What is a Homogenizer?
A homogenizer can be perceived as a specialized mixer, which forces substances through a narrow channel, at which time it can create a uniform and equal mix.
The force applied, either turbulence, cavitation, or high pressure, allows the contents of the solution to be homogenized at the same time.
Homogenizers have a positive displacement pump as part of their design and a special type of homogenizing valve. The pump will push material under pressure through a narrow channel between the valve seat and the valve assembly.
The pressure created, along with the flow through the system, will create turbulent flow inside the material. Most industries use homogenizers not only to mix products but also to manufacture stable, uniform, and consistent products.
Homogenizers are able to emulsify, suspend, grind, disperse and dissolve materials. The industries that rely on the homogenizers to improve quality and consistency are pharmaceuticals, beverages, and chemicals.
Homogenizers are often used with high shear mixers, batch mixers, and paddle mixers and are located down-stream to obtain more refined mixtures. However, some homogenizers have limiting factors in processing material with a very coarse particle.
This may result in increased energy consumption, decreased flow rate, excess heat, and undue wear on the material for a homogenizer. An upstream mixer will condition and prepare the material with premixing to begin processing with the homogenizers.
History
In the early 20th century, homogenizers were invented and developed when Auguste Gaulin introduced the first design for milk homogenization.
He had a positive displacement three pistons pump that forced fluid through capillary tubes at the outlet acting as throttling devices that transformed pressure into velocity.
As a result, the milk droplets against a concave valve downstream from the capillary tubes were forced through.
Subsequent refinements in homogenizer technology further simplified the designs by eliminating multiple capillary tubes and moving to one narrow tube.
The important technical innovation was in the narrow gap inside this tube; this was where the main homogenization process occurred and soon resulted in multiple gap geometries to increase the range of applications and greater efficacy of the equipment.
What are the Theories and Principles Behind Homogenizers?
The development of numerous theories has occurred over the years regarding the science reflecting the process of homogenization, particularly in relation to high pressure homogenizers.
Homogenization is an essential engineering process in a wide variety of industries and is driven through the desire of achieving repeatable fine particle/droplet size distribution of fluid mixtures: food and beverage production, pharmaceuticals, cosmetics, biotechnology, and chemical processing.
Globule disruption by turbulence and cavitation are currently the two most recognized theories that explain how homogenizers work, and these first principles show how homogenizers use mechanical forces to create/add stability to emulsions and suspensions.
The globule disruption by turbulence theory states that when a liquid jet passes through a small gap or valve, it is subjected to great turbulence, by creating numerous small eddies, otherwise referred to as micro whirls.
The induced turbulence will be intensified, and the velocity will increased, as the pressure is increased in the jet flow creating a greater mass of energy and velocity, coalescing droplets are formed.
Droplets will collide and when the droplets collide, the droplets will elongate, deform, and break up into a smaller, more even, droplet sizes.
The measure of the degree of reduction of droplets or particles in size, would be commonly measured as particle sizes distribution, generally, the degree of reduction is directly proportional to the pressure applied in homogenization.
Aims to optimize the use of pressure in relation to the product of interest for uniformity of product quality.
Cavitation theory describes the vapor bubbles or cavities formed by sudden pressures changes and collapses in liquid during homogenization or emulsification.
When bubbles or cavities cavitate/collapsed, vapor bubbles that explode into a high energy jet of liquid generate highest localized intensity of high speed micro jets that can rapidly impact suspended droplets, fat globules, and solid particles.
The uncontrolled release of energy imparted during cavitation, disperses similar particles, producing a finer size, more homogenous mixture.
Homogenization relies heavily on a combination of turbulence and cavitation to mechanically disrupt particles/beads either by mechanical homogenization through high pressure homogenizers, or by ultrasonic homogenizers.
Homogenizers have the general purpose of mixing and stabilizing emulsions and suspensions. An emulsion is a heterogeneous mixture of two or more immiscible liquids, such as oil and water, that do not easily intermingle due to factors such as interfacial surface tension, molecular polarity, and viscosity characterizing each phase.
