J.P.K. Seville

Interparticle forces in fluidisation: a review

Because of the balance of forces in fluidised beds, particle interactions can have a strong effect on their microscopic and macroscopic behaviour, leading to agglomeration and defluidisation. Three types of particle interactions are reviewed: van der Waals forces, liquid bridge forces and sintering. Sintering is qualitatively different in its effects because it is a time-dependent process. The observed effects of these three types of interactions on fluidisation behaviour are described and explained in terms of simple models.

Solids motion in bubbling gas fluidised beds

Abstract Gas fluidised beds are widely employed in the chemical, petrochemical, metallurgical, and pharmaceutical industries. Solids mixing in such devices plays a central role in controlling product quality and productivity. This paper presents some observations of particle motion in 3-D gas fluidised beds operated in the bubbling mode and at atmospheric pressure. The non-invasive positron emission particle tracking (PEPT) technique was used to observe and quantify particle trajectory, solids flow pattern, solids velocity, and solids circulation frequency. It is shown that for relatively deep beds (cylindrical columns, group B particles), particles move upward in the central region, and downward near the wall. The average upward particle velocity is ∼50% of the bubble velocity under the conditions of this study. Theoretical models are proposed to correlate the overall particle upward velocity and average particle velocity in the drift to the bubble velocity, and to estimate the duration of particle residence in the drift. Model predictions show reasonable agreement with experiments. It is suggested that solids motion in gas fluidised beds can be characterised by ‘jump’, ‘idle’ and ‘relaxation’ times. These times may also be linked to particle kinetic energy transfer in fluidised beds.

Solids motion in rolling mode rotating drums operated at low to medium rotational speeds

Abstract The industrial-scale rotating drums are usually operated in the rolling or slumping mode. For the rolling mode, the granular material bed can be divided into two regions, namely, a ‘passive’ region where particles are carried up by the drum wall, and an ‘active’ region where particles cascade down. As solids mixing mainly occurs in the active region, solids motion in this region and solids exchange between the active and passive regions are of prime importance for the overall performance of the drum. This paper reports some observations on particle motion in the transverse plane of a three-dimensional rotating drum operated at low and medium rotational speeds. The non-invasive PEPT (positron emission particle tracking) technique is used to follow particle trajectory and velocity. A mathematical model based on the thin-layer approximation is proposed to describe solids motion in the active layer. Reasonable agreement between the model predictions and experiments is obtained. A new parameter termed the ‘solids exchange coefficient’ is proposed to characterise particle exchange between the passive and active regions. A theoretical expression for this parameter is also derived. This expression, upon application of the thin-layer approximation, is reduced to give an explicit relationship between the solids exchange coefficient and drum operating parameters such as rotational speed and fill percentage, as well as the bed material rheological properties. The solids exchange coefficient is also shown to give a possible scale-up rule for rotating drums operated in a rolling mode.

Prediction of impeller torque in high shear powder mixers

Abstract An investigation was made of the factors that determine the impeller torque of vertical axis high speed mixers containing granular solids of low cohesion, the experimental material being sand. The diameter of the mixer bowls, which were constructed of stainless steel, ranged from 0.13 to 0.30 m . Disc impellers with both smooth and grooved surfaces were used. Two and three blade flat impellers were used with heights in the range 3– 12 mm and bevel angles ranging from 11° to 90°. The study was supported by the application of positron emission particle tracking (PEPT) to investigate the flow of the material in the mixer. A dimensional analysis was made of the data. The effects of the mass of powder, M , and the bowl radius, R , could be satisfactorily represented by the dimensionless torque group, T / MgR . In the case of disc impellers, the dimensionless torque was independent of impeller rotational speed. For the blade impellers, the dimensionless torque was found to be a function of the impeller Froude number and a dimensionless blade height. A powder mechanics analysis was made of the flow of material in the mixer fitted with both disc and blade impellers. The flow of the powder was modelled as ‘rigid’ body rotation and both frictional and inertial interactions with the impeller were accounted for. The analysis provides a first order representation of the effects of scale, mass fill, impeller rotational speed, blade height and blade bevel angle on the torque. The assumptions made in the model are critically discussed.

An investigation of the effects on agglomeration of changing the speed of a mechanical mixer

A study that focussed on the effects on agglomeration of changing the rotational speed of a vertical-axis high shear mixer is reported. The design of the mixer was such that, at high impeller speeds, the power input was high. The agglomeration behaviour was found to vary greatly with impeller speed. At low impeller speed, the extent of size enlargement increased with power input. At high impeller speed, the extent of size enlargement was low relative to the large power input. From examination of the changes in size distributions of the granules and of granule morphology, it was concluded that, at high speed, size enlargement was limited by granule breakage. The observation is important as it may provide an improved means of controlling the process.

