Co-Oxidation Effects of Methanol on Acetic Acid and Phenol in

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Co-Oxidation Effects of Methanol on Acetic Acid and Phenol in Supercritical Water Jie Zhang, Shuzhong Wang,* Yang Guo, Donghai Xu, Xuedong Li, and Xingying Tang Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China ABSTRACT: Methanol acting as a co-oxidation component was introduced in supercritical water oxidation experiments of acetic acid and phenol which were identified as the most common and refractory intermediates during supercritical water oxidation of many complex organics. Experiments were performed in a tubular flow reactor at 25 ± 0.5 MPa. The concentrations of acetic acid and phenol were constant at 0.17 and 0.067 mol/L, respectively, with varying methanol concentrations ranging from 0.0313 to 0.6563 mol/L. Computational simulations based on the elementary reaction model for the supercritical water oxidation of two binary mixtures were also carried out to understand the co-oxidation mechanism of methanol for these two compounds. For the binary mixture of methanol/acetic acid, the experimental results showed that the acetic acid conversions in the mixture were higher than those in the absence of methanol, and as the methanol concentration increased, the accelerating effect was more notable. For the methanol/phenol mixture experiment, a low concentration of methanol (0.0313 mol/L) was found to have no measurable effect on phenol conversion, while higher concentrations (0.1563 and 0.3125 mol/L) caused increases in phenol conversions. Calculation results based on the elementary reaction model could reasonably reproduce the experiment trends. Model analyses showed that the methanol-derived HO2·and OH· radicals seemed to play an important role in accelerating acetic acid or phenol decomposition.

1. INTRODUCTION Supercritical water oxidation (SCWO) has received a great deal of attention as an innovative and potential treatment technology in which hazardous organic compounds can be completely decomposed to CO2 and H2O.1−7 Supercritical water (SCW, Tc = 647 K, Pc = 22.1 MPa) can provide a homogeneous reaction environment for both organics and oxygen, thereby obviating interphase transfer and leading to fast reaction rates. In recent years, the SCWO reaction mechanisms for methanol have been intensively studied,8−12 and the attention has been focused on research for the application of methanol during the SCWO environment. Its significance stems primarily from its appearance as a readily degradable organic compound and release of high reaction heat during the SCWO process. At first, the higher concentrations of methanol aqueous solution (around 1.0−10 mol/L) were introduced into the research on supercritical water hydrothermal flames. When organic waste concentrations are high enough and temperatures are above the autoignition temperatures of the fuel compounds, oxidation in SCW takes place in the form of flames, called hydrothermal flames.13−20 On the other hand, several investigators have paid special attention to the methanol aqueous feed at lower concentrations, ranging between 1.0 × 10−5 and 1.5 × 10−2 mol/L, in the exploration of the SCWO co-oxidation mechanism.6,21,22 The co-oxidation is a process where the oxidation of labile, reactive species accelerates the rate of refractory, stable species oxidation decomposition in supercritical water. It is possible that methanol oxidizes much more rapidly and methanol oxidation would produce a concentration of reactive intermediates at shorter reaction times, a portion of which would then attack other stable species during the SCWO process. Phillip E. Savage et al.21 simulated © 2013 American Chemical Society

the oxidation of methanol/methane mixtures in supercritical water and suggested that methanol accelerated the disappearance rate of methane because methane largely consumed the methanol-derived reactive intermediates OH· and HO2· radicals. G. Anitescu et al.22 co-oxidized 4-chlorobiphenyl (4CB) in supercritical water in the presence of methanol. They concluded that the co-oxidation effects of methanol on the decomposition of halogenated compounds were determined by the H· radicals produced by methanol participating in the dechlorination reaction. Therefore, a lower concentration of methanol in the mixture system could generally play an enhancing role in the oxidation of other refractory species in SCW, having a co-oxidation effect. Acetic acid has been identified as one of the most refractory intermediates for the oxidation of more complex organics and is also considered to be a key rate-controlling compound during the overall SCWO process. 23−28 Besides, the SCWO mechanism of phenol has been elucidated extensively because phenol is generated as an intermediate of aromatic hydrocarbons in the SCWO process. Moreover, phenol SCWO produces many dimers such as furans and dioxins depending on the operating conditions.2,5,29−32 Jude A. Onwudili5 detected phenol as an intermediate product during the oxidation of both naphthalene and phenanthrene in SCW. Thomas D. Thornton et al.2 suggested that the primary product for SCWO of phenol 2-phenoxyphenol possibly accounted for the formation of dibenzo-p-dioxin. Also, another intermediate dibenzofuran was Received: Revised: Accepted: Published: 10609

March 4, 2013 July 6, 2013 July 8, 2013 July 24, 2013 dx.doi.org/10.1021/ie400704f | Ind. Eng. Chem. Res. 2013, 52, 10609−10618

