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.     

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.     

Mixing Time Studies in Multiple Impeller Agitated Reactors

Mixing time studies have been carried in a 0.3m diameter and 0.9m tall vessel equipped with three impellers. Conductivity measurement technique has been used for the measurements of mixing time. Effect of the various parameters i.e. tracer density, tracer volume, speed of rotation and impeller combination on mixing time has been studied for two impeller combinations used viz. PTD‐PTD‐PTD and PTD‐PTD‐DT. A compartment model (with one fitted parameter, the exchange flow rate Q_E) with single compartment per agitation stage has been used to predict the conductivity response and the exchange coefficients are calculated from the model parameter. An attempt has been made to explain the experimental results on the basis of the liquid phase axial dispersion coefficient and cell residence time, calculated from the model parameter Q_E     

Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

Role of sparger design in mechanically agitated gas-liquid reactors. Part II: Liquid phase mixing

Liquid phase mixing time was measured in 0.57, 1.0 and 1.5 m i.d. mechanically agitated gas-liquid reactors. Transient conductivity technique was used for the mixing time measurement. Pitched blade downflow turbine was employed. The design details of PTD impellers such as diameter (0.22 T to 0.5 T) and blade width (0.25 D to 0.35 D) were studied. The influence of sparger types and their design on mixing time has been investigated. For this purpose, pipe, ring, conical, and concentric ring spargers were employed. The design details of the ring sparger, i.e. ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important variable and, therefore, it was varied for practically all the spargers studied in this work. It was found that the liquid phase mixing time depends on the impeller design, sparger design, sparger location, impeller speed and superficial gas velocity. Correlations have been developed for the dimensionless mixing time.     

Mixing Time in a Short Bubble Column

Mixing time measurements have been carried out in a 0.2m I.D. short bubble column (Hc/D ? 5) with different spargers and for different clear liquid height to diameter (H_C/D) ratios. Superficial gas velocity has been varied in the range of 0.01m/s to 0.1m/s. Effect of bulk fluid viscosity on the mixing time has also been studied. The circulation cell model, with two fitted parameters viz. number of circulation cells, S and the inter‐cell exchange velocity, V_e, has been used to predict and explain the variation in mixing time and the flow pattern in the short bubble column for different types of spargers.     

Design of multiple impeller stirred tanks for the mixing of highly viscous fluids using CFD

The effect of multiple Intermig impeller configuration on hydrodynamics and mixing performance in a stirred tank has been investigated using computational fluid dynamics. Connection between impeller stages and compartmentalisation has been assessed using Lagrangian particle tracking. The results show that by a rotating the Intermig impeller by 45^° with respect to its neighbours, instead of a 90^° rotation as recommended by manufacturers, enables a wider range of operating conditions, i.e., lower Reynolds number flows, can be handled. Furthermore by slightly decreasing the distance between the lower two impellers, fluid exchange between the impellers is ensured down to Re=27.     

LIQUID PHASE MIXING IN MECHANICALLY AGITATED VESSELS

Liquid phase mixing and power consumption have been studied in 0.3, 0.57, 1.0 and 1.5 m i.d. mechanically agitated contactors. Tap water was used as liquid phase. The impeller speed was varied in the range 2-13.33 r/s. Three types of impellers namely disc turbine (DT), pitched turbine downflow (PTD) and pitched turbine upflow (PTU) were employed. The impeller diameter to vessel diameter ratio was varied in the range of 0.25 to 0.58. The effect of impeller clearance from tank bottom was also studied. Mixing time was measured using the transient conductivity measurement.

The PTD impeller was found to be the most energy efficient for mixing in liquid phase alone. Further, PTD (T/3) was found to be most energy efficient as compared with other impeller diameters. The effect of clearance was found to be design dependent and it was found to be diameter dependent in the case of pitched turbines.

Flow patterns of different impellers have been studied by visual observations (using guide particles). These observations were supported by the measurements using Laser Doppler Velocimetry. A model has been developed for the prediction of mixing time. In the case of all the three impeller designs, a fairly good agreement was found between the predicted and experimental values of mixing time.     

