Energy Fuels 2010, 24, 3970–3976 Published on Web 06/15/2010
: DOI:10.1021/ef100272e
Storage Stability of the Visbreaking Product from Venezuela Heavy Oil Na Zhang, Suoqi Zhao,* Xuewen Sun, Zhiming Xu, and Chunming Xu State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received March 9, 2010. Revised Manuscript Received May 25, 2010
The relationship between the structure and stability of the visbreaking product of Venezuela extra heavy oil has been studied. The number of resin layers adsorbed to asphaltene is proposed to describe the colloidal stability of Venezuela heavy oil and its visbreaking product. The results show that the colloidal stability of the visbreaking product is inferior to Venezuela heavy oil. The factors influencing the storage stability of the visbreaking product have been studied by analyzing the changes of kinematic viscosity, total acid number (TAN), weight loss, ratio of resins/asphaltenes, as well as free-radical content of visbreaking samples sealed and exposed to air or nitrogen for 90 days. It is found that the storage stability of the visbreaking product is influenced by oxidative condensation, evaporation, and non-oxygen condensation, among which oxidative condensation is the most significant factor. The free-radical reactions do not terminate after the thermal reaction, and the content of free radicals is reduced to different degrees for the storage cases. Antioxidants 2,6-di-tert-butyl-4-methylphenol and p,p-di-iso-octyl-diphenylamine were added to visbreaking products by 1 wt % to prevent oxidative condensation. When the TAN and content of free radicals of visbreaking samples with and without antioxidant are compared, it is demonstrated that the antioxidants play the role of generating stable free radicals, which favors the retention of stability.
Introduction
Table 1. Components of the Distillation Cut from Atmospheric Distillation of Venezuela Heavy Oil at 350 °C
With the increasing demand on oil products and depletion of conventional oil reserves, more and more attention is attracted to unconventional oil resources, such as extra heavy oil in Venezuela.1 The high viscosity, density, asphaltene, sulfur, metals, and concarbon residue (CCR) of heavy oils make them difficult to be transported and processed.2,3 With the features of low requirements for feedstock, mature technology, and less investment and operating costs, visbreaking4 was considered as an option for upgrading heavy oil for transportation purposes. For the visbreaking product (VisB), the reduced viscosity is favorable for transportation but instability makes them not beneficial for transportation and storage.5 The colloidal stability of heavy oil has been extensively studied. Mushrush and Speight6 and Asomaning and Watkinson7 used the colloidal instability index [CII = (asphaltene þ saturate)/ (resin þ aromatic)] to assess the instability of petroleum. Li et al.8 gave a more precise colloidal stability function based on the research of three Chinese vacuum residues. The formula is S = 1.36(resin/asphaltene) þ 3.11(aromatic) - 1.86(saturate),
content of components (wt %) 7.9 53.8 9.1 8.5 7.8 0.4 4.7 4.2 2.4 38.3 100
which explains quantitatively the contribution of each fraction in the colloidal stability of the residue. Matsushita et al.9 defined the ratio of the H/C of asphaltenes to the H/C of maltenes as a relative solubility index. It was found that the precipitation of asphaltenes is considered as the most severe threat to the colloidal stability of heavy oil. Andersen, Bartholdy, Buckley, and Aske et al. established a series of methods via optical instruments, such as UV-vis spectrometry,10,11 nearinfrared spectroscopy,12 and refractive index measurements,13 to detect the aggregation of asphaltenes during the destruction of the colloidal system. Mousavi-Dehghami et al.14 used the viscometry and interfacial tension methods to study the aggregation of asphaltenes. Licha and Herrera15 studied the
*To whom correspondence should be addressed: Telephone: 086-01089733743. Fax: 086-010-69724721. E-mail:
