Energy Fuels 2010, 24, 3093–3100 Published on Web 04/23/2010
: DOI:10.1021/ef100172r
)
Hydrogen Transfer and Coking Propensity of Petroleum Residues under Thermal Processing Aijun Guo,*,† Zhiqing Wang,†,‡ Huijun Zhang,†,§ Xuejun Zhang,†, and Zongxian Wang†
)
† State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum, Qingdao 266555, China, ‡Jiujiang Petrochemical Company, Jiujiang 332004, China, §Shengli Oil Field, Sinopec, Dongying 257000, China, and Institute of Daqing Petrochemical Company, Daqing 163714, China
Received February 11, 2010. Revised Manuscript Received April 7, 2010
Anthracene was used as a chemical probe to evaluate hydrogen donating abilities (HDAs) of two petroleum vacuum residues and their SARA fractions (i.e., saturates, aromatics, resins, and asphaltenes), and hydrogen donating kinetics of aromatics and resins were then analyzed. Also, 9,10-dihydroanthracene was used as a chemical probe to evaluate hydrogen accepting abilities of the asphaltenes of the residues. Coking propensities of both residues under thermal processing at 400 °C were evaluated by their coke induction periods. Results show that HDA of either residue proceeds to increase at first and then tends to decline under thermal processing, forming a maximum in the middle. The HDA peak value increases progressively with temperature increasing; the two residues may exhibit either different or similar HDAs at certain test conditions. When the four SARA fractions coexist in the form of a residue for further thermal processing, there exhibits synergism in HDA among the four SARA fractions. Hydrogen donating of aromatics and resins can be treated by first-order kinetics, and both rate constant and initial rate in hydrogen donating for resins show much higher values than those for aromatics. Asphaltenes accept substantially more hydrogens than the amounts they donate. A comprehensive analysis of the data thus obtained shows hydrogen transfer among the SARA fractions is essentially related to coking propensity of residue under thermal processing, where asphaltenes accept hydrogens from resins in the immediate neighborhood that are then supplemented by aromatics. Donatable hydrogens in asphaltenes alone appear insufficient to prevent asphaltenic radicals from combining to form coke. A residue whose asphaltenes accept more hydrogens with resins and aromatics releasing fewer hydrogens exhibits a higher coking propensity under thermal processing. Hydrogen donor/acceptor additives may serve to suppress/promote coke formation by influencing the coking rates of asphaltenes in the center by supplementing/depleting donatable hydrogens of the surrounding medium constituents, that is, resins and aromatics.
can only process relatively good feedstocks (Conradson carbon residue (CCR) less than 10 wt % and metals less than 100-150 ppmw), and residue fluidized catalytic cracking (RFCC) deals with even better ones of lower CCR and less metals.3-5 However, residue thermal processing is always accompanied by premature coking that causes deposit on the inner walls of furnace pipes and results in increased pressure drop, reduced heat transfer rates, hot spot formation due to uneven flow distribution, and corrosion by carbonization, leading to reduced run length, and high maintenance, operating, and utility costs.1,4,5 Even in other processes such as distillation, hydrogenation, RFCC, solvent deasphalting, and heat exchanging, premature coking and fouling are prone to occur during heating up/cooling down their residue feedstocks. Coking propensity has been vastly investigated under residue thermal processing conditions.6-13 Wiehe6 found that
1. Introduction Petroleum is generally getting heavier than ever, and its residue content is therefore increasing with less content of gasoline plus diesel.1-5 Furthermore, this residue tends to contain more heavy metals (such as Ni and V) and carbon residues.2,3 Efficient utilization of every drop of petroleum mainly lies in chemically converting its residue into distillable liquid. Thermal processing is a competitive commercial means to this end, and a major portion (about 63 wt %) of residue upgrading in the world is met by thermal processes.4,5 This is mainly because thermal processes such as delayed coking and visbreaking are considered suitable for feedstocks of varying metal content and carbon residue; residue hydroprocessing *To whom correspondence should be addressed. Telephone: 86-53286984615. Fax: 86-532-86981787. E-mail:
[email protected]. (1) Qu, G. Delayed Coking Technology and Engineering; Sinopec Press: Beijing, China, 2008; pp 1-134. (2) Liang, W. J. Heavy Oil Chemistry; Petroleum University Press: Dongying, China, 2000; pp 10-211. (3) Philips, G.; Liu, F. Advances in Resid Upgradation Technologies Offer Refiners Cost-Effective Options for Zero Fuel Oil Production. In: Proceedings of European Refining Technology Conference, Paris, 2002. (4) Joshi, J. B.; Pandit, A. B.; Kataria, K. L.; Kulkarni, R. P.; Sawarkar, A. N.; Tandon, D.; Ram, Y.; Kumar, M. M. Ind. Eng. Chem. Res. 2008, 47 (23), 8960–8988. (5) Sawarkar, A. N.; Pandit, A. B.; Samant, S. D.; Joshi, J. B. Can. J. Chem. Eng. 2007, 85 (1), 1–24. r 2010 American Chemical Society
(6) Wiehe, I. A. Ind. Eng. Chem. Res. 1993, 32 (10), 2447–2454. (7) Rahimi, P. M.; Gentzis, T. Fuel Process. Technol. 2003, 80 (1), 69–79. (8) Wang, Z. Q.; Wang, Z. X.; Guo, A. J.; Jiang, A. N.; Zhang, H. J. J. Fuel Chem. Technol. 2004, 32 (4), 429–434. (9) (a) Zhang, L.; Yang, G.; Que, G.; Yang, C.; Shan, H. Energy Fuels 2009, 23 (4), 2086–2089. (b) Wang, J.; Li, C.; Zhang, L.; Deng, W.; Que, G. Energy Fuels 2009, 23 (6), 3002–3007. (10) Asomaning, S.; Watkinson, A. P. Heat Trans. Eng. 2000, 21 (3), 10–16. (11) Li, S.; Liu, C.; Que, G.; Liang, W. J. Pet. Sci. Eng. 1999, 22 (1), 37–45.
