Critical Impeller Speed Research Articles

Numerical simulation study of the influence of stirred vessel structure on particle suspension characteristics in dilute solutions

The Euler-Euler/RANS approach has been used to simulate the particle suspension in solid–liquid stirred vessels, with experimental validation conducted through the Electrical Sensing Zone Method (ESZ). An innovative evaluation method has been introduced to quantify the critical impeller speed Nc required for particles to achieve a specific uniform suspension state in dilute solutions. The impact of different vessel bottom shapes (flat, round, and W-shaped) on particle suspension characteristics is examined. The results indicate that optimizing the bottom shape to align with flow streamlines markedly enhances particle suspension, a conclusion supported by the Online Micro-sized Particle Analyzer (OMiPA). Further analysis shows baffle configurations transform the tangential and radial velocities into the axial velocity, which is not conducive to providing initial conditions for particle suspension but improves overall suspension uniformity. Overall, effective particle suspension relies on the interaction between main flow and turbulent fluctuations, with tangential and radial flows playing critical roles.

Aqueous Two-Phase Separation (ATPS) Methods for Oleic acid extraction from Neem leaves

The aqueous two-phase separation system (ATPS) signifies an environmentally responsible approach for the extraction of bioactive compounds from a plants basis, as it is a liquid-liquid fractionation technique centred on the inconsistency of two aqueous solutions. In this investigation, various experimental parameters are optimized as the speed of agitation (200, 300 400 and 500 rpm) and solvent ratio (1:1, 2:3 and 3:2) with 20 % (w/w) of Ammonium Sulphate (AMS) salt composition and 30 % (w/w) of Polyethelyene Glycol (PEG). The obtained extract contains alkaloids, flavonoids, tannins, glycosides, acids and total phenolic compounds (TPC). The extracted Oleic acid by the ATPS method was measured with gallic acid equivalent (GAE) of TPC extracted from neem leaves powder. The determined concentration of oleic acid in the practice of TPC is 8.033 mg of GAE/g from the optimized experimental parameter. The optimized results can be cast off for a commercial process on an industrialized scale. Also, the mathematical modelling investigation was done to intent the critical impeller speed (Njs) with the Zwittering model. The identified model calculates the essential speed of agitation (rpm) for maximum extraction yield.

Bubble size distribution in aerated stirred tanks: Quantifying the effect of impeller-stator design

Bubble size is an important variable in aerated stirred tanks as it determines the surface area available for reactions. In the bubble break-up process, impellers play a key role. Despite this importance, research into the effect of impeller design on bubble size is scarce. In this work we study the effect of two impellers, with and without a stator, as well as the effect of airflow rate, impeller speed and surfactant concentration on bubble size.Results show that there is a critical impeller speed above which bubble size is not further decreased, regardless of the airflow. Operating at this critical speed results in the smallest bubble size possible without additional turbulence. The reduction in bubble size caused by a stator was quantified for the first time and, interestingly, it was found that the stator also reduced the critical coalescence concentration. The implications of these findings for the design, evaluation and optimisation of impellers are discussed.

Effects of surface vortex on the drawdown and dispersion of floating particles in stirred tanks

The effects of surface vortex on the drawdown and dispersion of floating particles in stirred tanks were investigated. Particle distribution and power consumption were analyzed by experiments and numerical simulations in both baffled and unbaffled tanks agitated by a Rushton impeller. In unbaffled tanks, a non-aggregation rule was applied and the average dispersion index was found to serve as a reasonable prediction of the full drawdown of floating particles. The critical impeller speeds in an unbaffled tank were higher than those in a tank with vertical baffles. At each immersion depth in a baffled tank, particles distributed more uniformly and more power was consumed. Comparison of snapshots of the baffled and unbaffled tanks shows that the surface vortex increases the drawdown speed while it decreases the particle distribution uniformity and power consumption. Therefore, the use of baffles to suppress the surface vortex provides for a more uniform particle distribution in stirred tanks.

