Effect of the attack angle on the roll and trailing vortex structures in an agitated vessel with a paddle impeller

The purpose of the present study is to observe the effect of the blade attack angle on the roll and trailing vortex structures in a stirred vessel via laser-Doppler velocimetry (LDV). In this investigation, four-bladed paddle impellers with four attack angles, which were 45^°, 60^°, 75^° and 90^°, respectively, were used. By synchronizing LDV with a rotary encoder coupled to the impeller shaft, angle-resolved measurements of all three velocity components were performed. This experimental method made it possible to capture the details of the vortical structure both behind the impeller blade and discharge region. Our study on the mean flow structure generated by three types of pitched blade turbines (45^°, 60^°, and 75^°, respectively) found that a single trailing vortex was formed around each turbine blade. Roll-up of the vortex sheet issuing from the blade tip was also observed, which indicated a major roll of trailing vortex generation mechanism for each pitched blade turbine.     

CFD simulation of impeller shape effect on quality of mixing in two-phase gas–liquid agitated vessel

Mixing efficiency in two-phase gas–liquid agitated vessel is one of the important challenges in the industrial processes. Computational fluid dynamics technique (CFD) was used to investigate the effect of four different pitched blade impellers, including 15°, 30°, 45° and 60°, on the mixing quality of gas–liquid agitated vessel. The multiphase flow behavior was modeled by Eulerian–Eulerian multiphase approach, and RNG k − ε was used to model the turbulence. The CFD results showed that a strong global vortex plays the main role on the mixing quality of the gas phase in the vessel. Based on the standard deviation criterion, it was observed that the axial distribution of the gas phase in the 30° impeller is about 55% better than the others. In addition, the results showed that the 30° impeller has a uniform radial distribution over the other impellers and the maximum gas phase holdup in the vessel. Investigation of the power consumption of the impellers showed that the 30° impeller has the highest power consumption among the other pitched blade impellers. Also, examine the effect of same power condition for pitched blade impellers showed that the 30° impeller has the best mixing quality in this condition.     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

Power Consumption in Dual Impeller Gas‐Liquid Contactors: Impeller Spacing,Gas Flow Rate,and Viscosity Effects

The dependence of power consumption on impeller spacing, and also in relation to gas flow rate and viscosity, in unaerated and aerated gas‐liquid contactors agitated by dual Rushton‐ and by dual pitched blade turbines was comparatively studied. In tap water the two Rushton impellers acted independently for spacings greater than ΔH = 1.65d, while in glycerol solutions the impellers acted independently on reaching an impeller spacing equal to 1.20d; the corresponding values for the two pitched blade impellers were 1.50d for tap water, 1.07d for relatively high viscosities, and 0.53d for very high viscosity values. The Newton number Ne decreases with increasing viscosity for the dual Rushton turbine systems, while an increase of Ne can be observed with increasing viscosity for the corresponding pitched blade systems. For the dual Rushton turbines, gas flow number Q has no effect on Ne, at very high values of viscosity, while at low and relatively high viscosity values a small effect of Q on Ne can be detected. As observed for the dual Rushton turbine systems, Ne is also not affected by Q for the corresponding pitched blade systems at very high viscosity values. Flow number Q does not significantly affect the Newton number for the water‐glycerol solutions with a relatively high viscosity agitated by dual pitched blade turbines, while for the aerated water systems a decrease of Ne can be observed at relatively small gas flow numbers; high values of Q do not affect the Newton number.     

