Computer modelling to improve polymerisation processes
The first phase in the production of commercially important polymers such as polypropylene from monomers is polymerization from the gas phase. During polymerization the monomers are joined together to form a long polymer chain of specific geometry using the Ziegler-Natta catalyst. In this process the monomer fragment is complexed at a metal centre. The complex binds to the polymer chain, which grows out of the metal centre and leaves a vacant site for further monomer complexation. The most widely established industrial gas phase technology is the fluidised bed reactor (FBR). Here the bed is kept in a bubbling regime by recycling reaction gases through the reactor. The heat of reaction is removed by cooling the circulation gas and sometimes by partial condensation. The reactor is operated either in transient or steady state mode. Optimal operating conditions of the fluidized bed reactor unit were studied in a model-based study. A generic mechanistic module for the Ziegler Natta gas-phase copolymerization in the reactor was developed to facilitate this. Optimal operating points for the formation of four different polymer grades were found by maximising the efficiency of monomer to polymer conversion. The time taken for polymer grade transition, and the consequent off-spec production was also explored and an optimal control structure formed. Further optimisation studies were performed for transient operation of the FBR The optimal operating conditions during the transition between four polymer grades was studied, and the best sequence of transition determined. Faster production, improved product quality, cost reduction and improved safety are all benefits from more efficient polymerisation as a result of modelling.
