condition, for the eccentric-shaft agitation scheme, TFB has the best suspension performance. For the concentric and eccentric agitation schemes, THPB impellers behave better than TFB. Under the lower-than- N js condition, increasing rotation speed can improve the suspension performance. Given the simple design and good suspension performance of eccentric-shaft agitation, it is promising to be applied in the industries. • Development of the eccentric-shaft agitation scheme. • Electrical resistance tomography (ERT) measurements of solid-liquid suspension performance. • Comparison of performances among the concentric, eccentric and eccentric-shaft agitation schemes. • Discussion on the effect of impeller type, rotation speed and solid loading.

Effects of Scale-Up and Impeller Types on Spherical Agglomeration of Dimethyl Fumarate

Spherical agglomeration (SA) allows for intensifying a crystallization process through simultaneous particle size enlargement and process efficiency enhancement, without a compromise between bioavailability and processability of active pharmaceutical ingredients (APIs). Inefficient collision intensity may encounter during scale-up of an SA process, while mixing intensity and flow patterns do play critical roles especially in a batchwise stirred tank. In the present study, the scalability of an SA process by operating at all the same configurations of different scales, 0.5, 2, and 10 L, using a common one-layered impeller with four pitched blades was examined through the use of dimethyl fumarate as a model API. Insights into the guidelines for scaling up of the SA process are also given. Furthermore, the effects of impeller types, including the one-layered impeller, the two-layered impeller with four pitched blades, and the maxblend impeller, on the size, size distribution, and roundness of dimethyl fumarate agglomerates were investigated carefully.

Effect of impeller type and scale-up on spatial distribution of shear rate in a stirred tank

The main spatial distribution features of shear rate in a stirred tank operated with five different radial and axial flow impellers were presented with particle image velocimetry (PIV) experiments. Not only the average shear rate in the whole tank but also the local value in vicinity of impeller increases linearly with impeller speed. Furthermore, the shear coefficient ( K s,imp ) at the impeller outlet is linearly related to the impeller flow number ( N q ) and decreases with the increase of N q in general at the constant power consumption per unit volume ( P v ). During scale-up based on the constant P v and geometric similarity, CFD results show that the volume-averaged shear rate ( γ avg ) for RDT decreases faster than that of other impellers with the impeller tip velocity ( U tip ). The novel multi-blade combined (MBC) impeller with the increased height-to-diameter ratio of the stirred tank is able to more effectively improve the distribution uniformity of shear rate at the same P v after scale-up. These studies provide a data basis for selecting the impeller types and improving the shear rate environment in large-scale stirred tank.

Generation of aqueous foams and fiber foams in a stirred tank

Generation of fiber foams with surface aeration was studied by simultaneous mixing of pulp fibers, surfactant, air and water in a lab-scale tank. The mixing took place in either transient or turbulent conditions. The effect of impeller type (RT-6 and BT-6), mixing speed, mixing power, and surfactant dosage on air content, bubble size and half-life time of the foam was investigated. Air content increased with increasing agitation power saturating to 50–75% depending on the SDS dosage. The dependence of bubble size on power was a power law with an exponent typical for processes where coalescence plays an important role. The relation between the half-life time and air content was well described by a power law. Power number of the BT-6 impeller was almost constant, and similar to that of pure water in turbulent conditions. The power number of the RT-6 impeller varied and was similarly to gas-liquid stirred tanks, in many cases clearly below that of water. The variability of the power number of RT-6 could be explained with the variation of the mixing Reynolds number.

