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Temperature-dependent mid-IR absorption spectra of gaseous hydrocarbons
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Journal of Quantitative Spectroscopy & Radiative Transfer 107 (2007) 407–420 www.elsevier.com/locate/jqsrt
Temperature-dependent mid-IR absorption spectra of gaseous hydrocarbons Adam E. Klingbeil, Jay B. Jeffries, Ronald K. Hanson Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Received 11 December 2006; received in revised form 20 February 2007; accepted 3 March 2007
Abstract Quantitative mid-IR absorption spectra (2500–3400 cm1) for 12 pure hydrocarbon compounds are measured at temperatures ranging from 25 to 500 1C using an FTIR spectrometer. The hydrocarbons studied are n-pentane, n-heptane, n-dodecane, 2,2,4-trimethyl-pentane (iso-octane), 2-methyl-butane, 2-methyl-pentane, 2,4,4-trimethyl-1-pentene, 2-methyl2-butene, propene, toluene, m-xylene, and ethylbenzene. Room-temperature measurements of neat hydrocarbon vapor were made with an instrument resolution of both 0.1 and 1 cm1 (FWHM) to confirm that the high-resolution setting was required only to resolve the propene absorption spectrum while the spectra of the other hydrocarbons could be resolved with 1 cm1 resolution. High-resolution (0.1 cm1), room-temperature measurements of neat hydrocarbons were made at low pressure (1 Torr, 133 Pa) and compared to measurements of hydrocarbon/N2 mixtures at atmospheric pressure to verify that no pressure broadening could be observed over this pressure range. The temperature was varied between 25 and 500 1C for atmospheric-pressure measurements of hydrocarbon/N2 mixtures (Xhydrocarbon0.06–1.5%) and it was found that the absorption cross section shows simple temperature-dependent behavior for a fixed wavelength over this temperature range. Comparisons with previous FTIR data over a limited temperature range and with high-resolution laser absorption data over a wide temperature range show good agreement. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mid-infrared hydrocarbon absorption spectra; Optical fuel diagnostic; Temperature-dependent absorption cross section
1. Introduction Optical absorption diagnostics are useful for probing species to measure concentration, temperature, and velocity [1,2] and have been demonstrated in a variety of environments including shock tubes, internal combustion engines, and pulse detonation engines [1–4]. Fixed-wavelength mid-IR laser diagnostics have been used to measure hydrocarbon concentration in such environments because they are sensitive, nonintrusive, and can provide fast time response [4,5]. Wavelength-tunable lasers using difference-frequency-generation make mid-IR absorption sensing even more attractive for obtaining sensitive, time-resolved hydrocarbon concentration measurements because the wavelength can be tailored to the hydrocarbon of interest [6]. Corresponding author. Tel.: +1 650 723 0941; fax: +1 650 723 1748.
E-mail address: [email protected] (A.E. Klingbeil). 0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.03.004
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However, to optimize the wavelength of these tunable devices, the pressure- and temperature dependence of the absorption cross section must be known over an extended range, and for the hydrocarbons of interest. The scientific community has been aware of the strong mid-IR absorption bands of hydrocarbons for more than 70 years [7,8], but much of the data have poor wavelength resolution and do not provide sufficiently quantitative absorption cross sections. For example, one source reports vapor-phase cross sections of 24 aromatic hydrocarbons, but the mid-IR absorption band suffers from interference by dichloromethane, which was mixed with the hydrocarbon to dilute the sample [9]. Because of this interference, the reported band intensities for the C–H stretch were 10–20% lower than other data cited in the article. There are some highquality databases available which present the infrared spectra of many hydrocarbons [10,11]; however, these data are only qualitative because the path length and thermodynamic conditions (i.e., temperature and partial pressure) vary from sample to sample and the concentration is not reported. The HITRAN database [12] provides detailed quantitative spectral information about many low-molecular-weight gaseous species and some low-temperature cross section data (temperatures range from 93 to 52 1C) of other high-molecularweight species. Many of these species are of interest in both atmospheric and combustion sciences. Unfortunately, this database includes only a few hydrocarbon species that do not contain chlorine or fluorine (e.g., methane, ethylene, acetylene, and formaldehyde). Other high-quality databases report measured quantitative spectra of many compounds, but these data cover a limited temperature range (typically from 5 to 50 1C) and are only available for a user fee [13–15]. Our continuing goal is to develop optical sensors and strategies to monitor fuel in combustion environments for both research and engineering applications that aid the development of practical combustion devices. For such quantitative sensing, absorption cross section data are needed as a function of temperature. In this work, we report temperature-dependent spectral absorption cross section data for a variety of complex hydrocarbons that are important components of commercial fuels, used in fuel surrogates, and/or present as residual unburned hydrocarbons in practical combustion devices. We expand on the information previously available by providing quantitative absorption spectra at temperatures as high as 500 1C. Experiments are first performed to determine the instrument resolution needed to resolve the spectral features of these hydrocarbons. Next, we investigate pressure broadening by comparing measurements at low pressure and atmospheric pressure, finding that the absorption spectra of these particular hydrocarbons are independent of pressure from 1 to 760 Torr (0.1–101 kPa). We then consider the effect of molecular structure on the spectra by comparing the mid-IR spectra of several classes of molecules (aromatics, olefins, and alkanes). Finally, we examine the temperature dependence of the spectra and find that the absorption cross section does not exhibit complex temperature-dependent behavior at a fixed wavelength for temperatures between 25 and 500 1C. We compare our measurements to data found in the spectral database maintained by Pacific Northwest National Labs [15]. This database provides quantitative spectral absorption cross sections for many vaporphase hydrocarbons for temperatures ranging from 5 to 50 1C and has been verified extensively. Additionally, the average band intensity of propene was reported by Brosmer and Tien for temperatures between 17 and 527 1C [16]. Their temperature-averaged band intensity data are compared to our own temperature-averaged band intensity data with good agreement. Moreover, in a previous paper, we report and validate temperaturedependent absorption cross section measurements at 2947.9 cm1 (3.39 mm) using laser absorption spectroscopy [17]. Comparison of these fixed-wavelength temperature-dependent measurements with our FTIR data at this wavelength also shows good agreement. Consistency between our measurements and other data, where available, provides confidence in our measurements where no other data are available. 2. Experimental setup The experimental setup of our absorption measurements is shown in Fig. 1. Measurements were made with a Nicolet 6700 FTIR spectrometer equipped with an XT-KBr beam splitter, IR light source, and HgCdTe detector. The spectrometer is sensitive to frequencies from 800 to 10,000 cm1 (wavelengths from 1 to 12.5 mm) and has a manufacturer-specified maximum resolution of 0.09 cm1 (FWHM). Linearization of the detector is accomplished through factory-calibrated electronics. The IR light from the source is focused through an iris, and then collimated. The collimated beam passes into a Michelson interferometer that causes
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Cell Empty
Cell Fill
409
Oven
HgCdTe Detector Michelson Interferometer Iris
Light Source
Fig. 1. Experimental setup for FTIR measurements in a heated cell.
