2008
Energy & Fuels 2006, 20, 2008-2012
Colloidal Stability Variation of Petroleum Residue during Thermal Reaction Longli Zhang,* Guohua Yang, Guohe Que, Qingxuan Zhang, and Pujiang Yang State Key Laboratory of HeaVy Oil Processing, College of Chemistry and Chemical Engineering, China UniVersity of Petroleum (East China), Dongying, Shandong ProVince 257061, P.R.China ReceiVed NoVember 15, 2005. ReVised Manuscript ReceiVed April 6, 2006
The linkages between coking characteristics, colloidal stability, and molecular characteristics of petroleum residue during thermal reaction were studied. First, the results reveal that the colloidal stability decreases sharply during the coke-induction period and changes little after that. This observation proves that the colloidal stability variation determines the coking characteristics of residue. Second, the asphaltene concentration increases as reaction time progresses, reaches its maximum at the end of the coke-induction period, and then declines thereafter. This result reveals that asphaltene aggregation happens when the colloidal stability of the residue decreases to its limit and that the aggregated asphaltenes will transform into coke to abate the worsening of the colloidal stability. Furthermore, the variation in fraction composition shows that the saturated solubility of asphaltenes in residue decreases as the reaction goes on after the coke-induction period. Moreover, both the VPO molecular-weight value and the mean dipole-moment value of asphaltenes show maxima at the end of the coke-induction period, which reveals that asphaltenes with larger MW values and more polarity prefer to aggregate and transform into coke.
1. Introduction Petroleum residues are found to be colloidal systems, and asphaltenes are considered to be the cores of them.1-6 The colloidal stability of petroleum residue plays an important role in its coking characteristics during thermal reaction. Many researchers pointed out that a decline in the colloidal stability of the residue during thermal reaction would cause asphaltene to aggregate and ultimately transform into coke.7,8 Wiehe7 made a detailed analysis about the asphaltene aggregation in residue. Moreover, Rahimi at al.8 had observed an asphaltene aggregating phase in petroleum residue during thermal reaction just before coking, which was a direct proof of this mechanism. Because the colloidal stability of the residue is difficult to describe quantitatively, there was no systematic report about the colloidal stability variation in petroleum residue during thermal reaction. There are few reports about the relationship between the colloidal stability variation and coking characteristics. Particularly, a report about the linkage between molecular characteristics and the colloidal stability of the residue is absent. This paper will focus on the two relationships. P. Fotland9,10 investigated asphaltene precipitation in crude oil by the mass fraction normalized conductivity method and * To whom correspondence should be addressed. E-mail: lizhang@ hdpu.edu.cn. (1) Dickie, J. P.; Yen, T. F. Anal. Chem. 1967, 39 (14), 1847-1852. (2) Yen, T. F. Fuel Sci. Technol. Int. 1992, 10 (4-6), 723-733. (3) Sheu, E. Y. Energy Fuels 2002, 16 (1), 74-82. (4) Storm, D. A.; Sheu, E. Y.; DeTar, M. M. Fuel 1993, 72 (7), 977981. (5) Storm, D. A.; Sheu, E. Y. Fuel 1995, 74 (8), 1140-1145. (6) Li, S.; Liu, C.; Que, G.; et al. Fuel 1996, 75 (8), 1025-1029. (7) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32 (11), 2447-2454. (8) Rahimi, P. M.; Gentzis, T.; Chung, K.; et al. Prepr. Am. Chem. Soc., DiV. Fuel Chem. 1997, 42(1), 146-152.. (9) Fotland, P.; Anfindsen, H.; Fadnes, F. H. Fluid Phase Equilib. 1993, 82, 157-164. (10) Fotland, P.; Anfindsen, H. Fuel Sci. Technol. Int. 1996, 14 (1-2), 101-115.
