Phase Separation and Colloidal Stability Change of Karamay Residue

May 20, 2009 - Present address: Center for Bioengineering and Biotechnology, China ... Stability Change of Asphaltene in the Residue during Slurry-Pha...
3 downloads 0 Views 3MB Size
Download PDF
3002

Energy & Fuels 2009, 23, 3002–3007

Phase Separation and Colloidal Stability Change of Karamay Residue Oil during Thermal Reaction Jiqian Wang,*,† Chuan Li, Longli Zhang, Wenan Deng, and Guohe Que State Key Laboratory of HeaVy Oil Processing, College of Chemistry and Chemical Engineering, China UniVersity of Petroleum, Qingdao, 266555, China ReceiVed December 31, 2008. ReVised Manuscript ReceiVed April 25, 2009

The thermal reaction of Karamay residue was carried out in a micro batch reactor under nitrogen atmosphere. The oil was taken out through an online sampling tube. The asphaltene and coke contents of cracked residue at different reaction times were determined. The phase separation of residue oil was observed by an optical microscope, and the colloidal stability of residue oil was determined quantitatively by the mass fraction normalized conductivity method. The alkyl side chain length of resins and asphaltenes was analyzed by infrared spectroscopy, and their structure parameters were calculated via density method. The macroscopic phenomena of phase separation, coke formation and colloidal stability deterioration were correlated with micro molecular structure. The results showed that the increase of asphaltene condensation degree and the decrease of peptizing ability of resins were the essential reasons for phase separation and coking during thermal reaction. At the initial coke formation point, the condensation degree of resins and asphaltenes during low temperature reaction.was lower than that of high temperature reaction.

1. Introduction Coking is the main obstacle for some petroleum residue upgrading processes, such as visbreaking and heavy oil hydrocracking. It causes equipment fouling and even plugging and catalyst deactivation. Researchers and engineers have analyzed the coke or sediment deposits on different positions of refinery facilities, such as reactor wall, filter, catalyst bed, and fraction tower wall. Their results showed that most of the coke and deposits were from asphaltene condensation.1-3 Elucidating the chemical mechanism of coking is a prerequisite to depress its formation. Residue oil thermal cracking is a series of sequential and parallel radical reaction. Saturates crack into volatiles; aromatics crack into saturates and volatiles, and condense into resins at the same time; resins crack into aromatics, saturates, and volatiles, and condense into asphaltenes; asphaltenes crack into resins, aromatics, saturates, and volatiles, and more importantly, condense into coke.4-7 Rahmani8 studied coking kinetics of asphaltenes by reacting five different asphaltenes in 1-methylnaphthalene and tetralin, with correlation of kinetics parameters with the chemical structure of asphaltenes. His kinetic model for coke formation incorporated phase separation and hydrogen transfer to the asphaltenes. The result showed * Corresponding author. Phone: 86-532-86981562; Fax: 86-53286981569; E-mail: [email protected]. † Present address: Center for Bioengineering and Biotechnology, China University of Petroleum, Qingdao 266555, China. (1) Gentzis, T.; Rahimi, P. M. Fuel 2003, 82, 1531–1540. (2) Stanislaus, A.; Hauser, A.; Marafi, M. Catal. Today 2005, 109, 167– 177. (3) Pang, W.-W.; Kuramae, M.; Kinoshita, Y.; Lee, J.-K.; Zhang, Y. Z.; Yoon, S.-H.; Mochida, I. Fuel 2009, 88, 663–669. (4) Que, G.; Liang, W. Fuel 1992, 71, 1483–1485. (5) Yue, C.; Watkinson, A. P.; Lucas, J. P.; Chung, K. H. Fuel 2004, 83, 1651–1658. (6) Wiehe, I. A. Ind. Eng. Chem. Res. 1992, 31, 530–536. (7) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32, 2447–2454. (8) Rahmani, S.; McCaffrey, W.; Gray, M. R. Energy Fuels 2002, 16, 148–154.

