Solids motion of calcium carbonate particles in a high shear mixer granulator: A comparison between dry and wet conditions

An experimental study has been carried out on the solids motion in a conical frustum-shaped vertical high shear mixer granulator by using the positron emission particle tracking (PEPT) technique. The mixer granulator has a vertical shaft, to which four sets of impellers are attached at different elevations. The shaft is operated at 3.9 Hz, 4.9 Hz and 5.8 Hz, which corresponded to the top impeller tip speed of 2.8, 3.5 and 4.1 m/s. The motion of calcium carbonate particles with and without a liquid binder is evaluated. Particles are observed to circulate in both the horizontal and vertical directions. The macroscopic solids circulation in the vertical direction reduces after adding the binder. There is a dominant solids motion in the tangential direction under both the dry and wet conditions with the maximum tangential velocity of 2.2 to 12.6 times that of the maximum axial and radial velocities. No obvious change is observed in the average axial and radial velocities when the impeller speed is changed under both dry and wet conditions, while the ratio of the maximum tangential velocity to the tip speed decreases with increasing impeller speed, suggesting a velocity-dependent behaviour. The three velocity components decrease in the magnitude after adding the binder at all tested agitation speeds except for the tangential velocity at a shaft speed of 3.9 Hz. The main difference between the dry and wet operations is that the decrease of tangential velocity in the near-wall zone under the dry condition is not observed under the wet condition.     

Surface velocity measurement in a high shear mixer

Particle velocity measurement in high shear granulation systems has been an area of interest in recent years. This paper gives an insight into the systemic variation of the bulk motion in a high shear mixer (HSM) equipped with a three-blade impeller. The velocities are measured using a high speed camera and particle image velocimetry (PIV) software. Fourier analysis of the velocity data shows a very regular pattern of velocity variation largely dependent on the impeller frequency. The effect of binder viscosity on the bulk flow is investigated as well. From these experiments it was possible to characterize, quantitatively, both the mean surface flow, and the fluctuations in this motion.     

Effects of process parameters on granules properties produced in a high shear granulator

Results of a study on the influence of process parameters such as impeller speed, granulation time and binder viscosity on granule strength and properties are reported. A high shear granulator (Cyclomix manufactured by Hosokawa Micron B.V., The Netherlands) has been used to produce granules. Calcium carbonate (Durcal) was used as feed powder and aqueous polyethylene glycol (PEG) as the binder. The dried granules have been analysed for their strength, density and size distribution. The results show that increasing the granulation time has a great affect on granules strength, until an optimum time has been reached. The underlying cause is an increase in granule density. Granules are consolidated more at higher impeller speeds. Moreover, the granule size distribution seems not to be affected significantly by an increase in impeller speed. Granules produced with high binder viscosity have a considerably lower strength, wide strength distribution due to poor dispersion of binder on the powder bed. Binder addition methods have showed no considerable effect on granule strength or on granule size distribution.     

Numerical analysis of agitation torque and particle motion in a high shear mixer

Numerical simulation of particle motion in a high shear mixer was conducted using a three-dimensional discrete element method (DEM). Torque of an agitator blade, particle velocity profiles and forces acting on a particle were calculated under various agitator rotational speeds. The agitation torque showed a good correlation with the agitator rotational speed and reflected the particle motion in the mixer. The simulated results revealed that particles received greater forces near the bottom of the vessel, as compared to the upper or middle height in the mixer. The particle flow changed with the agitator rotational speed, as experimentally observed, and indicated different velocity profiles. Particle kinetic energy determined the agitation torque and the particle behavior, regardless of the complexity of the changes in the particle flow. A significance of monitoring of the agitation torque in the mixer is also discussed here from the perspective of the particle kinetic energy.     

