Glucose Decomposition Kinetics in Water at 25 MPa in the

Feb 22, 2006 - Glucose decomposition kinetics was studied under supercritical pressure of 25 MPa in the temperature ranging from 448 to 673 K using a ...
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Ind. Eng. Chem. Res. 2006, 45, 1875-1879

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Glucose Decomposition Kinetics in Water at 25 MPa in the Temperature Range of 448-673 K Yukihiko Matsumura,* Satoru Yanachi, and Takuya Yoshida Department of Mechanical System Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527 Japan

Glucose decomposition kinetics was studied under supercritical pressure of 25 MPa in the temperature ranging from 448 to 673 K using a continuous reactor. It is usual that the reaction is assumed to be of the first order to determine the reaction rate coefficient, but in this study, reaction order was also determined from the experimental data, and reaction order less than unity was observed at a high temperature. Considering that reaction field changes from ionic to radicalic as temperature increases from 448 to 673 K, change in reaction mechanism is implied. Introduction Glucose is a monosaccharide readily observed in nature both as itself like in sugar-producing plants including sugar cane and sweet sorghum and as a monomer for starch and cellulose. Thus, it has been employed as a suitable model compound for lignocellulosics biomass. Especially to those who studied supercritical water gasification, its solubility in water provided ideal characteristics: Glucose solution behaved like actual biomass, while delivery to the high-pressure reactors was made easily with a conventional high-pressure liquid pump without plugging. Early study and fundamental study in hydrothermal treatment of biomass often treated glucose as a model compound.1-4 As early as 1968, it had been noticed that glucose could be converted (or degraded) into various smaller molecules in hot compressed water. Bobletter and Pape5 were one of the first researchers to determine the decomposition rate of glucose in hydrothermal condition; later, Amin et al.6 measured the decomposition rate of glucose. A more systematic study was made by Kabyemela et al.,7-9 and the reaction scheme as shown in Figure 1 was proposed. One interesting characteristic of the hydrothermal reaction field is its large change in polarity. At room temperature, the dielectric constant of water is as high as 80. With a temperature increase, it monotonically decreases and reaches 4. Dielectric constant is a good measure of ions in the reaction field, and when dielectric constant is high, ionic reaction easily takes place. High dielectric constant also means stability of polar and/or ionic intermediate for reactions. Antal et al.10 showed that both ionic and radical reactions take place in a hydrothermal reaction field. They showed that at a low-temperature region, ionic reaction takes place and that at a high-temperature region, radical reaction takes place for the same organic compound. A question arises: Is the reaction mechanism of glucose decomposition in the hydrothermal field the same for this wide temperature change? The purpose of this study was to determine the kinetics of glucose decomposition in hydrothermal conditions and find if there should be change in reaction mechanism with temperature. Experimental Section Figure 2 shows the reactor setup employed in this study. Figure 2a shows the whole reactor setup. The glucose solution * To whom correspondence should be addressed. Fax: +81-82-4227193. E-mail: [email protected].

was fed to the reactor by an HPLC pump (pump 1). Another HPLC pump (pump 2) fed water, which was delivered through a preheater and mixed with glucose solution at the inlet of the reactor. The reactor was stainless tubing with inner diameter of 1.0 mm, and the length was changed so that a desired residence time could be achieved within the range of the allowable flow rate of the HPLC pumps. After passing through the reactor, cooling water was added to the effluent and further cooled in the heat exchanger. The effluent flow was then depressurized with a back-pressure regulator and then sampled for liquid-phase analysis. The effluent was analyzed by HPLC for undecomposed glucose concentration. TOC analyses for the effluent showed that gas production in this study is negligible, and carbon balance was higher than 0.95 in most case. Figure 2b shows the detail of the mixing cross at the inlet and outlet of the reactor. The union on the left side of the figure is the inlet mixing cross, and glucose solution is fed from the right, and hot compressed water is fed from the left. After mixing, the flow is delivered to the reactor from the bottom part. The right-side union is for the exit. By making the connector part bored through, and extending the feeding line of the glucose solution to the middle of the cross, time required for mixing in the cross is minimized. The thermocouple was also placed in the cross, and temperature just after mixing could be measured. By inserting the thermocouple all the way down, it was shown that rapid mixing was possible by reducing dead volume. The concentration of the glucose solution ranged from 0.02 to 1.2 M at room temperature. The ratio of the flow rates of preheated water to that of glucose solution was 10:1. Reaction temperature was set at 448, 473, 498, 523, 573, 623, and 673 K, while reaction pressure for all runs was set at 25 MPa. The residence time for each temperature was determined so that change in glucose yield could be clearly found. Results and Discussions Figure 3 shows glucose yield change with time. The original data is also shown in Table 1 for reference purpose. Higher temperature results in higher reaction rate. First, to make a comparison with a previous study, we also conducted rate constant determination with the assumptions of reaction order being unity. The resulting parameters are shown in Table 2, and comparison with previous research was made in Figure 4. Regression of the parameters shown in Table 2 to

