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Thermogravimetric Analysis Studies on the Thermal Stability of Asphaltenes: Pyrolysis Behavior of Heavy Oil Asphaltenes Andre’ Hauser,*,† Dawoud Bahzad,‡ Anthony Stanislaus,‡ and Montaha Behbahani† Central Analytical Laboratory and Petroleum Research and Scientific Centre/Petroleum Refining Department, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait ReceiVed August 5, 2007. ReVised Manuscript ReceiVed September 11, 2007
Heavy feed upgrading in the atmospheric residues (AR) desulfurizatrion process suffers from rapid catalyst deactivation by coke deposition. The main source for the coke is unconverted, refractory asphaltenes that tend to coagulate and precipitate on the catalyst surface due to incompatibility with the hydroprocessed feed. Asphaltene fractions separated from three ARs, namely Kuwait export crude (KEC), Kuwait heavy crude (KHC), and Eocene (EOC), with an API gravity of 13.5°, 8.3°, and 7.4°, respectively, were pyrolised at 412 °C for 7 h under nitrogen. KEC-AR asphaltenes show the highest weight loss (26 wt %) followed by KHC-AR (21 wt %) and EOC-AR (18 wt %). KHC-AR (14 wt %) and EOC-AR (27 wt %) contain a significantly higher portion of refractory asphaltenes than KEC-AR (3 wt %). Applying the first- or second-order kinetic model, the rate constants of asphaltene conversion reflect this trend. Pilot plant tests revealed that KHC-AR and EOC-AR shorten the lifetime of a graded catalyst system typically used in a Kuwait refinery by 50% compared to KEC-AR. The structural changes in the asphaltenes occurring during pyrolysis were studied by solid-state 13C NMR and X-ray diffraction. Under pyrolysis, more scissioning of side chains occurs than cracking in a more distant position from the aromatic ring. Dealkylation/ hydrogenation is preferred over formation of larger polyaromatics. As a consequence, the average distance between the aromatic sheets and the stack height of the aromatic sheets in asphaltene aggregates reduce while the average diameter of the aromatic sheets hardly changes.
1. Introduction Asphaltenes are by definition the n-heptane insoluble fraction of crude oil and are the least reactive components of the feed that are mainly responsible for coke formation during catalytic hydroprocessing of heavy feedstocks. To estimate the impact that asphaltenes have on the deactivation of catalysts by coking, it is necessary to study the fate of the asphaltenes under hydroprocessing conditions. Various techniques, such as vapor phase osmometry (VPO), size exclusion chromatography (SEC), infrared spectroscopy (IR), nuclear magnetic resonance (NMR), and X-ray diffraction (XRD), were applied to examine the evolution of asphaltenes during hydroconversion of heavy oils. It is widely agreed upon that asphaltenes exert during catalytic hydroprocessing (i) a decrease of the portion of high molecular weight asphaltene components in favor of lower mass units due to hydrocracking,1 (ii) an increase of the H/C ratio due to hydrogenation,2 (iii) an increase of the asphaltene aromaticity due to dealkylation and condensation,1,3,4 and (iv) a decrease of the ratio of quaternary to tertiary aromatic carbon due to cracking reactions followed by hydrogenation.5 * To whom correspondence should be addressed. Fax: (+965)4989059. E-mail:
[email protected]. † Central Analytical Laboratory. ‡ Petroleum Research and Scientific Centre/Petroleum Refining Department. (1) Merdrignac, I.; Quoineaud, A. A.; Gauthier, T. Energy Fuels 2006, 20, 2028–2036. (2) Rana, M. S.; Ancheyta, J.; Maity, S. K.; Rayo, P. Catal. Today 2007, 25, 201–213. (3) Hauser, A.; Marafi, A.; Stanislaus, A.; Al-Adwani, A. Energy Fuels 2005, 19, 544–553. (4) Marafi, A.; Hauser, A.; Stanislaus, A. Energy Fuels 2006, 20, 1145– 1149. (5) Le Lannic, K.; Guibard, I.; Merdrignac, I. Pet. Sci. Technol. 2007, 25, 169–186.
