Energy & Fuels 1995, 9, 2-9
2
Articles Residual Oil Hydrodesulfurization Using Dispersed Catalysts in a Carbon-PackedTrickle Bed Flow Reactor Deuk Ki Lee,* Sung Koon Park,? Wang Lai Yoon, In Chul Lee, and Seong Ihl Woo* Energy Conversion Research Team, Korea Institute of Energy Research, Taedok Science Town, Taejon 305-343, Korea, Department of Chemical Engineering, Hanyang University, Sung Dong Ku, Haeng Dang Dong 17, Seoul, Korea, and Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Taedok Science Town, Taejon 305- 701, Korea Received April 25, 1994@
Heavy oil hydrodesulfurization (HDS) was carried out using dispersed catalyst which deactivated less strongly than the conventional supported catalyst. Various kinds of transitionmetal compounds containing either molybdenum, tungsten, nickel, or cobalt were tested as dispersed catalyst precursors individually or in combination. Experiments were performed in a continuous trickle bed reactor under high hydrogen pressure for the feedstock of atmospheric residual oil. To recover dispersed catalysts from the reactant mixture, activated carbon was packed into the reactor on which the dispered catalytic species happened to be deposited. Of the catalytic precursors examined, the compound containing molybdenum showed the highest conversions for sulfur and asphaltenes. A large HDS activity synergism was observed for the combined catalyst system consisted of two precursors of different metal kind. The Co-Mo catalyst showed the highest activity and selectivity for HDS reaction. Selectivity for hydrocracking reaction was the highest with the Ni-Mo catalyst. The Ni-W system showed the lowest activity in most of reactions. Co-Mo was considered as the most promising dispersed catalyst system for the HDS of heavy oil. It was proved that active carbon packed in the reactor was very effective in recovering the dispersed catalysts from reactant oils and that the dispersed catalysts deposited on activated carbon were also catalytically active, resulting in higher yield particularly for HDS.
Introduction HDS catalysts for heavy residual oil should have high activity and selectivity for HDS reaction to produce high-quality and low-sulfur oil with low operating cost. Although the conventional supported type catalysts have relatively high initial activity, they suffer from rapid deactivation particularly when applied for the heavy feedstocks containing large amounts of contaminants such as asphaltenes and metals. At most about 40% of the original activity of the catalyst survives to participate in the reaction. Furthermore, the development of a single supported type catalyst which has both high HDS activity and high metal tolerance appears to be unattainable, i.e., high catalyst activity (e.g., for HDS) and high metal tolerance (e.g., uptake capacity) are largely incompatib1e.l As an alternative to the use of the conventional supported type catalysts for the hydroprocessing of heavy feeds, several processes such as M-coke2 and
* To whom correspondence should be addressed.
+ Hanyang
University. Korea Advanced Institute of Science and Technology. Abstract published in Advance ACS Abstracts, October 15, 1994. (1)Dautzenberg, F.M.; De Deken, J. C. Cutul.Rev.-Sci.Eng. 1984, 26(3,4),421. (2) Bearden, R.;Aldridge, C. L. Energy Prog. 1981,1 , 44. @
Aurabon3 which utilize finely divided catalysts formed in situ have been developed. In these processes, catalyst particles of very small size are formed in situ in the reactor from precursor compounds containing a transition metal such as molybdenum (Mo-coke)or vanadium (Aurabon) which are dissolved or dispersed in feed oils before entering the r e a ~ t o r . A ~ dispersed catalyst system has a number of characteristic advantages over the supported type catalyst^,^^^ i.e, little deactivation problem enabling to process a heavy feed of very poor quality, high suppressibility of coke formation resulting from maximum interaction of oil and hydrogen with the catalysts of high population in reaction medium, and a high degree of catalytic metal utilization due to the absence of diffusional limitation of reactants. However, there are two disadvantages. First, since the dispersed catalyst system has been developed mainly for the purpose of the hydroconversion of heavier to lighter oils, there should be a need t o improve the activity and selectivity for the hydrotreating reaction such as HDS. (3) Adams, F.H.; Gatsis, J. G.;Sikonia,J. G. In The Future ofHeauy Crude Oils and Tar Sands, Proceedings of Unitur First International Conference, Edmonton, June 4-12, 1979;Meyer, R. F.,Steele, C. T., Eds.; McGraw-Hill: New York, 1981; p 632. (4) Kushiyama,S.;Aizawa, R.; Kobayashi, S.;Koinuma, Y.;Uemasu, I.; Ohuchi, H. Appl. Cutul. 1992,63,279. ( 5 ) Bianco,A. D.; Panariti, N.; Carlo, S.D.; Elmouchnino,J.;Fixari, B; Perchec, P. L. Appl. Cutal. 1993,94,1.
