Shock Tube Measurement of the High-Temperature Rate Constant for

Jul 31, 2015 - The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its ...
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Shock Tube Measurement of the High-Temperature Rate Constant for OH + CH3 → Products Shengkai Wang, Sijie Li, David F. Davidson,* and Ronald K. Hanson High Temperature Gasdynamics Laboratory, Mechanical Engineering Department, Stanford University, Stanford, California 94305, United States ABSTRACT: The reaction between hydroxyl (OH) and methyl radicals (CH3) is critical to hydrocarbon oxidation. Motivated by the sparseness of its high-temperature rate constant data and the large uncertainties in the existing literature values, the current study has remeasured the overall rate constant of the OH + CH3 reaction and extended the measurement temperature range to 1214−1933 K, using simultaneous laser absorption diagnostics for OH and CH3 radicals behind incident and reflected shock waves. tert-Butyl hydroperoxide and azomethane were used as pyrolytic sources for the OH and CH3 radicals, respectively. The current study bridged the temperature ranges of existing experimental data, and good agreement is seen between the current measurement and some previous experimental and theoretical high-temperature studies. A recommendation for the rate constant expression of the title reaction, based on the weighted average of the high-temperature data from selected studies, is given by k1 = 4.19 × 101(T/K)3.15 exp(5270 K/T) cm3 mol−1 s−1 ±30%, which is valid over 1000−2500 K. ature,4−10 whereas at T > 1000 K, rate constant data are limited to five shock tube measurements,11−15 with considerable scatter among them. Bott and Cohen11 studied reaction (1) near 1200 K and 1 atm via OH absorption at 309 nm using a UV discharge lamp. In their study, they shock-heated tert-butyl hydroperoxide (TBHP) and di-tert-butyl peroxide (DTBP) to produce OH and CH3 radicals, respectively, and measured the overall rate of reaction (1) to be 1.1 × 1013 cm3 mol−1 s−1. Krasnoperov and Michael12 employed a multipass cell incorporated with a UV lamp for OH resonance absorption at ∼308 nm and measured the overall rate at pressures of 50−940 Torr and over two temperature regions, 834−1176 K and 1817−2383 K; TBHP/ DTBP and methanol/methyl iodide were used as the pyrolytic precursors of OH/CH3 radicals, respectively. A two-parameter Arrhenius fit was found, valid over their entire temperature range: k1 = 1.05 × 1013 exp(915 K/T) cm3 mol−1 s−1, which was higher than the Bott and Cohen result near 1200 K by a factor of ∼2. Later, Srinivasan, Su, and Michael13 studied reaction (1) using a similar method and multipass UV absorption apparatus but with increased path length. They used TBHP and methyl iodide (CH3I) as the precursors for OH and CH3 radicals and reported the rate of OH + CH3 → 1 CH2 + H 2O over 1085−1348 K. Their values were significantly lower than those from the previous study of Krasnoperov and Michael (by a factor of ∼3), a discrepancy that would not be explained even after accounting for contributions from the other reaction channels not included

1. INTRODUCTION As a result of the abundance of OH and CH3 radicals in hydrocarbon flames, their reactions, particularly with each other, have attracted increasing attention in the study of combustion kinetics.1−15 De Avillez Pereira et al.,1 Xia et al.,2 and Jasper et al.3 have performed high-level theoretical calculations and recovered the following reaction channels for the OH + CH3 system: OH + CH3 → CH3OH∗ → CH3OH (1a) →1CH 2 + H 2O

(1b)

→cis‐HCOH + H

(1c)

→trans‐HCOH + H

(1d)

→CH 2O + H 2

(1e)

→CH 2OH + H

(1f)

→CH3O + H

(1g)

where CH3OH* denotes the vibrationally excited complex that is subsequently stabilized via collisions to form methanol 1a or decomposed to bimolecular products 1b−1g. At elevated temperatures, these authors also found that OH and CH3 could react via direct abstraction on the triplet surface: OH + CH3 → 3CH 2 + H 2O

(1h)

