Energy & Fuels 1995,9, 1011-1022
1011
Coprocessing: Elemental and Molecular Weight Distributions in Unconverted Vacuum Residues Marten Ternan,* Parviz Rahimi, Dirkson Liu, and Donald M. Clugston Energy Research Laboratories, CANMET, Natural Resources Canada, Ottawa, Ontario, K1A OGl, Canada Received May 22, 1995. Revised Manuscript Received July 10, 1995@
The difference between molecules that are converted to distillates during coprocessing (Cold Lake vacuum bottoms and Forestburg subbituminous coal) and those that are not has been studied. The following procedure was used. The fraction of the liquid product boiling above 525 "C was separated into five subfractions by preparative scale gel permeation chromatography (GPC). Each subfraction was weighed. Then the following measurements were made on each subfraction; molecular weight, elemental analyses (C, H, N, S, V, and Ni), and carbon-13 nuclear magnetic resonance. Five significant observations were made. (1)The molecules that were converted had larger WC atomic ratios and smaller N/C atomic ratios than the feedstock molecules. (2a) S, V, and Ni heteroatoms could be removed without causing much change in molecular weight. (2b) It was not possible to increase the WC atomic ratio or decrease the N/C atomic ratio without decreasing the molecular weight. (3) Generally, the unconverted +525 "C residue molecules became smaller as the processing severity increased. (4) The unconverted molecules retained their side chains at mild processing severities (425 "C). When the processing severity increased (450 "C), side chain carbon began to be removed. (5) If there was insufficient reaction time t o provide enough hydrogen, then molecules which were larger than the feedstock molecules were formed. Feedstock molecules that lost their hydrogen rich fragments (by cracking or by hydrogen transfer) without contacting enough hydrogen t o remove their nitrogen or metal heteroatoms may have oligomerized. If the reaction time was increased to allow more contact time with hydrogen, the oligomerized molecules which were larger than the feedstock molecules disintegrated. Finally, conversion of residue molecules to distillate molecules appeared to be limited by hydrogen addition. For conversion, hydrogen was required either t o hydrogenate aromatic rings or to remove nitrogen heteroatoms. There are other important requirements for hydrogen which are not primary steps in the conversion of large molecules to small ones. They include capping pyrolysis fragments and the removal of other heteroatoms (sulfur, metals).
Introduction One objective of coprocessing a mixture of coal and a hydrocarbon solvent (in this case Cold Lake vacuum bottoms) is to permit the hydrogen transfer molecules in the solvent to shuttle hydrogen from the gas phase to the coal1 (in this case Forestburg subbituminous coal). The purpose of this work was to identify the types of molecules that were not converted to distillates by the coprocessing reactions. The intention was t o compare the molecules having boiling points above 525 "C in the feedstock with those that remained unconverted in the reactor products. This approach to investigating coprocessing is patterned after a previous hydrocracking investigation that used Athabasca bitumen.2 The nomenclature used here makes a distinction between "unconverted" and "unreacted". 'Vnconverted" will be used t o describe +525 "C residue molecules that have not been converted to distillate molecules. Nevertheless, many reactions and chemical transformations have occurred to these unconverted molecules. How-
* Author to whom correspondence should be addressed. E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, September 1,1995. (1)Moschopedis, S. E.; Hawkins, R. W.; Fryer, J. F.; Speight, J. G. Fuel 1980,59,647-653. (2) Ternan, M.; Rahimi, P. M.; Clugston, D. M.; Dettman, H. D. Energy Fuels 1994, 8, 518-530. @
ever, none of these reactions will have caused the molecular weight of a particular +525 "C molecule t o decrease sufficiently for it to become converted to distillate molecules. Such a $525 "C residue molecule will be unconverted but not necessarily unreacted. The primary purpose of coprocessing is to convert the molecules in the solid coal and the molecules boiling above 525 "C in the residual oil 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 measurements are required to characterize the nondistillable +525 "C fraction of the liquid product. A preparative scale gel permeation chromatography (GPC)technique3 has been used in our laboratory for such separations. Each of the subfractions obtained by GPC was weighed and then analyzed to determine its molecular weight and other proper tie^.^,^ Other laboratories have employed similar technique^.^,' In this study vacuum residue from Cold Lake heavy oil was coprocesseds with different percentages of Forestburg subbituminous coal at different severities (temperature and space velocities). The work reported (3)Champagne, P. J.;Manolakis, E.; Teman, M. Fuel 1986,67,423425.
