Gasification of Glucose in Supercritical Water - Industrial

This work was supported by RaCER/Ministry of Commerce, Industry, and Energy of Korea. The authors thank Dr. M. J. Antal, Dr. X. Xu, and Dr. P. Takahas...
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Gasification of Glucose in Supercritical Water In-Gu Lee,†,‡ Mi-Sun Kim,‡ and Son-Ki Ihm*,† Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea, and Korea Institute of Energy Research, P.O. Box 103, Daejeon 305-343, Korea

Gasification of 0.6 M glucose in supercritical water was investigated at a temperature range from 480 to 750 °C and 28 MPa with a reactor residence time of 10-50 s. The yield of hydrogen among gaseous products increased very sharply with increasing temperature above 660 °C. On the other hand, the yield of carbon monoxide decreased with temperature, most probably due to the role of a water-gas shift reaction. Carbon gasification efficiency reached 100% at 700 °C. A simplified model was proposed for the reaction pathways related to hydrogen production. The rates for glucose conversion and COD degradation were obtained by assuming pseudo-firstorder kinetics. Introduction Pure water has unique properties above its critical point of 374 °C and 22.1 MPa. The major properties of supercritical water, such as density, viscosity, dielectric constant, and hydrogen bonding, are quite different from those of steam or liquid water.1 Supercritical water behaves like a nonpolar organic solvent under ambient conditions, and many kinds of organic compounds and gases are completely dissolved in supercritical water, resulting in a single phase.2 On the other hand, however, the solubility of inorganic compounds decreases dramatically in supercritical water.3 Accordingly, supercritical water has a lot of potential applications.4-11 Supercritical water gasification (SCWG), in which supercritical water is not only a solvent for organic materials but also a reactant, is one of the applications for producing fuel gas from organic resources. Organic compounds,8 sewage sludges,9 agricultural wastes, and food processing wastes11 were examined as major feedstocks for SCWG. These biomasses are not considered as promising materials for the conventional gasification processes because of their high content of moisture. Because the water contained in wet biomass is used as solvent as well as reactant in SCWG, the drying procedure is not required for the gasification of wet biomass. Meanwhile, the biomasses used in SCWG technology are easily rotten, thereby resulting in a lot of leachate with offensive odors at room temperature and pressure. Therefore, it is important to treat the organic matter completely as well as to produce gas fuel by SCWG. Biomass polymer is typically composed of cellulose, hemicellulose, lignin, and others. Cellulose is known as one of the most refractory components for dissolving in hot water.12 The complete conversion of cellulose to glucose and its oligomers can be achieved at temperatures as high as 400 °C in supercritical water conditions,13 while the decomposition rate of glucose was inhibited by the increase in reaction pressure in supercritical water.14 Glucose is also a refractory intermediate * Corresponding author. Phone: 042-869-3915. Fax: 042869-3910. E-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ Korea Institute of Energy Research.

formed during the gasification of biomass for the SCWG process, requiring a high pressure of over 25 MPa. Therefore, gasification of glucose in supercritical water can be considered as a good model for gasification of more complex cellulosic biomasses in supercritical water. Although glucose in supercritical water is expected to be gasified through a variety of reaction pathways, glucose steam reforming (eq 1) and water-gas shift reactions (eq 2) have received exclusive research attention because of the importance of their roles in determining the degree of gasification and composition of gaseous products.9,15-17

C6H12O6 + 6H2O f 6CO2 + 12H2

(1)

CO + H2O f CO2 + H2

(2)

A hydrogen-rich gaseous product was obtained from the catalytic gasification of glucose in water at 374 °C and 22.1 MPa, mainly through water-gas shift reaction with a low efficiency (∼20%) of carbon gasification.15 This result demonstrates that a higher temperature is necessary to reach the complete gasification of glucose in supercritical water. Recently, Holgate et al.17 reported that glucose with a concentration as low as 1.0 × 10-3 M was completely gasified at 600 °C and 24.6 MPa. However, much higher concentrations of biomass must be handled if SCWG technology can enjoy the commercial promise. Antal’s team9,16 was the first to study SCWG technology extensively for the purpose of hydrogen-fuel production from wet biomass. Yu et al.16 gasified glucose with a wide range of concentration in supercritical water. They reported that 0.1 M glucose was completely converted to hydrogen-rich synthesis gas but that glucose of higher concentrations experienced incomplete gasification at 600 °C, 34.5 MPa, and a reactor residence time of 34 s. The gas yield and composition were found to depend on the condition of the reactor wall and the reactant concentration. In a subsequent study, Xu et al.9 demonstrated that some activated carbon and charcoal were effective catalysts to improve the carbon gasification efficiency in SCWG of glucose solution. They also reported that the carbon gasification efficiency in the gasification of 1.0 M glucose at 34.5 MPa decreased rapidly from 98% to 51% as the temperature decreased from 600 to 500 °C. Further-