However, there are large discrepancies for the total rate constant (here denoted as k1) and the branching ratios between the three studies, and experimental data are required to validate and constrain these theoretical calculations. Much of the experimental work has been performed near room temper© XXXX American Chemical Society

Received: June 15, 2015 Revised: July 31, 2015

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DOI: 10.1021/acs.jpca.5b05725 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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A commercially available solution of TBHP (70% by weight) in water, supplied by Sigma-Aldrich, was used as the pyrolytic precursor for OH radicals. When shock-heated at temperatures above 1000 K and near-atmospheric pressure, TBHP decomposed almost instantaneously into OH, CH3, and acetone. Kinetic simulations have shown that the water vapor in the reactant mixture had little effect on the rate constant measurement. Azomethane (C2H6N2), prepared and purified in the laboratory using standard methods described in the literature,24−26 was used as the CH3 precursor. It is an efficient and clean source of CH3 at T > 1000 K, as the only byproduct N2 is chemically inert. Gas mixtures of the two radical precursors were prepared manometrically in a 40 L stainlesssteel mixing tank, following procedures described in previous studies,20,22,27 and diluted in research-grade argon (>99.999% purity) that was supplied by Praxair, Inc. The shock tube and the mixing assembly were routinely evacuated with a turbomolecular pump to 6 μtorr between experiments, to prevent contamination of the test mixture from any residual impurity. The concentration of OH radical was monitored using narrow-linewidth UV laser absorption near 306.69 nm, at the center of the R1(5) transition in the OH A-X (0,0) band. This strong and isolated transition was well-characterized by Herbon et al.28 and has been used successfully in several previous OH studies.16−20,22 A Spectra-Physics 380A tunable ring dye laser was used to generate the UV light. The dye laser was optically pumped by a 5 W Coherent Verdi laser at 532 nm, producing visible laser light at 613.38 nm, which was then intracavity frequency-doubled by an angle-tuned lithium triborate crystal to generate mW-level UV output at 306.69 nm. The output was captured with UV-enhanced Thorlabs PDA36A detectors (−3 dB bandwidth = 260 kHz, active area = 13.3 mm2). The concentration of CH3 radical was monitored using UV laser absorption near 216.6 nm, targeting the broadband feature in the CH3 B2A′1← X2A″2 system. Details of the hightemperature CH3 diagnostic have been described in the studies of Davidson et al.29 and Oehlschlaeger et al.,30 and they can also be found in previous kinetic studies involving CH3 measurement.31 The light source used in the current study was a Coherent Mira-HP frequency-quadrupled tunable Ti:sapphire pulse laser (picosecond version), which had a spectral linewidth (full width at half-maximum) less than 0.2 nm. The laser light was captured with New Focus 2032 detectors (−3 dB bandwidth = 150 kHz, active area = 26.4 mm2). All data were sampled at 2.5 MHz and recorded with a 14 bit National Instrument digital data acquisition system. Quantitative OH and CH3 profiles were calculated from the raw traces of fractional transmission using the Beer−Lambert law: I/Io = exp(−kχPL), where I and Io were the transmitted and incident laser intensities, respectively, k was the absorption constant, P was the total gas pressure, L was the diameter of the shock tube (15.24 cm), and χ was the mole fraction of the target species (OH or CH3) to be determined. Because of the intrinsic instabilities of the UV lasers (jittering of the dye jet and pulse-to-pulse variation of the Ti:sapphire laser), their output powers had rapid fluctuations (up to ∼10% peak-topeak changes), but the wavelengths remained stable. To remove these fluctuations in the detected laser signal, a common-mode-rejection scheme was applied, which enabled 1σ minimum detectable absorptions of 0.2% for OH and 0.9% for CH3. This translated to 1-σ detection limits of ∼2 ppm for OH and ∼30 ppm for CH3 at typical incident shock conditions

