V O L U M E 2 2 , NO, 9, S E P T E M B E R 1 9 5 0
1125
LITERATURE CITED
(3) Brattain, R. R., Rasmussen, R. S., and Cravath, A. M., J. Applied Phys., 14,418 (1943). (4) Crout, P. D., Trans. Am. Inst. Elec. Engrs., 60, 1235 (1941). (51 Kave. W. I.. and Otis. M. V.. ANAL.CHEM..20. 1006 (1948). (6) Vandenbelt, J. h l . , Forsythe, J., and Garrett,’ A., I ~ D ENQ. . CHEM., ANAL.ED.,17, 235 (1945).
(1) Ayres, G. H . , ANAL.C H E M . 21, , 652 (f949). (2) Barnes, R. B., Liddel, U.,and JVilliams, V. Z.. IND.ENG.CHEM., ANAL.ED.,15, 659 (1943).
RECEIVED March 3 , 1950. .Presented a t the Fifth Southwest Regional hleetSOCIETY, Oklahoma City, Okla., December ing of the AMERICANCHEMICAL 9 and 10,1949.
up the diethylbenzene-sec-butylbenzene analysis in the infrared, with an average absolute error of 0.5% for all components. The infrared is thus the region to be preferred for this analysis.
Infrared Analysis of Certain C,-C, Hydrocarbon Mixtures JAMES D. S T R O ~ P E Rohni & Haas Company, Philadelphia, Pa. Infrared analytical methods are developed for the direct determination of small amounts of ethane, propane, n-butane, and isobutane in natural and purified methane streams. Accuracies of =!=O.l% are obtained with the permanent gas absorption cell of the Beckman IR-2 spectrophotometer. An additional comparison cell set permits the direct determination of methane to within *O.;%. The methods have proved satisfactory in routine application for the past 2 years.
I
T HAS been necessary to provideaccurate, rapid, routine deter-
minations of very small amounts of ethane, propane, n-butane, and isobutane in methane streams both direct from the natural gas field and after continuous purification. Although the mutual interference situation suggested difficulties in the develop: ment of infrared methods for the analysis of such mixtures, the experience of others ( 4 ) in related problems encouraged sufficient exploratory work with a Perkin-Elmer 12-B infrared spectrophotometer to prove that it is possible to determine the above contaminants in methane to within =+=O.l%. On the basis of the preliminary results, a Beckman IR-2 spectrophotometer was specially adapted and calibrated and has performed satisfactorily in daily routine operation for the past 2 years. Success in this application of the methods of infrared analysis was possible because the compositions of normal “before and after” purification streams are such as to require few large interference corrections. Good instrumental stability Itnd careful calibration and operation permitted this favorable circumstance to be exploited to obtain high accuracy. It was expected and found that the composition of natural gas direct from the field varied only slightly day to day-the average methane stream containing about 4% ethane, 1.5y0propane, 0.3% n-butane, and 0.3% isobutane (plus pentanes). The purification process preferentially removes the contaminants of, higher molecular weight, leaving only traces of propane and small amounts of ethane. From the standpoint of the subsequent chemical processes propane is the critical component. Reference to Table I shows considerable n-butane interference a t bdth useful propane peaks; however, the concentration of n-butane is never more than a fraction of the propane content and can he most accurately determined, permitting appropriate correction of the propane dats. Mutual interference was not a serious problem in the case of any of the other contaminants, but the high relative concentrstion
Table I. Propane a t 13.3459 E t h a n e a t 12.2609 n-Butane a t 10.3059 Isobutane a t 8 . 4 9 0 ~ hlethane a t 7 . 6 4 6 ~ ProDane a t 9 . 3 4 5 ~
of methane necessitated particular care in the determination of its contribution a t all analytical wave lengths. Except for methane, the analytical nicthods were developed in the following way: From the 2.5 to 15p absorption spectra of the several c o m p e nents characteristic analytical wave lengths were selected and a program of appropriate slit widths and background filters was set up to permit straightforward determination of each component from separate samples loaded into the permanent (24.8-cm.) Beckman gas cell a t a working pressure of 740 mm. Accurate calibration data were obtained by expanding small, known, high pressure volumes of the pure components into the evacuated absorption cell. In the case of ethane, the pressure-broadening effect was empirically taken into account by externally preparing mixtures of ethane with nitrogen. Data were recorded in per cent transmittance, corrected for the effects of stray radiation, and calculated to absorbances which were then plotted against concentrations to give calibration charts permitting the direct graphical analysis of similarly handled unknowns by the method of successive approximations-the components being determined in the order methane, n-butane, isobutane, ethane, and propane. The determination of methane was handled differently. In this case the usual difficulties of determining the predominating component of a mixture were further complicated by the high intensity of the 7 . 