validated that the maximum of mass fraction normalized conductivity when adding normal alkanes to crude oil is the starting point of asphaltene precipitation. If experimental conditions are fixed, the amount of normal alkane needed to cause asphaltenes to aggregate is larger when the residue sample is more stable. So the ratio of added normal alkane to residue sample can be taken as the colloidal stability parameter of the residue sample. This method was used to study the variation in the colloidal stability of the residue during thermal reaction, and the relationship between colloidal stability and coking characteristics was studied as well. The polarity of asphaltene molecules plays an important role in the colloidal characteristics of the residue. Although petroleum residue contains polar molecules, which is accepted by most researchers, there have been few reports about the dipole moment values of petroleum residue molecules.11-14 In this study, residue samples are fractioned by liquid chromatography, the mean dipole moment values of asphaltene were measured, and the linkage between molecular characteristics and the colloidal stability of the residue is discussed. The relationship between colloidal stability and coking characteristics of petroleum residue during thermal reaction were studied. Furthermore, the fraction composition and the mean dipole moment of asphaltenes were measured to reveal their effect on the colloidal stability of the residue. 2. Experimental Section Five kinds of atmosphere residues were studied, the boiling points of which were more than 350 °C. The samples were (11) Carnahan, N. F.; Louis, J.; Anto´n, R.; et al. Energy Fuels 1999, 13, 3(2), 309-314. (12) Murgich, J.; Abanero, J. A.; Strausz, O. P. Energy Fuels 1999, 13, 3 (1), 278-286. (13) Maruska, H. P.; Rao, B. M. L. Fuel Sci. Technol. Int. 1987, 5 (2), 119-168. (14) Kelloma¨ki, A. Fuel 1991, 70, 1103-1104.
10.1021/ef050378o CCC: $33.50 © 2006 American Chemical Society Published on Web 08/08/2006
Colloidal Stability Variation of Petroleum Residue
Energy & Fuels, Vol. 20, No. 5, 2006 2009
Table 1. Properties of DGAR, MEAR, and THAR properties (m%)
DGAR
KLAR
MEAR
LHAR
THAR
C H S N ash oxygen
86.44 12.47 0.25 0.62 0.014 0.21
86.40 12.46 0.13 0.41 0.085 0.52
84.48 11.32 3.8 0.2 0.018 0.18
87.0 11.4 0.37 0.92 0.049 0.26
84.56 10.63 3.25 0.55 0.050 0.96
saturate aromatics resin C7-asphaltene
52.6 25.0 21.8 0.6
29.2 28.7 40.3 1.8
28.1 37.6 17.4 16.9
SARA Composition 50.4 37.2 22.2 46.0 27.2 14.2 0.2 2.6
DaGang AR (DGAR), Kalamayi AR (KAR), Middle East AR (MEAR), LiaoHe AR (LHAR), and TaHe AR (THAR). Their properties are shown in Table 1. Thermal reactions were performed under nitrogen, and the initial pressure was 1.0 MPa at 25 °C. During thermal reaction, samples were collected at various times by immerging on-line sampling equipment in the liquid. Through this sample collection method, samples from the same autoclave reaction can be compared. Because the critical temperature of light components was lower than the reaction temperature, the components were in the gaseous state under the reaction conditions and the boiling points of the samples were higher than 250 °C. To study the colloidal stability variation in samples collected at different thermal reaction time, we studied the samples directly without distillation, which included leaving the coke in them. Separation Scheme. Every sample collected from the thermal reaction was stirred to ensure equality and then divided into three parts. The samples were separated in order to measure the coke characteristics and colloidal stability parameters. The coke content is defined as the mass percentage of tolueneinsoluble material. The asphaltene was isolated by the addition of a 30:1 excess of n-pentane to the residuum at room temperature. The suspension was heated to boil and filtered after cooling down. The precipitate was extracted by n-pentane until the filtrate was colorless. The n-pentane was removed from the maltene by distillation; the maltene was then fractioned into five fractions by liquid chromatography, which included saturates in addition to light aromatics, heavy aromatics, light resins, middle resins, and heavy resins.15 The colloidal stability parameters were characterized through the mass fraction normalized conductivity method.9,10,16 The stability of the residue samples decreased with the addition of n-heptane, and the colloidal stability of the residue then decreased and collapsed. So the ratio of added n-heptane to residue sample can be taken as the colloidal stability parameter of the residue sample. A Hewlett-Packard impedance analyzer (HP4194A impedance/gain/phase analyzer) was used to measure the conductivity of residue solutions. The employed frequency was 1 kHz. The mass fraction normalized conductivity can then be calculated from the dilution ratio and the conductivity of residue solution. The conductivity is measured at 35 ( 0.1 °C. The conductivity of the residuum solution is measured after the addition of approximately 3 mL of n-heptane and mixing for about 5 min. The mass fraction normalized conductivity of the residue can be calculated from the conductivity and dilution ratio of the residue solution by formulas 1 and 2.
