Article pubs.acs.org/IECR
A Kinetic Analysis of Wood Degradation in Supercritical Alcohols Jeeban Poudel and Sea Cheon Oh* Department of Environmental Engineering, Kongju National University, 275, Budae-dong, Cheonan, Chungnam, 330-717, Korea ABSTRACT: The kinetic analysis method for degradation of wood in supercritical ethanol and methanol was proposed in this work. This method was applied to predict the degradation of wood in supercritical ethanol and supercritical methanol by a nonisothermal weight loss technique with heating rates of 3.1, 9.8, and 14.5 °C/min for ethanol and 5.2, 11.3, 16.3 °C/min for methanol. To verify the effectiveness of the kinetic analysis proposed in this work, the experimental values were compared with those of the numerical integration results using kinetic parameters obtained in this work. The kinetic analysis method proposed in this work gave reliable values of kinetic parameter for wood degradation in supercritical ethanol and supercritical methanol. To understand the effectiveness of the solvents as supercritical fluid, the calculation results of wood weight loss using the kinetic parameters obtained from this work were studied at a heating rate of 7 °C/min for both supercritical ethanol (SCE) and supercritical methanol (SCM). From this work, it can be seen that SCE is better solvent than SCM for wood degradation in supercritical alcohols.
1. INTRODUCTION Owing to the limitations of homogeneous and heterogeneous catalytic reactions for biodiesel production from biomass, a sustainable and environmental friendly technology using supercritical alcohol (SCA) has been receiving considerable attention. SCA technology is a noncatalytic process which makes the separation and purification of biodiesel relatively easy and simple. It has also been shown that this process only requires a small amount of reaction time to achieve a significant yield of biodiesel.11 Supercritical fluids have properties of both liquid and gas phases. Having a density close to that of a liquid, the supercritical fluid has the ability to dissolve many components, whereas the high diffusivity and low viscosity of the supercritical fluid also enables it to behave similar to gas. Such mobile properties of the supercritical fluid tend to maximize the yields of the product in the prescribed reaction time. Along with this major advantage of a supercritical fluid, others have been mentioned in the referred paper.2,3 Water offers a remarkable solvent tenability with its wide range of dielectric constants; its applications and potential as a new reaction medium in chemistry have been reviewed.4 The fact that the critical temperatures of alcohols (Tc = 512.6 K, 513.9 K; Pc = 80.9 bar, 61.4 bar; ρc = 0.272 g/cm3, 0.276 g/cm3 for methanol and ethanol, respectively) are quite low compared to that of water (Tc = 647.1 K; Pc = 220.6 bar; ρc = 0.322 g/ cm3) which suggests that hydrogen bonding is weaker in the alcohols than in water.5 The alcohols may be alternatives to water as supercritical solvents considering their less corrosive and aggressive chemical nature, the lower critical temperatures and pressures, and their reasonably high dielectric constants. Because these alcohols have lower critical temperatures and pressures than those of water, they can offer milder conditions for reaction. In addition, these alcohols are expected to readily dissolve relatively high molecular weight products from cellulose, hemicelluloses, and lignin because of their low dielectric constants when compared with that of water.6 However, there is sufficient literature which compared the effects of types of alcohol used in a noncatalytic SCA reaction, © 2012 American Chemical Society
