chapter 4 thermal hazard assessment of...

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CHAPTER 4

THERMAL HAZARD ASSESSMENT OF FIREWORKS

MIXTURE USING ACCELERATING RATE

CALORIMETER (ARC)

4.1 INTRODUCTION

Accelerating Rate Calorimeter (ARC) is one of the versatile

experimental tools available to study the self-propagating and thermally

sensitive reactions of fireworks mixtures. It operates in an adiabatic condition.

By using thermal techniques such as DSC and ARC in complement, it is

possible to address thermal hazard potentials of fireworks mixtures (Lightfoot

et al 2001, Brown and Gallagher 2003).

Although there are many reported studies on impact and friction

sensitivity of fireworks mixtures, studies of the thermal stability of fireworks

mixtures and their thermo kinetics are few.

In the present study, thermal hazards of fireworks mixtures were

studied using ARC to understand the thermal behavior of fireworks mixture.

ARC studies on fireworks mixtures were used to determine ceiling

temperatures and pressure limits for safe operation, storage and

transportation.

69

In this chapter, the thermal hazards of three cracker mixtures viz.

atom bomb cracker, Chinese cracker and palm leaf cracker and two tip

mixtures viz. flower pot tip mixture and ground spinner tip mixture by

adiabatic tests in ARC are studied. Further the water induced thermal hazards

of tip mixtures are also investigated by iso-ageing tests in ARC to deduce the

field conditions for triggering accidents.

4.2 MATERIALS AND METHOD

4.2.1 Accelerating Rate Calorimeter (ARC)

Accelerating Rate Calorimeter (ARC) has gained importance since

the 1980’s for studying the self heating reactions that cause thermal runaway.

It was employed (Aurbach et al 2003, Bunyan et al 1999, Carr 1983, Fenlon

1984, Fisher 1991, Iizuka and Surianarayanan 2003, Lee et al 2002, Liao et al

2011, Ottaway 1986, Roduit et al 2008, Smitha et al 2013, Sivapirakasam et

al 2006c, Surianarayanan et al 2001) for studying the runaway characteristics

of chemical reactions. ARC used in this study was an esARC supplied by

Thermal Hazard Technology, UK.

4.2.2 ARC Construction

Figure 4.1 illustrates the calorimeter part of ARC. It is a container

with its contents maintained at adiabatic conditions with respect to its

environment. This is accomplished by constant monitoring of its temperature

and suitably adjusting the surrounding temperature to minimize the heat gains

(or) losses from the container. In order to achieve an adiabatic environment

over a temperature range of ambient to 425°C, the ARC is equipped with a

sophisticated digital control for the heater system.

70

Shield

Bomb

Control Heater

Radiant Heater

Insulation Pressure Transducer

SampleThermocouple

ControlThermocouple

The calorimeter can be divided into three temperature control zones

viz. Top, middle and bottom with each of them equipped with its own control

instrumentation. The sample container or “bomb” is attached to a pressure

transducer on the top of the chamber for close monitoring of pressure

responses. The radiant heater located at the bottom of the adiabatic chamber,

is meant for heating the sample container at the start of the experiment.

Figure 4.1 Accelerating Rate Calorimeter chamber setup

4.2.3 ARC Principle

The working principle, design description, and operational details

of ARC were well cited in literature (Townsend and Tou 1980). ARC

measurements were made using a sample bomb, i.e. a metal sphere in a 2.5cm

diameter, typically made of Titanium. The sample mass usually (1-2 g) would

depend upon the expected energy release and type of sample container

71

(known as a bomb) used. The sample bomb was attached to the lid section on

the calorimeter assembly by a Swagelok pressure fitting and a pressure line

that led to the pressure transducer. A thermocouple was attached to the outer

surface of the bomb and the lid of the calorimeter positioned on the base

section.

The calorimeter has three separate thermal zones. The top (lid

section) contains two heaters and a thermocouple, the side zone of the base

section has four heaters and a thermocouple and the bottom zone at the base

section has two heaters and a thermocouple. After set up and connection, the

calorimeter was sealed with an explosion proof containment vessel. After

defining experimental conditions on the PC, the test commenced. Two types

of tests can be performed in ARC viz., heat-wait-search and iso-ageing

method. ARC experiments were performed both under heat-wait-search and

iso-ageing methods. The heat-wait-search method (Figure 4.2) is briefly

discussed below.

