Investigation of Coking Propensity of Narrow Cut Fractions from

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Energy & Fuels 1998, 12, 1020-1030

Investigation of Coking Propensity of Narrow Cut Fractions from Athabasca Bitumen Using Hot-Stage Microscopy† Parviz Rahimi,*,‡ Thomas Gentzis,‡ William H. Dawson,‡ Craig Fairbridge,‡ Chandra Khulbe,‡ Keng Chung,§ Vince Nowlan,§ and Alberto DelBianco| National Centre for Upgrading Technology, 1 Oil Patch Drive, Suite A202, Devon, Alberta, T9G 1A8 Canada, Syncrude Canada Ltd. Edmonton Research Centre, 9421-17 Avenue, Edmonton, Alberta, T6N 1H4 Canada, and Eniricerche S.P.A., Via F. Maritano 26, 20097 San Donato Milanese, Milan, Italy Received April 22, 1998. Revised Manuscript Received June 2, 1998

The incipient mesophase formation of 10 narrow cut fractions from Athabasca bitumen vacuum bottoms boiling above 525 °C (pitch) derived by supercritical fluid extraction (extraction) using pentane was performed using hot-stage microscopy. The experiments were performed in a stream of hydrogen at 750 psi (5.2 MPa) and 440 °C. The formation of mesophase for these pitch fractions correlated with their macromolecular carbon residue, molecular weight, and aromaticity. The higher the values of these properties, the sooner the mesophase appeared. The time for the mesophase formation was the shortest in a pitch fraction containing 88 wt % asphaltenes. In cases where the fractions contained low boiling point materials, the mesophase appeared significantly sooner than expected. This was rationalized in terms of the induction period where certain fractions of heavy oils, if present, delay mesophase formation.

Introduction Conversion of heavy oil and bitumen to transportation fuels by existing technologies is limited by retrograde reactions leading to coke formation. Any reduction in coke yield or any increase in distillate yield during heavy oil upgrading will have a significant impact on the economics of the processes. Attention has been given to new technologies that would upgrade bitumen and heavy oil with reduced coke formation.1 The work of Magaril and Aksenora2,3 and Magaril et al.4 showed that the formation of coke in high-temperature processes occurs via condensation and polymerization reactions that take place in a new solid phase. This phase forms from asphaltene precipitation. The study by Wiehe5 suggested that coke formation during thermal cracking of petroleum residua occurs via a mechanism of phase separation of asphaltenes and the formation of a phase that is lean in abstractable hydrogen. Coke is produced as a direct byproduct of thermal cracking of petroleum-based feeds through a series of reactions † This paper was presented, in part, at the Fuel Chemistry Division meeting at the 213th ACS National Meeting, San Francisco, CA, April 1997. * To whom the correspondence should be addressed. ‡ National Centre for Upgrading Technology. § Syncrude Canada Ltd. | Eniricerche S.P.A. (1) Storm, D. A.; Barresi, R. A. J.; Sheu, E. Y. Energy Fuels, 1995, 9, 168-176. (2) Magaril, R. Z.; Aksenora, E. I. Int. Chem. Eng. 1968, 8, 727729. (3) Magaril, R. Z.; Aksenora, E. I. Khim. Technol. Topl. Masel 1970, 7, 22-24. (4) Magaril, R. Z.; Ramazeava, L. F.; Aksenora, E. I. Int. Chem. Eng. 1971, 11, 250-251. (5) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 2, 2447-2454.

that involve polymerization, condensation, and aromaticity increase, in a direction from the lightest to the heaviest fractions as shown below:

oils f resins f asphaltenes f coke Asphaltenes tend to self-associate and aggregate. The most associated asphaltenes form coke by molecular weight growth, which is characterized by oligomerization.5 The recent study by Martinez et al.6 suggested two mechanisms for coke formation. The first involves the formation of primary coke, which originates from the stripping of the asphaltene core of its peripheral substituents. The second involves a sequence of polymerization and condensation steps from the lightest to the heaviest fractions. The kinetics of coke formation in hydrocarbon fractions of Athabasca bitumen and Cold Lake heavy crude in Canada have been studied extensively7 (and references therein). Results showed that the higher the aromaticity of the feedstocks, the greater the rate of coke formation. A tentative reaction scheme for coke formation was proposed by which the highly polycondensed aromatic and asphaltene fractions were responsible for the coke formation. The rate constants (min-1) of the fractions measured were as follows:

asphaltenes > resins > aromatics > saturates (6) Martinez, M. T.; Benito, A. M.; Callejas, M. Fuel 1997, 76, 899905. (7) Banerjee, D. K.; Laidler, K. J.; Nandi, B. N.; Patmore, D. J. Fuel 1986, 65, 480-484.