which is important as they influence the performance of the reaction significantly.7 There is a plethora of reported articles focusing on the supercritical methanol (SCM) process, while there is limited research on the supercritical ethanol (SCE) process. Very few researches have been conducted for comparative study of methanol and ethanol. SCM and SCE reactions were carried out by utilizing a single-variable experimental design to investigate the effect of alcohols on the yield of biodiesel.8 In this work, the degradation kinetics of wood using SCM and SCE has been studied. Wood and other forms of biomass can be used in various ways to provide energy through combustion, gasification, and pyrolysis, etc.9 Pyrolysis which converts wastes to fuel or useful hydrocarbons is a promising and economical process to produce liquid fuels that can be readily stored and transported.10 But it was never commercialized because of some problems to be tackled such as excessively long degradation time, tar formation, and the coking of reactants. So, new technologies using supercritical fluid are vital in the thermal treatment process of biomass. It is well known that a kinetic investigation is very important to get information for rationally designing the reactor for the degradation of wood in supercritical fluids. But unfortunately, such factors as hard reaction conditions, complicated compositions of degradation products, difficulty of continuous operation, etc. tend to limit the kinetic study on the degradation of wood in supercritical fluids. The overall target of this research is to generate the basic parameters for design of this system rather than developing the mechanism of operation. These parameters will help in further development of the system. The parameters are calculated using SCM and SCE which shows the comparative analysis of the two solvents too. The use of supercritical alcohols like ethanol and methanol shows some promising output for better Received: Revised: Accepted: Published: 4509
March 11, 2011 December 25, 2011 January 2, 2012 January 2, 2012 dx.doi.org/10.1021/ie200496b | Ind. Eng.Chem. Res. 2012, 51, 4509−4514
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condition, the heating of the vessel was interrupted. The solid residue was rinsed with alcohols to remove the absorbed liquid organic fraction and weighted after removal of the solvent in dry oven for 24 h. The procedure followed for ethanol and methanol are same except that the heating rate for methanol was 5.2, 11.3, and 16.3 °C/min and that for ethanol was 3.1, 9.8, and 14.5 °C/min. The conversion, α of wood was calculated as follows:
conditions. But, such factors as hard reaction conditions, complicated compositions of degradation products, and difficulty of continuous operation, etc. tend to limit the kinetic study on the degradation in supercritical fluids. The kinetics of wood decomposition has been analyzed using the nonisothermal weight loss technique on the Arrhenius form to obtain the kinetic parameters such as apparent activation energy, overall reaction order, pre-exponential factor, and order on the reaction temperature.
α=
2. EXPERIMENTAL SECTION 2.1. Materials and Apparatus. As the wood biomass sample, crushed chopstick wood was subjected to supercritical alcohol treatment. Proximate and elemental analysis of wood sample is shown in Table 1. From Table 1, the volatile material
dα = A(1 − α)n exp( − E /RT ) dt
elemental analysis
items
weight fraction (%)
elements
weight fraction (%)
initial moisture volatile matter fixed carbon ash
7.78 78.52 13.25 0.45
carbon hydrogen nitrogen oxygen sulfur
52.35 6.63 0.16 39.75 0.01
(1)
where Wo and W are the initial and final masses of the sample, respectively. 2.3. Kinetic Analysis. In general, the overall rate equation of conversion, α for thermal degradation is expressed in the Arrhenius form as
Table 1. Proximate and Elemental Analysis of Wood Sample proximate analysis
W0 − W W0
(2)
The pre-exponential factor A is not strictly constant but depends on the reaction temperature,11 and if the heating rate β = dT/dt is employed, it can be shown that
A dα = 0 T m(1 − α)n exp( − E /RT ) β dT
is major component of wood used in this work, and the weight percentage of carbon and oxygen are 52.35% and 39.75%, respectively. As solvents, methanol (99.5% purity) was purchased which was manufactured by Ducksan Chemical Co. (Korea), and ethanol (99.9% purity) produced by OCI Company Ltd. (Korea) was purchased. Figure 1 shows the
(3)
From eq 3, it can be shown that
A0 m dα T exp( − E /RT ) dT n = (1 − α) β
(4)
where on integrating and introducing the initial condition of α = 0 at T = T0 the following expression is obtained as
∫0
α
A0 T m dα T exp( − E /RT ) dT n = β T0 (1 − α)
∫
(5)
The integral approximation12 is to consider eq 5 when it can be shown that
A0 T m T exp( − E /RT ) dT β T0 A R ⎛ RT ⎞⎟ = 0 T m + 2⎜1 − (m + 2) exp( − E /RT ) ⎝ βE E ⎠
∫
(6)
and
∫0 Figure 1. Schematic diagram of the experimental apparatus.
