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As a product of petroleum refining, diesel accounts for a significant share in the fuel mix across various countries, and has become an important power fuel. With the accelerated development of the world economy and the increasing number of vehicles powered by diesel, demand for diesel is expected to rise over time. However, the exhaust gases produced by diesel combustion pose an increasingly serious threat to the environment. To this end, the major industrialized countries and regions around the world are imposing increasingly strict requirements on diesel quality. The future trend in diesel quality will be toward continuously reducing sulfur content until it becomes sulfur-free, further lowering the levels of aromatics and polycyclic aromatics as well as diesel density, while simultaneously increasing the cetane number. Reducing the sulfur content in diesel is key to improving its quality. The main technologies for lowering diesel sulfur content include hydrodesulfurization, oxidative extraction, biological desulfurization, and adsorption desulfurization; among these, hydrodesulfurization of diesel remains the primary and most effective technique for producing ultra-low sulfur diesel at present. Major oil companies around the world, including Yabao, Standard, Topsoe, Axens, Advanced Refining Technologies, and Sinopec, have actively developed diesel distillate hydrogenation technologies to produce diesel that meets Euro IV and higher emission standards. Notable among these technologies are: ExxonMobil’s DODD technology for ultra-deep desulfurization of diesel, Albemarle’s technology for ultra-deep hydrogenation desulfurization of diesel, Shell’s SMDH technology for hydrogenation of intermediate distillates, IFP’s Prime-D technology for deep hydrogenation desulfurization of diesel, as well as precious metal-based two-stage technologies for deep hydrogenation desulfurization of diesel developed by companies such as Topsoe, IFP, Criterion, UOP, ExxonMobil, Chevron, and Albemarle. In recent years, in the field of deep desulfurization of diesel, the Fushun Research Institute of Petrochemical Technology (FRIPP) and the Research Institute of Petrochemical Science (RIPP) under Sinopec have both developed several new process technologies, some of which have already been put into industrial use. The FCSH diesel reverse-flow hydrogenation process for deep desulfurization and dearomatization can operate in either a single-stage reverse-flow mode or in a series configuration with one stage in co-current operation and the second stage in reverse-flow operation. In this process, fresh hydrogen or hydrogen from the circulating hydrogen desulfurization tower enters the reactor from the bottom, while the feed oil enters from the top; the oil and gas come into contact in opposite directions to carry out the reaction. The upward flow of gas helps to remove the harmful gases H2S and NH3 generated during the reaction from the catalyst bed, allowing those substances that are most difficult to react with to do so under relatively clean reaction conditions, thereby **increasing the depth of desulfurization and dearomatization reactions. At the same time, in conventional gas-liquid co-current processes, the exothermic nature of the hydrogenation reaction causes the temperature of the reaction stream to increase as one moves toward the reactor outlet, which is unfavorable for deep dearomatization of diesel. In contrast, in counter-current reactors, cold hydrogen enters the reactor, resulting in a temperature decrease from top to bottom along the axial direction, thereby effectively overcoming this drawback. Therefore, reverse flow hydrogenation has now become an effective method for producing ultra-low sulfur diesel, and it is a new refining technology that oil companies around the world are competing to develop. Compared with conventional processes, the one-series process of FCSH offers significant advantages in terms of ultra-deep desulfurization efficiency, aromatic saturation capacity, density reduction, and increase in cetane number, enabling it to better meet the requirements for producing diesel that complies with Euro V emission standards. The FDAS two-stage deep desulfurization and dearomatization process: The FDAS two-stage deep desulfurization and dearomatization process developed by FRIPP is a new technique designed for catalytic diesel with high processing density, high aromatic content, and low cetane number. Its main objectives are to remove aromatics, reduce density, and increase the cetane number. FDAS technology enables hydrorefining in the first stage using conventional catalysts under moderate pressure, followed by deep hydrogenation and saturation in the second stage with non-precious metal catalysts. This approach achieves deep hydrogenation desulfurization and dearomatization of diesel, as well as an increase in its cetane number, resulting in low-sulfur, low-aromatic diesel. The adsorption active centers for nitrides formed during the hydrorefining process are the same as those for aromatics, and the adsorption strength of nitrides on these catalytic active centers is much greater than that of aromatics. This leads to competitive adsorption with aromatics, thereby inhibiting the saturation reaction of these aromatics. By using a two-stage process, the levels of organic nitrides, ammonia, hydrogen sulfide, etc. in the second stage are significantly reduced, which facilitates