3178
Energy & Fuels 2009, 23, 3178–3183
Effects of Supercritical Water in Vacuum Residue Upgrading Zhen-Min Cheng,* Yong Ding, Li-Qun Zhao, Pei-Qing Yuan, and Wei-Kang Yuan State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, People’s Republic of China ReceiVed February 23, 2009. ReVised Manuscript ReceiVed May 3, 2009
Vacuum residue (VR) upgrading was conducted in the environment of supercritical water (SCW) without oxygen addition in an attempt to yield a maximum of light oil. Simulated distillation of the liquid products from a set of orthogonal experiments shows that temperature should not be too high to restrict coke formation, and the most beneficial condition is found at (1) 420 °C for the temperature, (2) 0.15 g/cm3 for the water density, (3) 2 g/g for the H2O/oil ratio, and (4) 1 h for the reaction time. A simultaneous increase of the water density and H2O/oil would significantly improve the cracking behavior and the yield in light oil. Scattered coke particles between 10 and 100 µm were generated from VR cracking, which suggests the dispersion effect of SCW. The infrared spectrum analysis has indicated an increase in the H/C atomic ratio in the liquid product, which implies that hydrogen is generated from the condensation reactions rather than from water because no oxygen-containing group was detected.
1. Introduction Vacuum residue (VR) is the heavy fraction remaining in the crude oil after vacuum distillation, which is normally at 400-420 °C. VR normally constitutes 25-50% of the crude oil by weight; therefore, its effective conversion into light components is extremely important in view of the limited global oil storage. However, VR is hard to be refined because of its coarse quality. It has a high density of 0.93-1.0 g cm-3, a large molecular weight of 800-1000 g mol-1, and a low H/C atomic ratio of 1.4:1. Moreover, the contents of heteroatoms and metals can be extremely high; e.g., sulfur is as high as 5.4 wt %, nitrogen is about 1.0 wt %, and vanadium and nickel are in the range of 245 and 75 ppm, respectively. To obtain light oils, the large molecules, such as asphaltene and resin, should be cracked into small ones under severe conditions, such as fluid catalytic cracking, coking, and hydrocracking.1 It should be noted that VR is a heterogeneous system, which is characterized by the existence of asphaltene micelles dispersed in the paraffin and aromatics solution. Because asphaltene is typically defined as the component that is insoluble in n-heptane and soluble in toluene, asphaltene is normally associated with resins as a nuclei by static electrical attraction and the resin molecules are dispersed by aromatics and paraffin. Under the high temperatures of cracking conditions, the stability of the asphaltene micelle will be lost because of the cracking of the resins into aromatics, olefins, and alkanes. Because the asphaltene has a high weightaveraged molecular weight between 3200 and 36002 and the molecular polarity between asphaltene and paraffin is much different, the asphaltene tends to precipitate from the solution * To whom correspondence should be addressed. Telephone: +86-2164253529. Fax: +86-21-64253528. E-mail:
[email protected]. (1) Usuia, K.; Kidenaa, K.; Murataa, S.; Nomuraa, M.; Trisunaryantib, W. Catalytic hydrocracking of petroleum-derived asphaltenes by transition metal-loaded zeolite catalysts. Fuel 2004, 83, 1899–1906. (2) Acevedo, S.; Gutierrez, L. B.; Negrin, G.; Pereira, J. C. Molecular weight of petroleum asphaltenes: A comparison between mass spectrometry and vapor pressure osmometry. Energy Fuels 2005, 19, 1548–1560.
