Optimal operating policies for tailored linear polyethylene resins production

In this work, optimal operating policies for the ethylene polymerization in solution with a Ziegler-Natta catalyst in a series of tubular and stirred tank reactors are proposed. The polymer is specified through properties such as melt index, stress exponent and density. Usually such properties are predicted by means of a process model once the operating conditions are specified. However, the computation of appropriate operating conditions to match desired resins properties is a much more difficult and not yet industrially established task. This problem can be solved through optimization techniques, an efficient alternative to costly pilot plant or production scale tests. A formulation is proposed, where a stationary model of the flowsheet (DAE system) is cast into a multi-stage model with the spatial flow path coordinate as the independent variable. Discontinuities occur at the stage transitions because of model switches and reactants injection. Several studies, involving different polyethylene resin specifications, are carried out to present the high potential and versatility of the suggested procedure. © 2008 American Institute of Chemical Engineers AIChE J, 2008