Conradson Carbon Residue Conversion during Hydrocracking of

Energy & Fuels 1996,9, 549-559

649

Conradson Carbon Residue Conversion during Hydrocracking of Athabasca Bitumen: Catalyst Mechanism and Deactivation Emerson C . Sanford: Edmonton Research Center, Syncrude Canada Ltd., 9421 -1 7 Avenue, Edmonton, Alberta, Canada, T6N lH4 Received December 22, 1994@

The activity of promoted molybdenum on y-alumina catalysts toward residuum, Conradson carbon residue (CCR), and heteroatom conversions and solids formation in the reactor during hydrotreating and hydrocracking of Athabasca bitumen has been investigated during the first day on stream by using batch reactors and for up to 30 days on stream using a continuous flow pilot plant. It is proposed that, under hydrotreating conditions, catalytic CCR conversion and catalytic metals removal take place on different active sites from catalytic sulfur removal and that catalytic nitrogen removal takes place on different sites again. The active sites on the catalyst which promote conversion of CCR and heteroatoms through conventional hydrogenation reactions were lost in a matter of hours under cracking conditions. After the hydrogenation sites were deactivated, the catalyst remained active toward CCR and sulfur removal, presumably through a different mechanism. CCR conversion was strongly correlated with residuum conversion and initially there was no selectivity between conversion of CCR residuum molecules and non-CCR residuum molecules. Selectivity was introduced as the catalyst deactivated over 30-50 days with CCR conversion decreasing faster than residuum conversion. It is proposed that the main role of catalyst during hydrocracking is t o assist in the reaction of thermally generated phenyl radicals with molecular hydrogen, resulting in the addition of a hydrogen atom to condensed aromatic centers, and ultimately resulting in the decomposition of the condensed aromatic unit to give gases and distillate. Hydrogenation of aromatics does not play a significant role in the hydrocracking of Athabasca bitumen residuum. The catalyst was only indirectly involved in preventing solids formation in the reactor during hydroprocessing.

Introduction The earliest process for the conversion of petroleum residua into distillates was a simple thermal cracking process which produced as a byproduct an insoluble hydrocarbonaceous solid, commonly k n o w n as coke. Thus the process has come to be known as the coking pr0cess.l In order to predict the yield of coke which would be obtained from a coking process, two destructive distillation tests were developed, the Conradson carbon residue (CCR) and the Ramsbottom carbon residue (RCR) tests. can be used with either test t o predict the amount of coke which will be formed in a coking process, depending on the type of process used. The main limitation to the coking process, i.e., the formation of coke, can be overcome by carrying out the ~ the reaction in the presence of h y d r ~ g e n .However, hydrocracking processes do not give 100% residuum conversion and a pitch product is f ~ r m e d .The ~ pitch is + Present address: Department of Chemical Engineering, 536 Chemical-Mineral Engineering Bldg., University of Alberta, Edmonton, AB, Canada, TG6 2G6. Phone 403-492-7963. FAX 403-492-2881. E-mail: [email protected]. Abstract published in Advance ACS Abstracts, April 15, 1995. (1) Gary, J. H.; Handwerk, G. E. Petroleum Refining Technology and Economics; Marcel Dekker, Inc.: New York, 1984; pp 54. (2) Kirchen, R. P.; Sanford, E. C.; Gray, M. R.; George, Z. M.

then converted in a coker and the amount of coke formed in the coking step is a function of the amount of CCR left unconverted in the hydrocracking step. In the nonconventional crude oil industry, where the entire plant is devoted to the conversion of residuum to crude CCR conversion in the hydrocracking step is very important to the overall distillate yield of the process. A general reaction scheme was initially proposed6for the conversion of residuum into distillate, and centered around two classes of molecules in residuum, those which form residue in the CCR test, called CCR residuum, and those which do not, called non-CCR residuum. The proposed scheme was based on hydrocracking studies using an active catalyst, and catalytic runs were seldom of more than 1h duration. Additional studies utilizing these same short reaction time batch studies showed that residuum conversion was a function of both reaction severity and catalyst a ~ t i v i t y . ~ Recently, a detailed mechanism describing the role of hydrogen in preventing coke formation during hydrocracking has been p r o p o ~ e d . ~It, ~was suggested that, after the first 30-40% residuum conversion when the main reaction may be breaking of side chains, the rate-

@

AOSTRA J . Res. 1989,5 , 225-235. (3) Reference 1, p 151. (4)Bishop, W.; Smart, M.; James, L. C.; MeDaniel, N. K. NPRA Annu. Meeting, Sun Antonio, TX 1991,1 , AM-91-56,

0887-0624/95/2509-0549$09.00/0

(5) Hyndman, A. W.; Luhning, R. W. J. Can. Pet. Technol. 1991, 30, 61-71. (6) Sanford, E. C.; Chung, K. H. AOSTRA J . Res. 1991,7 , 37-45. (7) Sanford, E. C. AOSTRA J . Res. 1991,7 , 163-168. (8) Sanford, E. C. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1993, 38, 413-416. (9) Sanford, E. C. Ind. Eng. Chem. Res. 1994,33, 109-117.

