Ind. Eng. Chem. Res. 1991,30, 1745-1754
1745
Oxidation Kinetics of Ammonia and Ammonia-Methanol Mixtures in Supercritical Water in the Temperature Range 530-700 "C at 246 bar Paul A. Webley, Jefferson W. Tester,* and H. Richard Holgate Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Oxidation of industrial chemical and metabolic wastes in a supercritical water medium is an effective method for the treatment and disposal of these materials. Partial oxidation of nitrogen-containing organics in supercritical water leads to the formation of ammonia, which is the rate-limiting step in the overall oxidation to nitrogen. In this study, the oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water were experimentally determined in a packed and unpacked tubular plug flow reactor. The oxidation of ammonia was found to be partially catalyzed by the Inconel 625 (a nickel-chromium d o y ) reactor walls. In the unpacked reactor, the activation energy was 38 kcal/mol over temperatures ranging from 640 to 700 "C. Oxidation of ammonia in the packed reactor gave an activation energy of 7.1 kcal/mol over the temperature range 530-680 "C and a reaction rate approximately 4 times larger than the tubular reactor data. A power law model and a catalytic model were tested, and the catalytic model was found to represent the data well. In experiments with ammonia-methanol mixtures, the oxidation of ammonia was unaffected by the presence of methanol for experiments conducted in the tubular reactor and retarded by the presence of methanol for packed bed experiments. These observations are consistent with the hypothesis that ammonia oxidation proceeds predominantly as a catalytic reaction in the packed bed reactor at the temperatures investigated. Competitive adsorption of methanol and ammonia reduces the oxidation rate of ammonia in the packed bed reactor.
Introduction Oxidation in supercritical water is currently being used to treat a wide variety of hazardous chemical and metabolic wastes (Staszak et al., 1987;Modell, 1989;Webley and Tester, 1988). For example, future long-term space flights will require on-board treatment of waste and water recycling in a partially or fully enclosed life-support system. Similarly, environmental control technologies are needed to effectively deal with a large class of dilute organic aqueous wastes containing dissolved salts resulting from the manufacture of industrial chemicals and pharmaceuticals. In supercritical water oxidation, organics, air (or oxygen), and water are mixed a t pressures of approximately 250 bar and temperatures above 400 OC. At these conditions, organic oxidation is initiated spontaneously and the heat of combustion results in a temperature increase to 550-650 O C . Organics are destroyed rapidly with conversions to carbon dioxide in excess of 99.99% at reactor residence times of less than 1 min. Heteroatoms are oxidized and precipitated out of solution as salts by adding a base to the feed (Thomason and Modell, 1984). The oxidation of a variety of organics in supercritical water has been studied by several workers. Price (1982)eliminated 88-93% of the liquid total organic carbon (TOC) during oxidation of human metabolic waste in supercritical water and Modell (1985)oxidized several toxic chlorinated hydrocarbons in supercritical water, destroying at least 99.99% of the TOC. In a study of urea destruction, Timberlake et al. (1982)reported that, above 650 "C, urea was completely oxidized to nitrogen gas, carbon dioxide, and water at residence times of 20 s. Pure water is in a supercritical state if its temperature and pressure exceed 374.3 "C and 220 bar, respectively. At and above these conditions, the density is a strong function of both temperature and pressure and leads to large changes in the physical properties of water, particularly its solvation behavior (Josephson, 1982). Under *Author to whom correspondence should be addressed.
