Energy & Fuels 2008, 22, 2925–2932
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Comparative Study of Hydrodemetallization (HDM) Catalyst Aging by Boscan Feed and Kuwait Atmospheric Residue Andre’ Hauser,*,† Abdulazim Marafi,‡ Adel Almutairi,‡ and Anthony Stanislaus‡ Central Analytical Laboratory, and Petroleum Research and Scientific Centre/Petroleum Refining Department, Kuwait Institute for Scientific Research (KISR), Post Office Box 24885, Safat 13109, Kuwait ReceiVed May 8, 2008. ReVised Manuscript ReceiVed July 3, 2008
Two feeds, a high-metal feed, namely, Boscan heavy crude (BHC), and a conventional low-metal feed, such as atmospheric residue from Kuwait export crude (KEC-AR), were studied with regard to their capacity to deactivate a hydrodemetallization (HDM) catalyst typically used in Kuwait refineries. Feed properties, such as 1300 wt ppm metal (V + Ni), 10 wt % asphaltenes, and 12 wt% CCR, make BHC an ideal feed to accelerate the deactivation of catalysts in an aging test. The competitive adsorption of coke and metals shortens the lifetime of the HDM catalyst tested in a pilot plant experiment under operating conditions applied in Kuwait refineries by ≈80% compared to KEC-AR. During the start of the run phase (up to 240 h), the HDM catalyst accumulated ≈6 times more metals and ≈1.5 times more coke when aged with BHC instead of KEC-AR. At the end of run, the maximum metal on catalyst (MMOC) in both experiments, accelerated test with BHC (1944 h) and life test with KEC-AR (9720 h), was ≈24.5 g of metal (V + Ni)/100 g of fresh HDM catalyst. The coke deposition on catalyst was, however, about 10% higher for the BHC than in the case of KEC-AR. The higher concentration of both total asphaltenes and thermally stable entities (refractory asphaltenes) in BHC could be responsible for its higher coking propensity. The drastic shortening of the life of a HDM catalyst in an aging test with metal-rich feed, such as BHC, is the result of accelerated coking as well as metal poisoning.
1. Introduction Catalyst aging carried out in a benchtop or pilot plant scale is an important tool for refiners to simulate industrial operations and optimize catalysts and refining processes.1 With respect to upgrading of heavy oil, such as atmospheric residue (AR) or vacuum residue (VR), these tests are very lengthy (>300 days) and costly. Therefore, there is a vital interest to shorten the test to reduce costs. With regard to heavy oil refining to produce low sulfur fuel oil (LSFO), hydrodemetallization (HDM), hydrodesulfurization (HDS), and hydrodenitrogenation (HDN) catalysts are in the center of interest. The prediction of the catalyst performance under a certain operational regime while using a typical, industrial feedstock and meeting the market requirements for LSFO of 3 h), the metal take up of the catalyst aged with BHC is remarkably high and reaches about 18 wt % of the fresh (31) Wu, X.; Zilm, K. W. J. Magn. Reson. A 1993, 102, 205–213.
2928 Energy & Fuels, Vol. 22, No. 5, 2008
Figure 2. Metal (V and Ni) on HDM catalyst (MOC) versus TOS for aging with BHC and KEC-AR.
Hauser et al.
Figure 5. TPO graph of initial coke on HDM catalyst after aging with BHC.
Figure 6. TPO graph of initial coke on HDM catalyst after aging with KEC-AR.
Figure 3. Surface area (SA) and pore size distribution (PSD) of spent HDM catalyst after hydrotreating of Boscan feed.
Figure 4. Surface area (SA) and pore size distribution (PSD) of spent HDM catalyst after hydrotreating of KEC-AR feed.
