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D. R. SIMONSEN and H. W. CROUCH Eastman Kodak Co., Rochester, N. Y.
Monitoring of Solvent-Air Mixtures with Infrared Analyzers Vapors from organic solvents used in many processes may approach hazardous level of toxicity or explosibility. Use an infrared analyzer to control vapor concentrations
IN
MANY PROCESSES where organic solvents are used, solvent vapor-air mixtures are formed in which the concentration of vapor may approach a hazardous level of toxicity or explosibility. This is particularly true in situations where the solvents at a higher concentration are recovered for economical reasons. A number of instruments are available which will continuously monitor the concentration and thus provide the necessary control. The solvents used and the allowable concentrations vary over wide limits, and the most suitable type of instrument varies with these conditions.
These instruments are generally unsuitable for chlorinated solvents because of the corrosive action of the resulting chlorine and hydrochloric acid on the hot filament. Sulfur compounds and a few other elements or compounds also make the instrument unreliable as they poison the filament. Even with compounds containing only carbon, hydrogen, and oxygen the filaments slowly oxidize and eventually need to be replaced. Because of the aging, the filaments are checked about twice a week and should be replaced every few months when in continuous use. Thermal Conductivity Analyzer
Hot Wire Filament Analyzer
One of the instruments used is the so-called hot wire instrument where a sample of the combustible gas is passed over one of two similar heated filaments, The filaments, heated from either batteries or a rectifier connected to a power supply, are connected into a bridge circuit. When the solvent vapor comes in contact with the measuring filament, part of it is burned. The heat from this burning raises the temperature of the filament which unbalances the bridge; this is a measure of the combustibility of the vapor-air mixture. I n practice the filaments are often platinum or alloys of platinum which have a high temperature coefficient of resistance. Platinum has some catalytic properties which aid in the combustion. Instruments of this type are suitable for analyzing hydrocarbons and solvent mixtures primarily composed of hydrogen, carbon, and oxygen. Equal volumes of the lower explosive limits of these solvents produce approximately the same amount of heat when burned. The instruments are not equally efficient in the combustion of all of the solvents and thus, do not give the same indication for the lower explosive limits. Correction for this variation is normally made in the calibration.
The thermal conductivity cell can be used to measure solvent vapor concentrations. All of the common solvents have a lower conductivity than air. Many of the solvents of the hydrocarbon, alcohol, and ketone types give readings that are roughly proportional to the lower explosive limits, so the explosibility of the mixture is indicated by the thermal conductivity. This is generally not true when the solvent contains elements other than carbon, hydrogen, and oxygen. This correlation between explosibility and thermal conductivity can be considerably improved by passing the solvent-air mixture through a hot tube which completely burns the solvent and then measures the resulting concentration of carbon dioxide. This process has the added advantage that the error caused by chlorinated solvents and water vapor in the original mixture can be reduced. Thermal conductivity cells require very good temperature control and frequent calibration which reduces their general applicability of measuring explosibility. The response in the lower explosive limit varies with the ratio of the components in the mixture, and often the limits must be set unreasonably low to ensure safety. Another instrument, used for measur-
ing the concentration of solvent vapors where the ratio of the solvent vapors does not vary, compares the vapor density of the mixture with the density of air. This instrument measures the explosibility by the relative density of the gas. I t is unsuitable for solvents with a molecular weight about equal to air. Infrared Gas Analyzer
In recent years a number of continuous nondispersive infrared gas analyzers have appeared on the market. Infrared * analyzers require no burning of the solvent vapors and are not affected by chlorinated or sulfur compounds. Also, they are highly selective and have rapid response. Because of these advantages some work has been done to adapt the infrared analyzer for monitoring rather complex mixtures of solvent vapors of organic solvents including chlorinated solvents. Numerous references (7, 5, 7-9) in the literature describe the sensitization of continuous infrared analyzers. However, some of the commercially available nondispersive infrared gas analyzers are described briefly to give some background on how they work, and in particular, how they can be set up to obtain readings of the lower explosive limit of a vapor-air mixture containing more than one solvent vapor. Two general types of nondispersive infrared analyzers are those using nonselective receivers such as bolometers or thermopiles and those employing condenser microphone receivers that are selective in their response. The former are known as negative-type analyzers while the latter are positivetype analyzers. The terms negative and positive arose because, in the simpler cases, the receivers of negative analyzers measure the infrared radiation remaining after the infrared bands involved in the analysis have been removed, whereas the receivers of the positive analyzers measure only the radiation of the absorption VOL. 49, NO. 10
