The following presents a general procedure to be followed for a normal shutdown.
1. Gradually reduce the liquid feed and adjust the catalyst temperature downward, so that feed conversion remains at the desired level. When a very low feed rate is reached, stop all liquid feed to the reactor but continue to circulate recycle gas rate at the normal rate. As the liquid feed rate is cut, the reactor feed and effluent rates will be out of balance for short periods. The feed must be gradually reduced to prevent temperatures greater than the maximum design temperature occurring in the effluent/feed heat exchangers.
2. To strip off as much of adsorbed hydrocarbon as possible from the catalyst, raise the reactor temperature to the normal operating temperature after the feed is stopped. Circulate hot hydrogen until no more liquid hydrocarbon appears in the high-pressure separator. If the shutdown is for only brief maintenance, which does not require stopping the recycle compressor, continue circulation at the normal operating temperature and pressure. After the maintenance work is complete and unit is ready for feed, lower the reactor temperature and introduce the feed following the normal startup procedure.
3. If the shutdown is for catalyst regeneration, catalyst replacement, or maintenance that requires stopping the recycle compressor or depressurizing the reactor system, continue circulation at the normal operating pressure for 2 hours. Reduce the furnace firing rate and start gradually reducing the reactor temperature to 400°F. Any cooldown design restrictions for the reactor must be adhered to, to avoid thermal shock.
4. Add quench gas as required to evenly cool the reactor.
5. To maintain heater duty high enough for good control and speed up cooling after heater fires are put out, bypass reactor feed gas around the feed/effluent exchanger, as required.
6. While cooling, remove the bulk of hydrocarbon oil from the high-pressure separator by raising the water level and pressure to the low-pressure separator. Also, increase the pressure of any low-pressure liquid remaining in the H2S absorber to the amine section. Purge and block the liquid hydrocarbon line from the high-pressure separator. Block in the H2S absorber and drain the amine lines. Drain all vessels and the low points of all lines to remove hydrocarbon inventory.
7. If the shutdown is for catalyst regeneration, hold the reactor at 400°F.
8. If the shutdown is for catalyst replacement or maintenance that requires opening the reactor, determine the CO content of the recycle gas. If the CO content is below 30ppm CO, continue the cooling procedure; if more than 30ppm, proceed as follows:
a. If more than 30ppm CO has been detected in the recycle gas and the reactor is to be opened, the CO must be purged before cooling can continue to eliminate possible formation of metal carbonyls. Stop recycling gas circulation while the catalyst temperature is still above 400°F. Do not cool any portion of the bed below 400°F. Before depressurizing the system, check that all these valves are closed:
• Suction and discharge valves on charge pump and spares.
• Block lines on liquid feed lines.
• Block valves on the high-pressure separator, liquid product and water lines.
• Recycle and makeup compressor suction and discharge lines.
• Chemical injection and water lines.
• Makeup hydrogen line.
b. Depressure the system. Do not evacuate below atmospheric pressure, as there is the danger of explosion if air is drawn into the system. For effective purging of the reactor system, the nitrogen gas is injected at the recycle compressor discharge and follows the normal flow path through the catalyst beds.
c. Pressurize the system with dry nitrogen to 150 psig. Purge the compressor with nitrogen.
d. Depressurize the system to 5 psig and purge with nitrogen for 5 minutes.
e. Repressurize the system to 150 psig and purge all blocked lines as mentioned previously.
f. Pressurize the system with nitrogen from nitrogen header and recycle compressor.
9. Relight the furnace fires and maintain the reactor at a temperature of 400°F for at least 2 hours. Then analyze the recycling gas for CO content. If the CO content is still above 30ppm, repeat the preceding procedure.
10. If the CO content is below 30ppm, the reactor can be cooled below 400°F following the cooling rates restrictions for the hydrocracker vessel metallurgy. Stop the furnace fires and continue cooling by circulating nitrogen until the temperature has fallen to 120°F.
11. Stop nitrogen circulation and ensure that the system is blocked in preparation to depressurizing. Depressurize and purge the system again with nitrogen. Maintain the system under slight nitrogen pressure, so that no oxygen is admitted.
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