Thermodynamics-Based Process Sustainability Evaluation

Abstract

This article considers the problem of the evaluation of the sustainability of heterogeneous process systems, which can have different areas of focus: from single process operations to complete supply chains. The proposed method defines exergy-based concepts to evaluate the assets, liabilities, and the exergy footprint of the analysed process systems, ensuring that they are suitable for Life Cycle Assessment. The proposed concepts, evaluation framework and cumulative Exergy Composite Curves allow the quantitative assessment of process systems, including alternative solutions. The provided case studies clearly illustrate the applicability of the method and the close quantitative relationship between the exergy profit and the potential sustainability contribution of the proposed solutions. The first case study demonstrates how the method is applied to the separation and reuse of an acetic-acid-containing waste stream. It is shown that the current process is not sustainable and needs substantial external exergy input and deeper analysis. The second case study concerns Municipal Solid Waste treatment and shows the potential value and sustainability benefit that can be achieved by the extraction of useful chemicals and waste-to-energy conversion. The proposed exergy footprint accounting framework clearly demonstrates the potential to be applied to sustainability assessment and process improvement while simultaneously tracking different kinds of resources and impacts.

Exergy Footprint Assessment of Cotton Textile Recycling to Polyethylene

“…However, in many cases, the assignment of the weights or the scaling factors for calculating such an indicator is subjective. Instead of assigning arbitrary weights and factors, an exergy-based indicator can be used, as suggested by Varbanov et al [21]. In that work, exergy is used as a unified criterion based on the comparable workflow assumption that the waste outputs of a process system are brought to a specified neutral state, exploiting the patterns of material flow circularity and energy cascading through terrestrial systems.…”

“…The recent overhaul of the Exergy Footprint concept by Varbanov et al [21] has provided an accounting tool to evaluate both the thermodynamic feasibility of the process systems and their approximate sustainability contribution. It can be especially useful to apply Exergy Footprint as the link between multiple footprints.…”

“…(d) CE research is focused on maximising the process circularity, struggling to obtain measures for process improvement or how much circularity to apply. (e) Exergy profit and Exergy Footprint concepts have been formulated [21], building upon the previous Exergy Footprint ideas [33].…”

“…Exergy accounting is taken as the framework in the current method. The thorough reasoning for using the Exergy Footprint and exergy profit has been provided by Varbanov et al [21]. However, it can be summarised as follows:…”

“…Figure 2b illustrates the method followed in this study. Exergy assessment of the cycle is performed considering the physical exergy calculated following [21], and the chemical exergy is calculated using an online tool called Chemical Exergy Calculator [56]. It generates the exergy data based on the resource degradation models published in [57].…”