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Carbon catalyzed supercritical water oxidation of phenol
Journal of Supercritical Fluids 22 (2002) 149– 156 www.elsevier.com/locate/supflu
Carbon catalyzed supercritical water oxidation of phenol Yukihiko Matsumura a,*, Taro Urase a,1, Kazuo Yamamoto a, Teppei Nunoura b b
a En6ironmental Science Center, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -0033, Japan Department of Urban Engineering, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan
Received 1 February 2001; received in revised form 18 June 2001; accepted 15 August 2001
Abstract Activated carbon was employed as a novel catalyst for supercritical water oxidation of phenol. High-concentrations of phenol were treated in supercritical water at 673 K and 25 MPa with an equivalent amount of oxygen in a reactor packed with activated carbon. Although activated carbon itself was oxidized in the reaction field, its weight decrease was sufficiently slow for its catalytic effect on phenol oxidation to be observed. The catalytic effect of activated carbon consisted of an enhancement of the reaction rate, a decrease in the tarry product yield, and an increase in the gas yield. Under the condition used in this study, 65% of oxygen delivered into the reactor was effectively used for phenol oxidation while only 39% of oxygen was used when no catalyst was applied. This report is the first to indicate the catalytic effect of carbonaceous materials on supercritical water oxidation, and it demonstrates that supercritical water oxidation using lower operation temperatures and inexpensive carbon catalysts may be possible. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenol; Supercritical water oxidation; Activated carbon; Catalyst
1. Introduction Supercritical water oxidation of organic compounds has attracted great interest among researchers for its fast reaction rate, which easily * Corresponding author. Present address. Department of Mechanical System Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshimashi, Hiroshima 739-8527 Japan. Tel./fax: +81-824-24-7561. E-mail address: [email protected] (Y. Matsumura). 1 Present address. Department of Civil Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan.
allows complete decomposition of hazardous organics [1]. Even hazardous and stable materials such as polychlorinated dibenzo-p-dioxines and polychlorinated biphenyls can be easily decomposed by this method [2–5]. However, supercritical water oxidation technologies are also expensive due to their operation at high temperature and high pressure, as well as the substantial amount of power required to pressurize the oxygenating reagent. To reduce these costs, Operation at lower temperatures and reductions in the amounts of oxygen will be required to reduce costs, and this can only be achieved through an increase in the oxidation rate in supercritical wa-
0896-8446/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 8 9 6 - 8 4 4 6 ( 0 1 ) 0 0 1 1 2 - 7
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ter. Supercritical water oxidation processes thus far commercialized employ temperatures as high as 873 K to achieve complete oxidation. Since the yield stress of metals decreases rapidly with temperature, operation at a high temperature requires a high-pressure reaction vessel with a much thicker wall. The need for expensive corrosion-resistant metals also raises the cost. In addition, to achieve complete oxidation in a short residence time, the amount of oxidant must be much larger than the stoichiometric equivalence. A faster reaction rate reduces the amount of oxygen required to achieve complete oxidation within a specified reaction time. To improve the oxidation rate in supercritical water, numerous researchers have investigated the application of catalysts. Krajnc and Levec [6– 8] and Ding et al. [9–12] were among the first to investigate the catalytic effect in supercritical water. To date, precious metals such as platinum and metal oxides such as manganese oxide, chromium oxide and vanadium oxide have been studied. In recent studies showing the effectiveness of titanium oxide and manganese oxide catalysts, Professor Savage’s group at the University of Michigan conducted an analysis based on their catalytic reaction kinetics [13– 15]. They also evaluated the activity and stability of these catalysts [16]. A similar approach for kinetic analysis was also taken by Oshima et al. [17]. Thus far, however, only precious metals and metal oxides have been investigated as possible candidate catalysts for use in supercritical water oxidation. In this paper, we focused on the catalytic activity of carbonaceous material. It has been shown that carbonaceous materials have a catalytic effect on supercritical water gasification of organic compounds, including phenol and glucose [18 – 20]. The finding that carbonaceous materials interact with these organics suggests that carbonaceous materials might also effectively catalyze supercritical water oxidation. This expectation is partly supported by the findings of Tukac et al. [21,22], who reported successful application of a carbonaceous catalyst to wet air oxidation of organic compounds under a subcritical condition. Another example of interaction between organics and carbonaceous materials is found in the down-
draft gasifier for biomass gasification. In this gasifier, a product gas with high tar content is forced through a bed of heated and carbonated feed [23]. Tarry material is decomposed in the bed of carbonated feed and a tar-free producer gas is obtained. This effect, in which tar is decomposed to improve the degradation efficiency of feedstock, is also expected in supercritical water oxidation. Good resistance against acid or alkali corrosion is another advantage of carbon, since metal oxides suffer from corrosion under some pH conditions. Not only the intended organics, but also carbon itself should be oxidized in the supercritical water oxidation reactor. A supply of additional carbon catalyst is thus unavoidable. However, because carbonaceous materials are relatively inexpensive, supercritical water oxidation with carbonaceous catalysts should be economically feasible if the initial and operating costs can be reduced. For example, market price of manganese dioxide is $18/kg, while that of charcoal is $0.66/kg in Japan. Carbonaceous catalyst is feasibly replenished when consumption of the catalyst is 27 times as much as that of MnO2 catalyst. To date, there have been no reports on the catalytic activity of carbon in the supercritical water oxidation process, and quantitative evaluations of both the catalytic effect and oxidation characteristics of carbon are needed. Accordingly, we investigated the catalytic activity of carbon during supercritical water oxidation, as well as the oxidation rate of the carbon itself in the reaction field of supercritical water oxidation.
