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

Process design requires the simultaneous satisfaction of environmental, economic and social goals. This invariably requires some trade off between these objectives. The challenge for process design engineers is to develop synthesis and analysis tools, which support this requirement. Process System Engineering (PSE) techniques for multiple objective optimisation have to date focused typically on the optimisation of cost versus the potential for waste minimisation, with the recent inclusion of operability issues. The incorporation of environmental sensitivity into PSE approaches has been less than satisfactory. Much of this stems from the (seeming) difficulty in translating process information to environmental objectives. It is our argument that life cycle assessment (LCA), a methodology for quantifying the full 'cradle-to-grave' impact of industrial processes, can be used to assist in developing environmental objectives for process design and analysis. In this paper, we restrict our analysis to the multiple objective optimisation of environmental and economic objectives. Our approach is demonstrated for the case study of a nitric acid plant, modeled using Hysys(R). The general approach entails the transfer of mass and energy information from the Hysys(R) model 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 objectives with the LCA-based environmental objectives. The optimiser routine uses goal programming to identify the Pareto surface of non inferior solutions for this situation, thereby making explicit what trade-off between economic and environmental objectives results from any preferred operating condition. (C) 2000 Elsevier Science Ltd. All rights reserved.