CFD analysis of the flow dynamics of microorganisms in dilute cultures in stirred tank photobioreactors

The objective of the present work is to investigate the influence of the key factors such as the impeller speed in revolutions per minute and frequency of crossover of the particles between the light and the dark zones on the growth of microalgae in a stirred tank photobioreactor. To that end, the numerical simulations are performed for the culture in a stirred tank photobioreactor using computational fluid dynamics. The culture medium and the sparged air are considered to be two interpenetrating continua. Based on the light intensity profiles within, the reactor is divided into light and dark zones. Individual microorganisms are tracked. The fractional time spent by them in the dark zone, and the average frequency of crossover between the zones, are determined. The critical impeller speed for the maximum cell growth is compared against the experimental observation reported in literature and the maximum tolerable shear stress is identified.

Critical impeller speeds for a gas-inducing stirring tank loaded with solid particles

Abstract The influence of solid particles size, density and loading on the critical gas-inducing impeller speed was investigated in a gas–liquid–solid stirring tank equipped with a hollow Rushton impeller. Three types of solid particles, hollow glass beads with diameters of 300 μm, 200 μm, 100 μm, and 60 μm, silica gel and desalting resin, were used. It was found that the adding solid particles would change the critical impeller speed. For hollow glass beads and silica gel, whose relative densities were less than or equal to 1.5, the critical impeller speeds increased with the solid loading before reaching the maximum values, and then decreased to a value even lower than that without added solids. The size of the solids also had apparent influence on the critical impeller speed, and larger solid particles correspond to a smaller critical impeller speed. The experimental data also showed that the gas-inducing was beneficial to the suspension of the solid particles.

The Effect of Liquid Viscosity on Critical Impeller Speed in an Agitated Vessel

Background: Solid suspension in liquids is an important unit operation generally encountered in the process industry. In the present work solid−liquid suspensions are explored in an agitated vessel with different liquids using two radial impellers namely Rushton turbine and straight blade; two axial impellers namely pitched blade turbine and A320. The critical impeller speed (Njs) at which all solid particles get suspended and the relevant power requirements (Pjs) are evaluated. The effect liquid viscosity on these two parameters is investigated. Conclusion: Results show that critical impeller speed and power consumption decreases as liquid viscosity increases. The critical impeller speed was found to be highest for straight blade impeller and lowest for pitched blade turbine. A320 impeller showed strong dependence of viscosity on critical impeller speed. In addition, the power consumption was found to be least for A320 impeller among the impellers investigated in this study. Keywords: Impeller type, impeller clearance, liquid viscosity, solid suspension, critical impeller speed, wastewater treatment.

On the assessment of power consumption and critical impeller speed in vortexing unbaffled stirred tanks

Unbaffled stirred tanks are increasingly recognized as a viable alternative to common baffled tanks for a number of processes and bio-processes where the presence of baffles is undesirable. Notwithstanding the increasing industrial interest towards unbaffled tanks, available experimental information on their behaviour is still very poor, even for important parameters such as mechanical power drawn and critical impeller speed (Ncr) at which the transition between non-aerated (sub-critical regime) and aerated (super-critical regime) conditions occurs.In this work the influence of Reynolds and Froude numbers on power consumption characteristics of unbaffled stirred tanks is presented for tanks operating both in non-aerated (sub-critical regime) and aerated (super-critical) conditions. Influence of scale-up, impeller to tank size, liquid height aspect-ratio and presence or not of a top-cover is investigated in order to provide a general correlation able to assess (i) the critical rotational speed Ncr and (ii) the power number Np under any fluid dynamic regime.Experimental results obtained show that the proposed general correlation is fully validated when scaling-up the system. Moreover, the variation of geometrical features such as impeller to tank size, liquid aspect ratio and the presence or not of a top-cover do not modify the functional dependencies of power number on Re and Fr and only different (dimensionless) multiplying shape factors must be adopted. Finally, an overall general correlation for critical rotational speed (Ncr) assessment is also proposed.

