19.2 Flammability properties and terminology
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
19.2 Flammability properties and terminology
There are several properties of flammable materials that are frequently
reported. It should be remembered that most discussions concerning
flammable liquids usually consider air as the oxidant, but oxygen and
fluorine can also be used as oxidants for combustion, and they will result
in very different values.
The flammability limits in air are usually reported as the upper and lower
limits ( in volume percent at a certain temperature, usually 25C ), and
represent the concentration region that the vapour ( liquid HCs can not burn )
must be within to support combustion. Hydrocarbons have a fairly narrow range,
( n-hexane = 1.2 to 7.4 ), whereas hydrogen has a wide range ( 4.0 to 75 ).
The minimum ignition energy is the amount of energy ( usually electrical )
required to ignite the flammable mixture. Some mixtures only require a very
small amount of energy (eg hydrogen = 0.017mJ, acetylene = 0.017mJ ),
whereas others require more (eg methanol = 0.14mJ, n-hexane = 0.29mJ,
diethyl ether = 0.20mJ, acetone = 1.15mJ, dichloromethane = 133mJ @ 88C ),
and some require significant amounts, (eg ammonia = >1000mJ ).
The flash point is the most common measure of flammability today, especially
in transportation of chemicals, mainly because most regulations use the flash
point to define different classes of flammable liquids. The flash point of a
liquid is the temperature at which the liquid will emit sufficient vapours
to ignite when a flame is applied. The test consists of placing the liquid
in a cup and warming it at a prescribed rate, and every few degrees applying
a small flame to the air above the liquid until a "flash" is seen as the
vapours burn. Note that the flame is not applied continuously, but is
provided at prescribed intervals - thus allowing the vapour to accumulate.
There are a range of procedures outlined in the standard methods for
measuring flash point ( ASTM, ISO, IP ) and they have differing cup
dimensions, liquid quantity, headspace volume, rate of heating, stirring
speed, etc., but the most significant distinction is whether the space above
the liquid is enclosed or open. If the space is enclosed, the vapours will be
contained, and so the flash point is several degrees lower than if it is
open. Most regulations specify closed-cup methods, either Pensky-Martens
Closed Cup or Abel Closed Cup. It is important to remember that these methods
are only intended for pure chemicals, if there is water or any other volatile
non-flammable compounds present, their vapours can extinguish or mask the
flash. For used lubricants, this may be partially overcome by using the TAG
open cup procedure - which is slightly more tolerant of non-flammable
vapours. A material can be flammable, but may not have a flash point if other
non-flammable volatile compounds are present. For alkane hydrocarbons, flash
point increases with molecular weight.
There is an older measure, called the fire point, which is the temperature
at which the liquid emits sufficient vapours to sustain combustion. The fire
point is usually several degrees above the flash point for hydrocarbons.
The minimum autoignition temperature is the temperature at which a material
will autoignite when it contacts a surface at that temperature. The procedure
consists of heating a glass flask and squirting small quantities of sample
into it at various temperatures until the vapours autoignite. The only
source of ignition is the heat of the surface. For the smaller hydrocarbons
the autoignition temperature is inversely related to molecular weight, but it
also increases with carbon chain branching. Autoignition temperature also
correlates with gasoline octane ratings ( refer to Gasoline FAQ available in
rec.autos.tech, which lists octane ratings and autoignition temperatures for
a range of hydrocarbons.)
Flash Point Autoignition Flammable Limits
Temperature Lower Upper
( C ) ( C ) ( vol % at 25C)
methane -188 630 5.0 15.0
ethane -135 515 3.0 12.4
propane -104 450 2.1 9.5
n-butane -74 370 1.8 8.4
n-pentane -49 260 1.4 7.8
n-hexane -23 225 1.2 7.4
n-heptane -3 225 1.1 6.7
n-octane 14 220 0.95 6.5
n-nonane 31 205 0.85 -
n-decane 46 210 0.75 5.6
n-dodecane 74 204 0.60 -
n-tetradecane 99 200 0.50 -
Continue to:
My Books