Industrial & Engineering Chemistry Research

Article

organic solution preheater (1 m in length and 15 mm in internal diameter) and the reactor were packed up with insulation course and heated to the desired temperature by an electric heating wire which was measured by thermocouples. The organic solution was pumped from the feed reservoir to the preheater and then reactor by a high-pressure feed pump (9) whose flow rate was regulated by a frequency changer. For the oxidant line, oxygen was introduced into the supercharger by a pressure reducer at the beginning of each run, and then, oxidant was compressed to reach its reaction pressure by a high pressure metering pump (2), which pushed the piston of the supercharger. The oxygen is preheated to 200 °C. Its flow rate was calibrated through the volumetric flow meter before the subsequent experiments. Then, oxygen was injected into a mixer before the reactor inlet mixing with preheated organic solutions and introduced into the reactor. The Reynolds numbers in the tubular reactor during the reaction process ranged between 2467 and 2605, which was a transition regime. After reaction, the products were cooled down in a heat exchanger and depressurized by a back-pressure regulator to ambient temperature and pressure. The gas−liquid mixture was separated in a gas−liquid separator, and the volumetric flow rate of gaseous product was determined by a volumetric flow meter at gas outlet. All experimental conditions are given in Table 1a,b for SCWO experiments of methanol/acetic acid and methanol/

extremely resistant to oxidative degradation in SCW. It is obvious that acetic acid and phenol are the two typical resistant organic compounds that deserve special attention during the uncatalyzed SCWO process. However, to our knowledge, no research has been published concerning whether co-oxidation enhancement of methanol related to acetic acid and phenol in supercritical water existed. Methanol has higher oxidation reactivity in SCW than acetic acid and phenol. We postulated that the presence of methanol would accelerate decomposition of acetic acid or phenol during the mixtures reaction. Therefore, in this investigation, we initiated SCWO experiments with different amounts of methanol added into the acetic acid and phenol aqueous feeds, respectively, aiming to examine the presence of cooxidation effect of methanol on these two compounds. To validate the possible action mechanism, we simulated the SCWO reaction process for mixtures of methanol/acetic acid and methanol/phenol based on the detailed chemical kinetics models. This allowed us to make visual and deeper understanding about the interactions in the SCWO reaction process. The combination of experimental verification and simulation analysis makes the present work distinctive.

2. EXPERIMENTAL SECTION 2.1. Materials. The reagents used in this investigation were methanol (purity ⩾99.9%, Sinopharm Chemical Reagent Co.), acetic acid (purity ⩾99.5%, Xi’an Sanpu Chemical Reagent Co.), and phenol (purity ⩾94.11%, Tianjin Fuchen Chemical Reagent Co.). Aqueous solutions of methanol/acetic acid or methanol/phenol mixtures were prepared by resolving an appropriate amount of reagents into deionized water. 2.1. Apparatus and Experimental Procedures. The experimental setup employed in this work was the same one used in our previous study.33 The schematic diagram of the experimental setup is demonstrated in Figure 1. In brief, the reactor was a tubular flow reactor (1.7 m in length and 12.3 mm in internal diameter (i.d.)) made of Hastelloy C-276. The

Table 1. Experimental Reaction Conditions for (a) SCWO of Methanol/Acetic Acid Mixtures at 24.5 MPa and (b) SCWO of Methanol/Phenol Mixtures at 25.5 MPa (a) SCWO of Methanol/Acetic Acid Mixtures at Temperatures Ranging from 673 to 753 K and 24.5 MPa run ID

[CH3COOH]0 (mol/L)

[CH3OH]0 (mol/L)

ΦI

[O2]ceo1

1 0.17 0.0000 0.000 4.29 2 0.17 0.0313 0.188 4.29 3 0.17 0.0000 0.000 2.03 4 0.17 0.3125 1.875 2.03 5 0.17 0.0000 0.000 1.24 6 0.17 0.6563 3.938 1.24 (b) SCWO of Methanol/Phenol Mixtures at Temperatures Ranging from 673 to 753 K and 25.5 MPa run ID

[C6H5OH]0 (mol/L)

[CH3OH]0 (mol/L)

ΦII

[O2]ceo2

7 8 9 10 11 12

0.067 0.067 0.067 0.067 0.067 0.067

0.0000 0.0313 0.0000 0.1563 0.0000 0.3125

0.000 0.463 0.000 2.317 0.000 4.633

3.15 3.15 2.31 2.31 1.73 1.73

phenol mixtures, respectively. Each experimental run was conducted at temperatures ranging from 673 to 753 K with an increment of 20 K. The two parameters ΦI and ΦII represent the addition proportion for methanol in the two set binary mixtures, which are determined via dividing the initial concentration of methanol by that of the other compounds, as shown in eqs 1 and 2. Figure 1. Schematic diagram of tubular-flow reactor system. (1) Water tank, (2) high-pressure water pump, (3) supercharger, (4) oxygen bottle, (5) pressure reducer, (6) gas preheater, (7) liquid preheater, (8) feed tank, (9) high-pressure feed pump, (10) tubular-flow reactor, (11) electric heater, (12) quencher, (13) back-pressure regulator, (14) gas−liquid separator, (15) liquid product collector, (16) check valve, (17) thermocouples, and (18) volumetric flow meter. 10610

ΦΙ =

[CH3OH]0 [CH3COOH]0

(1)

ΦΙΙ =

[CH3OH]0 [C6H5OH]0

(2)

dx.doi.org/10.1021/ie400704f | Ind. Eng. Chem. Res. 2013, 52, 10609−10618


Article

Table 2. Supplementary Reactions for Phenol and Acetic Acid along with the Kinetics Parameters (k = ATn exp(−Ea/RT))a A

reaction

a

n

Ea

ref.

