Energy Fuels 2010, 24, 1844–1847 Published on Web 01/25/2010
: DOI:10.1021/ef901376j
Study of the Characteristics and Kinetics of Oil Sand Pyrolysis Yue Ma and Shuyuan Li* State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China Received November 17, 2009. Revised Manuscript Received January 5, 2010
The pyrolysis experiments on Buton oil sand, Indonesia, were carried out using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) at different heating rates of 15, 20, 25, and 30 °C min-1. The kinetic parameters (apparent activation energy E and frequency factor A) of oil sand pyrolysis were determined using the Friedman procedure. It is found that the apparent activation energy E ranges from 13 to 85 kJ mol-1, corresponding to the conversion of 10-90%. The values of activation energy increase gradually with the increasing conversion. At the higher heating rate, the pyrolysis reaction is performed at the higher temperature region. In addition, it is also shown that the plot of ln A versus E for oil sand pyrolysis becomes a linear line.
to oil shale. A lot of work on the characteristics of oil shale pyrolysis has been performed at home and abroad. The pyrolysis of oil shale was determined using column and gas chromatography under isothermal conditions (420-520 °C).3 The kinetics of pyrolysis of Turkish oil shales4 and Spanish oil shale5 were using isothermal and non-isothermal thermogravimetric analyses (TGAs). The kinetics models were studied by Li and Yue, including the regression function, integral method, differential method, Friedman procedure, maximum rate method, etc.6 The models of oil shale kinetics can be used for oil sand pyrolysis. In this paper, experimental methods used include the differential thermal method and thermogravimetric method. The Friedman procedure will be used to treat the experimental data to obtain the kinetic parameters (apparent activation energy E and frequency factor A) of Buton oil sand pyrolysis.
Introduction Oil sands, also known as tar sands or extra heavy oil, are a type of bitumen deposit. The sands are naturally occurring mixtures of sand or clay, water, and an extremely dense and viscous form of petroleum, called bitumen. Many countries in the world have large deposits of oil sands, including the United States, Russia, and various countries in the Middle East. However, two of the world’s largest deposits occur in two countries, Canada and Venezuela, which have oil sand reserves approximately equal to the world’s total reserves of conventional crude oil.1 The oil sand resources in the world are abundant. There is no thorough exploration and wellfounded estimation. Oil sand reserves have only recently been considered to be part of the world’s oil reserves, because higher oil prices and new technology enable them to be profitably extracted and upgraded to conventional fuels. Oil sands are often referred to as unconventional oil or crude bitumen, to distinguish the bitumen and synthetic oil extracted from oil sands from the free-flowing hydrocarbon mixtures known as crude oil. A large deposit of oil sand was found in Buton, Indonesia. There is not well-founded estimation. The bitumen content of oil sand in Buton is very high, up to 30%. There are three methods for oil recovery from sand, including hot water extraction, solvent extraction, and retorting. Because of the highly dense bitumen in the sand from Buton, the usual method of hot water extraction is not economical. Retorting is considered as a suitable approach for recovering oil from this type of oil sand. Therefore, the study of the pyrolysis kinetics of oil sand is important for the development of retorting technology. In the early 1950s, Hubbard and Robinson2 proposed the oil shale pyrolysis mechanism using bitumen as an intermediate. The oil sand is mainly composed of bitumen. Therefore, the principle for oil sand pyrolysis is considered to be similar
Experimental Section Sample. The sample used in this work was selected from Buton, Indonesia. The oil sand with a particle size of less than 0.076 mm was used in this experimental study. The contents of oil and bitumen are listed in Table 1, and the properties of Buton oil sand are listed in Tables 2 and 3, respectively. Apparatus and Procedure. Differential scanning calorimetry (DSC) and TGA were used to investigate the pyrolysis kinetics of Buton oil sand at different heating rates of 15, 20, 25, and 30 °C min-1. The experimental conditions are as follows: carrier gas, nitrogen with the purity of 99.99%; flow rate of carrier gas, 30 mL min-1; final pyrolysis temperature, 600 °C; and weight of oil sands, ∼45 mg. The thermogravimetry (TG) and differential thermogravimetry (DTG) curves are shown in Figure 1. From the TG and DTG cures, it was shown that, before the temperature of 200 °C, the weight loss is mainly caused by the (3) Skala, D.; Milan, B.; Jovan, J.; Rahimian, I. Pyrolysis of oil shale in a microretorting unit. Fuel 1993, 72, 829–835. (4) Dogan, O. M.; Uysal, B. Z. Non-isothermal pyrolysis kinetics of three Turkish oil shales. Fuel 1996, 75, 1424–1428. (5) Torrente, M. C.; Galan, M. A. Kinetics of the thermal decomposition of oil shale from Puertollano (Spain). Fuel 2001, 80, 327–334. (6) Li, S.; Yue, C. Study of different kinetic models for oil shale pyrolysis. Fuel 2003, 85, 51–61.
