Energy & Fuels 2005, 19, 1023-1028
1023
Estimating the Interaction Energy of Asphaltene Aggregates with Aromatic Solvents Yan Zhang, Toshimasa Takanohashi,* Takahiro Shishido, Sinya Sato, and Ikuo Saito Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569, Japan
Ryuzo Tanaka Central Research Laboratories, Idemitsu Kosan Co., Ltd., Sodegaura, Chiba 299-0293, Japan Received May 20, 2004. Revised Manuscript Received February 8, 2005
The molecular interactions between asphaltenes and solvents are an important parameter that dominates the processing behavior of asphaltenes. This study has developed a method for estimating the interaction energy of asphaltene (or resin) aggregates with aromatic solvent molecules. The thermal properties of three asphaltenes and one resin have been ascertained using differential scanning calorimetry (DSC) and thermogravimetry analysis (TGA), and the heats of solution (∆Hsolu) of the two species in quinoline, 1-methyl naphthalene (1-MN), and tetralin have been established using microcalorimetry. By combining the two techniques, the heat of mixing (∆Hmix) of the asphaltene or the resin with the solvents could be estimated. This not only evaluates the nature of interactions between asphaltene (or resin) aggregates and solvent but also provides a quantitative estimate of their magnitude.
Introduction Asphaltenes are the heaviest fraction in crude oils or vacuum residues (VRs), including condensed-ring aromatic compounds with heteroatoms and metals. To date, the characterization of asphaltenes has focused largely on chemical structural parameters, composition, and aggregate size.1-6 However, understanding some of the processes in which asphaltenes are involved requires information on their thermal properties, in addition to knowledge of their chemical structures and compositions. Thermal properties are particularly helpful in understanding the thermodynamic behavior of asphaltenes in various reactions. Asphaltene molecules form aggregates, and this process is responsible for the formation of coke precursors and the deactivation of catalytic reactions in upgrading and refining processes. The formation of such undesirable aggregates is mainly due to noncovalent interactions among asphaltene molecules, such as π-π interactions,7 hydrogen bonding,8-11 van der Waals forces,12 * Author to whom correspondence should be addressed. E-mail address:
[email protected]. (1) Su, Y.; Artok, L.; Su, Y.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 1265. (2) Strausz, O. C.; Mojelsky, T. M.; Lown, E. M. Fuel 1992, 71, 1355. (3) Leo´n, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (4) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290. (5) Sheu, E. Y.; Ling, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399. (6) Tanaka, R.; Hunt, J. E.; Winans, R. E.; Thiyagarajan, P.; Sato, S.; Takanohashi, T. Energy Fuels 2003, 17, 127. (7) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 17, 135.
and charge-transfer interactions.13,14 Recent computer simulations suggest that quinoline can break up the aggregated structure of asphaltenes, and autoclave experiments showed that the coke yield after pyrolysis was decreased when asphaltenes were pretreated in quinoline at 300 °C before pyrolysis, compared with the yield without pretreatment.15 Undoubtedly, molecular interactions have a significant role in both aggregate formation and relaxation of the aggregated structure. To date, attention has been focused on asphalteneasphaltene or solute-solute interactions, rather than asphaltene-solvent or solute-solvent interactions. However, a better understanding of asphaltene (or its aggregates)-solvent interactions may be more important and interesting for clarifying possible interactions between asphaltenes and other polar or nonpolar components contained in crude oils or VRs. Theory and Methodology When a liquid solute dissolves in a solvent, the heat of mixing (∆Hmix) reflects the essential interactions (8) Moschopedis, S. E.; Speight, J. G. Fuel 1976, 55, 187. (9) Ignasiak, T. M.; Strausz, O. P. Fuel 1998, 57, 617. (10) Tewari, K. C.; Galya, L. G.; Egan, K. M.; Li, N. C. Fuel 1978, 57, 245. (11) Acevedo, S.; Leon, O.; Rivas, H.; Marquez, H.; Escobar, G.; Gutierrez, L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1987, 32, 426. (12) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117, 201. (13) Wright, J. R.; Minesinger, R. R. J. Colloid Interface Sci. 1963, 18, 223. (14) Siffert, B.; Kuczinski, J.; Papirer, E. J. J. Colloid Interface Sci. 1990, 135, 167. (15) Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Pet. Sci. Technol. 2003, 21, 491.
