Determination of Trace Amounts of Methane in Air D. M. G. LAWREY and C. C. CERATO Research a n d Engineering Department, Sun Oil Co., Marcus Hook, Pa. ,From 1 t o 6000 p.p.m. by volume o f methane in a i r can b e determined by gas chromatography. A high sensitivity amplifier and a splitter column packed with a partially deactivated charcoal a r e used to gain the desired sensitivity and resolution. Results on 11 known blends ranging from 15 t o 6000 p.p.m. of methane gave an average deviation of f l 1%. Repeatability standard deviation was f2.37 p.p.m. a t the 30 p.p.m. level.
A
pollution and the safeguarding of air separation plants have focused attention on the importance of the trace constituents of air, particularly hydrocarbons. Kone of the gas chromatography columns described in the literature (1-6) are suitable for separating IR
traces of methane from air. This paper describes a gas chromatography method which is specific for such traces. EXPERIMENTAL
A schematic diagram of the equipment is shown in Figure 1. Readily available components n ere used: flow controller, Model 63-BLT, Moore Products Co., Philadelphia, Pa.; thermal conductivity cell, TE-I11 geometry, tungsten filaments, Gow-Alae Instrument Co., Madison, ?;. J.; flowmeter, taper tube size 1-15-6, n i t h steel float, Brooks Rotameter Co., Lansdale, Pa.; recording potentiometer, strip chart, 2.5-mv. span; and a modified high gain direct current amplifier, Leeds & S o r t h rup Co., Model 9836-A. Although room temperature seems optimum for this analysis, the system must be protected from small temperature changes and air
W
4
b Figure 1. Schematic d i a g r a m of chromatograph
m - MAIN COLUMN
I PREHEATER COLUMN ZI PRESEPARATOR COLUMN
Figure 2. Bridge and auxiliary circuit
v FINE ZERO CONTROL T C CELL --
* PRECISION WIRE WOUND RESISTORS 1 */e
lOO---lOT
POT
I
SAMPLE INLET
currents. The detector was embedded in a block of Styrofoam for thermal insulation, and the detector and column were enclosed in a draft-free space. Column. The apparatus was essentially a single column split into two sections; hence the term splitter column. It consisted of a 2- and a 4-foot section of 1/4-inch copper tubing, separated by a three-way valve and bypass leg; and packed with a 20 to 50-mesh charcoal (Pittsburgh Coke & Chemical Co.) wetted n j t h 1.5 weight % dinonyl phthalate. The three-11-ay valve permitted shunting the major portion of the air separated in the first stage directly to the detector to reduce the over-all time of analysis, eliminate 99+% of the air, and to effect the complete resolution of air and methane. Molecular Sieves. silica gel, and other activated charcoals were not as suitable as the column described for determination of traces of methane in air. Detector Circuit. The bridge circuit (Figure 2 ) was made simple and symmetrical, to eliminate heat-sensitive parts and transient currents. The filaments of the Gow-Mac cell are u-ired in series for maximum sensitivity and stability. The current amplifier was modified to perform as a high gain voltage amplifier. This circuit uses a standard 2.5-mv. recording potentiometer. Procedure. Once a flow rate of about 20 cc. per minute of helium carrier gas has been established, t h e column is purged for several hours t o remove a n y material absorbed b y t h e packing. Completion of this step is characterized by a relatively stable base line at high sensitivity settings of the amplifier. To perform an analysis, 20 cc. of air sample is iniected into the tor, of the column by a hypodermic syringk. Regulated helium carrier gas sweeps the sample through the first 2 feet of column, effecting preliminary separation of air and methane. The major portion of the air is bypassed to the detector and measured a t low sensitivity. At a predetermined time, usually 1 minute after the air peak has reached maximum deflection, the three-way valve is turned to pass the oncoming methanerich zone through the additional 4 feet of packing and into the detector. The methane concentration is measured with the amplifier a t the high srnsitivity setting. Concentrations are then calculated from the ratio of the area of the methane peak to the area of the air peak (both measured by polar planimeter), the ratio of amplifier settings and a predetermined calibration factor. Because peak area is proportional to concentration, VOL. 3 1 , NO. 6 , JUNE 1959
1011
x
XI
20.000
-
100
90
80 70
CHANQED
Figure 3.
