Januarv. 1926
INDUSTRIAL A N D ENGINEERING CHEMISTRY
43
Equilibrium Boiling Points’ By W. A. Whatmough FBIBRNWATCHAvS., NORTH FINCEUZY. LONDON, ENGLAND
T
HE equilibrium boiling points of motor fuels have been investigated by Barnard and Wilson,2 and, independently, by the writer3 in connection with a new system of dry-gas carburation, which involves boiling the fuel before its admixture as dry gas with air. Apparatus
The author’s first apparatus, made in 1919, consisted of a small iron boiler with float feed, which i t was (incorrectly) presumed maintained a constant level in the boiler itself. It was found that distillation of motor fuels took place at a regular rate within fairly narrow limits of temperature, but this varied somewhat when the rate of distillation was changed. A novel form of boiler was devised in 1921 as part of the Keith-Whatmough4 system of dry-gas carburation, and this was found to supply dry gas (unmixed with air) from heavy fuels, under varied running conditions on the road, a t temperatures well below the end point on the Engler distillation curve. T o investigate this phenomenon, a pressure-equalizing tube (P in Figure 1) was fitted between the top of the boiler and its float chamber, resulting in the equilibrium boiling point apparatus shown in Figure 1. This consisted of a boiler built up by welding 5-cm. (2-inch) pipe components (union, cap, and reducing piece) and had a capacity of about 65 cc. at normal level of the float feed chamber. The condenser consisted of a copper coil immersed in water. The function of the pressure-equalizing tube is to prevent variation between levels during vigorous ebullition. Otherwise, the disengagement of gas presses back the boiling liquid and lowers its level relative to the float chamber to an extent varying with the rate of distillation. The writer’s interest centered around the rapidity with which light and heavy motor fuels could be vaporized and their mean boiling range. With fuel fed at the same rate as it was vaporized the re-arranged apparatus gave constant boiling points for each fuel-i. e., though a change in the rate of boiling altered the boiling point temporarily, this always returned to an “equilibrium boiling point.”
Table I illustrates the course of temperature changes during a n equilibrium distillation. The first column gives the times in consecutive minutes, the second column t h e temperature of the gaseous distillate, and cross lines indicate change in rate of heating, which is reflected in amount of distillate shown in the third column. TabIe I-Temperature Changes during E uilibrium Distillation of Taxibus Petrol (No.3 l e t r o l ) ?me Temp. Distillate Time Temp. Distillate Minutes O C. cc. Minutes C. cc. 0 107 Starts 20 127 34 1 116 25 21 128 32 2 119 30 22 128 32 3 122 33 23 127.5 32
10 11 12 13 14 15
128 128 127.5 128 127.5 127
35 36 35 35 33 35
16
125
38
17
126
40
18
127 130
32 30
I9
30 31 32 33 34 35
1:;
36
40
128 128.5 128 127 126 127
36 39 34 34 33
127 127 126.5 127 127
34 34
18
35
34
The apparatus was loaned in 1922 to Ormandy and Craven,5 but their determinations of equilibrium boiling poi& were made quite independently. These investigators, who also determined latent heats at equilibrium boiling points, state
Procedure
The procedure to be followed in using this equilibrium boiling point apparatus depends upon the object in view. If the equilibrium boiling point alone is desired, it is sufficient to regulate heat from a Bunsen burner so that the rate of distillation will commence a t about 30 cc. per minute. When a steady boiling point is reached, increase the rate of boiling to about 40 cc. per minute, and continue until the constant boiling point is again attained, then decrease (to 20 cc. or so per minute) and re-observe. The equilibrium boiling point thus observed should not vary more than *lo C. The rate of distillation may be varied from 10 cc. to 45 cc. per minute, but the fuel will not keep in contact with the metal walls-i. e., spheroidal effects occur-if the rate of boiling is forced above this. The contents of the equilibration boiler are thus renewed every 1.5 to 6.5 minutes. 1 Received
*
*
May 21, 1925.
J . SOC.Aufomofiue Eng., 9, 313 (1921); IS, 287 (1923).
Keith and Whatmough, Proc. I n s f . Aufomobrle E n g . , 17, Pt. 1, 363 (1922). English Patent 184,266 (1922).
‘
Figure I-Equilibrium
Boiling P o i n t Apparatus w i t h Pressure Equalizing T u b e (P)
that extreme variation in equilibrium boiling point with Pratt’s No. 1 Petrol was 4’ C. (with thermometer in liquid), while varying rate of distillation from 8 to 32 cc. per minute did not make any difference. Further, they returned t h e equilibrium distillate to the supply tank, as this was “little 6 J . I n s l . Petrolcum Tech., 9, 371 (1923).
INDUSTRIAL A N D ENGI NEERING CHEiMISTRY
44
different to the original” and thus enabled determinations to be made with comparatively small samples. The author usually uses about 1liter of the fuel to be tested, but has made successful determinations with one quarter of this amount. Barnard and Wilson’s method of obtaining equilibrium solutions is in principle identical with that of the writer, the difference being that the use of a metal boiler with balance pipe permits a great saving in time and requires less care in the operating of an equilibrium boiling point apparatus. Results
Equilibrium boiling points of various fuels are given in Table 11. Generalizing from these results it would seem that the 80 to 85 per cent point on the Engler distillation curve is correct as regards equilibrium boiling points of kerosenes and American petrols. For the much more volatile petrols used in the British Isles the 60 to 70 per cent point is more usual. For aromatics the range is 60 to 85 per cent. -4zotropic admixtures containing alcohol show extreme variation between 50 and 90 per cent in a single series.
