Influence of Hydrogen and Catalyst on Distillate Yields and the

Energy & Fuels 1994,8, 1276-1288

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Influence of Hydrogen and Catalyst on Distillate Yields and the Removal of Heteroatoms, Aromatics, and CCR during Cracking of Athabasca Bitumen Residuum over a Wide Range of Conversions Emerson C . Sanford Syncrude Canada Ltd., 10120 17 Street, Edmonton, Alberta, Canada, T6P lV8 Received April 25, 1994. Revised Manuscript Received July 12, 1994@

The percent conversion of sulfur, nitrogen, vanadium, nickel, aromatics, and CCR (Conradson carbon residue) has been analyzed as a function of residuum conversion for five series of reactions carried out in a batch reactor under nitrogen, hydrogen, or hydrogen in the presence of a residuum hydrotreating catalyst, over residuum conversions ranging from 38 to 86%. Sulfur, nitrogen, and CCR conversions could be divided into three categories, up to 50% residuum conversion, 50-70% residuum conversion, and greater than 70% residuum conversion. Sulfur conversion in the first category was from the distillable liquids and resulted from a combination of thermal, hydrogenation, and hydrogen atom addition reactions. Nitrogen removal in the first category was due to incorporation of nitrogen into reactor solids in the early stages of the reaction and was therefore suppressed by hydrogen in the absence of a catalyst. In the presence of hydrogen, CCR conversion was largely due to hydrogen atom addition. Without hydrogen, CCR conversion was negative through the first two categories of conversion. Aromatics in the residua fractions increased steadily as residuum conversion increased, but increased faster in the coking case. Sulfur, nitrogen, and CCR conversions were all rapid after approximately 70% residuum conversion and were explained in terms of thermal reactions. Metals removal appeared to be due to thermal reactions with the solids in the reactor acting as a collector. Catalytic reactions did not appear t o play a role. Overall, the main role of catalyst was the removal of sulfur from distillable liquids produced in the early stages of the residuum conversion reaction, and in CCR conversion, again in the early stages of the reaction.

Introduction Current commercial methods for conversion of residua into distillable liquids, which on subsequent treatment, form the feedstock t o a refinery, can be considered to be either thermal or hydrothermal processes.' The most dramatic change between these two processes is the almost complete suppression of the reaction steps leading to carbonaceous solids formation in the presence of hydrogen. Complete suppression of this reaction requires high partial pressures of hydrogen and catalysts2 are normally used to allow the processes to be operated at much lower pressures. The common catalysts are metal sulfides, which result from the reaction of sulfur in the feed with finely divided metals in a variety of forms3 or more traditional residuum hydrotreating catalysts, such as Ni- or Co-Mo on y-alumina base.4 The residuum hydrotreating catalyst is generally assumed to play a major role in heteroatom removal and aromatics saturation (CCR reductionI5 as well as solids suppression. A possible mechanism for the suppression of solids formation by hydrogen during conversion of Athabasca Abstract published in Advance ACS Abstracts, September 1,1994. Handwerk, G. E. Petroleum Refining Technology and (1)Gary, J.H.; Economics; Marcel Dekker, Inc.: New York, 1984;Chapters 5 and 9. (2)Bearden, R.;Aldridge, C. L. US Patent 1979,4,134,825. (3)Dabkowski, M.J.; Shih, S. S.;Albinson, K. R. MChE Symp. Ser. 1991,87(282)53-61. (4)Beaton, W. I.; Bertolacini, R. J. Catal. Rev. 1991,33,281-317. (5)Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto,Y. J. Catalysis 1983, 83,371-383.

bitumen residuum has been proposed.6 Carbon-tocarbon bond breaking is thought to produce an aromaticcarbon aliphatic-carbon biradical intermediate and hydrogen transfer to the aromatic-carbon radical then prevents condensation which leads to coke formation. A hydrogen atom is formed from the reaction of hydrogen with the aromatic-carbon radical which can add to an aromatic ring, providing a pathway for decomposition of the aromatic ring. The aliphatic radical is expected to undergo fragmentation to produce gases and distillate. Gases are proposed to originate from hydroaromatic or naphthenic structures. Sulfur removal during hydrocracking is generally considered to consist of both thermal and catalytic reaction^.^,^ Thermal removal of sulfur most likely takes place from aliphatic sulfides and disulfides whereas catalytic reactions would be required for removal of sulfur from substituted thiophenes. Although there is some data available8 comparing desulfurization in coking and hydrocracking reactions in the presence of a residuum hydrotreating catalyst, hydrocracking is generally not carried out without catalysts and so a comparison with just hydrogen is not available. Catalytic desulfurization takes place mainly from the distillable liquids rather than the unreacted r e ~ i d u u m . ~

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(6)Sanford, E.C. Ind. Eng. Chem. Res. 1994,33,109-117. (7)Gray, M. R.;Jokuty, P.; Yeniova, H.; Nazarewycz, L.; Wanke, S. E.; Achia, U.; Krzywicki, A,; Sanford, E. C.; Sy, 0. K. Y. Can. J . of Chem. Eng. 1991,69, 833-843. (8) 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.