[email protected]. (1) Carbognani, L.; Gonzalez, M. F.; Pereira-Almao, P. Energy Fuels 2007, 21, 1631–1639. (2) Omole, O.; Olieh, M. N.; Osinowo, T. Fuel 1999, 78, 1489–1496. (3) Joshi, J, B.; Pandit, A. B.; Kataria, K. L.; Kulkarni, R. P.; Sawarkar, A. N.; Sawarkar, A. N.; Tandon, D.; Ram, Y.; Kumar, M. M. Ind. Eng. Chem. Res. 2008, 47, 8960–8988. (4) Allan, D. E.; Martinez, C. H.; Eng, C. C.; Barton, W. J. Chem. Eng. Prog. 1983, Jan, 85–89. (5) Zhang, L. L.; Yang, G. H.; Que, G. H.; Zhang, Q. X.; Yang, P. J. Energy Fuels 2006, 20, 2008–2012. (6) Mushrush, G. W.; Speight, J. G. Petroleum Products: Instability and Incompatibility; Taylor and Francis: Washington, D.C., 1995; pp 22-24. (7) Asomaning, S.; Watkinson, A. P. Heat Trans. Eng. 2000, 21, 10–16. (8) Li, S.; Liu, C.; Que, G.; Liang, W. J. Pet. Sci. Eng. 1999, 22, 37–45. r 2010 American Chemical Society
component alkane naphthene alkylbenzene tetralin indenes naphthalene naphthalenes acenaphthenes acenaphthylenes aromatic total
(9) Matsushita, K.; Marafi, A.; Hauser, A.; Stanislaus, A. Fuel 2004, 83, 1669–1674. (10) Andersen, S. I. Energy Fuels 1999, 13, 315–322. (11) Bartholdy, J.; Andersen, S. I. Energy Fuels 2000, 14, 52–55. (12) Aske, N.; Kallevik, H.; Johnsen, E. E.; Sj€ oblom, J. Energy Fuels 2002, 16, 1287–1295. (13) Buckley, J. S. Energy Fuels 1999, 13, 328–332. (14) Mousavi-Dehghani, S. A.; Riazi, M. R.; Vafaie-Sefti, M.; Mansoori, G. A. J. Pet. Sci. Eng. 2004, 42, 145–156. (15) Lichaa, P. M.; Herrera, L. SPE Tech. Pap. 5304, 1975.
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Table 2. Properties of Venezuela AR properties elemental analysis SARA components
density at 20 °C (g/cm3) CCR (wt %) molecular weight (g/mol) C (wt %) S (wt %) Ni (wppm) saturate (wt %) aromatic (wt %)
1.028 18.67 626 83.96 4.2 120 21.90 44.26
conductivity of oil to detect the aggregation of asphaltenes. Taylor16 studied the electric property of asphaltene by a homemade electrode deposition apparatus. Fotland et al.17-19 measured the conductivity change of several crude oils with the addition of the precipitation solvent (heptane). The results of microscopy and gravimetric analysis on the precipitation of asphaltenes were consistent with that of conductivity measurements.20 However, how to define the long-term storage stability of the heavy oil VisB that can be used as a refinery feedstock has not been reported up to now. This work studied the factors influencing the long-term storage stability of the VisB from Venezuela extra heavy oil by observing the changes of properties, structures, and compositions during 90 days to seek the possibility of shipping the VisB to China for further upgrading.
viscosity at 100 °C (mPa s) freezing point (°C)
2574 46
H (wt %) N (wt %) V (wppm) resin (wt %) C7 asphaltene (wt %)
9.92 0.64 450 23.06 10.77
Table 3. Properties of Venezuela Heavy Oil and Its VisB properties
heavy oil
VisB
viscosity at 50 °C (mPa S) API (deg) molecular weight (g/mol) resin (wt %) C7 asphaltene (wt %) freezing point (°C) flash point (°C) TAN (mg of KOH/g) N (wt %) S (wt %) H (wt %) C (wt %)
9665 9.0 498 19.14 8.72 26 228 2.94 0.56 4.0 10.12 83.72
110 11.8 422 9.32 12.84 1 111 0.44 0.73 3.5 9.76 84.98
140 mL of VisB was stored in a 150 mL beaker and the exposed fluid surface area was 20.50 cm2. Storage Stability Analysis. Kinematic viscosity was measured using a countercurrent capillary viscometer (ASTM D445). Resins and asphaltenes were obtained using the SARA method21 specified by the China Research Institute of Petroleum Processing (RIPP). The free-radical content was determined using an ER200D-SRC electron paramagnetic resonance instrument. The total acid number (TAN) was measured using the potentiometric titration method (ASTM D-664). Compositions and Structure Analysis. The molecular weight was determined using a Knauer vapor osmotic pressure apparatus using toluene as the solvent. Elemental analysis was conducted using a Flash EA 1112 element analyzer. The 