High-speed Impeller Research Articles

Abrasion of Particles by a Small Impeller in an Agitated Vessel

There are a number of operations that are carried out in the process industries by a small impeller in an agitated vessel in which particles are suspended, and thus, the possible abrasion of the particles should be considered precisely to understand the operation. To investigate the abrasion of particles in an agitated vessel, an aggregated spherical silica particle, whose diameter was set at about 2.0 mm or 1.0 mm, was prepared purposely, put into a vessel, and agitated by a Rushton turbine impeller (RT) or an edged turbine impeller (ET). The change in the particle diameter with time elapse was measured as an area-equivalent diameter by using a CCD camera. The frequency of the particle passing through the impeller-swept region was also measured by the 3-D PTV method. As a result, it was found that an ET is superior to a RT for the abrasion of particles at the same power input per unit volume. For both the impellers, the power efficiency of particle abrasion at a very high impeller speed was less than that at a low impeller speed. However, it was also found that with increasing impeller speed, the particle abrasion rate per circulation of the particle passing through the impeller-swept region, Rω, increased, especially at a high tip velocity in the case of an ET. At the same maximum impact energy, Rω in the case of an ET was more than that in the case of a RT.

A study on blending characteristics of axial flow impellers

Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Process Engineering, Technicka 4, 166 07 Prague 6, Czech Republic This paper presents an analysis of the blending characteristics of axial flow high-speed impellers under a turbulent regime of flow of an agitated low viscosity liquid. The conductivity method is used to determine the time course of blending (homogenisation) of miscible liquids in a pilot plant fully baffled mixing vessel, and a torquemeter is used for measuring the impeller power input in the same system. Four-blade and six-blade pitched blade impellers and three high efficiency axial flow impellers are tested for the given degree of homogeneity (98%). The experimental results and also the results of the authors´ previous study, in accordance with the theoretical approach described in the literature, show that there is a universal relationship between the impeller power number and the dimensionless blending time, taking into consideration the impeller-to-vessel diameter ratio, independent of the geometry of the axial flow impeller but dependent on the degree of homogeneity. This relationship is found to be valid on a pilot plant scale under a turbulent flow regime of an agitated liquid. Keywords: axial flow impeller, pitched-blade impeller, turbulent flow, impeller power input, blending time

The effect of the physical properties of the liquid phase on the gas-liquid mass transfer coefficient in two- and three-phase agitated systems

AbstractThe results of studies concerning two- and three-phase systems in an agitated vessel are presented. The aim of our research was to investigate the effect of the physical properties of the liquid phase on the value of the volumetric gas-liquid mass transfer coefficient in mechanically agitated gas-liquid and gas-solid-liquid systems. Our experimental studies were conducted in a vessel with an internal diameter of 0.288 m. The flat bottom vessel, equipped with four baffles, was filled with liquid up to a height equal to the inner diameter. The liquid volume was 0.02 m3. Three high-speed impellers of a diameter equal to 0.33 of the vessel diameter were used: Rushton turbine (RT), Smith turbine (CD 6), or A 315 impeller. The measurements were carried out in coalescing and non-coalescing systems. Distilled water and aqueous solutions of an electrolyte (sodium chloride) of two different concentrations were used as the liquid phase. The gas phase was air. In the three-phase system, particles of sea sand were used as solid phase. The measurements were conducted at five different gas-flow rates and three particle loadings. Volumetric gas-liquid mass transfer coefficients were measured using the dynamic method. The presence and concentration of an electrolyte strongly affected the value of the gas-liquid mass transfer coefficient in both two- and three-phase systems. For all agitators used, significantly higher k l a coefficient values were obtained in the 0.4 kmol m−3 and 0.8 kmol m−3 aqueous NaCl solutions compared with the data for a coalescing system (with distilled water as the liquid phase). The k l a coefficient did not exhibit a linear relationship with the electrolyte concentration. An increase in the sodium chloride concentration from 0.4 kmol m−3 to 0.8 kmol m−3 caused a considerable decrease in the volumetric mass transfer coefficient in both the two-phase and three-phase systems. It was concluded that the mass transfer processes improved at a certain concentration of ions; however, above this concentration no further increase in k l a could be achieved.

