**4. Conclusions**

This article reveals that the fundamental trade-offs between the various resource flows and environmental impacts—such as water–energy and water–energy–food nexuses—converge to the issues of material flow circularity and energy flow cascading. Based on this understanding, the concepts of exergy assets, exergy liabilities, and exergy profit/footprint are formulated, supplemented with a comprehensive evaluation framework.

Two case studies from completely different industrial domains are provided which illustrate the applicability of the framework for the seamless assessment of the energy/exergy needs of the process systems. These include the processes of acetic acid production and MSW treatment areas.

The results from the first case study show that the separation and reuse of the acetic-acid-containing purge stream are exergy-prohibitive and that it is not probable that such a solution would be sustainable. The follow-up analysis of the acetic acid production shows that the process requires a substantial external exergy input. Determining the degree of sustainability of such a process needs further analysis of the possible sources of providing such exergy.

The second case study clearly shows the sustainability potential of the MSW treatment for obtaining either useful energy directly or first extracting useful chemicals before the waste-to-energy process. The developed cumulative Exergy Composite Curves show a marginal advantage (less than 10%) of the chemical extraction route over the direct waste-to-energy route. The developed curves demonstrate that the proposed framework represents a useful toolset for evaluating process systems and alternative solutions.

The proposed concept can be further developed to create a complete framework which is capable of accounting for the thermodynamic irreversibility of processes. This will help us to reach a deeper understanding of the exergy flows, storages and losses and their relation to process sustainability.

Building on this, future work should incorporate economic metrics into the evaluation, leading to a complete toolset accounting for both the technical and economic performance of the considered process systems. This will make the tools suitable for decision-making in real engineering projects and for use by process managers and potential investors.

The correct selection of the system boundaries for the analysis of exergy footprints is key to the practical applicability of the concept. Full Life Cycle Assessment requires the collection of a large amount of information, which sometimes depends on subjective considerations. In many cases, not all stages of the life cycle are really significant with respect to the chosen criteria. In this context, further work should also be directed towards embedding this accounting framework within the Life Cycle Assessment framework, allowing for the scalability of the concepts and their adaptation to the modelling contexts.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1073/13/9/2132/s1.

**Author Contributions:** P.S.V.; conceptualisation and methodology formulation; H.H.C.; development of the case studies, formulation of the cumulative Exergy Composite Curves; A.-E.P.P.; state-of-the-art analysis, verification of the exergy models and calculations; S.B.: Introduction section development, feedback on the development of the concept in Section 2.3. proofreading. All authors have read and agree to the published version of the manuscript.

**Funding:** The EU-supported project Sustainable Process Integration Laboratory—SPIL funded as project No. CZ.02.1.01/0.0/0.0/15\_003/0000456, by Czech Republic Operational Programme Research and Development, Education, Priority 1: Strengthening capacity for quality research, based on the SPIL project, is gratefully acknowledged. The research was also supported by the Tomsk Polytechnic University (VIU-RSCABS-142/2019).

**Acknowledgments:** Alexandra Elena Plesu Popescu is a Serra Húnter fellow.

**Conflicts of Interest:** The authors declare no conflict of interest.