0887-0624/95/2509-0002$09.00/0 0 1995 American Chemical Society
Residual Oil Hydrodesulfurization Using Dispersed Catalysts
Energy &Fuels, Vol. 9, No. 1, 1995 3
Table 1. Properties of Atmospheric Residual Oil gravity, 1514 "C atomic WC ratio sulfur, wt % nitrogen, wt % vanadium, wt ppm nickel, wt ppm iron, w t ppm asphaltenes (n-pentane insolubles), wt % simulated distillation (ASTM D2887) initial b.p., "C middle b.p., "C final b.p., "C a
0.9865 1.53 3.9 (6.3P 0.5 20 (120)" 6 (3Ola 3 (16P 13.7 262 436 515
Concentrations of the correspondingelements in asphaltenes.
Second, the dispersed catalysts in situ generated from the precursors in reactant mixture are used once and are difficult to recover. Therefore, an effective way of recovering the dispersed catalyst components from the reaction medium should be developed to overcome this once-throughuse problem, which enables us to use more expensive transition metals as dispersed catalysts to improve hydroteating performance. De Agudelo and Galarragas ascertained that the metals of the active phase were deposited onto the surface of porous silica or silica-alumina particles, resulting in the in-situ formation of a supported catalyst which could be regenerated and reused. In this work, various kinds of metallic compounds as a precursor of dispersed catalyst containing a transition metal of either molybdenum, tungsten, nickel, or cobalt were tested individually or in combination in the presence of excess amounts of sulfur required to sulfide the catalyst. The combined catalyst system such as CoMo, Ni-Mo, or Ni-W, consisting of two dispersed precursors differing in metal kind, was examined to study the synergistic effect. Hydrotreatment reactions for a heavy residual oil feed were carried out continuously under high hydrogen pressure (6.9 MPa) and temperature (420 "C)using a trickle bed flow reactor. The reactor was packed with the porous extrudate material such as either activated carbon or y-Al2O3. Fine catalytic particles generated in situ from the reacting oil mass might be deposited onto the porous material. This attempt was made to recover the catalytic metals from the reaction mixture and to overcome the oncethrough use problem of the dispersed catalyst system. Catalytic performances of the dispersed catalysts, in terms of the conversions of sulfur, asphaltene, and metals, and the conversion of heavier to lighter oils, were compared with those of commercial supported type catalysts, i.e., CoMoly-Al203, NiMoly-Al203, and NiW/ Y-AlnOs.
Experimental Section Feedstock. The distillation residue obtained from the atmospheric distillation tower at Yukong R e h e r y Co. in Korea was used as feed oil. The residual oil originated from a crude mixture of Iranian Heavy (70%) and Geisum (30%). Its properties are shown in Table 1. Catalysts. Two types of catalysts, supported or dispersed, were evaluated in this study. The physcial properties of commercial catalysts of CoMo, NiMo, and NiW supported on (6) De Agudelo, M. M. R.; Galarraga, C. Development of a New Catalyst for the Hydroconversion of Heavy Oils, New Frontiers in Catalysis (Proceedings of the 10th International Congress on catalysis 1992 Budapest); Elsevier Science Publishers: New York, 1993;p 2511.
