TND USTRIAL A N D ENGINEERING CHEMISTRY
796
Vol. 15, No. 8
Direct Determination of Dew Points of Gasoline-Air Mixtures’ By W. A. Gruse MELLON INSTITUTE OF INDUSTRIAL RESEARCH, PITTSBCRGH, PA.
HE significance of the dew points, or tem-
T
peratures of initial condensation, of motor fuels when mixed with air in the proportions employed in internal combustion engines, has been pointed out by Wilson and Barnard293 arid by W i l ~ o n . ~For any one mixture of any one fuel the dew point is the lowest temperature a t which the entire fuel is completely vaporized. Additional heating in the manifold is not only unnecessary, but is undesirable, since it involves a loss in volumetric efficiency. Wilson and Barnard devised an ingenious but somewhat complicated method of determining the dew point of a fuel in any air mixture desired, through measurements of the vapor pressure of the liquid in equilibrium with the completely vaporized fuel. By determining an approximate average molecular weight for the original fuel, they find it possible to calculate from the vapor pressure curve of this equilibrium liquid the temperatures, not only of initial, but also of fractional condensation of the original fuel. The amount of work required renders the method unsuited for any but research purposes, and Wilson and Barnard were prompted to set up an arbitrary relationship for routine use. They found that the average boiling point of the equilibrium liquid was very close to the 85 per cent point on the distillation curve of the original fuel, and that by substracting a constant from this equilibrium boiling point, or from the 85 per cent point, a very close approximation to the dew point of the fuel in a certain mixture and a t a given total pressure could be obtained. This relationship depends, according to these investigators, on the approximate equality of the heats of vaporization of the different paraffin hydrocarbons, determining equal rates of change of vapor pressure with temperature. It may be predicted that the relationship would not hold for aromatic and alcohol blended fuels, except as a matter of chance. The present work is based on a belief in the fundamental significance of the dew point of a gasoline-air mixture, and has the object of devising a direct and more simple means of determining it.
EXPERIMENTAL
.
Much time and effort were spent in studying what might be called a ‘‘static” method of determining dew points. A weighed amount of gasoline was introduced into a vessel of known volume, containing dry air a t a known temperature and pressure. After allowing time for the vaporization and diffusion of the gasoline, a small metallic mirror set in the wall of a metal test tube was cooled by evaporating a suitable liquid from the test tube until dew was observed on the surface of the mirror, the temperature of the mirror being known from a thermometer in the evaporating liquid. First a glass flask and then a metal vessel with glass windows, set in an air bath, 1 Presented before the Division of Petroleum Chemistry at the 65th Meeting of the American Chemical Society, New Haven, Conn., April 2 to 7, 1923. * THISJOURNAL,13, 906 (1921). 8 J. SOC.Automotive Eng., la, 287 (1923). 4 Ibzd., 10, 6 (1922).
.
were employed. Several months’ experience with this method brought out a sufficient number of difficulties to cause its abandonment. The alternate procedure was then tried-of preparing a stream of mixed air and vaporized gasoline, the comDosition of which could bekept constant and predetermined approximately, and blowing the mixture against a metallic mirror cooled by the method already described. The temperature of appearance of fog on the mirror and of its disappearance could then be observed. At the end of the experiment the composition was calculated from the total volumes of gasoline and air used. This method proved to be reasonably accurate.
A method has been devised for the direct determination of the dew points of gasoline-air mixtures in the proportions required for in[ernal combustion engines. The method consists in blowing a fuel mixture of known composition against an internally cooled metallic mirror and observing the temperature of dew formation. A comparison of the results obtaihed with those deduced from the vapor pressures of the equilibrium liquids prepared from the gasoline shows that the results directly obtained are from 15 to 20 degrees higher than those obtained indirectly. The method is suggested as suitable for the comparison and evaluation of motor fuels.
