in the Chemical Laboratory
I CX.
Edited by NORMAN V. STEERE, 1 4 0 Melbourne Ave., S:E. Minneapolis, Minn. 55414
Linking Thermodynamics and Kinetics to
Predict Real Chemical Hazards* D. R. Stull. Ph.D., D o w Chemical Co., Midland, Mich
The hazard of present concern is the reactivity hazard leading to the risk, danger or peril resulting from handling, transporting, or processing o giuen chemical or eampositionof matter At the Fourth Lass Prevention Symposium 123). s method for the identification of potential chemical reaction hazards was presented. From the composition and heat of formation of a chemical compound (or mixture) and the thermodynamie properties of the decomposition (or reaction) products, parameters can he calculated which will distinguish potentially hazardOUJ materrals irom nunhanardour ones. Tho A'nli~nnl Tronspormtton Safer\ Hoard (17) defhrs a hwnrd ns follows: "A hazard is considered t o be a real or potent i a l condition, characteristic, or set of circumstances which can cause injury or death, or damage or loss of property or equipment, or cause an event which will lead to these losses." The hazard of present concern is the reactivity hazard leading to the risk, danger or peril resulting from handling, transporting, or processing a given chemical or eomposition of matter. Following the presentatian (23J, a member of the audience asked if any work was being done to correlate the degree of hazard with the Arrhenius equation. My reply noted that these two factors had not been correlated, but that my concern extended to this facet of the program as well.
THE NATURE OF CHEMICAL REACTIONS In the processing and handling of a chemical or a combination of chemicals, there are two main questions to be answered. First, will there be a reaction? Equilibrium chemical thermodynamics can provide an unequivocal answer to this question if the thermodynamic data for the system is at hand. The previous paper 'Reprinted with permission from CHEMICAL ENGINEERING PROGRESS. LOSS PREVENTION. Vol. 7. 67-73 (1973). (Presented at A.1.Ch.E. Meeting, November 28-30, 1972, New York City.)
(21)dealt with this aspect of the problem and was able t o identify those systems where energy releases were possible, and related the degree of "potential hazard" t o the magnitude of the energy release. Hawever, we do not live in a static, equilihrium world of patential hazards. It is necessary to couple the potential hazard evaluation with the rate of energy release by the reaetion, to evaluate the real extent of hazard. The chemical transformation and its associated energy release will range fmm well behaved t o violent depending upon the rate of the reaction. The second main question that must be considered is: what is the time rate of energy release? Once activated, potentially hazardous materials undergo a nonequilibrium chemical reaction farming the most stable products under the prevailing circumstances. The potentially hazardous systems are those capable of generating heat. They may require different levels of activation, hut the smaller the energy of activation, the more readily activated. Some materials (those capable of palymerization for example, see Appendix I) may he thermally activated by the ambient temperature of the system. Catalysis also plays a vital role in promoting low level activations. Regardless of the made of activation, if the heat generated by the reaction can he continuously transferred to the surroundings without creating an increase in temperature, the reaction will prdceed quietly in a well behaved manner. If the heat from the reaction is not all continuously transferred to the surroundings, the temperature of the reaction will increase, slowly a t f m t , but will finally reach a temperature where the reaction rate is catastrophic. Such thermal runaway reactions are referred to as "thermal explosions." These thermal explosion reaction types convert "potential" hazardous systems into "real" hazardous systems. Thus, it is necessary t o answer both of these two main questions to evaluate the real hazard af a system.
KINETICS Kinetic studies have been carried out in a variety of ways, and, of course, the basic fact observed is the rate constant K, =
I . ..
