Chapter 4 Thermal Hazard Assessment Of Fireworks Mixture -Books Pdf

CHAPTER 4 THERMAL HAZARD ASSESSMENT OF FIREWORKS MIXTURE
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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. 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. Insulation,Pressure Transducer,Control Control Heater. Thermocouple,Thermocouple,Radiant Heater, 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. 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. 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. 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,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,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, 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 C P 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. Incorporating the effects of thermal inertia the absolute. temperature rise is given by, 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, 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. dC d T T C dT,dt dt T T dt,m k C T 4 9, 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 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. 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. 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,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,1500 1600 1700 1800 1900 2000 2100 2200 2300 2400.
Figure 4 3 Temperature rise profile for different types of crackers. Atom bomb cracker Chinese cracker Palm leaf cracker. 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. 280 300 320 340 360 380 400 420,Temperature C, Figure 4 4 Self heat rate profile for different types of crackers. Atom bomb cracker Chinese cracker Palm leaf cracker. 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. 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. 280 300 320 340 360 380 400 420,Temperature C, Figure 4 5 Pressure rise profile for different types of crackers. Atom bomb cracker Chinese cracker Palm leaf cracker. 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 KNO 3. 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. 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.
The heat rates determined were compared with the experimentally. obtained heat rates in Figure 4 6 and they did show a good agreement between. 280 300 320 340 360 380 400 420,Temperature C, Figure 4 6 Self heat rate profile experimental and predicted for. different types crackers,Atom bomb cracker Chinese cracker Palm leaf.

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