APPARATUS-The apparatus is shown in Fig. 1. Gasoline is fed from a separatory funnel, A , through a needle valve, B ; drops fall regularly from a copper wire, C, and pass through tube X onto disks, E, made of 80-mesh copper wire gauze. Here they are met by a countercurrent stream of air, which has passed through the pressure regulator, R (a mercury trap device); a reservoir bottle, P , which equalizes the flow of air; the capillary flowmeter, 0, the wet displacement meter, N;6 and the warming coils, S (about 20 feet of thin metal tubing), which are immersed in the oil bath, L,in which the main apparatus is mounted. The temperature of the oil bath is so regulated as to effect very rapid vaporization of the gasoline; if any remains unvaporized, it will be detected in bottle M a t the end of the runs. Failure to vaporize all the gasoline would require a repetition of the experiment a t a higher bath temperature. Vaporization is entirely satisfactory with proper conditions of bath temperature and air supply. In the upper part of the vaporizing tube, D,above the end of tube X,are set three more gauze disks to prevent entrainment of unvaporized gasoline; they are entirely effective for this purpose. As an additional precaution, however, bottle M‘ is provided to receive any entrained gasoline carried through faulty operation of the device into the mixing chamber, F;a t no time has any liquid been detected in F o r M’. The mixing chamber may have a capacity of from 1to 2 liters; a baffle, Y,is provided, and additional screens may be inserted to cause thorough mixing. The mixture of air and completely vaporized gasoline then passes through tube G, made of Pyrex glass, against a plane mirror, S,soldered into the wall of a copper tube, H . This contains a volatile liquid (chosen for the temperature it is desired to reach) in which are immersed a thermometer, J , graduated in tenths of degrees, and an air-jet tube, K . The explosive mixture is then wasted out of doors. The mirror S is 8 x 15 mm., and is covered with a varnish of lampblack and shellac, except for an area 3 x 5 mm. The small mirror with black edge is easier on the eye than a large mirror, and the arrangement permits a change in case one surface is injured; moreover, the large mirror is easier to solder into the copper test tube than a small one. The mirror is illuminated through a round hole, U, of I-cm. diameter, cut a t the level of the mirror in the wall of the bath, L,and is observed through another hole, 2, about 3 cm. in diameter and a t the same level. The incandescent lamp best suited for the purpose was found to be one with a carbon filament, preferably an old smoked one or one of low power. Two mirrors were used, one of copper, gold plated, and the other of highly polished Monel metal.6 Both tarnished to some extent. The gold mirror, on long use, seemed to suffer a curious change, indicated by the fact that the gasoline deposited 6 This meter may be dispensed with and the volume of air obtained from the capillary flowmeter and the time of running. I t must be removed when dew points lying below the temperature of the meter are to be determined; otherwise, a water dew point is observed. 6 The use of Monel metal was suggested by W. S. James, of the Bureau of Standards.
I N D CSTRIAL A s D E,VGINEERI,VG CHEMISTRY
August, 1923
in relatively large droplets, rather than in a uniform fog-in other words, it did not wet the mirror readily. Washing with the ordinary solvents and polishing with dry rouge did not alter this condition. In order to equalize the flow of gasoline into the vaporizing tube, a pressure greater than atmospheric by 1.5 inches of mercury was applied above it by means of a very slow stream of air which was wasted through tube R. With one bubble of air passing through the mercury every 5 seconds, the amount of gasoline carried away could not have been appreciable. The joint, T , between the funnel and the needle valve, is of the stuffing-box type, the packing being wet with litharge-glycerbl mixture.
