Kinetic Study on Ammonia Oxidation Acceleration by Multi-Injection of

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Kinetic study on ammonia oxidation acceleration by multi-injection of methanol in supercritical water Eriko Shimoda, and Yoshito Oshima Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04357 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on January 6, 2017

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Industrial & Engineering Chemistry Research

Kinetic study on ammonia oxidation acceleration by multi-injection of methanol in supercritical water Eriko Shimoda and Yoshito Oshima*

Department of Environment Systems, Graduate School of Frontier Sciences,

The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan

*Corresponding author. E-mail: [email protected]. tel: +81 4 7136 4720. fax: +81 4 7136 4721.

KEYWORDS

supercritical water, oxidation, ammonia, methanol, co-oxidation effect,

multi-stage injection

ABSTRACT. The effectiveness of multi-stage methanol injection in supercritical water oxidation (SCWO) of an ammonia/methanol mixture as a method for utilizing the alcohol co-oxidation effect was studied at 530 °C and 25 MPa. A simple global reaction rate model was developed based on the results of kinetic experiments performed at various ammonia, methanol, and oxygen concentrations, yielding excellent agreement with experimental values. Ammonia conversion was predicted when the same molar flow rate of methanol was split

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between two points: a first injection at the entrance of a reactor and a second injection midway in the reaction stream. Two-stage methanol injection resulted in higher ammonia conversion than in the model calculation when methanol was injected once at the reactor entrance. Model calculations under the same experimental conditions showed that two-stage injection improves ammonia conversion, which aligns closely with predicted values. Moreover, our calculations suggest that ammonia conversion depends on injection timing, and on methanol concentration of the flow after injection as well.


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Introduction

Supercritical water is defined as water above its critical point (647.3 K and 22.12 MPa). The unique properties of supercritical water such as extremely low polarity and a diffusion coefficient two orders of magnitude higher than that of water at room temperature make organic solvents and oxygen miscible in supercritical water. Because supercritical water is a reaction field at high temperature, homogeneous oxidation reactions of organics with oxygen proceed at high rates. Supercritical water oxidation (SCWO) is a promising technique for treating hazardous industrial wastes. SCWO has advanced over recent years, with numerous publications describing theoretical and practical applications.1-7 Although SCWO has the potential of destroying organic molecules in a highly efficient way, phenol,8 benzene,9 ammonia,10 and other compounds have been reported as refractory materials. Because these compounds are commonly contained in industrial wastes, their destruction is one key industrial issue, not merely a rate-determining step in the SCWO. Kinetic studies of organic mixtures have attracted attention for treating practical wastes usually containing complex chemical mixtures, as well as studies on the co-oxidation of simple mixtures in supercritical water (see Table S111-32 in the Supporting Information). Most reports, including one recently from our group32 on the co-oxidation of alcohol with a compound, have shown that alcohol enhances conversion of the coexisting recalcitrant compound at 530 °C. In the SCWO of an ammonia/methanol mixture, ammonia conversion ACS Paragon Plus Environment

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improved upon methanol addition during methanol oxidation. Ammonia decomposition retarded after methanol oxidation. Detailed chemical kinetic modeling has revealed that radical chain reactions retard after methanol was completely oxidized. By devising a process to add a rate enhancer, the co-oxidation effect can be used more effectively for facilitating the destruction of recalcitrant compounds. The advantage of utilizing co-oxidation effect is to enable lowering the operation temperature, because oxidation of ammonia can proceed even at such low temperature as 530 °C, at which ammonia could not be decomposed without methanol addition. From the viewpoint of reactor engineering, intermittent injection, or the multi-injection of alcohol and oxygen, is one promising approach for improving reaction efficiency by the co-oxidation effect. The concept of multi-injection has been limited to multi-stage injection of oxygen30, 33-39 in previous studies. Muske et al.33 reported on the model-based control of an oxygen feed and estimation scheme using a reduced kinetic model. This model considers the amount of organic carbon (TOC), nitrate, ammonia, and oxygen to minimize the amount of aqueous nitrogen compounds in the effluent. A reactor possessing a two-stage oxygen injector was used in their work. García-Jarana et al.35 have reported on the SCWO of N,N-dimethylformamide (DMF) with multi-injection of an oxidant. Their experiments were performed by splitting the same amount of oxidant feed over two entry points, which demonstrated better conversion than that observed when using a single entry point for the oxidant. Although the effectiveness of multi-stage injection of oxygen has been proven, ACS Paragon Plus Environment