and deposit on the reactor wall or the catalyst surface, as demonstrated from a series of specialized research.3-5 To prevent the precipitation of asphaltene at the relatively high cracking temperature of over 400 °C, it is crucial to find a solvent to disperse VR, and supercritical water (SCW) should be a reasonable choice. It has been known that SCW, whose critical point is 374 °C and 22.1 MPa, has a dielectric constant approaching that of acetone at room temperature, and it has been found SCW has an excellent solubility for small organic molecules and good miscible behavior with many large organic compounds.6-10 Because of its special property being as a solvent for organics and a catalyst for acid or basic reaction, sub- and SCW has been considered as an efficient reaction medium in organic synthesis, fuel processing, biomass conversion, and hydrogen production in recent years.11-13 (3) Murgich, J.; Rodriguez, J. M.; Aray, Y. Molecular recognization and molecular mechanics of micelles of some model asphaltenes and resins. Energy Fuels 1996, 10, 68–76. (4) Fotland, P. Precipitation of asphaltenes at high pressuressExperimental technique and results. Fuel Sci. Technol. Int. 1996, 14, 313–325. (5) Deng, W.; Mu, B.; Que, G. Thermal reaction behavior of resins from Shengli vacuum residue in different dispersion medium. Acta Pet. Sin. 1998, 14, 6–10. (6) Siskin, M.; Katritzky, A. R. Reactivity of organic compounds in superheated water: General background. Chem. ReV. 2001, 101, 825–835. (7) Kruse, A.; Dinjus, E. Hot compressed water as reaction medium and reactant properties and synthesis reactions. J. Supercrit. Fluids 2007, 39, 362–380. (8) Griswold, J.; Kasch, J. E. Hydrocarbon-water solubilities at elevated temperatures and pressures. Ind. Eng. Chem. 1942, 34, 804–806. (9) Connolly, J. F. Solubility of hydrocarbons in water near the critical solution temperatures. J. Chem. Eng. Data 1966, 11 (1), 13–16. (10) Sutton, C.; Calder, J. A. Solubility of high-molecular weight n-paraffins in distilled water and seawater. EnViron. Sci. Technol. 1974, 8, 654–657. (11) Elliott, D. C. Catalytic hydrothermal gasification of biomass. Biofuels, Bioprod. Biorefin. 2008, 2, 254–265. (12) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Froling, M.; Antal, M. J.; Tester, J. W. Thermochemical biofuel production in hydrothermal media: A review of sub- and supercritical water technologies. Energy EnViron. Sci. 2008, 1, 32–65. (13) Savage, P. E. A perspective on catalysis in sub- and supercritical water. J. Supercrit. Fluids 2009, 47, 407–414.
10.1021/ef900132z CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
Effects of SCW in VR Upgrading
However, the mechanism for hydrocarbon cracking in the presence of water has been a topic of controversy for a long time. In many pyrolysis systems, such as bitumen hydrous pyrolysis, kerogen decomposition to bitumen, aquathermolysis of heavy oils, liquefaction of coal, and upgrading of extra heavy crude oil,14-21 water has been hypothesized to participate in the reaction and supply hydrogen to the system. In such processes, water is essential in the maturation reactions, which makes the aromatization reactions to the formation of cross-linked insoluble residue inhibited. However, the hydrogen donation mechanism of water was not supported by experiments with model compounds including the hydrous pyrolysis of n-C32H66,22 decomposition of tert-butylbenzene,23 and polyethylene conversion.24 The