INDUSTRIAL A N D ENGINEERING CHEMISTRY
942
VOI. 21, No. 10
Specific Heats of Petroleum Vapors' W. H. Bahlke and W. B. Kay STANDARD OILCOMPANY(INDIANA), WHITING.I N D
P T O the present time
U
The specific heats of the vapors of five petroleum distillates have been determined at substantially atmospheric pressure over temperature intervals varying from a temperature just above where they are completely vaporized to 350" C. The results can be expressed by the empirical equation
which delivered a steady and constant amount of distillate to a small pipe still, where it was completely vaporized and s u p e r h e a t e d . The superheated vapor was led through 0-S a heavily insulated pipe to a cp= 46450 702) (1) calorimeter, where d e l i c a t e 0-S thermocouples measured the c, = 4or 6450 ( t 670) (2) i n i t i a l temperature of the where C p = specific heat at atmospheric pressure, vapor and the temperature S = specificgravityat 15.55" C. (60"F.),and t = degrees difference p r o d u c e d b y a Centigrade in (1) and degrees Fahrenheit in (2). m e a s u r e d quantity of elecThese equations fit the experimental data with an trical energy. The vapor was average deviation of 1.33 per cent and represent distilthen condensed in a waterlates whose specific gravity at 15.55" C. varies from 0.9 cooled condenser and caught to 0.68. a n d weighed in a narrownecked receiver. SP. GR. A steady flow was obtained with the steam pump by pump74-76 gasoline 0 6859 ing an excess of the distillate into a constant-pressure vessel V. M. and P naphtha 0 7471 0 7848 Oleum spirits and withdrawing from the same the required amount of disGas oil No 1 0 8463 0 8911 Gas oil No. 2 tillate for vaporization. An orifice meter and valve in the and (2) an empirical equation which correlates the data for feed line to the still regulated this amount. The pipe still was heated by gas and its temperature conpractical purposes. trolled by regulating the gas pressure by means of a pressure Outline of Method gage and valve. It was thus possible to set the still to any The experimental method employed in this investigation desired temperature. The temperature of the vapor leaving was that known as the constant-flow method. In outline, i t the still was shown approximately by a thermometer in a well consisted in passing the superheated vapors of a given dis- in the exit pipe. This method of vaporizing and superheating tillate, a t a constnnt rate, through a calorimeter containing the distillate vapor was quite satisfactory. I n the runs with a heater to which a. known amount of electrical energy was distillates which could be completely vaporized without steam, imparted. Delicate thermocouples gave the initial tempera- it was possible to keep the temperature of the vapor constant ture and the temperature rise of the vapor through the calo- to within 0.1' C. for more than an hour. With steam the rimeter. If this temperature difference is AT and E heat units flow through the still was more unsteady but the maximum variation was never more than 0.3" C. of electrical energy have been supplied per hour. then The calorimeter is shown in detail in Figure 1. It was E assembled from 2-inch (5-em.) pipe with the appropriate m( AT f AT1) where C, is the specific heat, rn is the mass of vapor flowing fittings and fitted with an electrical heating coil, A , thermothrough per hour, and AT1 is a correction for the loss of heat couples B a n d B', and the baffles C, C', and C". Surrounding by radiation and conduction from the calorimeter a t the mean i t was the airjacket, D, which was supplied with heated air temperature of the vapor. The value of this last quantity from the electric heater, E. The heater A was made from 54 depends upon the temperature difference between the vapor feet (16.5 meters) of No. 15 gage chrome1 resistance wire in the calorimeter and the surrounding atmosphere and a p which had been wound in a coil and the coil, in turn, wound proaches zero as this difference is made smaller. It was around a hollow porcelain tube. The ends were bound to the determined in each experiment by measuring the drop in tube by wire and one of them was brought through the center temperature of the vapor through the calorimeter a t two of the hollow tube so that both terminated a t the same end of different temperatures, one below and the other above the the heater. Heavy flexible copper wire brazed to the ends mean temperature of the vapor. The required value was then led to the pressure-tight terminals constructed from spark plugs by threading the central electrode of the plug and found by simple interpolation. lengthening it by a collar and threaded rod. The copper Where the oils were not sufficiently low-boiling to completely vaporize them a t the desired temperature, they were wire was then fastened to this rod by a lock nut arrangement. vaporized with the aid of steam and the specific heat of the The heating unit fitted inside a large Pyrex glass tube, which mixture measured, this value being corrected for the specific was made to fit snugly inside the pipe by wrapping around it a few turns of asbestos tape attached by means of shellac. heat of steam to get the specific heat of the oil. I n order to secure a constant source of electrical energy to the Description of Apparatus heater, a motor-generator set was used. This, with a variable rheostat, gave a constant current which could be adjusted to The feed tank was a vessel of about 50 gallons (189 liters) capacity and was connected to a reciprocating steam pump any desired amount. The electrical energy was measured by a Weston wattmeter. Allowance was made for errors due 1 Received April 16,1929. Presented before the Division of Petroleum to residual magnetism in the coils by reversing the current Chemistry at the 77th Meeting of the American Chemical Society, Colum during the readings and taking the average of the two readings. bus. Ohio, April 29 to May 3, 1929. no d a t a h a v e been available on the specific heat of the vapors of mixtures of the various organic compounds present in petroleum distillates. Such data have long been needed in problems arising in the des i g n a n d t e s t i n g of plant equipment. T o supply such data this paper presents: (1) experimental results for the specific heat a t atmospheric pressure of the vapor of five r e p r e s e n t a t i v e distillates, name1.y:
