Energy & Fuels 2003, 17, 101-106
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Dissolution and Dilution of Asphaltenes in Organic Solvents Yan Zhang, Toshimasa Takanohashi,* Sinya Sato, Teruo Kondo, 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, Chiba299-0293, Japan Received May 8, 2002
The dissolution and dilution of asphaltenes in organic solvents was investigated using a microtwin-calorimeter at 25 °C. Quinoline, o-dichlorobenzene, 1-methylnaphthalene, tetralin, ethylbenzene, and toluene were used as solvents. Heats of solution were exothermic in all solvents and appeared to correlate with the solubility parameter of the solvents used, which suggested that dissolution is dominated by interactions between the asphaltene and solvent molecules. On the other hand, dilution of a concentrated asphaltene solution was endothermic, which could be attributed to dissociation of aggregates or micelles in solution.
Introduction Asphaltenes are thought to be responsible for the formation of coke-precursors and for the deactivation of catalytic reactions, both of which are serious problems encountered in attempts to upgrade and refine processes in the petroleum industry.1 Because these problems must be related to structure and composition, asphaltenes have been extensively characterized using various techniques2-6 including small-angle neutron scattering (SANS), surface tension and viscosity measurements, calorimetric measurements, and other methods.7-14 Information obtained with these methods has provided important insights into the properties or structures of asphaltenes in organic solvents. The heat of solution of an asphaltene in a solvent can be measured directly with calorimetric methods. To * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Sharma M. M.; Yen, T. F.; Chiligarian, G. V.; Donaldson, E. C. In Developments in Petroleum Science; Elsevier: New York, 1985; p 223. (2) Su, Y.; Artok, L.; Murata, S.; Nomura, M. Energy Fuels 1998, 12, 1265. (3) Groenzin, H.; Mullins, O. C. Energy Fuels 2000, 14, 677. (4) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Energy Fuels 1998, 12, 1290. (5) Calemma, V.; Iwanski, M.; Nali, M.; Scotti, R.; Montanari, L. Energy Fuels 1995, 9, 225. (6) Ravey, J. C.; Ducouret, G.; Espinat, D. Fuel 1988, 67, 1560. (7) Sheu, E. Y.; Ling, K. S.; Sinha, S. K.; Overfield, R. E. J. Colloid Interface Sci. 1992, 153, 399. (8) Thiyagaraian, P.; Hunt, J. E.; Winans, R. E.; Anderson, K. B. Energy Fuels 1995, 9, 829. (9) Sheu, E. Y.; De Tar, M. M.; Storm, D. A.; DeCanio, S. J. Fuel 1992, 71, 299. (10) Rogel, E.; Leon,; Torres, G.; Espidel, J. Fuel 2000, 79, 1389. (11) Leon, O.; Rogel, E.; Espidel, J.; Torres, G. Energy Fuels 2000, 14, 6. (12) Storm, D. A.; Barresi, R. J.; DeCanio, S. J. Fuel 1991, 70, 779. (13) Andersen, S. I.; Bridi, K. S. J. Colloid Interface Sci. 1991, 142, 497. (14) Andersen, S. I.; Cristensen, S. D. Energy Fuels 2000, 14, 38.
evaluate the critical micelle concentration (CMC) of asphaltenes in solution, Andersen et al.13,14 measured the heat of dilution of an asphaltene solution in the same solvent using a microcalorimetric titration technique. The change in enthalpy was attributed to the dissociation of asphaltene aggregates or micelles. Little information has been reported on the dissolution behavior of asphaltenes from the solid state into solution. Using microcalorimetry, we measured heats of solution for asphaltenes in several organic solvents and measured heats of dilution of the resulting concentrated solutions. Heats of solution or adsorption heats at the surface were also measured for a series of model compounds with different molecular weights or for graphite powder. The results of these studies allow us to estimate the contribution of several processes to the heats of solution or dilution for asphaltenes. Experimental Section Samples and Solvents. The asphaltene (AS) fractions were extracted from Iranian Light (IL), Khafji (KF), and Maya (MY) vacuum residues (VR), the materials left after 500 °C vacuum distillation of the crude oils. The VR (130 g) was mixed with heptane (1/20, wt/wt) and stirred for 60 min under 1 MPa of nitrogen at 100 °C in an autoclave. The product was allowed to stand overnight at room temperature and then separated on a membrane filter with an average pore size of 1.0 µm. Solvent was removed by rotary evaporation. The heptaneinsoluble fraction was dried at 90 °C overnight in vacuo. Chemical compositions and structural information for three asphaltenes are shown in Table 1. Quinoline, o-dichlorobenzene, 1-methylnaphathlene, tetralin, ethylbenzene, toluene, and n-heptane were purchased from Kanto Chem. Co., Inc., and used as received. Density of the solvents was measured at 25 °C by using a DMA4500 density meter. The measurements accuracy was (5 × 10-4 g/mL. The density and other properties of the solvents are listed in Table 2.