6 4 6 ~methane peak, its “sharpness,” and its marked pressure broadening ( 2 ) . In such a situation all experimental variables involved in single-cell methods become critical. In practice it has usually been satisfactory to obtain the methane concentration by difference, but it has also been possible to construct a comparison cell set-fitting into the liquid cell compartment of the Beckman IR-2 spectrophotometer-which permits the direct determination of methane to within =!=O.5yob. APPARATUS AND MATERIALS
Equivalent Parts Interference Data
Propane 1 22.6 50.5 22.5 39.7 1
Ethane 28.3
1 1170 288 61.3 2-10
n-Butane 1.12 13.8 1 51.1 9.7 11 0
Isobutane 73.4 8.8 57.5 1 20.7 17.2
Methane 607 146 2520 180 I
39;
The long wave-length infrared absorption peaks which distinguish ethane, propane, n-butane, arid isobutane from each other and from methane are not of inhercntly high absorptivity nor are they entirely free of serious mutual interference. To reach analytical accuracies of the order of *O.l% with commercially available spectrophotometers requires that a long path length and high operating pressure be co:nbined with precibe meas-
ANALYTICAL CHEMISTRY
1126
urements and suitahle nirans for the application of interference corrections. The original work on the system was carried out in the 1-meter absorption cell of a Perkin-Elmer 12-B spectro hotometer. The methods were later adapted t o make most ekcient use of the greater electronic and thermal stability, operating speed, a.nd accuracy of the Beckman IR-2 spectrophotometer. In the latter instrument the longest available path length is that of the permanent cell, which is an integral part of the optical system (24.8 cm.). The other gains more than compensated for the -fourfold loss in sensitivity due to the shorter path length and escept for the addition of a set of comparison cells for the determination of methane and the mounting of hairlined magnifying lenses over the per cent transmittance and wave-length scales to eliminate parallax and obtain greater reading accuracy, no changes in the commercially available equipment were found necessiuy. The wave-length scale was checked against the known spectra of carbon dioxide, ammonia, and water vapor. The component gases were obtained from the Matheson Company. Their 2.5 to 15, spectra were voinpared with the spertra of high purity standards ( 1 ) . S o identifiable contamination was detected in the stocks of n-butane and isobutane, claimed to he of -99 mole yo purity. One of two tanks of propane contained at least the 0.2 mole % limit of ethane and isohutane. A trave of carbon dioxide was noted in the methane, and strong ethylene ahsorption w%q observed a t 1 0 . 5 ~in the -9.5 mole yoethane. This impurity w m removed by a bromine water-sodium bisulfitephosphorus pentoside absorption train. With the exceptions noted above, the gas stocks of contaminants were assumed to be pure, because a t the very low concentrations used the effects of small traces of undetected impurities would be negligible. ANALYTIC4L METHODS AND THEIR C4LIBRATION
I-ChElO
-
where the a’ values-the respective “calibration coefficients” measured a t 13.345~a t 740 mm. in the permanent cell (a’ = a x 24.8)-are known and approximate knowledge of the concentration of each interfering component is available to permit its contribution to be subtracted from that of the mixture. The concentration of propane is then obtained directly from the calibration plot a t 13.345~. From the similar relations holding at each of the other peaks the approximation treatment proceeds through the following steps: first, a determination of the concentration of methane from its direct calibration data, then appropriate correction for methane interference made a t each peak, starting with n-butane and picking up additional approximate interferenee corrections for each component as determined in the order. n-butane, isobutane, ethane, and finally propane. A second series of similar graphical