INDUSTRIAL A N D ENGINEERING CHEMISTRY
Yovember, 1928
1169
gerated the conditions met in practice. Statistics compiled in this company’s laboratories indicate that approximately 40 per cent of the scrap tires received by the reclaimer contain reclaimed rubber. Tires containing reclaimed rubber have been reclaimed for a number of years without any decrease in the quality of the reclaim. In fact, the quality of reclaimed rubber is improving from year to year, and it is probable that, Table 111-Comparison of a Regularly Blended Scrap w i t h a Highin spite of the “reclaiming of reclaim,” this upward trend in R e c l a i m Scrap quality will continue as research leads to better methods of (Devulcanized 9 hours at 184.4’ C.) PRESS STRESS ELONGATIONmanufacture. KINDOF SOF- PLAYTICITY ACETONECUREAT AT AT Conclusions SCRAP TENER Y VALUB EXTRACT 141.1’c. BREAK BREAK the laboratory. Factory-blended whole-tire scrap was reclaimed in a similar experiment as a control. The amount of softener added to the high-reclaim scrap was reduced 50 per cent, because it was considered already high in softener content due to the reclaim. Tests on the resulting reclaims are shown in Table 111.
yo Regularly blended
25 to 40%
reclaim
Hours
%
Minutes
K g . / s q . cm.
yo
5
2.54
10.1
15 25
42 45
415 250
2.5
2.42
9.6
15 25
35 49
445
430
As far as could be ascertained, the reclaims were comparable in workability as judged by plasticity figures and hand examination. The reduction of softener proved to be in the right direction, since the acetone extracts of the two reclaims are practically the same. The reclaim made from scrap which was already fairly high in reclaim had a tensile . as good as and an elongation somewhat better than the control. The above experiment made it evident that a fairly high concentration of reclaim in the scrap had but little effect on the quality of the reclaim and even this concentration exag-
1-Reclaimed rubber, without the addition of new rubber in a subsequent vulcanization, cannot be repeatedly cured and reclaimed without a considerable deterioration in its physical properties. 2-The rubber scrap for reclaiming should be intelligently blended, and an endeavor made to keep the concentration of once-reclaimed rubber as low as is economically possible. 3-The chloroform extract and sulfur analyses of the uncured reclaimed rubber yield valuable information in regard to the quality of the product. Acknowledgment The writer wishes to express his appreciation to N. A. Shepard and H. F. Palmer for their many valuable suggestions relative to the experiments and the writing of this paper.
Critical Temperatures and Oil Cracking”’ Ralph H. McKee and Harold H. Parker COLUMBIA UNIVERSITY, NEW YORK,N. Y.
IL-CRACKISG processes have been generally classified by petroleum technologists as liquid- and vaporphase cracking. The extensive use of these terms in the literature of the industry, especially in patent specifications and drawings, makes it imperative that some dependable standard be established to differentiate between the two methods. Since the fundamental work on critical phenomena, by Cagniard de la Tour3 more than one hundred years ago, the temperature of change between the liquid and vapor states when a liquid, either pure or a binary mixture, is heated under pressure has been recognized as the critical temperature. There seemed to be no reason why this distinction could not likewise be applied to more complex liquid mixtures such as gas oils and other cracking stocks, and that a scientific basis for differentiating between liquid- and vaporphase cracking processes might be established. With this purpose in view an investigation was carried out to determine the critical temperatures of these complex mixtures, including both well- and shale-petroleum distillates. It was found that these oils have definite critical temperatures in just the same way as the simpler binary mixtures of organic compounds which have been previously reported in the literature. I n addition it was learned that these critical temperatures may be calculated from the
0 ’
1 Presented b y title before the Division of Petroleum Chemistry a t the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 to 16, 1927. Revised paper received July 6, 1928. This paper is part of the work presented by the junior author for the degree of doctor of philosophy in the Department of Chemical Engineering, Columbia University. a Ann. chim. 9hys., I21 21, 121 (1821).
