Positron Emission Particle Tracking Technique Research Articles

Density-modified tracers for positron emission particle tracking

Positron emission particle tracking (PEPT) is a powerful non-invasive technique used to study the underlying dynamics of granular and multiphase flowing systems. The essence of PEPT is to attach (chemically or otherwise) sufficient positron-emitting radioisotope to a particle chosen as representative of the bulk material under study, and hence used as a flow-following tracer. We report on standardised methods for creating density-modified tracer particles to expand the range of applications feasible to research with the PEPT technique.

DEM2GATE: Combining discrete element method simulation with virtual positron emission particle tracking experiments

Positron Emission Particle Tracking (PEPT) is a Lagrangian particle tracking technique useful for imaging flow in opaque granular media. The scientific and industrial systems studied using PEPT and the PEPT technique can also be simulated: Discrete Element Method (DEM) for granular systems and Monte Carlo for PEPT. Using the open-source DEM2GATE library, we quantify the reconstruction of particle trajectories and overall system behaviour by integrating particle trajectories from a rotating drum simulation with a PEPT detector simulation. The results show the main drivers of PEPT spatial error are particle acceleration and gamma-ray scattering. The accuracy of reconstructed flow fields increases logarithmically with measurement time. Measurements of higher-order, derived quantities require more time to reach a desired level of accuracy. Surprisingly, our results imply PEPT measurements relying on fluctuating velocity deviate substantially from the system's true behaviour. However, using DEM2GATE, these errors are predictable and can be corrected.

Novel Bio-compatible Tracers for Positron Emission Particle Tracking

To study the flow of objects in opaque flow processes, the PEPT (positron emission particle tracking) technique may be utilized. The PEPT technique is based on the same principle of positron emission tomography (PET). Both techniques detect the characteristic back-to-back gamma pairs caused by positron-electron annihilation. PEPT allows tracking of β+ - activated objects (tracers) with high spatial (microns) and temporal resolutions (milliseconds). This makes the technique promising in a number of medical applications that require observation of fast transient phenomena e.g. heart and brain hemodynamics, the aerodynamics of respiratory tract etc. However, a majority of PEPT-tracers used nowadays are tailored for industrial applications and are based on solid particles of a micrometric size, which are hardly suitable for in-vivo tests. In this contribution, we propose new bio-compatible tracers. The tracers are based on frozen radioactive solutions rather than activated solids. We produced a number of millimetre-sized ice tracers dispersing the droplets of radioactive solution in oil at negative temperatures. In this preliminary study, we demonstrate how the tracers behave in a settling column and in an agitated vessel. The PEPT-tracks are presented together with a supplementary numerical analysis that was conducted in order to estimate the freezing time of the droplets.

Convective flow speed of particles in a vibrated powder bed

When a powder bed is vibrated, the particles may move like a fluid, and various flow patterns and surface shapes can exist, depending on the vibration conditions — particularly the frequency and amplitude. A two-dimensional (2D) bed is often used so that the flow pattern in a vibrated powder bed can be directly visualized. Because three-dimensional (3D) fluidization phenomena are complicated, 2D observation is not sufficient. However, it is difficult to observe the internal flow of a 3D fluidized bed. A unique method for doing this is positron emission particle tracking (PEPT), a technique derived from the same physical phenomena as in positron emission tomography (PET). In the most common version of the PEPT technique, the flow in a bed is analyzed by following the behavior of a single radioactively labeled particle. We adopted the PEPT technique to analyze flow patterns in a cylindrical vibrated powder bed. From the particle trajectory data obtained with PEPT, some characteristic internal flow structures that cannot be understood from observing surface particle motion were successfully obtained. However, it is difficult to clarify the detailed mechanisms driving such flow patterns from experimental data alone. Therefore, we also carried out numerical simulations based on the discrete element method (DEM). Because the DEM is a Lagrangian method, its computational time depends on the number of particles, so that simulations were restricted to 2D. In order to compare the simulated results to the experimental ones, the average speeds of particles were obtained. Consequently, it was found that the simulated average speed of particles depends only on the amplitude of the applied vibration. In general, good agreement between simulated and experimental results was obtained, but agreement became poorer under conditions of high frequency and low vibration strength.

