Impeller Off-bottom Clearance Research Articles

Effect of NaX Zeolite Mass and Baffle Presence on Copper Sorption

Sorption is often conducted in stirred batch reactors without assessing the impact of mixing on yield and costs. Operating the mixing in the batch reactor at a state of complete suspension, <i>i.e.</i>, at just suspended impeller speed, <i>N</i><sub>JS</sub>, is a compromise between these conflicting goals. Therefore, copper sorption on NaX zeolite using a pitched blade turbine was conducted in both baffled and unbaffled batch reactors at <i>N</i><sub>JS</sub>. Maintaining constant impeller to reactor diameter ratio (<i>D</i>/<i>d</i><sub>T</sub> = 0.32), and impeller off-bottom clearance (<i>C/H</i> = 0.33), kinetic sorption experiments were performed. Besides kinetic sorption experiments and the kinetic analysis of data obtained, to gain insight into the hydrodynamic behaviour of the system, transient multiphase computational fluid dynamics simulations (CFD) were performed.<br /> The <i>N</i><sub>JS</sub> speed is related to the particle settling speed, wherein a higher zeolite suspension mass concentration requires a greater NJS speed, resulting in increased energy consumption. The differences in <i>N</i><sub>JS</sub> speeds for the investigated zeolite suspension mass concentrations were found to be insubstantial, and therefore, the increase in mixing intensity for the tested systems was not deemed significant. Regardless of the zeolite mass used, the values of <i>N</i><sub>JS</sub> and <i>P</i><sub>JS</sub> (mixing power consumption) were consistently higher in the unbaffled reactor. The hydrodynamic conditions employed were found to have no significant effect on the maximum amount of copper sorbed or process efficiency, but was significantly affected by zeolite mass. The kinetics were affected by both, and, generally, sorption occurred faster in the unbaffled reactor.

CFD research on the influence of geometry characteristic on flow pattern and the transition mechanism in Rushton turbine stirred vessels

Abstract In this paper, the transient MRF approach coupled with the standard k-ε and SST k-ω turbulence models was employed to study the effect of bottom shape, impeller diameter (D J) and bottom height (H 2) on critical impeller off-bottom clearance (C). It was found the bottom shape and bottom height (H 2) have obvious influence on the flow pattern transition from double-loop to single-loop of RT impeller. The flow pattern transition mechanism was inferred to relate to the relationship between the space required by the lower circulation zone and the actual space. The boundary conditions of critical C were further concluded to help distinguish the flow pattern and receive the expected one in the stirred vessel design.

Parametric numerical study and optimization of mass transfer and bubble size distribution in a gas-liquid stirred tank bioreactor equipped with Rushton turbine using computational fluid dynamics

Abstract Designing and optimizing a bioreactor can be an especially challenging process. Computational modelling is an effective tool to investigate the effects of various operating parameters on bioreactor performance and identify the optimum ones. In this work, a computational fluid dynamics-population balance model (CFD-PBM) was developed to elucidate the effect of different geometrical and operating parameters on the hydrodynamics and mass transfer coefficient of a batch stirred tank bioreactor. The validated model was projected to predict the effect of different parameters including the gas flow rate, the impeller off-bottom clearance, the number of agitator blades, and rotational speed of the impeller on the velocity profiles, air volume fraction, bubble size distribution, and the local gas mass transfer coefficient (K l a) in the bioreactor. Air bubble breakup and coalescence phenomena were considered in all simulations. Factorial experimental design approach was employed to statistically investigate the impacts of the aforementioned operating and geometrical parameters on K l a and bubble size distribution in the bioreactor in order to determine the most significant parameters. This can give an essential insight into the most impactful factors when it comes to designing and scaling up a bioreactor.

