Process simulation is a valuable tool to increase the profitability of polyester processes by reducing costs, increasing yield, and improving product quality. Although simulation has been heavily used in the chemical process industries for several decades, the polymer manufacturing industry has only begun to take advantage of this technology during the past five to ten years. Recently, the commercial development of polymer process simulation packages, such as Polymers Plus, has made it possible to simulate polymer processes in a simple and straightforward manner. In this discussion, we will examine the technical challenges that must be overcome to develop and validate a polyester process model. Several case studies will be presented. Finally, we will review how a rigorous process model can be leveraged to support additional business needs including operator training and process control. Presented at AIChE Spring 1999 Meeting, Houston Texas Session: Polyester Manufacturing Processes I Copyright © 1999 Page 2 of 19 Process Description Most polyethylene terephthalate (PET) is produced from purified terephthalic acid (TPA) and ethylene glycol (EG). Although many process configurations are found in the polyester industry, they all involve a series of three or more reactors. For demonstration purposes, we have developed a model of a typical five-reactor process, as shown in Figure 1. Solid terephthalic acid is mixed with ethylene glycol in carefully metered amounts. The mole ratio of ethylene glycol to TPA is a critical process variable, so many plants use control schemes to adjust the glycol feed rate to keep the paste density constant (the density is a good indicator of mole ratio). The resulting paste is fed to the first reactor, which is known as a primary esterification reactor or PE. Typically, the PE is operated at a pressure of 1-8 bar and a temperature of 255-280°C. At lower temperatures the reactor performance is limited by the solubility of TPA in the oligomer. The behavior of the reactor is highly non-ideal because the apparent reactor volume depends on the amount of solid TPA. At higher temperatures the reactor performance is limited by the solid-liquid mass-transfer rate. Under these conditions, the reactor performance depends on the TPA particle size. Oligomer from the primary esterifier is fed to the secondary esterifier (SE). The secondary esterifier is usually run close to atmospheric conditions with temperatures a bit higher than the PE. Frequently, the SE is divided into several chambers to enhance the effectiveness of the reactor. In our model, we assume the SE is divided into three equal-sized chambers. Each chamber is represented as an ideal CSTR reactor. These assumptions are justified by sensitivity studies that indicate that vapor back mixing between the stages has little influence on the model predictions.
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