PETpla.net Insider 06 / 2020

MATERIALS / RECYCLING PETplanet Insider Vol. 21 No. 06/20 www.petpla.net 43 sequence is performed for each cavity one after the other. In normal opera- tion, polymer is flowing through all four cavities. While one of the cavi- ties is cleaned, production continues through the other three. There are two challenges to improving this type of screen changer: to reduce downtime by extending the working life of the screens and thus reducing the frequency of required screen changes; and, to cut back on material loss by reducing both the amount of polymer expended per backflush cycle and the number of cycles needed. With a ton of PET bottle flake selling at EUR920 ($ 1,000 USD), material losses with a standard backflush screen changer, operating on a 24/7 basis, can amount to more than EUR100,000 ($ 108,467 USD) per year. To address these issues, Nordson has developed a filter stack composed of multiple so-called FlexDiscs which replaces the standard screen in each cavity, substantially enlarging filtration area while decreasing the melt loss caused by backflushing. Each filter stack consists of two to four cassettes, with two screen packs in each cassette. 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 fines 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 manu- ally, to systems that reduce downtime and operator intervention by providing continuous, automated filtration. Still more advanced is an automated, self- cleaning system that reduces overall pelletiser energy consumption by 10 to 17% by eliminating the need for a separate fines-removal system, which in standard systems requires a sec- ondary, dedicated water pump. Reductions in downtime and material loss with BKG FlexDisc filter stack for screen changers Competitor (8 cavities) BKG HiCon V-Type-250-3G with BKG FlexDisc Throughput 1,800 kg/hr 3,968 lbs/hr 1,800 kg/hr 3,968 lbs/hr Filter fineness 30 micron 30 micron Filter area 1,814 cm² 4,000 cm² Screen changes per day 2 to 3 ---- Screen changes per week 14 to 21 2 Screen working life [hours] 8 to 12 110 Maximum number of backflushes 20 120 Material loss per backflush cycle 11.33 kg (25 lb) 9.97 kg (22 lb) Material loss per day 680 kg (1,500 lb) 239 kg (528 lb) Material savings ---- 65% Source: Nordson Corporation Backflush screen changer FlexDisc Underwater Pelletiser In PET recycling, backflush screen changers combined with FlexDisc filter stacks have provided filtration as fine as 25-30 μ m, increased screen lifetime, and exhibited more than 50% less material loss vs. 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). Widely used for recycling, UWPs are claimed to provide several advantages over strand pelletisers. The UWP can achieve great through- put rates and is more capable of auto- mation, making possible linkage with the upstream process. Unfortunately a larger capital investment is required for realising these benefits. Unlike strand pelletising, underwater produc- tion of pellets is a closed process, minimising emissions as well as the generation of dust during production and in the final product. With constant melt flow (ideally ensured by a gear pump) and the flow uniformity pro- vided by a round die plate, underwater pelletising yields the most consistent pellet size distribution. The spheri- cal shape of pellets produced under water results in 5 to 10% greater bulk density compared to cylindrical pel- lets, and it reduces their tendency to