518
Energy & Fuels 1994,8, 518-530
The +525 "C Rwidue Before and After Hydrocracking with Bimodal Catalysts of Varying Macropore Volume Marten Ternan,' Parviz M. Rahimi, Donald M. Clugston, and Heather D. Dettman Energy Research LaboratorieslCANMET, Department of Natural Resources Canada, Ottawa, Ontario, K I A OG1, Canada Received August 25, 1993. Revised Manuscript Received January 11, 1994"
The +525 "C residues were examined to identify differences between those residue molecules that were converted to smaller distillate molecules and those that were not. Each +525 "C residue was divided into five subfractions by gel permeation chromatography. Molecular weight distributions, elemental (H, C, S, N, Ni, V, and Fe) distributions, and distributions of carbon atom types were determined. With the exception of 10-20%, most of the unconverted +525 "C molecules in the products were similar to the +525 "C feedstock molecules. The ones that were different appeared to have formed by a combination of dehydrogenation and molecular condensation reactions. The macropores in the bimodal catalysts provided diffusion paths for the large molecules to the reaction sites which dissociate hydrogen within the catalyst interior and thereby had a major effect on diminishing these condensation and dehydrogenation reactions. In contrast, the molecules that were converted to distillates were formed by a combination of hydrogenation (greater H/C ratios) and cracking (smaller molecular weights) reactions. An explanation for the same feedstock molecules reacting simultaneously by a dehydrogenationlcondensationsequence and a hydrogenation/cracking sequence is provided by the existence of two liquid phases at reaction conditions, as suggested by Shaw and co-workers. According to this concept, hydrogen is much more soluble in the nonpolar phase (where the hydrogenation/cracking reactions would occur) and much less soluble in the polar phase (where the dehydrogenation/condensaton reactions would occur).
Introduction The primary purpose of hydrocracking +525 "C vacuum residue feedstocks is to convert high boiling compounds to lower boiling distillate liquids that can be used in conventional transportation fuels. A number of established analyses, especially boiling point distribution, can be used to characterize distillable portions of the feedstock and hydrocracked products. Techniques other than boiling point are required to characterize the nondistillable portion. A preparative scale gel permeation chromatography (GPC) technique has been used in our laboratory1 for such separations. Each of the subfractions obtained by GPC was weighed and then analyzed to determine ita molecular weight and other properties.2J Other laboratories have employed similar technique^.^^^ In this study, vacuum residue from Athabasca bitumen was hydrocracked using several catalysts having different pore size distributions? The work reported here describes the characterization of GPC subfractions from the unconverted +525 "C fraction of the hydrocracked reaction product. Measurements included molecular weight distribution, distribution of carbon types (aromatics, aliphatics), and distribution of heteroatoms ( S , N, Ni, V, and Fe). Abstract published in Advance ACS Abstracts, March 1, 1994. (1)Champagne, P.J.;Manolakis, E.;Ternan, M. Fuel 1986,67,423426. (2)Nortz,R.L.;Balks, R. E.; Rahimi, P. Ind. Eng. Chem. 1990,29, 1968-1976. (3)Kyricacou,K. C.;Balks, R. E.; Rahimi, P.Fuel1988,67,109-113. (4)Woods,J. R.; Kotlyar, L. S.; Montgomery, D. S.; Sparks, B. D.; Ripmeester, J. A. Fuel Sci. Technol. Znt. 1990,8,149-171. (6) Cyr, N.; McIntyre, D. D.; Toth, G.; Strausz, 0. P.Fuel 1987. 66. 1709-1714. (6)Teman, M. Can. J. Chem. Eng. 1982,60,33-39. 0
0887-0624/94/2508-0518$04.50/0
These measurements were expected to provide information about the types of molecules in the unconverted residuum. In principle, the structure of molecules that had been converted could be deduced by comparing total product information with feedstock information. Such knowledge might suggest different strategies, for example, different processing conditions that would improve the chemical transformation of the Unconverted product molecules.
Experimental Section The preparation and properties of the catalysts have been described in detail previously.8 In brief, the catalysts were prepared by impregnating cobalt and molybdenum salts onto a support having two alumina components. When the catalysts were in their final states, one of the components in the support was y-alumina. The other alumina component contained macropores (MAP)that were much larger than the pores in y-alumina. The properties of the catalysts are summarized in Table 1. The vacuum bottoms feedstockshownin Table 2 was converted in a continuous flow fixed bed catalytic reactor. Reaction conditions were 10.6 MPa (1500psig),425 "C, 1.0-h-l liquidspace velocity, and a gas rate of 815 mL of HdmL of feed (3500 scf/ bbl). In this study, a commercial bimodal catalyst, 1442B from American Cyanamid, was the standard catalyst used for comparison with the laboratory prepared catalysts. The total liquid products were distilled into four fractions, ibp-176 O C , 176-343 "C, 343-525 OC,and+525OC,usingaHerzog Model MC 630 automated vacuum distillation apparatus in accordancewith ASTM method D-1160. The nondistillable +525 OC residue fractions were then Soxhlet extracted. All of these residue fractions were completelysoluble in the mixture of xylene isomers that was used as the extraction solvent.
Published 1994 by the American Chemical Society
+525
O C
Residue Before and After Hydrocracking
Energy & Fuels, Vol. 8, No. 3,1994 519
Table 1. Properties of the Fresh Catalysts surface area median (m2/g) pore dia. composition (&%) %MAP Hg (pm) (nm) MoOa COO component BET poro 256 5.9 0.25 16.3 3.4 stand 295 0 15 20 25 30
164 133 184 168 169
172 168 194 212 170
-
0.6 11 0.5 1.0
6.7 6.6 5.2 4.9 4.5
15.1 14.7 13.0 14.4 14.8
2.8 2.9 2.7 2.8 2.9
Table 2. Properties of Athabasca Vacuum Bottoms 82.1 Ti,& % 293 c,wt % 26.1 H,wt % 9.5 C5, insol w t % 1.25 s,wt % 6.06 tol, insol w t % 1.24 N,wt % 0.77 ash,wt % 1.059 Fe, ppm 1042 specific grav. 261 MCR,w t % 24.32 v, PPm 83.4 Ni,wt % 133 +525 O C w t % The second separation step was to divide the +525 "C fraction of each reaction product into five subfractions by preparative scale gel permeation chromatgraphy (GPC). A Water's preparative liquid chromatograph System 500A was used, with xylene as the solvent. The column was 1.2 m long and was filled with 100-A styragel. Each of the separated subfractions was weighed after evaporating the solvent. Several analyses were performed on the various subfractions. The number-average molecular weight of each subfraction was determined by vapor pressure osmometry (VPO)using a Knauer vapor pressure osmometer Type 11.0. These measurements were made using tetrahydrofuran solutions typically in the 0.1-0.5 wt % range. Carbon, hydrogen, and nitrogen were analyzed by combustion, reduction of nitrogen species to molecular nitrogen gas, and separation of the gasesby chromatography using a Perkin Elmer Model 2400 instrument. Sulfur was determined (ASTM D-1552) using a LECO SC-32 analyzer which employed hightemperature combustion followed by detection of SO2 gas by infrared absorption. Vanadium, nickel, and iron were measured by dissolving samples of the bitumen fractions in xylene and using a peristaltic pump to inject the solution into the flame of a Jarrell Ash Model 9OOO inductively coupled plasma (ICP) spectrometer. 1aC NMR and 1H NMR measurements were performed using a Varian XL-300 (automated) spectrometer at 293 "C operating at 75.429 MHz for carbon and 299.943 MHz for proton. The NMR samples were prepared by mixing approximately 150 mg of the subfraction with 700 pL of deuterochloroform (CDCl$. The proton spectra were collected with an acquisition time of 3 a, a sweep width of 4000 Hz, a pulse flip angle of 30.8" (8.2 ps), and no recycle delay. The spectra resulting from 16 scans were referenced to the residual chloroform resonance at 7.24 ppm. The quantitative l3C NMR measurements were acquired with an acquisition time of 0.938 a, a sweep width of 16000 Hz, a flip angle of 31.9" (5.6 p a ) , and a recycle delay of 4 a. Reverse gated waltz proton decoupling was used to avoid nuclear Overhauser effect enhancements of the carbon signals. Each spectrum was the result of 5000 scans. Exponential line broadening of 3 Hz was used to improve the signal to noise ratio. All the spectra were referenced to the CDC18 resonance at 77 ppm. The "distortionless enhancement by polarization transfer" (DEPT)spectra were collected using the pulse sequence supplied by Varian. The acquisition time and sweep width were the same as those for the quantitative carbon runs. A recycle delay of 2 s and a carbon-proton coupling constant of 140 Hz were used. The carbon 90" puhe was 15.8 pa while the proton 90' pulse was 21 pa. Four spectra were collected with 1264 scans each, with the following proton flip angles: 45O, 90°, 90°, and 135'. A line broadening of 2 Hz was used. The two-dimenhonal "hetero-correlation" (HETCOR) data were collected using the pulse sequence provided by Varian. The
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Figure 1. Average number molecular weight versus fraction of vacuum residuum feedstock. The open circles and dotted lines represent the feedstock. The solid circles and solid lime represent the product from a unimodal catalyst. carbon and hydrogen sweep widths were narrowed to include only the paraffinic regions of the spectra to obtain good spectral resolution. The data were collected with an acquisition time of 0.573 a, a carbon sweep width of 3574.0 Hz, a proton sweep width of 1426.9 Hz, and a carbon-proton coupling constant of 130 Hz. The carbon and hydrogen 90"pulses were as given for the DEPT experiments. Four scans were done prior to the start of data collectionfor each increment to &ow the magnetization to reach a steady-state value. There were 128 incrementa with 266 scans for each. The recycle delay was 1 a. The 4K X 128 data seta were zero-fiied to 4K X 266, prior to Fourier transformation. Ashifted sine-bell weighting function was used in both dimensions.