0887-0624/95/2509-1011$09.00/0Published 1995 by the American Chemical Society
1012 Energy &Fuels, Vol. 9, No. 6, 1995 Table 1. Properties of Forestburg Subbituminous Coal as dried received at 105°C MAJ? proximate analysis (wt %) 22.03 moisture 8.59 6.70 ash ultimate analysis (wt %) 51.73 66.35 72.58 carbon 4.42 4.84 3.45 hydrogen 1.19 nitrogen 1.53 1.67 0.49 0.54 0.38 sulfur 18.62 20.37 14.52 oxygen (by dim analysis of ash (wt % of ash) 38.07 Si02 16.92 A203 8.56 Fen03 0.40 Ti02 0.04 p203 15.62 CaO 3.02 MgO 10.99 so3 0.77 Na2O 0.34 K2O 0.30 BaO 0.11 SrO 0.03 NiO 4.75 loss
here describes the characteristics of materials that were not converted to distillates. Specifically, it was the GPC subfractions from the xylene-soluble portion of the unconverted +525 "C fraction of the coprocessed reaction product that were characterized. Measurements included molecular weight distribution, distribution of carbon types (aromatics, aliphatics) and distribution of heteroatoms (S,N, Ni, and V). An understanding of the unconverted material may suggest strategies to improve conversion.
Experimental Section The liquid products used in this study were obtained from coprocessing experiments that were performed in a continuous flow pilot plant, having a 10 L slurry upflow bubble column reactor (length to diameter ratio = 97). The reactions were performed a t a pressure of 13.9 MPa, temperatures of 425, 440,450, and 455 "C, weight hourly space velocities (based on weight of slurry) of 0.5 and 1.0 kg/(h.L), and coal concentrations in the slurry of 5 and 30 wt % coal. The feedstock slurries were premixed in a feed tank using Forestburg coal (Table 11, Cold Lake vacuum bottoms (Table 2), and an iron sulfate additive (which is transformed t o pyrrhotitegJOin the reactor). Nut-sized coal was dry ground t o -200 mesh ( 3000),shown in Table 4 are greater than those for any of the other product subfractions shown in Figures 9 and 10. However, most of them are smaller than the VIC and Ni/C ratios in the feedstock. This indicates that they may not have been formed from product molecules as suggested above. Rather, they may have been formed by the oligomerization of feedstock molecules which lost hydrogen-rich fragments (Figure 5) without contacting sufficient hydrogen to stabilize free radicals, to remove nitrogen, or to remove metals (Figure 9 and 10). The loss of hydrogen-rich fragments may have occurred by cracking or by hydrogen transfer. The product subfraction results for WC and N/C ratios were much different from those for S/C, V/C, and Ni/C ratios. WC and N/C ratios were independent of processing variables except for temperature which had only a minor influence. In contrast, processing variables, such as coal concentration, space velocity, and temperature, caused major changes to the S/C, V/C, and Ni/C ratios. The WC and N/C ratios could not be changed without causing a decrease in molecular weight. In contrast S, V, and Ni heteroatoms could be removed without the product subfractions disintegrating into smaller molecular weight species. All of the above reactions require hydrogen. If the hydrogen is used for hydrodesulfurization (HDS) or hydrodemetallization (HDM) little change in molecular weight occurs. If the hydrogen is used for hydrogenation (HYD) o r hydrodenitrogenation (HDN), then the molecular weight of the molecule decreases. An explanation for the effect of processing conditions being different for WC and N/C ratios than for S/C, VJC, and Ni/C ratios can be suggested in terms of the molecular structures shown in Figure 11. If a sulfur heteroatom is removed