Appreciate the Art of High Viscosity Mixing

Optimization requires considering crucial issues during specification and scale-up.

By Ken Langhorn and Tom Digiannurio, Charles Ross & Son Company, and George Lu, Ross Wuxi Equipment Company

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Many processing operations require mixing of high-viscosity materials. Unfortunately, there’re no “cookbook” answers for reliably matching an application to an appropriate mixer. You can’t simply go to a chart, select an application or a viscosity and find the appropriate mixer design. High viscosity mixing is an art, not a science. Mixer specification isn’t straightforward and predictable; many variables affect the specification process.

On the equipment side, we now have many agitator designs to consider — from high-speed, high-shear devices to low-speed anchors and close-tolerance planetary blades. In addition, numerous possible combinations of agitators working in tandem or independently exist; these can be stationary or move in various patterns inside the mix vessel as they rotate on their own axes.

On the materials side, the variables are less numerous but more subjective. They often interact in complex and subtle ways to profoundly influence mixer selection.

For example, viscosity may seem a straightforward measurement that should point to a particular type of mixer. However, in the real world of mixer specification, viscosity is not decisive. It’s just one of many co-dependent variables subject to interpretation.

Imagine two materials, each measuring 1 million cP on a viscometer at a specific shear rate. One is a common hair gel made with carbomers, gum thickeners and water; the other is a construction adhesive. We easily can mix the gel in a dual-shaft mixer with a stationary high-speed disperser and a slow-speed anchor agitator. However, the adhesive — because it’s a much stickier material — would require a much more powerful double planetary mixer.

The double planetary mixer (Figure 1) is designed to mix materials that are sticky and don’t flow easily. Its stirrers move through the batch in a planetary pattern while they rotate on their own axes. They physically transport material from the vessel walls into the vessel interior, mixing the batch thoroughly in only a few revolutions. This action promotes efficient dispersion — and, with bottom- and sidewall-scrapers added, efficient heat transfer as well.

Key Questions

These days, we can solve most mixing challenges with two or three different types of equipment. To find the optimal solution — not the only solution — the best course is to test a variety of equipment, not just the one that initially seems the most likely choice.

The selection process is intuitive. Although every formulation presents a unique set of mixing steps (e.g., high-solids dispersion or de-aeration), our process always begins with the same series of inter-related questions.

What is the viscosity, both peak and periodic? It’s important to specify the maximum viscosity of the mix materials during your process — and at key milestones during the mix cycle. A peak viscosity of 1 million cP, for example, suggests the need for either a planetary/disperser hybrid mixer or a double planetary mixer.

If a crucial dispersion takes place early in the cycle when viscosity is still quite low, we might consider combining a single planetary blade with a high-speed disperser blade. In this classic planetary/disperser hybrid (Figure 2), the high-speed disperser will disperse solids efficiently at viscosities up to 50,000 cP or so, and the planetary blade will motivate the batch later on when it’s heavily loaded and no longer flowable.

However, if the formulation demands dispersion when viscosity levels far exceed 50,000 cP — beyond the operating range of a high-speed disperser — a multi-shaft mixer or double planetary mixer may be a better choice. As viscosity increases, planetary blades become more and more effective at imparting shear and improving dispersions.

Is the material sticky or slippery? As we noted earlier, a sticky material at 1 million cP may require the power of a double planetary mixer while a slippery material may mix easily in a less costly multi-agitator mixer. For extremely sticky materials, we certainly would test a double planetary mixer with helical HV blades (Figure 3). As the helical planetary blades advance through the batch, they drive the sticky material forward and down, which prevents it from climbing the blades — a chronic problem with sticky materials.

Instead of applying the shear we need with high-speed devices, this mixer applies shear with close-tolerance planetary blades. Each time they pass one another and the vessel sidewall, the sliding action of the helical, swept-back blades imparts substantial shear.

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