ARTICLE pubs.acs.org/IECR
Kinetic Study of the Reaction System 4,6-DMDBT þ Naphthalene þ Carbazole, Using NiMo/Al2O3-SiO2(x) Catalysts Felipe Sanchez-Minero†,‡ and Jorge Ramírez*,†,§ †
UNICAT, Departamento de Ingeniería Química, Facultad de Química, UNAM, Cd. Universitaria, Mexico 04510, D.F., Mexico DIQP, ESIQIE, Instituto Politecnico Nacional, Zacatenco, Mexico 07738, D.F., Mexico § Instituto Mexicano del Petroleo, Eje central Lazaro Cardenas 152, Mexico 07730, D. F., Mexico ‡
ABSTRACT: Kinetic study of the reaction system 4,6-DMDBT þ naphthalene þ carbazole over NiMo/Al2O3-SiO2 catalysts (0 and 10 wt % of SiO2 in the support) was carried out using one, two, or three of these components in the reaction mixture. Kinetic equations that best fitted the experimental data were obtained. Then, kinetic parameters were estimated using Powell’s minimization. The results show that the catalyst containing silica present higher activity for the transformation of 4,6-DMDBT, naphthalene, and carbazole. It was found that the transformations of 4,6-DMDBT and naphthalene were strongly affected by the presence of carbazole, whereas the hydrodenitrogenation of carbazole in the presence of naphthalene and 4,6-DMDBT was similar to that observed for this compound alone. This behavior was due to the relative values of the adsorption constants of the three components of the reaction mixture.
1. INTRODUCTION Hydrotreatment of middle distillates requires deep hydrodesulfurization (HDS) to comply with strict environmental legislations.1 Thus, most countries are compelled to adopt changes in their refination system to produce diesel with ultralow levels of sulfur ( k6 > k2 > k1. Naphthalene is the most reactive compound (k4) since it does not contain a heteroatom like 4,6-DMDBT or carbazole. Carbazole presents higher reactivity than 4,6-DMDBT (k6 > k2), possibly because carbazole strongly bonds to the surface, preventing the adsorption and transformation of 4,6-DMDBT. Besides, the contribution of the direct desulfurization route (k1) to the total transformation of 4,6 DMDBT is small, confirming that the transformation of 4,6-DMDBT occurs mainly by the hydrogenation route (k2). Table 2 show the estimated values of the apparent adsorption constants (K) for the components studied. The values of K follow the decreasing order KC > K46 > KH2S > KNH3 > KN. Taking in to account that the KN/K46 ratio is 0.02 for NiMo-SAC 10 and 0.03 for NiMo-SAC 0, we can suggest that compared to 4,6DMDBT, naphthalene was weakly adsorbed over the catalytic surface. This indicates that the inhibition of the transformation of 4,6DMDBT by naphthalene is mainly due to its higher concentration in the reaction mixture rather than to the value of its adsorption constant (naphtahlene/4,6-DMDBT molar ratio was 6.25). These results are in good agreement with those of Farag21 using CoMo/C catalyst. 2675
dx.doi.org/10.1021/ie100692a |Ind. Eng. Chem. Res. 2011, 50, 2671–2677
Industrial & Engineering Chemistry Research
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Table 2. Apparent Reaction Rate and Adsorption Constants for the Complete Reaction Mixture (4,6-DMDBT, Naphthalene, and Carbazole) over NiMo-SAC Catalysts at 598 K and 4 MPa NiMo-SAC 0
NiMo-SAC 10
4,6-DMDBT k1 (h-1)
0.030
0.024
k2 (h-1)
0.129
0.179
k3 (h-1)
0.092
0.085
K46 (L/mol)
5.54
5.16
KH2S (L/mol)
1.13
0.98
Naphthalene 0.606 0.007
0.756 0.011
KN (L/mol)
0.18
0.09
Carbazole k6 (h-1)
0.604
0.584
k7 (h-1)
0.763
1.023
k8 (h-1)
0.336
KNH3 (L/mol)
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
k4 (h-1) k5 (h-1)
KC (L/mol)
are strongly affected by the presence of carbazole, whereas the transformation of carbazole does not show a significant change with respect the HDN of the pure compound. This is due to the significant difference between the estimated values of the apparent adsorption constants of 4,6-DMDBT and carbazole, which includes the effects of THC and OCHA. The reaction rate equations with low R2 values, close to 0.95, indicate that those reaction rate models need some improvement. We believe that the main changes should be in the terms associated with the products of carbazole HDN, particularly THC.
