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 obser o bserved ved to t o be 28.9°C, 28.9°C, 25° 25 °C, 21.75° 21 .75°C, 21.1° 21 .1°C C and 17.55° 17.55 °C, respecti re spectively vely.. 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 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 closedcup 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 T b 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,satfp(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 f raction, γ is the activity coefficient, and P isat 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 us ing 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.
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 Volume of Volume of (mol %) ethanol (ml) 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 Flash Average (mol %) Point (°C) (°C) 28 20 28.9 29.8 40 25 25 20.8 60 21.75 22.7 80 21.1 21.1 17.9 100 17.55 17.2 Ethanol (mol %) 20 40 60 80
Results and Discussion
100
Fire Point (°C) 28.6 31.1 25.9 21.5 23.6 22.4 18.2 17.5
Average (°C) 29.85 25.9 22.55 22.4 17.85
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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 stepby-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. 35
Wilson
C ⁰ 30 , t n 25 i o p h 20 s a l f 15 e r 10 u t x i 5 M
Experimental
0 0
0.5
1
1.5
Mole fraction of ethanol in mixture
Figure 1. Flash point temperature vs. mole fraction of ethanol in water.
Table 3. Percent errors between the experimental and theoretical values. mol Flash point fraction Experimental using the % diff of flash point Wilson ethanol equation 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
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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 8 th 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. 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.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
0.2
0.4
0.6
0.8
1
sat
sat new
T (⁰C)
A12
A21
γ
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
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
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
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
24.915
10.86839
0.599695
0.173103
1
24.915
P1
(mmHg)
P
(mmHg)
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