This article takes an in depth look at homogenizers.
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A homogenizer is a type of mixing equipment used to create a uniform and consistent mixture. It works by breaking the components and evenly distributing them throughout the solution. The components are either immiscible, have varying sizes, or are in different phases from each other. Aside from creating a homogenous mixture, a homogenizer serves additional functions similar to other mixing equipment. Some of these functions are emulsifying, suspending, grinding, dispersing, and dissolving.
Homogenizers are extensively used in food and beverage production, pharmaceuticals, chemical processing, agricultural products manufacture, and laboratory and testing.
Homogenizers are typically used in tandem with high shear mixers, batch mixers, and paddle mixers. They are typically installed downstream to create finer mixtures. Some types of homogenizers are unable to accept products with highly coarse components due to the risk of high energy consumption, decreased flow rate, heat generation, and increased material wear. Mixers upstream of the homogenizer are used to condition the premix fluid.
The development of homogenizers started in the early 1900s when Auguste Gaulin invented a piece of equipment for homogenizing milk. The equipment was composed of a three-piston, positive displacement pump with capillary tubes fitted at the discharge. The capillary tubes acted as a throttling device that converted fluid pressure into velocity. Downstream of the capillary tubes was a concave valve where the jet of milk droplets impinged.
Subsequent developments to the device lead to the replacement of the capillary tubes by a single small tube. The important aspect here was the small gap featured by the tube where the homogenization process happened. Different gap geometries have been developed and have further improved the applications of the equipment.
As discussed in the previous chapter, homogenizers are used to mix emulsions and suspensions. An emulsion is a mixture of two or more liquids that are normally immiscible due to their liquid to liquid phase separation. This liquid to liquid phase separation is brought by several physical mechanisms such as surface tension, polarity or repulsion, and viscosity. Homogenized emulsions are sometimes referred to as colloids, which is a term used to cover a broader mixture classification.
A suspension is a mixture composed of solid particles that settle down and cannot be dissolved completely in the mixture. The separation of the solid particles is brought about by their large size. Their size is around hundreds to thousands of times larger than that of the dispersed particles in a homogenous solution.
Knowing the types of heterogeneous mixtures processed by a homogenizer, it is evident how a homogenizer works. A homogenizer works by breaking or subdividing the dispersed components into smaller particles and then distributing them evenly throughout the mixture. The action created by the homogenizer continuously disrupts the formation of large particles due to immiscibility and precipitation.
The homogenization process happens within the homogenizer valve, which is the main component of the equipment. It was earlier mentioned that the first homogenizer valve was an assembly with a capillary tube and a concave valve. The capillary tube throttled the fluid pressure and converted it into kinetic energy. The concave valve served as an impact surface for the fluid jet. Modern designs replaced the capillary tube with a seat that mates with the valve at an appropriate clearance to create a small gap for throttling flow. Within this gap, the fluid experiences the right flow conditions for homogenization through different physical principles.
Homogenizing action is created by the combined effect of three main physical principles:
Shearing in fluids is primarily caused by friction between fluid molecules due to viscosity. In a no-slip condition, adjacent fluid molecules have the same velocity. However, when a disruption such as acceleration caused by a rotor-stator or deflection caused by an impact ring is present, different velocities develop because of the fluid‘s internal friction. At the boundary layer or the layer between the homogenizer surface and the fluid, the velocity of the fluid is zero. Away from the boundary layer, the velocity of the fluid approaches the same magnitude as when the fluid was in no-slip condition. Shearing is experienced by a large particle or droplet when it is caught between fluid layers with different velocities. The shear forces break down large particles and droplets into smaller sizes.
Cavitation happens when a fluid experiences a large pressure drop. Typically, upstream of a homogenizer valve is a pump that introduces the fluid at a higher pressure. The pressure of the fluid is converted into kinetic energy as it passes through the homogenizer valve. When the pressure drop is large enough, the vapor pressure of the fluid exceeds the absolute pressure inside the homogenizer. This allows the momentary formation of cavities from small pockets of vapor. Upon the collapse or implosion of these cavities, shockwaves are released, and they break the particles and droplets in the mixture.
The last physical principle involved in homogenization is turbulence. Turbulence occurs when high velocity is attained by the fluid. The high velocity creates irregular motions within the fluid. These irregular motions are a form of energy dissipation wherein the kinetic energy of the fluid is converted into internal energy in the form of eddy currents and some amount of heat. The eddies generated help break the particles into finer sizes.
The extent to which each physical effect contributes to the homogenizing process depends on the design of the homogenizer valve and fluid properties such as temperature, pressure, composition, and viscosity. Still, most studies and experiments indicate that the turbulence effect is the primary mechanism for creating homogenization.
The generation of shearing, cavitation, and turbulence effects is not limited to a homogenizer valve. The other types operate differently than the original homogenizer but produce the same effect.
