Infrared drying (IR) uses the energy from IR radiation to directly heat the bulk material of the polymer granule/flake. The delivered energy is applied directly to the granule with no other transfer medium. The applied energy causes internal heating and molecular oscillation and therefore heats the bulk material of the granule and any internal moisture. A stream of cooler ambient air surrounds the granule and the internal heat drives the moisture out into the cooler air stream that removes it from the process.
The driving force for IR drying is the difference in temperature between the vaporised water and the ambient air stream: this creates a partial pressure gradient from the inside of the granule to the outside and a strong driving force for moisture removal.
This system uses not only IR heaters but an additional applied high vacuum (down to − 980 mbar). The IR provides material heating and the vacuum further draws the water out of the material (as with standard LPD drying). This increases the process efficiency and reduces energy use.
These systems can be used for drying both hygroscopic and non-hygroscopic materials but are particularly suited to the drying of reprocessed PET. PET processing uses crystalline granules but the material becomes amorphous during processing – therefore PET regrind is amorphous and must be recrystallised before it can be processed again. Historically a separate recrystallisation process was used prior to drying and processing. In IR drying processes, the PET is raised above Tg and continuously agitated during the process, this effectively combines recrystallisation and drying in one pass.
IR systems (drum or IR + vacuum) offer dramatic improvements in energy efficiency.
Combine these with direct moisture measurement of the polymer (see Section 4.45) and significant energy use reductions in drying are possible.
Drying times for many hygroscopic materials can be dramatically reduced and, in the case of PET reprocessing, the recrystallisation process can be performed at the same time. The MOBY process (IR + vacuum) also provides a cleaning action to provide high-quality reprocessed PET for direct processing (‘Super-Clean’ for food applications) and can also be set up to improve the intrinsic viscosity (IV) of the PET from the system.
The drum system is fully continuous but the IR + vacuum system does need a buffer hopper.
The direct energy application at the point of use provides reduced process times and energy consumption is claimed to be 72–120 W/kg for PET recrystallisation and drying to final moisture contents of less than 0.005%. This is much lower than that achieved with traditional drying methods.
IR drying (either type) is suitable for high-volume throughput.
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