from a thiophenic ring in a very large molecule, a slight decrease in molecular weight occurs, but the structure of the original molecule remains intact. If a Ni or V heteroatom is removed from a metal porphyrin molecule, again a small decrease in molecular weight occurs, but the porphyrin molecule remains intact. However, if a nitrogen heteroatom is removed from a porphyrin molecule, the ring structure is destroyed to form a species containing double bonds. Olefins formed in this way could participate in further thermal bond cleavage via a free-radical mechanism. There is a difference between the requirement of hydrogen for removing heteroatoms and the requirement for hydrogen to decrease molecular weight. It was shown previously22that changing one catalyst property can simultaneously alter hydrogenation, sulfur removal
Energy h Fuels, Vol. 9, No. 6, 1995 1019
MW Distributions in Unconverted Vacuum Residues
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and nitrogen removal. This was also true in this study. However, this study indicates that sulfur (or vanadium or nickel) removal will not necessarily cause a change in molecular weight. In contrast, this study indicates that hydrogenating an aromatic ring or removing a nitrogen atom invariably causes a decrease in molecular weight. The influence of molecules in the coal feedstock should become apparent by comparing the 5%coal experimental results with those using 30%coal. Three categories of results have emerged: (1) xylene insolubles increased as the coal concentration increased, (2) the WC and N/C atomic ratios were independent of coal concentrations, and (3) the atomic ratios of other heteroatoms (S/C, V/C, and Ni/C) often decreased as the coal concentration increased. All of these results can be explained in terms of phase separation based on the polarity of the molecules. Polar molecules are less soluble in xylene than hydrocarbon molecules that do not contain heteroatoms. The coal feedstock molecules contained more nitrogen than the oil feedstock molecules (Tables 1 and 2) and are therefore more polar. Hence, (1)an increase in coal concentration would have been expected t o cause the increase in xylene insolubles that was observed. Only the less polar coal molecules would have been expected to be present in the xylene soluble phase. That could explain (2) the constant WC and N/C atomic ratios that were observed when the coal concentration changed from 5 to 30%. Constant WC and N/C atomic ratios in the distillate were also observed by Fouda et al. when the coal concentration varied from 5 to 30%. The most polar of the oil feedstock molecules would be expected t o be present in the xylene insoluble phase, together with the polar coal molecules. The most polar oil molecules are also the ones that have greater concentrations of S, V, and Ni. The movement of the polar oil molecules into the xylene insoluble phase would explain (3) the decrease in S/C, V/C, and Ni/C atomic ratios that were sometimes observed. The final result is that at (22) Ternan, M. J. Catal. 1989, 104, 256-257.
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Figure 12. Atomic fraction of carbon atoms in the 33.5-36.5 ppm spectral region (naphthenic carbon atoms attached to two hydrogen atoms, CH2 groups) vs molecular weight. The symbols represent the following: triangles, feedstock subfractions; circles, product subfractions; open circles, 425 "C reactor temperatures at all coal concentrations and all space velocities; solid circles, 450 "C reactor temperatures at all coal concentrations and all space velocities.
