Ind. Eng. Chem. Res. 2000, 39, 2807-2810
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Supercritical Water Oxidation of a Model Municipal Solid Waste Takehiro Mizuno,† Motonobu Goto,*,‡ Akio Kodama,‡ and Tsutomu Hirose‡ Core Technology Research Department, Central Research Laboratory, R&D Center, Meidensha Corporation, Tokyo 141-8565, Japan, and Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 860-8555, Japan
Supercritical water oxidation has been focused on as an environmentally attractive technology by which organic materials can be oxidized to carbon dioxide, water, and nitrogen gas. We applied supercritical water oxidation to the treatment of dog food as a model municipal solid waste. The reaction was carried out in a batch reactor with hydrogen peroxide as the oxidant over the temperature range of 673-823 K. The liquid reaction products were analyzed to determine the total organic carbon (TOC), organic acid, and ammonium ion contents. When the oxidant was stoichiometrically insufficient, TOC decreased with an increasing temperature and amount of oxidant. Acetic acid and ammonium ion were detected as major refractory intermediates. Ammonium ion was completely decomposed more easily than in sewage sludge, presumably because hydrochloric acid produced Cr ion by corrosion of the wall of the reactor. The activation energy was 97.2 kJ/mol for the reduction of TOC and 130.8 kJ/mol for the reduction of ammonium ion, when analyzed by first-order kinetics. 1. Introduction Waste treatment is one of the most important and urgent problems in environmental management around the world. Development of a zero-emission process for the efficient treatment of biomass waste would be extremely desirable. Biomass is an important source of energy and chemicals, which are synthesized by solar energy from carbon dioxide and water. It must be a desirable target to develop a process by which chemical resources can be recovered easily from biomass waste so that the circulation of carbon dioxide in the biosphere can be controlled.1 The development of such a treatment process for municipal solid waste is also needed. Supercritical water oxidation (SCWO) has attracted attention for the treatment of industrial waste, especially toxic and refractory waste. Research on SCWO has been reviewed by several authors,2-7 and there have been some reports on the application of SCWO for sludge and wastewater.8-10 SCWO is an environmentally acceptable technology that produces disposable clean liquids (pure water), clean solids (metal oxides), and clean gases (CO2 and N2). During the SCWO process, oxidation takes place in water above its critical point (647 K and 22.1 MPa). Supercritical water has unique features with respect to its density, dielectric constant, ion product, viscosity, diffusivity, electric conductance, and solvent ability. From the engineering point of view, kinetic analysis of the reactions involved in SCWO is very important. Li et al.11,12 have analyzed kinetic data on the changes of carbon compounds during subcritical and supercritical water oxidation. When nitrogen-containing compounds are processed, an ammonium ion is a refractory intermediate that is produced during SCWO. Webley et al.13 analyzed the decomposition of ammonia, while * To whom correspondence should be addressed. Phone: +81-96-342-3664. Fax: +81-96-342-3679. E-mail: mgoto@ kumamoto-u.ac.jp. † Meidensha Corp. ‡ Kumamoto University.
Dell’Orco et al.14 studied SCWO involving nitrate salts and ammonia. They proposed a reaction mechanism and evaluated kinetic parameters. Ding et al.15 used MnO2/ CeO2 as a catalyst for SCWO of ammonia, finding that the rate of conversion to nitrogen was increased by about 2 orders of magnitude when compared to that for noncatalytic oxidation. We have investigated the treatment of municipal excess sewage sludge and alcohol distillery wastewater in our previous studies.16-19 In the present study, we applied SCWO with hydrogen peroxide as the oxidant to the treatment of dog food as a model municipal solid waste, which is a common type of biomass waste containing proteins, fats, vitamins, fiber, and inorganic minerals. Raw garbage was not used because the composition of each batch would have varied, while the content of dog food is constant. The effects of temperature, oxidant concentration, and reaction time on the decomposition of dog food were investigated in a batch reactor, and the data obtained were analyzed using a single first-order reaction model. 2. Experimental Section Dog food (Vitaone soft, Japan Pet Food Co. Ltd.) was used as the raw material. The results obtained by elemental analysis are shown in Table 1. The main components of this dog food were protein (about 20.0%), fat (about 7.0%), fiber (about 3.0%), ash (about 7.0%), and other substances (about 63%). Initially, the dog food was mashed thoroughly to a powder with a uniform particle size and then was dried in an oven overnight at 323 K. A batch reactor was made from a stainless steel tube sealed with Swagelok caps. The reactor capacity was about 5 mL at temperatures lower than 723 K and about 7 mL at temperatures higher than 773 K. The reaction pressure was calculated from the reaction temperature and the amount of water in the reactor. The percentage of hydrogen peroxide added was based on the stoichiometric demand for oxygen to achieve complete oxidation to carbon dioxide of the calculated carbon content in the dog food, and the
