In this article, you will learn about high shear mixers. This comprehensive guide offers you the following:
- What is a high shear mixer?
- High shear mixers operating principles
- Attaining equilibrium mixing
- Comparison with high pressure homogenizers
- Applications of high-shear mixers
- And much more…
Chapter 1: What is a High Shear Mixer?
High shear mixers, also known as high shear reactors (HSRs), rotor-stator mixers, and high shear homogenizers, are used to emulsify, homogenize, disperse, grind and/or dissolve immiscible mixtures with components of the same or different phases. These machines have characteristics of high rotor tip speeds, high shear rates, localized energy dissipation rates, and higher power consumption than ordinary mixers.
Shearing forces mixing the components are generated by the relative motion of the rotating and stationary parts of the mixer. The rotating parts may be one or a set of impellers, paddles, or screws. The stationary parts, on the other hand, are the walls of the tank or chamber, and in some designs, baffles, and vanes.
High shear mixers are usually used in the manufacturing industry where different components or ingredients are mixed such as food manufacturing, pharmaceuticals, cosmetics, plastics, and so forth. Principle of high shear mixers can also be seen in chemical and petrochemical plants which are used to aid in the reaction process.
Chapter 2: Operating Principle
A fluid is any liquid or gaseous substance that is free to flow and is not bound nor restricted by any surface effects. The study of the behavior of fluids is known as fluid mechanics. Fluids, like solids, can experience force, stress, or pressure. A flowing fluid experiences shear stress, which is the driving principle of high shear mixers.
Shear stress in fluids is primarily caused by friction between the fluid molecules due to viscosity. Friction between fluids and a moving body also creates shear stress.
Note that at the surface, the moving body, and the fluid molecules directly in contact have the same velocity. This is known as the no-slip condition. Intermolecular forces act between the fluid molecules and the surface of the body, known as the boundary layer, resulting in attraction. It can also be noted that once steady state is attained, the velocity profile is linear. There is no more acceleration of the moving and thus, no more force to deform or shear the fluid. Motion is transferred across each layer of the fluid which is then countered by its viscosity.
Laminar flow happens when the fluid is flowing evenly without any disruption across these layers. Adding different bodies to cause shearing forces acting in different directions disrupts this flow. This type of flow, known as turbulent flow, is chaotic causing mass transfer across the fluid layers. Uneven flow across layers of fluid also causes mass transfer. This is effective in breaking up droplets suspended in the mixture causing emulsion, dispersion, and homogenization of the components.
High shear mixers have two main parts: the rotor and the stator. This assembly is known as the mixing head or generator. The rotor accelerates the fluid tangentially, but because of the inertia of the fluid, it does not completely flow together with the rotor. The fluid flows towards the shear gap or the region between the rotor tip and the stator. Inside the shear gap, high velocity differentials and turbulent fluid flow is present producing high shear rates.
The rotor and stator profile, their configuration relative to one another and other features such as holes and slots all contribute to creating the desired fluid flow depending on the application. Below are some of the processes performed by high shear mixers.
This requires liquid droplets to be uniform in size, completely distributed to create a mixture with one continuous phase. The liquid in the form of droplets are in the dispersed phase, while the liquid where the droplets are suspended is the continuous phase. In an emulsion, natural separation happens between the dispersed and continuous phases. This is particularly observed in immiscible liquids such as oil and water. Oil is nonpolar, and as a result they are not attracted by water molecules. Also, oils are usually lighter than water enabling them to float on the surface. Because of these properties, oil tends to naturally separate from water. The objective of the high shear mixer is to continuously break down these droplets before natural separation happens.
Another type of emulsion is a miscible liquid-to-liquid mixture but with different viscosities. Adding low viscosity droplets to a high viscosity solvent requires more mixing time and controlled component addition rates.
A suspension mixture has solid particles that are large enough to settle down which cannot be dissolved completely in the mixture. The objective is the same as the emulsion homogenization, to break down the large solid particles into smaller ones while evenly dispersing it into the medium.
One problem in this process is the difficulty in wetting the solid particles. These solid particles tend to form on the surface of the solvent. This is due to the surface tension of the liquid and the hydrophobic property of the particles. This will be discussed further in the topic of in-line high shear mixers.
Particle Size Reduction
In this application solid or semi-solid materials are milled down into finer particles either in a solution or fine suspension. The size reduction depends on the hardness of the product.
This involves mixing solid products with a binder or granulating liquid. As the powder and binder are blended, the mixture is continuously shaped producing high-density granules.
Most manufacturers advertise their equipment of being able to do all these functions. This is partially true; any mixer can do these but at low efficiencies. High shear mixers are carefully designed considering the phase of the dispersed particles, the fluid viscosity, the required particle size, and so on. Computational fluid dynamics (CFD) analysis is done to simulate the operation of the mixer to determine the optimum geometry of the rotating and stationary parts.