S0887-0624(98)00091-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/22/1998

Coking Propensity of Athabasca Bitumen

Other kinetic studies have concentrated on the conversion of the asphaltene fraction of petroleum residua, particularly as related to the coke-induction period.8-10 Coke forms much faster and at a higher rate when asphaltene is the reactant fraction, often without an induction period.5 When organic material such as pitch is heated to temperatures between 350 and 500 °C, the decomposition and polymerization reactions that take place result in the formation of polycondensed aromatic hydrocarbons. This formation is followed by the arrangement of these compounds in a fixed, oriented direction. The resulting mesophase or “liquid crystal” is formed because of the accumulation and stacking of these oriented polycondensed aromatic hydrocarbons. With further heating and increased interfacial forces, mesophase spheres form droplets, much like oil dispersed in water. The spheres grow in size and coalesce to form bulk mesophase if their viscosity and the viscosity of the medium remain low.5 Further heating of the pitch results in the formation of coke with either a mosaic or a fibrous texture. Using optical microscopy, it is possible to observe the incipient mesophase and coke formation and relate the resulting texture to the chemical and molecular composition of the feedstocks. Hotstage microscopy is a powerful technique developed to study the in situ thermal behavior and the dynamics of constituent molecules in pitches at high temperatures.11 Hot-stage microscopy (HSM) complements conventional, room-temperature microscopy and offers the advantage of allowing the observation of mesophase growth at the temperature of formation.12 HSM has been used routinely since the early 1970s to study the hydrocracking reactivity of heavy oil and bitumen feedstocks.12-15 Most of the earlier work using HSM was conducted in inert atmosphere (nitrogen) at pressures that ranged from 25 to 1900 psi (1.7 to 13.8 MPa). In nitrogen, mesophase size remained small (a few micrometers in diameter), and the resulting coke was very viscous and virtually impossible to deform even at 440 °C.15 In hydrogen, fluidity is enhanced and mesophase size is considerably larger. A pressure increase would promote random collision of mesophase spheres and result in unrestricted growth and formation of bulk mesophase. Hydrogen plays a noncatalytic role and increases miscibility with the more fluid secondary phase at high pressures.15 However, quantification of data in HSM is not possible; only direct physical representation of the reactions is possible. In addition, 1H- and13C-NMR (nuclear magnetic resonance) and ESR (electron spin resonance) have been (8) Schucker, R. C.; Keweshan, C. F. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1980, 25, 155-164. (9) Savage, P. E.; Klein, M. T.; Kukes, S. G. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 1169-1174. (10) Savage, P. E.; Klein, M. T.; Kukes, S. G. Energy Fuels 1988, 2, 619-628. (11) Uemura, S.; Hirose, T.; Takashima, H.; Kato, O.; Harakawa, M. Ext. Abstr. 14th Bienn. Conf. Carbon, Am. Carbon Soc. 1979, 7879. (12) Hoover, D. S.; Davis, A.; Perrotta, A. J.; Spackman, W. Ext. Abstr. 14th Bienn. Am. Conf. Carbon, Am. Carbon Soc. 1979, 393. (13) Lewis, R. T. Ext. Abstr. 12th Bienn. Am. Conf. Carbon, Am. Carbon Soc. 1975, 215-216. (14) Perrotta, A. J.; Henry, R. M.; Bacha, J. D.; Albaugh, E. W. High Temp.- High Pres. 1981, 13, 159-166. (15) Perrotta, A. J.; McCullough, J. P.; Beuther, H. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 1983, 633-639.