Impact of surface tension and viscosity on solids motion in a conical high shear mixer granulator

Surface tension and viscosity are the important properties of liquid binders affecting wet granulation processes. They could be used to control solids flow pattern and relative motion of particles for controlling wetting, granule growth, consolidation, and breakage. This study aims to investigate experimentally the impacts of the two properties with a conical high shear granulator. The results show significant effects of viscosity and surface tension on solids flow pattern and relative motion of particles. The relative importance of the two parameters, the surface tension and the viscosity, are found to vary with the axial and radial positions in the granulator. For example, the viscosity force decreases with an increase in the bed height in the axial direction (vertical plane). The viscosity force between particles coated with PEG4000 solution is in mN order, whereas that between particles coated with ethanol and water is in μN order. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Direct measurement of surface granular temperature in a high shear granulator

High speed images of the bulk motion in a flat disc high shear granulator have been analysed using particle image velocimetry (PIV) to obtain surface velocity fields. The fluctuations in the temporal surface velocity field are found to exhibit a Gaussian distribution, allowing direct measurement of the granular temperature from the velocity field. The spatial correlation of fluctuations is examined, indicating a high level of correlation at the length scale of granule diameter, which then decreases rapidly with increasing distance. This spatial correlation profile is shown to be consistent with the expected profile for uncorrelated motion, confirming that individual granules exhibit random fluctuations during bulk motion.     

高速混合制粒机制粒过程

应用高速混合制粒机对湿法制粒过程进行了研究。单因素实验结果表明,混合时间对混合均匀度影响较明显,混合时间过长,将出现混合均匀度下降的现象;随黏合剂质量分数增加大颗粒增加,小颗粒减少,颗粒度有所增加;剪切桨转速变化对粒度分布的影响较小;搅拌桨转速变化对粒度分布的影响较明显,随着搅拌桨转速的增大,目数大于20目与小于100目的颗粒都有明显减少的效果。     

Axial mixing in a ploughshare mixer

Motion is studied within a bladed 5 l ploughshare mixer for granular solids at two rotor frequencies, 2 Hz and 4 Hz, for fill levels between 12.5% and 70%. Positron emission particle tracking is used to follow the motion of a single radioactive tracer. Particles circulate around the bed in the transaxial plane in the direction of blade rotation. Axial motion perpendicular to this plane is significantly less and is largely contained in the region between the planes of rotation of adjacent blades. Axial mixing between these inter-plough regions is determined by calculating the transport rate across these planes. Transport is greater at 4 Hz than at 2 Hz, but decreases as fill level increases. Axial transport is further studied by recording the location of the tracer as it crosses each plane of blade rotation. These passages can be described as occurring either; (i), in defined regions of the bed; or (ii), by leading or lagging the blades by a constant angle. A transition from (i) to (ii) occurs as fill level increases. At 4 Hz or with oily rice, the transition occurs at lower fill levels.     

Wet granulation in a batch high shear mixer

This study deals with the wet granulation in a high shear mixer. The experimental apparatus is a laboratory scale ”Lödige” granulator, with a maximum volume of 20 l, equipped with a chopper and a pneumatic spraying system. The main objective of the study is to point out the effect of physico-chemical properties and operating conditions on the growth mechanisms and kinetics in this type of granulation device. Two kinds of alumina with different particle size distributions (alumina SH100 and alumina SH30) were granulated using various Newtonian liquids having different surface tension, viscosity, binder concentration, density, etc. (water, aqueous solutions of polyethyleneglycol or polyvinyl alcohol). Experimental results showed that the granulation process generally proceeds through three distinct growth regimes independent of the nature of the powder, the binder liquid or the operating conditions. However, the transition between different regimes depends on the physico-chemical properties of the solids and liquids, on operating conditions and on the experimental procedure. For the alumina powder used in this study the transition occurs when a degree of liquid saturation of about 68% is reached.     

