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NOTES Inlet Pressure Effects on the Collection Efficiency of Diffusion Scrubbers Purnendu K. Dasgupta’ and Per F. Llndgrent Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-106 1
w At constant mass flow rates commonly used for sampling, changes in inlet pressure result in a change in sample velocity and in turn in a change in collection efficiency. Significant errors can result unless this is corrected, especially for diffusion-based collectors that operate at low collection efficiencies. Virtually all designs will require appropriate inlet pressure correction if large changes in pressure, as in airborne Sam ling, are involved. An equation of the type f = 1- Ae-Epi where f is the collection efficiency, Pi is the inlet pressure, and A and €3 are constants fits the experimental data well and may be used for performing the necessary correction. Crider et al. ( I ) were apparently the first to suggest diffusion-based selective removal of a gas from a gasaerosol mixture. Since the exploitation of this concept by Fish and Durham (2) to measure the diffusion coefficient of SOz, the technique has become firmly established in trace atmospheric measurements, both for the determination of a trace gas collected on the walls of a coated tube or for the analysis of the aerosol penetrating the denuder, once the gas is removed. In a recently conducted “Nitrogen Species Methods Comparison Study” (3),eight separate groups resorted to diffusion-based collectors for trace HNOBmeasurement ( 4 ) . The general background and advantages of making diffusion denuder based measurements have been summarized by Ferm ( 5 ) . The diffusion scrubber, a membrane-based diffusion denuder, was developed to permit continuous collection and analysis of trace gases (6) and has since been shown to provide highly sensitive automated measurements for a variety of trace analyte gases (ref 7 and citations therein). Diffusion-based collection, either with a denuder or a membrane-based scrubber, is subject to the principles first laid out by Gormley and Kennedy (8). More recently, the Cooney-Kim-Davis solution (9) has been advocated for cases where the sink surface is less than 100% efficient (10). However, the sink efficiency has to be very substantially lower than unity before a departure from the Gormley-Kennedy solution is evident. This conclusion was also reached by Corsi et al. ( I I ) , who developed a numerical solution for computing the collection efficiency of membrane-based diffusion scrubbers. Other parameters remaining the same, the efficiency with which a given analyte gas is collected in a diffusionbased collector is dependent on the flow velocity (linearly related to the volumetric flow rate) through the device. For maintaining a constant sampling rate, the use of mass flow controllers is prevalent in the present practice of atmospheric analysis. As the name implies, such a device ensures a constant mass flow rate; thus whenever such a controller +Permanent address: Department of Analytical Chemistry, University of UmeA, UmeA, S901-87,Sweden. 0013-936X/89/0923-0895$0 1.50/0
is used, the flow velocity changes inversely with ambient pressure. Despite the widespread use of diffusion-based collectors, the change in collection efficiency due to changes in ambient pressure has not generally been considered. Admittedly, the change is not likely to be significant for ground-based denuders designed to collect 99+ % of the analyte gas under standard test conditions. However, the change is likely to be significant for denuders operated under conditions that result in subquantitative collection; this includes diffusion scrubbers (7) and transition-flow reactors (12,13)which typically operate in the subquantitative collection regime. Significant errors are likely for virtually all designs in airborne applications; the inlet pressure in this case may vary by over a factor of 2 from standard calibration conditions. We have recently developed a diffusion scrubber coupled ion chromatograph (14). In this instrument, the scrubber liquid effluent loads the injection loop of an ion chromatograph (suppressed hydroxide eluent anion chromatograph) slowly; the loop filling time is nearly equal to the time necessary for a single chromatographic run. The contents of the loop are injected in the chromatographic system and then the valve returns to the load position and the loop-filling process begins again. Thus, every 6 min (the time necessary for a single chromatographic run) the instrument produces a new chromatogram of the analytes it had collected. A variety of acid gases can be measured, with typical detection limits in the mid to low parts-pertrillion range. The instrument was flown aboard a National Center of Atmospheric Research aircraft at altitudes up to 5200 m above the mean sea level. Experimental verification of theoretical expectations, regarding the dependence of the collection efficiency on the inlet pressure, thus became necessary to interpret the data obtained. These experimental results are described in this note. Experimental Section
The experimental system is shown schematically in Figure 1. Pure dry air (zero-grade cylinder air) is metered by mass flow controller 2 into a permeation chamber containing a wafer-type permeation device PW emitting 20.9 ng/min sulfur dioxide at 30 “C (VICI Metronics, Santa Clara, CA). The permeation chamber effluent is diluted by more dry air metered by mass flow controller 1. The concentration of SO2 in the diluted air was held constant at 3.2 ppbv. A major fraction of the diluted calibrant is vented through needle valve N2. The remainder of the flow passes through an all-PTFE needle valve N1 (Gilmont Instruments, Great Neck, NY) through the air space of the diffusion scrubber DS (jacket i.d. 4.7 mm). The porous membrane tube M (400-pm i.d., 25-pm wall, 0.02-pm mean pore size, 40% surface porosity, polypropylene, Celgard X-20, Hoechst-Celanese Corp., Questar Division, Charlotte, NC) begins 10 cm past the inlet/outlet air connections to permit the full development
0 1989 American Chemical Society
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Table I. Dependence of Observed Signal Height as a Function of Sample Velocity A. Constant Mass Flow Rateo
o K
5
Torr
pi,
veloc,b cm/s
signal: nS/cm
collcn effic, f
-In (1 - f,
795 709 620 545 493 415 364
200 224 257 292 323 384 438
2340 2172 1908 1728 1620 1428 1308
0.159 0.147 0.130 0.117 0.110 0.097 0.089
0.173 0.160 0.139 0.125 0.116 0.102 0.093
W
B. Constant Inlet Pressured mass flow rate, veloc,b signal: SLPM cm/s nS/c,m
normlzd collcn signal' effic, f
W
128 193 257 321 388 453
1.00 1.50 Figure 1. Experimental arrangement: C, column; D, detector: DS, diffusion scrubber containing membrane tube M; E, eluent; MFC1, source dilution mass flow controller; MFC2, generation source mass flow controller; MFC3, sampling mass flow controller; N1, PTFE needle valve; N2, polyethylene needle valve; P1, chromatographic pump; P2, perlstaltlc pump for sample loop loading; P3, air sampllng pump; PM, pressure monitor: R, scrubber liquid reservoir, at positive pneumatic pressure: V, chromatographic injection valve.
