Analysis of Petroleum Products and Their Mixtures by Using Terahertz Time Domain Spectroscopy

Transcription

1 Journal of the Korean Physical Society, Vol. 53, No. 4, October 2008, pp Analysis of Petroleum Products and Their Mixtures by Using Terahertz Time Domain Spectroscopy Yun-Sik Jin, Geun-Ju Kim, Chae-Hwa Shon, Seok-Gy Jeon and Jung-Il Kim Terahertz Wave Research Laboratory, Korea Electrotechnology Research Institute, Ansan (Received 26 March 2008) We report on the feasibility of terahertz time domain spectroscopy (THz-TDS) for discriminating regular petroleum products and their mixtures. Basic optical properties of gasoline, diesel, kerosene and organic solvents, such as xylene, toluene and benzene, used as additives in producing a similar gasoline were analyzed in the spectral range of THz. A quantitative analysis on the mixtures of gasoline and the additives clearly shows that the optical properties of the mixtures dier from that of regular petroleum products in the THz spectral range. Furthermore, a variation in the mixing ratio results in systematic changes in the refractive index and the dielectric constant, indicating that THz-TDS is an eective means to analyze gasoline and its mixtures. PACS numbers: Re, f, p, Ci Keywords: Terahertz, Time-domain spectroscopy, Petroleum, Solvent, Liquid, Refractive index, Dielectric constant I. INTRODUCTION The price of petroleum products has increased 3 times over the past 5 years [1] and the nancial burden of an automobile driver for fuel is increasing every year. Under these circumstances, various kinds of illegal gasoline products and their mixtures have appeared at reduced prices. The similar gasoline is usually produced by mixing gasoline with organic solvents, such as xylene, toluene and benzene and a mixture of diesel and kerosene is also produced as an illegal fuel. In addition, a kind of gasoline substitute by the name of Cenox, which is made from a mixture of thinner, toluene and methanol, is another type of illegal mixed fuel for automobile. Irregular burning of such gasoline products causes a decrease in power and an increase in abnormal abrasion of automobile engine; furthermore, imperfect combustion of such gasoline products exhausts a variety of carcinogenic substances into the atmosphere. Therefore, an eective detection technique to identify illegal gasoline products and their mixtures should be provided. Pulsed THz time-domain spectroscopy (THz-TDS) is a coherent technique, in which both the amplitude and the phase of a THz pulse are measured. Since THz-TDS for water vapor was rst demonstrated by Exter et al. in 1989 [2], THz-TDS has been employed to investigate a wide variety of materials, including liquids [3], semiconductors [4{6], superconductors [7], explosive materials [8] and gases [9]. Al-Dousery et al. measured the optical ysjin@keri.re.kr; Fax: properties of petroleum products in the THz and farinfrared regions by using THz-TDS and Fourier transform infrared spectroscopy (FTIR) [10, 11] and showed the feasibility of using THz-TDS to discriminate between the regular petroleum products. However, if the dierence between petroleum products and their mixed fuels is to be classied, a systematic analysis of the properties of gasoline mixtures is crucial. In this research, we investigated the potential of THz- TDS as a quantitative analysis method for petroleum products and their mixtures with organic solvents such as xylene, toluene and benzene. The dierences in the optical properties, such as the refractive index, the absorption coecient and the dielectric constant, in the spectral range of THz allow us to identify the various petroleum products and their mixtures. II. EXPERIMENT 1. Sample Preparation and Calculations of Parameters The cell structure for containing the liquid samples is illustrated in Figure 1. The brass spacer is covered by 2-mm-thick poly-tetrauorethylene (PFTE; teon) windows at both sides. PFTE has a very low absorption coecient of less than 2.5 cm 1 and a constant reective index (n = 1.445) in the THz spectral range, which allows a higher transmission of THz radiation through the cell [12]. The cell diameter is 20 mm with a thickness of

