Andy Ingram

Twin screw wet granulation: the study of a continuous twin screw granulator using Positron Emission Particle Tracking (PEPT) technique.

In this paper, Positron Emission Particle Tracking (PEPT) techniques are utilised to track the trajectory of single particles through the mixing and conveying zones of a Twin Screw Granulator (TSG). A TSG consisting of conveying zones and mixing zones is used in this study. The mixing zones are arranged with kneading discs at an angle of 30°, 60° or 90°. Experiments were carried out using different mixing configurations with various screw speed and total mass flow rate. The PEPT data obtained were then utilised to obtain the residence time distribution (RTD) and the Peclet number in an attempt to gain some insight into the mixing of the process. The fill level of the granulator was also estimated to study the mechanism of granulation. As might be expected, it was shown that the residence time of the granulation process increases with decreasing screw speed. It also increases with increasing angle of the arrangement of kneading blocks in the mixing zones, but will decreases when powder feed rate is increased. The fill level of the mixing zone in particular increases when the screw speed decreases or when powder feed rate increases. Furthermore, the fill level of the granulator will increase when the mixing zone configuration changes from 30° to 90°. It is shown that the granulator is never fully filled, even using 90° mixer elements implying limited compaction which may explain why the granules produced are porous compared with those from a high shear mixer. Interestingly, the RTD analysis reveals that the extent of axial mixing in the mixing zone of the granulator does not change significantly for different configurations and process conditions. There is evidence of a tail in the RTD which implies some material hold up and channelling.

Application of Positron Emission Particle Tracking (PEPT) to validate a Discrete Element Method (DEM) model of granular flow and mixing in the Turbula mixer

The laboratory-scale Turbula mixer comprises a simple cylindrical vessel that moves with a complex, yet periodic 3D motion comprising of rotation, translation and inversion. Arising from this complexity, relatively few studies to obtain fundamental understanding of particle motion and mixing mechanisms have been reported. Particle motion within a cylindrical vessel of a Turbula mixer has been measured for 2mm glass spheres using Positron Emission Particle Tracking (PEPT) in a 2l blending mixing vessel at 50% fill level. These data are compared to results from Discrete Element Method (DEM) simulations previously published by the authors. PEPT mixing experiments, using a single particle tracer, gave qualitatively similar trends to the DEM predictions for axial and radial dispersion as well as for the axial displacement statistics at different operational speeds. Both experimental and simulation results indicate a minimum mixing efficiency at ca. 46 rpm. The occupancy plots also show a non-linear relationship with the operating speed. These results add further evidence to a transition between two flow and mixing regimes. Despite the similarity in overall flow and mixing behaviour measured and predicted, including the mixing speed at which the flow behaviour transition occurs, a systematic offset between measured and predicted result is observed.

Effect of packing fraction on granular jetting from solid sphere entry into aerated and fluidized beds

When a solid sphere impacts on a granular bed, a high-speed vertical jet can arise following the collapse of the cavity that is formed by the penetration of the sphere into the bed. The jet then becomes unstable and breaks into discrete clusters due to density inhomogeneities. In this study, the jetting process was observed using high-speed photography and determined to be a function not only of impact velocity and particle size, but also of the packing fraction in the bed during the impact. Experiments were performed for two different bed diameters, two bed heights, and two impact velocities. Under certain conditions, below a threshold packing fraction, the jet is seen to divide into two distinct parts: a thin upper section followed by a thick base. Geometrical constraints are also shown to be critical in determining the dynamics of the jet.

Instabilities in vertically vibrated granular beds at the single particle scale

The dynamics of granular motion have been studied in a vertically vibrated bed using positron emission particle tracking, which allows the motion of a single tracer particle to be followed in a noninvasive way. The particle movement is closely correlated with the oscillation of the bottom plate. Two types of granular motion have been observed in beds with heap formation: convection and fluctuation. The effects of system parameters, including vibration amplitude, frequency, and bed weight, have been studied. The particle cycle frequency was found to correlate well with the dimensionless acceleration. Cycle frequency appears to be inversely proportional to bed mass. The particle dispersion was determined by following the tracer particle trajectory. The system is highly anisotropic, as the horizontal dispersion is stronger than the vertical dispersion.

Asymmetric distribution in twin screw granulation

Positron Emission Particle Tracking (PEPT) was successfully employed to validate measured transverse asymmetry in material distribution in the conveying zones of a Twin Screw Granulator (TSG). Flow asymmetry was established to be a property of the granulator geometry and dependent on fill level. The liquid distribution of granules as a function of fill level was determined. High flow asymmetry at low fill level negatively affects granule nucleation leading to high variance in final uniformity. Wetting of material during nucleation was identified as a critical parameter in determining final granule uniformity and fill level is highlighted as a crucial control factor in achieving this. Flow asymmetry of dry material in conveying zones upstream of binder fluid injection leads to poor non-uniform wetting at nucleation and results in heterogeneous final product. The granule formation mechanism of 60°F kneading blocks is suggested to be primarily breakage of agglomerates formed during nucleation. Optimisation of screw configuration would be required to provide secondary growth. This work shows how fill dependent flow regimes affect granulation mechanisms.

Visualization of Powder Mixing in a High Shear Mixer using Positron Emission Particle Tracking

*1 Department of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, U.K. *2 Positron Imaging Centre, School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, U.K. (1) Present address: Environment & Process Technology Center, Nippon Steel Corporation, Futtsu 293-8511, Japan (2) Present address: Department of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, U.K. (3) Present address: School of Engineering, University of Warwick, Coventry, CV4 7AL, U.K.

Effect of operating parameters on fine particle grinding in a vertically stirred media mill

ABSTRACT This article evaluates the effects of operating parameters on fine particle grinding process with a vertically stirred media mill. The effects are investigated through size reduction under different operating conditions by changing solids content, tip speed and the configuration of the impeller. Solids content at 65% (w/w) is demonstrated to be more efficient than 75% (w/w). The maximum difference in product size () is about 2 . Velocity maps are described at tip speeds of 5.23 m/s, 6.54 m/s and 9.81 m/s by positron emission particle tracking (PEPT). The lower tip speed saves the specific energy 50–100 kWh/t. Over energy input 100–300 kWh/t, of product is reduced 1–2 by replacing standard impeller with new types of impeller.