Power consumption in a moving baffle oscillatory baffled reactor: A CFD study

Oscillatory baffled reactors (OBRs) can perform high-intensity mixing under low power consumption, and thus are viable replacements for stirred tank reactors in biological, chemical, and polymer processes. This study simulated the flow inside a moving baffle OBR with single orifice baffles using computational fluid dynamics (CFD). The effect of operational and geometrical parameters along with the fluid density and viscosity on average power consumption and maximum power consumption was investigated, and appropriate correlations for both average and maximum power consumption were obtained. It was found that average and maximum power consumption are independent of viscosity, and amplitude has a greater impact on maximum power consumption than on average power consumption. These correlations were then compared with available power models (that showed an acceptable level of discrepancies) in the literature. Lower power consumption values obtained from CFD results compared to those obtained from quasi-steady state model (QSM) and eddy enhancement model (EEM) models (developed for stationary baffle OBRs) under the same operating conditions, along with higher axial dispersion of moving baffle OBRs compared to stationary baffle types under the same operating conditions, indicated that a moving baffle OBR is a more efficient mixing device than a stationary baffle OBR in terms of power consumption. The ratio of average power consumption to maximum power consumption was proven to be independent of the type of fluid and a very weak function of oscillation frequency.     

多相流搅拌反应器研究进展

综述了近五年冷态气液固三相搅拌反应器的研究成果,阐明近期研究热点已转为热态多相流搅拌反应器的流体力学行为。概述了热态气液两相体系中通气功率、混合时间、局部及总体气含率的研究结果。指出今后研究重点应为热态的固液悬浮、气液传质过程及其机理,同时说明用计算流体力学（CFD）对气液固三相搅拌反应器的数值模拟仍属起步阶段。     

Simulation of the 200 MWe tonghae thermal power plant circulating fluidized bed combustor by using IEA-CFBC model

The 200 MWe Tonghae thermal power plant CFB boiler (2-units) is the largest boiler to fire a Korean anthracite coal for generation of electric power. The #1 unit CFB has been operated commercially since October 1998, and the #2 unit CFB, which will begin commercial operation in October 1999, is currently under construction. However, there is little operational data of the large CFB combustor firing anthracite coal, so it is necessary to predict the performance of the CFB combustor with variation of the operation conditions. Therefore, in this study, the simulation of the Tonghae CFB has been carried out to predict the performance with various nominal rates, coal particle size distribution and operating conditions. The simulation results with various loads could be fitted well to the design values at the given operation conditions. At the various operation conditions, the simulation results could explain the performance of the Tonghae CFBC well.     

Experimental and CFD investigation of power consumption in a dual Rushton turbine stirred tank

Power consumption of a mixing system is a key variable in chemical and bioprocess engineering, the determination of which is of interest of many processes. Besides, prediction of the flooding-loading transition in an aerated stirred tank is crucial for the correct design of aerated stirred tank reactors. In this research, laboratory investigation has been carried out on local and total power consumption of a single phase as well as gas-liquid phase systems in a fully baffled stirred tank equipped with dual six-blade Rushton turbines; moreover, the flow regime behavior of a gas-liquid system was investigated. Results have been compared with data obtained from CFD simulation of experimental setup and the data available in the literature. Reasonable agreement between the experimental and simulation results indicates the validity of the CFD model. Using predicted data some empirical correlations have been derived which present new relations in estimation of power consumption and flow regime transitions in stirred tanks with dual Rushton impellers.     

Characterization on the hydrodynamics of a covering-plate Rushton impeller

A modified Rushton impeller with two circular covering-plates mounted on the upper and lower sides of the blades was designed. There are gaps between the plates and the blades. The turbulent hydrodynamics was analyzed by the computational fluid dynamics (CFD) method. Firstly, the reliability of the numerical model and simulation method was verified by comparing with the experimental results from literature. Subsequently, the power consumption, flow pattern, mean velocity and mixing time of the covering-plate Rushton impeller (RT-C) were studied and compared with the standard Rushton impeller (RT) operated under the same conditions. Results show that the power consumption can be decreased about 18%. Compared with the almost unchanged flow field in the lower stirred tank, the mean velocity was increased at the upper half of the stirred tank. And in the impeller region, the mean axial and radial velocities were increased, the mean tangential velocity was decreased. In addition, the average mixing time of RT-C was shortened about 4.14% than the counterpart of RT. The conclusions obtained here indicated that RT-C has a more effective mixing performance and it can be used as an alternative of RT in the process industries.     

