操作条件对搅拌罐内液固混合过程的影响
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摘要
搅拌设备广泛应用于石油、化工、建筑等领域,液固混合的搅拌操作是比较常见的操作技术。目前,许多工作主要集中在气-固两相流、单相流的研究上,而对于液固混合过程以及颗粒的均匀悬浮研究较少。本文采用CFD技术对搅拌罐内液固两相混合过程进行数值模拟,进而对液固悬浮及搅拌罐内两相流场进行研究。首先根据流体力学、湍流理论建立起能够描述液固两相湍流流动的数学模型,采用标准k-ε湍流模型对流体相的湍流特性进行描述,利用多重参考系法对搅拌罐内液固两相流体动力学特性进行三维全流场数值模拟。其次利用已有的实验数据对模型进行验证,提出更符合实际的颗粒均匀悬浮判据。最后在以上工作的基础上,通过数值模拟着重研究搅拌桨转速、颗粒浓度和颗粒直径等操作条件对液固混合过程的影响。
Stirred tank is a common mixing equipment.In an stirred tank,mixed substance moves with the stirred paddle,acting with the fixed baffle,and forms complicated quasi-periodicity three-dimensional turbulent.In the present,Research on liquid-solid mixing has two methods:experimental measurement and numerical simulation. Experimental measurement is to measure liquid-solid flowing using several kinds of measurement devices through setting up experimental tables,numerical simulation is by setting up a mathematic model which describes the flow field in stirred tank,and undoes the Equations so as to simulate the flow field.However,the experimental table is limited by experimental technique and devices,such as:as for the tracking the high concentrated particle's pathway and observing the particle's moving speed,the conventional optical measurement will result in inactivation.By setting up a mathematical equation about the particle's or quasi-liquor's pathway and using the computer to undo the particle's pathway,the numerical simulation can obtain more accurately the flow field's complexor, speed,particle distribution,and so on.For example,for different concentrated operating modes,just adopting different theoretical models can do the simulations very well.This thesis is adopting the liquid-solid agitated mixed system in the stirred tank with a flat turbine as the subject investigated.From the point of numerical simulation,it does the research about the influence on the particle suspension's quality impacted by liquid-solid mixed process and operational conditions in the stirred tank.This thesis is divided into three main parts.
1.This thesis uses the particle dynamics theory,by setting up particle's compression stress and shearing strength to reflect the influence about particle's movement by particle's crash,and sets up Euler-Euler model which is applicable for liquid-solid turbulent:particles are assumed as quasi-liquor.It focuses on the two sets of equations about quasi-liquor and liquid and then connects the relational expression of the hydromechanics parameters(such as the state equation) to form a closed equations set in order to obtain the internal flow field information by undoing the equations set to get the quasi-liquor and liquid flowing movement.It uses this model to simulate the liquid-solid three-dimensional turbulent in the stirred tank,and makes a contrast with the experimental results.The diameter of the particles selected in this experiment is: d_p =264μm,the mediator water's density is:p_l 1000kg/m~3,and the viscosity is:μ= 0.001kg/m.s.The experiment is taken in the room temperature,the solid content (capacity's density ) is 10%,and the speed of rotation of the stirred paddle is 20rps.By comparing the capacity's density of the sample,it is found that the simulative result and the experimental result are consistent with each other very well.It indicates that Euler-Euler model of this thesis is able to reflect the liquid-solid turbulent flowing characteristics in the stirred tank,the numerical simulation can anticipate the fluid field distribution and some relevant research.