Research on Multi-Axis High Speed Machining Milling Force of Aeroengine Impeller

On the basis of the ball-end milling feature during high-speed machining impeller, the relationship between cutting force and chips is analyzed in this paper. The model of ball-end milling cutter cutting force is founded through differential method. And the coefficients solution of cutting force model is expounded. Besides, the coefficients solution and the cutting force model simulation are implemented by the software Matlab.

Effects of process parameters on granules properties produced in a high shear granulator

Results of a study on the influence of process parameters such as impeller speed, granulation time and binder viscosity on granule strength and properties are reported. A high shear granulator (Cyclomix manufactured by Hosokawa Micron B.V., The Netherlands) has been used to produce granules. Calcium carbonate (Durcal) was used as feed powder and aqueous polyethylene glycol (PEG) as the binder. The dried granules have been analysed for their strength, density and size distribution. The results show that increasing the granulation time has a great affect on granules strength, until an optimum time has been reached. The underlying cause is an increase in granule density. Granules are consolidated more at higher impeller speeds. Moreover, the granule size distribution seems not to be affected significantly by an increase in impeller speed. Granules produced with high binder viscosity have a considerably lower strength, wide strength distribution due to poor dispersion of binder on the powder bed. Binder addition methods have showed no considerable effect on granule strength or on granule size distribution.

Wet granulation in laboratory-scale high shear mixers: Effect of chopper presence, design and impeller speed

The effect of the main means of agitation in a high shear mixer has been investigated in this study. Granulation runs have been performed on a fine cohesive microcrystalline cellulose powder (Avicel 105, d 50 = 20 μm) often used as a pharmaceutical excipient in tablet formulations in two bowls of a Mi-Pro® laboratory high shear mixer with a capacity of 0.9 and 1.9 L, respectively. Torque curves recorded during granulation are found to allow good control of the process while increasing impeller speed is found to generally reduce granule size and the onset of breakage seems to occur for similar values of impeller tip speed. As a general rule, the chopper allows for better binder distribution in the Mi-Pro® and is found to be necessary for successful granulation at low to moderate impeller speeds. For high impeller speeds in excess of 4.4 m/s with or without a chopper, similar granule sizes and growth mechanisms are observed. Granule roundness was found to increase with impeller speed up to a certain speed after which granule roundness has been found to decrease with increasing impeller speed most probably because of increased breakage of the granules. Dry granule strength has been found to increase with increasing impeller speed, presenting only a slight decrease at the highest impeller speed studied.

Solid suspension and gas dispersion in gas-solid-liquid agitated systems

AbstractThe aim of the research work was to investigate the effect of superficial gas velocity and solids concentration on the critical agitator speed, gas hold-up and averaged residence time of gas bubbles in an agitated gas-solid-liquid system. Experimental studies were conducted in a vessel of the inner diameter of 0.634 m. Different high-speed impellers: Rushton and Smith turbines, A 315 and HE 3 impellers, were used for agitation. The measurements were conducted in systems with different physical parameters of the continuous phase. Liquid phases were: distilled water (coalescing system) or aqueous solutions of NaCl (non-coalescing systems). The experiments were carried out at five different values of solids concentration and gas flow rate. Experimental analysis of the conditions of gas bubbles dispersion and particles suspension in the vessel with a flat bottom and four standard baffles showed that both gas and solid phases strongly affected the critical agitation speed necessary to produce a three-phase system. On the basis of experimental studies, the critical agitator speed for all agitators working in the gas-solid-liquid systems was found. An increase of superficial gas velocity caused a significant increase of the gas hold-up in both coalescing and non-coalescing three-phase systems. The type of the impeller strongly affected the parameters considered in this work. Low values of the critical impeller speed together with the relatively short average gas bubbles residence time tR in three phase systems were characteristic for the A 315 impeller. Radial flow Rushton and Smith turbines are high-energy consuming impellers but they enable to maintain longer gas bubbles residence time and to obtain higher values of the gas hold-up in the three-phase systems. Empirical correlations were proposed for the critical agitator speed, mean specific energy dissipated and the gas hold-up prediction. Its parameters were fitted using experimental data.

