Off-bottom Clearance Research Articles

Experimental and CFD Analysis of Hydrodynamics in Dual-Impeller Crystallizer at Different Off-Bottom Clearances

Producing tailor-made crystals demands knowledge of the influence of hydrodynamics and nucleation kinetics. In this paper, the hydrodynamic conditions in a dual-impeller crystallizer and their influence on the key nucleation parameters of the batch cooling crystallization of borax at different impeller off-bottom clearances were investigated. Two different impeller configurations were used—a dual pitched-blade turbine (2 PBT) and a dual straight-blade turbine (2 SBT). Hydrodynamics was analyzed in depth based on the developed computational fluid dynamics model. The experimental results on mixing time and power input were used to validate the numerical model. The results show that the properties of the final product are affected by the impeller position in both dual-impeller configurations. An increase in the impeller off-bottom clearance in both systems results in a decrease in the mean crystal size. The hydrodynamic conditions generated at C/D = 1 in the 2 PBT impeller system and at C/D = 0.6 in the 2 SBT impeller system favored an earlier onset of nucleation compared to other impeller positions. It was found that the eddy dissipation rate and the Kolmogorov length scale correlate highly with the mean crystal size, suggesting that the size is affected by the shear stress in the vessel, rather than the overall convective flow.

Insights into the fluid dynamics of the Bach impeller

The Bach impeller is a novel impeller that was purposefully designed to reduce power consumption as well as shear stress levels in stirred bioreactors. Contrary to conventional impellers where stirring is achieved by the pushing action of the blades, the Bach impeller relies on a central spiral element which induces an up-pumping central vortex and discharges the flow in the tangential direction through vanes. This feature is new and relevant to the field of biochemical engineering where cell culture and microbial fermentation often make use of shear-sensitive, living organisms to produce biopharmaceuticals and medicines. This article offers a thorough characterisation of the fluid mechanics of this impeller by means of 2D phased- and time-resolved particle image velocimetry (PIV) experiments. Key flow dynamics parameters including velocity magnitudes, kinetic energy, and shear stresses were estimated for different combinations of impeller size, clearance, and speed. The standard impeller size, D/T=0.52, performed most promisingly in combination with an off-bottom clearance of C=0.6 T, as the elevated impeller height allowed to pull high momentum fluid to the upper part of the tank and therefore improved overall transport phenomena in the entire reactor volume. For this impeller diameter-clearance combination peak ensemble-averaged shear stresses at the vessel base were lower due to the impeller greater distance from the bottom. A distinctive feature of the Bach impeller was its "smooth operation" mode, where velocity fluctuations due to the vane passages are nearly negligible (∼5 % of ensemble-averaged kinetic energy). These aspects make of the Bach impeller an appealing option in the context of stem cell bioprocess applications.

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.

An empirical model for the prediction of cloud height

At high solids concentrations (>2 vol%), a clear interface appears between solid-rich and liquid-rich parts of a solid-liquid mixing tank. The height of this interface is known as cloud height. The cloud height is one of the key design parameters; therefore, accurate prediction of it plays a critical role in the design of solid-liquid mixing tanks. This study proposes an empirical model that predicts cloud height as a function of particle properties, solids concentration (XV), off-bottom clearance, and impeller speed. Experiments were performed in a tall tank (H=1.5 T) agitated with a 45° down-pumping PBT. The model was tested against two independent data sets. It successfully predicts the cloud height for XV between 2 to 8 vol% and off-bottom clearance between T/6 to T/3. The size and the density of the particles were varied between 123 to 712 µm and 2500 to 7600 kg/m3, respectively. A modified version of the model was also proposed for high solids concentrations.

Effect of Orifice Shape and Dip Tube Clearance on the Discharge Hydrodynamics and Residual Heel Volume in a Dispensing Bottle

In this work, the roles of the orifice shape and off-bottom clearance of the dip tube on the discharge flow rate of a complex fluid from a dispensing bottle and on the resulting residual “heel” volume left in the bottle were investigated. Particle Image Velocimetry (PIV) was used to monitor the discharge rate and the heel. The dip tube clearance and the orifice shape both affected the formation of the heel. Dip tubes provided with a flat cut orifice not only resulted in a smaller heel compared to angled cut orifices but also generated a higher flow rate at constant suction pressure. Reducing the dip tube clearance produced smaller heel volumes irrespective of the shape of the dip tube orifice. The results of this work were validated using the velocity contour maps obtained by PIV and, separately, with the heel profiles obtained from the PIV raw images.