2 Journal of the Korean Physical Society, Vol. 53, No. 4, October 2008 Fig. 1. Cell structure for containing the petroleum products and their mixtures. The brass spacer is covered by 2-mm-thick PFTE windows on both sides. Fig. 2. Schematics and notations for the transmission geometry of THz waves passing through an empty cell and a sample expressed as the n1 and n3, respectively, in the central area. t ij is the Fresnel transmission coecient. mm. The analysis of time-domain THz pulses was done by using the dierence between a THz reference pulse transmitted through the empty cell and a sample pulse transmitted through the same cell lled with the liquid under investigation. The optical constants are obtained by comparing the Fourier transforms of the sample pulse and the reference pulse. A schematic representation of a THz wave passing through the empty cell and sample is shown in Figure 2. The experimentally measured spectral components of the reference pulse and the sample pulse can be expressed as [13] ~S r (!) = C(!) t 12 p 2 t 21 p 1 t 12 p 2 t 12 ; ~S s (!) = C(!) t 12 p 2 t 23 p 3 t 32 p 2 t 12 ; (1) where t ij is the Fresnel transmission coecient expressed as t ij = 2ni n i+n j, p i is a factor representing propagation of radiation through a medium of refractive index ~n i and distance d, expressed as p i = e j ~ni!d=c0 and c 0 is speed of light in vacuum. The ratio of transmission spectra can be represented by ~S s (!) ~S r (!) = t 23 p 3 t 32 t 21 p 1 t 12 = ~n 3 ~n 1 ( ~n 1 + ~n 2 ) 2 ( ~n 2 + ~n 3 ) 2 e j( ~n3 ~n1)!d=c0 : (2) Using ~n 1 = 1:0 for air and ~n 2 n 2 = 1:445 for PFTE, ~n 3 = n 3 + j 3 can be calculated by using the observed spectra. In the calculation, n 3 3 is assumed because petroleum products in a liquid phase can be considered to be non-polar liquids with very low absorption: ~S s (!) j(!) = (!) e ~S r (!) (1 + n 2 ) 2 = n 3 (n 2 + n 3 ) 2 e 3!d=c0 e j(n3 1)!d=c0 : (3) From Eq. (3), ~n 3 = n 3 + j 3 is calculated and power absorption coecient of (!) and the complex dielectric constants of ~"(!) = " 0 (!) j""(!) can be derived by using the following equations: (!) = 2(!)!=c 0 ; (4) ~"(!) = [~n(!)] 2 : (5) 2. THz Time-Domain Spectroscopy System The transmission THz time-domain spectroscopic system for analysis is shown in Figure 3. The laser source is a commercial mode-locked Ti:sapphire laser (Spectra Physics Mai Tai) producing 100 fs pulses at 800 nm with a repetition rate of 80 MHz and an average power of 800 mw. The fs laser beam was divided into two beams, one for exciting the emitter antenna and the other for measuring the THz signal at the detector antenna. The

3 Analysis of Petroleum Products and Their Mixtures by { Yun-Sik Jin et al Fig. 3. Schematic diagram of the transmission mode THz-TDS system. The cell structure for containing the petroleum products and their mixtures as the sample is placed in the path of the THz wave. Fig. 4. (a) Measured THz pulses of reference and liquid samples. For clarity, the sample pulses are vertically displaced. (b) Corresponding amplitude spectra of the measured reference pulses and the sample pulses. THz emitter is a coplanar strip line antenna fabricated on a semi-insulating (SI) GaAs wafer. The gap distance of the emitter antenna is 100 m and is excited with a 10-mW focused laser beam. The bias voltage between the coplanar strip lines of the emitter antenna is 45 V (E bias = 4.5 kv/cm). An aplanatic hyper-hemispherical silicon lens is attached to the back of the THz emitter to increase the coupling eciency of the THz pulse from the antenna to free space. Four o-axis parabolic mirrors are used for guiding and focusing the THz beam to samples with a spot size of less than 2 mm diameter. The cell for the liquid sample is located at the focal point of THz beam. The detector is a photoconductive dipole antenna fabricated on a LT-GaAs wafer with a gap distance of 5 m. In order to prevent absorption by atmospheric water vapor, we enclose the THz generation and detection components in an acryl box supplied with dry air. The output signal from the detector antenna is monitored with a lock-in amplier and a computer. At each scan, 1024 data points, separated by 0.04 ps, were collected with a time constant of 100 ms. III. RESULTS AND DISCUSSION 1. Measurement of Pure Petroleum Products and Organic Solvents In the present study, ve kinds of petroleum products (gasoline, diesel, kerosene, thinner and cenox) and three

