Pitched Blade Turbine Research Articles

Complementary methods for the determination of the just-suspended speed and suspension state in a viscous solid–liquid mixing system

A poor understanding of mixing dynamics may lead to major economic losses in numerous industries due to poor yields and waste of raw materials. Then processes can be optimized through a better understanding of solid–liquid mixing dynamics.Previous studies have characterized the suspension of solid particles in a viscous medium using a standard high shear impeller (a pitched-blade turbine or PBT) and have measured the fraction of suspended particles using the pressure gauge technique (PGT). Since the PGT cannot be applied in a straightforward way to close-clearance impellers, we developed a novel technique based on the PGT to determine the fraction of suspended particles in a system using a close-clearance impeller such as a double helical ribbon (DHR).We studied a solid–liquid suspension in the laminar and transitional regimes with high particle loadings and a DHR. Since the dynamic pressure cannot be subtracted from the total pressure determined by the PGT, we developed two alternative methods. The first was the pressure difference method, which we used to determine a range of speeds where Njs is located. The second was the sedimentation-based method which we used to accurately quantify Njs and effectively eliminate the dynamic pressure. Their pertinence and applicability were demonstrated.

Mass Transfer in Dilute Solid–Liquid Stirred Tanks

This work deals with solid–liquid mass transfer in dilute solids suspension. The dissolution dynamics of salt particles in water are determined in two baffled tanks of similar geometry and different dimensions stirred by Rushton turbines, Pitched Blade Turbines and Lightnin A310 impellers. The liquid conductivity variation during the salt dissolution is measured locally by electrical resistance tomography. The mass transfer coefficient, kL, that ranges from 2.7 × 10–5 to 1.5 × 10–4 m/s, is obtained from the conductivity time traces as a function of the operating conditions and the stirred tank geometry. Stirring conditions aimed at energy savings are also considered. A dimensionless correlation for the mass transfer coefficient in dilute solid–liquid systems is proposed. It matches fairly well the experimental data collected in this and previous investigations, being the overall estimated error equal to 19% for tanks stirred by Rushton turbines, and its applicability can be easily extended to other impellers.

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.

Effect of mixing conditions on the wet preparation of ceramic foams

Ceramic foams are a promising alternative to conventional catalyst supports due to their macro-porosity, which should enhance mass transport properties during reactions. Whilst direct foaming is a straightforward production method, the use of kitchen mixers commonly reported in the literature to initially froth the ceramic slurry limits understanding of scale-up. This study reports a systematic experimental investigation of the impact of mixing parameters on the properties of the foams produced in an agitated baffled vessel of diameter, T=175mm, equipped with an up-pumping pitch blade turbine with diameter of either D=0.23T or 0.51T and a bottom round sparger with a diameter of 45mm. The flow conditions in the present study were in the low to mid transitional regime (50<Re<1000). Design of Experiments (DoE) was employed to generate a series of screening experiments by variation of sparging time, air flow rate, impeller speed and impeller diameter. The mixing behaviour was described as a function of relevant dimensionless groups (Re, Fr, Flg, etc.) whilst the gas–liquid flow regime was estimated by examination of a ceramic particles free system. The properties of the foams obtained were correlated with key dimensionless numbers, though the exponents obtained deviated from values in the published literature. In addition, the rheology of the foam was correlated to the bubble size distribution showing that rheology measurements have potential for at-line measurement to control the structure of the produced material.

Experimental study on the solid suspension characteristics of coaxial mixers

Coaxial mixers as novel mixers have the characteristics of high flexibility and wide adjustability. Three kinds of coaxial mixers consisting of an outer anchor and different inner impellers (six 45° pitched-blade turbine, propeller and Rushton turbine) and single-impeller mixers were investigated and compared in viscous systems. Effects of rotation and pumping modes, outer impeller speed, inner impeller diameter, liquid viscosity, solid volume fraction and particle diameter on the just-suspension impeller speed and power consumption were also studied. Results showed that the coaxial mixers were more energy efficient than the single-impeller mixers, and had greater advantage in systems of higher viscosity and solid volume fraction. Different from single-impeller mixers, adopting up-pumping mode was more energy efficient for coaxial mixers when the anchor speed was relatively high. In addition, the coaxial mixers with the inner up-pumping propeller and six 45° pitched-blade turbine had obvious superiority compared with the combination of anchor and Rushton turbine in terms of power consumption. Based on the experimental data, a new correlation for predicting just-suspension impeller speed of coaxial mixers was proposed.

