High pressure hydrogenation unit shutdown problem

As the temperature drops, the pressure also drops.

Briefly introduce our process: First cool down, then reduce the quantity, introduce diesel into the system and circulate, stop circulation, withdraw oil, lift hot hydrogen (maintain operating pressure before this), cool down, first reduce pressure, circulate hot hydrogen, stop heating furnace, circulate cold hydrogen, shut down, reduce pressure to normal pressure, and purge with nitrogen. Each shutdown has different processing requirements for the reaction system, and the shutdown steps will be slightly different. I hope this helps. This post was last edited by johnhwa on 2009-2-9 14:54 ]

When shutting down, a plan should be adopted to fully release the hydrogen energy stored during operation. For example, lower the pressure first and then lower the temperature.

The temperature is raised first and then the pressure is raised when starting the operation, and the pressure is lowered first and then the temperature is lowered when the operation is stopped. This is out of consideration for hydrogen-resistant steel equipment. The material of hydrogen-resistant steel equipment (such as 2.25Cr-1Mo) has temper brittleness problems. In order to avoid damage to the equipment when the pressure is too high, Therefore, the principle of first heating up and then increasing the pressure when starting work, and first lowering the pressure and then cooling down the temperature before shutting down is determined due to damage. As for the specific temperature at which the pressure can be raised or lowered, it needs to be determined according to the written documents issued by the equipment manufacturer and the design department. According to research, the temper brittleness problem is mainly caused by the material containing too much Si, P, Mn and other materials. Reducing these impurities can effectively eliminate the temper brittleness problem. Although the problem of temper brittleness exists, these possible accidents have not occurred in the hydrogenation unit and reforming unit so far.

Do not reduce the system pressure when shutting down to cool down the system. Reducing the pressure of the reaction system means reducing the amount of hydrogen circulating in the system. Since an important role of system circulating hydrogen is to bring out the reaction heat in the system, a reduction in system pressure will cause the reaction heat to not be taken out in time, which may cause the reactor to overheat. During normal shutdown operations, the amount of circulating hydrogen can be appropriately increased. Generally, after cutting off the feed and starting to strip the oil, the pressure of the reaction system drops to 14.0MPa, and the catalyst is stripped with the maximum amount of circulating hydrogen. After the stripping is completed, the temperature begins to drop. When the temperature of each bed is lower than 300°C, the pressure starts to drop at ≯1.5MPa/hour.

Adjustment of the reaction feed should be carried out strictly in accordance with the requirements of "increase the amount first and then increase the temperature, first lower the temperature and then decrease the amount". When the feed amount increases, the refining inlet temperature should be appropriately increased. Generally, after increasing the amount, you should wait for a period of time before increasing the amount and temperature. At least the previous stream of material has passed through one bed before continuing. This is mainly because after passing through one catalyst bed, the reaction heat has been released evenly, which can be controlled by cold hydrogen at the entrance of the next bed to avoid over-temperature accidents. Closely monitor all reactor and catalyst temperatures during shutdowns. If the temperature exceeds the normal temperature by 15°C, the 0.7MPa/min emergency pressure reduction system will be activated immediately. If the reactor temperature cannot be controlled and rises above the normal value of 28°C or reaches 425°C, the 2.1MPa/min pressure reduction system will be started.

Is it necessary to depressurize the reaction system before cooling down? In order to keep the entire system stable, the pressure should not drop. When does the pressure reduction operation usually start during normal shutdown? When the temperature drops, stop feeding, and then reduce the pressure after the oil cycle is completed.

First increase the volume and then increase the temperature, first cool down and then reduce the volume. This sentence has a premise and is a mantra in normal operation. In a shutdown, from my personal point of view, after the oil is stopped, the pressure and temperature must be reduced in a matching manner. In hydrogenation production, the most likely problem is shutdown. The temperature drops quickly and the system is prone to leaks, so the pressure must be lowered while cooling down.

