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Mixing In industrial process engineering, mixing is a unit operation that involves manipulation of a heterogeneous physical system with the intent to make it more homogeneous.process engineeringunit operationheterogeneoushomogeneous Mixing is performed to allow heat or mass transfer to occur between one or more streams, components or phases. The opposite of mixing is segregation. A classical example of segregation is the brazil nut effectsegregationbrazil nut effect

Mixing classification Liquid– liquid mixing Solid–solid mixing Gas–gas mixing Liquid– solid mixing Liquid–gas mixing Gas–solid mixing Multiphase mixing Laboratory mixing

Mixing of liquids occurs frequently in process engineering. The nature of liquids to blend determines the equipment used Single-phase blending tends to involve low- shear, high-flow mixers to cause liquid engulfment multi-phase mixing generally requires the use of high-shear low-flow mixers to create droplets Turbulent or transitional mixing is frequently conducted with turbines or impellers laminar mixing is conducted with helical ribbon or anchor mixersturbinesimpellers Advantages of Liquid–liquid mixing

Blending powders is one of the oldest unit-operations in the solids handling industries. For many decades powder blending has been used to homogenize bulk materials. Different machines have been designed to handle materials with various bulk solids properties. Nowadays the same mixing technologies are used to improve product quality, to coat particles, to fuse materials, to wet, to disperse in liquid, to agglomerate, to alter functional material properties, etc. Advantages of Solid– solid mixing

Disadvantages of Solid–solid mixing Requires a high level of knowledge Long time experience Extended test facilities to come to the optimal selection of equipment and processes

Advantages of Liquid–solid mixing Liquid–solid mixing is typically done to suspend coarse free-flowing solids or to break up lumps of fine agglomerated solids. An example is the mixing granulated sugar into water an example of the latter is the mixing of flour or powdered milk into water. Disadvantages of Liquid–solid mixing The mixer itself must destabilize the lumps Cause them to disintegrate.

This is especially important if the gas is expensive, such as pure oxygen, or diffuses slowly into the liquid. Useful when a relatively slow chemical reaction is occurring in the liquid phase Advantages of Liquid– gas mixing This reduces the driving force for mass transfer The gas accumulates in the low pressure zones behind the impeller blades which reduces the power drawn by the mixer. Disadvantages of Liquid–gas mixing

Gas–solid mixing Gas–solid mixing may be conducted to transport powders or small particulate solids from one place to another The size and shape of the particles is an important consideration Different particles have different drag coefficient and particles made of different materials have different densities

Schematic drawing of a fluidized bed reactor

Multiphase mixing Multiphase mixing occurs when solids, liquids and gases are combined in one step This may occur as part of a catalytic chemical process in which liquid and gaseous reagents must be combined with a solid catalyst such as hydrogenation This draws liquid upwards inside the plume and causes liquid to fall outside the plume. If the viscosity of the liquid is too high to allow for this an impeller may be needed to keep the solid particles suspended.

Laboratory mixing At a laboratory scale, mixing is achieved by magnetic stirrers or by simple hand- shaking. mixing in laboratory vessels is more thorough and occurs faster Magnetic stir bars are radial-flow mixers that induce solid body rotation in the fluid being mixed. This is acceptable on a small scale, since the vessels are small and mixing therefore occurs rapidly. The cylindrical stir bar can be used for suspension of solids, as seen in iodometry

magnetic stirrer

Selected Turbine Geometries and Power Numbers NamePower numberFlow direction Blade angle (degrees) Number of blades Blade geometry Rushton turbine4.6Radial06Flat Pitched blade turbine 1.3Axial45–603–6Flat Hydrofoil0.3Axial45–603–6Curved Marine Propeller0.2AxialN/A3Curved Turbines A selection of turbine geometries and power numbers are shown below. Industrial mixing equipment

Two main types of close- clearance mixers Anchors Anchor mixers induce solid- body rotation Do not promote vertical mixingHelical ribbons Rotated to push material at the wall downwards which helps circulate the fluid and refresh the surface at the wall Close-clearance mixers

High shear dispersers Such devices typically resemble circular saw blades and are rotated at high speed. Because of their shape they have a relatively low drag coefficient and therefore require comparatively little torque to spin at high speed. High shear dispersers are used for forming emulsions or suspensions of immiscible liquids and solid deagglomeration. High shear dispersers create intense shear near the impeller but relatively little flow in the bulk of the vessel. Static mixers Static mixers are used when a mixing tank would be too large too slow or too expensive to use in a given process.

Liquid whistles A kind of static mixer that pass fluid at high pressure through an orifice and subsequently over a blade. Subjects the fluid to high turbulent stresses and may result in mixing, emulsification, deagglomeration and disinfection.

Jet mixing Jet mixing employs a special dual nozzle assembly with entrainment ports.Tank liquid is pumped through a liquid line and then through the inner nozzle.High velocity is created by the inner nozzle forming a jet plume. This jet plume pulls tank liquid through the entrainment port and then discharges the liquid through the outer nozzle. Transfer of the high velocity stream to the surrounding liquid entrains additional flow increasing the pumping rate by as much as ten times the pumping capacity.

Jet mixing

Propeller mixer Propeller mixer is a mechanical device which is used to mix the liquid materials with the help of blades or impellers used. A propeller is a high-speed axial flow impeller generally used for liquids of low viscosity Propeller mixers have shafts that are mounted to a motor by way of an inferior coupling held by set screws, subjecting the motor and drive assembly to vibrations caused by mixing.

Blending of the water thin materails up to very large tanks Reaction and dispersion with intensive agitation upto 1500 gal High shear or emsulfing jobs upto 1000 gal Slurries upto 10% solids of minus 100 mesh Characteristics of propeller mixer Different types of blades used in propeller mixer

PHARMACEUTICAL APPLICATIONS The propeller mixer used in mixing the liquids upto 2000cp It has the capacity to mix low visocity emulsions Liquid phase chemical reaction The propeller mixer is used in mixing of suspensions with particles size upto 0.1 to 0.5 mm maximum with a drying residue of 10%

Top to bottom mixing can be achieved in propeller mixer. All sides of mixing is possible. ADVANTAGES The propeller mixer is high cost. Sensitvity in operation of vessel geomentry and in location within the tank. The propeller mixer is not used for rapid settling suspensions. DISADVANTAGES

Conical paddle mixer The conical paddle mixer is the newest generation of Hosokawa powder mixers It has been designed for mixing powders, granules and solids with liquids. The conical paddle mixer is a multi-purpose mixer for processes where high accuracy and fast mixing with limited product distortion are important.

Applications Mixing of fragile powders and ‘instant’ agglomerates Mixing of powders with liquids Sterilisation and pasteurisation of product batches High-temperature processing of powders and liquids Vacuum drying at low temperature with liquid recovery

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