Energy & Fuels 2004, 18, 283-284
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Communications Observation of Glass Transition in Asphaltenes Yan Zhang, Toshimasa Takanohashi,* Sinya Sato, and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 3058569, Japan
Ryuzo Tanaka Central Research Laboratories, Idemitsu Kosan Co., Ltd., Sodegaura, Chiba 2990293, Japan Received May 6, 2003 Asphaltenes are the heaviest fraction of crude oils and vacuum residues (VR). They contain poly-condensed aromatic compounds associated with heteroatoms and metals. Because of their extremely heavy and polar nature, asphaltene molecules commonly occur as colloidal particles that may form precursors of coke and also deactivate catalysts used in upgrading and refining processes. To successfully disperse asphaltene colloids, so as to facilitate such further upgrading processes, it is necessary to have an appreciation not only of asphaltenes’ chemical structure and composition, but also of their thermal properties, to provide insight into the thermodynamic behavior of asphaltenes under different conditions. Differential scanning calorimetry (DSC) has been widely used to study the glass transition and enthalpy relaxation of crude oils, bitumens, and pitches.1-6 By this technique, the glass transition temperatures (Tg) for asphaltenes from vacuum residues (VRs) were reported to be 294 °C by Kopsch.7 On the basis of this Tg value, the melting point temperature (Tm) for asphaltenes was estimated to be as high as 452 °C. In this study, we investigate the thermal behaviors of four asphaltenes from different VRs using different thermal analytical procedures, including DSC. It was found that asphaltenes started to soften at around 150 °C and attained a completely liquid state at temperatures between 220 and 240 °C. Moreover, repeated DSC scans showed a pronounced baseline shift in the 100-180 °C temperature range, which can be the recognized glass transition range for asphaltenes. Four asphaltenes (AS, n-heptane-insolubles) were used, which were obtained from fractionation of vacuum residues (VR) by vacuum distillation above 500 °C of Iranian Light (IL), Khafji (KF), Kuwait (KW), and Maya (MY) crude oils. The properties of the four asphaltenes are given in Table 1. All samples were dried at 80 °C overnight in vacuo before use. The thermal properties of * Corresponding author. E-mail:
[email protected]. (1) Claudy, P.; Le´toffe´, J.-M.; Chague´, B.; Orrit, J. Fuel 1988, 67, 58. (2) Hansen, A. B.; Larsen, E.; Pedersen, W. B.; Nielsen A. B.; RØenningsen, H. P. Energy Fuels 1991, 5, 914. (3) Claudy, P.; Le´toffe´, J.-M.; King, G. N.; Planche, J. P.; Bruˆle´, B. Fuel Sci. Technol. Int. 1991, 9, 71. (4) Masson, J.-F.; Polomark, G. M. Thermochim. Acta 2001, 374, 105. (5) Ehrburger, P.; Martin, Ch; Saint-Romain, J. L. Fuel 1991, 70, 783. (6) Lahaye, J.; Ehrburger, P.; Saint-Romain, J. L.; Couderc, P. Fuel 1987, 66, 1467. (7) Kopsch, H. Thermochim. Acta 1994, 235, 271.
Table 1. Properties of Asphaltenes elemental analysis (wt %) C H S N Oa ASIL ASKF ASKW ASMY a
83.2 82.2 82.7 82.0
6.8 7.6 7.6 7.5
5.9 7.6 8.3 7.1
1.4 0.9 0.8 1.3
2.7 1.7 0.6 2.1
H/C
Ni + V (ppm)
density g/cm3
fa
0.98 1.11 1.10 1.10
1590 750 ndb 2190
1.1669 1.1683 1.1919 1.1767
0.61 0.56 0.53 0.55
By difference. b Not determined.