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applied, often in conjunction with HSM, to the characterization of mesophase development in coal tar and petroleum pitches.16-19 The two magnetic resonance techniques are useful in studying the molecular motion and chemical changes in pitches during heat treatment. High-temperature 1H-NMR, in particular, is well suited for modeling the processes occurring prior to mesophase formation (the so-called mesophase “embryos”) which are not detectable optically16 as well as the in situ development of mesophase. The objective of this study was to investigate the coking propensities of extra heavy oil fractions from Athabasca bitumen vacuum bottoms (+525 °C) and to correlate them with the chemical properties of these fractions. To our knowledge, this is the first published study of the thermal behavior and coking propensity of Athabasca bitumen vacuum bottoms using the HSM technique. Experimental Section The details on the preparation of narrow fractions of petroleum residue using SCFE have been described elsewhere.20 The properties of the 10 narrow cut fractions are given in Table 1. Hot-stage microscopy was performed on all fractions as shown in Scheme 1. The first five fractions were very viscous and had to be heated prior to placement into the specially designed aluminum cup holders. The other fractions were solids that were crushed to a fine powder prior to placement into the holders. The differences in physical appearance attest to the differences in the chemical compositions of the subfractions. The amount of material used for HSM experiments varied between 5 and 12 mg depending on the nature of the fractions (liquid or powder form). The aluminum cups were carefully placed into the cells and covered by a set of soft copper O-rings and a YAG (yttrium-aluminum-garnet) crystal. This allowed for the observation of mesophase development and subsequent transformation. The cell was also tightened to prevent the escape of volatiles during heating and was properly placed in the heated stage of the Zeiss Axioplan microscope. The cell was connected through tubing to a gas system that supplied hydrogen or nitrogen at regulated pressures. In this study, the system was pressurized with H2 (750 psi-5.2 MPa) and the flow of the gas was maintained at 35 mL/min. The most critical factors in mesophase formation are temperature, residence time, heating rate, gas flow rate, and stirring rate.21 In this study, the fractions were heated from room temperature to 440 °C at a rate of 11 °C min-1 and held at that temperature for 3 h. The stated times are referenced to the beginning of the experiments (room temperature). Because of limitations of the experimental setup, the fractions were not subjected to stirring. The cooling effect of the flowing gas made it necessary to adjust the furnace temperature upward by approximately 50 °C to maintain the appropriate sample temperature. The appropriate sample temperature was confirmed by determining the melting points of K2CrO7 (actual mp ) 398 °C, measured mp ) 450 °C) and Zn (actual mp ) 419.47 °C, found (16) Azami, K.; Yokono, T.; Sanada, Y.; Uemura, S. Carbon 1989, 27, 177-183. (17) Azami, K.; Yamamoto, S.; Yokono, T.; Sanada, Y. Carbon 1991, 29, 943-947. (18) Parks, T. J.; Cross, L. F.; Lynch, L. J. Carbon 1991, 29, 921927. (19) Andersen, J. M.; Garcia, R.; Maroto-Valer, M. M.; Motnelo, S. R.; Snape, C. L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1996, 621-624. (20) Chung, K. H.; Xu, C.; Hu, Y.; Wang, R. Oil Gas J. 1997, 95, 66-69. (21) Honda, H. Carbon 1988, 26, 139-156.

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Table 1. Characteristics of Athabasca Bitumen Vacuum Bottom Fractions Obtained by SFCE Technique fraction no. pressure, MPa wt % of pitch density, g/mL at 20 °C molecular weight, Da sulfur, wt % nitrogen, ppm carbon, wt % hydrogen, wt % C/H (atomic) aromatic carbon,a fa nickel, ppm vanadium, ppm MCR, wt % saturates, wt % aromatics, wt % resins, wt % asphaltenes, wt % a

1

2A

2B

3

4

5

6

7

8

9

pitchb

4-5 12.7 0.9745 506 4 3080 84.5 11.5 0.612 0.26 12.8 30.7 5.6 26.8 57.2 15.9 0

5-5.5 9.8 0.993 755 4.5 4100 83.5 11.15 0.624 0.29 21.3 48.7 7.9 16.4 62.4 21.2 0

5.5-6 7.6 1.0061 711 5 4330 83.5 10.95 0.635 0.25 30.1 69.8 10.8 9.7 65.7 24.6 0

6-7 10.6 1.0228 799 5.4 5070 84.0 10.55 0.664 0.33 44.8 101 14.3 4.1 66.7 29.2 0

7-8 6.5 1.0427 825 6 6160 83.0 10.25 0.675 0.36 71.1 166 18.2 1.4 63.9 34.8 0

8-9 4.4 1.0543 948 6.2 6810 84.0 10.05 0.697 0.4 89.7 221 21.5 0.7 53.3 46.0 0

9-10 3.3 1.0646 1134 6.5 7370 83.0 9.8 0.706 0.37 123 300 24.7 0.6 45.4 54.0 0

10-11 2.6 1.0678 1209 6.8 7530 83.0 9.7 0.713 0.43 138 355 26.5 0.3 45.9 53.8 0

11-12 2.1 1.0736 1517 6.8 7900 82.5 9.5 0.724 0.43 162 409 28.7 0 40.8 59.2 0

>12 40.4 N/Ac 4185 7.6 10500 78.5 8.0 0.818 0.49 339 877 48.9 0 2.0 9.4 88.03

100 1.0868 1191 6.5 4600 82.7 9.0 0.766 0.41 148 364 26.7 6.3 33.0 29.4 31.4

C13-NMR. b Pitch ) +524 °C fraction. c N/A ) not applicable.