in their study. Recently, Vasudevan and co-workers remeasured the overall rate of reaction (1) behind reflected shock waves using narrow-linewidth OH laser absorption at 306.7 nm,14 at temperatures from 1081 to 1426 K and pressures near 1.6 atm. In the measurement, TBHP was used as the OH precursor, and in addition to methyl iodide, azomethane was also used as a CH3 precursor, providing a cleaner source of CH3. The resulting rate constant agreed in trend with the Krasnoperov and Michael data, and was consistent with the theoretical calculation from Jasper and co-workers. Most recently, Pang and co-workers re-evaluated the rate of OH + CH3 → 1CH2 + H2O15 and validated the Vasudevan et al. result with a neat TBHP pyrolysis experiment. However, in spite of the high accuracy of these two studies, their temperature ranges were limited to T < 1450 K, probably due to premature decomposition of TBHP at higher temperatures behind the incident shock wave. In fact, few experimental data exist in the 1450−2000 K region, where data are needed to resolve the discrepancies between the previous studies. The reaction between OH and CH3 also plays an important role in the rate constant measurements of OH + fuels, which often use TBHP as an OH precursor. As a result, the reaction of OH + CH3 → products, especially the product channel OH + CH3 → 1CH2 + H2O, has been a major interfering reaction and primary source of uncertainty in many shock tube measurements for the rate constant of OH + fuels that limited their measurement accuracies.15−20 Here, a new measurement was conducted for the overall rate constant of the reaction CH3 + OH → products (k1). The current study used azomethane as a clean CH3 precursor, exploited a new strategy of incident shock measurement to avoid premature chemical decomposition, and applied simultaneous OH and CH3 diagnostics to improve the measurement accuracy, with hopes to (1) reduce the scatter in the literature values for the target rate constant; (2) bridge the temperature regions of different studies; and (3) potentially help to reduce the uncertainties in future OH + fuels rate measurements.

2. EXPERIMENTAL SETUP Experiments were conducted in both the incident and the reflected shock regions of a high-purity shock tube. Detailed description of the Stanford shock tube facilities has been documented elsewhere, and here only a brief summary is given.21−23 The facility used in this experiment was a 15.24 cm inner diameter stainless-steel shock tube with a driven section of 10 m and a driver section of 3.7 m. Five PCBTM piezoelectric pressure transducers, located on the sidewall over the last 1.5 m of the driven section, measured the time intervals between the arrival of incident shock waves, from which the incident shock velocities were calculated and linearly extrapolated to the endwall location, and the incident shock attenuation rates were typically ca. 1.0%/m. In the incident shock measurements, the particle time (tp) and lab time (tl) were related by the density ratio of the gases before and after the incident shock wave (ρ2/ ρ1): tp/tl = ρ2/ρ1. No incident shock boundary layer model was used since the change in incident shock speed was less than 0.3% throughout the test time. Three pairs of optical windows, located 2 cm away from the endwall, enabled optical access for multiple laser absorption diagnostics. To extend the test time of the incident shock experiments, a modified endwall piece, providing an additional 1 cm distance to the endwall, was also used in the current study. B

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The Journal of Physical Chemistry A Table 1. Reaction Mechanisma A

reaction (1b) CH3 + OH → 1CH2 + H2O (2) TBHP → OH + tert-butoxy (3) tert-butoxy → CH3 + CH3COCH3 (4) OH + TBHP → H2O + O2 + tert-C4H9 (5) OH + TBHP → H2O + HO2 + iso-C4H8 (6) H + O2 → OH + O (7) OH + OH → O + H2O (8) CH3COCH3 + OH → H2O + CH2CO + CH3 (9) CH3COCH3 (+M) → CH3 + CH3CO (+M)c

