Energy & Fuels 1996,9, 928-935
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Residuum Hydrocracking with Supported and Dispersed Catalysts: Stable Hydrogen and Carbon Isotope Studies on Hydrogenation and Catalyst Deactivation Emerson C. Sanford* Syncrude Canada Ltd., 9421-17 Ave., Edmonton, AB T6N lH4, Canada
James G. Steer and Karlis Muehlenbachs Department of Geology, University of Alberta, Edmonton, AB T6G 2E3, Canada
Murray R. Gray Department of Chemical Engineering, University of Alberta, Edmonton, AB T6G 2G6, Canada Received March 15, 1995@
The generally accepted mode of action of Ni- or Co-MoIAlzOs catalysts during hydrocracking of residua is that the catalyst participates in the hydrogenation of aromatics which results in heteroatom removal and CCR conversion. Earlier studies have shown that under hydrocracking conditions with residua, the catalyst loses the ability to hydrogenate aromatics within hours, and thereafter catalysis proceeds by a different mechanism. The established techniques of stable isotope analyses have been used to provide additional support for the proposed mechanism. The isotope studies showed that in the absence of a catalyst, there was no incorporation of gaseous hydrogen into the residuum fraction and only a small amount into the distillates. With a fresh catalyst, hydrogen was incorporated into all fractions initially but hydrogenation of residuum lasted for only 4 h. When bitumen residuum was hydrogenated under mild conditions, the added hydrogen was not lost under hydrocracking conditions. Addition of gaseous hydrogen to the residuum was dependent on the concentration of catalyst for dispersed catalysts, and catalysts previously used under mild conditions in the pilot plant did not show any hydrogen transfer from the gas phase t o the residuum fraction. In all cases, carbon-to-carbon bond breaking was correlated with the incorporation of gaseous hydrogen.
Introduction The hydrocracking process as it is used commercially to convert Athabasca bitumen residuum t o distillable liquids1 has as one of its main objectives t o remove residue forming molecules (measured as Conradson Carbon Residue (CCR)) from the feed. Any CCRforming molecules which are not removed contribute to coke formation when the bottoms are processed in a fluidized bed coker.2 Of course, conversion of CCRforming molecules during hydrocracking is only of value if the molecules are converted to distillable liquids and not to gases. It is thought that the large condensed aromatic structures in residua contribute to coke formations and that during catalytic hydrocracking at least some of the rings are hydrotreated to the corresponding naphthenes, which can be converted in subsequent thermal reactions to distillable liquids. The thermal
* Author for correspondence Present address: Department of Chemical Engineering, 536 Chemical-Mineral Engineering Bldg., University of Alberta, Edmonton, AB T6G 2G6, Canada. Phone: 403492-7963. FAX: 403-492-2881. E-mail:
[email protected]. @Abstractpublished in Advance ACS Abstracts, August 1, 1995. (1)Bishop, W.; Smart, M.; James, L. C.; McDaniel, N. K. NPRA Annual Meeting, March 17-19, San Antonio, TX,1991, AM-91-56. (2) Kirchen, R. P.; Sanford, E. C.; Gray, M. R.; George, 2. M. AOSTRA J . Res. 1989,5,225-235. (3) Beaton, W. I.; Bertolacini, R. J. Catal. Rev.-Sci. Eng. 1991,33, 281-317.
conversions may take place either in the hydrocracker or in the coker. It has always been assumed that during residuum hydrocracking, a t least some of the residue forming aromatics (CCR forming molecules) are hydrogenated to the corresponding n a p h t h e n e ~ ,by ~ analogy with distillate hydrotreating, since the catalyst is chemically the same (pore volume, surface area, and metals level may Mer). However, there are many conditions present in residuum hydrocracking that would normally limit aromatic hydrogenation reaction^;^ for example, the high temperature required for cracking reactions (400450 "C) coupled with the heteroatom levels in the residuum and the coke forming tendency of the residuum itself would all tend t o deactivate the catalysts the high temperatures are thervery q u i ~ k l y .Also, ~ modynamically unfavorablefor aromatic hydrogenation6 and would in fact be expected to result in some dehydrogenation. Steer et al.7 developed a method to quantitatively determine the amount of coal incorporated into the (4) Sanford, E. C. Energy Fuels 1995, 9, 549-559. ( 5 ) Absi-Halabi, M.; Stanislaus,A.; Trimm, D. L. Appl. Catal. 1991, 72, 193-215. (6)Yui,S. M.; Sanford, E. C. Can. J . Chem. Eng. 1991, 69, 10871095. ~...
