Development and evaluation of a novel analyser for ISO14687 hydrogen purity analysis

https://iopscience.iop.org/article/10.1088/1361-6501/ab7cf3

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The instrument combined a Varian 450-GC-TCD/FID (gas chromatograph/thermal conductivity detector/flame ionisation detector) equipped with three columns with two OFCEAS instruments. The configuration of the GC is as follows:

• A Hayesep Q, 80–100 Mesh, 1.8 m × 1/8´´ × 2.0 mm connected to the TCD.
• A molecular Sieve 5A, 60–80 Mesh, 1 m × 1/8´´ × 2.0 mm connected to the TCD.
• A PoraBOND Q, 25 m × 0.53 mm × 10 µm connected to the FID.

The instrument has also three 6-port valves. A mass flow controller set to 500 Nml min⁻¹ allows the gas (hydrogen sample, Primary Reference Mixture (PRM) or helium) to reach the first 6-port valve where a sampling loop of 100 µl is connected between the ports 2 and 3 and the second 6-port valve having the same configuration but with a sampling loop of 500 µl. The sampling loops are filled during 10 s after which the 6-port valves are switched. The content of the first loop is introduced into the first packed column (Hayesep), while the content of the second loop is introduced into the PLOT-column. A third 6-port valve allows us to change the direction of the carrier gas in the second packed column. While H₂, He and Ar have reached the detector, O₂, N₂ and CO have reached the second column (Molecular Sieve), and CH₄ and CO₂ are still retained in the first column; the valve is switched causing the direction of the flow in the columns to change and the Molecular Sieve column to be by-passed. During this lapse of time, CH₄ and CO₂ reached the detector. Finally, the valve is switched again so O_2, CO and N₂ can be separated and detected.

Two custom-built optical feedback cavity enhanced absorption spectrometers (OFCEAS—ProCeas from AP2E) were developed for the measurement of O₂ and CO (instrument 1) and CO₂, H₂O and H₂S (instrument 2). Gas samples are introduced into the measurement cells at a flow rate of 100 ml min⁻¹ through a sulfinert-treated stainless steel restrictor using an internal pump. The measurement cell consists of a multi-path cavity equipped with highly reflective mirrors, allowing path lengths up to 10 km. The cell was kept at a pressure of 50 mbar and at a temperature of 40 °C. To minimize ingress of particulate matter into the instrument, a sulfinert-treated stainless steel particle filter with a 7 micron pore size was also installed upstream of the instruments. The instruments were pre-calibrated for O₂ concentrations from 1 µmol mol⁻¹ to 2000 µmol mol⁻¹, for CO₂ concentrations from 0.2 µmol mol⁻¹ to 2000 µmol mol⁻¹, for H₂O from 0.05 µmol mol⁻¹ to 500 µmol mol⁻¹, for H₂S from 0.001 µmol mol⁻¹ to 2 µmol mol⁻¹ and for CO from 0.002 µmol mol⁻¹ to 20 µmol mol⁻¹. For each compound, a three-point calibration was performed; at the lowest and highest values of the concentration range as well as in the middle of the range.

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