Review of Clean Diesel Hydrodesulfurization Technologies

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　　As an important fuel for vehicles, diesel combustion produces exhaust gases containing sulfur oxides (SOx), nitrogen oxides (NOx), and particulate matter, which are major causes of air pollution. The release of SOx into the atmosphere not only leads to acid rain, but also contributes to the formation of PM2.5 particles, thereby exacerbating smog. Therefore, diesel standards worldwide are becoming increasingly stringent, and the production of environment-friendly low-sulfur or ultra-low-sulfur diesel has become an issue that countries around the world as well as oil refining companies pay close attention to. The EU **implemented Euro V emission standards starting in 2009; these standards limit the sulfur content in diesel products to 10 μg/g or less. Our country has advanced the nationwide implementation date for National Standard V for vehicle gasoline and diesel to the end of 2016, which is one year earlier than the originally scheduled date. The National V standard for diesel fuel in vehicles also stipulates that the sulfur content in such fuel must not exceed 10 μg/g; this requirement is on par with the current EU standards. Beijing was the first region to implement its own emission standard, known as Jing V, which is equivalent to National V standards, starting in 2012. Nationwide, National V standards for diesel and gasoline will come into effect in January 2017, requiring that the sulfur content in diesel fuel not exceed 10 μg/g and that its cetane number be at least 51. 　　The sulfur compounds contained in diesel mainly include aliphatic sulfides, thioethers, dibenzothiophene (DBT), alkylbenzothiophenes, and alkyldibenzothiophenes. Oil desulfurization technologies are divided into non-hydrodesulfurization and hydrodesulfurization (Hydrotreating Desulfurization, HDS). Non-hydrodesulfurization technologies mainly include adsorption desulfurization, oxidation desulfurization, extraction desulfurization, and biological desulfurization. HDS technology involves using a desulfurization catalyst to carry out reactions under high temperature and pressure, thereby converting sulfides into H2S for separation. Compared to other desulfurization technologies, the HDS technology is more mature. For high-sulfur oils, this technology can significantly reduce the sulfur content. At the same time, HDS technology is flexible to operate, yields high-purity oil with excellent color, and can effectively remove difficult-to-remove sulfides such as thiophenes. 　　The main foreign diesel HDS technologies are listed below. 　　1 Advances in Overseas Diesel HDS Technology 　　1.1 Albemarle Corporation in the United States 　　In 2004, Albemarle Corporation acquired Akzo Nobel’s refining catalyst business in the Netherlands and renamed it Albemarle Catalysts, becoming the world’s largest producer of hydroprocessing and catalytic cracking catalysts. The company has developed STARS and NEBULA catalyst technologies. The STARS catalyst technology (type II active reaction centers) is suitable for producing low-sulfur or ultra-low-sulfur clean fuels with a sulfur content of less than 50 μg/g. The catalysts that have been put into industrial use include KF-757, which uses Mo-Co as the active metal component, and KF-848, which uses Mo-Ni as the active metal component. Both KF-757 and KF-848 exhibit high hydrogenation desulfurization activity under various operating conditions. KF-757 is suitable for producing products with a sulfur content of less than 50 μg/g under medium and low pressure conditions, while KF-848 is suitable for producing clean fuels with a sulfur content of less than 10 μg/g under medium and high pressure conditions. As soon as these two catalysts were introduced, they were widely used in the European and American markets, becoming the main catalysts for producing low-sulfur diesel. Recently, the company has continued to utilize STARS technology to promote the desulfurization of difficult-to-treat sulfur-containing compounds by controlling the formation of active Co-Mo phases. It has developed the Co-Mo catalyst KF-767 for producing ultra-low-sulfur diesel; its relative volumetric activity for deep hydrodesulfurization is 30% to 50% higher than that of KF-757, enabling the sulfur content in diesel to be reduced to below 10 μg/g and the nitrogen content to below 20 μg/g. The KF series of catalysts introduced by Albemarle have all been improved upon the original catalysts, enabling them to process heavy-duty feedstocks. Examples include KF-860; another example is KF-770, which boasts a longer service life—its service life is 1.35 times that of KF-757. 　　