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Energy & Fuels 2008, 22, 2512–2517
Molecular Assemblies in Asphaltenes and Their High-Temperature Coke Products. Part 1: Initial Molecular Organization Douglas L. Dorset* and Michael Siskin Corporate Strategic Research, ExxonMobil Research and Engineering Company, 1545 Route 22 East, Annandale, New Jersey 08801 ReceiVed March 24, 2008. ReVised Manuscript ReceiVed May 6, 2008
The physical nature of the coke product of pyrolyzed heavy hydrocarbon fractions has been of great interest to the petroleum industry for the ease of its removal from the drum in delayed coking. The coking process itself and chemical makeup of the residuum feed have been studied to judge how the easily removed, granular shot coke can be preferentially formed over an immobilized sponge coke product. In this work, electron diffraction was employed to study the progression of molecular alignment in asphaltenes during the formation of their coke byproduct. Asphaltenes exhibit the same glassy molecular assembly revealed previously by X-ray diffraction, indicating that a 10-9 sampled volume reduction betrays no increase of localized order. On the other hand, for cokes, the greatly reduced sampled volume reveals further structural details not detected in powder X-ray patterns, consistent with the ordering of “basic structural units” in early stages of graphitization. While the (002) stacking distance near 3.4 Å is commonly observed, variations of this spacing are somewhat insensitive to the early graphitization process, as are the lateral and stacked structural coherence. The most sensitive parameter for initial ordering appears to be the arcing of (002) reflections and the increasing number of its diffraction orders. The ordering seems to depend, in part, upon the time that the material is held at a higher temperature.
Introduction Crude petroleum comprises a chemical catalog of hydrocarbon species that can be distilled into characteristic boiling fractions that may have more unified molecular properties.1 Fractions that can be solidified at room temperature are similarly diverse. Relative amounts of subfraction chemical species can also depend upon the origin of the crude oil, where some might contain a large linear chain fraction, whereas others may include more branched chains and ring moieties. The presence of shorter chains and such chains on ring derivatives indicates that biodegradation has occurred. Of the various subfractions, the best understood, on a structural basis, are the waxes. When there is no chain branching, the rules for forming stable solid solutions have been extensively studied in terms of an underlying molecular stacking and crystal structure.2 Two types of solid solutions can be formed, one with an average lamellar structure and another with a “nematically” disordered arrangement of chains in the crystalline solid. Chainbranching influences the layer packing of the linear chain segments, possibly leading to an irregular lamellar interface. Nevertheless, the average crystal structures seem to obey the rules for molecular packing adopted by pure components. Other petroleum fractions can include molecules with a chain linked to a nonpolar aromatic or naphthenic ring moiety.1 However, the crystal packing of simple model compounds in this class have not been extensively studied. Although detergents and other amphiphilic molecules have been well-characterized, * To whom correspondence should be addressed. Fax: 908-730-3198. E-mail:
[email protected]. (1) Musser, B. J.; Kilpatrick, P. K. Energy Fuels 1998, 12, 715. (2) Dorset, D. L. Crystallography of the Polymethylene Chain. An Inquiry into the Structure of Waxes; Oxford University Press: Oxford, U.K., 2005.
