Keeping It Together: Reducing Powder Segregation in Vacuum Conveying

Vacuum-conveying technology is commonly used in the pharmaceutical industry to move materials through the processing line, blend ingredients in various solutions, and handle tablets. Because it is an enclosed system, vacuum conveying is safe, hygienic, and the preferred solution for many applications. The vacuum conveying of powders and granules, however, must be performed properly to avoid segregation, the separation of ingredients that results in an uneven mixture.

A costly quality problem
Segregation during the conveying of powders is a problem for the pharmaceutical industry. Various factors influence how solids separate during material transfer. These factors include particle size, shape, and density; speed of transfer; geometric variables such as the angle and length of the conveying distance; and the presence of static electricity. Regardless of its cause, segregation can be a costly challenge for manufacturers.

Because it uses an air stream to move particles, vacuum conveying presents the risk that ingredients will separate through sifting. Smaller particles will pass through the mixture of larger particles. Dense particles will drop below less dense particles. Many pharmaceutical products depend on an accurate powder-to-powder ratio, and end-product quality is in jeopardy if segregation is not mitigated. The problem should be addressed in advance because much raw material may be wasted even if the incorrect mixture is detected before end processing.

Lean-phase versus dense-phase conveying
Segregation is not exclusively a powder- and bulk-conveying problem. It can occur when a product is transferred from a mixer to a feeder or during mix discharge at the end of the conveying line, where heavier materials drop to the bottom of the mixer. But manufacturers can take steps to optimize the conveying process.

Pneumatic conveying encompasses several phases such as lean-phase conveying and dense-phase conveying. During lean-phase conveying, a great amount of air is mixed with a small quantity of powder, which results in high air velocity and high powder velocity. In this type of system, the volume and speed of air are high enough to keep particles continuously moving in suspension. These factors also increase the chance of segregation because light and less-dense particles flow faster than heavy and dense particles.

Dense-phase, or plug-phase, conveying occurs at a slow speed. A low volume of air in the conveyer moves material in plugs separated by small distances. This conveying technique creates fluidization, which converts powdered material into a fluidlike state. Sometimes used at the suction point and discharge point, fluidization lets compressed air pass through porous materials. Fluidized material is moved in waves at a slower speed than the compressed air.

Dense-phase conveying better maintains the mix’s desired ratio. Air speed is not as high in dense-phase conveying as in lean-phase conveying. Thus, the separation resulting from particle-size and weight variation is lower.

Empirical testing improves quality and content
Although segregation is a common problem, it requires a specific solution for each conveying application. Dense-phase conveying is the best strategy to mitigate segregation, but it cannot be applied the same way in every case. Different materials behave differently and should be tested and evaluated before the conveying process is adjusted. Likewise, different mixes of ingredients require different adjustments to the conveying application.

Pharmaceutical companies test their mixes regularly to ensure product quality and content. The conveying line should also be assessed, including how variables affect material transfer. For example, elements such as piping (e.g., pipe ends, bends, and sharp edges) change the material’s flow characteristics and affect segregation. The whole installation around the conveyor should therefore be assessed before developing a solution. 

Developing the optimal solution
New conveying products and systems will help pharmaceutical manufacturers reduce segregation. PIAB is currently developing a constant-speed conveyor. Setting the vacuum pump that provides flow in the conveyor at a consistent speed would ensure that, regardless of the product being conveyed, segregation would be an easier problem to solve. Typical pumps are affected by the pressure drops that occur throughout a system, but the pump PIAB is developing compensates for pressure drops by changing the flow (i.e., speed). A pump with a consistent speed is set for a fixed vacuum level and not a fixed flow.  

Peter Tell is the chief technical officer of PIAB, Sjöflygvägen 35 SE-183 04, Täby, Sweden, tel. +46 0 8 630 25 00, fax +46 0 8 630 26 90, info@piab.se.