EXPERIMENTAL TECHNIQUES
An Apparatus for Observing the Rate of Thermal Decomposition of a Gas-Evolving Solid Sidney Kleinberg and Fred P. Stein* Department of Chemical Engineering, Lehigh University, Bethlehem, Pa. 18016
An apparatus has been developed and successfully operated for the study of the kinetics of thermal decomposition of lead azide under vacuum. The nitrogen gas which was evolved during the decomposition of the solid lead azide in an isothermal furnace tube was quickly adsorbed in another part of the apparatus on cold activated charcoal that had been suspended from one arm of a recording microbalance. The record of weight increase of the charcoal with time provided the data from which the decomposition rates were determined. The maximum deviation from the mean of the amounts decomposed at a given time was 0.5%. The pressure in the apparatus during a complete decomposition was constant at approximately 1 0-4 Torr. The techniques and design are sufficiently versatile to be useful in kinetic studies of the decomposition of other solids which have a gas as one of the reaction products.
T h e thermal decomposition of a-lead azide proceeds as follows
PbSs(s) --+- Pb(s)
+ 352(g)
=Ibove approxiniat'ely 26OOC lead azide may explode before thermal decomposition is completed. Our st'udy of the kinetics of the slow thermal decomposition of pure d e a d azide and of d e a d azide containing selected impurities required the development, of an apparatus to meet' the following criteria, some of which m-ere lacking in apparat'uses used by previous investigators (Garner and Gomm, 1931; Griffiths and Groocock, 1957; Jach, 1963; Reitzner, et al., 1962): (1) teniperat'ure control had to be better than 0.1"C in the range 200-275OC for periods of as long as 48 hr; (2) the sample should not "see" temperatures other than the reaction temperature; (3) samples as small as 10 mg should be acceptable; (4) decomposition was to proceed under vacuum conditions; ( 5 ) a significant pressure rise during the reaction should not be permitted; aiid (6) personnel and expensive equipment, had to be shielded from harm from explosions. (Lead azide is an explosive sensitive to shock, abrasion, static electricity, and high temperatures.) We believe the concept,s and design used to meet these criteria are useful in st'udies of the rat'es of decomposition of other materials which have a gas as a reaction product. Our apparatus provides clear advantages over the standard pressure-rise technique, particularly a-here precise kinetic data are required. Aii important feature is that, the relative sensitivity of our apparatus is about the same over the whole range of decomposition and, in particular, there is no fall-off in sensitivity a t large fractions decomposed, as is experienced with pressure-rise t'echniques. I n addition, the rapid removal of product gases from the solid a t reaction temperat,ure to a cold region in anot'her part of the apparatus is certainly desirable in cases where the product gases are corrosive or are, themselves, subject to decomposition at higher temperatures. 134 Ind. Eng. Chern. Fundam., Vol. 11, No. 1, 1972
Apparatus
The principle of the apparatus is a gravimetric determination of the gas released from a decomposing solid. The nitrogen gas, which was released from the decomposing lead azide that had been dropped into the isothermal furnace tube of the evacuated apparatus, was adsorbed on cold activated charcoal that had been suspended from one arm of a recording microbalance. The continuous record of weight increase of the charcoal a i t h time provided the basic raw data from which the decomposition rates were determined. The apparatus is shown schematically in Figure 1. It consists of four sections: (a) microbalance and weighing section, (b) furnace aiid decomposition chamber, (e) sample-loading and delivery units, and (d) vacuum system. (a) Microbalance and Weighing Section. A Cahn Instrument Co. R G recording microbalance was built into the apparatus to determine t h e rate of nitrogen released b y the reaction and adsorbed on t h e charcoal suspended from its beam. T h e balance was used a t a sensitivity such that a weight change of 0.2 mg was represented by full scale on a 1-mi', 10-in.-chart potentiometric recorder. At this high sensitivity, the balance could be disturbed by slight environmental changes (e.g., by closing a door). T o miiiiniize these effects, the balance was firmly strapped to a 200-lb frame made of '/d-in. steel plate and 6-in., wideflange structural-steel beams. The balance was mounted so that its pivot coincided with the center of mass of t h e mounting frame. The assembly was bolted firmly t o a n earthbacked wall. K i t h this arrangement, the peak-to-peak noise of the balance was about 2 pg, and the balance was undisturbed by normal activities in the laboratory. Suspended from the balance was a 30-in. long, 30-gauge Sichrome wire which held a basket made from 200-mesh stainless steel screen. The basket was 10 m m in diameter aiid 25 m m long. The wire and basket weighed about 110 mg. The basket was filled with about 0.5 g of Pittsburgh BPL 12 X 30 activated charcoal which had a B E T surface
CAHN RG RECORDING MICROBALANCE L E V E L CONTROL
L I Q U I D Ne
Z A T I O N GAUGE VALVE
T R A N S I T E ENCLOSURE
2 " - SCHED 1 0 - 3 0 4 S S P I P E
\\\
FIBERGLASS
INSULATION
y ALUMINUM
BLOCK, 3 SECTIONS
LIQUID NITROGEN
T IN U M RES I STA N CE T H ER MOM E T E R P E R BLOCK ED CHARCOAL
ROTARY P U S H - P U L L F E E DT HROU GH
/ VAC .
