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INDUSTRIAL A N D ENGINEERING CHEMISTRY
(5) Holmberg, A n n . , 859, 90 (1908). (6) Mabery and Colldbordtors, Am. Chem. J . , 13, 233 (1891); 16, 83 (1891). 85, 404 (1936). (7) Ndik. J . Chem. Sor., 119, 379, 1231 (1921). (8) Thierry, l b r d . , la?’, 2756 (1925).
(9) (10) (11) (12) (13)
Wendt and Diggs, IND.END.CHEM.,16, 1113 (1924). Wood, Lowy, and Faragher. I b i d . , 16, 1116 (1924). Wood, Sheely, and Trusty, I b i d , 17, 798 (1925). Wood, Sheely. and Trusty, l b r d . , 18, 169 (1916). Youtz and Perkins, I b i d . , 19, 1217 (1927).
Composition of Paraffin Wax‘ S. W. Ferris, H. C. Cowles, Jr., a n d L. M. Henderson THE ATLANTIC REPINISGCo., PHILADELPHIA, PA.
HE many variations in the crystalline behavior of the divergence increased with increasing molecular weight. After solid constituents of petroleum have given rise to repeated recrystallizstions from ethylene dichloride, however, three important theories as to the composition of these a t temperatures from 35” to 45” C., the fractions were found waxes: (1) the proto-pyro-paraffin theory of Zaloziecki (9), to show close agreement, in melting point-molecular weight which postulates the existence in petroleum of an amorphous relationships, with the pure compounds of Krafft. They wax (probably branch-chain) which upon distillation cracks therefore believed that the recrystallizztions had removed a to the crystalline (straiglit-chain) variety; (2) the theory very small amount of “soft wax” impurity, and concluded, advanced by Gurwitsch ( J ) , which attributes differences in not only that all petroleum waxes belong to the C.H2,+1 crystal behavior to tfheproperties of the crystallizing medium; homologous series, but that they are straight-chain comand (3) the “soft wax” postulate of Buchler and Graves ( I ) . pounds. These writers attributed all crystalline peculiarities t o the Thus, while there is abundant evidence that waxes contain effect of an “impurity” which they termed “soft wax.” saturated straight-chain hydrocarbons, there is very little t o Many investigators have attempted to establish the com- indicate the presence of isomers, either branch-chain or cyclic. position of wax by obtaining fractions of a high degree of Carpenter ( 2 ) did, however, suggest tliis idea, and reported purity and comparing their properties with those of synthetic properties for two purified fractions which appeared partially straight-chain hydrocarbons. There has resulted a mass of to substantiate it, although the author himself points out that evidence indicating the presence of members of the CnHzn+z the accuracy of the molecular weight determinations was homologous series, and these observations have been borne probably of the order of 2 per cent. His fractions were: out by x-ray investigations (3, 6, 8). FRACTION MELTING POINT MOLECULAR WEIGHT
T
1 2
c.
66 5 70 0
463 450
I n view of the apparently well-accepted idea t h a t the liquid constituents of petroleum comprise many isomers, not only of chain but also of cyclic structure, it would seem t o be a rather strange contradiction that the solid constituents should be solely straight-chain saturated compounds. The present work was, therefore, undertaken t o investigate further the nature of the compounds present in paraffin wax.
Buchler and Graves examined the entire range of waxes from Salt Creek, Wyo., crude, including paraffin wax (slack wax), slop wax, petrolatum wax, and rod wax. The present investigation embraces only the “paraffin” waxes, from midcontinent (although not Salt Creek) crude, but to aid in correlating their conclusions with the indications reported here, a brief resume of their work is given below: Slack wax was freed .of oil by repeated recrystallizations from ethylene dichloride at 4.4” C., and then subjected to careful vacuum fractionation. When the physical properties of the resulting fractions were plotted, i t was found that they did not agree with the properties of the synthetic straightchain hydrocarbons prepared by Krafft (J), and that the Received April 16, 1920. Presented before the Division of Petroleum Chemistry at the 77th Meeting of the American Chemical Society, Columbus, Ohio, April 29 to May 3, 1929. I
REFRACTIVE
INDEX CBO’C
Figure 2
Preparation and Distillation of Oil-Free Wax Oil-free wax was recovered from various paraffin intermediates (derived from midcontinent petroleum) by successive recrystallizations from ethylene dichloride at - 18” C. These waxes were fractionally distilled a t an absolute pressure
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of less than 1 mm. Hg, using a &foot (1.5-meter) column packed with jack-chain. The fractions had boiling ranges, from over to dry point, of approximately 15" C. The boiling ranges of the individual fractions were determined by vacuum assay distillations ( 1 ) a t 10 nim. pressure. If the boiling ranges were expressed, as they frequently are, in terms of the temperature change a t the top of the fractionating column while the fraction was being collected, they mould have been much shorter-i. e., averaging about 2-3" C.--varying from less than 0.5" C. to about 7 " C. /
