Advances in ultra-deep desulfurization technology for producing clean gasoline

Thread Content

This post was last edited by I am kind on 2017-4-19 at 15:35. 1 Introduction: The sulfur compounds in fuel oil, which are converted into sulfur oxides (SOx) during engine combustion, not only poison the three-way catalyst but also cause acid rain, thereby severely polluting the environment. As awareness of the environment grows, many countries around the world have introduced increasingly stringent standards for fuel oil emissions. The United States and Europe required that the sulfur content in gasoline be 10 ug/g by 2009; in contrast, China’s standard for sulfur content in gasoline is lower, at only 150 ug/g (Table 1). However, the depletion of crude oil resources has led to an increasing sulfur content in crude oil today. To resolve this contradiction, researchers have/are developing various ultra-deep desulfurization technologies for gasoline. Given that our country imports large quantities of crude oil with high sulfur content from the Middle East each year, it is particularly important to develop new and efficient desulfurization technologies in our country. This paper will provide a review of gasoline desulfurization technologies from the perspectives of hydrodesulfurization and non-hydrodesulfurization. 2 Ultra-deep desulfurization technology 2.1 Types of organic sulfides present in gasoline Gasoline generally refers to straight-run gasoline, catalytic cracking gasoline (FCC gasoline), coker gasoline, and their blend components. China’s crude oil is heavy-grade, and up to 80% of its gasoline comes from FCC gasoline. FCC accounts for a smaller proportion in gasoline in the United States and Western Europe, at 36% and 27% respectively (Table 2). Numerous studies have shown that the types of organic sulfides in gasoline include thiols (RSH), thioethers (RSR), disulfides (RSSR’), thiophenes, and their derivatives. 2.2 Ultra-deep hydrodesulfurization technology for gasoline: Ultra-deep desulfurization of gasoline refers to reducing the sulfur content in gasoline to below 30 ppmw. Over 90% of the sulfides in gasoline originate from FCC gasoline; therefore, the production of clean gasoline focuses primarily on ultra-deep desulfurization of FCC gasoline. Currently, FCC gasoline desulfurization primarily relies on the traditional hydrodesulfurization (HDS) technology. Figure 1 shows the hydrodesulfurization activity of various organic sulfides in fuel oil. It can be seen that as the molecular size of organic sulfides increases, their hydrodesulfurization activity decreases. Furthermore, the sulfides in FCC gasoline (thiols, thioethers, disulfides, thiophene sulfides) possess high hydrodesulfurization activity, and it is not technically difficult to remove them through hydrodesulfurization. However, the high amount of olefins present in FCC gasoline (10–35 V%) gets saturated during hydrodesulfurization, resulting in a significant decrease in gasoline octane number and an increase in hydrogen consumption. To achieve both ultra-deep desulfurization and the preservation of octane number, in addition to improving HDS technology through catalyst modification (selective hydrodesulfurization), optimization of the operational process, and design of new reactors, researchers have also employed catalytic distillation, aromatization followed by HDS, and isomerization after HDS. However, HDS technology has disadvantages such as high investment costs, high operating expenses, and high hydrogen consumption. 2.3 Ultra-deep hydrodesulfurization technologies for gasoline: In addition to the various improvements to HDS technologies mentioned above, oil companies and research institutions around the world are competing to develop hydrodesulfurization techniques that can significantly reduce desulfurization costs, and certain results have already been achieved. These include adsorption desulfurization, oxidation desulfurization, extraction desulfurization, alkylation desulfurization, and biological desulfurization. They are introduced below separately. 2.3.1 Adsorption Desulfurization Adsorption desulfurization (ADS) is a desulfurization technique that relies on solid adsorbents to selectively adsorb organic sulfides in FCC gasoline. The desulfurization efficiency of adsorption desulfurization is primarily determined by the following factors: the adsorption capacity of the adsorbent, its selectivity for organic sulfides, as well as the service life of the adsorbent and the number of times it can be regenerated. Adsorption desulfurization is divided into reaction-adsorption desulfurization and adsorption desulfurization based on the mechanism of interaction between organic sulfides and the adsorption sites. During the reaction-adsorption desulfurization process, the C-S bonds in organic sulfides break, 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. Depending on the different ways in which organic sulfides interact with the surface of solid adsorbents, the adsorption desulfurization mechanisms are divided into desulfurization via π-bond bonding and desulfurization via S–M bond bonding (Figure 3). The π-bonding mechanism involves the outer empty f-orbitals of Cu+ adsorbed on the surface of Y zeolite accepting the lone pairs of