Article pubs.acs.org/EF
Optical Characterization of the Principal Hydrocarbon Components in Natural Gas Using Terahertz Spectroscopy Li N. Ge,†,‡,∥ Hong L. Zhan,‡,§,∥ Wen X. Leng,§ Kun Zhao,*,§ and Li Z. Xiao† †
State Key Laboratory of Petroleum Resources and Prospecting and ‡Beijing Key Laboratory of Optical Detection Technology for Oil and Gas, China University of Petroleum, Beijing 102249, China § Key Laboratory of Oil and Gas Terahertz Spectroscopy and Photoelectric Detection, China Petroleum and Chemical Industry Federation (CPCIF), Beijing 100723, China ABSTRACT: A rapid technique is necessary to detect the natural gas which is a more and more significant fuel resource in modern industry. Terahertz (THz) technique was employed in this research to detect the principal hydrocarbon components of natural gas including methane, ethane, and propane. Two- and three-component mixtures were measured by THz setup, respectively. The amplitude ratio and time delay deviation of THz peaks between samples and reference were calculated. Phase projection pictures were obtained between the component concentrations and the amplitude ratio as well as time deviation. The phase figures evidently reflected the concentration dependent THz response, and a greatly different distribution was located in the whole phase projection area. In addition, back-propagation artificial neural networks method was utilized for the quantitative determination of components concentration and total pressure, and the correlation coefficient of the prediction set was proved to be 0.9859. Therefore, THz technique can satisfy the increasing need of rapid and efficient detection in the natural gas industry.
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INTRODUCTION Natural gas is an extremely vital fuel resource and will play a more and more significant role in worldwide industry because of the larger and larger energy consumption. As a fuel resource, natural gas was found to possess large reserves and great exploration values.1−3 In brief, natural gas is a complicated mixture mainly composed of hydrocarbons. The chief components of the hydrocarbon in natural gas include methane (CH4) along with ethane (C2H6) and propane (C3H8). To realize more efficient exploration and detection, a rapid technique is necessary to realize the identification of the principal hydrocarbon components in the natural gas industry.4,5 Terahertz (THz) spectroscopy is a newly developed spectral technique due to the rapid development of ultrashort pulse lasers, semiconductors, and optical detectors, which has received increasing attention in many fields in recent years. THz spectroscopy ranging from 0.1 to 10 THz bridges the gap between microwave and infrared spectroscopy. As a newly developed spectral technique, THz spectroscopy is a very promising method for natural gas detection because of unique properties. THz spectroscopy can provide rich intermolecular and intramolecular vibration modes and give the amplitude as well as phase information on the sample simultaneously. In addition, THz is little sensitive to thermal background radiation and the scattering effect in the gas, and scarcely causes any damage to the tested organic gas. Generally, the high signal-tonoise ratio (>1000) makes it an effective tool for both qualitative and quantitative method. THz time-domain spectroscopy (THz-TDS) is a normal and significant THz method based on the THz electric field with time resolution which is generated by a femtosecond laser pulse. After employing the fast Fourier transform, the frequency dependent spectra can be obtained. Some spectral features can be observed from the THz © 2015 American Chemical Society
spectra and can used as the standard to qualitatively and quantitatively determine the natural gas. THz-TDS can be used to characterize the principal components of natural gas in a simple measurement condition in that the samples do not need any pretreatment.6−12 Several reports were found to study the THz response of gas and gas mixtures.13−19 The humid air was mostly investigated and was observed to possess several absorption characteristic peaks in virtue of the water vapor in air.13−15 In addition, polar molecules reflected great absorption effect in THz range; in brief, nonpolar molecules has a relative small absorption of THz pulse.16−19 The research about the principal hydrocarbon ingredients of natural gas has been very significant in actual industry and has been found little using THz technique in previous work. In this research, the principal hydrocarbon components of natural gas, including CH4, C2H6, and C3H8, were qualitatively and quantitatively analyzed using THz-TDS. The two- and three-component mixtures were discussed, respectively. The two-component system, such as CH4 + C2H6, was compounded as follows: the gas cell was filled by CH4 in 1.0 atm and then C2H6 was introduced at the pressure intervals of 0.1 atm, so the pressure ratios of CH4/C2H6 in this system equaled 1:0, 1:0.1, 1:0.2, ..., 1:2. Other systems were obtained similarly. The twocomponent systems, such as CH4 + C2H6, C2H6 + CH4, C3H8 + CH4 and CH4 + C3H8, were initially measured; then, the THz-TDS of the three-component systems were also scanned with THz setup. The amplitude and the delay of samples and reference were extracted to calculate the amplitude ratio and the delay time deviation. The ratio and deviation were found to be related to the concentration of different components, and Received: December 16, 2014 Revised: February 28, 2015 Published: March 2, 2015 1622
DOI: 10.1021/ef5028235 Energy Fuels 2015, 29, 1622−1627
Article
Energy & Fuels the plotted phase projections between concentrations and ratio as well as deviation obviously reflected the concentration dependent THz response. Moreover, back-propagation artificial neural networks (BPANN) method was used to build a quantitative model between the THz spectra and concentrations as well as the total pressure of three-component system. These results indicate that THz is a promising tool for the qualitative and quantitative detection of natural gas.