The main purpose of homogenization is to provide vendor consistency and evenness in dispersivity of droplets as well as reduction in droplet size, from a emulsion to a colloidal solution, including colour, stability over time and texture of the product, as well as shelf life.
Examples of emulsions are emulsion of milk, mayonnaise, paints, and creams. Homogenization is a common processing step in dairy and food production, when micro or nano-emulsions are produced; in order to meet safety and quality specifications.
A suspension is a mixture containing solid particles degree of aggregation that does not dissolve and separate over time. As the particles settle, phase separation may occur.
In suspensions, particle size can range from a visible aggregate of particles, to particulate sizes often in the hundreds or thousands of times larger than true solutions.
Non-homogeneous mixtures such as, pharmaceutical suspensions, pigment dispersions, and wastewater treatments are produced with in production lines that often require homogenizers to reduce particles size and create relative uniformity of the product by preventing sedimentation, separation, and agglomeration.
Particle sizes in suspensions vary widely from very large to small ones, some large enough to be visible to the naked eye. By design, a homogenizer can add sufficient mechanical shear, impact, and high pressure to disperse all particle types or sizes (including coarse particles).
All solids will be better integrated with liquids as a result of homogenization and will benefit overall product consistency, improve bioavailability, increase process yield; which are real benefits in industrial chemical, biotechnological and laboratory sample preparation processes.
Understanding the types of heterogeneous mixtures—emulsions, suspensions and colloids—that homogenizers process is helpful for appreciating the value of homogenizers across different applications.
Homogenization technology works by breaking down the dispersed phase particles (hopefully by some type of micronization or nano-milling process) into very small particles, and their more uniform and rapid distribution through the continuous phase; thus, the homogenizer is preventing the particles from agglomerating and precipitating over time, resulting in a more stable product (in terms of uniform viscosity and texture) and performance characteristics.
Homogenization happens with the homogenizer valve, as it is the main mechanical part that regulates product consistency and efficiency.
The homogenizer valve, which originally via capillary tube and concave to restrict fluid flow, turned pressure into high velocity kinetic energy and created an impact zone for the disintegration to happen.
Now with high pressure homogenization, there are new valve designs including flat, conical or multi-orifice valves to regulate the gap opening and optimize flow parameters relative to the viscosity of the product and size reduction of the dispersed particles.
Each of those hydraulic flow variables is one parameter in creating a bespoke homogenizing scenario, while also producing efficiencies and improved cleaning processes.
Homogenization’s physical mechanisms depend on and proceed based on three fundamental principles.
- Shearing
- Cavitation
- Turbulence
1. Shearing
Shearing in liquids arises from friction between neighbouring molecules, limited by the viscosity of the liquid.
As these molecules flow, especially between two fixed surfaces and moving fluid layers at the rotor boundary (such as rotor-stator homogenizers), the differences in velocity cause high shear force to occur.
Large droplets or aggregates that are caught between fluid layers moving at different velocities are stretched and broken down to smaller particles by mechanical and hydraulic shear forces.
There is improved dispersion and reduced particle size which is needed for stable emulsions and suspensions, such as those required for pharmaceutical formulations, as well as with nanoemulsions and cream-like products.
2. cavitation
The homogenizer valve causes cavitation as a result of rapid changes in pressure within a fluid as liquid is transported under high velocity.
An industrial pump forces the fluid under pressure; when liquid enters the narrow gap of the homogenizer, energy from the pressure translates to kinetic energy, which may drop below the vapor pressure of the fluid.
High speed vapor cavities may form due to this decreased pressure, which only last for very short periods of time. When the vapor cavity implodes, high speed shockwaves are generated which separate suspended particles and droplets apart, moving them away from each other.
This accelerates homogenization or emulsification. If cavitation can be efficiently generated and sustained, this will improve efficiencies in cell rupture (lysis), lump-free blending and size reductions that are important to biotechnology and nanotechnology systems.
3.Turbulence
The final principle of turbulence. When applying high flow velocities through free passage restrictive valve types, fluid motion becomes extremely chaotic, irregular, and filled with random gyrations (turbulent flow).
The turbulent swirls dissipate kinetic energy via collisions and fluctuations, acting with physical forces that further break apart droplets and particles.
