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.
The size and shape of the openings in the stator (often referred to as the stator geometry) and the clearance between the rotor blade tips and the stator inner wall determine the flow pattern and the machine's shear rates. For example, a stator with round holes gives an exceptionally vigorous mixing action, particularly well-suited for disintegrating solids and preparing gels, suspensions and solutions. Slotted holes produce a more scissor-like shearing action, appropriate for disintegrating elastic or fibrous materials. Fine screens would be used where a high degree of particle- or globule-size reduction is required and for preparation of fine colloidal suspensions and emulsions. With simple reconfiguration, such as a switch of stator, a single machine can carry out duties that otherwise would require several different pieces of processing equipment.
As with sawtooth dispersers, the effectiveness of in-tank rotor/stator mixers depends somewhat upon batch size, as the power requirement for a high-speed unit capable of circulating large volumes can be uneconomical. Despite this, specialized units (such as bottom-entry disintegrators) can handle batches in excess of 20,000 gal.
Closed-rotor mixers fall somewhere between agitators and high-shear devices. These high-speed units feature a mixing head with rotors or pumping elements at the top or bottom of the head that draw material into the head. There, the materials are subjected to hydraulic shear forces before being driven out of the head by centrifugal force. Further hydraulic shear is applied to the material as it is projected back into the body of the mixture. Designs range from sanitary single-piece mixing heads to more complex units that feature stator assemblies or paddles attached to the shaft. Typically, closed-rotor mixers handle powder/ liquid dispersing, dissolving and gelling applications, batch sizes up to 2,000 gal. and viscosities up to 25,000 cP.
The question of batch size raises an important issue. Mixing equipment that operates immersed in the mixture performs two separate functions: the mixing task itself, be it disintegrating and solubilizing solids, creating fine emulsions or suspensions, dispersing powders or blending liquids of widely different viscosity and, secondly, circulating the contents of the vessel. Simple agitators can handle very large batches but only are effective in producing flow, promoting heat transfer and maintaining in-tank uniformity. In certain circumstances, achieving desired results might require combining agitators with other in-tank mixer types. However, as the batch size increases so does the power requirement, which then can make an immersion mixer uneconomical. One solution to this is to decrease the speed , but this also reduces the efficiency of the machine. Some applications that require an extremely high power-to-volume ratio simply could not be achieved using a large-scale immersion mixer. A cost-effective approach to this problem is to employ an in-line unit.
Figure 1. An anchor can handle fluids up to 500,000 cP and frequently is used with another type of mixer.
These devices operate outside of the vessel in a pipe that can be configured for single-pass processing, recycling material around a vessel, or passing it between two vessels. This means that material can be processed in a defined amount. The design should make bypassing impossible, so all the contents of the vessel pass through the mixer.
The simplest in-line devices are static mixers. They consist of a series of baffles that divert material flow, blending the mixture as it passes through the pipe. Static mixers have no moving parts and are by nature very low maintenance. They are ideal for applications such as in-line blending of like-viscosity liquids. On the downside, they are very low-shear units that require feeding by an auxiliary pump. They also can present some problems where process hygiene is an issue because the baffles may be difficult to clean.
In-line agitators or turbines also are relatively simple. They consist of an impeller of one of a variety of designs mounted on a short shaft set in a chamber in the pipe. They can put more work into the material than static mixers and can handle slightly more demanding, simple blending duties, such as dealing with liquids of dissimilar viscosity. Each unit still requires an auxiliary pump.
In-line rotor/stator mixers offer high-shear processing across a range of viscosities, from up to 5,000 cP as a self-pumping unit to more than 100,000 cP with additional pump assistance. These mixers employ a workhead, similar to that of their in-tank counterparts, mounted in a small chamber in the pipe. The machine's effort is concentrated on the small amount of material inside the workhead at any given moment; therefore, power is not wasted moving large volumes. Consequently, a unit can process capacities that would require a much larger immersion-type mixer. A single rotor/stator mixer can handle a variety of jobs simply by changing the stator geometry to alter mixing characteristics. Multistage units, in which the workhead features a series of concentrically mounted rotors set within a corresponding series of stators, are increasingly used for applications requiring intense high shear.