Λ ) κ(1 + xF)
(1)
x ) VF/M
(2)
where Λ is the mass fraction normalized conductivity of residuum (S/m), κ the measured conductivity of residuum (S/ m), x the ratio of solvent to the weight of residuum (g/g), F the density of solvent (0.685 g/cm3 for n-heptane), V the volume of n-heptane added (cm3), and M the mass of the residue sample (g). Because the viscosity of the solution decreases as a result of the dilution, the mass fraction normalized conductivity of the residue sample does not decrease except when asphaltene aggregation happens. The maximum of the mass fraction normalized conductivity of the residue sample is a sign of asphaltene aggregation. So the ratio of added n-heptane to residue sample when asphaltene aggregation happens can be taken as the colloidal stability parameter of the residue sample, which is shown as formula 3.
P ) Mh/Mr
(3)
where P is the colloidal stability parameter of the residue sample (g/g), Mh the mass of added n-heptane to cause asphaltene aggregation (g), and Mr the mass of the residue sample (g). The molecular weights of separated fractions from residue or thermal samples were measured by a Knauer vapor pressure osmometer at 45 °C, using benzene as the solvent. In addition, the mean dipole moments of asphaltenes were calculated by the method developed by Guggenheim and Smith.17,18 They developed a method for gaining the apparent polarization and dipole moment of a polar solute in a nonpolar solvent by measuring the dielectric constant and refractive index of the solutions. The formulas are as follows
(
P2µ ) 3M2V1
)
V R - 2 2 (1 + 2) (n1 + 2)2
(4)
∂12 ∂w2
(5)
∂n122 V) ∂w2
(6)
R)
where w2 is the weight fraction of solute, 12 the dielectric permittivity of solution, n12 the refractive index of solution, M2 the molecular weight of the solute, and V1 the specific volume of the solvent. The fractions were diluted separately by benzene, which is a nonpolar solvent. The dielectric permittivities and refractive indexes of these solutions were measured. The dielectric permittivity of the solutions was measured by using a HP4194A analyzer. The refractive index of solutions was measured by using a refractive meter. The mean dipole moment of fractions could then be computed. The relationship between coking characteristics and colloidal stability variation of all five residua during thermal reaction were studied, and three residua were selected to study the linkage between molecular characteristics and the colloidal stability. 3. Results and Discussions 3.1. Relationship between Colloidal Stability Parameters (CSP) Variation and Coking Characteristics. First, the relationship between colloidal stability parameters (CSP) and (15) Liu, C. G.; et al. Acta Pet. Sin. 1987, 3(1), 90-98. (16) Zhang, L. L.; Yang, G. H.; Huanget, Q. S.; et al. Prepr. Am. Chem. Soc., DiV. Pet. Chem. 2002, 47(1), 26-28. (17) Guggenheim, A. Trans. Faraday Soc. 1949, 45, 714-720. (18) Smith, J. W. Trans. Faraday Soc. 1950, 46, 394-399.
2010 Energy & Fuels, Vol. 20, No. 5, 2006
Figure 1. Variation in CSP and coking characteristics of DGAR during thermal reaction at 405 °C.
Zhang et al.
Figure 5. Variation in CSP and coking characteristics of KLAR during thermal reaction at 400 °C.
Figure 2. Variation in CSP and coking characteristics of MEAR during thermal reaction at 400 °C. Figure 6. Composition variations in DGAR during thermal reaction.
Figure 3. Variation in CSP and coking characteristics of THAR during thermal reaction at 400 °C. Figure 7. Composition variations in MEAR during thermal reaction.
Figure 4. Variation in CSP and coking characteristics of LHAR during thermal reaction at 400 °C.