Beside mass distribution and structure, the thermal stability of asphaltenes is another important characteristic that governs the convertibility of the n-heptane insoluble fraction in heavy oils. It is essential that, during hydroprocessing, resins and asphalteners are converted at a similar rate. Otherwise, unconverted asphaltenes tend to precipitate due to incompatibility between the refractory asphaltenic entities and their hydroprocessed oil matrix.3,6 Asphaltenes of atmospheric residues (AR) from three feeds, namely Kuwait export crude (KEC), Kuwait heavy crude (KHC), and Eocene (EOC), were thermally degraded and characterized using thermogravimetric analysis (TGA) as well as elemental and metal analysis (EA, MA), 13C NMR, and XRD. The TGA findings are related to the impact that these ARs have on the lifetime of the graded catalytic system that is typically used in an atmospheric residue desulfurization (ARDS) process. This study is part of a wider investigation on hydroprocessing of Kuwait crude that is undertaken in joint projects between the Kuwait Institute for Scientific Research (KISR) and the Japan Cooperation Center, Petroleum (JCCP), carrying the KISR project numbers PF010C and PF025C. 2. Experimental Section 2.1. Materials. 2.1.1. Atmospheric Residues. Three ARs were obtained from the following crudes, KEC, KHC, and EOC, respectively. The bulk properties of the ARs are presented in Table 1. 2.1.2. Asphaltene Separation. Before the asphaltene separation was performed, the feedstock samples have been homogenized by warming them up to 50 °C and thoroughly shaking by hand for 3 min For each sample, duplicate separations and subsequent quantification have been carried out. The asphaltenes were separated (6) Ancheyta, J.; Rana, M. S.; Furimsky, E. Catal. Today 2005, 109, 3–15.
10.1021/ef700477a CCC: $40.75 2008 American Chemical Society Published on Web 11/08/2007
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Table 1. Bulk Properties of Atmospheric Residues from Three Kuwait Crudes origin of AR property
unit
KEC
KHC
EOC
density @ 15 °C API gravity kinematic viscosity @ 100 °C conradson carbon residue (CCR) saturates aromatics resins asphaltenes (preparative) asphaltenes (COSMO) carbon contents hydrogen contents sulfur contents nitrogen contents vanadium contents nickel contents
g/mL deg cSt wt % wt % wt % wt % wt % wt % wt % wt % wt % wt % ppm ppm
0.9791 13.5 65 12.3 27.91 60.46 6.35 5.28 4.92 83.8 11.0 4.6 0.3 69 21
1.002 8.3 187 16.8 20.35 62.92 6.88 9.85 9.71 83.5 11.0 5.75 0.4 73 35
1.003 7.4 148 16.2 23.56 60.35 6.29 9.79 9.47 83.9 11.0 5.4 0.3 72 29
Figure 1. Three-lump model for thermal asphaltene decomposition under nitrogen in a TGA apparatus.
Table 2. Elemental and Metal Contents in Asphaltenes Separated from AR Fractions of Three Kuwait Crudes origin of asphaltenes parameter
unit
KEC-AR
KHC-AR
EOC-AR
carbon contents hydrogen contents sulfur contents nitrogen contents vanadium contents nickel contents
wt % wt % wt % wt % ppm ppm
84.6 7.5 6.7 1.1 777 275
82.8 7.4 8.6 1.1 514 309
83.9 6.9 8.1 1.1 923 318
Table 3. Key Characteristics of the Graded Catalyst System CAT-B catalyst system CAT-B property
unit
A
typea
B
HDM HDM catalyst bulk density g/mL 0.4–0.6 0.4–0.6 surface area m2/g 120–220 120–220 pore volume cm3/g 0.7–1.2 0.5–1.0 avg pore large large Diameter metal content low medium
C
D
E
HDS 0.5–0.8 220–270 0.3–0.8 medium
HDS 0.7–0.9 120–220 0.3–0.8 small
HDS/HDN 0.7–0.9 120–220 0.3–0.8 small
medium high
high
a
HDM: hydrodemetalization. HDS: hydrodesulfurization. HDN: hydrodenitrogenation.
