*4.3. Interview Procedure*

All interviews in the study were semi-structured interviews that were conducted face-to-face. This enabled good communication as well as the possibility to discuss printouts of flow charts in detail. In addition, semi-structured interviews enable a good combination of structure as well as flexibility with respect to opportunities for follow-up questions and discussion during the interviews [42]. To our knowledge, this method has not been used before for discussing and investigating technical aspects related to implementation of heat integration retrofit measures. The interviews were conducted with technical staff with significant knowledge about operational and technical aspects of the refinery. Most interviews were about one hour long, but there was no time limit. The interviews were conducted in Swedish and all material was transcribed afterwards.

The interview procedure was the same for all technical staff responsible for the process units included in the study. HEN retrofit proposals were shared in advance to give the interviewees an opportunity to prepare for questions and check anything uncertain about the affected part of the process unit. The same set-up of open questions was used to discuss all retrofit proposals. Firstly, open questions were asked about the interviewee's thoughts about potential consequences of implementing the retrofit proposal. For all issues that were identified, solution suggestions were requested and discussed. Following the open questions, more specific questions were asked about operability aspects considered in the design phase of the retrofit proposal. To conclude, the interviewee was asked to list the top three obstacles and grade the retrofit proposals implementation potential from one (low) to four (high). The interviews with mechanical engineers, control engineers, and the process utility system engineer started with a general discussion about their expertise related to process integration. Retrofit proposals that were discussed were sent beforehand. This allowed the interviewees to collect necessary information about the processes affected by the proposals and less experienced engineers could discuss the proposals with more experienced engineers beforehand. The interviews with mechanical engineers, control engineers, and the process utility system engineer also provided an opportunity to verify anything unclear brought up in the previous interviews with operations and process engineers regarding equipment, utility systems, or control systems. Table 4 lists the content discussed in each interview.

Finally, results from the interviews were summarized and presented at a validation seminar which was attended by several of the interviewed engineers as well as managers responsible for process development. The results and main conclusions from the interviews were presented to the refinery experts involved in the study. The refinery experts confirmed and clarified the results. Consequently, a comprehensive and systematic in-depth coverage of the included topics was achieved.


**Table 4.** List of interviewees and content discussed in the interviews.

## **5. Observations and Insights**

In this section, the main observations and insights from the interviews are presented and discussed. The discussion refers to the retrofit proposals that are presented in Appendix A.
