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What optical methods can be used to measure the concentration of reacting gases in a reactor during an exothermic and fast reaction? I need to monitor and measure the local concentration of my gases (in the methane production reaction) along a reactor bed in order to be able to clarify the kinetics of this reaction. Which optical techniques are able to make such measurement?

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There are a number of techniques but what you would use depends on how you plan to start the reaction. What use to be called Flash-Photolysis and now called Pump-Probe Spectroscopy is the most general and can be used from the femtosecond to multi-second time range. This is a general method as it measured the absorption of all species produced. It works by using a suitable duration and wavelength flash of light, usually from a laser, to initiate the reaction and then at various time delays after this, the absorption spectrum of any species produced is measured. Absorption can be in the uv, visible or ir ranges. Sometimes Raman scattering or fluorescence can be used instead of absorption. What is used depends on equipment available and the species you need to measure.

If you cannot start the reaction with light then mixing methods can be used, but these are relatively slow, longer than microsecond time scale and more like milliseconds. In the Stopped-Flow method the reactant gases are mixed as rapidly as possible in the chamber so as to start the reaction then this is monitored with absorption etc. as above. Additionally, there are flow methods whereby the gaseous reactants are mixed at the end of a long flow tube and are monitored spectroscopically at various points as they flow along a tube. The gases are pumped to cause the flow and so distance along the tube is an alias for reaction time. Each of these methods can be used with gases or liquids.

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  • $\begingroup$ The reaction is a catalytic reaction and is very fast and exothermic (accompanied with hotspot formation) and we use packed bed reactors for applying the catalyst in it. my purpose was measuring the local concentration of the gas species (methane, carbon mono oxide, carbon dioxide, water, hydrogen) with some optical method e.g. Raman spectroscopy. But I am not quit sure about the details of the system that could actually work. $\endgroup$ – SarvF Aug 4 '17 at 12:13
  • $\begingroup$ It is quite routine to measure on the nanosecond timescale, is this fast enough? Focussed nanosecond lasers are intense and can produce good raman spectra from the focal region so if you can get your laser to the region you need then this is a possibility. However, sensitivity is low. Have a look therefore at the technique called CARS (coherent anti-stokes raman scattering) and related methods. The book 'Laser Spectroscopy, basic concepts and instrumentation' 2nd ed by Demtroder although written some time ago is still good for details on all aspects of lasers and detectors etc. $\endgroup$ – porphyrin Aug 4 '17 at 16:37

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