Safety
• Determine conditions where fires and /or
explosions can occur.
• Develop estimates for upper/lower
flammability limits in mixtures
• Utilize inerting to prevent fires/explosions.
Combustion/Fire/Explosion
CH 4  2O2  CO2  2H 2O  Energy
rCH 4
  Ea 
 kpCH 4 pO2 , k  A exp

 RT 
Where Does Reaction Occur?
• In gas phase where ignition source, oxygen
and fuel coexist.
• Can be autocatalytic under certain
conditions.
• May not need ignition source if temperature
is high enough.
Types of Reactions
• Slow Oxidation
– Energy can be absorbed by surroundings without
increase in temperature.
• Fire
– Energy released can be dissipated by environment
with an increase in temperature to a stable point.
• Deflagration/Explosion
– Energy released cannot be fully dissipated by
environment and temperature continuously
increases.
Definitions
• Flash Point Temperature
– Enough fuel exists in air to create a flammable
mixture. Will “burn out”.
• Fire Point Temperature
– Enough fuel exists in air to create a sustainable
flammable mixture.
• Flammability Limits
– Volume percent ranges of fuel in air where burning
occurs.
rCH 4
  Ea 
 kpCH 4 pO2 , k  A exp

RT


• LFL Lower Flammability Limit
– Partial pressure of fuel is too low to keep
reaction going
• UFL Upper Flammability Limit
– Partial pressure of oxygen is too low to keep
reaction going
Sources for LFL/UFL
• MSDS sheets where data was obtained
experimentally.
• Mixtures of Fuels
– Can be calculated with known LFL/UFL of all
components
Calculating LFL/UFL of Mixtures
LFL 
UFL 
1
yi
 LFL
i
1
yi
 UFL
i
yi  mole fraction of i on combustable basis
20:80 Hexane/Heptane Liquid at 25 oC
• Assume Liquid is in equilibrium with air in
headspace
• Calculate mole fraction of each component
using Raoult’s Law or suitable model.
• Calculate LFL/UFL of mixture
B
, Tin K
T C
Hexane : A  15.8366, B  2697.55, C  48.78
Heptane : A  15.8737, B  2911.32, C  56.51
ln p*  A 
*
*
pHexane
 151.3 mm Hg , pHeptane
 45.9 mm Hg
0.2 151.3
 0.040, yHeptane  0.048
760
0.040
yHex 
 0.45, yHep  0.55
0.040  0.048
1
LFL 
 1.20%
0.45 0.55

1.20 1.20
1
UFL 
 7.1%
0.45 0.55

7.5
6.7
yMixture  0.040  0.048  0.088  8.8%
yHexane 
Temperature Dependence of LFL/UFL
0.75
LFLT  LFL25 
T  25
H C
0.75
UFLT  UFL25 
T  25
H C
where :T
 kcal 
C , H C 
 : Net Heat of Combustion
 gmole 
 
o
T = 20
oC
LFLHex  1.21, UFLHex  7.49
LFLHep  1.21, UFLHex  6.69
LFLMix  1.21, UFLMix  7.05
yMix  0.54  5.40%
Pressure Effects
UFLP  UFL  20.6log10 P  1
where P is in Megapascals, absolute
Flammability Diagrams
• Flammability Diagrams
• Compression and Ignition
40% Nitrogen
40% Fuel
20% Oxygen
Original Mixture
40% Nitrogen
40% Fuel
20% Oxygen
Dilute with Air
Original Mixture
40% Nitrogen
40% Fuel
20% Oxygen
Dilute with Air
Air Added
Original Fuel
Constructing Flammability Diagram
1. Draw Air Line
Fuel + zO2  CO2 + H2O
2. Enter LFL & UFL
3. Determine z
4. LOC = zLFL
(use data, if available)
UFL
LFL
Constructing Flammability Diagram
5. Add Stoichiometric
Line
6. Get Pure Oxygen LFL
and UFL (if available)
UFL
LFL
LOC
Fuel + zO2  CO2 + H2O
Stoich. 
z
 100
1 z
Constructing Flammability Diagram
7. Construct Curve
LOC
Flammable
Region
Fuel + zO2  CO2 + H2O
Stoich. 
z
 100
1 z
Compression of Gases
 Pf 
T f  Ti  
 Pi 
where :
 1

T f , Ti are final and initial temperatures, absolute
Pf , Pi are final and initial pressures, absolute

Cp
Cv
Acrylic Acid Process
Compressor Section
1.4 1
1.4
5
T f  300  
 475 K  202 oC
1
o
Autoignition Temperature for Propylene  458 C
Safety (MSDS) data for hexane
Physical data
Appearance: colourless liquid
Melting point: -95 C
Boiling point: 69 C
Vapour density: 3 (air = 1)
Vapour pressure: 132 mm Hg at 20
C
Specific gravity: 0.659
Flash point: -10 F
Explosion limits: 1.2% - 7.7%
Autoignition temperature: 453 F