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Flash Point Determination of an Ethanol-Water Mixture

Beriña, Robert Lloyd N., Galang, Duane Lemuel Q., Perez, Jose Fernando O.

Department of Chemical Engineering, College of Engineering

University of the Philippines, Diliman, Quezon City, Philippines

Submitted and Received August 17, 2010

Abstract

Flash point is defined as the temperature at which the substance emits sufficient vapor

in order to form a combustible mixture with air, while the fire point is the lowest

temperature at which a substance can sustain a flame for more than five seconds. In

the experiment, the flash points at 20%, 40%, 60%, 80% and 100% ethanol in water

were observed to be 28.9°C, 25°C, 21.75°C, 21.1°C and 17.55°C, respectively. The

theoretical values calculated using the Liaw Model and Wilson Equation were lower

than the experimental values. The percent error ranges from 33.2% to 61.4%, the

highest being exhibited by the pure ethanol solution. It is recommended that the set-up

be improved, i.e. performing in a darker environment as to see the flash point more

clearly and minimize human error.

Keywords: flash point, flame point, open-cup method, Liaw Model, Wilson equation

Introduction

Flammability is an important factor to

consider in developing safe methods for

storing and handling solids and liquids.

Laboratories and industries commonly use

flammable substances. Corresponding

mixtures are used in order to carry out

certain experiments and processes. With

this in mind, it is important to take note of

the physical properties of the substances in

order to avoid any of the hazards

associated with them. Flash point and fire

point are two of those properties. Flash

point is defined as the temperature at which

the substance emits sufficient vapor in order

to form a combustible mixture with air, while

the fire point is the lowest temperature at

which a substance can sustain a flame for

more than five seconds. Usually, the fire

point is a few degrees above the flash point.

Different processes handle mixtures at

certain temperatures and pressures so it is

very important to be mindful of the

flammability properties as it can be used to

assess the risk level associated with each

process. It is important to note that

predicted values for these properties are not

accurate and known values may be specific

for certain types or brands. Thus,

experimental values are favored over

theoretical or predicted values.

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There are two basic methods in

determining the flash point of a certain

substance: the open-cup and the closed-

cup method. In the latter, vapors are

prevented from escaping the container thus

resulting in a flash point greater than the

open-cup method. In the experiment,

however, the open-cup method was used.

Mixtures are much more difficult to work

with because their physical attributes are

not as easy to calculate, even with the use

of the mixing rule. Various correlations from

journals may be used to calculate the flash

point of pure hydrocarbons and mixtures of

hydrocarbons. For pure hydrocarbons, the

flash point id calculated using the equation:

(1)

where Tf is the flash point (in °C) and Tb is

the boiling point (in °C). Another correlation,

for estimating the flash points of organic

compounds and petroleum mixtures

(2)

where Tf and Tb are the flash point and

boiling point, respectively, in Kelvin, and a,

b, c are constants evaluated by nonlinear

regression using the Gauss-Jordan iteration

method. This non-linear exponential

correlation was found to be able to predict

the flash point of substances within an error

margin of 1% when tested with over 1220

compounds.

An alternative method, using the boiling

point temperature and chemical structure of

the substance, uses the following equations:

(3)

(4)

(5)

where C, S, H, X and O are the number of

carbon, sulfur, hydrogen, halogen and

oxygen atoms are present in the compound

and temperatures are in Kelvin.

In determining the flash point of a

mixture, what is being calculated in reality is

temperature at which the saturated vapor

pressure is equal to the lower flammability

limit (LFL) composition of the mixture.

(6)

where Pi,sat

fp(Tf) is the saturated vapor

pressure at the flash point temperature and

P is the ambient pressure. Using Chatelier’s

principle, the relation between the two

components in a mixture is found to be

(7)

Substituting the modified Raoult’s Law

into the equation, we get

(8)

where x is the liquid more fraction, γ is the

activity coefficient, and Pisat is the vapor

pressure. Setting the first component to be

water and the second component ethanol,

all those with a subscript of 1 in equation (8)

can be cancelled out because water is a

non-flammable liquid. The final equation

would then be

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(9)

Psat can be calculated using Antoine’s

equation and the activity coefficients may be

estimated by using equations such as

Margules, van Laar and Wilson.

