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Energy & Fuels 2009, 23, 2086–2089
Dipole Moment Variation of a Petroleum Residue during Catalytic and Thermal Upgrading Longli Zhang,* Guohua Yang, Guohe Que, Chaohe Yang, and Honghong Shan State Key Laboratory of HeaVy Oil Processing, China UniVersity of Petroleum, Dongying, Shandong ProVince 257061, People’s Republic of China ReceiVed September 6, 2008. ReVised Manuscript ReceiVed January 17, 2009
Polarity and compositional characterization of fractions separated from catalytic upgrading and thermal reaction samples were performed to elucidate their similarities and differences. The colloidal stability of residues presented an inflection point at the end of the coke-induction period during both catalytic and thermal upgrading, which reveals that the colloidal stability of the residue has an intrinsic relationship to its coking characteristics. The parameters affecting the colloidal stability of samples were elucidated, including fraction composition, mean molecular weight, and mean dipole moment. In comparison to the thermal reaction, the asphaltenes content during catalytic upgrading increased more slowly during the coke-induction period. The asphaltenes that were extracted from catalytic reaction samples had both less molecular weight (MW) and less polarity than that of the thermal reaction. The results proved that the catalyst and hydrogen restrained asphaltene condensation. For the two kinds of reactions, the dipole moment of asphaltenes increased during the cokeinduction period and decreased with the reaction time going on after that, which reveals that asphaltenes having more polarity preferred to aggregate and transform into coke.
1. Introduction Asphaltenes, which are the heaviest and most polar fraction in a residue, are the cores of the colloidal systems of petroleum residues1-3 and play a key role in the processing of heavy oil. Asphaltenes would not transform into coke until they separated from solution during the reaction, and the study of petroleum thermal processing validated the occurrence of liquid-liquid phase separation.4 The residue contains the most polar molecules, and the polarity of them plays an important role on the characteristics of the residue. The fraction composition of the residue, which was based on the polarity difference, played an important role on the colloidal stability, the reaction characteristics, and the emulsification performance.5-7 From the solubility difference and chromatography separation, the former researchers found that the polarity of residue fractions increased one by one as the following sequence: saturates and light aromatics, heavy aromatics, light resins, middle resins, heavy resins, and asphaltenes.8 Although the polarity of the petroleum residue contains polar molecules, which was accepted by most researchers, there were few reports about the dipole moment values of petroleum residue molecules.9-11 * To whom correspondence should be addressed. Fax: 86-546-8391971. E-mail:
[email protected]. (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.; De Tar, M. M. Fuel 1993, 72 (7), 977– 981. (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) Liu, C. G.; et al. Acta Pet. Sin. 1987, 3 (1), 90–98. (9) Paul Maruska, H.; Rao, B. M. L. Fuel Sci. Int. 1987, 5 (2), 119– 168. (10) Kelloma¨ki, A. Fuel 1991, 70, 1103–1104.
In this study, polar and compositional characterization of fractions from catalytic upgrading and thermal reaction samples were performed to elucidate their similarities and differences. The residue samples are fractioned by liquid chromatography, and the mean dipole moment values of fractions were measured. The relationship between colloidal stability and polarity of fractions of the petroleum residue during the reaction was studied. 2. Experimental Section The reaction characteristics of a Middle East atmosphere residue (ME-AR) were studied. Its properties were shown in Table 1. Thermal reactions were performed under nitrogen, and the initial pressure was 1.0 MPa at ambient temperature. During the thermal reaction, samples were collected at various times by online sampling equipment immerging in the liquid. The samples were studied directly without distillation as described previously.12 Catalytic reactions were performed under hydrogen, and the initial pressure was 3.0 MPa at ambient temperature. The inorganic nickel compound was dispersed in the residue as a catalyst, and the nickel concentration was 800 ppm. The reaction was terminated when the reaction time was reached. Then, the liquid samples were studied further. Separation Scheme. Every sample was stirred to ensure equality and then divided into three parts. The samples were separated to measure the coke characteristics and colloidal stability parameters and to prepare fractions separately. The coke content is defined as the mass percentage of toluene-insoluble material. The 0 min sample implied that the sample gained when the reaction temperature was reached. Because this sample had been suffering from the temperature increasing process for about 1.5 h, it was different from the ME-AR. (11) Wattana, P.; Fogler, H. S.; Yen, A.; Del Carmen Garcı`a, M.; Carbognani, L. Energy Fuels 2005, 19 (1), 101–110. (12) Zhang, L.; Yang, G.; Que, G.; Zhang, Q.; Yang, P. Energy Fuels 2006, 20 (5), 2008–2012.