Influence of the Impeller Diameter and Off-Bottom Clearance on the Flow Velocity Distribution Characteristics Near the Bottom inside a Flotation Machine

The solid particle suspension inside a flotation machine is significantly dependent on the flow field, particularly the flow hydrodynamics characteristics near the bottom of the flotation machine. In this study, a laser Doppler anemometer (LDA) was utilized to investigate the influence of the impeller diameter and the impeller off-bottom clearance of a flotation machine on the flow velocity distribution characteristics near its bottom. The results showed that centripetal, centrifugal, and transitional spiral ascending vortexes were generated for different cases of the impeller variables. The impeller diameter and the off-bottom clearance were found to have a significant and interactive influence on the flow pattern, radial and axial velocities, velocity vector distribution, and axial fluctuating root mean square (RMS) velocity characteristics. When the centripetal flow was generated with a large impeller diameter and a small off-bottom clearance, the vortex stability was improved, the low-velocity distribution area was reduced near the bottom center, and the high axial RMS velocity distribution area was extended and became more consistent. The latter provided an advantageous condition for the momentum transfer between the liquid flow and the solid particles, as well as the airflow. However, the axial RMS velocity in the centrifugal flow formed in other cases of the impeller variables was less than that in the centripetal flow. Although the increase in the impeller off-bottom clearance contributed to increasing the velocity magnitude, this is certainly disadvantageous to the service life of the impeller blades, as expected from the high-velocity area extension. These results may provide a reference for the impeller design and optimization of a KYF (Kuang Yuan Flotation) flotation machine, as well as a basis for further investigation on the behavior of the dispersed phases inside a flow field.

Analysis of immiscible liquid-liquid mixing in stirred tanks by Electrical Resistance Tomography

In this work, the dispersion of diesel fuel in water in a baffled tank stirred by a Rushton turbine is investigated. The impeller speeds corresponding to the initial drawdown, the just dispersed regime and the completely dispersed regime are identified by suitable variables obtained by means of the Electrical Resistance Tomography technique. The effect of the dispersed phase volume fraction and of the impeller off-bottom clearance on the different dispersion regimes is considered. The impeller Froude number, the Archimedes number and a dimensionless distance between the liquid-liquid interface position and the impeller elevation are adopted for the data interpretation. For the investigated system, the local volume fraction profiles show that the completely dispersed condition is practically achieved above the just dispersed impeller speed, with negligible gradients of the local volume fraction distribution in the axial direction.

CFD Study on the Flow Field and Power Characteristics in a Rushton Turbine Stirred Tank in Laminar Regime

Abstract A computational fluid dynamics (CFD) simulation was performed to study the hydrodynamics characteristics in a Rushton turbine stirred tank in laminar regime. The effects of operating condition, working medium and geometrical parameter on the flow field and power number characteristics were investigated. It is found that the two-loop flow pattern is formed in laminar regime when the impeller is not very close to tank bottom, while its shape and size vary with Reynolds number and impeller diameter. For a given geometrical configuration, the flow pattern, power number and dimensionless velocity profile are mainly depended on Reynolds number, and do not change with working medium and scale-up for a constant Reynolds number. When impeller off-bottom clearance is too low and Reynolds number is relatively high, the fluid flow would transit from two-loop flow pattern to sing-loop flow pattern as that occurs in turbulent regime. Power number falls for larger impeller in laminar regime. Surprisingly, in laminar regime, power number in the baffled tank with small impeller is almost identical to that in the unbaffled tank.

Effect of Impeller Off-bottom Clearance on Crystal Growth Kinetics of Borax in Dual-impeller Batch Cooling Crystallizer