C6H5OH + O2 = C6H5O· + HO2 ·

1.00 × 109

0

35 000

31

C6H5OH = C6H5O· + H ·

2.69 × 10

0

88 838

31

C6H5OH + H · = C6H5O· + H 2

1.90 × 10−10

0

12 393.5

45

0

−2582

47

0

43 000

47

19

CH3OO· + HO2 · = CH3OOH + O2

7.69 × 10

−14

CH3OOH = CH3O· + OH ·

4.00 × 10

15

Units are cm, mol, s, and cal.

the residence times in the experiments at 673, 693, 713, 733, and 753 K were 47, 37, 32, 29 and 27 s, respectively. The data obtained from the gas chromatograph was used to calculate the gas yields defined as the amount of gaseous products divided by the total organic carbon molar concentration in the feed solution, as shown in eq 6.

Oxidation coefficient, [O2]ceo, is defined as the ratio of O2 added to that needed for the complete oxidation of mixtures, which is demonstrated in eqs 3 and 4. [O2 ]ceo1 =

[O2 ]0 SCH3OH[CH3OH]0 + SCH3COOH[CH3COOH]0 (3)

[O2 ]ceo2 =

gas yield =

[O2 ]0 SCH3OH[CH3OH]0 + SC6H5OH[C6H5OH]0

(6)

Additionally, the conversions of acetic acid, phenol, and TOC for SCWO reactions are given as follows:

(4)

where [O2]ceo1 and [O2]ceo2 are the oxidant coefficients for mixtures CH3OH/CH3COOH and CH3OH/C6H5OH, respectively. S is the stoichiometric ratio for each model compound (SCH3OH = 1.5; SCH3COOH = 2; SC6H5OH = 7). 2.3. Analytical and Calculation Methods. The identification and quantification of gaseous products were conducted in a gas chromatograph (Shanghai, GC-112A) equipped with a thermal conductivity detector (TCD), and helium was used as the carrier gas. A 3m × 3 mm TDX-01 packed column was used and operated at the column temperature of 323 K, while the TCD detector was operated at 333 K. The method of analysis was simple and effective for the detections of H2, CO2, CO, CH 4 , N 2 , and O 2 . The total organic carbon (TOC) concentrations in the liquid samples were monitored using a TOC analyzer (ET1020A). Liquid products of unreacted acetic acid or phenol were determined by a high-performance liquid chromatography (model LC-3000) with an ultraviolet detector (model UV3000) and two high pressure pumps (model P3000). Acetic acid was quantified with an aqueous 0.1% H3PO4 solution serving as the mobile phase and UV detection at 210 nm, while phenol with 60% methanol was detected at 270 nm. Prior to the analysis, we established the quantitative correlations between peak areas and acetic acid or phenol concentrations, respectively, and verified that both correlations were linear over the range of concentrations of interest. In this study, the residence time was calculated from the equation below. ⎛ V ⎞ ⎛V ⎞ τ = ⎜⎜ r ⎟⎟ × ⎜ 0 ⎟ ⎝Q0 ⎠ ⎝ V ⎠

the amount of gaseous product total organic carbon concentration in feed

XHOAC =

[CH3COOH]0 − [CH3COOH]f × 100% [CH3COOH]0

(7)

XPhOH =

[C6H5OH]0 − [C6H5OH]f × 100% [C6H5OH]0

(8)

X TOC =

[TOC]0 − [TOC]f × 100% [TOC]0

(9)

where[CH3COOH]0, [C6H5OH]0, and [TOC]0 are the initial concentrations for acetic acid, phenol, and total organic carbon in feed steams, respectively, while [CH3COOH]f, [C6H5OH]f, and [TOC]f are the residual values in liquid effluents.

3. REACTION MODELS DESCRIPTION The detailed chemical kinetic models for SCWO have been reported for various species regarding H2, CO,34,35 methane,36 methanol,10 ethanol,1 benzene,37 methylphosphonic acid,38 and methylamine.39,40 Moreover, several investigators6,41 have successfully employed the detailed chemical kinetic models to analyze the co-oxidation mechanism for binary mixture reactants during SCWO processes.6,21,41 These studies have shown that the SCWO chemistry is analogous to that of gasphase combustion proceeding via radical reaction mechanisms, except that the SCWO reaction occurs at a lower temperature than conventional combustion. In this paper, we adopted the mechanism and kinetics parameters for methanol reported by Rex T. Skodje42 involving 93 elementary reaction steps and 18 chemical species. By comparing this model with supercritical water oxidation reaction kinetics for methanol from Rice,43 Tester,44 and Brock,8 we have validated the feasibility of this model in the condition of supercritical water oxidation. The kinetic models for phenol was obtained from the literature37 where Joanna L. DiNaro developed a reduced elementary reaction mechanism for benzene oxidation in SCW. We extracted the reactions for phenol and the kinetic parameters. Sudhama Gopalan et al.31 analyzed the systematic paths of phenol SCWO reaction by comparing the kinetics data for several possible competing

(5)

where Vr is the reactor volume (0.2 L); Q0 and V0 are the volumetric flow rate and specific volume of liquid effluent under room temperature and atmospheric pressure conditions, respectively; V is the specific volume of the feed solution under reaction condition, which is obtained with the calculation software for physical properties of water (approximate evaluation under low organic concentrations) when the reaction temperature and pressure are given. Furthermore, 10611



