Energy & Fuels 1992,6, 540-544
540
Stable Isotope Analysis of Hydrogen Transfer during Catalytic Hydrocracking of Residues James G. Steer and Karlis Muehlenbachs Department of Geology, University of Alberta, Edmonton, Alberta, T6G 2E3 Canada
Murray R. Gray* Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, T6G 2G6 Canada Received November 20, 1991. Revised Manuscript Received June 1, 1992
Four Alberta residues (424 "C+) were hydrocracked at 430 "C and 13.9 MPa in a continuous flow reactor over Ni/Mo catalyst. Feed and product fractions were analyzed for deuterium and hydrogen by isotope ratio mass spectrometry to determine where gas-phase hydrogen had been added. The distillate fractions were significantly enriched in hydrogen from the gas phase, relative to the residue fractions. Hydrogen addition to the 525 "C+ residue fraction depended on the feed source; Athabasca and Peace River were hydrogenated while Cold Lake and Lloydminster gave net dehydrogenation.
Introduction
The critical role of catalysts in hydrocracking of residues is to transfer hydrogen to liquid-phase c0mponents.l To optimize process design with respect to both catalyst performance and thermal reactions, we must know where in the liquid product the additional hydrogen appears. If all the product fractions contain the same amount of gasphase hydrogen, then either the catalyst has broad activity in all boiling ranges or the exchange of hydrogen between liquid-phasecompounds is very rapid. On the other hand, a catalyst which is ineffective for promoting reactions of high-molecular-weight componenta would preferentially transfer hydrogen to the distillate products. Such insight into the distribution of the added hydrogen cannot be obtained directly from a balance on hydrogen content. For example, consider the naphtha fraction from hydrocracking of residue. When the naphtha has more hydrogen than the feed residue, we cannot assume that this difference is simply due to transfer from the gas phase because in general the elemental composition of any cracked fragments will difffer from the original residue. Similarly,comparison of the hydrogen content in naphtha from thermal versus catalytic processing is complicated by differingyields and heteroatom contentsof each product fraction. One method for measuring where the gas-phase hydrogen is incorporated is to use a label that can be detected in the liquid products. For example, Kabe et al.2-4used tritium as a label to study the distribution of hydrogen from the gas phase into the products of coal liquefaction.
* Author for correspondence.
(1) Miki, Y.; Yamadaya, S.; Oba, M.; Sugimoto, Y. J. Catal. 1983,83, 371-383. (2) Kabe, T.;Nitoh,O.;Funatsu,E.;Yamamoto,K.FuelProcess. Technol. 1986,14,91-101. (3) Kabe, T.; Nitoh, 0.;Funatsu, E.; Yamamoto, K. Fuel 1987, 66, 1326-1329. (4) Kabe,T.; Kimura, K.; Kameyama, H.; Ishihara, A.; Yamamoto, K. Energy Fueb 1990,4, 201-206.
0887-0624/92/2506-0540$03.00/0
Tritium labeling was used to quantitate the transfer of gas-phase hydrogen to solvent, exchange reactions with the gas phase,2 and the role in Ni-Mo on A1203catalyst in hydrocracking of coaL3s4 Tritium labeling suffers from two drawbacks: the introduction of radioactive labels into most test reactors is undesirable for safety reasons, and the tritium reactions may be subject to a strong kinetic isotope effect. The use of stable isotopes (deuterium and hydrogen) that occur naturally in the feed is much simpler. This approach has been used to study coprocessing of coal and oil5 and hydrogen transfer from Tetralin to coale6 In each case, the natural abundance of deuterium was sufficiently different between the gas and liquid phase to allow material balance calculations. The objective of the present study was to use the difference in deuterium concentration between the hydrogen gas and liquid feeds to determine the distribution of gas-phase hydrogen, as a function of boiling range, in the liquid products from catalytichydrocracking of residue. Isotopic analysis of feeds and products from experiments on four different Alberta residues was used to assess the importance of hydrogen exchange between liquid-phase components (i.e., scrambling of the isotopes) and the effectiveness of the catalyst in promoting hydrogenation of the different liquid fractions. Materials and Methods Residues from Athabasca (ATH),Cold Lake (CL),Lloydminster (LL),and Peace River (PR,all 424 OC+ fractions) were hydrocrackedover 8 g of commercial Ni/Mo catalyst in a 150-mL continuous-flow stirred reactor. Details of operation, residue composition, reactor performance, and feed and product com(5) Steer, J. G.; Ohuchi,T.; Muehlenbachs, K. Fuel Process. Technol. 1987,15,429-438. (6) Kamo,T.; Steer, J. G.; Muehlenbachs, K. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1991,36 (3), 1259-1265.
