T H E RECRACKIXG OF A CRACKED OIL PRODUCED FROM PETROLEUM BY GUSTAV EGLOFF A N D THOMAIS J. TTT’OMEY
Introduction In the cracking of petroleum, either for gasoline or aromatics, by the gas phase method, a cracked oil representing a percentage of the oil pumped into the cracking tube is recovered in the condensers and in the carbon trap. n‘hether the operation has been conducted for either gasoline or aromatics, the cracked oil is distilled to 170’ C. The distillate up to this point contains the gasoline or the aromatic hydrocarbons. benzene, toluene and xylenes produced. The question arose as to the disposition of the residue above 170’ C. This residue from commercial operations contains, besides oil, approximately 2 0 percent pitch and one to two percent of naphthalene. Would it be advisable to crack it again for the production of aromatics or gasoline, and what yields would be obtained in comparison with those obtained by the first decomposition of the petroleum oil? By experience i t has been learned that a petroleum oil which has a relatively high pitch content-in this case represented by the residue with its 2 0 percent pitch content-is not especially desirable as a starting oil for the production of benzene and toluene on account of the resulting carbon formation. Whenever possible, the carbon formation as controlled by the composition of the starting oil is avoided. For this reason, the residue is distilled and an oil is obtained free from pitch. The problem then limited itself to the advisability of using the distillate from the cracked oil for further cracking and to ascertaining the yields of either gasoline or aromatics which might be obtained. Seeking this information constituted the grounds for the present communication. The questions which were to be answered were as follows: I . What value has a cracked oil for either the production of gasoline or aromatic hydrocarbons?
5 98
Gustar EgloJ alzd Thomas
J . Twowtey
2 . Will the recracking of a cracked oil result in amounts of carbon sufficient to make commercial operations not feasible?
.
Theoretical There can be but one answer as to results which will be produced by thermolizing a cracked oil. These are deduceable from the nature of a cracked oil itself and from data obtained in the cracking of different types of oils. Depending upon the extent to which the decomposition produced by temperature and pressure has taken place, a recovered oil is obtained which contains aliphatic and aromatic hydrocarbons in proportions corresponding to these conditions. If the reaction is carried far enough, it is possible to obtain a recovered oil which consists almost entirely of aromatic compounds. The cracked oil studied in this paper was made under conditions which gave a mixture of these two types. The aromatic hydrocarbons resulted from the breaking down of the aliphatic compounds present in the petroleum: the aliphatic compounds are composed of low boiling compounds produced by the cracking and high boiling compounds unaffected by the cracking reaction due to incomplete equilibrium. In any distillate from a cracked oil boiling above zooo C-similar to the one employed in these experimentsit is not a simple matter to show chemically that there are aromatic hydrocarbons present. In this case, the presence of naphthalene and anthracene, the determination of which is simple, was not considered. A physical constant, however, such as that of specific gravity, may be safely used as a qualitative indication. This method is recommended by Engler. The specific gravity of aromatic compounds is so much higher in value than those of the aliphatic compounds that mixtures of these types of hydrocarbons are shown by a gravity midway between the two extremes. The gravity of the distillation cut from the cracked oil is between that known for similar distillation cuts from many petroleums, and that generally associated with dead or creosote oils from which the Engler-Hofer: Das Erdol, 1914.
Recrackivg o j a Cracked Oil
5 99
naphthalene and anthracene salts have been removed. On these grounds, the conclusion was drawn that the distillate derived from the cracked oil was a mixture of aliphatic and aromatic hydrocarbons or, perhaps better stated, a mixture of unchanged petroleum and aromatic hydrocarbons produced by cracking. There are sufficient data a t hand to show what happens when a petroleum or aromatic oil is cracked by itself. The literature in regard to the decomposition of the first type of oil is extensive, the tendency of this reaction being as follows: Aliphatic to aromatic hydrocarbons to hydrogen and carbon. On the other hand, there are less accessible data on the cracking of aromatic compounds boiling above 200' C. In a series of cracking experiments,' using dead oils, i. e., distillation cuts from coal tar above 200' C, it was found that these types of oils-almost aromatic entirely in composition-did not tend to break down to any considerable extent to lower boiling hydrocarbons. An oil containing such a mixture of aliphatic and aromatic hydrocarbons will break down into lower boiling compounds only in proportion to the amount of aliphatic hydrocarbons -unchanged petroleum-present . Since the cracked oil only contains a percentage of unchanged petroleum, the cracking will not produce as much gasoline or aromatic hydrocarbons such as benzene and toluene as did the cracking of the original petroleum oil. It is also to be expected that more carbon will be produced because whatever cracking of the aromatic constituents takes place tends to produce carbon and gas.
