Visualized Kinetic Aspects of Decomposition of a Wood Block in Sub

Mar 25, 2005 - Quantitative kinetic analysis was performed for “hinoki” (a coniferous tree, Chamaecyparis obtusa) as a typical wood sample at a te...
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Ind. Eng. Chem. Res. 2005, 44, 2975-2981

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Visualized Kinetic Aspects of Decomposition of a Wood Block in Sub- and Supercritical Water Daisuke Shoji,† Kazuko Sugimoto,† Hiroshi Uchida,† Kiyoshi Itatani,† Makoto Fujie,‡ and Seiichiro Koda*,† Department of Chemistry, Faculty of Science and Technology, Sophia University, Kioi-cho 7-1, Chiyoda-ku, Tokyo 102-8554, Japan, and Joint Research Center for Supercritical Fluids, Japan Chemical Innovation Institute, Nigatake, Sendai 983-8551, Japan

Decomposition of a wood block in sub- and supercritical water was studied by directly observing the change in the size and shape of the wood block in a flow-type reaction cell through sapphire windows attached to the cell. Quantitative kinetic analysis was performed for “hinoki” (a coniferous tree, Chamaecyparis obtusa) as a typical wood sample at a temperature of 523-703 K and a pressure of 10-25 MPa. The phenomenological rate of the size shrink could be analyzed on the basis of two first-order reaction terms. Together with the measurements of the emission of total organic carbons (TOC), it was concluded that the wood block shrank with emitting TOC, and then it shrank at a smaller rate with emitting much smaller amounts of TOC. In the subcritical region, the phenomenological first-order rate constant for the initial shrink increased with the reaction temperature and approximately obeyed the Arrhenius equation with an activation energy of ca. 120 kJ mol-1. However, the rate constant decreased suddenly near the critical point and again increased with the temperature at the higher temperatures. The phenomenological kinetics was apparently determined by a certain chemical reaction. When the decomposition reaction was assumed to be proton-catalyzed, probably hydrolysis, the rapid change in the vicinity of the critical point was reasonably understood by taking into account the remarkable decrease of the ionic product. Introduction Supercritical water (SCW) technology, in particular supercritical water oxidation (SCWO) technology, is now going to be practically applied for decomposing organic materials and for energy recovery from low-quality fuels.1-3 Kinetics of homogeneous reaction of SCWO have been well-documented. Indeed, several detailed chemical kinetic models derived as an extension from combustion reaction models have been applied with considerable success to the analysis of SCWO of simple organic compounds such as methanol4 and benzene.5 However, kinetics of solid substances in supercritical water is rarely studied in both the presence and the absence of oxygen, which may be composed of mass transport and heterogeneous reactions as well as homogeneous reactions. In our previous publications on SCWO of solid substances,6,7 carbon particle was used as a model substance because a lot of knowledge has been accumulated on the combustion of carbon under ordinary pressures. By means of direct observation of the reaction progress with shadowgraph as well as Schlieren photography, being aided by computational fluid dynamics (CFD) calculation, the importance of the flow field surrounding the solid substances was revealed. However, in addition to the research for the simple material (i.e., carbon), more complex solid substances should be also studied for practical purposes. * To whom correspondence should be addressed. Tel: +81-3-3238-3377. Fax: +81-3-3238-3361. E-mail: s-koda@ sophia.ac.jp. † Sophia University. ‡ Japan Chemical Innovation Institute.

Biomass is a renewable organic resource and can produce recoverable useful chemicals, fuels, and heat; thus, it is expected to use as an alternative resource of energy of desiccation fuels such as oil and coal. Biomass treatment technology has been developed, such as pyrolysis,8 enzymatic hydrolysis,9 and acid hydrolysis.10-13 Recently, sub- and supercritical water have received much attention as a reaction solvent for biomass treatment in both the presence and the absence of oxygen. Because of the unique properties of water near the critical point, such as widely changeable ionic product with temperature, together with the potential thermal effect, it is expected that biomass is effectively and uniquely converted in sub- and supercritical water to fuels and chemicals. Not only woods themselves14,15 but also the main components of woods such as cellulose, hemicellulose, lignin, and their model compounds16-21 are interested. So far, sub- and supercritical water treatments such as gasification and liquefaction22-24 have been studied mainly on the basis of analysis of products. However, fundamental kinetic investigations are indispensable for near future practical applications of the supercritical water treatment technology. In the present study, we directly observed the time variation of the size and shape of a wood block in suband supercritical water in the absence of oxygen by means of shadowgraph imaging with a reaction cell, which is similar to the previous one employed in the SCWO study of carbon particles.6,7 Although direct visual observation is very promising for reaction engineering design, such trials have been rarely attempted. In the present work, phenomenological kinetic aspects were studied for “hinoki” (a coniferous tree, Chamaecyparis obtusa). The size change of a small wood block