10.1021/ie010066i CCC: $22.00 © 2002 American Chemical Society Published on Web 01/31/2002

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Figure 1. Schematic of supercritical-water tubular-flow reactor (1, balance; 2, feed tank; 3, pump; 4, union cross; 5, rod heater; 6, thermocouple; 7, cooling water jacket; 8, coiled heater; 9, Hastelloy C-276 tube; 10, furnace; 11, external thermocouple; 12, filter; 13, pressure transducer; 14, back-pressure regulator; 15, gas-liquid separator; 16, wet test meter).

more, the water-gas shift reaction with the high concentration of glucose was reported to be very sensitive to a small change in the experimental conditions at 600 °C. Therefore, further investigations are desirable for better understanding of the temperature-dependence of the two reaction pathways involved in SCWG of glucose. In this work, gasification of glucose solution was carried out using a SCW tubular flow reactor at a temperature range between 480 and 750 °C and a constant pressure of 28 MPa. The effect of the temperature-dependence in the two reaction pathways (eqs 1 and 2) for hydrogen production was investigated with a glucose solution of 0.6 M as a feed, which could minimize the side reactions such as char formation. The reaction pressure of 28 MPa was applied because SCWG of glucose does not seem to depend significantly upon the pressure in the range of 25-35 MPa.9 Pseudo-firstorder kinetics were assumed as a lumping kinetic analysis for decomposition of glucose and chemical oxygen demand (COD), respectively. Experimental Apparatus and Procedures A schematic of the apparatus used in this experiment is shown in Figure 1. This supercritical water flow reactor system was first developed by Antal et al.9 A glucose solution of 0.6 M was introduced by a highpressure pump (Waters, model 515 HPLC pump) from the feed tank into the reactor. The reactor was constructed of a Hastelloy C-276 tube 9.53 mm o.d., 6.22 mm i.d., and 670 mm long and was used for more than 50 h before being employed in this work. Two coiled heaters were employed at the entrance and exit of the reactor, respectively. The coiled heater was made by wrapping a long rod heater (4.6 mm diameter and 127 mm heated length) around the reactor’s outer wall. The length of the coiled heater toward the axial direction of the reactor was about 55 mm. One internal rod heater (3.18 mm diameter and 30 mm heated length) was also installed at the entrance region of the reactor for the rapid heating of the reactant. Reactor temperature was maintained at a desired value by a furnace and the heaters. A type K thermocouple was employed inside of the reactor to measure the reaction temperature. Axial temperature profiles along the reactor length were measured by eight type K thermocouples mounted on the reactor’s outer wall. The reactor temperature mentioned in this paper was the averaged value of the axial temperatures measured along the reactor functional

Figure 2. Temperature dependence of water density and thereby variations in reactor residence time at 28 MPa.

