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Energy & Fuels 2002, 16, 96-101
A Kinetic Approach to the Temperature-Programmed Pyrolysis of Turkish Oil Shales in a Fixed Bed Reactor: Determination of Kinetic Parameters for n-Paraffins and 1-Olefins Evolution Levent Ballice* and Mithat Yu¨ksel University of Ege, Faculty of Engineering, Department of Chemical Engineering, 35100 Bornova, Izmir, Turkey Received March 30, 2001. Revised Manuscript Received July 14, 2001
The thermal degradation of both Go¨ynu¨k oil shales (GOS) and Beypazari oil shales (BOS) has been investigated under nonisothermal conditions. The recovery of organic carbon as 1-olefins and n-paraffins was determined by temperature-programmed pyrolysis of oil shales. A fixed bed reactor under argon flow was used to pyrolyze small samples of oil shales. A special gas-phase sampling technique was used to determine the composition of products eluted from the reactor as a function of temperature and time. Hydrocarbon evolution data have been analyzed by CoatsRedfern and Chen-Nuttall combinations. It must be emphasized that the evaluation of temperature-programmed pyrolysis data by combined use of Coats-Redfern and Chen-Nuttall methods provide satisfactory mathematical approaches to obtain kinetic parameters for 1-olefins and n-paraffins formation from thermal degradation of Turkish oil shales. Using this method, it is possible to identify every stage of pyrolysis and derive values for kinetic parameters.
Introduction The most important reaction in oil shale processing is that leading to shale oil. It is possible to develop global rate constants for this reaction that predict the timing of oil generation. These rate constants can help determine processing times and consequently, reactor volume, since reactor volume depends on throughput times the processing time. Rate constants can be also developed for secondary reactions that affect both oil quality and quantity. The kinetics of decomposition of oil shales have been studied by many investigators, and various suggestions for the decomposition mechanisms have been reported.1,2 Although some attempts have been made to understand the complex nature of decomposition of oil shale, involving numerous organic compounds, some authors have found it sufficient to consider a global firstorder kinetic expression to represent the overall decomposition rate of oil shale.1-4 Overall kinetics can be easily obtained by measuring the change in weight of a sample with time based on isothermal or nonisothermal thermogravimetric data.1,2,5 The processing of oil shale involves numerous chemical reactions and in addition to desired products, the chemical reactions also generate byproducts that can lead to environmentally undesirable emission. An understanding of all these chemical reactions and their rates can help design processes that minimize the total * Corresponding author. E-mail:
[email protected]. (1) Scala, D.; Kopsch, H.; Sokic, M.; Neumann, H. J.; Jovanovic, J. A. Fuel 1987, 66, 1185. (2) Scala, D.; Kopsch, H.; Sokic, M.; Neumann, H. J.; Jovanovic, J. A. Fuel 1990, 69, 490.
processing cost, including the cost associated with meeting environmental regulations.6 In this work, a kinetic study on the thermal decomposition of Go¨ynu¨k oil shale (GOS) and Beypazari oil shale (BOS) is presented. The thermal degradation of the oil shales is investigated under nonisothermal conditions. An innovative approach to the data obtained by temperature-programmed pyrolysis of GOS and BOS7,8 led us to determine the energies of activation for linear 1-olefins and n-paraffins formation. In the present study, the contribution of the activation energies of linear 1-olefins and n-paraffins formation to the apparent activation energy of overall degradation was determined. Theory Pyrolytic Decomposition of Kerogen and Hydrocarbon Generation. Thermal breakdown of kerogen in oil shale embodies three broad classes of reaction. These are decarboxylation reactions (involving principally the decomposition of -COOH groups), major breakdown of kerogen to form oil and gas, with hydrocarbons as the main products and carbonization of the aromatic char.9 Previous investigations have led to the (3) Yang, H. S.; Sohn, H. Y. Fuel 1985, 64, 1511. (4) Braun, R. L.; Burnham, A. K.; Reynolds, J. G.; Clarkson, J. E. Energy Fuels 1991, 5, 192. (5) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Schulz, H.; Hanogˇlu, C. Fuel 1995, 74, 1618. (6) Burnham, A. K. NATO ASI-Akcay-Tu¨rkiye, 1993, UCRL-JC114129, preprint. (7) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Schulz, H. Fuel 1996, 75, 453. (8) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Schulz, H. Fuel 1997, 76, 375.
