characterization and deactivation studies of spent resid catalyst from ebullating bed service

I n d . Eng. Chem. Res. 1995,34, 755-762 755

Characterization and Deactivation Studies of Spent Resid Catalyst from Ebullating Bed Service

Per Zeuthen* and Barry H. Cooper Haldor Topsae Research Laboratories, DK-2800 Lyngby, Denmark

Fred T. Clark and David Arters Research and Development Department, Amoco Oil Company, Naperville, Illinois 60566

Two series of bimodal CoMo/AlzOs catalyst samples have been analyzed and characterized using porosimetry, temperature-programmed oxidation and NMR spectroscopy. The samples are withdrawn after varying times on stream of up to 120 days. The results show that coke and metals are deposited rather rapidly and that the pore volume of the catalyst at equilibrium is almost filled with coke and/or metals. With increasing time on stream, the coke in both the first- and third-stage samples becomes more hydrogen and sulfur deficient as evidenced by increasing concentration of aromatic carbon and a lowering in concentration of S associated with the coke. There is evidence that the sulfur in the coke is associated with the upper layers of coke and that nitrogen may adsorb preferentially during the initial coke laydown. Further- more, it is shown that vanadium catalyzes the oxidation of coke. Model compound activity measurements show that the HDS activity is less sensitive to the effects of coke deposition than the HDN and hydrogenation activities. It is also shown that the initial metals deposits have a stronger effect on the loss of activity for HDS than for HDN and hydrogenation, indicating that different sites are involved in these hydrotreating reactions.

Introduction

Catalysts for resid upgrading operate under severe ' process conditions which require the catalyst to be active for heteroatom removal while undergoing deposition of relatively large amounts of carbon and feed metals. Deactivation of resid catalysts is a complex process which has been studied by Bartholdy and Cooper (19931, Diez et al. (1990), Fleisch et al. (19841, Hannerup and Jacobsen (1983), Khang and Mosby (19861, Thakur and Thomas (19851, and Wolf and Alfani (1982). Further work is required to fully model the deactivation of these catalysts.

The purpose of this paper is t o describe the nature of carbon and feed metals deposition on aged resid catalyst. A further goal is to characterize the deactivating role of the deposits with respect to the individual catalytic reactions: HDS, HDN, and hydrogenation using model compound reactions.

To this end a unique series of samples has been investigated. For the first time a series of aged resid hydrotreating catalysts from a three-stage expanded bed pilot plant has been withdrawn "on stream" from two different positions in the reactor. It is significant that the exact age of each of these samples is known. They have been studied using nitrogen and mercury porosimetry, temperature-programmed oxidation, solid state NMR spectroscopy, and elemental analysis. These techniques combined with model compound activity studies were used to obtain more insight into the properties of the deposits on the working catalyst as well as to characterize the relations between activity functions and the carbonaceous and metal deposits.

Experimental Section

The series of aged Co/Mo-promoted alumina resid hydrotreating catalysts was obtained from a 100 kglday

* Fax +45 45 27 29 99; e-mail [email protected].

three-stage expanded bed pilot plant designed to dupli- cate commercial conditions (Beaton et al., 1986; and Boening et al., 1987). The catalysts were proprietary formulations developed by Amoco and prepared com- mercially by Haldor Topsae A/S. The catalysts consisted of 4.5% Moo3 and 0.7% COO supported on a high surface area y-alumina. On-stream catalyst withdrawals from the first and third reactor were performed during the first 21 days of the run and at the conclusion of the run on day 120. Initially, the three-stage pilot plant was charged with fresh catalyst in each reactor.

The high-pressure, high temperature pilot plant used in previous deactivation studies (Myers et al., 1989) was operated isothermally at conditions of commercial severity (Beaton and Bertolacini, 1991).

Conversion of 538f "C material to lighter products was 80-85% for the first 21 days. The feed was a blend of 90 vol % Khafji vacuum resid and 10 vol % of a proprietary lower-boiling diluent. Feed properties are given in Table 1.

Catalysts described in this paper are limited to those samples obtained from the first and third stages. Catalyst samples were washed with gasoil, toluene Soxhlet extracted to a clear end point, then dried under nitrogen at 107 "C.

