**1. Introduction**

There are currently many driving forces to incentivize increasing energy efficiency in industry. For example, the European Union's Energy Efficiency Directive [1] has resulted in national laws requiring large companies to perform energy audits to identify measures for energy efficiency. Environmental legislation, various incentives, and policy support programs for energy efficiency, as well as economic and environmental concerns from customers and business partners, all motivate a stronger focus on energy efficiency in process industry companies.

However, technical, economical, and organizational barriers often hinder implementation of energy efficiency measures. Fleiter et al. [2] stress the importance of distinguishing between different types of energy efficiency measures when discussing barriers for energy efficiency. For example, the risk of production disruption is one of the most important barriers when the energy efficiency measures can affect the core process. Dieperink et al. [3] discussed difficulties associated with implementing energy efficiency measures that affect the core process for selected industrial sites in the Netherlands. Rhodin et al. [4] presented an example from the Swedish foundry industry where technical risks such as production disruptions was the second largest barrier for implement energy e fficiency measures. Thollander and Ottosson [5] discussed an example from the Swedish pulp and paper industry in which technical di fficulties were also ranked as a large obstacle for implementing energy e fficiency measures. Cagno and Trianni [6] also addressed the importance of considering barriers for specific energy e fficiency measures, rather than assuming that barriers are the same for all types of measure. This implies that research is needed that addresses a variety of industrial sectors, as well as di fferent types of energy e fficiency measures, in order to thoroughly evaluate and investigate which factors a ffect the implementation potential. Much research has been conducted concerning drivers and barriers in small and medium-sized enterprises. Johansson and Thollander [7] discussed drivers and barriers and suggested recommendations for in-house energy managemen<sup>t</sup> procedures for several industrial sectors in Sweden, including both small and medium-sized enterprises and energy-intensive industry. Cagno et al. [8] presented a framework for assessing non-energy benefits and non-energy losses that is targeted at industrial decision-makers and covers both technical and managemen<sup>t</sup> perspectives. However, only a few studies have focused on large energy-intensive industrial plants, such as the Swedish pulp and paper industry [5], the Swedish iron and steel industry [9], and the German steel industry [10], and the lack of studies from the petrochemical process industry is noteworthy.

Changes to an industrial process can have major e ffects on process operability. It is therefore imperative that operational issues are considered when planning such changes. Process operability includes di fferent operational aspects such as flexibility, controllability, reliability, availability, and start-up and shutdown of the process [11]. For example, if a process is not flexible it cannot adapt to di fferent operating conditions, such as varying feedstock, change of product specifications and/or product mix, and varying ambient conditions. Equipment reliability/availability issues can cause expected and unexpected operational disruptions and controllability problems can lead to major safety issues and production disruptions. Therefore, it is important to investigate how energy e fficiency can affect operability. Furthermore, energy e fficiency measures can also improve operability, for example by reducing the load on a process capacity-limiting furnace, leading to valuable non-energy benefits for the process. Non-energy benefits refer to benefits other than the direct energy cost savings from the energy e fficiency improvement, e.g., reduced carbon dioxide emissions, increased production, and better work environment [12].

Although there are several options to increase energy e fficiency in industry, thermal energy is used in large quantities in chemical process plants and heat recovery is therefore one important option to improve energy e fficiency in such plants. In previous research, many case studies have identified large techno-economic potentials for energy savings by heat integration in existing industrial plants. To evaluate feasibility of new processes and increased energy e fficiency through increased heat recovery, a better estimation of the techno-economic potentials of process heat integration measures is necessary as well as a better understanding of the drivers and barriers a ffecting the implementation potential. Rebuilding an existing industrial plant to increase heat integration a ffects the process in several ways. In particular, the number of interdependencies between di fferent parts of the process increases. In previous studies it has been repeatedly discussed that the risk for operability or control problems is strongly connected to the number of interdependencies and interconnections within a process. Subramanian and Georgakis [13] investigated plant-wide steady-state operability issues for an integrated process plant. Setiawan and Bao also discussed the connection between an integrated process and operability issues, both in a study considering interactions between process units [14] and in a study that investigates interaction e ffects connected to operability [15]. Such operability problems may lead to production disruptions, which must be avoided since they are extremely costly. This underlines the importance of considering operability of heat integration measures at an early stage when investigating retrofits of industrial energy systems.

Heat integration analysis can be based on mathematical programming or graphical insights (e.g., pinch analysis) (see e.g., [16]). A wide variety of case studies have shown a large potential for increased energy e fficiency through retrofitting of heat exchanger networks (HENs) at di fferent industrial process sites. There are many different methodologies to identify HEN retrofit designs that achieve high energy savings at low cost, each of which has their own benefits and drawbacks (see e.g., [17] for a review of HEN retrofit methodologies and applications). It is common that several HEN designs can be identified that achieve approximately the same energy saving at similar costs. However, such HEN designs can vary significantly regarding network complexity, placement of new heat exchangers, as well as utility heaters and coolers for target temperature control, etc. It is thus clear that technical and operational factors need to be considered together with investment cost and fuel cost savings when investigating HEN retrofit options.

