Effect of Process Conditions and Catalyst Properties on Catalyst

Chapter 17

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 23, 2016 | http://pubs.acs.org Publication Date: June 6, 1996 | doi: 10.1021/bk-1996-0634.ch017

Effect of Process Conditions and Catalyst Properties on Catalyst Deactivation in Residue Hydroprocessing M. Absi-Halabi and A. Stanislaus Petroleum Technology Department, Petroleum, Petrochemicals, and Materials Division, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait

A comprehensive study of catalyst deactivation during hydroprocessing of Kuwait vacuum residue in trickle-bed reactors was carried out. The influence of selected process and catalyst parameters including temperature, hydrogen pressure, liquid hourly space velocity, presulfiding and catalyst pore size on coke and metals deposition was investigated. Increasing reactor temperature increased both coke and metal deposition on the catalyst, while increasing pressure decreased coke deposition. Vanadium deposition on the other hand increased with increasing pressure. Increasing feed flow rates increased the rate of deactivation by metals, but decreased coke deposition. Catalyst pore size distribution had a significant effect on catalyst deactivation. The rate of deactivation by both coke and metals deposition was found to be higher for catalysts having predominantly narrow pores. Presulfiding of the catalyst reduced coking and led to better distribution of foulant metals within the catalyst pellet. The effect of the studied parameters on surface area and pore volume of the catalyst was determined. Mechanistic arguments are presented to explain the results.

Rapid catalyst deactivation is a serious problem in heavy oil upgrading by hydroprocessing. It is generally accepted that the deactivation is caused by the accumulation of carbonaceous residues and metal (V, Ni and Fe) deposits (1,2). Coke is believed to be primarily responsible for the rapid decline in catalyst activity during the early period of the run (3-6). After this initial period, coking slows down while metals gradually build up on the catalyst throughout the run. Both coke and metals deposition contribute to diffusional resistance to the reacting molecules in the heavy oils by constricting the catalyst pore diameter and thereby reducing effective catalyst life (1,4,7).

0097-6156/96/0634-0229$15.00/0 © 1996 American Chemical Society

O'Connor et al.; Deactivation and Testing of Hydrocarbon-Processing Catalysts ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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DEACTIVATION AND TESTING OF HYDROCARBON-PROCESSING CATALYSTS

Since rapid catalyst deactivation is undesirable to refiners from an economic point of view, intensive efforts have been directed towards controlling and minimizing the deactivation problem. Studies have shown that the mechanism of catalyst deactivation is very complex, particularly, in residual oil processing where both coke and metals deposition contribute to deactivation (1,2,8). The key to controlling catalyst activity decline is to understand the factors that influence the deposition of coke and metal foulants. A recent review on the subject indicates that studies related to the factors influencing catalyst deactivation in residual oil hydroprocessing are very limited, and the available information is often incomplete or conflicting (8). In the present study, attention was focused on determining the extent of catalyst fouling by coke and metal deposition under varying operating conditions during hydroprocessing of Kuwait vacuum residue. The influence of catalyst pore size on the deposition of the foulant materials was also investigated. In addition, the effect of presulfiding of the catalyst in suppressing catalyst deactivation was examined as part of the study. Experimental The experiments were conducted in a fixed bed reactor using Kuwait vacuum residue as feedstock (API gravity = 6.8; S = 5.2 Wt.%; N = 0.44 Wt.%; V = 94 ppm; Ni = 26 ppm; asphaltenes = 9.2 Wt.%; CCR = 19.2 Wt.%). A 50 ml sample of the catalyst, diluted with an equal amount of carborundum, was charged into a tubular reactor. Thermocouples inserted into a thermowell at the center of the catalyst bed were used to monitor the reactor temperature at various points. After loading the catalyst, the system was purged with nitrogen, and the temperature was increased to 150 °C gradually. Then the system was purged with hydrogen and pressurized to 120 bar. Under these conditions the presulfiding feed (recycle gas oil) was fed and presulfiding was carried out using standard procedures (9). When presulfiding was completed, the feed (Kuwait vacuum residue) was injected and the conditions were adjusted to desired operating temperature, pressure, hydrogen flow and LHSV. Run duration was 240 hours. In a few selected experiments the feed was introduced without formal catalyst presulfiding. At the end of each run, the aged catalyst was removed, Soxhlet extracted with toluene, then dried. The washed and dried spent catalyst samples were analyzed for carbon and vanadium. The BET surface areas of different catalyst samples were measured with a Quantasorb Adsorption unit. A mercury porosimeter (Quantachrome Model-Autoscan 60) was used to determine pore volume and pore size distribution. In selected samples, the distribution of the major metal foulant (i. e. vanadium) within the pellet was determined using a scanning electron X-ray microprobe analyzer (Camebax). The properties of the used catalysts reported in the results and discussion section are averaged values of composite samples representative of the total bed.