Carbon, hydrogen and nitrogen contents were determined by combustion analysis. Vanadium and nickel were determined by XRF.

Regeneration. Decoking of the spent catalyst was performed by heating the latter in air at a rate of l"C1 min to 400 "C for approximately 4 h in order t o burn off the carbon. The metal deposits are primarily converted to oxides.

Porosity. Catalyst porosity was determined by nitrogen desorption using the BJH method on a Mi- cromeritics 2400 instrument and by mercury intrusion using an Autopore 9200 I1 instrument and assuming a contact angle of 130". Measurements were performed

0888-588519512634-0755$09.00/0 0 1995 American Chemical Society

766 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995

Table 1. Resid Feed Properties sulfur, wt % nitrogen, wt 8 Ramsbottom carbon, wt 5% API gravity 538+ “C, wt % nickel, ppm vanadium, ppm 538+ “C

WC (molar) N/C (molar)

WC (molar) N/C (molar)

asphaltenes

5.00 0.42 20.5 4.17 82.7 46 153

1.41 0.0051

1.06 0.0125

both on whole pellets and powdered catalyst pellets-no difference was found.

Temperature-Programmed Oxidation. The temperature-programmed experiments were carried out in a fixed bed reactor consisting of an 8 mm diameter quartz tube. The experimental conditions employed were 0.1-0.3 g of crushed (20-30 mesh) sample, 10 “C/ min linear heating rate, 30 cm3/min flow rate and a temperature of up to 600 “C. Some of the samples were repeated at a lower temperature rate (2 “C/min.). The oxidation gas was obtained from a gas cylinder (20% 0, in Ar), dried using molecular sieves, and introduced via a mass flow meter. Part of the effluent stream was monitored for desorbed species with a quadrupole mass spectrometer (Balzers QMG 420).

lSC NMR. Carbon-13 NMR analyses were performed using cross-polarized, magic angle spinning (CP/MAS) on a Chemagnetics M-100 spectrometer. The magnetic field strength was 2.35 T, and the spin rate was 4.1 kHz. Spectra were obtained by using a 5.5 ps lH 90” pulse, a 1 ms cross polarization contact time, and a relaxation delay of 0.5 s. Chemical shifts were referenced to tetramethylsilane.

Activity Measurements. To measure the effect of carbonaceous deposits and metal deposits on the activities for the different samples, model compound activity studies were performed. After f is t sulfiding the samples, HDS, HDN, and hydrogenation (HYD) activities were measured simultaneously using a reaction mixture containing dibenzothiophene (DBT), indole (IN), and naphthalene (NAP). Fresh catalyst activities were also measured using the same procedure.

The activity studies were carried out in an isothermal fixed-bed reactor using 0.3 g of crushed (20-30 mesh) catalyst diluted with 0.3 mm glass beads in a 1:l ratio. Liquid feed and hydrogen were introduced through an evaporation section packed with 3.0 mm glass beads. The catalyst samples were initially presulfided for 4 h at 400 “C, 5.0 MPa, by passing hydrogen and a solution of 2 wt % CS2 in n-heptane through the reactor. The catalyst activity was measured using a reaction mixture consisting of 3 wt % dibenzothiophene, 0.5 wt % indole, 2 wt % naphthalene, and 2 wt % CS2 in n-heptane. Test conditions were 390 “C, 9 MPa total pressure, and gas SV = 68 h-l. Measurements were first taken after the 8 h of operation at these conditions in order to ensure steady state. Analysis of the reactor effluent was carried out on condensed liquid samples by direct injection via an automatic injector into the gas chro- matograph, which was equipped with a 50 m cross- linked methylsilicon capillary column and a FID detec- tor. Measurements were taken over 16 h. The catalyst activity was calculated from measured conversions of DBT, IN, and NAP. No deactivation was observed during the activity test.

Results and Discussion

Physical and Chemical Analyses. Analyses of the aged catalyst samples are shown in Table 2. All figures are given on a fresh catalyst basis. Apart from the day 120 sample, the catalysts from the third reactor con- tained only small amounts of deposited Ni and V. The coke levels and coke aromaticities (by 13C NMR) were generally higher for the third reactor samples. Also, both the coke level, the aromaticity and the WC ratio equilibrated more slowly for the third-stage samples than for the first-stage samples. However, this may have been an artifact of the sequential deactivation of fresh catalyst in the first two stages. Aromaticity of the coke is defined below.