Table 4. Typical Process Conditions of the ARDS Process at Kuwait’s Refineries parameter
unit
process condition
operating temperature range hydrogen H2/oil ratio at outlet maximum pressure operating pressure water addition fuel-oil sulfur level
deg C % m3 H2/m3 oil MPa MPa vol % wt %
380–426 80 670 14.5 13.6–13.7 4 0.7
from the ARs applying the ASTM method D6560. The elemental and metal contents of the asphaltenes are presented in Table 2. 2.1.3. Catalysts. The life tests under ARDS process conditions were carried out with a graded catalyst system (CAT-B), typically used in the ARDS process at Kuwait refineries, which consists of five commercially available catalysts. The general properties of the catalysts are listed in Table 3. 2.2. Methods. 2.2.1. ThermograVimetric Analysis (TGA). The thermogravimetric analysis of approximately 30 mg of sample placed in a macro platinum cell was carried out on a Shimadzu TGA-50. The samples were heated at 10 °C/min up to 412 °C and then kept at a constant temperature for about 7 h. The weight loss versus time was recorded. During the experiment, the reaction chamber was purged with nitrogen to avoid oxidation and to remove
Figure 2. TGA graph of asphaltene conversion under nitrogen at 412 °C; weight loss in percent of solid matter.
volatile reaction products from the chamber. The flow rate of the gas was 50 mL/min. 2.2.2. Nuclear Magnetic Resonance (NMR). Solid-state NMR measurements were carried out on a Bruker Avance 300 spectrometer (7.0463 T) equipped with a cross-polarization magic angle spinning (CP/MAS) probe and a fully automated pneumatic unit for sample spinning. 2.2.2.1. SPE/MAS 13C NMR. The single pulse excitation (SPE) spectra were obtained from ground asphaltene samples using a 4 mm multinuclear CP/MAS probe. Here, 13 kHz MAS was chosen to reduce the spinning side bands (SSB) to approximately 3% of the center signal and to move them to the margins of the spectral range. A pulse length of 1.7 µs was used, corresponding to the 30° 13C flip angle. The protons were inverse gated decoupled with maximum power (120 W). The recycling delay was 20 s. 2.2.2.2. CP/PI/MAS 13C NMR. The cross-polarization pulse inversion (CP/PI) experiments7 were carried out under Hartman– Hahn conditions at a moderate spinning rate of 4 kHz to minimize overlapping between the main signals (aliphatic and aromatic carbon) and the SSB; 400 µs as contact time and 1.5 s as recycling delay were chosen. For more details see the work of Hauser et al.8 All spectra were measured with a sweep width of 55 kHz, and the free induction decay (FID) was sampled with 4 096 data points. 2.2.3. X-ray Diffraction (XRD). X-ray diffraction patterns of powdered dry asphaltenes were obtained using a Philips PW 1830 automated diffractometer operating at 40 kV and 35 mA with a Cu anode tube (λCu ) 1.54056 × 10-10 m). The step size was 0.02 (2θ) starting at 6° and ending at 80°. Using the available software (Profit version 1.0c), the diffractograms were smoothed and deconvoluted to determine the band position and width. 2.2.4. Life Test. The tests were carried out under operating conditions similar to those applied in industrial ARDS units in Kuwait. The temperatures of the reactors were continuously adjusted to maintain a sulfur level in the product at around 0.6%, which is equivalent to 0.7% S in fuel oil. The tests were terminated when the temperature of the downstream reactor reached 412 °C. Three tests were conducted (7) Wu, X.; Zilm, K. W. J. Magn. Reson. A 1993, 102, 205–213. (8) Hauser, A.; Stanislaus, A.; Marafi, A.; Al-Adwani, A. Fuel 2005, 84, 259–269.
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Energy & Fuels, Vol. 22, No. 1, 2008 451
Table 5. TGA Data of Asphaltene Fractions from Atmospheric Residues of Three Kuwait Crudes kinetic parameter first-order (10-3 min-1) origin of asphaltenes KEC-AR KHC-AR EOC-AR
second-order (10-5 min-1)
TWLa wt %
k*
k1
k2
k*
k1
k2
Cb wt %
26 21 18
9.0 4.9 3.3
2.3 1.1 0.8
6.6 3.7 2.5
12.0 8.7 5.7
3.8 2.3 1.4
8.2 6.4 4.3
2.7 14.1 27.0
a Total weight loss of the sample after 7 h at 412 °C under nitrogen. first-order approach.
b
Unconverted asphaltenes (CAS) after 7 h at 412 °C under nitrogen using a
removed from the reaction chamber; therefore, the formation of coke by secondary cracking of oil can be neglected as indicated in Figure 1. The rate of asphaltene conversion can be expressed as n1 n2 + k2CAS –(dCAS ⁄ dt) ) k1CAS
(1)
The yield of oil and gas (O + G) and coke (C) can be obtained by the following relations n1 (dCO+G ⁄ dt) ) k1CAS
(2)
Figure 3. Asphaltene conversion under nitrogen at 412 °C, first-order approach.