0 1994 American Chemical Society

Influence of Hz and Catalyst on Distillate Fields

Nitrogen removal is considered to be harder to achieve than sulfur r e m ~ v a lmainly ,~ because most if not all of the nitrogen is contained in heteroaromatic rings which require hydrogenation prior to nitrogen removal. Because of the requirement for the ring saturation step, nitrogen removal is thought to be "definitely a catalytic pro~ess".~ Studies by Gray and co-workers' were based on a comparison of four feeds using catalysts of different activities, including the catalyst base material with no added metals. A detailed comparison of coking, hydrocracking, and hydrocracking in the presence of a hydrotreating catalyst is not available. Nickel and vanadium are present in all heavy petroleum fractions and are considered to be mainly in petroporphyrin compound^.^ Distillable liquids produced as a result of cracking of residua do not contain metals except to the extent that heavy ends may be carried over during distillation. During the coking-toextinction processes, all metals ultimately end up in the coke fraction. Whether or not the metals are removed from their porphyrin structures in purely thermal reactions has not been reported. Similarly, hydrocracking with or without finely divided metal sulfide catalysts results in a pitch fraction which contains most of the metals originally present in the feed. Again, there are no reports of the nature of the metals in the hydrocracker bottoms and whether or not the metals are still in porphyrin compounds or present as metal sulfides, suspended in the pitch. Metals removal during hydrocracking in the presence of a hydrotreating catalyst (see Reynoldsg and Gray et a1.8) is one of the most widely studied aspects of hydrocracking, due to the fact that the metals deposit on the catalyst and therefore are thought to deactivate the catalyst with respect to hydrogenation and hydrogenolysis reactions. Probably because the metals deposit on the catalysts, the demetallization reactions are assumed to be catalytic, and no comparisons of metals removal during hydrocracking in the presence and absence of a catalyst have been reported. Extensive modeling work has been done on the hydrodemetallization reaction (see ref 41,and catalysts have been designed specifically for metals removal. Thermal cracking of aromatic rings is reported to start only a t temperatures greater than 1000 OC.l0 If this were to hold for cracking of residua, aromatics would not be converted under normal hydrocracking conditions. However, hydrogenation of aromatics during residua conversion would make the residua more reactive to thermal conversion as well as make sulfur, nitrogen, and metals removal from heteroaromatic compounds feasible. Despite the obvious desirability of aromatics saturation of r e ~ i d u a the , ~ reaction is not normally studied directly, mainly because measuring aromatics in such a complex mixture is not a routine laboratory procedure. Gray et al.7 have shown that the CCR content of a residue can be correlated with the fraction of aromatics in the sample. The removal of compounds which contribute to Conradson or Ramsbottom carbon residue (CCR or RCR) is thought to be due to aromatics saturation and is an indirect way of studying aromatics saturation. This reaction is felt to be very important4 since it is a measure of the extent of coke formation in (9) Reynolds, J. G. Chem. Ind. 1991,August, 570-574. (10)Poutsma, M.L. Energy Fuels 1991,4, 113-131.

Energy & Fuels, Vol. 8, No. 6, 1994 1277

thermal reactions.'l If CCR conversion is high during hydrocracking, the assumption is that this has overall benefits for the process. Whether or not CCR conversion during hydrocracking leads to distillate products or just produces more refinery gas has not been determined. Several studies have indicated that CCR conversion results from both thermal and catalytic r e a ~ t i o n s , ~ , ~ J ~ but a direct comparison of the thermal and hydrocracking processes has not been made. Experimental data is presented here on distillate yields, heteroatom removal, and aromatic saturation during reactions under identical conditions except that nitrogen or hydrogen was used as the reaction atmosphere and some reactions with hydrogen were carried out in the presence of a residuum hydrotreating catalyst. Residuum conversion varied from 38 to 86%.

Experimental Section The procedure for the experiments reported here has been described previously.6 All experiments were carried out at 400 "C in a stirred batch autoclave with reaction times varying from 30 t o 480 min. The coking experiments were carried out under 11 MPa of nitrogen partial pressure. Otherwise, hydrogen was used at the same partial pressure. Five series were carried out. Coking runs were done in a nitrogen atmosphere. Hydrocracking runs were identical except that hydrogen was used. Hydrogenolysis hydrocracking runs were the same as hydrocracking runs except that a fresh commercialresiduum hydrotreating catalyst was present. The runs with a spent catalyst were the same as the above hydrogenolysis hydrocracking runs except that the catalyst had been discharged from a commercial hydrocracker used to process Athabasca bitumen. In the final series,fresh catalyst was used as above in the hydrogenolysis hydrocracking runs except that nitrogen was used instead of hydrogen. For the hydrocracking runs with catalyst (hydrogenolysis hydrocracking, HHC),the catalyst (75 g of a commercial CoMo on gamma alumina residuum hydrotreating catalyst per 300 g of feed or an equivalent weight of spent catalyst) 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 new catalyst was not presulfided but was converted t o the sulfided form as a result of reaction with the feed during the heat-up period. The feed was Athabasca bitumen residuum, prepared by distilling whole bitumen to a 524 "C end point. Redistillation of the residuum feed on an analytical D-1160apparatus gave a composition of 91.0% 524 "CS with 9% heavy gas oil boiling in the 343-525 "C range. The residuum feed contained 5.50% sulfur, 7047 ppm nitrogen, 81.53% carbon, 9.36% hydrogen, 383.8ppm vanadium, 143.5ppm nickel, and 24.77% CCR, and 35.5% of the carbon atoms were in aromatic structures as determined by 13C NMR. After the specified reaction time, a sample of gas was removed for laboratory analyses and the remainder vented. The contents of the reactor was then slurried in methylene chloride. The methylene chloride slurry was filtered through a 45 pm filter followed by a 30 pm filter. The filtrate was then centrifuged for 40 min at 1500 rpm, decanted and filtered through a 20 pm filter. The solvent was removed from the combined filtrates on a rotary evaporator. The total liquid product was distilled, first by spinning band distillation at atmospheric pressure to remove the C-5 t o 195 "C fraction, and then by D-1160,first at 20 mm pressure and then at 1 mm pressure to remove the 195-343 "C cut and the 343-524 "C cuts, respectively. (ll)Kirchen, R. P.; Sanford, E. C.; Gray, M. R.; George, 2. M. AOSTRA J. Res. 1989,5,225-235. (12)Sanford, E.C.;Chung, K. H. AOSTRA J.Res. 1991,7,37-45.

1278 Energy & Fuels, Vol. 8, No. 6, 1994

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Figure 1. Liquid products present in the reaction mixture during coking of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion: (B)total liquid product, (A)naphtha, (0)light gas oil, (*I heavy gas oil, and

Figure 2. Liquid products present in the reaction mixture during hydrocracking of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 1.

( x ) total distillate.

The relative rates of residuum conversion were the same within experimental error in all cases.6 The plots in this report are based on average percent conversions at specified reaction times for all five series, a total of 21 runs, and are as follows: (one standard deviation, four t o five runs) 30 min 38 f 4, 60 min 47 k 7,120 min 61 f 8,240 min 82 k 5, and 480 min 86 i 4. All conversions were calculated as follows: (weight of feed x composition) - (weight of total liquid product x composition)/(weight of feed x composition) x 100.

