Kinetic desulfurization

Effect of HjS, Carbon Oxides, Etc. Hydrogen sulfide in the treat gas has an inhibiting effect on the kinetics of hydrotreating. Being a product of the desulfurization reactions, HjS must diffuse from the catalyst surface into the bulk gas stream. Any HjS present beyond that formed, further slows down the rate of diffusion with a consequent decrease in the amount of desulfurization for a given amount of catalyst. Therefore, additional catalyst would be required. 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

Effects of Sulfur Coverage. The kinetic results reported in the previous section are from reactions performed on surfaces that are initially clean. The surfaces after reaction were examined by AES and shown to be covered with carbon and sulfur at coverages close to a monolayer. It is interesting to note that although this is the case even after reaction times on the order of minutes the reaction rate is constant for a period of approximately one hour. Either the reaction is occurring on top of this oarbon/sulfur layer or these species are present as sulfur containing hydrocarbon fragments that are intermediates in the desulfurization process. 

Wang P, AE Humphrey, S Krawiec (1996) Kinetic analysis of desulfurization of dibenzothiophene by Rhodo-coccus erythropolis in continuous cultures. Appl Environ Microbiol 62 3066-3068. 

A strain P. delafeildii R-8 was reported to desulfurize DBT giving 2-HBP as the end product [92], This strain was isolated from sewage pool of Shanghai oil field. The report described the effect of cell density, oil/water phase ratio, which was very similar to that of IGTS8. These parameters will be discussed in Section 2.2.10. A Michaelis-Menten model was used to describe the kinetics and the Vmax and Km were reported for DBT to be 13mmol/kg dcw/h and 1.3 mM, respectively. 

The study did model and fit all data using a kinetic model given in Eqn. (5). The experimental data was fit to obtain values of a and (3. The utility of this model is limited because it is not based on independent parameters. However, the model does show how the desulfurization activity depends on the growth rate and cell density for various experimental conditions. 

Use of a N. globerula R-9 strain was demonstrated for desulfurization of straight run diesel oils. Sulfur reduction from 1807 to 741 mg/dm3 was reported at a desulfurization rate of 5.1 mmol/Kgdcw/h. The desulfurization of model oils containing DBT and 4,6 dimethyl DBT was studied and Michaelis-Menten kinetic parameters were reported. 

Wang, P., and Krawiec, S., Kinetic analyses of desulfurization of dibenzothiophene by Rhodococcus erythropolis in batch and fed-batch cultures. Applied and Environmental Microbiology, 1996. 62(5) pp. 1670-1675. 

Kobayashi, M. Horiuchi, K. Yoshikawa, O., et al., Kinetic analysis of microbial desulfurization of model and light gas oils containing multiple alkyl dibenzothiophenes. Bioscience Biotechnology and Biochemistry, 2001. 65(2) pp. 298-304. 

Marcotrigiano, G. Peyronel, G. Battistuzzi, R. 1972. Kinetics of desulfuration of S-35-labeled thiourea in sodium hydroxide studied by chromatographic methods. /. Chem. Soc., Perkin Trans. 2 1539-1541. 

Vestal, M.L. and Johnston, W.H., "Desulfurization Kinetics of Ten Bituminous Coals", Report No. SRIC 69-10. Baltimore Scientific Research Instruments Corp., 1969. 

It was observed that the overall kinetics for the desulfurization could be described by lumping the rate constants for the individual sulfur species into four reactivity groups as shown in Fig. 6. These groups are listed in decreasing order of reactivity in Fig. 7. The relative contributions of the four groups were 39, 20, 26, and 15% for groups 1, 2, 3, and 4, respectively. Thus, if the new sulfur standards are to be met with this feed, groups 1, 2,... 

Fig. 7. Kinetic reactivity groups in desulfurization (360°C, 2.9 MPa). Reprinted with permission from Ref. 14, Ma et at. (1994). Copyright 1994 American Chemical Society.

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

The kinetics of the reactions of triphenyl phosphine with a - and trans- 2-butene sulfides and with l-butene sulfide have been measured and each of the reactions was found to be bitnolecular, first on lor ki each reactant. Further, the rates were found to be relatively insensil iv<-to large changes in the dielectric constant of the medium. Such result suggest that charge separation is not important in the transition stair for the rate-coDtoolbng step, and that the desulfurization proceeds by... 

Residuum desulfurization kinetics are generally not first order. Figure 5 illustrates this with a first-order plot for desulfurization of Arabian light residuum. On this type of plot a first-order reaction would yield a straight line with a slope corresponding to the reaction rate constant. The over-all desulfurization reaction is not therefore first order and can in fact be represented by second-order kinetics. However, the figure shows that it may also be considered as the sum of two competing first-order reactions. The rates of desulfurization of the oil and asphaltene fractions are reasonably well represented as first-order reactions whose... 

A tandem 1,4-addition-Meerwein-Ponndorf-Verley (MPV) reduction allows the reduction of a, /i-unsaturated ketones with excellent ee and in good yield using a camphor-based thiol as reductant.274 The 1,4-addition is reversible and the high ee stems from the subsequent 1,7-hydride shift the overall process is thus one of dynamic kinetic resolution. A crossover experiment demonstrated that the shift is intramolecular. Subsequent reductive desulfurization yielded fiilly saturated compounds in an impressive overall asymmetric reductive technique with apparently wide general applicability. 

Barton and coworkers exploited this strategy in the preparation of overcrowded ethylenes456 usually the desulfurization of a thiirane is accomplished by one equivalent of tertiary phosphine, mainly triphenylphosphine. However, spontaneous loss of sulfur from thiiranes substituted by aryl or halogen has sporadically been reported. Huisgen has reviewed this subject455 and performed many kinetic studies. He found that the desulfurization step can be accomplished by catalytic thiolates and also by thiobenzophenone or other thioketones, although in this case the reaction is slower (equation 131). 

Trithiepine 47 was dissolved in DCCI3 and the photolysis was monitored by 111 NMR spectroscopy to determine the kinetic parameter of the reaction. In the spectra, the decrease of the intensity of signals of 47 and the appearance and increase of the intensity of new signals of 48 were observed. The plot of ln(trithiepine) versus the reaction time reveals that the desulfurization of 47 to 48 follows a first-order kinetics with respect to the substrate concentration. The rate constant and the half-life period of this photoreaction were calculated to be 7 = (2.82 1.11) x 10-4 and /1/2 = 41.0min <2000TL1801>. 

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