The NEBU-LA catalyst technology adopts a completely new concept that is entirely distinct from traditional hydroprocessing catalyst technologies, and it offers hydrodesulfurization, hydrodenitration, and hydrodearomatization activities that are difficult to achieve with conventional hydroprocessing techniques; it is designed specifically as a catalyst for producing ultra-low sulfur diesel (ULSD). The NEBU-LA catalyst has been industrially applied in multiple plants around the world. The representative catalysts of NEBU-LA are mainly NEBULA-1 and NEBULA-20 catalysts. Among them, the hydrodesulfurization, nitrogen removal, and aromatic hydrocarbon conversion activities of the NEBULA-1 catalyst are much higher than those of other catalysts; it can operate at lower temperatures while maintaining the same hydrogenation activity as the KF catalyst. NEBULA-20 features a higher airspeed and more gentle operating conditions. The NEBULA series of catalysts is expensive, and it is often used in combination with KF series catalysts to achieve optimal hydrogenation results. 　　The core of the catalyst grading system developed by Yabao Company lies in the division of different reaction zones within the hydrorefining reactor. Appropriate catalysts are installed in each reaction zone based on the characteristics of that zone, and a series of catalysts tailored to the specific needs of each zone are developed in order to achieve a synergistic effect in catalytic activity. 　　 　　1.2 Haldor Topsoe A/S　The Danish company Topsoe has its own technological advantages in the two-stage combined process of ultra-deep desulfurization and dearomatization. Since the discovery of the Co–Mo–S nanoscale active structure, significant progress has been made in the development of this company’s catalysts. Through joint research by Topso Company, Aarhus University in Denmark, and the Danish Technical University, atomic-resolution images of the active Co-Mo-S (or Ni-Mo-S) structures were provided for the first time, demonstrating the hydrogenation capability of hydrogenation catalysts and explaining the importance of metal-support interactions as well as the origin of type I and type II active centers. Based on these theories, the BRIM technology was developed; this technology not only enhances the hydrogenation activity of the active sites at the edge but also increases the number of active sites for direct desulfurization. The combination of high hydrogenation activity and high direct desulfurization activity makes BRIM technology highly suitable for the production of ULSD. Topso has recently introduced two new hydrogenation catalysts manufactured using BRIM proprietary technology. Both the edge active centers for hydrogenation and the corner active centers for direct desulfurization of these two new catalysts have been optimized. The first batch of industrially produced BRIM technology catalysts are TK-558BRIM (Co-Mo) and TK-559BRIM (Ni-Mo). The recently developed TK-576BRIM is a catalyst used for producing ULSD; it is specifically designed to remove sterically hindered dibenzothiophene sulfides, thereby facilitating the hydrogenation and desulfurization of such sulfides. It is capable of reducing the sulfur content in feed oil with a level of 1,100 μg/g to below 50 μg/g. Compared with the previous generation catalyst TK-574, the activity of the TK-576 catalyst is increased by about 40%, which is equivalent to a reduction in reaction temperature of 7 °C. This catalyst has been used in multiple industrial plants to produce ULSD. Furthermore, the two-stage deep desulfurization and dearomatization combined process using TK-555 and TK-907 is suitable for producing clean diesel with ultra-low sulfur and low aromatic content. The two sections can be used separately, making them suitable for modifying existing devices as well. The first section is the desulfurization section, using a Ni-Mo catalyst, while the second section employs a sulfur-resistant precious metal catalyst; the final product contains almost no sulfur, and the aromatic content can be reduced to below 5%. 　　 　　