(375m) Multiobjective Synthesis of Heat Exchanger Networks Minimizing the Cost and the Environmental Impact

The use of heat exchanger networks (HEN) has produced significant economics and environmental benefits for the industry. For the synthesis of HEN, sequential techniques based on the pinch point method are one of the most widely used, in this case the problem is decomposed in several easer subproblems through a set of heuristic rules; however, the optimal solution of the problem is not guaranty. On the other hand, the simultaneous techniques consider all components for the total annual cost of the HEN through a mathematical formulation yielding the optimal solution. In addition, most of the methods reported to solve the synthesis of HEN problem have been based on the selection of only one type of cold and one type of hot utility; however, in the industrial practice there are available several types of utilities with different temperatures and unitary costs. In addition, the papers reported at this moment have not considered the environment degradation and they have considered exclusively the minimization of the total annual cost. Recently have been reported methodologies to determine the overall environmental impact for a process or product through the life cycle analysis technique (LCA), producing an eco-indicator to quantify the overall environmental impact. Therefore, present paper presents a multiobjective optimization model for the synthesis of HEN considering the minimization of the total annual cost and the minimization of the environmental impact. The total annual cost considers the capital costs for the exchangers and the utility cost, considering the optimal selection of different types of utilities. The environmental impact is measured through the eco-indicator 99 for the different types of utilities. A new superstructure is implemented to model adequately the optimal location for the different types of utilities available, and the model consists in a multiobjectie mixed integer non linear programming problem. A solution strategy is proposed and it is applied to several examples without numerical complications. Results show that the solutions with the minimum costs usually correspond to the solutions with the maximum environmental impact, and vice versa, therefore there exist two objectives that contradict each other and the designer must select the solution that satisfies both objectives depending of the specific case.