Limiting Lompositions in S. W. FERRIS
Data are presented indicating that: Regardless of extrema variations in type of stock, pressure, temperature, and rate of reaction, and regardless of the presence or absence of catalysts, whenever petroleum hydrocarbons are subjected to physical conditions capable of changing their chemical composition, the reactions proceed toward the formation of a synthetic crude exhibiting a definite boiling range, defined as the limiting band. Attainment of that boiling range requires a certain amount of hydrogen chemically combined in the liquid products of the reaction. This boil-
The Atlantic Refining Company, Philadelphia, Penna.
EVERAL investigators of petroleum cracking have pointed out that the reaction appears to travel toward a n equilibrium. They do not agree, however, either upon the nature of the final product or upon the controlling factors. For example, Snelling ( 1 4 , in a patent filed May 5, 1915, states: I have found that the action of heat and pressure upon oils does not, as is usually presumed, go only one way; . under pro er conditions an equilibrium appears in which breaking down anzreconstruction actions go forward until a certain balance obtains between the concentration or amount of the gases, of the low-boiling oils, and of the high-boiling oils in the space afforded for action. Being a true equilibrium it can be approached from either side; that is, I can heat a heavy oil under proper definite conditions . . . . and produce gases together with . . . . a synthetic crude oil . . . . (or) I can produce this same equilibrium by heating light oils, or even hydrocarbon gases, under suitable heavy pressure and at a high temperature . . . , . the equilibrium obtained . . . . . depends upon the ratio of the volume of the original liquid and the capacity of the container.
S
35.4 per cent. (The pressure and temperature conditions for the experiments covered by Figures 1 to 11, 13 to 18, and 21 t o 26, inclusive, are found in Table I.) The family of curves in Figure 1 strongly suggests that the cracking reaction proceeds toward the production of a synthetic crude characterized by a definite boiling range. These and similar data indicate that the limit toward which the assay distillation moves is the line representing the following figures:
A review (6) of three papers by H. A. Wilson reports that he found the products of cracking to consist largely of paraffin and olefin hydrocarbons, which reach a state of equilibrium in the cracking unit, and that maximum amounts of any given hydrocarbon may be obtained by proper choice of temperature and pressure. Discussing the chemical and physical nature of cracking in his handbook, Cross (4)says:
Initial 5%
10 20
In the treating of a great variety of oils for the production of a synthetic crude oil, there is always a tendency toward a chemical equilibrium among the different series of hydrocarbons as well as among the different hydrocarbons of different boiling points and gravities of the same series. This tendency toward equilibrium is of course quantitatively different under different conditions of heat and pressure. . . . I n treating a typical heavy residual stock a t a temperature of 800” F. and in pressure equilibrium with the evolved vapors and gases . . . . the yield of gasoline hydrocarbons reaches a maximum after a certain period of time.
ll5O F. 135 137 205
% ;; 80 90
2QS‘ F. 415 605
710
I n order that i t may be distinguished from assay distillations representing specific samples, and in order approximately to indicate the reproducibility of the distillation test, a “limiting band” has been constructed. It is defined by two lines, one lying 1.5 per cent (on the vertical axis) above and the other a n equal distance below the line corresponding to the data given in the above table. This band is shown on all subsequent charts of assay distillations. Before proceeding to other examples, i t is well to define the synthetic crude and to indicate to what extent similar assay distillation curves may be considered to indicate similar composition. Synthetic crude, as used here, includes all the liquid material resulting from a reaction, under such conditions of condensation that the vapor pressure will approximate 10-15 pounds per square inch absolute. Roughly this means that the synthetic crude contains considerable butane but little propane and only minor quantities of ethane and methane. Solid constituents such as coke are obviously not included. I n modern cracking units a synthetic crude, as the term is here employed, exists only in the cracking coils and is not recovered as such, since it is split into gasoline, recycle stock, and tar. In several of the examples taken from the literature, only these narrower boiling fractions were reported, and it was therefore necessary to “reconstruct” the assay distillation of the synthetic crude. However, many trials have demonstrated that this procedure does yield the boiling range of the synthetic crude with all the accuracy required in the present discussion.
