production of o 3 , no, and n 2 o in a...

Production of OB, NO, and N20 in a Pulsed Discharge at 1 Atm

Kevin G. Donohoe, Fredrick H. Shah,’ and Oliver R. Wulf

Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 9 1 725

The rates of formation of the oxides of nitrogen (NO, N20) and of ozone have been studied in a pulsed electrical discharge in air at atmospheric pressure. The “hybrid” discharge consisted of a brush discharge in which O3 and N20 were produced and of hotter filaments which formed NO. At lower flow rates the effect of NO2 poisoning of the ozone production was found to be important. A diffusion model of the flow in the discharge tube was developed to describe the observed results and to predict the onset of NO2 poisoning. Using these results an estimate is made of the contribution from natural point discharges at the surface of the earth to the N20 content of the at- mosphere. It appears to be very small.

Introduction Interest in atmospheric pressure electrical discharges has

increased in recent years due to the development of the TEA laser and to the economic advantages of operating commercial plasma processing devices at atmospheric pressure (Beaulieu, 1971). The role played by electronegative gases in such discharges is also of interest, from the point of view of how electron attachment processes affect the breakdown of such gases and from the viewpoint of how electronegative impurities (or reaction products) can significantly change the operating and performance characteristics of a discharge. This is of interest, of course, for discharges operating at any pressure. At high pressure, one would in general expect negative ion formation rates to increase because high-pressure discharges typically operate at low values of E / p (electric field to pressure ratio) relative to low-pressure discharges. This means that the electrons are less energetic on the average, and since attachment cross sections generally peak at low values of electron energy, the result would be a higher rate of formation of the negative ions. On the other hand, the higher collision frequency at atmospheric pressure should tend to increase the detachment rates, particularly for negative molecular ions. These mechanisms have been discussed in some detail (Loeb, 1965; Nasser, 1971; McDaniel, 1964). The use of the various freons and SF6 for the suppression of spark and corona breakdown in high-voltage circuits is perhaps the most com- mon application of the effects of electronegative gases on a plasma (Devins and Sharbaugh, 1961). Baker et al. (1971) have investigated the effect of impurity level concentrations of electronegative compounds (and others) on the N-atom production in low-pressure microwave and condensed electrode discharges in N2.

We have investigated the oxidation of nitrogen and the formation of ozone in a pulsed atmospheric pressure discharge in air with the purpose of examining the effect of the presence of NOz, an extremely electronegative species, on the electrical and chemical characteristics of the discharge. The N2-02 system was chosen because the phenomenon of NO2 “poisoning” of the ozone production occurs in it (first reported in 1906 by Warburg and Leithauser). Relatively speaking, it is a “simple” chemical system in that a limited number of stable reaction products are formed. In addition, the homogeneous gas-phase kinetics of the reactions of nitrogen oxides with themselves and with ozone are well documented (Crutzen, 1971; Johnston, 1951). The oxidation of nitrogen in electrical discharges was reviewed up to 1939 by Glockler and Lind. We also report the use of a new type of discharge circuit which can produce volumetric glow discharges or the “hybrid” combi- nation of brush (nonequilibrium) and filamentary (con- stricted, and “hotter” than the brush) discharges in gases a t

atmospheric pressure, depending on the circuit parameters and electrode geometry. The discharge under discussion here is different from those used in previous work in that its time scale is nearly that of the formative time of a streamer breakdown in a gas at atmospheric pressure.

Experimental Section Discharge Cell and Flow System. The discharge cell was

constructed from a 6 in. length of aluminum tubing (1 in. i.d. X 3/16 in. wall) with a 0.020-in. diameter copper wire electrode along its central axis. An oxide layer quickly formed on the copper and was present during all of the experiments discussed here. The wire was initially held in place with Teflon end plates which were subsequently changed to Lucite (poly(methy1 methacrylate)) to permit visual observation of the discharge. Neither changes in the electrical characteristics of the discharge nor changes in the amount of oxides of nitrogen produced were observed when the Lucite was intro- duced. Thus, for the conditions investigated, the Lucite had a negligible influence on the results. Untreated Linde com- pressed air (less than 3 ppm of HzO) was metered with a needle valve, passed through the discharge tube, and into a scrubber filled with 4-mm Raschig rings. The scrubber’s efficiency was checked by flowing known NO2-air mixtures through it and measuring the amount of nitric acid formed. These tests indicated that the scrubber’s efficiency for absorption of NO2 was in excess of 90% for the range of flow rates used. In order to vary the amount of time that the discharged gas spent in flowing from the discharge tube to the scrubber, the tube and the scrubber were connected in two different ways: direct connection with Pyrex tubing (of 2 cm3 of volume between the discharge and the water in the scrubber) and secondly, the addition of a 3-L spherical flask (of approximately 3500 cm3 of volume between the discharge and the scrubber). These two conditions are referred to in subsequent figures as “no flask” and “3-L flask.”

Electrical. The circuit used is a modified version of the one originally designed by Nikola Tesla in the late 1880’s (Martin, 1894). It consists (Figure 1) of a variable 60-cycle high-voltage supply (Variac and neon sign transformer), which charges the capacity in an LC resorant circuit to a voltage determined by the spark gap electrode separation. The spark gap firing rate is then controlled by the magnitude of the transformer primary voltage. During the time that the spark gap is conduct- ing, the LC circuit resonates with a frequency of approximately 1 / a , damped by the spark gap resistance with a 30 ws time constant. This oscillation excites the secondary of the Tesla transformer, resulting in the application of a large amplitude damped sinusoidal voltage across the discharge cell electrodes. The Tesla primary is a coil of 14 turns of Y8-h. di-

208 Ind. Eng. Chem., Fundam., Vol. 16, No. 2, 1977

SPARK GAP

r v - m

NEON Sll3N TRANSFORMER

- T E S L A CELL I

TRANSFORMER

Figure 1. Schematic of discharge circuit.

ameter copper tubing, 8 in. in diameter and 14 in. long. The secondary, a coil of 636 turns of no. 30 cotton-covered wire wound on a 3 in. diameter x 16 in. long phenolic tube, is coaxial to the primary. With the 12000 pF capacity used (6 2000-pF ceramic capacitors), the circuit resonates a t 0.45 MHz, and the transformer efficiency (no-load voltage gain/ turns ratio) is 20%. The spark gap electrodes are faced off lengths of 3/8-in. diameter copper rod and permit stable operation of the circuit for up to 10 h without requiring any ad- justment. The small spark gap electrode separations used (0.010-0.020 in.) were adjusted with a feeler gauge. Voltage and current waveforms 'were monitored with Tektronix P6015 high-voltage probes and a Pearson Model 4100 current probe. Waveforms were displayed with a Tektronix 465 oscillo- scope.

