I
0. B. CECIL’ and W. E. KOERNER Organic Chemicals Division, Monsanto Chemical
Gas and Heat h o l u t i o n from.
Co., St.
Louis 77, Mo.
..
i
Thermally Unstable Materials Reliable gas and heat evolution data, easily obtainable with this device, can be valuable aids in achieving safe engineering design
Q
UANTITATIVE heat and gas evolution data are needed in engineering design to facilitate correct sizing of equipment and to ensure proper choice of safety monitoring systems. A recent report from this laboratory described quantitative measurements of heat evolution rates from thermally unstable materials using a simple thermal stability device (7). Highly exothermic decompositions are dangerous because the self-heating which can produce a “runaway” reaction is often accompanied by copious gas evolution. This can result in severe rupture hazards in closed vessels or those which are vented primarily through complex process piping. The need for proper evaluation of hazards associated with gas evolution accompanying thermal decompositions led to development of a method for concurrently measuring gas heat evolutjon from unstable materials. A similar approach was reported earlier (Z),but the emphasis was not on obtaining data which could be scaled u p directly to plant-size units. This report describes equipment which provides accurate, quantitative data on gas and heat evolution rates from thermally unstable materials. The data are independent of sample size and rate of heating. Calibration checks showed that the slight change in cell geometry over that previously used (7) had no detectable effect on calculated heat evolution rates. Thus, one thermal stability determination yields both rates. If process temperatures could exceed the maximum temperature utilized in the gas evolution experiment, a special experiment in other equipment should be conducted a t the maximum anticipated temperature to make certain that a n unexpected gas-producing decomposition mechanism with a high activation energy is not encountered in the plant.
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.,
Present address, Texas Instruments, Inc., P. 0. Box 312, Dallas, Tex.
Experimental
Equipment. Since the basic device has been described elsewhere ( I ) , only the modifications necessary for measuring pressure build-up in the sample cell are described. Glass had originally been chosen for cell construction because it was chemically inert to many materials and could be fabricated so easily that cells could be discarded after each experiment. The desire to retain these advantages greatly influenced the choice of a pressure measuring system. Both metal foil and wire strain gages cemented on the glass cells were considered for the pressure measurement. However, danger of cell rupture which would destroy the element and large changes in operating temperature, with the possibility of frequent strain gage calibration, made these choices unsatisfactory. The final choice was a liquid-
filled hydraulic system in which the liquid leg extended well into the heated zone of the furnace. This design provided a simple means of transmitting cell pressure without introducing any cold spot which would allow material being tested to distill from the cell. The most difficult problem to solve was the development of a simple, pressure-tight, metal-to-glass coupling. An epoxy resin metal-to-glass seal satisfactorily withstood pressures u p to 900 p.s.i.g., but the seal was sensitive to thermal shock. A Bridgman-type seal using two 0 rings-one each side of the glass flat-was satisfactory (p. 476, top). (The lower 0 ring over tip H is not shown.) The seal requires only a fingertight plus one half to three fourths turn on nut D to achieve a satisfactory seal. The metal tubing used in the hydraulic system (p. 476, bottom) was l/ginch 20-gage stainless steel with stand-
-Applications The equipment described will undoubtedly b e ufilized in a variety o f other applications. T o date, the authors h a v e been primarily concerned with fhermal stability a n d correlation o f heat evolution with gas evolution. The following types o f decompositions h a v e been encountered in the course of routine use:
e
Exothermic decompositions with accompanying gas evolution. Most decomposing materials fall into this group and represent the most dangerous situation, since “auto-heating” can take place. e Endothermic decompositions with associated gas evolution. This type can be dangerous unless closely watched, since there would be little advance warning if the reactor temperature was being intentionally raised. This situation demands rate of pressure rise monitoring devices. Gas evolution but no heat evolution. This type of decomposition can be even more dangerous than the second type, since even less warning is available. e Exothermic decompositions with little or no accompanying gas evolution. This type can be expensive if safety aspects of plant design were based on rates of heat evolution alone and a considerable amount of overdesign or expensive auxiliary equipment were included.
