- Email: [email protected]
Critical ignition temperatures of chemical substances
Critical ignition temperatures of chemical substances Takashi Kotoyori Research Institute of Industrial Safety, Ministry of Labour, j-35-1, Shiba, Minatoku, Tokyo 108, Japan Critical ignition temperatures have been found for eight self-heating or thermally unstable chemical substances, using an adiabatic self-heating test. From the experiments, the self-heating behaviour of the substances has been classified into two groups. The first group has been termed ‘T-C’ (thermal combustion) type and the second termed ‘A-C’ (autocatalytic combustion) type. Five substances were classified as ‘T-C’ type, and the critical ignition temperatures ranged from 329 K for C Y, cr’-azobisisobutyronitrile to 351 K for dinitrosopentamethylenetetramine. Ignition of the ‘A-C’ type substances ultimately occurred after 60 days at 314 K for nitrocellulose, 315 K for lauroyl peroxide, and 351 K for p-toluenesulphonylhydrazide. Such values agree with those of previous workers. (Keywords: explosion; control systems; process choice)
Incidents involving ignition or explosion of thermally unstable substances occur due to failure of temperature control in process vessels, e.g. reactors or distillation columns, or in storage/transportation conditions, e.g. in warehouses, where the normal temperature has been slightly exceeded. Such incidents can be -prevented by keeping the substances well below the critical ignition temperature. The critical ignition temperature, T,, is vital information with regard to the selection of safety margins for accident prevention. However, very little reliable T, data are currently available for bulk quantities of materials. The methods available to determine T, values can be divided into two broad groups: direct experiment; and calculation. A typical experimental method can be defined as: ‘a known quantity, in a defined form, of the sample is placed in a test cell and maintained at a constant temperature for a definite time period and any ignition point determined’. The procedure is repeated for a number of temperatures. This method of repeating tests at different constant temperatures is expensive in terms of resources. Also relatively large samples are employed and disposal of the resulting products may pose problems. A specific example of an experimental method is the BAM (Bundesanstalt fiir Materialpriifung, Berlin) Warmestau Lagerung (Heat-accumulation storage) test I. This determines the SADT (Self-Accelerating Decomposition Temperature). The SADT ’ is defined as the minimum temperature of the air environment for an Received 7 September 1988 0950-4230/89/010016-06s3.00 @ 1989 Butterworth 81 Co. (Publishers) Ltd 1 6 J. Loss Prev. Process Ind.. 1989. Vol2. Januarv
unstable substance at which auto-accelerative decomposition begins, just after 1 week when the substance is packaged in its largest commercial container and placed in the testing facility. In the BAM test ‘, 400 ml of a sample is placed in a 0.5 1 Dewar vessel. A thermocouple in a glass sheath is suspended from the lid of the vessel, so that its soldered joint is 60 mm above the bottom of the vessel. The filled Dewar is placed in the test oven, which is maintained at a constant temperature. The storage test is repeated with a fresh sample at a number of constant temperatures until the SADT is found. Many of the calculation methods are based on the work of Frank-Kamenetskii on the critical condition for thermal explosion 2.
L-1
6 = AHEr’A -E C XR T: exp RT,
(1)
As long as the thermal quantities AH, E, A and X are known, then T, can be calculated. The resulting T, values tend to give reasonable agreement with those found from direct experiment. However, problems arise due to the lack of appropriate thermal data. For instance, the values of E and A may be found by thermal analysis, or by measuring the rate of exothermic decomposition using very small sample sizes and hence the values obtained may not reflect the relevant conditions which lead to ignition on the large scale. The experimental condition of forced external heating, and the rapid temperature rise are distinctly different from those applicable at the initial stages of ignition i.e. spontaneous heating and the slow temperature rise. AH
Critical ignition temperatures of chemical substances: T. Kotoyori
measurements are usually taken for the complete reaction while the relevant A H for the initial stages of ignition is for incomplete reaction, involving the possible formation of some active or unstable intermediate products. To avoid some of the above problems, previous workers have used
1
AHEA ,,, [-] ScT,’ =g+ln -
r2 c [ hR This is obviously a re-arrangement of Equation (1). The following steps are carried out: l
the T, values are found for various sized samples
.ln6c7f.IS plotted against l/T, to obtain a straight
[ r= I
line l the ‘r’ value is extrapolated to that of the practical container 0 the T, value for relevant bulk sizes of substances is found This estimation method is based on the extrapolation of Tc vaiues obtained previously by experiment, and hence it is not surprising that a measure of agreement exists between the extrapolated value and that observed in experiments involving practical-sized samples. Again, the method is resource dependent i.e. it is timeconsuming and labour intensive, even though small sized samples are employed in the experimental pro-
cedures .
When the sample cell is held under adiabatic conditions, the temperature at the centre of the cell and that on the outside of the cell can be regarded as equal i.e. no temperature gradient within the cell. Hence
1
Cp s= AHA exp e; [
Equation (6) is a basic differential equation, which describes the relationship between the heat released per unit volume per unit time and temperature rise rate under adiabatic conditions. Figure I shows a diagrammatic profile of the heating process, which is normally found when any self-heating substance is maintained under adiabatic conditions. It can be seen that a very slow heating process (the socalled induction period) lasts for a considerable time in the initial stages of ignition. Eventually, heat or active products accumulate to such an extent, that rapid heating ensues, followed by the actual ignition. During the induction period (especially within a narrow range of temperature near T, as seen in Figure I) it can be assumed that the rate of temperature rise, dT/dt remains virtually constant hence the value of the right- . hand side of Equation (7) must also be constant = constant (Cl) hence
s
(7)
I>
I,
(‘3)
dr=-&, T’ dT s 71
so that
The above indicates that difficulties occur in the measurement of T,, whether this is based on direct experimental measurements or on the acquisition of reliable thermo-analytical data on the reaction. In order to minimize the difficulties, a new adiabatic self-heating ignition testing apparatus and procedure have been developed.