Results and Discussion The molecular weight distributions in Figure 1represent the total feedstock (dashed lines and open circles) and the +525 "Cresidue fractions of a reaction product (solid line and solid circles) obtained using a unimodal catalyst. The ordinate indicates that some of the product subfractions have greater molecular weight than the corresponding feedstock subfraction. The abscissa indicates that some of the residuum feedstock molecules have been converted to nonresiduum product molecules. Each bar in Figure 1 shows the amount of a subfraction and its average molecular weight. The single dot at the top center of each bar is a more concise but less informative representation of the same data. The dot presentation of data allows more distributions to be represented on the same graph, as is shown in Figure 2. The molecular weights measured by VPO will only be accurate if the molecules are present in the tetrahydrofuran solution as individual molecules and not as micelles or associated aggregates of molecules. Recently, Woods et aL4showed that the apparent molecular weight of a heavy maltene fraction decreased by a factor of 2, as its concentration in solution decreased from 18 to 3 w t 5%. Their observation is consistent with earlier work7 that showed molecular weight to be a function of solution concentration. To avoid this problem we have followed
Ternan et al.
520 Energy & Fuels, Vol. 8, No. 3, 1994 1 .o
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M 0 LEC U LAR WE I G HT Figure 2. Average number molecular weight versus weight fraction of vacuum residuum feedstock. The open circles and dotted lines represent the feedstock. For the products, symbols (catalysts) are listed with the description of the catalyst in the brackets following the symbol: solid circles (unimodal catalyst), open triangles (15% MAP catalyst), open squares (20% MAP catalyst),solid squares (26% MAP catalyst),solid triangles (30% MAP catalyst), and inverted solid triangles (standard catalyst). The solid l i e s are drawn through the unimodal catalyst and the 20% MAP catalyst data points. the method of Chung et al.? who recommended the use of much more dilute solutions. Solution concentrations of 0.1,0.2,and 0.45 w t % were used in this work. A leastsquares line was drawn through the four data points (the three solution concentrations plus the origin which is the experimental datum point obtained during calibration) plotted as instrument reading versus solution concentration. If the data points fall on a concave line rather than a straight line, it suggests that some of the molecules may still be associated in the solution and that the molecular weight determined from the slope of the best fit straight line may be too large. In the past, molecular weights were found to vary with the solvent used7 and with the methode used (VPO or freezing point depression (FPD)). Using the method of Chung et al.? we have obtained consistent moleculer weights using different solvents and using different methods.' Furthermore, the molecular weights obtained by this procedure are reasonably consistent with those determined by field ionization mass spectrometry
(FIMS).2*3 Molecular weights above 2000 were not observed by Boduszynskilo in his characterization work on heavy petr'oleums. However,he did not investigate heavy crudes as heavy as the Athabasca bitumen used in this work. Nevertheless, the number-average molecular weights of the feedstock subfractions in Figure 1 are all less than 2000. However, many of the product subfractions had molecular weights which exceeded 2000. It must be emphasized that in these cases the VPO data formed (7) Moechopedis, S.E.;Fryer,J. F.; Speight, J. G. Fuel 1976,55,227232. (8) Chung,K. E.;Anderson,L. L.; Wiser, W. H.Fuel1979,58,847-852. (9) Speight J. G.;Moschopedis, S. E. Eke1 1977,56,344-345. (10) Boduszyneki, M. M. Energy Fueh 1987, I, 2-11.
straight lines and were not concave. As a result, the molecular weights reported here are considered to be reasonably accurate. Six product data sets (presented as points) plus the distribution for the total feedstock (open circles)are shown in Figure 2. The open and solid circle data points in Figure 2 are the same as those in Figure 1. A dotted line has been drawn through the points representing the subfractions obtained from the feedstock. Again, it is apparent that some of the product subfractions have moleclular weights that are greater than the corresponding feedstock subfractions. Solid lines have been drawn through two of the data sets. The product distribution containing the subfraction having the greatest molecular weight was obtained with the unimodal catalyst that did not contain any of the MAP component (solid circles). The product distribution consisting of the smallest resid molecules was obtained with the catalyst containing 25% MAP (solid squares). Since the objective was to produce smaller molecules, the catalyst containing 25% MAP would be favored on the basis of these product molecular weight distributions. Presumably less hydrocracking would have to be performed on small resid molecules than on large ones in order to obtain distillate molecules. The unimodal catalyst (solid circles in Figure 2) which does not have any large macropores was the one that produced the residuum fraction having the greatest molecular weight. The catalysts with macroporesll will permit the largest molecules to diffuse rapidly to the interior of the catalyst where unsaturated radical groups in the carbonaceous molecules are likely to encounter dissociated hydrogen from the Co-Mo-S sites on the catalyst surface. After they are hydrogenated, there is less possibility for them to oligomerizewith other molecules and produce higher molecular weight products. Unimodal catalysts without macropores would not permit rapid diffusion of large carbonaceous species to the catalyst interior. Therefore, less dissociated hydrogen would be available to radical groups. That would increase the possibility for oligomerization reactions which in turn would explain the larger molecular weight species that were obtained with the unimodal catalyst in Figure 2. Because the chemical composition of all the catalysts was identical, differences in reaction products cannot be explained by catalyst compositiion-only by differences in catalyst pore structure. The hydrogen to carbon ratios (H/C) of the subfractions for the feedstock and products are shown in Figure 3, as a function of molecular weight. The greatest H/C ratios were produced by the standard catalyst (solid inverted triangles). The lowest H/C ratios were produced by the unimodal catalyst (solid circles). The H/C ratios for the products obtained from the MAP bimodal catalysts were generally between the two lines described above. There is less variation in H/C ratio with molecular weight for the feedstock subfractions than for the product subfractions. H/C ratios are normally expected to increase as the molecular weight decreases. This was certainly true for all of the distillate fractions obtained during this study. Figure 3 shows that for the fractions having the greatest molecular weights the H/C ratio increases- the molecular weight decreases, as expected. However, the H/C ratio (11) Ternan M.; Menashi, J. Proceedings of t 10th International Congress on Catalysis; Guczi, L., Solymozi,F., T auyi, P., Eda.; Stud. Surf. Sci. Catal. 1993; Vol. 75, pp 2387-2390.
