Transcript Lecture 3
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 20.6log10 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