PRODUCTION OF OZONE B Y THE USE OF A PLASMA JET C H A R L E S S. S T O K E S A N D L E O N A . S T R E N G T h e Research Institute of Temple University, 4150 Henry Ave., Philadelphia, P a .
Ozone has been synthesized by a new method involving the use of an inert gas plasma jet quenched by liquid oxygen. Yields of 0.46 wt. % and production rates as high as 1.17 pounds per hour have been obtained at powers up to 10 kw.
forms from oxygen in various types of electrical This article describes a n investigation of the feasibility of producing ozone by use of a plasma jet. Since, thermodynamically, the stability of ozone is favored a t low temperatures, the production of oxygen radicals at high temperatures and then rapid quenching of these species with oxygen molecules may well produce ozone. This scheme is the basis of these investigations as the plasma jet can easily produce radicals. ZONE
0 discharges in small yields.
Apparatus
The plasma generator (1J) consists of a water-cooled 3/16-inch 27, thoriated tungsten cathode with an annulus for feeding gas. T h e copper anode, which has a ‘/r-inch jet opening, is also water-cooled. A nylon cathode enclosure insulates the cathode from the anode. An Airco Bumblebee 750-amp. d.c. welder is used as the power source. The electrical meters shown in Figure 1 permit regulation of the operation. Starting the arc is accomplished by means of a high frequency starter built into the welder or by “shorting out” the electrodes with a 1/16-inch diameter rod of graphite. A liquid-oxygen feeding ring, attached directly to the plasma generator, consists of a 4-inch diameter stainless-steel ring 5/8 inch thick with a removable annulus insert which contains sixteen l/s*-inch entrance ports. The liquid oxygen enters perpendicularly to the plasma-jet stream.
The ozone-recovery system is made of stainless steel throughout. The unit consists of a chamber of 2-inch i.d. tube, 1 foot long. A Dewar adapter ring connects the chamber to a 2liter stripped Dewar. The chamber is attached directly to the liquid-oxygen feeding ring. A schematic drawing of the complete apparatus is shoivn in Figure 2. Preparation of Ozone
The preparation of ozone by use of plasma jet was accomplished by passing liquid oxygen into a n inert-gas plasma jet. The plasma jet was set up n i t h the liquid-oxygen feeding ring and a reaction tube as shown in Figure 2. The liquid oxygen (LOX) was alloived to flow fast enough through the ring to be collected in a Dewar flask at the bottom of the reaction tube. The concentration of ozone in the liquid oxygen collected in the Deuar flask was 0.4 =t0.05 wt. 70. The determination of the ozone content of the liquid oxygenozone mixture \\as made by the evaporation method in the first fen experiments. The vapor pressure and boiling point of the concentrated sample nere identical \kith those of ozone. The amount of ozone in the gas from the plasma jet collected beyond the liquid ozone-oxygen collection system varied from 0.006 to 0.05% 0 3 . ILhich showed that the ozone in the exit gas is not a major portion of the ozone produced.
Airco Bumblebee 600 Welder
To Transformer, Rectifiers
240 V.
Figure 1. 36
Electrical schematic of plasma jet
I&EC P R O D U C T RESEARCH A N D D E V E L O P M E N T
Method of Analysis
T h e following method was used for determining quantitatively the amount of ozone in liquid ozone-oxygen solutions by use of a Beckman spectrophotometer with a liquid nitrogen cooled cell.
Plasma Jet-Them1 Insulators
Determine per cent transmission of liquid oxygen alone (yoTo2)a t 600 mm. Determine per cent transmission of liquid ozone-oxygen solution [y0T,,,+ O t ) ] a t 600 m m . Then, % = % T < 0 3 +on) (100 - % To2) Determine D by: 100 D = log __ % TO8
1
\
+
Determine concentration of 0 C (moles/liter) =
3
in 0
2
Chamber
solution
D 2 (specific for cell used'
or Wt.
