Modeling of the Dehydration Process Coupled with Solid-State Polymerization in Nylon 6 Drying Column

Drying of wet Nylon 6 chips is a significant process for dehydration after granulation. During the process, the flow pattern of gas-solid phases is countercurrent flow, while the mass transfer and solid-state polymerization (SSP) are deeply coupled with each other. In this paper, the mass transfer behavior for dehydration at drying conditions has been investigated by single pellet experiments using Thermogravimetric Analyzer (TGA). A diffusion model for dehydration has been developed and the diffusion coefficient has been estimated by TGA test data. Besides, the SSP reaction characteristic has been independently studied in a polymerization tube at different temperatures. The reaction kinetics parameters, including the rate constants and equilibrium constants for both forward and reverse reactions, have been obtained by fitting the experimental data. Based on that, an integrated model combining the mass/heat transfer and polymerization reaction has been presented for the industrial continuous Nylon 6 drying process. The influences of drying conditions including the initial number average degree of polymerization, inlet nitrogen temperature, and flow rate, on distribution profiles of temperatures, water contents, and degree of polymerization of chips along the column height have been simulated. The integrated model can be used to predict the industrial drying process.