Energy & Fuels 2006, 20, 1157-1160
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Extraction of Tumuji Oil Sand with Sub- and Supercritical Water Meng Meng, Haoquan Hu,* Qiumin Zhang, and Ming Ding State Key Laboratory of Fine Chemicals, Institute of Coal Chemical Engineering, Dalian UniVersity of Technology, 129 Street, Dalian 116012, People’s Republic of China ReceiVed December 15, 2005. ReVised Manuscript ReceiVed February 13, 2006
Tumuji oil sand from Inner Mongolia was subjected to sub- and supercritical water extraction on a semicontinuous apparatus. The experiments were carried out to investigate the effect of temperature on extract yield, formation rate, and product components at different pressures. The results indicated that extract yield increases with the increase of pressure and a maximum extract yield of 81.1 wt % could be obtained at 30 MPa. The formation rate of extract has a maximum with the variation of temperature. With increase of pressure from 20 to 30 MPa, the maximum extract formation rate increases from 32.99 × 10-5 to 58.37 × 10-5 s-1 and the temperature corresponding to the maximum extract formation rate moves from 412 to 390 °C. The extract was fractionated into oil, asphaltene, and preasphaltene, and the oil was further fractionated into saturates, aromatics, and resin. The results show that formation rates of saturates, aromatics, and resin all have a maximum with the variation of temperature. The temperature corresponding to the maximum formation rates of saturates and aromatics decreases, while that of resin increases with increase of pressure. The gas yield is quite low, and the main gas components are CO2, CH4, and H2.
Introduction With the decrease of conventional crude oil reserves and increase of crude oil price, oil sand has gradually been considered as an inexpensive substitute of crude oil. Oil sand primarily consists of quartz sand, clay, water, and heavy bitumen with a little amount of metals. At present, some processing methods may be applied to recover bitumen from the sand, for example, organic solvent extraction, hot water extraction, cold water extraction, vacuum pyrolysis, and hydrogenation coking, etc.1-7 China is rich in oil sand reserve, and some oil sand mines have already been explored in Inner Mongolia, Xinjiang, Sichuan, and Qinghai provinces. Supercritical fluid is an ideal solvent in extraction processes because it combines the advantages of both gas and liquid as solvent.8 Near the critical point, the properties of supercritical fluids are strong functions of temperature and pressure.9 Isothermal extraction has deserved a lot of attention from investigators. Many experiments with respect to supercritical isothermal extraction of oil sand have been done so far. The solvents used in experiments included ethane, propane, water, and carbon dioxide.10-15 However, as compared to isothermal * Corresponding author. Tel./Fax: +86-411-88993966. E-mail: hhu@ chem.dlut.edu.cn. (1) Kirkbride, C. G.; Doyle, J. A.; Hildebrandt, F. U.S. Patent, 116,319,395, 2001. (2) Pakel, H. S.; Roy, C. Energy Fuels 2003, 17, 1145-1152. (3) Qi, D.; Keng, H. C. Fuel 1996, 75, 220-226. (4) Ahmad, N.; Williams, P. T. The 1996 ICHEME Research Event/ Second European Conference for Yong Researchers, 1996; pp 259-261. (5) Duyvesteyn, W. P. C.; Budden, J. R.; Picavet, M. A. U.S. Patent, 5,968,349, 1999. (6) Li, S. Y.; Wang, J. Q.; Tan, H. P.; Wu, Z. L. Fuel 1995, 74, 11911193. (7) John, V.; Deo, M. D.; Hanson, F. V. Fuel 1995, 74, 311-316. (8) Taylor, L. T. Supercritical Extraction; Wiley: New York, 1996. (9) McHugh, M.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth: Stoneham, MA, 1994. (10) Hu, H. Q.; Guo, S. C.; Hedden, K. Fuel Process. Technol. 1998, 53, 269-277.
