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Publication Title | Two-Phase Model for Continuous Final-Stage Melt Polycondensation of Poly(ethylene terephthalate). III. Modeling of Multiple Reactors with Multiple Reaction Zones

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Two-Phase Model for Continuous Final-Stage Melt Polycondensation of Poly(ethylene terephthalate). III. Modeling of Multiple Reactors with Multiple Reaction Zones

In Sun Kim,1* Boo Gon Woo,1† Kyu Yong Choi,1 Chang Kiang2

1Department of Chemical Engineering, University of Maryland, College Park, Maryland 20742 2Rhodia-ster Fibras e Resinas Ltda., Alameda Poliester, 1000, PoC ̧ os De Caldas, MG 37701-970, Brazil

Received 29 October 2002; accepted 22 January 2003

ABSTRACT: A steady-state two-phase model has been developed for a continuous finishing stage of the melt poly- condensation process that consists of two rotating-disk re- actors in series. Each reactor has multiple reaction zones with different types of rotating disks to establish plug flow profiles and to facilitate the removal of volatile reaction byproducts. The effect on reactor performance of varying the mass transfer parameter was found to be small for the reaction conditions used. The simulation results show that the use of two reactors offers increased flexibility in reactor

INTRODUCTION

Poly(ethylene terephthalate) (PET) is manufactured commercially by melt polycondensation polymeriza- tion processes using dimethyl terephthalate or tereph- thalic acid as a starting monomer with ethylene glycol. With growing competition with other thermoplastic polymers for many new and existing applications, improving PET quality and the efficiency of PET po- lymerization technology is a very important issue to PET manufacturers.

A finishing-stage PET reactor is typically a large horizontal cylindrical reactor equipped with such in- ternals as cages, screws, or disks to generate large vapor–liquid interfacial areas for mass transfer and to provide a certain desired fluid flow pattern. Wiped film reactors also are used to make PET polymer. The prepolymer feed to the finishing reactor is generally a low-molecular-weight polymer produced in the

Correspondence to: K. Y. Choi (choi@eng.umd.edu).

*On leave from Dongyang Technical College, Seoul, Ko- rea.

†Current address: LG Chem Research Park, LG Chemical Ltd., 104-1 Moonji-dong, Yusong-gu, Taejon 305-380, Korea.

Contract grant sponsor: Rhodia-ster, Sao Paulo, Brazil.

Contract grant sponsor (to In Sun Kim): Dongyang Tech- nical College, Seoul, Korea.

Journal of Applied Polymer Science, Vol. 90, 1088–1095 (2003) © 2003 Wiley Periodicals, Inc.

operations to obtain the desired polymer properties. Al- though the proposed model has not been fully validated with experimental or plant data, it has illustrated that the complex multizone reactor system can be easily modeled by the two-phase modeling technique and that added physical insights can be made through numerical model simulations. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1088–1095, 2003

Key words: polymerization; poly(ethylene terephthalate); reactor modeling

upstream transesterification and prepolymerization stages using stirred tank-type reactors. The finishing reactors are designed and operated for a highly vis- cous polymer melt to flow without forming any stag- nant zones. A high-molecular-weight polymer should also be obtained in a short reaction time, with minimal production of side products such as diethylene glycol and acetaldehyde. In a rotating-disk reactor a fraction of the polymer melt is dragged upward as the shaft rotates, forming a thin layer of polymer melt on the disk surface. Ethylene glycol, a major condensation byproduct, is mostly removed by diffusion from the polymer layers on the disk surface to the vapor phase. The polymer layer, after being exposed to a vapor phase for a short time, is mixed again with the bulk polymer melt.

As polymer melt flows toward the outlet of the reactor, the polymer molecular weight increases and hence does the melt viscosity. In the relatively low- viscosity zones near the inlet of the reactor, maintain- ing the plug flow profile is important for minimizing the back-mixing that lowers the conversion of func- tional end groups and hence the polymer molecular weight. On the other hand, in the high-viscosity zones near the outlet of the reactor, minimizing flow resis- tance becomes more important. Therefore, in many industrial PET processes, different types of disks are used in a reactor. For example, some disks may have holes of different shapes and sizes, and most designs of such disks are proprietary. It is also not uncommon

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