of laminar flow. The inlet pressure to the DS was controlled by the adjustment of needle valves N1 and N2 and measured by pressure monitor PM (Baratron, MKS Instruments, Inc., Andover, MA). With both needle valves fully open, the inlet pressure to the DS is essentially ambient pressure (-620 Torr during these experiments). By closing needle valves N1 and N2 (one at a time), subambient and superambient inlet pressures were respectively attained. The airflow through the DS [l-3.5 standard liters per minute (SLPM)] was maintained by suction pump P 3 and controlled by mass flow controller MFC3. Scrubber liquid (1 mM H202)is aspirated from a pressurized (2-4 psi, higher pressures used at superambient DS inlet pressures to prevent air bubble formation within the membrane) reservoir R through membrane tube M and through the loop of the high-pressure injection valve V by peristaltic pump P2. The chromatographic components include the high-pressure pump P1 (QIC, Dionex Corp., Sunnyvale, CA) pumping 28 mM FaOH eluent (E) through V, chromatographic column C (Dionex AS5A, 4 X 250 mm), a membrane suppressor S (1-m filament-filled helical externally resin packed single-membrane suppressor, Nafion 020, see ref 15 for construction; 4 mM H2S04regenerant, 6 mL/min, regenerant flow not shown in figure), and a conductivity detector D (Dionex QIC). Collected SOz was completely oxidized by the H2OZ scrubber liquid prior to chromatography; the sulfate peak was used for quantitation. The blank value (zero air being sampled) of the sulfate peak was 40 f 5 nS/cm.
Results and Discussion The effect of inlet pressure should be possible to interpret solely in terms of the velocity of the sampled fluid. The dependence of collection efficiency on the sample velocity should be identical for two independent data sets, one obtained at constant mass flow rate with variable inlet pressure and one obtained at constant inlet pressure with variable mass flow rate. The dependence of the instrument response on sampling velocity is shown in Table I. In part A (constant mass flow rate), the collection efficiency is computed based on its linear relationship to the signal intensity, whereas in part B (variable mass flow rate), the 896
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2.00 2.50 3.00' 3.5d
3360 2400 1908 1574 1352 1179
1680 1800 1908 1968 2028 2064
0.228 0.163 0.130 0.107 0.092 0.080
-In (1 - f, 0.259 0.178 0.139 0.113 0.096 0.083
OAt 2 SLPM standard liters per minute (SLPM). *Calculated as 100QT760/(?rrh2273Pi), where Q is mass flow rate in SLPM, T i s ambient temperature in K, rh is the hydraulic radius of the diffusion scrubber, and Pi is inlet pressure in Torr. 'Corrected for blank, dAt 620 Torr except as indicated in footnote f. 'Calculated as sienal x2/mass flow rate in SLPM. fInlet Dressure, 615 Torr.
2000-
1000
!
100
I
260 300 Velocity (cm/s)
I
400
I
Flgure 2. Dependence of the instrument response to sample velocity (see text).
