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摘要: 本文针对化工过程中在线检测丙烯的需求，研究了基于调制吸收光谱技术（TDLAS）的检测技术，提出了一种独立于光谱线型特征的数值仿真方法，考虑实际激光光源宽线宽对吸光度的影响，通过对比仿真和实验的光谱幅度变化规律，确定了丙烯气体分析装置的设计参数和技术方案，选择中心波长为1 628.5 nm的宽调谐DFB激光器，采用差分方案去除解调光谱的直流偏置，采用多元回归模型降低化工过程的背景气体光谱干扰。在模拟实际环境的气体实验中，该装置在0~1% 量程内的最大相对误差为0.55%。对0.2% 的丙烯进行3小时连续测量，标准差为9.3×10−6；Allen方差分析发现在积分时间为221.9 s 时，极限标准差可达1.33×10−6。在抗干扰测试中，当背景气体甲烷、乙烯的浓度变化时，丙烯的测量误差最大仅为19.17×10−6。调制吸收光谱技术克服了色谱和软测量等传统方法的不足，TDLAS装置可检测有复杂光谱特征的重烃分子，展示了测量精度高、稳定性好、抗背景光谱干扰能力强等优点。
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关键词:
- 可调谐二极管激光吸收光谱 /
- 丙烯测量 /
- 过程分析 /
- 多元线性回归
Abstract: To satisfy the need for propylene measurement in the olefin production process, Tunable Diode Laser Absorption Spectroscopy (TDLAS) was studied to improve analytical performance. In this paper, a numerical simulation approach is proposed using absorbance from a spectral database to obtain the optimized design parameters, which is independent of spectral features. In the simulation, the effect of a wider linewidth laser on the absorbance profile was considered. Through the comparison of simulation results and experimental collection, the TDLAS-based propylene analysis apparatus was developed correspondingly. It has a 1 628.5 nm center wavelength broad-tuning DFB laser. A differential method was utilized in demodulated spectral acquisition to eliminate bias voltage. The multivariate linear regression model was employed to reduce the strong spectral interference from the background components in the analysis. Based on the simulated field test, the max relative error is 0.55% in the 0~1% range for the step test. For the long-term test, the standard deviation (1σ) is 9.3×10−6 for 0.2% propylene concentration. The best standard deviation is 1.33×10−6 at 221.9 s of integration time through Allen variance analysis. In the anti-interference test, the max error of 19.17×10−6 is demonstrated for 0.2% propylene concentration while methane and ethylene concentrations vary. The disadvantages of traditional methods such as the Gas Chromatogram (GC) and soft measurement methods are overcome by modulated absorption spectroscopy. The TDLAS system for heavy hydrocarbon detection with complex spectral features was demonstrated to have distinct advantages in precision, stability and interference suppression through multivariate regression modeling. -
图 1 数据库中C3H6、CH4、C2H4、C2H6在1 628 nm附近的吸光度
Figure 1. Gas absorbance of C3H6、CH4、C2H4 and C2H6 near 1 628 nm in database
图 2 丙烯光谱的3种线型拟合结果。（a）线型拟合；（b）拟合残差
Figure 2. Fitting results of three line types of propylene spectrum. (a) Absorption profile; (b) fitting residual
图 3 激光器的线宽对丙烯光谱的影响。 （a）线宽对吸光度的影响；（b）线宽对丙烯仿真2f光谱的影响
Figure 3. The influence of laser linewidth on propylene spectrum. (a) Absorbance of propylene; (b) simulation 2f spectra of propylene
图 4 不同调制幅度下C3H6的吸收光谱。（a）数值仿真；（b）实验采集
Figure 4. Absorption spectra of C3H6 with different modulation amplitudes. (a) Numerical simulation; (b) experiment
图 5 不同调制幅度下C3H6的2f光谱峰值强度
Figure 5. 2f spectra intensity of C3H6 at different modulation amplitudes
图 9 分析装置采集到的C3H6、CH4、C2H4吸收光谱和N2背景下光功率变化图
Figure 9. Absorption spectra of C3H6、CH4、C2H4 and optical power in N2 background acquired through the analyzer
图 10 （a）步进测试过程中分析装置丙烯浓度读数曲线；（b）丙烯浓度设定值与分析装置测量值线性关系曲线
Figure 10. (a) Analyzer reading of C3H6 concentration in step test；(b) the linear response of C3H6 analyzer measurement vs. setting gas concentration
图 11 稳定性测试结果。（a）丙烯浓度固定时分析装置浓度读数曲线; （b）Allan方差分析
Figure 11. Stability test results. (a) Analysis device reading curve when propylene concentration is fixed; (b) Allan deviation analysis
图 12 CH4、C2H6背景气体浓度变化情况下装置的C3H6浓度读数
Figure 12. C3H6 concentration reading varying with the concentration of the background gas CH4 and C2H6
表 1 步进测试的丙烯测量精度
Table 1. Measurement error of C3H6 concentration in step test
(×10−6) Setting
concentrationMeasured
concentrationStd.
DeviationAbsolute
error0 −12.12 6.20 12.12 1 000 1 020.09 19.47 20.09 2 000 2 022.57 41.73 22.57 5 000 5 055.13 51.52 55.13 10 000 10 014.32 58.67 14.32 
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表 2 抗干扰测试结果
Table 2. Results of anti-interference test
(10−6) Section Interfering CH4 Interfering C2H6 Measured concentration Std. deviation Absolute error 1 2 000 0 1 997.58 33.96 −2.42 2 1 000 10 2 003.46 33.84 3.46 3 500 100 1 993.64 33.63 −6.36 4 0 300 2 019.17 35.64 19.17
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