The test conditions were a start and end temperature and choosing

the size of ‘heat steps', 'wait time' and detection sensitivity'. The system will

heat to the start temperature. A small heater in the calorimeter, the radiant

heater was used to heat the sample, bomb and its thermocouple. The

calorimeter was cooled and the temperature difference observed by the three

calorimeter thermocouples. The system then applied power to the calorimeter

heaters to minimize the temperature difference. This was to continue as the

temperature rose to the start temperature. When this start temperature was

reached the system would go into a wait period and during this time no heat

was provided by the radiant heater. This allowed the temperature differences

within the calorimeter to be reduced to zero. The calorimeter operates

adiabatically allowing the calorimeter temperature to track sample

72

temperature. These wait periods (typically 10-15 minutes) was followed by a

search or seek period. Again during this period (typically 20 minutes) no heat

was provided by the radiant heater, and any temperature drift, upwards or

downwards, was observed. If there was upward temperature drift it was

caused by a self-heating reaction. The heat-wait-seek procedure, the normal

mode of operation of the ARC was to continue until an upward temperature

drift observed an exothermic reaction, greater than the selected sensitivity

(normally 0.01-0.02°C min-1). The system automatically switched to the

exothermic mode; it would apply heat to the calorimeter jacket to keep its

temperature the same as the bomb/sample.

The adiabatic control is the key feature of the ARC. The system

continues in the exotherm mode until the rate of self heating is less than the

chosen sensitivity and at this stage the heat-wait-seek procedure resumes.

When the end temperature is reached (or an end pressure is reached) the test

automatically stops and cooling, by compressed air, begins.

The aim of the ARC is to complete the test to get a full time,

temperature, and pressure profile of the exothermic reaction in a safe and

controlled manner. In iso-ageing method the sample is screened for

exothermic activity at a specified temperature, on initiation of exothermic

activity, the self heating was followed under adiabatic control mode. The

difference between iso-ageing method and heat-wait-search method is that, in

the iso-ageing method the sample is subjected to remain at a pre determined

temperature. On exothermic onset ARC follows the exothermic reaction

adiabatically as in heat-wait-search method.

73

Figure 4.2 Heat-wait-search operation of ARC

Following data plots can be obtained from an ARC experiment.

Self heat rate vs. temperature

This plot provides information on the onset temperature of the

exothermic activity and qualitative indication of the rate of energy liberation.

Temperature vs. time

It provides information on the vigour of the exothermic reaction

and also the available time span from the onset of exothermic activity to the

end of the reaction.

Pressure vs. temperature

Information on the rate of pressure and temperature rise will be

most useful for estimating the vent area required for the safe operation of a

reaction mixture.

74

4.2.4 Adiabatic Thermo Kinetics

The first assumption in the interpretation of ARC experimental data

is the representation of concentration in terms of temperature differences

(Townsend (1977 and 1991), Townsend and Tou (1980)). The equivalence of

temperature and concentration for a simple well-defined chemical reaction is

established using the ratio (Bunyan et al 1999, Davis et al 1991, Fisher 1991,

Kossoy et al 1994, Lee and Back 1986, Nalla et al 2012, Ottaway 1986,

Smitha et al 2013, Wedlich and Davis 1990,):

= = (4.1)

Here C is the concentration of the reacting substance, and T the temperature.

The subscript ‘0’ indicates some initial condition, and F a final state in which

the substance has been consumed. Then T = TF –T0 is the temperature rise

for the reaction. It is also equal to the ratio of enthalpy to average specific

heat. As the reaction proceeds, the disappearance of the reacting species

produces a proportional increase in the heat energy. The heat of reaction, Hr,

can be calculated from

H = mC T (4.2)

where PC , is the average heat capacity, m the mass of the sample.

The heat generated in an exothermic reaction is used to heat the

material, the container or bomb, and the surroundings. The heat being used up

in heating the sample mass depends on specific heat. The proportion of heat

used in heating the container is called thermal inertia ( ), expressed as

=Heat capacity of sample(s)and container or bomb(b)

Heat capacity of sample(or)

75

=m C + m C

m C(or) (4.3)

= 1 +m Cm C

(4.4)

Incorporating the effects of thermal inertia ( ), the absolute

temperature rise is given by

Tab= T (4.5)

The corrected heat of reaction, Hr, is calculated using

Hr = mC T (4.6)

The question that is basic to the study of relationship of time to

explosion is the measurement, and extrapolation of data. Extrapolation must

involve a concept of concentration since no material can continue to self-heat

forever. The time dependence of concentration for an nth order reaction rate is

expressed as follows

dCdt

= kC (4.7)

Where C is the concentration, k the rate coefficient, and t the time. When

Equations (4.1) and (4.7) are used, additional temperature dependence appears.

dCdt

= Cddt

(T T)T

=CT

dTdt

(4.8)

m =dTdt

= k(T T

T) . C T (4.9)

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Here mT , is defined as the rate of temperature increase (or slope of the graph

of T vs. t), i.e. the self-heat rate. To remove this extra temperature

dependence, a modified rate is defined as the pseudo rate constant, k*.

k = k. C =m

T. (

TT T

) (4.10)

It is defined in such a way that its dimensions for any order reaction

are reciprocal of time. In practice, k* is evaluated from experimental data

using the right hand side expression. With the proper choice of N, k* has the

same temperature dependence as k, and yields a straight-line graph.