The shutdown procedure of the hydrogenation unit of my unit is written like this, and it is summarized as follows:: 1. Cool down the reaction system while reducing the system capacity (normal processing capacity is 25t/h, reduce the capacity to 15t/h) ； Purpose: Avoid cooling down first and causing too much unqualified oil, ensure the reaction depth and airspeed, and try to produce more qualified products during the shutdown process. 2. The reaction system + fractionation system is put into long circulation and the reaction feed pump (low pressure) is stopped. ； Purpose: The second reverse catalyst of this device has requirements for the nitrogen content of the hydrogenated oil generated by the first reverse. First, the qualified oil of the fractionation system is put into the reaction system for circulation, so as to avoid (or reduce) the first cooling and causing the second reverse catalyst to be in a high nitrogen oil feed state for a long time and reduce the impact on the second reverse catalyst. 3. Introduce the shut-down diesel oil into the reaction system, and at the same time, the fractionation system sends circulating oil to the unqualified line to ensure 15t/h feed of the entire system and cool down the reaction system. ； Purpose: Use shutdown diesel to replace the hydrogenated oil in the entire system and perform preliminary cleaning of the catalyst. 4. When it is detected that all the fractionation system is diesel, the diesel feed will be stopped and the delivery of unqualified oil will be closed. ； 5. Cut off the reaction system + fractionation system circulation, withdraw oil from the system, and stop the new hydrogen compressor ； 6. The full amount of circulating hydrogen is circulated, and the system is heated up to carry out hot hydrogen extraction. ； 7. Cool down the temperature and pressure of the reaction system ； 8. Stop the reaction heating furnace 9. Stop the circulation hydrogen compressor 10. The system is reduced to normal pressure 12. Nitrogen replacement purge of the reaction system. I don’t know if this is appropriate? Please give me some advice from all the hydrogenation seniors! Thank you:handshake. This post was last edited by ぁ红蓝 pencilぁ on 2009-2-23 20:48 ]

my opinion: 2. The reaction system + fractionation system is put into long circulation, and the reaction feeds into the high-pressure pump ； Cut off the external supply, cycle for 3 to 4 hours, and reduce external delivery. 3. Stop the reaction system + fractionation system and put it into long circulation, stop the reaction feed high-pressure pump, and cool down the reaction system until the furnace is shut down. ； Runs for 8 hours. Purpose: The oil is circulated to bring out the qualified oil in the system. 4. Remove oil from the system and stop the new hydrogen compressor. ； 5. Stop the cycle hydrogen compressor 6. Lower the system to normal pressure 7. Replace and purge the reaction system with nitrogen

The first step you can take before stopping work: Maintain normal operating conditions and reduce the capacity (if allowed, the capacity can be reduced from the normal processing capacity of 25t/h to 15t/h) ； I guess you can't cycle with a processing capacity of 25t/h. Step 2: The reaction system + fractionation system is put into circulation (the circulating oil is not enough to supplement the raw materials or even diesel), and the raw materials can be pumped less or not at all. ； In this way, only the oil stored in the system is unqualified oil, and it has basically no impact on the secondary reverse catalyst. The third step is to cool down the reaction system ； You can start while changing the cycle. The next step is to introduce the diesel fuel, remove the oil, etc., and there are no big changes. I have a question. In the plan given by LZ, the reaction feed high-pressure pump is stopped first. Then how does the stopped diesel fuel enter the reaction system? I would like to know if there are other high pressure pumps. It is recommended that the so-called substandard oil and shutdown diesel oil can be stored separately as oil for the next start-up.

johnhwa is a great reminder! Sorry, instead of stopping the high-pressure feed pump first, the raw material feed pump should be stopped. This has been corrected. Thanks for the advice! :handshake

study* Yes, I always thought that cooling down first and then lowering the pressure was to prevent the material from vaporizing and damaging the catalyst! ?