calculations, using in each case the above preliminary concentration values for each interfering component, permits sufficientaccuracy to make a third approximation step unnecessary. The process is simple and takes only a few minutes a t most. In Table IV the results of typical routine infrared analysis of a half dozen representative hydrocarbon samples are compared with mass spectrometer data thought to be accurate to within *0.5% for methane and ethane and * O . l % for propane, nbutane, and isobutane plus (25’s. The first three are laboratory samples used to test the original calibration work; the others were obtained as checks a t intervals during the routine operation of the equipment. DISCUSSION
In general, the two independent analytical methods appear to give closely comparable results, except for the case of ethane,
where the maSs spectrometer values are systematically higher. On an abSam le Sam le solute basis the calibration data are felt 5, 6, ~ to be a t least adequate to reach acMS IR MS IR curacies of *0.1% in the determination 93.4 ... 9 5 . 5 . . . 4.3 4.0 2.2 1.8 of propane, n-butane, and isobutane. 1.2 1.3 0.8 0.6 0.4 0 . 3 0.5 0.5 Routine experimental u n c e r t a i n t i e s 0 . 1 0.3 0 . 4 0.3 have been shown not to cause variations greater than *0.02% for these components and there appears to belittle likelihood of serious undetected interference; therefore relative infrared results are thought to be satisfactory to limits of *0.05%. Experimentally the direct comparison cell infrared determination of methane should be accurate to within 10.2% methane on a relative basis. Because the absolute standard for this work was a methane supply found by mass spectrometer analysis to contain -1.5% nonhydrocarbon impurities, there is a greater uncertainty in the absolute values. The infrared results for small concentrations of ethane are consistently lower than the mass spectrometer values. Laboratory mixtures 1, 2, and 3 were made up from the same stocks used in the calibration work and it is unlikely that unidentified contamination is responsible for the higher mass spectrometer results. The infrared calibration data provided by direct external mixtures of ethane with nitrogen and with methane and by similar mixtures for which independent combustion (carbon equivalent) analyses were available (Figure 1) must and does adequately sa& isfy the linear absorption law after straightforward corrections are made for stray radiation and pressure-broadening effects. If the mass spectrometer points are correct, an empirical calibration curve would have to depart considerably from linearity in the region close to the origin where any real effects are known to be a t least significant. I t would appear either that t h e , *0.5% limits of accuracy for mass spectrometer measurements of ethane are consistently positive in the low concentration range or that ethane is not completely absent from the methane used in calibrating its significant interference a t 12.260~. This discrepancy is unimportant in the application for which the methods were developed. Unfortunately, it is within the limits of uncertainty of alternative independent analytical methods. Relative ethane determinations by infrared absorption have not varied more than +=0.05% owing to purely experimental factors; therefore it does not seem unreasonable to assume that on an absolute basis the values approach error limits of *O.lo/o of the total sample.
8
8
ACKNOWLEDGMENT
The author wishes to acknowledge the permission of the Rohm & Haas Company to publish this paper. He wishes also to thank -4. G. Knox for his assistance in the original prospecting work and C. H. Beckworth for his careful application of the methods and development of many practical shortcuts for routine operation. LITERATURE CITED (1) American Petroleum Institute, Research Project 44, Spectrogram Serial Nos. 59, 373, 374, 528, 529. (2) Coggeshall, N.D., and Saier, E . L., J. Chem. Phy.9., 15, 65 (1947). (3) Hughes, H. K . , Bull. SOC.Applied Spectroscopy, 4 , No. 2 (1949); Progress Rept. 2 from Joint Committee on Nomenclature. (4) Meier, H . H., and Draeger, A. A . , Humble Oil and Refining Co.; Carr, D. E . , and Beckwith, L. B., Union Oil Co.: Humphreys. C. J., National Bureau of Standards: Heigl, J.. Esso Laboratories; and Fenake, M. R., Pennsylvania State College, private communications. (5) Nielsen, J. R.. and Smith, D . C., IND. ENG.CHEM.,ANAL.ED., 15, 609 (1943).
RECEIVED April 11. 1950. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, February 1950.
