Explosive Characteristics of
Hfdrogen Peroxide Vapor CHARLES N. SATTERFIELD, GEORGE M. KAVANAGH’, AND HYMAN RESNICK Department of Chemical Engineering, Massachusetts Znnstitute of Technology, Cambridge, Mass. These studies were made to provide a quantitative knowledge of the explosive characteristics of hydrogen peroxide vapor-a subject of technical interest deriving from the increasing manufacture and use in recent years of highly concentrated hydrogen peroxide solutions. At atmospheric pressure, vapors containing 26.0 mole % or more hydrogen peroxide can be exploded by initiation with a hot wire or spark gap. The ignition limit is unchanged by the system of ignition used, by varying the temperature of the hot wire between 1350’ and 1750’ K., or by changes in the hydrogen peroxide-wygen ratio in the diluent gas. The ignition limit increases to 33 mole qo hydrogen peroxide at 200 mm. of mercury total pressure, and to 55 mole 90 hydrogen peroxide at 40 mm. Vapors lying within the explosive composition region at 1 atmosphere total pressure may be exploded by contact with catalytically active materials initially at mom temperature or by “noncatalytic” materials like aluminum at temperatures of 150’ C . or higher. The explosion mechanism is believed to involve straight chains only, and to be essentially thermal in nature. The resulta are of present practical interest wherever hydrogen peroxide vapor may be generated or used or where concentrated hydrogen peroxide solutions may exist at elevated temperatures.
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LTHOUGH i t has long been recognized that explosions may occur on heating concentrated hydrogen peroxide, until recently there has been little knowledge of the conditions involved or even of whether the explosion occurs in the liquid or in the vapor phase. Mixtures of concentrated hydrogen peroxide with organic substances may be powerful and sensitive explosives. If solutions of hydrogen peroxide in a container become severely contaminated, the heat released by the exothermic decomposition can produce a self-accelerating decomposition reaction and a simultaneous pressure riee which can lead to rupture of the veseel, even if it is vented. %cent studies of the explosive characteristics of pure aqueous hydrogen peroxide solutions a t room temperatures ( 1 , I, 6 , 8 , 1 2 )have shown, however, that it is apparently impossible to obtain a propagating detonation in pure solutions containing up t o 99.6 weight yo hydrogen peroxide, if they are confined in relatively thin-walled containers. Even in heavywalled containers and at ,elevated temperatures there is only a slight enhancement by 90 weight % hydrogen peroxide of the explosive effect of the initiator, and in all cases the effect has been far less violent than that obtained from the same quantity of high explosive. It recently has been suspected that the explosions which have been encountered on heating a pure, unconfined, concentrated aqueous hydrogen peroxide solution to temperatures near the boiling point have occurred in the vapor phase. This suspicion was reinforced in simple experiments by the authors in which 1
Preuant addreas, Tmoerlab, Ino., Boston, Mw.
drops of 00 to 98 weight % hydwgca peroxide were allowed t C J fall onto a heated metal plate. No explosions were observed until plate temperatures corresponding approximately t o the boiling point-about 160” C.--of the liquid were attained. Recently Hart (a) reported observations of flames and explosions in vapor containing hydrogen peroxide and preliminary studies by the present authors ( 9 )established the existence of a reproducible ignition limit at a vapor composition of 26 mole % hydrogen peroxide a t a total pressure of 1 atmosphere. All other studies of the vapor decomposition reported in the literature have been made a t low partial pressures of hydrogen peroxide ; under such conditions, even in the most inert veswls, the decomposition is largely if not wholly due to reaction on the wall. The objectives of the present studies therefore have been: first, to provide a better knowledge of the conditions under which hydrogen peroxide vapor a t higher concentrations may be safely handled, so that experiments and processes may be intelligently planned for advance into regions hitherto considered doubtfully , safe; and second, to help provide an understanding of the decomposition phenomena taking place at these relatively higher temperatures and pressures. The studies have included measurvment of the ignition limits at atmospheric and subatmospheric pressures and an investigation of the effect of surface conditionn on the initiation of vapor explosions within the explosive region. The term “ignition limit” as used hew is defined as the minimun~ concentration of hydrogen peroxide in a vapor mixture under specified conditions, below which powerful external ignition cnnnot, propagatc an explosion. EXPERIMENTAL
HYDROGEN PEROXIDE BOILER.Even the most inert surfaces, such as Carefully prepared glass, cause a measurable rate of decomposition of the liquid or vapor. Therefore a flow system was used in order to establish accurately the composition of the vapor a t the time of explosion. A hydrogen peroxide solution was fed to a boiler at a constant rate and the vapor thus produced passed through a n explosion bulb and then to a condenser. Since the vapors produced in the boiler may lie within the explosive region, the boiler must be carefully designed t o avoid premature explosions durjng use. During the long period of development, it was found that the auccessful production of a vapor containing a bUbstantial quantity of hydrogen peroxide at atmospheric pressure necessitates a boiler which meets the following requirements:
All-glass construction in order to provide relativdy inert Burfaces throughout the boiler and thus decrease decomposition and eliminate boiler ex losions. The use of submerged aluminum and stainless steel suAces for heat transfer purpose8 was satisfactory although the amount of decomposition in the boiler increased. No boiler explosions were encountered unless the metal surfaces became exposed t o the concentrated vapor region abovc. the liquid level. Rapid boiling rate and small residence time in order to decrease decomposition in the boiler, and t o reduce the time required to attain steady-state operation, 2507
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Maintenance of a constant liquid level in order to give a constant output of vapor. The design which best met these qualifications is the modification of a climbing film evaporator shown in the lower portion of Figure 1.
