RETROFIT DESIGN OF NEW UNITS INTO AN EXISTING PLANT - CASE-STUDY - ADDING NEW UNITS TO AN AROMATICS PLANT

The design of a process plant generally follows recursive steps of syn thesis, analysis, and evolution. Retrofit design makes changes or addi tions to the plant in order to achieve expanded and/or more economical operation. A retrofit design algorithm which was developed in this st udy, uses a process synthesis approach with heuristic rules based on e ngineering experience, detailed process calculations and detailed econ omical evaluations leading to an optimal design. This approach was tes ted extensively and successfully on the flowsheet of an existing aroma tics plant. The intervention of experienced engineering judgment leads to improved and more profitable retrofit design or to the optimal cho ice between alternate downstream process additions. Cascading distilla tion towers and reactors, maximal utilization of processing unit capac ities, and the reuse of superseded equipment improves the economics of the design.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 e xpanding it gradually. Economical failure can result from poor guesses of future prices of feedstocks and products. It was shown for the pla nt studied in this paper by redesign under historical conditions that the added value of the chemicals in the good years made up for the poo rer performance in bad years