Desulfurization of gas oil

The present review summarizes contemporary views of the problems, achievements, and prospects involved in the deep desulfurization of gas oils, including identification and reactivity of sulfur species in the feed, the reaction pathways and mechanisms, activity and selectivity of the conventional catalysts, and concerns of fluorescence color production. Process schemes and guidelines for the development of the next-generation catalysts for improved deep desulfurization technology based on these discussions are also proposed. The structure and nature of the active sites of current catalysts will not be extensively covered in this review, because several excellent reviews have been published on these subjects within the past two years (1-3). 

Some major problems associated with deep desulfurization of gas oil are as follows ... 

Samadi-Maybodi A, Teymouri M, Vahid A, Miranbeigi A (2011) In situ incorporation of nickel nanoparticles into the mesopores of MCM-41 by memipulation of solvent-solute interaction and its activity toward adsorptive desulfurization of gas oil. J Hazeu d Mater 192 1667-1674... 

Abbad-Andaloussi, S. Warzywoda, M., and Monot, F., Microbial desulfurization of diesel oils by selected bacterial strains. Oil Gas Science and Technology-Revue Institut Francais Du Petrole, 2003. 58(4) pp. 505-513. 

The conditions employed for the hydrodesulfurization of the heavier feedstocks may be similar to those applied to the hydrodesulfurization of gas oil fractions but with the tendency to increased pressures. However, carbon deposition on, and metal contamination of, the catalyst is much greater when residua and heavy oils are employed as feedstocks and, unless a low level of desulfurization is acceptable, frequent catalyst regeneration is necessary. 

Figure 4-3 Effects of analyst bed length on the desulfurization (u) and denitrogenation (h of gas oils first-order kinetic plots based on the holdup model (after Paraskas et til.1 ).

Meats23-2 showed that the catalyst bed-length effect observed during de-nitrogenation of gas oils in pilot-scale reactors can be correlated on the basis of an axial dispersion effect on the reactor performance. Montagna and Shah29 showed that the bed-length effect observed in desulfurization reaction with 22 percent KVTB and 36 percent KATB (see Fig. 4-4) can also be explained on the basis of an axial dispersion effect on the reactor performance. 

Sano and coworkers reported an interesting work on adsorptive desulfurization of real gas oil over AC with surface area from 683 to 2972 m2/g.165 They found that using the AC materials can remove sulfur and nitrogen species from gas oil and the AC materials with the larger surface area and higher surface polarity have the better adsorptive performance. They also found that the adsorption pretreatment of gas oil over the AC material significantly improved the HDS performance of the gas oil. 

Sano, Y., Choi, K.H., Korai, Y., and Mochida, I. Effects of nitrogen and refractory sulfur species removal on the deep HDS of gas oil. Applied Catalysis. B, Environmental, 2004, 53, 169 Sano, Y., Sugahara, K., Choi, K.H., Korai, Y., and Mochida, I. Two-step adsorption process for deep desulfurization of diesel oil. Fuel, 2005, 84, 903. 

Finally, the new environmental regulations limiting the sulfur content of gas oil used for vehicle transportation involves a new and increasing effort to understand the mechanisms of desulfurization of the most refractive compounds. These molecules are mainly DBTs with alkyl groups near the sulfur atom (fourth and sixth positions) (160). The reasons for their low HDS reactivity is still intensively discussed and two hypotheses have been proposed. The first proposal suggests that the transformation of 4-alkyl- and 4,6-dialkyl-DBT is limited by the adsorption step by the sulfur atom, which would be sterically hindered by the presence of the alkyl groups (161). The second hypothesis supposes a fiat adsorption of the DBT compounds on the catalyst (162). In this case, sterical factors do not hinder the first step of adsorption on the catalyst but more an intermediate step of elimination. 

Interestingly, the major source of sulfur nowadays is via the desulfurization of crude oil and natural gas — testimony to the enormous scale on which these materials are refined, since they only contain a few per cent of sulfur-containing compounds. These compounds have to be removed because they tend to poison the catalysts used in later (downstream) reactions and pollute the environment. 

For example, in the case of light Arabian crude (Table 8.16), the sulfur content of the heavy gasoline, a potential feedstock for a catalytic reforming unit, is of 0.036 weight per cent while the maximum permissible sulfur content for maintaining catalyst service life is 1 ppm. It is therefore necessary to plan for a desulfurization pretreatment unit. Likewise, the sulfur content of the gas oil cut is 1.39% while the finished diesel motor fuel specification has been set for a maximum limit of 0.2% and 0.05% in 1996 (French specifications). 

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