Effect of shaft eccentricity on the laminar mixing performance of a radial impeller

The effects of impeller eccentricity and Reynolds number on the mixing performance were studied for Rushton turbine stirred tank systems operated in the laminar regime (Re < 10). A digital image analysis of an acid–base decolourization reaction was used to characterize the mixing efficiency. Results show that both parameters have remarkable effects on the destruction of the toroidal segregated regions surrounding the Rushton turbine in laminar flow. Criteria are given to prevent the formation of these segregated regions in a tank. It is also shown that shaft eccentricity does not impact on the impeller power consumption.     

Effect of the shaft eccentricity and rotational direction on the mixing characteristics in cylindrical tank reactors

Strategy of the shaft eccentricity is introduced to enhance the mixing characteristics in a flat bottomed cylindrical vessel without baffles. The mixing is ensured by a six-curved blade impel er. Three solutions which are models of food emulsions are used as working fluids. These solutions have a shear thinning behavior modeled by the power-law. The effects of fluid properties, stirring rates, impeller rotational direction and impeller eccentricity on the 3D flow fields and power consumption are investigated. Three values of impeller eccentricity are consid-ered, namely 0%, 24%and 48%of the vessel diameter. It is found that the opposite clockwise rotational direction reduces the power consumption, compared with the clockwise rotational direction. Also, the obtained results show that an impeller placed at an eccentric position between 24%and 48%of the vessel diameter and at the third of the vessel height may ensure the best mixing characteristics.     

偏心射流-刚柔组合桨搅拌器内混沌混合行为研究

搅拌反应器内普遍存在混合隔离区,是实现高效混合的一大障碍。流场耦合诱发流体的混沌现象,可减少混合隔离区,提高流体混合效率。结合Matlab软件,探究偏心空气射流-单层刚柔组合桨体系的混合行为演变规律,对比分析了不同偏心率下桨叶类型、桨叶离底高度、空气射流量以及转速对流体混沌混合的影响。结果表明,刚柔组合桨通过其自身刚-柔-流的多体运动与偏心空气射流的流场耦合,破坏了流体混合过程中出现的对称性流场,使更多的流体进入混沌状态。刚-柔组合桨（RF-RDT、RF-IRDT）比刚性桨（RDT、IRDT）的LLE值大,其中RF-RDT相比于其他3种类型的搅拌桨（IRDT、RDT、RF-IRDT）,其LLE值分别提高了约42.8%,27.0%、6.9%;空气射流的偏心率等于0.6时,其最大LLE值相比于其他偏心率（0.8、0.4、0.2、0）,依次提高了6.5%、2.4%、17.6%、25.1%。该研究结果可为刚柔组合桨的优化设计提供理论依据。     

偏心空气射流双层桨搅拌反应器流场结构的分形特征

搅拌槽内流场分为混沌混合区和隔离区。为提高搅拌槽内流体的混合效率、降低搅拌过程的能耗,调控流场结构特征是重要的途径。结合图像处理软件,实验研究了偏心空气射流双层桨搅拌槽内空气-水体系的流场结构分形维数的变化规律。实验结果表明,流场结构分形维数受搅拌转速和空气流速的共同作用;偏心空气射流能改变流场结构分形维数,使流体混沌混合特性增强;机械搅拌转速增大,能改变射流场的拟序结构,提高气液混合效率。     

A CFD model for predicting the flow patterns of viscous fluids in a bioreactor under various operating conditions

Computational fluid dynamics simulation is becoming an increasingly useful tool in the analysis and design of simultaneous saccharification fermentation (SSF) and saccharification followed by fermentation process (SFF). To understand and improve mixing and mass transfer in a highly viscous non-Newtonian system, it was necessary to simulate the flow behavior in this bench scale bioreactor (BioFlo 3000). This study focused on designing a high concentration medium agitation system for such a process using the commercial computational fluid dynamics package Fluent (V. 6.2.20) and its preprocessor Mixsim (V. 2.1.10). The objective of this study is to compare performance of various designs of a bioreactor and identify the flow pattern and related phenomena in the bench scale tank. The configuration of the physical model for simulating a mixing tank with a Rushton impeller consists of an ellipsoidal cylindrical tank with four equally spaced wall mounted baffles extending the vessel bottom to the free surface, stirred by a centrally located six-blade Rushton turbine impeller. Simulations were performed with the original and a modified design in which the lower bottom shaft mounted a Lightnin A200 impeller. The results suggest that there is a potential for slow or stagnant flow between top impellers and bottom of the tank region, which could result in poor nitrogen and heat transfer for highly viscous fermentations. The results also show that the axial velocity was significantly improved for the modified geometry in the bottom of the tank.     

