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

Abstract

To achieve different end-use properties of polymers, an industrial plant must produce several grades of the product through the same process under different operating conditions. As molecular weight distribution (MWD) is a crucial quality index of polymers, grade transition based on MWD is of great importance. Dynamic optimization of the grade transition process using MWD is a challenging task because of its large-scale nature. After analyzing the relationships among state variables during polymerization, a novel method is proposed to conduct the optimal grade transition using dynamic optimization with a small-scale moment model, combined with a steady-state calculation of the MWD. By avoiding expensive computation in dealing with dynamic MWD optimization, this technique greatly reduces the computational complexity of the process optimization. The theoretical equivalence of this simplification is also proved. Finally, an industrial high-density polyethylene slurry process is presented to demonstrate the efficiency and accuracy of the proposed strategy.

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].…”

“…In order to address this type of problem in a systematic manner, continuing studies are motivated on developing adequate dynamic models and involving dynamic optimization for the polymer grade transition process. [3][4][5][6][7][8][9][10][11][12] A nonlinear control system that integrates an off-line optimizer and a nonlinear MPC controller is developed by Wang et al 4 to perform optimal grade transition operations at the industrial polyolefin reactors. Cervantes et al 6 presented an optimal control policy for an industrial low-density polyethylene plant based on average molecular weight using a simultaneous dynamic optimization strategy.…”

“…Moreover, the cumulative method when PSSA holds in short time intervals is still time consuming. Though some recent novel dynamic MWD modeling strategies 11,21,22 have been developed, based on the Flory method to improve the computational efficiency, they cannot remove the limitations of this method. Another challenge is that the dynamic optimization problem involving the time evolution of MWD is difficult to solve due to its large scale, high nonlinearity, and strong coupling features, especially for the complex grade transition process.…”

A multi‐scenario nonlinear model predictive control approach for robust product transitions

“…Product or grade transition problems are still important and hard to solve tasks in the processing industry . The subject of product transitions has been widely studied in the research literature owing to its industrial relevance . In grade transition problems, starting from a given operating point, we are interested in reaching a final or target process condition in the best possible way holding a set of process constraints.…”

“…[1] The subject of product transitions has been widely studied in the research literature owing to its industrial relevance. [2][3][4][5][6][7][8][9] In grade transition problems, starting from a given operating point, we are interested in reaching a final or target process condition in the best possible way holding a set of process constraints. The starting point can be a steady-state (for continuous production) or an initial processing condition (for batch/semibatch facilities).…”