A Non‐Conventional Approach for the Make‐Down of Concentrated Suspensions

Concentrated suspensions made of delaminated kaolin clay in water were prepared using two approaches, namely steady stirring and dynamic perturbation operating modes. In the first approach, the impeller was rotated at constant speed in one direction. For the second approach, the flow into the tank was dynamically perturbed by rotating the impeller cyclically in both directions. Two high‐shear impellers were used: a Sevin impeller and a Cowles turbine. During the mixing, the evolution of torque was measured and samples were taken at regular intervals. All the results were compared in terms of the mixing energy consumption, particle average size and distribution, as well as the shear rheology. Suspensions prepared with the dynamic perturbation approach were found to require less energy and exhibited, in general, smaller particles and lower viscosity than with conventional mixing, which indicates that the proposed method could be an efficient alternative to conventional mixing.     

Coaxial Mixer Hydrodynamics with Newtonian and non‐Newtonian Fluids

The performance of several combinations of a wall scraping impeller and dispersing impellers in a coaxial mixer operated in counter‐ and co‐rotating mode were assessed with Newtonian and non‐Newtonian fluids. Using the power consumption and the mixing time as the efficiency criteria, impellers in co‐rotating mode were found to be a better choice for Newtonian and non‐Newtonian fluids. The hybrid impeller‐anchor combination was found to be the most efficient for mixing in counter‐rotating or co‐rotating mode regardless of the fluid rheology. For both rotating modes, it was shown that the anchor speed does not have any effect on the power draw of the dispersing turbines. However, the impeller speed was shown to affect the anchor power consumption. The determination of the minimum agitation conditions to achieve the just suspended state of solid particles (N_js) was also determined. It was found that N_js had lower values with the impellers having the best axial pumping capabilities.     

Transitional Mixing of Shear‐Thinning Fluids in Vessels with Multiple Impellers

The mixing efficiency of shear‐thinning fluids was evaluated using carboxymethylcellulose sodium salt (Na‐CMC) aqueous solutions of varying mass concentrations and three types of impellers (Rushton turbine (RT), six‐flat‐blade turbine (FBT), six‐pitched‐down‐blade turbine (PBT)) which were mounted on a common shaft in combinations of three, four, and five impellers. The mixing time proved to be dependent on the number of impellers as well as on the distance between. The Reynolds number has a significant influence on the mixing time for all studied systems. The results of power consumption allowed to choose the impeller system with the best efficiency.     

Effect of impeller type on continuous‐flow mixing of non‐Newtonian fluids in stirred vessels through dynamic tests

Mixing of non‐Newtonian fluids with axial and radial flow impellers is prone to a significant extent of nonideal flows (e.g., dead zones and channelling) within the stirred reactors. To enhance the performance of the continuous‐flow mixing of pseudoplastic fluids with yield stress, close‐clearance impellers were utilised in this study. We explored the effects of various parameters such as the type of close‐clearance impeller (i.e., the double helical ribbon (DHR) and anchor impellers), impeller speed (25–500 rpm), impeller pumping direction, fluid rheology (0.5–1.5% xanthan gum solution), fluid flow rate (3.20–14.17 L min^?1) and the locations of outlet (configurations: top inlet–top outlet, top inlet–bottom outlet) on the dynamic performance of the mixing vessel. The performance of the DHR impeller was then compared to the performance of various types of impellers such as axial‐flow (Lightnin A320) and radial‐flow (Scaba 6SRGT) impellers. The dynamic tests showed that the DHR impeller was the most efficient impeller for reducing the extent of nonideal flows in the continuous‐flow mixer among the impellers employed in this study. In addition, the mixing quality was further improved by optimising the power input, increasing the mean residence time, decreasing the fluid yield stress, using the up‐pumping impeller mode and using the top inlet–bottom outlet configuration. © 2011 Canadian Society for Chemical Engineering     

Mixing of a lignin‐based slurry fuel

Lignin‐based slurry fuels are a potential alternative to fossil fuels in kraft pulp mills. Lignogels — mixtures of lignin, fuel oil, water and surfactant — are non‐Newtonian fluids, with shear‐thinning and thixotropic behaviour. Their mixing was investigated in tanks with volumes of 3 and 30 L. An A310 hydrofoil impeller was used in all experiments. Results were compared with measurements in Newtonian fluids, used to characterize the impeller over a broad range of Reynolds numbers (1–500 000). An aqueous CMC solution was also used for characterization of the impeller and estimation of the Metzner‐Otto constant. Results in the transition region were corrected by introduction of two empirical parameters.     

