*5.3. De-Bottlenecking*

Several retrofit proposals turned out to provide opportunities for removing bottlenecks in the production process. De-bottlenecking increases the flexibility of the process. Retrofit proposals that result in load reduction in process furnaces that currently constitute production bottlenecks were ranked higher than other proposals. These debottlenecking implications were identified by the interviewees even though they were not intentionally included in the retrofit proposals during the design procedure. The possibility to increase production or yield of desirable products was declared as important and recurrently discussed in the interviews. For example, the operations engineer in interview 3 stated that

*"Well, I am very interested in this, if we can reduce the energy consumption here* ... *it will not only be an energy aspect, but I think we can increase the flow through the unit as well"*

*Operations engineer Unit C*

Regarding the process furnace, HTR-C, included in retrofit proposals 4A–C.

## *5.4. Controllability and Flexibility*

The e ffect on flexibility and controllability from increased interconnections and complexity was mentioned as a potential issue, but often needs further investigation to evaluate its significance. For retrofit proposal 1B (see Appendix A), it is suggested to heat a distillation column reboiler by internal heat exchanging instead of with utility steam. The retrofit proposal contains several new interconnections, both within the process unit (Unit B) and between Unit A and Unit B. It was considered a potential problem that the reboiler would become dependent on other parts of the unit. Whether the increased number of interdependencies would have a significant e ffect on reboiler operability needs to be further investigated. Similar issues were discussed regarding retrofit proposal 4C (see Appendix A) in which a stream split is included. Stream splits are not used to a grea<sup>t</sup> extent in the process units for which the interviewed process and operations engineers are responsible and they therefore had no clear opinion about possible impacts on operability. The control engineer, on the other hand, stated that the stream split is possible but new control valves and measurements are needed, as well as a more thorough analysis of the control structure. However, at the validation seminar it was acknowledged that almost all refineries have several well-functioning stream splits in the crude oil pre-heating unit. Both examples (the integration of the reboiler in 1B and the stream split in 4C) show that a large increase in interdependencies might cause operability issues, but to know whether this is the case, and how it then can be managed, a more thorough analysis is needed. Issues to be investigated include, for example, modeling and simulation and potentially more advanced control structure design.

Another important aspect discussed concerning flexibility and controllability is that temperatures in several process units change over time due to the deactivation of reactor catalysts. Because of the changing temperatures during the catalyst cycle, it is important to consider the HEN design for more than one operational point.

Large negative effects on flexibility or controllability caused by heat exchange between process units were not discussed to any grea<sup>t</sup> extent during the interviews. Units A + B are almost always operated simultaneously, but a back-up solution for heating/cooling needs to be available when one of the units is not in operation. During the validation seminar, the same aspects were discussed, and it was confirmed that it is possible to heat exchange between two process units without any significant decrease in flexibility or controllability. However, this is only the case for units with similar operation patterns and, as previously stated, if there is a back-up solution available when heat exchange is not possible. Heat exchange between two process units that are not operated according to similar schedules was considered very unlikely to be feasible.