coking characteristics during thermal reaction of DGAR, KAR, MEAR, LHAR, and THAR were studied. It can be seen in Figures 1-5 that there exists a cokeinduction period for all residue samples studied. For example, they are 80 min for DGAR, 120 min for MEAR, and 80 min for THAR. During this period, no coke formation is observed. However, coke forms rapidly after that. On the contrary, for all residue samples studied, the colloidal stability of the residue samples decreases upon increasing thermal reaction time but shows little dependence on it after the coke-induction period is reached. This phenomenon reveals that the coking characteristics
of a residue are determined by its colloidal stability. As thermal reaction progresses, the colloidal stability of the residue declines to its utmost, and asphaltene solubility reaches the saturation state. It could be inferred that the asphaltene aggregation happens, and aggregated asphaltenes would ultimately transform into coke. 3.2. Fraction Composition Variation of Residue during Thermal Reaction. The colloidal stability variation of residue came from the shift in fraction composition and the molecular characteristics of every fraction. Wiehe7 analyzed the SARA (saturates, aromatics, resins, and asphaltenes) composition of fractions during thermal reaction and found that the fraction composition played an important role in the coking characteristics of a residue. In this study, the toluene-soluble parts of reacted residue samples were separated into n-pentane asphaltenes and n-pentane soluble maltenes. The composition variations with thermal reaction time are shown in Figures 6-8. It can be seen from Figures 6-8 that asphaltene concentration in residue increases quickly during the coke-induction period, reaches a maximum at the end of the coke-induction period, and then decreases slowly during coke production. This phenomenon proves that asphaltenes aggregate and transform into coke at the end of the coke-induction period.
Colloidal Stability Variation of Petroleum Residue
Energy & Fuels, Vol. 20, No. 5, 2006 2011 Table 5. Mean Dipole Moments of Residue Samples during Thermal Reaction reaction time (min) DGAR µ (Debye) MEAR µ (Debye) THAR µ (Debye)
Figure 8. Composition variations in THAR during thermal reaction. Table 2. Molecular Weights of Fractions of DGAR during Thermal Reaction reaction time (min)
F1
F2
F3
F4
F5
C5-asp
DGAR 40 80 120 160
660 470 440 360 350
1020 1030 750 670 600
1560 1110 920 870 930
1680 940 650 570 1310
1300 1100 780 680 610
4740 4800 5250 6580 5050
Table 3. Molecular Weights of Fractions of MEAR during Thermal Reaction reaction time (min)
F1
F2
F3
F4
F5
C5-asp
MEAR 40 80 100 160
290 290 270 270 250
510 470 320 320 340
930 580 540 450 490
610 570 580 480 420
600 570 530 540 510
4610 4670 4170 3540 2050
Table 4. Molecular Weights of Fractions of THAR during Thermal Reaction reaction time (min)
F1
F2
F3
F4
F5
C5-asp
THAR 40 80 120 160
400 450 380 350 520
510 330 400 380 360
960 590 610 470 660
570 460 380 380 580
1890 580 490 490 740
7700 16130 8930 6820 3520
The proportion of maltene decreases sharply during the cokeinduction period and decreases slowly after that. The enhancement of asphaltenes and abatement of the maltene portion deteriorate the colloidal stability of the residue samples. So the colloidal stability of the residue decreases sharply during the coke-induction period and changes little thereafter. It can be concluded that the variation in asphaltenes and the maltene proportion has an intrinsic relationship to the colloidal stability variation. During the coke-induction period, the increase in asphaltene content and decrease in maltene content caused the colloidal stability to deteriorate and resulted in the saturation of asphaltene. In other words, asphaltenes stay in the saturated state after the coke-induction period. The excess of asphaltene would aggregate and transform into coke. The decrease in asphaltene content can ameliorate the colloidal stability of the residue, so the colloidal stability parameter does not decrease after the coke-induction period. 3.3. Molecular-Weight Variation in Fractions during Thermal Reaction. During thermal reaction, the colloidal stability variations come from the composition of the fractions and their characteristics. The molecular-weight values are important parameters for the characteristics of residue fractions. The molecular-weight values of every fraction were measured by the VPO method. The MW data of fractions at different reaction times are shown in Tables 2-4. First, for almost all the samples, the MW values of the fractions of maltenes decrease as the reaction goes on. This phenomenon comes from the thermolysis of residue molecules
0
40
80
11.7 14.3 17.6
15.6 16.4 27.7
15.3 12.9 22.0
100
120
160
16.3
10.8 9.7 9.8
12.4 22.9