Mechanism for the effect of agitation on the molecular weight of hyaluronic acid produced by Streptococcus zooepidemicus

Various views persist about the effect of agitation on hyaluronic acid (HA) molecular weight, and the mechanism remains unclear. In this paper, the effect of agitation on HA molecular weight was investigated. The HA molecular weight increased with impeller speed, due to a mass transfer enhancement; however, higher impeller speed caused a decrease in the HA molecular weight. According to the experimental results, it is proposed for the first time that reactive oxygen species (ROS) under higher impeller speed in the bioreactor, which were generated by NADH oxidase, could cause the reduction in the HA molecular weight. ROS generation increased remarkably during the exponential growth phase when the impeller speed increased from 450 rpm to 1000 rpm, and the NADH oxidase activity increased correspondingly. Finally, it was proved that the addition of salicylic acid could increase the HA molecular weight, but inhibit the efficiency of the HA synthesis.

Evaluation of platelet activation in patients supported by the Jarvik 2000 ∗ high–rotational speed impeller ventricular assist device

The Jarvik 2000 (Jarvik Heart, Inc, New York, NY) is a thumb-sized high-speed impeller pump that is used as a ventricular assist device in patients with terminal heart failure. Because the Jarvik 2000 is designed for long-term use, it is a central question whether the mechanical forces inside the pump affect blood components. This study evaluated the potential association of the high rotational speed of the Jarvik 2000 with platelet activation, which may result in thromboembolic events. The study group comprised patients with terminal heart failure who were supported with the Jarvik 2000. All were men and received 100 mg aspirin daily. In 8 patients, soluble platelet activation markers (P-selectin and sCD40L), platelet counts, and hemolysis markers (haptoglobin and lactate dehydrogenase levels) were determined. In 5 patients, P-selectin expression and platelet receptor glycoprotein IIb/IIIa activation were determined with flow cytometry and compared with a control group of 5 healthy men. Platelet activation was measured at various rotational device speeds. After Jarvik 2000 implantation, increased hemolysis was observed, but platelet activation markers and platelet counts were not affected. Increased rotational speed (8000 to 12,000 rpm) of the device also did not result in increased platelet activation. The Jarvik 2000 was not associated with detectable platelet activation, despite high rotational impeller speeds.

Characterisation of granule structure and strength made in a high shear granulator

Results of a study of the influence of impeller speed on the strength, structure and morphology of granules produced in a type of high shear mixer granulators are reported. Calcium carbonate particles (Durcal 65) have been granulated in a Cyclomix with a capacity of 5 L. An aqueous solution of polyethylene glycol was used as the binder. The granules produced have been dried and their structure visualized using X-ray micro-tomography equipment, Nanotom, with a resolution of less than 1 μm. It is shown that the operation of the granulator at high impeller tip speeds produces granules with a higher strength and lower porosity than those produced at medium and low impeller speeds. Two different granule micro-structures and morphologies are produced at high and low impeller speeds. Structure descriptors such as phase volume fraction (as representative of porosity), chord length distribution and auto-correlation function (as indices of homogeneity of structure) are used to quantify the internal structure of granules in 3D, which in turn affects the granule strength.

