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Experimental and simulation studies on the role of fluid velocity during particle separation in a liquid solid fluidized bed A K Mukherjee a B K Mishrab a R accepted 29 November 2006 Available online 16 January 2007 Abstract The fluidization technique has been in use for particulate material processing operations for many years It has been widely believed that the ratio of size and density of the particulate components controls the separation efficiency In this paper it is demonstrated that fluid velocity during fluidization could assume an overriding significance when an improvement in separation efficiency is required This was at first experimentally established by analyzing simpler particulate systems and later a simulation scheme was adopted to study a wider range of particulate systems The numerical scheme known as the discrete element method DEM that incorporates both the solid and hydro dynamic components of the interactive forces served as an important tool in understanding the separation behavior of binary particulate systems in fluidized beds It has been established that mere fluidization does not necessarily guarantee an optimal separation especially when the particles differ widely in density and size 2006 Elsevier B V All rights reserved Keywords Solid liquid fluidization Fluid velocity Particle segregation 1 Introduction The fluidization technique is widely used in industry for its different useful applications Primarily it has been used as a mixing process in the chemical industry where enhanced reaction combustion and heat transfer rates are sought In the mineral industry this technique is used for separation of mineral particles having different physical properties Jig hydrosizer and floatex are examples of commonly used machines for separating minerals using the fluidization technique In recent times there has been a renewed research interest Steiner 1996 Mishra and Mehrotra 2001 Galvin et al 2002 in understanding the fluidization behavior of particles in greater detail to improve the efficiency of separation of particles Here fluidization involves liquid solid particle inter action and is nonbubbling in nature Liquid solid interaction can be explained at different levels starting with a spherical particle settling in a fluid medium in isolation The force balance on the particle leads to the concept of terminal settling velocity ut as deduced here pd3 6 gqf C0 5 D qfu2 t pd2 4 pd3 6 gqs 1 where CDis the drag coefficient d is the diameter of the particle g is the acceleration due to gravity and fand s arethedensities ofthefluidandparticle respectively Theinteraction between asingleparticleandfluidcanbe Int J Miner Process 82 2007 211 221 Corresponding author E mail address akmukherjee A K Mukherjee 0301 7516 see front matter 2006 Elsevier B V All rights reserved doi 10 1016 j minpro 2006 11 006 clearly distinguished and correlated for a wide range of Reynolds number Re However when the processes involve multiple particle system with a varying partic ulateassemblage themathematicalexpressionsforthese systems become difficult due to the complex nature of fluid flow The first important work dealing with an assembly of particles was done by Richardson and Zaki 1954 They proposed the relationship between the superficial fluid velocity u the terminal settling velocity ut andthevoidage foramulti particle fluidized system as given below u ut en 2 The value of the exponent n is a function of particle Reynolds number Numerous publications relating to assemblies of particles that followed after the work of Richardson and Zaki show no agreement as to how the particles interact with liquid in a fluidized bed The prediction of fluidization behavior requires an a priori knowledge of the effective forces acting on a moving particle These forces are mainly the gravity buoyancy particle contact forces and drag force The prediction of the drag force is difficult While evaluating the drag force on a particle in a multi particle system voidage function f is considered such that FD FD0f e 3 where FDis the drag force on a particle when taking into consideration the effect of neighboring particles and FD0is the drag force exerted on the same particle when falling freely under the effect of the liquid force and gravity force only The function f which we shall refer to as the voidage function is defined as follows f e 1 e b 4 where is the solid volume fraction and is a parameter that is closely related to the particle properties and Reynolds number In many numerical studies this exponent was assumed to be constant or a function of the particle physical properties Here we have followed the work of Di Felice 1994 1995 for multi particle systems which served well for our liquid particle system In liquid particle fluidization processes the drag force is the predominant force for mixing or segregation of particles The drag force depends on voidage particle properties and the fluid velocity and it can be used to predict the minimum fluidization velocity Gauthier et al 1999 claimed that the minimum fluidization velocity for a mixture of particles having a Gaussian size distribution could be predicted more accurately com pared to a binary particulate system This indicates that binary mixtures of particles possess a very different hydrodynamic behavior Asif and Ibrahim 2002 studied the hydrodynamic behavior of binary particulate systems to determine the minimum fluidization velocity They found that the fluidization state for a binary particulate system at minimum fluidization velocity depends largely on particle properties viz size ratio and density ratio However there is no established way of quantifying the range of size and density ratio required for the mixing and segregation state to occur in a binary system Additionally the situation is compli cated by the role of fluid velocity very little information is available on the progress of fluidization with an increase