Optimal process operation for the production of linear polyethylene resins with tailored molecular weight distribution

Abstract

An optimization model is presented to determine optimal operating policies for tailoring high density polyethylene in a continuous polymerization process. Shaping the whole molecular weight distribution (MWD) by adopting an appropriate choice of operating conditions is of great interest when designing new polymers or when improving quality. The continuous tubular and stirred tank reactors are modeled in steady state by a set of differential-algebraic equations with the spatial coordinate as independent variable. A novel formulation of the optimization problem is introduced. It comprises a multi-stage optimization model with differential-algebraic equality constraints along the process path and inequality end-point constraints on product quality. The resulting optimal control problem is solved at high computational efficiency by means of a shooting method. The results show the efficiency of the proposed approach and the benefit of predicting and controlling the complete MWD as well as the interplay between operating conditions and polymer properties.

A novel strategy for dynamic optimization of grade transition processes based on molecular weight distribution

“…Al‐haj‐Ali et al addressed the issue of MWD in a single reactor during gas‐phase ethylene polymerization and concluded that a broad MWD could be achieved by proper control of the hydrogen to monomer ratio inside the reactor. Pontes et al presented an optimization model for determining the optimal operating policies for tailoring polyethylene with desired MWDs in a continuous ethylene polymerization process; this model used a Ziegler–Natta catalyst in a complex configuration of continuous stirred tank reactors (CSTRs) and plug flow reactors. However, many studies that deal with target MWDs only minimize the deviation of the calculated MWDs from the specified profile but do not care about the computational efficiency.…”

Dynamic optimization for grade transition processes using orthogonal collocation on molecular weight distribution

“…1, 4-7, and 22-29), population balance equations of all the other species (Eqs. [10][11][12][13][14][15][16][17][18][19], and dynamic MWD calculation (Eqs. [30][31][32].…”

“…A few papers focus on determining optimal operating policies of polymerization processes from target MWDs. Pontes et al 14 developed a steady-state multistage optimization model of continuous stirred tank and plug flow reactors with the spatial coordinate as the independent variable; optimal operating conditions were determined for tailoring polyethylene with desired MWDs. Asteasuain et al 15 implemented a steady-state model of the high-pressure ethylene polymerization in a tubular reactor using gPROMS, which incorporated the prediction of the complete MWD and branching indices as a function of the reactor axial distance.…”

Process analysis and optimization mapping through design of experiments and its application to a polymerization process

“…The present paper, however, presents a comprehensive approach that subdivides the optimization problem into two levels: objective function mapping through DOE followed by a rigorous optimization (Pontes et al, 2008(Pontes et al, , 2010. The paper shows that the DOE can be successfully used prior to a rigorous optimization not only to map the feasible optimal region, but also to ascertain more precisely the degrees of freedom of the problem, reducing its complexity.…”