4500
Ind. Eng. Chem. Res. 1999, 38, 4500-4503
Kinetic Analysis for Ammonia Decomposition in Supercritical Water Oxidation of Sewage Sludge Motonobu Goto,* Daisuke Shiramizu, Akio Kodama, and Tsutomu Hirose Department of Applied Chemistry and Biochemistry, Kumamoto University, Kumamoto 860-8555, Japan
Supercritical water oxidation was applied to the destruction of municipal excess sewage sludge. The reaction was carried out in a batch reactor with hydrogen peroxide as an oxidant in the temperature range of 723-823 K. Ammonia and acetic acid are found to be refractory intermediates in supercritical water oxidation of organic wastes. Ammonia concentration produced during the reaction was measured as a function of reaction time. The dynamic data were analyzed by a first-order kinetics. The reaction rate constant coincides with those reported in the literature. Introduction
Table 1. Composition of Sewage Sludge
Waste treatment is one of the most important and urgent subjects in environment preservation throughout the world. Development of efficient treatment technology for industrial/municipal wastes as well as toxic and refractory wastes has been desired. Supercritical water oxidation (SCWO), where reaction takes place in water above its critical point (647 K, 22.1 MPa), has been focused on as an environmentally acceptable technology for the treatment of various wastes. The research on SCWO was reviewed in several pieces of literature.1-6 SCWO had been applied to various sludges and wastewaters.7-9 We have applied SCWO to the treatment of municipal excess sewage sludge. The effects of temperature, oxidant amount, and reaction time on the decomposition of the sludge in a batch reactor were reported in our previous papers.10-12 Kinetic analysis of the decomposition rate in SCWO is important to design the process. Since the feed is generally a complex mixture, the reactant and product consist of a multicomponent system and the reaction mechanism is complex. A global reaction model has been extensively used to analyze SCWO. There are two elements, carbon and nitrogen, which should be focused on in kinetic analysis because destruction of the refractory intermediate products, acetic acid and ammonia, is often rate-controlling in the SCWO process. In our previous paper,12 we have analyzed the carbon-containing component represented by total organic carbon (TOC) by using first-order kinetics. In this work we focused on the decomposition of ammonia produced as a refractory intermediate. We measured the ammonium ion concentration in liquidphase product for SCWO of sewage sludge with hydrogen peroxide as an oxidant by using a batch reactor. The experimental kinetic data were analyzed by firstorder kinetics.
water content (%)
96.51
carbon (%) nitrogen (%) hydrogen (%) others (%)
38.06 6.95 6.04 48.96
of dried raw material are shown in Table 1. The sewage sludge mainly consisted of lipid (about 10%), protein (about 40%), carbohydrate, lignin (about 17%), and ash. A batch reactor made of a stainless steel tube sealed with Swagelok caps (about 4 mL in volume) was used. Hydrogen peroxide (about 30%) was used as an oxidant. The reactor fed with sludge, hydrogen peroxide, and water was heated to supercritical conditions (reaction temperature, 723-823 K; estimated reaction pressure, 30.0 MPa) by placing it in a preheated molten salt bath. The time required to heat up the reactor was 20-30 s. After a certain reaction time, the reactor was cooled to room temperature by immersing it in a water bath. The liquid-phase product was analyzed for nitrogen components. The ammonium ion concentration was measured by the indophenol blue method.13 An ion chromatograph was also used to measure nitrogen-containing components. The solid product was weighed and analyzed by elemental analysis. The experimental procedure is described in detail in our previous paper.10 The percentage amount of hydrogen peroxide charged in a reactor was based on the stoichiometric demand of oxygen for complete oxidation to carbon dioxide of carbon calculated from the carbon content in the feed. The amount of hydrogen peroxide used was 200% of the stoichiometric demand. The reaction pressure was calculated from the reaction temperature, amount of water in the reactor, and reactor volume. The nitrogen and carbon contents in the feed material calculated from elemental analysis were 1.89 and 9.72 kg/m3, respectively.
Experiments Municipal excess sewage sludge was used as a raw material. The properties obtained by elemental analysis
Reaction Kinetics
* To whom correspondence should be addressed. Phone: +81-96-342-3664. Fax: +81-96-342-3679. E-mail: mgoto@ kumamoto-u.ac.jp.
It has been recoganized that acetic acid and ammonia are refractory reaction intermediates for SCWO of organics containing carbon and nitrogen. A simple
10.1021/ie990407g CCC: $18.00 © 1999 American Chemical Society Published on Web 10/01/1999
Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999 4501
reaction pathway was proposed by Li et al.14 for carbonand nitrogen-containing organic materials.