The aim of this work was to experimentally investigate the influence of impeller off-bottom clearance on crystal growth kinetics of disodium tetraborate decahydrate (borax) in a dual-impeller batch cooling crystallizer. Examinations were carried out in a crystallizer of 15 dm3 with liquid height to crystallizer diameter ratio (H/dT) of 1.3. In this research, two pitched blade turbines (PBT) with diameter D/dT=0.33, were mounted on the same shaft with a constant spacing between impellers (s/D=1.0). Mixing was performed at the impeller speed which ensured the state of complete suspension in the system (N=NJS). The impeller off-bottom clearance (C/D) was varied from 0.2 to 1.3. Crystallization was carried out by a controlled cooling of the solution saturated at 30°C. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over time were determined by optical microscope and image analysis software. Experimental results indicate that impeller off-bottom clearance influences the change of crystal size over process time. But, regardless of the impeller off-bottom clearance, crystal growth is integration limited. The value of an order of crystal growth rate changes insignificantly with C/D ratio, while the crystal growth rate constant is affected by the change in hydrodynamics. Obtained results are a consequence of the hydrodynamic conditions in the dual-impeller crystallizer. In order to analyze the overall fluid flow pattern, in which an interaction of the flows generated by each of the impellers occurred, photographs of the flows were taken and simulations by VisiMix 2000 Turbulent were made. Taking into account the values of kinetic parameters of crystal growth, characteristic of obtained crystals, power consumption and the overall fluid flow pattern, it is possible to define the impact of impeller off-bottom clearance on borax crystal growth kinetics.

Using tomograms to assess the local solid concentrations in a slurry reactor equipped with a Maxblend impeller

The solid-liquid mixing operation has extensive applications in many industrial processes. The aim of this investigation was to assess the mixing performance of a Maxblend impeller in a slurry reactor. The Maxblend impeller is one of the most efficient types of the new generation of the close clearance impellers with a unique geometry. The mixing efficiency of the Maxblend impeller for solid-liquid mixing operation was compared to those measured for the A200 (an axial-flow impeller) and the Rushton turbine (a radial-flow impeller). Electrical resistance tomography (ERT) as a non-intrusive/non-invasive visualisation technique was employed to quantify the degree of solid suspension within the vessel. The signals received from four ERT sensor planes (each plane containing sixteen electrodes) were used for the reconstruction of the tomograms through the non-iterative linear back projection algorithm. The tomography images were employed to assess the particles' distribution inside the slurry reactor. The particle concentration profiles were utilized to evaluate the extent of homogeneity and mixing index for the suspensions of the particles. The tomography data enabled us to examine the effects of several significant parameters such as impeller speed, particle size, solid loading, use of baffles, and impeller off-bottom clearance on the mixing performance of the Maxblend impeller. The results from this study showed that the use of baffles enhanced the extent of homogeneity attained by the Maxblend impeller. The extent of homogeneity and mixing index in the system enhanced with a raise in the agitation speed. Nonetheless, after attaining the maximum homogeneity level, further increment in the agitation speed had an adverse impact on the extent of homogeneity and mixing index. Hence, the assessment of the optimum impeller speed is extremely important to enhance the local mixing quality in the mixing vessel. The results also demonstrated that the maximum attainable homogeneity for the Maxblend impeller increased with a raise in solid loadings. The maximum homogeneity attained by the Maxblend impeller was higher than those of the A200 and the Rushton impellers.

CFD prediction of mean flow field and impeller capacity for pitched blade turbine

This work focused on exploring a computational fluid dynamics(CFD)method to predict the macro-mixing characteristics including the mean flow field and impeller capacity for a 45° down-pumping pitched blade turbine( PBT)in stirred tanks. Firstly, the three typical mean flow fields were investigated by virtue of three components of liquid velocity. Then the effects of impeller diameter(D)and off-bottom clearance(C)on both the mean flow field and three global macro-mixing parameters concerning impeller capacity were studied in detail. The changes of flow patterns with increasing C/D were predicted from these effects. The simulation results are consistent with the experimental results in published literature.

Experimental and CFD studies of solid–liquid slurry tank stirred with an improved Intermig impeller

The improved Intermig impeller has been used in the seed precipitation tank in China, which could enhance the mixing and suspension of Al(OH)3 particles and the power consumption declined largely. The flow field, solids hold-up, cloud height, just off-bottom speed and power consumptions were investigated in solid–liquid mixing system with this new type of impeller by CFD and water experiment methods. Compared with the standard Intermig impeller, the improved one coupled with specially sloped baffles could promote the fluid circulation, create better solids suspension and consume less power. Besides lower impeller off-bottom clearance is good for solid suspension and distribution. The just-off-bottom speed was also determined by a power number criterion. Meanwhile, the predicted results were in good agreement with the experimental data.