a
t-
-
dc/dt, the time rate of reactant conversion. Many of the kinetic measurements reported in the literature have been carried out in a n isothermal manner. K, has been measured a t two or more constant temperatures and the data represented by the Arrhenius equation
where dc/dt = K, = t h e rate constant in moles/sec. A = a frequency factor in reciprocal sec. E. = the activation energy in keal/mole R = the gas constant = 1.9872 cal/mole T = Kelvin temp. Kinetic behavior has often been explained in terms of the theory of the activated state, graphically shown in Figure 1. The reactants in a potential energy valley are raised to the metastable activated state by absorbing the energy of adivation. The reactants in the activated state can then form stable products by releasing both the activation energy and the reaetion energy E,. Reactions of the thermal explosion type are very real hazards, and do not take place in an isothermal manner. They take place in a continuously increasing temperature regime finally reaching enormous rates where heat losses are small compared to the heat generated, and the system approaches adiabatic behavior. Reaction rates a t temperatures higher than the isothermal studies are commonly obtained by extrapolating the Arrhenius parameteis t o higher temperatures. Those who interpret the kinetics of explosive reactions are divided into two groups; those who believe that the isothermal data taken a t lower temperatures can be confidently applied over a wide range of temperatures, and those who feel that extension of the law temperature isothermal measurements to higher temperatures leads to questionable conclusions (15). Several attempts have been made to measure reaction rates a t the elevated temperatures required by applied problems (5, 8, 11, 12, 16, 18, 29, 30, 311. These have usually involved experimental and evaluational compromises that made the interpretation of the data mare difficult. Moreover, there are relatively few compounds reported in the open literature far which Arrhenius activation energies have been measured in a nonisothermal manner. In addition, the Arrhenius activation energies for a given material measured by isothermal and nonisothermal methods display a noticeable discordance, as shown in Table 1. In every case, the values re(Continued onpageA22J Volume 5 1 . Number 7, January 7974
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Safety
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ported from nonisothermal measurements are much less than the values for the same compound measured by isothermal methods at lower temperatures. A unified collection of kinetic data on gas phase unimolecular reactions measured by isothermal methods has recently been presented by Benson and O'Neal ( I ) . Their values have been supplemented by additional values from the literature. As this study progressed, activation energies by hoth isothermal and nonisothermal methods were used in parallel calculations and led to virtually the same conclusions. However, since the isothermal measurements are mare numerous, and can provide useful results now, they have been made the basis of this presentation.
I
ENERGY
I Figure 1. Schematic Energy Diagram of the Activated State.
Table 1. Arrhenius Activation Energies in KCAL/Mole Measured by Isothermal and Nonisothermal Methods
CH4 C2H2 C2H4 ClH40 CgHa C8H60 C&O? CnHlo CeHs
Methane Acetylene Ethylene Acetaldehyde Ethane Propylene Oxide Ethylacetate Butane Cyclahexane
LINKING KINETIC AND THERMODYNAMIC DATA The evaluation of real hazards as they actually exist in the world of today require hoth an assessment of the potential hazard (by chemical thermodynamic calculations), and the time rate of energy release (requiring kinetic rate constants). Considerable thought was devoted to the problem of how to link kinetic and thermodynamic data in a manner that would contribute to the evaluation of real hazards. It soon hecame apparent that definitive results are beyond the present state of the art, and methodology utilizing facts at our disposal would have to serve our present needs. The results should, hopefully, provide additional understanding of real hazards as opposed to potential hazards.
Isothermal
Nonisothermal
Benson & O'Neal ( I )
Penner & Fenn & Mullins (16) Calcote (5)
103.0 40.5 46.5 48.0 89.5 58.0 48.0 86.3 64.1
29.0 31.0
26. 20.
Referring to Figure 1, it will be noted that two energy quantities are involved; the Arrhenius activation energy and the energy generated by the reaction. In the potential hazard evaluations described earlier (W), the maximum adiabatic temperature reached by the pmducts of a decomposition reaction was named Td (the decomposition temperature). Since the energy generated by the reaction is very closely paralleled by Td, it was decided to replace the reaction energy by the substantially equivalent decomposition temperature. The problem then became that of linking the Arrhenius activation energy E, with the decomposition temperature Td to give a single value for a given material. Various schemes were tried before the nomograph shown in Figure 2 was developed. A square with Td increasing upward from 0 to 3000'K was plotted on the left side, while E . was plotted on the right side with values from 0 to 100 kcal/male increasing in the opposite direction (downward). The diagonal line connecting the zeros of the two scales was named the Reaction Hazard Index, or RHI for short. The end of the RHI line at 0°K would unquestionably be at minimum hazard (or zero index), while the end of the RHI line a t 0 keal/mole (zem activation energy) must surely correspond to maximum hazard (assigned an index value of 10). Thus, RHI values from 0 to 10 cover the whole range from minimum to maximum hazard. The intercept an the RHI line is given by the simple relationship RHI = 10 Td/ NoMoGRaPH
*
Td WITH _E.
Eo Td *K
Reaction Hozard Index R H I =
KCALIMOLE
io Td Td + 3OE,
Figure 2. Nomograph Linking the Decompasition Temperature Td 'K with the Arrhenius Activation Energy Ea in kcal/moie.
~ 2 /2 Journal ot Chemical Education
EXAMPLE O F K E L F REACTION & & ! ! ? X O &x Eo
K c ~ L ! MOLE
Figure 4. Correlation of the NFPA Reactivity Rating with the Reaction Hazard index.
Figure 3. Exampie of Use of Reaction Hazard index with Methane. Ethylene, and Acetylene.
