Preparation of liquid Ozone and OzoneG. M. PLATZ
AND
C. K. HERSH
Armour Research Foundation o f lllinois lnstitute of Technology, Chicago 76, 111,
Since 1948 the Armour Research Foundation has been applying certain principles for the production of 1 0 0 ~ o gaseous or liquid ozone f r e e of sensitizing elements a n d compounds. This article presents a description of the research leading to the discovery of means for preparing ozone free of sensitizers. T h e method for producing nonsensitized ozone consists of purification of t h e process oxygen by high temperature decomposition of sensitizing organic materials present in the f e e d oxygen and diluent oxygen, and maintaining a system such a s precludes any possible recontamination. The principal compounds which sensitize ozone a r e organic a n d must b e maintained below a concentration of 20 p.p.m., expressed a s carbon dioxide, to ensure production of stable ozone
FOR
the last 15 years the Department of Chemistry and Chemical Engineering of Armour Research Foundation has been conducting research on the utilization of ozone as an industrial heavy chemical. Of course, the pot,ential of ozone as a rocket oxidizer \vas kept in mind, and now sufficient information has been accumulated to warrant its evaluation as such an oxidizer. Inasmuch as it is impractical to handle and work with concentrated liquid ozone mixtures TT-ithout first having learned to make "safe" ozone, this art,icle deals with the requirements for the manufacture of safe ozone and the preparation of bulk quantities for use as a rocket propellant. Past history of ozone research is riddled with reports of spontaneous and unpredicted explosions, and countless researcherE have said that 100 yoozone, as a gas or liquid, could not be attained, let alone handled in any useful manner (4,7 , 9, 1%). Thus, if ozone is ever to become a useful commodity, the cause for its reported instability must be understood. In the investigation, conducted under the sponsorship of the Air Reduction Co., Inc., and carried out by Armour Research Foundation, oxygen, as opposed to air, was used in the generation of ozone. In the initial stages of the investigation the ozone was generated, by electric discharge methods, using oxygen taken directly from oxygen cylinders. There m r e periods Tvhen all ozone generated seemed very stable and other periods vhen it ivae impossible, because of spontaneous explosions, to obtain a sample of 100% liquid ozone. I t appeared that the stability of the ozone varied from cylinder to cylinder of oxygen gas and that the stability of ozone was poorest when the oxygen gas in the cylinder dropped to but a fen- hundred pounds pressure. Consequently, it 1%-assuspected that the liquid phase equilibrium acetylene content, accumulated in the separat,ion of liquid air may have been the culprit; thus, an attempt was made to analyze the cylinder oxygen for acetylene by absorpt,ion in ethyl alcohol at atmospheric pressure and -70" C. A modified Ilosvay (3') test, on the absorbing solution mas negative. Perhaps this should have been expected, inasmuch as the concentration of acetylene in the cylinder oxygen was in all likelihood less than the equilibrium vapor concentration of acet'ylene over ethyl alcohol a t atmospheric pressure and -70" C.
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At this time, it was discovered that the probabilit'y of making safe ozone could be improved if the cylinder oxygen gas was passed through a cold trap refrigerat,ed wit,h liquid oxygen. Ozone made in this manner and subsequently condensed could, on occasion, be boiled and evaporated without the occurrence of spontaneom explosions. This indicated that the sensitizing compounds sometimes could be removed by low temperature condensation. I t \vas then proved that neither carbon dioxide nor m t e r sensitizes ozone t'o spontaneous inst'antaneous decomposition. When an unpredict,ed explosion of liquid ozone did occur, it m-as usuallj- during the boiling or evaporation of liquid mixtures, which again pointed t i organic materials as the sensitizing compounds. References to translations of certain German literature relating to the manufacture of oxygen stated that the lubricants used in air compressors are a source of danger. Oils v,Vhich crack in t,he compressor form many volatile products which are extremely difficult to trap out, often moving completely through the air-fractionating column. Also, the contents of low boiling hydrocarbons (especially acetylene) in the air are seldom high enough so that they precipitate in the heat exchangers before liquefaction of the air takes place. In the German literature i t is stated also that acet'ylene is soluble in liquid oxygen to 3 p.p.m. by weight; any acetylene in excess of this figure is present as solid floating particles ivhich are particularly sensitive to percussion or friction. Assuming t'hat acetylene could be the sensitizing agent it x a s decided to investigate the sensitivity of ozone prepared from oxygen initially IOT in acetylene. The oxygen was purified by passing through silica gel sorption beds refrigerated mith liquid nibrogen. I n this manner, the concentration of acetylene in the oxygen gas was reduced to 0.003 p.p.m. This, holvever, did not reduce the frequency of explosions. Obviously, there were other sensitizers besides acetylene. The oxygen gas was examined then for other impurities and found to contain appreciable amounts of combustible material above and beyond that Tvhich could be accounted for by the acetylene known to be present. The analyses mere made by passing the oxygen through a combustion furnace and analyzing, gravimetricallj-, for the carbon dioxide that was produced. The combustion furnace was packed with copper oxide, causing any organic or carbon-con-
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ROCKET PROPELLANTS taining combustible material t o be oxidized to carbon dioxide and water. Thus, the carbon dioxide in the effluent gas became an index of the degree of initial contamination of the oxygen. It did not, however, tell anything of the specific nature of the organic contaminant.