Table 2. Specification of the Catalysts Supported on y-&@s
BET
composition (wt %) area catalvsts NiO COO Moon WOn- (mzle) . Mo (lab-made) 9.2 140 NiW (Shell 454) 5.2 23.5 180 NiMo (Shell 324) 2.7 13.2 165 CoMo (Amocat 1A) 2.5 9.8 235 _I
median pore diameter"
(A) ~~
~,
145 102 116 102
Table 3. Specifications of the Dispersed Catalyst Precursors Studied metal composn Diecursor twe notation manufr ( w t %) solvent molybdenum naphthenate MoNP Shepherd 6 Mo oil-soluble molybdenyl acetylMoAA Aldrich 29.4Mo ethanol acetonate phosphomolybdic acid PMA Aldrich 64.0 Mo ethanol phosphotungstic acid PTA Aldrich 75.3W ethanol nickel acetylacetonate NiAA Strem 19.3Ni ethanol cobalt acetylacetonate CoAA Aldrich 19.3Co ethanol and a Moly-AlzOs catalyst are given in Table 2. Commercial catalysts supplied as cylindrical extrudates were crushed and sieved into particles of 35/45 mesh size (mean diameter 0.42 mm). These oxide catalysts were presulfided in situ with gas mixutre of H2S (10 vol %) and Hz before reaction at 450 "C for 3 h. The specifications of dispersed catalyst precursors used in this study are listed in Table 3. The dispersion of the oil-soluble precursor such as molybdenum naphthenate (MoNP) was done by adding a desired amount of precursor to the feed oil during agitation, whereas, the dispersion of oil-insoluble precursor compounds such as acetylacetonates and the inorganic acids were achieved by first dissolving the precursor in ethanol, and then adding the resulting solution to the feed oil followed by evaporating ethanol a t 100 "C during agitation. The mixture of feed oil, ethanol, and catalytic precursors was finally heated t o 250 "C in a silicon oil bath and maintained a t this temperature for 2 h for the complete evaporation of ethanol from feed oil. For the combined catalyst system, inorganic acid of molybdenum (PMA) or tungsten (PTA) was used as an active component and the acetylacetonate of nickel (NiAA) or cobalt (CoAA) as a promoter. The atomic ratio ((promoter)/(promoter active metal)) of combined system of dispersed catalysts was fixed at 0.3. The total metal concentration of a dispersed catalyst system in the feed oil was maintained a t 300 ppm. Three combined systems of catalysts, Co(62.5)-Mo(237.5), Ni(62.5-Mo(237.5), and Ni(36.3)-W(263.7), were prepared. The values in parentheses indicate ppm weight. Apparatus and Reaction Procedure. Experiments were carried out in a 11mm i.d. and 500 mm length stainless steel tube reactor, which was specially designed to avoid channeling and axial d i ~ p e r s i o n .Detailed ~ description of the reaction system can be found elsewhere.s The packing configuration of supported type catalysts is shown in Figure 1A. Catalyst particles (3.2 g) mixed with ceramic beads of mean diameter of 0.4 mm were loaded into the middle section of the reactor. Total volume of catalyst bed was 10 mL. Ceramic balls of mean diameter of 1.8 mm were placed in the top and bottom parts of the reactor to obtain even distribution of reactant flow. For the experiments where dispersed catalysts were used, porous extrudate material was packed into the reactor. The packing materials employed in this study were microporous y-Al203
+
(7) Doraiswamy, L. K.; Tajbl, D. G . Catal. Reu.-Sci. Eng. 1974, 10, 177. (8)Lee, D. K.; Lee, I. C.; Woo, S.I. Appl. Catal. A: Gen. 1994,109,
195.
Lee et al.
4 Energy &Fuels, Vol. 9,No.1, 1995
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40 I
11 4
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1
7
Sieve screen Ceramic beads
Catalysts and ceramic beads
c Activaled carbon 0
Alumina
Ceramic beads Q -
c .-
2
J 1 -
s
t
0
0 ( A I
(5)
Figure 1. Schematic diagram of the reactor packing pattern for the supported type catalysts (A) and for the dispersed catalyst with porous extrudate packing material (B). Table 4. Physical Properties of the Porous Extrudate Materials for Immobilizing Dispersed Catalyst Particles porous materials properties manufacturer BET surface area, m2/g pore volume, cm3/g mean pore diameter, A particle size, mesh
activated carbon Darco 712 0.98 54 16-20
Y-&o3
Nishio KHA 150 0.54 141 16-20
Table 5. Reaction Conditions in the Trickle Bed Flow Reactor System type of catalyst experimented reaction conditions supported dispersed reaction temperature, "C 420 420 reaction pressure, psi 1000 1000 weight of catalyst charged, g 3.2 catalyst bed volume, mL 10 packing volume of 15 immobilizer, mL feed oil flow rate at 60 "C, mIJh 20 30 reaction time, h 0.5 (1LHSV) 0.83 (space time) 500 hydrogedoil feeding ratio, 500 L (STP) of H A of oil activated carbon and macroporous y-A1203. Their physical properties are listed in Table 4. Activated carbon has several advantages over y - A l 2 0 3 , namely, less expensive and combustible to a metal-rich ash for metal recovery and reduced volume of material for d i s p o ~ a l . Fifieen ~ milliliters of the porous extrudate material was packed into five different beds as shown in Figure 1B. The particle size of the mateiral was 1620 mesh (mean diameter = 1.0 mm). Between the porous material beds, there existed an inert bed of volume 5 mL which consisted of the nonporous ceramic beads of mean diameter 1.3 mm, and the void fraction of the bed was estimated to be 0.4. Before reaction, the reactor was heated a t 350 "C for 2 h in a Ns flow. Reaction conditions are summarized in Table 5. Liquid product samples were periodically taken to measure the contents of sulfur and asphaltenes (n-pentane insolubles), and the chromatographic distribution of hydrocarbons constituting deasphalted oil fractions, Le., maltenes. These analytic pro(9)Rankel, L. A.Energy Fuels 1993,7 , 937.