2. Experimental The experimental apparatus used in this study was identical to that used previously for measuring the oxidation characteristics of high-concentrations of phenol [24], with the exception that a packed bed reactor of activated carbon was employed rather than a tubular reactor. Briefly, two HPLC pumps were used to feed phenol and hydrogen peroxide aqueous solutions into the reactor heated by the molten salt bath. The effluent from the reactor was rapidly cooled down to
Y. Matsumura et al. / J. of Supercritical Fluids 22 (2002) 149–156
room temperature using a water jacket, and depressurized by a back-pressure regulator. Liquid effluent was collected in a conical flask, and gas effluent was collected in a gas-collecting bag connected to the conical flask. The packed bed reactor was composed of 3/8 in. stainless tubing (inner diameter, 7.53 mm; length, 51 mm) and Swagelok connectors. The volume inside the reactor that was not occupied by activated carbon was 2.6 ml. This was identical to the corresponding volume in the tubular reactor used previously. Sintered stainless filters with an effective pore size of 10 mm were placed at the inlet and exit of the reactor to prevent carbon catalyst from being washed out of the reactor. The reactor was heated to the reaction temperature with water feed under the operation pressure, and then the phenol solution, but not the hydrogen peroxide solution, was delivered to the reactor. After the phenol concentration at the exit reached stability, hydrogen peroxide solution was delivered to the reactor, and the effluent was sampled with time. To measure the oxidation rate of activated carbon, several runs were conducted in which oxidation was stopped after a specific time, the reactor was cooled down, and the amount of activated carbon left in the reactor was measured. We used phenol as a model compound to be decomposed in the supercritical water oxidation process. Phenol is easily dissolved in water and then delivered to the reactor, and it is one of the most difficult compounds to decompose, even in supercritical water. Supercritical water oxidation of phenol has been widely studied [25– 32], and there is a large body of published material on the kinetics, reaction mechanism, and catalytic effect of other materials. In this study, the phenol concentration in feed was set at 2 wt.%. This concentration was high compared to that in previous research, and it was not suitable for kinetic analysis due to the possible temperature increase inside the reactor caused by the exothermic oxidation reaction. However, this is the practical concentration for supercritical water oxidation, and to detect the expected advantage in tarry material decomposition, a high concentration of phenol is desirable as feed [24]. The hydrogen peroxide solution was prepared so that the equivalence
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ratio of oxygen to phenol was unity. The residence time of the phenol solution ranged from 6.5 to 26 s. The reaction temperature was measured at the inlet and outlet of the reactor, and set at 673 K. Operation pressure was measured before the back-pressure regulator, and set at 25 MPa. As catalyst, 0.9 g of activated carbon from a coconut shell (10-32 mesh; Nakaraitesque, Tokyo, Japan) was packed into the reactor. Product gas was analyzed by GC-TCD analysis (GC/8A; Shimadzu), and the gas generation rate was calculated from the dilution factor of air. The liquid effluent was analyzed with an HPLC analyzer (HP-1100; Hewlett-Packard) for unreacted phenol concentration and with a TOC analyzer (TOC-500; Shimadzu) for TOC concentration. Reaction intermediate was extracted from the liquid phase using dichloromethane, and analyzed by GC-MS analysis (HP-6890, HP-5973; HewlettPackard). Relative errors associated with these analyses are expected to be at most 5%, judging from the reproducibility of the analytical result.
3. Results and discussion
3.1. Effect of adsorption onto acti6ated carbon Since activated carbon adsorbs phenol at room temperature, the adsorption characteristics of activated carbon in supercritical water were determined first. Prior to the experimental runs to decompose phenol by supercritical water oxidation, a breakthrough curve was determined at 673 K, 25 MPa by delivering only phenol solution and water, each at 0.5 cm3 min − 1, to the reactor and measuring the effluent phenol concentration with time. The residence time of the solution in the reactor was 26 s for this measurement. The breakthrough curve thus obtained is shown in Fig. 1. The change in phenol concentration in the effluent for the run with no activated carbon is also shown. The two curves are identical within the experimental error, and it was concluded that adsorption of phenol onto activated carbon was negligible under this operating condition. After 2 h, the phenol concentration in the effluent was the same as that in the feed solution.
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This indicates that no pyrolysis of phenol took place even in the presence of activated carbon catalyst under this condition. Xu et al. [18] made a similar observation when they reported an only 80% conversion of phenol in supercritical water even at a temperature as high as 873 K. At a lower temperature of 673 K, there should be no or much less phenol decomposition. The fact that it takes 2 h for the effluent to reach the feed concentration means that the system itself is not actually a plug flow reactor. Back mixing should have been caused by the combination of 1/16 in. tubing and the reactor of 3/8 in. o.d., as well as by the sintered stainless filters. This does not affect the following discussion, but this discrepancy from the plug flow reactor should be noted when discussing the change in effluent concentration with time.
3.2. Oxidation rate of acti6ated carbon
Fig. 2. Change of activated carbon weight with time during carbon-catalyzed phenol oxidation.
of a pseudo-steady state. Activated carbon is known to react with supercritical water to produce hydrogen and carbon dioxide, but its reaction rate, as measured by Matsumura et al. [33], shows its effect is negligible at 673 K. In the following section, the phenol decomposition efficiency is discussed based on the activated carbon oxidation rate observed here.
