Chemical Engineering October 2021 - 27

a sensing chamber containing a specific
gas, the only wavelengths that
are absorbed are the wavelengths
that match the chemical bonds
in that gas. The rest of the light is
transmitted through the chamber
without hindrance. As most chemical
compounds absorb at several
different frequencies, IR absorbance
can be used for measuring the
gas concentration.
FIGURE 4. The detection mechanism of metal-oxide
semiconductor sensors involves the measurement
of conductivity as the target gas is oxidized
on the sensing element
are available to detect ammonia
vapor and other toxic or combustible
gases within industrial applications.
The following sections look at the
operating principles behind these
technologies with an eye toward
understanding their strengths and
potential weaknesses.
Optical infrared technologies
Optical infrared (IR) sensors have
been used for decades to monitor
combustible and toxic gas leaks,
including ammonia (Figure 1). The
key benefits of IR sensors are their
high specificity to ammonia and reduced
(or even eliminated) need for
calibration adjustment. IR detectors
have a wide dynamic range and
are not degraded or consumed by
exposure to high concentrations of
ammonia. Their main limitations are
the physical size of the detector assembly,
the need to protect the detector
against the potential effects
of fluctuating temperature and humidity
and higher cost compared to
other detectors.
Differential infrared. Differential
IR sensors measure gas as a function
of the absorbance of infrared
light. Molecules consist of atoms
that are held together by chemical
bonds. The bonds in a particular
type of molecule (such as ammonia)
absorb energy at selected wavelengths.
When a chemical bond
absorbs infrared light, it continues
to vibrate at the same frequency,
but with greater amplitude after the
transfer of energy. In other words,
molecules that can absorb energy
at that wavelength are heated to a
higher temperature than molecules
that are not able to absorb light at
that wavelength.
When IR radiation passes through
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TDL absorption spectroscopy
Alternatively, for some molecules,
it can be possible to find an absorbance
peak at a specific wavelength
that is not shared by other
molecules that may be present. In
this case, absorbance at a particular
wavelength can be used to provide
a substance-specific measurement
for a specific molecule. This is possible
for some molecules, including
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OCTOBER 2021
27

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Chemical Engineering October 2021

Table of Contents for the Digital Edition of Chemical Engineering October 2021

Contents
Chemical Engineering October 2021 - Cover1
Chemical Engineering October 2021 - Cover2
Chemical Engineering October 2021 - Contents
Chemical Engineering October 2021 - 2
Chemical Engineering October 2021 - 3
Chemical Engineering October 2021 - 4
Chemical Engineering October 2021 - 5
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Chemical Engineering October 2021 - Cover3
Chemical Engineering October 2021 - Cover4
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