VOL. 53, NO. 6
JUNE 1961
475
Glass sample cell was modified from an earlier design so that pressure build-up could be measured A. Heat evolution cell: B. 3.5 inches o f 8-mm.
heavy-walled tubing; c. 1.5-mm. glass capillary, 6-mm. O.D.; D. 0.25-inch Swagelok plug drilled to fit 6-mm. O.D. capillary; E. stainless steel follower from No. 400 0.25-inch Swagelok fitting; F. 0.25-inch silicone rubber 0 ring; G. Glass flat, 9-mm. O.D.; H. tip, 5 mm. long, 4.5-mm. O.D.; and 1. 8-mm. filling arm
ard Swagelok tube fittings (Crawford Fitting Co., Cleveland 10, Ohio). The system was designed to have minimum volume so that gallium could be used. if necessary, as the hydraulic fluid a t a reasonable cost. I n the work reported here, mercury was used as the hydraulic fluid. A standard needle valve (No. 122, ‘ / 8 inch NPT, female, straight body; Whitey Research Tool Co., Oakland, Calif.) was modified by drilling a small hole through the side of the valve body into the high pressure cavity of the valve and silver soldering a length of tubing into the hole. The other components of the hydraulic system were connected to the modified valve to provide a continuous path between the cell and pressure transducer. With this connection the valve needle could provide access for nitrogen in pressure tests prior to an experiment and permit withdrawal of decomposition gases for analysis after an experiment. The modified valve was utilized in preference to a tee and valve arrangement because the svstem could be more readilv filled comaletelv with hydraulic fluid, ‘ a system designed f& miniI
,
mum it is important to minimize trapping of air bubbles when the system is filled with hydraulic fluid. Otherwise, compression of the voids as pressure increases during a decomposition experiment ill move the hydraulic fluid meniscus out of the
‘J
A
B
C
”
)IG
Hydraulic system for pressure cell was designed with minimum volume for using gallium as hydraulic fluid if desired. Mercury was used in these experiments, however B. vacuum line connection; C. vent to atmosphere; D. mercury reservoir; E. valve; F. Swagelok ‘/*-inch stainless steel tee; G. strain gage; H. electrical probe to detect passage of H g meniscus (Nichrome wire cemented into F with epoxy resin); 1. connection to strain gage; K. connection to cell block; L. connection to nitrogen source and cell vent line; M. valve and connection to cell vent line; N. valve and connection to nitrogen tank; P. cell block; Q. thermal stability pressure cell; and R. furnace A. Air supply;
.
476
INDUSTRIAL AND ENGINEERINO
CHEMISTRY
furnace and cause sample distillation. To compensate for small voids, a short section of larger tubing was included between the capillary and the cell proper, and the hydraulic fluid meniscus was positioned in the larger tubing at the beginning of each experiment. A pressure cell (P6146TCb-750-350, Serial No. 2328; Statham Laboratories, Los Angeles 64, Calif.) with a range of 0 to 750 p.s.i.g. was used as the sensing element. The output of the gage was measured with a 0- to 10-mv. recorder. The power supply to the pressure cell measuring bridge was adjusted io give a calibrated output equivalent to 50 p.s.i. per mv. Procedure. T o convert pressure data to rate of gas evolution, the available gas space or “free volume” in the cell must be known. Prior to assembly in the thermal stability cell, free volume measurements at room Lemperatuie were made on all filled cells wirh a special device (right). This device !cas connected to a capillary manometer, and the rubber coupling at the top was connected to the sample cell, thus essentially forming a glass-to-glass connection between cell and measuring equipment. The manometer was read, the volume of the system increased by 5 cc. (the volume of the syringe), and the manometer read again. This procedure was repeated with the sample cell removed and the equipment plugged at the rubber coupling. By means of Boyle’s law. the free volume of the cell plus measuring equipment was determined from the f i s t set of data and the volume of the measuring equipment only from the second set The difference between rhe two volumes i: rhe sample cell free volume, measured to the tip of the glass flat. A calculated correction could be made for the amount of space occupied by the mercury in the capillary leg during the run. Alternatively, the volume occupied by mercury could be measured after the heavy-walled tubing and capillary were cut from the cell proper (for repeated use) a t the conclusion of an experiment. Slight reaction of sample vapors with mercury inevitably left a slight stain to mark the position of the mercury meniscus. The procedure for filling the sample cell and assembling the equipment was the same as previously described (7), except that the pressure cell was attached to the hydraulic system a t block P; which is located directly beneath the furnace. Sample cell and hydraulic system were pressure tested with 400 p.s.i.g. nitrogen pressure via the piping shown in the hydraulic system prior to each thermal stability test. If no leaks were found, the pressure was vented, the system closed to the atmosphere, and the mercury level adjusted to approxi-
THERMALLY UNSTABLE MATERIALS The direct observations from a thermal stability experiment were time records of sample temperature, cell pressure, and differential temperature. The differential temperature and sample temperature were used to calculate the heat evolution rates as reported earlier (7). Similarly, the cell pressure and sample temperature records were combined to yield gas evolution rates in terms of moles of gas per unit time per unit sample weight. This method of reporting the data eliminated dependence on sample weight and rate of heating, and the data could thus be used directly for scale-up calculations. Steps involved in converting from cell pressure observations to rate of gas evolution can be summarized as follows:
A
1
Free volume measuring device provided data for converting pressures to gas evolution rates A. To sample cell; E. rubber couplings; C. to
D. T 12/5 ball joint; manometer; capillary; and F. 5-cc. syringe
4
4
E. 1 -mm.
mately 1 inch below the point of attachment of the hydraulic leg to the sample cell. Then the thermal stability run was started in the customary manner ( I ) . The run was normally stopped automatically at 300' C., at A T between sample and reference of 10' C., or at cell pressure of 400 p.s.i.g. After the sample had cooled, any residual gas could be vented into an evacuated sample container for analysis. Infrared gas analyses have been used to identify gaseous products of decomposition in a number of cases.
