Mixing (process engineering)

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.

Single-phase blending tends to involve low-shear, high-flow mixers to cause liquid engulfment, while multi-phase mixing generally requires the use of high-shear, low-flow mixers to create droplets of one liquid in laminar, turbulent or transitional flow regimes, depending on the Reynolds number of the flow.

Blending in a more viscous liquid, such as honey, requires more mixing power per unit volume to achieve the same homogeneity in the same amount of time.

Blending times using dry ingredients are often short (15–30 minutes) but are somewhat dependent upon the varying percentages of each component, and the difference in the bulk densities of each.

Many products including pharmaceuticals, foods, chemicals, fertilizers, plastics, pigments, and cosmetics are manufactured in these designs.

A wide variety of horsepower-and-speed combinations and optional features such as sanitary finishes, vacuum construction, special valves and cover openings are offered by most manufacturers.

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 behavior.

In the first case, the particles can be lifted into suspension (and separated from one another) by bulk motion of the fluid; in the second, the mixer itself (or the high shear field near it) must destabilize the lumps and cause them to disintegrate.

[5]The degree of homogeneity of a solid-liquid suspension can be described by the RSD (Relative Standard Deviation of the solid volume fraction field in the mixing tank).

[9] For ‘good’ quality suspensions, some examples of useful correlations can be found in the publications of Barresi (1987),[10] Magelli (1991),[11] Cekinski (2010) [12] or Macqueron (2017).

[6][13] Very fine powders, such as titanium dioxide pigments, and materials that have been spray dried may agglomerate or form lumps during transportation and storage.

In some ways, deagglomeration of solids is similar to the blending of immiscible liquids, except for the fact that coalescence is usually not a problem.

An everyday example of this type of mixing is the production of milkshakes from liquid milk and solid ice cream.

When a tank and impeller are used, the objective is typically to ensure that the gas bubbles remain in contact with the liquid for as long as possible.

Mixing in a tank is also useful when a (relatively) slow chemical reaction is occurring in the liquid phase, and so the concentration difference in the thin layer near the bubble is close to that of the bulk.

If there is a (relatively) fast chemical reaction in the liquid phase, it is sometimes advantageous to disperse but not recirculate the gas bubbles, ensuring that they are in plug flow and can transfer mass more efficiently.

A common unit operation the process industry uses to separate gases and solids is the cyclone, which slows the gas and causes the particles to settle out.

Since mixers operate in the turbulent regime, many of the equations are approximations that are considered acceptable for most engineering purposes.

Magnetic stir bars are radial-flow mixers that induce solid body rotation in the fluid being mixed.

The cylindrical stir bar can be used for suspension of solids, as seen in iodometry, deagglomeration (useful for preparation of microbiology growth medium from powders), and liquid–liquid blending.

Moving mixers are powered with electric motors that operate at standard speeds of 1800 or 1500 RPM, which is typically much faster than necessary.

Planetary mixers are ideal for mixing and kneading viscous pastes (up to 6 million centipoise) under atmospheric or vacuum conditions.

Planetary blades each rotate on their own axes, and at the same time on a common axis, thereby providing complete mixing in a very short timeframe.

[22] Large industrial scale planetary mixers are used in the production of solid rocket fuel for long-range ballistic missiles.

They are used to blend and homgenize the components of solid rocket propellant, ensuring a consistent and stable mixture of fuel & oxidizer.

[23][24] ResonantAcoustic mixing (RAM) is able to mix, coat, mill, and sieve materials without impellers or blades touching the materials, yet typically 10X[25]-100X[26] faster than alternative technologies by generating a high level of energy (up to 100 g) through seeking and operating at the resonant condition of the mechanical system - at all times.

Anchor mixers induce solid-body rotation and do not promote vertical mixing, but helical ribbons do.

Because of their shape, they have a relatively low drag coefficient and therefore require comparatively little torque to spin at high speed.

Liquid whistles are a kind of static mixer which pass fluid at high pressure through an orifice and subsequently over a blade.

[32] This subjects the fluid to high turbulent stresses and may result in mixing, emulsification,[33][34] deagglomeration and disinfection.