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Molecular Size and Size Distribution of Petroleum Residue Zhentao Chen,* Suoqi Zhao, Zhiming Xu, Jinsen Gao, and Chunming Xu* State Key Laboratory of Heavy Oil Processing and Faculty of Chemical Science and Engineering, China University of Petroleum, Changping, Beijing 102249, People’s Republic of China ABSTRACT: The size of residue molecules is crucial to catalyst design and petroleum use. An Athabasca tar sand vacuum residue was fractionated into 13 narrow fractions and an end-cut by supercritical fluid extraction and fraction (SFEF). Average molecule diameters and size distributions of the residue and its five SFEF cuts were determined from bulk-phase diffusion coefficients, which were measured at 308 K by a diaphragm cell. All five cuts show obvious polydispersity in size, with the end-cut possessing the broadest size distribution. A strong tendency of asphaltenes to aggregate suggests that the large size of the end-cut results from the aggregation of asphaltene molecules. The average hydrodynamic diameter of the end-cut was estimated to be 4.7 nm, as opposed to a range of 1.11.7 nm for the four narrow fractions. The average diameters of all five cuts can be correlated with their average molecular weight. In comparison to the size range of 1.14.7 nm for the narrow cuts, the feedstocks of the whole residue have a smaller size distribution of 1.43.9 nm. The attraction of heavier molecules at the beginning of the diffusion run and the disequilibrium at the end result in the smaller size distribution for the whole residue.
’ INTRODUCTION Stimulated by the need for processing heavy petroleum feedstocks and residue, there has been considerable interest in the structure of reacting species. These materials are characteristically more difficult to process than petroleum distillates because of larger molecule size and higher amounts of heteroatoms. Although the chemical structures of residue and asphaltenes have been extensively studied, there is still some debate about the size and structure of these materials because of their complicated compositions. Two main methods have been used extensively to investigate the size and structure of residue and asphaltene molecules. First, the molecular structure and size of heavy oil have been determined through the measurement of hydrodynamic properties, such as solution viscosity and diffusion coefficient. This methodology was developed from the study of biological molecules and polymers. Sakai et al.,1 Wargadalam et al.,2,3 and Nortz et al.4 obtained molecule size from the bulk-phase diffusivity and intrinsic viscosity of fractioned petroleum pitches, coal extracts, and residue fractions, respectively. They also deduced the molecule structure from their experimental data. Takeshige5 calculated three different semi-axes of asphaltenes from their viscosities. The results of these studies conclude that the diameters of the heavy petroleum molecules are in the range of 1.22.5 nm. Second, a great variety of analytical techniques have been employed to investigate the molecular size of heavy oil. Asphaltene molecular diameters were obtained in the range of 1.22.4 nm by fluorescence depolarization (FD)6,7 and in the range of 2.02.5 nm by fluorescence correlation spectroscopy (FCS)8 techniques. The average diameter of asphaltenes was estimated at about 3 and 2.5 nm from 1H diffusion-ordered spectroscopy (DOSY) nuclear magnetic resonance (NMR)9 and transmission electron microscopy (FFTEM)10 techniques, respectively. Furthermore, asphaltenes have a strong tendency to aggregate. Small-angle X-ray scattering (SAXS) and small-angle r 2011 American Chemical Society
neutron scattering (SANS) have been employed to investigate asphaltene aggregation. Most of the studies show characteristic sizes of asphaltene aggregates range from 3 to 10 nm.11,12 In the past few decades, the size of asphaltenes and their fractions were often described in terms of a mean value deriving from the methods described above. As is well-known, the residue and its fractions are highly complicated systems, which contain thousands of different molecules. The molecules behave differently during processes of reaction and transfer. Thus, it is imperative to obtain the size distribution of the entire residue. As mentioned above, the bulk-phase diffusivity of heavy petroleum molecules has been used to determine the size and structure. In this study, we show that the bulk-phase diffusivity distribution can be obtained by using a diaphragm diffusion cell and the size distribution of the residue and its fraction could be calculated from the measurement of the bulk-phase diffusivity. To our knowledge, there have been few diffusion studies into the size distribution of the residue or its fractions. As is well-known, the resistance to Brownian motion of a solute can be equated to the hydrodynamic drag on a particle of equivalent size in free solution. The Brownian motion of the solute in a dilute solution solely because of the thermal fluctuations of the movements around it can be described by the wellknown StokesEinstein equation Db ¼