Power Requirement when Mixing a Shear‐Thickening Fluid with a Helical Ribbon Impeller Type

The optimal design of close clearance impellers requires the knowledge of the power demand of the mixing equipment. In non‐Newtonian mixing, this can be readily obtained using the Metzner and Otto concept [1]. In this work, this concept and the determination of the K_s value for an atypical helical agitator (PARAVISC system from Ekato firm) have been revised in the case of shear‐thinning fluids and a shear‐thickening fluid. For poor shear‐thinning fluids, it has been shown that for our mixing system the K_s value does not vary strongly with the flow behavior index, and may be regarded as a constant for the mixing purpose design. By contrast, for the shear‐thickening fluid, power consumption measurements indicate that the relationship between the K_s values and the flow behavior index is much more complex due to a partial solidification of the product around the impeller.     

Unconventional Configuration Studies to Improve Mixing Times in Stirred Tanks

Dynamic perturbations and off‐centered single and dual mixing impeller configurations have been investigated to reduce mixing time with viscous fluids. Mixing times, measured with a color‐discoloration technique based on a fast acid‐base reaction, reveal the presence of both segregated and dead zones. A statistical design approach has been used to evaluate the effect of the impeller position as well as the dynamic conditions. Homogenization is significantly enhanced when a radial flow impeller is used under both off‐centered and dynamic perturbation conditions. In the case of an axial flow impeller, a combination of long clockwise times and short counter‐clockwise times give better mixing times. An enhanced homogenization is also observed when a dual impeller configuration is used.     

Borax Crystallization Kinetics in a Pitched‐Blade Turbine/Straight‐Blade Turbine Dual‐Impeller Crystallizer

The influence of hydrodynamic conditions on crystallization kinetics and properties of borax crystals obtained in a dual‐impeller batch cooling crystallizer was investigated. The two impellers used, i.e., pitched‐ and straight‐blade turbines, were mounted on the same shaft. Hydrodynamics was analyzed by means of the mixing time values and specific fluid flow patterns generated. Results indicate that wider metastable zones were generally observed at impeller positions characterized by longer mixing times. In those cases, the growth rate constants were lower, resulting in a formation of smaller but more regularly shaped crystals. These findings imply that the dual‐impeller position should be taken into account in order to produce crystals of desired characteristics.     

Influence of Impeller Diameter on Crystal Growth Kinetics of Borax in a Mixed Dual‐Impeller Batch‐Cooling Crystallizer

The influence of impeller diameter on crystal growth kinetics of borax decahydrate in a batch‐cooling crystallizer of non‐standard aspect ratio was evaluated. The dual‐impeller configuration consisted of a pitched‐blade turbine which was mounted below a straight‐blade turbine on a single shaft. Three different impeller‐to‐tank diameter ratios were investigated. In all experiments, mixing was conducted at just‐suspended impeller speed. To examine hydrodynamic conditions, mixing times were measured. The fluid flow pattern and velocity distribution were determined by computational fluid dynamics. Results showed that the smallest but also more regularly shaped crystals were produced in a system with standard diameter impellers. Product yield and power consumption were highest in this case.     

Flow Patterns in Rheologically Evolving Model Fluids Produced by Hybrid Dual Mixing Systems

The flow patterns produced by two dual mixing systems composed of independently driven impellers were studied. The dual impellers included a turbine rotating at high speed (Rushton or Smith) and a slowly rotating helical ribbon agitator (HR). Visualizations and power input were used to evaluate mixing performance. The influence of the rotational speed ratio on the flow patterns was evaluated. For high shear‐thinning fluids, N_T/N_HR modifies the flow patterns considerably. Three typical behaviors were found with shear thinning fluids: segregation of two principal flow patterns (N_T/N_HR < 10), turbine dominance (N_T/N_HR > 10), and a well‐distributed flow pattern throughout the tank (N_T/N_HR = 10). For low‐viscosity fluids, the motionless HR reduced the vortex length and the T‐HR systems eliminated vortex when the impellers rotated in opposite directions at N_T/N_HR = 10. Finally, a relationship between the dimensionless vortex length and the Froude number is proposed for individual turbines as well as for the turbine‐motionless HR systems.     

CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part II Vessel with Three and Four Impellers

This article deals with CFD simulations of flow inside stirred vessels equipped with three and four radial or axial impellers mounted on the same shaft. A comparison was made between simulated data and experiments for one‐ and two‐impeller systems and was presented in Part I [1]. The effect of the lowest impeller off‐bottom clearance, number of impellers used, and impeller type on the tracer distribution was studied. The simulations were mainly focused on the grid size and type and the analysis of the concentration curves in each impeller section. The predicted velocity fields, power and pumping numbers, concentration curves, and mixing times were validated with experimental data. The simulation results show the significant influence of the grid density on the velocity profiles and power and pumping numbers in contrast to the low impact on the concentration curves. A better prediction of the concentration curves was reached when radial impellers were used; the mixing times were generally over‐predicted.     