supercritical conditions, water behaves like a dense gas with a high solubility of organics (Connolly, 1966) and complete miscibility in all proportions with oxygen and air (Japas and Franck, 1985),light organic gases (de Loos et al., 1983,and carbon dioxide (Todheide and Franck, 1963). In addition, supercritical water exhibits high diffusivities (Lamb et al., 1981),low viscosity (Sengers and Watson, 1986), and low solubility and dissociation of inorganics, particularly salts (Martynova, 1976). These solvation properties together with the high temperature and pressure make supercritical water an ideal medium for oxidation of organic wastes, since organics and oxygen can be intimately mixed in a single homogeneous phase and inorganic salta can be readily removed from solution by precipitation. Although the presence of organics and dissolved gases in water can significantly affect the critical properties of the mixture, supercritical water oxidation is practically defined as oxidation occurring above the critical temperature and pressure of p u r e water. Although the technical feasibility of the supercritical water oxidation (SCWO) process has been demonstrated, there is little information available on the reaction kinetics and mechanisms responsible for the oxidation rates of even simple waste compounds. Such information is useful for design and development of a SCWO process and for the evaluation of alternative process configurations. Although real wastes contain many complex compounds, the ratedetermining step in the oxidation of these compounds is frequently the oxidation of lower molecular weight organic compounds that are themselves partial oxidation products of the initial waste mixture. These partial oxidation products can include ethanol, methanol, formaldehyde, carbon monoxide, and ammonia. Previous work by Helling and Tester (1987,1988)determined the oxidation kinetics of carbon monoxide and ethanol in supercritical water. Webley and Tester (1988,1989, 1991) determined the oxidation kinetics of methane and methanol in supercritical water, and more recently, Holgate and Tester (1991) measured the oxidation kinetics of hydrogen in supercritical water. A related and important problem in the
0888-5885191 2630-1745$02.50/0 0 1991 American Chemical Society
1746 Ind. Eng. Chem. Res., Vol. 30, No. 8, 1991
fl c r
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I
Ammonium Hydroxide Feed Saturator
Oxygen Saturator
High Pressure
Oxygen Helium
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1
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I1 I Preheater Sandbath
200 - 500°C
Exchanger
Regulator
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Liquid
Figure 1. Schematic of supercritical water oxidation apparatus.
treatment of industrial wastes in the SCWO process is the formation and removal of salts in the reaction mixture. The fundamentals of salt nucleation, precipitation, and growth in supercritical water are currently being studied by Armellini and Tester (1990). In this paper, the oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water are presented and discussed. Experimental Apparatus and Procedure Two reactors were used for measuring the oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water. The first (referred to as the tubular reactor) consisted of 4.71 m of Inconel 625 tubing with an inside diameter of 1.71 mm and an outside diameter of 6.35 mm. The second reactor was a packed bed tubular reactor (referred to as the packed reactor) used to test for potential catalytic effects of the reactor wall material on the oxidation kinetics. The packed bed reactor consisted of 0.62 m of 1.429 cm 0.d. X 0.912 cm i.d. Inconel 625 tubing packed with -60+100-mesh (150-250 pm) Inconel 625 beads. The void volume in the packed bed reactor was calculated in two ways. In the first, the weight of a known volume of packing was recorded and the particle volume calculated from the density of Inconel 625 as supplied by the manufacturer. Using this technique, a void fraction of 0.39 was obtained. The second method used to determine void volume was to add a known volume of water to a known volume of dry beads and measure the volume taken up by the water. Using this technique, a void fraction of 0.41 was obtained. The value of 0.39 was considered more reliable and used in the calculation of residence time. The packed bed reactor provided a surface to volume ratio about 30 times greater than the tubular reactor while retaining a similar void volume. Reactions that are catalytic should therefore exhibit higher reaction rates in the packed bed reactor than in the tubular reactor for the same temperature, pressure, and concentrations. A detailed description of both reactor systems is given by Webley (1989). The reactor (either packed or tubular) was placed in a fluidized sand bath for temperature control. A maximum operating temperature of 700 "C was possible. A schematic of the experimental reactor system is shown in Figure 1. Oxygen was fed to the reactor aa a dilute aqueous solution, prepared by saturating purified water with oxygen in a high-pressure agitated saturator. Ammonia was fed to the reactor in the form of an ammonium hydroxide (NH40H) solution, prepared by diluting a standard 30 wt ?% aqueous solution of ammonium hydroxide. The ammonia-water saturator was slightly pressurized with helium at 1 bar to
