Onset of gas induction and power consumption in an agitated tank having shortened narrower baffles

The onset speed of gas induction and the power consumption after gas induction were studied in a gas-induced type agitated tank. In such an agitated tank, four unique baffles which were shorter and narrower than the conventional ones were adopted. Two 45° downward pitched blade turbines (PBT) in series were used as impellers. Gas induction could be formed by the dual impellers operated above the onset impeller speed. Several geometric factors and operation parameters were considered by theoretical aspects and the dimensionless analysis to study the onset of gas induction and the power consumption. In all experiments, air and tap water were used as working fluids. Results revealed that a lower agitation speed and less power consumption were needed for the present tank than a narrower full-length baffled agitated tank. In the present tank, the least specific power consumption was found when the liquid level was about 1·9 times the tank diameter. Two correlation equations were obtained to predict, respectively, the onset speed of gas induction and the power consumption after gas induction. Comparison between the present type and various other gas-inducing type agitated tanks were also discussed. © 1998 SCI     

搅拌槽中非稳态温度场分布的研究

在对称装有4组垂直加热列管、直径为500mm的搅拌槽中使用长薄叶(CBY)桨进行搅拌,采用液晶(LC)测温技术测量搅拌槽内温度场的分布。分别研究了搅拌槽中轴向温度分布随加热时间、叶轮雷诺数以及桨的安装高度不同而变化的情况。结果表明,列管加热时间的长短对槽内温度分布的趋势没有影响,温度分布的趋势主要取决于所用搅拌桨产生的流型;随着叶轮雷诺数的增加,槽内温度分布相对趋于均匀;搅拌桨的安装高度对温度场的分布影响也很显著。     

Effects of Geometrical Factors on Liquid Mixing in a Gas-Induced Agitated Tank

An agitated tank having two 45° pitched blade downward turbines in series and four thin baffles which were narrower than the conventional ones was studied. At higher impeller speeds, a gas and liquid vortex induces the gas above the liquid surface vertically downward into the turbines, which in turn cuts the gas stream into bubbles which then circulate in the liquid. Consequently gas-induction increases the gas utilization ratio as well as simplifying the ancillary equipment otherwise required to recycle the escaped gas. The relationship between tank geometrical factors and the liquid mixing time was also investigated. The experimental variables investigated were the impeller speed, the baffle width, the turbine diameter, the space between two impellers, the liquid level, and the clearance between the lower turbine and the bottom of the tank. The results showed that all but the clearance between the lower turbine and the bottom of the tank had significant effects on the mixing time under normal gas-induction, revealing results in contrast to the conventional agitated tank. © 1997 SCI.     

多层桨搅拌槽内的液-液分散特性

在直径为0．476m的椭圆底圆柱形搅拌槽内,以水／航空煤油及水／环己烷为实验体系,研究了Rushton涡轮式搅拌桨(RT-6)、六半椭圆管涡轮式搅拌桨(HEDT)、折叶轴流式搅拌桨及翼形轴流式搅拌桨(CBY)的6种不同组合桨的液-液分散特性,用取样法测定了分散相体积分数的轴向分布及体系澄清时间。结果表明,组合桨中的底桨在液-液分散中发挥了主要作用。单位体积输入功率相同时,底桨为CBY的组合桨进行液-液分散后,液滴的平均滴径最小,体系澄清时间较长;底桨为HEDT的组合桨的分散效果次之;底桨为RT-6的组合桨因滴径分布较宽,虽然平均滴径最大,但澄清时间也较长。     

搅拌罐（釜）的工程放大方法和经济分析

介绍了搅拌罐（釜）的工程放大原理和方法，并通过实例进行了经济分析。     

不同桨型的搅拌槽中非稳态温度场分布的研究

在搅拌槽中,由于搅拌浆型式的不同,会导致流体温度分布的不同。本文在直径为0．5米的搅拌槽中分别使用CBY,带副叶CBY以及PBT三种浆进行研究,三种浆的搅拌雷诺分别为267,251和203,在搅拌槽中设置了四组加热列管（兼作挡板）进行加热,将热色液晶均匀地分散在甘油中,而显示温度的分布。实验研究了相同功率下三种浆的瞬时温度分布,结果表明,搅拌槽中的温度分布取决于搅拌浆产生的流型,当流体被加热时,流场内漩涡中温度梯度通常大于其它区域,漩涡的存在不利于槽内温度分布的均一性,在三种浆中,PBT浆的轴向温差最小。     

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.     

