The Hydrodesulfurization Reaction

Hydrodesulfurization (HDS) is the reaction through which sulfur is removed from petroleum feedstocks in refineries hy their interaction with hydrogen over solid catalysts under rather severe conditions of temperature and pressure, according to the generic transformation represented hy Eq. 1.1  

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Step 1 represents adsorption of ammonia and step 2 its activation. The irreversible step 3 is obviously not elementary in nature, but unfortunately much information on the level of elementary steps is not available. Step 4 describes water formation and step 5 is the reoxidation of the site. Step 6 describes the blocking of sites by adsorption of water. The model thus relies on partially oxidized sites and vacancies on an oxide, similarly to the hydrodesulfurization reaction described in Chapter 9. The reactions are summarized in the cyclic scheme of Fig. 10.15. 

Can the hydrodesulfurization reaction also be considered to be a Mars-van Krevelen reaction ... 

The sulfidation mechanisms of cobalt- or nickel-promoted molybdenum catalysts are not yet known in the same detail as that of M0O3, but are not expected to be much different, as TPS patterns of Co-Mo/A1203 and Mo/Al203 are rather similar [56J. However, interactions of the promoter elements with the alumina support play an important role in the ease with which Ni and Co convert to the sulfidic state. We come back to this after we have discussed the active phase for the hydrodesulfurization reaction in more detail. 

The thermodynamics of the hydrodesulfurization reaction has been evaluated from the equilibrium constants of typical desulfurization or partial desulfurization reactions such as ... 

The near completely random motion of the catalyst bed virtually ensures an isothermal operation, but the efficiency of the hydrodesulfurization reaction tends to suffer because of the back mixing of the product and feedstock. Hence, to effect sulfur removal at over 75% efficiency, it may be necessary to operate with two or more reactors in series. The need for two or more of these units to effectively desulfurize a feedstock may be cited as a disadvantage of the reactor, but the ability of the reactor to operate under isothermal conditions as well as the onstream catalyst addition-withdrawal system and the fact that the reactor size required for an expanded catalyst bed is often smaller than that required for a fixed bed can be cited in support of such a unit. 

Thus, if the hydrogen is not recycled, the process economics are unfavorable and, in addition, the efficiency of the hydrodesulfurization reaction may be adversely affected because of the possible competing reactions outlined above. 

The extent of the hydrocracking is, like the hydrodesulfurization reaction, dependent upon the temperature, and both reaction rates increase with increase in temperature. However, the rate of hydrocracking tends to show more marked increases with temperature than the rate of hydrodesulfurization. The overall effect of the increase in the rate of the hydrocracking reaction is to increase the rate of carbon deposition on the catalyst. This adversely affects the rate of hydro-desulfurization hydrocracking reactions are not usually affected by carbon deposition on the catalyst since they are more dependent upon the noncatalytic scission of covalent bonds brought about by the applied thermal energy. 

In addition, it has been found that isomerization sites are completely poisoned by the carbon deposit and that the hydrogenation reaction is more sensitive to poisoning than the hydrodesulfurization reaction indicating either the existence of different sites or of different reaction mechanisms. 

In this paper factors controlling the catalytic activity in the hydrodesulfurization reaction (HDS) are discussed. The SiOa-supported phosphormolybdenum heteropolyacid (HPMo) is used as a model catalyst. Two types of the catalyst deactivation have been shown. The reversible deactivation effect is related with changes in the molybdenum valence, its 0- and 0,S-surrounding and adsorbtion of the S-containing reaction products. The HDS activity is irreversibly changed when the transformation of the catalyst phase composition is carried out ... 

Your supervisor at Kleen Petrochemical wishes to use a hydrodesulfurization reaction to produce ethylbenzene from a process waste stream. You have been assigned the task of designing a reactor for the hydrodesulfurization reaction. Focus reactor design. 

In this work a study about the effect of external difiiision on the hydrodesulfurization reaction of straight run gas oil is presented. The experimental study was carried out at typical HDS operating conditions over a commercial catalyst using an iqiflow and downflow trickle-bed microreactor (TBMR). The experimental results showed that upflow is the best system for conducting kinetic studies in order to avoid interphase mass transfer limitations. 

As an alternative to the hydrodesulfurization reaction (HDS, Sec. VI.D.l.), the removal of refractory sulfur-containing compounds can be accomplished by oxidation with hydrogen peroxide in the presence of a catalyst. Thus Palomeque et al. found that dibenzothiophene can be fully oxidized to sulfone in a niffile solvent over activated MgAl-LDO at 60°C (585a), where the resulting sulfone can be extracted by the solvent. 

The ability to tailor metal oxide systems for physical and chemical applications represents an obvious advantage for improved performance. Interesting examples are cobalt and nickel oxides, which find applications in many oxidation reactions and also as promoters of Mo and W oxide catalysts for the hydrodesulfurization reactions of middle distillates. NiO and CoO can be mixed in all proportions to form homogeneous solid solutions of the type C0jcNii xO, 0 < x < 1, in a well-ordered rock salt crystal structure in the bulk material. When used in redox reactions in alkaline media [38], these solid solutions exhibit electrochemical properties. The reason why CoO and NiO form solid solutions lies in the close match of... 

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