on taking natural logarithms and r earranging
Derivation of an equation to calculate T,
where At = time required for the sample temperature to rise by AT from T . Equation (10) is applicable to adiabatic self-heating
If the heating reaction is assumed to be zero order’, homogeneous and simple as defined by FrankKamenetskii, then the basic combustion theory equation becomes
at=-& AT=Eexp[&-]
(9)
1
A TCp In Al = $T+ In [ AHA
(10)
Ignition
1
Cp g= div (X grad T - CpvT) + AHA exp -$ (3) [ where v = flow velocity In spontaneous ignition, the medium is generally regarded as static and the above equation reduces to Cp z= div h grad T + AHA exp -$ [ I If the dependence of the thermal conductivity on temperature is neglected, then
1
Cp g= APT+ AHA exp R$ [ where v= div grad = Laplacian operator.
(9
1
- Induction period Time Figure 1 A diagrammatic profile of the heating process of selfheating substances under adiabatic conditions
J. Loss Prev. Process lnd., 1989, Vol2, January
17
Critical ignition temperatures of chemical substances: T. Kotoyori ‘for temperatures of a few degrees above T. The gra dient, a, and intercept, b of an empirical formula
(11)
In Ar=%+b
calculation of T, are the thermal diffusivity and the experimental terms, a, b and AT, which can be obtained from an adiabatic test apparatus. The values of r and 6, can be chosen in accordance with Frank-Kamenetskii theory.
are a=iand b=ln s
[
Experimental procedure
3
Values of a, b and AT can be found experimentally and used to calculate T,. Incidentally Frank-Kamenetskii also tried to express the adiabatic induction period Tad (Ref. 2). It was expressed as
(12) which, after expansion of the exponent and integration, gives Cp RT;
Tad=zE
E-1 E
(13)
Cxp RTo
If the relationship for temperature-rise under stationary conditions’ below the explosion limit is substituted into Equation (13) and T* = TO for spontaneous ignition’: RT; A T RT, -z ,or A T = E T* E Equation (13) can be rewritten in the same form as Equation (9). Frank-Kamenetskii’s critical condition for thermal explosion is based on a balance between the heating process and the heat loss process under non-adiabatic conditions. Equation (10) is only concerned with the heating process, since it involves adiabatic conditions. However, both equations deal with the identical slow heating process in the subcritical state prior to ignition i.e. the physical quantities appearing in Equations (1) and (10) are identical. Taking natural logarithms of Equation (1) and rearranging ln
Tc+e=ln r+fln[F)] c
c
(15)
Self-heating substances can be classified into two groups. The experimental procedure for ‘T-C’ type substances is given below.
Equipment A detailed description of the adiabatic self-ignition testing (SIT) apparatus has been given previously3.4. Figure 2 depicts the open-type sample cell, which is useful for oxidatively heating substances (e.g. sawdust). For the present study, a closed sample cell with an inside volume of 2 ml was devised. The closed system suppressed vaporization phenomena of the sample or decomposition products, which are accompanied with endothermic effects, and evolution of corrosive and/or toxic gases (see Figure 3). The closed SIT enables the adiabatic self-heating behaviour of chemicals to be studied safely and with a high degree of sensitivity.
Procedure A known quantity of material is placed in the cell. The exact quantity is dependent on the nature of the material e.g. 450 mg for nitrocellulose to 3 g for high test hypochlorite. A thermocouple is inserted into the sample. The sample cell assembly is placed in an adiabatic jacket. The starting temperature is selected on the basis of the nature of the material to give an estimated rate of temperature rise of = 1.25 K h-’ and ranged from 393 K for nitrocellulose to 323 K for lauroyl peroxide. The air bath heater is switched on. The sample temperature is gradually increased until the starting temperature is reached (see Figured). At this point a zero-suppression procedure is adopted and then adiabatic control and recording of the heating process are commenced.
if ‘a’ and ‘b’ are substituted into Equation (IS), then l n T,+&=ln r+i[,n(y) -b]
Thermocouple 1
(16)
Thermal conductivity can be expressed in terms of the thermal diffusivity and heat capacity per unit volume i.e. X=CrC/J
hence In T,+&=ln r+i [Infe) - b ]
(17)
From Equation (17) it can be seen that it is possible to dispense with the individual values of AH, E, A, X or C of the substance. The only data required to enable the
10 J. Loss Prev. Process Ind., 1989, Vol2, January
Inert gas Figure 2 Block diagram of SIT:l, resistance thermometer; 2, glass wool; 3, sample cell assembly; 4, silica tube; 5, glass wool; 6. wind guide plate; 7, fan; 8. heater
Critical ignition temperatures of chemical substances: T. Kotoyori
I
a
Time Figure4 Schematic heating behaviour shown by T-C type chemical substances: a, sample inserted into the air bath at temperature T; b, the sample temperature goes on rising slowly beyond T, when adiabatic control and recording of the heating process begin
i i
-
The thermal diffusivity of the substance is measured at two temperatures using the stepwise h e a t i n g method developed by Araki’. The package is assumed to be an infinite cylinder, the 6, value of which is 2.0, 40.64 cm (16 inch) in diameter. T, is calculated by substituting the numerical values of the paiameters into Equ&ion (17).
