**4. Methodology**

In this section, the framework for the interview study is described as well as the methods used for the interviews and HEN retrofit design.

Although scientific literature is scarce on the subject of operability related to heat integration measures, many experienced engineers and operators in industry possess a deep knowledge and understanding of their processes and the way they operate under various conditions. To be able to tap into this extensive knowledge base, a case study approach based on interviews was adopted. As discussed by Sovacool in references [39,40], this approach provides a broader and more detailed perspective of process operation compared to simulation using a computer model which includes only known parameters and variables. Since limited research is available on the operability aspects that are most important to consider in a HEN retrofit study, a mapping is needed which can thereafter provide guidance for future HEN retrofit evaluations.

An overview of the methodology used for the interview study is shown in Figure 3. As the figure shows, HEN retrofit proposals were designed specifically for the case study process (see Section 4.2), based on a literature review of operability issues related to heat integration measures [10]. The process

data for the retrofit proposals were taken from a previous energy targeting study at the refinery [37]. The proposals were discussed with refinery experts in eleven interviews (see Section 4.3). The results were then summarized and presented to the refinery experts again for confirmation and further discussion at a validation seminar.

**Figure 3.** The work flow for the interview study.

#### *4.1. Inventory of Possible Process Operability Implications Related to Heat Integration Measures*

Implementation of new heat recovery measures involves many changes to the process equipment and operation, ranging from new and modified heat exchanger units to changes in pressures, steam balances and interactions between different parts of the process. In this paper, we refer to these changes as "process implications".

In order to discuss different aspects of operability during the interviews, a number of heat integration retrofit proposals were designed that cover different process implications related to operability. To ensure an exhaustive coverage of process implications and operability aspects, a list of potential process implications was compiled based on literature examples and experience from previous process integration projects. The implications included on the list were matched with the operability aspects that were most likely to be affected (see Table 1). After the first round of interviews with plant staff, the list of possible implications was extended if new process implications were identified. Table 1 was also used to design the list of questions to be addressed during the interviews, as described in Section 4.3.


**Table 1.** The assumed relations between the process implications of heat integration retrofit measures and the most affected operability factors and practical

#### *4.2. Design of Retrofit Proposals*

Nine HEN retrofit proposals were designed for discussion during the interviews. The proposals were designed to include specific process implications connected to operability and technical implementation aspects. The proposed HEN retrofits were designed within selected process units at the refinery, using a pinch technology approach [41] based on stream data representing normal operating conditions and information about the current placement of heat exchangers.

Previous studies at the refinery, also based on pinch analysis, showed that five process units account for 90% of the current hot utility use and also have the greatest potentials for utility savings [38]. One of these units has been rebuilt since the data was collected and pinch analysis targeting was conducted. Therefore, the remaining four units were chosen for this study. To be able to investigate operability aspects of heat integration between process units, two units located close to each other were grouped together. Current hot utility usage and theoretical minimum heat demand for the chosen units are listed in Table 2. The analysis and design were conducted for a single operating point which represents normal refinery operation. It should be noted, however, that process operation and ambient conditions vary over time. Although the study includes a single operating point, flexible operation for other process conditions was discussed in the interviews.

**Table 2.** Current and minimum heat demands for process units included in this paper. For the pinch analysis, the following minimum temperature differences contributions were considered; ΔTmin/2 = 10 K for condensing/boiling hydrocarbons, ΔTmin/2 = 5 K for water, ΔTmin/2 = 2.5 K for boiling water, and ΔTmin/2 = 15 K for other process streams.


The design of retrofit proposals was based on the list of implications presented in Section 4.1. Each retrofit proposal was designed to investigate the e ffect of some of the specific implications. The retrofit proposals were also designed so that all implications are covered, which can be seen in Table 3. All retrofit proposals are described in detail in Appendix A.

For Unit A + B, the main objective was to include heat exchange between two process units (Implication #9) in the retrofit proposals (Retrofit proposal 1A–C and 2). All proposals for Unit A + B include reduced load on the same furnace, but with di fferent paths for stream pre-heating. The stream pre-heating configurations di ffer with respect to complexity, additional heat exchanger area requirements, and heat source (hot process streams or hot flue gases). Another aspect included in Retrofit proposal 1B is the replacement of steam heating in a distillation column reboiler by heating by internal heat exchange within the process units. For Unit C, three di fferent ways of increasing the pre-heating before a process furnace were considered. The first retrofit proposal, 4A, involves heat recovery from other process streams currently cooled with overhead air fans. An excess of low-pressure (LP) steam is available at the refinery during most of the year. In Retrofit proposal 4B, excess LP steam is used for the pre-heating, decreasing the number of process interconnections. Retrofit proposal 4C also uses LP steam for pre-heating, but the proposal includes a stream split. For unit D, two retrofit proposals were designed. The retrofit proposals for unit D involve heat savings in two di fferent process furnaces that also result in a reduction in high-pressure steam production. The furnace in Retrofit proposal 5 is placed prior to an exothermic reactor and is suggested to be taken out of operation. Both retrofit proposals in unit D also include process streams with high pressures and heat exchangers with large pressure di fferences between the process streams.


**Table 3.** Implications included in each retrofit proposal.
