By Doug Cohen and Ken Langhorn
As competition intensifies and becomes more global, chemical manufacturers must scramble to remain competitive. On the process line, they must strive to produce more and, ideally, higher-quality product in faster mix cycles.
- The motivating force needed to generate adequate flow within the vessel;
- The peak viscosity anticipated during each phase of the mixing cycle;
- The level of shear required to accomplish essential mixing functions during the course of the mix cycle; and
- The materials' tolerance to heat and shear.
For simplicity, we will address these issues one by one, but they are thoroughly intertwined. Heat is a consequence of applying intense shear. Flow/turnover is the result of energy input, agitator design and speed, viscosity, and the vessel's working capacity. Viscosity, in turn, can be influenced by batch temperature, shear and numerous other factors. It's impossible to consider one factor alone. Nevertheless, in everyday practice, we focus on each variable separately to allow us to assign priorities in performance and agitator selection and find the correct balance for each application.
FlowAll batch mixing requires a vigorous flow of material throughout the mixing vessel. Energetic flow fosters complete batch turnover and a high level of top-to-bottom homogeneity as the batch material circulates through the high-shear zones.
Flow also helps to disperse the heat created by the high-shear agitators. Hot spots formed near those agitators can degrade many materials, and vigorous flow helps to thwart this.
At first, the importance of vigorous flow seems straightforward: flow simply carries heat away and prevents thermal damage. However, the value actually goes much further and may allow you to push the process envelope. Even a small improvement in flow and heat transfer may open an opportunity to apply greater shear. An increase in shear often can provide tremendous leverage to shorten the mix cycle, raise the output efficiency of the mixer and lower per-batch processing costs.
ViscosityViscosity and product density always have been important and have become even more so today. Virtually all industries are trending toward higher viscosities in their mixing processes. Environmental regulations seeking to minimize discharge of volatile organic compounds (VOCs) have encouraged companies to reduce or eliminate solvents from many products. This creates higher solids content -- and increased viscosity. At the same time, many manufacturers are finding that their efforts to meet customer demand for improved product performance are also driving their formulations toward higher viscosities.
The result of this general trend is that multi-agitator mixers today should be optimized for operation near the high end of their viscosity range. Subtleties, such as anchor profiles and the use of sidewall/bottom scrapers, are more important than ever before. In addition, drives must be specified with wide speed ranges to accommodate a broad range of working viscosities in the typical mix cycle.
Of course, viscosity greatly impacts flow, which significantly influences the operation of all three agitators in the mixer. High viscosity and limited flow can restrict the speed and shear of both the HSD and HSM. For this reason, we often modify the viscosity during the batch cycle to allow the application of greater shear without risk of heat degradation. We then adjust viscosity near the end of the cycle to produce the final product. We also watch viscosity carefully and operate the agitators in combinations that are appropriate for the current viscosity of the batch.
ShearIn most mixing operations these days, the rule of thumb is to apply as much shear as possible without overstressing the batch with excessive shear or heat. This usually is the most direct way to accelerate the mix cycle. We can boost shear by:
- Raising the speed of the high shear devices;
- Reducing the tolerances between the rotor and stator; or
- Switching to a stator design that is more restrictive. This increases mechanical and hydraulic shear, but decreases flow.
In our test center, the impact of higher shear usually is observed in any of three ways: smaller particle/droplet sizes; more-uniform particle-size distribution; and faster cycle.
In some circumstances, achieving just one of these improvements alone can provide significant benefit. For example, by adding a second disperser blade to a shaft, we certainly will boost the energy applied to the batch, but will not increase shear meaningfully or affect particle size markedly. We will stimulate greater circulation in the vessel and perhaps allow a faster cycle time than would be possible with only one disperser blade. A cycle time gain could lead to a valuable increase in per-batch production.
A typical applicationThe following summary of a polymer dispersion test provides a step-by-step look at a typical multi-agitator application -- and a good indication of the flexibility possible with this type of mixer.
Trials were run in a triple-shaft mixer with a jacketed "change can," with a batch working capacity of 300 gals. The anchor is 55 in. in diameter and has PTFE scrapers. It has a 20 hp drive and can reach speeds up to 36 rpm. The HSD has a 14-in. diameter blade, a 30 hp drive and can achieve 1,090 rpm. The rotor/stator HSM is 7 in. in diameter, has a slotted stator, a 30 hp drive that allows speeds to 1,800 rpm and includes a high-speed powder/liquid induction system. All the drives are independently controlled and offer a 10:1 variable speed ratio.
Phase 1, mixing: 150 gals. of base liquid (oil) are added to the vessel and the temperature is raised to 350 Degrees F. As the base liquid begins to warm, we immediately add 300 lbs. of solid rubber polymer and start the anchor at 25 rpm and the HSD at 1,090 rpm.
Phase 2, mixing: Once the rubber polymer has been broken down sufficiently, we start fine grinding with the HSM at 1,800 rpm. Once the batch has reached its target temperature, all three agitators run. The HSM is now doing most of the high shear work; the HSD is mainly contributing to batch circulation. The anchor is constantly moving material from the vessel perimeter to the batch interior -- where the two high shear devices pull it into their localized flow patterns.