Scheme 1. Hot-Stage Microscopy Analysis of Athabasca Bitumen VB and Its Fractions

mp ) 470 °C) under identical conditions used for bitumen fractions. The process was recorded by a video cassette recorder (VCR) for further observations. Temperature and pressure data were recorded and displayed on the monitor. The growth of mesophase diameter with time was measured periodically using a microscaling device. Photomicrographs were taken in polarized light under cross polars to show the optical texture of mesospheres and isochromatic regions. Although the microscope is equipped with interchangeable objectives, the 20× objective was chosen for convenience. The combined magnification of the system was 200×. The classification of textural components used in this study is similar to that published in Barriocanal et al.22

Results and Discussion Hot-Stage Microscopy Results. The general description of the hot-stage microscopy results for the fractions are given below and pictorially in Figures 1-6. Fraction 1. Because of the high volatility of this fraction, no mesophase was observed. This behavior is typical of a feed containing mainly light hydrocarbon fractions. The experiment was terminated after 82 min. Fraction 2A. The behavior of this fraction was similar to that of fraction 1. The surface of the sample showed evidence of fluidity at approximately 65 °C. Within the first 60 min, the sample had volatilized to a large extent, and the cell window was covered with (22) Barriocanal, C.; Hanson S.; Patrick, J. W.; Walker, A. Fuel 1994, 73, 1842-1847.

condensation rims. The fraction formed mesophase after 72 min, which was more evident in areas near the edge of the cup. Areas that developed domain anisotropy were also observed occasionally. Fraction 2B. The development of mesophase spheres and their subsequent coalescence to form large domain anisotropy regions characterized the fraction. Tiny amounts of mesophase appeared after approximately 65 min. The diameter of mesospheres grew from 4 µm at 70 min to 30 µm at 84 min (Figure 1a, Table 2). Growth continued and the diameter exceeded 50 µm at 94 min. Between 94 and 105 min, the coalescence of the mesophase accelerated and large isochromatic areas (>150 µm) showing domain anisotropy developed (Figure 1b). At this time smaller mesospheres (50 µm mean size) with mesophase spheres still embedded in a fine-grained mosaic matrix. A second run resulted in similar optical textures, but the time of mesophase formation was different. Mesospheres first appeared after only 65 min, approximately 40 min earlier than in the first run. A third run resulted in mesophase formation after 84 min. This inability to reproduce the timing of mesophase formation even with the same fraction requires further investigation because the inability to reproduce the timing indicates that the time of mesophase appearance should be used with some caution when correlating coking behavior with the chemical composition of the fraction. Fraction 4. The fraction appeared to flow slowly at 60 °C. Volatilization continued up to 400 °C, often in the form of bubbles escaping periodically and traversing the field of view. Mesophase first appeared after 105 min. The growth of the mesophase spheres was observed as follows: 4 µm at 106 min, 9 µm at 116 min,


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Figure 2. Fraction 4: (a) Growth of anisotropic mesospheres from isotropic matrix after 136 min. (b) Continuous growth and coalescence of mesophase after 150 min. (c) Transformation of mesophase to bulk mesophase having a “donut” shape. Mesospheres of various sizes still form in the isotropic pitch, even after 166 min. (d) Same field of view as (c). The isotropic matrix has become anisotropic upon cooling. Note the presence of desiccation cracks in the mosaic of the matrix. (e) Mesophase fuses to form an isotropic matrix upon reheating. Desiccation cracks have disappeared. (f) A second cycle of cooling converted the previously isotropic matrix to anisotropic.

and 33 µm at 127 min (Table 2). Growth and coalescence of mesophase spheres resulted in the formation of larger mesophase (>50 µm) at 136 min (Figure 2a) and even larger at 150 min (Figure 2b). After 166 min, the mesophase particles had formed a donut-shaped structure surrounded by mesospheres of various sizes and shapes (Figure 2c). The experiment was terminated after 170 min. When the residue was cooled and then reheated to 440 °C using a higher rate of heating (i.e., 20 °C min-1), the same behavior shown by fraction 2B was observed.

In the cooling cycle, the isotropic matrix became anisotropic with a fine-grained mosaic texture (Figure 2d). Upon reheating, the fine-grained matrix appeared to redissolve, and the matrix became completely isotropic again after about 28 min. In addition, no desiccation cracks could be seen (Figure 2e). A second cycle of heating did not appear to have an effect on the size of the matrix mosaic (Figure 2f). Fraction 5. Fluidity was evident at 130 °C while volatilization was continuous up to 440 °C. Large vacuoles were observed bursting throughout the heat-



Figure 3. Fraction 6: (a) Growth of mesophase spherules from isotropic matrix after 120 min. (b) Same field of view as (a). Mesophase spheres continue to grow after 130 min. (c) Mesophase coalescence takes place after 135 min. (d) Same field of view as (a) and (b) a few seconds after the start of cooling. Previously isotropic matrix has been converted to granular anisotropic.

