Computers and Chemical Engineering 23 (1999) 479 – 495 A case study for reactor network synthesis: the vinyl chloride process A. Lakshmanan, W.C. Rooney, L.T. Biegler * ,1 Chemical Engineering Department, Chemical Mellon Uniersity, Pittsburgh, PA 15213, USA Received 11 February 1998; received in revised form 3 July 1998 Abstract A key objective of the integrated reactor network synthesis approach is the development of waste minimizing process flowsheets (Lakshmanan & Biegler, 1995). With increasing environmental concerns in process design, there is a particularly strong need to maximize conversion to product and avoid generation of wasteful byproducts within the reactor network. This also avoids expensive treatment and separation costs downstream in the process. In this study, we present an application of the mixed integer nonlinear programming (MINLP)-based reactor network synthesis strategy developed by Lakshmanan and Biegler (1996a). Here we focus on applying these reactor network synthesis concepts to the vinyl chloride monomer production process. Vinyl chloride is currently produced by a balanced production process from ethylene, chlorine and oxygen with three separate reaction sections: oxychlorination of ethylene; direct chlorination of ethylene; and pyrolysis of ethylene dichloride. The hydrogen chloride produced in the pyrolysis reactor is used completely in the oxychlorination reactor. Byproducts such as chlorinated hydrocarbons and carbon oxides are generated by these reaction sections. These are studied using reaction kinetic models for the three reaction sections. The case study results in optimal reactor networks that improve the conversion of ethylene to vinyl chloride and minimize the formation of byproducts. These results are used to generate an improved flowsheet for the production of vinyl chloride monomer. Moreover, an overall profit maximization, that includes the effect of heat integration, is presented and a set of recommendations that improve the selectivity of vinyl chloride production are outlined. Finally, the optimal reactor structures, overall conversion and annual profit are shown to be only mildly sensitive with respect to small changes in the kinetic parameters. © 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction In most chemical processes the selective conversion of raw materials in the reactor determines the composition and the amount of waste products produced, and the reactor network has a significant influence on the recycle structure and downstream processing steps. Hence, re- action systems and reactor design often determine the character of the flowsheet and better performance of the reactor system improves the performance of the entire process. In the past decade, several researchers have worked on problems of reactor network synthesis and reactor design. This problem involves nonlinear reaction models, uncertain rate laws and numerous possible reactor types and networks. Hence, researchers in this area have achieved limited success. Motivated by these thoughts Lakshmanan and Biegler (1996a) proposed a reactor network synthesis strategy that combines princi- ples developed for MINLP optimization of prepostu- lated reactor superstructures and geometric techniques for establishing the reactor network. In particular, geometric techniques construct an at- tainable region, which represents the limits that can be achieved by the processes of reaction and mixing. This region includes the performance of any reactor network structures in terms of conversion of raw materials to desired products and the amount of waste products that are produced. Glasser, Crowe and Hildebrandt (1987) and Hildebrandt, Glasser and Crowe (1990) proposed constructive techniques to map the attainable region for reactor network synthesis problems in two or three dimensions. These techniques are useful to develop new reactor structures and to understand the reactor net- work synthesis problem, but it is difficult to map these regions in more than three dimensions by using the geometric approach. On the other hand, the optimiza- tion based approach of Lakshmanan and Biegler * Corresponding author. 1 Present address: Aspen Technology, Cambridge, MA02141, USA. 0098-1354/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 9 8 - 1 3 5 4 ( 9 8 ) 0 0 2 8 7 - 7