each wavelength to be modulated. This modulated beam is aligned through an optical cell and focused onto the detector. The detector signal is analyzed with the manufacturer’s software (OMNIC Ver. 7.3), providing spectrally resolved intensity of the modulated beam at the detector. This software automatically controls the aperture size and converts the detected signal into frequency-dependent relative intensity. Many FTIR spectrometers are bundled with software that enables baseline corrections and spectral subtraction. The data reported here have not required such corrections because the samples were carefully prepared to minimize condensation and adsorption on the windows and a stable light source minimized intensity fluctuations. Furthermore, spectral subtraction of ambient interferences was not required because the interference from water and CO2 is small for the wavelength range studied. Thus, the drift in ambient concentrations resulted in negligible variation of this interference. FTIR measurements were made using Mertz phase correction and boxcar apodization without zero filling. Measurement time was limited to 2 min (100 scans at 1 cm1 or 25 scans at 0.09 cm1 resolution) except at 500 1C where measurement times were limited to 15 s to avoid thermal decomposition during the measurement. The 15.9-cm-path-length optical cell has a stainless-steel body with brazed sapphire windows. The sapphire windows limit the spectral range of our measurements to 2000–10,000 cm1 (1–5 mm), which is sufficient to obtain spectral information for the region near 3000 cm1 that encompasses the fundamental C–H absorption bands. The maximum temperature and pressure of the cell are 500 1C and 13 atm (1317 kPa), respectively. The cell is completely enclosed inside an oven to provide uniform heating. The oven has two CaF2 windows that allow the beam to pass through the oven walls. A K-type thermocouple inside the cell measures gas temperature. Because the cell and inlet plumbing are enclosed in the oven and are not in physical contact with the heating elements, the temperature nonuniformity inside the cell is assumed to be negligible. Pressure is measured using two pressure gauges, an MKS instruments 0–100 Torr (0–13 kPa) manometer and a Setra Systems 0–13,000 Torr (0–1730 kPa) manometer. The low-pressure manometer is used to verify that the system is evacuated, to measure the partial pressure of the hydrocarbons in the mixture, and to measure the pressure in the cell for the low-pressure measurements. The high-pressure manometer measures the total pressure above 100 Torr (13 kPa). Mixtures were prepared in a stainless-steel mixing tank with a magnetically actuated mixing system. The mixing tank was heated to 100 1C to prevent condensation or adsorption of the hydrocarbons. Details about the mixing process can be found in Ref. [17]. The mixing tank and manifold were first evacuated to a pressure of less than 100 mTorr (0.013 Pa). The tank was then filled with 1–50 Torr (0.13–6.7 kPa) of the hydrocarbon to be studied. For low-vapor-pressure hydrocarbons (e.g., n-dodecane), a liquid sample was injected through a septum, but for high-vapor-pressure hydrocarbons (e.g., n-heptane), the hydrocarbon vapor was extracted
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directly from a flask of the neat liquid, under vacuum. Peak absorbances ranged from 0.2 for lowvapor-pressure species to 1–2 for high-vapor-pressure species. Next, the mixing tank was backfilled with N2 to 1800 Torr (240 kPa), closed, and allowed to mix for 10 min. During this mixing time, a baseline intensity measurement was recorded in the evacuated cell. After the sample was thoroughly mixed, it was introduced into the evacuated cell and the transmitted intensity was recorded. Then the cell was evacuated and filled again with a second sample to be measured. Finally, the cell was evacuated and the baseline intensity was measured again. By measuring the baseline before and after the sample measurements, we could check for intensity drift of the light source. By making two measurements of the same mixture, we could verify repeatability of the measurement and average the measurements for increased signal-to-noise ratio (SNR). A sensitivity analysis was performed to estimate the uncertainty in our cross section measurement and to determine the primary sources of uncertainty. In general, the cross section uncertainty was dominated by uncertainty in the mixture composition and uncertainty in the sample temperature. For many of the measurements, the cross section uncertainty was calculated to be 2%. For species with the lowest vapor pressure (i.e., n-dodecane and m-xylene), uncertainty in the concentration creates a larger uncertainty in the cross section (6% for n-dodecane and 5% for m-xylene). The reproducibility of the data reported here was within the calculated uncertainty. 2.1. Optical absorption by a gaseous mixture Optical absorption is a line-of-sight technique that can be used to measure the mole fraction of a target species. Beer’s law describes absorption of a monochromatic source with frequency n in a uniform gaseous mixture: I ¼ expðsn;T N i LÞ, (1) I0 n where ðI=I 0 Þn is the fractional transmission of the gas at frequency n, sn;T is the temperature-dependent absorption cross section of the absorbing species (cm2 mol1) at n, Ni is the molar density of the absorbing species (mol/cm3) and L is the path length (cm). For a uniform sample, optical absorption measurements can be used to measure concentration of a species over a given path length, but the absorption cross section must be known for the temperature and pressure of interest. In the infrared, absorption bands are the result of rovibrational transitions (i.e., the molecule undergoes a simultaneous change in the rotational and vibrational quantum number). Each rovibrational band is centered at a characteristic vibrational frequency and the band has a width that can be associated with the various rotational levels that are populated. Characteristic rotational energy spacing for these molecules is on the order of a few Kelvin, corresponding to an energy spacing of 1 cm1, and thermodynamic equilibrium dictates that many rotational levels are populated at room temperature. As the temperature increases, more rotational levels are populated and the rovibrational band ‘spreads out’. However, C–H vibrational energies are on the order of several thousand Kelvin (energy spacing of 3000 cm1), and at our moderate temperatures (25–502 1C), nearly all of the population remains in lowest C–H stretch energy level. Because the population in the lower vibrational level is not sensitive to temperature at these low temperatures, we can integrate the frequency-dependent cross section over the entire absorption feature and expect that this integrated band intensity will remain independent of temperature [18]: Z sn;T dnaf ðTÞ. (2) C2H vibrational bands
This result is useful because we can quantitatively compare measurements made at different temperatures by comparing integrated band intensities. These data also enable evaluation of potential uncertainty in the mixture composition at low temperatures where vapor condensation or wall adsorption may be a problem or at high temperatures where thermal decomposition of the hydrocarbon may cause uncertainty.
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2.2. Spectral resolution FTIR resolution can be limited by several spectrometer components, including the iris and the interferometer [19]. A small iris provides a more collimated beam and enables higher wavelength resolution, but at the cost of reduced light throughput. The Michelson interferometer has one fixed and one moving mirror. The distance traveled by the moving mirror affects the instrument resolution. If the mirror moves farther, resolution is increased at the expense of longer test times and lower SNR. To insure accurate cross section measurements, it is necessary to establish that the instrument resolution is sufficient to resolve the structure of the absorption features. The instrument resolution can be verified by measuring an absorption feature that is spectrally narrow compared to the instrument resolution. For this purpose, we made measurements of optical absorption by a carbon monoxide mixture at low pressures (0.25% CO in N2 at 25 kPa). For these conditions, the linewidth of the CO absorption features is 0.028 cm1 (FWHM), whereas the minimum linewidth observed with our FTIR spectrometer was 0.09 cm1 (FWHM), thereby providing a measure of the maximum instrument resolution. After the instrument resolution was established, it was necessary to determine the minimum required resolution for the set of hydrocarbons studied. Because the experiment test time increases and SNR decreases when measuring at high resolution, it is desirable to make measurements at a low resolution. Conversely, sufficiently high resolution must be retained to insure the absorption spectrum is accurately measured. To determine the required resolution, each of the hydrocarbons was measured at low pressure (o20 Torr, 2.7 kPa) and at 0.1 and 1 cm1 resolution. An instrument resolution of 1 cm1 was found to be sufficient to resolve the spectral features of the hydrocarbons studied here, with the exception of propene, which required measurements at 0.1 cm1 resolution. Additionally, no pressure broadening was observed when these spectra were compared to the spectra measured at atmospheric pressure. 3. Absorption cross section measurements Absorption cross sections were measured for 12 hydrocarbon compounds in five structural classes (straight alkanes, branched alkanes, straight olefins, branched olefins, and aromatics). Table 1 lists each hydrocarbon, the spectral resolution employed, and the ranges of temperature and mole fraction studied. Comparison of the spectra of species within a class and across classes confirms the structural groups (i.e., CH2 or CH3 groups) that are responsible for absorption peaks at specific wavelengths. Interestingly, our data show that the absorption cross sections vary with temperature in a nearly linear manner at most wavelengths. The integrated band intensity is independent of temperature within the accuracy of our measurements, enabling useful comparisons with data measured at different temperatures. Good agreement is found when the current Table 1 Summary of hydrocarbons measured and range of measurement conditions Name
Formula
Molecular class
FTIR resolution (cm1)
Minimum T (1C)
Maximum T (1C)
Mole fraction (%)
n-Pentane n-Heptane n-Dodecane 2-Methyl-butane 2-Methyl-pentane 2,2,4-Trimethyl-pentane Propene 2-Methyl-2-butene 2,4,4-Trimethyl-1pentene Toluene m-Xylene Ethyl-benzene
C5H12 C7H16 C12H26 C5H12 C6H14 C8H18 C3H6 C5H10 C8H16
Straight alkane Straight alkane Straight alkane Branched alkane Branched alkane Branched alkane Straight olefin Branched olefin Branched olefin
1 1 1 1 1 1 0.1 1 1
27 26 51 27 28 26 27 27 27
502 501 501 503 502 501 502 501 451
0.4–1.3 0.3–1.1 0.07–0.1 0.2–1.1 0.4–1.3 0.3–1.6 1–6 0.06–2.3 0.3–1.3
C7H8 C8H10 C8H10
Aromatic Aromatic Aromatic
1 1 1
27 27 26
501 502 502
1–1.5 0.4–0.6 0.3–0.6
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temperature-dependent data are compared with other available broadband data at limited temperatures [15,16] and also with fixed-wavelength, temperature-dependent laser absorption measurements [17] of hydrocarbon/nitrogen mixtures. The three straight alkanes studied were n-pentane, n-heptane, and n-dodecane. n-Pentane is present in high concentrations in gasoline, n-heptane is an important component of gasoline and diesel surrogate mixtures, and n-dodecane is a common component of jet fuel surrogate mixtures. As noted in Table 1, each hydrocarbon was investigated at temperatures as high as 500 1C and similar temperature-dependent trends were observed for all of the species studied. The absorption cross section peaks decrease and spread out with increasing temperature, as expected. The absorption spectrum of n-pentane, measured at 50 1C, is shown in Fig. 2 and compared to the data measured by Sharpe et al. for the same temperature. Instrument resolution for our measurements was 1 cm1 and the cited data were measured with 0.1 cm1 resolution. The two spectra show good agreement, confirming that 1 cm1 resolution is sufficient for this measurement. When we compared our measured spectra to those of Sharpe et al. for the other species, we consistently found good agreement at all wavelengths within this absorption band. However, throughout the rest of this paper, we report only comparisons of the band intensity, calculated from Eq. (2), and integrated from 2500 to 3400 cm1 (see Table 2). The final column in Table 2 shows the percent difference between the two sets of data. Our integrated intensities agree well with the data from Sharpe et al. and are within the combined uncertainties of the two studies. The absorption spectra of n-pentane, n-heptane, and n-dodecane are shown in Fig. 3(a–c), respectively. Computed three-dimensional (3-D) geometries are also included to illustrate the influence of molecular structure on the absorption spectrum. The dark (red) spheres represent the hydrogen atoms and the light (yellow) spheres represent the carbon atoms. Details of the 3-D modeling can be found in Ref. [20]. The absorption spectrum of n-pentane was measured at nine discrete temperatures ranging from 27 to 502 1C with an estimated cross section uncertainty of 2%. In the experiments at 500 1C, it was noted that the peak absorbance decreased steadily with time after the cell was filled with the mixture. Methane absorption peaks were subsequently observed in the spectrum. This behavior was interpreted as thermal decomposition and, by noting the change in peak absorbance with time, the decomposition rate was estimated to be 2%/min. By averaging fewer scans (10 scans rather than 100), test times were reduced to 15 s for this hightemperature condition, and decomposition was limited to less than 0.5%. Absorption peaks for n-pentane are located at 2881, 2940, and 2966 cm1. The spectrally sharp absorption features exhibited near room temperature are smoothed out at high temperatures. At higher temperatures, the cross section decreases at the peaks and increases away from the peaks as the features spread. This high-temperature spectral broadening of the C–H stretch bands was evident for all of the hydrocarbons studied. The band intensity (i.e., the cross
Cross Section [cm2/mole]
5x105
Sharpe et al. This Work
4x105 3x105 2x105 1x105 0x105 2800
2900
3000
3100
Frequency [cm-1] Fig. 2. Measured absorption spectrum of n-pentane at 50 1C compared to data from Sharpe et al. at 50 1C [15].
