function of thgcocktail composition, because we did not observe loss of 14Cor 3H activity from aqueous solutions mixed with the dioxane base cocktail described above. Our experience indicates that with certain cocktails, polyethylene vials may not be satisfactory containers. Care should be exercised in the selection of counting cocktails and vials to avoid measurement errors and contamination problems due to migration of activity through the vial walls.
Literature Cited (1) Weimer, W. C., Rodel, M. G., Armstrong, D. E., Enuiron. Sci. Technol., 9 (lo),966 (1975). (2) Iverson, R. L., Bittaker, H. F., Myers, V. B., Limnol. Oceanogr., 21,756 (1976). ( 3 ) Budnitz, R. J., Health Phys., 26,165 (1974). (4) Cantelow, H. P., et al., ibid., 23,384 (1972).
Receiued for review August 27, 1976. Accepted December 20,1976.
Further Developments in Oxidation of Methane Traces with Radiofrequency Discharge Daniel L. Flamm’’ Department of Chemical Engineering, Texas A&M University, College Station, Tex. 77843
Theodore J. Wydeven NASA Ames Research Center, Moffett Field, Calif. 94035
The radiofrequency discharge, previously shown to oxidize trace levels of methane in oxygen, was studied with contaminated air a t 50,600, and 760 torr. As with oxygen, the concentration of methane traces could be reduced by several orders of magnitude, and no organic reaction products were detected in the effluent; however, substantial concentrations of NO, (0.1-6%) were formed during treatment. The concentration of NO, was decreased by using a large diameter electrode. There is evidence that the process will oxidize N2 and NO as well as organic impurities in oxygen or oxygenlinert gas atmospheres. Recently, we reported that a radiofrequency glow discharge could be used to effect almost complete removal of contaminative methane traces from oxygen ( I ) over a wide range of pressure (50 torr to 1atm) and concentration (70-8000 ppm). Unlike many other methods of purification, the fraction of methane removed was insensitive to concentration within this range, even at high degrees of removal (99%) and very low concentration. The device was proposed as a means for removing trace contaminants from closed environments such as spacecraft or from “zero gas” used as a standard for monitoring equipment. In the previous investigation, only contaminated oxygen was studied. We have now extended that work to air, which contains methane traces, and report the formation of NO, in air which is so treated. An analysis of the thin film which forms on the reactor wall ( I ) is also given.
Experimental The basic apparatus and reactor have been described in detail (1,2).A mixture of dry air or dry oxygen and methane is formed in a dynamic dilution system (Figure 1)and metered into the reactor. The reactants and products have been analyzed by use of a gas chromatograph with a dual thermal conductivitylflame ionization detector. In addition to this analysis, the product stream from the air discharges was routinely analyzed for total nitric oxides. For
this purpose, a sample of the reactor effluent was collected in an evacuated bomb containing 25 cm3 of 0.003 N HzS04/0.3% H2Oz solution and chilled to 0 “C with an ice bath. The samples collected at subambient pressure were back-filled to atmospheric pressure with oxygen and set aside for 24 h. NO and NO2 are thus converted into nitrate in solution. To each sample was added 0.5 ml of 5 M KF ionic strength adjuster, and an Orion Model 93-07 ion selective electrode was used to determine the nitrate concentration. An attempt was made to analyze ozone from two oxygen discharges a t atmospheric pressure by iodometry (Table I). The treated stream passed through an extra coarse gas dispersion tube immersed in 300 ml of 0.2 M KI solution buffered with 0.1 M boric acid. The iodine thus formed was titrated with 0.1 N Na2S203. During these experiments the TC filament detector was operated at high current giving increased sensitivity over that of our previous investigation. This enabled us to detect and thus confirm the presence of COz, CH4, and NO2 with the TC detector, although these chromatograms were not quantitative or entirely reproducible due to the consequent oxidation of the filament.
NEEDLF VALVE VENT
OUTER ELECTRODE
C EE LN EC T1 R TA RL ODE
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1
REACTOR
VALVE
Present address, Bell Laboratories, Room 63-216, Murray Hill, N.J. 07974.