The Arrhenius relationship for determining the rate coefficients is

k = Ae (4.11)

where T is the absolute temperature in Kelvin, E the activation energy, R the

universal gas constant, and A is the pre-exponential factor

First order model (i.e. N=1) kinetics was assumed for the

decomposition of fireworks tip mixture.

For N =1 Equation (4.10) becomes

k = k =m

(T T)(4.12)

Pseudo rate constants (k*) were calculated using Equation (4.12).

Then E/R, and pre-exponential factor were found out by plotting ln k*

versus inverse of temperature. The slope of the plot was equal to E/R, and

the intercept gave the pre-exponential factor, A.

77

4.3 RESULTS AND DISCUSSION

4.3.1 ARC Studies for Cracker Mixtures

Under adiabatic conditions the atom bomb cracker mixture

decomposed slowly (Figure 4.3) until 1750 min (285°C) and beyond this, the

temperature rise is sudden and sharp until the end of the exothermic activity.

The entire activity is recorded within a time span of 300 minutes. The sharp

and sudden rise in temperature shows the vulnerability of this mixture to

undergo violent decomposition. The discontinuity in both the self heat rate

plot and time versus temperature plot is indicative of multiple exothermic

activities.

Figure 4.3 Temperature rise profile for different types of crackers

(Atom bomb cracker ( ), Chinese cracker ( ), Palm leaf cracker ( ))

1500 1600 1700 1800 1900 2000 2100 2200 2300 2400

280

300

320

340

360

380

400

420

Time, (min)

78

The self heat rate plot for thermal explosive decomposition of atom

bomb cracker consisting of KNO3, S and Al in the ratio of 60:20:20 is shown

in Figure 4.4 and the data summarized in Table 4.1. The onset for thermal

explosive decomposition was observed at 275°C and extended up to 340°C.

The self heat rate plot shows a maximum heat release rate of 1088.1°C min-1

at 320°C. The observed large heat release rate confirms the vigour of

exothermic explosive process of atom bomb cracker mixture.

Figure 4.4 Self heat rate profile for different types of crackers


The exothermic activity is accompanied by a considerable quantity

release of gaseous components as shown in Figure 4.5. It is interesting to

note that one gram of sample could contribute to a peak pressure rise of 25.9

bar at 342.3°C. The adiabatic temperature rise for this process was

66.7°C.The heat of reaction for the exothermic activity was calculated as

504.2 Jg-1. The ARC data showed that the fireworks mixture decomposition

process under adiabatic condition was vigourous and therefore dangerous.

280 300 320 340 360 380 400 420

0.1

1

10

100

1000

Temperature, (°C)

79

Although explosive potential was required for it to be a cracker, the same

property could be dangerous if the event occurred during handling,

manufacturing, storage or transport. Evidence is available (Chapter 6) that the

onset temperature attainment could be possible due to mechanical stimulus.

Figure 4.5 Pressure rise profile for different types of crackers


The ARC results for Chinese cracker and palm-leaf crackers are

shown in Figures 4.3-4.5. Both Chinese and palm-leaf crackers show delayed

onset temperatures of 295°C and 290°C respectively as compared to the atom

bomb cracker mixture. The delay in initiation of the exothermic activity can

be related to their mixture, especially the quantity of the oxidiser KNO3.

While the percentage of KNO3 in palm-leaf is more or less equal to that of

Chinese cracker, early onset is perhaps due to the presence of another oxidiser

Ba(NO3)2 and a large quantity of different grades of aluminium. Palm-leaf

280 300 320 340 360 380 400 4200

5

10

15

20

25

30

Temperature, (°C)

80

cracker also shows multiple exothermic activities extended up to 420°C. Both

these mixtures liberate peak heat rates more than 100°C min around 320°C.

The time versus temperature plot (Figure 4.3) shows that the exothermic

activity is sudden and sharp as observed with atom bomb cracker mixture.

Although the heat rates of the second exothermic activity of palm-leaf

mixture were within 1 °C min-1, the decomposition process raises the system

temperature suddenly to 410°C from 345°C. The decomposition process

contributes to a system pressure rise up to 15 bar. The ARC data showed that

the Chinese cracker and palm-leaf cracker decompositions under adiabatic

conditions were vigourous and therefore dangerous.