17. ABSI-HALABI & STANISLAUS

Catalyst Deactivation

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Results And Discussion Temperature Effect. The data presented in Figure 1 show that the amount of both carbon and vanadium deposited on the catalyst increases steadily with increasing operating temperature in the range 380- 450 °C. Coke deposition on the catalyst is particularly high at temperatures above 410 °C. Reactor temperature is known to enhance the rates of various hydrotreating reactions (10). In fixed bed industrial hydrotreating the reactor temperature is normally increased with the time on stream to compensate for the loss of catalyst activity. Bartholdy and Cooper (11) observed that more coke builds up and deactivation by coke occurs each time the operating temperature is increased to compensate for the loss of activity during the process. Our results indicate that in addition to coking, metal deposition is also severe at higher temperatures. Electron microprobe analysis of the distribution of the deposited vanadium within the catalyst pellets showed a relatively high concentration of vanadium near the outer surface of the pellet for the high temperature (>430 °C) runs. Such dense accumulation of vanadium deposits near the external surface of the catalyst is detrimental to the catalyst life as it can cause pore mouth plugging (4,6,12). A remarkably large loss in the catalyst's surface area (Figure 1) and pore volume is consistent with this. Pressure Effect The operating pressure was varied between 70 bar and 135 bar to study its influence on both carbon and metals deposition on the catalyst. The results presented in Figure 2 indicate that increasing pressure has a favorable effect in suppressing coke formation. It is interesting to note that the pressure effect in reducing coke deposition is more pronounced up to 100 bar above which the effect is negligible. In contrast the amount of vanadium deposited on the catalyst continues to increase with increasing pressure. These results are in accordance with the enhancing effect of pressure on the hydrogenation and HDM reactions (7,10). The surface area is found to improve with increasing operating pressure, despite the increase in vanadium deposition at higher pressures, indicating that the surface area loss of the catalyst is more influenced by coke than by the vanadium deposits (Figure 2b). Coke on the catalyst is, thus, largely responsible for catalyst deactivation by loss of surface area, and this could be rninirnized by increasing the hydrogen pressure. However, increasing pressure has been reported to increase vanadium deposition more near the exterior surface of the catalyst pellet (13,14). In essence, an increase in the hydrogen pressure has a beneficial effect in suppressing coke formation, but can lead to shorter catalyst life due to rapid accumulation of vanadium at pore mouths. LHSV Effect Figure 3 illustrates the influence of feed space velocity on the deposition of carbon and vanadium on the catalyst during hydroprocessing of Kuwait vacuum residue. It is seen that the vanadium on the catalyst increases while the amount of carbon decreases with increasing feed space velocity. The loss in catalyst surface area is substantially high at low feed flow rates (Figure 3b), presumably due to increased carbon deposition. Considering these results, it is reasonable to conclude



232


350

400

450

500

350

400

450

500

T e m e r a t u re ( ° C )

T e m p e r a t u re ( ° C )

Figure 1. Effect of operating temperature on (a) coke and metal deposition, and (b) catalyst surface area (Run duration = 240 h).

E o> o

1

•s 3 75

100

125

P r e s s u re ( b a r )

50

100

125

Pressure

75

(bar)

150

Figure 2. Effect of hydrogen pressure on (a) coke and metal deposition, and (b) catalyst surface area (Run duration = 240 h).




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that increasing feed space velocity in residue hydroprocessing operations increases the rate of catalyst deactivation by metals, but has an opposite effect on the deactivation by coke. The linear relationship observed between feed space velocity and the catalyst's vanadium content (Figure 3a) could possibly be used for predicting catalyst life based on short duration runs. It is possible to calculate the amount of feed that could be processed over a given catalyst from the amount of vanadium accumulated during the short duration test, provided the catalyst maximum tolerable capacity for metals is known. Influence of Catalyst Presulfiding. Presulfiding of hydrotreating catalysts has been widely practiced by refiners in distillate hydrotreating operations. It is generally believed that presulfiding of hydrotreating catalysts plays an important role in creating the essential surface requirements for optimum activity. In residue hydroprocessing, studies on the influence of presulfiding on catalyst performance are relatively scarce, despite the industrial importance of the process. In the present work, a detailed investigation of the effect of presulfiding on catalyst deactivation during hydroprocessing of Kuwait vacuum residue was undertaken. Catalysts with and without formal presulfiding were used to examine the effect of sulfidation on coke and metals deposition. The results revealed that presulfiding reduces the extent of early deactivation of the catalyst by coke deposition (15). Furthermore, the presulfided catalysts showed improved distribution of the major metal foulant vanadium within the pellet (15). The exact reasons for this improvement are not clear. It is likely that passivation of the highly active acidic sites by presulfiding reduces the coke forming tendency. In addition, the small amount of coke deposited on the catalyst during the sulfidation process may also have a passivating effect on the highly active hydrogenation and hydrogenolysis sites allowing deeper diffusion of the metal-bearing molecules in the feedstock into the pellet. Table I. Meso- and Macro-Pore Distributions in Various Catalysts Catalyst

Pore Meso-Pore Distribution %) Volume (ml/g) 30-100A 100-250 250 -500

Macro-Pore Distribution %)

