Processes, Comparison and Types of High Shear Mixers

High Shear Mixer: Types, Processes and Comparisons

Introduction

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, rotor-stator mixers, or high shear homogenizers, are highly specialized devices designed for emulsifying, homogenizing, dispersing, grinding, and dissolving immiscible mixtures. These tools function at high rotor tip speeds and shear rates, delivering concentrated energy dissipation and utilizing more energy compared to standard industrial mixers.

Shearing forces are the stresses exerted by mixing blades or impellers on the liquids, solids, and other materials being processed. In high-speed mixing applications, a rotating rotor forces the material against a non-moving stator, producing shear by moving various portions of the material in opposing directions on the same plane. This unique technique allows for the blending of liquids, solids, or gases that ordinary mixing methods cannot effectively achieve.

High shear mixers are indispensable for creating consumer products with immiscible components, including salad dressings, paints, cosmetics, detergents, shampoos, and various ointments. They serve a multitude of functions such as emulsifying liquids, blending liquids with varying viscosities, incorporating solids into liquids, dispersing powders into liquids, and particle size reduction. These mixers are vital in industrial operations, playing an integral role in manufacturing pharmaceuticals, chemicals, healthcare products, and cosmetics.

Chapter 2: What is the operating principle of high shear mixers?

A fluid is any substance in a liquid or gaseous state that can flow freely and is not constrained by surface effects. The behavior and properties of fluids are studied under fluid mechanics. Like solids, fluids can experience force, stress, or pressure. When a fluid flows, it encounters shear stress, which is the fundamental principle behind high shear mixers.

Shear stress in fluids arises mainly from friction between the fluid molecules, attributed to viscosity. Additionally, shear stress is generated by the friction between the fluid and any moving body within it.

The moving body and the fluid molecules in direct contact with it share the same velocity, a phenomenon known as the no-slip condition. Intermolecular forces between the fluid molecules and the body's surface, referred to as the boundary layer, create an attractive force. Once a steady state is reached, the velocity profile becomes linear. At this point, there is no further acceleration or force needed to deform or shear the fluid. Motion is transferred across each fluid layer and is opposed by its viscosity.

Laminar flow occurs when the fluid moves smoothly and evenly without disturbances across these layers. Introducing different bodies that create shearing forces in various directions disrupts this flow, leading to turbulent flow. Turbulent flow is chaotic and facilitates mass transfer across fluid layers. This uneven flow helps break up droplets suspended in the mixture, resulting in emulsion, dispersion, and homogenization of the components.

High shear mixers typically consist of two primary components: the rotor and the stator, collectively known as the mixing head or generator. The rotor accelerates the fluid tangentially, creating inertia that prevents the fluid from moving at the same speed as the rotor. As a result, the fluid is directed towards the shear gap, the space between the rotor tip and the stator. Within this shear gap, high velocity differentials and turbulent flow generate intense shear rates.

The profile and configuration of the rotor and stator, along with features like holes and slots, play a crucial role in shaping the fluid flow according to the specific application. Below are some of the processes facilitated by high shear mixers.

Emulsion Homogenization

To create a uniform mixture with a single continuous phase, the liquid droplets must be evenly sized and distributed. In this setup, the droplets represent the dispersed phase, while the liquid in which they are suspended is the continuous phase. Emulsions often experience natural separation between these phases, particularly in immiscible liquids like oil and water. Since oil is nonpolar and not attracted to water molecules, and because oil is typically less dense than water, it tends to float and separate from the water. The high shear mixer’s goal is to continually break down these droplets to prevent this natural separation.

Another type of emulsion involves mixing miscible liquids with different viscosities. When adding low-viscosity droplets to a high-viscosity solvent, it’s essential to use more mixing time and carefully control the rates at which components are added.

Suspension Homogenization

A suspension mixture contains solid particles large enough to settle out and that cannot be fully dissolved in the liquid. The goal, similar to emulsion homogenization, is to break down these large solid particles into smaller ones and evenly disperse them throughout the medium.

One challenge in this process is effectively wetting the solid particles, which often tend to float on the surface of the solvent due to the liquid’s surface tension and the hydrophobic nature of the particles. This issue will be further explored in the section on in-line high shear mixers.

Particle Size Reduction

In this application, solid or semi-solid materials are ground into finer particles, either in a solution or a fine suspension. The extent of size reduction depends on the hardness of the material.

Granulation

This process involves combining solid products with a binder or granulating liquid. As the powder and binder are mixed, the mixture is continuously formed into high-density granules.

