Ind. Eng. Chem. Res. 2001, 40, 131-135
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Catalytic Hydroprocessing of Aromatic Compounds: Effects of Metal Sulfide Deposits Formed in Commercial Residuum Hydroprocessing Mitsugu Yumoto,†,‡ Simon G. Kukes,§ Michael T. Klein,† and Bruce C. Gates*,†,| Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716, Cosmo Research Institute, Satte, Saitama, Japan 340-01, Amoco Research Center, Post Office Box 3011, Naperville, Illinois 60566, and Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616
A Ni-Mo/Al2O3 hydroprocessing catalyst that was used for a year in a continuous commercial process for the hydroprocessing of Arabian Light and Arabian Heavy petroleum residua underwent deactivation associated with deposits of nickel and vanadium sulfides and coke. The used (spent) catalyst and a regenerated catalyst formed by burning off the coke deposits in the spent catalyst were tested for hydroprocessing of naphthalene, dibenzothiophene, or quinoline in a batch reactor at 350 °C and 165 atm. The conversion data were analyzed to determine approximate reaction networks, and the pseudo-first-order rate constants in these networks were determined for each reactant, for the fresh, spent, and regenerated catalysts. The spent catalyst, which contained 1.1 wt % Ni and 2.7 wt % V (based on the mass of the fresh catalyst) retained only very little of its initial activity, and most of the activity loss is attributed to coke deposits, as the activity of the regenerated catalyst was only slightly less than that of the fresh catalyst. The performance of the regenerated catalyst was nearly the same as that of a model catalyst with the same Ni and V contents that was made by treating the fresh catalyst with nickel and vanadium naphthenates to form metal sulfide deposits. The data show that the performance of the model catalyst provides a good approximation of the performance of the regenerated catalyst. Thus, such model catalysts are inferred to provide a good basis for developing correlations to predict the effects of deposited nickel and vanadium sulfides on catalyst aging. However, the results are restricted to catalysts with relatively small amounts of metal sulfide deposits and do not account for the effects of pore plugging that result when large amounts of deposits form. Introduction Catalytic hydroprocessing is the major process for the upgrading of heavy fossil fuels, and the catalysts, typically sulfided forms of materials represented as CoO-MoO3/Al2O3 or NiO-MoO3/Al2O3, undergo deactivation as a result of deposition of coke and metal sulfides (primarily nickel and vanadium sulfides), the latter formed from feed organometallic components, which, in petroleum, are largely present in the asphaltene fraction (Topsøe et al., 1996; Tamm et al., 1981). Air-quality regulations are forcing increasingly higher fractional removals of sulfur from fuels, and there is a trend toward treatment of heavier residua (which typically have higher contents of nickel and vanadium than lighter residua) and increasing severity of the processes. Thus, the issues of catalyst deactivation and, specifically, the deposition of nickel and vanadium sulfides are becoming increasingly important in technology. * Address correspondence to B. C. Gates, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Phone: (530) 752-3953. E-mail:
[email protected]. † University of Delaware. ‡ Cosmo Research Institute. § Amoco Research Center. | University of California, Davis.
Our goal was to gain further information about catalyst deactivation in residuum hydroprocessing and to evaluate procedures for modeling the deactivation resulting from the deposition of nickel and vanadium sulfides. We report data showing the performance of a sulfided NiO-MoO3/Al2O3 catalyst that had been deactivated by one year’s continuous operation in largescale hydroprocessing of petroleum residua (resulting in deposition of coke and metal sulfides). For comparison, we report the performance of the fresh catalyst and the used catalyst after it had been regenerated by burning off the coke without removal of the metal sulfide deposits. This work complements work with catalysts that had been deactivated by deposition of coke in the near absence of metal sulfide deposits (Diez et al., 1990) and work with catalysts that had been deactivated by deposition of metal sulfides in the near absence of coke (Yumoto et al., 1996). The metal sulfide deposits in the latter catalysts were formed from metal naphthenates and not from petroleum, and so the comparison with the data presented here allows an assessment of how well the catalysts prepared from the model compounds represent the catalyst used in an industrial process. The catalyst performance was measured with a batch reactor for hydroprocessing of the pure compounds naphthalene, quinoline, and dibenzothiophene (DBT).