Experimental study on gas–liquid dispersion and mass transfer in shear-thinning system with coaxial mixer

The effects of impeller type, stirring power, gas flow rate, and liquid concentration on the gas–liquid mixing in a shear-thinning system with a coaxial mixer were investigated by experiment, and the overall gas holdup, relative power demand, and volumetric mass transfer coefficient under different conditions were compared. The results show that, the increasing stirring power or gas flow rate is beneficial in promoting the overall gas holdup and volumetric mass transfer coefficient, while the increasing system viscosity weakens the mass transfer in a shearing–thinning system. Among the three turbines, the six curved-blade disc turbine (BDT-6) exhibits the best gas pumping capacity; the six 45° pitched-blade disc turbine (PBDT-6) has the highest volumetric mass transfer coefficient at the same unit volume power.

Optimisation of stirred vessel geometry for the drawdown and incorporation of floating solids to prepare concentrated slurries

This paper reports on a Design of Experiments (DoE) approach to optimise the geometric configuration for effective drawdown and incorporation of floating solids to prepare high solid content slurries. The impeller speed and power draw required to ensure all dry powder is incorporated within four seconds of addition to the vessel free surface, NJI and PJI, were used as metrics to determine incorporation performance. Mixed flow pitched blade turbines at D/T=0.5 were used. The main parameters considered were the impeller pumping direction (up versus down), impeller submergence, eccentricity, and angle of tilt. DoE was used to examine both the independent effects of the main parameters and their interactions.Pumping mode was found to be the most significant parameter, with down-pumping impellers generally providing the best drawdown and incorporation performance. This is related to the strong interaction between pumping mode and all other parameters, where adding tilt or eccentricity reduced drawdown performance for up-pumping impellers, yet caused improvement in the case of down-pumping impellers.The optimal geometry from the DoE was found using a down-pumping PBT, 10° tilt, 10% of the vessel diameter eccentricity and placed at an initial submergence of half the liquid height. This geometry is shown to reduce the time required to prepare a 50wt% slurry by two thirds compared to a generic Rushton turbine design, emphasising the benefits of rational impeller and vessel design.

Assessment of k – ε models using tetrahedral grids to describe the turbulent flow field of a PBT impeller and validation through the PIV technique

Abstract In turbulence modeling, the RNG and Realizable models have important improvements in the turbulent production and dissipation terms in comparison to the Standard. The selection of the appropriate turbulence model has an impact on the convergence and solution in STRs, and they are used in mixing, multiphase modeling or as starting solution of transient models as DES and LES. Although there are several studies with the pitched blade turbine (PBT) impeller, most of them used the Standard model as representative of all k–e models, using structured hexahedral grids composed of low number of cells, and in some cases under axial symmetry assumptions. Accordingly, in this work the assessment of the Standard, RNG and Realizable models to describe the turbulent flow field of this impeller, using the Multiple Reference Frame (MRF) and Sliding Mesh (SM) approaches with tetrahedral domains in dense grids, is presented . This kind of cell elements is especially suitable to reproduce complex geometries. Flow velocities and turbulent parameters were verified experimentally by PIV and torque measurements. The three models were capable of predicting fairly the pumping number, the power number based on torque, and velocities. Although the RNG improved the predictions of the turbulent kinetic energy and dissipation rate, the Realizable model presented better performance for both approaches. All models failed in the prediction of the total dissipation rate, and a dependence of its value on the number of cells for the MRF was found.

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.

Kinetics and Optimization of Lipophilic Kojic Acid Derivative Synthesis in Polar Aprotic Solvent Using Lipozyme RMIM and Its Rheological Study.

The synthesis of kojic acid derivative (KAD) from kojic and palmitic acid (C16:0) in the presence of immobilized lipase from Rhizomucor miehei (commercially known as Lipozyme RMIM), was studied using a shake flask system. Kojic acid is a polyfunctional heterocycles that acts as a source of nucleophile in this reaction allowing the formation of a lipophilic KAD. In this study, the source of biocatalyst, Lipozyme RMIM, was derived from the lipase of Rhizomucor miehei immobilized on weak anion exchange macro-porous Duolite ES 562 by the adsorption technique. The effects of solvents, enzyme loading, reaction temperature, and substrate molar ratio on the reaction rate were investigated. In one-factor-at-a-time (OFAT) experiments, a high reaction rate (30.6 × 10−3 M·min−1) of KAD synthesis was recorded using acetone, enzyme loading of 1.25% (w/v), reaction time of 12 h, temperature of 50 °C and substrate molar ratio of 5:1. Thereafter, a yield of KAD synthesis was optimized via the response surface methodology (RSM) whereby the optimized molar ratio (fatty acid: kojic acid), enzyme loading, reaction temperature and reaction time were 6.74, 1.97% (w/v), 45.9 °C, and 20 h respectively, giving a high yield of KAD (64.47%). This condition was reevaluated in a 0.5 L stirred tank reactor (STR) where the agitation effects of two impellers; Rushton turbine (RT) and pitch-blade turbine (PBT), were investigated. In the STR, a very high yield of KAD synthesis (84.12%) was achieved using RT at 250 rpm, which was higher than the shake flask, thus indicating better mixing quality in STR. In a rheological study, a pseudoplastic behavior of KAD mixture was proposed for potential application in lotion formulation.

