Anal. Chem. 1981, 53, 817-820
817
Gas Chromatographic Determination of Gases Formed in Catalytic Reduction of Nitric Oxide Willlam C. Hecker and Alexis T. Bell" Department of Chemical €ngineering, Unjversity of California, Berkeley, California 94720
The construction and performance are described of a gas chromatographic system for the analysis of gas mlxtures formed during the catalytic reduction of NO with CO, Hz, and NH,. Reactants and products are separated by using three packed columns COntaiNed within two isothermally operated gas chromatographs. Thie system operatlon, data acquisition, and data redudion are accomplished by means of a microprocessor. Quantitative ianalysis of NO, NH,, N,, N,O, CO, and COP can be obtained down to levels In the range of 10-35 ppm, and H2 can be ainalyzed down to 400 ppm. Water present in the sample is reparated from the other components but is not quantified.
The catalytic reduction of NO by H2,CO, and NH3has been studied extensively as a means for abating NO emissioqs from both automobiles and stationary sources (1-3). The reduction products usually contain all or some of the following components: NO, NH3, N2, N,O, GO, C 0 2 , Hz, and H20. The analysis of such complex gas mixtures has proven difficult m d has usually required the use of multiple analytical techniques (e.g., nondispersive infrared spectroscopy, gas chromatography, mass spectrometry, and wet chemical analysis) in order to obtain a complete determination of gas composition. The alternative approach has been to analyze for only a portion of the products and to determine the remainder through the assumption of a mass bialance. The danger in using this approach is that unanticipated products may be missed. Such a situation can arise, for example, when solid as well as gaseous products are formed during the catalytic reduction of NO by CO and H2 ( 4 ) . The use of gas chromatography (GC) to analyze the products of NO reduction has great appeal since the required instrumentation is simple', relatively inexpensive, and easy to calibrate. However, the development of such a procedure has proven difficult and, thus far, has led to only a single paper dealing with this subject. In a study by Landau and Petersen (5)it was demonstrated that mixtures containing N2, 02,Hz, NO, NH3, and NzO could be separated by using a pair of chromatographic columns each operated isothermally. The first consisted of a segment packed with Porapak R followed by a segment packed with Porapak R coated with 10% POlyethylenimine. The second column was packed with 5-W molecular sieves. Quantitative analysis of all components except NH3 was possible down to a few hundred parts per million. NH3 could be quantified down to only 2500 ppm due to the severe tailing of the NH3 peak. The analysis of CO and COB was also shown to be possible; however, the analysis for COZ would only be carried out in the absence of NH3 since the peaks for NH3 and GO2 overlapped. The present paper describes the construction and performance of a gas chromatographic system for the analysis of mixtures containing NO, NH3, N2, N20, CO, COz, H2,and HzO. This system uses two isothermally operated gas chromatographs equipped with thermal conductivity detectors and a total of three columns. Operation of the system, data ac-
quisition, and data reduction are accomplished by means of a microcomputer. Quantitative analysis of NO, NH3, Nz, N20, CO, and COz can be obtained down to levels in the range of 10-35 ppm, and H2 can be analyzed down to 400 ppm. Water is separated from the other components but is not detected.
