混合原理及设备

1、混合原理与设备课程论文 存档日期： 存档编号: 北京化工大学研究生课程论文课程名称： 混合原理及设备 课程代号： ChE522 任课教师： 000 完成日期： 2014年11月28日 专 业： 化学工程与技术 学 号： 000 姓 名： 000 成 绩： Research progress of stirred vessel through CFD simulationAbstract: Computational fluid dynamics (CFD) is a simulation tool, which used powerful computer and applied mathem

2、atics to model fluid flow situations for the prediction of heat, mass and momentum transfer and optimal design in stirred vessels. Based on four main mixture systems, the research progress of CFD simulation of gas-liquid, solid-liquid, liquidliquid and gas-liquid-solid flow field information of stir

3、red vessel was reviewed.Key words: CFD, simulation, stirred vesse91. IntroductionComputational fluid dynamics (CFD) used powerful computers and applied mathematics to model fluid flow situations. The yardstick of success is how well the results of numerical simulation agree with experiment in cases

4、where careful laboratory experiments can be established, and how well the simulations can predict highly complex phenomena that can not be isolated in the laboratory1. As a developing science, CFD has received extensive attention throughout the international community since the advent of the digital

5、 computer. Since the late 1960s, there has been considerable growth in the development and application of CFD to all aspects of fluid dynamics2. As a result, CFD has become an integral part of the engineering design and analysis environment of many companies because of its ability to predict the per

6、formance of new designs or processes before they are ever manufactured or implemented3. Researchers, equipment designers and process engineers are increasingly using CFD to analyze the flow and performance of process equipment, such as baking ovens4, refrigerated display cabinets5, stirred tank6, he

7、at exchangers7 and some other equipment. In design and development, CFD programs are now considered to be standard numerical tools which predict not only fluid flow behaviour, but also the transfer of heat, mass (such as in perspiration or dissolution), phase change (such as in freezing, melting or

8、boiling), chemical reaction (such as combustion or rusting), mechanical movement (such as an impeller turning, pistons, fans or rudders) and stress or deformation of related solid structures (such as a mast bending in the wind). Based on four main mixture systems, the research progress of CFD simula

9、tion of gas-liquid, solid-liquid, liquidliquid, gas-liquid-solid flow field information of stirred vessel was reviewed in this paper.2. Advantages of using CFDCFD has grown from a mathematical curiosity to become an essential tool in almost every branch of fluid dynamics. It allows for a deep analys

10、is of the fluid mechanics and local effects in a lot of equipment. Most of the CFD results will give an improved performance, better reliability, more confident scale-up, improved product consistency, and higher plant productivity8. Some design engineers actually use CFD to analyze new systems befor

11、e deciding which and how many validation tests need to be performed. The advantages of CFD can be categorized as : It provides a detailed understanding of flow distribution, weight losses, mass and heat transfer, particulate separation, etc. Consequently, all these will give plant managers a much be

12、tter and deeper understanding of what is happening in a particular process or system. It makes it possible to evaluate geometric changes with much less time and cost than would be involved in laboratory testing. It can answer many what if questions in a short time. It is able to reduce scale-up prob

13、lems because the models are based onfundamental physics and are scale independent. It is particularly useful in simulating conditions where it is not possible to take detailed measurements such as high temperature or dangerous environment in an oven.Since it is a pro-active analysis and design tool,

14、 it can highlight the root cause not just the effect when evaluating plant problems.3. CFD simulation of stirred vessel3.1 Gas-liquid stirred vesselGas-liquid stirred vessels are widely used in chemical process industry to carry out gas-liquid reactions. In many industrial applications, vessels equi

15、pped with impellers are used. the stirred vessel offers more degrees of freedom for controlling the gas dispersion as well as the bulk flow of liquid phase. Different fluid dynamic characteristics can be obtained in a vessel depending on the equipment and the operating parameters, such as impeller d

16、esign, impeller spacing, rotational speed and volumetric gas flow rates. These different fluid dynamic characteristics lead to different rates of transport and mixing processes. Despite the widespread use of stirred vessel,the complex relationship between fluid dynamics, operating parameters and rea

17、ctor hardware is not adequately understood, and design engineers are left to use empirical information. This is especially true for multiphase flows in tall stirred vessels. It is therefore, essential to have better understanding of the influence of the reactor hardware configuration and operating p

18、arameters on prevailing fluid dynamics, to optimize and have better control on the reactor performance.Computational fluid dynamics (CFD) was used to investigate the influence of parallel, merging and diverging flow configurations on the gas dispersion operation in stirred vessel. Most of these stud

19、ies were restricted to single impeller system. Simulations with multiple impeller system are much less common due to increased computational complexity and the lack of experimental data. Khopkar et al.9have simulated the gas-liquid flows in a stirred vessel equipped with three down-pumping pitched b

20、lade turbines. They have studied the influence of prevailing flow regimes on the liquid phase mixing. However, they did not carry out quantitative validation of flow characteristics due to lack of experimental data. Recently, Kerdouss et al.10have simulated gas-liquid flows generated by dual Rushton

21、 turbine operating with parallel flow configuration. They have compared the predicted gas holdup distribution and bubble size distribution with the available experimental data. Their results show under prediction of the gas holdup values in the upper part of the vessel. They have attributed this to

