Challenges and opportunities of light olefin production via thermal and catalytic pyrolysis of end-of-life polyolefins: Towards full recyclability

Abstract

Is full recyclability of polyolefins via chemical recycling a dream, or can it become a reality? The main problem in recycling plastic waste is that its composition is highly heterogeneous while sorting and purifying solutions to obtain mono-streams are complex and require large investments, thereby hampering the economy of scale. Ideally, novel chemical recycling processes are designed to have mixed plastic wastes as input and higher value products are produced such as C₂–C₄ olefins or aromatics instead of a low value oil. In this review we show the directions how we can realize these objectives. Classical thermal pyrolysis offers some possibilities but requires very high temperatures exceeding 800 °C to transform the plastic waste back into the desired temperatures. Nevertheless, because of its robustness, thermal pyrolysis of polyolefinic plastic waste is currently intensively studied and the first industrial applications are operated at low to medium temperature range to maximize oil as the main product. Catalytic pyrolysis is still under development, but under ideal lab-scale conditions around 85 wt.% of C₂–C₄ olefins can be produced when pure polyolefin feeds are used. With improved catalyst design it should be possible to get this number further up without affecting the catalyst stability. As the yield of light olefins in pyrolysis is impacted by both the process design (reactor type, the efficiency of plastic sorting prior to conversion, flexibility towards feed composition) and experimental parameters (temperature, catalyst type, catalyst/feed ratio, contact mode, residence time, addition of inert or reactants) also further improvements are possible in this respect. To industrialize pyrolysis of plastic waste, short residence times (<1 s) are crucial to avoid secondary reactions and by-products such as methane, coke, and aromatics. Pyrolysis reactors that are designed according to these principles, such as downers, spouted fluidized bed, and vortex reactors, are envisaged to result in optimal yields of C₂–C₄ olefins. However, coke formation seems to be inevitable and the reactor designs need to be sufficiently robust to allow for in-situ coke removal. For future research it will be crucial for the industrial viability of plastic waste pyrolysis to improve the purification of the plastic waste stream, optimize both the catalysts selectivity and stability, and design a suitable industrial reactor. It is envisaged that further innovations in these three areas will eventually allow reaching the 90 wt.% target.