''
+
+
October, 1929
INDUSTRIAL AND ENGINEERING CHEMISTRY
The thermocouples B and B' were of S o . 24 gage ironconstantan wire and were clamped between the faces of the flanges F and F ' , from which they were insulated by asbestos gaskets soaked with shellac. The couples for measuring the temperature rise of the vapors were of the multiple type arranged so that the voltage per degree was multiplied to four times that of a single couple. The initial temperature of the vapors was measured by a single couple. All couples were in direct contact with the vapor. Before the couples were installed they were tested for uniformity a t the boiling point of naphthalene (218.0" C. a t 760 mm.), and those used for the multiple couple were carefully matched. The melting point of ice was used as the temperature of the cold junction for the single couple measuring the initial temperature of the vapors, and the e. m. f . measurements were made with a Type K Leeds and Sorthrup potentiometer capable of measuring to 0.001 millivolt. The baffles C, C', and C", consisting of alternate rings and disks, were also clamped in the flanges, and served both to protect the couples from direct radiation from the heater and to mix the vapor and bring it to a uniform temperature before coming in contact with the couples. The pressure gage, G, gave the pressure of the vapor in the calorimeter and H, a differential gage, gave the pressure difference across the calorimeter. This last quantity, which never amounted to more than 0.2 inch ( 5 mm.) Hg, and therefore had little effect on the experimental results, was not taken into account in the final calculation. The apparatus was carefully lagged with asbestos pipe covering and, to reduce the radiation loss further, the whole was surrounded by an insulated jacket, D, through which was passed air heated to within a few degrees of the temperature of the vapor. An electrical heater similar to the one described above was used t o heat the air. The temperature was kept fairly well under control by means of a rheostat in series with the heater and an orifice with a manometer attached to regulate the flow of air. I n order to prevent channeling of the heated air, it was introduced through a perforated annular ring a t the bottom of the jacket and a spiral baffle forced it t o rise in a spiral manner around the calorimeter to the top, where i t escaped through a narrow annular opening. The temperature of the jacket, indicated by three thermocouples, J , J ' , and J " , decreased from t h e bottom to the top, but the average temperature throughout a run was kept constant to within 1 " C. The condenser, in which the superheated vapors were condensed, consisted of 7 turns of a 1-inch (2.5-cm.) pipe bent into a helical coil about 15 inches (38 cm.) in diameter. This was cooled by t a p water. The condensed vapors were caught in a narrow-necked receiver and weighed.
943
Readings of the temperature of the entering vapor and the temperature drop through the Calorimeter were taken and recorded every 10 minutes and finally every 5 minutes, until the temperature and temperature difference became constant. The current to the vapor heater mas then turned on and readings were taken a t regular intervals until temperature equilibrium was again reached, when a weighed receiver was placed under the condenser and a stop watch started. Readings of the temperature of the vapor, the temperature rise, electrical energy to the heater, and the still temperature were taken every 5 minutes for 80 minutes, while, in addition, a t 10-minute intervals the temperature of the air bath a t three points mas taken and the rheostat to the air heater adjusted, when necessary, to maintain constant temperature. The time of the actual experiment was noted on the stop watch and the quantity of vapor passing through the calorimeter during this time determined by weighing the condensate in the receiver. Usually about 60 pounds (27 kg.) of oil were passed through the calorimeter. A check determination was made in nearly every run. At the conclusion of the run the current to the vapor heater was turned off and the fire to the still increased until the initial vapor temperature was slightly
Experimental Procedure
The apparatus and experimental procedure were tested by making several determinations of the specific heat of superheated steam a t various temperatures a t atmospheric pressure. With new and carefully matched couples, i t was possible to check the specific heat to within 0.7 per cent of the accepted value. However, i t was found that after the couples had been exposed to petroleum vapors, even of highly refined distillates, they gave values for the specific heat of steam which were too large and lyhich increased with the time of exposure. This change was measured by frequent determinations of the specific heat of steam and the percentage deviation taken into account when the data for the oils were calculated. I n making a determination, the still and calorimeter were brought u p to t h e desired temperature with steam, then the steam was turned off and the distillate pumped into the still.