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initiation steps; they recognized that the oxidation of phenol was initiated by bimolecular reactions between phenol and oxygen forming the phenoxy radical C6H5O· and peroxy radical HO2·. We added this initiation reaction to the detailed chemical kinetic model for phenol. The calculation results show that this reaction led to the postponement of phenol decomposition, probably due to the fact that oxygen was less reactive than the hydroxyl and peroxy radicals in attacking chemical bonds and consequently decelerated the overall reaction rate for phenol. Moreover, we complemented the other two reactions C6H5OH = C6H5O· + H· and C6H5OH + H· = C6H5O· + H2 which were obtained from the studies.31,45 The three reactions are summarized in Table 2. Lori Torry Boock et al. offered a detailed mechanism for the supercritical water oxidation of acetic acid in literature,46 and each reaction step was assigned a constant A factor and Polanyi relationship where the universal values of α were given as 0.5. In this paper, the mechanism steps for acetic acid and the kinetic parameters were both estimated on the basis of Lori Torry Boock’s investigation. Except that the kinetic data for the termination step CH3OO· + HO2· = CH3OOH + O2 and the decomposition step CH3OOH = CH3O· + OH· were obtained from NIST kinetics database.47 The two reactions mentioned above are also listed in Table 2.

Figure 2. SCWO experimental results of acetic acid conversion as a function of temperature with and without methanol at different oxidation coefficients: [CH3COOH]0 = 0.17 mol/L, P = 24.5 MPa.

tration of methanol would cause larger enhancement of the CH3COOH disappearance rate. It is supposed that, in the methanol/acetic acid reaction system, methanol oxidation at shorter reaction time would produce a higher concentration of reactive intermediates. Some of these reactive intermediates would then attack acetic acid, thus accelerating the rate of acetic acid disappearance kinetics. Accordingly, which intermediate species contributed to the co-oxidative mechanism in methanol/acetic acid mixtures needed to be explored. Several researchers6,22,41,48,49 have reported that the SCWO destruction of organic compounds occurs primarily through free radical pathways rather than the ionic pathways. This is because supercritical water is highly aprotic and nonpolar with a low dielectric constant.50 Therefore, free radical reactions dominate during SCWO processes. Krajnc and Levec23 reported that the oxidation of CH3COOH was initiated via hydrogen abstraction reactions. One of the reaction processes was decarboxylation, in which hydroxyl radical or oxygen abstracted hydrogen from carbonyl first, as shown in Reactions 10 and 11. Maharrey et al.27 studied the SCWO mechanism for acetic acid and successfully measured the HO2· species expected to be important in the overall oxidation mechanism. It is supposed that the initiation step of the hydrogen abstraction reaction for acetic acid oxidation primarily developed by Reaction 11; that is, O2 species rather than the OH· radicals attack CH3COOH molecules. However, with more OH· generated in the system, which has greater reactivity and less selectivity than that of oxygen in attacking chemical bonds, the initiation reaction involving OH· may become more favorable, and Reaction 10 behaved significantly within the early reaction steps. Lori Torry Boock46 reported that the oxidation of acetic acid was terminated at the radicals recombination reaction between HO2· and CH3OO·, as shown in Reaction 12, where most radicals were consumed. In Reaction 12, TP represented the termination products. On the basis of the above discussion, it seems that OH· and HO2· played a substantial role in the SCWO kinetics of acetic acid. Correspondingly, we postulate that the presence of methanol accelerated the oxidation of acetic acid in SCW because methanol oxidation could generate a larger concentration of reactive intermediates OH· or HO2·at earlier times than SCWO

4. RESULTS AND DISCUSSION On the basis of the experimental results, the carbon balance was performed to check whether the effluent carbon accounted for all of the carbon fed into the reactor. The carbon balance was equal to the carbon amount at the reactor outlet divided by that at the reactor inlet. The carbon fed to reactor was determined by the total organic carbon (TOC) of the organic solutions, while the carbon flowing out of the reactor was identified by the total carbon (TC) in the liquid sample and CO2, CO, CH4, etc. in gas products. For instance, at reaction temperature of 753 K in Run 7 experiments, the inlet carbon flow rate was 1.0452 mol/h, while the flow rates for carbon in liquid and gas products were 0.3607 and 0.6292 mol/h,respectively. Thus, the carbon balance for this point was 94.7%. Overall, the carbon balance in all experiments was covered within 87−106%, which indicates that the identified and quantitatively determined products account for all of the carbon introduced into the reactor. 4.1. Oxidation of Methanol/Acetic Acid. The experimental results for the decomposition of acetic acid in supercritical water under the conditions described in Table 1a are presented in Figure 2, which demonstrates the acetic acid conversions as a function of reaction temperature ranging from 673 to 753 K with a varying methanol addition ratio ΦI. Experiments for Runs 1, 3, and 5 were conducted with acetic acid alone, while experiments for Runs 2, 4, and 6 were performed with increasing concentrations of methanol. Comparing Run 1 (ΦI = 0.0) and Run 2 (ΦI = 0.188), acetic acid conversions with [CH3OH]0 = 0.0313 mol/L were slightly higher than those without a methanol presence of about 6− 12%. Obvious increases of 17−43% and 22−60% in acetic acid conversions can be seen from the results of Run 4 (ΦI = 1.875) and Run 6 (ΦI = 0.0) when compared to the results of Run 3 (ΦI = 0.0) and Run 5 (ΦI = 0.0), respectively. A greater increase in CH3COOH conversion was obtained with higher concentrations of methanol, which confirms that the presence of methanol can accelerate the decomposition rate of acetic acid during the SCWO process. Furthermore, a higher concen10612



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of acetic acid without methanol, which would be supplied to acetic acid degradation reaction at later reaction time. CH3COOH + OH· = CH3COO· + H 2O

(10)

CH3COOH + O2 = CH3COO· + HO2 ·

(11)

CH3OO· + HO2 · = TP

(12)

CH3OH + O2 = CH 2OH + HO2 ·

(13)

CH3OH + O· = CH 2OH + OH·

(14)

In order to test the above postulate, we simulated the oxidation of a methanol/acetic acid mixture in SCW by the elementary reaction model. Figure 3 displays the calculated Figure 4. HO2· concentrations predicted for SCWO of methanol/ acetic acid mixtures at T = 733 K, P = 25 MPa, [CH3COOH]0= 0.17 mol/L, and [O2]ceo1 = 2.03.