that the rate of cracking of asphaltenes in 1-methylnaphthalene corresponding to the content of aliphatic sulfur, and the yield coefficient for coke corresponding to aromaticity. Rahmani9 also investigated the solvent effects on the formation of coke from asphaltenes during thermal cracking. The most important characteristics of the liquid phase are hydrogen donating ability and the ability to initiate cracking reactions. Phase separation and coke formation are formed during the complicated chemical and physical interactions of residue oil molecules in a sequence of pyrogenation, polymerization, and condensation reactions. Wiehe6,7 postulated that phase separation was an important step of the coking process. Any disturbing affecting asphaltene aggregation or phase separation may restrain the coke formation, such as the addition of fine solid particle to absorb or disperse asphaltenes and coke precursors.10-14 It is known that residue oil is an organic colloidal system with special characteristics like typical colloids. Li15 depicted that asphaltenes coaggregated with the heavier resins of higher polarity and formed mixed micelles. The remaining fractions including the lighter resins, aromatics, and saturates acted as the dispersing phase in the colloid. If the petroleum residue colloid is destroyed by thermal effect or diluted with alkane solvent, then asphaltene will precipitate. The colloidal stability of residue oil will deteriorate with the cracking reaction. When the new phase of precipitating asphaltene appears, the colloidal (9) Rahmani, S.; McCaffrey, W.; Dettman, H. D.; Gray, M. R. Energy Fuels 2003, 17, 1048–1056. (10) Gentzis, T.; Rahimi, P.; Malhotra, R.; Hirschon, A. S. Fuel Process. Technol. 2001, 69, 191–203. (11) Rahimi, P.; Gentzis, T.; Fairbridge, C. Energy Fuels 1999, 13, 817– 825. (12) Tanable, K.; Gray, M. R. Energy Fuels 1997, 11, 1040–1043. (13) Wang, S.; Chung, K.; Masliyah, J. H.; Gray, M. R. Fuel 1998, 77, 1647–1653. (14) Storm, D. A.; Barresi, R. J.; Sheu, E. Y.; Bhattacharya, A. K.; DeRosa, T. F. Energy Fuels 1998, 12, 120–128. (15) Li, S.; Liu, C.; Que, G.; Liang, W.; Zhu, Y. Fuel 1996, 75, 1025– 1029.

10.1021/ef801149q CCC: $40.75  2009 American Chemical Society Published on Web 05/20/2009