some processing conditions, increasing the coal concentration can improve heteroatom removal (S, Figure 8; V, Figure 9; Ni, Figure 10) from the unconverted xylenesoluble material. l3C NMR spectra were measured for all of the subfractions. The general features of the spectra were identical to those reported previously.2 The spectra were divided into the spectral regions shown in Table 3 and given the same assignments that were used previously.2 The weight fractions of the carbon atoms in selected spectral regions have been compared to understand the transformations that have occurred to the unconverted residues. Naphthenic carbon atoms bonded to two hydrogen atoms are responsible for much of the broad hump between 10 and 50 ppm in 13C NMR spectra of hydrocarbons. There are several peaks superimposed on this hump, each of which is caused by a specific type of carbon atom, which is not necessarily naphthenic. There are two spectral regions of this naphthenic hump, 24-28.5 and 33.5-36.5 ppm, which do not contain peaks caused by other species. These regions can be ascribed exclusively to naphthenic carbon atoms.2 Figure 12 shows that for the larger molecular weight subfractions in the 33.5-36.5 ppm spectral region, the unconverted residues have less naphthenic carbon than the feedstock. For a fixed temperature all of the data fall near a single line regardless of space velocity or coal concentration. Temperature is the only processing variable that has an effect, and the difference it causes between 425 and 450 "C is small. At the higher temperature, 450 "C,there are fewer naphthenic carbon atoms than at 425 "C. This observation is consistent with a shift in equilibriumls with increasing temperature, making dehydrogenationmore favorable (less naphthenic carbon with increasing temperature). Naphthenic carbon atoms bonded to a single hydrogen atom resonate in the 41-60 ppm spectral region. The fraction of these carbon atoms in the product subfrac-
1020 Energy & Fuels, Vol. 9, No. 6, 1995
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Figure 13. Atomic fraction of carbon atoms in the 41-60 ppm spectral region (naphthenic carbon atoms attached to one
hydrogen atom, CH groups)vs molecular weight. The symbols represent the following: triangles, feedstock subfractions; circles, product subfractions; open circles, 425 "C reactor temperatures at all coal concentrations and all space velocities; solid circles,450 "C reactor temperatures at all coal concentrations and all space velocities. tions is shown in Figure 13. For a given temperature, the data at all space velocities and all coal concentrations fall near the same line. Only a small change in naphthenic carbon occurred between 425 and 450 "C. The product subfractions having molecular weights greater than 1000 contained fewer CH naphthenic carbon atoms than the feedstock subfractions. At molecular weights slightly less than 1000, the amounts of CH naphthenic carbon atoms in some of the product subfractions are scattered about the value in the feedstock subfraction. Aromatic carbon atoms atttached to one hydrogen atom resonate in the 112-130 ppm spectral region. The amounts of these carbon atoms in the product subfractions are shown in Figure 14. For a given temperature, the data at all space velocities and all coal concentrations fall near the same line. Only a small change in the C-H aromatic carbon occurred between 425 and 450 "C. The product subfractions have more aromatic carbon atoms than the feedstock. Furthermore, there are more C-H aromatic carbon atoms at the higher temperature, 450 "C, than at 425 "C, showing an increase in aromatics as the temperature increases. The fractions of aromatic carbon atoms bonded to one hydrogen (aromatic CH) in the oligomerized molecules, shown in Table 4, are similar to those of the larger product molecules ( M W = 3000) in Figure 14. The fractions of aromatic CH carbon atoms (Figure 14)in the smallest molecules (MW < 1000)are much greater than the values in Table 4. Again this indicates that the molecules which oligomerized may have been related to the larger product molecules, rather than the smallest molecules. However, that suggestion would not be consistent with the V/C atomic ratio data. The evidence from the aromatic CH carbon atom data supports the data for WC, N E , and metal removal data. Both are consistent with these very large molecules (3000< MW < 6000) being formed by the oligomerization of dehydrogenated feedstock molecules, which lost
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Figure 14. Atomic fraction of carbon atoms in the 112-130 ppm spectral region (aromatic carbon atoms attached to one
hydrogen atom, CH groups)vs molecular weight. The symbols represent the following: triangles, feedstock subfractions; circles, product subfractions; open circles, 425 "C reactor temperatures at all coal concentrations and all space velocities; solid circles, 450 "C reactor temperatures at all coal concentrations and all space velocities. 0.40
