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Chemical Process Technology

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Monday, August 31, 2009

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Lower flammable limit (LFL) or Lower Explosive Limit (LEL) is minimum vapor concentration in air which a mixture will burn when an ignition source is present. Upper flammable limit (UFL) or Upper Explosive Limit (UEL) is maximum vapor concentration in air which a mixture will burn when an ignition source is present. Concentration of mixture of vapor in air below LFL/LEL (too lean) or above UFL/UEL (too rich), mixture will not burn even an ignition source is present. Therefore, flammable range or explosive range is concentrations between LFL/UFL and UFL/UEL.

Component LEL & UEL
LFL/LEL and UFL/UEL for some common gases are indicated in table below. Some of the gases are commonly used as fuel in combustion processes.

Fuel Gas (LFL/LEL)
(%)
(UEL/UFL)
(%)
Acetaldehyde 4 60
Acetone 2.6 12.8
Acetylene 2.5 81
Ammonia 15 28
Arsine 5.1 78
Benzene 1.35 6.65
n-Butane 1.86 8.41
iso-Butane 1.80 8.44
iso-Butene 1.8 9.0
Butylene 1.98 9.65
Carbon Disulfide 1.3 50
Carbon Monoxide 12 75
Cyclohexane 1.3 8
Cyclopropane 2.4 10.4
Dimethyl Ether3.4
27
Diethyl Ether 1.9 36
Ethane 3 12.4
Ethylene 2.75 28.6
Ethylene Oxide
3.6
100
Ethyl Alcohol 3.3 19
Ethyl Chloride 3.8 15.4
Fuel Oil No.1 0.7 5
Hydrogen 4 75
Isobutane 1.8 9.6
Isopropyl Alcohol 2 12
Gasoline 1.4 7.6
Kerosine 0.7 5
Methane 5 15
Methyl Alcohol 6.7 36
Methyl Chloride 10.7 17.4
Methyl Ethyl Ketone 1.8 10
Naphthalene 0.9 5.9
n-Heptane 1.0 6.0
n-Hexane 1.25 7.0
n-Pentene 1.65 7.7
Neopentane 1.38 7.22
Neohexane 1.19 7.58
n-Octane 0.95 3.20
iso-Octane 0.79 5.94
n-Pentane 1.4 7.8
iso-Pentane 1.32 9.16
Propane 2.1 10.1
Propylene 2.0 11.1
Silane 1.5 98
Styrene 1.1 6.1
Toluene 1.27 6.75
Triptane 1.08 6.69
p-Xylene 1.0 6.0

Note : The limits indicated are for component and air at 20oC and atmospheric pressure.

Mixture LFL/LEL & UFL/UEL
A mixture is combustible / flammable within mixture LFL/LEL and UFL/UEL. Common units for both limits is mole (or volume) percent fuel in air [moles fuel/(moles fuel + moles air)]. A mixture LFL/LEL and UFL/UEL limits can be calculated using the equations first proposed by Le Chatelier in 1891 :




Example
A vapor contains of 20 vol% of Methane (C1), 20 vol% of Ethane (C2) and 60 vol% of Propane (C3). Find LEL of this mixture at 20 degC and Atmospheric pressure (101325 kPaA).

LEL
C1 = 5 vol% at 20 degC & 101.325 kPaA
LELC2 = 3 vol% at 20 degC & 101.325 kPaA
LELC3 = 2.1 vol% at 20 degC & 101.325 kPaA

LELMix = 1 / [ 0.2/5 + 0.2 / 3 + 0.6 / 2.1 ]
LELMix = 2.55 vol% at 20 degC & 101.325 kPaA

Above LEL may be linked to MOC as discussed in "Minimum Oxygen Concentration (MOC) for Flare Purge". Vapor mixture flammability & explosivity at Operating P & T discussed in this post.

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posted by Webworm, 2:16 AM

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