Optical spectroscopy techniques for gas analysis

Several Tunable Diode Laser gas analyser systems have been reported recently, in quite different applications! This is a review of the different systems, to illustrate the scope of the technology.

Recently we have seen several applications quoted for Tunable Diode Laser gas analyser systems, in quite different applications, and price levels! These are those known as at September 2007.

** The Yokogawa product launch in Europe of the ‘TruePeak’ tunable diode laser analyser, for gas analytical measurements, described the background to the technology: this unit uses a beam transmitted directly across a pipeline or flue.

Straightaway this gives a major advantage over GC and similar gas analysers, which have to use an extracted sample, which inevitably undergoes cleaning and conditioning: this can seriously affect the chemicals and relative concentrations that are present in the stream, before they reach the analyser.

The TruePeak is an in-situ optical transmission system, inserted into the process line, so it avoids the need for a sample, and also gives a reading in a few seconds, rather than the minutes or hours that sampling systems might require.

This unit is offered for monitoring O2, CO, CO2 and moisture on-line, in situ, using measurements at narrow absorption peaks specific to these gases, compared to reference measurements made at wavelengths away from absorption peaks, to give the zero reference.

This is where the tuning is required.

Once again the instrumentation industry benefits from telecommunications: without the mobile phone industry we would not have the components available that have made radar level measurement possible, or for that matter, wireless adaptations of pressure transmitters.

This time the tunable lasers that are used by this Yokogawa technology have been made available because of the need for tunable lasers for near infra-red fibre-optic transmission of telephone conversations.

It is pure serendipity that these devices can be tuned across these useful gas analysis absorption lines.

The near IR region is a good region for monitoring dual element gases, like H2O, CO, CO2, and the technique has been extended to H2S and Hydrocarbons as well.

Yokogawa admit that all the research and basic work has come from Dow Chemicals: the pioneers there have hundreds of these units installed, in Dow processes across the world.

The obvious application for these analysers is in combustion gas analysers, to ensure the boiler or furnace is operating at peak efficiency, with the correct fuel / air mixture.

But the Dow applications have gone further, their units are also used for monitoring water vapour in chlorine plants, to stop corrosion: and critically for the monitoring of acetylene at 1-2ppm in ethylene plants, which allows the plant efficiency to be much improved.

Further serendipity comes to answer the query that the more alert of you readers will be asking: how come a near IR analyser can measure O2 then, as it is not a dual element gas? Apparently this is one of nature’s quirks, that oxygen is paramagnetic, both outer electrons spin in the same direction, which is what makes oxygen so important maybe! But it produces a near IR absorption band, when it should not have one! Such analysers get immediate application in monitoring incinerators etc, preventing harmful emissions, because the unit can work at temperatures up to 1500C.

This TruePeak is not cheap, but it is not an inordinately expensive unit: and it must be one of the best of the ‘Boy’s Toys’ available to satisfy any Process Gas Analytical Research Chemist this Christmas!.

** Vaisala also introduced an oxygen monitoring system using a similar tunable diode laser detector earlier in the year: this is called the Vaisala Spectracap Oxygen Transmitter.

Again this can be flange-mounted directly into a process without sampling or sample conditioning equipment, but this unit uses a stainless steel probe in the flow stream, which tolerates chemicals and excessive amounts of moisture.

This Vaisala probe is quoted as suitable for process gas monitoring, in terms of monitoring oxygen in gas generation systems, inert gas blanket monitors, and monitoring the gas levels in composting and biological processes.

Their unit is not intrinsically safe, and I have the impression it would not be suitable for insertion into flue gas streams.

But a sampler system is available, to extract a small gas flow for analysis.

Currently Vaisala only offer this unit tuned for oxygen measurements.

** Offshore Europe in Aberdeen turned up yet another application of a Tunable Diode Laser (TDL) in a gas detection system from Allison Engineering.

Apparently their TDL system is used as a perimeter gas detection alarm, with the laser beam bounced around a whole area, like a chemical plant or a compound.

The typical gases detected this time are HF or H2S, in the atmosphere traversed by the light beam.

The concentration in the atmosphere is monitored to provide an alarm system to initiate a process shutdown of the plant should a major escape of these gases be seen, detected by the TDL, to protect the surrounding population, and environment.

Such systems as these are apparently in operation around critical areas on chemical plants in Coryton and Grangemouth.

** A Manchester based company, surprisingly called TDL Sensors, announced a process tunable diode laser spectrometer in 2006.

This product was quoted as designed primarily for the in-situ measurement of gases such as O2, CO, CO2, H2O, HCl, HF and NH3 from ambient temperatures up to 1500C, with both cross duct and single access point configurations available.

** Water vapour measurement in natural gas is particularly important as high moisture levels can encourage the formation of methane hydrate, a solid that forms in the pipeline and reduces flow or can even block the line completely.

The heart of the water vapour sensing system from IMA for this duty is a small tunable laser diode (about 2mm2) that produces a very narrow and specific wavelength of light tuned to a harmonic of the water vapour molecule in the near-infra-red band.

The light causes the molecule to vibrate and therefore absorb energy.

The laser is scanned through the specific wavelength of interest and, by comparing the light energy being absorbed at a water vapour wavelength to the light energy at surrounding wavelengths, a very precise measurement can be made.

What sets The IMA tunable diode laser (TDL) systems apart from other NIR analysers is the ability to get down to parts-per-million levels.

**Siemens has a Process Analyser that appears to use TDL techniques.

Up to three in-situ cross duct sensors, optionally available in an intrinsically safe version for operation in Ex zones, can be connected with their LDS analyser.

The list of gases able to be measured with their NIR diode lasers includes gases like O2, NH3, HCl, HF, H2O, CO, CO2, H2S, CH4, …and it is continuously growing with the state of the development in semiconductor laser technology.

Even traces of NH3, HCl or HF concentrations can be determined in wet process gases in front of any gas cleaning step.

See the LDS6 on https://pia.khe.siemens.com/index_process_analytics.products_solutions.products_systems.continuous_process_gas_analytics.in-situ-10231.htm.

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