Optimization of the performance of the hydrodynamic parameters on the flotation performance of coarse coal particles using design expert (DX8) software

Hydrodynamic parameters play an important role in flotation efficiency. In the present work, the effects of some key hydrodynamic factors such as impeller speed (600, 700rpm), impeller off-bottom clearance (0.3H, 0.4H), solids percent (12%, 14%), frother type (Pine oil and methyl isobutyl carbinol (MIBC)) and their concentrations (140, 170ppm) on the flotation performance of coarse coal particles were investigated using factorial experimental design. The DX8 software was used to analyze the results of the experiments. According to DX8 software, it is concluded that until 700rpm, the effects of the studied parameters follow the order: impeller off-bottom clearance>impeller rotational speed>frother concentration>the interaction between impeller off-bottom clearance and impeller speed. But for higher impeller speeds, the impeller rotational speed has the most effects. The positive effect of impeller speed before 700rpm (i.e. 97397.38<Re<119589.22, 1.21<Fr<1.53 and 0.0035<Ca<0.0038 for 0.01226<μp<0.01299) and its negative effect beyond (i.e. Re>113609.61, Fr>1.53 and 0.0033<Ca<0.0035 for μp=0.01299) concluded that the optimum value for impeller speed is 700rpm. Also, it was concluded that the recovery of coarse coal particles is increased by increasing pulp solids percent because of increasing medium viscosity.

Morphological characteristics of poly(styrene-co-divinylbenzene) microparticles synthesized by suspension polymerization

Abstract The synthesis of poly(styrene-co-divinylbenzene) microparticles by the suspension polymerization technique requires the use of some protective agents to reduce the coalescence/agglomeration of the microparticles during the polymerization process. Since the effect of each protective agent may vary, depending on the type and proportion of the other reactants in the polymerization mixture, the influence of certain factors affecting the protective properties of tricalcium phosphate on the mean particle size, the particle size distribution, and the agglomeration of microparticles was studied using the design of experiments methodology. The factors investigated were the ammonium hydroxide and surfactant concentrations and impeller off-bottom clearance. The models thus obtained indicate that the absence of NH4OH in the polymerization mixture, or a low level of the surfactant agent, leads to agglomeration of the microparticles. It was also observed that a low value of the impeller off-bottom clearance factor and a high value of the sodium dodecyl sulfate (SDS) concentration favor the formation of microparticles with a narrower particle size distribution. Additionally, microparticles with the smallest mean size were obtained using the highest value of the impeller off-bottom clearance factor.

Effect of impeller type and position in a batch cooling crystallizer on the growth of borax decahydrate crystals

The effects of impeller type and position in a batch cooling crystallizer on the crystal growth kinetic as well as the shape, agglomeration and crystal size distribution of disodium tetraborate decahydrate (borax) were investigated in detail. Three types of impellers applied in this work generated completely different fluid flow patterns in the crystallizer allowing investigation of the influence of the liquid flow on the kinetic parameters as well. The ratio of impeller off-bottom clearance, C, to the liquid height, H, was varied from 0.1 to 0.5 for all impeller types used. Results indicate that the impeller type and off-bottom clearance influence the overall crystal growth rate and the crystal size distribution of the final product. It was also found that agglomeration of crystals produced was less pronounced as the radial fluid flow in the crystallizer became more dominant. In that system borax crystals tend to be smaller, but more regularly shaped.

