Pneumatic conveying methods provide sanitary conveying options for sensitive powders and achieve distances mechanical conveying systems cannot. However, homogeneity issues can arise in blended powders that are transported using the wrong pneumatic conveying system. Such issues can be costly endeavours resulting in scrapped product, downtime, and concerns around brand standards and reputation. This column will discuss the two main categories of pneumatic conveying systems: Dense and dilute (also known as lean) phase and highlight dense as the best option for blended powders.
Pneumatic conveying systems work by generating a gas flow (generally air) in a pipe combined with a pressure difference between the pickup and receiving points. Product moves from a point of higher pressure to a point of lower pressure.
Dilute Phase Conveying Methods In a dilute phase conveying system, product is dropped into a continuous stream of air that is generated either by a blower at the front end (pressure) or back end (vacuum) of the process where typical air velocities can range from 3,000 to 7,000 ft/min. Dilute phase conveying is an excellent way to move robust powders, but not so much for fragile, instantized, or blended powders. With the high conveying velocities, fragile or instantized products are broken down. When dropping a blended product into a fast-moving airstream, the first thing the product does is separate based on particle size, shape, and density. You will lose the homogeneity you worked so hard to achieve.
Dense Phase Conveying Methods A dense phase system moves slugs of powder through the convey line separated by cushions of air where typical convey velocities will range from 150 to 2,500 ft/min. These slower convey velocities, as well as the higher concentration of powders, makes it less likely for fragile powders to be broken down or blended powders to de-mix. A dense phase conveying system can be set up to meet almost any production requirement remembering that the goal of this system is to move the product as slowly as possible, without plugging the lines and keeping up with required production. A dense phase system can be set up to run either as a batch system or continuous, depending on the production requirements, and can achieve rates up to 20 tn/hr.
Dense phase conveying can further be subdivided into a pressure system where compressed air is used to pressurize a transporter vessel to push the powder, or a vacuum system where a vacuum pump is used to draw a vacuum on the receiving vessel and pull the powder. A pressure-dense phase system can move powder up to 325 ft with an elevation increase of over 75 ft, and a vacuum dense phase system can move powder over 160 ft with an elevation increase of 60 ft.
When the production requirements are outside the limitations of a dense phase system, semi-dense phase conveying is a good alternative. By adding a little more air, hence moving the product a bit faster, the overall distance (horizontal and vertical) can be increased without overly affecting the product’s integrity or homogeneity.
When developing a powder blending system, it is important to not only spend effort on the design of the blending process, but on the post-blending handling process. It is advisable to for producers to test their chosen solution in a pilot plant to confirm the breakage rate, de-mixing, and other important characteristics. Test centers specialized in powder handling solutions let producers simulate any kind of powder handling process to test the system on sensitive products and validate line performance. This extra effort will go a long way because the wrong powder handling process can easily negate the efforts to develop your blending system.