catalyst after 240 h, while KEC-AR contributes in same period of time only 4 wt % (see Figure 2). The total deposit (coke + metal) on HDM catalyst after 240 h of aging with BHC is about 57 wt % of fresh catalyst compared to about 29 wt % from KEC-AR. The severe catalyst fouling during SOR by BHC is also reflected by the losses in surface area and pore volume (Figure 3) For comparison, Figure 4 shows the losses observed when aged with KEC-AR. 3.1.2. Coke Classification. Previous studies26 have shown that, during SOR-coking, two types of coke are formed on metal-
sulfide-supported catalysts.3,32,33 The first type, nonreactive (refractory) coke, is strongly adsorbed at the support surface, and its generation is catalyzed by the acidic sites of the support.25 The second type, reactive coke, represents coke that is associated with the active metal phase on the catalyst34 that could catalyze the oxidation of carbon and lower the burn off temperature (400 °C).35 Figures 5 and 6 display the TPO profiles of 1, 48, and 240 h aged catalysts, respectively. All CO2 profiles show two overlapping peaks, with one maximum 400 °C. At the beginning of initial coking with BHC, predominately nonreactive coke is formed (Figure 5; 1 h profile). With elapsing TOS, the high-temperature peak maximum shifted from 530 to 415 °C and the low-temperature maximum shifted from 450 to 340 °C. In fact, the oxidation temperature of coke correlates with the metal loading on catalyst, indicating that the deposited vanadium has an accelerating effect on coke oxidation. After 240 h TOS, both peaks, the reactive and nonreactive coke, coincide. Aging with KEC-AR, as seen in Figure 6, results also in a fast build up of nonreactive coke that outweighs the reactive coke. Likewise for the BHC coke, the two peaks shifted to lower temperatures with increasing metal loading on the catalyst. Because MOC after 240 h of aging is much higher when BHC is used, the downshift for instance of the high-temperature peak is about twice as much for BHC coke (∆T ≈ 130 °C) as for KEC-AR coke (∆T ≈ 70 °C). (32) Li, C.; Chen, Y. W.; Yang, S. J.; Yen, R. B. Appl. Surf. Sci. 1994, 81, 465–468. (33) Furimsky, E.; Massoth, F. E. Catal. Today 1993, 17, 537–659. (34) Van Doorn, J.; Moulijn, J. A. Fuel Process. Technol. 1990, 26, 39–51. (35) Bartholdy, J.; Zeuthen, P.; Massoth, F. E. Appl. Catal., A 1995, 129, 33–42.
HDM Catalyst Aging by Boscan and Kuwait AR
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Figure 7. ss-SPE/NMR spectra of coke on the HDM catalyst. Right column, aging with BHC; left column, aging with KEC-AR. fa ) Car/C.
3.1.3. Coke Structure. Solid-state SEP/MAS 13C NMR spectra of the SOR coke from both feeds, BHC and KEC-AR, are shown in Figure 7. The spectra demonstrate that the aromaticity (fa ) Car/C), representing the ratio of aromatic to total carbon, of the KEC-AR coke increases somewhat with TOS and reaches after 240 h ≈0.70 compared to ≈0.60 of BHC coke regardless of the duration of coking. Applying SEP/MAS and PI/CP/MAS techniques by solid-state 13C NMR further structural parameters were determined (Table 5) and the following characteristics of the SOR coke structure depending upon the feed are observed: (i) the percentage of aromatic carbon (Car) in coke increases from 58% to 62% (BHC) and from 56 to 69% (KEC-AR), respectively, with TOS; (ii) the BHC coke is richer in quaternary, aromatic carbon (Car;q) than the KEC-AR coke; (iii) the BHC coke contains a remarkably high level of sulfur, and in consequence, the aromatic carbon attached to heteroatoms is equally high; (iv) the alkyl-substituted, aromatic carbon (Car;R) as well as the degree of substitution (σ) is always higher in the BHC coke than in the KEC-AR coke; and (v) the degree of condensation (γ) lies around 0.2 for the coke from both feeds. 