Suspension of solid particles in multi‐impeller three‐phase stirred tank reactors

Solids suspension characteristics in gas—liquid–solid three‐phase stirred tanks with multi‐impellers were experimentally examined. Minimum impeller speeds for ultimately homogeneous solid suspension have been measured stirred tank reactors. Three impellers were installed: two four‐pitched blade downflow disk turbines and one Pfaudler type impeller chosen to provide good gas dispersion and to accomplish off‐bottom suspension of solid particles, respectively. Gas dispersion causes an increase in particle sedimentation associated with a decrease in power consumption and as a result, minimum impeller speeds for ultimately homogeneous solid suspension increase with increasing gas flow rates. A correlation was developed to predict minimum impeller speeds for ultimately homogeneous solid suspension. The proposed correlation, which agrees satisfactorily with the experimental results, is expected to be useful in design and scale‐up.     

CFD Investigation of the Mixing of Yield‐Pseudoplastic Fluids with Anchor Impellers

The study was carried out to simulate the 3D flow domain in the mixing of pseudoplastic fluids possessing yield stress with anchor impellers, using a computational fluid dynamics (CFD) package. The multiple reference frames (MRF) technique was employed to model the rotation of the impellers. The rheology of the fluid was approximated using the Herschel–Bulkley model. To validate the model, the CFD results for the power consumption were compared to the experimental data. After the flow fields were calculated, the simulations for tracer homogenization were performed to simulate the mixing time. The effects of impeller speed, fluid rheology, and impeller geometry on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (impeller clearance to tank diameter) and w/D (impeller blade width to tank diameter) ratios were determined on the basis of minimum mixing time.     

LDA Velocity Measurements of High‐Viscosity Fluids in Mixing Vessel with Vane Geometry Impeller

The object of this work was to measure the velocity field in non‐Newtonian fluids inside mixing vessel. The six‐bladed vane rotor used for mixing was designed from rotating vane geometry of a sensor system, commonly used for rheometrical measurements of complex fluids (Barnes and Nguyen, J. Non‐Newtonian Fluid Mech. 98 , 1‐14 (2001); Schramm, 1994). During mixing, the viscosity was determined by measuring the torque at different impeller speeds, and compared to rheologically obtained shear dependent viscosity. The velocity field was determined by LDA measurements at twelve places inside mixing vessel. It was observed that axial and radial component of the velocity were insignificant at all measurement points. On the other hand, the results showed the periodic nature of tangential component of the velocity, which was confirmed with computer‐aided visualization method.     

Turbulent forward–reverse mixing characteristics in vessel with multiple‐turbine impellers

BACKGROUND: Mixing in unbaffled vessel with multiple‐turbine impellers was studied. The mixing time and mixing power were evaluated in relation to the distance between impellers and the number of impellers. RESULTS: It has been confirmed that frequency of oscillation has no influence on the mixing time and mixing power values or on drag and added mass coefficients. The coefficients were greater when distance between impellers was smaller. Moreover added mass coefficient was dependent on Reynolds number (n_i > 2). Compared with unidirectional mixing conditions, for systems with one type of impeller, the power requirement was about 38% higher for forward‐reverse mixing. Despite the fact that the power demand was greater, the mixing time was not shorter, but about 30% higher than unidirectional mixing in a baffled vessel. However, the forward‐reverse mixing mode exhibits a higher level of homogeneity which it achieved faster than unidirectional mixing. CONCLUSION: The power requirements and mixing time for forward‐reverse mixing mode were higher in comparison with unidirectional mixing. Despite this, higher values of homogeneity were achieved faster. Higher levels of shear rate and better homogeneity indicate that forward‐reverse mixing can be beneficial for multi‐phase mixing in vessels with multiple impellers. © 2012 Society of Chemical Industry     

Gas Dispersion in Rheologically‐Evolving Model Fluids by Hybrid Dual Mixing Systems

Gas dispersion experiments (0.18 ≤ Fr ≤ 0.71, 0.02 ≤ F1 ≤ 0.09) were carried out using a hybrid dual mixing system, which included a helical ribbon impeller and either a Smith or a Rushton turbine. Newtonian and non‐Newtonian model fluids were used as rheologically‐evolving fluids to evaluate changes in gas dispersion performance. A motionless helical ribbon agitator was used as a baffle in low‐viscosity Newtonian fluids. Both Smith and Rushton turbines produced a vortex, which was eliminated by the motionless helical ribbon impeller. Gas dispersion in low‐viscosity fluids was enhanced when the helical ribbon agitator and turbine of the dual hybrid mixing system was kept at a rotational speed ratio of 10 (N_T/N_HR = 10), which allowed dispersion at a lower Fr than the turbine alone. For moderate‐viscosity Newtonian fluids, gas dispersion was achieved at Fr ≤ 0.71 and F1 ≤ 0.05. Flow properties of non‐Newtonian fluids played an important role in gas dispersion; transition from dispersing to flooding stages was observed for the fluids that were more shear‐thinning (n ≤ 0.38).     

