**7. Conclusions**

This paper presented the results of a comprehensive inventory of potential technical barriers and process operability constraints related to the implementation of heat recovery measures in industrial process plants. Unlike previous studies, which mainly focus on single implementation barriers or operability aspects of heat integration measures, we presented a complete inventory of such technical constraints including aspects related to flexibility, controllability, start-up/shutdown, reliability/availability, and other practical considerations related to the implementation of measures.

We also suggested a mapping of these potential technical issues to certain design characteristics that may appear in HEN retrofit designs. This mapping could be used to identify the issues that are most likely to be of importance when evaluating a certain heat recovery design. As such, it was proposed to support a new approach for investigating operability and implementation constraints in the early-stage screening of candidate heat recovery measures in industrial process plants. The approach is based on qualitative analysis by means of an interview study, in which operability and technical implementation issues related to specific heat recovery projects are identified, characterized and described with respect to their impact on implementation potential.

The results from interviews at a large oil refinery used as a case study indicated that technical and practical constraints in the plant have a major influence on how the potential for implementation of the heat integration retrofits are ranked. The process considerations that were highlighted most often in the interview study were spatial limitations and maintenance requirements. Another conclusion from the interviews is that non-energy benefits of an energy-saving project can be far more important than the energy savings as such and may also outweigh potential technical difficulties. An example of this is when the heat savings create an opportunity to remove a production bottleneck.

Based on the ranked importance of technical constraints and productivity benefits, it can be concluded that it would be valuable to take such process aspects into consideration at an earlier design stage than usual when constructing HEN retrofits for increased heat integration. If operability, non-energy benefits and practical implementation issues were considered in earlier-stage techno-economic assessments and screenings of heat recovery measures, several issues could be avoided, and large process benefits could be achieved. The inclusion of those factors would also lead to a better estimation of techno-economic potentials for heat integration measures and thereby a more accurate screening process for different energy efficiency and climate mitigation options.

Future work including additional case studies is needed to further confirm the impacts of operability and other technical constraints on the feasibility and cost-efficiency of HEN retrofit measures, prior to suggesting ways to include the effect of these at an earlier design stage and better quantify their values. It would also be of grea<sup>t</sup> value to study how these aspects have affected the evaluation of previously implemented and rejected heat integration measures.

**Author Contributions:** Conceptualization, S.M. and E.S.; Data curation, S.M.; Formal analysis, S.M.; Funding acquisition, E.S.; Investigation, S.M.; Methodology, S.M. and E.S.; Project administration, E.S. and S.H.; Resources, S.H.; Supervision, E.S. and S.H.; Validation, E.S. and S.H.; Visualization, S.M.; Writing—original draft, S.M.; Writing—review and editing, E.S. and S.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Preem AB through Chalmers Energy Area of Advance.

**Acknowledgments:** A number of participants from Preem are gratefully acknowledged for their time, input data and participation in interviews.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **Appendix A Heat Exchanger Network Retrofit Proposals**

In this appendix all retrofit proposals included in the interview study are presented.
