(304e) Segregation During Pneumatic Conveying of Blended Plastic Pellets

Authors: 
Vasquez, N. A., The Dow Chemical Company
Jacob, K., The Dow Chemical Company
Walia, P., The Dow Chemical
Voets, A., RWTH Aachen University


The transport of plastic pellets in dilute phase pneumatic conveying is largely used in the chemical industry and it has been the subject of study in numerous publications. Many of these studies have focused on the analysis of particle dynamics and collision models, showing how the pellets travel along the conveying line. Thus it has been shown that some materials convey significantly slower than others due to intense bouncing. However, the transport of blended materials, in which the component pellets travel at very different velocity when conveying individually, has rarely been analyzed. The following work focuses on the potential segregation that can occur during the pneumatic conveying of two or three components blends during the transport along the pipeline and at the receiver.

In order to analyze whether segregation developed during pneumatic conveying, a series of conveying trials were performed with blends of two and three components as well as with the individual materials. High speed video analysis was used to measure individual particle velocities. The results of the study showed that:

- Harder materials show very little contact with the pipe wall. - Soft (elastomeric) materials show intense bouncing and slow velocity when they are conveyed individually. - When conveyed as a blend, all components (hard and soft) move with a very similar conveying velocity. The mixture reduces the bouncing of the soft pellets, allowing them to travel faster than in pure state. - No detectable segregation was found during conveying in either the two component or the three component blends. - Once the system is in steady state, segregation will not be an issue unless long conveying distances are used. In this case transient effects could become important and should be studied in more detail.

The segregation at the reception hopper was analyzed by testing two different geometries for the receiver inlet and three types of conveying line materials. The key conclusions were:

- Segregation is highly affected by the development of static charge. - The conveying line material has a strong effect on the development of static charges. - In terms of receiver design, the center inlet with pellet distributor showed the best performance in preventing segregation. - The tangential inlet promotes segregation due to the formation of an internal vortex that intensifies the bouncing of the particles and produces an inclined level of solids. - Reducing the air velocity, without reaching the saltation condition at the pickup point, reduces the segregation at the receiver due to less static charge build up and less bouncing of the pellets.