【Technology】Progress in ultra-deep desulfurization technology to produce clean gasoline

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【Technology】Progress in ultra-deep desulfurization technology to produce clean gasoline 【Technology】Technology progress in ultra-deep desulfurization technology to produce clean gasoline. 1. Preface. The sulfur oxides (SOx) produced by the combustion of sulfides in fuel oil not only poison the three-way catalyst, but also cause acid rain and seriously pollute the environment. 　　[Technology] The progress of ultra-deep desulfurization technology to produce clean gasoline. In recent years, air pollution prevention and control has attracted attention from various industries. This article will introduce the progress of ultra-deep desulfurization technology to produce clean gasoline. The content is as follows: 　　1. Introduction The sulfur oxides (SOx) produced by the combustion of sulfides in fuel oil in engines not only poison the three-way catalyst, but also cause acid rain and seriously pollute the environment. As people's environmental awareness increases, many in the world * * Increasingly stringent fuel oil emission standards have been introduced one after another. The United States and Europe required the sulfur content in gasoline to be 10ug/g in 2009. In contrast, my country's gasoline sulfur content standard is lower, only 150ug/g (Table 1). However, the depletion of crude oil resources has led to increasing sulfur content in today's crude oil. To resolve this contradiction, researchers have/are developing various gasoline ultra-deep desulfurization technologies. Since my country imports a large amount of high-sulfur crude oil from the Middle East every year, the development of new high-efficiency desulfurization technology is particularly important in our country. This article will review gasoline desulfurization technology from two aspects: hydrodesulfurization and non-hydrodesulfurization. 　　2. Ultra-deep desulfurization technology 2.1 The types of organic sulfur compounds present in gasoline generally refer to straight-run gasoline, catalytically cracked gasoline (FCC gasoline), coked gasoline and their blending components. my country's crude oil is heavy crude oil, and up to 80% of gasoline comes from FCC gasoline. The proportion of FCC in gasoline in the United States and Western Europe is smaller, 36% and 27% respectively (Table 2). A large number of studies have shown that the types of organic sulfur compounds in gasoline include mercaptans (RSH), thioethers (RSR), disulfides (RSSR'), thiophene and their derivatives. 　　2.2 Ultra-deep hydrodesulfurization technology for gasoline Ultra-deep desulfurization of gasoline refers to reducing the sulfide content in gasoline to less than 30 ppmw. More than 90% of sulfides in gasoline originate from FCC gasoline, so the production of clean gasoline mainly focuses on ultra-deep desulfurization of FCC gasoline. Currently, FCC gasoline desulfurization mainly uses traditional hydrodesulfurization technology (HDS). Figure 1 shows the hydrodesulfurization activity of various organic sulfides in fuel oil. It can be seen that as the molecular size of organic sulfide increases, its hydrodesulfurization activity decreases. In addition, the sulfides (mercaptans, thioethers, disulfides, and thiophene sulfides) in FCC gasoline have high hydrodesulfurization activity, and it is not technically difficult to remove them through hydrodesulfurization. However, the large amount of olefins (10-35V%) contained in FCC gasoline will be saturated during the hydrodesulfurization process, resulting in a significant reduction in the gasoline octane number and increased hydrogen consumption. In order to achieve the goal of ultra-deep desulfurization while maintaining the octane number, in addition to improving HDS technology from the aspects of catalyst modification (selective hydrodesulfurization), optimization of operating processes, and new reactor design, researchers will also use catalytic distillation, aromatization and then HDS, and HDS and then isomerization technology. However, HDS technology faces shortcomings such as high investment, high operating costs, and large hydrogen consumption. 　　Figure 1 Hydrodesulfurization activity of organic sulfur compounds in fuel oil 2.3 Ultra-deep non-hydrodesulfurization technology for gasoline In addition to the various improved HDS technologies mentioned above, domestic and foreign oil companies and research institutions are competing to study non-hydrodesulfurization technologies that can significantly reduce desulfurization costs, and have achieved certain results. Including adsorption desulfurization, oxidative desulfurization, extraction desulfurization, alkylation desulfurization and biological desulfurization. They are introduced below. 　　2.3.1 Adsorption desulfurization Adsorption desulfurization (ADS) is a desulfurization technology based on solid adsorbents that can selectively adsorb organic sulfides in FCC gasoline. The desulfurization rate of adsorption desulfurization is mainly determined by the following aspects:: The adsorption capacity of the adsorbent, the selectivity of the adsorbent to organic sulfides, the service life of the adsorbent and the number of regenerations. Adsorption desulfurization is divided into reaction-adsorption desulfurization and adsorption desulfurization based on the interaction between organic sulfides and adsorption centers. During the reaction-adsorption desulfurization process, the CS bond of organic sulfides is broken, and sulfur is fixed on the adsorbent (Figure 2). Adsorption desulfurization is based on the physical adsorption or weak chemical adsorption of organic sulfides on the surface of solid adsorbents. 　　