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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 19. N o . 6
The Petroleum Waxes’ By C . C. Ruchler a n d G. D. Graves STANDARD OIL COMPANY (INDIANA),CASPER,WYO
HREE waxes are known in refinery practice-paraffin,‘ bons or iso-paraffins. In addition, Marcusson showed that which is pressed from the lighter wax distillates and ceresin wax is identical with the amorphous wax of petroleum, and by the slow distillation of ceresin he obtained a crystalline sweated free from oil; slop wax, which is present in wax, Zaloziecki’s pyro-paraffin. the heavier wax distillates commonly considered unpressOn the other hand, by means of photomicrographs using able; and petrolatum ’wax, characteristic of residual stocks. polarized light, Gurwitsch4 has shown that petroleum waxes have been neither distilled nor purified by crystallization ceresin, as refined from the naturally occuring ozokerite, which from a solvent are crystalline, even including the waxes present is a fourth petroleum wax* It is probably essentially the same in vaseline. Furthermore, he has shown that as the viscosity as rod wax which separates from crude in the wells around of the crystallizing medium increases the wax crystals become the sucker rods. smaller and less well-defined. Gurwitsch therefore concludes t h a t the crystallization of petroleum waxes is dependent prias found in refinery stocks exhibit markedly marily These upon the viscosity of the oil-crystallizing medium, and also different crystallizing behavior. Petrolatum wax forms possibly upon the presence of some unknown colloidal material. H e does not agree with Zalosuch ill-defined crystals that ziecki that it is necessary t o i t c a n n o t b e pressed hut p o s t u l a t e the existence of m u s t b e cold-settled or Paraffin, slop wax, p e t r o l a t u m wax, and rod wax have proto-paraffin. T h e v i e w centrifuged from the cylinthat petroleum w a x e s a r e been s e p a r a t e d into a series of pure f r a c t i o n s belonging der stocks in which it ocpresent in crude as such and to t h e same homologous series of paraffin hydrocarbons t h a t good crystallization is curs. It will not sweat and ranging f r o m C18H38 to C43H88. P h o t o m i c r o g r a p h s s h o w inhibited by the viscosity of is commonly thought of as that each pure c u t crystallizes in plates w h e t h e r frozen the oil or the presence of some amorphous or s o m e t i m e s unknown colloidal substance f r o m the m o l t e n s t a t e or f r o m solution. In purificais supported by the work of microcrystalline. t i o n t w o i m p u r i t i e s were removed-oil a n d an u n k n o w n Rakusin,6 Gordon,6 S a c h a m i x t u r e called “soft wax.” To it is d u e t h e f o r m a t i o n The waxes present in disn e n , and Padgett, Hefley, of needle crystals in i m p u r e waxes and the f o r m a t i o n tillate oils form c r y s t a l s and Henriksen.8 A number of investigators of vaselines w h e n high m e l t i n g waxes are mixed w i t h larger than those of petrohave shown t h a t the hydrooil. latum wax. The lighter wax carbons present in paraffin distillates contain paraffin wax belong to the CnHzn-2 wax, which forms large cryshomologous series of parafin tals well suited for-filter-pressing and sweating. The wax hydrocarbons. Mabery8 obtained a series of fractions from comparaffin wax melting from 48 O to 63’ C. (118.4 to 145.5’ found in heavier distillates, on the contrary, does not crys- mercial F.). However, as Gurwitsch points out, the melting points are tallize so well and has been designated as slop wax. Since a too low and the specific gravities too high, compared with the redistillation under cracking conditions greatly improves the series of synthetic straight-chain hydrocarbons prepared by pressing qualities of these distillates, it has been assumed that Kraff t. l o Francis, Watkins, and Wallington” have isolated seven fractions from Scottish shale wax which x-ray analysis12 has the slop wax “cracks” to paraffin. A distillate which does not pure shown to be straight-chain hydrocarbons. Dunstan and KewleyI3 press properly or a slack wax which does not sweat well is said and CarpenterL4have isolated hydrocarbons from t o C3( from to contain slop wax and to be insufficiently cracked. It has Burma crude. Considerable work has been done on the crystalline nature of even been suggested that paraffin itself does not exist in waxes. Gurwitsch4 shows photomicrographs of crude but results entirely from the cracking of slop wax. petroleum both natural and artificial vaselines taken by polarized light t o
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Historical Review The reasons for the different behavior of the petroleum waxes are no more clearly explained in the literature than they are understood by refiners. Two distinct theories have been advanced. From his extensive investigations on petroleum waxes, including ozokerite, Zaloziecki* concluded that there is present in petroleum a n amorphous a s well as a crystalline variety of wax. Since distillates always contain the crystalline variety, Zaloziecki believes that the amorphous variety, or “protoparaffins,” when subjected t o distillation crack into the crystalline variety, “pyro-paraffins.” Zaloziecki suggests that this conversion is possibly one of branched-chain into straightchain hydrocarbons which crystallize in well-formed crystals. The work of Hofer, Engler and Bohm, and Boleg seems t o be in agreement with the proto-pyro-paraffin theory of Zaloziecki. More recently the work of Marcusson3 further substantiates Zaloziecki’s theory. Marcusson prepared pure samples of ceresin wax and paraffin wax of the same melting point and found that the values of molecular weight, specific gravity, and index of refraction were all higher for the ceresin wax, and therefore concluded that ceresin consists of branched-chain hydrocar1 Received February 14, 1927. Presented before the Division of Petroleum Chemistry a t the 73rd Meeting of the American Chemical Society, Richmond, Va., April 11 t o 16, 1927. 2 Z . angew. Chem., 3, 126, 261, 318 (1888). 3 Cham.-Zip., 38, 73 (1914); 39, 578, 613 (1915).
