Kinetic Rate Modeling for Moisture Extraction in Polyesters
Polyester moisture extraction follows temperature-dependent Fickian diffusion, requiring dew points below minus forty degrees to prevent melt hydrolysis.
Pellet Drying Rate
Meaning
Dehumidification kinetic metrics quantify the mass of water removed from a unit weight of polymer resin per unit time under specific temperature and airflow conditions. Tracking pellet drying rate enables moulding technicians to establish optimum residence times in drying hoppers before processing moisture-sensitive resins. The rate progresses through an initial constant-rate drying phase governed by surface evaporation, followed by a falling-rate phase controlled by internal moisture diffusion through the solid polymer.
Accurate measurement prevents under-drying that causes hydrolysis, as well as over-drying that causes thermal degradation and energy waste.
Mass Transfer
Moisture removal mechanics depend upon gas dew point, dry air velocity, and granule geometry. Internal Fickian diffusion governs water migration from the center of the granule to its exterior surface during the falling-rate regime. A steep pellet drying rate during initial processing rapidly strips surface moisture, but internal resistance slows the process as the core moisture content drops toward target specifications.
Smaller pellet sizes increase total surface area per unit mass, accelerating diffusion and reducing required hopper residence time.
Thermal Diffusion
Heat transfer from drying air elevates pellet temperature, providing the activation energy required to break hydrogen bonds between water molecules and polymer chains. Raising drying air temperature increases internal diffusion coefficients, accelerating moisture departure from the polymer matrix. Excessive thermal input causes sticky pellet surfaces and agglomeration inside the hopper base.
Processing Boundary
Airflow rate increases no longer improve drying speed once internal moisture diffusion becomes the sole rate-limiting step. Regrind particles with irregular shapes dry unevenly compared to uniform spherical or cylindrical virgin pellets, producing unpredictable moisture levels in the hopper output.