Figure 2 Reaction-Adsorption desulfurization principle According to the different interaction modes between organic sulfides and solid adsorbent surfaces, the adsorption desulfurization mechanism is divided into π bonding desulfurization and SM bonding desulfurization (Figure 3). The π bonding mechanism is that the Cu+ outer layer empty f orbital loaded on the Y molecular sieve surface accepts the lone pair of electrons in the thiophene sulfide. At the same time, the Cu+ outer layer filled d orbital electrons enter the anti-π bond orbit of the thiophene sulfide to form a π bond. The SM bonding mechanism is that the empty orbitals of metal ions loaded on the surface of Y molecular sieve accept the lone pairs of electrons in thiophene sulfides to form SM bonds. 　　As early as many years ago, there were reports on the use of adsorption desulfurization technology to desulfurize model compounds. in recent years. With the increasingly stringent environmental regulations and potential applications in fuel cells and other aspects, research on adsorption desulfurization has gradually moved from model compounds to real oil products, and the research direction has become increasingly complex. This article introduces each type of adsorbent. 　　 2.3.1.1 Research on molecular sieve adsorbents based on molecular sieve adsorbents with FAU topological structure began in the 1990s. Salem et al. found that 13X molecular sieve can efficiently remove sulfide from low-sulfur oil products at room temperature. Yang et al. prepared Cu+ and Ag+ exchanged Y molecular sieve and used it to desulfurize model compounds of gasoline. Experiments found that the adsorbent can remove all thiophene sulfides in the model compound, and it is believed that thiophene sulfides are combined with Cu+ and Ag+ in a π bonding manner. However, when it is used to desulfurize real oil products, its desulfurization effect drops significantly. The reason is that real oil products contain a large number of olefins and aromatic hydrocarbons, which have π electrons with thiophene sulfides and compete for adsorption on the surface of the solid adsorbent, thus reducing the desulfurization ability of the adsorbent. In order to solve this problem, Song et al. prepared Ce4+-exchanged Y molecular sieve and used it for desulfurization of real oil products. They found that the sulfur capacity of the adsorbent was still as high as 10 mgS/g even in the presence of large amounts of aromatic hydrocarbons. It is believed that thiophene sulfide and Ce4+ are combined by SM bonding. This is because thiophene sulfide molecules have S atoms that aromatic hydrocarbon molecules do not have. They can occupy the empty d orbital of Ce4+ to form SM bonds through the lone pair of electrons on the S atom, regardless of the presence or absence of aromatic hydrocarbons. Redeau et al. studied the desulfurization performance of HFAU molecular sieve and found that thiophene sulfides were adsorbed on the surface of HFAU molecular sieve by combining with the acidic sites on its surface. 　　The use of molecular sieve adsorbents based on MFI topology for desulfurization also began in the early 1990s. Weitcamp et al. used ZSM-5 molecular sieve to remove impurity thiophene from benzene. Luo Guohua and others exchanged Cu ions to the surface of ZSM-5 molecular sieve to remove thiophene sulfide from coked benzene. However, this type of adsorbent has a very poor effect on removing sulfides in real oil products. Because of the restriction of the molecular sieve pores, organic sulfide molecules containing multiple benzene rings in the molecules cannot diffuse into the molecular sieve pores and cannot be removed. 　　 2.3.1.2 Oxide adsorbents: This type of adsorbent is mainly obtained by using alumina as a precursor and subjecting it to appropriate modifications. A typical representative is transition metal-loaded alumina used in the IRVAD process. Its adsorption mechanism is based on the polarity of sulfur-containing organic compounds. The desulfurization ability is limited by the adsorption capacity and affinity for organic sulfides. Klabunde et al. investigated the adsorption desulfurization ability of nano-alumina loaded with silver ions. The study found that the active center of adsorption desulfurization is Ag+ bonded with carbonate rather than free Ag+, and the introduction of Lewis acid can improve its desulfurization ability. Some scholars have introduced Zn and Fe oxides on γ-alumina, and then added an appropriate amount of CeO2. This mixed oxide adsorbent has excellent removal capabilities for organic sulfides in real gasoline. In addition, Song et al. loaded metallic nickel on porous silica gel and proposed the concept of selective adsorption desulfurization (SARS), which can completely remove organic sulfides in gasoline. 　　 2.3.1.3 Other adsorbents Sano et al. used activated carbon with large specific surface area and high surface polarity as an adsorbent to remove sulfur-containing compounds from DC gas oil, with a maximum adsorption capacity of up to 98 mgS/g. When the activated carbon surface is further oxidized and heat treated, the sulfur adsorption capacity can be greatly increased. Jiang Zongxuan and others also found that after oxidation treatment of activated carbon, its sulfur capacity increased from 24 to 53 mgS/g. They believed that the increase in sulfur capacity was attributed to the increase in the mesopore volume and the number of surface oxygen-containing groups on the surface of activated carbon after oxidation treatment. 　　