Generation of Clean Air to Permit Gas Chromatography without Cylinders W. H. King, Jr. Analytical and Information Division, Esso Research and Engineering Company, P.O. Box 121, Linden, N.J. 07036
THECOST AND ANNOYANCE of gas cylinders can be eliminated in the gas chromatography laboratory where flame ionization detectors (FID) are employed by using a combination of techniques. The FID uses three gas supplies: the carrier gas, the hydrogen fuel, and air to support combustion. The most often changed cylinder i? the support air because it is consumed at a high rate of 300-500 cc/min per FID. The house compressed air cannot be used directly since it contains hydrocarbons in varying amounts, and this causes a high and variable background signal. It is this problem which was solved so that house air can be used. The “heatless dryer” invented by Skarstrom (1) was adapted to prepare hydrocarbon-free air for use by the FID. The heatless dryer is so named because it regenerates the drying beds without heat. This is accomplished by taking advantage of the chromatographic principle that a front moves through a bed (or column) in proportion to the volume of gas that flows. The heatless dryer contains two beds; one bed is under full line pressure of 8 atmospheres while it is drying the compressed air that passes through it. Assume one bed volume of air at 8 atmospheres flows through in a given period and the dry product air is expanded to 1 atmosphere; then 3 of the resulting 8 bed volumes at one atmosphere are taken as useful product while the remaining 5 volumes are used to backflush the other bed. At the end of the time period (usually 30 seconds), the roles of the two beds are interchanged by solenoid valves. In this way, each bed passes one volume in the drying mode but receives 5 volumes of backflush in the regeneration mode. The action is such that each cycle produces air that is,dryer than the previous cycle. In a test starting with soaking wet beds, air with 1 ppm H 2 0was produced within 2 days; in 2 weeks the air was so dry that water could not be detected since the Frost point was less than -9O’C. See the Skarstrom patent for other details. Although Skarstrom’s original work was aimed at drying air, he has shown that the device can perform other functions, and a better title would be “heatless fractionator.” Skarstrom recommended activated carbon for our problem. In this work the heatless fractionator beds were prepared by packing with CX 640 8-12 mesh coconut charcoal (Matheson Coleman and Bell). To obtain good long-term hydrocarbon rejection efficiency, the purge or backflush volume must be 5 or more times the feed volume, and the bed length should be 20 inches or longer. The product will contain a steady state level from 0.03 to 0.5 ppm methane and less than 2 ppb of C2or other higher hydrocarbons; the particular level will depend on feed, bed size, and operating conditions. Heatless dryers are commercially available from a number of licensees. Several units used in this investigation were obtained from the Pure Gas Equipment Co., 1445 Strongs Road, Copiague, N. Y . 11727. Model H F 200 using 105 psig inlet air gave 2 liters per minute (STP) of acceptable product. The methane concentration reduces with reduction in product rate. The 100 psig compressed air line in our building supply contains a fairly steady concentration of 1.5 ppm methane;
it is saturated with compressor oil vapor and varying amounts of Cz to C, materials. Since all but a small amount of C1 is rejected, the flame background is not only low but steady. Compared to cylinder air, the flame background is reduced 4- to 5-fold with the use of heatless fractionated air. Besides obtaining low flame background and lower noise levels with the fractionator, an additional benefit can be derived by use of oxygen enrichment which can be attained by using Linde 5 A molecular sieves in addition to the CX 640 carbon. In a unit with 1-inch diameter beds containing 10 inches of 5 A sieves plus 10 inches of CX 640 carbon, 17 liters per minute purge, 0.35 liter per minute product, and 100 psig feed air, a dry hydrocarbon-free product was obtained with 23.8x oxygen. Since the feed was 21.1 % 0 2 , this represents a 13% increase in 02 concentration. Increasing O2 increases the FID sensitivity up to a point. When the 0 2 concentration is too high, the FID becomes nonlinear. For example, at 100% 02,the FID produces 10-fold higher signals for 10 ppm hydrocarbon, but 5-fold lower signals for 10-ppb samples than the same FID at 21 02. The signals at 21.1 and 23.8% O2 are linear functions of hydrocarbon concentrations in the low ppm range. This is not to be confused with nonlinear behavior previously reported as being due to mechanical design (2). Thus one must be careful in making this or other modifications to the FID. Using enriched super clean support air, house nitrogen carrier gas, and electrolytically derived hydrogen with a PerkinElmer F-11 chromatograph, a noise level of 5 X ampere was attained with a flame background level of 3.5 X 10-13 ampere. This noise level is close to that of the electrometer alone with the flame out. It was important to seal the detector with No. 27 Scotch tape and lead the exhaust through a lis-in. X 6-in. tube to keep the flame under slight pressure in order to exclude the influence of hydrocarbons in the room air. Hydrocarbons in ambient air at ppb levels were analyzed directly using a 10-ml sample injection loop with a Porapak Q column l/s-in. X 5 ft at 100 “C. A 3-inch high peak (1.5 X 10-13 ampere) was recorded for 8 ppb prcpane when the noise level was *0.1 inch. Thus 0.5 ppb determinations were obtainable. The unit has run for over two years continuously using the house nitrogen carrier on the Porapak column without adverse effects. The author need not extol the advantages of electrolytic hydrogen. A heatless fractionator operated for one year on carbon without any observable deterioration in the outlet concentration of hydrocarbons. The unit was purposely terminated and repacked with the carbon plus sieves to gain oxygen enrichment; this too has enjoyed 1’12 years of faithful operation. Thus if house nitrogen and an electrolytic generator are available, the addition of a heatless fractionator will permit the operation of a gas chromatograph on a cylinder-free basis.
(1) C . W. Skarstrom, U.S. Patent No. 2,944,627.
(2) W. H. King and G. D. DuprC, ANAL.CHEM., 41, 1936 (1969).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, J U N E 1971
RECEIVED for review February 11, 1971. Accepted March 25,1971.