3.2. EOR Deactivation. Toward the EOR, the metal take up capacity of the catalyst reaches its maximum (MMOC) and the carbonaceous deposition is converted into highly condensed hydrogen-deficient coke. Both types of deposit, metal and coke, cause blockage of the catalyst pore structure and the active sites at the catalyst surface become inaccessible for the feed.36 As demonstrated in Figures 3 and 4 (EOR), at total deactivation with either of the feeds, the surface area and pore size have dropped by about 95% compared to the juvenile catalyst. At this point of operation, the contents of
contaminants (S, V, Ni, and asphaltenes) in the product increase suddenly despite further temperature increase.19 The catalyst has reached the end of its lifetime, and further conversion of feed is merely due to thermal cracking. Table 6 shows the MMOC and elemental composition of the coke on HDM catalyst at the end of the life test with BHC and KEC-AR, respectively. The catalysts from both life tests carry about the same amount of metals. Because the deposited metals are present as sulfides (vanadium or nickel sulfides), the sulfur content of the deposit is remarkably high. However, according to the higher sulfur content in the BHC feed compared to KEC-AR, the sulfur in BHC coke is by ≈10% higher than in KEC-AR coke. As seen in Figure 8, the TPO CO2 profiles of the EOR coke also show two peak maxima, one < 400 °C and another one at 400 °C. Because of the high metal concentration in the deposit and the fact that metals catalyze the coke oxidation, the dividing line between reactive and nonreactive coke has moved to lower temperatures (≈400 °C for SOR coke) and classification of EOR coke with regard to two types of coke becomes somewhat doubtful. Regardless of the feed, the peak maxima of both profiles nearly coincide. Concerning the coke structure (Table 7), the EOR coke from the life tests with BHC is more aromatic than the one formed by KEC-AR. In detail, with BHC coke contains 69% aromatic carbon compared to 56% in KEC-AR coke. The portion of Car;q is about 46% in BHC coke and 36% in KEC-AR coke. Furthermore, the Car;q consists of 25 and 19% bridged, aromatic carbon (Car;b), 14 and 12% Car;R, and 6 and 5% aromatic (36) Vogelaar, B. M.; Berger, R. J.; Bezemer, B.; Janssens, J. P.; van Langeveld, A. D.; Eijsbouts, S.; Moulijn, J. A. Chem. Eng. Sci. 2006, 61, 7185–7564.
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Table 5. Structural Parameters of SOR Coke on HDM Catalysta percent carbon in structureb
g/100 g of fresh catalyst
degreeb
Car;q
Car;b
Car;R
Car;X
γ
σ
31.3 29.9
10.9 10.7
9.4 12.2
10.9 6.9
0.20 0.19
0.26 0.32
33.8 27.6
10.8 13.8
13.5 9.9
9.5 3.9
0.18 0.23
0.34 0.24
15 16
5 h/6 hc 62.8 34.9 57.0 25.2
10.5 12.6
15.7 8.9
7.0 3.7
0.18 0.22
0.36 0.22
37 69
15 17
10 h/12 hc 50.8 34.6 62.1 26.1
4.3 13.1
17.3 9.8
15.1 3.3
0.08 0.21
0.49 0.22
0.73 0.81
38 70
14 18
57.8 62.9
42.8 29.3
16.0 12.0
16.6 11.4
10.2 6.0
0.28 0.19
0.52 0.25
20.0 17.3
0.72 0.84
35 61
12 19
57.0 68.8
42.5 32.4
6.5 15.0
19.5 12.1
17.0 5.2
0.11 0.22
0.55 0.25
BHC KEC-AR
16.1 18.5
0.73 0.86
33 64
4 17
57.1 73.0
46.0 27.6
13.7 17.8
18.0 8.6
18.0 3.2
0.21 0.24
0.65 0.16
BHC KEC-AR
16.3 19.8
0.68 0.87
38 64
2 15
62.0 69.2
44.8 22.2
19.6 11.6
14.7 6.8
9.8 2.0
0.33 0.16
0.46 0.15
feed
C
C/H
C/N
C/S
car
BHC KEC-AR
6.4 13.1
0.47 0.71
31 85
8 15
57.8 55.7
BHC KEC-AR
7.4 15.2
0.47 0.79
54 81
8 18
62.2 57.9
BHC KEC-AR
17.2 13.5
0.64 0.77
39 79
BHC KEC-AR
18.5 15.3
0.68 0.81
BHC KEC-AR
18.7 16.7
BHC KEC-AR
1h
3h
24 h
48 h
120 h
240 h
c
a To remove feed, product, and coke precursors, the catalysts were extracted with toluene. First value for aging with BHC and second value for aging with KEC-AR.
b
For parameter definition, see the footnote of Table 2.