average boiling points, in turn obtained from the ordinary standard analysis (A. S. T. M. method), by the following equation: te = 1.05tb
+ 160
(1)
This also represents the relationship between the boiling points and critical temperatures of normal, straight-chain aliphatic hydrocarbons. Further, this work shows that this equation holds for mixtures of straight- or branched-chain saturated or unsaturated aliphatic hydrocarbons, and for naphthenes, but not for aromatic compounds. The equation was derived from the boiling point-critical temperature graph for the straight-chain aliphatic compounds, t , being the critical temperature in degrees Centigrade, and t b the average boiling point. If Fahrenheit temperatures are used, 288 is substituted for the figure 160. Future workers, by the use of this equation, which checks the observed critical temperatures quite accurately for well-petroleum and shalepetroleum oils, can avoid an experimental determination of the critical point. The experimental data reported in this paper cover distillates from five widely separated fields in this country, varying in boiling point from that of petroleum ether to that of gas oil. Zeitfuchs’s experimental values4listed in Table I check with the writers’ equation. Furthermore, the standard A. s. T. M. analyses reported herein were made by three different persons. It would thus seem that the equacion was applicable under a wide variety of laboratory conditions and that the method was essentially free from factors of personal error. 4
IND. END.CHEW.,18, 79 (1926).
INDU8l’RIAL A,YD ENGINEERISG CHEMISTIZY
1170 Table I-Calculated
Vol. 20, No. 11
Values f r o m Work of Zeitfuchs on California Petroleum Oils Av. BOILILG CRITICAL TEMPERATURE GRAVITY POINT Obsd. Calcd.
as definite dividing lines between liquid and vapor states. although certain workers6 have failed recently to see a conOIL nection between this and the existence of a critical temperaA . P. I. c. c. c. ture for petroleum distillates. Landolt and Bornstein? list Petroleum ether 80.9 52 211 216 Gasoline 53.9 136 298 303 some thirty-nine binary mixtures for which critical data Naphtha 48.2 156 348 324 W. W. distillate have been reported Hartman* has published an elaborate 40.8 203 376 372 Gas oil 27.8 286 459 461 treatise on the phase relations involved, and Straus and Pawlewskij have proposed a simple volume rule with which to Critical Temperatures of Cracking Oils calculate the critical temperatures of the mixtures from those The work also revealed the fact that the critical tempera- of the components. Forty-three per cent of the binary mixtures of the oils used as cracking stocks, especially those tures listed in Landolt and Bornstein obey this rule. Excontaining lighter distillates, as would be the case in actual amples are air for gases and mixtures of ethyl ether and operation, were lower than the temperatures used in some benzene for liquids. No work has been done with members of the more import,ant, so-called liquid-phase processes such of the same homologous series, but the present writers’ work as the Cross and the “tube and tank.’’ I n plant operation with gas oil, gasoline, and kerosene shows that such mixtures of these the average temDerature of the oil leaving the heat- of these hydrocarbons obey this rule. ing tubes is 454”-C.(85bo Very little work has been F.). The critical temperapublished for mixtures of ture of the charging stock is m o r e t h a n t w o compoThis investigation was carried out to determine the generally less than this and nents. Zeitfuchs4 has recritical temperatures of complex mixtures such as is lowered as cracking occently made some measuregasoline or gas oil, with a view to using them as a basis curs, owing to the formaments on California petrofor classifying different cracking processes. tion of non-condensable gas leum oils, and Guidog has It is shown that the critical temperatures of these and lighter fractions. This studied the effect of comproducts are definite temperatures, and that they can is well illustrated by the position on the critical tembe readily calculated from the results of the ordinary data in Table 11, which perature of lemon oil. A. S. T. M. distillation; further, that