Reliability refers to

*"* ... *the ability to operate a process without unexpected equipment failure."*

Whereas availability on the other hand refers to

*"* ... *the expected operating time for equipment during a time period that also includes planned maintenance."*

Practical considerations are not per se strictly related to operability only but are nevertheless included in the analysis, given their importance related to implementation of HEN retrofit measures [16]. Examples of practical implementation issues include space for new equipment, time availability for retrofitting during major process maintenance shut-down periods and accessibility for erecting new equipment.

#### **3. Industrial Case Study Plant**

To thoroughly discuss operability of heat integration measures, a single plant was considered in the case study, which gave the opportunity to design and evaluate retrofit proposals based on real process data. Large industrial plants include many interconnected process units and extensive utility systems. A comprehensive data collection and analysis is thus essential to obtain the details necessary to identify candidate HEN retrofit measures and related operability aspects. In this work, a single process plant was investigated in detail, which provided the opportunity to design HEN retrofits (see Section 4.2) that include many of the aforementioned operability aspects and discuss the proposed retrofit measures in detail with refinery sta ff. This level of detail would not have been possible if several plants were included in the study.

The case study was conducted at one of the most modern and energy e fficient complex oil refineries in Europe, with a crude oil capacity of 11.5 million tons per year and total CO2 emissions of 1.6 million tons in 2017 [35]. The main products are petrol, diesel, propane, propylene, butane, and bunker oil.

The heat demand of the refinery is satisfied mainly by direct fired process furnaces and by steam that is produced in steam boilers, flue-gas heat recovery boilers and process coolers. The process furnaces and steam boilers are fired by fuel gas that consists mainly of non-condensable gases from the refinery distillation columns. Liquefied Natural Gas (LNG) is used as make-up fuel when the non-condensable gas fuel stream is insu fficient. An overview of the main material and energy flows is presented in Figure 2.

The refinery steam network consists of four main pressure headers, which are connected by let-down valves and turbines. The turbines are used in direct drive configuration to operate compressors and pumps, a number of which can be switched to electric motor drive. There is no electrical power generation on site. The refinery regularly has an excess of low-pressure steam. During 25% of the year, the excess of low-pressure steam is particularly high due to an excess of non-condensable gases from the refinery processes. Flaring is strictly regulated, and the refinery has no storage capacity for the non-condensable gases, thus excess gas is combusted in the steam boilers, leading to an excess of steam that is vented.

**Figure 2.** Major material and energy flows of the studied refinery. Data for shaft work from steam turbines and electricity to pumps/compressors was not collected at the same time as the process stream data. The refinery steam system is described in detail in reference [36].

In connection to earlier research projects, energy targeting [37] and retrofit studies [38] have been carried out for the case study refinery. In this work, stream temperature and heat load data were collected for the majority of the refinery heat exchangers. Process stream data was collected 23 April 2010 from production data control room screen shots and data logs. The date was chosen in collaboration with refinery engineers to represent stable full capacity operation. For these operating conditions, the process hot utility demand that was covered by process furnaces was determined to be 409 MW [37]. Minimum utility requirements were determined for the different process units using pinch analysis. Details about the results of this energy targeting are presented in Section 4.2.
