Techno-Economic Process Analysis of the Chemical Recycling of Nylon 6 Using Phosphoric Acid

Chemical recycling has increasingly gained attention in order to counteract environmental pollution and support the transition to a circular economy. Today, only around two percent of polyamides are chemically recycled each year. This is mainly due to the economic performance of the process. The conventional depolymerisation process of polyamide 6 using phosphoric acid was simulated to analyse the techno-economic potential and identify its existing constraints. A conceptual process design was carried out using Aspen Plus and Python. The results show that the process becomes profitable at 4600 t/year of caprolactam. Assuming a reasonable plant capacity of 12.6 kt/year, the payback time would be between three and seven years in the best- and worst-case scenarios. The disposal of the waste stream containing unreacted polyamide 6, oligomers and phosphoric acid, as well as phosphoric acid raw material costs together contribute 47% of the total production cost. Accordingly, a sensitivity analysis showed that the total annualised costs depend significantly on the amount of phosphoric acid raw material used. Overall, the results indicate great industrial potential for the depolymerisation of polyamide 6. Still, the focus of further research on this conventional process should rely on phosphoric acid and its efficient recovery along with the treatment of the waste stream.