Infrared Analysis Applications

Infrared (IR and NDIR) Absorption Analysis


Plots of electromagnetic energy absorption between 2 and 15 µm are known as infrared (IR) absorption spectra.

The number of IR absorption bands and the intensity of the absorption are directly related to the molecular structure. However there are molecular-specific absorptions exhibited within a series of compounds. For example, all organic compounds tend to exhibit IR absorption at about 3.4 µm, the group frequency of the ‘C-H’ bond vibration. Thus it may not be possible to discriminate among a mix of hydrocarbons at this wavelength and other absorptions bands should be utilized.

Process IR analyzers are dedicated quantitative analysis devices designed to monitor a single key component in a multicomponent stream. A wavelength is selected for which the component of interest (analyte) strongly absorbs infrared and background components are transparent. Process infrared analyzers do not use prisms or gratings and thus are defined as nondispersive infrared (NDIR) analyzers.



Common applications:

  • A typical process application is the measurement of 0 to 1% methane from primary and secondary reformers in an ammonia synthesis plant. The analysis is performed at 3.3 µm where methane absorbs infrared energy and background components (CO2, H2O, CO and N2) are transparent.
  • Hydrogen plants typically monitor process streams for total carbon oxides for catalyst protection.
  • In a ethylene oxide plant, NDIR analyzers are used to monitor ethylene feed gas purity (methane and ethane contaminants), ethylene in the reactor feed and recycle streams, CO2 in the reactor inlet and outlet streams…
  •  The measurement of carbon monoxide in air to meet air-quality standards is often performed by NDIR analyzers
  • Continuous emission monitoring systems are used on stacks for measurement of SO2, NOX and other pollutants.
  • Sensors for water quality analysis, such ad total carbon (TC) and total organic carbon (TOC) are infrared analyzers.
  • Liquid applications involve the measurement of water in petrochemicals liquids, such as 0 to 200 ppm water in ethylene dichloride.



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