A curve shows this maximum to be between 55 and 60 per cent of 437” F. end point material in the synthetic crude. The purpose of this paper is to present data bearing on the same question; while developing the theories here presented, the author has referred to “equilibria”. He has found, however, that this word connotes, in the minds of others, many specific characteristics and relations which are not borne out by the data presented and form no part of the accompanying theories. I n an attempt to avoid this sort of confusion, no further use of the word “equilibrium” will be made, and it is hoped that the expressions used will not, as did equilibrium, convey impressions quite foreign to the author’s thinking. Thermal Cracking Figure 1 presents the assay distillations of synthetic crudes resulting from single-pass high-pressure thermal cracking of a paraffinic gas oil. The severity of cracking may be judged by the amount of gas (by weight of stock) indicated on each curve. When 27.5 per cent of gas was produced, there was a maximum of low-boiling material and the curve was but slightly affected when the gas production was increased to 752
Hydrocarbon Conversion J
ing range will persist under further treatment until, by loss of hydrogen (as hydrogen gas, methane, and ethane), the specific gravity at 60° F. of the synthetic crude becomes higher than about 0.93. Beyond this point aromatics, chiefly benzene and naphthalene, are formed. Natural crudes appear to be reacting during geologic time toward the attainment of the same boiling range. The only method of forcing the reaction of a heavy stock beyond the limiting band seems to be the provision of large amounts of hydrogen in the presence of a catalyst.
An assay distillation affords information with respect to the percentage of materials boiling within certain ranges. It can show nothing more. It is possible, for example, to prepare a mixture of pure aromatics which, in terms of an assay distillation, would be identical with a mixture of pure paraffins. When, therefore, various synthetic crudes are shown here to be approaching the same limiting band, there is no implication that the various synthetic crudes are chemically identical. On the contrary, data will be presented to show that they vary over rather wide limits. Molecular weight does, however, follow the property of boiling point rather closely, and it can thus safely be assumed that a synthetic crude showing the same assay distillation would be found to contain a similar distribution of components of various molecular weights. The distillation curves in Figure 2 also resulted from thermal cracking; but the stock (the residual from a naphthenic crude) was very different from that of Figure 1. Again,
however, the synthetic crude appears to approach the same limiting band. A still heavier stock was chosen for the runs shown in Figures 3 and 4. I n the former the cracking pressures were very high (3000 to 4800 pounds); in the latter they were low (50 to 300 pounds). The course of the reaction (as judged by distillation curves) seems, however, to have been little affected and in each instance to be seeking the same proportion of low- and high-boiling compounds. The curves of the stocks so far reported have fallen to the right of the limiting band; that in Figure 5 was of such boiling range that it crossed the band. The route of the curve was necessarily greatly different, but it appeared to proceed toward the same band. Thermal reforming utilizes a light stock which crosses the limiting band at a low temperature. Figure 6 presents the results of high-pressure thermal reforming. Following the curves in order, the first part of the reaction (3.1per cent gas) produces low-boiling constituents; their concentrations are increased until the lower part of the limiting band is reached, and during this period only relatively small amounts of heavy compounds are formed. Once the lower portion has reached the band, however, the reaction seems to be largely polymerization, with the result that the upper portion of the assay of the synthetic crude proceeds rapidly toward the limiting band. Statements such as “the reaction seems to be largely one of polymerization” should not be rigorously construed. So far as the assay reveals, the net result is that of polymerization. Some cracking doubtless occurs a t the same time, but not enough to increase the proportion of low-boiling constituents. The same stock was thermally reformed a t atmospheric pressure. I n general (Figure 7), the course seems to be very similar with the exception that much larger quantities of gas are produced; 60.2 per cent of the entire stock was converted to gas before the reaction proceeded far enough to show definitely that the synthetic crude was becoming heavier and approaching the band.