On the hydrodynamics characterization of the straight Maxblend impeller with Newtonian fluids

The hydrodynamics generated by the straight version of the Maxblend^® impeller with Newtonian fluids in a baffled stirred vessel under the transitional and turbulent regime has been experimentally characterized by means of the particle image velocimetry technique. The flow fields obtained with the Maxblend were compared with those obtained with a double stage classical pitched blade turbine (PBT) and a double Ekato Intermig^® impellers under the same specific power draw. It is shown that these open impellers induce complex local flows in the radial and axial direction, with an intensity decreasing away from the blades. By contrast, the Maxblend impeller generates a more regular circulation pattern, with efficient top-to-bottom pumping.     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

Impact of impeller type on methyl methacrylate emulsion polymerization in a batch reactor

The emulsion polymerization of methyl methacrylate (MMA) was carried out in a lab‐scale reactor, which was equipped with a top‐entry agitator, four wall baffles, a U‐shaped cooling coil, and a temperature controller. Potassium per sulfate and sodium dodecyl sulfate as were used as the initiator and the surfactant, respectively. The experimental investigation demonstrated the impact of the impeller type (45° six pitched‐blade turbine and Rushton impeller), number of impellers (single and double impellers), and impeller speed (100–350 rpm) on the monomer conversion, polymer particles size, molecular weight, and glass transition temperature. The results revealed that the effect of the impeller speed on the characteristics of the polymer attained using the pitched‐blade turbine was more prominent than that for the Rushton turbine. It was also found that the impact of the impeller speed on the polymer characteristics was much more pronounced for the double pitched‐blade turbines rather than for the double Rushton turbines. However, more uniform size distribution was achieved with the Rushton turbine. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40496.     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

CFD simulation of impeller shape effect on quality of mixing in two-phase gas–liquid agitated vessel

Mixing efficiency in two-phase gas-liquid agitated vessel is one of the important challenges in the industrial processes. Computational fluid dynamics technique (CFD) was used to investigate the effect of four different pitched blade impellers, including 15°, 30°, 45° and 60°, on the mixing quality of gas-liquid agitated vessel. The multiphase flow behavior was modeled by Eulerian-Eulerian multiphase approach, and RNG k-ε was used to model the turbulence. The CFD results showed that a strong global vortex plays the main role on the mixing quality of the gas phase in the vessel. Based on the standard deviation criterion, it was observed that the axial distribution of the gas phase in the 30° impeller is about 55% better than the others. In addition, the results showed that the 30° impeller has a uniform radial distribution over the other impellers and the maximum gas phase holdup in the vessel. Investigation of the power consumption of the impellers showed that the 30° impeller has the highest power consumption among the other pitched blade impellers. Also, examine the effect of same power condition for pitched blade impellers showed that the 30° impeller has the best mixing quality in this condition.     

Studies in multiple impeller agitated gas-liquid contactors

Experiments have been performed to study the effect of the density and the volume of the tracer pulse on the mixing time for two impeller combinations in the presence of gas in a 0.3 m diameter and 1 m tall cylindrical acrylic vessel. The tall multi-impeller aerobic fermenters, which require periodic dosing of nutrients that are in the form of aqueous solution, is a classic case under consideration. Conductivity measuring method was used to measure the mixing time. Two triple impeller combinations; one containing two pitched blade downflow turbines as upper impellers and disc turbine as the lowermost impeller (2 PBTD-DT) and another containing all pitched blade downflow turbines (3 PBTD) have been used. Other variables covered during experiments were the density and the amount of the tracer pulse, the impeller rotational speed and the gas superficial velocity. Fractional gas hold-up, Power consumption and mass transfer coefficient have also been measured for both the impeller combinations. Influence of aeration and impeller speed on the mixing time has been explained by the interaction of air induced and impeller generated liquid flows. Three different flow regimes have been distinguished to explain the hydrodynamics of the overall vessel (i.e., multiple impeller system). A compartment model with the number of compartments varying with the flow regimes have been used to model liquid phase mixing in these flow regimes. A correlation for the prediction of the dimensionless mixing time in the loading regime has been proposed in order to account the effect of the density and the amount of the tracer pulse on the mixing time. Correlations have also been proposed to predict fractional gas hold-up and k_La.     

