Hydrogenation reactor

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The hydrogenation reactor is the core equipment of a hydrocracking unit; it operates under conditions of high temperature, high pressure, and in the presence of hydrogen (including H2S), and the materials fed into the reactor often contain impurities such as sulfur and nitrogen. Due to the harsh operating conditions of hydrogenation reactors, the development of such reactors has primarily focused on improving their safety. Therefore, whether in design or manufacturing, apart from emphasizing performance, it is also necessary to emphasize safety performance. 1. Factors affecting the hydrogenation process
1.1 Hydrogen partial pressure
Increasing the hydrogen partial pressure is beneficial for the progression of the hydrogenation reaction, thereby accelerating the reaction rate. At a constant reaction temperature and other conditions, pressure has a positive effect on the degree of conversion. The quality of the product is greatly affected by the hydrogen partial pressure. 1.2 Reaction temperature affects the reaction rate, as well as the distribution and quality of the products. 1.3 Space velocity Space velocity affects the volume of the reactor and the amount of catalyst required; reducing the space velocity is beneficial for increasing the conversion rate in the hydrogenation process. 1.4 Hydrogen-to-oil ratio The effect of the hydrogen-to-oil ratio on the hydrogenation process mainly manifests in three aspects: it affects the reaction process ； Affecting the catalyst's service life ； An excessively high hydrogen-to-oil ratio will increase the operating costs and equipment investment of the facility. 2. What are the main types of damage that can occur in hydrogenation reactors? 2.1 High-temperature hydrogen corrosion Under high-temperature and high-pressure operating conditions, hydrogen that penetrates and diffuses into steel undergoes a chemical reaction with dissolved carbon or unstable carbides, forming methane ； That is, Fe3C + 4 → CH4 + 3Fe. The main factors affecting high-temperature hydrogen corrosion include the effects of temperature, pressure, and exposure time, the influence of alloying elements and impurity elements, the effect of heat treatment, and the effect of stress. 2.2 Hydrogen embrittlement Hydrogen embrittlement is a phenomenon of embrittlement caused by the retention of hydrogen in steel. In steel materials that have undergone hydrogen embrittlement, the elongation and reduction of area decrease significantly. 2.3 Corrosion by H2S at high temperatures: In the presence of both hydrogen sulfide and hydrogen, the corrosion of steel is more severe and intense than when hydrogen sulfide is present alone. Its corrosion rate generally increases with rising temperature. 2.4 Polysulfide stress corrosion cracking: A cracking phenomenon that occurs due to the combined effect of polysulfides (H2SxO6, x = 3–6) and tensile stresses present in the material subjected to their action. 2.5 Tempering brittleness of Cr-Mo steel: When Cr-Mo steel is maintained at temperatures between 325–575°C for an extended period of time or cooled slowly from these temperatures, its fracture toughness deteriorates. This occurs because trace impurity elements and alloying elements in the steel accumulate at the boundaries of the original austenite grains, thereby reducing the cohesion between those grain boundaries. 2.6 Delamination of the austenitic stainless steel surfacing layer: The Cr-Mo steel used in the reactor body and the austenitic stainless steel employed for the surfacing layer have different hydrogen solubilities and diffusion rates, resulting in very high hydrogen concentrations on the surfacing layer side of the transition zone ； Under high-temperature and high-pressure operating conditions, hydrogen penetrates into the reactor walls; during shutdown, hydrogen escapes from these walls. This leads to the delamination of the austenitic stainless steel surfacing layer. 2. Design methods for hydrogenation reactors. The main design methods include conventional design and analytical design. 2.1 Conventional design method: Conventional design is based on the elastic failure criterion; applicable codes include ASME’s Boiler and Pressure Vessel Code, Volume VIII, Part 1, in the United States, as well as China’s GB150–2011 Standard for Pressure Vessels. The main computer-aided software for conventional design includes: PVELite-2017 for ASME codes, and SW6-2011 for GB150 standards. 2.2 Analytical design method: This approach is based on plastic failure criteria; the relevant standards include Volume II of Part VIII of the American ASME Boiler and Pressure Vessel Codes, as well as China’s JB4732 \"Steel Pressure Vessels – Standards for Analytical Design\". ““Analytical design” requires a detailed calculation of stresses at relevant parts of the reactor, classification of these stresses based on their nature, and evaluation of each type of stress as well as their combinations. Additionally, it imposes stricter requirements on materials, manufacturing, and inspection compared to “conventional design”. This, in turn, enhances the accuracy and reliability of the design; however, it also leads to an increase in design costs. The main computer-aided software for analysis and design includes ANSYS Classic and the Workbench finite element stress analysis software. 3. What considerations are necessary when designing a hydrogenation reactor? 