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Table 2 Temperature-averaged integrated band intensities for C–H vibration (integrated from 2500 to 3400 cm1) and measurement uncertainties for the hydrocarbons studied here compared to data from Sharpe et al. [15] Current study
n-Pentane n-Heptane n-Dodecane 2-Methyl-butane 2-Methyl-pentane 2,2,4-Trimethylpentane Propene 2-Methyl-2-butene 2,4,4-Trimethyl-1pentene Toluene m-Xylene Ethyl-benzene
Cross Section [cm2/mole]
6x105 5x105
Sharpe et al. Uncertainty (%)
Integrated area (1 106 cm1 cm2 mo1l)
Uncertainty (%)
39.0 51.7 83.3 37.0 43.3 53.7
2 2 6 3 2 2
39.6 51.7 85.7 37.1 43.8 53.6
3 3 7 3 3 3
1.6 0.0 2.8 0.4 1.1 0.1
10.0 24.7 39.8
2 2 2
10.3 25.0 40.3
3 3 3
2.3 1.0 1.4
13.2 18.5 19.4
2 5 5
13.2 19.0 20.2
3 7 7
0.5 2.5 4.1
7x105
T=27°C T=251°C T=502°C
4x105 3x105 2x105 1x105 0x105 2800
Difference (%)
Integrated area (1 106 cm1 cm2 mol1)
Cross Section [cm2/mole]
Name
2850
2900
2950
Frequency
3000
3100
T=26°C T=251°C T=501°C
5x105 4x105 3x105 2x105 1x105 0x105 2800
2850
2900
[cm-1]
1.4x106 Cross Section [cm2/mole]
3050
6x105
1.2x106
2950
Frequency
T=51°C T=251°C T=501°C
3000
3050
3100
[cm-1]
c
1.0x106 0.8x106 0.6x106 0.4x106 0.2x106 0.0x106 2800
2850
2900
2950
3000
3050
3100
Frequency [cm-1] Fig. 3. Measured absorption spectrum of (a) n-pentane, (b) n-heptane, and (c) n-dodecane from 2750 to 3100 cm1 at discrete temperatures (25oTo501 1C). Instrument resolution for these measurements was 1 cm1. Triangle markers in (b) indicate selected frequencies from Fig. 5.
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section integrated from 2500 to 3400 cm1) was found to vary less than 72% over the entire temperature range. The integrated band intensity results for all the species studied are listed in Table 2. Note that, for each species, the total variation in measured band intensity with temperature was less than the estimated uncertainty of the individual measurements. Thus, the band intensity values listed in Table 2 are average values for all of the temperatures measured. These data are compared to the measurements of Sharpe et al. which were typically measured at 5, 25, and 50 1C and were averaged over all of the available temperatures for this comparison. The average integrated band intensity for n-pentane in the present work is within 1.6% of the value reported by Sharpe. The absorption spectrum of n-heptane was measured as a function of temperature from 25 to 500 1C in 501 increments. Three sample spectra are shown in Fig. 3(b) for temperatures of 26, 250, and 501 1C. Absorption peaks for n-heptane are located at 2874–2882, 2936, and 2968 cm1. The measured band intensity, integrated from 2500 to 3400 cm1, was observed to be constant within 72% between 25 and 500 1C. The average band intensity agrees very well with the Sharpe data (measured from 5 to 50 1C). Additionally, the FTIR-measured cross sections at 2948 cm1 are compared to our previous laser absorption measurements at 2947.9 cm1 [17] in Fig. 4 for temperatures ranging from 25 to 500 1C. The agreement is well within the estimated uncertainty of the FTIR measurements (2%) over the temperature range of the measurements, providing confidence in the temperature-dependence of our spectral cross section measurements. While Fig. 3 shows trends in the absorption spectrum as temperature increases, it is interesting to examine the variation in cross section with temperature at fixed optical frequency (or wavelength). Fig. 5 shows the temperature dependence of the absorption cross section for n-heptane at five selected frequencies. (These frequencies are also marked in Fig. 3 for reference.) These data reveal that, at all of these frequencies, the variation in absorption cross section is approximately linear with temperature over the temperature region investigated. Furthermore, this linear variation was observed at most frequencies for all of the hydrocarbons studied here, with the exception of propene. By employing this approximation, the absorption cross section at an arbitrary temperature may be estimated from a limited database. For species with cross sections that do not vary linearly with temperature, one might consider approximating the temperature dependence with a more complex equation, such as the one described in Ref. [21]. The absorption spectrum of n-dodecane was measured at eight temperatures between 50 and 501 1C. The minimum temperature of the measurement was limited by low-vapor-pressure constraints below 50 1C. At 501 1C, thermal decomposition was evident. Thus, fewer scans were averaged, limiting the measurement time to 15 s, over which time decomposition was negligible. Spectra at selected temperatures are displayed in Fig. 3(c). Again, the integrated band intensity was found to be consistent with data from the literature within the estimated uncertainty of our measurements.
Cross Section [cm2/mole]
5.0x105
HeNe laser at 2947.9 cm-1 FTIR at 2948 cm-1
4.8x105 4.6x105 4.4x105 4.2x105 4.0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 4. Comparison of the measured n-heptane absorption cross section using FTIR data at 2948 cm1and helium–neon laser absorption data at 2947.9 cm1 [17]. Note the suppressed zero on the y-axis.
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Cross Section [cm2/mole]
8x105
2969 cm-1
2956 cm-1
2901 cm-1
2881 cm-1
415
2935 cm-1
6x105
4x105
2x105
0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 5. Temperature-dependent absorption cross sections of n-heptane at specific frequencies.
Cross Section [cm2/mole]
8.0x105
HeNe laser at 2947.9 cm-1 FTIR at 2948 cm-1
7.5x105 7.0x105 6.5x105 6.0x105 5.5x105 5.0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 6. Temperature-dependent absorption cross sections of n-dodecane using FTIR data at 2948 cm1and helium–neon laser absorption data at 2947.9 cm1 [17]. Note the suppressed zero on the y-axis.
FTIR measurements of the n-dodecane cross section at 2948 cm1 are compared with laser absorption data in Fig. 6, revealing that the HeNe data are 15% lower than the FTIR measurements. We attribute this discrepancy to the extremely low-vapor-pressure of dodecane (0.1 Torr at 296 K), which poses difficulty in diluting the mixture without causing some condensation in the mixing tank. We have recently found evidence that suggests that some dodecane was condensing from the mixture during the original HeNe measurements. However, by diluting the mixture very slowly (diluted to 1:1000 dodecane:nitrogen in 10 min rather than the usual 1 min), we were able to minimize condensation for the current FTIR measurements. These FTIR data are in good agreement with PNNL data that have been recently obtained using a special procedure [22,23]. The good agreement between the two laboratories using two different procedures implies that condensation was not likely for these measurements. We also tested our modified dilution method with m-xylene, the species with the next lowest vapor pressure from our group of 12 hydrocarbons. Here we found good agreement in the spectra using both our initial and the modified dilution methods. These findings suggest that the 10 min dilution procedure is only required for gases with a vapor pressure less than that of m-xylene (8 Torr at 298 K). For n-dodecane, peaks are located at 2866, 2933, and 2970 cm1. These peak locations are close to those of n-pentane and n-heptane. It should also be noted that the peak at 2970 cm1 has a similar cross section
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magnitude for n-pentane, n-heptane, and n-dodecane, which suggests that these peaks are caused by the two CH3 groups that are common to each of these molecules. The peaks at 2933 and 2866 cm1 have significantly higher cross sections for n-dodecane than for n-heptane or n-pentane. Because n-dodecane has 10 CH2 groups, n-heptane has five CH2 groups, and n-pentane has three CH2 groups, we can infer that these peaks are sensitive to absorption by the CH2 groups. These interpretations are consistent with data from the Handbook of Spectroscopy [24] which reports resonant frequencies for CH3 groups ranging from 2875 to 2925 cm1 and 2950 to 3000 cm1 and resonant frequencies for CH2 groups ranging from 2850 to 2900 cm1 and 2925 to 2975 cm1. The absorption spectra of three branched alkanes were also measured at discrete temperatures. Sample spectra for 2-methyl-butane, 2-methyl-pentane, and 2,2,4-trimethyl-pentane are shown in Fig. 7(a–c), respectively. Note that 2-methyl-butane is present in high concentrations in gasoline exhaust [25], and that 2methyl-pentane is a significant component of gasoline [26]. 2,2,4-Trimethyl-pentane is present in gasoline and is used extensively in gasoline surrogate mixtures. For 2-methyl-butane, there are two absorption peaks located at 2890 and 2965 cm1, for 2-methylpentane the two primary absorption peaks are located at 2885 and 2965 cm1, and for 2,2,4-trimethyl-pentane the peaks are located at 2883, 2912, and 2965 cm1. 2-Methyl-butane has three CH3 groups, one CH2 group, and one CH group. 2-Methyl-pentane has three CH3 groups, two CH2 groups, and one CH group. 2,2,4Trimethyl-pentane has five CH3 groups, one CH2 group, and one CH group. Comparing the spectra of 2,2,4trimethyl-pentane with those of 2-methyl-butane, 2-methyl-pentane, and n-pentane, we can see that the peak at 2965 cm1 scales with the number of CH3 groups.
