Optimal Grade Transitions for a Propylene Polymerization Loop Reactor

Polyolefin plants operate under continuous production, but they need to supply many types of polymer grades (polymer with different specification, used in different applications). The operating conditions are maintained constant during the production of a certain grade and need to change periodically to produce another grade. Grade transitions allow the production of different polymer grades in a single reactor, but as they require changes in the operating conditions, there is a large production of off-specification polymer. Therefore, a desired transition drives the polymer properties to the new grade in a short period of time, producing a small amount of off-specification polymer. Efficient strategies to operate the transition can be obtained by solving a problem of dynamic optimization. In this work, a quadratic integral objective function was minimized by using dynamic sequential techniques for solving optimization. The results were optimized for the copolymerization of propylene with ethylene. The results show that to increase the MI was necessary to reduce the concentration of hydrogen in the feed and increase the comonomer flow rate; and to decrease the density was necessary to increase the comonomer flow rate. It was observed that the instantaneous properties have faster and aggressive dynamics compared to the cumulative properties. The trajectories of the transition properties towards grade 1 - grade 2 showed different behavior compared to grade 2 – grade 1 transition, concluding that the profile of a transition depends strongly on the direction of change in the properties.