only one alkyl benzene crystal structure is known3 but none has been determined for alkyl derivatives of single-ring cycloalkanes/naphthenes. Although an alkyl benzene contains no strongly polar moiety, its packing is again similar to, e.g., the non-ionic alkyl glucose detergents in that the cross-sectional area of the ring system packing is matched by the alkyl chain subcell packing via chain tilt and interdigitation.2 The better understood crystal structures of cholesteryl esters might also give insights into the packing of more complicated naphthenic molecules, particularly because some of these do not include a well-defined methylene subcell region in the unit cell. The crystal structures of some cholesteryl ester solid solutions have also been determined,2 but multicomponent solids of alkyl benzenes, etc., have not been characterized. Even less understood are the asphaltenes containing a rich array of aromatic and saturated ring moieties that can be linked to or via linear chains.4,5 In wide-angle X-ray scattering studies of this petroleum fraction, it is postulated that a diffraction peak due to residual linear chain paraffinic components can co-exist with a nearby peak because of the π-π stacking of aromatic rings suggestive of a precursor graphitic assembly.6–9 Low-angle peaks detected by small-angle X-ray scattering (SAXS) or small(3) Merz, K. Acta Crystallogr., Sect. E: Struct. Rep. Online 2002, 58, 450. (4) Strausz, O. P.; Majelsky, T. W.; Lown, E. M. Fuel 1992, 71, 1355. (5) Siskin, M.; Kelemen, S. R.; Eppig, C. P.; Brown, L. D.; Afeworki, M. Energy Fuels 2006, 20, 1227. (6) Ebert, L. B.; Scanlon, J. C.; Mills, D. R. Liq. Fuels Technol. 1984, 2, 257. (7) Sadeghi, M.-A.; Chilingarian, G. V.; Yen, T. F. Energy Sources 1986, 8, 99. (8) Tanaka, R.; Sato, E.; Hunt, J. E.; Winaus, R. E.; Sato, S.; Takanohashi, T. Energy Fuels 2004, 18, 1118. (9) Andersen, S. I.; Jensen, J. O.; Speight, J. G. Energy Fuels 2005, 19, 2371.
10.1021/ef800210c CCC: $40.75 2008 American Chemical Society Published on Web 06/24/2008
Molecular Assemblies in Asphaltenes
angle neutron scattering (SANS) were originally attributed to a micellar-like assembly of globules.8,10 More recently, this model has been revised in favor of a microphase separated structure (arrested spinodal decomposition).11 Aliphatic functionalization in asphaltenes is lost upon heating as they are transformed into cokes. Presumably, the simplification of the average molecular species in the assembly would promote graphitization, as evidenced in the early X-ray studies of graphitizing carbons. Because electrons are more efficiently scattered by matter than X-rays, e.g., 106-fold in terms of scattered intensity,12 electron diffraction has provided a clearer view of molecular packing in various petroleum waxes, enabling old models for wax assemblies to be revised.2 Although electron diffraction has been used often for the study of graphites and their precursors,13–15 there seems to be no carefully executed studies where this technique has probed asphaltene fractions as they are transformed into cokes. An initial attempt to begin such a systematic characterization is described in this paper. Materials and Methods Materials. An attempt was made to study materials at three levels of organization. First, asphaltenes from Maya crude oil and Cold Lake bitumen were considered. The resid deasphalting was carried out by the following procedure. A mixture of vacuum resid feedstock and n-heptane were added to a 250 mL round-bottom flask in a ratio of 1 part of feed stock to 8 parts of n-heptane and allowed to stir for 16 h at room temperature. The mixture was then filtered through a coarse Buchner funnel to separate the precipitated asphaltenes. The solids were dried in a vacuum oven at 100 °C overnight and weighed. The n-heptane was evaporated from the oil/heptane mixture to recover the deasphalted oil. Samples for electron diffraction were solubilized in xylenes and evaporated to dryness on a carbon-covered 300-mesh copper electron microscope grid. Alternatively, they were sonicated in n-heptane after being crushed to a powder, and the dispersion was then evaporated on the grid surface. At the second level of complexity, a vacuum residuum from a sweet crude [also termed vacuum tower bottoms (VTB)] was subjected to a microcarbon residue test5 (MCRT) (ASTM D453003 standard test for determination of carbon residue (micro method); ASTM D189-05 standard test for conradson residue for petroleum products) in which approximately 2 g of the sample was heated to 100 °C over 10 min under a nitrogen flow of 66 cc/min. The temperature was then increased from 100 to 300 °C at the same nitrogen flow rate and then from 300 to 500 °C at a reduced nitrogen flow (19.5 cc/min). The total heating time from 100 to 500 °C was 30 min. The sample was then held at 500 °C for a variable time (t ) 0, 0.5, 2, 3, 4, 5, 10, 15, and 30 min) at 19.5 cc/min nitrogen flow and finally quenched to room temperature while maintaining the same nitrogen flow. The solid was again ground in a mortar and pestle and sonicated in n-heptane for dispersion on a grid surface. Finally, cokes from various commercial processes were examined. These were produced as delayed-coker sponge or shot cokes or the cokes from a Flexicoker or Fluid coker.16 For the delayed (10) Mansoori, G. A. Arabian J. Sci. Eng. 1996, 21, 707. (11) Sirota, E. B. Energy Fuels 2005, 19, 1290. (12) Vainshtein, B. K. Structure Analysis by Electron Diffraction; Pergamon: Oxford, U.K., 1964. (13) Finch, G. I.; Wilman, H. Proc. R. Soc. London, Ser. A 1936, 155, 345. (14) Ogawa, T.; Moriguchi, S.; Isoda, S.; Kobayashi, T. Polymer 1994, 35, 1132. (15) Ayer, R.; Scanlon, J. C.; Ebert, L. B. J. Phys. Chem. 1995, 99, 9639. (16) Hammond, D. G.; Lampert, L. F.; Mart, C. J.; Massenzio, S. F.; Phillips, G. E.; Sellards, D. L.; Woerner, A. C. Sixth Topical Conference on Refining Processing, Tutorial on Resid Upgrading, AIChE 2003 Spring National Meeting.