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studies are limited to the injection of oxygen, the process for multi-injection of oxygen has yet to be precisely described from the viewpoint of reaction mechanisms and optimization. Therefore, developing a quantitative reaction rate model that predicts the co-oxidation effect is critical for considering multi-injection as a method to take full advantage of the co-oxidation effect. This work aims to clarify the effectiveness of multi-injection using experimental and model-based simulation results for the conversion of a recalcitrant compound. We chose an ammonia/methanol mixture as the model case for the mixture of a recalcitrant compound with alcohol. In this work, experiments were performed to determine the global reaction rate expressions as a function of concentrations. The effectiveness of the two-stage injection on the SCWO of the mixture is also discussed based on the ammonia conversion obtained by both calculation and experiment.

Experimental methods

SCWO experiments were conducted in an isothermal, isobaric tubular flow reactor maintained at 530 °C and 25 MPa. The reactor tube was made of Hastelloy C-276 with an i.d. = 0.80 mm. The reagents, procedure, and analysis method were similar to those reported in a previous study.32 The different points related to the preparation of reagents, experimental procedures, and analytical method are described below. ACS Paragon Plus Environment

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Reagents. During single-stage oxidation, the concentrations of ammonia, methanol, and oxygen were changed (see Table 1). The initial oxygen concentrations were 0.79 to 2.2 times the stoichiometric concentration for the complete oxidation of ammonia and methanol based on the reactions below: 2NH + 2O → N O + 3H O 2CH OH + 3O → 2CO + 4H O

Procedure. Two types of experimental apparatus were used in this study: 1) one for single-stage oxidation (Figure 1a), and 2) the other for two-stage oxidation (Figure 1b). The tank, pumps, check valve, and a section after the heat exchanger for the cooling water were common. The apparatus for the single-stage oxidation was the same as that in a previous study.32 Figure 1b shows the length of the first reactor as 20 m, and that of the second reactor as 16 m. The preheaters for the ammonia/methanol mixture solution are 7.0 m (i.d. = 0.50 mm), and those for H2O2 solution in the first stage are 3.0 m (i.d. = 1.0 mm). The preheater for methanol solution in the second stage is 6.0 m (i.d. = 1.0 mm). All preheaters were made of SUS316.


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Analysis. CO, CH4, CO2, and N2O were analyzed using a gas chromatograph equipped with a thermal conductivity detector (Shimadzu Corp., GC-8A, hereinafter GC-TCD) and a Shincarbon ST 50/80 column (Shinwa Chemical Industries Ltd.).

Results and discussion

Experimental results. Table S2 in the Supporting Information shows the experimental results of the single-stage oxidation experiments performed under the conditions shown in Table 1. Ammonia conversions from l.5 % to 16 % were obtained for the residence times under each set of conditions. The C-total ranged between 0.66 and 1.10. Ammonia conversion and N2O yield was almost same, which suggests that N2O was the main product in this study. Figure 2a shows the time profile of methanol conversion, CO yield and CO2 yield under condition #10 (refer to Table 1). This figure shows methanol was oxidized to CO, and finally oxidized to CO2. In methanol single oxidation, methanol oxidation proceeds via consecutive reactions (CH3OH→HCHO→CO→CO2).40 The oxidation behavior of methanol even with ammonia seemed to be the same as in the case of methanol single oxidation. Methane was detected by GC-TCD in most runs of this study. Methane can be regarded as a minor product because it amounted less than 3.1 % carbon-base. Figure 2b shows the pseudo-first order plot of ammonia decomposition. Compared with Figure 2a, Figure 2b shows the hypothesized characteristics of ammonia/methanol oxidation, that is, ammonia ACS Paragon Plus Environment

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decomposition continues during methanol co-oxidation but ammonia decomposition retards as methanol co-oxidation reaches completion. This is consistent with our earlier study on the kinetics of the SCWO of ammonia/methanol mixtures.32