kT 3πηd
ð1Þ
where Db is the bulk-phase diffusion coefficient, κ is Boltzmann’s constant, T is the absolute temperature, η is the solvent viscosity, and d is the diameter for the spherical solute or the equivalent hydrodynamic diameter for the non-spherical solute. According to eq 1, an average hydrodynamic diameter of a solute can be Received: January 22, 2011 Revised: March 18, 2011 Published: March 22, 2011 2109
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Energy & Fuels determined using the bulk-phase diffusion coefficient at infinite dilution. Note that asphaltenes are known to readily aggregate in an organic solvent. The values of critical micelle concentrations (cmc’s) for asphaltenes in various organic solvents have been presented in the range of 516 g/L.13 Oh et al.14 observed that the critical aggregation concentrations (cac’s) lie in the range of 38.2 g/L. However, recent studies have suggested that asphaltenes tend to aggregate in lower concentrations. Evdokimov et al.15 observed an onset of aggregation at an asphaltene concentration of about 10 mg/L. Andreatta et al.16,17 have suggested that large cmc’s of asphaltenes correspond to a higher level of aggregation. They observed critical nanoaggregate concentrations (cnac’s) at ∼100 mg/L. In the present study, the bulk-phase diffusion coefficients were determined using a diaphragm diffusion cell for a series of supercritical fluid extraction and fraction (SFEF) cuts of Athabasca tar sand vacuum residue (AVR). The average diameters and size distributions of the five SFEF cuts were obtained from their diffusivities. The size of the AVR determined from the five cuts was also compared to that of the entire residue. The results show that the four SFEF fractions vary in size, but the variations among the fractions are quite small. The end-cut and the entire AVR are more polydisperse in size. These experiments provide a size range of narrow fractions that is in close agreement with previous studies. The size of the end-cut is shown to be significantly larger than the monomer size of asphaltenes in the literature.18,19
’ EXPERIMENTAL SECTION Feedstock Preparation. The AVR was obtained from the Syncrude oilsands plant in Fort McMurray, Alberta, Canada. The residue was separated into 13 narrow fractions and an end-cut by SFEF. The separation details and operating procedure have been reported elsewhere.20,21 A major advantage of SFEF technology is that it can be used to prepare sufficient quantities of narrow fractions of residue for further in-depth studies. It is known that the properties of SFEF fractions vary gradually with an increased SFEF yield. Four narrow fractions (SFEF-3, SFEF-6, SFEF-9, and SFEF-12), the end-cut, and the AVR were chosen to be the feedstocks of the diffusion experiments. The AVR and the corresponding five SFEF cuts (four fractions and the end-cut) were subjected to various analyses. The average molecular weight was determined in toluene at 45 °C using a Knauer vapor pressure osmometer (Knauer Instruments, Germany). Saturates, aromatics, resins, and asphaltenes (SARA) analyses were performed according to the procedures described by Liang.22 The diffusion experiments were performed under ambient pressure. Toluene was used as the solvent for the diffusion experiments. The endcut solution was filtered through a 2 μm pore size filter before preparation. A 10 g/L solution of each feedstock in toluene was agitated at room temperature for 24 h. Diaphragm Diffusion Cell. The diaphragm diffusion cell, a key instrument to our study, is shown schematically in Figure 1. The diaphragm diffusion cell contains two glass chambers, which are clamped together with a membrane between them. Teflon-coated magnetic stirring bars are mounted in chambers and driven by the rotation of an external magnet. To eliminate boundary layer resistance, a blade fixed near each of the stirring bars was placed about 2 mm away from the membrane surface. The speed of rotation can be varied from 50 to 500 revolutions per minute (rpm) and controlled at any fixed value. To keep the temperature constant, the cell was placed in a thermostatic bath during each experiment.