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Figure 15. Atomic fraction of carbon atoms in the 130-150
ppm spectral region (aromaticcarbon atoms attached to only to other carbon atoms) vs molecular weight. The symbols represent the following: triangles, feedstock subfractions; circles, product subfractions; open circles, 425 "C reactor temperatures at all coal concentrations and all space velocities; solid circles,450 "Creactor temperatures at all coal concentrations and all space velocities. hydrogen-rich fragments without contacting sufficient hydrogen to remove metals or to remove nitrogen. Aromatic quaternary carbon atoms, those that are bonded only to other aromatic carbon atoms, resonate in the 130-150 ppm spectral region. The amounts of these carbon atoms in the product subfractions are shown in Figure 15. For a given temperature, the data a t all space velocities and all coal concentrations fall near the same line. An increase in the quaternary aromatic carbon occurred between 425 and 450 "C. The aromatic content of the subfractions can have an effect on their free-radical concentrations. I t has been
h4W Distributions in Unconverted Vacuum Residues
recognized that some catalysts provide hydrogen to (1) stabilize free radicals and (2) add hydrogen to the It~has been suggested that the specific hydrocarb~n.~ ,~~ role of the catalyst is to dissociate the hydrogen.25 Freeradical concentrations can be measured by electron spin resonance (ESR). Provine et aLZ6 performed short residence time (250 s) coal liquefaction experiments on two coals having WC atomic ratios of 0.89 and 1.01. Their ESR measurements at room temperature showed that as the aromatic content of the residues increased (WC ratio decreased) the free-radical concentration of the unconverted reactor residues increased. Ibrahim and Seehra2' reported room temperature ESR measurements on coal-derived liquid residues. As the WC ratio of their samples decreased from 1.11to 0.89 the freeradical concentration increased which is consistent with the report by Provine et a1.26 However, as the WC ratio of their liquid residues decreased further, from 0.89 t o 0.83, the free-radical concentration decreased. Their maximum free-radical concentration occurred when 0.87 < WC < 0.94. Most WC ratios reported in this work, were larger than the above values. Nevertheless, as shown in Figures 14 and 15, the aromatic contents of the unreacted product subfractions were greater than those of the feedstock. According to the studies referred to above, this increase in aromatic content may have caused the free-radical concentration to increase and thereby may have increased the initiation of thermal reactions. A transition from naphthenic carbon t o aromatic carbon can be observed from the information in Figure 12 to 15. The fraction of naphthenic carbon decreased in the following order, feedstock residue > 425 "C residue > 450 "C residue, as can be seen in Figure 12 and 13. The fraction of aromatic carbon increased in the following order, 450 "C residue > 425 "C residue > feedstock residue, as can be seen in Figure 14 and 15. This evidence shows very clearly that processing conditions do have a major influence on the structure of the unconverted residue. Carbon atoms in hydrocarbon chains attached to cores of condensed aromatic rings are included in the sharp peak that occurs in the 28.5-31 ppm spectral region. This peak is caused by CH2 groups that are y or more (three or more carbon atoms) from terminal CH3 groups on alkane chains. For example, in long-chain alkanes the y carbon in CH2 groups are in the 29.5-30.5 ppm region.28 The weight fractions of y carbon atoms are shown in Figure 16. For a given temperature, the data at all space velocities and all coal concentrations fall near the same line. Only a small change in the aliphatic CH2 carbon occurred between 425 and 450 "C. The product subfractions obtained at 425 "C have a distribution of y carbon with molecular weight that is generally similar to the feedstock subfractions. One (23)Burgess, C. E.; Artok, L.; Schobert, H. H. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel. Chem. 1991,36, 462-469. (24) DelBianco, A,; Panariti, N.; DiCarlo, S.; Beltrame, P. L.; Caniti, P. Energy Fuels 1994, 8, 593-597. (25)Walter, T. D.; Casey, S. M.; Klein, M. T.; Foley, H. C. CataE. Today 1994,19, 367-380. (26)Provine,W. D.; Jung, B.; Jacintha, M. A.; Rethwisch, D. G.; Huang, H.; Calkins, W. H.; Klein, M. T.; Scouter, C.; Dybowski, C. R. Catal. Today 1994, 19, 409-428. (27) Ibrahim, M. M.; Seehra, M. S. Catal. Today 1994, 19, 337352. (28) Stothers, J. B. Carbon-13NMR Spectroscopy; Academic Press: New York, 1972.