0 1992 American Chemical Society
Catalytic Hydrocracking of Residues
Energy & Fuels, Vol. 6, No. 5, 1992 541
positions have been reported previously.' The catalyst was presulfided in place using an H&H2 mixture. Each experiment lasted 6-8 h, during which time the sulfur content of successive product sampleswaa checkedto verify that steady-stateoperation had been achieved. Liquid and gas feed and product flows were measured continuously. Liquid densities were measured by an Anton Parr Density Meter (ModelOMA 02C).The gas products were routed to a gas chromatograph(HP Model 5840A) equipped with two detectors and automatic switching of columns for refinery gas analysis. Hydrogen was detected by TCD in the hydrogen/helium carrier gas. c1-C~hydrocarbons were detected by flame ionization detection (FID). Hydrogen sulfide was measured by absorption in 1 N NaOH, followed by iodometric titration. The overall gas composition was calculated by normalizing the Hz and C& concentrations to make 100 mol % when added to the measured HzS concentration. Feeds and products were distilled to give four fractions: naphtha (initial boiling point (IBP) 177 "C), middle distillate (177-324°C),gaaoi1(324-525 "C) andresidue (525OC+). Carbon, hydrogen, nitrogen, and oxygen analysis of feed and product fractions was done by using a Perkin-Elmer elemental analyzer (Model 240). Oxygen was measured directly. Sulfur content was determined using a Leco sulfur determinator (Model SC132). Elementalanalyseswere normalized to 100%. Conversion of nitrogen was based on analysis of feeds and products by an Antek nitrogen analyzer (Model 711). Molecular weights were determined by vapor-pressure osmometry in benzene. The feeds and products were analyzed for D/H ratio by conventional methods of stable isotope geochemistry, following Steer et al.,S 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 2024 h and then cooled gradually over 24 h. Water of combustion was removed from the breakseal by heating a mineral oil bath at 80 O C . The evolved water was frozen into a sample tube containing0.5 g of Zn metal and then converted to hydrogen gas for mass spectrometricanalysis. The data are reported as parts per thousand (ppt) relative to the international standard of standard mean ocean water (SMOW)by the following equation: (1) SD = ([D/Hl- [D/HI,Q,M,W)/[D/HI,Q,M~W X 1000
where [D/H]s~ow= 1.558 X lo4. The error of replicate analyses was *3 ppt.
Results The overall 6D values for the feeds were -132 to -137 ppt, while the products had overall values of -129 ppt for LL, -134 ppt for CL, -176 ppt for ATH, and -178 ppt for PR. These product values were based on the data for individual boiling fractions, added as follows: 6D for whole sample = ZfiHi6Di/ZfiHi
(2)
where the fraction of each boiling cut was fi, and the hydrogen content was Hi. Representative feed and product compositions from Athabasca residue are listed in Table I. The elemental analysis calculated from the sum of the boiling fractions (Table I) was identical to the results from independent analysis of the unfractionated oils to within the error of the elemental analysis. The bonds between deuterium and carbon are stronger than the bonds between hydrogen and carbon; therefore, a product or fraction that has undergone a net loss of hydrogen will become enriched in deuterium, and will give ~
(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 . Chem. Eng. 1991,69,833-843.