?
+525 O C Residue Before and After Hydrocracking 1.600
1.500
1
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-
Energy & Fuels, Vol. 8, No. 3, 1994 621 r
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"
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Figure 4. Molecular weight versus boiling point for selected pure compounds: solid circles are aromatics, solid squares are naphthenes, and solid triangles are paraffins.
passes through a maximum and decreases for the smallest resid molecules. The existence of small resid molecules having low H/C ratios was not expected, although very recently Woods and co-workers4reported a similar finding during their GPC investigation of maltenes. In this work, as the molecular weight decreased from 5000 to 1000 the H/C atomic ratio reached a maximum of 1.5. This value is just slightly below the 1.6 H/C atomic ratio that was obtained for the heavy gas oil fractions (343-525 "C) produced during this study. One explanation for the small value of the H/C ratio at small molecular weights is provided in Figure 4. It shows plots of molecular weight as a function of boiling point for a number of pure compounds. The upper plot is for paraffinic compounds. The lower plot is for aromatic compounds. The dotted line near the paraffin line is for naphthenic compounds. One can consider a boiling point separation made at 525 OC in terms of this graph. The aromatic molecules that boil just above 525 OC will have the same molecular weight as paraffins and naphthenes that are in the -525 OC distillate fractions. Therefore, the lowest molecular weight fraction in Figure 3 will be more enriched in aromatic compounds. According to Figure 4 the naphthenic compounds having the same molecular weight will be in the distillate fractions. This is one of the factors that contribute to the subfractions in Figure 3 with the lowest molecular weights having lower than expected H/C atomic ratios. It may be the predominate factor for the maximum in the H/C ratio of feedstock subfraction shown in Figure 3. Increasing the hydrogen content in Figure 4 does not cause much of an increase in molecular weight, as shown by the progressions from naphthalene "1" to tetralin "2"
to decalin "3" and from anthracene "4" to dihydroanthracene "5"to hexahydroanthracene "6". However, these changes do cause a significant change in boiling point. This explains how the aromatic molecules having boiling points that are slightly greater than 525 "C can be converted to naphthenic molecules having boiling points below 525 OC by only adding hydrogen, without the requirement of breaking any bonds. The weight percent of aromatic carbon atoms determined by 13C NMR is given in Figure 5. As expected it is the inverse of Figure 3. The subfractions from most of the MAP bimodal catalysts are bracketed by the standard catalyst and the unimodal catalyst. However, the subfractions with the lowest molecular weights that had the least aromatic carbon atoms were obtained using the MAP bimodal catalysts. The minimum in Figure 5 was also observed by Woods et alS4during their study of maltenes. Figure 6 compares the nitrogen to carbon atomic ratio (N/C) for the subfractions of the products and feedstock. The shapes of these curves are similar to those for aromatic carbon shown in Figure 5. The product subfractions of middle molecular weight have essentially the same N/C atomic ratios as their correspondingfeedstock subfractions. However, others contain more nitrogen than their corresponding feedstock fractions. This indicates that material having lower nitrogen concentrations is preferentially transformed into a different molecular weight fraction, leaving some resid subfractions enriched in nitrogen. For distillate fractions, the N/C ratio is known to decrease as the molecular weight decreases. This relationship was also observed in Figure 6 for those resid subfractions with the greatest molecular weight. However, the N/C ratio
Ternan et al.
522 Energy & Fuels, Vol. 8, No. 3, 1994
passed through a minimum and increased for the resid molecules that had the smallest molecular weight. This observation was not expected. Nevertheless, the recent data reported by Woods et a1.4 for maltenes showed the same phenomenon. For the lowest molecular weight residue subfraction, the combination of large N/C atomic ratio (Figure 6)and small H/C atomic ratio (Figure 3) is consistent with molecules having condensed aromatic rings that contain nitrogen heteroatoms. This was explained above in terms of separation by boiling point being different than separation by molecular weight. However, there are two additional explanations. It is often suggested that side chains are removed from from the condensed aromatic ring cores in residue molecules to create new molecules which are predominantly condensed aromatic ring structures without side chains. This is easy to visualize in terms of the most recent molecular structure proposed by Strausz, Mojelsky, and Lown.12 The observation that the lowest molecular weight product subfractions are more aromatic (Figure 3) than the corresponding feedstock subfraction is consistent with the removal of the hydrogen-rich side chains. Since the side chains have relatively little nitrogen compared to the rings, side chain removal would increase the nitrogen content of the remaining molecule. The observation that the lowest molecular weight product subfractions have more nitrogen that the correpsonding feedstock subfraction is also consistent with the removal of side chains. The third possibility is that molecules containing aromatic rings and polar atoms such as nitrogen may have been adsorbed by the GPC column and may have been eluted in the last subfraction instead of their correct molecular weight fraction. The last subfraction to be eluted is normally the lowest molecular weight subfraction. If adsorption by the GPC column was the predominate effect, the molecular weight of the last subfraction eluted would be larger than expected. The molecuar weights of the last subfractions to be eluted from the GPC column covered the range 570-768,with 650being a typical value. In contrast, a typical molecular weight value for the second last subfraction is 870. If larger molecules were selectively adsorbed by the GPC column, the subfraction eluted last would have a molecular weight closer to or greater than the 870value corresponding the subfraction eluted second last. Since an average molecular weight of 650 is quite reasonable for the smallest molecular weight fraction, it can be concluded that while some adsoption on the GPC column undoubtedly does occur, it does not appear to have a major influence on the results. In summary, there are three different possible explanations for the lowest molecular weight subfraction having high aromatics and high nitrogen content. The first is that separation by boiling point and by molecular weight is different. For molecules boiling near 525 OC , some of the saturated hydrocarbons are in the heavy gas oil fraction, while the aromatic molecules of corresponding molecular weight are in the lowest molecular weight subfraction of the residue. The experimental observation that the feedstock residue subfraction containing the smallest molecules has a smaller H/C atomic ratio and a larger N/C atomic ratio than the feedstock residue subfraction containing the next smallest molecules suggests that this (12) Strausz, 0. P.;Mojelsky, T.W.; Lown, E.M. Fuel 1992,71,13551363
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Figure 7. Sulfur to carbon (S/C) atomic ratio (XlOa) versu8 average number molecular weight. Symbolsas given in Figure 2. Solid lines through unimodal and standard catalysts. effect could be operating. The second explanation is that side chains are removed from some of the residue molecules leaving highly aromatic cores that contain large nitrogen concentrations. The experimental observation that the lowest molecular weight subfractions of the product residues have smaller H/C atomic ratios and larger N/C atomic ratios than the corresponding feedstock values suggests that this effect is also operative. The third explanation is that the molecules which are highly aromatic with large nitrogen contents are retained on the GPC column longer than nonaromatic nitrogen-free molecules and therefore do not elute with their correct molecular weight fraction. While there is no doubt that this effect occurs, based on the differences in average molecular weight of the two residue subfractions of lowesermolecular weight, it does not appear to have had a major influence. The S/C atomic ratios in Figure 7 show somewhat larger ratios at both large and small molecular weights. Large sulfur concentrations in the largest and smallest molecules have also been reported by Sughrue et al.13 The variation in S/C atomic ratio among the subfractions within any single residuum fraction is much smaller than the corresponding variations in other (e.g., N or 0)heteroatom to carbon ratios (HA/C). The variation of S/C ratio between the lines in Figure 7,representing subfractions produced by different catalysts, decreased in the following order: feedstock subfractions, subfractions from the unimodal catalyst, subfractions from the MAP bimodal catalysts, and finally subfractions from the standard bimodal catalyst. This indicates that at least some hydrodesulfurization (HDS) of the resid fraction could occur without decreasing the size of the resid molecule from which the sulfur was removed. For other heteroatoms such as nitrogen, the HA/C ratio in the product subfractions was equal to or greater than the ratio in the corresponding feedstock subfraction. Sulfur was different. Its S/C atomic ratio was smaller in the product than in the feedstock. The two solid lines drawn through the V/C atomic ratios in Figure 8 indicate elevated ratios a t both large and small molecular weights. Large vanadium concentrations in both the largest and smallest molecules have also been reported by Reynolds et al.14 Both lines have the same general shape as the lines for the N/C atomic ratios shown in Figure 6. The upper solid line through the subfractions from the (13) Sughrue, E.L.;Hauler, I).W.; Liao, P.C.; Stope,D.J. I d . Erg. Chem. Res. 1988,27,397-401. (14) Reynolds, J. G.; Jones, E.L.;Bennett,J.A.;Bigga, W.R.Fuel Sci. Technol. Znt. 1989, 7,625-642.
