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Vol. 17, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
Dew Points of Gasoline-Air Mixtures' By D. P. Barnard and R. E. Wilson MASSACHUSETTS INSTITUTE os TECHNOLOGY, CAMBRIDGE?, MASS.
N TWO previous papers213the writers presented the results
760 to 15 mm. it was believed that solutions prepared of an experimental method for determining the minimum at 130 mm. would be very close in composition to those temperatures of complete vaporization of motor fuels in which might be prepared at 15 mm., and that solutions made air mixtures. This method consisted essentially in the meas- under atmospheric pressure would be accurate enough for urement of the vapor pressure of a hydrocarbon mixture, so most practical purposes. It appears, however, that not prepared as to be in equilibrium with the completely vapor- enough work was done to be sure that the difference between ized fuel, and the subsequent calculation of the dew points.4 760 and 130 mm. was only 3" C., and furthermore that, I n all, about fourteen fuels were investigated by this method, while these experiments covered over 80 per cent of the the fuels ranging from a special aviation gasoline to several pressure range, they embraced less than half of the temperasamdes of kerosene. Since then Gruse6 has presented the ture range between 760 and 15 mm. The relative volatilities of the hydrocarbons resuits of a series of direct would, therefore, vary more dew point determinations The discrepancies noted between the dew points obbetween 130 and 15 mm. made by observing the temserved by Gruse and the writers are due to the difference in perature of fogging of a than between 760 and 130 the composition of the equilibrium solution prepared at mm. cooled mirror surrounded atmospheric pressure with one prepared at the low partial In order to determine defby a dry gasoline-air mixpressures corresponding to those prevailing in the maniture of known composition. initely the source of any fold. The vapor pressure method will give correct values Gruse's method gave dew errors present in the equilibif the equilibrium solution is prepared at these low partial rium method, and particpoints 20" to 30" C. higher u l a r l y if e r r o r s existed pressures, but this is experimentally difficult. The direct than had been reported by which were due to too short method of Gruse is simple and accurate. It does not, the present writers for idenhowever, yield all the information given by the vapor a time allowance to obtain tical fuels. A consideration pressure method. true equilibrium, or to exof the direction and magnicessive condensation of light The 85 per cent point is still believed to constitute the tude of probable errors in ends of the vapor when s t o p best single measure of the effective volatility of a fuel from manipulation indicated that the standpoint of manifold distribution and crank-case ping the distillation, experithe discrepancies between ments were conducted on dilution. The dew point may be obtained with accuracy the results of the two b i n a r y m i x t u r e s whose sufficient for most purposes by comparing the 85 per cent methods could not be exv a p o r p r e s s u r e relations plained on that basis. The point with the accompanying Figure 2. were a l r e a d y known. writers have therefore studToluene-acetone- solutions ied the theoretical asDects were employed for this purpose, and it was found that, of both methods in an effort to account for the differences. Since there are no obvious fundamental flaws in the direct while equilibrium solutions prepared a t atmospheric presmethod of Gruse, it was decided to re-examine carefully the sure (60" to 90" C.) gave results closely checking existing basis of the method used by the writers. As a result, it ap- data, they varied 5" to 7" C. from those prepared at 40 mm. peared that the only uncertainty was whether an equilibrium (-15" to -3" C.). This behavior indicated that the method solution prepared a t a high pressure (130 to 760 mm.) had itself was sound, but that the difficulty in the previous dew substantially the same composition as a similar solution point results was probably due to changes in relative volatility prepared a t a low pressure (15 mm.) corresponding to the with temperature. While it might be expected that two dissimilar hydrocarbons, such as toluene and acetone, would partial pressure of the fuel in a combustible mixture. vary more in relative volatility than the similar hydrocarExperlmental bons present in gasolines, the very wide boiling range of the It was recognized at the outset of the work previously latter would tend to accentuate such differences. This is reported that the composition of the equilibrium solution shown by the fact that the ratio of vapor pressure of penmight change with the temperature of preparation, and it tane and decane a t 150"C. is 33 as compared to 66 at 100°C. was therefore attempted to prepare these solutions a t pres- and 140 a t 50" C.6 This indicates that equilibrium solutions sures corresponding to actual manifold conditions. This prepared a t higher temperatures should contain a larger prowas found impractical with the vacuum pumps and cooling portion of lighter hydrocarbons than those made at lower apparatus generally available, and in the case of gasoline temperatures. The former solutions would have higher 130 mm. was the lowest pressure that could be readily reached. vapor pressures and would give correspondingly lower dew Comparative vapor-pressure measurements were made on points than the latter. The followingexperiments were carried equilibrium solutions prepared a t 760 and 130 mm., and out with great care to determine as closely as possible the magit was found that the two curves differed by only 3" C. nitude of this effect with commercial hydrocarbon mixtures. As samples of the fuels previously reported were not As this covered over 80 per cent of the pressure range from available in sufficient quantities for this work, a quantity of 1 Presented before the Division of Petroleum Chemistry at the 68th motor gasoline was procured from a local filling station. TWO Meeting of the American Chemical Society, Ithaca, N. Y., September 8 to 13, equilibrium solutions were made from this fuel, as follows: 1924.