Colloid mills employ the rotor/ stator principle in that they consist of both rotating and stationary elements within a chamber. This chamber is larger than that for a rotor/stator mixer and allows more fluid to be retained in the mixing zone; retention times are longer, imparting a great deal of energy to the material with each pass. Retention times and shear rates also can be controlled by varying the clearance or "grinding gap" between the rotor and stator, which can have coarse or fine teeth, or other configurations. These devices require pump feeding and primarily handle duties such as the preparation of viscous emulsions and fines de-agglomeration. Typical applications include manufacture of toothpastes, ointments, putties and lubricating greases. A colloid mill, however, only offers limited throughput. Flow capacity of the largest units scarcely exceeds a few hundred gallons per minute.
A media mill relies on a "grind charge," such as ceramic or metal beads or another inert medium, inside a relatively large mixing chamber, which usually is agitated by some sort of impeller. Material (often a concentrated premix) is pumped through the chamber where shear and attrition produce rapid dispersion of the mixture. Upon discharge from the mill, the material passes through a mesh that retains the grinding medium. Premixes are then "let down" to their final working concentration and viscosity. This type of mixer primarily finds use in applications requiring dispersion and de-agglomeration of high proportions of solids into liquids, for example, for inks, paints, coating compounds and pesticides.
Pressure homogenizers are one of the most energy-intensive forms of mixers, but can achieve the finest particle/droplet size. To accomplish this, material is pumped into a valve chamber, in which considerable pressure is generated. As material is forced through the restricted aperture in the valve, the pressure and flow velocity increase dramatically. Then, as the material passes out of the valve, the pressure and velocity decrease, causing intense cavitation and turbulence in the material. The pumping rate or valve clearances can be adjusted to fine-tune the unit's performance. However, these systems incur high operating and maintenance costs and offer limited throughput. Pressure homogenizers also are less effective on materials with a viscosity of more than 500 cP. The devices are most common in food applications, such as homogenizing milk and producing flavor and other fine sub-micron emulsions.
Figure 2. Coupling the toothed design with high-speed rotation creates powerful hydraulic shear forces.
Making the choice
Some applications demand certain mixer types, whereas many others can be carried out by a number of methods. For example, neutralizations or other chemical reactions require intimate contact between liquid/liquid or liquid/solid constituents. This can be achieved with adequate results using a static mixer or in-line agitator. These probably would be the cheapest option initially, but not necessarily the most economical in the long run. If the mixture contains fluids of widely differing viscosities or agglomerates that require breaking down, then an in-line rotor/stator mixer would be more efficient because its intense high shear disperses the fluids more finely, accelerating the reaction and increasing yield.
An application common in many industries is the dispersion and hydration of rheology modifiers, such as emulsifying and stabilizing agents. Most powders of this type tend to float or "raft" on the liquid's surface and thus require a mixer capable of creating a powerful vortex to incorporate them into the liquid. Closed-rotor mixers are ideal for this application if the agglomerates that form once the powder is added to the liquid are readily broken down. However, an in-tank rotor/stator mixer would be a better option for hard agglomerates that require more intense shear to break down. An alternative is to use an agitator to wet out the powder, together with an in-line rotor/stator mixer to provide the high shear. Obviously viscosity is a major factor here. Some materials maintain a relatively low viscosity, whereas others, such as carbopols, form on neutralization viscous gels that demand a stirrer/scraper to ensure adequate in-tank movement.
Emulsions can be produced by a variety of means, all of which must provide a degree of shear to sufficiently reduce droplet size to create a stable emulsion. The pressure homogenizer is most commonly used, especially in food applications. However, a colloid mill or a multistage rotor/stator mixer would be more suitable for handling higher-viscosity materials. Where particle size is less critical, a rotor/stator mixer offers economic and speed benefits. It also can be employed in the preparation of pre-emulsions, which then can be fed through the homogenizer or colloid mill at a much faster rate.
Remember, there is no "one-size- fits-all" solution to mixing challenges. Therefore, properly assessing all options is crucial. After all, taking guesswork out of the mix can be the key to successful processing.
Chris Ryan is a technical adviser for Silverson Machines, Chesham, England. He is responsible for the preparation of application reports and training publications. E-mail him at email@example.com.