Flow,suspension and mixing dynamics in DASGIP bioreactors,Part 2

This work aims to characterize the mixing and suspension dynamics occurring within two commercially available DASGIP bioreactor configurations, equipped with a two-blade paddle impeller with large impeller to tank diameter ratio, D/T = 0.97. Both continuous and intermittent agitation modes were employed to determine the impact that agitation strategy has upon mass transfer and microcarrier settling/suspension. This paper builds upon the flow dynamics data presented in Part 1 for a flat bottom DASGIP bioreactor and shows how intermittent agitation can break-up regions of slow mixing observed during continuous agitation, therefore substantially increasing the mixing efficiency of the system. Similarly, it was found that microcarrier characteristics might significantly affect the level of suspension when the impeller is in dwell status when intermittent agitation modes are used.     

Effect of eccentricity on laminar mixing in vessel stirred by double turbine impellers

Laminar mixing is often conducted in industrial processes, for example in polymerization reactors or in biotechnological processes. The laminar flow conditions caused problems of inefficient mixing due to some mixing anomalies like occurrence of the isolated mixing regions (IMR), segregation or compartmentalization phenomena. In this paper, flow visualization experiments are used to examine the size, positions and structure of the IMR regions as a function of Reynolds number and eccentricity ratio in the vessel equipped with double turbine impellers. It was found that the eccentricity brings deformation and reduction of the IMR volume. Moreover another benefit of using eccentrically located impeller systems is an improvement of axial flow. Two types of IMR regions are found: undulated IMR (UIMR) and ribbon-like IMR (RIMR). The structure of IMR depends on the eccentricity ratio defined as E/R. At the low eccentricity values the structure of single filament wrapped around core of the IMR is found. Additionally, the IMR region is inclined to the impeller plane.     

The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels

A new stirred tank geometry, the confined impeller stirred tank (CIST), was designed to provide repeatable testing of the effect of mixing on the performance of chemical additives at the bench scale. The CIST (T = 0.076 m, H = 3T) is filled with five or six impellers. Three impeller geometries were tested: A310, Rushton and Intermig. This paper presents the following hydrodynamic characteristics of the CIST: power number, flow number, momentum number, velocity profiles at different locations in the tank and the transition point from fully turbulent to transitional flow. Based on the scaled velocity profiles, the CIST was able to keep the flow turbulent at Re < 2000 for Rushton turbines and 3200 for Intermigs. The ratio ?_max/?_average was lower for the CIST than for a conventional stirred tank, indicating that the energy dissipation is more uniformly distributed in the CIST. The CIST consistently maintains turbulent flow down to a Reynolds number 10× smaller than that needed in a conventional stirred tank.     

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.     

Characterization of the continuous-flow mixing of non-Newtonian fluids using the ratio of residence time to batch mixing time

Continuous-flow mixing of pseudoplastic fluids possessing yield stress is a complex phenomenon exhibiting non-ideal flows within the stirred vessels. Electrical resistance tomography (ERT), a non-intrusive technique, was employed to measure the mixing time in the batch mode while dynamic tests were performed to study the mixing system in the continuous mode. This study attempts to explore the effects of the operating conditions and design parameters on the ratio of the residence time (τ) to the mixing time (θ) for the continuous-flow mixing of non-Newtonian fluids. To achieve these objectives, the effects of impeller types (four axial-flow impellers: A310, A315, 3AH, and 3AM; and three radial-flow impellers: RSB, RT, and Scaba), impeller speed (290–754 rpm), fluid rheology (0.5–1.5%, w/v), impeller off-bottom clearance (H/2.7–H/2.1, where H is the fluid height in the vessel), locations of inlet and outlet (configurations: top inlet-bottom outlet and bottom inlet-top outlet), pumping directions of an axial-flow impeller (up-pumping and down-pumping), fluid height in the vessel (T/1.06–T/0.83, where T is the tank diameter), residence time (257–328 s), and jet velocity (0.317–1.66 ms⁻¹) on the ratio of τ to θ were investigated. The results showed that the extent of the non-ideal flows (channeling and dead volume) in the continuous-flow mixing approached zero when the value of τ/θ varied from 8.2 to 24.5 depending on the operating conditions and design parameters. Thus, to design an efficient continuous-flow mixing system for non-Newtonian fluids, the ratio of the residence time to the mixing time should be at least 8.2 or higher.     

