Method and device for the production of polyamide 6 pellets

A method for the production of polyamide 6 pellets, the method comprising:

• a) providing a melt of polyamide 6;
• b) feeding the melt into an underwater pelletizing system being operated with a fluid and producing pellets from the melt;
• c) transporting the pellets from the underwater pelletizing system with the fluid into an upper side of a vertical vessel having a cylindrical section and a tapered bottom section connected to a pipe comprising a screw and/or a rotary gate valve for conveying the pellets;
• d) extracting of cyclic dimers and oligomers from the pellets;
• wherein
• the extracting takes place in the underwater pelletizing system,
• after sedimentation by gravity into the tapered bottom section of the vertical vessel, the pellets together with residual amounts of the fluid are conveyed by the screw and/or the rotary gate valve through the pipe from the vertical vessel;
• the fluid in the underwater pelletizing system and the vertical vessel comprises an aqueous solution of ε-caprolactam,
• the fluid in the underwater pelletizing system is held at a temperature between Tg+X·(Tm-Tg) and y·Tm, wherein Tg and Tm are a glass transition temperature and a melting temperature of the polyamide 6, x=0.5-0.8 and y=0.95-1.0,
• the tapered bottom section of the vertical vessel is cooled,
• a pressure between 4 bar and 12 bar is maintained in the underwater pelletizing system and the vertical vessel, and
• the fluid is circulated between the underwater pelletizing system and the vertical vessel wherein a fraction of the fluid is withdrawn from circulation and the residual amounts of fluid leaving the vertical vessel with the pellets via the pipe and the withdrawn fraction are of the fluid is replenished with a fresh fluid.