04-
2 .!It,'
.1
4
5
I D ? A L FITiCTION W O U T 17 * , N G
'
ing the fourth recrystallization. Since four recrystallizztions had been used previous t o the original fractional distillations, each final fraction on the chart was recovered, as a crystal, a t least eight and in most cases between twelve and fourteen times. After the fourth crystal-fractionation, some of the separations were made by crystallizing one or two fractions and recovering the remainder from the filtrate by removing the solvent. It was assumed that, if the fractions consisted essentially of true paraffins contaminated with a "soft wax," the numerous recrystallizations would result in gradual separation /o II 12. 6 7 8 9 and purification of these two constituents. It is 0 ' O ! N G I c ! A T P S T f i A T F R O C T J O i ? W 4 . S R E C d Y S T A L L I Z E D 64 O . , W J T U ~ E , FW R E C R ~ , ~ A ~ L I Z A T G O N~ f ~ ~ ~ ~ J O N IO+-^ S , N ~ ~ Cseen ~ ~ ~ a~t once, however, thnt no such tendency was *.- - - - - - _ _ _ found, but that six different fractions were finally ..---- ---P-- - - - -----secured, indicated on Figure 3 as fraction9 A to F . The relative purity of these products is iiidicated by 56 the last crystallization, which slion s, In each case, a very narrow split, compared with thc original separations, between the portion crystallized and the portion remaining in solution.
&-"
Composition of Fractions
If we were to assume thnt the fractions A to F were composed of various mixtures of a high-melt36 ing constituent and one of low melting point, A and F might be supposed to be relatively pure but C and D would be highly impure. The test crystallization just mentioned indicates that thi. is not the case, but it is more definitely established by . the cooling curves given in Figure 4, in which fractions to F arecompsred ntith fractions of s i m i l a r b o i l i n g ranges obtained from the fractional distillition of the oil-free wax, whose properties are shown in Figure 1. It is a p p m n t that all the final fraction$, resulting from recrystallization, are more pure than the material from which they were derived. This evidence, then, would strongly indicate the presence of several classes of compounds. Some physical properties of fractions A t o F are given in Table I and plotted on Figure 5. I t will he noted that the 50 per cent boiling points are subFtantidly the same, but that what variation does exist is in the opposite direction from 40
I I
2
3
4
5
NUI? B E
R
6 OF
I
I
I
I
7
8
9
lo
I //
I /Z
R E C R 15 T A L L I Z A TICNS
Figure 3
The properties of these fractions are presented in Figures 1 and 2. The boiling points in Figure 1 mere also determined for each fraction by means of the vacuum assay distillation. Recrystallization of Mixtures of Fractions
It will be noted t h i t fractions having identical 50 per cent boiling points or refractive indices varied widely in melting points. The obvious explanation would be that the several fractions were of different composition. All the properties might, however, be explained upon the assumption that the cuts (of the same boiling point) represented various mixtures of saturated straight-chain hydrocarbons with a single additional class of compounds which could be called "soft wax." I n order to test the two hypotheses stated above, two waxes were prepared, which might be termed "hard" and "soft" wax concentrates, respectively, by mixing distillat e fractions as indicated on Figure 1. Each of these mixes was refractionated under the same conditions as the previous distillation, and the middle cut selected for recrystallization experiments. They were designated as waxes I and 11, respectively (see Figure 3). The solvent used for recrystallization was ethylene dichloride. The fraction was put into solution, and then cooled until a portion of the wax had crystallized. The crystals were recovered by filtration; the filtrate was further cooled and another portion crystallized, and so on. I n every case the final crystallization mas a t -18" C. or slightly lower. Each fraction so obtained was freed of solvent and its melting point determined. If two or more fractions were obtained whose melting points were reasonably close together, they were combined and the mixture subjected to further fractional crystallization; many fractions, however, were individually recrystallized, as shown in Figure 3. It should be emphasized that all fractions were recovered as crystals U D to and includ-
AI
Figure 4
that which would be anticipated from the melting points, which vary from 59.9" to 29.4" C. The maximum difference in molecular weight is slightly less than two carbon atoms, and again the highest melting fractions exhibit the lowest molecular weights. The molecular weights were determined by the boiling point method using benzene as a solvent.