the common were calculated from disOf interest to the petrointerpretation of several of the commonly used oilt i l l a t i o n analyses of the l e u m technologist is the cracking processes as “liquid-phase” processes is charging stock and of oil compilation by Wilson and probably without justification. leaving the heating tubes Bahlke’o of critical data for Although asphaltic materials are non-volatile in the in R cracking unit of the saturated, normal aliphatic ordinary sense of the word, they are soluble in highly Cross type. hydrocarbons. compressed vapors. It will be observed that It seems unreasonable to the cracking occurring in s u p p o s e that these more the tubes produces a sufficomolex hvdrocarbon mixcient quantity of gas and lighter oil fractions to lower the crit- tures (gas oils, kerosenes, and gasoline’s) shohd differ materiical temperature of the charging stock 20” C. (36” F.) This ally in their behavior from the above binary mixtures. value is further decreased as the cracking proceeds in the Phase rule relations demand that there be one definite temreaction chamber. I n the “tube and tank” process the per- perature a t which the mixture is homogeneous. Further, centage of the cracking taking place in the tubes is even the experimental work of the writers has proved beyond larger than in the Cross process. question that there is a definite temperature a t which the Further confirmation of this lowering of the critical tem- liquid phase vanishes. perature is furnished by the experimental work with mixtures Experimental Procedure of gas oil and gasoline. (Table VI) Such hydrocarbon mixtures apparently obey the rule of Straus and Pawlewski,6 The determinations were made in sealed tubes of Pyrex which states that the critical temperature of the mixture is glass 10 to 12 cm. long. These were heated in an electric proportional to the volume per cent of the components. This furnace like the one described by Z e i t f u c h ~with , ~ sufficient empirical relationship is discussed elsewhere in this paper. variable resistance in the circuit, to maintain a constant temperature a t any point over the range 250” to 500” C. A Table 11-Average Critical Temperature for Different Distillates furnace in which the tubes could be placed horizontally was SAME WITH 30% BY CROSSPLALTDATA not found satisfactory, since t,he meniscus could not be VOL. OF Charging Oil at end of STOCK Ossd. GASOLINE stock tubes readily seen. The glass tubes should have a bore large c. C. O F . ’ C. ’F . enough (1 to 3 mm.) that the meniscus may be readily obKerosene 405 376 444 831 424 795 served, and a side wall a t least as thick as the inside diameter Mineral seal or “300” oil 458 413 to avoid any danger of breakage a t the high pressure. This Gas oil 475 425 pressure is for the gasolines near 400 pounds per square inch The temperature dividing lines listed in Table I1 were (28.2 kg. per sq. em.) in the vicinity of the critical point. calculated by this rule. They show the approximate limit It decreases as the boiling point of the oil is increased. The of the operating temperature above which the oil cannot tubes are preferably sealed by using an oxy-gas or oxgexist as a liquid. These average critical temperatures acetylene flame while the filled end of the tube is immersed are given for different charging stocks and are based upon in an ice and salt mixture. the above plant and laboratory evidence. Accurate figures The critical temperature, the point a t which the meniscus may be calculated for any particular oil from the distillation 6 Kleinschmidt, Gasoline Products Co. vs. American Refining Co., U.S. analysis by the above equation. District Delaware; Equity 571, p. 348. Little, I b i d . . p. 370 (1926). Wilson, 0
0
Critical Temperature of Binary Mixtures
Critical temperatures of pure compounds, of binary mixtures, and also of solutions have been generally recognized I
J . Russ. Phrr.-Chem. SOC.,18, 207 (1880)
U. S. UP. Standard Oil Co. (Ind.), et al., Northern District Illinois, Equity 4131, p . 2731 (1926). 7 Tabellen, Vol. I (1922). 8 J. Phys. Chem., 5, 425 (1901). 8 Giorn. chim. ind. aofilicata, 4, 8 (1921). 10 I N D .E N G .CHBM., 16, 115 (1924).
