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Journal of The Institution of Engineers, Singapore Vol. 45 Issue 6 2005

SYNTHESIS AND DESIGN OF CHEMICAL PROCESSES
Lim Kuan Howe1, Dennis1, N.V.S.N. Murthy Konda1 and G.P Rangaiah1* ABSTRACT The goal of this work was to critically examine two hierarchical procedures used by Douglas [1] and Stanislav [2] for the conceptual design of chemical processes. Using the Hydrodealkylation of Toluene(HDA) process as a case study, it was found that the flowsheet developed using Stanislav’s procedure was more economical than the flowsheet developed using Douglas’ procedure. The main advantage of Stanislav’s procedure is that it is rich enough to allow all reasonable alternatives to surface, hence eliminating the possibility of letting the best flowsheet slip through the mind of the engineerduring the design process. However, the drawback is that the process is more time consuming as more options have to be enumerated. This disadvantage was circumvented in this study by the formulation of a HYSYS-EXCEL interface using Visual Basic programming, which essentially combines the simulation power of HYSYS with the spreadsheet capabilities of EXCEL, thereby greatly shortening the time needed toevaluate a particular flowsheet. INTRODUCTION Process design involves not only technical knowledge but also a good dose of creativity. With the increasing complexity of chemical processes and the invention of novel operations (e.g. reactive distillation, membrane separations), it is not surprising that numerous alternatives can be generated. The goal of the chemical engineer then, is to choose thealternative that brings the largest economic benefits to the company. However, even with the advent of powerful process simulation tools, it still remains a challenge to develop an entire process flowsheet from the drawing block. Hence arises the need for a systematic approach to plant design that would help the chemical engineer to arrive at the best design in a logical fashion. Douglas [1]suggested a hierarchy of decisions (Figure 1), which shall be referred to in this paper as the conventional design procedure. Using this procedure to develop the process flowsheet will help the designer to arrive at the final design using a stage-bystage method. At each stage, he generates new alternatives and evaluates those using selected economic criteria. He then chooses the best alternative andproceeds to the next stage where he will have to make another decision among the new alternatives that are generated. Finally, he will arrive at the last stage where the final choice will lead to the ‘best’ alternative. Stanislav [2] suggested a new hierarchy, also shown in Figure 1, which shall be referred to in this paper as the modified design procedure. There are two key modifications from
1Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 119260. Author for correspondence; Email: chegpr@nus.edu.sg. Fax: (65) 6779 1936; Phone: (65) 6874 2187

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Journal of The Institution of Engineers, Singapore Vol. 45 Issue 6 2005

the conventional procedure. Firstly, the reactor design is considered as a separate stage of the designprocedure rather than as part of the recycle structure. Secondly, the decision to recycle is left to the last stage of the hierarchy, just before the heat integration.

Figure 1: Conventional and Modified Design Procedures The objective of this study is therefore to present a comparison of the conventional and modified design procedures, the aim being to see which design hierarchy would help theengineer to arrive at the ‘best’ process flowsheet in the shortest time. The HDA process is used as the example in this study, which involves the extensive use of a process simulator as well as sizing, costing and profitability analysis. FLOWSHEET EVALUATION In order to apply either of the two procedures listed in Figure 1, it is necessary to evaluate the profitability of the different flowsheets...