10.1021/ie050830r CCC: $33.50 © 2006 American Chemical Society Published on Web 02/22/2006

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Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006

Figure 1. Glucose decomposition pathways in hydrothermal conditions.

Figure 2. Experimental apparatus: (a) whole reactor setup, (b) detail of the mixing cross.

determine the Arrhenius parameter resulted in a preexponential factor of 1.33 × 1010 s-1 and activation energy of 121 kJ‚mol-1. Our data are in good agreement with Bobleter and Pape5 and Amin et al.6 where measurement was made at temperatures less than 498 K. However, much lower values than were obtained by Kabyemela et al.7 resulted in the higher temperature region. The data points are on the straight line in the low-temperature range, but deviation from the Arrhenius equation is larger in

the high-temperature range. This deviation led us to the further analysis using an nth order reaction rate equation. To determine the reaction order and reaction rate constant in the glucose-decomposition reaction rate equation:

dC ) -kC n dt

(1)

we integrated the above equation for n not equal to unity from

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Figure 3. Decomposition of glucose with time: (a) 448 K, (b) 473 K, (c) 498 K, (d) 523 K, (e) 573 K, (f) 623 K, (g) 673 K.

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Table 1. Experimental Results

expt

initial glucose concn in reactor (M)

050203-1 050203-1 050203-1 050203-1 050203-1 050203-2 050203-2 050203-2 050203-2 050203-3 050203-3 050203-3 050203-3 050203-3 050205-1 050205-1 050205-1 050205-1 050205-1 050205-2 050205-2 050205-2 050205-2 050205-2 041027-2 041027-2 041027-2 041027-2 041027-2 041123-1 041123-1 041123-1 041123-1 041123-1 041123-2 041123-2 041123-2 041123-2 041123-2 041123-3 041123-3 041123-3 041123-3 041123-3 041123-4 041123-4 041123-4

1.81E-02 1.81E-02 1.81E-02 1.81E-02 1.81E-02 9.07E-02 9.07E-02 9.07E-02 9.07E-02 2.72E-01 2.72E-01 2.72E-01 2.72E-01 2.72E-01 4.53E-01 4.53E-01 4.53E-01 4.53E-01 4.53E-01 5.44E-01 5.44E-01 5.44E-01 5.44E-01 5.44E-01 1.76E-02 1.76E-02 1.76E-02 1.76E-02 1.76E-02 8.81E-02 8.81E-02 8.81E-02 8.81E-02 8.81E-02 2.64E-01 2.64E-01 2.64E-01 2.64E-01 2.64E-01 4.41E-01 4.41E-01 4.41E-01 4.41E-01 4.41E-01 5.29E-01 5.29E-01 5.29E-01

temp (K)

residence time (s)

glucose yield (-)

448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 448 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473 473

42.5 68.8 95.4 114.6 228.9 45.4 67.3 94.7 228.9 44.7 66.8 94.9 113.7 230.8 44.6 68.7 95.4 114.5 229.5 44.7 69.2 95.9 114.6 229.5 44.6 68.4 91.0 114.1 218.9 42.7 55.7 95.1 170.2 325.8 42.6 56.9 83.4 111.5 209.0 42.6 50.9 83.4 103.7 166.5 42.6 55.2 82.6

