**6. Discussion**

Many of the insights presented in Section 5 have been highlighted in previous research. However, previous studies have only investigated the related aspects individually and separately and have not presented a comprehensive overview of operability and practical implementation issues. Lundberg et al. [43] identified the positive effects of de-bottlenecking the recovery boiler at a Kraft pulp mill if heat integration measures are implemented simultaneously when rebuilding the plant. Dhole and Buckingham [44] proposed a methodology to simultaneously consider pinch analysis and column targeting (modification of column design to fit thermodynamic profiles obtained from pinch analysis) for a refinery, in order to achieve de-bottlenecking without increasing the existing furnace load. See also Li et al. [45] for a description of combining de-bottlenecking and pinch analysis in oil refining industry. These examples indicate that the importance of de-bottlenecking highlighted in the interview study is applicable for other cases than the selected oil refinery. Similar comparisons can be made for technical difficulties such as controllability or flexibility. Although it has previously been shown that non-energy benefits and technical and practical difficulties will have an impact on decision making process for selecting new projects to implement, the results in this paper present a wider perspective. The results show these aspects together rather than separately which enables a discussion of their relative importance. For example, the results indicate that non-energy benefits can outweigh the negative effects of technical and practical difficulties for heat integration retrofit proposals. Economic considerations are included in traditional pinch analysis based design but were not in focus in the retrofit proposals in this work. In traditional pinch design, the profitability of heat integration rebuilds is assumed to depend primarily on the energy cost savings and the investment cost for new heat exchangers. Operability considerations are likely to affect both the operating and investment costs for heat integration retrofit measures. Traditional pinch analysis is conducted for steady-state operation. In order to achieve good dynamic operability, additional equipment might be required, such as advanced control systems and/or over-capacity or back-up systems for flexible production. Additionally, if flexibility is considered, the heat savings can vary for different operating scenarios which change the expected heat savings, affecting the cash flows and the expected profitability. Non-energy benefits also affect the profitability of the heat integration retrofit proposals by increasing the revenues or decreasing the capital costs.

Practical issues discussed during the interviews indicate that spatial limitations should be considered earlier in the screening of alternative investment options. It is usually only the investment cost for a new heat exchanger or for extension of an existing heat exchanger that is included in the analysis. Since practical issues, especially spatial restrictions, were a major part of the issues discussed during the interviews, this is an important parameter to include in the early energy targeting analysis. One way to take this into account is to include spatial restrictions in the choice of ΔTmin. The optimal ΔTmin value is affected by investment and operating costs as well as operability considerations [30]. For retrofit studies where space is limited, spatial restrictions should be reflected by a higher value of ΔTmin in the screening phase.

Steam balances were discussed in several of the interviews. At large refineries, and other chemical industrial plants, utility systems are often large and complicated. The fuel balances at the studied refinery vary during the year, and during 25% of the year the refinery has an excess of fuel gas for the steam boilers and process furnaces. As a consequence, the studied refinery often has an excess of low-pressure steam, which is considered as free. For steam at higher pressure levels it is more difficult to know how the overall steam and fuel balances are affected by changes. Since it is not obvious how changes in steam production and consumption affect the overall steam and fuel balances at the refinery, a steam model for the refinery was developed after the interview study and applied to the retrofits that included the steam system (see [36]).