10.1021/ie040263s CCC: $30.25 © 2005 American Chemical Society Published on Web 03/25/2005

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Figure 1. Reaction apparatus: (1) water reservoir, (2) syringe pump, (3) preheater, (4) cell block, (5) wood block sample, (6) sapphire window, (7) cooling fins, (8) Hasteloy rod, (9) outside magnet, (10) inside magnet, (11) light source, (12) CCD camera, (13) cooler, (14) back-pressure regulator, and (15) TOC analyzer.

together with the emission of total organic carbons (TOC) under various reaction conditions was kinetically analyzed. Only qualitative study was attempted for “sina” (a broad-leaved tree, Tilia japonica) and “kaba” (a broad-leaved tree, Betula maximowicziana). Experimental Section The experimental apparatus is shown in Figure 1, which consists of the reaction cell and flow systems. The cell is made of a Hasteloy cylindrical block of 60φ × 65 mm with a sample-transfer mechanism. Inside of the cell is a vertical cylindrical hole (8 mm in diameter) that is crossed at the center by two horizontal cylindrical holes at right angles with each other. The holes are equipped with four sapphire windows at their ends. The cell is controlled at a desired temperature using rod heaters and a thermocouple embedded in the Hasteloy block. The cell is connected to the other cylinder through a ceramic insulator and cooling fins, owing to which the temperature of the lower cylinder can be kept below 373 K. The visual observation is possible through the sapphire windows. The shadowgraph photographic system is consisted of a white light source and a CCD camera (Toshiba IK-645). A rectangular-shaped wood block of several mm side length is connected to one end of a Hasteloy rod (1.6 mm in diameter) by using a 0.1 mm diameter Pt wire that penetrates the center of the wood block. The other end of the Hasteloy rod is attached to a magnet. The magnet is coupled with the other magnet outside of the cylinder for moving the rod so as to transfer the sample to the center of the reaction cell at the initiation of the reaction. The distance between the fluid inlet and the center of the cell where the wood block is observed through the windows is 20 mm. A syringe pump (Isco 100DM) supplies water through a preheater to the reaction cell via a narrow inlet (1 mm in diameter) of the reaction cell. The inlet temperature of the water flow should be somewhat lower than that of the reaction cell itself, although the connection line between the preheater and the cell is protected against the heat loss by the surrounding insulators. The water flow exits upward through the vertical cylinder of the Hasteloy block. The pressure inside of the reaction cell is controlled by the back-pressure regulator (Jasco SCF-Bpg). Experimental procedure was as follows. First, the cell was filled with water to a desired pressure with water flow. After a sufficient period for stabilizing the condi-