The simplest stirred tank for laminar mixing: Mixing in a vessel agitated by an off‐centered angled disc

We study the mixing structure, mixing performance, and short term dynamics in round bottomed laminar tanks agitated by an eccentrically located angled disc. We define eccentricity (E = e/R) as the ratio of the distance of the axis of rotation from the center line of the tank (e) and the tank radius (R). The structural and dynamic features observed at different eccentricity values were compared using planar laser‐induced fluorescence techniques and computational fluid dynamics calculations. A Poincaré analysis demonstrates the chaotic nature of the flow induced by eccentricity. Practically globally chaotic conditions are observed for E = 0.42 and E = 0.50, with mixing times of 5–8 min at Re = 416. We study the effect of different injection points on the short‐term mixing dynamics and we calculate axial flow rates and Power numbers. Stirred tanks agitated by an eccentrically located angled disc are a simple and cost effective system for laminar mixing applications. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3092–3108, 2013     

CFD simulations of gas-liquid-solid stirred reactor: Prediction of critical impeller speed for solid suspension

In this work, simulations have been performed for three phase stirred dispersions using computational fluid dynamics model (CFD). The effects of tank diameter, impeller diameter, impeller design, impeller location, impeller speed, particle size, solid loading and superficial gas velocity have been investigated over a wide range. The Eulerian multi-fluid model has been employed along with the standard k-ε turbulence model to simulate the gas-liquid, solid-liquid and gas-liquid-solid flows in a stirred tank. A multiple reference frame (MRF) approach was used to model the impeller rotation and for this purpose a commercial CFD code, FLUENT 6.2. Prior to the simulation of three phase dispersions, simulations were performed for the two extreme cases of gas-liquid and solid-liquid dispersions and the predictions have been compared with the experimental velocity and hold-up profiles. The three phase CFD predictions have been compared with the experimental data of Chapman et al. [1983. Particle-gas-liquid mixing in stirred vessels, part III: three phase mixing. Chemical Engineering Research and Design 60, 167-181], Rewatkar et al. [1991. Critical impeller speed for solid suspension in mechanical agitated three-phase reactors. 1. Experimental part. Industrial and Engineering Chemistry Research 30, 1770-1784] and Zhu and Wu [2002. Critical impeller speed for suspending solids in aerated agitation tanks. The Canadian Journal of Chemical Engineering 80, 1-6] to understand the distribution of solids over a wide range of solid loading (0.34-15 wt%), for different impeller designs (Rushton turbine (RT), pitched blade down and upflow turbines (PBT45)), solid particle sizes (120-) and for various superficial gas velocities (0-10 mm/s). It has been observed that the CFD model could well predict the critical impeller speed over these design and operating conditions.     

桨型和挡板对自浮颗粒三相体系混合的影响

在直径３８６ｍｍ的搅拌釜内,考察了多种搅拌桨和挡板组合对自浮三相体系的搅拌功耗,气含率和釜底部颗粒含量的影响。实验表明,自浮三相体系的搅拌混合上层桨宜采用上推式桨;当液高与釜径比为１．６时,三层桨的混合参数优于两层桨;简易型轴流桨的混合效果好于涡轮桨。此外还对优异的搅拌桨和挡板组合进行了桨间距的优化,并回归了关联式,可供过程放大时参考。     

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.     

Effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred tanks

The aim of this work is to investigate the effect of the shaft eccentricity on the hydrodynamics of unbaffled stirred vessels. The difference between coaxial and eccentric agitation is studied using a combination of experiments carried out by particle image velocimetry, that provide an accurate representation of the time-averaged velocity, and computational fluid dynamics simulations, that offer a complete, transient volumetric representation of the three-dimensional flow field, once a proper modelling strategy is devised. The comparison of the experimental and simulated mean flow fields has demonstrated that calculations based on Reynolds-averaged Navier-Stokes equations are suitable for obtaining accurate results. Depending on the position of the shaft, steady-state or transient calculations have to be chosen for predicting the correct flow patterns. Care must be exerted in the choice of turbulence models, as for the unbaffled configurations the results obtained with the Reynolds stress model are superior to that of the k-ε model.     