Hydrodynamics performance of Newtonian and shear-thinning fluids in a tank stirred with a high shear impeller: Effect of liquid height and off-bottom clearance

The hydrodynamics performance of a ring-style Hockmeyer© high-shear impeller (HSI) in an unbaffled tank with Newtonian and shear-thinning fluids at Reynolds numbers (Re) between 20 and 300 is addressed. Power measurements of this study were used to validate computational fluid dynamics simulations. Key hydrodynamic parameters for the HSIs performance, i.e., pumping and power numbers, and viscous dissipation are analyzed. The effect of the off-bottom clearance (C), working fluid height (Z) (in the range 0.18 ≤C/T ≤ 0.38 and 0.8 ≤Z/T ≤ 1, where T is the tank diameter), and fluid rheology, on these parameters were investigated. Results show that the examined C/T and Z/T ratios do not significantly affect the power number. The fluid rheology, C/T and Z/T had, on the contrary, a significant impact on the remaining hydrodynamic parameters. For 45 ≤Re≤ 300, the shear-thinning fluid showed better performance for dispersion processes, i.e., higher pumping number and viscous dissipation, and lower power number than the Newtonian fluid.

Zeolite NaX Mass and Propeller Agitator Speed Impact on Copper Ions Sorption

Sorption is often carried out in stirred batch reactors without any consideration of how much mixing is sufficient to avoid the effect of diffusion without compromising yield and cost due to overmixing. Therefore, the focus of this work was to study how the maximum sorption capacity, removal efficiency, kinetics and power consumption (P) of the studied process are affected by different mixing speeds, i.e., impeller speed/minimum impeller speed for complete suspension (N/NJS) ratio values and zeolite suspension mass concentrations. Experiments were conducted in a baffled reactor with the propeller at a standard off-bottom clearance. In addition to the experimental studies, numerical modelling approaches were carried out to investigate the sorption process using a transient multiphase computational fluid dynamics model and fitting selected kinetic models. The results show that an increase in zeolite mass leads to a slight increase in the NJS and consequently PJS. The impeller speed affects the velocities, power consumption, kinetics, final amount and removal efficiency of copper sorbed. The experimentally determined kinetic data fit Ritchie’s kinetic model well. However, for two experiments, performed at N/NJS ratios of 0.8 and 0.6, Mixed kinetic model fits better, suggesting that the second-order reaction is suppressed by diffusion. Due to the influence of diffusion, the experimentally determined sorption efficiency decreased from 59.377% to 54.486% and 46.372% for N/NJS ratios of 0.8 and 0.6, respectively.

Numerical Simulation of Dense Solid-Liquid Mixing in Stirred Vessel with Improved Dual Axial Impeller

Computational fluid dynamics (CFDs) were adopted in order to investigate the solid suspending process in a dense solid–liquid system (with a solid volume fraction of 30%), agitated by a traditional dual axial impeller and a modified dual axial impeller, otherwise known as a dual triple blade impeller (DTBI) and a dual rigid-flexible triple blade impeller (DRFTBI), respectively. The effects of rotational speed, connection strap length/width, and off-bottom clearance on the solid distribution were investigated. The results show that the proportion of solid concentration larger than 0.4 in the DTBI system was 26.56 times of that in the DRFTBI system. This indicates that the DRFTBI system can strengthen the solid suspension and decrease the solid accumulation in the bottom of the tank. Furthermore, the velocity and turbulent kinetic energy in the DRFTBI system were promoted. In addition, for an optimal selection, the optimum length of connection strap was 1.2 H1, the optimum range of connection strap width was D/7–D/8, and the off-bottom clearance selected as T/4 was better.

Optimization studies on mixing of curd and ingredients during Lassi (StirredCurd) manufacturing

Consumption of the fermented milk is on the rise due to its therapeutic benefits to human health. Several processes and technologies evolved to further enrich the fermented foods. In the present study, an Indian fermented milk product Lassi (sweet stirred curd) was selected for the experimental trials and optimized for the different process conditions. Lassi was prepared by adding sugar and water in curd and mixing them in the prefabricated mixing vessel. Three different impellers were tested in the preparation of Lassi. Two independent factors, namely off-bottom clearance (OBC) and impeller speed (IS) were selected to optimize the process conditions to yield a uniform good quality product. The quality of the finished product was assessed by sensorial and rheological attributes. Performance of the impeller was estimated by evaluating the mixing time, mixing index and power consumption. Three levels of OBC (3, 6, 9 cm) and IS (300, 400, 500 rpm) respectively were selected. Experiments were designed as per response surface methodology and accordingly trials were conducted. Constraints were optimized by setting up the goal for each of them. Based upon the solutions provided by the software, the product was prepared keeping the impeller positioned at 4.8 cm OBC and rotational speed of 389 rpm. The prepared product was validated against the experimental values and found no significant difference between theoretical and experimental values. Results indicated that the multivane churn impeller was found to be suitable for the preparation of good quality lassi. Mixing time and power consumption were optimized at 160±17.32 and 34.21±0.70, respectively. Overall acceptability of the product was 8.16±0.153 on 9-point hedonic scale. It can be concluded that the developed unit and impeller was able to deliver a uniform quality product with minimum mixing time and power consumption.

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.

