Solvent-based dye removal in polyester recycling : A process evaluation and modelling study

The transition towards sustainable textile approaches, such as textile recycling, requires im-proved methods for handling dyes. Currently, no real industrially implemented processes exist that remove dyes without damaging them and this thesis investigates this issue together with Rewin Textiles AB. This company recycles polyester through depolymerisation and therefore, the implementation of a pre-treatment step using a solvent for dye-extraction purposes is evaluated. Five solvents are assessed: DMSO, benzyl alcohol, Cyrene, supercritical carbon dioxide, and ethylene carbonate with tetramethylurea. DMSO exhibits the most fa-vourable overall trend, taking into consideration toxicity, process practicality, extraction efficiency and cost. Rewin aims at recycling 5000 kg/h polyester and according to experimental values found, 25,000 kg/h DMSO is needed to extract 87 % of dyes after one cycle of 30 minutes at 100 °C. This constitutes the basis of the recommended flowsheet, which instead includes three cycles implemented through a counter-current batch process. After dye extraction, polyester is cleaned with water, whilst the dye is separated from DMSO through precipitation. To improve the precipitation behaviour of dyes, Aspen Plus was used to model a single evaporator representing a three-stage evaporator, achieving a 90 % removal rate of DMSO at 0.16 bar and 130 °C. Followingly, 660 kg/h of water is mixed in the DMSO-dye solution to trigger the dye to precipitate, which is then filtered out. Lastly, DMSO and water are recovered through distillation. The solvent cost is 351 million SEK per year with a recirculation rate of 95 %, which represents a higher cost than what can be saved through the process. Overall, the thesis showed that a solvent-based pre-treatment step is a promising process to separate dyes from polyester. However, due to many assumptions and limitations, the presented results should be taken with precautions, and further research is needed.