The amount of turbulence, which is set by the homogenizer settings, primarily influences the final particle size, emulsion stability, and consistency of the product.
These characteristics are some of the most important variables in the quality assurance of food processing, chemical engineering, and pharmaceutical manufacturing.
The relative levels of influence exhibited by shearing, cavitation, and turbulence on any given homogenizing effect differs based on several primary factors including: homogenizer valve designs, process pressure, viscosity of the fluid, control of the temperature and feed components.
Most of the literature suggests that turbulence is the dominant mechanism for homogenization, especially in high-pressure homogenizers, but some of the more sophisticated ultrasonic and microfluidizer systems may prioritize different characteristics for particular applications.
While its essential to understand that there are physical impacts that we often see with traditional piston-type or high-pressure homogenizers, there are several other homogenizing technologies like ultrasonic homogenizers, bead mills, and high-shear mixers that have entirely different mechanical actions, but achieve similar effects on particle size reduction, cell disruption and emulsion formations.
Selecting the optimum homogenizer and operative parameters will allow manufacturers to deliver products with optimized quality, efficiently, and cost-effectively to fit the needs of their industry.
High-pressure homogenization (HPH) is one of the earliest used developed and most widely recognized industrial homogenizers available today.
The HPH is well known for its performance, efficiency, power, and versatility and has since become a popular option in several industries for the manufacturing pharmaceutical drugs and preparing food products around the world where producing proper dispersion, particle size reduction, and emulsification is essential for product quality and consistency.
High-pressure homogenizers, alternatively known as piston or high-shear homogenizers, are composed of a strong high-pressure pump coupled with a precision engineered homogenization valve.
The positive displacement reciprocating pump in these machines is effective in processing viscous liquids, suspensions, and emulsions due to its ability to maintain process consistency when flow rate or pressure varies, without sacrificing mechanical efficiency.
Industrial HPH applications will contain an excessive number of pistons/blowers (greater than 3 initially), which work together to stabilise fluid delivery, reduce pulsation amplitudes, reduce oscillatory vibration for longer equipment life, and to produce a consistent fluid delivery.
The majority of commercial HPH operating are typically between (8,000 psi to 40,000 psi) (550 bars – 2,750 bars) which allows manufacturers to achieve ultra-fine sized particles and droplets.
For example, the food industry depends on HPH’s for producing smooth dairy, sauces, and beverages with osmotically desirable textures; whereas the drug industry depend on HPH’s for processes such as cell lysis, nanoemulsifying, and to disperse an API (active pharmaceutical ingredient) into a carrier drug for further use.
The main processing piece of equipment at the core of any HPH system is the homogenization valve assembly including a seat, valve, and impact (or wear) ring.
When processing fluids the particles in the flow stream (solids or immiscible) and the fluids themselves are displaced at a very high flow rate through the narrow gap between the valve and the seat which creates extreme turbulence and powerful shear forces (for processing purposes) to break particulates down, disrupt cell walls, and produce highly stable emulsions and dispersions.
Additionally, cavitation (the rapid formation of microbubbles that rapidly implode to create a high energy state to break down small particles and or surface tension between immiscible particles) will occur upon exiting the high-pressure gap due to the differing energy flows from high to low energy for the processed fluid; the flow difference produces shockwaves across the microbubbles that disassociate particles even smaller and homogenises the entire mixture.
The homogenisation process is essential for product stability, maximum shelf-life, bioavailability, and quality concerning texture of consumer products and immiscible pharmaceutical drug applications.
Selecting the correct homogenizer valve is important to optimize potential results and energy usage. Each homogenizer valve will have distinct advantages, performance abilities, and be applicable to certain raw materials, or homogenization goals.
The following contains the most common homogenizer types, as these are the major contributors to industry advances in mixing and high-shear processing.
Radial Diffuser Valve
The radial diffuser valve, often referred to as the “standard” valve for high-pressure homogenization, is extensively used in a wide variety of industries, including dairy, beverage, chemical, and cosmetic.
The design of this valve incorporates a plug with a seat, and many of the newer designs allow for the adjustment of the seat providing accurate control of this critical gap, one of the key process parameters regarding particle size and emulsion stability.
