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An Approach to Design and Performance Testing of an Impeller-Driven Single-Use Mixer
Bojan Isailovic, Bruce Rawlings
BioProcess International, Vol. 9, No. 8, September 2011, pp. 60–69
 
CRITICAL REQUIREMENTS FOR AN IMPELLER MIXER

In our studies on traditional mixer design, we identified the basic engineering principles and requirements that would be critical in development of a versatile, high-performance, mechanically driven impeller mixer for single-use applications. The most important properties (and their significance) can be summarized as follows.

Power Input: Mixing efficiency is strongly influenced by power input (transfer of energy into the mixing process). The benefit of an impeller directly coupled to a motor is that power input is maximized and can be precisely controlled and adjusted. Factors influencing power input include fluid density, impeller power number, speed, and diameter.

Flow Patterns: A flow pattern that distributes fluid rapidly and efficiently throughout a container is an important requirement. This pattern must ensure an absence of dead zones, especially in the container extremities. Flow must accommodate mixing of low-density powders on a liquid surface and of high-density solids in the base of the container, as well as liquid–liquid mixing and high-viscosity fluids. These are very different and demanding requirements.

Flow regimes are defined by turbulent and laminar-flow components. Strong turbulent flow is desirable when using turbine impellers and can be advantageous for many mixing applications. On the other hand, some impellers operate well in a laminar flow regime (e.g., close-clearance impellers in high-viscosity fluids).

Shear forces carry out mixing processes. In an impeller system, shear forces represent shear stress and are induced primarily by a combination of two effects:



•Fluid velocity and relative motion of liquid layers within a container (the dominant component for impeller mixers in most, but not all, fluids)

•Shear forces in the immediate vicinity of impeller blades (where a small amount of fluid experiences high shear).

Container Geometry: The shape and dimensions of a container are critical properties that can substantially influence flow patterns and mixing performance. Most stainless steel, stirred tanks are cylindrical with a liquid height:diameter ratio greater than one (H/T > 1), and they often have dished bottoms to prevent accumulation of solids in their corners (Figure 5). This cylindrical shape can induce undesirable tangential flow or swirling of the liquid resulting in reduced power draw by the impeller and lower mixing efficiency, but such effects can be minimized by the introduction of wall baffles.


Figure 5:


By contrast, many single-use containers have a rectangular cross-section (Figure 6), with a 3D profile that is either cuboid (H/T = 1) or rectangular (H/T < 1). This noncylindrical profile can be beneficial for two reasons: First, the reduced height can improve top-to-bottom, axial-flow distribution; and second, the rectangular profile can minimize undesirable tangential flow, the corners acting like a baffle to break up the regular flow pattern and promote vertical flow.


Figure 6:


We considered other properties and features in our design approach, as well: for example, dynamic sealing of an impeller shaft, access for liquids and solids, external hardware (tote), materials of construction, and control systems. Those aspects are described below.

APPLYING CRITICAL REQUIREMENTS FOR DESIGN AND FUNCTION

First and foremost, we wanted to ensure that our new design would be based on proven engineering principles for mixer performance. We first approached this objective through a detailed research study into the scientific and engineering principles of mixing processes and system design. This study involved a substantial amount of engineering expertise and experience within our working environment, together with the application of good engineering practice (GEP) during development and testing (5).

Container Design: We based our container design on an existing three-dimensional (3D) single-use biocontainer, using the same film material as it used. This approach enabled us to benefit from proven performance and qualification studies established for the existing product. However, that container's geometry — a rectangular cross-section, a shorter height than width, and a flat bottom — is different from that of a conventional cylindrical mixing vessel. Those characteristics required that special features be incorporated to ensure achievement of the required mixing properties and performance.

Impeller Type: Designing an ideal impeller for the wide range of intended mixing duties was a critical part of this project. To help explain the basis for our final decision, we summarize below the main types and properties of general-purpose impellers, which fall into three main categories: radial flow, hydrofoil, and axial flow.

Radial-Flow Impellers: An example of a radial-flow impeller is a paddle stirrer with rotating vertical flat blades. This design discharges fluid predominantly radially (horizontally) toward the vessel wall. It can create higher shear and turbulence at the blade surface than some other types will, but it also provides low pumping.