10.1021/ie0000109 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/29/2000
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Table 1. Catalyst Compositions and Physical Properties
catalyst
added Ni content, wt %a
fresh spentb regeneratedc modeld
0.0 1.1 1.1 1.1
V content, wt %a
surface area, m2/g
pore volume, cm3/g
0.0 2.7 2.7 2.7
250 79 191 216
0.58 0.19 0.48 0.52
a Amount accumulated by fresh catalyst during commercial operation; values based on mass of fresh catalyst. b Conditions of commercial operation stated in text. c Conditions of regeneration stated in text. d Catalyst impregnated with nickel and vanadium naphthenates (Yumoto et al., 1996).
Experimental Methods Reagents and Catalyst. The reagents DBT, naphthalene, quinoline, cyclohexane, CS2, and H2 were as described before (Yumoto et al., 1996). The catalyst (Table 1) was an industrial NiO-MoO3/Al2O3 (1/16-in. extrudate) used commercially by Cosmo Oil Co. Ltd. for hydroprocessing of Arabian Heavy and Arabian Light residua, under the following conditions: temperature, 350-400 °C; H2 partial pressure, 105 atm; and liquid hourly space velocity, 0.18-0.23. In this commercial operation, the typical removal of sulfur from the feedstock by hydrodesulfurization (HDS) was about 90%. The spent (used) catalyst contained 2.7 wt % V (based on the mass of fresh catalyst) and 1.1 wt % added Ni (based on the mass of fresh catalyst). The fresh catalyst contained 3.9 wt % Ni (the added Ni represents the amount deposited during operation of the catalyst for residuum hydroprocessing) and approximately 40 wt % C (based on the mass of fresh catalyst). A sample of the spent catalyst was regenerated by burning off the coke under the following conditions (which were determined in thermal gravimetric analysis and differential thermal analysis experiments to lead to full coke removal): The catalyst in a fixed bed was treated in flowing dry N2 at 1 atm as the temperature was ramped from room temperature to 220 °C at a rate of 10 °C/min and then held for 1.5 h under dry N2. The sample was then heated in a flow of 6 vol % O2 in N2 as the temperature was ramped from 220 to 450 °C at a rate of 23 °C/h, held at 450 °C for 12 h, and then heated to 500 °C at a rate of 5 °C/min and held for 3 h at 500 °C before being cooled to room temperature in the O2N2 mixture. Catalyst Characterization and Analysis. The catalyst samples were characterized by standard N2 adsorption and mercury penetration methods to determine surface area/pore volume characterizations. Catalyst compositions were determined by inductively coupled plasma analysis. Fourteen individual catalyst particles from different sections of the commercial reactor (near the midsection axially) were characterized by electron microprobe analysis with a JEOL model JXA733 instrument to determine profiles of V, Ni, and Mo. Line profiles across the circular cross sections were measured with wavelength dispersive spectroscopy; the current was 40 nA, and the voltage was 25 kV. Batch Reactor Experiments. The performance of the fresh, spent, and regenerated catalysts was evaluated with a 1-L batch autoclave reactor with the catalyst particles confined in a basket. The pressure was kept at 165 ( 3 atm, the H2 partial pressure was 50 ( 2 atm, and makeup H2 was introduced to hold the pressure nearly constant. Liquid reactants were injected after the