The carbon level increases logarithmically with catalyst age in third-stage samples but increases only slowly after day 1 in the first-stage samples. Presumably, this is associated with the higher level of metal deposits in the first reactor samples.

These differences in rate and amount of coke laydown are the result of several combined factors. The lower ultimate coke laydown on first stage samples is probably a function of the hydrogenating effect of deposited metals and the higher hydrogen partial pressure in the first reactor. The faster initial laydown rate may be a function of the higher reactivity of the fresh feed relative to the downstream stages. A “sequential” deactivation of the stages may also protect the downstream catalyst from coke-forming components.

Another result of interest in Table 2 concerns changes in the nitrogen and hydrogen content of the coke as a function of reactor stage and catalyst age. Initially, NIC ratios for both first-stage and third-stage catalysts are high as compared with the feed in Table 1, indicating that nitrogen is strongly adsorbed to acid sites (Absi- Halabi et al., 1991; Trimm, 1990) or to the active phases on the catalyst surface (Zeuthen and Jacobsen, 1989). As the catalyst ages, however, differences are noted. In stage 1, the N/C ratio stays reasonably constant, whereas in stage 3, the N/C ratio falls to levels typically found in product asphaltenes (Amoco, unpublished results).

Hydrogen, on the other hand, indicates a coke “hard- ening mechanism in both fist- and third-stage samples. A high initial WC ratio drops with time to levels that are lower than typical WC ratios found in product asphaltenes (Amoco unpublished results). The variation in WC ratio in third-reactor samples seems to be larger than for the first-reactor samples, probably due to the larger variation in third-stage coke levels with time. Variations in WC ratio can, however, still be seen if background H present as OH or SH groups is sub- tracted. Catalyst samples, sulfided by H2S/H2 prior to Soxhlet extraction, typically contain 0.55 wt % H.

It is also interesting to note from Table 2 that the rate of metals deposition for Ni and V in the first reactor is similar and almost constant throughout the run.

13C NMR Spectroscopy. Figure 1 compares normalized I3C NMR spectra of the first stage vs third stage spent catalysts after 21 days on oil. All spectra contain peaks due to “aromatic” carbon, “aromatic” carbon spinning sidebands (SSBs), and “aliphatic” carbon as shown in Figure 1. The concentration of aromatic carbon in the coke, values of which are listed in Table 2 for selected catalysts, is defined as follows:

Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995 757

Table 2. Elemental Analysis of Spent Catalyst (Fresh Basis, Normalized to Mo) ~~ ~~

reactor age, days V, wt % Ni, wt% S , w t % Fe, wt % C/C,,,a% WC, M N/C, M NMR-CA, %

1 1 0.404 0.125 2.94 0.016 34.0 1.359 0.0417 68.8 1 1 1 1 1 1 3 3 3 3 3 3

3 4 5 8

21 120

1 3 4 8

21 120

1.092 1.350 2.903 2.728 7.100

30.750 0.001 0.004 0.003 0.006 0.033 7.014

0.346 0.428 0.866 0.829 2.112 9.000 0.003 0.005 0.008 0.022 0.128 2.704

3.36 3.61 4.55 3.97 7.27

24.00 2.64 2.52 2.62 2.79 2.70 6.80

0.023 0.025 0.035 0.036 0.085 0.302 0.016 0.014 0.019 0.025 0.035 0.078

43.6 47.3 50.7 51.4 52.4 62.2 13.7 22.7 41.5 57.8

100.0 146.6

1.166 1.118 1.091 0.981 0.936 0.551 2.674 1.706 1.046 0.815 0.600 0.434

C,, is defined as the weight of carbon deposited on the catalyst in the third stage after 21 days on oil.