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Results Distillation Cuts. The distribution of liquid products into naphtha (C-5-195 "C),light gas oil (195-343 "C), heavy gas oil (343-524 "C),total distillate (C-5524 "C),and total liquid product for coking, hydrocracking, and hydrocracking in the presence of an active residuum hydrotreating catalyst is shown in Figures 1-3 as a function of residuum conversion. Only small differences show up when the total amounts of distillate produced in each series are compared. The coking reaction (Figure 1) produced less distillate than the hydrogenolysis hydrocrackingreaction (Figure 3), which produced less distillate than hydrocracking (Figure 2). There was very little difference in the amount of distillate formed in the three series up to 40% conversion. The result from the hydrogenolysis hydrocracking runs may be influenced by the batch system in that a significant amount of the feed (up to 24%16is deposited on the catalyst in the form of insoluble hydrocarbonaceous solid. Such deposits would still occur in a flow system but would not constitute such a large proportion of the feed. In two of the three cases, liquid yields were at a maximum around 70% residuum conversion and decreased slightly at longer reaction times. The amount of heavy gas oil was at a maximum between 50 and 70% residuum conversion in the three series and decreased as reaction times became longer. As the amount of heavy gas oil decreased, the amount of light gas oil and naphtha both increased slowly, except for the hydrocracking run. More heavy gas oil was produced in this run and it decomposed faster in secondary reactions toward the end of the run, producing larger quantities of naphtha. When the distillate fractions were compared by first subtracting the amount of gas oil present in the feed

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Figure 3. Liquid products present in the reaction mixture during hydrocracking of Athabasca bitumen residuum in the presence of a fresh catalyst at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 1.

from the total amount of distillate and then expressing the amount of distillate in each cut as a percent of the total distillate, there was a general trend toward increasing yields of naphtha and light gas oil and a corresponding decrease in the yield of heavy gas oil. These changes are likely due to secondary reactions. The results are plotted in Figures 4 and 5 as a function of residuum conversion. Sulfur Conversion. Sulfur removal from the total liquid product is plotted as a function of residuum conversion in Figure 6. The extent of sulfur removal in the early stages of the reactions varied considerably, depending on whether or not hydrogen was used and whether or not a fresh catalyst was used. In all series with hydrogen, sulfur removal appears to level off between 60 and 80%residuum conversion and increases rapidly again after 80% conversion of the feed residuum. The reaction in the presence of fresh residuum hydrotreating catalyst is appreciably faster than the other two, throughout the range studied. The reactions with hydrogen alone, hydrogen with spent plant catalyst, and nitrogen with fresh catalyst are significantly faster than the coking reaction, up to approximately 80%residuum conversion. The shape of the curves indicate that sulfur

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removal is likely a primary reaction in the presence of hydrogen, but not necessarily in the absence of hydrogen. The distribution of sulfur among the total liquid product and the reactor solids for three of the series are plotted as a function of residuum conversion in Figures 7-9. Sulfur not accounted for in the liquids and solids is plotted as losses and is expected to represent sulfur lost as hydrogen sulfide in the gas phase. For the coking reaction (Figure 71, there is very little sulfur removed from the liquid product up to 50% residuum conversion, and the sulfur that is removed is mainly in the gas phase. After 50%residuum conversion, the main source of sulfur loss from the liquid product is to the solids. Sulfur loss to the gas phase is nearly linear with residuum conversion up to 80% residuum conversion, when it increases rapidly. In the presence of hydrogen (Figure 8), only a small amount of solids are formed and the sulfur in this product represents only a small fraction of the total sulfur lost. Most of the sulfur removed from the liquids is in the gas phase. In the presence of a fresh catalyst

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Figure 7. Sulfur mass balance in products from coking of Athabasca bitumen residuum at 400 "C,plotted as a function of residuum conversion: (W) total liquid product, (+I reactor solids, (*) losses.

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Figure 8. Sulfur mass balance in products from hydrocracking of Athabasca bitumen residuum at 400 "C,plotted as a function of residuum conversion. Symbols as for Figure 7.

(Figure 9), losses are plotted as the difference between sulfur lost from the liquid product and that present in the small amount of reactor solids formed. Sulfur on

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Figure 9. Sulfur mass balance in products from hydrocracking in the presence of fresh catalyst of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion: (m) total liquid product, (+) reactor solids, (*) losses not including the catalyst, (0)catalyst.

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Figure 10. Sulfur concentration in the naphthdight gas oil fractions from coking,hydrocracking,and hydrocracking in the presence of fresh catalyst of Athabasca bitumen residuum at 400 "C,plotted as a function of residuum conversion. Symbols as for Figure 5. the catalyst is plotted separately. Up to 40%residuum conversion, all of the sulfur lost to the gas phase goes on the catalyst in the form of either metal sulfides or sulfur in the carbonaceous solids. Between 40 and 80% residuum conversion, the amount of sulfur on the catalyst remains constant, and losses from the liquid product go to the gas phase, and amount to an additional 10% of the sulfur in the feed a t 80% residuum conversion. The sulfur concentrations in the naphtha/LGO fractions (C-5-343 "C) recovered from the coking, hydrocracking, and hydrogenolysis hydrocracking (fresh catalyst) runs are plotted as a function of residuum conversion in Figure 10. Sulfur concentrations for the two hydrocrackingruns follow similar trends in that the concentration of sulfur in the distillable liquids decreases as the reaction time increases. The actual concentration of sulfur in the product from the hydrocracking runs with catalyst is significantly lower than in the hydrocracking runs without catalyst, as exp e ~ t e d . ~The , ~ coking run is quite different. Sulfur levels are initially considerably lower than for the hydrocracking series without catalyst, being similar to

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Figure 12. Sulfur concentration in the residuum fractions from coking, hydrocracking,and hydrocracking in the presence of fresh catalyst, hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C,plotted as a function of residuum conversion. Symbols as for Figure 6.