4 Journal of the Korean Physical Society, Vol. 53, No. 4, October 2008 Fig. 5. (a) Refractive index, (b) absorption coecient and (c) dielectric constant of the petroleum products as functions of the frequency. kinds of organic solvents (benzene, toluene and xylene) were used as liquid samples. The THz reference pulse transmitted through the empty cell and the sample pulse transmitted through the same cell lled with the liquid were recorded in the time domain and the corresponding frequency spectra were obtained by using numerical Fourier transforms. The measured reference and sample (gasoline, toluene) pulses and the corresponding Fourier transformed spectra are shown in Figures 4 (a) and (b). Every pulse curve is the average of two individual measurements in order to increase the signal-to-noise ratio (SNR) and the SNR of the THz-TDS system was estimated to be higher than For all liquid samples, no absorption peak appeared within the frequency range of 0.2 THz to 3 THz. The refractive index (n(!)) (frame a), the power absorption coecient ((!)) (frame b) and the real part of the dielectric function - dielectric constant (" 0 (!)) (frame c) of the pure petroleum products are shown in Figure 5. The refractive index n(!) and the dielectric constant " 0 (!) in all samples show small variations of less than 3 % in the spectral range of THz. The refractive indices at 1 THz are distributed from to with highest value for diesel and the lowest value for thinner. The absorption coecient of gasoline is higher than that of diesel or kerosene by ve times. Gasoline contains appreciable quantities of aromatic hydrocarbons, such as toluene, etylbenzene and xylene, as main ingredients ( 25 % in total), moderately absorbing THz wave owing to the high polarizability of electrons in aromatic orbitals [14]. The clear dierences in the refractive index and the absorption coecient imply that it is possible to identify several kinds of petroleum products by using THz-TDS. Especially, gasoline and cenox, which is a kind of gasoline substitute, can be distinguished clearly in terms of their absorption properties. Furthermore, it is possible to maintain the quality of petroleum products by monitoring the refractive index and the absorption properties in the THz spectral range. Figure 6 shows the optical properties for the organic solvent samples. On the whole, the refractive indices and the dielectric constants of the organic solvents have higher values than those of the petroleum products. For example, the refractive indices of benzene (1.502) and toluene (1.497) are separated from gasoline (1.407) or diesel (1.455) by more than The dierence of 0.04 in the refractive index is large enough to be detected by the THz-TDS system. However, the power absorption

5 Analysis of Petroleum Products and Their Mixtures by { Yun-Sik Jin et al Fig. 6. (a) Refractive index, (b) absorption coecient and (c) dielectric constant of the organic solvent samples analyzed in the spectral range of THz. organic solvents. The absolute value of the dielectric constants " 0 (!) for toluene and benzene are in good agreement with those of Rnne et al. at room temperature [15], which conrms the accuracy of our measurements. 2. Measurement of Petroleum Products and Their Mixtures with Organic Solvents Fig. 7. Comparison of the refractive index between kerosene, diesel and a 50 : 50 mixture of kerosene and diesel in the spectral range of THz. coecient of the two liquid groups shows very similar variation patterns. From these observations, we can say that the refractive index and the dielectric constant are good parameters to distinguish petroleum products from Using the individual optical properties of the petroleum products and the organic solvents, we performed the quantitative analyses on mixtures. First, mixtures of diesel and kerosene were analyzed. Kerosene is frequently mixed with diesel to make an illegal fuel for diesel engines. As shown in Figure 7, the refractive index of the mixture has an intermediate value of the two component liquids (n Kerosene = 1:438, n Diesel = 1:455, n Kero:+Die: = 1 THz). The dielectric constant of the mixture showed a similar variation as that of the refractive index. The results indicate that THz-TDS is an eective tool to discriminate pure fuel from a mixture with other type of petroleum products. Another type of illegal fuel is a mixture of gasoline and organic solvents. Benzene and toluene as organic