10.1021/ef010077o CCC: $22.00 © 2002 American Chemical Society Published on Web 01/16/2002
Temperature-Programmed Pyrolysis of Turkish Oil Shales
conclusion that the pyrolysis of oil shale kerogen could be explained as k1
kerogen 98 bitumen + gas + coke k2
bitumen 98 coke + oil + gas
(1) (2)
The formation of intermediate bitumen is a considerably more rapid step compared to the decomposition of bitumen at temperatures below 482 °C10,11 and 487 °C.10,11 The first reaction can be omitted from the present investigation since this covers a temperature range of 265 to 400 °C. The exclusion of kerogen from consideration assume the validity of Allred’s suggestion.5,10 Therefore it is accepted that all of the kerogen has been converted to bitumen, gas, and carbonaceous residue and no carbonaceous matter was formed after that time.10 Therefore, the pyrolysis reaction can simply be considered as follows: k
bitumen 98 hydrocarbons
Energy & Fuels, Vol. 16, No. 1, 2002 97
n-paraffins and 1-olefins distribution as a function of temperature. Typical gas chromatograms for pyrolysis from GOS and BOS are shown in Figure 1.7,8 The effect of temperature or time on the rate of total product evolution,7,8 1-olefins and n-paraffins formations, are shown in Figures 2, 3, and 4, respectively. The rate of decomposition of oil shale is equivalent to the rate of evolution of decomposition products. The amount of total carbon recovered as 1-olefins and n-paraffins can be determined by numerical integration of Figures 3 and 4 for each desired temperature as a function of time. The mathematical procedure used in the analysis of temperature-programmed pyrolysis data for determination of 1-olefins and n-paraffins formation kinetics corresponds to the integral method used by many other investigators.1,5,15-17 Assuming first order for the rate of mass loss during the formation of 1-olefins and n-paraffins, the following well-known rate expression can be written.1,5,15-17
dx ) kf(x) dt
(3)
The bitumen is of high boiling point range and remains in the particle for significant periods of time. It becomes subject to two competing processes: heavy oil production and intraparticle (liquid-phase) coking. The released heavy oil is further subjected to thermal cracking in the vapor phase surrounding the particles. Carbonization of the aromatic char occurs between about 500 and 1200 with evolution of hydrogen and this process although contributing very little to the total mass loss. Our investigation was performed in the temperature range of 280-500 °C. This temperature covers hydrocarbon generation from oil shale. Pyrolysis products include straight- and branched-chain paraffins and olefins from methane to C26 and besides small aromatic hydrocarbons, poly-aromatic compounds also present in pyrolysis products. In this study, the present analysis method did not allowed poly-aromatics in the pyrolysis products to be characterized but the data obtained by temperature-programmed pyrolysis of GOS and BOS7,8 led us to determine the contribution of the activation energies of linear 1-olefins and n-paraffins formation to the apparent activation energy of overall degradation. Determination of the Energies of Activation for 1-Olefins and n-Paraffins Formation by Temperature-Programmed Pyrolysis Data. Numerous attempts dealing with the mechanistic and kinetic points of view have been made to understand the process occurring during oil shale pyrolysis. The rate of decomposition can be studied best when the temperature increases at a constant rate during pyrolysis. In our investigation we used the apparatus described by previous studies.7,8,12-14 The temperature-programmed pyrolysis of GOS and BOS was investigated using a new, highly efficient technique. In particular, the temperature that produced the maximum evolution rate for volatile products has been determined, and organic product composition has been characterized in terms of (9) Hoare, I. C.; Stuart, W. I. Thermochim. Acta 1987, 111, 53. (10) Berber, R.; Okan, Y. In Proceedings of International Conference on Alternative Energy Sources-VI; Veziropi, T. N., Ed.; Hemisphere Publications: Bristol, PA, 1985, 3, 449. (11) Allred, V. D. Chem. Eng. Prog. 1966, 62, 55.
