Development and Implementation of a Quantum Cascade Laser based Gas Sensor for sub-ppm H 2 S Measurements in petrochemical Process Gas Streams Harald Moser 1 , Johannes Ofner 1 , Bernhard Lendl 1 1 Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria www.cta.tuwien.ac.at/cavs Sensor setup Financial support was provided by the Austrian research funding association (FFG) under the scope of the COMET program within the research Ŷetwork Process AŶalytical Cheŵistry ;coŶtract # 8Ϯϱϯϰ0Ϳ Introduction Sensitive detection of hydrogen sulfide (H 2 S) is essential for production control and environmental monitoring purposes in the field of petrochemical, paper and pulp or biotechnological processes. Despite a variety of online monitoring options for gaseous hydrogen sulfide, its reliable quantitative determination still remains a challenge in the field of chemical sensors. In the aspect of laser spectroscopy the constant improvement of quantum cascade lasers (QCLs) has led to their application as reliable sources of coherent light ranging from the mid-infrared (MIR) to the terahertz spectral region for sensitive detection of molecular species on their fundamental vibrational bands. A sensitive, selective and industrial fit gas sensor based on second harmonic wavelength modulation spectroscopy (2f-WMS) employing a 8 µm continuous wave distributed feedback quantum cascade laser (CW-DFB-QCL) was developed for detecting H 2 S at sub-ppm levels in petrochemical process gas streams. References (1) NIOSH Pocket Guide to Chemical Hazards, National Institute for Occupational Safety and Health, 2007. (2) Pearson, C.D. & Hines, W.J., 1977. Determination of hydrogen sulfide, carbonyl sulfide, carbon disulfide, and sulfur dioxide in gases and hydrocarbon streams by gas chromatography/flame photometric detection. Analytical Chemistry, 49(1), pp.123–126. (3) Hodgkinson, J. & Tatam, R.P., 2013. Optical Gas Sensing: A Review. Measurement Science and Technology, 24(1), p.012004. (4) Rieker, G.B., Jeffries, J.B. & Hanson, R.K., 2009. Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments. Applied optics, 48(29), pp.5546–60. (5) Linnerud, I. et al., 1998. Gas monitoring in the process industry using diode laser spectroscopy. Applied Physics B: Lasers and Optics, 305(3), pp.297–305. QC Laser based 2f-WMS of H 2 S H 2 S sensor architecture Results The sensitivity and linear response of the H 2 S sensor was investigated at different H 2 S concentrations. A recorded calibration curve yielded a limit of detection (LOD) of H 2 S better than 150 ppbV. An exemplary online purge gas process spectrum of the hydration reaction plant containing ~300 ppmV H 2 S alongside with continuous purge gas H 2 S monitoring during defined feed change events of a hydro-desulphurization run of straight-run oil batches are shown in Figure 4. The H 2 S sensor was tested at a petrochemical research hydrogenation platform (OMV AG). In order to meet with the on-site safety regulations the H 2 S sensor platform was installed in an industry rack and equipped with the required safety infrastructure meeting the ATEX directive for hazardous and explosive environments. The H 2 S sensor rack combines a purge and pressurization system with intrinsic safety electronic devices achieving a versatile explosion prevention and malfunction protection. 12.2016 Mid-Infrared laser based wavelength modulation spectroscopy is a very sensitive technique allowing measurements of target gases in the sub-ppm concentration range. The use of QCL technology assures both selectivity and sensitivity by targeting single, strong absorption lines of the analytes. Conventional direct absorption laser spectroscopy techniques are not sufficient to achieve sensitivities and detection limits required for many industrial monitoring and process control applications. Advanced wavelength modulation spectroscopy techniques improve on the signal-to-noise contrast by encoding and demodulating the absorption signals at high frequencies, where 1/f noise levels are sufficiently low for sub-ppm detection (Figure 1). For spectral