Using Positron Emission Particle Tracking (PEPT) to Study Mixing in Stirred Vessels
Mostafa Barigou, Paulina Pianko-Oprych, Luke Adams, Antonio Guida, Fabio Chiti, and Alvin W. Nienow. Department of Chemical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom
Positron Emission Particle Tracking (PEPT) is unique in flow visualisation terms, being able to examine flow phenomena quantitatively in three dimensions in systems that cannot be studied by techniques such as PIV and LDA because they do not allow the passage of laser light. Slurries are a typical case. They are found in many industries including minerals, food processing, household and personal care products. Indeed, some 80% of products in the chemical industry, for example, are of this type. The suspension of solids in low viscosity fluids is one common example and is critical for promoting chemical reaction between the two phases, or for enhancing crystal growth. In the latter, a wide size range is found and establishing their spatial distribution as function of size has proved difficult though it is important. PEPT is able to do this and also determine the velocity of each of the two phases. In many other cases, especially in food processing, household and personal care products, slurries exhibit non-Newtonian, paste-like behaviour. Little work has been done to understand how to mix them effectively. The reason for this is their opaque nature. Therefore, it has been assumed that knowing the rheology of the slurry enables the prediction of its behaviour, based on studies on transparent polymer solutions with similar rheological properties. Again, PEPT can tackle such issues, giving a full flow field, viz., the three velocity vectors throughout the vessel. Such work has been carried out on paper pulp (a yield stress fluid), a personal care product (yield stress and viscoelastic) and on a china clay slurry (shear thickening}. All this work will be summarized, along with a critique of the technique and a study to establish its accuracy, based on a comparison with the extensive literature on Rushton turbines in water.