Fractional gas hold-up in gas inducing type of mechanically agitated contactors

Fractional gas hold up was measured in gas inducing type of mechanically agitated contactors (GIMAC) with single and multiple impellers. Three vessels of 0.57, 1.0 and 1.5 m i.d. were used. For the multiple impeller system, six different impeller designs were employed. The impeller speed was varied in the range of 0.30 to 15.45 s^?1. The ratio of impeller diameter to tank diameter (D/T), the submergence (S) of the upper impeller, the clearance of the lower impeller from the tank bottom (C₁) and the impeller spacing (C₃) were varied over a wide range. The design of the lower impeller was optimized in terms of diameter (D), blade width (W), blade angle (Bø), number of blades (n_b) and the blade thickness (t_b). An optimum design has been proposed for the multiple impeller system. Rational correlations have been proposed.     

刚柔组合搅拌桨强化搅拌槽中流体混沌混合

搅拌槽内普遍存在着两种不同类型的混合区域：混沌混合区和规则区。增大混沌混合区,是提高流体混合效率、降低搅拌过程能耗的重要途径。而合理设计搅拌桨有助于流体形成适宜的流动状态,实现混沌混合。柔性体与刚性体组合,可设计出具有多体运动行为的刚柔组合搅拌桨,可强化流体混沌混合行为。结合Matlab 软件,实验研究了双层桨搅拌槽内自来水体系的最大Lyapunov指数（LLE）和多尺度熵（MSE）的变化规律,对比分析了刚性桨和刚柔组合桨两种桨叶对流体混沌混合的影响。结果表明,刚柔组合桨强化流体的运动特性,使更多流体进入混沌混合状态。在转速为210 r·min^-1时,流体的混沌混合达到最佳状态,刚性桨体系的LLE为0.041,而刚柔组合桨体系的LLE为0.048;刚柔组合桨可有效耗散能量,使整个槽体的能量分布均匀,刚柔组合桨在150 r·min^-1时的多尺度熵率与刚性桨在210 r·min^-1时基本相近;刚柔组合桨体系的混合时间均低于刚性桨体系,在转速为120 r·min^-1时,刚柔组合桨使流体的混合时间缩短了26%左右。刚柔组合桨可改变流场结构和能量耗散方式,强化流体混沌混合,实现高效节能操作。     

Role of sparger design in mechanically agitated gas-liquid reactors. Part I: Power consumption

Power consumption (Part I) and liquid phase mixing time (Part II) were measured in 0.57, 1.0 and 1.5 m i.d. vessels. A pitched blade downflow impeller (PTD) was used. Design details of the PTD impeller such as diameter (0.22T to 0.5T), blade width (0.25D to 0.4D) and blade thickness (2.8, 4.3 and 6.4 mm) were studied. The effect of sparger type and geometry on power consumption has been investigated. For this purpose, pipe, ring, conical and concentric ring sparger were employed. Design details of the ring sparger such as ring diameter, number of holes and hole size were also studied in depth. Sparger location with respect to the impeller was found to be the most important parameter and was therefore varied for nearly all the spargers studied. A correlation for the power consumption has been developed.     

CFD modeling of flow,macro-mixing and axial dispersion in a bubble column

The flow pattern in a bubble column depends upon the column diameter, height, sparger design, superficial gas velocity and the nature of gas–liquid system. In this paper, the effect of some of these parameters have been simulated using Computational Fluid Dynamics (CFD). The relationship of these parameters with the interphase force terms has been discussed. A complete energy balance has been established. Using this methodology, the flow patterns reported by Hills (1974), Menzel et al. (1990), Yao et al. (1991) and Yu and Kim (1991) have been simulated. Excellent agreement has been shown between the CFD predictions and the experimental observations. The above model has been extended to homogenization of an inert tracer. In order to confirm this model, mixing experiments were carried out in a 200 mm i.d. bubble column. A radioactive tracer technique was used for the measurement of mixing time. Tc-99m (^99m Tc), in the form of sodium pertechnate salt, was used as the liquid phase tracer. Good agreement has been shown between the predicted and the experimental values of mixing time. The model was further extended for the estimation of axial dispersion coefficient (D_L). Excellent agreement between the simulated and the experimental values of the axial dispersion coefficient confirms the predictive capability of the CFD simulations for the mixing process.     