catalyst-solvent mixture had been brought to the desired reaction temperature, 350 ( 1 °C. In a typical experiment, a sample of catalyst was introduced into the reactor with 500 mL of cyclohexane solvent. The catalyst mass, excluding the deposits, was 2.00 g. CS2 (2.0 mL) was added to the mixture so that the catalyst was presulfided upon the addition of H2. The reactor was then brought to a pressure of 42 atm with H2, heated over a period of 2 h to 350 °C, and held for 2 h at 350 °C. The following reactant mixtures were used: naphthalene + H2, DBT + H2, quinoline + H2, and naphthalene + DBT + quinoline + H2. When a mixture of the three organic components was used, the amount of each was the same as stated above. The liquid products were analyzed by gas chromatography. Details are provided elsewhere (Yumoto et al., 1996). Results Physical Characterization of Catalysts. Catalyst composition, surface area, and pore volume data are summarized in Table 1. Both the surface area and the pore volume decreased as a result of the commercial operation in residuum hydroprocessing. A typical average pore diameter in the fresh catalyst was about 76 Å; the pores were substantially filled by deposits in the spent catalyst. Data not shown indicate that the micropore volumes were negligibly small. The electron microprobe analyses of the spent catalyst show that vanadium was deposited nonuniformly in the cylindrical catalyst particles, being located preferentially near the particle edges, i.e., the effectiveness factor for hydrodemetallization to remove vanadium from the residua was substantially less than unity. The data of Figure 1A are typical, but we emphasize that there were substantial particle-to-particle variations in the profiles determined by electron microprobe analysis, as expected, and data were obtained from too few particles to justify a statistical analysis. In contrast, the microprobe data characterizing the nickel-containing deposits gave nearly flat profiles, with slightly higher amounts of nickel near the particle edges (e.g., Figure 1B). The molybdenum profiles show slightly lower amounts near the particle edges where the amounts of vanadium (and nickel) were relatively high (e.g., Figure 1C). Catalyst Performance Data. The data determining the activities and selectivities of the fresh, spent, and regenerated catalysts for conversion of naphthalene, DBT, and quinoline are summarized in the following paragraphs. The raw data are not shown; they are similar to and essentially the same in quality as those shown by Yumoto et al. (1996). The activities and selectivities of similar catalysts that were reused several times remained unchanged within experimental error (Yumoto et al., 1996). The product composition vs time profile for naphthalene hydrogenation in the batch reactor was found to be similar to those published (Yumoto et al., 1996). Other results, such as the mass balance closures, are nearly the same as those reported (Yumoto et al., 1996). Naphthalene Conversion. The reaction network was observed to be a sequential hydrogenation of naphthalene to form tetralin, which is hydrogenated to give Decalins (Yumoto et al., 1996). The conversion data characterizing the fresh catalyst were used to determine the complex network reported before (Figure 2A, Yumoto et al., 1996). The method of data fitting was as before
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 133 Table 2. Rate Constants in the Naphthalene Hydrogenation Network rate constant, L/(g of catalyst × h) catalyst fresh spent (by use in residuum hydroprocessing) regenerated (as coke was burned off the catalyst) model (prepared from nickel and vanadium naphthenatesa)
10 × k1 102 × k3 103 × k4 10 × k5 4.0 0.2
2.9 0.0
8.2 0.0
0.7 0.0
3.4
2.9
9.5
0.1
3.9
3.5
11
4.5
a Ni and V contents the same as those in the spent and regenerated catalysts (1.1 wt % added Ni and 2.7 wt % V) (Yumoto et al., 1996).
Figure 2. Approximate reaction networks for hydroprocessing of (A) naphthalene, (B) dibenzothiophene, and (C) quinoline. The reactions are represented as pseudo first order in the organic reactant. The forward rate constants, represented by the ki values, are summarized in Tables 2-4. The reactant H2 is omitted from the depiction to simplify the representation. Figure 1. Electron microprobe profiles of spent catalysts: (A) vanadium, (B) nickel, and (C) molybdenum. See text for details.