2.This thesis is using the method of numerical simulation to present the more effective particle suspension's judgment criterion.Particle suspension degree includes incomplete suspension,off-bottom suspension and uniform suspension.Some industrial process demands the particle suspend uniformly in the stirred tank,that is to say,every particle's capacity density in the stirred tank is nearly the same.For this kind of liquid-solid flowing system,it is essential to find out the judgment criterion of the uniformed suspension of particles.The two judgment criterions of solid suspension research are:setting up the average density of the sample at the bottom of the stirred tank and setting up the average density of the sample at the 90%height of the stirred tank. When the selected surface is reaching the uniform density,it is believed that the solid in the stirred tank is suspending uniformly.This thesis is doing the simulation using the density of 8%,10%and 15%liquid-solid mixed systems,sets up the average density in the selected surface at the bottom and at the 90%height,and builds eight selected points to monitor the average density in the stirred tank.When the selected surface is reaching the average density,then the solid in the stirred tank is reaching the uniform suspension. Calculating the two density standard deviation of the two kinds of judgment criteria can compare the solid uniform suspension degree.Therefore,we can tell the advantages and disadvantages of the two judgment criteria.The simulative experimental results indicate that the two kinds of judgment criteria are both applicable for the high density liquid-solid mixed system.The differences of the two solid suspension degree are little. With the increase of the density,the gap is becoming smaller.It is a little better to adopt the bottom selection judgment.According this,adopting the bottom selection judgment can fulfill the requirements of simple operation,fair accuracy and wide application in the industrial application.This becomes the basis of the following research and an innovation of this thesis.
3.The influence of liquid-solid mixing process in the stirred tank by operating conditions.The operating conditions include the rotating speed of stirred paddle,particle density and particle diameter.Based on the mathematical model and solid suspension judgment,this thesis studies the influence of the mixing process by operating conditions through the simulative liquid-solid mixing process in the stirred tank.
1) The thesis studies the influence by ten kinds of rotate speeds of stirred paddles N=2.5～40rps and observes the solid suspension quality and mixng time under different rotating speeds.The conclusion is that solid suspension quality is increasing as the speed increasing.However,when the speed is higher than N=16.1rps,the influence is becoming smaller;the mixing time is decreasing as the speed increasing,and the decay degree is becoming smaller.
2) This thesis studies the influence of the speed and suspension degree by different particle densities:8%,10%and 15%.Through the observation of the combined speed, axial speed,radial speed and tangential speed's change along with the radial speed,it is known that the increase of the particle density will bring in the dramatic decrease of the axial speed and combined speed,a small decrease of the tangential speed and no change of the radial speed.Through the observation of the three density distribution of the three axial lines:R/T=0.4、0.6、0.9,it is known that solid suspension quality is becoming smaller as the particle density increasing.
3) This thesis studies the influence of solid speed and suspension degree by particle diameters of three sizes:0.264mm、0.400mm、0.750mm.Through the observation of the changes of the combined speed,axial speed,radial speed and tangential speed,it is known that the increase of the particle's diameter will bring in the dramatic decrease of axial speed and combined speed,a small decrease of tangential speed and no change of the radial speed.Through the observation of the density distribution of the three axial lines:R/T=0.4、0.6、0.9,we know that solid suspension quality is decreasing as the particle's diameter is increasing.Through the observation of the density distribution of the three radial lines:z=30、120、240,it is known that there is no remarkable change of the radial density as the increase of particle's diameter.
There are five chapters in this thesis.Chapter One introduces the research background and the present situation of the liquid-solid mix study,and gives a brief introduction of the research contents and methods in this thesis.Chapter Two introduces the stirred tank briefly,What's more,the discussion of the liquid-solid system includes the aim of liquid-solid mix.finally the research methods,simulative theory and the software under use are introduced.Chapter Three mainly includes the introduction of the computer simulation of this thesis,which includes the distribution methods of simulative calculation thought,simulative theoretical model,physical model-setting process and physical graticle,and parameter setting of the software and the setting up of the parallel calculation platform.Chapter Four includes the examination of the simulative model,the presentation of the judgment criteria of the solid suspension and the influence of the mixed process by the operational conditions.Chapter Five is the conclusion part of the thesis which includes the achievements obtained,the deficiency of this thesis and the anticipation of the future research.
