间歇与半间歇反应热失控危险性评估方法

针对间歇、半间歇反应器中频繁更换物系和/或反应工艺,受时间和经济条件限制往往不可能对发生在其中的反应动力学进行深入研究的特点,建立适用于它们的热失控危险评估方法具有重要意义。本文对此做了简要的综述并且认为,尽管迄今还没有普适性的方法,但是对于某些类型的重要反应,安全界限图方法和反应失控情景分析方法还是较为适用的。前者虽然涉及参数较多,且没有考虑高温下物料二次分解的可能性,但能按照反应结果将量纲1操作参数平面划分为无积累、反应失控、未充分引发及无害4个子区域,从而可以先验地获取更多的信息;后者虽然忽略了均相与非均相反应的不同,但该方法可对一定工艺条件下目标反应失控和二次分解反应进行分析评估,简单易行,考虑更全面。     

Multi-scale modeling of a mixing-precipitation process in a semibatch stirred tank

In this work the influence of turbulent mixing on the course of barium sulfate precipitation is investigated for a process carried out in a semibatch stirred tank reactor. A time-scale analysis for the controlling process mechanisms is presented to highlight the multi-scale nature of the process. A detailed CFD based process model is presented which accounts for all relevant phenomena from the micro- to the macro-scale and interactions between mechanisms on different scales. Computational results of the models for various operating conditions, i.e., different agitation rates, reactant concentrations and feed addition modes, are compared with experimental data and simulation results obtained using a simpler mechanistic model, thus highlighting the strengths of the complex model.     

CFD modelling of stirred tank chemical reactors: homogeneous and heterogeneous reaction systems

The presented work is a part of studies carried out to develop a practical method to prevent runaway events in chemical reactors. The use of computational fluid dynamics (CFD) techniques is proposed here to indicate in advance the local hot-spots in the reaction mixture, so as to elaborate an efficient and sensitive method for early warning detection of runaway. The CFD method has been chosen to support, elaborated by Bosch et al. (Comput. Chem. Eng. 28 (2004) 527), the on-line runaway detection method in batch reactors, which is based on the divergence criterion. Series of CFD simulations have been executed for two exothermic reactions, being the esterification reaction of the 2-butanol and the propionic anhydrite catalysed by the sulphuric acid (homogeneous system) and the hydrolysis of the propionic anhydrite catalysed by the sulphuric acid (heterogeneous system). For both the considered reactions the results of calculations have been verified with the results of experimental measurements performed in a bench scale RC1 Mettler-Toledo reaction calorimeter. The elaborated conclusions can be further employed in the parametric sensitivity analysis (Comput. Chem. Eng. 28 (2004) 527; Am. Inst. Chem. Eng. J. 45 (1999) 2429) to indicate a number of temperature sensors and their location inside the reactor, which will help to apply efficiently the divergence criterion method.     

搅拌反应器内计算流体力学模拟技术进展

综述了计算流体力学(CFD)技术应用在搅拌反应器的进展情况。重点对搅拌反应器内流动场模拟的各种处理方法,包括"黑箱"模型法、内外迭代法、多重参考系法和滑移网格法,进行了介绍与评价,指出了各种方法所具有的特点及存在的问题。阐述了搅拌反应器内CFD技术的发展方向,并就国内的研究现状进行了简单概述。     

Current problems in the modelling of chemical reactors

Modelling of chemical reactors is reviewed with an emphasis on process development and scale-up. A distinction is made between modelling of chemical kinetics, of rate processes in volume elements and of whole reactors. Examples are mainly taken from papers presented at the Sixth International Symposium on Chemical Reaction Engineering. Special attention is given to the modelling of single phase reactors, fixed beds, trickle beds, fluid beds, and gas bubble reactors.     

Bubbling process in stirred tank reactors I: Agitator effect on bubble size, formation and rising

The study of the hydrodynamics generated by impellers and its effect on the generation of bubbles and on their rising and dispersion is of key importance to improve the knowledge about the contact between phases and the mass transfer rates, particularly in cases where it is the limiting step. CFD simulations and high-speed video techniques are used to study the hydrodynamics developed by five different impellers, each located at three different positions above the dispersion device. Furthermore, two dispersion devices with one and two holes, respectively, are also used. The effect of the impellers on the characteristics of the bubbles and of the dispersions generated has been analysed. Bubbles generated under stirring are smaller than those generated in stagnant fluids. It is also shown that the initial bubble size at the orifice determines the contribution of the impeller and the perforated plate to the Sauter mean diameter. Although bubble formation is chaotic, the formation period is predictable based on three variables: the location of the impeller, its rotational speed and the gas flow rate. Bubble mean diameter was correlated to classical equations based on Kolmogorov's theory. Only when impellers are capable of breaking the bubbles, Kolmogorov's theory is completely verified.     