10.1021/ef0201073 CCC: $25.00 © 2003 American Chemical Society Published on Web 12/12/2002
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Table 1. Properties of Asphaltenes elemental analysis (wt%) C ASIL 83.2 ASKF 82.2 ASMY 82.0
H
S
N
Oa
6.8 7.6 7.5
5.9 7.6 7.1
1.4 0.9 1.3
2.7 0.98 1.7 1.11 2.1 1.10
H/C
Ni + V Fb (ppm) (g/cm3) 1590 750 2109
fa
Mnc
1.1669 0.61 706 1.1683 0.56 903 1.1767 0.55 787
a
By difference. b Density at 25 °C. c Number average molecular weight.
Lithium bromide, graphite powder (particle sizes ) 200 µm, Kanto Chemical Co., Inc.), and the model compounds or polymers 1,5-dimethylnaphthalene, phenanthrene, benzo[ghi]perylene, dibenzothiophene sulfone, thianthrene, dinaphthyl, 9,10,-diphenylanthracene, 2,5-di-(4-biphenyl)oxazole, 2,5-bis(5′tert-butyl-2′-benzoxazolyl) thiophene, octaethyl porphyin, N,N′bis (2,6-dimethylphenyl-3,4,9,10-perylenetetracarboxylic diimide, tetraphenylporphine, vanadyl 5,14,23,32-tetraphenyl-2,3-naphthaocyanine, triphenylmethane, nickel (II) tetrakls (4-cumylphenoxy) phthalocyanine, and polystylene (MW ) 418, 500, 2800, 5400, 9500, and 19600) were commercial products obtained from either Tokyo Kasei Kogyo Co., Ltd. or Aldrich Chemical Co. The molecular weight, chemical structure, and heat of solution of each compound in quinoline are listed in Table 3. Surface Tension Measurements. Surface tensions of the solution or solvent were measured by a Wilhelmy plate method at 25 °C using an automatic surface tensiometer (CBVP-Z, Kyowa Interface Science Co. Ltd.). 13 C NMR Analysis. 13C NMR spectra were measured in DCCl3 with TMS as the internal standard on a Lambda 500 FT-NMR spectrometer at a resonance frequency of 600 MHz for protons. Molecular Weight Measurement. Gel Permission Chromatography (GPC) was carried out on a HPLC system equipped with a JASCO PU-986 pump, two polystyrene/ polydivinylbenzene columns (Mixed-D type, 300 × 7.5 mm, Polymer Laboratories Ltd., U.K.), and a JASCO RI-930 differential refractometer detector, using tetrahydrofurane (THF) as an eluent. Operation was performed at a flow rate of 1 mL/min at room temperature. A series of standard polystyrene samples were used as the calibration of molecular weight. Apparatus and Procedures. Heats of solution or dilution were measured using a micro-twin-calorimeter (MPC-11, Tokyo Riko Co., Ltd.) at 25 °C. The schematic diagram of the apparatus is shown in Figure 1. It consists of two identical transparent holding cells equipped with a pair of thermometers with the same temperature coefficients, a sample and a reference cell, a pair of ampule breaking devices, ampule holders, and a pair of stirrers. All parts are installed in an aluminum block. A sealed glass ampule containing the solid sample, or a concentrated solution of the solid sample in the respective solvent, and a blank were prepared and immersed into sample and reference cells that contained 20 mL of the respective solvent. The two cells were set in the measuring and reference holding cells, which were then filled with 4-5 mL of liquid paraffin. The calorimeter was left overnight to reach thermal equilibrium, after which the measurement was started by simultaneously breaking the two ampules within the two cells. The difference in temperature between the two holding cells was recorded on a chart recorder. The temperature change on dissolution or dilution was obtained according to Challoner’s method.15 Electrical calibration was also performed after either the dissolution or dilution experiments, and the temperature change was obtained in the same way. In each dissolution or adsorption measurement, the heat of solution was determined by mixing about 30 mg of solid sample with 20 mL solvent. The final solution had a concentration of ca. 1.5 g/L for each system.