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coke induction period is a common feature of residue thermal conversion kinetics. He reported that maltene and asphaltene fraction (Asp) had 90 and 0 min induction periods, respectively, whereas their parent residue with Asp content of 25 wt % had a 45 min induction time. Thus, coke formation from Asp is significantly delayed with the aid of maltenes. Rahimi and Gentzis7 further proved that it was hydrogen donation from maltene that inhibited the conversion of Asp to coke the most and distillate plus gas as well. On the other hand, many groups8,9 found that the coke induction period was closely related to colloidal stability of the reacting system, which depended on fraction composition, mean molecular weight, mean dipole moment, and peptizability of resin fraction (Re) upon Asp. The colloidal stability index (CSI) was defined as the mass ratio of added nheptane to residue sample when asphaltene aggregation was triggered based on a mass-fraction-normalized conductivity method. Many other authors considered CSI from the viewpoint of residue’s SARA composition: Re and/or aromatic fraction (Ar) contribute to CSI while saturate fraction (Sa) and Asp undermine CSI.10-12 Considering a reduction in coke yield as an obvious indication of hydrogen transfer, Sanford and Xu13 used coke yields resulted from reaction under a variety of conditions as the main indicator of hydrogen transfer. Sanford and Xu13 investigated the influence of hydrogen donor/acceptor additives on coke formation of Athabasca bitumen under thermal processing. When hydrogen gas or tetralin (good hydrogen donors) were used, coke formation was cut in half (i.e., 4.2 wt %), and the combination of hydrogen gas and tetralin decreased coke formation by one-half again (i.e., 2.0 wt %). In contrast, when ethylene gas (a good hydrogen acceptor) was used, coke formation increased by 67 wt %. Therefore, hydrogen transfer appears critical in controlling initial coking, or coking propensity, of petroleum residue under thermal processing. This study makes an effort to elucidate the functioning scheme on how these hydrogen donor/acceptor additives influence coking propensity of petroleum residue. By quantifying hydrogen donating ability (HDA) and hydrogen accepting ability (HAA) of organic materials, hydrogen transfer has been widely reported to influence the coking characteristics of coal upon carbonization14-21 and coprocessing behavior of the coal-residue slurry.22,23 However, study on HDA and HAA of petroleum residue is
Interestingly, petroleum residue is sparsely documented. recently reported to contain high concentrations of natural hydrogen donors.25 Thus, by using anthracene (ANT) and 9,10-dihydroanthracene (DHA) as chemical probes to evaluate HDA and HAA, respectively,14,18,19 of two petroleum residues and their SARA fractions, this study aims to analyze hydrogen transfer behavior of petroleum residues and reveal its relationship with coking propensity under thermal processing. 2. Experimental Section 2.1. Materials and Characterization. Vacuum residues originating from Liaohe oil field (LH-VR) and Gudao oil field (GDVR) were used as the petroleum residues in this study. These residues were separated into SARA fractions by a similar method in the literature,26 which is briefly described here. First, n-pentane Asp were obtained by filtering off the precipitate from a slurry of residue, benzene, and n-pentane (1:1:40 in g/mL/mL). After evaporating the added solvents from the filtrate, the maltene was obtained. The maltene was then chromatographically separated on a neutral alumina column (with a water content of 1 wt %), sequentially eluting with petroleum ether, benzene, and mixed solvent benzene/ethanol to get Sa, Ar, and Re, respectively. The SARA fractions were evaporated from solvents, dried in a vacuum drying oven, and weighed. Elemental analysis was accomplished by using a Vario EL III elemental analyzer from Elementar Company for contents of carbon, hydrogen, nitrogen, and sulfur. Density of residues was measured with a pycnometer at 20 °C,27 and density or specific gravity of the SARA fractions could be calculated from eq 1 in the literature:28 d 20 4 ¼ 1:4673 - 0:0431 ðH, wt%Þ
ð1Þ
