Selection of the most suitable mixing equipment for a particular task is the key to maximising process efficiency and obtaining a consistent, high-quality product, time after time.
For example, conventional agitators can give satisfactory results across the broadest range of capacities on viscosities, from that of water to highly viscous slurries in many applications, but their effectiveness tends to be limited to simple duties such as blending liquids of similar viscosities, maintaining in-tank uniformity, and promoting heat transfer.
With more demanding duties such as the formation of an emulsion, or the suspension, dispersion and hydration of powders, such as thickeners and stabilisers, and blending of liquids with widely differing viscosities, an agitator is at best only effective as a ‘process aid’, supplementing the action of equipment with a more positive mixing action.
In the food industry, a high shear rotor/stator mixer is probably the most efficient option for the above applications. These highly efficient mixers are supremely versatile and made of a robust and simple construction, which keeps maintenance and downtime to a minimum.
The advantages of the rotor/stator mixer over conventional agitators stem from the multistage mixing/shearing action as materials are drawn into the workhead by the high speed rotation of the rotor blades, subjected to intense hydraulic and mechanical shear, then forced out through the stator at great speed and projected radially back into the mix.
This positive mixing action results in dramatically shorter mixing times—in some applications, a 90 percent reduction is achievable. Other benefits include greatly improved product quality and consistency, and more easily reproducible results. The rotor/stator principle also offers unrivalled versatility.
A single machine can mix, emulsify, homogenise, solubilise, suspend, deagglomerate, disperse and disintegrate solids—duties that in the past may have required several different pieces of processing equipment. This is because the mixing characteristics of most rotor/stator type mixers can easily be modified by the use of interchangeable workheads or stator screens, each of which imparts different flow patterns and shear rates.
A workhead with round holes gives an exceptionally vigorous mixing action, ideal for general purpose use and particularly suitable for the disintegration of solids, such as frozen ingredients, fruit, vegetables, and nuts, and the preparation of suspensions and solutions, such as brines, syrups and gum solutions.
Slotted holes produce a more scissor-like shearing action, suitable for disintegration of fibrous or elastic materials. Some manufacturers offer a wider range of stator screens, including square hole screens, which are used in applications where particle size reduction is required, and for the preparation of emulsions and fine colloidal suspensions. There are also dedicated emulsor screens, often available in a range of perforation sizes.
In Or Out
The types of rotor/stator mixer available can be divided into two categories: those which operate immersed in the vessel, and external or ‘in-line mixers’. These are both generally available in a range of sizes, from laboratory mixers up to large production-scale units.
Some manufacturers specialise in the smaller scale, but many offer a full range of mixers, with the added advantage that results from the ability of the laboratory or test kitchen to be scaled up reliably to full production. A wide range of ‘standard’ models is available, as well as many specialised designs, and due to the versatility of the rotor/stator principle, they can readily be modified and even custom built to accommodate clients’ specific process requirements.
Immersion or batch mixers are generally suitable for volumes of up to 1,000 litres, depending on viscosity, and can be either vessel mounted or used with a mobile floor stand. This allows a mixer to be repositioned during operation to give the optimum mixing pattern, for example, the mixer can be positioned so as to create a vortex for rapid incorporation of powder into the surrounding liquid. Once this has been achieved, the unit can be lowered to decrease surface movement, reducing aeration to a minimum.
Bottom entry mixers also fall into the in-tank mixer type. They are widely used in the food industry, especially with higher viscosity mixes or products that increase in viscosity upon cooking. Typically, this would be in a hemispherical vessel (often jacketed for heating) in conjunction with a stirrer/scraper.
The high shear mixer and stirrer/scraper are complementary to one another; the bottom entry rotor/stator mixer provides high shear processing of the product, which the low shear scraper unit cannot impart, while the scraper unit assists by distributing the output of the high shear mixer uniformly throughout the vessel.
Recent advances in high shear technology include the use of more powerful bottom entry mixers, which can operate without the need for the stirrer scraper. These are typically used for the disintegration of solids and incorporation of powders.
Another major benefit of bottom entry mixers is the elimination of immersed frame arms and the mixer shaft, reducing product contact parts to an absolute minimum. This offers considerable advantages in process hygiene. Indeed, these mixers can be designed and manufactured to an ‘ultra hygienic’ specification, and are increasingly specified in the food industry for this reason.
Clean By Design
The demand for improvements in process hygiene is undoubtedly one of the most significant trends in food processing equipment, and has been the major driver in the development of high shear mixing technology in recent years.
The concept of ‘ultra hygienic’ mixing equipment has revolutionised approaches to design and manufacture, resulting in a new generation of mixers that meet and often exceed recognised standards and guidelines, such as 3-A, the European Hygienic Engineering and Design Group (EHEDG), FDA and cGMP.
As such, the food industry is second only to the pharmaceutical sector in the demands it places on the equipment supplier. When approaching design from the hygiene point of view, the first consideration is the choice of materials.