0 1995 American Chemical Society

Sanford

550 Energy & Fuels, Vol. 9, No. 3, 1995

determining step is the thermal breaking of a carbonto-carbon bond to give an aliphatic radical and a n aromatic radical. This first step is independent of whether or not hydrogen is present. The aliphatic radical can undergo rapid fragmentation to produce gases and distillate whereas the aromatic radical can either undergo condensation reactions leading to coke formation or react with molecular hydrogen t o produce a carbon-to-hydrogen bond and a hydrogen atom. The hydrogen atom can react with condensed aromatic centers leading to the decomposition of these centers, giving again distillates and gases. Although this proposed mechanism gives a plausible explanation for the reaction of hydrogen in preventing coke formation, it does not deal specifically with the role of catalyst. On a commercial scale, expanded bed hydrocrackinglO is normally carried out in the presence of a residuum hydrotreating catalyst of the Ni-Mo or Co-Mo on y-alumina type. The catalyst is thought to participate in CCR removal, heteroatom removal, and aromatics saturationlo and may participate in the hydrogentransfer reactions which control coke formation in the reactor. In the nonconventional crude oil industry, it is CCR conversion which is of paramount importance. The current picture of how CCR-forming molecules are removed during catalytic hydrocracking is by analogy with catalytic hydrotreating of distillates. Distillate hydrotreating is a technology which is practised world*wide and is generally well underst0od.l' The major difference between distillate- and residuum hydrotreating is that distillate hydrotreating is generally carried out under conditions which minimize cracking and that the feeds contain a very low concentration of metals and CCR-forming molecules. It is believed that the CCRforming molecules in residua consist of condensed aromatic rings and that CCR reduction comes about through catalytic hydrogenation of the aromatic rings, followed by thermal cracking of the naphthenic rings produced by hydrogenation.1° Beaton and BertolacinilO also indicate that there is evidence that the overall reduction of CCR is coupled thermally and catalytically. The role of the catalyst during hydrocracking has been studied by comparing side-by-side hydrocracking reactions with and without catalyst12 and by comparing several feeds using several catalysts of different reactivities.13 It was found that the catalyst participated in all of the expected hydrogenation and hydrogenolysis reactions and was especially important in transferring hydrogen to the heavy oil fraction to prevent carbonization. In each case studied, the reaction time was 6-8 h, and although catalysts of different reactivity were used, it was assumed that catalyst deactivation during the run was small. Previously deactivated catalysts gave lower conversions than active catalysts. Under essentially noncracking conditions, Sanford and Chung6 have shown that CCR-forming molecules are catalytically hydrotreated to form a non-CCR containing residuum, and it is expected that a t least in the early stages of hydrocracking, when the catalyst is (10) Beaton, W. I.; Bertolacini, R. J. Catal. Reu. 1991,33,281-317. (11)Reference 1, p 130. (12) Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto, Y. J . Catal. 1983. 83, 371-383.

(13)Gray, M. R.; Khorasheh, F.; Wanke, S. E.; Achia, U.; Krzywicki, A,; Sanford, E. C.; Sy, 0 .K. Y.; Ternan, M. Energy Fuels 1992,6,478485.

active, that catalytic hydrogenation of CCR-forming molecules takes place. Deactivation of the catalyst with respect t o hydrotreating may take place rapidly under hydrocracking conditions since it has been found that the amount of CCR in the residuum fraction correlates with the amount of residuum, both in the presence14 and absencel4.l5of catalyst; i.e., conversion of residuum and conversion of CCR both appear to be thermal reactions. Such a correlation would not be expected in the presence of an active hydrotreating catalyst,' since CCR conversion would be catalytic and residuum conversion mainly thermal. The data of Ternan and KrizI4 show that CCR conversion is faster in the presence of a catalyst than without a catalyst even if the catalyst is highly deactivated with respect to hydrotreating. The results suggest that the role of catalyst in CCR conversion during hydrocracking is by some mechanism other than hydrotreating. The available information indicates that during the first few hours of a hydrocracking reaction, catalytic hydrogenation reactions take place. Thereafter, the catalyst appears to be deactivated toward hydrotreating reactions but still participates in CCR conversion reactions. The intent of this paper is to discuss the mechanism of catalytic action during hydrocracking starting with the initial few hours and extending to several days. The findings will be related t o the proposed mechanismg for the reaction of hydrogen in coke prevention.

Experimental Section The apparatus used for the batch experiments reported here has been described previously.6 All experiments were carried out at 350-440 "C under 3-27 MPa hydrogen partial pressure with reaction times varying from 60 t o 240 min. Enough hydrogen gas was added to the reactor a t room temperature to give a pressure close to the desired pressure when the temperature reached reaction temperature. As soon as reaction temperature was reached, the pressure was adjusted to reaction pressure and hydrogen was added as needed to maintain the pressure, except for the runs reported in Figure 1. For these runs, no additional hydrogen was added and the pressure was allowed to decrease as the reaction proceeded. I n the cases where gas generation was greater than gas consumption, the pressure was allowed to increase until the pressure limit of the reactor (34 MPa) was reached. If the reaction pressure approached the pressure limit of the reactor, some gas was released from the system. For the hydrocracking runs under 8.3-10.3 MPa hydrogen in the presence of a catalyst, a commercial Co-Mo or Ni-Mo 1'-alumina residuum hydrotreating catalyst, similar to the catalysts described by Beaton and Bertolacini,lo was used. Any of the commercial catalysts, either Co-Mo or Ni-Mo, will produce similar results under these conditions. Seventy-five grams (per 300 g of feed) was predried for 4 h at 107 "C and placed in a stainless steel wire mesh basket immersed below the level of the liquid in the reactor. The catalyst was not presulfided in most cases but was sulfided by reaction with the feed during the heat-up period. For the runs with presulfided catalyst, the catalyst was heated a t 195 "C for 12 h under 8.3 MPa hydrogen in the presence of a hydrotreated kerosene containing a n excess butane thiol ( 5 wt 92 butane thiol). Heating was continued for another 12 h a t 343 "C, the reaction mixture was cooled, and the catalyst was rinsed with methylene chloride to remove the kerosene and dried in air 114)Ternan, M.; Kriz, J. F. AOSTRA J . Res. 1990, 6, 65-70. (15)Bearden, R.; Aldridge, C. L. Presented at the AIChE 90th National Meeting, April 5, 1981.

Energy & Fuels, Vol. 9, No. 3, 1995 551

Hydrocracking of Athabasca Bitumen

60 Sulfur

30

m Nitrogen m CCR m Vanadium m

20

Residuum

50

n

8

40

W

v)

