Direct use of mixing data for modeling high‐viscosity, melt‐phase, condensation polymer reactors

Abstract

SYNOPSISMathematical models for simultaneous reaction and mass transfer occurring in the manufacture of high-viscosity condensation polymers are considered. Particle tracking experiments are used to estimate convective flow rates and mixing volumes in a disc-ring reactor configuration. These results are incorporated directly into a mixing-cell model without resorting to the use of restrictive assumptions regarding the convective mixing. Both a penetration theory model and a flash evaporation model are used to simulate the transport a t the liquid-vapor interface. Although widely used in previous studies, the penetration theory model is ultimately rejected because it underpredicts the overall reactivity. Model results predict interactions between agitation rate, residence time, and the overall reaction rate for commercial-scale systems producing poly(ethy1ene terephthalate). The model is partially verified by comparison with degassing data. 0 1995 John Wiley & Sons, Inc. I NTRO D U CTlO NManufacture of condensation polymers in a meltphase process often requires continuous removal of the condensation product from the reaction mass to drive the reaction to high conversion. This occurs when the polycondensation reaction is reversible. At high conversion, the high viscosity of the melt decreases the rate of transport of the condensation product to the liquid surface and thereby reduces the overall reaction rate. A primary reactor design problem is to generate large amounts of surface area within the polymerization reactor in order to reduce the effective distance over which the condensation product must diffuse. A second problem is to ensure adequate mixing between the bulk of the reaction mass and the surface. This design challenge is currently met using a variety of equipment configurations. Examples of commercial polycondensation reactor designs include twin-screw and multiplescrew extruders, twin-shaft paddle mixers, horizontal disc-ring reactors, and the more traditional (and less capable) stirred tank with an anchor agitator.

Modeling high‐viscosity, melt‐phase, condensation polymer reactors with a time‐scales analysis

“…These data can be used to calculate the flow rates between each pair of cells and the volumes of the cells. 6 The time scale of mixing is chosen to be the "renewal time" of a fluid element leaving cell 3, traveling to cell 2 and possibly to cell 1 (where evaporation of condensation product occurs) and then again back to cell 3. The choice for the definition of the time scale of mixing is somewhat arbitrary.…”