Prepare a table of pressure and time or temperature readings (from strip chart records) and convert from p.s.i.g. to p.s.i.a. o Correct each pressure observation in the table for fixed gas pressure change caused by temperature change (Charles' law) and for mercury vapor pressure. Prepare a semilogarithmic plot of the corrected pressures us. reciprocal of the absolute temperature. The curve will most likely consist of an initial straight line segment with deviation toward higher pressures at higher temperatures as decomposition begins. The straight line portion approximates the vapor pressure curve of the sample. A curve of this type, obtained with p-nitrophenol, is shown (below). 0 Extrapolate the vapor pressure curve to higher temperatures and correct the observed pressures for sample vapor pressure.
0 Convert the observed decomposition
gas pressure to millimoles of gas per kilogram of sample by using the free volume determination and sample weight. 0 Plot millimoles of gas per kilogram of sample us. time and calculate the slopes of tangents drawn at several points. The slopes are gas evolution rates with units of millimoles of gas per minute per kilogram of sample weight. 0 Plot gas evolution rates us. reciprocals of absolute temperatures. Implicit in this method of plotting gas evolution rates and using a straight line function for interpolation and extrapolation is the assumption that only a small fraction (10% or less) of the total sample decomposed during the experiment and that a single decomposition mechanism predominates. If this is the case, this Arrhenius-type plot of the data is indicated, as discussed earlier (7). If heat evolution and gas evolution are a result of the same phenomena, then the slope of the rate of heat evolution us. the reciprocal of the absolute temperature curve should be parallel to a similar plot of gas evolution rate, as is the case for p-nitrophenol (below). In this particular case, the rate of heat evolution is a more sensitive detection of decomposition, but this is merely a result of the sensitivity of the particular equipment used. The intermediate numerical values involved in converting the experimental data to amount of decomposition gas per unit weight of sample are summarized for p-nitrophenol in the table (p. 478). The slopes of the chosen tangents to the
Calculations
To make fullest use of the pressure measuring equipment used for thermal stability measurements, it was desirable that both pressure and rate of heat evolution data be obtained from a run. Identical heat evolution rates were obtained for three different compounds when run in the old and new design sample cells (assuming the effective weight of the new glass cell was 20 grams). Thus, the previously determined correction factors for calculating heat evolution rates (7) were still adequate, and the slight change in geometry created by the new sample cell design was not significant.
0.1 I8
RECl P R O C A L T E M P E R A T U R E (IOOO/TD K I
RECIPROCAL
TEMPERATURE
?i;oo/&'K,
Observed gas pressures from p-nitrophenol decomposition
Variation of heat and gas evolution rates from p-nitrophenol as function of temperature
Straight line approximates vapor pressure curve of sample
Parallel slopes show that heat and gas evolution result from the same phenomena
VOL. 53. NO. 6
JUNE 1961
477
Gas Evolution Data and Calculations for p-Nitrophenol Hg
Time, AIin. 250 260 270 280 290 300 310 3 15 320 325 330 335 340 345 350 353
Observed Temp.,
Observed Pressure, P .s.1.B .
Permanent Gas Correction, P.S.I.A.
161.5 170 178.5 187.5 197 206 215.5 220.5 225.5 230.5 236 241.5 247.5 254 261.5 266.5
58 60 62 63 65 67 69 71 73 76 82 84 115 153 222 295
58 59 60.3 61.5 62.1 63.9 65.2 65.9 66.5 67.2 67.9 68.7 69.5 70.3 71.3 72.0
c.