Effect of Impeller Clearance and Liquid Level on Critical Impeller Speed in an Agitated Vessel using Different Axial and Radial Impellers

The effect of impeller clearance and liquid level on the critical impeller speed (Njs) for various radial and axial flow impellers in 0.29 m ID agitated vessel has been studied. Five types of radial impellers: Rushton turbine (RT), Straight blade (SB), Curved blade (CB), Curved blade with disc (CBWD) and R130 impeller and four types of axial impellers: Rushton turbine 45o angle (RT 45), Pitched blade (PBT), A320 and HE3 impeller were used. Tap water and resin particle of 0.506 mm were used as liquid and solid phases, respectively. The impeller clearance to vessel diameter (T) was varied between 0.17 and 0.41. The liquid level (H) was also varied as H/T=0.5, H/T=0.75 and H/T=1. The R130 impeller and A320 impeller was found to be more efficient among radial and axial impellers respectively. A new expression for Zwietering constant ‘S’ was developed to predict critical impeller speed, considering impeller clearance and liquid level for all the impellers. The results obtained here show that the ‘S’ values increase with increase in clearance, and decrease with liquid level for all impellers and it also depends on the type of impeller.

Influence of different factors on momentum transfer in mechanically agitated multiphase systems

Abstract A comparative analysis concerning the influence of different factors on momentum transfer in mechanically agitated systems was carried out on the basis of experimental results for solid-liquid, gas-liquid and gas-solid-liquid systems. The effects of the impeller - baffles system geometry, scale of the agitated vessel, type and number of impellers and their off-bottom clearance, as well as physical properties of the multiphase systems on the critical impeller speeds needed to produce suspension or dispersion, power consumption and gas hold-up were analysed and evaluated.

Effect of impeller type and density difference on the draw down of low density microspheres

Experiments on dispersion of floating solids into continuous liquid medium were carried out with low density microspheres with particle density of 680kg/m3 and size 100, 230 and 325μm in different liquids having density varying from 778 to 1830kg/m3 and kinematic viscosity varying from 0.3 to 19cP. A four-bladed radial impeller and axial paddle impellers with upward and downward flow directions were employed. The critical impeller speed (Ncrit) required for uniform dispersion of particles throughout the continuous medium was experimentally determined for various conditions. Experimental results indicate that the radial impeller with impeller location near the surface required minimum impeller speed for uniform dispersion. Strong effect of the density difference between the particles and the liquid and submergence of the impeller was noted. Based on the experimental results, a correlation in terms of relevant dimensionless groups was proposed for calculating Ncrit for the three impellers. The overall correlation indicates that the critical impeller speed is not significantly affected by liquid viscosity and particle diameter but that it is strongly influenced by the impeller diameter and the density difference. These results are in agreement with trends reported in the literature.

Optimum solids concentration for solids suspension and solid–liquid mass transfer in agitated vessels

The effect of solids concentration on specific impeller power consumption and solid–liquid mass transfer coefficient was investigated in a 0.2 m diameter baffled agitated vessel with standard six-bladed Rushton turbine for solids concentration up to 0.40 (v/v). It was found that the increase of solids concentration significantly increases the mass transfer coefficient up to an optimum solids concentration and decreases thereafter when the system is operated at a just-suspended condition. The increase in mass transfer coefficient with an increase of solid concentrations is mainly due to the increase in Njs (critical impeller speed) with increasing solids concentration, thereby leading to an upsurge of turbulence around the particles. The solids concentration at which the highest mass transfer coefficient is obtained is designated as the effective solids concentration. In a geometrically similar 0.3 m diameter tank, the trends in impeller energy efficiency and solid–liquid mass transfer coefficient values with increasing solids concentration are similar. The experimental data are satisfactorily correlated using the concept of the Kolmogoroff's theory of isotropic turbulence to develop an equation to estimate the solid–liquid mass transfer coefficient in agitated vessels.

Principle and Performance of Gas Self-inducing Reactors and Applications to Biotechnology.

Gas-liquid contacting is an important unit operation in chemical and biochemical processes, but the gas utilization efficiency is low in conventional gas-liquid contactors especially for sparingly soluble gases. The gas self-inducing impeller is able to recycle gas in the headspace of a reactor to the liquid without utilization of additional equipment such as a gas compressor, and thus, the gas utilization efficiency is significantly enhanced. Gas induction is caused by the low pressure or deep vortex at a sufficiently high impeller speed, and the speed at which gas induction starts is termed the critical speed. The critical impeller speed, gas-induction flow rate, power consumption, and gas-liquid mass transfer are determined by the impeller design and operation conditions. When the reactor is operated in a dead-end mode, all the introduced gas can be completely used, and this feature is especially favorable to flammable and/or toxic gases. In this article, the principles, designs, characteristics of self-inducing reactors, and applications to biotechnology are described.

