On-line optimization of a high-pressure low-density polyethylene tubular reactor

Low-density polyethylene (LDPE) is a high-volume commodity polymer that is produced in high-capacity tubular and autoclave reactors. The extreme operating conditions employed in the high-pressure free-radical polymerization of ethylene, and the tight profit margins of the LDPE market provide a strong economic incentive to seek the optimal operation of a high-pressure LDPE reactor. In the present work, an on-line multi-level optimization procedure is developed for a high-pressure tubular LDPE reactor based on a comprehensive steady-state mathematical model of the process. Following the well-established separation hypothesis, the problem is hierarchically decomposed in two phases: (i) the parameter estimation phase where certain key model parameters are adjusted, based on selected process measurements, in order to eliminate any process/model mismatch; (ii) the optimization phase where the adapted model is optimized by minimizing a specified objective function defined in terms of the ethylene conversion and the "quality" of the final polymer product. The proposed optimization algorithm is successfully tested against the operation of a large-scale industrial reactor. The most prominent features of the procedure are highlighted. These include the development of a comprehensive mathematical model of the process, the disturbance oriented selection of the adjustable model parameters, the appropriate spatial decomposition of the parameter estimation problem into a number of subproblems and the conditional structuring of the optimization problem.