PETpla.net Insider 03 / 2016

MATERIAL / RECYCLING 11 PET planet insider Vol. 17 No. 03/16 www.petpla.net The EcoSphere technology resolves this dilemma by providing cost savings and quality at the same time. (Fig.3) Polymetrix says that the energy savings available by using EcoSphere are significant; the extent is revealed when examining the way that tem- perature develops in each process step. A conventional SSP heats PET pellets from room temperature to about 210°C, some time after the PET melt has been cooled down from about 290°C. The EcoSphere technology restricts cooling to the crystallisa- tion temperature, around 150°C. This enables an SSP plant to cut energy consumption by more than half, compared to conventional processes. (Fig.4) A range of quality advantages result from the lower starting IV. Higher IV build-up in the solid state leads to lower oligomer contents, lower acetaldehyde (AA) values and much lower 2-Methyl-1,3-dioxolane (MDO) values. MDO is formed from AA and ethylene glycol (EG), when PET is treated under air especially. In contact with water it decomposes to EG and AA. This means that MDO is, essentially, a hidden form of AA; this is often missed in PET resin quality control. The reduced shear in the melt phase eliminates the formation of gels and black spots. Residence time in a solid state reactor is not a main cost driver. It facilitates reduced catalyst concentra- tions, which is important in markets where antimony concentrations are restricted. Together with the reduced shear, this leads to higher L (bright- ness) values and lower yellowness. The quality advantages become more obvious when the PET is recycled. While intermediate storage pre- vents energy savings it does provide the flexibility to run the CP and the solid phase process independently of each other. At the same time, the wider operation window resulting from transition at a lower starting IV increases overall plant reliability. Eco- Sphere is said to provide the best of both worlds: greater flexibility and the established robustness of a conven- tional SSP plant. (Fig.5) An important process advantage with EcoSphere technology is the uninterrupted operation of the CP plant at moderate viscosity targets; typically below 0.6dl/g. It also offers an option to run the CP and solid phase plants independently of each other. EcoSphere retains the usual flex- ibility in plant layout that allows the use of either candle filters or screen changers and leaves room to position cutters in a way that facilitates access to the equipment for operation, main- tenance and upgrade work. Fig 5: Operation flexibility and reliability An aspect crucial to the long term success of a PET manufacturing plant is the yield of grade A quality material. Fig.4: Temperature profile: conventional CP/SSP compared with EcoSphere technology