Effects of vessel baffling on the drawdown of floating solids

AbstractThe effects of baffling of an agitated vessel on the production of floating particles suspension are presented in this paper. Critical agitator speed, needed for particles dispersion in a liquid agitated in a vessel of the inner diameter of 0.295 m, was determined. The just drawdown agitator speeds were defined analogously to the Zwietering criterion. Specific agitation energy was calculated from the power consumption experimental data obtained by means of the strain gauge method. The experiments were carried out for twelve configurations of the baffles differing in number, length and their arrangement in the vessels. The following high-speed impellers were used: up- and downpumping six blade pitched blade turbines, Rushton turbine, and propeller. The impeller was located in the vessel in the height equal to two-thirds or one-third of the vessel diameter from the bottom of the vessel. The results were described in the form of a dimensionless equation.

The flotation of fine particles using charged microbubbles

One of the major problems in the flotation of fine particles is the decreased probability of collision between particles and bubbles, which can be improved by a reduction in bubble size. Additionally, smaller particles, having lower momentum, may not be able to break through the liquid barrier surrounding a bubble. In order to overcome this limitation, flotation exploiting electrostatic interactions has potential. Colloidal gas aphrons (CGAs) are charged microbubbles generated using a high speed impeller, typically of the size 50 μm. In this work, CGAs generated using the anionic surfactant sodium dodecyl sulphate (SDS) have been used to separate copper oxide and silica powder (−10 μm). By the manipulation of pH (therefore zeta potential), particles can be recovered by bubbles of opposite charge. Agglomeration between particles of different charge is a potential problem, however, initial results indicate a good separation using a novel system of froth recovery.

Mixing time in an agitated multi‐lamp cylindrical photoreactor using electrical resistance tomography

AbstractBACKGROUND: There is scarce information on the application of electrical resistance tomography (ERT) in UV photoreactors, in which mixing and mass transfer are important. Therefore, the feasibility of an ERT system in an agitated multi‐lamp UV photoreactor was investigated to monitor the mixing process.RESULTS: The locations of the UV tubes had a significant impact on the mixing time, particularly at the lower impeller speeds (45 and 150 rpm). Also, at the higher impeller speeds (250, 350, and 500 rpm) and the same radial position (r), changing the angle θ from 15 to 45°, resulted in only a slight variation of the mixing time. Finally, the maximum mixing time occurred when UV tubes were positioned at r = 13 cm (r/R = 0.68) and θ = 0°, while minimum mixing time occurred at location r = 16 cm (r/R = 0.83) and θ = 45°.CONCLUSION: The experimental results demonstrated the feasibility of the ERT system to monitor the mixing process in the UV photoreactor. The ERT results also indicated that the locations of the UV tubes had a significant effect on the mixing performance of the photoreactor. Furthermore, the mixing time varied inversely with the rotational speed, and this effect was more pronounced at lower speeds. Copyright © 2008 Society of Chemical Industry

Scale-up synthesis of lipase-catalyzed palm esters in stirred-tank reactor

Lipase-catalyzed production of palm esters by alcoholysis of palm oil with oleyl alcohol in n-hexane was performed in 2 L stirred-tank reactor (STR). Investigation on the performance of reactor operation was carried out in batch mode STR with single impeller mounted on the centrally located shaft. Rushton turbine (RT) impellers provide the highest reaction yield (95.8%) at lower agitation speed as compared to AL-hydrofoil (AL-H) and 2-bladed elephant ear (EE) impellers. Homogenous enzyme particles suspension was obtained at 250 rpm by using RT impeller. At higher impeller speed, the shear effect on the enzyme particles caused by agitation has decreased the reaction performance. Palm esters reaction mixture in STR follows Newtons’ law due to the linear relation between the shear stress ( τ) and shear rate (d υ/d y). High stability of Lipozyme RM IM was observed as shown by its ability to be repeatedly used to give high percentage yield (79%) of palm esters even after 15 cycles of reaction. The process was successfully scale-up to 75 L STR (50 L working volume) based on a constant impeller tip speed approach, which gave the yield of 97.2% after 5 h reaction time.

