Hydrodesulphurization (HDS) is a standard process for removing sulphur compounds in fuel oils in industry. HDS is effective to remove simple aliphatic sulphur compounds while less effective to remove thiophenes, dibenzothiophenes, and their derivatives because of sterically hindered adsorption on catalyst surface. Application of ionic liquids (ILs, a new class of compounds) substituting for traditional volatile organic solvents in extractive desulphurization (EDS) or oxidative desulphurization (ODS), have been being studied intensively in the latest decades, and many very promising results have been obtained, showing a good prospect as complement method to HDS. In this chapter, these fresh research results of EDS and ODS using ILs are summarized along with comprehensive discussions on diversified desulphurization factors along with some potential problems. It can be inferred that ILs are a class of potential ideal solvents to realize clean fuel oil in future although some problems come too.
TopIntroduction1
Deep desulphurization of fuels has become a very important research subject worldwide, due to environmental concern and the upcoming stricter sulphur legislative regulation. Oil refineries are finding ways to get around a bottleneck caused by the presence of heterocyclic sulphur compounds in fuels, which emits oxides of sulphur (SOx) as environmental pollutants during combustion (Mei et al, 2003). The task to minimize sulphur content in fuel has hastened in recent years due to growing environmental awareness from the public and more importantly, demanding statutory limit of these compounds in fuels. Europe, the US and other mature economies have been forced to lower the content in fuel through legislation (Figure 1a and 1a for previous, current and proposed sulphur levels in diesel and gasoline in developed countries). In 1998, the European directive on transportation fuels limited the sulphur content to 150 ppm for gasoline and 350 ppm for diesel and just five years later, that number was reduced to 50 ppm. Now, it stands at <10 ppm. The United States and Japan have put tax on transportation fuel exceeding 10 ppm Sulphur content (Essar et al, 2004).
Figure 1.
Previous, current, and proposed sulphur limitations in diesel (a) and gasoline (b) in different countries
Fuel oils contain variety of sulphur compounds. These sulphur compounds are both aliphatic (thiols, sulfides, disulfides etc) and aromatic (TS, BT, DBT and their derivatives).The structures of those sulphur compounds are shown in the Figure 2.
Figure 2. Sulphur compounds in fuel oils
The hydrodesulphurization (HDS) method is widely used in industry for the desulphurization of diesel fuels, where sulphur compounds are catalytically converted into H2S and corresponding hydrocarbons, which are subsequently separated from oils and catalytically oxidized into elemental sulphur in the Claus process. HDS achieved by catalytic processes operates at elevated temperatures (300 °C to 340 °C) and pressures (20 to 100 atm of H2) with Co-Mo/Al2O3 or the Ni-Mo/Al2O3 catalyst (Kabe et al, 1993 & Pedarnera et al, 2003). The HDS process is highly efficient in removing thiols, sulfides, and disulfides but is less effective for aromatic thiophenes and thiophene derivatives. Thus, the sulphur compounds that remain in the transportation fuels are thiophene (TS), benzothiophene (BT), dibenzothiophene (DBT), and their alkylated derivatives. The reactor size of HDS unit needs to be increased by factors of 5 to 15. The severe operation conditions, expensive equipment, high hydrogen consumption are the other disadvantages hamper the application of HDS in deep desulphurization (F.-L. Yu, Wang, Liu, Xie, & Yu, 2014). Unfortunately, more severe conditions result not only in a higher level of desulphurization but also in undesired side reactions. When fluid catalytic cracking (FCC) gasoline is desulphurization at higher pressure, many olefins are saturated and the octane number decreases. Higher temperature processing leads to increased coke formation and subsequent catalyst deactivation. It is also important to note that in practice the severity of the operating conditions is limited by the HDS unit design (Babich & Moulijn, 2003 and Zhang et al, 2007).). Faced with the severely high costs of compliance, a surprising number of petroleum refiners are seriously considering reducing or eliminating production of transportation fuels (Yang, Hernández-Maldonado, & Yang, 2003). To address these drawbacks, the alternative methods such as extraction, oxidation, adsorption, and bio desulphurization are under considerations. Status and details about these technologies are mentioned in section “technologies for desulphurization of fuel oils”.