A Knauer K-7000 VPO was used to measure the molecular weights of the residues and fractions.27 Benzil was used as the standard to calibrate the instrument with four concentrations of between 20 and 50 g solute/kg toluene. The test temperature was 80 °C, and the solvent used was toluene (of analytical purity). An estimate of the molecular weight of any sample was repeated three times, and experiment error was minimized. The compositions and properties of the residues and their SARA fractions are listed in Table 1. The elemental analysis data presented in Table 1 are normalized results. 2.2. Evaluation of Hydrogen Transfer Ability. To evaluate HDA and HAA of petroleum residues and their SARA fractions, ANT and DHA were used as the hydrogen accepting probe and the hydrogen donating probe, respectively.14,18,19 Mixtures of ANT (or DHA) and oil sample (1:1, wt/wt, total amount 1.0 g) were loaded into a microautoclave with a nitrogen atmosphere of 3.0 MPa at room temperature. Then, after the reactor was preheated in a tin-bathed heater at 320 °C for 10 min, it was immediately plunged into a second tin-bathed heater to bring about the hydrogen-transfer reaction at an elevated temperature for varying soaking times. The reaction was quenched by immersing the reactor in cold water. The reactor was then opened, and the condensed contents were quantitatively transferred for extraction with chloroform, and then the extracts were analyzed by capillary GC.17-21 The GC analyses were done on a Varian 3400 gas chromatograph equipped with FID. Separations were performed by using a WCOT fused-silica capillary column (30 m 0.25 mm I.D.) coated with CP-SIL 8CB with temperature programmed from 100 to 280 at 15 °C/min, nitrogen as carrier gas
(12) Liu, C.; Zhu, C.; Jin, L.; Shen, R.; Liang, W. Fuel Process. Technol. 1999, 59 (1), 51–67. (13) Sanford, E. C.; Xu, C. M. Can. J. Chem. Eng. 1996, 74 (3), 347–352. (14) Yokono, T.; Marsh, H.; Yokono, M. Fuel 1981, 60 (7), 607–611. (15) Yokono, T.; Obara, T.; Iyama, S.; Sanada, Y. Carbon 1984, 22 (6), 623–624. (16) Yokono, T.; Obara, T.; Sanada, Y.; Shimomura, S.; Imamura, T. Carbon 1986, 24 (1), 29–32. (17) Kidena, K.; Murata, S.; Nomura, M. Energy Fuels 1996, 10 (3), 672–678. (18) Dı´ ez, M. A.; Domı´ nguez, A.; Barriocanal, C.; Alvarez, R.; Blanco, C. G.; Canga, C. S. J. Chromatogr. A 1999, 830 (1), 155–164. (19) Bermejo, J.; Canga, J. S.; Guillen, M. D.; Gayol, O. M.; Blanco, C. G. Fuel Proc. Technol. 1990, 24 (2), 157–162. (20) Menendez, J. A.; Pis, J. J.; Alvarez, R.; Barriocanal, C.; Fuente, E.; Dı´ ez, M. A. Energy Fuels 1996, 10 (6), 1262–1268. (21) Menendez, J. A.; Pis, J. J.; Alvarez, R.; Barriocanal, C.; Canga, C. S.; Dı´ ez, M. A. Energy Fuels 1997, 11 (2), 379–384. (22) Rahimi, P.; Dawson, W. H; Kelly, J. F. Fuel 1991, 70 (1), 95–99. (23) Wang, S. L.; Curtis, C. W. Energy Fuels 1994, 8 (2), 446–454. (24) (a) Uemasu, I.; Kushiyama, S. J. Chromatogr. 1986, 368 (2), 387– 390. (b) Wang, Z.; Guo, A.; Zhang, H.; Que, G. Prepr. Pap. - Am. Chem. Soc., Div. Fuel Chem. 1998, 43 (3), 530–532. (25) Gould, K. A.; Wiehe, I. A. Energy Fuels 2007, 21 (3), 1199–1204.
(26) Yang, C.; Gu, K.; Wu, W. Analytical Methods for Petrochemicals (RIPP Tests); Science Press: Beijing, China, 1990; pp 31-33. (27) Wang, C. G.; Tu, Y. S.; Yang, C. H. Experimental Manual for Petroleum Refining Engineering; Petroleum University Press: Dongying, China, 1997; pp 9-144. (28) Liang, W. J. Petroleum Chemistry; Petroleum University Press: Dongying, China, 1995; pp 15-288.
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Table 1. Compositions and Properties of Different Residues and Their Group Fractions a
sample
content, wt % density @ 20 °C, g/cm3 carbon hydrogen nitrogen sulfur H/C (atomic ratio) MWb (by VPO) nickle, μg/g vanadium, μg/g CCRc, w% viscosity @100 °C, mm2/s
GD-VR
GD-Sa
GD-Ar
GD-Re
GD-Asp
LH-VR
LH-Sa
LH-Ar
LH-Re
LH-Asp
100 0.9756
22.3 0.8548
32.1 0.9880
32.1 1.0290
13.5 1.0760
100 1.0013
18.8 0.8768
26.4 0.9898
36.9 1.0471
17.9 1.0734
84.61 11.22 1.06 3.10 1.58 810 48.0 2.2 15.6 3375
85.79 14.21 0 0 1.97 550
87.21 11.19 1.24 0.36 1.53 990 122.6 2.9 19.0 1710
86.30 13.70 0 0 1.89 540
87.52 11.08 0.84 0.56 1.51 720
87.68 9.75 2.11 0.46 1.32 1300
88.23 9.14 2.26 0.37 1.23 5200
Elemental Analysis, wt % 84.03 85.12 82.75 11.12 10.17 9.08 0.85 1.46 1.52 4.00 3.25 6.65 1.58 1.42 1.31 710 1060 3440
a In naming the sample names, LH and GD before the hyphen denote parent materials sourced from Liaohe oil field and Gudao oil field, respectively, and VR, Sa, Ar, Re, and Asp denote vacuum residue, and its saturate, aromatic, resin, and asphaltene fractions, respectively. b Molecular weight. c Conradson carbon residue.