Energy & Fuels, Vol. 22, No. 4, 2008 2513 coker cokes, the residuum feed was heated to 500 °C for 1 to 2 min, followed by a residence in the coker drum from 1 to 12 h at 425 °C (vacuum residua, such as Maya or Cold Lake, might produce shot coke in a delayed coker,5 while a sponge coke would result from the coking of sweet VTB). For the other two (i.e., Flexicoker or Fluid coker) cokes, there was an average residence time of 8 min at 530 °C on the surface. The coke bead onto which the feedstock had been sprayed had previously seen a burner at 600 °C for 2 to 3 min. In the Flexicoker case, there is an additional 1-3 h of average residence time in the gasifier at 900-950 °C. Samples for electron diffraction were prepared again from the n-heptane dispersion as before. Electron Diffraction. Selected area electron diffraction experiments were carried out at 300 kV with a Philips/FEI CM-30 electron microscope. For radiation-sensitive samples, a 1.27 µm selected area diameter was chosen. Incident electron beam current density to the samples was diminished partially by use of a spot size of 5. For samples relatively stable in the electron beam, a 0.25 µm diameter was chosen at an incident beam spot size of 3. Electron diffraction patterns were recorded on a Kodak BioMax MS X-ray film. The dispersed crushed samples were observed as thin areas of several micrometers in diameter and were estimated to be around 100-200 Å thick. Diffraction spacings were calibrated against a gold powder standard. Because electron diffraction is a microscopic technique, there is a practical concern with the visualization of a statistically meaningful sample, compared to bulk measurement techniques, such as powder X-ray diffraction. In principle, such a limitation has been recognized since at least the 18th century.17 For this reason, at least 25 different patterns were obtained from any given specimen, and these data were evaluated to seek internal consistency. On the other hand, the benefit of electron diffraction is that local ordering can be observed when it might not be so easily discerned by powder diffraction techniques. Most electron diffraction experiments were carried out at ambient temperature. In one set of experiments on asphaltenes, an Oxford liquid nitrogen stage was used in the electron microscope to permit observations at -90 °C.
Results Asphaltenes from Vacuum Residua. Interpretation of the electron diffraction results from the asphaltenes is rather straightforward. When the materials are taken up in xylene and resolidified by evaporation, it is possible to find microcrystalline inclusions of a paraffinic wax component in both Maya and Cold Lake samples (Figure 1), evidenced by the characteristic hk0 pattern.2 More typical is the continuous diffuse profile (Figure 1) with an edge near 3.4-3.5 Å. When the solid asphaltenes are crushed and sonicated in n-heptane for dispersion on the grid surface, only the continuous diffuse profile is seen. Cooling the samples to -90 °C does not affect the appearance of the continuous diffraction pattern. X-ray and neutron scattering patterns from the Cold Lake asphaltene18 also show the characteristic broad peak that includes characteristic γ and (002) components observed in other studies.9 There is also direct evidence for a wax component within the residua (Figure 1). Characteristic hk0 electron diffraction patterns19,20 from untilted paraffin chains were sometimes observed from materials recrystallized from xylene solution. (17) Baker, H. The Microscope Made Easy, or the Nature, Uses, and Magnifying Powers of the Best Kind of Microscope Described, Calculated and Explained; R. Dodsley: London, U.K., 1742; Chapter 15: Cautions in viewing objects. (18) Sirota, E. B. Personal communication. (19) Vainshtein, B. K.; Lobachev, A. N.; Stasova, M. M. SoV. Phys. Crystallogr. 1958, 3, 452. (20) Dorset, D. L. Acta Crystallogr., Sect. A: Found. Crystallogr. 1976, 32, 207.
