Electrochemical and thick-film sensors are fine for measuring many gases, but more challenging applications are often better tackled using infrared photometry.
Traditional gas analysis applications are well catered for in terms of reliable, cost-effective technologies. For example, measurement of CO, CO2 and hydrocarbons can be carried out with analysers based on thick film or electrochemical sensors. Some of these technologies have limitations when the sample gas is flammable, corrosive or toxic, in which case alternative technologies have to be adopted. Infrared photometry can be adaptable to demanding process applications, depending on the detailed design of the analyser.
Infrared analysis works on the principle that molecules of many gases absorb energy in the infrared region of the electromagnetic radiation spectrum. Specific wavelengths of IR energy cause the bonds within the molecules to vibrate; each type of molecule will be excited by different wavelengths and, therefore, absorb energy from infrared light at their specific wavelengths when it passes through the gas sample. However, the important point is that each gas has a unique absorption spectrum, which can be likened to a photochemical fingerprint. So by passing infrared light through a sample containing the target gas and by measuring the reduction in the received energy, it is possible to gain an indication of the gas concentration within the sample.
Turning this principle into a gas analyser suitable for use within an industrial environment is highly complex and a number of alternative approaches can be used to overcome particular difficulties. For example, a system using a single beam of light and a single wavelength, known as SBSW, can be subject to zero drift and sensitivity drift, as there is no reference against which the measurement can be compared. Such analysers therefore tend to require frequent calibration if they are to remain accurate.
Another technique is the 'classic' dual-beam configuration, where energy from a single source is split into two separate paths, one containing a sealed reference cell and the other a cell containing the sample to be measured. However, such techniques do have limitations, especially under harsh process conditions. The Luft or microflow detector used to measure the received energy from the two cells can leak, the reference cell can leak, and Luft cells are particularly susceptible to vibration. Furthermore, the gold plating used in the cells to reflect the light can be damaged by corrosive gas samples, and most cells have factory-sealed windows, making cleaning and maintenance impossible for the operator.
To overcome these problems, a single beam dual wavelength (SBDW) or single beam multiple wavelength (SBMW) arrangement can be used - as in the Servomex 2500 and 2550 process gas analysers. This has the advantage that there is a single optical path and a robust solid-state pyroelectric detector instead of the Luft or microflow detector. A motor-driven chopper wheel alternately places two filters into the optical path. The measure filter is designed so that a relatively high proportion of the energy passing through will be absorbed by the gas to be measured, while the reference filter passes infrared energy that will be largely unaffected by either the target gas or the other gases present in the sample cell.
The pyroelectric detector measures the received energy and, because the measurement and reference readings are continually being compared with each other, any build-up of dirt on the optics or reduction in energy from the infrared source will be automatically compensated for (zero drift and sensitivity drift are therefore both inherently low). Onboard electronics convert the received energy levels to a reading of the gas concentration. Note that the Servomex 2500 and 2550 process gas analysers are designed in such a way that the sample cell can be easily removed for cleaning and maintenance. As a result, this type of SBDW instrument is accurate and reliable for process control, plant safety and legislative compliance applications.
For measuring low gas concentrations, or for applications where there is a strong possibility of cross-interference from other gases in the sample, an alternative is to use gas filter photometry - or gas filter correlation (GFC), the term used by Servomex for the technology as applied in the 2510 analyser. This technique is often used for gases such as CO, NO, CO2, SO2 and HCl that have a clearly defined fine structure absorption spectrum. In addition, HCl is a notoriously weak absorber of infrared energy, making low-level measurements difficult using traditional infrared photometry.
Gas filter photometry is similar to SBDW except that instead of the reference and measure optical filters, the chopper wheel is fitted with a pair of sealed cuvettes, one containing nitrogen and the other containing a closely controlled sample of the measurement gas. An optical bypass filter is also positioned permanently in the optical path to minimise saturation and further improve cross-interference rejection. As with SBDW, the pyroelectric detector measures and compares two energy absorption values, which results in a reading that is largely unaffected by any build-up of dirt on the optics.
Two strengths of gas filter photometry are selectivity and sensitivity in an instrument that is reliable in an industrial environment. For instance, the technology can be used for measuring low-level CO in the presence of CO2, which would normally cause problems due to cross-interference. It can also be employed to achieve a rejection ratio of target gas to background gases in the order of 100,000:1 when measuring CO in crack gas streams in ethylene plants. Nevertheless, one of the reasons behind the success of the Servomex 2510 gas analyser is that Servomex has developed a way of ensuring that the cuvettes remain sealed, as otherwise any leakage would lead to measurement inaccuracies and a loss of sensitivity.
While the above descriptions outline the technologies involved, there are some classes of applications for which infrared photometry offers additional benefits. For example, the Servomex 2550, with its SBMW (single beam, multi-wavelength) technology, can be used for multi-component (multi-gas) measurements by using multiple optical filters. Clearly the suitability will depend on the gases to be measured, their relative absorption rates and the ability of the software to deal with cross-channel interference (cross-talk). For those applications where multicomponent measurements are technologically feasible, the potential savings in purchase and installation costs, as well as ongoing maintenance costs, can be substantial. Typical multicomponent measurements would be for CO, CO2 and hydrocarbons.
One of the design highlights of the Servomex infrared process analysers is the design of the sample cell, which allows for easy maintenance, combined with an analyser design that ensures the sample gas remains isolated from the electronics. This means the instruments lend themselves well to the analysis of corrosive, hazardous and toxic gases.
Furthermore, the design of the sample cell can be adapted to the measurement of certain liquids. Care has to be taken to ensure that the sample cell is mounted horizontally so that any entrained bubbles can escape and, in addition, the sample cell flow characteristics are designed to avoid the creation of vortices and cavitation effects that could otherwise affect the measurement. The technology can be used extremely successfully for applications such as the measurement of trace water in EDC (ethylene dichloride).
Another point to be aware of in liquid applications is temperature sensitivity: for water in EDC applications, one degree C increase in temperature can cause the reading to fall by approximately 1 ppm (part per million), which is a significant amount when the measurement range of interest is typically 0-20 ppm. Whereas earlier designs of liquid analysers used heating systems in an attempt to maintain a constant sample temperature, the modern approach is to use real-time temperature compensation to correct for any changes in reading due to sample temperature changes.
Other challenging applications for which specialised infrared photometry is suitable include the measurement of phosgene on TDI (toluene di-isocyanate) plants, as well as the measurement of methyl chloride and methylene chloride. This is partially due to the readily configurable nature of the technology in terms of the wavelengths, optics and filters. Indeed, custom-configured instruments offer high reliability without compromising accuracy, which can result in a lower cost of ownership than might be the case if a standard instrument utilising another measurement technology was used, which had to be frequently maintained, repaired or replaced.
Servomex has extensive experience in supplying infrared analysers into these applications. Contact Servomex for more information about these or the Servomex 2500 series infrared gas analysers, as well as other Servomex gas analysers, all of which are available and fully supported worldwide.