In the existing literature, there are many studies presenting methods for accounting for specific practical considerations and associated costs in HEN retrofit studies. For example, Becker and Maréchal [18] presented a method to consider heat exchange restrictions using mixed integer linear programming, and Cerda and Westerburg [19] presented a study of HEN synthesis with restricted stream matches. Polley and Kumana [20] suggested dividing larger networks into a number of smaller networks to deal with large heat integration projects. Practical considerations and associated costs are especially important when considering integration at large sites or even across company boundaries, which is the case, for example, for piping and pressure drops. To include plant-specific factors such as piping costs, pressure drop, and heat losses, Bütün et al. [21] proposed a mixed integer linear programming framework. Hiete et al. [22] also included piping costs when considering energy integration between industrial plants with different owners. Jegla and Freisleben [23] also considered pressure drops in their practical method for energy retrofit, but in addition they also considered available heat exchanger space. Reddy et al. [24] presented an optimization method for retrofits of cooling water systems including pressure drops, cooling tower operation, and piping costs. Hackl and Harvey [25] developed a methodology for identifying cost-effective retrofit measures in a chemical cluster adopting a total site perspective. Nemet et al. [26] developed methods for included piping costs, pressure drops, and temperature drops in total site analyses.

Other methods have also been proposed to account for certain specific operability considerations such as flexibility and controllability in network design. Escobar et al. [27] suggested a method to include flexibility and controllability consideration in HEN synthesis. Another method for including operability and observability in HEN design was recently proposed by Leitod et al. [28]. Andiappan and Ng [29] presented a methodology to consider energy systems operability, feasibility and debottlenecking opportunities connected to retrofit design. Abu Bakar et al. [30] suggested including operability in addition to investment and utility cost savings in the choice of ΔTmin for HEN design. Several authors have used mixed-integer programming for multi-period optimization to consider flexibility in process integration problems, for example, for integration of utility systems [31], flexible HEN design [32], and integration of biomass and bioenergy supply networks [33]. Bütün et al. [34] presented an approach for including multiple investment periods for a longer time horizon for energy integration.

Although many studies have focused on specific individual aspects of process operability, there is, to the best of our knowledge, no scientific literature that provides a comprehensive overview of the wide variety of process operability issues and that systematically investigates their impact on decision processes related to implementation of HEN retrofit measures. Furthermore, operability issues are traditionally not considered at the early conceptual design stage for techno-economic ranking of alternative heat integration measures. One common approach in HEN retrofit studies is to identify pinch rules violations in the existing HEN, and thereafter attempt to remove or reduce such violations starting with the largest violation. At this early design stage, it is unusual to consider costs other than heat exchanger and utility costs. Operability issues are usually not included until the pre-feasibility or feasibility study phases of the decision-making process for heat integration projects, see Figure 1. However, since energy efficiency measures for increased heat integration are closely connected to the core process of industrial plants, technical aspects can be assumed to be important barriers for their implementation. By considering possible technical barriers and operability issues at an earlier stage of the screening process of energy efficiency options, it could be possible to avoid spending resources on detailed design and feasibility studies of projects that are highly unlikely to be implemented. This is crucial to enable a rapid and relevant screening process of energy efficiency measures and to be able to estimate accurate technical and economical potentials of heat integration. To enable more explicit consideration of operability issues earlier in the screening process, it is, however, important to know which operability factors are most important to consider in the techno-economic evaluation.

**Figure 1.** Decision-making process for process development projects.

The aim of this paper is to present a comprehensive overview of operability and technical implementation issues related to heat integration measures by mapping, discussing and clarifying how such measures relate to a comprehensive set of key operability factors. This approach differs from previous studies that have primarily investigated these issues individually. The work was conducted in the form of an interview study at a large oil refinery in Sweden, and as such suggests a new approach for inclusion of operability considerations at an early stage of screening of alternative heat integration options. The paper aims to present an in-depth discussion of the theoretical definitions of operability and the practical considerations of heat integration by investigating its relevance in a real industrial process plant. The case study contributes to expanding the knowledge base for operability and practical implementation issues related to heat integration retrofits.

#### **2. Definition and Categorization of Operability**

The following definitions were proposed in previous work by the authors, based on a review of the literature in the area of operability issues related to heat integration [10]. Operability is defined as

*"* ... *the ability to operate equipment, process units and total sites at di*ff*erent external conditions and operating conditions, without negatively a*ff*ecting safety or product quality and quantity. This includes both steady-state and dynamic aspects of operation."*

It was also proposed to distinguish between a number of operability aspects that can be sorted into the following sub-categories: flexibility, controllability, feasibility of start-up/shutdown transitions, reliability, availability, and other practical considerations. These sub-categories were based on the following considerations:

Flexibility:

*"A flexible process has the ability to maintain feasible operation for di*ff*erent operating scenarios. For oil refining processes, flexibility includes, for example, being able to handle di*ff*erent crude recipes, product mixes and ambient conditions. Flexibility also includes the ability for the operation to handle long-term variations within the process, such as decreased reactivity in catalyst beds and decreased heat transfer due to fouling."*
