(397d) Synthesis of Recycle/Reuse Networks Based On Properties Considering Simultaneously Economic and Environmental Concerns

The proper use of the resources is one of the major concerns of the chemical and process industry. Resources in the form of mass and energy constitute the main raw materials for the industry that impact the processes from both economic and environmental considerations. Based on the well-known ideas for energy integration, several strategies have been recently proposed for the reuse of the wastewater streams to minimize fresh streams usage and wastewater discharged to the environment. Particularly, the synthesis of recycle and reuse mass integration networks constitutes a powerful tool that enables the simultaneous minimization of fresh water usage and wastewater discharge through the direct recycle of wastewater streams and/or their treatment and reuse in process equipments. Several techniques have been reported for the synthesis of mass integration networks. Most of them have been based on the composition of streams; however, many recycle streams are constituted by several components and are difficult to characterize based on composition. To address this problem, Ponce-Ortega et al. (2009) have recently proposed a model formulation for the optimization of direct recycle networks together with wastewater treatment processes to satisfy a given set of environmental regulations. This formulation incorporated constraints based on stream properties (e. g. pH, toxicity, chemical oxygen demand, color and odor) in addition to the composition constraints. The properties balances considered by Ponce-Ortega et al. (2009) are based on the concept developed by El-Halwagi et al. (2004) for the property integration defined as a functionality-based holistic approach for the allocation and manipulation of streams and processing units. Ponce-Ortega et al. (2010) extended their previous model to include a property interception network within the process considering both process and environmental constraints. However, even when those formulations incorporate environmental constraints, they are based on simple objective functions such as minimizing the consumption of fresh sources. If one performed a more global analysis, it could be assessed whether this type of solution yielded an increase on the environmental burdens somewhere else in the life cycle. For example, the treatment units required for the process sources may increase the pollution to the environment, or the use of one type of fresh source may reduce the pollution in the plant, but the pollution caused during the production of this fresh source may be higher than the one avoided in the plant. Therefore, life cycle assessment (LCA) provides a more suitable approach to evaluate the overall environmental loads associated with a process, product or activity that identifies and quantifies the materials and energy used as well as the wastes released to the environment. This paper presents a multiobjective optimization model for the synthesis of property-based recycle and reuse mass exchanger networks considering simultaneously economic and environmental impacts. The economic objective function considers the fresh sources costs, the treatment costs and the piping costs, whereas the environmental objective function is measured through the eco-indicator 99, which is based on the life cycle analysis methodology. The problem gives rise to a multiobjective mixed integer non linear programming model (moMINLP). A superstructure that yields a convex problem is proposed to guaranty a global optimal solution. Several examples are solved to show the applicability of the proposed model.

References: El-Halwagi, M. M., Glasgow, I. M., Qin, X. Y., & Eden, M. R. (2004). Property integration: Componentless design techniques and visualization tools. AIChE Journal, 50 (8), 1854-1869. Ponce-Ortega, J. M., Hortua, A. C., El-Halwagi, M. M., Jiménez-Gutiérrez, A. (2009). A property-based optimization of direct recycle networks and wastewater treatment processes. AIChE Journal. 55 (9): 2329-2344. Ponce-Ortega, J. M., El-Halwagi. M. M., Jiménez-Gutiérrez, A. (2009). Global optimization of property-based recycle and reuse networks including environmental constraints. In press in Computers and Chemical Engineering.