Using CFD analysis to accurately simulate actual conditions is complex and requires scholar level type of study. This makes approach on the design of high shear mixers empirical; focused on development through application specific testing for different products and manufacturing setups.
Chapter 3: Types of High Shear Mixers
This chapter categorizes high shear mixers according to their configuration. All these types can perform different mixture processes as described previously, each with its own degree of efficiency. Moreover, mixer configurations are application dependent. For example, batch shear mixers may not be applicable to pharmaceutical industries due to its susceptibility to contamination; nor an ultra-high shear mixer for paint or ink production.
Batch High Shear Mixers
This type mixes all components by batch and in large volumes in a tank or vessel. Charging of the components is usually done at the top of the vessel. Batch high shear mixers can also be configured to have only one mixing head that can be lifted and suspended serving several vessels. Batch mixing is said to process faster than in-line high shear mixers with the same power rating. One problem in this system is the cleaning process between batches with varying formulations. This problem is particularly evident in viscous mixtures. Residues from the previous batch can become a contaminant to the next batch. To solve this problem, clean in place (CIP) systems are employed.
In-line High Shear Mixers
This type involves mixing the components in a chamber with an inlet and an outlet. Since high shear mixers create centrifugal force, it then acts as a pump to drive the mixture through the chamber. The chamber is always closed making it less prone to contamination, in contrast to the batch type. Moreover, since the inline mixer is part of the product stream, it is more controlled than batch mixers. The product flowing from the mixer can be monitored continuously which enables process operators to modify parameters in real-time.
In a typical setup of an in-line high shear mixer, a mixing pot or vessel is used to collect and combine all raw materials. This vessel may be a simple mixing chamber or a batch mixer.
Through a static head, the pre-mixed materials are then transferred into the in-line high shear mixer. The static head is important since most in-line mixers does not have self-priming capabilities. As the in-line high shear mixer homogenizes the materials, the mixture is either transported to downstream equipment or recirculated back into the mixing chamber. Once the desired particle size and a continuous phase is attained, recirculation is stopped. The material is then diverted to downstream processes.
Powder Induction High Shear Mixers
This type of high shear mixers uses vacuum systems to draw powdered components directly into the mixing head. A vacuum is generated on the rotor-stator assembly drawing the powder from a hopper. This solves several problems that arise when dealing with difficult to process powders.
When charged into the mixing chamber, some powders quickly agglomerate upon contact with the liquid. Clumps tend to form on the surface of the liquid which will then require higher mixing speed to form a vortex. The vortex is required to draw the powders to the mixing head. To mitigate this, these powdered components need to be added carefully and slowly to prevent agglomeration on the surface. However, adding too slowly can cause the continuous phase to reach its target parameters without thoroughly combining dispersed particles that are still undissolved and floating on the surface. In a mass production scale, this directly impacts throughput and in turn, profit margins.
Other scenarios that could happen are the irreversible changes in viscosity and degradation by heating. Breaking the particles further by over shearing it can cause the mixture to become thinner or thicker than the desired viscosity. This is evident in non-Newtonian liquids. Shear-thinning liquids become less viscous when subjected to shear, while shear-thickening liquids do the opposite. A property of non-Newtonian liquids is thixotropy where the liquid becomes thinner as it is being sheared. This time-dependent, thinning property of the liquid makes the mixing time critical, else the product will be off-specifications.
Degradation happens when the components become overly heated, creating unwanted chemical reactions. This is prevalent to batch type mixers since the chamber is a closed system. Mixing adds mechanical energy into the system creating friction between fluid molecules. This friction then becomes heat that can degrade the system.
High Shear Granulators
This process involves converting fine powders into strong, dense agglomerates called granules. This is done by mixing the powdered components and a binding liquid, aided by agitation as provided by an impeller. This process is known as wet granulation. High shear mixers, compared to ordinary mixers, also provide the means to break down the powder into finer particles.
Wet granulation can be broken down into three processes: wetting, growth, and breakage. Wetting happens when the powder encounters the binder liquid. This forms large agglomerates or powder masses known as nuclei. These nuclei then collide with one another resulting in consolidation and growth. As this happens, the large agglomerates become denser. The resulting granules are not uniform in size which are then broken down by the mixer. Shearing and impact forces break the granules to its final particle size.
Aside from adding a liquid binder, dry powders can act also as a binder. This powder melts due to the increase in temperature from the mixing itself or through heaters. This solves problems from liquid binders such as clogging of pumps and nozzles due to high viscosity.
Ultra-High Shear Mixers
These types are designed to run at very high speeds aimed to produce a very fine particle size distribution. This causes dispersed solids and liquids to be homogenized faster into a continuous phase. The rotor is specially contoured to create high pumping capacity and shear intensity. Vortices are created both above and below the mixing head. These vortices draw the mixture into the mixing head which is then expelled radially through the stator slots. Also, the vortex is capable of drawing agglomerates floating on the surface.
Chapter 4: What is Equilibrium Mixing?