Fig. 2 shows the change of activated carbon weight with time during carbon-catalyzed phenol oxidation in supercritical water. For this and the following experimental runs, flow rates of phenol solution and hydrogen peroxide were both set at 0.80 cm3 min − 1. Activated carbon is oxidized at a constant rate, resulting in a linear weight decrease. Three hours were required for complete oxidation and gasification of 0.9 g of carbon. This was a sufficient duration compared to the residence time in the reactor (26 s), and the system in the reactor could be analyzed under assumption
Fig. 3 shows the phenol decomposition efficiency for the supercritical water oxidation of phenol as a function of time on stream. The decomposition efficiency for the case of no catalyst is indicated by a dashed line. Early in the trial, when the original amount of activated carbon was left in the reactor, the phenol decomposi-
Fig. 1. Breakdown curve for phenol adsorption onto activated carbon packed bed in supercritical water (673 K, 25 MPa).
Fig. 3. Phenol decomposition efficiency for supercritical water oxidation of phenol as a function of time on stream.
3.3. Catalytic effect of acti6ated carbon on supercritical water oxidation of phenol
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by, and the temperature at the outlet was always maintained at 673 K. Gopalan and Savage [28] measured the global reaction rate of phenol oxidation as rate= 102.34 9 0.28exp(−
12.49 1.0 )[OH]0.85 9 0.04 RT
[O2]0.50 9 005[H2O]0.42 9 0.05
Fig. 4. TOC conversion change with time on stream.
tion efficiency was 0.75, much higher than the efficiency of 0.5 obtained in the absence of a catalyst. Over time, the decomposition efficiency decreased with the decrease in activated carbon in the reactor (Fig. 2). After 200 min of time on stream, the phenol decomposition efficiency became stable at a value close to that in the absence of a catalyst. This was in accordance with the complete gasification of carbon in the reactor. Fig. 4 shows the TOC conversion, which was defined as the ratio of the TOC value of effluent to that of the phenol feed. The same tendency was observed as for the phenol decomposition efficiency. From this result, it is clear that the presence of activated carbon enhanced the decomposition of phenol. There are three possible explanations for the improvement in phenol decomposition efficiency. The first is that the improvement was due to the catalytic effect, as originally expected. The second is that the oxidation of activated carbon raised the temperature inside the reactor, thereby enhancing the reaction rate of supercritical water oxidation of phenol. And third, the surface temperature of the activated carbon, rather than the temperature inside the reactor, may have become locally high due to the oxidation of activated carbon, and phenol may have reacted at a faster reaction rate in the surface pores. In regard to the second possibility, a bed temperature increase to 693 K was actually observed at a position close to the inlet when the oxygen was first introduced to the reactor. However, the temperature decreased as time on stream passed
(1)
where R and T denote the gas constant and temperature, respectively. Calculation based on this rate equation requires that the whole reactor be heated to 773 K in order to attain a phenol decomposition efficiency of 75% at a residence time of 26 s. This is by far higher than the observed temperatures. Thus the increase in phenol decomposition efficiency cannot have been due to the temperature increase in the reactor. In regard to the third possibility, the heat balance was calculated using the Ranz equation [34],
hdp Cv = 2.0+ 0.6 p uf uf
1/3
9dpG v
1/2
,
(2)
to determine the heat transfer coefficient between activated carbon particles and supercritical water. In this equation, uf, Cp, v, and G are the thermal conductivity, heat capacity, viscosity, and mass flux of supercritical water; dp is the diameter of activated carbon particles; and h is the heat transfer coefficient between particle and supercritical water. The assumption is that heat generation by oxidation of activated carbon is balanced by the heat removal by the supercritical water passing through the packed bed. Namely, hnpap(Ts − Tf)= racDH,
(3)
where np, ap, Ts, Tf, rac, and DH are number of particles in the packed bed, surface area of a single particle, surface temperature, temperature of supercritical water, combustion rate of activated carbon, and heat of combustion for activated carbon, respectively. The surface temperature needed to attain this heat transfer rate was calculated to be 673.4 K when the temperature of supercritical water was 673.0 K. Thus, the temperature at the surface of activated carbon could not have been high enough to attain the observed phenol decomposition efficiency. From
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the above discussion, we conclude that the improvement of the phenol decomposition efficiency must have been due to the catalytic effect of the activated carbon. Fig. 5(a–b) compares the effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation with and without activated carbon. Interestingly, the increase in residence time did not show a substantial effect on the decomposition efficiency of phenol. This indicates that the oxidation reaction of phenol occurred relatively quickly, and thus oxygen, fed only by the equivalence ratio, was consumed within B5 s. This is in accordance with the observed absence of oxygen in the effluent gas. However, if this was the case, only a part of the packed bed reactor would have been needed for the phenol decomposition, and phenol decomposition efficiency should not have changed with time on
Fig. 6. (a) Yield of each dimer as a function of residence time for supercritical water oxidation of phenol without activated carbon. (b) Yield of each dimer as a function of residence time for supercritical water oxidation of phenol with activated carbon.