C

'E0

el

9

s 10

n

Fresh

Batch Pilot Catalysts

None

Figure 1. Comparison of hydrotreating reactions of Athabasca bitumen a t 370 "C with catalysts deactivated to varying degrees, carried out under conditions of varying system pressure. Fresh: unused oxidic catalyst. Batch: Used in a 0.33 h batch hydrocracking run a t 400 "C. Pilot: used in a 28 day pilot run a t 430 "C. None: hydrocracking with no added catalyst. a t 100 "C for 1h. When a spent catalyst was used, the weight was corrected to a fresh catalyst basis. The feed was Athabasca bitumen which contained 4.2% sulfur, 4430 ppm nitrogen, 83.1% carbon, 10.2% hydrogen, 196 ppm vanadium, 74 ppm nickel, and 14.4% CCR. After the specified reaction time, the reactor was cooled and a sample of gas removed for analysis. The remainder of the gas was vented and the contents of the reactor were either submitted directly to the analytical laboratory for analyses or slurried in methylene chloride for solids determination. Any solid which adhered to the reactor walls or internals was carefully scraped off and added to the methylene chloride slurry. The methylene chloride slurry was then filtered through a 45 pm filter followed by a 30 pm filter. The filtrate was centrifuged for 40 min a t 1500 rpm, decanted and filtered through a 20 pm filter. The solids were combined, dried, and weighed. The solvent was removed from the combined filtrates on a rotary evaporator and the total liquid weighed. The feed was also slurried with methylene chloride and filtered as above to determine the amount of methylene chloride insoluble solids in the feed. During a run with catalyst, part of the feed is deposited on the catalyst in the form of carbonaceous solids which contain sulfur, nitrogen, and metals. The increase in weight of the catalyst was determined by a loss-on-ignition measurement after removing toluene-soluble materials in a Soxhlet extractor. Catalyst deactivation was investigated in the batch system in a series of 2 h runs. After each run, the catalyst was removed and cleaned by Soxhlet extraction with toluene until the extract was clear ( x 4 h). The catalyst was then air-dried and weighed and reused with fresh bitumen. The pilot plant tests were carried out in a continuous upflow expanded bed reactor a t a feed rate of between 50 and 100 L per day. The catalyst (described above for the batch system) was presulfided with a stream of hydrogen sulfide in hydrogen prior to introduction of the bitumen feed. The weight hourly space velocity was between 0.5 and 1.5 h-l and the temperature was 380 "C for the hydrotreating runs and between 430 and 450 "C for the hydrocracking runs. The

hydrogen partial pressure was similar to that used in the batch experiments. Active catalyst was not added during the run. The control system was such t h a t the pilot plant could be run uninterrupted generally for 30-40 days, unattended most of the time.

Results Catalyst Age < 1 day. The results from using a fresh promoted molybdenum on y-alumina catalyst, from using the same catalyst which had been used in the batch reactor previously for 0.33 h at 400 "C, from using the same catalyst which had been removed from a 4 week pilot plant run, and with no catalyst are given in Figure 1. It is apparent that the catalyst is appreciably deactivated with respect to CCR and heteroatom removal even after 0.33 h in the batch reactor and that the catalyst at the end of a pilot plant run is only slightly different from having no catalyst at all, with respect to its ability to catalyze hydrotreating reactions under these conditions. The fresh catalyst used in the run in Figure 1was in the oxidic form. The used batch catalyst as well as the pilot plant catalyst would be in the sulfidic form as a result of prior reaction with the sulfur in the bitumen. A comparison of two samples of the same catalyst used t o treat bitumen at three temperatures, one in the oxidic form and one presulfided, is shown in Figure 2. Data is given for sulfur, nitrogen, and CCR removal. Differences tend to be small and random. Catalyst deactivation in the early stages of the hydrocracking reaction was explored in a series of 2 h batch run@ where the catalyst was used repeatedly (16) Sanford, E. C. Presented at the Symposium on Resid Upgrading Processes, AIChE Houston Spring National Meeting, April 4, 1991.

Sanford


400

370

420

Reaction Temperature (OC) I

I

1

Sulfur-Oxide ffJ$J Nitrogen-Sulfide

Sulfur-Sulfide CCR-Oxide

Nitrogen-Oxide CCR-Sulfide

1

1

Figure 2. Comparison of sulfur, nitrogen, and CCR conversion from Athabasca bitumen with fresh oxidic and presulfided residuum hydrocracking catalysts at three temperatures.

Sulfur

-c

Nitrogen

I

CCR

+ Vanadium *

i

I

Nickel t Residuum

Figure 3. Effects of catalyst age, up to 12 h at 400 "C, on

residuum, CCR, and heteroatom conversions from Athabasca bitumen. with fresh feed. Plots of sulfur, nitrogen, CCR, vanadium, nickel, and residuum conversions as a function of catalyst age a t 400 "C are given in Figure 3. There is a relatively small change in residuum conversion as a result of catalyst deactivation as observed previously.s The heteroatom conversions decrease rapidly in the first 8 h and gradually level off. Metals and nitrogen removal appear to decrease faster than sulfur and CCR conversions. Despite the fact that the catalyst deactivates rapidly in the first few hours, the conversions level off at a significant level, for instance, CCR conversion begins a t 65% and levels off at 40%. After the 2 h run, the weight of the catalyst had increased by 28% corresponding to 7% of the feed for that run. Over the 2-12 h period, the catalyst weight increased gradually to a maximum of 34% increase, which corresponded t o 1.5% of the total feed.

CCR and residuum conversion data under a variety of conditions for batch reactor runs using oxidic catalyst, together with the corresponding heteroatom conversions, are given in Table 1. The effects of pressure as a function of temperature a t constant reaction time on CCR, heteroatom, and residuum conversions are given in series 1 , 2 , and 3. In general, higher hydrogen partial pressures result in increases in all conversions, including residuum conversions, with the ratio of CCR to residuum conversion remaining generally greater than 2 a t the lower temperatures. In the series at 420 "C, the ratio drops to between 0.4 and 0.5. CCR conversion is much lower than the corresponding residuum conversion at this temperature, resulting in a low CCR to residuum conversion ratio. When reaction time was varied at constant reaction temperature in two series at different pressures (series 4 and 5 ) ) in general, all conversions increased and a t the low temperature used (370 "C), the ratio of CCR to residuum conversion was generally greater than 2. For the lower pressure series, (series 4),the ratio did tend to decrease with reaction time, and for the longest run, the ratio was less than 2. When the temperature was varied a t constant reaction time and two pressures (series 6 and 71, residuum conversion increased steadily, whereas CCR conversion increased at first and then decreased. Because CCR conversion did not increase as fast with increasing temperature as residuum conversion, the ratio of CCR conversion to residuum conversion decreased steadily from 3.3 to 0.4. Sulfur conversions tended to follow CCR conversion, first increasing and then decreasing. Nitrogen, vanadium, and nickel conversions all increased with temperature, although not as fast as residuum conversion. CCR conversion as a function of average residuum conversion for coking and hydrocracking of Athabasca



Table 1. Summary of Batch Stirred Tank Reactor Experimental Conditions and CCR and Heteroatom Conversions

series

hydrogen press. (MPa)

temp ("C)

time (min)

sulfur (% conv)

nitrogen (% conv)

CCR (% conv)

V (o/o conv)

Ni (% conv)

residue (% conv)

CCR convfres conv

3.4 10.3 3.4 10.3 20.7 26.9 3.4 10.3 20.7 10.3 10.3 10.3 10.3 26.9 26.9 26.9 26.9 3.4 3.4 3.4 10.3 10.3 10.3 10.3 10.3