ing. After about 61 min, the first indentations on the sample surface became visible. These indentations were followed by the appearance of mesophase at 84 min. Growth and coalescence of mesospheres were fast, growing to more than 70 µm after 87 min and to more than 130 µm after 95 min (Table 2). However, the coking behavior was not consistent throughout the sample. For example, large areas of coke with domain anisotropy appeared to be concentrated near the periphery of the cup holder while the central region of the cup was devoid of any sample. A second run produced similar results. Mesophase appeared after 66 min, which is 18 min later than the induction period of the first run. Individual mesophase spheres exhibited a characteristic cross-extinction pattern (Maltese cross) and coalesced to form larger areas of domain anisotropy. As was noted with fraction 4, coke was preferentially concentrated at the edge of the cup rather than in the middle. Upon cooling, this fraction behaved in a fashion similar to fraction 4. Fraction 6. The fraction started to flow at 67 °C, while volatilization was first noticed at 154 °C. Occasionally, large bubbles, almost 100 µm in diameter, formed in the matrix and burst only seconds later. Three runs resulted in mesophase formation after 6770 min. A fourth run indicated that mesospheres formed after 86 min. Mesophase spheres grew slowly, attaining a diameter of only 4 µm at 93 min and 27 µm at almost 120 min (Figure 3a) and 125 min (Figure 3b;

Table 2). Coalescence continued and large areas (>100 µm) of coke showing domain anisotropy developed after 135 min (Figure 3c). After cooling and while the cell was still pressurized, the isotropic matrix became anisotropic with a fine granularity (Figure 3d). This behavior is consistent with the behavior of fractions 4 and 5. Fraction 7. This fraction showed evidence of fluidity at approximately 80 °C. Small vesicles appeared in the 150-300 °C range. The times of mesophase formation were 58 min and 70 min in the two runs conducted. Mesophase spheres grew from 3 µm in diameter at 74 min to 21 µm at 98 min (Figure 4a) and finally formed large coke areas with flow domain anisotropy after 125 min (Figure 4b) and 144 min (Figure 4c; Table 2). The anisotropic domains enclosed smaller patches of isotropic matrix, which in itself contained numerous tiny mesospheres (1-2 µm in size) (Figure 4d). The disklike inclusions of isotropic pitch surrounded by anisotropic pitch are similar to the results reported by Hu¨ttinger et al.23 Complete conversion of this isotropic phase to coke could take place only after a long time at 440 °C. Flow domain, composed of elongated isochromatic areas often curved, was observed only in this fraction (Figure 4e and 4f). The behavior of this fraction warrants further investigation. Fraction 8. The fraction started to flow at about 90 °C. Volatilization was continuous from 150 to 440 °C, (23) Hu¨ttinger, K. J.; Bernhauer, M.; Christ, K.; Gschwindt, A. Carbon 1992, 30, 931-938.


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Figure 4. Fraction 7: (a) Formation of mesophase after 98 min. Note size and cross-extinction pattern of spherules. (b) Formation of coke with domain anisotropy after 125 min. Mesospheres of various sizes are embedded in the isotropic matrix. (c) Coke has covered almost the entire field of view after 145 min. (d) Development of flow domain and domain anisotropy after 163 min. Note the inclusion of round isotropic matrix and tiny mesospheres in the coke structure. (e) Flow domain coke typical of this fraction. Isotropic matrix converted to anisotropic following cooling. (f) Similar features as in (e) a few seconds after the start of cooling.

and condensation droplets formed on the bottom of the cell window. At 200 °C, fluidity increased considerably and the sample surface developed a “pitted” texture. The first mesophase spherules appeared at 71 min The sample eventually formed coke with domain anisotropy. Fraction 9. Fluidity became evident at 165 °C, and volatilization was first noticed at 385 °C, which was much later than in any of the other fractions examined. In the two runs conducted, the first mesophase spheres were noticed after 49 min and 57 min, which was faster than for any other fraction. Large droplets escaped to

the surface after 75 min. These droplets often contained tiny spherules of mesophase (Figure 5a). The boundary between these droplets and the surrounding matrix was sharp. Small spheres clustered around the edge of the droplet, and the spheres continued to grow in size. Gas bubble percolation occasionally disrupted the sample and elongated some of the anisotropic areas. Trains of spheres 15 µm in diameter could be seen traversing the surface. These trains appeared to form at the intersections of upwelling convection sites. Where three or more convection areas intersected, large zones (up to 0.5 mm)



