Pressure and temperature regulation in the hydrogenation system

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Discussion on heating and pressurizing hydrogenation units: Recently, many colleagues from Sichuan have asked questions regarding the heating, pressurizing, cooling, and depressurizing of hydrogenation systems. This process may seem simple, but in fact, many accidents in hydrogenation units are more or less related to it. Let’s discuss together the operations of heating, cooling, pressurizing, and depressurizing in hydrogenation reactions. The requirements for increasing temperature and pressure are primarily constrained by the materials used in the equipment. After the installation is completed, it is necessary to discuss issues related to low-temperature toughness with the material suppliers, and then develop appropriate operating procedures; it is absolutely not permissible to simply adopt the operating procedures of other hydrogenation units! ! ! ! A concept that must be mentioned here is hydrogen embrittlement. Hydrogen embrittlement occurs when hydrogen dissolved in steel combines to form hydrogen molecules, resulting in stress concentration that exceeds the steel’s strength limit; this leads to the formation of tiny cracks within the steel, also known as white spots. Hydrogen embrittlement can only be prevented, not cured. Once hydrogen embrittlement occurs, it cannot be eliminated. Trace amounts of hydrogen (on the order of 10 to the negative sixth power) that enter the steel during its smelting process and during the manufacturing and assembly of parts (such as electroplating and welding) can cause the material to become brittle or even crack under the effect of internal residual stresses or external stresses. The properties of the steel can be restored through dehydrogenation treatment (for example, by heating to over 200°C for several hours to reduce internal hydrogen) before cracking occurs. Therefore, internal hydrogen embrittlement is reversible. The occurrence of hydrogen embrittlement is an important cause of damage to hydrogenation equipment. Procedures for Pressurization/Depressurization and Temperature Adjustment of the Reactor. These procedures were established because the shell of the hydrogenation reactor is primarily made of Cr-Mo steel, and over time, under high temperatures and in a hydrogen-rich environment, it may develop varying degrees of temper embrittlement, which can lead to cracking incidents during the start-up and shutdown phases of the plant. To ensure the safe operation and extended service life of the hydrogenation reactor, these procedures have been formulated. Procedures for Pressurization/Depressurization and Temperature Adjustment of the Reactor. These procedures were established because the shell of the hydrogenation reactor is primarily made of Cr-Mo steel, and over time, under high temperatures and in a hydrogen-rich environment, it may develop varying degrees of temper embrittlement, which can lead to cracking incidents during the start-up and shutdown phases of the plant. To ensure the safe operation and extended service life of the hydrogenation reactor, these procedures have been formulated. 1. Scope of Application 1.1 These regulations apply to the operational restrictions regarding pressure and temperature changes in the hydrogenation reactor during the start-up and shutdown phases of hydrogenation units, as well as the related requirements. 1.2 These regulations apply to the general pressure and temperature control limitations for operating reactors made of 1.25Cr-0.5Mo-Si or 2.25Cr-1Mo steel in hydrogenation and reforming units of oil refineries, as well as in coal liquefaction units. Similar reactor operations in other refineries can also be referred to for implementation. 2. Regulations for pressurization, depressurization, and temperature adjustment of the reactor 2.1 Pressurization limits for the reactor 2.1.1 During the first start-up of the reactor, at any stage – whether it is the gas-tightness testing, drying, sulfidation, or normal operation phase – the operating pressure shall not exceed 1/4 of the design pressure until the lowest temperature on the reactor wall (as measured by surface thermocouples) reaches 50°C. Only after this temperature is reached may the operating pressure be gradually increased to the normal operating level, with an increase rate not exceeding 2.8 MPa/h (i.e., the MPT for this stage is 50°C). 2.1.2 When starting up again after one operating cycle, during the pressurization process, the operating pressure shall not exceed 1/4 of the design pressure until the lowest temperature on the reactor wall (as measured by surface thermocouples) reaches 93°C. Only after this temperature is reached may the operating pressure be gradually increased to the normal operating level, with an increase rate not exceeding 2.8 MPa/h (i.e., the MPT for this stage is 93°C). 2.2 Reactor depressurization limits 2.2.1 During the first shutdown of the reactor, before the lowest temperature of the reactor wall (measured by surface thermocouples) drops to 50°C, the operating pressure must be reduced to less than 1/4 of the design pressure. 2.2.2 After one operating cycle and before shutting down, the operating pressure must be reduced to less than 1/4 of the design pressure until the lowest temperature on the reactor wall (as measured by surface thermocouples) drops to 93°C. 2.3 Reactor temperature rise limit: To avoid high thermal stress differences, the temperature difference between any two surface thermocouples must be strictly controlled. If the distance between surface thermocouples is less than 2.5 (R×T)1/2, the maximum temperature difference should not exceed 28°C. In the formula: R—reactor radius ； T—Reactor wall thickness. The above conditions can generally be met by controlling the heating rate of the feed and the feed temperature, as shown in Table 2.3. Table 2.3 Control of the heating rate and feed temperature. Minimum temperature on the reactor wall (measured by surface thermocouples): 50°C. Heating rate of the feed