Vol. 18, No. f
boiling point is a better index of mixture stability, as there is undoubtedly a close connection between deposition temperature or “fog point” of a fuel and its equilibrium boiling point. Finlaysons comments upon the close agreement between the writer’s “fog points,” which were observed under running conditions on the road, and Wilson and Barnard’s condensation temperatures (as modified in a private communication from these investigators to Finlayson). It will be observed from Table I1 that the equilibrium boiling point tends towards the 85 per cent point as a limit for paraffinoid distillates, and this may be sufficiently accurate when applied to American petrols. However, 7 0 to 80 per cent points are usual with motor fuels on the British market, owing to their higher content of aromatic constituents. Greater still is the divergence with mixed fuels containing alcohol. I
1
I
I
I
I
I
I
Boiling Points of Various Fuels Approx. percentage Engler. fractionation Equilibrium point at boiling oint equilibrium boiling point OBSERVER
Table 11-Equilibrium
FUEL British: Aviation Shale National benzol mixture (50:50 petrol-benzol) Pratt’s No. 1 Shell Taxibus (No. 3) American : Special aviation Domestic aviation Socony gasoline Red Crown gasoline High end-point gasoline British: Light (for cleaning) Heavv (lamo oil) American f
No. 3
8
Pctrols
-
Socony
102 111
70 65
Whatmough Ormandv and Craven
115 120
70 70
Whatmough Ormandv and Cravein Whatmough
121 127 93 103 157 173
187 Kerosenes 225 242
85 85
I
it}
227 244
Barnard and Wilson
Whatmough Barnard and Wilson
Naphihenes Russian kerosene
243
65
Whatmough
75
Ormandv and
Aromatics Motor benzol
97
Crude benzol (white spirits) Crude light (tar) oil Asiatic Petroleum Co.’s intermediate distillate (Miri benzene) 164 Borneo kerosene ‘6o O 1 whatmough 227 Edeleanu extract (Ricardo’s‘ heavy aromatics) 240 70 J Alcohol Miriures (Made by Volume with 95 Per ceni Alcohol) Alcohol 207’ petrol 80% 116 101 Alcohol 33lA%, petrol 664/a% Whatmough Alcohol 50%, petrol 50% a8 80 Discol (alcohol, benzol.. and vaporizing paraffin) . 100.5 Discol Ricardo’s H.L.H. ( = high latent heat) 73 Ormandy and Discol Ricardo’s H.H.V. ( = high heat value) 71 55 Craven Power methylated spirit (P. 75 M. S. No. 2) 55 J Kerosene 40%. white spirits 40%. alcohol 20% 150 Kerosene 33l/n%, white 6o alcohol spirits 331/3%, Whatmougb 148 60 33’/s% Miri benzene 66*/a%, alcohol
i
1
11
331/3%
128
60
The writer differs from Barnard and Wilson2*’as regards the 85 per cent point in the Engler distillation curve being the best indication of the distribution characteristics in the manifold of a n internal combustion engine. The equilibrium *Automobile E q . , 11, 282 (1921) TRISJOURNAL, 17, 428 (1925).
7
Figure 2-Engler Distillation Curves of Petrol-Alcohol Mixtures Dotted line joining equilibrium boiling points indicates marked lowering of equilibrium boiling points
A series of Engler distillation curves marked with equilibrium boiling points indicates certain characteristics of mixed fuels. A sudden departure from a regular slope in the Engler distillation curve of a mixed fuel denotes a tendency towards the formation of a mixture of minimum or maximum boiling point. The Engler distillation curves for mixtures of petrol and alcohol (Figure 2) provide examples of mixtures with additive or increasing vapor tension (and minimum boiling points), the theory being that the vapors are not mutually soluble (or associated) and thus obey Henry’s law of partial pressures. The broken lines joining the determined equilibrium boiling points show where the boiling points of fuels intermediate in composition will fall, so that three or four determinations serve to identify a particular series of mixtures. Paraffin-benzol mixtures show a tendency in the opposite direction to alcohol. Hydrocarbon fuels (petrols, gasolines, or kerosenes) have regular and comparatively flat Engler distillation curves, and admixtures with lighter aromatics give a series of fuels with equilibrium boiling points arranged on a curve concave upwards. Consequently, such fuels proved disappointing in practice as regards volatility and mixture stability owing to the high temperature needed to keep the combustible mixture dry, this in turn rendering necessary more of the expensive aromatic to prevent detonation. On the other hand, alcohol markedly depresses the equilibrium boiling point and permits of comparatively low induction-pipe temperatures with kerosenes. A mixture of equal parts of kerosene, white spirits, and alcohol in a n equilibrium boiler evaporates the heaviest kerosene fractions a t a temperature below that of the most volatile portion of the kerosene. U. S. -4.cracked distillates and flast Indian aromatic fuels are available in enormous quantities, but are of little value because not sprayable; however, they are equally effective in a dry gas carburetor as petrol a t ordinary mixture temperatures (40’to 60’ C.) on reducing equilibrium boiling point with the aid of alcohol. 8
Aufomobilc Ens.. 16. 121 (1925).