by slowing or cutting off the current of air through the cooling liquid. The temperature of disappearance of the fog was taken, and the mean of the two temperatures regarded as the dew point. This procedure was repeated as many times as possible while the measured amount of gasoline was run through the apparatus. In order to avoid eye fatigue, one observer did not make over two or three observations, changing off with another who regulated and recorded the temperature of the mirror. Temperature changes were made very slowly, and it was often possible to observe the appearance and disappearance of fog within two or three tenths of a degree. Large ranges were, however, frequent. A moderately close and an average series are given to show the usual range of observations: D e w on
08 5 ERV ER
FIG. 1
PROCE.DURE-The oil bath was maintained ordinarily a t 80" to 90" C. by means of an electric immersion heater; for fuels containing heavy ends a temperature of 100" to 110" C. was employed. The bath was agitated occasionally by hand, using a stirrer with a handle projecting through the bath cover. Twenty-five or 50 cc. of the gasoline, of known specific gravity, were measured into the container A , after filling the needle valve and the neck of the bulb A to a mark and morking out air bubbles. The mirror was washed with acetone or ether, and polished with dry rouge on cotton wool. The copper test tube was filled with the cooling liquid to the level of the top of the mirror. This liquid soon evaporated down, leaving part of the mirror uncovered. A tendency to deposit dew more readily on the bottom of the mirror then existed, and a slight advantage was gained through a comparison of the two parts of the mirror. After setting the displacement meter a t zero, and recording the time, gasoline w-as allowed to flow into the apparatus a t a rate of 50 drops in 15 seconds. To get different mixtures this rate could be adjusted or the rate of air flow could be changed; both methods were employed. The apparatus was allowed to run 2 minutes, and then air was bubbled very slowly through the cooling liquid (for this work ethylene dichloride, b. p., 82" to 85" C., was used; it is practically noninflammable) until fog or tiny droplets were observed on the face of the mirror. By adjusting the lamp so that the image of one filament lay across the mirror, the observation was made easier. The temperature of dew forination was noted, and the mirror was allowed to warm
Av.
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Av.
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7 2 . 5 7 2 ' 1 1 7 2 . 2 7 2 .' 40 / i 2 . 2 72.3 Avrra~e--72.2' C.
Dewoff
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Dew on
68.6
Dewoff
69.4) 68.3 68.6
U
797
(69.0
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"'O j69.l 69.2 68.7 J 68.7 68.8
69'3 169.4 69.6
Average-68.9'
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68'6 168.6
68.6 C.
To eliminate personal errors and variations in mixture composition all readings were averaged. At the end of the run the air stream through the apparatus was stopped, time noted, meter reading observed, and drainage flasks inspected for signs of incomplete vaporization. The corrected meter reading was then used for calculating the mixture composition. This mixture may be predetermined approximately, but not exactly, and the best way to compare two fuels is to make several determinations for each and plot two curves of dew point against mixture composition. Comparison of definite points can then be made. I n order to determine the accuracy of the method, the dew point of purified toluene in the mixture of 5.5 parts of air to 1 part of hydrocarbon was measured and was found to be 31.5" C. By calculation from the results of Kahlbaum on the vapor pressure of toluene, as given by Landolt-Borhstein, the dew point of a 5.5 :1 air-toluene mixture is 32.3" C. It was also observed that air, approximately saturated with water a t 22" C., showed a dew point of 21.2" C. The fog of water on the mirror is quite different in appearance from that of a hydrocarbon. The following precautions are important: 1-The mirror should be cleaned before each series of observations. 2-The bath temperature should be high enough to vaporize all the gasoline; a few drops of unvaporized material may lower the dew point several degrees. 3-The rate of gasoline feed should be observed frequently and the capillary flowmeter watched for variations in air supply. 4-The outside of the tube G and the surface of the glass of the incandescent lamp bulb should be kept clean. Overheating the bath oil to the extent of vaporizing its light constituents should be avoided, for these condense on the tube G, giving a fogged appearance t o the mirror. 5-A heavy dew appears only some distance below the true dew point, and will not disappear until the temperature is considerably above the true dew point. 6-One observer should not attempt to carry out the whole observation, both for the sake of accuracy in reading temperatures, and to avoid the effect of self-deception.
RESULTS The distillation curves of the three fuels studied are given in Fig. 2. They are commercial fuels, bought in the open market in Pittsburgh. Their differential distillation curves-percentages between 10-degree intervals-are given in Fig. 3. It will be observed from these figures that Purol (Motor) and Good Gulf have approximately the same initial and dry point, but that the former has a higher percentage of heavy naphtha fractions with a sharp increase near the end point,
79s
I S D USTRIAL A-VD ENGINEERING CHEXIXTRY
vol. 15, No. 8
24 0
IO
20
30
40
50
FIG. 2
FIG.
