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

EnergyOHCOOCH 2224 22

RT

EAkpkpr a

OCHCH exp,244

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.

• 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

RT

EAkpkpr a

OCHCH exp,244

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

1

1

i

i

i

i

i

LFLy

LFL

UFLy

UFL

y 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

*

* *

ln ,

: 15.8366, 2697.55, 48.78

: 15.8737, 2911.32, 56.51

151.3 , 45.9

0.2 151.30.040, 0.048

7600.040

0.45, 0.50.040 0.048

Hexane Heptane

Hexane Heptane

Hex Hep

Bp A Tin K

T CHexane A B C

Heptane A B C

p mm Hg p mm Hg

y y

y y

5

11.20%

0.45 0.551.20 1.20

17.1%

0.45 0.557.5 6.7

0.040 0.048 0.088 8.8%Mixture

LFL

UFL

y

Temperature Dependence of LFL/UFL

25

25

0.7525

0.7525

: , :

TC

TC

oC

LFL LFL TH

UFL UFL TH

kcalwhere T C H Net Heat of Combustion

gmole

T = 20 oC

1.21, 7.49

1.21, 6.69

1.21, 7.05

0.54 5.40%

Hex Hex

Hep Hex

Mix Mix

Mix

LFL UFL

LFL UFL

LFL UFL

y

Pressure Effects

absolute ls, Megapascain is P where

PUFLUFLP 1log6.20 10

Flammability Diagrams

• Flammability Diagrams

• Compression and Ignition

40% Nitrogen40% Fuel20% Oxygen

Original Mixture40% Nitrogen40% Fuel20% Oxygen

Dilute with Air

Original Mixture40% Nitrogen40% Fuel20% Oxygen

Dilute with Air

Air Added

Original Fuel

Constructing Flammability Diagram

1. Draw Air Line

2. Enter LFL & UFL

LFL

UFL

3. Determine z4. LOC = zLFL

(use data, if available)

Fuel + zO2 CO2 + H2O

Constructing Flammability Diagram

LFL

UFL

Fuel + zO2 CO2 + H2O

LOC5. Add Stoichiometric

Line6. Get Pure Oxygen LFL

and UFL (if available)

. 1001

zStoich

z

Constructing Flammability DiagramFuel + zO2 CO2 + H2O

LOC . 1001

zStoich

z

7. Construct Curve

FlammableRegion

Compression of Gases1

:

,

,

ff i

i

f i

f i

p

v

PT T

P

where

T T are final and initial temperatures, absolute

P P are final and initial pressures, absolute

C

C

Acrylic Acid ProcessCompressor Section

1.4 1

1.45300 475 202

1

458

of

o

T K C

Autoignition Temperature for Propylene C

Safety (MSDS) data for hexane 

Physical dataAppearance: 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