method, nonisothermal technique can give more detailed information, such as the gas, liquid yield, and composition of the products variation with temperature in an individual run, which proves that it is an efficient way to study the mechanism of oil sand extraction. Meanwhile, sub- and supercritical water extraction has caught more attention because of its interesting physical and physicochemical properties. Some studies on supercritical water extraction in nonisothermal have been carried out, for instance, extraction of oil shale and lignite with supercritical water.10,16 This paper mainly reports the effects of temperature and pressure on extract yield, extract formation rate, and component of extraction products through a nonisothermal extraction experiment of oil sand with sub- and supercritical water. Experimental Section Materials. The oil sand sample studied in this work was from Inner Mongolia, and its analysis data are listed in Table 1. TG and DTG. To estimate the pyrolysis behavior of oil sand, an experiment was carried out in a thermogravimetric analyzer (Mettler Toledo TGA/SDTA851e). About 10 mg of sample was placed in a ceramic crucible and pyrolyzed under 30 mL/min N2 flow at a heating rate of 5 K/min from 25 to 600 °C. Apparatus and Procedures of Extraction. The extraction experiments were carried out in a semi-continuous apparatus.10,16 In each experiment, about 80 g of oil sand sample was charged to a fixed bed reactor that could be heated by an electrical oven outside. The solvent was pumped into an electrical heater and then (11) Rose, J. L.; Monnery, W. D.; Chong, K.; Svrcek, W. Y. Fuel 2001, 80, 1101-1110. (12) Walter, E. R.; Tejraj, M. A. Energy Fuels 2000, 14, 464-475. (13) Subramanian, M.; Hanson, F. V. Fuel Process. Technol. 1998, 55, 35-53. (14) Kamimura, H.; Takahashi, S.; Kishita, A.; Moriya, T. Fuel Chem. 1998, 43, 741-745. (15) Ayhan, D. Pet. Sci. Technol. 2000, 18, 771-781. (16) Hu, H. Q.; Zhang, J.; Guo, S. C.; Chen, G. H. Fuel 1999, 78, 645651.
10.1021/ef050418o CCC: $33.50 © 2006 American Chemical Society Published on Web 03/21/2006
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Table 1. Analyses of Oil Sand Sample proximate analysis, wt % moisturea ash volatile matter fixed carbon bitumenb ultimate analysis, wt % daf C H N S+Oc H/C fraction of oil sand bitumen, wt % saturates aromatics resin asphaltened preasphaltenee
1.55 84.62 13.51 0.32 13.55 79.16 10.65 0.35 9.84 1.61 55.81 16.68 12.86 6.96 7.69
Figure 1. TG and DTG of oil sand sample.
a Measured according to Dean-Stark. b THF solubles in oil sand obtained by Soxlet extraction with THF. c By difference. d Cyclohexane insolubles but toluene solubles. e Toluene insoluble but THF solubles.
to the reactor by a high-pressure-metering pump. The reactor system and the entering pressurized solvent were slowly heated from room temperature to 500 °C. The liquid solvent flow rate of 6.5 mL/ min, the heating rate of 4 K/min, and the pressure were held constant during extraction. At different temperatures, liquid samples were collected with the help of tetrahydrofuran (THF) to dissolve the extract products stuck on the wall of the condenser after depressurization in time, and gas products were collected in a gasbag. Clay brought out from the oil sand in liquid sample was eliminated using a centrifugal machine, and then water and THF were removed from the liquid sample in a rotary evaporator to obtain the extract sample. Subsequently, the extract was separated by Soxlet extraction with cyclohexane, toluene, and THF into oil (cyclohexane solubles), asphaltene (toluene solubles but cyclohexane insolubles), and preasphaltene (THF solubles but toluene insolubles). The oil was further separated into saturates, aromatics, and resin with a gel chromatograph using a refluxing solvent of cyclohexane, toluene, and a mixture of toluene and ethanol, respectively. From the weight of extract and extract fractions, the extract yield, the extract fraction content, and the extract formation rate at different temperatures can be determined. The formation rate of extract or extract fractions at temperature Tj was calculated as the ratio of the amount of extract or its fractions in a time interval to the initial oil sand weight, which can be expressed with the following formula: FRi(Tj) )
∆wi W(t2 - t1)
where FRi(Tj) is the formation rate of i at temperature Tj, i represents extract, saturates, aromatics, or resin in this work; Tj is the average temperature of the temperatures at time t1 and t2; ∆wi is the mass of extract or its fractions obtained in a time interval between t1 and t2; and W is the initial oil sand mass, in dry and ash-free (daf) basis. The bitumen in oil sand was separated by Soxlet extraction with THF as solvent and was determined as the percentage of THF solubles in oil sand. The fractions of bitumen in the oil sand sample were also separated with the same procedures as for the extract. The composition of gas product was analyzed using gas chromatography (model 7890 with thermal conductive detector).