Power Consumption in the Turbulent Regime for a Coaxial Mixer

The power consumption of a new coaxial mixer composed of an anchor impeller and a pitched‐blade turbine impeller, and a series of rods operated in a contra‐rotating mode has been characterized experimentally in the turbulent regime. It is shown that both the power curve and the turbulent power number vary significantly with the speed ratio between the impellers. Likewise with single impeller mixers, the transition regime starts at a Reynolds number above 100 and the turbulent regime between 10³ and 10⁴ irrespective of the definition of the Reynolds number used.     

LIQUID DISPERSION MECHANISMS IN AGITATED TANKS: PART II. STRAIGHT BLADE AND DISC STYLE TURBINES

The dispersion of oil in water in an agitated vessel was studied for two types of radial discharge impellers, straight blade and disc style turbines. Two different dispersion mechanisms, ligament stretching and turbulent fragmentation, were observed to occur in the vortex systems of the impeller discharge. Although these two dispersion mechanisms were similar to pitched blade turbine performance, differences in the velocity magnitudes and vortex interactions were observed with the radial flow impellers. The ligament stretching mechanism was observed between the vortex formation regime and the transition to the fragmentation regime. The turbulent fragmentation mechanism was observed only in highly turbulent flow.

Blade thickness was found to influence the ligament stretching mechanism. A thin blade on the straight blade turbine created higher vortex velocities and smaller drop sizes than a thick blade for the same tip speed and processing time. The consequences of this blade thickness effect could be significant when laboratory data are used to design large process equipment for liquid-liquid dispersion.     

Critical impeller speed for solid suspension in multi-impeller three phase agitated contactors

The critical impeller speed for solid suspension in gas-liquid-solid systems has been measured in multi-impeller agitated contractors of 0.15 and 0.30 m and ID and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were used. Air, deionised water and sand particles were used as the gas, liquid and solid phases, respectively. The superficial gas velocity and solid loading were varied in the ranges 0–15 mm/s and 0.5 to 10% w/w, respectively. The effects of impeller type and its diameter, particle size and loading and gas flow rate were studied. Some measurements of gas hold-up and mixing time were also made in order to get some insight of the hydrodynamic behaviour of the reactor. The critical impeller speed for solid suspension in the presence of gas (n_isg) was found to be more than that in the absence of the gas and the increase of critical speed correlated well with the gas flow rate. The influence of particle—liquid parameters on solid suspension speed in the gassed system was similar to but relatively weaker than that in the ungassed condition.     

Impeller power draw during turbulent operation in solid‐liquid suspensions

Experimental measurements with six impeller types in solid‐liquid suspensions indicate that impeller power draw in the turbulent regime is approximately proportional to the solid‐liquid suspension density when the solids are distributed throughout the liquid; however, the accuracy of this approach is limited and there are clear differences in the behaviours of the various impellers. In general, power draw increases are less than suspension density increases for impellers with large blade‐trailing vortices, while power draw increases are equal to or greater than suspension density increases for impellers with smaller blade‐trailing vortices. The power draw data is well‐described using linear relations between the impeller power number and the density difference correlating parameter proposed by Micheletti et al.,^[9] with the slope of the relation being dependent on impeller type. More extensive testing with a pitched‐blade turbine, using a greater variety of solids, found that the relation between the impeller power number and the density difference correlating parameter is independent of particle size for particles as large as 1 mm (1000 microns). For particles larger than 1.7 mm (1700 microns), in addition to suspension density, the solid volume fraction affects the pitched‐blade turbine power number; however, it is difficult to determine if this effect exists at all scales or if it is a result of the large particle size relative to the impeller dimensions in the experimental system. For large particles, the power draw is increased by the addition of neutrally‐buoyant particles that do not change the suspension density, with the magnitude of the increase being dependent on impeller type.     

