47 TRADE SHOW REVIEW PETplanet Insider Vol. 22 No. 10/21 www.petpla.net in reverse direction back through the screen, carrying away contaminant for removal from the system. The sequence is performed for each cavity one after the other. In normal operation, polymer is fl owing through all four cavities. While one of the cavities is cleaned, production continues through the other three. There are two challenges to improving this type of screen changer: fi rst, to reduce downtime by extending the working life of the screens and thus reducing the frequency of required screen changes; and second, to cut back on material loss by reducing both the amount of polymer expended per backflush cycle and the number of cycles needed in a given period of time. With a ton of PET bottle fl ake selling at € 920 (US$1,000), material losses with a standard backfl ush screen changer, operating on a 24/7 basis, can amount to more than €100,000 (US$108,467) per year. To address these issues, Nordson has developed a fi lter stack composed of multiple so-called FlexDiscs which replaces the standard screen in each cavity, substantially enlarging fi ltration area while decreasing the melt loss caused by backfl ushing. Each fi lter stack consists of two to four cassettes, with two screen packs in each cassette. In PET recycling, backfl ush screen changers combined with FlexDisc fi lter stacks have provided fi ltration as fi ne as 25-30 μm, increased screen lifetime, and exhibited more than 50% less material loss versus comparable screen changers. Comparative data from trials at one recycling company appear in Table 1. Pelletising systems Two types of pelletising system are used in PET recycling - strand pelletisers and underwater pelletisers (UWPs). A third type, water ring pelletisers, is not appropriate for lower-viscosity materials like PET. Widely used for recycling, UWPs provide several advantages over strand pelletisers. The UWP can achieve by far the greatest throughput rates and is more capable of automation, making possible linkage with the upstream process. (Unfortunately a larger capital investment is required for realising these benefi ts.) Unlike strand pelletising, underwater production of pellets is a closed process, minimising emissions as well as the generation of dust during production and in the fi nal product. With constant melt fl ow (ideally ensured by a gear pump) and the fl ow uniformity provided by a round die plate, underwater pelletising yields the most consistent pellet size distribution. The spherical shape of pellets produced under water results in 5-10% greater bulk density compared to cylindrical pellets, and it reduces their tendency to clog in subsequent post-pelletising equipment as well as in hoppers and feed throats. Downstream of the pelletiser are the pellet dryer and the tempered water system. After process water has been used in the pelletiser cutting chamber and for transporting pellets to the dryer, it needs to be cleaned of fi nes and cooled to within a strictly maintained temperature range for reuse in the pelletising cycle. Fines-removal components in tempered water systems range from a woven wire mesh screen that periodically must be removed and cleaned manually, to systems that reduce downtime and operator intervention by providing continuous, automated fi ltration. Still more advanced is an automated, self-cleaning system that reduces overall pelletiser energy consumption by 10-17% by eliminating the need for a separate fi nes-removal system, which in standard systems requires a secondary, dedicated water pump. Energy-saving inline crystallisation For the bottle-to-bottle recycling process, systems have been developed that reduce energy costs by combining the pelletising and crystallisation step in a single integrated process. This technology uses the thermal energy of the molten polymer in PET pelletising for subsequent crystallisation, avoiding the need to cool PET after pelletising and then reheat it for crystallisation. The inline crystallisation system developed by Nordson, called CrystallCut, is designed to work with the spherical pellets generated by UWPs and is an integrated network incorporating pelletiser, dryer, and crystallising unit. rPET pellets achieve a crystallisation level of up to 32% (in production of virgin polymer, by comparison, the fi gure is approximately 40%, depending on the initial quality of the material). Subsequently, a solid state polycondensation (SSP) reactor raises the intrinsic viscosity of the polymer and carries out a fi nal decontamination step by removing trace substances like acetaldehyde, which have a deleterious effect on taste in food applications. Pellet temperature in the CrystallCut system is maintained at 140-180 °C. The system eliminates the intervening step of holding pellets in a silo at a maximum 60 °C (to avoid pellets sticking together) and a separate pre-crystallisation step before the pellets are transferred to the SSP reactor. Alternatively, by adjusting process parameters, it is possible to bypass the CrystallCut step or to produce amorphous polymer. Compared with a traditional operation with UWP and SSP operating at 2.5 t/h, the CrystallCut system provides thermal energy savings of 30-50 kWh/t, electrical
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