Characterization of helical impellers by circulation times

Measurements of the circulation time during batch mixing of Newtonian and non-Newtonian fluids were used to analyse and compare the performance of six different helical-ribbon impellers and a screw impeller inside a draft coil. The distribution of circulation times obtained from two hundred repeated measurements is shown to depend markedly on the impeller geometry. Mixing time data with the helical-ribbon impellers are correlated with the mean value and the reduced standard deviation of the circulation times. Effective (rapid) mixing corresponds to a large circulation capacity and a wide distribution of the circulation times. The mixing mechanism of Newtonian fluids with the helical-ribbon impellers is qualitatively described by the Voncken model. The helical impellers' circulation capacities are not affected by the shear-thinning properties of non-Newtonian fluids. However, in highly shear-thinning fluids, the presence of important stagnant zones is responsible for much longer mixing times which consequently do not correlate with the circulation parameters. The relative efficiencies of the different impellers in Newtonian fluids are compared using a criterion based on the the total energy required to achieve a certain degree of mixing. The wide blade impeller is the most efficient of the helical-ribbon impellers but is considerably less efficient than the screw impeller in a draft coil.     

最大叶片式桨在假塑性流体中的搅拌流场模拟

为研究最大叶片式桨在高黏假塑性流体中的搅拌流动行为,以黄原胶溶液为研究体系,采用计算流体力学方法重点研究了釜内流体的功耗特性、速率分布、剪切速率、表观黏度分布和总体流动状况。结果表明:最大叶片式桨具有与大多数径流桨相似的"双循环"流型结构,且预测的功耗特性与实验数据一致性良好。最大叶片式桨适用于高黏假塑性流体的混合,而对于高黏牛顿流体的混合则效果不佳。釜内的剪切速率分布较宽泛,且受转速影响较大。转速可作为该桨改善黄原胶体系混合效率的重要参数之一。     

Unsteady Mixing Characteristics in a Vessel with Forward‐Reverse Rotating Impeller

Usually, mixing is carried out in a vessel with four baffles and a single impeller. In some applications, however, the use of a baffled vessel is not recommended. One of the stirring methods used instead is unsteady agitation with forward‐reverse rotating impellers. The aim of this work was to characterize the agitation characteristics in a baffled and an unbaffled vessel with a turbine impeller. Mixing time and mixing power were evaluated in relation to the presence of baffles and the frequency of forward‐reverse rotation. It was found that the frequency of oscillation does not affect either the mixing time and mixing power values or the drag and added mass coefficients. Power requirements and mixing time were higher compared to the steady mixing conditions in a baffled vessel. The results showed that it is not recommended to use baffles because they have no influence on unsteady mixing.     

Improving the dynamic performance of continuous-flow mixing of pseudoplastic fluids possessing yield stress using Maxblend impeller

The strategic approach of this article is to characterize the continuous-flow mixing of pseudoplastic fluids possessing yield stress in a stirred reactor with the Maxblend impeller. Dynamic experiments were carried out through the frequency-modulated random binary input of a brine solution to determine the extent of non-ideal flows. Mixing quality was determined on the basis of the extent of channeling and fully mixed volume. The effects of important parameters such as impeller speed (25–500 rpm), absence of baffles, fluid rheology (0.5–1.5%), fluid flow rate (3.20–14.17 L min⁻¹), and the locations of inlet/outlet on the dynamic performance of the continuous-flow mixing vessel were explored. The performance of the Maxblend impeller was then compared to the performances of various types of impellers such as close-clearance (an anchor), axial-flow (a Lightnin A320), and radial-flow (a Scaba 6SRGT) impellers. It was found when the channeling approached zero and the fully mixed volume approached the total fluid volume in the vessel, the power drawn by the A320 impeller and the Scaba impeller were about 2.9 and 4.3 times greater than that of the Maxblend impeller. Thus, the Maxblend impeller was able to drastically improve the performance of continuous-flow mixing with huge power savings. The mixing quality was further improved by optimizing the impeller speed, decreasing the fluid flow rate, decreasing the fluid concentration, and using bottom inlet- top outlet configuration. The flow non-ideality of the mixing system increased in the absence of the baffles. Thus, better mixing quality and more energy savings can be achieved by employing the findings of this study.