Multi-objective multi-period synthesis of energy efficient processes under variable environmental taxes

Multi-objective optimization yields a set of Pareto solutions. Although Pareto curves provide useful information, deeper insights are needed for identifying more advanced process solutions. This study presents a methodology for analysing Pareto process solutions considering variations in environmental tax rates. Three new combined economic-environmental indicators are defined that measure the relationship between loss of economic value when deviating from the economic optimum, and savings originating from environmental impact reduction at a given environmental tax rate. It is shown that investing in processes or technologies for the reduction of environmental footprints can reduce emissions without worsening economic performance, provided that investors renounce the savings arising from reduced emissions. A multi-period synthesis of a process flow sheet is performed by targeting the economic performance at a given tax rate to the optimal level without any environmental tax. In a case study of Heat Exchanger Network synthesis, emissions were reduced by 8.7% after implementing carbon emissions tax at 60 €/t CO2, in comparison with the optimal design obtained at zero carbon emissions tax. However, the net present values of both solutions remained the same. In this way, more environmentally friendly yet economically attractive process designs can be generated.