350 350 370 370 370 370 420 420 420 370 370 370 370 370 370 370 370 350 370 420 350 370 400 420 440

60 60 60 60 60 60 60 60 60 60 120 180 240 60 120 180 240 60 60 60 60 60 60 60 60

40.2 46.6 54.6 59.4 63.8 69.3 47.5 59.7 70.8 59.4 68.2 69.7 71.3 69.3 77.8 79.8 84.0 40.2 54.6 47.5 46.6 59.4 64.3 59.7 57.0

21.6 20.6 31.0 25.0 34.3 41.8 36.2 43.1 38.0 25.0 30.5 22.5 29.4 41.8 54.7 55.8 58.5 21.6 31.0 36.2 20.6 25.0 35.3 43.1 46.9

20.7 30.7 24.0 40.7 44.3 53.4 23.8 33.7 59.3 40.7 44.1 47.4 43.4 53.4 67.3 71.3 74.6 20.7 24.0 23.8 30.7 40.7 47.7 33.7 40.6

33.7 69.6 79.6 80.2 88.3 94.5 79.5 86.1 88.3 80.2 90.4 91.7 93.2 94.5 96.8 98.0 99.1 33.7 79.6 79.5 69.6 80.2 75.9 86.1 94.2

26.2 65.0 78.3 78.9 90.3 89.3 78.0 84.0 90.3 78.9 87.4 89.0 91.7 89.3 92.5 95.7 97.9 26.2 78.3 78.0 65.0 78.9 77.9 84.0 93.5

9.0 9.4 19.8 16.0 20.9 21.1 61.9 70.7

2.3 3.3 1.2 2.5 2.1 2.5 0.4 0.5

16.0 22.2 23.6 31.3 21.1 24.9 26.9 30.0 9.0 19.8 61.9 9.4 16.0 46.3 70.7

2.5 2.0 2.0 1.4 2.5 2.7 2.7 2.5 2.3 1.2 0.4 3.3 2.5 1.0 0.5

1

2

3 4

5

6 7

6o 'OI I 5

501

d

I -3v

20

-

30

50 60 70 80 Residuum Conversion (")

40

90

100

Figure 4. CCR conversions for six series of runs at 400 "C with Athabasca bitumen residuum, with and without hydrogen and catalyst, with reaction times varying from 30 to 480 min. COKING-N2: under nitrogen. HC-NO CATALYST: under hydrogen. HC-FRESH CATALYST: hydrocracking with fresh oxidic catalyst. HC-SPENT CATALYST: hydrocracking with spent catalyst from the commercial plant. FRESH CATALYST-N2: fresh oxidic catalyst under nitrogen. HCCOKED CATALYST: hydrocracking with a catalyst treated with bitumen at 400 "C.

bitumen residuum has been presented previ0us1y.l~The data is replotted for convenience in Figure 4 as a function of individual residuum conversions. One additional data point is added, that of CCR conversion with a presulfided, precoked catalyst. Two data points stand out, those at low residuum conversion (0.5and 1.0 h reaction time) with a fresh catalyst, attributed17 to catalytic hydrogenation. A least-squares regression line through the remaining 20 data points gives a good linear correlation between CCR conversion and residuum conversion, with a regression coefficient R2 of 0.84. The single data point (Figure 4, HC-COKED CATALYST) with a catalyst which had been heated with bitumen under a hydrogen pressure up to 400 "C (17)Sanford, E.C.Energy Fuels 1994, 8, 1276-1284.

-

-

-

-

and then cooled immediately, as a means of presulfiding and precoking the catalyst, did not exhibit any catalytic activity with respect to CCR conversion. For all except short reaction time runs ( < lh) with a fresh oxidic catalyst, CCR conversion is well correlated with residuum conversion. Catalyst Age 1' day. In continuous flow systems, a line-out period of from a few hours to 1day is normally used to allow the system to "stabilize" or reach "equilibrium". In the pilot plant runs reported here, time zero is normally 18-24 h after feed in at reaction temperature. This distinction will become very important in the discussion of the results. The effects of catalyst deactivation on sulfur, nitrogen, and CCR conversions in pilot plant runs are shown in Figure 5. The data from catalyst A was from a hydrotreating run under conditions which gave a residuum conversion of 25%. Under these relatively mild conditions using an active catalyst, sulfur conversion was approximately 70%and CCR conversion approximately 45%, comparable to the batch run in Figure 1with fresh catalyst. The % CCR to % residuum conversion ratio was 1.8. The remaining four groups (catalysts B-E) were from hydrocracking runs. Conditions were chosen such that the residuum conversion was nearly the same in each case (60%),but catalyst activities varied widely. Data from the run using catalyst B was from day 2 of a hydrocracking run, so the catalyst had been at hydrocracking conditions no more than 1 day. The % CCR to % residuum conversion ratio was 0.9. The data using catalyst C was from the same run after 30 days of operation. Sulfur, nitrogen and CCR conversion had dropped significantly but were still appreciable (% CCR conversion/% residuum conversion = 0.8). Data from the run using catalyst D was from a similar pilot plant run using discharged spent catalyst from the commercial plant (% CCR conversion/% residuum conversion = 0.6) and the data using catalyst E was a hydrocracking run without a hydrotreating catalyst (% CCR conversion/% residuum conversion = 0.6). A

Sanford


90

1

Sulfur

80-

m

Nitrogen

70n

s .-6

E s CCR

60-

€€El

v

Residuum

50-

2 Q)

> 40-

C

s 30-

ZOl 10 0

L

E

A

Catalysts Figure 5. A comparison of CCR and heteroatom conversions in pilot plant runs at low residuum conversion (380 "C) with fresh catalyst (A) and at 60% residuum conversion (430 "C) with fresh catalyst (B), 30 day old pilot plant catalyst (C),spent commercial plant catalyst (D), and unsupported metal sulfide catalyst (E).

dispersed transition metal sulfide was added a t approximately 200 ppm on feed t o prevent coke formation in the reactor. It is expected that the additive, a t this level, would not have any impact on heteroatom, CCR and residuum conversions.18 At the temperatures and feed rates required to give 60% residuum conversion, the conversions of sulfur, nitrogen and CCR are only higher than for the run which gave 25% residuum conversion (catalyst A) for the catalyst that was 1 day old (catalyst B). For all other runs, CCR and heteroatom conversions are the same or lower, although the severity was increased enough to give more than double the residuum conversion. At the higher temperatures employed for the catalyst B-E runs, the catalytic runs would be expected to give much higher conversions due to the higher temperatures but conversions would decrease if catalyst deactivation were significant. Residuum, CCR, and heteroatom conversion rate constants are plotted in Figures 6-8 for a pilot plant run in which bitumen was fed t o the reactor under hydrotreating conditions (380 "C) for 12 days (catalyst A, Figure 5), after which the catalyst (standard residuum hydrocracking catalyst) was replaced with fresh catalyst and the run continued, with the heavy product from the first run (some light naphtha removed) being used as feed for the second run. The rate constants for residuum (Figure 6), CCR (Figure 6), and metals conversion (Figure 8) were all similar in that there was relatively little change throughout the duration of the run, for either the first or second run, and the rate of conversion was slower in the second run compared to the first. (18)Heck, R. H.; Rankel, L. A,: DiGuiseppi, F. T.Fuel Process Technol. 1992,30, 69-81.

