Supercritical Water Oxidation of Methylamine - American Chemical

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Supercritical Water Oxidation of Methylamine Kenneth M. Benjamin and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

Methylamine was oxidized in supercritical water in a Hastelloy tubular flow reactor at 249 atm and temperatures between 390 and 500 °C. The major carbon-containing products were CO2 and CO, with trace amounts of CH3OH. The major nitrogen-containing products were NH3, N2O, and N2. A reaction network consistent with all the results has been constructed. Ammonia appears to be the exclusive nitrogen-containing intermediate between methylamine and the final products, N2O and N2. The disappearance of ammonia during methylamine SCWO is markedly faster than that during oxidation of ammonia alone. Approximately 3-4 times more N2O is produced than N2, whereas the N2O/N2 ratio is essentially zero when ammonia is oxidized alone in SCW. We attribute these differences in the rate of and selectivity from ammonia oxidation in supercritical water in the present experiments to the presence of methylamine in the reaction environment and catalytic chemistry on the reactor walls. Introduction Supercritical water oxidation (SCWO) is a process by which organic compounds are oxidized in an aqueous environment above the critical point of water (Tc ) 374 °C, Pc ) 218 atm).1 Typical operating conditions for a SCWO reactor are 400-650 °C and 250 atm. SCWO exploits the unique properties of supercritical water to provide enhanced solubilities of organic reactants and permanent gases (like oxygen and carbon dioxide) and a single-phase environment free of interphase mass transfer limitations. The high temperature gives fast reaction kinetics and a high selectivity to complete oxidation products.2 These features make SCWO an attractive alternative to technologies such as wet-air oxidation or incineration, which suffer from mass-transfer limitations and hazardous byproduct formation from incomplete combustion, respectively. For incineration of a nitrogen-containing organic molecule, byproducts may include nitrogen oxide (NO) and nitrogen dioxide (NO2). At the lower temperatures of SCWO, however, thermodynamics favors nitrogen conversion into N2 and N2O, rather than NO and NO2.3 In addition, whatever NOx is formed will remain in the supercritical water, rather than be emitted to the atmosphere. In this manner, SCWO is self-scrubbing and is considered by the EPA to be a totally enclosed treatment method.4 While fewer in number than investigations into carbon chemistry, investigations into nitrogen chemistry in SCWO have covered a range of compounds and conditions. These investigations have largely probed homogeneous (free-radical) chemistry in supercritical water,5-21 but some intentional catalytic studies have been conducted as well.6,21-23 This body of literature provides some initial guidance in the areas of kinetics, reaction pathways, and product distribution for the SCWO of nitrogen-containing molecules. The literature * To whom correspondence should be addressed. Tel.: (734) 764-3386. Fax: (734) 763-0459. E-mail: psavage@ umich.edu.

suggests that the dominant reaction pathway for nitrogen atoms during SCWO is

nitrogen-containing reactant f NH3 f N2 In only three instances13,19,23 was another major end product, N2O, observed along with N2. This article presents results from the oxidation of a model nitrogen-containing molecule, methylamine, in supercritical water. There have been no previous reports on the SCWO of methylamine. Experimental Section We oxidized methylamine in supercritical water between 390 and 500 °C and at 249 atm in a nominally isothermal, isobaric, plug-flow reactor. The reactor was a coiled tube made of Hastelloy C-276. Two different reactor sizes were used during the experiments. One reactor had an internal diameter of 0.216 cm and a length of 75 cm, while the other had an internal diameter of 0.267 cm and a length of 308 cm. Volumetric flowrates through these tubular reactors ranged from 0.13 to 2.88 cm3/s (at the reaction conditions). These two reactor sizes and the range of volumetric flowrates allowed us to explore residence times between 1 and 130 s at various temperatures and water densities. The experimental apparatus and procedure have been described in detail elsewhere,30 so what follows is a brief summary. Water was distilled, deionized, and degassed prior to use. Methylamine feed solutions were prepared by diluting a 40 wt % aqueous solution (from Aldrich). The oxygen for reaction was produced from the decomposition of hydrogen peroxide (Fisher, 30 wt % solution) in a preheater line. Streams of the organic and oxidant solutions were preheated to the reaction temperature separately by flowing through Hastelloy C-276 lines immersed in an isothermal fluidized sand bath (Techne, model #SBL-2D) controlled to (2 °C. The preheater lines had an internal diameter of 0.108 cm and a length of 2.5-3 m. The two streams (organic and oxidant) were mixed at the reactor entrance. The shorter (75 cm) flow reactor system employed a mixing tee that provided a