The heats of solution and dilution, which are denoted by ∆Hm and ∆Hd, respectively, were derived from the following equation:
∆Hm (or ∆Hd) ) ∆Hc(∆Ts/∆Tc)
(1)
where ∆Hc is the heat quantity for electrical calibration and ∆Ts and ∆Tc are the temperature changes in dissolution (or dilution) and in the calibration experiment, respectively. The heat of dilution of asphaltenes was measured by mixing 0.5 mL of concentrated asphaltene-quinoline solution (at ca. 60 g/L) with 19.5 mL of pure quinoline. The final concentration of the solution was nearly the same as that of dissolution system. Accuracy and Precision. To assess the accuracy and precision of our method, we remeasured an exothermal (LiBr in water) and an endothermal (triphenylmethane in benzene) system for which results have been reported:16,17 the heats of solution of the two systems and experimental values reported are listed in Table 4. Certain absolute differences were found between our results and those in the literature; we consider them to be acceptable because both the apparatus and the experimental method used were different. Good reproducibility was found in both systems.
Results and Discussion Solvent Dependence of the Heat of Solution for Asphaltenes. Heats of solution of the three asphaltenes are summarized in Table 5. All heats of solution were exothermic in the range -20.2 ∼ -8.1 J/g. The solvent solubility parameter δ can be directly determined by experiments or calculated with the semiempirical equation18
δ ) 4.1(γ/V1/3)0.43
(2)
where γ and V are the surface tension and molar volume of the solvent, respectively. As shown in Table 2, for quinoline, ethylbenzene, tetralin, and toluene, the calculated solubility parameters δcal are in excellent agreement with the experimental values δexp taken from literature.19 Because of this agreement, it seems appropriate to use δcal values for other two solvents for which no experimental values are available. A plot of the heat of solution of the three asphaltenes vs δcal of the solvents is shown in Figure 2. The three asphaltenes showed a similar tendency that the value of ∆Hm increased with an increase in δcal of the solvents, which indicates that the dissolution behavior of asphaltenes may depend on solvent solubility properties. Heat of Solution of the Model Compounds in Quinoline. At first glance, the exothermic values for dissolution of asphaltenes are counterintuitive. In general, the heat of solution for a nonpolar solute in a nonpolar solvent must be endothermic, because solutesolvent interactions are primarily the result of London dispersion force.20,21 Results for the dissolution of triphenylmethane in benzene determined in this study (15) Challoner, A. R.; Gundry, H. A.; Meethnmy, A. R. Philos. Trans. R. Soc. London, A 1955, 247, 553. (16) Hill, J. O.; Ojelund, G.; Wadso, I. J. Chem. Thermodyn. 1969, 1, 111. (17) Timofeev, A. A. Chem. Zentralbl.. 1905, 76II, 429. (18) Sinoda, K. Z. Solution and Solubility; Maruzen: Tokyo, 1974. (19) Van Krevenlen, D. W. Fuel 1965, 44, 229. (20) Hildebrand, J. H.; Scott. R. L. The Solubility of Nonelectrolytes; Reinhold: New York, 1950. (21) Lewis, G.; Johnson, A. F. J. Chem. Soc. A 1969, 1816.
Asphaltenes in Organic Solvents
Energy & Fuels, Vol. 17, No. 1, 2003 103 Table 2. Properties of the Solvents (25 °C)
a
solvent
purity (%)
Mwa
Fb (g/cm3)
γc (dyne/cm3)
δcald (cal/cm3)1/2
δexpd (cal/cm3)1/2
quinoline o-ichlorobenzene 1-methylnaphthalene tetralin ethylbenzene toluene
>95.5 >99.0 >97.0 >98.0 >98.0 >99.5
129 147 142 132 106 92
1.08897 1.30044 1.01891 0.96486 0.86260 0.87368
44.3 36.9 38.6 35.3 28.5 28.2
10.56 9.82 9.72 9.38 8.69 8.76
10.40 nde nd 9.50 8.80 8.91
Mw: Molecular weight. b Density. c Surface tension.
d
Solubility parameter. e Data not found in literature.