500-

1000-

>3000A

A

A

p

0.53

38

60.5

1.5

IOOO A 0

3000 A 0

0

Q R

0.60

4

11

27

15

43

0

0.73

7

34

19

6

16

18

S

0.75

55

8

8

6

21

2

Influence of Catalyst Pore Size. In the present work, four Ni-Mo/Al 0 catalysts with different pore size distributions were used to assess the effect of catalyst pore size on deactivation by coke and metals deposition. Table I summarizes the pore size distribution of the four catalysts used in the present work. The amount of carbon and 2


3


234


vanadium deposits formed on various catalysts together with the surface area and pore volume losses are included in Table n. The highest amount of carbon deposition is found for the bimodal pore Catalyst (S) that has a large percentage (55%) of very narrow pores ( R > P > Q. The pore volume loss is generally caused by the deposition of coke and metals in the pore. In catalysts with a large amount of small pores the deposited metals can cause pore mouth plugging leading to a large loss in pore volume and surface area. The highest loss of pore volume and surface area observed for the predominantly small pore Catalyst S is therefore not surprising. Similarly, in catalysts with macropores, the metals penetration into the catalyst pellet is high and they are deposited evenly on the pore surface throughout the catalyst pellet without causing pore blockage. The lowest loss of pore volume and surface area noticed for the large pore Catalyst Q is in agreement with this. Summary and Conclusions The influence of some important process parameters, such as operating temperature, hydrogen pressure, liquid hourly space velocity, and selected catalyst parameters, such as catalyst pore size distribution and presulfiding, on catalyst fouling by coke and metals deposition was investigated during hydroprocessing of Kuwait vacuum residue. Increasing reactor temperature increased both coke and metals deposition, while increasing hydrogen pressure decreased coke deposition. Vanadium deposition on the other hand increased with increasing pressure. Coke deposition contributed more to the surface area loss than the metals deposition. Increasing feed flow rates enhanced the rate of deactivation by metals, but decreased coke deposition. Catalyst pore size distribution had a significant effect on catalyst deactivation. The deposition of both coke and metals was found to be higher for catalysts having predominantly narrow pores. Presulfiding of the catalyst reduced coking and led to better distribution of foulant metals within the catalyst pellet. References 1. 2. 3. 4. 5. 6. 7.

Absi-Halabi, M., Stanislaus, A., and Trimm, D. L., Appl. Catal., 72, 193(1991) Thakur, D. S., and Thomas, M. G., Appl. Catal., 45, 197(1985) Ternan, M., and Kriz, F., Stud. Surf. Sci. Catal., 6, 283(1980) Sie, S. T., Stud. Surf. Sci. Catal., 6, 545(1980) Gaulda, G. and Kasztelan, S., Stud. Surf. Sci. Catal., 88, 145(1994) Bridge, A. G., Stud. Surf. Sci. Catal. 53, 363(1990) Hannerup, P., and Jacobson, A. C., Preprints, Div. of Petrol. Chem., ACS, 28(3), 576 (1988) 8. Bartholomew, C. H., In "Catalytic Hydroprocessing of Petroleum and Distillates" (Edited by M. C. Obella and S. S. Shin), Marcel Dekker, New York, (1994), pp 1-32.





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9. Absi-Halabi, M., Stanislaus, A., Owaysi, F., and Khan, Z., Stud. Surf. Sci. Catal. 53, 201(1990) 10. Stanislaus, A., Absi-Halabi, M., Owaysi, F., and Khan, Z., "Effect of Temperature and Pressure on Hydroprocessing of Kuwait vacuum Residues". KISR Publication No. 2754 (1988) 11. Bartholdy, J., and Cooper, B. H., Preprints, Div. of Petroleum Chemistry, 205th National ACS Meeting, (1993) 386-390. 12. Quan, R. J., Ware, R. A., Hung, C. W., and Wei, J., Advances in Chem. Eng. 14, 95(1988) 13. Pazos, J. M., Vonzalex, J. C., and Salazar, A. J. Ind. Eng. Chem. Process Des. Dev. 22, 653 (1983). 14. Tamm, P. W., Harnsberger, H. F., and Bridge, A. G., Ind. Eng. Chem. Process Des. Dev. 20, 262 (1981). 15. Absi-Halabi, M., Stanislaus, A., Qamra, A., and Chopra, S. Paper Presented at the 2nd International Conference on Catalysts in Petroleum and Petrochemical Industries, April 22-26, 1995, Kuwait. 16. Ternan, M., Can. J. Chem. Eng., 61,689(1983) 17. Baltus, E W., and Anderson, J. L., Chem. Eng. Sci., 38,1959(1988) 18. Speight, J. G., Stud. Surf. Sci. Catal., 19, 551(1984) 19. Stanislaus, A., Absi-Halabi, M., Mughni, T., Khan, S., and Qamra, A., Proceedings of Joint Kuwaiti-Japanese Symposium on Catalytic Processes for the Petroleum Refining and Petrochemicals Industries ( J. Bishara, H. Qabazard, and M. Absi-Halabi, Editors), Kuwait Institute for Scientific Research, Kuwait, (1993), pp. 13-25.


Effect of Process Conditions and Catalyst Properties on Catalyst

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