Granular motion in rotating drums: bed turnover time and slumping–rolling transition

Theoretical models are presented for calculation of the bed turnover time in both the slumping and rolling modes and the slumping-to-rolling transition in rotating drums with less than 50% volumetric fill of free-flowing granular materials. It is suggested that the transition from slumping to rolling occurs when the two turnover times are equal. The model for the bed turnover time in rolling beds is compared with the data reported in the literature. Very good agreement has been obtained. The bed turnover time is also employed to define a new Froude number for constructing the bed behaviour diagram. It is shown that the bed behaviour diagram based on the new Froude number brings the data points for sand and limestone into single curves. The implications of the bed turnover time for solid mixing and heat transfer within particle beds are also discussed.

Modelling of sintering in high temperature gas fluidisation

An investigation was made of the phenomenon of defluidisation caused by visco-plastic sintering. This occurs when the fluidised bed is operated at a temperature sufficiently high for material at the surface of the particles to flow. Previous theoretical treatments of the problem have employed force and energy balances to express the ability of sintered aggregates to withstand the hydrodynamic forces imposed by the fluidised bed. A new conceptual approach is proposed which is based on a comparison of the characteristic times for quiescent motion of particles in a bed and for the growth of sinter necks. The theory gives a quantitative relationship between the gas velocity required to maintain fluidisation and the temperature of the bed in terms of the surface viscosity of the particles. Experimental results are reported on the defluidisation and sintering behaviour of a model material (low density poly-ethylene). The data are used to test the theory and to evaluate the potential of the new approach.

The effect of binder viscosity on particle agglomeration in a low shear mixer/agglomerator

Abstract A study is reported of the effects of changing the binder viscosity in rotating drum granulation of a narrow size fraction of an irregularly shaped sand. Silicone fluids, having viscosities in the range 20–500 mPa s, were used as binders. The size distribution of granules was determined by analysis of microscope images and the granule morphology by examination of sections of granules. The compressive strength of granules was also measured. It was found that the viscosity of the binder affected both the rate of size enlargement and the mechanism of size enlargement. The growth rate increased with increase in binder viscosity up to maximum at a viscosity of about 100 mPa s. Enlargement occurred by a layering mechanism. With binders of viscosity greater than 100 mPa s, layering was not observed and growth was found to be by coalescence. Stokes number analyses of the internal deformation on impact and of the adhesion on impact of surface-wet granules were made and found to account, in part, for the effects of changing binder viscosity.

Discrete element simulations of a high-shear mixer

Abstract Particle motion in a vertical-axis mixer was studied using discrete element method (DEM) simulations and positron emission particle tracking experiments. The mixer was fixed with a circular disk rotating in a horizontal plane or a simple paddle. In the DEM simulations, linear springs, dashpots and frictional sliders were used to model the contact mechanics between the particles and the particles and the walls. Quantitative comparisons were made between the numerical calculations and the experimental measurements. The DEM prediction captures the major features of the flow patterns in the mixer when the mixer was fixed with a disk impeller rotating at 100 r.p.m., although the predicted particle velocities are higher than experimental measurements when using physically reasonable simulation parameters (normal stiffness = 1000 and 10000 N/m; coefficient of restitution = 0.9; internal friction coefficient = 0.2, 0.3 and 0.45; wall friction coefficient = 0.2, 0.25 and 0.3). However, when the mixer was fixed with the paddle impeller, the calculated results using physically reasonable simulation parameters were different from the measurements. The calculated particle velocity was as high as 2 m/s, while the averaged particle velocity from measurement was about 0.1 m/s.

Attrition of porous glass particles in a fluidised bed

Fluidisation is frequently accompanied by unwanted attrition of the bed material. This paper focuses on the mechanical aspects of fines creation by attrition in fluidised beds supported by multi-orifice distributor plates. The attrition rates of low-density porous glass particles were measured; these particles show abrasive wear behaviour rather than breakage. Positron emission particle tracking (PEPT) was used to follow particle motion in three dimensions within the fluidised bed. For a single orifice distributor with background fluidisation, the attrition rate increased exponentially with increasing orifice gas velocity. For a multi-orifice distributor, however, attrition rates were roughly proportional to excess gas velocity, except near to a critical ratio of particle to orifice diameter; as this ratio approached 2, attrition was observed to increase by an order of magnitude. A method is proposed for estimating attrition rates from a combination of small-scale experimental results and theoretical calculations of distributor jet entrainment rates.