WT,,+ 30 E.). A single value of the Reaction Hazard Index between 0 and 10 Provides a numerical index far a given material. Figure 3 shows an example of use comparing the reaction hazard of three hydrocarbon gases, methane, ethylene and acetylene, with the values of T d r Ea. and RHI given in Tahle 2. The RHI figures correctly place methane as least reactive, acetylene as mast reactive, and ethylene roughly midway between the two. The earlier study (23) correlated potential hazard values with National Fire Protection Assn. Chemical Reactivity Rating (6). The values for these three hydrocarbon gases are in accord and are compared in Table 2. Of their reactivity ratings, NFPA says, "This material is not an official standard of the NFPA; it is only a compilation of data from various authoritative sources presented for information as a guide." Their ratings are based on the realities of fighting fires, on reactions of hazardous materials, and represent the collected human experience to date. Linking kinetic and thermodynamic data in the manner suggested here (or in some other way) could strengthen and support the experience of the past, and make possible its extension into the unknown areas represented by materials where experience is lacking. An extended effort was made to find data for as many chemical compounds as possible where the NFPA Reactivity Rating could be compared directly with the Reaction Hazard Index. Tahle 3 lists 80 compounds where the direct comparison could be made, and includes all of the basic data necessary ta calculate the Reaction Hazard Index. The 80 compounds are grouped into the five NFPA Reactivity Ratings as fallows: 38 campounds with 0 rating (the largest and best established group), 13 campounds in rating 1 and in rating 2, 6 compounds in rat-
ing 3, and 10 compounds in rating 4. A cross plot of the NFPA Reactivity Rating versus the Reaction Hazard Index is presented in Figure 4. The average RHI for each NFPA rating is given in Tahle 3, and is plotted as a + in Figure 4. The best dotted straight line t h r o q h the average indices shows a remarkable accord between, these two methods of reactivity hazard rating. The NFPA Reactivity Ratings represent the consensus of a committee of experts, and because of the number of factors taken into consideration the values agreed upon might in some instances border on the next higher (or lower) value, and thus be indeterminate by one unit. Hence, acetylene (No. 72) rightfully carries the maximum hazardous NFPA Reactivity Rating of 4, and based on the evidence in Table 3, it is not unreasonable that vinylacetylene (No. 68) should also. The Reaction Hazard lndex method presented here is based on experimentally ohserved quantities representing the Arrhenius energy of activation for the decomposition process and the energy generated by the decomposition as indicated by the adiabatic decomposition temperature reached by the decomposition products. The method used develops a single numerical value that is characteristic of the reaction hazard for the compound measured and represents the real hazard. Minimum definite measurements can be prescribed for compounds where handling experience is lacking. This objective method can he used to evaluate the real degree of hazard for a single compound or for a mixture of chemicals, and can be used to develop a Reaction Hazard lndex system that will effectively rate all chemical reaction hazards with respect to each other.
CONCLUSION Combination of appropriate (known and tabulated) thermodynamic data with the heat of formation of a chemical compound (or mixture) and the Arrhenius activation energy for the decomposition of the com-
Table 2. Comparison of NFPA Reactivity Rating with the Reaction Hazard lndex
l -"K
"K
kcal/mole
A
CHI CzHn C2Hr
Methane Ethylene Acetylene
-
Trl
298 1005 2898
RHI
Reactivitv Rating
0 9 0 46.5 4.2 2 40.5 7.1 4 (Continued on page A 2 4
103
Volume 51. Number 1, January 1974
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Table 3. Data Sources. Reaction Hazard Index, and National Fire Protection Association Reactivity Rating
No.
Formula
Gaseous Com~ound
AH8 298 kcal/mole Ref. (24)
Activation Energy for Decomposition
Ta "K
kcal/mole
Ref.
Reaction NFPA Hazard ReacIndex tivity R H I Ratine
Chloroform Formic Acid Methyl Chloride Methane Methylamine 1,l-Dichloroethane 1,2-Dichloroethane Ethvlbromide ~th;lchloride Ethane Dimethy m i n e Ethylamine Cyclopropane Acetone l-Chloropropane Propane l-Butene Cyclobutane 2-Bntanone Ethylacetate l-Bromohutane l-Chlorobutane Butane 2-Methylpropane tert-Butyl Alcohol Ethyl Ether Iso-propylacetate 2,2-Dimethylpropane %Methylbutane Pentane 2,2-Dimethylpmpanol Cvclohexane ~utylacetate Toluene Methylcydohexane tert-Amylacetate Ethylbenzene 4-Xylene Average of 38 compoulids
2.89
0
Average of 13 compounds
3.58
1
Average of 13 compounds
4.27
2
Methylhydrazine Acetic Acid 1,l-Dimethylhydrazine Allylbromide Pro~ionitrile Propylene Propionaldehyde Acetic Anhydride Diethyl Carbonate Iso-propyl Ether Benzyl Chloride Dicyclopeutadiene-endo Dicyclopentadiene-ex0
Ethylene Ethylene Acetaldehyde Propylene Oxide Dioxolane 1,3-Butadiene 1,3-Butadiene Crotonyl Alcohol Vinyl Ethyl Ether Vinyl Ally1 Ether Styrene
* For the polymerization process 83 64
A24
CsHn N;H4
Vinyl Cyclohaane Hydrazine
/ Journal of Chemical Educah'on
Table 3 Data Sources. Reaction Hazard Index, and National Fire Protection Association Reactivity Rating (Continued)
No.
