Refractory Sulfur Compounds

Hydrofining has all the advantages of acid treating without the disadvantages. For example, acid treating does not readily remove refractory sulfur compounds such as thiophene the treated products must be rerun to remove polymers with a consequent yield loss and disposal of the acid sludges is a serious problem. 

In a practical HDS process for gas oil, both aromatic species existing in the feed and various types of sulfur compounds compete for the active sites on the catalyst surface. Moreover, H2S and some other hydrocarbons produced in the early stages of the desulfurization appear to inhibit the HDS of the less reactive sulfur species. The reactivities of refractory sulfur compounds and the effects of inhibitors in gas oils need to be fully understood for the development of an improved economical desulfurization process. 

As this gas oil was desulfurized with a conventional CoMo/A1203 catalyst, the most refractory sulfur compounds were found to be 4-methyldibenzo-thiophene and 4,6-dimethyldibenzothiophene, consistent with previous reports (5-8) on the relative reactivity of various sulfur species. 

The ODS of hydrocarbon fuels consists of the conversion of sulfur compounds in the fuels by oxidation to element sulfur, sulfur oxides, sulfoxides, and/or sulfones followed by adsorption or abstraction separation of the oxidized sulfur compounds from the hydrocarbon fuels. The potential advantages for ODS are (a) the process does not need to use H2 gas (b) ODS takes place at relatively mild operating conditions in comparison with HDS or even at ambient conditions and (c) ODS is able to remove some refractory sulfur compounds, such as 4,6-DMDBT, that are difficult to be removed by HDS or selective adsorption. 

Isoda, T., Ma, X.L., Nagao, S., and Mochida, I. Reactivity of refractory sulfur compounds in diesel fuel. Part 3. Coexisting sulfur compounds and by-produced H2S gas as inhibitors in HDS of 4,6-dimethyldibenzothiophene. Journal of the Japan Petroleum Institute, 1995, 38, 25. 

Recently, investigations demonstrated that sulfur compounds remaining in diesel fuels at sulfur level lower than 500 ppm are dominantly the DBTs with alkyl substituents at the 4- and/or 6-position, and are lower in HDS reactivity.These species are termed refractory sulfur compounds. Both steric hindrance and electronic factor are responsible for the observed low reactivity of 4- and 6-substituted DBTs. ... 

Thiophenic compounds are naturally present in crude oil. Although hydrodesulfurization targets them and converts toward hydrocarbon and hydrogen sulfide [7] still significant quantities are present after this treatment. The reactivities of the 1- to 3-ring compounds decreases in the order thiophenes > benzothiophenes > dibenzothiophenes. In gasoline benzothiophenes are present, in jet fuel - benzothiophenes and dibenzothiophenes whereas in diesel fuel dibenzothiophenes and 4, 6-dimethyldibenzothiophene (4,6-DMDBT) are found. The latter is considered as the most refractory suffur compound towards HDS. The most common refractory sulfur compounds in liquid fuels are presented in Fig. 4. 

Among these blendstocks, the alkyl dibenzothiophenes with alkyl groups at the four- or/and six-positions are typical representatives of the refractory sulfur compounds, which are difficult to remove using the conventional HDS process. Thus, to satisfy the environmental regulations, deep desulfurization and ultra-deep... 

Not fully competent to refractory sulfur compounds (e.g., DBT, 4-MDBT, 4,6-DMDBT)... 

The most refractory DBTs have substitutions at the 4 and 6 positions, which are adjacent to the sulfur moiety and are believed to sterically hinder access of the sulfur atom to the catalyst surface (8,9). In middle distillate (diesel range) fractions, the sulfur that remains after aggressive HDS treatment is typically in the form of substituted DBT compounds, particularly those substituted at the 4 and 6 positions. As regulations on sulfur levels in fuels become stricter, more of the HDS-refractory compounds must be removed. This requires increasingly more severe and inherently more costly HDS. As a result, HDS-refractory sulfur compounds represent a significant barrier to reaching very low sulfur levels in middle and heavy distillate range fuels. 

An opposite situation happens when the liquid phase is very dilute with B, while gas-phase reactant A is highly soluble (or at high pressures). A common example of such a situation is when slurry systems are employed for hydrodesulfurization of oil fi-actions, in which ppm levels of refractory sulfur compounds (such as dibenzothiophenes and alkyl diben-zothiophenes) may be present in fuel oils like diesel. In comparison, the gas phase is pure hydrogen, which at high pressures behaves like a fairly soluble gas in the hydrocarbon liquid. In such a case, the liquid-phase concentration of the gas-phase reactant is fairly constant throughout (and much higher than the concentration of the liquid-phase reactant B), including inside the catalyst pellets. The effective depletion rate of B (Eq. 6.4) reduces to... 

Figure 10.50. Dibenzothiophene (DBT). Aikylated DBT with alkyl (e.g., methyl) groups at the 4-and 6-positions, that is, 4-MDBT and 4,6-DMDBT, are the refractory sulfur compounds that are most difficult to remove by HDS or sorbents aiming at bonding with S due to steric hindrance.

Schulz, H. Bohringer, W., Ousmanov, F., and WaUer, F. Refractory Sulfur Compounds in Gas OUs. Fuel Processing Technol., 1999, 61, 5 2. 

Isoda, T. Ma, X. and Mochida, . Reactivity of Refractory Sulfur Compounds in Diesel Fuel (Part 2) Inhibition of Hydrodesulfurization Reaction of 4,6-Dimethyldibenzothiophene by Aromatic Compound. J. Japan Petrol. Inst. 1994, 37, 506. Farag, H. Sakanishi, K. Mochida, L Whitehurst, D. D. Kinetic Analyses and Inhibition by Naphthalene and H2S in Hydrodesulfurization of 4,6-Dimethyldibenzothiophene (4,6-DMDBT) over CoMo-Based Carbon Catalyst. Energy Fuels 1999, 13,449. 

Such a relationship was obtained from experimental information of model compounds and it is basically an expression with adjustable parameters (Narasimhan et al., 1999). Equation 11.67 indicates that the reactivity of sulfur compounds decreases monotonically as TBP of fraction increases, that is, light fractions contain the most reactive sulfnr componnds while heavy fractions are concentrated with the most refractory sulfur compounds (San et al., 1997). The continuous kinetic approach allows for continuous description, bnt although not determined, the number of sulfur compounds is finite in the petroleum mixture. The kinetic behavior of each sulfur compound must remain invariant even if its distribution is described as a continuum function, so that in order to keep the consistence between discrete and continuous descriptions a factor must be introduced. The factor or Jacobian to change from 0- to k-coordinates was assumed to be (Narasimhan et al., 1999) ... 

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