The Results of This Paper Experimental data were obtained directly in accord with the theoretical considerations. From a study of the results produced by the thermal decomposition of a petroleum oil and a cracked oil derived from the petroleum oil, i t is possible to show that a cracked oil is not so well adapted for the production of gasoline as is the petroleum oil; that the cracked 1
Rittman and Egloff: Met. and Chem. Eng., 24, jz (1916).
L
Gustaz EgloSj atid Thomas J . Twome?
600
oil is adapted for the production of aromatic hydrocarbons, but then only yielded much smaller percentages of these compounds as compared to those made from the petroleum oil, and that carbon formation is greater when the cracked oil is recracked than when the petroleum oil is subjected to the same factors of decomposition.
Experimental a . Oils [ s e d The two oils employed for the cracking experiments were ( I ) a distillate or gas oil from Pennsylvania Crude Petroleum, and ( 2 ) the distillate between 170 and 360' C from the recovered oil produced by the cracking of the distillate from the Pennsylvania Crude Petroleum. The analysis of these two oils by distillation in a standard Engler flask of IOO cc capacity and by specific gravities follows: DISTILLATION ~~NALYSIS
9 = Petroleum Oil B = Cracked Oil
Sp. gr.
51 SP. gr. I % SP.gr ________---
x
B
The main difference in these two oils is that the gravity of distillation cuts of the cracked oil are considerably higher than those of the distillate from the Pennsylvania Crude Petroleum. This indicates, as has already been mentioned, that the cracked oil is a mixture of unchanged petroleum and aromatic hydrocarbons which were formed by the first cracking.
b.
Furnace Employed
The cracking of the two oils was conducted in an electrically heated furnace which has been described. Briefly, it
Recracki?zg oJ a Cracked
Oil
60 I
consisted of a one and one-half inch diameter steel tube, thirty-two inches in length, and which was operated in a vertical position. To the top of the tube was attached a lubricator through which the oil was admitted to the tube. Connected a t the bottom was a condenser and a receiver. The tube was heated over a length of 2 2 inches by conducting I I O volt D. C. current through 18 gage nichrome wire wound three times to the inch and insulated from the tube by three layers of asbestos paper. The temperature of the cracking was recorded by a pyrometer inserted through a weld and a stuffing box into the center of the tube. A natural gas line conducted through a compressor led into the top of the tube. The gases formed during the cracking were led out by means of a pipe line to the gasometer from the top of the receiver. c.
Experiments Conducted
The oils were cracked at atmospheric and 1 5 0 lbs. pressure a t temperatures 550, 600, 650 and 700' C, under identical conditions. The temperature of the tube as recorded by the pyrometer was controlled accurately by means of resistance in the electrical circuit and varied only *5' C during an experiment. The oil was admitted from the lubricator at a constant rate of three drops per second which represented a flow of approximately 246 grams per hour. The regularity of the rate was controlled by comparison with a metronome which oscillated three times to the second. When operating a t 150 lbs. pressure, the pressure in the system was built up by means of compressed natural gas. For each experiment 546 grams of oil were used. Three or more runs were made a t each temperature and pressure in order to obtain checking results and for the purpose of obtaining sufficient oil for analysis. In the experimental data the average of these runs are reported.
The cracked oil recovered from the receiver was filtered to remove any suspended carbon and then analyzed for benzene,
Gustav Egloj avd Thomas J . Tttlonzey
602
toluene and xylenes by a method devised by the authors.' The naphthalene and anthracene salts were frozen from the distillation cuts and thoroughly dried before weighing. The cut to 170' C was taken as indicative of the amount of gasoline produced. All analytical results are expressed in percents by weight. Specific gravities were taken by means of a Westphal balance a t 15.5' C.