5
10
15
20
25
Time on-stream (hr) Figure 2. Conversions of sulfur and asphaltenes by activated carbon and y-Al203. cedures were reported in detail elsewhere.s Metal contents in oils were determined by inductively coupled plasma spectrophotometry (ICP) and scanning electron microscopy (SEM) was used to analyze the activated carbon-deposited catalytic metals.
Results and Discussion Preliminary Experiments. In order to select an appropriate porous extrudate material to support the dispersed catalyst, noncatalytic thermal hydroprocessing reactions were performed in a trickle bed reactor packed with either activated carbon or y-alumina. Figure 2 shows noncatalytic thermal conversions of sulfur and asphaltenes with reaction time on-stream. Activated carbon is more reactive than y-alumina, indicating that activated carbon displays a catalytic activity as compared with y-alumina although it is small in magnitude. Some reports indicated that the carbon-supported catalysts generally gave higher HDS activty10-18 and less coking propensity16J9than the alumina-supported catalysts. Laine et a1.20 reported that carbon itself behaved as an HDS catalyst, i.e., exhibiting significant HDS activity and a decrease in coking tendency when presulfided. Theyz0also suggested through a temperature-programmed reduction study that the nature of carbon-incorporated sulfur was chemically bonded to (10)Groot, C. IC; De Beer, V. H. J.; Prins, R.; Stolarski, M.; Niedzwiedz, W. S. I d . Eng. Chem. Prod. Res. Dev. 1986,25,522. (11)Amoldy, P.; Van Oers, E. M.; De Beer, V. H. J.; Moulijn, J. A,; Prins, R.Appl. Catal. 1989,48,241. (12)Duchet, J. C.;Van Oers, E. M.; De Beer, V. H. J.; Prins, R. J .
Catal. 1988.80.386. (13)Vissers, V. H. J.;Scheffer, B.; De Beer, V. H. J.; Moulijn, J. A.; Prins, R. J . Catal. 1987,105,277. (14)Scheffer,B.; Amoldy, P.; Moulijn, J. A.J . Catal. 1988,112,516. (15)Hillerova,E.;Vit, Z.; Zdrazil, M.; Shkuropat, S. A,; Bogdanets, E.N.;Startsev, A. N. Appl. Catal. 1991,67,231. (16) Drahoradova,A.; Vit, Z.; Zdrazil, M. Fuel 1991,71, 455. (17)Vit, Z. Fuel 1993,72, 105. (18)Van Veen, J. A. R.; Gerkema, E.; Van Der Kraan, A. M.; Knoester, A. J . Chem. SOC.,Chem. Commun. 1987,1684. (19)Scaroni, A. W.; Jenkins, R. G.; Walker, Jr., P. L. Appl. Catal. 1986,14,173. (20) Laine, J.; Severino, F.; Labady, M.; Gallardo, J. J . Catal. 1992, 138,145.
Energy &Fuels, Vol. 9, No. I, 1995 5
Residual Oil Hydrodesulfirization Using Dispersed Catalysts 70 I
I
It 0
MolAIZO3
Q MoNP300ppm
h PMA300ppm
Wlth
0
sulfur
NlAA(62 5 ppm) .PMA(237 5 ppm) without sulfur
C
.f 0
5
10
15
20
40
25
Time on-stream (hr)
Figure 3. Effects of elemental sulfur addition to Ni-Mo dispersed catalyst on the conversions of sulfur and asphaltenes.