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tained a steady level, varying only with the amount of bubbling present. The degree of bubbling depended on the concentration of the hydrogen peroxide being used and on the amount of decomposition taking place, but a t no time was it violent enough to disturb the satisfactory operation of the boiler. An unstabilized solution, containing 90% h drogen peroxide by weight and a maximum of a few parts per milEon of impurities, was acquired from the Buffalo Electrochemical Co. and diluted to the appropriate concentration for each run. The rate of decomposition was relatively high in the initial trials of the boiler, in spite of the short residence time and attempts to keep all surfaces scrupulously clean. It waa subsequently reduced t o a negligible amount by using conductivity water instead of distilled water as a diluent for the concentrated hydrogen peroxide and by adding approximately 15 p.p.m. of phos horic acidwhich acts as a stabilizer-to the boiler. If no jecomposition occurs in the boiler after steady state has been reached, the concentration of the vapor is the same as that of the entering liquid feed, but the liquid in the boiler, being in equilibrium with the vapor, is much more concentrated in hydrogen peroxide, No explosions occurred in the operation of this boiler a t feed and vapor concentrations u to 65 weight % hydrogen peroxide, corresponding to 87% hygogen peroxide in the liquid phase in the boiler, provided that carefully cleaned glass surfaces were used throughout and that no metal was allowed to come in contact with the vapor. In several cases attempts t o operate at higher concentrations led to explosions of considerable violence after steady-state operation for as long as 1 hour. The entire experimental apparatus was shielded behind steel plates.
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After trying various cleaning procedures during the course of work, the following standard procedure was adopted. A11 glass surfaces were thoroughly washed with soap and water and then placed in a hot bath of concentrated sulfuric acid for 24 hours. After rinsing thoroughly with distilled water, the glassware was soaked in concentrated hydrogen peroxide for 24 hours and then rinsed thoroughly with conductivity water.
TO POWER SUPPLY
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J
FEED FROM LEVELING DEVICE
Figure 1. Ignition Limit Apparatus
It consisted of a 500-ml. borosilicate glass flask with an atOne le of the U, tached glass U approximately 10 inches Ion constructed of 25-mm. tubing, entered the t o t t o m of the flask. The other leg consisted of 12-mm. tubing and was wound for 8 inches with 15 feet of 0.0625 x 0.0125 inch Chrom$ A ribbon. The heating element was kept from direct contact a i t h the glass surface by means of ceramic thermocouple tubes and was insulated by several layers of asbestos tape. This heated arm extended through the side of the flaak and over to the mouth of the other ann, opening downward in such a way a3. to eliminate entrainment. Below the heating coil was attached a small diameter feed inlet connected t o a leveling device, similar t o that shown in Figure 3, adjusted t o produce a level about 4 inches below the bottom of the flask. A drain a t the bottom of the U was used for emptying the boiler after runs. During operation, heated liquid and vapor rose in the heated arm, while cooler liquid flowed down through the unheated arm; a rapid circulation was thus set up which improved mixing and heat transfer characteristics. The vapor produced flowed out of the top of the boiler and was then available for any purpose. In this apparatus it flowed to the explosion bulb and thence to a condenser. The heat input was maintained in the range of 600 to 800 watts with a resulting condensate rate of 9 to 15 ml. per minute. Attempts to operate at a higher heat flux caused the boiler arm t o fuse. Under these operating conditions, it required approximately 25 minutes to reach a steady state, as shown by constancy of all concentration and rate measurements. In operation there was a continuous stream of liquid flowing out of the top of the boiling arm into the downflow arm, with no entrainment or fog visible in the flask. The liquid in the downflow arm main-
With this procedure, negligible decomposition of the hydrogen peroxide occurred on boiling; however, it is the opinion of the authors that considerable simplification of this procedure would have resulted in only a slightly higher degree of decomposition. STUDIES AT ATMOSPHERIC PRESSURE.The vapor produced was passed through the explosion bulb shown in Figure 1. Because of the physical configuration of the apparatus, the linear velocity past the wire was not known exactly, but was estimated to be approximately 5 cm. per second a t the boiling rates used. The ignition surface consisted of a loo of platinum wire, 0.01 inch in diameter and 5 cm. long, welde$across two 0.04-inch diameter platinum leads, which were sealed through a glass plug. The loop of wire was located approximately in the center of the 500-ml. explosion bulb. It was connected in series with 22 ohms acrose a IlO-volt direct current line