I N D U S T R I A L A N D ENGINEERING CHEMISTRY
September, 1923
963
Calculation of Flash Points for Pure Organic Substances' By Edward Mack, C. E. Boord, and H. N. Barham2 OHIO STATEUNIVERSITY, COLUMBUS, OHIO
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HE work a c c o m Thorton has shown that in a mixture of air and a combustible are Purely they give good results for henplished by Burgess gas or vapor, the minimum explosive mixture contains about twice lene, and with slight modiand Wheeler3 and 0s many oxygen atoms and the maximum mixture about one-half fieation for acetylene, ethylby Thorton4 in attempting a s many oxygen atoms a s are required for complete combustion. ene, Propylene, butylene, to determine the relationI t has seemed worth while to use this rule to calculate flash points carbon dim ship between the limits of for (I number of pure substances. This is done by calculating from sulfide, methyl alcohol and inflammability of gaseous vapor pressure data the temperatures to which the substances in alcohol, and many substances and their physquestion must be brought in order that their saturated vapors will ical and chemical properties furnish mixtures with air which arejust explosive. Unfortunately, Other different pounds* has a direct bearing on the the vapor pressure data are very meager. Flash points calculated Having found the per subject of flash points. In in this way agree well with the observed d u e s . cent by volume of the infact, by making use of the flammable substance in air relationship discovered by Thorton, the writers have been able to calculate the flash which just gives an explosive mixture, the partial pressure points of pure liquid and solid organic compounds when their of the inflammable substance may be calculated. For vapor pressures are known in the region of the flash-point example, in the case of pentane, mentioned in the last paratemperature. Thorton determined the relationship at both graph, the partial pressure would be 1.31 per cent of 760 the upper and lower limits of inflammability. It may be mm., or 9.96 mm. To obtain the flash point of pentane, it is then only necessary to find the temperature to which the stat$d as follows: liquid pentane must be brought in order that its saturated I n mixtures of paraffin hydrocarbons with air, there is twice have a pressure Of 9*96mm* This temperature the volume of inflammable gas at the upper limit of inflamma- vapor bility as in the mixture required for complete combustion; or, may be read off directly from the vapor pressure curve of there are one-half the number of atoms of oxygen as are required pentane. I n the tables given below are listed a number for perfect combustion. At the lower limit of inflammability of flash points in this way. ignition fails when there are twice as many oxygen atoms per molecule of substance as are needed for perfect combustion, and first becomes possible when there is one less than this.
OBSERVED FLASH POINTS When seeking a comparison between the calculated and If M equals the number of molecules of the inflammable observed flash points, one is confronted with the considerable substance and N equals the number of oxygen atoms for disagreement of values obtained by the various flash-point testers complete combustion, then the upper limit mixture is 2 M in common use. Such disagreeN , and the lower limit M (2 N - 1). For example, let us consider methane, which burns to ment is not at all surprising. Aside from the varying degrees carbon dioxide and water according to the equation of purity of the sample to be CHI 202 = COz 2H20 tested, a large part of it is due in Here, M = 1 and N = 4. In the upper-limit mixture, the different cases, to (1) the emtherefore, there are two molecules of methane to every four ployment of different of atoms, or two molecules, of oxygen. The per cent by volume ignition-it by no of methane in such a mixture is given by the expression that an inflammable liquid 2 X 100 per cent where 4.85 is the number of volumes of flash at the same temperature for 2 (2 X 4.85) ' an electric spark, as it will for a air which will hold one volume of oxygen. The result is match flame, or a wick flame; (2) 17 per cent methane, as compared with an observed value to the unsaturated condition of of 14.8. For ethane the calculated and observed per cents the vapor where ignition are 10.5 and 10.7; for propane, 7.6 and 7.35; for butane, in some &&-point testers the 5.9 and 5.7; for pentane, 4.9 and 4.5. vapor may be almost saturated, I n the lower-limit mixture, with which one is more directly in others it is far from saturation. concerned in the calculation of flash points, there are 2 N - 1, A simple apparatus was conor 7 atoms of Oxygen for every molecule of methane. The structed to determine actual flash per cent by volume of methane is given by the expression points, having in mind these two 2 X 100 per cent and equals 5.5 per cent, as compared points. The ap(7 X 4.85) ' 2 paratus is shown in Fig. 1. The with an observed value of 5.6. For ethane the calculated substance to be tested is placed and observed per cents are 3.08 and 3.1; for propane, 2.12 in the large glass test tube, T, -S and 2.17; for butane, 1.62 and 1.55; for pentane, 1.31 and and the platinum points, S, form1.35. While the two formulas 2 M N and M (2 N - 1) ing a spark gap, are adjusted so that they are very near to the 1 Received March 19, 1923. 2 T h e experimental part of this paper is abstracted from the thesis surface Of the liquid. Air which presented by H. N. Barham to the Graduate School of the Ohio State Unihas been dried by passage though versity in partial fulfilment of the requirements for the degree of Master of a tube is allowed Science. to flow slowly over the surface 8 J. Chem. Soc , 99, 2013 (1911). 4 Phil. Mag., 33, 190 (1917). of the liquid, and serves the FIG.1
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964
INDUSTRIAL AND ENGINEERING CHEMIXTRY
double purpose of washing out the oxidation products of the preceding test, and of supplying oxygen. The rate of air flow is not great enough to disturb the liquid surface. The temperature of the substance is regulated so that the flash point can be approached from below, using either a Dewar flask with solid carbon dioxide and acetone for the outside bath, or a beaker of water heated with a Bunsen burner to the proper temperature, and testing a t intervals of one degree or even fractions of a degree in the near neighborhood of the flash point itself. The temperature at which an almost invisible blue flame passes over the surface of the liquid is the flash point. It is best to view the flame against a dark background. With every pure substance the flash point is sharp and distinct, and is reprpducible within quite narrow limits (0.5 degree). Below the minimum explosive mixture the color of the spark is light blue, accompanied by no flash. The color of the spark above the maximum mixture is red (with no flash), the red color possibly being due to a luminescence of carbon set free ,by the partial combustion of the substance near the electrodes, in a small nonspreading zone of oxidation which exists there, Chemical reaction often takes place when the liquid is several degrees below the flash point, but such action is confined strictly to a small region about the spark gap, and the reaction does not propagate itself throughout the whole mixture. For instance, in the case of carbon disulfide, it is easy to see that chemical action is taking place in the spark gap at temperatures of -40" and -35" C., and the odor of sulfur dioxide is very Vtreqg; but no pelf-propagating flame is formed until the temperature has been raised to about -26" C. The manner in which the flash point depends on the nature of the source of ignition is as yet not fulIy understood. In electric spark, which because of its reproducition to employ in
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