OF EXPERIMENTS INCLUDED IN THE FIQURES TABLE I. CONDITIONS
Fig No.’ 1
Designation 3 . 6 % gas 7.8% 17.2T0 21.S70
2 3
2.6% 5.9% 12.5%
i:% I!:% 1.5% 5.1% 6.37 16.48
7
3.1% 15.2% 35.5
60.2
Gage Pressure Lh/Sq. Ih.
Temp, 0 F.
3000 3000 3000 3000 3000 5000
875 929 925 1000 1015 975
5000 5000 5000 3000 3000 3000 4500 sn 300 300 3000
900 925 950 975 1035 920 925 946 812 984 1000
3000 3000 3000 0
925 1000 1026 1170 1235 1300
_ I
0 0
Fig No. 8 9
10
Designation 7 . 8 % oatalyst deposit 13.8%
....
1 2 7 4 5
11 13 14 15
16
1
a
1315-4410 1400-4970 880-4100 590-4340 370-4640
... ...
750 750 750 750 3000 3000
Temp., F. 850 850 850 806
* * *
Fig. No. 21 22
SO6 824 842 842
68-77 450 392 850
0 50
4
17
20 20 20
0 100
3
1s
Gage Pressure, Lb./Sq. In.
23
24
212 572 572 302
25
925 902
26
Designation 0 % carbon 2.4% 5.3% 3.7% 7.57 15.3%
0 %
9.6% 13.270 20.0% 24.6% 2 5 7 10: 1% 16.270
%:%
36.2% 38.8%
Gage Pressure, Temp., Lb./Sq. In. O F. 162 74 250 0 338 74
932 1112 1112 1382 1112 1382
0 74 338 338 74 250 74 162 162 74 74 338 250 74 162
932 932 932 1022 1112 1112 1382 1382 842 932 1112 1022 1112 1382 1382
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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C' l
l
l
SINQLE-PASS
2 SINQLE- PASS
THERMAL CRACKINQ
THERUAL
l
l
l
Vol. 33, No. 6
l
l
l
l
l
~
~
I L
240
SINGLE-PASS
E O L
T H E R M A L CRACKINQ
FIGURES 1 TO 7
Processes Other than Thermal Cracking Even if it is accepted, as the data already presented strongly suggest, that for thermal cracking, variations of pressure and temperature seem not to change the boiling range composition toward which the synthetic crude proceeds, it would still seem highly probable that a real change would be effected by the presence of a catalyst. Figure 8, however, records the results of catalytic cracking as reported by Houdry and co-workers (9). I n this instance the catalyst deposit may be taken as roughly proportional to the extent of cracking, and when 13.8 per cent of deposit was produced (from a 17.5 per cent mid-continent crude bottoms), the resulting assay distillation rather closely approximated the limiting band. The other curve is from a 35 per cent West TexasNew Mexico bottoms. The results of the same process as applied to a gas oil stock are given in Figure 9. Candea and Sauciuc (3) record a large number of experiments on the destructive hydrogenation of a Bucsani crude residuum having an overpoint of about 626' F. From their results five curves were picked a t random which seem to indicate that this process also follows a similar path (Figure 10). Curve 5, lying almost on the limiting band, was the lightest reported.
Autodestructive alkylation with a charge stock consisting of pure 2,2,4trimethylpentane seems radically different from the processes thus far considered. The results of Ipatieff and Grosse (11) show, however, that even this process produces a synthetic crude which, if taken far enough, would apparently reach the limiting band (Figure 11). Birch and co-workers ( 1 ) secured a product not far from the limiting band by alkylation in the presence of sulfuric acid, starting with isopentane and diisobutane (Figure 12). It is obvious that in polymerization the resulting products will lie closer to the limiting band than the charge. I n commercial operation the reaction is controlled so that little material is produced boiling above 400" F., but Ipatieff and Egloff (10)carried out a run (Figure 13) where the polymerization proceeded so far that the limiting band was almost attained. While the stocks thus far considered have ranged from gaseous materials to very heavy residual products, they have all been hydrocarbons. The catalytic water-gas reaction operates upon carbon monoxide and hydrogen; that the assay distillation of the product nevertheless seeks the same resting point is demonstrated by the curve for Kogasin oil (Figure 14) as reported by Egloff et al. (6). In its relation to the limiting band, catalytic reforming, as
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
June, 1941
CATALYTIC CRACKING
CATALYTIC CRACKINO
HOUDRY ET AL.