Phase Separation and Stability of Karamay Residue Oil

system is totally destroyed. Therefore, the coking process can be studied conveniently by monitoring the change of colloidal stability and phase separation of the residue. To estimate the stability of the residue oil colloidal system, several methods have been proposed by former researchers. Mushrush16 and Asomaning17 used colloidal instability index (CII) to assess the instability of petroleum, which was given as CII ) (asphaltene + saturate)/(resin + aromatic). Li18 gave a more precise colloidal stability function on the basis of three Chinese vacuum residues. The function is: S ) 1.36(Resin/ Asphaltene) + 3.11(Aromatic) - 1.86(Saturate), which explains quantitatively the contribution of each fraction in the colloidal stability of residue. Matsushita19 defined the H/C ratio of asphaltenes to H/C of maltenes as a relative solubility index. Quality and quantity of coke on the spent catalysts were found to be affected by the relative solubility index. Asphaltene is the dispersed phase and resin is the dispersing agent in residue oil. Stable residue oil must have the proper weight ratio between resin and asphaltene. If the weight ratio is too small, asphaltene would deposit for the lack of peptizing by resin. Therefore, the ratio of resin/asphaltene is an important index to colloidal stability. Aromatic component acts as a positive factor in the colloidal stability, whereas the saturated component acts as a negative factor. Different techniques have been developed in recent years to investigate the colloidal stability of residue oil. Andersen, Bartholdy, Buckley, and Aske established a series of methods via optical instruments, such as UV-vis spectrometer,20,21 nearinfrared spectroscopy,22 and refractive index measurement,23 to detect the aggregation of asphaltenes during the destruction of colloidal system. Mousavi-Dehghami has discussed the viscometry and interfacial tension force methods to study the aggregation of asphaltenes.24 Since asphaltene contains sulfur, nitrogen, oxygen, nickel, and vanadium, it has a large dipole moment. Licha and coworkers had studied the conductivity of oil as early as 1975.25 They applied a homogeneous direct current field across two electrodes immersed in oil, and then migration and deposition on one of the electrodes was found. Taylor26 also studied the electric property of asphaltene by a homemade electrode deposition apparatus. His experiment result showed that asphaltene is positive charged and resin is negative, that is, asphaltene behaves like organic ions, which possesses net charge. If this is the case, asphaltene should conduct current as do ions. Fotland et al.27-29 measured the conductivity change of several crude oils with the addition of precipitation solvent (16) Mushrush, G. W.; Speight, J. G. Petroleum Products: Instability and Incompatibility; Taylor & Francis: Washington DC, 1995. (17) Asomaning, S.; Watkinson, A. P. Heat Trans. Eng. 2000, 21, 10– 16. (18) Li, S.; Liu, C.; Que, G.; Liang, W. J. Pet. Sci. Eng. 1999, 22, 37– 45. (19) Matsushita, K.; Marafi, A.; Hauser, A.; Stanislaus, A. Fuel 2004, 83, 1669–1674. (20) Andersen, S. I. Energy Fuels 1999, 13, 315–322. (21) Bartholdy, J.; Andersen, S. I. Energy Fuels 2000, 14, 52–55. (22) Aske, N.; Kallevik, H.; Johnsen, E. E.; Sjo¨blom, J. Energy Fuels 2002, 16, 1287–1295. (23) Buckley, J. S. Energy Fuels 1999, 13, 328–332. (24) Mousavi-Dehghani, S. A.; Riazi, M. R.; Vafaie-Sefti, M.; Mansoori, G. A. J. Pet. Sci. Eng. 2004, 42, 145–156. (25) Lichaa, P. M.; Herrera, L. SPE 5304. (26) Taylor, S. E. Fuel 1998, 77, 821–828. (27) Fotland, P.; Anfindsen, H.; Fadnes, F. H. Fluid Phase Equilib. 1993, 82, 157–164. (28) Fotland, P.; Anfindsen, H. Fuel Sci. Technol. Int. 1996, 14, 101– 115. (29) Fotland, P. Fuel Sci. Technol. Int. 1996, 14, 313–325.

Energy & Fuels, Vol. 23, 2009 3003 Table 1. Properties of Karamay Atmospheric Residue Oil density (20 °C) g/m3 viscosity (100 °C) mm2/s condensation point,°C element analysis, wt % C H H/C S N

0.9442 108.7 2.0 86.60 12.50 1.73 0.13 0.41

carbon residue, wt % ash content, wt % molecular weight toluene insoluble, wt % SARA analysis, wt % saturate aromatic resin heptane asphaltene

