Catalyst Deactivation during Hydroprocessing of Maya Heavy Crude Oil

The effect of reaction temperature on catalyst deactivation during hydroprocessing of Maya heavy crude oil was studied. Experiments were carried out i...
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Energy & Fuels 2003, 17, 462-467

Catalyst Deactivation during Hydroprocessing of Maya Heavy Crude Oil. (II) Effect of Temperature during Time-on-Stream J. Ancheyta,*,†,‡ G. Betancourt,† G. Centeno,† and G. Marroquı´n† Instituto Mexicano del Petro´ leo, Eje Central La´ zaro Ca´ rdenas 152, Me´ xico 07730 D. F., Mexico, and Instituto Polite´ cnico Nacional, ESIQIE, Me´ xico 07738 D. F., Mexico Received September 4, 2002

The effect of reaction temperature on catalyst deactivation during hydroprocessing of Maya heavy crude oil was studied. Experiments were carried out in a fixed-bed pilot plant at constant pressure, hydrogen-to-oil ratio, and space-velocity. Reaction temperature during 1120 h timeon-stream was varied from 400 to 430 °C. Catalyst deactivation was monitored by the changes in asphaltenes, sulfur, metals (V and Ni), and Rambottom carbon contents in the hydrotreated products. Fresh and spent catalysts were characterized by textural properties, metals, and carbon contents, and scanning electron microscopy. Coke and metals depositions were around 18.5 and 25%, respectively.

1. Introduction The study of catalyst deactivation during hydroprocessing of heavy oil fractions is one of the most important aspects to improve the catalytic performance in petroleum refining processes. The main causes of catalyst deactivation have been considered to be the accumulation of carbonaceous and metallic depositions and the structural changes of the catalyst components.1 Coke is formed very rapidly during the first hours of time-on-stream, and deactivation of catalyst by this material appears to rapidly reach a pseudo steady-state level, while metals in the feed (mainly V and Ni) are converted to their sulfides, which deposit within the pores and irreversibly deactivate the catalyst. Deactivation of the catalyst by metals takes a longer time period. During hydroprocessing of heavy oil fractions, coke deposition of up to 25 wt % of the weight of the original catalyst and large loss of catalyst specific surface area (50-60%) have been reported in the literature.2 Metals, mainly vanadium, build up until the pore becomes pore mouth plugged. With a low-metals feedstock, there is an initial activity loss as coke is deposited on the catalyst. This is followed by a more gradual loss in activity as the metals deposit. However, with a highmetals feedstock, there is a more rapid and severe deactivation after the initial coke deposition. The rapid falloff in activity begins at the onset of pore mouth plugging, and finally the catalyst is deactivated to the level where the run must be terminated.3,4 * Author to whom correspondence should be addressed. Fax: (+5255) 3003-8429. E-mail: [email protected]. † Instituto Mexicano del Petro ´ leo. ‡ Instituto Polite ´ cnico Nacional, ESIQIE. (1) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381. (2) Absi-Halabi, M.; Stanislaus, A.; Trimm, D. F. Appl. Catal. A 1991, 72, 193. (3) Furimsky, E. Appl. Catal. A 1998, 171, 177. (4) Beaton, W. I.; Bertolacini, R. J. Catal. Rev. Sci. Eng. 1991, 33, 281.

For this reason, during commercial operation, reaction rate is increased by constantly raising reaction temperature. The purpose of this raise in temperature is to compensate for the loss of catalyst activity. Of course, there is a final period of the run in which the temperature cannot be increased sufficiently to keep up with deactivation, and the operation has to be stopped. Catalyst deactivation during hydroprocessing of residua has been the subject of many investigations.5-11 However, deactivation of catalysts when heavy crude oils, such as Maya, are hydroprocessed has not received too much attention. In a previous work, we reported a deactivation study during hydroprocessing of Maya heavy crude oil which was carried out in a high-pressure pilot plant at constant operating conditions for 490 h time-on-stream. Taking into account the abovementioned common approach to compensate for catalyst deactivation, in this second part we study the effect of reaction temperature on product quality during run of operation as well as the changes in catalyst properties after reaction. 2. Experimental Section Deactivation studies were carried out in a high-pressure pilot plant. The reactor (2.54 cm inside diameter and 143 cm total length) operates in isothermal mode. All experiments were conducted in once-through hydrogen in the down-flow (5) Callejas, M. A.; Martı´nez, M. T.; Blasco, T.; Sastre, E. Appl. Catal. A 2001, 218, 181. (6) Thakur, D. S.; Thomas, M. G. Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 349. (7) Song, C.; Nihonmatsu, T.; Nomura, M. Eng. Chem. Res. 1991, 30, 1726. (8) Kobayashi, S.; Kushiyama, S.; Aizawa, R.; Koinuma, Y.; Keiichi, I.; Shimizu, Y.; Egi, K. Ind. Eng. Chem. Res. 1987, 26, 2245. (9) Beuther, H.; Larson, O. A.; Perrotta, A. J. In Catal. Deact.; Delmon, B., Froment, G. F., Eds.; Elsevier: Amsterdam, 1980; p 271. (10) Nakamura, N.; Togari, O.; Ono, T. 45th API Midyear Meeting, Houston, TX, May 1980; Vol. 201, p 22. (11) Wiwel, P.; Zeuthen, P.; Jacobsen, A. C. In Catal. Deact.; Bartholomew, C. H., Butt, J. B., Eds.; Elsevier: Amsterdam, 1991; p 257.