(4)
If k is substituted in terms of activation energy and frequency factor, eq 4 can be rewritten as
dx ) A exp(-E/RT)(1 - x) dt
(5)
where A ) frequency factor (min-1), E ) activation energy (J/mol), R ) gas constant (J mol-1 K-1), T ) temperature (K), and where
x)
(W0 - Wt) (W0 - Wf)
(6)
where W0 is the initial weight of carbon in the original sample (mg-C), Wt is the carbon weight at any time (mgC), and Wf is the final weight of carbon in solid residue after the pyrolysis of the oil shale (mg-C), k is the specific rate constant, and f(x) ) (1 - x) for first-order reactions. Equation 5 can be rewritten as
dx dT ) A exp(-E/RT)(1 - x) dT dt
(7)
A dx ) exp(-E/RT)(1 - x) dT b
(8)
or
()
where b is dT/dt.8,12 The integration of eq 8 and rearranging gives
(
ln
)
1 AR E + 2RT E ln ) ln 2 1 x b RT T
(9)
This equation is linear in form and known as the ChenNuttall Model.1,5,15 Coats and Redfern developed a graphical method to determine the kinetic parameters for solid decomposi(12) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Schulz, H.; Reimert, R. Fuel 1998, 77, 1431. (13) Ballice, L. Energy Fuels 2001, 15, 659. (14) Ballice, L.; Yu¨ksel, M.; Sagˇlam, M.; Reimert, R.; Schulz, H. Energy Fuel 1998, 12, 925. (15) Thakur, D. S.; Nuttall, H. E., Jr. Ind. Eng. Chem. Res. 1987, 26, 1531.
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Ballice and Yu¨ ksel
Figure 1. Gas chromatogram of organic products at maximum evolution temperatures during pyrolysis of GOS and BOS.
Temperature-Programmed Pyrolysis of Turkish Oil Shales
Figure 2. Total product evolution rate of GOS and BOS as a function of temperature and time.
Figure 3. n-Paraffins and 1-olefins evolution rate for GOS as a function of temperature and time.
Energy & Fuels, Vol. 16, No. 1, 2002 99 to be evolution path type I for Go¨ynu¨k oil shale and type II for Beypazari oil shale.5 The results of elemental analysis and Fisher assay of the oil shale have previously been reported.5,7 For the pyrolysis experiments, the oil shale was crushed and ground in a jaw mill until the desired particle size was obtained. The sample was sieved to obtain a 0.9 for all lines).
Figure 6. Analysis of temperature-programmed pyrolysis data of n-paraffins and 1-olefins formation from BOS by CoatsRedfern/Chen-Nuttall model combinations (r > 0.9 for all lines).
cording to this model, a plot of -(ln[-(ln(1 - x))/T2]) vs 1/T should result in a straight line of slope E/R. The Combined Use of Coats-Redfern and ChenNuttall for Determination of the Apparent Activation Energies of n-Alkane and 1-Alkene Formation. The Coats-Redfern analysis was performed by plotting -(ln[-(ln(1 - x))/T2]) vs 1/T, the decomposition of oil shale was assumed to be first-order with respect to oil shale decomposition. The rate of decomposition of the oil shale is equivalent to the sum of the rates of gas and volatile products (olefins, paraffins, dienes, aromatics) formation and coke. The rate of formation for n-paraffins and 1-olefins are shown in Figures 3 and 4. The amount of 1-olefins and n-paraffins formation is proportional to the integral of the rate of 1-olefins (or n-paraffins) generation (dm/dt) from t ) 0 to desired t value. Where m and t are the amount of 1-olefins or n-paraffins (mg-C) and time, respectively. The amount of 1-olefins and n-paraffins were determined by numerical integration of the curves in Figures 3 and 4 for each desired temperature range. The conversion values for 1-olefins and n-paraffins formation were calculated by eq 6, respectively. Where the oil shale sample weight in total carbon basis at any time and temperature (Wt, mg-C) is the difference between the initial weight of the total carbon in oil shale sample (Wo, mg-C) and the cumulative amount of 1-olefin (or n-paraffins) in carbon basis calculated by the integration of curves in Figures 3 and 4 for each desired temperature. The conversion values of 1-olefins and n-paraffins for oil shale samples calculated by eq 6 were used to produce a graph from the Coats-Redfern equation and hence for determining the slope and intercept of linear parts of the plot. The lines for each oil shale type (Figures 5, and 6) indicate 1-olefins and n-paraffins formation occurred in the temperature range 280-500 °C. Activation energies were calculated from the slopes of these lines and these activation energy values and a conversion that was calculated at any selected temperature were substituted in eq 9 in order to calculate the frequency factor. The kinetic parameters computed for 1-olefins and n-paraffins formation from oil shales (GOS, BOS) are listed in Table 1. In our investigations, the decomposition of oil shale was assumed to be first order with respect to oil shale decomposition. Karabakan et al. also investigated the kinetic parameters of the pyrolysis reactions of Turkish-