EXPERIMENTAL SECTION Chromatographic System. Figure 1 shows a schematic of the chromatographicsystem. The principal components are two isothermal gas chromatographs (Varian 90P3, Varian Associates, Palo Alto, CA) equipped with thermal conductivity detectors and three valves, each of which is operated by a motor-driven actuator (Carle Model 4201, Carle Instruments, Inc., Anaheim, CA). Three columns are required to separate all of the Components of interest. Column A (3.2 mm X 3.05 m) is packed with Chromsorb 106, chosen for its ability to separate COPand N20 (6). Column B (3.2 mm X 2.8 m) is packed with Chromsorb 103 to permit good resolution of an NH3peak (6). Finally, column C (4.8 mm X 3.05 m) is packed with 5-A molecular sieves. This packing is used to separate Hz, N2, NO, and CO (4). Columns A and B are both placed in the oven of the first gas chromatograph, GC1, and are maintained at 118 "C. This temperature represents a compromise, allowing a good separation between GO2and N20at the same time suppressing the tailing of the NH3 peak. Column C is located in the oven of the second gas chromatograph, GC2, and is maintained at 150 OC, a near optimum temperature for resolution of NO and CO. To achieve a complete analysis, two two samples, each 1 cm3 in volume, are injected into the chromatographic system using valve 1 (Whitey SS-43Y 6F 52, Oakland Valve and Fitting, Pleasant Hill, CA). The first sample passes through valve 2 (Carle 2013, CARLE Instruments, Inc., Anaheim, CA) and into column A which separates COzand N20 from a composite peak containing all of the light components (i.e., H2, N2, NO, and CO). The composite peak elutes from column A first and is directed via valve 3 (Whitey 43Y F2, Oakland Valve and Fitting, Pleasant Hill, CA) to column C in GC2. A complete resolution of the components present in the composite peak is achieved in GC2. To avoid the introduction of C02 and N2O onto column C, which would strongly adsorb these gases, valve 3 is switched 3 to its bypass position once the composite peak has entered GC2. The surge in the pressures across the detectors of both chromatographs when valve 3 is turned is minimized by the presence of two needle valves which simulate the flow resistances of columns A and C. The second sample injection is made 4.5 min after the fiist and 2 min after switching valve 2. The 2 min delay following actuation of valve 2 is needed to regain a flat base line from GC1 which is upset by the reversal in flow direction through column B. The sample passes thrbugh column B where it is resolved into an NH3 peak, one composite peak consisting of COZ and N20,and a second composite peak consisting of the light components. In addition to selecting between columns A and B, valve 2 serves a second function. Examination of Figure 1shows that the column not in service is backflushed by an auxilliary flow of carrier gas which also passes through valve 2. Backflushing of the columns facilitatesremoval of H20 (and NH3from column A) which would otherwise take 20-30 min to elute. Experience showed that in order to achieve consistent NH3 peaks free of excessive tailing it was necessary to introduce a small amount of NH3into the helium carrier gas. Addition of 600-1200 ppm of NH3 increased the NH3 peak height by almost 10-fold over that observed using an ",-free carrier, greatly suppressed peak tailing, and made it possible to attain reproducible peaks.
0003-2700/01/0353-0817$01.25/00 1981 Amerlcan Chemical Society
818
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
TO VENT
TO VENT
GCI
Column Dimensions A(2mmx3m) B(2 m m x 2.8 m )
GC2 C(3mmx31-n)
packing Chromsorb 106 Chrornsorb 103
5A Mol. Sieve
Carrier Rate (SSCM) 30
FIOW
30
Column T(°C) lie lie
150
Detector Current (mA) 225
D:":"($
150
240
165
Figure 1. Schematic of the chromatographic system.
Table I. Calibration Equations species COZ NZO NZ
NO
co "3
H2
a
range, P P ~
equationa
3tco2 = (1.042 X 10-')Rco, = (1.124 X 1 0 - 6 ) R ~ , o X N ~= (1.700 X 1 0 T 6 ) R ~ , xNO = ,(4.331 x 1 0 - 6 ) ~ N 0 0 . 9 w XN,O
(2.160 X 1 0 - 6 ) R ~ 0 0 ' 9 Q 3
XNO
xco = (2.500 X 10-6)Rco X", = (1.138 x 10-')(450 t R"3)1'933 X N H ~= XH,
(1.190 X
t (4.080 X 1 0 - 6 ) R ~ ~ 3
= (2.250 X 1 0 - S ) R ~ 2 1 * P a
Ri = peak height in mV,xi = mole fraction.
