Hydrodesulfurization Model

For modeling HDS, it was assumed that a huge amount of sulfur compounds in reactant mixtnre is present. A relationship between dimensionless TBP temperature and sulfur compound reactivity has been derived by Sau et al. (1997)  

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)  

The derivative of 9 with respect to HDS reactivity coefficient can be obtained from Equation 11.67. 

If only HDS reactions of different sulfur compounds are considered to take place, the differential mass balance in a plug-flow reactor can be written for each compound as 

According to previous reports, the reaction order has been assumed to be 1 (Ho, 1991). 

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J-Bonded metal complexes, hydrodesulfurization models with benzothiophene, 1, 769 with dibenzothiophene, 1, 769 Bonding studies energetics, 1, 285 overview, 1, 573—603 ring size effects, 1, 396 strength, 1, 609... 

Research into cluster catalysis has been driven by both intrinsic interest and utilitarian potential. Catalysis involving "very mixed -metal clusters is of particular interest as many established heterogeneously catalyzed processes couple mid and late transition metals (e.g., hydrodesulfurization and petroleum reforming). Attempts to model catalytic transformations arc summarized in Section II.F.I., while the use of "very mixed -metal clusters as homogeneous and heterogeneous catalysis precursors are discussed in Sections I1.F.2. and I1.F.3., respectively. The general area of mixed-metal cluster catalysis has been summarized in excellent reviews by Braunstein and Rose while the tabulated results are intended to be comprehensive in scope, the discussion below focuses on the more recent results. 

Figure 4.23. Infrared spectra of NO probe molecules on sulfided Mo, Co, and Co-Mo hydrodesulfurization catalysts. The peak assignments are supported by the IR spectra of organometallic model compounds. These spectra allow for a quantitative titration of Co and Mo sites in the Co-Mo catalyst.

Here we illustrate how to use kinetic data to establish a power rate law, and how to derive rate constants, equilibrium constants of adsorption and even heats of adsorption when a kinetic model is available. We use the catalytic hydrodesulfurization of thiophene over a sulfidic nickel-promoted M0S2 catalyst as an example ... 

Figure 7.18. Dependence of the rate of thiophene hydrodesulfurization on the partial pressures of thiophene at different temperatures, along with fits according to the Langmuir-Hinshelwood model, Eq. (32). [Fron A. Borgna and J.W. Niemantsverdriet, to be published (2003).]...

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. 

Hydrodesulfurization (HDS) is a very important large-scale process used in refineries to remove sulfur from oil products. It is actually one of the largest catalytic processes. As a model system for this process we shall consider the HDS of thio-... 

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

The general reaction occurring in hydrodesulfurization has been described in Section 2.1.1. The most studied model compound is DBT. The reactivity towards hydrogenation of the phenyl substituents already mentioned (Section 2.1.1) is also observed in the hydroprocessing of sulfur compounds. The reactivity towards hydrogenolysis of the C-S bond masks the effects associated to aromatics hydrogenation. The DBT reaction network is sketched in Fig. 8 the pseudo-first-order reaction constants measured by Houalla [68] have been included. 

Coulier, L. Kishan, G. van Veen, J. A. R., and Niemantsverdriet, J. W., Surface science models for CoMo hydrodesulfurization catalysts Influence of the support on hydrodesulfurization activity. Journal of Vacuum Science Technology A Vacuum, Surfaces, and Films, 2001. 19(4) pp. 1510-1515. 

Perez De la Rosa, M., Trader, S. Berhault, G., et al., Structural studies of catalytically stabilized model and industrial-supported hydrodesulfurization catalysts. J. Catal., 2004. 225 pp. 288-299. 

These metals form chalcogenolate complexes in several oxidation states, and from the application-oriented point of view manganese compounds have been synthesized as models for hydrodesulfurization processes and rhenium and technetium derivatives as models for radiopharmaceuticals. 

P.R. Raithby (eds), vol 2, Wiley-VCH, Weinheim, 1999, 741 (b) R.A. Sanchez-Delgado, Organometallic Modeling of the Hydrodesulfurization and Hydrodenitrogenation Reactions, Kluwar Academic Publishers, Dordrecht, 2002. 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

We illustrate the use of RBS with a study on the sulfidation of molybdenum hydrodesulfurization catalysts supported on a thin layer of Si02 on silicon [21], As explained in connection with the SIMS experiments on this model system (Fig. 4.8), the catalyst is sulfided by treating the oxidic Mo03/Si02 precursor in a mixture of H2S and H2. RBS is used to determine the concentrations of Mo and S. 

LEIS has been applied to study the surface composition of Co-Mo and Ni-Mo hydrodesulfurization catalysts [46-48], Fe-based Fischer-Tropsch [49] and ammonia synthesis catalysts [50], and model systems such as Pt evaporated on Ti02 [51]. The review of Horrell and Cocke [52] describes several applications. 

What is the structure of this Co-Mo-S phase A model system, prepared by impregnating a MoS2 crystal with a dilute solution of cobalt ions, such that the model contains ppms of cobalt only, appears to have the same Mossbauer spectrum as the Co-Mo-S phase. It has the same isomer shift (characteristic of the oxidation state), recoilfree fraction (characteristic of lattice vibrations) and almost the same quadrupole splitting (characteristic of symmetry) at all temperatures between 4 and 600 K [71]. Thus, the cobalt species in the ppm Co/MoS2 system provides a convenient model for the active site in a Co-Mo hydrodesulfurization catalyst. 

The data analysis in Table 9.3 summarizes the crystallographic information of the Co-Mo-S phase active for hydrodesulfurization. The Co-S distance in Co-Mo-S is 0.22 nm, with a high sulfur coordination of 6.2 1.3. Each cobalt has on average 1.7 0.35 molybdenum neighbors at a distance of 0.28 nm. Based on these distances and coordination numbers one can test structure models for the CoMoS phase. The data are in full agreement with a structure in which cobalt is on the edge of a MoS2 particle, in the same plane as molybdenum. 

Data of a pilot plant hydrodesulfurizer (Sherwood, 1963) are to be fitted to a Gamma model. That equation is rearranged into a linear form as ln[trE(tr)] = ln[nn/r(n)] + n In[trexp(-tr)]... 

Mo(CO)6 and Co(CO)3NO NaY zeolite Adsorption from vapor phase and H2S treatment Intrazeolite Co2Mo2S i clusters, model hydrodesulfuration catalyst [25]... 

The promoting action of cobalt on the activity for hydrodesulfurization has been shown already in the pioneering work of Byrns, Bradley and Lee (14). This promoting action might be linked with the sulfiding step, since the actual catalyst is the sulfided form of cobalt- or nickel-molybdenum-alumina. Voorhoeve and Stuiver (15) and Farragher and Cossee (16) demonstrated the promoting action for the unsupported Ni-WS2 system. Their intercalation model was based on these experiments. 

One marked effect of a hydrodesulfurization process is the buildup of hydrogen sulfide and the continued presence of this reaction product in the reactor reduces the rate of hydrodesulfurization. Thus, using the two first-order models, the effect of hydrogen sulfide on the process can be represented as ... 

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