Satety Considerations in Industrial use of
ORGANIC PEROXIDES J. B. ARMITACE H. W. STRAUSS
peroxides are used industrially to iniMosttiate .organic free radical polymerizations. In fact, this application accounts for 90-95% of the total U. S. consumption. The present discussion of safety testing and procedures applies to all those organic materials which are used as sources for free radicals, including azo compounds as well as organic peroxides. Examples of the most popular classes of initiators are shown in Figure 1. The principal considerations in industrial use are urnally the utility of the particular peroxide as a free-radical source, the compatibility of the products of decomposition, and of course, cost. However, before the peroxide can be used on a commercial scale, industrial management must have assurance that adequate safety precautions will be taken in handling the material. The types of hazards associated with peroxides are explosion, fire, and toxicity. The most unusual and lethal hazard is that of explosion. FiOl
The procedures which are involved in handling the peroxides are: receipt of shipment storage as received storage in process equipment use in process disposal It is with these necessary procedures in mind that the tests for safety which will be described in this article have been designed. Procedures for receiving and storing of the shipment are related since both involve handling of the material in containers supplied by the vendor. It is in this area that the vendor has had the most experience. He has designed the container and has stored the material in the containers in maintaining his inventory. There are many considerations which determine his choice of the shipping container and form of its contents. Dilution of the ueroxide with a suitable solvent is the most common method of reducing the explosion hazard. A solvent which does not burn. or which burns at nearly the same rate as the peroxide and is low in volatility, will reduce the fire hazard. The explosion hazard can also be minimized by reduction of the degree of confinement (using an easily broken package), limitation of quantity in a single container, and separation and insulation of each unit of a multicontainer box. Vendors package as little as a few grams or as much as 100 lb. of peroxide in a single container. Fire hazards are sometimes reduced by using nonflammable absorbent packing material and by provisions for prevention of static charge. Habilify Tedr
Establishment of the temperature at which self-heating occurs in the vendor’s container is imperative. A responsible vendor will supply ~- . the test data from which this temperature was determined. This is usually a test in 28
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1
which the peroxide or its solution is heated in its container to a given temperature with warm air and then held at the given temperature for a week or 10 days to determine if self-heating and consequent explosive decomposition or fire occur at the particular conditions. Tests are continued until self-heating o c z r s or the temperature is higher than anticipated during the hottest summer days. In southeast Texas, a temperature of 110' F. is usually chosen as maximum. The temperature at which self-heating occurs, of course, dictates the temperature at which storage is allowed and at which temperature alarms must be set, and so forth. The demonstration of the results of self-heating indicates whether the major hazard is explosion or fire. An example of test results is shown in Table I. It is essential that the tests be conducted in the type of containers that are shipped, filled to the same level with peroxide, because of the effects of shape and size on the temperature at which self-heating occurs. It was in 1939 (7) that D. A.
Frank-Kamenetskyproposed a criticality criterion (Equation l) to determine whether a self-heating chemical would heat to self-destruction with time, or warm a few degrees and then reach a steady temperature distribution. The criterion is a dimensionless number which is a function of the physical and chemical properties of the material, the concentration of the material, and the temperature and dimensions of the assembly. It clearly illustrates the need for keeping all variables the same as in practice if the data are to be valid. When considering application of data to another vessel, safety dictates that the criterion only be used when storage data for a given concentration of peroxide in a particular vessel configuration are available. For this particular case, the relationship can be simplified (Equation 2). The effect of quantity of solution and the container size on the autoignition temperature of a peroxyester is illustrated in Table 11. Laboratory tests with small quantities of peroxides can be misleading. Shock Tests
.s grca
Its criticality criierio activation energy heat of reaction F molar dens?
1
=
h
=
frequency f constant gas constan absolute ter thermal cor
cure or assemmy vity of the materi; characteristic dimension of the a3 radius of sphere or cylinder, etc.; i a sphere > reg, ' ight circular cylir e > infinitely lor inder > infinite 5
.. ... . .. .
If-lif of h in'
J . B. Armitage is Senior Supervisor, Polyolcfins Division of the Sabine Riuer Laboratory, E. I. du Pont de Nemours and Go., Orange, Tex. H. W. Slrauss is Senior Research Engineer, at the same location.