tions, the wood block sample prepared as shown later was transferred into the cell (at the moment t ) 0) using the sample transfer mechanism, and the shadow image was started to be observed by a video camera. The ranges of experimental conditions were temperature 523-703 K, pressure 10-25 MPa, and the water flow rate 8.3 × 10-3-8.3 × 10-2 cm3 s-1 at normal pressure and room temperature. The effluent after the backpressure regulator was sampled in a 10 s interval when desired and analyzed using a TOC analyzer (Shimadzu TOC-VCPH). The elemental analysis was performed by means of a conventional analyzer (Perkin Elmer Series II CHNS/O analyzer 2400) for the dried wood block and also for the residue after being dried for 1 h in a drying oven at 100 °C. Small rectangular-shaped blocks of hinoki, sina, and kaba woods were at first dried at 100-120 °C for 1 h. Then the sample wood blocks were cut out from the blocks as a rectangular shape of 1.5 × 1.5 × 3.0 (sample S) or 3.0 × 3.0 × 4.5 mm (sample L). A hole of 0.1 mm diameter was drilled through which the Pt wire penetrated for attaching it to the Hasteloy supporting rod. The obtained sample was immersed in water under ambient condition for 1 day before use. It was confirmed that the geometrical size did not change after several hours passed since the immersion. Results and Discussion Visual Observation of the Wood Block Shrink in Sub- and Supercritical Water. When a hinoki wood block was transferred into the cell center at a high temperature around 300 °C, the block immediately started to shrink together with emission of some colored materials into the surrounding water. The surrounding water became colored, but the transparency was not completely lost under any adopted conditions. In most experiments, a small residue remained even after a very long reaction time such as 1 h in the case of 300 °C. The sina wood was expanded first and then shrank with maintaining approximately the original shape. The qualitative kinetic features were very similar to what was found for the case of hinoki. In the case of kaba wood, a very prompt decrease of the block size as if it melted was observed with emission of so dense materials that the visual observation became very difficult. The different behavior dependent on the kind of wood is expected to be caused by the different reactivity of the wood components. In particular, the character of the lignin component is different between a coniferous tree and a broad-leaved tree. The behavior of hinoki seemed to be simplest, typical, and thus most adequate for studying the applicability of the present new visualization method for understanding the kinetic features. The following analysis is limited to the case of hinoki. As mentioned above, the hinoki wood started to decrease the size after the transfer to the cell center. The shadowgraph images for a typical experimental run are shown in Figure 2. The shape decreases gradually. The side length D along the horizontal direction mainly decreases. In the present case, the vertical direction is parallel to the lead pipe of the tree. In experiments when the lead pipe direction was settled horizontally, the side shrink mainly proceeded along the vertical direction (i.e., along the right angle direction to the lead pipe). The shrinking rate for the right-angle direction to the lead pipe was eventually independent of the direction of the wood block location in the flow field. Thus the

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Figure 2. Time evolution of the shape of hinoki wood sample L. Reaction conditions: 310 °C, 25 MPa, and flow rate of 5 × 10-2 cm3 s-1.

Figure 3. Side length D change against time. Reaction conditions: 25 MPa; flow rate of 5 × 10-2 cm3 s-1; temperature (0) 300, (b) 350, and (O) 400 °C; sample L.

asymmetric shape change was principally caused by the inhomogeneous character of the wood block itself but not by the flow field asymmetry. In the previous SCWO study of carbon particles,6,7 the spherical shape of the particle was kept during the size change, which indicated that the asymmetric flow field did not appreciably affect the shape change. Temperature Evolution within the Wood Block. Before carrying out quantitative analysis of the size change, the time necessary for the wood block to reach the reaction temperature after the transfer to the center of the cell was estimated as follows. Assuming a sphere in place of the rectangular block for simplicity, the temperature evolution of the sphere is described by the following partial differential equation:

(

)

∂T ∂2T 2 ∂T )R 2 + ∂t r ∂r ∂r

(1)

The thermal diffusivity R is related to the thermal conductivity λ, density F, and heat content Cp as

R ) λ/(CpF)

(2)

By employing typical values of hinoki wood25 (i.e., λ ) 3.67 × 10-1 kJ m-1h-1K-1, F ) 5.30 × 102 kg m-3, and Cp ) 1.76 kJ m-3K-1 at an ambient temperature), R is evaluated to be 1.1 × 10-7 m2 s-1. This value is close to the value of water, which is 1.7 × 10-7 m2 s-1 at 100 °C and 20 MPa.26 The initial and boundary conditions solving eq 1 are

T ) T0 at t ) 0 T ) Ts at r ) R, and

∂T ) 0 at r ) 0 ∂r

(3) (4)

Ts is the surface temperature of the particle that is equal to the fluid temperature in the reaction cell, and T0 is that at the sample transfer cylinder. R is the radius of the sphere. By solving the equation for R ) 1.5 mm, the temperature of the center of the sphere reaches its 90% value of the settled reaction temperature, i.e.

(T at sphere center - T0)/(Ts - To) ) 0.9

(5)

within 4 s, when R ) 1.7 × 10-7 m2 s-1 was adopted as a representative value during the heat transfer process.

Figure 4. Common logarithmic change of D and its division to two exponential terms. Reaction conditions: 25 MPa; flow rate of 5 × 10-2 cm3 s-1; temperature (0) 300, (b) 350, and (O) 400 °C; sample L.

