PETpla.net Insider 06 / 2020

MATERIALS / RECYCLING PETplanet Insider Vol. 21 No. 06/20 www.petpla.net 42 Pelletising and melt-delivery systems Producing virgin-like rPET by Frank Asmuss, Business Development Manager, and Stefan Wöstmann, Application Engineer, Nordson Corporation The worldwide boom in demand for recycled PET (rPET) has generated new challenges for polymer processing systems used to transform molten polymer from PET flake into high- quality material for reuse. The biggest challenge involves bottle-to-bottle (B2B) recycling where regulatory and marketing mandates call for a dramatic increase in bottle collection and use of rPET in new packaging. Downstream of the extruder, the key equipment in rPET production lines includes so-called “melt delivery” components - gear pumps and screen changers; pelletising systems; and, as required, systems for crystallising pelletised material. Among the major post-extrusion challenges faced by processors of PET flake are intensifying require- ments for:  High levels of purity: Bottle flake feedstock includes considerable amounts of contaminants which must be filtered out by the screen changer.  Melt pressure stability: A gear pump provides the uniform melt flow and pressure required for the process to operate efficiently and with uniformly high-quality prod- uct. The pressure requirements for each component downstream of the extruder must be calcu- lated to determine the appropri- ate extruder drive rate and type of gear pump.  Fineness of melt filtration: A fine- ness of 25-30 μ m is essential for bottle-to-fibre applications in order to extend the working life of the screen packs on the spinning line. In B2B recycling, finenesses are now increased from a typical former level of 60 μ m or more up to 40 μ m or less. By comparison, a young human eye cannot distin- guish particles finer than 50 μ m.  Minimal losses of polymer: The widely used “backflush” screen changer ejects some of the poly- mer melt as part of the process of removing contaminants. Over time, the amount of material loss can be substantial. One goal of development in screen changers is to reduce such losses.  Energy efficiency: In conventional underwater pelletising systems, three phases where an energy input is required are 1) main- tenance of the tempered water system circulating through the pelletiser; 2) drying of pellets; and 3) crystallisation. Reduction of energy consumption is one goal.  High productivity: The maximum throughput now considered the state of the art in producing rPET pellets for bottle applications is approaching 5 t/h. For post- extrusion components, achieving these rates depends on three fac- tors: 1) component designs that yield streamlined flow properties; 2) enhanced control over melt pressure and temperature; and 3) reduction in downtime for trouble- shooting and maintenance. Gear pump The gear pump shifts part of the job of building melt pressure from the extruder, thereby reducing stress on the extruder and on the processed material while delivering melt at the pressure level required by the screen changer and the fol- lowing downstream equipment. In addition, the gear pump provides uniform flow, reducing surges and other process variations that can negatively affect the pelletising process and pellet uniformity. With use of a gear pump, the degassing system of an extruder can be made more reliable, and the extruder length can be reduced. Screen changer While typically deployed down- stream of the extruder and upstream of the screen changer, the gear pump may be preceded by an additional screen changer that provides “pre- filtration” at a level down to 1,000 μ m, removing large particles that can cause excessive wear to the gear pump. At this particle size, the con- tamination level is less than 0.1%, provided that there has been efficient washing and sorting prior to extrusion. Otherwise, the contamination level is larger. For the main filtration step in PET processing, a piston-actuated system with a self-cleaning feature based on a hydraulically powered process called backflushing is state-of-the-art. In a screen changer with four screen cavities, for example, melt flow is split into four streams. Backflushing starts when the pressure differen- tial caused by contaminant build-up increases to a pre-set level. Melt is then compressed and discharged in reverse direction back through the screen, carrying away contaminant for removal from the system. The Recycling Special Gear pump