Two different types of discharge occur in the tube: a brush discharge near the wire occurs each time the spark gap fires, and bright filamentary discharges which cross the gap occur less frequently. For reasons that will be discussed below, it was necessary to count the number of filamentary discharges which occurred during the 15-min duration of each experi- ment. A 1.7 in. focal length lens situated 2 in. from the inlet end of the discharge tube was used to focus the light from the filaments and brush into a 6-ft length of fiber optics tubing. This optical signal drove a photomultiplier tube whose output was first amplified and then attenuated to a level such that the larger signals associated with the filaments could trigger a monostable multivibrator while the weaker signals from the brush could not. The multivibrator pulses were then counted with a Beckman Model 522B counter. The amplifier-photomultiplier circuit was adequate for counting these events; however, its frequency response did not permit quantitative measurement of the amount and duration of the radiation from the discharges.

Chemical. In the scrubber, aqueous hydrogen ion concentrations were measured by NaOH titration of a boiled al- iquot of scrubber solution to a phenolphthalein end point. The sample was boiled prior to titration to remove any excess ozone from solution (the indicator solution contains ethanol) and to convert any remaining nitrous acid to nitric. The overall stoichiometry is

3N02 + I 3 2 0 = 2HN03 + NO ( 7 ) (a)

The possibility of the NO released reacting with 0 3 or 0 2 was ruled out by bubbling ,air-NOz and N r N O mixtures through water saturated with ozone. The NO2 gave the 2:3 ratio predicted by reaction a, iind the NO produced a hydrogen ion concentration that Waf3 negligible compared to that obtained for NO2.

Total NO, (NO, NO2, N205) from the discharge was measured with a Beckman 951 NO/NO, analyzer. The discharge effluent was passed through a 100 cm X 1 cm diameter Pyrex tube heated to 250 'C:. This decomposed all of the N205 to NO2 and destroyed any excess 0 3 . The cooled mixture was then analyzed on the 'Beckman analyzer.

SPARK GAP VOLTAGE FREQ.: 0.55 MHz

SCHEMATIC NO-LOAD

I- 2r K C . 1

BREAKDOWN WAVEFORMS

Figure 2. Discharge voltage waveforms.

N20 concentrations were measured with an Analytical In- struments Development, Inc. electron-capture gas chroma- tograph equipped with a tritium detector heated to 150 'C. A 20 f t X '&in. Porapak Q column was used to separate the NzO from the sample (Wentworth and Freeman, 1973). Ni- trogen carrier gas flowing at 60 cm3/min (STP) was used. Chromatograms were analyzed with a Spectra Physics Au- tolab System I digital integrator. Calibration by CSTR dilu- tion (Drivas et al., 1972) before and after the experiments indicated that the calibration factor remained constant to within 2% and that the low level of sensitivity was 0.5 ppm of N2O.

Results and Discussion The pale blue brush discharge mentioned earlier occurs in

the central part of the tube along the highly stressed central electrode and the brighter radial discharges, here called filaments, cross the gap. Visually, the brush extends less than 0.5 cm radially from the wire and occurs uniformly along its entire length. The filaments appear as bright blue channels about 1 mm in diameter. Although they do not branch, they do have a slightly zigzagged shape similar to the return stroke of a lightning discharge which follows the path of the leader stroke that precedes it. The filaments occur randomly with respect to angular orientation and axial position at the highest flow rates used but tend toward the pink in color and occur for the most part within a 30-45' wedge of angle a t the lowest flow rates. The effect of this change in the filament location will be discussed below. A detailed description of the operating characteristics of the circuit is available (Donohoe, 1976) and will not be considered here. Its behavior is outlined schema- tically in Figure 2. The no-load applied voltage (the voltage that would be applied if no discharge occurred) takes the form of a damped sine wave and begins at alternating polarity for successive spark gap firings. This is because the spark gap, driven by a 60-Hz signal, is fired 120 times per second, as shown in the top of Figure 2. Because of the circuit characteristics (Donohoe, 1976), the first half-wave of applied voltage is smaller in magnitude than the second. The peak no-load voltage is 43 kV, preceded by a 25 kV peak half-wave. This signal is independent of initial polarity (center of Figure 2). When the discharge tube is in the circuit, the voltage waveform depends on the initial polarity of the applied voltage, as indicated in the breakdown waveforms shown in Figure 2. For the wire initially positive (dashed line), the first half-wave of the applied voltage is loaded down to 12 kV by the brush current and no discontinuity occurs. The lack of a disconti-

Ind. Eng. Chem., Fundam., Vol. 16, No. 2 , 1977 209

Figure

I50 * “10” AS N205 0 3 LITER FLASK 0 NO FLASK

a SPAQK G A P ~ O . 0 2 0 m h ~ l

I R I T E - 120/rce.

GAS FLOWRATE - cm3Isec.