Investigation of impeller modification and eccentricity for non-Newtonian fluid mixing in stirred vessels

A shear thinning fluid (1% carboxymethyl cellulose) was used to investigate mixing under laminar flow conditions in an unbaffled vessel. The effects of impeller modification in addition to eccentricity were studied. Quantitative measurements such as percentage of uncovered area and coefficient of variance (CoV) of a tracer solution distributed inside the vessel were obtained using planar laser-induced fluorescence (PLIF) method. Increased eccentricity was found to be more effective than increasing rpm alone in reducing isolated mixing regions size (determined by the percentage of uncovered area). The dual-flow pitched blade turbine (DF-PBT), which was the modified version of a standard pitched blade turbine (PBT), was designed to provide both upward and downward flow at the same time to induce more chaotic flow. Though numerical analysis showed this type of flow generated, DF-PBT did not return lower values for the percentage of uncovered area and CoV than PBT did. Power consumption data were also compared between the two impeller types and eccentric locations. Further analyses focusing on the interactions between the impeller blades and fluid rheology is needed to improve laminar mixing in stirred vessels by impeller modification.     

Experimental determination and numerical simulation of mixing time in a gas-liquid stirred tank

In this work, mixing experiments and numerical simulations of flow and macro-mixing were carried out in a 0.24 m i.d. gas-liquid stirred tank agitated by a Rushton turbine. The conductivity technique was used to measure the mixing time. A two-phase CFD (computational fluid dynamics) model was developed to calculate the flow field, k and ε distributions and holdup. Comparison between the predictions and the reported experimental data [Lu, W.M., Ju, S.J., 1987. Local gas holdup, mean liquid velocity and turbulence in an aerated stirred tank using hot-film anemometry. Chemical Engineering Journal 35 (1), 9-17] of flow field and holdup at same conditions were investigated and good agreements have been got. As the complexity of gas-liquid systems, there was still no report on the prediction of mixing time through CFD models in a gas-liquid stirred tank. In this paper, the two-phase CFD model was extended for the prediction of the mixing time in the gas-liquid stirred tank for the first time. The effects of operating parameters such as impeller speed, gas flow rate and feed position on the mixing time were compared. Good agreements between the simulations and experimental values of the mixing time have also been achieved.     

Effect of eccentricity on transitional mixing in vessel equipped with turbine impellers

In this paper, the results of the experimental studies of the mixing time, as well as the power consumption and baffle presence in the stirred tank with dual eccentrically located impellers are presented. The experiments were carried out in an unbaffled flat-bottomed cylindrical vessel. Three types of impellers were used: Rushton turbine, six pitched blade turbine and six flat blade turbine. The obtained data show that eccentricity of dual impeller systems leads to reduction of mixing time. Moreover, the experimental data confirmed the enlargement of power consumption in such systems. In the paper the analysis of relation between eccentricity ratio and mixing time has been performed.     

10种石灰石浆液箱搅拌机的选型与计算

通过对搅拌机各组成部分的选型分析与介绍,针对脱硫脱硝中10种常见规格的石灰石浆液箱,进行搅拌机工艺选型上的优化设计,分析在脱硫脱硝石灰石浆液箱中如何进行搅拌机的优化选型,从而达到最优的使用效果和工艺要求。目前,搅拌机的选型和内构件的设计很大程度上依赖试验和实际工程经验。产品设计的优劣可使搅拌设备的效益相差十分悬殊,因此本文在明确石灰石浆液物料性质的基础上,针对搅拌设备的各个要素,例如叶轮的形状、叶轮直径、布置层数、转速、搅拌轴大小、挡板的尺寸和个数等进行一一优化。充分利用已有业绩对比分析,对10种常见的石灰石浆液箱规格型号在设计选型上完成优化,使机械搅拌机的设计理论更加完善,也能满足日常脱硫脱硝行业中从业者对石灰石浆液箱搅拌机的选型需求。     

Study of solid–liquid mixing in agitated tanks through electrical resistance tomography

Electrical resistance tomography (ERT), which is a non-intrusive flow visualization technique, was used to investigate the solid–liquid mixing in an agitated tank equipped with a top-entering axial-flow impeller. The signals obtained from eight ERT planes were utilized to reconstruct the tomograms by using the linear back projection algorithm. The ERT measurements were correlated to solid concentration profiles by which the degree of homogeneity was quantified. In this study, the effect of important parameters such as impeller type (Lightnin A100, A200, A310, and A320 impellers), impeller speed (250–800 rpm), impeller off-bottom clearance (T/5–T/2, where T is the tank diameter), particle size (210–1500 μm), and solid concentration (5–30 wt%) on the degree of homogeneity were explored. The results showed that the degree of homogeneity in the solid–liquid mixing was improved with increasing the impeller speed. However, after reaching the maximum achievable homogeneity, further increase in impeller speed was not beneficial but might be detrimental. Hence, the measurement of the optimal impeller speed as a function of operating conditions and design parameters has vital role in achieving maximum homogeneity in a solid–liquid mixing system.