FIRES AND EXPLOSION

1. FIRES AND EXPLOSION LECTURE 10

2. Fire Triangle • Fuels: Liquids:gasoline,acetone Solids: plastic,wood dust,fibers Gases: acetylene,propane,hydrogen • Oxidizers: Gases: oxygen,fluorine,chlorine Liquids:hydrogen peroxide,nitric acid Solids: metal peroxides,ammonium nitrite • Ignition sources: Sparks,flames,static electricity,heat

3. Difference between fires and explosions • Rate of energy release • Fires release energy slowly, explosions release energy rapidly • Fires can result from explosions, explosions can result from fires • Analogy example: automobile tire. Compressed air within tire contain energy. If energy is released slowly through nozzle, tire is harmlessly deflated. But if tire ruptures suddenly and all energy within the compressed tire releases rapidly, the result is dangerous explosion

4. Definition • Combustion or fire: chemical reaction which a substance combines with an oxidant and releases energy • Ignition: ignition of a flammable mixture may be caused by flammable mixture coming in contact with a source of ignition with sufficient energy or the gas reaching a temperature high enough to cause the gas to autoignite • Autoignition temperature: a fixed temperature above which adequate energy is available in the environment to provide an ignition source • Flash point: flash point of a liquid is the lowest temperature at which it gives off enough vapor to form an ignitable mixture with air

5. Flammability limits: Vapor-air mixtures will ignite and burn only over a well-specified range of compositions. Mixtures will not burn when composition is lower than the lower flammable limit (LFL). Mixture is also not combustible when it is above the upper flammable limit (UFL). Mixture is flammable only when the composition is between LFL and UFL • Explosion:rapid expansion of gases resulting in a rapidly moving pressure or shock wave. The expansion can be mechanical or can be the result of a rapid chemical reaction • Deflagration: an explosion in which the reaction front moves at a speed less than the speed of sound in the unreacted medium • Detonation: an explosion in which the reaction front moves at a speed greater than the speed of sound in the unreacted medium

7. Flammability characteristics of liquids and vapors Liquids • flash point temperature is one of the major quantities used to characterize the fire and explosion hazard of liquids • Flash points can be estimated for multicomponent mixtures if only one component is flammable and if flash point of the flammable component is known Gases and vapors • Flammability limits for vapors are determined experimentally in a specially designed closed vessel apparatus (see pg 255)

8. Vapor mixtures • Frequently LFL and UFL for mixtures are needed. These mixture limits are computed using the equation: • LFLi is the lower flammable limit for component i • yi is the mole fraction of component I on combustible basis • N is the number of combustible species

9. Example 6.2 What are the LFL and UFL of a gas mixture composed of 0.8% hexane, 2.0% methane, and 0.5% ethylene by volume?

10. Flammibility limit dependence on temperature net heat of combustion (kcal/mole)

11. Flammibility limit dependence on pressure • Pressure has little effect on LFL except at very low pressure (<50 mm Hg absolute)

12. Estimating flammability limits

13. Cst = stoichiometric concentration • Stoichiometric concentration for most organic compounds is determined using the general combustion reaction

15. Example 6.4 Estimate the LFL and UFL for hexane, and compare the calculated limits to the actual values determined experimentally

16. Limiting oxygen concentration and inerting • Explosions and fires can be prevented by reducing the oxygen concentration regardless of the concentration of the fuel • Below the limiting oxygen concentration (LOC) the reaction cannot generate enough energy to heat the entire mixture of gases (including the inert gases) to the extent required for the self-propagation of the flame • LOC=MOC ~ minimum oxygen concentration • This concept is the basis for a common procedure called inerting (Chapter 7)

17. Ignition energy • Minimum ignition energy (MIE) is the minimum energy required to initiate combustion. All flammable materials (including dusts) have MIEs. • MIE depends on specific chemical or mixture, concentration, pressure and temperature • MIE decrease with increase of pressure • MIE of dusts is in general, at energy levels somewhat higher than combustible gases • An increase in nitrogen concentration increases MIE • Many hydrocarbons have MIEs about 0.25mJ • Static discharge by walking across rug = 25mJ • Electrostatic discharges, as a result of fluid flow, also have energy levels exceeding MIE of flammable materials and can provide an ignition source  plant explosion

18. Autoignition • Autoignition temperature (AIT) of vapor=spontaneous ignition temperature (SIT) • Temperature at which the vapor ignites spontaneously from the energy of environment • Depends on concentration of vapor, volume of vapor, pressure of system, presence of catalytic material and flow conditions

19. Auto-oxidation • Process of slow oxidation with accompanying evolution of heat, sometimes leading to autoignition if the energy is not removed from the system • Liquids with relatively low volatility are susceptible to this problem • Liquids with high volatility are less susceptible to autoignition because they self-cool as a result of evaporation

20. Adiabatic compression • Gasoline and air in an automobile cylinder will ignite if the vapors are compressed to an adiabatic temperature that excceeds the autoignition temperature • It is the reason some overheated engines continue to run after the ignition is turned off • Several large accidents have been caused by flammable vapors being sucked into the intake of air compressor – subsequent compression resulted in autoignition

21. The adiabatic temperature increase for an ideal gas is computed from the thermodynamic adiabatic compression equation: Tf is the final absolute temperature Ti is the initial absolute temperature Pf is the final absolute pressure Pi is the initial absolute pressure

22. Example 6.6 What is the final temperature after compressing air over liquid hexane from 14.7 psia to 500 psia if the initial temperature is 100 ̊F? The AIT of hexane is 487 ̊C (from Appendix B) and  for is 1.4

23. Explosions

24. Explosion results from rapid release of energy. Energy release must be sudden enough to cause local accumulation of energy at the site of explosion • Energy is dissipated~formation of pressure wave, projectiles, thermal radiation, acoustic energy • Damage from explosion is caused by dissipating energy

25. If explosion occurs in gas, the energy causes the gas to expand rapidly, forcing back the surrounding gas and initiating a pressure wave that moves rapidly outward from the blast source • Pressure wave contains energy~damage to surroundings • For chemical plants much of the damage from explosions is due to pressure wave • A pressure wave propagating in air is called blast wave because the pressure wave is followed by strong wind • Shock wave or shock front results if the pressure front has an abrupt pressure change~highly explosive material-TNT

26. Detonation and deflagration • The damage effects from an explosion depend highly on whether the explosion results from a detonation or a deflagration • The difference depends on whether the reaction front propagates above or below the speed of sound in the unreacted gases • In some combustion reactions, the reaction front is propagated by a strong pressure wave, which compresses the unreacted mixture in front of the reaction front • This compression occurs rapidly, resulting in an abrupt pressure change or shock in front of the reaction front - This is classified as detonation, resulting in a reaction front and leading shock wave that propagates into the unreacted mixture at or above sonic velocity

27. For deflagration, the energy from the reaction is transferred to the unreacted mixture by heat conductiion and molecular diffusion. • These processes are relatively slow, causing the reaction front to propagate at speed less than sonic velocity