Dynamic optimization of low-density polyethylene production in tubular reactor under thermal safety constraint

Abstract

A commercial low-density polyethylene (LDPE) which is produced by the polymerization process of ethylene in the presence of initiators in a long tubular reactor is the most widely used in polymer industry. The highly exothermic nature of the LDPE polymerization process and the heating-cooling prerequisite in the tubular reactor can lead to various problems, particularly safety in terms of thermal runaway and productivity, i.e., decreasing monomer conversion. Therefore, model-based optimization of an industrial LDPE tubular reactor under thermal safety consideration is required to be implemented. A first principle model for this process is developed and validated using industrial data. Mass and energy balances have been derived from kinetics of LDPE polymerization. Thereafter, an expression of reactor temperature under critical condition is developed and incorporated in the reference model for the thermal safety study. In order to ensure the process is thermally safe and meets the desired product grade, the constrained dynamic optimization is proposed to maximize the conversion of the monomer using orthogonal collocation (OC). The dynamic optimization result shows that the maximum reaction temperature under critical condition constraint can be satisfied by optimizing the reactor jacket. Moreover, it is achieved without jeopardizing the monomer conversion and the product grade.