Flammability issues of mixtures:
If you hold a match over pure MeOH in air a fire is likely because the MeOH
vapor over the liquid is within the flammability limits. Looking at the other end if you
have 99 per cent water - 1 per cent MeOH by weight and you hold a match over the
liquid in air what happens? Nothing I expect. Question is what happens between 1% and
100% MeOH. Adding to the problem are the different evaporation rates of the MeOH and
water....
The following values were used in the calculation,
For Methanol:
Flash point = 12.22 °C
Upper Flammability Temperature = 43 °C
Lower Flammability Temperature = 7 °C
Upper Flammability Limit = 36.5% (by volume)
Lower Flammability Limit = 6 % (by volume)
Ignition Temperature = 350 °C
(Source: The Methanol Institute, Washington, DC 20006 - http://www.methanol.org)
The Vapor-Liquid Equilibrium data for Methanol – Water binary is given by
VLE of Methanol-Water Binary
Methanol Fraction in Vapor
1.2
1
0.8
0.6
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1
1.2
Methanol Fraction in Liquid
(Source: Vapor-Liquid Equilibria calculation program for Methanol – Water system by
Shuzo Ohe - http://www.s-ohe.com/Methanol_Water_cal.html)
The Vapor Pressure vs. Temperature plot for methanol is
(Source: Vapor Pressure
ohe.com/methanol.html)
Data
for
Methanol,
Shuzo
Ohe
–
http://www.s-
The VLE data of Methanol – Water Binary was fitted to the following equation,
y = −16.695 x 6 + 58.50 x 5 − 82.079 x 4 + 59.404 x 3 − 23.978 x 2 + 5.824 x + 0.133
where ‘y’ is the mole fraction of methanol in the vapor phase and ‘x’ is the mole fraction
of methanol in the liquid phase.
Similarly, the Vapor Pressure vs. Temperature data of methanol was also fitted to the
following equation,
V .P = 41.52e 0.0427T
where ‘V.P’ is the vapor pressure of methanol at temperature ‘T’.
Using the VLE data and the Vapor Pressure Data, composition of methanol vapor over
the methanol – water binary was calculated for various temperatures and the plot is given
below,
For example, at 30 °C, a composition of 6% Methanol and up would produce a
combustible vapor above its surface and if there is a source near the surface whose
temperature is more than the ignition temperature (350 °C) of methanol, fire is likely.
2. Given a round gallon can of liquid open to air. Start with a 50/50 mix and show what
happens to concentration and volume with time. Also, seal a half full can. What is the
pressure and composition of the vapor at say 150 F.
Let us take a mixture of 2 moles of methanol (64g) and 2 moles of water (36g) in
a round gallon can whose radius is 5 cms. Let us assume that the temperature and
pressure are 65.5 °C (150 F) and 1 atm respectively. The plot mole fraction of methanol
vs. time is given below followed by the total volume of the mixture vs. time,
If the can were sealed, the pressure of the methanol vapor would be the equal to the vapor
pressure of methanol at 150 F (65.5 °C) times the mole fraction of methanol in vapor as
given by the VLE data. The pressure in this case would be 0.8343 atm and composition
of the vapor would be 79.26% methanol and the rest water vapor.