Global reaction rate in NH3/CH3OH co-oxidation. Our previous study32 revealed that ammonia conversion increased as initial methanol concentration increased, and pseudo-first order plot of ammonia conversion was fitted by a straight line, that is, ammonia decomposition proceeds via first order kinetics. Assuming that the initial methanol and oxygen concentrations affect the ammonia decomposition rate during methanol oxidation, the ammonia decomposition rate is expressed by the following equation: NH = − NH O CH OH

(1)

where NH is ammonia concentration and O is oxygen concentration. CH OH is the initial methanol concentration, is the rate constant, and a and b are parameters. Figures 2a and 2b indicate that ammonia decomposition proceeds rapidly until 14 s. Our previous study32 showed that ammonia decomposition retards as methanol oxidation to reach completion, namely CO2 yield reaches 1. In other words, methanol contributes to ammonia decomposition until methanol is completely oxidized. To express this decomposition behavior, equation (1) was changed to the following equation: NH = − NH O ( CH OH (1 − ))


(2)

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The term CH OH (1 − )

is carbon concentration working effectively on the radical

chain mechanism. Our experimental results suggest that when CO yield is between 0.050 and 0.10, ammonia decomposition starts to retard and the pseudo-first order plot can be roughly fitted by straight lines. In this study, the time when CO yield reaches 0.050 is defined as the boundary time for the complete oxidation of methanol. A straight line can fit the pseudo-first order plot of ammonia conversion after 14 s. Here, the period when the pseudo-first order plot of ammonia can be fitted by a straight line is defined as the latter stage of the reaction. The ammonia decomposition rate in the latter stage of the reaction can be expressed a simplified form of (3): NH = − ′ NH

(3)

where ′ is the rate constant, a function of the initial methanol concentration and initial oxygen concentration because the variances in the methanol and oxygen concentrations are very small at the latter. We extracted the data when CO yield is lower than 0.050 from the data shown in Table S2 in the Supporting Information and obtained ′ by calculating the inclination of the pseudo-first order plot in every condition. We then determined the correlation coefficient of methanol and oxygen initial concentrations. The correlation coefficient is 0.71, indicating that the two variables are highly correlated. Therefore we assumed that ′ can be expressed as follows: ACS Paragon Plus Environment

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′ = CH OH +

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(4)

using only initial methanol concentration as an explanatory variable. We determined the variable values to be

= 2.38 × 10$% and = −1.32 × 10$% by regression analysis,

and hypothesis testing was performed to verify the validity of these values. The null hypothesis & is = 0, and the critical t-value for the one-sided test is 2.31 ['⁄ () − ) = .* (10 − 2) = 2.31] at a 95% confidence level. Because t-value for is 3.32, null hypothesis is rejected. Therefore, ′ can be obtained by the following equation: ′ = 2.38 × 10$% CH OH − 1.32 × 10

%$(5)

The methanol oxidation rate follows pseudo-first order kinetics40 because the methanol oxidation in supercritical water proceeds via consecutive reactions (CH3OH→HCHO→CO→ CO2). Based on this, the global rate expressions for methanol oxidation are described by the following equations: CH OH + = −( + ′ NH ) O , CH OH HCHO + = ( + ′ NH ) O , CH OH -

(6)

(7)

− ( + ′ NH ) O HCHO .

CO = ( + ′ NH ) O . HCHO /

(8)

− ( + ′ NH ) O CO 0

These equations assume simple pseudo-first order kinetics. Here, because ammonia conversion was comparatively small (maximum value 0.16), the ammonia concentration


10

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terms in equations (6) to (8) were fixed as initial ammonia concentration [NH3]0. Because CO2 was the final product of methanol oxidation, its concentration was calculated as follows:

CO = CH OH − CH OH − HCHO − CO

(9)

The oxygen consumption as a function of the ammonia decomposition and methanol oxidation follows the chemical equations below: 2NH3 + 2O2 →N2O + 3H2O

(10)

2CH3OH + O2 → 2HCHO + 2H2O

(11)

2HCHO + O2 → 2CO + 2H2O

(12)

2CO + O2 → 2CO2

(13)

When using equation (3) to calculate ammonia decomposition rate, oxygen consumption by ammonia was negligible compared to consumption by methanol because the ammonia oxidation rate was also negligible for every run.