Experimental Data The results found from the cracking of the distillate oil from Pennsylvania Crude Petroleum and the cracked oil derived from it, under similar conditions of temperature and pressure are shown in the following manner :
Table I and Figs. I avld 2.--The comparison of the effect of temperature a t atmospheric and 150 lbs. pressure on the percent of recovered oil and the specific gravity of the recovered oil from the petroleum and cracked oils. Table 2 and Figs. 3 i l z d I.-The comparison of the effect of temperature a t atmospheric and 150 lbs. pressure on the percent of benzene, toluene, xylenes. naphthalene and anthracene in the recovered oils produced from the petroleum and cracked oils. Table 3 a n d Figs. .j utzd 6.-The comparison of the effect of temperature a t atmospheric and 150 lbs. pressure on the percent of benzene, toluene, xylenes, naphthalene and anthracene on the basis of petroleum and cracked oils used for production. Table 4 uizd Figs. 7 und K.-The comparison of the effect of temperature a t atmospheric and 150 lbs. pressure on the percent of distillate to 170' C, on the specific gravity of the distillate to 170' C, and on the distillation cuts of the recovered oils from the petroleum and cracked oils.
1
Rittman, Twomey- and Egloff: Met. and Chem. Eng., 13, 682
(1915)
Recruckivg ot' a Crucked Oil
603
Fig. I The effect of temperature a t atmospheric pressure upon the percent of recovered oil, and the specific gravity
604
Gustav Egloff aiid Thomas J . Twouney
Temperafure -Degrees CenTigrade Fig. 2 The effect of temperature a t 150 pounds pressure upon the percent of recovered oil and the specific gravity
Recracking o j a Cracked Oil
TABLEI A comparison of the effect of temperature a t atmospheric and I j o lbs. pressure on the percent of recovered oil, and the specific gravity of the recovered oil from the Petroleum and Cracked Oils Atnzospheric Pressure
I Temp ( " C )
,
yc recovered I Sp. gr. recovered oil 1
'
I
600
I
Cracked
Pet oil
oil
851 o 820
~
I
855 o 89j
43.1 o 860
I I
1
6:
i24
I
650
I
";c recovered Sp. gr. recovered oil
1
550
6j j o 965
1 I
I
1
Cracked oil
74 0
0
922
j 00
26.7 o 899
~
j 2 2
I
009
I j o Lbs.
I
600
550
I
Fc recovered Sp. gr. recovered oil ! I ~
1
60.9 0.818
~
6j o
69.0 0.921
, ~
1 I
45.2 0.888
1I 7 00
jj.0
0.964
Gustar EgloJ aizd Thomas J . Twomey
606
TABLE 2 A comparison of the effect of temperature a t atmospheric and I j o lbs. pressure on the percent of Benzene, Toluene, Xylenes, Saphthalene and Anthracene in the recovered oils produced from the Petroleum and Cracked Oils A twospheric Pressure 600
550
Temp. ( " C )
Pet. oil
Benzene 70 Toluene ye Xylene Naphthalene 7@ Anthracene
0 0
0.i
0 3 O 0 0 0
1
Cracked oil
Pet. oil
0 1
0.8 2.3
I
I ' I
1 5 1 9
1
1.4 2.8 3.2
1.j
0 0
0.0
0 0
0.0
Cracked oil
~
0.0
0.0
6j o
% Benzene
ye Toluene % Xylene
Xaphthalene
0.0
yo Benzene 7oToluene yc Xylene yc h-aphthalene Anthracene
'
1
1
~
4 5
I
I
I
600
550
yo Benzene % Toluene
2.6
0.0
7GAnthracene
o j
IO 0
2
1 0 2
0
'
'
4 5 6 2
IO. 2
I'
4 6 9 2
o o
I
Trace Trace
I
9 6 9 0
3 0 2 9
4.2 3.6 13.3 2.6
Recracking o j a Cracked Oil
607
Temperature -Degrees Centigrade Fig 3 The effect of temperature a t atmospheric pressure on the percent of benzene, toluene, xylenes, naphthalene and anthracene in the recovered 011
608
35
1/50Ibs. Pressure‘
I
I
I
30 ___ Solid Linos -used for Recovered Oils from Petroleum Oil Doped Lines’) ’) ’’ Cracked It
9 )
Note- Where Curve3 coincide Dots D a b Lines are used
25
20
o = X Benzene 0= % Toluene
-0 =%Xylene
X = X NaphThalene += X Anthracene
15
10
5,
0.