Go¨ynu¨k and US Green River oil shales, carbonate-free shale, and silicate-free shale. The orders of reaction were calculated approximately as 1 (between 0.9 and 1.3).19 Thermogravimetric data were analyzed by Dogˇan et al. in order to obtain kinetic parameters for overall degradation of Turkish oil shales.20 The activation energies were reported vary between 12 and 40 kj/mol depending on the type of shale and temperature interval. Studies of the kinetics of oil shale pyrolysis can be dated to the early nineteenth century, when Engler first examined the pyrolytic mechanism of oil shale. Since then a number of investigations have been made by different researchers. Among these investigations, that by Hubbard and Robinson was regarded as the most extensive. Their experimental data on Colorado oil shale pyrolysis have been widely used for verifying various kinetic models and the values previously reported in the literature for the activation energies of kerogen decomposition vary from 5.5 to 57.1 kcal/mol.10,21-23 Based on the previous experimental evidence, Allred11 proposed the following reaction sequence: k1
k2
kerogen 98 bitumen 98 oil and gas The reaction sequence explains the consecutive nature of the decomposition reaction. The suggestion of Allred prompted Lin et al. to propose a more sophisticated model. Their model can be represented by k1
kerogen 98 bitumen k2
bitumen 98 oil and gas k3
kerogen + bitumen 98 bitumen + bitumen k4
kerogen + bitumen 98 bitumen + oil and gas k5
kerogen + bitumen 98 bitumen + carbon residue The activation energies for these parallel reactions have (19) Karabakan, A.; Yu¨ru¨m, Y. Fuel 1998, 77, 12. (20) Dogˇan, M. O ¨ .; Uysal, B. Z. Fuel 1996, 75, 1424. (21) Lin, S. H. J. Chem. Eng. Jpn. 1991, 24, 673. (22) Hubbart, A. B.; Robinson, W. E. U.S. Bur. Mines, Rept. Invest., No. 4744 (November 1950). (23) Herrel, A. Y.; Arnold, C., Jr. Sandia Lab. Energy Report, Sand75-0344, Albuquerque, NM, October 1975.
Temperature-Programmed Pyrolysis of Turkish Oil Shales Table 1. Apparent Kinetic Parameters for 1-Olefins and n-Paraffins Formation 1-olefins
n-paraffins
materials
E, kJ/mol
ln A, min-1
E, kJ/mol
ln A, min-1
GOS BOS
37.3 ( 2 48.2 ( 3
5.2 ( 0.02 7.2 ( 0.01
28.1 ( 1 33.2 ( 2
3.6 ( 0.02 4.5 ( 0.01
been reported as 20.3, 37.4, 33.8, 28.2, and 31.0 kcal/ mol, respectively. The pyrolysis kinetics of oil shales from Yugoslavia, North Korea, and Russia were investigated under nonisothermal conditions by Scala et al.2 They performed their investigations using kerogen concentrate, bitumen originally present in oil shale, extracted by toluene and kerogen concentrate and the activation energies for nonisothermal decomposition of these materials have been reported as 52.7 (2 K/min), 78.9 (10 K/min), and 39.6 kJ/mol (2 K/min). Primary kerogen-to-petroleum and secondary oil-togas conversion processes have been investigated by temperature-programmed pyrolysis of Toarcian shale at heating rates 0.1, 0.7, and 5 K/min.24 The activation energies have been reported as 52 kcal/mol (for oil) and 53 kcal/mol (for gas).24 Karabakan et al. investigated the kinetic parameters of the pyrolysis reactions of Turkish-Go¨ynu¨k and US Green River oil shales and the activation energies of pyrolysis reactions of Go¨ynu¨k and Green River oil shales have been reported as 158.2 and 363.2 kJ/mol, respectively.19 It can be seen that the examination of the literature data leads to an incredible diversity of kinetic models (reaction networks and rate expression) for conversion of kerogen to oil and gas and the values of activation energies reported in the literature vary in a