While in operation, the premix fluid travels axially until it is redirected at 90° by the plug and requires the fluid to travel radially through a designer gap.
The fluid stream will then impinge on the annular surface or the impact or wear ring where additional mechanical energy will continue to aid in the size reduction of fine particles and droplet size uniformity.
The primary benefit with this type of valve is the ability to control the homogenizing pressure through the gap and thus the continuous ability to operate in a true constant flow mode.
Also, this valve functions especially well in controlling the homogenization efficiency. It is particularly useful in process environments where consistency, repeatability, and flexibility of formulation and the process is required.
Axial Flow Valve
An axial flow valve operates on the same principle as an orifice or venturi valve, using a small precisely manufactured gap (usually made with either an orifice plate, venturi constriction, or short section of tubing) to create controllable shearing conditions.
Some of these valves have a movable needle that is aligned axially with the flow, so the operator can adjust the gap in real-time to account for changes in process pressure or viscosity.
The variability built into axial flow valves allows a fine-tuning of homogenization intensity, which can be advantageous in certain segments of biotechnology, nutraceutical manufacturing, and nanoemulsion preparation.
As the premix fluid flows axially through the orifice, shear, cavitation, and turbulence are all introduced into the narrow confinement of the orifice.
As opposed to radial diffuser valves, the axial flow configuration does not use an impact ring, which results in a directed jet with a predictable velocity profile for ongoing processing (Henry, 2021).
Axial flow valve designs can be either dynamically or statically controlled. Static designs allow for adjustments to the pressure through the modulation of flow, while designs with a needle that moves vertically can allow for a much more precise changing of the orifice width to maintain optimal process pressure and product consistency with sensitive bio-based or high viscosity formulations.
Counter Jet Valve
When a counter jet valve sprays existing premix fluid, the premix fluid is delivered through advanced microchannel technology allowing the two (or more) streams created to be high kinetic energy streams splitting the incoming premix directionally toward one another within an interaction chamber causing a head-on collision of the streams.
The mechanisms utilized for high shear mixing rely on maintaining dynamic energy and a high degree of particle disruption without any moving parts, making counter jet valves particularly viable in the areas of dependability needed for applications like continuous pharmaceutical production and advanced material processing.
There are no moving parts, so all the wear is removed and maintenance is greatly reduced, increasing overall equipment availability.
However, counter jet valves need a minimum flow rate to transfer energy effectively, which can limit the maximum homogenization pressure able to be achieved in systems working with low-volume or very viscous products.
The counter jet valve performs equally well in terms of scalability and throughput, due to their ability to provide highly uniform particle size distribution, and consistent cell disruption rapidly, critical in applications where accurate scaling of the output product is desired.
Microfluidizer
A microfluidizer is a high-shear homogenizing device that moves product from an inlet reservoir into a proprietary pumping device that can apply pressures up to 30,000 psi through microchannels in an interaction chamber.
These shear rates and impact forces are extraordinarily high and promote highly efficient cell disruption, nanoemulsification, nanomilling, and particle size reduction.
Microfluidizers are essentials in the Pharmaceutical R&D, Nanotechnology, Cosmetics, and Food Science fields for innovation.
Inside a microfluidizer, product that is being moved through the microchannels collides with both the chamber walls and each other.
The product is homogenized as it moves through microchannels resulting in tight particle size profiles (narrow particle distribution) for enhanced bioavailability of active ingredients while still remaining smooth, stable, and reproducible – important aspects that meet regulatory and quality control standards in commercial environments.
When considering homogenizer valve technologies, make sure to contemplate: fluid properties, desired particle or droplet size, batch or continuous mode, maintenance costs, lifetime efficiency, and potential value/benefit of the paths to least resistance.
Working with experienced process engineers can help develop and evaluate your options to positively influence process output and decant with negotiate the scalability for usage in high growth sectors.
What are mechanical homogenizers?
Mechanical homogenizers function via mechanical work to break down components of the premix in a manner similar to high shear mixers.
The premixed fluid may be introduced into the system at atmospheric, low, or medium pressures that are much lower than the operating pressures for pressure homogenizers.
