Whitepapers

Since most industrial processes don't take place in a beaker, we must always consider the real-world behavior of high-shear batch mixers in large vessels. Thinking on a molecular level, we ask, "How many times does each particle or droplet pass through the high shear zone?" Backing up to see the process from a wider perspective, we ask, "How consistent aremy results? How uniform is the distribution of particle or droplet sizes in my batch?" In many applications these are critical questions because they can profoundly influence the properties of your end product. The daily challenge in high-shear rotor/stator (HSM) mixing is to reach the target droplet or particle size and achieve a satisfactory particle-size distribution in the most cost-effective manner.

This requires a careful balance of rotor/stator design, cycle time at arequired batch size, capital cost, and per-cycle operating costs. Any batch mixing process — whether the process goal is particle dispersion, particle-size reduction or emulsification — generates a Gaussian distribution of results. The greater the ratio of product volume to HSM throughput, the broader the distribution will be. Of course, the goal is usually to produce the narrowest distribution possible with an equipment solution that meets the site-specific process and business requirements. The question is simply, "What is the most effective, economical, and practical way to produce the required particle - or droplet size distribution?"

At mixing equilibrium we have reached the target average particle size. Additional processing will gradually narrow the distribution curve, but extending the process for this purpose alone almost always amounts to a substantial waste of time and energy. An alternative strategy is to increase the size of the HSM. This will increase the ratio of HSM throughput-to-product- volume and narrow the curve. But this will also increase both the initial capitalinvestment and the ongoing energy costs. The attractiveness of this solution depends heavily on the value of the product being manufactured, its competitive strength, and the overall business case for investing heavily in equipment.

A radical strategy for narrowing the particle-size distribution would be to use two tanks and a conventional inline HSM. You would pump directly from the first tank, through the inline HSM, and into the second tank. The process would then be reversed: pumping from the second tank, through the inline HSM, and back into the first tank. This cycle can be repeated over and over until the product reaches the desired characteristics. With this method, the shear history of the product could be more closely monitored than in customary batch or batch-recirculation processes. Of course, the expense of purchasing and tying up two tanks is usuallyprohibitive. This technique would also be extremely time-consuming, and it would require either constant operator intervention or complex automation.