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Classification of hydrocracking process flows: The processes of fixed-bed hydrocracking can be generally divided into three categories: two-stage hydrocracking process, single-stage hydrocracking process, and one-stage in-series hydrocracking process. Processes such as one stage of hydrocracking followed by two stages (with unconverted oil separately) and other processes have evolved from these three basic processes. ⑴Two-stage hydrocracking process: The two-stage hydrocracking method utilizes two reactors, and it has been used for the hydrocracking of coal and its derivatives since the early 20th century. The crude oil is first subjected to hydrorefining (HDS, HDN, HDO, olefin saturation HDA along with partial conversion) in the first reactor, after which it enters a high-pressure separator for gas/liquid separation ； The hydrogen-rich gas separated from the top of the high-pressure section is reused in the first stage, while the effluent from the bottom of the high-pressure section enters a distillation tower where it is separated into products such as naphtha, jet fuel, and diesel ； The unconverted oil at the bottom of the tower enters the second reactor for hydrocracking ； The reaction effluent from the second stage enters the high-pressure section of this stage for gas/liquid separation; the hydrogen-rich gas obtained from the top of this section is recycled in the second stage ； The effluent at the bottom of the second high-pressure section and the effluent at the bottom of the first high-pressure section enter the same distillation column for product separation. The two-stage hydrocracking process has the following characteristics: ① The reactors of the first and second stages, as well as the high-pressure fractionator and recycled hydrogen (including the recycle compressor), form an independent system ； ②Shared hydrogen booster and two sections of the product fractionation tower ； ③The process flow is relatively complex, with higher investment and energy consumption ； ④It has strong adaptability to crude oil, high production flexibility, and a long operational cycle. (2) Single-stage hydrocracking process: The single-stage hydrocracking method uses one reactor to carry out both HDS, HDN, HDO, olefin saturation, and HDA reactions on the feed oil, as well as the hydrocracking reaction itself. It can be operated either by one-pass processing or by cyclic cracking of unconverted oil. Its characteristics are: ① Simple process flow, with a relatively high space velocity ； ②The catalyst should have strong resistance to compounds such as S, N, and O ； ③The nitrogen content of the crude oil should not be too high, and the distillate should not be too heavy ； ④The reaction temperature is relatively high, and the operation cycle is relatively short. (3) One-stage series hydrocracking process: The one-stage series hydrocracking process involves the operation of two reactors in series. After deep hydrodenitration of the crude oil in the first reactor (refining section), its reaction stream enters directly the second reactor (cracking section) for hydrocracking. The stream at the outlet of the cracking section undergoes heat exchange and air/water cooling before entering the high-pressure and low-pressure separators for gas/liquid separation. The hydrogen-rich gas separated from the top of the high-pressure separator is recycled, while its liquid distillate goes to the low-pressure separator for further gas/liquid separation. The liquid effluent with a low boiling point is sent to the fractionation system for product separation and fractionation; the unconverted oil at the bottom of the tower is returned (or partially returned) to the cracking section for cyclic cracking, or it is discharged from the plant as a feedstock for downstream units. See Figure 4-3. A series of hydrocracking units is characterized by the following: ① The catalysts in the refining section should possess high hydrogenation activity (especially HDN activity) ； ②The catalyst in the cracking section should have resistance to H2S and NH3 ； ③Good product quality, high production flexibility, long operating cycle per cycle ； ④Compared to a conventional FCC unit, it has a greater tolerance for feedstocks, with relatively lower space velocity and reaction temperature ； ⑤Compared with two-stage hydrocracking, its investment and energy consumption are relatively lower. (4) One stage followed by two stages (unconverted oil separately) of hydrocracking: This involves one refining reactor and two cracking reactors; the refined oil is subjected to hydrocracking in the first cracking reactor, while its unconverted portion is hydrocracked in the second cracking reactor, representing a special form of one stage followed by multiple stages. Whether one or two stages of cracking are used in series mainly depends on the scale of the plant, as well as the manufacturing and transportation conditions of the reactors. If the refinery is located near the coast, and the size of the reactors is not constrained by related transportation conditions, it is more economical to use two large reactors in series. If constrained by transportation conditions such as ports, docks, roads, bridges, and culverts, using two-stage cracking with three reactors can save 5–10% in investment compared to using two parallel series (a total of 4 reactors). At that time, Union Oil believed that a single-stage cracking process was suitable for plants with a capacity of less than 1 million tons per year ； For plants with a capacity of 1–1.5 million tons per year, it is appropriate to adopt a single-stage in-series (two-stage cracking) process. In recent years, with the research and development of hydrocracking technology, some changes have taken place in the existing process flows; the diagram below shows a typical such flow. HyCycle hydrocracking process flow