Monitoring Trace Hydrocarbons in Air by Catalytic Oxidation and Nondispersive Infrared Analysis E. J.
ROSENBAUM, R. W. ADAMS, and
H. H. KING,
Jr.
Research and Engineering Deparfrnenf, Sun Oil Co., Marcus Hook, Pa.
b The concentration of hydrocarbons in air a t the low parts per million level i s continuously determined b y combining catalytic oxidation with a nondispersive infrared analyzer to determine the resulting carbon dioxide. Known blends of methane in oxygennitrogen mixtures are used to test catalyst activity and calibrate the combination of catalyst and analyzer. The limit of detectability i s below 1 p.p.m. and the calibration curves are linear up to a concentration of 300 p.p.m. The equipment has operated under plant conditions for over two years with no loss of catalytic activity and only routine maintenance.
I
N THE
low temperature distillation
of liquid air, substantial quantities of liquid oxygen are produced. It is necessary for safety to prevent contact between liquid oxygen and trace hydrocarbons in the feed air Tvhich might be concentrated during the fractionation of the air. The continuous determination of hydrocarbons in the feed air of a large scale ammonia plant was desired to detect promptly any abnormal rise in hydrocarbon level, including quantitative analysis a t concentrations down to a few parts per million. It !vas also essential that thoroughly reliable equipment be used. Initially. no instrumentation suitable for this purpose was knonn and a nondispersive infrared gas analyzer of the selective detector type was chosen. To achieve its selectivity, this instrument uses the absorption of infrared radiation in the detector compartment by the sample component to be determined, Such an instrument is most accurate and sensitive when the detector is sensitized to a single compound. Because the hydrocarbons in the atmosphere may include many compounds in varying proportions, they were first oxidized and the resulting carbon dioxide was determined. This gives a quantitative measure of the total carbon content of the feed air. The oxidation step also results in a gain in sensitivity of about a factor of 5 , because carbon dioxide is a much stronger absorber of infrared radiation than hydrocarbons. A further gain is
1006
ANALYTICAL CHEMISTRY
achieved because the combustion of any hydrocarbon other than methane produces tn-o or more molecules of carbon dioxide for every hydrocarbon niolecule which is oxidized. A source of difficulty is the presence of carbon diovide as a normal component of the atmosphere, I n this particular plant. the atmospheric carbon dioxide in the feed air is removed almost completely by caustic scrubbing. The trace remaining is compensated by the use of a reference absorption cell in the analyzer through which a portion of the scrubbed air is passed. [After this paper 1%-assubmitted for publication, attention was called to two applications of this method (1, 4 ) to gas chroniatography detectors. Jn these cases the effluent hydrocarbons from the chroniatography column are oxidized to carbon dioxide, m-hich is determined by laboratory-type nondispersive infrared analyzers.]
blending presumably because of adsorption of carbon diovide on the nalls of the container. The full-scale concentration \vas varied by varying the sample pressure or the amplifier gain setting of the instrument. .4t a pressure of 100 p.s.i.g. and relatively high gain, a full-scale chart reading corresponds to 5 p.p.m. of carbon dioxide and the calibration curve is linear. The limit of detectability is less than 1 p.p.m. When the pressure is dropped to atmospheric and the gain is reduced, the range OUTLET A
STAINLESS STEEL INCONEL LINER
~
\
11'
=.D. I ,
APPARATUS
The analyzer used was the ListonBecker RIodel 21 infrared gas analyzer (Beckman Instruments, Inc.). The detector contains carbon dioxide. The absorption cells, 41 inches long with sapphire windows which transmit to about 6 microns, could be pressurized a t 150 p.s.i.g. The analyzer was enclosed in a n explosion-proof housing. The detector output was recorded on a recording potentiometer. The oxidation catalyst used was Baker Division, Engelhard Industries, Type F pelleted catalyst. It was loaded into a stainless steel, cylindrical chamber with a n Inconel lining to avoid carbon pickup from the steel (Figure 1). The chamber was placed in a n electric furnace and maintained a t a temperature of 950' F. X versatile manifold was constructed for passing samples of calibrating gases into either of the two cells of the infrared analyzer, with or without passage through the catalyst chamber (Figure 2).
INLET
--+
THERMOCOUPLE
Figure 1. furnace
Catalyst
chamber
1
CATALYST FURNACE
1..
1 ZERO
and
II
REFERENCE
I/
CALIBRATION AND TESTING
The infrared analyzer was calibrated over a range of carbon dioxide concentrations by passing through the sample cell mixtures of known composition of carbon dioxide in nitrogen (3). K t r o gen was used in the reference cell. Mixtures in the parts per million range could not be prepared by quantitative
f
+dREFERENCE CELL 1 Figure 2. studies
Manifold for
Y
oxidation
becomes 100 p.p.m. with good linearity. For concentrations above about 300 p.p.m. the calibration curve is nonlinear because of excessive absorption. The activity and life of the oxidation catalyst were studied hy passing known mixtures of hydrocarbons in nitrogcnoxygen mixtures (simulating air) over the catalyst and through the infrared analyzer to determine the carbon dioxide formed. A portion of the known mixture was passed through the reference cell to compensate for residual traces of carbon dioxide in the gases used to make up the mixture. Methane was used for most of this work because it is the most difficult of all the lieht hvdrocarhons to oxidize.
