MODEL-PREDICTIVE CONTROL OF A SLURRY POLYMERIZATION REACTOR

This paper addresses the problem of melt index (MI) control in the pol ymerisation of propylene in slurry reactors. Good MI control performan ce is usually hampered by measurement delays and a low sampling freque ncy. The traditional solution to the control problem is to control the hydrogen concentration in the reactor (H-2 serves as the chain transf er agent in the reaction and therefore has a good correlation with the MI). The H-2 concentration in the gas cap of the reactor is relativel y easy to measure and therefore chosen to close a SISO control loop. H owever, this concentration is not always a good measure for the H-2 co ncentration in the slurry phase where the reactions occur. Consequentl y, MI variations can still be large at good control of H-2 in the gas cap. In the present paper, the use of a soft sensor for the H-2 concen tration in the slurry phase is investigated by means of simulation. Wh en used in combination with a model predictive control method, it is s hown that plant performance significantly improves both at disturbance s and at grade changes.