Mechanical homogenization is performed with rotating elements (like cones, blades and paddles) and with no valve. In mechanical homogenization, a stator is functionally engaged with the rotor to provide proper homogenization conditions.
Mechanical homogenizers use mechanical tearing through the mechanical moving parts similar to high shear mixers; however, the underlying methods of particle disruption remain constant.
Following are some of the more common types of mechanical homogenizers.
Colloid Mill
A colloid mill is a type of homogenizer with conical rotor and stator. The rotor is mounted with a stator, with a narrow gap in between for the premix to flow under shear and centrifugal forces, which applies controlled agitation.
The premix is delivered to the rotor-stator unit via a container hopper. The rotor propels the premix outward through exit slots or holes.
The rotor rotates at very high speeds, between about 3,000 – 15,000 rpm, causing shear forces to dissipate the premix composition. The rotor also provides very high velocities to the fluid which causes very high turbulent forces.
Control of shearing in a colloid mill can be managed with the adjustment of the gap between the rotor and stator. Reducing the gap does compromise the flow rate of the product, limiting the reduction in fineness of particle size associated with high-pressure or ultrasonic homogenizers.
Colloid mills work well with highly viscous materials, or products with large amounts of suspended solids.
Rotor-Stator Homogenizers
These homogenizers are built much like high-shear mixers. They consist of a rotor-stator unit (also called a mixing head, generator, or probe) that is located in the container where the premix fluid will be homogenized (typically a batching tank or vessel).
Rotor-stator homogenizers function by accelerating the fluid outwards in a tangential manner. However, the fluid has inertia; therefore, it does not match the speed of the rotor completely, but is guided to the shear gap between the rotor’s edge and the stator.
The result is high velocity differentials and turbulent flow across the shear gap with high shear rates. The size of the particles produced will be dictated on the rotor and stator types (and potential designs), including the gap between the rotor and stator and also – in the case of high capacity rotor-stator homogenizers – the holes and slots.
The material will be drawn into the rotor-stator system in a spiral motion due to the high speed. The fluid will be processed due to a combination of suction and high shaving forces within the shear gap.
Rotor-stator homogenizers are not appropriate for solid samples, and they can be time-intensive when homogenizing multiple samples.
Bead Mill
Bead mills, also called ball mills, employ the use of beads as grinding media to mechanically crush and reduce large particles that are suspended in the premix fluid. The beads supply significant impact and shear forces to reduce particle size.
The beads are placed into the container and places in contact with the premix fluid. They are agitated either by internal rotating components like paddles and blades, or through centrifugal spinning of the container at extremely high speeds.
The agitation through rotating components is generally found in larger homogenizers that follow the production stream. Agitation through centrifugal action is often found in laboratories that prepare homogenized product samples.
Blade Type Homogenizers
These homogenizers use blades for their rotors and unlike colloid mills and rotor-stator homogenizers there is no shear gap created with a stator.
Instead, the shearing action is only produced by the high-speed rotation of blades. Their design and use is not complicated and they are essentially blenders.
Blade-type homogenizers generally have lower homogeneatization efficiency than rotor-stator type homogenizers.
This is particularly the case when we are discussing size reduction of monodisperse, emulsion, or disperse systems to smaller particle sizes, as with all homogenizers, blade-type homogenizers still produce emulsified and dispersed mixtures properly.
Because blade-type homogenizers generally produce larger particle size distributions than other homogenizing equipment, abrasive agents (e.g., beads, sand) can be used to increase the homogenizing action of the blade-type homogenizers.
What are ultrasonic homogenizers?
Ultrasonic homogenizers, often called sonicators or sonic disruptors, are based on ultrasonic cavitation principles. Ultrasonic homogenizers produce sound waves operating at ultrasonic frequencies, usually above 20 kHz.
The process of cavitation occurs in the production of sound waves by the cycling of compression and rarefaction, where cavitation is the primary mechanism of disruption of any of the components.
Vapor cavities are created during the rare factional phase of ultrasonic wave root propagation while cavities collapse during the compression phase of the coupling.