Modelling of dense and complex granular flow in high shear mixer granulator—A CFD approach

In the aspect of granulation process control, the numerical simulations appear to be a cost-effective and flexible tool to investigate the flow structure of granular materials in mixer granulators of various configurations and operating conditions. Computational fluid dynamics (CFD) is used in this study to model the granular flow in a vertical high shear mixer granulator. The simulation is based on the continuum model of dense-gas kinetic theory [Gidaspow, D., Bezburuah, R., Ding, J., 1992. Hydrodynamics of circulating fluidized beds, kinetic theory approach. In: Fluidization, vol. VII, Proceedings of the 7th Engineering Foundation Conference on Fluidization, Brisbane, Australia, pp. 75-82] with consideration of inter-particle friction force at dense condition [Schaeffer, D.G., 1987. Instability in the evolution equations describing incompressible granular flow. Journal of Differential Equations 66 (1), 19-50]. This study aims to verify this numerical method in modelling dense and complex granular flows, where the solids motion obtained from the simulation is validated against the experimental results of positron emission particle tracking (PEPT) technique [Ng, B.H., Kwan, C.C., Ding, Y.L., Ghadiri, M., Fan, X.F., 2007. Solids motion of calcium carbonate particles in a high shear mixer granulator: a comparison between dry and wet conditions. Powder Technology 177 (1), 1-11]. In general, the Eulerian based continuum model captures the main features of solids motion in high shear mixer granulator including the bed height and dominating flow direction (the tangential velocity). However, the continuum based kinetic-frictional model is not capable of capturing the complex vertical swirl pattern. Quantitative comparison shows over-predictions in the tangential velocity and stiff drops of the tangential velocity at the wall region. These results demonstrate the deficiency in transmitting forces in the bed of granular materials which indicate the necessity to improve the constitutive relations of dense granular materials as a continuum.     

Solids behaviour in a gas-solid two-phase mixture flowing through a packed particle bed

This paper reports the solids behaviour in a dilute gas-solid two-phase mixture flowing through a packed bed. The positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the data gave solids velocity, the residence time of suspended particles, bed tortuosity in terms of solids motion, as well as solids occupancy in the cross-section of the packed bed. The results suggest that the wall effect on the motion of suspended particles is limited to approximately one packed particle diameter under the conditions of this work. Both the average axial and radial velocities of suspended particles, normalised by the superficial gas velocity, change periodically with radial position, but the periodicity does not correspond exactly to the packed particle diameter. The peak and trough values of the average axial velocity of the suspended particles in the bulk region are, respectively, ∼25% and ∼15% of the superficial gas velocity under the conditions of this work and the superficial gas velocity shows little effect. The peak and trough values of the average radial velocity of the suspended particles in the bulk region are, respectively, +5% (positive) and -5% (negative) of the superficial gas velocity. The results of the residence time and tortuosity of the suspended particles show an approximately Gaussian distribution with the peak residence time and tortuosity increasing with decreasing superficial gas velocity. The occupancy data suggest that particles spend more time in an annular region close to the wall, indicating a non-uniform particle distribution across the packed bed cross-section.     

Solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels

This paper reports, for the first time, the solids behaviour in a dilute gas-solid two-phase mixture flowing through monolith channels. The non-intrusive positron emission particle tracking (PEPT) technique was used in the work, which allowed investigation of three-dimensional solids motion at the single suspended particle level. Processing of the PEPT data gave solids velocity and occupancy in the monolith channels. The results showed a non-uniform radial distribution of both the solids velocity and concentration. The highest axial solids velocity occurred in monolith channels located in the central part of the column, whereas the highest solids concentration took place at a position approximately 0.7 times the column radius. The axial distribution of the axial solids velocity showed an entrance region with a length of approximately 33 times the hydrodynamic diameter of a monolith channel under the conditions of this work. Analysis of the PEPT data also gave distributions of particle residence time and tortuosity in terms of solids motion. The distributions were approximately Gaussian-type with the tortuosity distribution more skewed toward the right hand side. The peak residence time and tortuosity decreased with increasing superficial gas velocity and the distributions were broadened at lower superficial gas velocities. The results of this work also provided a possible explanation to our previously observed early laminar-to-turbulent flow transition in monolith channels.     

