NOTES
Cryogenic Separation of Methane from Other Hydrocarbons in Air John C. Cooper,' Harvey E. Birdseye, and Russell J. Donnelly Department of Physics, University of Oregon, Eugene, Ore. 97403
Methane has a higher vapor pressure than any other hydrocarbon. Because of this, methane can be separated from the other hydrocarbons in ambient air by condensing the heavier ones on a suitably cold surface. A methane separator based on this idea has been constructed and tested with bottled gas mixtures. At the present stage of development, the device traps more than 95% of all the nonmethane hydrocarbons used and has no measurable effect on methane. When used with a total hydrocarbon analyzer, the separator establishes a baseline for nonmethane hydrocarbons, obviating the problem of setting a n accurate hydrocarbon zero level.
Many of the hydrocarbon contaminants in air take part in the photochemical reactions t h a t form smog, but methane, which occurs naturally a t relatively high levels, does not. Studies (Environmental Protection Agency, 1970) have shown t h a t methane typically is present a t levels above 940 pg/m3 (1.4 ppm) and constitutes perhaps half of the total hydrocarbon level in an urban atmosphere. The ratio of methane to nonmethane hydrocarbons varies so widely, however, that total hydrocarbon measurements do not accurately reflect smog generation potential. For this reason the Environmental Protection Agency (EPA) has set a national primary and secondary ambient air standard for hydrocarbons (Federal Register, 1971) specifying t h a t the 6-9 a.m. 3-hr average of nonmethane hydrocarbons shall not exceed 160 pg/m3 (0.24 ppm) more than once per year. Methane in ambient air can be measured by gas chromatography using flame ionization detectors (Stevens et al., 1970). Systems t h a t will automatically sample ambient air once each 5-15 min, measuring separately methane and total hydrocarbons, are commercially available and this method has been approved by the EPA (Federal R P ~ ister, 1971). There are, however, reasons for considering other possible methods of measurement. Gas chromatography is inherently a batch process, requiring a few minutes per analysis, thus it cannot follow the more rapid hydrocarbon fluctuations that occur in urban atmospheres. Also, since this system and the requirement to correct for methane is fairly new, many air pollution agencies have total hydrocarbon analyzers but no method of discriminating between methane and other hydrocarbons. For them a relatively inexpensive modification to their sampling system would be preferable to purchase of a complete new instrument. This paper describes a device based on a cryogenic t r a p t h a t provides a useful separation of methane from other hydrocarbons. Results of tests performed in our laboratory are included. Evaluation of performance of this instrument for a wide range of ambient air conditions will be part of a comprehensive field test program planned for the next year.
Device Description
Examination of vapor pressure data for hydrocarbons (some extrapolated from higher temperatures) indicated t h a t a t temperatures near 77°K it should be possible to t r a p most hydrocarbon compounds to such an extent t h a t their concentrations would be reduced to negligibly small levels in an air sample. At the same time, methane, at typical ambient air concentrations of a few parts per million, should pass through the trap. Thus a separation appeared possible using a cryogenic trap a t liquid nitrogen temperature. The basic methane separation device is a cooled tube (trap) through which air flows before entering a total hydrocarbon analyzer (THC). At the optimum temperature, methane concentration in the air is unchanged, while concentrations of the heavier hydrocarbons and many other air contaminants are reduced, most to negligibly small amounts. The THC then measures the methane level with very little interference from other compounds. A temperature around 80°K will cause good separation between methane and other hydrocarbons. Liquid nitrogen boils a t 7-0 i i K, is readily available and not expensive, so it was used as the coolant in our work to date. To operate this device with ambient air for reasonable lengths of time, several additions to the basic trap were made. The flow diagram of the current version of a methane separator is shown in Figure 1. Timer-controlled valves have been added alternately to supply ambient air or processed air to the THC. Thus the THC alternately measures total hydrocarbons or methane. Levels of hydrocarbons. methane corrected, are obtained by subtraction. Oxygen will condense from ambient air a t 77°K and will eventually fill the trap with liquid. This can be avoided by operating the trap a t a partial vacuum (less than about 24 in. of mercury absolute). The pump on the outlet and
iDL VALVE
I
SAMPLE IN
To whom correspondence should be addressed.
T O THC ANALYZER
Figure 1. Schematic diagram of the methane separator
Volume 8 , Number 7, July 1974
671
Table I. Percent of Various Hydrocarbons That Pass Through Trap Dew Point of I n p u t Gas Stream, "C Component
1
25
100
100
33
< 2a
100 2.5
17 28 33 33
