Many experiments nowadays, be it on the bench scale, pilot scale or industrial scale, involve the mixing of one or more reagents to either produce a reaction or solubilize solid particulates into a liquid medium.
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The effectiveness of the mixing process can be key to determining whether the overall process is viable on any scale and can be the difference between a reaction working and the difference between a high and low yield. Here, we dissect the principles behind the fluidic mixing process and look at a few main types of equipment that are used in both academic and industrial laboratories.
Mixing Principles
The effectiveness of a mixing process is determined by the flow rate of the liquid which in turn determines how much of a reagent/additive is solubilised into the system. There are two main types of fluidic flow- laminar (uniform and non-uniform) and turbulent flow.
The type of flow exhibited within a system is determined by one of two main parameters, fluid volumetric flow rate and mass flow rate, depending on the type of mixing. Regarding these flow parameters, fluid volumetric flow concerns itself with solutions and is governed by the amount of solution flowing past a defined point at a given time, whereas mass flow rate involves the amount of mass movement that flows past a given point.
There are also many factors that can affect the flow rate. The main parameters which could affect the flow rate are the viscosity of the liquids, density of the liquids and the friction of the liquid in contact with the mixing vessel.
Depending on the size and type of the vessel, the type of flow-derived mixing can be very important. In most cases, it is beneficial to achieve a turbulent flow as it allows for better mixing both laterally and vertically in a mixing vessel. But this is not always possible, so laminar mixing sometimes must be used.
Laminar Flow
Laminar type mixing is the least effective mixing method and is often used in large pilot scale equipment, such as food mixers, or in magnetic stirring approaches. Magnetic stirring through stirrer hotplates can reach turbulent flow, but at lower speeds only produces a laminar flow.
A laminar flow is where the liquid undergoing mixing flows in layers which pass over each other. These can either be uniform or non-uniform, and lead to axisymmetric and asymmetric flows, respectively.
The fluid in a laminar flow follows a smooth path and the layers of fluid never interfere with each other, hence, why this type of mixing is not always effective. However, in laminar-type mixing, the velocity of the fluid is constant at any point within the fluid.
Turbulent Flow
A turbulent flow ensures a greater mixing efficiency and is often produced at high speeds, be it from a stirrer hot plate, vortex mixer, ball mill mixer or a large-scale mixer with high speed capabilities.
Unlike the laminar flow, a turbulent flow distorts the interface between the fluidic layers in the mixing vessels, breaking them down, and allows for mixing in both lateral and vertical dimensions. Turbulent mixing often forms whirlpools- if the reaction vessel you are using has not formed a whirlpool, then you have not achieved a turbulent flow.
This mixing type is especially useful for solubilisation and the mixing of reactants as it allows for all the components to mixed in all 3 dimensions of the vessel. A turbulent flow occurs at much higher speeds as the energy required to break the interface is high, but, the velocity of the fluid is not constant at every point. The point at which a flow becomes turbulent is often defined by Reynolds number.
Reynolds Number
Reynolds number is a dimensionless value used to predict the flow patterns in a fluid. This extends to general fluidic flow and the flow during a mixing process. Reynolds number combines the parameters of the fluids density, the fluids velocity, a linear dimension measured in metres, the dynamic viscosity of the fluid and the kinematic velocity of the fluid to output a number that corresponds to whether a fluid has a laminar or turbulent flow pattern.
In simple terms, Reynolds number is governed by ratio of the inertial forces and the viscosity forces within the fluid. The value to achieve turbulent flow depends on the type, size and shape of the particulates in the fluidic solution, so is a variable quantity depending on the process.
Bulk Mixers
Bulk mixers come in varying sizes, from small scale magnetic stirrer hot plates, to pilot scale food mixers and industrial scale chemical mixers. With these types of mixers, the flow rates of the mixing process generally determine the type of flow within the vessel, and both laminar and turbulent mixing is possible.
In the bulk mixer class, the most common kind found in a scientific establishment is the Stirrer hotplate. These come in many forms and manufacturers, including Stuart Equipment, Thermo Fischer Scientific, Camlab, Corning and IKA, to name a few. These are only useful for mixing on a small-scale.
For mixing applications, both a magnetic stirrer plate and a stirrer hotplate can be used. For reactions that require mixing and heating simultaneously, a stirrer hotplate is the best choice, but for general mixing applications, either can be used.
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These stirrer plates use a magnetic stirrer bar, commonly coated with Teflon, to mix the solution. The speed of the mixing will determine the efficiency of the mixing, but, at high speeds the magnetic stirrer can become unbalanced in the magnetic field and not provide any further spinning/mixing. So, finding the optimum speed is key before leaving a vessel alone to be mixed.
These are widely used across all of the scientific disciplines and are commonly found in most academic laboratories and small-scale R&D laboratories. Thermometers can also be built in to keep the solution temperature constant and multiple stirrer plate mixers can also be purchased.
For larger mixing applications, pilot scale mixers can be used. However, with a larger bulk, and a greater bulk flow, in the reaction vessel, many larger scale mixers produce a laminar type flow rather than a turbulent flow which sometimes leads to problems in scaling-up a process due to the two scales having different mixing dynamics. However, larger mixing vessels can produce greater shear forces which help to compensate for the lack of turbulent flow.
These mixers come in a wide variety and are common for both chemical mixing and food research applications. For food R&D applications, the most common type is a Winkworth mixer. Food research mixers generally don’t have the same shear capability as chemical mixers due to the fragility of some food products.
Common pilot scale equipment for general chemical mixing are produced by Kevin Process Technologies, Silverson, IKA and Ross, to name a few manufacturers. Industrial scale mixers, i.e. those involved in commercial scale production, are produced from a wide range of companies, including Morton Mixers, Hosokawa Micron Ltd and John R Boone, amongst some of the other manufacturers already mentioned.
Vortex Mixers
Vortex mixers, also known as vortexers, are commonly used in laboratories to achieve efficient mixing using high speeds in a low volume environment. The result is a turbulent flow that can effectively mix a solution/disperse a solid, homogenise a solution and disrupt cells. Vortex mixers use specific low volume tubes that fit easily onto the unit.
Vortex mixers use an electrical motor with a vertical drive shaft and a rubber piece on top. When the motor is turned on, which occurs when a sample vial is placed upon the mixer, the circular motion is transferred to the liquid sample, and at sufficient speeds forms a vortex.
Vortex mixers are only used in small scale research labs, most commonly for biological applications but also for some chemical and engineering-based samples. There are many vortex mixers on the marketplace, with the common manufacturers being Stuart, VWR, Corning, IKA, Fischer Scientific and SciQuip.
Ball Mill Mixers
A ball mill mixer is less commonly found in a laboratory, but can be useful for mixing, homogenisation, cell disruption and DNA recovery. Ball mills, in essence, are a high-speed shaker. If you were to take a small sample vial and shake the contents along its vertical axis then you would do the same job as a ball mill mixer, albeit with a much-reduced efficiency.
Ball mill mixers are generally used for small-scale R&D laboratories, be it in an academic or industrial setting. They use impact and friction energy to efficiently mix a sample. Ball mill mixers are generally high energy mixers, with the amount of energy exerted depending on the equipment purchased, and are suitable for dry, wet and cryogenic samples.
The G-force in the high-speed ball mill mixers is enough to break up, and disperse, rock and ceramic samples. More than one sample can be run at once, with most machines accommodating two simultaneous samples per machine. Common manufacturers of ball mill mixers are Retsch, SPEX SamplePrep and Horiba.
Sources and Further Reading
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