Figure 3. SCWO of methanol and acetic acid at 733 K and 25 MPa, with [CH3COOH]0 = 0.17 mol/L. Symbols are experimental data (hollow, with methanol; solid, without methanol), and the curves are model prediction results. Figure 5. OH· concentrations predicted for SCWO of methanol/ acetic acid mixtures at T = 733 K, P = 25 MPa, [CH3COOH]0= 0.17 mol/L, and [O2]ceo1 = 2.03.

temporal variations of the acetic acid conversions at the same conditions as the acetic acid experimental runs presented in Table 1a at a temperature of 733 K. The experimental results appear as discrete points with the same color as the corresponding run. It can be observed by comparing conversion profiles of Run 1 with Run 2, Run 3 with Run 4, and Run 5 with Run 6, respectively, that the more the methanol addition ΦI, the more obvious is co-oxidative enhancement of methanol in acetic acid conversions, which is consistent with the experimental observation. As shown in Figure 3, the predicted values with pure acetic acid were lower than the experimental results. The possible reason causing this deviation was attributed to the reaction mechanism employed in this investigation, from Lori Torry Boock’s study,46 where acetic acid oxidized at a lower temperature of 653 K. Additionally, both calculated results for Run 4 and Run 6 with the presence of methanol have reached saturation at 30 s while the experimental conversions were 95% and 100%, respectively. However, the employed model could accurately predict the trends of co-oxidation influence for methanol on acetic acid decomposition during the SCWO process. Figures 4 and 5 illustrate the calculated temporal variations of the two key reactive intermediates, HO2· and OH· radicals, at 733 K, 25 MPa, [CH3COOH]0 = 0.17 mol/L, and [O2]ceo1 = 2.03. The peak profiles of HO2· concentration in Figure 4 indicate that HO2· radicals accumulated at first and then were consumed by intermediate organics, and the peak height showed a notable dependence on the initial methanol

concentration; that is, the maximum in the HO2· concentrations shifted to higher values and appeared at shorter times as the methanol concentration increased. An increment in peak HO2· concentration can be observed with ΦI increasing from 0.0 to 0.2, while with ΦI further increasing to 1.0 and 1.5, the maximum values of HO2· concentrations were about 1.5 and 2.0 times higher than that in the absence of methanol (ΦI = 0.0). The time profiles of the OH· concentration in Figure 5 were similar to those for HO2·, that is, with the presence of methanol, the peak values of OH· concentrations were elevated as the methanol concentration increased. However, regardless of initial methanol concentration, the profiles of OH· concentration were always several orders of magnitude lower than those of HO2·. Reactions 13 and 14 were two radicalproducing reactions from methanol oxidation employed in the binary reaction kinetics. Clearly, an increase of methanol could facilitate the two reaction equilibriums to move in the forward direction and promoted the generation of reactive HO2· and OH·. As a consequence, it is the HO2· and OH· produced by methanol oxidation at lower reaction time that accelerated the acetic acid decomposition during the SCWO of methanol/ acetic acid mixtures. Even with a small addition of methanol (ΦI = 0.188), the reactive intermediates generated by CH3OH reactions were sufficient for co-oxidation of CH3COOH. The 10613



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Table 3. Reaction with the Largest Normalized Sensitivity Coefficients for HO2· and OH· during SCWO of Methanol/Acetic Acid Mixtures at 733 K, 25 MPa, [CH3COOH]0 = 0.17 mol/L, [O2]ceo1 = 2.03, ΦI = 1.5, and τ = 3.2 s normalized sensitivity coefficient reaction

reaction number

H · + O2 (+ M) = HO2 ·(+ M)

RI9

OH · + HO2 · = O2 + H 2O

RI13

HO2· 0.500 −0.251

H 2O2 (+ M) = 2OH ·(+ M)