1.3 US Standard Catalyst Company US Standard Catalyst Company possesses core technologies for highly active diesel hydrogenation catalysts, with catalyst series such as CENTINEL, ASCENT, and CENTERA. Among them, the CENTINEL series of catalysts treat active metals using the proprietary \"locked position\" impregnation technique, resulting in a highly dispersed metal composition that facilitates the conversion of metal oxidation states into sulfide states. CENTINEL technology has been upgraded by increasing the loading and dispersion of active metals, resulting in more type II active centers and a significant enhancement in hydrogenation activity. Typical CENTINEL diesel deep hydrodesulfurization catalysts include the Mo-Co-type catalysts DC-2118 and DC-2318, as well as the Mo-Ni-type catalysts DN-3110 and DN-3120. Among them, the DC-2318 catalyst has low hydrogen consumption, can be used to process low-quality raw materials, and is easy to regenerate. CENTINEL GOLD is a class of catalysts with Type II Co–Mo–S and Ni–Mo–S active centers; typical examples include the Mo–Ni catalyst DN-3330 and the Mo–Co catalyst DN-2318. In 2004, the company introduced the ASCENT technology by improving the techniques for preparing the carriers and for impregnating them. The catalyst surface is a mixture of Type I and Type II active centers, which increases the dispersion of the active metal and the mechanical strength of the catalyst, thereby reducing hydrogen consumption. Its representative is the DC-2531 catalyst, which not only exhibits extremely high activity but also has excellent regenerability; after regeneration, it can regain over 90% of its activity, allowing it to be used for multiple cycles without the need for replacement. The CENTERA technology was introduced in 2008. This series of catalysts comes in two models: DC-2618, suitable for medium and low-pressure systems, and DC-3630, suitable for high-pressure systems. They enable the production of diesel with extremely low sulfur content even at low catalyst loadings and high space velocities. These new technologies meet the needs of producing ULSD. 　　 　　1.4 French Institute of Petroleum Research The Prime-G hydrogenation technology developed by the French Institute of Petroleum Research first separates the oil into light fractions rich in olefins and heavy fractions containing large amounts of sulfur compounds through distillation. Subsequently, selective hydrogenation using two types of catalysts is applied to the heavy fractions, enabling a 100% yield of liquid product and reducing the sulfur content in the oil to below 10 μg/g. The Prime-G technology features process stability, low hydrogen loss, high removal efficiency of sulfur compounds, and minimal octane loss. The HR series of diesel deep hydrodesulfurization catalysts developed by the French Petroleum Research Institute are represented by the Mo-Co series HR-416 catalyst and the Mo-Ni series HR-448 catalyst. The HR-416 catalyst is suitable for processing straight-run diesel to achieve ultra-deep desulfurization. When catalytic diesel or coker diesel is used as the feedstock, in addition to deep desulfurization, if it is also necessary to remove aromatics, improve stability, and increase the cetane number, then the HR-448 catalyst is employed. For example, when using Middle Eastern straight-run diesel with a boiling range of 217–358 °C, whose sulfur content is 13,100 μg/g and aromatic content is 16.7%, hydrogenation using this technology reduces the sulfur content in the diesel to less than 50 μg/g and the aromatic content to less than 10%. 　　 　　1.5 ExxonMobil of the United States ExxonMobil in the United States has developed the SCANFining new hydrogenation technology. This technology still relies on traditional hydrogenation processes, but it uses the RT-225 catalyst, thereby achieving maximum retention of octane rating while reducing hydrogen consumption. The RT-225 catalyst is a highly dispersed catalyst in the Co-Mo series, exhibiting excellent hydrogenation activity and stability. The 2nd-generation SCANFining technology consists of two stages, with H2S removal between them; it is suitable for processing high-sulfur feedstocks, allowing for thorough desulfurization while minimizing losses in octane rating. The second stage can be added as needed. Compared to the 1st generation SCANFining technology, the sulfur content in the product can be reduced to 10–20 μg/g, with a loss in octane number of only 1–1.5 units. 　　 　　