Table I. Example Feed and Product Composition for the Athabasca Residue ~~~
naphtha mid dist gas oil residue IBP-177 "C 177-324 O C 324-525 O C 525 O C +
sum
Feed Residue wt%
c,wt % H,wt % 0,wt % N,wt % s,wt % bD, ppt
avMW
29.6 84.74 10.69 0.95 0.22 3.39 -138h2.5 (n=4) 460
70.4 82.41 9.58 1.41 0.91 5.63 -129 -129 1690
100 83.10 9.91 1.32 0.70 4.97 -132'
38.1 86.60 10.94 0.50 0.38 1.59 -175 -175 -172 365
33.7 85.53 8.88 1.67 0.98 3.94 -151 -152
100 85.56 10.90 0.88 0.55 2.11 -176'
Products C,wt % H,wt % 0,wt % N, w t % s,wt % 6D,bppt
8.7 84.66 14.38 0.70 0.04 0.22 -202 -203
av MW
110
wt%
19.6 85.71 12.78 0.33 0.37 0.81 -194 -194 -193 200
1160
bD for sum of fractions calculated from eq 2. Results are shown for repeat measurements on each sample. a
6D values of >-130 ppt. Enrichment of isotopes by this means is called the kinetic isotope effect. For example, thermal cracking of Athabasca bitumen at 430 OC and 8.9 MPa of hydrogen pressure gave 6D values ranging from -170 ppt for naphtha to -106 ppt for toluene-insoluble residue (Steer and Muehlenbachs, unpublished data). The naphtha composition was much closer to the feed under low-pressure pyrolysis conditions (6Dnaphth= -142 ppt) where hydrogen shuttling would be negligible. In the present experiments, the 6D value for the gas phase was -330 ppt, so that net exchange between the liquid and the gas would shift the products to more negative values (Le., more depleted in deuterium relative to SMOW). The overall6D values of the product streams, therefore, indicated net hydrogen addition to Athabasca and Peace River and almost no net change for the CL and LL feeds. The 6D data for the product boiling fractions are plotted in Figure 1 as a function of molecular weight. All of the feed fractions had 6D values in the range of -130 to -150 ppt, while the hydrogen gas had a value of -330 ppt. The data shown in Figure 1 for feed fractions (gas oil and residue) are means of three or more determinations. Where the 95% confidence interval was greater than the size of the symbol, error bars are shown. The data for products are the result of individual measurements on product fractions. In every case the 6D value of the products decreased with molecular weight from the residue fraction to the naphtha. The decrease was most pronounced for Athabasca, which gave a naphtha a t -202 ppt, compared to ca. -150 to -170 ppt under similar conditions in the absence of a catalyst (unpublished data). Although the residues from Athabasca and Peace River were more enriched in deuterium, they were still depleted relative to the feed, which indicated net hydrogen transfer from the gas phase. In contrast, Cold Lake and Lloydminster showed a net enrichment of deuterium in the residue and gas oil fractions (larger 6D values relative to the feed), which indicates that these fractions gave up hydrogen. The middle distillate was only slightly depleted relative to the feed 6D values.
Steer et al.
542 Energy & Fuels, Vol. 6, No. 5, 1992 -110 1
I
Table 11. Material Balance on Hydrocracking
Experiments
-120 - 1 30
-140
feed liquid, g/min Hz gas,O L/min product liquid, g/min gas, L/min mass Balance, % product gas, mol %
-150 P 0.