Figure 1-Calorimeter for Measuring Specific Heats of Petroleum Vapors
above its mean temperature during the actual experiment. Readings were again taken until thermal equilibrium was reached and the initial temperature and the temperature drop through the calorimeter recorded. I n the case of the heavy distillates steam was used in order to vaporize them completely a t temperatures where no cracking took place. The procedure was the same except that the oil and water were separated and their individual weights determined.
INDUSTRIAL A N D ENGI,VEERI,VG CHEMISTRY
944
VOI.
21, KO. 10
lighter distillates, but on account of the difficulty of maintaining temperature equilibrium when steam was used, it probably amounts t o about 2.5 t o 3.0 per cent for the heavy distillates. An examination of Figures 2 to 6 shows that the experimental points fit the calculated results with an average deviation of 1.33 per cent.
0.54 \r
e
4 060
s
P
Y OJ6
0 32
Table I-Properties
of Distillates Studied
9 48
A. S.
SP. GR. 9 44
DxsTILLATE IO0
100
200
Figure 2-74-76
c,
AT l 5 . j o
(60'
350
F.1
c.
Gasoline 74-76 Gasoline 0.6859 Oleum spirits 0.7846 V . M. a n d P. naphtha 0.7471 G a s oil No. 1 0.8463
V&l
G a s oil S o . 2
062
37
0.5911
T.M. DISTILLATION
Initial boiling/ point
F.
98
Final boiling point
153 218
c.
149
301
102
215 169 426 378 502 Above 400
219 261
308 424
F.
337 712
Above 7.50
I
MOL.
WT.
88 134 110 220 260
\
8
a
Discussion of Results
OW
U
2 2 OM (r
3 056 034 PO0
225
250
Figure 3-V.
273
300
325
300
325
M . a n d P. Naphtha
111 correlating the data obtained in this investigation as well as data for the lower molecular weight hydrocarbons ( l ) , several empirical equations were developed which express the relationship between specific heat, temperature, and some other more easily determined property, such as specific gravity. However, the equation which appears t o have greatest utility was obtained by a method of analysis similar t o that used by Fortwh and Whitman for liquids. Thus i t
.(r
2 069 I ,
5
060
b
058 0 $6 0 9
200
225
250
Figure 4-Oleum
2 75
Spirits
Results
The rewlts are shown graphically i n Figures 2 to 6. The solid line in these figures is considered the most represeiitative line through the data, and is so drawn as to give equal veight to each experimentally determined point. The dashed line was calculated from an empirical equation and nil1 be referred t o later. As will be seen in Figures 2, 3, and 4, the data for 74-76 gasoline, V. 31. and P. naphtha, and o!eum spirits fall almost on a straight line. However, in the case of the two ga5 oils (Figures 5 and 6 ) the variation of the data ib conciderably greater than for the lighter distillates. This is due not only to the fact that it was necessary to use steam to vaporize these heavy distillates, in which case it was found .oiiien-h:it more difficult t o maintain temperature equilibrium, but alio to the fact that the oil had to be separated froin the c o n d e n 4 steam and thuq the possibility of an error in weight n a 3 introduced. Xevertheless, the average deviation of the data from the straight line amounts to only 2.56 per cent for the lighter arid 2.06 per cent for the heavier gas oil. Some properties of the five distillates studied are given in Table I. Accuracy of Results
As has been previously stated, it was possible to check the specific heat of superheated steam consistently to within 0.7 per cent when the thermocouples were new. Since the couples were tested frequently, the error in the correction for their change did not produce an error of more than 0.2 per cent in the value for the specific heat. The maximum experimental error \Todd therefore be about 1.0 per cent for the
25 0
300
375
Figure 5-Gas
Oil No. 1
Figure 6-Gas
Oil No. 2
3 :i
A P O R , STATE-___ VLUlUlD STATE-----
074
a 70 u.0
$
066
u
5 2
062 056
054
0 50 IS0
Figure 7-Specific
200
2.50
Heat-Specific Gravity-Temperature Relationship
250
I,VDUSTRIAL A S D ENGINEERING CHEMISTRY
October, 1929
was found that the relationships could be expressed by the following equations:
or
C,
=
c,
=
(4.0s) ~ ~ (1.8t 5+ 0 702)
~
(4.0 - S) 6460 (t
+ 670)
where C, = specific heat at atmospheric pressure S = specific gravity at 16 5' C (6!" F ) t = temperature [ " C in (1) and F. in (2)]
Values for the specific heat calculated by these equations are in good agreement with the experimental ones as will be seen in Figures 2 to 6 where the results calculated from the equations are s h o ~ ~by i i the dashed line. For the low inolecular-weight hydrocarbons the calculated values agree fairly well with the observed values a t the average temperature of the determination ( 2 ) as is shown in Table 11. Table 11-Specific
Heats a t 50' C.