6x105
7x105
T = 27°C T = 252°C T = 503°C
Cross Section[cm2/mole]
Cross Section [cm2/mole]
7x105
5x105 4x105 3x105 2x105 1x105 0x105 2800
2850
2900
2950
3000
3050
3100
6x105 5x105 4x105 3x105 2x105 1x105 0x105 2800
Frequency [cm-1]
Cross Section [cm2/mole]
1.2x106 1.0x106
T = 28°C T = 252°C T = 502°C
2850
2900
2950
3000
3050
3100
Frequency [cm-1]
T= 26°C T= 251°C T = 501°C
0.8x106 0.6x106 0.4x106 0.2x106 0.0x106 2800
2900
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For these three hydrocarbons, we find excellent agreement between our band intensity data and the data available in the literature (see Table 2). Note that our measurements were made at temperatures ranging from 25 to 500 1C, while most previous spectral data were limited to temperatures below 50 1C. Furthermore, we find good agreement between our FTIR measurements of 2,2,4-trimethyl-pentane at 2948 cm1 and our laser absorption data for the same molecule, which are compared over a wide temperature range in Fig. 8. Temperature-dependent absorption measurements were also conducted for the aromatics toluene, m-xylene, and ethylbenzene, and sample spectra are shown in Fig. 9(a–c). These three aromatics are all present in gasoline, and in addition, toluene is included in many gasoline surrogates. The absorption spectrum of toluene was measured from 25 to 500 1C. The structure of toluene is that of a benzene ring with one CH3 group attached to one of the ring carbons. For toluene, the peaks at 2884 and 2940 cm1 are much smaller than the corresponding peaks of the alkanes because toluene only has one CH3 group and does not have any CH2 groups. The additional peaks at 3040 and 3073 cm1 are due to the C–H bonds on the benzene ring [24]. m-Xylene is a benzene ring with a CH3 group on the first and third carbon atoms of the benzene ring. Thus, the absorption peaks of m-xylene at 2882 and 2938 cm1 are approximately twice as strong as those of toluene because there are two CH3 groups. Likewise, the absorption features at 3033 and 3069 cm1, which are caused by the C–H bonds attached directly to the aromatic ring, are approximately the same intensity for m-xylene and toluene. Comparing our band intensity data to those of Sharpe et al., we find that the data agree within 5%, which is within the estimated uncertainty of the two sets of measurements. Ethylbenzene, the third aromatic studied here, is a benzene ring with a CH2–CH3 chain attached to one of the ring carbons. Ethylbenzene has multiple peaks located at 2886, 2942, 2975, 3039, and 3075 cm1. The peak at 2886 cm1 can be associated with the CH3 group, and the two peaks at 3039 and 3075 cm1 can be associated with the ring CH vibrations. The absorption spectra of two branched olefins (2-methyl-2-butene and 2,4,4-trimethyl-1-pentene) and one straight olefin (propene) were measured at discrete temperatures ranging from 25 to 500 1C. The spectra of 2-methyl-2-butene and 2,4,4-trimethyl-1-pentene provide useful sample spectra for branched olefins, and these hydrocarbons are also important because they are present in small quantities in gasoline. Furthermore, 2-methyl-2-butene can be found in moderate concentrations in the exhaust gas of internal combustion engines [25]. Propylene is an important intermediate combustion species in many hydrocarbon reactions and is present in high concentrations in the unburned hydrocarbons of gasoline engines [25]. The absorption spectra of these hydrocarbons are shown in Fig. 10(a–c). Note that the numerous, spectrally narrow peaks in the absorption spectrum of propene spread out and the spectrum transforms into one continuous feature at high temperatures. The absorption spectra of 2-methyl-2-butene and 2,4,4-trimethyl-1-pentene are also similar to those of the other hydrocarbons. 2-Methyl-2-butene has peaks located at 2887, 2938, and 2988 cm1, while
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-1]
Fig. 9. Measured absorption spectra for (a) toluene, (b) m-xylene, and (c) ethylbenzene at select temperatures.
2,4,4-trimethyl-1-pentene has peaks at 2879, 2920, 2967, and 3082 cm1. Band intensities for these two measurements are in good agreement with the data from the literature, as shown in Table 2. The narrow absorption features of propene required the use of 0.1 cm1 spectrometer resolution to faithfully recover the spectrum. Table 2 shows good agreement between our data and those of Sharpe et al. Propene is one of the few hydrocarbons with previously reported high-temperature spectra, and our data agree well with the average band intensity value of 10.5 106 cm1 cm2 mol1 [16] reported by Brosmer and Tien for measurements made at 17, 277, and 527 1C. 4. Summary Infrared spectra were measured for 12 hydrocarbons at temperatures ranging from 25 to 500 1C. The hydrocarbons were selected because of their importance in research related to practical fuels such as gasoline and jet fuels. The stable combustion intermediate propene is also included because of its importance in hydrocarbon chemistry. Uncertainties in the measurements range from 2% to 8%, depending on the vapor pressure of the species. We found that the mid-IR spectra of all of the species become broadened at high temperatures, but the integrated band intensity is insensitive to temperature. It was also shown that our measurements of integrated band intensity agree well with previous measurements near room temperature. Furthermore, we noted that the absorption cross sections at most frequencies are nearly linear with temperature. These data will be useful for mid-IR spectral analysis of gaseous mixtures containing hydrocarbons and will provide guidance for wavelength optimization of laser-based absorption diagnostics for hydrocarbons.
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Acknowledgments Support for this work was provided by the Air Force Office of Scientific Research with Julian Tishkoff as technical monitor, the Army Research Office with Ralph A. Anthenien Jr. as program manager, and by Nissan Motor Company, Ltd. References [1] Sanders ST, Baldwin JA, Jenkins TP, Baer DS, Hanson RK. Diode-laser sensor for monitoring multiple combustion parameters in pulse detonation engines. Proc Combust Inst 2000;28(1):587–93. [2] Philippe LC, Hanson RK. Laser diode wavelength-modulation spectroscopy for simultaneous measurement of temperature, pressure, and velocity in shock-heated oxygen flows. Appl Opt 1993;32(30):6090–103. [3] Jeffries JB, Schulz C, Mattison DW, Oehlschlaeger MA, Bessler WG, Lee T, et al. UV absorption of CO2 for temperature diagnostics of hydrocarbon combustion applications. Proc Combust Inst 2005;30(1):1591–9. [4] Tomita E, Kawahara N, Nishiyama A, Shigenaga M. In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 mm infrared laser absorption method: application to an actual engine. Meas Sci Technol 2003;14(8):1357–63. [5] Klingbeil AE, Jeffries JB, Hanson RK, Hinckley K, Dean A. 3.39 mm laser absorption sensor for ethylene and propane measurements in a pulse detonation engine. In: 44th AIAA aerospace sciences meeting and exhibit, Reno, Nevada; 2006 [Paper AIAA-2006-925]. [6] Klingbeil AE, Jeffries JB, Hanson RK. Tunable mid-IR laser absorption sensor for time-resolved hydrocarbon fuel measurements. Proc Combust Inst 2007;31(1):807–15. [7] Kettering CF, Sleator WW. Infrared absorption spectra of certain organic compounds, including principle types present in gasoline. Physics 1933;4:39–49.
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[8] Hunter WH, Northey EH. Absorption spectra of a series of organic molecular compounds. J Phys Chem 1933;37:875–87. [9] Etzkorn T, Klotz B, Sorensen S, Patroescu IV, Barnes I, Becker KH, et al. Gas-phase absorption cross sections of 24 monocyclic aromatic hydrocarbons in the UV and IR spectral ranges. Atmos Environ 1999;33(4):525–40. [10] Pouchert CJ. The Aldrich library of FT-IR spectra. Milwaukee, WI: Aldrich Chemical Co; 1981. [11] /http://webbook.nist.gov/chemistry/S. [12] Rothman LS, Jacquemart D, Barbe A, Benner DC, Birk M, Brown LR, et al. The HITRAN 2004 molecular spectroscopic database. JQSRT 2005;96(2):139–204. [13] Chu PM, Guenther FR, Rhoderick GC, Lafferty WJ. The NIST quantitative infrared database. J Res Natl Inst Stand Technol 1999;104(1):59–81. [14] The QASoft Database contains quantitative reference spectra of 392 vapor phase compounds. More information is available at /http://www.infraredanalysisinc.com/database.htmS. [15] Sharpe SW, Johnson TJ, Sams RL, Chu PM, Rhoderick GC, Johnson PA. Gas-phase databases for quantitative infrared spectroscopy. Appl Spectrosc 2004;58(12):1452–61. [16] Brosmer MA, Tien CL. Thermal radiation properties of propylene. Combust Sci Technol 1986;48(3–4):163–75. [17] Klingbeil AE, Jeffries JB, Hanson RK. Temperature- and pressure-dependent absorption cross sections of gaseous hydrocarbons at 3.39 mm. Meas Sci Technol 2006;17:1950–7. [18] Edwards DK. Thermal radiation by combustion gases. Int J Heat Mass Transfer 1973;16(1):25–40. [19] Griffiths PR, De Haseth JA. Fourier transform infrared spectrometry. New York: Wiley; 1986. [20] /http://webbook.nist.gov/chemistry/3d-structs/S. [21] Wakatsuki K, Fuss SP, Hamins A, Nyden MR. A technique for extrapolating absorption coefficient measurements to high temperatures. Proc Combust Inst 2005;30:1565–73. [22] Personal communication with Steven Sharpe from Pacific Northwest National Lab. [23] Johnson TJ, Sharpe SW, Covert MA. Disseminator for rapid, selectable, and quantitative delivery of low and semivolatile liquid species to the vapor phase. Rev Sci Instrum 2006;77(9):94103-1. [24] Robinson JW. Handbook of spectroscopy, vol. 2. Clevland, OH: CRC Press; 1974. [25] Kaiser EW, Siegl WO, Henig YI, Anderson RW, Trinker FH. Effect of fuel structure on emissions from a spark-ignited engine. Environ Sci Technol 1991;25(12):2005–12. [26] Dempster NM, Shore PR. Investigation into the production of hydrocarbon emissions from a gasoline engine tested on chemically defined fuels. In: SAE international congress and exposition, SAE technical paper series, Detroit, MI, USA; February 26–March 2, 1990.