Consulting the above calculation, the real reaction temperature in the wood block should be considered to be slightly lower than the surrounding temperature. However, the disagreement is not severe because the reaction proceeds at the rate of minutes range as shown later. The outline of the time evolution of the wood block after it is introduced into the water flow is thus expected to be pursued without taking into account the heat transfer limitation. Analysis of the Decreasing Rate of Sample Side Length D. The side length change along the time is plotted in Figure 3 for the hinoki wood under several different temperatures (300, 350, 400 °C) at 25 MPa. The side length D decreases very rapidly in the initial stage and then slowly at later stages. At the same time, it is worthy to note that the side length reduction is more rapid at 350 °C than at 400 °C. To take out valuable kinetic information, D/D0 is plotted in a logarithmic scale against the time in Figure 4, where D0 is the initial side length. For the kinetic description, it might be better to adopt the volume change. However, we have adopted the side length D itself as the dependent valuable to describe the kinetics, to avoid the possible error accumulation through the procedure for evaluating the volume from the measured side length. The behavior in Figure 4 can be fairly well analyzed by using two first-order reaction terms, i.e.

D/D0 ) A exp(-k1t) + B exp(-k2t)

(6)

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Figure 5. Effect of pressure on reaction rate k1. Reaction conditions: temperature 300 °C and flow rate of 5 × 10-2 cm3 s-1. (b) Sample S; (O) sample L.

Figure 7. Arrhenius temperature dependence of the rate constant k1 of the first kinetic term. Reaction conditions: 25 MPa and flow rate of 5 × 10-2 cm3 s-1. (b) Sample S; (O) sample L.

Figure 6. Effect of flow rate on reaction rate k1. Reaction conditions: temperature 300 °C and 25 MPa, sample S.

The fitting procedure is visualized in the same figure. The second stage might correspond roughly to the formation of char. If we adopt a larger number of terms, it may result in a better fitting. At the same time, another way of description that does not use the summed exponential terms may be possible. However, it is clear that the two terms description by eq 6 is fairly satisfactory, and thus we have adopted the present two terms description for simplicity. By the described procedure, the two phenomenological firstorder rate constants have been determined. The contribution B of the second term is usually small, and the determined k2 value should be less reliable due to the possible error accumulation. We mainly discuss the kinetic behavior based on the first term rate constant k1. The rate constant k1 is not appreciably dependent on the pressure as well as the flow rate as shown in Figures 5 and 6, respectively. The negligible dependence on the flow rate suggests that the kinetics is not limited by mass transfer between the bulk and the wood block. Concerning the temperature dependence, a peculiar behavior has been found as already mentioned and also described more clearly in Figure 7 where the logarithmic value of k1 is plotted against the inverse temperature. In the subcritical region, the k1 rate constant increases with the reaction temperature obeying the Arrhenius equation with an activation energy of ca. 120 kJ mol-1. However, the rate constant drops suddenly near the critical temperature and again starts to increase further with the temperature at the higher temperatures. The initial size of the wood block seems not to affect the kinetic behavior for the different pressure and temperature.

Figure 8. Analysis of TOC time evolution on the basis of shape change. Reaction conditions: 300 °C, 25 MPa, and flow rate of 5 × 10-2 cm3 s-1. (O) Impulse response function f(t); (9) experimentally observed TOC; (]) estimated Φ(t) value.

TOC Emission, Size Shrink, and Carbon Mass Balance. For the more detailed understanding of the kinetics, quantitative production rates of individual chemicals are desirable. At the beginning, we have measured the total organic carbon (TOC). To check the relationship between the TOC emission and the size shrink, we have observed the TOC time profile when the wood block is transferred to the central location and kept here only for 15 s. The observed TOC output is drawn in Figure 8. This can be regarded as the impulse response function f(t) approximately, the distribution of which mainly corresponds to the dispersion due to the flow disturbance up to the back-pressure regulator. The expected profile Φ(t) of the TOC when the wood block is kept at the central location is obtained by convoluting the shrinking rate of the wood block volume V(t) with the impulse response function, according to the following equation:

Φ(t) )

∫0∞

dV(τ) f(t - τ) dτ dτ

(7)