4.3.2 Thermo Kinetics of Cracker Mixtures

Pseudo rate constants (k*) were calculated using Equation (4.12).

Then ln k* versus inverse of temperature was plotted. The straight line

obtained confirms the assumption that the pyrotechnic mixture follows first

order kinetics. The slope of the plot is equal to E/R. The activation energy

was calculated as 352.17 kJ mol-1, 403.34 kJ mol-1 and 513.82 kJ mol-1 for

atom bomb cracker, Chinese cracker and palm leaf cracker respectively. The

pre-exponential factor was evaluated as 1.38X1030, 8.35X1033, and 1.05X1044

min-1. Thus the Arrhenius rate law for thermal decomposition of atom bomb

cracker, Chinese cracker and palm leaf cracker mixture can be given as

Equations (4.13), (4.14), and (4.15) respectively.

k = 1.38X1030exp (-352.17 /RT) (4.13)

k = 8.35X1033exp (-403.34/RT) (4.14)

k = 1.05X1044exp (-513.82/RT) (4.15)

81

The heat rates determined were compared with the experimentally

obtained heat rates in Figure 4.6 and they did show a good agreement between

the two.

Figure 4.6 Self heat rate profile experimental and predicted for

different types crackers

(Atom bomb cracker ( ), Chinese cracker ( ), Palm leaf

cracker ( ), Predicted ( )

280 300 320 340 360 380 400 420

0.1

1

10

Temperature, (°C)

83

4.3.3 ARC Studies of Ground Spinner Tip and Flowerpot Tip

Mixtures

The ARC results for ground spinner tip mixture are presented in

Figures 4.7-4.9. Compared to DSC results, adiabatic calorimeter showed

interesting results for ground spinner tip mixture and ground spinner tip

mixtures with varying water content. The ground spinner tip mixture without

added water is marked as 0% in Figures 4.7-4.9. The time temperature curve

(Figure 4.7) shows two discontinuous exothermic activity at 160-190°C and

220-320°C. The first self heating rate process proceeds slowly compared to

the second event.

Figure 4.7 Time vs. Temperature plot for ground spinner tip mixture

and ground spinner tip mixture with different % of water

content

(0% ( ), 10% ( ), 20% ( ), 30%( ), 40%( ))

200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 300060

80

100

120

140

160

180

200

220

240

260

280

300

320

340

360

380

400

Time, (min)

84

Figure 4.8 Temperature vs. Self heat rate plot for ground spinner tip mixture and ground spinner tip mixture with different % of water content

(0% ( ), 10% ( ), 20% ( ), 30%( ), 40%( ))

The self heating process for ground spinner tip mixture started at

220°C and peaked at 320°C with a heating rate of 100°C min-1. The self

heating process ended at this point perhaps due to complete depletion of the

sample. The sudden jump in self heat rate from 10-50°C and finally 100°C

min-1 (in a discontinuous manner) indicated the explosive nature of the

process. As the water content increased the exothermic onset point decreased.

The mixture with 40% water content showed the lowest onset point of 80°C.

The time required for exothermic initiations under dynamic heat-wait-seek

pattern was 300 minutes. Once the exothermic initiation occurred, in less than

100 minutes, the temperature rose to 300°C. Although the onset temperature

(120°C) for the mixture with 30% water content was higher, the behavior of

the exothermic event was similar to that of the mixture with 40% water

content, but with a higher end point i.e. 340°C. The mixtures with no water

60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 4000.01

0.1

1

10

100

Temperature, (°C)

85

and 10-20% water (w/w) content underwent multiple exothermic events in the

temperature range of 140°C-380°C.

The self heat rate pattern observed for fireworks tip mixture

presented in Figure 4.8 showed the vigour of exothermic behavior of these

samples. The neat sample showed a maximum self heat rate of 100°C min-1at

320°C. The sample with 40% water (w/w) content released a maximum heat

of 40°C min-1at 300°C. The maximum heat releases for mixture containing

30, 20, 10% water (w/w) content were within 15°C min-1.

Figure 4.9 Temperature vs. Pressure plot for ground spinner tip

mixture and ground spinner tip mixture with different % of

water content

(0% ( ), 10% ( ), 20% ( ), 30%( ), 40%( ))

The pressure release profiles for mixtures are presented in

Figure 4.9. The self heating exothermic process for 40 and 30% water (w/w)

containing samples accompanied a maximum pressure release of 65, and 75

75 100 125 150 175 200 225 250 275 300 325 350 375 400

0

10

20

30

40

50

60

70

80

Temperature, (°C)

86

bar respectively. The samples with less than 30% water (w/w) content showed

pressure release of less than 20 bar, though that was comparatively less than

these of samples with 30 and 40% water (w/w) content.