7.01 0.085 470 0 50.4 22.2 27.2 0.2

(heptane). Because the viscosity of oil solution dropped with the addition of solvent, the mobility of asphaltenes increased, and the mass fraction normalized conductivity showed an initial increase. At the onset of asphaltenes precipitation, the conductivity curve reached its maximum. After that, the curve was dropping down to zero conductivity, which was almost the same as pure heptane. Both microscopy and gravimetric analysis were consistent with the conductivity measurements. Fotland considered that the conductivity of asphaltene was strongly related to the charge transport mechanism. In solvent with low dielectric constant, such as crude oil or alkane, organic salts tend to associate to ion pairs or even clusters. Nitrogen and oxygen are likely to be found as amines and carboxylic acids, respectively. These functional groups have the capacity to become ionized and thereby carry charge in an electric field. In Fotland’s research, the conductivity was determined at a frequency of 1500 Hz. Measurements of conductivity showed no significant deviation as a function of frequency when the frequency was below 500 kHz. However, the conductance was observed to increase at the frequencies of 500 kHz or above. In a recent study, Sheu and Mullins30 determined the conductance change with the increase of frequency. Their result also showed that 0.56 MHz was a critical frequency. When it was below 0.56 MHz the conductivity was almost independent of frequency, and increased drastically when it was above 0.56 MHz. The conductance of asphaltenes might evolve a chargetransformation mechanism named the Mott hopping mechanism. Compared with light transmission, interfacial tension, and gravimetric analysis methods, an electrical conductivity method is more convenient to proceed. The oil system does not require the sample to be diluted enough to be as transparent as it is in the light transmission methods. Interfacial tension and gravimetric analysis are time-consuming and difficult to perform. Thus, the electrical conductivity method with some improvements was chosen to evaluate the change of residue oil colloidal stability in thermal reaction. The details can be found in ref 31. The aim of this study is to understand profoundly the chemical mechanism of colloid system destruction and coke formation of residue oil thermal reaction by correlating the phase separation with molecular structure. 2. Experimental Section 2.1. Materials. All the solvents, including toluene, benzene, heptane, and carbon tetrachloride were analytic grade and were used as received. The residue oil used in this study was Karamay atmospheric residue (Karamay AR), originated from China Xinjiang oilfield. Its main properties are listed in Table 1. 2.2. Thermal Reaction. The thermal reaction of Karamay AR was carried out in a high-pressure autoclave with a sampling apparatus, through which liquid products were taken out online at different reaction times. The autoclave has a reactor with volume of 500 mL. A 350 mL portion of residue oil was inputted, and (30) Sheu, E. Y.; Mullins, O. Energy Fuels 2004, 18, 1531–1534. (31) Zhang, L.; Yang, G.; Que, G.; Zhang, Q.; Yang, P. Energy Fuels 2006, 20, 2008–2012.

3004

Energy & Fuels, Vol. 23, 2009

30∼45 mL of sample was withdrawn each time. The stability and phase separation were observed quasionline. The autoclave was purged 5 times with nitrogen at 1 MPa before reaction. The initial nitrogen pressure was 3.0 MPa at room temperature. The temperature was raised at about 8 °C/min, and the reaction temperature was reached in about 50 min. The reaction time was defined as the zero point when the reaction temperature reached the setting point. The oil was sampled at the reaction time of 0, 20, 40, 60, 80, and 100 min. Experiments were carried out at 390, 405, and 420 °C, respectively. About 3 g of sampled oil was mixed with 90 mL of heptane, then refluxed for 30 min to ensure adequate mixing. The mixture was kept in darkness overnight and filtered. The insoluble was placed in a Soxhlet extractor and extacted with heptane to remove all the heptane-soluble components. Then, the solid was extracted with toluene to remove the toluene soluble components, asphaltene. Asphaltene was acquired after toluene was evaporated. Toluene insolubles and asphaltene were dried in a vacuum drying oven and weighed. 2.3. Phase Separation and Colloidal Stability Analysis. The phase separation of the oil sample was monitored via an Olympus BH-2 optical microscope at room temperature. The oil sample was daubed on a glass slide, and the oil layer should be thin enough to allow the light to transmit. Colloidal stability of oil samples was analyzed through a mass fraction normalized conductivity method. The conductivity was determined via a modified DDS-11A conductivity meter at 35 ( 0.1 °C. Because the conductivity value of oil and its heptane solution was very low, a homemade conductivity cell was used instead of commercial conductivity cells. The homemade cell had four identical copper plates with the diameter of 30 mm, and each one was spaced out 1.5 mm apart parallelly. Plates No. 1 and No. 3 were connected as one electrode, and plates No.2 and No.4 were the other one. The cell constant was calibrated and was given the value of 0.966 m-1. The conductivities of oil samples were determined with heptane titration. The mass ratio of heptane to oil at asphaltene precipitating point was defined as the colloidal stability parameter (CSP). 2.4. Molecular Structure Analysis. Resins and asphaltenes were separated by the method recommended by China Research Institute of Petroleum Processing (RIPP).32 Molecular weight of asphaltene and resin was determined in benzene by Knauer vapor osmotic pressure molecular weight measuring apparatus at 45 °C. The structural parameters were calculated by density method (RIPP 12-90). The IR spectra of resins and asphaltenes were obtained on a Magna 750 FT-IR spectrograph with samples dissolved in carbon tetrachloride and dropped on a KBr crystal slide for solvent volatilization and formation of sample thin film. The absorption values were controlled between 0.25 and 0.75 by adjusting the sample concentration. The scanning speed was 60 times per minute, and the wavelength range was 400-4000 cm-1.