PRELIMINARY STUDIES
Early work with the combination of catalyst and infrared analyzer gave anomalous results which were puzzling until it was realized that the catalyst is a n excellent adsorbent for carbon dioxide. Fresh catalyst when placed in the chamber a t a temperature of about 950" F. and purged with pure oxygen gave off several hundred parts per million of carbon dioxide for nearly a week. The blank value then dropped to zero. When the catalyst was exposed to the atmosphere, it again showed high carbon
dioxide values on purging, although for a shorter time than t b r initial cleanup. When a known mixture m s passed over the catalyst at 950" F. long enough to give a constant carbon dioxide value and the catalyst temperature was then dropped by about 50" F., the apparent carbon dioxide content of the exit gas first dropped to zero. When the temperature was again constant, the carbon dioxide reading returned t o its original value. When the temperature was raised, the carbon dioxide reading first increased and then returnrd t o the original value when temperature constancy mas again reached and a n equilihrium was established a t each temperature between the carbon dioxide in the gas stream and that adsorbed on the catalyst. For this reason the tcmperature was held within fairly narrow limits (*IO0 F.),although the exact value was not important. The adsorbent properties of the c a b alyst caused another initial difficulty. Although a filter was inserted between the catalyst chamber and the infrared analyzer, i t was not completely effective and some catalyst fines settled as a dust on the walls of the sample cell. This not only caused a zero drift, but also resulted in erratic readings which varied with the previous sample in the cell. This Nits eliminated by installing sintered-strel filters, 20-micron porosity,
Table I. Oxidation of Methane (Concentrations in p.p.m.) Net CHI COI Blend Concn. Conen. 3G
6
Main uncertainty in these mixtures is in CH, concentration. When results were obtained, no independent method ior determination of CHI wa8 available. Determined by gas chromatography (1).
Table 11. Space Velocity Data (Catalyst volume 0.0005 cu. ft.; temp. 1075" F.: CH, concentration 51 . mom.) _. Flow COS ConSpace Rate, Velocity, Conen., version, SCFH H ~ . - L r.i'.nf. T~
in the lines leading t o the sample and reference cells. When a methane mixture was first passed over a fresh cleaned-up catalyst, the carbon &oxide reading rose slowly (over a I s m i n u t e period) t o a value close to the known concentration of methane. With continued use, the response time of the catalyst was appreciably shortened and a constant reading for a methane mixtiire was ohtained in 2 or 3 minutes. RESULTS
The calibration curve for methane in air is linear. The limit of detectability is below 1 p.p.m. Typical data for the oxidation of methane in oxygen-nitrogen mixtures are presented in Tahle I. The results of a study of limiting space velocities over the catalyst are presented in Tahle 11. The data show that oxidation of methane is complete up to a space velocity of 6000 hour-'. The space rate actually used in the plant installation is far helow this value. T o determine whether there is an upper limit t o the concentration of methane which could he oxidized by the catalyst, a special mixture containing approximately 6000 p.p.m. was passed over the catalyst under normal conditions. The concentration of methane in the exit gas, as determined by gas chromatography, was less than 1 p.p.m. Figure 3.
Chart showing hydrocarbon content of f e e d air Full scale equals 50 p.p.m.
(8).
T o learn if lubricating oil, which might accidentally come from the feed VOL. 31, NO. 6. JUNE 1959
1007
air compressors, would deactivate the catalyst, some oil was introduced into the catalyst chamber. The expected formation of large concentrations of carbon dioxide occurred for a while, but there was no loss of catalytic activity. The entire system has been in continuous operation for over tvio years. Except for routine maintenance and periodic checks of calibration with known mixtures, no attention has been required and there has been no sign of
catalyst deactivation. Figure 3 is a typical record of the hydrocarbon content of feed air in terms of parts per million of methane. The rapid response to changes in concentration is clearly evident. ACKNOWLEDGMENT
Clyde AIcKinley and Joseph T. Bernstein, .4ir Products, Inc., were helpful in the early stages of this jvork.
LITERATURE CITED
(1) Heaton, 1 ~ B., . ~ ~ J. T., ~ ANAL.CHEJI.31, 349 (1959). (2) Lawrey, D. M. G., Cerato, C. C., Ibzd., 31, (1959). (3) Loveland, J. W.,Adams, R. IT., King, H. H., Koxak, F. rl., Cali, L. J., Ibid., 31, 1008 (1959). ( 4 ) Martin, A. E , Smart, J., Nature 175,
422 (1955).