hydrous pyrolysis of n-C32H66 demonstrated that no detectable direct deuterium exchange occurs between D2O and organic aliphatic hydrogen via a radical pathway, which means that the only pathway for the deuterium exchange between water and hydrocarbons is by an ionic rather than a free-radical mechanism. In the decomposition of tert-butylbenzene in SCW, thermal decomposition of hydrocarbons in SCW was found to follow the free-radical mechanism predominantly, in which process no hydrogen donation and no specific solvent effect of water were found; instead, water was only acting as a hydrogentransfer medium. The solvent effect was evidenced from n-C16 and polyethylene conversion in SCW because it was found that the pyrolysis rate and the product distribution of n-C16 in SCW were almost the same as that in an argon atmosphere; however, PE pyrolysis results with and without SCW were clearly different. Because of the special physicochemical property of water, upgrading of heavy oil in the presence of hot and compressed water or steam has been an important subject to the academic and industrial community. Catalytic effects of some mineral components in the oil reservoir on the aquathermolysis of the heavy oils under steam injection conditions have been observed to accelerate the breakdown of organosulfur components presented in the heavy oil,25-27 which has increased the generation of saturated and aromatic compounds. Meanwhile, (14) Lewan, M. D. Experiments on the role of water in petroleum formation. Geochim. Cosmochim. Acta 1997, 61, 3691–3723. (15) Stalker, L.; Farrimond, P.; Larter, S. R. Water as an oxygen source for the production of oxygenated compounds (including CO, presursors) during kerogen maturation. Org. Geochem. 1994, 22, 477–486. (16) Schlepp, L.; Elie, M.; Landais, P.; Romero, M. A. Pyrolysis of asphalt in the presence and absence of water. Fuel Proc. Technol. 2001, 74, 107–123. (17) Sato, T.; Adschiri, T.; Arai, K.; Rempel, G. L.; Ng, F. T. T. Upgrading of asphalt with and without partial oxidation in supercritical water. Fuel 2003, 82, 1231–1239. (18) Ogunsola, O. M.; Berkowitz, N. Removal of heterocyclic S and N from oil precursors by supercritical water. Fuel 1995, 74, 1485–1490. (19) Ovalles, C.; Hamana, A.; Rojas, I.; Bolivar, R. A. Upgrading of extraheavy crude oil by direct use of methane in the presence of water. Fuel 1995, 74, 1162–1168. (20) Ovallesa, C.; Filgueirasa, E.; Moralesa, A.; Scottb, C. E.; GonzalezGimenezc, F.; Embaidc, B. P. Use of a dispersed iron catalyst for upgrading extra-heavy crude oil using methane as source of hydrogen. Fuel 2003, 82, 887–892. (21) Amestica, L. A.; Wolf, E. E. Catalytic liquefaction of coal with supercritical water CO solvent media. Fuel 1986, 65, 1226–1232. (22) Leif, R. N.; Simoneit, B. R. T. The role of alkenes produced during hydrous pyrolysis of a shale. Org. Geochem. 2000, 31, 1189–1208. (23) Ederer, H. J.; Kruse, A.; Mas, C.; Ebert, K. H. Modelling of the pyrolysis of tert-butylbenzene in supercritical water. J. Supercrit. Fluids 1999, 15, 191–204. (24) Watanabe, M.; Hirakoso, H.; Sawamoto, S.; Adschiri, T.; Arai, K. Polyethylene conversion in supercritical water. J. Supercrit. Fluids 1998, 13, 247–252. (25) Clark, P. D.; Hyne, J. B.; Tyrer, J. D. Chemistry of organosulfur compound type occurring in heavy oil sands. 1. High-temperature hydrolysis and thermolysis of tetrahydrothiophene in relation to steam stimulation processes. Fuel 1983, 62, 959–962.