Figure 1. Plot of HDA of GD-VR against time under thermal processing at a range of temperatures.
achieved by heating at about 8 °C/min from room temperature. Online sampling of 30-45 g of the reactant/product mixture was done at different reaction times. Then the contents of toluene insolubles were analyzed.
(inlet pressure of 13 psi), splitter ratio of 1:65, and injector and detector temperatures of 300 and 330 °C, respectively. Chromatographic quantification was achieved by taking fluorene as an internal standard.18-21 The reaction of oil samples with ANT gave two hydrogenated products, DHA and 1,2,3,4-tetrahydroanthracene (THA), which were identified by comparison of retention time with those of standard compounds and confirmed by GC-MS analysis. The HDA of oil samples was calculated from the amount of DHA and THA formed from ANT and expressed as milligrams of hydrogen per gram of oil, or mg H/g oil. By assuming THA as the product of DHA, the HAA was estimated as the difference between the amount of ANT and the THA formed after thermal treatment.18 The HAA is also expressed as mg H/g oil. The data are the average of at least three chromatographic determinations, and typical errors are within 0.0010 mg H/g oil. 2.3. Test for Coking Propensity. Coking propensities of a petroleum residue were achieved by thermally processing the residue in a high-pressure autoclave and subsequently analyzing the series of yields of coke (i.e., toluene insolubles) formed with time. Three hundred and fifty grams of residue was loaded into the autoclave, which has a reactor volume of 500 mL and also online sampling equipment. The autoclave was purged with nitrogen before reaction, and the initial nitrogen pressure was 7.0 MPa at room temperature. Reaction temperature was
3. Results and Discussion 3.1. HDA of Residue and Its SARA Fractions. Reaction temperature and residence time are important operation parameters for petroleum residue thermal processing. The effects of temperature and time on hydrogen donating abilities of GD-VR and LH-VR are shown in Figures 1 and 2. It is shown that for thermal processing residue at different temperatures, its HDA proceeds to increase at first and then tends to decline, forming a maximum in the middle. This phenomenon may be attributed to an overall reversible reaction between residue and ANT under thermal processing (eq 2): VR
ANT T DHA þ THA
ð2Þ
At the early stage of the reaction, the concentration of ANT’s hydrogenated products (i.e., DHA and THA) is low and the residue is relatively rich in donatable hydrogens, and that of 3095
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Figure 2. Plot of HDA of LH-VR against time under thermal processing at a range of temperatures. Table 2. Time (min) Needed to Reach HDA Peak Values of Both Residues at a Range of Temperatures
Table 3. Comparison of the HDA Peak Values of Both Residues at a Range of Temperatures
temperature (°C)
360
380
410
430
temperature (°C)
360
380
410
430
GD-VR LH-VR
90 60
60 50
15 45
10 40
HDA of GD-VR (mg H/g oil) HDA of LH-VR (mg H/g oil) DDa
0.5591 0.3944 1.42
0.6296 0.4153 1.52
0.6884 0.5606 1.23
0.6997 0.7296 0.96
the free radicals resulted from residue cracking is also low. Thus, DHA and THA are hardly consumed, and the overall reaction moves forward, leading to a rising HDA. At the later stage, however, DHA and THA molecules becomes abundant and the residue is relatively lean in donatable hydrogens, and the number of the free radicals resulting from residue cracking is large. Thus, DHA and THA are evidently consumed, and the overall reaction moves backward, leading to a falling HDA. The HDA peak value occurs just when the reaction is in equilibrium. Therefore, if residue is thermally coprocessed with anthracenestructure-related aromatic solvent, the solvent may be converted to hydroaromatics at an early stage, which is equivalent to “store” the HDA of the residue for later use. With thermal processing going on, these hydroaromatics serve as hydrogen donors and provide the residue fragments with active hydrogens, potentially preventing large aromatic cores from condensing to form coke. It is worth noting that when the upgrading exceeds the time for HDA peak value, concentration of transferable hydrogens dwindles and that of highly aromatic free radicals increases, so coke induction period6 may end during this stage. For the range of temperatures for thermal processing of both residues, the time for HDA to reach peak value is summarized in Table 2. When the reaction temperature is raised from 360 to 430 °C, the time corresponding to HDA peak value dwindles from 90 to 10 min for GD-VR, in contrast to the time dwindling in a narrower window (i.e., from 60 to 40 min) for LH-VR. Thus, with either residue the time corresponding to HDA peak value at a higher temperature is shorter than that at a lower one. This is more sensitive and evident for GD-VR; the sensitivity difference might be attributed to the variations in the SARA composition and heteroatom content of both residues (Table 1).
a DD denotes the ratio of the HDA peak value of GD-VR to that of LH-VR at a certain temperature.