electrons from thiophene sulfides; simultaneously, the electrons in the filled d-orbitals of Cu+ move into the anti-π bond orbitals of the thiophene sulfides, thereby forming π bonds. The S-M bonding mechanism involves the empty orbitals of metal ions adsorbed on the surface of Y zeolite accepting the lone pairs of electrons in thiophene-based sulfides, thereby forming S-M bonds. Many years ago, there were already reports on the use of adsorption desulfurization technology to remove sulfur from model compounds. In recent years. With increasingly stringent environmental regulations and potential applications in areas such as fuel cells, research on adsorption desulfurization is gradually shifting from model compounds to actual crude oils, making the research directions more complex. This article introduces them separately according to the type of adsorbent. 2.3.1.1 Research on molecular sieve adsorbents based on the FAU topology began in the 1990s; Salem and others discovered that at room temperature, 13X molecular sieves can effectively remove sulfides from oils with low sulfur content. Yang et al. prepared Cu+ and Ag+-exchanged Y molecular sieves and used them for the desulfurization of model compounds in gasoline. Experiments showed that these adsorbents could remove all thiophene sulfides from the model compounds, and it was concluded that thiophene sulfides bind to Cu+ and Ag+ through π-bonds. However, when it is used for desulfurization of real crude oils, its desulfurization efficiency drops significantly. The reason is that real crude oils contain large amounts of olefins and aromatics, which, along with thiophene sulfides, possess π electrons and compete for adsorption on the surface of the solid adsorbent, thereby reducing the desulfurization capacity of this adsorbent. To address this issue, Song et al. prepared Ce4+-exchanged Y molecular sieves and used them for the desulfurization of real crude oils. They found that the sulfur capacity of these adsorbents remained as high as 10 mg S/g even in the presence of large amounts of aromatics, suggesting that thiophene sulfides bind to Ce4+ through S–M bonds. This is because thiophene sulfide molecules possess an S atom that aromatic molecules do not have; this S atom can use its lone pair of electrons to occupy the empty d orbitals of Ce4+, thereby forming an S-M bond, independent of the presence or absence of aromatic compounds. Rechardeau and colleagues studied the desulfurization performance of HFAU molecular sieves and found that thiophene sulfides are adsorbed on their surface by binding to the acidic sites on the surface of the HFAU molecular sieves. The use of molecular sieve adsorbents with an MFI topology for desulfurization also began in the early 1990s; Weitcamp and others employed ZSM-5 molecular sieves to remove the impurity thiophene from benzene. Luo Guohua and others exchanged Cu ions onto the surface of ZSM-5 molecular sieves to remove thiophene sulfides from coked benzene. However, such adsorbents have extremely poor performance in removing sulfides from actual crude oils; due to the restrictive effect of the molecular sieve pores, organic sulfur compounds containing multiple benzene rings cannot diffuse into these pores and thus cannot be removed. 2.3.1.2 Oxide-based adsorbents: These adsorbents are primarily obtained by appropriately modifying alumina as a precursor. A typical example is the alumina loaded with transition metals used in the IRVAD process; its adsorption mechanism is based on the polarity of sulfur-containing organic compounds, and its desulfurization capacity is limited by the adsorption capacity and affinity for organic sulfides. Klabunde et al. investigated the adsorption desulfurization capacity of silver-ion-loaded nanoalumina. Their research found that the active center for adsorption desulfurization is Ag+ bonded to carbonate ions, rather than free Ag+, and the introduction of Lewis acids can enhance its desulfurization ability. Some scholars have also introduced oxides of Zn and Fe onto γ-alumina, followed by the addition of an appropriate amount of CeO2; such mixed-oxide adsorbents exhibit excellent ability to remove organic sulfides from real gasoline. Furthermore, Song et al. loaded metallic nickel on porous silica gel and proposed the concept of selective adsorption desulfurization (SARS); this adsorbent is capable of completely removing organic sulfides from gasoline. 2.3.1.3 Other adsorbents: Sano et al. used activated carbon with a large specific surface area and high surface polarity as an adsorbent to remove sulfur-containing compounds from straight-run gas oil, achieving a maximum adsorption capacity of up to 98 mg S/g. The sulfur adsorption capacity can be greatly increased by further oxidizing the surface of activated carbon and subjecting it to heat treatment. Jiang Zongxuan and others also found that after oxidation treatment, the sulfur capacity of activated carbon increased from 24 to 53 mg S/g, attributing this increase in sulfur capacity to the rise in the pore volume of the surface pores and the number of surface oxygen-containing groups in the activated carbon following the oxidation treatment. 