Results and Discussion Pyrolysis of Oil Sand. Figure 1 shows the TG and DTG curves of oil sand obtained with a thermogravimetric apparatus. It can be seen that the pyrolysis of oil sand could be divided into two steps. The first step is at below 350 °C where water and a relatively light organic substance volatilize. The second step is at a temperature between 350 and 500 °C where a
Figure 2. Relations between formation rate of extract, oil fractions, and temperature at 30 MPa.
relatively heavy organic substance cracks. A deep peak appears at 450 °C. According to the result of TG and DTG, 500 °C was chosen as the ending temperature of sub- and supercritical water extraction. Effect of Extraction Temperature. Figure 2 shows the formation rates of extract and its fractions as a function of extraction temperature at 30 MPa. The extraction vessel was heated from room temperature; only a little extract flows out until the temperature is above 210 °C. Practically, extraction mainly takes place in the temperature range of 350-465 °C. With the increase of temperature, the extract formation rate increases and reaches a peak value at about 390 °C, and then decreases with the further increase of temperature. The phenomenon is the result of two opposite effects of temperature on extraction. At low temperature, little organic substance can be extracted because of the low solubility of water to relatively heavy organic substances in oil sand for water’s high dielectric constant at low temperature.17 With an increase of temperature, the dielectric constant of water increases and its solubility to heavy organic substances in oil sand increases too. Meanwhile, large molecular substances in oil sand decompose to form smaller molecular substances. All of these make the extract formation rate increase. On the other hand, with a further increase of temperature, the density of water will decrease and its extraction capacity for bitumen from oil sand decreases. At about 390 °C, the opposite effects of temperature on extract formation rate balance each other, and consequently a peak value is obtained. The formation rate of extract fractions follows a trend similar to that of the extract and has a peak value at different temperatures; see Figure 2. Saturates and aromatics are formed at a relatively lower temperature with a maximum (17) Heger, K.; Uematsu, M.; Franck, E. U. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 758-762.
Extraction of Tumuji Oil Sand with Water
Energy & Fuels, Vol. 20, No. 3, 2006 1159
Table 2. Gas Composition from Different Extraction Temperature Ranges at Different Pressuresa pressure, MPa
temperature range, °C
H2
20
100-374 374-450 450-500 100-374 374-450 450-500 100-374 374-450 450-500
7.56 19.43 21.59 4.78 17.32 26.78 3.28 16.46 28.34
25 30
gas composition, vol % CO CO2 CH4 C2H6 9.16 8.53 7.11 14.02 10.78 6.20 14.61 12.53 5.50
76.00 40.11 20.40 81.20 36.21 14.34 82.11 33.30 13.77
C2H4
7.28 25.72 33.75
2.11 5.32
4.10 11.83
27.89 36.98
2.98 4.87
4.82 10.83
28.38 38.04
2.53 3.70
6.80 10.65
a Fixed extraction conditions: oil sand weight 80 g; water flow rate 6.5 mL/min; heating rate 4 K/min; final temperature 500 °C.
Figure 3. Relations between extract formation rate and temperature at different pressures.