Controlling granule size through breakage in a novel reverse-phase wet granulation process; the effect of impeller speed and binder liquid viscosity

The feasibility of a novel reverse-phase wet granulation process has been established previously highlighting several potential advantages over the conventional wet granulation process and making recommendations for further development of the approach. The feasibility study showed that in the reverse-phase process granule formation proceeds via a controlled breakage mechanism. Consequently, the aim of the present study was to investigate the effect of impeller speeds and binder liquid viscosity on the size distribution and intragranular porosity of granules using this novel process. Impeller tip speed was found to have different effects on the granules produced by a conventional as opposed to a reverse-phase granulation process. For the conventional process, an increase in impeller speed from 1.57 to 3.14ms−1 had minimal effect on granule size distribution. However, a further increase in impeller tip speed to 3.93 and 4.71ms−1 resulted in a decrease in intragranular porosity and a corresponding increase in mean granule size. In contrast when the reverse-phase process was used, an increase in impeller speed from 1.57 to 4.71ms−1 resulted in increased granule breakage and a decrease in the mean granule size. This was postulated to be due to the fact that the granulation process begins with fully saturated pores. Under these conditions further consolidation of granules at increased impeller tip speeds is limited and rebound or breakage occurs. Based on these results and analysis of the modified capillary number the conventional process appears to be driven by viscous forces whereas the reverse-phase process appears to be driven by capillary forces. Additionally, in the reverse-phase process a critical impeller speed, represented by the equilibrium between centrifugal and gravitational forces, appears to represent the point above which breakage of large wet agglomerates and mechanical dispersion of binder liquid take place. In contrast the conventional process appears to be difficult to control due to variations in granule consolidation, which depends upon experimental variables. Such variations meant increased impeller tip speed both decreased and increased granule size. The reverse-phase process appears to offer simple control over granule porosity and size through manipulation of the impeller speed and further evaluation of the approach is warranted.

CFD simulation of complete drawdown of floating solids in a stirred tank

AbstractComputational fluid dynamics (CFD) simulations were carried out to find a CFD methodology to predict the critical impeller speed (NCS) for the complete drawdown of floating solids in a stirred tank with an up‐ and down‐pumping pitched blade turbine (PBTU and PBTD). CFD models along with six mixing parameters predicted that NCS was 7.5 rps and 5.8 rps, respectively, for PBTU and PBTD, when impeller submergence (S) was equal to half of the liquid height (H/2). Reasonable agreement was observed between CFD simulations and published empirical correlation. NCS was smaller for PBTD relative to PBTU at S = H/2, which agreed with results in the literature. The drawdown mechanisms based on the mean drag, turbulent fluctuations or large scale eddies failed to explain the difference in NCS between PBTU and PBTD. The flow patterns based on a simulated velocity field were further investigated and successfully accounted for this difference.

Behaviour of bubble clusters in a turbulent flotation cell

The rate of capture of particles decreases as the particle size increases in froth flotation. It has been postulated that the upper size range of particles that can be recovered in conventional machines could be extended by the use of bubble clusters [1].This study is concerned with the behaviour of bubble clusters in turbulent flotation cell. The breakup and re-formation of clusters and the effect of bubble size and impeller speed on the behaviour of clusters have been investigated. The apparatus used was essentially a laboratory flotation cell, agitated by a Rushton turbine. The cell was modified to allow pre-formed clusters to rise out of a fluidized bed and into the path of the rotating impeller. The events were captured using a digital camera, and the images were analysed to give the sizes of the bubbles and clusters.In the first part of the investigation, a collector was used but no frother. Under these conditions, the bubble diameter was effectively controlled by the collector concentration, and it varied considerably. It was found that the sizes of clusters decrease with increasing shear rate at low impeller speeds, and at higher speeds the clusters are broken up into bubbles and particles.In the second part, frother was used at a concentration above the critical coalescence concentration, to control the bubble size, which remained essentially constant at this concentration. The bubbles were too small to be broken by the action of the impeller, so they always remained at the same size. In this case it was found that when the impeller speed was increased, two stages of formation were observed, the fragmentation and equilibrium stages. In the fragmentation stage, at low impeller speeds, the clusters were loose and filamentous, and as the energy input increases, they rupture and re-form. In the second stage, above a critical impeller speed, dense clusters formed whose size was relatively insensitive to the energy input.