n2 (dCC ⁄ dt) ) k2CAS
(3)
Table 6. Comparison of Time On Stream Periods for Catalyst System CAT-B Processing KEC-AR, KHC-AR, and EOC-AR
3.1.1.1. First-order Reactions. Assuming first-order reactions with n1 ) n2 ) 1 and k* ) k1 + k2, the expression for asphaltene conversion would be
and
feedstock
time on stream period (h)
refractory asphaltenes (wt %)
KEC-AR KHC-AR EOC-AR
7383 2722 4117
3 14 27
0 exp(–k*t) CAS ) CAS
(4)
0 is the concentration of asphaltenes in the sample where CAS when the temperature of 412 °C is reached and the yield of coke in the sample equals 0 (k2 ⁄ k*)[1 - exp(–k*t)] CC - CC0 ) CAS
(5)
where CC0 is the concentration of coke in the sample when the temperature of 412 °C is reached. Assuming the formation of coke during the heat-up phase is negligible (CC0 ≈ 0), the mass of the sample in the TGA apparatus at a given time (t) can be obtained by summing up eqs 4 and 5. 0 {[1 - (k2 ⁄ k*)][exp(-k*t)] + (k2 ⁄ k*)} (6) CAS + CC ) CAS
3.1.1.2. Second-Order Reactions. Assuming a second-order reaction with n1 ) n2 ) 2 and k* ) k1 + k2, the kinetic expression for asphaltene conversion would be Figure 4. SPE/MAS 13C NMR spectra of asphaltenes originated from KEC-AR (a) before and (b) after TGA.
0 0 ⁄ (CAS k*t + 1) CAS ) CAS
using KEC-AR, KHC-AR, and EOC-AR. A summary of typical process life test conditions is shown in Table 4.
and with the assumption that the formation of coke during the heat-up phase can be neglected (CC0 ≈ 0), the expression for the formation of coke would be
3. Results and Discussions
0 2 0 ) k2t] ⁄ [CAS k*t + 1] CC ) [(CAS
3.1. Thermal Asphaltene Decomposition. As soon as the feed enters the reactor, its components are subjected to thermal and catalytic cracking beside other reactions. As reported by Martinez et al.,9 under thermal cracking conditions, asphaltenes convert into gas and oil as well as coke. In order to investigate the thermal degradation, the asphaltene fractions from ARs of three Kuwait crudes were pyrolized under nitrogen using a thermogravimetric analyzer. 3.1.1. Kinetic Model of Asphaltene Decomposition. Thermal decomposition of asphaltenes follows a three-lump model.9,10 Under TGA conditions, the volatile reaction products are
The mass of the sample in the TGA apparatus can be obtained by summing up eqs 7 and 8. 0 0 0 CAS + CC ) {CAS [CAS k2t + 1]} ⁄ [CAS k*t + 1]
(7)
(8)
(9)
3.1.2. TGA of Asphaltenes. The weight loss of the asphaltene samples obtained from KEC-AR, KHC-AR, and EOC-AR with time at 412 °C under nitrogen in a TGA apparatus is plotted in Figure 2. (9) Martinez, M. T.; Benito, A. M.; Callejas, M. A. Fuel 1997, 76, 871–877. (10) Wang, J.; Anthony, E. J. Chem. Eng. Sci. 2003, 58, 157–162.
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Table 7. Solid-State 13C NMR Data of Asphaltenes Before and After Thermal Decomposition origin of asphaltenes parameter
abbrv Elemental
carbon contents hydrogen to carbon ratio sulfur contents nitrogen contents ratio of carbon in CH3 groups to carbon in saturated building blocks carbon in aromatic building blocks tertiary aromatic carbon quaternary aromatic carbon substituted peripheral aromatic carbon and double-bridged peripheral aromatic carbon triple-bridged internal aromatic carbon a
C H/C S N Solid-State CCH3/Cal
unit
KEC-AR
KHC-AR
EOC-AR
84.6/85.0 1.08/0.81 6.7/8.0 1.1/1.3
82.8/84.1 1.07/0.89 8.6/8.5 1.1/1.2
83.9/86.4 0.99/0.73 8.1/7.1 1.1/1.2
0.15/0.32
0.15/0.36
0.36/0.36
Analysisa wt% wt% wt% 13C
NMRa
Car Car-t Car-q Car-s+b2
% % % %
53/75 27/41 26/34 22/30
54/76 25/38 29/38 26/32
59/80 28/33 31/47 28/41
Car-b3
%
4/4
3/6
3/6
First value before TGA; second value after TGA.