(7) Steer, J. G.; Ohuchi, T.; Muehlenbachs, K. Fuel Process. Technol. 1987,15, 429-438.
0887-0624l95I2509-0928$09.0QlQ 0 1995 American Chemical Society
Residuum Hydrocracking with Catalysts
synthetic oil generated by coprocessing Alberta coal and Athabasca bitumen, using 13CP2Cratios in the coal and bitumen. The method of using stable isotopes was extended to study hydrogen transfer during coal liquefaction8 by measuring the natural 2Hcontents of the hydrogen gas, tetralin, and coal. The method was also used by Steer et al.9 t o measure hydrogen transfer during catalytic hydrocracking of residua from four Alberta sources. The latter studies showed that both the distillate fractions and the residuum fraction from Athabasca bitumen were significantly enriched in hydrogen from the gas phase when processed in the presence of a residuum hydrocracking catalyst. The authors concluded that "scrambling" of isotopic data due to rapid exchange reactions was relatively slow under the conditions used and that the method could be used t o determine the distribution of hydrogen added to product fractions. The objective of the present study was to use the techniques developed in the previous studies to determine the extent of addition of hydrogen from the gas phase to the distillable fractions and the residuum during hydroprocessing of Athabasca bitumen, when catalysts of different activities and types were used under different conditions. Carbon isotope ratios were measured as well to provide information on carbon-tocarbon bond breaking which may take place in conjunction with hydrogenation. The extent of hydrogenation and bond breaking as a function of catalyst activity was expected to provide valuable information on the role of the catalyst during hydrocracking.
Experimental Section The apparatus used for the batch experiments reported here has been described previously.1° All experiments were carried out a t 370 or 400 "C under 10 MPa hydrogen partial pressure with reaction times varying from 60 to 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. In the cases where gas generation was greater than gas consumption, normally in the later stages of the reaction when most catalytic reactions were nearly complete,1° 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 in the presence of a catalyst, the catalyst (75g of a commercial Ni-Mo y-alumina residuum hydrotreating catalyst 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 as a result of reaction with the feed during the heat-up period. 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. m e r the specified reaction time, the reactor was cooled and a sample of gas removed for analysis. The remainder of the (8)Kamo, T.; Steer, J. G.; Muehlenbachs, K. Prepr. Pup.-Am. Chem.
Soc., Diu.Fuel Chem. 1991,36,1259-1265. (9) Steer, J. G.; Muehlenbachs, K.; Gray, M. R. Energy Fuels 1992, 6, 540-544. (10)Sanford, E.C.; Chung, K. H. AOSTRA J. Res. 1991,7,37-45.
Energy & Fuels, Vol. 9, No. 5, 1995 929 gas was vented and the contents of the reactor were submitted directly to the analytical laboratory for analyses, except for the runs without catalyst or with added molybdenum sulfide. For these runs, the total liquid product was slurried with methylene chloride and filtered. The solvent was then removed from the filtrate on a rotary evaporator. The runs with finely dispersed molybdenum sulfide were carried out by adding a preprepared concentrated mixture of phosphomolybdic acid in bitumen to regular feed bitumen to give a mixture resulting in either 250 or 125 ppm Mo in the bitumen. The molybdenum was presumed t o become sulfided by reaction with the sulfur in the bitumen. The catalyst from the catalyst ageing runs was removed from the reactor after the first 2 h run and cleaned by extracting with toluene in a Soxhlet extractor. The cleaned and dried catalyst was then used for the second run with fresh bitumen and the procedure repeated. The two-stage runs were carried out by first hydrotreating bitumen with fresh catalyst a t 370 "C for 4 h, with low deuterium hydrogen in one case and regular hydrogen in the other case, then cooling to room temperature, and removing