Chapter 1 Introduction
Chapter 1
Introduction and scope of the work
Continuously and batch stirred vessels are the most common mixing equipment in the chemical industry, frequently encountered in chemical processes which involve liquid blending, chemical reaction, gas dispersion in liquids, solid suspensions in liquids, as well as heat and mass transfer.
For this reason the study and the comprehension of the mechanisms behind mixing are of primary importance. For fast and competitive reactions, which occurs in liquid phase, the effect of an improvement in the mixing of the system, directly leads to higher selectivity and yield of the reactions, thus to a lower by-product formation. Mixing has direct effects on the economy of the production. The complex nature of the flow field inside stirred tanks reactors, is the reason for the great number of studies that have been done, in order to understand all of the effects on process operations.
This work is concerned with the characterization of the hydrodynamic of stirred baffled vessel, studying the differences in the flow due to the change in the pumping configuration. The use of impellers in the up-pumping configuration rather than the commonly used down-pumping configuration, changes drastically the dynamics within the vessel. This radical choice in the flow configuration may leads to significant advantages. (Chapman C.M et al., 1983). (RIPARTIRE DA QUA )
In the present work, mixing is achieved through the use of a 45° 6 blades Pitched Blade Turbine (PBT), focusing the attention on the differences in the flow, due to the well established and commonly used down-pumping configuration and the less known and thus studied possibility of adopting the up-pumping. The power draw together with the power number, have been evaluated to provide the same power input per unit of mass in both the up-pumping and down up-pumping configurations, in order to obtain comparable properties for the two set ups.
Chapter 1 Introduction
The flow field on which are based the observations has been obtained with the use of a two-dimensional particle image velocimetry (PIV), in the next chapters is fully described the principles behind this technique and its limits.
The most effective zones within a stirred tank, where to obtain a complete mixing in short time are located in the most energy dissipative areas. Due to the small length scales were the turbulent kinetic energy dissipates, is not possible to fully measure its values. As is further explained in the next chapters, different methods, based on different approaches exist for its estimation, and so a comparative study on the energy dissipation calculation and its effects on mixing have been carried out. Moreover a variable integral length scale has been evaluated through the use of an autocorrelation.
The use of a synchronizer together with a trigger has made possible to acquire images with PIV, at certain blade angles. The angle resolved analysis technique leads to fully evaluate the actual flow around the impeller, highlighting the information that otherwise would have been hidden by the time averaged analysis. The chosen degree of separation, gives higher spatial resolution compared to the previous work (Kahn F.R, 2005).
Angle resolved vorticity has been evaluated as function of the angle with the blade, to measure the distance of the center of the first loop vortex from the centre of the impeller.