HOUDRY ET AL.
755
D E S T R U C T I V E HYDROQENATION CANDFA
a
WJCIUC
.eye CATALYST DEPOSll
103
200
303
400
500
8oo°F.
x
80
80
c 60
80
HALL 8 NASH
3
:
40 C A T A L Y Z C WATER GAS REACTION
20
EQLOFr E T AL.
t 10i0 L200 &300& 400M5005 WU O 700 ~F. L ~
0
FIQURES 8 TO 16
described by Peterkin et al. ( I $ ) , is not essentially different from thermal reforming, as a comparison of Figures 15 and 6 shows. The work of Hall and Nash (7) indicates that when ethylene is polymerized with the aid of aluminum chloride or aluminum chloride and aluminum granules, products are formed which parallel and appear to approach the limiting band (Figure 16). Curve 1 was obtained at 212' F., the others at higher temperatures. It may be that a t low temperatures the reaction is almost solely polymerization and that this is accompanied by decomposition a t higher temperatures, carrying the synthetic crude toward the limiting band.
Specific Tests of the Limiting Band The stock shown in Figure 17 was deliberately chosen to give a n unusual assay distillation curve. It consisted of 60 per cent by volume of cracked tar and 50 per cent straight-run naphtha. The tendency for the assay curve to move toward the limiting band and for those portions most distant to move most rapidly is apparent. Another special stock (Figure 18) was a blend of cracked tar with a light straight-run naphtha, and in this instance the two separate branches of the assay curve move in opposite directions toward the band.
Table I1 presents another test. A naphthenic crude oil was thermally cracked to yield 8.6 per cent gas. This synthetic crude was recracked to yield an additional 8.7 per cent gas (based on the charge to the second pass). The charge to the third pass was the second synthetic crude, and the charge t o the fourth pass, the third synthetic crude. After the first pass the synthetic crude showed an assay very close to the limiting band, and the three succeeding passes failed to effect any material change in the assay distillation.
TABLE11. MULTIPASS CRACKING OF NAPHTHENIC CRUDEOIL Gas formed % b wt. Gravity O k P Sp. gr. i t 60' F: Assay distillation, F. Imtial b. p. 5% over 10 30 50 70 80 End point yo at end
f.
Synthetic Crude after: 2 passes 3 passes 4 passes
Charge
1 pass
37:6 0.837
8.6 38.6 0.832
8.7 36.2 0.844
8.1 33.7 0.857
8.2 31.7 0.887
112 168 196 316 501 673 784 969 96
105 146 170 262 360 603 596 644 89
96 138 165 254 346 503 598 647 88
102 148 172 254 348 498 600 646 88
115 165 179 259 363 504 606 656 88
INDUSTRIAL AND ENGINEERING CHEMISTRY
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I n the experiment recorded in Table 111, a naphthenio straight-run naphtha was blended with a heavily cracked tar in such proportions that the assay distillation of the blend closely approximated the limiting band. Thermal cracking with the formation of 4.2 per cent gas caused only minor changes in the assay distillation.
The evidence thus far presented appears to show that, starting with a material lying wholely t o the right of the limiting band, it is impossible to force the assay of the synthetic crude to the left of the band. The conception of distillate crudes is well established; they are believed to represent the light fraction of a normal crude, which has “distilled” and been trapped in another formation. Those crudes lying t o the left of the band may therefore be said to be distillate crudes. TABLE 111. CRACKING OF BLENDCONTAINING NAPHTHEXIC It seems reasonable to assume that the crudes deficient in * STRAIGHT-RUN NAPHTHA AND HEAVILY CRACKED TAR light material are residual crudes; that is, the material remaining in the formation after some geologic freak has Synthetic Charge Crude enabled the lighter material to escape.
...