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'' t2
by its molecular weight according to the equation of Lorentz and Lorenz: n2 - 1 ( M R = m
*
+)
The results are given in Table I, together with values calculated from the atomic refractions of carbon (2.418) and hydrogen (1.100). The close 46 agreement between the observed and calculated x fror S W E A T O I L valuw indicates that all the fractions, with the 42 possible exception of F,are saturated hydrocarbons. Thus, while there seems to be much evidence, apart 36 from the molecular refractivities, for the existence of isomeric waxes, we have as yet found nothing to 34 indicate any appreciable amounts of either un3o saturated or cyclic compounds. 30 The yield percentages given apply to the particu26 lar mixture, selected for recrystallization, which, 26 ~ x ,260 270 2 ~ 0 290 3w 310 14280 143m 14320 (4m 14360 i f s o 1R E F R A C T I V E I N D E X e BO'C as explained, was derived from several wax inter50% B O I L I N G P O i N T @ l O m r - 'C Figure 5 mediates. The previous history of these materials was not known with sufficient accuracy to permit The solubilities of the final materials are shown in Figure 6. a calculation giving the relative concentrations of fractions It will be noted that F is nearly seven hundred times more A to F i n the original crude. The work is now being extended, soluble a t 14" C. than is A . Furthermore, the solubilities however, to the quantitative examination of the wax-bearwere taken by the "cloud point" method; that is, with a ing fractions of a definite crude. known amount of wax in solution, the temperature was noted Tab1e.I-Data on Paraffin Wax Fractions a t which the first crystals separated. It is apparent, there- Fraction A B C D E F fore, that even small quantities of A in the other fractions Melting: point, C. 59.9 55.2 47.1 40.5 35.2 29.4 would have very markedly lowered the observed solubilities. g$,:g~p$nt-269.5 269.5 272.0 272.0 272.0 2 7 3 . 5 Most of the crystallizations of the higher melting fractions R2F$;e index1.4303 1.4306 1.4330 1.4350 1.4359 1.4380 were a t 20' C. or higher. A study of the solubility curves Specific gravity800 c. 0.770 0.773 0.779 0 . 7 8 3 0.786 0.792 will therefore show that, had C or D been composed of a Molecular weight 366 367 379 389 385 377 mixture of A and F , they would certainly have been sepa- S o t ; i ' ~ ~ ~ ;ihA1:Z, ~$~: grams per 100 cr. 0.115 0.218 0.82 2.4 5.7 70.3 rated.
\ f ,, yo , ,
,I , , , ,fi
~~
503
Molecular refraction of wax (liquid state). observed Av. value of n in formula CnlIzn Molecular refrac&n of wax, calculated Percentage yields
122.7 26
122.8 26
122.3 59
122.3 6
126.5 27 126.9 7
129.8
128.4
126.0
27.6
27.4
26.8
129.7 9.5
128.7 9 5
126.0 8
Conclusions
Despite the fact that t'he liquid constituents of petroleum have long been considered to be complex mixtures of numerous compounds differing in structure, many investigations have strongly indicated, and it has been widely believed, that the solid constituents comprise only straight-chain, saturated hydrocarbons. It is believed that the evidence here presented is sufficient to warrant the conclusion that petroleum wax contains, in addition to these, a number of other types of hydrocarbons which are prohably branch-chain paraffins rather than unsaturated or cyclic compounds. 6
r
I
I111111~
I
I l l l l l l ~
I
I Illlld
Figure G also demonstrates why the majority of waxes examined in the past have been similar to A or B. Recrystallization has almost invariably been used for purification. Buchler and Graves, for example, first removed oil by repeated crystallization from ethylene dichloride a t 4.4"C., a t which temperature F and E are very markedly soluble and D and C appreciably soluble. I n the later recrystallizations a t 3545" C. obviously only A and B could be recovered. It seems highly probable that their soft waxes consisted essentially of wax B a n d possibly C. The molecular refractions of the molten waxes were determined by multiplying the specific refractive power of each
Literature Cited (1) Buchler and Graves, IND. ENG.CHEM.,19, 718 (1927). (2) Carpenter, J. Inst. Petvaleam Tech., 12, 288 (1926). (3) Francis, Watkins, and Wallington, J . Chem. Soc., 121, 496, 1529, 2801 (1922). (4) Gurwitsch, "Scientific Principles of Petroleum Technology," pp. 15 and 450, Chapman & Hall, London, 1926. (5) Krafft, Ber., 19, 2223 (lS86); 29, 1323 (1896); 40, 4783 (1907). (6) Muller and Saville, J , Chem. SOC..127, 599 (1925). (7) Peterkin and Ferris, I N D . ENC.CHEM., 17, 1218 (1925). ( 8 ) Piper, Brown, and Dyment, J . Chem. Sac., 127, 2194 (1925). (9) Zaloziecki, Z. a n g e v . Chem., S, 126, 318 (1888).
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