Chemical Recycling Processes of Nylon 6 to Caprolactam: Review and Techno-Economic Assessment.

[Display omitted] • Chemical recycling of polyamide 6 can become profitable at the industrial scale. • Alkaline and acidic hydrolysis and alcoholysis are the most promising pathways. • A payback period of less than 5 years is required. • A non-volatile solvent catalyzing the backbiting depolymerization is most promising. • An integrated reaction and separation zone results in the lowest costs. Chemical recycling has increasingly gained attention in order to mitigate environmental pollution and facilitate the transition to a circular economy. Yet only a minor fraction of the polyamide 6 waste is chemically recycled to its monomer ε-caprolactam. The present paper provides a systematic overview of chemical polyamide 6 re-monomerization pathways, an analysis of each pathway's reaction mechanism, a comprehensive simulative techno-economic process comparison of the alcoholysis and neutral, acidic, and alkaline hydrolysis as well as process opportunities for the chemical re-monomerization of polyamide 6. For this, a conceptual process design was carried out using a connected Aspen Plus and Python model. The results indicate that alkaline hydrolysis is the most energy-efficient process due to its simple integration of the reaction and purification section. In contrast, neutral hydrolysis using subcritical water generates by-products, which require energy-intensive separation, leading to the highest total annual costs. Although the conventional pathway using phosphoric acid requires less energy than the alcoholysis using supercritical isopropanol, the expensive hazardous waste treatment and raw material costs make it less economically attractive. Nevertheless, a cash flow analysis shows that both alkaline and acidic hydrolysis, as well as alcoholysis, become profitable within five years at the chosen scale of 12 kilotons of caprolactam per year. The results suggest that a non-volatile solvent or catalyst that catalyzes the chain-end back-biting depolymerization mechanism of polyamide 6 using an integrated reaction and separation zone with straight distillation or stripping of caprolactam would be the most economically and environmentally attractive process for chemical recycling of polyamide 6. [ABSTRACT FROM AUTHOR]