Reactor performance with high velocity surface feed

Reagents have traditionally been fed into the impeller region of stirred tank reactors to utilize the high levels of turbulence, improve mixing rates and hence reduce mixing limitations. There are; however, numerous operational problems associated with submerged feed with dip tubes so surface feed is still preferred by operations staff. One alternate way of adding reagents into a stirred tank is to feed at the liquid surface using a high velocity jet. The ability of a high speed jet mixed feed stream to reduce mixing limitations has been investigated in this study. While most previous studies report results in the laminar flow regime (even for mesomixing limited operation), the present investigation presents data for much higher feed velocities.Experiments revealed that with reduced feed time, there is a local maximum in byproduct formation and the product quality initially improves with faster feed once this point is reached. However, other factors (primarily macromixing issues) limit the maximum improvement that can be achieved and in fact at very short feed times the byproduct formation increases again. Analysis of the design based on mixing and turbulent jet constraints shows that it is very difficult to obtain a fully turbulent jet because this leads to very short feed times (and associated convective stoichiometric limitations), very high feed stream velocities (where the feed stream overshoots the impeller) or very small feed pipe diameters (giving a large pressure drop through nozzle). Feed jets at high velocities are difficult to achieve under reasonable operating conditions and do not provide any substantial improvement in performance.     

生物反应器内不同桨叶组合计算流体力学模拟

运用计算流体力学(CFD)数值模拟方法研究在相同工况下(搅拌桨转速为400 r/min、通气速度为0.86 vvm和操作温度为15 ℃),4种不同桨叶组合方式对5 L气液生物反应器流场、氧传质系数kLα、空气体积分数、气含率(体积分数)和功率的影响,评判各桨叶组合综合性能.计算结果表明:1号方案上下档均为径向桨,具有最...     

搅拌生物反应器的结构模型、放大及搅拌器改型

搅拌生物反应器（ＳｔｉｒｒｅｄＢｉｏｒｅａｃｔｏｒ－ＳＢＲ）是最重要的生物反应器，随着生物高新技术的发展，在医药、食品、化工及环境等领域对其提出了更高的要求。为此详细评述了最近１０年在ＳＢＲ的结构模型、放大及搅拌器改型方面的进展及存在的问题，特别强调了ＳＢＲ性能研究的总体战略及具体的方法步骤。还就本学科发展的研究工作提出了建议。     

丙烯直接环氧化溶剂回收过程的能耗模拟与优化

为了提高1500 t/a环氧丙烷中试装置的能量利用水平,在建立了热力学模型的基础上,采用PRO/II流程模拟软件对装置的溶剂回收塔进行计算,并考察了操作条件对再沸器加热蒸汽消耗量的影响。经中试装置验证,采用优化操作条件后实际蒸汽耗量可从10 t/tPO下降到9.4 t/tPO,降幅为6%。鉴于单塔节能的挖潜效果欠佳,本文设计了溶剂回收的双效精馏流程。模拟计算的结果表明,可使蒸汽消耗量从单塔优化后的9.27 t/tPO下降到5.68 t/tPO,节能率达到38.6%,为万吨级装置的设计提供参考依据。     

变频刚柔搅拌反应器强化锰矿浸出及除铁过程

传统锰矿浸出过程采用刚性搅拌反应器,存在矿粉悬浮效果差、锰矿利用率低、浸出液中铁含量较高的问题。本文采用变频刚柔搅拌反应器强化锰矿的浸出过程,实验对比分析了不同桨叶类型、搅拌桨操作方式和变频间隔时间对锰矿浸出效果、搅拌功耗的影响,同时分析了变频刚柔桨耦合空气射流在除铁过程的效果。结果表明,变频刚柔桨通过柔性钢丝绳对流体的扰动作用以及变频操作对稳定流场的更新作用,提高浸出液中Mn²⁺含量;同时,由于体系变频操作的频率变化,降低了搅拌功耗。相同功耗下,刚柔组合桨体系浸出液中Mn²⁺含量相对于刚性桨体系提高了10.8%;相同转速下,刚柔组合桨体系在浸出过程的功率消耗相对于刚性桨体系提升了25.4%。变频间隔时间为30min时,变频刚柔桨体系浸出液中Mn²⁺含量相对于刚柔组合桨体系降低了2.17%,但相应搅拌功耗降低了28%。调节体系pH后,采用变频刚柔桨耦合偏心空气射流操作,可有效降低浸出液中Fe²⁺含量。在通气60min后,浸出液中Fe²⁺浓度已至较低水平,低于0.15mg/L。     