α
1 − (1 − α)1 − n dα = (1 − α)n 1−n
=− ln(1 − α)
schematic diagram of the batch-type reactor manufactured by Parr Instrument Co. (USA) with a volume of 25 mL. The permissible reactor conditions are 500 °C and 55 MPa. 2.2. Procedure. Initially, the wood sample was put in a 105 °C oven for at least 24 h to make the sample dry. At room temperature, a total of 1 g of the wood sample was charged in a stainless steel autoclave with 16 g of methanol or ethanol. The pressure inside the vessel was monitored by a pressure gauge. When the reaction temperature and time got to the preset
for n ≠ 1
for n ≠ 1
(7) (8)
From the logarithm of eqs 6, 7 and 8, the following results are obtained:
⎡ 1 − (1 − α)1 − n ⎤ E ⎥=F− ln⎢ RT 1−n ⎦ ⎣
ln[− ln(1 − α)] = F − 4510
E RT
for n ≠ 1 (9)
for n ≠ 1
(10)
dx.doi.org/10.1021/ie200496b | Ind. Eng.Chem. Res. 2012, 51, 4509−4514
Industrial & Engineering Chemistry Research
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the conversion was obtained when the reaction temperature was attained at around 400 °C. Also from the comparison of TG data in nitrogen atmosphere with those of supercritical alcohols, we can see that higher conversion in SCE yields at lower temperature than those of SCM and TG data while the conversion in SCM and TG data showed similar tendency. It has been reported that the low reaction temperature can minimize the formation of undesirable hydrocarbon gases and char.13 So it can be seen that SCE can provide an alternative approach to lower operation temperature of wood degradation reactions. Figures 3, 4, and 5 show the application of the kinetic analysis method proposed in this work at heating rates of 3.1,
where
⎡A R ⎛ RT ⎟⎞⎤ F = ln⎢ 0 T m + 2⎜1 − (m + 2) ⎥ ⎝ ⎣ βE E ⎠⎦
(11)
The plot of ln[(1 − (1 − α)1‑n/(1 − n))] or ln[−ln(1 − α)] versus 1/T should give a straight line with the slope determining the activation energy E. Also,A0 and m can be calculated from the activation energy E and the intercept F on the y-axis by using the least-squares method.
3. RESULTS AND DISCUSSION Figure 2 shows the weight loss curves of wood biomass in SCM and SCE, respectively. Also it includes the thermogravimetric
Figure 3. Application of the kinetic analysis method at the heating rate of 3.1 °C/min for SCE.
Figure 2. Weight loss curves of wood degradation in supercritical alcohols including TG in nitrogen atmosphere.
(TG) data of wood in nitrogen atmosphere at heating rate of 3.1, 10, and 16.3 °C/min. From Figure 2, it was found that the weight loss curves of wood degradation in supercritical methanol, ethanol and nitrogen atmosphere were displaced to higher temperatures due to the heat transfer lag with an increased heating rate. In a comparison of the TG data and the wood degradation data in SCM and SCE as in Figure 2, toward the lower temperature, the TG data and the wood degradation data in SCM and SCE showed similarity. But as the temperature increases, the difference between the two data seems to have significance which shows the justification of using supercritical alcohols in wood degradation. At the inception of the critical condition, the conversion is slow which tremendously increases at around 300 °C. Almost 99% of
Figure 4. Application of the kinetic analysis method at the heating rate of 9.8 °C/min for SCE. 4511
dx.doi.org/10.1021/ie200496b | Ind. Eng.Chem. Res. 2012, 51, 4509−4514
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Figure 5. Application of the kinetic analysis method at the heating rate of 14.5 °C/min for SCE.
Figure 7. Application of the kinetic analysis method at the heating rate of 11.3 °C/min for SCM.
9.8, and 14.5 °C/min for SCE, and Figures 6, 7 and 8 show the application of the kinetic analysis method for SCM at heating
Figure 8. Application of the kinetic analysis method at the heating rate of 16.3 °C/min for SCM. Figure 6. Application of the kinetic analysis method at the heating rate of 5.2 °C/min for SCM.
