Charles Segar, market manager at Servomex looks at a better approach to the detection of trace oxygen in high-purity nitrogen production processes
High-purity nitrogen is widely used in applications where contamination of the nitrogen by oxygen can lead to considerable losses through rejected product and downtime.
Suppliers of high-purity nitrogen therefore need to be certain that the nitrogen they are producing is of an acceptable quality. Furthermore, should any contamination be detected, a fast response is required so as to minimise the amount of contaminated nitrogen.
Finally, the recovery time needs to be quick so that the downtime is no longer than is absolutely necessary. Traditionally wet electrochemical sensing technology has been employed for detecting trace oxygen contamination, but its limitations have been recognised by nitrogen producers who wish to improve their process efficiency.
Although wet electrochemical technology can operate at the low ppm range required, the response time can be in the region of 30 seconds or more - which is long enough to allow quantities of sub-standard nitrogen to be produced.
Furthermore, electrochemical technology can suffer from 'oxygen shock' after contamination with oxygen, requiring recovery times of tens of minutes or even hours, depending on the cell structure.
If production is delayed because of this, it has a direct impact on the producer's profitability.
For many years solid electrolyte zirconia has been considered as an alternative sensing technology, but these sensors can show a high cross-sensitivity to trace combustibles that are often present in nitrogen.
Standard platinum-based solid electrolyte zirconia sensors operate at around 700C and the catalytic oxidation of the combustible impurities consumes oxygen (for example, hydrogen and oxygen react to produce water) and leads to the cell indicating a lower level than the true oxygen value.
As such, standard platinum electrode zirconia cells have proved to be unsuitable for trace oxygen measurement in nitrogen purity applications, despite several attempts to reduce the cross-sensitivity.
However, a new approach uses alternatives to platinum, such as silver or gold, for the electrodes.
These metals are able to operate at a lower temperature without significantly degrading the other important performance parameters, thereby opening the door for such zirconia sensors to be used in nitrogen purity applications.
One example of these inhibited catalyst sensors is the cell found within the Servomex 4100 analyser.
This has been thoroughly tested by several industrial gas producers, with extremely good results.
For instance, in a test performed by an American gas producer, a sensor was stabilised with house nitrogen (with less than 1ppm oxygen), then introduced to 8.8 ppm oxygen in balance nitrogen. The sensor T90 response time was measured as less than five seconds, compared with 31 seconds for a quality wet electrochemical sensor tested at the same time.
After stabilising at 8.8ppm oxygen, the sensors were reintroduced to house nitrogen and the T90 times were measured.
Again, the inhibited catalyst sensor exhibited a recovery time some 25 seconds shorter than that of the wet electrochemical sensor.
Further tests were also performed to assess the accuracy of the inhibited catalyst sensor in the presence of trace combustibles.
The strong performance of the new technology was clearly demonstrated, and stability over a 28-day period was also found to be excellent.
For users of wet electrochemical sensors who are seeking to improve their process efficiency, inhibited catalyst sensors offer a very attractive alternative.
Not only do they deliver the required accuracy and stability, but they also react and recover faster, with less vulnerability to oxygen shock and low cross-sensitivity to trace combustibles.