BELLOWS SEAL VENT VALVES RMOCOUPLE GAUGE OPTICAL CYROBAFFLE
BAY L EY T E M P E R AT U R CONTROLLER E L E M E N C E N T R A L COPPER BLOCK
THREE STAGE
Figure 1 . Apparatus for studying decomposition of lead azide
area of over 1100 ni2 g-'. A counter weight of similar mass was suspended from the other arm of the beam. The hangdown wire and basket were suspended in the 1.5411. diameter inner tube of a concentric-tube condenser (outer wall, 4-in. diameter). The outer .ivall of the double-pipe condenser aiid, hence, the charcoal were cooled by a liquid nitrogen bath. The iiitrogeii level was controlled b y a partially submerged gas-filled temperature-sensing bulb which activated an oiioff solenoid valve. 1 secoiid, den-ar-type inside condenser, cooled rrith liquid nitrogen, n'as provided between the furnace and balance to trap a n y moisture, carbon dioxide, or other impurities that might be evolved from the surface of the sample before they reached the charcoal. (b) Furnace a n d Decomposition Chamber. T h e decomposition was conducted b y dropping t h e lead azide sample into a depression in a copper block located a t t h e center of t h e furnace tube. T h e depression was a 1-in. deep X 2-in. wide groove machined into a 4-in. diameter X 5-in. long copper cylinder. ,211 18-in. length of 2-in. schedule-10 stainless steel pipe mas silver soldered t o either side of the copper block; standard 2-in. stainless steel flanges were used for connections. Eighteen chroniel-alumel thermocouples n ere cemented to the pipe and the copper block along its length. The primary temperature measurement originated in the copper block from a platinum resistance thermometer which was embedded just beneath the sample. N o s t of the mass of the furnace consisted of three 6-111. diameter cylinders of aluminum which were machined to fit snugly around the pipe and the copper block. The central cylinder, which was 14 in. long, was isolated a t its ends from
the other tIvo, which were each 6.5 in. long, by a 1/4-in. gap filled with fiber glass. Strip heaters embedded in heat-conducting cement were fastened t o the outside of the aluminum cylinders. Each aluminum cylinder became an independent, heat zone with its own variable transformer. ,111 three zones were controlled by a single temperature controller with the independent variable transformers acting t o balance the system. The heaters were wrapped with several layers of aluminum-foil-backed-by-fiber glass insulation, and the entire assenibly was contained in a n asbestos-board box. Under the worst conditions a sample in the depression in the copper block could "see" a niaximuni of 10 in. of the top of t'he furnace tube. Thus, it was t>he objective of the design to keep the 14-in. central section of the furnace tube isothermal. The copper block with high thermal conductivity was used to keep the decomposition chamber uiiiiornl in temperature; the stainless steel pipe, rritli relatively thin walls and lower thermal coiiduct'ivity, was used to limit heat transfer from the center t o the ends of the furnace tube; and the aluminum blocks of high mass aiid high thernial conductivity were used t o maintain an isothermal environment around the tube. Temperature cont,rol was accomplished with a Bayley Instrument Co. controller lvhich utilized a nickel resistance element' embedded in the heat transfer cement between the strip heaters on the central aluminum cylinder. When first built, a n auxiliary heater, coiisistiiig of resistance wire wrapped around the copper block, was installed, aiid this heater was also controlled by a Bayley controller. However, subsequent use of the auxiliary system was not necessary when the furnace controller was able to maintain a constant Ind. Eng. Chem. Fundam., Vol. 1 1 , No. 1, 1972
135
TO REACTION CHAMBER
TO VACUUM
SYSTEM
DELIVERY
'EEDT HROUGH
Figure 2. Top view of sample-loading and delivery units