1 I

I I 1 I I I I t I I 300 250 200 150 100 50 0 -50

PPm Figure 1. 13C NMR spectra of spent catalysts withdrawn after 21 days on oil from (a) third-stage reactor CA = 87.6% and (b) first-stage reactor CA = 77.6%, (1) aromatic carbon, (2) aliphatic carbon, (3) aromatic carbon spinning sidebands.

c, = area under aromatic carbon peaks + aromatic carbon SSBs

total peak area (aromatic + SSBs + aliphatic)

Figure 1 shows that the carbonaceous material deposited on the third-stage catalyst after 21 days on stream is significantly more aromatic in character than the material deposited on the first-stage catalyst. Aro- matic carbon concentrations are 87.6% vs 77.6%, respectively.

An inspection of Table 2 shows that spent catalysts from the third stage have consistently higher concentrations of aromatic carbon as compared with spent catalysts from the first stage after equivalent time on stream. Also, as shown in Table 2, the aromaticity of the coke on both the first- and third-stage samples increases with time on stream. However, the rate of this increase differs depending on the stage. Aroma- ticity of third-stage samples increases steadily over the course of the 120 days from 76.6% to 87.6% CA, whereas first-stage samples show a rapid equilibration after the first 21 days on stream from 68.8% to 77.6% CA with little change in aromaticity after this period due to sequential deactivation of the first two stages.

The NMR results of Table 2 and Figure 1 agree with results of similar earlier studies. For example, in the core electron energy loss spectroscopy (CEELS) analyses of deactivated resid hydrotreating catalyst from an earlier Maxi pilot plant run, Myers observed increasing aromaticity of the coke on the catalyst as the stage number increased and also as the catalyst itself aged (Myers et al., 1989). 13C NMR studies of aged resid demetalization catalysts and aged bitumen hydrocrack- ing catalysts by Egiebor showed that the aromaticity

0.0345 0.0330 0.0230

0.0462 77.6 0.0507 77.5 0.0631 76.6 0.0454 0.0359 82.8 0.0340 0.0282 87.6 0.0210 90.2

of the organic residues on the catalyst increased with increasing process severity Egiebor et al. (1989). Sa- jkowski observed increasing aromaticity of the carbon on catalyst with increasing process severity in deactivation studies of coal liquefaction catalysts (Sajkowski et al., 1988).

Temperature-Programmed Oxidation. Tempera- ture-programmed oxidation studies were carried out in order to characterize the deposits on the spent catalysts. Oxidation of carbon gave C02 and CO; oxidation of nitrogen and sulfur gave NO and S02, respectively. Note that NO2 was not formed.

All TPO spectra are normalized to fresh catalyst and the relative intensities are comparable for the two series.

TPO COZ Analyses. The oxidation of the carbon on the samples typically gave a single broad peak for both CO (mass 28) and C02 (mass 44). The position of the peak maximum for the first-reactor samples varied from 380 "C to 460 "C with lower temperatures being obtained the older the sample (Figure 2a). For samples from the third reactor, the position of the peak maximum occurred at about 510-520 "C except for the day 120 sample, where the peak maximum was 410 "C (Figure 2b).

The older samples from the third reactor were oxi- dized both at slow (2 "C/min) and fast (10 "C/min) temperature rates but gave the same results.

TPO studies on vanadium-free catalysts by Zeuthen (1989) have shown that coke burnoff is independent of coke type but dependent on the metals loading of the catalysts. Yoshimura et al. (1988) have shown that the temperature at which the C02 peak maximum occurs increases with increasing aromaticity of the coke. However, the NMR studies show that for the first-stage samples, the aromaticity increases with age even though the temperature of the CO2 peak decreases. The explanation seems to be that the vanadium catalyzes the oxidation of the coke, and in fact a good correlation exists between amount of vanadium on the catalyst and temperature of coke burn-off (Figure 2c). These results are consistent with those of Zeuthen (1989) and Massoth (1981).

