Take guesswork out of the mix

Selecting the most efficient mixer for a particular task can be the key to successful processing. These days, when ingredients and even formulations have to be declared, manufacturing technique is playing an increasingly vital role in maintaining competitive advantage and profit margins. That said, choosing the right mixer for the job can be a somewhat complex task. The huge variation in applications has led to an equally diverse array of mixing equipment.

For liquid/liquid and solid/liquid mixing applications, units fall into two broad categories: "in-tank" units such as agitators, sawtooth-blade, rotor/stator and closed-rotor mixers; and "in-line" devices such as static mixers, colloid and media mills, and pressure homogenizers.

Some tasks naturally suggest certain types of equipment. However, many other applications can be handled with equal success using mixers of various types. But which would be the most efficient, cost effective, reliable or versatile?

Technology increasingly is playing a role in helping make the right selection. Computational fluid dynamics (CFD) can provide detailed predictions of mixer performance. It can be a useful tool for design and selection of conventional agitators. However, present CFD technology cannot simulate the more complex flow patterns of other types of mixers.

Therefore, for more demanding tasks, such as particle-size reduction and the formation of emulsions and suspensions, practical testing remains the most reliable means of ensuring the right choice. Most mixer manufacturers can offer some form of testing service, which may include providing loan machines and in-house demonstration facilities for carrying out anything from laboratory trials to full-scale production runs. Additional technology, e.g., particle-size analysis equipment, also can offer insights.

Keep in mind that manufacturers are prepared to modify and customize units for particular jobs

The starting point
Where should you start when selecting a mixer for a particular process? The most common option is a top-entry in-tank mixer, also called an immersion or batch mixer. These come in the broadest variety of designs and can handle the widest range of capacities and viscosities. Many also can be supplied as either bottom- or side-entry units. These variants generally are more complex designs; they eliminate the problems associated with immersed shafts and difficult-to-clean scraper blades, frame arms, etc., and offer improvements in process hygiene.

The simplest form of in-tank mixer is an agitator, which consists of a rotating shaft with an impeller attached to its end. In some cases, the shaft may contain more than one impeller. Impellers come in an array of types, each of which is designed to maximize flow while minimizing the power requirement. They generally are designed to produce a certain type of movement in the vessel or for certain viscosity ranges. Axial turbines feature impeller blades pitched at an angle of about 45 Degrees that create upward or downward movement in the vessel. These are ideal for pulling light powders down from the top of the vessel, drawing dense materials upward from the base of the mixture, maintaining solids in suspension, etc. Radial turbines have blades parallel to the shaft. This arrangement draws material in from top and bottom and forces it out toward the vessel walls, which is good for promoting heat transfer. Both types can handle viscosities up to 75,000 cP.

Gate or anchor stirrer/scrapers (Figure 1) are more appropriate for higher-viscosity materials up to 500,000 cP. They frequently are teamed with another mixer type. For example, many processes using an anchor also include a sawtooth-blade disperser or rotor/stator mixer (see below), which actually do the job. The stirrer/scraper merely ensures that the output from the other unit is distributed uniformly throughout the vessel.

An important difference
There is an important distinction between the terms agitator and mixer. Agitators are low-shear devices best described as process aids because their main functions, i.e., producing flow in a vessel, promoting heat transfer and ensuring in-tank uniformity, are secondary to the process. Processing ingredients, which includes dissolving solids, dispersing powders into liquids, breaking down agglomerates, or combining two immiscible liquids, requires a mixer with a more positive action, such as the following designs.

Dispersers or sawtooth-blade mixers (Figure 2) consist of an impeller mounted at the end of a rotating shaft, but they generally operate at much greater speeds than agitators. The impeller is a disc edged with angular teeth, resembling a rotary-saw blade. The combined action of the teeth and high-speed rotation create powerful hydraulic shear forces, which are further increased by the fluid's flow resistance. So, this type of device is most effective on high-viscosity mixtures, typically between 10,000 and 50,000 cP. This includes the dispersion of powders and pigments into pastes, inks and paints. Batch sizes are limited to about 1,000 gal. due to power requirements and low flow generation; supplementary in-tank agitation often is required with high-viscosity materials. For this reason, many manufacturers offer sawtooth-blade mixers as part of a complete system that includes a vessel, slow-speed scraper/stirrer and other types of in-tank agitation required for the process.

Rotor/stator mixers consist of a high-speed centrifugal-type rotor mounted within a stator that is held in place by three or four frame arms. During operation, high-speed rotor revolution creates a powerful suction that draws both liquid and solid materials into the center of the workhead assembly. There, they are subjected to intense high shear. Centrifugal force then drives the materials to the periphery of the workhead, where they encounter milling action in the clearance between the rotor blade tips and the stator inner wall. Intense hydraulic shear follows as the materials are forced out through the openings in the stator and are projected radially at great velocity back into the body of the mixture.