length. A back-pressure regulator (Tescom, model 261762-24) was employed between the pressure transducer and the gas-liquid separator to reduce the pressure from 28 MPa to atmospheric. The product flow was isolated in a gas-liquid separator. The gas flow rate was measured by a wet test meter (Sinagawa, model W-NK-0.5A). Liquid product was collected at the bottom of the separator and measured for its flow rate. The temperature and reactor residence times were the main experimental variables studied in this paper. The glucose feed concentration and operating pressure were fixed at 0.6 M and 28 MPa, respectively. The reactor residence time was defined as the reactor volume divided by volumetric flow rate of water at the reactor temperature and pressure. The volumetric flow rate of water was given by mass flow rate of water divided by density of water at the reaction conditions. Figure 2 shows the temperature-dependence of water density and, thereby, the variations in reactor residence time at 28 MPa and several mass flow rates of water.18 For example, the temperature range of 480-750 °C results in reactor residence times of 35-19 s at a water flow rate of 240 g/h. Meanwhile, the water flow rate range of 120-480 g/h lead to reactor residence times of 1042 s at 700 °C. Glucose and all of the reagents for the analysis of liquid effluent were purchased from Junsei Chemical Co. The glucose reactant solution was prepared with distilled and deionized water. Gaseous products were analyzed with a gas chromatograph (Donam Instruments, model DS 6200). A thermal conductivity detector was employed for the detection of hydrogen, carbon monoxide, carbon dioxide, and methane and a flame ionization detector for light hydrocarbons. A 80/100 mesh carbosphere molecular sieve-packed column was used, operating at 35 °C for 4.2 min, heating at 15 °C/min to 227 °C and 30 °C/min to 350 °C, and being held at 350 °C for 2 min. A mixture of 8% hydrogen in helium was used as a carrier gas. Liquid products were analyzed for glucose and COD concentrations and for pH. The glucose concentration was measured by the dinitrosalicylic acid (DNS) method.19 The COD concentration was determined by the closed reflux titrimetric method,20 and acidity was measured using a pH meter (Orion, model 290A). Results and Discussion The Hastelloy C-276 tube employed in this work as the reactor has been known to affect glucose gasification

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Table 1. Product Distributions in 0.6 M Glucose Gasification in Supercritical Water at 28 MPa Experimental Conditions reactor temperature (°C) 480 600 feed flow rate (g/h) 240 120 residence time (s) 35 50 H2 CO CO2 CH4 C2H4 C2H6 C3H6 C3H8 carbon hydrogen oxygen glucose COD PH

Gas Yield (mol/mol) 0.08 2.63 0.47 0.59 0.40 1.72 0.03 0.71 0.01 0.01 0.01 0.32 0.01 0.02 0.00 0.10 Gasification Efficiency (%) 16.5 67.3 4.0 91.3 21.4 67.0

600 360 16 0.52 1.30 0.32 0.21 0.04 0.08 0.06 0.03

750 240 19 4.78 0.27 3.52 1.26 0.00 0.46 0.00 0.00

39.2 26.2 32.4

99.7 145.4 121.2

Liquid Effluent Destruction (%) 82.1 99.9 91.5 38.6 86.7 62.7 2.7 2.7 2.5

100 99.8 3.8

in supercritical water when it is exposed to aqueous salts such as NaCl solution before gasification.16 Meanwhile, a series of previous experiments in our laboratory showed that the extent of gasification and the yield of hydrogen product increased slightly with the increase in total operating time of the Hastelloy C-276 tubing reactor, although the reactor had never been exposed to salts solutions before gasification. However, the continuous glucose gasifications yielded very stable and reproducible results as the total operating time of the reactor was over 40 h. Therefore, the Hastelloy C-276 tube which had been used in supercritical water conditions for more than 50 h was employed as the reactor in this whole work. It is noted that the wall effect of the reactor on SCWG of glucose may still remain during the experiments but may not be significant in comparison with the other reaction conditions, such as temperature and reactor residence time. Table 1 lists typical results of glucose gasification in supercritical water under some of extreme experimental conditions. In all cases, the gaseous product was composed of hydrogen, carbon monoxide, carbon dioxide, and methane as major components and ethane, ethene, propane, and propene as minors. Ethine, propine, C4-, and higher hydrocarbons were not detected. The yields (the moles of gas divided by the moles of glucose fed into the reactor) of the major gaseous products varied significantly with both reactor temperature and residence time. The gasification efficiencies (percentage of the total moles of C (H, O) atom in gaseous products per moles of C (H, O) atom of glucose feed) and liquid effluent destruction also depended greatly upon the reaction conditions. Especially, hydrogen yield and hydrogen gasification efficiency increased dramatically with the temperature varying from 480 to 750 °C. The effect of temperature and reactor residence time on the gasification results are discussed in more detail. Effect of Temperature. Figure 3a-d shows results of the 0.6 M glucose gasification in supercritical water at a reactor temperature range of 510-740 °C, 28 MPa, and a 30 s reactor residence time. Two SCWGs of glucose were conducted at intervals of 20 °C from 620 to 740 °C. The lines in parts a and b indicate a trend of the experimental data. The yields of hydrogen, carbon dioxide, and methane in Figure 3a have a tendency to increase with increasing temperature. The carbon mon-