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bands of the material being analyzed. The positive-type analyzers can also be negatively sensitized or set up in a similar way to the negative type of analyzers. The optical units of negative-type infrared analyzers operate on the same principle but differ in details such as cell arrangement and type of receiver. All of the analyzers have tw-o beams so that the measurement depends upon the difference between the two beams. In negative-type instruments one beam from the single source passes through an interference or contaminant cell, the sample cell, and a sensitizing cell called the filter cell, before striking one receiver. The other beam likewise passes through the first two cells and then through a different sensitizing cell called the compensating cell before striking the other receiver. The analyzer responds to a given material by placing that material in the filter cell. For example, carbon dioxide in the filter cell would cause the analyzer to respond to carbon dioxide because the receiver in the filter beam would be affected less by carbon dioxide in the sample cell than would the receiver in the reference or compensating beam, If carbon dioxide placed in the filter cell causes the analyzer to respond positively (upscale) to carbon dioxide! then carbon dioxide in the compensating cell would cause the analyzer to respond negatively (downscale) to carbon dioxide. The compensating cell is used to make the analyzer insensitive to infrared absorbing materials which have absorption bands overlapping those of the material being analyzed. The interference cell cuts across both beams, and so an infrared absorbing material placed in that cell will reduce the sensitivity of both beams to that material. The interference cell reduces the response of the analyzer to infrared absorbing materials having infrared absorption bands that overlap slightly with those of the material being analyzed. It is also useful for adjusting the linearity of response of the analyzer. All positive-type infrared analyzers use condenser microphone detectors. These detectors consist of two chambers filled with gas separated by a flexible diaphragm, which is one plate of a condenser microphone. When the gas in one chamber absorbs more infrared radiation than the gas in the other chamber, a differential pressure is produced causing the diaphragm to move. To increase both stability and sensitivity the two beams are chopped SO that changes rather than absolute values of capacitance are measured. Various commercially available analyzers differ in cell arrangement, chopping speed, and whether they are deflection or null instruments. A pure sample of the gas component to be analyzed is placed in the condenser microphone detector. The two beams are chopped, and if the radiation in each beam is not equal
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the metallic membrane Ivhich is one plate of the condenser microphone will oscillate due to heating of the gas. This oscillation is a measure of the concentration of the infrared absorbing material in the sample cell. The negative sensitization principle can also be used in the positive-type analyzer. This is used when more correction is needed for interfering materials than is possible with a single filter cell. It is accomplished by putting a sensitizing or filter cell in each beam. Sensitization of Jnfrared Gas Analyzers
The problem of sensitizing an infrared analyzer so that it will respond in terms of the lower explosive limit to one or a number of solvent vapors in air becomes more complicated as the number and chemical variety of the solvents increase. Infrared gas analyzers are easily set up to monitor mixtures of air and a single solvent since all solvent vapors absorb energy in the infrared region of the spectrum, whereas the elemental gases such as nitrogen and oxygen do not absorb infrared radiation. Some of the solvent vapor must be placed in the sensitizing or detector cell to sensitize the instrument to that vapor. An analyzer so sensitized may be sensitive to water and other vapors. This sensitivity can be reduced by the proper selection of window materials and/or the use of filter cells. To sensitize the infrared analyzers to mixtures of two solvent vapors so that the analyzer responds in terms of the lower explosive limit of the mixture in air is more difficult. For example, if a solvent vapor-air mixture contained acetone and benzene it would be necessary for the analyzer to give the same response to 2.5% by volume of acetonethe lower explosive limit of acetone in air-as to l.4yo of benzene-the lower explosive limit of benzene in air. Furthermore, the response should be suffi-
Table 1.