10.1021/ef800742y CCC: $40.75 2009 American Chemical Society Published on Web 02/12/2009
Dipole Moment Variation of a Petroleum Residue
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Table 1. Properties of ME-AR properties
ME-AR
C (m %) H (m %) S (m %) N (m %) ash (m %) oxygen (m %) SARA composition saturate (m %) aromatics (m %) resin (m %) C7 asphaltene (m %)
84.48 11.32 3.8 0.2 0.018 0.18 37.2 46.0 14.2 2.6
The separation scheme, mean molecular weight, and the mean dipole moment measuring methods were the same as described previously.12,13 The colloidal stability parameters were characterized through a mass-fraction-normalized conductivity method.12,14,15 The ratio of added n-heptane/residue sample when asphaltene aggregation happens could be taken as the colloidal stability parameter of the residue sample. The molecular weight (MW) of separated fractions was measured by a Knauer vapor-pressure osmometer at 45 °C, using benzene as the solvent. Although the vapor-pressure osmometry (VPO) method was usually used in heavy oil research, it could only give an relative mean value of the aggregate weights of asphaltenes, because the requisite concentrations of asphaltenes greatly exceeded the critical nanoaggregate concentrations, which were on the order of 100 mg/ L.16 Therefore, the MW data of asphaltenes derived from the VPO method were further more than that of field-ionization mass spectroscopy (FIMS), laser desorption mass spectroscopy (LDMS), and matrix-assisted laser desorption ionization (MALDI).17,18 The fractions were diluted by benzene separately. First, a stock solution of fraction was prepared. Then, the stock solution was distributed in vials, and benzene was added to each vial to achieve the preconcerted weight percentage. The solution was placed at least for 24 h before measurement to ensure stabilization. Then, the same solutions were used to measure dielectric permittivity, refractive index, and mean molecular weight step by step. The use of the same solutions was to guarantee the fraction molecules, especially the asphaltenes, to be at the same dispersibility level.
3. Results and Discussion 3.1. Comparison of the Colloidal Stability Parameter (CSP) Variation and Coking Characteristics. The CSP of samples obtained at different reaction times were examined to investigate their relationship to the coking characteristics. The variation of CSP and coking characteristics of ME-AR during catalytic upgrading at 420 °C was shown in Figure 1. The coke-induction period is defined as the reaction time when the coke content reached 0.1 wt %. It can be seen in Figure 1 that the coke-induction period is 30 min for ME-AR during catalytic upgrading at 420 °C. The variation of CSP and coke content during the thermal reaction at 400 °C is shown in Figure 2 for comparison conveniently,12 and it is shown that the coke(13) Zhang, L. L.; et al. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 2003, 48 (4), 330–331. (14) Fotland, P.; Anfindsen, H.; Fadnes, F. H. Fluid Phase Equilib. 1993, 82, 157–164. (15) Fotland, P.; Anfindsen, H. Fuel Sci. Technol. Int. 1996, 14 (1/2), 101–115. (16) Asphaltenes, HeaVy Oil, and Petroleomics; Mullins, O. C., Sheu, E. Y., Hammami, A., Marshall, A. G.; Schlumberger: New York, 2007; Chapter 3. (17) Miller, J. T.; Fisher, R. B.; Thiyagarajan, P.; Winans, R. E.; Hunt, J. E. Subfractionation and characterization of Mayan asphaltene. Energy Fuels 1998, 12 (6), 1290–1298. (18) Hortal, A. R.; et al. On the determination of molecular weight distributions of asphaltenes and their aggregates in laser desorption ionization experiments. J. Mass Spectrom. 2006, 41, 960–968.
Figure 1. Variation of CSP and coking characteristics of ME-AR during the catalytic upgrading at 420 °C.
