improved measurements of hydrogen sulphide in … 17...chromatogram for h 2 s measurements on the...
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IMPROVED MEASUREMENTS OF HYDROGEN SULPHIDE IN NITROGEN REFERENCE GAS MIXTURES
Nompumelelo Leshabane
Gas Scientist
Test and Measurement 2019 Conference and workshop, Muldersdrift, Gauteng
17 September 2019
Overview
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Introduction
Challenges of hydrogen sulphide measurements
Gravimetric Preparation of hydrogen sulphide reference gas mixtures
Stability and adsorption study of Hydrogen sulphide
Techniques used for verification of gas mixtures
Results and discussions
Conclusion
Introduction
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Hydrogen Sulphide (H2S)
• Colourless gas with a rotten egg odour, highly poisonous, flammable gas and very corrosive in wastewater application
• Occurs both naturally (e.g. swamps, natural gas) and from man-made processes (e.g. pulp and paper mills, petroleum refineries and power plants).
• It can depletes oxygen in air, thus inhibiting oxygen from reaching vital organs such as the brain.
• Mainly monitored for occupational health and safety and indoor air quality monitoring
Aim• To accurately produce stable and reliable measurements of hydrogen
sulphide reference gas mixtures
• To benchmark hydrogen sulphide measurements through participation in international key or regional comparisons
• To support the Indoor air quality monitoring industry, in accordance to the set regulations of the air quality act
Challenges of H2S measurements
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Measurements require continuous improvement• Improved or better accuracy
• Improved uncertainty of measurements
• Detailed purity analysis of high pure gases or source material
• Precise and sensitive analysis techniques
Stability of H2S gas mixture
• Adsorption/desorption:
• Nature of the component: very sticky component tend to be adsorbed on transfer line and inner surfaces of gas cylinder
• Passivation process in the cylinders
Reactive and corrosive
• 2H2S(g) + 3O2(g) 2H2O(l) + 2SO2(g)
International equivalence In 2009 APMP.QM-41
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Gravimetric preparation of H2S reference gas mixtures1
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Production diagram of H2S
Each mole fraction level of H2S was
diluted with high pure nitrogen gas
Purity
analysis
of high
pure
gases
Old Mass comparator balance
➢ Balance with a suspension pan
➢ Buoyancy effect
➢ Manually weighing the gas
cylinders
➢ One cylinder weighed at a time
➢ Ergonomically challenges to
laboratory personnel
New Automated Weighing System
➢ Faster and precise measurements
➢Readability of 1-2 mg
➢ Automatic switching of cylinders,
substitution method
➢Reduced uncertainty of measurement
❖No handling of cylinders
❖Adsorption on cylinder surface
➢ Four cylinders can be weighed within 20 minutes
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Gravimetric preparation of H2S reference gas mixtures2
Techniques for verification of gas mixtures1
Old Instruments used
The Fischer Rosemount System:
❑One instrument, consisted of 5 analysers
❑Analyse only one component per
mole fraction range at a time
❑One sampler box, possible contamination
❑Longer turn around time of six weeks
Gas chromatography with Pulsed DischargedHelium Ionisation (GC- PDHID)→Multipoint calibration→Analyse one component per mole fraction range
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New Instruments:
Analyser per component
NDUV Analyser Conditions:
Sample flow: 200ml/min
Stabilisation time:180 seconds
Sample readings: 90
Number of cycle repeats: 4
UV Fluorescence Conditions:
Response time:10 minutes
Span gas: 10 µmol/mol standard
Number of samples: 50 readings
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Techniques for verification of gas mixtures2
Non dispersive ultraviolet
analyser (NDUV)
Ultraviolet (UV) fluorescence
analyser
Techniques for verification of gas mixtures3
New Agilent Gas Chromatography Instruments
Analysis of gas mixtures using gas chromatography coupled with three detectors, sulphur chemiluminescence detector (SCD), thermal conductivity detector (TCD) and Pulsed discharged helium Ionisation detector (PDHID), GC-SCD/TCD/PDHID
➢ Specific and sensitive detectors for analysis of H2S and other components
➢ Fast analysis of sulphur compounds such as H2S
➢One point calibration: One gas mixture was used as the reference standard
𝐶𝑠𝑎𝑚𝑝𝑙𝑒 =𝐴𝑆𝑎𝑚𝑝𝑙𝑒
𝐴𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒× 𝐶𝑟𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒
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Analytical conditions:
Agilent 7890B GCDetector: PDHID @ 150°C
Column: Hayasep Q, 80/100, 2 m,
ID(2.1 mm), 1/8’’
Sample loop size: 1 ml
Sample flow (Mass flow controller: 35
ml/min
Oven temperature: 120°C isothermal
GC-PDHID channel was used for H2S
Stability and adsorption study of Hydrogen sulphide reference gas mixture1
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The adsorption/desorption study of H2S on the inner surface of the aluminium cylinder:
➢ Equal division method was used for adsorption study
➢ Mole fraction of 10 µmol.mol-1 with a pressure of 9.0 MPa was selected and transferred to two empty cylinders.
➢ The Transferring process was done with care to prevent the Joules Thompson effect.