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fully turbulent cross-flow for the two reactant streams.31 This improved design reduces mixing times to 1 s or less. A thermocouple in the mixing tee measured the reactor temperature. We verified experimentally that methylamine does not undergo appreciable hydrothermal decomposition as it flows through the preheater line. This result is supported by previous research on methylamine reactivity in supercritical water.32 Therefore, methylamine reacts only in the tubular reactor itself, after mixing with the oxidant stream. Adjusting the flow rates of the organic and oxidant streams (while maintaining a constant ratio) varied the residence time in the reactor while maintaining the nominal feed concentrations. Immediately following the tubular reactor are two tube-in-tube heat exchangers, which use cold water to cool the reactor effluent. Then, a pressure reducing regulator is used to lower the pressure of the effluent. Finally, the stream enters a vapor-liquid separator, where gas and liquid phases are separated for subsequent analyses. The gas-phase portion of the reactor effluent went directly to a Hewlett-Packard Model 5890 Series II Gas Chromatograph (GC) for on-line analysis with a thermal conductivity detector (TCD). A 1/8 in.-15 foot stainless steel column, packed with 60/80 mesh Carboxen 1000 (Supelco), was used to separate and identify permanent gases. Henry’s law was used to determine the relative amounts of carbon monoxide, nitrogen, and nitrous oxide in the liquid-phase portion of the reactor effluent. Due to the presence of methylamine and ammonia in the liquid phase, quantifying carbon dioxide in the liquid phase required a more advanced treatment. This thermodynamic analysis will be discussed in the Results section. The liquid phase was collected and analyzed on an Agilent 6890 GC using a flame ionization detector (FID) for methylamine and methanol and a thermal conductivity detector for ammonia. Separation of methylamine, ammonia, methanol, and water in the GC was accomplished with an 8-ft long, 1/4-in. stainless steel column packed with 28% AT-223 and 4% KOH on 80/ 100 GasChrom R (Alltech Associates). Standard solutions were prepared using the aqueous methylamine solution, methanol (Fisher), and 29.4 wt % aqueous ammonium hydroxide solution (Mallinckrodt). Analysis of the standards provided the data for a calibration curve for each compound. Molar yields were calculated as the molar flow rate of product formed divided by the molar flow rate of methylamine entering the reactor. In both the liquid- and gas-phase GC analyses, helium was used as the carrier gas. No attempt was made to determine nitrate/nitrite yields. These are not likely to be important products of methylamine SCWO given the results of previous investigations on the SCWO of nitrogen-containing compounds.16,17 Additional support, to be presented in full later, is that nitrogen atom balances (based upon measured N2, N2O, and NH3 yields) were always greater than or equal to unity. Results for Methylamine SCWO Methylamine was oxidized in supercritical water at 249 atm and 390-500 °C. The initial concentrations of methylamine and oxygen were nominally 3.3 and 17.9 mmol/L, respectively, and their run-to-run variability was less than 9%. The oxygen concentration represents

Table 1. Molar Yields (%) of Major Species during Methylamine SCWO temp (°C)

τ (s)

CH3NH2

390

0 15 17 20 24 30 40 124 0 1.8 2.1 8.7 9.8 13 20 25 35 84 0 1.0 1.3 1.6 1.9 2.4 3.2 7.3 9.6 15 19 29 63 0 1.1 1.3 1.6 2.2 2.8 6.0 7.3 12 15 21 55