3. Results and Discussion 3.1. Coke and Asphaltene Content at Different Reaction Time. The micrographs of oil samples under different reaction temperatures are shown in Figures 1-3 with respect to their reaction times. The coke and asphaltene contents of the oil samples with different reaction times at three reaction temperatures are shown in Figure 4. It can be deduced from Figure 4 that both asphaltene and coke have almost the same formation tendency at different reaction temperatures. The asphaltene content increased in the coke induction period until it reached the coking onset point. At the initial coking point, the oil has the maximum asphaltene content, and then the asphaltene content begins to decrease with the coke formation, as has been reported by others.5,7,31 The only difference among (32) Standard method No. SH/T0509-92 recommended by China Research Institute of Petroleum Processing.

Wang et al.

Figure 1. Liquid phase separation of thermal reaction at 390 °C Photographs of Karamay residue samples at different reaction times: A, B, C, D, E, and F are 0, 20, 40, 60, 80, and 100 min, respectively.

Figure 2. Liquid phase separation of thermal reaction at 405 °C Photographs of Karamay residue samples at different reaction times: A, B, C, D, E, and F are 0, 20, 40, 60, 80, and 100 min, respectively.

Figure 3. Liquid phase separation of thermal reaction at 420 °C Photographs of Karamay residue samples at different reaction times: A, B, C, D, E, and F are 0, 20, 40, 60, 80, and 100 min, respectively.

these processes is the length of the coke induction period. At lower temperature (390 °C), the asphaltene content increased before 100 min with no coke forming, which means its coke induction period is longer than 100 min, but at middle temperature (405 °C) and higher temperature (420 °C), coke induction periods were 40 and 20 min, respectively. The phase separation phenomena from microscopic analysis, in which the coke can be seen as small isolated amorphous black points, also confirmed the results. At 390 °C, the oil samples were in homogeneous phase even after reaction for as long as 100 min, but at 405 and 420 °C, neophases emerge at 40 and 20 min, respectively. The coke yields in correlation to reaction time is worth specially explaining. As seen in Figure 4, the coke content had a maximum according to reaction time. The coke content maximum was caused by the on-line sampling apparatus that can only take a liquid sample out of the autoclave. The coke particles were growing up in the reaction system, while the viscosity of oil decreased with the progress of cracking reaction. Most of the coke deposits on the reactor bottom could not be taken out with the liquid any more. Therefore, the coke content in liquid

Phase Separation and Stability of Karamay Residue Oil

Figure 4. Asphaltene and coke contents of Karamay residue along with reaction time during thermal reaction at different reaction temperatures. The dotted lines are changes in asphaltene content at 390 °C (line 1), 405 °C (line 2), and 420 °C (line 3); the solid lines are changes in coke content at 390 °C (line 1′), 405 °C (line 2′), and 420 °C (line 3′).

product increased at the initial period and decreased with the reaction extending. The asphaltene content of residue oil increased in the coke induction period, at the end of which the content reached the maximum. That is to say, the asphaltene content maximum point was consistent with the coking onset point. These phenomena can be explained by the molecular structural changes of resins and asphaltenes and the balance between their contents. In the reaction system, as will be shown below, resins cracked into light molecules and condensed into asphaltenes at the same time.4,5,7 Since resins are the main component to peptize asphaltenes,33 there is a limit of the ability to hold a certain quantity of asphaltenes in a given oil sample. When the asphaltene concentration exceeds this limit, the quantity of asphaltenes is too many to be peptized thoroughly by resins. Thus, the phase separation occurs. The asphaltene formed a neophase, and a second liquid phase as Li7 and Wiehe15 defined appears. In this new phase, the asphaltenes are more prone to condense, that is, the coking reaction begins, which is consistent with Wiehe’s former study.6 What is worth noting is the maximum asphaltene contents are different under three reaction temperatures. The content of lower temperature is larger than that of relative higher temperature. These phenomena can be explained by the molecular structure of resins and asphaltenes at different reaction temperatures, which will be given according to colloidal stability in the following part. 3.2. Colloidal Stability Change in Thermal Reaction. Mass normalized fraction conductivity method can be used to characterize the colloidal stability quantitatively. The curves of CSP against reaction time at three temperatures are showed in Figure 5. It can be seen that the colloidal stability deceased sharply in the coke induction period. At 390 °C, although with no phase separation, the colloidal stability ran down with reaction time extension. At 405 and 420 °C, the colloidal stability was deteriorated rapidly in the initial reaction period. After the coking onset, the CSP declined slowly and leveled off to a rather low value. With larger content of the asphaltene, it is more difficult to be held in the oil, and the colloidal stability (33) Leo´n, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18, 5106–5112.