2.3.2 Oxidative desulfurization Oxidative desulfurization (ODS) was proposed in the 1990s and generally involves two steps: ① Oxidation of organic sulfides in the oil; ② Removal of sulfur oxides from the oil. Compared with HDS, ODS has advantages such as mild reaction conditions, high selectivity, and low operating costs. At present, hydrogen peroxide is primarily used as an oxidant, but due to its strong oxidizing properties, it is highly prone to explosion during storage, transportation, and use; therefore, efforts are being made to find alternatives to it. Thermodynamic and kinetic calculations indicate that the direct oxidation of thiophene sulfides by air or oxygen to produce SO2 and hydrocarbons is feasible. Using water and air as the hydrogen source and oxidant respectively is a desulfurization approach with great application potential. 2.3.3 Extraction desulfurization Extraction desulfurization refers to the use of a solvent that is immiscible with the oil to extract organic sulfides from the oil, thereby achieving desulfurization of the oil. The specific process is shown in Figure 4: gasoline and fresh solvent come into full contact in the mixer, and due to the higher solubility of organic sulfides in the solvent, they transfer from the oil phase to the solvent phase. Separation of the solvent from gasoline is carried out in a secondary separator; the solvent containing organic sulfides is subjected to distillation to separate the organic sulfides from the solvent. The selected extractant must meet the following requirements: it must be inexpensive, organic sulfides should have a higher solubility in it, and the extractant and organic sulfides must have different boiling points. The greatest advantage of extraction desulfurization is its simple operating conditions, the fact that the chemical structure of the various components in gasoline remains unchanged, and the lack of special requirements for the equipment. 2.3.4 Alkylation desulfurization Alkylation desulfurization (OATS) refers to the alkylation reaction of thiophene sulfides and long-chain alkenes present in gasoline, which converts them into alkyl-substituted thiophenes with higher boiling points; these compounds are then separated from gasoline through distillation, with the high-boiling-point organic sulfides accumulating at the bottom of the distillation tower to form a sulfur-rich heavy fraction. BP uses OATS in a newly improved process to remove sulfides from FCC gasoline. The OATS process consists of three steps: raw material pretreatment, alkylation reaction, and product separation. In the alkylation reactor, an acid catalyst is used to promote the reaction between thiophene and long-chain olefins. The gasoline that has passed through the alkylation reactor is separated in a distillation tower into sulfur-free light fractions and sulfur-rich heavy fractions. The OATS process can remove over 99.5% of organic sulfides from gasoline. However, there are no reports yet of the commercialization of the OATS process. 2.3.5 Biological desulfurization Biological desulfurization refers to a method of removing organic sulfides from gasoline through a series of enzyme-catalyzed reactions under mild conditions. Compared with traditional HDS technology, biological desulfurization technology has the following advantages: it can operate at normal temperature and pressure, requires low investment, offers great operational flexibility, does not consume hydrogen, and does not produce carbon dioxide. Bio-desulfurization technology has a research history of over 70 years, but there have been no reports of its commercialization; the reason for this is that during the desulfurization process, some of the hydrocarbons in gasoline are consumed by microorganisms as a carbon source. However, due to its many aforementioned advantages, research on this desulfurization technology has not been abandoned. In recent years, with the development of biotechnology, research on biological desulfurization technology has remained very active in the United States, Japan, and European countries, and significant progress has been made. 3 Conclusion Social demands and technological advancements drive the development of new, efficient, and energy-saving desulfurization technologies. Over 90% of the sulfides in gasoline originate from FCC gasoline; therefore, the key to ultra-deep desulfurization of gasoline lies in finding effective methods to remove sulfides from FCC gasoline efficiently. HDS technology is capable of producing clean gasoline, but it still has the disadvantages of high operating costs, high equipment investment, and high hydrogen consumption. Non-hydrodesulfurization technologies, especially adsorption desulfurization, represent the future development direction for clean gasoline production technologies. However, adsorption desulfurization still faces many challenges both from a technical and theoretical perspective. In the future, research on adsorption desulfurization should focus primarily on the following three aspects: how to improve the sulfur capacity and selectivity of adsorbents ； The mode of interaction between organic sulfides and adsorption active centers ； To adapt to large-scale industrial production, optimization of the adsorption desulfurization agent under fixed-bed conditions and its associated process parameters was carried out.