100 86 92 90 86 63 44 9.1 100 92 94 79 9.1 3.0 5.6

410

450

500

100 82 84 78 75 29 7.8 4.4 2.8 20 17 100 1.1 1.3 1.4 3.1 11

4.0

CO

CO2 CH3OH N2O

4.7 3.4 4.4 5.7 26 40 54

1.4 1.3 1.3 1.3 1.9 2.3 1.8

17 36 48 42 30 18

3.4 3.1

4.0 8.9 19 58

1.3 6.4 17 34 64 21 15 11 5.9 5.2 1.6

53 57 63 68 68 74

22 19 16 20 13 5.3 2.0 0.8 0.3 0.1 0.1

67 77 71 69 78 86 86 90 88 89 95

18

N2

NH3

0.1 0.1 0.8 0.6

14 8.2 9.7 14 0.6 37 1.1 53 2.2 86

8.3 11 4.5 9.3 3.1 3.0

0.3 0.9 3.5 6.9 9.7 14

7.6 5.7 19 90 93 82 79 73 61

0.9 1.0 1.2 1.5 2.1 1.9 2.7 2.2 6.6 4.9 1.9 4.0

0.1

0.2 0.2 0.2 0.2 0.2

2.5

1.2 2.6 3.4 4.0 5.1

1.0 3.4 1.1 4.5 5.5 4.7 6.5 5.7

18 16 21 23 67 75 64 57 45 36 50 31

6.3 2.7 6.0 9.2 7.4 7.7 7.1 8.5 7.0 7.1 10

53 60 57 39 42 50 56 46 56 57 33

0.1 0.8 1.1 5.1 15 17 21 17 19 21 17 17 15 20 16 17 15 19 15 14 21

0.3

a 140% excess amount for the complete oxidation reaction:

9 1 5 CH3NH2 + O2 f CO2 + N2 + H2O 4 2 2 Table 1, which contains all of the experimental data, shows that the rate of methylamine disappearance increases with increasing temperature. Also, there appears to be an induction time, which is most noticeable at 390 °C. For about the first 24 s, the methylamine yield is steady at about 90%, but after that time it decreases steadily. Induction times have been observed in other SCWO studies,33 and they can arise from freeradical chain reaction mechanisms. It should be noted that the scatter present in some data sets in Table 1 (such as the methylamine yields at 450 °C and the methanol yields) is likely due to random error (run-torun variability). A measure of the kinetics of methylamine disappearance during SCWO can be deduced from the data in Table 1. Assuming methylamine disappearance after the induction time (τind) follows pseudo-first-order kinetics, we can write the following expression for the methyl-

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Table 2. Pseudo-First-Order Rate Constants and Induction Times for Methylamine SCWO temp (°C)

k (s-1)

tind (s)

390 410 450

0.021 ( 0.002 0.19 ( 0.07 1.1 ( 0.2

8.0 ( 3.8 1.2 ( 3.8 1.2 ( 0.4

amine concentration (C) as a function of reactor residence time, τ:

ln

( )

C(τ) ) [-k(τ - τind)] C0

(1)

C0 is the feed concentration and k is the pseudo-firstorder rate constant. By fitting eq 1 to the experimental data for methylamine disappearance, one determines the rate constant and induction time at each temperature. We employed an unweighted, linear regression to fit the data to eq 1. Table 2 lists the values of the kinetics parameters (where the uncertainties are standard errors). Linear regression of the rate constant data to the Arrhenius equation leads to the following expression for the pseudo-first-order reaction rate constant (where the uncertainties are standard errors):

(-61 ( 15RTkcal/mol)

k′ ) 1018.6(4.6(s-1) exp

(2)