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EXPERIMENTAL METHODS
As shown in Figure 1, the measurement system was built based on a commercial transmission THz-TDS setup with a multipass cell which
Figure 2. THz-TDS of CH4, C2H6, and C3H8 with the pressure of 1 atm.
components should be considered and quantitatively characterized in the two- or three-component system. As shown in Figure 3a, the reference spectra (black line, ref.) is initially measured, which indicates the THz field amplitude as a function of time after the transmission of the THz pulses through a gas cell with vacuum phenomenon (pressure < 0.0001 atm). The gas cell filled by CH4 in 1.0 atm is scanned as the first sample and then filled out with ethane at the pressure intervals of 0.1 atm so that 21 groups of the CH4 + C2H6 mixture are obtained and measured by the THz-TDS setup. Similarly, the C2H6 + CH4, C3H8 + CH4, and CH4 + C3H8 mixtures at the intervals of 0.1 atm are then scanned by THzTDS setup one by one. The THz-TDS of the four systems are depicted in Figure 3a−d, respectively. A special tendency is observed with regard to the time delays, which increase gradually with the increasing input of the second component in all mixture systems, such as C2H6 in the CH4 + C2H6 system, CH4 in the C2H6 + CH4 system, CH4 in the C3H8 + CH4 system, and C3H8 in the CH4 + C3H8 system. The spectral phenomenon also proves the stability of the setup performance. Depending on the system stability and measurement conditions, the three-component hydrocarbon system is also analyzed and discussed with THz technique in this research. The gas cell filled by CH4 in 0.6 atm is scanned as the first sample, and then filled out with ethane (0.6 atm) so that the volume ratio of CH4/C2H6 equals 1:1. C2H6 gas is continuously introduced into the cell at the pressure intervals of 0.1 atm. Finally, 20 samples are obtained, and every sample is scanned in the terahertz range. Similarly, another two systems of CH4, C2H6, and C3H8 mixtures, including C3H8 + C2H6 + CH4 and C2H6 + CH4 + C3H8, are manufactured with the different input order of gases. The THz-TDS of the three three-component systems were plotted in Figure 4a−c, respectively. Similar tendency is found among them that the time delays regularly change with the gradual input of the second and third components with the different pressures. It is noted that the waveforms of the samples are quite similar in Figures 3 and 4 with the input of new gas in all systems, but the THz responses seem different from each other. To discuss the THz responses of samples with the different components and concentrations, the amplitudes and time delays of the THz pulses are extracted and correlated with certain component concentrations. Here the amplitude ratio (Isample/Ireference) and delay deviation (Δt, Tsample − Treference) are used for the sake of removing the small differences of references caused by different
Figure 1. Sketch map of THz-TDS setup. has been discussed in our previous report. In brief, a mode-locked Tisapphire laser, whose central wavelength was 800 nm, was used as the source to generate and detect terahertz waves.20 The laser beam was split into pump and probe beams. The pump beam (∼100 mW) was focused onto the surface of a biased GaAs photoconductive antenna for terahertz generation and the probe beam for electrooptic detection. In this study, a 50 cm long multipass cell was designed and built so as to fit the default optical path.21 The cell was attached on the top of a vacuum chamber. By the insertion of two flat mirrors, the THz wave was reflected into the cell so that the total path length of the sample equaled 1 m. The cell was made of an airtight cylinder, and two vacuum ports were attached on the sidewall. The pressure in the cell was monitored by a capacitance manometer. The cell was sealed with O-rings and 10 mm thick polytetrafluoroethylene windows fixed on the entrance and the exit of the THz beam, which had little reflection at THz frequencies. The sample chamber surrounding the THz path is purged by a continuous flow of nitrogen gas to reduce the effect of water-vapor absorption. In this research, the principal hydrocarbon gases, including methane (CH4), ethane (C2H6), and propane (C3H8) with the purity of 99.95%, were used