Results and discussion The chemical substances tested were.as follows: nitrocellulose (NC, nitrogen content: 12.27%); -1auroyl 99%); pperoxide content: (I-PO. toluenesulphonylhydrazide (TSH, content: over 95%); cy, 01’ -azobisisobutyronitrile ( A I B N , c o n t e n t : 98%), benzoyl peroxide (BPO, content: 98%); high test hypochlorite (HTH, Ca(OCl)z content: ca 70%); p, p’-oxybisbenzenesulphonylhydrazide (OBSH, content: o v e r 9 5 % ) ; and dinitrosopentamethylenetetramine (DPT, content: 98%). NC is an explosive; LPO, AIBN and BP0 are polymerization initiators; TSH, AIBN, OBSH and DPT are foaming agents and HTH is a powerful bleaching agent and the only inorganic substance among the tested substances. The samples were all of industrial purity. Figure 3 Sealed cell assembly of SIT: a, stainless steel sheath CA thermocouple, 0.5 mm in diameter; b, silica tube; c, glass wool; d, aluminium round plate; e, cap nut; f, washer; g, Teflon sheet; h, inorganic adhesives; i, glass sheath flange for thermocouple; j, O-ring; k, glass sealed cell ; I, Teflon sheet; m, washer; n, sample: o. gold powder
Colddon of T, for T-C type substances Adiabatic self-heating curves for each substance are recorded at several appropriate starting temperatures. The time *At(min) required for the sample temperature to rise by a AT value of 1.25 K from the starting temperature is determined from each heating curve. The In A1 versus 1 /T plot is made for each substance The values of the coefficients ‘a’ and ‘b’ of Equation (I 1) are determined from the plot by a least squares program.
Two groups classijkation Self-heating chemical substances have been classified into two groups. These have been tentatively named ‘T-C’ (thermal combustion) type and ‘A-C’ (autocatalytic combustion) type after the modes previously described by Frank-Kamenetskii’. Figure 4 depicts ‘T-C’ type behaviour. It occurs when a sample reaches the temperature of its surroundings and its temperature then continues to rise slowly in accordance with the Arrhenius equation. Figure 5 describes ‘A-C’ type behaviour. In this mode, the sample reaches the surrounding environment temperature slowly but as soon as the temperature is reached a sudden and rapid rise occurs. * Thermal combustion behaviour only occurs in exothermic reactions. During the thermal induction period the accumulated heat causes the temperature of the
J. L OSS Prev. Process In& ?989, Vol2, January, 19
Critical ignition temperatures of chemical substances: T. Kotoyori
reactive substance to increase2. It is possible to neglect the reaction rate at the initial sample temperature. After starting, the reaction rate depends exponentially on the temperature and on a power law of the concentration. From the above, it can be seen that the FrankKamenetskii critical condition for thermal explosion is only applicable to chemical substances of the ‘T-C’ type. T, values of five substances of ‘T-C’ type are presented in Table 1. They were calculated by applying Equation (17) and each was assumed to be packed in a sealed infinite cylinder, 40.64 cm in diameter. Autocatalytic combustion is possible only with autocatalytic or chain branching reactions. In autocatalytic combustion, propagation takes place by diffusion i.e. the transport of active substances. Consequently autocatalytic combustion can really only be observed under isothermal conditions’. An abrupt change in the behaviour of the reaction rate corresponds to chain expansion *, and the transition occurs from stationary to non-stationary conditions. The total induction period is regarded as the sum of the kinetic and thermal periods. The kinetic induction period is defined as the time to reach the optimum concentration of active products before the maximum reaction rate is reached, and the thermal induction period as the adiabatic induction period at this optimum temperature. However, according to FrankKamenetskii, the rise in temperature may not be taken into account before the optimum concentration is reached, and heat transfer is not taken into account afterwards *. This situation gives some agreement with the experimental results on substances of the ‘A-C’ type (see Figure 5). In the case of autocatalytic reactions, the position of the explosion limit can be regarded as being a function of the induction period. An empirical formula of the type In At=%+ b’
(18)
was found to hold, where At = time elapsed prior to an abrupt change in reaction rate following exposure of
sample to an environment temperature, T (see Figure 5). On applying the above equation, it is possible to calculate a T value, corresponding to an arbitrary At value, for each ‘A-C’ type substance. Such Tvalues can be used as a critical value to prevent ignition accidents. Examples of the calculations are given in Table 2. Relative ignition potential of eight chemical substances The graphs for the eight substances are presented in Figure 6. The graphs indicate the relative potential of
g
T .
AIBN BP0 HTH OBSH DPT
21,058 34,732 13,747 17,468 15,747
b
a
E
- 57.362 9 3 . 5 6 1 - 34.293 -42.199 -38.514
0.07 0.05 0.13 0.06 0.06
175 289 114 145 131
P
TC
0.55 0.64 1.36 0.59 0.64
56 72 73 78 78
20 J. Loss Prev. Process lnd., 7989, Vol2, J a n u a r y
.
.
.
.._................._.......... _
.
.
Time
Table 2 Experimental parameters of slow autocatalytic reaction and critical ignition temperatures of three chemical substances of A-C type
Chemical substance a’ NC LPO TSH
11,709 48,415 31,238
b’
P
- 25.931 - 142.293 -77.488
0.20 0.55 0.57
7 30 60 6 1 2 2 4 h h h days days days 95 87 60 54 52 51 102 98 95
60 46 87
48 43 81
41 42 78
Each T, value shown in the lower column was calculated, applying Equataon (18). After the elapsed tome pernod. the temperature is that recorded when the substance begins to heat-up spontaneously. a’ and b’. coefficients are temperature defined by Equation (18). respectively; T,, critical ignition (OCI; p. iniual packing density in the sample cell Igcm-3) (the density becomes almost doubled after the sample melted, in the case of LPO and TSHI.