*To whom correspondence should be addressed. E-mail: syli@ cup.edu.cn. (1) Alberta’s Oil Sands: Opportunity, Balance. Government of Alberta, Canada, March 2008, ISBN 978-07785-7348-7. (2) Hubbard, A. B.; Robinson, W. E. Bur. Mines Rep. Invest. 1950, 4744. r 2010 American Chemical Society
1844
pubs.acs.org/EF
Energy Fuels 2010, 24, 1844–1847
: DOI:10.1021/ef901376j
Ma and Li
Table 1. Yield of Oil (Fischer Assay) and Content of Bitumen (Dean-Stark) in Buton Oil Sand Fischer assay
Dean-Stark
oil (%)
water (%)
semi-coke (%)
gas (%)
bitumen (%)
moisture (%)
sand (%)
22.19
4.87
68.29
4.65
29.74
4.36
65.90
Table 2. Proximate Analysis of Buton Oil Sand volatile (%)
ash (%)
moisture (%)
fixed carbon (%)
40.52
52.52
2.58
4.38
Figure 3. dx/dt-T curves of Buton oil sand at different heating rates.
heating rate. Corresponding to the heating rate, the maximum value moves to a high-temperature region gradually. Figure 1. Weight loss versus temperature for Buton oil sand at the heating rate of 10 °C min-1.
Mathematical Model The following differential equation will be used to calculate the pyrolysis rate of oil sand:7 " # dx 6 1 2 ðTÞ ¼ 2 ðTj þ 1 -TÞ -ðTj þ 1 -TÞ xj dT hj hj " # 6 1 2 þ 2 ðT -Tj Þ - ðT -Tj Þ xj þ 1 hj hj " # 1 3 2 þ ðTj þ 1 -TÞ -2ðTj þ 1 -TÞ mj hj hj " # 1 3 2 - 2ðT -Tj Þ - ðT -Tj Þ mj þ 1 ð1Þ hj hj where h = Tjþ1 - Tj, the temperature interval. In eq 1, assume that the second derivative is 0 when the conversion x tends to 0 or 1. Using experimental data of conversion versus temperature, the pyrolysis rate of oil sand can be determined (see Figure 3). The thermal decomposition of oil sand can be described using the following first-order reaction model:8 dx E ð1 -xÞ ð2Þ ¼ A exp dt RT
Figure 2. Conversion versus temperature for Buton oil sand at different heating rates.
moisture removal from oil sand. From 200 to about 350 °C, the evolution of adsorbed light hydrocarbon contributes to the main weight loss. The main pyrolysis temperature ranges from 350 to 500 °C. A series of complex reactions occur when most of the hydrocarbon substances are heated. Between 350 and 500 °C, the heavy organic macromolecules begin to crack and produce the smaller molecules, including the gaseous hydrocarbon, inorganic gas, and oil. The pyrolysis involved rupture of the C-C bond, the dehydrogenation of condensed nuclei aromatics, etc. It is obvious that the above curves have the same trend at different heating rates. At the higher heating rate, the pyrolysis reaction is performed at the higher temperature region, which could be caused by the heat conduction and the mass-transfer rate. The results from Figure 2 show almost not change of conversion after the temperature of approximately 500 °C. The pyrolysis completed until the temperature reached about 500 °C. In Figure 3, the dx/dt curves increased first and then decreased. The maximum values of dx/dt increase with the increasing
After taking the natural logarithm of eq 2, the following eq 3 can be obtained: dx E ln ¼ ln½Að1 -xÞ ð3Þ dt RT
(7) Wen, S. Applied Numerical Analysis, 1st ed.; Petrolic Industry Publishing: Beijing, China, 1999. (8) Li, S.; Yue, C. Study of different kinetic models for oil shale pyrolysis. Fuel Process. Technol. 2004, 85, 55–56.
