Diesel fuel desulfurization

The main justification for diesel fuel desulfurization is related to particulate emissions which are subject to very strict rules. Part of the sulfur is transformed first into SO3, then into hydrated sulfuric acid on the filter designed to collect the particulates. Figure 5.21 gives an estimate of the variation of the particulate weights as a function of sulfur content of diesel fuel for heavy vehicles. The effect is greater when the test cycle contains more high temperature operating phases which favor the transformation of SO2 to SO3. This is particularly noticeable in the standard cycle used in Europe (ECE R49). 

Finally, sulfur has a negative effect on the performance of the catalyst itself. One sees for example in Figure 5.23 that the initiation temperature increases with the sulfur level in the diesel fuel, even between 0.01% and 0.05%. Yet, in the diesel engine, characterized by relatively low exhaust temperatures, the operation of the catalyst is a determining factor. One can thus predict an ultimate diesel fuel desulfurization to levels lower than 0.05%. 

Yu, G.X., Lu, S.X., Chen, H., and Zhu, Z.N. Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon. Carbon, 2005, 43, 2285. 

An important example are alumina-supported Co—Mo and Ni—Mo sulfides, which constitute the active phases in catalysts for hydrotreating of middle distillates (403). It appears that in such catalysts, mostly pseudoboehmite-derived Y-AI2O3 is used as the support. According to fiter-ature data, catalysts for fluid catalytic cracking (FCC) gasoline desulfurization, which is performed at 260—340 °C and 5—30 atm, may contain 5—11 wt% molybdenum and 2-3 wt% cobalt supported on AI2O3 with a surface area of220—240 w (405). Catalysts for diesel fuel desulfurization to low-sulfur... 

As we have shown previously, obtaining both good cold operation characteristics and sufficient cetane numbers constitutes the principal objective for the refiner in the formulation of diesel fuel. To this is added the need for deep desulfurization and, perhaps in the future, limitations placed on the chemical nature of the components themselves, e.g., aromatics content. 

Following 1 October 1996, diesel fuel should be desulfurized to a level of 0.05% while the maximum sulfur content of home-heating oils will stay provisionally at 0.2 %. 

In any and all cases, desulfurization of diesel fuel is a necessary condition for attaining very low particulate levels such as will be dictated by future regulations (Girard et al., 1993). 

Desulfurization will become mandatory when oxidizing catalysts are installed on the exhaust systems of diesel engines. At high temperatures this catalyst accelerates the oxidation of SO2 to SO3 and causes an increase in the weight of particulate emissions if the diesel fuel has not been desulfurized. As an illustrative example, Figure 5.22 shows that starting from a catalyst temperature of 400°C, the quantity of particulates increases very rapidly with the sulfur content. 

The elimination of lead, the reduction of aromatics in gasoline, and the desulfurization of diesel fuels are oing to require significant reformulations of these products that will irripiy development of specific additives that allow the refiner to optimize costs while meeting the required specifications. 

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

Song, C., an Overview of New Approaches to Deep Desulfurization for Ultra-Clean Gasoline, Diesel Fuel and Jet Fuel. Catal. Today, 2003. 86 pp. 211-263. 

Song, C., and Ma, X., New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. 

Table 13. Relative rates of desulfurization of diesel fuels by various biocatalysts...

Villasenor, F. Loera, O. Campero, A., and Viniegra-Gonzalez, G., Oxidation of dibenzothiophene by laccase or hydrogen peroxide and deep desulfurization of diesel fuel by the later. Fuel Processing Technology, 2004. 86(1) pp. 49-59. 

Pacheco, M. A. Lange, E. A. Pienkos, P. T., et. al., Recent advances in desulfurization of diesel fuel, in National Petroleum and Refiners Association, Annual Meeting. 1999. San Antonio, Tex. National Petroleum and Refiners Association. NPRA AM-99-27, 1-26. 

Gas oils Utilized as straight-run distillate after desulfurization. Lighter atmospheric and vacuum gas oils are often hydrocracked or catalytically cracked to produce gasoline, jet, and diesel fuel fractions heavy vacuum gas oils can be used to produce lubestocks or as fluid catalytic cracking (FCC) feedstock... 

Wasserscheid and coworkers were the first to attempt to use ILs for the desulfurization of model solutions (dibenzothiophene [DBT], in n-dodecane) and real diesel fuels [41]. For extraction, the authors used ILs with l-alkyl-3-methylimidazolium cations ([C CiIm], n = 2, 4, 6) and various anions. Also, binary mixtures of l-alkyl-3-methylimidazolium chloride with AICI3 (Lewis-acidic ILs), the equimolar mixture of cyclohexyldiethylammonium and tri-butylammonium mefhanesulfonates (Brnnsted-acidic IL) and the equimolar mixture of cyclohexyldiefhylmethylammonium and tributylmefhylammo-nium methanesulfonafes were tested. 

Bosmann, A., Datsevich, L., Jess, A., Lauter, A., Schmitz, C., Wasserscheid, P, Deep desulfurization of diesel fuel by extraction with ionic liquids, Chem. Commun., 2494-2495, 2001. 

Time will show if there is need for desulfurization of high speed Diesel fuels. There is no doubt that many companies are actively studying desulfurization processes so as to be in a position to make the best choice. Of those for which published information is available for full scale operation, the most attractive seemed to be sulfur dioxide and furfural extraction, the latter having merit because it has successfully handled high sulfur catalytically cracked recycle stocks. The previously mentioned extraction process using anhydrous hydrogen fluoride also seems attractive because of the low treatment and high yield of refined product, but, until the results of commercial operation are fully known its merits cannot be established. 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Studies have also been reported in which the type of zeolite was varied. ZSM-5 was compared with HY as an acid function in composites with Co and NiMo/Al203 (33, 149). As might be expected, the ZSM-5 additive did little to improve the HDS of 4,6-DMDBT, as the pore size is too small to allow facile entry to 4,6-DMDBT. Mechanistic studies were conducted at higher temperature (360°C), at which cracking was severe. More than 80% of the desulfurization of 4,6-DMDBT was found to occur via center cracking of the C-C bond joining the aromatic rings. Toluene was a major product. Unfortunately, if such a scheme were to be applied to gas oil or diesel fuel, the losses in desired product would be excessive. 

If new investment is considered, then there are many new approaches to reducing the sulfur level in gas oils and diesel fuels by means of sequentially staged operations. The first stage can be a mild-severity selective process to modify the feed so as to make desulfurization more facile, or it may be a severe process to lower the sulfur level but produce a product that must be further treated to meet some other specification. 

It is probable that catalysts of this type have also been used for the desulfurization of Diesel fuels and crude oils (36). Operating pressures of 50 atm. with 0.2 cbm H2/kg. of feed and throughput of 2 kg./(liter catalyst) (hr.) are reported with catalyst-regeneration cycles of up to 150 hr. Table XIII shows the reported results temperature and yield were not given. 

CED [Conversion Extraction Desulfurization] A process for reducing the sulfur content of diesel fuel. Peroxyacetic acid oxidizes the organic sulfur compounds to sulfones, which are removed by solvent extraction. Developed in 2000 by Petro Star. 

---