for THz measurement. A reference spectrum was first measured using the vacuum cell evacuated below 10−4 atm by a turbo molecular pump. The reference spectrum was obtained by the averaging of five scans. The concentration ratio of components in the mixtures system was calculated by controlling the pressure of each ingredient, displayed by the capacitance manometer. In this research, the THz spectra of samples were measured in common room condition. To avoid vapor absorption in air and enhance the signal−noise ratio, the setup, including the THz spectrometer and gas cell, was covered with dry nitrogen. The detailed measurement temperature and relative humidity can be tested by a sensor in the THz setup and displayed in the computer, and were 295.1 ± 0.4 K and 0−0.3% in the experiment, respectively.
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RESULTS AND DISCUSSION In the actual natural gas industry, both pressure and hydrocarbon components have been significant parameters to be characterized. It is noted that THz is not only sensitive to pressure effects but also to the components. Figure 2 showed the THz-TDS of the CH4, C2H6, and C3H8, all pressures of which were 1 atm. Therefore, both the pressure and 1623
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Figure 3. THz-TDS for two-component systems: (a) CH4 + C2H6; (b) C2H6 + CH4; (c) C3H8 + CH4; (d) CH4 + C3H8.
Figure 4. THz-TDS for three-component systems: (a) C3H8 + CH4 + C2H6; (b) C3H8 + C2H6 + CH4; (c) C2H6 + CH4 + C3H8.
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concentration, but this trend cannot remain when C3H8 is introduced as the last component. In this case, the amplitude ratio increases with the CH4 and C2H6 concentrations; on the contrary, it decreases with the increasing of the C3H8 content. However, an obvious phenomenon can be observed that Δt regularly changes with the input of all components, including CH4, C2H6, and C3H8 both in two- and three-component systems. Therefore, results based on Figure 5 indicate the probability of the THz technique for the identification of the principal components and their mixtures in the natural gas industry. Furthermore, in order to study the THz response of CH4, C2H6, and C3H8 systems with different concentrations, a phase diagram was employed to build a relationship between the THz parameter and components’ concentrations. As shown in Figure 6, the phase projection pictures of amplitude ratio (a) and delay deviation (b) were graphically described between samples and reference in the CH4 + C2H6 + C3H8 system, respectively. An interesting tendency was observed that the smallest amplitude ratio and the largest Δt were located in the same field of phase projection in Figure 6a,b. In this field, the C3H8 concentration approximated to 60% and both CH4 and C2H6 concentrations equaled about 20%. It can be concluded that with the input of C3H8, the THz response of the system gradually changed, which was related to the intrinsic nature of C3H8. C3H8 is an asymmetric top with C2ν symmetry, of which the dipole moment is small but strong enough to make the rotational spectrum be observed obviously. Consequently, C3H8 provided the torsional and rotational spectra in the terahertz region.22,23 For CH4, C2H6, and other nonpolar gases, the absorption coefficient α is too little to show any fingerprint spectra in the THz region, in spite of the increase of partial pressure. According to the rule of the phase diagram, the sum of the coordinates of any point should equal 1. The aim of Figure 6 is to correctly describe the relationship between the THz response and the hydrocarbon components. Actually, pressure is another significant parameter in actual natural gas industry and has influence on THz response. In this research, pressure was not a constant throughout the experiment and was quantitatively determined along with the components’ concentration by BPANN, which will be discussed later. Consequently, the amplitude ratio and Δt reflected a greatly different distribution in the whole phase projection area, indicating that the phase figures can be selected as the standard to identify or predict the classification and stability of gas mixtures. The results provided an appropriate suggestion for gas detection in the natural gas industry.
