Chapter 1: Introduction
3
1
Introduction
1.1 Motivation
Mixing in stirred tank reactors is one of the most common operations carried out in the manufacture of fine chemicals, pharmaceuticals, polymers, food, and in water treatment. The term “mixing” refers to the processes used to reduce the degree of non-uniformity, or gradient of a property in a system such as concentration, viscosity, temperature and so on.
Even in a geometrically simple stirred tank the flow, under most operative conditions, is fully turbulent and the hydrodynamics is very complex.
At present, process design is usually based on laboratory or pilot-plant experiments from which scaling up is performed and involves the choice of a series of parameters.
Nevertheless, for this purpose, more modern approaches such as Computational Fluid Dynamics have been developed but, even in this case, the physical models used require information on local flow characteristics. Chemical reactions present an additional problem due to the interaction between the hydrodynamics and the chemistry of the system, which determines the physical properties of the final products. In these situations mixing plays a critical role.
Reliable measuring techniques are therefore needed as well in academia as in industry for the rational description and the design of multiphase reactors.
Chapter 1: Introduction
4 In recent years Particle Image Velocimetry contributed significantly to mixing study by enabling a deep insight into the velocity fields in stirred tanks.
However, a continuous progress is the key to the complete understanding and prediction of such phenomena.
1.2 Objectives
Two new PIV techniques have been developed to obtain qualitative and quantitative information on the hydrodynamics of stirred vessels.
The former involved a liquid-solid system and was focused on the set-up of the parameters for a good data acquisition and processing. Most of the typical problems commonly encountered in analysing such systems are fully explained and discussed.
In the latter data concerning the flow fields of the impeller swept volume have been acquired with a completely new High Frame Rate PIV system and the structure of the trailing vortices generating from behind the blades both of a Rushton turbine and of a Pitched Blade turbine have been figured out.
Chapter 1: Introduction
5
1.3 Thesis Layout
Each chapter of this thesis is divided into two main parts: the first one regarding the study of the multiphase system, and the second one regarding the new High Speed PIV.
Chapter 2 provides an introduction and a literature survey to the topics previously explained.
Chapter 3 gives a full description of the experimental equipments and techniques employed during the all works.
The results obtained are reported, firstly, in Chapter 4, with a brief analysis and then in Chapter 5, where they are compared with those present in the literature.
In Chapter 6, finally, conclusions are given with recommendations for future works.