Parameter determination. Parameters for rate constants (kN, k1, k2, k3) and the reaction orders (a to h) in equation (2) and equations (6) to (8) were obtained by simultaneously fitting the experimental results to these equations to minimize the sum of the differences between experimental values and predicted values. The differential equations (2), (3), and (6) to (8) were solved using the explicit Euler method in which step value ∆ equals 0.001 s. The calculated and experimental data of NH3, CH3OH, CO, and CO2 were compared. When C-total was outside the range of 0.90 to 1.10, only the data for NH3 was compared. ACS Paragon Plus Environment

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To elucidate the validity of the fitting results, the predicted conversions of ammonia and methanol, yields of CO and CO2 are plotted against experimental ones obtained from all the tests as shown in Figures 3a to 3d. These figures show that the prediction data obtained using the model equations were in good agreement with the experimental data. The data in Figures 3a to 3d are shown in Table S3 in the Supporting Information. In Figure 3b the prediction data of methanol obtained using the model equations were not in good agreement with the experimental data. In theory, the data would include a lower methanol conversion. However, the range of methanol conversion in this study was biased in the higher region, causing the disagreement with the experimental data for the fitting. We think it is inevitable in this study because the methanol decomposition rate is much faster than that of ammonia.

The parameters obtained (see Table 2) suggest that the ammonia decomposition rate while the ammonia oxidation was promoted by methanol coexistence is 0.71 times relative to

CH OH (1 − ) and 0.35 times relative to oxygen concentration. The parameters also suggest that ammonia does not affect methanol oxidation under these reaction conditions though methanol promotes ammonia oxidation. Kinetic analysis of ammonia decomposition in ammonia/methanol mixtures has not been extensively studied in the past; consequently, the comparison of our results with previous reports has proved challenging. However, numerous reports on the kinetic analysis of methanol oxidation exist such as that reviewed by Vogel et al.41 Our previous study32 ACS Paragon Plus Environment

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demonstrated that co-existing ammonia increased methanol conversion when initial oxygen concentration is in large excess. Nevertheless, compared with the values calculated using the model on methanol single oxidation, “2. low methanol feed concentration PFR data”, in Vogel’s review and the induction time of 0.538 s suggested by Brock et al.40, our model produced lower methanol conversion. The methanol conversion obtained experimentally was mostly higher than 0.80 in our study, with a longer residence time. Prevention of radical accumulation by coexisting ammonia likely explains this discrepancy. Kinetic analysis of methanol decomposition and investigating of the effect of ammonia coexistence on the induction period of methanol decomposition is a subject of further study.

Investigation of multi-injection

Effect of second methanol injection. We studied the effect of a two-stage methanol injection and compared it with the results of a single-stage methanol injection. Single-stage injection requires methanol to be injected only at the entrance of the reactor, whereas two-stage injection requires methanol to be injected twice. We compared ammonia conversion while maintaining the molar flow rates of carbon, nitrogen, and oxygen fixed. To confirm the effect of the second-stage injection, we performed an experiment under the conditions shown in Table 3, #B. The ammonia conversion under the conditions shown in Table 3 #A and #B was predicted using the global reaction rate expressions mentioned in the ACS Paragon Plus Environment

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previous section. Figure 4 illustrates a comparison of ammonia conversion under conditions in Table 3 #A and #B, with the total residence time (defined as sum of residence times in both reactors) of 21 s. The figure shows that improvement in ammonia conversion was confirmed experimentally.

Prediction using global reaction rate expressions. We attempted to obtain the optimum conditions for the desired ammonia conversion using global rate expressions. The concentration for the second methanol injection was varied while maintaining both the total methanol molar flow rate and the timing of the second injection constant. Table 4 shows the details of the conditions studied. Figure 5 illustrates the ammonia conversion at a total residence time of 30 s. The figure shows that ammonia conversion is higher when methanol concentration is higher. The fitting results discussed in the previous chapter indicate that ammonia conversion increases to 0.71 times relative to CH OH (1 − ), which seems to be the main reason for the observed effect of the second methanol concentration on the ammonia conversion. Ammonia conversion was calculated by varying the timing of the second methanol injection while the second methanol concentration and molar flow rates remain constant. Figure 6 shows the time profiles of the ammonia conversion, and indicates that adding methanol too early will result in a low ammonia conversion. What is more, this result