Figure 2. Variation of CSP and coking characteristics of ME-AR during the thermal reaction at 400 °C.
induction period is 100 min. The variation of CSP and coke for both the catalytic upgrading and thermal reaction show similar phenomena. During the coke-induction period, no coke formation is observed. However, coke forms rapidly after that. On the contrary, the colloidal stability of residue samples decreases upon the increase of the reaction time but shows little dependence on it after the coke-induction period is reached. This phenomenon reveals that the coking characteristics of the residue were determined by its colloidal stability, during both the catalytic upgrading and thermal reaction. 3.2. Fraction Composition Variation of the Residue during the Thermal Reaction or Catalytic Upgrading. The colloidal stability variation of the residue came from the shift of the fraction composition and the molecular characteristics of every fraction.7,12 A summary of the fractionation results of samples obtained from the thermal reaction or catalytic upgrading was presented in Tables 2 and 3. It can be seen from Table 2 that the asphaltene concentration in the residue increased quickly as the reaction advanced and reached 8.65 wt % at 100 min, just at the end of the cokeinduction period. Then, asphaltenes content began to decrease as the reaction continued. This phenomenon shows that the colloidal stability decreases as the asphaltene content increases during the coke-induction period and declines to its maximum. Then, the excessive asphaltenes would aggregate and transform into coke; therefore, asphaltene content gained its maximum at the end of the coke-induction period. It can be seen from Table 3 that the asphaltene concentration in the residue during catalytic upgrading increased as the reaction continued, which was different from the thermal reaction. It can be seen from Figure 1 that the coke-induction period of the ME-AR catalytic upgrading at 420 °C was 30 min. The asphaltenes content in the residue during the cokeinduction period was about 6 wt %, far less than that of the
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Table 2. Fraction Composition of Samples of the ME-AR Thermal Reaction at 400 °C (wt %)
Table 5. Molecular Weights of Fractions of ME-AR Thermal Reaction Samples
reaction time saturates and heavy light middle heavy (min) aromatics aromatics resins resins resins asphaltenes
reaction time saturates and heavy light middle heavy C5 (min) light aromatics aromatics resins resins resins asphaltenes
ME-AR 40 80 100 160
67.52 69.81 72.80 72.39 70.30
11.09 4.81 5.41 5.12 5.09
6.98 7.28 7.79 6.83 6.89
2.72 2.50 2.37 2.27 2.41
6.62 7.69 3.26 4.73 7.25
5.07 7.90 8.36 8.65 8.05
ME-AR 40 80 100 160
344 291 268 272 254
522 471 316 321 345
683 570 578 476 417
708 567 526 545 506
751 581 540 454 490
4350 4667 4173 3539 2052
Table 3. Fraction Composition of Samples of the ME-AR Catalytic Upgrading at 420 °C (wt %)
Table 6. Molecular Weights of Fractions of ME-AR Catalytic Upgrading Samples
reaction time saturates and heavy light middle heavy (min) aromatics aromatics resins resins resins asphaltenes
reaction time saturates and heavy light middle heavy C5 (min) light aromatics aromatics resins resins resins asphaltenes
ME-AR 0 30 60 90 120 150
67.52 71.05 67.19 71.86 69.78 68.01 67.61
11.09 10.70 14.74 11.66 12.08 13.76 12.57
6.98 5.94 7.00 6.18 6.80 6.56 6.69
2.72 2.77 2.62 2.14 2.20 2.24 2.25
6.62 3.60 2.48 2.22 2.16 2.05 1.90
5.07 5.94 5.96 5.93 6.98 7.38 8.97
thermal reaction. This phenomenon proves that hydrogen and the catalyst depresses asphaltene production. After the cokeinduction period, the asphaltenes content continued to increase and reached 8.9 wt % at 150 min, which was more than that of the thermal reaction. After the coke-induction period, asphaltenes would stay saturated and the redundant would aggregate and transform into coke. This phenomenon proves that hydrogen and catalyst depressed asphaltene condensation; therefore, the asphaltenes produced during catalytic upgrading were easier to disperse in the residue than that of the thermal reaction. The summary of saturates or light aromatics and heavy aromatics was about 80 wt % all through the catalytic upgrading process, which was more than that of the thermal reaction. This phenomenon proves that hydrogen and the catalyst depressed asphaltene condensation and accelerated asphaltene rupture. The characteristics of fractions were studied furthermore. 3.3. Molecular-Weight Variation of Fractions during the Catalytic Upgrading or Thermal Reaction. The molecular-weight values are important parameters for the characteristics of residue fractions. The MW data of fractions were measured by the VPO method. The MW data and the error bar of fractions of ME-AR were shown in Table 4. The MW data of fractions of reaction samples were shown in Tables 5 and 6. First, for the two kinds of reactions, the MW values of fractions of maltenes decreased as the reaction continues. The down trend of the saturate will pick up with the colloidal stability deterioration as reactions progressed. Second, for the thermal reaction, the MW values of asphaltenes increased at first, reached a maximum at the end of the coke-induction period, and then decreased as the reaction advances. On the contrary, the MW values of asphaltenes decreased as the reaction advanced during the catalytic reaction and went down to 1690 Da when the reaction time was 150 min. Third, the MW values of fractions of maltenes decreased as the reaction continued for the two kinds of reactions. However, it decreased more markedly during the catalytic upgrading, because the presence of hydrogen restrained the condensation of asphaltene molecules and enhanced alkyl chain rupture.