The values of V(t) can be estimated from the measured side length D change. The estimated Φ(t) values are plotted for a typical case at 300 °C in Figure 8, where TOC is assumed to emit proportionally to the volume reduction of the wood block all through the shrinking process. The experimentally observed TOC profile is always smaller than the calculated profile by eq 7. This is more clearly shown in Figure 9, where the ratio between the experimental

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Figure 9. Ratio of the experimentally observed TOC against the predicted TOC Φ(t). The corresponding data are derived from Figure 8. Table 1. Carbon Balance of Wood Block Treated in Suband Supercritical Watera T (°C)

total TOC (g/g)

carbon content in residue (g/g)

total (g/g)

250 300 350 430

0.47 0.86 0.88 0.55

0.51 0.17 0.05 0.08

0.98 1.03 0.93 0.63

a The listed values are the rate of carbon content of the indicated portion to that of the initial wood block.

TOC and the estimated TOC Φ(t) is plotted. It is thus probable that the TOC emission is relatively larger for the earlier stage of the shrinkage and becomes smaller at later stages. The carbon mass balance was calculated by summing up the total TOC and the carbon content of the residue by the elemental analysis. The results are tabulated in Table 1. A satisfactory carbon balance was obtained at 250 and 300 °C. The recovered amount of carbon became smaller when the temperature became higher. The missing part is considered to be due to the gas evolution, which is expected to be larger at higher temperatures. Relationship of the Rate Constant and Ionic Product: Reaction Mechanism. The facts that the k1 value is not dependent on the bulk flow rate (in Figure 6) and that the different initial size of the wood block seems not to strongly affect the kinetic behavior suggest that the mass transfer process plays not such a decisive role for the k1 process as was found in the SCWO of active carbon particles.6,7 However, the rate constant obtained through the described procedure should not be considered to be exactly corresponding to a certain elementary reaction itself. Considering that the cellulose hydrolysis, as one example, is reported to proceed within 1 s under certain conditions,27 the present rate constant in the order of 10-1 s-1 at the largest may not correspond to such an elementary reaction only. Some complex rate processes, in some cases, being somewhat connected even with mass or heat transfer processes and also dissolution of wood components are expected to be responsible. Even though, a certain chemical reaction itself and/or some processes related directly to the chemical reaction are expected to mainly control the whole rate process. Thus, we will search for the relevant chemical processes in the following discussion. As one possible interpretation of the peculiar temperature dependence, the reaction was assumed to be induced by the coexisting proton with a rate constant

Figure 10. Arrhenius plot of the corrected rate constant k1′ for the water ionic product change. Reaction conditions: 25 MPa and flow rate of 5 × 10-2 cm3 s-1. (b) Sample S; (O) sample L. Dotted line: ionic product at 25 MPa.

proportional to its concentration, i.e.:

k1 ) k1′[H+]

(8)

The logarithmic value of the rate constant k1′ is accordingly

1 ln k1′ ) ln k1 - ln[H+] ) ln k1 - ln Kw 2

(9)

Here Kw is the ionic product, and its temperature and pressure dependences are taken from ref 26. The obtained k1′ values and Kw values are plotted against the inverse temperature in Figure 10. The Arrhenius law with an activation energy of 141 kJ mol-1 roughly obeys in subcritical region where the change of the ionic product against the temperature is relatively small. Above the critical temperature, the k1′ value is apparently on another straight line with a larger activation energy of 714 kJ mol-1. This indicates that the large drop of the k1 value may be attributed to the decrease of the ionic product, but that the higher reaction rate at the higher temperatures is induced by a different reaction mechanism from the subcritical mechanism. Whether the catalyst is indeed the proton is, however, not conclusively determined. This is because the same dependence as described in eq 9 can be derived even if OH- anion is the true catalyst. The ionic product does not change strongly against the pressure adopted in Figure 5, and thus even if the reaction is catalyzed by the coexisting proton or OH- anion, the rate constant is not expected to change appreciably against the pressure change. The main components of wood are hemicellulose, cellulose, and lignin. The typical fractions are reported to be 22, 47, and 30 wt %, respectively.28 If the phenomenological size shrink of the wood block is eventually controlled by a certain chemical reaction, the observed activation energies in the present study are to be compared with those of individual components. Various kinetic studies for the three components in water have been published. Among them, the recent thermogravimetric study for the components from corn stalk12 showed that hemicellulose decomposition occurred at relatively lower temperatures whereas cellulose decomposition occurred at relatively higher temperatures. Lignin decomposition covered the entire region. The reported activation energies for the decomposition of hemicellulose and lignin were 51 and 18,