measurement. The left pictures of parts a−d of Figure 5 reflect the amplitude ratio as a function of the CH4 concentration in
Figure 5. Volume concentration of different gas component dependent amplitude ratio (left) and Δt (right): (a) CH4 proportion dependence of amplitude ratio (left) and Δt (right) in two-component systems; (b) CH4 proportion in three-component systems; (c) C2H6 proportion in three-component systems; (d) C3H8 proportion in three-component systems.
the two-component system and the CH4, C2H6, and C3H8 concentrations in the three-component system. Besides, the right pictures of parts a−d of Figure 5 indicate the Δt as a function of the same component concentration with that in the left pictures of parts a−d of Figure 5. According to the information in Figure 5, it can be observed that the amplitude ratios are basically unchanged with the increasing of certain
Figure 6. Phase projection picture of amplitude ratio (a) and delay deviation (b) between samples and reference in the CH4 + C2H6 + C3H8 system. 1625
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propagation. More interesting is that multiple variables of the target samples can be determined simultaneously so that it is very promising for detecting natural gas in the petroleum industry.24 In this research, BPANN was used to build a quantitative model between the THz technique and the gas properties with the input of THz-FDS over the range from 0.2 to 1.5 THz and without any spectral pretreatment. In order to efficiently determine the subsequently given spectra in the prediction set, the number in the training set should exceed that in the prediction set. Within all of the 60 samples, 15 groups were randomly selected as the prediction set and the remaining 45 samples were the training set. According to the significant response of both pressure and hydrocarbon components in the THz range, which were reflected in Figures 2−4, the pressure and components’ concentrations were characterized simultaneously. The quantitative results were depicted in Figure 8,
In addition, quantitative determination of component concentration and total pressure is always a necessary part to evaluate the quality and safety of gases in the natural gas industry. Also, synchronous characterization plays a very significant role to improve the detection efficiency because multiple properties are often difficult to be detected simultaneously. To realize the synchronous determination of the total pressure as well as the concentration of components, including CH4, C2H6, and C3H8, BPANN was employed with the input of THz frequency domain spectra (THz-FDS), which was calculated from the THz-TDS of the three-component system in Figure 4 after the application of fast Fourier transform. Figure 7 showed the THz-FDS of selected samples
Figure 7. THz-FDS of the four selected samples in three-component systems.
in the three-component system, and the concentration of the components was also displayed in this figure. It is noted that the difference can be obviously observed in frequency dependent THz spectra among the mixtures with different pressure and components concentration. The THz response reflected in Figure 7 was in good agreement with that in Figures 2−4. In addition, BPANN, a mathematical nonlinear dynamics system simulating structure and function of biological neural networks in the human brain, was also used. No priori models were required for ANN owing to capturing the inherent information from the considered variables and learning from the existing data, even when noise was present. Neurons, the elemental information processing units of the ANN structure, were linked up through synaptic weights to organize into several layers. Based on searching an error surface using gradient descent for a point with minimum error, the BP learning algorithm stored a lot of input−output mapping relationships without prior revealing of the mathematical equation and was composed of an input, hidden, and output layer. In the present study the THz frequency domain spectra data were introduced into the input layer and calculated in the hidden layer; then the results would be finally obtained in the output layer. The three-layer network is enough to simulate the complicated functions. In order to identify all of the subsequently given spectra correctly in the prediction set, the number of training sets should exceed half of the sample numbers. BPANN can store a large number of input−output mapping relationships without prior giving of the mathematical equation which describes the mapping relationships. Its learning rule is to use the method of steepest descent, to constantly adjust the network weights and threshold by back-
Figure 8. Quantitative model of CH4, C2H6, and C3H8 concentrations as well as the total pressure built by BP-ANN. Circle and star points reflect the training set and prediction set, respectively.