Fig. 5. (a) Effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation without activated carbon. (b) Effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation with activated carbon.
stream as long as a sufficient amount of activated carbon was left. We do not yet have any reasonable explanation for this phenomenon, and further investigation is needed. The decrease in the dimer yield by application of activated carbon was remarkable. The carbon balance was also much better when activated carbon catalyst was employed, indicating a decrease in the production of tarry material. Since the color of the effluent liquid was much lighter and suspended solid particles were not observed in the supercritical water oxidation of phenol using activated carbon, clearly the activated carbon suppressed the production of the tarry material. It is also noteworthy that the gas yield was largely increased by the activated carbon. Note that this gas yield does not include gas generation from the activated carbon itself. The correction was made based on the weight decrease curve of the activated carbon shown in Fig. 2. This enhanced gasification is important from the viewpoint of wastewater treatment, in which the TOC
Y. Matsumura et al. / J. of Supercritical Fluids 22 (2002) 149–156
or BOD value of effluent should be reduced to less than the limit determined by the regulations. Fig. 6(a–b) compares the yield of each dimer as a function of residence time. Note that the vertical scales are different between these figures. Not only was the yield of each dimer decreased, but the composition of the dimer products was affected by the activated carbon catalyst. By assuming that activated carbon oxidation produces only carbon dioxide, the fractions of feed oxygen used for activated carbon oxidation and phenol oxidation can be determined. For the operation at a residence time of 26 s, 65% of oxygen was used for phenol decomposition while 35% of oxygen was used for activated carbon oxidation. On the other hand, only 39% of oxygen fed into the reactor was used for phenol oxidation when no activated carbon was used. Thus it is concluded that although a portion of the oxygen fed into the reactor was used in activated carbon oxidation, the addition of activated carbon increased the oxygen utilization efficiency for phenol decomposition.
4. Conclusion The effect of activated carbon catalyst on supercritical water oxidation of phenol was investigated using a packed bed reactor at 673 K and 25 MPa, and the following conclusions were obtained. Adsorption of phenol onto activated carbon is negligible in supercritical water at 673 K, 25 MPa. Activated carbon is oxidized as well as phenol, although the carbon is oxidized more slowly. Activated carbon enhances the phenol decomposition efficiency, but this enhancement decreases as the amount of activated carbon decreases by oxidation. This enhancement of the phenol decomposition efficiency is due not to the temperature increase caused by the oxidation of carbon, but to the catalytic effect of activated carbon. Activated carbon is also effective for suppressing tarry material formation and increasing gas yield. Both the yield and composition of dimer products are affected by activated carbon.
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Acknowledgements This study was supported by the Earth Foundation. References [1] P.E. Savage, S. Gopalan, T.I. Mizan, C.J. Martino, E.E. Brock, Reactions at supercritical conditions: Applications and fundamentals, AIChE J. 41 (1995) 1723. [2] K. Hatakeda, Y. Ikushima, S. Ito, N. Saito, O. Sato, Supercritical water oxidation of 3-chlorobiphenyl using hydrogen peroxide, Chem. Lett. 303 (1997) 245. [3] K. Hatakeda, Y. Ikushima, O. Sato, T. Aizawa, N. Saito, Supercritical water oxidation of polychlorinated biphenyls using hydrogen peroxide, Chem. Eng. Sci. 54 (1999) 3079. [4] T. Sako, Decomposition of dioxins with supercritical water oxidation, Kagaku (Kyoto) 52 (1997) 31. [5] T. Sako, T. Sugeta, K. Otake, M. Sato, M. Tsugumi, T. Hiaki, M. Hongo, Decomposition of dioxins in fly ash with supercritical water oxidation, J. Chem. Eng. Jpn. 30 (1997) 744. [6] M. Krajnc, J. Levec, Catalytic oxidation of toxic organics in supercritical water, Appl. Catal. B 3 (1994) L101. [7] M. Krajnc, J. Levec, The role of catalyst in supercritical water oxidation of acetic acid, Appl. Catal. B 13 (1997) 93. [8] M. Krajnc, J. Levec, Oxidation of phenol over a transition-metal oxide catalyst in supercritical water, Ind. Eng. Chem. Res. 36 (1997) 3439. [9] L. Jin, Z. Ding, M.A. Abraham, Catalytic supercritical water oxidation of 1,4-dichlorobenzene, Chem. Eng. Sci. 47 (1992) 2659. [10] Z. Ding, S.N.V. Aki, M.A. Abraham, Catalytic supercritical water oxidation; Phenol conversion and product selectivity, Environ. Sci. Technol. 29 (1995) 2748. [11] S.N.V. Aki, Z. Ding, M.A. Abraham, Catalytic supercirical water oxidation: stability of Cr2O3 catalyst, AIChEJ. 42 (1996) 1995. [12] Z.Y. Ding, M.A. Frisch, L. Li, E.F. Gloyna, Catalytic oxidation in supercritical water, Ind. Eng. Chem. Res. 35 (1996) 3257. [13] J.L. Yu, P.E. Savage, Kinetics of catalytic supercritical water oxidation of phenol over TiO2, Environ. Sci. Technol. 34 (2000) 3191. [14] J. Yu, P.E. Savage, Catalytic oxidation of phenol over MnO2 in supercritical water, Ind. Eng. Chem. Res. 38 (1999) 3793. [15] J. Yu, P.E. Savage, Phenol oxidation over CuO/Al2O3 in supercritical water, Appl. Catal. B 28 (2000) 275. [16] J. Yu, P.E. Savage, Catalyst activity, stability, and transformations during oxidation in supercritical water, Appl. Catal. B 31 (2001) 123. [17] Y. Oshima, K. Tomita, S. Koda, Kinetics of the catalytic oxidation of phenol over manganese oxide in supercritical water, Ind. Eng. Chem. Res. 38 (1999) 4183.