Computational and experimental investigation of laminar flow mixing system in a pitched-blade turbine stirred tank

Computational and experimental investigation of laminar flow mixing system in a pitched-blade turbine stirred tank

Computational and experimental investigation of laminar flow mixing system in a pitched-blade turbine stirred tank

Computational and experimental investigation of laminar flow mixing system in a pitched-blade turbine stirred tank

Impact of HSPBT blade angle on gas phase hydrodynamics in a gas–liquid stirred tank

For gas dispersion applications in stirred tanks, standard pitched blade turbine (PBT) with 45° blade angle is widely used. However, the standard PBT suffers from low pumping efficiency as well as high torque instability when a large volume of gas is to be dispersed. Thus, high solidity pitched blade turbine (HSPBT), which has a higher blade to impeller area has been developed. The performance of HSPBT relies on the impeller blade angle that governs impeller shear, break up of bubbles, and pumping efficiency. For HSPBT, detailed experimental data is not available, which hinders its design optimization. This study investigates the effect of blade angle (30°, 45°, and 60°) on gas holdup, power consumption, and bubble properties (size, frequency, and velocity) in a pilot-scale stirred tank using optical probe measurements. Increasing the blade angle from 30° to 60° resulted in increase in overall gas holdup and power consumption. For 30° blade angle, the bubble frequency was approximately three times higher at impeller trailing edge compared to the tank center and wall; whereas average bubble velocity was 15–20% lower at the trailing edge. Increasing the blade angle to 60° resulted in further increase in bubble frequency and drop in bubble velocity at the trailing edge. Wide chord length distribution (1–5cm) was observed at the tank center; whereas narrower distribution in chord length (0–1cm) was observed at the impeller trailing edge. Further analysis of bubble population shown that the contribution of small bubbles to overall gas holdup was highest using HSPBT45°. Higher gas holdup and finer bubbles were observed for HSPBT45° compared to HSPBT60° at reasonably lower power input.

Coupling simulation of fluid structure interaction in the stirred vessel with a pitched blade turbine

Abstract The interaction between fluid and a down-pumping pitched blade turbine fixed with a flexible shaft in the stirred vessel, as a typical fluid structure interaction phenomenon, was simulated by coupling the Computational Fluid Dynamics and Computational Structural Dynamics. Based on the verification of the simulated impeller torque and dimensionless shaft bending moment with experimental result, the dimensionless shaft bending moment and various loads acting on impeller (including lateral force, axial force and bending moment) were discussed in detail. By separating and extracting the fluid and structural components from those loads, the results show that the shaft bending moment mainly results from the lateral force on impeller although the axial force on impeller is much larger. The impeller mass imbalance increases the shaft bending moment and the lateral force on impeller, but has little influence on the axial force and bending moment acting on impeller. The dominant frequencies of impeller forces are macro-frequency, speed frequency and blade passing frequency, and are associated with the impeller mass imbalance.

Synergistic and interference effects in coaxial mixers: Numerical analysis of the power consumption

Abstract This paper is concerned with the design and application of coaxial mixers with the aid of analysis of interaction between each individual impeller. Two types of coaxial mixers pitched blade turbine (PBT)-helical ribbon (HR) and inner-outer HR operated in laminar regime were studied experimentally and numerically. The interaction implies synergistic and interference effects, which was revealed through the investigation of axial circulation rate, energy dissipation rate and power consumption. The influence factors including rotational speed ratio, rotating mode and impeller configuration were explored systematically. Quantitative analysis of power consumption involves three parameters: rate of variation in power consumption, interactive mode and ratio of power consumption. Analysis indicated that some important properties were embodied in the power curve. These properties are one-way and two-way interactions, critical speed ratio and dominant impeller. Finally, a new suggestion for power estimation was given.