AUTHOR
Another important consideration in handling peroxides is the shock sensitivity. The degree of sensitivity to shock is affected by both the physical state and temperature. The effects of physical state on the explosive characteristics of substances are not sufficiently well understood to permit prediction of the behavior of any one compound. Generalizations concerning any aspect of the explosion initiation problem can seldom be made. For this reason it is important in testing that all possible physical states and temperatures be considered. Various tests for shock sensitivity include: drop-weight test drop test-using vendor's container bullet impact test influence tests pendulum friction test The first test, the drop-weight test, consists of dropping a 5 kg. weight from various heights onto a piston which then rams into a light-weight cup holding about 0.1 gram of the material under test. The height is varied to the point at which 50% of the drops result in decompositions. If the material is not shock sensitive, the maximum drop (55 inches in some equipment) does not result in decomposition. An example of test results is shown in Table 111. The second type of test for shock sensitivity consists of dropping several fully filled containers of the peroxide or solution from a height of about 20 feet onto a stone surface. In the bullet impact test, caliber 0.30 rifle bullets are fired into 0.5 lb. or larger quantities of the peroxide. An example of test results is shown in Table IV. Any evidence of explosive decomposition in any of these tests is taken as proof of shock sensitivity. I n many cases, the influence tests and pendulum friction tests are considered secondary to the three other shock sensitivity tests previously described. In influence tests, the sample is subjected to the stimulus from the detonation of a high explosive charge. The measure of relative sensitivity is the minimum spacing distance beVOL 56
NO. 1 2 D E C E M B E R 1 9 6 4
29
Tests for safety must be designed keeping in mind all possible tween the primary charge and the test sample which can be maintained without initiating the sample. The pendulum friction test is a Bureau of Mines test in which samples on a steel anvil are subjected to a series of frictional impacts by a shoe attached to a pendulum rod. This test is particularly suitable for solid peroxides. Evidence of sparks, flames, or audible reports are taken as proof of shock sensitivity. Fire and Toxicity Hazards
Most peroxides will burn very readily and are fire hazards. When they decompose they are a source of heat, and therefore can cause fires. They contain a large amount of active oxygen which-can directly support combustion even when air is excluded. Any fire hazards of a peroxide which are not directly associated with explosion hazards are usually caused by decomposition products. Therefore, an analysis of the decomposition products and recognition of the flammability of these decomposition products are advisable. The toxicity of both the peroxide and its products of decomposition is an important consideration ( 3 ) . Most peroxides are irritating to the skin, eyes, and mucous membranes. It is usually necessary to assure adequate ventilation and to avoid contact of the peroxide with the eyes and skin. However, as with other chemicals, each case must be examined separately. Receipt and Storage
The descriptions of the explosion, fire, and toxicity hazards of the peroxide obtained by the various tests which have been described thus far suggest the handling procedures which must be adopted in receiving and storing shipments of the peroxide. In many cases, it is best that peroxides be shipped in special refrigerated trucks. These trucks are usually equipped with recording thermocouples. When the truck arrives at the industrial plant site, the temperature record can be examined to ensure that at no time since leaving the vendor's plant has the peroxide been exposed to temperatures which could initiate self-heating and consequent delayed explosion. Another consideration, when solutions of peroxides are handled, is assurance that temperatures are not so low that a second phase of higher peroxide concentration could separate. The place of storage on the industrial plant, amount of storage, and distance from operating facilities and public roads and so forth are other considerations. A separate storage building of special design is usually recommended. The amount in storage and the distance from operating facilities are usually governed by factors based both on safety and cost. The storage facilities are, of course, designed for a temperature considerably below the temperature of self-heating. I n fact, depletion of assay due to slow decomposition can become an important factor. 30
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
The National Board of Fire Underwriters Report Number 11 lists these recommendations for building construction (2) : --Noncombustible materials -Adequate ventilation for removal of products of decomposition -Suitably grounded and equipped with lightning rods -Roof, door, or wall which is easily displaced with low pressure -Deluge-type automatic sprinkler system (unless water is too warm) and source of large quantity of water -Refrigeration and suitable insulation and high temperature alarms TABLE I .
SELF-HEATING TEST-LUPERSOL
NO. 11
tert-Butyl peroxypivalate solution in 5 gal. polyethylene bottle with plastic screw cap. I
Test IYO. Oven temperature, F. Solution Time elapsed, hr. Temperature, F. .4ssay, yo
2
7
I
72-76
I
i
18-19 72-76
0 ;:,3
..,
85
I
i
28-168 78-80 65.1
'
~
10
0 16 85
28 >200
..
..
...