0.161

,^

I

I

Residuum +

I

0.14- " I 0.12-1

m

~

CCR

I

-

0.041

o,021:,~::--~"'

__.:_

0

0

5

10 15 Mass Balance Period

20

1

25

Figure 6. First-order rate constants for the conversion of

residuum and second-order rate constants for the conversion of CCR during pilot plant processing (380 "C) of Athabasca bitumen (mass balance periods 1-11) and hydrotreated Athabasca bitumen (mass balance periods 13-21). The rate constants for sulfur and nitrogen removal (Figure 7) were similar for the first run in that there was a rapid decrease in rate over the first five days of the run, with a leveling out after that. The two series were quite different in the second run in that the sulfur conversion rate constants were similar to the first series, and showed a similar decrease in rate. For the nitrogen series, the rates were slightly higher than the lined out rates in the first series, and there was little evidence of catalyst deactivation throughout the second series. Residuum conversion and the corresponding CCR conversions for an 18 day pilot plant run under cracking conditions are shown in Figure 9. The general trend was for 5% CCR conversion and 5% residuum conversion to decrease as a function of time (catalyst age) a t

Hydrocracking

Energy &Fuels, Vol. 9, No. 3, 1995 555

Athabasca Bitumen

of

0,121

1

7

1

'-1

Sulfur

65o.08!

5 600' 'E

E

0.06-

s

0.04-

55-

5045-

_____

-

04

5

0

, i

1

10

20

15

25

Mass Balance Period

40

Figure 7. Second-order rate constants for the conversion of sulfur and first-order rate constants for the conversion of nitrogen during pilot plant processing (380 "C) of Athabasca bitumen (mass balance periods 1-11) and hydrotreated Athabasca bitumen (mass balance periods 13-21). 1 I Vanadium

L

00

5

1

I

10

20

15

25

Mass Balance Period

Figure 8. First-order rate constants for the conversion of vanadium and the conversion of nickel during pilot plant processing (380 "C) of Athabasca bitumen (mass balance periods 1- 11) and hydrotreated Athabasca bitumen (mass balance periods 13-21).

5

48:

- \+

4 2 - , 0 2

A

+ + +

46 444 4

,

6

w

I

8

I

I

10 12 Run days

'

14

'

16

+I I

18

45

50 55 60 Residuum Conversion (%)

I

20

Figure 9. Percent residuum and % ' CCR conversions as a function of r u n days in a pilot plant hydrocracking run. Constant conditions (430 "C) for the first 12 r u n days, varying severity for run days 13-18. relatively constant temperature and feed rate (up to mass balance 12), with % CCR conversion decreasing faster. After mass balance 12, the severity in the reactor was changed, but CCR conversion continued to follow residuum conversion very closely, similar to the

65

70

Figure 10. Percent CCR conversions as a function of the corresponding % residuum conversions for two pilot plant runs at a n average of approximately 50% (430 "C) and 60% (445 "C) residuum conversions.

Table 2. Regression Data for CCR and Heteroatoms as a Function of Residuum Conversion and Catalyst Age constant std err of Y est R2 no. of observations degrees of freedom X coefficient (res conv) std err of coeff Xcoefficient (cat age) std err of coeff

A

0.051

401

CCR

sulfur

nitrogen

13.2 1.42 0.93 37 34 0.76 0.05 -0.012

30.3 2.15 0.9 37 34 0.76 0.07 -0.017 0.001

3.5 7.77 0.15 37 34 0.35 0.26 -0.009 0.005

0.001

batch runs (Figure 4) using residuum as feed. The changes in CCR conversion and in residuum conversion resulted in a steady decrease in the % CCR conversion t o % residuum conversion ratio from 1.02 to 0.86. This trend has been observed in many pilot plant runs, with the % CCR conversion to % residuum conversion always being close to 1 at the beginning of the run (day 2). Percent CCR conversion for two pilot plant runs at different severities, which resulted in average % residua conversions of 50% and 60%, are plotted against % residuum conversion in Figure 10. For each run, % CCR and % residuum conversion are well correlated. It is apparent that both residuum conversion and CCR conversion are related to each other and to catalyst age (kg of bitumen per kg of catalyst). Regression parameters for correlating % CCR, % sulfur, and % nitrogen conversion with % residuum conversion and catalyst age, for these two runs, are given in Table 2. Figure 11 shows a plot of calculated vs measured % CCR conversion. By including catalyst age in the regression correlation, CCR and sulfur can be represented as a function of residuum conversion and catalyst age, with a correlation coefficient R2 of 0.9. However, it must be remembered that the independent variables are not truly independent since part of the residuum conversion is dependent on catalyst age. In Figure 9, residuum conversion goes from 57 to 48% as a result of catalyst ageing. At the same time, CCR conversion goes from 58 to 43%. Since the residuum fraction of Athabasca bitumen contains 25% CCR, conversion of 15% of the CCR consists of an overall residuum conversion of 3.75%, if each CCR molecule went entirely to residue


-- 1

Sanford

The effect of catalyst age on solids formation in the reactor was estimated by carrying out a series of runs in the batch reactor at 400 "C and reusing the catalyst from the first run in the second run. After 4 h, no reactor solids were formed and the catalyst increased in weight from 100 to 138 g. When this catalyst was used for another 4 h, the catalyst increased in weight to 157 g and 7.9 g of reactor solids was formed. Repeating the reaction one more time resulted in 16.8 g of reactor solids, a t a catalyst age of 12 h. This amount of reactor solids is similar to the amount formed in the presence of hydrogen with no catalyst. The catalyst gained another 9.6 g of solids.

