improved measurements of hydrogen sulphide in … 17...chromatogram for h 2 s measurements on the...

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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Non dispersive ultraviolet

analyser (NDUV)

Ultraviolet (UV) fluorescence

analyser


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.


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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


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The stability study of H2S reference gas mixtures:


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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


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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


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


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Cylinder Number D67 9403

Gravimetric mole


10.2975

Average

verification mole


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.

TQM-5134-2

improved measurements of hydrogen sulphide in … 17...chromatogram for h 2 s measurements on the...

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