Journal of Quantitative Spectroscopy & Radiative Transfer 107 (2007) 407–420 www.elsevier.com/locate/jqsrt
Temperature-dependent mid-IR absorption spectra of gaseous hydrocarbons Adam E. Klingbeil, Jay B. Jeffries, Ronald K. Hanson Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA Received 11 December 2006; received in revised form 20 February 2007; accepted 3 March 2007
Abstract Quantitative mid-IR absorption spectra (2500–3400 cm1) for 12 pure hydrocarbon compounds are measured at temperatures ranging from 25 to 500 1C using an FTIR spectrometer. The hydrocarbons studied are n-pentane, n-heptane, n-dodecane, 2,2,4-trimethyl-pentane (iso-octane), 2-methyl-butane, 2-methyl-pentane, 2,4,4-trimethyl-1-pentene, 2-methyl2-butene, propene, toluene, m-xylene, and ethylbenzene. Room-temperature measurements of neat hydrocarbon vapor were made with an instrument resolution of both 0.1 and 1 cm1 (FWHM) to confirm that the high-resolution setting was required only to resolve the propene absorption spectrum while the spectra of the other hydrocarbons could be resolved with 1 cm1 resolution. High-resolution (0.1 cm1), room-temperature measurements of neat hydrocarbons were made at low pressure (1 Torr, 133 Pa) and compared to measurements of hydrocarbon/N2 mixtures at atmospheric pressure to verify that no pressure broadening could be observed over this pressure range. The temperature was varied between 25 and 500 1C for atmospheric-pressure measurements of hydrocarbon/N2 mixtures (Xhydrocarbon0.06–1.5%) and it was found that the absorption cross section shows simple temperature-dependent behavior for a fixed wavelength over this temperature range. Comparisons with previous FTIR data over a limited temperature range and with high-resolution laser absorption data over a wide temperature range show good agreement. r 2007 Elsevier Ltd. All rights reserved. Keywords: Mid-infrared hydrocarbon absorption spectra; Optical fuel diagnostic; Temperature-dependent absorption cross section
1. Introduction Optical absorption diagnostics are useful for probing species to measure concentration, temperature, and velocity [1,2] and have been demonstrated in a variety of environments including shock tubes, internal combustion engines, and pulse detonation engines [1–4]. Fixed-wavelength mid-IR laser diagnostics have been used to measure hydrocarbon concentration in such environments because they are sensitive, nonintrusive, and can provide fast time response [4,5]. Wavelength-tunable lasers using difference-frequency-generation make mid-IR absorption sensing even more attractive for obtaining sensitive, time-resolved hydrocarbon concentration measurements because the wavelength can be tailored to the hydrocarbon of interest [6]. Corresponding author. Tel.: +1 650 723 0941; fax: +1 650 723 1748.
E-mail address: [email protected] (A.E. Klingbeil). 0022-4073/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jqsrt.2007.03.004
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However, to optimize the wavelength of these tunable devices, the pressure- and temperature dependence of the absorption cross section must be known over an extended range, and for the hydrocarbons of interest. The scientific community has been aware of the strong mid-IR absorption bands of hydrocarbons for more than 70 years [7,8], but much of the data have poor wavelength resolution and do not provide sufficiently quantitative absorption cross sections. For example, one source reports vapor-phase cross sections of 24 aromatic hydrocarbons, but the mid-IR absorption band suffers from interference by dichloromethane, which was mixed with the hydrocarbon to dilute the sample [9]. Because of this interference, the reported band intensities for the C–H stretch were 10–20% lower than other data cited in the article. There are some highquality databases available which present the infrared spectra of many hydrocarbons [10,11]; however, these data are only qualitative because the path length and thermodynamic conditions (i.e., temperature and partial pressure) vary from sample to sample and the concentration is not reported. The HITRAN database [12] provides detailed quantitative spectral information about many low-molecular-weight gaseous species and some low-temperature cross section data (temperatures range from 93 to 52 1C) of other high-molecularweight species. Many of these species are of interest in both atmospheric and combustion sciences. Unfortunately, this database includes only a few hydrocarbon species that do not contain chlorine or fluorine (e.g., methane, ethylene, acetylene, and formaldehyde). Other high-quality databases report measured quantitative spectra of many compounds, but these data cover a limited temperature range (typically from 5 to 50 1C) and are only available for a user fee [13–15]. Our continuing goal is to develop optical sensors and strategies to monitor fuel in combustion environments for both research and engineering applications that aid the development of practical combustion devices. For such quantitative sensing, absorption cross section data are needed as a function of temperature. In this work, we report temperature-dependent spectral absorption cross section data for a variety of complex hydrocarbons that are important components of commercial fuels, used in fuel surrogates, and/or present as residual unburned hydrocarbons in practical combustion devices. We expand on the information previously available by providing quantitative absorption spectra at temperatures as high as 500 1C. Experiments are first performed to determine the instrument resolution needed to resolve the spectral features of these hydrocarbons. Next, we investigate pressure broadening by comparing measurements at low pressure and atmospheric pressure, finding that the absorption spectra of these particular hydrocarbons are independent of pressure from 1 to 760 Torr (0.1–101 kPa). We then consider the effect of molecular structure on the spectra by comparing the mid-IR spectra of several classes of molecules (aromatics, olefins, and alkanes). Finally, we examine the temperature dependence of the spectra and find that the absorption cross section does not exhibit complex temperature-dependent behavior at a fixed wavelength for temperatures between 25 and 500 1C. We compare our measurements to data found in the spectral database maintained by Pacific Northwest National Labs [15]. This database provides quantitative spectral absorption cross sections for many vaporphase hydrocarbons for temperatures ranging from 5 to 50 1C and has been verified extensively. Additionally, the average band intensity of propene was reported by Brosmer and Tien for temperatures between 17 and 527 1C [16]. Their temperature-averaged band intensity data are compared to our own temperature-averaged band intensity data with good agreement. Moreover, in a previous paper, we report and validate temperaturedependent absorption cross section measurements at 2947.9 cm1 (3.39 mm) using laser absorption spectroscopy [17]. Comparison of these fixed-wavelength temperature-dependent measurements with our FTIR data at this wavelength also shows good agreement. Consistency between our measurements and other data, where available, provides confidence in our measurements where no other data are available. 2. Experimental setup The experimental setup of our absorption measurements is shown in Fig. 1. Measurements were made with a Nicolet 6700 FTIR spectrometer equipped with an XT-KBr beam splitter, IR light source, and HgCdTe detector. The spectrometer is sensitive to frequencies from 800 to 10,000 cm1 (wavelengths from 1 to 12.5 mm) and has a manufacturer-specified maximum resolution of 0.09 cm1 (FWHM). Linearization of the detector is accomplished through factory-calibrated electronics. The IR light from the source is focused through an iris, and then collimated. The collimated beam passes into a Michelson interferometer that causes
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Fig. 1. Experimental setup for FTIR measurements in a heated cell.
