Particle motion in relatively thin fluidised bed models

Bubble motion is the primary means of inducing particle motion in fluidised beds. The objective of this study is to link bubble motion to the particle motion. The technique used is positron emission particle tracking (PEPT), which enables a single particle tracer to be tracked in milliseconds and in 3D movement. Features investigated include particle velocity and particle “jump” behaviour.     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

Positrons as in situ probes of transient phenomena

Techniques such as positron emission tomography (PET), positron emission particle tracking (PEPT) and positron emission profiling (PEP) are currently used to perform in situ studies of physical and chemical phenomena that occur within chemical reactors and other processing equipment under process conditions. PEPT is capable of measuring the flow patterns inside mixers and fluidized‐bed reactors by tracking a labelled particle. Currently a particle as small as 500 μm moving at 0.1 m/s can be continuously located with a resolution of 5 mm. The ability to perform such measurements greatly aids in the development and optimisation of such systems. PET and PEP are being employed to perform measurements of the concentration of molecules labelled by ¹¹C, ¹³N and ¹⁵O inside packed‐bed reactors. The measured concentration distributions, as a function of position and time, are used as input for mathematical models based on the kinetics of the physical (e.g., diffusion, adsorption) or chemical processes occurring. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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.     

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

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.     

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.     

PEPT for agglomeration?

Positron emission particle tracking, generally known as PEPT, is an imaging method for following the motion of a single tracer particle in a dense medium; this particle may be as small as 100 μm. The imaging method has been applied to many operations, among these being the mixing of dry powders and granular materials. There is one class of equipment in which mixing is imparted by the use of blades that has much in common with equipment used for agglomeration. From studies on dry mixing, it is possible to deduce some of the mechanisms governing behaviour and a physically based approach to design and operation is now becoming possible. Little is presently known from applying PEPT to agglomeration. However, PEPT will be of direct value in determining the occupancy of material in equipment as a function of position and time; it will provide similar information on the velocity field. It is capable of development for use on commercial scale agglomerators. While it does not allow direct observation of the collision processes that are critical for growth and breakage, PEPT does provide a quantitative basis from which logical inferences are possible.     

On trajectory and velocity measurements in fluidized beds using positron emission particle tracking (PEPT)

Positron emission particle tracking (PEPT) is a non-invasive technique that can be used for following the trajectories of particles in fluidized beds, so increasing understanding of solids motion in fluidized bed processes. We describe how PEPT is applied, how its performance is optimized, and how trajectory information can be built up into instantaneous and time-averaged measures of particle movement. Choices and pitfalls in data processing are explained and illustrated by reference to the travelling fluidized bed (TFB) collaboration initiated by Professor John Grace.     

PEPT combined with high speed digital imaging for particle tracking in dynamic foams

Positron emission particle tracking (PEPT) has been combined with high speed digital imaging to track a particle within a foam column. The tracer was sufficiently large (diameter 2.5 mm) to allow visual verification of the tracer trajectory recorded with PEPT. This enables validation of the technique for use with smaller, less visible, tracers in the future. A difference in recording rates of PEPT and the high speed camera necessitated the use of a weighting function to interpolate the discrete PEPT data set into a continuous function. A kernel width of 200 ms was used to ensure a confidence level of 95% that the tracer position calculated from PEPT and measured visually were the same, within error limits of ±2.7 mm. The largest contribution to the error was the resolution of the images. Images of dynamic foam structure can now be paired with PEPT measurement to observe the tracer trajectory relative to individual bubbles.     

Measurement Techniques to Monitor and Control Fluidization Quality in Fluidized Bed Dryers: A Review

Fluidized bed dryers (FBD) are commonly employed in many industries to dry particulate solids. FBDs provide good solids mixing, high rates of heat and mass transfer, and relative ease of material handling. For efficient operation, it is important to be able to monitor and control the fluidization regime, particle size distribution (PSD), moisture content, and bulk density as well as product chemical properties. This review provides an overview of the trends in the application of different experimental techniques to monitor and control the hydrodynamic conditions of FBDs which influence the particle physiochemical properties. This review covers a wide range of measurement techniques, including infrared moisture sensor (IR), near infrared (NIR) spectroscopy, analysis of pressure fluctuations, optical imaging techniques, acoustic emission (AE), electrical capacitance tomography (ECT), spatial filter velocimetry (SFV), Raman spectroscopy, focused beam reflectance measurement (FBRM), microwave resonance technology (MRT), triboelectric probes, positron emission particle tracking (PEPT), and some novel techniques for monitoring and control of FBDs. The present review summarizes the use of the diverse techniques and outlines their merits and limitations. Prospects for future research in this area are also identified. The measurement techniques can be used for research, development, and operation of fluidized bed equipment used in non-drying applications as well.     