Intensification on drawdown process of floating particles by circle package fractal impellers

Abstract The floating particle drawdown and dispersion characteristics in a stirred tank with four pitched-blade impellers, fractal impellers, and circle package fractal impellers were studied using computational fluid dynamics (CFD) simulation. The effects of impeller type, impeller speed, impeller spacing, impeller submergence, initial floating particle concentration, and floating particle diameter on the floating particle dispersion quality were investigated. Results showed that the floating particle dispersion degree was improved with an increment in the impeller speed, in term of axial solid concentration profile, solid concentration distribution, and cloud height. The impeller spacing of T2/3 and impeller submergence of T/6 were appropriate for the floating particle drawdown process in this work. Higher initial floating particle concentration was easier to achieve a higher floating particle dispersion degree. Smaller particle diameter resulted in smaller buoyancy and more uniform distribution. Meanwhile, circle package fractal impeller can reduce the power consumption compared with four pitched-blade impeller and fractal impeller at the same impeller speed, and enhance the solid integrated velocity and the level of homogeneity for floating particle dispersion process under the constant power consumption. In addition, the just drawdown speed (Njd) and correlation equation for Njd in the three different impeller systems were obtained.

Effect of impeller type on mixing of highly concentrated slurries of large particles

Many chemical engineering processes deal with dense suspensions of large solid particles in liquid. Behaviors of such systems are difficult to predict because the many solid particles strongly affect the hydrodynamics produced by the impeller. The main objective was to assess the performances of the PBT, A310, and PF3 impellers in agitating highly concentrated slurries of large particles. Electrical resistance tomography was used to determine the degree of homogeneity and solid particle distribution within the slurry reactor. To gain more insight into the complex behavior of the flow inside the dense suspensions, computational fluid dynamics simulations were conducted using a Eulerian‒Eulerian multiphase model. Both qualitative and quantitative analyses were performed to obtain details on the dispersed solid phase distribution and its effect on the mixing quality. We found that the PBT impeller was the most efficient in terms of consuming power and generating velocity and turbulent kinetic energy within the mixing tank.

PIV Measurement and CFD Simulation of Liquid-Liquid Mixing in Mixer Settler with Rigid-Flexible Impeller

Abstract Particle image velocimetry (PIV) was used to measure the flow field in the mixing chamber of a mixer settler. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate liquid-liquid two-phase flow, turbulent flow, and impeller rotation in the mixer settler, respectively. The effects of impeller type, impeller clearance and baffle configuration on the flow field were analyzed. Results showed that rigid-flexible impeller can spread the impeller energy more uniformly in the mixing chamber and increase the velocity of the continuous phase through the multi-body motion of flexible sheet compared with rigid impeller. The flow numbers of RF-RT, RF-PBT and RF-CBT systems were increased by 46 %, 54 % and 43 %, respectively, compared with RT, PBT and CBT systems. Meanwhile, the suction capacity of impeller was enhanced with a decreased impeller clearance, and the backflow was eliminated by the installation of baffles.

Effect of impeller type on preparing spherical and dense Ni1−x−yCoxMny(OH)2 precursor via continuous co-precipitation in pilot scale: A case of Ni0·6Co0·2Mn0·2(OH)2

The spherical and dense Li[Ni1−x−yCoxMny]O2 particles work excellently as the cathode material for Li-ion secondary batteries. The key to their preparation lies in synthesizing the spherical and dense Ni1−x−yCoxMny(OH)2 precursor via co-precipitation in a mechanically stirred tank reactor. The effect of impeller type on the precursor property was investigated by taking Ni0·6Co0·2Mn0·2(OH)2 as an example. Four different types of impellers were tested and compared, which were the propeller turbine (PT), pitched blade turbine (PBT), flat blade turbine (FBT) and Rushton turbine (RT). Only with the PT could the spherical and dense precursor be prepared, whose tap density exceeded 2.0 g/mL. As expected, the LiNi0·6Co0·2Mn0·2O2 obtained in this case exhibited the best electrochemical property, which delivered a discharge capacity of 182.3 mAh/g at 0.2C rate between 2.7 and 4.3 V. The capacity retention was 97.9% after 200 cycles at 2C rate. The different properties of the precursors resulted from the different flow fields in the reactor. The single-loop flow field excited by the PT caused the nuclei to grow into flat and thick flaky primary particles, which then could assemble tightly to form the dense secondary particles. Additionally, the circulation of secondary particles in the whole reactor further enabled them to grow, consolidate, become spherical and smooth by adsorbing the free nuclei and primary particles. On the other hand, the double-loop flow field generated by the FBT or RT caused the primary particles to be thin and curve, which could only pile up loosely to form the secondary particles. What's worse, the secondary particles were entrapped in the lower loop, having little chance to grow and consolidate by adsorbing the nuclei and primary particles that were mainly formed in the upper loop.

