Simulation of continuous solid-phase polymerization of nylon 6,6 (II): processes with moving bed level and changing particle properties

Abstract

Solid-phase polymerization (SPP) reactors are used to remove moisture from nylon 6,6 prepolymer particles and to increase the degree of polymerization (DP), at low temperatures where undesirable thermal degradation reactions do not occur. In this article, reactor start up, shut down, and step changes in properties of the incoming prepolymer particles are simulated using VLUGR2, a finite difference code for solving partial differential equations (PDEs) in time and two spatial dimensions. Some of the reactor operating situations simulated result in dynamic changes in bed level, leading to moving boundary conditions at the top surface of the bed. To permit solution of this moving-boundary problem using VLUGR2, the model equations are transformed using a dimensionless vertical coordinate. A transformation in the radial coordinate is also introduced, so that changes in incoming particle diameter can be simulated. Sharp changes in the properties of particles entering the reactor result in steep fronts, moving downward through the bed with a predictable velocity. A stretching transformation is introduced to aid in simulation of reactor operation with these moving steep fronts. The stretching transformation tracks the movement of the front through the bed, automatically increasing the relative number of nodes near the moving front, and leading to improved simulation accuracy for a given number of total nodes.