Optimization of grade transition for high-density polyethylene continuous slurry process
A model of grade transition is presented for the industrial high-density polyethylene (HDPE) continuous slurry process, which has two reactors and one pelletizer connected in series.The object of the model is to minimize the quantity of off-specification product and the time of grade transition.By the optimization of the objective function, the trajectories of both the controlled variables and the quality target of polymer product can be obtain, such as polymerization temperature, hydrogen to ethylene ratio([H_2]/[C_2]), co-monomer butylene to ethylene ratio([C_4]/[C_2]), catalyst feed rates, the quality target of polymer product in the two reactors and the polymer’s melt index(MI) in the pelletizer.The result showed that the polymer grade was determined by the properties of both the polymer slurry from the reactors and the polymer pellet from the pelletizer.Therefore, the grade transition strategy must be optimized from a plantwide viewpoint.The factor of pelletizer was added into the objective function, which effectively restrained the sharp change of controlled variable and ensured the security of the grade transition process.