Oxidation of ammonia is the slowest reaction for complete decomposition of wastes into carbon dioxide, molecular nitrogen, and water. Thus, kinetics analysis for ammonia in SCWO is very important for design and analysis of the SCWO plant as well as wet air oxidation. There are many works on reaction kinetics for the carbon component in the literature as summarized by Li et al.15 However, decomposition kinetics for the nitrogen component have been reported only by a few researchers.16,17 Webley et al.16 analyzed the oxidation of ammonia and ammonia-methanol mixtures in SCWO in the temperature range of 803-973 K at 24.6 MPa by using a packed or unpacked tubular reactor. They found the oxidation of ammonia was partially catalyzed by the Inconel 625 reactor wall. Ding et al.17 studied SCWO of NH3 over a MnO2/CeO2 catalyst in the temperature range of 683-743 K at 27.6 MPa. They found the conversion of ammonia was very low (less than 3%) without the catalyst at the residence time of 0.55 s. Therefore, a longer residence time is required for the complete decomposition of ammonia without a catalyst. Since the decomposition of ammonia is much slower than that of acetic acid, kinetic analysis of ammonia decomposition in SCWO is a crucial factor for the design of the SCWO process. Thus, ammonia concentration during SCWO of the sludge was analyzed to obtain the reaction rate constant. In the development of kinetic models, most researchers have adopted the global rate equation for an overall oxidation reaction,
-d[A]/dt ) k0 exp(-Ea/RT)[A]m[O]n
(2)
where [A] and [O] indicate the concentration of the reactant and oxidant, respectively. The global kinetic analyses for various organic substances in subcritical and supercritical water oxidation were summarized by Li et al.15 In subcritical water oxidation, oxygen mass transfer between the gas and liquid phases may limit the reaction rate, resulting in the reaction orders for oxygen, n, having a half- to first-order dependence on the oxygen concentration. On the other hand, since oxygen and the reactant exist in a single supercritical phase in SCWO and oxygen is stoichiometrically in excess, the reaction rate becomes independent of the oxygen concentration, so that the reaction can be treated as a psuedo “mth”-order reaction with respect to ammonia. Most researchers have adopted unity as the reaction orders for the concentration of organic substances, m ) 1, for both subcritical and supercritical water oxidation. Thus, eq 2 reduces to the following first-order reaction:
-d[A]/dt ) k0 exp(-Ea/RT)[A]
(3)
Integration of eq 3 with initial conditions ([A] ) [A]0 at t ) 0) gives
ln([A]/[A]0) ) -k0 exp(-Ea/RT)t
(4)
Figure 1. Semilog plot of ammonia decomposition curves in SCWO of sewage sludge.
which indicates a plot ln [A] versus time gives a straight line with a slope given by k ) k0 exp(-Ea/RT). In this work, we have applied eq 4 to the decomposition of ammonia. Results and Discussion In our previous paper,12 we measured TOC (total organic carbon) and organic acids for the liquid phase of the reaction product. From the analysis of TOC in the liquid-phase product, we have obtained reaction rate constant for the carbon component. High-performance liquid chromatography analysis of the liquid phase revealed a large fraction of organic compounds was converted to organic acids and the major component was acetic acid as reported in our previous paper.10,11 In a shorter reaction time (∼60 s) small amounts of lactic acid and propionic acid were detected. In this work, we focused on nitrogen-containing components measured by ion chromatography and the indophenol blue method for the ammonium ion. Since most of the nitrogen-containing components were detected as the ammonium ion, we used the ammonium ion concentration in the following kinetic analysis. Since it is difficult to differentiate which species, ammonia or ammonium ion, is oxidized in SCWO, we used the term “ammonia” below. Figure 1 shows the ammonia remaining in the liquid phase as a function of reaction time at four temperatures. The decomposition of sludge to ammonia is much faster than further decomposition of ammonia to nitrogen. As shown in an enlarged plot in Figure 1, the normalized ammonia concentration starts from near unity; that is, the ammonia concentration is close to C0 which corresponds to an imaginary initial ammonia concentration, assuming that all the nitrogen components in the sludge are converted to ammonia. At higher temperature some of the ammonia produced was already decomposed at the shortest time. Therefore, ammonia formation is fast enough at any temperature to neglect the time for heating, so that the analysis for ammonia decomposition is not influenced by the formation rate of ammonia.