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Figure 16. Atomic fraction of carbon atoms in the 28.5-31 ppm spectral region (aliphatic carbon atoms attached t o two hydrogen atoms - CH2) versus molecular weight. The symbols represent the following: triangles, feedstock subfractions; circles, product subfractions; open circles, 425 "C reactor temperatures at all coal concentrations and all space velocities; solid circles, 450 "C reactor temperatures a t all coal concentrations and all space velocities.
difference is that the 425 "C product subfractions contain molecules of smaller molecular weight with less y carbon than the feedstock subfractions. This shows that for most unconverted molecules, the side chains are not removed from the unconverted cores of condensed aromatic rings at a reaction temperature of 425 "C. This experimental observation is contrary to the traditional view that side chains are removed during hydroprocessing. However, this observation is consistent with a similar observation2 made at reasonably mild temperatures, during residue hydrocracking experiments. Even though side chains are not removed from most unconverted molecules, it is entirely plausible that side chains are removed from molecules that are converted to distillates. For example, Sanfordz9has shown that gas formation during residue hydrocracking is a secondary reaction. This implies that the residue molecules are first converted to distillate molecules and that gases are subsequently formed from these distillate molecules. The larger molecules in the product subfractions obtained at 450 "C have had some side chain carbon removed as indicated in Figure 16. This indicates that as the temperature increases, side chains are removed from unconverted molecules. However, even at 450 "C generally more than half of the side chain carbon atoms still remain. The lowest molecular weight product subfractions obtained at 450 "C contain molecules of smaller molecular weight and fewer side chain carbon atoms than either the corresponding product subfractions obtained at 425 "C or the feedstock subfractions. The oligomerized molecules in the 450 "C product subfraction in Table 4 also have less side chain carbon than the feedstock subfractions. In summary, many of the unconverted molecules have the same amount of side chain carbon as the feedstock. The two exceptions at 450 "C are small residue molecules: ( M W < 700) and oligomerized molecules (MW > 2000). (29)Sanford,E. C. Ind. Eng. Chem. Res. 1994,33, 518-530.
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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 than the corresponding feedstock subfraction is also consistent with the removal of side chains.
Conclusions Elemental and molecular weight distributions have been measured in the unconverted portion of liquid products obtained by coprocessing subbituminous coal (Forestburg) and vacuum tower bottoms (Cold Lake heavy oil). Five significant observations were made. (1) The molecules that were converted had larger WC atomic ratios and smaller N/C atomic ratios than the feedstock molecules. (2a) S, V, and Ni heteroatoms could be removed without causing much change in molecular weight. (2b) It was not possible to increase the WC atomic ratio or to decrease the N/C atomic ratio without decreasing the molecular weight. (3)Generally, the unconverted f 5 2 5 "C residue molecules became smaller as the processing severity increased. (4) The unconverted molecules retained their side chains at mild processing severities (425 "C). When the processing severity increased (450 "C) side chain carbon began to
be removed. ( 5 ) If there was insufficient reaction time to provide enough hydrogen, then molecules which were larger than the feedstock molecules were formed. Feedstock molecules that lost their hydrogen-rich fragments (by cracking or by hydrogen transfer) without contacting enough hydrogen to remove their nitrogen or metal heteroatoms may have oligomerized. If the reaction time was increased to allow more contact time with hydrogen, the oligomerized molecules which were larger than the feedstock molecules disintegrated. Finally, conversion of residue molecules to distillate molecules appeared to be limited by the rate of hydrogen addition. The hydrogen was necessary either t o hydrogenate aromatic rings or to remove nitrogen heteroatoms.
Acknowledgment. The authors gratefully acknowledge the contributions of Mr. Bart Young who performed the gel permeation chromatography separations, and the staff of the analytical laboratory who peformed the elemental analyses. This work was supported in part by external sponsors (Amoco Canada Petroleum, Rheinbraun AG, and the Alberta Oil Sands Technology and Research Authority) and in part by the federal Program on Energy Research and Development (PERD). EF9500954