Investigation of crystal growth of borax in single and dual impeller batch cooling crystallizer

Abstract

The purpose of this work was to investigate and compare the influence of fluid flow in a single and dual impeller batch cooling crystallizer on crystal growth kinetics of borax decahydrate. Examinations were conducted in a crystallizer of 15?dm³ stirred by a single pitched blade turbine and straight blade turbine as well as their dual configurations. Kinetics parameters of crystal growth determined at applied mixing conditions were correlated with hydrodynamic conditions. In this paper, hydrodynamics was characterized by mixing time, which was experimentally determined, and by fluid flow patterns, which were simulated by the means of computational fluid dynamic (CFD). It was found that although the crystal growth in all systems investigated was controlled by the integration mechanism, the crystal growth rate constant changed significantly with impeller configuration. Regarding the characteristics of the final product, a dependence of the crystal size distribution on the fluid flow pattern was noticed while the number of impellers did not affect the product properties. On the other hand, mixing efficiency differed significantly with the type and number of impellers.     

THE EFFECT OF VISCOSITY ON PARTICLE SUSPENSION IN AN AERATED STIRRED VESSEL WITH DIFFERENT IMPELLERS AND BASES

The agitator speed required to suspend solids under gassed conditions, N _JSg , has been studied in water and in corn syrup of 0.01 and 0.1 Pas giving Reynolds numbers from the full turbulent region down to ～10³. Of the impellers tested, the down-pumping, three-blade, axial flow hydrofoil impellers are generally unsuitable for this duty, and although six-blade, mixed flow down-pumping impellers require the lowest mean specific energy dissipation rates to suspend the solids, (? _T ) _JSg , at low gas flow rates, they are still prone to flow instabilities and torque fluctuations. The latter poor characteristics are made worse by reducing the size of the impeller relative to the vessel and by increasing viscosity and gas flow rate, Q _GV . Thus, they are of limited use for such systems. The Ekato InterMIG impeller has the highest (? _T ) _JSg and tends to cause vessel vibrations when dispersing the gas, and this weakness is also enhanced by increasing viscosity and gas flow rate. Again, they are generally not appropriate for three-phase systems. The radial flow Rushton turbine is quite stable and able to suspend the solids in all the fluids. However, it requires the second highest (? _T ) _JSg , and both (? _T ) _JSg and N _JSg increase substantially with increasing Q _GV . The up-pumping six-blade, mixed flow impeller of approximately half the vessel diameter is able to suspend the solids and is very stable in all the fluids. In addition, both (? _T ) _JSg and N _JSg are very insensitive to Q _GV , with (? _T ) _JSg generally being the lowest at the highest Q _GV . It is thus the preferred agitator among those tested. As in ungassed systems, modifying the base of the vessel can significantly lower (? _T ) _JSg and N _JSg for a given impeller type in water compared to a flat base. The concept of keeping constant torque as a means of maintaining suspension has been tested and found not to be valid in this work. Another approach to generalizing the results is also suggested.     

Improved power and mass transfer correlations for design and scale-up of multi-impeller gas-liquid contactors

The impeller power and volumetric mass transfer coefficient were measured in a pilot-plant single-, double- and triple-impeller vessels of inner diameter 0.6 m. The experimental conditions corresponded with those used earlier in geometrically similar laboratory scale vessel of inner diameter 0.29 m [Fujasová et al., 2007, Chem. Eng. Sci. 62, 1650-1669]. The same impeller types and their combinations were used as well as the experimental techniques and forms of the data treatment/correlations, which distinguish bottom and upper section behaviour. Concretely, 23 combinations of the following impeller types were used: Rushton turbine (RT), six-pitched-blade impeller pumping upwards (PBU) and downwards (PBD), Lightnin A315 (LTN) impeller, and Techmix 335 pumping upwards (TXU) and downwards (TXD). Distilled water, representing a low-viscosity coalescent batch, was used as the liquid phase.It was found that the correlations established on the basis of the laboratory scale data might be used to describe the transport characteristics in the pilot-plant vessel. The more precise correlations, based on the data from both the laboratory and the pilot-plant scale vessels have also been established. The specific powers dissipated by impellers under gassed conditions (P_g) were within the interval from 10 to 8500 W m⁻³ in the experiments. General correlations of the relative power down under aeration (P_g/P₀) are presented separately for the bottom and upper sections of the vessel. k_La were measured by dynamic pressure method in the individual vessel sections simultaneously. Their values moved within the interval from 0.002 to 0.21 s⁻¹. The best fit provided correlating the single- and the multi-impeller (double and triple) vessels data separately. Correlation of the k_La data measured in the middle height of the triple-impeller vessel, the method often used in literature, is also included.Of the triple-impeller configurations, 3RT gave the best mass transfer performance. The configurations utilizing the same impeller type have shown that the radial flow impellers provide higher (20 up to 50%) mass transfer coefficients than the axial flow impellers. The combined configurations (i.e., those with an RT impeller in the bottom section) do not achieve the mass transfer performance of 3RT. The k_La values produced by RT+2PBD and RT+2PBU were only 15-20% lower than those achieved using 3RT at the same power input. The 3LTN and RT+2LTN configurations provided the poorest mass transfer coefficients at the same power input, both being up to 40% lower than those of 3RT.     