1H nuclear magnetic resonance (NMR) was conducted using a VARIAN INOVA 200 MHz nuclear magnetic resonance apparatus. The structural parameters were calculated using the B-L method22 based on 1H NMR, molecular weight, and elemental composition. X-ray diffraction (XRD) analysis on asphaltenes was carried out using a Bruck D8 Advance instrument. Cu KR (1.54056 A˚) was selected as the monochromic energy radiation. The diffraction angles (2θ) were scanned from 5° to 60° at a 1°/min scanning rate with a 0.05° increment. The asphaltenes was mortared into fine powders prior to XRD analysis at room temperature. The heteroatoms in Venezuela heavy oil and the VisB were analyzed using Bruker apex-ultra negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) equipped with a 9.4 T superconducting magnet. Negative ions were generated by infusing sample solution to a microelectrospray source equipped with a 50 μm inner diameter fused silica micro ESI needle at a flow rate of 150 μL/h by a syringe pump. Typical negative-ion ESI conditions were þ2.5 kV emitter voltage, -3.0 kV capillary column front end voltage, and 320 V capillary column end voltage. Ions accumulated for 0.1 s in a hexapole with 2.4 V direct-current (DC) voltage and 300 Vp-p radio-frequency (RF) amplitude.
Experimental Section Materials. The feedstock was Venezuela extra heavy oil, which was subjected to atmospheric distillation at 350 °C. The distillation cut and atmospheric residue (AR) were obtained. The components of the distillation cut are listed in Table 1. The properties of Venezuela AR are shown in Table 2. It is a typical extra heavy oil with high viscosity, freezing point, density, CCR, asphaltene, sulfur, nickel, and vanadium. Visbreaking Reaction. A total of 350 mL of the AR sample was processed by visbreaking in a 500 mL autoclave, which was purged 3 times with nitrogen at 1 MPa before heating. The visbreaking reaction of AR was carried out at atmospheric pressure. The autoclave was heated at 8 °C/min and reached the set reaction temperature of 420 °C, followed by a 30 min reaction. The degree of the visbreaking reaction was determined according to the previous research, in which the coke yield was below 0.1 wt %, to avoid the onset of coking reactions.5 The produced gas and liquid products from the outlet were cooled in a condenser and collected. After the end of the reaction, the autoclave was cooled by water to room temperature quickly to prevent the residue from further cracking. The visbreaking residue from the bottom of the autoclave was blended with cracking fraction oil at 50 °C. The cracking gas, fraction oil, and visbreaking residue were obtained after the reaction. The total liquid yield is 97 wt % under the above conditions. The liquid product from AR visbreaking was blended with the distillation cut obtained from the atmospheric distillation at the ratio of 86:14 (w/w), and the blend was denoted as the VisB. To determine the storage stability of the VisB, it was stored respectively under three different conditions, including exposure to air (air), exposure to nitrogen (N2), and sealed in nitrogen (N2C) and open to daylight at room temperature. The VisB stored under three different conditions was denoted as the visbreaking sample (VB). To study the effect of evaporation,
Results and Discussion Visbreaking Reaction. The properties of Venezuela heavy oil and its VisB are listed in Table 3. It is found that visbreaking significantly reduces the viscosity of Venezuela heavy oil
(16) Taylor, S. E. Fuel 1998, 77, 821–828. (17) Fotland, P.; Anfindsen, H.; Fadnes, F. H. Fluid Phase Equilib. 1993, 82, 157–164. (18) Fotland, P.; Anfindsen, H. Fuel Sci. Technol. Int. 1996, 14, 101– 115. (19) Fotland, P. Fuel Sci. Technol. Int. 1996, 14, 313–325. (20) Wang, J. Q.; Li, C.; Zhang, L. L.; Deng, W. A.; Que, G. H. Energy Fuels 2009, 23, 3002–3007.
(21) Standard Method SH/T0509-92. China Research Institute of Petroleum Processing, Beijing, China. (22) Brown, J. K.; Ladner, W. R. Fuel 1960, 39, 87–96.