Also, the HDA peak values of both residues for the range of temperatures for thermal processing are summarized in Table 3. It is shown that the HDA peak value increases progressively with temperature increasing for either residue. Furthermore, a parameter DD is defined as the ratio of the HDA peak value of GD-VR to that of LH-VR at a certain temperature. This ratio is high (1.4-1.5) at lower temperatures and tends to decrease to unity at 430 °C, showing that the two residues may exhibit either different or similar HDAs at certain test conditions. It is interesting that Rahimi et al.22 reported that heavy oils/bitumens sampled from different geological locations exhibit similar HDA by using sulfur as the evaluating probe, and Kidena et al.17 showed that coals of various ranks also exhibit similar HDA by using naphthacene as a probe, whereas others14-21 reported that heavy materials such as coal and petroleum cokes and pitches from coal and petroleum exhibit HDA by using ANT as a mild probe at moderate testing conditions. Petroleum residue is a highly complex mixture of organic hydrocarbons possibly with heteroatoms of sulfur and nitrogen, which is generally grouped into several pseudocomponents based on molecular structure similarity, and in-depth study of residue processing frequently involves the chemistry of these pseudocomponents, such as the famous SARA fractions.1-12 Therefore, it is necessary to delve further into the HDAs of the SARA fractions of residues (Table 4). Among the SARA fractions, Ar, Re, and Asp constantly exhibit substantial HDA at low or high temperatures, which is in contrast to Sa (Table 4). It can be noted that all the four SARA fractions donate more hydrogens with temperature 3096
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Table 4. Comparison of the HDA (mg H/g oil) of the SARA Fractions and Their Source Residues GD
LH
material source and test temperature
380 °Ca
430 °Ca
380 °Ca
430 °Ca
Sa Ar Re Asp SARA summationb VR
0.0603 0.3626 0.4879 0.2293 0.3174 0.6296
0.1847 0.4707 0.5585 0.5186 0.4416 0.6997
0.0402 0.3593 0.2826 0.3723 0.2733 0.4153
0.2923 0.7803 0.4893 0.7529 0.5763 0.7296
a Test time was the one where HDA of the corresponding residue reached peak values, as shown in Figures 1 and 2 and in Table 2. b Physical summation is based on the SARA composition as shown in Table 1.
Figure 3. Comparison of the synergism index (SI) of residue at different temperatures. SI is the ratio of HDA of a residue to the physical summation of the HDAs of its SARA fractions.
increasing. In comparison with other fractions, Sa hardly donates any hydrogens at a low temperature of 380 °C. GDRe has a greater HDA than LH-Re at either temperature. If the HDA of the SARA fractions is summed up based on the SARA composition as shown in Table 1, it can be seen that the summation HDA is significantly lower than the corresponding HDA of VR. This means that when the SARA fractions are mixed to be a residue for further thermal processing, there exhibits synergism in HDA among the four SARA fractions. The underlying mechanisms leading to this synergism are probably multiple. When the SARA fractions coexist, the reactive free radicals from the pyrolysis of Re and Asp, whose thermal reactivity are relatively high,6 come close to any C-H bonds of naphthenes and even alkanes,23,29 leading to the loosening and activation of these bonds. Thus, the hydrogens on the activated C-H bonds may be reactive enough for addition to probe ANT molecules, showing additional HDA. Alternatively, H2S can be released from Ar, Re, and Asp. Thus, through free radical pathway, such as H2 S þ ANT f DHA þ THA þ HS 3
Figure 4. Plot of HDA of GD-Ar and GD-Re against time under thermal processing temperatures.
ð3Þ
GD-Re against time under thermal processing temperatures. It is clear that at a certain temperature, HDA of Ar rises fast first and then less fast until its relaxation to an equilibrium maximum, whereas HDA of Re rises fast first to a peak value and then drops a little to a constant level. The maximum HDA is greater for Ar than for Re at a set temperature, which is probably due to higher H/C atomic ratio and more naphthenic rings for Ar. For short reaction times, HDA of either fraction increases with temperature, which is similar to that of the residues (Figures 1 and 2). In the presence of probe ANT, hydrogen donating reactions of Ar and Re at the HDA rising stage may be simplified as
HS 3 þ alkaneðcycloalkaneÞ f alkeneðcycloalkene plus aromatic hydrocarbonÞ þ H2 S ð4Þ H2S is able to shuttle hydrogens in Sa (alkanes plus cyclalkanes) and even in the side chain of the other fractions onto ANT, again showing additional HDA. Now synergism index (SI) is defined as the ratio of HDA of a residue to the physical summation of the HDAs of its SARA fractions, and SI of both residues is shown in Figure 3. It is clear that SI for the SARA fractions of GD-VR is greater than that for the LH-VR counterparts at either temperature, and this is in agreement with the evidence of more Sa and more sulfur contained in GD-VR than LH-VR (Table 1). On the other hand, SI decreases with temperature increasing. This can be attributed to the more thermal activation of both ANT and Sa or the other fractions at higher temperatures, causing hydrogen transfer reaction between ANT and potential species to be more complete. 3.2. Hydrogen Donating Kinetics of Residue Ar and Re. Ar and Re are the principal fractions of both residues (Table 1) and they also exhibit remarkable HDA at different temperatures (Table 4). Therefore, these two fractions of one of the residues were used as representatives to study hydrogen donating kinetics. Figure 4 shows HDA of GD-Ar and
ANT
Arðor ReÞ sf hydrogen atoms þ Ar0 ðor Re0 Þ
ð5Þ
where Ar0 and Re0 are the derivative products of Ar and Re after active hydrogens are donated to probe ANT, respectively. In this way, the donatable hydrogens stored in Ar and Re can be donated or released progressively. This process is attempted to be treated by first-order kinetics, thus dc ¼ kc dt On integration from time 0 to t, it becomes c0 ln ¼ kt c
ð6Þ
ð7Þ
Where c0 (mg H/g oil) is the initial concentration of donatable hydrogens in fraction Ar or Re at a certain temperature, c (mg H/g oil) is the concentration at time t (min), and k (min-1) is the reaction rate constant. Because HDA remains fairly high
(29) Clark, J. W.; Rantell, T. D.; Snape, C. E. Fuel 1984, 63 (10), 1476–1478.
