Retrofit design of new units into an existing plant: Case study: Adding new units to an aromatics plant

Abstract

The design of a process plant generally follows recursive steps of synthesis, analysis, and evolution. Retrofit design makes changes or additions to the plant in order to achieve expanded and/or more economial operation. A retrofit design algorithm which was developed in this study, uses a process synthesis approach with heuristic rules based on engineering experience, detailed process calculations and detailed economical evaluations leading to an optimal design. This approach was tested extensively and successfully on the flowsheet of an existing aromatics plant. The intervention of experienced engineering judgement leads to improved and more profitable retrofit design or to the optimal choice between alternate downstrem process additions. Cascading distillation towers and reactors, maximal utilization of processing unit capacities, and the reuse of supersede equipment improves the economics of the design. A grassroot design of the whole plant will in most cases be more economical than constructing an initial small plant and then expanding it gradually. Economical failure can result from poor guesses of future prices of feedstocks and products. It was show for the plant studied in this paper by redesign under historical conditions that the added value of the chemicals in the good years made up for the poorer performance in bad years.