-.-.-.
5. 0
-.-.-
*600
650
700
TemperaTure-Degrees Cenbgrade
Fig I The effect of temperature a t 150 pounds pressure on the percent of benzene, toluene, xylenes, naphthalene and anthracene in the recovered oil
el' il
Recrackiiig
C'ruckcd Oil
609
T.IBLE3 -A comparison of the effect of temperature a t atmospheric and 1 j 0 lbs pressure on the percent of Benzene, Toluene, Xylenes, Saphthalene and .Anthracene on the basis of Petroleum and Cracked Oils used for production -4t i nos plier ic Press it re
I
550
Temp
i
'Cj
Pet
600
Cracked,
oil
011
Pet
Cracked
~
011
011
_____ I
LT Benzene cc Toluene Xvlene cc Saphthalene r6 *Anthracene
0 0
0 1
0 6 0 3
1 3
0 0
0 0
5 ' 4
I 0 2 1
0
1 6
1
0
2 4
0 0 0 0
0 0 0 0
0 0
650
0 0
700
-
Benzene Toluene yG Xylene Saphthalene .Anthracene OC
;'
2 j
' 9
3 3
3 1
2
2
14 0 0 0 0
2
1
2 7
2
2 1
3 0
' 9
'1
0 0 0 0
0 0 0 0
1 4 0 6
I j o Lhs. 550
yc Benzene ';c Toluene yc Xylene Saphthalene cc Anthracene
'>
1
600
0 7
0 4
4 5
-7
1 4 I 8
46 3 0
0 0 0 0
0 0
i-
2 0 0 0 0 0
I
0 0
6j o
% Benzene Toluene Xylene yc Saphthalene pc -Anthracene
7 6
2 9 2 8 2 8
'
Trace Trace
1
1 4 9
0 0
j
3 5
I
0 0 0 0
j 00
3 2 2
2
3 1
~
6 8 2
1
2
' 4
I
3 3 ' 3 I
2
2
2
42
0
2
0 8
Gustar Egloj and Thomas J . Twomey
610
1 3.5.
I
I
I
Artnospheric Pressure I
I
-
I
Solid Lines used for Potroleurn Oils Doped Lines- *' I' Cracked 9s
550
600
-
650
700
Temperafur0 Degrees Cenfigrade Fig. 5 T h e effect of temperature a t atmospheric pressure on the percent of benzene, toluene, xylenes, naphthalene and anthracene on the basis of oil used for production
Recracking of a Cracked Oil
1
I
I
/50lbs. Pressure I
I
I
Solid Lines -used f o r Petroleum oil Dored Lines- '' *' Cracked H
a0 Note
-
- Where Curves coincide Dot Dash Lines are
used
8
70
6.0
5.0
4.0
3.0
3
m
e
2.0
3
Q
5 1.0
4
0,
550
1
I
I
I
700
Temperature -Degross Cedigrade Fig. 6
1
1
I
650
600
j /{toluene,xylenes, naphthalene and anthracene on the basis of oil used for production
'I'ABLE 4 of temperature at atmospheric and I j o lbs. pressure on the Distillate t o 170' C, on the specific gravity of the Distillate to 1 7 0 ' C, and on the specific gravity of the distillation cuts from the Petroleum and Cracked Oils *4twospheric Pressitre
X comparison of the effect
I
Temp. ("Cj
% to 1 7 0 ° c Sp. gr. t o 170' C Sp. gr. t o 9j O C Sp. gr. 9j-120 Sp. gr. 120-1 j 0
% to 170° C Sp. gr. t o 1 7 0 ' C Sp. gr. t o 9 j 0 C Sp. gr. 9j--120 Sn rr u p. 5 'r .
Sp. gr. Sp. gr. Sp. gr. Sp. gr.
