Homogenizer or Overhead Stirrer?
Selecting the proper equipment for your high speed, high shear mixing application.
Typical applications requiring high speed and high shear mixing include dispersion, emulsification, and homogenization. Determining the appropriate equipment can be confusing ‑ even equipment and application names can overlap. The following information can be a guide to the equipment best suited for your mixing goals.
Some definitions that are helpful:
- Homogenization ‑ a process resulting in a mixture’s components being uniformly distributed, such as homogenizing tissue samples
- Dispersion ‑ a homogeneous distribution of particles in a continuous phase of another material. Two familiar types of dispersions are described below:
- Liquid particles in a different liquid – called an Emulsion. Oil in vinegar dressing is a well‑known example.
- Solid particles in a liquid – commonly just called a dispersion. Example is Milk of Magnesia. In our conversations, ink and paint formulators will also call this a Grind.
Regardless of equipment used, be mindful with high speed and high shear mixing:
- Heat will be generated, which may impact temperature sensitive raw materials and formulations.
- The shear may be too extreme for some high molecular weight polymers such as carbomers, risking degradation.
Rotor-Stator Mixer
Omni Homogenizer GLH 850, used with permission
Also called a rotor-stator homogenizer or homogenizer, the most recognizable batch rotor-stator design is the toothed or slotted version. The high-speed spinning rotor has a close tolerance within the stationary stator tube, resulting in both mechanical and fluid force high shear. Top speed settings can be 30,000 rpm and higher. The stator creates high shear but also restricts flow and pumping, so the maximum viscosity for effectiveness is generally around 10,000 to 20,000 cP. When used for solid in liquid dispersions, it is recommended solids content be less than 15% w/w. Favoured for micro and nano-emulsions, especially when requiring a narrow size distribution of the liquid particles. Also effective for gums and gels agglomerate breakdown, and tissue or cell homogenization.
Omni Family of Rotor Stator Impellers, used with permission
Overhead Stirrer with Dispersion Blade
High speed overhead stirrer systems are commonly called dispersers when assembled with a high shear impeller. When the mixture is drawn into contact with the rapidly spinning impeller blade, it is subjected to high hydraulic shear forces.
Various names for this style of blade include:
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Caframo 6015 dispersing powders
Depending on the model, top speed settings can be 2000, 3000 rpm, even up to 6000 rpm. One characteristic of dispersion blade mixing is the creation of a vortex, which is helpful for incorporating solids. Ideal flow for paint formulators is sometimes called a doughnut roll, appearing like a doughnut with the spinning blade forming the hole. The higher flow that occurs also enables a wider range of viscosities, from water-like to 50,000 cP. Formulations with 15‑65% w/w solids can be mixed with a dispersion blade. Caframo’s high torque stirrers are a favoured choice for this type of dispersion, occurring widely in paint and ink formulations. Other successful applications include emulsions for adhesives and ceramics, microencapsulation of essential oils, and food gums and pectin premixes.
Caframo Lab Solutions has expertise in high speed and high shear mixing. Contact us today to see how an overhead stirrer system can meet your application needs.
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References:
Banaszek, C. (2015) High-Speed Mixers for Coatings and Inks. PCI Magazine. Retrieved Aug 2018 from: https://www.pcimag.com/articles/100125-high-speed-mixers-for-coatings-and-inks
Charles Ross & Son Company (n.d.) High speed mixing: saw-tooth dispersers versus rotor/stator mixers. Mixing Technology Insight #10. Retrieved Aug 2018 from: https://www.mixers.com/insights/mti_10.pdf
Hawkins, G.B. (2013) Process Engineering Guide GBHE-PEG-MIX-709: ‘High Shear’ Mixers. GBH Enterprises, Ltd. Retrieved Aug 2018 from: https://www.slideshare.net/GerardBHawkins/high-shear-mixers
Hockmeyer, H. (2005) A Practical Guide to High Speed Dispersion. In A. A. Tracton (Ed.), Coatings Technology Handbook (3rd ed., pp. 45-1 – 45-7). Boca Raton, FL: CRC Press.
Lab Manager (n.d.) Homogenization Technologies Pros and Cons. Infographic. Retrieved Sept 2018 from: https://www.labmanager.com/multimedia/2017/03/homogenization-technologies-pros-and-cons#.W5LXJEZKi70
Lubrizol (2007) Dispersion Techniques for Carbopol® Polymers. TDS-103.
Ryan, C. (2004) Take guesswork out of the mix. Chemical Processing. Retrieved Aug 2018 from: https://www.chemicalprocessing.com/articles/2004/385/
Shechter, D. (2017) Emulsions vs. Dispersions: What is the difference? Retrieved Sep 2018 from: http://www.beei.com/blog/emulsions-vs-dispersions#
Wikipedia. (n.d.) Colloid. Retrieved Aug 2018 from: https://en.wikipedia.org/wiki/Colloid
Wikipedia. (n.d.) Dispersion (chemistry). Retrieved Aug 2018 from: https://en.wikipedia.org/wiki/Dispersion_(chemistry)
Wikipedia. (n.d.) Homogeneous and heterogeneous mixtures. Retrieved Aug 2018 from: https://en.wikipedia.org/wiki/Homogeneous_and_heterogeneous_mixtures