Effect of scale of operation on granule strength in high shear granulators

Results of a study on the influence of operation scale and impeller speed of high shear mixer granulators on the strength of granlues are reported in this paper. Calcium carbonate particles have been granulated in four scales of a geometrically similar high shear granulator (Cyclomix) with 1, 5, 50 and 250 L capacities. For the smallest scale, the effect of a small deviation from geometric similarity was also investigated. An aqueous solution of polyethylene glycol was used as the binder. Three scaling rules of constant tip speed, constant shear stress and constant Froude number have been used to determine the impeller speed for the different scales of the granulators. The granules produced in these experiments have been dried and tested for strength using side crushing test method. The data have then been analysed and compared. Operation of granulators according to the constant tip speed rule produces granules with a similar strength for all four scales, followed by a similar trend for the constant shear stress rule, albeit to the less extent. The constant Froude number rule produces a heterogeneous strength distribution and is not a suitable criterion for scaling-up of high shear granulators. The distribution of granule strength has been fitted to the normal, log-normal and Weibull distributions. Weibull distribution fits the data well for the constant tip speed operations.     

Mixing Agglomeration in a High‐Shear Mixer with a Stirred Mixing Vessel

This paper deals with the agglomeration process in a high‐shear mixer. High‐shear mixers rotate with a very high mixing tool speed such that not only a mixing effect, but also a grinding effect is achieved. The parameter study reported here was carried out to determine the parameters influencing mixing agglomeration. The results will help the user to decide which parameters have to be considered for an optimum mixing agglomeration. This article will highlight some of the findings obtained from the comprehensive parameter study.     

Solids motion in flowing froths

Flotation is a widely used process within the minerals processing industry, as well as being used for water treatment and de-inking of recycled paper. The froth phase and its role in the separation achieved is as yet ill understood. A fundamentally based model of the behaviour of solids within a flowing froth allows for a fuller understanding of the froth phase of flotation vessels and process optimisation.This paper outlines a model for the motion of solids within a flowing froth. It builds on earlier work on the modelling of bubble and liquid motion within a flowing froth and includes all the effects of same phenomena that effect liquid motion, as well as including the effect of solids concentrations on liquid motion.The solids are divided into two classes for the purposes of modelling, namely the attached material, which follows the bubbles, and the unattached material, which mainly follows the liquid, but can move relative to the water by means of hindered settling and geometric and Plateau border dispersion. The attached material consists of hydrophobic particles, while the unattached material can consist of both hydrophobic and hydrophilic particles. Attached particles can become unattached due to coalescence or bursting.Results from simulations are shown to illustrate the movement and concentration of the solids from the pulp-froth interface to the upper, bursting surface and overflowing the weir.     

LDA measurements of near wall powder velocities in a high shear mixer

In this study, a new method has been developed to measure particle velocity distributions in the near wall region of a high shear mixer by using Laser Doppler Anemometry (LDA). The velocities along the side of the granulator have been measured at different impeller speeds and it has been found that it is possible to obtain tangential and axial velocity data in the dense powder flow up to 4 mm in depth. Moreover, it has been found that the tangential velocity component increases slightly with distance from the wall in the near wall region, indicating a partial slip boundary condition for the solid phase at the vessel wall. It is also shown that the tangential velocity decreases with increased vertical distance to the impeller. The velocity fluctuations, represented by root mean square (rms) velocities, also decrease with increased vertical distance to the impeller and the tangential and axial rms components are found to be of the same order of magnitude.     

Structure of powder flow in a planetary mixer during wet-mass granulation

Wet massing granulation, a widely used industrial process, is difficult to monitor and control and the structure of the flow is poorly understood. Flow patterns in a planetary mixer were investigated using positron emission particle tracking. Both dry and wet powders of a model pharmaceutical formulation were studied to develop understanding of the influence of moisture content on the flow structure during granulation. The flow structure was characterised using the distributions of the velocity components in different cross-sections of the mixer. Fourier analysis showed that the dry system is essentially dissipative and disordered whereas the wet system, being more inertial, shows signs of being more ordered with a periodic recirculation within the bowl. In both systems, radial and axial displacements are strongly correlated. For the dry system, within a central radial core region, the behaviour of the particle was determined by the rapid movement of the agitator, forming a single toroidal recycling cell. The radial and axial velocities of the tracer were up to two orders of magnitude lower than the tangential component. However, in the regions close to the wall, the particle was found to exhibit small movements dictated by the planetary rotation. For wet systems these two main regions were again observed. However, velocity field and velocity distribution showed the presence of two toroidal circulation loops, one above the other. In the wall region, the small movements governed by the planetary motion were again found, but with the amplitude of the displacements reduced by an order of magnitude.     