which represented the predicted values versus the actual data, including the total pressure and the concentration of CH4, C2H6, and C3H8. The correlation coefficient between the actual and the predicted values was calculated as 0.9859. Moreover, most data both in the training set and the prediction set were found to be very close to the reference lines (two black lines), which represented zero residuals between the actual and the predicted properties. Therefore, the results show that there exists a special rule between the THz technique and the gas resource. The research aimed to discuss the THz responses of principal ingredients mixtures in natural gas and realize the determination of the components’ concentrations and the total pressure by the combination of THz technique and mathematical method. It was of interest that great differences were observed between any two different two- or threecomponent systems. With the input of C3H8, the THz response gradually changed and the tendency can be obviously found in phase projection figures. Moreover, the concentration and pressure were quantitatively predicted with a very high correlation coefficient. The phase diagrams and the quantitative models provided a proof to identify the natural gas qualitatively and quantitatively. Actually, the more classification and larger numbers of gases that were employed to analyze with THz technique, more abundant qualitative and quantitative models can be obtained. Consequently, THz technique possesses a broadly promising application in the natural gas industry. 1626
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(12) Jin, W. J.; et al. Experimental measurements of water content in crude oil emulsions by terahertz time-domain spectroscopy. Appl. Geophys. 2013, 10, 506−509. (13) Yang, Y. H.; et al. Measurement of the transmission of the atmosphere from 0.2−2 THz. Opt. Express 2011, 19, 8830−8838. (14) Exter, M. V.; et al. Terahertz time-domain spectroscopy of water vapor. Opt. Lett. 1989, 14, 1128−1130. (15) Mittleman, D. M.; et al. Gas sensing using terahertz timedomain spectroscopy. Appl. Phys. B: Lasers Opt. 1998, 67, 379−390. (16) Harde, H.; Zhao, J. THz time-domain spectroscopy on ammonia. J. Phys. Chem. A 2001, 105, 6038−6047. (17) Foltynowicz, R. J.; et al. Terahertz absorption measurement for gas-phase 2,4-dinitrotoluene from 0.05 THz to 2.7 THz. Chem. Phys. Lett. 2006, 431, 34−38. (18) Naftaly, M.; et al. Terahertz transmission spectroscopy of nonpolar materials and relationship with composition and properties. Int. J. Infrared Millmeter Waves 2005, 26, 55−64. (19) Lattanzi, V.; et al. THz spectrum of monodeuterated methane. J. Quant. Spectrosc. Radiat. Transfer 2008, 109, 580−586. (20) Löffler, T.; et al. Larger-area electro-optic ZnTe terahertz emitters. Opt. Express 2005, 13, 5353−5362. (21) Leng, W. X.; et al. Pressure-dependent terahertz optical characterization of heptafluoropropane. Chin. Phys. B 2014, 23, No. 107804. (22) Lide, D. M. Microwave spectrum, structure, and dipole moment of propane. J. Chem. Phys. 1960, 33, 1514−1518. (23) Drouin, B. J.; et al. THz measurements of propane. J. Mol. Spectrosc. 2006, 240, 227−237. (24) Jiang, S.; et al. Application of BPANN for prediction of backward ball spinning of thin-walled tubular part with longitudinal inner ribs. J. Mater. Process. Technol. 2008, 196, 190−196.
CONCLUSION In summary, the practicability was demonstrated about the THz technique being applied to qualitatively and quantitatively detect the principal hydrocarbon components of natural gas, including CH4, C2H6, and C3H8. Both two- and threecomponent systems were analyzed, and the amplitude ratio as well as delay deviation were extracted and related to the components’ concentrations. The phase projection pictures obviously indicated the THz response of gas mixtures with different content ratio. Moreover, BPANN was employed to quantitatively determine the concentrations and the total pressure of the three-component system with the input of THz-FDS. Results showed that simple and efficient models were built between THz spectra and gas mixtures and evidently reflected in phase and BPANN pictures. Therefore, THz technique is a new selection to realize the rapid determination of principal hydrocarbon components in the natural gas industry.
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AUTHOR INFORMATION
Corresponding Author
* Tel.: +86-10-89732270. E-mail:
[email protected]. Author Contributions ∥
L.N.G. and H.L.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge the National Key Basic Research Program of China (Grant 2014CB744302) and the Specially Founded Program on National Key Scientific Instruments and Equipment Development (Grant 2012YQ140005) for the financial support of this work.
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