Response of lnfrared
Solvent Vapor
Leeds and Northrup. Approximate value.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Special Nine-Solvent Application
An application was encountered in which it was desired to determine the concentration of a mixture of nine solvent vapors in air in terms of the per cent of the lower explosive limit. For praciical and safety reasons it was assumed that the solvent vapor in any given sample could be made with any combination possible of the nine different solvents. The solvents were methyl, ethyl, isopropyl, and n-butyl alcohols, acetone, cyclohexane, and methylene, ethylene, and propylene chlorides. The lower explosive limits of the vapors of these solvents in air are shown in Table I. The problem was to sensitize an analyzer so that it Ivould give not only the same response to these concentrations of the various single solvents! but also the same relative response to these solvent vapors lvhen present in mixtures. To get the same relative response to the individual solvents when alone or in mixtures it is necessary that the response of the an-
Gas Analyzer
Methyl alcohol Ethyl alcohol Isopropyl alcohol %-Butyl alcohol Acetone Cyclohexane Methylene chloride Ethylene chloride Propylene chloride Nitrogen saturated with water a t room temperature a
ciently linear so that the responsc to mixtures would be additive as the concentrations of the components of a mixture are additive when expressed in per cent of the lower explosive limir. The analyzer is first sensitized to give maximum response to that solvent which produces the least response. Then, methods are investigated to reduce the response of the other solvent. Spectrophotometric curves aid in this step. Kext, the response of the analyzer to water vapor is checked so that the analyzer is made insensitive to water vapor particularly if the humidity of the sample is subject to change. It ma)- be necessary to repeat these steps several times to get The best compromise between maximum signal and closest agreement in the desired response to the individual solvents. For mixtures of more than two solvents the difficulties rapidly increase, but the satne techniques are used.
to Solvent Vapor-Air Mixtures
Lower Explosive Limit in Air, % by T’ol. 7.0 3.5 2.5
1.7 2.5 1.3 15b 6.2 3.5
Response of L & XIL Analyzer, Microvolts 80% L.E.L. 40% L.E.L. 109 96 103 103 99 89 91 100 97
57 51 57 51 54 57 61 54 52 -4
S A F E T Y IN C H E M I C A L I N D U S T R Y alyzer be reasonably linear. I n addition to adjusting the response of the analyzer to nine solvents it was also desirable that the response to water vapor should be negligible. The sensitizing of an infrared analyzer for the special application was worked on cooperatively with most of the manufacturers of commercially available instruments. The results indicated that these analyzers could be sensitized satisfactorily for this nine-solvent application providing the proper source temperature and window materials are used. I t was found that some compensation or negative desensitization was necessary to make the response of the analyzer to water be negligible. This required that the sample cell be placed in both beams of the analyzer in all analyzers except the Liston-Becker Model 30, which has two detectors, the second of which gives a negative response and thereby compensates for the response to water vapor. The sensitization of the Leeds and Northrup analyzer for the nine-solvent application is described as an example of the sensitizing problem of any commercially available analyzer. The analyzer was used with a source temperature of approximately 800’ F. and with cell windows of calcium fluoride. First, the response of the analyzer to the vapors of the nine individual solvents was determined when sensitized to each of these nine solvents. Then some related compounds which are gases at room temperature-for example, ethyl chloride, methane, and propane-were tried out for sensitizing the analyzer. The response of the analyzer followed a pattern in which the solvents fell into the following groups :