the catalytic case, but increase with conversion until the concentrations are similar to the noncatalytic hydrocracking series after about 80% conversion. There is a relatively small change in the concentration of sulfur in the HGO fraction (Figure 11)as a result of the extent of residuum conversion for all three series. The concentrations initially decrease and then increase rapidly after 80% residuum conversion up to levels higher than those present in the low residuum conversion cuts. Overall, there is considerably less sulfur in the products from the series with fresh catalyst and hydrogen compared to the two series with just hydrogen or no hydrogen, which have similar concentrations of sulfur in the heavy gas oil fractions a t any given residuum conversion. Sulfur concentrations in the residuum fractions from all series are plotted as a function of residuum conversion in Figure 12. There is a considerable amount of scatter in the data, and in general, no significant difference in the sulfur concentrations as a result of having hydrogen or hydrogen plus a hydrotreating


Energy & Fuels, Vol. 8, No. 6, 1994 1281 9080-

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Figure 13. Nitrogen conversion during coking, hydrocracking, hydrocracking in the presence of fresh catalyst, hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum a t 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 6.

catalyst present compared to the coking runs. The trend appears to be for the sulfur concentrations t o be lowest in the hydrogenolysis hydrocracking runs, with the coking runs second, and the highest sulfur concentrations in the runs with hydrogen and no fresh catalyst. Sulfur concentration in the residuum is generally less than the sulfur concentration in the feed. Nitrogen Conversion. Overall nitrogen conversion (Figure 13) increased as residuum conversion increased with nitrogen removal being higher in the presence of hydrogen and a hydrotreating catalyst compared to the coking series. In the hydrocracking series without catalyst, there was not any appreciable nitrogen conversion up to approximately 60% residuum conversion, at which time nitrogen removal became rapid, and by 80% residuum conversion, nitrogen removal was essentially the same in all three series. Nitrogen removal with fresh catalyst in an atmosphere of nitrogen gave the same nitrogen removal as with fresh catalyst in an atmosphere of hydrogen. For both the coking and the hydrocracking reactions in the presence of catalyst, the loss of nitrogen from the total liquid product is accounted for by the nitrogen in the solids formed, either solids in the reaction mixture (Figure 14) or solids on the catalyst (Figure 15). As was the case for sulfur, losses in Figure 15 are plotted independently of the nitrogen on the catalyst. However, it can readily be seen that all of the losses are accounted for by nitrogen on the catalyst. For the series with just hydrogen, nitrogen is lost from the system after approximately 60% residuum conversion ( Figure 16). In both the coking and hydrogenolysis hydrocracking reactions (Figure 13), nitrogen removal was initially rapid, then leveled off between 50 and 75% residuum conversion, and then increased rapidly as residuum conversion approached 80%. The shape of the curves in all series indicates that these are secondaryreactions. Nitrogen in the naphthdLGO cut (Figure 17) showed the opposite trend to the corresponding sulfur data (Figure 10) in the sense that more nitrogen containing molecules which boil in the naphtha/LGO range are produced at higher residuum conversions. However, one unusual aspect that was similar to the sulfur case

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Figure 14. Nitrogen mass balance in products from coking of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 7.

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Figure 16. Nitrogen mass balance in products from hydrocracking in the presence of fresh catalyst of Athabasca bitumen residuum a t 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 9.

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Figure 16. Nitrogen mass balance in products from hydrocracking of Athabasca bitumen residuum a t 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 7.

is that a t residuum conversions up t o approximately 80%, coking produces a product which contains less nitrogen than the products from the hydrocrackingruns which were similar with respect to nitrogen levels.

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Figure 19. Nitrogen concentration in the residuum fractions from coking, hydrocracking, and hydrocracking in the presence of fresh catalyst, hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 6. 100,

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Figure 18. Nitrogen concentration in the heavy gas oil fractions from coking, hydrocracking, and hydrocracking in the presence of fresh catalyst of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 5.

The concentration of nitrogen in the HGO fraction from all three series increases (Figure 18)as residuum conversion increases, similar to the naphtha/LGO cut (Figure 17). Although there is considerable scatter in the data, the run with hydrogen and no catalyst appears to have higher concentrations of nitrogen compared t o the other two runs under comparable conditions. Nitrogen concentrations in the residuum fractions (Figure 19) are similar to the nitrogen concentrations in the heavy gas oil fractions (Figure 18) in that concentrations tend to increase as residuum conversion increases and there is scatter in the data. The data from all of the series tend to scatter about the same line which increases steadily up to 80%residuum conversion and then increases rapidly after that. Metals Conversion. Vanadium and nickel conversions from all five series are plotted as a function of residuum conversion in Figures 20 and 21. In each case, the liquid product was filtered through a 20 pm filter after centrifuging. Any metal sulfides which had been removed from the organic matrix and were suspended in the liquid product would be removed, provided that the suspension was coarser than approximately 20 pm.

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Figure 20. Vanadium conversion during coking, hydrocracking, hydrocracking in the presence of fresh catalyst, hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 6.

The nickel and vanadium conversions from the coking runs, hydrocracking runs with spent catalyst, and the coking runs in the presence of fresh catalyst increase steadily with residuum conversion. In the presence of hydrogen and a residuum hydrotreating catalyst, conversions are initially faster but are the same as the coking series after approximately 60% residuum conversion. With hydrogen in the absence of a catalyst, the rate of removal of both metals is slower than with a catalyst or in the absence of hydrogen. The shapes of the curves indicate that metals removal is a secondary reaction. After 80% residuum conversion, metals removal is the same in all cases. During runs such as those under discussion, metals can deposit on reactor solids, on any catalyst that is present or on reactor internals. Vanadium mass balance data for the three series are given in Figures 2224. Mass balance data for nickel was very similar in every respect. For the coking runs (Figure 22), the sum of the vanadium in the reactor liquid and solids was close to 1008 of the metals in the feed. Similarly for

Influence of H2 and Catalyst on Distillate Fields

Energy & Fuels, Vol. 8, No. 6, 1994 1283

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Figure 21. Nickel conversion during coking, hydrocracking, hydrocracking in the presence of fresh catalyst, hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 6.

