Desulfurization sulfur-containing molecules

There is a variety of sulfur-containing molecules in a residuum or heavy crude oil that produce different products as a result of hydrodesulfurization reaction. Although the deficiencies of current analytical techniques dictate that the actual mechanism of desulfurization remain largely speculative, some attempt... 

Yet another application of solid-state catalysis occurs in the desulfurization of petroleum. Natural petroleum includes various molecules that contain sulfur atoms. Combustion of this petroleum produces S02, which must be removed from the exhaust to prevent air pollution. One way to prevent pollution by S02 is to remove the sulfur from the petroleum before it is used for fuel—the desulfurization of petroleum. One type of sulfur containing molecules found in petroleum are thiols, which can be written R—SH, where R represents a molecular fragment containing a long chain of carbon atoms. In desulfurization the goal is to remove the sulfur from this molecule to produce a hydrocarbon (R—H) ... 

The molecular size distributions and the size-distribution profiles for the nickel-, vanadium-, and sulfur-containing molecules in the asphaltenes and maltenes from six petroleum residua were determined using analytical and preparative scale gel permeation chromatography (GPC). The size distribution data were useful in understanding several aspects of residuum processing. A comparison of the molecular size distributions to the pore-size distribution of a small-pore desulfurization catalyst showed the importance of the catalyst pore size in efficient residuum desulfurization. In addition, differences between size distributions of the sulfur- and metal-containing molecules for the residua examined helped to explain reported variations in demetallation and desulfurization selectivities. Finally, the GPC technique also was used to monitor effects of both thermal and catalytic processing on the asphaltene size distributions. 

Since residuum hydroprocessing involves both demetallation and desulfurization reactions, the residuum metal and sulfur-containing molecules and their sizes are important. As shown in Table V and illustrated in Figure 5 for the Arabian Light vacuum asphaltene, the sulfur, nickel, and vanadium compounds for the asphaltene from any given residuum have similar sizes ... 

Catalyst selectivity differences have been found for sulfur and metals removal in residuum hydroprocessing (19). Mass transfer limitations are believed to be important (20). The data reported here show that the metal-containing molecules are larger consequently, they should be more subject to diffusion restrictions than the sulfur-containing molecules. Therefore, it will be more difficult for a small pore catalyst to demetallate a residuum than to desulfurize it. 

Application of this model to a residuum desulfurization gave a linear relationship. However, it is difficult to accept the desulfurization reaction as a reaction that requires the interaction of two sulfur-containing molecules (as dictated by the second-order kinetics). To accommodate this anomaly, it has been suggested that, as there are many different types of sulfur compounds in residua and each may react at a different rate, the differences in reaction rates offered a reasonable explanation for the apparent second-order behavior. For example, an investigation of the hydrodesulfurization of an Arabian light-atmospheric residuum showed that the overall reaction could not be adequately represented by a first-order relationship. However, the reaction could be represented as the sum of two competing first-order reactions and the rates of desulfurization of the two fractions (the oil fraction and the asphaltene fraction) could be well represented as an overall second-order reaction. 

The types of organosulfur compound present in petroleum feedstocks are alkyl and aryl thiols (RSH), thioethers (RSR ), disulfides (RSSR ), and thiophenic compounds (Fig. 1). The ease with which sulfur is abstracted depends very much on the nature of the sulfur-containing molecule aliphatic compounds (thiols, thioethers) are usually desulfurized much more easily than heteroaromatic (e.g. thiophenes, benzothiophenes, dibenzothiophenes). Among the latter, reactivity decreases in the order thiophene > benzothiophene > dibenzothiophene. The presence of aliphatic substituent groups can sometimes alter reactivity. The sterically hindered compound 4,6-dimethyldibenzothiophene is, for example, very difficult to desulfurize. 

The sulfur-abstracting capacity of [(Cp )2Mo2Co2(CO)4( 3-S)2(/i4-S)] (64) vis-a-vis sulfur-containing molecules other than thiophene has been studied. For example, whereas sulfur abstraction from thiophene produces a mixture of hydrocarbons cracked to various extents (but notably no butadiene, which is a product of HDS of thiophene over heterogeneous catalysts), a completely clean reaction is seen for thiols (RSH) which are desulfurized to Thiophenol (PhSH) is desulfurized... 

In the presence of a vapor phase with sulfur-containing molecules, metal surfaces become sulhdized. A familiar class of sulfidic catalysts are M0S2 and WS2- They are usually promoted with NiS or CoS. Such catalysts are very active in desulfurization and hydrodenitrogenation catalysis. 

In natural gas fuel, some of the sulfur compounds are present naturally (from the wellhead) while other compounds are added as an odorant for leak detection. In other fuels such as anaerobic digester gas or coal bed methane, all sulfur present is naturally occurring. In either case, many different types of inorganic and organic sulfur-containing molecules may be present and sulfur levels must be reduced to sub-ppm level. A desulfurization system for natural gas and other fuels depends on the concentration as well as the nature of the sulfur compounds. Desulfurization can be accomplished by any of the following processes ... 

A major problem in the catalytic hydrodesulfurization of residual oils is the deactivation of the catalyst by metal-containing asphaltenic species in the feed. As can be seen from the results of a typical desulfurization experiment presented in Fig. 1, the catalyst shows a rapid initial decline which is attended with a fast build-up of coke on the catalyst. At a relatively low catalyst age 0, as defined in Section IV, a stationary coke level is reached. In contrast, the deposition of the inorganic remnants of the hydro-cracked asphaltenes (mainly vanadium and nickel sulfides) continues and gradually clogs the pores in the outer zone of the catalyst particles, as confirmed by electron microprobe analyses of spent catalyst samples (see Fig. 2). This causes a slow further loss in desulfurization activity over a longer period of time. Ultimately, the catalyst becomes totally inactive for desulfurization because the - still active - inner core has become completely inaccessible to the sulfur-bearing molecules. 

---