THE GROWTH OF CAVERNS FORMED AROUND ROTATING IMPELLERS DURING THE MIXING OF A YIELD STRESS FLUID

The growth of caverns, formed around rotating impellers in a yield stress fluid during mixing in a stirred vessel, has been studied by observing impeller speeds at which fluid motion was first observed at the vessel's wall and base, and at the free liquid surface. The effect of impeller geometry has been studied with a disk turbine (DT), a two bladed paddle (2BP), a pitched blade turbine (PBT) and a marine propeller (MP).

The presence of four baffles (10%) was found to increase the impeller speed at which the cavern reaches the vessel wall by 9% on average over that observed without baffles. After the cavern has reached the vessel walls, impeller type had a small effect upon the vertical expansion of the cavern with increasing impeller speed. Radial flow impellers (DT and 2BP), on average, performed better than an axial flow impeller (MP), with a mixed flow impeller (PBT) in between. Baffles significantly reduce the rate of this vertical expansion of the cavern. Clearance of the impeller from the vessel base had little effect upon the growth of the cavern above the impeller.     

IMPROVEMENTS IN GAS INDUCING IMPELLER DESIGN

Rate of gas induction, static pressure, mixing time and power consumption have been measured in 0.57 m i.d. vessel. Different types of impellers namely shrouded disc turbine, shrouded curved blade turbine and pitched blade turbine were used. The impeller diameter was varied from 0.15-0.25 m and the impeller speed was varied from 3 to 20 r/s.

The pitched blade turbine was found to give 30-60 per cent higher rates of gas induction as compared with the best design reported in the literature. The mixing time was found to be lower by a similar magnitude. Moreover in the case of pitched blade turbine it was found that the gas was getting induced radially as well as axially. This eliminates the necessity of the diffuser and hence reducing the complexities in the mechanical structure.     

CRITICAL IMPELLER SPEED FOR SOLID SUSPENSION IN MULTI-IMPELLER AGITATED CONTACTORS: SOLID-LIQUID SYSTEM

The critical impeller speed for suspension of solid particles (N_js) has been measured in multi-impeller mechanically agitated contactors of 0.15 and 0.30m id and 1.0 m height. Three types of impellers, i.e. disk turbine (DT), pitched turbine downflow(PTD) and pitched turbine upflow(PTU) were used. The number of impellers used in the 0.3 m and 0.15 m id reactors were three and four, respectively. The distance maintained between two impellers was equivalent lo the tank diameter. The effect of impeller type and diameter, particle size and loading, and clearance of the bottom impeller from the reactor bottom was studied and results compared with those of single impeller agitated contactors. PTD impeller was found to be more efficient for solid suspension. The N_js values obtained in reactors with multiple impeller are essentially the same as those observed in single impeller reactor and the bottom impeller plays dominant role in determining the N_js, values. An empirical correlation has been proposed for estimation of N_js and an attempt has been made to explain the mechanism of suspension in multi-impeller agitated reactor.