Crystallization-Based Separation of ε-Caprolactam from a Depolymerization Reaction Mixture – Fundamentals and Potential Separation Strategies

Depolymerization and subsequent capture of monomers is of interest for process efficiency and sustainability. Polyamide-6 (PA-6) can be depolymerized in aqueous solution using a homogeneous catalyst. To purify the ε-caprolactam (CPL) monomer and recycle the catalyst, two potential crystallization-based strategies are studied: direct crystallization from the reaction mixture and liquid-liquid extraction prior to crystallization. Solubilities in water and promising organic solvents were measured and compared to predictions using perturbed-chain statistical associating fluid theory (PC-SAFT) or conductor-like screening model for real solvents (COSMO-RS). Detailed solid-liquid equilibria (SLE) of the CPL/water system were investigated utilizing DSC and PXRD. This work provides an overview of important factors of PA-6 depolymerization including recycling of tungstophosphoric acid (TPA) and purification of CPL monomers. A first conceptual process design to be further developed is derived.