T ? n - T' cJ" n
IL"
26 0 742 0 71'
";;'c to I 70' C Sp. gr. t o 170' C Sp. gr. t o 9j0 C Sp. gr. 9j--120 Sp. gr. 120-150
j
o 762 0
i,
0
806 784
0
824
0
0
n 99: "."&J
0 j72 0 734 0 809
1 I,
19
766
0
792
728
~
0.870 0.869
0
n "
42 T o 8jo n 9;n Y . " J"
9 :n "J"
595 i35
809 o 826
3 809 762 822 832
o 8j6
794 o 846
0 ~
0
1
0
I
,
" . " , V
27
i33 o 815 o 8j2 o 850
6j o
0.867
I7 o o o
0
I
o 816
I
2
0 0 so2
771
31 9
t o 170' C to 9 j 0 C 9j-120 120-150
~~
600
I
550
0 0
1
0
835 800
o 858
700
o 8j6 845 0.860 0
0.866
0.867 0.868 0.868
0.868 0.864 0.868 0.868
Rccrackirzg q i a Cracked Oil
613
a 0=Distillation Cwt from Petroleum Oil
Dq0.2,
x=
I;i 1
9p
,.
Cracked
*I
i 1 1
Disfillafe t o I7O0C.
0
550
11
650
600
700
Temperature -Degrees Cenfigradb Fig. 7
The! effect of temperature a t atmospheric pressure on the distillate to 170' C, on the specific gravity of the distillate t o 1 7 0 ' C, and on the distillation cuts from the petroleum and cracked oils
Gustav Egloj and Thomas J . Twomey
614
v 9, pll e,10
w
_II
O=Dlshllsrlon Cvt from Pafrolourn Oil ~
X=
*'i
'9
~
1'
Distillate to l70'C.
3 0.1 2
e5 0
550
650
600
TtzmperaTure-Degrees Cenfigrada
, ~I
700
Fig. 8
The effect of temperature a t I jo pounds pressure on the distillate to 170' C, on the specific gravity of the distillate t o 17oO C, and on the distillation cuts from the petroleum and cracked oils
Recrackiwg
0.i
a Cracked Oil
615
Discussion of the Experimental Results a . T h e comparisoTi qt tlze eflect o j temperature at atnzosplzeric atzd I - j O lbs. pressure o n the percent 0-i recovered oil and the specific gra;'itj qf the rciozered oil j r o w tlze petroleuw a n d cracked oils ,4t all temperatures and pressures employed, the percent of recovered oil obtained by the decomposition of the cracked oil is larger than that from the petroleum oil. Since the cracked oil is the product of primary thermal decomposition at approximately the same temperatures and pressures employed in these experiments, it does not break up to gases and lower boiling compounds to the same extent as does the original distillate oil. It contains aromatic hydrocarbons formed by temperature and pressure effect which are, therefore, more or less stable when again subjected to the same cracking conditions. The decomposition which took place may be attributed in the main part to the presence of the unchanged petroleum. This is very similar to what has been shown in the cracking of dead oils' from coal tar, where the percent of recovered oil was larger than that obtained from petroleum oil cracked under the same conditions. The decomposition of the cracked oil proceeded in this case t o a larger degree than was found with dead oils, but yet not the extent noticed with a petroleum oil. There are no new points of particular interest in regard to the specific gravities of the recovered oils except that the gravity of the recovered oil from both sources increased fairly regularly with increase of temperature and pressure. There is another regularity which seems to be characteristic of these data. The percent of recovered oil and the specific gravity of the recovered oil from both the petroleum and cracked oils produced a t atmospheric pressure and 600' C corresponds to that produced a t 150 lbs. pressure and 550' C ; likewise, the results at atmospheric pressure and 650" C are similar to those found at 150 lbs. pressure and 600' C. LOC.cit.
