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

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, and a comprehensive simulation and techno-economic comparison of alcoholysis, and neutral, acidic, and alkaline hydrolysis. Furthermore, process opportunities for the chemical re-monomerization of polyamide 6 are elaborated. 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.