almost completely while from the EOC-AR-asphaltenes remain still at 27% in the solid matter (see Figure 3 and Table 5). Assuming second-order kinetics, the remaining asphaltenes range between 17% and 30%. Under thermal cracking conditions, the trends, that are apparent, can be summarized as follows: (i) the formation of coke is about 3 times faster than the oil and gas formation for all three types of asphaltenes, (ii) the kinetic constants (k2) for coke formation range in the sequence (k2)KEC-AR > (k2)KHC-AR > (k2)EOC-AR
(10)
and (iii) the global rate constants k* for asphaltene conversion follow the sequence / / / > kKHC-AR > kEOC-AR kKEC-AR
Figure 5. Tentative mechanism for thermal asphaltene conversion.
Assuming first-order reactions, the rate constants k1 and k2 (see Figure 1) can be determined by curve fitting using eq 6 (solid line in Figure 2). The correlation coefficients (R2) for the first-order kinetic plots are close to 1 (left side in Figure 2) indicating that the first-order model describes satisfactorily the pyrolysis of the asphaltenes. Using the second-order equation (eq 9) for curve fitting (dashed line in Figure 2), the correlation coefficients (right side in Figure 2) are as good as those for the first-order case. The kinetic data together with the total weight loss (TWL) and the contents of refractory asphaltenes (CAS) in the solid matter of each sample are compiled in Table 5. Comparing the TGA results from the three asphaltenes, the curve fitting indicates that both models, first- and second-order reactions, apply satisfactorily to the TGA data. Using the firstorder approach, KEC-AR-asphaltene converts within 400 min
(11)
indicating that the asphaltenes from EOC-AR are the most refractory ones. 3.2. Impact of the Nature of Asphaltenes on the Deactivation of Catalyst System CAT-B. In order to estimate the impact that the nature of asphaltenes might have on the life performance of catalyst system CAT-B in an industrial ARDS process while hydrotreating KHC-AR or EOC-AR, pilot plant tests were conducted under conditions as shown in Table 4. For comparison, a run with KEC-AR, which is currently processed in Kuwait refineries, was conducted as well. One of the main concerns with the processing of KHC-AR or EOCAR, which contain a high level of asphaltenes (≈10 wt %), is their propensity to form coke. Coke laydown is most extensive during the very early stages of operation.11–16 The initial coking is reported to cause a large loss in surface area on the catalysts and a fairly rapid deactivation of the catalyst.17 Temperature raise during operation to compensate for the loss of activity causes even more coke buildup and accelerates deactivation.18 (11) Marafi, M.; Stanislaus, A. Appl. Catal., A 1997, 159, 259–267. (12) Absi Halabi, M.; Stanislaus, A.; Owaysi, F.; Khan, Z. H.; Diab, S. Stud. Surf. Sci. Catal. 1989, 53, 201–212. (13) Absi-Halabi, M.; Stanislaus, A.; Trimm, D. L. Appl. Catal. 1991, 72, 193–215. (14) Beuther, H.; Schmid, B. K. Reaction mechanism and rates in residue HDS. In Proceedings of the 6th World Petroleum Congress; John Wiley and Sons: New York, 1963; Vol. 3., pp 297–306. (15) De Jong, K. P.; Reinalda, D.; Emeis, C. A. Stud. Surf. Sci. Catal. 1994, 88, 155–166. (16) Marafi, A.; Hauser, A.; Stanislaus, A. Catal. Today 2007, 125, 192– 202. (17) Marafi, A.; Stanislaus, A.; Hauser, A.; Matsushita, K. Pet. Sci. Technol. 2005, 23, 385–408. (18) Bartholdy, J.; Cooper, B. H. Prepr. Pap.––Am. Chem. Soc., DiV. Fuel Chem. 1993, 38, 386–390.