Formula
Gaseous Compound Ethylene Oxide Nitroethane 1-Nitropropane Vinylacetylene tert-Butyl Peroxide Cellulose Nitrate
AHte 298 kcal/mole Ref. (24)
-12.58 -24.20 -29.80 f72.80 -81.50 -229.80
Td
O K
1062 1161 1046 2317 850 2213
Activation Energy for Decomposition kcal/mole Ref. 57.4 45.0 47.7 28. 37.4 46.7
(1) (1) (1) (3) (1) (14)
Average of 6 compounds 71 72 73 74 75 76 77 78 79 80
CH3N01 C2Hz CIHIOs C1H5NO3 C8H,Ns09 C4He0, CAHLOO, CnHwO. CsHlsOl CsHlzOz
Nitromethane Acetylene Peracetic Acid Ethyl Nitrate Nitroglycerine Acetyl Peroxide tert-Butyl Hydroperoxide Diethyl Peroxide Di-tert-hutyl Peroxide Cumene Hydroperoxide
-17.86 4-54.19 -97.73 -36.8 -73.20 -116.1 -62.9 -46.1 -81.5 -21.9
2621 2898 976 2094 2895 983 919 968 850 989
59.0 40.5* 32.0 39.9 40.3 29.5 37.8 37.3 37.5 29.0
Average of 10 compounds
NFPA Reactivit,y Rating 3 3
Index RHI 3.81 4.62 4.22 7.33 4.31 6.12
3
3 3 3
5.07 (1) (21) (1) (21) (1) (1) (1) (1) (25) (25)
5.97 7.05 5.04 6.36 7.05 5.26 4.48 4.64 4.30 5.32 5.55
* F o r t h e polymerization process
pound (or mixture) will permit the evaluation of a Reaction Hazard Index for that chemical compound (or mixture). Merging kinetic and thermodynamic data should promote new studies in the area of technology common to these two important disciplines.
APPENDIX 1 POLYMERIZATION The process of polymerization is a heat producing reaction that may get out of control and create (or be contributory to) a thermal explosion. Styrene (2) which has a heat of polymerization of 17 kcal/male at 25°C will he used as the example for this discussion. At high rates of polymerization (catalyzed perhaps) in a low heat loss environment (adiabatic), the manamer can he all vaporized causing pressure rupture of a normal container, and the temperature can reach the vicinity of the critical temperature (37373. Even though this is well below the spontaneous ignition temperature in air (490'C). a pressure rupture could lead to a vapor release in air that could form a composition in the explosive range (1 to 6 volume % styrene in air). When a sizable quantity of monostyrene is subjected to a large heat flux (as in a fire), the heat of polymerization joins in producing an elevated temperature where monostyrene (or its polymers) undergo thermal decomposition. The thermal decomposition of mono and polystyrene is also an exothermic process twice as energetic as the polymerization process, and the thermal decomposition proceeds more or less to com~letionand can reach Td = 720°C. Each substance that can polymerize must be studied separately since the result depends upon the magnitudes of the heat quantities involved.
For materials that hecome progressively Less stable as the temperature is raised, the process of polymerization can act in a triggering manner. The decomposition kinetics generate heat which accelerates polymerization, and the polymerization kinetics also generate heat which accelerates the decomposition. Whether the initial process is decomposition or polymerization, the end result is the same, namely conversion to the products of decompasition.
16. S. S. Pcnnm and B. P. Mullma, '"Erpioeiona, Deto-
natiam, Flammability and Ignition." Pergaman h a , N. Y. 1959. B. P. Mullins. Fuel 32, 211-252. 327-379.46492 (1953). 11. J. H. Reed. 0 . M. Laurel, F. H. MeAdam.. L. M. T h a w , and I. A. Burms, R i k Concrpts in Dangerous G d a Tranlpartstim Rcsllatims." Report No.: NTSB.STS-71-1, Natl. Trans. Seflty Ed.. Wash.. 0. c.. ranvary n,1971. 18. R. N. Rngcn. S. K. Yesuda, and J. Zlnn. A n d C k m . 32i61.672-678 i l w ) . 19. G. R. Sehultu and G. Wasasrmann. Z. Ekkfrockm 41.77k77R