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Table 5. Arrhenius Parameters for Hydrogen Donating and Initial Hydrogen Donating Rate (r0a) from Ar and Re to ANT material
temperature (°C)
k (min-1)
R
Ea (kJ/mol)
A (min-1)
r0a (mg H/g oil/min)
GD-Ar GD-Ar
360 380
0.0017 ( 0.0001 0.0043 ( 0.0001
0.9769 0.9958
159
2.43 1010
0.0015 0.0039
GD-Re GD-Re
360 380
0.0053 ( 0.0005 0.0101 ( 0.0009
0.9887 0.9879
110
7.33 106
0.0040 0.0082
a
r0 = k c0.
in the later reaction stage (Figure 4), the highest HDA value throughout the reaction at any temperature can be taken as c0, and thus c = c0 - HDA. The Arrhenius parameters for hydrogen donating from Ar and Re are therefore calculated and summarized in Table 5. Initial hydrogen donating rate (r0, mg H/g oil/min) can further be calculated by multiplifying k by c0, and the values of r0 are also listed in Table 5. It can be seen from Table 5 that the regression coefficients for obtaining k are high and therefore first-order kinetics treatment is reasonable. Both k and r0 for Re show much higher values than those for Ar at either temperature, and activation energy for Re is accordingly much lower. Therefore, Re donates hydrogens much faster than Ar does. Furthermore, rising temperature causes hydrogen donating reactions to be even faster for both Ar and Re. 3.3. HAA of Asphaltenes. From Table 1 it is shown that the Asp fraction has the lowest hydrogen content among the SARA fractions of either residue. Asp molecules are widely reported to be coke precursors1,2,6,9 and their low-hydrogencontent moiety will transform into coke when a petroleum residue is thermally processed, although prominent HDA has been detected for Asp (Table 4). On the other hand, coke formation is a dehydrogenation process. If enough hydrogen atoms (i.e., hydrogen radicals) are provided for capping the aromatic radicals from Asp, as in residue hydroprocessing, these radicals will be hard to recombine and thus less or even no coke will form. Therefore, a study of HAA of Asp is also specially warranted here (Figure 5). Here, test temperatures were 380 and 430 °C, and the test times were 60 and 10 min, respectively. For comparison purpose, these test conditions of temperature and time were the ones where HDA of GDAsp (Table 4) was evaluated. It is shown in Figure 5 that both Asp fractions accept substantially more hydrogens than the amounts they donate (Table 4) under thermal processing. This means that Asp molecules thermally rupture easily and give a high concentration of free radicals. Or, the average number of the fused aromatic rings of Asp might be at least four. These Asp may be the case because Kidena et al.17 found that napthacene, a compound of four fused aromatic rings, abstracted hydrogens much more easily than ANT. High concentration of highly aromatic free radicals will make it easy for recombination reaction to occur to form coke. Furthermore, Asp HAA varies much with both residue source and test conditions. At identical test conditions, LH-Asp accepts more hydrogens from DHA than does GD-Asp. It is interesting that although HDA of Asp increases with temperature (Table 4), HAA of Asp decreases with temperature. 3.4. Relationship between Hydrogen Transfer and Coking Propensity. Coking propensity of residue under thermal processing has been reported to be measured by the length of coke induction period, which can be obtained by microscopic9,30 or gravimetric6,9 analysis of coke formed (such as toluene insolubes, solid product, etc). In this study, a gravimetric analysis of coke (i.e., toluene insolubles) formation at 400 °C is presen-
Figure 5. Hydrogen accepting ability (HAA) of GD-Asp and LHAsp measured under thermal processing temperatures.