For the comparative work for wood degradation in SCM and SCE, same degradation condition for highlighting the performance of the solvent was needed. Figure 9 shows the experimental values and the calculated values from eqs 9 and 11, by using the kinetic parameters of Table 2. It is seen that the calculated values agree very well with the experimental values, and the kinetic analysis method used in this work gives reliable values of the kinetic parameters for the degradation of wood in supercritical alcohols. Figure 10 shows the comparison between SCE and SCM for a heating rate of 7 °C/min. Also, the figure incorporates the TG
rate of 5.2, 11.3, and 16.3 °C/min, respectively. The best fit values were determined by employing the overall reaction order values n from −2.0 to 2.0 at intervals of 0.25. The negative value of reaction order at heating rates of 9.8 and 14.5 °C/min for SCE was obtained due to its mathematical complexity and the relation between parameters in equation. It was also found that the apparent activation energy decreased as the heating rate was increased. The best overall fit values for different parameters were obtained as shown in Table 2. 4512
dx.doi.org/10.1021/ie200496b | Ind. Eng.Chem. Res. 2012, 51, 4509−4514
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Table 2. Summary of the Kinetic Parameters Obtained in This Work
reaction order, n
activation energy, E [kJ mol−1]
factor, Ao [min−1 K−m] × 10−8
order, m
0.75 −0.25 −0.75
62.8 48.5 44.5
3.2891 3.2644 1.9375
−1.45 −1.96 −2.00
5.2 11.3 16.3
1.25 0.50 0.25
78.8 55.7 53.7
5.0421 4.9926 4.9901
−0.67 −1.71 −1.96
3.1 10.0 16.3
0.50 0.00 −0.50
50.7 44.5 36.4
2.7705 1.4127 0.1885
−2.00 −2.00 −2.00
heating rate, β [min−1] SCE
3.1 9.8 14.5
SCM
TG
Figure 10. Comparison of wood degradation in supercritical alcohols (calculated) and TG data (real) at a heating rate of 7 °C/min.
Hoffmann and Conradi have studied hydrogen bonds in SCM and SCE. From their work, when the hydrogen bonding of methanol and ethanol is examined, the temperature dependence of the data extrapolated to a constant liquidlike density implies enthalpies of hydrogen bonding of 12.8 and 16.8 kJ/mol in methanol and ethanol, respectively. But, at the supercritical region, the degree of hydrogen bonding of ethanol is slightly weaker in comparison to that in methanol.14 As the temperature rises, the degree of hydrogen bonding is less, which shows the possibility that ethanol can work as a better solvent at supercritical conditions. In accordance with the result obtained in this work, the work by Madras at al. also highlights that during the transesterification of sunflower oil the conversion in supercritical ethanol is higher than that in supercritical methanol.8 This is because the solubility parameter of ethanol is lower than that of methanol and is closer to the solubility parameter of the oil.8 But at the same time, the cited work highlights that the yield obtained by using SCM is higher than that obtained by using SCE although the difference is much less.7 According to the work by Madras, the activation energies, determined from the slope of the regressed lines of the Arrhenius plot, are 3 and 2 kJ/mol for transesterification of sunflower oil using supercritical methanol and ethanol, respectively, which shows the effectiveness of SCE over SCM.8 The ethanol being less toxic is more likely than methanol to be used as a solvent in biodiesel synthesis. The above results from several authors were obtained for other materials than wood and at different working conditions. An analysis of many researches going on14−16 shows that the difference in SCM and SCE for wood degradation is still ambiguous. So, this field still needs deeper and concrete comparative studies. But, in an environmental point of view, the requirement of methanol makes the current biodiesel product not totally 100% renewable as methanol is derived from fossilbased products. Ethanol, on the other hand, can be produced from agricultural biomass via fermentation technology and is already easily available in the market at a high purity.17 So, we
Figure 9. Real and calculated weight loss of wood degradation in supercritical alcohols..