Thermal Stability of Asphaltenes
Energy & Fuels, Vol. 22, No. 1, 2008 453
Figure 6. XRD pattern of asphaltenes originating from KEC-AR (a) before and (b) after TGA. Table 8. XRD Analysis Data of Asphaltenes Before and After Thermal Decomposition origin of asphaltenesa parameter distance between aliphatic chains distance between neighboring aromatic sheets size of aromatic sheet height of aromatic stack no. of aromatic rings per sheet no. of aromatic sheets per stack a
abbreviation dγ dm La Lc Nar Nas
unit
KEC-AR
KHC-AR
EOC-AR
10-10 m 10-10 m 10-10 m 10-10 m
4.5/3.8 3.5/2.9 10.1/10.4 16.3/16.5 4/4 6/7
4.5/3.6 3.5/2.9 9.8/8.5 15.8/16.9 4/3 5/7
4.1/3.7 3.5/2.9 10.1/9.4 21.7/15.3 4/4 7/6
First value before TGA; second value after TGA.
Hydroprocessing the two ARs, KHC-AR or EOC-AR, it is anticipated that the ARDS unit must be operated at a distinctly higher temperature because of the high sulfur contents (g5.5 wt %) and to achieve the desired level of desulfurization (e0.6 wt %).19,20 This factor, in addition to the high contents of refractory asphaltenes in both feeds (KHC-asphaltenes 14 wt %; EOC- asphaltenes 27 wt %), will cause the catalyst system to deactivate at a fast rate.21,22 Table 6 shows the time on stream period of the catalyst system CAT-B using KHC-AR or EOC-AR and meeting the criteria (i) product oil sulfur e0.6 wt % and (ii) second reactor temperature e412 °C. For comparison, the lifetime of the catalyst system is about twice as long if KEC-AR is processed under identical conditions. The drastic shortening of the lifetime of catalyst system CAT-B while processing KHC-AR or EOCAR instead of KEC-AR correlates well with the thermal stability of the asphaltenes in the feed. Merdrignac et al.23 observed, as they hydrodemetalized (HDM) and hydrodesulfurized (HDS) AR Arabian Medium, that there is a portion of high molecular weight asphaltenes that are not effected by HDM or HDS while the low molecular weight entities are converted to resins and aromatics. 3.3. Solid-State 13C NMR. In order to investigate the nature of structural changes occurring during thermal cracking of (19) Stanislaus, A.; Fukase, S.; Koide, R.; Al-Baroud, A.; Marafi, A.; Al-Jassem, F.; Absi-Halabi, M. Prepr. Pap.––Am. Chem. Soc., DiV. Fuel Chem. 1999, 44, 827–831. (20) Sie, S. T. Appl. Catal., A 2001, 212, 129–151. (21) Seki, H.; Yoshimoto, M. Fuel Proc. Technol. 2001, 16, 229–238. (22) Seki, H.; Yoshimoto, M. Sekiyu Gakkaishi 2001, 44, 102–108. (23) Merdrignac, I.; Truchy, C.; Robert, E.; Guibard, I.; Kressmann, S. Pet. Sci. Technol. 2004, 22, 1003–1022.
asphaltenes, the three AR-asphaltenes and their residues after thermal treatment were examined by solid-state 13C NMR. The SPE/MAS technique24 was applied to determine the ratio of aromatic to aliphatic carbon, and the CP/IP/MAS8 technique was used to estimate the ratio of tertiary to quaternary, aromatic carbon. Figure 4 shows the SPE/MAS spectra of the original asphaltenes and their solid matter after thermal decomposition. Table 7 summarizes the average structural parameters deduced from the solid-state 13C NMR spectra together with elemental analysis data. From the solid-state 13C NMR spectra, the most obvious effect of thermal cracking of the asphaltenes is the remarkable increase of the ratio of aromatic to aliphatic carbon in the samples. This is accompanied by a decline of the atomic H/C ratio. Both findings indicate that the asphaltenic material undergoes at 412 °C drastic structural changes. As shown by Speight,25 in the temperature range 350–400 °C, cracking of thermally labile bonds results in a rapid increase in the yields of benzeneinsoluble materials and volatile products. Ancheyta et al.6 identified these weak bonds in asphaltenes as those between carbon atoms in multicarbon alkyl chains, between sulfur and carbon in mercapto ethers, and between aromatic and adjacent naphthenic rings. As result of side-chain cleaving, the NMR signal at 30 ppm and the atomic ratio between hydrogen and carbon decrease while the aromaticity of the nonvolatile residue increases. The N/C and S/C ratio stays about the same as in the (24) Fonseca, A.; Zeuthen, P.; Nagy, J. B. Fuel 1996, 75, 1363–1376. (25) Speight, J. G. Prepr. Pap.––Am. Chem. Soc., DiV. Fuel Chem. 1987, 32, 413–418.