ci
Lo
- 1 60
-170 -180 /
-210
1
Hz HzS
Ath CL LL PR
I
A 0 A 0 0 0
Feed Product
I
100
Cl-CS e146 as wt
1
1000 Molecular Weight
Figure 1. Measured 6D values of feed and product boiling fractions as a function of molecular weight. The molecular weights are mean values for each boiling range (naphtha,middle distillate, gas oil, and residue). The trend of decreasing 6D with molecular weight was much larger than would be expected on the basis of hydrogen addition due to cracking. If 1 mol of hydrogen atoms is added to each mole of product to cap the radical intermediates, then the 6D of the naphtha fraction would shift by only -12 ppt (to ca. -141 ppt), and the middle distillate by -7 ppt to ca. -137 ppt. The overall material balance for these steady-state experiments ranged from 97.5 to 100.1%,calculated as total products (gas + liquid) over total feeds (Table 11). Elemental balances were in the same range, for example, for Athabasca the overall balance on carbon was 100.17%, on hydrogen 98.7 5% (no correction for ammonia or water formation), and on sulfur 99.1 7%. The total addition of gas-phase hydrogen to the liquids can be estimated from a material balance on hydrogen in the reactor. Total hydrogen consumption was calculated by balance, as follows: total H, consumed, mol/kg =
Qin
- QoutX~z
22.4mfeed
(3)
where Q i n and Qout are the volumetric flows of gas entering and leaving the reactor, X His~the mol fraction hydrogen in the gas, and m f .is the mass flow rate of feed liquid. The data for liquid and gas streams and the composition of the gas product are listed in Table 11. To estimate the total hydrogen added to the liquid products, the total consumption must be corrected for formation of HzS, NH3, HzO, and c&3 gases. Table I11 gives estimates of gas-phase H2 consumed for forming gases, assuming 1 mol H2/mol HzS, 1.5 mol Hdmol NHB, 1 mol Hz/mol H20, and 0.5 mol H2/mol c146 gases. Hydrogen for H2S and c& gases was calculated directly from the gas-phase composition (Table 111,while hydrogen consumption for NH3 and H2O was based on liquid-phase elemental compositions. The hydrogen addition to the liquid phase was then calculated as the difference between total consumptionand consumption for gas products, NH3, and H20. This method, based on analysis of gas flows and compositions, was more accurate than a balance on hydrogen from elemental analysis of the liquids, because the organic hydrogen in the feed was 5-10 times the hydrogen consumption.
% of
feed liquid feed S, wt % S conversion feed N, wt % N conversion feed 0, w t % 0 conversion
Peace River
Athabasca (ATH)
Cold Lake (CL)
Lloydminster (LL)
1.547 1.035
1.527 1.011
1.538 1.122
1.575 1.076
1.463 0.858 99.8
1.419 0.953 100.1
1.416 0.971 98.2
1.443 0.842 97.5
91.41 3.89 4.70 3.3
90.94 3.16 5.90 4.8
91.90 2.93 5.18 4.1
90.02 4.26 5.72 3.9
5.14 0.61 0.56 0.26 1.17 0.37
5.10 0.63 0.45 0.29 0.97 0.57
4.69 0.66 0.53 0.29 0.99 0.55
7.02 0.66 0.63 0.24 1.09 0.45
(PR)
Gas flow at STP.
Table 111. Hydrogen Addition to the Liquid Products Cold
Peace
Athabasca Lake Lloydminster River (ATH) (CL) (LL) (PR) Hz consumed, mol/kg of feed totala as HzSb 88 NH3‘ as HzOC as C 1 4 s d Hz to liquids: mol/kg feed product H,I wt % predicted S I P actual SD,ppt
1.2 0.96 0.16 0.24 0.58 5.3
4.2 0.88 0.14 0.38 0.82 2.0
6.7 0.83 0.16 0.32 0.73 4.6
9.0 1.02 0.16 0.29 0.68 6.9
11.1 -151 -176
11.2 -139 -134
11.3 -154 -129
11.4 -159 -178
a Calculated from HZbalance on gas in and out of reactor. b From HzS analysis of gas product. From analysis of feed and product liquids. Assuming 0.5 mol of gas-phase Hdmol of C1-Ce gases. e Calculated by difference; total consumed - Hz to gases (including NH3 and HzO). f Hydrogencontent by direct measurement of product liquid. 8 Predicted value of SD calculated from eq 4.