2514 Energy & Fuels, Vol. 22, No. 4, 2008
Dorset and Siskin
Figure 3. Electron diffraction, delayed coking.
Figure 4. Electron diffraction, MCR test, anneal 0.5 min at 500 °C.
Figure 1. Electron diffraction of asphaltenes.
Figure 2. Coke electron diffraction.
Commercial Cokes. Fluid or Flexicoke. Electron diffraction patterns from these cokes are shown in Figure 2. They can be interpreted21 in terms of a graphitic d(002) spacing near 3.5 Å and another arc corresponding to the (100) reflection near 2.1 Å. At times (as will be illustrated below), ring patterns containing the graphitic (100) and (110) reflections are observed. (21) Oberlin, A.; Goma, J.; Rouzaud, J. N. J. Chim. Phys. 1984, 81, 701.
As the bulk material is examined, many cycles through the coker might have been involved in its production. Delayed Cokes. Electron diffraction patterns from these cokes are shown in Figure 3. They are more ordered than the ones depicted in Figure 2. The patterns shown contain at least 3 orders of the graphitic (002) reflection, with d(002) near 3.5 Å and two arcs corresponding to the (100) and (110) reflections, respectively, near 2.1 and 1.2 Å. Ring patterns are also observed just with the graphitic (100) and (110) reflections. Microcarbon Residue Tests. Materials prepared from a vacuum residuum according to the protocol outlined above vary only in dwell time at a temperature after 500 °C is reached. By diffraction, samples immediately quenched resemble the asphaltenes described above. Generally, one observes (a) hk0 patterns from intact paraffin wax domains, (b) composites of the paraffin wax domain and the continuous profile characteristic of the asphaltene itself, or (c) the continuous profile itself (e.g., similar to Figure 1). Again, the edge of the continuous profile occurs near 3.4 Å. Material held at 500 °C for 0.5 min begins to show an emergent (002) ring [d(002) ) 3.40(6) Å] in some selected microareas (Figure 4), while the typical asphaltene pattern is observed elsewhere. At 2 min at this temperature, the (002) diffraction ring is seen almost constantly, where d(002) ) 3.46(5) Å. These results are observed with a 1.27 µm diameter selected area aperture. If a 0.25 µm diameter aperture is used instead, arced patterns are commonly observed for the sample annealed for 2 min (Figure 5). On the other hand, arced, ring, or “aphaltene”-like patterns are found for the sample annealed for 0.5 min when the smaller aperture is employed. If the heated vacuum residuum is allowed to remain at 500 °C for 5 min, the patterns shown in Figure 6 are obtained. The (002) reflections are arced, somewhat like those from the Fluid or Flexicoke samples. One can also obtain ring patterns including the (100) and (110) reflections. The average d(002) ) 3.46(6) Å. When the gap between 2 and 5 min annealing
Molecular Assemblies in Asphaltenes
Energy & Fuels, Vol. 22, No. 4, 2008 2515
Figure 5. Electron diffraction, MCR test, anneal at 0.5 and 2 min, sampled with a 0.25 µm aperture diameter. Figure 8. Electron diffraction, MCR test, annealing 30 min at 500 °C (1.27 µm aperture).
when the material is held at a temperature for 30 min, an ordered, directed pattern is again observed, where d(002) ) 3.54(5) Å, along with the characteristic ring pattern (Figure 8). Discussion
Figure 6. Electron diffraction, MCR test, 5 min at 500 °C (1.27 µm aperture).