Methodology

The experiment studied the flash point

property of mixtures of ethanol and water.

The open-cup method was employed for all

mixtures. Materials used in the experiment

include the cup apparatus, Bunsen burner,

taper flame, and a thermocouple.

Five mixtures of ethanol-water were

prepared. The molar concentration of

ethanol in water ranged from 20 to 100%,

with 20% increments. A sample calculation

can be found in the appendix. The 5

mixtures were then maintained in an ice

bath.

The cup was filled with the prepared

concentration of the mixture. The Bunsen

burner was placed below the cup the same

time the taper flame was ignited and placed

directly above the mixture and kept in a

continuous motion. The flash point of the

sample is reached when a large blue flame

appears over the entire sample. The fire

point often soon follows when the entire

sample remains ignited after 5-10 seconds.

After each trial, the cup was thoroughly

cooled before washing and drying. The

same procedure was done for each of the

remaining four concentrations.

Results and Discussion

The volume amounts for water and

ethanol for different molar concentrations

were calculated. The densities of ethanol

and water used were 0.7876 g/mL and 1

g/mL, respectively.

Table 1. Summary of experimental values of

flash point and fire point for different

concentrations of ethanol in water.

Ethanol

(mol %)

Volume of

ethanol (ml)

Volume of

water (ml)

20 44.9115 55.0885

40 68.4943 31.5057

60 83.0266 16.9734

80 92.8796 7.1204

100 100 0

The flash points and fire points for the

different mixtures of ethanol and water are

tabulated below.

Table 2. Summary of experimental values of

flash point and fire point for different

concentrations of ethanol in water.

Ethanol

(mol %)

Flash

Point (°C)

Average

(°C)

20 28

28.9 29.8

40 25 25

60 20.8

21.75 22.7

80 21.1 21.1

100 17.9

17.55 17.2

Ethanol

(mol %)

Fire Point

(°C)

Average

(°C)

20 28.6

29.85 31.1

40 25.9 25.9

60 21.5

22.55 23.6

80 22.4 22.4

100 18.2

17.85 17.5

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In order to get the theoretical value of

the flash point, the Liaw model, which was

discussed in the introduction, was used. For

the calculation of gamma, the Wilson model

was employed. Because of the dependence

of the value of gamma on the temperature,

several iterations must be made until the

value for Psat converges, and the resulting T

would be the theoretical flash point. A step-

by-step calculation for the theoretical flash

points is included in the appendix.

The theoretical and experimental values

of the flash points are plotted in Figue 1. As

seen from the graph, there is a clear

difference between the two. The

experimental values are well above the

theoretical values. More models may be

used but due to the lack of constants and

coefficients, only one model was used for

comparison in the experiment. Also shown,

Table 3 tabulates the percent error between

the two values.

Figure 1. Flash point temperature vs. mole

fraction of ethanol in water.

Table 3. Percent errors between the

experimental and theoretical values.

mol fraction

of ethanol

Experimental flash point

Flash point using the Wilson

equation

% diff

0.2 28.9 21.68920171 33.24603

0.4 25 18.30082892 36.60583

0.6 21.75 16.01772103 35.78711

0.8 21.1 13.65114866 54.56575

1 17.55 10.86839252 61.47742

The e flash point of pure ethanol has a

literature value of 13°C, still far from the

experimental 17.55°C and calculated

10.87°C. this may be due to the inaccuracy

in the used model, or non-accordance to

certain assumptions of the model.

One cause of error for this experiment is

the method of drying the cup. It is important

to wash the cup after each trial and dry it

thoroughly. It is possible that some water is

still present in the cup contributing to

variations in the flash point and fire point

measurement. It should also be noted that

the temperature reading in the

thermocouple may not correspond to the

appearance of the flash point because of its

speed. There is a delay in the

measurement. What it measures is the

temperature below the temperature of the

fuel bath. Errors in filling the flash point cup

are also common problems. Too much

liquids in the cup will result in the test

ignition flame applied too closely to the

surface of the liquid, therefore, obtaining

lower observed flash points. This condition

may be possible in the experiment.