0.976 0.973 0.976 0.972 0.965 0.993 0.994 0.996 0.972 0.973 0.994 0.970 0.986 0.975 0.996 0.993 0.974 0.986 0.928 0.997 0.997 0.986 0.986 0.980 0.939 0.904 0.892 0.910 0.823 0.958 0.960 0.937 0.926 0.816 0.953 0.968 0.907 0.823 0.857 0.989 0.982 0.955 0.981 0.927 0.953 0.969 0.974

expt

initial glucose concn in reactor (M)

041123-4 041123-4 041027-3 041027-3 041027-3 041027-3 041027-3 041129-1 041129-1 041129-1 041129-1 041129-1 041129-2 041129-2 041129-2 041129-2 041129-2 041129-3 041129-3 041129-3 041129-3 041129-3 041129-4 041129-4 041129-4 041129-4 041129-4 040714-3 040714-3 040714-3 040714-3 040714-3 040617-1 040617-1 040617-1 040617-1 040617-1 050113-1 050113-1 050113-1 050113-1 050113-1 050113-2 050113-2 050113-2 050113-2 050113-2

5.29E-01 5.29E-01 1.71E-02 1.71E-02 1.71E-02 1.71E-02 1.71E-02 8.53E-02 8.53E-02 8.53E-02 8.53E-02 8.53E-02 2.56E-01 2.56E-01 2.56E-01 2.56E-01 2.56E-01 4.26E-01 4.26E-01 4.26E-01 4.26E-01 4.26E-01 5.12E-01 5.12E-01 5.12E-01 5.12E-01 5.12E-01 1.65E-02 1.65E-02 1.65E-02 1.65E-02 1.65E-02 8.04E-02 8.04E-02 8.04E-02 8.04E-02 8.04E-02 1.64E-02 1.64E-02 1.64E-02 1.64E-02 1.64E-02 8.21E-02 8.21E-02 8.21E-02 8.21E-02 8.21E-02

Table 2. Reaction Parameters Obtained by Assuming First-Order Reaction k (s-1) temp (K)

optimum detd by least-squares

95% reliability

448 473 498 523 573 623 673

8.52E-05 5.36E-04 2.21E-03 4.98E-03 2.34E-01 5.04E-01 4.92E+00

8.30E-05 ( 3.60E-05 5.80E-04 ( 1.03E-04 2.19E-03 ( 2.68E-04 4.97E-03 ( 1.13E-04 2.34E-01 ( 2.12E-02 5.04E-01 ( 2.85E-02 5.08E+00 ( 4.15E-01

temp (K)

residence time (s)

glucose yield (-)

473 473 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 498 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523 523

110.9 164.5 42.1 65.5 87.7 106.6 214.6 39.8 66.9 109.2 159.8 382.5 40.8 63.1 82.8 111.0 194.9 41.6 62.8 80.3 108.3 160.1 41.8 61.4 77.9 107.9 164.7 4.3 6.4 13.4 20.8 26.8 6.0 12.7 26.5 29.2 44.9 4.3 6.5 8.9 10.7 21.2 4.3 6.3 10.7 16.1 32.1

0.957 0.937 0.873 0.755 0.735 0.663 0.542 0.934 0.855 0.736 0.722 0.745 0.955 0.854 0.847 0.805 0.648 0.872 0.846 0.801 0.771 0.757 0.940 0.881 0.797 0.774 0.729 0.847 0.824 0.710 0.676 0.616 0.811 0.746 0.666 0.654 0.561 0.809 0.801 0.786 0.737 0.635 0.905 0.904 0.840 0.797 0.701

expt

initial glucose concn in reactor (M)

040701-2 040701-2 040701-2 040701-2 040714-1 040714-1 040714-1 040714-1 040714-1 040915-1 040915-1 040915-1 040915-1 040915-1 040714-2 040714-2 040714-2 040714-2 040708-2 040708-2 040708-2 040708-2 040603-1 040603-1 040603-1 040603-1 040603-1 040708-3 040708-3 040708-3 040708-3 040708-3 040707-1 040707-1 040707-1 040707-1 040707-1 040602-1 040602-1 040602-1 040602-1 040602-1 040708-1 040708-1 040708-1 040708-1 040708-1