Radial action produces flow patterns (Figure 7) with two circulating flow loops, one above and one below the impeller. Mixing occurs between those loops. Such a flow pattern is not ideal because it causes stratification or compartmentalization in a tank, resulting in poor homogeneity of the bulk fluid. Although the flow pattern can be converted to a strong top-to-bottom flow using suitable baffles, we did not consider this type of impeller to be ideal for a general-purpose, single-use system.


Figure 7:


Hydrofoil impellers contain tapered, twisted blades. Their flow pattern has a greater vertical (top to bottom) distribution than flat-blade impellers do, and the flow is very uniform and streamlined, with lower shear than other types have. Although these properties are favorable for many mixing applications, the flow pattern cannot be reversed by changing the rotation of the hydrofoil. For reasons discussed below, a reverse-flow feature was an important requirement for our mixer, so we did not select a hydrofoil impeller.

Axial-Flow Impellers: An example of an axial flow impeller is a pitched-blade impeller with angled blades (Figure 8). This generates both axial and radial flow in low- to medium-viscosity fluids, as shown in Figure 9 with a down-pumping flow. The impeller produces slightly higher shear at its blade surface than a hydrofoil does, which gives a good balance between pumping and shear action. So this is considered to be a good general-purpose impeller. Therefore, we chose a pitched-blade design with four blades for our development program.


Figure 8:



Figure 9:


Impeller Blade Pitch Angle: The angle of a blade's pitch influences several mixing properties, from the balance between axial and radial flow to the pumping capability and shear rate. These combined effects affect both mixing performance and flow patterns of a mixer.

One other key factor helped determine our final selection of pitch angle: namely the requirement to operate a mixer in both down-flow and up-flow pumping directions by reversing the motor and shaft rotation. This would provide greater flexibility for use. To ensure the same pitch angle for both pumping directions, we incorporated a 45° angle. The benefits of this feature are discussed below.

Blade Diameter: The ratio of blade diameter to vessel diameter (D/T) is an important parameter because it affects flow pattern and power input — and consequently mixing efficiency. D/T values of 0.33–0.50 have been studied and used in design and operation of conventional stirred tanks. Larger blade diameters (higher D/T ratios) provide higher power input at lower speeds, which improves mixing. However, it also increases radial flow at the expense of axial flow, which changes the flow pattern and can negatively affect mixing performance. With D/T ratios of 0.55 and higher, flow becomes predominantly radial and not ideal for general-purpose mixers.

As with many variables, it is a matter of arriving at the optimum balance for a given application. For example, we sized the blade for a 200-L rectangular container to give a D/T ratio of 0.41 and 0.55 for the respective wider and narrower container dimensions we offer. That range ensures good power input and sufficient axial flow.

Impeller clearance from the tank bottom needs to be taken into consideration in a mixer design. The ratio of clearance to liquid height (C/H) is an important parameter that ranges typically from 1/3 to 1/6. For down-flow pumping impellers, a very small clearance can cause predominantly radial flow with high shear and reduced pumping. But that may be beneficial for sweeping solids from the tank bottom — at the expense of bulk homogeneity.

For our single-use system, we found a C/H ratio in the region of 1/4 to be optimal for mixing performance in the wide range of duties defined by user requirements. Nevertheless, we set the ratio at 1/7. Lowering the impeller nearer to the base of the container allows for use of smaller working volumes without exposing the impeller above the liquid surface. Such exposure can generate excessive turbulence and undesirable effects on sensitive products. In-house mixing tests at this clearance showed satisfactory mixing efficiency. This feature can also assist in the drainage phase of a mixing process to ensure homogeneity of the whole solution and good product recovery.


Figure 10:


Pumping Direction: We considered the ability to operate an impeller in a down-flow or up-flow pumping direction to be beneficial for extending the range of mixing duties that our system could fulfill. By reversing the motor rotation, our pitched-blade impeller with its 45° angle could provide this function.

In many applications, counterclockwise rotation is preferred for down-flow pumping. It has the specific benefit of creating an area of high turbulence below the impeller to lift solid particles that may rest in the container bottom.



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