Energy & Fuels, Vol. 23, 2009 3179
the average molecular weight and viscosity of heavy oils were reduced. Although aqueous thermolysis at a temperature of less than 250 °C is significant in improving the flowing property of the crude oil, the long reaction time and low degree in cracking of the heavy molecule are not acceptable in upgrading of VR in considering its much different and refractory property. Unfortunately, the reports on the upgrading of heavy oil or VR in SCW are not sufficient in providing the essential experimental and theoretical understanding for this process. In the report by Zhao et al.28 for VR upgrading in SCW, it shows that the viscosity of VR was lowered from 1160 to 6.2 mPa s by 179 times, the average molecular weight was reduced from 1860 to 646 g mol-1, and the contents of the heteroatoms S, N, Ni, and V were reduced by 32, 15, 83, and 85%, respectively. However, almost all of the upgrading runs were carried out in the temperature range of 380-460 °C and at a fixed condition of 25.0 MPa and H2O/VR of 0.78:1 by weight. Recently, Kokubo et al.29 carried out heavy oil upgrading in an autoclave apparatus under 25 MPa from 400 to 470 °C with and without a supply of SCW. Results of their experiments showed that gas and reformed oil was selectively recovered by SCW at a rate proportional to temperature. In their experiments, only the dependence of heavy oil cracking on temperature was under investigation. The status of the current investigation on VR cracking in SCW demonstrates that it is required to have more detailed research to provide a better understanding on the experimental factors and more specifically on the contribution of water to the reaction. 2. Experimental Section The experiments were carried out in two different reactors. One is a stirred autoclave (PARR 4571, 1000 mL, 500 °C, and 35 MPa), which is to give a precise measurement of pressure and temperature and also to provide enough capacity for the product collection. Another is a stainless-steel bomb reactor, constructed of 316 L stainless steel with 11.5 mm inner diameter and a capacity of 12.2 mL, being immerged in a molten salt bath. The bomb reactor is only used in higher temperature experiments for the sake of safety. For the bomb reactor, the heating up time is considered to be within a few minutes, because this reactor was immerged in the molten salt bath with very good thermal conductivity. For the autoclave reactor, the heating up process was following a declined heating rate procedure. For example, to reach a reaction temperature at 420 °C, the heating rate was decreased from 200 to 40 °C h-1 and a total of 180 min was required. The feedstock VR was supplied from Shanghai Petroleum and Petrochemicals Co., Limited. Both reactors were carried out in the batch mode under vibration or agitation. In all of the experiments, the reactants, including VR and deoxygenated water, were first loaded into the reactor and then the reactor was purged by nitrogen 3 times before the reactor was closed. When a preset reaction time was reached, the bomb reactor was quenched in a water bath and the product was collected by washing with n-heptane for component analysis. In the autoclave run, when the reaction time was reached, the reaction mixture was cooled to the ambient temperature within 10 min by strong coolant (26) Fan, H.; Zhang, Y.; Liu, Y. The catalytic effects of minerals on aquathermolysis of heavy oils. Fuel 2004, 83, 2035–2039. (27) Katritzky, A. R.; Murugan, R.; Balasubramanian, M.; Siskin, M.; Brons, G. Aqueous high temperature chemistry of carbo- and hetero-cycles. Part 16. Model sulfur compounds study of hydrogen sulfide generation. Energy Fuels 1991, 5, 823. (28) Zhao, L. Q.; Cheng, Z. M.; Ding, Y.; Yuan, P. Q.; Lu, S. X.; Yuan, W. K. Experimental study on vacuum residuum upgrading through pyrolysis in supercritical water. Energy Fuels 2006, 20, 2067–2071. (29) Kokubo, S.; Nishida, K.; Hayashi, A.; Takahashi, H.; Yokota, O.; Inag, S. Effective demetallization and suppression of coke formation using supercritical water technology for heavy oil upgrading. J. Jpn. Pet. Inst. 2008, 51, 309–314.
3180
Energy & Fuels, Vol. 23, 2009
Cheng et al.
Figure 1. Analysis procedure for VR and the upgraded product.