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Figure 1. Schematic diagram of the diaphragm diffusion cell. The lower and upper chambers hold a volume of 48.1 and 55.3 mL, respectively, which were measured by weighing the cell before and after filling with deionized water. A track-etched Nuclepore polycarbonate membrane with a nominal pore diameter of 1000 nm (Whatman Plc) was used in the diffusion experiments. This type of membrane has been extensively used in diffusional transport because of its ideal pore geometry.2325 Diffusion Coefficient Measurements. The diffusion coefficient measurements were performed using a diaphragm diffusion cell (Figure 1). The procedure used for the diffusion experiments was similar in each case. Initially, the lower chamber was filled with solution, and the upper chamber was filled with pure solvent. Such a design can reduce convective transport.26 For diffusion experiments of AVR and its five cuts, discrete samples (about 2 mL) were withdrawn from the upper chamber at appropriate time intervals. Then, an equal volume of pure toluene was added to the upper chamber to keep the volume constant. The diffusion experiment ended when equilibrium was approached. The concentrations of the samples and the final solutions in the upper and lower chambers were determined by evaporating the solvent and weighing the solids. The concentration in the lower chamber was determined from mass balance. In the pseudo-steady state, the flux across the membrane in time dt equals the change in the amount of solute in a chamber; thus dcL S ¼ D ðcL cU Þ dt l
ð2Þ
dcU S ¼ D ðcL cU Þ dt l
ð3Þ
VL
VU
where cL and cU refer to solute concentrations in the lower and upper chambers of the diaphragm cell, VL and VU are the volumes of the lower and upper chambers, S is the effective diffusion area of the membrane pores, and l is the effective diffusion path length. There are two main assumptions behind these equations. First, diffusion should approach steady state when one measures the diffusion coefficient. This can be achieved after preliminary diffusion in our experiments according Gordon’s correlation.27 Second, the diffusion coefficient of the solute does not vary much with the concentration of the residue cuts. In a recent study, Durand et al.9 reported that the diffusivities of petroleum fractions varied slightly in the 010 wt % range. Also, our results 2110
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Table 1. Properties of the AVR and Its Five SFEF Cuts properties
AVR
accumulative yield (% mass)
SFEF-3
SFEF-6
SFEF-9
SFEF-12
end-cut
17.3
34.0
50.3
66.7
100
molecular weight
1147
643
748
903
1599
8061
saturates (% mass)
7.80
14.78
5.07
0
0
0
aromatics (% mass)
41.52
68.03
63.32
61.43
48.15
5.80
resins (% mass)
32.60
18.16
31.41
38.13
58.72
18.84
asphaltenes (% mass)
18.09
0
0
0
0
75.86
indicated that the diffusivities of the SFEF cuts are similar at concentrations ranging from 10 to 50 g/L. When eqs 2 and 3 are combined, one obtains d ðcL cU Þ ¼ βDðcL cU Þ dt
ð4Þ
where β is the constant of the diaphragm cell and is given by eq 5. S 1 1 þ ð5Þ β¼ l VL VU Integrating eq 4 between the jth and (j þ 1)th samplings, we have cL, j þ 1 cU, j þ 1 ln ¼ βDj Δtj, j þ 1 ð j ¼ 0, 1, 2, :::Þ ð6Þ cL, j cU, j In this equation, the subscripts j and j þ 1 refer to the sampling numbers, t is the time of sampling, and Dj is the average diffusion coefficient of the solute transporting through the pores between the jth and ( j þ 1)th samplings. The mass balances were checked for the diffusion experiments. The results show that the mass of the solute in the lower chamber at the beginning of the experiment equaled the mass of the solute in both chambers at the end of the experiment. The differences in the mass measurements are less than 0.6%. Calibration of the Diaphragm Cell Constant. To apply the diaphragm cell method for measuring diffusion, the stirring speeds and the diaphragm cell constant need to be determined. A stirring rate of 105 rpm was selected, and an aqueous solution of potassium chloride at 25 °C was used to determine β in this study. The method is described elsewhere,2830 and the diaphragm cell constant is provided in our previous study.31