oxide yield increased with increasing temperature, reaching the highest value at 660 °C, and then rapidly decreased with increasing temperature. Particularly, the carbon monoxide yield was much higher than those of the other gaseous products at a temperature range of 510-660 °C. Xu et al.9 obtained similar results from 1.0 M glucose gasification in supercritical water at temperatures of 500-600 °C and 34.5 MPa. These demonstrate that a significant amount of glucose was converted to carbon monoxide and remained quietly stable over the temperatures of 510-660 °C. Some of the glucose carbons might be gasified directly to carbon monoxide by a pyrolysis reaction. According to other researchers’ reports,14-17 however, most fractions of glucose in supercritical water are first decomposed to many kinds of water-soluble organic compounds before being converted finally to gaseous products. As the temperature increased beyond 660 °C in Figure 3a, the increasing rates of the hydrogen and carbon dioxide yields were accelerated. The carbon monoxide yield showed exactly the opposite behavior at the same temperature range. It is believed from this result that the water-gas shift reaction played an important role as well as the steam reforming reaction at temperatures above 660 °C for producing hydrogen. Meanwhile, the amount of methane produced increased steadily with temperature up to 740 °C. Methane is believed as a very stable compound under SCWG conditions. Webley and Tester21 also reported that methane was not converted in supercritical water at 652 °C, 24.6 MPa, and a reactor residence time of 14.8 s. The gas mixture produced from SCWG of wet biomass generally consists of a number of components including C-2 and C-3 hydrocarbons, as displayed in Table 1. Information about the composition of gaseous products may be important when a specific gas species should be separated. For example, pure hydrogen can be separated from the gas mixture, probably with a pressure swing adsorption (PSA). Figure 3b shows variations in the composition of the gaseous products with the reactor temperature in SCWG of 0.6 M glucose. The carbon monoxide has the highest molar fraction of 0.65 at 560 °C and lowest of 0.05 at 741 °C, while the hydrogen composition seemed to be steadily enhanced with an increase in the temperature. The abrupt enhancement in hydrogen and carbon dioxide compositions and the corresponding drop in carbon monoxide composition with increasing temperature beyond 660 °C demonstrated again that a fast-type water-gas shift reaction occurred in the temperature range. It is wellknown that the water-gas shift reaction rate is slow and that its equilibrium constant decreases with increasing temperature in gas phase.22 In this study, we observed a different type of water-gas shift reaction in supercritical water environments. Other researchers9,17 also found the fast-type water-gas shift reaction in their own studies of the glucose gasification in supercritical water. Meanwhile, the thermochemical equilibrium compositions of the gaseous products were theoretically calculated at the experimental temperatures and the pressure by the STANJAN computer program employing the elemental potential method. The prediction indicates that the molar fractions of hydrogen and carbon monoxide increase steadily with increasing temperature at 28 MPa in supercritical water, while those of carbon dioxide and methane steadily decrease at the same conditions. A very small amount of other gases is

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Figure 3. Effect of reactor temperature on 0.6 M glucose gasification in supercritical water at 28 MPa and a 30 s reactor residence time: (a) gas yields, (b) molar fractions of gaseous products, (c) gasification efficiencies, (d) glucose and COD conversions.

predicted to be present at equilibrium over the experimental conditions. A comparison of the experimental data with the theoretical data indicated that the carbon monoxide molar fraction from the experiments was higher than the predicted value, while opposite results were true for the other gaseous products through all of the experimental temperatures. In particular, the compositions of hydrogen and carbon monoxide were far from those predicted at low temperatures but approached abruptly to the theoretical values as the temperature was over 700 °C. For example, the compositions of hydrogen and carbon monoxide are 0.08 and 0.57 at 510 °C and 0.55 and 0.05 at 740 °C, respectively in Figure 3b. Meanwhile, the theoretical data for hydrogen and carbon monoxide are 0.28 and 0.003 at 510 °C and 0.59 and 0.03 at 740 °C, respectively. Figure 3c shows variations in the gasification efficiencies of carbon, hydrogen, and oxygen with the reaction temperature. The gasification efficiencies of both carbon and hydrogen increased with increasing temperature. The carbon gasification efficiency of 100% at temperatures above 700 °C indicates that the glucose was converted completely into gaseous products. The hydrogen gasification efficiency increased sharply with increasing temperature and reached 158% at 740 °C. This value (higher than 100%) demonstrates that the supercritical water was used as a hydrogen source as well as a solvent for glucose gasification. It is noted that the gasification efficiency does not give any information about a specific reaction pathway. The liquid effluent