3. “ 0 3 and NO, production vs. flow rate.

nuity indicates that the gap is not completely broken down. On the next half-wave, the voltage reaches 25 kV (wire negative) and drops sharply to zero in 0.025 ps . For the case in which the wire is initially negative (solid line in Figure 2), the brush current loads the first half wave to 18 kV. The filamentary discharge which occurs on the second half-wave loads down the voltage a t 18 kV; the voltage drops to zero in 0.020 p s . The filamentary discharge consumes the energy that was stored in the capacitors and the circuit stops ringing. The peak pulse current of the filaments was 2.5 A with a 0.02 p s risetime. Since this is the risetime of the current probe, the 2.5 A represents a lower bound for the current, corresponding to a peak power density (1-mm diameter filament) of 2 MW/cm3. The brush current could not be measured above the noise level but is less than 2.5 A at its maximum value. The voltage measurements did not vary with flow rate.

From these measurements, it is evident that the discharges in the tube produce reaction products that are distributed in space and in time in a very nonuniform manner. The duty cycle (fraction of time on) of the filaments is 2.4 X and each filament occupies about 0.01% of the tube volume. The duty cycle for the brush is less than 1.8 X and it occupies less than 15% of the tube volume. Consequently, strong concentration gradients of the species produced in the discharges occur.

The gas leaving the discharge was analyzed for total NO, (NO, NOz, and NzO5), N20, and for its ability to produce nitric acid in the scrubber. Of these, only the last depended on the presence or absence of the 3-L flask. Figure 3 shows the results of the hydrogen ion (nitric acid) and total NO, measurements as a function of flow rate. The ordinate is normalized on a per filament basis because the hydrogen ion production was found to vary directly with the number of filaments counted. In addition, operation of the discharge at a 0.005-in. spark gap (a voltage a t which no filaments occurred) produced NO, at a rate of 0.5% of that obtained a t the higher filament-producing voltage. Roughly then, the maximum amount of NO, produced in the brush at the 0.020-in. spark gap is 2% of the total.

The upper curve in Figure 3 designated “NO, as N205,” indicates that each filament produces a constant amount of NO, independent of flow rate for flows greater than 25 cm3/s. Since these data were taken with the Beckman NO, meter as concentration measurements, it was necessary to use the average filament frequency to convert the concentration measurement to the units of pmol/lO5 filaments. The error bars represent the variation in filament frequency which occurred during the concentration measurements. The level of this curve is coincident with that of the “3-L flask” curve for flows between 25 and 70 cm3/s. This means that enough ozone is produced by the discharge to oxidize all of the NO to N205 if the 3-L flask is used to provide enough time for the oxidation to go to completion. It is possible that, for flows above 70 cm3/s, the relative amount of O3 produced dropped but there is no particular reason for this to occur, especially in view of

GAS RESIDENCE TIME - sec.

Figure 4. N20 production vs. residence time.

the constant NO production in the filaments and, as will be discussed below, the constant NzO production in the brush. Also, operation of the tube with pure 0 2 under the same circuit conditions produced 0 3 a t a constant rate of 0.7 pmol/s over a flow rate range of 66-220 cm3/s (residence time from 1.2 to 0.35 s).

For flows less than 25-30 cm3/s, the nitric acid production drops sharply for the case in which the 3-L flask is present and less sharply for the case in which it is not. This implies that the amount of ozone relative to the NO is beginning to decrease and that it becomes less than stoichiometric at the point where the 3-L flask data drop off. Although it is not evident from the figure, the two curves coincide for flows less than 17 cm3/s down to 9 cm3/s (the lowest flow rate used). This decrease is coincident with the tendency of the filaments to become more restricted with respect to angular orientation in the tube. The fact that the curves coincide for flows less than 17 cm3/s indicates that no reactions occur in the 3-L flask for these flows, i.e., that no ozone is leaving the tube.

The N20 production by the discharge is shown in Figure 4 (residence time is the tube volume (77 cm3) divided by the volumetric flow rate of the air). Since the NzO concentration has a linear dependence on residence time, a constant amount of it is formed each time a discharge occurs and no decomposition at the lower flow rates is observed. The linearity of the curve suggests that the NzO is formed by the action of the brush since the brush occurs each time the spark gap fires. To check this, the spark gap was reduced to a point at which the filament rate depended strongly on the spark gap firing rate. At a spark gap of 0.010 in. (no-load voltage = 21 kV peak), the filament rate varied from about 1 per second (for a spark gap rate of 6O/s) to 410 per second (for a spark gap rate of 540/s). If the NzO were formed by the filaments, its concentration should increase rapidly as the spark gap firing rate is increased under these conditions. On the other hand, if it were formed by the brush, its concentration should increase linearly with the spark gap rate. Figure 5 shows the results of this measurement and indicates that the NzO is formed by the brush. I t is noteworthy that the curve in Figure 4 shows the NzO production per brush discharge as constant, even though the production of NO and of O3 starts to drop rather sharply a t residence times greater than 2.5 s (from Figure 3). This means that the NzO is formed via a mechanism which is not affected by changes in the amounts of NO, and O3 leaving the discharge.

In summary, the NO is formed in the filaments at a constant amount per filament for flows greater than 25 cm3/s. NzO, formed by the brush, is produced at a constant amount per discharge for all flow rates used, and is formed a t a rate of approximately ‘k; that of the NO a t high flow rates. The gas analysis for NO, and the hydrogen ion measurements of the scrubber solution indicate that enough ozone is produced in the brush to oxidize all of the NO to NzO5 for flows greater than 25-30 cm3/s, but it is not present in high enough concentration to allow the oxidation to go to completion in the 2


Table I 14 - 1 FEW FILAMENTS-MANY FILAMENTS

/ e 1

I SG: 0.010 mches I

/ AIR FLOW: 40.3 cc/secq 2 c -A- 4 : 0 L -

0 l2C 240 360 480 600 720

SPARK GAP FIRING RATE- sec.-l

Figure 5. NiO productioii vs. spark gap firing rate.

cm3 of volume present when the 3-L flask was not used. The decrease in nitric acid production per filament at low flows occurs as the filaments become pink in color and become more restricted with respect to angular orientation in the tube, but the voltage waveforms do not change as this occurs, In an ef- fort to understand these results, a diffusion model describing the mixing of reaction products in the discharge tube was developed.

