D.J. Parker

Positron emission particle tracking - a technique for studying flow within engineering equipment

Abstract The use of the Birmingham positron camera for tracking positron-emitting particles within engineering structures has been investigated, using particles containing typically 4 MBq (100 μCi) 18 F or 22 Na. An algorithm for discarding γ-ray trajectories corrupted by scattering, etc., and using the remaining trajectories to compute the particles location in three dimensions has been developed, and its performance explained. A slowly moving particle can be located to within 1 mm several times per second, while a particle moving at 1 ms −1 can be located to within 5 mm 50 times per second, through a considerable thickness of surrounding material. An example of results obtained from tracking a particle in a rotating drum of powder illustrates the potential value of the technique.

Positron emission particle tracking studies of spherical particle motion in rotating drums

Positron emission particle tracking has been used to track the motion of a single radioactively labelled tracer particle within a bed of similar particles in a partially filled horizontal rotating drum. Runs were performed using 1.5 mm glass spheres in a 136 mm diameter drum and using 3 mm glass spheres in 100 and 144 mm diameter drums, at drum rotation speeds from 10 to 65 rpm. An active surface layer approximately two-thirds as thick as the underlying bed layer was apparent in all cases. Considerable slip of the bed at the walls was observed in most runs, which is attributed to a rolling motion of the outermost layer of spheres. The axial dispersion coefficient was determined for each run and was found to be proportional to the frequency at which particles circulate around the bed, and to be strongly dependent on particle size but independent of drum diameter.

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.

Positron emission particle tracking using the new Birmingham positron camera

Abstract Since 1985 a positron camera consisting of a pair of multi-wire proportional chambers has been used at Birmingham for engineering studies involving positron emitting radioactive tracers. The technique of positron emission particle tracking (PEPT), developed at Birmingham, whereby a single tracer particle can be tracked at high speed, has proved particularly powerful. The main limitation of the original positron camera was its low sensitivity and correspondingly low data rate. A new positron camera has recently been installed; it consists of a pair of NaI (Tl) gamma camera heads with fully digital readout and offers an enormous improvement in data rate and data quality. The performance of this camera, and in particular the improved capabilities it brings to the PEPT technique, are summarised.

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.

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.

A phenomenological study of a batch mixer using a positron camera

Abstract Particulate motion within a Lodige mixer has been analysed using a refinement of the nuclear imaging technique, positron emission tomography. The detailed tracking of a single, in this case large, radioactive tracer particle has revealed information on the detailed flow behaviour in the mixer, by determining not only axial, radial and angular locations but also axial, radial and angular velocity distributions. The mixer had a five litre mixing bowl containing two ploughshare blades and two end scraper blades mounted upon a horizontal mixing shaft rotating at a fixed frequency of 4Hz. Three active mixing regions separated by boundaries coinciding with the axial positions of the ploughshare blades were found, and residence time distributions in these regions of the mixer determined. Assessment was performed for eight powder fill proportions ranging from 7% to 70% by volume; the results show a great influence of detailed mixer geometry on behaviour. This application of the positron emission particle tracking technique demonstrates its potential for powder mixing studies.

Granular flow over a flat-bladed stirrer

Understanding the mixing of powders is vital because of its major impact on the performance of many unit operations and the behaviour of many products. However, little is known about the behaviour of particles within process equipment. The motion of particles in a vertical cylindrical mixer with two opposed flat blades was studied throughout the bed using positron emission particle tracking, this yielding the location of a tracer particle many times per second. At the wall the particles in the path of the blades were displaced upward, forming a heap, and then moved over the blade. Away from the wall a part of the upward flow moved away from the blades down the faces of the heaps. These particles also moved radially, producing three-dimensional recirculation regions. Recirculating particles took up to 60 blade passes to cross between the blades, compared with less than 3 blade passes outside these regions. The overall velocity field varied linearly with blade speed for the range of speeds studied. However, the size of the recirculation regions and the number of blade passes that the particles spent therein decreased as fill level was increased or as blade speed was increased.

Fluid trajectories in a stirred vessel of non-newtonian liquid using positron emission particle tracking

Abstract The technique of positron emission particle tracking (PEPT) enables the position of a small radioactive particle tracer to be detected many times per second and its trajectory followed. The technique was employed to determine the flow field of viscous non-Newtonian CMC solutions inside a 290 mm diameter vessel using a 600 μm almost neutrally buoyant tracer. Two axial flow impellers, Lightnin A410 and A320, were used. Their mixing effectiveness was characterised qualitatively by a study of the flow patterns generated within the vessel, and quantitatively by a statistical analysis of the trajectories of the Lagrangian tracer tracked over a test period of about an hour. Zones of effective fluid agitation as well as zones of stagnation and poor mixing were clearly identified. Analysis of particle trajectory-length distributions showed that an increase in agitation speed gave rise to a wider trajectory distribution in the tank. Both agitators behaved predominantly as radial flow impellers as the fluid viscosity was increased. This feature seems to be specific to highly viscous fluids. The ratio of the mean fluid-circulation velocity and the impeller tip velocity is proposed as an index of agitation.

Positron emission tomography for process applications

After a brief general discussion of radioisotope emission tomography, this paper discusses the principal types of detector systems which are used in medicine for positron emission tomography. It then describes the Birmingham positron camera and the various ways in which this has been used for studying industrial processes.