30 50 70 80 E n d point a t end
4.2
38.3 0.833
37.2 0.839
117 145 173
106 130 150 259 372
266 380 532 630
650 83
;I lOOr
526 642 650 82
1. RUMANIA 2-OULAHONA 3-WEST VIRGINIA 4-UANSAS 5 S ,-IGl -E R CALIFORNIA YINI 7,S,10- T E X I S
20
Application to Natural Crude Oils
If it is true that substantially all thermal reactions of petroleum hydrocarbons tend to produce a synthetic crude lying near or within the limiting band, it should follow that natural crudes seek the same boiling range during geologic time. T o test this hypothesis, examination was made of a large number of crude assays as reported in various publications of the Bureau of Mines (3). The majority of crudes gave assay distillation curves roughly parallel to, and in some cases coinciding with, the limiting band. This is shown by the full lines (samples 1-6) of Figure 19. However, a smaller number was found whose assay distillations exhibited a steeper slope, as indicated by the broken lines. Furthermore, the latter appeared to fall into two classes: first, heavy crudes containing little or no light material (samples 7-12), and secondly those consisting principally of light material and falling definitely above the limiting band (samples 13-15).
FIGURES 17 AXD 18
0
--
0 - LOUIS1INA 12- WYOYINO 13- OANADA 14- ITALY 15- SUMATRA
I I00
200
WO
400
500
600
700
OF.
FIGURE19 In an attempt to strengthen this latter assumption, the percentage of distillate a t 527” F. was read from the assays of 904 crude oils as reported by the Bureau of Mines. These were segregated into bands and are plotted in Figure 20. If all crudes were approaching the limiting band in exactly the same fashion, Figure 20 would be expected to approximate closely a probability curve. It does have some resemblance to a probability curve, but the number of crudes showing 71 per cent or more a t 527” F. appears t o be abnormal. Similarly the left-hand portion of the charts appears abnormal. The three probability curves of Figure 20 represent the actual figures rather accurately. This of itself might be a coincidence, but reference to Figure 19 shows that distillate crudes should be characterized by yielding more than 70 per cent of distillate at 627” F. This is precisely where the small probability curve labeled “distillate crudes” (Figure 20) starts. The heavy crudes showing steep assay curves on Figure 19 exhibit from 4 t o 44 per cent distillate a t 527’ F. Again these are the limits between which the probability curve labeled “residual crudes” is drawn (Figure 20). We would expect to find more residual crudes than distillate crudes, because it may safely be assumed that many of the distillates which have escaped from the original formation have been totally lost. This is borne out by Figure 20. Also, Texas and Louisiana have long been known to yield many heavy crudes which are here characterized as residual. Only recently, however, has deeper drilling demonstrated that distillate crudes also exist in that field. If the suggestions thus far have been correct, it follows that either a distillate or residual crude, once formed, will take up its journey toward the limiting band, just as gases and heavy stocks, when cracked, proceed in opposite directions toward the band. The Sumatra crude shown in Figure 19 may therefore be a crude which was once separated from the natural crude as a distillate, but which has subsequently developed by a slow process of polymerization the heavy constituents characteristic of a normal crude. It can scarcely be more than pure conjecture t o say which of the curves represent the younger crudes. Pennsylvania
June, 1941
INDUSTRIAL A N D ENGINEERING CHEMISTRY SO4
CRUDE
OILS
PER CENT AT 527'F
FIGURE 20
oils are frequently considered very old, and they show high percentages a t 527"F. It may be, therefore, t h a t the German crude on Figure 19 is young. It has not yet been determined whether this relation does agree with the geologic indications, but in any event the relation would be proportional, not to time, but rather to time-temperature units. If the past temperature of a formation could be approximated, it is possible that a guess a t the age of crudes could be made.
