Gas analyzers for oxygen, nitrogen and argon production on air separation units

The principal precautions against hydrocarbon contaminant build up in the cryogenic liquid in the ASU are well established and focus on carbon dioxide analysis at the warm end of the ASU and hydrocarbons analysis in the cryogenic liquid oxygen sump. Analysis of carbon dioxide after the pre-purification unit (PPU) using a gas analyzer such as the Uras26 is important to ensure that this gas does not enter the ASU because it solidifies and causes blockages. Furthermore, detection of carbon dioxide break-through from the PPU is used to warn about the possibility that hydrocarbons such as methane or ethane are not being removed by the PPU and are also entering the ASU. This serves as an early warning system.

Analysis of hydrocarbons in the cryogenic liquid where there is the highest potential concentration of hydrocarbon contaminants (generally by extraction of liquid oxygen from the main reboiler sump) is the second line of defence. For the THC measurement, many instrument engineers and plant operators will be familiar with flame-ionisation-detector (FID) gas analyzers such as the Fidas24. This technology has an established history in this application and can be operated either with, or without a methane cutter to provide THC and non-methane HC (NMHC) readings. Whilst the technique is robust, it does require specialty gases grades of high purity hydrogen and instrument air for its operation to create the flame. Cylinder change overs of these consumable gases are required in addition to occasional calibration using a certified specialty gas mixture.

For additional speciation within the hydrocarbons our Uras26 NDIR instrument is suitable. It can be configured to measure up to four separate IR-active species, for example: methane, ethane, ethylene and propane. These are some of the main hydrocarbon contaminants that have the potential to accumulate in the liquid oxygen in the ASU main reboiler sump.

Regular acetylene analysis can be performed by taking liquid oxygen samples and sending them to a specialised accredited offsite contract laboratory. Most operators are looking to detect acetylene at less than 0.5 VPM and reliable and precise continuous gas analysis techniques for acetylene measurement in the sub-VPM level are not common. But, we believe that our LGR-ICOS analyzer is up to the task. This enhanced laser technique is capable of accurate trace hydrocarbon measurements in a complex background gas matrix.

As a bonus, the Uras26 and LGR-ICOS instrumentation above require no consumable gases and have minimal calibration gas mixture requirements. This simplifies gas analyzer maintenance and reduces their cost of ownership.  It all adds up to safe and productive ASU operations.