Hydrodesulfurization Reactions

Experimental data of the sulfur curves, that is, boiling temperature versus sulfur content, was used together with Equation 11.71 and the hydrocracking model parameters previously determined to calculate the model parameters for the HDS reaction. The values of the three parameters are = 7.5h, = 0.08h , and P = 2. 

Regarding the shapes of the sulfur curves of the feed and the hydrotreated product, they are typical ascending curves, which indicate that the heavier the fraction, the more sulfur content that they contain. For the snlfnr curve of the product, it is observed that sulfur in light fractions is almost totally removed, while in the heavy fraction, that is, vacuum residue, sulfur content although drastically reduced is still present. In the heavy fraction, the reduction of sulfur content can be attributed essentially to proceed via hydrocracking, because the sulfur compounds that this fraction possesses are harder to react, that is, refractory in nature, under the operating conditions studied, which is corroborated by the low value of It is therefore 

FIGURE 11.19 Comparison of experimental data (symbols) of HDS and simulated results (lines). ( ) Feedstock sulfur distribution, (o) product sulfur distribution. T = 382°C, P = 9.8 MPa, LHSV = 0.2h-, and H -to-oil ratio = 5800SCF/bbl. 

Modeling of Processes and Reactors for Upgrading of Heavy Petroleum 

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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 ... 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

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. 

Among these sulfides, only the ordinary cobaltfll) sulfide, CoS has commercial applications. It is used as a catalyst for hydrogenation or hydrodesulfurization reactions. Cobalt(II) sulfide is found in nature as the mineral syco-porite. The mineral linneite is made up of C03S4, tricobalt tetrasulfide. 

The olefin (369) is thought to arise by reaction of the intermediate diradical (368) with hydrogen before subsequent hydrogenation of 369 to 370.767-769 When benzo[6]thiopheno[3,2-6]benzo[6]thiophene (154) is hydrodesulfurized, bibenzyl, (rans-stilbene, and 2-phenyl-benzo[6]thiophene are obtained in amounts determined by the catalyst and reaction conditions.768 This result lends support to the view that removal of sulfur precedes hydrogenation in hydrodesulfurization reactions. 

The picture here is even less clear than for reduced catalysts. In spite of the fine studies by Delmon and workers on the bulk mixed sulfides, and that of Schuit and De Beer and workers on various hybrid catalysts, it is not conclusively proved that bulk sulfides are the active ingredients for hydrodesulfurization reactions for the mildly sulfided catalysts employed in industry. 

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

Table 4-2 Heats of Reaction for Selected Hydrodesulfurization Reactions... 

The data indicate the types of reactions that can occur during the hydrode-sulfurization reaction and include those reactions that will occur at the upper end of the temperature range of the hydrodesulfurization process whether it is a true hydrodesulfurization reaction or a cracking reaction. Even though some of the reactions given here may only be incidental, they must nevertheless be taken into account because of the complex nature of the feedstock. The several process variations (Chapter 9) which (in addition to the fact that the overall hydrodesulfurization process is exothermic (Table 4-2) also contribute to the complexity of the product mix. 

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... 

Table 4-3 Hydrodesulfurization Reactions for Typical Sulfur Compound Types in Petroleum...

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 a study of the deactivation by coking of an atmospheric residue HDM catalyst, we have been able to obtain coked catalysts almost free from metal deposits in batch reactor and coked catalysts containing small amounts of metal sulfide deposits in continuous flow reactor using a Safaniya atmospheric residue under similar experimental conditions (30). We report in this paper a study of the deactivating effects of the deposits using toluene hydrogenation, cyclohexane isomerization and thiophene hydrodesulfurization reactions. 

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 ... 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

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. 

Hydrodesulfurization reactions occur when hydrogen reacts with the sulfur atom and forms hydrogen sulfide. The rate of the reaction depends on the type of sulfur compounds present, namely, aliphatic and aromatic thiols, sulfides, disulfides, various thiophenes and thiophene derivatives. The order of reactivity among these... 

Product distribution (wt. %) and liquid yield in the DBT hydrodesulfurization reaction... 

Because of these special characteristics the complex [(triphos)Ir(Ti -benzene)]BF4 has been successfully used as a precursor in modeling a complete stepwise hydrodesulfurization reaction in solution by the sequential addition of thiophene, H and H as shown in Fig. 4.3 [27],... 

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