Material Drying via Venting/Outgassing
Polycondensates such as PET or PA must be dried prior to processing to prevent hydrolitic decomposition during processing. Usually, moisture is removed from the pellets by means of special dryers – a sometimes time-consuming and energy-intensive process step. For instance, trying of PA6 with a high moisture content can take up to 48 h. The energy required for this may be as high as 0.6 kWh/kg of material.
Against this backdrop, a research project was conducted at the IKV to determine whether the moisture could be removed from the material via melt outgassing. The result of the trials: melt outgassing on a single-screw extruder yields a product quality similar to that achieved with drying in advance of processing. Since the melt in the plasticating zone still contains moisture, hydrolytic decomposition processes occur. For this reason, the extruder screw should be designed such that the melt proceeds as quickly as possible to the venting/outgassing zone.
Materials with a high moisture content should remain in the venting zone. At high throughputs, however, the residence times in the venting zone are short, which means that the moisture cannot be removed completely. To meet especially high quality requirements, the IKV researchers thus recommend combining processes: the material should be first dried to a defined residual moisture level during brief preliminary dying and then processed on an extruder equipped for melt outgassing.
The power consumption of an extruder with melt outgassing depends largely on the moisture content of the resin. This means that the overall energy required for processing can be reduced significantly. According to the information from the IKV, eliminating preliminary drying can reduce overall energy consumption by up to 50%.
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