Throughout the process industries, manufacturers rely on high shear mixing to produce emulsions, dispersions and solutions. Many of today's chemicals, adhesives, pharmaceuticals, foods, plastics, coatings, composites and other products can no longer be produced efficiently using traditional intermittent mixers such as propellers, turbines and serrated dispersers. This can be attributed not only to new raw materials and formulations, but also to an increasingly competitive market that places a high value on process efficiency and lean manufacturing. Long mixing times, multiple transfer steps, batch-to-batch variations and other issues that manufacturers simply deal with have become unacceptable. Therefore, we present some ideas for selecting the right in-line high shear mixer for your process.
A rotor/stator mixer, also known as a high shear mixer, consists of a rotor rotating at high speed inside a stationary stator. As the blades rotate, material is continuously drawn into one end of the mixing head and discharged at high speed through an opening in the stator. The resulting hydraulic shear promotes rapid mixing, breaks down solids attachments and reduces the size of suspended droplets.
Although high-shear mixers were originally developed as intermittent, top-entry mixing devices, they are now just as widely used in in-line configurations where they behave similarly to centrifugal pumps. In-line rotor/stator mixers are not self-priming and therefore require either hydrostatic (gravity feed) or positive pressure (pump feed) to introduce the material into the mixing chamber. Gravity usually feeds the product into the rotor/stator assembly, as the mixer is usually located on the floor or on a platform below the liquid level. Depending on the shear-thinning characteristics of the formulation, most in-line high-shear mixer models can easily move material up to about 10,000 centipoise (cP). Using an auxiliary pump, products of medium viscosity can be handled in the rotor/stator mixer.
In-line high shear mixers are a practical solution for batch tanks equipped with gentle mixing devices. They provide supplemental shear and agitation that can significantly reduce cycle times. A major advantage of the in-line mixer is that it is easy to install without disturbing existing equipment. Its configuration eliminates the difficulty of trying to squeeze a top-entry mixer into a vessel with pre-existing agitators, baffles and other obstructions.
Unlike batch mixers that require high horsepower motors to produce sufficient circulation in large vessels, the in-line mixer can handle a 2,500-gallon mixing tank as easily as a 25-gallon batch. In fact, with proper piping, a single in-line mixer can serve multiple batch tanks of different sizes. A typical installation uses a simple valve to transfer the finished product downstream or to immediately switch from one source vessel to another.
Q: Can single-stage rotor/stator mixers produce fine dispersions and emulsions with submicron particles or droplets?
A: Yes, standard high shear mixers are capable of producing sub-micron droplets or particles. However, to a large extent, this is also highly dependent on the formulation itself. In addition to mixing strength, several other factors can affect particle or droplet size reduction, including the inherent properties of the raw materials, their interaction or chemistry, the presence and type of surfactant, operating temperature, shear sensitivity, etc. Therefore, particle size distribution can only be accurately determined by mixer tests in which actual raw materials are used and simulate real line conditions as closely as possible. In some cases, even the same raw material from different suppliers can produce significantly different
Conventional rotor/stator mixers
(1) four-bladed rotor; (2) fine-screen stator head; (3) round-hole disintegrating stator head; (4) slotted stator head; (5) product inlet The basic single-stage inline high-shear mixer consists of a four-bladed rotor that rotates at high speed within a stationary stator. Typical rotor blade tip speeds range from 3,000 to 4,000 feet per minute.
Rotor/stator mixers offer a variety of interchangeable stator designs to fine tune particle size reduction, depolymerization, emulsification, flow rate and temperature rise.
In-line high shear mixers are piped to batch tanks for recirculation and downstream pumping. The disintegration head, with round holes or large squares, is well suited for general purpose mixing and generating intense flow. It rapidly reduces the size of larger solid particles and accelerates dissolution. The slotted head offers the most popular combination of high shear and efficient flow. It is ideal for emulsions and many medium viscosity mixing applications. The fine sieve head applies the highest peak shear of all single-stage high shear mixers. Depending on the size of the mixer and its application, the openings in this stator can be made of reinforced screens or fine holes. It is typically used for lower viscosity emulsions and fine dispersions.
Our high shear mixers are 3A certified. Liquid and solid materials can be mixed in a batch tank and then recirculated through an in-line rotor/stator mixer. When the desired level of dispersion and homogenization is achieved, the recirculation line is closed and the mixer is used as a transfer pump to move the finished mixture to a nearby collection tank or downstream process.
Another delivery method suitable for liquid-liquid and gas-liquid applications is to meter the ingredients into the high shear mixer for true continuous operation.
In-line mixers designed for rapid powder injection
In-line high shear mixers with sub-surface powder injection capability are well worth considering when large quantities of powder need to be added quickly to liquids or when hard-to-disperse solids take a long time to mix completely.
In early powder injection systems, the pump would push the liquid stream into the injector, thereby creating a vacuum. The powder fed through the header tube would be drawn into the injector by this vacuum, and the liquid stream would be added to the injector. The resulting "premix" was moved to the rotor/stator mixer, where shear and mixing action was applied to break up the adhesions and transport the mixture downstream. At the time, this system provided a useful tool for powder sensing. The in-line system eliminated the floating solids problem of batch systems and provided more precise control of the mixing process. But this setup also had some serious limitations. With three separate units connected in series, maintenance - in terms of labor, required expertise and spare parts - is intensive.
Today's in-line rotor/stator mixers with integral powder injection capability are designed to be easier to operate and less costly to maintain. These new generation high shear mixers no longer require the use of centrifugal pumps or ejectors to generate suction for powder injection. One of the pioneers of this technology is the Ross Solid/Liquid Injection Manifold (SLIM) system.
In addition to ease of operation and maintenance, the SLIM offers significant benefits in optimizing raw material usage and reducing cycle times by up to 80% based on user experience. the SLIM mixer provides similar shear levels to a standard rotor/stator, but its ability to combine solids and liquids in areas where intense mixing occurs has a significant impact on dispersion quality. Most of the agglomerates and "fish eyes" typically found when adding solids from the top of the batch are prevented from forming in the first place.