provide a positive pressure for the high-pressure feed pump. The feed solutions were pumped separately with a high-pressure HPLC-type pump through preheater tubing (immersed in a preheater sandbath) and through additional tubing in the primary sandbath before mixing at the reactor inlet. An Inconel sheathed thermocouple was sealed in the mixing tee and placed in direct contact with the entering fluids. This thermocouple provided the primary measurement of reactor temperature. Thermocouples placed in the sandbath were usually within 2 "C of the reactor temperature. Upon exiting the reactor, the effluent was rapidly quenched in a water-cooled double pipe heat exchanger. The pressure was reduced from 245.8 bar (typical operating pressure) to atmospheric pressure through a back pressure regulating valve, and the resulting liquid and vapor phases were separated in a gas/liquid separator. Both liquid and gas flow rates were measured. The composition of the gas phase (containing nitrogen, unreacted oxygen, traces of hydrogen, and, in the case of methanol-ammonia mixtures, carbon monoxide) was determined by gas chromatography. Two gas chromatographs were used. The first used helium as the carrier gas and was equipped with a flame ionization and thermal conductivity detector. This GC was used for separation and detection of oxygen, nitrogen, carbon monoxide, carbon dioxide, the nitrogen oxides N20,NO,and NO2,and methanol. Two columns were used in series to effect the separation. A stainless steel 60/80-mesh Poropak T was connected in series to a stainless steel 80/ 100-mesh molecular sieve. A pneumatically operated switching valve switched the order of the columns during the analysis, thereby preventing carbon dioxide and nitrous oxide from entering the molecular sieve column (and adsorbing to the packing) while sending other compounds through the column a second time, effectively doubling the column length. A temperature program was employed which ramped the temperature from 40 to 100 "C over the duration of the analysis. The second GC used nitrogen as the carrier gas and was used for detection of hydrogen. A thermal conductivity detector was used. An 8 ft X 1/8 in. stainless steel molecular sieve column was used to effect the separation between hydrogen and other light gases such as oxygen. The large difference in thermal conductivities of hydrogen and nitrogen led to a high sensitivity for both helium and hydrogen. The mole fraction of ammonia in the gas phase was calculated from the concentration of ammonia in the liquid phase and the partial pressure of ammonia above a solution of that concentration. In all cases, the ammonia in the gas phase constituted less than 0.1% of the total ammonia leaving the reactor. The dissolved gases (nitrogen, carbon monoxide, oxygen) leaving in the liquid phase were accounted for by assuming equilibrium in the gas/liquid separator and using the Henry's law coefficient to calculate the concentration of dissolved gas. The assumption of equilibrium in the gas-liquid separator was found to be a good one by performing tests on the saturator system at ambient reactor conditions (Webley, 1989). Ammonia concentrations in both the liquid feed to the reactor and the liquid product from the reactor were determined by analysis with an ion-specific electrode (HNU, Model ISE-10-10-00). A small drift in the electrode response (f5 mV) led to relatively large errors (3-6% of the absolute concentration) in the ammonia concentration measurement. This is reflected directly in the calculation of the ammonia conversion and the first-order rate constant, which depends on both inlet and exit concentrations.
Ind. Eng. Chem. Res., Vol. 30, No.8, 1991 1747 Table I. Experimental Rerults for Oxidation of Ammonia in Supercritical Water [021,1b ammonia [NH3lo: g-mol/L X g-mol/L X converreactor type temp, "C residence time, s sion, % 10-3 10-3 3.4 640 11.8 4.41 0.58 tubular 650 4.8 3.4 3.49 2.92 tubular 1.27 650 11.6 5.5 4.03 tubular 660 11.2 4.1 6.85 2.50 tubular 660 11.3 5.9 tubular 4.25 1.95 670 11.0 5.4 6.70 2.44 tubular 680 10.9 10.9 tubular 2.73 2.50 680 10.9 8.3 1.92 2.43 tubular tubular 680 10.8 7.2 4.71 2.60 680 10.8 6.9 tubular 6.39 2.37 680 10.9 9.7 tubular 3.70 1.87 680 11.0 3.99 1.22 8.8 tubular 680 11.1 5.6 tubular 3.92 0.51 690 10.6 4.89 2.57 8.2 tubular 700 4.4 8.9 tubular 2.91 2.43 700 10.5 10.1 4.85 tubular 2.53 700 10.5 1.1 7.03 tubular 0.05 531 9.5 13.9 2.58 packed 3.52 680 15.7 42.5 4.37 packed 1.53 680 15.7 32.4 packed 4.13 1.06 680 15.2 1.2 4.21 0.009 packed 680 5.9 4.09 1.31 24.4 packed packed 680 15.8 31.8 4.40 1.11
av rate, g-mol/(L.s) x 104 1.28 2.48 1.90 2.51 2.21 3.27 2.75 1.47 3.15 4.06 3.29 3.18 1.99 3.76 5.94 4.68 0.76 3.80 11.90 8.53 0.33 17.1 8.85
ln -5.8 -5.0 -5.3 -5.6 -5.2 -5.3 -4.5 -4.8 -5.0 -5.0 -4.7 -4.8 -5.2 -4.8 -3.8 -4.6 -6.8 -4.2 -3.4 -3.7 -7.1 -3.0 -3.7
nitrogen material balance$ % 98.3 92.6 83.2 92.3 98.4 93.0 98.9 96.7 99.0 96.0 86.9 87.3 95.9 93.9 99.9 93.1 98.2 97.8 94.8 99.3 96.7 85.5 98.4
a Ammonia feed concentration at reactor T and P. Oxygen feed concentration at reactor T and P. e Ln of first-order rate constant with respect to ammonia. dClosures indicated as total recovered N2 and unreacted NH, relative to amount of NH3 fed.