RI15

HCO· + O2 = CO + HO2 ·

RI24

CH 2O + OH · = HCO· + H 2O

RI37

CH3· + O2 = CH 2O + OH ·

RI43

CH3· + HO2 · = CH3O· + OH ·

RI44

−0.204

CH3O· + O2 = CH 2O + HO2 ·

RI60

0.010

CH3COOH + O2 = CH3COO· + HO2 ·

RI78

0.252

co-oxidative enhancement of methanol stems from two reasons: increasing oxidation rate constant and shortening the induction time. It is reasonable to propose that both increment of oxidation rate constant and reduction of the induction time occurred during co-oxidation reactions for methanol and acetic acid. First, the time that HO2· and OH· radicals reached the peak concentrations was reduced with the addition of methanol. The accumulation time for radicals was shortened, and induction time decreased. Second, the peak HO2· and OH· concentrations obviously were elevated as methanol addition increased. According to the collision theory of reactions, an important factor that determines the reaction rate is the percentage of collisions between activated molecules. The obvious increment in HO2· and OH· concentrations in the presence of methanol can promote the collision probabilities of acetic acid molecules and these active radicals and thus enhance the reaction rate constant rate. Therefore, the two effects mentioned above accelerated the acetic acid decomposition after addition of methanol. Additionally, as demonstrated in Figures 4 and 5, when acetic acid was oxidized without methanol (ΦI = 0.0), HO2· and OH· concentrations accumulated with residence time in the initial stage of the reaction. It is noted that HO2· radicals were generated in Reaction 11 while OH· radicals were consumed in Reaction 10, but on the basis of the calculated profiles for OH· concentration in Figure 5, OH· radicals increased at first. The above-mentioned contradiction suggests that decarboxylation of acetic acid cannot undergo Reaction 10 but can undergo Reaction 11 where oxygen abstracted hydrogen from the acetic acid molecules forming HO2· radicals, which was consistent with conclusions reported by Sean P. Maharrey et al.27 A sensitivity analysis for methanol/acetic acid mixture reactions was performed to determine the key elementary steps for the productions of HO2· and OH·. Reactions with the largest sensitivity coefficients appear in Table 3. The accumulation of HO2· was sensitive to the hydrogen abstraction reactions by the O2 (RI9, RI24, RI60, and RI78), whereas the sensitivity coefficients of reactions RI13 and RI44 were negative for the formation of HO2·. For the OH· radicals, the reaction involving the dissociation of H2O2 to supply OH· contributed significantly to the production of OH· radicals, and reactions RI13 and RI37 led to the consumption of OH·. Figure 6 depicts the influence of methanol concentration on gas yields and TOC removal rate for SCWO of methanol/ acetic acid mixture solution at 713 K. The gas yield of one mole

OH· −0.314 0.676

0.232 −0.309 0.049 0.256

Figure 6. Yields of different gaseous products and TOC removal efficiency for SCWO of methanol/acetic acid mixture solutions (T = 713 K, [CH3COOH]0 = 0.17 mol/L, and [CH3OH]0 = 0.0, 0.0313, 0.3125, and 0.6563 mol/L).

of organic carbon generally increased when acetic acid was cooxidized with increasing amounts of methanol, and the TOC removal rate was kept in the same changing trend with the gas yield. TOC removals increased with the initial methanol concentration. It indicates that a higher methanol addition can lead to a larger proportion of organic carbon degradation in the liquid phase. Figure 6 shows that the yield of CO2 increased with initial methanol concentrations while the yields of CH4, CO, and H2 decreased, revealing that the methanol-derived reactive species HO2· and OH· would accelerate the complete disappearance of gaseous intermediates, as shown in the Reactions 15−20. CH4 + HO2 · = CH3· + H 2O2

(15)

CH4 + OH· = CH3· + H 2O

(16)

CO + HO2 · = CO2 + OH·

(17)

CO + OH· = CO2 + H·

(18)

H· + HO2 · = OH· + OH·

(19)

H 2 + OH· = H 2O + H·

(20)

4.2. Oxidation of Methanol/Phenol. Figure 7 demonstrates the phenol conversion for SCWO of phenol and 10614



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Figure 8. SCWO of methanol and phenol at 733 K and 25 MPa, with [C6H5OH]0 = 0.067 mol/L. Symbols are experimental data (hollow, with methanol; solid, without methanol), and the curves are model prediction results.

Figure 7. SCWO experimental results of phenol conversion as a function of temperature with and without methanol at different oxidation coefficients: [C6H5OH]0 = 0.067 mol/L, P = 25.5 MPa.

12 for CH3OH/C6H5OH mixtures are dotted lines. The corresponding experimental results appeared as discrete points with the same color. The hollow and solid points are respective results with and without methanol. It seems that the model slightly overpredicted the reactivity of phenol. Nevertheless, the mechanism-based model is generally in accordance with the experimental data for phenol conversions at 733 K. When comparing the calculated profiles for Runs 7 and 8, it is noticed that, before the reaction time of 33 s, methanol with an initial concentration of 0.0313 mol/L inhibited the conversion of phenol, while after the reaction time of 33 s, the phenol disappearance rate in Run 8 became higher than that in Run 7. This phenomenon is interpreted in terms of the reaction mechanism discussed later. When ΦII increased to 2.3 (Run 10) and 4.6 (Run 12), the phenol conversion profiles were significantly higher than those in the absence of methanol, and the co-oxidation enhancement was promoted with increasing [CH3OH]0 concentrations. Overall, the simulation results are in good agreement with the experimental trends. We calculated the temporal variations of two key radicals HO2· and OH· at 733 K and 25 MPa with varying [CH3OH]0/ [C6H5OH]0 ratios (ΦII), as depicted in Figures 9 and 10,