2.3.2 Oxidative desulfurization Oxidative desulfurization (ODS) was proposed in the 1990s and is generally divided into two steps.: ①Oxidize organic sulfides in oil ② Remove sulfur oxides from oil. Compared with HDS, ODS has the advantages of mild reaction conditions, high selectivity, and low operating cost. At this stage, hydrogen peroxide is mainly used as the oxidant. However, hydrogen peroxide has extremely strong oxidizing properties and is prone to explosion during storage, transportation, and use. Therefore, people are trying to find its substitute. Thermodynamic and kinetic calculations show that it is feasible to directly oxidize thiophene sulfides with air or oxygen to generate SO2 and hydrocarbons. Using water and air as hydrogen source and oxidant respectively will be a promising desulfurization approach. 　　2.3.3 Extractive desulfurization Extractive desulfurization refers to using a solvent that is incompatible with the oil to extract organic sulfides from the oil, thereby achieving the purpose of desulfurizing the oil. The specific process is shown in Figure 4. Gasoline and fresh solvent are fully contacted in the mixer. Due to the higher solubility of organic sulfides in the solvent, they are transferred from the oil phase to the solvent phase. The solvent and gasoline are separated in the re-separator, and the solvent containing organic sulfide is separated from the solvent through distillation. The selected extraction agent must meet the following requirements: The extraction agent must be cheap, have a higher solubility of the organic sulfide in it, and have a different boiling point than the organic sulfide. 　　Figure 4 Simplified flow chart of extractive desulfurization The biggest advantage of extractive desulfurization is that the operating conditions are simple, the chemical structure of each component in the gasoline will not change, and there are no special requirements for equipment. 　　2.3.4 Alkylation desulfurization Alkylation desulfurization (OATS) refers to the alkylation reaction of thiophene sulfides and long-chain olefins contained in gasoline into alkyl-substituted thiophenes with higher boiling points, which are then separated from gasoline through distillation. The high-boiling point organic sulfides accumulate at the bottom of the distillation tower to form sulfur-rich heavy fractions. BP uses OATS to remove sulfur compounds from FCC gasoline in a new and improved process. The OATS process includes three parts: raw material pretreatment, alkylation reaction, and product separation. In the alkylation reactor, an acid catalyst is used to promote the reaction of thiophene with long-chain olefins. The gasoline passing through the alkylation reactor is divided into a sulfur-free light fraction and a sulfur-rich heavy fraction in the distillation column. The OATS process can remove more than 99.5% of organic sulfides in gasoline. However, there are currently no reports of commercialization of the OATS process. 　　2.3.5 Biological desulfurization Biological desulfurization refers to a method that uses a series of enzyme-catalyzed reactions to remove organic sulfur compounds in gasoline under mild conditions. Compared with traditional HDS technology, biological desulfurization technology has the following advantages:: It operates under normal temperature and pressure, has low investment, high operating flexibility, does not consume hydrogen and does not produce carbon dioxide. Biological desulfurization technology has been studied for more than 70 years, but there have been no commercial reports. The reason is that during the desulfurization process, part of the hydrocarbons in gasoline will be consumed by microorganisms as carbon sources. However, due to its many advantages mentioned above, people still have not given up research on this desulfurization technology. In recent years, with the development of biotechnology, research on biological desulfurization technology in the United States, Japan and European countries is still very active, and important progress has been made. 　　3. Conclusion Social needs and technological progress promote the development of new high-efficiency, energy-saving desulfurization technology. More than 90% of the sulfides in gasoline come from FCC gasoline. Therefore, the key to ultra-deep desulfurization of gasoline is how to use effective means to efficiently remove sulfides in FCC gasoline. HDS technology has the ability to produce clean gasoline, but it still has the disadvantages of high operating costs, high equipment investment and large hydrogen consumption. Non-hydrodesulfurization technology, especially adsorption desulfurization, is the future development direction of clean gasoline production technology. However, adsorption desulfurization still faces many challenges from both a technical and theoretical level. Future research on adsorption desulfurization should mainly focus on the following three aspects:: How to improve the sulfur capacity and selectivity of adsorbents; the interaction between organic sulfides and adsorption active centers; in order to adapt to large-scale industrial production, optimization of adsorption desulfurization agents and adsorption desulfurization process conditions under fixed bed conditions is carried out.