Calculation of emergy flows within complex chemical production systems

Calculating emergy flows within a typical chemical production system is challenging because it is made up of various interconnected reaction, separation, heating and cooling units with extensive recycle of material and energy. The complexity of the interior structure of a chemical production system, if not carefully considered, could pose problems in emergy calculation. The difficulty is further compounded by the fact that the rules of emergy algebra are somewhat opaque and incomplete. This article shows that complex chemical production systems may be decomposed into four basic structures: series, parallel, bypass and feedback, which can greatly facilitate the process of emergy calculation. Systematic procedures for calculating the emergy flows of the four basic structures without violating the basic rules of emergy algebra are given in detail. Special attention has been paid to the feedback structure, which is analyzed using a novel concept known as virtual emergy flow. An illustrative example based on a hypothetical system made up of the four basic structures and a case study based on an actual polypropylene production system are used to demonstrate the efficacy of the emergy calculation procedures presented in this study.

► Different basic structures of chemical system were analyzed by improved emergy algebra. ► Set up the virtual emergy flow to solve the existing problem in feedback of chemical system. ► Emergy algebra was firstly applied to the complex chemical production system.