methanol mixture solution with variable initial methanol concentrations at a reaction temperature ranging from 673 to 753 K. Runs 7, 9, and 11 were for the results with pure phenol, and Runs 8, 10, and 12 were for the reaction process with methanol addition. The specific experimental conditions are displayed in Table 1b with a constant initial phenol concentration of 0.067 mol/L in all cases. As shown in Figure 7, for [CH3OH]0 = 0.0313 mol/L (Run 8), the phenol conversion was very close to the data obtained without methanol (Run 7). As the initial methanol concentration increased to 0.1563 and 0.3125 mol/L, the phenol disappearance rate was obviously higher than that with [CH3OH]0 = 0 mol/L. For example, at the reaction temperature of 673 K, the phenol conversions were 47.5% and 53.0% at ΦII ratios of 2.317 and 4.633, respectively, relative to the phenol conversions of 39.9% and 34.6% in the absence of methanol. It seems that a lower concentration of methanol had no appreciable improving effect on phenol conversions, and conversely, a higher concentration of methanol can lead to a notable increase in the phenol decomposition rate. During the SCWO reaction process for methanol/phenol binary solution, methanol was capable of accelerating phenol oxidation, but there appeared to be a threshold [CH3OH]0 concentration below which there was no perceptible co-oxidative effect. A possible explanation for the above phenomenon is that, with a lower concentration of methanol added, the reactive intermediates generated from methanol oxidation at shorter reaction time were consumed by methanol at first, leaving behind a far insufficient amount for phenol reaction. Therefore, the co-oxidative advantage was not perceptible. At higher methanol concentrations, ranging from 0.1563 mol/L (Run 10) to 0.3125 mol/L (Run 12), methanolderived reactive intermediates at the earlier time were available to supply phenol oxidation and thus accelerated phenol disappearance kinetics. To understand how methanol co-oxidized phenol in supercritical water, simulations corresponding to the SCWO conditions in Table 1b for methanol/phenol mixtures were also implemented on the basis of the detailed chemical kinetics models. Figure 8 displays the model predictions of phenol conversions for the reaction conditions used in the experiments at 733 K. The profiles of Runs 7, 9, and 11 for SCWO of phenol alone are expressed as solid lines, while Runs 8, 10, and

Figure 9. HO2· concentrations predicted for SCWO of methanol/ phenol mixtures at T = 733 K, P = 25 MPa, [C6H5OH]0 = 0.067 mol/ L, and [O2]ceo2 = 1.73. 10615



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the degradation of phenol. The second feature for HO2· and OH· profiles is that the HO2· concentration with ΦII = 0.5 was higher than that with ΦII = 0; conversely, OH· radicals with ΦII = 0.5 were relatively lower. As shown in Figure 8, for calculated results for phenol conversions, we find that phenol decomposed in Run 8 with ΦII = 0.5 more slowly than that in Run 7 with ΦII = 0.0 before 33 s, but after 33 s, the phenol conversion with ΦII = 0.5 was higher than that with ΦII = 0.0. Accordingly, this leads us to speculate whether there is a causal relationship between the observation of phenol conversions and the second feature for HO2· and OH· profiles. Sudhama Gopalan et al. have proposed a detailed reaction mechanism for supercritical water oxidation of phenol.31 They reported that oxidation of phenol was initiated by a bimolecular reaction between phenol and oxygen to form phenoxy radical. Following initiation, HO2· and OH· radicals in the mixture began to attack phenol molecules. Reactions 21−23 illustrate these steps. In the SCW environment, the free radical process dominated over the ionic process, and phenoxy radical, which was a stable delocalized radical, did not favor oxygen addition. Therefore, the radical−radical process became essential for the subsequent reaction of phenoxy radicals, and the ring-opening reaction for phenoxy radicals proceeded via HO2· addition to the phenoxy radical. Combining the SCWO reaction mechanism for phenol, we postulate that in the case of lower methanol addition (ΦII = 0.5) OH· radicals in the system were mostly consumed by methanol, leading to less OH· available for phenol initiation reaction than that with pure phenol oxidation, thus restraining decomposition of phenol at earlier reaction steps before 33 s, as shown in Figure 8. Meanwhile, HO2· radicals were important during the ring-opening of the aromatic ring at a later reaction step, and thereby, more HO2· in the presence of methanol would accelerate the phenol disappearance rate after 33 s.

Figure 10. HO· concentrations predicted for SCWO of methanol/ phenol mixtures at T = 733 K, P = 25 MPa, [C6H5OH]0 = 0.067 mol/ L, and [O2]ceo2 = 1.73.

respectively. Both figures show that no peaks appeared in HO2· and OH· profiles at [CH3OH]0 = 0.0313 mol/L (ΦII = 0.5), which were elevated and then leveled off to the given values with the same order of magnitude as that for SCWO of phenol alone. As ΦII increased to higher values of 2.0 and 3.0, the shape of HO2· and OH· concentration profiles changed and substantial peaks appeared at shorter reaction time, which was synchronous to the appearance of significant co-oxidation effect on phenol conversions with greater initial methanol addition. Therefore, it is reasonable to propose that the oxidation of methanol with a lower concentration cannot generate adequate HO2· and OH· radicals for refractory phenol decomposition, and thus, the co-oxidation effect at this [CH3OH]0 was not substantial. Accordingly, there was a threshold for ΦII triggering an appreciable increment of phenol decomposition by adding methanol. On the other hand, the co-oxidative enhancement of methanol for phenol decomposition was negligible at the [CH3OH]0 concentration of 0.0313 mol/L. It seems that reactive radicals generated by methanol during the induction period were far insufficient to reach the critical concentrations. However, notable increases in phenol decomposition were obtained as [CH3OH]0 increased to 0.1563 and 0.3125 mol/L. It is suggested that the reaction of methanol with higher concentration obviously accelerated the concentrations of HO2· and OH· radicals to reach the critical values during the induction period, thereby shortening the induction time. Correspondingly, in the case of methanol/phenol reactions, the reduction of induction time was more important for cooxidative effect of methanol. The temporal plots in Figures 9 and 10 of HO2· and OH· concentration with variable ΦII ratios represent two conspicuous features. The first one is that peaks emerged with larger ΦII of 2.0 and 3.0, and the maximum of HO2· and OH· concentrations shifted to higher values and appeared at a shorter time as the initial methanol concentrations increased. Specifically, increasing the ΦII from 2.0 to 3.0 improved the peak values of HO2· and OH· concentration by almost 1 order of magnitude while bringing the peak point forward temporally by 10 s. This notable characteristic confirms that, during the SCWO reaction of methanol/phenol mixtures, methanol oxidation proceeded at much shorter times producing reactive intermediates HO2· and OH·, which remained at a later time and would participate in phenol oxidation, thereby accelerating

