(142ai) Dynamic Modeling of Solid-State Polymerization of Bisphenol a in a Moving Packed Bed Reactor

Solid-state polymerization (SSP) is used to manufacture high molecular weight condensation polymers such as polycarbonate (PC), poly(ethylene terephthalate) (PET) and nylons at a temperature above the polymer's glass transition temperature but below its melting point. Low molecular weight amorphous polycarbonate prepolymer prepared by melt polymerization must be partially crystallized before solid-state polymerization. Experimental and theoretical model simulations show that the rate of SSP and polymer molecular weight are strongly dependent on the mole ratio of reactive end groups, reaction environment, and physical shape of the polymer particles. It is also shown that the polymer chain length distribution can be quite broad even in the polymer that has a very high molecular weight average. In this work, a dynamic model is also developed for a continuous moving packed bed polymerization process in which prepolymer particles and heated purge gas flow countercurrently or cocurrently. For small prepolymer particles, intraparticle mass transfer and heat transfer effects are insignificant but for large prepolymer particles, such intraparticle transport effects can be significant. In some extreme cases, only the polymers near the particle surface react to higher molecular weight. To account for the particle-level and reactor-level reaction, mass and heat transfer limitations, a heterogeneous dynamic model has been derived. Finite element model simulations using FEMLAB® indicate that internal temperature nonuniformity is present in the reactor in both the axial and radial directions. The temperature nonuniformity has a significant effect on the rate of solid-state polymerization because it causes the change in crystallization rate and the nonuniformity in the end group concentrations and polymer molecular weight. For example, the presence of low temperature zones near the top section of the reactor during the transient period lowers the overall rate of molecular weight increase, which then results in longer reaction time to reach a certain targeted high molecular weight. The model simulator is also used for various reactor design and operating conditions to devise the reactor configuration that offers the shortest possible reaction time to obtain high molecular weight polycarbonate.