The cavity is created and destroyed multiple times in cycles of compression followed by rare faction cycles.
While these cavities are extremely small and not perceptible during operation, they are extremely localized high-energy areas with the ability to achieve very high temperatures and extremely high pressures.
Ultrasonic homogenizers are composed of three components:
- Generator: The generator is the component that takes on electrical energy and transforms it into a usable format to power the transducer at a predetermined frequency. The frequency of electrical power by public utility power systems is 50 and 60 Hz (low frequency). Ultrasonic frequencies are characterized as being 20 kHz and above frequency range. Therefore, it is necessary to change the power supply frequency into the appropriate level for each application. The power supply frequency is dictated by the characteristics of the premix fluid, and levels must be adjusted to produce the proper sized cavities.
- Transducer: The transducer utilizes the high-frequency oscillating electrical current power supplied by the generator and converts into ultrasonic vibration. The transducer most likely used is a piezoelectric type. Piezoelectric transducers operate through inverse-piezoelectricity, or the characteristics of a material to elongate and contract upon the application of an electric current.
- Probe: The probe is the component in contact with the premix fluid. One end of the probe is held in contact with the transducer, which causes the probe to vibrate at a desired frequency. The probe’s vibration is transferred to the premix fluid and allows cavitation to occur.
Ultrasonic homogenizers are alike to high-pressure homogenizers with respect to reduction in size of particles and energy efficiency.
The ultrasonic homogenizers operate at atmospheric pressure, which is a distinct advantage. Furthermore, the level of disruption is controllable by changing the electrical power at the generator, as well as the temperature of the premix fluid, all without any moving parts.
What extra functions can a homogenizer perform?
Homogenizers are used not only for making emulsions and suspensions via particle size reduction and mixing but also for additional purposes, especially in the food and pharmaceutical industries.
Their potential in mechanically disrupting microorganisms and natural products expands the opportunity for its use as process equipment.
However, these activities will be limited to high-pressure, ultrasonic, and bead mill homogenizers. In fact, the nominated homogenizers can disrupt particles to the nanoscale, which is from 50 to 500 nm.
- Microbial Inactivation: Microbial inactivation is a core process in food and pharmaceutical production. Microbial inactivation works by finely fracturing the cellular structure of dispersed organisms, thus mitigating risk of microbial growth and allowing for a longer shelf life of the product in question since homogenization processes use mechanical action to rupture the cellular structure of microorganisms, it serves an important alternative route to heating or pasteurizing, which can damage or alter the final product.
- Cell Fractionation: This process obliterating a single cell, with the goal of preserving intracellular components of interest. By regulating the degree of homogenization, one can disrupt the cell and preserve any intracellular components. Recovery of intracellular components is commonplace in the biotechnology industry for making agricultural and pharmaceutical bioproducts.
- Enzyme Activation/Inactivation: High-pressure homogenization can also be used to restructure and render enzymatic activity. Enzymes are proteins that function as catalysts, speeding up biological activity. Homogenization provides an controllable pressure, which can be used to inactivate or activate enzymes. This aspect has potential applications in beverage and liquor production.
- Compound Extraction: Extracting these compounds using homogenizers is known as Homogenizer-assisted Extraction, or HAE. The dynamic pressure exposed to the biological material during a homogenization process allows for a more stable extractability and efficacy in extracting high value compounds like, polyphenols, flavonoids, lycopene, etc.
Conclusion
A homogenizer is a type of mixer used to make mixtures homogeneous and consistent. They can perform the mixing process by breaking the components and dispersing them evenly throughout the solution.
Homogenizers were first invented by Auguste Gaulin for the homogenization of milk. The equipment consisted of a three-piston positive displacement pump with capillary tubes fitted into the discharge.
The process of homogenization happens inside the homogenizer valve, which is the main component of the homogenizer.
The homogenizing action occurs as the combined effect of the three main physical principles: shearing, cavitation, and turbulence.
The three main types of homogenizers are high pressure (HPH), mechanical, and ultrasonic.
Homogenizers can perform other functions besides emulsifying and dispersing. Some examples are microbial inactivation, cell fractionation, enzyme activation/inactivation, and extraction of compounds.