the saturation reaction of aromatics. The desulfurization efficiency, aromatic saturation capacity, and increase in cetane number of the FDAS two-stage deep desulfurization and dearomatization process are all significantly higher than those of the conventional hydrorefining process. Moreover, its aromatic saturation capacity and cetane number increase are also greater than those of the MCI single-stage two-agent process for maximizing cetane number improvement. The SRH diesel liquid-phase cyclic hydrogenation process: The downflow-type SRH diesel liquid-phase cyclic hydrogenation technology developed by FRIPP does not include a hydrogen circulation system in its reaction section; instead, it relies on the dissolved hydrogen carried into the reaction system through the large-scale circulation of liquid-phase products to provide the hydrogen required for the hydrogenation reaction. The reactor used in this process has a structure similar to that of a trickle-bed reactor. The advantage of the SRH liquid-phase cyclic hydrogenation technology is that it can eliminate the influence of the catalyst’s wetting factor. Due to the high specific heat capacity of the circulating oil, it **reduces the temperature rise in the reactor, improves the efficiency of catalyst use, and can minimize side reactions such as cracking. It features fewer high-pressure components, resulting in lower heat loss; both its investment costs and operating expenses are lower than those of conventional hydrorefining. It is one of the technologies that enables the improvement of oil quality at low cost. The continuous liquid-phase cyclic hydrogenation process RIPP – the continuous liquid-phase cyclic hydrogenation technology developed in this context – has a main reaction mechanism and process range that are similar to those of the SRH upflow liquid-phase hydrogenation technology developed by FRIPP. There are two main differences: 1) an upflow reactor is used; 2) a heat and high-pressure stripping separator is added. (1) Upward-flow reactor: In continuous liquid-phase hydrogenation processes, in order to ensure the smooth progress of the hydrogenation reaction while reducing the flow rate of the circulating oil, it is necessary to maximize the saturation of dissolved hydrogen in the liquid phase of the reactor; therefore, a small amount of hydrogen needs to be present in gaseous form at the reactor outlet. At this point, the liquid phase in the reactor is the continuous phase, while the gas phase is the dispersed phase. To prevent the gas in the dispersed phase from accumulating in certain areas of the reactor and affecting the uniformity of the flow of the gas and liquid reactants, an upward-flowing reactor is the best choice. In an upward-flow reaction, the gas and liquid phases of the reactant stream flow through the catalyst bed from bottom to top; the flow direction of the medium is consistent with the gas diffusion direction, which minimizes the possibility of local accumulation of gas within the reactor and helps to distribute a small amount of hydrogen evenly. At the same time, compared with downward-flow reactors, upward-flow reactors have a smaller bed spacing, require fewer internal components, possess a higher catalyst loading capacity, entail less work for maintenance and installation, thus saving on equipment investment and operating costs; moreover, they have a lower pressure drop, which reduces energy consumption. (2) Hot-high-pressure stripping separator: In continuous liquid-phase hydrogenation technology, the products of the hydrogenation reaction enter the hot-high-pressure stripping separator directly after leaving the reactor, without going through heat exchange for cooling. This not only reduces heat loss during the heat exchange process but also ensures a high solubility of hydrogen at high temperatures. By adding a small number of specially designed trays to conventional thermal high-pressure separators, and optionally using hot hydrogen stripping when necessary (such as when processing high-sulfur crude oils to produce ultra-low-sulfur diesel products), the levels of H2S and NH3 in the circulating oil, which inhibit reactions, can be reduced. Meanwhile, due to the certain skirt height of the thermal high-pressure stripping separator, the reaction product circulation pump at its bottom will not experience cavitation issues. RTS Diesel Ultra-Deep Hydrodesulfurization Process: The RTS diesel ultra-deep desulfurization technology developed by RIPP utilizes one or two non-precious metal hydrorefining catalysts, and employs a two-stage, single-pass process to carry out the ultra-deep hydrodesulfurization of diesel in two reactors. The first reactor carries out deep desulfurization and denitration reactions at higher temperatures, with the removal of most sulfides that are easy to remove and almost all nitrogen compounds taking place in this first reactor. The second reactor, which has had the nitrogen compounds removed from it, operates at lower temperatures to completely hydrogenate and saturate the remaining sulfides, thereby improving the color of the oil product. By utilizing RTS technology and optimizing the process flow and operating conditions, ultra-low sulfur diesel products with a sulfur content of less than 50 μg/g, or even less than 10 μg/g, can be produced from feedstocks primarily consisting of high-sulfur straight-run diesel at an space velocity that is more than 50% higher than that of conventional hydrorefining processes.