Or, the amount of decomposition produced by increasing the pressure I j o lbs. corresponds to that produced by increasing the temperature j o O C. U-hile no quantitative data were recorded as to the amount of carbon and gas produced, it was noted for particular reasons that the cracking of the cracked oil yielded more carbon and less gas than did the cracking of the petroleum oil. On account of the carbon formation, when the cracked oil was recracked, an extra amount of time and labor was required t o remove the carbon from the tube after each run. No such difficulties were experienced in operating with the petroleum oil. Xeither was it necessary to release the excess gases formed in order to keep the pressure constant when the cracked si1 was recracked as often as it was when the petroleum oil was thermolized. b. Tlzc compuiisoii of tiae e j e c t 01 temperature at atmospheric a n d I jo lhs. ;hicssiire 011 tlae pcrceiit ot bciizeiw, toluene, xyletzes, naplzihaleiie aiid untlzraLciie tii tlac reco2ered oils produced -/row ihc petroleum uiid ciucked ozls The percentage of benzene, toluene and xylenes in the recovered oil from the petroleum oil is almost uniformly twice as large as that found in the oil resulting from the cracked oil. Naphthalene and anthracene formation are apparent exceptions to this observation. At atmospheric pressure, and 5 j o and 600' C, it appears from the data that the cracked oil gives a recovered oil which contains more aromatic hydrocarbons than does the petroleum oil, but the differences are so slight that no conclusions may be deduced. In all cases the petroleum oil produces the larger quantities. The conclusion may, therefore, be drawn that the cracked oil is not so well adapted for aromatic formation as is the petroleum oil. The effect of temperature on the formation of benzene, toluene, xylenes, naphthalene and anthracene a t atmospheric pressure has been shown in a previous communication.' A part of the data in this table, that of the 1
Egloff and Twomey: Jour. Phys. Chem.,
20, 122
(1916).
Rcciackiiig o j a Cracked Oil
617
petroleum oil, is used in the present connection. The data from the cracked oil at atmospheric pressure also illustrate the points which have been previously pointed out, but in a manner more in accord with the theory of the cracking reaction. At temperatures j jo, 600 and 6jo" C, there is more xylene than toluene, but more toluene than benzene. A t 700' C , the percent of benzene is the largest. At this temperature in this series of experiments, toluene should have been present in a larger quantity than xylene, but the experimental data do not show this. The fundamental reaction of xylene to toluene to benzene to naphthalene to anthracene is well illustrated by the data at I j o lbs. pressure from both the petroleum and cracked oils. The results from the cracked oil are slightly better than those from petroleum. At the lower temperatures employed, j j o and 600" C , the percentages of xylene and toluene exceed that of benzene. Above these temperatures, benzene increased very rapidly, apparently at the expense of the toluene and xylene, and this increase was accompanied by naphthalene formation. Higher temperatures than 700' C were not used in the present work because it was judged from the data obtained that these temperatures would have only produced results similar to those already found at atmospheric pressure. That is, higher temperatures would have produced recovered oils in which the percentage of benzene has fallen off together with that of the toluene and xylene, and in which the amounts of naphthalene and anthracene predominate. It might be well at this time to call attention to the effect of pressure on the aromatic formation. This has been done by another investigator, but the results expressed in distillation cuts are only qualitative indications of the direct effect of pressure on the benzene and toluene formation. Pressure increases the aromatic formation. m'hile all the data show the effect, that at 650" C will be mentioned. At atmospheric pressure there was formed from the petroleum oil 5.0 percent benzene as compared to 24.5 percent at 150 lbs., 7.0 percent
618
.