in fluid velocity Therefore it is difficult to predict the fluidization state at and above the minimum fluidization velocity even for a simple binary system Rasul et al 2000 illustrated elegantly the simplicity of various mechanisms of separation of particles during fluidization and in particular layer inversion They showed that a binary mixture of particles could be subdivided into two groups mixing or segregation type depending on the size and density ratioof the particles in the two components The group of particles that mixes and the other group that segregates are referred here as type I and type II binary particulate systems respec tively In addition an intermediate group of particles that could either segregate or mix will be termed a type III binary system This group would be quite sensitive to the fluid velocity Fig 1 shows the mixing and segregation regime for a binary particulate system as proposed by Rasul et al This figure suggests that particles of a certain combination of density ratios and size ratios will mix type I whereas another increasing of combination of density ratios and size ratios will always segregate type II Additionally the intermedi ate group of particles type III is marked in Fig 1 Typically at any fixed density ratio the type II particles will have a larger size ratio Rasul et al like other researchers have used experimental methods to identify the boundary between the type I and type II binary particulate systems The research work has great significance in mineral processing operation since it has the potential to predict the possibility of separating particles based on size and density For example separation of gangue low density from the mineral high density would be possible only when the binary particulate system falls in the type II region during fluidization However it appears that this method of prediction to be accurate cannot just consider the 212A K Mukherjee B K Mishra Int J Miner Process 82 2007 211 221 particle properties in isolation without explicitly con sidering the effect of fluid velocity The research work presented in this paper is mainly concerned with two issues relating to the fluidization of liquid particle system First we recognize that the classification of particulate systems based on their response during fluidization must explicitly include the effect of fluid velocity Second we also recognize that the earlier research work was confined to a few par ticulate systems due to the inherent limitations in the operating range of the process parameters that could be varied experimentally In the analysis presented here we consider fluid velocity as well as the particle properties to predict the fluidization state We also use a simulation tool based on the discrete element method DEM that allows us to predict the fluidization state of a variety of binary particulate systems at different fluid velocities Within the framework of DEM the fluid behavior was modeled according to Di Felice s correlation and the spring dashpot type contact model described particle particle interaction DEM analysis gives the number of contacts for any particle at any instant This information was used to estimate the voidage around any particle The voidage was in turn used in the fluid model to compute the drag force Thus in this manner the com plete hydrodynamic behavior of the particulate system was characterized The model was validated against the experimental results and subsequently used for predict ing the fluidization state of a variety of binaryparticulate systems at different fluid velocities Finally the knowl edge gained through this study was applied to specific unit operations in mineral beneficiation 2 Experimental The experimental set up for fluidization tests consists of a cylindrical Plexiglas column of 1 m height and 6 0 cm in diameter as shown in Fig 2 A homogeni zation chamber is fitted at the bottom of the fluidization column to keep the fluid flow uniform across the cross section Water was pumped to the fluidization column from the bottom through a rotameter A manual valve fixed to the rotameter and a bypass line were used to control the flow of water into the fluidization column The experimental set up also has provision to measure the flow rate the bed height and the pressure drop across the bed during fluidization The rotameter was used to measure the flow rate The bed heights were read visually with the help of a ruler placed along the length of the column The pressure drop across the bed was measured using a manometer Prior to the start of the experiment the density volume fraction and voidage of both constituent particle types of the binary system were measured The test results were then compared and analyzed on the basis of superficial fluid velocity of water bulk density of the slurry and dynamic voidage of the column Binary mixtures of particles real as well as artificial corresponding to the type I type II and type III classes were tested As special case type IV binary system was Fig 1 Classification of a particle population based on their response to fluidization 213A K Mukherjee B K Mishra Int J Miner Process 82 2007 211 221 studied to follow size segregation during fluidization Type IV is a pure size variant system while in other systems particle components differ in size as well as density The constituent properties of these types of particle mixtures are given in Table 1 All the ex periments were carried out in two steps In the first step individual particle types were placed in the column and tested separately The slurry bulk density and pressure drop were measured at different fluid velocity for component 1 the heavier of the two Then the particles were replaced by the particles of component 2 and tested independently In the second step both the components were tested as a binary system keeping component 1 at the bottom of the column at the start of the experiment The binary mixture of particles was expected to show a different fluidization