’ RESULTS AND DISCUSSION Properties of the Residue and Its Five SFEF Cuts. The properties of the residue and the five chosen SFEF cuts are summarized in Table 1. There are large variations in the properties of the four SFEF fractions and the end-cut. The molecular weight increases as the fractions become heavier. The average molecular weight of the end-cut is much larger than that of the four SFEF fractions. Asphaltenes, which are the heaviest component in the residue, are entirely concentrated in the end-cut. Bulk-Phase Diffusion Coefficients of the Five SFEF Cuts. The bulk-phase diffusion coefficient of a solute is required to determine its hydrodynamic size when the StokesEinstein equation is used. As mentioned above, the diameters of heavy petroleum molecules are in the range of 1.22.5 nm. The pore size of 1000 nm is large enough for residue molecules to diffuse freely through the membrane. The results of our previous study showed that the diffusion coefficients of residue molecules through 1000 nm pore size membranes approached the bulk diffusion coefficients.31 The bulk-phase diffusion coefficients of the four SFEF fractions and the end-cut at 308 K are plotted against the experimental time in Figure 2.
Figure 2. Diffusion coefficients of SFEF cuts of AVR in 1000 nm pore size membranes. The curves represent the power-law fits.
As can be seen from Figure 2, the bulk-phase diffusion coefficients of the five cuts are all time-dependent. The diffusion coefficients of each cut decrease gradually as the experiment proceeds. This illustrates that these cuts are polydisperse mixtures and contain a large number of substances with different diameters. The smaller species diffuse more readily through the membrane pores, resulting in the decrease of the diffusion coefficients as the experiment progresses. A comparison of the curves shows that the diffusion coefficients of the five SFEF cuts through the same membrane decrease as they become heavier. The decreasing trend of diffusion coefficients between two adjacent cuts is greater as the cuts become heavier. This is in agreement with the variation trend of the molecular weight of the five cuts, which is shown in Table 1. Figure 2 also shows that the experimental time approaching the diffusion equilibrium increases as cuts become heavier. Just as deduced from eq 1, the Brownian motion of a smaller solute is more active. Thus, the time needed to attain the diffusion equilibrium is longer for heavier cuts. In recent years, there have been several studies on the measurement of asphaltene diffusivity in toluene. Durand et al.32 found the infinite diffusion coefficient for Buzurgan asphaltenes to be about 2.4 106 cm2 s1 by means of 1H DOSY NMR. Lisitza et al.33 observed a continuous decrease of the diffusion coefficient from 2.9 106 cm2 s1 below 0.3 g/L to 1 106 cm2 s1 at 2.1 g/L for UG8 asphaltenes. As stated by Lisitza et al., the change in the diffusion coefficient is due to the change in the size between monomers and aggregates. Fluorescence correlation spectroscopy showed that the translational diffusion coefficients of several petroleum asphaltenes were about 3.5 106 cm2 s1 at room temperature.8,34 The diffusion coefficients of the four SFEF fractions are larger than the diffusion coefficients of asphaltenes in the literature mentioned above. This seems reasonable because of the distinct 2111
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compositions and properties between maltene and asphaltenes. Different diffusion coefficients among the four fractions and asphaltenes for heavy crude oil have previously been determined by DOSY NMR.9 The diffusion coefficient of the residue fraction is more than 3 times as large as that of asphaltenes in their study. Furthermore, asphaltenes have a strong tendency to aggregate, and the end-cut is rich in asphaltene components. Numerous techniques have revealed that asphaltenes can aggregate at concentrations below 1 g/L. Thus, the low diffusion coefficient of the end-cut might result from aggregation of asphaltene molecules that it contains. Size Distributions of the Five SFEF Cuts. As mentioned above, the hydrodynamic diameter of a solute can be determined from its