(131i) Advanced Recycling of Plastics in a Circular Economy: State of the Art and Major Challenges for Complementary Chemical Recycling Technologies

The chemical industry is more and more confronted with social and political expectations to increase process efficiency and to provide new process technologies, allowing for the implementation of the sustainable development goals; for that purpose, circular economy principles in terms of circularity strategies must be considered. Besides the utilization of circularity strategies such as product re-design (which can be driven e.g. by means of simplified recycling approaches) and the avoidance and/ or substitution of crude oil derivatives and other fossil resources, the circularity strategies include recycling processes. In the chemical industry, such processes aim at the reintegration of material flows into specific production processes, e.g. the integration of pyrolysis oils in steam crackers as a naphtha substitute [1] or the re-integration of monomers obtained from advanced plastics recycling in polymer production [2]. Regarding plastics in particular, advanced recycling technologies are currently strongly focussed: The complementary – rather than competitive [3] – reintegration of material flows from both mechanical and chemical recycling processes aims at substituting crude oil derivatives and allowing for closed material cycles, thus reducing waste volumes in the genuine sense of a circular economy.

Chemical recycling processes are classified according to their basic operating principle into cracking, gasification and depolymerization processes [4, 5], whereas cracking processes can be further divided into pyrolysis which is associated with thermal and/ or catalytic cracking of polymers in the absence of oxygen; the other mechanism is the so called liquefaction, which combines pyrolysis and the evaporation of liquefied polymer components in a plastic melt [6]. Product characteristics of all chemical recycling processes do not only depend on the actual operating principle, but strongly on feedstock characteristics and process conditions (reactor types, temperatures, residence times, catalysts, etc.) as well [7]. Both the resulting diversity of chemical recycling processes themselves, and – for most of the respective process technologies – their low level of technical maturity, point out at fundamental challenges for chemical recycling technologies that must be considered to develop a plausible and knowledge-based complementary strategy for both mechanical and chemical recycling technologies. The major challenges are #1 the lack of a knowledge-based methodological approach for a higher-level evaluation of chemical recycling technologies, allowing for the identification of specific promising processes that should be considered in detail regarding the development and elaboration of a data base for in-depth evaluation methods such as LCA. This identification procedure is crucial for focusing the available research resources on promising technologies, rather than applying a watering can-like focus on all possible, or at least on many different technology scenarios regardless of their actual potential. Respective evaluation approaches have been proposed lately that consider technical evaluation criteria on a higher level such as TRL, process sensitivity towards feedstock contamination, range of treatable polymer types and others [8]. This approach allows for the identification and selection of suitable processes, when an actual waste problem in terms of feed characteristics, socio-local waste management issues, desired throughput, etc. is defined; further, it allows for the identification of process limitations, and, thus, for focusing of research activities. Another approach connects the planetary boundaries concept with LCA-orientated environmental measures such as GHG emissions, freshwater use, and others, by deriving limits for the mentioned measures [9]. By that, this approach allows an assessment of whether a particular process or combination of technologies has a potential to be compatible at all with a circular economy and with the planetary boundaries, respectively. Within the contribution, these novel approaches will be analyzed and discussed in-depth to point out both their limitations and possible applications in terms of process evaluation and selection.

Challenge #2 is the fact that there are still fundamental challenges in process development to be addressed and clarified, i.e. in terms of suitable approaches for the determination of the real-world process energy demand including actual conversions steps and the downstream effort, and appropriate balancing methods. For example, the minimum energy demand of the degradation of polystyrene (PS) is typically determined using the educt and product difference of the standard enthalpies of formation (e.g., [10]), although it is known that exothermic regions in the degradation of PS exist [11] which lower the overall energy demand for the degradation step. Further, the energy demand for downstream operations must be considered since it can be in a similar order of magnitude as the actual pyrolysis step, as it was shown by the author and co-workers for the chemical recycling of PS [2]. Additionally, a systematic methodology for the implementation and combination of both experimental data - especially regarding kinetics of the conversion steps - and model-based approaches for the design and scaling of reactors for chemical recycling technologies are missing, although the availability of a design tool for reactors is crucial for knowledge-based engineering.

And #3, the suitability and possible application scenarios of different chemical recycling process technologies are often limited to “plastics-to-plastics” approaches such as the derivation of naphtha by recycled oils obtained from e.g. pyrolysis of plastic waste. This narrowed perspective, on the one hand, often underestimates the downstream effort in terms of purification of pyrolysis oils, which is typically necessary to get rid of impurities such as heteroatoms and high olefin contents if a re-integration of pyrolysis oils into steam crackers on an industrially relevant level is considered. On the other hand, the actual idea behind a circular plastics economy, which is in a genuine sense the reduction of waste amounts and the idea of closing carbon cycles, and thus leading to a “waste-to-product” perspective, is threatened to get out of sight.

In summary, this review-like contribution gives a detailed overview on the state of the art of chemical recycling technologies and derives the major challenges for the establishment of a knowledge-based complementary pathway for chemical recycling technologies, with a strong focus on the implementation of respective technologies on an industrially relevant level. For that, #1 concrete examples and methods allowing for e.g. the knowledge-based evaluation of entire processes including downstream operations, #2 perspectives and specific tasks for process development and #3 methodological approaches for the identification of suitable re-integration routes of chemical recycling products are discussed in-depth. Based on that, the applicability and usability of discussed methods and their transferability regarding an industrial implementation of chemical recycling processes as a complementary technology for the realization of a circular plastics economy are evaluated.

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