Anomalies in Flash Points of Liquid Mixtures Melvin Gerstein” and William
B.
Stine
Department of .Tfechanical Engineering, Cniversity of Southern California,Los Angeles, Calif. 90097
Results of analysis and experiments show how a liquid mixture of a flammable hydrocarbon and an inert inhibitor can have no flash point using standard test procedures but still be flammable. This “anomaly” results from the differences between the vapor space mixture within the test apparatus and that resulting from air dilution outside of the apparatus. A simple expression i s derived to describe these phenomena analytically, and the above behavior i s predicted when this expression i s related to flammability limits data. A limited amount of experimental data have been obtained to verify the conclusions. Implications of this finding to product safety are considered
F l a s h point measurements of various types are used as one measure of the flammability of liquid materials. -4lthoug.h the various methods differ in specific details of apparatus and procedure, each method provides a technique for determining the liquid temperature a t which a flammable vapor-air mixture is produced. The flash point is also related t o the lean flammability limit which measures the minimum content of combustible in a combustible-air mixture which will propagate flame (Mullins and Penner, 1969). Allthought h e flash point and flammability techniques do riot produce identical results, flammability limit data can be used t o assist in t h e interpretation of flash point measurements. I n a typical flash point measurement, t h e temperature of the liquid is increased until t h e application of a test flame causes ignition of the vapor-air mixture within a cup containing the liquid aiid the vapor-air mixture. Since the flammable gas phase is approached from the lean side of the flammable zone, further dilution by air of t h e mixture leaving t h e cup or by diffusion of air into t h e cup does not produce a flammable mixture when t h e fluid is below the flash point. A n anomaly seems to exist in t h e case of mixtures of some inhibiting halogenated hydrocarbons (e.g., CCl,) and flammable hydrocarbons and alcohols. Certain mixt,ures of such compounds will not exhibit a flash point in t h e standard performance of the test alt’hough the vapor will ignite above t h e cup on mixing with ambient air or within the cup if the cup is left uncovered and air is permitted to enter. Generally such flames do not propagate t o t h e liquid surface as required by standard procedures, but ignition of the vapor does nevertheless occur. The anomalous ignition of the vapor-air mixture under conditions differing from those specified precisely in standard procedures may not alter t h e validity of the test, but t h e fact t,hat ignition can be observed above a fluid which is presumably below its flash point does create some confusion. It is also important t o understand t h a t some liquids which exhibit no flash point may still explode and/or burn in use. The anomalous behavior of certain mixtures is explained in this paper. h limited amount of experimental data have been obtained t o verify t h e conclusions. KOattempt has been made t o provide precise flash point data. The compounds used were chosen for convenience, not for any practical significance. ,1relatively complete survey of experimental data of dilution flammability limits of mixtures of gases and vapors is contained in Coward and Jones (1952) and Burgoyne and Williams-Leir (1948a). By correlating flammability limit con-
centrations with liquid vapor pressures, Uurgoyne a i d Williams-Leir (1949) were able to predict flash points aiid got good agreement with experimental results in most cases. They also discussed the possibility of using this technique for liquid mixtures and mentioned the possibility of the subject anomaly. Zabetakis (1965) indicated that if bot,h componeiitR of a liquid mixture are flammable, Le Chatelier’s rule may he applied t o t h e vapor mixture above the liquid Lvith the use of Raoult’s law. In this study we analytically describe the flash point test for liquid mixtures and show how the application of the techniques described above predicts the previously described anomalous behavior. Theoretical Analysis