At temperatures inuch different from this, however, the agreement with the equation proposed by Lewis and McAdams is not so good, because the temperature coefficient of the two equations is different. The equation proposed here should be considered to apply to distillates froin midcontinent crude a t atmospheric pressure and in the temperature range from that a t which the distillate is just completely vaporized to temperatures where thermal decomposition sets in. I t s applicability t o distillate, froin crudes other than midcontinent has not been shown. HOWever, it would probably give results not much in error. It will be noted that the equation is similar to that of Fortsch arid Whitman ( 1 ) for the specific heat of the liquids. The comparison of the calculated values for liquid and vapor is shown in Figure 7 . Ackncwledgment
The writers are indebted to W.H. hlcAdams aiid W.G. Whitman for suggestions in the development of the empirical equations.
LEWISA N D MCADAMS BAHLKEA X D K A Y Propane Butane
0 434 0 410
0 428 0 420
945
Literature Cited (1) Fortsch and Whitman, INDEVG CHEM 18, 795 (1926) ( 2 ) Lenis and Ivlc4dams, Chem .Vel Eng , 36, 336 (1929)
Thermal Treatment of Natural Gas' D. S. Chamberlin and E. B. Bloomz LEHICHUNIVERSITY, BETHLEHEX, Pa.
vv
cubic meters in a horizontal Natural gas was thermally treated in tubes of silica, 'HILE studying some steel, copper, iron, nickel, monel, and clay. Between reaction tube of quartz, 3 cin. of the characterisinside diameter. Copper extemperatures of 500" and 900" C., the various tubes tics of natural gas erted little catalytic effect on had different effects on converting methane to benzene, as found in various parts of the decomposition; with an naphthalene, anthracene, acetylene, etc. All these this country, it was observed iron tube no c o n d e n s a b l e materials except nickel gave yields of benzene varying that when natural gas was p r o d u c t s of decomposition from 4.00 to 40.00 liters per thousand cubic meters gas thermally treated clouds of oil were obtained. The yield of treated. Nickel catalyzed methane, causing a breakvapors were formed and that aromatic hydrocarbons dedown to carbon and hydrogen. benzene as one of the chief creased as the hydrogen diluThe effects of temperature, surface, dilution, and constituents of the treated gas tion increased. other factors were studied. Various units are discussed could be recovered by adsorpBetween 1000" aiid 1200" ranging from laboratory-size apparatus to a semition. C., Stanley arid S a s h (3)have commercial installation. Bone and Coward ( I ) made concluded that comparatively This work shows that carbon which is formed in the these observations in their long heating periods and t h e treatment tubes is responsible for the specific catalytic study of the decomposition presence of such active mateeffect in changing natural gas to aromatic hydrocarof methane, but little regard rials as iron and nickel cause bons. The CH residue is not necessary in the conwas given to this mist formainethane to deco1npo.e into version but no doubt takes place in the polymerization tion. carbon and hydrogen. On to aromatics. Fischer ( 2 ) found that a t the other hand. shorter heattemperatures around 1000 O C. and- a high gas velocity satisfactory yields of tar and light ing periods and the presence of such materials ab porcelain, oil were obt>ained. Quartz and porcelain were suitable tubes silica, and beryllia are much less active in proriiotiiig this for this synthesis, while iron and copper tubes favored the decomposition. This investigation on the heat treatment of iiatural gas, formation of carbon. On diluting methane with other gases, a high temperature of conversion mas necessary to bring which has been under way for the last four years, has conabout the same degree of conversion. The tar contained sidered the practical treatment of natural gas in apparatus naphthalene, phenanthracene, and the light oil, benzene and of large table size as well as semi-conimercial uiiitq, to examine the mechanism of thermal treatment as regarding toluene. \\-heeler and Wood (4)found that benzene was an iinpor- surface, materials of construction and conrersion, and t h e t a n t product of the pyrolysis of methane between 8i5" and economic thermal conditions necessary to give a practical 1000" C. The optimum temperature for the productioii of conversion. benzene was about 1050" C., yielding 27 liters per 1000 Materials, Apparatus, and Method Presented before the Division of Gas and 1 Received April 11, 1929. Fuel Chemistry at t h e 77th Meeting of t h e American Chemical Society, Columbus, Ohio, April PO to M a y 3, 1929. 2 Columbidn Carbon Fellow, Lehigh University.
The natural gas as used in these experiments was obtained from near Charleston, W. Va., and from the Monroe field of Louisiana. These gases were compressed a t the wells into