ME-AR 0 30 60 90 120 150
344 324 351 308 303 290 288
522 550 483 468 398 346 337
683 704 502 517 474 472 467
708 756 600 511 488 474 442
751 623 618 555 533 481 453
4350 4371 4221 2934 2419 2129 1692
3.4. Dipole Moment Variations of Fractions during the Thermal Reaction and Catalytic Upgrading. The polarity of fractions is tightly related to the colloidal stability of residue samples. The mean dipole moments of fractions during both the thermal reaction and catalytic upgrading were measured to elucidate their similarities and differences. The mean dipole moments of fractions were measured by the method developed by Guggenheim and Smith.9-11,19,20 They developed a method to gain 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 fractions were diluted by benzene separately, which is a nonpolar solvent. The dielectric permittivity and refractive index of these solutions were measured. The dielectric permittivity of the solutions was measured by a HP4194A analyzer. The refractive index of solutions was measured by a refractive meter. Then, the mean dipole moment of fractions can be approached. The polarity of fractions is tightly related to the colloidal stability of residue samples. The mean dipole moments of fractions during both the thermal reaction and catalytic upgrading were measured to elucidate their similarities and differences. The dipole moment and the error bar of fractions of ME-AR are shown in Table 7. The dipole moment data of fractions derived from reaction samples are shown in Tables 8 and 9. It can be seen from Figure 2 that the coke-induction period of the ME-AR thermal reaction at 400 °C was 100 min. Table 8 displayed the dipole moment of the fraction of ME-AR thermal reaction samples at different reaction times. For all of the samples, the mean dipole moment values of the fractions increased one by one as the following sequence: saturates and light aromatics, heavy aromatics, light resins, middle resins, heavy resins, and asphaltenes. The dipole moment values of fractions are consistent with the solvent solubility used in liquid chromatography separation.8 This result proves that the dipole moment measurement method is feasible. Moreover, the mean dipole moments of asphaltenes kept increasing as the thermal reaction advanced, reached their maximum, and decreased thereafter. The time to gain the maximum is approximately equal to the coke-induction period. This result reveals that the
Table 4. Molecular Weights of Fractions of ME-AR frations
saturates and light aromatics
heavy aromatics
light resins
middle resins
heavy resins
C5 asphaltenes
MW relative error
344 ( 66 0.19
522 ( 24 0.05
683 ( 67 0.10
708 ( 96 0.14
751 ( 161 0.21
4350 ( 271 0.06
Dipole Moment Variation of a Petroleum Residue
Energy & Fuels, Vol. 23, 2009 2089
Table 7. Dipole Moments of Fractions of ME-AR frations
saturates and light aromatics
heavy aromatics
light resins
middle resins
heavy resins
C5 asphaltenes
dipole moment (Debye) relative error
0.5 ( 0.19 0.38
1.4 ( 0.73 0.53
2.5 ( 0.24 0.09
3.2 ( 0.71 0.23
4.5 ( 0.88 0.19
16.8 ( 1.49 0.09
Table 8. Dipole Moments of Fractions of ME-AR Thermal Reaction Samples (Debye) reaction time saturates and heavy light middle heavy C5 (min) light aromatics aromatics resins resins resins asphaltenes ME-AR 40 80 100 160
0.5 0.82 0.58 0.68 0.62
1.4 1.73 1.23 2.48 1.04
2.5 2.30 2.71 2.91 2.13
3.1 2.34 3.36 3.77 3.69
4.5 3.63 3.72 3.44 4.01
16.8 18.03 15.65 17.41 12.20
Moreover, the asphaltene content during catalytic upgrading was more than that of the thermal reaction when time was long, because, after the coke-induction period, the asphaltenes reached the saturated content and the superfluous asphaltenes would aggregate and transform into coke. This result reveals that asphaltenes with less polarity were easier to disperse in samples steadily. 4. Conclusions
Table 9. Dipole Moments of Fractions of ME-AR Catalytic Upgrading Samples (Debye) reaction time saturates and heavy light middle heavy C5 (min) light aromatics aromatics resins resins resins asphaltenes MEAR 0 30 60 90 120 150
0.5 0.64 0.84 0.49 0.25 0.20 0.55
1.4 1.43 0.90 0.85 1.36 0.54 0.76
