(598a) Operability Analysis for Grade Transitions in the High-Density Polyethylene Process

    According to customer’s various and complicated demands, polymer plants have to produce many polymer grades with frequent grade transition operations. Numerous studies have demonstrated that polymerization processes always exhibit highly nonlinear dynamic behaviors, including input/output multiplicities, limit cycles, sustained oscillations, hysteresis, and chaos. These nonlinearities may adversely affect process operation and process controller performance and may lead to unsafe operations^1,2. The grade transitions operation, resulting in changes of operating/design parameters or topology structure (e.g. parallel mode and series mode), will alter and complicate the overall nonlinearity and internal dynamic behavior of the polymerization process. Therefore, it is of critical importance to assess the effects of the nonlinear dynamics of the plant-wide process and the operability (e.g. stability and controllability) of feasible operating regions and different topology structures for producing required polymer grades.

    The major objective of this paper is to analyze the operability for grade transitions in a high-density polyethylene process (HDPE) from a network perspective. The framework contains two parts. First, a plant-wide model of an HDPE process is established for steady-state simulation. Second, based on open-loop stability/instability and minimum/nonminimumphase behavior, the operability of the process for producing different polymer grades can be efficiently probed. An attempt is made with the help of this article to illustrate how to clarify the roots of the poor operability that arise in the grade transition to provide helpful guideline for design of the HDPE processes operation parameters and the control system.

References

1. Yuan ZH, Chen BZ, Zhao JS. Phase Behavior Analysis for Industrial Polymerization Reactors. AIChE J. 2011; 57: 2795-2087.
2. Seider WD, Brengel DD, Widagdo S. Nonlinear dynamics found in polymerization processes — a review. Chem. Eng. Sci. 2000; 55:275-290.