diffusion coefficient. The distributions of the bulk-phase diffusion coefficients presented in Figure 2 are used to calculate size distributions of the five cuts via the relation in eq 1. The results are listed in Table 2, in which di,j is the sphere equivalent hydrodynamic mean diameter of the ith fraction in the jth interval of sampling. There are different size distributions between the four SFEF fractions and the end-cut. The variation range of size for each SFEF fraction is lower than 0.15 nm. However, the variation range of size for the end-cut is 3.75 nm, indicating that the endcut is more polydisperse in size than the four SFEF fractions. There are also two fractions between every two adjacent fractions chosen as feedstocks in this study. Then, it can be deduced from Table 2 that the size will overlap between the adjacent cuts of residue. Zhao et al.20 have demonstrated that the separation of the residue by SFEF is based on molecular weight and polarity. Therefore, there is inevitable overlapping in the size between the adjacent cuts because of the complicated components. It can also be seen that the size of the four fractions range from 1.01 to 1.75 nm. As mentioned above, the size of petroleum asphaltenes found in most previous studies range between 1.2 Table 2. Size Distribution of the Five SFEF Cuts as a Function of the Time Interval cut
and 2.5 nm. It has been shown that the residue has a continuum of chemical constituents and resins have a long wavelength tail extending to the spectral range where asphaltenes are most prominent.18,35 Furthermore, the resin has shown no aggregation in similar conditions.36,37 On the basis of this, the size of the four fractions seems to be reasonable because they are slightly lighter than asphaltenes. Otherwise, the size distribution of the endcut, which is from 3.26 to 7.01 nm, mostly ranges in the size domain of asphaltene aggregates.36,38 Kawashima et al.39 found that the diameters of asphaltenes and its aggregates were about 1.5 and 5.0 nm at 0.1 and 10 g/L, respectively. The concentration of asphaltenes for the end-cut solution is slightly lower than 10 g/L according to the SARA data in Table 1. Thus, it indicates that the aggregation of asphaltenes might contribute to the large measured size of the end-cut. To visualize the results, size distributions of the five cuts varied with the accumulative yield were plotted in Figure 3. Here, the accumulative yield for each cut is based on the SFEF result (Table 1) and the diffusion experiment (Table 2). There is a continuous increase in the molecular size of residue cuts with respect to the accumulative yield. The increase is gradual, except for the largest size value in the end-cut. More recent data indicate that there are two states in the aggregation process of asphaltenes: nanoaggregate of asphaltenes can occur at lower concentrations (below 150 mg/L), and asphaltene nanoaggragates can form clusters when concentrated at several grams per liter.32,37,40 As seen in recent reviews by Mullins,41 the former aggregate process was termed the “primary aggregation” and the latter aggregate process was termed the “secondary aggregation”. In combination with these results, the abrupt increase of the size for the end-cut might result from the contribution of the secondary aggregation. Average Diffusivity and Average Diameter of the Five SFEF Cuts. To compare the diffusivities and sizes of different cuts, the average diffusion coefficient of each SFEF cut was defined as follows: n
di,1 (nm) di,2 (nm) di,3 (nm) di,4 (nm) di,5 (nm) di (nm)
SFEF-3
1.01
1.05
1.09
1.09
1.11
1.08
SFEF-6
1.13
1.17
1.20
1.21
1.21
1.19
SFEF-9
1.26
1.28
1.32
1.36
1.41
1.36
SFEF-12
1.62
1.70
1.77
1.79
1.80
1.75
end-cut
3.26
3.40
3.89
4.04
7.01
4.68
Di ¼
∑ ðDi, jmi, j Þ j¼1 n
∑
j¼1
ði ¼ 3, 6, :::, end-cut;
mi, j
j ¼ 1, 2, :::, nÞ
ð7Þ
Figure 3. Size distribution of SFEF cuts of AVR as a function of the accumulative yield. The dashed line represents the power-law fit of the four fractions. 2112
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Figure 4. Relations between average diffusion coefficients, average size, and molecular weight of the SFEF cuts. The curves represent the exponential fits.