2.5 2.35 1.20 1.29 1.16 1.56 1.02
3.1 3.83 3.02 2.82 2.22 2.05 2.60
4.5 5.48 4.28 4.67 2.92 4.32 3.44
16.8 17.95 18.83 13.11 12.11 10.21 8.30
asphaltenes having more polarity tending toward aggregation and transform into coke during the thermal reaction, which made the polarity of asphaltenes kept in the residue to decrease after the coke-induction period. Table 9 shows the variation of the mean dipole moments of fractions during catalytic upgrading. For the samples studied, the mean dipole moment values of the fractions increased one by one from saturates and light aromatics to asphaltenes. For the asphaltenes extracted from catalytic upgrading samples, the mean dipole moments showed a little increase as the reaction advanced, reached their maximum at the end of the cokeinduction period, and decreased quickly thereafter. The maximum was close to that of the thermal reaction. The dipole moment values decreased sharply after the coke-induction period. For example, it was 8.3 Debye when the reaction time reached 150 min, which was less than that of the thermal reaction. This result reveals that the presence of the catalyst and hydrogen depressed the condensation of asphaltenes; therefore, the polarity of asphaltenes decreased remarkably. Moreover, the variation of the difference between the hydrogen upgrading and thermal reaction indicates that the action of the catalyst and hydrogen needs time. On the one hand, they were effortless during the coke-induction period; therefore, the difference of asphaltene polarity was tiny. On the other hand, they were effective after the coke-induction period; therefore, the difference of asphaltene polarity were distinctive. It can be concluded from Tables 8 and 9 that the mean dipole moments of asphaltenes during catalytic upgrading were less than that of the thermal reaction after the coke-induction period. (19) Guggenheim, A. Trans. Faraday Soc. 1949, 45, 714–720. (20) Smith, J. W. Trans. Faraday Soc. 1950, 46, 394–399.
The characteristics of the catalytic upgrading and thermal reaction were analyzed at different levels, including coking characteristic, fraction composition of samples, MW of fraction, and polarity of fractions. The relationships between them were studied. The conclusions are as follows. During either the catalytic upgrading or thermal reaction, the colloidal stability of residues decreased sharply during the cokeinduction period and changed little thereafter. This phenomenon reveals that the colloidal stability of the residue has an intrinsic relationship to its coking characteristics. During thermal reaction, the asphaltene concentration in the residue increased quickly during the coke-induction period and gained its maximum at the end of the coke-induction period. However, the asphaltene concentration in the residue during the catalytic upgrading increased as reaction advanced. The summary of saturates or light aromatics and heavy aromatics during the catalytic upgrading were more than that of the thermal reaction, which reveals that hydrogen and the catalyst depressed asphaltene condensation and accelerated asphaltene rupture. The MW values of asphaltenes gain their maximum at the end of the coke-induction period during the thermal reaction. On the contrary, the MW values of asphaltenes decreased markedly as the reaction advanced during the catalytic reaction, which indicated that the hydrogen and catalyst restrained the condensation of asphaltene molecules and enhanced alkyl chain rupture. For both kinds of reactions, the mean dipole moments of asphaltenes gain their maximum almost at the end of the cokeinduction period, which reveals that the asphaltenes having more polarity prefer to aggregate and transform into coke. At the initial reaction stage of catalytic upgrading, the mean dipole moments of asphaltenes were close to that of the thermal reaction and the difference was more and more distinct as the reaction advanced. Acknowledgment. This work was partially supported by the National Basic Research Program (973 Program, 2006CB202505) and the National Science Fund Committee of China (20506017 and 20776160). EF800742Y