To investigate further the catalytic role of vanadium, the day 4 sample from reactor 3 was impregnated with 1.5% V as vanadyl oxalate by pore volume impregnation. The temperature for the C02 peak was lowered significantly due to the catalyzed oxidation. The peak temperature is lower than expected from Figure IC for two possible reasons. First, the impregnated vanadium may have been evenly distributed over the surface enhancing the oxidation, whereas in the unimpregnated samples, the deposited vanadium may be unevenly distributed


1 I \ a -021 COZ Reactor 1

Temperature, 'C

C .$

8 g! -

B 2 'e -

0 100 200 300 400 500 600 700 Temwrature'C

0 100 200 300 400 500 600 700

Temmrature 'C

0 Reactor 1 A Reactor3 Q Reactor 3 - Day 4 @ Reactor 3 ~ Day 4 +I .5%V 0 N 8 500

u.u I u.1 1 1u 1 vu %V

Figure 2. C02 profiles for TPO of (a) first-stage samples, (b) third- stage samples, and (c) temperatures for maximum COZ production in the TPO experiments: model samples; third-stage samples; first-stage samples; samples with impregnated vanadium.

with a relatively lower content of vanadium in the coke as discussed below. Second, the impregnated V may have decomposed to VzO5 at a lower temperature than the decomposition temperature of vanadium sulfides (Yoshimura et al., 1991) which would also enhance the oxidation. The results for the pure carbon samples with a surface area of 220 m21g (powdered graphite, powdered graphite mixed with the fresh catalyst) are shown in the same plot. These model samples give the temperature for noncatalyzed oxidation of carbon which is above 500 "C.

TPO SO2 Analyses. Figure 3 shows the TPO profiles for SO2 (mass 64) in the first reactor and third reactor, respectively. From the TPO of sulfided fresh catalyst it is seen that the oxidation of Co/Mo sulfides to SO2 occurs between 200 and 250 "C. In the spent catalyst samples there is an SO2 peak occurring in the same temperature range. In all the reactor 1 samples (Figure 3a1, the intensity of this low-temperature peak is greater than that for fresh catalyst with maximum intensity exhibited by the 21 day sample. Peak height for the third reactor samples (Figure 3b) is very similar to that obtained with fresh catalyst.

I \

I00 200 300 400 500 600 Temperature'C

Figure 3. SO2 profiles for TPO of (a) first-stage samples and (b) third-stage samples.

A possible explanation of the low-temperature SO2 peak in the spent catalyst samples is that it emanates from the oxidation of deposited metal sulfides that are not covered by coke since there is no corresponding CO, peak at the same temperature, as confirmed in other related studies (Absi-Halabi et al., 1991; Yoshimura et al., 1991). Judging from the peak intensities, this explanation leads to the conclusion that the major part of the deposited metals are "exposed". There may also be some interaction between the deposited metals and catalyst metal sulfides. The relatively low peak intensity of the 120 day samples reflects less exposed metal despite higher total deposited metals, indicating that the exposed metals become covered by coke as the catalyst ages. Another possibility is that the low- temperature peak arises from metal-catalyzed oxidation of absorbed sulfur species with the degree of catalysis increasing as metal deposits increase.

The spent catalysts also exhibit a high temperature SO, peak occurring between 400 and 500 "C which is not present in the fresh catalyst. Since similar catalysts regenerated at 500 "C have Alz(SO4)3 present as shown by XRD Clark et al. (19911, this high-temperature SO2 peak could be due to thermal decomposition of the sulfate. Even though handbooks give 770 "C as the value for thermal decomposition of A12(S04)3, high levels of V205 present on the catalyst promote the decomposition of Alz(SO413 at lower temperatures (Clark et al., 1993). However, at low NifV levels, the high-temperature peak is due to organic sulfur as previously discussed by Zeuthen et al. (1991). The position of the peak coincides with the front edge of the corresponding C02 peak. The peak heights are greater for reactor 1 samples than reactor 3 samples, but the differences are less pronounced than for the low-temperature peak.

Both these facts indicate that the high-temperature SO, peak in samples containing less than 1 wt % Ni + V is due to oxidation of organic sulfur associated with the coke rather than inorganic sulfur bound to deposited metals. The organic sulfur is thought to be only


NO Reactor 1 -120 .021 a

0 100 200 300 400 500 660 700 TemperatureOC

0 100 200 360 400 500 600 700 Temperature'C

Figure 4. NO profiles for TPO of (a) first-stage samples and (b) third-stage samples.

associated with the upper layer(s) of coke since the evolution of SO2 occurs at the same temperature as the initial oxidation of carbon. This would also explain the almost constant sulfur level found for reactor 3 samples days 1 to 21 (which are essentially devoid of deposited metals) despite the large variation in carbon level.