Shannon entropy for local and global description of mixing by Lagrangian particle tracking

If 100 dice cannot be cast simultaneously, one single die can be cast 100 times. On the basis of this simple principle, the experimental technique of positron emission particle tracking has been used to develop and implement a new methodology for quantifying the local and global mixing characteristics within a mechanically agitated fluid batch system. This Lagrangian technique uses a single positron-emitting particle as flow follower. Using a high data acquisition rate, such a tracer is continuously tracked in 3D space and time to accurately determine its trajectory over a considerable period of time. By partitioning its long trajectory, the single particle tracer can be regarded as thousands of simultaneously tracked particles which are instantaneously, locally and non-invasively injected in the mixing system at varying feed positions. A large amount of PEPT data were collected for impeller rotational speeds ranging from 100 to 500 rpm which allowed new statistical tools derived from information theory, such as Shannon entropy and uncertainty, to be implemented in the data analysis. Thus, measurements of entropy mixing indices were obtained as a function of position, time and impeller speed. The method also allowed the determination of characteristic time parameters including the macroscale mixing time which agreed very well with correlations of the dimensionless mixing time available in the mixing literature. Detailed local information is provided on minimum mixing time positions for feed and withdrawal of material, which can be used to optimise the design or operation of stirred batch mixing systems.     

EFFECT OF IMPELLER DESIGN ON LIQUID PHASE MIXING IN MECHANICALLY AGITATED REACTORS

Liquid phase mixing time (θ_mix) was measured in mechanically agitated contactors of internal diameter 0.57 m, 1.0 m and 1.5 m. Tap water was used as the liquid phase. The impeller speed was varied in the range of 0.4-9.0 r/s. Three types of impellers, namely disc turbine (DT), pitched blade downflow turbine (PTD) and pitched-blade upflow turbine (PTU) were employed. The ratio of impeller diameter to vessel diameter (D/T) and the ratio of impeller blade width to impeller diameter (W/D) were varied over a wide range. The effects of impeller clearance from the tank bottom (C), the blade angle (φ), the number of blades (n_b), the blade thickness (k) and the total liquid height (H/T) were studied in detail. Mixing time was measured using the conductivity method.

Mixing time was found to have a strong dependance on the flow pattern generated by the impeller. Mixing time was found to decrease by decreasing the impeller clearance in the case of DT and PTU. However in the case of PTD it increases with a decrease in the impeller clearance. Similar trend of the effect of impeller clearance on θ_mix, was observed for all the other PTD impellers with different diameter, number of blades and blade angle (except 60^° and 90^°). All the impeller designs were compared on the basis of power consumption and on this basis optimum design recommendations have been made. For PTD impellers, a correlation has been developed for the dimensionless mixing time.     

错位六弯叶搅拌槽内假塑性流体的混合特性

在直径为0.21 m的搅拌槽内,使用FLUENT?软件对错位六弯叶桨的流动场及混合性能进行了数值模拟.工作介质分别选用牛顿流体(水)和假塑性流体1.0%(wt)黄原胶水溶液,计算采用标准κ-ε湍流模型,并将牛顿流体的速度场分布与粒子图像测速仪(PIV)实验结果进行了比较.通过与标准六弯叶桨进行对比分析,阐述了错位桨用于假塑性流体搅拌时在混合速率和混合效率方面的特性.结果表明:速度矢量的计算值与 PIV 实验数据吻合较好,湍流模型的计算结果可靠;混合过程与流场结构密切相关,监测点位置对浓度变化有较大影响.错位六弯叶桨的混合时间数明显小于标准六弯叶桨,混合速率更高,同时错位六弯叶桨的混合效率大大高于标准六弯叶桨,单位体积混合能只有标准六弯叶桨的52%,具有节能功效,体现出错位桨的优越性.     