and the transformation of components. The downtrend of saturate will pick up with the colloidal stability deterioration as the thermal reaction progresses. Second, the MW values of the asphaltenes increase at first, reach a maximum at the end of the coke-induction period, and then decrease as the reaction advances. During the cokeinduction period, the condensation and aggregation reactions tend to increase the measured MW value of them. At the end of the coke-induction period, the large molecules prefer to aggregate and transform into coke, accompanied by a decrease in the asphaltene MW value. The result is consistent with the conclusion that asphaltenes having a larger conglomerating tendency prefer to transform into coke. Third, the variation in the MW values of the fractions is consistent with the colloidal stability change. The fraction of resin has an important protective ability to the asphaltene fraction, and the protection ability is more prominent when the difference between them is less. During the coke-induction period, the uptrend of asphaltene MW and the downtrend of resin MW would weaken the protection ability of resins, which is consistent with the rapid deterioration of colloidal stability. Furthermore, the downtrend of asphaltene MW and resin MW is consistent with the slow deterioration of colloidal stability after the coke-induction period. 3.4. Dipole Moment Variations of Fractions during Thermal Reaction. The polarity of asphaltenes is closely related to the colloidal stability of the residue samples. The mean dipole moments of the asphaltenes were measured. The dipole moments of asphaltenes are shown in Table 5. Table 5 shows the variation in the mean dipole moments of the asphaltenes during thermal reaction. For the three residues studied, the mean dipole moments of the asphaltenes kept increasing as the thermal reaction advanced; the dipole moments then reached their maximum and decreased thereafter. The time to gain the maximum is approximately equal to that of the cokeinduction period. This result reveals that the asphaltenes with more polarity tend to aggregate and transform into coke, which made the polarity of the asphaltenes kept in the residue decrease after the coke-induction period. As the polarity of the asphaltenes decreases, their solubility should increase if the peptization ability of maltene was unchanged. But the data in this research showed that the content of asphaltenes decreased after the coke-induction period, which illuminated that the peptization ability of maltene decreased or that the compatibility between asphaltenes and maltenes declined after the coke-induction period was reached. 4. Conclusions The relationship between the colloidal stability variation and the coking characteristics of petroleum during thermal reaction was studied. The linkage between asphaltene molecular characteristics and the colloidal stability of the residue was discussed. The conclusions are as follows. The colloidal stability of the residues decreases sharply during the coke-induction period and changes little thereafter. This
2012 Energy & Fuels, Vol. 20, No. 5, 2006
phenomenon reveals that the colloidal stability of the residue has an intrinsic relationship to its coking characteristics. Asphaltene concentration in the residue increased quickly during the coke-induction period and reached a maximum at the end of the coke-induction period. It can be concluded that the variation in the asphaltene and maltene proportion has an intrinsic relationship to the colloidal stability variation and that the deterioration of colloidal stability will cause asphaltenes to aggregate and transform into coke. The MW values of asphaltenes reached their maximum at the end of the coke-induction period, but MW values of resins decreased as the thermal reaction continued. This phenomenon revealed that the characteristics of residue influenced the colloidal stability of the residue very much. The mean dipole moments of asphaltenes reach their maximum at the end of the coke-induction period, which reveals that the asphaltenes with more polarity preferentially aggregate and transform into coke. Generally, the linkage between molecular characteristics, the colloidal stability variation, and the coking characteristics of petroleum during thermal reaction can be described as follows. As the thermal reaction occurs, the asphaltene concentration increases and the maltene concentration decreases, which causes the colloidal stability of the residue to deteriorate. At the same time, the polarity of the asphaltene molecules increases, which signifies a decrease in the colloidal stability of the residue. As the colloidal stability of the residue decreases, the asphaltene
Zhang et al.
concentration can be dispersed. When the colloidal stability of the residue decreases to a critical value, the asphaltene solubility limit will be exceeded. The excessive asphaltenes then aggregate to form a second liquid phase and transform into coke thereafter. This triggers the formation of coke at the end of the cokeinduction period and causes a decrease in the asphaltene concentration from its maximum value. Because the asphaltene molecules with more polarity prefer to aggregate and transform into coke, the dipole moment of asphaltenes in the residue decreases from its maximum value. Moreover, the colloidal stability of the residue changes little thereafter, because the concentration and polarity of the asphaltene molecules decrease after the coke-induction period. Acknowledgment. This work was partially supported by the National Science Fund committee of China (20506017), the open fund from the State Key Laboratory of Heavy Oil Processing of China (2004HD-04), and the doctor’s fund of China University of Petroleum (Y030427). The authors thank the Petrophysics Laboratory of Well Logging Company of ShengLi Petroleum Bureau for supplying the HP 4194A and senior engineer Bingkai Liu for enthusiastic help. The authors are grateful to Shengchuang Yang and Liping Huang, who did many measurements. The authors are grateful to Professor E. Y. Sheu of Vanton Research Laboratory for critical review of this paper. EF050378O
