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

• Published in: Process safety and environmental protection Vol. 136; pp. 56 - 65
• Main Authors: Yi, Chunwang, Yang, Ce, Li, Jie, Chen, Juan, Zhang, Shen, Sun, Huan
• Format: Journal Article
• Language: English
• Published: Rugby Elsevier B.V 01.04.2020; Elsevier Science Ltd
• Subjects: Agglomeration; Caprolactam; Caprolactam concentrates; Cyclic dimer; Dimers; High temperature; Industrial plants; Low temperature; Morphology; Morphology evolution; Multilayers; Optimal process model; Optimization; Polyamide resins; Polyamides; Precipitates; Recovery; Structure; Cyclic dimer; Agglomeration; Morphology evolution; Structure; Optimal process model; Caprolactam concentrates
• ISSN: 0957-5820; 1744-3598
• DOI: 10.1016/j.psep.2020.01.026

•Cyclic dimer is essential to the aggregation of caprolactam solution concentrates.•Cyclic dimer showed higher solubility in liquid caprolactam than hot water.•Adding fresh caprolactam effectively avoids blockages of solution concentrates.•Aggregation mechanism of caprolactam solution concentrates was illustrated.•An energy and cost saving process model was introduced to avoid aggregation. 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.