(Yumoto et al., 1996). Two or three sets of conversiontime data for each catalyst were used to estimate the pseudo-first-order rate constants. The order of each reaction in H2 is approximated as zero, and the order of each reaction in the organic reactant is approximated as unity, as before (Yumoto et al., 1996). The data determine the back-reaction rate constants only rela-
tively inaccurately because the concentrations of products were low in most of the analyses. Consequently, we report only the forward rate constants. The spent catalyst had a very low activity, and no Decalin formation was observed. The pseudo-first-order rate constants are shown for each catalyst sample in Table 2. The reaction network of Figure 2A provides a good fit of the data and is regarded as a satisfactory representation, but we infer that there were too few data
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Table 3. Rate Constants in Dibenzothiophene Hydroprocessing Network
Table 5. Rate Constants for Mixture Relative to Those for Single Componentsa
rate constant, L/(g of catalyst × h)
rate constant ratio
catalyst
10 × k1
10 × k2
102 × k3
102 × k4
102 × k6
10 × k8
fresh spent regenerated modela
2.9 0.2 2.0 1.6
1.1 0.3 1.5 1.3
2.1 0.5 0.3 4.3
1.4 0.1 1.2 0.4
5.4 5.1 4.9 5.9
0.1 0.1 1.0 1.0
a
Ni and V contents the same as those in the spent and regenerated catalysts (1.1 wt % added Ni and 2.7 wt % V); see Table 1 (Yumoto et al., 1996). Table 4. Rate Constants in Quinoline Hydroprocessing Network rate constant, L/(g of catalyst × h) catalyst
k1
fresh spent regenerated modela
2.5 0.8 4.9 3.2
10 × 10 × 10 × 10 × 10 × k3 k4 k5 k6 k7 3.4 0.3 2.9 3.2
1.4 0.1 6.3 1.5
7.7 0.0 47 6.4
4.5 0.2 1.0 3.0
3.9 0.8 4.2 4.4
k8 1.0 7.6 1.3 1.2
10-1 × 10 × k9 k10 2.9 15 3.4 1.4
0.1 1.0 0.4 0.0
a Ni and V contents the same as those in the spent and regenerated catalysts (1.1 wt % added Ni and 2.7 wt % V); see Table 1 (Yumoto et al., 1996).
and too many parameters needed for a good fit to determine uniquely the best model of the network or to determine error bars on the rate constants. DBT Conversion. The data are also similar to those reported for the fresh catalyst (Yumoto et al. 1996). The observed products were biphenyl, cyclohexylbenzene, bicyclohexyl, hydrogenated dibenzothiophenes [inferred on the basis of earlier reports (Girgis and Gates, 1991) to be hexahydrodibenzothiophene and tetrahydrodibenzothiophene], and byproducts including toluene and ethylbenzene. The byproducts were not fully resolved by the gas chromatographic analysis. These results were used to determine the reaction network of Figure 2B, with the same methods as stated above for the naphthalene network (Yumoto et al., 1996). The data were used to estimate the pseudo-firstorder rate constants for the forward reactions; the parameters are summarized in Table 3. The limitations of the network are as stated above for the naphthalene conversion network. Quinoline Conversion. The observed products were 1,2,3,4-tetrahydroquinoline, 5,6,7,8-tetrahydroquinoline, 2-propylaniline, n-propylbenzene, and n-propylcyclohexane. Decahydroquinoline was not detected. The closures of the mass balances were not as good as those for naphthalene and DBT conversion. The uncertainty is typical for quinoline conversion experiments in a batch reactor (Girgis, 1988); evidently, not all of the products were accounted for in the gas chromatographic analysis. The data determined for the quinoline network (Figure 2C) are less reliable than the data for the others. We infer, as before (Yumoto et al., 1996), that the network is too complex to be established well by the data, but the network fits the data better than the simplified network reported earlier (Yumoto et al., 1996). The best-fit parameter values are summarized in Table 4. We emphasize that the error bounds in some of the parameters were large. Conversion of a Mixture. The rate constants k1 representing the primary reactions of several reactants in a mixture (Table 5) show that the effects of the metal
catalyst
naphthalene hydrogenation (k1)mixt/k1
DBT HDS (k1)mixt/k1
quinoline HDN (k1)mixt/k1
fresh spent regenerated modelb
0.40 0.48 0.47 0.39
0.55 0.74 0.67 0.69
0.81 0.91 0.90
a
The subscript mixt refers to the value for the compound in the mixture of reactants; see text for details of composition; k1 refers to the rate constant with that name in the appropriate reaction network. b Ni and V contents the same as those in the spent and regenerated catalysts (1.1 wt % added Ni and 2.7 wt % V); see Table 1 (Yumoto et al., 1996).