Second Limiting Composition Even though a synthetic crude has "reached" the limiting band, i t is manifestly not stable in the sense that it is able to resist further chemical changes. The examples given in Tables I1 and I11 show that further pyrolysis of such synthetic crudes effects changes in chemical composition, as indicated by the changes in specific gravity, without materially affecting the boiling range. The final equilibrium for hydrocarbons is presumably carbon andrydrogen; it is therefore natural to wonder how long
757
the distribution of compounds resulting in the limiting band can "hold out", as more and more of the stock is transformed to materials which are not included in a normal assay distillation. The work of Rittman, Dutton, and Dean, reported in 1916 (IS), gives a preliminary answer to this question. They carried out a t the Bureau of Mines a large amount of work, one of whose principal objects was to produce aromatic hydrocarbons. They succeeded in doing so, and since they used rather low pressures (varying from vacuum to 24 atmospheres), low pressure has long been believed to favor the formation of aromatics. Figure 21 presents the results of several of their runs on Pennsylvania refined burning oil. The amount of carbon formed is indicated on each synthetic crude assay curve. They drove a Pennsylvania oil to the limiting band when 5.3 per cent carbon was formed. When, however, they cracked this stock even harder (Figure 22), the curves no longer followed the band but appeared to break into an inverted S-form. The two short vertical lines drawn above the limiting band on Figure 22 show the boiling points of benzene and naphthalene, respectively. The evidence seems reasonably good that the second limiting composition is benzenenaphthalene. Figure 23 records their work with Oklahoma fuel oil residuum. Substantially limiting-band composition was obtained when 9.6 per cent carbon was deposited. The same bending of the curves with additional carbon formation is, however, noted in Figure 24, and again the curves appear to be approaching benzene-naphthalene. The stock for the work shown in Figure 25 was California, crude oil, and it was found impossible to bring this stock to the limiting band. When, however, they produced extremely large quantities of carbon (Figure 26), the typical inverted S-curves approaching benzene-naphthalene were obtained. The California crude oil was less rich in hydrogen than the other two stocks. We are immediately led to suppose that the attainment of the limiting band or the maintenance of i t requires a t least a certain amount of hydrogen present in the liquid synthetic crude. The problem of the formation of
-100
200
330
400
500
600'6
IO0
200
SO0
400
SOO'F
80
40
20
1 0 O%CAROON
100
200
300
4W
S O T
700
200
300
400
500
FIGURES 21 TO 26
BOO'F:
INDUSTRIAL AND ENGINEERING CHEMISTRY
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aromatics in large quantities by thermal means would therefore seem to be not one of choosing particular operating conditions, but rather of carrying the reaction so far that the available hydrogen becomes diminished to the point where only aromatic linkages are possible. (Hydrogen in methane, for example, is no longer “available”.) The following table gives the specific gravities of a number of synthetic crudes to which reference has been made:
B:b
Description of Crude Rumania Up er ourve c a d y s t deposit
5.3 carbon 9.6 carbon 7.57 carbon 13.23 carbon 27.6 carbon 15.39 20.0%
:E:
Figure No. 19 10
Sp. Gr. a t 60e F. 0.802
21 23 22 24 26 22 24
0.926 0.931
Remarks
8 1
The limiting-band conditions can apparently be satisfied by crudes ranging in specific gravity from 0.800 to 0.930, but gravities above the latter figure indicate a n extent of hydrogen impoverishment which demands that the reaction proceed toward the production of benzene and naphthalene.
Process Independent of Limiting Band It has just been suggested that the existence of the limiting band is somehow related to the availability of hydrogen; hence it is not surprising to find, as the work of Haslam and Russell (8) shows (Figure 27), that when large amounts of hydrogen are supplied to the reaction in the presence of a catalyst capable of making it “available”, it is possible to cause the boiling range of a stock to change with apparently
FIGIJR~ 27
little regard to the limiting band. However, it is a bit startling, if true, that of the myriad processes for changing the boiling range of hydrocarbons, only hydrogenation appears to change the tendency of the synthetic crude to assume a definite boiling range.
