High shear mixer

-A high shear mixer disperses, or transports, one phase or ingredient (liquid, solid, gas) into a main continuous phase (liquid), with which it would normally be immiscible. It is used in the adhesives, chemical, cosmetic, food pharmaceutical and plastics industries for emulsification, homogenization, particle size reduction and dispersion.^[1]

How it works

Fluid undergoes shear when one area of fluid travels with a different velocity relative to an adjacent area. A high shear mixer uses a rotating impeller or high speed rotor, or a series of such impellers or inline rotors, usually powered by an electric motor, to "work" the fluid, creating flow and shear. The velocity, or tip speed of the fluid at the outside diameter of the rotor will be higher than the velocity at the centre of the rotor, and it is this that creates shear.

A stationary component may be used in combination with the rotor and is referred to as the stator. The stator creates a close clearance gap between the rotor and itself and forms an extremely high shear zone for the material as it exits the rotor. The rotor and stator combined together are often referred to as the mixing head, or generator. A large high shear rotor-stator mixer may contain a number of generators..^{[2] [3]}

Key design factors include the diameter of the rotor and its rotational speed, the distance between the rotor and the stator, the time in the mixer and the number of generators in the series. Variables include the number of rows of teeth, their angle, and the width of the openings between teeth.

Batch high shear mixers

In a batch high shear mixer, the components to be mixed (whether immiscible liquids or powder in liquid) are fed from the top into a mixing tank containing the mixer on a rotating shaft at the bottom of the tank. A batch high shear mixer can process a given volume of material approximately twice as fast as an inline rotor-stator mixer of the same power rating, and continue to be used used where faster processing by volume is the major requirement, and space is not limited.^[4] When mixing sticky solutions, some of the product may be left in the tank, necessitating cleaning. However there are designs of batch high shear mixers that clean the tank as part of the operating run.^[5] The entire mixing array is normally removed from the tank for cleaning at regular intervals.

Inline high shear mixers

In an inline high shear rotor-stator mixer, the rotor-stator array is contained in a housing with an inlet at one end and an outlet at the other, and the rotor driven through a seal. The components to be mixed are drawn through the generator array in a continuous stream, with the whole acting as a centrifugal pumping device. Inline high shear mixers offer a more controlled mixing environment, take up less space, and can be used as part of a continuous process. Equilibrium mixing can be achieved by passing the product through the inline high shear mixer more than once.^[4]

High shear granulators

A high shear granulator is a process array consisting of an inline or batch high shear mixer and a fluid bed dryer. In a granulation process, only the solid component of the mixture is required. Fluid is used only as an aid to processing. The high shear mixer processes the solid material down to the desired particle size, and the mixture is then pumped to the drying bed where the fluid is removed, leaving behind the granular product.^[6]

Ultra high shear inline mixers

In an ultra high shear inline mixer, the high shear mixing takes place in a single or multiple passes through a rotor-stator array. The mixer is designed to subject the product to higher shear and a larger number of shearing events than a standard inline rotor-stator mixer, producing an exceptionally narrow particle size distribution.Sub micron particle sizes are possible using the Ultra High Shear technology.^[7] To achieve this, the machine is equipped with stators with precision machined holes or slots through which the product is forced by the rotors. The rotor-stator array can also include a mechanism whereby the momentum of the flow is changed (for example by forcing it sideways through the stator), allowing for more processing in a single pass.^[8]

Equilibrium mixing

High shear mixers are used in industry to produce standard mixtures of ingredients that do not naturally mix. When the total fluid is composed of two or more liquids, the final result is an emulsion; when composed of a solid and a liquid, it is termed a suspension and when a gas is dispersed throughout a liquid, the result is a lyosol^[9]. Each class may or may not be homogenized, depending on the amount of input energy.

To achieve a standard mix, the technique of equilibrium mixing is used. A target characteristic is identified, such that once the mixed product has acquired that characteristic, it will not change significantly thereafter, no matter how long the product is processed. For dispersions, this is the equilibrium particle size. For emulsions, it is the equilibrium droplet size. The amount of mixing required to achieve equilibrium mixing is measured in tank turnover - the number of times the volume of material must pass through the high shear zone.^[4]

Uses of high shear mixing technology

High shear mixers are used throughout the chemical process industries, wherever it is necessary to produce standardized mixtures of ingredients that do not naturally mix. These include

• Pharmaceuticals, preparations of suspensions and granular products
• Paper manufacture, bleaching and preparation of paper pulp^[10]
• Food preparation, emulsions for sauces and dressings
• Manufacture of cosmetics and toiletries^[11][12]

External sources

"High shear smashing homgenizer" (in English (trans from Chinese)). Smash equipment series. Dongshen. http://www.qddsjx.com/english/pro2.htm. Retrieved 2009-06-07. 

References