PETpla.net Insider 04 / 2014

MATERIAL / RECYCLING 22 PET planet insider Vol. 15 No. 04/14 www.petpla.net Ohl process makes it safe for food PET recycling takes a tumble At last year’s K trade fair in Düsseldorf, ProTec Polymer Processing was present- ing the Ohl process in which flakes extracted from used PET bottles and washed are treated to create PET that exhibits material properties equivalent to those of virgin PET. The tumble reactor will also be presented at Chinaplas (see page 42). This bottle-to-bottle recycling is based primarily on two process steps: a continuous extrusion process and the subsequent discontinuous treat- ment of the extracted pellets in what is called the Ohl tumble reactor. This is supported by upstream, in-between and downstream auxiliary processes with the corresponding plant compo- nents. On display for the first time at the trade show is a new version of the tumble reactor featuring optimised energy consumption and drive tech- nology. This coincides with innova- tions in the process control that aim to minimise the inevitable material degeneration (IV drop) in the extruder during treatment of the PET flakes. Energy consumption The Ohl tumble reactor, a rota- tion reactor that can be heated and evacuated, is a key system compo- nent in the multistage treatment pro- cess. PET recycling involves purging (decontaminating) the regrind of any volatile pollutants on the one hand, while allowing for simultaneous post- condensation (increasing the intrinsic viscosity IV) to occur on the other. For this purpose, the reactor consisting of a double shell is heated using heat transfer oil. The structurally optimised version of the reactor presented at the trade show uses a smaller volume of oil to achieve the heat transfer required to control the process. This also results in lower energy consump- tion. Together with the weight optimi- sation of the reactor container overall as well as the optimisation of the drive and control technology, energy consumption for this process step has been reduced by around 20%. Due to the general susceptibility of PET to hydrolysis, moisture is criti- cal for the treatment in the extruder because of the associated material degeneration (IV drop). The flakes stored in silos or in big bags must therefore be pre-dried before entering the extruder. This cannot be compen- sated for by more intensive degassing of the extruder alone. Drawing on its many years of expertise in material processing, ProTec Polymer Process- ing has now enhanced this process step with a two-stage procedure. This is achieved by connecting a Somos dry air dryer downstream of the warm air dryer generally used to remove surface moisture from the washed flakes. This pre-drying of the mate- rial down to residual moisture in the ppm range significantly reduces IV drop during extrusion. Reducing the IV drop reduces the need to use large amounts of energy to post-conden- sate the material again afterwards. Consequently, the process duration required in the tumble reactor can also be shortened. Precise control over the para- meters temperature, vacuum and time ensures that granulates – recyclates or new product – in the tumble reac- tor are individually processed to the required quality depending on the technical specifications and require- ments of the intended use. As a result, PET pellets that can be reused to manufacture PET beverage bottles must meet minimum requirements particularly with regard to IV values: for beverages without carbon dioxide, these IV values are at least 0.75dl/g, for carbonated beverages, IV values of 0.78 to 0.82dl/g – and for technical applications, in part even significantly higher IV values are required. The rPET has an acetaldehyde content of ≤1ppm and is said to exhibit material properties just like virgin PET product. The non-objection letter from the FDA (Food and Drug Administration) veri- fies that the PET recyclates produced in this manner are safe when in direct contact with foodstuffs. Through batch operations, the entire content in the reactor has exactly the same reten- tion time, through which reproducible, consistent product characteristics are attained. www.sp-protec.com