T e m p e r a t u r e (“C) 115
3t Tc values were calculated, applymg Equation I1 7). assuming each substance to be packed in a sealed infinite cylinder, 40.64 cm (16 inch) in diameter. E, apparent activation energy (kJ mol.‘); P. packing density in the sample cell (g cmm3); Tc, critical ignition temperature I’C); for the other factors, refer to the notation at the end of the paper
.
Figure 5 Schematic heating behaviour shown by A-C type c h e m i c a l subtances: a , s a m p l e i n s e r t e d i n t o t h e a i r b a t h a t temperature T; b. the sample temperature begins a sudden, rapid rise
I a
.
a
Table 1 Adiabatic experimental parameters of slow self-heating reaction and critical ignition temperatures of five chemical substances of T-C type Chemical substance
F---il....
100
I
’
, 2.6
I 2.7
a5
70
I
I
I
I
2.8
2.9
60
I
200
I 3.0
10317 Figure 6 Plots of In At versus l/T for 8 c h e m i c a l s u b s t a n c e s . Solid lines represent T-C type substances. plotted In accordance with Equation I1 1). Dotted lines represent A-C type substances, plotted in accordance with Equation (181.
Critical ignition temperatures of chemical substances: T. Ko to yori
the substances to self-heat, or ultimately to ignite. NC appears to be relatively hard to ignite, since its data are situated at the higher temperature side. However, extrapolation to moderate induction periods yields unexpectedly lower temperature values due to the smaller gradient of the graph.
200r G 175e 150e 125-
I-
Comparison of T, values found with those found by
other workers Table 3 provides a comparison of data from this work with those of previous workers for seven of the substances. Data could not be found for NC at the appropriate temperatures. The low temperature values for NC may account for the many previous incidents caused by the spontaneous ignition of celluloid or other products made from nitrocellulose. The data in Table 3 indicate good agreement except for OBSH.
25-
I
I 100
Figure 7
I I 250 300 r (cm) (T, - r) diagram on BP0
Figure 8
r (cm1 Partial magnification of Figure 7
50
0
I 150
I 200
I 350
I 400
I 450
_ 500
6
(Tc-r) diagram for ‘T-C’ type substances T, is a function of r in Equation (17) and hence a (T, - r) diagram for a ‘T-C’ type substance will allow a T, value to be found for any arbitrary r value. Figures 7 and 8 provide such information for BPO.
Table 3 Comparison of Tc values of seven kinds of chemical substances calculated in this work with corresponding temperature values reported by other researches
Chemical substance LPO TSH AIBN BP0 HTH OESH OPT
Calc. in this work (‘0
Reported by others(OC)
46a e7= 56’ 69’ 75d 78’ 78’
45ae ii:: 6gcg 76*h ;$
‘The temperature recorded when the sample has been exposed to the atmosphere and begins to heat spontaneously after a 7 day period (cited from Table 2: these are SADT) *T, values calculated with SC-SIT, assuming each substance to be packed in a sealed infinite cylinder, 40.64 cm in diameter (cited from Tab/e I) =A Tc value calculated in this work, with open cell-SIT, assuming the substance to be packed in an open infinite cylinder, 26 cm in diameter ‘A T, value calculated in this work, with SC-SIT, assuming the substance to be packed in a sealed infinite cylinder, 35.4 cm I” diameter eA temperature value observed with a heat-accumulation storage testing apparatus of BAM type. applying the same procedure as that described in Ref. 6. Therefore this value is also a kind of SADT ‘Temperature values observed, wirh open cell-SIT, applying a procedure similar to those described in Refs. 1 and 2. These are also a kind of SADT values’ gA Tc value calculated, using an adiabatic storage testing apparatus, which has an open cell and was made by A. Miyake. applying the same procedure as that described in this work, applying Equation I1 6) in this paper, assuming the substance to be packed in an open infinite cylinder, 26 cm in diameter’ ‘A TC value calculated, applying Equation (2) in this paper, assuming the substance to be packed in a sealed infinite cylinder, 35.4 cm in diameter’
References I The United Nations Committee of Experts on the Transport of Dangerous Goods, ‘Transport of Dangerous Goods’, Intereg. Chicago, 1981, pp. 270-274, 284-288 2 Frank-Kamentskii, D. A., in ‘Diffusion and Heat Transfer in Chemical Kinetics’, translated from Russian by J.P. Appleton, Plenum Press, New York-London, 2nd ed., 1969, p. 3, 7-8, 25, 301-2, 333, 346-7, 352, 357, 379, 412 3 Kotoyori, T., Proceedings of the 1st International Symposium on Fire Safety Science, Hemisphere Publishing Corporation, Washington, 1985, p. 463 4 Kotoyori, T., and Manna, M., Thermochim. Acre 1Y83, 67, 35 5 Araki, N., lnternofional Journal of Therrnophysrcs 1984, 5, 53 6 Itoh, A., et al., Proceedings of the 16th Symposium on Safety Engineering, Japan Society for Safety Engineering, Yokohama, 1986, p. 231 (in Japanese) 7 Okuse K., er ol., A Technical Report, unofficially issued from Eiwa Chemical Industries Co. Ltd., Kyoto, 1980 (in Japanese) 8 Miyake, A., ef ol., Proceedings of the Annual Convention, the Industrial Explosives Society, Japan, Tokyo, 1986. p. 80 (in Japanese) 9 Uehara, Y., Uematsu, H., and Saito, Y. Combusr. &%‘lorne 1978, 32, 85
Nomenclature frequency factor of zeroth order reaction (mol cm- ’ min coefficient defined by Equation (I I) coefficient defined by Equation (18) coefficient defined by Equation (I I) coefficient defined by Equation (18) molar heat capacity (J mol-’ K-‘) Molar density (mol cm-‘) heat capacity per unit volume (J cm-” K-l) apparent activation energy (J mol-‘)
’)
molar heat of reaction (J mol- ‘) gas constant (J mol-’ K-‘)
radius of a body in Frank-Kamenetskii’s critical condition for thermal explosion (cm) starting temperature (K) critical ignition temperature (K) time (min) thermal diffusivity (cm’ min-‘) Frank-Kamenetskii’s critical condition for thermal explosion thermal conductivity (J cm-‘min-’ K-‘) adiabatic induction period
J. Loss Prew Process lnd., 1989, Vol2, January 21