Binder addition methods and binder distribution in high shear and fluidised bed granulation

Fluidised beds and high shear mixers are both important in industrial granulation. The binder addition method (pouring, melt-in, spraying) affects the growth and properties of granules and is therefore of vital importance to the fundamental understanding of this detailed process. Non-uniformity of binder distribution is well known in high shear melt granulation, however, there is limited literature surrounding binder distribution in fluidised bed granulation. It was therefore the aim of the paper to compare the binder distribution using alternative addition methods in both high shear mixer and fluidised bed.In this work two binder addition methods, ‘wet’ and ‘dry’, in a fluidised bed and high hear mixer were used to successfully produce granules with a typical pharmaceutical size, 150-300 μm. The granules were analysed for final binder distribution in different size classes using Patent V blue dye and ultra-violet spectrometry.All binder addition methods supported previous work showing non-uniformity of binder distribution throughout the size classes. High shear mixer results show great similarity in binder content whichever binder addition method was chosen. This is likely to be due to the same mechanisms occurring due to the impeller forces in the process, mean while the fluidised bed results show little similarity. The binder distribution by mass is also investigated and shows that although most studies show a relative higher binder content in the larger size classes that actually the majority of binder can instead be found around the mean size of the batch.     

A narrow size distribution on a high shear mixer by applying a flux number approach

High shear granulation is a common technology for particle size enlargement, but generally the product properties are badly affected by the broad size distribution generated in the process. A recently published approach by Michaels et al. [J.N. Michaels, G. Wang, L. Farber, K.P. Hapgood, J.H. Chou, S. Heidel, and G.I. Tardos, 2006, One-dimensional scale-up of high-shear granulators, Paper 243c, World Congress Particle Technology 5, Orlando (FL)] employs low binder solution spray rates and long granulation times, whilst the solids are kept in roping flow, to avoid coarse formation. The present work applies this approach to a two-component binder system with a dry powder gum and water spray as activation agent. Similarities with fluidised bed granulation and coating processes are explored. The work shows that indeed narrow size distributions of fine granules can be achieved with ease. Dimensionless numbers for spray fluxes are useful to identify operating regimes and to steer optimisation efforts. Comparison of flux numbers for different systems shows that they are not useful (yet) for detailed product and process design. Further work on material-specific quantities controlling nucleation and growth, e.g. particle wetting, is recommended.     

Measurement of the velocity field and frictional properties of wet masses in a high shear mixer

In order to develop predictive process models and to enhance process understanding in high shear granulation, there is an ongoing search for non-intrusive methods for measuring the wet mass velocities in the mixer. In this study, a high speed CCD camera is used in combination with software for particle image velocimetry (PIV) calculations to obtain information about the wet mass velocities. The focus has been on obtaining good spatial and angular resolution for the velocities along the glass bowl wall. In a Jenike shear cell, both internal and wall frictional properties have been measured and together with velocity data, this information is used for prediction of the impeller torque. It has been shown that the near wall velocities are strongly dependent on the coefficient of wall friction, which decreases during liquid addition. The decrease in the coefficient of wall friction results in increased wet mass velocities close to the bowl wall. It is also found that the wet mass velocity has a strong angular dependence, resulting in a high frequency pulsing bed behaviour which cannot be detected by visual inspection. The predictive impeller torque model developed by Knight et al. [2001. Prediction of impeller torque in high shear powder mixers. Chemical Engineering Science 56, 4457-4471] has been generalized to account for cohesive materials and with frictional and velocity data, the level of the impeller torque is well predicted. However, the model is based on crude assumptions regarding the velocity distribution and hence, it cannot capture the dynamics in the measured torque curve satisfactorily.