INDUSTRIAL AND EXGINEERING CHEMISTRY
November, 1928
disappeared on heating and appeared on cooling, was measured with a platinum, platinum-rhodium therinocouple in conjunction with a Leeds and Korthrup portable pot,entiometer. The glass furnace is sliown in Figure 1. Accuracy of Measurement The accuracy of the measureruent depends upon sccrirate temperature-measuring instruments, elimination of lag between oil and furnace temperature, provision for having the contents of the tube homogeneous at the critical point, :md correct observation of the critical point. Correct temperature n~easurementwas insured by using the thermocouple above mentioned. The set,-op had been recently standardized and was accurat,e to *2' C. Temperature lag between the oil iii the thick-walled glass tube and the air of tho furnace might introduce considerable error. This would be most easily eliminated in ordinary work by maintenance of constant temperature for some time in the vicinity of the critical point. However, appreciable cracking occurs with these materials if maintained at a high temperature for 10 t,o 15 minutes. This changes the composition of the liquid and introduces an error into the measurement. T o exclude this, experiments were first run with an equimolal rnixturc of sodium and potassium nitrates, which was inserted in the furnace uiider conditions similar to those under which the oil was inserted. It was found that a rate of about 5" C. per minute near the critical point gave no appreciable lag. No cracking was evident under these conditions except with the mineral seal aiid gas oils having critical temperatures near 460" and 480' C., respectively. To determine the magnitude of the error introduced by this cracking, the critical temperature of a Bradford, Pa., gas oil was ascertained as in the previous runs; the tube was then allowed bo cool and a second determination made using the same tube. I n like manner a third value o[ the critical temperature was obtained. If cracking occurred during those runs, successive values should be lower than the previous one, and the amount of this lowering would depend upon the quantity of the lighter fractions present, which is a measure of the amount of cracking. The extent of this cracking is governed by the time a t the high temperaturesio this case the time that the tube was kept above 375' C., b e low which theamount of cracking would not beappreciable as shown byrecent workof Leslieand Potthoff." The first time noted in Table I11 was only that required to heat the tube from375" C. to the critical point, or 9 minutes. The 7 and 8 minutes included both the time of cooling from the first observed critical point to below a temperature at which any appreciable cracking occurs and also the time of reheating to detcrmine the critical point of the now partially cracked oil.
as compared with 397" C . reported in the literature. This lower value may be due to the slight decomposition which plainly occurs when bromobenzene is heated in a sealed tube to the critical point. Different values are obtained when different amounts of oil are used in the tube, as shown by the data in Table IV. The ratios refer t.o tho volume of the tube compnred with the volume of the oil at. room temperature. A high rat,io gives a high result, which agi'ees widi work recently published by Zeitfuchs' for California petroleum oils. The error may be as large as 40" C. This variation with binary mixtures led early workers t o doubt the existenceof a definite critical point, but tlieerperim e n t s of Young,'* Ramsay,js and Villard14 clearly showed that it was due to differences in the composition of the liquid and vapor phases. This error, due to I not having the cont e n t s of t h e t u b e ~ i ~ u l--Purnncefor r e Critical Temperatuse Measurement homogeneous at the critical point, ivav avoided in this work by filling the tubes so that, allowing for expansion, they would he practically full near the critical ooint. It was imnossiblc to remove all error in this way, as the liquid must always be a t least a short distance from the top in order to observe the ' C. if meniscus. IIowever, this error is not more than 4 a tube-liquid ratio of 1.9 is used-that is, with the tube about one-half full-as the expansion ratio between the liquid at the critical point and the liquid at room temperature is near 1.7. A difference in ratio should give a result ahout 4" C. too high based on interpolation from the data in Table IV. Table IV-Effect OIL
First Second Third
-
TGYPBR*T"RB
c.
475 470 403
c.
A
A €3
n
n
0 s CnacxrNc Minulem 9
8 7
From each of these determinations the lowering of the critical temperature of the oil in the tube per minute of cracking may be roughly evaluated. This effect averages near 7" C. for 9 minutes of cracking. It is accordingly apparent that we have here a source of error which gives, with materials such as gas oils, which crack at this temperature, an observed critical temperature only a few degrees lower t,han the true value of the starting material. Error due to incorrect observation of the critical point was avoided by determining the critical temperature of carefully purified bromobensene. The value found was 393" C. IT 1x0.