response to the alcohols without materially affecting the response to cyclohexane and the chlorinated solvents can be solved another way. This involves turning the light trimmer in the compensating beam into the beam until the response to the alcohols equals that of the cyclohexane and the chlorinated solvents. This method works if the analyzer is sensitized with materials which have infrared absorption bands corresponding to the compounds that require maximum response (in this case, cyclohexane and the chlorinated solvents), and if the interfering materials have relatively strong absorption bands which are in other regions of the spectrum and/or widely overlap the bands of the sensitizing materials. A number of workers (2-4, 6) have referred to this technique of reducing the response to interfering compounds. This method is particularly adaptable to the negativetype infrared analyzer. I n discussing the nine-solvent application so far the response to the chlorinated solvents and cyclohexane was controlled by the materials used in the sensitizing cell. The response of the alcohols was corrected by the compensator trimmer adjustment. The response of the analyzer to acetone could be controlled independently because it is the only solvent which has the strongly absorbing carbonyl group. I t was found that a small amount of acetone vapor placed in the sensitizing cell took care of the acetone. This includes all of the vapors affecting the response of the analyzer except water vapor. The response to water vapor was affected strongly by the trimmer adjustment, and if the trimmer was turned in sufficiently to correct for the response to the alcohols, the
produced a higher response to the chlorinated solvents than the vapors of the chlorinated solvents themselves. This was undoubtedly due to the high concentration possible with the gases as contrasted to the solvent vapors. In a representative solution to the sensitizing problem the analyzer was set up as follows: The compensating sensitizing cell was filled with 1.01% isopropyl alcohol and 5.oYc methyl alcohol in argon at atmospheric pressure. The sensitizing cell was filled with 5.0% propylene chloride, 0.16% acetone, 25.170 propane, and 36.6% methane in argon a t atmospheric pressure. The contaminant cell was filled with nitrogen bubbled through water at 44’ C. The compensating trimmer was turned in 16 complete turns. The response in microvolts to the solvent vapors in SOYo and 40% of their lower explosive limits for a 2.5-inch sample cell is shown in Table I. The linearity of response could be improved by using a shorter sample cell, but a shorter cell would also reduce the level of response. The 2.5-inch sample cell was selected as a good compromise between linearity and level of response. The success of the application of an infrared analyzer for monitoring an industrial process, particularly for safety purposes, depends upon the reliability of the instrument as much if not more than upon the sensitization of the analyzer. The reliability includes zero and calibration stability, and freedom from maintenance. Furthermore, the analyzer should be set up so it fails on the safe side. An evaluation of all of these factors requires considerable time and depends on the application that is to be made of the infrared analyzer. literature Cited
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IV
Methyl alcohol Ethyl alcohol Isopropyl alcohol n-Butyl alcohol
Cyclohexane
Acetone
Methylene chloride Ethylene chloride Propylene chloride
With the analyzer operating with its beams balanced the alcohols gave too high a response relative to the other solvents even when the analyzer was sensitized with the other solvents. By putting sufficient methyl alcohol in the compensating or negative sensitizing cell the response to it and the other alcohols could be reduced relative to the chlorinated solvents until their response in lower explosive limits was equal. However, if the response of the alcohols is reduced in this way with the beams balanced, the response to cyclohexane is reduced too much. Reducing the
response of the analyzer to water vapor could be negative. This problem was overcome by placing just enough alcohol vapor in the compensating sensitizing cell to bring the alcohols in line with the other solvents after the trimmer had been turned in as far as the water-vapor response would permit. The exact composition of the mixtures in the sensitizing cells and the position of the trimmer screws are influenced by the purity, particularly dryness, of the solvent vapors and the characteristics of the optics of the infrared analyzer. A mixture of methane and propane
(1) Gray, W. T., ZSA Journal 2, 189 (1955). (2) Hollander, L., Martin, G. A., Skarstrom, C. W., IND.END. CHEM.46, 1377 (1954). (3) Koppius, 0. G., Anal. Chem. 23, 554 (1951). (4) Leeds & Northrup Go., Philadelphia, Pa., “Infrared Gas Analyzer,” Bull. 7804, p. 26. (5) Martin, R. L., Thomas, B. W., IND. ENG.CHEM.46,1393 (1954). ( 6 ) Ogden, G. W., private communication (Nov 19, 1954). (7) Smith, L. N., Instruments 26, 421 (1953). ( 8 ) Wall, R. F., Giusti, A. L., Fitzpatrick, J. W., Wood, C. E., IND. ENC. CHEM.46.1387 (1954). (9) Waters, J.’ L., Hartz,’ N. w., ZSA Journal 25, 57 (1952). RECEIVED for review .4pril 27, 1957 ACCEPTED July 19, 1957 Division of Industrial and Engineering Chemistry, Symposium on Safety in the Chemical Industry, 131st Meeting, ACS, Miami, Fla., April 1957. VOL. 49, NO. 10
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