The ARC results of flowerpot tip are presented in Figures 4.10-

4.12. Compared to DSC results, adiabatic calorimeter showed interesting

results for flowerpot tip mixture and mixture with 40% water content. The

neat sample showed an onset of 170.62°C but the sample with 40% water

content showed the lowest onset point of 95.71°C. The time required for

exothermic initiations under dynamic heat-wait-seek pattern was 475 minutes.

Once the exothermic initiation occurred, in less than 100 minutes, the

temperature rose to 210°C. Both the samples underwent multiple exotherm at

two different temperatures as shown in Figure 4.10.

Figure 4.10 Time vs. Temperature plot for flowerpot tip mixture and

flowerpot tip mixture with 40% of water content

(Flowerpot tip ( ), Flowerpot tip with 40% water ( ))

400 600 800 1000 1200 1400 1600 1800 200080

100

120

140

160

180

200

220

240

260

280

300

320

340

360

Time, (min)

87

The self heat rate pattern observed for flowerpot tip mixture

presented in Figure 4.11 showed the vigour of exothermic behavior of

these samples. The neat sample showed a maximum self heat rate of

143.48 °C min-1 at 339°C. The sample with 40% water (w/w) content

released a maximum heat of 4.19 °C min-1at 147.65 °C.

Figure 4.11 Temperature vs. Self heat rate plot for flowerpot tip mixture

and flowerpot tip mixture with 40% of water content


The pressure release profiles for mixtures are presented in

Figure 4.12. The self heating exothermic process for neat sample and sample

with 40% water (w/w) containing samples accompanied a maximum pressure

release of 32, and 20 bar respectively.

80 100 120 140 160 180 200 220 240 260 280 300 320 340 3600.01

0.1

1

10

100

Temperature, (°C)

88

Figure 4.12 Temperature vs. Pressure plot for flowerpot tip mixture and

flowerpot tip mixture with 40% of water content


4.3.4 Thermo Kinetics of Ground Spinner Tip Mixtures

Thermo kinetics for the thermal behavior of ground spinner tip for

dynamic heat-wait-search, and iso-ageing ARC tests were determined. The

thermo kinetic data are presented in Tables 4.2 and 4.4 respectively. The

reliability of Arrhenius kinetics has been checked by predicting self heat rate

by employing Arrhenius equation.

The activation energy was calculated for fireworks tip with 0%,

10%, 20%, 30%, and 40% water (w/w) are 81.04, 266.35, 164.94, 196.09, and

164.94 kJ mol-1 respectively. The pre-exponential factor was evaluated as

6.78 x 1020, 3.57 x 1023, 1.47 x 1013, 6.68 x 1021, and 9.17 x 1020 min-1. Thus

the Arrhenius rate law for thermal decomposition of ground spinner tip for

different quantities of water (w/w) 0%, 10%, 20%, 30%, and 40% can be

given as Equations (4.16) to (4.20) respectively.

80 100 120 140 160 180 200 220 240 260 280 300 320 340 3600

5

10

15

20

25

30

Temperature, (°C)

89

k = 6.78 x 1020 exp (-81.04/RT) (4.16)

k = 3.57 x 1023 exp (-266.35/RT) (4.17)

k = 1.47 x 1013 exp (-164.94/RT) (4.18)

k = 6.68 x 1021 exp (-196.09/RT) (4.19)

k = 9.17 x 1020 exp (-164.94/RT) (4.20)


obtained heat rates in Figure 4.13 and they did show a good agreement

between the two.

Figure 4.13 Experimental and predicted self heat rates of thermal

decomposition of ground spinner tip mixture with different

% of water

(0% ( ), 10% ( ), 20% ( ), 30%( ), 40%( ), Predicted( ))

80 100 120 140 160 180 200 220 240 260 280 300 320 340 360

0.1

1

10

Temperature, (°C)

91

4.3.5 Thermo Kinetics of Flowerpot Tip Mixtures

Thermo kinetics for the thermal behavior of flowerpot tip for

dynamic heat-wait-search, and iso-ageing ARC tests were determined. The

thermo kinetic data are presented in Table 4.3 and 4.5. The reliability of

Arrhenius kinetics has been checked by predicting self heat rate.

