Within the current problem of plastic residues management, waste from electrical and electronic equipment (WEEE) are becoming one of the hot topics in the scientific community. The consumption of this type of equipment has increased significantly in recent years, as they contribute to high levels of life expectancy and their large-scale production makes them affordable to a large part of society. But being short-lived products, they quickly become waste and, due to their chemical and structural heterogeneity, current management systems have become practically unsustainable.
The technologies available for plastic waste management include mechanical and chemical recycling, incineration and, as a final option, landfill disposal. Within thermochemical recycling, pyrolysis is currently being explored as the main alternative to produce fuels or other valuable chemicals from plastics that cannot be recycled by other ways.
Pyrolysis involves the thermal decomposition of plastics in the absence of oxygen at temperatures in the range of 400-600°C, typically producing up to four fractions: gas, oil, waxes and a carbonaceous solid (char). By optimising the reaction conditions and in the presence of a catalyst, oil yields can be maximised. The role of the catalyst in this case is twofold: boosts cracking reactions, allowing further depolymerisation of the feedstock into lighter hydrocarbons, and, on the other hand, narrows the composition of the oil into specific and valuable chemical products (e.g. monoaromatics).
Among the catalysts investigated in the pyrolysis process so far, zeolites exhibit excellent performance due to their unique acidity and pore architecture. In particular, zeolite ZSM-5 is recognised as one of the most promising catalysts due to its high cracking activity and lower coke generation than other zeolites thanks to its pore structure. In addition, its acidic and accessibility properties can be modified to further improve its reaction performance.
Electronics cable recycling
Studies of catalytic pyrolysis with model plastics are numerous, however, there are fewer examples when it comes to real samples with a variety of polymers in their composition, additives and inorganic material. Samples containing PVC are particularly problematic due to their high chlorine content (up to 57% by weight), which has to be almost completely removed for introduction into the refineries.
In this context, the Thermochemical Processes Unit of Institute IMDEA Energy together with researchers from the University of Calabria have studied the conversion of real plastics, mixtures of polyethylene (PE) and PVC compounds, from the recycling of cables from the electronics sector.
The strategy followed by the two institutions is based on a two-stage pyrolysis process. In the first stage, which is exclusively thermal, the plastic waste is subjected to a relatively low temperature (350 ºC) and most of the chlorine contained in the PVC molecules is released in the form of hydrochloric acid gas, which is safely retained in an aqueous solution after the reaction. The resulting plastics are then subjected to a second stage at a higher temperature, converting the plastic into vapours at the reaction temperature and passing them through a bed of modified ZSM-5 zeolite catalyst to improve both their accessibility and acidity properties.
The results obtained demonstrate that this combination is an efficient strategy for the production of a significant fraction of oil with high aromaticity and octane number. Furthermore, the chlorine content has been found to be low enough to be finally processed in refinery units, both to produce transport fuels and chemical products.
More information: 10.1016/j.cattod.2021.11.033