Numerical Simulation of Gas-Liquid Dispersion in A Stirred Tank Agitated by Punched Rigid-Flexible Impeller

Abstract The hydrodynamics of gas-liquid dispersion process in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers were investigated using a combined computational fluid dynamics (CFD) and population balance model (PBM) approach. A classical Eulerian-Eulerian approach coupled with standard k-e turbulence model was employed to simulate gas-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, flexible connection piece length, aperture size/ratio, impeller speed and superficial gas velocity on the local gas holdup and bubble size distribution for the gas-liquid dispersion process were investigated. Results showed that a long flexible connection piece length was conductive to the gas-liquid dispersion process. The optimum aperture ratio and aperture diameter were 12 % and 8 mm, respectively, for the gas-liquid dispersion process. Punched rigid-flexible impeller could reduce the differential pressure ahead and behind the impeller blade, decrease the size of low-pressure zone, and improve the water shear strain rate compared with rigid impeller and rigid-flexible impeller at the same Pg,v. It was found that punched rigid-flexible impeller was more efficient in terms of gas dispersibility than rigid impeller and rigid-flexible impeller.

Influence of Impeller Geometry on Hydromechanical Stress in Stirred Liquid/Liquid Dispersions

Hydromechanical stress is a crucial parameter for a broad range of multiphase processes in the field of (bio)chemical engineering. The effect of impeller type and geometry on hydromechanical stress in stirred tanks is important. The present study aims at characterizing conventional and new impeller types in terms of particle stress. A two-phase liquid/liquid noncoalescing dispersion system is employed, and the drop breakage is monitored in-line in a stirred tank. The published effects of agitation on drop deformation are confirmed and expanded significantly for five modified new impeller types. Radial impellers are advantageous for applications where low shear conditions are desired. A modified propeller with a peripheral ring and the developed wave-ribbon impellers present remarkable results by producing significantly low and high hydromechanical stress, respectively. The results obtained are correlated in terms of mean and maximum energy dissipation rate, as well as circulation frequency in the impeller...

Size distribution of bubbles in agitated viscous Newtonian and non‐Newtonian solutions

AbstractThe effects of impeller type, Newtonian shear thinning liquid viscosities, and noncoalescing conditions on the impeller gas cavities, impeller power consumption, and Sauter mean bubble diameter (d32) in a multiphase agitated vessel were systematically studied. An underwater recording technique was used to study gas cavities and bubble size in various operating and design conditions. The gas cavity images were used to relate d32 to gas cavity structure. It was observed that d32 values increase with an increase in Newtonian and shear thinning liquid viscosities. Results also indicate that impellers operating in shear thinning liquids consume higher gassed power at the same operating conditions compared with those in Newtonian fluids. In addition, the d32 values in noncoalescing liquids are substantially smaller compared with those in coalescing solutions at the same Pg/V. Mathematical correlations are proposed for estimating the d32 in the impeller discharge stream and Pg/V in viscous Newtonian and non‐Newtonian solutions.

Selective flotation of fine-grained pentlandite from low grade polymetallic ore

The main aim of this study is to test a new type of an impeller in selective flotation of a low grade polymetallic Ni-Cu-PGE ore. The effect of impeller type and speed on bubble size and nickel grade and recovery was investigated. It was noticed that bubble size was independent of impeller type and speeds. However, the highest volume of small bubbles was measured using a new type of impeller. The results showed that a pitched blade turbine impeller has a positive effect on nickel grade and recovery at higher impeller speed.