G'ustu? E g l o j ayid Thonias J . Twomeq'
toluene compared to 10.3 percent, 3.2 percent xylenes as compared to 4.6 percent and 0.0 percent naphthalene as compared to 9.2 percent. Pressure is favorable for aromatic formation, since the reaction which results in the formation of the aromatic hydrocarbons is without doubt one primarily of polymerization. c. The conzparisoiz of the e j e i t oj temperature at atwiospheric aud I j O lbs. pressure oti the perceiit oj beiize$ze, toluene, xylenes, tiaphthaleiie aud a d w a c e ? i e ow the basis o j petroleuwr a n d cracked oils ased .tor produi tioii The results in this table are obtained by multiplying the percent of aromatic hydrocarbons in the recovered oil by the percent of recovered oil. The figures represent the number of pounds of aromatic compounds which are produced from roo pounds of oil subjected to the cracking. The petroleum oil yields more aromatic hydrocarbons on this basis than does the cracked oil. The data simply bring out what has already been stated-the cracked oil is not so well adapted for aromatic formation as is the petroleum oil. d . The cowzparisou 01 the ej7eit oJ t c m p e r a t w e at atnzospheric aiid I j o lbs. pressure oti the dzstillatt~to 170' c', the specific graeitj' oj the distillate to 170' C', uiid the specijic giazities o j the distillation cuts jronz the petroleun? aud cracked oils Cp to the present we have been dealing entirely with the question of aromatic formation. The data in this section permit the consideration of the possibility of gasoline formation from a cracked oil, as compared with that from the petroleum oil. The gasoline from the recovered oils in both cases are represented by the percent of distillate to 170' C, and by the specific gravities. The gasoline formation is greater by far from the petroleum than from the cracked oil; in fact there was formed approximately twice as much in one case as in the other. Strictly speaking, the experiments at 550 and 600' C are the only ones which may be taken as typifying the production of gasoline. These results indicate that the cracked oil even a t low temperatures does not form much gasoline. The main
Recvackiug oj a Cracked Oil
619
change is one producing aromatic hydrocarbons. The results of these experiments should not be taken as indicating that in commercial operations it would not be possible to produce gasoline by the second cracking of the oil. The cracked oil used in these experiments is the result of a cracking reaction mainly for the production of aromatic hydrocarbons, where a large change into aromatic hydrocarbons is produced. This does not happen when the cracking is conducted for the production of gasoline. U‘hen cracking for gasoline, the distillate from the cracked oil which would be used for recracking will contain only a very small percentage of aromatic hydrocarbons and will differ in composition but slightly from the starting petroleum. Consequently, gasoline will be formed on the recracking, but in slightly less amounts than from the original petroleum oil, due to the small change produced by the first cracking reaction in the composition of the oil. The number of times that recracking will produce gasoline may only be determined by actual experiment for each individual oil. With each recracking, the distillate from the recovered oil or the residue becomes more and more aromatic in composition and, therefore, less and less valuable as a starting oil for gasoline production The data in these experiments represent in a measure the extreme which would be obtained IYhen cracking for gasoline similar conditions would probably result after two recrackings of the recovered oil from which the gasoline has been removed by distillation. Similar results in regard to the value of residues from cracked oils for the further production have been found by Hall‘ in commercial operations. He states “The liquid residues are usually run through a second time, but they are seldom of the same value for the purpose as the original oil.” In an excellent paper on studies of pressure distillation of petroleum hydrocarbons. A. P. Bjerregaard? produced 4.8 percent gasoline boiling up to I 50’ C at 400 lbs. pressure a t a cracking temperature of 433 O C . The residue was recracked *
The Inst Petroleum Technologists, 1914 Jour Ind Eng Chem 7 5 j 3 ( 1 9 r j )
620
Gustaz EgloJ- aiid Thomas J . P t o n z e j
and a yield of 4.5 percent was obtained. In this case, there was but little difference between the amounts produced from the original petroleum oil and the residue. In any commercial operation, however, the first cracking to be of commercial value must yield several times 4.8 percent. Urhen the cracking has taken place t o this extent, the residue, it will be found, will have changed enough to make it less valuable for the second cracking. The results of Bjerregaard are what would be expected, as the first cracking produced but very little cracking as judged by the gasoline formation, and hence but very little change in the composition of the starting petroleum oil.
Summary Under identical conditions, the cracking of a petroleum oil and a cracked oil derived from this petroleum oil gave results from which the following conclusions may be drawn: I . The cracked oil does not decompose to the same extent as does the petroleum oil. 2 . The cracked oil is not so well adapted for either aromatic or gasoline production as is the petroleum oil. 3. That whatever cracking takes place t o lower boiling hydrocarbons from a cracked oil may be attributed to a large extent to the unchanged petroleum in the cracked oil and not t o its aromatic constituents. 4. There is a limit t o the number of times which an oil can be recracked, because the tendency of the reaction is t o form aromatic compounds which decompose neither to gasoline nor to members of the benzene series appreciably, but toward the ultimate products, carbon and hydrogen.
