A continuously operating modified Cuette-type shearing device has been developed for

in-situ measurements to estimate the average particle size during size enlargement

processes in fluidized bed granulator. It was proven by experiments that well-defined

correlation exists between the mean torque and the average particle size being in the

device. DEM simulations revealed interesting aspects of this method

Dencs, B.

Gyenis, János

Szépvölgyi, J.

Tardos, G. I.

English

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Torque Measurements and DEM Simulations in a Cuette-type Devicewith Application to Particle Size MeasurementsB. Dencs1, J. Szepvolgyi1, G. I. Tardos2 and J. Gyenis11 University of Kaposvar, Research Institute of Chemical & Process Engineering,Egyetem u. 2, H-8200, Veszprem, Hungary2 The City College of the City University of New York, Department of ChemicalEngineering, Convent Ave. and 140 Street, New York, NY 10031, USAAbstractA continuously operating modified Cuette-type shearing device has been developed forin-situ measurements to estimate the average particle size during size enlargementprocesses in fluidized bed granulator. It was proven by experiments that well-definedcorrelation exists between the mean torque and the average particle size being in thedevice. DEM simulations revealed interesting aspects of this method.Key words: granular flow, torque measurement, particle size, Cuette device, simulation1. IntroductionDuring granulation or size enlargement processes, one of the most important features ofthe treated solids is particle size. There are a number of different methods and devices tomeasure and control the size and size distribution of the product. These can be used on-or off-line, in situ or employing separate measuring tools. One of these methods isbased on force measurements during continuous shearing of particle bed, carried out ina Cuette-type shearing device. To study the forces acting on a slowly rotating roughcylinder in a dry frictional powder, Tardos and co-workers [1] used such a piece ofequipment (see Fig.1). During the work reported here, this type of device was used toestimate the actual particle size during a size enlargement process. Other workers [3-4]also used similar devices for continuous shearing of fine fluidized powders.During this work, a Cuette-type device was applied in a special fluidized-bedgranulator, which recovers solids directly from solutions or suspensions [5]. In suchequipment, shown in Fig.2, a constant and narrow size distribution can be achieved byensuring equilibrium between the effects of simultaneous attrition and granulation.Attrition of larger granules in this equipment takes place due to the breaking effect ofnotched grinding rolls rotating and orbiting along the wall of the equipment. At thesame time, the size enlargement of smaller particles is the consequence of simultaneousagglomeration and surface layering due to spraying solution of binding material into theparticle bed. On-line size measurement may help to control the equilibrium granule sizeduring these concurrent processes, by setting the operational parameters to appropriatevalues. Details of the operation of this equipment can be found elsewhere [5].2. ExperimentalFor the present work, a modified Cuette-type device (Fig.3), similar to that employedby Tardos et al. [1, 2] was used to study the effect of particle size on the measuredtorque during continuous shearing of dry, granulated sucrose particles.The experimental device consisted of an inner rotating, and an outer stationary cylinder.Both were covered by coarse sandpaper to ensure that wall friction be comparable to theinternal friction of the particle bed. The inner cylinder could be rotated between n=60and 240 RPM (vp=0.16-0.65 m/s peripheral speed) by an electronically controlled DCmotor. The torque was continuously measured and recorded by a data acquisitionsystem connected to a PC. The maximum measurable torque was 0.44 N-m.The diameters of the outer stationary cylinder and that of the inner rotating cylinderwere D=106 mm and Dc=52 mm, respectively. Thus, the width of the gap between thetwo cylinders was lg=26 mm. This gap was about two times wider, compared to theearlier experiments of Tardos et al. [2], thus the shear region do not extended to the wallof the outer standing cylinder. The heights of the particle bed and the rotating cylinderwere 100 mm and 80 mm, respectively.The main characteristics of the granular material prepared from sucrose by granulationused for the experiments are given in Table 1. The mean particle sizes were variedbetween d=0.3 and 1.4 mm.Table 1. Characteristics of the particlesSucrose  sizerange  mmSucrose meanparticle sizemmForm ofparticlesAngleofreposeFlowabilitycm3/sBulkdensitykg/m31 0.20-0.40 0.30 prismatic* 39.0 13.3 7402 0.40-0.63 0.52 prismatic* 39.1 12.8 7603 0.63-0.80 0.72 rounded prism 39.8 11.9 7704 0.80-1.00 0.90 near spherical 40.9 11.0 7605 1.25-1.60 1.43 near spherical 40.9 8.9 790* without sharp tips and edges3. Results and discussionThe curves, shown in Fig.4, represent typical torque signals obtained for five differentmean particle sizes (0.30, 0.52, 0.72, 0.9, 1.43 mm respectively). The sixth curve nearthe bottom refers to the torque measured in the empty device, i.e. with no particles. Thisrepresents only the friction of bearings and a small brake force that was necessary toadjust the basic level of torque. During evaluation, this latter value was subtracted fromall measured torque data. From this Figure, it is evident