Hazard factors and preventive measures during the shutdown of hydrogenation units
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1. Cool down and reduce the flow rate of the reaction system – The hydrogenation unit is shut down by first cooling down and reducing the flow rate of the reaction system. In this process, the principle of cooling first and then reducing the amount should be followed. A decrease in the feed rate to the reaction system leads to a reduction in space velocity, an increase in the temperature rise within the hydrogenation reactor, and an increased risk of reaction \"temperature runaway\". The so-called “rapid temperature rise” refers to the phenomenon of the reactor temperature increasing rapidly to an uncontrollable level. 2. Replace the entire system with low-freezing-point feedstocks. The feed oil used in hydrogenation units is generally heavy and has a high freezing point; it tends to solidify within catalysts, pipelines, and equipment during shutdowns. To avoid the above situations, the system should be replaced with low-viscosity oil before shutdown; the low-freezing-point oil used is generally normal paraffin oil. 3. Stop the feed pump for the reaction mixture; when shutting off the reaction feed, it is important to ensure that the temperature of the reactor remains appropriate, so as to prevent any significant rise in temperature in the cracking reactor. 4. Oil and hot hydrogen circulation in the reaction system for stripping: After stopping the reaction feed, the reaction heater is heated, and hot circulating hydrogen is used to remove the oil remaining in the catalyst. The temperature for hot hydrogen stripping should be determined based on the requirements of the catalyst; it is generally around 200°C. The temperature must not be too high, in order to prevent the catalyst from being reduced by the hot hydrogen. 5. Cool down and reduce pressure of the reaction system: The hydrogenation reaction system is cooled down and its pressure is reduced at the required rate. 6. Reaction system N: Substitution. The reaction system is denoted by N, which is substituted with N: Environment, thereby changing the hydrogen and hydrocarbon concentration in the system
Risk factors during shutdown include high-pressure and high-temperature substances remaining in heaters, reactors, and other equipment, which can lead to explosions and fires； The catalyst may oxidize or catch fire on its own during shutdown periods ； Pipes and equipment may be at risk of corrosion and leakage. To prevent these dangers, the following measures need to be taken: 1. Ensure that the temperature and pressure inside the equipment are reduced to safe levels, by gradually decreasing the supply volume and turning off the heating system. 2. Use low-freezing-point crude oil to flush the entire system in order to prevent the crude oil from freezing during shutdowns. 3. When stopping the reaction feed, maintain an appropriate reactor temperature to prevent uncontrolled temperature rises. 4. After cutting off the feed, use hot cyclic hydrogen to carry away the oil remaining in the catalyst, ensuring that the catalyst is not reduced by hot hydrogen. 5. Cool down and reduce pressure at the required rate to ensure that the system enters a shut-down state safely and stably. 6. Use nitrogen to purge the reaction system with a nitrogen atmosphere, reducing the concentration of hydrocarbons to