collection efficiency is computed based on its linear relationship to the normalized signal intensity. The normalization (with respect to a sampling rate of 2 SLPM) in the latter case is necessary because at a constant mixing ratio, changing the mass flow rate proportionately increases the number of SOz molecules sampled. In Figure 2, the results are shown plotted as signal height (or normalized signal height), directly proportional to collection efficiency, as a function of sample velocity. For all practical purposes, the two independent data sets represent the same dependence. The collection efficiency tends to be marginally higher for the constant mass flow rate case at high sampling velocities compared to the constant inlet pressure
case. The difference is real; the individua. variances of the data shown in Figure 2 are substantial y below the dimensions of the points plotted. While the maximum difference between the two curves within I he velocity range studied is not large (53%),we sought a rational explanation. Variations in the amounts of water evaporated from the scrubber cannot result in significant actual changes in the sampled mass flow rate. It has been previously shown (7) that with dry inlet air, a diffusion scrubber of the presently used dimensions loses only 6.6 ML/min water through evaporation (1 SLPM air, inlet pressure 675 Torr, 22 "C). Thus, changes in water evaporation can, at worst, cause a -0.5% error in the mass flow rate. Similarly, changes in the rates of water vapor efflux from the membrane surface are not likely responsible; it has been shown (7) that the collection efficiency does not change (no span error occurs) as the inlet relative humidity is varied from 15 to 85% with a fixed SOz concentration. It is possible that the scrubber liquid-air interface protrudes further into the pores toward the outer surface of the membrane as external pressure is decreased. This would reduce the diffusion distance to the interface and thereby increase collection efficiency slightly. The absolute collection efficiency of an identical DS device was determined at a flow rate of 2 SLPM and an inlet pressure of 680 Torr (velocity 234 cm/s) to be 0.142 under otherwise identical test conditions, with a corresponding signal height of 2090 nS/cm. This allowed us to compute the collection efficiencies (f, reported in Table I and evaluate the relationship between In (1 - f ) and the reciprocal of the sample velocity, which is expected to be linear for the diffusion-based collectors. The best linear least-squares regression line for the nonlinear equation -In (1 - f ) = m / V + b yields the best fit values m = 29.98 and b = 0.024 (r = 0.999), where Vis the velocity in cm/s. The equation resulting from the data in part B is quite similar: -In (1 - f ) = 31.27/V + 0.015, ( r = 1.000); if pooled together, the slope and intercept for the regression line for all the data are 30.78 and 0.019, respectively (r = 0.999). The inlet pressure dependence of collection efficiency (Table IA) can be more directly addressed because the sampling velocity is directly related to the reciprocal of pressure (at constant T ) , leading to a readily usable equation: f = 1 - Ae-BPi (1) For the data in Table IA the best fit values (r = 0.999) are A = 0.997 and B = 1.89 X Such equations are thus useful for performing inlet pressure corrections. For situations that involve a significant change in temperature from calibration conditions, a modified form of eq 1 where the Pi term is replaced by a Pi/Ti term should be appli-
cable. While the ranges of pressure variation in our experiments were quite large in order to interpret the aircraft data, which will be published elsewhere, it is obvious from the experimental data that calibrating a diffusion-based collection device in one location and then using it for measurement in another location can lead to appreciable errors even for ground-based measurement if a significant change of atmospheric pressure is involved. Acknowledgments We wish to thank the National Center of Atmospheric Research for the use of its Research Aviation Facility where these experiments were conducted. In particular, the help and assistance of Gregory L. Kok is gratefully acknowledged. Registry No. SOz, 7446-09-5.
Literature Cited (1) Crider, W. L.; Barkley, N. 0.;Knott, M. L.; Slater, R. Anal. Chim. Acta 1969, 47, 237-241. (2) Fish, B. R.; Durham, J. L. Environ. Lett. 1971, 2, 13-21. (3) Lawson, D. R. Atmos. Environ. 1988, 22, 1517. (4) Hering, S. V.; Lawson, D. R.; Allegrini, I.; Febo, A.; Perrino, C.; Possanzini, M.; Sickles, J. E., 11;Anlauf, K. G.; Wiebe, A.; Appel, B. R.; et al. Atmos. Environ. 1988,22,1519-1539. (5) Ferm, M. Concentration Measurements and Equilibrium
Studies of Ammonium, Nitrate and Sulphur Species in Air and Precipitation; University of Goteborg Press: Goteborg, Sweden, 1986. (6) Dasgupta, P. K. Atmos. Environ. 1984, 18, 1593-1599. (7) Dasgupta, P. K.; Dong, S.; Hwang, H.; Yang, H.-C.; Zhang, G. Atmos. Enuiron. 1988, 22, 949-964. (8) Gormley, P. G.; Kennedy, M. Proc. R. Ir. Acad., Sect. A 1949,52A, 163-169. (9) Cooney, D. 0.;Kim, S.; Davis, E. J. Chem. Eng. Sci. 1974, 29, 1731-1738. (10) Murphy, D. M.; Fahey, D. W. Anal. Chem. 1987, 59, 2753-2759. (11) Corsi, R. L.; Chang, D. P. Y.; Larock, B. E. Environ. Sci. Technol. 1988,22, 561-565. (12) Durham, J. L.; Ellestad, T. G.; Stockburger, L.; Knapp, K. T.; Spiller, L. L. J. Air Pollut. Control Assoc. 1986, 36, 1228-1232. (13) Ellestad, T. G.; Knapp, K. T. Atmos. Environ. 1988,22, 1595-1600. (14) Lindgren, P. F.; Dasgupta, P. K. Anal. Chem. 1989, 61, 19-24. (15) Gupta, S.; D isgupta, P. K. J. Chromatogr. Sci. 1988,26, 34-38.
Received for review September 16, 1988. Revised manuscript received February 13, 1989. Accepted March 6, 1989. This research was supported by the Electric Power Research Institute through Grant RP 1630-55.
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