搅拌槽中酯化反应热失控的CFD模拟

热失控是化工过程中常见的安全风险之一。在间歇釜式反应器中,桨叶的机械转动可以增强流体的循环流动、湍流强度、混合程度以及传热,进而有效防范热失控。防控效果与反应器结构和搅拌桨型密切相关。针对丙酸异丙酯酯化反应,采用计算流体力学模拟研究了桨型(Rushton桨、30o PBT桨及60o PBT桨)、转动方向和挡板对釜式反应器内温度演化的影响,从流动结构方面分析了原因。基于散度的失控判据比较了三种搅拌桨抑制热失控的能力,抑制能力为Rushton桨＞30° PBTD桨＞60° PBTD桨。本研究可为搅拌反应器热失控的优化设计提供一定的理论依据。     

The modeling of batch and continuous emulsion polymerization reactors. Part II: Comparison with experimental data from continuous stirred tank reactors

A detailed model for emulsion polymerization is compared with extensive data from continuous stirred tank reactors. Model predictions are compared with both steady-state and dynamic data, for polymerization of styrene, methyl methacrylate, and vinyl acetate. Good agreement between model and experiment is achieved with one set of parameters and without any data fitting. The results show that the model is capable of predicting all of the experimentally observed phenomena including steady-state multiplicity, sustained oscillations, ignition and extinction dynamics, and overshoot during start-up.     

An assessment of different turbulence models for predicting flow in a baffled tank stirred with a Rushton turbine

This work presents a comprehensive study of different turbulence models, including the k–ε, SST, SSG–RSM and the SAS–SST models, for simulating turbulent flow in a baffled tank stirred with a Rushton turbine. All the turbulence models tested predict the mean axial and tangential velocities reasonably well, but under-predict the decay of mean radial velocity away from the impeller. The k–ε model predicts poorly the generation and dissipation of turbulence in the vicinity of the impeller. This contrasts with the SST model, which properly predicts the appearance of maxima in the turbulence kinetic energy and turbulence energy dissipation rate just off the impeller blades. Curvature correction improves the SST model by allowing a more accurate prediction of the magnitude and location of these maxima. However, neither the k–ε nor the SST model is able to properly capture the chaotic and three-dimensional nature of the trailing vortices that form downstream of the blades of the impeller. In this sense, the SAS–SST model produces more physical predictions. However, this model has some drawbacks for modelling stirred tanks, such as the large number of modelled revolutions required to obtain good statistical averaging for calculating turbulence quantities. Taking into consideration both accuracy and solution time, the SSG–RSM model is the least satisfactory model tested for predicting turbulent flow in a baffled stirred tank with a Rushton turbine.     

Kinetic study of the catalytic cracking of polypropylene in a semibatch stirred reactor

A lumped kinetic model including both thermal and catalytic cracking and catalyst decay has been developed for the cracking of polypropylene in a semibatch stirred reactor. Two decay equations in where the catalyst decay is either a function of time on stream or function of coke on catalyst have been tested. The kinetic model fits very well the experimental results and is able to simulate the process in a wide range of operating conditions.     

Characterisation and comparison of the micromixing efficiency in torus and batch stirred reactors

The mixing at a molecular scale (micromixing) plays an important role on selectivity, yield and quality of final products of a large range of competing fast chemical reactions. In this study, we have compared, by the use of iodide–iodate reaction tests, the micromixing in two reactors, the first one is the standard batch stirred reactor and the second is the torus reactor. Various conditions of agitation and feed locations were used for this study. A comparative analysis of the micromixedness ratio (α) in the two reactors was carried out on the basis of the local rate of specific energy dissipation.     