1.6 Axens, France　　Axens’ diesel hydrotreating technology is centered around its catalyst technology; it also includes process optimization, internal reactor components, and process control systems. Collectively, these are referred to as the Prime-D technology. In 2003, Axens introduced the ACE (Advanced Catalytic Engineering) technology, which enhances desulfurization activity by increasing the number of mixing centers and improving denitrification activity. For a synergistic effect to occur, the active metal atoms and promoter atoms must be brought into close proximity to one another. By using this technology to upgrade the HR400 catalyst, the HR500 series of catalysts were produced. In addition to the ACE technology, the HR 500 series catalysts have been improved in other aspects: first, a new type of Al2O3 carrier was developed, which increases its surface area and pore volume, optimizes the pore distribution, and allows for acid modification according to the requirements of hydrogenation processes; second, the metal loading amount has been increased, by about 20% compared to the HR400 series. The HR500 series catalysts can stably produce diesel with a sulfur mass fraction of less than 50 μg/g and less than 10 μg/g. No catalyst deactivation occurred during the 7 months of operation of the device. Furthermore, the TiO2 titanium catalyst jointly developed by Axens and Chiyoda Corporation of Japan exhibits high activity in hydrogen desulfurization applications; it can also react vigorously with nitrogen-containing aromatic compounds. Therefore, compared to Al2O3 catalysts, it can be more effectively utilized for hydrogen denitration (HDN). It is estimated that this characteristic will further expand its application scope in the future, for the hydrotreating of various distillates with higher nitrogen content. 　　 　　1.7 Advanced Refining Technologies, Inc. Advanced Refining Technologies, Inc. is a joint venture between the American company Chevron and Grace Davison. It has conducted thorough research on various combinations of catalysts with different active metal components for the deep hydrodesulfurization of diesel. The research results show that when producing low-sulfur diesel with a sulfur content of less than 50 μg/g, Mo-Ni catalysts are superior to Mo-Co catalysts. To meet the requirements for producing clean diesel, the company has developed the SMART Catalyst SystemTM technology for deep hydrodesulfurization of diesel. Utilizing SMART technology, a series of dual catalysts was jointly developed. Given the differences between the desulfurization reactions of DBT and 4,6-DMDBT in diesel fuel, these catalysts operate separately for desulfurization—each catalyst removes one type of sulfide. One is a highly active Co-Mo catalyst that directly desulfurizes DBT-type compounds; the other is a specially designed Ni-Mo catalyst that first hydrogenates the sterically hindered compound 4,6-DMDBT and then desulfurizes it. 　　 　　1.8 UOP’s MQD Unionfining process is a representative desulfurization and upgrading process developed by UOP. In the single-stage process, non-precious metal catalysts (such as Co-Mo/Ni-Mo) are used for deep desulfurization of diesel, reducing the initial sulfur content of 1.85% to 350 μg/g or 50 μg/g through hydroprocessing. The AS-250 precious metal catalyst used in the two-stage process has a modifying effect; it enables deep hydrogenation and saturation of the aromatics in diesel as well as selective hydrocracking, thereby reducing the sulfur content to 50 μg/g while increasing the cetane number from 49 to 51. This process has been applied in hundreds of units. 　　 　　1.9 DuPont’s ISO-therming technology DuPont’s ISO-therming is a fully liquid-phase circulation hydrogenation technology that does not require a hydrogen circulation system. It relies on high-temperature circulation oil pumps to circulate a certain proportion of hydrogenation products to the top of the reactor, thereby supplying the dissolved hydrogen needed for the reaction. This approach also reduces the number of high-pressure devices, which in turn decreases the space required. It is an excellent technology for reducing operating costs while improving the quality of oils. 　　 　　In summary, the main trends in the development of diesel hydrotreatment technologies abroad are as follows: (1) Developing hydrogenation catalysts with higher activity; (2) Improving the design of reactors and internal components such as distributors, as well as optimizing the process flow and automatic control systems, in order to enhance the economic efficiency of producing ultra-low sulfur diesel; (3) Developing integrated technology sets that meet quality requirements regarding aromatic content, cetane number, density, and cold flow properties. 　　 　　2 Progress in diesel HDS technology in China