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Figure 24. Vanadium mass balance during hydrogenolysis hydrocracking of Athabasca bitumen residuum at 400 "C, plotted as a h c t i o n of residuum conversion. Symbols as for Figure 16,except crossed box, catalyst.

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Figure 22. Vanadium mass balance during coking of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion: (B) total liquid product, (A) reactor solids, and (0)totals.

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Figure 23. Vanadium mass balance during hydrocracking of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 16. the catalytic runs (Figure 241, the metals on the catalyst plus the metals in the liquids and solids generated was close to 100%although the totals dropped noticeably and

Figure 25. CCR conversion during coking, hydrocracking, hydrocrackingin the presence of fresh catalyst,hydrocracking in the presence of spent catalyst, and coking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 6.

smoothly at high residuum conversions. As expected, most of the metals removed from the feed in the presence of a catalyst are deposited on the ~ a t a l y s tThe .~ hydrocracking runs in the absence of a catalyst are quite different. Only a small amount of solids are formed and only a small percentage of the metals removed from the liquids are accounted for by metals in the solids. Although metals removal is significant a t residuum conversions greater than 50%, the metals which are removed from the feed are lost (Figure 231, i.e., they are not contained in reactor products. CCR Conversion. The CCR conversions from all series are plotted as a function of residuum conversion in Figure 25. In the hydrocracking cases with fresh residuum hydrotreating catalyst, the CCR conversion increases as residuum conversion increases, leveling off between 50 and 70% residuum conversion and appears to become rapid a t residuum conversions greater than 80%. In the coking case, CCR conversion is negative in the early stages of the reaction, up to 50% residuum conversion, then increases to the same level as in the other two hydrocracking cases after approximately 80%


Sanford

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Figure 26. Fraction of carbon that is in aromatic rings in the residuum fractions during coking, hydrocracking, and hydrocracking in the presence of fresh catalyst, of Athabasca bitumen residuum at 400 "C, plotted as a function of residuum conversion. Symbols as for Figure 5.

residuum conversion. CCR conversion in the presence of a fresh residuum hydrotreating catalyst is faster than CCR conversion without a catalyst and remains above the level of all other series up to residuum conversions of approximately 80%. CCR concentration in the heavy gas oil samples from the three series was small (less than 2%) and the amount appeared to be relatively random with respect t o residuum conversion. The amount present is likely a function of the efficiency of the distillation, and would reflect the amount of 524 "C+ residue which was contained in the gas oil. Aromatics Conversion. The fraction of carbon atoms that are in aromatic rings in the residuum fraction as determined by I3C NMR (Figure 26) increases sharply as residuum conversion increases. Clearly, the nonaromatic portions of the molecules are being converted t o distillate, gas, and coke faster than the aromatic portions. The residuum from the coking runs contain a higher fraction of aromatic carbons than the residuum from the hydrocracking runs, and this result was not expected, since the aromatics can form coke. Molecular Weights. An indication of the relative molecular weights of the residua cuts is given by the GPC curves in Figure 27. Throughout the range of the GPC traces, there is a shift to lower molecular weights as the reaction time, and therefore extent of conversion, increases. The GPC curves shown are for the coking runs. Similar curves for the two hydrocracking runs were superimposable on the curves for the coking runs. Average molecular weights, measured by vapor pressure osmometry in o-dichlorobenzene, gave a feed residuum molecular weight of 1486 compared to molecular weights of between 681 and 838 for the residua fraction from selected runs.13

Discussion There are several possible routes by which heteroatoms and aromatics can be removed from the total liquid product (methylene chloride soluble) during the type of (13)Srinivasan, N., unpublished results.

Figure 27. Cumulative gel permeation chromatography traces of the residuum fractions produced during coking of Athabasca bitumen residuum at 400 "C.

conversions being described here. In all five series, residuum is converted to products (gases, distillable liquids, reactor solids, and solids on catalyst) largely in thermal reactions, so that there is always a thermal route possible for removal of heteroatoms. Methylene chloride insoluble reactor solids are also produced in each series,14although in much larger quantities in the coking series6 and incorporation of heteroatoms and aromatics into the carbonaceous solids provides a route for removal of these species from the total liquid product. When a residuum hydrotreating catalyst is present, two more routes are available. Nitrogen, sulfur, metals, and aromatics can all be catalytically removed in the early stages of reaction with fresh catalyst,14under the conditions used. In addition, there is a significant buildup of carbonaceous solids on the fresh catalyst, and these solids provide another route for removal of heteroatoms. The other possible route for heteroatom and aromatics removal is through the decomposition of aromatics by hydrogen atom addition.6 Hydrogen atom addition was proposed as a plausible route leading to the fragmentation of aromatics during hydrocracking and could also lead to heteroatom removal. This reaction is also accelerated by residuum hydrotreating ~ata1ysts.l~ In all except the runs carried out in a nitrogen atmosphere without any added catalyst, there is a rapid removal of sulfur in primary reactions (0-60% residuum conversion) followed by a period of virtually no removal (60-80% residuum conversion) followed by very rapid removal at high residuum conversion ('80%). Most of the sulfur conversion which is achieved, in all four hydrocracking series, occurs during the first 50% of residuum conversion, which is the stage of the reaction when it is believed that side chains are being broken and whole molecules are largely being converted to distillatea6 It should be recalled that in these batch reactions, each data point represents a reaction started with fresh catalyst each time, in reactions where fresh catalyst was used, and that sulfiding and coking of the catalyst occurred each time. When the reaction is carried out in a nitrogen atmosphere (coking) without a catalyst, the observed sulfur conversion of approximately 10%during the first 50% residuum conversion probably represents thermal (14)Sanford, E. C.; J. Catal., submitted for publication.

