While it is the ideal solution for many applications due to the hygienic properties of using a safe, enclosed system, vacuum conveying of powders and granules must be done properly to avoid the separation of ingredients that can lead to an uneven mixture. This separation is also known as segregation, and it affects most manufacturers transferring and combining ingredients with varied characteristics.

The issue of segregation in conveying is one that pervades the powder and bulk handling industry. How solids separate during material transfer is dependent on a variety of factors: particle size, shape and density; speed of transfer; geometric variables like angle and length of conveying distance; even static electricity. Regardless of the how and why, segregation can prove to be a costly challenge for manufacturers in a variety of industries.

When conveying in an air stream, manufacturers face the risk of ingredients separating through sifting. Smaller particles will find their way through the mixture of larger particles. Dense particles will drop below less dense particles.

Clearly this issue effects production. Whether it is a food product that requires the correct mixture of ingredients, or a pharmaceutical product that depends on an accurate powder-to-powder ratio, the quality of an end product is in jeopardy if the situation is not assessed. Even if the incorrect mixture is caught before end-of-line processing, a vast amount of raw material may have been wasted.

Of course, the problem is not exclusive to powder and bulk conveying only; segregation can occur on the way from the mixer to the feeder, as well as during mix discharge at the end of the conveying line, where heavier materials drop to the bottom. However, when dissecting the problem, there are steps that can be taken to optimise the conveying process itself.

There are different phases in pneumatic conveying; from lean phase conveying to dense phase conveying. When conveying with a low phase density, or lean phase density, a lot of air is mixed with a little powder, resulting in high-velocity conveying. In this type of system, the volume and speed of air is high enough to keep particles continuously moving in suspension. This also increases the chance of segregation.

Conversely, dense phase, or plug phase conveying, is a slow-down in speed. With a lower volume of air in the conveyer, material can be moved in plugs with a bit of distance between them. Fluidisation allows for compressed air to pass through porous materials. This fluidised material is pulled along at a slower speed than the compressed air, in intermittent waves. The mix is more likely to remain true to the desired ratio when dense phase conveying is utilised.

Empirical testing

While segregation may be a common and somewhat general problem, the solution is not a general one. Each segregation issue is unique as each mixture is unique. The issues must be treated as such because of the importance and gravity of the situation.

Take as an example a drug manufacturer in the pharmaceutical industry. Here is a condition where the content of the final product may be a life or death matter. While a consumer receiving pain reliever capsules with not enough medication in them may just continue to have a headache, the consumer receiving too much medication may have a dangerous overdose.

Because the consequences can be dire, the situation must be addressed empirically on the spot. Many companies producing delicate and potentially hazardous goods will test their mixes on a regular basis to ensure product quality and content. The next step is to assess the conveying line and test how different variables affect material transfer.

What a vacuum partner can contribute to the powder and bulk handling process is a recommendation for controlled speed. When the speed of conveying is kept low, and denser phase conveying is employed, the end result will be better.

Recently, PIAB worked with a company producing antacid tablets that was conveying about six tons of powder per hour. The manufacturer faced a segregation problem, whereby the ingredients, including citric acid and calcium carbonate, were separating out during high speed conveying. The powders were brought to our testing facilities, where we simulate the conveying process using different variables, such as speed and distance, for our manufacturing partners. The results this time were clear: by reducing the speed, the problem was solved.

Product and system updates and innovations in conveying will help manufacturers in the food and pharmaceutical industry with segregation concerns. For instance, we at PIAB are currently in development with a constant-speed conveyor. Controls to set the vacuum pump providing flow in the conveyor at a consistent speed would ensure that regardless of the product being conveyed, segregation is a much easier adversary to beat. v

Peter Tell is Chief Technology Officer of PIAB and developer of COAX technology, Where. www.piab.com