The longer induction time at 390 °C is likely due to both the time required to build up a pool of radicals (a chemical effect) and poorer mixing in the mixing tee used in these experiments (a physical effect). However, given the uncertainties in the calculated induction times (especially at 410 °C), we cannot be dogmatic about any trends. In addition to reactant disappearance, it is also of interest to explore the temporal variations of the yields of different products and the influence of temperature thereon (as displayed in Table 1). We first consider CO, an intermediate on the reaction pathway to the complete oxidation product CO2. At 410 and 450 °C, both the formation and subsequent decay of CO was captured as evidenced by the existence of a maximum yield. Naturally, the maximum CO yield shifted to shorter residence times as the reaction temperature increased. At the highest temperature investigated, 500 °C, the disappearance of CO is dominant over its formation. Next, we examine the effect of temperature on the CO2 yield. At the lowest temperatures, 390 and 410 °C, less CO2 is formed, probably because of the slow oxidation of CO. At 410 °C, CO2 first appears at about 20 s, the same time at which the CO yield reaches its maximum. As the CO yield decreases at longer times the CO2 yield increases. This behavior suggests that CO is an intermediate to CO2. At the two higher temperatures, 450 and 500 °C, oxidation of CO proceeds more rapidly and higher yields of CO2 appear. At 500 °C, the CO2 yield is nearly 100%, which indicates that nearly all of the carbon fed to the reactor is converted to CO2. Complete conversion of organic carbon to CO2 is one of the chief goals of SCWO. We note here that approximately 40% of the CO2 formed in the laboratory reactor resides in the liquid phase after postreaction quenching. This substantial partitioning is due to the complex chemical equilibria present in amine-CO2-H2O and NH3-CO2-H2O mixtures.34-37 Reactions in the aqueous solution lead to the

Figure 1. Temporal variation of NH3 yields during methylamine SCWO.

formation of carbamate ions (RNH2COO-), which act as a sink to draw more CO2 into the liquid phase. In this work, the combined phase and chemical equilibria for amine-CO2-H2O and NH3-CO2-H2O mixtures were modeled with ASPEN PLUS 11.1.37,38 Methanol yields obtained during methylamine SCWO also are reported in Table 1. Though yields of 8-11% appeared at 410 °C, the molar yield of methanol was more typically around 2-3% for the temperatures and residence times investigated. We suspect that methanol formed in a side reaction, such as hydrolysis,32 rather than in the oxidation reaction pathway. Considering the yields of N2O and N2, respectively, as functions of temperature one notes that higher temperatures and longer residence times lead to increased yields of these products. It is interesting to note here that we find higher yields of N2O than N2. This finding differs from previous investigations into homogeneous nitrogen chemistry in SCWO.3,6,9,10,13,14,16-18 Last, we examine the effect of temperature on the NH3 yield. Ammonia was always the major nitrogencontaining product observed. In our analytical work, the GC peak areas for ammonia were small because of the low initial concentrations of methylamine used and the lower sensitivity of a TCD (compared to an FID). These small peaks made precise quantification difficult and caused the ammonia yields determined by GC to give nitrogen atom balances greater than 100%. We have greater confidence in the accuracy of the yields of the other nitrogen-containing compounds, so the molar yields of ammonia reported herein were calculated as the difference between a nitrogen atom balance of unity and the sum of the yields of methylamine, nitrogen, and nitrous oxide. The trends in the ammonia yields were the same regardless of whether they were calculated directly from the GC analysis or indirectly by difference. The ammonia yields reported in Table 1 also appear graphically in Figure 1, to facilitate analysis and discussion. The error bars in Figure 1 and all subsequent figures represent relative uncertainties at the 95% confidence interval level estimated via propagation of errors. Figure 1 shows that the rate of formation of NH3 is faster at the three highest temperatures than at 390 °C. Also, the NH3 yield reaches a maximum and then decreases at these three higher temperatures. The temporal variation of the NH3 yields suggests that NH3

Ind. Eng. Chem. Res., Vol. 44, No. 14, 2005 5321 Table 3. Rate Constants for Pseudo-First-Order Global Model for Methylamine SCWO k1 (s-1) k2 (s-1) k3 (s-1) k4 (s-1) k5 (s-1) k6 (s-1) k7 (s-1)

390 °C

410 °C

450 °C

2.1 × 10-2 6.2 × 10-4 1.4 × 10-4 1.0 × 10-1 7.0 × 10-3 2.3 × 10-3 4.0 × 10-34

1.9 × 10-1 2.5 × 10-3 5.7 × 10-3 7.8 × 10-2 3.7 × 10-2 1.7 × 10-2 8.9 × 10-3

1.14 1.4 × 10-2 3.4 × 10-2 1.51 1.0 × 10-1 4.3 × 10-3 2.0 × 10-1

for methylamine SCWO. One such set of reaction paths is k1

CH3NH2 98 NH3 + CI Figure 2. First-rank Delplot for methylamine SCWO, 390-450 °C.

