ROTORS AND STATORS

Looking at previous milling methods, it is obvious that the future holds newer ideas. Though the Rotor & Stator design works well for a "shearing" method, a new "pressure" method might be in store.

Abstract:
Looking at previous milling methods, it is obvious that the future holds newer ideas. Though the Rotor & Stator design works well for a "shearing" method, a new "pressure" method might be in store.

Detail: [By: Mark Drukenbrod]

Those of you who read this column regularly know that I have, in the past, been an outspoken advocate of rotor/stator mixers in certain applications. As with most pieces of machinery though, I quickly pointed out that the R/S was a machine much like the basket mill, that is, one which has applications limited by rheology. Surely, if you are willing to reformulate your coating to fit within the rheological profile of the machine, both machines can perform with the best of "old" technology, giving several benefits beside. The Rotor/Stator has generally been thought of as a very high speed device, most running at many times the speed of even a high speed disperser. The theory behind these machines up to this point has been to semi-isolate a quantity of the material under process and simply shear it with intense energy, developing intense velocity and energy incongruities in the material, causing the agglomerates in is to pretty much explode. For the ability to expose material to this level of energy, several things were given up. First was the ability to move material in and out of the rotor/stator head, therefore making the equipment ineffective at circulating material in the tank. To remedy this, several manufacturers began mount auxiliary pumping devices on their R/S shafts. Unfortunately, their devices were poorly suited to running at the speeds necessary for the R/S head, and the efficiency of the combination suffered. For instance, if we take a marine turbine that is designed to run at 300 rpm, and all of a sudden start running it 3000 rpm, we have a device that uses roughly 28 times more power and has a tendency to cavitate. The second ability we gave up in using a rotor/stator was the ability to control the heat in the batch. Those of you who have run a standard high speed disperser know that you can "play around" with the viscosity of the material you are making by increasing and decreasing the speed of the machine, and therefore the amount of energy that is input as heat. The heat is generated very close to the surface of the blade or stator, and is dissipated in the batch by circulation. Because the rotor/stator has a comparative deficiency in circulation power, and has the capability to input more energy than the material can absorb, the two characteristics can cause uncontrollable thermal rise in the batch.

In general, the moral to the story for most rotor/stators is "keep it thin". They can be very useful devices if used in the correct application by the correct operator. Most are not as flexible as high speed dispersers, but have a much more refined and specialized design, and cannot be expected to be as broadly applicable.

In order to solve the major problems of rotor/stator applicability, we must look beyond the current method of applying energy, which is hydraulic shear, and look for other methods of energy input. I have long been a fan of high pressure homogenizers for deagglomeration and the pulling of emulsions (the liquid/liquid equivalent to deagglomeration.) My major stumbling block here was cost. They are very good, but very, very expensive on a dollar per volume processed basis.

Earlier this year, I met a guy who changed my thinking on rotor stator technology by shifting my brain from "shear" mode to "pressure" mode. The twist here was that he generates this pressure gradient with a device that looks deceptively like, well, a rotor/stator. I have several admissions to make up front. First, the guy is British. Not that I have anything against the people, but in my errant youth I was the victim of British technology in the form of an MGB, a Triumph TR-4 and Jaguar XK-120 (which I still own), and most recently a Red Label Alvis. Frankly, they are wonderful machines, but I can't say I've ever taken a trip in any of them without having to get the tool kit out. Second, my company became so impressed with the technology that we have undertaken to begin manufacturing the machines here in the good old USA in the very near future. Third and finally, I am not writing this column to promote the machine but to explore the technology – notice at no time do I even mention a name.

O.K., caveats out of the way, let’s take a look at the technology. The major part if it is in the pressure plates, which make up the top and bottom of the rotor/stator housing. These pressure plates make up the stator, and look like a magnified cross section of a mill bastard file, with a jagged washboard-like area made up of acute triangular surfaces radiating from the shaft out. The rotor looks like a typical pitched turbine with flow straightening strakes on the outside diameter. The material is induced at the top and bottom and moves radially away from the shaft as it is repeatedly compressed and allowed to rare fact because of the radial "ramps" in the stator. The interesting part here is that if you look at the outside of the package, it looks like anybody else's shear-based R/S.

So, what does it do, and how does it do it? Well, on low viscosity materials, it works as well as the shear rotor/stators that I have experienced. But what sets this design apart is its performance on materials that are generally not considered to be within the R/S purview. I've seen this machine work on materials in excess of a million centipoises. No, I wasn't on elephant tranquilizers at the time. The design will circulate materials in the million centipoises range as efficiently as it does water. If we take a look back at the design, we can see why this is true. Instead of having what is in effect a radial turbine flailing the material through holes in a stator as in the typical shear design, we have an axial turbine (already better at circulating high viscosities) which is incidentally applying energy to the process material in steps (the pressure wedges), pumping material much like a multi-stage pump would. The mass of the material is not used as in a centrifugal pump. I guess if you were drawing a pump analogy, you would opt for the progressive cavity type, where small volumes of material are pumped at a time, but back to back and continuously. Another benefit is that they are relatively low speed, high torque devices, as opposed to the high speed low torque shear types. Modern power transmission technology is much more easily harnessed in high torque, low speed applications…or at least you have a lot more options.

So there you have it. I've finally found something that I vowed and declared for years did not exist. A rotor/stator for high viscosities. What'll they invent next? Mills that don't use media? Well…that's another column.

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