PLASMA DISCHARGE
SAMPLE ANALYSIS
Figure 1. Apparatus
Results The degrees of methane removal achieved in air (Figure 2) are comparable to those obtained using an oxygen atmosphere. However, nitric oxides were formed in air, and the observed concentrations (Figures 3 and 4) would be unacceptable for an atmospheric purification process. The iodometric analyses were positive (Table I), but not large enough to indicate the presence of ozone (see Discussion section). In any case, traces of ozone would not detract from the utility of this process since they are readily removed by heterogeneous catalytic decomposition. The chemical analysis did not differentiate between various nitric oxides (NO, Nos, etc.). At 600 and 760 torr, the bomb samples generally were amber in color which indicates N02; however, at the observed levels (0.1-7% total NO,), NO is rapidly oxidized to NO2. Thus, the presence of NO2 in the effluent air stream does not imply that it is produced within the discharge. As was previously reported for oxygen ( I ) , the inner glass wall of the outer electrode developed a thin, transparent, brownish film which appeared to quickly attain a steady-state thickness. This coating was visibly denser when it formed in the air discharges, especially at 600 and 760 torr, but it tend-
ded to lighten when an oxygen discharge was subsequently operated. As before, the electrical operating characteristics were constant with time throughout each experiment (20 min to 3 h) and were insensitive to flow rate and methane concentration. A quartz reactor unit used in the experiments with a 0.079-cm electrode was sectioned to characterize the film formed on the inside surface. Samples of the inner quartz wall were taken from two different longitudinal locations and were scanned for carbon, chromium, iron, nitrogen, and silicon with a Hewlett-Packard Model 5950A x-ray photoelectron spectrometer. One section of the tube was taken from a location which was always in the discharge zone and where the tint of the film was darkest. Another section was taken from a position where the edge of the plasma zone had been and where the film was faintest. Except for silicon, the above elements were detected in almost the same abundance ratio in both samples (C:Cr:Fe:N = 1.3:0.77:3.6:0.32). The lighter sample indicated silicon in a relative abundance of 0.13 (referred to the above), whereas the more intense signal from the darker sample did not show a noticeable silicon peak. There were two oxidized forms of carbon (286.4 and 288.4 eV), as well as a principal peak a t 285.0 eV (relative abundance 0.14:0.22:1).
Table 1. Nominal Residence Times for Typical Conditionsa 50 W, rns
50 Torr
240 W, ms
4-10
10-10
F = 6875 cm3/min
600 torr
600 Torr 3-6 15-60 F = 533 cm3/min 760 Torr No data 20-80 F = 452 cm3/min a T = V/F was computed using approximate volumes, V, filled by visible
z
140 W
W
v
-
I
-
discharge with a 0.318-cm electrode and a feed rate of 420 cm3-stp/min at room temperature and discharge pressure (F). Lower figures are for air, and higher values for oxygen. To a approximation, T was inversely propwtional to feed rate with other variables held constant.
120
I-
.I
o,o 0.079-cm ELECTRODE 0.318-Cm ELECTRODE
0 , A,
1
w
"
I l l i i i l l
:r
Figure 3.
I
-
600 torr
700cm3- stplmin
2
z- cIt "i -#
X
0
\
z
L
3 0
Nitric oxides formation vs. air flow rate
v
5 0 torr
.I
I000
100 GAS FLOW, cm3-stp/min
IO
600 torr
l
-
P
I .Ol
L
W
a 4-
' 6 0 0 torr
.
O
I 2' .,
IO
O
I
~
-
l
' ' 100
1
1
1
1
;
I
0 0.318 cm ELECTRODE
1
o,n,A0.079Cm ELECTRODE
1000
G A S FLOW, cm3-stp/min
Figure 2. Methane removal vs. flow rate (A,A ,~ ) a i r(El, ; 0)oxygen. Open points for 0.079-cm central electrode Shaded points (A,U) denote 0.318-cm central electrode
1
0
Figure 4.