22、the limitations of the drag model used. The influence of different techniques for simulation of impeller motion (the multiple reference frames and sliding mesh models) on predictions of gas-liquid flow field and tracer distribution was investigated by M. Jahoda et al11. For the solution, a simplifie

23、d numerical setup of mono-dispersed bubbles and the k- mixture turbulence model have been applied. Despite the assumed simplifications, the numerical predictions exhibit a good agreement with the experimental data.3.2 Solid-liquid stirred vesselSolid-liquid mixing within vessels agitated by stirrers

24、 can be easily encountered in many industrial processes.The distribution of solid particles in a stirred vessel is a quite complex function of velocity field, turbulence characteristics and liquid-particle interactions. Thus, the soundness of the former approximation depends on several factors, such

25、 as geometrical configuration and suspension properties. for example radial impellers provide larger concentration gradients than axial impellers, also, the higher the particle size and concentration, the higher the concentration gradients .The present work is devoted to the investigation via CFD of

26、 the particle distribution in a dense suspension ranging from partial to complete suspension conditions. In particular, the CFD model by Tamburini et al.12 is the only one purposely developed to deal with partial suspension conditions. It has been fully validated in previous works and found to relia

27、bly predict integral data. Such essential data concerning the particle suspension phenomenon,however, do not provide any information on local details since they are intrinsically lumped.A. Tamburini et al13 successfully applied the CFD model to the prediction of the sediment amount and shape was ado

28、pted here to simulate the particle distribution under partial-to-completesuspension conditions. Both transient and steady state RANS simulations were carried out for the case of a flat bottomed baffled tank stirred by a Rushton turbine.Results show that the model can reliably predict the experimenta

29、l particle distribution at all investigated impeller speeds. Transient simulations were found to predict slightly better the experimental data with respect to steady state simulations. Radial gradients of solids concentration, usually neglected in the literature, where found to be significant in the

30、 presence of unsuspended solid particles settled on the vessel bottom3.3 Liquidliquid stirred vesselIn the previous part of work14, a multi-block model was introduced to consider the inhomogeneous characteristics of a stirred tank. It was shown that a stirred tank could easily be divided into subreg

31、ions according to the knowledge of the flow properties. The model is capable of using the local properties of the flow and it is thus believed that it results in a more fundamental understanding of the drop breakage and coalescence properties. Ville Alopaeusa et al15tested the model in a parameter f

32、itting procedure. The drop breakage and coalescence parameters are fitted against drop size measurements from dense liquid-liquid dispersions, which were assumed fully turbulent. Since the local turbulence and flow values of a stirred tank are used in the present model, the fundamental breakage and

33、coalescence phenomena can be examined more closely. Furthermore, the model is capable of predicting inhomogeneities occurring in a stirred tank. It is also to be considered as an improved tool for process scale-up, compared to the simple vessel-averaged population balance approach, or use of correla

34、tions of dimensionless numbers only. The model can use two sources of data for fitting parameters in the drop rate functions. One is to use transient data of the measured drop size distribution as the impeller speed is changed. The other is to use time-averaged data measured at different locations o

35、f the stirred tank. It is shown in that the different flow regions can be chosen from the CFD simulations in a straightforward manner. CFD flow simulation results can be used to select the flow regions when no experimentally obtained flow conditions are available. 3.4 Gas-liquid-solid stirred vessel

36、sMechanically agitated reactors involving gas, liquid and solid phases have been widely used in the chemical industries and in mineral processing, wastewater treatment and biochemical industries.Despite their widespread use, the design and operation of these agitated reactors remain a challenging pr

37、oblem because of the complexity encountered due to the three-dimensional (3D) circulating and turbulent multiphase flow in the reactor.The designing and scaling up of mechanically agitated reactors are generally based on semi-empirical methods. because of even additional complexities associated with

38、 the three phase systems, practically no published information is available on the CFD simulations of three phase systems.In addition, experimental data for the local velocities and the local gas and solid phase hold-ups are also not available which are useful for the validation of CFD models.In the

39、 case of a three-phase stirred tank system, the solid suspension process depends upon the quality of gasliquid dispersion in the absence of solids and the quality of solidliquid dispersion in the absence of gas. It has been observed that the CFD model could well predict the critical impeller speed o

40、ver these design and operating conditions16. Ranganathan et al17used CFD simulations to study solid suspension in gasliquidsolid mechanically agitated contactor using multi-fluid approach along with standard k- turbulence model. A multiple frame of reference (MFR) has been used to model the impeller

41、 and tank region. The CFD model predictions are compared qualitatively with the literature experimental data and quantitatively with experimental data. The present study also involves the effects of impeller design, particle size and gas flow rate on the critical impeller speed for solid suspension

42、in gas-liquid-solid mechanically agitated contactor. The values predicted by CFD simulation for critical impeller speed agrees well with experimental data for various operating conditions.4. ConlusionComputational fluid dynamics (CFD) has been widely used to model fluid flow situations for the predi

43、ction of heat, mass and momentum transfer and optimal design in stirred vessels.With the development of computer technology, in the future,CFD can use more powerful computer technology ,and it will be a more useful simulation tool.The flow field information of stirred vessel need further research.5.

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