Figure 5. Fraction 9: (a) The dynamism of mesophase formation is evident after 75 min. Zones of coke have formed near convection cells. (b) Coalescence of mesophase spheres to form bulk mesophase and isochromatic areas with domain anisotropy after 90 min. (c) Formation of mesophase isochromatic areas with domain anisotropy after 95 min. (d) Same field of view as (c) after 100 min. Growth of mesophase and fluidity of isotropic matrix are apparent. (e) Note the serrated-edge appearance of the isochromatic areas after 104 min and the absence of any “flow” domain. Mesophase spherules are still forming from isotropic matrix. (f) Same field of view as (e) a few seconds following the start of cooling. Isotropic matrix has been converted to anisotropic.

of mesophase formed. Individual spheres in the zones gradually grew in size by absorbing the smaller spheres that were pushed in by convection. Movement within the material gradually ceased as pockets of smaller mesophase were incorporated into the overall structure. Mesophase growth was fast, and the isochromatic areas increased to a mean size of >60 µm (Figure 5b). After 95 min the fraction had developed large isochromatic areas (>40 × 10 µm in size) with domain anisotropy and rounded margins (Figure 5c; Table 2). Mesophase spheres of up to 15 µm in diameter were present in the

isotropic matrix. The same area was watched for the next 7 min to observe the dynamics of the process. After 102 min mesospheres of only slightly larger size could be seen floating in an isotropic matrix (Figure 5d). In other locations of the sample surface, the resulting coke was angular with serrated edges (Figure 5e). Overall, mesosphere formation was very dynamic, much more so than in fractions 7 and 8. Upon cooling, these isotropic areas became anisotropic with a mosaic texture (Figure 5f), which was consistent with the previous fractions.


Rahimi et al.

Figure 7. Mesophase formation and growth.

Figure 6. Athabasca bitumen vacuum bottom: (a) mesophase enclosed by coke with domain anisotropy following cooling.

Athabasca Bitumen Vacuum Bottom. Fluidity was noticed at about 115 °C and was followed by the evolution of volatiles. Mesophase formation occurred after 69 min, which is in the middle of the range for all individual fractions. At 150 min the residue consisted of large (25 µm in diameter) and small mesospheres as well as coke with domain anisotropy (Figure 6a). No flow domain texture was formed. Discussion of Mesophase Formation. Fraction 1 contains relatively low boiling point saturated materials (26.8 wt %; Table 1), including straight chain paraffins. The fraction volatilized completely and did not form mesophase at the reaction conditions employed. At moderate hydrogen flow, lighter fractions of the pitch present in the feed and products formed after heating were carried out of the reaction cell. Very little coke is expected to form from fraction 1; the rate of coke formation is very slow and has been reported to be less than one-tenth of the rates associated with aromatic fractions.7 Incipient mesophase formation for fraction 2B in the hot-stage run occurred earlier than expected. Fraction 2B contains 16.4 wt % saturates, 24.6 wt % resins, and 65.7 wt % aromatics. On the basis of previous studies, this fraction should have had a longer mesophase formation time than fractions 4 and 7. The anomalous behavior of fraction 2B was attributed to the fact that this fraction was relatively volatile and, under hot-stage conditions, evaporated significantly and left the most refractory components behind. The work of Wiehe5 revealed that the asphaltene fractions showed no induction period compared to full residua. In addition, heptane soluble fractions in pitch showed the longest mesophase induction period and inhibited the formation of coke by the asphaltenes. Furthermore, Rodriguez et al.24 showed that asphaltenes form mesophase much faster and the optical texture is much smaller compared to aromatics and polar aromatics. Finally, Kershaw et al.25 stated that lower molecular weight components (MW) in pitch inhibit mesophase formation. The efficient removal of low MW species allows mesophase formation to commence earlier and to proceed more (24) Rodriguez, J.; Tierney, J. W.; Wender, I. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1991, 36, 1081-1087. (25) Kershaw, J. R.; Black, K. J. T.; Jaeger, H. K.; Willing, R. I.; Hanna, J. V. Carbon 1995, 33, 633-643.