Influence of Hz and Catalyst on Distillate Fields conversion of the sulfur which is present in Athabasca bitumen residuum in non-thiophenic form, most likely sulfides and disulfides,15 and is present in a portion of the molecule which allows it t o be easily removed. A small amount of carbonaceous solids is formed during this period (approximately 3% of feed) and some of the sulfur is incorporated in these solids (Figure 7). As the coking reaction proceeds beyond 50%residuum conversion, reactor solids increase rapidly to nearly 25%of the feed, and much of the sulfur found in the products is in the solids. Despite this, there is no change in the slope of the curve representing sulfur removed as hydrogen sulfide, indicating that the two reactions are independent. It is somewhat surprising that the hydrocracking series with no added catalyst has higher sulfur removal than the coking series since there is no obvious route for removal of sulfur other than thermal reactions, which are expected to be the same in both cases. The difference between the amount of gas-phase sulfur in the coking reactions and the hydrocracking reactions without catalyst must be due to reaction with hydrogen. The proposed route of residuum decomposition through hydrogen atom addition6J4must be contributing t o the additional sulfur removal, and the present data suggest that this mechanism of hydrogen atom addition may take place even in the early stages of the residuum hydroconversion reaction. If sulfur removal in these reactions is due to a hydrogen atom addition reaction, there must be selective addition to sulfur-containing aromatics to account for the additional sulfur removal when overall residuum conversion is the same in both series. The additional removal of sulfur in the presence of hydrogen and a catalyst would then represent the sulfur in the product liquids which could be readily removed as hydrogen sulfide in catalytic reactions. Since the catalyst was not presulfided, these experiments do not allow one to distinguish between catalytic removal of sulfur as hydrogen sulfide and directly sulfiding the catalyst through reaction of the metal oxides with sulfur in the feed. Most of the sulfur is removed in the first hour of reaction (50% residuum conversion) and ends up on the catalyst (Figure 9). Since each run is started with fresh catalyst, the same reactions would occur in the first hour of each run. As reaction times are extended beyond 1 h, very little additional sulfur is removed from the liquids, and what is removed ends up as gas-phase sulfur, likely hydrogen sulfide. The catalyst appears to have lost its sulfur removal active sites in the first hour, in keeping with previous findi n g ~ The . ~ ~removal of sulfur after the first hour is likely due to the same hydrogen atom mechanism proposed for the reaction with hydrogen above. If the amount of hydrogen sulfide formed in the coking reaction (Figure 7) is subtracted from the amount of hydrogen sulfide formed in the hydrocracking reaction (Figure 81, the difference is comparable to the amount of hydrogen sulfide formed in the latter stages of the hydrogenolysis hydrocracking reaction (Figure 9). The series with spent catalyst is more representative of what would happen in a continuous reactor in that (15) Strausz, 0. P. Bitumen and Heavy Oil Chemistry. In AOSTRA Technical Handbook on Oil Sands, Bitumens and Heavy Oils; Hepler,L.G., Hsi, C., Eds.; AOSTRA, Edmonton, 1989.

Energy & Fuels, Vol. 8, No. 6, 1994 1285 the fresh feed is in contact with a deactivated catalyst. In this case, the catalyst is significantly deactivated and sulfur removal from the liquids is the same as for hydrogenation reactions with no catalyst (Figure 6). For a run in a continuous reactor starting with a fresh catalyst, the results would be expected to be intermediate between the fresh and spent catalyst data in Figure 6 since only the activity for conventional hydrotreating would be lost in the first hour.14 Sulfur removal using a fresh catalyst with a nitrogen atmosphere is similar to the hydrogen data with no catalyst but is likely due to coke buildup on the catalyst, comparable to sulfur in the coke on the catalyst in the presence of hydrogen. The difference between the sulfur concentrations in the N/LGO distillable liquids produced in the presence of nitrogen and those produced in the presence of hydrogen without a catalyst clearly demonstrate that the distillable liquids are produced by a different mechanism as suggested above. The early stages of coking reactions is expected to mainly break side chains which would be low in sulfur and produce a low sulfur distillate, as observed. If distillable liquids are produced by hydrogen atom addition to condensed aromatic centers, then more thiophenes could be included in distillate fractions, resulting in a higher sulfur distillate. During the period between 50 and 80% residuum conversion (Figure 61, very little additional sulfur is removed from the reactor liquids in the presence of hydrogen. Residua molecules which are being converted are thought to be highly condensed naphthenic and aromatic compounds and gas formation increases relative to distillate production.6 The data presented here indicate that the molecules which are converted during this stage are not the thiophenic aromatics but rather hydrocarbons. In keeping with the low conversion of sulfur overall during this period, the sulfur concentration of the distillable liquids continues to drop (except for naphtha/LGO from the coking run, as discussed above). During the 50430%residuum conversion stage, thiophenic sulfur would appear to inhibit homolytic bond cleavage in sulfur-containing molecules. During this period, one would expect the concentration of sulfur in the residuum fraction of the liquid products to increase (Figure 12). However, there is too much scatter in the data to determine if this is so. Above 80% residuum conversion, the residuum is highly aromatic (70-75% of the total carbon is in aromatic rings, Figure 26) and sulfur conversion appears to increase rapidly. This increase indicates that destruction of aromatics containing sulfur must be taking place, probably in thermal reactions (through hydrogen atom addition) since the relative rate of sulfur conversion appears to be the same for all series. It is possible that at this stage of residuum conversion all molecules contain at least one sulfur atom, so that any conversion of residuum results in sulfur conversion. There is a noticeable increase in the sulfur concentration of the heavy gas oil fraction during this period of residuum conversion. When the residuum fractions are considered (Figure 121, it is not clear that there is any catalytic removal of sulfur from this fraction, although the scatter in the data does not rule out some catalytic reaction. Clearly, most of the sulfur which is removed in catalytic reactions is from the distillate fractions, as observed previ-