temperature to bet,ter than 0.05"C for more than 24 hr. The temperature profile along the central 12 in. of the furnace tube was flat to within 0.2OC. (c) Sample-Loading and Delivery Units. T h e primary purpose of the sample-loading and delivery units was t'o provide deconiposibion samples t o the apparatus with a minimum disturbance of the apparatus, particularly the vacuum. A top view of t'he sample-loading and delivery units is shown in Figure 2 . When loading fresh samples from outside, the crosses Ivere isolated from the main apparatus and the vacuum system b y vacuum valves and were purged with a slight posit'ive pressure of an inert gas to minimize entry and adsorption of water vapor on the 1%-alls.The port of the cross which contained the sample-holding tray was quickly and easily removed and replaced. The loading unit was then opened directly to the vacuum system and evacuated overnight before the valve to the main apparatus was opened. The IO-mg samples were carefully weighed into 1-ml Teflon beakers which were pressed into brass cups of approximately t'he same size for rigidity. Five such samples were placed in depressions in the loading tray which was bolted to a flange that sealed one of the ports of the crosses. (In work with nonexplosives it might be desirable to load more than five samples a t one time.) The end of the transfer feedthrough, which had a cone motion in addition to push-pull and rotary capability, was fitted with a ring t,hat could be maneuvered over and tightened around the sample cup, thus enabling it to be transferred from the tray to a similar ring on the delivery feedthrough. The ring on t h e transfer feedthrough was then loosened and retracted a bit, freeing the sample cup to be pushed into the furnace tube on the end of the delivery feedthrough. With this system five experiments could be conducted without any break in any part of the vacuum. 136 Ind. Eng. Chem. Fundam., Vol. 1 1, No. 1, 1972
(d) Vacuum System. T h e vacuum system used was a qtandard 2-in. system built around a three-stage air-cooled oil-diffusion pump. h nitrogen cooled optical cryobaffle was used. The blank off pressure achieved was 2 X 10+ Torr. There was little point in achieving a lower pressure since experiments were carried out a t about 10-4 Torr, the partial pressure of nitrogen over charcoal cooled to 77°K. Procedure
I n preparation for a decomposition experiment , a sample was loaded into the delivery feedthrough and the apparatus was evacuated overnight. When the liquid nitrogen traps were filled, approximately 1 hr was allowed t o cool the activated charcoal because heat transfer was rather slow b y radiation a t 77" K. The weighing and decomposition sections of the apparatus were isolated, and a leak rate mas determined over a 6-hr period. This leak rate m-as usually between 1 and 3 pg/hr. The rate was reproducible between runs and corrections were always made t o the raw data for t h e leak rate. After determining the leak rate, the system was again pumped down for at least 1hr. T o start a decomposition, the delivery feedthrough was inserted approximately 30 in, into the furnace until it was centered automatically above the depression in the copper block by a stop on the feedthrough rod. The cup was then inverted t o dump the sample into the depression, and the feedthrough was quickly withdrawn. Immediately, the system containing the decomposition chamber and weighing section was isolated b y snapping shut the full-port toggle valve. Simultaneously, the microbalance recorder chart and a timer were set into operation. This charging operation took about 15 see, a time which was insignificant relative to the 6 hr or more of a typical experiment. The nitrogen released by the lead azide in the furnace tube was adsorbed b y the
TIME OF REACTION, minutes
Figure 3. Thermal decomposition of a-lead azide