Conclusion

Although the experimental and theoretical

values of the flash points exhibit the same

trend, the difference between two flash

0

5

10

15

20

25

30

35

0 0.5 1 1.5

Mix

ture

flas

h p

oin

t, ⁰

C

Mole fraction of ethanol in mixture

Wilson

Experimental

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points in one specific molar concentration is

too large. The flash point determination

experiment still accounts for some human

error. The experiment may be improved by

having more accurate thermocouples, or by

performing it in a darker environment.

References

Perry, R.H. (2008) ‘Perry’s Chemical Engineers’ Handbook 8th Edition’ USA:

McGraw-Hill

Hristova, M., Tchaoushev, S. ‘Calculation of flash points and flammability limits of

substances and mixtures’ Journal of the

University of Chemical Technology and Metallurgy, 41, 3 (2006) 291-296. <http://www.uctm.edu/journal/j2006-3/04-

Hristova-291-296.pdf>

Hristova, M., Tchaoushev, S. ‘Calculation of flash points and flammability limits of substances and mixtures’ Journal of the

University of Chemical Technology and Metallurgy, 41, 3 (2006) 291-296. <http://www.uctm.edu/journal/j2010-

1/2_Mariana_19-24.pdf>

http://www.ilpi.com/msds/ref/flashpoint.html

Appendix

1. Calculation of volumes for ethanol-water mixture

For 20% ethanol

Let x = volume of ethanol (Assay: 99.5% v/v)

Total volume of mixture = 100 mL

Solving for x, x = 44.91149623 mL ethanol

100-x = 55.08850377 mL water

2. Sample calculation with iteration of flash point temperature using Wilson equation, with x2 = 0.2

i)

ii)

iii)

iv)

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v)

vi)

vii)

viii) Repeat steps iii-vii, setting as

until

3. Table of excel iteration to determine flash point temperature.

x2 P1sat (mmHg) T (⁰C) A12 A21 γ Psat

new (mmHg)

0.2

124.575 38.55683 0.696779 0.182191 2.435552 51.14858

51.14858 22.50701 0.640928 0.177073 2.550146 48.85015

48.85015 21.7297 0.638192 0.176815 2.555977 48.7387

48.7387 21.69121 0.638057 0.176802 2.556266 48.73318

48.73318 21.6893 0.63805 0.176801 2.556281 48.73291

48.73291 21.68921 0.638049 0.176801 2.556281 48.7329

48.7329 21.6892 0.638049 0.176801 2.556281 48.7329

48.7329 21.6892 0.638049 0.176801 2.556281 48.7329

48.7329 21.6892 0.638049 0.176801 2.556281 48.7329

0.4

62.2875 25.89279 0.652812 0.178186 1.545691 40.2975

40.2975 18.52674 0.626891 0.175741 1.566457 39.7633

39.7633 18.30766 0.626117 0.175667 1.56709 39.74725

39.74725 18.30104 0.626093 0.175664 1.567109 39.74676

39.74676 18.30084 0.626093 0.175664 1.567109 39.74675

39.74675 18.30083 0.626093 0.175664 1.567109 39.74675

39.74675 18.30083 0.626093 0.175664 1.567109 39.74675

39.74675 18.30083 0.626093 0.175664 1.567109 39.74675

0.6

41.525 19.02075 0.628637 0.175908 1.198837 34.63775

34.63775 16.06447 0.618174 0.174904 1.202267 34.53893

34.53893 16.01846 0.618011 0.174888 1.202321 34.53738

34.53738 16.01773 0.618008 0.174888 1.202321 34.53735

34.53735 16.01772 0.618008 0.174888 1.202321 34.53735

34.53735 16.01772 0.618008 0.174888 1.202321 34.53735

34.53735 16.01772 0.618008 0.174888 1.202321 34.53735

0.8

31.14375 14.36337 0.612136 0.174319 1.045757 29.78106

29.78106 13.65456 0.609617 0.174074 1.045982 29.77466

29.77466 13.65117 0.609605 0.174073 1.045983 29.77463

29.77463 13.65115 0.609605 0.174073 1.045983 29.77463

29.77463 13.65115 0.609605 0.174073 1.045983 29.77463

1 24.915 10.86839 0.599695 0.173103 1 24.915