Many manufacturers claim that their equipment can perform all these functions. While it is true that most mixers can handle these tasks, they often do so with lower efficiency. High shear mixers are specifically engineered with considerations for the dispersed particle phase, fluid viscosity, required particle size, and other factors. Computational Fluid Dynamics (CFD) analysis is used to simulate the mixer’s operation and determine the optimal design for both the rotating and stationary components.

Accurately simulating real-world conditions with CFD analysis is complex and requires advanced expertise. As a result, the design of high shear mixers often relies on empirical methods, focusing on development through application-specific testing for various products and manufacturing setups.

Leading Manufacturers and Suppliers

Chapter 3: What are the different types of high shear mixers?

High shear mixers are classified by their configurations, including batch, in-line, powder induction, and ultra-high shear types. Although the methodologies may differ, the fundamental shearing process remains consistent across all types. Manufacturers use high shear mixers to blend viscous materials into homogeneous products and to agglomerate materials that retain their integrity without breaking down.

Batch High Shear Mixers

Batch high shear mixers process all components in large volumes within a tank or vessel, with components typically added at the top. These mixers can be equipped with a single mixing head that can be lifted and used across multiple vessels. Batch mixing is often faster than in-line high shear mixing when comparing mixers with the same power rating. However, a major challenge is cleaning between batches with different formulations, especially for viscous mixtures. Residues from previous batches can contaminate subsequent ones. To address this issue, Clean-In-Place (CIP) systems are employed.

In-line High Shear Mixers

In-line mixers are integrated into a processing system, allowing ingredients to flow through the mixer during the mixing process. The rotor-stator assembly of an in-line shear mixer is positioned perpendicular to the flow of ingredients, generating an exceptionally intense shearing force.

In-line mixers consist of a chamber with an inlet and an outlet and utilize centrifugal force as a pump to drive the mixture through the chamber. The chamber is tightly sealed to prevent contamination. Because in-line mixers are part of the continuous product stream, the mixing process is closely controlled and monitored. Operators can adjust the mixing parameters in real time, as the product flowing from the mixer is continuously observed.

With an in-line high shear mixer, a mixing pot or vessel is used to collect and combine all raw materials, which can be a simple mixing chamber or a batch mixer.

In-Line High Shear Mixer Recirculation Set-Up

Materials are transferred from the static head into the in-line high shear mixer, which benefits from the self-priming provided by the static head. As the in-line high shear mixer homogenizes the materials, the mixture is either sent to downstream equipment or recirculated back into the mixing chamber. Once the desired particle size and continuous phase are achieved, recirculation stops, and the mixed materials are directed to downstream processes.

Positioning an in-line high shear mixer within the processing system ensures a continuous flow of ingredients, facilitating seamless and uninterrupted mixing. The rotor-stator assembly, positioned perpendicular to the ingredient flow, enhances precision, making it ideal for industries such as pharmaceuticals, cosmetics, and personal care.

With their compact design, in-line shear mixers offer a small footprint for easy integration into processing systems, making them a space-saving solution. Despite their compact size, they are designed to be exceptionally efficient and productive.

Powder Induction High Shear Mixers

This type of high shear mixer employs a vacuum system to draw powdered components directly into the mixing head. A vacuum created by the rotor-stator assembly pulls the powder from a hopper. This approach addresses several challenges associated with processing difficult-to-handle powders.

When powders are added to the mixing chamber, they can quickly agglomerate upon contact with the liquid, forming clumps on the surface. This requires higher mixing speeds to create a vortex that draws the powders to the mixing head. To prevent surface agglomeration, powdered components need to be added carefully and slowly. However, if added too slowly, the continuous phase may reach its target parameters before all dispersed particles are fully integrated, impacting throughput and profit margins in mass production.

Additional challenges include irreversible changes in viscosity and degradation due to heating, which can alter the mixture's viscosity from the desired level. This is particularly noticeable in non-Newtonian liquids. For example, shear-thinning liquids become less viscous under shear, while shear-thickening liquids become more viscous. Thixotropy, a property of some non-Newtonian liquids, causes the liquid to become thinner as it is sheared. This time-dependent thinning requires precise control of mixing time to avoid off-specification products.

Degradation occurs when components are exposed to excessive heat, leading to unwanted chemical reactions. This issue is particularly common in batch-type mixers, where the chamber is a closed system. The mechanical energy introduced during mixing generates friction between fluid molecules, which in turn produces heat that can degrade the materials in the system.