C6H5OH + O2 = C6H5O· + HO2 ·

(21)

C6H5OH + HO2 · = C6H5O· + H 2O2

(22)

C6H5OH + OH· = C6H5O· + H 2O

(23)

Additionally, from the sensitivity analysis for HO2· and OH· radicals during SCWO of methanol/phenol mixtures, as shown in Table 4, the production of HO2· at an early time was sensitive to the hydrogen abstraction reactions by O2 from C6H5OH, CH2OH·, and CH3OH (reactions RII20, RII94, and RII117). The decomposition of H2O2 to OH· was the most sensitive reaction for accumulation of OH· radicals, which was the same as the reactions for methanol/acetic acid mixtures, but the reactions RII22, RII86, and RII115 negatively affected the formation of OH·. Figure 11 denotes the effect of methanol concentration in the yield of various gas constituent and total organic carbon removal for SCWO of methanol/phenol mixtures at 713 K. Apparently, the total gas yield was nearly in step with that of TOC removal efficiency. Both of them were slightly reduced with increasing ΦII from 0.0 to 0.463, which confirms the threshold concentration of [CH3OH]0 for notable co-oxidation effect of methanol on phenol conversion from another perspective. As ΦII increased up to 2.317, the total gas yield and TOC removal both increased significantly. Moreover, with further increasing ΦII to 4.633, the CO yield decreased, conversely leading to an obvious increment in the CO2 yield, with the total gas yield increasing as a result. This indicates that, 10616



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model demonstrated the same trends as the experimental observation for both acid/methanol and phenol/methanol mixtures. The calculated temporal profiles for HO2· and OH· radicals confirmed that the co-oxidative correlation between methanol and the two compounds could be attributed to methanol oxidizing more rapidly and generating more reactive intermediates HO2· and OH· at shorter reaction time. Then, a portion of these radicals attacked the other refractory compound of the binary mixtures, especially contributing to the HO2· and OH· consumption at initial reactions of both acetic acid and phenol, as well as to the reactions with intermediate radicals CH3OO· and C6H5O· at later reaction steps, respectively. Furthermore, the time profiles of HO2· and OH· for both mixtures show that, as methanol was added into feed, the temporal advance and increment of HO2· and OH· were both sensitive to the initial methanol concentration, which was positively related to [CH3OH]0. This confirms that greater perceptible co-oxidation enhancement for acetic acid or phenol oxidation in SCW can be obtained with a higher initial methanol concentration.

Table 4. Reaction with the Largest Normalized Sensitivity Coefficients for HO2· and OH· during SCWO of Methanol/ Phenol Mixtures at 733 K, 25 MPa, [C6H5OH]0 = 0.067 mol/L, [O2]ceo2 = 1.73, ΦII = 3.0, and τ = 2.5s normalized sensitivity coefficient reaction

reaction number

H · + O2 (+ M) = HO2 ·(+ M)

RII9

HO2·

OH·

0.089

H 2O2 (+ M) = 2OH ·(+ M)

RII15

C6H5OH + O2 = C6H5O· + HO2 ·

RII20

C6H5OH + OH · = C6H5O· + H 2O

RII22

HCO· + O2 = CO + HO2 ·

RII24

CH 2O + OH · = HCO· + H 2O

RII86

CH3O· + O2 = CH 2O + HO2 ·

RII94

CH3OH + OH · = CH3O· + H 2O

RII115

CH3OH + O2 = CH3O· + HO2 ·

RII117

0.021

CH3OH + HO2 · = CH3O· + H 2O2

RII119

−0.409

0.999 0.266 −0.203 0.056 −0.033 0.808 −0.366

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20090201110010).

■

REFERENCES

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Figure 11. Yields of different gaseous products and TOC removal efficiency for SCWO of methanol/phenol mixtures solution (T = 713 K, [C6H5OH]0 = 0.067 mol/L, and [CH3OH]0 = 0.0, 0.0313, 0.1563, 0.3125 mol/L).

above the threshold of [CH3OH]0 concentration, the CO2 yield varied positively with methanol addition, whereas the CO yield varied inversely, revealing that the increment of initial methanol concentration would obviously accelerate complete degradation of phenol and methanol mixtures during the SCWO process.

5. CONCLUSIONS In this work, methanol acting as the co-oxidation reagent was added into acetic acid and phenol solutions. The SCWO of methanol/acetic acid and methanol/phenol mixtures were conducted in a tubular flow reactor under different conditions. For SCWO of the methanol/acetic acid mixture, it is observed that more notable co-oxidative enhancement of acetic acid decomposition was obtained as CH3OH concentration increased. Moreover, the SCWO experiments for phenol/ methanol solution showed a conspicuous difference that notable co-oxidation effect of methanol on phenol disappearance required the methanol concentration to be higher than a threshold value. Simulation based on an elementary reaction 10617



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Co-Oxidation Effects of Methanol on Acetic Acid and Phenol in

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