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indicates that the timing of the second injection determines the total residence time when ammonia reaches the targeted conversion. Figure 7 shows the total residence times when the ammonia conversion reaches 0.10, 0.12, and 0.15. This figure also shows optimum timing to obtain a targeted ammonia conversion in shorter residence time. These results indicate that predicting the optimum conditions for the effective decomposition of ammonia is possible by using the global rate expressions for the SCWO of ammonia/methanol mixtures. The simulation of two-stage methanol injection was performed using a detailed chemical kinetic model (DCKM), which was also used in our previous study32. The simulation results suggested that the timing of the injection and concentration of methanol after injection affect ammonia conversion, which was the same as the prediction using the global rate expressions aforementioned. The net reaction rate analysis in this simulation by DCKM suggested that the timing of the second injection and the methanol concentration in the second reactor contribute to time dependence of OH accumulation during methanol oxidation and the acceleration of radical chain mechanism, respectively.

Conclusions

The use of multi-stage injections of easily degradable compounds as a radical source for decomposition of refractory compounds was proposed. We experimentally confirmed that multi-stage injection is effective in controlling reaction conversion. The SCWO of an ACS Paragon Plus Environment

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ammonia/methanol mixture was conducted at 530 °C, 25 MPa by varying the initial concentrations of ammonia, methanol, and oxygen as parameters. We then developed a model using global reaction rate expressions for ammonia decomposition and methanol oxidation by fitting experimental results to the rate equations defined in this paper. An experiment on two-stage methanol injection was also performed, demonstrating an improvement in ammonia conversion. The predicted values obtained using the model under the same experimental conditions revealed an excellent correlation between prediction and experiment. We also calculated ammonia conversion when methanol was added to reaction flow. The results suggest that timing of the injection and concentration of methanol after injection affect ammonia conversion. The work presented herein confirms that reaction control in SCWO is possible by optimizing reactor design and conditions based on kinetic analysis. This method may be applied to other mixtures of recalcitrant compounds and alcohols such as phenol/methanol, and acetic acid/methanol.


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Figure captions

Figure 1. Experimental apparatus of (a) single-stage oxidation (b) multi-stage oxidation. (1) NH3/CH3OH solution feed tank (2) CH3OH solution feed tank (3) H2O2 solution feed tank (4) He gas (5) pump (6) check valve (7) pre-heating line (8) thermocouple (9) tubular reactor (10) sand-bath (11) cooling water (12) pressure gauge (13) back pressure regulator (14) liquid sample (15) gas liquid separator (16) gas sample.

Figure 2. (a) Time profiles of methanol conversion, CO and CO2 yields under reaction condition #10 from Table 1. (○) methanol conversion, (△) CO yield, (◆) CO2 yield. (b) Pseudo-first order plot of ammonia conversion under reaction conditions #10 from Table 1. The fitted line determined by least-square method was also indicated.

Figure 3. Parity plots of the (a) ammonia conversion, (b) methanol conversion, (c) CO yield, and (d) CO2 yield.

Figure 4. Effect of two-stage methanol injection during ammonia conversion with a total residence time of 21 s. The conditions corresponding to each element were 2 (calculation): #A, 1+1(experiment): #B, 1+1 (calculation): #B shown in Table 3. A bar indicates standard deviation.

Figure 5. Dependence of the second methanol concentration on the ammonia conversion at 30 s under the conditions shown in Table 4.


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Figure 6. Effect of the second injection timing on the time profile of ammonia conversion. (· - ·-) 1.0 s, (― ―) 3.0 s, (···) 5.0 s, (- - -) 10 s, (− −) 20 s, (-− -−) 25 s. First injection: [CH3OH]0 = [NH3]0 = 6 mmol/L, [O2]0 = 24 mmol/L, flow rate = 4.0 mL/min. Second injection: [CH3OH]0 = 48 mmol/L, flow rate = 0.50 mL/min.