Diffusion Model for the Discharge Tube Spatially, the NO is produced by the radial filaments

throughout the tube. I t is assumed that the NO production in each filament is independent of radial position. Because of the alternating polarity of successive breakdowns (firing rate of 120/s), variations in the production by positive and negative wire filaments are unimportant.

The radial distributions of brush-produced 0 3 and NzO are more complicated to understand. Rideal and Kunz (1929) studied the steady-state ozone concentration profile in a coaxial corona in 0 2 a t atmospheric pressure. They observed that most of the 0:3 was located in the outer region of the tube (away from the bright corona near the central wire) and suggested that ultraviolet radiation from the discharge photochemically destroyed 0 3 in the central region. In the pulsed brush discharge of interest here, the time scales for electron impact and photochemical reactions are approximately the same and are of the order of less than s. Consequently, electron impact dissociation of 0 2 and photochemical dissociation of 0 3 are indistinguishable sources of 0 atoms. For the condition of atmospheric pressure air with NO and/or NO2 present, the fastest O(3P) atom reaction rate is the formation of 0 3 via 0 + 0 2 + M = 0 3 + M (Axworthy and Benson, 1959; Kaufman, 1961). This reaction has a time constant of approximately s. (Because O(’D) is quenched to O(3P) so quickly at atmospheric pressure (DeMore and Raper, 1962), we need only to examine O(3P) reactions.) Thus, in a pulsed brush, the radial production of 0 3 should depend on the radial and temporal variations in the electron energies in the gap. Analysis of the characteristic electron energy as a function of the undistorted appliled field (McDaniel, 1964) shows that this energy is less then 8j/2 eV for distances greater than 0.1 cm from the central wire. Thus, most of the 0 atom production shvuld occur within this region. The radial production of N20 (which is treated as inert) will be shown to be similar to this below.

To summarize, the NO is produced in radial columns which cross the gap, and the O3 is produced in a small region near the wire. As the NO and 0 3 mix via radial diffusion, they react to form NO2 and then N 2 0 5 if enough 0 3 is present. Since a numerical solution to t he equation of continuity would require a detailed description of the flow field (including entrance effects) and the spatial/temporal profiles of discharge-produced NO and 03, a simplified model based on the characteristic times associated with the diffusion, convection, and reactions which occur in the tube was developed.

Rate constant, 295 K crn”/s-] Reaction (Crutzen. 1971)

Table 11. Characteristic Times for Processes in the Discharge Tube

Process Characteristic time Value

Electrical Duration of current 410-6 s) discharge flow

time Mean gas residence V/Q 0.37 - 8 s

Radical diffusion R2/3.580 (Carslaw 3 s

NO oxidation ( 0 3 ) l/kd(M1 - 1) [Nolo and Jaeger, 1959)

2 x 10-4 s (MI = 2 )

NO:! oxidation l / k e ( 2 M 2 - 1) [NO210 2.9 X lo-’ s ( 0 3 ) (M2 = 1)

N205-NO reaction l / k N 2 0 5 - N 0 (Smith -(lo0 s) and Daniels, 1947)

(England and Corcoran, 1975)

NO oxidation ( 0 2 ) l l k N O - o n [02][NO]o 4 5 0 s)

The kinetics of NO-03 reactions are of particular interest in atmospheric chemistry and are’ reviewed in considerable detail (Crutzen, 1971). For the present system, the set of reactions given in Table I is adequate to describe the oxidation kinetics. Since reaction b goes essentially to completion before reactions c-f begin ( k b / k , = 266), the kinetics can be simplified by allowing (b) to go to completion separately. Defining M1 as [03]0/[NO]o, the initial ratio of 03 to NO, reaction b gives, forM1 z 1

Applying the steady-state assumption to [N205] and [NOB], the NO2 conversion from reactions c-f is

where M2 = M I - 1 and [NO& = [Nolo. A numerical integration of the rate equations, obtained by

considering the five reactions simultaneously, gives about 15% agreement with the above analysis for the N205 concentration as a function of time. The characteristic times for the NO and NO2 oxidations were calculated with eq 1 and 2, using the local filament concentration (I-mm filaments). These are listed in Table 11, along with the times associated with the other processes which occur in the tube.

It is evident from the table that the discharges themselves represent virtually instantaneous sources of NO and 0 3 . Also, the oxidation kinetics are fast compared to radial diffusion. Finally, the radial diffusion is fast compared to the residence time at low flow rates (residence time greater than 3 s) and slow a t high flows. So for flow rates greater than -477 cm3/3 s = 25.7 cm3/s), the oxidation of NO to Nz05 is controlled by the radial diffusion of 0 3 .

The ozone is treated as being produced in an effective brush discharge radius R,; with time, it diffuses radially outward. In the steady state, the 0 3 will then be present in a cone- shaped volume of radii Ro at the tube inlet and R1 at the exit.

Ind. Eng. Chem., Fundam., Vol. 16, No. 2, 1977 211

150 in

0

w 100 a U

+ I

w v) 50

2 Y 0 1 - 1 , 1 .A i o 50 100 I50 200 250

GAS FLOWRATE-cm 3/sec.

Figure 6. Variation of HNOB production with Ro model calculation.

R1 varies with the gas flow rate and is calculated by averaging the laminar velocity profile over the cone-shaped volume. The final radius R1 = VIR (where r) = r / R ) is

V I R = Ro + ~‘(3.585Dt) ( 3 ) where D, the Fickian diffusion coefficient of O3 in air, is 4 . 1 5 cm2/s. The average radius of the cone-shaped volume of 03, (o ) , is approximately equal to 271/3. The average velocity in the cone is obtained by averaging the velocity profile (Bird et al., 1960) over the cone-shaped volume

where K is the ratio of the wire radius to the tube radius R. The time for radial diffusion is then

t = L / ( V , ) (5)

where L is the tube length (15.24 cm). Solving eq 3-5 simultaneously (numerically) permits calculation of the volume of the diffusive cone of ozone. Entrance effects were neglected. Because the kinetics of the NO and NO2 oxidations by ozone are fast, we assume that within the cone volume, all the NO is oxidized to N205. Since the amount of NO produced per filament is constant, the assumption of a constant filament rate of 108 per second gives the total NO in ppm as NO = 4542/Q, where Q is in cm3/s. Since the cone volume is

SL 3

V, = - (Ro2 + (71R12 + RRo71)

the N205 concentration leaving the discharge tube is (tube volume = 77 cm3)

also V 4542

[NOIppm= ( I + ) ( T )

The hydrogen ion production is then calculated by as- suming a value for the initial ozone concentration and per- mitting homogeneous reaction between the NO and O3 (reactions b-f) to occur in the 2 cm3 of tubing which is treated as a plug flow reactor.