1.65 3.57 1.26 2.30 2.49 1.04 3.57 2.95 7.24 1.70 2.22

(10) C2H6N2 → CH3 + CH3 + N2 a

× × × × × × × × × × ×

1013 b 1013 1014 1013 1013 1014 104 1013 1025 d 1064 e 1039

m

E

reference

0.00b 0.00 0.00 0.00 0.00 0.00 2.40 0.00 −2.72d −12.64e −7.99

0.00b 35.7 15.3 5.21 5.27 30.3 −2.10 4.58 87.7d 89.5e 51.5

15 15 34 35 36 37 37 38 39

32

40 33

The JetSurF 1.0 mechanism (194 species, 1459 reactions) is used as the base mechanism (a different base mechanism, USC-Mech II with 111 species and 784 reactions, yields essentially identical results). Here, only those reactions are given that are not parts of JetSurF 1.0 or are used with a different set of rate parameters. Rate constants are given in the form of k = ATm exp(−E/RT) (units kcal, cm3, mol, s, and K). All reactions, except R9 and R10, are originally in the mechanism of Pang et al.15 bRate constant expression valid over 799−1316 K. cPressure-dependent rate constant expressed in the Troe falloff form, with Fcent = 2 × 10−5 T + 0.144. dRate constant expression for kinf. eRate constant expression for k0.

of 1500 K and 0.4 atm, and ∼1 and 12 ppm for CH3 at typical reflected shock conditions of 1300 K and 0.7 atm.

using the Pang et al. mechanism. Figure 2 indicates that the title reaction OH + CH3 is the dominant reaction affecting both the OH and CH3 profiles, yet interferences from other reactions prevent accurate determination of its rate constant from this pure TBHP pyrolysis expriment. As suggested in the study of Vasudevan et al.,14 an additional source of CH3 should be introduced into the reaction system to provide good chemical isolation of the target rate.

3. VALIDATION OF THE PREVIOUS TBHP MODEL The existing TBHP submechanisms are first validated. A representative TBHP mechanism is given by Pang and coworkers,15 which has incorporated the latest re-evaluation of the OH + CH3 = 1CH2 + H2O rate and been adopted in several recent studies.16,17,19,20 Details of the reaction mechanism are listed in Table 1. In the current study, we validated the Pang et al. mechanism with simultaneous OH and CH3 measurement in TBHP pyrolysis behind reflected shock waves. A representative example is shown in Figure 1, where ∼60 ppm TBHP is

4. CURRENT HIGH-TEMPERATURE MEASUREMENT To measure the rate constant of OH + CH3 → products, we used azomethane (C2H6N2) as the additional CH3 precursor. When shock-heated at T > 1000 K, a C2H6N2 molecule decomposes almost instantaneously into two CH3 radicals and a N2 molecule: C2H6N2 → CH3 + CH3 + N2

(10)

In the current study, we observed significant premature decomposition of TBHP behind the incident shock wave at high temperatures (T > 1450 K). This may have limited the temperature ranges of previous studies of Vasudevan et al.14 and Pang et al.15 to T < 1430 K. The sparseness of experimental data between 1400 and 2000 K in the literature has motivated the current study to use a different measurement strategy by exploiting the incident shock wave region. To measure the target rate constant, we monitored the OH time histories during the pyrolysis of azomethane/TBHP mixtures behind incident shock waves. As shown by the representative example in Figure 3, the current OH measurements had very high signal-to-noise ratio, which was ideal for rate constant determination. The overall rate constant of OH + CH3 was inferred from best-fitting the model prediction to match the first 150 μs of the measured OH time histories by varying the target rate constant. Also included in Figure 3 were the simulated OH time histories corresponding to the best-fit rate perturbed by a factor of 2, illustrating strong sensitivity of the OH time history to the target reaction. In the current study, we also performed simultaneous CH3 measurements to help confirm the initial CH3 concentration. In spite of higher noise in the CH3 signal resulting from weaker absorption, it is worth noting that useful information about the initial CH3 concentration can still be obtained. The latter part of the CH 3 profile is dependent on the initial CH 3

Figure 1. Example OH and CH3 time histories in the pyrolysis of 60 ppm TBHP/275 ppm H2O/Ar at 1328 K, 0.71 atm.

pyrolyzed at 1328 K, 0.71 atm. Excellent agreement can be seen between the current measurement and the numerical simulation using the Pang et al. model (performed through the CHEMKIN-PRO software, under constant volume and constant internal energy constraints). Because of its high overall accuracy, we chose to use the Pang et al. mechanism as the base mechanism in the current study. To investigate the influnces of individual reaction rates ki to the OH and CH3 time histories, we calculated the OH and CH3 sensitivities (shown in Figure 2), defined as (∂χOH/∂ki)/(χOH/ki) and (∂χCH3/∂ki)/(χCH3/ki), C

DOI: 10.1021/acs.jpca.5b05725 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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Figure 2. Sensitivity plots for the TBHP pyrolysis experiment at 1328 K. (left) OH sensitivity; (right) CH3 sensitivity.