The predicted 6D value of the products was calculated by adding the estimated amount of hydrogen gas a t ~ D H , = -330 ppt to the original hydrogen in the feed (bDfd), as follows: predicted 6D = [26DHzRnL,z/10+ 6Dfd(Hpr, - 2nL,2/10)1/ffp,, (4)
where R was the ratio of kilograms of feed to kilograms of product, nLHzwas the amount of hydrogen added to the liquids (mol of Hdkg of feed, from Table 111) and Hpr,,,j was the total hydrogen content of the product liquids in weight percent (Table 111). The factor 2/10 converted the units to a weight percent basis. This estimate was the predicted result due to addition reactions only. A value for the product in excess of the value predicted from hydrogen addition reactions would indicate the presence of exchange reactions, which give isotopic exchange but no change in total hydrogen content. Since the gas phase is depleted in deuterium, isotope exchange would deplete the liquids and enrich the gas phase. The Athabasca and
Energy & Fuels, Val. 6, No. 5, 1992 543
Catalytic Hydrocracking of Residues Table IV. Eetimated Fraction of Total Hydrogen Exchanged with Gas Pham
Fraction naphtha middle distillate gas oil residue
fraction of liquid H added plus exchanged with gas-phase H2 ATH CL LL PR 0.15 0.12 0.30 0.34 0.32 0.07 0.0 0.22 0.22 0.11
-0.02
-0.05
-0.07 -0.08
0.24 0.14
Peace River products gave a larger shift in 6D, for the overall product mixture, than predicted on the basis of hydrogen addition, while Cold Lake and Lloydminster gave smaller values. The large shifts observed for the distillates suggest significant addition and exchange of hydrogen between the gas and the liquids. The fraction of hydrogen added and exchanged in each product fraction was estimated using the data for the 6D for the initial residues and the gas phase. Equilibration of the isotopic composition with an excess of gas-phase H2, Le., 100% exchange plus addition] would give a 6D value of -330 ppt. At the other extreme, no isotopic exchange or addition of gas-phase H2 would give the same value as the feed residue. An estimate of the fraction of the H in each product that is exchanged and added is given by fraction exchanged = (6Dprd- 6D,,)/(6DH2
- 6Dfd) (5)
where SD,,d was the measured value for the specific product fraction (Figure 1). The estimates range from 34% for ATH naphtha to -8 % for LL residue (Table IV). The former result indicates that over one-third of the original hydrogen atoms had been exchanged with the gas phase or added, while the negative result for Lloydminster residue indicates net dehydrogenation.
Discussion The isotopic analysis clearly shows a fundamental difference in behavior between CL and LL on one hand and ATH and PR on the other. The products were obtained under identical conditions of catalytic activity, flow rate, temperature, pressure, and hydrogen gas. Furthermore, all the feeds had very similar 6D values. The products were dramaticallydifferent, however, in that all the ATH and PR product fractions were depleted in deuterium relative to the feed, while the CL and LL products fell both above and below the feed isotope ratio. The results for all feeds showed more depletion of deuterium (negativeshift in 6D) in the naphthaand middle distillate products, consistent with the higher hydrogenation activity of the Ni/Mo on y-alumina catalyst toward these lower boiling fractions.8 Catalytic hydrogenation of aromatics, removal of heteroatoms, and catalytic exchange reactions would account for the depletion of deuterium in these fractions relative to the feed material. The heavier fractions of Athabasca were also depleted in deuterium (morenegative 6D value),consistent with a combination of addition and exchange between the liquids and the hydrogen gas. The products from PR followed a very similar pattern to ATH. The estimates for addition of hydrogen to the liquid products (Table 111) suggested that ATH and PR products should have more (8) Trytbn, L. C.; Gray, M. R.; Sanford,E.C. Ind. Eng. Chem. Res. 1990,29,725-730.