RECEIVED for review June 12, 1958. Accepted March 2, 1959. Second Delaware valley ~ ~ bfeeting, ~ ACS,i Philadelphia, Pa., February 1958.
Spectrophotometric Titration of Parts Per Million of Carbon Dioxide in Gases J. WEST LOVELAND, ROBERT W. ADAMS, H. H. KING, Jr., FRANCES A. NOWAK, and LAWRENCE J. CALI Research and Engineering Department, Sun O i l Co., Marcus Hook, Pa.
b In the spectrophotometric titration of traces of carbon dioxide in a gas stream, the carbon dioxide is absorbed in dilute sodium hydroxide and the excess caustic i s back-titrated with dilute hydrochloric acid. The titration is followed at 555 mp on spectrophotometer using phenolphthalein as an indicator. A special absorption bulb gives a complete recovery of all carbon dioxide up to gas flow rates of 100 cc. per minute. Sensitivity i s to about 1 p.p.m. of carbon dioxide for 20 liters of gas. The accuracy i s to about 1 p.p.m. and the repeatability standard deviation i s to 0.4 p.p.m. in the 1 to 10 p.p.m. range. Other acidic gases interfere.
A
of carbon dioxide in the range of 1 to 50 p.p.m. in gas streams containing no other acidic constituent TTas needed in conjunction with a long-path infrared nondispersive analyzer during studies of the oxidation of traces of hydrocarbons. Chemical methods for concentrations of less than 1% generally involve absorption of the carbon dioxide by a n alkaline reagent with several finishing steps available. A gravimetric analysis using ilscarite absorption tubes was described by Kolthoff and Sandell ( 2 ) . Others have used either the p H of a sodium bicarbonate absorber solution ( 1 ) or a continuous photometric method using phenol red as an indicator in aqueous bicarbonate (3). The latter method covered the range of 0.06 to 12% carbon dioxide. Spector and Dodge (6) report that a photometric method using the color of phenolphthalein in dilute sodium hydroxide is senDETERMINATIOS
1008
ANALYTICAL CHEMISTRY
sitive in the parts per million range. Pieters (4) recommends absorption in barium hydroxide solution and backtitration with hydrochloric acid which is sensitive to 0.1 mg. of carbon dioxide or about 5 p.p.m. for a 10-liter sample. Attempts to use Spector and Dodge's (6) method rvere not successful in the very 1017 parts per million range because of fading of the phenolphthalein color with time. Too frequent calibrations were necessary. The direct titration method of Pieters (4) appeared the most promising, although detection of the end point n-as difficult in the parts per million concentration range. However, this \vas overcome by using a spectrophotometer for the end point detection. Since the completion of this work, Toren and Heinrich (?) have reported a method n-hich measures pH a t the equilibrium point of saturated solutions of a n alkaline earth and carbon dioxide at concentrations from 1 p.p.m. to 1007, carbon dioxide in the gas streams. The method appears to have sensitivity and accuracy comparable to the spectrophotometric titration procedure. The titration method should be of use to laboratories where special infrared equipment is not available. The analysis requires that all the carbon dioxide in a k n o w volume of gas sample be absorbed in a given quantity of alkaline solution. During this work, it was found that absorption of carbon dioxide by a single bubbler is quantitative only when extrenielp lorn rates are employed. A number of absorption bubblers ere tried to find a single efficient bubbler to reduce handling of sample.
Table I gives the data obtained with these bubblers as rrell as the approximate dimensions and type of dispersing element for each bubbler. About 100 ml. of 0.0001.1' sodium hydroxide n-as used in each. For the first bubbler, from 70 to 300 y of carbon dioxide, measured in a gas pipet, ITas sir-ept through the caustic solution with nitrogen, using a total volume of sweep gas of about 3 liters a t a rate of about 70 cc. per minute. The amount of carbon dioxide absorbed was determined by spectrophotometric titration of the caustic solutions. For the second and third bubblers, k n o m blends of carbon dioxide in nitrogen were used for testing the efficiency a t about the same flow rate. Using the titration cell as the absorber or the longer column with a smooth surface gave 1011- recoveries. Apparently, the contact time was insufficient for the caustic to react completely lvith the traces of carbon dioxide. The best results were obtained with a Vigreus-type column. The restrictions in the column provided a stirring effect and a much longer residence time for the tarbon dioxide to be absorbed in the solution. A series of determinations with the Wgreux-type scrubbing system used varying flow rates of sample gas (Table 11). The sample used was a certified blend of 8 p.p.ni. carbon dioxide (Matheson Co., Inc.). About 10 liters of gas was used in each test. The flow rate is critical and should be maintained a t a rate below 100 cc. per minute. DISCUSSION OF PROCEDURE
The spectrophotometric titration of
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