Halogenated compounds such as those nieiitioned previously decrease the flammable range when added to combustibleair mixtures. some critical concentration of halogenated inhibitor, no flame propagation occurs in a typical flammability limit experiment. -4 typical inhibitor effect is illustrated in Figure 1 (see Lewis and von Elbe (1961) for similar curves). Here pi and p i represent t h e partial pressure of inhibit,or and fuel, respect’ively, and intercepts on the abscissa represent the normal lean and rich flammability limits of the fuel. The remainder of the curve represents the narrowing of the flamniable range due to the addition of inhibitor. The maximum of the curve represents the quantity of inhibitor necessary to make inert all possible mixtures of the components represented. Similar curves would be obtained for other combustibles and other inhibitors. Figure 1 can be used to interpret flash point data for t h e combinat’ion of fuel and inhibitor represented. Ahsumea mixture of halogenated inhibitor and combustible. .Issume further that the liquid obeys Raoult’s law and that the vapor is a perfect gas. It is also convenient to assume t h a t the ClausiusClapeyroii equation can be used with constant heat of vaporization. None of these assumptions are required, but they do simplify t h e analysis. Assume a liquid binary mixture where ni and ni are the mole fractions of fuel and inhibitor, respectively, ill the liquid phase. The partial pressures of each constituent, in the vapor phase are pionr
(1)
p ., - p . lo n l
(2)
pr
=
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CRITICAL FLASH LINE NON-FLAMMABLE
1
/
/’
REQION
a m I
-z-
n
C
p, (FUEL) p, (FUEL)
Figure 1. Representative flammability limit curve for mixtures of fuel, halogenated hydrocarbon inhibitor, and air
ENVELOPE
pf (FUEL)
Figure 2. Use of eq 5 to predict flash points. Different liquid mixtures are denoted b y ni,l-ni,4
where pf and pi represent the partial pressures of fuel and inhibitor in the mixed vapor phase, respectively, and pfo and pio represent t h e vapor pressure of pure fuel and inhibitor. The change in pio and pio with temperature at 1 a t m can be calculated using the Clausius-Clapeyron equation
where AH,,f and AHVj are the enthalpy of vaporization of fuel and inhibitor and Tb,f and Tb,i are t’he boiling points of fuel and inhibitor a t atmospheric pressure, respectively, R is the universal gas constant, and T is the temperature. I n this form the enthalpy of vaporization is assumed constant and the pressure is in atmospheres. Combining eq 1-4 provides a relationship between pf and p i which is nonlinear, Le.
Note t h a t all parameters in this expression are predefined by t h e flash point experiment except for pi and pi. As noted in the previous equations, both pr and pi increase as the temperature of the liquid increases in a typical flash point experiment. Each mixture composition produces a curve which can be plotted on the same coordinates as the flammability curve of Figure 1. Whenever the curve of eq 5 crosses t h e flammability envelope an ignition should occur. As a first approximation these iiitersectiori points are close to flash points. They are not exactly flash points because the experimental conditions for 254
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Figure 3. Illustration of how dilution by air can produce a flammable mixture from one which did not exhibit a flash point
measuring flash point and for measuring flammabiIity limit do not coincide exactly. The nature of t h e solution for flash point is illustrated in Figure 2. The flammability envelope of Figure 1 has been reproduced as a solid line while typical curves representing eq 5 for varying values of nl have been shown as dashed lines. The flash points are represented by circles. The curves for different liquid mixtures indicated by ni ni ,*, and ni , 3 would have observable flash points. The curve for mixture ni,4 shows that it has been heated until a state indicated by A in Figure 2 is achieved inside of the test cup without obtaining a flash point. If one ignores a small effect due to slight differences in the diffusion coefficient of the two species, dilution of the mixture represented by A outside of the cup by excess air causes a change in p i and pf but preserves the ratio pi/pf. Simple dilution of t h e vapor mixture at A with air can be represented by a straight line connecting A with the origin. The equation of this line is pi = pf(pi/pf)A
(6)
where the term in parentheses may be evaluated a t state A. A dilution curve illustrating eq 6 is shown returning to the origin from A in Figure 3. When sufficient dilution has occurred so t h a t the mixture is within the flammable envelope, Le., a t point B, burning will occur if an ignition source is present. The burning will occur inside or outside of the flash point cup depending on where the dilution with air occurs. I n a n open-cup apparatus, the anomalous ignition is likely t o occur above the apparatus, while in a closed-cup apparatus, t h e anomalous ignition is likely t o occur near the upper surface inside the cup or in the vicinity of the opening during the test. A straight line through the origin and tangent t o t h e flammability envelope has been drawn in Figure 3 and called the critical flash line. Any liquid mixture for which its vapor passes below this line and to t h e right of the point of tangency upon heating can show anomalous ignition by dilution. Burgoyne and Williams-Leir (1948b) have described this tangent as demarking “limiting safe mixtures” in their study of premixed vapor-gas dilution limits. Experimental data and calculations for mixtures of a commercial kerosene and carbon tetrachloride are shown in Figure 4. The flash points were obtained using a Pensky-Martens Closed Tester per ASTlLl D93-66. Partial pressure data for commercial kerosene from AEG (1968) and carbon tetrachloride from Weast (1968) were used along with eq 1 and 2 t o describe compositions of the saturate vapor. The flamma-