An experimental study of double-to-single-loop transition in stirred vessels

The velocity characteristics of the flows in a fully baffled vessel of diameter T = 290 mm stirred by a Rushton impeller of diameter D = T/3 were investigated by means of laser-Doppler anemometry measurements. The effects of clearance and rotational speed on the flow patterns in the vessel were studied. It was found that at impeller clearances from the bottom of the vessel (C) around 0.2 T the characteristic double-loop flow pattern undergoes a transition to a single-loop one with the impeller stream direction becoming partly axial and being inclined at around 25 to 30° to the horizontal. The impeller stream inclination varied with radial distance from the impeller, as well as with angular position between blades (blade angle). Impeller speed was found to have no effect on the flow pattern or the mean velocities and turbulence levels normalized by V_tip for C/T > 0.20 or C/T ≤ 0.15. The flow structure measured with C = 0.15T is described in detail and the implications of the data for fluid mixing in stirred vessels are discussed.     

Process Design Aspects of Jet Mixers

In the present work, mixing time measurements have been made with jet mixers over a wide range of jet velocities, liquid levels and tank sizes. The nozzle was kept along the axis of the vessel and the nozzle clearance was varied over a wide range. It was observed that the mixing time decreases with an increase in the jet path length (nozzle clearance) for a given tank size and given amount of liquid. The reasons for this behaviour are explained with the help of CFD modelling. The effect of tank diameter has been studied independent of the jet path length. A correlation has been developed for process design. Jet mixers have been compared with impeller‐stirred tanks in terms of their energy efficiency. Reasons for the observed behaviour have been provided.     

Influence of Floating Suspended Solids on the Homogenization of the Liquid Phase in a Mixing Vessel

The impact of floating suspended solids on the homogenization of the liquid phase in a stirred vessel was studied. The experiments were performed in a tank with an internal diameter of 0.32 m, equipped with a 45° pitched four-blade turbine (PTD) placed at varying positions in the vessel. Tap water was used as the liquid phase and polyethylene particles (PEHD) were used as the solid phase. The impeller speed was varied from N = 200–900 rpm. The mixing time of the suspended system was measured by a conductivity technique using a sodium chloride solution as the tracer, whereas power consumption was measured by the torque table. The influence of mean concentration of the suspended floating solids, average particle size, surface tension at the liquid/air interface and impeller diameter and its position on the mixing time and power consumption were analyzed.     

半圆管曲面涡轮搅拌槽内混合特性的数值模拟

在商业化软件ANSYS CFX 10.0平台上,采用多重参考系法来解决挡板与桨叶之间的相对转动问题,由标准k-ε模型对半圆管曲面涡轮搅拌槽内流动和混合过程进行了详细的数值模拟,本模拟所得的功率准数和设计值以及相关文献值吻合良好。结果表明：当搅拌桨离底距离由搅拌槽直径的1/2处变为1/3处时,搅拌槽内的流型均为典型的“双循环流型”,而当搅拌桨离底距离由搅拌槽直径的1/3处降低至1/6处时,槽内流型由典型的“双循环流型”转变为“单循环流型”;通过对不同时刻不同桨叶离底距离下的示踪剂浓度分布图分析表明槽内的混合过程与流动场密切相关;加料点位置对于最终的流场混合效果有着显著影响,对于混合时间数据的采集应注意不同加料位置时监测点的选取。CFD模拟结果表明本文所采用的模型可以很好的预测半圆管曲面涡轮搅拌槽内的混合特性,为进一步改进和优化半圆管曲面涡轮的设计提供了一定的参考。     

Effects of Physical Property Differences on Blending

Within this study, the effects of viscosity differences between added and bulk liquids on mixing times were investigated. This was carried out in stirred tanks of diameter T = 0.31, 0.61, 1.83 m to study the effect of scale. Different impeller types (hydrofoils, disc turbines, and pitched blade turbines) and sizes (D = T/2 and T/3) were employed. Operating conditions for which mixing time correlations for similar property liquids could be used were identified at scales relevant to industrial applications. Recommendations are made for improving blending under operating conditions where these correlations are not applicable as the mixing times are too long.