’ ACKNOWLEDGMENT The authors are grateful for financial support through projects DGAPA-UNAM IN-101406 and CONACyT 49479. J.R. acknowledges a sabbatical grant from DGAPA-UNAM.
0.368
78.26
74.82
0.83
0.81
Carbazole presents the higher adsorption strength. The KC/ K46 ratio was 14.5 for NiMo-SAC 10 and 14.1 for NiMo-SAC 0. Previously, Koltai22 reported a value of 10.01 for the carbazole/ 4,6-DMDBT apparent adsorption constants ratio. Therefore, the presence of carbazole inhibits the transformation of 4,6-DMDBT and naphthalene because all the components compete for the same type of adsorption sites, but carbazole is strongly adsorbed over the active surface, preventing the adsorption of the sulfur and aromatic compounds. Since the partially hydrogenated carbazole products, THC and OCHA, were not considered in the model equations, the adsorption constant of carbazole represents in fact an apparent adsorption constant, which contains the effects of the adsorption of THC and OCHA. A similar approach was taken previously by Ho.23 Finally, the apparent adsorption constants of H2S and NH3 show that these compounds can inhibit the adsorption of the reactants studied, mainly the adsorption of naphthalene. The KH2S/K46 and KNH3/K46 ratios were close to 0.2 and 0.15, respectively. Therefore, H2S and NH3 can affect slightly the adsorption of 4,6-DMDBT. Carbazole adsorption was not affected by the presence of H2S and NH3.
4. CONCLUSIONS From the experimental results and the modeling of the reaction system 4,6-DMDBT-naphthalene-carbazole over NiMo-SAC catalysts, the following conclusions can be drawn: Catalyst containing silica present a higher activity than the catalyst without silica during the transformation of 4,6-DMDBT, naphthalene, and carbazole. This is because the support with silica improves the hydrogenation capacity of the catalyst and the reactants studied are mainly transformed by means of the hydrogenation route. The simultaneous transformation of 4,6-DMDBT, naphthalene, and carbazole over NiMo-SAC catalysts presents different levels of inhibition. The conversion of 4,6-DMDBT and naphthalene
’ NOTATION C46 = 4,6-DMDBT concentration CDMDP = DMDP concentration CMCHT = MCHT concentration CDMDCH = DMDCH concentration CN = naphthalene concentration CT = tetralin concentration CD = decaline concentration CC = carbazole concentration CTHC = THC concentration CCHB = CHB concentration CDCH = DCH concentration CH2S = H2S concentration CNH3 = NH3 concentration k1 = apparent reaction constant for the direct desulfurization of 4,6-DMDBT k2 = apparent reaction rate constant for the hydrogenation of 4,6DMDBT k3 = apparent reaction constant for the hydrogenation of MCHT k4 = apparent reaction constant for the hydrogenation of naphthalene k5 = apparent reaction constant for the hydrogenation of tetralin k6 = apparent reaction constant for the hydrogenation of carbazole k7 = apparent reaction constant for the hydrogenation of THC to CHB k8 = apparent reaction constant for the hydrogenation of THC to DCH K46 = apparent adsorption constant of 4,6-DMDBT KN = apparent adsorption constant of naphthalene KC = apparent adsorption constant of carbazole KH2S = apparent adsorption constant of H2S KNH3 = apparent adsorption constant of NH3 R2 = square of the sample correlation coefficient between the outcomes and their predicted values ’ REFERENCES (1) Knudsen, K. G.; Cooper, B. H.; Topsøe, H. Catalyst and process technologies for ultra low sulfur diesel. Appl. Catal., A 1999, 189, 205. 2676
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