10.1021/ef0498770 CCC: $30.25 © 2005 American Chemical Society Published on Web 04/08/2005
1024
Energy & Fuels, Vol. 19, No. 3, 2005
Zhang et al.
Table 1. Properties of Asphaltenes and Resin C REMY ASMY ASKF ASIL a
82.4 82.0 82.2 83.2
Elemental Analysis (wt %) H N S
Oa
H/C
aromaticity, fa
number-average molecular weight, Mn
9.5 7.5 7.6 6.8
1.3 2.1 1.7 2.7
1.38 1.10 1.11 0.98
0.39 0.55 0.56 0.61
770 787 930 706
1.1 1.3 0.9 1.4
5.7 7.1 7.6 5.9
By difference.
between solute and solvent molecules directly. If the mixing process occurs at a constant volume, then the heat of mixing is given by the Van Laar-Hildebrand equation:16
∆Hmix ) kTχhn1ø2
(1)
where n1 is the moles of solvent, ø2 is the volume fraction of solute, and χh is a parameter that reflects the interaction between the solute and the solvent. According to this equation, ∆Hmix reflects the nature and magnitude of the interaction of a liquid solute with a solvent. Calorimetric measurements are the most reliable and represent one direct method of obtaining such interaction information. However, when a solid solute dissolves in a solvent, ∆Hmix must be measured above the melting point (Tm) or glass-transition temperature (Tg) of the solute. At such a high temperature, calorimetric measurements become more difficult, and the thermal stability of the solute and the solvent can cause problems. Therefore, when a solid solute dissolves in a solvent, the heat of solution (∆Hsolu) does not provide a direct interaction parameter, insofar as it might be influenced by enthalpy changes that result from phase transitions, such as those which occur in the dissolution of glasses17,18 or changes in crystal structure.19 In this case, ∆Hsolu can be expressed as the sum of the enthalpy changes due to any phase transition (∆Htran). For example, in the case of completely crystalline materials,
∆Hsolu ) λ + ∆Hmix
(2)
where λ represents the heat of fusion of the crystalline structure. For ideal glass polymers,
∆Hsolu ) ∆Cp(T - Tg) + ∆Hmix
(3)
where T and Tg are the arbitrary temperature at the solid state and glass-transition temperatures of the polymers, respectively, and ∆Cp is defined as the difference in the heat capacity of the glass and solid states (∆Cp ) Cp,glass - Cp,solid) and is positive. The glass enthalpy is always exothermic and can be represented as two forms: ∆Cp(T - Tg) or -∆Cp(Tg - T). In eqs 2 and 3, ∆Hsolu can be determined directly using a microcalorimeter, whereas the heat of fusion and the glass enthalpy can be obtained from DSC measurements. Therefore, for a solid material, the value of ∆Hmix can be estimated by combining microcalorimetry and DSC measurements. In this study, the thermal properties of asphaltenes and a resin fractionated from Maya (MY), Khafji (KF), and Iranian Light (IL) VRs, and their heats of solution in solvents, were determined using differential scanning calorimetry (DSC) and microcalorimetry. By combining the two techniques, a quantitative method for estimat-
ing the interaction energy between asphaltene aggregates and solvent was developed. Experimental Section Materials and Chemicals. Maya (MY), Khafji (KF), and Iranian Light (IL) VRs (after the vacuum distillation of the crude oils at 500 °C) were used as the starting materials for asphaltene (AS) and resin (RE) preparations. Asphaltene was extracted as an n-heptane-insoluble fraction by adding nheptane to the VR. We have described the fractionation procedures previously.20 The resin was separated from maltene (n-heptane-soluble) using a column chromatography method. The maltene was adsorbed