In China, fluid catalytic cracking (FCC) is used for the deep processing of heavy oil. However, there are problems with diesel quality, such as poor quality and high sulfur content, particularly a high level of hard-to-remove sulfides. To meet the demand for processing larger amounts of high-sulfur straight-run diesel and low-quality secondary-treated diesel, to produce low-sulfur diesel that meets standards, to achieve deep desulfurization of diesel, and at the same time increase its cetane number, the Fushun Research Institute of Petrochemical Technology (FRIPP) under Sinopec has successively developed new process technologies such as hydroprocessing of low-quality diesel using MCI and FHUDS catalysts, selection and grading technologies for S-RASSG catalysts, and liquid-phase cyclic hydroprocessing of SRH diesel. The Research Institute of Petroleum Processing (RIPP) under Sinopec has developed hydroprocessing technologies such as RICH for increasing diesel’s cetane number while reducing its density, MHUG for medium-pressure hydroprocessing of diesel, and RST for producing ultra-low-sulfur diesel. The Research Institute of Petroleum Processing (PRI) under CNPC has developed the PHF-101 catalyst-based hydrorefining technology. 　　2.1 Technologies Developed by FRIPP 　　2.1.1 MCI Technology 　　The core of the MCI technology developed by FRIPP is to maximize the cetane number of low-quality catalytic diesel, thereby enabling the improvement of such diesel. An important feature of the MCI process is its ability to control the ring-opening of naphthene-based aromatics without causing chain scission; it improves the cetane number while ensuring a good yield of diesel, and represents a process that lies between hydrorefining and medium-pressure hydrocracking. In the catalytic diesel modified by this process, the levels of bicyclic and tricyclic aromatic hydrocarbons in the produced oil are significantly reduced, the content of naphthenes remains essentially unchanged, while the levels of monocyclic aromatic hydrocarbons and alkanes increase substantially. The MCI process can be carried out under conditions similar to those used in conventional hydrorefining. Its advantages lie in the fact that it not only enables thorough hydrodesulfurization and hydrodenitration of the oil, as well as the hydrogenation saturation of olefins and aromatic compounds with more than two rings and their ring-opening to increase the cetane number of diesel, but also ensures a high yield of diesel (over 95%) while reducing the consumption of chemical hydrogen. This technology employs a one-stage series process of hydrogenation refining and reforming using dual catalysts; refineries such as FH-5, FH-5A, and FH-98 are used as catalysts in the refining stage, while a dedicated MCI reforming catalyst is utilized in the reforming stage to modify diesel. The catalyst specifically designed for the modification stage features high catalytic activity and a high degree of aromatic conversion; it can selectively open cycloalkanes without causing chain scission, thereby effectively addressing the challenge of improving the cetane number while maintaining diesel yield. Furthermore, specialized modification catalysts have a high resistance to impurities, enabling them to work with low-quality raw materials; their good stability ensures long-term operation. The MCI modified catalysts include the 1st-generation 3963 catalyst, as well as the 2nd-generation FC-18 and FC-20 catalysts. The second-generation catalyst uses modified FMA alumina as a carrier, modified molecular sieves as cracking components, and tungsten-nickel metals as active centers for the hydrogenation reaction. Thanks to a newly developed production process, the desulfurization and denitration capabilities of this catalyst are significantly higher than those of the first-generation catalyst. The second-generation catalyst can increase the relative removal rate of aromatics by 15% and raise the cetane number of diesel by 2 units. 　　Currently, MCI technology has been successfully applied industrially in subsidiaries under Sinopec and PetroChina. The results show that when this technology is used to process diesel with a sulfur content of 7,000 μg/g, the sulfur content can be reduced to 5.8 μg/g, achieving a desulfurization rate of up to 99.1%. Meanwhile, the cetane number increases by more than 10 units, and the density of the feedstock can be reduced by 0.04 to 0.045 units. 　　 　　