Diesel desulfurization

Desulfurization processes are absolutely necessary for producing clean fuels. Possible strategies to realize ultradeep suffiirization currently include adsorption, extraction, oxidation, and bioprocesses. Oxidative desulfurization (ODS) combined with extraction is considered one of the most promising of these processes [13]. Ultradeep desulfurization of diesel by selective oxidation with amphiphilic catalyst assembled in emulsion droplets has given results where the sulfur level of desulfurized diesel can be lowered from 500 ppm to about 0.1 ppm without changing the properties of the diesel [12]. 

In this section, we will begin by discussing overall process designs and process alternatives. Most of the designs come from processes patented by EBC, although other players have contributed recently as well. The alterations to processes come from variations in the raw material to be desulfurized, (diesel vs. crude oil), or from point of application perspective (before or after HDS, in oil field vs. in refinery, etc.) or from changes to reaction schemes (complete desulfurization vs. stopping at an intermediate... 

Another Pseudomonas strain P. delafieldii R-8 was reported to remove 90.5% sulfur from highly desulfurized diesel oil [259], The biocatalyst achieved desulfurization via a pathway similar to the 4S pathway. The rate of desulfurization was reported to be 11.25 mmol sulfur/kg dcw/h, with the sulfur being reduced from 591 to 56 mg/L. This was achieved via two biocatalyst treatments lasting 20 hours each, although the biocatalyst was active only for first 6h in each treatment. Up to C4-DBTs were reported to be removed. Almost 100% of Q and C2 DBTs were removed and about 94% C3 DBTs and 97% C4 DBTs were removed. This strain of Pseudomonas thus appears to have a mechanism to uptake up to C4 DBTs through its cell membrane. 

It should be noted that Diversa was involved in a BDS study from 2003 to 2006 in collaboration with PetroStar, Inc. They acquired many of the strains developed by EBC and investigated the potential of desulfurizing diesel using these strains and conducted further genetic engineering and host strain manipulation to develop better biocatalysts. 

Figure 6.8.7 Deep H DS of two pre-desulfurized diesel oils [360°C, 30 bar lines indicate second order with respect to total S, see Eq. (6.8.8)] (Schmitz, 2003 Schmitz, Datsevich, and Jess, 2004).

With respect to the increasing demand for practically S-free fuels (<10ppm), S-species with a very low reactivity like dibenzothiophene derivatives have to be converted (Figure 6.8.6), typically in a second deep HDS step, which leads to additional high investment and operating costs. The enormous expenses needed today for such a deep HDS of already hydrotreated (pre-desulfurized) diesel oil can be illustrated by typical industrial data [MiRO-refinery 2004 (oral communication) Wache et al. (2006)] Deep HDS reactors have a volume of up to 500 m to treat about 400 of oil per h. The feed rate of fresh H2 is about 40 m (NTP) per m oil, the recycle rate is about three, and for a typical pressure of 6 MPa the pressure drop of about 1 MPa has to be compensated by a huge recycle compressor (1 MW). 

ASR-2 A process for desulfurizing diesel fuel by oxidation with hydrogen peroxide and formic acid. The sulfur compounds are oxidized to sulfones that are removed by adsorption on alumina. Developed by Unipure and Texaco and demonstrated at Valero Energy s refinery at Krotz Spring, LA, from 2002. 

Figure 8.6 Sulfur-specific GC-FPD chromatograms of prehydrotreated diesel, prehydrotreated diesel after oxidation, and desulfurized diesel.

Original diesel Oxidized diesel Original diesel Desulfurized diesel ... 

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 %. 

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). 

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. 

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%. 

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). 

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. 

Coal, energy Coal-oil mixtures Coal-water slurry Gas turbine coal fuel Diesel coal fuel Fuel heneficiation, desulfurization... 

Hydrofining is employed to desulfurize high sulfur diesel stocks, both virgin and cracked. The stability of cracked diesel stocks is also improved. In the diesel range, operating conditions become more severe. Compared to naphthas, temperatures are increased from the 550-600°F level to 700°F. 

Desulfurization of FCC feedstocks reduces the sulfur content of FCC products and SOX emissions. In the United States, road diesel sulfur can be 500 ppm (0.05 wt%). In some European countries, for example in Sweden, the sulfur of road diesel is 50 ppm or less. In California, the gasoline sulfur is required to be less than 40 ppm. The EPA s complex model uses sulfur as a controlling parameter to reduce toxic emissions. With hydroprocessed FCC feeds, about 5% of feed sulfur is in the FCC gasoline. For non-hydroprocessed feeds, the FCC gasoline sulfur is typically 10% of the feed sulfur. 

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. 

Rhee S-K, JH Chang, YK Chang, HN Chang (1998) Desulfurization of dibenzothiophene and diesel oils by a newly isolated Gordona strain CYKSl. Appl Environ Microbiol 64 2327-2331. 

ExxonMobil extended the Hydrofining technology to produce a 200 ppm diesel, with the Diesel Oil Deep Desulfurization technology, DODD. The reactor is packed with multiple beds of different catalysts. A preceding history of commercial experience provided data to build a model for deep HDS and pave the way to a new technology, MAK Fining. 

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. 

Grossman et al. [75,76] identified two strains as Arthrobacter and deposited them as ATCC 55309 and ATCC 55310. These were later reclassified as Rhodococcus. These strains were mostly used for studying extent of diesel desulfurization, which is covered in Section 2.2. 

Desulfurization using purified enzymes Investigations into enzymatic desulfurization as an alternative to microbial desulfurization has revealed several enzymes capable of the initial oxidation of sulfur. A study reported use of laccase with azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) as a mediator for oxidation of DBT [181]. The rate of this reaction was compared to hydrogen peroxide-based phosphotungstic acid-catalyzed oxidation and the latter was found to be about two orders of magnitude higher. The authors also oxidized diesel oil sulfur to no detectable levels via extraction of the oxidized sulfur compounds from diesel. In Table 9, the enzymes used in oxidation of DBT to DBTO are reported. 

Improving stability of microbial strains in organic solvents is another challenge for enabling commercialization of desulfurization processes for gasoline and diesel applications. This was a target for a project in Klibanov s group [231], whose approach... 

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