onto activated alumina particles and then washed with n-heptane and toluene; resin was obtained by desorption from the particles using a mixed solvent (methanol/tetrachloromethane, 50/50 v/v). In the following, the asphaltenes from Maya, Khafji, and Iranian Light VRs are denoted ASMY, ASKF, and ASIL, respectively. The resin from Maya VR is denoted REMY. The main properties of the asphaltenes and resin are summarized in Table 1. The asphaltenes and resin are all solid powders at room temperature. Quinoline, 1-methyl naphathlene (1-MN), tetralin, and the other solvents described above were purchased from Kanto Chemical Co., Inc., and used as received. N,N-dimethylnaphthylamine (DMNA) was supplied by Tokyo Kasei Kogyo Co., Ltd. The purity and melting point of this compound are >98% and 41 °C, respectively. Structural Analyses. The carbon and hydrogen contents were measured using a CHN-O-Rapid analyzer (Elementar); the sulfur, nitrogen, and oxygen contents were measured using an AQS-6W sulfur tester (Tanaka Scientific Instruments), an ANTEK7000 (Antek), and a CHN-O-Rapid (Heraeus) analyzer, respectively. Densities were measured in conformity with Japanese Industrial Standard JIS K 7112, using a DMA45 apparatus (Paar). The aromaticity (fa) of the samples were obtained by 13C NMR. 13C NMR spectra were measured in CDCl3 with TMS as the internal standard on a Lambda 500 FT-NMR spectrometer at a resonance frequency of 600 MHz for protons. Differential Scanning Calorimetry (DSC) Measurement. The thermal properties of the samples were measured with a Seiko model DSC 120 calorimeter. The temperature and heat capacity were calibrated with high-purity tin. In a typical run, a 6-10 mg sample was heated at a rate of 10 °C/min from 8 °C to 300 °C, under a 50 mL/min nitrogen flow. Repeated scans of the same sample were obtained by heating under the same conditions after the sample had been quenched quickly to 8 °C. Thermogravimetric analysis (TGA) was performed using a Shimadzu model TGA-50H analyzer under the same experimental conditions as those used for the DSC measurements. (16) Hidebrand, J. H.; Scott, R. L. The Solubility of Nonelectrolytes; Reinhold: New York, 1950; Chapter 20. (17) Bianchi, U.; Pedemonte, E.; Rossi, C. Makromol. Chem. 1966, 92, 114. (18) Cottam, B. J.; Cowie, J. M. G.; Bywater, S. Makromol. Chem. 1965, 86, 116. (19) Schreiber, H. P.; Waldman, M. H. J. Polym. Sci., Polym. Phys. Ed. 1967, 5, 555. (20) Zhang, Y.; Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 18, 101.
Interaction Energy of Asphaltene Aggregates
Figure 1. Microcalorimetry results for N,N-dimethylnaphthylamine (DMNA) in quinoline.
Energy & Fuels, Vol. 19, No. 3, 2005 1025
Figure 2. Differential scanning calorimetry (DSC) thermogram for DMNA.
Microcalorimetry Measurements. The heat of solution (∆Hsolu) of the samples was measured using a micro-twincalorimeter (MPC-11, Tokyo Riko Co., Ltd.). The detailed procedures and an examination of the accuracy and reproducibility of the calorimetric measurements have been reported elsewhere20 and are only summarized here. A sealed glass ampule containing the solid sample and a blank were prepared and immersed in sample and reference cells that contained solvent. The calorimeter was left overnight, to allow it to attain thermal equilibrium, and then the measurement was started by breaking the ampules in each cell simultaneously. In each dissolution measurement, the ∆Hsolu value was determined by mixing ∼30 mg of solid sample with 20 mL of solvent. The maximum operating temperature of this instrument is 60 °C.