injection. The suitable temperatures for the autoclave and the bomb reactor were, respectively, 380-420 and 420-460 °C. To make sure the conditions in both reactors are under SCW conditions, the water density should be above 0.208 g cm-3 for the temperature between 380 and 400 °C or 0.113 g cm-3 for the temperature above 400 °C. The contents of saturates, aromatics, resins, and asphaltenes (SARA) in the feedstock and products were measured by the standard analysis procedure illustrated in Figure 1. The determination of viscosity was conducted in a capillary tube viscometer, and the gel permeation chromatography (GPC) method was for the average molecular-weight measurement. The total sulfur content analysis was by the conventional tubular furnace method, and nitrogen content was measured by the element analysis instrument (Elementar vario EL III, Germany). Both Ni and V contents were measured by atomic absorption spectrometry (AAS IRIS 1000). The components in the raw VR and the product are classified into three groups: maltene, asphaltene, and coke. They are respectively defined as the class of hydrocarbons soluble in n-heptane (HS), the class of hydrocarbons insoluble in n-heptane and soluble in toluene (HI-TS), and the class of hydrocarbons insoluble in toluene (TI). Furthermore, the maltene is divided into saturates, aromatics, and resin through washing with different solvents in an activated Al2O3 chromatographic column. As shown in Figure 1, the VR or the upgraded product was first dissolved in n-heptane prior to the chromatographic solvent washing. Asphaltenes and coke were separated by dissolving the n-heptane-insoluble precipitant in toluene. Following the procedure described in Figure 1, it shows that the fraction of coke in VR is 4.30 wt % and the fraction of liquid is 95.7 wt %. In the liquid fraction, the SARA components of saturates, aromatics, resin, and asphaltene are, respectively, 21.2, 42.6, 21.4, and 14.8% by weight.
3. Results and Discussion The cracking of VR is subjected to a number of factors, and the most important one will be the temperature, because initiation of the cracking reaction requires the breaking of the C-C bond, which has an activation energy of around 60-80 kcal mol-1. After the initiation step, the radical reaction is propagated through (1) a β-scission reaction of the alkyl radical with an activation energy of 30 kcal/mol, with an olefin generated in this reaction, and (2) H-abstraction reactions between an alkyl radical and a paraffin, with an activation energy of 12-16 kcal/mol by abstracting one hydrogen from the paraffin to the alkyl radical. The termination is a reaction between two radicals with an activation energy of 0.30,31 Because (30) Ford, T. J. Liquid phase thermal decomposition of hexadecane: Reaction mechanisms. Ind. Eng. Chem. Fundam. 1986, 25, 240–243. (31) Khorasheh, F.; Gray, M. R. High-pressure thermal cracking of n-hexadecane in tetralin. Energy Fuels 1993, 7, 960–967.
Figure 2. SARA contents in products of vacuum residuum pyrolysis in SCW.
the H-abstraction reaction has a lower activation energy than the β-scission reaction, its reaction rate will be much faster than the β-scission reaction and lead to olefins and small molecules; therefore, equimolar distribution of n-alkanes and low selectivity to olefins and gases will be obtained. 3.1. Influence of Temperature. The influence of the temperature is first investigated in the high-temperature range from 420 to 460 °C, and the bomb reactor was employed. Figure 2 gives the product distribution for VR cracking at 420 and 460 °C under a fixed H2O/VR ratio of 2:1 and a water density of 0.15 g/cm3. Although in these experiments the pressure in the reactor was not recorded, it could be estimated to be 25-27 MPa under 0.15 g/cm3 of water density, which is greater than 22.1 MPa of the critical pressure of water. From Figure 2, it shows the reaction was very rapid in the first 5 min at both