The accepted standard for most applications is 316L stainless steel. This grade offers greater resistance to pitting and corrosion and is suitable for use over a wide range of temperatures.
New manufacturing techniques and design features, such as crevice free construction, self draining designs with zero retention, electropolishing and the selection of raw materials and components from the US Food & Drug Administration Master List are also key to improving hygienic construction.
The preparation of Data Dossiers to meet regulatory requirements is becoming the norm in the food industry, whereas even a few years ago, this was a rarity.
Getting In Line
These advances in hygienic design have also led to the increased use of in-line mixers as opposed to in-tank devices. As the name suggests, in-line mixers operate outside of the vessel, usually recirculating product around the vessel.
They can also be used for continuous single-pass processing with some products, or for passing the product backwards and forwards between two vessels. The latter gives the ultimate control as the product can be processed by a defined amount.
In-line mixers are generally suitable for clean-in-place (CIP) duties and in some cases, can be supplied to a specification suitable for sterilisation-in-place (SIP). The new generation of ‘ultra hygienic’ mixers are generally self-draining and feature crevice-free design and special shaft seals and elastomers, with a range of motor options and other modifications being available to suit the application.
Erik Anestad, US
In-line high shear mixers can be added to an existing process with a minimum of expense, while providing a dramatic increase in productivity—and product quality. Often, an in-line mixer can be installed in place of a centrifugal pump, since the unit provides its own self-pumping capacity.
Equally, they can replace traditional low-shear devices such as in-line agitators and static mixers—a series of baffles inside the pipeline designed to mix merely by disruption of flow. Since both of these devices require pump feeding, the benefits of the high shear mixer are considerable.
The in-line mixer is based on the same rotor/stator principle as immersion mixers; however, the workhead is mounted in an enclosed chamber in a pipeline. As the processing vessel, pipework and in-line mixer form a closed system, aeration is eliminated and bypassing is impossible—the entire contents of the vessel must pass through the mixer.
A further advantage of this system is that the machine’s effort is concentrated on the small volume of material inside the mixing chamber at any given moment, and power is not wasted moving large volumes of liquid. Consequently, a relatively small in-line unit can process volumes, which would require a much larger immersion type mixer.
Most in-line high shear mixers also produce a non-positive pumping action, which is sufficient for most process requirements, although larger batches may require an auxiliary agitator to ensure in-tank uniformity.
Another type of in-line device is the so-called shear pump. These are generally low-cost units based on a centrifugal pump, modified to impart a degree of shear. While they may offer an advantage over conventional static mixers or agitators, ironically, the design modifications can often result in a reduction in the efficiency of the pump, without a significant mixing capability, and as such, they are not considered as true high shear mixers within this article.
Incorporating powders into liquids is one of the most challenging mixing applications. Additives such as gums and thickeners and other ‘functional ingredients’ are by nature liable to form agglomerates, which must be completely broken down to achieve a smooth end product, and maximise yield and thickening or stabilising effect.
Light powders, which tend to float or ‘raft’, can form a ‘scumline’ around the top of the vessel, and partially hydrated material tends to stick to parts of the agitator and vessel walls. Rather than try to incorporate the powder by adding it straight into the vessel, an effective means of overcoming these problems is to add it to a stream of liquid, as with the venturi principle, which can greatly reduce the formation of agglomerates, but cannot produce a completely lump-free dispersion.
High shear mixers have been specially developed to overcome these difficulties, some improving on the venturi principle, others using completely different approaches. A wide range of powder/liquid high shear mixers is available from many manufacturers.
Typically, the liquid part of the formulation is recirculated from the process vessel through the mixer, and the powder is sucked from a hopper into the liquid stream and then passes directly into the high shear mixer’s rotor/stator assembly, which subjects the mixture to intense but targeted high shear before pumping the product into the process vessel or subsequent manufacturing stages.
This ensures a homogeneous, agglomerate-free dispersion of powder with minimised aeration and foaming. Again, as with in-line mixers, these powder/liquid mixers can be supplied to an ‘ultra hygienic’ specification, with CIP and even SIP capability.
As mentioned above, apart from the broad categorisation of mixers into in-tank and in-line types, the rotor/stator principle is readily adapted to suit various process requirements and can be modified to suit a wide range of specific applications and installations.
This can vary from a simple task such as sizing a mixer for an existing vessel, to a more complex project such as an automated system. As a result, equipment suppliers are increasingly working ever more closely with their raw material and component suppliers, as well as the end user, to develop mixers custom built to suit each application.
A broad application knowledge as well as close links with the regulatory bodies also ensure the correct balance is struck between the requirements of the physical task—ie: the mixing job itself—and the conditions under which that task must be carried out.
Consequently, equipment suppliers are being selected increasingly for their ability to provide innovative solutions to new problems, a trend which is set to continue.