was analyzed for both glucose and COD concentration. The COD concentration was measured as an alternative to represent the concentration of total organic compounds in the liquid effluent. Because only glucose and pure water were used in the gasification, the glucose and its reaction intermediates in the liquid product were unique materials affecting the COD analysis. The glucose destruction in Figure 3d was less than 70% at temperatures below 600 °C and abruptly increased with increasing temperature to reach 100% at 663 °C. The COD destruction at 510 °C was only 23% and increased steadily with increasing temperature to reach 85% at 663 °C and almost 100% at 700 °C. The color of the liquid product was almost red at 510 °C and varied to dark brown f orange f yellow as the temperature increased up to 600 °C. The liquid product became clear at 680 °C, at which point the COD destruction was over 95%. No solid materials such as char were detected in the liquid products at any temperatures, although a very small amount of black solid (char) was obtained during cleaning the reactor that was operated at low temperatures below 680 °C. The pH of the liquid effluent became higher with increasing temperature and had a range of 2.2-3.6. Figure 4 shows the reactor temperature profiles along with the reactor length of 0.7 m together with the furnace temperature profiles. The thermocouple position was numbered from the entrance region of the reactor heated. The reactor temperature was described as a bold letter on the left side of the reactor temperature line.

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Figure 4. Comparison of reactor wall temperature with furnace temperature for 0.6 M glucose gasification in supercritical water at 28 MPa and a 30 s reactor residence time. (Solid line: reactor wall temperature. Dash-dot line: furnace temperature.)

Figure 5. Effect of reactor residence time on the yields of gaseous products from 0.6 M glucose gasification at 28 MPa in supercritical water at 600 and 700 °C, respectively.

The furnace employed in this work consisted of three split zones, and each zone had its own thermocouple and heating system. All of the furnace temperatures were much higher than the reactor temperatures at the reactor temperature of 511 °C. This reveals that endothermic reactions were predominant through the whole reactor at this reactor temperature. Meanwhile, the temperature of the furnace zone 3 at the reactor temperature of 564 °C was slightly lower than the corresponding reactor temperatures, indicating that exothermic reactions occurred at the downstream zone of the reactor. As the reaction temperature increased over 660 °C, the temperature of the furnace zone 2 became significantly lower than the reactor temperatures. These phenomena lead to a conclusion that the exothermic reaction occurred from downstream of the reactor and proceeded toward upstream of the reactor as the reactor temperature was enhanced. The watergas shift reaction, which is exothermic, is believed to play an important role on this result. Effect of Reactor Residence Time. The effect of reactor residence time on the gasification of 0.6 M glucose in supercritical water was investigated at 600 and 700 °C, respectively. The variation in the reactor residence time at each temperature resulted from the change in the feed flow rate. As shown in Figure 5, the yields of all of the gases remained almost constant at 700 °C, being independent of the residence time except for the shortest residence time of 10.4 s. The high hydrogen yield and the low carbon monoxide yield at 700 °C must be due to the role of water-gas shift reaction at this temperature. The methane yield at 700 °C was higher than that obtained at 600 °C at every residence time. Kinetics. We found that a significant amount of carbon monoxide was produced from SCWG of the glucose and that its production rate increased with increasing temperature up to 660 °C at 28 MPa and a 30 s residence time. Although the carbon gasification efficiency reached 100% at 700 °C, the hydrogen and carbon dioxide yield continued to increase with increasing temperature beyond 700 °C. Most of the carbon monoxide produced was expected to come from watersoluble organic compounds that were the first products from glucose gasification. Meanwhile, Holgate et al.17 detected about 26 organic compounds in the liquid effluent from the glucose hydrolysis in supercritical