4.79E-01 4.79E-01 4.79E-01 4.79E-01 1.52E-02 1.52E-02 1.52E-02 1.52E-02 1.52E-02 9.49E-02 9.49E-02 9.49E-02 9.49E-02 9.49E-02 4.26E-01 4.26E-01 4.26E-01 4.26E-01 1.27E-02 1.27E-02 1.27E-02 1.27E-02 6.25E-02 6.25E-02 6.25E-02 6.25E-02 6.25E-02 3.75E-01 3.75E-01 3.75E-01 3.75E-01 3.75E-01 3.41E-03 3.41E-03 3.41E-03 3.41E-03 3.41E-03 1.67E-02 1.67E-02 1.67E-02 1.67E-02 1.67E-02 9.82E-02 9.82E-02 9.82E-02 9.82E-02 9.82E-02

temp (K)

residence time (s)

glucose yield (-)

523 523 523 523 573 573 573 573 573 573 573 573 573 573 573 573 573 573 623 623 623 623 623 623 623 623 623 623 623 623 623 623 673 673 673 673 673 673 673 673 673 673 673 673 673 673 673

6.4 12.9 22.2 32.3 0.81 1.62 0.53 2.72 3.41 0.51 0.81 1.70 2.40 4.97 0.52 0.79 1.58 3.35 0.43 0.68 1.00 1.32 0.46 0.64 1.23 2.08 4.94 0.44 0.66 1.01 1.31 2.15 0.11 0.15 0.18 0.36 0.60 0.13 0.14 0.18 0.37 0.52 0.12 0.15 0.18 0.31 0.36

0.858 0.817 0.780 0.719 0.810 0.629 0.855 0.488 0.422 0.929 0.812 0.689 0.527 0.362 0.872 0.841 0.736 0.584 0.513 0.329 0.274 0.171 0.513 0.445 0.287 0.186 0.072 0.580 0.552 0.436 0.392 0.304 0.213 0.149 0.059 0.014 0.014 0.364 0.362 0.265 0.105 0.049 0.314 0.238 0.277 0.141 0.125

assume glucose concentration at t ) 0 to be unity to avoid possible error caused by a temperature change at the inlet of the reactor, despite all the effort for quick heating by mixing

time t1 to t2 for the corresponding concentration C1 and C2 to obtain

C2 ) {C1-n - (1 - n)k(t2 - t1)}1/1-n 1

(2)

Each data point shown in Figure 3 is subject to experimental error. By least-squares method for the glucose yield, the values for k and n were determined for each temperature. We did not

Figure 4. Comparison of reaction parameters obtained by assuming firstorder reaction.

Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006 1879

Figure 5. Change in reaction order of glucose decomposition with temperature. Table 3. Obtained Reaction Parameters as n-th Order Reaction optimum detd by least-squares

95% reliability

temp (K)

k

(M1-n‚s-1)

n (-)

k

(M1-n‚s-1)

n (-)

448 473 498 523 573 623 673

1.09E-04 4.10E-04 1.75E-03 4.98E-03 1.20E-01 2.43E-01 1.44E+00

1.13 0.87 0.90 0.77 0.84 0.73 0.76

1.06E-4 ( 2.08E-5 3.95E-4 ( 5.36E-5 1.73E-3 ( 1.43E-4 4.97E-3 ( 1.13E-4 1.20E-1 ( 3.09E-3 2.43E-1 ( 8.48E-3 1.40E+0 ( 9.06E-2

1.26 ( 0.24 0.89 ( 0.16 0.93 ( 0.09 0.77 ( 0.01 0.84 ( 0.01 0.73 ( 0.02 0.76 ( 0.01