Table 3. Results of Tumuji Oil Sand Extraction with Water at Different Pressuresa pressure, MPa extract yield, wt % daf saturates, wt % aromatics, wt % resin, wt % asphaltene, wt % preasphaltene, wt % gas yield, 10-2 mL/g H2, vol % CO, vol % CO2, vol % CH4, vol % C2H6, vol % C2H4, vol %
20
25
30
73.82 72.00 12.86 12.00 1.38 1.76 4.85 13.88 7.65 45.48 23.44 3.00 6.55
76.21 66.34 16.23 13.77 1.60 2.06 4.48 18.72 9.92 39.42 24.32 2.36 5.26
81.10 63.51 20.45 11.60 1.79 2.65 4.38 18.88 9.46 37.12 25.72 2.41 6.41
a Fixed extraction conditions: oil sand weight 80 g; water flow rate 6.5 mL/min; heating rate 4 K/min; final temperature 500 °C.
peak at about 390 °C, but resin is formed at a relatively higher temperature with a peak at 412 °C. Table 2 shows the gas composition from different temperature ranges at different pressures. It can be seen that under experimental pressure neither C2H6 nor C2H4 appears before 374 °C. With the increase of temperature, the gas is mainly from the decomposition of oil sand, and more H2, CH4, and C2 can be formed, so the volume percentages of H2, CH4, C2H6, and C2H4 increase while the volume percentages of CO and CO2 decrease. Effect of Extraction Pressure. Pressure is an important parameter in the extraction process. It affects the solubility of substances in the supercritical fluid. In this work, experiments were carried out at different pressures (20, 25, and 30 MPa). The extraction results are presented in Table 3. The extract yield increases from 73.8 to 81.1 wt % daf with pressure variation from 20 to 30 MPa. With increase of pressure, more asphaltene and preasphaltene will be formed, and the percentage of saturates in extract decreases while that of aromatics increases, and the resin changes a little. The gas yield is very low, and the highest gas yield is only 0.0485 mL/g. Pressure has little effect on the gas yield. The volume percentages of H2 and CH4 in gas increase, while that of CO2 decreases with an increase of pressure. Figure 3 presents the relationship between the extract formation rate and temperature at different pressures. It can be observed that the general features of the extract formation rate are similar at different pressures, but the locations of the peak value are different. The temperature corresponding to the maximum formation rate of extract increases from 390 to 412 °C, and the maximum formation rate of extract decreases from 58.37 × 10-5 to 32.99 × 10-5 s-1 with the pressure decrease
Figure 4. Relations between formation rate of oil fractions and temperature at different pressures.
from 30 to 20 MPa. This indicates that the extraction could be carried out at lower temperature under higher pressure. In other words, to extract the substances at lower pressure, the larger molecular substances must be decomposed to form smaller molecular substances at a relatively higher temperature. Figure 4 shows the relations between formation rates of extract fractions and temperature at different pressures. It can be seen that with an experimental pressure increase, the peaks of saturates and aromatics formation rates move toward the low-temperature region, but the peak of resin, which is relatively heavy, formation rate shifts toward the high-temperature region. The higher is
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Table 4. Analyses of Liquid Extraction Products from Different Temperature Rangesa temperature, °C 325-375 375-400 400-425 425-450 450-475 pressure, 20 MPa fractions of extract, wt % saturates 72.46 64.43 aromatics 11.90 17.25 resin 12.44 15.27 asphaltene 1.51 1.43 preasphaltene 1.69 1.62 ultimate analysis of extract, wt % daf C 81.05 80.69 H 11.62 11.30 N 0.37 0.32 O+Sb 6.96 7.69 H/C 1.72 1.68 pressure, 25 MPa fractions of extract, wt % saturates 58.10 64.50 aromatics 20.63 17.27 resin 18.36 15.29 asphaltene 1.10 1.26 preasphaltene 1.81 1.68 ultimate analysis of extract, wt % daf C 80.84 80.82 H 11.25 11.61 N 0.29 0.28 O+Sb 7.62 7.29 H/C 1.67 1.72 pressure, 30 MPa fractions of extract, wt % saturates 56.25 64.61 aromatics 20.33 23.91 resin 18.49 7.28 asphaltene 2.14 1.82 preasphaltene 2.79 2.38 ultimate analysis of extract, wt % daf C 81.84 81.17 H 11.25 11.70 N 0.22 0.28 b O+S 6.69 6.85 H/C 1.65 1.73
