Gas analysis in processes

Gas analysis is now a key element used for the control of many industrial processes BY Andrew Armitage

There was a time when the function of gas analysis in industrial processes was almost entirely that of safety monitoring.

In this role, the gas analyser tended to be seen purely as a cost item – an unwelcome but unavoidable addition to the overhead which was necessary to avoid risks. Typically, oxygen analysers were used to control the level of oxygen present during the unloading of oil tanks, or during the conveying of bulk sugars, and in dozens of other circumstances where oxygen is undesirable or dangerous.

Over the past 10 years or so, the level of sophistication involved in industrial processes has substantially increased, requiring a growing use of gas analysis as a vital control mechanism to improve efficiency and quality.

Gas analysers have been developed which meet the needs for faster, more specific and precise measurements in a range of industrial processes. The need to meet increasingly stringent environmental and legislative requirements has also led to the development of analysers to measure the concentrations of a broad range of gases, such as carbon dioxide, hydrochloric acid, nitrogen dioxide and sulphur dioxide.

Efficiency and quality

The process used to produce Polymer grade Terephthalic Acid (PTA), a raw material for the manufacture of plastic bottles, is a good example of the increased dependence process operators have on gas analysis.

Global production of PTA is continually increasing, as demand for the polymer grows steadily throughout the world, particularly in the fast-expanding economies of South-East Asia. Manufacturers are therefore looking increasingly for process efficiency and product quality improvements as ways to meet this growth in demand.

PTA is made from p-xylene by precisely controlled air oxidisation in a reactor at high pressure and temperature. The process uses liquid acetic acid as a solvent for the reaction. The crystalline PTA product is then separated into crystalliser vessels, from which it is recovered and purified. Gas analysers are used both in the oxidisation reactors and in the crystallisers.

In the oxidisation reactors, the reaction which converts p-xylene into terephthalic acid leaves some oxygen unreacted, and this level of oxygen must be maintained at around 4-5%. If the level gets too high, a dangerous runaway reaction could develop, possibly resulting in an explosion.

If the level of oxygen drops, insufficient oxidisation occurs, leading to poor efficiency and low product yield. In the crystallisers, similar risks apply if the oxygen level is not controlled.

This critical oxygen analysis must be carried out in hazardous areas, and is made using analysers, such as Servomex’s 1100A or its Xendos 1900 series.

Keeping the party clean

Many processes whose efficiency requires constant monitoring of a gas stream inherently produce extremely dirty gas samples. An example of this is the monitoring and control of blast furnaces.

In this classic iron production process, the accurate determination of the concentrations of carbon monoxide, carbon dioxide and hydrogen in the gases emanating from the furnace is crucial to the efficiency of production.

But blasting superheated air at a mixture of iron ore and coke produces complex reactions and a great deal of sooty deposit on anything in contact with the gas to be analysed.

The key to preventing the high dust content of the gas from reducing the efficiency of the analyser and endangering the process lies in effective sample conditioning.

This is a process which extracts samples of the gases coming from the furnace, filters them, cools them, dries them to a standard humidity and delivers the resulting cleaned gas sample to the analyser.

Modern sample conditioning systems, like the Servomex 4995, are extremely compact integrated units which fit, like the analyser, in a standard 19in rack and occupy very little space.

With a sample conditioning system in place, the Xendos 2500 infra-red analyser stays spotlessly clean and on-going accuracy of the analyses is assured.

In the pharmaceutical industry, accurate control of oxygen levels is often crucial to the efficiency, or even the viability of processes. Many pharmaceutical manufacturing processes rely on fermentation.

Those with any experience of such activities will know that there are two essential types of fermentation – aerobic and anaerobic. As the names suggest, they are, in a sense, opposites – aerobic reactions requiring oxygen to proceed and anaerobic reactions needing the absence of oxygen to take place.

The manufacture of antibiotics is a typical example of an aerobic fermentation process. The fermenter broth contains a mixture of proteins, sugars and enzymes to produce the required antibiotic. When the reaction is in progress, oxygen is consumed and carbon dioxide levels in the fermenter increase correspondingly.

Monitoring of the oxygen consumption by the process gives an indication of enzyme activity, and the measurement of the carbon dioxide concentration can be used to confirm enzyme performance.

The efficiency of the process as a whole is often confirmed by comparing the level of oxygen in the head space above the reaction with that in the feed gas.

In anaerobic fermentation, air is excluded from the process since excess oxygen inhibits the reaction. As well as monitoring the fermenter to ensure that oxygen is not present, it is often necessary or desirable to establish the concentrations of other gases specifically involved in the fermentation process. Examples include carbon dioxide, ethanol, ammonia and ethylene.

In some processes, it is necessary to feed the process by introducing specific gases, such as methane. In these cases, accurate measurement of the concentration of that gas is essential to the efficiency of the process.

Deciding exactly how to go about monitoring the gases involved is a job for an expert in gas analysis.

Growing expertise

The techniques of gas analysis have, during the past 10 years or so, become dramatically more complex and varied than they were.

The new developments are not only in the analysers, particularly in the areas of specificity and speed of response, but also in the sample preparation. Advances in the design of sampling probes, the techniques of sample conditioning and the methods of interfacing gas analysers with production control computers have all made major steps forward.

* The Author is with Servomex.