10.1021/ie0001117 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/21/2000
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Table 1. Composition of the Dog Food Used in This Study carbon (%) nitrogen (%) hydrogen (%) all other elements (%)
44.03 4.81 6.76 44.40
carbon concentration in the reaction mixture was adjusted to 37 800 mg/L for each experiment. The mass ratio of dog food to water was less than 10%. After the treated dog food, hydrogen peroxide as the oxidant, and water were added, the reactor was heated to supercritical conditions (a reaction temperature of 673-823 K and an estimated reaction pressure of 28 MPa) in a molten salt bath. After 30 min (the time when no residual TOC was detected after adding 200% of the oxidant stoichiometrically required), the reactor was cooled to room temperature by immersing it in a cool water bath. The reaction mixture was analyzed to determine the residual total organic carbon (TOC) using a TOC analyzer (Shimadzu TOC-5050A), while organic acids were detected by HPLC using the BTB reaction and the ammonium ion was measured by UV detection of a coloration reaction with phenol compounds. The kinetic data for TOC and the ammonium ion were obtained using 200% of the oxidant stoichiometrically required.
Figure 1. Effect of the oxidant amount and temperature on residual TOC.
3. Results and Discussion 3.1. Effect of the Oxidant Amount. The reaction products included solid, liquid, and gaseous components. The solid component was obtained as sediment because the residual solids settled rapidly. It was difficult to analyze the composition of this component because of the insufficient amount available. The main gases produced were probably carbon dioxide and nitrogen, as reported by other authors. The carbon content of the solid residue was negligible when oxidation was sufficient, and its color became paler as the amount of oxidant added to the reactor was increased. The liquid product was completely odorless when the amount of oxidant added was more than 100% of the stoichiometric requirement at temperatures higher than 723 K and more than 150% at 673 K. The liquid produced by the reaction was transparent and colorless when the amount of oxidant was more than 100% of the stoichiometric requirement at temperatures higher than 723 K and more than 150% at 673 K. Figure 1 shows the effect of the amount of oxidant on TOC in the liquid product. The reaction temperature ranged from 673 to 823 K, while the pressure was constant at 28 MPa. TOC decreased as more oxidant was added, and it was below the detection limit with over 100% oxidant at temperatures higher than 723 K and over 150% oxidant at 673 K. TOC was lower for reactions at a higher temperature. Various organic acids were produced during the reaction as intermediate products. Figure 2 shows the organic acids found in the reaction mixture at 723 K. The major component of the organic acids was acetic acid, which is reported to be the major intermediate of subcritical and supercritical water oxidation because of its refractory nature. The total amount of organic acid in the liquid phase (the acetic acid content was about 85-90%) increased to 40% and then decreased as the amount of oxidant added was increased further. Propionic acid was the second largest component, while a
Figure 2. Organic acids in the liquid phase after reaction.
small amount of iso-valeric acid was also detected at temperatures of less than 723 K and with less addition of oxidant. Organic acids were almost undetectable when over 100% oxidant was added at 723 K. 3.2. Kinetic Analysis. Kinetic analysis of the decomposition of wastes to carbon dioxide, nitrogen, and water by SCWO is useful for assisting the design of an effective waste treatment plant. However, waste generally contains various components, so the reactions involved in SCWO will be complex. Acetic acid and the ammonium ion are refractory intermediates during SCWO of organic waste containing carbon and nitrogen. The oxidation mechanism in SCWO has been studied by a number of researchers. Li et al.12 proposed the following simplified reaction pathway including carbon- and nitrogen-containing intermediates:
It is often sufficient to adopt a global rate model equation for representation of the overall oxidation reaction. Most researchers have used first-order kinetics for the reactant concentration, when the rate is independent of the oxidant concentration, as proposed by Goto et al.18 This gives
-d[A]/dt ) k0 exp(-Ea/RT)[A]
(2)
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Figure 3. Semilog plot of the ratio of TOC to the initial carbon content versus reaction time.
Figure 5. Semilog plot of the ratio of the residual ammonium ion to the initial nitrogen content versus reaction time.
