Simulation of Hydrolytic Polymerization of Nylon-6 in Industrial Reactors: Part I. Mono-Acid-Stabilized Systems in VK Tube Reactors

Abstract

Nylon-6 polymerization in a VK tube (vertical column) reactor is largely not understood because of the complex internal structure of the reactor and the unavailability of essential industrial data on polymerization. In the present study, the hydrolytic polymerization of nylon-6 has been simulated in an industrial VK tube reactor for concentrations of various species. The model, which assumed a flat velocity profile in the radial direction, predicted the industrial results with high accuracy. The internal design of the reactor appears to be responsible for getting close to plug-flow conditions inside the VK tube. An empirical relation based on temperature and pressure was used to determine the water profile along the vertical axis of the reactor. Because the pressure increased along the axis of the reactor and the temperature first increased and then decreased, the model predicted a lowest water content (LWC) point at near the highest temperature point. The concentration of water at the LWC point was found to be critical in determining the properties of the end product. The model suggested three main zones in a VK tube reactor: a very small top turbulent zone, where most of the water was lost, followed by a large middle zone, which was a vaporizing plug-flow tubular zone, and a bottom zone, which was a nonvaporizing plug-flow tubular zone. The vaporizing zone ended and the nonvaporizing zone began where the LWC point was achieved. The small turbulent zone at the top did not appear to affect the end polymer properties of polymerization. This model was found to be useful in carrying out process optimization and suggesting improvements in VK tube designs for higher productivity.