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Figure 1. Average structures of resins and asphaltenes in Venezuela heavy oil and its VisB.
Table 4. Structural Parameters of Resins and Aspahltenes in Venezuala Heavy Oil and Its VisB asphaltenes
input
unit sheet
molecular formula
C (wt %) H (wt %) S (wt %) N (wt %) M hA hR hβ hγ fA n usw CA* RT* RA* RN * CT* HT* CN * CS* CP * fN * fP * RA*/RN
resins
heavy oil
VisB
heavy oil
VisB
81.70 7.67 5.40 1.00 6186 0.0682 0.1800 0.5438 0.2080 0.48 4.49 1377.17 44.55 19.68 13.52 6.16 93.76 105.62 18.47 49.21 30.74 0.20 0.33 2.20 C421H474S10N4
83.26 6.30 5.10 1.10 4665 0.1221 0.2022 0.5117 0.1640 0.60 3.53 1322.04 55.17 23.50 17.06 6.44 91.73 83.28 19.33 36.55 17.23 0.21 0.19 2.65 C323H293S7N3
81.67 9.02 4.20 1.10 814 0.0705 0.1984 0.5533 0.1757 0.39 1.16 701.57 18.41 7.91 4.80 3.10 47.75 63.28 9.31 29.34 20.03 0.20 0.42 1.55 C55H73S1N0.6
83.34 8.17 3.90 1.40 809 0.1150 0.2493 0.5064 0.1293 0.48 1.49 544.18 18.12 7.50 4.71 2.80 37.79 44.46 8.39 19.67 11.28 0.22 0.30 1.68 C56H66S1N0.8
and freezing point, improves API, and decreases the molecular weight slightly. The amount of asphaltenes in VisB increases by 4 wt %, and the amount of resins decreases by 10 wt %, which show that 60% of resins is cracked to light oil components and 40% of resins is condensed to form asphaltenes. The amount of corrosive compounds, carboxylic and
naphthenic acids, are effectively reduced after visbreaking. Table 3 shows that the TAN of Venezuela heavy oil is reduced from 2.94 to 0.44 mg of KOH/g. The structure parameters of resins and asphaltenes in Venezuela heavy oil and its VisB are shown in Table 4. The sulfur species in heavy oil contain 70% thiophenes and 3972
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Figure 3. Structure of the asphaltene cluster. Table 5. Crystalline Parameters of Asphaltenes in Venezuela Heavy Oil and Its VisB
Figure 2. XRD patterns of asphaltenes of Venezuela heavy oil and its VisB.
30% sulfides, whereas in the VisB, all sulfur species are thiophenes.23,24 The nitrogen species are assumed to be non-basic nitrogen compounds. The average structures of resins and asphaltenes in Venezuela heavy oil and its VisB are shown in Figure 1. The structures of resins before and after the visbreaking are quite similar. However, the asphaltene in the VisB has more aromatic cores than those in Venezuela heavy oil. The XRD patterns of asphaltenes in Venezuela heavy oil and its VisB are shown in Figure 2. The XRD pattern of asphaltene has primarily three characteristic bands: the γ band, the 002 band, and the 100 band at 2θ =20°, 25°, and 44°, respectively.25 The structural data of the asphaltene clusters, the average distance between the aromatic sheets (dm), the height of the crystallite (Lc), and the average diameter of the aromatic sheets (La), respectively, were obtained from the 002 band position and the width of the 100 and 002 bands at half-maximum. According to Bragg and Warren’s equations,26 the following parameters can be obtained:27-29 Average distance between aromatic sheets, dm (A˚) λ dm ¼ ð1Þ 2 sin θ002
0:9λ ω cos θ002
ð2Þ
Average diameter of the aromatic sheets, La (A˚) 0:9λ ω100 cos θ100
dγ (A˚)
dm (A˚)
Lc (A˚)
M
La (A˚)
RA*
resin layers