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[18] X. Xu, Y. Matsumura, J. Stenberg, M.J. Antal Jr., Carbon-catalyzed gasification of organic feedstocks in supercritical water, Ind. Eng. Chem. Res. 35 (1996) 2522. [19] X. Xu, M.J. Antal Jr., Gasification of sewage sludge and other biomass for hydrogen production in supercritical water, Environ. Prog. 17 (1998) 215. [20] M.J. Antal Jr., S.G. Allen, D. Schulman, X. Xu, R.J. Divilio, Biomass gasification in supercritical water, Ind. Eng. Chem. Res. 39 (2000) 4040. [21] V. Tukac, J. Hanika, Catalytic wet oxidation of substituted phenols in the trickle bed reactor, J. Chem. Technol. Biotechnol. 71 (1998) 262. [22] V. Tukac, J. Vokal, J. Hanika, The effect of oxygen transport and hydrodynamics on the phenol oxidation in a trickle-bed reactor, Chem. Listy 94 (2000) 449. [23] T.B. Reed (Ed.), Biomass gasification— Principles and technology, Noyes Data, Park Ridge, 1981. [24] Y. Matsumura, T. Nunoura, T. Urase, K. Yamamoto, Supercritical water oxidation of high concentrations of phenol, J. Hazard. Mater. B73 (2000) 245 –254. [25] T.D. Thornton, P.E. Savage, Kinetics of phenol oxidation in supercritical water, AIChE J. 38 (1992) 321. [26] T.D. Thornton, P.E. Savage, Phenol oxidation pathways in supercritical water, Ind. Eng. Chem. Res. 31 (1992) 2451.
[27] R. Li, T.D. Thornton, P.E. Savage, Kinetics of CO2 formation from the oxidation of phenols in supercritical water, Environ. Sci. Technol. 26 (1992) 2388. [28] S. Gopalan, P.E. Savage, Reaction mechanism for phenol oxidation in supercritical water, J. Phys. Chem. 98 (1994) 12646. [29] S. Gopalan, P.E. Savage, A reaction network model for phenol oxidation in supercritical water, AIChE J. 41 (1995) 1864. [30] S. Gopalan, P.E. Savage, Phenol oxidation in supercritical water From global kinetics and product identities to an elementary reaction model, ACS Symp. Ser. 608 (1995) 217. [31] M. Krajnc, J. Levec, On the kinetics of phenol oxidation in supercritical water, AIChE J. 42 (1996) 1977. [32] Y. Oshima, K. Hori, M. Toda, T. Chommanad, S. Koda, Phenol oxidation kinetics in supercritical water, J. Supercrit. Fluids 13 (1998) 241. [33] Y. Matsumura, X. Xu, M.J. Antal Jr., Gasification characteristics of an activated carbon in supercritical water, Carbon 35 (1997) 819. [34] W.E. Ranz, Friction and transfer coefficients for single particles and packed beds, Chem. Eng. Prog. 48 (1952) 247.
Carbon catalyzed supercritical water oxidation of phenol Yukihiko Matsumura a,*, Taro Urase a,1, Kazuo Yamamoto a, Teppei Nunoura b b
a En6ironmental Science Center, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -0033, Japan Department of Urban Engineering, Uni6ersity of Tokyo, 7 -3 -1 Hongo, Bunkyo-ku, Tokyo 113 -8656, Japan
Received 1 February 2001; received in revised form 18 June 2001; accepted 15 August 2001
Abstract Activated carbon was employed as a novel catalyst for supercritical water oxidation of phenol. High-concentrations of phenol were treated in supercritical water at 673 K and 25 MPa with an equivalent amount of oxygen in a reactor packed with activated carbon. Although activated carbon itself was oxidized in the reaction field, its weight decrease was sufficiently slow for its catalytic effect on phenol oxidation to be observed. The catalytic effect of activated carbon consisted of an enhancement of the reaction rate, a decrease in the tarry product yield, and an increase in the gas yield. Under the condition used in this study, 65% of oxygen delivered into the reactor was effectively used for phenol oxidation while only 39% of oxygen was used when no catalyst was applied. This report is the first to indicate the catalytic effect of carbonaceous materials on supercritical water oxidation, and it demonstrates that supercritical water oxidation using lower operation temperatures and inexpensive carbon catalysts may be possible. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Phenol; Supercritical water oxidation; Activated carbon; Catalyst
1. Introduction Supercritical water oxidation of organic compounds has attracted great interest among researchers for its fast reaction rate, which easily * Corresponding author. Present address. Department of Mechanical System Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshimashi, Hiroshima 739-8527 Japan. Tel./fax: +81-824-24-7561. E-mail address: [email protected] (Y. Matsumura). 1 Present address. Department of Civil Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan.