The temperature at which the peak maximum occurs seems to be a function of vanadium concentration in much the same way as the C02.

The three samples with the highest level of metal deposits (day 21 and 120 reactor 1 and day 120 reactor 3) also exhibit SO2 peaks between 260 and 380 "C. It is probable that these peaks are due to oxidation of deposited metal sulfides but more work is needed to identify the species involved.

TPO NO Analyses. The NO (mass 30) profiles shown in Figure 4 for the two series also show the appearance of two peaks, indicating that at least two types of nitrogen exist on aged catalysts. The high- temperature peak occurs at temperatures between 500 and 600 "C, which according to Zeuthen et al. (1991) is characteristic of nitrogen associated with the coke. In addition, a small low-temperature component peak seems to be present. Since this peak occurs before the appearance of the C02, it is thought to stem from nitrogen associated with the active CoMoS phase. In Zeuthen et al. (1991) evidence is given to the effect that the peak is attributed to strongly absorbed NH, species.

The intensity of the low-temperature peak in both the first- and third-stage series corresponds very closely to the results for the low-temperature SO2 peak and gives added support to the idea that these peaks reflect the amount of exposed metal sites. The intensity of the high-temperature peak increases with the amounts of carbonaceous species present but occurs at a higher temperature than the C02 peak maxima and the high- temperature SO2 peak.

Reactor 1 I l a A' Fresh

Relative Pore Radius


I C Day 120


Figure 5. Pore size distribution of (a) first-stage samples, (b) third-stage samples, and (c) catalysts withdrawn after 120 days from both stages.

Furthermore, although the position of the peak is to some extent dependent on the vanadium level, the dependency is very different from that of C02 and S02. These facts tend to indicate that the nitrogen is not evenly distributed throughout the coke but more likely to be found beneath the more condensed coke or in separate structures deposited on the catalyst. This is also supported by the data in Table 2, where high initial N/C ratios suggest that nitrogen may have been preferentially adsorbed to acid sites, as reported in Absi- Halabi et al. (1991).

Porosity. Figure 5a,b shows pore size distributions of fresh and spent catalysts withdrawn after varying time on stream from the first- and third-stage reactors, respectively. The bimodal character of the catalysts is evident from the figures, which are plotted on a spent catalyst basis. Coke and metals deposit significantly more rapidly on the first-stage catalyst after 1 day on oil (Figure 5a), whereas the third-stage catalyst just shows a minor loss in pore volume after the first day on stream (Figure 5b). By day 8, however, the third- stage sample has lost significantly more pore volume than the first-stage sample. Table 3, showing porosity analyses of the catalysts on a spent basis, also confirms these trends. By day 8, the first-stage catalyst retained 71% of the initial surface area as compared with only 50% retention in the third-stage catalyst. Average


Table 3. Porosity Analyses, Spent Catalyst Basis age % BET surface % pore vol retained in pores avmesopore % pore vol retained in

reactor (days) area retained ~ 1 2 0 0 A diameter (N2D BJH) diameter (4 V/A), A pores '1200 A diameter (Hg) 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3

0 1 3 4 8

21 29

120 0 1 3 4 8

21 120

100 84 80 79 71 85 61 34

100 90 80 81 50 34 23

100 70 61 54 56 84 43 23

100 87 76 68 33 28 15

mesopore diameter (4 x PVBET) decreases steadily over most of the run for both the first- and third-stage samples as seen in Table 3. Also, interestingly, macropore volume in pores > 1200 h; diameter decreases more rapidly in third-stage samples as compared with first-stage samples as seen in Table 3, indicating the predominance of lower density coke in the third-stage samples and the predominance of higher density metal sulfides in the first-stage samples.

Coke densities, calculated from changes in third-stage sample porosities, averaged 0.75 g/cm3, which is similar t o the value reported by Myers et al. (1989). The relatively low value-as compared with various aromatic molecules-indicates the presence of some microporosity.

