Optimal grade transitions in a gas phase polyethylene reactor



Using gas-phase technology many grades of polyethylene can be produced in a single reactor. For a series of three polyethylene products, model-based dynamic optimization is used to determine optimal grade changeover policies. Optimal manipulated variable profiles are determined for hydrogen and butene feed rates, reactor temperature setpoint, gas bleed flow, catalyst feed rate, and bed level setpoint. It is shown that large transitions in melt index are hampered by slow hydrogen dynamics, and that the time required for such a transition can be reduced by manipulating the temperature setpoint and the bleed stream flow. Reduction of the bed level and catalyst feed rates during changeovers can significantly reduce the quantity of off-specification polymer produced. It is demonstrated that melt index and density are not sufficient to characterize the properties of polymer produced during grade transitions, and that the shape of the cumulative copolymer composition distribution is very sensitive to the grade changeover policy used. Optimal transition policies should not be implemented without feedback control. Disturbances and model mismatch can result in product property trajectories which deviate significantly from the nominal optimal trajectory.