INDUSTRIAL Ai\-D ENGIiYEERISG CHEMISTRY
January, 1926
79
Specific Heats, Heats of Vaporization, and Critical Temperatures of California Petroleum Oils' By Edward H. Zeitfuchs STANDARD OIL Co. (CALIF.), RICHMOND, CALIF.
T
HE investigation of the specific heats, heats of v a p o r i z a t i o n , and critical temperatures of California petroleum oils presented in this paper were carried out primarily to obtain reliable data for the use of the refinery engineer. These data were especially needed for high temperatures (up to 400' C., 752' F.) for use in designing new equipment and for studying new processes in the course of d e v e l o p ment. Heat of Liquid and Specific Heat
The experimental procedure for determining the quantity of heat absorbed for a given temperature change was as follows: A known weight of the oil was sealed up in a light steel cylindrical capsule of about 15 cc. capacity, shown at A , Figure 1. This was suspended from a light wire in a n electric tube furnace, B, and heated. When temperature equilibrium was reached, the capsule was dropped from the furnace into a Dewar flask, C, containing 850 grams of water, and the temperature rise of the water measured. The
1 ETER
ERNOCWPLE
STEEL CAPJULE
Q = - HI
-
H2 (1) di in r h i c h Q = heat given up by 1 gram of oil in cooling from temperature of furnace to temperature of water in flask Hi = heat given up by steel capsule and ill grams of oil in cooling from temuerature of furnace to temperature of water in flask HZ = heat given up by steel capsule alone in cooling from temperature of furnace to temperature of water in flask
The steps involved in the reduction of the observations made a t one temperature are shown by the following example :
STEEL CAPSULE
Figure 1-Apparatus
a well passing through the middle of the capsule. The heat capacity of the Dewar flask containing 850 grams of water, shown by Curve 3, Figure 2, had been determined by measuring the electrical energy required to raise the tempemture through a measured range. Figure 3 shows the a p p a r a t u s b y means of which the calibration was made: A is a 1000cc. Dewar flask, B a coil of No. 22 B. & S. gage nichrome resistance wire, C a Reckm a n t h e r m o m e t e r , D an ammeter, and E a voltmeter. The source of current was an Edison storage battery of six cells. The total heat given up by the empty steel capsule when cooled from various temperatures of the furnace had been determined by dropping the empty capsule into the Dewar flask in exactly the same manner as when oil was present. The result of this calibration is shown in Curve 2, Figure 2. The error introduced by the heat given up by the oil vapor condensing in the capsule and by the difference between the heat capacity of the vapor and liquid was made negligible by keeping the volume of vapor above the oil as small as possible. Having determined the amount of heat given up by the oil and capsule combined and knowing the heat capacity of the capsule empty, the heat given up by the oil alone in cooling from the furnace temperature to the temperature of the water in the Dewar flask was calculated by means of the following equation:
The specific heats of California petroleum oils covering a wide range of specific gravity have been experimentally determined by dropping the heated oil sealed in a light steel bomb into a mass of water. Measurements were made up to 371.1' C. (700"F.). The critical temperatures of these oils were determined by observations of the behavior of the meniscus of the oils heated i n sealed glass capillary tubes. Curves showing the heat of vaporization as a function of the temperature have been prepared for these oils by the aid of Trouton's rule, the known slope of the curves, a t about 38" C. (100' F.) above the average boiling temperature a t atmospheric pressure and the critical temperature. The measurements were made for the purpose of securing reliable data sufficiently accurate for regular petroleum oil refining practice.
for D e t e r m i n i n g H e a t of L i q u i d a n d Specific H e a t
temperature of the oil in the capsule before dropping was measured with an accurately calibrated iron constantan thermocouple, the hot junction of which was suspended in 1 Received July 24, 1925. Presented before the joint session of the Division of Petroleum Chemistry and the Section of Gas and Fuel Chemistry a t the 70th Meeting of the American Chemical Society, Los Angeles, Calif., August 3 t o 8. 1925.
