Mix It Up

The drive to remain competitive also requires an ongoing effort to eliminate redundant equipment and streamline processes wherever possible. The new breed of multi-agitator mixers can help meet this challenge because of their versatility and efficiency. In fact, multi-shaft mixers, by integrating several separate mixing steps within one vessel, often enable manufacturers to retire specialized pieces of equipment.

Multi-agitator mixers feature several independent agitators and a flexible control system, so these mixers can readily switch gears to accommodate a fast-changing variety of products. This presents an obvious advantage for manufacturers focused on short campaigns and fast changeover. It also offers benefits to companies that simply are looking for greater flexibility on the plant floor to enhance their responsiveness to changes in market demand and other business pressures.

Evolution
Today's multi-shaft mixers are far more sophisticated than those made only a few years ago. Advances in agitator design, powder/liquid induction, heat transfer and controls have enhanced capabilities and efficiency. This makes the mixers particularly suitable for today's more demanding product formulations. In our test center, working with hundreds of customers each year, we have seen a steady increase in formulations that involve higher peak viscosities, more precise solid/liquid additions, faster and more complex cycles and tighter control over critical parameters such as shear and batch temperature.

Multi-agitator mixers enable you to operate two or three different agitators in an endless variety of functional combinations while charging ingredients, mixing and then discharging the finished product. Typically, the agitators are independently controlled and each is powered by an electronic variable-speed drive.

For simple applications where utmost versatility is not required, dual-shaft mixers will suffice. They usually combine a slow-speed, low-shear anchor agitator and a high-speed disperser (HSD) that applies moderate shear.

The addition of a third agitator dramatically broadens the mixer's functionality. This advantage is especially pronounced when the third agitator is a rotor/stator high shear mixer (HSM), because its capabilities complement those of the high-speed disperser so well. For most processes, the triple-shaft mixer is the wisest choice. We will focus solely on triple-shaft mixers, such as the one shown in Figure 1. The operation of such a unit is depicted in Figure 2.

The individual elements
To understand the operation and capabilities of a multi-shaft mixer, it's best to start by examining each agitator individually.

The anchor agitator. This is a low-shear agitator that promotes gentle mixing and stimulates radial and axial flow. Running at tip speeds from 25 to 525 fpm, it promotes efficient heat transfer and batch homogeneity, and "feeds" material to the high-shear agitators.

The anchor helps to disperse heat within the vessel by stimulating mass flow and constantly removing stagnant material from the tank walls and bottom and pushing it toward the interior. Scrapers mounted on the agitator prevent an insulating layer from accumulating on the interior surface. These scrapers can be made of nonstick material like PTFE.

The basic anchor generally features three horizontal arms and vertical flights. For applications that require enhanced top-to-bottom flow, a helical ribbon can be added to promote axial flow and prevent stratification (Figure 3).

In large batches, and especially as viscosity rises, mass flow in a multi-shaft mixer almost always requires the action of an anchor agitator. Except at very low levels of viscosity, the high-speed agitators alone cannot stimulate enough flow to achieve homogeneity. For this reason, the anchor in a multi-agitator mixer is virtually always running (though speeds may vary substantially).

Running alone, the anchor is particularly effective during cooling cycles. Providing slow agitation, it adds minimal energy. At the other end of the temperature scale, it also is used alone during the early stages of many processes -- whenever solids need to be melted and brought up to an elevated temperature before other mixing functions can begin. Finally, capitalizing on the gentle action of this agitator, it is often used alone to safely disperse delicate ingredients such as glass microspheres.