water at 500 °C. They found that the stability of organic compounds produced decreased sharply with increasing temperature except for several species, including 5-hydroxymethylfurfural, acetic acid, and acetaldehyde. They reported that these temperature-resistant compounds were converted finally to methane or carbon dioxide but not to carbon monoxide. Kabyemela et al.14 also experienced that glucose was rapidly converted into the aqueous organic compounds in supercritical water at 400 °C. Because we conducted glucose gasification at higher temperatures and residence times than those of the previous researchers, it is hypothesized that the water-soluble intermediates that formed from glucose were more rapidly converted to the gases. On the basis of aforementioned description, we summarized major reaction pathways associated with hydrogen production from glucose gasification in supercritical water. Glucose in supercritical water is first converted to a lot of water-soluble intermediates, and then most of the intermediates formed are contributed to produce carbon monoxide. Meanwhile, some of the intermediates are converted to carbon dioxide and hydrogen by a steam reforming pathway. The carbon monoxide formed is finally converted to carbon dioxide and hydrogen through the water-gas shift reaction. The rate of carbon monoxide formation is faster than that of the water-gas shift reaction at low temperatures. However, the water-gas shift reaction becomes very fast at temperatures as high as 700 °C. SCWG of organic matters in our work has two important goals: one is to produce gas fuel such as hydrogen, the other is to destroy the organic matters completely so that no other apparatus is necessary for further treatment of liquid effluent from the reactor. In this session, the rates of glucose and COD decomposition were obtained as a measure of the organic matter destruction rate. To obtain the rate of glucose decomposition, we further gasified 0.6 M glucose in supercritical water at a wide range of temperatures and residence times, and the results are displayed in Table 2. Pseudo-first-order kinetics were assumed for glucose decomposition in supercritical water at 480-700 °C and 28 MPa. The ratio of functional length to inner diameter of the tubular-flow reactor employed in the experiment was more than 110, and water occupied 98.8 mol % of the reactant solution at standard conditions. Furthermore, the tubular flows at two extreme temperatures

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1187 Table 2. Experimental Data on the Kinetics for 0.6 M Glucose Gasification in Supercritical Water at 28 MPaa run no.

T (°C)

τ (s)

Xg (mol %)

Xc (mol %)

ln kg (s-1)

ln kc (s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

480 483 500 501 504 512 518 524 531 536 544 546 555 569 574 587 599 600 601 601 601 602 616 631 649 700

34.7 34.1 32.1 32.1 31.6 31.0 30.1 29.7 29.2 28.8 28.1 28.1 27.4 26.4 26.1 25.5 24.8 19.5 48.6 32.4 24.3 16.2 23.9 23.4 22.6 20.8

70.1 72.9 66.2 67.3 72.0 64.7 71.4 70.8 71.7 71.4 73.2 85.1 76.5 81.3 82.7 87.2 93.0 89.1 99.8 99.2 94.5 85.8 94.0 98.3 99.6 99.8

38.6 44.1 36.3 37.0 70.1 37.0 39.8 70.0 39.0 72.5 44.1 50.9 74.7 51.7 54.2 62.1 68.1 65.9 86.7 83.3 73.1 62.7 71.4 80.0 85.5 96.6

-3.36 -3.26 -3.39 -3.36 -3.21 -3.39 -3.18 -3.18 -3.14 -3.13 -3.06 -2.69 -2.94 -2.76 -2.70 -2.52 -2.23 -2.17 -2.04 -1.91 -2.13 -2.12 -2.14 -1.75 -1.41 -1.18

-4.27 -4.07 -4.27 -4.24 -3.24 -4.21 -4.08 -3.21 -4.08 -3.10 -3.88 -3.68 -2.99 -3.59 -3.51 -3.27 -3.08 -2.89 -3.18 -2.90 -2.92 -2.80 -2.95 -2.68 -2.46 -1.82

a

Figure 6. First-order Arrhenius plot for glucose degradation in supercritical water at 28 MPa.

τ/Ci ) ∫X0 dX/-r, -r ) kCi[1 - X].

adopted in this work satisfied most of the criteria for plug-flow idealization, which are summarized by Cutler et al.24 Therefore, our tubular-flow reactor was modeled as a plug-flow reactor. For the plug-flow reactor operated at isothermal and isobaric conditions, the following reactor design equation is written for glucose decomposition in supercritical water:

τ ) Cgi

∫0X

g

dXg -rg

(3)

where τ ) reactor residence time (s), Xg ) conversion of glucose (mol/mol), Cgi ) initial concentration of glucose (M), and -rg ) reaction rate of glucose. Under the assumption of the pseudo-first-order kinetics for rg, a simplified design equation can be given as follows:

kg ) -ln(1 - Xg)/τ

(4)