Using the high-speed induction system, we charge the vessel with: 100 lbs. of fumed silica in about 5 min., 25 lbs. of carbon black in 15-30 sec., and 20 gals. of minor liquids in 15-30 sec.
To complete this mixing phase, we pull a 29.5 in. Hg vacuum and deaerate. Because we are dealing with a batch material whose viscosity is strongly temperature dependent, deaeration is most efficient during this phase -- before the product cools in the following phase of the cycle. The viscosity is now about 10,000 cP.
Phase 3, cooling: The first step in this phase is to shut down the HSM and reduce the speed of the HSD to 110 rpm to minimize energy input. Meanwhile, the anchor runs at 20 rpm. The HSM will not run again in this cycle. This is for two reasons: we will not need to apply intense shear again; and, with the batch temperature lowered by 180 Degrees F, viscosity will rise to 100,000 cP. This is beyond the range of the HSM, even with the added flow generated by the anchor and HSD.
When the batch has cooled, we add 50 lbs. of a fine polymer solid. Because the powder/liquid induction system can't be used at this viscosity, we charge the polymer directly to the batch surface, pull vacuum again to deaerate the powder and rely on the vortex created by the HSD to draw the powder into the batch. To accomplish this, we accelerate the HSD to 1,090 rpm.
Phase 4, discharge: Vacuum is released and the finished product is discharged through a flush tank valve while the anchor turns at 10 rpm. Scrapers clean the vessel sidewall and bottom as the product level falls. When discharge is complete, the walls and bottom have been scraped clean.
Don't accept the status quoAs we have highlighted, a multi-agitator mixer may offer enhanced flexibility and versatility in your process. The sidebar provides some telltale signs of when to consider such a unit. But the opportunities certainly are even broader.
Doug Cohen is vice president of technical services and Ken Langhorn is product manager for multi-agitator mixers at Charles Ross & Son Co., Hauppauge, N.Y.
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.
Figure 1.This includes a three-wing anchor with scraper, a two-blade high-speed disperser and a rotor/stator high-shear mixer. It also features a dished cover with angled ports for adding solvent.
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.
Figure 2.The three separately driven agitators work in concert to produce a thorough mixture. The agitators can handle materials that have a peak viscosity of about 500,000 cP.
The individual elements
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).
Figure 3.Adding a helical ribbon to an anchor, such as in the 1,000 gal. mixer shown, is recommended for applications requiring enhanced top-to-bottom flow.
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.
High-speed disperser. This agitator applies moderate shear while generating substantial flow and a vortex both above and below the blade. It can be configured with many different blade styles and is simple to use and easy to clean.
Working alone, it easily handles viscosities up to about 50,000 cP. When used in combination with the anchor, its operating range can be extended by a factor of 10, to at least 500,000 cP.
The HSD is a simple and effective tool for mixing many materials. During the last few years, however, we have seen increasing demand for smaller particle sizes and more uniform particle-size distribution, because an improved dispersion almost always leads to a significant improvement in end-product quality. This trend has contributed to a decline in the popularity of the HSD as a stand-alone mixer.
In the multi-agitator mixer, however, the HSD plays a vital new role. Because its capabilities complement those of the high-shear mixer, it often functions as a flow generator, pre-mixer and simple powder-induction device.
One of the agitator's advantages is that it can disperse and reduce the size of solid materials that are too large for a rotor/stator HSM. Once these large chunks are broken apart by the disperser, the HSM can disintegrate them.
Perhaps the most important advantage of the HSD is that it creates a vortex above and below the blade, pulling material from both the top and bottom of the batch into the high-shear zone. In some cases, this provides a simple way to get powders into the batch. However, this method also can create serious problems, because the surface vortex also draws a great deal of air into the batch. This often necessitates a deaeration phase later in the cycle. New injection technology can introduce powders without undue aeration. Another alternative is to draw in powders via the surface vortex, but only after pulling a vacuum in the vessel.
Rotor/stator HSM.This unit uses a high-speed rotor/stator generator to apply intense mechanical and hydraulic shear. The blades of the rotor run at tip speeds of 3,000 to 4,000 fpm within a fixed stator. As the blades rotate past each opening in the stator, they shear particles and droplets, expelling material at high velocity into the surrounding mass. As material is expelled, more is drawn from beneath into the high-shear zone of the rotor/stator, promoting continuous flow and fast droplet/particle size reduction. Units are available in a variety of configurations (Figure 4). The large square-hole disintegrating head (a) suits applications that require extremely energetic flow. The disintegrating head (b) with large round holes works best for general purpose mixing. It generates vigorous flow and rapidly reduces the size of large particles. The slotted head (c) provides the most popular combination of high shear and efficient flow rate. It often is chosen for emulsions and medium viscosity materials. The fine screen head (d) gives the highest shear, but at the expense of a slower feed rate. It is most suitable for low-viscosity emulsions and fine dispersions.
This HSM's intense shear works well to reduce droplet or particle size for homogenization, dissolution, solubilization, emulsification, grinding and dispersion. Operating alone, the HSM is most suitable for mixing materials with a maximum viscosity of 10,000 cP. Used in tandem with the anchor, it can handle viscosities up to about 200,000 cP.