Equilibrium mixing is the point where the mixture has acquired the target characteristics. This may be the mixture’s viscosity, dispersed particle size, granule density, and so forth. For dispersions, it is the mixture’s equilibrium particle size, while for emulsions, it is the equilibrium droplet size. Further exertion of effort to mix the components will not change these parameters.
This concept is important for scaling up the volume of a given rotor-stator mixing head. From empirical tests, a particular configuration of a mixing head may reach equilibrium mixing for a given time. However, upscaling the volume may lead to different results. Equilibrium can be reached much faster in a small-scale mixing head than a scaled-up full production unit.
Chapter 5: Comparison with High Pressure Homogenizers
High pressure homogenizers or mixers force a liquid mixture into a stream under high pressure into a system designed to reduce particle size and homogenize the components. Unlike high shear mixers, these types do not use rotor-stator mixing heads or probes. A high-pressure homogenizer operates using a combination of shearing, impact, and cavitation.
A high-pressure homogenizer usually consists of high-pressure tanks containing each component of the mixture. The pressure in the tanks is usually around 15 to 40 bars. The mixture is then forced into a valve or channel with narrow slits establishing high shear stress. The resulting droplet or particle size can be adjusted by changing the pressure and power input. An advantage of this technology is microbial safety because of its closed system. Charging of components is done in separate vessels lessening the risk of product contamination. Another benefit is its ability to achieve precise control and repeatability, real-time response to parameter changes and the ability to charge the components separately through individual pumps. A downside to this set-up, however, is the large initial cost as compared to high shear mixers. Also, more than one pumping unit is necessary, meaning a larger area is occupied.
Chapter 6: Applications of High Shear Mixers
High shear mixers can be seen from all industries requiring ingredients to be combined. Below are applications of high shear mixers.
There is a wide range of high shear mixer applications under this category. High shear mixers used in the food industry can create emulsions, suspensions, powders, and granules. A popular application is the manufacture of sauces, dressings, and pastes. Most of the ingredients are composed of solid particles, and immiscible liquids such as oil and water.
Some ingredients are more difficult to process such as ketchups, mayonnaise, and doughs. These liquids and semi-solids have viscoelastic properties which require a minimum force before creating flow. This requires specialized rotor-stator mixing heads.
Pharmaceuticals and Cosmetics
Like in the food industry, pharmaceuticals deal with different types of mixtures. Inline high shear mixers are used due to its closed system eliminating any intrusion of contaminants. All pharmaceutical products such as tablets, syrups, suspensions, injection solutions, ointments, gels, and creams go through a high shear mixer, all of which have varying viscosity and particle size.
Paints and Coatings
Paints (latex) are known to be a non-Newtonian, thixotropic liquid. This makes paints difficult to process. Paint thins as it is being sheared, either by processing or by end-use. Mixing time for these fluids are carefully controlled to prevent over shearing.
Inks and Toners Manufacture
Viscosity of inks (printer) is the opposite of paints. Inks are considered rheopectic. Rheopectic fluids thicken as it is being sheared, making the mixing process time dependent.
Applications under this category include combining resins and solvents for casting or injection molding, modifying oil viscosity, emulsifying waxes, asphalt production, and so forth.
- High shear mixers are used to emulsify, homogenize, disperse, grind and/or dissolve components of a mixture that is too difficult, expensive, or time consuming to be processed by ordinary mixers.
- These machines operate by shearing the mixture composed of a dispersed phase and a continuous phase. During the shearing process, the dispersed particles or droplets are reduced to smaller sizes making it easier to be dissolved and combined creating a homogenized, continuous phase.
- The two main parts of a high shear mixer are the rotor and the stator. This assembly is known as the mixing head or generator. The region between the rotor and the stator, known as the shear gap, is where the mixture is being sheared.
- An emulsion is created when two immiscible liquids are mixed, such as oil and water. Aside from being hydrophobic, oils are lighter than water which enables them to float on the surface. To homogenize this mixture, the dispersed droplets must be broken down into smaller ones preventing their natural separation.
- A suspension is different from an emulsion where the dispersed particles are solid. The function of the high shear mixer is also to break down the dispersed solids into smaller particles.
- Particle size reduction and granulation are other two functions performed by high shear mixers. High shear mixers can help bind components into larger and denser agglomerates. These agglomerates are then broken down into finer particles until the target size is attained.
- There are four types of high shear mixers: batch, in-line, powder injection, and granulators. Batch and in-line high shear mixers are the two main types, while the other two are modifications. Batch high shear mixers can process large volumes in a shorter period. In-line mixers, on the other hand, are less prone to contamination and can be controlled effectively.
- Powder injection high shear mixers are a modification of the first two wherein a vacuum system draws in the powdered components directly into the mixing head to facilitate better mixing.
- High shear granulators mix fine powdered products by using a binder liquid to form large agglomerates. These agglomerates are then broken down into granules with the desired particle size.
- Equilibrium mixing is the point where additional effort by the mixer does not change the properties of the product.