state that is segregation and mixing at different fluid velocities Thus in this manner it was possible to closely observe and analyze the fluidization state at different fluid velocities for all four types of binary system 3 Results and discussion In a fluidized bed the bulk density of the bed of particles varies with the fluid velocity from its initial value corresponding to a static state through the dynamic state till complete fluidization Fig 3 shows the variations of bulk density with increase in fluid velocity for each component magnetite and glass beads of the type III binary system It is seen that the slurry bulk density of magnetite was higher than that of the glass beads at a lower fluid velocity regime Conversely at a higher velocity slurry bulk density of magnetite was lower than that of glass beads In the intermediate velocity regime the slurry bulk densities for both the components were comparable This implies that for the given binary mixture normal segregation would be observed in a fluidized bed at a lower fluid velocity In other words heavier magnetite particles will get distributed at the bottom and lighter glass beads at the top of the fluidized column With an increase in fluid velocity the binary particulate system would show complete mixing At still higher velocity the fluidized column would show a reverse segregation that is lighter glass beads at the bottom and heavier magnetite particles at the top of the fluidized column The above phenomenon can be explained by considering the dynamic voidage and its effect on the drag force Our experimental results have shown that the Table 1 Particulate types used in the fluidization tests Particulate typeCompositionsSize mm Mixing typeSand and coal particles Sand 0 5 Type I Coal 2 0 Segregation typeGlass beads and coal particles Glass beads 4 5 Type II Glass beads 8 0 Intermediate typeMagnetite and glass beads Magnetite 2 0 Type III Glass beads 4 5 Special caseGlass beads of two different sizes Glass beads 3 0 Type IV Glass beads 4 5 Fig 3 Effects of fluid velocity on slurry bulk density for each of the two components of a type III binary system Fig 2 Schematic of the experimental set up 214A K Mukherjee B K Mishra Int J Miner Process 82 2007 211 221 voidage of glass beads is much higher than the magnetite particles We know from the literature that the drag force would be less for a system of particles having higher voidage Therefore with an increase in fluid velocity the glass beads experienced less drag force compared to the magnetite particles This resulted in more bed expansion for magnetite particles leading to a much higher rate of decrease of the slurry bulk density of the magnetite particles than the glass beads A stage was reached where the slurry bulk densities for both components were the same and show complete mixing Later at a still higher fluid velocity the slurry bulk density for magnetite particles dropped below that of glass beads resulting in reverse segregation In order to obtain direct evidence of the reverse segregation the entire sequence of events leading to reverse segregation was photographed Fig 4 shows photographs of the particle bed at different stages of fluidization starting from normal segregation Fig 4a through mixing Fig 4c till reverse segregation Fig 4e Fig 4e shows the fluidization state where the relative position of the two types of particles has reversed compared to their initial position At this stage the fluid velocity was instantaneously reduced to zero The particles immediately settle forming a static bed of segregated particles Fig 4h However the segregation is now reversed compared to the initial state Fig 4a In a similar manner the fluid velocity can be adjusted to produce a segregated bed where the light particles lie above the heavy particles in the fluidized state Thus it is clear that the fluid velocity plays an important role in determining the fluidization state of a binary particulate system It has been found that a binary mixture of type III particles will change from the normal segregation state to the mixing state and then to the reverse segregation state only by increasing the fluid velocity in steps In contrast test results on type I and type II particles indicate it is not possible simply by changing the velocity to change the fluidized state of the bed from segregation to mixing or vice versa However the effect of fluid velocity on the degree of mixing or segregation was evident Monitoring the pressure drop across a fixed bed height is a common practice in fluidization studies Fig 5 shows a typical plot of pressure drop against fluid velocity for a mixture of particles with a size and density ratio close to unity and it follows the ideal trend The pressure drop across a fixed bed height increases with an increase in fluid velocity till the bed becomes com pletely fluidized At this point the fluid velocity reached the incipient fluidization state where the pressure drop p is simply the ratio of the weight of the particles less the upward thrust and the bed cross sectional area It is defined as Dp thA 1 e qs qf gb A h 1 e qs qf g 5 The above relation shows that the pressure drop p across the bed height is a function of the density of the particles the voidage in the particulate system and the Fig 4 Progress of fluidization 215A K Mukherjee B K Mishra Int J Miner Process 82 2007 211 221 fluid density Thus a binary system as shown in Fig 4a will experience a higher pressure drop across the bed of magnetite particle layers than across the glass particle layers This is because the two different particle layers have different voidages and particle densities In fact in the magnetite glass system it was observed that early fluidization of glass beads lying above the magnetite particles led to channeling of fluid Therefore the relationship between pressure drop and fluid velocity for this type of binary system type III would differ from the ideal case It should be mentioned that most of the bi