each wavelength to be modulated. This modulated beam is aligned through an optical cell and focused onto the detector. The detector signal is analyzed with the manufacturer’s software (OMNIC Ver. 7.3), providing spectrally resolved intensity of the modulated beam at the detector. This software automatically controls the aperture size and converts the detected signal into frequency-dependent relative intensity. Many FTIR spectrometers are bundled with software that enables baseline corrections and spectral subtraction. The data reported here have not required such corrections because the samples were carefully prepared to minimize condensation and adsorption on the windows and a stable light source minimized intensity fluctuations. Furthermore, spectral subtraction of ambient interferences was not required because the interference from water and CO2 is small for the wavelength range studied. Thus, the drift in ambient concentrations resulted in negligible variation of this interference. FTIR measurements were made using Mertz phase correction and boxcar apodization without zero filling. Measurement time was limited to 2 min (100 scans at 1 cm1 or 25 scans at 0.09 cm1 resolution) except at 500 1C where measurement times were limited to 15 s to avoid thermal decomposition during the measurement. The 15.9-cm-path-length optical cell has a stainless-steel body with brazed sapphire windows. The sapphire windows limit the spectral range of our measurements to 2000–10,000 cm1 (1–5 mm), which is sufficient to obtain spectral information for the region near 3000 cm1 that encompasses the fundamental C–H absorption bands. The maximum temperature and pressure of the cell are 500 1C and 13 atm (1317 kPa), respectively. The cell is completely enclosed inside an oven to provide uniform heating. The oven has two CaF2 windows that allow the beam to pass through the oven walls. A K-type thermocouple inside the cell measures gas temperature. Because the cell and inlet plumbing are enclosed in the oven and are not in physical contact with the heating elements, the temperature nonuniformity inside the cell is assumed to be negligible. Pressure is measured using two pressure gauges, an MKS instruments 0–100 Torr (0–13 kPa) manometer and a Setra Systems 0–13,000 Torr (0–1730 kPa) manometer. The low-pressure manometer is used to verify that the system is evacuated, to measure the partial pressure of the hydrocarbons in the mixture, and to measure the pressure in the cell for the low-pressure measurements. The high-pressure manometer measures the total pressure above 100 Torr (13 kPa). Mixtures were prepared in a stainless-steel mixing tank with a magnetically actuated mixing system. The mixing tank was heated to 100 1C to prevent condensation or adsorption of the hydrocarbons. Details about the mixing process can be found in Ref. [17]. The mixing tank and manifold were first evacuated to a pressure of less than 100 mTorr (0.013 Pa). The tank was then filled with 1–50 Torr (0.13–6.7 kPa) of the hydrocarbon to be studied. For low-vapor-pressure hydrocarbons (e.g., n-dodecane), a liquid sample was injected through a septum, but for high-vapor-pressure hydrocarbons (e.g., n-heptane), the hydrocarbon vapor was extracted
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directly from a flask of the neat liquid, under vacuum. Peak absorbances ranged from 0.2 for lowvapor-pressure species to 1–2 for high-vapor-pressure species. Next, the mixing tank was backfilled with N2 to 1800 Torr (240 kPa), closed, and allowed to mix for 10 min. During this mixing time, a baseline intensity measurement was recorded in the evacuated cell. After the sample was thoroughly mixed, it was introduced into the evacuated cell and the transmitted intensity was recorded. Then the cell was evacuated and filled again with a second sample to be measured. Finally, the cell was evacuated and the baseline intensity was measured again. By measuring the baseline before and after the sample measurements, we could check for intensity drift of the light source. By making two measurements of the same mixture, we could verify repeatability of the measurement and average the measurements for increased signal-to-noise ratio (SNR). A sensitivity analysis was performed to estimate the uncertainty in our cross section measurement and to determine the primary sources of uncertainty. In general, the cross section uncertainty was dominated by uncertainty in the mixture composition and uncertainty in the sample temperature. For many of the measurements, the cross section uncertainty was calculated to be 2%. For species with the lowest vapor pressure (i.e., n-dodecane and m-xylene), uncertainty in the concentration creates a larger uncertainty in the cross section (6% for n-dodecane and 5% for m-xylene). The reproducibility of the data reported here was within the calculated uncertainty. 2.1. Optical absorption by a gaseous mixture Optical absorption is a line-of-sight technique that can be used to measure the mole fraction of a target species. Beer’s law describes absorption of a monochromatic source with frequency n in a uniform gaseous mixture: I ¼ expðsn;T N i LÞ, (1) I0 n where ðI=I 0 Þn is the fractional transmission of the gas at frequency n, sn;T is the temperature-dependent absorption cross section of the absorbing species (cm2 mol1) at n, Ni is the molar density of the absorbing species (mol/cm3) and L is the path length (cm). For a uniform sample, optical absorption measurements can be used to measure concentration of a species over a given path length, but the absorption cross section must be known for the temperature and pressure of interest. In the infrared, absorption bands are the result of rovibrational transitions (i.e., the molecule undergoes a simultaneous change in the rotational and vibrational quantum number). Each rovibrational band is centered at a characteristic vibrational frequency and the band has a width that can be associated with the various rotational levels that are populated. Characteristic rotational energy spacing for these molecules is on the order of a few Kelvin, corresponding to an energy spacing of 1 cm1, and thermodynamic equilibrium dictates that many rotational levels are populated at room temperature. As the temperature increases, more rotational levels are populated and the rovibrational band ‘spreads out’. However, C–H vibrational energies are on the order of several thousand Kelvin (energy spacing of 3000 cm1), and at our moderate temperatures (25–502 1C), nearly all of the population remains in lowest C–H stretch energy level. Because the population in the lower vibrational level is not sensitive to temperature at these low temperatures, we can integrate the frequency-dependent cross section over the entire absorption feature and expect that this integrated band intensity will remain independent of temperature [18]: Z sn;T dnaf ðTÞ. (2) C2H vibrational bands
This result is useful because we can quantitatively compare measurements made at different temperatures by comparing integrated band intensities. These data also enable evaluation of potential uncertainty in the mixture composition at low temperatures where vapor condensation or wall adsorption may be a problem or at high temperatures where thermal decomposition of the hydrocarbon may cause uncertainty.
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2.2. Spectral resolution FTIR resolution can be limited by several spectrometer components, including the iris and the interferometer [19]. A small iris provides a more collimated beam and enables higher wavelength resolution, but at the cost of reduced light throughput. The Michelson interferometer has one fixed and one moving mirror. The distance traveled by the moving mirror affects the instrument resolution. If the mirror moves farther, resolution is increased at the expense of longer test times and lower SNR. To insure accurate cross section measurements, it is necessary to establish that the instrument resolution is sufficient to resolve the structure of the absorption features. The instrument resolution can be verified by measuring an absorption feature that is spectrally narrow compared to the instrument resolution. For this purpose, we made measurements of optical absorption by a carbon monoxide mixture at low pressures (0.25% CO in N2 at 25 kPa). For these conditions, the linewidth of the CO absorption features is 0.028 cm1 (FWHM), whereas the minimum linewidth observed with our FTIR spectrometer was 0.09 cm1 (FWHM), thereby providing a measure of the maximum instrument resolution. After the instrument resolution was established, it was necessary to determine the minimum required resolution for the set of hydrocarbons studied. Because the experiment test time increases and SNR decreases when measuring at high resolution, it is desirable to make measurements at a low resolution. Conversely, sufficiently high resolution must be retained to insure the absorption spectrum is accurately measured. To determine the required resolution, each of the hydrocarbons was measured at low pressure (o20 Torr, 2.7 kPa) and at 0.1 and 1 cm1 resolution. An instrument resolution of 1 cm1 was found to be sufficient to resolve the spectral features of the hydrocarbons studied here, with the exception of propene, which required measurements at 0.1 cm1 resolution. Additionally, no pressure broadening was observed when these spectra were compared to the spectra measured at atmospheric pressure. 3. Absorption cross section measurements Absorption cross sections were measured for 12 hydrocarbon compounds in five structural classes (straight alkanes, branched alkanes, straight olefins, branched olefins, and aromatics). Table 1 lists each hydrocarbon, the spectral resolution employed, and the ranges of temperature and mole fraction studied. Comparison of the spectra of species within a class and across classes confirms the structural groups (i.e., CH2 or CH3 groups) that are responsible for absorption peaks at specific wavelengths. Interestingly, our data show that the absorption cross sections vary with temperature in a nearly linear manner at most wavelengths. The integrated band intensity is independent of temperature within the accuracy of our measurements, enabling useful comparisons with data measured at different temperatures. Good agreement is found when the current Table 1 Summary of hydrocarbons measured and range of measurement conditions Name
Formula
Molecular class
FTIR resolution (cm1)
Minimum T (1C)
Maximum T (1C)
Mole fraction (%)
n-Pentane n-Heptane n-Dodecane 2-Methyl-butane 2-Methyl-pentane 2,2,4-Trimethyl-pentane Propene 2-Methyl-2-butene 2,4,4-Trimethyl-1pentene Toluene m-Xylene Ethyl-benzene
C5H12 C7H16 C12H26 C5H12 C6H14 C8H18 C3H6 C5H10 C8H16
Straight alkane Straight alkane Straight alkane Branched alkane Branched alkane Branched alkane Straight olefin Branched olefin Branched olefin
1 1 1 1 1 1 0.1 1 1
27 26 51 27 28 26 27 27 27
502 501 501 503 502 501 502 501 451
0.4–1.3 0.3–1.1 0.07–0.1 0.2–1.1 0.4–1.3 0.3–1.6 1–6 0.06–2.3 0.3–1.3
C7H8 C8H10 C8H10
Aromatic Aromatic Aromatic
1 1 1
27 27 26
501 502 502
1–1.5 0.4–0.6 0.3–0.6
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temperature-dependent data are compared with other available broadband data at limited temperatures [15,16] and also with fixed-wavelength, temperature-dependent laser absorption measurements [17] of hydrocarbon/nitrogen mixtures. The three straight alkanes studied were n-pentane, n-heptane, and n-dodecane. n-Pentane is present in high concentrations in gasoline, n-heptane is an important component of gasoline and diesel surrogate mixtures, and n-dodecane is a common component of jet fuel surrogate mixtures. As noted in Table 1, each hydrocarbon was investigated at temperatures as high as 500 1C and similar temperature-dependent trends were observed for all of the species studied. The absorption cross section peaks decrease and spread out with increasing temperature, as expected. The absorption spectrum of n-pentane, measured at 50 1C, is shown in Fig. 2 and compared to the data measured by Sharpe et al. for the same temperature. Instrument resolution for our measurements was 1 cm1 and the cited data were measured with 0.1 cm1 resolution. The two spectra show good agreement, confirming that 1 cm1 resolution is sufficient for this measurement. When we compared our measured spectra to those of Sharpe et al. for the other species, we consistently found good agreement at all wavelengths within this absorption band. However, throughout the rest of this paper, we report only comparisons of the band intensity, calculated from Eq. (2), and integrated from 2500 to 3400 cm1 (see Table 2). The final column in Table 2 shows the percent difference between the two sets of data. Our integrated intensities agree well with the data from Sharpe et al. and are within the combined uncertainties of the two studies. The absorption spectra of n-pentane, n-heptane, and n-dodecane are shown in Fig. 3(a–c), respectively. Computed three-dimensional (3-D) geometries are also included to illustrate the influence of molecular structure on the absorption spectrum. The dark (red) spheres represent the hydrogen atoms and the light (yellow) spheres represent the carbon atoms. Details of the 3-D modeling can be found in Ref. [20]. The absorption spectrum of n-pentane was measured at nine discrete temperatures ranging from 27 to 502 1C with an estimated cross section uncertainty of 2%. In the experiments at 500 1C, it was noted that the peak absorbance decreased steadily with time after the cell was filled with the mixture. Methane absorption peaks were subsequently observed in the spectrum. This behavior was interpreted as thermal decomposition and, by noting the change in peak absorbance with time, the decomposition rate was estimated to be 2%/min. By averaging fewer scans (10 scans rather than 100), test times were reduced to 15 s for this hightemperature condition, and decomposition was limited to less than 0.5%. Absorption peaks for n-pentane are located at 2881, 2940, and 2966 cm1. The spectrally sharp absorption features exhibited near room temperature are smoothed out at high temperatures. At higher temperatures, the cross section decreases at the peaks and increases away from the peaks as the features spread. This high-temperature spectral broadening of the C–H stretch bands was evident for all of the hydrocarbons studied. The band intensity (i.e., the cross
Cross Section [cm2/mole]
5x105
Sharpe et al. This Work
4x105 3x105 2x105 1x105 0x105 2800
2900
3000
3100
Frequency [cm-1] Fig. 2. Measured absorption spectrum of n-pentane at 50 1C compared to data from Sharpe et al. at 50 1C [15].