Table 3. Structure and Heat of Solution of Model Compounds or Polymers in Quinoline
scope of the present paper. Some of the solvents used in this study, such as quinoline and o-dichlorobenzene, are generally classified as polar solvents. The heats of solution of a series of model compounds, which were in the solid state before dissolution, or polymers in quinoline are summarized in Table 3. Heats of solution for the monomeric compounds (1-9, Table 3) were endothermic, but most of the polymeric compounds (10-19) had exothermic heats of solution. A plot of ∆Hm as a function of molecular weight for these compounds is shown in Figure 3. The heat of solution seems to depend both on the molecular weight and chemical structures. Proposed Dissolution Mechanism for Asphaltenes in Organic Solvent. The transfer of asphaltenes from the solid to the solution phase involves very complex processes. We assume that at least three processes occur during dissolution: (1) adsorption of solvent on the solid surface of the asphaltenes, which can take place at the start of mixing (exothermic, ∆Hads); (2) dispersion or dissociation of asphaltene from the solid state into free molecules, micelles or colloid particles (endothermic, ∆Hdis); and simultaneously (3) interactions between solvent molecules and the dissolved asphaltenes (exothermic, ∆Hint)s the solute-solvent interaction (affinity) term that may be represented in terms of solvation energy. The total heat of solution (∆Hm) should be the simple sum of the enthalpy of each process:
∆Hm ) ∆Hads + ∆Hdis + ∆Hint
(3)
However, the exothermic changes observed here for ∆Hm of asphaltenes suggests the following inequality:
∆Hads + ∆Hint . ∆Hdis
(Table 4) are clear evidence for this conclusion. A discussion of the polarity of asphaltenes is beyond the
(4)
Of course, if only ∆Hads is generated, solid molecules could not dissolve in the solvent. In that case, it is expected that the thermal effect of adsorption may be small because the interaction area with solvent molecules is restricted to the solid surfaces. For example, we examined graphite powder in quinoline and ASIL in heptane. The heat changes in the two systems may be caused primarily by adsorption of the solvent molecules on the surface of graphite or ASIL powder. Both systems yielded very small exothermic values of ∆Hm: only -2.2 J/g for graphite powder/quinoline system and -4.6 J/g for ASIL/heptane system (Table 5). Thus, we presume that the heat change during adsorption makes only a small contribution to the total heat of solution of ASIL. For an exothermic ∆Hm, then, the solute-solvent interaction energy must be dominant during dissolution of asphaltenes in these solvents. This suggestion is in accord with the excellent relation between ∆Hm and solubility parameter δ (Figure 2).
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Figure 1. Schematic diagram of micro-twin-calorimeter. (a) Aluminum block. (b) Sample cell. (c) Holding cell. (d) Ampule holder. (e) Thermostat. (f) Ampule breaking device. (g) Stirrer. (h) Reference cell. (i) Glass ampule. (j) Solvent. (k) Liquid paraffin. (REC) Recorder. Table 4. Heat of Solution of LiBr in Water and Triphenylmethane in Benzene (25 °C) (a) LiBr/H2O System (∆Hm ) -48.8 kJ/mol in Literature16)
LiBr (g)
H2O (mL)
final concentration (g/L)
0.02173 0.02080 0.02125
20 20 20
1.09 1.04 1.06
∆Hm (kJ/mol) -46.2 -45.9 -46.7 -46.3 ( 0.5
(b) Triphenylmethane/Benzene System (∆Hm ) 17.6 kJ/ mol in Literature17) triphenylmethane (g)
benzene (mL)
final concentration (g/L)
0.01682 0.01700 0.01704
20 20 20
0.84 0.85 0.85
∆Hm (kJ/mol) 18.9 19.2 19.5 18.9 ( 0.4
Table 5. Heats of Solution of Asphaltenes and Adsorption Heat of ASIL or Graphite Powder in Organic Solvents (25 °C) ∆Hm (J/g) system
ASIL
ASKF
ASMY
graphite powder
quinoline 1-methylnaphathalene tetralin ethylbenzene o-dichlorobenzene toluene n-heptene
-20.2 -12.8 -10.8 -9.8 -14.2 -8.1 -4.6a
-19.2 -16.2 -11.2 -12.3 nd nd nd
-22.1 -18.3 -15.6 -13.8 nd nd nd
-2.2a ndb nd nd nd nd nd
a
Adsorption heat. b nd: Not determined.