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respectively. On the other hand they were 102 (at lower temperature 512-589 K) and 266 (at higher temperature 587-627 K) kJ mol-1 for cellulose. A higher activation energy such as 126 kJ mol-1 was reported for the acid-catalyzed hydrolysis of the prehydrolyzates of broad-leaved trees (hardwood) hemicellulose.10 The lignocellulose from sorghum straw was recently studied and reported to decompose through acid-catalyzed hydrolysis with an activation energy of around 180 kJ mol-1.11 An activation energy of 179 kJ mol-1 was reported for high solids acid hydrolysis of lignocellulose at 200-225 °C.12 A similar value of 172 kJ mol-1 13 was found for a dilute acid hydrolysis of cellulose originated from municipal solid wastes (pH 0.34-0.85. temperature 200-240 °C) with obeying a first-order dependence on [H+]. Sasaki et al.20 reported that the hydrolysis rate of cellulose slurry changed suddenly in the vicinity of the critical point. Their activation energy in subcritical region was ca. 130 kJ mol-1. The apparent effect of the ionic product, together with the comparison of relevant activation energies as mentioned above, suggests that the lower temperature mechanism is ionic. Probably a certain kind of hydrolysis reaction is rate-determining. On the other hand, the large activation energy at the higher temperatures seems to support nonionic mechanism, probably of a homolytic reaction above the critical temperature, although why again k1′ seems to obey the Arrhenius law is not explainable. The value of the very large activation energy itself may be not so much reliable due to the difficulty in precisely observing the rapid size change in the initial stage. Although hemicellulose is usually decomposed at the lowest temperature with the emission of acetic acid, it is difficult to identify individual components from the present study. The observed activation energy of 141 kJ mol-1 in subcritical region seems to agree with the hydrolysis mechanism of lignocellulotic materials. It is natural that ionic-type reactions dominates the process at a high water density or lower temperatures. Conclusions Decomposition of a wood block of hinoki (a coniferous tree, Chamaecyparis obtusa) in sub- and supercritical water was studied by directly observing the change of its size and shape under the temperature of 523-703 K and pressure of 10-25 MPa. The phenomenological rate of the size shrink could be analyzed on the basis of two first-order reaction terms. The wood block shrank with emitting TOC, and then it shrank at a smaller rate with emitting much smaller amount of TOC. The rate constant of the first process decreased suddenly near the critical point. The above phenomenological kinetics are considered to be mainly determined by relevant chemical reactions. By assuming that the decomposition is proton (or OHanion)-catalyzed, the rapid change in the vicinity of the critical point is attributed to the remarkable decrease of the ionic product. The relevant reaction may be hydrolysis in the subcritical region. Acknowledgment The present study was supported in part by a grant provided by NEDO (via JCII) based on the project “Research & Development of Environmentally Friendly Technology Using SCF” of the Industrial Science Technology Frontier Program (METI), which is greatly appreciated.