10.1021/ef0201883 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/06/2003

Catalyst Deactivation during Hydroprocessing of Crude Oil mode of operation. The pilot plant, catalyst loading, experimental procedure, and catalyst sulfiding were reported in detail elsewhere.12,13 Maya crude oil was used for deactivation experiments. The main properties of this crude oil are: 20.9° API gravity, 3.44 wt % sulfur, 10.5 wt % Ramsbottom carbon (RBC), 12.4 wt % asphaltenes in nC7, 299 wppm V, and 55 wppm Ni contents. The catalyst employed during the tests was a Ni-Mo commercial sample (2.54 mm diameter). The catalyst (75 mL) was in-situ activated by sulfiding with hydrodesulfurized naphtha containing 0.8 wt % CS2 at the following conditions: pressure of 54 kg/cm2, H2-to-oil ratio of 2000 ft3/bbl, temperature of 230 °C, and liquid-hourly space-velocity (LHSV) of 3.2 h-1. The sulfiding time was 18 h. The deactivation study was carried out at the following constant operating conditions: 70 kg/cm2 of total pressure, 5000 ft3/bbl of H2-to-oil ratio, and 1.0 h-1 of LHSV. The startof-run temperature was 400 °C and it was increased at 420 °C and then up to 430 °C in order to compensate for the deactivation of the catalyst. It should be emphasized that temperature was raised during the time-on-stream, and effects on product properties would come from both the change of temperature and time-on-stream, not just the temperature alone. It means that conducting experiments in this way does not really decouple the effects of temperature and time-on-stream. However, this is the way HDT commercial plants compensate for deactivation of the catalyst. Product quality was monitored during 1120 h time-onstream. The first product was obtained at 4 h. Then, product samples were recovered every 50 h. Physical and chemical properties of the feed and products were determined with the following methods: total sulfur, ASTM D-4294; Ramsbottom carbon, ASTM D-524; asphaltenes in nC7, ASTM D-3279; metals (Ni and V), ASTM D-5863. The catalyst unloading procedure is very important in determining changes in properties of a catalyst when it finishes its operation. We have followed the procedure reported in a previous work.14 The spent catalyst was washed with toluene and dried at 80°C before characterization. Catalysts were analyzed before and after hydroprocessing reaction by using the following methods: metals content (nickel and vanadium) was determined by an atomic absorption spectrometer; specific surface area, pore volume, and pore size distribution were measured by nitrogen adsorption at 77 K; carbon content was measured by combustion with an infrared detector; scanning electron microscopy of spent catalyst was performed in a JSM-35 CF JEOL model electron microscope working at 20 kV. The microscope is equipped with a Sigma 2 Kevex model disperse energy system. The samples were deposited on a carbon holder and covered with a thin carbon film.

3. Results and Discussion 3.1. Product Quality. Results of hydrotreated product properties (S, V, Ni, RBC, and asphaltenes) are shown in Figures 1 and 2 as a function of time-onstream. Decay of the catalyst is clearly observed from these figures. The well-known rapid catalyst deactivation period during the first hours of run (0-100 h) is also observed. Asphaltenes content in the product considerably in(12) Ancheyta, J.; Betancourt, G.; Marroquı´n, G.; Pe´rez, A.; Maity, S. K.; Cortez, M. T.; del Rı´o, R. Energy Fuels 2001, 15, 120. (13) Ancheyta, J.; Maity, S. K.; Betancourt, G.; Centeno, G.; Rayo, P.; Go´mez, M. T. Appl. Catal. A 2001, 216, 195. (14) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquı´n, G.; Alonso, F.; Garciafigueroa, E. Energy Fuels 2002, 16, 1438.

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Figure 1. Sulfur, vanadium, and nickel contents in product as a function of time-on-stream.