Other observations: Sample 1, test terminated after 7 days. Sample 2, at 28 hours, large ball of black smoke and some flame. At 33 hours, burning again lasting 45 minutes. Loose fitting oven cover not displaced either time. Testsperformed by Lucidol Division of Wallace and Tiernan, Inc. TABLE
II.
VARIATION O F A U T O I G N I T I O N PERATURE-LUPERSOL NO. 11
1
Container
Autoignitzon Tenijerature, O F. >78 < 85 169
Vendor's filled 5-gal. bottle 8 Grams in 80-1111. bomb TABLE
111.
D R O P - W E I G H T TEST-DIISOPROPYL PE R O X Y D I CA RBONATE
Peroxide Form 50% Solution 75ycSolution 90% Solution 1 0 0 ~ Liquid o
TABLE
IV.
TEM-
1
50YGPoint, Inches No decomposition below 55 22 10.6 7.5
BULLET IMPACT TEST-DIISOPROPYL PEROXYD !CARBONATE
__
50% Solution 757( Solution
~
~~~
None None None
combinations of conditions which may occur during use In-Process Storage
*
d
In general, the considerations in storage in process equipment are the same as in the bulk storage. The major difference is the emphasis on alarms and relief devices. In process storage, two temperature sensing devices are usually used. As the temperature in the equipment rises, the signal from one thermocouple first activates an alarm and then automatically dumps the peroxide solution to a drain where it is flushed with large quantities of water. If for some reason this first thermocouple fails to activate the dump system, the second activates a secondary dump system at a higher temperature. The last safeguard is the rupture disk or disks on the process storage vessel. The rupture disks are usually set to burst at so low a pressure that in case of a runaway reaction and bursting of the disks, there is a possibility that enough time remains for manual dumping of the vessel contents before the more violent decomposition is encountered. The quantity and concentration ofperoxide solution are restricted to ensure that a runaway decomposition will not result in extensive damage. I t would be preferable to run safety tests in equipment exactly the same as in the plant installation with each peroxide which is used in the facility. However, in many cases this is not practical. Field tests can be run with only one or two of the most violent peroxides which are used. Evaluation of all other peroxides are then based on laboratory tests which are related to the field tests. T o be acceptable commercially, an experimental peroxide must be equal or lower in violence of decomFigure 2.
position than the worst peroxide which was evaluated in field tests. The field test consists of slowly heating the proposed charge of peroxide in equipment exactly like the plant installation to a temperature at which runaway reaction occurs. The rate of heating is greater than the highest rate anticipated in the plant facility. The laboratory test consists of sealing the sample within a cylindrical, 80 to 500 ml. closed vessel equipped with an external electrical heating mantle. The temperature of the vessel is increased at about 10" C./min. until decomposition of the sample occurs. The bomb is instrumented with a thermocouple and a pressure transducer which in conjunction with an oscilloscope and suitable amplifiers permits pressure-time relationships of any rapid decompositions to be measured. The rate of pressure rise, maximum pressure generated, and autoignition temperature are recorded. These data are compared at equal loading densities. Table V shows that the benefits of dilution in reducing explosibility are not the same for all peroxides. They also illustrate effects of different solvents. Strictly speaking, the effectiveness of a diluent in minimizing the severity of an explosion is directly related to its heat capacity only if all other considerations remain unchanged, and this is not the usual case. Another consideration in storage of peroxides in process equipment is the possibility of contamination of the peroxide with some foreign material. Materials of construction are chosen so that corrosion rates are extremely small. Some metals are apt to reduce the decomposition temperature. In some commercial operations, there is
Equipment used in laboratory tests to determine the largest site of tubing in which an explosive decomfiosition will not propagate
/RUPTURE
DISK HOLDER
E l.ECTRIC HEAT ER
AMPLIFIER
OSClllOSCOPE
CAMERA
STRAIN CELL
-THERMOCOUPLE
LINE FOR PROPAGATION TESTS
I
VOL 56
NO. 1 2
DECEMBER 1964
31
the possibility that one peroxide will be mixed with another or with any solid decomposition products of another. Therefore, these mixtures must be evaluated in laboratory thermal stability tests. This is necessary because in most cases one cannot predict with certainty whether any interaction is possible. An example of test results is shown in Table VI. Here the violence of explosion was greater than predicted from data on the individual peroxides. With most of the mixtures examined, the explosive properties were milder than predicted. In-Process Use
Use in a process such as in initiation of polymerization requires still more considerations. The peroxide is usually transferred from the storage vessel to the reactor in tubing or pipe. Since there is always the possibility that unexpected heat will be applied to the tubing (such as would occur from a steam leak, for example), it is important to ensure that an explosive decomposition of the initiator will not propagate through the tubing or pipe back to the storage vessel. The degree to which such a decomposition will propagate is determined in part by the line diameter, since heat losses to the walls of small diameter tubing may be large enough to reduce the severity of, or even quench, an explosion. Similarly, TABLE
V. LABORATORY T H E R M A L S T A B I L I T Y T E S T Calculations for fully loaded condition
Max. Rate of Pressure ~
Rise,
R Pressure, isZS.1.