Discussion The author does not insist that the data he has collected definitely prove the inferences drawn. If a satisfactory explanation could be advanced for the regularities of behavior, the case would be much strengthened, but the author cannot now supply such a theory. Since surprising points of similarity had been noted in the products obtained under strikingly different conditions by diverse processes normally considered fundamentally different, reams of data were sifted in order to disprove the indicated generalities. Additional data seemed only to strengthen the suppositions, however, and such s u p
Vol. 33, No. 6
positions seemed to throw considerable light on the processes involved. The relations pointed out are manifestly empirical but are presented in the present form in the hope that others may either disprove the theories advanced or strengthen them by supplying a fundamental basis. The author feels that the data presented constitute rather good support for the following statements. Most commercial processes fall far short of reaching the limiting assay. I n cracking, for example, an undue amount of coke would be formed before that boiling range could be established; in polymerization or reforming it would be undesirable to make the conditions sufficiently rigorous to produce the high-boiling compounds contained in the product defined by the limiting band. Whether or not the stated generalities are accepted, most cracking plants are operated as if they were. The suggested mechanism renders i t impossible to produce, from a stock lying to the right of the limiting band, a synthetic crude containing more than about 58 per cent of material boiling below 400” F. If such a synthetic crude were further processed, the net effect would be to strip gaseous fragments from low- and high-boiling constituents alike without chatlging the boiling range. If, however, such a synthetic crude is split into various fractions, some heavier and some lighter than the limiting band, each alone may be made to alter its boiling range without high gas production, and in altering its boiling range it proceeds again to the limiting band. Commercial plants produce a synthetic crude containing 20-30 per cent boiling below 400” F., and then recover from it heavy stocks for further cracking and light stocks for polymerization. It would obviously be advantageous t o process a heavy stock in one step t o the point where all the hydrocarbons are liquid and boil below 400” F. Except in the case of hydrogenation, however, this seems impossible. Emphatically, attainment of the assay distillation curve representing the limiting band does not connote that a point has been reached at which reaction ceases. This is shown in Table I1 where, after the first pass, the assay distillation of the synthetic crude was substantially that of the limiting band, and its specific gravity was 0.832. While subsequent exposure to cracking conditions in the second, third, and fourth passes did not change the distillation curve, they did progressively raise the specific gravity to 0.867 which clearly indicates that the chemical composition was changing. It may well be objected that the assay distillations presented do not present the entire products of the reaction, in that they omit coke, possibly some butane, considerable propane, and substantially all methane, ethane, and hydrogen. It is certainly true that if all these gases were included in the assay distillation, as they could be, the resulting curves would not show the same regularities as the ones actually presented. For example, if the 60.2 per cent gas had been shown in Figure 7, the entire curve would have fallen far above the limiting band. Coke, ethane, and methane may be dismissed as nonreactive and therefore removed, to all intents and purposes, from the sphere of the reaction. The same cannot be said for butane and propane, inasmuch as they certainly react in the polymerization process. We can only say, therefore, that while it would appear that propane and butane should affect any limiting composition toward which a pyrolysis reaction may be progressing, no evidence can be found that they do. Surprise may be occasioned by the fact that less than one per cent of natural crudes is found to have reached the limiting-band assay during geologic ages. Reaction rates a t formation temperatures are infinitesimally low, and this, together with the fact that few high-temperature processes reach the limiting band, should make it seem less unusual.
INDUSTRIAL AND ENGINEERING CHEMISTRY
June, 1941
On the other hand, the low percentage of limiting-assay crudes may merely strengthen the arguments of those who, in scouting the predicted exhaustion of petroleum, contend that the production of crude oil is by no means a finished process, and that new crudes are always being formed within the earth.
Acknowledgment The author is indebted to his associates, particularly to D. W. Gould and 8. J. Macuga, for supplying much of the original data, and to the former for locating several of the references cited. Both contributed helpful criticism and discussion of the theories advanced.
Literature Cited (1) Birch, 8. F., Dunstan, A. E.,Fidler, F. A., Pim, F. B., and Tait, T., IND. ENQ.CHEM., 31, 1079 (1939). (2) Bur. of Mines, Bull. 291 (1928),401 (1937); Tech. Papw 346 (1925); Circ. 6014 (1926); Repts. Znveatigations 2202, 2235, 2290, 2293 (1921); 2322, 2364, 2416 (1922); 2595, 2608 (1924); 2807, 2808, 2824, 2846 (1927); 3253 (1934); 3346 (1937).
