Tank Geometry and Impeller Design

There is not only one optimal or unique tank design for each kind of process, since several designs may satisfy the process specifications [65]. In order to simplify design and minimize costs, standard reactor designs are usually considered sufficient for most processes. Based on experience, it has been found 

Jakobsen, Chemical Reactor Modeling, doi 10.1007/978-3-540-68622-4.7, Springer-Verlag Berlin Heidelberg 2008 

The axial impeller discharges fluid mainly axially, parallel to the impeller shaft. The fluid is pumped through the impeller, normally towards the bottom of the tank. Since the flow make a turn at the bottom, the velocity vectors fan out radially at approximately D/2 beneath the impeller. Then, the flow moves along the bottom and rises near the tank wall. Analyzing the flow pattern, one can see that a back-flow eddy region is formed directly under the impeller. Upon examining the flow around the axial impeller one can also observe that a large contribution to the inlet flow enters radially at the tip of the impeller blade. This pattern is shown in Fig 7.1. 

Examples of axial impellers are marine propeller and pitched blade turbine, shown in Fig 7.2. A three bladed marine t3rpe propeller is similar to the propeller blade used in driving boats. The propeller can be a side entering type in a tank or be clamped on the side of an open vessel in an off-center position. Axial flow impellers are used in blending and mixing of miscible liquids. 

In turbulent mixing unwanted phenomena such as solid body rotation and central surface vortices may occur. In solid body rotation the fluid rotates as if it was a solid mass, and as a result little mixing takes place. At high impeller rotational speeds the centrifugal force of the impeller moves the fluid out to the walls creating a surface vortex. This vortex may even reach down to the impeller resulting in air entrainment into the fluid [87]. 

However, the standard tank does not reflect an optimized geometry for all processes performed in stirred tanks. The determination of an optimal geometry for a given process is very difficult, hence the standard geometry has been viewed as a reference geometry and as a point of departure for novel studies. For turbulent mixing, a liquid height to tank diameter ratio (H/T) equal to 1 is often used as a 

Disk Style Flat Blade Turbine Commonly Referred to as the Rushton Impeller 

Sweptback or Curved Blade Turbine (a Bpirat Turbine) 

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In addition, the turbulent fluctuations set up a microscale type of shear rate. Microscale mixing tends to affect particles that are less than 100 /xm in size. The scaleup rules are quite different for macroscale controlled process in comparison to microscale. For example, in microscale processes, the major variables are the power per unit volume dissipated in various points in the vessel and the total average power per unit volume. In macroscale mixing, the energy level is important, as well as the geometry and design of the impeller blades and the way that they set up macroscale shear rates in the tank. 

Mass-Transfer Models Because the mass-transfer coefficient and interfacial area for mass transfer of solute are complex functions of fluid properties and the operational and geometric variables of a stirred-tank extractor or mixer, the approach to design normally involves scale-up of miniplant data. The mass-transfer coefficient and interfacial area are influenced by numerous factors that are difficult to precisely quantify. These include drop coalescence and breakage rates as well as complex flow patterns that exist within the vessel (a function of impeller type, vessel geometry, and power input). Nevertheless, it is instructive to review available mass-transfer coefficient and interfacial area models for the insights they can offer. 

Many elements must be taken into consideration designing mixing devices, basically determining the geometry and size of the tank, what type of impeller to use, the size of the impeller, the size and geometry of heat transfer equipment and if baffles are needed. 

The geometry of the system is first chosen, e.g., tank diameter, impeller design, D/T ratio, clearance, submergence, sparger size, impeller speed, and others. 

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