Stirred tank is a common mixing equipment.In an stirred tank,mixed substance moves with the stirred paddle,acting with the fixed baffle,and forms complicated quasi-periodicity three-dimensional turbulent.In the present,Research on liquid-solid mixing has two methods:experimental measurement and numerical simulation. Experimental measurement is to measure liquid-solid flowing using several kinds of measurement devices through setting up experimental tables,numerical simulation is by setting up a mathematic model which describes the flow field in stirred tank,and undoes the Equations so as to simulate the flow field.However,the experimental table is limited by experimental technique and devices,such as:as for the tracking the high concentrated particle's pathway and observing the particle's moving speed,the conventional optical measurement will result in inactivation.By setting up a mathematical equation about the particle's or quasi-liquor's pathway and using the computer to undo the particle's pathway,the numerical simulation can obtain more accurately the flow field's complexor, speed,particle distribution,and so on.For example,for different concentrated operating modes,just adopting different theoretical models can do the simulations very well.This thesis is adopting the liquid-solid agitated mixed system in the stirred tank with a flat turbine as the subject investigated.From the point of numerical simulation,it does the research about the influence on the particle suspension's quality impacted by liquid-solid mixed process and operational conditions in the stirred tank.This thesis is divided into three main parts.
1.This thesis uses the particle dynamics theory,by setting up particle's compression stress and shearing strength to reflect the influence about particle's movement by particle's crash,and sets up Euler-Euler model which is applicable for liquid-solid turbulent:particles are assumed as quasi-liquor.It focuses on the two sets of equations about quasi-liquor and liquid and then connects the relational expression of the hydromechanics parameters(such as the state equation) to form a closed equations set in order to obtain the internal flow field information by undoing the equations set to get the quasi-liquor and liquid flowing movement.It uses this model to simulate the liquid-solid three-dimensional turbulent in the stirred tank,and makes a contrast with the experimental results.The diameter of the particles selected in this experiment is: d_p =264μm,the mediator water's density is:p_l 1000kg/m~3,and the viscosity is:μ= 0.001kg/m.s.The experiment is taken in the room temperature,the solid content (capacity's density ) is 10%,and the speed of rotation of the stirred paddle is 20rps.By comparing the capacity's density of the sample,it is found that the simulative result and the experimental result are consistent with each other very well.It indicates that Euler-Euler model of this thesis is able to reflect the liquid-solid turbulent flowing characteristics in the stirred tank,the numerical simulation can anticipate the fluid field distribution and some relevant research.
2.This thesis is using the method of numerical simulation to present the more effective particle suspension's judgment criterion.Particle suspension degree includes incomplete suspension,off-bottom suspension and uniform suspension.Some industrial process demands the particle suspend uniformly in the stirred tank,that is to say,every particle's capacity density in the stirred tank is nearly the same.For this kind of liquid-solid flowing system,it is essential to find out the judgment criterion of the uniformed suspension of particles.The two judgment criterions of solid suspension research are:setting up the average density of the sample at the bottom of the stirred tank and setting up the average density of the sample at the 90%height of the stirred tank. When the selected surface is reaching the uniform density,it is believed that the solid in the stirred tank is suspending uniformly.This thesis is doing the simulation using the density of 8%,10%and 15%liquid-solid mixed systems,sets up the average density in the selected surface at the bottom and at the 90%height,and builds eight selected points to monitor the average density in the stirred tank.When the selected surface is reaching the average density,then the solid in the stirred tank is reaching the uniform suspension. Calculating the two density standard deviation of the two kinds of judgment criteria can compare the solid uniform suspension degree.Therefore,we can tell the advantages and disadvantages of the two judgment criteria.The simulative experimental results indicate that the two kinds of judgment criteria are both applicable for the high density liquid-solid mixed system.The differences of the two solid suspension degree are little. With the increase of the density,the gap is becoming smaller.It is a little better to adopt the bottom selection judgment.According this,adopting the bottom selection judgment can fulfill the requirements of simple operation,fair accuracy and wide application in the industrial application.This becomes the basis of the following research and an innovation of this thesis.
3.The influence of liquid-solid mixing process in the stirred tank by operating conditions.The operating conditions include the rotating speed of stirred paddle,particle density and particle diameter.Based on the mathematical model and solid suspension judgment,this thesis studies the influence of the mixing process by operating conditions through the simulative liquid-solid mixing process in the stirred tank.
1) The thesis studies the influence by ten kinds of rotate speeds of stirred paddles N=2.5～40rps and observes the solid suspension quality and mixng time under different rotating speeds.The conclusion is that solid suspension quality is increasing as the speed increasing.However,when the speed is higher than N=16.1rps,the influence is becoming smaller;the mixing time is decreasing as the speed increasing,and the decay degree is becoming smaller.