Effects of type and number of impellers and liquid viscosity on the power characteristics of mechanically agitated gas—liquid systems

AbstractEffect of the type and number of high-speed impellers installed on a common shaft on the power characteristics was investigated in water and glucose solutions of different concentration. Different configurations of the Rushton or Smith turbines, pitched blade turbines, propeller, and A 315 impeller were tested. Measurements of power consumption were carried out within the transitional and turbulent regime of the fluid flow using the strain gauge method. Baffled agitated vessels with inner diameter of 0.288 m and 0.634 m were used for the experiments. Liquid height in the vessels was equal to the vessel diameter or it was twice higher. The relative power consumption was compared for different configurations of the impellers.

GAS-LIQUID MASS TRANSFER CHARACTERISTICS OF LARGE-SCALE IMPELLERS: EMPIRICAL CORRELATIONS OF GAS HOLDUPS AND VOLUMETRIC MASS TRANSFER COEFFICIENTS IN STIRRED TANKS

ABSTRACT Gas dispersion with large-scale impellers consisting of modified large paddle impellers in stirred tanks, with rather large ratios of both impeller diameter and impeller height to tank diameter, was experimentally examined in transition and turbulent mixing ranges. Gas holdups and volumetric gas-liquid mass transfer coefficients with large-scale impellers, i.e., Maxblend and Fullzone impellers, were measured in 0.31 and 0.6 m I.D. stirred tanks, and the gas dispersion performance of large-scale impellers was compared with that of double conventional small-scale high-speed impeller systems, i.e., double four-flat blade disk turbine impellers and double four-flat paddle impellers. The gas holdups of the large-scale impellers were comparable with those of the small-scale impeller systems at a given rotational speed. The volumetric gas-liquid mass transfer coefficients for large-scale impellers were also similar to those of the small-scale impeller systems. It was found that the large-scale impellers are not more energy efficient than the small-scale impellers in obtaining good gas dispersion. Empirical correlations for gas holdups and volumetric gas-liquid mass transfer coefficients were developed. They fit the experimental data in transition and turbulent mixing ranges reasonably well, with correlation factors greater than 0.84.

Effect of mixing protocol on formation of fine emulsions

Emulsions are usually stabilised with a mixture of surfactants with different hydrophilicity. The initial partitioning of surfactants between the dispersed phase and continuous phase, and how these phases are brought into contact, can significantly affect the emulsification processes. Dynamic-phase behaviour maps were prepared to allow for a systematic investigation of the effects of emulsification routes on emulsion properties. Six semibatch modes of additions with constant surfactant concentration across the routes were selected. For a target cyclohexane-in-water emulsion using a pair of polyoxyethylene nonylphenyl ether surfactants with a specified HLB and water volume fraction, fine droplets could form only if water dissolving the water-soluble surfactant was added to the oil dissolving the oil-soluble surfactant. This route allowed the transitional inversion to occur and as a result fine droplets were formed due to an ultra-low interfacial tension. The addition of water dissolving the water-soluble surfactant to oil dissolving the oil-soluble surfactant, direct emulsification method, produced by far large droplets because of a rather high interfacial tension. In a series of experiment, the semibatch direct and phase-inversion emulsification method, were assimilated in situ. The impeller location was used as a variable that controls which phase is added as the dispersed phase. The location of impeller in relation to the interface did not affect the emulsion drop size at a high agitation rate, but it did at a low agitation rate. Under low agitation speed and when the impeller was placed in the oil phase, the oil layer progressively, but slowly, dragged the water phase and eventually inverted to an oil-in-water emulsion, indicating that transitional-phase inversion has locally occurred in the oil layer. At a high agitation speed the mechanical energy provided by the impeller homogenised the emulsion instantaneously and did not allow the optimum formulation and the associated ultra-low interfacial tension to be reached regardless of location of the impeller. A high impeller speed increased drop size by transforming the transition inversion mechanism to a catastrophic mechanism under which the size of drops is mainly determined by the mechanical energy provided. This paper aims to show how some of the complexities involved in emulsification processes can be explained by consulting with dynamic-phase maps.