1286 Energy & Fuels, Vol. 8, No. 6, 1994 0us1y.~The general trend toward lower sulfur in the residuum as residuum conversion increases probably reflects the sulfur conversion in the early stages of the reaction. Nitrogen is present in Athabasca bitumen residuum in aromatic rings, namely pyrrole and pyridine structures.l5 Little if any nitrogen from such molecules would be expected t o be removed in noncatalytic reactions to produce ammonia. However, nitrogen is removed from the total liquid product (Figure 13) in the early stages of coking reactions. Since the main mechanism for nitrogen removal in this case is expected to be through selective incorporation of nitrogen-containing molecules into carbonaceous solids which likely form from aromatics, and since nitrogen is contained in aromatic rings, the loss of nitrogen is expected to be thorough coke-forming reactions. This expectation is confirmed in Figure 14 which shows that all of the nitrogen which is removed from the liquid product during coking reactions is accounted for by nitrogen in the reactor solids. At the same time, the distillate which is produced is lower in nitrogen than the original residuum (Figures 17 and 181, again indicating the selectivity for nitrogen containing aromatics to form solids. With hydrogen and a hydrotreating catalyst, nitrogen can be removed through catalytic reaction12 as well as through carbonaceous solids being formed on the catalyst. During this initial period of residuum conversion, the buildup of carbonaceous solids on the catalyst is rapid6 and exceeds the amount of carbonaceous solids formed in the coking reaction. This buildup of solids would likely deactivate the catalyst toward nitrogen rem0va1.l~It seems unlikely that there would be any actual catalytic removal of nitrogen under these conditions, and the data in Figure 15 confirms this assumption. All of the nitrogen which is removed from the liquid product is accounted for by the selective incorporation of nitrogen into the solids in the reactor and on the catalyst. Nitrogen removal from the total liquid product in the presence of a fresh residuum hydrotreating catalyst in a nitrogen atmosphere is the same as in the presence of hydrogen (Figure 131, in keeping with the mechanism being one of deposition of solids on the catalyst, rather than any hydrogenation reaction. The heavy gas oil produced in the two series under discussion contained similar concentrations of nitrogen (Figure 18) as did the residua (Figure 19). However, the naphthaLG0 fraction contained more nitrogen when the reaction was carried out in the presence of a catalyst. This result again indicates that catalytic removal of nitrogen is not important in these reactions. The higher concentration of nitrogen in the lighter distillate may be the result of a different mechanism in the presence of hydrogen compared to nitrogen as discussed for the sulfur case. When hydrogen is added to the system without a catalyst (Figure 131, carbonaceous solids formation is suppressed and simultaneously the route for removal of nitrogen is also removed. There is virtually no nitrogen removal up to 60%residuum conversion. The lack of nitrogen removal from the total liquid product is reflected in the HGO cut which is higher in nitrogen than the corresponding cuts from the other runs.

Sanford

As overall residuum conversion increases beyond 60%, nitrogen removal increases rapidly and at the same time, the nitrogen concentration in all three liquid products increases. The mechanism is likely the same as for sulfur removal during this period, i.e., thermal conversion of residuum molecules, especially aromatics, through the reaction of hydrogen atoms.6 However, for both the coking case and the catalytic hydrocracking case, nitrogen removal from the total liquid product is still accounted for by nitrogen in the solids formed. In the case of hydrogen alone, nitrogen is lost from the system (Figure 161, presumably as ammonia. With a spent plant catalyst, nitrogen removal is intermediate between that observed for fresh catalyst and no catalyst and probably reflects some buildup of solids on the catalyst. Although vanadium and nickel removal (Figures 20 and 21) appear to be similar to nitrogen removal, there is one significant difference. In addition to removing metals through carbonaceous solids formation, there is potentially a purely thermal route available to remove metals from the organic matrix which would result ultimately in the formation of metal sulfides. These metal sulfides could remain suspended in the reaction medium or become deposited on reactor solids, including catalyst. In the coking reactions, mass balance data (Figure 22) show that the vanadium concentration in the solids formed corresponds with metals removal from the liquids. The low removal of metals in the first 35% of residuum conversion corresponds t o the low rate of formation of solids during this period. The implication is that metals are removed through a mechanism of incorporation into the carbonaceous solids. In the presence of a spent catalyst in a hydrogen atmosphere or a fresh catalyst in an inert atmosphere, metals removal from the liquids is very similar to the coking case. However, solids formation is somewhat different. With a fresh catalyst in an inert atmosphere, solids formation on the catalyst is rapid at the beginning of the reaction. With a spent catalyst which already contains a significant amount of coke, solids buildup is more uniform over the entire reaction. The coking run and the two catalyst runs taken together indicate that metals are removed from the liquid product in thermal reactions which are independent of hydrogen. The solids in the reactor, whether they are carbonaceous solids or catalyst, merely act as collectors for the metals, which are probably in the form of metal sulfides. With a fresh catalyst and hydrogen, there is some indication of catalytic action in the first half hour of reaction (40% residuum conversion). Because of the nature of the batch system, reaction can take place on the fresh catalyst during the heat-up period, before significant catalyst deactivation takes place. After 1 h reaction time (50% residuum conversion), metals removal is the same as for the coking and spent catalyst cases. Metals removed from the total liquid product are largely accounted for by metals on the catalyst. Again it seems likely that the catalyst is mainly acting as a collector for vanadium and nickel which is removed from the organic matrix in thermal reactions. In the absence of a significant amount of solids in the reactor (hydrocracking, no catalyst), metals removal appears to be slower than in the other cases (Figures