show their crystalline structure. Padgett, Hefley, and Henriksens show pictures to the same effect. Distilled waxes show larger crystals which may be either of plate or needle form, this latter type being desirable in pressing distillates and sweating waxes. l6 The factors favorin4 needle formation are, however, unknown. Carpenter14 noted t h a t needles were obtained on crystallizing at about 39.4” C. (103” F.) from glacial acetic acid while plates were formed at a low temperature from ether without a change in the molecular weight of the wax which separated. He believes that a definite transition temperature exists at which one type changes t o the other. Myrick16 proposes the same idea, presenting photomicrographs of the two types Padgett, Carpenter, and Myrick all note the formation of needles‘ followed by plates from waxes containing considerable oil. 4 “Wissenschaftliche Grundlagen der Erdolverarbeitung.” pp. 1 6 and 218, Julius Springer, Berlin, 1924. 6 J . Russ. Phyr.-Chem. SOL.,46, 1544 (1914). 8 J . Soc. Chem. I n d . , 42, 4051‘ (1923). 7 Petroleum Z.,22, 484 (1926). 8 THIS JOURNAL, 18, 832 (1926). a A m . Chem. J . , 53, 251 (1905). 10 Ber., 19, 2223 (1886); 29, 1323 (1896); 40, 4783 (1907) 1 1 J . Chem. SOC.(London),121, 496, 1529, 2804 (1922). 1 2 Piper, Brown, and Dyment, I b i d . , 127, 2194 (1925). 1s J . I n s f . Petroleum Tech., 10, 503, 511 (1924). 14 Ibid., 12, 288 (1926). 1 5 Pbyala, 7th Intern. Cong. Applied Chem., Pet. 4, 1392. 1 6 Unpublished report from Whiting Laboratory, Standard Oil Company (Indiana), February, 1918.
IYDUSTRI-AL A S D ENGINEERIh'G CHEMISTRE-
June, 1927
Purpose of This Investigation
Obviously, neither the observations of the refiner nor the published information are adequate to explain the differing behavior of the petroleum waxes, important as it is to the processes of pressing, sweating, cold-settling, and centrifuging. These are still the conflicting hypotheses of Zaloziecki and Gurwitsch. For this reason an investigation was directed to determine, first, whether all petroleum waxes belong to the same homologous series of hydrocarbons, and second, the correct explanation of the difference in crystalline form noted by all investigators working with petroleum waxes. EXPERIMENTAL
Such physical properties as melting or solidifying point, index of refraction, and density of members of a homologous series show a regular increase with increasing molecular weight. It was therefore determined to isolate a series of pure fractions from paraffin, slop wax, petrolatum wax, and rod wax, and to plot the above-mentioned physical properties. The formation of smooth curves would prove the existence of a homologous series. Since the physical properties as determined give average values for mixtures, it is unnecessary to work with individual compounds, narrow cuts serving as well to prove the point Preparation of Materials
The oil-free wax was decolorized either with fuller'. earth or decolorizing carbon. The resulting oil-free wax was then subjected to very careful vacuum fractionation under a pressure of less than 1 mm. of mercury. The flask, which was internally electrically heated, was a modified Claissen provided with a fractionating column of the Skinner type about 25 cm. in length. For the higher boiling waxes it was found advantageous to lag the fractionating column with heavy asbestos rope in order to eliminate excessive refluxing. For the lover boiling waxes a receiver consisting of two bulbs with a stopcock sealed between was found most convenient. The side arm of the distillation flask entered the upper bulb, which received one fraction, while the prel'rious one was drawn off from the bottom bulb. The bottom bulb was then exhausted by a second vacuum pump before being connected
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Among other solvents which are available for the purification of petroleum waxes from oil may be mentioned acetone and methylethyl ketone for the paraffin wax range, while n-butyl alcohol and alcohol-benzene mixtures serve for slop and petrolatum waxes. However, ethylene dichloride is far superior. in that it is miscible in all proportions with oil, which thereby eliminates any possibility of fractionation of the oil during crystallization with resultant contamination of the recrystallized wax. Furthermore, readily filterable crystals are always obtained even from petrolatum wax.
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The following products furnished the source of raw materials: paraffin wax (slack wax) as removed by pressing wax distillate a t -17.8' C. (0' F.); slop wax as present in heavy distillate from the coking process; petrolatum wax centrifuged from residual stocks a t -17.8' C. (0" F a ) ; and rod wax as collected from the sucker rods in the field. A11 represent fractions from Salt Creek, Wyo., crude. Each wax was recrystallized from ethylene dichloride a t 4.4" C. (40" F.) until a wax of constant melting point and index of refraction wa? obtained, which was used as a criterion of complete removal of accompanying oil. Uqually three to four crystallizations were necessary.
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