is an intermediate between the reactant, methylamine, and the other nitrogen-containing products, N2O and N2. Observing ammonia as a key intermediate during oxidation is consistent with prior work on the SCWO of nitrogen-containing compounds.7,8,12-15

k2

NH3 98 N2 k3

NH3 98 N2O k4

CI 98 CO k5

Global Reaction Network for Methlyamine SCWO Given the experimental data for methylamine SCWO, one can construct a global reaction network. A useful tool for this analysis is the Delplot technique.39 A firstrank Delplot (selectivity vs conversion) identifies primary products because those alone possess a positive y-intercept (initial selectivity). From Figure 2, which shows a first-rank Delplot for NH3 and CO, it is evident that NH3 is a primary product. With an initial selectivity equal to unity, NH3 appears to be the exclusive nitrogen-containing product arising directly from methylamine. CO and all other products detected (though not shown in Figure 2) have initial selectivities equal to zero. The variation of selectivity with conversion provides some additional insight into the reaction network. The NH3 selectivity is unity until the methylamine conversion reaches and exceeds 90%. At the conditions leading to higher conversions, the NH3 selectivity is lower, indicating that NH3 reacts to form other nitrogencontaining products (i.e., N2 and N2O). The CO selectivity starts at zero, increases to about 0.7, and then decreases at high conversions. This behavior is consistent with CO being formed from an intermediate product (not directly from methylamine) and then reacting further to form another product (i.e., CO2). The discussion above suggests the global reaction network CH3NH2 f NH3 f N2, N2O for the nitrogen chemistry and CH3NH2 f CI f CO f CO2 for the carbon chemistry, where CI is an unidentified, carboncontaining intermediate between methylamine, the reactant, and CO, a secondary product. We infer the existence of this unidentified product from the Delplot analysis. Formaldehyde, a compound difficult to detect and analyze by GC but known to form from methanol SCWO 40 and methylamine oxidation in the gas phase,24,25 is one candidate for the identity of CI. Armed with the essential features in the global reaction network, one can propose a set of reaction paths that can serve as the basis for a mathematical model

CO 98 CII k6

CII 98 CO2 k7

CO 98 CO2

(R1) (R2) (R3) (R4) (R5) (R6) (R7)

Reactions R5 and R6 represent water-gas shift chemistry, where CII is a carbon-containing intermediate (likely HCOOH41) in the pathway between CO and CO2. Reaction R7 represents direct oxidation of CO to CO2. A system of first-order ordinary differential equations was generated from the model above (assuming plugflow behavior in the reactor and pseudo-first-order kinetics for each pathway after the induction times), and the model was fit to the experimental SCWO data. Table 3 provides the best-fit values of the rate parameters k1k7. Since reactions (R1)-(R7) are global/lumped reactions the parameters k1-k7 are lumped rate parameters and not rate constants for elementary steps. As such, they need not exhibit Arrhenius behavior. Figures 3-5 compare experimental product yields with those calculated from the reaction model. The residence times used in Figures 3-5 are the experimental residence times minus the induction times in Table 2. The simple model does a very good job of capturing the trends in nitrogen and carbon speciation at both 390 and 410 °C (Figures 3 and 4) and only a slightly poorer job at 450 °C (Figure 5), where the reaction is much faster. The SCWO chemistry at 500 °C was not modeled because the disappearance of methylamine (reaction R1 of the global reaction network) was too fast to be quantified accurately (see Table 1). The product distribution and reaction network for methylamine SCWO differ from those for gas-phase combustion. The chief difference is the absence of products containing both carbon and nitrogen from SCWO, whereas such products (e.g., H2CNH, HCN) appear during gas-phase oxidation.24,25,29 There is exclusive production of NH3 as the nitrogen-containing primary product from SCWO, which indicates more

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Figure 3. Experimental and calculated concentration profiles at 390 °C: a. nitrogen-containing compounds and b. carbon-containing compounds.