Esa. C t r a ~ . 18. . 776 (1920)
4.0 2.7 1.9 3.0 2.3 1.9
320
4.0 2.8 2.0
420 408 392
YO9
288 306 290 282
Kerosene:
C
c
TIM*
OBSERYSB
CX~TZCIL Taxl~nn~ruae a,.
A
(Gas oil. Bradford, Pa., field) RBITINO
of Tube-Liquid Ratio
RATIOV,:Vx
Gasoline:
Table 111-Effect of Cracking on Critical Temperature Measurement OBSD. CBITIC*L
1171
c
This difficulty witb binary mixtures has been overcome by former workers by rotating the tube in the heating medium. According to Nernst,I3 very well defined values can be obtained in this way, and Centnersswer and Zoppi'6 report measurements accurate to 0.1' C. Both references are tu binary mixtures. Experimental difficulties due to the cracking of the oil at the high temperature made it impracticable to use such a set-up in this work. The over-all accuracy of these experiments is no doubt well within 10" C . limits. The maximum difference between J . Cham. SOC.(London), TlT, 447 (1897). 2.$hyrik. Chcm., 14.486 (1894). Ann. rhim. P k w . 171 10, 387 (1897). "Theoretical Chemistry." 8th to 10th German editions, revised by Codd. 1923. p. 74. Macmillsu and Co..Ltd. 3. E . ghyiik. Cham.. €4,089 (1906). I*
1:
I*
INDUSTRIAL AND ENGINEERING CHEMISTRY
1172
the readings obtained on heating and cooling was around 0.06 millivolt or approximately 6" C. and the average deviation was 0.03 millivolt or 3' C. The error due to not having
the tube completely full makes the measurement too high by about 4" C. Adding to this a maximum error in temperature reading of 2' C. makes the total possible error nearly 10" C. The agreement between the observed and calculated values of the critical temperature is well within the requirements, as the differences may be due to errors in the experimental work.
boiling point ratio as commented upon by Rittman, Dutton, and Dean.17 I n support of this it was found that the critical temperatures of these oils listed in Table VI are intermediate between calculated values by the above equations. I n other words, the last equation above should give the true critical temperature of a mixed distillate only if the oil contained 100 per cent of aromatic compounds. The other equation should hold if no aromatics were present. This critical temperature work may thus be used to indicate the chemical nature of the substances present in the oil.
General Discussion
Table VI-Observed
The empirical equation (1) developed in the course of this study expresses the critical temperatures of the oils as a straight-line function of their boiling points. These are calculated by averaging the initial temperatures with those recorded for each of the 10-cc. cuts in the standard A. S. T. M. distillation. The temperature corresponding to 100 cc. of distillate was obtained by extrapolation. An example of this method of calculation is given below: Bradford, Pa., Gasoline; Gravity 61.8' A. P. I. Distilled
CC.
DISTILLATION ANALYSIS Temp. Distilled
Temp.
cc.
c.
e
Then, by equation (IF-
c.
O
+
+
t c = 1.05t. 160 = 1.05 X 130' 160 = 296' C. Observed critical temperature = 298' C.
This method does not, of course, give the true average boiling point of the oil if considered from the ideal case in which the distillation curve is a straight line. It has been used in this work because of its wide acceptance in previous literature, and it has the further advantage that it makes use of an established method of analysis, the A. S. T. M. distillation. As noted above, this equation was derived from a consideration of the boiling points and critical temperatures of normal, straight-chain aliphatic hydrocarbons. It may also be used to calculate critical temperatures for unsaturated compounds, of which some typical ones are listed in Table V. Temperatures of Unsaturated a n d Aromatic Hydrocarbons CRITICAL TEMPERATURE HYDROCARBON BOILINGPOINT Obsd. Calcd.
OIL
c.
O
c.