Figure 5a,b shows a discontinuous increase in the mesopore diameter on day 29 for the first-stage sample and on day 21 for the third-stage sample, respectively. This discontinuity may indicate a partial rearrangement of the coke and metals in the pores perhaps associated with changes in coke morphology or dehydrogenation. However, WC ratios in Table 2 did not indicate any unusual compositional changes occurring to the coke over this period.

Figure 5c compares the pore size distributions of the equilibrium end-of-run catalysts withdrawn from the first stage (top) and the third stage (bottom). The third- stage catalyst clearly has less pore volume in both mesopores and macropores as compared with the first- stage as seen in Figure 5c and Table 3. The mesopores in both the first- and third-stage spent catalysts of Figure 5c are characterized by a peak or shoulder at about 20 h; radius as well as a larger peak centered at about 40 h; radius. A comparison of Figure 5c with 5a or 5b shows that the mesopores of the equilibrium catalyst are almost completely filled with coke and/or metal deposits.

The 20 h; peaks may indicate an intricate network of mesoporous channels formed by the deposited coke. Similar 20 h; radius pores are found in coked catalysts from related studies. In one study, LC-fining CoMo catalyst was pyrolyzed with styrene at 425 "C at various coke loadings up to 18 wt %. At coke loadings above 10 wt %, a similar porous network of 20 A mesopores was formed within the catalyst Baumgart et al. (1990). Interestingly, the same 20 A mesopores were also found in y-aluminas pyrolyzed in cyclohexene or ethene at 600-700 "C (Vissers et al., 1988). Both of these results support our findings despite our use of real resid feeds. Other studies report similar pore radii as being due to physical phenomena taking place during the experiments Gregg and Sing (1982).

154 127 117 113 107 115 109 105 154 148 146 130 102 123 99

100 80 77 52 54 47 53 49

100 56 70 51 50 56 31

Activity Measurements. Figure 6 shows the activities for the samples relative to the fresh catalyst as a function of age. In these three-dimensional plots, the relative activities (HDS, HDN, and HYD) are shown as a function of the carbon content (normalized t o day 21 samples) and the vanadium content (logarithmic scale).

Figure 6 also shows the activities of decoked (regenerated) first-stage samples. Because of the somewhat higher activities of these samples, the activity tests were carried out at 350 "C instead of at 390 "C.

The deactivating effect of coke is not the same for all the reactions. This is seen most clearly for the third reactor samples, days 0-8, which contain very small amounts of deposited metals. The HDN and HYD functions decrease rapidly as the coke level increases. The curves can be fitted by an expression of the type of activity = flexp(-%C)), which suggests a fouling-type deactivation mechanism by the coke. The HDS function deactivates quite differently with only slight (approximately 20%) deactivation up to a coke level of 55 wt % followed by an extremely sharp decline in activity as the coke level increases. The HDS activity fits an exp- (-age) type function reasonably well.

The difference in activity decline for HDS and for HDN and HYD indicates that there are two different sites for these reactions as also discussed by previous authors (Muralidhar et al., 1984). Another possible explanation is that the activity decline is due to diffu- sional limitations (Lee et al., 1991; Lee et al., 1991). As seen from Table 3, the average mesopore diameter for these samples is reduced by about 50% during the run. However, increased diffusion limitations arising from the buildup of coke in the pores will cause the most rapid decline in activity for the reaction having the highest activity. The activity ratios for the three reactions HDS/HDN/HYD on fresh catalyst are 1/0.5/ 0.4. Thus, although part of the activity decline may be attributable to diffusion limitations, the difference in behavior of HDS and HDN/HYD cannot be explained by this.

For the first-reactor samples, the situation is more complex. The carbon level increases only slightly after day 3, and the spent catalyst samples contain much larger amounts of deposited metals. The deactivation is more rapid for all functionalities.

