Retrofit of complex and energy intensive processes II: stepwise simultaneous superstructural approach

Simultaneous parameter and structural optimization of an existing complex and energy intensive continuous process have been studied using rigorous models. The method that was recently developed to sequentially optimize retrofits has been extended to a stepwise simultaneous superstructural approach, using available process simulators and optimization software capabilities. An extended procedure has been employed for retrofits using a three-step approach: (i) generation of a process superstructure by pinch analysis; (ii) formulation of a mixed integer nonlinear programming (MINLP) model and its simplification into a relaxed nonlinear programming (NLP) model; (iii) simultaneous optimization, first by a process simulator and than by the NLP algorithm. Pinch analysis offers alternative retrofit designs for postulating the superstructure. An advantage of the relaxed NLP formulation over the MINLP one is that it can be easily generated. Poor local optima due to nonlinear interconnections between alternative solutions have been avoided by applying a direct search complex algorithm using the simulator. The approach has been illustrated by maximizing the annual profit of an existing methanol plant. The optimal solution yields additional savings of 5.17 MUSD per annum (6.75% of the total income) which is 6.5 times the savings obtained by the earlier sequential optimization method. © 2000 Elsevier Science Ltd. All rights reserved.