Dynamic Performance of Continuous-Flow Mixing of Pseudoplastic Fluids Exhibiting Yield Stress in Stirred Reactors

The core objectives of this work were to characterize and optimize the continuous-flow mixing of pseudoplastic fluids exhibiting yield stress in stirred reactors. To achieve these objectives, the effects of impeller type (for the seven axial-flow impellers A100, A200, A310, A315, A320, 3AH, and 3AM and the four radial-flow impellers R500, RSB, RT, and Scaba), impeller speed (50–800 rpm), impeller diameter (T/3.2–T/1.6, where T is the tank diameter), impeller off-bottom clearance (H/3.4–H/1.7, where H is the fluid height in the vessel), inlet and outlet locations (for the four configurations top inlet–top outlet, top inlet–bottom outlet, bottom inlet–bottom outlet, and bottom inlet–top outlet), pumping directions for axial-flow impellers (upward and downward pumping), fluid height in the vessel (T/1.06–T/0.83), residence time (257–328 s), and jet velocity (0.317–3.24 m s–1) on the dynamic performance of the mixing vessel were explored. To identify nonideal flows, dynamic tests were conducted using the frequency-modulated random binary input of a brine solution with the feed. The mixing quality in the vessel was substantially improved by increasing the impeller diameter, increasing the residence time, optimizing the impeller off-bottom clearance, decreasing the fluid height, optimizing the jet velocity, and using the up-pumping axial-flow impeller. Applying these findings will lead to improved quality of products and more efficient use of power in continuous-flow mixing of yield-pseudoplastic fluids.

Using electrical resistance tomography images to characterize the mixing of micron-sized polymeric particles in a slurry reactor

Electrical resistance tomography (ERT) was employed to study the solid–liquid mixing in a slurry reactor equipped with a top-entering axial-flow impeller. The ERT data were used to reconstruct the tomogram using the linear back projection algorithm and calculate the degree of homogeneity. In this study, the effects of the impeller speed (252–400 rpm), impeller pumping mode (downward and upward), impeller type (A310, A100, A200, A320, A315, 3AM), impeller off-bottom clearance ( T/3.8– T/2.5, where T is the tank diameter), impeller diameter to tank diameter ratio ( D/ T = 0.29–0.47), particle size (5.2–9.1 μm), and solids concentration (15–30 wt%) on the degree of homogeneity of micron sized latex particles in a slurry reactor were explored. The results showed that the level of homogeneity in a solid–liquid mixing system improved with the increase in impeller speed. However, after achieving the maximum level of homogeneity, any further rise in the impeller speed had a detrimental effect on the level of homogeneity. A310 impeller, with D/ T ratio of 0.31, demonstrated the highest level of homogeneity while the upward pumping direction was found to be more efficient than the downward one. In addition, a clearance of T/3 proved to create the highest level of homogeneity. The results also showed that a rise in the size and concentration of particles decreased the level of homogeneity. Thus, 5.2 μm latex particles with the concentration of 15 wt% demonstrated the highest level of homogeneity.

Study of Solid−Liquid Mixing in Agitated Tanks through Computational Fluid Dynamics Modeling

Solid−liquid mixing is one of the most important mixing operations due to its vast applications in many unit operations such as crystallization, adsorption, solid-catalyzed reaction, suspension polymerization, and activated sludge processes. In this study, a computational fluid dynamics (CFD) model was developed for solid−liquid mixing in a cylindrical tank equipped with a top-entering impeller to investigate the effect of impeller type (Lightnin A100, A200, and A310), impeller off-bottom clearance (T/6−T/2, where T is tank diameter), impeller speed (150−800 rpm), particle size (100−900 μm), and particle specific gravity (1.4−6) on the mixing quality. An Eulerian−Eulerian (EE) approach, standard k−e model, and multiple reference frames (MRF) techniques were employed to simulate the two-phase flow, turbulent flow, and impeller rotation, respectively. The impeller torque, cloud height, and just suspended impeller speed (Njs) computed by the CFD model agreed well with the experimental data. The validated CFD...