thus giving a 5-ampere heating current, which was turned on to initiate an explosion. To avoid condensation the upper part of the boiler and the explosion bulb were completely insulated with glass wool, except for a small sighthole for viewing the ignition source. In performing the experiment, hydrogen peroxide vapors were passed continuously through the explosion flask, condensed, and analyzed until the measurements showed that steady state had been obtained. At this time an attempt was made to explode the vapor by passing current through the wire. The current was then shut off and sufficient time for the system to reach its original state was allowed to elapse before ignition was again attempted. The minimum time interval was found experimentally to range from 30 seconds to 2 minutes, depending on the pressure level in the explosion bulb. If the vapor concentration were substantially above the explosive limit, an audible pop was heard. At higher concentrations, some of the explosions were sufficiently violent to destroy the apparatus. With a decrease in vapor concentration the phenomena became successively attenuated, and the question as to whether or not a n explosion occurred was decided by close observation of the fog formed in the condenser. On heating the wire, it was possible to distinguish clearly two cases: either the fog wa$ expelled suddenly from the condenser,
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Ir==== Liquid-Ail
v
CartesianDiver Surge Tank
Condensate
Figure 2.
Low Pressure Ignition Limit Apparatus
indicating a sharp increase in pressure, or there was no noticeable disturbance there. If there was a sudden expulsion of all the fog from the condenser a n explosion was considered to have occurred even though there was no audible report. The effect of the ratio of water vapor to oxygen in the diluent gas was observed by introducing oxygen gas through a frittedglass plate inserted in the wall of the boiler at the base of the heated arm. A desired vapor composition was obtained by adjusting the amount and concentration of hydrogen peroxide fed to the boiler to the amount of oxygen introduced through the fritted-glass disk. When the boiler reached steady state, ignition attempts were made as described above. The effect of the ignition wire temperature on the explosion limit was determined by measurement with a special, high magnification optical pyrometer (constructed and loaned by H. C. Hottel of the Massachusetts Institute of Technology Combustion Laboratory) having a very fine resistance wire which could be compared accurately with the ignition wire. The i q t i o n wire temperature was slowly increased in successive initiation attempts by varying a resistance in series with the wire. I n other experiments at atmospheric pressure, the system was modified by the introduction of a spark-gap initiator instead of the wire. The initiation system consisted of two aluminum electrodes projecting into the explosion flask in the same position previously occupied by the hot wire. The voltage necessary for the 5-mm. spark was supplied by a Ford spark coil; the-power dissipated in the spark was approximately 6 watts. STUDIES AT SUBATMOSPHERIC PRESSURE. To determine the effect of pressure, the apparatus was modified by adding a n en-
closed condensate receiver to the end of the condenser and by using a n arrangement whereby a sample of condensate could be withdrawn from the evacuated device without disturbing the steady operation of the system. The pressure within the explosion bulb could be varied from about 30 to 760 mm. mercury and was controlled by a manostat opeSating in conjunction with a surge tank and vacuum pump, as shown in Figure 2. The minimum operating pressure of 30 m.waa determined by the characteristics of the leveling device. At low pressures the sudden pressure increase produced by the explosion forced a portion of the hydrogen peroxide in the reservoir into the leveling device which consequently produced a sudden increased flow of hydrogen peroxide into the boiler, thereby lowering the boiler concentration. A considerable time interval would then elapse before steadystate operation could again be attained. In order t o avoid this surge of liquid into the boiler, a side arm was connected to the leveling device slightly above the normal liquid level so that the liquid forced from the reservoir would be drawn off into an overflow receiver rather than interfere with the boiler operation. I n operation of the equipment, the reservoir was filled with hydrogen peroxide solution and the system closed. After the entire system was evacuated t o the desired pressure the hydrogen peroxide was allowed t o flow into the boiler, heating was begun, and boiling proceeded until steady state was attained, at which point explosions were attempted. The occurrence of an explosion was indicated in the same manner as a t atmospheric pressure. However, because of the decreased violence of the explosions a t low pressures, it became increasingly difficult to determine the exact composition a t which explosion ceased and rapid decomposition began. The rate of boiling used was such
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Figure 3.
