Handbook of Solvents - George Wypych - ChemTech - Ventech!

52 George Wypych
downward and horizontal propagation of flame. Flame propagation in these directions generally
requires a higher vapor concentration than it is required for the upward flame propagation
used to determine flammability limits. The flame in flash point determination is at
some distance from the surface where the vapor concentration is at its highest (because vapors
have higher density than air) than exists on the liquid surface thus flush analysis
underestimates concentration of vapor.
An increased vapor pressure typically increases the upper limit of flammability and reduces
the lower limit of flammability. Pressures below atmospheric have little influence on
flammability limits. An increase in temperature increases the evaporation rate and thus decreases
the lower limit of flammability.
There are a few general rules which help in the estimation of flammability limits. In
the case of hydrocarbons, the lower limit can be estimated from simple formula: 6/number
of carbon atoms in molecule; for benzene and its derivatives the formula changes to: 8/number
of carbon atoms. To calculate the upper limits, the number of hydrogen and carbon atoms
is used in calculation.
The lower flammability limit of a mixture can be estimated from Le Chatelier’s Law:
LFL
mix
100
=
φ1φ2 φn
+ + �+
LFL LFL LFL
1
2
n
[2.3.9]
where:
φi fraction of components 1, 2, ..., n
LFLi lower flammability limit of component 1, 2, ..., n
2.3.9 SOURCES OF IGNITION AND AUTOIGNITION TEMPERATURE
Sources of ignition can be divided to mechanical sources (impact, abrasive friction, bearings,
misaligned machine parts, choking or jamming of material, drilling and other maintenance
operations, etc), electrical (broken light, cable break, electric motor, switch gear,
liquid velocity, surface or personal charge, rubbing of different materials, liquid spraying or
jetting, lightning, stray currents, radio frequency), thermal (hot surface, smoking, hot transfer
lines, electric lamps, metal welding, oxidation and chemical reactions, pilot light, arson,
change of pressure, etc.), and chemical (peroxides, polymerization, catalysts, lack of inhibitor,
heat of crystallization, thermite reaction, unstable substances, decomposition reactions).
This long list shows that when making efforts to eliminate ignition sources, it is also
essential to operate at safe concentrations of volatile, flammable materials because of numerous
and highly varied sources of ignition.
The energy required for ignition is determined by the chemical structure of the solvent,
the composition of the flammable mixture, and temperature. The energy of ignition of
hydrocarbons decreases in the order alkanes > alkenes > alkynes (the presence of double or
triple bond decreases the energy energy required for ignition). The energy requirement increases
with an increase in molecular mass and an increase branching. Conjugated structure
generally requires less ignition energy. Substituents increase the required ignition energy in
the following order: mercaptan < hydroxyl < chloride < amine. Ethers and ketones require
higher ignition energy but an aromatic group has little influence. Peroxides require extremely
little energy to ignite.