Pneumatic mixers sometimes called air-mix mixer or air-drive mixer is a mixer with a motor that uses compressed air or air bubbles instead of electricity. The blender consists of a mixing silo and in some cases a central conveying tube with an inverted conical deflector at the top for spreading material. Pneumatic blenders are equipped with aerators mounted around a housing cone. Compressed air or gas is introduced intermittently at high velocities through nozzles present at the bottom or side of the silo, to mix powder materials that exhibit expansion characteristics when aerated. Particles are reoriented in relation to one another as a result of the expansion of a powder bed by gas. The nozzles are arranged in a manner that escaped air stream in vertical motion gives a chance for powers to settle. To achieve mixing, the system employs blower or compressor to generate air flow. The solid particles rise due to the drag force of the air injected and an increase in air velocity causes agitation in the bed, resulting in the formation of bubbles which causes blending to take place.
Factors that determine the amount of air required to fluidized the system and the minimum fluidized velocity are;
- Size of the particles
- Particle density
- Bulk density of the solid
- Density of the gas used for fluidization
Although pneumatic mixed may differ from one another in vessel geometry, air nozzle type and air nozzle configuration no pneumatic mixer subclass has been identified.
Pharmaceutical uses of pneumatic mixers
- Pneumatic blenders are best used for non-critical blends, where particle sizes and densities are similar among all the components of the mix so that fines are not carried to the top by the air.
- It is used in mixing dry and free flow powders with particle size of about 50
Advantages of Pneumatic Mixers
- There is less risk of failure due to misapplication.
- They are safe for use with flammable solvents.
- They are easy to clean and maintain.
- Contamination of material is minimal since there are no moving parts.
- The blending process is extremely gentle.
- Less handling of materials is required with pneumatic blender.
- Pneumatic blender is insensitive to heat.
- The compressed air expanding in the air engine cools the agitator.
- Air is practically available everywhere in unlimited qualities.
Disadvantages of Pneumatic Mixers
- Compressed air requires good preparation and constant piston speed.
- Compressed air is economically only up to a certain force requirement.
- The exhaust air is loud.
CCPS (Center for Chemical Process Safety) (2010). Guidelines for Safe Handling of Powders and Bulk Solids. American Institute of Chemical Engineers, New York.