The error associated with the calculation of average reaction rate is somewhat smaller, since gas chromatographic measurements (considered more reliable than electrode measurements) of N2 produced from the ammonia oxidation are used for this calculation. Experimental Conditions The oxidation of ammonia in supercritical water was investigated over the temperature range 530-700 "C at a pressure of 246 bar (3550 psi) for reactor residence times of 4.4-15.8 s. Sixteen oxidation runs were performed in the tubular reactor and five in the packed bed reactor. Two pyrolysis runs were conducted-one in the tubular and one in the packed reactor. In these two runs, oxygen was excluded from the reaction mixture although contamination of the saturators with air is possible. The concentration of oxygen is typically 2 orders of magnitude lower than in a typical oxidation run. For the oxidation runs,feed concentrations of ammonia at reactor conditions ranged from 1.92 X lo9 to 6.85 X g-mol/L and oxygen concentrations ranged from 5 X 10" to 3.5 X lb3g-mol/L. Feed ratios of oxygen to ammonia ranged from 0.13 to 1.4. For ammonia oxidation to molecular nitrogen, the stoichiometric ratio of oxygen to ammonia is 0.75. The feed ratios used in the experiments therefore covered the range from substoichiometric to excess oxygen. However, ammonia conversions in all the experiments were low enough that there was always unreacted oxygen remaining in the reactor product. Oxygen never limited the conversion of ammonia to nitrogen even with substoichiometric feeds. Obvious exceptions were the two cases in which pyrolysis of ammonia was attempted. In these runs, oxygen was deliberately excluded from the reactor. Experimental Observations Ammonia was oxidized to nitrogen in the tubular reactor above 640 "C at residence times ranging from 4.4 to 10.8 s with conversions ranging from 3 to 11% (based on disappearance of ammonia). Average reaction rates, based on a differential reactor analysis, ranged from 1.3 X to 6 X 10" g-mol/(L-s). At temperatures below 640 "C, for residence times of 10.5 s and less, no significant con-
version (> k2 over the temperature range 64Ck700 "C, the equation for surface coverage of oxygen (13) indicates
Ind. Eng. Chem. Res., Vol. 30, No. 8,1991 1751 cient data to characterize each pathway. Experiments on ammonia oxidation in the packed bed reactor with a range 5/ 1 of Inconel 625 bead sizes (from very fine to relatively coarse) will change the surface/volume ratio over a wide range, thereby changing the contribution of the heterogeneous reaction to the overall rate. In this way, the two reaction pathways can be separated. These experiments need to be conducted over a range of temperatures since the heterogeneous reaction is expected to become more dominant as the temperature is decreased.
0 , 0
1
2
,
3
4
5
Observed Rate, gmole/l.s X 1 0 5
-
Figure 6. Predicted oxidation rate from catalytic model.
8 1. This is expected in view of the aggressive oxidizing environment of supercritical water-oxygen mixtures. In terms of the mechanism presented in (8)-(12), the ratelimiting step is the reaction of an ammonia molecule with an adsorbed, dissociated oxygen species, (9). Adsorption and dissociation of the oxygen on the surface is therefore rapid. The reaction rate is determined by the ammonia concentration and is approximately independent of oxygen concentration, in agreement with experimental observations. The reactor wall in the present study is Inconel 625. Nickel is the major component ( 5 5 4 8 %) with lesser amounts of chromium (20-23%), molybdenum (&lo%), niobium (3-4%), and iron ( < 5 % ) . Trace amounts (