ted in Figure 6. Because coke formation onset point is usually recognized as 0.1 wt % of coke,6,9 coke induction periods at 400 °C for LH-VR and GD-VR are 95 and 115 min, respectively. Thus, LH-VR is more prone to coking than GD-VR under thermal processing. Since coking is chemically the growth of aromatic cores through condensation reaction,6,7,31 hydrogen transfer may play a large part in the coking propensity of petroleum residue under thermal processing. It is well-known that Asp molecules are the precursors of coke. If the prominent HAA of Asp is taken to be an indication of potentially high concentration of highly aromatic radicals, then a model involving hydrogen transfer among residue SARA fractions is postulated to rationalize coking propensity. Initial coking index (ICI) of a residue is therefore attempted to be defined as HAA of Asp ðAsp wt%Þ ICI ¼ P ðHDA of X ðX wt%ÞÞ
ð8Þ
where X denotes one or more of the SARA fractions, or even the residue itself. Larger ICI indicates the corresponding residue has a higher coking propensity with a relative short coke induction period. The ICI values thus obtained for both residues at different processing temperatures are summarized in Table 6. As examples, when considering solely Asp fraction whose HDA contributes to ICI, the calculations based on eq 8 for deriving the ICI values at 380 °C of GD-VR and LH-VR are presented in eqs 9 and 10, respectively: ICI ¼ ICI ¼
1:7223 ð13:5%Þ 0:2325 ¼ ¼ 7:51 0:2293 ð13:5%Þ 0:03096
1:9609 ð17:9%Þ 0:3510 ¼ ¼ 5:27 0:3723 ð17:9%Þ 0:06664
ð9Þ ð10Þ
(30) Rahimi, P.; Gentzis, T.; Dawson, W. H.; Fairbridge, C.; Khulbe, C.; Chung, K.; Nowlan, V.; DelBianco, A. Energy Fuels 1998, 12 (5), 1020–1030. (31) Magaril, R. Z.; Ramazeava, L. F.; Aksenora, E. I. Int. Chem. Eng. 1971, 11 (2), 250–251.
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Figure 6. Plot of initial coking of residues against time under thermal processing at 400 °C. Table 6. Initial Coking Index (ICIa) of Residue Based on Hydrogen Transfer Ability ICI at 380 °C
Figure 7. The microemulsion/colloidal structure of petroleum residue, containing solutes, As (asphaltenes), dispersants, Re (resins), solvents, Ar (aromatics), and nonsolvents, Sa (saturates).
ICI at 430 °C
row number
material whose HDAb contributes to ICI
GD-VR
LH-VR
GD-VR
LH-VR
I II III IV V VI VII
Asp Ar Re Ar, Re Ar, Re, and Asp Sa, Ar, Re, and Asp VR
7.51 2.00 1.48 0.85 0.76 0.73 0.37
5.27 3.70 3.37 1.76 1.32 1.28 0.85
2.60 1.20 1.01 0.55 0.45 0.41 0.26
2.04 1.33 1.52 0.71 0.53 0.48 0.38
Asp molecules are coated with Re multilayers32-34 or partly interact with Re molecules,35,36 and thus statistically Asp molecules are at the center with Re molecules surrounding, which are then dispersed in the oil medium (i.e., Ar plus Sa). A prevalent presentation of the physical structure is shown in Figure 7.4,5,37 Furthermore, it is shown that Re molecules donate hydrogens much faster than Ar molecules do through hydrogen donating kinetics analysis (Tables 5). So, Re molecules mainly donate hydrogens to Asp free radicals directly and Ar supplements hydrogens to Re. In this way, Ar protects Re from deteriorating too fast by donating hydrogen. When Asp cannot accept enough hydrogens timely, chance of condensation reactions leading to coking increases. For inhibiting coking of Asp, if the Re fraction is called the primary “fighting” agent, the Ar fraction is then the secondary. Sanford and Xu13 showed that hydrogen donor agents— tetralin and hydrogen gas—suppressed coke formation, whereas a hydrogen accepting agent—ethylene gas—promoted coke formation. These agents may thus supplement or deplete donatable hydrogens of Ar and Re, thus influencing the coking rates of Asp in the center. Data in rows II∼VII (Table 6) also shows that the ICIs measured for both residues at a lower temperature differ greatly, although they differ much less when measured at a higher temperature. This indicates that coke induction periods for different residues at lower temperatures are obtained more precisely than at higher temperatures. Taking a relatively low temperature of 400 °C to measure the coke induction periods in this study and in the literature6,31 is therefore reasonable. Interestingly, if HAA of Asp fractions from various residues is approximated to be identical (cf., Figure 5), and HDA of both Ar and Re fractions is also approximated to be identical (cf., Table 4), then a simplified ICI parameter can be defined:
a ICI is attempted to be defined by eq 8. HAA data for calculation are from Figure 5. b HDA at either temperature was measured at the time where HDA of the corresponding residue reached peak values, as shown in Figures 1 and 2 and Table 2, and the HDA values are listed in Table 4.