heavy oil VisB
0.36 0.44
6.06 5.95
3.53 3.51
18.45 19.28
6.2 6.5
28.21 40.09
10.6 15.0
4.5 1.4
ð5Þ
where ω is the width of the band at half-maximum, λ is the X-ray wavelength, and θ is the diffraction angle or reflection angle. The X-ray crystallite parameters of asphaltenes in Venezuela heavy oil and its VisB are presented in Table 5. It is noticed that the aromatic factor (fA) of asphaltenes determined by XRD is lower than that determined by NMR, which is consistent with the previous findings.25,30 The explanation of it is a small fraction of aromatic carbon in the structures of asphaltene is not stacking and cannot be detected by XRD. The average numbers of aromatic sheets of asphaltenes (M) for Venezuela heavy oil and the VisB are 6.2 and 6.5, respectively. According to XRD crystalline parameters, the structure of the asphaltene cluster can be described as shown in Figure 3. Because of the peptization of the colloidal system, the colloidal system of Venezuela heavy oil can be stable. When the content of resins decreases, it is possible to cause the aggregation of asphaltene, especially active asphaltene, and then the precipitation of asphaltene. Then, the colloidal stability of heavy oil declines. A indicator is proposed in this research as the number of resin layers adsorbed to asphaltene to predict the colloidal stability of heavy oil. Asphaltenes are believed to form a layer or sheet structure, and the condensed aromatic sheets have a tendency to stack, bearing naphthenic and alkyl systems on their periphery (Figure 3). Condensed aromatic sheets of resin are postulated to adsorb on that of the asphaltene cluster. When a monolayer adsorption of resins onto the asphaltene sheet surface is assumed, the number of resin layers adsorbed to asphaltene can be determined by eq 6
Average number of aromatic sheets per crystallite, M Lc M ¼ þ1 ð3Þ dm
La ¼
fA
Number of aromatic rings in the unit sheet La RA ¼ 2:667
Height of the crystallite, Lc (A˚) Lc ¼
sample
ð4Þ
n ¼ (23) Zhao, S.; Kotlyar, L. S.; Woods, J. R.; Sparks, B. D.; Chung, K. H. Pet. Sci. Technol. 2000, 18 (5 and 6), 587–606. (24) Zhao, S.; Xu, Z.; Xu, C.; Chung, K. H.; Wang, R. A. Fuel 2005, 84 (6), 635–645. (25) Simon, I. A. Energy Fuels 2005, 19, 2371–2377. (26) Kaufman, H. S.; Fankuchen, I. Anal. Chem. 1949, 12 (1), 24–29. (27) Schwager, I.; Farmanlan, P. A.; Kwan, J. T.; Weinberg, V. A.; Yen, T. F. Anal. Chem. 1983, 55, 42–45. (28) Yen, T. F.; Erdman, J. G.; Pollak, S. S. Anal. Chem. 1961, 33, 1587–1594. (29) Bouhadda, Y.; Bormann, D.; Sheu, E.; Bendedouch, D.; Krallafa, A.; Daaou, M. Fuel 2007, 86, 1855–1864.
Mn, asp RA, asp 1 mres 2 masp Mn, res RA, res
ð6Þ
where n is the number of resin layers adsorbed to asphaltene, mres is the weight of resin, masp is the weight of asphaltene, Mn,asp is the molecular weight of asphaltene, Mn,res is the molecular weight of resin, RA,asp is the aromatic carbon number of asphaltene, and RA,res is the aromatic carbon number of resin. (30) Ebert, L. B. Fuel Sci. Technol. Int. 1990, 8 (5), 563–569.
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Figure 4. Changes of viscosity of VB exposed to air during storage.
Figure 5. Evaporation of VBs exposed to air or nitrogen during storage.