the catalyst basket. Bitumen was allowed to drain from the basket into the reactor. The bitumen in the reactor was then heated under regular hydrogen for 2 h a t 400 "C. The feeds and products were analyzed for carbon' and hydrogens isotopes by conventional methods of stable isotope geochemistry, using isotope ratio mass spectroscopy. Vycor breakseals containing 10-20 mg of hydrocarbon, 2.5 g of CuO, and small strips of pure-Cu and Ag (to remove sulfur and nitrogen oxides) were evacuated to Torr and sealed with a torch. The breakseals were placed in a furnace preheated to 800 "C for 20-24 h and then cooled gradually over 24 h. Gases in the tube were extracted in a vacuum line by freezing the sample in liquid nitrogen and then breaking the seal. Noncondensable gases were pumped away. Carbon dioxide was separated from water vapor by immersion of the break seal in a dry ice ethanol trap and refreezing the carbon dioxide in a liquid nitrogen trap. The 13C/12Cratio of the carbon dioxide gas sample was measured by mass spectrometry. Water of combustion was removed from the breakseal by heating in a mineral oil bath at 80 "C. The evolved water was frozen into a sample tube containing 0.5 g of Zn metal and then converted to hydrogen gas for mass spectrometric analysis. The data for hydrogen are reported as parts per thousand (ppt) relative to the international standard of standard mean ocean water (SMOW) by the following equation:
where [D/H]s~ow = 1.558 x The error of replicate analyses was f3 ppt. The carbon data are reported with respect to the PDB7 standard for carbon where 13C/12C = 1123.8 x The hydrogen used for most of this work was an electrolytic hydrogen which was very low in deuterium (6D = -480 ppt) relative to ordinary hydrogen prepared from natural gas (6D = -330 ppt). Both have low deuterium relative to the natural abundance of deuterium in Athabasca bitumen (6D = -122 to -134 ppt).
Results The conditions for the batch hydrocracker runs and heteroatom and residuum conversions are given in Table 1. Runs 14-16 were carried out at 400 "C for 2 h in the presence of a used catalyst. Residuum conversion for the three runs was 52 f 2%. Similarly, runs 5 and 17 to 19,under the same conditions gave residuum conversions of 53 f 3%. Run 2, carried out under the same conditions without any catalyst, had a residuum
930 Energy & Fuels, Vol. 9,No.5, 1995
Sanford et al.
Table 1. Reaction Conditions and Heteroatom, CCR, and Residuum Conversions for Batch Stirred Tank Reactor Runs run no.
feed catalyst catalyst temp time sulfur nitrogen CCR V Ni residuum (g) type (g) ("C) (min) (%) (%I (%I (%I (%) (%I 1 400 400 60 11.8 8.7 7.5 0.9 4.3 28.1 0.8 3.2 50.1 54.8 43.2 2 300 400 120 17.8 400 240 19.2 -1.2 9.3 43.7 41.8 59.0 3 400 4 400 CHC" 100 400 60 75.9 35.7 55.6 95.2 92.1 37.2 5 400 CHC 100 400 120 79.5 41.0 69.1 99.3 100.0 51.1 6 400 CHC 100 400 240 74.2 45.6 79.8 99.5 98.1 52.8 7 300 Twostageb 75 370 240 8 300 HC-nocat 400 120 55.9 20.7 48.9 66.9 71.1 52.4 9 300 TwostageC 75 370 240 10 300 HC-nocat 400 120 54.8 19.2 43.4 81.7 79.4 47.0 11 400 CHC 100 370 240 72.2 34.8 62.9 98.0 95.1 31.1 12 300 DispMoSd 15 400 120 28.0 7.8 15.6 46.9 50.3 38.4 1.1 15.3 38.3 44.6 40.5 13 300 DispMoS 9 400 120 23.2 Z-1SP 100 400 120 28.2 8.7 5.5 74.7 70.7 53.4 14 300 15 300 Z-2NDf 100 400 120 30.3 4.7 69.8 66.9 52.5 16 300 SpentCHCs 100 400 120 15.7 7.3 3.5 67.0 68.0 48.9 17 400 CHCh 400 120 66.9 21.3 63.1 97.8 75.2 57.2 18 400 CHCh 400 120 64.4 11.6 51.6 84.7 87.1 52.3 19 400 CHCh 400 120 63.4 9.2 54.1 80.3 83.2 52.1 CHC: hydrocracking reaction in the presence of Ni-Moy-AlzO3 catalyst. Two stage: bitumen was hydrotreated in the first stage (run 7) with low D hydrogen, then hydrocracked in the second stage (run 8 ) without any catalyst and normal hydrogen. Normal hydrogen used in both stages. Disp MoS: dispersed molybdenum sulfide. e Z-1ST: catalyst from first stage of pilot plant hydrotreating run. f Z2ND: catalyst from second stage of pilot plant hydrotreating run. g Spent CHC: deactivated catalyst recovered from the commercial plant. Series in which the catalysst was used repeatedly with fresh bitumen (first run is run 5).
