Equation-Oriented Optimization on an Industrial High-Density Polyethylene Slurry Process with Target Molecular Weight Distribution

Abstract

High-density polyethylene (HDPE) is widely used as the main material in the film, pipe, and container industries. To meet the different market requirements, a process is expected to be capable of producing a number of grades of HDPE products using the same equipment. Given that molecular weight distribution (MWD) is crucial to the product quality, the process operating conditions with different specifications on MWD is highly significant. The current study develops an approach for the optimization of an industrial HDPE slurry process with target MWD. To exploit fully the advantages of nonlinear programming (NLP) algorithms, a complete equation-oriented (EO) model including rigorous kinetic mechanism, thermodynamics, and MWD calculation is first established. Model validation is conducted with real plant data to show the good accuracy of calculated MWD. The good performance of the EO model in convergence is also demonstrated through simulations. On the basis of the EO model, productivity optimization with a specified MWD as a constraint is proposed and solved by using an efficient simultaneous approach. Case studies of bimodal and unimodal MWD curves for the process with two reactors in a series are presented. Further study shows that the optimization also works for a different process configuration with reactors in parallel. This systematic method shows good potential to optimize the productivity for different product quality demands.

A multi-thread parallel computation method for dynamic simulation of molecular weight distribution of multisite polymerization

“…We presented two cases with different model details to elucidate the acceleration effects. The first case (Soares and McKenna, 2012) includes only the kinetic model with six types of active sites; the second case (Zhang et al, 2013) exhibits a complicated process structure. The thermodynamic and kinetic models were included; and five active sites exist.…”

“…Both the kinetic and thermodynamic models were included; five active sites existed (N S = 5). The description of the model was previously reported (Zhang et al, 2013).…”

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].…”

“…By the optimization of multistage formulation, the grade transition time is further shortened to 9.18 h, the reduction of which reaches to 25% on the basis of step change. Moreover, the multistage formulation reduces the amount of off-spec production from 16,474.75 kg to 12,599.00 kg, by about 24%. Figure 12 presents the MWD results immediately after the transition to Grade B completes.…”

“…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. Zhang et al 16,17 established an equation-oriented (EO) model for a high-density polyethylene slurry process with target MWDs. Using this EO model as a basis, a fully connected process superstructure was generalized, with which the optimal reactor network configurations with specifications on MWDs were determined.…”