4
60
70
80
SO
100
5
FIG.3
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while the latter has a higher percentage of light fractions, somewhat low, because of the possibly incomplete vaporizaand a corresponding deficiency of heavy ends. Gasoline X is tion just mentioned, but they are not likely to be too high. Through the courtesy of D. P. Barnard and of W. S. James, low in both light and heavy ends and much higher in middle fractions than either of the others.’ Its initial boiling point is samples were obtained of three of the fuels studied by Wilson the same and its dry point 10 degrees lower than those of the and Barnard in their later workb3 The dew points of these fuels, as directly determined, were found to be as follows: other fuels. The curves showing the dew points of these three gasolines Dew Point FUEL Mixture c. for different mixture compositions are given in Fig. 4. It 17 Domestic Aviation . . , . . , , , . . . . . . . , . . . . will be noted that Gasoline X and Good Gulf possess about {E;; 11, approx. the same “effective” volatility over the whole range of mixCooperative Fuel Research Gasoline 12 1 53.7 tures used in internal combustion engines, although they are A (A-2). ... . . . . . . . . . . {I,:, 51.5 15.4 1 Above 76 strikingly different in make-up. Purol (Motor) has a much D................................. 16.4.1 74.5 higher range of dew points, but a flatter curve; its dew point does not rise as rapidly for an enriched mixture as do the DISCUSSION others. The relation of these measurements to those of Wilson and I n Fig. 5 are shown the effects of adding 1 and 2 per cent, respectively, of kerosene to Gasoline X and Good Gulf. Barnard merits discussion. If the curve given by them3 is The experiments were made to determine the sensitiveness of used to find the dew points which the three fuels in the first the dew point to the presence of small amounts of heavy ends. group would have by their method, calchlated from the 85 It will be seen that increases of from 4 to 6 degrees are effected, per cent point, divergences are noted, as follows: differences which can easily be detected. It will be observed ---12 17 - 1 5 1 - 7 85%oPoint Calcd. F y n d Calrd. Found also that 1 per cent of kerosene in Gasoline X seems to cause c. c. c. c. c FUEL Gasoline X . . . . ...*. 167 38 64 33 56 a larger increase than 2 per cent in Good Gulf. This may be Purol (Motor), . , . , . . 186 50 72 45 65 due either to the points on the Good Gulf-kerosene curve Good G u l f . . .... . . . . . 173 42 64 37 56 being too low because of failure to vaporize small traces of the It mill be noted that in general their figures are somewhat kerosene, or to the fact that Gasoline X is high in middle fractions, which, with the added kerosene, tend to run the over 20 degrees lower than those determined directly, and that curve u p rapidly with an enriched mixture. It should be the change in dew point corresponding to a change from a noted that dew points determined by this method might be 12:l to a 15:l mixture is of the order of 7 or 8 degrees as measured directly, while Wilson and Barnard found a uniform 7 This gasoline was bought as a “6&-70” product, b u t was found t o have difference of 4 to 5 degrees for a number of different fuels. a BaumC gravity of 60 8’.
1
INDUSTRIAL A N D SNGINEERING CHEMISTRY
August, 1923
The differences are larger than the degree of accuracy indicated by Wilson and Barnard. A comparison of the figures for the second group of fuels, studied directly by both methods, shows that direct determination gives dew points in all cases higher than those determined from the equilibrium mixture, although the differences are not quite as consistent as those shown by the first group of fuels. -DEW
Indirect Domestic Aviation.
.........
Cooperative Fuel Research Gasoline A (A-2)
Mixture
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17 11 53.7 51.5
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Above 75
their vapor pressure determinations too high, might account for the discrepancy. The following features should be observed concerning the method here described: 1-The apparatus can be constructed in the ordinary laboratory and machine shop. 2-With a little practice the dew points may be read with fair accuracy and reproduciblity. 3-It is believed sufficiently direct to be free of large errors. 4-It applies to volatile fuels of any nature.