Positron Emission Particle Tracking of Granular Flows.

Positron emission particle tracking (PEPT) is a noninvasive technique capable of imaging the three-dimensional dynamics of a wide variety of powders, particles, grains, and/or fluids. The PEPT technique can track the motion of particles with high temporal and spatial resolution and can be used to study various phenomena in systems spanning a broad range of scales, geometries, and physical states. We provide an introduction to the PEPT technique, an overview of its fundamental principles and operation, and a brief review of its application to a diverse range of scientific and industrial systems.

Non-invasive studies of multiphase flow in process equipment. Positron emission particle tracking technique

Positron emission particle tracking (PEPT) is a novel experimental technique for non-invasive inspection of industrial fluid/particle flows. The method is based on the dynamic positioning of a positron-emitting, flowing object (particle) performed through the sensing of annihilation events and subsequent numerical treatment to determine the particle position. The present paper shows an integrated overview of PEPT studies which were carried out using a new PET scanner in the Bergen University Hospital to study multiphase flows in different geometric configurations.

The use of positron emission particle tracking (PEPT) to study milling of roll-compacted microcystalline cellulose ribbons

Milling is a critical process for controlling the properties of the granules produced by roll compaction. In the current study, the positron emission particle tracking (PEPT) technique was used to examine the milling kinematics of roll-compacted ribbons at various milling speeds. Microcrystalline cellulose (MCC, Avicel PH-102) was used as the model feed material and a radioactive particle (tracer) was mixed with the MCC powder and roll-compacted to form sample ribbons. They were then milled using an oscillating mill at various speeds and the kinematics of the ribbons (trajectory, velocity, and occupancy) were quantitatively determined using PEPT. A close examination of the PEPT data reveals that, for milling MCC PH-102 ribbons using the oscillating mill considered in this study, the milling speed plays an important role: at low values, the milling process is dominated by cooperative motion of the ribbons with the blade (i.e. the speeds of the ribbons and the blade are similar, and the ribbons move along with the blade) and the ribbons are milled primarily by abrasion; as the speed increases the ribbons undergo more random motion involving collisions that results in an increase in ribbon breakage and hence an increase in the milling efficiency. It is shown that the PEPT technique is a useful technique for examining milling kinematics of roll-compacted ribbons.

Maximizing energy transfer in vibrofluidized granular systems.

Using discrete particle simulations validated by experimental data acquired using the positron emission particle tracking technique, we study the efficiency of energy transfer from a vibrating wall to a system of discrete, macroscopic particles. We demonstrate that even for a fixed input energy from the wall, energy conveyed to the granular system under excitation may vary significantly dependent on the frequency and amplitude of the driving oscillations. We investigate the manner in which the efficiency with which energy is transferred to the system depends on the system variables and determine the key control parameters governing the optimization of this energy transfer. A mechanism capable of explaining our results is proposed, and the implications of our findings in the research field of granular dynamics as well as their possible utilization in industrial applications are discussed.

Investigation of granular impact using positron emission particle tracking

We present results from an experimental study of granular impact using a combination of high-speed video and positron emission particle tracking (PEPT). The PEPT technique exploits the annihilation of photons from positron decay to determine the position of tracer particles either inside a small granular bed or attached to the object which impacts the bed. We use dense spheres as impactors and the granular beds are comprised of glass beads which are fluidised to achieve a range of different initial packing states. For the first time, we have simultaneously investigated both the trajectory of the sphere, the motion of particles in a 3-D granular bed and particles which jump into the resultant jet, which arises from the collapse of the cavity formed by the impacting sphere.