Gas-liquid mixing performance, power consumption, and local void fraction distribution in stirred tank reactors with a rigid-flexile impeller

In this work, the mixing performance of gas–liquid was characterized by the largest Lyapunov exponent (LLE), relative power demand (RPD), power number NP and local void fraction in a rigid-flexible impeller reactor equipped with a tube sparger. The effects of impeller types, gas flow rates, agitation speed, flexible piece lengths, axial spacing distances lh and angles θ were investigated. Meanwhile, global variable analysis (GVA) based on the mixing index (MI) was proposed to characterize the mixing process. The results show that the increases in flexible piece length, agitation speed, and gas flow rate could enhance the mixing degree. Meanwhile the reductions of axial spacing, and angle were also able to improve the mixing performance of gas–liquid two phases to a certain extent. These findings were in conformity with the results of investigation into MI. The results of this study could provide theoretical guidance for the optimal design and engineering scale-up of STRs.

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine (BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine (PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.

Concentrated slurry formation via drawdown and incorporation of wettable solids in a mechanically agitated vessel

This article describes the effect of vessel configurations on the drawdown and incorporation of floating solids to prepare concentrated alumina slurries in stirred tanks. The impeller speed and power draw required to incorporate all dry powder within four seconds, NJI and PJI, are used to evaluate incorporation performance. The effect of impeller type is assessed, with pitched blade impellers proving to be the most effective across the full range of solid contents considered. At higher solids content the energy demand is shown to increase dramatically, with a 100‐fold increase in energy required to add 1% w/w more solid at 50% by weight compared to 1% by weight. Analysis of impeller power numbers show this coincides with a transition from constant power number to a region where power number increases linearly with decreasing Reynolds number. Contrary to studies at low solids content, the presence of baffles is shown to inhibit drawdown. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1885–1895, 2018

Numerical simulation of solid-liquid suspension in a stirred tank with a dual punched rigid-flexible impeller

The hydrodynamics of solid-liquid suspension process in a stirred tank with a dual rigid impeller, a dual rigid-flexible impeller, and a dual punched rigid-flexible impeller were investigated using computational fluid dynamics (CFD) simulation. A classical Eulerian-Eulerian approach coupled with standard k-ε turbulence model was employed to simulate solid-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, impeller speed, flexible connection piece width/length of dual rigid-flexible impeller, aperture size/ratio of dual punched rigid-flexible impeller, particle diameter, and liquid viscosity on the homogeneity degree of solid-liquid system were investigated. Results showed that the homogeneity degree of solid-liquid system increased with an increase in impeller speed. A long and wide flexible connection piecewas conductive to solid particles suspension process. Larger particle diameter resulted in less homogenous distribution of solid particles. An increase in liquid viscosity was beneficial to maintain solid particles in suspension state. The optimum aperture ratio and aperture diameter were 12% and 8mm, respectively, for solid particles suspension process. It was found that dual punched rigid-flexible impeller was more efficient in terms of solid particles suspension quality compared with dual rigid impeller and dual rigid-flexible impeller under the same power consumption.

Design of impeller blades for efficient homogeneity of solid-liquid suspension in a stirred tank reactor

Computational fluid dynamics (CFD) was used to investigate the hydrodynamics of solid-liquid suspension process in a stirred tank with rigid impellers, rigid-flexible impellers and punched rigid-flexible impellers. The effects of impeller type, impeller speed, flexible connection piece length, impeller spacing, particle size, and aperture size/ratio on the mixing quality were investigated. Results showed that the degree of solid-liquid homogeneity increased with an increase in impeller speed. A long flexible connection piece was conductive to solid particles suspension process. The solid particles could not obtain enough momentum to suspend to the upper region of stirred tank with small impeller spacing. Larger particle size resulted in less homogenous distribution of solid particles. The optimum aperture ratio and aperture diameter of punched rigid-flexible impeller were 12% and 8mm, respectively, for solid particles suspension process. It was found that punched rigid-flexible impeller was more efficient in suspending solid particles compared with rigid impeller and rigid-flexible impeller at the same power consumption. In addition, less impeller power was consumed by punched rigid-flexible impeller compared with rigid impeller and rigid-flexible impeller at the same impeller speed.