2) This thesis studies the influence of the speed and suspension degree by different particle densities:8%,10%and 15%.Through the observation of the combined speed, axial speed,radial speed and tangential speed's change along with the radial speed,it is known that the increase of the particle density will bring in the dramatic decrease of the axial speed and combined speed,a small decrease of the tangential speed and no change of the radial speed.Through the observation of the three density distribution of the three axial lines:R/T=0.4、0.6、0.9,it is known that solid suspension quality is becoming smaller as the particle density increasing.
3) This thesis studies the influence of solid speed and suspension degree by particle diameters of three sizes:0.264mm、0.400mm、0.750mm.Through the observation of the changes of the combined speed,axial speed,radial speed and tangential speed,it is known that the increase of the particle's diameter will bring in the dramatic decrease of axial speed and combined speed,a small decrease of tangential speed and no change of the radial speed.Through the observation of the density distribution of the three axial lines:R/T=0.4、0.6、0.9,we know that solid suspension quality is decreasing as the particle's diameter is increasing.Through the observation of the density distribution of the three radial lines:z=30、120、240,it is known that there is no remarkable change of the radial density as the increase of particle's diameter.
There are five chapters in this thesis.Chapter One introduces the research background and the present situation of the liquid-solid mix study,and gives a brief introduction of the research contents and methods in this thesis.Chapter Two introduces the stirred tank briefly,What's more,the discussion of the liquid-solid system includes the aim of liquid-solid mix.finally the research methods,simulative theory and the software under use are introduced.Chapter Three mainly includes the introduction of the computer simulation of this thesis,which includes the distribution methods of simulative calculation thought,simulative theoretical model,physical model-setting process and physical graticle,and parameter setting of the software and the setting up of the parallel calculation platform.Chapter Four includes the examination of the simulative model,the presentation of the judgment criteria of the solid suspension and the influence of the mixed process by the operational conditions.Chapter Five is the conclusion part of the thesis which includes the achievements obtained,the deficiency of this thesis and the anticipation of the future research.
引文
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[2]陈志平.搅拌与混合设备设计选用手册[M].北京:化学工业出版社,2004.
[3]Zwietering,T.N.H.Suspending of solids particle in liquid by agitators[J].Chemical Engineering Science 1958,8:244-253.
[4]Barresi.A,Baldi.G Solids suspension in an agitated vessel[J].Chemical Engineering Science.1987,42(12):2949-2956.
[5]Armenante.P.M,Nagamire.E.U.Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks[J].Chemical Engineering Science.1997,53(9):1757-1775.
[6]A.Ochieng,M.S.Onyango,A.Kumar,et al.Mixing in a tank by a rushton turbine at a low clearance[J].Chemical Engineering and Processing.2008,47:842-851.
[7]A.Ochieng,Alison.E.Lewis.CFD simulation of solids off-bottom suspension and cloud height[J].Hydrometallurgy.2006,82:1-12.
[8]Montante.G,Micale.G,Magelli.F,Brucanto.A.Experimental and CFD prediction of solid particle distribution in vessel agitated with four pitched blade turbines[J].Transactions of Chemical Engineers.2001,79A:1005-1010.
[9]Derksen.J.J.Numerical simulation of solids suspension in a stirred tank[J].American Institute of Chemical Engineer.Journal,2003,49(11):2700-2714.
[10]A.Ochieng,Maurice.S.Onyango.Drag models,solid concentration and velocity distribution in a stirred tank[J].Powder Technology.2008,181:1-8.
[11]单岩,谢龙汉.CATIAV5机械设计应用实例[M].北京:清华大学出版社,2004.
[12]宁贵欣.CATIAV5工业造型设计实例教程[M].北京:清华大学出版社,2004.
[13]Gambit Help[Z].Fluent inc.
[14]Fluent Help[Z].Fluent inc.
[15]钟丽,黄雄斌,贾志刚.固-液搅拌槽内颗粒离底悬浮转速的CFD模拟[J].北京化工大学学报.2003,30(6):18-22.
[16]Mersmann.A,Werner.F,Maurer.S,Bartosch.K.Theoretical prediction of the minimum stirrer speed in mechanically agitated suspensions.Chemical Engineering and Processing.1998,37(6):503-510.
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