Measurements and modelling of free-surface turbulent flows induced by a magnetic stirrer in an unbaffled stirred tank reactor

Measurements and numerical simulations of turbulent flows with free-surface vortex in an unbaffled reactor agitated by a cylindrical magnetic stirrer are presented. Measurements of the three mean and fluctuating components of the velocity vector are made using a laser Doppler velocimetry in order to characterise the flow field at different speeds of the stirrer. A homogeneous Eulerian-Eulerian multiphase flow model coupled with a volume-of-fluid method for interface capturing is applied to determine the vortex shape and to compute the turbulent flow field in the reactor. Turbulence is modelled using a second-moment differential Reynolds-stress transport (RST) model, but for some cases the k-ε/k-ω based shear-stress transport (SST) model is also used. The predictions obtained using the ANSYS CFX-5.7 computational fluid dynamics code are compared with the images of the vortex and the measured distributions of mean axial, radial and tangential velocities and turbulent kinetic energy. The predicted general shape of the liquid free-surface is in good agreement with measurements, but the vortex depth is underpredicted. The overall agreement between the measured and the predicted axial and tangential velocities obtained with the RST model is good. However, the radial velocity is significantly underpredicted. Predictions of the turbulent kinetic energy yield reasonably good agreement with measurements in the bulk flow region, but discrepancy exists near the reactor wall where this quantity is underpredicted. The SST model predictions are generally of the same quality as those of the RST model, with the latter model providing better predictions of the tangential velocity distribution.     

Numerical optimization for blades of Intermig impeller in solid–liquid stirred tank

The multiphase flow in the solid-liquid tank stirred with a new structure of Intermig impeller was analyzed by computational fluid dynamics(CFD).The Eulerian multiphase model and standard k-ε turbulence model were adopted to simulate the fluid flow,turbulent kinetic energy distribution,mixing performance and power consumption in a stirred tank.The simulation results were also verified by the water model experiments,and good agreement was achieved.The solid-liquid mixing performances of Intermig impeller with different blade structures were compared in detail.The results show that the improved Intermig impeller not only enhances the solid mixing and suspension,but also saves more than 20% power compared with the standard one.The inner blades have relatively little influence on power and the best angle of inner blades is 45°,while the outer blades affect greatly the power consumption and the optimized value is 45°.     

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.     

Age distribution and the degree of mixing in continuous flow stirred tank reactors

New development of mean age theory is discussed for quantitative analysis of mixing and age distribution in steady continuous flow stirred tank reactors. A new relationship between the moments of age and the moments of residence time are derived. With this new relationship the variance of residence time distribution can be computed much more efficiently and accurately. The relationships of three existing variances of age are described and a new set of variances and the degree of mixing are defined. The theory is used to characterize mixing performance in a CFSTR with different layouts of an inlet and an outlet. Mean age and higher moments of age in the reactors are obtained from CFD solutions of their steady transport equations. The spatial distribution of mean age reveals details of the spatial non-uniformity in mixing. Variances of age and the degree of mixing discussed by Danckwerts and Zwietering are computed for the first time in the literature for non-ideal stirred tank reactors. It is found that although these measures are useful, certain key features in non-uniform mixing are not reflected accurately. Results show that the new set of variances and the degree of mixing more accurately characterize the non-uniform mixing in the reactors.     

Emulsion copolymerization of vinyl esters in continuous reactors: Comparison between loop and continuous stirred tank reactors

The high solids emulsion copolymerization of vinyl acetate and veova 10 was studied in a continuous loop reactor and in a continuous stirred tank reactor (CSTR) in an attempt to elucidate the similarities and differences between these reactors. Reactions were carried out under comparable conditions, namely, similar macromixing and the same feed composition and space time. The behavior of both reactors was almost the same when the heat generation rate was low; otherwise, thermal runaway occurred in the CSTR whereas the loop reactor temperature was easily controlled. © 1995 John Wiley & Sons, Inc.     

大涡模拟搅拌槽中的液相流动

采采用大涡模拟湍流模型对有档板的Rushton 桨搅拌槽进行了数值模拟研究。控制方程采用控制容积法进行离散,对流项用三阶QUICK格式,扩散项是二阶中心差分。压力 速度耦合方程在交错网格上采用SIMPLE算法进行求解。小尺度流动的模化采用动力学（dynamic）亚格子模型。搅拌桨与档板之间的相互作用采用改进的内外迭代法进行处理。计算结果和文献值吻合得很好。     

Drag models, solids concentration and velocity distribution in a stirred tank

Drag force influences both the particle suspension and solids concentration distribution in a stirred tank. The influence of drag models on the prediction of solids suspension in a tank stirred by a hydrofoil impeller was studied in the present work using computational fluid dynamics (CFD) and experimental techniques. A comparison was made between the drag models based on Reynolds number only and those that take solid volume fraction into account or those that account for the effect of the free stream turbulence. One of the drag models investigated was a function of the energy dissipation rate, and therefore, the influence of the methods of determining the energy dissipation rate on the prediction of solids suspension was investigated. It was shown that a better agreement between the CFD simulation and experimental results can be obtained using drag models based on solids volume fraction than those that are based on Reynolds number only.