temperatures, although they are different by 40 °C. After 5 min, the product composition only changed gradually as a result of the completion of the decomposition of the reactants at 60 min. The primary difference between the two experiments on the content of the saturates fraction is obvious, e.g., from about 75 wt % at 420 °C to 85 wt % at 460 °C. From the tendency of the profiles, it is realized that the increase in the saturates fraction is corresponding to the decrease of the other three components, especially the aromatics. However, this does not mean the increase in saturates is fully from the decomposition of alkylaromatics because the mechanism is more complicated than it was imagined. The total reaction would involve the decomposition of asphaltenes and resins into saturates and aromatics and the simultaneous condensation of polynuclear aromatics into coke and release of hydrogen. Because no oxygen-containing compound was detected, the extra hydrogen from the increased amount of saturated paraffin can only come from the hydrogen released
Effects of SCW in VR Upgrading
Energy & Fuels, Vol. 23, 2009 3181
Table 1. Amount of Coke Generated in the Process of VR Pyrolysis (wt %) H2O/VR ) 2:1 (wt), water density ) 0.15 g/cm3 reaction time (min)
reaction temperature (°C)
0
5
15
30
60
120
420 460
4.3 4.3
1.33 1.77
2.22 2.45
2.31 6.99
2.66 7.77
3.64 12.36
Table 2. Orthogonal Experimental Results of VR Upgrading run temperature water density H2O/oil time weight loss e350 °C number (°C) (g/cm3) (g/g) (h) (wt %) 1 2 3 4 5 6 7 8 9 optimal
k1 k2 k3 R
380 380 380 400 400 400 420 420 420 420
22.77 42.87 68.32 45.55
0.1 0.15 0.2 0.1 0.15 0.2 0.1 0.15 0.2 0.15
2:1 3:1 4:1 3:1 4:1 2:1 4:1 2:1 3:1 4:1
44.73 47.44 41.79 5.65
0.4 0.7 1 1 0.4 0.7 0.7 1 0.4 1
18.93 20.13 29.24 41.88 45.19 41.54 73.37 77.01 54.58 83.33
45.83 38.86 49.27 10.41
Figure 3. Simulated distillation profiles of VR and the pyrolysis products according to experiments in Table 2.
39.57 45.01 49.38 9.81
from condensation reactions. To provide related information on this mechanism, the production of coke was measured, as shown in Table 1. It shows from Table 1 that the amount of coke decreased in the first 5 min, possibly because of its dissolution into water and decomposition into small fragments. At 5 min later, the coke amount increased monotonically with time because of the coking reaction at high temperatures. It should be noted that, in comparison to the production of the saturates, the amount of coke produced at 420 °C is relatively low but it seems too much to be accepted at 460 °C. Therefore, to obtain as much liquid product as possible, 420 °C can be regarded as a reasonable temperature. 3.2. Factor Analysis and Optimal Condition. It has been shown that the temperature should not be too high to keep the coke formation to the lowest level and 420 °C was a reasonable temperature. It should be noted that, in addition to temperature, there are still some other important factors, including the water density, the water/oil ratio, and the reaction time. Therefore, there are totally four factors to be studied together. Assuming that each factor has three levels to be investigated, the total experimental run number will be 64. To reduce the experimental duty and simultaneously identify the contribution of every factor, orthogonal experiment design was employed preferably. For the experiments in this work with four factors at three levels, a Table of L9(34) was used and the total amount of experimental number was reduced from 64 to 9. In Table 2, the quality of the product was evaluated by simulated distillation expressed in terms of weight loss before 350 °C. Obviously, the more the weight loss, the more liquid lighter than 350 °C is obtained. From the weight loss, it shows that the reaction at 420 °C is much superior to those at 380 and 400 °C. The importance of the four factors are discriminated
Figure 4. FTIR spectrum of VR and the cracking products.