H 2 S assessment a CW-DFB-QCL emitting at ~8.0 μŵ was employed, generating up to 35 mW of coherent optical radiation. In order to perform selective and sensitive H 2 S 2f-WMS measurements, strong absorption lines in the 1250-1245 cm -1 region were targeted and the QCL operating temperature was set in the range of 0-20 °C. The mid-IR laser radiation of the CW-DFB-QCL was passed through an optical isolator, overlaid with a visible 532 nm DPSS trace laser beam, collimated with a plano-convex lens and coupled into an astigmatic Herriott multipass gas cell with a total path length of 100 m (AMAC100, Aerodyne Inc.). The laser radiation exiting the multipass sample cell containing the spectral information of the target analytes was focused onto an optically immersed TE cooled MCT detector (PCI-2TE-12, Vigo Systems) and the signals were demodulated and further processed. The H 2 S sensor platform has been able to provide sensitive and selective measurements of hydrogen sulfide in petrochemical process gas streams with fast detector response while performing under the imperative on-site safety regulations for hazardous and explosive environments. PC FQ PE PC N2 CAL Process plant FC PI PI Purge QCL DET Analyzer Purgegas Q-H2S Q-H2 Power supply PI 1 bar 1 bar 3 bar 0,5 bar 40bar 0,35bar 0,02bar 100 mbar abs. Atm. 0,1-1 l/min 0,1-1l/min C1 C2 C3 C4 C5 C6 V1 V2 V3 V4 V5 V6 V8 V7 V9 V10 V11 V12 R4 R5 R1 R2 S1 S2 N2 Sample in Sample out Purge out FC PS >0,1 bar 3,6 l <900 mbar abs. V14 1 1 2 2 1 3 CO4 CO1 CO2 CO3 PS F1 0,1 μ F2 F3 F4 QCL-H2S-Analysator Applikation für Technikum Version 1.6 / 17.7.2014 / W.Pölz MRDIQO-S Vacuum unit Multipath cell 100 m Figure 3: Piping, instrumentation and safety flow diagram (left), the optical layout (middle) and the on-site H 2 S sensor rack (right). Figure 4: Calibration curve of the H 2 S sensor in the range of 0-50 ppmV H 2 S in N 2 (top left) and an online purge gas process spectrum downstream the hydration reaction plant containing ~300ppmV H 2 S (top right). Continuous purge gas H 2 S monitoring during defined feed change events of a hydro-desulphurization run of straight-run oil batches (bottom). 0 5 10 15 20 25 30 35 40 45 50 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 5 10 15 20 25 30 35 40 45 50 -0.04 -0.02 0.00 0.02 0.04 2f Peak Height (A.U.) H 2 S Concentration (ppmV) LOD (3) = 150 ppb H 2 S Residuals H 2 S Concentration (ppmV) 0 100 200 300 400 500 600 700 800 900 1000 -0.02 0.00 0.02 0.04 0 100 200 300 400 500 600 700 800 900 1000 -0.10 -0.05 0.00 0.05 0.10 0.15 2f Signal (A.U.) H 2 S Reference Cell Process #240 2f Signal (A.U.) Spectral Index (-) cH 2 S=292.7 0.15 ppmV 0 12 24 36 48 60 72 200 400 600 800 1000 1200 H 2 S Concentration (ppmV) Time (h) H 2 S Concentration Purge HD4 Feed Change #1 Feed Change #2 Feed Change #3 Feed Start 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 200 400 600 800 1000 1200 H 2 S Concentration (ppmV) Time (h) GC Sampling Figure 2: H 2 S absorption spectrum and QC laser tuning range (left). QC laser tuning and optical power characteristics (right). 0.50 0.55 0.60 0.65 0.70 0.50 0.55 0.60 0.65 0.70 1246 1247 1248 1249 1250 1251 0.50 0.55 0.60 0.65 0.70 0 5 10 15 20 25 30 Wavenumber (cm -1 ) 0°C 5°C 10°C 15°C 20°C Optical Power (mW) QCL Injection Current (A) Detector Signal, D (a.u.) Intensity (a.u.) Figure 1: WMS implements a slow scan of emission wavelength over absorption features (Hz regime) with a superimposed fast sinusoidal small wavelength modulation (kHz regime). FM to AM conversion occurs due to non-linear absorption features and giving rise to multiple harmonics in the detector signal D - represented by the 3D space curve (black) embedded in the 3D surface representation of absorption features (t - ν - D space). The projection of the 3D space curve onto the t–D plane is the detector signal as a function of time. Demodulation / envelope extraction of n th harmonics of the t-D Signal with lock-In amplifier / FFT+iFFT: nf-WMS . Σ Slow Scan Current Ramp (Hz) Fast Modulation Sinusoid (kHz) I 0 I t L, α(ν) 0 exp[ (,)]; () () t I I tL N