PEPT - Positron Emission Particle Tracking

PEPT refers to the technique of Positron Emission Particle Tracking. This is particularly applied to the study of multiphase interactions in hydrocyclones. In this project we combine high performance computing for the simulation of hydrocyclones together with the experimental study of the performance of the hydrocylones using PEPT. This dual computational-experimental approach allows the bench- marking of the code which encapsulates the understanding of the key parameters and concepts affecting cyclone performance and optimisation. There is IP encapsulated in the physics and engineering concepts and their embodiment in code developed by us and also in the preparation of hard, sub-millimetre PEPT sources using direct irradiation at iThemba LABS. The iThemba LABS Tandem Accelerator in Gauteng is the only facility in the country where the direct irradiation required can be carried out, leading to PET isotopes pre-encapsulated in ultra-hard materials with exceptionally high specific activity.

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  1.   1.  Introduction
  2.   2.  PEPT Sources
  3.   3.  CFD Modelling
  4.   4.  Hydrocyclone apparatus

1.  Introduction


A cyclone separator is used in the metals and minerals processing industry. Its primary purpose is to separate solid (metal/mineral) particles by size (diameter) or density from a slurry (liquid-solids mixture). Another use is for the removal of particulates (solid/liquid particles) from a gas or liquid stream. The gas or liquid stream with entrained particles are pumped into the inlet. The large diameter or high density particles are forced to the outer walls of the cyclone and out through the underflow. The smaller or less dense particles are carried out the overflow. Positron Emission Particle Tracking (PEPT) is a technique used to study the flow field or particulate trajectories inside a system (usually engineering equipment). PEPT is predominantly used to investigate particulate and granular flows in process equipment such as hydrocyclones and ball mills as in references, respectively. However, it can be used to investigate fluid flow in general. PEPT is based on Positron Emission Tomography (PET). In the case of PEPT, a radioactively labelled tracer particle is released into the flow field with other non-labelled particles. The tracer particle undergoes beta-plus-decay, thus leading to the emission of a positron from the nucleus. The positron annihilates with an electron, releasing energy most often in the form of two back-to-back emitted 511 keV γ-rays. Detectors mounted around the system detect the γ-rays. Thus, a line is reconstructed between the detectors, known as a Line of Response (LOR). LORs are reconstructed within a small time step (on the micro-second scale). Using various tomographic algorithms, the position of the particle is determined for each small time-slice. Ultimately, the full 3D trajectory of the particle is detected. By differentiating the trajectory the full 3D velocity field can be obtained. The 3D density distribution can be established by integrating the trajectories.

2.  PEPT Sources

One aspect of this work is the production of a PEPT source in a 50 μm, insoluble and hard ceramic material using direct irradiation to produce tracer particles (sources). These PEPT sources must be hard as they are to be used in process equipment such as hydrocyclones where the particle can breakup or grind down due to collision or friction forces, respectively. Ideally the tracers should have a relatively short half life, ideally between twenty minutes and one hour. This will ensure that a new PEPT experiment can be done, safely, by the next day without any radiation safety concerns. Our PEPT micro-sources have been prepared using activation via a microprobe coupled to an EN-Tandem Accelerator where the target is a laser machined cylinder cubic boron nitride of diameter and length 50 μm. The activation is via the 14N(p,α)11C reaction, and the maximum cross-section is 200 mb (at Ep ~ 7 MeV). Irradiation is performed with a beam diameter of 20 μm and a beam current of 100 nA. The activity achieved in about 100s of irradiation was 100 μmCi. The source was there 11C and it had a half life of 20 min, leading to an ideal PEPT source for these measurements.

11C PEPT Source

3.  CFD Modelling

CFD Modelling

The aim of the research project is to derive and propose a new or modified physical and mathematical model capable of accurately describing the effect of solid particles on the gas or liquid flow field (particle- fluid interaction), particle-particle interaction and particle-wall interaction in gas-solid or liquid-solid flows applicable to cyclone separators. The model should provide a microscopic and a macroscopic description of the flow field. The models should be applicable to flows with small and large solids volume fractions. The model should be computationally tractable so that it can be used to simulate industrial sized cyclone separators. PEPT plays a pivotal role in validating these models as it provides a more detailed description of the flow field and particle trajectories. Thus, the model will be experimentally validated using PEPT.

4.  Hydrocyclone apparatus

The cyclone test rig has modifications to enable PEPT measurements safely.

Multotec Hydrocyclone modified for PEPT experiments