carbon via sulfhydryl groups (-SH) which have been assumed to play an important role in HDS.21,22 In contrast, alumina was r e p ~ r t e d to ~ ~be, ~essentially ~ inactive for HDS and t o produce practically the same amount of coking whether presulfided or not. Therefore, it was decided t o use activated carbon as a support material. The effect of elemental sulfur addition to the Ni-Mo dispersed catalyst system on the conversion of sulfur and asphaltenes was investigated in the reactor packed with activated carbon. The amount of elemental sulfur was twice as much as the stoichiometric amount needed for the sulfidation of the metals to the corresponding sulfides of Ni3S2 and MoS2. As shown in Figure 3, conversions of sulfur and asphaltene were significantly increased by the addition of elemental sulfur. The increase in activity with the addition of elemental sulfur might be ascribed to the enhanced degree of activation of the dispersed catalyst precursors by sulfidation. The metals contained in the precursors would be in situ converted to the catalytically active metal sulfides such as MoS2, WS2, COgs8, and NisS2 under the reaction conditions at 420 "Cand hydrogen pressure of 1000 psig (6.9 MPa). At these reaction conditions, precursors were decomposed to react with available sulfur-containing compounds. Kushiyama et al.4 suggested that hydrogen sulfide is essential for the catalyst species to be sufficiently sulfided and activated without suffering from possible coke poisoning. It was also reported4 that the added elemental sulfur was easily converted to hydrogen sulfide at temperatures above 240 "C, which could accelerate the decomposition of the precursors, leading to the formation of sulfided active catalyst species but that the sulfur compounds included in the feed contributed little to the catalyst sulfidation. The catalytic activities for HDS and HDM increased (21)Topsoe, N.Y.;Topsoe, H. J. Catal. 1993,139,641. (22)Prins, R.;De Beer, V. H. J.; Somorjai, G. A. Catal. Rev.-Sci. Eng. 1989,31(1,2),1. (23)Brito, J.;Gdding, R.; Severino, F.; Laine, J. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1982,27,762.
Time on-stream (hr)
Figure 4. Comparison of the dispersed molybdenum precursor compounds in the conversions of sulfur and asphaltenes.
with increase in the amount of elemental sulfur up to an atomic ratio of sulfudmetal slightly higher than l.4 The amount of elemental sulfur added in the present study (atomic ratio of sulfur/catalyst metal is 2.8) is enough for the complete sulfidation of the catalyst precursors. Comparison of Molybdenum Precursor Compounds in the Conversions of Sulfur and Asphaltenes. In order to investigate the effects of the molybdenum precursor compound on the reaction performances of the dispersed catalysts, MoNP, PMA, and M O Mwere chosen and tested. The concentration of the dispersed molybdenum precursor, MoNP, PMA, and M O M ,in the feed oil was 300 ppm on molybdenum basis. Catalytic performances of these dispersed catalysts were compared with those of Moly-AlzO3. The conversions of sulfur and asphaltenes with time on-stream of the reactor-packed activated carbon are shown in Figure 4. The difference in the sulfur conversions among the catalyst precursors during the initial period of reaction may be attributed mainly to the relative difference in the degree of dispersion of the precursors. Because MoNP is an oil-soluble compound, its degree of dispersion in oil is thought to be better than that of oilinsoluble PMA or MOM, resulting in HDS activity higher than for PMA and M O M . Actually, Dabkowski et al.24suggested that dispersed catalyst performance could be correlated with the solubility of the metalcontaining compounds in the oil. Consequently, organic complexes are preferred to inorganic salts. However, Bearden and Aldridge2 reported that aqueous PMA showed performance comparable to MoNP and molybdenum hexacarbonyl, which are both oil-soluble. The increase in sulfur conversion with the dispersed catalysts with time on-stream is due to the gradual increase in catalytic species concentration inside the reactor by the deposition of the active species onto the reactorpacked active carbon. It is also observed that the rate of increase in sulfur conversions was much higher for (24)Dabkowski, M.J.; Shih,S. S.; Albinson, K. R. AIChE Symp. Ser., Tar Sand Oil Upgrading Technol. 1991,87,53.
*
Lee et al.
6 Energy &Fuels, Vol. 9,No. 1, 1995 70 1
1 PMA300ppm
6o 50
0 PTA300 ppm
40 30
3 0
3
A
.
CoAA(62 5 ppm) PMA(237 5 ppm) PMA 300 ppm C a m 300 ppm
10
60
n
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z z
s
20
0
5
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20
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0
25
5
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Time on-stream (hr)
Time on-stream (hr)
Figure 5. Comparison of the dispersed tungsten with dispersed molybdenum in the conversions of sulfur and asphalt-
Activity promotion in the Co-Mo dispersed catalyst
system.
enes. 50
r
I
I
8o
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E fi
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.-
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9
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.-
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A
PMA(300 ppm) CoAA(62.5 ppm)/PMA(237.5 ppm)
-
20 0
5
10
15
20
25
Time on-stream (hr)
Figure 8. Dependence of asphaltene conversion on the concentration of dispersed molybdenum. Time on-stream (hr)
Figure 6. Comparison of the dispersed nickel and cobalt with dispersed molybdenum in the conversions of sulfur and as-
phaltenes.