As shown in Table 5 and Figure 2, ASMY showed higher ∆Hm values than those of ASKF and ASIL in quinoline, 1-methylnaphathlene, tetralin, and ethylbenzene, which should reflect the difference in certain characteristics among the three asphaltenes. Comparing the conventional parameters shown in Table 1, the density parameter seems to show a better correlation with the heats of solution than the others; the asphaltenes with the high density show the large ∆Hm values.
The density may be considered as a parameter reflecting the aggregated structure of asphaltenes, having a close connection with both chemical compositions and molecular structures. However, as described above, the dissolution of asphaltenes is a very complex process containing both phase transition (from solid to solution) and a variety of interactions between asphaltene and solvent molecules. It is not easy to find a satisfied parameter applicable to all dissolution processes. Although a certain correlation was found between the density and the heat of solution for asphaltenes, the validity of correlation may be limited only to materials having similar molecular structures or physical properties. Heat of Dilution (∆Hd) of Asphaltenes in Quinoline. We were also interested in the heats of dilution of asphaltene solutions. Figure 4 shows a typical heat trace for dilution in the ASIL/quinoline system; a trace for dissolution of ASIL in quinoline is included for comparison. The results for ASKF and ASMY were not shown here, because they showed a similar pattern in heat trace and also a similar magnitude in the heat of dilution in quinoline (in the range of 2-3 J/g). The heat trace was slightly exothermic as dilution began, but then became endothermic (Figure 4b). This suggests that at high concentrations asphaltene molecules might retain partly solidlike structures. Comparing Figure 4a,b, it can be seen that the heat trace for the dissolution system was much larger in magnitude than that for dilution: ∆H for dissolution was -20.2 J/g (exothermic), but only 2.3 J/g (endothermic) for dilution. The endothermic changes in dilution can be attributed to the dispersion or dissociation of aggregates in solution, which should dominate the contribution to ∆Hd. Thus, ASIL molecules are more associated in the initial (high) concentrations used. Our previous surface tension measurements confirmed that self-association of ASIL occurs at extremely low concentrations in organic solvents.22 A surface tension profile of ASIL in quinoline (22) Zhang, Y.; Takanohashi, T.; Sato, S.; Tanaka, R. Association Behavior and Structure of Vacuum Residue related Materials in Solution. In Proceedings of the AIChE’s 2002 Spring Meeting, New Orleans, 2002; Vol. 48a.
Asphaltenes in Organic Solvents
Energy & Fuels, Vol. 17, No. 1, 2003 105
Figure 2. Heats of solution of three asphaltenes vs calculated solubility parameters of the solvents.
Figure 3. Heats of solution of model compounds or polymers as a function of logarithm of the molecular weight.
Figure 4. Heat traces of ASIL during dissolution (a) and dilution (b) in quinoline.
is shown in Figure 5 as an example. A discontinuity point observed at ca. 0.006 g/L (log C ) -2.22) on the surface tension-log C curve suggested that aggregation of some components in asphaltenes might start from the
concentration lower than 0.01 g/L. Because of this, it is thought that ASIL is dissolved in the form of micelles or aggregates rather than as free molecules at the concentration used for dilution (60 g/L).
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state were exothermic in all solvents, which was judged to be caused primarily by interactions between asphaltene and solvent molecules. The heat of solution of asphaltenes correlated fairly with solvent solubility parameter. Dilution of a concentrated solution of asphaltenes in quinoline was endothermic, which can be attributed to the dispersion or dissociation of structures that are associated in concentrated solution. However, dissociation of micelles or aggregates during dilution had only a small thermal effect.
Figure 5. Surface tension vs logarithm of concentration for ASIL in quinoline at 25 °C.
Conclusions Dissolution of the three asphaltenes in organic solvents and dilution of their concentrated quinoline solutions have been investigated by measuring heat changes during each process. Heats of solution for dissolution of the three asphaltenes from the solid to the solution
Acknowledgment. Financial support for the work was supplied by the Proposal-Based International Joint Research Program, New Energy and Industrial Technology Development Organization. The authors thank Dr. R. E. Winans and Dr. J. E. Hunt of Argonne National Laboratory, Professor M. R. Gray of University of Alberta, and Dr. X. Ma of The Pennsylvania State University, for all useful discussions. EF0201073