The activation energy was calculated for flowerpot tip mixture and

flowerpot tip mixture with 40% water (w/w) are 171.99 and 167.54 kJ mol-1

respectively. The pre-exponential factor was evaluated as 3.11 x 1014 and

2.35 x 1021 min-1 respectively. Thus the Arrhenius rate law for thermal

decomposition of flowerpot tip mixture and mixture with 40% water (w/w)

can be given as Equations (4.21) and (4.22) respectively.

k = 3.11 x 1014 exp (-171.99 / RT) (4.21)

k = 2.35 x 1021 exp (-167.54 / RT) (4.22)


obtained heat rates in Figure 4.14 and they did a good agreement between the

two. For kinetic treatment, the initial portion of exotherm alone is sufficient;

since the interest lies in knowing the Arrhenius constants for the onset of

exothermic activity and not for the entire exothermic region. Furthermore, as

the reaction progresses, complexity may arise due to onset of secondary

decompositions.

92

Figure 4.14 Experimental and predicted self heat rate plot for flowerpot

tip mixture

(Flowerpot tip ( ), Flowerpot tip with 40% water ( ),

Predicted ( ))

80 100 120 140 160 180 200 220 240 260 280 300 320 340 3600.01

0.1

1

10

100

1000

Temperature, (°C)

94

4.3.6 ARC Iso-Ageing Studies for Ground Spinner Tip Mixture

The ARC outputs for ground spinner tip mixture containing 40%

water (w/w) were subjected to iso-ageing test in ARC in the temperature

range of 40-75°C (Figures 4.15-4.18). The temperature range chosen here was

based on the dynamic ARC results presented in the previous section and

involvement of these mixtures in frequent accidents in the industry.

Figure 4.15 Time vs. Temperature plot for ground spinner tip mixture

at various iso-ageing temperatures

(40°C ( ), 47.5°C ( ), 50°C ( ), 55°C ( ), 60°C ( ), 65°C

( ), 75°C ( ))

A simple view of the time versus temperature profile shown in

Figure 4.15 at various iso-ageing temperatures confirmed the vigourous

thermal behavior of the mixtures. Irrespective of the iso-ageing (thermal

history) conditions all the samples show a single sharp rise to a final

temperature within a short duration from the onset point. The onset point for

0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000

40

60

80

100

120

140

160

180

200

220

240

260

280

Time, (min)

95

the thermal event was dependent on the initial iso-ageing temperature. Lowest

onset of 40°C was recorded for the mixtures. The time needed to initiate the

event depended on the initial iso-ageing temperature. At the lower iso-ageing

temperature (40°C), longer time was taken to initiate the event.

Thus, this study, perhaps for the first time, could successfully

correlate the accident occurring conditions. More or less close to ambient

conditions considered here exist in the southern districts of Tamilnadu, India,

where a major firework manufacturing is carried on.

Figure 4.16 Self heat rate plot for ground spinner tip mixture at various

iso-ageing temperatures

(40°C ( ), 47.5°C ( ), 50°C ( ), 55°C ( ), 60°C ( ), 65°C

( ), 75°C ( ))

The self heat rate plots presented in Figure 4.16 depict the vigour of

the exothermic event. Interestingly the experimental runs at lower iso-ageing

temperatures (although too long to initiate the event) showed maximum self

heat release rates i.e. 40°C is 37°C min-1, 47.5°C is 30°C min-1 and 50°C is

40 60 80 100 120 140 160 180 200 220 240 260 280

0.1

1

10

Temperature, (°C)

96

25°C min-1. The self heat rate release curves also showed multiple steps in

their pattern; the first trend from onset to maximum heat release rate followed

by a drop at the temperature range 100 - 140°C, the second slower exothermic

activity picking up from 110 - 120°C to reach a final temperature range of

185- 260°C. This behavior was a clear indication of operation of two different

reaction mechanisms for the exothermic events.



between the two.

Figure 4.17 Experimental and predicted self heat rates plot for ground

spinner tip mixture at various iso-ageing temperatures

(40°C ( ), 47.5°C ( ), 50°C ( ), 55°C ( ), 60°C ( ), 65°C

( ), 75°C ( ), Predicted ( ))

40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120

0.1

1

10

Temperature, (°C)

97

Figure 4.18 confirmed the pressure release during the exothermic

events. In fireworks industry, the unused water mixed with ground spinner tip

mixture was left overnight at the end of the day for next day use. Explosive

accidents were reported after shutting down the factory. The iso-ageing

studies could be correlated to blast accidents in fireworks industry involving

ground spinner tip mixture.

