Separation of caprolactam from water by membrane processes

Abstract

Caprolactam (C6H11NO, CPL) is the precursor to nylon 6. Production of caprolactam generates wastewater streams that contain a small amount of caprolactam. The objective of this study is to evaluate appropriate separation techniques to treat such wastewater and recover caprolactam from the wastewater for reuse. Nanofiltration (NF) process was shown to be applicable to treat this caprolactam-containing wastewater, but the highest rejection of caprolactam reached only 70%. Operating conditions (e.g., transmembrane pressure, and caprolactam feed concentration) were shown to affect the performance of the NF process significantly. An increase in operating pressure improved the permeation rate, while both permeation rate and caprolactam rejection decreased with increasing feed caprolactam concentration. Surface fouling of the membrane also severely impeded NF operation. On the other hand, vacuum membrane distillation (VMD) offers a promising alternative that can filter out a high concentration of caprolactam from an aqueous solution. A higher operating temperature significantly increased the water permeation rate, while increasing feed caprolactam concentration lowered the permeation flux. Membrane fouling occurred during the concentration of caprolactam because the caprolactam solute deposited on the membrane surface, resulting in a decline in the VMD performance.