Modeling and Optimization of Grade Changes for Polyethylene Reactors
Researchers: Per-Ola Larsson, Niklas Andersson (Chem. Eng), Johan Åkesson, Tore Hägglund, Bernt Nilsson (Chem. Eng)
In this project, which is a collaboration with Borealis AB, we are considering grade changes for a chain of three polyethylene reactors and a gas recycle system consisting of three distillation columns. In chemical industry today, the market is changing rapidly both in raw material pricing but also in product demand. To have the ability to perform fast, safe and economical grade changes and thus adapting to the market is highly advantageous.
One part of the project is to develop physical models of the three polyethylene reactors and recycle system at Borealis' Stenungsund site, see Figure 1. Using both well known physical laws and empirical relations, together with models derived for a non-linear MPC controller used at site today, a model library in the Modelica languange has been constructed. The library includes both reactor and distillation column models and framework models for optimization and validation.
The second part of this project is optimization of grade changes using the models in the library. In the optimization, which is non-convex, both limitations of the reactors and columns, but also economics, safety, time, and parameters defining the desired grade needs to be considered. Using JModelica.org, which is a framework for dynamic optimization of Modelica models, grade change optimization problems can be solved. JModelica.org incorporates Optimica, which extends the Modelica language with constructs to encode optimization problems.2009
During 2009, the core of the library has been built. A first optimization of a grade change for a single reactor has been performed successfully and submitted for publication. Model calibration using measurements has been initiated at the Chemical Engineering Department using a computer cluster.2010
The work performed during 2010 mainly includes three parts. First, optimization of grade change with all three reactors has been performed with success and submitted for publications. Secondly, modeling of the recycle part, i.e., the distillation columns, has begun with support from measurement data from Borealis AB. Grade change optimization with both reactors and recycle system is under ongoing work. Thirdly, static model calibration using measurement data has been performed successfully at the Chemical Engineering Department.2011
During 2011, optimizations with all three reactors and recycle system have been performed using both quadratic cost functions, but also, more importantly, cost functions that considers plant economy. The latter cost functions take into consideration the grade definition, sell price of the polymer as well as the cost of raw material. The results from the optimizations have shown that economically optimal grade transitions can be divided into three phases with distinguishing features.
In November, Per-Ola Larsson finished his Ph.D. thesis entitled "Optimization of Low-Level Controllers and High-Level Polymer Grade Changes", which includes the above results.
At the Department of Chemical Engineering, calibrations of the plant model have been performed using both stationary and dynamic measurement data.
During the year a graphical user interface for the model calibration and grade change optimizations has been developed in a M.Sc. thesis. The graphical user interface also includes an analysis of the economics of grade changes.Links
Per-Ola Larsson, Johan Åkesson, Niklas Andersson: "Economic Cost Function Design and Grade Change Optimization for a Gas Phase Polyethylene Reactor". In 50th IEEE Conference on Decision and Control and European Control Conference, Orlando, Florida, USA, December 2011.
Per-Ola Larsson: Optimization of Low-Level Controllers and High-Level Polymer Grade Changes. PhD Thesis ISRN LUTFD2/TFRT--1088--SE, Department of Automatic Control, Lund University, Sweden, November 2011.
Per-Ola Larsson, Johan Åkesson, Staffan Haugwitz, Niklas Andersson: "Modeling and Optimization of Grade Changes for Multistage Polyethylene Reactors". In 18th IFAC World Congress, Milano, Italy, August 2011.
Niklas Andersson, Per-Ola Larsson, Johan Åkesson, Staffan Haugwitz, Bernt Nilsson: "Calibration of a polyethylene plant for grade change optimisations". In 21st European Symposium on Computer Aided Process Engineering, Chalkidiki, Greece, June 2011.
Per-Ola Larsson, Niklas Andersson, Johan Åkesson, Staffan Haugwitz: "Modeling and Optimization of Grade Changes for a Polyethylene Reactor". In Reglermöte 2010, Lund, Sweden, August 2010.
Per-Ola Larsson, Johan Åkesson, Staffan Haugwitz, Niklas Andersson: "Modeling and Optimization of Grade Changes for Multistage Polyethylene Reactors". In 16th Nordic Process Control Workshop, Lund, Sweden, August 2010.
Per-Ola Larsson, Niklas Andersson, Johan Åkesson, Staffan Haugwitz: "Modelica Based Grade Change Optimization for a Polyethylene Reactor". In 9th International Symposium on Dynamics and Control of Process Systems, Leuven, Belgium, July 2010.