conversion of 43%, and the 12th and 13th runs, again under the same conditions except with a molybdenum sulfide additive, gave residuum conversionsof 39 f 1%. The catalyst appears t o increase the overall residuum conversion as has been observed previously in pilot plant ~ t u d i e s .Interestingly, ~ the two-stage runs (runs7-10), which had catalyst present only during the initial hydrotreating stage, where there is only approximately 20% residuum conversion, gave an overall conversion similar to the runs with catalyst. This may be due to a combination of conversion during the hydrotreating stage and hydrogenation of aromatics which are converted later in the second stage. Similar results have been observed in previous studies of two-stage processing.ll The first three runs were carried out without added catalyst at 400 "C for times varying from 60 t o 240 min. Sulfur conversions approached 20% under these conditions but nitrogen and CCR conversions were low without any catalyst. Metals normally collect on the catalyst and conversions can then be measured. Without any catalyst, metals in the form of finely divided sulfides remain suspended in the liquid producP as was the case for run 1. Filtration collects at least some of the metals which are removed from the bitumen in thermal reactions. Measured conversions in these cases (runs 2 and 3) were approximately 50%. Runs under similar conditions with fresh oxidic catalyst (runs 4-6) gave much higher conversions for all heteroatoms and for CCR, as expected. Sulfur conversions were close to 80% and nitrogen approximately 40%, and CCR conversions ranged from 56 to 80%. Metals removal was essentially complete under these conditions. Both in the presence and absence of catalyst, reaction time was not a significant factor in heteroatom and CCR conversions, indicating that these conversions take place rapidly near the beginning of the run. (11)Beret, S.; Reynolds, J. G. Fuel Sci. Technol. Int. 1990,8, 191219.
(12)Sanford, E. C . Energy Fuels 1994,8, 1276-1288.
The one run under hydrotreating conditions (run 11) gave conversions which were comparable to the runs at 400 "C with catalyst. Most of the catalytic reactions appear to take place early in the experiment and the results here, together with the time series discussed above, support this conclusion. By the time the reactor reaches 400 "C, most of the catalytic reactions have taken place. The two two-stage runs (runs 7-10) would be expected to be similar to run 11 since the first stage of the runs was the same. However, heteroatom and CCR conversions were lower in the two-stage runs for no apparent reason. Sulfur and CCR conversions were higher in the presence of the dispersed molybdenum sulfide catalyst (runs 12 and 13)than without any catalyst (run 21, but considerably lower than with a fresh residuum hydrocracking catalyst (run 5). Nitrogen and metals removal were comparable to the no-catalyst runs. The two catalysts which were used under hydrotreating conditions in the pilot plant (runs 14 and 15)showed higher sulfur and metals conversions than the runs with no catalyst but otherwise were comparable. The spent catalyst from the commercial plant (run 16)was comparable to no catalyst, except for metals removal which was slightly higher. The four runs in the series with the same catalyst (runs 5 and 17-19) showed steadily decreasing conversions for heteroatoms and CCR while residuum conversion remained reasonably constant, indicating increasing catalyst deactivation over an 8 h period. The ratio of percent CCR conversion to percent residuum conversion4changed from 1.35t o 0.99in only 4 h of catalyst use under cracking conditions. Deuterium isotope data for the series of runs are given in Table 2 and the corresponding carbon isotope data are given in Table 3. Repeat data on the samples are given where available, and an average and standard deviation is calculated. In general, standard deviations are f 6% or less but occasionally go as high as f 11%. When the data are plotted, smooth curves are generally observed, but occasionally data are significantly off the line and appear to be outliers. The reason for these
Energy & Fuels, Vol. 9, No. 5, 1995 931
Residuum Hydrocracking with Catalysts Table 2. GDeuterium Isotope Data for Batch Stirred Tank Reactor Runsa Nb LGO"
