Issues to Consider During Emergency Shutdown of Hydrocracking Units

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Hydrocracking units operate under high temperature, high pressure, and in the presence of hydrogen, resulting in demanding production conditions with many factors that can affect stable operation. All operators of the facility must be fully aware that the hydrocracking catalyst bed must maintain a balance between the heat generation rate and the heat release rate. When the heat generation rate exceeds the heat release rate, it leads to an abnormal rise in temperature of the bed; this increased temperature in turn accelerates the hydrocracking reactions as well as the heat generation rate. These reactions continue indefinitely in a spiral pattern, causing the temperature of the catalyst bed to rise above 800°C within minutes (i.e., a runaway temperature situation). This results in severe coking and damage to the catalyst, and may even destroy the reactor or lead to unimaginable consequences. Therefore, the hydrocracking unit is the most hazardous Class A unit in refineries in terms of explosions and fires. The purpose of developing accident response plans is to improve the emergency response capabilities of operators and managers at all levels in the event of major accidents in production facilities, so as to better protect human safety as well as the safety of catalysts and production equipment, and to ensure the safe, stable, continuous, full-capacity, and efficient operation of the facilities. 2.1 General Provisions 1) This device is equipped with an automatic interlock protection program, namely the SIS emergency shutdown system. Operators should have a thorough understanding of the operating principles of the SIS emergency shutdown procedure, so as to be able to use it skillfully, promptly, and accurately in the event of an accident. 2) Operators should be proficient in the operating procedures for their positions, and know the steps and precautions for handling emergency and routine faults. 3) Operators must be meticulous and careful during operation, constantly monitoring changes in various parameters. When abnormalities occur, it is necessary to identify the causes promptly and take effective measures to nip the incident in the bud. 4) Backup equipment should always be in standby mode. The unconverted oil line must remain unobstructed at all times. 5) The emergency venting systems at 0.7 MPa/minute and 2.1 MPa/minute (including manual valves) are always in good condition. 2.2 Principles for accident handling 1) The handling of equipment accidents is aimed at ensuring the safety of people’s lives and property as well as restoring production as soon as possible. 2) When shutdown is required due to an accident, try to follow the normal shutdown procedures as much as possible, provided that conditions permit. 3) During accident handling, the following points should be noted: (1) Prevent fires and explosions. (2) For equipment related to high and low voltage, strict precautions must be taken to prevent voltage transference. (3) The heating furnace should be shut down immediately to prevent overheating at the reactor inlet or coking of the furnace tubes. (4) Maintain gas flow within the reactor bed to protect the catalyst, and strictly prevent the bed from overheating and damaging the catalyst. (5) After cutting off the feed, the product should be promptly directed to the defective product line or put into circulation to avoid contaminating the product tanks and prevent quality incidents. (6) Maintain a certain system pressure as much as possible to restore normal operation as quickly as possible. (7) During the processing, priority should be given to ensuring the safety of the unit and maintaining the proper functioning of the compressor’s dry gas seal system, in order to prevent the compressor from entering a surge condition ； Ensure that the unit’s SIS is in good condition; in the event of a failure, initiate an emergency shutdown manually if necessary, and ensure that the unit’s lubrication system operates properly. 4) The plant operators must monitor the temperature changes of the reactor at all times; in the event of an abnormal temperature rise, every effort must be made to control the bed temperature and prevent any incidents of runaway temperature. 5) It is necessary to strictly control the pressure difference across each reactor so that it does not exceed 0.4 MPa. Especially when activating the emergency venting system at a rate of 2.1 MPa per minute, it is important to prevent any changes in the pressure difference between the different bed layers, in order to avoid damage to the internal components of the reactors and the catalysts due to excessive pressure differences. 6) When an accident occurs, the shift leader should determine the cause of the accident as quickly as possible and make the following decisions: (1) Determine the scope affected by the accident and the potential duration it may last. (2) Notify the dispatcher, department duty officer, and relevant supervisors. (3) Determine how to handle the situation based on the accident conditions. 7) During the handling of any accident, unified command must be provided by the team leader to strengthen communication between different positions and between various units. 8) Accidents in hydrocracking units develop rapidly, and no time should be wasted. The shift supervisor has the authority to take emergency action without seeking approval in the event of such accidents (but should contact the production control department to secure prompt external support for handling the accident, thereby preventing delays or further escalation of the situation)