The method is offered tentatively as suitable for the direct determination of the “effective” volatility of motor fuels, with the idea that it may be useful in studying specifications and blending operations and for the control of distillation tests and other methods of evaluation.
74.5
ACKNOWLEDGMENT
It may be observed that the range of temperature between the dew points a t 12:l and 15:l varies considerably. ’ The possible errors of the method employed by the author are, as has been mentioned, such as to make the results obtained more probably low than high; some consistent error in the work of Wilson and Barnard, such as errors in the preparation of the equilibrium solution, which would tend to make
799
The author wishes to express his gratefulness for the helpful suggestions received from W. F. Faragher, Senior Fellow, Petroleum Fellowship, Mellon Institute, and also to S. P. Marley for assistance in the construction of the apparatus used, and to S. P. Marley and R. W. Henry, for assistance in making observations.
Determination of Phenols in Coal-Tar Oils and Crude Carbolic Acid’ By J. Bennett Hill THEBARRETT Co., 40 RECTORST., NEWYORK,N. Y.
of steam distillation of the The various methods for defermination of phenols or tar acids in original oil, a somewhat great many methods coal-tar oils, such as crude carbolic acid, dip oil, coal-tar flotation elaborate procedure for conhave been advocated oils, etc., have been investigated, special attention being given to centrating the phenols in for determination of actual the contraction method of Weiss, in which the contraction of the the steam distillate into a phenols or tar acids in coaloil on extraction with caustic soda is taken as the per cent phenols. small quantity of benzene, tar oils, such as creosote oil, The claim that this method gives high results has been shown to be and final measurement by dip oil, etc., and in crude without foundation. The liberation methods, in which the tar acids shaking with caustic soda carbolic acid. Where only are extracted from the oil with caustic soda, liberated from the solution and measuring a rough determination has carbolate with acid and measured, give high results when carefully the increase in volume. been required, almost any run. multiplying the result by of these methods is suita factor empirically deterable. A demand for a greater accuracy of determination and closer check between mined, which happens to be the reciprocal of the specific producer, and consumer is, however, frequently made, and gravity of the phenols. Among the liberation methods are included those of it was to meet this demand that this investigation of some Weiss2 (Test H-12), Allen,4 and most of the methods in use by of the existing methods was made. The successful methods for determination of phenols in various industrial laboratories. The methods in general oils and crude acids are all based on their solubility in soda differ only in detail, the liberation method of Weiss differing and their extraction from the oils with a solution of sodium hy- also from the others in that the phenols after liberation are dissolved in refined coal-tar naphtha before measurement. droxide. They may be divided into two main classesIt has been generally believed that the contraction method first, those which use the contraction in the volume of the oil or the increase in the volume of the aqueous soda as the gave results higher than the correct values, and the liberameasure of the phenols; and second, those which liberate the tion methods results lower than the actual but much closer phenols from the carbolate solution obtained and measure the than those obtained by the contraction methods, as is noted volume of liberated acids. The first class may be called the by Weiss in his description of the two methods. The author’s “contraction” methods and the second class the “liberation” experience has failed to bear out this impression, however, methods. Among the detailed methods belonging to the and it was therefore hoped that the present work would throw first class may be mentioned those of Weiss (Test H-11)2 real light on the relative accuracies of the various methods. Since the publication of the contraction and liberation and of C h a ~ i n .Weiss ~ uses a simple repeated extraction of the distilled oil in a specially graduated separatory funnel methods of Weiss, he has found it advisable to make a few and takes the contraction in volume of the oil as the volume changes in the details of their operation, which he has not of phenols contained in it. The method of Chapin consists published. Thus, in the contraction method (H-11) where repeated extraction with 50 cc. of 10 per cent soda is recomReceived March 16, 1923. mended, it has been found simpler and safer to make in every * THISJOURNAL, 10, 913 (1918).
ROM time to time a
F
8
Bzw. Anzmul I n d u s t r y , Bull, 107.
4
Allen, “Commercial Organic Analysis,” Vol. 111, p . 375 (1909).