20 and 21) and the metals which are removed from the liquid product are lost from the system (Figure 231, presumably by depositing on the reactor walls or internals. In view of the above discussion, it seems reasonable that metals are removed in this series in thermal reactions similar to the other series. The difference may be that in the absence of an efficient collector, finely divided solids pass through the filter and remain in the liquid and give the appearance of a slower reaction. When CCR conversion is plotted as a function of residuum conversion (Figure 251, the shape of the plot with hydrogen and a fresh catalyst is similar to the plots for heteroatom removal in that there is an initial rapid increase in CCR removal in the two hydrocracking runs (0-45% residuum conversion) followed by a period with very little conversion (45-75% residuum conversion) and again followed with very rapid conversion. Under these conditions, the 10% conversion in the hydrogen without catalyst series probably represents thermal reaction with hydrogen and may be a reflection of solids formation in the reactor and on the catalyst in the early stages of the reaction. The 35% CCR conversion in the hydrogen plus catalyst series would represent catalytic hydrogenation plus thermal reaction. The catalytic portion of the conversion is confounded by the fact that large amounts of carbonaceous solids are deposited on the catalyst during this period,6and part of these solids are most likely from CCR-forming molecules, thus contributing to overall CCR removal. The mechanism of the conversion in the presence of hydrogen (no catalyst) has been discussed previously6 and is likely due to hydrogen atom addition to aromatics in the latter stages of the reaction. Without hydrogen, CCR conversion likely reflects solids formation. It is perhaps unexpected that during the first 50% of residuum conversion in the coking series, when approximately 10%of the feed is converted to solids,6CCRforming molecules are actually generated in the reaction. The expectation is that solids are formed from CCR-forming molecules. After 50% residuum conversion, CCR-forming molecules are destroyed rapidly, as in the two hydrocracking series. Mazza and Cormack16 have shown that the resin and asphaltene fractions of Athabasca bitumen are interchangeable under thermal conditions so it is not surprising that CCR-forming molecules are produced. In the absence of hydrogen, the catalyst does not have much effect on CCR conversion. The presence of hydrogen with fresh catalyst, or no catalyst, suppresses the reactions which lead to formation of CCR-forming molecules (dehydrogenation), probably through radical capping reactions, as well as allows hydrothermal conversion of the CCR species already present.6 The increase in CCR-forming molecules in the coking series is supported by the concentration of aromatics in the residuum fraction which is higher for the coking series relative to the hydrocracking reactions in the presence and absence of a catalyst. The literature on thermal and hydrocracking processes often refers to “coke precursors” as intermediates which are produced during the sequence of reactions which leads to the formation of coke. For example, Lott and Cyr17 propose a model for the formation of coke that (16)Mazza, A. G.; Cormack, D. E. AOSTRA J.Res. 1988,4, 193208.

Energy & Fuels, Vol. 8, No. 6,1994 1287 involves “formation of a new liquid phase rich in coke precursors”. Since “coke” which is generated during upgrading of petroleum residua is generally defined in terms of its solubility in the reaction medium or a specified solvent, one would expect “coke precursors” to be large, high molecular weight molecules which are on the verge of being insoluble in the reaction medium. An example of the GPC chromatograms of the residuum fractions from the coking experiments described here (Figure 27) shows a continuous decrease in molecular size as conversion increases, and the GPC traces were virtually identical for all three series, i.e., coking, hydrocracking and hydrogenolysis hydrocracking. There is no evidence for the formation of any high molecular weight intermediates.

Conclusions There was only small differences in the amounts of distillable liquids produced in the three series carried out under nitrogen, hydrogen, or hydrogen in the presence of a fresh hydrotreating catalyst. Distillate yields appeared to reach a maximum a t around 70% residuum conversion. The amount of heavy gas oil produced decreased as the reaction time became longer, resulting in corresponding larger amounts of lighter distillable liquids. The removal of sulfur, nitrogen, and CCR can be divided into three phases: up to 50% residuum conversion, 50-70% residuum conversion, and greater than 70% residuum conversion. Most of the sulfur removal takes place during the first 50% of residuum conversion in the presence of hydrogen and mainly takes place from the produced distillable liquids with little difference in sulfur removal from the residuum fraction for the coking and hydrocracking series. Sulfur removal during coking is due to a combination of thermal reactions which produce hydrogen sulfide (up to 12% of the feed sulfur at 70% residuum conversion) and condensation of sulfur containing molecules which eventually form reactor solids. Sulfur removal in the presence of hydrogen with no catalyst or with a spent catalyst is a combination of thermal removal which produces hydrogen sulfide as in the coking reaction, and a hydrogenation reaction, probably through a hydrogen atom addition mechanism, accounting for 20%of the total sulfur at 70%residuum conversion. In the presence of a fresh residuum hydrotreating catalyst, 35-40% of the sulfur in the feed is removed early in catalytic reactions, and all of the sulfur remains on the catalyst as metal sulfides or sulfur in coke on the catalyst. After these early reactions, an additional 10% of the sulfur in the feed is removed, probably through-hydrogen atom reactions, similar to the case without catalyst. After 70% residuum conversion, sulfur is removed rapidly, probably in hydrothermal reactions. In purely thermal reactions, nitrogen is removed through preferential incorporation into coke with the hydrocarbon portion of the molecules giving a naphtha which is lower in nitrogen. In the presence of hydrogen, (17)Lott, R.; C y , T. J. In Proceedings, International Symposium on Heavy Oil and Residue Upgrading and Utilization; Chongren, H., Chu, H., Eds.; International Academic Publishers: Beijing 100044, Peoples Republic of China, 1992; pp 309-315.


there is no pathway for removal of nitrogen and all liquid fractions contain more nitrogen compared to the coking and hydrogenolysis hydrocracking reactions. Nitrogen removal in the presence of residuum hydrotreating catalyst is thought to be due largely to nitrogen incorporation into solids deposited on the catalyst. Nitrogen removal in all three series mimics sulfur removal after 80% residuum conversion. There was no evidence for catalytic nitrogen removal. Vanadium and nickel removal from the residua during cracking appears to be as a result of thermal reactions with the metals depositing on reactor solids including catalyst, and reactor internals. Some catalytic reaction appears to take place in the early stages of the reaction. The amount of CCR formed from the various residua increased initially in the coking reactions, then de-

Sanford creased, whereas in the catalytic hydrocracking reactions, CCR removal was initially rapid, followed by a period of no removal, followed again by a period of rapid removal. The hydrotreating catalyst may increase CCR conversion in the early stages of the reaction. There was no evidence to support the formation of high molecular weight “coke precursors” in the liquid products from any of the reactions.

Acknowledgment. The author is indebted to Mary Chan, Ken Douglas, and Kevin Pollitt for their thorough and careful experimental work. The assistance of Milan Selucky and Naras Srinivasan with the NMR, molecular weight, and GPC data is acknowledged. The author thanks Syncrude Canada Ltd. for permission to publish and the Alberta Hydrogen Research Program of the Department of Energy for a funding contribution for a portion of this work.

Influence of Hydrogen and Catalyst on Distillate Yields and the

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