Figure 3. Example species time histories in the pyrolysis of TBHP/azomethane mixture. (left) OH; (right) CH3.

was dominated by the title reaction. Analysis using the individual channel rates from Jasper et al.3 suggetsed that OH + CH3 → 1,3CH2 + H2O were the primary channels contributing to the OH removal rate at the experimental conditions of the current study. As a numerical simplification in calculating the overall rate constant k1 using the CHEMKINPROTM software, we lumped different channels of the title reaction into one channel, OH + CH3 → 1CH2 + H2O. And we found this lumped treatment to be accurate enough (within 5% uncertainty, according to calculations using the individual channel rates from Jasper et al.3) for determining the total rate of OH + CH3. This was because the secondary contributions from reactions between the products of the title reaction, for example, from the thermal dissociation of CH3OH generated via (1a): OH + CH3 → CH3OH (active at T < 1500 K), were very small. For the other reactions shown in Figure 4, their reaction rates were relatively well-known, and therefore the measurement uncertainties derived from their addition to the mechanism were low. A detailed uncertainty analysis was performed to quantify the uncertainty contributions from all sources in the measurement, and the results are shown in Figure 5. The overall uncertainty of the measurement at 1513 K, calculated from the root-meansquare of the individual sources, was estimated to be ±30%, with the primary contributors being the mixture composition uncertainty and the fitting errors of the OH profiles. As for the mixture composition uncertainty, we found in the experiment that the measured initial CH3 concentrations were less than the

concentration and relatively insensitive to the OH + CH3 rate. Therefore, by fitting the entire CH3 profile a relatively accurate determination of the initial CH3 concentration could be achieved with a 1-σ uncertainty of less than 30 ppm. Also of note is that at elevated temperatures, especially T > 1400 K, acetone starts to decompose and produce additional CH3 radicals. This is captured by reaction 9 in the mechanism, whose pressure-dependent rate constant is taken from Saxena et al.39 and validated in the latest study of Wang et al.41 OH sensitivity analysis was performed in the similar way as in Section 3. As shown in Figure 4, the OH sensitivity spectrum

Figure 4. OH sensitivity of TBHP/azomethane pyrolysis. D

DOI: 10.1021/acs.jpca.5b05725 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A Table 2. Summary of the Current Rate Constant Measurement

Figure 5. Uncertainty analysis.

nominal values calculated from the manometric mixture compostion (by 20% in extreme cases), possibly due to walladsorption in the mixing manifold before the gas mixture entered the shock tube. However, this uncertainty was considerably reduced when we used the experimentally measured CH3 concentration for the rate constant determination. Nonetheless, a conservative estimation of 20% was assigned to the mixture composition uncertainty and included in the overall uncertainty analysis. Three secondary reactions, all related to the CH3 chemistry, also had minor contributions to the uncertainty of the OH + CH3 rate constant. They were: (1) the pressure-dependent reaction of acetone decompostion, CH3COCH3 (+M) = CH3 + CH3CO (+M) (uncertainty factor = 2, affecting the target rate by +7/−14%); (2) the pressuredependent reaction of methyl recombination into ethane, CH3 + CH3 (+M) = C2H6 (+M) (uncertainty factor = 2, affecting the target rate by +8/−4%); and (3) the radical product channel of the methyl recombination reaction, CH3 + CH3 = C2H5 + H (uncertainty factor = 2, affecting the target rate by +7/−4%). Experimental uncertainties from other sources, including the incident shock temperature and pressure, the OH absorption coeffieicnt, and the particle time, were found to be small (