GENERATOR
TO VENT
)_Iow REGULATOR
COLD TRAP
REFRIGERANT CONDENSED OZONE
PURIFIER
C Y OXYGEN
L
d
Figure 1. Minimum equipment for manufacture and condensation of bulk 100% liquid ozone
I n an effort again t o determine the effect of acetylene on the stability of ozone, electrolytic cylinder oxygen was substituted for regular cylinder oxygen. The method of production of electrolytic oxygen precludes the presence of acetylene. Ozone made with electrolytic oxygen continued to be susceptible t o unpredicted explosions. Therefore, i t was postulated that the sensitizing elements were introduced, as suggested in the German literature, during the pressure filling of the respective cylinders. Attempts to generate safe ozone directly from evaporated commercial liquid oxygen also proved unsatisfactory. This may have been due t o the presence of acetylene or contamination in the liquid oxygen, said contamination having been accumulated during the previous history of the container. However, out of this investigation did come a method for the production of relatively stable ozone. The method finally evolved consisted of treating the cylinder oxygen gas by passing it through a copper oxide catalyst bed maintained a t 1300' F. to convert all organic or carbonaceous materials to carbon dioxide and water. This method for the production of relatively stable ozone is described in a patent (1). Liquid ozone produced from oxygen purified in this manner has displayed remarkable thermal, impact, and vibrational stability. It has been dropped from great heights without causing an explosion due to impact, and refrigerated samples have been subjected, without incident, t o vibrations of 60 cycles per second for very long periods of time. I n making liquid ozone mixtures diluted with liquid oxygen it has been shown that it is equally important that the diluent oxygen be purified in the same manner, and that the system remain closed to possible contamination. Coincident with the investigation of the purification of oxygen, a similar investigation was being conducted on cleaning procedures for the equipment to be used in the ozone-handling program. For borosilicate glass equipment, cleaning with standard chromic acid cleaning solution, followed by a thorough distilled water rinse and drying in a contaminant-free atmosphere suffices. Since i t is impractical to clean metals by this method, metal equipment may be cleaned by rinsing the equipment well with technical grade carbon tetrachloride and air drying in a contaminant-free atmosphere. Passivation of equipment with a n ozone gas atmosphere is a very strong deterrent t o unstability. All equipment used, whether borosilicate glass, metallic, or plastic, is subjected to a gas atmosphere of 10 to 50% ozone prior t o use. This treatment guarantees the cleaning and passivation of any surfaces and crevices not thoroughly cleansed by liquidcleaning methods. There are many reports, both oral and written, that 100% liquid and gaseous ozone are treacherously
April 1956
unstable. It has also been said that 100% gaseous ozone cannot be attained (10). These have probably been the experiences of researchers who have not reduced the organic content of the oxygen gas to less than 20 p.p,m. by weight expressed as carbon dioxide and have not properly cleaned and conditioned the equipment for ozone. Actually, 100% liquid ozone and all liquid ozone-oxygen mixtures, if properly prepared and very carefully handled, are relatively stable. All explosions, even in the liquid phase, are initiated in the gas phase, hence it is this phase which requires careful handling. A long study of gaseous ozone decompositions has been made and the approximate speeds of initial and normal decomposition rates have been established (11). Table I lists the various velocities and energies for initiated decompositions of gaseous and liquid ozone-oxygen mixtures. I n the gas phase testing, pointed electrodes having a '/sZ-inch gap and pomred with a 3000-volt transformer were used. I n the liquid phase, pointed electrodes with a 0.020-inch gap and charged with 15,000 volts were used. The duration of the spark used in these tests was less than a millisecond. The data reported in Table I were obtained with the test samples under 1 atm. absolute pressure. Additional data with respect to velocity for initiated decompositions of gaseous and liquid ozone-oxygen mixtures can be found in the literature (3, 5 , 6).
Table I. Explosive Limits for Gaseous and Liquid OzoneOxygen Mixtures Using Electric Spark Initiation Ozone Concn. Vol. Wt.
%
%
11 14.3 25 38
15.6 20 33.3 47.9
Initial State Gaseous Gaseous Gaseous Gaseous
44
54.1
Gaseous
40
50
Liquid
Initiating Energy Calories 0.023 0.004 0.0001 0,00001
... ...
Decompn. Wave Velocity, Cm./Sec. 1 2 51 24,000 215,000
.. .
Explosion Characteristics Soft, smooth Soft, smooth Soft, smooth Sonic velocity, ruptures glassware Detonation, pulverizes glassware Violent explosion, destroys glassware
The values and limits shown in this table are higher than those reported by Schumacher (8) whose values reflect the influence of improper purification procedures.
02
.