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Table 2 Temperature-averaged integrated band intensities for C–H vibration (integrated from 2500 to 3400 cm1) and measurement uncertainties for the hydrocarbons studied here compared to data from Sharpe et al. [15] Current study
n-Pentane n-Heptane n-Dodecane 2-Methyl-butane 2-Methyl-pentane 2,2,4-Trimethylpentane Propene 2-Methyl-2-butene 2,4,4-Trimethyl-1pentene Toluene m-Xylene Ethyl-benzene
Cross Section [cm2/mole]
6x105 5x105
Sharpe et al. Uncertainty (%)
Integrated area (1 106 cm1 cm2 mo1l)
Uncertainty (%)
39.0 51.7 83.3 37.0 43.3 53.7
2 2 6 3 2 2
39.6 51.7 85.7 37.1 43.8 53.6
3 3 7 3 3 3
1.6 0.0 2.8 0.4 1.1 0.1
10.0 24.7 39.8
2 2 2
10.3 25.0 40.3
3 3 3
2.3 1.0 1.4
13.2 18.5 19.4
2 5 5
13.2 19.0 20.2
3 7 7
0.5 2.5 4.1
7x105
T=27°C T=251°C T=502°C
4x105 3x105 2x105 1x105 0x105 2800
Difference (%)
Integrated area (1 106 cm1 cm2 mol1)
Cross Section [cm2/mole]
Name
2850
2900
2950
Frequency
3000
3100
T=26°C T=251°C T=501°C
5x105 4x105 3x105 2x105 1x105 0x105 2800
2850
2900
[cm-1]
1.4x106 Cross Section [cm2/mole]
3050
6x105
1.2x106
2950
Frequency
T=51°C T=251°C T=501°C
3000
3050
3100
[cm-1]
c
1.0x106 0.8x106 0.6x106 0.4x106 0.2x106 0.0x106 2800
2850
2900
2950
3000
3050
3100
Frequency [cm-1] Fig. 3. Measured absorption spectrum of (a) n-pentane, (b) n-heptane, and (c) n-dodecane from 2750 to 3100 cm1 at discrete temperatures (25oTo501 1C). Instrument resolution for these measurements was 1 cm1. Triangle markers in (b) indicate selected frequencies from Fig. 5.
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section integrated from 2500 to 3400 cm1) was found to vary less than 72% over the entire temperature range. The integrated band intensity results for all the species studied are listed in Table 2. Note that, for each species, the total variation in measured band intensity with temperature was less than the estimated uncertainty of the individual measurements. Thus, the band intensity values listed in Table 2 are average values for all of the temperatures measured. These data are compared to the measurements of Sharpe et al. which were typically measured at 5, 25, and 50 1C and were averaged over all of the available temperatures for this comparison. The average integrated band intensity for n-pentane in the present work is within 1.6% of the value reported by Sharpe. The absorption spectrum of n-heptane was measured as a function of temperature from 25 to 500 1C in 501 increments. Three sample spectra are shown in Fig. 3(b) for temperatures of 26, 250, and 501 1C. Absorption peaks for n-heptane are located at 2874–2882, 2936, and 2968 cm1. The measured band intensity, integrated from 2500 to 3400 cm1, was observed to be constant within 72% between 25 and 500 1C. The average band intensity agrees very well with the Sharpe data (measured from 5 to 50 1C). Additionally, the FTIR-measured cross sections at 2948 cm1 are compared to our previous laser absorption measurements at 2947.9 cm1 [17] in Fig. 4 for temperatures ranging from 25 to 500 1C. The agreement is well within the estimated uncertainty of the FTIR measurements (2%) over the temperature range of the measurements, providing confidence in the temperature-dependence of our spectral cross section measurements. While Fig. 3 shows trends in the absorption spectrum as temperature increases, it is interesting to examine the variation in cross section with temperature at fixed optical frequency (or wavelength). Fig. 5 shows the temperature dependence of the absorption cross section for n-heptane at five selected frequencies. (These frequencies are also marked in Fig. 3 for reference.) These data reveal that, at all of these frequencies, the variation in absorption cross section is approximately linear with temperature over the temperature region investigated. Furthermore, this linear variation was observed at most frequencies for all of the hydrocarbons studied here, with the exception of propene. By employing this approximation, the absorption cross section at an arbitrary temperature may be estimated from a limited database. For species with cross sections that do not vary linearly with temperature, one might consider approximating the temperature dependence with a more complex equation, such as the one described in Ref. [21]. The absorption spectrum of n-dodecane was measured at eight temperatures between 50 and 501 1C. The minimum temperature of the measurement was limited by low-vapor-pressure constraints below 50 1C. At 501 1C, thermal decomposition was evident. Thus, fewer scans were averaged, limiting the measurement time to 15 s, over which time decomposition was negligible. Spectra at selected temperatures are displayed in Fig. 3(c). Again, the integrated band intensity was found to be consistent with data from the literature within the estimated uncertainty of our measurements.
Cross Section [cm2/mole]
5.0x105
HeNe laser at 2947.9 cm-1 FTIR at 2948 cm-1
4.8x105 4.6x105 4.4x105 4.2x105 4.0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 4. Comparison of the measured n-heptane absorption cross section using FTIR data at 2948 cm1and helium–neon laser absorption data at 2947.9 cm1 [17]. Note the suppressed zero on the y-axis.
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Cross Section [cm2/mole]
8x105
2969 cm-1
2956 cm-1
2901 cm-1
2881 cm-1
415
2935 cm-1
6x105
4x105
2x105
0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 5. Temperature-dependent absorption cross sections of n-heptane at specific frequencies.
Cross Section [cm2/mole]
8.0x105
HeNe laser at 2947.9 cm-1 FTIR at 2948 cm-1
7.5x105 7.0x105 6.5x105 6.0x105 5.5x105 5.0x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 6. Temperature-dependent absorption cross sections of n-dodecane using FTIR data at 2948 cm1and helium–neon laser absorption data at 2947.9 cm1 [17]. Note the suppressed zero on the y-axis.
FTIR measurements of the n-dodecane cross section at 2948 cm1 are compared with laser absorption data in Fig. 6, revealing that the HeNe data are 15% lower than the FTIR measurements. We attribute this discrepancy to the extremely low-vapor-pressure of dodecane (0.1 Torr at 296 K), which poses difficulty in diluting the mixture without causing some condensation in the mixing tank. We have recently found evidence that suggests that some dodecane was condensing from the mixture during the original HeNe measurements. However, by diluting the mixture very slowly (diluted to 1:1000 dodecane:nitrogen in 10 min rather than the usual 1 min), we were able to minimize condensation for the current FTIR measurements. These FTIR data are in good agreement with PNNL data that have been recently obtained using a special procedure [22,23]. The good agreement between the two laboratories using two different procedures implies that condensation was not likely for these measurements. We also tested our modified dilution method with m-xylene, the species with the next lowest vapor pressure from our group of 12 hydrocarbons. Here we found good agreement in the spectra using both our initial and the modified dilution methods. These findings suggest that the 10 min dilution procedure is only required for gases with a vapor pressure less than that of m-xylene (8 Torr at 298 K). For n-dodecane, peaks are located at 2866, 2933, and 2970 cm1. These peak locations are close to those of n-pentane and n-heptane. It should also be noted that the peak at 2970 cm1 has a similar cross section
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magnitude for n-pentane, n-heptane, and n-dodecane, which suggests that these peaks are caused by the two CH3 groups that are common to each of these molecules. The peaks at 2933 and 2866 cm1 have significantly higher cross sections for n-dodecane than for n-heptane or n-pentane. Because n-dodecane has 10 CH2 groups, n-heptane has five CH2 groups, and n-pentane has three CH2 groups, we can infer that these peaks are sensitive to absorption by the CH2 groups. These interpretations are consistent with data from the Handbook of Spectroscopy [24] which reports resonant frequencies for CH3 groups ranging from 2875 to 2925 cm1 and 2950 to 3000 cm1 and resonant frequencies for CH2 groups ranging from 2850 to 2900 cm1 and 2925 to 2975 cm1. The absorption spectra of three branched alkanes were also measured at discrete temperatures. Sample spectra for 2-methyl-butane, 2-methyl-pentane, and 2,2,4-trimethyl-pentane are shown in Fig. 7(a–c), respectively. Note that 2-methyl-butane is present in high concentrations in gasoline exhaust [25], and that 2methyl-pentane is a significant component of gasoline [26]. 2,2,4-Trimethyl-pentane is present in gasoline and is used extensively in gasoline surrogate mixtures. For 2-methyl-butane, there are two absorption peaks located at 2890 and 2965 cm1, for 2-methylpentane the two primary absorption peaks are located at 2885 and 2965 cm1, and for 2,2,4-trimethyl-pentane the peaks are located at 2883, 2912, and 2965 cm1. 2-Methyl-butane has three CH3 groups, one CH2 group, and one CH group. 2-Methyl-pentane has three CH3 groups, two CH2 groups, and one CH group. 2,2,4Trimethyl-pentane has five CH3 groups, one CH2 group, and one CH group. Comparing the spectra of 2,2,4trimethyl-pentane with those of 2-methyl-butane, 2-methyl-pentane, and n-pentane, we can see that the peak at 2965 cm1 scales with the number of CH3 groups.