Results and Discussion Elemental and Structural Differences between Asphaltenes and Resin. Table 1 lists the elemental and structural properties of the three asphaltenes and one resin (REMY). The resin is the second heaviest species in crude oils and VRs, and it gathers around asphaltene colloidal particles.21 Comparisons of the elemental compositions and structural parameters of the two fractions indicate that asphaltenes are more aromatic and contain more heteroatoms than the resin. There is little doubt that asphaltenes have larger condensed aromatic unit structures than the resin, because the former have higher fa values and lower H/C ratios. Verification of Dissolution Theory Using Microcalorimetry and Differential Scanning Calorimetry (DSC). To verify the reliability of the theory and methodology, N,N-dimethylnaphthylamine (DMNA) was used as a reference. This compound was selected because its melting point is 41 °C below the operating limit of the microcalorimeter (60 °C), allowing a direct comparison of the determined and estimated values of ∆Hmix. First, the ∆Hmix value for this compound was determined directly at temperatures above its melting point using microcalorimetry. As an example, Figure 1 shows microcalorimeter results for DMNA in quinoline at different temperatures. The mixing of DMNA with quinoline at 45 °C was exothermic, with ∆Hmix ) -3.7 J/g. This value represents the enthalpy change during the mixing of liquid DMNA with quinoline, which should reflect their interaction energy. Second, the value of ∆Hmix can be estimated from eq 2. As seen in Figure 1, when the microcalorimeter measurement was performed at 25 °C, i.e., below the melting point of DMNA, the process was endothermic (21) Pfeiffer, J. P.; Saal, R. N. J. J. Phys. Chem. 1939, 43, 139.
Figure 3. (a) DSC thermogram and (b) heat capacity of REMY resin.
with ∆Hsolu ) 58 J/g. DMNA remained completely solid at 25 °C. Consequently, the value of ∆Hsolu at 25 °C could be used to estimate the value of ∆Hmix. Figure 2 shows the DSC thermogram of DMNA, which is typical of that obtained from a completely crystalline material. The heat of fusion (λ) was obtained by integrating the area of the endothermic peak on the DSC thermogram, giving a value of 61.4 J/g. From eq 2, ∆Hmix ) -3.4 J/g (exothermic), which is almost the same as the analyzed value of -3.7 J/g. The similarity of the results demonstrates that a combination of DSC and microcalorimetry is useful for determining the ∆Hmix of solid materials, such as resin and asphaltenes, in solvents. Interaction Energy between Resin and Solvents. Compared with DMNA, a resin is a very complex mixture of molecules that have different structures and molecular weights. Figure 3a shows the DSC thermogram of REMY, which proved very similar to that of semicrystalline materials in that it had both an endothermic peak and a baseline shift.22,23 This behavior suggests that REMY contains both melting and glassy phases. When dissolving such a material in a solvent, (22) Wunderlich, B.; Boller, A.; Okazaki, I.; Ishikiriyama, K.; Chen, W.; Pyda, M.; Park, J.; Moon, I.; Androsch, R. Thermochim. Acta 1999, 330, 21. (23) Wunderlich, B.; Okazaki, I.; Ishikiriyama, K.; Boller, A. Thermochim. Acta 1998, 324, 77.
1026
Energy & Fuels, Vol. 19, No. 3, 2005
Zhang et al.