4502
Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999
Conclusion The decomposition rate of ammonia which produced in SCWO of municipal sewage sludge as a refractory intermediate was measured. The rate obeyed first-order kinetics and the reaction rate constant agreed well with the literature data for SCWO of ammonia. The activation energy evaluated was 139 kJ/mol. Acknowledgment This work was supported by a Grant-in-Aid for Scientific Research (No. 09450294, No. 10141238) from the Ministry of Education, Science, Sports and Culture, Japan. Nomenclature Figure 2. First-order reaction rate constants in an Arrhenius plot in comparison with those reported in the literature. Table 2. Reaction Rate Constants k (s-1)
k0 (s-1) Ea (kJ/mol)
T ) 723 K T ) 773 K T ) 823 K T ) 873 K
3.28 × 10-5 1.31 × 10-4 4.34 × 10-4 1.82 × 10-3 3.16 × 105 139 (R2 ) 0.995)
The decomposition rate of ammonia is very slow at lower temperature and decomposition was about 30%, even in 4 h of reaction at the temperature of 723 K. In the semilog plot of ammonia vs time, the data at each temperature lie on a straight line, indicating that the reaction obeys first-order kinetics. The rate constants, k, obtained from these regression lines are listed in Table 2. The rate constants for sewage sludge were compared in Figure 2 with those reported by Webley et al.16 for SCWO of ammonia in a tubular flow reactor where ammonium hydroxide was used as a reactant and oxygen was used as an oxidant. The temperature range in this work was lower than those for the literature. The data by Webley et al.16 are for the decomposition rate of ammonia, whereas the data in this work are for the decomposition rate of ammonia which was produced as an intermediate product in SCWO of sewage sludge. The reactor used in this work is a batch reactor whereas Webley et al.16 used a flow reactor. A drawback for a batch reactor is that the reaction during heating to the desired temperature cannot be avoided. However, this influence is negligible in the reaction of ammonia because heating-up time in our work is about 30 s, which is much shorter than the time for the decomposition of ammonia. The difference in the oxidant, hydrogen peroxide and oxygen, may not be influential because the decomposition of hydrogen peroxide to oxygen and water occurs much faster18 than the decomposition of ammonia. Thus, oxygen may play a role as an oxidant for the oxidation of ammonia in SCWO. Although the reaction temperature and the reactant are different, the decomposition rate of ammonia agreed well between this work and the literature. The activation energy was evaluated from the slope in the Arrhenius plot and shown in Table 2. The activation energy evaluated in this work was 139 kJ/mol (R2 ) 0.995), while the data by Webley et al.16 gave 157 kJ/mol.
A ) concentration (mol/L) k ) first-order reaction rate constant (1/s) k0 ) pre-exponential factor (1/s) Ea ) activation energy (kJ/mol) m, n ) order of the reaction O ) concentration of the oxidant (mol/L) R ) gas constant, 8.314 (J/mol‚K) T ) temperature (K) t ) time (s)
Literature Cited (1) 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. (2) Gloyna, E. F.; Li, L.; McBrayer, R. N. Engineering Aspects of Supercritical Water Oxidation. Water Sci. Technol. 1994, 30, 1. (3) Caruana, C. M. Supercritical Water Oxidation Aims for Wastewater Cleanup. Chem. Eng. Prog. 1995, April, 10. (4) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2. (5) Gloyna, E. F.; Li, L. Supercritical Water Oxidation Research and Development Update. Environ. Prog. 1995, 14, 182. (6) Levec, J. Wet Oxidation Processes for Treating Industrial Wastewaters. Chem. Biochem. Eng. Q. 1997, 11, 47. (7) Modell, M.; Larson, J.; Sobczynski, S. F. Supercritical Water Oxidation of Pulp Mill Sludge. Tappi J. 1992, June, 195. (8) Shanableh, A.; Gloyna, E. F. Supercritical Water Oxidations Wastewater and Sludges. Water Sci. Technol. 1991, 23, 389. (9) Blaney, C. A.; Li, L.; Gloyna, E. F.; Hossain, S. U. Supercritical Water Oxidation of Pulp and Paper Mill Sludge as an Alternative to Incineration. In ACS Symposium Series 608; American Chemical Society: Washington, DC, 1995; p 444. (10) 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. (11) 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. (12) Goto, M.; Nada, T.; Kodama, A.; Hirose, T. Kinetic Analysis for Destruction of Municipal Sewage Sludge and Alcohol Distillery Wastewater by Supercritical Water Oxidation. Ind. Eng. Chem. Res. 1999, 38, 1863. (13) Japanese Industrial Standards Committee, NH4+, Testing Methods for Industrial Wastewater JIS K0102-1986; Japanese Standards Association: Tokyo, 1986; p 134. (14) Li, L.; Chen, P.; Gloyna, E. F. Kinetic Model for Wet Oxidation of Organic Compounds in Subcritical and Supercritical Water. In ACS Symposium Series 514; American Chemical Society; Washington, DC, 1993; p 305. (15) Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37, 1687.
Ind. Eng. Chem. Res., Vol. 38, No. 11, 1999 4503 (16) 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. (17) 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.
(18) Croiset, E.; Rice, S. F.; Hanush, R. G. Hydrogen Peroxide Decomposition in Supercritical Water. AIChE J. 1997, 43, 2343.
Received for review June 2, 1999 Revised manuscript received August 19, 1999 Accepted August 20, 1999 IE990407G