70 0
3
=
2.130
This method was quantitative for determining the wt. % 0 3 in liquid ozone-oxygen solutions. I t was checked by using several solutions u p to 0.6 wt. yo 0 3 and proved to be best for determining the ozone concentration in the liquid ozoneoxygen mixtures obtained from the plasma jet.
Dewar Adapter Stream
Factors Varying the Production of Ozone
03-02 Mixture
Production of Ozone in the Helium-Plasma Jet at Varying Power Levels. Power level is defined as follows: Power level =
(kilowatts to arc) (14.34 kcal./min. kw.) (helium flow liters/min.) (9.0907 moles/liter)
+- -I I I------- I J
Dewar
- - --I---
~
Table 1.
Arc characteristics Voltage 29 Amperage 320 Kilowatts 9.3 Helium flow, liters/min. 1 5 . 2 Kcal. /mole He 119 L O X flow Liters/min. 3 Ozone yield Wei ht yo 0.46 Lb. fhr., collected 1.13
28.7 29 315 310 9.05 9.0 15.2
115
3
3
0.40
...
Schematic of apparatus used for ozone production
Production of Ozone in the Plasma Jet Varying Power level
27.5 31 300 225 8.3 7.0
15.2
116
Figure 2.
15.0 108
29.5 28.5 26.5 28 26 28 27.8 27.5 280 350 350 320 340 360 340 315 8.3 10.0 9.3 9.95 8.85 10.05 9.45 8.65
15.2
15.2
15.2
15.2
18.0
12.2
14.1
16
28.8 300 8.65
30 280 8.4
18.9
21.7
23.5
90
106
128
119
97
141
138
115
88
76
69
2.9
3
3
3
3
3
3
3
3
3
3
3
0.38
0.40
0.14
0.23
0.29
0.23
0.35
0.24
0.20
0.29
0.235
0.23
0.26
1.17
...
0.35
0.57
0.77
0.69
1.07
0.78
0.49
0.71
0.62
0.61
0.50
~~
Table II.
Production of Ozone in the Plasma Jet Varying Liquid-Oxygen Flow
Arc . . .- characteristics ...-. ... . ~ ~~
~~~~
Voltage Amperage Kilowatts Helium flow. liters/min. Kcal./mole He ' LOX flow Liters/min. Ozone yield Wei ht % Lb.fhr., collected
27 340 9.2 16 111
27 340 9.2 16 111
26.8 345 9.25 16 111
26.8 3 45 9.25 16 111
29 310 9.0 15.2 115
1.25
2
4
5
2
3.2
0.23 0.35
0.23 0.63
0.21 1.04
0.20 1.Ol
0.16 0.43
0.27 0.88
VOL. 4
29 310 9.0 15.2 115
NO. 1 M A R C H 1 9 6 5
37
or Table 111.
Effect of Higher Power on Ozone Production
Voltage Amperage Kilowatts Power level Helium flow, liters/min. LOX flow, liters/min. Ozone yield, lb./hr., collected
29 310 9 115 15.2 3.2
0.88
30 400 12 116 20 4
26.8 345 9.25 111 16 4 1.04
1.10
Power level
29.5 410 12.1 117 20 4
=
kcal./mole H e
Power-level values were plotted us. ozone production in pounds per hour. The resulting curve is shown in Figure 3 (least square plot). Table I lists some of the experimental runs. The ozone yields in Table I are given in pounds per hour of ozone produced and refer to the actual amount of liquid ozone-liquid oxygen mixture collected during the run.
1.09
1.2
1.0
0.8
i
’
\
0.6
0 0
0.4
0
0.2 70
Figure 3.
90
80
100
110
Kcal/mole
-
120
130
140
150
He
Power level vs. ozone production for constant liquid oxygen flow
0.1-
2
1
3 LOX
Figure 4. 38
4
5
FLOW l i t e r s / m i n .
