PREFORM PRODUCTION PETplanet Insider Vol. 24 No. 05/23 www.petpla.net 36 The PET material dryer - an often unrecognised key point for economical preform production by Stefan Bock, Managing Director at ReduPET GmbH In most cases, a dryer is an energy guzzler and/or a process killer because it rarely runs at the optimum operating point. The energy content in the PET granulate cannot be directly determined economically during preparation for processing in the production plant, so an average of 40Wh is lost unnoticed per kilogram of PET raw material processed - largely through the heat exchanger, connected to a cooling machine, which requires a further 12Wh/kg. For a preform system with, for example, 96 cavities and an average throughput of 900 kg/h, this amounts to almost 400,000 kWh/a - or, depending on the price of electricity or gas US$ 44,000-88,000 per production system. A lost profit that can be easily and precisely realised with a clear effort. How it works The problem in almost all preform production plants is that hardly any operator really knows how his dryer uses energy. If the material is sufficiently dried, there is general satisfaction - the real energy consumption is usually not asked about, as the understanding of these figures is complex and therefore difficult to classify anyway. But how is a dryer set correctly so that it runs optimally in terms of energy? In almost all cases, this is left to the dryer manufacturer - the easiest way at first. However, if the operator stands at the dryer and really tries to understand the energy management, he quickly reaches the limits. The setting values are based on rough experiences in the past that have led to good preforms - or have been recommended by the dryer manufacturer. The end result need not be doubted: if enough dried process air is blown through the material, the preparation of the material is usually sufficient and the process satisfactory. But then usually far too much air is blown through the material: 30-50% too much is not uncommon - and the excess energy disappears via the heat exchanger into the cooling machine. When processing rPET, it is very likely that the material will turn yellow due to the higher dwell time at the heat maximum. Another strategy employed by operators is to keep the energy input at the dryer as low as possible - as long as the desired degree of dryness is achieved below 50 ppm. In this case, it is easy for the material to still be 25-40% short of thermal energy when it is heated in the drying hopper, despite the degree of dryness achieved. This missing energy then has to be added to the plasticising process - usually relentlessly in conjunction with increased friction and a significantly higher energy input. The preform quality then also suffers, which shows itself with an increased appearance of bubble formation and “black specks”. In addition, this is generally accompanied by increased acetaldehyde (AA) cleavage and IV degradation. Continuous dryer monitoring can do even more The above explanations clearly show that a dryer has an ideal operating point which, however, the operator finds difficult to determine due to a lack of sufficient information - certainly not continuously and adapted to the current processing procedure. It is also not possible to determine instabilities during the drying process in retrospect - and then to assign them to quality problems on the preform. These can occur, for example, if the material feed does not continuously and reliably supply the hopper with PET material. It is also not possible to determine when a drying hopper needs maintenance and cleaning, especially in the area of the air filters. For this reason, the filter is also cleaned here - often as a precaution - usually far too early - or far too late. Both mean losses in preform production. How can a dryer be continuously and reliably monitored? As explained at the beginning, the energy content in PET material cannot be measured economically. The erroneously circulated “bucket method” of pouring the PET material from the drying hopper into a bucket to measure the temperature of the PET granules is misleading, as it only measures the temperature of the hot air between the highly insulating PET
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