Figure 7. Dependence of the second injection timing on the total residence time when ammonia reaches to the targeted conversions. (▲) 0.10, (○) 0.12, (■) 0.15. The conditions are same as Figure 6.

Table captions

Table 1. Concentration conditions.

Table 2. Parameters calculated for Equations (2), (6), (7), and (8).

Table 3. Conditions of single-stage injection and two-stage injection of methanol.

Table 4. Conditions for comparison of the second methanol concentration. Flow rate of the stream and that of the methanol concentration were varied to maintain the added methanol molar flow at a constant rate.


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Figures


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Figure 1. Experimental apparatus of (a) single-stage oxidation (b) multi-stage oxidation. (1) NH3/CH3OH solution feed tank (2) CH3OH solution feed tank (3) H2O2 solution feed tank (4) He gas (5) pump (6) check valve (7) pre-heating line (8) thermocouple (9) tubular reactor (10) sand-bath (11) cooling water (12) pressure gauge (13) back pressure regulator (14) liquid sample (15) gas liquid separator (16) gas sample.


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Figure 2. (a) Time profiles of methanol conversion, CO yield and CO2 yield under reaction condition #10 from Table 2. (○) methanol conversion, (△) CO yield, (◆) CO2 yield. (b) Pseudo-first order plot of ammonia conversion under reaction conditions #10 from Table 2. The fitted line determined by least-square method was also indicated.


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Figure 3. Parity plots of the (a) ammonia conversion, (b) methanol conversion, (c) CO yield, and (d) CO2 yield.

Figure 4. Effect of two-stage methanol injection during ammonia conversion with a total residence time of 21 s. The conditions corresponding to each element were 2 (calculation):


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#A, 1+1(experiment): #B, 1+1 (calculation): #B shown in Table 3. A bar indicates standard deviation.

Figure 5. Dependence of the second methanol concentration on the ammonia conversion at 30 s under the conditions shown in Table 4.


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Figure 6. Effect of the second injection timing on the time profile of ammonia conversion. (· - ·-) 1.0 s, (― ―) 3.0 s, (···) 5.0 s, (- - -) 10 s, (− −) 20 s, (-− -−) 25 s. First injection: [CH3OH]0 = [NH3]0 = 6 mmol/L, [O2]0 = 24 mmol/L, flow rate = 4.0 mL/min. Second injection: [CH3OH]0 = 48 mmol/L, flow rate = 0.50 mL/min.


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Figure 7. Dependence of the second injection timing on the total residence time when ammonia reaches to the targeted conversions. (▲) 0.10, (○) 0.12, (■) 0.15. The conditions are same as Figure 6.


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Tables

Concentration conditions

Table 1.

[CH3OH]0

[NH3]0

[O2]0

(mmol/L)

(mmol/L)

(mmol/L)

1

5.9

2.9

25

2

6.0

3.0

13

3

2.9

3.0

7.8

4

2.8

2.9

5.6

5

12

2.9

20

6

6.0

11

23

7

5.8

1.6

12

8

3.1

1.4

6.4

9

5.6

5.1

16

10

11

5.2

29

11

2.7

5.2

11

#

Table 2.

Parameters calculated for Equations (2), (6), (7), and (8)

a

b

c

d

e

f

g

h

0.35

0.71

0.48

0.79

0

1.0

0

0.26

kN

k1

k1 ’

k2

k2 ’

k3

k3 ’

0.0021

0.14

0

40

0

0.50

0.11

Table 3.

Conditions of single-stage injection and two-stage injection of methanol


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

First reactor

Second reactor Timing

Concentration[mmol/L]

#

methanol ammonia

oxygen

A

5.39

2.64

13.3

B

3.03

2.97

15.0

Table 4.

Page 28 of 48

Mass flow

of 2nd

rate[L/min]

injection [s]

0.004

Concentration

Mass flow

[mmol/L]

rate of the stream

methanol

added [L/min]

-

-

-

12.4

24.04

0.0005

Conditions for comparison of the second methanol concentration. Flow rate of the

stream and that of the methanol concentration were varied to maintain the added methanol molar flow at a constant rate. Second reactor

First reactor

#

[CH3OH]0 (mmol/L)

[NH3]0

[O2]0

(mmol/L) (mmol/L)

flow rate at

timing of

mass flow rate

initial

second

of the stream

reactor

injection

added

[mL/min]

[s]

[mL/min]

added [CH3OH]0 (mmol/L)

1

0.20

120

2

0.50

48

1.0

24

2.0

12

5

4.0

6

6

8.0

3

3 4

6.0

6.0

24

4.0

12.4

AUTHOR INFORMATION

Corresponding Author


28

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*E-mail: [email protected]. tel : +81 4 7136 4720. fax: +81 4 7136 4721.