Particle flow visualization in quartz slurry inside a hydrocyclone using the positron emission particle tracking technique

For the past 120years, hydrocyclones have been used a wide variety of industrial applications, with their main use in mineral processing being as a classifier. Hydrocyclone characterization relies heavily on empirical and phenomenological models. There is a need to develop a method by which the flow patterns can be quantified under industrial conditions. Positron emission particle tracking (PEPT), developed by the University of Birmingham in the late 1980s, has proven to be a powerful in situ visualization tool for engineering applications. This paper presents data on the motion of quartz particles in a two-inch hydrocyclone using the PEPT technique. Quartz tracer particles were labeled using the direct activation technique. The particle size range was between −2000 and +150μm which illustrates the flow pattern of particles reporting to the underflow.

Using positron emission particle tracking (PEPT) to study the turbulent flow in a baffled vessel agitated by a Rushton turbine: Improving data treatment and validation

Positron emission particle tracking (PEPT) is a relatively new technique allowing the quantitative study of flow phenomena in three dimensions in opaque systems that cannot be studied by optical methods such as particle image velocimetry (PIV) or laser Doppler anemometry (LDA). Here, velocity measurements made using PEPT in two sizes of baffled vessel (∼0.20 m and ∼0.29 m diameter) and two different viscosity fluids agitated by a Rushton turbine are compared for the first time directly in depth with some studies reported in the literature made by LDA for the turbulent regime in the equivalent geometry. Initially, the paper considers how the Lagrangian data obtained by PEPT can be converted into Eulerian in order to make the comparison most effective. It also considers ways of data treatment that improve the accuracy of both the raw PEPT data and the velocities determined from it. It is shown that excellent agreement is found between the PEPT and literature results, especially for the smaller vessel, except for the radial velocity just off the tip of the blade in the plane of the disc of the Rushton turbine. This difference is attributed to the very rapid changes in both magnitude and direction that occurs in that region and also to the different way of ensemble averaging in the two techniques. In addition, the results for the absolute velocities normalised by the impeller tip velocity for all the rectangular cross-section toroidal cells in each size of vessel and each fluid and a range of agitator speeds are compared in the form of frequency histograms. In this analysis, the velocities for each run are obtained from PEPT based on tracking a particle for 30 min and the mean and mode of the velocities each decrease slightly with decreasing scale and Reynolds number. The possible reasons for this variation in the mode and the mean are discussed. Overall, it is concluded that for the radial flow Rushton turbine the PEPT technique can be used to obtain accurate velocity data throughout the entire complex three-dimensional turbulent flow field in an agitated, baffled vessel except very close to the impeller in the radial discharge stream.

Positron emission particle tracking (PEPT) compared to particle image velocimetry (PIV) for studying the flow generated by a pitched-blade turbine in single phase and multi-phase systems

Abstract Positron emission particle tracking (PEPT) is a new technique allowing the quantitative study of flow phenomena in three dimensions in opaque systems that cannot be studied by techniques based on optical methods such as particle image velocimetry (PIV) or laser Doppler anemometry (LDA). It has previously been used for studying solid particle motion in various systems used in particulate processing. Here, for the first time, velocity measurements made using PEPT with a down-pumping pitched blade turbine (PBTD) are compared directly with those made by PIV in water in the same equipment. It is shown that excellent agreement is found between the two methods except just below the impeller in the discharge. However, this difference is attributed to the different type of data collected and the different way of ensemble-averaging in the two techniques. Similar results were found at higher agitator speeds with both the PBTD and an up-pumping PBT (PBTU) where a small amount of surface aeration occurred. Measurements in solid liquid systems with surface aeration at 0.5 wt% solids or higher were not possible with PIV, but excellent results were obtained with PEPT for both the PBTD and PBTU in a 5 wt% suspension. It is concluded that this calibration study shows that the PEPT technique can be used to obtain accurate velocity data throughout all of the complex three-dimensional flow field in a range of mechanically agitated, turbulent, multi-phase systems previously not amenable to quantitative analysis.