The high-pressure homogenizer (HPH) is the earliest type of industrial homogenizer developed and is the one frequently described in this article. Its versatility and homogenizing efficiency make it some of the most common homogenizer equipment in industrial and manufacturing setups.
A high-pressure homogenizer consists of two main parts: a high-pressure pump and a homogenization valve. The high-pressure pump is typically a positive displacement reciprocating type since these types are inherently suitable for viscous fluids and remain efficient while experiencing variations in flow and pressure. It usually consists of three or more pistons or plungers. Selecting a reciprocating pump with more plungers stabilizes the delivery of fluid into the homogenizer valve and reduces equipment vibration.
The pressure delivered by the pump depends on the type of fluid. Most homogenizers used in food and pharmaceutical industries operate at around 8,000 up to 40,000 psi (550 to 2,750 bars).
The homogenization valve, as previously described, is composed of a seat, valve, and impact ring. As the premix fluid passes through the valve, it increases its velocity; this creates turbulence. The turbulence, in turn, develops eddies that break down the components of the premix. The small gap between the seat and the valve helps create strong shearing forces, which also contribute to the disruption of components. Upon exiting the gap, cavitation occurs. The implosion from the cavitating fluid creates shockwaves that break the components. Cavitation further improves the efficiency of the homogenizing process.
The homogenizer valve is the most important part of a homogenizer assembly. There are different types of homogenizer valves. Each type has its set of advantages and drawbacks; these make them suitable for a particular application. Below are the types of homogenizer valves.
The radial diffuser valve is also known as the standard valve since it is the most extensively used in various industries. It consists of a plug and a seat. Typical design features a movable seat for adjusting the gap between the two parts.
In this type of homogenizer valve, the premix fluid initially flows axially and is deflected at a 90° angle by the plug. This forces the fluid to flow radially along the small gap. After exiting the gap, the fluid stream hits an annular surface called an impact or wear ring.
The main advantage of a radial diffuser valve is its ability to control the homogenizing pressure by adjusting only the gap size while the flow rate is kept constant.
An axial flow valve resembles an orifice valve. The small gap is created by an orifice, a venturi, or a short tube. Other axial flow valve designs feature a moving needle that is used to adjust the gap. The gap between the needle and the seat is oriented axially, hence the name. In a way, this is similar to a radial diffuser valve, which uses a plug.
The premix remains flowing axially as it passes through the orifice. Shearing, cavitation, and turbulence also take place within the small gap. Upon exiting the gap, the fluid jet is expanded without an impact chamber, unlike what is seen in a radial diffuser valve.
The design of axial flow valves varies depending on how the homogenizing pressure is controlled. Designs that only use static components control the homogenizing pressure by varying the flow rate. Designs that feature a moving needle control the pressure by adjusting the gap size.
In a counter jet valve, the incoming stream of premix fluid is divided into two or more streams using microchannels. These microchannels serve as the gap for homogenizing the premix fluid. Upon exit, the streams are at a high velocity and are made to impinge on one another. The microchannels direct the streams into a small area called the interaction chamber.
The benefit of using a counter jet valve is operation without moving parts. Thus, they have greater reliability than radial diffuser valves. Moreover, no impact ring tends to wear over time due to the continuous impingement of the fluid.
However, the drawback is its dependence on flow rate to control the homogenizing pressure. On top of that, since it divides the premix stream using multiple channels, it needs a high flow rate to operate properly. This limits the homogenizing pressure attained by the valve.
Mechanical homogenizers use mechanical work as the main source of energy for breaking the premix components. They function similarly to a high shear mixer. The premixed fluid or feed can be introduced at atmospheric pressure, low, or medium pressure, much lower than that of a high-pressure homogenizer. Instead of using a valve, rotating parts are used such as cones, blades, and paddles. The rotors are mated with an appropriate stator to create the desirable conditions for homogenization. The homogenization process relies on the mechanical tearing caused by the moving parts. Nevertheless, the previously mentioned physical principles involved in disrupting the particles still apply to mechanical homogenizers.
Below are the most popular types of mechanical homogenizers.
A colloid mill is a homogenizer composed of a conical rotor and stator. The rotor and the stator are separated by a small clearance where the premix will flow due to shear and centrifugal forces. As the premix is gravimetrically fed into the rotor-stator assembly by a hopper, it is thrown outward towards the exit slot or holes. The high rotating speed (around 3,000 to 15,000 rpm) of the rotor causes a tremendous amount of shearing, which breaks the components of the premix fluid. Moreover, since the fluid is accelerated by the rotor, high fluid velocities can be achieved. With enough velocity, turbulence is also developed.
The magnitude of shearing can be adjusted by varying the clearance between the rotor and stator. However, decreasing the clearance will negatively affect the flow rate of the product. This limits the resulting particle size; it is not as fine as those made by high pressure and ultrasonic homogenizers.
Colloid mills are used for highly viscous products or products with high amounts of suspended solid particles.