High Shear Granulators

This process involves converting fine powders into strong, dense agglomerates known as granules. It is achieved by mixing the powdered components with a binding liquid, with agitation provided by an impeller. This process, called wet granulation, is enhanced by high shear mixers, which not only form granules but also break down the powder into finer particles compared to ordinary mixers.

Wet granulation consists of three stages: wetting, growth, and breakage. During the wetting stage, the powder interacts with the binder liquid, forming large agglomerates known as nuclei. These nuclei then collide and consolidate, resulting in larger, denser agglomerates. The granules produced are initially not uniform in size, so the mixer further processes them. Shearing and impact forces are used to break the granules down to the desired final particle size.

In addition to liquid binders, dry powders can also serve as binders. These powders can melt due to the heat generated by mixing or through external heating sources. This method helps avoid issues associated with liquid binders, such as clogging of pumps and nozzles due to high viscosity.

Ultra-High Shear Mixers

These mixers are designed to operate at very high speeds to achieve a fine particle size distribution. The high speed allows for rapid homogenization of dispersed solids and liquids into a continuous phase. The rotor is specially contoured to generate high pumping capacity and intense shear. Vortices are created both above and below the mixing head, drawing the mixture into the head and expelling it radially through the stator slots. Additionally, these vortices can draw agglomerates floating on the surface into the mixing process.

Bottom Entry High Shear Mixer

Bottom entry high shear mixers are installed at the base or side of a container or tank and are used in conjunction with a slow-speed anchor stirrer or scraper units. The high shear mixer provides intense homogenization, while the stirrer helps distribute the output throughout the vessel or container. The rotor-stator assembly is connected directly to the motor via a shaft, which rapidly rotates the rotor and stator to draw materials into the mixing zone.

The high-speed rotation of the rotor blades in a bottom entry high shear mixer creates strong suction, pulling liquid and solid materials downward into the center of the workhead. The centrifugal force generated pushes these materials toward the edge of the workhead, where they are milled between the rotor and stator. This process generates intense hydraulic shearing forces as the material is forced through the stator and projected radially at high speeds toward the sides of the mixing container. As mixing progresses, new material is continuously drawn into the workhead to sustain the mixing cycle.

Chapter 4: What is Equilibrium Mixing?

Equilibrium mixing in high shear mixing is used to determine the characteristics of the finished mixture. It represents the point at which further mixing is unnecessary, as additional processing will not alter the product’s properties. Achieving equilibrium is the primary objective of high shear mixing and signals when mixing can be stopped.

High shear mixers are designed to blend ingredients that are naturally immiscible. When mixing two or more types of liquids, the result is known as an emulsion. Combining a solid with a liquid produces a suspension, while dispersing a gas in a liquid forms a lyosol, which includes micellar solutions and aqueous biopolymer solutions. The energy required for mixing determines whether the combination of solids, gases, and liquids is adequately homogenized.

To achieve a consistent mixture with a high shear mixer, equilibrium mixing helps identify when the reactants will no longer change. For dispersions, equilibrium is defined by particle size, while for emulsions, it is defined by droplet size. The amount of mixing required to reach equilibrium depends on the number of times materials pass through the high shear zone.

Determining whether a mixture has reached equilibrium involves assessing whether the mixture has achieved its target characteristics, such as viscosity, particle size, and granule density. Once equilibrium is achieved, additional mixing will not improve the mixture and may be counterproductive.

The concept of equilibrium is crucial for scaling up the volume of a rotor-stator mixing head. Smaller mixing heads reach equilibrium faster than larger, full-production units. Thus, the design of the mixing head affects how quickly equilibrium is achieved.

In high shear mixing, equilibrium is typically reached within 5 to 10 passes through the rotor-stator. Exceeding this number of passes is inefficient, may damage the mixer, waste energy, and increase equipment wear and tear.

Chapter 5: How do high shear mixers compare to high pressure homogenizers?

High pressure homogenizers or mixers apply high pressure to a liquid to force it through a membrane or valve that has narrow slits. The process causes high shear, large pressure drop, and cavitation that homogenizes the sample. A high pressure homogenizer uses a combination of shearing, impact, and cavitation. The term high pressure homogenizer refers to any homogenizer that forces a liquid in order to reduce particle sizes.

A high-pressure homogenizer consists of high-pressure tanks that hold the materials to be mixed, with pressures ranging from 15 to 40 bars. As the materials are forced through a valve or channel with narrow slits, high shear stress, pressure drop, and cavitation occur, collectively contributing to the homogenization of the mixture.