preheated water. Instead, we assumed that there is a time t1 for the glucose yield to be unity that is specified for each series of runs for the same temperature and initial glucose concentration, and the value of t1 was also determined by the least-squares method. This means that we assumed that error caused by initial heating of the solution, if any, is the same if reaction temperature and initial glucose concentration is the same. This may be controversial assumption, but it was the best assumption we could make, and the resulting value of t1 was close to zero. To determine the glucose concentration in the reactor, we assumed that density of the reaction fluid is the same as that of water at the same temperature and pressure. The reaction parameters thus obtained are shown in Table 3 with their 95% reliability interval. To determine 95% interval, optimal values for each parameter is determined by each data point. The average of these optimal values is also shown. Ideally, this value should be identical to the optimum value obtained by the least-squares method. Note that these values are sufficiently close. In determining the reaction parameters, the following step was taken. First, reaction parameters were determined by using all data points at the temperature. Then, experimental data were compared with the predicted value using the obtained rate equation. Differences between these two values were calculated, and standard deviation was calculated. Then data points whose difference were larger than twice of thus obtained standard deviation were omitted, and the reaction parameters were recalculated. The points that were omitted are also shown in Figure 3 as marks with slash on them. Figure 5 shows thus a determined order of reaction for glucose decomposition in supercritical water. As can be seen clearly, reaction order changed from unity to lower values as temperature increased. Since reaction order is directly affected by the reaction mechanism, and considering that reaction field changes from ionic to radicalic as temperature increases from 448 to 673 K,

this change in reaction order implies a change in reaction mechanism with the change of reaction temperature under 25 MPa. Glucose decomposition is a set of parallel reactions as shown in Figure 1. Each reaction is of different activation energy, and the main reaction pathway may shift from one to another with temperature. So far, the authors have not elucidated from which reaction to which reaction the main path moved. However, the authors believe that this change in main reaction path is of importance in order to understand the fundamentals of glucose gasification under hydrothermal condition. The fact that the data of Kabyemela et al.7 data were rather well expressed with a primary reaction equation in temperature range above 573 K while ours were not should be discussed. It is to be noted that they conducted experiments at 30, 35, and 40 MPa under 673 K. They also conducted reaction rate study at 25 MPa, but only under 573 and 623 K. Thus, all their experimental conditions are at density higher than that at 673 K and 25 MPa. Considering that we also obtained a reaction order close to unity for the condition where water density is higher, and in this high-density region our results were also in good agreement with previous research, it is probable that reaction order of glucose decomposition in water is close to unity when water density is high, but deviates from unity when water density gets lower. Conclusions Reaction rate for glucose decomposition in water under 25 MPa was determined in the temperature range of 448-673 K. The reaction order decreased with temperature from values around unity at 448 K to 0.7 at 673 K. Preexponential factors for each temperature were also determined. Acknowledgment This study was supported by NEDO International Grant and JSTS Grant-in-Aid. Literature Cited (1) Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen production by steam reforming glucose in supercritical water. Energy Fuels 1993, 7, 574. (2) Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J., Jr. Carboncatalyzed gasification of organic feedstocks in supercritical water. Ind. Eng. Chem. Res. 1996, 35, 2522. (3) Sinag, A.; Kruse, A.; Schwarzkopf, V. Key compounds of the hydropyrolysis of glucose in supercritical water in the presence of K2CO3. Ind. Eng. Chem. Res. 2003, 42, 3516. (4) Matsumura, Y.; Harada, M.; Li, D.; Komiyama, H.; Yoshida, Y.; Ishitani, H. Biomass gasification in supercritical water with partial oxidation. J. Jpn. Inst. Energy 2003, 82 (12), 919. (5) Bobleter, O.; Pape, G. Hydrothermal decomposition of glucose. Monatsh. Chem. 1968, 99, 1560. (6) Amin, S.; Reid, R. C.; Modell, M. Reforming and decomposition of glucose in an aqueous phase. ASME Pap. 1975, No. 75-ENAs-21. (7) 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. (8) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K.; Ohzeki, H. Rapid and selective conversion of glucose to erythrose in supercritical water. Ind. Eng. Chem. Res. 1997, 36, 5063. (9) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: detailed reaction pathway, mechanisms, and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888. (10) Antal, M. J., Jr.; Brittain, A.; DeAlmeida, C.; Ramayya, S.; Roy, J. C. Heterolysis and homolysis in supercritical water. ACS Symp. Ser. 1987, No. 329, 77.

ReceiVed for reView July 13, 2005 ReVised manuscript receiVed January 24, 2006 Accepted January 26, 2006 IE050830R