Assessment of Nylon‑6\nDepolymerization for\nCircular Economy: Kinetic Modeling, Purification, Sustainable Process\nDesign, and Industrial Practice

Nylon-6 is a commonly produced plastic that has found\na wide range\nof applications in carpets, textiles, electronics, and automotive\nparts. Chemical depolymerization to form the nylon-6 monomer caprolactam\n(CL) has been practiced to recover oligomers and postindustrial nylon-6\nwaste. There is a growing interest in developing and improving processes\nfor closed-loop recycling of postconsumer nylon-6 waste. This work\npresents a comprehensive assessment of the three primary types of\nnylon-6 depolymerization processes: liquid-phase hydrolysis, steam\nstripping, and solvent-free depolymerization. This study is unique\nin providing a thorough assessment of both the academic literature\nand industrial patents. We develop depolymerization kinetic models\nand suggest further experimental measurements for determining reaction\nmechanisms and discuss industrial practice of crude CL purification\nmethodologies and waste management strategies. We develop for the\nfirst time detailed Aspen Plus simulation models incorporating thermodynamic\nand kinetic modeling, impurity separation, product purification, and\nwaste minimization to provide the quantitative foundation for sustainable\ndesign of all three types of depolymerization processes. We identify\nwhere there are uncertainties in design and modeling and where gaps\nin data currently exist. Finally, we implement heat integration and\nprocess intensification to reduce utility demand and CO<sub>2</sub> emission, enhance the sustainable design, and provide quantitative\ncomparison of depolymerization process options in terms of energy\ndemands and CO<sub>2</sub> emission, among others.