Table 6. MMOC and Elemental Composition of Coke at the End of the Lifetime of the HDM Catalysta metal feed BHC KEC-AR a
coke
g/100 g of fresh catalyst
g/100 g of fresh catalyst
atomic ratios
V
Ni
MMOC
C
H
N
S
total
C/H
C/N
C/S
23.2 22.2
1.1 2.6
24.3 24.8
9.1 13.1
1.3 1.7
0.1 0.2
44.4 31.8
54.9 46.8
0.58 0.64
106 64
0.5 1.1
To remove feed, product, and coke precursors, the catalysts were extracted with toluene.
Table 7. Structural Parameters of EOR Coke on HDM Catalysta percent carbon in aromatic structureb feed BHC KEC-AR
Car
Car;q
69.0 56.0
EOR: 1944 h/9000 hc 45.8 25.2 14.0 36.0 18.5 12.0
Car;b
Car;R
degreeb
Car;X
γ
σ
6.0 5.5
0.36 0.33
0.38 0.37
a To remove feed, product, and coke precursors, the catalysts were extracted with toluene. b For parameter definition, see footnote of Table 2. c First value for aging with BHC and second value for aging with KEC-AR.
Figure 8. TPO graph of EOR coke on HDM catalyst after life test with BHC or KEC-AR.
carbon attached to heteroatoms (Car;X), respectively. Although the aromaticity of KEC-AR coke is by about 10% lower, the degrees of condensation (γ) and substitution (σ) reflect a similarity of the carbonaceous deposits from both feeds. 4. Discussion Catalyst testing with high metal feed, especially HDM catalysts, aims to determine MMOC within the shortest possible period of time. Two observations usually serve as an indicator that the MMOC is reached: (i) under isothermal conditions, the HDS rate constant goes toward zero;28 and (ii) under constant
product sulfur regime, the sulfur in the product increases despite a further temperature increase.18 If either of the observations is made, the catalyst is considered as fully deactivated and the test can be terminated. Simultaneously, to metal take up, however, deactivation by coke deposition takes place. For predicting the lifetime of a catalyst based on accelerated aging tests with high metal feed, such as BHC, it is also necessary to determine the propensity of the feed to form coke and to compare it with a typical ARDS feed, such as KEC-AR. Asphaltenes, by definition the n-heptane-insoluble fraction of the feed, are the least reactive components that are primarily responsible for coke formation during catalytic hydroprocessing of heavy feedstocks.37 As Hauser et al.4 could show, beside the (37) Maity, S. K.; Perez, V. H.; Ancheyta, J.; Rana, M. S.; Centeno, G. Pet. Sci. Technol. 2007, 25, 241–249.
HDM Catalyst Aging by Boscan and Kuwait AR
Figure 9. TGA graph of asphaltene (AS) conversion under nitrogen at 412 °C; 2 and [ weight loss in wt % of solid matter (CAS, asphaltenes; CC, coke) versus pyrolysis time; and solid lines curve fitting with k1 ) 2.7 × 10-3 min-1 and k2 ) 1.8 × 10-3 min-1 (BHC-AS) and k1 ) 2.3 × 10-3 min-1 and k2 ) 6.6 × 10-3 min-1 (KEC).
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Figure 11. Asphaltene (AS) conversion under nitrogen at 412 °C; firstorder kinetics (CAS, asphaltenes; CC, coke; CO+G, oil and gas); and origin of asphaltenes BHC.
Figure 10. Three-lump model for thermal asphaltene decomposition under nitrogen in a TGA apparatus.
concentration of asphaltenes in the feed, their structural characteristics significantly affect the propensity of the feed to form coke. For instance, hydrotreated feeds generated more coke than straight-run AR, although the contents of asphaltenes in the feeds were lower than that in the straight-run AR by 33% (HDM feed) and 76% (HDM/HDS feed), respectively. The higher propensity in coke formation using hydotreated feeds is due to the condensed polynuclear aromatic structure of the modified asphaltenes and their high stability against hydrogenation and cracking. A recent study on the thermal stability of asphaltenes38 showed that the more refractory asphaltenes in the feed the faster the deactivation of the catalysts. When the two feeds are compared with respect to their asphaltene fractions (Tables 1 and 2), BHC contains about 3 times as much asphaltenes as KEC-AR. Concerning the thermal stability of asphaltenes, pyrolysis of asphaltenes, Hauser et al.38 showed that the asphaltenes from atmospheric residues of heavy crudes consist of a bimodal composition. During pyrolysis at 412 °C under nitrogen (see Figure 9), BHC asphaltenes revealed as well a bimodal composition. Using a three lump model39,40 (Figure 10) and first-order reaction kinetic38 after 220 min at 412 °C, the residue of refractory asphaltenes in the reaction chamber is for BHC asphaltenes ≈3 times higher than for KECAR asphaltenes (Figures 11 and 12.) Catalyst life tests in a pilot plant scale demonstrated that the higher the ratio of refractory to easily pyrolized entities in the asphaltenes the faster an industrial ARDS catalyst system deactivates.38 (38) Hauser, A.; Bahzad, D.; Stanislaus, A.; Behbahani, M. Energy Fuels 2008, 22, 449–454. (39) Martinez, M. T.; Benito, A. M.; Callejas, M. A. Fuel 1997, 76, 871–877. (40) Wang, J.; Anthony, E. J. Chem. Eng. Sci. 2003, 58, 157–162.