The COD destruction rate as a measure of the degradation of all the organic compounds in the liquid effluent from SCWG of glucose was also investigated through the same pseudo-first-order kinetics. In this case, subscript c was used for COD instead of g. Table 2 lists 26 data runs at the experimental conditions and reaction rate constants calculated by eq 4 for the glucose and COD degradation, respectively. The reaction rate constant has a range of 0.29-0.85 s-1 for glucose and 0.01-0.55 s-1 for COD. Kabyemela et al.14 obtained reaction rate constants of 0.45-15.8 s-1 from the gasification of 0.007 M glucose in supercritical water at 300-400 °C, 25-40 MPa, and an extremely short residence time. They obtained the rate constants for glucose decomposition from the experiment conducted using a stainless steel reactor which was different from ours. Their operating conditions, especially the reactant concentration (0.007 M), were quite different from ours (0.6 M). The activation energies and pre-exponential factors were obtained through Arrhenius plots with the

Figure 7. First-order Arrhenius plot for COD degradation in supercritical water at 28 MPa.

reaction rate constants. Figure 6 shows the Arrhenius plot for glucose decomposition in supercritical water at 28 MPa. The error bars are the estimated values of experimental data at the 95% confidence level. The solid line obtained by regression can be said to be in good correlation with experimental data. At 600 °C, the errors in the reaction rate constants are only 10% for different reactor residence times. Ramayya and Antal25 suggested that the possible maximum error in reaction rate constant due to the incorrect application of the plugflow idealization to parabolic laminar-flow data would be about 30%. The reaction rate for glucose conversion can be given as follows:

-rg ) 103.09(0.26 exp(-67.6 ( 3.9/RT)Cg

(5)

Figure 7 shows the first-order assumed Arrhenius plot for COD degradation during SCWG of glucose. With the activation energy and pre-exponential factor predicted from Figure 7, the reaction rate for the first-order degradation of COD concentration is given as follows:

-rc ) 102.95(0.23 exp(-71.0 ( 3.9/RT)Cc

(6)

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Even if a better correlation could be obtained with non-first-order kinetics, it is hoped that these simplified reaction rate expressions provide basic information for the design of supercritical water gasification reactors under similar reactor system and experimental conditions. Conclusion The gasification of 0.6 M glucose in supercritical water was investigated using tubular-flow reactors operated at a reactor temperature range of 480-750 °C and 28 MPa with a reactor residence time of 10-50 s. Carbon monoxide product was present with high concentrations at temperatures of 510-660 °C and a 30 s residence time, and its yield increased with increasing temperature over the temperature range. The hydrogen yield increased sharply with increasing temperature over 660 °C, while the carbon monoxide yield decreased with temperature. It is believed that the water-gas shift reaction occurred significantly at temperatures over 660 °C. Methane was identified as a very stable compound in supercritical water at temperatures as high as 700 °C. Carbon gasification efficiency remained 100% at 700 °C for a wide range of reactor residence times of 10-50 s. A simplified model was proposed for the reaction pathways related to hydrogen production. Pseudo-firstorder kinetics were obtained for each of glucose and COD degradations by assuming a plug-flow. The activation energy was 67.6 kJ/mol for glucose decomposition and 71.0 kJ/mol for COD degradation, respectively. Acknowledgment This work was supported by RaCER/Ministry of Commerce, Industry, and Energy of Korea. The authors thank Dr. M. J. Antal, Dr. X. Xu, and Dr. P. Takahashi, (HNEI) for their valuable discussions on SCWG. Literature Cited (1) 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, (Dec), 26. (2) Josephson, J. Supercritical Fluids. Environ. Sci. Technol. 1982, 16, 548A. (3) Wofford, W. T.; Dell’Orco, P. C.; Gloyna, E. F. Solubility of Potassium Hydroxide and Potassium Phosphate in Supercritical Water. J. Chem. Eng. Data 1995, 40, 968. (4) El harfi, K.; Bennouna, C.; Mokhlisse A.; Ben chanaˆa, M.; Leme´e, L.; Joffre, J.; Amble`s, A. Supercritical Fluid Extraction of Moroccan (Timahbit) Oil Shale with Water. J. Anal. Appl. Pyrolysis 1999, 50, 163. (5) Goemans, M. G. E.; Tiller, F. M.; Li, L.; Gloyna, E. F. Separation of Metal Oxides from Supercritical Water by Crossflow Microfiltration. J. Membr. Sci. 1997, 123, 129. (6) Matsumura, Y.; Nonaka, H.; Yokura, H.; Tsutsumi, A.; Yoshida, K. Co-Liquefaction of Coal and Cellulose in Supercritical Water. Fuel 1999, 78, 1049. (7) Gloyna, E. F.; Li, L. Supercritical Water Oxidation: An Engineering Update. Waste Manage. 1993, 13, 379.