Ozone production vs. liquid oxygen flow at constant power level
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
~
Power, kw. 10.7 Helium flow, liters/min. 1 5 . 2 Oxveen flow. liters/min. 137 v0i.y % 0 8 ’ 0.0015 Ozone yield, grams/hr. 0.28
~~~~
~
~
Table IV. Ozone Production under l o w LOX Flows 10.5 10.6 10.7 10.1 10.0 10.5 15.2 15.2 15.2 15.2 15.2 15.2 191 216 218 268 340 520 0,011 0.020 0.025 0.045 0.057 0.059 2.54 5.26 6.42 15.0 22,6 36.3
10.5 15.2 585 0.068 48.6
10.5 15.2 558 0.078 49.7
10.5 15.2 485 0,085 48.3
using increasingly smaller liquid-oxygen flows, constant power levels, and constant helium flows. By using a water-cooled chamber, the effluent liquid oxygen-ozone mixture was vaporized, and the mixture was collected in polyethylene bags for analysis. Table IV gives the results of these investigations, and Figure 5 compares the results with the liquid runs. The maximum gas flow obtainable before liquid oxygen started to flow from the water-cooled chamber was 600 liters per minute. This maximum flow leaves a gap between the gas and liquid experiments of some 500 liters per minute (gas flow). Figure 6 shows that the slope of the gas-production curve deviates from the extension of the slope of the liquidoxygen flow curve (expressed in gas flow, units, liters per minute). This may be due to the fact that in the LOX case, oxygen is lost by evaporization. Conclusions
GLKm OXYGEN CQLECTION 10
LITER I MIN O2
(BASED ON GAS1
Figure 5. Comparison of small vs. large liquid oxygen flow on ozone production
T h e maximum yield of ozone under optimum operating conditions was 0.46y0 by weight or approximately 1.1 pounds per hour of ozone.
Production of Ozone in the Plasma Jet with Varying Liquid-Oxygen Flow. By varying the liquid-oxygen flow and keeping the power level constant, the weight per cent of 0 3 produced varies appreciably, as shown in Figure 4. T h e production of ozone in pounds per hour appears to reach a maximum for this particular configuration of the plasma jet. Table I1 lists several of the experimental runs.
The Effect of Higher Power (Kw.) with Proportional Increases in Liquid-Oxygen Flow. T h e experimental results, several sets of which are listed in Table 111, show that the use of higher powers and proportional increases in liquidoxygen flow causes a corresponding increase in ozone production. Ozone Production under Low LOX Flows. A series of experiments was carried out to determine the ozone yield by
A new method has been found for the production of ozone by using a plasma jet apparatus. The plasma jet device provides a convenient method for producing oxygen atoms which in turn react with 0 2 to form Os. T h e ozone formed is rapidly removed from the reaction zone and quenched by the excess liquid oxygen. T h e apparatus described in this paper is capable of producing ozone a t the rate of over 1 pound per hour continuously and is relatively simple to fabricate. The preparation of ozone by using a plasma jet is not restricted to the apparatus reported herein. Generally speaking, ozone can be produced from any similar jet device. The important factor in producing ozone by this method is the rapid quenching of the reaction product, 0 3 , by the excess liquid oxygen. Also, either a x . or d.c. power could be used to operate the jet. This new technique for producing ozone would find a practical utility where small concentrations of gaseous ozone in oxygen a t reasonably large flow rates or liquid “ozonated” oxygen are required. Acknowledgment
The authors thank A. V. Grosse for his many helpful suggestions for carrying out this work and A. G. Streng for carrying out some of the experimental investigations. This work was carried out under the sponsorship of the Welsbach Gorp.? Philadephia, Pa., and the authors are indebted to T. C. Manley for his interest in the studies. literature Cited
(1) Leutner, H. W., Stokes, C. S., Znd. Eng. Cham. 53, 341 (1961). (2) Stokes, C. S., Knipe, W. W., Zbid., 52, 287 (1960).
RECEIVED for review June 23, 1964 ACCEPTED January 15, 1965
VOL. 4
NO. 1
MARCH 1965
39