454 Energy & Fuels, Vol. 22, No. 1, 2008
original asphaltenes. Calemma et al.,26 who analyzed the gaseous products from asphaltene pyrolysis in the temperature range 330–360 °C, showed that alkanes and olefins make up to 80% of the volatile products, which is in line with our NMR results. Hamid27 reported another interesting aspect of the thermal treatment of asphaltenes. He found that above 400 °C the concentration of free radicals present in the nonvolatile material increases abruptly. This indicates that the chemistry of thermal cracking of asphaltenes involves the occurrence of aromatic radicals that can in turn either recombine to polyaromatics or react with hydrogen radicals to dealkylated aromatics (Figure 5). The inspection of the solid-state NMR data of the original asphaltenes and their pyrolysis products demonstrates that (i) the ratio between carbon in CH3 groups and total aliphatic carbon changes little and (ii) the percentage of tertiary, aromatic carbon increases drastically while the percentage of triplebridged, internal, aromatic carbon is about the same in both samples. These two facts suggests that the scission of the entire side chain occurs rather than cracking in a more distant position from the aromatic ring and that dealkylation/hydrogenation is preferred over formation of larger polyaromatics. 3.4. X-ray Diffraction Analysis. Figure 6 displays both XRD patterns, the original asphaltenes and their pyrolysis products, together for a direct comparison. The graphs show clearly that thermal cracking has a significant effect on the bulk structure of these samples. In accordance with the solid-state NMR findings, X-ray diffraction analysis also demonstrates that during cracking the asphaltenes lost their alkyl chains, which in turn reduces the average distance between the aromatic sheets (dm) and the stack height of the aromatic sheets (Lc) (see Table 8). The average diameter of the aromatic sheets (La), however, has hardly changed. The latter is evident for dealkylation rather than condensation of the asphaltenic material under thermal cracking. Conclusions Thermal degradation of asphaltenes, obtained from atmospheric residues of heavy crude of different origins, revealed (26) Calemma, V.; Rausa, R. J. Anal. Appl. Pyrolysis 1997, 40–41, 569– 584.
Hauser et al.
that the asphaltenes consist of a bimodal composition. There is a fraction of refractory thermally stable asphaltenes, and there are other entities that are more easily converted. Depending on the origin of the asphaltenes, the ratio between both components varies. Life test experiments on a pilot-plant scale demonstrated that heavy AR feeds, which contain a high percentage of refractory asphaltenes, deactivate an industrially used the ARDS catalyst system twice as fast as a typical ARDS feed. Pyrolysis of the asphaltenes at 412 °C has resulted in a remarkable increase of the ratio of aromatic to aliphatic carbon (from ∼1 to ∼3) accompanied by a decline of the atomic H/C ratio (from ∼1.1 to ∼0.8). The facts that (i) the ratio between carbon in CH3 groups and total aliphatic carbon remained unaffected and (ii) the percentage of tertiary, aromatic carbon increased drastically while the percentage of triple-bridged, internal, aromatic carbon stayed about the same suggest that during pyrolysis the scission of the entire side chain occurs rather than cracking in a more distant position from the aromatic ring and that dealkylation is preferred over the formation of larger polyaromatics. Consequently, the average distance between the aromatic sheets and the stack height of the aromatic sheets in asphaltene aggregates reduce while the average diameter of the aromatic sheets hardly changes. Acknowledgment. The authors would like to acknowledge the financial support of the Kuwait Institute for Scientific Research (KISR) and the Japan Cooperation Center, Petroleum (JCCP), a Japanese organization supported by Japan Ministry of Economics, Trade and Industry (METI). The authors also give their special thanks to the management of the Japan Energy Corporation (JEC) for their support and technical assistance in pursuing this study. The authors appreciate the Central Analytical Lab at KISR for their assistance. This project bears the KISR Project No. PF025C. EF700477A
(27) Hamid, S. H. Pet. Sci. Technol. 2000, 18, 871–888.