Figure 4. Arrhenius plot of the reaction rate constant for reduction of TOC.
Figure 6. Arrhenius plot of the reaction rate constant for the reduction of the ammonium ion.
Table 2. Reaction Rate Constants for the Reduction of TOC
Table 3. Reaction Rate Constants for the Reduction of the Ammonium Ion
T ) 673 K T ) 723 K T ) 773 K
k (1/s) k0 (1/s) Ea (kJ/mol)
1.97 × 10-4 6.27 × 10-4 1.87 × 10-3 6.86 × 104 97.2 (R2 ) 0.999)
k (1/s)
k0 (1/s) Ea (kJ/mol)
where [A] is the reactant concentration. Integration of eq 2 with the initial condition ([A] ) [A0] at t ) 0) gives
ln([A]/[A0]) ) -k0 exp(-Ea/RT)t
(3)
which indicates that a plot of ln [A] versus time will yield a straight line with a slope given by k ) k0 exp(-Ea/RT). 3.2.1. TOC Analysis. Figure 3 shows a semilog plot of TOC in the liquid phase as a function of reaction time at 673, 723, and 773 K. TOC decreased faster at higher temperatures and was almost zero within 30 min even at the lowest temperature. The data for each temperature formed a straight line on the semilog plot, indicating that this was a first-order reaction. The reaction rate constant, k, obtained from each regression line is listed in Table 2. The activation energy was 97.2 kJ/mol. In the Arrhenius plot shown in Figure 4, these reaction rate constants are compared with those for municipal sewage sludge and alcohol distillery wastewater containing molasses.18 The activation energies were 76.3 and 64.7 kJ/mol for sewage sludge and alcohol distillery wastewater, respectively.
T ) 673 K T ) 723 K T ) 773 K T ) 823 K
7.10 × 10-5 1.78 × 10-4 9.56 × 10-4 4.75 × 10-3 7.47 × 105 130.8 (R2 ) 0.971)
The activation energy for dog food was higher than that for sewage sludge or wastewater, and this difference may have been related to differences of the coexisting inorganic components or pH. 3.2.2. Ammonium Ion Analysis. Figure 5 shows the effect of reaction time on the ammonium ion level in the liquid phase at temperatures of 673, 723, 773, and 823 K. The rate of reduction of the ammonium ion was faster than that for municipal sewage sludge19 and was also faster at higher temperatures. The ammonium ion was completely decomposed within 20 min at temperatures higher than 773 K. The data obtained at each temperature were approximately on a straight line in a semilog plot, with some scatter. The reaction rate constant, k, obtained from each regression line is listed in Table 3. The activation energy was 130.8 kJ/mol. An Arrhenius plot is shown in Figure 6 that provides a comparison with municipal sewage sludge, which had an activation energy of 139.0 kJ/mol for ammonium ion decomposition.19 The activation energy of dog food was lower than that of sewage sludge. This difference may have been related to differences in the composition and pH of the
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raw material. Hydrochloric acid that was formed by chlorides in the dog food seemed likely to indirectly enhance the decomposition of the ammonium ion. We analyzed pH, chloride ion, and metal ion levels in the reaction products. Dog food contained chloride compounds, and the pH of the filtrate of the original sample was 4.2. Because of ammonium ion production, the pH of the product was in the range of 8.4-9.0 after reaction for 30-3600 s at 723 K. After complete removal of the ammonium ion at 7200 s, the pH decreased to 6.4. The chloride ion concentration in the liquid phase was almost constant at about 500 ppm independent of the reaction time. Metals in the reaction product were investigated by measuring Fe and Cr. The Fe ion was not detected in the liquid phase, while the Cr ion level was 5, 19, 71, and 155 ppm after reaction at 723 K for 300, 1200, 3600, and 7200 s, respectively. These results indicate the effect of corrosion of the reactor wall by hydrochloric acid. Such corrosion was not observed with sewage sludge. The existence of hydrochloric acid may lead to corrosion of the reactor wall and production of the Cr ion, which may act as a catalyst in the decomposition rate of the ammonium ion. However, it is unclear how hydrochloric acid directly contributes to the oxidation of the ammonium ion in the supercritical water reactor. Therefore, the chloride ion or Cr ion may catalytically influence the oxidation reaction. 4. Conclusion Dog food was used as simulated garbage in a supercritical water batch reactor. Both color and odor were completely removed from the liquid phase when there was a sufficient amount of oxidant. When there was insufficient oxidant, TOC of the liquid phase decreased at higher temperatures and with a larger amount of oxidant. Acetic acid was a major component of the residual organic acids in the liquid phase, while propionic acid and isovaleric acid were also detected. After reaction with a sufficient amount of oxidant, acetic acid and the ammonium ion were the major refractory intermediates. A first-order reaction model was applied to analyze the kinetics of TOC and the ammonium ion. The decomposition rate of the ammonium ion was greater than that for sewage sludge, presumably because hydrochloric acid produced the Cr ion by corrosion of the reactor wall. The activation energies were 97.2 and 130.8 kJ/mol for the destruction of TOC and the ammonium ion, respectively.
Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research (No. 11128240) from the Ministry of Education, Science, Sports and Culture of Japan. Literature Cited (1) Arai, K. Biomass conversion in supercritical water for chemical recycling. Energy Resourc. 1995, 16, 2, 37. (2) Barner, H. E.; Huang, C. Y.; Johnson, T.; Jacobs, G.; Martch, M. A.; Killilea, W. R. Supercritical water oxidation: An emerging technology. J. Hazard. Mater. 1992, 31, 1. (3) Gloyna, E. F.; Li, L.; McBrayer, R. N. Engineering aspects of supercritical water oxidation. Water Sci. Technol. 1994, 30, 1. (4) Caruana, C. M. Supercritical water oxidation aims for wastewater cleanup. Chem. Eng. Prog. 1995, April, 10. (5) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet air oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (6) Gloyna, E. F.; Li, L.; McBrayer, R. N. Supercritical water oxidation research and development update. Environ. Prog. 1995, 14, 182. (7) Levec, J. Wet oxidation processes for treating industrial wastewaters. Chem. Biochem. Eng. Q. 1997, 11, 47. (8) Modell, M.; Larson, J.; Sobczynski, S. F. Supercritical water oxidation of pulp mill sludges. Tappi J. 1992, June, 195. (9) Shanableh, A.; Gloyna, E. F. Supercritical water oxidations wastewater and sludges. Water Sci. Technol. 1991, 23, 389. (10) Blarney, C. A.; Li, L.; Gloyna, E. F.; Hossain, S. U. Supercritical water oxidation of pulp and paper mill sludge as an alternative to incineration. ACS Symp. Ser. 1995, 608, 444. (11) Li, L.; Chen, P.; Gloyna, E. F. Generalized kinetic model for wet oxidation of organic compounds. AIChE J. 1991, 37, 1687. (12) Li, L.; Chen, P.; Gloyna, E. F. Kinetic model for wet oxidation of organic compounds in subcritical and supercritical water. ACS Symp. Ser. 1993, 514, 305. (13) Webley, P. A.; Tester, J. W.; Holgate, H. R. Oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water in the temperature range 530-700 °C at 246 bar. Ind. Eng. Chem. Res. 1991, 30, 1745. (14) Dell’Orco, P. C.; Gloyna, E. F.; Buelow, J. Reactions of nitrate salts with ammonia in supercritical water. Ind. Eng. Chem. Res. 1997, 36, 2547. (15) Ding, Z. Y.; Li, L.; Wade, D.; Gloyna, E. F. Supercritical water oxidation of NH3 over a MnO2/CeO2 catalyst. Ind. Eng. Chem. Res. 1998, 37, 1707. (16) Goto, M.; Nada, T.; Kawajiri, S.; Kodama, A.; Hirose, T. Decomposition of municipal sludge by supercritical water oxidation. J. Chem. Eng. Jpn. 1997, 30, 813. (17) Goto, M.; Nada, T.; Ogata, A.; Kodama, A.; Hirose, T. Supercritical water oxidation for the destruction of municipal excess sludge and alcohol distillery wastewater of molasses. J. Supercrit. Fluids 1998, 13, 277. (18) Goto, M.; Nada, T.; Kawajiri, S.; Kodama, A.; Hirose, T. Kinetic analysis for destruction of municipal excess sludge and alcohol distillery wastewater by supercritical water oxidation. Ind. Eng. Chem. Res. 1999, 38, 1863. (19) Goto, M.; Shiramizu, D.; Kodama, A.; Hirose, T. Kinetics analysis for ammonia decomposition in supercritical water oxidation of sewage sludge. Ind. Eng. Chem. Res. 1999, 38, 4500.
Received for review January 26, 2000 Accepted June 8, 2000 IE0001117