that, after a short initialtransient period, the mean value of the measured torque signals remained essentiallyconstant with some stochastic fluctuations.As seen from Fig.5, the mean value of the torque increases with the mean particle size,for different rotation speeds (n=80 and 120 rpm) and for different preset break forces.This dependence was more pronounced at lower rotation speed, especially below d=0.8-0.9 mm granule diameters for the given geometry and design of the measuring device.The preset break force (i.e. the basic level of torque) had no significant influence.Measurements were also performed with particles moving downwards within the gapbetween the cylinders. A sliding velocity of vs=0.001m/s, corresponding to 2.0-2.2 kg/hmass flow rate of particles was chosen to be negligibly small, relatively to the vp= 0.22m/s peripheral velocity of the rotating cylinder. This sliding velocity ensured continuousreplacement of the material being in the measuring cell with not too long time delay. Itwas resulted that this vertical motion had no observable influence on the torque values.Experiments were also performed with particle systems having much broader sizedistribution compared to the narrow granule fractions given in Table 1. Two particlesystems with broad size distribution were tested in this respect. One of them wasblended from the first three fractions of Table 1, resulting in a size distribution betweend=0.20-0.63 mm and d=0.45 mm mean particle size. By blending the last two particlefractions of Table 1, d=0.63-1.60 mm size distribution and 1.1 mm mean particle sizewas obtained. The results of torque measurements are shown in Fig.6. From this, itbecame evident that torque values obtained by these fractions did not show broaderscattering than obtained for the narrow particle size distributions. Deviation between thereal mean particle sizes and those calculated from torque values remained below 10 percent.To simulate experimentally the transition from one particle size to another in acontinuously working granulator, sucrose particles with d=0.40-0.63 mm size range(mean particle size 0.52 mm) were initially filled into the fluidized bed. The modifiedCuette-type device was connected to the granulator as shown in Fig.7. Continuous flowof particles was ensured through the measuring cell by a screw feeder with vs=0.001 m/ssuperficial velocity. Subsequently, larger particles with d=0.80-1.00 mm size range(mean particle size 0.90 mm) were continuously introduced into the fluidized bed, andthe torque was continuously measured and recorded. The mean particle sizes calculatedfrom these torque values are plotted in Fig.8 for about 800 s period. To check thereliability of this method, mean particle size in the granulator was measured at thebeginning and at the end of the measuring period by sieve analysis. It is clearly seenfrom this Figure that the transition from the smaller mean particle size to the larger onecould be monitored by this method with appropriate accuracy. Deviation between thedata calculated from torque signals and measured by sieve analysis was below 8%.To compare these results to earlier theory and experiments, some discussion is needed.Tardos et al. [1, 2] proposed the following general expression to evaluate this kind ofexperimental data −=mfBq UURgLM 1sin22 φπρ (1)According to this equation, the torque Mq is proportional to L2 and R2 (where L and Rare the height of the sheared layer and the radius of the inner cylinder, respectively).Linear dependence is found on bulk density ρB, on the inner friction coefficient of theparticle bed, sinΦ, and on (1-U/Umf), where U and Umf are the actual superficial gasvelocity and the minimum fluidization velocity, respectively. However, no directrelation is shown with particle diameter.There is no clear explanation of the discrepancy between the experimental findingswhere torque depended on particle size and the above equation where there is nodependence. It is true that Eqn.1 was obtained for shearing particle beds in a relativelynarrow gap between the surfaces of the stationary and rotating cylinders. In this case,the effect of shear has extended over the whole particle region in the gap, from the innerrotating to the outer stationary wall, independently of particle size. But, in theexperiments presented here, much wider gaps were used between the two cylinders.Therefore, the torque measured on the shaft of the rotation cylinder might depend onfriction forces between the moving and stationary particle regions, which in turn couldbe influenced by the sizes of the particles and geometric parameters. This possibleexplanation was supported by DEM simulation.DEM simulationDEM (Distinct Element Modeling) simulation, based on the method developed andapplied to a number of systems by Tsuji et al. [6], was carried out for the particle systemand Cuette-type device used for the experiments. By this way, both the instantaneousposition and velocity of all particles could be determined, as well as the direction andmagnitude of forces acting on them. By summing of the tangential forces along a givencylindrical surface within the sheared particle bed, the friction acting against rotationcould be calculated.Fig.9 shows snapshots on the positions of particles in a cross section of the simulatedCuette device at different time intervals (t=0.0, 0.4, 0.8 and 1.2 s). From this, it isclearly seen that particles which, directly or indirectly, get into contact with the rotatingsurface are moving in the same direction as the inner cylinder (but with lower velocitydue to the slip between them). Fig.10 shows the mean angular velocity of particles, forthree different particle diameters as a function of their distance from the center ofrotation, i.e. from the axis of the rotating cylinder. These plots show decaying angularvelocity as the distance from the cylinder surface increases.Fig.11 shows the time signal of the summarized frictional forces acting on the particlessituated in the particle layer adjacent to the surface of the rotating cylinder. Averages ofthese time signals after achieving quasi steady state conditions are plotted vs. particlediameter in Fig.12. The tendency is very similar to that obtained by the experiments: theshear forces between the surface of the rotating cylinder and the contacting particles areincreasing as the particle diameter is increasing. It means that these results seem toconfirm the experimental findings and give an explanation of the measured behavior.4. ConclusionsA modified Cuette-type shearing device with a relatively small rotating inner cylinderand wide gap for the sheared particle bed was constructed. The effect of particle size onthe torque acting on the shaft of the driving motor was studied. For five differentsucrose particle fractions with mean diameters ranging from 0.3 to 1.4 mm, it wasobserved that the measured torque has significantly increased as a function of the meanparticle size. This finding was also validated for sliding particle beds and for broadersize distribution. This method could also be applied under non steady-state conditions.AcknowledgementWork for this paper was partially supported by the US-Hungarian Fund (JF No 593/96project number) and by the Hungarian National Foundation for Fundamental Research(OTKA T029313). The authors would like to acknowledge this help.References1. Tardos, G. I., Khan, M. I., Schaeffer, D. G.: Forces on a slowly rotating roughcylinder in a Couette device containing a dry, frictional powder. Physics of Fluids,10 (2) (1998) 335.2. Tardos, G. I., Khan, M. I., Mort, P. R.: Critical parameters and limiting conditions inbinder granulation of fine powders. Powder Technol., 94 (1997) 245.3. Cheremisinoff, N. P., Cheremisinoff, P. N.: Hydrodynamics of Gas-FluidFluidization, Gulf, Houston, Texas, 1984, 127.4. Anjaneyulu P., Khakhar D.V.: Rheology of a gas-fluidized bed. Powder Technol. 83(1995) 29.5. Dencs, B., Ormós, Z.: Particle size control in fluidized bed spray-dryer andgranulator during the recovery of solids from liquids. Hung. J. Ind. Chem., 21 (1993)225.6. Tsuji, Y.: Discrete Particle Simulation of Gas-Solids Flows (From Dilute to DenseFlow) Review, KONA, 57 (1993) 11.Figure 1. Schematic diagram of the Cuette-shearing device used by Tardos et al. [2]Figure 2. Schematic diagram of a continuously operating grinding granulator [3]Figure 3. Schematic diagram of the modified Cuette device used for these experimentsh                                                                                                                                                                                                                                                                                                                                                ParticlesFluidizing AirBinderIntroductionNormalStressSensorPurge AirGranules OuterStationarCylinderDistributorRoughSurfaceInnerRotatingCylinderHot AirCleaned AirFluidized bedAir distributorGrinderSpraying nozzleJet-filterProduct GranulesSolid MaterialScrew feederSolution  orSuspension DC Motor withControllable RPMInner RotatingCylinder Coveredwith Sand PaperOuter StationaryCylinder Coveredwith Sand PaperGap to ShearParticle BedBuilt-in Torque MeterConnected to PC DataAcquisition SystemFigure 4. Torque signals versus time measured with different mean particle sizesFigure 5. Average torque values versus mean particle sizeFigure 6. Average torque values measured versus time for a broad size distributiony = -0.161x2 + 0.4587x - 0.007800.050.10.150.20.250.30.350.40 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6d, (mm)M (average), (Nm)n=80 rpm,  Mo=0.043 Nm n=80 rpm,  Mo=0.092 Nmn=120 rpm, Mo=0.057 Nm00.10.20.30.40.50 10 20 30 40 50t, (sec)M, (Nm)Without particlesParticle size d=0.2-0.63mmParticle size d=0.63-1.6mm00.10.20.30.40.50 30 60 90 120 150 180t, (sec)M, (Nm)d=0.3 mmd=0.5 mmd=1.4 mmd=0.7 mmd=0.9 mmFigure 7. Cuette-type measuring cell, connected to fluidized bed granulatorFigure 8. Transition of particle size data calculated from torque valuesFigure 9. Snapshots of particle positions during shearing of the particle bed      Signal to computer1234561-Fluidized bed2-Sliping bed3-Rotated cylinder4-Torque meter5-Screw feeder6-MotorParticle flow(with sand paper P30)Diameter: 52 mmHeight: 80 mm    0.30.40.50.60.70.80 200 400 600 800 1000t, (sec)d, (mm)d (calculated)d (measured)Figure 10. Average angular velocity of particles around the inner cylinderFigure 11. Tangential frictional forces acting on the particles around the rotatingcylinderFigure 12. Average of frictional forces as a function of particle diameter0,00,51,01,52,02,50,025 0,030 0,035 0,040Distance from the centre of the vessel [m]Angular velocity, rad/s d=3mmd=2.4mmd=2mmd=1.6mm0,40,50,60,70,80,91,01,11,21,31,40,0 0,5 1,0 1,5 2,0Time [s]Force [N]d=1.6mmd=2.0mmd=2.4mm0,60,70,80,91,01,11,21,31,2 1,4 1,6 1,8 2,0 2,2 2,4 2,6Particle diameter, mmForce, N

Torque Measurements and DEM Simulations in a Couette-type Device with Application to Particle Size Measurements

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