Liquid-phase mixing time in boiling stirred tank reactors with large cross-section impellers

Liquid-phase mixing times in boiling systems were measured in a 0.2 m i.d. stirred tank reactors with large cross-section impellers, i.e. Maxblend and Fullzone impellers. For reference, a triple-impeller system consisting of three six-flat blade disk turbines was also used. The impeller speed and vapor generation rate were varied from 0.83 to 7.5 s −1 and 0.011 to 0.045 m/s, respectively. Nucleation occurred at the impeller instead of the heater at higher impeller speeds, whereas vapor was mainly generated from the heater at lower impeller speeds. The mechanical power consumption decreased due to vapor generation. Although the gas hold-ups increased with increasing vapor generation rate, for the large cross-section impellers the gas hold-ups in the boiling systems at higher gas flow rates and lower impeller speeds changed only slightly with increasing impeller speeds and were rather larger than those in the cold systems. This finding for the large cross-section impellers was quite different from the result for the triple-impeller system. The mixing times for larger-scale impellers decreased with increasing vapor generation rate and were almost independent on the impeller speed at low rotational speeds of impeller. At higher impeller speeds, the mixing time decreased with impeller speed. The experimental results for dimensionless mixing times were reasonably correlated with the inverse of the power number.

High shear wet granulation modelling—a mechanistic approach using population balances

A population balance approach based on splitting the coalescence kernel into two factors, the first describing the collision frequency of particles and the second describing the collision efficiency, is applied to modelling wet granulation in a high shear mixer. Four different expressions for the collision frequency are compared and discussed. The kernels are the size independent kernel, the shear kernel proposed by Smoluchowski [M. Smoluchowski, “Versuch einer matematischen Theorie der Koagulationskinetic kolloider Lösungen”, Z. Physik. Chem. 92, (1917) 129–168.] and the two kernels proposed by Hounslow [M.J. Hounslow, “The population balance as a tool for understanding particle rate processes”, Kona (1998) 179–193.], i.e. the EKE kernel and the less used kernel based on equipartition of fluctuating translational momentum (ETM kernel). Microcrystalline cellulose (mcc) is granulated under different process conditions and it is found that the ETM kernel best describes the granulation at higher impeller speeds, whereas the EKE kernel gives better agreement at lower impeller speeds. The collision efficiency is assumed to be a function of the liquid saturation. By using this assumption, it was possible to detect similar trends for the remaining part of the collision efficiency regardless of process conditions.

Methods and milliliter scale devices for high-throughput bioprocess design

Based on electromagnetic simulations as well as on computational fluid dynamics simulations gas-inducing impellers and their magnetic inductive drive were optimized for stirred-tank reactors on a 10 ml-scale arranged in a bioreaction block with 48 bioreactors. High impeller speeds of up to 4,000 rpm were achieved at very small electrical power inputs (63 W with 48 bioreactors). The maxima of local energy dissipation in the reaction medium were estimated to be up to 50 W L(-1) at 2,800 rpm. Total power input and local energy dissipation are thus well comparable to standard stirred-tank bioreactors. A prototype fluorescence reader for 8 bioreactors with immobilized fluorometric sensor spots was applied for online measurement of dissolved oxygen concentration making use of the phase detection method. A self-optimizing scheduling software was developed for parallel control of 48 bioreactors with a liquid-handling system for automation of titration and sampling. It was shown on the examples of simple parallel batch cultivations of Escherichia coli with different media compositions that high cell densities of up to 16.5 g L(-1) dry cell mass can be achieved without pH-control within 5 h with a high parallel reproducibility (standard deviation<3.5%, n=48) due to the high oxygen transfer capability of the gas-inducing stirred-tank bioreactors.