Mean residence time of gas in mechanically agitated gas–liquid contactors

In the present study of gas–liquid contactors, mean residence/contact time was calculated from knowledge of superficial velocity and the gas phase hold-up, for various gas rates and impeller geometry and speeds, and compared with values obtained from RTD measurements. A new correlation, involving Flow Number, Froude Number, system geometry and the physical properties, is proposed. This uses the authors data and those available in literature.     

Relation of mechanical power to gas holdup and mass transfer in an agitated vessel

Two devices were developed, mechanical and electrical, to measure the mechanical power relative to superficial gas velocity and stirring speed in a mechanically agitated reactor. Two bubble regimes were demonstrated. The study of gas holdup, obtained from the residence time distribution, and the bubble size, determined by interfacial area measurement, confirm these two regimes of flow. Our results show that the gas holdup, the interfacial area, and the liquid-side mass transfer coefficient are increasing functions of the energy dissipated in the solution.     

Power and mean flow characteristics in mixing vessels agitated by hyperboloid stirrers

The mean flow and energy consumption in vessels powered by hyperboloid stirrers was investigated. The Newton number followed an inverse linear law for Reynolds numbers below approximately 200, which had values more than twice higher the corresponding Newton number for a standard Rushton turbine. At high Reynolds number flows the Power number varied between 0.5 for a D/T = 0.78 impeller to 0.95 for a D/T = 0.24 impeller, as compared to a value of 5 for the standard Rushton stirrer, and to values of 0.31 and 1.58 for the Chemineer and Prochern hydrofoils. The power consumption did not change with the fluid height and was double for the double-stack configuration. The shear ribs below the impeller were the main contributor to the increased energy loss relative to a non-shear ribbed impeller and the small clearance had no major effect upon the power consumption. For the D/T= 1/3 hyperboloid stirrer the flow in the whole tank was rather gentle, defining a circulation number of 0.57, thus leading to a circulation efficiency more than 7 times lower than that of the hydrofoils.     

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     

电厂除盐水箱聚脲防腐涂层的设计与应用

介绍了电厂除盐水箱的技术参数、所处的腐蚀环境以及聚脲防腐涂层的设计要求。应用验收结果表明,防护涂层外观平整、光滑,顶部厚度1.02 mm,侧面厚度1.56 mm,底部厚度1.52 mm,电火花通过率100%,拉伸强度12.6 MPa,撕裂强度46 kN/m,断裂伸长率260.5%。水箱出水口水质符合要求,除盐水箱使用寿命可达30年。     

Experimental and modelling study of gas dispersion in a double turbine stirred tank

Gas dispersion in a double turbine stirred tank is experimentally characterised by measuring local gas holdups and local bubble size distributions throughout the tank, for three liquid media: tap water, aqueous sulphate solution and aqueous sulphate solution with PEG. For all these media, bubble coalescence generally prevails over breakage. Where average bubble size decreases, this can be attributed to the difference in slip velocity between different sized bubbles. Most of the coalescence takes place in the turbine discharge stream.A compartment model that takes into account the combined effect of bubble coalescence and breakage is used to simulate gas dispersion. The model predicts spatial distribution of gas holdup and of average bubble size, with average bubble size at the turbines as an input. Reasonable agreement between experiment and simulation is achieved with optimisation of two parameters, one affecting mainly the slip velocity, the other related mainly to the bubble coalescence/breakage balance. Different sets of parameters are required for each of the three liquid systems under study, but are independent of stirring/aeration conditions. The model only fails to simulate the smaller average bubble diameters at the bottom of the tank.     