The two adjustable parameters in this model are Ro, the initial ozone radius (or the effective brush radius), and the amount of ozone produced. Visual observation of the discharge limits Ro to 0.5 cm maximum. A lower limit is estimated by calculating the distance from the central wire electrode at

Z 3 0

150 in

0

a E5 100

+ I

W m 50

A Y O L U L - - - - 1 _ . I i o 50 100 150 200 250

GAS FLOWRATE-cm3/sec.

Figure 7. Variation of “ 0 3 production with M model calculation.

which the peak applied field falls below 30 kV/cm (the nom- inal breakdown voltage of air). This gives the length of the ionizing sheath as 0.15 cm and represents a lower limit on Ro. A value of Ro = 0.3 cm was chosen somewhat arbitrarily (the sensitivity of the results of the calculation with respect to Ro will be discussed). Because at high flow rates the NO and NzO production is constant with respect to filament and brush frequencies, respectively, it was assumed that each brush discharge in air produced a constant amount of 03, just as it does in an oxygen discharge. Indeed, the data do not permit any more complicated assumptions abdut the O3 production. If M is defined as the ratio of the O3 produced in the brush to the NO produced in the filaments, i.e., the ratio [03]/[NO] that would leave the tube if no reaction between the two took place, then this assumption makes M constant for flow rates greater than 25-30 cm3/s.

Figure 6 shows the results of varying Ro for M = 2. The low flow cutoff point of each curve is the flow rate a t which = 1, i.e., the flow rate at which the ozone diffuses to the tube wall at the end of the tube. This point depends on Ro and is independent of M. Figure 7 shows the variation of the nitric acid production as a function of M for Ro = 0.3 cm. The lower dashed curve is the calculated nitric acid production that would occur if no reactions took place in the tubing between the discharge and scrubber. The values Ro = 0.3 cm and M = 2 fit the data. However, a decrease in Ro would be compen- sated for by an increase in M. The reason that Ro = 0.3 cm is chosen is that its low flow cutoff point is consistent with the onset of the NO2 poisoning (which decreases the rate of oxidation of nitrogen and the ozone production at low flow rates).

The operation of a dc brush discharge in an “ozoneless” mode was first reported by Warburg and Leithauser (1906). They found that this discharge produced an NO2 concentration of 130 ppm and that (1909) N2O was produced in the brush in both modes with an N20/NO,) ratio of about 0.2 in both a poisoned positive sphere-to-plane brush and an ozone-producing brush of opposite polarity. In addition they observed that, in a Siemens tube (ozonizer), N20 could be produced from a mixture of NO2 and Nz (presumably via the reaction NO2 + N = NzO + 0) but that only traces of N2O were produced from a mixture of N205 and 0 2 in the same tube. Zabolotskii (1950), working with atmospheric pressure discharges in air, found that the addition of NO to the feed often caused the light from the discharge to go out. He interpreted this as an increase in the breakdown potential of the gas caused by the presence of NOz. The particular mechanism by which these discharges became poisoned is not clear, but it involves changes in the composition and therefore the electrical characteristics of the gas mixture. A short set of experiments in this laboratory using a Siemens ozonizer with air indicated that the O3 concentration leaving the discharge dropped from 0.02% to zero when small amounts of NO, N02, SFs, or CF3Br were added to the feed. Clearly, the presence


of these gases can inhibit the discharge production of ozone.

For mixtures of NO:! in 0 2 , Jones and Wulf (1937), in ' re- porting the visible absorption spectrum of NO3, found that NO2 could not be observed spectroscopically in an N205-03 mixture in 0 2 until both the NO3 and the O3 bands disap- peared. In a discharge, then, one would expect poisoning by NO2 to begin when the N205 concentration reaches a level a t which its thermal decomposition (and decomposition by the discharge) becomes large enough to significantly affect the ozone concentration, according to a reaction scheme like the one proposed by Johnston (1951)

Nz05 + M = NzO5* + M

N2O5* + M = Nz05 + M

N205* = NOz + NO3

NO2 + NO3 = NnO,=,

2(N02 + 0 3 = NO:3 + 0 2 )

NO3 + NO3 = 2N02 + O2

Net: 2 0 3 = 302

Once this catalytic destruction of ozone begins to occur at a rate comparable to the O3 production by the discharge, the lack of 0 3 permits NO:! to build up (a similar scheme, proposed by Wofsey et al. (1975) in their evaluation of the effect of freons on stratospheric ozone describes a similar catalytic destruction of 0 3 by ithe presence of halogen radicals). The choice of Ro = 0.3 cm in the diffusion model is consistent with this poisoning scheme in that once the 0 3 diffuses across the entire tube cross secti,on, its concentration is lowest because of mixing and reaction, and the N205 concentration is large. These two conditions make the onset of poisoning most probable. The gradual decrease, observed here, in the 0 3 concentration leaving, the discharge is due in part to the fact that the filaments teind to occur over a limited angular orientation at the lower flow rates, resulting in a poisoning which initially occupies this small (-45') wedge and diffuses around the entire tube cross section at the lowest flow rates.