Concentration apparatus Figure 4.
where A is peak area, S is the scale multiplier setting of the amplifier, and K a constant. K is evaluated by running known methane blends in air. Multiplying both sides of the equation by p.p.m.v. CHq'K gives: P.p.m.v. CHa = A C H , / A ~X~ ~ SCH'/Snir
x Ilk'
Table I. Repeatability of Method (Known blend containing 30 p.p.ni. CHa) Analysis, Difference P.P.M. CHI from Average 27.16 -2.84 26.56 -3.44 31.34 +1.34 30.87 +0.87 29.24 -0.76 32.91 +2.91 29.26 -0.74 32.69 +2.64 Av. 30.00 Av. dev. 1.95 Std. dev. = S~2.37p.p.m. = 7.90y0 Table II. Accuracy of Method CHa, P.P.hI. Difference, % Found Exptl. - Known Present 15.8 18.8 19 20.2 15.9 - 21 30.2 29.4 -3 45 0 49.2 c9 80 2 87.1 i 9 125 0 143.0 14 250.2 289 2 16 523 0 468.0 -7 858.0 982.0 14 5200.0 5688.0 +9 6000.0 6057.0 Dev. of av. $ : . 5 % Av. dev. 11,lY0
+
++ +
Table 111.
Analysis Using Auxiliary Concentration
CH,, P.P.11. Gas Chromatography 2 8 11 5
Infrareda Difference 0 7 3 5 11 0 -0 5 11 4 7 1 4 3 15 8 13 3 2 5 16 6 14 6 2 0 a Data obtained with Franklin Institute long-path infrared instrument.
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ANALYTICAL CHEMISTRY
Auxiliary Concentration Procedure.
K h e r e samples contain less than 5 p.p.ni. b y volume of methane, the sample may be concentrated approaimately tenfold by using t h e apparatus shown in Figure 3. -40.1-cu. foot portion of sample is passed through the coiicentration coil immersed in dry ice. The coil consists of 2 feet of '/,-inch diameter copper tubing packed in the same manner as the chromatographic column. The methane is desorbed from the concentration coil by placing the coil in a 350' F. oil bath and pumping off the methane by a Toeppler pump (150-cc. volume). Impurities should be desorbed by this procedure before the first concentrate is prepared. Figure 4 shows that oxygen and nitrogen in air are not resolved. This is not necessary for the determination of methane. Holyever, complete separation is effected between the methane and air peaks. The method has been satisfactory for approximately 2 years on a routine basis. REPEATABILITY AND ACCURACY
Repeatability was tested by repeated analyses of a 30 p.p.m. by volume blend of methane in air (Table I). The standard deviation of an observation was *2.37 p.p.m. by volanie, which is =t7.9% of the 30 p.p.m. by volume in the sample. Table I1 compares concentrations found to the known conipositions of 11 blends in the 15 to 6000 p.p.ni. by volume range. The average deviation n-as about 11% of the amount of methane present in the blends. If the results of Table I1 were plotted. they would show that the calibration is linear down to approximately the 15 p.p.m. by volume level. T o test the auxiliary concentration step, five samples containing less than 15 p.p.m. by volume of methane were analyzed. The results were compared to those obtained by long-path infrared spectroscopy at the Franklin Institute, Philadelphia, Pa. (Table 111). TTThilethe gas chromatography method
Chromatogram of methane
appears to give somen hat lower results, the agreement is good. APPLICATION
OF METHOD
These procedures hare been used to analyze over 1000 samples from the air separation plant area in the range from 5 to 1000 p.p.m. by volume of methane. Simultaneously, ethane and heavier hydrocarbons m-ere determined by a freezeout and a mass spectrometer procedure. The latter was a modification of the method of Quiram and Biller ($), The results aided in interpretation of the records of the continuous infrared stream analyzers ( 5 ) . CONCLUSIONS
Combining this method for methane with the methods for determining ethane and heavier hydrocarbons permits a complete light hydrocarbon analysis of air or air separation plant streams. This in turn allows liondispersive infrared analyzers to be used in the plant on a continuous basis. ACKNOWLEDGMENT
The authors thank J. F. Paulson for constructive suggestions about the apparatus. Phillip Jarrline who obtained much of the experimental data, and A. B. Kent and J. C. S. Kood, under n-hose supervision the work \vas done. LITERATURE CITED
(1) Bennet, C. E., Sogare, S. D., Safran-
ski, L. W.,Lewis, C. D., ANAL.CHEM. 30,898 (1958). (2) Eggertsen, F. T., Nelsen, F. RI., Ibid., 30, 1040 (1958). (3) Gaulin, C. A., Nichaelson, E. R., Alexander, A. B., Jr., Sauer, R. W., Chem. Eng. Progr. 56, Yo. 9, 49 (1958). (4) Quiram, E. R., Biller, W. F., ASAL. CHEK 30, 1166 (1958). (5) Rosenbaum, E. J., Adams, R. W., King, H. H., I b i d , 31, 1006 (1959). (6) West, P. W.,Sen, Buddhadev, Gibson, N. A., Ibzd., 30, 1390 (1958). RECEIVEDfor review June 12, 1958. Sccepted January 6, 1959. Delaware J'alley Regional Rleeting, ACS, Philadelphia, Pa., February 1958.