759
Candea, C., and Sauoiuc, L., Refiner Natural Gasoline Mfr., 18.
434 (1939). Cross, Roy, Kansas City Testing Lab., BuU. 25, 284 et seq.
.----,
11928>.
Egloff, Gustav, Lowry, C. D., Jr., and Schaad, R. E., J. Inst. Petroleum Tech., 16,133 (1930). Egloff, Gustav, Nelson, E. F., and Morrell, J. C., IND.ENO. CHEM.,29,555 (1937). Hall, F. C.,and Nash, A. W., J . Inst. Petroleum Tech., 24, 471 (1938). Haslam, R. T.,and Russell, R. P., IND. ENO.C H ~ M22, . , 1030 (1930). Houdry, E., Burt, W. F., Pew, A. F., Jr., and Peters, W. A. Jr., Proc. Am. Petroleum Inst., 111, 19,133 (1938). Ipatieff, V. N.,and Egloff, Gustav, Natl. Petroleum News, May 16,1935. Ipatieff, V. N., and Grosse, A. V., IND.ENO.CHEM.,28, 461 (1936). Peterkin, A. G., Bates, J. R., and Broom, H. P., preprint of paper presented before Am. Petroleum Inst., Nov. 17, 1939. Rittman, W.F.,Dutton, C . B.,and Dean, E. W., Bur. of Mines, BUZZ. 114,58-64 (1916). Snelling, W. O., U. S. Patent 1,624,848(April 12,1927). PRISINTIUD under the title "Petroleum Equilibria" before the Division of Petroleum Chemistry at the 100th Meeting of the Amerioan Chemioal Society, Detroit, Mioh.
Heat Capacities of Organic Vapors CARROLL J. DOBRATZ, University of Cincinnati, Cincinnati, Ohio
The method of calculating heat capacities of organic vapors from valence-bonding frequencies as developed by Bennewitz and Rossner ( I ) has been modified by the assumption of rotation within the molecule so as to give more accurate results. Frequencies have been assigned to valence bondings with halogens, nitrogen, and sulfur which, when used with those originally assigned to carbon, hydrogen, and oxygen bondings, enable the calculation of the heat capacities of the vapors of practically all organic compounds formed from these elements. The values of the Einstein functions at different temperatures corresponding to the assigned frequencies have been fitted to equations of the form, C A BT CTp,so as to simplify computations. An illustration shows the method of calculating heat capacities, and comparison of calculated and experimental data indicates that an accuracy within 5 per cent can be expected in most cases.
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+
+
CCORDING to the work of Bennewits and Rossner ( I ) , the heat capacities of the vapors of organic compounds containing carbon, hydrogen, and oxygen can be calculated by the following equation:
A
(Cv)p
-
o =
3R
+
+ 3n -zqi-
Z*iC&
(1)
where R = gas constant per mole n = number of atoms in molecule = number of valence bonds of ith type yi, Csj = Einstein functions for a given bond having characteristic frequencies vi and bi.
8
The Einstein function to be used in this case is the derivative of energy with respect to temperature:
where X = hv/lcT h = Planck's constant v = characteristic frequency IC = gas constant Eer molecule T = temperature, K. e = base of natural logarithms The Y frequencies were evaluated from Raman data, and the 6 frequencies were determined empirically from experimental data previously obtained ( 1 ) . I n order to simplify the computations, values of Einstein functions corresponding to the given frequencies were listed at 40" intervals from 290" to 690" K. The expression,
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which is derived from Berthelot's equation of state ( I J ) , was used to convert values of (CJP o t o the more commonly used form (CJ, P. Heat capacities calculated by this method were found to agree within 5 per cent with the experimental data for a large number of compounds containing carbon, hydrogen, and oxygen, mostly obtained a t the same temperature, 410' K. However, when the heat capacities of propane, butane, and pentane were calculated and compared with the experimental data of Sage, Webster, and Lacey ( I @ , the differences were much greater than 5 per cent a t low temperatures (Table I). This error is probably due to the fact that this method takes no account of rotation within the molecule. The assumption
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