negative 6D values than CL, because more gas-phase Hz was added overall. On this basis, however, the isotopic composition of LL should be similar to ATH. On the basis of the comparison between actual and predicted 6Dvalues in Table 111, the unusual characteristic of the ATH and PR feeds was significant exchange with the gas phase over and above the addition reactions. The estimated fraction of hydrogen atoms added plus exchanged in the naphtha products (Table IV) ranged from 34% for ATH to 12% for LL. These values would overestimate the actual extent of addition and exchange if cracked fractionsare depleted in deuterium due to kinetic isotope effects. Noncatalytic hydrocracking of ATH gave naphtha of 6D = -150 to -170 ppt, compared to -129 ppt for the feed residue, but interaction with the gas-phase Hz cannot be ruled out. A challenge for calibrating this isotopic technique is to systematically correct for kinetic isotope effects so that the effect of catalytic reactions on 6D, versus thermal reactions, can be quantitated. The Cold Lake and Lloydminster residue products were enriched in deuterium relative to the feed (more positive 6D values in Figure 1); therefore, any addition of lH from the gas phase was counteracted by reactions giving an enrichment in deuterium. This process would require net dehydrogenation of the feed molecules in the residue fraction. The most likely reaction would be dehydrogenation of hydroaromatic or sterane-typealicyclic structures to give internal hydrogen transfer. The differencein 6D behavior between CL and LL versus ATH and PR did not coincide with any significant differences in the composition of the feed residue. The aromatic carbon content of CL was 32% of total carbon and LL contained 34%; both values were intermediate between ATH and PR. The ratio of H,/C,, which gives some indication of condensation of aromatic rings, was 0.26for LL and 0.28 for CL compared to 0.29for ATH and 0.32 for PR. More detailed analysis of aromatic rings groups would be required,therefore]in order to understand and verify the differences in deuterium depletion. All of the bitumens were distinctly different in their behavior when compared to coal liquids. Kabe et alez+ used tritium as a label to study hydrogen addition and exchange reactions. They observed some differences between coals, but in general the catalytic transfer from the gas phase favored exchange with the coal liquids, as opposed to the solvents such as tetralin. Exchange was highest for the highest molecular weight fraction, increasing in the order oils < asphaltenes < preasphaltenes < insolubles. Similar results were noted by Kamo et al.6 based on the analysis of stable isotopes. The exchange of label between the coal and the solvent was favored at temperatures over 400 OC, in accord with the hydrogen shuttling mechanism of solvents such as Tetralin. The bitumens gave much different results; little shuttling of deuterium was observed between the light fractions and the heavy fractions in the case of Cold Lake, because the latter were almost unchanged from the feed. Furthermore the exchange that did occur was more pronounced for the lighter distillates, in opposition to the trend noted by Kabe et al. for coal. The above differences likely stem from the different chemistry of the two hydrocarbons. Coal liquids are primarily aromatic, with short bridges between aromatic groups (i.e., biphenyl linkages) and methyl substitution of aromatic rings. The chemistry of hydrogen interaction
544 Energy & Fuels, Vol. 6, No. 5, 1992
with these materials, therefore, will be dominated by T bonds between aromatic carbons and u-bonds linking aromatic and aliphatic carbons. The chemistry of the bitumens, on the other hand, is dominated by the paraffmic and naphthenic groups. The other difference was the temperature of reaction, which was 430 "C for the bitumens and 350-400 "C for the coal liquids. Analysis of bitumen products from reactions in the same temperature range would be more directly comparable to the work of Kabe et al. Although "scrambling" of isotopic data due to rapid exchange reactions would obscure patterns in hydrogen addition, the present data indicate that for bitumen and heavy oil reactions exchange must be relatively slow. The significant differences between the different product fractions and the differences in 6D between the same distillate range from different feeds both suggest that exchange is slow. Exchange reactions must contribute some of the observed 6D change from feeds to products;
Steer et al. nevertheless, the present results indicate that isotopic analysis using 6D values can be used to determine the distribution of hydrogen added to product fractions.
Conclusions Hydrogen exchange and addition to the light distillates produced by catalytic hydrocracking was extensive, consistent with better access to the catalyst than the unconverted residue. Major differences in product 6D were observed as a function of the feed material; residues from CL and LL lost 'H, while the ATH and PR residues gained 'H. The range of 6D values between product fractions and the gas phase indicated that stable isotopes can be used to track where gas phase hydrogen is added to residue hydrocracking products. Registry No. Deuterium, 7782-39-0.