Figure 7. Electron diffraction, MCR test, annealing at 500 °C for 3 and 4 min and sampling with two selected area diameters.
time was filled, samples held at 500 °C for 3 and 4 min were also examined. With either aperture diameter, arced patterns are generally observed (Figure 7), although the extent of (002) reflection arcing is less when the area is sampled with the 0.25 µm diameter. In either case, the average d(002) is 3.47 Å. If the heated vacuum residuum remains at 500 °C for 10 min, patterns reveal a much more ordered sample, somewhat reminiscent of the delayed cokes. In this case, d(002) ) 3.47(6) Å. The ring pattern shown is much sharper, but others with broader peak profiles are also observed. The vacuum residuum kept at 500 °C for 15 min gives similar patterns. There are always variations in ordering among microcrystalline areas. In this case, d(002) ) 3.49(5) Å. Finally,
There have been numerous X-ray diffraction studies of asphaltenes to reveal broad peaks that can be separated into putative paraffinic components (γ peak) and the (002) peak from stacked aromatic rings.4–9 There is essentially nothing new that can be gleaned from electron diffraction patterns of asphaltenes (Figure 1), except that, at an approximately 10-9 reduction of sampled volume, the material still has the same glassy nature. On the other hand, it is clear that aromatic ring compounds are partially ordered in stacks with a nearly common orientation, so that these materials can be easily graphitized.22 The process of graphitization has also been extensively studied by powder X-ray diffraction,23–25 revealing a precursor stacking of aromatic rings accounting for an interlayer spacing between 3.44 and 3.45 Å, always larger than that of the graphite c/2 ) 3.354 Å distance. More information about molecular alignment is gained from electron diffraction and transmission electron microscopic investigations as shown by Oberlin and co-workers.21,22,26–28 A working model for the graphitization process26 was proposed, where with increasing heat treatment, basic structural units (BSUs) of stacked rings grow toward a more complete graphite structure through at least four stages of organization. The characteristic electron diffraction patterns from these various stages22,27 reveal the gradual increase of order within the solid. As mentioned above, single-crystal electron diffraction studies of graphite itself have also been carried out (in addition to X-ray crystallographic studies of its polymorphs), including the quantitative determination of its crystal structure from such data.14 The common crystal structure of graphite refers to space group P63/mmc,29,30 where a ) 2.46 Å and c ) 6.70 Å. A rhombohedral modification, in space group R3jm, also exists,29,30 accounting for stacking disorders. In no case are true graphite structures encountered for the cokes considered in this work, although these materials can be heated to the more ordered (22) Rouzaud, J. N.; Oberlin, A. Carbon 1989, 27, 517. (23) Franklin, R. E. Acta Crystallogr. 1950, 3, 107. (24) Franklin, R. E. Acta Crystallogr. 1951, 4, 253. (25) Monthioux, M.; Oberlin, M.; Oberlin, A.; Bourrat, X.; Boulet, R. Carbon 1982, 20, 167. (26) Oberlin, A.; Bonnamy, S.; Bourrat, X.; Monthioux, M.; Rouzaud, J. N. ACS Symp. Ser. 1986, 303, 85. (27) Oberlin, A. Chem. Phys. Carbon 1989, 22, 1. (28) Oberlin, A. Carbon 2002, 40, 7. (29) Lipson, H.; Stokes, A. R. Proc. R. Soc. London, Ser. A 1942, 181, 101. (30) Laves, F.; Bashin, Y. Z. Kristallogr. 1956, 107, 337.
2516 Energy & Fuels, Vol. 22, No. 4, 2008
Dorset and Siskin Table 1. Domain Sizes (Å) of Coke Samples, as Estimated by Electron Diffraction Line Broadening
Figure 9. Indexing of zonal electron diffraction from various views of a graphitic material (after Oberlin et al.21).