2.1.2 S-RASSG Technology and FHUDS Series Catalysts To meet the needs of oil refining companies to produce clean diesel that complies with National Standard IV and National Standard V, FRIPP has developed the FHUDS series of catalysts, which are suitable for various feedstocks and utilize W–Mo–Ni (Mo–Ni) and Mo–Co as active metals, respectively. Based on the different operating conditions and reaction characteristics of the catalyst beds in hydrogenation reactors, as well as the desulfurization mechanisms of different types of catalysts, S-RASSG catalyst grading technology has been developed for the production of ultra-low sulfur diesel. The S-RASSG technology makes use of the synergistic effect of active sites to increase the pore volume and specific surface area of the Al2O3 carrier, thereby improving the dispersion of active metals. By regulating the interaction between the active metals and the carrier, it is possible to sulfide the active metals more completely, resulting in the formation of more highly active edge active centers that enhance the hydrogenation desulfurization activity of the catalyst. 　　The results of industrial applications of over 40 large-scale diesel hydrogenation units both domestically and internationally show that by using the S-RASSG technology along with the accompanying FHUDS-2/FHUDS-5 catalyst systems as well as the new generation of FHUDS-6/FHUDS-5 catalyst systems, it is possible to produce low-sulfur diesel with a sulfur content of less than 50 μg/g, meeting the National IV standards. This can be achieved when processing mixtures of atmospheric pressure diesel with about 40% by weight of high-sulfur catalyzed diesel or coker diesel/low-pressure diesel, at a reactor inlet pressure of 8.0 MPa, a volumetric space velocity of the main catalyst of 1.85–2.25 h-1, and an average reaction temperature of 350 °C. For mixtures of atmospheric pressure diesel with about 30% by weight of low-pressure diesel and a small amount of coker diesel, it is possible to produce ultra-low-sulfur diesel with a sulfur content of less than 10 μg/g, meeting the Euro V standards, under similar conditions. 　　 　　2.1.3 SRH Technology The SRH diesel liquid-phase cyclic hydrogenation technology, developed in collaboration between FRIPP and Luoyang Engineering Company of Sinopec Corporation, features relatively low operational energy consumption and investment costs. In 2009, this technology was first applied in China Petrochemical Corporation’s Changling Branch, where the country’s first industrial demonstration plant for liquid-phase cyclic hydrogenation with a capacity of 200 kt/year was built by upgrading existing facilities, and it was successfully put into use. Subsequently, liquid-phase cyclic hydrogenation plants with capacities of 1.5 Mt/year were established at the Jiujiang Branch, 2 Mt/year at Zhanjiang Dongxing, and 1 Mt/year at the Shengli Petrochemical Complex, all of which were brought online for industrial use. Industrial application results show that the sulfur mass fraction in diesel can be reduced to below 50 μg/g. 　　 　　2.2 Technologies Developed for RIPP 　　2.2.1 RICH Technology 　　The RICH technology, developed for RIPP, enables the deep hydroprocessing of diesel fuel by opening the rings of polycyclic aromatic hydrocarbons without breaking the carbon chains, thereby producing clean diesel fuel with low sulfur content, low density, and a high cetane number. The advantage of RICH technology is its ability to simultaneously carry out desulfurization, denitration, olefin and aromatic saturation, as well as selective ring-opening cracking reactions. This technology uses a RIC bifunctional catalyst; an appropriate amount of molecular sieve is added to the Al2O3 carrier to provide the necessary acidity, while W and Ni metals are loaded onto the carrier as the active components of the catalyst. The catalyst thus produced exhibits high selectivity for desulfurization and ring-opening reactions in diesel. This process reduces the density by more than 0.035 g/cm3 while ensuring an increase in diesel yield; the cetane number increases by over 10 units, and the diesel yield remains above 95%. The new RIC-1 catalyst is the key to the success of the RICH technology. In June 2001, the 800 kt/a diesel hydrotreating unit at the Luoyang Branch of Sinopec Corporation utilized this catalyst for hydroprocessing. Operating results showed that under conditions of a reaction pressure of 7.4 MPa, a reaction temperature of 366 °C, and a space velocity of 1.0 h-1, the cetane number of catalytic diesel could be increased from 32.2 to 42.4; meanwhile, the diesel yield exceeded 95%. Compared with the RIC-1 catalyst, the second-generation RIC-2 catalyst produced by this technology exhibits a 25% increase in space velocity under identical operating conditions, along with better improvements in diesel cetane number and lower density. 　　