PMA and MoAA than for MoNP and that there was little difference in the conversion among the Mo precursors when the time on-stream of reaction was longer than 15 h. This is attributed to the poor dispersion of PMA and MoAA in the feed oil as compared to MoNP. Hence, the catalytic effect provided by Mo species deposited onto carbon is more highly pronounced for PMA and MoAA than for MoNP. Contrary to the sulfur conversion, the asphaltene conversion was higher in the dispersed catalysts than in the supported catalyst. It is inferred that there is a better contact between asphaltene molecules and catalytically active species in the dispersed catalysts than in the supported catalysts. This is plausible in that there can be a pore diffusion limitation for the access of asphaltenes to the catalytically active sites in the supported catalysts, while no diffusional limitation
exists in the dispersed catalysts. The asphaltene conversion for the dispersed catalysts approached a constant level with time on-stream, while for the supported catalyst it decreased continuously due to the possible physical blockage of its active centers by coke deposition. Activities of Tungsten, Nickel, and Cobalt Precursors. The reaction performances of dispersed tungsten, nickel, or cobalt were compared with molybdenum in terms of sulfur and asphaltene conversions. Figure 5 shows the results for W precursor of PTA in reference to the corresponding Mo precursor of PMA, while Figure 6 shows those for nickel and cobalt compared with molybdenum, where the precursor compounds are in the form of acetylacetonate. In Figures 5 and 6, the dispersed catalysts of tungsten, nickel, or cobalt resulted generally in low activities for HDS and asphaltene conversion compared to the molybdenum catalyst. Because the precursors tested were oil-insoluble inorganic acids or acetylacetonates, this result might be attributed to lower intrinsic activities of tungsten, nickel, or cobalt than that of molyb-
Energy & Fuels, Vol. 9,No. 1, 1995 7
Residual Oil Hydrodesulfurization Using Dispersed Catalysts h
$
I
O'
Time on-stream : 13 hr
E
6o
C6-ClO 'Cll-C20'C21-C30'C31
-
A
+
Oil fraction by carbon number
COAA(62.5 ppm) PMA(237.5 ppm)
[s NiAA(62.5 ppm) PMA(237.5 ppm)
20
Dispersed Mo-based catalysts in their ppm concentrations :
=
F n d 01
0(CoM-PMAI 300
PMA 300
10
PMA
100
29
I NiMolAl203 NiW/AI203 A CoMo~AI203 i-1 NiAA(62.5 ppm) - PMA(237.5 ppm) 12NiAA(36.3 ppm) PTA(263.7 ppm) A CoAA(62.5ppm) PMA(237.5 ppm)
1
10
15
25
20
Time on-stream (hr)
Figure 11. Comparison of asphaltene conversion in the combined systems of dispersed catalysts.
E
h
-
80
5
0
Figure 9. Dependence of the conversion of heavier to lighter oil fractions on the concentration of dispersed molybdenum.
""
I
Time on-stream : 13 hr
Y L
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denum. Pecoraro and Chianelli25reported in the HDS of dibenzothiophene over the unsupported transitional metal sulfide catalysts that a higher sulfur conversion CogS8. was obtained in order of MoS2 > WS2 > Ni& Our results are in good agreement with the results of Pecoraro and ChianelLz5 HDS Activity Synergy in the Pair of Dispersed Catalyst Precursors. The conversion of sulfur catalyzed over a dispersed catalyst mixture Co-Mo was much higher than those catalyzed over individual catalyst of Co or Mo, as shown in Figure 7. However, this kind of synergistic effect was not observed for asphaltene conversion. The same pattern of activity promotion was also observed for the dispersed catalyst mixture systems of Ni-Mo and Ni-W. It has been widely accepted for the supported catalyst that the HDS activity promotion arises from the intrinsic activity increase of the active metal sulfide through the chemical structure like C O - M O - S . ~ ~ -However, ~~ in the present experiments, the possibility that such chemical synergy phase as Co-Mo-S, Ni-Mo-S, or Ni-W-S was formed to a certain degree is thought to
-
(25)Pecoraro, T. A.; Chianelli, R. R. J . Cutul. 1981,67,430. (26)Topsoe, H.;Clausen, B. S.; Candia, R.; Wivel, C.; Morup, S. J . Catul. 1981,68,433. (27)Wivel, C.; Candia, R.; Clausen, B. S.; Morup, S.; Topsoe, H. J . Cutul. 1981,68,453. (28)Topsoe, H.;Clausen, B. S. Cutul. Rev.-Sci. Eng. 1984,26(3,4), 395.