curve are plotted for comparison with heat evolution rates (p. 477, bottom right). For a solid material which decomposes prior to melting, sample vapor pressure corrections can usually be neglected because of the lowvaluesinvolved. Discussion
Several approximations are involved in the method of calculating gas evolution rates as described. I n theory. the free volume will change slightly during a run because the density of material in the cell changes with changing temperature. Usually this error is minor and can be neglected. If sufficient data are available to enable suitable corrections to be calculated, this should be done. A second error is related to the arbitrary adoption of a fixed glass weight used in calculating rates of heat evolution (7). Because of the long glass stem: which apparently does not contribute to the specific heat of sampleplus-container because of its close proximity to the furnace wall, it is impractical to weigh each cell with that part of the stem which might contribute to the effective heat capacity. One cell was cut off a t that position on the stem to which the mercury lwel is adjusted. The weight of this cell and portion of stem was 20 grams. This value was adopted arbitrarily as an average sample cell Iueight. The insensitivity of the calculations to small changes in sample cell Iveight and the excellent reproducibility in size of cells made by a skilled glass blower justifies the use of an assumed 20-gram cell weight in all experiments. Perhaps of more importance than the above considerations are the possible effects on decomposition caused by mercury vapor. Calibration checks failed to show evidence for any such effect with the materials tested, but the possibility that mercury vapor could
478
Vapor
Vapor
+
Pressure Decomposition Correction, Gas Pressure, P.S .I.a. P.S.I.A.
... ...
0.2 0.2 0.3 0.4 0.6 0.6 0.8 0.9 1.0 1.2 1.4 1.6 2.0 2.2
0 1 1.5 1.3 2.6 2.7 3.2 4.5 7.3 7.9 13.1 14.1 44.1 81.1 148.7 220.8
enhance a decomposition must not be overlooked. The original plans were to use gallium as the hydraulic fluid because of its extremely low vapor pressure, and the system nras designed as a low volume system in order to conserve on gallium. Howevrr, until more operating experience is gained, mercury will continue to be used as the hydraulic fluid. Autocatalysis may be encountered occasionally. A continually increasing slope will usually be observed when the gas evolution rates are plotted against reciprocals of absolute temperatures. Furthcr confirmation of autocatalysis can be obtained by rerunning the same sample to see if higher gas or heat evolution rates occur at corresponding temperatures or by holding a new sample a t a temperature at which a moderate decomposition rate was observed to see if autocatalysis occurs under isothermal conditions. If reacti1.e decomposition gases are produced, their subsequent reaction rate may be enhanced by confinement under pressure in the cell. When significant gas evolution rates are observed, decomposition gases should be identified so that their reactivity can be considered properly in interpreting results of the experiment. Another factor affecting absolute accuracy of gas evolution rates obtained is the problem of decomposition gas solubility in the sample. The detection of sas solubjlitv errors of large magnitude can be approached in two ways:
o Comparison of the sample cell pressure before and after cooling at the conclusion of the experiment will indicate the type of decomposition gas produced. When permanent gases which are virtually insoluble in the sample are produced, the pressure drop observed on cooling is very close to the value predicted by Charles’ law. When the decomposition gases are readily condensable vapors, the
INDUSTRIAL AND ENGINEERINGCHEMISTRY
Vapor
Pressure Correction, P.S.I.A.
...
... ... ... ...
... ...
4.5 5.0 5.6 6.4 7.3 8.5 9.9 10.8
Decomposition GasiUnit Decomposition Weight, Gas Pressure, Gram-mAloles/ Kg. Sample P 3.I.8.
... *.. ... ... ... ... ... ...
2.8 2.9
7.5 7.7 36.8 72.6 138.2 210.0
...
... ...
... ... ... ... ... 2.7 2.7 7.0
7.I 33.5 65 I22 184
pressure drop observed on cooling will be much larger than that predicted bv Charles’ law. Usually readily condensable decomposition gases will also exhibit considerable solubility in the sample. Considerable deviations from Charles’ law behavior indicate that calculated gas evolution rates may be low by a significant amount. 0 Infrared examination of the decomposition gases ofcen provides positive identification. T h e order of magnitude of the solubilitv of known gases in the sample can often be estimated and, consequently, the magnitude of possible errors. If the identity of the decomposition gas can be established, it would also be possible to measure actual gas solubilities as a function of temperature and correct the observed values. Pressure tests showed that the glass cells could normally withstand 600 p.s.i. Similar but not identical cells would not Lvithstand such high pressures. Consequently. adoption of the pressure test described herein should include a critical examination of pressure limits desired, coupled with the ability of the sample cell design used to achieve the desired pressure level. Acknowledgment
The authors express their appreciation to Harold Luebke for many of the experimental measurements. Consultation and aid in construction of the glassto-metal seal and electrical circuitry associated Xvith the strain gage, by R. H Munch, L. Fowler, 1Valter Trump. and R. G. Rose. are also gratefully acknowledged. Literature Cited (1) Deason, W. R., Koerner, W. E., Munch, R . H., IISD.EKG. CHEM.51, 997 (1959). ,- - , (2) Rapean. J. C., Pearson, D. C., Sello, H., Ibid., 51, 77 A (February 1959).
RECEIVED for review November 14. 1960 ACCEPTED March 22, 1961
