Shutdown for maintenance of gasoline hydrogenation unit (catalyst replacement required) and precautions upon restart

The main steps during shutdown include: reducing temperature and flow rate, shutting off feed, circulating hot hydrogen, further reducing temperature and pressure, purging with nitrogen, carrying out pre-maintenance procedures (such as switching to nitrogen isolation for protection and performing neutralization and cleaning), and removing the catalyst and cleaning the reactor. Many of the operational principles during shutdown are the same as those during startup; for example, during the process of reducing flow rate and temperature, it is necessary to prevent excessive temperature rises, while also paying attention to the speed of temperature and pressure reduction, temperature limits, and monitoring of liquid level in the high-pressure section. The main steps for starting up a new catalyst are: loading the catalyst, removing air from the reaction system, ensuring airtightness of the reaction system using nitrogen, drying the catalyst, ensuring hydrogen-based airtightness of the reaction system, sulfiding the catalyst, feeding low-nitrogen oil into the reaction system, passivating the catalyst, and then switching to normal feed. The startup process varies depending on the condition of the catalyst. The key points during catalyst loading are to ensure the quality of the loading as well as the quality of the installation of the internal components, in order to prevent uneven fluid distribution such as channeling and wall flow within the bed after feeding begins. The consequences of this are uneven contact between the reaction stream and the catalyst, inconsistent reaction rates, the formation of local hot spots within the bed, and also the inability of some catalyst particles to function due to the liquid taking short circuits. Ultimately, it affects the progress of the reaction and can easily lead to excessive temperature. Before maintenance, nitrogen isolation protection measures should be applied to the equipment that does not need to be opened; temporary facilities must be installed to carry out the protection process, while the equipment that needs to be opened should be neutralized and cleaned. Main reasons: 1) During the hydrogenation process, iron sulfide is generated within the system, and it can catch fire easily when exposed to air (especially in the catalyst bed) ； 2) Protect equipment from stress corrosion cracking caused by polyoxysulfuric acid

The same precautions apply during shutdown as during startup: during the process of reducing volume and temperature, it is necessary to prevent excessive temperature rises; attention must be paid to the speed at which temperature and pressure are reduced, to the temperature limits associated with different pressure levels, and to monitoring the level of liquid in high-pressure vessels

To prevent the catalyst from catching fire during desulfurization, operations must be carried out under anaerobic conditions

Main process of shutdown: Reduce flow rate — Cut off feed — Heat hydrogen with oil present — Lower temperature and pressure — Displace with nitrogen — Isolate with blind flanges before maintenance. Catalyst removal: Must be carried out under anaerobic conditions; the removed old catalyst must also be stored under anaerobic conditions to prevent spontaneous combustion. Main steps for starting up with a new catalyst: Load the catalyst — Displace air from the reaction system — Make the reaction system airtight using nitrogen — Dry the catalyst — Make the reaction system hydrogen-tight — Sulfurize the catalyst — Feed low-nitrogen oil into the reaction system — Passivate the catalyst — Switch to feed

Should it be part of the catalyst that is replaced, or the entire catalyst?

1. Precautions during shutdown include preventing overheating in the hydrogenation reaction section, preventing overpressure in the low-pressure distillation section, and maintaining a slight positive pressure in the reactor using nitrogen for protection. 2. Thoroughly master the operating procedures, the principles of reducing temperature and flow rate, hydrogen purging (hydrogen sulfide content in circulating hydrogen), and measures to prevent nitrogen asphyxiation among personnel during shutdown processes. 3. To remove the catalyst, prepare dry ice or sodium carbonate in advance to prevent the spent catalyst from catching fire on its own. 4. When draining oil, purge each pipeline sequentially in accordance with the procedure to prevent oil leakage and cross-flow.

Some time ago we carried out maintenance work (replacing catalysts in the pre-reaction and reaction sections). If the new employees were incorrect, please point it out. 1. For the pre-reaction and reaction sections, reduce the temperature and flow rate; stop feeding raw materials into the unit, establish a circulation system to analyze the bromine value, and then reduce the temperature and remove the oil. Use nitrogen to displace water and push the oil out (in some cases where catalysts are replaced, it’s not allowed for water to enter the reactor, so ensuring thorough purging without any dead corners is difficult and time-consuming). 2. In the hydrodesulfurization unit, establish a circulation system to analyze the bromine value (add sulfur during this process to prevent catalyst reduction); reduce the temperature, stop adding sulfur, remove the steam drum, shut down the ammonia washing system, remove the oil, use hot hydrogen for stripping, displace with nitrogen, and isolate the reactor (the catalysts in the hydrodesulfurization tower do not need to be replaced). I remember that isolating the reactor was quite troublesome at that time – there was oil and gas in the pipelines, as well as water. There was also a passivation process during that time, with passivators being added, but I’ve forgotten when exactly that was done. . . . For replacement, we carry out a partial replacement here: first, the explosive gas is replaced through analysis, and once it meets the standards, the large cover is opened to introduce nitrogen for an oxygen-free replacement. This is for new employees only; some information may be incorrect, so please feel free to point out any errors.