|Laser Doppler anemometer testing of a proprietary impeller design. Courtesy of Philadelphia Mixing Solutions.|
The most common error in selecting liquid mixing/blending equipment is a lack of understanding of fluid rheology and properties. Customers tend to simplify blending scenarios, for example only describing them in terms of a single parameter, like viscosity or specific gravity. Its especially important with viscous blending to understand the complete fluid rheology, or at least the relation between shear rate and viscosity, Wyczalkowski notes. Most of the tests we do for customers are to determine fluid rheology. All too often when talking with them they dont even understand what were asking for.When problems persist, Philadelphia Mixing will suggest that customers run a blend at the vendors site, which has a twofold benefit. The customer learns about mixing, and we learn about the process, says Wyczalkowski.Philadelphias lab provides a controlled environment for testing and observing, through fully monitored glass mixing vessels, what is actually occurring during the blend. One of the companys pet analytical techniques is conductive tomography, which uses process fluid conductivity to construct a three-dimensional picture of the blend in-process. The technique works for any pair of liquids, or any solid-liquid combination, that alters the electrical conductivity of the blend as a function of time.Bill Scott, president of Scott Turbon Mixer (Adelanto, Calif.) is even more of a power-pusher than Wyczalkowski. Scott differentiates blending from mixing based on raw horsepower: the former is low-energy, the latter high-energy. Theres a huge difference in effect between one horsepower per 10,000 gallons, which is stirring, to one horsepower per gallon, the kind of power used to mix dough and polymers, he says. Just where the cutoff exists between blending and mixing is difficult to say. Scott defines blending at about one horsepower per 100 gallons or less about the energy required to combine two light, dissimilar liquids, or water and an easily miscible powder (such as sugar or salt).With powders, the minimum objective is preventing the solids from sinking to the bottom of the vessel. How much energy that takes depends on many factors: liquid viscosity and specific gravity, density of the powder, and vessel size, to name a few. Solids that tend to clump, like gums and starches, or oil-water emulsions, may require 20 to 50 hp per 1,000 gallons to disperse and hydrate. Adding a solid to the emulsion takes about 75 hp per 1,000 gallons. Higher viscosity and specific gravity for staring liquids increases power requirements even more. And with pastes the sky is the limit, says Scott. Those could take one horsepower per gallon, which is huge.It might sound simple to anyone who uses kitchen-counter blenders, but industrial practitioners will tell you that liquid mixing/blending is not as straightforward as it appears. Mixing is not what you learned in college, says Scott. You can take a class in hydrodynamics, but when it comes to liquid mixing, success is all about experience.Its the solids, stupid
In fact, one could say that the cutting edge of liquid mixing/blending is formulations involving solids. Contract manufacturer Patheon (Mississauga, Ont.) does a fair amount of semi-solid manufacturing, which Dr. Anil Kane, associate director for formulation development, defines as any multi-phase blending involving at least one liquid component. Examples include oil/water (for emulsions), solid/liquid dispersions, and the range of formulations that are the basis of gels, creams, and ointments.Semi-solid mixing is especially challenging with very potent active ingredients. The key is choosing a vehicle in which the active exhibits the proper partition coefficient (assuming at least two liquid components), so the API is solubilized or dispersed uniformly throughout the final product. Patheon often uses combinations of hydrophilic and lipophilic vehicles to achieve the right partition balance for example, mixtures of glycerol, propylene glycols, lipids, solubilizers and surfactants. The choice of co-solvent blends is also critical, Kane notes.Sometimes the semi-solid blend needs a bit of help. Kane recalls one project where everything went well until the introduction of one vehicle component, which caused the API crystals to grow in size. Since the vehicle in question was essential, Patheon introduced wet milling to reduce crystal particle size. Similar problems may arise when API crystals form hydrate or semi-hydrate crystals.Sounding off
Sonic mixing/blending, an interesting alternative to mechanical agitation, uses sound waves to create micro-sized vapor pockets in liquids. During this process, known as cavitation, tiny, highly energetic bubbles implode, causing a nearly infinite number of tiny shock waves within the fluid. Instantaneous temperatures reach the 5,000 K range, which can be exploited for chemical catalysis. Thanks to cooling rates approaching 100,000 K per second, sonication may also be used for liquid-liquid and liquid-solid processing.Advanced Sonic Processing (Oxford, Conn.), develops sonic process equipment mostly to accelerate chemical reactions, but for mixing and preparing crystals for milling before blending into liquids as well. In mixing mode, theres no degradation of ingredients, assures president David Hunicke. One setup, for example, accelerates enzymatic reactions while sparing delicate proteins. The trick is to use the precise ultrasonic dose, he says.Ultrasonics is very good at micro-mixing dry and wet additives. When clusters are acoustically activated in a liquid, they naturally de-agglomerate to uniformly wet particles, then uniformly disperse throughout the liquid. For liquid-liquid mixing, say for emulsions, the material must first be macro-mixed with a mechanical mixer. Most processors think of ultrasound as a polishing operation, says Hunicke. Emulsion tightness is directly proportional to the uniformity of constituents entering the sonic reactor.
|LIQUID BLENDING: TIPS FROM THE PRO|
Bill Scott, head of Scott Turbon Mixer, offers manufacturers a unique blend of advice and wisdom:
- Aim for tank diameter-to-height ratio of 1:1 to 1.25:1, especially for emulsions and dispersions. Tough blends and short tanks dont mix, says Scott.
- Select dished (rounded-bottom) tanks for liquid mixing. Flat or sloped bottoms give dead spots.
- Know your materials of construction. Customers tend to overspend in this department, Scott says for example, specifying 316 or 316L stainless steel. Customers want it to be able to go to the moon and back, which is crazy, Scott says. 304 stainless is fine. Biotech sometimes requires 316 or 316L, but thats the exception.
- Know the limitations of your blending equipment: a 1-hp mixer can only do one horsepowers worth of work.
- Throw a lot of horsepower at difficult mixing jobs. The processing time saved, and the lower costs from reduced stopping, testing and re-starting, will more than pay for the extra investment.
- Add dry ingredients quickly, even simultaneously. Processors can cut batch times by 80% or more through rapid solids addition, provided theres enough energy to do the job. Theres no reason why the batch should not be ready for filling a short time after the last ingredient is added, Scott says.