from the maximum difference analysis expressed by R in the first column on the last line, which suggests: temperature > H2O/ oil > time > water density. As shown in Table 2, the values given for ki (i ) 1, 2, and 3) were obtained, which indicate the average of the total contributions of a specific factor under all of the experimental combinations at level i. For example, the k1 value of 22.77 for the temperature was obtained from the average of the total contributions at level 1 of 380 °C, which are 18.93, 20.13, and 29.24. In this regard, the optimal level of each factor could be obtained from the biggest one among k1, k2, and k3, and they are determined to be (1) 420 °C for the temperature, (2) 0.15 g/cm3 for the water density, (3) 4 g/g for the H2O/oil ratio, and (4) 1 h for the reaction time. Under the optimal condition, the VR upgraded oil could reach a weight loss of 83.33 wt %. To illustrate the boiling point range of VR and its pyrolysis products under different conditions, the corresponding simulated distillation profiles were shown in Figure 3, where it is clear to find the different effect because of operating conditions. It shows that the gas oil fraction (boiling point below 350 °C) in the feedstock VR is only 6.96 wt %; as a comparison, it was increased to 83.33 wt % at the optimal cracking conditions in SCW. Table 3 compares the property of this particular VR and the liquid product after upgrading in SCW. It shows that, after the reaction, the density of VR was reduced from 0.993 to 0.899
Table 3. Properties of VR and the Product after VR Upgrading in SCWa SARA (wt %)
elemental analysis of SARA (wt %)
sample
F420 (g/cm3)
Mn/Mwb
dynamic viscosity (mm2/s)
S
Ar
R
As
C
H
S
N
H/C (mol)
VR product
0.9930 0.8993
1700/1860 595/646
1116 (80 °C) 6.2 (50 °C)
21.2 40.1
42.6 30.0
21.4 24.8
14.8 5.1
84.79 85.94
10.42 10.99
2.30 1.08
1.112 0.93
1.475:1 1.535:1
a Reacting conditions: 420 °C, 25 MPa, 1 h, water/oil ) 2, and water density ) 0.15 g/cm3. weight-averaged molecular weight.
b
Mn/Mw ) number-average molecular weight/
3182
Energy & Fuels, Vol. 23, 2009
Cheng et al.
Figure 5. SEM images of coke generated from VR pyrolysis in SCW. Table 4. Coke Produced from VR Pyrolysis in SCW and Nitrogen coke formation (wt %)
reaction time (min)
0
5
15
30
60
120
in SCWa innitrogen
4.3 4.3
1.33 4.57
2.22 6.09
2.31 6.63
2.66 8.65
3.64 14.9
a Conditions: 420 °C, H O/VR ) 2:1 (wt), and water density ) 0.15 2 g/cm3.
g/cm3, the weight-averaged molecular weight was from 1860 to 646 g/mol, and the kinematic viscosity was from 1116 to 6.215 mm2/s. It is interesting to find from the elements analysis that the atomic ratio of H/C has increased by 4% from 1.475:1 to 1.535:1. 3.3. Effect of Water. As shown in the above paragraph, cracking of VR in the environment of SCW has brought about a high yield in light oil with only a small coke yield of less than 3 wt % at a high conversion in 1 h. Besides, the H/C atomic ratio has increased by 4%. Therefore, it is desirable to know what the effect of water is during the VR cracking. 3.3.1. SolVent Effect. Table 2 shows that both water density and the H2O/oil ratio are affecting the VR cracking; however, their functions may be different to a certain degree. As a matter of fact, water density is related to its vapor pressure and solubility; i.e., increasing the water density will increase the number of water molecules per volume, and as a result, more oil molecules will be interacted with water and the solubility is increased. On the other hand, the H2O/oil ratio is a measure of the proportion of the molecules of the two reactants; therefore, increasing this ratio is also favorable to the dissolving of oil, which will prevent the phase separation. A simultaneous increase of the water density and H2O/oil will inevitably improve the cracking behavior, which is in agreement with the water density effect on lignin gasification over supported noble metal catalysts in SCW.32,33 Nevertheless, it may not be economic if too much water is used. The solvent effect is clearly evidenced by comparing experiment number 8 and the optimal one; both are under the same conditions, except for the different H2O/oil ratios. It is found that, by increasing the H2O/oil ratio from 2:1 to 4:1, the yield of light oil with a boiling point lower than 350 °C was increased from 77 to 83 wt %. To better understand the role of water in this reaction, infrared spectra of the raw and product are provided in Figure 4. It shows that the carboxyl group (CdO) in VR and the product was not detected at its typical absorption peak of 1700 cm-1. The absence of CdO in VR could be explained from the decom(32) Osada, M.; Sato, O.; Watanabe, M.; Arai, K.; Shirai, M. Water density effect on lignin gasification over supported noble metal catalysts in supercritical water. Energy Fuels 2006, 20, 930–935. (33) Penninger, J. M. L.; Rep, M. Reforming of aqueous wood pyrolysis condensate in supercritical water. Int. J. Hydrogen Energy 2006, 31, 1597– 1606.