form.31 Achievement of the true graphite structure (Oberlin’s stage 426) would be betrayed by electron diffraction patterns with well-defined spots, in accordance with unit cell symmetry. Only at this stage do graphitic layers achieve a stacking registry to form the ordered unit cell. As shown by Oberlin,27,28 intermediate patterns from incompletely graphitized carbons (e.g., Figures 2 and 3) can be understood if one begins with the electron diffraction pattern from a single graphite layer. A single layer would result in a continuous diffraction line in the direction perpendicular to the sheet itself. If there is an in-plane rotational disorder of neighboring sheets (i.e., a “turbostratic” array), including rotational disorder of stacked sheets, to form a two-dimensional texture, then the 3D diffraction pattern will be a continuous cylinder (along c*) for the lateral (100) and (110) reflections and the only set of reflections with discrete values along c* (turbostratic sheet stacking) will be the 00l row. This theoretical pattern (Figure 9) follows from any elementary consideration of layer-stacking disorder.32 From well-known Fourier transforms of limited crystal sizes,33 it is also possible to estimate the lateral and stacking domain sizes for the various materials, respectively, from the widths of (00l) and (100) lines.27 Electron diffraction patterns from the coke products described in this paper as well as previously described heat-treated cokes27,31,34,35 also match those anticipated by theory. The identification of the ring patterns in a [001] projection (e.g., Figures 5 and 6) is obvious and measured diffraction spacings correspond to the theoretical values based on the graphite unit cell.29,30 Patterns from the Flexi- and Fluid cokes, in addition to those from the microcarbon residue test that had been held at 500 °C for 5 min, seemingly correspond to the first stage of graphitization proposed by Oberlin.26 Domain sizes, estimated from diffraction broadening, are 32 Å (lateral) and 34 Å (layering). At greater heating times, the lateral domain can occasionally grow to >100 Å, although more typical values near 30 Å are also observed. Other cokes, including the shot and sponge examples and materials held for a longer time at high temperature seem to fit with either Oberlin’s26 stage 2 or 3, i.e., by a visual match of electron diffraction patterns. For shot coke, the estimated lateral domain size is ca. 84 Å and the stacking domain size is 51 Å. For sponge coke, these figures are, respectively, 35 and 31 Å, corresponding also to measurements for the solid Flexicoker and Fluid coker products. Specific results are summarized in Table 1. Actually, there is a broad distribution (31) Kuroda, H. Bull. Chem. Soc. Jpn. 1959, 32, 728. (32) Harburn, G.; Taylor, C. A.; Welberry, T. R. Atlas of Optical Transforms; Cornell University Press: Ithaca, NY, 1975. (33) Dorset, D. L. Structural Electron Crystallography; Plenum: New York, 1995; p 8. (34) Sanders, J. V.; Spink, J. A.; Pollack, S. S. Appl. Catal. 1983, 5, 65. (35) Pieck, C. L.; Jablonski, E. L.; Parera, J. M.; Frety, R.; Lefebvre, F. Ind. Eng. Chem. Res. 1992, 31, 1017.
coke
L(a) [lateral, from (100)]
L(c) [layer, from (001)]
MCRT, 0.5 min MCRT, 2 min MCRT, 5 min MCRT, 10 min MCRT, 15 min MCRT, 30 min sponge shot flexicoker fluid coker
17 36 32 36 38 20 35 84 33 38
12 34 32 53 43 31 51 33 38
of values, showing that these parameters are not particularly sensitive to the early stages of graphitization nor are any trends noted that can be correlated to relatively short annealing times. All values correspond to the initial stages of local molecular orientation.27 Variations in the (002) spacing and domain size are also rather insensitive parameters to assess the early stages of graphitization (Figure 10) and are not useful in themselves for determining the ordering of basic structural units.20 From the mean values plotted in Figure 10, there appears to be an increase of lamellar stacking distance with increasing annealing time. This is counterintuitive. However, the large standard deviations of these data points indicate that the curve can be equally well-fit by a flat straight line, i.e., no variation with annealing time. This seems to be the correct interpretation of these data. More insight into structural changes can be obtained from the arcing angle of the (002) reflections22 (the measured value used is one that corresponds to the most ordered micro-area observed during an experiment on a given annealed specimen). Using this parameter, the time sequence of microcarbon residue test samples can be understood. At increasing hold time of the residuum at 500 °C, this angle decreases, viz 180°, 180°, 62°, 46°, 58°, 45°, 40°, and 36°, respectively, for 0.5, 2, 3, 4, 5, 10, 15, and 30 min (Figure 11). Take note that these values correspond only to 1.27 µm selected area diameters. The Flexicoker and Fluid coker cokes (angles 85° and 66°, respectively) are much less ordered, by this criterion, than are the shot and sponge cokes (angle 38°) which were held at lower temperatures (∼425 °C) but for longer times (hours versus minutes), betrayed also by the increased number of (00l) reflection orders for the latter two specimens. From these measured arc angles, the cokes all correspond to the initial stage of graphitization according to Rouzaud and Oberlin,22 consistent
Figure 10. Stacking distance of layers (Å), d(002), with increasing annealing time in the MCR test.