Table 6. Changes of the H/C Ratio and fA of Asphaltenes in VBs during Storage storage time (day) structure parameters H/C air N2 N2C fA air N2 N2C
0
30
60
0.85 0.85 0.85
0.91 0.88 0.89
0.78 0.80 0.80
0.64 0.64 0.64
0.60 0.62 0.62
0.69 0.67 0.67
The calculated number of resin layers adsorbed to asphaltene decreases from 4.5 to 1.4 after visbreaking. Because resins are considered as the key stabilizer for the heavy oil colloidal system, decreasing the number of resin layers adsorbed to asphaltene means that the colloidal stability of heavy oil declines. This indicates that the VisB may be unstable. Hence, the long-term storage stability of the VisB is studied thoroughly as follows. Storage Stability of the VisB. The kinematic viscosity of VB exposed to air as a function of the storage time is shown in Figure 4. Although its colloidal stability is lower than that of Venezuela heavy oil, the viscosities of the VBs withdrawn from the top and bottom of the beaker are similar over a 2 month period. This suggests that the precipitation of asphaltenes is not obvious and the VisB is still a stable colloidal system. However, the viscosity of the VB significantly increases after 30 days. The H/C ratio of asphaltenes of VBs under various storage conditions decreases, and fA rises after 30 days, as shown in Table 6, which indicates that aggregation of asphaltene happens. The changes of the H/C ratio and fA of asphaltene are consistent with the changes of viscosity (Figure 4). The data in Table 7 show that the CCR of VBs decreases slightly over 3 months of storage, which is independent of the storage conditions. The above results suggest that the aggregation of asphaltene is in mild severity during storage. Evaporation of VBs was determined by their weight loss in the beaker. The curves in Figure 5 demonstrate that evaporation of light components of VBs occurs when they were exposed to air or nitrogen. In the case of nitrogen, the evaporation of VisB terminates after 100 days. The weight losses caused by evaporation exposed to air or nitrogen after 200 days are 2.5 and 1.25%, respectively. The ratio of resins/asphaltenes of VBs is monitored as a function of the storage time, as shown in Figure 6. The results
Figure 6. Changes of the resins/asphaltenes ratio of VBs during storage. Table 7. Changes of CCR of VBs during Storage CCR (%) storage time (day)
air
N2
N2C
1 73 89
21.08 20.35 20.07
21.08 19.21 18.07
21.08 20.17 18.22
show that the ratio of resins/asphaltenes decreases with the increase of the storage time and the ratio when exposed to air is smaller than that exposed to nitrogen. As known, the smaller the ratio of resins/asphaltenes of VisB, the more unstable VisB. It can be explained that oxygen could trigger the oxidative condensation reactions that convert resins into asphaltenes. Oxidative condensation reactions are known to have a negative effect on the stability of VisB.31,32 The results in Table 3 show that the TAN of Venezuela heavy oil decreases by 85% after visbreaking (TAN of heavy oil is 2.94 mg of KOH/g, and TAN of VisB is 0.44 mg of KOH/g). The negative-ion ESI FT-ICR mass spectrum in Figure 7 shows that most of the oxygen species in Venezuela heavy oil are removed by visbreaking, such as C22H21O2 and C20H25O2S, which means that visbreaking reduces the TAN of heavy oil by the decarboxylation reaction. The details will be discussed in a following paper. To determine the effect of oxidation, the TAN of VBs was measured using (31) Joshi, J. B.; Pandit, A. B.; Kataria, K. L.; Kulkarni, R. P.; Sawarkar, A. N.; Tandon, D.; Ram, Y.; Kumar, M. M. Energy Fuels 2008, 47, 8960–8988. (32) Aksoy, P.; Gul, O.; Cetiner, R.; Fonseca, D. A.; Sobkowiak, M.; Miller, S. F.; Miller, B. G.; Beaver, B. Energy Fuels 2009, 23, 2047–2051.
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Figure 7. Expanded mass spectra at 329 Da of ESI FT-ICR MS analysis of Venezuela heavy oil and its VisB.
Figure 9. Extent of the viscosity increase of VBs during storage.
Figure 8. Changes of the TAN of VB exposed to air during storage.