CFD investigation of the pipe transport of coarse solids in laminar power law fluids

A numerical parametric study of the laminar pipe transport of coarse particles in non-Newtonian carrier fluids of the power law type has been conducted using an Eulerian–Eulerian computational fluid dynamics (CFD) model. The predicted flow fields have been successfully validated by experimental measurements of particle velocity profiles obtained using a positron emission particle tracking technique, whilst solid–liquid pressure drop has been validated using relevant correlations gleaned from the literature. The study is concerned with nearly-neutrally buoyant particles flowing in a horizontal or vertical pipe. The effects of various parameters on the flow properties of such mixtures have been investigated over a wide range of conditions. The variables studied are: particle diameter (2–9 mm), mean solids concentration (5–40% v/v), mean mixture velocity (25–125 mm s −1), and rheological properties of the carrier fluid ( k=0.15–20 Pa s n ; n=0.6–0.9). A few additional runs have been conducted for shear thickening fluids, i.e. n>1. Whilst the effects of varying the power law parameters and the mixture flowrate for shear thinning fluids are relatively small over the range of values considered, particle size and solids concentration have a significant bearing on the flow regime, the uniformity of the normalised particle radial distribution and of the normalised velocity profiles of both phases, and the magnitude of the solid–liquid pressure drop. The maximum particle velocity is always significantly less than twice the mean flow velocity for shear thinning fluids, but it can exceed this value in shear thickening fluids. In vertical down-flow, particles are uniformly distributed over the pipe cross-section, and particle diameter and concentration have little effect on the normalised velocity and concentration profiles. Pressure drop, however, is greatly influenced by particle concentration.

Modification of ilmenite surface chemistry for enhancing surfactants adsorption and bubble attachment

In this study, microwave irradiation is used to modify ilmenite surface chemistry to enhance the adsorption of surfactants and the air bubble attachment. The results indicate that microwave irradiation can increase ilmenite flotation recovery by 20%. A positron emission particle tracking technique is used to study the dynamic behaviour of ilmenite particles in a Denver cell. The data shows that the poor flotation recovery of ilmenite is not only due to the reduce probability of ilmenite being captured by air bubbles, but also the short residence time of the particles remaining in the froth phase. The ilmenite particles can be frequently captured by air bubbles, but dropped to the bulk liquid from the froth phase, normally over 15 s. Microwave irradiation changes the ilmenite flow pattern in the Denver cell. The average time of ilmenite remaining in froth phase is increased from 11.5 to 29.1 s.

Positron emission particle tracking—Application and labelling techniques

The positron emission particle tracking (PEPT) technique has been widely used in science and engineering to obtain detailed information on the motion and flow fields of fluids or granular materials in multiphase systems, for example, fluids in rock cracks, chemical reactors and food processors; dynamic behaviour of granular materials in chemical reactors, granulators, mixers, dryers, rotating kilns and ball mills. The information obtained by the PEPT technique can be used to optimise the design, operational conditions for a wide range of industrial process systems, and to evaluate modelling work. The technique is based on tracking radioactively labelled particles (up to three particles) by detecting the pairs of back-to-back 511 keV γ-rays arising from annihilation of emitted positrons. It therefore involves a positron camera, location algorithms for calculating the tracer location and speed, and tracer labelling techniques. This paper will review the particle tracking technique from tracking algorithm, tracer labelling to their application.

Granular flow fields in vertical high shear mixer granulators

AbstractThis study aims to elucidate some basic features of the granular flow field in a conical frustum‐shaped vertical high shear mixer granulator by using the Positron Emission Particle Tracking technique. The tested range of particle size (d50 = 23, 43, and 60 μm) and shape as quantified by the angle of internal friction (26° and 38°) have a negligible effect on the granular flow field in the mixer granulator. Density of particle imposes great influence on the flow field where reducing density from 2600 to 700 kg m−3 increase the tangential velocity by 1.95 time. Decreasing fill level from 3.5 to 0.9 kg increases the tangential velocity by a factor of 1.19. No obvious discrepancy in the solids motion is seen between machines of 1 and 5 L capacity and both machines have similar mixing and dispersion with constant tip speed criterion. © 2007 American Institute of Chemical Engineers AIChE J, 2008

Prediction of bubble behaviour in fluidised beds based on solid motion and flow structure