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Apparatus for Determining Et€ect of Surface Conditions on Iriitiation of Explosions
that condensate was roduced a t 5 t o 10 ml. per minute; the linear velocities past tge ignition wire were estimated t o be about 10 cm. per second at 200 mm. of mercury pressure and 40 cm. per second a t 50 mm. of mercury pressure.
STUDIES ON EFFECT OF SURFACE CONDITIONS.The apparatus used to Rtudy the effect of the temperature and chemical nature of an initiating surface on the occurrence of explosions is shown in Figure 3. The initiatirig surface here waa a tube having an outaide diameter of 0.125 inch, and bent into a U shape for insertion into the explosion flask, with a total length of 9.375 inches exposed to the vapors. Ignit,ion tubes of three different inaterials were used-Type 2 S 0 aluminum, Type 304 stainless steel, and fine silver. Aluminum and stainless steel are relatively inert to hydrogen peroxide while silver is an active catalyst. The tube in each case was heated by rapid internal circulation of a heavy mineral oil, which served to heat the tube uniformly and was also an efficient means of carrying away heat liberated at the tube surface by vapor decomposition, thus preventing an uncontrollable temperature rise. The temperature of the tube was measured b thermocouples inserted in the circulating oil immediately begre it entered and after it left the explosion ahamber. It was assumed that the tube surface temperature was essentially the same as that of the oil, an assumption 'ustified by consideration of the heat transfer coefficients involved. In operating the equipment, the tube was maintained at a low temperature until vapor of a constant composition was produced at a steady rate. The tube temperature was then increased a t the rate of 2 O t o 3 C . per minute until an explosion occurred. ANALYTICALMETHOD. The vapor to be analyzed was condensed and its exact analysis obtained in either of two ways.
In the first method, a wet test meter was used t o measure the rate of oxygen leaving the condenser. The condensate was analyzed by titration with potassium ermanganate using the analytical method recommended by huckaba and Keyes (6).
Combining the ox gen rate measurement with the amount and concentration of t i e condensate gave a complete analysis of the vapor. The second method consisted of a calculation of the vapor composition from an analysis of the liquid feed and condensate, using a material balance and assuming stead state conditions. Analyses obtained by the two met{& always checked very closely. The usual procedure wm t o use the second method became of ita greater simplicity and the results were checked occasionally by use of the wet test meter. RESULTS
IGNITION LIMITS. Figure 4 shows that the ignition limit obtained by the hot-wire method a t 1 atmosphere total pressure, 26.0 mole % hydrogen peroxide, is not changed by a considerable variation in the ratio of oxygen to water vapor in the diluent gas; it also indicates the high degree of reproducibility of the phenomenon. Figure 5 shows that the ignition limit is not changed by variation in wire temperature over the range of 1350' to 1750' K. and indicates that the ignition limit occurs a t about 25.6 mole yo rather than 26.0%; this discrepancy is attributed to excess decomposition caused by platinum vaporized onto the surface of the glass bulb and is discussed in more detail below. At temperatures above 1750" K., the platinum wire became soft and broke. All hot-wire ignition studies other than those reported in Figure 5 were made a t wire temperatures of about 1350' K. With the use of spark ignition, the limit remained at 26.0 mole % hydrogen peroxide. The reproducibility of this limit is indicated by the points shown in the upper portionof Figure 6. The effect of total pressure on the ignition limit is shown in Figures 6 and 7 which represent the use of hot-wire and spark-gap initiatlon, respectively. The temperature of the vapor studied is equal t o that of the liquid in the boiler in equilibrium with it and therefore it will vary with the total pressure and the hydrogen peroxide concentration. Vapor temperatures at the ignition
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Experimental measurements of the vapor temperature in the apparatus agreed closely with those calculated above. One of the difficulties of operating the hot-wire equipment was caused by the catalytic activity of the platinum wire. At all pressures studied, sufficient decomposit