Table 2. Thermal Properties of Asphaltenes and Resin
REMY ASMY ASKF ASIL
glass-transition temp, Tg (°C)
glass enthalpy, ∆Cp(T - Tg) (J/g)
heat of fusion, λ (J/g)
42 119 115 112
-1.8 -11.8 -9.6 -9.3
4.6 5.6 4.7 4.9
∆Hsolu at 25 °C (J/g) REMY/quinolin REMY/tetralin
the heat of solution should contain contributions from both the heat of fusion (λ) and the glass enthalpy (∆Cp(T - Tg)), i.e.,
∆Hsolu ) λ + ∆Cp(T - Tg) + ∆Hmix
Table 3. Heat of Solution, and Determined and Estimated Heat of Mixing, for REMY in Quinoline and Tetralin
(4)
However, there are no reports on how to determine the contributions of the heat of fusion and glass enthalpy from DSC curves for a resin. In this study, we divided the DSC thermogram in two parts. Area R in Figure 3a corresponds to the heat of fusion of the melting phase. The glass enthalpy must be calculated from a heatcapacity (Cp) determination, as shown in Figure 3b, which corresponds to the area surrounded by abe or estimated by multiplying ∆Cp and ∆T (the area surrounded by abcd). The glass-transition temperature Tg was defined as the midpoint of the baseline shift on the DSC or Cp curve using a general method that has been described previously.24-26 Using the DSC thermogram and heat-capacity measurement, the glass-transition temperature Tg, glass enthalpy ∆Cp∆T, and heat of fusion λ of REMY can be obtained, and the results are given in Table 2. Specific explanations about heat of fusion should be given here. The term “heat of fusion” is the most befitting to those pure crystalline substances such as DMNA; however, it is anxious about whether the same term is unquestionably applicable to those very complex mixtures such as resins (or asphaltenes, which will be discussed later). For this reason, there is a need to call attention about the different meanings of the expression “heat of fusion” between DMNA and resin (or asphaltenes) studied in this study. The heat of fusion of DMNA means the enthalpy change originated from the melt of pure DMNA molecules upon heating, whereas heat of fusion of resin (or asphaltene) means “transition enthalpy of the melting structures or phases in resin or asphaltene during heating”. Resins (or asphaltenes) are very complex mixtures. All structural parameters or thermal properties of them (such as aromaticity fa, H/C atomic ratio, heat of fusion λ, heat of solution ∆Hsolu, etc.) only express an average aspect. In other words, although the heat of fusion of resins or asphaltenes may be originated from a part of the structures or phases in them, it is unavoidable to present it in a bulk basis (in units of J/g-sample). This difficulty may lead to apparent absurdness in some cases. As shown in Table 2, the heat of fusion of resin was only 4.6 J/g, which is an absurd value, out of the range of common organic compounds (50-200 J/g), but is similar to those of bitumen: ∼7 J/g and 13.17 J/g, as reported by Masson and Polomark27 (24) Claudy, P.; Le´toffe´, J.-M.; King, G. N.; Planche, J. P.; Bruˆle´, B. Fuel Sci. Technol. Int. 1991, 9, 71. (25) Claudy, P.; Le´toffe´, J.-M.; Chague´, B.; Orrit, J. Fuel 1988, 67, 58. (26) Hansen, A. B.; Larsen, E.; Pedersen, W. B.; Nielsen, A. B.; Røenningsen, H. P. Energy Fuels 1991, 5, 914. (27) Masson J.-F.; Polomark, G. M. Thermochim. Acta 2001, 374, 105.
a
6.1 9.3
∆Hmix (J/g) determineda estimatedb 4.3 7.1
3.3 6.5
Determined directly, using a microcalorimeter at 45 °C. from eq 4.
bEstimated
and Jime´nez-Mateos et al.,28 respectively. Despite the aforementioned difference and difficulty between DMNA and resin or asphaltene, for convenience, we would still like to use an identical term, “heat of fusion”, for our discussion in the present study. Table 3 shows the dissolution behavior of REMY in quinoline and tetralin at 25 and 45 °C. As indicated by DSC, the phase transition of the resin occurred at ∼42 °C, which makes it possible to determine the heat of mixing ∆Hmix directly using the microcalorimeter at the resin liquid state above 42 °C. The mixing of the resin with quinoline and tetralin at 45 °C is endothermic, suggesting that resin molecules are