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
January, 1926
Wilson and Barnard9 regard the 85 per cent point on an Engler distillation curve as the best indication of the manifold distribution characteristics of a fuel. The author prefers a n actual determination of the equilibrium boiling point (at pressures approximately atmospheric) as a definite guide J . Soc. Aulomofivc Eng. 9, 313 (1921);
@
THISJOURNAL, 17, 428 (1925).
45
to the volatility of a fuel under the variable running conditions of a road motor (since back-firing due to fuel deposition occurs with open throttle). The relationships between equilibrium boiling points and condensation temperatures (or dew points) of working mixtures of motor fuels with air be the subject Of a subsequent
Latent Heats of Vaporization of Distillates from Paraffin-Base Petroleum’ By E. H. Leslie, J. C. Geniesse, T. W. Legatski, and L. H. Jagrowski UNIVERSITY OF h f I C H I O A N ,
RECENT search of the literature, made in connection with the preparation of manuscript for the section of the International Critical Tables dealing with petroleum products, showed that available data on latent heats of vaporization were few and in poor agreement. The work described in the present article was done to supply, in part a t least, the desired but missing information. Figure 1 graphically summarizes present knowledge of the latent heat of petroleum products vaporized a t a pressure of one atmosphere. Curve 1 is established by fragmentary data2 that are clearly out of line with the more complete series of experimental or calculated values. Gurwitsch3 furnishes the information on which Curve 2 is based. The fractions were from a Baku crude petroleum. Curve 3 is plotted from data calculated by Graefe.4 He determined the molecular weights of several oils by the freezing point method using stearic acid as a solvent, and the boiling point by averaging initial, final, and nine intermediate temperature readings taken during Engler distillations. The latent heat was then calculated by Trouton’s rule using 20 for the value of the constant. Graefe’s curve is so widely different from all the others that i t must be concluded that his molecular weight determinations were in error. The data from which Curve 4 is plotted are those of Syniewskis on fractions from a Galician crude petroleum. Three of them are definitely above the curve drawn through the other four, and the curve is therefore shown passing through four points only. Syniewski calls attention to the fact that his higher boiling fractions cracked, and that this was responsible for the high latent heats. Curve 5 is plotted from values calculated by Ogrodzinski and Pilat.6 Benzine fractions, each boiling within a range of 1’ C., were separated from a Boryslaw (Galician) crude petroleum by use of a Glinsky stillhead. The molecular weights were then determined by the freezing point method using benzene as the solvent, and the latent heats of vaporization calculated by Trouton’s rule, apparently using the value 20.7 for the constant. Wilson and Bahlke’ have recently calculated the latent heats of vaporization of the paraffin hydrocarbons by use of Hildebrand’s6 modification of Trouton’s rule, and have
A
1
-4
2
*
Received August 24, 1925. Burrell, Biddison, and Oberfell, Bur. M i n e s , Buli. 120, 49. “B’issenschaftliche Grundlagen der Erdol Bearbeitung,” p. 8 3 , Berlin.
1913. 4
Petroleum, 6, 569 (1910).
8
Z.angew. Ckem., 27, 621 (1898).
6
Petroleum, 8,1181 (1913). THISJOURNAL, 16, 116 (1924). J. .4m.Chem. Soc., 37, 970 (1915).
8
A N N ARBOR,MICH.
discussed the merits of different methods of calculating latent heats. Their calculated values are presented as Curve 6. The results of the writers’ experimental work are given as Curve 7. The method by which these data were obtained will be briefly described. Experimental
Since it was the writers’ purpose to measure the latent heats of petroleum fractions by direct heat input, the first requisite was a series of fractions of such narrow boiling range that the heat required to raise the temperature of the body of the liquid as vaporization progressed was small as compared with the heat involved in the change of state. These were prepared from a Cabin Creek, W. Va., crude petroleum, a high-grade paraffin-base oil. Ten liters of this petroleum were first roughly cut into a series of fractions by distillation from a steel still fitted with a 1-inch diameter column 5 feet long, packed with I/c-inch Lessing sheet-metal rings, and supplied with carefully controlled reflux. The fractions obtained from the first distillation were then systematically distilled in a smaller column apparatus
Boiling point of fraction--o C. Figure 1
capable of fractionating so closely as to give practically the true boiling point curve. The column of this apparatus was 12.5 to 12.8 mm. diameter, packed with 5-mm. Lessing rings for a distance of 115 em., supplied with reflux liquid in an amount susceptible of exact control, and doubly jacketed and heated in such manner as to be nearly adiabatic in operation. Fourteen close-boiling fractions, and three fractions of wider boiling range, were selected from among those prepared. The properties of these fractions are given in Table I. Closer boiling cuts could easily have been taken,