Once NO2 is present, its large electron affinity affects the electrical properties of the gas. The electron affinity of NO*, nearly 2.4 eV (Herbsit et al., 1974; Berkowitz, et al., 1971), is large compared to that of the other constituents of the mixture: 0 3 (1.96 eV), 0 (1.47 eV), NO (0.91 eV), and 0 2 (0.44 eV) (Chemical Rubber Co., 1970). The presence of a strongly electronegative gas has the effect of decreasing the first Townsend coefficient a by an amount 1, the number of electrons lost by attachment per centimeter of electron drift. Cross sections for electron capture (McDaniel, 1964; Rapp and Briglia, 1965) indicate that is a maximum for low electron energies (or at low values of Elp) and is smaller for large values of electron energy at which dissociative attachment (XU t e- - X + Y-) becomes important. I t has been shown (Geballe and Reeves, 1953) that for large values of p d , the asymptotic value of Elp below %which breakdown will not occur corre- sponds to the Elp at which 7 and a are equal. Consequently, the presence of an electronegative gas in a mixture decreases the rate of Townsend ionization.

Because of the time-varying nature of the electric field in a highly overvolted pulsed discharge, the effect of electron attachment (Le., of VI) on the measured voltage waveforms is more complex. The voltage waveforms measured here do not change as the NO2 regime begins. This is because even though the Townsend avalanche that leads to streamer breakdown occurs a t a slower rate if NO2 is present in small amounts, the tip field of the midgap streamer increases so rapidly with time

(or distance) a t threshold that the electron multiplication which depends on (a - 7) is virtually identical with that which depends on (a). The effect of 7 is smaller because 7 decreases with E / p and, in air, a l p varies with ( E / P ) ~ (McDaniel, 1964). Consequently, the voltage or the time at which the tip field causes the unstable condition of electron growth which leads to the current loading of the voltage waveform is not strongly affected by the presence of the NO2. If the applied pulse voltage had been the minimum required for breakdown (this depends, of course, on the pulse shape), the presence of NO2 would have prevented the breakdown from occurring. The electron density and energy distribution is expected to be affected, however, since the low energy part of the distribution is available for electron attachment, and to a lesser degree, the higher energy part is available for dissociative attachment. The details of the NO2 electron capture cross section are not known. However, the effect of the electron capture by NO2 on the energy distribution and density of the electrons should cause changes in the electron impact reaction rates.

At flow rates less than 17 cm3/s, the production rate per filament of nitric acid in the scrubber is constant a t 30% of what it was at the high flow rates. Since the presence of the 3-L flask does not affect the amount of acid formed, no 0 3 leaves the discharge tube a t these flow rates.

It is most likely that the NO is formed via the reaction N2(A3Z) + 0 2 = 2N0. This reaction, first suggested by Brocklehurst and Downing (1967) involves the metastable A3Z state which, as Noxon (1962) has shown, has a lifetime of the order of 1 s in pure nitrogen a t atmospheric pressure. Fur- thermore (Young and St. John, 1969), almost all of the emis- sion during N atom recombination terminates on this level and direct recombination into the A state is, by statistical weight, 1/3 of the total recombination flux. The production of NO via reactions such as N + 0 2 = NO + 0 is unlikely because the faster reaction N + NO = Nz + 0 becomes equal to it in rate (in air) at an NO concentration of approximately 25 ppm (Kistiakowsky and Volpi, 1957). Since the minimum bulk concentration of NO, observed here was -20 ppm, the local NO concentrations near the small filaments are in excess of this value. Consequently, these reactions do not contribute substantially to the NO production.

The mechanism by which the NzO is produced must be one whose rate is unaffected by the NO2 poisoning of the discharge (Figure 4), and by the filament frequency (Figure 5). This indicates that the NzO is not a secondary product of NOz, i.e., that it is not formed by the reaction (Kistiakowsky and Volpi, 1957) N + NO:, = NzO + 0. Some limited evidence exists for the formation of N20 via a three-body reaction between oxygen atoms and nitrogen in its ground state. Harteck and Dondes (1954) have demonstrated that a mixture of Nz and ozonized oxygen reacts to form N20 at 295 O C , presumably via the reaction O(3P) + N2 + M = NzO + M. Groth and Schi- erholz (1957) found that photochemically produced oxygen atoms (O(lD) and O(3P) were produced) reacted with N2 to form N20 with an efficiency of 1 N20 molecule per 104 0 atoms. The mechanism of this reaction is discussed by De- More and Raper (1962). If this reaction occurred here, its reported efficiency with respect to 0 atoms ( 1:104) is too low to account for the amount of NzO observed. For example, a t a flow rate of 9.6 cm3/s, the N20 concentration was 30 ppm. If lo4 O(lD) atoms were required for each N20 molecule formed, then each brush discharge would have had to dissociate 2.7% of the oxygen in the brush volume; this value seems unrea- sonably large.

One reaction which has not been reported in the literature is suggested here as a possible path for the NzO formation. This is the reaction O(3P) + Nz* + M = N20 + M, where N2* represents vibrationally excited Nz( lZ). Recent work by McNeal et al. (1974) has shown that the rate a t which O(3P)


quenches Nz* (u = 1,2) is anomalously large compared to that predicted by vibrational relaxation theory. This is not understood and no measurements of possible chemical products for this system have been reported. However, the production of vibrationally excited Nz is known to occur readily in discharges; indeed, this excitation permits resonant energy transfer to occur between Nz and COz in the COz laser (Beaulieu, 1971). Cross sections for vibrational excitation of Nz by electron impact indicate that Nz* is expected to be formed (Schulz, 1966). Because of the low efficiency of the O(lD)-Nz reaction, the reaction O(lD) + N2* + M = N20 + M probably does not occur.