potentiometric titration. The TAN of the VB exposed to air rises with the increase of the storage time, as shown in Figure 8. This indicates that VisB is unstable and easy to oxidize. In all, with the increase of the storage time, the TAN, extent of evaporation, and fA of VBs increase and the H/C and resin/asphaltene ratio decrease. The results prove that oxidative condensation, light-component evaporation, and nonoxygen condensation happen. The increase of viscositiy of VBs caused by oxidative condensation, evaporation, and non-oxygen condensation after every 10 days is showed in Figure 9. It can be observed that the three factors all bring up the viscosity of the VBs with time extending, but for different factors, the increase of viscosity in every 10 days is different. The effect of oxidative condensation on the increase of viscosity with storage time is more significant than the others. Especially after 60 days, the increase of viscosity caused by oxidative condensation in every 10 days is 4-5-fold those caused by evaporation and non-oxygen condensation. Therefore, it is concluded that oxidative condensation is the most significant factor influencing the long-term storage stability of the VisB. Electron spin resonance (ESR) was used to measure the number of unpaired electrons, namely, spin concentration. Free-radical concentration can be expressed by the unpaired electron number per gram or milliliter of sample, namely, spins/g. It is showed in Table 8 that the content of free radicals in the VisB is 5.81 1018 spins/g and those in all VBs decrease with the increase of the storage time. It is inferred that the free-radical reaction does not terminate after the
Table 8. Changes of the Free-Radical Content of VBs during Storage content of free radicals (1018) (spins/g) storage time (day)
air
N2
N2C
0 30 90
5.81 2.74 1.75
5.81 1.18 1.01
5.81 2.50 1.57
thermal reaction. Free radicals are very active and can react with each other at room temperature. Because the contents of free radicals of VBs are different under different storage conditions, it is deduced that the free radicals are consumed not only by the combination reaction between free radicals but also by other reactions, such as the addition reaction with olefin and the reaction with oxygen. Therefore, the long-term storage stability of the VisB is affected by free radicals. Because oxidative condensation is found to be the most significant factor influencing the long-term storage stability, it can be concluded that the interaction of free radicals and oxygen results in the great growth of viscosity of VBs. Antioxidant To Prevent Oxidation. To further verify the effect of oxidation, antioxidants 2,6-di-tert-butyl-4-methylphenol (T501) and p,p-di-iso-octyl-diphenylamine (DPA), commonly used in commercial lube, were added to VisBs at 1 wt %33 and the changes of TAN and content of free radicals were studied. As shown in Figure 10, the TAN of VBs with T501 and DPA added are smaller than the TAN of VBs without (33) Cook, C. D. J. Org. Chem. 1953, 18 (3), 261–266.
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of free radicals of VBs without antioxidants. It means that the antioxidants react with free radicals and then become stable free radicals. The antioxidants also inhibit free radicals from reacting with free radicals, oxygen, and olefin. The result is in agreement with the previous findings.33,34 The result also testifies that active free radicals and oxygen decrease the long-term storage stability of the VisB. Conclusions The properties and structures as well as colloidal structures of Venezuela extra heavy oil and its VisBs were studied using NMR, XRD, and other conventional analysis methods. It is found that the number of resin layers adsorbed to asphaltene after visbreaking decreases from 4.5 to 1.4, which corresponds to the decrease of the colloidal stability. The number of resin layers adsorbed to asphaltene can be used to characterize the colloidal stability of heavy oil. With the increase of the storage time, the kinematic viscosity, fA of asphaltenes, weight loss, and TAN of VBs under different conditions increase and the H/C ratio of asphaltenes and ratio of resins/asphaltenes decrease. The factors influencing the storage stability of the VisB are oxidative condensation, light-component evaporation, and non-oxygen condensation, among which oxidative condensation is the most significant factor. Visbreaking reduces the amount of acid components significantly. However, the TAN of VB exposed to air increases greatly during storage. The free-radical reaction does not terminate after the thermal reaction. They are still active and tend to react further at room temperature, especially with the existence of oxygen. Active free radicals and oxygen decrease the long-term storage stability of the VisB.
Figure 10. Changes of TAN of VBs with T501 and DPA added during storage.
Figure 11. Content of free radicals of VBs with T501 and DPA added.
antioxidants. It is shown that antioxidants inhibit the oxidation of the VisB to form acid compounds. After 65 days, the TAN of the VB with DPA added is larger than that exposed to air. It can be explained that the DPA is consumed after 65 days and cannot prevent the VisB from further oxidation. It can be seen in Figure 11 that the contents of free radicals of VBs with T501 and DPA added are more than the contents
Acknowledgment. The authors are thankful for the funding supported by the National Key Basic Research Development Program of China (973 Program, 2010CB226901) and the Program of Introducing Talents of Discipline to Universities (B07010). (34) Howard, J. A.; Ingold, K. U. Can. J. Chem. 1962, 40, 1851–1864.
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