Figure 4.18 Temperature vs. Pressure plot for ground spinner tip

mixture at various iso-ageing temperatures

(40°C ( ), 47.5°C ( ), 50°C ( ), 55°C ( ), 60°C ( ), 65°C

( ), 75°C ( ))

40 60 80 100 120 140 160 180 200 220 240 260 280-5

0

5

10

15

20

25

30

35

40

45

50

55

60

65

Temperature, (°C)

99

4.3.7 ARC Iso-Ageing Studies for Flowerpot Tip Mixture

The ARC outputs for flowerpot tip mixture containing 40% water

(w/w) were subjected to iso-ageing in the temperature range of 40 °C

(Figures 4.19-4.21). A simple view of the time versus temperature profile

shown in Figure 4.19 at iso-ageing temperature confirmed the vigourous

thermal behavior of the mixtures.

Figure 4.19 Time vs. Temperature plot for flowerpot tip mixture with

40% water at 40°C iso-ageing temperature

The self heat rate plots presented in Figure 4.20 depict the vigour of

the exothermic event. Interestingly the experimental runs at 40°C iso-ageing

temperatures (although too long to initiate the event) showed maximum self

heat release rates of 30.62 °C min-1. The self heat rate release curves also

showed multiple steps in their pattern; the first trend from onset to maximum

3580 3600 3620 3640 3660 3680 3700 3720

40

60

80

100

120

140

160

180

200

220

240

Time, (min)

100

heat release rate followed by a drop at the temperature range 80-120°C, the

second slower exothermic activity picking up from 120-220°C to reach a final

temperature range of 220-230°C. This behaviour was a clear indication of

operation of two different reaction mechanisms for the exothermic events.

Pressure profile in Figure 4.20 confirmed the pressure release during the

exothermic events.

Figure 4.20 Self heat rate plot and pressure plot for flowerpot tip

mixture with 40% water at 40°C iso-ageing temperature

(Self heat rate ( ), Pressure ( ))

40 60 80 100 120 140 160 180 200 220 240

0.1

1

10

100

Temperature, (°C)

0

5

10

15

20

25

30

35

101

Figure 4.21 Experimental and predicted self heat rate plot for flowerpot

tip mixture with 40% water at 40°C iso-ageing temperature

(Self heat rate ( ), Predicted ( )



between the two. Only the initial portion of the exothermic onset is

considered for prediction of the exothermic behaviour of the mixture.

The ARC studies confirmed the vigour of water induced

exothermic behavior of flowerpot tip mixtures.

40 60 80 100 120 140 160 180 200 220 240

0.1

1

10

Temperature, (°C)

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4.4 PROCESS SAFETY

4.4.1 ARC Results and their Impact on Process Safety of Cracker

Mixtures

Fireworks mixtures are vulnerable to thermal hazards. ARC data

are used for determining the ceiling temperature for processing, handling and

transportation of hazardous materials. The practice adopted is that the

process/handling temperature should be 100°C below the onset temperature

observed in ARC. This rule has been in practice in process chemical industry

for safe and successful operation of process plants, storage systems and

transportation. On these considerations, in case of atom bomb cracker,

Chinese fire cracker and Palm leaf cracker the ceiling temperature should

never exceed 175°C, 195°C, and 191°C respectively. Although there is no

possibility of reaching this temperature during normal mixing and packing

process of these mixtures, the chances of thermal explosions cannot be ruled

out. This is because the onset temperature can be achieved under situations

like heat radiation from neighbouring area or ignition from unknown sources,

subjecting these mixtures to impact (dropping the mixture containing boxes

leading to impact stimulus) and friction (dragging the mixture compositions

leading to the generation of sparks as a result of friction stimulus). During

such abnormal situations the cracker mixture is vulnerable to hazard.

Further, impact and friction sensitivity can also lead to triggering of

explosive decompositions. As of now, there is no direct correlation available

between thermal, impact and frictional sensitivity, either to predict one or the

other or to predict which of these forces can come together to trigger a

thermal explosion. We hypothesize that impact or frictional stimulus onsets

the thermal stimuli for the fireworks mixture to undergo thermal explosion.

Under severe impact or friction stimuli, thermal stimuli can occur

104

immediately, and this can lead to a catastrophic thermal explosion.

Irrespective of the nature of the stimuli, explosion occurs through thermal

mechanism only. This means that, for these mixtures, the impact or any other

stimulus can only initiate the thermal mechanism by providing the minimum

threshold energy needed/necessary to raise the onset temperature observed

experimentally for thermal explosion in ARC. Therefore it is possible to relate

the mechanical form of energy with the threshold energy ( E) observed in the

ARC. ARC study provides a means of suggesting a predictive correlation in

such explosive systems. Nevertheless the degree of explosivity also depends

on other factors such as compactness, particle size and shape and other

environmental conditions.

4.4.2 ARC Results and their Impact on Process Safety of Tip

Mixtures

The ARC studies for fireworks tip mixture confirmed that the

trigger of exothermic activity could be possible at ambient conditions. The

fireworks tip mixture is involved in triggering accidents often in fireworks

industry, when the remaining quantities of these mixtures are left over night.

