Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.3 Basic physical and chemical properties 51
Flash point, o C
100
80
60
40
20
0
alcohols
ketones
-20
0 50 100 150 200 250
Vapor pressure, mm Hg
Figure 2.3.12. Flash point vs. vapor pressure of
solvent.
0
-20 0 20 40 60 80 100 120
Flash point = alog p + b [2.3.8]
Lower flammability limit, %
The constants a and b are specific to each group of solvents. Figure 2.3.12 shows that estimation
of flash point from vapor pressure of solvent is less accurate than its estimation from
boiling point.
2.3.8 FLAMMABILITY LIMITS
Two limits of solvent flammability exist. The lower flammability limit is the minimum concentration
of solvent vapor in oxidizing gas (air) that is capable of propagating a flame
through a homogeneous mixture of the oxidizer and the solvent vapor. Below the lower
flammability limit the mixture is too lean to burn or explode. The upper flammability limit
is the maximum concentration of solvent vapor in an oxidizing gas (air) above which propagation
of flame does not occur. Mixtures with solvent vapor concentrations above the upper
flammability limit are too rich in solvent or too lean in oxidizer to burn or explode.
The flammable limits depend on oxygen concentration, concentration of gases other
than oxygen, the inert gas type and concentration, the size of the equipment, the direction of
flame propagation, and the pressure, temperature, turbulence and composition of the mixture.
The addition of inert gases to the atmosphere containing solvent is frequently used to
reduce the probability of an explosion. It is generally assumed that if the concentration of
oxygen is below 3%, no ignition will occur. The type of inert gas is also important. Carbon
dioxide is more efficient inert gas than nitrogen. The size of equipment matters because of
the uniformity of vapor concentration. A larger head space tends to increase the risk of
inhomogeneity. The cooling effect of the equipment walls influences the evaporation rate
and the vapor temperature and should be used in risk assessment.
The flash point is not the temperature at which the vapor pressure in air equals the
lower flammable limit. Although both parameters have some correspondence there are
large differences between groups of solvents. There is a general tendency for solvents with a
lower flammability limit to have a lower flash point. The flash point determination uses a
7
6
5
4
3
2
1
Flash point, o C
alcohols
aromatic CH
esters
ketones
Figure 2.3.13. Lower flammability limit of solvents vs
their flash points.