Energy & Fuels, Vol. 23, 2009 3005

Figure 5. Colloidal stability of Karamay residue along with reaction time during thermal reaction at different reaction temperatures Colloidal stability parameter is the mass ratio of heptane to oil at the asphaltene precipitating point.

is deteriorated. Because the amount of asphaltene is increasing before the coking onset, the CSP decreases accordingly. After the coking onset, the colloidal stability is completely destroyed, with CSP also stabilized at a low value. 3.3. Average Molecular Structure of Resin and Asphaltene. In the colloidal model accepted by most researchers, asphaltenes and resins are accepted as colloid micelles and peptizing agents, respectively. Resins are absorbed on peptized asphaltenes in the dispersing media formed by saturates and aromatics. Zhang31 also studied the colloidal stability variation of petroleum residue during thermal reaction through the mass fraction normalized conductivity method and correlated the colloidal stability with molecular-weight value and the mean dipole-moment value of asphaltenes. The result revealed asphaltenes with larger MW values and more polarity at the end coke-induction period and condensed into coke. Unfortunately, his did not take the molecular character of resin and the alkyl side chain of asphaltene into account. Because resin acts as the dispersing agent for asphaltene, and the alkyl side chain determines the interaction between asphaltene and dispersing medium, and both of them are important to the colloidal stability of residue oil. We determined the average molecular structure of resins and asphaltenes before the coking onset, at the coking onset, and after the coking onset and listed them in Table 2. It is known from Table 2 that the aromatic carbon ratio, condensation index (CI) of asphaltenes, and resins increased in reaction progress. Meanwhile, the molecular weight, naphthenic ring number, and H/C atomic ratio decrease. It can be deduced that resins were condensed and might lose the ability to peptize asphaltene; asphaltenes were more aromatic and apt to condense into coke. Especially after 20 min at 420 °C and after 60 min at 405 °C, H/C and naphthenic ring number of asphaltenes are less than 1, which means asphaltenes are almost aromatic cores. They are very easy to aggregate into coke without the protection of resins. To characterize the length of asphaltenes and resin aliphatic chains, FT-IR is a useful method. Many researchers had correlated different vibration frequencies with the ratio of nCH2/ nCH3 and acquired good linear relationships between them. (34) Liang, W. J.; Que, G.; Chen, Y.; Liu, C. In Asphaltene and Asphalt; Yen, T. F. Chilingarian, G.V., Eds.; Elsevier: New York, 2000; p 287. (35) Liu, D.; Wang, Z.; Zhou, J.; Deng, W.; Liang, S.; Que, G. Prepr. Pap. Am. Chem. Soc., DiV. Petrol. Chem. 2001, 46, 256–258.

3006

Energy & Fuels, Vol. 23, 2009

Wang et al.