Energy &Fuels, Vol. 8, No.3, 1994 623
+525 "C Residue Before and After Hydrocracking 2.5 0
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Figure 8. Vanadium to carbon (V/C)atomic ratio (XlOS) versus average number molecular weight. Symbols as given in Figure 2. Solid lines through the unimodal and standard catalysts. 0.8
0
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MOLECULAR WEIGHT
Figure 9. Nickel to carbon (Ni/C) atomic ratio ( X W ) versus average number molecular weight. Symbols as given in Figure 2. Solid lines through unimodal and standard catalysts. unimodal catalyst is also typical of the MAP bimodal catalysts but not the standard catalyst. The product subfractions obtained using the standard catalyst had lower V/C atomic ratios than all of the other products. This suggests that, using the standard catalyst, vanadium removal is like sulfur removal in that some of the molecules were so stable that they remained within their original residuum subfraction after heteroatom removal, rather than being converted to a lower boiling species. Somewhat less conversion of residuum to distillates occurred with the standard catalyst than with the others. The stabilization of devanadized molecules by the standard catalyst may explain the lower conversion achieved with the standard catalyst. The Ni/C atomic ratios are shown in Figure 9 as a function of molecular weight. The minimum in the Ni/C ratio in Figure 9 is similar to the minimum in the N/C ratios in Figure 6. Many of the product subfractions appear to have Ni/C atomic ratios which are similar to the feedstock subfractions of corresponding molecular weight. I t should be emphasized that a substantial portion of the +525 "C feedstock has been converted (40-45%). Nevertheless, the resid that remains unconverted still has a distribution of nickel concentration that is similar to that of the feedstock. This suggests that if the Ni is removed from a molecule, then that molecule is simultaneously converted to one or more species having molecular weights that are not within the residuum subfraction. In other words, denickelized molecules do not remain in their original residuum subfraction of the same molecular weight. The Fe/C atomic ratio is shown in Figure 10as a function of the molecular weight. It is apparent that the Fe/C
Figure 10. Iron to carbon (Fe/C) atomic ratio (XlOa) versus average number molecular weight. Symbols as given in Figure 2. Solid lines through unimodal and standard catalysts. atomic ratios in the feedstock subfractions and in the product subfractions from the unimodal Catalystare almost identical. After processing with bimodal catalysts, the Fe appears to have become more concentrated in all of the other product subfractions than it was in the corresponding feed subfraction. This suggests that the parta of a residuum molecule that do not contain iron are more readily cracked to lower boiling fractions than the parts of the residuum molecule that do contain iron. The aromatic portion of a 13C spectrum for typical residuum subfractions is shown in the lower curve of Figures 11. The aliphatic region of a typical 13Cspectrum for the proton-containing carbon atoms is shown in Figure 12. The spectra were divided into the regions described below and were assigned to different types of carbon atoms. The assignments were based on the results of distortionless enhanced polarization transfer (DEPT) experimenta (Figures 11and 12) and on two-dimensional hetero-correlation (HETCOR) experiments (Figure 131, as well as the assignments previously reported by Cyr et ale: Petrakis and Allen,'S Thiel and Gray,16Suzuki et al.,17 FormlEek et al.,l8 and Stothers.lg The assignments are summarized in Table 3. The aromatic carbon resonances between 100 and 170 ppm in Figure 11were divided into three regions. DEPT analyses showed that resonances between 112and 130ppm were from carbon atoms in CH groups. This is evident in the spectrum a t the top of Figure 11,which is for carbon atoms that have one hydrogen bonded to them. Those resonances greater than 130 ppm were from quaternary carbons (carbons not attached to hydrogen atoms). The 130-150-ppm region was assigned to quaternary carbon atoms, bonded to other carbon atoms. The 15&160- ppm region was assigned to quaternary carbon atoms that were adjacent to heteroatoms. The aliphatic region, from 0 to 60 ppm, of the DEPT spectra is shown in Figure 12. The bottom spectrum shows all the resonances from aliphatic carbons bonded to one or more hydrogens whereas the middle spectrum shows those which have only one bound hydrogen atom. In the top spectrum the resonances with positive intensity (above (15) Petrakis, L.;Allen, D. NMR for Liquid Fossil Fuels; Elsevier: Amsterdam, 1987. (16) Thiel J.; Gray, M. R. AOSTRA J. Res. 1988,4 , 63-73. (17) Suzuki, T.; Itoh, M.; Takepami, Y.; Watanabe,Y. Fuel 1982,61, 402-410. (18) FormAhk, V.;Desnoyer, L.; Kellerhala, H. P.; Keller, T.; Clerc, J. T. Bruker '3C NMR Data Book, Vol. 1; Bruker-Physik Germany, 1976. (19) Stothers, J. B. Carbon-13 NMR Spectroscopy; Academic Preaa: New York, 1972.
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624 Energy & Fuels, Vol. 8, No. 3,1994
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Figure 11. Distortionlesse n h a n d polarizationtransfer (DEPT) graphs for the aromatic portion of subfraction 4,from the 25%
MAP catalyst, which shows (at the top) the intensitiesof signals from carbon atoms in CH groups and (at the bottom) the intensities of resonances from all C atoms.
0 8 II I
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Figure 13. Dimensionlessheteronuclear correlation (HETCOR) diagram for subfraction 4 from the 25% MAP'catalyst, which shows the relationship between the '9c and 'H NMR spectra.
Table 3. Assignments of '43 NMR Spectral Regions hydrocarbon group in which C atom is locatad
spectral region I
PPm 0-13.5 13.5-18 18-21 21-22 22-24
24-28.5 28.5-31
CHa groups in rotoevaporator grease terminal CHa groups on alkane chains adjacent CHa groups on aromatic or naphthenic rings CHs groups a to aromatic rings 50% CHa groups on the sides of alkane chains (isoalkanes) 50% CH2 groups that are a to terminal CHa groups in
Chain8 CH2 groups in naphthenic rings CH2 groups y or more from brminal CHs groups in
chains
III/I,II,
,,,,~,,,,
60
I I I 1 I I I I I
50
I""~""l""~""l""~""I 30 20 1 0 PPM 0
, I , , ) , , , ,
40
Figure 12. Distortionlesse n h a n d polarizationtransfer ( D E W graphs for the alifihaticportion of subfraction 4 from the 25% MAP catalysts, which shows (at the top) the intensity of signals from atoms in CH, and CH groups (pointingup) and CH2 groups (peakspointing down),(inthe middle) the intensityof CH groups (pointingup), and (at the bottom) the intensities of resonances from all C atoms.
