Development and Implementation of a Quantum Cascade Laser based Gas Sensor
for sub-ppm H2S Measurements in petrochemical Process Gas Streams
Harald Moser1, Johannes Ofner1, Bernhard Lendl1
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 etwork Process A alytical Che istry co tract # 8 0
Introduction
Sensitive detection of hydrogen sulfide (H2S) 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 H2S 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 H2S H2S sensor architecture
Results
The sensitivity and linear response of the H2S sensor was investigated at different H2S
concentrations. A recorded calibration curve yielded a limit of detection (LOD) of H2S better
than 150 ppbV. An exemplary online purge gas process spectrum of the hydration reaction
plant containing ~300 ppmV H2S alongside with continuous purge gas H2S monitoring during
defined feed change events of a hydro-desulphurization run of straight-run oil batches are
shown in Figure 4.
The H2S sensor was tested at a petrochemical research hydrogenation platform (OMV AG). In
order to meet with the on-site safety regulations the H2S 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 H2S 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 H2S assessment a CW-DFB-QCL emitting at ~8.0 μ was employed, generating
up to 35 mW of coherent optical radiation. In order to perform selective and sensitive H2S
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 H2S 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
Sam
ple
in
Sam
ple
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 TechnikumVersion 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 H2S sensor rack (right).
Figure 4: Calibration curve of the H2S sensor in the range of 0-50 ppmV H2S in N2 (top left) and an
online purge gas process spectrum downstream the hydration reaction plant containing
~300ppmV H2S (top right). Continuous purge gas H2S 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 45 500.0
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 P
eak H
eig
ht (A
.U.)
H2S Concentration (ppmV)
LOD (3) = 150 ppb H2S
Resid
uals
H2S 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 S
ignal (A
.U.)
H2S Reference Cell
Process #240
2f S
ignal (A
.U.)
Spectral Index (-)
cH2S=292.7 0.15 ppmV
0 12 24 36 48 60 72
200
400
600
800
1000
1200
H2S
Concentr
ation (
ppm
V)
Time (h)
H2S Concentration Purge HD4
Feed Change #1 Feed Change #2 Feed Change #3Feed Start
1.0 1.5 2.0 2.5 3.0 3.5 4.00
200
400
600
800
1000
1200
H2S
Co
nce
ntr
atio
n (
pp
mV
)
Time (h)
GC Sampling
Figure 2: H2S 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
Wa
ve
nu
mb
er
(cm
-1)
0°C
5°C
10°C
15°C
20°C
Op
tica
l P
ow
er
(mW
)
QCL Injection Current (A)
De
tect
or
Sig
na
l, D
(a.u
.)
Inte
nsi
ty (
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 nth harmonics of the t-D Signal with lock-In
amplifier / FFT+iFFT: nf-WMS .
Σ
Slow Scan Current Ramp (Hz)
Fast Modulation Sinusoid (kHz)
I0 ItL, α(ν)
0 exp[ ( , ) ]; ( ) ( )t
I I t L N