6 Journal of the Korean Physical Society, Vol. 53, No. 4, October 2008 Fig. 8. Comparison of the refractive index between gasoline, toluene, benzene, a 50 : 50 mixture of gasoline and toluene and a 50 : 50 mixture of gasoline and benzene in the spectral range of THz. solvents were mixed with gasoline at the ratio of 50 : 50, respectively. A comparison of refractive index between the pure liquids and the mixed ones is shown in Figure 8. The refractive index of the mixtures was almost equal to an average of the refractive indices of the component liquids (n Gas:+T ol: = 1:452, (n Gas: + n T ol: )=2 = 1:452, n Gas:+Ben: = 1:453, (n Gas: +n Ben: )=2 = 1 THz). The dielectric constant of the mixture was also measured to be equal to an average of each component liquid within a 0.2 % error (" 0 Gas:+T ol: = 2:110, (" 0 Gas: + "0 T ol: )=2 = 2:111, " 0 Gas:+Ben: = 2:114, ("0 Gas: + "0 Ben: )=2 = 1 THz). Based on the measurement results for the 50 : 50 mixtures, more quantitative analysis for dierent mixing ratios was carried out. The refractive indices and the dielectric constants of mixtures of toluene and gasoline were measured, where the toluene content was varied from 0 to 100 %. As shown in Figure 9, both the refractive index and the dielectric constant changed linearly with toluene content. The absolute values of the refractive index and the dielectric constant can be represented by two linear equations: n mix = 1: x and " 0 mix = 1: : x, where x is volume % of toluene. The linear variations of the optical constants indicate that THz-TDS is a strong quantitative, nondestructive and contactless method for analyzing mixtures of petroleum products and solvents. The liquid samples studied in this study do not have a specic absorption peak or ngerprint in the frequency region between 0.1 to 2.5 THz, as shown in Figure 4(b). Gasoline and xylene have absorption peaks at frequencies above 4.5 THz [10], so it is not easy to dierentiate the exact components of mixtures with the present THz-TDS system when two or more unknown petroleum products and solvents are mixed. For a more complete measurement, it is desirable for the THz TDS to be used in combination with other complementary tools, such as Fig. 9. Variations of the refractive index and the dielectric constant as functions of the toluene content in mixtures of gasoline and toluene at a frequency of 1 THz. FTIR. If the components of the mixture are identied by FTIR, a quantitative analysis would be possible with THz-TDS based on the database of individual samples in the terahertz spectral range. IV. CONCLUSION In this research, the feasibility of using transmission THz -TDS to discriminate pure petroleum products from similar ones was investigated. Measurements of the refractive indices, the absorption coecients and the dielectric constants in the THz range revealed that pure petroleum product and pure organic solvents could be identied by using THz-TDS. A quantitative analysis on mixtures of gasoline and additives clearly showed a linear change in the refractive index and the dielectric constant with the mixing ratio. These results indicate that THz-TDS can be used for the identication and the discrimination of pure petroleum products, organic solvents and their mixtures. REFERENCES [1] http: // [2] M. van Exter, C. Fattinger and D. Grischkowsky, Opt. Lett. 14, 128 (1989). [3] T. Ikeda, A. Matsushita, M. Tatsuno, Y. Minami, M. Yamaguchi, K. Yamamoto, M. Tany and M. Hangyo, Appl. Phys. Lett. 87, (2005). [4] Tae-In Jeon and D. Grischkowsky, Appl. Phys. Lett. 72, 3032 (1998). [5] S. Nashima, O. Morikawa, K. Tanaka and M. Hangyo, J. Appl. Phys. 90, 837 (2001). [6] U. W. Kim, S. J. Oh, I. Maeng, C. Kang and J.-H. Son, J. Korean Phys. Soc. 50, 789 (2007).

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