solvophobic to the two solvents. Alternatively, using the λ and ∆Cp∆T values determined by DSC (Table 2) and the heat of solution ∆Hsolu determined by microcalorimetry at 25 °C, ∆Hmix can be estimated from eq 4. As shown in Table 3, the estimated values (3.3 J/g in quinoline and 6.5 J/g in tetralin) are similar to the values determined directly using microcalorimetry at 45 °C (4.3 J/g in quinoline and 8.1 J/g in tetralin). This suggests that the method combining DSC and microcalorimetry for estimating ∆Hmix is applicable to complex materials such as resin. Interaction Energy between Asphaltene and Solvents. Asphaltene is a heavier fraction than resin. The three asphaltenes studied had similar thermal properties and DSC patterns. More detailed information has been reported in a previous paper.29 Figure 4 shows the DSC thermograms (Figure 4a) and heat capacity (Figure 4b) of ASMY as examples. Compared with resin thermograms, the asphaltene DSC thermograms were more complex: three endothermic peaks (indicated by arrows) were observed on the first scan but disappeared on the second. Some additional experimental runs were also performed to visually check the state change of the sample at various temperatures upon heating. Asphaltenes were observed to soften at ca. 150 °C and become tarlike at ca. 230 °C.29 The three endothermic peaks observed on the first DSC scan are related to two heating behaviors: (1) the evaporation of volatile components or the release of adsorbed gases, and (2) the softening of some components in asphaltenes. If the evaporation effect contributes to the DSC scan, a weight loss should be observed during heating. Figure 5 shows the TGA for ASMY. On the first heating run, ASMY started to lose weight at 100 °C. A broad peak was recorded from 100 °C to 200 °C on the first derivative thermogravimetry (DTG) curve at a temperature range similar to that of the second endothermic peak observed on the first DSC scan (see Figure 4a). The weight loss observed here might result in the (28) Jime´nez-Mateos, J. M.; Quintero, L. C.; Rial, C. Fuel 1996, 75, 1691. (29) Zhang, Y.; Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. Energy Fuels 2003, 18, 283.
Interaction Energy of Asphaltene Aggregates
Energy & Fuels, Vol. 19, No. 3, 2005 1027 Table 4. Heats of Solution and Estimated Heats of Mixing for Asphaltenes in Aromatic Solvents
a
Figure 4. (a) DSC thermograms and (b) heat capacity of ASMY asphaltene.
Figure 5. Thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) curves for ASMY asphaltene.
endothermic effects observed on the first DSC scan. These results indicate that both the evaporation of light components and the melting of some of the components in asphaltenes contribute to the first DSC scan in the temperature range studied. Such overlapping effects make it difficult to obtain accurate information about the thermal properties of asphaltenes using the first DSC scan. In contrast, an obvious baseline shift observed on the Cp curve (see Figure 4b) between the temperature ranges below 100 °C and above 250 °C provides clear evidence that asphaltenes undergo a glass transition. Simultaneously, the remaining broad endothermic peak at ∼180 °C (see Figure 4a) indicates that asphaltenes contain both glassy and melting phases. Because of the contribution of weight loss on the first DSC scan, the thermal properties of the three asphaltenes were determined from their second DSC runs and heat-capacity determinations. The results are also shown in Table 2. Unlike with resins, ∆Hmix could not be determined directly using microcalorimetry for asphaltenes, because DSC showed that the Tg of each asphaltene is above 110 °C (see Table 2), which is much higher than the operating limit of the microcalorimeter. For such materials, ∆Hmix can be estimated using a combination of
system
∆Hsolu (J/g)
∆Hmixa (J/g)
ASMY/quinoline ASMY/1-MN ASMY/tetralin
-22.1 -18.3 -15.6
-15.9 -12.1 -9.4
ASKFquinoline ASKF/1-MN ASKF/tetralin
-19.2 -16.2 -11.2
-14.3 -11.3 -6.3
ASIL/quinoline ASIL/1-MN ASIL/tetralin
-20.2 -12.8 -10.8
-15.8 -8.4 -6.4
Estimated from eq 4.
