Measurement of Ethylene Concentrations at High Pressure Based on Tunable Diode Laser Absorption Spectroscopy near 1620 n

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Journal of Applied Spectroscopy, Vol. 87, No. 4, September, 2020 (Russian Original Vol. 87, No. 4, July–August, 2020)

MEASUREMENT OF ETHYLENE CONCENTRATIONS AT HIGH PRESSURE BASED ON TUNABLE DIODE LASER ABSORPTION SPECTROSCOPY NEAR 1620 nm T. Zhang, G. Zhang, X. Liu, G. Gao, and T. Cai*

UDC 543.42;621.378.8

A system for detection of ethylene (C2H4) at high pressure is developed based on tunable diode laser absorption spectroscopy using a distributed feedback laser near 1620 nm. To eliminate the influence of spectral line overlap under high pressure, a differential absorption (peak-minus-valley) scheme is adopted. The peak and valley wavelengths used for the measurement correspond to 6174.64 and 6174.45 cm–1, respectively. Absorption cross sections of ethylene are measured for the selected peak and valley wavelengths. The measured concentration agrees with the known concentration, and the maximum of the standard deviation is 0.746% for all measurements. In addition, long-term continuous measurements indicated good stability of the system. The sensitivity of the system is ~18 ppm with an optimum averaging time of 110 s. All the experimental results validate the applicability of the system in ethylene trace detection. Keywords: tunable diode laser absorption spectroscopy, differential absorption, ethylene, concentration, high pressure. Introduction. Ethylene (C2H4) is one of the most important products in chemical raw materials. It is widely used in the production of hydrocarbons and many other intermediate products [1]. Ethylene can also be mixed with air to form explosive mixtures. When exposed to open fire, high heat, or contact with oxidants, there is a risk of fire and explosion [2]. Therefore, it is of great significance to develop an ethylene gas detection technology. There are various studies on the detection of ethylene. Platz et al. described sub-Doppler overtone spectroscopy of C2H4 with optoacoustic and optothermal spectra in the range of 6147–6170 cm–1 [3]. Hai Pham-Tuan et al. presented an automated capillary gas chromatographic system to measure ethylene in biological materials [4]. Sgro examined the solubility in water of combustion generated organic particulate matter by comparing the ultraviolet visible spectra (UV-Vis) observed in situ in rich ethylene air flames with the spectra of extra situ sampled material trapped in water [5]. However, these methods all have some disadvantages. For example, the detection cycle of a gas chromatograph is long, which is not suitable for real-time monitoring; the UV-Vis is susceptible to water vapor interference. In order to solve these problems, the technology of tunable diode laser absorption spectroscopy (TDLAS) based on the Beer–Lambert law was used in this paper, and the gas concentration was obtained by analyzing changes in the selected absorption line [6]. This technique is widely used in trace gas detection due to its advantages of noninvasiveness, high spectral resolution, and high selectivity [7–10]. It has also been used for the detection of C2H4. For