Process Section – Daily Hydrogenation Process Knowledge Question No. 018

Since the 2.25Cr-1Mo steel used in manufacturing hot-wall hydrogenation reactors is a type of Cr-Mo steel with a high sensitivity to temper embrittlement, and the operating temperature of such reactors remains within the temper embrittlement range of 325–575°C for extended periods of time. Therefore, once a hot-wall hydrogenation reactor is put into operation, temper embrittlement of its materials is inevitable. During the start-up and shutdown of the reactor, when the wall temperature is low, the toughness of the wall material can decline significantly due to the combined effect of hydrogen embrittlement and temper embrittlement. At this point, if the stress level in the reactor walls is high, it is possible to trigger a brittle failure accident. To prevent such accidents, the usual measure taken is to set a minimum pressure rise temperature for the reactor. That is, when the temperature inside the reactor is below the minimum pressure-raising temperature, the internal pressure cannot exceed the preset pressure limit. For hydrocracking reactors, it is generally specified that when the bed temperature is below 135°C, the pressure must not exceed 1/3 of the reactor’s design pressure. During the operation of a hot-wall hydrogenation reactor, the degradation of its material gradually increases as operating time progresses, which means that the pressure-limiting and temperature-raising measures that were considered safe at the beginning of operation can become dangerous later in the reactor’s service life. Therefore, it is highly important to accurately assess the safety status of reactor operation based on the degradation condition of its materials, and to determine an appropriate minimum pressurization temperature, in order to ensure the long-term safe use of hot-wall hydrogenation reactors.

Why is the reactor heated first and then pressurized? I don’t understand what back pressure means.

Since the 2.25Cr-1Mo steel used in manufacturing hot-wall hydrogenation reactors is a type of Cr-Mo steel with a high sensitivity to temper embrittlement, and the operating temperature of such reactors remains within the temper embrittlement range of 325–575°C for extended periods of time. Therefore, once a hot-wall hydrogenation reactor is put into operation, temper embrittlement of its materials is inevitable. During the start-up and shutdown of the reactor, when the wall temperature is low, the toughness of the wall material can decline significantly due to the combined effect of hydrogen embrittlement and temper embrittlement. At this point, if the stress level in the reactor walls is high, it is possible to trigger a brittle failure accident.

Since the 2.25Cr-1Mo steel used in manufacturing hot-wall hydrogenation reactors is a type of Cr-Mo steel with a high sensitivity to temper embrittlement, and the operating temperature of such reactors remains within the temper embrittlement range of 325–575°C for extended periods of time. Therefore, once a hot-wall hydrogenation reactor is put into operation, temper embrittlement of its materials is inevitable. During the start-up and shutdown of the reactor, when the wall temperature is low, the toughness of the wall material can decline significantly due to the combined effect of hydrogen embrittlement and temper embrittlement. At this point, if the stress level in the reactor walls is high, it is possible to trigger a brittle failure accident.

Since the 2.25Cr-1Mo steel used in manufacturing hot-wall hydrogenation reactors is a type of Cr-Mo steel with a high sensitivity to temper embrittlement, and the operating temperature of such reactors remains within the temper embrittlement range of 325–575°C for extended periods of time. Therefore, once a hot-wall hydrogenation reactor is put into operation, temper embrittlement of its materials is inevitable. During the start-up and shutdown of the reactor, when the wall temperature is low, the toughness of the wall material can decline significantly due to the combined effect of hydrogen embrittlement and temper embrittlement. At this point, if the stress level in the reactor walls is high, it is possible to trigger a brittle failure accident.

Also, pressure is easy to control, while temperature is not. Because the pressure gauge responds quickly, while the temperature has too strong a lag

The chromium-molybdenum steel used in the hydrocracking unit prevents the occurrence of temper brittleness, and a hot-state start-up and shutdown procedure is adopted, that is, temperature is increased first followed by pressure increase during start-up, while pressure is reduced first followed by temperature reduction during shutdown. For 21/4Cr-lMo alloy steel, the minimum heating temperature is 93.

Since the 2.25Cr-1Mo steel used in manufacturing hot-wall hydrogenation reactors is a type of Cr-Mo steel with a high sensitivity to temper embrittlement, and the operating temperature of such reactors remains within the temper embrittlement range of 325–575°C for extended periods of time. Therefore, once a hot-wall hydrogenation reactor is put into operation, temper embrittlement of its materials is inevitable. During the start-up and shutdown of the reactor, when the wall temperature is low, the toughness of the wall material can decline significantly due to the combined effect of hydrogen embrittlement and temper embrittlement. At this point, if the stress level in the reactor walls is high, it is possible to trigger a brittle failure accident. To prevent such accidents, the usual measure taken is to set a minimum pressure rise temperature for the reactor. That is, when the temperature inside the reactor is below the minimum pressure-raising temperature, the internal pressure cannot exceed the preset pressure limit. For hydrocracking reactors, it is generally specified that when the bed temperature is below 135°C, the pressure must not exceed 1/3 of the reactor’s design pressure. During the operation of a hot-wall hydrogenation reactor, the degradation of its material gradually increases as operating time progresses, which means that the pressure-limiting and temperature-raising measures that were considered safe at the beginning of operation can become dangerous later in the reactor’s service life. Therefore, it is highly important to accurately assess the safety status of reactor operation based on the degradation condition of its materials, and to determine an appropriate minimum pressurization temperature, in order to ensure the long-term safe use of hot-wall hydrogenation reactors.

Due to the prolonged operation of chromium-molybdenum steel at 370–575°C, the material becomes brittle. Therefore, this type of steel is prone to brittle fracture at temperatures below 121°C. So generally, when the temperature is below 121°C, the pressure in chromium-molybdenum steel equipment is limited to a range such that the resulting stress does not exceed 20% of the material’s yield strength. Considering the difference between the temperature inside the reactor and the temperature of its outer wall, this temperature was increased to 135°C.