Abstract It has been demonstrated that the non-intrusive positron emission particle tracking (PEPT) could be a potential technique for observing bubble flow pattern, measuring bubble size and rise velocity in bubbling fluidised beds according to the solid motion in bubble and its wake. The results indicate that the behaviour of air bubbles varies greatly with the bed materials and superficial gas velocity. Three types of bubbling patterns (namely A, B and C) have been reported in this study, in which the pattern C is observed when the polyethylene fluidised bed is operated at the superficial gas velocity ( U − U mf ) of 0.25–0.5 m/s and the ratio of bed height to bed diameter is unity. After the comparison of the results measured by the PEPT technique with the values calculated by using a number of empirical correlations, two modified correlations are recommended to calculate the bubble size based on the PEPT data.

The effect of bed materials on the solid/bubble motion in a fluidised bed

Gas fluidisation provides good mixing and contact of the gas and particle phases as well as good heat transfer. These attractive features are achieved by the high degree of bubble-induced particle circulation within the bed. Bubble and particle motion vary with bed materials and operating conditions, as investigated in the present study, by the use of the non-intrusive positron emission particle tracking (PEPT) technique. The selected materials were spherical polyethylene and glass particles. The data obtained by the PEPT technique are used to determine the particle velocities and circulation pattern. Bubble rise velocities and associated sizes can be inferred from the particle velocity data, since particles travel upwards mostly in the bubble wake. The results indicate that the flow structure and gas/solid motion within the fluidised beds were significantly different, even at the same value of the excess gas velocity, U - U mf . The solid circulation pattern within the beds differ: if for glass beads, a typical UCDW-pattern existed (upwards in the centre of the bed, downwards near the wall), the pattern in the polyethylene bed is more complex combining a small zone of UWDC movement near the distributor and a typical UCDW-pattern higher up the bed. Transformed data demonstrate that at the same value of excess gas velocity, U - U mf , the air bubbles in the polyethylene fluidised bed were smaller and rose more slowly than in the fluidised bed of glass beads, thus yielding a longer bubble residence time and improved gas/solid contact. For polyethylene beads, the size and rise velocity of air bubbles did not increase monotonically with vertical position in the bed as would be predicted by known empirical correlations, which however provide a fair fit for the glass beads data. Bubble sizes and solid circulation patterns are important parameters in the design of a fluidised bed reactor, and vary with the bed material used.

Avalanching of granular material in a horizontal slowly rotating cylinder: PEPT studies

A detailed analysis of the motion af a single particle in a slowly rotating horizontal cylinder in the avalanching mode has been carried out using the positron emission particle–tracking technique to follow the motion of a tagged particle. Cylinders of diameters 240, 390, and 488 mm, and sand of mean diameter 500 μm were used; the tracer was an irradiated sand particle. It was found that a tracer particle rotates with the bed in rigid body motion and then traverses the active surface region in a series of discrete avalanches before being absorbed once again into the rigid body rotation. The mean number of avalanches required to traverse the active region decreased from 3.5 to 4 at low rotational speed to 1 at higher rotational speed at the transition to the rolling mode.

Experimental verification of the scaling relationships for bubbling gas-fluidised beds using the PEPT technique

An experimental verification of the scaling relationships for bubbling gas-fluidised beds has been carried out by measuring particle cycle frequency. The Positron Emission Particle Tracking technique was used, which enabled non-intrusive measurements of solids motion in three dimensions with a spatial resolution of ∼2 mm under the conditions of this work. Three cylindrical beds (70, 141 and 240 mm ID) equipped with multiple orifice-type distributors were tested. It was shown that for geometrically similar beds, the pair of Froude numbers based on the minimum fluidisation and excess gas velocities was sufficient for similarity at the viscous limit. The experimental results showed little influence of the gas-to-particle density ratio, but a small effect of particle-to-bed diameter ratio when the change in the ratio was significant. The experimental results also indicated a possible relaxation of the viscous limit up to a particle Reynolds number of about 100. More experimental studies are needed to confirm the validity of such a relaxation.