Incidents involving ignition or explosion of thermally unstable substances occur due to failure of temperature control in process vessels, e.g. reactors or distillation columns, or in storage/transportation conditions, e.g. in warehouses, where the normal temperature has been slightly exceeded. Such incidents can be -prevented by keeping the substances well below the critical ignition temperature. The critical ignition temperature, T,, is vital information with regard to the selection of safety margins for accident prevention. However, very little reliable T, data are currently available for bulk quantities of materials. The methods available to determine T, values can be divided into two broad groups: direct experiment; and calculation. A typical experimental method can be defined as: ‘a known quantity, in a defined form, of the sample is placed in a test cell and maintained at a constant temperature for a definite time period and any ignition point determined’. The procedure is repeated for a number of temperatures. This method of repeating tests at different constant temperatures is expensive in terms of resources. Also relatively large samples are employed and disposal of the resulting products may pose problems. A specific example of an experimental method is the BAM (Bundesanstalt fiir Materialpriifung, Berlin) Warmestau Lagerung (Heat-accumulation storage) test I. This determines the SADT (Self-Accelerating Decomposition Temperature). The SADT ’ is defined as the minimum temperature of the air environment for an Received 7 September 1988 0950-4230/89/010016-06s3.00 @ 1989 Butterworth 81 Co. (Publishers) Ltd 1 6 J. Loss Prev. Process Ind.. 1989. Vol2. Januarv
unstable substance at which auto-accelerative decomposition begins, just after 1 week when the substance is packaged in its largest commercial container and placed in the testing facility. In the BAM test ‘, 400 ml of a sample is placed in a 0.5 1 Dewar vessel. A thermocouple in a glass sheath is suspended from the lid of the vessel, so that its soldered joint is 60 mm above the bottom of the vessel. The filled Dewar is placed in the test oven, which is maintained at a constant temperature. The storage test is repeated with a fresh sample at a number of constant temperatures until the SADT is found. Many of the calculation methods are based on the work of Frank-Kamenetskii on the critical condition for thermal explosion 2.
L-1
6 = AHEr’A -E C XR T: exp RT,
(1)
As long as the thermal quantities AH, E, A and X are known, then T, can be calculated. The resulting T, values tend to give reasonable agreement with those found from direct experiment. However, problems arise due to the lack of appropriate thermal data. For instance, the values of E and A may be found by thermal analysis, or by measuring the rate of exothermic decomposition using very small sample sizes and hence the values obtained may not reflect the relevant conditions which lead to ignition on the large scale. The experimental condition of forced external heating, and the rapid temperature rise are distinctly different from those applicable at the initial stages of ignition i.e. spontaneous heating and the slow temperature rise. AH
Critical ignition temperatures of chemical substances: T. Kotoyori
measurements are usually taken for the complete reaction while the relevant A H for the initial stages of ignition is for incomplete reaction, involving the possible formation of some active or unstable intermediate products. To avoid some of the above problems, previous workers have used
1
AHEA ,,, [-] ScT,’ =g+ln -
r2 c [ hR This is obviously a re-arrangement of Equation (1). The following steps are carried out: l
the T, values are found for various sized samples
.ln6c7f.IS plotted against l/T, to obtain a straight
[ r= I
line l the ‘r’ value is extrapolated to that of the practical container 0 the T, value for relevant bulk sizes of substances is found This estimation method is based on the extrapolation of Tc vaiues obtained previously by experiment, and hence it is not surprising that a measure of agreement exists between the extrapolated value and that observed in experiments involving practical-sized samples. Again, the method is resource dependent i.e. it is timeconsuming and labour intensive, even though small sized samples are employed in the experimental pro-
cedures .
When the sample cell is held under adiabatic conditions, the temperature at the centre of the cell and that on the outside of the cell can be regarded as equal i.e. no temperature gradient within the cell. Hence
1
Cp s= AHA exp e; [
Equation (6) is a basic differential equation, which describes the relationship between the heat released per unit volume per unit time and temperature rise rate under adiabatic conditions. Figure I shows a diagrammatic profile of the heating process, which is normally found when any self-heating substance is maintained under adiabatic conditions. It can be seen that a very slow heating process (the socalled induction period) lasts for a considerable time in the initial stages of ignition. Eventually, heat or active products accumulate to such an extent, that rapid heating ensues, followed by the actual ignition. During the induction period (especially within a narrow range of temperature near T, as seen in Figure I) it can be assumed that the rate of temperature rise, dT/dt remains virtually constant hence the value of the right- . hand side of Equation (7) must also be constant = constant (Cl) hence
s
(7)
I>
I,
(‘3)
dr=-&, T’ dT s 71
so that
The above indicates that difficulties occur in the measurement of T,, whether this is based on direct experimental measurements or on the acquisition of reliable thermo-analytical data on the reaction. In order to minimize the difficulties, a new adiabatic self-heating ignition testing apparatus and procedure have been developed.
on taking natural logarithms and r earranging
Derivation of an equation to calculate T,
where At = time required for the sample temperature to rise by AT from T . Equation (10) is applicable to adiabatic self-heating
If the heating reaction is assumed to be zero order’, homogeneous and simple as defined by FrankKamenetskii, then the basic combustion theory equation becomes
at=-& AT=Eexp[&-]
(9)
1
A TCp In Al = $T+ In [ AHA
(10)
Ignition
1
Cp g= div (X grad T - CpvT) + AHA exp -$ (3) [ where v = flow velocity In spontaneous ignition, the medium is generally regarded as static and the above equation reduces to Cp z= div h grad T + AHA exp -$ [ I If the dependence of the thermal conductivity on temperature is neglected, then
1
Cp g= APT+ AHA exp R$ [ where v= div grad = Laplacian operator.
