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