the baseline) are from either CH or CH3 carbon atoms, whereas the resonances with negative intensities (below the baseline) are from CH2 carbon atoms. Using the DEPT spectra and literature assignments, the aliphatic carbon region was divided into 14 bands. A peak in the 0-5-ppm region appeared consistently in the lowest molecular weight subfraction. I t was the last subfraction to elute from the GPC column. Carbon atoms in CH3 groups in the silica grease used in the rotoevaporator for removing solvent are in this region. The 0-13.5-ppm spectral region was assigned to these CH3 groups. The 13.5-18-ppm region was assigned to terminal CH3 groups in aliphatic chains.ls DEPT experiments indicated that CH3 groups were also in the 18-21-ppm region. They were assigned to o-dimethyl groups on aromatic or naphthenic rings.'9 The 21-22-ppm region was assigned to CH3 groups
31-32.5 32.5-33.5 33.5-36.5 36.5-38.5 38.5-41 41-60 112-130 130-150
CH2 groups that are B to terminal CHs groups in chains CH groups that are a to naphthenic rings CH2 group in naphthenic rings CH2 groups that are a to aromatic rings 30% CH2 groups in naphthenic rings 70% CH groups in naphthenic rings CH groups in naphthenic rings CH groups in aromatic rings C atoms in aromatic rings that are bonded only to
other carbon atoms
150-160
C atoms in aromatic ringe that are adjacent to heteroatoms and are bonded only to other carbon
atoms
which are a to ar0mati~s.l~The 22-24-ppm region contained two components: 50% of the carbon atoms were assigned to CH3 groups attached to an aliphatic chain (isoalkanes) and 50% of the carbon atoms were assigned to CH2 groups that are a to terminal CH3 groups in aliphatic chains.lg The basis for the 50150 division is apparent in Figure 12. The bottom spectrum which shows the resonances from all ofthe aiiphatic carbons has a strong peak a t 22.8 ppm. However, that peak is not seen in the top spectrum. The positive resonances from CH3 groups have essentially cancelled the negative resonances from the CH2 groups, indicating that both groups contained an identical number of carbon atoms. The 24-28.5-ppm region is part of the region assigned to CH2 groups in
+525 OC Residue Before and After Hydrocracking
naphthenes. For example,the CH2 groups in cyclohexane resonate at 27.7 ppm.lg Much of the broad hump between 22 and 50 ppm can be ascribed to CH2 groups in naphthenes. In contrast, the sharp peaks in this region are caused by carbons in straight chains. The 24-28.5ppm region is one of the regions that does not have any signals from carbon atoms in straight-chain compounds. The 28.5-31-ppm region was assigned to CH2 groups that are y or more from terminal CH3 groups on alkane chains. For example, in long chain alkanes the y or more CH2 groups are in the 29.5-30.5-ppm region.lg The 31-32.5ppm region was assigned to CH2 groups that are fl to terminal CH3 groups.19 The 32.5-33.5-ppm region was assigned to CH groups in chains that are (IL to naphthene~.~8Jg The 33.5-36.5-ppm region was assigned to CH2 groups in naphthene rings.lg It is another region of the broad hump for naphthenes that does not contain any sharp peaks. The 36.5-38.5-ppm region was assigned to carbon atoms in alkane chains that are in CH2 groups located a to aromatic rings.ls The 38.5-41-ppm region was assigned to both CH2 groups (30%) and CH groups (70%) in naphthenic rings.lsJg The 30/70 assignment was obtained from the 38.5-41-ppm region in Figure 12. The relative areas in the top spectrum (CH2 groups) and those in the middle spectrum (CH groups) have a 30/70 ratio. The 41-60-ppm region was assigned to CH groups in naphthenic rings.18J9 More information on the molecular stuctures present in the subfractions was obtained from two-dimensional HETCOR NMR data. Each of the contour lines in the HETCOR plot in Figure 13is the "aerial" perspective (top view) of the peak resulting from the correlation of the carbon resonance of a particular carbon species with the resonance(s) of ita attached hydrogen(s). Thus, this twodimensionalspectrum gives both the carbon and hydrogen chemical shifts of each of the carbon species. Carbons which do not have attached hydrogens are not detected with this technique. The spectrum shows that the methyl residue whose carbon resonance is a t 14.3ppm has a peak for ita hydrogens at 1.0 ppm. This confirms the assignment that this is a terminal methyl of an aliphatic chain. Similarly, a comparison of model compoundsto the carbon and proton chemical shifts of the CH2 carbon resonance a t 29.8 ppm assigns it, as expected, to the methylene residues which are y or greater from an end of an aliphatic chain. Of interest, however, is the occurrence of two proton correlations for the carbon resonance a t 22.8 ppm. The DEPT spectrum showed that this resonance actually consists of two species, a methyl (CH3) and a methylene (CH2). The HETCOR shows that the methyl hydrogens resonate a t 0.98 ppm, which assigns them to an isobutyl-type methyl rather than a methyl which is (IL to an aromatic ring, as may be expected. The methylene species a t 22.8 ppm can be assigned as being at the @ position from the CH3 end of an aliphatic chain. The signal to noise ratio of the HETCOR spectrum is sensitive to the line width (relaxation rate) of the carbon species. As molecules get larger, their relaxation rates increase and their line widths get broader. In this sample, the molecules are relatively large, so those carbon species which are part of the ring structures of the molecules will have broad resonances. This is seen both in the aromatic region as well as for the aliphatic CH species whose resonances are between 40 and 60 ppm. It is not possible
Energy & Fuekr, Vol. 8, No. 3,1994 625 Table 4. Atoms of Hydrogen pew Molecule: A Comparison of Calculations Based on Elemental Analyses and NMR
sample feed
0% MAP
catalyst
no. of H atoms per molecule measured by fraction mol elemental NMR % difference no. wt analyses analyses element-" 1 1642 146 159 -8.6 2 3 4 5 1 2 3 4
5 15% MAP
catalyst
20% MAP
catalyst
25% MAP catalyst
30% MAP
catalyst
standard catalyst
1 2 3 4
5 1 2 3 4 5 1 2 3 4
5 1 2 3 4
5 1 2 3 4 5
1473 1185 912 688 4986 1931 1378 872 623 2792 1406 1332 812 688 3545 2138 1173 801 668 2562 1389 924 646 605 3656 2304 1645 927 768 2677 1667 1086 810 570
150 117 95 66 393 169 132 85 48 242 132 135 82 58 302 205 121 80 53 220 131 96 62 46 326 212 160 93 66 249 168 114 80 46
151 118 94 62 433 187 138 84
50 267 140 138 83 60 314 206 123 78 58 229 141 93 60 47 358 237 173 96 72 252 202 113 81
-
-0.4 -1.0 +0.9 +5.8 -10.3 -10.4 -4.3 +0.9 4.0 -3.9 -5.7 -2.6 -0.7 4.0 -3.9 -0.4 -1.9 +2.4 -10.3 4.2 -7.9 +2.8 +2.6 -2.6 -10.1 -11.9 -7.8 -3.0 -9.0 -1.5 -20.4 +0.5 -1.1
-
to obtain a HETCOR spectrum for these regions. The aliphatic species whose HETCOR spectrum was obtained have resonances which are relatively narrow. This indicates that these species are more mobile than the aromatic residues. This is expected if these residues are in flexible chains, possibly, though not necessarily, attached to the core ring structures. A hydrogen material balance is shown in Table 4. Using one molecule as a basis, hydrogen content is calculated in two ways. From the elemental analysis of hydrogen, one can calculate the number ofhydrogen atoms in an average molecule for that fraction. For the second method, the elemental analysis of carbon is used to calculate the number of carbon atoms in the molecule. The NMR data are then used to calculate the number of carbon atoms in each of the spectral regions. The DEPT spectra in Figure 12 show which of the regions have none, one, two, or three hydrogen atoms bonded to each of the carbon atoms. This NMR information was applied to each spectral region and used to calculate the number of hydrogen atoms in each average molecule. The percent difference between the two calculation methods is also shown in Table 4. With one exception, the greatest difference among the 34 subfractions is 125%. More than half of the subfractions had differences less than 4 % . The results in Table 4 show that the l3C NMR data are consistent with the elemental analysis. The types of carbon in several spectral regions have been plotted to illustrate the types of reactions that are
Ternan et al.