I
J . SOC.Aulomoliwc Eng., 9,313 (1921). Ibid., 12, 287 (1923). 4 Since the foregoing papers were published, i t has come to the writer's attention that this method had been previously used by Rosanoff in studying binary mixtures. THISJOURNAL, 16, 796 (1923). 2
Motor Gasoline
8
Equilibrium solution 1
2 6
Pressure in still Atmospheric 160 mm.
Equilibrium temperature in still 150' C. 970 c .
Wilson and Bahlke, THIS J O U R N A L , 16, 115 (1924).
April, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Owing to the presence of a considerable amount of highly volatile material, it was impractical to prepare the solution a t a pressure lower than 150 mm. The distillation curves for fuel and solutions are given in Figure 1. Two solutions were also made for Socony kerosene. I n this instance their vapor pressures were also determined and the dew points calculated. Equilibrium solution 1 2
Socony Kerosene Equilibrium temperature Pressure in in still still Atmospheric 244' C. 164' C. 40 mm.
429
higher than would be compatible with the observed distribution characteristics of the fuel. On the other hand, in the equilibrium method such contamination would result in a very slow rise in the apparent equilibrium temperature which would indicate a gradual accumulation in the flask of the heavy material. If the solutions were removed and tested /
1
Dew point of 12:l mixture
950 c. 117' C.
Discussion of Results
From these results it is evident that, owing to the variation in relative volatilities with temperature of the various hydrocarbons, the composition of the equilibrium solution may vary markedly with temperature of preparation. This is shown in Figure 1, the early part of the distillation curve of the 150-mm. equilibrium solution for motor gasoline lying approximately 15' C. above that for the solution prepared a t atmospheric pressure. For mixtures of narrow boiling range this effect is small. For motor gasolines, however, the boiling range is upward of 175' C. and the change in relative volatility is large enough to cause the discrepancies noted between the observations of Gruse and those of the writers in which the equilibrium solutions were prepared a t atmospheric pressure. The vapor pressure method used by the writers will give the correct dew point if the equilibrium solution is prepared a t a pressure equal to the partial pressure of the fuel in the air mixture. Such procedure, although entirely possible with high capacity vacuum pumps or special condensing means, is not usually p r a c t i c a ble with ordinary laboratory apparatus. When the dew point only is desired, t h e direct method of Gruse would probably be preferable as beingeasierof manipulation. If, however, it is desired to determine the composition of the initially condensing material or to determine the temperature a t which intermediate percentages of the fuel are vaporized, the l a t t e r m e t h o d , modified to allow a definite percentage of the feed to accumulate in the flask, is indispensable. These intermediate points are probably of more practical importance than the temperature of complete vaporization. For example, the utility of the equilibrium method in determining the starting volatility of a fuel (20 or 25 per cent vaporized) has been described in a previous papers7 A possible difficulty with the direct dew point method is that it may be too sensitive to contamination by traces of heavy fractions. For example, 0.5 per cent of a lubricating oil in the gasoline would probably give R very high dew point"What Constitutes True Volatility?" Proceedings, American Petroleum Institute, Chicago Meeting, December, 1921. 7
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The United States Tariff Commission will resume its investigation of magnesite, confining the investigation to the consideration of differences in costs of production of crude and caustic calcined magnesite.