Table 2. Structure Parameters of Resins and Asphaltenes at Different Reaction Timea sample

MWb

C wt %

H wt %

H/C

fac

CId

Rte

Caf

Rag

Rnh

390-80 Ri 390-100 R 405-20 R 405-60 R 405-100 R 420-0 R 420-20 R 420-100 R 390-80 Aj 390-100 A 405-20 A 405-60 A 405-100 A 420-0 A 420-20 A 420-100 A

750 920 770 610 410 1090 560 480 1750 1550 1640 1380 1070 1280 1170 1035

85.85 85.83 86.03 85.21 85.64 85.99 85.55 86.85 84.33 84.4 84.71 85.97 86.96 83.34 82.73 85.01

9.85 9.64 9.76 9.63 8.96 9.85 9.06 7.85 7.23 6.95 7.26 6.81 6.33 7.90 6.40 6.23

1.37 1.34 1.35 1.35 1.25 1.37 1.26 1.08 1.02 0.98 1.02 0.94 0.87 1.13 0.92 0.87

0.378 0.399 0.389 0.394 0.469 0.379 0.458 0.600 0.650 0.683 0.649 0.710 0.771 0.565 0.735 0.770

0.255 0.262 0.259 0.259 0.284 0.255 0.280 0.322 0.328 0.336 0.329 0.346 0.361 0.305 0.343 0.356

7.83 9.61 8.14 6.61 5.15 10.97 6.59 6.59 21.17 19.29 20.03 18.08 14.99 14.56 14.81 14.04

20.24 26.25 21.45 17.04 13.72 29.59 18.25 20.84 79.84 74.35 75.11 70.13 59.74 50.17 59.25 56.44

4.56 6.06 4.86 3.76 2.93 6.90 4.06 4.71 19.46 18.09 18.28 17.03 14.44 12.04 14.31 13.61

3.27 3.55 3.28 2.85 2.22 4.07 2.52 1.88 1.71 1.21 1.76 1.05 0.56 2.51 0.50 0.43

a The structure parameters are calculated through density method (RIPP 12-90) recommended by China RIPP. b MW: molecular weight. c fa: aromatic carbon ratio. d CI: condensation index. e Rt: total ring number. f Ca: aromatic carbon number. g Ra: aromatic ring number. h Rn: naphthenic ring number. i 390-80R: resins of oil sample after 80 min at 390 °C, the rest may be deduced by analogy. j 390-80A: Asphaltenes of oil sample after 80 min at 390 °C, the rest may be deduced by analogy.

Table 3. nCH2/nCH3 of Resins and Asphaltenes at Different Reaction Time Calculated via Eq 1 sample

nCH2/nCH3

sample

nCH2/nCH3

sample

nCH2/nCH3

390-0 R 390-20 R 390-40 R 390-60 R 390-80 R 390-100 R 390-0 A 390-20 A 390-40 A 390-60 A 390-80 A 390-100 A

1.8777 1.8534 1.7399 1.5553 1.5401 1.5064 1.7070 1.5816 1.3944 1.3634 1.1670 1.1352

405-0 R 405-20 R 405-40 R 405-60 R 405-80 R 405-100 R 405-0 A 405-20 A 405-40 A 405-60 A 405-80 A 405-100 A

1.7681 1.6051 1.5091 1.4168 1.4013 1.2827 1.6835 1.5106 1.2643 1.1965 1.1511 1.0826

420-0 R 420-20 R 420-40 R 420-60 R 420-80 R 420-100 R 420-0 A 420-20 A 420-40 A 420-60 A 420-80 A 420-100 A

1.5843 1.5204 1.5117 1.6088 1.2880 1.2480 1.5427 1.1856 1.1425 0.9741 0.8430 0.6270

Therefore, nCH2/nCH3 is an effective index to characterize the length of aliphatic side chain quantitatively. Liang34 and Liu35 used relative intensities of the alkane bend deformation vibrational frequencies between 1380 and 1460 cm-1. Coelho,36 Gonzales,37 and Kadim38 used 2920 and 2950 cm-1, as well as 2927 and 2957 cm-1, corresponding to the symmetric and asymmetric stretching frequencies of the methylene and methyl groups, respectively. In this work, eq 1, recommended by Liu et al.,20 is used to calculate the nCH2/nCH3 of resins and asphaltenes. The caculated results are listed in Table 3. nCH2/nCH3 ) 2.93A1460/A1380-3.70