facile C-N bond cleavage in the SCWO experiments than in the gas phase. This enhanced C-N bond breaking in the primary reaction path suggests that the gas-phase free-radical combustion mechanism might not be dominant during methylamine SCWO in our reactor system. Rather, it seems likely that the reactor walls (Hastelloy C-276) may be catalytically active for methylamine oxidation, as they are thought to be for pyridine SCWO.8,23 The surface science literature supports this hypothesis. Methylamine adsorbs on nickel,42-48 which constitutes 57% of Hastelloy C-276. Moreover, nickel facilitates C-N bond cleavage in methylamine both thermally44,46,47 and oxidatively.49 Therefore, methylamine SCWO may involve a heterogeneous component in the Hastelloy reactor system. Ammonia Oxidation during Methylamine SCWO Ammonia is an expected intermediate during methylamine SCWO, because it appears during methylamine oxidation in the gas phase24,25 and SCWO of other nitrogen-containing compounds.7,8,12-15 The literature on SCWO of nitrogen-containing compounds, however, would lead one to expect ammonia to be refractory at typical SCWO conditions.5,6,11,15,16,18 Our results suggest otherwise. The disappearance of ammonia during reaction is evident in Figure 1 and consistent with the subsequent appearance of N2O and N2 (see Table 1). Table 4 uses rate equations reported in the literature for ammonia SCWO to summarize its reactivity. The last column displays the rate of NH3 disappearance calculated at 450 °C and 3.3 mmol/L. Calculating these

Figure 4. Experimental and calculated concentration profiles at 410 °C: a. nitrogen-containing compounds and b. carbon-containing compounds.

rates required extrapolating the literature rate equations to either temperatures6,18 or concentrations11 lower than those used in the experiments. It is evident that the rate of ammonia disappearance is higher in our system than in any of these earlier studies. The rate is 2-3 orders of magnitude higher than previously observed in other open tubular reactors and about five times higher than the rate obtained in a packed bed reactor with a much higher surface area. In addition to the enhanced rate of ammonia oxidation, there also exists a difference in product selectivities. As mentioned earlier, we find more N2O than N2 as a product of methylamine SCWO and, by inference, ammonia oxidation in supercritical water. This finding also differs from all other SCWO investigations of nitrogen-containing compounds,6,9,10,13,14,16-18 which report exclusive production of N2. The main differences between the studies outlined in Table 4 and the present work are the use of Hastelloy C-276 as the reactor material and methylamine as the feed compound. It is possible that ammonia is oxidized catalytically on Hastelloy C-276. Both Segond et al.18 and Webley et al.6 suggested that ammonia SCWO is catalyzed by the reactor wall materials. Also, other investigations8,23 have suggested that Hastelloy C-276 may be catalytically active for pyridine SCWO. The catalytic oxidation on Hastelloy might be faster than on stainless steel or Inconel, and the catalytic mechanism on Hastelloy might shift the product distribution to favor N2O instead of N2. An alternate or complementary explanation for the enhanced reactivity of NH3 in our experiments can be built on the presence of methylamine in the system. The oxidation of ammonia might

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tion is enhanced by the presence of methylamine (or products therefrom) in the reaction environment, rather than by the presence of Hastelloy. If the dominant mechanism is homogeneous, ammonia oxidation might be accelerated by the presence of methylamine or its associated radicals. Co-oxidizing methanol with ammonia did not increase the rate of ammonia oxidation,6 but it is possible that larger amounts of methanol might have been required to see an effect.50 If the dominant mechanism is heterogeneous, methylamine might be required to place the nitrogen atoms on the catalytic surface, for their subsequent conversion to N2O and N2. This scenario would require that methylamine adsorbs more readily than ammonia on Hastelloy. Previous work45 on methylamine adsorption on nickel, the major component of Hastelloy, suggests that this is the case. Conclusions

Figure 5. Experimental and calculated concentration profiles at 450 °C: a. nitrogen-containing compounds and b. carbon-containing compounds.