C31 +
Dispersed catalyst pairs of 300 ppm concentration :
=
Time on-stream (hr)
Figure 10. Comparison of sulfur conversion in the combined systems of the dispersed and the supported catalysts.
C11 -C20 C21 - C30
Oil fraction by carbon number 0 (CoAA.PMA10 (NiAA.PTA]
INAA.PMA)
Figure 12. Comparison of the conversion of heavier to lighter oil fractions in the combined systems of dispersed catalysts. Table 6. Metal Concentrations of Product Oils Sampled at the Time On-Stream (20 h) concentrations (ppm) catalysts [NiAA-PMA] [NiAA-ITA] [COAA-PMA]
V 10.0 11.6 8.9 53.00
Fe 0.1 0.3 0.1 2.8"
Ni
3.6 4.1 2.2 14.3"
Co
-
Mo 0.4
-
-
0.2 1.3"
0.5 0.6"
W -
0.2 -
-"
Concentrations of the correspondingelements in asphaltenes.
be very rare, because most of the dispersed catalyst precursors should be separately present in the oil. This indicates that the HDS activity promotion appeared in this experiment was not attributed to the Co-Mo-S structure. The remote control mechanism proposed by Karroua et al.29-31may be the most plausible route of HDS activity promotion exhibited in this experiments. The remote control mode129-31makes it possible to explain the activity promotion between the separate phases of cogs8 and MoS2. For the catalyst mixture of Co-Mo, the promotion mechanism by this model is that a reactive hydrogen atom forms on a Cogs8 by a molecular hydrogen dissociation, migrates (spillover) (29)Karroua, M.; Centeno, A.; Matralis, H. IC;Grange, P.; Delmon,
B.Appl. Cutul. 1989,51,L21. (30)Karroua, M.;Matralis, H.; Grange, P.; Delmon, B. J . Cutul.
1993,139, 371. (31) Karroua, M.; Grange, P.; Delmon, B. Appl. Cutul. 1989,50,L5. (32)Aimoto, K.;Nakamura, I.; Fujimoto, K. Energy Fuels 1991,5, 739.
8 Energy & Fuels,
Vol. 9,No.1, 1995
Lee et al.
I
I
6CNT
18.22KEU
lBaV/ch
A
EDmX
Figure 13. SEM for a carbon particle sampled from the reactor after the 17 h reaction over the dispersed catalyst MoNP (600 ppm Mol.
onto the surface of the other catalytic phase MoS2 and reacts with it for creating HDS active sites. In the case
of Ni-Mo and Ni-W mixture systems, Ni& and WS2 might play the role of cogs8 and MoS2, repectively.
Residual Oil Hydrodesulfurization Using Dispersed Catalysts
Energy &Fuels, Vol. 9, No. 1, 1995 9
In the experiments of thiophene HDS using the on-stream. The trend of increasing HDS activity is due mechanical mixture of CoSJC and MoSdAl203, Karroua to the gradual increase in the concentrations of the et al.29suggested that the spillover of hydrogen occurs catalyst species participating in the reaction. As the across the surfaces of carbon and alumina. Such a feed oil containing the dispersed catalyst precursors was similar hydrogen transfer by spillover was also proposed steadily fed into the reactor, the finely dispersed catain the hydrocracking reaction of heavy oil using a Ni/C lytic particles generated in situ could be continuously catalyst.32 In our experiments, such a hydrogen spilldeposited onto the reactor-packed activated carbon, over might occur across the surfaces of the activated resulting in increase of the catalytic species concentracarbon packed in the reactor when the metal sulfides tion. It can be seen from Table 6 that product oils came were deposited onto the activated carbon. Even in the from the reactor nearly without the catalytic metals reacting oil medium, the hydrogen spillover might occur originally dispersed, indicating that most of catalytic via an intermediate of large carbonaceous molecule such metals were captured by activated carbon bed. Figure as asphaltene or coke if the carbonaceous molecule was 13 is a SEM picture and an EDX result for a typical present in association with both of the two catalytic activated carbon particle sampled from the upper packr ~ ~ the species. In view of a discussion by T e i ~ h n e that ing position of the reactor after feeding MoNP (Mo 600 spilled-over hydrogen that migrated t o the support ppm) precursor for 17 h. SEM analysis was taken after surface from catalyst was not reactive toward hydrogenwashing activated carbon particle with CHzCl2 to reation of unsaturated hydrocarbons, the large carbonmove oil or asphaltene deposits followed by drying at aceous molecules could play a role of vehicle for the 100 "C. The analysis result confirms that the impregspillover of hydrogen. nation of dispersed molybdenum species onto the surAs