73.96 12.68 8.86 1.76 2.74
79.43 9.02 8.78 1.14 1.63
72.16 12.45 13.94 0.65 0.80
79.94 11.60 0.28 8.18 1.74
80.32 11.78 0.32 7.58 1.76
81.45 11.54 0.24 6.77 1.70
65.48 15.86 14.66 1.82 2.18
67.90 14.84 12.59 2.12 2.55
75.13 13.22 7.53 1.84 2.28
80.50 11.61 0.27 7.62 1.73
80.45 11.80 0.32 7.43 1.76
79.35 11.77 0.23 8.65 1.78
56.54 23.34 15.80 2.03 2.29
71.08 15.29 9.82 1.69 2.12
73.26 16.14 6.57 1.86 2.17
81.01 11.61 0.27 7.11 1.72
80.88 11.88 0.27 6.97 1.76
81.47 12.01 0.31 6.21 1.77
a Fixed extraction conditions: oil sand weight 80 g; water flow rate 6.5 mL/min; heating rate 4 K/min. b By difference.
the operation pressure, the bigger are the maximum formation rates of the three fractions. These prove that high pressure avails supercritical water extraction of oil sand. Analyses of Extract. During the experiment, the fractions and their ultimate analyses of extracts obtained at different temperature ranges under different pressures were made. Table 4 presents the analysis results of the extracts. The following
features can be summarized from the results: (a) In comparison with Table 1, the percentages of saturates in extracts at different temperature ranges are higher than that in oil sand, but the percentages of asphaltene and preasphaltene are lower than that in oil sand. (b) The contents of C and H in extracts from different temperature ranges are higher than those in the oil sand sample. (c) Extracts have a higher H/C ratio than the original sample. The first feature proves that saturates are relatively easy to extract using water. As for the latter two features, it can be understood that these extracts contain more relatively hydrogenrich components and less relatively heavy components than bitumen in oil sand. Generally, oil sand has properties similar to oil shale, but the extract yield and extract properties are different if we compare the results in this work with those from oil shale extracted by supercritical water previously.16 For the oil sand, an extract yield of about 81 wt % daf can be obtained at 30 MPa, and the main extract fraction is oil, about 96 wt % in extract; while for oil shale, the highest extract yield is only about 57 wt % daf obtained at 30 MPa and 400 °C, and the oil fraction in extract is only 50 wt %.16 These results indicated that the recovery of oil with supercritical water extraction from oil sand is much easier and more feasible than that from oil shale. Conclusions From the experiment results and discussion, it can be concluded that a high liquid extract yield of oil sand can be obtained by using sub- and supercritical water extraction. With increase of pressure, the extract yield increases. At 30 MPa, the extract yield reaches 81.1 wt % daf. The formation rates of extract and its fractions all increase to a maximum and then decrease with the increase of temperature. The temperature corresponding to the maximum of extract formation rate moves to lower temperature region with the increase of operation pressure. The variation of saturates and aromatics formation rates with pressure is similar to that of the extract, but the resin is opposite. The pressure affects the extract composition. The percentage of saturates in extract at low pressure (20 MPa) is higher than that at relatively high pressure (25 MPa, 30 MPa). The hydrogen-rich components are extracted during extraction of oil sand with water at the experimental conditions studied. The sub- and supercritical water extraction of oil sand produces little gas that is mainly composed of CO2, CH4, and H2. Both C2H6 and C2H4 do not appear until the temperature is above 374 °C. EF050418O