The products of methylamine SCWO at 249 atm and 390-500 °C include CO2, CO, CH3OH, NH3, N2O, and N2. Complete conversion of the organic carbon to CO2 occurred at 500 °C with about 50 s of residence time. A reaction model that involves cleavage of the C-N bond in the primary reaction step is quantitatively consistent with the experimental data. Ammonia appears to be the exclusive nitrogen-containing intermediate between methylamine and the end products, N2O and N2. The subsequent oxidation of ammonia during our experiments appears to be due to heterogeneous chemistry. The presence of methylamine appears to be the variable that increased both the rate of ammonia oxidation and the selectivity to N2O instead of N2 to levels higher than those observed for SCWO of ammonia alone.

Table 4. Kinetics Comparison for Ammonia SCWO at 450 °C and 3.3 mmol/L

Acknowledgment

reactor

reactor material

Goto et al.11 Webley et al.6 open tube Webley et al.6 packed bed Segond et al.18 Segond et al.18 this work

SS Inconel Inconel SS SS Hastelloy

rate of NH3 area/vol. disappearance (mol/L/s) (cm-1) NA 23 690 18.5 40 15/18.5

9.45 × 10-8 4.81 × 10-8 2.97 × 10-5 9.38 × 10-7 1.44 × 10-6 1.58 × 10-4

be faster in the presence of the various intermediates that could form from the parent methylamine molecule. Such acceleration of the rate of one compound by the presence of a second has been demonstrated for homogeneous SCWO.50 We conducted a SCWO experiment with ammonia alone in the feed stream to determine whether the presence of methylamine or Hastelloy was the more important variable. SCWO of Ammonia Alone We performed an ammonia SCWO experiment at 249 atm and 410 °C. The initial ammonia and oxygen concentrations were 5.2 and 18.5 mmol/L, respectively. The ammonia conversion was less than 1% at a residence time of 78 s, in agreement with previous work.6,18 The pseudo-first-order rate constant for ammonia disappearance is about 9.7 × 10-5 s-1, which is much lower than the pseudo-first-order rate constant of 8.2 × 10-3 s-1 (sum of k2 and k3 from Table 3) for ammonia conversion during the methylamine SCWO experiment at 410 °C. These results indicate that ammonia oxida-

We thank Corey Grice for experimental assistance. This work was supported, in part, by the National Science Foundation (CTS-9903373). Literature Cited (1) Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hong, G. T.; Barner, H. T. Oxidation of Hazardous Organic Wastes in Supercritical Water: A Review of Process Development and Fundamental Research. In Emerging Technologies in Hazardous Waste Management III; Tedder, D. W., Pohland, F. G., Eds.; ACS Symposium Series 518, 1993; p 35. (2) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at supercritical conditions: applications and fundamentals. AIChE J. 1995, 41, 1723. (3) Killilea, W., R.; Swallow, K.; Hong, G. T. The Fate of Nitrogen in Supercritical Water Oxidation. J. Supercrit. Fluids 1992, 5, 72. (4) Thomason, T.; Hong, G.; Swallow, K.; Killilea, W. The MODAR Supercritical Water Oxidation Process. In Innovative Hazardous Waste Treatment Technology Series: Thermal Processes; Freeman, H., Ed.; Technomic Publishing: Lancaster, 1990; p 31. (5) Helling, R. K.; Tester, J. W. Oxidation of Simple Compounds and Mixtures in Supercritical Water: Carbon Monoxide, Ammonia, and Ethanol. Environ. Sci. Technol. 1988, 22, 1319. (6) Webley, P.; Tester, J. W.; Holgate, H. R. Oxidation Kinetics of Ammonia and Ammonia-Methanol Mixtures in Supercritical Water in the Temperature Range of 530-700 °C at 246 bar. Ind. Eng. Chem. Res. 1991, 30, 1745. (7) Lee, D. S.; Gloyna, E. F. Hydrolysis and Oxidation of Acetamide in Supercritical Water. Environ. Sci. Technol. 1992, 26, 1587. (8) Crain, N.; Tebbal, S.; Li, L.; Gloyna, E. F. Kinetics and Reaction Pathways of Pyridine Oxidation in Supercritical Water. Ind. Eng. Chem. Res. 1993, 32, 2259.

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Received for review September 1, 2004 Revised manuscript received December 21, 2004 Accepted December 29, 2004 IE0491793