DSC and microcalorimetry, as indicated by eq 4. Table 4 shows the heats of solution of the three asphaltenes in the four solvents at 25 °C, along with the estimated ∆Hmix values. The exothermic nature of ∆Hmix suggests that asphaltene molecules are solvophilic to these solvents. The magnitude of ∆Hmix decreased in the following order: quinoline > 1-methyl naphthalene (1MN) > tetralin. Quinoline has the largest interaction energy with asphaltene, compared to the other solvents (see Table 4). Computer simulation results also suggested that quinoline had a larger relaxing effect for the aggregated asphaltene structure than 1-MN.15 The methodology used to estimate the interaction energy of asphaltenes with solvents in this study is just an attempt. The reliability of the data was based on the fact that the same method provided sufficient agreements between estimated and determined heats of mixing for both DMNA and REMY. However, some limitations and assumptions must be acknowledged here. Most importantly, there may be a doubt about how much the ∆Hmix estimated for asphaltenes reflects molecule-molecule interactions, because asphaltenes are known to aggregate at very low concentrations, as we and some other researchers have reported.20,30,31 Our previous results suggested that the aggregation of asphaltenes in quinoline started from low concentrations near 10 mg/L. Therefore, the asphaltene concentration of 30 mg/20 cm3 (1.5 g/L), which was used for microcalorimetry measurements in this study, was much higher than the initial aggregation concentration of asphaltene (10 mg/L). Obviously, it can be better if the microcalorimetry measurements could be conducted at the concentrations below 10 mg/L, so that information regarding the so-called “molecule-molecule interaction” could be obtained. Unfortunately, because of the limit of our current instrument, it was difficult to obtain reliable data when the experiments were performed at such low concentrations. Despite such difficulty, some efforts were put forth to evaluate the magnitude, with respect to the heat of dilution attributed to dissociation of aggregates in solution. In the previous paper,20 an experimental test related to the heat of dilution was done by diluting a concentrated asphaltene (ASIL)/ quinoline solution (30 mg/0.5 cm3, 60 g/L) to 19.5 cm3 quinoline. The result was compared with that obtained when 30 mg of solid ASIL was dissolved to 20 cm3 of quinoline. The two systems have the same final con(30) Zhang, Y.; Takanohashi, T.; Sato, S.; Saito, I.; Tanaka, R. J. Jpn. Pet. Inst. 2004, 47 (1), 32. (31) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; Decanio, S. J. Fuel 1992, 71, 299.
1028
Energy & Fuels, Vol. 19, No. 3, 2005
centration, 1.5 g/L. It was observed that the dissolution of ASIL in quinoline was largely exothermic, with a value of -20.2 J/g, as can be also observed in the present paper, whereas the dilution of the concentrated ASIL/ quinoline solution was endothermic, with a value of 2.3 J/g. The former mainly reflected asphaltene-quinoline interactions, whereas the latter could be attributed to the dispersion or dissociation of structures that were aggregated in the concentrated solution. Comparison of the magnitudes of ∆Hsolu and the heat of dilution suggested that dissociation of micelles or aggregates during dilution had only a small thermal effect. On the other hand, microcalorimetry measurements indicated that the heat changes during dissolution or dilution were almost over after 2 h. This means that, even though dissociation of large aggregates of asphaltenes could easily be slow and some heat changes could occur continuously during slow dissociation, the magnitude of these changes was too small to be detected by microcalorimetry. Thus, we took little notice of the effect of aggregation on the ∆Hmix in this study. What we want to stress here is that, although there is an unknown level of asphaltene association in the microcalorimetry experiments, the focus of this study was to obtain an understanding of the thermodynamic properties of both asphaltene and resin in different surrounding mediums (organic solvents). Further efforts should be taken to clarify how these properties correlated with their processing behaviors.
Zhang et al.
The magnitude of solute-solvent interactions may dominate the behavior of solutes during practical processes, such as solubility, adsorption/desorption, and swelling. The combination of DSC and microcalorimetry can provide quantitative information on the interactions between a solid solute and a solvent. Conclusions Differential scanning calorimetry (DSC) and microcalorimetry were used to quantify the interactions of asphaltenes with quinoline, 1-methyl naphthalene (1MN), and tetralin. According to DSC thermograms, the asphaltenes contained both glassy and melting phases. Based on the heat of solution determined by microcalorimetry and the thermal properties obtained by DSC, the estimated heat of mixing was between -16 J/g and -6 J/g in the three aromatic solvents used. The results indicated that asphaltene interacted more strongly with quinoline than with the other two solvents. The combination of DSC and microcalorimetry is a useful tool for estimating the interaction energy of asphaltenes with organic solvents. Acknowledgment. This work was supported financially by the Proposal-Based International Joint Research Program, New Energy, and Industrial Technology Development Organization. EF0498770