Figure 12. Asphaltene (AS) conversion under nitrogen at 412 °C; firstorder kinetics (CAS, asphaltenes; CC, coke; CO+G, oil and gas); and origin of asphaltenes KEC-AR.
Taking into account that asphaltenes play a crucial role in catalyst deactivation especially during the SOR phase and considering the differences in concentration, structure, and thermal stability of the asphaltenes in both feeds, it can be anticipated that BHC will deactivate a catalyst system much faster than KEC-AR not only because of the high concentration of organometallic compounds in BHC but also because of the nature of BHC asphaltenes. In summary, the accelerated deactivation of the HDM catalyst observed when BHC is hydroprocessed instead of KEC-AR occurs through the simultaneous fouling of the catalyst by coke and metal (V and Ni) deposition, and there is no definite indicator for quantifying the contribution of each of the two processes causing the overall deactivation. The MMOC determined by using either of the feeds resulted in nearly the same metal take up (KEC-AR, 24.8 g/100 g of fresh catalyst; BHC, 24.3 g/100 g of fresh catalyst) but a difference in coke deposition (KEC-AR, 46.8 g/100 g of fresh catalyst; BHC, 54.9 g/100 g of fresh catalyst), indicating that the coke build up is not hampering the metal deposition or vise versa. Apperently, the drastic shorter life cycle of a HDM catalyst in an aging test with rich metal feed, such as BHC, is the result of accelerated coking as well as metal poisoning.
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5. Conclusions Two feeds, BHC and KEC-AR, were studied with regard to their capacity to deactivate a HDM catalyst typically used in Kuwait refineries. BHC differs from KEC-AR by a higher concentration in metals (≈15 times), asphaltenes (≈3 times), and thermally stable asphaltenes (≈3 times). These characteristics make BHC an ideal feed to accelerate the deactivation of catalysts in an aging test. The lifetime of a HDM catalyst tested in a pilot plant experiment with BHC under operating conditions applied in Kuwait refineries shortens by ≈80% compared to KEC-AR. After a start of run test (up to 240 h), the HDM catalyst carried ≈6 times more metals (V and Ni) and ≈1.5 times more coke when aged with BHC instead of KEC-AR. Regardless of the differences between BHC and KEC-AR, two types of coke, soft and refractory coke, are found on the catalyst. Concerning the coke structure, however, differences between BHC coke and KEC-AR coke are observed.
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After a pilot plant lifetime test (total deactivation), the maximum metal on catalyst (≈25 g/100 g of fresh catalyst) was nearly the same for aging with either of the feeds but the coke deposition is higher in the case of BHC (≈55 g/100 g of fresh catalyst) compared to KEC-AR (≈47 g/100 g of fresh catalyst). Both coking and metal poisoning contribute to the drastic deactivation of the HDM catalyst in an accelerated aging test with a metal-rich feed, such as BHC. Acknowledgment. The authors acknowledge the financial support of the Kuwait Institute for Scientific Research (KISR) and the Japan Cooperation Center, Petroleum (JCCP), a Japanese organization supported by the Japan Ministry of Economics, Trade, and Industry (METI). The authors also give their special thanks to the management of the Japan Energy Corporation (JEC) for their support and technical assistance in pursuing this study. The authors appreciate the Central Analytical Laboratory at KISR for their assistance. It bears the KISR Project PF010C. EF800298Q