(9
1
- Induction period Time Figure 1 A diagrammatic profile of the heating process of selfheating substances under adiabatic conditions
J. Loss Prev. Process lnd., 1989, Vol2, January
17
Critical ignition temperatures of chemical substances: T. Kotoyori ‘for temperatures of a few degrees above T. The gra dient, a, and intercept, b of an empirical formula
(11)
In Ar=%+b
calculation of T, are the thermal diffusivity and the experimental terms, a, b and AT, which can be obtained from an adiabatic test apparatus. The values of r and 6, can be chosen in accordance with Frank-Kamenetskii theory.
are a=iand b=ln s
[
Experimental procedure
3
Values of a, b and AT can be found experimentally and used to calculate T,. Incidentally Frank-Kamenetskii also tried to express the adiabatic induction period Tad (Ref. 2). It was expressed as
(12) which, after expansion of the exponent and integration, gives Cp RT;
Tad=zE
E-1 E
(13)
Cxp RTo
If the relationship for temperature-rise under stationary conditions’ below the explosion limit is substituted into Equation (13) and T* = TO for spontaneous ignition’: RT; A T RT, -z ,or A T = E T* E Equation (13) can be rewritten in the same form as Equation (9). Frank-Kamenetskii’s critical condition for thermal explosion is based on a balance between the heating process and the heat loss process under non-adiabatic conditions. Equation (10) is only concerned with the heating process, since it involves adiabatic conditions. However, both equations deal with the identical slow heating process in the subcritical state prior to ignition i.e. the physical quantities appearing in Equations (1) and (10) are identical. Taking natural logarithms of Equation (1) and rearranging ln
Tc+e=ln r+fln[F)] c
c
(15)
Self-heating substances can be classified into two groups. The experimental procedure for ‘T-C’ type substances is given below.
Equipment A detailed description of the adiabatic self-ignition testing (SIT) apparatus has been given previously3.4. Figure 2 depicts the open-type sample cell, which is useful for oxidatively heating substances (e.g. sawdust). For the present study, a closed sample cell with an inside volume of 2 ml was devised. The closed system suppressed vaporization phenomena of the sample or decomposition products, which are accompanied with endothermic effects, and evolution of corrosive and/or toxic gases (see Figure 3). The closed SIT enables the adiabatic self-heating behaviour of chemicals to be studied safely and with a high degree of sensitivity.
Procedure A known quantity of material is placed in the cell. The exact quantity is dependent on the nature of the material e.g. 450 mg for nitrocellulose to 3 g for high test hypochlorite. A thermocouple is inserted into the sample. The sample cell assembly is placed in an adiabatic jacket. The starting temperature is selected on the basis of the nature of the material to give an estimated rate of temperature rise of = 1.25 K h-’ and ranged from 393 K for nitrocellulose to 323 K for lauroyl peroxide. The air bath heater is switched on. The sample temperature is gradually increased until the starting temperature is reached (see Figured). At this point a zero-suppression procedure is adopted and then adiabatic control and recording of the heating process are commenced.
if ‘a’ and ‘b’ are substituted into Equation (IS), then l n T,+&=ln r+i[,n(y) -b]
Thermocouple 1
(16)
Thermal conductivity can be expressed in terms of the thermal diffusivity and heat capacity per unit volume i.e. X=CrC/J
hence In T,+&=ln r+i [Infe) - b ]
(17)
From Equation (17) it can be seen that it is possible to dispense with the individual values of AH, E, A, X or C of the substance. The only data required to enable the
10 J. Loss Prev. Process Ind., 1989, Vol2, January
Inert gas Figure 2 Block diagram of SIT:l, resistance thermometer; 2, glass wool; 3, sample cell assembly; 4, silica tube; 5, glass wool; 6. wind guide plate; 7, fan; 8. heater
Critical ignition temperatures of chemical substances: T. Kotoyori
I
a
Time Figure4 Schematic heating behaviour shown by T-C type chemical substances: a, sample inserted into the air bath at temperature T; b, the sample temperature goes on rising slowly beyond T, when adiabatic control and recording of the heating process begin
i i
-
The thermal diffusivity of the substance is measured at two temperatures using the stepwise h e a t i n g method developed by Araki’. The package is assumed to be an infinite cylinder, the 6, value of which is 2.0, 40.64 cm (16 inch) in diameter. T, is calculated by substituting the numerical values of the paiameters into Equ&ion (17).