Particle flow in a hydrocyclone investigated by positron emission particle tracking

The flow of a particle through a hydrocyclone acting on water has been studied by positron emission particle tracking (PEPT). The positron-emitting radioactive tracer was ¹⁸F. It was found that the activity on an ion-exchange resin particle labeled with ¹⁸F did not leach out into the water during the duration of an experiment. In the state-of-the-art PET camera it is shown to be possible to locate the centroid of the tracer particle with a standard deviation of only about 0.2 mm once per ms, making both the temporal and spatial resolution high enough to trace the particle in its very fast motion through the hydrocyclone. The design of the hydrocyclone was a modified Stairmand high-efficiency geometry with a long cone. The results are, among other things, shown as spatial tracks of the tracer particle as it moves through the hydrocyclone. Several interesting features were seen. The particle path, although the particle was much larger than the cut size of the cyclone, exhibited excursions into the inner, upwardly directed, part of the vortex giving rise to recirculatory loops. Moreover, at a particular position low in the cyclone, the particle exhibited a complicated flowpattern moving up and down repeatedly across this position. Careful analysis of the motion is presented, particularly of the motion low in the hydrocyclone, on basis of which it is made likely that this position represents the end of the vortex in the hydrocyclone.     

The Use of Positron Emission Particle Tracking in the Study of Multiphase Stirred Tank Reactor Hydrodynamics

This paper describes the use of positron emission particle Tracking (PEPT) in the analysis of local particle and fluid velocities in solid‐liquid stirred tank reactors agitated with a Rushton turbine and an upward‐pumping pitched blade turbine. PEPT captures the full three‐dimensional characteristics of hydrodynamics and mixing in stirred vessels, allowing the analysis of the two‐phase flow fields. Furthermore, by comparing the liquid and particle velocities, the spatial and temporal variation of the relative particle‐liquid velocity can be estimated. Such information reveals considerable heterogeneity in the vessel and facilitates the evaluation of impeller design, particularly with the aim of minimizing mass transfer limitations.     

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.     

Using the discrete element method to predict collision-scale behavior: A sensitivity analysis

Discrete element method (DEM) simulations have recently been used to investigate collision-scale measurements such as collision frequency and impact velocity distributions. These simulations are typically validated against particle velocity fields using experimental techniques such as particle image velocimetry or positron emission particle tracking. An important question that has not been addressed is whether validation of a macroscopic velocity field or solid fraction field also implies a validation of collision-scale measurements such as collision frequency. In this study, DEM measurements of solid fraction, shear rate, collision frequency, and impact velocity are made in a small region just beneath the free surface in a rotating drum. The effects of periodic drum length, particle stiffness, coefficient of restitution, and particle size are investigated. The solid fraction and shear rate do not vary with particle stiffness or coefficient of restitution over the range of values studied. However, the collision rate increases with increasing particle stiffness and coefficient of restitution. In addition, the average collision speed decreases as particles become stiffer or less elastic. The shear rate varies with particle size, but the average collision velocity remains constant. These findings indicate that validation against particle velocity and solid fraction fields does not necessarily imply validation of collision frequency and impact velocity. Indeed, the velocity and solid fraction fields were found to be relatively insensitive to a range of DEM contact stiffnesses and coefficients of restitution while the collision distributions were sensitive.     

Solids motion in a conical frustum-shaped high shear mixer granulator

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 attached to which are 4 sets of impellers at different elevations. The shaft is operated at 3, 6 and 12 Hz, which correspond to the top impeller tip speed of 2.1, 4.1 and 8.3 m/s. Particles are observed to circulate in both the horizontal and vertical directions. The period of horizontal circulation is short and is in the order of seconds, whereas that of the vertical circulation takes tens of seconds and often consists of lots of higher frequency fluctuations. There is a dominant solids motion in the tangential direction at all impeller speeds with the maximum tangential velocity 2.2-5.3 times that of the maximum axial and radial velocities. The maximum values of the three velocity components increase with increasing impeller speed, but the ratios of the maximum velocity to the tip speed decreases with increasing impeller speed, suggesting a rate-dependent behaviour. The particle flow pattern shows the presence of swirling flows at a position depending on the impeller speed. The results also suggest the existence of an optimal impeller speed that gives the best macroscopic mixing characterised by the vertical solids circulation.     

Development of positron emission particle tracking for studying laminar mixing in Kenics static mixer

Positron emission particle tracking (PEPT) is a flow visualisation technique that has found application in a wide range of processes. In this work, PEPT has been used to study laminar flow of a high viscosity Newtonian and non-Newtonian fluid in a Kenics static mixer (KM). Through analysis of the trajectories of many hundreds of passes of the tracer particle through the mixer, it is possible to compute the overall flow field and to visualise how the fluid twists and folds as it passes along the mixer. Eulerian velocity maps plotted for the Newtonian and non-Newtonian fluids showed that the length required for the flow to develop is shorter for the non-Newtonian fluid than the Newtonian. The stretching and folding mechanism of mixing was observed by grouping the trajectories into clusters according to whether the trajectory passes to the left or right of the blade at the transition between elements. Those trajectories making the same L–R–L decision tended to remain in the same striation through two or three elements until that striation became stretched and folded back on itself, sandwiching other layers. It is clear that the PEPT data is rich and powerful. We are hopeful that the techniques we develop for the flow and mixing in the Kenics mixer will be applicable to studying more complex laminar flows.     

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.