Model-based optimization of economical grade changes for the Borealis Borstar® polyethylene plant

Economical grade changes are considered for a Borealis Borstar® polyethylene plant model, incorporating two slurry-phase reactors, one gas-phase reactor and a recycle area with three distillation columns. The model is constructed in the Modelica language and the JModelica.org platform is used for optimization. The cost function expresses the economical profit during a grade change and is formulated using on-grade intervals for seven polymer quality variables such as melt index, density and reactor split factors. Additionally, incentives to produce polymer with quality variables on grade target values, not only inside grade intervals, are added. Twelve inflows and three purge flows are used as decision variables. Two grade changes are thoroughly reviewed, showing the effect of using a cost function that regards plant economy. Resulting trajectories can be divided into three phases with distinguishing features, and the synchronization of inflows and usage of recycle area off-gas flows are important in the grade changes.

► Grade change optimization for a polyethylene plant is performed.
► Modeling is performed in the Modelica language.
► Cost function regards plant economy.
► Optimal transitions may be divided into three distinguishable phases.