*5.5. Safety Aspects*

Safety aspects were discussed in many interviews, especially regarding retrofit proposal 5 (see Appendix A) which involves taking a process furnace (HTR-D1) out of operation since it is not needed from an energy point of view. Increased internal heat exchange could easily replace the heat provided by the furnace. The process furnace is placed immediately upstream of an exothermic reactor which is very sensitive to inlet temperature. During the interviews it was clearly stated that the retrofit proposal would not create a controllability issue with respect to the stabilizing function of the reactor inlet temperature during normal operation. The temperature control would not be affected since the existing control is located upstream of the furnace. However, it was very clear during the interviews that a safety issue could occur. In all interviews regarding retrofit proposal 5, it was explained that it is necessary to be able to rapidly lower the reactor inlet temperature if a runaway reaction occurs. This is currently achieved by shutting down the furnace. If the furnace is to be taken out of operation, another solution would be necessary to stop potential runaway reactions. Possible solutions were discussed during the interviews, but the safety control for the retrofit proposal needs to be thoroughly investigated if the retrofit proposal is to be implemented.

## *5.6. Start-Up and Shutdown*

In the interviews, operability aspects related to start-up and shut-down of the process were mostly discussed concerning heat exchange between different process units and taking furnace HTR-D1 out of operation in retrofit proposal 5 (see Appendix A). Regarding heat exchange between different process units, the interviewees stated that the process units are not usually started up and shut down simultaneously. Consequently, back-up solutions for heating/cooling during start-up/shutdown are required for heat integration designs in which heating is provided from a different process unit during normal operation. Regarding retrofit proposal 5, the process furnace is not needed during normal operation. However, HX-D1 in the proposal is dependent on the hot reactor effluent. To start the reaction, heat needs to be added to the process, making the process furnace necessary during start-up.