6x105
7x105
T = 27°C T = 252°C T = 503°C
Cross Section[cm2/mole]
Cross Section [cm2/mole]
7x105
5x105 4x105 3x105 2x105 1x105 0x105 2800
2850
2900
2950
3000
3050
3100
6x105 5x105 4x105 3x105 2x105 1x105 0x105 2800
Frequency [cm-1]
Cross Section [cm2/mole]
1.2x106 1.0x106
T = 28°C T = 252°C T = 502°C
2850
2900
2950
3000
3050
3100
Frequency [cm-1]
T= 26°C T= 251°C T = 501°C
0.8x106 0.6x106 0.4x106 0.2x106 0.0x106 2800
2900
3000
3100
Frequency [cm-1] Fig. 7. Measured absorption spectra at discrete temperatures for (a) 2-methyl-butane, (b) 2-methyl-pentane, and (c) 2,2,4-trimethylpentane.
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Cross Section [cm2/mole]
5.4x105
417
HeNe at 2947.9cm-1 FTIR at 2948 cm-1
5.2x105 5.0x105 4.8x105 4.6x105 4.4x105 0
100
200 300 400 Temperature [°C]
500
600
Fig. 8. Temperature-dependent absorption cross sections of 2,2,4-trimethyl-pentane using FTIR data at 2948 cm1 and helium–neon laser absorption data at 2947.9 cm1 [17]. Note the suppressed zero on the y-axis.
For these three hydrocarbons, we find excellent agreement between our band intensity data and the data available in the literature (see Table 2). Note that our measurements were made at temperatures ranging from 25 to 500 1C, while most previous spectral data were limited to temperatures below 50 1C. Furthermore, we find good agreement between our FTIR measurements of 2,2,4-trimethyl-pentane at 2948 cm1 and our laser absorption data for the same molecule, which are compared over a wide temperature range in Fig. 8. Temperature-dependent absorption measurements were also conducted for the aromatics toluene, m-xylene, and ethylbenzene, and sample spectra are shown in Fig. 9(a–c). These three aromatics are all present in gasoline, and in addition, toluene is included in many gasoline surrogates. The absorption spectrum of toluene was measured from 25 to 500 1C. The structure of toluene is that of a benzene ring with one CH3 group attached to one of the ring carbons. For toluene, the peaks at 2884 and 2940 cm1 are much smaller than the corresponding peaks of the alkanes because toluene only has one CH3 group and does not have any CH2 groups. The additional peaks at 3040 and 3073 cm1 are due to the C–H bonds on the benzene ring [24]. m-Xylene is a benzene ring with a CH3 group on the first and third carbon atoms of the benzene ring. Thus, the absorption peaks of m-xylene at 2882 and 2938 cm1 are approximately twice as strong as those of toluene because there are two CH3 groups. Likewise, the absorption features at 3033 and 3069 cm1, which are caused by the C–H bonds attached directly to the aromatic ring, are approximately the same intensity for m-xylene and toluene. Comparing our band intensity data to those of Sharpe et al., we find that the data agree within 5%, which is within the estimated uncertainty of the two sets of measurements. Ethylbenzene, the third aromatic studied here, is a benzene ring with a CH2–CH3 chain attached to one of the ring carbons. Ethylbenzene has multiple peaks located at 2886, 2942, 2975, 3039, and 3075 cm1. The peak at 2886 cm1 can be associated with the CH3 group, and the two peaks at 3039 and 3075 cm1 can be associated with the ring CH vibrations. The absorption spectra of two branched olefins (2-methyl-2-butene and 2,4,4-trimethyl-1-pentene) and one straight olefin (propene) were measured at discrete temperatures ranging from 25 to 500 1C. The spectra of 2-methyl-2-butene and 2,4,4-trimethyl-1-pentene provide useful sample spectra for branched olefins, and these hydrocarbons are also important because they are present in small quantities in gasoline. Furthermore, 2-methyl-2-butene can be found in moderate concentrations in the exhaust gas of internal combustion engines [25]. Propylene is an important intermediate combustion species in many hydrocarbon reactions and is present in high concentrations in the unburned hydrocarbons of gasoline engines [25]. The absorption spectra of these hydrocarbons are shown in Fig. 10(a–c). Note that the numerous, spectrally narrow peaks in the absorption spectrum of propene spread out and the spectrum transforms into one continuous feature at high temperatures. The absorption spectra of 2-methyl-2-butene and 2,4,4-trimethyl-1-pentene are also similar to those of the other hydrocarbons. 2-Methyl-2-butene has peaks located at 2887, 2938, and 2988 cm1, while
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Cross Section [cm2/mole]
1.4x105 1.2x105
2.0x105
T = 27°C T= 251°C T = 501°C
Cross Section[cm2/mole]
418
1.0x105 0.8x105 0.6x105 0.4x105 0.2x105 0.0x105 2800
2900
3000
3100
3200
T = 27°C T = 251°C T = 502°C
1.5x105
1.0x105
0.5x105
0.0x105 2800
Frequency [cm-1]
Cross Section [cm2/mole]
3.0x105 2.5x105
3000
2900
3100
3200
Frequency [cm-1]
T = 26°C T = 251°C T = 502°C
2.0x105 1.5x105 1.0x105 0.5x105 0.0x105 2800
2900
3000 Frequency [cm
3100
3200
-1]
Fig. 9. Measured absorption spectra for (a) toluene, (b) m-xylene, and (c) ethylbenzene at select temperatures.
2,4,4-trimethyl-1-pentene has peaks at 2879, 2920, 2967, and 3082 cm1. Band intensities for these two measurements are in good agreement with the data from the literature, as shown in Table 2. The narrow absorption features of propene required the use of 0.1 cm1 spectrometer resolution to faithfully recover the spectrum. Table 2 shows good agreement between our data and those of Sharpe et al. Propene is one of the few hydrocarbons with previously reported high-temperature spectra, and our data agree well with the average band intensity value of 10.5 106 cm1 cm2 mol1 [16] reported by Brosmer and Tien for measurements made at 17, 277, and 527 1C. 4. Summary Infrared spectra were measured for 12 hydrocarbons at temperatures ranging from 25 to 500 1C. The hydrocarbons were selected because of their importance in research related to practical fuels such as gasoline and jet fuels. The stable combustion intermediate propene is also included because of its importance in hydrocarbon chemistry. Uncertainties in the measurements range from 2% to 8%, depending on the vapor pressure of the species. We found that the mid-IR spectra of all of the species become broadened at high temperatures, but the integrated band intensity is insensitive to temperature. It was also shown that our measurements of integrated band intensity agree well with previous measurements near room temperature. Furthermore, we noted that the absorption cross sections at most frequencies are nearly linear with temperature. These data will be useful for mid-IR spectral analysis of gaseous mixtures containing hydrocarbons and will provide guidance for wavelength optimization of laser-based absorption diagnostics for hydrocarbons.
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3.0x105
T = 27°C T = 251°C T = 502°C
1.0x105
Cross Section [cm2/mole]
Cross Section[cm2/mole]
1.2x105
0.8x105 0.6x105 0.4x105 0.2x105 0.0x105 2800
2900
3000
3100
3200
2.5x105
T = 27°C T = 251°C T = 502°C
2.0x105 1.5x105 1.0x105 0.5x105 0.0x105 2800
2850
Frequency [cm-1]
Cross Section[cm2/mole]
6x105 5x105
419
2900
2950
3000
3050
3100
Frequency [cm-1]
T = 27°C T = 250°C T = 451°C
4x105 3x105 2x105 1x105 0x105 2800
2850
2900
2950
Frequency
3000
3050
3100
[cm-1]
Fig. 10. Temperature-dependent absorption cross sections for (a) propene, (b) 2-methyl-2-butene, and (c) 2,4,4-trimethyl-1-pentene.
Acknowledgments Support for this work was provided by the Air Force Office of Scientific Research with Julian Tishkoff as technical monitor, the Army Research Office with Ralph A. Anthenien Jr. as program manager, and by Nissan Motor Company, Ltd. References [1] Sanders ST, Baldwin JA, Jenkins TP, Baer DS, Hanson RK. Diode-laser sensor for monitoring multiple combustion parameters in pulse detonation engines. Proc Combust Inst 2000;28(1):587–93. [2] Philippe LC, Hanson RK. Laser diode wavelength-modulation spectroscopy for simultaneous measurement of temperature, pressure, and velocity in shock-heated oxygen flows. Appl Opt 1993;32(30):6090–103. [3] Jeffries JB, Schulz C, Mattison DW, Oehlschlaeger MA, Bessler WG, Lee T, et al. UV absorption of CO2 for temperature diagnostics of hydrocarbon combustion applications. Proc Combust Inst 2005;30(1):1591–9. [4] Tomita E, Kawahara N, Nishiyama A, Shigenaga M. In situ measurement of hydrocarbon fuel concentration near a spark plug in an engine cylinder using the 3.392 mm infrared laser absorption method: application to an actual engine. Meas Sci Technol 2003;14(8):1357–63. [5] Klingbeil AE, Jeffries JB, Hanson RK, Hinckley K, Dean A. 3.39 mm laser absorption sensor for ethylene and propane measurements in a pulse detonation engine. In: 44th AIAA aerospace sciences meeting and exhibit, Reno, Nevada; 2006 [Paper AIAA-2006-925]. [6] Klingbeil AE, Jeffries JB, Hanson RK. Tunable mid-IR laser absorption sensor for time-resolved hydrocarbon fuel measurements. Proc Combust Inst 2007;31(1):807–15. [7] Kettering CF, Sleator WW. Infrared absorption spectra of certain organic compounds, including principle types present in gasoline. Physics 1933;4:39–49.
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