allows complete decomposition of hazardous organics [1]. Even hazardous and stable materials such as polychlorinated dibenzo-p-dioxines and polychlorinated biphenyls can be easily decomposed by this method [2–5]. However, supercritical water oxidation technologies are also expensive due to their operation at high temperature and high pressure, as well as the substantial amount of power required to pressurize the oxygenating reagent. To reduce these costs, Operation at lower temperatures and reductions in the amounts of oxygen will be required to reduce costs, and this can only be achieved through an increase in the oxidation rate in supercritical wa-
0896-8446/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 8 9 6 - 8 4 4 6 ( 0 1 ) 0 0 1 1 2 - 7
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Y. Matsumura et al. / J. of Supercritical Fluids 22 (2002) 149–156
ter. Supercritical water oxidation processes thus far commercialized employ temperatures as high as 873 K to achieve complete oxidation. Since the yield stress of metals decreases rapidly with temperature, operation at a high temperature requires a high-pressure reaction vessel with a much thicker wall. The need for expensive corrosion-resistant metals also raises the cost. In addition, to achieve complete oxidation in a short residence time, the amount of oxidant must be much larger than the stoichiometric equivalence. A faster reaction rate reduces the amount of oxygen required to achieve complete oxidation within a specified reaction time. To improve the oxidation rate in supercritical water, numerous researchers have investigated the application of catalysts. Krajnc and Levec [6– 8] and Ding et al. [9–12] were among the first to investigate the catalytic effect in supercritical water. To date, precious metals such as platinum and metal oxides such as manganese oxide, chromium oxide and vanadium oxide have been studied. In recent studies showing the effectiveness of titanium oxide and manganese oxide catalysts, Professor Savage’s group at the University of Michigan conducted an analysis based on their catalytic reaction kinetics [13– 15]. They also evaluated the activity and stability of these catalysts [16]. A similar approach for kinetic analysis was also taken by Oshima et al. [17]. Thus far, however, only precious metals and metal oxides have been investigated as possible candidate catalysts for use in supercritical water oxidation. In this paper, we focused on the catalytic activity of carbonaceous material. It has been shown that carbonaceous materials have a catalytic effect on supercritical water gasification of organic compounds, including phenol and glucose [18 – 20]. The finding that carbonaceous materials interact with these organics suggests that carbonaceous materials might also effectively catalyze supercritical water oxidation. This expectation is partly supported by the findings of Tukac et al. [21,22], who reported successful application of a carbonaceous catalyst to wet air oxidation of organic compounds under a subcritical condition. Another example of interaction between organics and carbonaceous materials is found in the down-
draft gasifier for biomass gasification. In this gasifier, a product gas with high tar content is forced through a bed of heated and carbonated feed [23]. Tarry material is decomposed in the bed of carbonated feed and a tar-free producer gas is obtained. This effect, in which tar is decomposed to improve the degradation efficiency of feedstock, is also expected in supercritical water oxidation. Good resistance against acid or alkali corrosion is another advantage of carbon, since metal oxides suffer from corrosion under some pH conditions. Not only the intended organics, but also carbon itself should be oxidized in the supercritical water oxidation reactor. A supply of additional carbon catalyst is thus unavoidable. However, because carbonaceous materials are relatively inexpensive, supercritical water oxidation with carbonaceous catalysts should be economically feasible if the initial and operating costs can be reduced. For example, market price of manganese dioxide is $18/kg, while that of charcoal is $0.66/kg in Japan. Carbonaceous catalyst is feasibly replenished when consumption of the catalyst is 27 times as much as that of MnO2 catalyst. To date, there have been no reports on the catalytic activity of carbon in the supercritical water oxidation process, and quantitative evaluations of both the catalytic effect and oxidation characteristics of carbon are needed. Accordingly, we investigated the catalytic activity of carbon during supercritical water oxidation, as well as the oxidation rate of the carbon itself in the reaction field of supercritical water oxidation.
2. Experimental The experimental apparatus used in this study was identical to that used previously for measuring the oxidation characteristics of high-concentrations of phenol [24], with the exception that a packed bed reactor of activated carbon was employed rather than a tubular reactor. Briefly, two HPLC pumps were used to feed phenol and hydrogen peroxide aqueous solutions into the reactor heated by the molten salt bath. The effluent from the reactor was rapidly cooled down to
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room temperature using a water jacket, and depressurized by a back-pressure regulator. Liquid effluent was collected in a conical flask, and gas effluent was collected in a gas-collecting bag connected to the conical flask. The packed bed reactor was composed of 3/8 in. stainless tubing (inner diameter, 7.53 mm; length, 51 mm) and Swagelok connectors. The volume inside the reactor that was not occupied by activated carbon was 2.6 ml. This was identical to the corresponding volume in the tubular reactor used previously. Sintered stainless filters with an effective pore size of 10 mm were placed at the inlet and exit of the reactor to prevent carbon catalyst from being washed out of the reactor. The reactor was heated to the reaction temperature with water feed under the operation pressure, and then the phenol solution, but not the hydrogen peroxide solution, was delivered to the reactor. After the phenol concentration at the exit reached stability, hydrogen peroxide solution was delivered to the reactor, and the effluent was sampled with time. To measure the oxidation rate of activated carbon, several runs were conducted in which oxidation was stopped after a specific time, the reactor was cooled down, and the amount of activated carbon left in the reactor was measured. We used phenol as a model compound to be decomposed in the supercritical water oxidation process. Phenol is easily dissolved in water and then delivered to the reactor, and it is one of the most difficult compounds to decompose, even in supercritical water. Supercritical water oxidation of phenol has been widely studied [25– 32], and there is a large body of published material on the kinetics, reaction mechanism, and catalytic effect of other materials. In this study, the phenol concentration in feed was set at 2 wt.%. This concentration was high compared to that in previous research, and it was not suitable for kinetic analysis due to the possible temperature increase inside the reactor caused by the exothermic oxidation reaction. However, this is the practical concentration for supercritical water oxidation, and to detect the expected advantage in tarry material decomposition, a high concentration of phenol is desirable as feed [24]. The hydrogen peroxide solution was prepared so that the equivalence
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ratio of oxygen to phenol was unity. The residence time of the phenol solution ranged from 6.5 to 26 s. The reaction temperature was measured at the inlet and outlet of the reactor, and set at 673 K. Operation pressure was measured before the back-pressure regulator, and set at 25 MPa. As catalyst, 0.9 g of activated carbon from a coconut shell (10-32 mesh; Nakaraitesque, Tokyo, Japan) was packed into the reactor. Product gas was analyzed by GC-TCD analysis (GC/8A; Shimadzu), and the gas generation rate was calculated from the dilution factor of air. The liquid effluent was analyzed with an HPLC analyzer (HP-1100; Hewlett-Packard) for unreacted phenol concentration and with a TOC analyzer (TOC-500; Shimadzu) for TOC concentration. Reaction intermediate was extracted from the liquid phase using dichloromethane, and analyzed by GC-MS analysis (HP-6890, HP-5973; HewlettPackard). Relative errors associated with these analyses are expected to be at most 5%, judging from the reproducibility of the analytical result.