I
determinations of mixtures of halogenated hydrocarbons and combustible liquids. Strictly speaking these anomalous ignitions should not be reported as flash points since the phenomena would not be defined as such by a strict interpretation of procedures. Severtheless, the fact t h a t mixtures which do not exhibit a flash point can burn and/or explode when diluted by air merits serious consideration and suggests t h a t additional studies to define these anomalous ignitions are warranted. Acknowledgment
p,(KEROSENE)-PSIA Figure 4. Results of experiments using mixtures of commercial kerosene and carbon tetrachloride. Per cent by volume of CCI, for mixtures 1-8 i s 0, 2, 4, 6, 8, 10, 1 1 , and 20, respectively
bility envelope is estimated from the flash points as there are no known flammability data for this particular mixture. Liquid compositions of mixtures 1-8 in Figure 4 were 0, 2, 4, 6, 8, 10, 11, and 20% carbon tetrachloride by volume, respectively. It is noted t h a t when testing mixture 8 some burning took place around the test flame a t 140’F and the entire immersion port was in flame a t 160°F, giving the appearance of a bright yellow diffusion flame.
The authors wish to acknowledge Mr. William Rockwell and Professor E. Kent Springer, who brought this problem to their attention. literature Cited
AEG, “Aircraft Engine Data,” Report AEG 215 4/68 (,l5M), 0 4-27, General Electric Aircraft Engine Group, _ . Cincinnati, Ohio, 1968. Burgoyne, J. H., Williams-Leir, G., Fuel, 27, 118 (1948a). Burgoyne, J. H., Williams-Leir, G., Proc. Roy. SOC.,Ser. A , 193, 52.5 fl948h). -_-__ _
_
_
\
Burgoyne, J. H., Williams-Leir, G., Fuel, 2 8 , 145 (1949). Coward, H. F., Jones, G. W., U . S. Bur. M i n e s , Bull., No.503 /,n?n\ (luafi).
Lewis, B., von Elbe, G., “Combustion, Flames and Explosions of Gases,” 2nd ed, p 702, Academic Press, New York, N. Y., 1961. Mullins, B. P., Penner, S. S., “Explosions, Detonations, Flammability and Ignition,” AGARDograph Yo. 31, Pergamon Press, London, 1959. Weast, R. C., Ed., ‘‘Handbook of Chemistry and Physics,” 49th ed, p E-26, Chemical Rubber Publishing Co., Cleveland, Ohio, ~~
1968.
Concluding Remarks
Zabetakis, M. G., U . S . Bur. M i n e s , Bdl., No. 627 (1965).
A mechanism has been suggested t o explain the anomalous ignition phenomena which are observed during flash point
RECEIVED for review February 20, 1973 ACCEPTEDJune 16, 1973
Hypothesis of Relative Advantage Robert F. Hunter Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto 181, Canada
A common experience in life is to correct one problem only to find another has been created. Such experience does not appear to be limited to natural processes, as evidenced in the scientific and engineering community, but is a “way of life.” This paper presents a general treatment of this observation in the form of a hypothesis. Possible techniques of applications are described. Definitions
The terminology advantage and disadvantage appears to be reasonably self-descriptive and is therefore employed in the hypothesis. They can be specifically defined, however, as follows. Advantage. An advantage is a result or property (dependent variable) which is considered equivalent to or better than a specific desired result or criterion. Disadvantage. A disadvantage is a result or property
(dependent variable) which is considered worse than a specific desired result or criterion. The amount of advantage or disadvantage is related to the distance from the criterion level. From these definitions, i t can be seen that the judgment of advantage or disadvantage is relative to a desired result or requirement in a system. This desired result can be either entirely subjective or entirely objective with respect to required performance for a specific function. Hypothesis
When an advantage for a system is implemented, the potential for the production of one or more new disadvantages is created and a new disadvantage will become a reality if a specific critical amount of advantage is obtained. The hypothesis has several implications. For example, it infers that it is possible to obtain a wide distribution for the Ind. Eng. Chem. Prod. Res. Develop., Vol. 12, No. 3, 1973
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