rapidly. Therefore, the removal, by volatilization, of low MW species and heptane soluble components that are present in fraction 2B has shortened the mesophase induction period. Fraction 2B behaved in a manner similar to that of the asphaltene-rich fraction 9. Overall, the results of HSM are consistent with the autoclave experiments for fractions 4, 7, and 9 (which formed progressively more coke26). Fraction 9, which is rich in asphaltenes, showed rapid mesophase formation, while fractions rich in resins and aromatics (i.e., 4 and 7) were characterized by longer mesophase formation times. The mesophase induction time was shorter for fraction 7 than fraction 4. Fraction 7 contained 53.8 wt % resins and 45.9 wt % aromatics whereas fraction 4 contained fewer resins (34.8 wt %) and more aromatics (63.8 wt %) (Table 1). This timing of mesophase formation of fractions 4 and 7 is also in agreement with the results of other workers.7 Fraction 9 also showed the fastest coalescence. Compared to fractions 2B and 4, molecules in the aromatics-rich fraction 9 require shorter heating time to become lamellar and, eventually, to become involved in mesophase formation. This behavior indicates that the presence of asphaltenes initiates and promotes mesophase formation. The presence of spherical particles of liquid crystalline material, which is a precursor to coke, is a direct piece of evidence for coke formation that involves a liquid-liquid phase separation step. The overall results confirm those of Bannayan et al.27 and Martinez et al.6 who proposed that asphaltenes are responsible for coke formation at high temperatures during hydrocracking processes. It has also been suggested by Mochida et al.28 that the fragmentation of large molecules to form smaller components (saturates) during hydrocracking decreases the solubility of asphaltene molecules and thus facilitates their precipitation. Mesophase Growth Rate. Mesophase growth rates in four fractions were compared: 2B, 4, 7, and 9. The relative growth rate of mesophase and coke formed can be related to the chemical composition of the fractions. Mesophase growth rate (size) as a function of time was recorded for a number of fractions (Figure 7). The intent was to demonstrate that mesophase induction period and mesophase growth rates differ for a number of fractions isolated from the same parent pitch material. (26) Rahimi, P. M.; Gentzis, T.; Chung, K.; Nowlan, V.; DelBianco, A. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1997, 42, 146. (27) Bannayan, M. A.; Lemke, H. K.; Stephenson, W. K. Catalysts in Petroleum Refining and Petrochemical Industries; Absi-Halabi, M., et al., Eds.; Elsevier Science B. V: Amsterdam, 1996, pp 273-281. (28) Mochida, I.; Zhao, X.; Sakanishi, K. Ind. Eng. Chem. Res. 1990, 29, 2324-2327.


In fraction 2B the isothermal mesophase growth rate was slower, between 10 and 45 min at 440 °C compared to the rate after 45 min. Similarly, in fraction 4 the isothermal growth rate was slower between 65 and 75 min but was followed by a faster growth beyond 75 min but was followed by a faster growth beyond 75 min. In contrast, fraction 7 showed a continuous linear growth rate. Fraction 9, the most refractory according to the chemical characteristics (Table 1), showed the fastest growth compared to the other three fractions examined (Figure 7). The results for fraction 9 are in agreement with the results of Banerjee et al.,7 who observed that asphaltenes from Athabasca bitumen showed higher rates of coke formation than resins. The growth rate of fraction 2B initially behaved like that of fractions 4 and 7. Since this sample was volatile and became refractory with time, the growth rate increased after about 85 min. For similar time intervals, the optical texture of mesophase in fraction 2B was considerably smaller than the texture in fraction 9. The above data indicate that a pitch rich in asphaltenes not only produces more coke but also at a greater rate compared to pitches that contain less asphaltenes. The rapid increase of mesophase formation in fraction 9 during the first 20 min at final temperature of 440 °C (Figure 7) is characteristic of petroleum pitches and resembles first-order reaction kinetics.7,19 Similar results have been obtained by Martinez et al.6 in a study of the kinetics of asphaltene hydroconversion of a coal residue at 450 °C. To the contrary, fractions 2B and 4 showed faster rate of mesophase formation within the first 20 min, followed by a slower rate of formation (Figure 7). This dual behavior of fractions 2B and 4 needs further investigation. Other Observations. The behavior of fractions 2B, 4, 5, 6, and 9 upon cooling and reheating is worthy of discussion. The mesophase spheres reappear at the same physical positions during the cooling part of the temperature cycle (within 10 °C), which suggests that certain spots in the pitch act as nucleation sites.13,29 During reheating, mesophase spheres present at room temperature dissolve in the isotropic pitch as higher temperatures are attained. Lewis13 has reported the above feature for a variety of pitches including petroleum- and coal-derived pitches, and Honda21 has reported the same feature for a hydrogenated coal-tar pitch. The isotropic phase always has a lower viscosity than the mesophase. The magnitude of the difference in viscosity between the two phases can vary considerably, depending on the nature of the precursor pitch. The behavior observed in this study is typical of samples containing a relatively low-viscosity mesophase or of samples having a large difference between the viscosities of the mesophase and the isotropic matrix.13 When these samples are cooled from temperatures around 440 °C, an additional 5-10% mesophase in the form of small spherules appears in the isotropic matrix. This behavior shown by the fractions examined indicates that the changes from an isotropic phase to an anisotropic phase and back to an isotropic phase are reversible with temperature. The above feature also confirms the presence of a soluble and fusible mesophase in these (29) Srinivasan, N. S.; McKnight, C. A. Fuel 1994, 74, 1511-1517.


