Technical Articles

Rotor Stator Mixer Design Shifts Into High Gear: A new high-energy, high-shear mixer rewrites the rulebook...again.


Multi-stage rotor/stator mixers are excellent choices for many high-quality dispersions and emulsions. They are also extremely versatile, since the rotor/stator head can be changed to accommodate a variety of product requirements. Results typically include a fast mix cycle and end-point particle/droplet sizes in the range of 1-5 microns in most applications.

The “X” factor - and a quantum improvement

The design of early rotor/stator generators followed a predictable path. Batch mixers got bigger. Inline mixers recirculated longer. Single-stage generators evolved into multi-stage generators. Shear rates went up and particle/droplet sizes went down. But the first quantum advance in several decades of rotor/stator mixing didn’t arrive until the late 90s. By applying far more intense shear, the new kind of generator - dubbed the “X-Series” by its creators at the Ross Test & Development Center - began to produce sub-micron dispersions and emulsions that rival those produced with more expensive equipment such as colloid mills.

The new generator does not include a rotor (or several rows of rotors) with conventional blades. Instead, the rotor and stator are comprised of many concentric rows of intermeshing teeth. The mix material begins at the center of the generator and moves outward through radial channels cut in the rotor/stator teeth.

As the mix material migrates toward the outlet, passing through many concentric rows of teeth, it is subjected to intense mechanical and hydraulic shear - and thousands of shearing events in each pass.

The total shear applied is further intensified because this generator operates at extremely high tip speeds - up to 18,000 fpm. (Tip speed is commonly used as an indicator of shear and overall rotor/stator performance. Conventional single-stage rotor/stator mixers generally operate with tip speeds in the range of 3,000 – 4,000 fpm.)

X-Series units are appropriate for applications in which sub-micron particle/droplet size is essential to achieve the desired product stability, visual appeal and texture. Applications typically include such products as mayonnaise, mustard, greases and lubricants, silicone elastomers and silica dispersions.

The X-Series rotor/stator generator generally operates with an unaided flow rate of 30-50 gpm, and some units reach 125 gpm. Because all of its power is focused on high-energy shearing rather than pumping, an auxiliary pump is often required for recirculation and for moving the mixed material downstream.

MegaShear increases the shear and the flow - all in one pass

In terms of rotor/stator generator design, the MegaShear represents an ingenious step forward. But in operation, it is a hybrid - the first to combine:

  • High flow rates: up to 500 gpm without an auxiliary pump
  • high tip speeds: over 11,000 fpm
  • Ultra-high shear: comparable to that of an X-Series High Shear Mixer

The combination of high flow and high shear is an important improvement over previous rotor/stator designs, because gains in throughput can produce direct gains in overall production, operating efficiency, and bottom-line profitability. Even for those who are already using an ultra-high shear rotor/stator mixer to produce sub-micron emulsions and dispersions, the MegaShear offers many manufacturers a tool to produce more end-product, faster.

But perhaps the most significant advantage that the MegaShear offers is its ability to disintegrate large particles and droplets in a single pass. With virtually 100% particle/droplet disintegration on the first pass, the MegaShear produces an exceptionally uniform particle/droplet size distribution – which is critically important for many fine emulsions and dispersions. A extremely narrow size distribution contributes to superior product stability and texture, faster solubilization, greater efficacy in the case of pharmaceuticals, and greater efficiency for catalyzed chemical reactions.

By eliminating the need for inline recirculation, the mixer also delivers an important improvement in process control. Unlike recirculating systems, in which passes through the rotor/stator were estimated statistically, the MegaShear operates with exactly one pass through the rotor/stator - a statistical certainty. The MegaShear is especially well-suited for applications that include high-quality chemical dispersions, emulsions, suspensions and solutions. Food applications include condiments, sauces, dressings and marinades.

Making sense of all the choices - test!

Today, there are more choices in rotor/stator mixing than ever before. In many applications, more than one rotor/stator design will do the job. The question is, which meets your process requirements (for mean particle/droplet size and uniformity, for example) and matches your business priorities (for cost vs. performance).

The only way to know for certain is to test a wide variety of designs in a well-equipped laboratory. Don’t rely solely on intuition to judge performance: test using your own ingredients in the lab, and measure the results with analytical equipment.

The next step is usually to test in your own plant. Reputable manufacturers will invite you to rent the equipment you plan to purchase for an in-plant trial before you buy it. Ultimately, this is the best assurance you can get that your mixing solution will meet all of your goals for production and profitability.


Click for Articles