Purification of Air for Use in Gas Chromatography as a Carrier Gas and in a Flame Ionization Detector F. W . Williams and H. G. Eaton Chemical Dynamics Branch. Naval Research Laboratory, Washington, D. C. 20375 An economical and convenient source of pure air is needed in gas chromatography to support combustion for the flame ionization detector (FID) and as a carrier gas ( I ) . The use of purified laboratory air and electrolytic hydrogen would eliminate the use of bottled gases if air were used as a carrier gas. In addition, a previous publication (2) showed that using purified air and electrolytic hydrogen improved the sensitivity of the FID by an order of magnitude. King ( 3 ) has developed a batchwise process for air purification using a modified version of the heatless dryer (4). This technique, using carbon as a purification medium, does not eliminate methane from the air which can occur naturally at a concentration u p to 10 ppm by volume. Some “laboratory” compressed air supplies have over 200 ppm of methane present ( I ) . A few ppm of methane in the oxidant supply for the FID would cause some background noise and limit the sensitivity. More significantly, the same quantity of methane in the carrier gas would cause a much higher background noise. This is due to burner design and diffusion flame considerations-that is, the methane in the oxidant supply does not reach the “ionization” sensitive portion of the combustion region. For the specific analysis of methane, quantities below the background would result in negative peaks. Carrier gas supplies must also be well regulated and constant if reliable retention times are to be obtained. Thus, unless a large ballast were used, a batch processor for air would not have a constant delivery. A gas chromatograph has been developed, the NRL Total Hydrocarbon Analyzer ( I ) , which uses air as a carrier gas and incorporates the principles of backflush chromatography ( 5 ) . The chromatograph, developed for use on nuclear submarines, uses ships’ air for the carrier gas and oxidant for the FID. Since this air source contains significant concentrations of methane ( I ) and halogenated hydrocarbons ( 6 ) , treatment to supply pure and well-regulated air is mandatory for this application. Studies conducted on air purification using catalytic oxidation show that chlorinated hydrocarbons as a group of compounds are difficult to oxidize (7, 8). This publication describes an air purification system employing the t.echnique of catalytic oxidation which delivers a constant flow of air essentially free of organic contaminants.
ite, a mixture of oxides of manganese and copper, 6 mesh (Mine Safety Appliances Corporation). Two different types of catalytic burners were employed, one with a special preheater design (9) which is shown in Figure 1 and a Hewlett-Packard Model H P 18 helium purifier, replacing the catalyst normally used in the helium purifier with the test catalysts. Halogen acids produced in the catalytic burner were removed with a bed of lithium carbonate in the gas stream. The catalytic air purifier shown in Figure 1 is designed with the heater in the middle and next to the catalyst bed. The space outside the bed is the region in which the air to be purified is preheated. The preheating increases the effective volume of the catalyst bed. The test gases contained 200 ppm by volume each of methane and dichlorodifluoromethane (R-12).Some of the tests were conducted with 200 ppm by volume of 1,2-dichloro-1,1,2,2-tetrafluoroethane (R-114)in addition to the above two gases. The standards were obtained from Matheson Company. The effluent products from the catalytic bed were monitored with an NRL Total Hydrocarbon Analyzer and a Beckman GC-5 gas chromatograph. Both chromatographs were coupled to a Hewlett-Packard 2116C computer through a Hewlett-Packard 3370 electronic integrator to facilitate data handling.
RESULTS AND DISCUSSION Two types of tests were conducted on each catalyst: (1) lifetime and (2) minimum temperature required for complete oxidation of the contaminants. The severity of the test gas that challenged the catalyst was dictated by the projected source of air for the Submarine Analyzer application. Various refrigerant type gases and methane can be found in most laboratory and general air supplies. The platinum catalyst (Test No. 2, Table I) removed methane and R-12 a t a much lower temperature (180 “C) than the other catalysts; methane is harder to remove than R-12. The table also shows that at the minimum EXHAUST
,,,THERMOCOUPLE
EXPERIMENTAL The catalysts studied were 0.5% palladium on Ys- and %e-inch alumina pellets (Engelhard Industries, Inc.), 0.3% platinum on %-inch alumina pellets (Baker and Company, Inc.) and hopcal(1) H. G. Eaton, M . E. Umstead, and W . D. Smith, J. Chromafogr. SCi.. 11,275 (1973). (2) F. W . Williams, F. J. Woods. and M . E. Umstead, J. ChfOmafOgr. Sci., 10, 570 (1972). (3) W . t i King. J r . , Anal. Chem., 43, 984 (1971). (4) C. W . Skarstrom, U.S. Patent No. 2,944,627. (5) V . Schwenk and E. Weber, Fresenius’ Z. Anai. Chem., 164, 159 (1958). (6) F . W . Williams, M . E . Umstead, and J. E. Johnson, NRL Report 6964. 6 Nov. 1969, Naval Research Laboratory, Washington, D.C. 20375. (7) R . H . Johns, Chem. Eng. Prog., Symp. Ser., 62, (63) 81 (1966). (8) J K. Musick, F. S . Thomas and J . E. Johnson. Ind. Eng. Chem.. Process Des. Develop.. 11, 350 ( 1972).