电站锅炉气体不完全燃烧损失对供电煤耗的影响

将燃煤锅炉烟气中的可燃气体简化成CO气体,对中国电站100～1000 MW煤粉锅炉(PCB)和50～300 MW循环流化床锅炉(CFBB)的气体不完全燃烧损失(q3)和由此引起的供电煤耗提高值(Δb)进行了计算。结果表明:当烟气中的CO质量浓度从50 mg/m3增加到400 mg/m3时,随着动力煤干燥无灰基挥发分Vdaf的提高,煤粉锅炉和循环流化床锅炉的q3在0.01%～0.18%之间变化;燃烧褐煤的q3损失最大;PCB和CFBB的q3引起的Δb变化范围为0.03～0.65 g/kWh;相对于PCB,随着Vdaf的提高,CFBB的q3和Δb的波动幅度较小;q3和Δb随着烟气中的CO质量浓度的提高基本上线性提高;PCB和CFBB的q3、Δb随着锅炉蒸汽压力的提高逐步降低。     

CFD modeling of gas entrainment in stirred tank systems

In the present work, CFD modeling was used to study the phenomenon of gas entrainment in stirred tank systems. Two types of impellers (DT, PBTD) were simulated. VOF method was used as surface tracking technique along with LES model to study interfacial behavior at the onset of gas entrainment. Simulations were performed to study cause of entrainment and underlying interfacial mechanism at the location of entrainment. CFD simulations clearly showed differences in onset and non onset conditions in terms of the magnitudes of interfacial turbulence. As per the predictions, phenomenon of surface aeration in stirred tank systems was characterized by exchange of momentum across the interface from water side to air side. Magnitudes of instantaneous axial velocities on air side, strain rates on air side and vorticities on air side exhibited a threshold at the onset of entrainment and reduced substantially after the onset.     

Measurement of gas and liquid flows in stirred tank reactors with multiple agitators

The gas flow in a 3:1 aspect ratio vessel agitated by triple Rushton turbines has been measured by an ultrasound Doppler probe and by means of residence time studies. Strong recirculation around each impeller is found which fits in well with the compartmentalisation found in earlier liquid mixing studies. Surprisingly, when two axial A315 impellers above a Rushton turbine were used, gas recirculation around each impeller was still found. Study of the liquid phase mixing by a decolourisation technique confirmed that the gas flow essentially destroyed the strong axial liquid flow expected. Indeed, even under unaerated conditions, compartmentalisation was found between each impeller.     

搅拌槽内桨叶高度对流场结构和功率消耗影响的数值模拟

运用多重参考系法和滑移网格法处理搅拌桨与挡板之间的相对运动,结合剪切应力输运模型建立了描述流体运动的控制方程组,考察了搅拌槽内桨叶高度对流场结构和功率消耗的影响,并将模拟结果与文献报道结果进行了比较.结果表明,当桨叶高度逐渐降低时,槽内流型由双环流结构转变为单环流结构,功率准数也随之急剧下降,计算流体力学模拟结果与文献报道结果有较好的一致性.     

CFD modeling of gas dispersion and bubble size in a double turbine stirred tank

In the present paper, gas dispersion in a double turbine baffled stirred tank is modeled using a commercial computational fluid dynamics (CFD) code FLUENT 6.1 (Fluent Inc., USA). A bubble number density equation is implemented in order to account for the combined effect of bubble break-up and coalescence in the tank. In the proposed work, the impellers are explicitly described in three dimensions using multiple reference frame model. Dispersed gas and bubbles dynamics in the turbulent water are modeled using an Eulerian-Eulerian approach with dispersed k-ε turbulent model and modified standard drag coefficient for the momentum exchange. The model predicts spatial distribution of gas holdup, average local bubble size and flow structure. The results are compared with experimental and numerical finding reported in the literature and good agreement between the present model and measurements of Alves et al. [Gas liquid mass transfer coefficient in stirred tanks interpreted through bubble contamination kinetics. Chemical Engineering Science, 2002, 57, 487-496] is achieved.