Since the cross section for the electron impact formation of N2(A32) is maximum a t 7.6 eV (McDaniel, 1964), both it and 0 atoms should be formed in the filaments and in the brush. However, Nz0 is observed to form in the brush and not in the filaments. The most probable reason for this is that the larger NO and NO2 concentrations in the filaments remove the 0 atoms via (Kaufman, 1961) 0 + NO + M = NO2 + M and 0 + NO2 = NO + 0 2 . This can be seen by considering the relative volumes of the brush and the filaments, and the relative amounts of NO produced by each. Using 0.1 cm and 0.6 cm (2Ro) as the filament and brush diameters, respectively, the ratio of their volumes is 430. As mentioned previously, the NO production by the brush is about 2% of the total. If we assume that the number of 0 atoms produced by each discharge is proportional to the peak current (the ratio of the filament current to the brush current is greater than 2.5), then the relative reaction rates between 0 and NO in each region can be evaluated. The ratio is ([O][NO])brush/( [O]-

the 0 atoms, not consumed by NO or NO2 in the brush as quickly as they are in the filaments, are available for the presumably slower NzO formation.

The poisoning of the filaments decreases the rate of NO production. This is probably caused by a decrease in the number of electrons with energies capable of forming Nz(A3Z) (the gas-phase catalytic destruction of O3 by N2O5 decomposition initiates the poisoning). This means that the number of higher energy electrons required to dissociate 0 2 has decreased as well. The N20 production remains constant because of the slow radial diffusion of 0 3 produced in the brush. This slow diffusion in the radial direction prevents the region near the central wire in which the brush occurs from being poisoned by the N205 decomposition. I t is expected that a t flow rates much lower than those used here, the buildup of the NO2 concentration in the tube would eventually lead to the poisoning of the brush as well, resulting in a decrease in the ozone and in the N20 production in the then-poisoned brush discharge.

An additional comment may be made about the nature of NO2 poisoning in a discharge. In this work, the amount of oxidized nitrogen produced decreased as the discharge became poisoned. I t is not clear that this always happens. Shulz and Wulf (1940) found that in an ozonizer operated at constant voltage with air, the yield of oxidized nitrogen a t a low flow rate initially increased in the temperature range 20-100 "C, dropped sharply from 100-200 "C, and finally increased almost linearly with temperatures from 200 to 700 "C. Since no O3 or N2O5 was present at these high temperatures, it is clear that the discharge operated in an "ozoneless" mode but still produced substantial amounts of oxidized nitrogen. Part of this behavior may be due to the variation of the operation of the ozonizer discharge with temperature. However, it is evident from their work that the effect of operating a discharge in an ozoneless mode does not always cause the production of oxidized nitrogen to decrease.

The Production of Atmospheric NzO by the Point Discharges from the Earth's Surface. A current problem

[NO])filamenb = (1/(2.5 X 430)) (0.02/430) = 4.3 X lo-'. Thus

in atmospheric chemistry is that the sources and sinks of N20 are not completely understood (Johnston and Selwyn, 1975). Since NzQ has been produced in a brush discharge in this work, it is of interest to estimate the total NzO production that results from the point (i.e., brush) discharges which occur a t the surface of the earth. Unfortunately, the global point discharge current is not well known. An estimate (Chalmers, 1967) of this current gives a global value of the order of lo3 A.

The N20 formation rate observed in this work was scaled to this total current. An upper bound is calculated by treating the brush discharge current as a square wave, 0.5 A in magnitude and 0.1 ps in duration. This gives an N20 production efficiency of 9.85 X mol of NzO/A-s. For a global current of lo3 A, this would result in the production of approximately 1500 tons of Nz0 per year. Bates and Hays (1967) estimate that at least lo7 tons of NzO per year are produced a t the earth's surface. Consequently, it would appear that point discharges from the earth's surface do not constitute an important source of global N20. This conclusion is based on the assumption that it is appropriate to use the N2O production efficiency evaluated here and on the estimate of the global point discharge current.

Summary The action of a "hybrid" pulsed electrical discharge in air

a t atmospheric pressure has been studied with respect to the formation of NO,, (NO, NOz, NzOs), NzO, and 03. At high flow rates, the amount of NO, formed depended only on the number of filamentary discharges which occurred. The NzO formation by the brush discharge occurred a t a constant rate for all flow rates, including the low flows at which the discharge became poisoned by NOz, thereby causing the NO, production to drop to 30% of its high flow rate value and the net O3 production to drop to zero. A diffusion-controlled model of the radial mixing of the discharge products in the tube was developed by comparing the characteristic times associated with the transport and reaction processes in the discharge tube. This model was used to calculate the flow rate at which poisoning would begin and to calculate the amount of O3 produced by the discharge. The change in the character of the discharge due to poisoning by NO2 is interpreted in terms of the large electron affinity of NO2 compared to those of the other constituents of the gas. The onset of poisoning in an air discharge is apparently due to the catalytic gas phase consumption of O3 by decomposing Nz05, as well as by the decomposition of NzO5 by the discharge.

Literature Cited Axworthy, A. E., Benson, S. W.. Adv. Chem., Ser., No. 21, 388 (1959). Baker, R. R., Jacob, A., Winkler, C. A,, Can. J. Chem., 49, 1671 (1971). Bates, D. R., Hays, P. B., Planet. Space Sci., 15, 189 (1967). Beaulieu, J. A,, Proc. E€€, 59, 667 (1971). Berkowitz, J., Chupka, W. A,, Gutman, D., J. Chem. fhys., 55, 2733 (1971). Bird, R . B., Stewart, W. E., Lightfwt, E. N., "Transport Phenomena," Wiley, New

Brocklehurst, B.. Downing, F. A,, J. Chem. fhys., 46, 2976 (1967). Carslaw. H. S., Jaeger, J. C., "Conduction of Heat in Solids," Chapter 8. Oxford

University Press, 1959. Chalmers, J. A. "Atmospheric Electricity." p 301, Chapter 11, Pergamon Press,

New York, N.Y., 1967. Chemical Rubber Company, "Handbook of Chemistry and Physics," 50th ed,

Chemical Rubber Co.. Cleveland, Ohio, 1970. Crutzen, P. J., J. Geophys. Res., 76, 7311 (1971). DeMore. W.. Raper, 0. F., J. Chem. Phys., 37, 2048 (1962). Devins, J. C., Sharbaugh, A. H., flectro-technology, 104 (1961). Donohoe, K. G., Ph.D. Thesis, Califwnia Institute of Technology, Pasadena, Calif.,

Drivas, P. J., Simmonds, P. G., Shair. F. H., Environ. Sci. Techno/., 6, 609

England, C., Corcoran, W. H., lnd. fng. Chem., Fundam., 14, 55 (1975). Geballe, R., Reeves, M.. Phys. Rev., 92, 867 (1953). Glockler. G., Lind, S. C.. "The Electrochemistry of Gases and Other Dielectrics,"

Groth, W. E., Schierholz, H.. J. Chem. Phys., 27, 973 (1957). Harteck, P., Dondes, S.. J. Chem. Phys., 22, 758 (1954).