I
OZONE GENERATOR
02+O3
I
i
TO VENT FLOW REGULATOR
IL
OXIDIZER REFRIGERANT
PURIFIER
CYLINDER& t OXYGEN
Figure 2.
'
Basic equipment for preparation of bulk liquid ozone-oxygen solutions
The preparation of 100% liquid ozone, using available commercial ozonizers, is straightforward. Figure 1 shomx the basic equipment requirements for this process. Gaseous cylinder oxygen, a t a regulated pressure of approximately 10 lb. per sq. inch gage, is first passed through a flow regulator and the oxygen purifier before going into the ozonizer. The effluent of the ozo-
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nizer, containing from 1 to 6 weight per cent ozone, is passed into a refrigerated vessel. The ozone, xhich condenses a t - 111.9” C. and 1 atm. partial pressure, is retained, whereas the gaseous oxygen is passed on to a vent. Transfer of the liquid ozone thus condensed to another vessel or to the point of utilization is straightforward.
FLOW REGULATOR
n
gaseous oxygen. Also, since the cold vaporized and unconverted oxygen gas contains usable refrigeration, two heat exchangers are installed as shown. The use of heat exchangers as shown has three pronounced effects on the process. First, less purified liquid oxygen is evaporated from the oxidizer reservoir; secondly, less make-up refrigerant is required; thirdly, the over-all efficiency of the ozone generator is improved by operating a t subambient temperatures. The instrumentation and control required for these processes have been omitted from these figures in the interest of ease of interpretation; they are not unique. It has been the experience of the staff of l r m o u r Research Foundation t h a t highly concentrated ozone, as either a gas or a liquid, can be produced and handled in pilot plant quantities n i t h care and ‘Piithout mishap. One of the prime objectives of this presentation has been to explain n hy other researchers have been unsuccessful in their attempts t o vork n i t h ozone and to present the first necessary step in any ozone investigationnamely to ensure that production of ozone, free of sensitizers, is utilized. Acknowledgment
CYLINDER OXYGEN
Figure 3. Basic equipment recommended for preparation of liquid ozone-oxygen mixtures
The preparat’ion of liquid ozone-oxygen mixtures (Figure 2 ) does not differ greatly from the preparation of 100% liquid ozone. The principal difference is that the ozonizer effluent is bubbled through an initial charge of purified liquid oxygen. The ozone condenses, and the oxygen gas which bubbles through is vented to the atmosphere. This procedure is followed until the concentration of ozone in the oxidizer solution has been brought to the desired level. Obviously in condensing liquid ozone some of the initial purified liquid oxygen contained in the oxidizer reservoir is vaporized and vented to the atmosphere. It may be necessary t o replace this viith freshly condensed purified oxygen, hence there is need for the liquid nitrogen heat exchanger. Basically, this is the minimum equipment for the production of liquid ozone-oxygen mixtures. Since it is wasteful to purify oxygen and discard the unconverted and vaporized gas to the atmosphere, recycle of this gas with the aid of a squirrel cage blower as shown in Figure 3 is recommended. The recycle of this gas reduces the cost of the only raw feed material-namely,
This work was under the direction of C. E. Thorp, with major contributions by L. C. Kinney, R. F. Remaly, 8. J. Gaynor, and T. A. Erikson, all of the Armour Research Foundation. Special mention must be made of the technical assistance and guidance given by L. I. Gilbertson and F. R. Balcar of the Air Reduction Co. toward the successful solution of a difficult and baffling problem. Literature Cited (1) Air Reduction Co., Inc., U. S. Patent 2,700,648 (Jan.25, 1955). (2) Hirschfelder, J. O., Curtiss, C. I?., Campbell, D., J . Phys. Chem.
57, 409-14 (1953). (3) Ilosvay, Be?. 32, 2697 (1899) (Beilstein. “Organische Chemie,” Band I ilcyclishe Reihe, p. 238). (4) Ladenburg, A . , Ibid., 34, 631, 1834 (1901). ( 5 ) Lewis, B., Feitknecht, W., J . Am. Chenz. SOC. 53, 2910--34 (1931). (6) Lewis, B., Von Elbe, G., J . C h e m P h y s . 2 , 537-46 (1934). (7) Riesenfeld, E. H., Schwab, G. AI., S a t w w i s s . 10,470-1 (1922). (8) Schumacher, H. J., Anales asoc. quim. urgentina 41, 230-48 (1953). (9) Schwab, G . A I . , 2. p h y s i k . Clzenz. 110, 599-625 (1924). (IO) Thorne, P. C. L., Roberts, E. R., ”Ephraim’s Inorganic Chernistry,” p. 122, Nordeinan Publ. Co., New York, 1943. (11) Thorp, C. E., “Bibliography of Ozone Technology,” voi. 2, Armour Research Foundation, Chicago, 1955. (12) Vosman. A , “Ozone: Its Manufacture, Properties and Uses,” Van Nostrand, New York, 1916. RECEIVED for review October 21, 1955.
ACCEPTED.\larch 2, 195G.
Liquid propellant rocket catapult test
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