(1)

where nCH2 is the number of methylene, and nCH3 is the number of methyl. A1460 is the IR absorption at 1460 cm-1, and A1380 is the IR absorption at 1460 cm-1. Although the nCH2/nCH3 ratio of resins and asphaltenes decreases in the reaction progress, the nCH2/nCH3 ratio of higher reaction temperature is larger than that of lower reaction temperature with the same reaction time. The interaction between aliphatic chains and oil medium ensures asphaltenes and their absorbed resins dispersing. According to Table 3, when the nCH2/nCH3 ratio is less than 1.2, the coke induction periods ends and coke emerges. From the above results, the coking mechanism can be briefly discussed below. (36) Coelho, R. R.; Hovell, I.; de Mello Monte, M. B.; Middea, A.; de Souza, A. L. Fuel Process. Technol. 2006, 87, 325–333. (37) Buenrostro-Gonzalez, E.; Groenzin, H.; Lira-Galeana, C.; Mullins, O. C. Energy Fuels 2001, 15, 972–978.

The thermal reaction of resins and asphaltenes goes in two opposite directions, which are cracking into little molecules and condensing into big molecules. As for asphaltenes, the cracking reaction causes the dealkylation and breaking up of naphthenic ring, and these fragments change into lighter fractions of saturates, aromatics, and resins, and the aromatic cores condense into coke. As for resins, some condense into asphaltenes and others crack into light fractions. The content balance between asphaltenes and resins is disturbed, which is the essential reason of phase separation and coke formation. Since the alkyl side chains are shortened and the naphthenic rings have ruptured or condensed into aromatic rings, the newly formed asphaltenes are smaller and more aromatic than the original asphaltenes, which are too difficult to be stabilized and apt to separate from the oil system. The newly formed resins also have the structural features like asphaltenes, such as short alkyl side chains and high aromatic carbons ratios and CI. Thus, their ability to peptize asphaltenes is weakened, which is another reason for phase separation and further coke formation. The molecular structures of resins and asphaltenes are important to the colloidal stability in addition to the ratio balance between them. The changes in resins and asphaltenes molecular structures are the essential reason of colloidal deterioration during thermal reaction. The small alkyl molecules formed by broken side chains and naphthenic rings are a disadvantage for colloidal stability, because they act as short-chained alkanes used to precipitation asphaltenes. Comparing the chemical structural parameters of asphaltens and resins at coking onsets at different reaction temperatures, it was found that the parameters are not the same. The CI and fa of lower reaction temperature are smaller than those of higher reaction temperature, while the H/C and nCH2/nCH3 are larger than those of higher reaction temperature. That is to say, the resins and asphaltenes produced under higher temperature were more condensed than those under lower temperature. This also elucidates the larger maximum asphaltene content of lower temperature than that of high temperature, which is the reason that the temperature and time are not mutually compensatory (38) Khadim, M. A.; Sarbar, M. A. J. Pet. Sci. Eng. 1999, 23, 213– 221.

Phase Separation and Stability of Karamay Residue Oil

completely. Cracking reaction at high temperature and short time is more severe than that at low temperature and long time. 4. Conclusions During the thermal reaction of Karamay residue, the asphaltene concentration of liquid products was increasing gradually until reaching the maximum, after which the asphaltene content leveled off to a certain constant. The maximum of asphaltene content was in correspondence with the coke formation onset during thermal reaction. The maximum asphaltene content at lower temperature was larger than that of higher temperature. Macroscopically, the colloidal stability parameter of residue decreased gradually in the coke induction period, and leveled off to a very low value after coking onset. The tendency was consistent with the change of asphaltene concentration.

Energy & Fuels, Vol. 23, 2009 3007

The CI of resins and asphaltenes increased and alkyl side chains were shortened with the reaction progress. The asphaltenes form highly aromatic cores with the loss of side chains, and the peptizing ability of resins becomes weaker with the loss of side chains of resins. These two changes cause the phase separation and further coke formation. The fa, H/C, and nCH2/nCH3 decreased during the reaction and did not level off to certain constant values. The resins and asphaltenes of higher temperature were more condensed than those of lower temperature. As to coke formation reaction, the temperature and time are not mutually compensatory completely. Acknowledgment. The authors thank for the funding of National Natural Science Foundation of China (No. 20506017). EF801149Q