data of wood degradation at the same heating rate. From Figure 10, there is a higher conversion in SCE yields at lower temperature than those of SCM and TG data, while the conversion in SCM and TG data are similar. It was also found that the final residues in SCM and SCE were lower than that of TG data. But, the degradation of wood in SCE was higher than that of SCM for the same work condition. So, it can be seen that the use of supercritical ethanol as a reaction medium provides an alternative approach for the synthesis of biodiesel from wood. 4513
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(8) Madras, G.; Kolluru, C.; Kumar, R. Synthesis of biodiesel in supercritical fluids. Fuel 2004, 83, 2029−2033. (9) Liu, Q.; Wang, S.; Wang, K.; Luo, Z.; Cen, K. Pyrolysis of wood species based on the compositional analysis. Korean J. Chem. Eng. 2009, 26, 548−553. (10) Kim, S.; Eom, Y. Estimation of kinetic triplet of cellulose pyrolysis reaction from isothermal kinetic results. Korean J. Chem. Eng. 2006, 23, 409−414. (11) Turns, S. R. In An Introduction to Combustion: Concepts and Applications; McGraw-Hill: New York, 1996. (12) Coats, A.; Redfern, J. Kinetic Parameters from Thermogravimetric Data 1964. (13) Lee, S. B.; Hong, I. K. Depolymerization Behavior for cisPolyisoprene Rubber in Supercritical Tetrahydrofuran. J. Ind. Eng. Chem. 1998, 4, 26−30. (14) Hoffmann, M. M.; Conradi, M. S. Are there hydrogen bonds in supercritical methanol and ethanol? J. Phys. Chem. B 1998, 102, 263− 271. (15) Barlow, S. J.; Bondarenko, G. V.; Gorbaty, Y. E.; Yamaguchi, T.; Poliakoff, M. An IR study of hydrogen bonding in liquid and supercritical alcohols. J. Phys. Chem. 2002, 106, 10452−10460. (16) Zhang, Y.; Yang, J.; Yu, Y. X.; Li, Y. G. Structural and hydrogen bond analysis for supercritical ethanol: A molecular simulation study. J. Supercrit. Fluids 2005, 36, 145−153. (17) Gui, M. M.; Lee, K. T.; Bhatia, S. Supercritical ethanol technology for the production of biodiesel: Process optimization studies. J. Supercrit. Fluids 2009, 49, 286−292.
think that the ethanol is a better solvent for the wood degradation in supercritical alcohols.
4. CONCLUSION The kinetic analysis method for degradation of wood was studied in supercritical ethanol and methanol to obtain the comparative study of SCM and SCE. From this study, at the supercritical region, it was found that higher conversion in SCE occurs at a lower temperature than that of SCM. So, it can be seen that SCE can provide an alternative approach to lower operation temperature of wood degradation reactions, although further research works will still be obligatory. Also to verify the effectiveness of the kinetic analysis method proposed in this work, the experimental values were compared with the computed values by using kinetic parameters obtained in this work. The experimental values and the computed values from the model in this work totally agree, and the kinetic analysis method used in this work gives reliable values of the kinetic parameters for the degradation of wood in supercritical alcohols.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 82-41-521-9423. Fax: 82-41-552-0380. E-mail: ohsec@ kongju.ac.kr.
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ACKNOWLEDGMENTS The authors thank Kongju National University, South Korea, for its research grant and the continuous support throughout the project.
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NOMENCLATURE A = pre-exponential factor [min−1] Ao = factor in eq 3 [min−1 K−1/2] E = apparent activation energy [J mol−1] M = order on the reaction temperature, dimensionless [−] n = overall reaction order, dimensionless [−] R = gas constant, 8.314 [J mol−1 K−1] T = temperature [K] t = time [min] W = final mass of a sample [g] Wo = initial mass of a sample [g] α = conversion, dimensionless [−] β = constant heating rate [K min−1]
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REFERENCES
(1) Demirbaş, A. Biodiesel from vegetable oils via transesterification in supercritical methanol. Energy Convers. Manage. 2002, 43, 2349− 2356. (2) Oh, S. C.; Kwak, H.; Bae, S. Y. A kinetic analysis of polymer degradation in supercritical fluid. J. Chem. Eng. 2006, 39, 1004−1009. (3) Hwang, G. C.; Kim, B. K.; Bae, S. Y.; Yi, S. C.; Kumazawa, H. Degradation of polystyrene in supercritical acetone. J. Ind. Eng. Chem. 1999, 5, 150−154. (4) Shaw, R. W.; Brill, T. B.; Clifford, A. A.; Eckert, C. A.; Franck, E. U. Supercritical water: A medium for chemistry. Chem. Eng. News 1991, 69, 26−39. (5) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987. (6) Yamazaki, J.; Minami, E.; Saka, S. Liquefaction of beech wood in various supercritical alcohols. J. Wood Sci. 2006, 52, 527−532. (7) Tan, K. T.; Gui, M. M.; Lee, K. T.; Mohamed, A. R. An optimized study of methanol and ethanol in supercritical alcohol technology for biodiesel production. J. Supercrit. Fluids 2010, 53, 82−87. 4514
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