Study of solid–liquid mixing in agitated tanks through electrical resistance tomography

Electrical resistance tomography (ERT), which is a non-intrusive flow visualization technique, was used to investigate the solid–liquid mixing in an agitated tank equipped with a top-entering axial-flow impeller. The signals obtained from eight ERT planes were utilized to reconstruct the tomograms by using the linear back projection algorithm. The ERT measurements were correlated to solid concentration profiles by which the degree of homogeneity was quantified. In this study, the effect of important parameters such as impeller type (Lightnin A100, A200, A310, and A320 impellers), impeller speed (250–800 rpm), impeller off-bottom clearance ( T/5– T/2, where T is the tank diameter), particle size (210–1500 μm), and solid concentration (5–30 wt%) on the degree of homogeneity were explored. The results showed that the degree of homogeneity in the solid–liquid mixing was improved with increasing the impeller speed. However, after reaching the maximum achievable homogeneity, further increase in impeller speed was not beneficial but might be detrimental. Hence, the measurement of the optimal impeller speed as a function of operating conditions and design parameters has vital role in achieving maximum homogeneity in a solid–liquid mixing system.

Axial Force at the Vessel Bottom Induced by Axial Impellers

This paper deals with the axial force affecting the flat bottom of a cylindrical stirred vessel. The vessel is equipped with four radial baffles and is stirred with a four 45° pitched blade impeller pumping downwards. The set of pressure transducers is located along the whole radius of the flat bottom between two radial baffles. The radial distribution of the dynamic pressures indicated by the transducers is measured in dependence on the impeller off-bottom clearance and impeller speed.It follows from the results of the experiments that under a turbulent regime of flow of an agitated liquid the mean time values of the dynamic pressures affecting the bottom depend not on the impeller speed but on the impeller off-bottom clearance. According to the model of the flow pattern of an agitated liquid along the flat bottom of a mixing vessel with a pitched blade impeller, three subregions can be considered in this region: the liquid jet streaming downwards from the impeller deviates from its vertical (axial) direction to the horizontal direction, the subregion of the liquid flowing horizontally along the bottom and, finally, the subregion of the liquid changing direction from the bottom upwards (vertically) along the wall of the cylindrical vessel, when the volumetric flow rates of the liquid taking place in the downward and upward flows are the same.

NUMERICAL SIMULATION OF LAMINAR SOLID-LIQUID TWO-PHASE FLOW IN STIRRED TANKS

Laminar flow of solid-liquid suspension has been rarely reported in the literature. In this article, the laminar solid-liquid two-phase flow in a lab-scale stirred tank is measured with digital particle image velocimetry (DPIV) and numerically simulated with the improved inner-outer iterative method. The simulation results show good agreement with the present data. In the range of low solid volume concentration under investigation, solid particles have minor impact on two-phase flow, and the simulated flow fields of both phases are similar to the counterpart of single-phase laminar liquid flow. The comparison of simulation with experiments of single-phase laminar flow shows reasonable agreement. These observations suggest that the numerical method employed in the present study is reliable for analysis of stirred tanks. The influence of impeller off-bottom clearance on velocities is found significant: the lower the impeller, the smaller the radial and axial velocities, and the higher the solid concentration.

Solid particle distribution of moderately concentrated suspensions in a pilot plant stirred vessel

Detailed particle distribution of a solid–liquid suspension in the just suspended state was measured by means of a conductivity probe. A total of eight experimental settings were investigated, involving two particle diameters (0.14 and 0.35 mm), two average particle concentrations (5 and 10 vol.%) and two impeller off-bottom clearances (H2/d = 1 and 0.5). A pilot plant stirred vessel of 1 m in diameter stirred with flat six pitched-blade turbine was used. A particle-filled layer of suspension was observed in all these experiments, and was particularly pronounced at the solid concentration of 10 vol.% and impeller off-bottom clearance H2/d = 0.5. The local solid concentration profiles within this layer were investigated. The axial and radial concentration gradients and their standard deviations were determined. The results indicate the presence of a concentration gradient in the radial direction within the particle layer, and of significant concentration fluctuations at the interface between the particle layer and the clear liquid layer in the upper part of the stirred vessel.