Adjusting the pressure and power input changes the size of the droplets or particles in the mixture.

Being a closed system, high-pressure homogenizers are shielded from microbes and external contaminants. Component charging is done in separate containers, further reducing the risk of contamination.

High-pressure homogenizers offer precise control for flexibility and repeatability, real-time response to parameter changes, and component charging through individual pumps. They efficiently process large volumes of materials, making them well-suited for the dairy industry. Their powerful processing capability and ability to recycle material streams enable the achievement of very small, submicron particle sizes.

Due to their high cost—units typically exceeding $10,000—and the need for multiple pumping units for efficient operation, high-pressure homogenizers are suitable only for high-volume processing. They are heavy and require a significant amount of space. Additionally, to prevent cross-contamination, thorough cleaning is necessary after each cycle.

Chapter 6: What are the applications of high shear mixers?

High shear mixers are essential in many industrial processes due to their capability to execute a range of mixing functions, including reduction, dispersion, homogenization, deagglomeration, and wet milling. These processes enhance product quality and improve production efficiency. As technology advances, high shear mixers are increasingly replacing traditional mixers that use propellers and turbines.

Food Manufacturing

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.

Certain ingredients, like ketchup, mayonnaise, and dough, are challenging to process due to their viscoelastic properties, which require a minimum force to initiate flow. These materials demand specialized rotor-stator mixing heads to handle their unique characteristics effectively.

Pharmaceuticals and Cosmetics

In the pharmaceutical industry, various types of mixtures are processed, similar to the food industry. Inline high shear mixers are preferred for their closed system, which prevents contamination. Pharmaceutical products, including tablets, syrups, suspensions, injection solutions, ointments, gels, and creams, all undergo high shear mixing. These products have diverse viscosities and particle sizes, making high shear mixers essential for ensuring consistency and quality.

Paints and Coatings

Paints, such as latex, are non-Newtonian and thixotropic liquids, which can make them challenging to process. These paints thin out when sheared, whether during processing or in use. Therefore, mixing times for these fluids must be carefully controlled to avoid over-shearing and ensure the desired consistency.

Inks and Toners Manufacture

Inks used in printers exhibit the opposite behavior of paints; they are rheopectic. Rheopectic fluids thicken as they are sheared, making the mixing process highly time-dependent.

Petrochemicals

Applications in this category include combining resins and solvents for casting or injection molding, modifying oil viscosity, emulsifying waxes, and producing asphalt, among others.

Chapter 7: What should you consider when purchasing a high shear mixer?

The mixing process is central to many industrial applications, making high shear mixers crucial for producing high-quality products efficiently. Selecting the right high shear mixer for a specific application requires thorough study and research. Mixer manufacturers provide extensive literature to guide the purchasing process and offer sales assistance through highly trained and knowledgeable personnel.

Product

The first consideration when purchasing a high shear mixer is the type of product being processed. High shear mixers handle products with viscosities ranging from 1 centipoise (CPS) to 10,000 CPS, where 1 CPS is equivalent to the viscosity of water. Products with viscosities above 10,000 CPS are more resistant to flow and present greater mixing challenges.

Product Requirements

High shear mixers are versatile and can perform various mixing applications, such as particle size reduction, blending, gelling, dispersion, and emulsification. Before selecting a high shear mixer, it is crucial to understand the specific results required for the product and its consistency. Modern high shear mixers are highly adaptable and can be tailored to meet the needs of different products effectively.

Sizing

Sizing refers to the daily production rate, which can range from several hundred to several hundred thousand units. The efficiency of a high shear mixer is influenced by factors such as downtime, mixing duration, material supply, and transport of raw materials. For high-volume operations, multiple mixers may be necessary to achieve production goals, as large-scale production often requires several units to maintain efficiency.

Costs

Upfront costs and long-term amortization are key considerations for manufacturers. While purchasing a highly efficient and reliable high shear mixer may involve significant initial investment, it can offer substantial benefits, including reduced downtime, efficient production, and high-quality output. The cost of high shear mixers varies, and while some equipment can be quite expensive, the focus should be on the value it provides in terms of product quality and operational efficiency.

Customization

Customization is increasingly important in modern manufacturing, as many industries develop innovative and unique products requiring specialized equipment. While custom solutions can be costly, they can significantly enhance productivity and performance. To develop equipment tailored to specific needs, it is essential to collaborate with experts in the mixer industry for planning, engineering, and designing a mixer that meets the exact requirements of the product.

Conclusion

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.