Material-tar, specific gravity 0.9440 (gravity A. P. I. 18.4') Mass of capsule and tar, 42.335 grams Mass of capsule empty, 31.835 grams M = Mass of tar in capsule, 10 500 grams Temperature of tar in capsule before dropping, 371.1' C. (700' F.) Temperature of water in Dewar flask after dropping capsule, 28.2' C. T = total temperature rise of water in Dewar flask, 4.06' C. Hi = total heat absorbed by flask and water, 3696 calories HZ = heat given up by capsule in cooling from 371.1" to 26.2' C. (700" t o 7'9.3' F.), 1510 calories Then from Equation 1
I N D U S T R I A L A N D ENGUNEERING CHEMISTRY
80
dQ at The values of Q determined in this manner for the individual samples are given in Table I. Heat Given Up b y Oils In Cooling f r o m Furnace Temperature (Q) Temp. of capsule before dropping 7-c. O F. Cal./gram B. t. u . / ~ . Lubricating Oil Distillate (sg. gr. 0.9065; A. P. I. 24.6') 220 396 171 308 133 240 100 180 50 90 207 373 159 286 106 191 195 351 75 135 T a r ( s p . gr. 0.9440;A. P.I. 18.4') 324 615 171 308 274 525 142 256 264 507 126 227 199 390 89 160 98 208 29 66 37 52 84 183 _Gas Oil ( s p . gr. 0.8883; A. P. I. 27.8') 377 712 215 387 339 643 190 342 189 372 87 157 130 267 53 96 Naphtha ( s p . gr. 0.7874; A. P.I. 48.2') 276 530 144 249 480 137 196 385 99 106 129 265 59 70 Penetration Asghalt (sg. gr. 1.013) 312 595 163 294 306 583 159 286 196 386 90 161 121 250 49 88
Table I-Total
REDUCTION OF OBSERVATIONS-Heat of Liquid. The temperature of the water in the Dewar flask at end of period of rise ranged between 24' and 26" C., depending upon the temperature of the oil before dropping. To simplify the reduction of the observations, a mean value of 25' C. was taken, the error introduced by this procedure being well within the experimental error. I I r l
I
I
I
I
I
I
I
I
I
'
I
I
I
Vol. 18, No. 1
= a
+ 2b(t - 25)
I n the experimental method used for determining Q, the vapor space was small; the composition of the liquid therefore remained nearly constant and cooling occurred under approximately equilibrium conditions. Q and dQ/dt are therefore the heat of liquid and the specific heat along the saturation curve. I I The values of a and b determined as described a b o v e w e r e p l o t t e d against the specific g r a v i t y a n d curves were drawn as shown in Figure 5 . I n making use of the a, b curves for calculating the specific heats of California pet r o 1e u m oils, it m u s t be a s sumed that the specific heat is a function of the specific g r a v i t y . This is not exactly true, because it is known that different oils of the STIRRER NOT SROWN same specific gravities contain hydrocarbons of several series in vari- D E W A R ous proportions. The values taken from the 'ococc~ curves will, however, be found Figure 3-Apparatus for D e t e r m i n i n g accurate for use in en- H e a t Ca a c i t y of t h e Dewar Flask Congineering calculations, t a i n i n g !& G r a m s Water I n Table I1 are shown the calculated specific heats of oils varying in specific gravity from 0.75 to 0.95 and for temperatures between 25" and 400' C. '
I
-
Table 11-Specific H e a t s of California Petroleum Oils (Calculated from the equation dQ a f 2b(t 25) and the values of dt a and b taken from the curves shown in Figure 5) Specific gravity 0.75 0.80 0.85 0.90 0.95 Gravity A. P. I. (degrees) 5 7 . 2 45.4 35.0 25.7 17.50 a 0.564 0,531 0.495 0.462 0.429 0.375 0.219 0.311 b X 101 0.419 0.453
-
250 c. 1000 C. 2000 c. 300' C. 400' C. CALI0R4lION CURVES USCD IN DITERMINING TU€ SPECIFIC HEAT OF PETROLEUM OILS
The values of Q (heat of liquid) Table I, plotted against - 25), are shown in Figure 4,which presents the results in the most convenient form for use in engineering caIculations. Calculation of Specific Heat. The curves in Figure 4 were represented by an equation having the form Q = a(t - 25) b ( t - 25)2 (2) in which Q = calories absorbed by one gram of oil referred (t
+
t
to 2 5 O c. = temperature,
O
C.
a, b = constants determined for each curve by the
equation
Q ) m = a -I- b(t - 25) The specific heat obtained by differentiating Equation 2 is:
0.564 0.597 0.641
. .. ...
-
0.531 0.578 0.640 0.702
. ..
0.495 0.551 0.626 0.701 0.776
0.462 0.525 0.609 0.692 0.776
0.429 0.497 0.588 0.678 0.769
Critical Temperature and Volume
A knowledge of the critical temperature-that is, the temperature a t which the oil can no longer exist as a liquid in the ordinary sense-was desired as one of the points on the curve, showing the heat of vaporization as a function of the temperature. The heat of vaporization a t the critical temperature is zero. The critical temperatures were experimentally determined by means of the apparatus shown in Figure 6. The oils were sealed up in Pyrex glass capillary tubes of approximately 2 mm. bore, 8 mm. outside diameter, and from 80 to 120 mm. long, and heated electrically as shown in Figure 6 until the meniscus disappeared. The volumes of the oils in the tubes were determined at 15.58' C. (60' F.) and at the critical temperature by noting the position of the meniscus on the capillary tubes which were graduated. The critical temperature was found to depend on the volume of the vapor space above the liquid oil, as illustrated by the following data:
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
January, 1926
which these data3 were available. The slopes of the tangents to these curves a t their atmospheric boiling points were found to differ only slightly. Curves were now drawn for each oil tangent to a line a t the mean boiling point having the average of the slopes found for these hydrocarbons. The curves were extended to the ,
Crude Naphtha Sample 3
T,
VC
vt
=
Vls.as
VVEtZ
Sample 4 Vlh.66
= about 4.0
Vls6
Sample 1
Vi
Vt
= = = =
-
V O