Molecular Assemblies in Asphaltenes
Figure 11. Variation of (002) reflection arcing angle (rad.) with an increasing annealing time at 500 °C (MCR test). Results are plotted for two selected area diameters.
with the temperatures used for their formation. Trends are seen therefore in the ordering process at increasing annealing times for a single feedstock. This does not give a complete picture of the ordering process however. The occurrence of (002) ring patterns at short annealing times prompted a further study with a 0.25 µm aperture (Figure 5), as well as a comparative study with two aperture diameters for samples annealed for 3 and 4 min (Figure 7). Although reduction of the sampled area revealed no change in diffraction from asphaltenes, would further reduction of the sampled area affect diffraction results from the cokes? As indicated above, it certainly does. While a powder pattern can still be observed often (but not always) at 0.5 min annealing time, arced (002) reflections are always observed for all longer annealing times with the smaller aperture. The increased ordering at smaller selected area diameter indicates that mutually aligned stacked molecular domains form at very short annealing times and that the extent of this alignment increases at longer annealing periods, so that no difference in the arc lengths will be noted eventually for differing aperture diameters. Thus, for annealing times, 0.5, 2, 3, and 4 min, the arc angles resulting from a 0.25 µm aperture are reduced to 44°, 28°, 34°, and 29°, respectively (again, there is actually a distribution of angles so that the ones cited are the smallest observed in a given experiment). From Oberlin’s description of electron diffraction and dark field transmission electron microscope imaging techniques used to study carbons36 and the subsequent charac(36) Oberlin, A. Carbon 1979, 17, 7.
Energy & Fuels, Vol. 22, No. 4, 2008 2517
terization of graphitized materials,21,22,25–28 it is not clear what selected area diameters were used in these pioneering studies. It appears that the selected areas used in this study may be somewhat smaller than those used before. In any case, nuances of the initial alignment of stacked rings are now detected for the first time. Although the number of orders of the (002) reflection generally denotes increased molecular alignment, it does not distinguish between the formation of shot and sponge cokes. On the other hand, different mosaic texture sizes are noted by cross-polarized (visual light) optical microscopy for these two cokes.5 In other words, the increased crystallographic order in delayed cokes, compared to other materials, is already a property of submicrometer regions, where the distinctions are made below a 1.27 µm diameter. It may also be reasonable to expect that differing results would be anticipated depending upon the nature of the original vacuum residual feedstock. On the other hand, coking is a great equalizer after the initial chemical response (removal of aliphatic chains) at the 52 kcal/mol step. Holding the liquid phase at longer time at high temperature allows for equilibration to the lowest energy ordered state of aromatic nuclei before solidification of a mesophase. Therefore, the coking process leads to a product where there is much more order (e.g., in the aromatic layer packing) than was present original thermally untreated asphaltene fraction (in solution or as an isolated solid). Again, the production of graphite itself never occurs within the temperature range of the treatments considered in this study. In an early paper from Oberlin’s group,36 where asphalts were heated to very high temperatures (e.g., 3000 °C) to express the most periodic structures, it was found that only certain ones could approach a graphite-like structure, as measured by the final d(002) spacing (approaching the 3.35 Å value; see ref 24). These were the materials that had the lowest heteroatom content (mainly oxygen and sulfur). Because the expression of shot coke versus sponge coke depends upon the relative heteroatom content,5 the end product would seem to be directly related to the ability to express long-range lateral order. Acknowledgment. Thanks to Mr. David J. Moser for the preparation of the sample series with the microcarbon residue test. Drs. Manuel Francisco and Bernard G. Silbernagel are thanked for helpful discussions. EF800210C