低高径比外循环气升式生物反应器带渣发酵生产有效霉素

Fermentation experiments to produce validamycins from crude substrates by Streptoniyces hygroscopi-cus were carried out in an external-loop airlift bioreactor (0.0115 m3 ) with a low ratio of height to diameter of the riser of 2.9 and a ratio of riser to downcomer diameter of 6.6. The influences of gas flow rate and liquid volume on fermentation of validamycins were investigated. Comparisons of validamycin fermentation were made among the external-loop airlift bioreactor, a mechanically stirred tank bioreactor (0.010m3 ) and shaking flasks. Under the same operation conditions including fermentation medium composition, inoculum ratio and culture temperature, the fermentation time in the external-loop airlift bioreactor (45 h) was shorter than that in the shaking flasks (100 h) and the same as that in the mechanically stirred tank bioreactor. After a total fermentation time of 45 h under optimized operation conditions, average validamycin concentration obtained in the external-loop airlift bioreactor was     

Viscoelastic Liquids in Stirred Vessels – Part I: Power Consumption in Unaerated Vessels

Different shear‐thinning and elastic fluids (STE fluids) have been stirred under unaerated conditions, in vessels equipped with Rushton disc turbines. Their power consumption has been evaluated over a wide range of stirring rates and their Metzner‐Otto constant (k_s) has been measured. A correlation has then been proposed to predict k_s values for a Rushton turbine operating in non‐Newtonian solutions. Power curves of STE fluids have been drawn and compared with reference curves (Newtonian, shear‐thinning inelastic and elastic with constant shear viscosity fluids). The STE fluids have thus been divided into two categories. The STE fluids of the first category (STE I fluids), which are concentrated viscous solutions of polymers (guar, CMC) reducing the power consumption at the beginning of the transitional region and connecting with the Newtonian reference at higher Reynolds numbers. In contrast, STE solutions of the second category (STE II fluids), which are solutions of drag reducing polymers (PAA), are less viscous and more elastic. They reduce the power consumption at the end of the transitional region and do not connect with the Newtonian reference, at least until Re = 6000. A general correlation has finally been proposed to model the power curve of STE fluids stirred by a Rushton turbine from the laminar to the turbulent regions, as a function of their elasticity.     

MIXING CHARACTERISTICS IN SLURRY STIRRED TANK REACTORS WITH MULTIPLE IMPELLERS

Hydrodynamic parameters such as power consumption, gas holdup, critical impeller speed for solid suspension and mixing time were measured in slurry stirred tank reactors with multiple impellers. The experiments were mainly conducted in a stirred tank of 0.2mi.d. with baffles. It contained two four-pitched blade downflow turbines for gas dispersion and one Pfaudler type impeller for solid suspension. As a part of scaling studies, additional experiments were also carried out in a larger stirred tank reactor (0,8m i.d.) geometrically similar to the smaller one. Glass beads and polymeric particles were used as a solid phase. Solid concentration was in the range of 0-20% (K/K). Tap water and methanol were used as a liquid phase

The power consumption decreased due to an introduction of gas and the presence of solids caused a decrease in the extent of reduction in power consumption. A correlation for power consumption in aerated slurry systems was proposed, It was found that the presence of solids is responsible for a decrease in gas holdup. A new correlation for gas holdup in gas-liquid-solid three-phase stirred tank reactors was developed. It fit the present experimental data reasonably. The critical impeller speed for solid suspensions increased with increasing gas flow rate. However, its increase was rather smaller as compared with the predictions of the correlations available in the literature. We proposed a correlation of the critical impeller speed for solid suspension in the presence of gas. The mixing time complicatedly increased or decreased depending on gas flow rate, impeller speed, solids type and concentration.     