Power consumption for an agitated vessel equipped with pitched blade turbine and short baffles

Power characteristics for an agitated vessel equipped with planar short baffles of length L and pitched blade turbine of pitch β are presented in the paper. The studies were carried out in the vessel of inner diameter D = 0.6 m, where the baffles were located in the distance p from the vessel bottom (p + L = H). Torque was measured using strain gauge method within the turbulent regime of the flow of Newtonian liquid in the agitated vessel. The effects of the pitch β and geometrical parameter p/H on the power number Ne were determined mathematically. The results showed that, for the assumed value of the angle β, the function Ne = f (L/H) decreases with the decrease in the baffle length L (i.e. with the increase in the parameter p). Moreover, for the assumed value of the baffle length L, the function Ne = f (β) increases with the increase in the angle β of the inclination of the impeller blade.

Visualization of Wormlike Micelle Fluid Flow in a Mixing Tank

In this study, particle image velocimetry was applied to investigate flow fields of wormlike micelle (WLM) fluids in a mixing tank equipped with a four-blade, down-pumping, pitched blade turbine. First, the rheology of WLM fluids was investigated, and the Carreau model was utilized to describe their apparent viscosity. Then, the effects of the rotation speeds and rheological properties on the flow fields were studied using different WLM fluids at different rotation speeds. The results revealed that with increasing Reynolds number, the discharge angle decreases and the carven size increases. Furthermore, elastic effects of WLM fluids lead to a more curved flow and a smaller carven.

CFD simulation of the mixing and dispersing of floating particles in a viscous system

Abstract Based on the Gidaspow model, the distributions of velocity, turbulence intensity, and solid concentration in stirred vessels equipped with a down-pumping propeller (TXL),a six flat-blade disc turbine (Rushton), or a down-pumping six 45° pitched-blade turbine (PBTD-6) in a viscous system were simulated. The power curve of the TXL propeller and the dimensionless solid concentrations of one sampling point in the vessel at different agitation speeds were obtained by simulation and experiment, which were in good agreement with each other. The results showed that: (1) both the tangential velocity and turbulence intensity on the liquid surface caused by a Rushton turbine were the highest of the three conditions at the same agitation speed; (2) the turbulence intensity on the azimuth of 90° behind the baffle near the shaft on the liquid surface was relatively larger than that in other regions; (3) the uniformity of solid concentration distribution in the stirred vessel equipped with a Rushton or PBTD-6 turbine was better than that with a TXL impeller at the same agitation speed.

Computational fluid dynamics simulation of hydrodynamics in an uncovered unbaffled tank agitated by pitched blade turbines

Computational fluid dynamics (CFD) simulations were applied for evaluating the hydrodynamics characteristics in an uncovered unbaffled tank agitated by pitched blade turbines. A volume of fluid (VOF) method along with a Reynolds stress model (RSM) was used to capture the gas-liquid interface and the turbulence flow in the tank. The reliability and accuracy of the simulations are verified. The simulation results show that the vortex can be divided into central zone and peripheral zone, and flow field in the tank can be divided into forced vortex flow region and free vortex flow region. With the increase of impeller speed, the vortex becomes deeper, while the critical radius of the two zones keeps almost unchanged. The impeller clearance and the rotational direction have little effect on the vortex shape. The vortex becomes deeper with increasing of the impeller diameter or the blade angles at the same rotational speed. Power number is little influenced by the impeller speed, and decreases by about 30% when impeller diameter varies from 0.25T to 0.5T. When blade angle varies from 30° to 90°, power number increases by about 2.32-times. Power number in uncovered unbaffled tank is much smaller than that in baffled tank, but is very close to that in a covered unbaffled tank. The discrepancy of power number in uncovered unbaffled tank and that in covered unbaffled tank is less than 10%.

Hydrodynamic characteristics of an aerated coaxial mixing vessel equipped with a pitched blade turbine and an anchor

AbstractBackgroundIn this work, computational fluid dynamics (CFD) simulation of an aerated coaxial mixing vessel composed of a centered impeller and a wall scraping anchor was conducted to investigate the effects of speed ratio, rotation modes (co‐rotating and counter‐rotating), and fluid viscosity on the local and global gas holdup values, flow pattern within the vessel, and turbulent kinetic energy. To validate the developed model, simulated gas holdup and gassed power uptake were compared with the measured experimental values. To gather experimental gas holdup values, an electrical resistance tomography technique was utilized.ResultsThe results demonstrated that the co‐rotating coaxial mixer with a speed ratio higher than 10 provided a higher gas volume fraction within the vessel. It was also shown that the turbulent kinetic energy attained in the counter‐rotating mode was lower than those for the co‐rotating coaxial mixer in most regions within the mixing tank, especially near the vessel walls. The size and the number of circulation loops developed within the coaxial mixer were affected by the speed ratio.ConclusionIt was demonstrated that speed ratio and rotation mode of the impellers affected the hydrodynamics developed within the aerated coaxial mixer. © 2017 Society of Chemical Industry