Regenerated Catalyst Activity. To help separate the effects of coke and metals on the first-reactor samples, activity measurements were also made on regenerated (Le., decoked) stage 1 samples. As seen in Figure 6, the HDS activity declines quite rapidly as the


Table 4. Activity Data, Aged Catalysts Relative to Fresh Catalyst a Sulfur Activity

Relative to Fresh

0 Reactor 1 Reactor 3

o Decoked Reactor 1

b Nitrogen Activity Relative to Fresh


o Decoked Reactor 1

75

1 .o 0

CICeq, %

c Hydrogenation Activity Relative to Fresh


0 Decoked Reactor 1

75

Figure 6. Relative fresh-catalyst activities with carbon content as normalized to 21 days of exposure and vanadium deposits (logarithmically): (a) sulfur removal activity; (b) nitrogen removal activity; (c) hydrogenation activity.

amount of deposited metal increases, whereas the activity decline is much slower for HDN and HYD reactions.

The results obtained for the original first-reactor samples are, to a first approximation, the sum of the deactivating effects for metal deposition as found for the decoked samples and coke deposition as found for the reactor-3 samples. To explain the measured activities of the decoked samples in terms of different active phases, artificially aged samples were prepared. Fresh catalysts were impregnated by pore volume impregnation with 6% V and 1.5% Ni, respectively, and the activities were determined in the same way as for the decoked samples. Table 4 shows the results which are normalized to the fresh catalyst activity.

The effect of 1.5% Ni on the fresh catalyst is seen as an increase in activity possibly owing to formation of a

HDS HDN HYD fresh 100 100 100 fresh + 1.5% Ni 192.6 153.8 152.3 fresh + 6% V 9.8 80.7 53.6 decoked 21-day 2.1% Ni, 7.1% V 17.8 72.3 79.1

NiMoS phase Topsee et al. (1990). HDS increases more than HDN and HYD. Contrary to this it is seen that 6% V reduces the activity. The HDS activity is reduced more than HDN and HYD. On the basis of these results, the measured activities of the regenerated catalysts can be explained qualitatively in terms of the promoting effect of deposited nickel and the deactivating effect of deposited vanadium (e.g., VS,).

Conclusions

Each reactor of a three-stage ebullating pilot plant was loaded with fresh bimodal CoMo/AlzOs catalyst and operated under typical commercial vacuum resid hydroprocessing conditions over a 120-day time period. Small samples of the catalysts were withdrawn from the first and third reactor of the pilot plant during the run, each sample having a well-defined age in the unit.

In the first-stage samples, catalyst mesopores were filled initially with both coke and Ni + V contaminant metals; but after day 1, coke levels increased only slowly. In the third-stage samples, coke levels increased logarithmically with catalyst age with relatively little metals pickup over the first 21 days. Catalyst porosity measurements may indicate an extensive network of mesoporous channels associated with the equilibrium coke in the end-of-run third-stage sample.

Overall coke levels and aromaticities (13C NMR) were higher in the third-stage samples. Initially, coke N/C ratios were high as compared with the feed in both first- and third-stage samples, indicating that nitrogen was strongly adsorbed, presumably to acid sites on the catalyst. As the catalysts aged, N/C ratios decreased in the third-stage samples but remained relatively constant in the first-stage samples. In both first- and third-stage samples, the coke on the catalyst "hardened" with age, exemplified by decreasing WC molar ratios and increasing aromaticities. The effect was more pronounced in the third-stage samples, however.

Temperature programmed oxidation (TPO) confirmed the presence of two types of adsorbed nitrogen in the coke: NH, species associated with the CoMoS phase and a more strongly adsorbed nitrogen associated with the coke. TPO studies also indicated the strong catalytic effect of vanadium in the oxidation of the coke to COz and S02, the latter attributed to both organic sulfur in the coke as well as to the decomposition of metal sulfides and sulfates.

Model compound activity tests at 9 M Pa (1330 psig) indicated that HDS may occur on a different site than the HDN or hydrogenation reactions. HDS activity was less sensitive to coke laydown but more sensitive t o Ni + V metals deposition as compared with the HDN and hydrogenation reactions. The high activity of the decoked first-stage samples was interpreted in terms of the weighed average of activity-enhancing phases such as Co-Mo-S, Ni-Mo-S or other phases associated with the deposited nickel as well as activity-retarding phases such as those associated with the deposited vanadium (e.g., VS,).


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Received for review June 8 , 1994 Accepted November 7, 1994 @

IE940364Y

@ Abstract published in Advance ACS Abstracts, February 1, 1995.

characterization and deactivation studies of spent resid catalyst from ebullating bed service

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