2.2.2 MHUG Technology 　　 RIPP developed the MHUG technology for the production of clean diesel through medium-pressure hydrogenation reforming in the early 1990s. This technology can be used in hydrogenation processes that use straight-run diesel, catalytic cracking diesel, vacuum light fractions, coker diesel, or mixtures thereof as raw materials; it enables the production of low-sulfur diesel or clean diesel with low sulfur and low aromatic content at medium pressure through selective ring-opening and aromatic cracking, and under appropriate conditions, it can even be used to produce 3# jet fuel. Similar to single-stage or two-stage series hydrocracking units, the reaction system, fresh hydrogen system, recycled hydrogen system, and distillation system constitute the process flow of this technology. The MHUG production technology operates under medium-pressure conditions, using a single-stage two-agent series configuration in which a hydrorefining reactor equipped with a highly active catalyst is connected in series with a hydrocracking reactor fitted with a catalyst possessing high selectivity for ring-opening reactions, while both operate simultaneously. Based on the feed conditions and the operating characteristics of the unit, the ratio of the two catalysts is adjusted accordingly. The series-connected hydrorefining process achieves desulfurization, denitration, and demetallization, improves the color of the oil, and enhances its quality. The series-connected hydroprocessing step saturates and cleaves the bonds in bicyclic aromatics; after cracking, their molecular weight decreases. The cracking products are monocyclic aromatics and alkanes. This production process not only yields clean diesel with good production stability, characterized by low viscosity, low sulfur content, and low aromatic compounds, but also provides high-quality raw materials for ethylene cracking. Due to the different properties of raw materials in the diesel processing process, two sets of units are usually required for hydrorefining and the upgrading of low-quality diesel. In response, based on its diesel hydrotreating technology and the diesel hydroprocessing MHUG technology, RIPP has proposed the flexible hydroprocessing MHUG-II process. This process features multiple reaction zones; for diesel feedstocks with different properties, hydrogenation is carried out by feeding them into respective zones. This approach meets the requirements for both the hydro-upgrading of catalytic cracking diesel and the hydrorefining of straight-run diesel in refineries. It also enhances the selectivity for producing clean diesel while reducing equipment investment costs. Currently, MHUG technology has been applied in 16 refineries, and the diesel produced meets the Euro V standard. 　　2.2.3 RST technology By studying the effects of nitrides and polycyclic aromatic hydrocarbons on ultra-deep hydrodesulfurization, RIPP proposed the concept of controlling different reaction zones, and developed a targeted RST technology for ultra-deep hydrodesulfurization of diesel oil, which removes various types of sulfides in different zones. The specific technical route involves using one or two non-precious metal hydrotreating catalysts to accomplish the ultra-deep hydrodesulfurization of diesel oil through two reaction zones. The first reaction zone is a high-temperature, high-space velocity reaction zone that removes most of the easily removable sulfides and almost all of the nitrides; the second reaction zone is a low-temperature, high-space velocity reaction zone that completely removes the remaining sulfides, carries out hydrogenation saturation of polycyclic aromatic hydrocarbons, and improves the color of the oil. The newly built 2.6 Mt/year diesel hydroprocessing unit at the Yanshan branch utilizes the RST technology for ultra-deep hydrogenation desulfurization of diesel. The unit operates smoothly, with the sulfur content in the diesel product remaining no higher than 10 μg/g, and the properties of the product remain stable. 　　