(1)
The effects of other components on the calibration equations were less than 1%and as a result were neglected. The lowest practical limits at which the components described by eq 1 could be detected were between 10 and 25 ppm. The calibration data obtained for NO were not linear in NO mole fraction. As a result two expressions of the form XNO =
NORN NO^
10 11 17 35 25 25 400
Corresponds to Ri = 10 mV.
For the present work an NH3concentration of 850 ppm was found to be most suitable. It should be noted that the presence of NH3in the carrier leads to the observation of negative as well as positive NH3peaks. When a sample containing less NH3 than the carrier gas is analyzed, a low concentration zone is created which propagates through the column at the same speed as a positive peak would. When this zone reaches the detector, it produces a negative peak since its thermal conductivity is greater than that of the carrier gas. Calibration, The relationships between component peak heights, Ri, obtained from the chromatographs and the mole fractions of each of the components, xi,in the sample are expressed in terms of calibration equations. These relations are listed in Table I and were obtained by correlating the responses observed for standard gas mixtures (Matheson Gas Go., Newark, CA) diluted with helium. Accurate dilution of the standard gas mixtures was facilitated by the use of mass flow controllers (Precision Flow Devices, Inc., San Jose, CA). As shown in Table I the calibration data for COz, NzO, Nz, and GO were fit by linear equations of the form xi = aiRi
0-20000 0-20000 0-5000 0-5300 5300-20000 0-60000 0-1200 (R" 2 0) 1200-20000 ( 3 ~ " ~ 2 0 ) 0-20%
lowest detectable limit,b ppm
(2)
were used to characterize the relation between response and sample concentration. The average deviation between eq 2 and
the data was less than 1% The effects of GOz and HzO on the NO calibration equation were explored but found to be less than 1%. The presence of NH3 in the sample did decrease the NO response slightly, but this deviation did not exceed 2.6%and hence no correction for NH3 was imposed. The response relationship for Hz is also nonlinear. The larger value of aH2compared to the values of ai for the other componenb reflects the low sensitivity of the detector for Hz. This results from the fact that the difference in thermal conductivity between Hz and He is smaller than that for the other components. Development of a calibrationequation for NH3was much more difficult than for any of the other gases. This was due in part to the fact that both positive and negative responses occurred with this component and in part due to the influence of NO, HzO, and GOz in the sample on the magnitude of the NH3 response. To account for these latter effects, we obtained extensive NH3 calibration data with and without each of the three Components present. These data were then correlated to produce correction functions which were used to modify the basic NH3calibration equations. A flowsheet of the complete algorithm used to calculate xNHs is shown in Figure 2. Since a considerable number of computations were required, the whole calculation was programmed for a microprocessor. In the positive response range ( R m t 0), the algorithm is fairly simple, As shown in Figure 2, three independent corrections are calculated and then applied to the uncorrected calibration equation which is that of a straight line. The correction for NO was found to be only a function of XNO and thus fairly simple. This correction is normally relatively small except at high concentrations of NO and low concentrations of NH3where it becomes quite significant relative to the value of x"~. The correction function for HzOis also rather small and only becomes significant at fairly high concentrations of both NH3and HzO. The HzO mole I
ANALYTICAL CHEMISTRY, VOL. 53, NO. 6, MAY 1981
818
TWO-PEN RECORDER
n
BUFFER
I
FILTERS
I
0@
Ill
L I N E POWER FOR VALVE ACTUATORS
Figure 3. Interconnections between the PET microprocessor and the chromatographic system.
0 "NH~. ' N H ~
IRCC= R C + I . S ~ X I O ~exp(-924xNHs)I X~,~
1 Figure 2. Algorithm used io calculate xNH3. fraction, xHz0,required for this correction is calculated frcm the stoichiometry of the system. The correction function for COz is somewhat more complicated than those for NO and HzO. A function of the form cxco,o.@ AXNHs
=
1 + dR":
(3)
was found to fit the Calibration data quite well. At various levels of xCoZand RNHo X N H ~was fit with an average deviation of 0.9%. In the negative response range ( R N H ~
