Assessment of Nylon-6 Depolymerization for Circular Economy: Kinetic Modeling, Purification, Sustainable Process Design, and Industrial Practice

Nylon-6 is a commonly produced plastic that has found a wide range of applications in carpets, textiles, electronics, and automotive parts. Chemical depolymerization to form the nylon-6 monomer caprolactam (CL) has been practiced to recover oligomers and postindustrial nylon-6 waste. There is a growing interest in developing and improving processes for closed-loop recycling of postconsumer nylon-6 waste. This work presents a comprehensive assessment of the three primary types of nylon-6 depolymerization processes: liquid-phase hydrolysis, steam stripping, and solvent-free depolymerization. This study is unique in providing a thorough assessment of both the academic literature and industrial patents. We develop depolymerization kinetic models and suggest further experimental measurements for determining reaction mechanisms and discuss industrial practice of crude CL purification methodologies and waste management strategies. We develop for the first time detailed Aspen Plus simulation models incorporating thermodynamic and kinetic modeling, impurity separation, product purification, and waste minimization to provide the quantitative foundation for sustainable design of all three types of depolymerization processes. We identify where there are uncertainties in design and modeling and where gaps in data currently exist. Finally, we implement heat integration and process intensification to reduce utility demand and CO2 emission, enhance the sustainable design, and provide quantitative comparison of depolymerization process options in terms of energy demands and CO2 emission, among others.