position of this group under the high temperature of vacuum distillation, although thermal decomposition of carboxylic acids, ketones, aldehydes, and esters require extended times at temperatures above 400 °C. Because CdO was not detected, its decomposition into CO and the subsequent reaction of CO with water to give hydrogen through the water-shift reaction will not exist, and therefore, the condensation reaction through coke formation will be the only source of hydrogen. 3.3.2. Dispersion Effect. Scanning electric microscopy (SEM) was used to characterize the coke morphology generated in SCW cracking. As shown in Figure 5, the shape of the coke is irregular, with its size distributed in the range from 10 to 100 µm. A closer observation under a scope of magnification by 2000 times shows that the coke particle has a porous structure being composed of holes of 5-10 µm in diameter. The scattered small coke particle and the coke porous structure indicate the dispersion effect of water, which means, despite the fact that SCW is an excellent solvent for many organic molecules, it is impossible to dissolve asphaltenes and resins completely. Therefore, the small coke particles are the consequence of the condensation within the individual coke precursor droplets, which are dispersed in the SCW. More evidence on the dispersion effect of SCW is shown in Table 4 through a comparison of coke formation from VR pyrolysis in SCW and nitrogen. Obviously, the coke formation in nitrogen increased with time monotonically, besides the amount of coke was several times of that in SCW. The difference for the above result could be explained from the mechanism of coking, which is generally a second-order free-radical reaction between two molecules, whereas the polymer cracking, such as polyethylene, is a first-order thermal free-radical reaction.24 In the environment of nitrogen, the asphaltene, which is in the form of the condensed phase, will have a high concentration and there will be no mass-transfer resistance because the reaction is homogeneous. On the contrary, when the reaction is in SCW, the asphaltene will be partially dissolved and dispersed as an emulsion; this will not only reduce the asphaltene concentration for coking, but also the coking reaction will be slowed down because of the mass-transfer resistance between different emulsion droplets. 4. Conclusion Petroleum residue is a colloidal suspension system, which is unstable at high cracking temperatures after initial cracking reactions of alkylaromatics because of the separation of the asphaltenes from the bulk phase. The separated asphaltenes will easily convert into coke and thus reduce the production of light oil and deposit on the reactor wall or catalyst surface. However, a solution to this problem is not well-developed by the present industrial technology. In this regard, SCW was proposed as a novel medium, which not only provides a high light oil (boiling
Effects of SCW in VR Upgrading
point lower than 350 °C) yield of 83.3 wt % but also a low coke production of 3.64 wt %. The role of water in VR cracking was preliminarily discussed. Hydrogen donation from water was not confirmed because CdO is not present in the VR molecule; therefore, the effect of water is mainly in the physical meaning. From a series of experimental work conducted, it shows SCW is a unique cracking medium which bears the following advantages: (1) The solvent effect. Because SCW has a high diffusivity coefficient and a large solubility to many organic molecules, the asphaltene cracking product (light oil) can easily dissolve and diffuse into the supercritical fluid phase; therefore, more liquid product is produced, and the reaction rate is increased.
Energy & Fuels, Vol. 23, 2009 3183
(2) The dispersion effect. In view of its large molecular structure, asphaltene can only partially dissolve in SCW, which constitutes highly dispersed micro-emulsions. The formation of emulsion has decreased the rate of coking, which follows a second-order reaction rate equation under its much reduced concentration. On the other hand, SCW has increased the light oil production in view of the large specific surface area of the emulsion, which is favorable to mass transfer. Acknowledgment. This work was carried out under support from the Shanghai Scientific and Technical Committee on Fundamental Research Projects (03JC14024 and 07DJ14002) and partial support from the Natural Science Foundation of China (20876043). EF900132Z