Figure 2. Ramsbottom carbon and asphaltenes contents in product as a function of time-on-stream.

creases in this period after which it is maintained at a constant value (Figure 2). Being the major coke precursor, this behavior in asphaltenes content indicates that coke is the main source of catalyst deactivation during the initial period of time-on-stream. Rambottom carbon follows a trend similar to that of asphaltenes, which is correct since RBC also indicates the tendency of the feed to form coke. On the other hand, metals (Ni and V) follow a behavior different from that of asphaltenes and RBC (Figure 1). The contents of Ni and V in the product show a linear increase with run of operation. Sulfur content incremented at a rate similar to that of vanadium content. After 280 h of run, it was decided to adjust the reaction temperature from 400 to 420 °C. The main purpose of raising the temperature is to increase the reaction rate, thus to compensate for the loss of catalyst activity. Contents of all contaminants immediately decrease at initial values similar to those observed at the first temperature level. As imentioned before, this increase in reaction temperature was done during time-on-stream, which means that all the behavior observed at temperatures higher than the initial values is due to both the temperature and time-on-stream. When the temperature was increased, we were thinking that another period of carbon deposition was occurring due to the similarity with the catalyst deactivation

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Figure 3. Catalyst stability for HDS (9), HDAsp (O), HDNi (0), HDV (4), and RBC removal (b) at different temperatures. Table 1. Properties of Fresh and Spent Catalysts physical properties m2/g

specific surface area, pore volume, cm3/g mean pore diameter, Å pore size distribution, vol % 500 Å

fresh catalyst

spent catalyst

175 0.56 127

54 0.18 130

9.8 69.1 15.0 6.1

36.4 58.3 4.7 0.6

chemical properties

fresh catalyst

spent catalyst

molybdenum, wt % nickel, wt % sodium, wt % vanadium, wt % iron, wt % carbon, wt % sulfur, wt %

10.66 2.88 0.041 0 0 0 0

4.56 2.69 0.22 7.15 0.29 18.5 9.70

trend during first hours of run; however, when results of carbon on catalyst during different times-on-stream were compared, we found that for 490 h (part I of this series14) and for 1120 h (this study) carbon deposited on catalyst is essentially the same (18.3 and 18.5 wt %, respectively). It indicates that even with an increase in temperature coke is only deposited during the first hours of operation. V removal was higher during all time-on-stream at 420 °C compared to 400 °C, which is a consequence of the higher hydrocracking of asphaltenes activity commonly observed at elevated temperature. Also, removal of vanadium was much faster than nickel removal, indicating that the vanadium moieties are the most reactive. At this temperature level, those metal-containing compounds located at the internal part of the asphaltene molecule are released and hence they can be easily removed. At 700 h of run, it was again decided to increase the reaction temperature from 420 to 430 °C to compensate again for deactivation of the catalyst. The same behavior observed with the first increase was presented, and all contaminants contents in the product decrease at similar values found at previous temperatures. With run of operation, asphaltenes content continues being more or less the same values as the first temperature, which confirms that coke deposition is mainly present at short times-on-stream. On the contrary, nickel and vanadium contents are higher than in previous temperature stages.

Figure 4. Variation of specific surface area (b) and pore volume (O) as function of time-on-stream.

Figure 5. Metals deposition on the catalyst during hydroprocessing of Maya crude oil.

The main feature of this third stage of temperature is that the product exhibited higher content of all contaminants than previous stages. This indicates first that only a 10 °C temperature increase is not enough to compensate for deactivation of the catalyst. Second, deactivation by metals is becoming more important. In addition, the exothermality of the reactions at higher temperatures causes an increase in deactivation rates. 3.2. Stability of the Catalyst. Stability of the catalyst can be directly observed from Figures 1 and 2 as an increase in contaminant contents during run of operation. To have a quantitative value of this stability, the following definition was adopted in this work. Asphaltenes removal (HDAsp) was taken as an example:

HDAsp Stability )

HDAsptf HDAspt0

where HDAspto is asphaltenes removal at the beginning of the first temperature stage (initial conversion) and HDAsptf is the corresponding one at the end of each temperature. For comparison purposes we have applied eq 1 with the same time-on-stream for each temperature, 270 h, since for 400 °C we have data only up to this time. Initial conversions were taken at 50 h run, which is more representative than shorter times because of the

Catalyst Deactivation during Hydroprocessing of Crude Oil

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Figure 6. Intraparticle V profiles in the longest and shortest extremes of tetra-lobe spent commercial catalyst.

dramatic increase in conversion observed during the few hours of run. By this means, HDAsp stability can be calculated for the three temperatures on the same time-on-stream basis. The complete results of these calculations for all reactions are shown in Figure 3. It is observed that, in general, stability of the catalyst was higher at 420 °C, which means that the 20 °C increase in temperature was enough to compensate for catalyst deactivation, and activity and stability observed at 400 °C were reestablished. In fact, activities were maintained during more time of operation. Stability for asphaltenes, vanadium, and RBC removals was higher, while for nickel and sulfur removals it

was similar to those observed at 400 °C. At 430 °C catalyst stability substantially decreased due mainly to metals deposition. 3.3. Characterization of Fresh and Spent Catalysts. Specific surface area, pore volume, mean pore diameter, and pore size distribution of fresh and spent catalysts are shown in Table 1. The same behavior discussed in Part I14 about changes of textural properties of catalyst after reaction was observed, which is summarized as follow: • Losses of specific surface area and pore volume of catalyst were almost 70%. • Percentages of pore volume having pore diameter > 250 Å exhibited a very important decrease (75%).

