Augicet 1952
Instrumentation Infrared analyzers can solve many continuous analysis problems ; however, the prospective user must understand their operating principles b@ Ralph H. Micnch
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are becoming more and more familiar with the use of infrared absorption spectroscopy for the analysis of complex mixtures of compounds in research and production control laboratories. Some type of infrared spectrometer is used for the work done in such laboratories. Basically, a spectrometer consists of a light source supplying light of all wave lengths in the range to be used, a sample cell, a prism or grating monochromator to select any dwired wave length from those supplied by the source, and a detector, usually a thermocouple or bolometer with an appropriate ampliHEMISTB
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Figure 2. Positive Filter Type
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Figure 3 . e a
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Filter Type
fier and recorder to measure the intensity of the radiation. With such an instrument, a plot of per cent transmission as a function of wave length, commonly known as the absorption spectrum of the sample, can be prepared. For example, Figure 1 shows the absorption spectra of carbon dioxide and methane. The wave lengths a t which absorption occurs and the relative intensities of the absorption bands are (Continued on page 82 A )
Figure 1. Absorption Spectra of Carbon Dioxide and Methane
Figure 4.
August 1952
Absorption Spectra of Hydrogen Cyanide and Acetylene
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characteristic of the sample. T h e intensity of the absorption is a function of the effective thickness of the sample layer. These facts are the basis for qualitative and quantitative analysis by infrared methods. While satisfactory control of some processes can be achieved b y occasional analysis performed in the laboratory, others require instruments which can give a continuous record of the concentrations of certain critical materials. Infrared spectrometers of the type used in the laboratory are not at all adapted for installation in a plant. They a r e large, complex, expensive, and subject t o damage b y humid atmospheres. To avoid these disadvantages, two filter types of infrared photometers (Figures 2 and 3) have been designed for continuous analysis of flowing plank streams. Figure 3 shows a schematic diagram of' the negative filter or nonselective detector type of infrared gas analyzer. The light source, A , in this instrument is a coil of' Nichrome wire heated to a red heat b y passing a suitable current through it. Radiation from the source is focused through two symmetrical paths onto the arms B and B of a bolometer by concave mirror Ma Both beams pass through sample cell S and interference cell I , while one beam passes through the cell, F , and the other passes through the compensator cell, C. Now, if a mixture of nitrogen which has no infrared absorption and carbon dioxide which has the spectrum shown in Figure 1 is passed through the sample cell, equal amounts o f radiation will be absorbed from both beams. To sensitize the instrument t o carbon dioxide, one atmosphere of carbon dioxide is introduced into filter cell F. This results in all the radiation capable o f being absorbed by carbon dioxide being removed from the beam falling on bolometer arm B. Now if a sample containing carbon dioxide is passed through the sample cell, the intensity of the beam falling on bolometer arm B' will be reduced while that falling on B will be unchanged. rhus the electrical output from the bolom2ter bridge will be a function of the amount 2f carbon dioxide in the sample cell. The nstrument will not be affected by other gases unless they possess absorption bands werlapping those of carbon dioxide. For nstance, it could be used to record the :oncentration of carbon dioxide in methme, since as can be seen from Figure 1 the ibsorption bands of the two gases d o lot overlap.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 44, No. 8
b y placing acetylene in filter cell F , t h e instrument would b e almost as sensitive t o hydrogen cyanide as t o acetylene. T h e sensitivity t o hydrogen cyanide could be largely eliminated b y filling t h e interference cell with t h a t gas, t h u s filtering out of both beams all light t h a t can b e absorbed by hydrogen cyanide. If t h e instrument still showed a residual sensitivity t o hydrogen cyanide, reverse sensitizing b y placing just t h e right hydrogen cyanide concentration in t h e compensator cell, C , could be used t o reduce t h e sensitivity t o t h a t gas to a negligible value. T h e technique of sensitizing a gas analyzer is empirical. T h e spectra serve as a guide b u t d o n o t tell t h e whole story because t h e fine structure of the bands is not resolved. The other t y e of infrared gas analyzer, the positive &ter or selective receiver type, is shown schematically in Figure 2. Light from identical sources A and A' passes through identical cells S and C to receivers R and R'. The radiation in the two paths is interrupted simultaneously a t a frequency of 12 cycles per second by the shutter driven b y motor M . This style of instrument is sensitized by placing the gas to be measured in R and R', which are separated by diaphragm D adjacent to grid G. When the shutter permits radiation to reach R and R', energy absorbed by the gas filling raises its temperature, increasing its pressure to the same extent in each so that no movement of the diaphragm results. When the beams are occulted, both receivers cool and decrease in pressure to the same extent. Now if a Sam le containing the gas used to fill R and R P.is introduced into sample cell S, less radiation of a wave length t o which R is sensitive will reach it and there will be a differential pressure across the diaphragm causing it to move with respect to grid G . This vanes the electrical capacitance between the diaphragm and G so that an electrical signal can be obtained which is a function of the amount of absorbing gas in S. If there were overlapping absorption by another constituent of the sample a cell filled with the interfering substance placed in both beams could be used to eliminate the interference.
This - is u FISH stow! -
Correspondence concerning this column will be forwarded promptly if addressed to the author, % Editor, INDUSTRIAL AND ENGINEERING CHEMISTRY,1155--16th St.,N.W.,Washington& D. C.
August 1952
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Over 3,000,000 Fish Processed Daily in a well known southern plant using.. ~
These two types of infrared gas analyzers each have specific advantages a n d limitations. W e will a t t e m p t t o describe these next month.
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@EVAPORATORS Perhaps in your industry, too, increased profits and lower costs may result f r o m consultation w i t h O B Process Engineers.
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CHEMISTRY
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