discussed above, because the spilled-over hydrogen face of activated carobn has occurred throughout the from the dispersed cogs8 species contributes to creating reaction period. It could be seen that part of the the HDS active sites on the codispersed MoS2 ~ p e c i e s ~ ~ - ~ molybdenum l species had penetrated into the pores and but has not enough reactivity to directly hydrogenate deposited on the inner surfaces of carbon particles. the unsaturated hydrocarbon^,^^ the promoted activity The catalytic metal species in the deposited state should be selectively effected on the HDS reaction. This could contribute to the hydrotreating reaction as reis consistent with the experimental result that the ported by De Agudelo and Galarraga6 that the metals activity promotion was observed in HDS reaction but of active phase were deposited onto the surface of porous not in asphaltene conversion, as shown in Figure 7. It silica or silica-alumina particle, resulting in the in situ is shown in Figures 8 and 9 that asphaltene conversion formation of a supported catalyst which could be regenor the conversion of heavier to lighter oil fractions was erated and reused. not affected by the presence of cobalt species but was dependent only on the dispersed concentration of molybdenum species. The catalyst mixture of CoAA-PMA, Conclusion of which the Mo concentration was 237.5 ppm, revealed Dispersed catalyst precursor compounds containing the lowest conversions for asphaltenes and the heavy molybdenum showed better hydrotreating performance oil fractions. in terms of sulfur and asphaltene conversions than did The reaction performances of the pair-dispersed cataother precursors containing tungsten, nickel, or cobalt. lyst systems Co-Mo, Ni-Mo, and Ni-W, were comA large HDS activity synergism was observed for the pared in terms of sulfur conversion, asphaltene convercombined catalyst system consisting of two different sion, the conversion of heavier to lighter oil fractions, precursors. In general, the Co-Mo system showed the and concentrations of metals in product oils, as ilhighest activity and selectivity for HDS reaction. Howlustrated in Figures 10-12 and Table 6,respectively. ever, selectivity for hydrocracking reaction was the The Co-Mo catalyst showed the highest conversion in highest in the Ni-Mo system, while the Ni-W system HDS reaction. For asphaltene conversions, the Co-Mo and Ni-Mo systems are similar but higher than the showed the lowest activity in the overall reaction. Ni-W system. For the conversion of heavier to lighter Consequently, Co-Mo was considered as the best oil fractions, a higher conversion is obtained with the dispersed catalyst system for the HDS of heavy residual Ni-Mo than the Co-Mo system. It is observed from oil. It was also confirmed that the reactor-packed Table 6 that a higher hydrodemetallization (HDM) are activated carbon was very effective in recovering the achieved in order of Co-Mo > Ni-Mo > Ni-W. In view dispersed catalysts from reactant oils and that the of these results, it can be said that the Co-Mo system catalysts in the deposited state onto activated carbon is more reactive for HDS and asphaltene conversion took a part in the reaction, contributing to the improvewhile the Ni-Mo system is more reactive toward ment in reaction performance, particularly for HDS. hydrocracking. Accordingly, the best pair of the disIt is clear that the existence of the HDS activity persed catalysts for heavy oil HDS is thought to be promotion between codispersed catalyst precursors and cobalt and molybdenum. the possible recovery of the catalyst species by the Immobilization of Dispersed Catalysts on the reactor-packed carbon make it promising to utilize the Activated Carbon. It is apparent in Figures 4 and dispersed catalyst system in the heavy oil HDS in stead 10 that alumina-supported catalysts showed a rapid of the conventional supported type catalysts which are deactivation trend with time on-stream of reaction deactivated rapidly. notwithstanding their very high initial HDS activity. On the other hand, the dispersed catalysts showed a Acknowledgment. Financial support provided by general trend of incrasing sulfur conversion with time the Ministry of Trade, Industry and Energy, Republic (33) Teichner, S. J. J. Cutal. 1989, 115, 591. of Korea, is gratefully acknowledged.