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• Pores in the range of 100-250 Å showed less decrease (15%), while pores 500 Å are due to partial or total blockage of large pores, which become smaller pores. This also explains the increase in pores smaller than 100 Å which may be not blocked by the large heteroatoms-containing compounds present in heavy crude oils. • Metals and coke deposits not only block the pore mouth but also help to increase pores with smaller size. On the basis of these latter observations the following representation for pores blockage is suggested:

Experiments in Part I14 were conducted at 400 °C constant temperature up to 490 h time-on-stream, and experiments in this second part at variable temperature (400-420-430 °C) during 1120 h of operation. Despite this difference in temperature, the effect of time of operation on specific surface area and pore volume of catalyst was plotted in Figure 4. It is observed that the longer the time-on-stream, the higher the losses in specific area and pore volume of catalyst. The rate of this loss is lower at high time of operation, and after 1000 h it seems that these catalyst properties reach constant values. 3.4. Metals and Coke Deposits. Coke deposited on the catalyst, measured as carbon content, was of 18.5 wt %, which is mainly formed from asphaltenes. Balances between metals in feed and products were done and hence the total amount of metals removed and deposited on the catalyst was calculated as a function of time-on-stream. The accumulated values are shown in Figure 5. The total amount of metals deposited on the catalyst during 1120 h time-on-stream on the basis of fresh catalyst was around 25%. It we compare the amount of metals deposited in our previous experiment carried out at constant temperature of 400 °C during 490 h14 with those deposited at the same time-on-stream but with different temperature (400-420 °C), we found the same percentage of metals, that is about 10%. The same deposition trend is also observed in both experiments. It implies that deposition rate of metals is reestablished when temperature is increased during run of operation. Scanning electron microscopy (SEM) analyses were used to determine how metals are being deposited on the catalyst. Two profiles were examined considering the longest and shortest extremes of the tetra-lobe commercial particle as can be observed in Figure 6. It is seen that V profiles of the two selected zones are very similar. This behavior is different from that reported

Figure 7. Similarity between HDS of refractory compounds and metals deposition in tetra-lobe commercial catalyst.

in Part I.14 The main differences in both SEM analyses are the time-on-stream and reaction temperature. For 490 h at 400 °C constant temperature (Part I), the longest extremes of particle had higher V content on the external surface, and for 1120 h at 400-420430 °C temperature (Part II) vanadium accumulation profile is more or less the same for both the longest and shortest catalyst extremes. In the first case, V deposits were reported to be higher in the lobes of the catalyst, which can be attributed to both the shape of the commercial catalyst and the complex nature of the metal-containing compounds. This type of different deposition patterns in lobes and between them in commercial catalyst can be explained in a way similar to that of hydrodesulfurization of refractory sulfur compounds, such as 4,6-dimethyldibenzothiophene (4,6-DMDBT) as shown in Figure 7. In the case of 4,6-DMDBT, the methyl groups located in 4 and 6 positions hinder the sulfur atom from seeing the active catalyst site, making this molecule very hard to desulfurize. In the case of tetra-lobe commercial catalyst, the lobes, which act as the methyl groups in 4 and 6 positions, hinder the points between lobes, which at the same time act as a sulfur atom. Hence, when complex metals-containing compounds approximate the catalyst, these points are harder to access and accumulation is then higher in the lobes.

Catalyst Deactivation during Hydroprocessing of Crude Oil

However, when temperature is increased, as in the case of this study, hydrocracking of asphaltenes is favored and some of them crack to form smaller molecules. Thus, these new smaller metals-containing compounds can access the points between lobes in the commercial catalyst. This is the reason for having very similar V content profiles in both SEM analyses corresponding to the extremes of the catalyst. Conclusions The increase of temperature during time-on-stream compensates for catalyst deactivation, and both coke and metals deposition rates are maintained almost constant at the operating conditions reported in this study. The former is present only during the first hours

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of operation and the latter continue having a linear trend even with the increase in temperature. The activity of the catalyst is completely compensated when the temperature is increased from 400 to 420 °C, and its stability is maintained for a longer time. Specific area and pore volume of catalyst were reduced as the time-on-stream was higher and after 1000 h or run rate of these losses reached constant values. Hydrocracking of asphaltenes found at high temperature was the reason for having the same vanadium depositing pattern in the longest and shortest extremes of the tetra-lobe commercial catalyst. Acknowledgment. The authors thank Instituto Mexicano del Petro´leo for its financial support. EF0201883