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Herbst, E., Paterson, T. A,, Lineburger, W. C., J. Chem. Phys., 81, 1300

Johnston, H. S., J. Am. Chem. SOC., 73, 4542 (1951). Johnston, H. S., Seiwyn, 0. S., Geophys. Res. Lett., 2, 549 (1975). Jones, E., Wulf, 0. R., J. Chem. Phys., 5, 873 (1937). Kaufman. F., Prog. React. Kinet., 1, l(1961). Kistiakowsky. G. B., Volpi, G. G.. J. Chem. Phys., 27, 1141 (1957). Loeb, L. B., "Electrical Coronas--Their Basic Physical Mechanisms," University

of California Press, Berkeley, Calif. 1965. Marlin, T. C., "The Investions. Researches and Writings of Nikila Tesla." Mcllroy

& Emmet Press, New York, N.Y.. 1894. McDaniei, E. W., "Collision Phenomena in Ionized Gases," Wiley, New York,

N.Y., 1964. McNeal, R. J.. Whitson, M. E., Jr., Cook, G. R., J. Geophys. Res., 79, 1527 (1974).

Nasser. E., "Fundamentals of Cjaseous Ionization and Plasma Electronics," Wiiey. New York, N.Y., 1971.

Noxon, J. F., J. Chem. Phys., 36, 926 (1962). Rapp, D., Briglia, D. D., J. Chiem. Phys., 43, 1480 (1965).

(1974). Rideal, E. K., Kunz, J., J. Chem. Phys., 24, 379 (1920). Schulz, G. G., Phys. Rev., 135A, A988 (1966). Schulz, J. R., Wulf, 0. R., J. Am. Chem. SOC., 62, 2980 (1940). Smith, J. H., Daniels, F., J. Am. Chem. SOC., 89, 1735 (1947). Warburg, E., Leithauser, Ann. Phys., 20, 743 (1906). Warburg, E., Leithauser, Ann. Phys., 28, 313 (1909). Wentworth, W. E., Freeman, R. R., J. Chromatogr., 19, 322 (1973). Wofsy, S. C., McElroy, M. B., Sze, N. D., Science, 187, 535 (1975). Young, R. A., St. John, G. A., "Chemical Reactions in Electrical Discharges."

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Zabolotskii. T. V., Zh. Obshch. Khim., 20, 1384 (1950)

Receiued for reuiew December 19, 1975 Accepted November 9, 1976

This work was supported in part through USAEC Grant AT(04-3)-767 under Project Agreement No. 1.

Simultaneousl Absorption of Two Gases Which React between Themselves in a Liquid

Harua, Hikita,' Satoru Asai, and Haruo lshlkawa

Department of Chemical Engineering, University of Osaka Prefecture, Sakai, Osaka, Japan

The problem of simultaneous absorption of two gases A and B accompanied by an irreversible (m,n)th-order chemical reaction between themselves, A + vB - products, has been considered. Approximate analytical solutions for the reaction factor have been derived on the basis of the film and penetration theories and have been compared with the numerical or exact analytical solutions. It has been shown that the present approximate solutions are in good agreement with the numerical or exact solutions.

In roduc ion Simultaneous absorption of two gases which react between

themselves in a liquid medium is often encountered in the chemical industry. A practical example is the Solvay process where carbon dioxide and ammonia are absorbed into water to form ammonium carbamate. Another example is the manufacture of ethyl chloride by the simultaneous absorption of ethylene and hydrogen chloride into ethyl chloride solution. Theoretical analysis of the chemical absorption processes of this type was first presented by Roper et al. (1962), who obtained a numerical solution based on the penetration theory for the case of an irreversible (1,l)th-order reaction. However, the range of variables covered in their numerical calculations was limited. Teramoto et al. (1970) presented a numerical solution for a wider range of the variables. On the other hand, approximate analytical solutions based on the film theory have been derived by Ramachandran and Sharma (1971), Teramoto et al. (1971)1, and Juvekar (1974) for absorption with a (1,l)th-order reaction and Chaudhari and Doraiswamy (1974) for absorption. with an (rn,n)th-order reaction. A numerical solution based on the film theory was also presented by Teramoto et al. (1'971) for absorption with a (1,l)th-order reaction. The approximate analytical solutions described above, however, deviate from the numerical solutions, as will be shown later.

In this paper the approximate analytical solutions which conform very closely to the numerical or exact analytical solutions are presented for gas absorption accompanied by an irreversible (rn,n)th-order chemical reaction on the basis of both film and penetration theories.

Film Theory Analysis Film Theory Solution. Let us consider the case where two

solute gases A and B dissolve into a liquid phase and then react between themselves, according to the following reaction scheme

A + vB - products (1)

The reaction is assumed to be irreversible and to be (m,n)th-order, Le., mth- and nth-orders with respect to the gases A and B, respectively. The differential equations describing the diffusion of A and B in the liquid, based on the film theory, are as follows

d2B dx

D g 7 = ukArnBn (3)

In this paper, the gas B is assumed to have a finite concentration in the bulk of the liquid. Thus, the boundary conditions for the above eq 2 and 3 can be written as

x = 0; A = Ai, B = Bi (4)

x = xf; A = 0, B =Bo ( 5 ) The concentration profiles of A and B in the liquid may be

obtained by solving eq 2 and 3 under the conditions 4 and 5. The reaction factors for A and B can then be obtained from


production of o 3 , no, and n 2 o in a...

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