the asphaltenes were measured with a Seiko DSC 120 calorimeter. In a typical run, 6-10 mg of sample was heated at a rate of 10 °C/min from 8 to 300 °C in a nitrogen flow of 50 mL/min. Repeated scans of the same sample were obtained by heating under the same conditions once the sample had been quenched quickly to 8 °C. To make sure that the baseline was flat and horizontal over the temperature range studied, the slope and balance of the baseline were adjusted according to the actual analysis condition. After properly adjustment by using two aluminum cells, analyses were performed with pure lead (>99.999%) to obtain a processing baseline, which is shown in Figure 1a. Thermogravimetric analysis (TGA) was carried out using a Shimadzu TGA-50H analyzer. Experimental conditions were the same as those used in the DSC measurements. The DSC thermograms of four asphaltenes are shown in Figure 1. All four asphaltenes showed similar calorimetric signals in the temperature range studied. The three endothermic peaks at about 130 °C, 180 °C, and 230 °C on the first scan relate to two heating behaviors: (1) the evaporation of volatile components or the release of adsorbed gases, and (2) the softening of some components in the asphaltenes. If the evaporation effect contributes to the DSC scan, a weight loss should be observed during heating. Figure 2 shows the TGA and its firstderivative (DrTGA) curves for ASIL. On the first heating run, ASIL started to lose weight at the comparatively low temperature of about 100 °C, and the total weight loss up to 300 °C was about 0.6 wt %. A broad peak was recorded from 150 to 230 °C on the first DrTGA curve at a similar temperature range to the second endothermic peak recorded on the first DSC scan (Figure 1). The weight loss below 300 °C is probably mainly due to the evaporation of low-molecular-weight components or the release of adsorbed gases, including water, in the as-
10.1021/ef0301147 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/25/2003
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Figure 1. DSC thermograms of (a) ASIL, (b) ASKF, (c) ASKW, and (d) ASMY.
Figure 2. TGA and first-derivative curves for ASIL.
phaltenes; both processes can produce endothermic records on the first DSC scan. The state of the samples at various temperatures was observed visually and each was seen to soften at and above 150 °C and to become wholly tar-like above 230 °C. The combined results indicate that both evaporation of light components and melting of the crystallized phase in asphaltenes contribute to the first DSC scan over the analytical temperature range. Such an overlapping of effects makes it difficult to get accurate information about the glass transition of asphaltenes on the basis of a single first DSC scan. Glass transition phenomena are generally accompanied by a sudden change in the specific heat capacity, Cp. As a result, a baseline shift should be observed on DSC thermograms or Cp-temperature curves that should prove reversible on repeated heating (or cooling) runs. As shown in Figure 1, although there is still a broad endothermic contribution centered at about 180 °C on the second and third scans, an obvious baseline shift is present at temperatures below 100 °C and above 180 °C, and this provides clear evidence that each of the four asphaltenes undergoes a glass transition. The glass transition temperature (Tg) can be determined by the general methodology described in references 1-6. As shown in Figure 1, all four asphaltenes show a similar Tg value near 120-130 °C. In addition, no weight loss is observed on the second TGA or its DrTGA curve (Figure 2). Thus, the endothermic contribution near 180 °C on
the second and third DSC scans cannot arise from the evaporation of volatile materials in the asphaltenes, but must be due to the melting of a crystalline phase such as occurs in crude oils1,2 or semicrystalline polymers.8,9 Nalwaya et. al.10 reported that more polar fractions of asphaltenes exhibit a crystalline appearance, while less polar fractions look like amorphous solids. Although the experimental methods were different between them and us, there was a good agreement in conclusion that asphaltenes had both crystalline and amorphous (showing glass feature) phases. It should be noted that the Tg values of asphaltenes observed in this study were much different from those reported by Kopsch.7 It may be impossible to make a direct comparison with Kopsch’s results, because of the differences in both structural features of the samples used and temperature ranges studied. For example, four asphaltenes used in this study have similar H/C atomic ratios (0.98-1.1), almost similar as many asphaltenes from different geologic origins.11-13 The asphaltenes used by Kopsch had a lower H/C ratio of ca. 0.87 (ca. 1.14 in C/H ratio). In addition, Kopsch presented the DSC thermograms of asphaltenes from 200 to 550 °C, where a large pyrolysis peak might be emphasized. In conclusion, the glass transition temperatures of four asphaltenes were determined as lying near 120-130 °C, on the basis of repeated DSC thermograms. The baseline shift and the endothermic contribution observed on second and third DSC thermograms point to an overlapping between the glass transition and the melting of crystalline phases in asphaltenes within the temperature range of 100 to 230 °C. EF0301147 (8) Wunderlich, B.; Boller, A.; Okazaki, I.; Ishikiriyama, K.; Chen, W.; Pyda, M.; Park, J.; Moon, I.; Androsch, R. Thermochim. Acta 1999, 330, 21. (9) Wunderlich, B.; Okazaki, I.; Ishikiriyama, K.; Boller, A. Thermochim. Acta 1998, 324, 77. (10) Nalwaya, V.; Tangtayakom, V.; Piumsomboon, P.; Fogler, S. Ind. Eng. Chem. Res. 1999, 38, 964. (11) Long, R. B.; Speight, J. G. Rev. Inst. Fr. Pet. 1990, 45, 553. (12) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530. (13) Storm, D. A.; DeCanio, S. J.; DeTar, M. M.; Nero, V. P. Fuel 1990, 69, 735.