Results and discussion The chemical substances tested were.as follows: nitrocellulose (NC, nitrogen content: 12.27%); -1auroyl 99%); pperoxide content: (I-PO. toluenesulphonylhydrazide (TSH, content: over 95%); cy, 01’ -azobisisobutyronitrile ( A I B N , c o n t e n t : 98%), benzoyl peroxide (BPO, content: 98%); high test hypochlorite (HTH, Ca(OCl)z content: ca 70%); p, p’-oxybisbenzenesulphonylhydrazide (OBSH, content: o v e r 9 5 % ) ; and dinitrosopentamethylenetetramine (DPT, content: 98%). NC is an explosive; LPO, AIBN and BP0 are polymerization initiators; TSH, AIBN, OBSH and DPT are foaming agents and HTH is a powerful bleaching agent and the only inorganic substance among the tested substances. The samples were all of industrial purity. Figure 3 Sealed cell assembly of SIT: a, stainless steel sheath CA thermocouple, 0.5 mm in diameter; b, silica tube; c, glass wool; d, aluminium round plate; e, cap nut; f, washer; g, Teflon sheet; h, inorganic adhesives; i, glass sheath flange for thermocouple; j, O-ring; k, glass sealed cell ; I, Teflon sheet; m, washer; n, sample: o. gold powder
Colddon of T, for T-C type substances Adiabatic self-heating curves for each substance are recorded at several appropriate starting temperatures. The time *At(min) required for the sample temperature to rise by a AT value of 1.25 K from the starting temperature is determined from each heating curve. The In A1 versus 1 /T plot is made for each substance The values of the coefficients ‘a’ and ‘b’ of Equation (I 1) are determined from the plot by a least squares program.
Two groups classijkation Self-heating chemical substances have been classified into two groups. These have been tentatively named ‘T-C’ (thermal combustion) type and ‘A-C’ (autocatalytic combustion) type after the modes previously described by Frank-Kamenetskii’. Figure 4 depicts ‘T-C’ type behaviour. It occurs when a sample reaches the temperature of its surroundings and its temperature then continues to rise slowly in accordance with the Arrhenius equation. Figure 5 describes ‘A-C’ type behaviour. In this mode, the sample reaches the surrounding environment temperature slowly but as soon as the temperature is reached a sudden and rapid rise occurs. * Thermal combustion behaviour only occurs in exothermic reactions. During the thermal induction period the accumulated heat causes the temperature of the
J. L OSS Prev. Process In& ?989, Vol2, January, 19
Critical ignition temperatures of chemical substances: T. Kotoyori
reactive substance to increase2. It is possible to neglect the reaction rate at the initial sample temperature. After starting, the reaction rate depends exponentially on the temperature and on a power law of the concentration. From the above, it can be seen that the FrankKamenetskii critical condition for thermal explosion is only applicable to chemical substances of the ‘T-C’ type. T, values of five substances of ‘T-C’ type are presented in Table 1. They were calculated by applying Equation (17) and each was assumed to be packed in a sealed infinite cylinder, 40.64 cm in diameter. Autocatalytic combustion is possible only with autocatalytic or chain branching reactions. In autocatalytic combustion, propagation takes place by diffusion i.e. the transport of active substances. Consequently autocatalytic combustion can really only be observed under isothermal conditions’. An abrupt change in the behaviour of the reaction rate corresponds to chain expansion *, and the transition occurs from stationary to non-stationary conditions. The total induction period is regarded as the sum of the kinetic and thermal periods. The kinetic induction period is defined as the time to reach the optimum concentration of active products before the maximum reaction rate is reached, and the thermal induction period as the adiabatic induction period at this optimum temperature. However, according to FrankKamenetskii, the rise in temperature may not be taken into account before the optimum concentration is reached, and heat transfer is not taken into account afterwards *. This situation gives some agreement with the experimental results on substances of the ‘A-C’ type (see Figure 5). In the case of autocatalytic reactions, the position of the explosion limit can be regarded as being a function of the induction period. An empirical formula of the type In At=%+ b’
(18)
was found to hold, where At = time elapsed prior to an abrupt change in reaction rate following exposure of
sample to an environment temperature, T (see Figure 5). On applying the above equation, it is possible to calculate a T value, corresponding to an arbitrary At value, for each ‘A-C’ type substance. Such Tvalues can be used as a critical value to prevent ignition accidents. Examples of the calculations are given in Table 2. Relative ignition potential of eight chemical substances The graphs for the eight substances are presented in Figure 6. The graphs indicate the relative potential of
g
T .
AIBN BP0 HTH OBSH DPT
21,058 34,732 13,747 17,468 15,747
b
a
E
- 57.362 9 3 . 5 6 1 - 34.293 -42.199 -38.514
0.07 0.05 0.13 0.06 0.06
175 289 114 145 131
P
TC
0.55 0.64 1.36 0.59 0.64
56 72 73 78 78
20 J. Loss Prev. Process lnd., 7989, Vol2, J a n u a r y
.
.
.
.._................._.......... _
.
.
Time
Table 2 Experimental parameters of slow autocatalytic reaction and critical ignition temperatures of three chemical substances of A-C type
Chemical substance a’ NC LPO TSH
11,709 48,415 31,238
b’
P
- 25.931 - 142.293 -77.488
0.20 0.55 0.57
7 30 60 6 1 2 2 4 h h h days days days 95 87 60 54 52 51 102 98 95
60 46 87
48 43 81
41 42 78
Each T, value shown in the lower column was calculated, applying Equataon (18). After the elapsed tome pernod. the temperature is that recorded when the substance begins to heat-up spontaneously. a’ and b’. coefficients are temperature defined by Equation (18). respectively; T,, critical ignition (OCI; p. iniual packing density in the sample cell Igcm-3) (the density becomes almost doubled after the sample melted, in the case of LPO and TSHI.
T e m p e r a t u r e (“C) 115
3t Tc values were calculated, applymg Equation I1 7). assuming each substance to be packed in a sealed infinite cylinder, 40.64 cm (16 inch) in diameter. E, apparent activation energy (kJ mol.‘); P. packing density in the sample cell (g cmm3); Tc, critical ignition temperature I’C); for the other factors, refer to the notation at the end of the paper
.
Figure 5 Schematic heating behaviour shown by A-C type c h e m i c a l subtances: a , s a m p l e i n s e r t e d i n t o t h e a i r b a t h a t temperature T; b. the sample temperature begins a sudden, rapid rise
I a
.
a
Table 1 Adiabatic experimental parameters of slow self-heating reaction and critical ignition temperatures of five chemical substances of T-C type Chemical substance
F---il....