fractions and suggests that temperature fluctuations could have a direct effect on mesophase formation during heavy oil and bitumen upgrading.12 The fine cracks formed during quenching (Figure 2d) developed after the temperature dropped below the softening point of the mesophase.30 Another observation was the ability of fraction 7 to form flow-domain coke. This fraction developed extended flow domain characterized by homogeneous anisotropic structures. Large domain sizes (50-200 µm) correspond to higher degrees of graphitizability.24 This type of texture is characteristic of high-quality needle coke, which makes the fraction a suitable feedstock for carbon fiber production. Suitable feedstocks ideally should be highly aromatic, with most components boiling at or above 425 °C. Such materials include thermal tar, decant or slurry oil, and residues of catalytic cracking, but not asphaltene-rich oils. The latter tend to raise the coefficient of the thermal expansion of the product graphite.31 Flow domain size usually increases in a pitch having a high content of aromatic fractions and decreases when polar and asphaltene fractions dominate.24 However, based on the available data, the behavior of fraction 7 cannot be explained. Experimental Limitations. There are a number of limitations related to the surface tension of the samples. For example, a few of the samples dispersed toward the edge of the cup, thus forming a thicker layer near the edges and a thinner layer in the center. This phenomenon caused coke texture near the center of the cup holder to be different than at the margins. These variations in optical texture within such a small distance (the diameter of the cup is only 0.2 cm) may also be explained in terms of sample inhomogeneity or uneven heating of the cup. Some of the above problems might be solved if the sample could be agitated during the experiment. When hydrogen is the carrier gas, the fluidity of the pitch is enhanced with increasing pressure; this enhancement is believed to be caused by the increased noncatalytic hydrogen addition and increased miscibility with the more fluid secondary phase. At the moderate pressures of our experiments (750 psis5.2 MPa), spheroidal mesophase growth is expected to be relatively uniform because of some restrictions in movement and coalescence.15 On the basis of observations made by the above authors, we would expect a relatively homogeneous optical texture in our experiments. However, the above limitations may explain the observations, which are contrary to the results of Perrotta et al.15 The second limitation was the inability to reproduce the times of incipient mesophase formation for most of the fractions. The differences between repeated runs ranged from 8 min for fraction 9, to 12 min for fraction 7, and to 18 min for fraction 5. The reproducibility was poorer for the more volatile fractions. For example, the difference between repeated runs was 40 min for fraction 3 and 45 min for fraction 2B. To minimize this variation, an attempt should be made to use the same heating cell throughout the set of experiments as well (30) White, J. L., and Buechler, M. Petroleum-Derived Carbons; Bacha, J. D., et al., Eds.; Am. Chem. Soc., Symp. Ser. 303, 1986, 6284. (31) Zimmer, J. E.; White, J. L. Am. Carbon Soc. 1975, 23-224.


as the same amount of sample. Powdered samples should be melted prior to being loaded into the cup. An attempt was made to use the same cell in as many runs as possible and to maintain the sample weight between 10 and 12 mg. Finally, the data presented should be used to interpret the overall trend of the fractions. More work is necessary and has been planned in order to narrow the variability in mesophase formation time. Conclusions Following the heat treatment of 10 fractions of Athabasca bitumen vacuum bottoms using hot-stage microscopy, the following conclusions were made: 1. There is a relationship between the incipient mesophase formation and the rate of growth of mesospheres and the chemical composition of the fractions. The higher the asphaltene content in a fraction, the shorter the time interval for incipient isothermal mesophase formation and the faster the mesophase growth. 2. The poor reproducibility of mesophase formation time for certain fractions was attributed to differential volatilization, which resulted in different amounts of

Rahimi et al.

refractory material left behind at different time intervals. Agitation of the sample during heat treatment should improve the reproducibility considerably and should eliminate the preferential dispersion of the sample caused by surface tension phenomena. 3. The asphaltene-rich fraction 9 shows a continuous mesophase formation at a fast rate, which is typical of petroleum pitches. In contrast, the more volatile fractions 2B and 7 show a much slower rate of mesophase formation that has a dual behavior over time. 4. The dissolution and reappearance of mesophase spheres upon heating and cooling cycles point to how temperature affects the behavior of the mesophase and suggest the presence of fusible and soluble mesophase in the fractions examined. This observation supports the theory of coke formation in pitch through the process of nucleation. 5. Overall, hot-stage microscopy is very useful in studying the thermal behavior of extra-heavy oil fractions. EF980091+

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