When specifying a process gas analyser and associated sample system one of the first decisions to make is whether the analysis should be done on a ‘wet’ basis or a ‘dry’ basis.
While making the measurement on the wet basis will give the true gas concentrationit requires the sample system and analyser cell to operate at a higher temperature in order to avoid problems associated with condensation. On the other handto make the analysis on a dry basis – at a lower temperature – requires a sample system that removes the condensatesand the reading for the gas concentration needs to be adjusted to take account of those condensates.
David Fahlevice president of hydrocarbon processing at Servomexstates: “Clearly both approaches have their advantages and disadvantagesbut careful consideration of a particular application usually steers the specifier towards one basis in preference to the other. In a recent visit to key customers in the Middle Eastit was clear that careful evaluation of the total stream composition needed to be looked at when specifying and installing a new gas analysis system.”
Take the example of the measurement of oxygen concentration as a means of reducing the explosion risk of a flammable process gas – such as in the inerting of centrifuges or reactors.
The sample gas will typically include various flammable organic solvents and/or water at a temperature above ambientas well as nitrogen usually introduced for inerting – reducing the oxygen level to below the level that ignition can occureven in the presence of flammable vapours. Cooling the gas before analysis will condense and remove some or all of the solvents or watergiving a higher oxygen measurement; on rising oxygen levelsthis will generate an alarm prematurely.
Howeversince this gives enhanced safetythe dry analysis technique will usually be acceptable to the plant operator. It should also be mentioned that often in centrifuge applications when the solvents and or water have been driven offthere is dry powder left in the centrifuge (the product) which may itself be flammableso the low level of oxygen must still be maintained by the injection of nitrogen in order to prevent ignition.
Another example is the reporting of emissions from combustion processes. This often requires measurement to a standard set of conditionswhich may include making the analysis on a dry basis.
The usual method is to condense the water vapour from the sample gas before measuring the pollutants. Alternatively the analysis can be performed at a high temperature without removing the liquid water – a wet analysis – with a separate measurement of the water content to enable the pollutant concentrations to be corrected to give a dry basis equivalent reading. Although this approach is not usually cost-effectivedue to the extra analyser required to measure the waterit is occasionally necessary if one of the components to be measuredperhaps hydrogen chlorideis soluble in water and would be removed with the condensate.
In safety-critical applications it is acceptable to err on the side of safetybut greater accuracy may be demanded where the analysis is performed as part of a process control systemwith implications for product quality or yield. In such casesif the sample gas has a high dew point the measurement must be made without cooling the sample as this would lead to an error due to the condensing of components. An analyser is therefore used with a measuring cell heated to a temperature above the dew point.
A typical example is the air oxidation of cyclohexane to cyclohexanolwhich is a stage in the production of caprolactam used for the manufacture of Nylon6where it is necessary to measure the oxygen content at various points to determine process safety and efficiency.
Iffor instancethe reactor product stream has a dew point of about 103°C and is cooled to ambient temperaturesay 15°Cfor analysisvirtually all the cyclohexanol and cyclohexanone will condense out. This will lead to a large change in sample composition andconsequentlya large and unacceptable error in the oxygen analysis.
To prevent this error the measurement needs to be made without cooling. Instruments such as the Servomex2200H analyser are suitable as a result of the measuring cell that can be heated to either 110 or 135°C (Fig.1). In additionthe analyser is connected to the process using heated sample lines and a heated sample conditioning system.
Howeveras some processes can polymerise at high temperaturesthe temperature of the analyser cell must be carefully controlled to maintain the sample above its dew pointwhile ensuring that the sample is not heated to the polymerisation temperature.
The technology used within an analyser sensor cell clearly has an impact on its suitability for use at elevated temperatures or with samples containing solvents. Magnetodynamic paramagnetic cellsas used in the Servomex2200 series gas analysers for measuring oxygen concentrationbenefit from excellent sensitivity and linearityas well as being virtually immune to cross-interference from other gases and capable of operating at elevated temperatures. Since the measurement principle is purely physicalwith no electrolyte or chemicals consumedthe cell has a virtually unlimited life with minimal maintenance requirements.
For monitoring process reaction component concentrationsphotometric (infrared) analyserssuch as the established Servomex2500 seriesare often used. Heated sensor cells can be used up to 180°C for high dew point sample gases and the selectivity and sensitivity can be specified to suit the process gas.
Fahle adds“Clearly assessing the intended use of the analysis data is critical in deciding on the right type of analysisbe it wet or dry. Howevereven more important is choosing the right technology that will provide long-term peace of mind.”
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