cold charcoal in t.he weighing section and recorded as a weight gain b y the microbalance. A decomposition \Vas coiitiiiued until 110 further weight gain above t'he leak rat'e was detect'able. Since all operat'ions of the apparatus, including data recording and filling cold traps, ivere automatic, only occasional at'tent'ioiimas required by t h e apparatus during a run. Occasionally explosions of the IO-mg samples were set off intent'ionally a t temperatures above 260°C. No sound was heard outside the apparatus, and the explosion was detected only by rapid fluctuations and weight gain displayed on the recorder of the microbalance. There was insufficient disturbance to cause motion of the charcoal-cont,aining basket, and there was no detectable damage t o t,he apparatus. Immediately after explosion, the pressure probably rose t o about 0.1 Torr. limitations and Characteristics
The maximum utilit'y of the apparatus is obtained wit'h reactions for which the gaseous product is well defined so t h a t proper interpretation of the weight' of the adsorbed gas is possible. T h e apparatus was designed to handle very small samples. T h e residue from each decomposition was normally left in the decomposition chamber. Residue could be removed only with difficult'y. However, a sample handling system has recently been designed which is compat,ible with the present apparatus and which will remove the residue after each decomposition. This technique was successful wit'h lead azide decomposition because the rate of adsorption of nitrogen on charcoal a t 77°K was rapid compared t'o the rate of decomposition. T h a t such was indeed the case was demonstrated by measuring system pressures periodically during a decomposit'ion reaction. No significant pressure rise could be detected even when the decomposition rate was at its maximum. Calculations were made to determine the time for a powdered sample to reach a reaction temperature of 250°C after i t had been dumped into the furnace. Assuming heat transfer only by radiat'ion and a 10-mg hemispherical pile of sample, calculated time of heat-up was on the order of 1 sec.
Calculations were made t'o determine t'he quantit'y of residual gas in the system a t the end of a decomposition, Torr and a system based on a final pressure of 5 X volume of about 5 1. This quantity of residual gas was about 0.05% of the t'otal gas usually released during a react'ion. For runs that involved explosions the measured decomposition averaged 99.7 i 0.5% based on the weight of nitrogen adsorbed on the charcoal and the weight' of lead azide delivered to the reactmionchamber. These dat'a show that t,he apparatus was capable of det'ecting essentially all of the nitrogen released and t'hat' the lead azide was stoichiometric, PbKe. Sample Data
The experimental data were recorded on a 10-in. strip chart on which 200 fig was full scale. Thus, the chart mas traversed about 15 times for a complet,e run. From these recordings and the total weight change a t complete decomposition, data in the form of fraction decomposed us. time of react'ioii can easily be extracted. Figure 3 shows such data for lead azide for several t,emperat'ures between 210 and 244'C. The maximum deviation from the mean of t'he amounts decomposed at' a given time was about 0.5%. Such agreement is very good coilsidering: that the decomposition rate of powders is strongly influenced by part'icle size. Stat'istical deviat'ioiis on particle-size distribution were t o be expected in the duplicate small samples decomposed. Acknowledgment
Guidance on the safe handling of lead azide was provided b y the personnel of the Explosives Laboratory of Picatimy Arsenal. literature Cited
Garner, W. E., Gomm, A. S., J . Chenz. SOC.2123 (1931). Griffiths, P. J. F., Groocock, J. RI., J . Chem. SOC.3380 (1057). Jach, J., Trans. Faraday SOC.59, 947 (1963). Reitzner, B., Kaufman, J. V. R., Bartel, E. R . , J . Phys. Chem. 66, 421 (1962). RECEIVED for review March 23, 1971 ACCEPTEDSeptember 23, 1971 This work was supported by the Explosives Laboratory, Picatinny Arsenal, Dover, N. J. and by the Army Research Office (Durham). Ind. Eng. Chem. Fundam., Vol. 11, No. 1 , 1972
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