526 Energy & Fuels, VoL. 8,No. 3, 1994
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2000
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M 0 LEC U LAR W El G HT
Figure 14. Quaternary carbon atoms in aromatic structures (atom % in the 130-150 ppm spectral region) versus average molecular weight of a subfraction. occurring. Figure 14 shows the amount of quaternary carbon atoms (carbon atoms that are only bonded to other carbon atoms and are not bonded to hydrogen-the 13& 150-ppm spectral region) in aromatic structures plotted as a function of the average molecular weight in the subfraction. The distributions for the feedstock, the unimodal catalyst, and the bimodal catalyst with 15% MAP are shown. It is apparent that for the same molecular weight all three have similar amounts of quaternary carbon atoms. It has already been indicated that condensation reactions occurred with the unimodal catalyst and that product molecules larger than the feedstockmoleculeswere formed. Figure 14 also shows that these molecules have more aromatic quaternary carbon atoms than either the feedstock or any of the product molecules formed with the bimodal catalyst. This indicates that dehydrogenation has occurred to the same molecules that were produced by oligomerizationlcondensation reactions. Figure 15 shows the amount of aromatic CH carbon atoms (carbon atoms that are bonded to one hydrogen atom-the 112-130-ppm spectral region). For the same molecular weight, all three distributions show similar amounts of CH aromatic carbon. However, the largest molecular weight subfractions which were produced with the unimodal catalyst had more CH aromatic carbon than the feedstock subfractions. By combining Figures 14 and 15, it is apparent that the low molecular weight and mid molecular weight product subfractions are similar to the feedstock subfractions. However, the largest molecular weight product subfractions have molecules which are larger and more aromatic (more aromatic quaternary carbon and CH carbon) than the feedstock molecules. Figure 16shows the amount ofCH carbon atoms (carbon atoms in the 41-60-ppm spectral region) in naphthenic structures. For the same molecular weight, all three distributions show similar amounts of CH naphthenic carbon. As noted above, the unimodal catalyst produced molecules which were larger than the feedstock molecules. Figure 16 shows that these molecules contained fewer CH naphthenic carbon atoms than the feedstock molecules. The CH2 naphthenic carbon atoms are in the broad hump under the various peaks that are in the 15-40-ppm spectral region. It is difficult to separate the CH:!
1000
2000
3000
4000
5000
M 0 L E C U LAR WE IG HT
Figure 15. CH carbon atoms in aromatic structureswhich form a bond with only one hydrogen atom (atom % in the 112-130ppm spectral region) versus average molecular weight of a subfraction. 0.12
n
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0
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1000
2000
3000
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Figure 16. CH carbon atoms in naphthenic structures which form a bond with only one hydrogen atom (atom % in the 4160-ppm spectral region) versus average molecular weight of a subfraction. naphthenic carbons from the others because there is overlap with several peaks. Figure 17 shows only the CH2 naphthenic carbons in the 24-28.5-ppm spectral region because it is one of the few regions where there is no overlap with any peaks. Again, for the same molecular weight all three distributions show similar amounts of CH2 naphthenic carbon in this limited region. However, the molecules in the subfraction having the greatest average molecular weight have less CH2 naphthenic carbon than the feedstock molecules. The data in Figures 14 to 17 can be summarized as follows. For the mid molecularweight residuum molecules, it is evident that the feedstock and product molecules have similar aromatic and naphthenic carbon contents. However, there were major changes in both the highest and lowest molecular weight subfractions. Figures 14and 15show that there is an increase in aromatic carbons (both quaternary and CH aromatics). Figures 16 and 17 show
+525 O C Residue Before and After Hydrocracking
Energy & Fuels, Vol. 8, No. 3, 1994 527
0.07
0.18
0.1 6
w
v,
z u
Z
0
m 0.06 U
I
V
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w Z
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0.04 2000
3000
4000
51
1000
MOLECULAR WEIGHT
Figure 17. CH2 carbon atoms in naphthenic structures which form a bond with only one hydrogen atom (atom % in the 2428.5-ppm spectral region). that there is a decrease in the naphthenic carbons (both CH and CHz). This strongly suggests that hydrogen transfer reactions have occurred.z0 The results are consistent with those reported by Sullivan, Boduszynski, and FetzerZ1 for hydrocracking vacuum gas oil. They observed that a small fraction of their process stream was converted to polyaromatic hydrocarbons which had the highest possible degree of condensation. In the process of transferring hydrogen, naphthenes have been converted to aromatics. These data do not identify the species that received the transferred hydrogen. However, it is likely that they are in the distillate fractions. One might speculate that the hydrogen has been used in several ways, in saturating free radicals, in hydrogenating olefins to produce paraffins, or in being added to other aromatic rings which may have been subsequently opened. Figure 18 shows the CHZaliphatic carbon (in straight chains, the 28.5-31-ppm spectral region). For the same molecular weight, all three distributions show similar amounts of CH2 aliphatic carbon. This is true for all of the different molecular weight subfractions. Even the high molecular weight subfraction produced with the unimodal catalyst had a CHZaliphatic carbon similar to that of the feedstock high molecular weight subfraction. In general, the data in Figure 18 show that the product residue molecules have just as many CHZgroups in side chains as do the feedstock residue molecules. The exception in Figure 18is that the smallest molecular weight product subfractions have slightly fewer side chains than the corresponding feedstock molecules. There are a t least two possible explanations for this observation. First, it may be that the side chains were not cracked from the large unconverted molecules. This would indicate that the conventional mechanism of cracking alkanes by a radical chain mechanismzzhas not been operating on most of the unconverted residue molecules. There is no reason to suggest that side chains were not removed from the residue molecules that were converted, by the conventional (20)Eiech, J. J.; Sexsmith, S. R.; Singh, M. Energy Fuels 1989,3, 761-762. (21)Sullivan, R. F.; Boduszynski, M. M.; Fetzer, J. C. Energy Fuels 1989,3,603-612. (22)Poutama, M. L.Energy Fuels 1990,4,113-131.
2000
3000
4000
5000
MOLECULAR WEIGHT
Figure 18. CH2 carbon atoms in aliphatic chains (atom % in the 28.5-31-ppm spectral region). cracking mechanism or some other mechanism such as solvent-mediated hydrogen~lysis.~~ However, these data indicate that the majority of residue molecules that were not converted also have their side chains intact. The second possibility is that the number of original side chains cracked from unconverted molecules is equal to the number of new side chains formed by ring opening. This seems less likely, because the data in Figure 18are for y-carbons (ones that are three or more carbon positions from the terminal carbon atom of a carbon chain). This means that several rings would have to open to replace side chains carbons that had been removed. Another reason it would be unlikely is that it would be necessary for the ring opening to occur without an appreciable change in molecular weight. In previous thermal cracking or thermolysis studies performed with pure compounds having a side chain attached to a ring, some of the molecules were converted and some were not. Mushrush and Hazlettz4 studied n-tridecylcyclohexane, 1-phenylpentadecane, and l-phenyltetradecane. Blouri et al.26studied 1-phenyldodecane. Savage and Klein26*27 studied pentadecylbenzene, tridecylcyclohexane, and 2-ethyltetralin. Wu, Klein, and Sandlerz8 studied benzyl phenyl ether. Savage and coworkers2w1studied 1-dodecylpyrene,2-(3-phenylpropyl)naphthalene, and 1,3-bis(l-pyrene)propane. These studies were performed in the absence of a hydrogen and in the absence of a catalyst. In general, the reaction products consisted primarily of a methyl group attached to a ring (e.g., toluene) plus an n-alkene or a vinyl group attached to a ring (e.g., styrene) plus an n-alkane. When the (23)Malhotra R.;McMillen, D. F. Energy Fuels 1993,7,227-233. (24)Mushrush G.W.; Hazlett, R. N. Ind. Eng. Chem. Fundom. 1984, 23,288-294. (25) Blouri, B.;Hamdan, F.; Herault, D. Ind. Eng. Chem. Process. Des. Dev. 1986,24,30-37. (26)Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1987,26,488-
--
AQA-.