搅拌槽内气液两相混沌混合及分散特性

传统Rushton刚性桨常应用于过程工业中搅拌反应器内的气液分散过程,但由于桨叶背后易形成较大的气穴,气液混合效果较差。为了提高搅拌槽内气液两相的混合效果,提出了一种刚柔组合桨强化气液两相的分散过程。利用LabVIEW软件处理刚性桨和刚柔组合桨体系中气液混合过程的压力脉动信号,通过Matlab软件编程计算最大Lyapunov指数（LLE）,分析气液混合体系的混沌混合行为,同时,对刚性桨和刚柔组合桨体系中的相对搅拌功耗、整体气含率、局部气含率进行测量。结果表明,在功耗为170 W,通气量为10 m³·h^-1条件下,与刚性桨相比,刚柔组合桨能够通过刚-柔-流的耦合作用促进桨叶能量的传递过程,提高搅拌体系的混沌混合程度,刚柔组合桨体系的LLE提高了8.89%。同时,在相同操作条件下,与刚性桨相比,刚柔组合桨能够有效提高相对搅拌功耗以及搅拌槽内的整体气含率和局部气含率,且搅拌槽内气体分散更为均匀。     

搅拌生物反应器混合特性的数值模拟与实验研究

以工程流体计算软件CFX-4.4为工具,对不同规模的机械搅拌生物反应器的混合特性进行数值模拟,研究了不同操作条件下反应器混合时间的变化规律. 采用pH电极在位监测[H+]的方法实验测定混合时间. 模拟结果与实验测定值之间的误差随反应器容积增大而逐渐减小,对容积为25 m3的反应器误差小于11.6%.     

Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass trans-fer performance under al operating conditions. At high uG and a given power input, the impel er combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.     

振动筛板槽中液滴分散动力学的研究——(Ⅱ)实验结果及Monte-Carlo模拟技术的应用

本文根据前文(I)报的理论分析,采用Monte-Carlo方法对实验数据进行模拟.液滴直径的测量采用照相法及透光法.所测得的液滴直径分布数据及平均液滴直径的数据,在微机上进行拟合归纳,得到了振动筛板槽中液滴直径分布模型及液滴平均直径的关联式.模型计算结果与实测值拟合较好.应用Monte-Carlo模拟技术对液滴破碎和凝聚这一随机过程进行了模拟及参数估计,得到了液滴破碎速率及凝聚速率参数.     

Application of KHX Impeller in a Low-shear Stirred Bioreactor☆

In our previous work, a low-shear stirred bioreactor was explored. With a pitched blade turbine impeller downflow (PBTD) used, the shear stress generated is high compared with that in some low shear ax...     

STUDY OF THE TURBULENT FLOW IN AN UNBAFFLED STIRRED TANK BY DETACHED EDDY SIMULATION

Detached eddy simulation (DES) of the liquid-phase turbulent flow in an unbaffled stirred tank agitated by a six-blade, 45°-pitched blade turbine was performed in this study. The tank wall is cylindrical with no baffle and the fluid flow problem was solved in a single reference frame (SRF) rotating with the impeller. For the purpose of comparison, computation based on large eddy simulation (LES) was also carried out. The commercial code Fluent was used for all simulations. Predictions of the phase-averaged turbulent flow quantities and power consumption were conducted. Results obtained by DES were compared with experimental laser Doppler velocimetry (LDV) data from the literature and with the predictions obtained by LES. It was found that numerical results of mean velocity and turbulent kinetic energy profiles as well as the power consumption are in good agreement with the LDV data. When performed on the same computational grid, which is under-resolved in the sense of LES, DES allows better accuracy than LES in that it works better in the boundary layers on the surface of the impeller and the stirred tank walls. It can be concluded that DES has the potential to predict accurately the turbulent flow in stirred tanks and can be used as an effective tool to study the hydrodynamics in stirred tanks.     

常温及热态通气条件下多层桨三相搅拌反应器中局部气含率分布

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration (a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT+2WHU) recommended in previous work has been used in this work. The operating temperatures were 24°C and 81°C, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are investigated and discussed. Results show that the increasing of agitator speed or gas flow rate leads to an increase in local void fraction at the majority of measurement points in both cold and hot systems. However, the uniformity of gas dispersion does not always increase as the rais-ing of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase stirred tanks.