In terms of construction, these homogenizers are the closest to high shear mixers. Their rotor-stator assembly is sometimes called a mixing head, generator, or probe. The assembly is lowered into a batching tank, vessel, tube, or container where the premix fluid is homogenized.
Rotor-stator homogenizers work by accelerating the fluid tangentially, but because of fluid inertia, 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 are present; this produces high shear rates. The rotor and stator profile, their separation distance, and other features such as holes and slots all contribute to controlling the resulting particle size.
Bead mills (sometimes referred to as ball mills) are homogenizers that employ beads for mechanically grinding and breaking large particles dispersed in the premix fluid. The beads are grinding media that reduce particle size by strong impact and shearing forces.
The beads are loaded inside the container and are in contact with the premix fluid. They are then agitated by internal, rotating components such as paddles and blades. Agitation can also be done by centrifugally spinning the container at extremely high speeds. Agitation using rotating components is typically seen in larger homogenizers that are in line with the production stream. Agitation through a centrifugal action is commonly used in laboratories for preparing homogenized product samples.
These homogenizers use blades as their rotor. Unlike the colloid mills and rotor-stator homogenizers, they do not have a shear gap formed with a stator. The shearing effect is developed only by the high-speed rotation of the blade. Their construction and operation closely resemble that of a blender.
Blade-type homogenizers are less efficient than rotor-stator types. Their homogenizing ability is sufficient in creating a well emulsified and dispersed mixture. However, the resulting particle size is not as fine as those produced by the other types of homogenizers. To increase their homogenizing efficiency, abrasive media such as beads are used.
Ultrasonic homogenizers, also known as sonicators or sonic disruptors, take advantage of a physical principle called ultrasonic cavitation. Hence, the primary cause of component disruption is the cavitation effect. Cavitation is induced by creating alternating rarefaction and compression periods at ultrasonic frequencies. Ultrasonic frequencies are sound waves vibrating at 20 kHz or higher.
The rarefaction period creates the vapor cavity, while the compression period causes it to implode. These periods happen in one cycle of an ultrasonic wave. The voids created are microscopic in scale and cannot be seen during operation, but these are high-energy, localized regions that can reach extremely high temperatures and pressures.
Ultrasonic homogenizers are composed of three parts:
The generator is the part that receives electrical power and converts it into a suitable form for energizing the transducer at the desired frequency. The standard electrical frequency of power utility systems is 50 and 60 Hz. Since ultrasonic frequencies range from 20 kHz and above, the power supply frequency must be changed to the appropriate range. The power supply frequency depends on the properties of the premix fluid and must be fine-tuned to create properly sized cavities.
The transducer uses the high frequency, oscillating electrical current supplied by the generator and converts it into ultrasonic vibration. The most common type of transducer is the piezoelectric type. Piezoelectric transducers operate through inverse-piezoelectricity, which is the ability of a material to elongate or contract upon the application of an electric current.
The probe is what is in contact with the premix fluid. One end of the probe is connected to the transducer, which causes it to vibrate at the desired frequency. The vibration of the probe is transferred to the premix fluid where cavitation is developed.
Ultrasonic homogenizers are comparable to high-pressure homogenizers in terms of particle size reduction and energy efficiency. The main advantage of ultrasonic homogenizers is their operation at atmospheric pressure. Moreover, the degree of disruption can be easily varied by manipulating the electrical power supplied by the generator and the temperature of the premix fluid. All this is accomplished without using any moving parts.
Aside from producing emulsions and suspensions through particle size reduction and mixing, homogenizers also perform other functions, particularly in the food and pharmaceutical industries. Their ability to mechanically disrupt microorganisms and natural compounds extends their viability as processing equipment. However, these roles are limited to high pressure, ultrasonic, and bead mill homogenizers. These homogenizers can disrupt particles down to the nanoscale, ranging from 50 to 500 nm.
Microbial inactivation is one of the main process objectives in food and pharmaceutical manufacturing. Homogenizers induce this by finely breaking the cell structure of dispersed microorganisms. This potentially eliminates the risk of microbial growth, prolonging the shelf life of the product. Since homogenization relies on mechanical actions to disintegrate microorganisms, it becomes a particularly important alternative to heat treatment or pasteurization, both of which can degrade product quality.
Cell fractionation is the process of rupturing a cell while keeping its internal components intact. Controlling the degree of homogenization allows cell disruption and the preservation of intracellular components. The recovery of intracellular components is widely used in the biotech industry for making agricultural and pharmaceutical bioproducts.
High-pressure homogenization can modify the structure of enzymes. Enzymes are proteins acting as catalysts that speed up biological processes. By carefully controlling the homogenization pressure, enzymes can be targeted for activation or deactivation. This function presents a potential application in beverage and liquor production.
Compound extraction using homogenizers is called Homogenizer-assisted Extraction or HAE. The dynamic pressure experienced by biological matter through a homogenizer improves the stability and extractability of high-value compounds such as polyphenols, flavonoids, lycopene, and so on.
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