(27)Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1988,27,13481356. (28)Wu, B.C.;Klein, M. T.;Sandler, S.I. AIChE J. 1990,36,11291136. (29)Savage, P. E.;Jacobs, G. E.; Javanmardian,M. Ind. Eng. Chem. Res. 1989,28,645-654. (30)Smith, C . M.; Savage, P. E. Ind. Eng. Chem. Res. 1991,30,331339. (31)Smith, C.M.; Savage, P. E. Energy Fuels 1991,5,146-165.
528 Energy & Fuels, Vol. 8, No. 3, 1994
temperatures used in these previous studies were similar to the temperature used in this work, substantial fractions of the reactant molecules remained unconverted, with their side chains still attached to the ring, at the end of the reaction period. That is quite consistent with the data in Figure 18in which the side chains are still attached to the unconverted molecules. When thermal cracking reactions are performed at high pressures, the product distribution obtained is different from that obtained at atmospheric pressure. Khorashed and Grays2have summarized some of the earlier studies. One of the differences is that branched alkanes having greater molecular weights than the reactant molecules are formed, presumably by the addition of radicals to a-olefins produced by thermal cracking reactions. In high-pressure thermal cracking studies using hexadecane as the reactant in the presence of toluene,33n-alkylbenzenes were formed having greater molecular weights than the reactant molecules. This was attributed to the addition of a-olefins to benzyl radicals that were formed from the toluene. If side chains are being cracked to form a-olefins and methyl groups attached to ring structures, on the basis of the results reported by Khorashed and a t least some of the a-olefins could become re-attached to the methyl groups to re-form the side chains. There has been a limited amount of work performed on noncatalytic hydrocracking or hydrogenolysis reactions. A study by Zhou and C r y n e ~ 3a t~350 "C used dodecane as a reactant. They found that the function of hydrogen was to saturate hydrocarbon radicals and suppress coupling reactions. Penninger and ~ o - w o r k e r s 3used ~ ~ ~indan, ~ tetralin, n-butylbenzene, and n-propylbenzene as reactants a t 500-540 OC. They reported ring opening plus radical saturation by hydrogen transfer primarily from the indan. Less than 25 % of the hydrogen transfer was from the gas phase. For the work reported here, the catalyst macropores allowed large molecules closer access to the catalyst sites where the hydrogen was dissociated and made available for transfer. I t was the unimodal catalyst without macropores that produced the largest product molecules by failing to suppress coupling. Many of the hydrocracking studies have been performed using aromatic ring compounds in the presence of a catalyst. The compounds studied include d e ~ a l i n ?an~ thracene,a*39phenanthrene,- perylene,44pyrene,&chry(32)Khorasheh,F.; Gray, M. R. Ind. Eng. Chem. Res. 1993,32,18531863. (33)Khorasheh,F.; Gray, M. R. Ind. Eng. Chem. Res. 1993,32,18641876. (34)Zhou, P.; Crynes, B. L. Ind. Eng. Chem. Process. Des. Deu. 1986, 25,508-514. (35)PenningerJ. M.L.; Slotboom, H. W. Recl. Trav. Chim. Pays-Bas 1973,92,1089-1094 (36)Penninger, J. M. L.; Versluis, K. Fuel 1982,61, 283-290. (37)Shabtai, J.; Ramakrishnan, R.; Oblad, A. G. In Thermal Hydrocarbon Chemistry. Oblad, A. G., Davis, H. G.,Eddinger,R. T., Eds. Am. Chem. SOC., Adv. Chem. Ser. 1979,183,297-328. (38)Bloom, P. W. E.; Dekker, J.; Fourie, L.; Kruger, J. A.; Potgieter, H. G . J. J . S.Africa Chem. Inst. 1976,28,130-138. (39)Nakatauji, Y.;Fujioka, S.; Nomura,M.; Kikkawa,S. Bull. Chem. SOC.Jpn. 1975,50,3406-3408 (40)Wu, W. L.; Haynes, H. W. in Hydrocracking and Hydrotreating; Ward, J. W., Qader, S. A., Eds. ACS Symp. Ser. 1975,20,65-81. (41)Huang, C. S.;Wang, K. C.; Haynes, H. W. in Liquid Fuels from Coal; Ellington, R. T., Ed.;Academic Press: New York, 1977;pp 63-78. (42)Lemberton J. L.;Guisnet, M. Appl. Catal. 1984, 13,181-192. (43)Lemberton,3.L.;Touzeyidio, M.; Guisnet, M. Appl. Catal. 1989, 54,91-loo. (44)Dekker, J.; Nell, B. C. K.; Potgieter, H. G. J. Fuel 1978,57,361364. (45)Ting, P. S.;Curtiss, C. W.; Cronauer, D. C. Energy Fuels 1992, 6, 511-518.
Tenan et al. sene,& and fl~oranthene.~'In general, the reactions proceeded by hydrogenation of the terminal rings of the compound, isomerizationto form a methylcyclopentylring, ring opening, and dealkylation (side chain removal)!' Girgis and GatesG have compiled an extensive review of these reactions. As mentioned above, several rings on a molecule would have to open to replace the long side chains that have been identified in residue molecules.12 It seems more likely that the side chains being removed from these compounds would be short ones, and therefore their removal would result in the production of gas. A plot of product yield versus conversion will be tangent to the ordinate for a primary reaction and tangent to the abscissa for a secondary reaction. Sanford's49 data have clearly shown that gas formation is a secondary reaction whereas distillate formation is a primary reaction. This indicates that side chain removal from aromatic ring compounds is not a primary reaction. All of the above considerations apply to molecules that have been converted. In those studies cited above which were performed at the same temperature used in this work, there was always a significant part of the reactant that was not converted. Therefore one category of product residue molecules would be those that are the same as the feedstock residue molecules. It is also likely that some of the feedstock residue molecules may have reacted, but not sufficiently to form distillate molecules. They would be a second category of product residue molecules. Differences between the smallest, middle sized, and largest molecular weight subfractions are apparent in Figures 14-18. Both the smallest and the largest residue molecules in the products are different than the residue molecules in the feedstock. For the smallest residue subfractions, some of the product molecules had smaller H/C and larger N/C atomic ratios. This would be consistent with these molecules being the aromatic cores of feedstock molecules that had their side chains removed. In contrast, some of the feedstock residue molecules became larger from condensation reactions. These same molecules were dehydrogenated by hydrogen-transfer reactions to become enriched in aromatic ring structures and diminished in naphthenic ring structures. These changes were observed in the product residue subfractions of the largest molecular weight. Finally, most of the middle sized molecules in the unconverted residuum products are similar to the residuum molecules in the feedstock. For example, they have virtually the same number of carbon atoms in side chains as the original feedstock molecules. The most important observation is that although some of the feedstock residue molecules reacted, approximately half of them were completely unaffected. One explanation for the conversion of some residuum molecules but not others might be based on hydrogen availability. In this work, the distillate molecules had greater H/C atomic ratios (H/C = 1.6 for 343-525 "C gas oil and H/C = 1.9 for ibp-176 "C naphtha) than the unconverted residuum molecules (H/C atomic ratios varied from 1.1to 1.5, as shown in Figure 3). This suggests that molecules that did not obtain additional hydrogen were (46)Nakatauji, Y.; Kubo, T.; Nomura, M.; Kikkawa, S. Bull. Chem. SOC.Jpn. 1978,51,61&624. (47)Lapinas, A. T.; Klein, M. T.; Gates, B. C.; Macris, A.; Lyons,J. E. Znd. Eng. Chem. Res. 1987,26,1026-1033. (48)Girgis, M.J.; Gates, B. C. Ind. Eng. Chem. Res. 1991,30,20212058. (49)Sanford, E. C. Znd. Eng. Chem. Res. 1994,33,109-117.