Computational fluid dynamics analysis to effects of geometrical design and physical property on complete suspension of floating solids in stirred tanks

ABSTRACTThe computational fluid dynamics methodology was developed to qualitatively study effects of geometrical design and physical property on the mixing quality of floating solids in stirred tanks with an up‐pumping pitched blade turbine. The multiple reference frame approach was used to model the relative motion between rotating impeller and stationary baffles. Two important mixing parameters, power number (Np) and standard deviation (σ), were used to predict critical impeller speed (NCS) for complete suspension of floating solids and compare the mixing efficiency. Predicted results were compared with the experiment results in the open literature and were explained with published mechanisms or theories concerning solid suspension. Reasonable agreements were achieved. Copyright © 2014 Curtin University of Technology and John Wiley & Sons, Ltd.

Numerical simulation of macro-mixing in liquid–liquid stirred tanks

Numerical simulations of turbulent immiscible liquid–liquid mixing processes in cylindrical stirred tanks driven by a Rushton turbine are carried out based on an Eulerian–Eulerian approach using in-house codes. An isotropic standard k−ε turbulence model and an anisotropic two-phase explicit algebraic stress model (EASM) are used for flow field simulations. Quantitative comparisons of the homogenization curve and mixing time predicted by the EASM are conducted with reported experimental data and other predictions by the standard k–ε model and large eddy simulation (LES). The comparisons show that the EASM predictions are in satisfactory agreement with experimental data and better than the k–ε model ones. The variation of the continuous phase mixing time with impeller speed can be an effective method to determine the critical impeller speed for complete dispersion of oil phase. The key features of the complex liquid–liquid mixing processes in stirred tanks have been successfully predicted by the EASM, which can be an alternative tool for practical engineering applications with economical computational cost and good accuracy.

Impact of liquid driving flow on the performance of a gas-inducing impeller

The present work was conducted to improve the gas-induction performance over a preliminary gas-inducing impeller designed in our previous work. For the first time, a series of liquid-inlet holes with different diameters were drilled separately so as to examine their effects on the gas-induction process and also to find the optimum size. The critical impeller speed (NCG) and the overall gas hold-up (ɛG) were measured experimentally in order to evaluate the properties of the gas-inducing impeller. A mathematical model, which takes into account the geometrical parameters of the gas-inducing impeller, was proposed to predict NCG of the gas-inducing impeller with or without liquid-inlet holes. The predictions are in good agreement with the experimental results. The results also show that the optimum gas-induction properties would be obtained with respect to the lowest value of NCG and the highest value of ɛG, when the diameter of the liquid-inlet hole is approximately 0.5 times the diameter of the gas-inducing pipe.

Solid Suspension in Stirred Tank Equipped with Multi-Side-Entering Agitators

The flow field and solid distribution in a stirred tank equipped with four side-entering agitators were investigated using both experimental measurements and CFD simulations. Experiments were carried out to determine the regularity of the solid sediment distribution on the tank bottom and the critical impeller speed for off-bottom suspension. The CFD simulation was performed using the Eulerian-Eulerian two-phase approach, the standard k-ε turbulence model and the multiple reference frame (MRF) approach. The predicted critical impeller speeds were compared with the experimental data to validate the CFD model. The effects of the solid loading and some installation parameters including inclined angles of the impeller, plunging length and mounting height of the shaft on the critical impeller speed were investigated. The predicted results showed reasonably good agreement with the experimental data for both the distribution of solid accumulation and critical impeller speeds. The solid loading has a greater influence on the critical impeller speed in the side-entering stirred tank than that in the top-entering tank. On the basis of the CFD simulation determined critical impeller speeds under different conditions, the above installation parameters of the agitator were optimized. The results and discussion here will have useful implications for design and optimization of the solid-liquid suspension in side-entering stirred tanks.