Table 3. Average Diameters of the Five Cuts from the Experiment and Predictions fraction
molecular weight
d (nm)a
d (nm)b
di (nm)
SFEF-3
643
1.58
1.31
1.08
SFEF-6
708
1.69
1.41
1.19
SFEF-9
908
1.84
1.54
1.36
SFEF-12 end-cut
1599 8061
2.38 4.92
2.02 4.40
1.75 4.68
a
Relationship from ref 42; d = 0.86M0.45 Å. b Relationship from ref 4; d = 0.586M0.48 Å.
where Di is the average diffusion coefficient of the ith cut and mi,j is the amount of the ith cut through the membrane in the jth interval of sampling. Then, the average sphere equivalent hydrodynamic diameter can be calculated from the average diffusion coefficient by the StokesEinstein equation. The average hydrodynamic diameters of the five SFEF cuts are listed in the last column of Table 2. The average diameters increase as the cuts become heavier. The size variations between two adjacent fractions are 0.11, 0.17, and 0.29 nm, respectively. The size variation between SFEF-12 and the end-cut is 2.93 nm, which is an order of magnitude larger than that between adjacent SFEF fractions. The trend in size variation is identical to that in molecular-weight variation, as shown in Table 1. The molecular weight is one of the most important properties of the residue and asphaltenes. As seen from Tables 1 and 2, the increasing trend of the molecular weight and size intensifies as cuts become heavier. The average diffusion coefficients and the average sphere equivalent hydrodynamic diameters versus the molecular weight of the five cuts are presented in Figure 4. The results here clearly reveal that both the diffusion coefficient and the size show a consistent dependence upon the molecular weight. Diffusion measurements of asphaltene molecules have been considered to play a decisive role in resolving the molecular-weight controversy.32,41 Furthermore, the size of asphaltenes have been observed to follow a power law with the molecular weight.4,42 Regression analyses of the results between average diameters and molecular weights of this study yield the following relationship: d ¼ 0:025M 0:58
ð8Þ
Figure 5. Diffusion coefficients and diameters of the residue as a function of the experimental time.
Equation 8 fits the experiment data well, with a R2 value more than 0.99. As mentioned above, the measured average diameter of the end-cut might stand for that of aggregates. The results of vapor pressure osmometry (VPO) reveal that the aggregated state of the asphaltenes lies in the 16 g/L concentration range.43,44 Asphaltenes in the diffusion experiment stay at the similar aggregated state as in the VPO measurement because of similar underlying conditions. Thus, the end-cut does not show much deviation from the relationship of residue fractions between the size and molecular weight. Furthermore, the 0.58 power dependence between d and M is slightly larger than the values reported in the literature.4,42,45 Here, we use molecular weight (Table 1) and relations presented in the literature4,42 to predict diameters of the five SFEF cuts. The results of our experiment and the predictions are listed in Table 3. A comparison of these results shows that the average diameters from the diffusion experiments are lower than values from predictions for the four fractions but that the average diameter of the end-cut is close to its predicted values. The cited two correlations were both obtained from asphaltenes. As listed in Table 1, the end-cut is rich in the asphaltene component. Asphaltenes have a more complicated structure and larger polarity than the other components. Thus, the different configurations and degrees of solvation between the four fractions and asphaltenes may explain the results. Diffusivity and Size of the Residue. Figure 5 depicts the diffusion coefficient variation of the entire AVR with