Environmental life cycle impact as a tool for process optimisation of a utility plant

The purpose of this paper is to minimize environmental life cycle impact when a detail process modelling is available. A methodology is presented to calculate the optimum operating conditions of an ethylene process utility plant. The overall environmental impact is calculated as a weighted sum of global warming, acidification, eutrophication, photochemical oxidation, ozone depletion, human toxicity and ecotoxicity. The battery limits of the plant are extended to include the relevant environmental impacts corresponding to the imported electricity generated in thermoelectric, hydroelectric and nuclear plants. A mixed integer non-linear programming problem is formulated and solved in GAMS. Significant reductions in environmental impact particularly in global warming, the most relevant category, are obtained choosing the pressure and temperature of high, medium and low pressure steam headers together with the selection of optional drivers and boilers. Improvements achieved simultaneously in natural gas and electricity consumption and operating cost are also reported.