P.S.I./Sec.
75y0tert-Butyl peroxide in
4,200
5,200
cyclohexane 55y0 tert-Butyl peroxide in cyclohexane
2,900
1,600
13,900
300,000
Vapor press. only
Vapor press. only
75y0tert-Butyl
Disposal of Peroxides
hydroperoxide
in benzene 50y0tert-Butyl hydroperoxide in benzene
50% tert-Butyl hydroperoxide
690
3,380
in cyclohexane
TABLE VI. EFFECT O F M I X T U R E S Calculations for fully loaded condition 25% diethyl cu,cu’-azobisisobutyrate
Composition Bomb Data
Actual-2 Stages
Autoignition temp., ’ C. Max. pressure rise, p.s.i. Max. rate of pressure rise, p.s.i./sec.
TABLE
wall thickness will affect explosive characteristics to the extent that this parameter affects the heat capacity of the line. Therefore, a laboratory test for propagation has been devised using the largest diameter tubing in peroxide service. Figure 2 shows the type of laboratory equipment used. The tube is filled with solution and a known loading density is established in the bomb. The tubing is then heated with an open flame near the end. Since heat capacity is quite important, the temperature at which tests are performed can be as low as the maximum expected in process. .Any peroxide solution which will propagate in decomposition is not used or consideration is given to a smaller tubing size or more dilution. An example of test results is shown in Table VII. In many cases chilled water is used for cooling process lines and compressors. This is to prevent explosive decomposition and loss of assay. This is a particularly important point during compression. If the peroxide solution is not cool enough at the suction of a compressor, the heat of compression may be large enough to increase the temperature in the cylinder above the autoignition point with an explosive decomposition in the pump cylinder. Still another consideration is the physical form of the peroxide in its pure form. If the peroxide is used as a solution in a volatile solvent, a problem may occur at any leaking joint in the piping. Around the point of a small leak, the volatile solvent will evaporate leaving the peroxide (with its lower vapor pressure) deposited in the pure form. Since dilution is the major method used for reducing the hazard of explosive decomposition or fire, in many cases a peroxide is preferred which in its pure state is a liquid rather than a solid. The liquid, of course, has the advantage that it will drain away from the point of leakage and evaporate.
~
Calculated
190 1910 1290
131 1750 1090
VI I. PROPAGATION DATA T E R T - B U T Y L
PEROXYACETATE inch S. S. Tubing X 0.035 inch I Peroxide Concentration, % Propagation I 75 9 Ft. in 15 sec.
Disposal of peroxides can be a problem. I t is advisable to dispose of any excess material as soon as possible. Methods of disposal are dependent to a large extent upon the physical and chemical characteristics of the specific peroxide. Usually, the easiest method for disposing of peroxides which are slightly soluble in water is to feed the peroxide to a large flowing stream of water so that no fire or explosive hazard exists. .Another common method is to spread the peroxide over a large area on the downwind side of a pit which is used for burning waste solvent. TYaste solvent is then fed to the pit from a remote position and the solvent is ignited from a remote position. The burning solvent will ignite or decompose most peroxides without any violence since the peroxide is not confined and is low in quantity at any point. It is clear that the major safety emphasis in industrial use of organic peroxides must be on the explosion hazard. Most of the chemical industry is already well acquainted with fire and toxicity hazards and methods for control.
3/g
i
50
32
No
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
RE FER ENCES (1) Frank-Kamenetsk D. A . Z h u r . Fir. Khim. 13, No. 6, 738 { 1 9 3 9 ) ; Sparks, John R., J. Ckem. I%ysm 3 k , -26 (1961). (2) National Board of Fire Underwriters Report No. 11, “Fire and Explosion Hazards of Organic Peroxides,” 1956. (3) S a x , N. I., “Dangerous Properties of Industrial Materials,” Reinhold Publishing Corp., New York, 1957.