UNSATURATED
201.2 191.6 150.7 234.4 227,3 304,S
202 197 154 222 230 290
288.5 320 6 358 3 345.6 344.4 280
244 276 309 305 302 244
40
Amylene Isoamylene Isobutylene Diallyl Diisopropyl Octylene
59.5 87 124.6
Benzene Toluene o-Xylene m-Xylene p-Xylene Hexamethylene
80.1 110.7 142 139 137 81
-:3
A.
(2)
288 319 350 347 345 289
Aromatic cmpounds, some of which are listed in this table, do not obey this rule. The relationship in this case may be expressed by the following equation using Centigrade temperatures and the same symbols as before: tc
=
tb
+ 208
P.I.
C.
179 240a
ii7
135 131 137 138 130 140 140
C.
358 433 43s 301 29s 302 307 310 298 307 322
O
C
348 412 io4 302 298 304 305 296 307 307
351 405 406 405 384 458 458 458 487 439
182 232 234 232 213 283 284 284 311 266.
...
...
City, November, 1926.
Observed and calculated critical temperatures for the different oil distillates are compared in Table VI. The shale-petroleum distillates possess slightly higher critical points than the corresponding well-petroleum materials, perhaps owing to the presence of aromatic compounds. Both classes exhibit very definite transition temperatures. The critical temperatures of different percentage-byvolume mixtures of gas oil and gasoline are shown in Table VII. The calculated values are based on the rule of Straus and P a w l e w ~ k i . ~Thus for the mixture containing 10 per cent by volume of gasoline having an observed critical temperature of 298" C., the critical temperature is 0.1 X298" 0.9 X 478"=459O C. The observed value was 468" C.
+
Table VII-Critical
Temperatures of Mixtures of Gasoline a n d Gas Oil OBSD. CALCD. OIL
Bradford gas oil Bradford gas oil containing 10% by volume of gasoline Same, 207, gasoline Same 33'3 gasoline BurbAnk "300" oil and Burbank gasoline, 50% by volume
c.
478 468 467 425 380
= c.
487 459 443 418 381
Effect of Asphaltic Materials
AROMATIC
(1)
a n d Calculated Critical Temperatures Av. CRITICAL BOILINGTEMPERATURE ORIGINO F CRUDE GRAVITYPOINT Obsd. Calcd.
Gasoline Utah shale 47.4 Kerosene Utah shale 34.0 Gas oil Utah shale 26.6 Gasoline No. l b New York City 54.5 Gasoline No. 2b New York City 59.5 Gasoline No. 3b New York City 60.6 Gasoline No. 4b New York City 58.0 Gasoline 57.2 Burbank, Okla. Gasoline Bradford, Pa. 61.3 Gasoline Pioneer, Texas 56.3 Naphtha Vinton, La. 50.3 "Turpentine substitute" Vinton, La. 39.4 Kerosene Burbank 42.1 Kerosene Bradford 45.3 Kerosene Pioneer 42.9 33 5 Kerosene Vinton 300" oil Burbank 37 0 "300" oil Bradford 40.5 "300" oil Pioneer 38.1 Bradford Gas oil 38.2 Vinton Gas oil 28.0 Transformer oil Vinton 24.0 N o distillation analysis. b Four typical gasolines on sale in New York
Table V-Critical
c.
Vol. 20, No. 11
(2)
I t s use is illustrated by the calculated values in the column with the heading (2) in Table V. The presence of these aromatic compounds in the Vinton, La., and shale distillates is indicated by the high gravity-
Finally, the question arose as to whether asphaltic materials soluble in the oil to give a liquid, similar to a charging stock, remained in solution above the critical point. Work with such solutions showed no precipitate even when the liquid was kept for a long time above the critical temperature proving that the asphalt was soluble in the highly compressed vapor. This agrees with work18 previously reported in the literature €or solutions of various non-volatile solutes in organic solvents. Acknowledgment
The authors wish to acknowledge the kindness of W. F. Coover and F. E. Brown in placing the facilities of the Chemistry Laboratory, Iowa State College, Ames, Ia., at their 18
Chem. News, 41, 103 (1880).