The ICI data in Table 6 can be grouped into two groups based on the relative extent of the ICIs of the two residues. Row I falls into the first group: ICI for GD-VR is greater than that for LH-VR, where Asp alone is used as the hydrogen donating material in deriving the ICI values. In contrast, rows II∼VII fall into the second group: ICI for GD-VR is lower than that for LH-VR, where Ar and/or Re play a very important part in the composition of the hydrogen donating material in deriving the ICI values. Data in the first group is in contradiction with the coking propensity of the residues shown in Figure 6, whereas data in the second group is in good agreement with that coking propensity. Thus, HDA of Asp possibly plays at most a minor part in coke formation inhibition; the hydrogens provided by Asp itself are insufficient to effectively cap the asphaltenic free radicals to delay coke formation. Thermal coke induction period of Asp is usually reported to be zero.6,31 Hydrogen donation for ICI calculation in rows III∼VII is based on Re and possibly other fractions, and the relative ICI extent explains coking propensities of the residues. Therefore, hydrogen donation of Re appears to be the main factor controlling coking propensity, and that of the other fractions is solely conducive to the control. Although the ICI based on hydrogen donation of Ar is also in agreement with coking propensity, Ar is considered to be supplementary to Re in controlling coking propensity. In crude oil and its residue, Asp and Re are at least partly immediate neighbors because
ICI ¼
Asp wt% Ar wt% þ Re wt%
ð11Þ
(34) Speight, J. G. Oil Gas Sci. Technol. 2004, 59 (5), 467–477. (35) Mullins, O. C. Energy Fuels 2010, DOI: 10.1021/ef900975e. (36) Graham, B. F.; May, E. F.; Trengove, R. D. Energy Fuels 2008, 22 (2), 1093–1099. (37) Wiehe, I. A.; Liang, K. S. Fluid Phase Equilib. 1996, 117 (1,2), 201–210.
(32) Le on, O.; Contreras, E.; Rogel, E.; Dambakli, G.; Acevedo, S.; Carbognani, L.; Espidel, J. Langmuir 2002, 18 (13), 5106–5112. (33) Gutierrez, L. B.; Ranaudo, M. A.; Mendez, B.; Acevedo, S. Energy Fuels 2001, 15 (3), 624–628.
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Table 7. Initial Coking Index (ICIa) of Residue based on SARA Composition material
ICI
GD-VR LH-VR
0.211 0.283
Ar, Re, and Asp constantly exhibit substantial HDA at the low or high temperatures, which is in contrast to Sa. All four SARA fractions donate more hydrogens with temperature increasing. GD-Re has constantly greater HDA than LH-Re at the temperatures tested. Furthermore, when the SARA fractions coexist in the form of a residue for further thermal processing, there exhibits synergism in HDA among the four SARA fractions, showing a higher HDA value than expected from the summation based on SARA composition; this synergism decreases with increasing temperature. In comparison, both Asp fractions accept substantially more hydrogens than the amounts they donate under thermal processing. Hydrogen donating of Ar and Re can be treated by firstorder kinetics. Both rate constant and initial rate in hydrogen donating for Re show much higher values than those for Ar, and activation energy for Re is accordingly much lower. On the basis of the hydrogen transfer studied above and also the SARA compositions of residues, ICI was defined. Hydrogen donation of Re appears to be the main factor controlling coking propensity, and that of the other fractions may be solely conducive to the control. Specifically, Ar mainly provides Re with a donatable hydrogen pool in controlling coking propensity, and Asp plays possibly at most a minor part in the control. A relatively low temperature of about 400 °C to measure the coke induction periods is recommended. If hydrogen transfer abilities of the SARA fractions of different residues are assumed to be similar, ICI may be simplified as (Asp wt %)/(Ar wt % þ Re wt %), suggesting that both hydrogen transfer among the SARA fractions and the physical phase stability of the reacting system cogovern coking propensity. In addition, hydrogen donor/acceptor additives may serve to suppress/promote coke formation by influencing the coking rates of asphaltenes in the center by supplementing/depleting donatable hydrogens of the surrounding medium constituents, i.e., resins and aromatics.
a ICI is defined as the ratio of Asp content to the total contents of Ar and Re.
The ICI values thus solely based on SARA composition are presented in Table 7. These values (0.211 and 0.283 for GDVR and LH-VR, respectively) are also in good agreement with coking propensity in the form of coking induction periods (115 min and 95 min for GD-VR and LH-VR, respectively, cf., Figure 6). So the simplified ICI, which is solely based on SARA composition, is both easily available and useful for predicting coking propensity. Liu et al.12 treated (Asp wt %)/(Ar wt % þ Re wt %) of 40 reacted residues as an instability measure, and they found that it was in loosely direct proportion to coke yields for 30 of the residues under thermal processing. In this way they concluded that coke formation also depended on the phase separation of asphaltenes from the colloidal system of residues in thermal processes. Asomaning et al.10 also successfully used similar instability measure to correlate coking rates inside heat exchangers. Therefore, coking propensity of petroleum residues under thermal processing appears to be the result of both hydrogen transfer among the SARA fractions and the physical phase stability of the reacting system. 4. Conclusions Under thermal processing conditions, HDAs of GD-VR and LH-VR proceed to increase at first and then tends to decline, forming a maximum in the middle. When reaction temperature is raised from 360 to 430 °C, the time corresponding to HDA peak value dwindles from 90 to 10 min for GDVR, in contrast to the time dwindling in a narrower window (i.e., from 60 to 40 min) for LH-VR. The HDA peak value increases progressively with temperature increasing for either residue; the two residues may exhibit either different or similar HDAs at certain test conditions. Among the SARA fractions,
Acknowledgment. The authors thank both PetroChina (grant numbers W2008 E-1502/02, W2008 E-1503/01, and W2008 E-1503/03) and the Fundamental Research Funds for the Central Universities (grant number 09CX04055A) for partial financial supports.
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