---%++Figure 1. Stainless steel constructed catalytic special preheat zone
air
purifier with a
(9) F. W. Williams and J. E . Johnson, U.S. Patent No. 3.607.131
Table I . A Comparison of Properties of Catalysts Used to Purify Air Catalyst Test
Volume, cm3
Type
0.5% Pd
1
Weight, gram
Size, in.
Tyw. Ca
Gas flow cm3,/min, STP
Vol. of gas processed/ Wt of Lifetime, catalyst, I./gram hours
26
27
'la
300
300
17.7
26
0.3% Pd
50
44
'I8
180
400
65.5
120
on alumina Hopcalite
50
0.5% Pd
28
56 24
680 31 0
475 300
32.6 63.7
64 86
145
104
325
360
63.1
304
on alumina 2 3 4
on
mesh
1'1 6
alumina
0.5% Pd
5
6
I
1 '1 6
on alumina
Test gases
+ R-12 CHd + R-12 CH4
CHI CH4
+ R-12
+ R-12
CH4, R-12,
and R - 1 1 4
a Minimum temperature required to completely scrub the test gas
z 0
2
0
~
~
~
~
~
~
-
-
56
-
-
100
IO0
2
C C12F2
_ _ 2
8
I
I
14
20
26
32
38
44
50
TIME, HRS
Figure 2. Continuous catalytic oxidation of methane and dichlorodifluoromethane by 2 7 grams of 0.5% palladium on Ys-inch alumina pellets at 325 "C. Methane oxidation is incomplete after 2 5 hours temperature required to remove R-12 and methane for equal size pellets (ys inch), the lifetime of the platinum is 3.7 times that of palladium. Unfortunately, the platinum would only remove 40% of the R-114 even a t 450 "C. The hopcalite (Test 3) required a catalyst bed temperature of 680 "C to completely remove R-12 and methane. This is about 100" above the workable temperature of the catalyst (10); the workable temperature being defined as (10) W. A. Cannon and C. E. Welling, lnd. Eng. Chern, Prod. Res. Develop., l , 152 ( 1 962).
180
the limit above which serious deterioration of the catalyst would result through a transformation (10). The palladium catalyst (Test 5) removed methane, R114, and R-12 completely a t 325 O C with the preheater burner. As expected, the surface area of the pellet (size) plays an important role in the lifetime of the catalyst; 4~ and2 5 in0 the~Table. The y16~ compare ~ Test ~ 1 to ~ Tests ~ inch pellets will process 3.6 times as much gas as the l / g inch pellets in the same burner. A pictorial representation of Test 1 is shown in Figure 2. Palladium removed R-12 completely a t 315 "C and the catalyst has a lifetime of 26 hours. After the catalytic bed was spent, additional heating of the catalyst had little effect on its ability to remove additional methane (see the Figure). A higher temperature was required for the palladium to remove R-12 than for methane, which is just the opposite of the platinum. It is also interesting that the lifetime of the palladium catalyst was not appreciably affected by the addition of R-114 to the test gas. We currently operate catalytic air purifiers using 0.5% palladium on alumina on all our laboratory air supplies and the catalytic beds have not required changing in over two years. One precaution has been taken to prolong the life of the catalyst; an oil aerosol filter manufactured by Deltech Engineering Corp. has been installed ahead of the catalytic burners. Received for review June 8, 1973. Accepted August 22, 1973.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1, J A N U A R Y 1974