/

i

60,

/ = = 0.93

.,/squared

>

6a

50-

0 0

8 45-

d,= n

40 40

45

50

55

60

65

70

% CCR Conv. (measured)

Figure 11. A comparison of % CCR conversions measured in two pilot plant runs a t a n average of approximately 50% and 60% residuum conversions (Figure 10)and the corresponding values calculated from residuum conversion and catalyst age data.

in the CCR test. If each CCR residuum molecule formed 50% residue in the CCR test, then the conversion of 15% of the CCR would account for 7.5 of the 9% change observed in overall residuum conversion. It is likely that a significant proportion of the change in residuum conversion is simply due to the change in CCR conversion. Reactor Solids Formation. The suppression of coke formation by hydrogen, hydrogen plus finely divided metal sulfide catalysts, and hydrogen donor solvents is well-known,15J8and it is expected that the promoted Mo-y-alumina catalysts may also play a role in preventing solids formation during hydrocracking. Several batch runs were carried out a t 400 "C for 4 h in order to determine the effects of hydrogen, a hydrogen donor solvent, and fresh and deactivated catalysts on reactor solids formation. Data is given in Table 3. With the conditions used, under a nitrogen atmosphere, approximately 30 g of solids were formed from 400 g of bitumen. When the gas was changed to hydrogen from nitrogen or if the hydrogen donor solvent tetralin was used, the amount of solids formed was approximately one-half. If both hydrogen and tetralin were used, the effect was additive, and less solids were formed. With spent catalyst from the commercial plant in the presence of hydrogen, there was little, if any, change in the amount of solids formed in the reactor compared t o the runs without catalyst. However, some additional solids were formed on the catalyst and appeared t o be proportional t o the amount of catalyst present. Since these were spent catalysts, they contained more than 25% solids compared to a fresh catalyst, at the start of the reaction. When active commercial catalyst was used with hydrogen, the amount of reactor solids decreased substantially, depending on the amount of catalyst used. Again there was an appreciable amount of solids deposited on the catalyst, which amounted to approximately 40% of the weight of the catalyst, except for the smallest amount of catalyst. When nitrogen was used with the fresh catalyst instead of hydrogen, the amount of solids which formed in the reactor and on the catalyst was nearly identical to the corresponding run with hydrogen.

Discussion When Athabasca bitumen is brought into contact with a catalyst in the batch reactor under hydrocracking conditions, there is a rapid buildup of solids on the catalyst which is independent of whether or not hydrogen is present. This reaction is most likely an acidcatalyzed polymerization induced by acidic sites on the alumina support.lg Once these solids form on the catalyst, no other solids are formed in the reactor. The indication is that there is a certain portion of the feed bitumen which is highly susceptible to solids formation and that once that is removed, solids do not form under hydrocracking conditions. As the catalyst increases in weight up to 50%,it gradually loses the ability to induce polymerization on the catalyst surface due to deactivation of the acid sites. When coke no longer forms on the catalyst from fresh feed, coke formation in the reactor begins. Coke formation in the reactor can be controlled by hydrogen transfer using highly dispersed, unsupported, transition metal sulfides,20 but there is no indication that the catalyst in the present case plays an active role in this reaction. If such active sites for hydrogen transfer exist on supported catalysts, they are lost very rapidly as a result of carbonization of the catalyst as described above. In the batch tests, a long reaction time (4 h) was chosen such that a significant amount of solids would be formed in the reactor for convenience and reproducibility in measuring the amount formed. It was also desirable that a significant change would occur on the introduction of hydrogen but the amount of solids formed would not decrease to zero so that the effect of catalyst could be measured. In the commercial plant and in the pilot plant, severity must be such that the reactors are operated below the "threshold of coking"20 since coke formation does not take place to any significant extent and the above discussion indicates that the catalyst does not play a direct role in preventing coke formation in the reactor. As solids are building up on the catalyst, hydrogenation and hydrocracking reactions are also taking place. The role of catalysts in these reactions can be discussed most readily by reference to the role of hydrogen in the hydrocracking of Athabasca bitumen residuum, as described in ref 9, and which is outlined in schematic (19) Berteau, P.; Delmon, B.; Dallons, J.-L.; Van Gysel, A. Appl. Catal. 1991, 70, 307-323. (20) Kriz, J. F.;Ternan, M. In Progress in Catalysis; Smith, K. J., Sanford, E. C., Eds.; Elsevier Science Publishers B.V.: Amsterdam, 1992; Vol. 73, pp 31-33.



Table 3. Amounts of Reactor Solids Formed from 400 g of Bitumen in a Stirred Batch Reactor at 400 "C for 4 h with Different Catalysts and Tetralin catalyst tvpe

tetralin amount

(go

-

new new new new new new 4h 8h spentb spentb spentb

100 100 50 25 10 100

13gd 156d 75 100 200

40 40 -

-

gas nitrogen nitrogen nitrogen hydrogen nitrogen hydrogen nitrogen hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen hydrogen

press. (MPa) 7.24 8.27 8.83 8.62 8.27 7.24 8.55 8.55 8.55 8.55 8.55 8.55 8.55 8.55 8.62 8.62 8.62

reactor solidsa (g) 30.0 29.1 31.3 15.4 16.4 7.7 1.2 -0.1 2.8 7.2 15.1 -0.1 7.9 16.8 15.7 14.7 13.4

solids on catalyst (g) -

39.9 38.0 20.0 10.3 7.9 38.0 19.1 9.6 1.1 2.2 6.4

total solids (g) 30.0 29.1 31.3 15.4 16.4 7.7 41.1 37.9 22.8 17.5 22.4 37.9 27.0 26.4 16.8 16.9 19.8

Corrected for solids in the feed (1.7 g/400 g feed). Spent cat,alyst from the commercial plant. New catalyst from the commercial plant. Increase in weight due to coking of the catalyst. a

Gas + Distlllate +

LmEL

f--

Figure 12. A schematic representation of a proposed mechanism of residuum conversion during hydrocracking showing possible roles of hydrogen and catalyst.

