Q4 2009 / Faster Zirconia oxygen sensor technology boosts nitrogen purity

The production of high purity nitrogen requires rapid response sensors that can protect production lines from any accidental oxygen ingress along with an ability to deliver accurate measurements of oxygen impurities.

For this application trace oxygen analysers must provide acceptable accuracy levels of 0.1 ppm oxygen with a rapid response to limit the risk of oxygen contamination in the gas line as well as rapid recovery from any exposure to high oxygen concentrations to allow for quick resumption of production.

The analyser used must also provide reliable performance in the presence of the typical trace impurities found in high purity nitrogen including hydrogen, methane and carbon monoxide.

Traditionally, wet electrochemical sensors were chosen over solid electrolyte zirconia technology. The reason was that although zirconia was capable of offering an excellent performance with respect to response time and recovery from oxygen shock, it showed a high cross-sensitivity to the presence of trace combustibles.

New developments in technology, however, have led to the development of a modern inhibited catalyst zirconia sensor which counters these problems. These new sensors offer a significant reduction in cross-sensitivity to trace combustibles.

Let’s look at some of the features of the traditional platinum-electrode solid electrolyte sensor and then compare it to the new inhibited catalyst zirconia sensor.

Platinum-based zirconia sensors

Platinum-based solid electrolyte zirconia sensors operate at elevated temperatures typically in excess of 700°C. The standard platinum-electrode induces a catalytic reaction with trace combustibles common in high purity nitrogen. The reactions are explained in the reactions equations on the right:

The catalytic oxidation of the combustible impurities consumes oxygen and causes the zirconia cell output to indicate a lower level than the real oxygen value present in the gas. This renders the platinum-electrode zirconia cells unsuitable for trace oxygen measurement in nitrogen purity applications. For many years this was a significant barrier.

Several approaches were attempted to circumvent the catalytic oxidation of combustible impurities. The most common approach was to introduce modifications that reduced the operating temperature of the sensor using alternate solid electrolytes and to make changes to the platinum electrode itself. Unfortunately, these attempts had limited success due to the close relationship between the electrode’s catalytic oxidation with combustibles and its primary function of promoting dis-association and re-association of molecular oxygen in the gas and ionic oxygen in the solid electrolyte lattice.

Progress stalled. The relationship between the operating temperature and the catalytic activity of the electrode material was commonly understood (Maskell 1991¹). The complexity of producing a solid electrolyte (typically zirconia) which offered both low combustibles cross-sensitivity and maintained the high performance required by these sensors for high purity nitrogen production required a new approach.

New inhibited catalyst zirconia sensor

It was the development of inhibited catalyst electrodes (Goffe and Mason 1981²) that allowed zirconia sensors finally to be used in the nitrogen purity application. Inhibited catalyst electrodes were originally designed to replace expensive platinum electrodes using less expensive alternative metals such as silver or gold and at the same time retain the ability to operate at lower temperatures without degrading performance parameters.

Although there are many examples of commercially available platinum-based zirconia solid electrolyte sensors, there are relatively few commercially available examples of the inhibited catalyst sensor. One is the Servomex trace oxygen zirconia sensor (Kocache & Holman 1982³) fitted in the SERVOPRO 4100 and new SERVOPRO MultiExact analyser.

The performance of this type of sensor compared to traditional wet electrochemical cells was demonstrated by the results of tests conducted by a number of industrial gas producers.

Why rapid response analysis is vital

If a process event leads to higher oxygen levels, the trace oxygen analyser must respond quickly to speedily initiate prompt corrective action and ensure that the high purity nitrogen gas line is not contaminated. Obviously, the more rapid the response, the higher the product quality maintained in production.

Zirconia solid electrolyte sensors have demonstrated the ability to respond faster to oxygen than most other methods of measuring oxygen. In a test by a major U.S. industrial gas producer, the Servomex inhibited catalyst zirconia sensor was introduced to 8.8 ppm oxygen in balance nitrogen after stabilizing with house nitrogen (containing less than 1 ppm oxygen). The sensor T90 response time was measured as less than 5 seconds, compared to 31 seconds for a quality wet electrochemical sensor tested at the same time (Figure 1) ? an exceptionally improved reaction time.

Fast recovery means less downtime

A trace oxygen analyser that can quickly return to its full operating performance with ppm levels of oxygen after exposure to high concentrations of oxygen minimizes downtime at a cryogenic plant after inadvertent contamination.

Wet electrochemical sensors suffer from oxygen shock when exposed to air or percentage levels of oxygen. Depending on cell structure, such a shock may take from ten minutes up to several hours for recovery. The kinetics of wet electrochemical sensors are inherently slower than for the higher temperature inhibited catalyst zirconia sensors. This means that after shocking, the dynamics for achieving equilibrium with ppm levels of oxygen (with percentage levels of oxygen) is much longer for wet electrochemical sensors than for solid-electrolyte zirconia sensors. In short, the inhibited catalyst sensor exhibits a faster recovery time when exposed to high ppm levels of oxygen compared to a wet electrochemical sensor.

In a comparison test by a major U.S. industrial gas producer, the Servomex inhibited catalyst

 zirconia sensor was allowed to stabilize at 8.8 ppm oxygen and was then introduced to house nitrogen (containing less than 1 ppm oxygen). The T90 recovery time was 6 seconds, some 25 seconds faster than a quality wet electrochemical sensor tested at the same time.

Accuracy in the presence of trace combustibles

The accuracy of the inhibited catalyst zirconia sensor measuring ppm oxygen in the presence of trace amounts of combustibles was validated in testing by a number of major industrial gas producers. Test results from industrial gas producers based in the Germany and the U.S. are shown in the tables above. The trace oxygen analyzer with inhibited catalyst zirconia sensor was calibrated using a reference gas mixture containing ppm oxygen and known amounts of trace hydrocarbons (Tables I and II). In fact, the analyser performed well and demonstrated stability over a full 28-day period.

In the final reel

The modern inhibited catalyst zirconia oxygen sensor provides the required accuracy for nitrogen purity monitoring in the presence of trace hydrogen, methane, and carbon monoxide, while offering the advantages of rapid response and quick recovery times, plus less vulnerability to oxygen shock.

All in all, inhibited catalyst zirconia technology offers an attractive solution to the needs of nitrogen purity applications – especially when viewed with the practical solutions it offers in terms of faster reaction to contamination, reduced exposure to contamination, and reduced production downtime.

About the author

Charles Segar – Charles Segar is a Business Unit Manager, for the Industrial Instrumentation Group of Servomex Group Ltd and can be contacted direct at CSegar@servomex.com or on +44 (0)1892 603213. For more information about Servomex products, including the SERVOPRO 4100 and SERVOPRO MultiExact, visit: www.servomex.com.