2.3 Technologies for RIP development 　RIP, in collaboration with China University of Petroleum (Beijing), developed the ultra-low sulfur diesel hydrofining catalyst PHF-101. This catalyst features a porous structure with appropriate acidity; the active metals are evenly distributed on its surface. It possesses two types of active centers for direct and indirect hydrogenation desulfurization. The protective additives and structured carriers work together to facilitate the removal of large molecular sulfur-containing compounds. It also enables the saturation of both olefins and aromatics, simultaneously removing sulfur, nitrogen, and aromatics. In 2014, it was introduced in 7 enterprises including Sinopec Dagang Petrochemical Company and Sinopec Daqing Petrochemical Company, and all of the products met the requirements of National Standard V. In 2015, it was applied in the 3 Mt/year diesel hydrotreating unit of China National Petroleum Corporation Lanzhou Petrochemical Company, producing qualified products; the sulfur content in the refined diesel was below the standard of 40 μg/g for National IV diesel, thus meeting the requirements of the technical agreement. The PHF-101 catalyst provides a technical foundation for the production of National IV and National V clean diesel thanks to its excellent activity and stability. The DBS-10 ultra-low sulfur diesel hydrorefining catalyst, jointly developed by RIP and China University of Petroleum, has been applied in the 1.2 Mt/year hydrorefining unit at the Daqing Petrochemical Company’s refinery in China. The sulfur content in the feedstock ranges from 1,191 to 1,242 μg/g, while that in the product is between 43 and 45 μg/g, meeting the standards for National IV diesel. The average temperature of the catalyst reactor was approximately 336 °C, the inlet pressure was 6.95 MPa, and the space velocity was 2.48 h-1. 　　 　　From the above analysis, it can be seen that the development trends of diesel catalytic hydrogenation technology in China are characterized by the following aspects: (1) Developing hydrogenation catalysts with excellent performance at high space velocities; (2) Developing highly active catalysts to reduce operating costs; (3) Since inferior catalytic diesel accounts for over 1/3 of the components used in diesel blending, there is a strong need to improve the cetane number of the final diesel product, which requires the development of high-quality catalysts for diesel hydrogenation and upgrading. 　　 　　 　　3 Conclusion 　Producing low-sulfur or ultra-low-sulfur diesel has become a major trend in the world’s modernized refining industry to meet increasingly stringent environmental regulations. Developing high-quality clean diesel fuel production technologies is one of the main challenges in the field of petroleum refining, and low sulfur content is the direction for the future development of diesel fuels for use in vehicles. HDS technology remains the mainstream approach for producing ultra-low sulfur diesel at present. The development of efficient and stable hydrodesulfurization catalysts, as well as the upgrading of existing hydrodesulfurization processes, are the main focuses of research in this field. To meet the requirements for upgrading the quality of clean diesel, although domestic research institutions have carried out some work and made progress in reducing sulfur content and aromatic content in diesel as well as increasing its cetane number, continued development is still needed to satisfy the ever-changing market demands. Improving the various properties of diesel hydrogenation catalysts by focusing on aspects such as the catalyst carrier, additives, and active metal components will be the main development direction for such catalysts in China in the future. To address the challenges faced by HDS technology in deep desulfurization, developing a desulfurization technique based on a combination of HDS technology and other separation methods holds good prospects for development. 　　 　　References: Yang Ying et al., Advances in Hydrodesulfurization Technology for Clean Diesel, Petrochemical Technology and Economics, June 2015; Hu Junli et al., Current Research Status of Diesel Hydrogenation Technology, Energy & Chemical Engineering, February 2016; Liu Wei et al., Advances in Ultra-Deep Hydrodesulfurization Technology for Diesel, Refining Technology and Engineering, Volume 45, 2015