3.1 High-temperature hydrogen corrosion ① In terms of material selection. a) Select materials that can resist hydrogen corrosion correctly, strictly based on the latest version of the Nelson curve ； b) Minimize the impurity elements in steel that have an adverse effect on hydrogen corrosion (such as Sn, Sb) ； c) The main material shall be treated by normalizing (accelerated cooling is allowed) + tempering; quenching is not permitted. ②In heat treatment. The equipment must undergo post-weld heat treatment. ③In terms of operating instructions. a) It is strictly prohibited for the equipment to overheat ； b) Control the level of applied stress (avoid sudden temperature increases or decreases, as well as sudden changes in pressure). 3.2 Hydrogen embrittlement ① The sensitivity to hydrogen embrittlement generally increases as the strength of the steel increases ； The strength of the steel shall not exceed the values specified in the design. It is also important to reduce the hardness of the weld heat-affected zone; therefore, the design documents should specify the hardness of the weld and its heat-affected zone. ②The microstructure of steel has an impact on hydrogen embrittlement, and macroscopic defects are eliminated through non-destructive testing. ③Residual stress has an impact on hydrogen embrittlement, and residual stress is eliminated through heat treatment. 3.3 High-temperature H2S corrosion: The inner wall is surfacing-welded with TP309L+TP347 austenitic stainless steel (austenitic stainless steel has excellent resistance to high-temperature H2S corrosion). 3.4 Stress corrosion cracking in polydisulfate environments ① Use ultra-low carbon or stabilized stainless steels, such as Tp321 and Tp347. ②In structural design, structures prone to stress concentration should be avoided as much as possible. 3.5 Tempering brittleness of Cr-Mo steel: During design, it is necessary to strictly control the impurity elements (such as P, Sn, As, Sb) and certain alloying elements (such as Si, Mn, etc.) in the material’s chemical composition. 3.6 Delamination of the surfacing layer in austenitic stainless steel: ① In terms of material selection. Improvements are made in terms of the material properties; for example, the carbon content in the Cr-Mo steel base material is reduced, and elements such as V are added to compensate for any possible decrease in strength resulting from this reduction in carbon content, while the appropriate amount of Nb in the Tp.347 surfacing metal is controlled, etc ； 4. What considerations are needed in the manufacturing of hydrogenation reactors? 4.1 High-temperature hydrogen corrosion: In manufacturing, alkaline electrodes should be used, the moisture content of the electrodes must be controlled, and the welding materials require tests for diffused hydrogen in the deposited metal. During welding, preheating before welding is necessary, the interpass temperature must be controlled, and degassing is required after welding. The equipment must undergo post-weld heat treatment. 4.2 Hydrogen embrittlement In manufacturing, a) minimize the strain amplitude, reduce thermal stress, and avoid stress concentration. For example, increase the chamfering, use a smoothly transitioning structure, and control the weld bead height. b) Try to maintain high ductility in the surfacing metal or welded metal of Tp.347. Control the δ-ferrite content in the surfacing metal of Tp.347 stainless steel to be less than 10% (with a lower limit of greater than 3%) ； For areas prone to hydrogen embrittlement, TP.347 must be applied after PWHT. Specify appropriate post-weld heat treatment specifications to minimize the amount of δ-ferrite that transforms into σ-phase. 4.3 High-temperature H2S corrosion: In terms of manufacturing, the manufacturer shall re-test the sulfur and phosphorus content in the materials, strictly control the hardness of the welds and the heat-affected zones of the base metal, and measure the hardness values both before and after heat treatment. After the equipment is welded, in addition to a 100% RT inspection of the weld joints, a 100% UT inspection should also be carried out to ensure that there are no defects in the welds. 4.4 Stress corrosion cracking in polydisulfuric acids: In manufacturing, residual stresses caused by cold working and welding should be eliminated or reduced as much as possible, and care should be taken to design the structure in such a way that no stress concentrations occur, or that any existing stress concentrations are kept as small as possible. Stamping on the welds is not allowed; forced assembly is strictly prohibited. Appropriate heat treatment processes should be employed to eliminate welding stresses and cold working stresses. 4. Tempering brittleness of Cr-Mo steel: Efforts should be made to minimize the number of heat treatment steps during manufacturing, in order to reduce the impact of these steps on the tempering brittleness of Cr-Mo steel. At the same time, simulated heat treatment specimens that reflect the entire heat treatment process should be used to evaluate the various mechanical properties of the material, thereby assessing the damage caused by this entire heat treatment process to the material. 4.6 Peeling of the austenitic stainless steel surfacing layer: Conduct process evaluations for surfacing manufacturing, select an appropriate surfacing method, and use a surfacing technique with high current and high welding speed. (Research results indicate that weld overlays produced by surfacing at a high welding speed exceeding 20 cm/min, using an appropriate dilution rate of approximately 25%, exhibit good resistance to spalling. In short, efforts should be made to avoid the formation of coarse grains during surfacing. Appropriate post-weld heat treatment conditions should be selected (the post-weld heat treatment temperature and holding time have a significant impact on the resistance to peeling) (isothermal dehydrogenation).