Agglomeration behaviour of caprolactam solution concentrates triggered by cyclic dimers in the recovery process: Characterisation, mechanism, and process optimisation

Abstract

The cyclic dimer 1,8-diazacyclotetradecane-2,9-dione is one of the most important components of caprolactam solution concentrates and triggers significant agglomeration during the recovery process in industrial polyamide-6 plants. For this reason, the agglomeration behaviour and morphology changes of cyclic dimers in solution concentrates were investigated. Precipitates separated from an ∼80wt.% solution concentrate were explored. Cyclic dimers aggregated in the solution concentrates at lower temperatures to form larger, thicker, and compacted structures or multi-layers structures. An appropriate increase in the solution temperature and addition of fresh caprolactam aid in avoiding agglomeration, but high temperatures (over 140 ℃) initiate the hydrolytic polymerisation of caprolactam in the solution concentrate, which aggravates blockages. Based on these results, an optimised process model is introduced to avoid aggregation.