Mixing (process engineering)
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In industrial process engineering, mixing is a unit operation that involves manipulating a heterogeneous physical system, with the intent to make it more homogeneous. Familiar examples include pumping of the water in a swimming pool to homogenize the water temperature, and the stirring of pancake batter to eliminate lumps.
Solids mixing 
Blending powders is one of the oldest unit-operations in the solids handling industries. For many decades powder blending has been used just to homogenise bulk materials. Many different machines have been designed to be able to handle materials with various bulk solids properties. On the basis of the practical experience gained with these different machines, engineering knowledge has been developed to construct reliable equipment and to predict scale-up and mixing behaviour. Nowadays the same mixing technologies are used for many more applications: to improve product quality, to coat particles, to fuse materials, to wet, to dispers in liquid, to agglomerate, to alter functional material properties, etc. This wide range of applications of mixing equipment requires a high level of knowledge, long time experience and extended test facilities to come to the optimal selection of equipment and processes.
Mixing mechanisms 
In powder mixing two different dimensions in the mixing process can be determined: convective mixing and intensive mixing. In the case of convective mixing material in the mixer is transported from one location to another. This type of mixing process will lead to a less ordered state inside the mixer, the components which have to be mixed will be distributed over the other components. With progressing time the mixture will become more and more randomly ordered. After a certain mixing time the ultimate random state is reached. Usually this type of mixing is applied for free-flowing and coarse materials. Possible threat during macro mixing is the de-mixing of the components, since differences in size, shape or density of the different particles can lead to segregation. In the convective mixing range, Hosokawa has several processes available from silo mixers to horizontal mixers and conical mixers. When materials are cohesive, which is the case with e.g. fine particles and also with wet material, convective mixing is no longer sufficient to obtain a randomly ordered mixture. The relative strong inter-particle forces will form lumps, which are not broken up by the mild transportation forces in the convective mixer. To decrease the lump size additional forces are necessary; i.e. more energy intensive mixing is required. These additional forces can either be impact forces or shear forces.
Mixing Calculations 
The level of mixing is determined by the pumping effect or dynamic response that the mixer imparts into the fluid. When a mixing impeller rotates in the fluid, it generates a combination of flow and shear. The impeller generated flow can be calculated by using the following equation:
Flow = (Flow_Number * RPM * Impeller_Diameter^3) / 231
Output is in Gallons / Minute
Flow numbers for impellers have been published by the North American Mixing Forum, Post Mixing, and Fusion Fluid Equipment.
An online mixing calculator is available http://www.fusionfluid.com/FusionFluidEquipmentLLC/html/calculator_guide.html
Laboratory mixing 
At a laboratory scale, mixing is achieved by magnetic stirrers or by simple hand-shaking.
Mixing in Microfluidics 
When scaled down to the microscale, fluid mixing behaves radically different. This is typically at sizes from a couple (2 or 3) millimeters down to the nanometer range. At this size range normal convection does not happen unless you force it. Diffusion is the dominate mechainism whereby two different fluids come together. Diffusion is a relatively slow process. Hence a number of researchers had to devise ways to get the two fluids to mix. This involved Y junctions, T junctions, 3 way intersections and designs where the interfacial area between the two fluids is maximized. Beyond just interfacing the two liquids people also made twisting channels to force the two fluids to mix. These included multilayered devices where the fluids would corkscrew, looped devices where the fluids would flow around obstructions and wavy devices where the channel would constrict and flare out. Additionally channels with features on the walls like notches or groves were tried.
One way to tell if mixing is happening due to convection or diffusion is by finding the Peclet number. It is the ratio of convection to diffusion. At high Peclet numbers, convection dominates. At low Peclet numbers, diffusion dominates.
Peclet = flow velocity * mixing path / diffusion coefficent
Industrial mixing 
At an industrial scale, efficient mixing can be difficult to achieve. A great deal of engineering effort goes into designing and improving mixing processes. Mixing at industrial scale is done in batches (dynamic mixing) or with help of static mixers.
Typical example of a mixing process in the industry is concrete mixing, where cement, sand, small stones or gravel and water are commingled to a homogeneous self-hardening mass, used in the construction industry. Another classical mixing process is mulling foundry molding sand, where sand, bentonite clay, fine coal dust and water are mixed to a plastic, moldable and reusable mass, applied for molding and pouring molten metal to obtain sand castings that are metallic parts for automobile, machine building, construction or other industries.