+525 "C Residue Before and After Hydrocracking HYD
Energy & Fuels, Vol. 8, No. 3, 1994 529
CRACK
and [CMI is the concentration of any carbonaceous molecule in the polar or nonpolar phases. Since hydrogen + has greater solubility in the nonpolar phase, it would be HIC- 1.W-1.W NIC-0.0008-0.0048 more likely that carbonaceous molecules in that phase would be hydrogenated (HYD) and subsequently cracked > SIMILAR CM HIC 1.37 HIC 1.33-1. A 4 (CRACK) to form distillate liquids and gases. This is the NIC -0.0055 NIC 0.00834.0088 conventional concept used to describe hydrocracking CONDENS vacuum residuum. The same carbonaceous molecule in ICMI, -> DeHyCM > LARGERCM the polar phase would not have as much hydrogen HIC-1.3 Figure 19. Residuum hydrocracking reaction mechanism NIC-0.011 inavailable. Either no reactions would occur at all or dehydrogenation (DEHYD) via hydrogen-transfer reacdicatingthe presence of two liquid phases at reaction conditions. tions would occur. This could be followed by condensation (CM), and (CM), represent carbonaceousmolecules in the polar (CONDENS) reactions to form larger carbonaceous moland non-polar phases, respectively. HyCM and DeHyCM repecules. This would explain our observation of condensaresent hydrogenated and dehydrogenated carbonaceous moletion and dehydrogenation reactions for some molecules. cules, respectively. It also offers a mechanism for some moleculesto encounter hydrogen and become converted, while other identical not converted and remained intact in their original state. molecules in the other phase would not encounter hydrogen and therefore would not become converted. This obserFor most unconverted molecules there was no change in vation of the formation of larger molecular weight the types of molecular structures and no change in the molecules in a second liquid phase may be the beginning side chain carbons. The exceptions were the unconverted of a transition to mesophase and eventually to solid coke molecules of highest molecular weight, which were prodeposits.53 Such a concept is definitely consistent with duced from dehydrogenation and condensation reactions. the studies by WieheMJj5on coke formation by phase The residuum molecules that were converted would have separation. become either distillate liquids or gases. Both of these The nitrogen data in Figure 6 are also consistent with products contain more hydrogen than the feedstock the presence of two liquid phases at reaction conditions. molecules, indicating that hydrogen is essential for conFigure 6 shows that all of the product subfractions having version. This explanation suggeststhat conversion appears greater molecular weights than the feedstock subfractions to be limited by hydrogen availability. also have larger N/C atomic ratios than the feedstock The above findings are consistent with the co-existence subfractions. The only way the molecular weight could of two liquid phases in the reactor, a concept suggested have increased is if two or more molecules originating in by Shaw, Gaikwad, and Stowe.50 One phase would be a the feedstock combined. The only way the new coupled "polar" liquid that would have relatively larger concenmolecules could have larger N/C atomic ratios is if the trations of polar compounds and condensed ring comparticular molecules participating in the coupling reaction pounds, plus a comparatively small solubility of hydrogen. originally had larger than average N/C atomic ratios. If The other phase, a "non-polar" liquid, would have relatively molecules with greater than average N/C atomic ratios larger concentrations of both naphthenic compounds and segregated in a separate dispersed "polar" liquid phase, hydroaromatic compounds, plus a comparatively larger the opportunity for them to react with one another would hydrogen concentration. I t must be emphasized that both increase. Furthermore, if that phase solubilized an naphthenic and polar compounds would exist in both inadequate amount of hydrogen to saturate hydrocarbon phases. However, their concentrations in the two phases could be substantially different. Shaw and c o - w o r k e r ~ ~ p ~ ~radicals, the possibility of coupling reactions would increase. The physical presence of a second liquid phase have reported experimental data at hydroprocessing reaction conditions showing the existence of two liquid was not identified in this work, although Shaw and cophases of different composition. They studied mixtures workers have identified two liquid phases in their experiof tetralin and pyrene50as well as Athabasca bitumen and ments measurements. Nevertheless, the data obtained in Venezuelan heavy oil vacuum bottoms.51 They noted that this work can plausibly be explained by the presence of hydrogen is 8 times more soluble in tetralin than in pyrene. two liquid phases. Shaw52 also developed a correlation in which hydrogen solubility is inversely related to solvent density. On this Conclusions basis, hydrogen would also be less soluble in the polar phase. Approximately 40-45 3' % of the residuum feedstock The 13C NMR data reported here are consistent with molecules were converted to distillate molecules which the existence of two liquid phases under reaction condihad substantially greater H/C atomic ratios than the tions. Figure 19illustrates the concept. The concentration feedstock. Most of the unconverted residuum molecules of a particular species of carbonaceous molecule in each were similar to the feedstock molecules, which clearly of the phases would be affected by the equilibrium indicated that they had not been hydrogenated (i.e., no constant, K , where net change in hydrogen content). Only a small percentage of the unconverted molecules (the largest and the smallest molecules) were different from the feedstock molecules. It was apparent that the larger molecules had been formed (50) Shaw, J. M.; Gaikwad, R. P.; Stowe, D. A. Fuel 1988,67, 1554by condensation and dehydrogenation reactions. The 1559. macropores in the bimodal catalysts provided access to (51) Dukhedin-Lalla,L.;Yushun, S.;Shaw, J. M.; Rahimi, P. M. Fluid (CMI,,
->
HyCM
->
SMALLER CM lDlSTlLLATES GASES)
cy7 -
-
I
Phase Equilib. 1990,53,415-422. (52) Shaw, J. M. Can. J. Chem. Eng. 1987,65,293-298. (53) Kriz J. F.; Ternan, M. In Progress in Catalysis; Smith, K. J., Sanford, E. C., Eds. Stud. Surf. Sci. C a d . 1992, 73,31-33.
(54) Wiehe, I. A. Ind. Eng. Chem. Res. 1992,31,530-536. (55) Wiehe, I. A. I n d . Eng. Chem. Res. 1993, 32, 2447-2454.
530 Energy & Fuels, Vol. 8, No. 3, 1994
the reaction sites for the large molecules and thereby had a major effect on diminishing these condensation and dehydrogenation reactions. None of the paraffinic side chains had been removed from the unconverted residuum molecules, suggesting that gas formation occurs only by cracking side chains from molecules that are converted. All of these observations were explained by the existence of two liquid phases at reaction conditions, which was originally proposed by Shaw, Gaikwad, and Stowe.60 The larger hydrogen solubility in the nonpolar phase would provide the hydrogen to convert residue molecules to products having greater H/C atomic ratios. In contrast, residue molecules in the polar phase, having less hydrogen solubility, may not even have enough hydrogen to stabilize
Ternan et al.
thermal radicals. Oligomerization/condensationof radicals would explain the experimental observation that some of the product residue molecules had greater molecular weights than the feedstock residue molecules.
Acknowledgment. The authors gratefully acknowledge the contributions of Stan Soutar, who performed the reaction experiments; Mylene Fleurant, who performed the gpc separations; and Rachelle Yazdani, who peformed the vpo measurements. This work was supported in part by a group of industrial companies (Suncrude, PetroCanada, and Imperial Oil) and in part by the Federal Program on Energy Research and Development (PERD).