form in Figure 12. The mechanism in Figure 12 is what is expected at greater than 30-40% residuum conversion, i.e., after most of the easy-to-break bonds have been broken in thermal reactions. The feed molecule contains two functional groupings, condensed naphthenic rings and condensed aromatic rings, which were proposed to play a major role in hydrocracking. In addition to the conversion route initiated by thermal bond cleavage (path I), a catalytic hydrogenation route is shown6 (path V) which could lead to CCR conversion, through a hydrogenated residuum molecule which could be subsequently decomposed by a thermal route t o give gas and distillate. This is in fact the route to CCR conversion which is believed t o be operational in hydrocracking reactions carried out in the presence of a residuum hydrotreating catalyst.1° The data presented in this study and in previous studies,6 for reactions carried out for less than 1 day, or for reactions carried out under hydrotreating conditions in the pilot plant, show that hydrogenation of residua molecules does indeed take place with fresh catalyst. In the two phases of the pilot plant hydrotreating run, the ratio of % CCR conversion to % residuum conversion remains at 1.7 throughout, a good indication that CCR conversion is taking place through catalytic hydrogenation. The active sites on the catalyst for CCR conversion (Figure 8) and metals conversion (Figure 8) show only

slight deactivation throughout the time frame studied, and in each case, the rate constant is lower in the second run compared to the first. If one were working with a pure compound, and the catalyst was not being deactivated, the rate constant would not change throughout either run. If the catalyst was being deactivated, the rate constant would decrease throughout the first run and then increase at the beginning of the second run to be equal to the rate constant at the beginning of the first run. When a mixture of compounds is present, and the compounds in the mixture are hydrotreated at different rates, the overall rate constant would remain unchanged throughout the first run in the absence of catalyst deactivation or would decrease with catalyst deactivation. In the second run with active catalyst, the rate constant would be lower than for the beginning of the first run, since many of the easy to treat compounds would already have reacted but would be higher than at the end of the first run if catalyst deactivation was a factor in the first run. The data for CCR, vanadium, and nickel conversion appear to fit the case where there is a range of compounds that are hydrotreated at different rates and there is little catalyst deactivation through the range studied. CCR conversion and metals removal may be similar to each other because both may involve highly delocalized aromatic type structures, condensed aromatics in the case of CCR and porphyrins in the case of the metals. Sulfur conversion is quite different. The rate constant for sulfur removal (Figure 7) shows a steady decline throughout the first run, then increases in the second run to a value similar to the rate constant at the beginning of the first run, and then declines again. This behavior would be expected for a pure compound in a situation where the catalyst is being deactivated. The likely situation with bitumen is that the sulfur which is being removed catalytically is contained largely in one type of environment, likely substituted thiophenes. The observation that the catalyst loses activity with respect t o sulfur removal and not with respect to CCR and metals is an indication that these reactions take place on different active sites on the catalyst. Nitrogen again presents a different situation (Figure 7). There is a clear decrease in the rate constant as the first run progresses. In the second run, the rate con-


stant a t the beginning is higher than the rate constant a t the end of the first run. The observations are consistent with a range of reactivities in the nitrogen containing molecules, combined with catalyst deactivation in the first run. There is little evidence of catalyst deactivation in the second run, suggesting that the substances which deactivate the catalyst toward nitrogen removal are no longer present in the hydrotreated feed. Active sites on the catalyst for nitrogen removal appear to be different from those for sulfur removal and for CCR and metals removal. Despite the fact that catalytic hydrogenation of bitumen does take place with a fresh catalyst, the hydrogenation activity of these commercial residua hydroprocessing catalysts is effectively lost within a matter of several hours a t 400 "C in a batch reactor (Figures 1 and 3). Percent CCR conversions of up to 3 times % residuum conversions rapidly decrease to ratios of 1and less. Catalysts which show ratios of approximately 0.6 under cracking conditions show little if any activity when tested under hydrotreating conditions (Figure 1). Even in the batch system, where fresh catalysts are used with each run, the catalysts deactivate rapidly as a function of reaction temperature, with respect to sulfur and CCR conversion (Table 1,series 7). Despite the apparent loss of hydrogenation activity within a few hours, the catalysts do promote all conversions, although a t a lower level, as is apparent in Figure 3. Under normal pilot plant hydrocracking conditions (430-450 "C), the catalytic sites for hydrogenation of aromatics would deactivate so quickly that path V in Figure 12 would not play a significant role in CCR conversion. The loss of hydrogenation activity should not be surprising since in catalytic hydrotreating of distillates temperatures are generally kept below 400 0C,21even a t the bottom of the last catalyst bed. The inclusion of a high-boiling distillate tail from the residuum in heavy gas oil is often thought to be detrimental to the maintenance of catalyst activity, since the high-boiling distillate may lead to coke deposition on the catalysts and may contain small amounts of vanadium. When bitumen is heated up to 400 "C in the presence of hydrogen and a commercial Ni-Mo on y-alumina hydrocracking catalyst and immediately cooled, the catalyst, after extracting with toluene, gains approximately 20% in weight due to deposited carbon. Vanadium is known to deactivate hydrotreating catalysts22and catalysts which contain in excess of 1%vanadium are normally not considered worthy of regeneration. Catalysts which are removed from commercial hydrocrackers often contain up to 30-40% vanadium, depending on the feed and the condition^.^^ The finding that catalysts lose their hydrotreating activity rapidly when in contact with residuum a t 430-450 "C is in keeping with everything that is known about hydrotreating catalysts. In continuous pilot plant units, the behavior of the catalyst during the first few hours is normally not observed. When the first samples are taken 16-24 h after adding feed at cracking reaction temperature, most of the hydrotreating activity has been lost, and there is (21) Thakur, D. s.;Thomas, M. G. Appl. Catal. 1985,15,197-225. Massoth, F. E.; Furimsky, E. Fuel Process. Technol. (22) Kim, (2.7s.; 1992,32,39-46. (23) McKnight, C. A,; Nowlan, V. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1993,38,391-397.