3. Results and discussion
3.1. Effect of adsorption onto acti6ated carbon Since activated carbon adsorbs phenol at room temperature, the adsorption characteristics of activated carbon in supercritical water were determined first. Prior to the experimental runs to decompose phenol by supercritical water oxidation, a breakthrough curve was determined at 673 K, 25 MPa by delivering only phenol solution and water, each at 0.5 cm3 min − 1, to the reactor and measuring the effluent phenol concentration with time. The residence time of the solution in the reactor was 26 s for this measurement. The breakthrough curve thus obtained is shown in Fig. 1. The change in phenol concentration in the effluent for the run with no activated carbon is also shown. The two curves are identical within the experimental error, and it was concluded that adsorption of phenol onto activated carbon was negligible under this operating condition. After 2 h, the phenol concentration in the effluent was the same as that in the feed solution.
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This indicates that no pyrolysis of phenol took place even in the presence of activated carbon catalyst under this condition. Xu et al. [18] made a similar observation when they reported an only 80% conversion of phenol in supercritical water even at a temperature as high as 873 K. At a lower temperature of 673 K, there should be no or much less phenol decomposition. The fact that it takes 2 h for the effluent to reach the feed concentration means that the system itself is not actually a plug flow reactor. Back mixing should have been caused by the combination of 1/16 in. tubing and the reactor of 3/8 in. o.d., as well as by the sintered stainless filters. This does not affect the following discussion, but this discrepancy from the plug flow reactor should be noted when discussing the change in effluent concentration with time.
3.2. Oxidation rate of acti6ated carbon
Fig. 2. Change of activated carbon weight with time during carbon-catalyzed phenol oxidation.
of a pseudo-steady state. Activated carbon is known to react with supercritical water to produce hydrogen and carbon dioxide, but its reaction rate, as measured by Matsumura et al. [33], shows its effect is negligible at 673 K. In the following section, the phenol decomposition efficiency is discussed based on the activated carbon oxidation rate observed here.
Fig. 2 shows the change of activated carbon weight with time during carbon-catalyzed phenol oxidation in supercritical water. For this and the following experimental runs, flow rates of phenol solution and hydrogen peroxide were both set at 0.80 cm3 min − 1. Activated carbon is oxidized at a constant rate, resulting in a linear weight decrease. Three hours were required for complete oxidation and gasification of 0.9 g of carbon. This was a sufficient duration compared to the residence time in the reactor (26 s), and the system in the reactor could be analyzed under assumption
Fig. 3 shows the phenol decomposition efficiency for the supercritical water oxidation of phenol as a function of time on stream. The decomposition efficiency for the case of no catalyst is indicated by a dashed line. Early in the trial, when the original amount of activated carbon was left in the reactor, the phenol decomposi-
Fig. 1. Breakdown curve for phenol adsorption onto activated carbon packed bed in supercritical water (673 K, 25 MPa).
Fig. 3. Phenol decomposition efficiency for supercritical water oxidation of phenol as a function of time on stream.
3.3. Catalytic effect of acti6ated carbon on supercritical water oxidation of phenol
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by, and the temperature at the outlet was always maintained at 673 K. Gopalan and Savage [28] measured the global reaction rate of phenol oxidation as rate= 102.34 9 0.28exp(−
12.49 1.0 )[OH]0.85 9 0.04 RT
[O2]0.50 9 005[H2O]0.42 9 0.05
Fig. 4. TOC conversion change with time on stream.