1845
Energy Fuels 2010, 24, 1844–1847
: DOI:10.1021/ef901376j
Ma and Li
Table 3. Composition Analysis of Ash from Oil Sand (%) SiO2
Al2O3
Fe2O3
TiO2
CaO
MgO
K2O
Na2O
MnO2
P2O5
SO3
15.51
4.14
2.36
0.20
72.49
1.46
0.43
0.48
0.04
0.11
1.79
Table 4. Kinetic Parameters Calculated Using the Friedman Procedure x (%) 10 15 20 25 30 35 40 45 50
-1
E (kJ mol ) 12.79 16.49 16.96 19.04 23.56 29.99 35.78 40.05 46.12
A (min-1) -2
8.16 10 2.54 10-1 9.04 10-2 3.77 10-1 1.42 100 9.63 100 5.32 101 1.83 102 1.01 103
R
x (%)
E (kJ mol-1)
A (min-1)
R
0.9967 0.9996 0.9966 0.9987 0.9992 0.9981 0.9921 0.9886 0.9946
55 60 65 70 75 80 85 90
49.59 53.42 56.74 58.90 61.16 60.45 72.96 84.96
2.63 103 7.45 103 1.77 104 3.08 104 5.35 104 4.24 104 9.04 105 1.26 107
0.9937 0.9915 0.9904 0.9969 0.9909 0.9982 0.9969 0.9988
Figure 5. Activation energy versus ln A for oil sand pyrolysis.
Figure 4. Activation energy versus conversion of oil sand pyrolysis.
consequent reactions, the organic macromolecules begin to crack to produce smaller molecules, including the rupture of the C-C bond, the dehydrogenation, the condensation of polycyclic aromatics, the removal of carboxyl, etc. These reactions give the main contribution to oil production and coke formation on the sand surface. With the increasing conversion, oil sand pyrolysis becomes more difficult, resulting in higher activation energy. The results obtained in this work are significant to the development of oil sand retorting technology. From Figure 5, it is found that the plot of ln A versus E for oil sand pyrolysis becomes a linear line with a regression coefficient of 0.998. The regressed linear equation is written as
For the same conversion x, the temperature T and corresponding rate dx/dt are different for the different heating rates. A series of different values of fractional conversion x (x1, x2, x3, ...) are chosen as the constants. According to eq 3, the linear regression of ln(dx/dt) as a function of 1/T will determine the apparent activation energy E and frequency factor A for a specific x. This kinetic approach was called the Friedman procedure.9 Results and Discussion All regression coefficients in Table 4 are greater than 0.99. This implied that the Friedman procedure can be reasonably used to describe oil sand pyrolysis. The values of apparent activation energy E have the range of about 70 kJ mol-1, from 13 to 85 kJ mol-1, corresponding to the different conversions of 10-90% (Figure 4). Prior to conversion of about 30%, the values of activation energy are less than 24 kJ mol-1. In this stage, the pyrolysis with a lower activation energy was concerned mainly with the evolution of adsorbed volatile hydrocarbon and the rupture of weaker chemical bonds, such as, C-O and C-S bonds. In this process, the main pyrolysates are H2O, CO2, H2S, some light hydrocarbons, etc. The rupture of weaker chemical bonds gives a lower activation energy. This process contributed mainly to the gas production. In the
E ¼ 21:69574 þ 3:67609ln A
ðR ¼ 0:998Þ
ð4Þ
Conclusions (1) The pyrolysis experiments on Buton oil sand, Indonesia, were carried out using DSC and TGA. The kinetics of oil sand pyrolysis was investigated. The contents of oil or bitumen in Buton oil sands are about 22 and 30%, respectively. (2) The oil sand pyrolysis can be reasonably interpreted as follows. Before the temperature of 200 °C, the main reaction process is the removal of moisture in oil sand. From 200 to about 350 °C, the evolution of volatile hydrocarbon and the rupture of weaker chemical bonds (C-S and C-O bonds) occur.
(9) Friedman, H. L. Kinetics of thermal degradation of charforming plastics from thermogravimetry. Application to a phenolic plastic. J. Polym. Sci., Part C: Polym. Lett. 1964, 61, 183–195.
1846
Energy Fuels 2010, 24, 1844–1847
: DOI:10.1021/ef901376j
Ma and Li
Between 350 and 500 °C, the main pyrolysis reactions occur and contribute to oil production. (3) At the higher heating rate, the pyrolysis reaction occurs at the higher temperature region. The pyrolysis reaction is almost completed when the temperature reached 500 °C. (4) The Friedman procedure can be suitably used to describe the oil sand pyrolysis. The values of apparent activation energy E range from 13 to 85 kJ mol-1, corresponding to the different conversions of 10-90%. (5) The plot of ln A versus E for oil sand pyrolysis becomes a linear line.
Nomenclature A = frequency factor (s-1) E = activation energy (kJ mol-1) h = temperature interval R = regression coefficient T = pyrolysis temperature (°C) t = pyrolysis time (s) x = fractional conversion dx/dT = pyrolysis reaction rate (K-1)
1847