(8) Elliot, D. C.; Phelps, M. R.; Sealock, L. J., Jr.; Baker, E. G. Chemical Processing in High-Pressure Aqueous Environments. 4. Continuous-Flow Reactor Process Development Experiments for Organics Destruction. Ind. Eng. Chem. Res. 1994, 33, 566. (9) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J., Jr. Carbon-Catalyzed Gasification of Organic Feedstocks in Supercritical Water. Ind. Eng. Chem. Res. 1996, 35, 2522. (10) Cocero, M. J.; Alonso, E.; Torio, R.; Vallenlado, D.; FdzPolanco, F. Supercritical Water Oxidation in a Pilot Plant of Nitrogenous Compounds: 2-Propanol Mixtures in the Temperature Range 500-750 °C. Ind. Eng. Chem. Res. 2000, 39, 3707. (11) Lee, I. G.; Lee, J. S.; Kim, M. S. Hydrogen Production by the Gasification of Biomass in Supercritical Water. Presented at the 5th Korea-Japan Joint Symposium ‘99 on Hydrogen Energy, Taejon, Korea, November 1999; 365. (12) Allen, S. G.; Kan, L. C.; Zemann, A. J.; Antal, M. J., Jr. Fractionation of Sugar Cane with Hot Compressed Liquid Water. Ind. Eng. Chem. Res. 1996, 35, 2709. (13) Sasaki, M.; Kabyemela, B.; Adschiri, T.; Malaluan, R.; Hirose, S.; Takeda, N.; Arai, K. Cellulose Hydrolysis in Supercritical Water. The 4th International Symposium on Supercritical Fluids, Sendai, Japan, 1997; 583. (14) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Kinetics of Glucose Epimerization and Decomposition in Subcritical and Supercritical Water. Ind. Eng. Chem. Res. 1997, 36, 1552. (15) Amin, S.; Reid, R. C.; Modell, M. Reforming and Decomposition of Glucose in an Aqueous Phase. The Intersociety Conference on Environmental Systems, San Francisco, CA, 1975; The American Society of Mechanical Engineers (ASME); New York, 1975; ASME Paper No. 75-ENAs-21, 1. (16) Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen Production by Steam Reforming Glucose in Supercritical Water. Energy Fuels 1993, 7, 574. (17) Holgate, H. R.; Meyer, J. C.; Tester, J. W. Glucose Hydrolysis and Oxidation in Supercritical Water. AIChE J. 1995, 41, 637. (18) Haar, L.; Gallagher, J. S.; Kell, G. S. NBS/NRC Steam Tables: Thermodynamic and Transport Properties and Computer Programs for Vapor and Liquid States of Water in SI Units; Hemisphere Publishing Corp.: Bristol, PA, 1984. (19) Ghose, T. K. Measurement of Cellulase Activities. Pure Appl. Chem. 1987, 59, 257. (20) Franson, M. A. H. Standard Methods for the Examination of Water and Wastewater, 18th ed; American Public Health Association: Washington, DC, 1992. (21) Webley, P. A.; Tester, J. W. Fundamental Kinetics of Methane Oxidation in Supercritical Water. Energy Fuels 1991, 5, 411. (22) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed; McGraw-Hill: New York, 1993. (23) Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X. Biomass Gasification in Supercritical Water. Ind. Eng. Chem. Res. 2000, 39, 4040-4053. (24) Cutler, A. H.; Antal, M. J., Jr.; Jones, M., Jr. A Critical Evaluation of the Plug-Flow Idealization of Tubular-Flow Reactor Data. Ind. Eng. Chem. Res. 1989, 27, 691. (25) Ramayya, S. V.; Antal M. J., Jr. Evaluation of Systematic Error Incurred in the PlugFlow Idealization of Tubular Flow Reactor Data. Energy Fuels 1989, 3, 105.

Resubmitted for review June 19, 2001 Revised manuscript received November 16, 2001 Accepted November 21, 2001 IE010066I