tion efficiency was 0.75, much higher than the efficiency of 0.5 obtained in the absence of a catalyst. Over time, the decomposition efficiency decreased with the decrease in activated carbon in the reactor (Fig. 2). After 200 min of time on stream, the phenol decomposition efficiency became stable at a value close to that in the absence of a catalyst. This was in accordance with the complete gasification of carbon in the reactor. Fig. 4 shows the TOC conversion, which was defined as the ratio of the TOC value of effluent to that of the phenol feed. The same tendency was observed as for the phenol decomposition efficiency. From this result, it is clear that the presence of activated carbon enhanced the decomposition of phenol. There are three possible explanations for the improvement in phenol decomposition efficiency. The first is that the improvement was due to the catalytic effect, as originally expected. The second is that the oxidation of activated carbon raised the temperature inside the reactor, thereby enhancing the reaction rate of supercritical water oxidation of phenol. And third, the surface temperature of the activated carbon, rather than the temperature inside the reactor, may have become locally high due to the oxidation of activated carbon, and phenol may have reacted at a faster reaction rate in the surface pores. In regard to the second possibility, a bed temperature increase to 693 K was actually observed at a position close to the inlet when the oxygen was first introduced to the reactor. However, the temperature decreased as time on stream passed
(1)
where R and T denote the gas constant and temperature, respectively. Calculation based on this rate equation requires that the whole reactor be heated to 773 K in order to attain a phenol decomposition efficiency of 75% at a residence time of 26 s. This is by far higher than the observed temperatures. Thus the increase in phenol decomposition efficiency cannot have been due to the temperature increase in the reactor. In regard to the third possibility, the heat balance was calculated using the Ranz equation [34],
hdp Cv = 2.0+ 0.6 p uf uf
1/3
9dpG v
1/2
,
(2)
to determine the heat transfer coefficient between activated carbon particles and supercritical water. In this equation, uf, Cp, v, and G are the thermal conductivity, heat capacity, viscosity, and mass flux of supercritical water; dp is the diameter of activated carbon particles; and h is the heat transfer coefficient between particle and supercritical water. The assumption is that heat generation by oxidation of activated carbon is balanced by the heat removal by the supercritical water passing through the packed bed. Namely, hnpap(Ts − Tf)= racDH,
(3)
where np, ap, Ts, Tf, rac, and DH are number of particles in the packed bed, surface area of a single particle, surface temperature, temperature of supercritical water, combustion rate of activated carbon, and heat of combustion for activated carbon, respectively. The surface temperature needed to attain this heat transfer rate was calculated to be 673.4 K when the temperature of supercritical water was 673.0 K. Thus, the temperature at the surface of activated carbon could not have been high enough to attain the observed phenol decomposition efficiency. From
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the above discussion, we conclude that the improvement of the phenol decomposition efficiency must have been due to the catalytic effect of the activated carbon. Fig. 5(a–b) compares the effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation with and without activated carbon. Interestingly, the increase in residence time did not show a substantial effect on the decomposition efficiency of phenol. This indicates that the oxidation reaction of phenol occurred relatively quickly, and thus oxygen, fed only by the equivalence ratio, was consumed within B5 s. This is in accordance with the observed absence of oxygen in the effluent gas. However, if this was the case, only a part of the packed bed reactor would have been needed for the phenol decomposition, and phenol decomposition efficiency should not have changed with time on
Fig. 6. (a) Yield of each dimer as a function of residence time for supercritical water oxidation of phenol without activated carbon. (b) Yield of each dimer as a function of residence time for supercritical water oxidation of phenol with activated carbon.
Fig. 5. (a) Effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation without activated carbon. (b) Effect of residence time on phenol decomposition efficiency and yields of gas and phenol dimers for supercritical water oxidation with activated carbon.
stream as long as a sufficient amount of activated carbon was left. We do not yet have any reasonable explanation for this phenomenon, and further investigation is needed. The decrease in the dimer yield by application of activated carbon was remarkable. The carbon balance was also much better when activated carbon catalyst was employed, indicating a decrease in the production of tarry material. Since the color of the effluent liquid was much lighter and suspended solid particles were not observed in the supercritical water oxidation of phenol using activated carbon, clearly the activated carbon suppressed the production of the tarry material. It is also noteworthy that the gas yield was largely increased by the activated carbon. Note that this gas yield does not include gas generation from the activated carbon itself. The correction was made based on the weight decrease curve of the activated carbon shown in Fig. 2. This enhanced gasification is important from the viewpoint of wastewater treatment, in which the TOC
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or BOD value of effluent should be reduced to less than the limit determined by the regulations. Fig. 6(a–b) compares the yield of each dimer as a function of residence time. Note that the vertical scales are different between these figures. Not only was the yield of each dimer decreased, but the composition of the dimer products was affected by the activated carbon catalyst. By assuming that activated carbon oxidation produces only carbon dioxide, the fractions of feed oxygen used for activated carbon oxidation and phenol oxidation can be determined. For the operation at a residence time of 26 s, 65% of oxygen was used for phenol decomposition while 35% of oxygen was used for activated carbon oxidation. On the other hand, only 39% of oxygen fed into the reactor was used for phenol oxidation when no activated carbon was used. Thus it is concluded that although a portion of the oxygen fed into the reactor was used in activated carbon oxidation, the addition of activated carbon increased the oxygen utilization efficiency for phenol decomposition.
4. Conclusion The effect of activated carbon catalyst on supercritical water oxidation of phenol was investigated using a packed bed reactor at 673 K and 25 MPa, and the following conclusions were obtained. Adsorption of phenol onto activated carbon is negligible in supercritical water at 673 K, 25 MPa. Activated carbon is oxidized as well as phenol, although the carbon is oxidized more slowly. Activated carbon enhances the phenol decomposition efficiency, but this enhancement decreases as the amount of activated carbon decreases by oxidation. This enhancement of the phenol decomposition efficiency is due not to the temperature increase caused by the oxidation of carbon, but to the catalytic effect of activated carbon. Activated carbon is also effective for suppressing tarry material formation and increasing gas yield. Both the yield and composition of dimer products are affected by activated carbon.
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