wide range. The comparison between apparent activation energies of n-alkanes and 1-alkenes formation during Turkish oil shales degradation in our studies and literature values shows consistent kinetic parameters. Our investigation was performed in the temperature range 280-490 °C. This temperature range is also in quite good agreement with literature values. The temperature range for the decomposition reactions of oil shales was also indicated as 300-500 °C in the literature.5,6,10,15 Thakur and Nuttall investigated the decomposition process of oil shale by evaluating the thermogravimetric data which were divided into two regions: region 1, corresponding to the first step of the reaction in a temperature range of 300-380 °C; and region 2, corresponding to the second step at 380-500 °C.10-12,15 In our investigations, the temperature range is 300500 °C and in quite good agreement with literature. The low activation energy for n-paraffins formation of GOS and BOS indicated quite good agreement with (24) Dieckmann, V.; Schenk, H. J.; Horsfield, B.; Welte, D. H. Fuel 1998, 77, 23. (25) Bar, H.; Ikan, R.; Aizenshtat, Z. J. Anal. Appl. Pyrolysis 1987, 10, 167. (26) Wallman, P.; Tamm, P. W.; Spars, B. G. Am. Chem. Soc. Symp. Ser. 1981, 93. (27) Burnham, A. K.; Ward, R. L. Am. Chem. Soc. Symp. Ser. 1981, 79.
Energy & Fuels, Vol. 16, No. 1, 2002 101
the literature data. According to the mechanism of alkene and alkane formation, alkene formation is a unimolecular reaction and alkane formation is a bimolecular reaction. The activation energies of the first-order reactions are greater than second-order reactions.25-27 The activation energies of 1-olefins and n-paraffins formation from BOS is greater than that of GOS and it was explained by the higher mineral contents of BOS than GOS. The diffusion of organic matter through the carbonate matrix required a high temperature and relatively more energy. Berber and Okan also found a lower activation energy value for pyrolysis of decarbonated oil shale than untreated oil shale in their investigation.10 It is clear that the calculations of kinetic parameters using the Chen-Nuttall model needs repeated regression analysis.5,17 This method requires an initial guess of E in order to calculate the left side of eq 9 and an iteration until the desired accuracy for E and A (frequency factor) was achieved. The determination of activation energy by the Coats-Redfern method is very easy and it provides an accurate assessment of E (activation energy) for Chen-Nuttall model and there is no need for iteration steps of the repeated regression technique. This combined method was also used for our previous studies on determination of global kinetic parameters for Turkish oil shales by using thermogravimetric data.5 Conclusion The thermal degradation of both GOS and BOS have been investigated under nonisothermal conditions. Hydrocarbon evolution data were evaluated by the combined use of Coats-Redfern and Chen-Nuttall models so as to determine the kinetic parameters for 1-olefins and n-paraffins formations. It is clear that the calculations of kinetic parameters using the Chen-Nuttall model needs repeated regression analysis. This method required an initial guess of E in order to calculate the left side of eq 9 and an iteration until the desired accuracy for E and A (frequency factor) was achieved.5,17 The determination of activation energy by the Coats-Redfern method is very easy and it provides an accurate assessment of E for the Chen-Nuttall model. It would appear that the kinetic analysis of the temperature-programmed pyrolysis data by the combined method of Coats-Redfern and Chen-Nuttall model is an effective method and this method provides new and satisfactory mathematical approaches to obtain kinetic parameters for 1-olefins and n-paraffins formation of degradation of oil shales. Using this method, it is also possible to identify every stage of pyrolysis and derive values for kinetic parameters. Acknowledgment. The authors thank the Department of Petroleum, Gas and Coal of the Engler-Bunte Institute, University of Karlsruhe, and German Academic Exchange Service-(Deutscher Akademischer Austauschdienst-DAAD) for financial support. EF010077O
