Process synthesis and optimisation tools for environmental design: methodology and structure

Process design requires the simultaneous satisfaction of environmental, eco nomic and social goals. This invariably requires some trade off between the se objectives. The challenge for process design engineers is to develop syn thesis and analysis tools, which support this requirement. Process System E ngineering (PSE) techniques for multiple objective optimisation have to dat e focused typically on the optimisation of cost versus the potential for wa ste minimisation, with the recent inclusion of operability issues. The inco rporation of environmental sensitivity into PSE approaches has been less th an satisfactory. Much of this stems from the (seeming) difficulty in transl ating process information to environmental objectives. It is our argument t hat life cycle assessment (LCA), a methodology for quantifying the full 'cr adle-to-grave' impact of industrial processes, can be used to assist in dev eloping environmental objectives for process design and analysis. In this p aper, we restrict our analysis to the multiple objective optimisation of en vironmental and economic objectives. Our approach is demonstrated for the c ase study of a nitric acid plant, modeled using Hysys(R). The general appro ach entails the transfer of mass and energy information from the Hysys(R) m odel to the optimisation algorithm. Environmental objectives, based on the Hysys(R) model, are formulated first using a life cycle assessment toolbox. The multi-objective formulation of the process combines economic objective s with the LCA-based environmental objectives. The optimiser routine uses g oal programming to identify the Pareto surface of non inferior solutions fo r this situation, thereby making explicit what trade-off between economic a nd environmental objectives results from any preferred operating condition. (C) 2000 Elsevier Science Ltd. All rights reserved.