The water in the mixture slowly dries; the exothermic activity induced during

drying of water appears to be responsible for accidents. Most of the fireworks

industry accidents (reported in media) have occurred either late at night or

early in the morning. Therefore, from this study the following precautions are

given to the fireworks industry.

The unutilized fireworks tip mixture should be appropriately

disposed off at the end of each day.

During the manufacture of fireworks, it should be ensured that

the ambient temperature should be less than 30°C.

105

Effect of impact and friction stimulus should be analysed, as

these could trigger accidents.

As ARC studies presented here have shown such acts have the

potential to initiate exothermic activity at ambient conditions. For the first

time the reasons for accident triggering at closer to ambient conditions and the

culprit mixture have been identified.

4.5 SUMMARY

ARC studies of the atom bomb cracker, Chinese cracker and palm

leaf cracker have revealed that all the above mixtures are susceptible to

thermal decomposition. The thermal decomposition contributes to substantial

rise in system pressure. The onset temperature for explosive decomposition

observed in accelerating rate calorimeter is however the minimum

temperature for triggering an accident. In practice this temperature can be

attained by thermal and mechanical stimulus. Therefore these mixtures are to

be handled carefully. The thermo kinetic studies revealed that the mixtures

follow first order Arrhenius Kinetics and the kinetic data are validated by

comparing the predicted self heat rates with the experimental data.

The sharp and sudden rise in temperature shows the

vulnerability of these mixtures to undergo violent

decomposition. The discontinuity in both the self heat rate plot

and time versus temperature plot is indicative of multiple

exothermic activities.

The self heat rate plot shows a maximum heat release rate of

1088.1°C min-1 at 320°C. The observed large heat release rate

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confirms the vigour of exothermic explosive process of atom

bomb cracker mixture.

One gram of atom bomb cracker mixture can contribute to a

peak pressure rise of 25.9 bar at 342.3°C.

The ARC data showed that the fireworks mixture

decomposition process under adiabatic condition was vigourous

and therefore dangerous. Although explosive potential is

required for it to be a cracker, the same property can be

dangerous if the event occurs during handling, manufacturing,

storage or transport.

Both Chinese and palm-leaf crackers show delayed onset

temperatures of 295°C and 290°C respectively as compared to

the atom bomb cracker mixture. The delay in initiation of the

exothermic activity can be related to their composition,

especially the quantity of the oxidiser KNO3.

While the percentage of KNO3 in palm-leaf is less than that of

Chinese cracker, early onset is perhaps due to the presence of

another oxidiser Ba(NO3)2 and a large quantity of different

grades of aluminium

The time versus temperature plot shows that the exothermic

activity is sudden and sharp as observed with atom bomb

cracker mixture.

ARC studies of the fireworks tip mixture have revealed that the

above mixture is susceptible to thermal decompositions at ambient conditions.

The thermal decomposition contributes to substantial rise in system pressure.

107

The onset temperature for explosive decomposition observed in accelerating

rate calorimeter is however the minimum temperature for triggering an

accident. In practice this temperature can be attained by thermal and

mechanical stimulus. Therefore, this mixture is to be handled carefully. The

thermo kinetic studies revealed that the mixture follows first order Arrhenius

kinetics and the kinetic data are validated by comparing the predicted self heat

rates with the experimental data.

A second self heating process for the ground spinner tip mixture

started at 220°C and peaked at 320°C with a heating rate of

100°C min-1; the self heating process ended at this point perhaps

due to complete depletion of the samples. The sudden jump in

self heat rate from 10 to 50 °C min-1and finally 100°C min-1 (in

a discontinuous manner) indicated the explosive nature of the

process.

The time vs. temperature plot of ground spinner tip shows two

discontinuous exothermic activities 160-190°C and 220-320°C.

The first self heating rate process proceeds slowly compared to

the second event.

Thus the self heating nature of the ground spinner tip mixture

and its reactive thermal hazards with water necessitated the

study of the behaviour of these samples under iso-ageing

conditions.

The neat mixture showed a maximum self heat rate of 143.48

°C min-1 at 339°C. The sample with 40% water (w/w) content

released a maximum heat of 4.19°C min-1at 147.65 °C.

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The self heating natures of the ground spinner tip mixture and

its reactive thermal hazards with water under iso-ageing

conditions showed a lowest onset temperature of 40°C.

The self heating exothermic process for neat sample and sample

with 40% water (w/w) containing samples accompanied a

maximum pressure release of 32, and 20 bar respectively.

chapter 4 thermal hazard assessment of...

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