What does pellet size have to do with drying performance?

Drying

What does pellet size have to do with drying performance?

Resin manufacturers typically provide recommendations for the temperature and length of time required for resin drying. Between two samples of the same hygroscopic material of equal weight, however, differences in the physical size and shape of resin particles mean that one of the samples may dry more quickly than the other.

Drying virgin material. Virgin pellets may come in a variety of shapes including cylindrical, round, oval, and square cut, depending on the type of material and pelletizer they were produced on.

Why does pellet size matter? Go back for a moment to the four fundamentals of resin drying (Temperature, Time, Dewpoint, and Airflow). Start by considering the factors of Temperature and Time:

  • Temperature. Plastic pellets must be thoroughly heated in order for the water molecules to diffuse from the polymer chains and migrate to the surface of the pellet.
  • Time. Because plastics are not good conductors of heat, it takes time for the heat from the drying air to migrate to the center of the pellet. Only then will the polymer release the water molecules so they can migrate to the pellet surface.

With this understanding, it is easy to see that larger pellets require more time for heat to travel the longer distance to the pellet center. Then, once heated, larger pellets also require more time for the water molecules to migrate to the surface. Therefore, larger pellets may require a relatively longer time to dry.

So, if as a rule, larger pellets tend to dry more slowly, it stands to reason that smaller pellets tend to dry faster.

Just how fast smaller pellets dry depends on a third drying fundamental: airflow. Here’s why: it is the flow of hot, dry air through the drying hopper that heats the pellets and strips moisture away.

Smaller pellets, however, tend to pack together, leaving less free air space. Less free air space between pellets means restricted airflow. Reduced airflow, in turn, limits the amount of heat that can be delivered to the pellets in a given time. Therefore, drying capacity can be reduced in much the same way as if a dryer filter becomes blinded by dust, causing a restriction in airflow.

Drying regrind. Pellet size and shape are also a major consideration when reclaiming scrap and drying regrind materials. In this case, drying airflow is affected not only by pellet/flake size, but also by the flow characteristics of the regrind material.

  • Compared to regrind, virgin pellets tend to be more uniformly shaped and more free-flowing. Irregularly shaped regrind pellets/flakes/particles are more likely to flow in a non-uniform way, resulting in bridging or differences in material density at different points in the drying hopper. Fines in the regrind can sift into small spaces between the pellets/flakes, reducing space, restricting air flow and reducing drying efficiency.
  • Similarly, regrind from flat sheet or thin-wall bottles may tend to nest or layer as it passes through a drying hopper, reducing airflow in some areas and forcing drying air to channel through others, again resulting in some reduction of drying system performance.

If you have drying problems associated with pellet size or shape, there are two basic solutions: 1) Extend drying times to ensure that all material in the hopper is exposed to adequate airflow, heating, and drying, and 2) inspect your granulation equipment to ensure that its design is appropriate for the scrap being processed and that knives are sharp and screens are clean, since these factors are essential to produce consistent regrind with a minimum of fines.

If you’ve got questions about dryers, or need advice about improving dryer performance, give us a call at 724-584-5500.

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