Literature Cited (1) Savage, P. E. Organic reactions in supercritical water. Chem. Rev. 1999, 99, 603. (2) Gloyna, E. F.; Li, L. Supercritical water oxidation research and development update. Environ. Prog. 1995, 14, 182. (3) Tester, J. W.; Cline, J. A. Hydrolysis and oxidation in subcritical and supercritical water: connecting process engineering science to molecular interactions. Corrosion 1999, 55, 1088. (4) Oshima, Y.; Bijanto, B.; Koda, S. PFR and CSTR analyses of supercritical water oxidation of methanol. J. Chem. Eng. Jpn. 2000, 35, 507. (5) DiNaro, J. L.; Howard, J. B.; Green, W. H.; Tester, J. W.; Bozzelli, J. W. Elementary reaction mechanism for benzene oxidation in supercritical water. J. Phys. Chem. A 2000, 104, 10576. (6) Sugiyama, M.; Kataoka, M.; Ohmura, H.; Fujiwara, H.; Koda, S. Oxidation of carbon particles in supercritical water: rate and mechanism. Ind. Eng. Chem. Res. 2004, 43, 690. (7) Sugiyama, M.; Tagawa, S.; Ohmura, H.; Koda, S. Supercritical water oxidation of a carbon particle by Schlieren photography. AIChE J. 2004, 50, 2082. (8) Bridgwater, A. V.; Peacocke, G. V. C. Fast pyrolysis process for biomass. Renewable Sustainable Energy Rev. 2000, 4, 1. (9) Chang, V. S.; Holtzapple, M. T. Fundamental factors affecting biomass enzymatic reactivity. Appl. Biochem. Biotechnol. 2000, 84, 5. (10) Kim, S. B.; Lee, Y. Y. Hydrolysis of hemicellulose by solid superacid. Biotechnol. Bioeng. Symp. 1985, 15, 81. (11) Tellez-Luis, S. J.; Ramirez, J. A.; Vazquez, M. Modelling of the hydrolysis of sorghum straw at atmospheric pressure. J. Sci. Food Agric. 2002, 82, 505. (12) Church, J. A.; Wooldridge, D. Continuous high-solids acid hydrolysis of biomass in a 1 1/2-in. plug flow reactor. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 371. (13) Malester, I. A.; Green, M.; Shelef, G. Kinetics of dilute acid hydrolysis of cellulose from municipal solid wastes. Ind. Eng. Chem. Res. 1992, 31, 1998. (14) Saka, S.; Ebara, K. Supercritical fluids to biomass research (II). Cellul. Commun. 2002, 9, 137. (15) Ehara, K.; Saka, S.; Kawamoto, H. Characterization of the lignin-derived products from wood as treated in supercritical water. J. Wood Sci. 2002, 48, 320. (16) Saisu, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Conversion of lignin with supercritical water-phenol mixtures. Energy Fuels 2003, 17, 922. (17) Yoshida, T.; Matsumura, Y. Gasification of cellulose, xylan, and lignin mixtures in supercritical water. Ind. Eng. Chem. Res. 2001, 40, 5469. (18) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Low-temperature catalytic gasification of lignin and cellulose with a ruthenium catalyst in supercritical water. Energy Fuels 2004, 18, 327. (19) Minowa, T.; Zhen, F.; Ogi, T. Cellulose decomposition in hot-compressed water with alkali or nickel catalyst. J. Supercrit. Fluids 1998, 13, 253. (20) Sasaki, M.; Kabyemela, B.; Malaluan, R.; Hirose, S.; Takeda, N.; Adschiri, T.; Arai, K. Cellulose hydrolysis in subcritical and supercritical water. J. Supercrit. Fluids 1998, 13, 261. (21) Sato, T.; Osada, M.; Watanabe, M.; Shirai, M.; Arai, K. Gasification of alkylphenols with supported noble metal catalysts in supercritical water. Ind. Eng. Chem. Res. 2003, 42, 4277. (22) Holliday, R. L.; King, J. W.; List, G. R. Hydrolysis of vegetable oils in sub- and supercritical water. Ind. Eng. Chem. Res. 1997, 36, 932. (23) Song, C.; Hu, H.; Zhu, S.; Wang, G.; Chen, G. Nonisothermal catalytic liquefaction of corn stalk in subcritical and supercritical water. Energy Fuels 2004, 18, 90. (24) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. Hydrothermal gasification of biomass and organic wastes. J. Supercrit. Fluids 2000, 17, 145.

Ind. Eng. Chem. Res., Vol. 44, No. 9, 2005 2981 (25) Murata, K.; Sadoh, T. Heat absorption and transfer in softwoods and their knot surfaces. Mokuzai Gakkaishi 1994, 40, 1180 (in Japanese). (26) Wagner, W.; Pruss, A. The IAPWS formulation 1995 for the thermodynamic properties of ordinary water substance for general and scientific use. J. Phys. Chem. Ref. Data 2002, 31, 387. (27) Sasaki, M.; Adschiri, T. Arai, K. Kinetics of cellulose conversion at 25 MPa in sub- and supercritical water. AIChE J. 2004, 50, 192.

(28) Mokuzai Kogyou Handbook (Handbook of Wood Industry); Forestry and Forest Products Research Institute, Eds.; Maruzen Co.: Tokyo, 2004; p 139 (in Japanese).

Received for review October 20, 2004 Revised manuscript received February 17, 2005 Accepted February 18, 2005 IE040263S