1
I
100
I
200 POWER, watts
1
i 1
i
I
300
Nitric oxides formation vs. power Volume 11, Number 5, May 1977
515
Discussion The presence of carbon and nitrogen is consistent with our previous identification of the film as a polymeric deposit ( I ) ; the nitrogen apparently was incorporated during the air discharges. Evidently, there is a plasma-chemical mechanism whereby iron and chromium are transported from the central electrode to the quartz wall (and in the reverse direction since the film thickness does not increase indefinitely). The nitric oxide concentration increases with decreasing flow rate, increasing pressure (hence, increasing residence time), and increasing power, but appears to approach a steady concentration as the residence time is increased a t a fixed pressure and power level. Nominal residence times (based on room temperature and the actual volume of active discharge-see ref. I ) for typical conditions are presented in Table I. Since the electrode size does not have much effect on the degree of methane oxidation ( I ) ,a large electrode diameter can be used to minimize the production of nitric oxides in the presence of nitrogen. S O , is an undesirable by-product when the objective is to remove trace contaminants from air; however, the oxidation of nitrogen is a positive result when applying the process to produce “zero” oxygen or for the purification of oxygenlinert gas atmospheres in the absence of nitrogen. Since a minor component, methane, and a major constituent, nitrogen, are both oxidized by the discharge, it seems reasonable to expect that traces of nitrogen will also be oxidized. The oxygen used in these experiments (aviator’s breathing oxygen) contained approximately 0.05%S P(Ar 0.2%, CH4 14 ppm, water 3.2 ppm, CO2 0.8 ppm, N20 0.6 ppm), but we made no attempt to measure nitric oxides in the effluent of the treated oxygen. Nevertheless, the studies of Malt’sev, Eremin, and their coworkers ( 3 , 4 )provide support for our suggestion that nitrogen will be oxidized when it is a minor component of the feed stream. These investigators found that NO, (reported as NO) was formed from N2/02 mixtures in glow discharges a t low pressure (50-400 torr) and that the fraction of oxidized S2 increased from about 4%when Np:02 = 4 to 25% when N2:Op = 0.25. The iodometric analyses of the treated oxygen stream (Table 11) indicates the presence of SO2 and/or 0.3.S O is nearly insoluble and would not be detected by this analysis. However, SO2 will produce a response equivalent to 10-30% of an equimolar quantity of ozone ( 5 ) .The complete oxidation of the S2 impurity in aviator’s breathing oxygen would produce [SOP] 0.1% and thereby give an iodometric response equivalent to 0.01-0.03% of 0 3 , the range of our results. If nitrogen emerged from the reactor as NO and if [0:3] 0.01%, this SO would be entirely converted to SO2 by the fast reaction (k:ioo = 1.65 X cm3/s):
-
-
-
-
--
-
03
+ NO = NOp + 0 2
(1)
or to higher oxides (6). We may therefore exclude the possibility that 0 3 and S O were both present. During these ab-
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Environmental Science & Technology
Table II. Oxidant Analysis at 1 atm and 700 cc-stp/ min Power, W
Equivalent ozone concn, %
102
0.015
230
0.028
sorption experiments the reactor effluent was vented into the laboratory, but the odor of ozone was not detected as it almost certainly would have been a t the concentrations in Table I. Therefore, the data are consistent with 300-1000 ppm SO2 in the effluent. The probable presence of NO2 rather than NO is significant because the latter is relatively inert, whereas SO2 and higher oxides are effectively absorbed by certain molecular sieves (7-9). Thus, the discharge may be useful for removing NO as well via oxidation to SOz. Conclusions Contaminative methane traces are oxidized in air as well as in oxygen. Unfortunately, the removal is accompanied by the formation of NO,; unless this by-product can be minimized by changes in reactor design and operating conditions, the utility of the discharge purification process will be restricted to atmospheres in which nitrogen is not a major component. This oxidation of nitrogen can be a useful result in applying the process to produce “zero” oxygen or for the purification of oxygenhnert gas atmospheres. Acknowledgment We thank James C. Carver for performing the ESCA analyses. Literature Cited (1) Flamm, D. L., Wydeven, T. J., Enuiron. Sci. Technol., 10, 591 (1976). (2) Flamm, D. L., Ind. Eng. Chem. Fundam., 14,263 (1975). (3) Pollo, I., Mal’tsev, A. N., Eremin, E. N., Russ. J . Phys. Chem., 37, 1130 (1963). (4) Krykhtina, L. M., Mal’tsev, A. N., Eremin, E. N., ibid., 40, 1497 (1966). ( 5 ) Hodgeson, J. A,, Int. J . Enuiron. Anal. Chem., 2, 113 (1972). (6) Singh, T., Sawyer, R. F., Starkman, E. S., Caretto, L. S., J . Air Pollut. Control Assoc., 18,102 (1968). 17) Joithe. W.. Bell. A. T.. Lvnn. S.. Ind. Enp. Chem. Process Des. D e u , 11,434 (1972). (8) Bartok. W.. Crawford. A. R.. Hall. H. J.. Mannv. E. H.. Skom. A,. “Systems Study of Nitrogen Oxide Control Methods for StaGonary Sources”, NTIS Report P B 184479, May 1969. (9) Fornoff, L. L., AIChE S y m p . Ser., 68 (1261, 111 (1972). Y
Received for revieu April 28, 1976. Accepted January 4 , 1977. Financial support from N S F Grant GK-37469, N A S A Grant NCA2OR773-501, and support of one of the authors (D.L.F ) by a N A S A / A S E E Summer Facult) Fellowship.