Simulation of flow field characteristics in scheelite leaching tank with H2SO4–H3PO4

Abstract Digesting scheelite by using H2SO4–H3PO4 is an environment-friendly and low-cost technology. The key approach to achieving efficient scheelite decomposition involves providing a good environment with uniform material composition for the growth of calcium sulfate. Therefore, numerical simulation of gypsum particle suspensions in a square stirred tank with a frame-type agitator for leaching scheelite was investigated. Simulated optimized results showed that the homogeneity of a multiphase flow system increased with the speed of the agitator. Reducing off-bottom clearance eased the dispersion of gypsum into the liquid. Adding baffles increased turbulence intensity and axial velocity in the tank, which eased solid suspension. The suspension improved, together with increases in the torque and power requirements of the agitator when the speed changed and baffled were added. However, when the solid suspension improved, the stirring torque and power slightly decreased, under a different off-bottom clearance of the agitator. Meanwhile, with residence time distribution as an evaluation criterion, the experimental results verified that the flow characteristics of the solid particles improved after optimization. This study can provide a theoretical basis and guidance for the optimization of the design and enlargement test of the stirred tank for leaching scheelite with sulfuric–phosphorous mixed acid.

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.

Investigation of cell geometry effect on the turbulence characteristics and flotation performance using particle image velocimetry technique

The flow patterns and turbulence characteristics of the flow fields in a mechanical flotation cell have important effects on the hydrodynamic and metallurgical responses. In this study, the effect of cell geometry (off-bottom clearance (C), cell aspect ratio (H/T) and impeller diameter (D) on the turbulence properties and flotation performance using pure quartz particles of different sizes in a lab-scale Denver flotation cell was investigated. The results showed that the change in cell geometry changed the local turbulence characteristics. The change of C (from 0.2 T to 0.1 T), D (from T/2 to T/3) and H/T (from 1.2 to 1.5) did not change the flow pattern. The lower C was recommended to improve flotation performance. The larger D (T/2) was more useful for particle size of +38 μm. At the same Reynolds number, higher H/T is suitable for coarse particles, while lower H/T is recommended for particls size of −150 μm.

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.

Experimental investigation on solid suspension performance of coaxial mixer in viscous and high solid loading systems

The coaxial mixers consisting of an inner Rushton turbine or propeller and an outer anchor were combined first with the just off-bottom suspension in viscous and high solid loading systems. In this study, a new method based on RGB brightness analysis was proposed to determine the cloud height, and the measures to promote cloud height were also discussed. In addition, the effects of rotation modes, pumping modes and solid loading on the just-suspension power were investigated. The results showed that the co-rotation coaxial mixers with an up-pumping inner impeller could achieve the just suspension at lower power consumption and were recommended. The combination of anchor and up-pumping propeller was the most energy efficient among the investigated mixers, whereas its cloud height was relatively low. The high off-bottom clearance and the combined inner impeller could promote the cloud height with little power variation, but the enlargement of inner impeller had no obvious effect.

Mechanisms for drawdown of floating particles in a laminar stirred tank flow

Particle image velocimetry (PIV) experiments on a laminar stirred tank flow with floating particles at just drawdown conditions were performed. Careful refractive index matching of the two phases allowed to resolve the flow around the particles. The maximum solids volume fraction was 4%. The impeller was a pitched blade turbine with upward and downward pumping modes and with different off-bottom clearances, and the impeller-based Reynolds number ranged from about 50 to 120. Computational fluid dynamics (CFD) coupled to a discrete phase model (DPM) and discrete element method (DEM) was used to predict the flow field, with an emphasis on the tangential velocity component. The liquid velocity profiles predicted by the CFD simulations are in good agreement with the PIV experimental data. The drawdown process and local particle accumulation simulated by the DPM-DEM model agrees with the experimental phenomena as well. Tangential velocity and particles collisions around the shaft trigger the onset of the drawdown of the floating particles.

Experimental and numerical investigation of hydrodynamics and mixing in a dual-impeller mechanically-stirred digester

The aim was to investigate the influence of mixer design and operating conditions in an anaerobic digester designed for BioH2 production. Distributive mixing was studied through different dual-impeller configurations with various impeller size, clearance and types. Advanced optical experimental techniques, such as Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) were used to determine mixing time and the flow pattern in the bioreactor. PLIF, decolorization and conductimetric methods were compared for mixing analysis. CFD was used to achieve a more detailed analysis of hydrodynamics and mixing. Experiments and simulations were limited to power input lower than 10W/m3, which is a necessary condition to achieve sustainability and corresponds to 50–150rpm in this work. Nine designs of dual-impeller configurations were compared in an unbaffled tank. Experimental results suggest that mixing was strongly modified by changing the geometry of the lower impeller and, to a lesser extent, the off-bottom clearance and the injection position for dual-impeller devices. Macromixing time tm derived from the conductimetric technique always agreed with PLIF, but was higher than values from chemical decolorization when an axial-tangential flow pattern validated by PIV and CFD data was reported. CFD was able to predict the mean flow and the turbulence features, so that their respective role on mixing could be compared. Finally, a non-conventional mixer dual-impeller device was shown to counterbalance lower turbulence and power input by an enhanced axial flow circulation that made tm weakly dependent on the position of the injection and favored dispersive mixing.

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.