the experiment proceeding. For convenience of comparison, the range of the vertical axis is set to be the same as that of Figure 4. Figure 5 shows that the variation trend of the diffusivities of the AVR with experimental time is similar to that of the five SFEF cuts presented in Figure 2. The range of the diffusion coefficients is broader than any cut, which means that it is more polydisperse in size. The size of the AVR obtained from diffusivity ranges between 1.42 and 3.93 nm. It can be seen that the size range of the AVR is narrower than that of the five SFEF cuts summed together by comparing Figures 4 and 5. The diameter of the AVR determined from the first interval, 1.42 nm, is larger than the average diameter of the SFEF-3. The reduced diffusion caused by the attraction of heavier components in the residue might result in the larger size. The diameter measured from the final interval, 3.93 nm, is smaller than the average diameter of the end-cut. It is 2113
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’ CONCLUSION There has been extensive research devoted to determining the size of asphaltenes. In view of the importance of efficient use of the residue, it seems necessary to obtain the size information of the entire residue as asphaltenes. The results presented suggest that the SFEF cuts and the AVR are all polydisperse in size. The size distributions of the five SFEF cuts and the residue can be determined from their diffusion coefficients. The size variation of the end-cut is an order of magnitude broader than the four fractions, which means the end-cut has a higher degree of polydispersity than the fractions. The average diameters of the five SFEF cuts increase as the cuts become heavier. A similar aggregation state of asphaltene molecules can occur between diffusion and VPO experiments. A power-law relationship between the average diameters and mean molecular weight of the five SFEF cuts was demonstrated. The size distribution of the whole residue is narrower than the entire size range of the five SFEF cuts. ’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ86-10-8973-3392. Fax: þ86-10-6972-4721. E-mail:
[email protected] (Z.C.);
[email protected] (C.X.).
’ ACKNOWLEDGMENT This research was supported by the National Key Basic Research Development Program of China (973 Program) (2010CB226901 and 2010CB226902), the Program of Introducing Talents of Discipline to Universities (B07010), and the National Natural Science Foundation of China through the program for Distinguished Young Scholar (Grant 20725620). The authors thank Dr. Quan Shi at the China University of Petroleum for his advice. ’ REFERENCES (1) Sakai, M.; Yoshihara, M.; Inagaki, M. Carbon 1981, 19, 83–87. (2) Wargadalam, V. J.; Norinaga, K.; Iino, M. Energy Fuels 2001, 15, 1123–1128. (3) Wargadalam, V. J.; Norinaga, K.; Iino, M. Fuel 2002, 81, 1403–1407. (4) Nortz, R. L.; Baltus, R. E.; Rahimi, P. Ind. Eng. Chem. Res. 1990, 29, 1968–1976. (5) Takeshige, W. J. Colloid Interface Sci. 2001, 234, 261–268. (6) Groenzin, H.; Mullins, O. C. J. Phys. Chem. A 1999, 103, 11237–11245. (7) Badre, S.; Carla Goncalves, C.; Norinaga, K.; Gustavson, G.; Mullins, O. C. Fuel 2006, 85, 1–11. (8) Andrews, A. B.; Guerra, R. E.; Mullins, O. C.; Sen, P. N. J. Phys. Chem. A 2006, 110, 8093–8097. (9) Durand, E.; Clemancey, M.; Quoineaud, A.-A.; Verstraete, J.; Espinat, D.; Lancelin, J.-M. Energy Fuels 2008, 22, 2604–2610. (10) Acevedo, S.; Zuloaga, C.; Rodríguez, P. Energy Fuels 2008, 22, 2332–2340. (11) Xu, Y.; Koga, Y.; Strausz, O. P. Fuel 1995, 74, 960–964. (12) Gawrys, K. L.; Kilpatrick, P. K. J. Colloid Interface Sci. 2005, 288, 325–334. (13) Mohamed, R. S.; Ramos, A. C. S.; Loh, W. Energy Fuels 1999, 13, 323–327.
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