100
I
’
, 2.6
I 2.7
a5
70
I
I
I
I
2.8
2.9
60
I
200
I 3.0
10317 Figure 6 Plots of In At versus l/T for 8 c h e m i c a l s u b s t a n c e s . Solid lines represent T-C type substances. plotted In accordance with Equation I1 1). Dotted lines represent A-C type substances, plotted in accordance with Equation (181.
Critical ignition temperatures of chemical substances: T. Ko to yori
the substances to self-heat, or ultimately to ignite. NC appears to be relatively hard to ignite, since its data are situated at the higher temperature side. However, extrapolation to moderate induction periods yields unexpectedly lower temperature values due to the smaller gradient of the graph.
200r G 175e 150e 125-
I-
Comparison of T, values found with those found by
other workers Table 3 provides a comparison of data from this work with those of previous workers for seven of the substances. Data could not be found for NC at the appropriate temperatures. The low temperature values for NC may account for the many previous incidents caused by the spontaneous ignition of celluloid or other products made from nitrocellulose. The data in Table 3 indicate good agreement except for OBSH.
25-
I
I 100
Figure 7
I I 250 300 r (cm) (T, - r) diagram on BP0
Figure 8
r (cm1 Partial magnification of Figure 7
50
0
I 150
I 200
I 350
I 400
I 450
_ 500
6
(Tc-r) diagram for ‘T-C’ type substances T, is a function of r in Equation (17) and hence a (T, - r) diagram for a ‘T-C’ type substance will allow a T, value to be found for any arbitrary r value. Figures 7 and 8 provide such information for BPO.
Table 3 Comparison of Tc values of seven kinds of chemical substances calculated in this work with corresponding temperature values reported by other researches
Chemical substance LPO TSH AIBN BP0 HTH OESH OPT
Calc. in this work (‘0
Reported by others(OC)
46a e7= 56’ 69’ 75d 78’ 78’
45ae ii:: 6gcg 76*h ;$
‘The temperature recorded when the sample has been exposed to the atmosphere and begins to heat spontaneously after a 7 day period (cited from Table 2: these are SADT) *T, values calculated with SC-SIT, assuming each substance to be packed in a sealed infinite cylinder, 40.64 cm in diameter (cited from Tab/e I) =A Tc value calculated in this work, with open cell-SIT, assuming the substance to be packed in an open infinite cylinder, 26 cm in diameter ‘A T, value calculated in this work, with SC-SIT, assuming the substance to be packed in a sealed infinite cylinder, 35.4 cm I” diameter eA temperature value observed with a heat-accumulation storage testing apparatus of BAM type. applying the same procedure as that described in Ref. 6. Therefore this value is also a kind of SADT ‘Temperature values observed, wirh open cell-SIT, applying a procedure similar to those described in Refs. 1 and 2. These are also a kind of SADT values’ gA Tc value calculated, using an adiabatic storage testing apparatus, which has an open cell and was made by A. Miyake. applying the same procedure as that described in this work, applying Equation I1 6) in this paper, assuming the substance to be packed in an open infinite cylinder, 26 cm in diameter’ ‘A TC value calculated, applying Equation (2) in this paper, assuming the substance to be packed in a sealed infinite cylinder, 35.4 cm in diameter’
References I The United Nations Committee of Experts on the Transport of Dangerous Goods, ‘Transport of Dangerous Goods’, Intereg. Chicago, 1981, pp. 270-274, 284-288 2 Frank-Kamentskii, D. A., in ‘Diffusion and Heat Transfer in Chemical Kinetics’, translated from Russian by J.P. Appleton, Plenum Press, New York-London, 2nd ed., 1969, p. 3, 7-8, 25, 301-2, 333, 346-7, 352, 357, 379, 412 3 Kotoyori, T., Proceedings of the 1st International Symposium on Fire Safety Science, Hemisphere Publishing Corporation, Washington, 1985, p. 463 4 Kotoyori, T., and Manna, M., Thermochim. Acre 1Y83, 67, 35 5 Araki, N., lnternofional Journal of Therrnophysrcs 1984, 5, 53 6 Itoh, A., et al., Proceedings of the 16th Symposium on Safety Engineering, Japan Society for Safety Engineering, Yokohama, 1986, p. 231 (in Japanese) 7 Okuse K., er ol., A Technical Report, unofficially issued from Eiwa Chemical Industries Co. Ltd., Kyoto, 1980 (in Japanese) 8 Miyake, A., ef ol., Proceedings of the Annual Convention, the Industrial Explosives Society, Japan, Tokyo, 1986. p. 80 (in Japanese) 9 Uehara, Y., Uematsu, H., and Saito, Y. Combusr. &%‘lorne 1978, 32, 85
Nomenclature frequency factor of zeroth order reaction (mol cm- ’ min coefficient defined by Equation (I I) coefficient defined by Equation (18) coefficient defined by Equation (I I) coefficient defined by Equation (18) molar heat capacity (J mol-’ K-‘) Molar density (mol cm-‘) heat capacity per unit volume (J cm-” K-l) apparent activation energy (J mol-‘)
’)
molar heat of reaction (J mol- ‘) gas constant (J mol-’ K-‘)
radius of a body in Frank-Kamenetskii’s critical condition for thermal explosion (cm) starting temperature (K) critical ignition temperature (K) time (min) thermal diffusivity (cm’ min-‘) Frank-Kamenetskii’s critical condition for thermal explosion thermal conductivity (J cm-‘min-’ K-‘) adiabatic induction period
J. Loss Prew Process lnd., 1989, Vol2, January 21