sulfide deposits in the regenerated catalyst and in the model catalyst were nearly the same. Discussion The data show that the spent catalyst had lost almost all of its activity for naphthalene hydrogenation (Table 2), DBT conversion (Table 3), and quinoline conversion (Table 4). The strong deactivation is attributed to a combination of the effects of coke and deposited metal sulfides. The data characterizing naphthalene hydrogenation with the regenerated catalyst (Table 2) show that most of the activity returned after the coke was burned off, which implies that most of the deactivation can be attributed to the coke. We emphasize that, in the commercial residuum hydroprocessing, the severity was not high, and the catalyst accumulated only a relatively small amount of metal sulfide deposits. Thus, these results pertain to what is sometimes called a second-stage hydroprocessing operation and not to one that involves heavy metal sulfide accumulation by the catalyst, as in a first-stage operation or a guard-bed operation. The regenerated catalyst was almost as active for naphthalene hydrogenation as the fresh catalyst (e.g., k1, Table 2) but was not as active as the model catalyst that had been deactivated by incorporation of nickel and vanadium from the naphthenates (Yumoto et al., 1996) (Table 2). Similarly, the regenerated catalyst was almost as active for DBT conversion as the fresh catalyst and hardly different in activity from the model catalyst (Table 3). Similar statements also pertain to quinoline conversion (Table 4), but we reemphasize the large uncertainty in the parameter values characterizing the reactions in the quinoline network. An important conclusion following from these comparisons is that there is relatively good agreement between the data characterizing the regenerated catalyst and those characterizing the model catalyst (which has the same nickel and vanadium contents). The results representing the regenerated and model catalysts are not expected to be identical because of the nonuniformity of the vanadium sulfide deposits in the regenerated catalyst and the near uniformity of such deposits in the model catalyst (Yumoto et al., 1996). A practical conclusion is that, to a good first approximation, the model catalysts predict the effects of the metal sulfide deposits and provide a basis for justifying the use of such catalysts (Quann et al., 1988) to develop correlations accounting for deactivation by relatively small amounts of metal sulfides. The data of Table 5,
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showing the effects of the deposits on relative rate constants for reactions in mixtures relative to those of the compounds reacting alone (with H2), suggest that the model catalysts may be better for correlating relative activities than absolute activities. The data reported for the model catalysts (Yumoto et al., 1996) show that the effects of nickel sulfide and vanadium sulfide deposits are different from each other, and the fact that the nickel sulfide deposits are nearly uniform in the catalyst particles whereas the vanadium sulfide deposits are not [as is typical (Tamm et al., 1981)] implies that the results obtained with the model catalysts are likely to give a better basis for processing correlations to predict effects of nickel sulfides than to predict effects of vanadium sulfides. It is perhaps surprising that the agreement between the data characterizing the model catalyst and the regenerated catalyst is as close as it is considering the nonuniformity of the vanadium sulfide deposits in the regenerated catalyst. Thus, we emphasize that the conclusions stated here are limited to catalysts containing only relatively small amounts of vanadium sulfide deposits, and they do not account for the pore mouth plugging that can result when large amounts of vanadium sulfide deposits form. Acknowledgment We thank Scott Stark for help in the data fitting and Cosmo Research Institute, Satte, Japan, for supporting
M. Yumoto’s work at the University of Delaware. We thank Cosmo Research Institute and Amoco Oil Co. for supporting the research. Literature Cited (1) Diez, F.; Gates, B. C.; Miller, J. T.; Sajkowski, D. J.; Kukes, S. G. Deactivation of a Ni-Mo/γ-Al2O3 Catalyst: Influence of Coke on the Hydroprocessing Activity. Ind. Eng. Chem. Res. 1990, 29, 1990. (2) Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High-Pressure Catalytic Hydroprocessing. Ind. Eng. Chem. Res. 1991, 30, 2021. (3) Quann, R. J.; Ware, R. A.; Hung, C. W.; Wei, J. Catalytic Hydrodemetalation of Petroleum. Adv. Chem. Eng. 1988, 14, 95. (4) Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Effects of Feed Metals on Catalyst Aging in Hydroprocessing. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 262. (5) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis. In CatalysissScience and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1996; Vol. 11, p 1. (6) Yumoto, M.; Kukes, S. G.; Klein, M. T.; Gates, B. C. Catalytic Hydroprocessing of Aromatic Compounds: Effects of Nickel and Vanadium Sulfide Deposits on Reactivities and Reaction Networks. Ind. Eng. Chem. Res. 1996, 35, 3203.
Received for review January 6, 2000 Revised manuscript received September 14, 2000 Accepted September 21, 2000 IE0000109