Sanford

no selectivity between conversion of CCR residuum molecules and non-CCR residuum molecules (Figure 5, ratio of percent conversions = 1). However, the hydrotreating activity can be observed by running the unit a t lower temperature, where catalyst activity loss is much slower (Figures 6-8). As the catalyst ages under cracking conditions, selectivity is introduced in the reactions, and CCR residuum molecules decompose slower than non-CCR residuum molecules. In the pilot plant, using conditions which give residuum conversions of around 60%, it takes up to 40-50 days before the catalyst activity decreases to the point that % CCR conversion is comparable t o the case without catalyst. The active sites on the catalyst which promote CCR and heteroatom removal, after deactivation of the hydrotreating sites, deactivate relatively slowly under hydrocracking conditions. The pilot plant data indicate that under cracking conditions, after the catalyst has lost its hydrotreating activity, % CCR conversion is directly related to % residuum conversion and will never be much higher than % residuum conversion. When the catalyst age is zero, Le., with 1 day old catalyst, both CCR conversion and overall residuum conversion most likely arise through the same rate determining step, which does not involve hydrogen or c a t a l y ~ t ,and ~ , ~which is likely a thermal bond breaking (Figure 12, step I). This conclusion is in keeping with the results of Ternan and h i d 4 and Beaton and Bertolacini'O who found that CCR conversion was related to residuum conversion. As the catalyst ages, selectivity is introduced and CCR-forming molecules are not converted as fast as other residuum molecules. Sulfur conversions are similar to CCR conversions, except that there appears to be a significant removal of sulfur prior to much residuum conversion. Since the hydrotreating (aromatics saturation) activity of the catalyst has already been removed, this sulfur removal is probably due to thermally labile sulfur and may include some catalytic sulfur removal, but by some mechanism other than hydrogenationhydrogenolysis. The remaining sulfur shows a stronger dependence on catalyst activity than does CCR removal as indicated by the coefficient (Table 2) of the catalyst age term. Nitrogen conversions tend to be low and contain a lot of scatter and are not correlated with residuum conversion and catalyst age. Previous studied7 have shown that nitrogen is removed during hydrocracking mainly through solids formation. In keeping with the mechanism proposed in Figure 12, the role of catalyst in hydrocracking is seen as one of accelerating the reaction between molecular hydrogen and a carbon radical (step 111). It is proposed that hydrogen adds to a carbon radical contained in a condensed aromatic center (the CCR portion of the residuum) and that the hydrogen atom generated adds t o the aromatic center, probably in a concerted manner (steps I11 and IV combined), to form a cyclohexadienyl radical intermediate which leads to decomposition of the condensed aromatic unit, forming gases and distillate^.^ It is reasonable that decomposition of molecular fragments by such a route would lead t o some removal of sulfur and nitrogen. Sulfur removal is greatly favored over nitrogen removal (Figure 31, which suggests a selectivity in the addition of hydrogen atoms t o sulfur-


containing condensed aromatics (substituted thiophenes). It is unlikely that hydrotreating, where the hydrogen is catalytically added across double bonds in an aromatic ring to give a saturated ring, plays a significant role in hydrocracking. As the catalyst deactivates, both CCR conversion and overall residuum conversion decrease. Since the CCRforming molecules are part of the residuum, it is likely that the decrease in overall residuum conversion is due to the change in CCR conversion. From the point of view of the mechanism in Figure 12, deactivation of the catalyst must be affecting step IV. That is, the CCRforming portion of the molecule is no longer being converted to products and would remain as part of the residuum fraction. If loss of step I11 was involved in catalyst deactivation, then one would expect coke to form in the reactor, which is not observed. This mechanism suggests that the key role of the promoted Mo on y-alumina catalyst during hydrocracking of Athabasca bitumen residuum is supplying hydrogen atoms for step IV, the formation of a cyclohexadienyl radical, which provides a pathway for the conversion of condensed aromatic molecules to gases and distillates. The main difference between supported and highly dispersed catalysts in these cracking reactions may be that with the dispersed catalysts, a t the low concentrations normally used, only step I11 takes place, leading to overall low CCR conversion. The discussion presented here suggests two separate roles for supported catalysts in residuum hydroprocessing. The first is conventional hydrotreating of bitumen under essentially noncracking conditions, leading to aromatics saturation and heteroatom removal, through catalytic hydrogenation and hydrogenolysis reactions. It is further suggested that these hydrogenation reactions take place on three separate sites on the catalyst. As the temperature is increased from hydrotreating to hydrocracking conditions, coke builds up on the catalyst and the hydrotreating sites are rapidly lost. van Doorn and MoulijnZ4have suggested that coking of catalysts does not remove the active sites for hydrogen activation and these sites are kept free of coke. The loss of hydrotreating activity may be due to the loss of sites for the aromatic molecules t o come into contact with the 124) van Doorn, J.; Moulijn, J. A. Fuel Process. Technol. 1993,35, 275-28 7 .


catalyst. Once the hydrotreating activity is lost, it is proposed that the mechanism changes to one of reaction of activated hydrogen with carbon radicals. During residuum processing, these active hydrogenation sites are lost over a period of several weeks. Considering the amount of contamination of the catalysts, residuum molecules probably do not come into contact with the active sites on the catalyst. Hydrogen atoms or activated hydrogen molecules are likely formed at active sites and migrate to the surface of the catalyst, perhaps through a spillover mechanism, and reaction likely takes place close to the external surface of the catalyst. The observed deactivation of the catalyst in extended pilot plant runs is considered to be a slow loss of the active sites for the activation of hydrogen toward carbon radicals. Since catalyst are known to be deactivated by both carbon deposits and metal^,^^,^^ the logical assumption is that it is the metals which are deposited uniformly over time which deactivate the catalyst with respect to hydrogen transfer. However, these catalysts which contain large amounts of metals in the form of nickel and vanadium sulfides can be regenerated without metals r e m o ~ a l ,to~ give ~ , ~activi~ ties comparable to the fresh catalyst. This result argues against deactivation by metals and suggests that the active sites for hydrogen atom transfer t o carbon radicals must be lost by continued coke deposition on the catalyst.

Acknowledgment. The author acknowledges the careful experimental work of Mr. Ken Douglas, Mr. Kevin Pollitt, and Mr. Arthur Lemke and the involvement of Dr. Vince Nowlan and Mr. Craig McKnight in the pilot plant runs. Financial assistance from the Alberta Hydrogen Research Program of the Department of Energy for a portion of this work is gratefully acknowledged. The author thanks Syncrude Canada Ltd. for permission to publish this paper. EF940229Y (25)Trimm, D. L. Catalysts in Petroleum Refining, D. L. Trimm, et. al. editors, Elsevier Science Publishers 1989,41-60. (26) Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. Ind. Eng. Chem. Process Des. Dew. 1981,20,262-273. ( 2 7 )Clark, F. T.; Hensley, A. L.; Shyu, J. Z.; Kaduk, J. A,; Ray, G. J. In Catalyst Deactiuation; Bartholomew, C. H., Butt, J. B., Eds.; Elsevier Science Publishers, B.V.: Amsterdam, 1991; pp 417-423. (28) Carruthers, J. D.; Brinen, J. S.; Komar, D. A,; Greenhouse, S. Symposium on Hydroprocessing of Petroleum Distillates, AIChE Spring Meeting, March 28-April 3, 1993; No. 73a.

Conradson Carbon Residue Conversion during Hydrocracking of

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