Stability and adsorption study of Hydrogen sulphide reference gas mixture2
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➢The three gas mixtures were analysed using Limas 11 UV analyser
➢Uncertainty of adsorption/desorption on the inner surface of the cylinders was calculated to be 0, 03%.
➢Therefore no adsorption of H2S on the inner surface of the cylinder
Cylinder numbers Gravimetric
Concentration
Means of analyser
response
%RSD % Difference
(Mother cylinder)
D62 6475
10.01 9.985 0.144
(Daughter 1)
D67 9548
10.01 9.958 0.326 0.272
(Daughter 2)
D67 9397
10.01 9.999 0.049 -0.136
Stability and adsorption study of Hydrogen sulphide reference gas mixture3
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The stability study of H2S reference gas mixtures:
Stability and adsorption study of Hydrogen sulphide reference gas mixture4
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The stability study of H2S reference gas mixtures:
❖The long-term stability results show a % difference of the new hydrogen sulphide reference gas mixture prepared to be (-0.50 %).
❖This indicate that the gas mixture is stable for a period of 2 years within the measurement uncertainty of 1 %.
Cylinder
number
Mole Fraction
(µmol/mol)
Means % RSD % Difference Preparation
dates
D67 9596 9.99 9.94 0.271 -0.501 09/10/2018
D19 4914 10.01 9.99 0.1465 - 09/03/2016
Results and Discussion1
For GC-PDHID and UV Fluorescence:
The verification was done using the one point calibration (Reference-Sample-Reference) method, where the reference standard was analysed before andafter the sample:
For NDUV:
The verification was done using the multi-point calibration method. Referencestandards used for the verification ranged from 8 to 12 µmol/mol.
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Cylinder Number Mole Fraction
(µmol/mol)
Expanded uncertainty
(µmol/mol), k=2
D67 9551 7,9777 0,0033
D67 9596 9,9860 0,0036
D67 9392 8,9689 0,0032
D67 9504 10,9815 0,0040
D67 9342 11,9977 0,0043
Results and Discussion2
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UV Fluorescence and NDUV H2S reference gas mixture verification
results
Cylinder Number D67 9403
Gravimetric mole
fraction (µmol/mol)
10.2975
Average verification
mole fraction
(µmol/mol)
10.3415
Standard deviation
(µmol/mol)
0.01685)
Estimated standard
deviation (ESDM)
0.0075
Combined
uncertainty
0.0406
U(K=2) 0.0813
% Relative
expanded
uncertainty (%REU)
0.7895
NDUV analyser results Cylinder Number D67 9403
Gravimetric mole
fraction (µmol/mol)
10.2975
Average verification
mole fraction
(µmol/mol)
10.3027
Standard deviation
(µmol/mol)
0.0093
Estimated standard
deviation (ESDM)
0.0054
Combined
uncertainty
0.0023
U(K=2) 0.0046
% Relative expanded
uncertainty (%REU)
0.045
UV Fluorescence analyser results
Results and Discussion3
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Cylinder Number D67 9403
Gravimetric mole
fraction (µmol/mol)
10.2975
Average
verification mole
fraction (µmol/mol)
10.3310
Standard deviation
(µmol/mol)
0.0220
Estimated
standard deviation
(ESDM)
0.0083
Combined
uncertainty
0.0016
U(K=2) 0.0231
% Relative
expanded
uncertainty
(%REU)
0.2243
GC-PDHID results
Chromatogram for H2S measurements on the
GC-PDHID
Conclusion
➢Our measurement uncertainty results show that the gravimetric value, internal consistency, homogeneity, adsorption and stability gave a relative uncertainty of less than 1.0 % compared to previous results of 3 %.
➢ The new techniques from both gravimetric preparation and analysis gave better results than old techniques used.
➢No adsorption/desorption on the inner surface of the gas cylinders for H2S gas mixtures.
➢ Stability study for hydrogen sulphide reference gas mixtures indicated that the mixture will be stable over a period of 2 years with a measurement uncertainty of 1%.
Future Plans:
• Improve H2S measurement uncertainty to less than 0.5 %.
• Improve the certification period for the measurements of hydrogen sulphide reference gas mixtures
• Development of other sulphur compounds such as Carbonyl sulphide (COS), dimethyl sulphide (DMS), ethyl mercaptan (CH3CH2SH) and tetrahydrothiophene (THT)
• Development low mole fraction of hydrogen sulphide in nitrogen reference gas mixtures at nanomole per mole (ppb) level.
➢ Relative uncertainty was less than 1 % compared to previous results of 3 % relative uncertainty.
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➢Relative uncertainty was less than 1 % compared to previous results of 3 % relative uncertainty.
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Acknowledgements
This work was supported by National Metrology Institute of South Africa (NMISA) under the
NMISA cross-cutting projects for measurement technology solving green economy.
Gas laboratory staff at NMISA
My supervisor and co-supervisor ( Dr James Tshilongo and Prof. SJ Moja) for their
encouragement through my Master’s degree project
THANK YOU
➢Relative uncertainty was less than 1 % compared to previous results of 3 % relative uncertainty.
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