Investigating practical aspects of the exergy based multi-objective optimization of chemical processes

摘要

When optimizing chemical processes, conflicting objectives are often present (Logist et al, 2011). In industry, the trend towards sustainable development highlights this challenge because it encompasses three pillars: economy, society, and environment. A common example is the search for higher profitability while improving the safety of operation and reducing the energy consumption. In this framework, the exergy concept is highly relevant since it is understood as the confluence of energy, environment, and sustainable development (Rosen and Dincer, 2001). This contribution presents a novel approach to exploit the synergies of the exergy analysis and the gradient-based multi-objective optimization (MOO) of processes. An interface with Aspen Plus has been developed to exploit the added value of this approach, aiming to minimize the lost work in the context of conflicting objectives. The interface links the process simulator to Matlab, which is used as platform to run CasADi, a symbolic framework for algorithmic differentiation and numerical optimization (Andersson, 2013). The traditional butyl acetate production process is chosen as case study. It represents a common set-up of the chemical industry, where the conditions in the separation section are critical regarding quality but also yield and process intensity. Tri-objective constrained problems were formulated to address sustainability goals. The traditional approach based on energy consumption is compared to the case considering lost work minimization. The developed approach reveals how the minimum energy and lost work result from the partial increase in the duty for one of the distillation columns. This reflects the enhanced process understanding obtained, for which the exergy based optimization proves to be more informative, with slightly better results, and highly favorable trade-offs between objectives.