Notes The authors declare no competing financial interest.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge via the Internet at http://pubs.acs.org/.

ACKNOWLEDGMENTS This study was supported by the Sasakawa Scientific Research Grant from The Japan Science Society. All of this support is greatly appreciated. We also thank Masaatsu Aichi, PhD, for helping us to fit the parameters to the global reaction rate expressions, Makoto Akizuki, PhD, for reviewing our manuscript and commenting on this study, and Prof. Freeman, for helping us English proofread.

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Figure 1. Experimental apparatus of (a) single-stage oxidation (b) multi-stage oxidation. (1) NH3/CH3OH solution feed tank (2) CH3OH solution feed tank (3) H2O2 solution feed tank (4) He gas (5) pump (6) check valve (7) pre-heating line (8) thermocouple (9) tubular reactor (10) sand-bath (11) cooling water (12) pressure gauge (13) back pressure regulator (14) liquid sample (15) gas liquid separator (16) gas sample. 239x166mm (96 x 96 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Experimental apparatus of (a) single-stage oxidation (b) multi-stage oxidation. (1) NH3/CH3OH solution feed tank (2) CH3OH solution feed tank (3) H2O2 solution feed tank (4) He gas (5) pump (6) check valve (7) pre-heating line (8) thermocouple (9) tubular reactor (10) sand-bath (11) cooling water (12) pressure gauge (13) back pressure regulator (14) liquid sample (15) gas liquid separator (16) gas sample. 248x171mm (96 x 96 DPI)


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Figure 2. (a) Time profiles of methanol conversion, CO yield and CO2 yield under reaction condition #10 from Table 2. (○) methanol conversion, (△) CO yield, (◆) CO2 yield. (b) Pseudo-first order plot of ammonia conversion under reaction conditions #10 from Table 2. The fitted line determined by least-square method was also indicated. 78x69mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. (a) Time profiles of methanol conversion, CO yield and CO2 yield under reaction condition #10 from Table 2. (○) methanol conversion, (△) CO yield, (◆) CO2 yield. (b) Pseudo-first order plot of ammonia conversion under reaction conditions #10 from Table 2. The fitted line determined by least-square method was also indicated. 78x69mm (300 x 300 DPI)


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Figure 3. Parity plots of the (a) ammonia conversion, (b) methanol conversion, (c) CO yield, and (d) CO2 yield. 78x69mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



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Figure 4. Effect of two-stage methanol injection during ammonia conversion with a total residence time of 21 s. The conditions corresponding to each element were 2 (calculation): #A, 1+1(experiment): #B, 1+1 (calculation): #B shown in Table 3. A bar indicates standard deviation. 74x39mm (300 x 300 DPI)



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Figure 5. Dependence of the second methanol concentration on the ammonia conversion at 30 s under the conditions shown in Table 4. 78x69mm (300 x 300 DPI)


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Effect of the second injection timing on the time profile of ammonia conversion. (∙ - ∙-) 1.0 s, (― ―) 3.0 s, (∙∙∙) 5.0 s, (- - -) 10 s, (− −) 20 s, (-− -−) 25 s. First injection: [CH3OH]0 = [NH3]0 = 6 mmol/L, [O2]0 = 24 mmol/L, flow rate = 4.0 mL/min. Second injection: [CH3OH]0 = 48 mmol/L, flow rate = 0.50 mL/min. 80x73mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Dependence of the second injection timing on the total residence time when ammonia reaches to the targeted conversions. (▲) 0.10, (○) 0.12, (■) 0.15. The conditions are same as Figure 6. 78x69mm (300 x 300 DPI)


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Kinetic Study on Ammonia Oxidation Acceleration by Multi-Injection of

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