IXDUSTRIAL A N D ENGINEERING CHEMISTRY
82
Av. mol.
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SD.er. 0.6662 0.7632 0.8774 0.8212 0.8883
OIL
Petroleum ether Gasoline Naphtha W. W. distillate Gas oil
Gravity A. P. I. wt. 79 80.9 114 53.9 128 48.2 151 40.8 216 27.8
Mean boiling temperature =
Table IV-Heat of Vaporization of California Petroleum Oil8 Av. boiling Crit. temp. at I-Heat of vaporization, cal./gram temp. 760 mm. Crit. C. O C. 50° C. 5 2 O C. 100' C. 136'C. 156'C. 20O'C. 202'C. 286" C. 300'C. 400'C. temp. 0 32 75 211 52 84 84 0 79 74 62 298 136 87 41 0 77 69 62 348 156 85 65 65 47 0 so 374 202 88 67 53 51 33 0 459 286
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1713 = 156' C. or 429' A. 11
Heat of vaporization at mean boiling temperature Trouton's rule is
= 20.5
in which M = mean molecular weight; naphtha = 128 T = temperature, absolute, at boiling point at one atmosphere = 429" A. L = molal heat of vaporization, 20.5 = a constant Then, heat of vaporization of naphtha at mean boiling temperature = 429 I
'
20'5 = 68.6 cal. per gram (123 B . t. u, per lb.) 128 I
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The calculation of the heat of vaporization a t atmospheric pressure by Trouton's rule depends on accurately determined molecular weights. The average molecular weights of California petroleum oils have been determined over a wide range of specific gravity by the freezing point lowering method, using benzene as the solvent, and also by the Victor Meyer method on the lighter fractions, including gasoline, in this laboratory. Distillation of a number of sampIes by the Bureau of Mines method with a thermometer in the vapor space and a thermocouple in the liquid showed that the temperature of the liquid wasfrom3" to 20" C. higher than that of the vapor, depending upon the specific gravity of the stock. The samples tested in this way ranged in specific gravity from 0.666 to 0.904 (80.9" to 24.9"A. P. I.). The difference in the heat of vaporization calculated by Trouton's rule on the basis of temperatures measured in liquid and vapor was not greater than 1 to 4 per cent, the difference increasing with increase in specific gravity of the stock. Acknowledgment
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The author is grateful to R. A. Halloran, manager of the Development Department; and to J. B. Terry, chief chemist of the Standard Oil Company of California, for helpful advice during the course of this investigation.
Use of Methyl Salicylate in a Flowmeter' By R. H. K. Foster GASOLINR RECOVERY CORP., CHARLESTON, W. VA.
ARIOUS liquids have been suggested for use in measuring the differential pressure through the capillary of a flowmeter, among them water, mercury, and glycerol. These all have certain advantages and disadvantages. The principal disadvantage with water is its inability to wet the tube if there happens to be a film of grease on the tube, thus causing an uneven meniscus and subsequent error in reading. The same is true of glycerol, and in addition glycerol, being an extremely viscous liquid, is not so sensitive to sudden fluctuations. Methyl salicylate (oil of wintergreen) is being used in this laboratory as the flowmeter liquid, and offers the following advantages: ( a ) It dissolves any oil or grease film on the tube, thereby forming a perfect meniscus. ( b ) It has a very low vapor pressure and will last indefinitely. (c) Because of its low rate of evaporation the diffusion of its vapor into the air or gas stream is negligible. ( d ) It is easier to fill the flowmeter with methyl salicylate and much easier to remove air bubbles from the inverted U-bend, than when water is used.
Since methyl salicylate dissolves rubber, a flowmeter of the construction shown in the illustration is preferable. Once 1
Received November 24, 1923.
the flowmeter has been filled and the proper adjustment made, no further attention should be necessary, except a n occasional check on the calibration. It should be o b s e r v e d t h a t t h e density will change with the temperature and therefore it must be calibrated a t different temperatures. Soap, rather than stopcock grease, s h o u l d be u s e d t o lubricate the stopcock. If a rubber tube and screw clamp are used instead of the glass stopcock as shown, it will be necessary to readjust the zero level of the liquid every day a t least. An appreciable amount of rubber may dissolve in the methyl salicylate before its density has changed sufficiently to affect the results on ordinary work. The density of methyl salicylate is 1.186 and can thus be used for all purposes for which water is suited without introducing some of the less desirable features which the use of water incurs.
