CRYSTAL BEHAVIOR OF PARAFFIN WAX S. W. ’FERRIS AND H. C. COWLES The Atlantic Reflning Company, Philadelphia 1, Pa. Evidence is presented in support of the following theory: Petroleum waxes consist of mixtures of hydrocarbons belonging to various homologous series. The members of each series crystallize similarly, as either plates, mal crystals, or needles. If but one type (plate, mal, or needle) is present, the crystal form remains the same regardless of such factors as amount or kind of solvent. If the types
is slowly cooled t o form a new cake. Here the microscope shows that satisfactory sweating occurs only when the wax forms needle crystals. Yet the wax originally crystallized from the distillate for pressing was the same wax chilled to cake form for sweating. Centrifuging, particularly from naphtha solution, seemed t o be a t its best when the wax aaaumed no definite crystalline form except that so long referred t o as amorphous. Several investigators have determined the chemical composition of petroleum waxes and almost unanimously report them aa composed solely of straight-chain paraf3in hydrocarbons. A multitude of theories were advanced to explain the Jekyll-Hyde character of wax, and, with composition accepted aa constant, this factor was largely ignored. Without attempting to present a complete review of
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are mixed, and if the solubility relations are such that more than one type can crystallize simultaneously, either the needle or mal can impress its form on the plate. If, on the other hand, sufficient solvent is present to maintain needles and mals in .solution until plates are well established, mals ali.d needles can then deposit upon, and thus take the form of plates.
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Nove~mber,1945 PLATE
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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sidered needles t o consist of plates characteristically formed in layers. Later, amplifying the suggestions of Carpenter, Kat2 (‘7) introduced the concepts of “concentration of change C:’ and ‘%empersture of chan e T,”;both “changes” were from late t o needles, and thereverse cfange was held t o be im ossible. %e found that T, is constant for any given wax, substantiaT1y independent of solvent,
and 5-10 ’C. below the melting point. C, isinfluenced by both wax and solvent; it is the concentration of wax in any given solvent at which the transformation from plate t o needle takes place. Wax concentrations above C, produce needles, and concentrations below C,, plates. Therefore according t o K a t 9 theory, a wax which crystaliizes as a needle in dilute solution is an impossibility. Ivanovszky (6) came t o a conclusion diametrically opposed t o that of Rhodes and co-workers, showing that the removal of amorphous matter from paraffin wax permits the growth of very large needles whch pass or grow into plates.
When a number of partially confirmatory, partially contradictory explanations are advanced on the basis of good observations for the same phenomenon, either an erroneous assumption has been made or some important variable has I been uncontrolled. I n this instance the present writers believe the erroneous assumption is t h i t petroleum waxes are chemically homogeneous; the uncontrolled variable is actually a rather wide variation in chemical composition of the various fractions chosen for crystal study. Two previous publications (3 4) were devoted principally to the presentation of evidence that the comDosition of wax is. in fact. complex. While the authors did not demonstrate ’the actuai structure of those constitutents which were not straight chain, they reported nine oil-free waxes, each of substantially the same molecular weight, but varying in melting point from 63.6’ to 34.0’ C.
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1056
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 37, No. 11
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
November, 1945
1057
A theory was proposed t o explain the occurrence of at least three types of wax crystals. I n brief, the theory depended upon the assertion that there were three classes of wax; each class, when pure, exhibited one of the three well-known types of crystal form. That the theory did not, receive wide acceptance has been demonstrated by subsequent publications of other investigators. There was one rather adequate basis for lack of credence; all but one of the photomicrographs presented were of wax- c r y s talliaed from dilute solutions. Still more pertinent was the fact t h a t the vast majority of the crystals were obtained from nonpetroleum solvents such as nitrobenzene and ethylene dichloride. Since proposing the theory the authors have been able to test it more rigorously, and have found it helpful in the study of a wide variety of problems related t o wax refining. The purpose of the present communication is to restate the theory and t o present additional evidence in ita support.
tions, such as 5F, C, and BG,into plates and needles have been unavailing. That the waxes other than straight-chain (plate) do not occur as impurities, is shown by the central portion of Figure 2, also taken from the previous paper (4). I n the “front end” of paraffin distillate all the crystalliaable materials appear t o be of the plate type, but in the “back end” the sum of needle and mal crystal types is greater than 50%. These curves indicate that in higherboiling distillates the percentage of plate waxes would be small, and this is borne out both by rough observation of their crystal habit and by careful experimentation. The proportions of the various types of waxes probably vary t o some extent from crude t o crude; nevertheless the authors believe that the generalities covered by Figure 2 will be found applicable to the vast majority of the wax-bearing petroleums.
COMPOSITION
Both needle and mal crystalline waxes have marked powers, under the proper conditions, of impre&ng their own form on plate waxes. The most important factor appears to be the relative solubilities of the various types of waxes in whatever solvent may be present. Figure 3 shows the solubility, a t a single temperature, of s m ples embracing pure plate, needle, mal crystalline, and commercial waxes, respectively, in four solvents. Figure 3 leads to the generalization that the solubility of a wax in a given solvent is substantially independent of type, but varies with melting point. I n other words, waxes 4,5F, E, and 6J (Figure 1). would be soluble to substantially the same extent in any solvent which the authors have investigated. Figure 4 shows the solubility of a purified plate wax a t various temperatures in a number of solvents. This information is similar to that found for commercial waxes by Sachanen (11),and by Sullivan, McGill, and French (18);that is, solubility is high in naphtha or benrene, lower in oils of higher boiling ranges, and very low in nitrobenzene and isopropyl alcohol.
SOLUBILITY
Figure 1, reproduced from a previous publication (4), presents boiling point and melting point data on a number of highly purified fractions obtained from a mid-continent slack wax. Methods of purification and physical properties have already been presented ( 9 , 4 ); suffice it to say here that each fraction is narrow with respect both to distillation range and t o melting range. Samples 6B and 6J, for example, have substantially the same boiling points and molecular weights; yet their melting points differ widely, and there are considerable differences between their refractive indices and specific gravities (liquid), respectively. The only reasonable explanation is that their structures differ markedly. Samples BZ and 8J were subjected t o careful combustion analyses by Mair and Schicktana (8),who reported them t o be composed chiefly of cyclic hydrocarbons. Figure 1 was drawn by representing, in lieu of the customary points, the crystal form of each fraction as obtained from ethylene dichloridq. (Fractions 1, 2, and 4 were not studied in this particular solvent.) Three general types appeax-plate, needle, and (for want of a better term) “mal-formed crystal”. Furthermore, these three types fall into bands. For a given boiling range, those of highest melting points (straight-chain paraffin hydrocarbons) crystallize as plates; those of the lowest melting points (cyclic hydrocarbons), as needles; those of intermediate melting point (possibly branched-chain paraffins) form mal crystals. The most persistent efforta t o resolve mal crystalline frac-
BASIS FOR T H E THEORY
The theory, as stated in the synopsis a t the beginning of this article, rests upon the behavior of wax crystals as observed under the microscope. The authors, as well as many others, have recorded their findings by means of photomicrographs. It is extremely difficult, however, to obtain significant photomicro-
Figures 5 to 11. Crystallization of Purified Plate Waxes FXQUBS
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Figures 12 to 18. Crystallization of Purified Mal Waxes WAX CONCN., G./lOO ML.SOLVENT
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SOLVSNT USSD Nitrobensene Ethylene dichloride Nitrobenmne Nitrobensene Aoetio aoid Nitrobenrene
Crystalllaationof PurifUed Needle Waxes WAX
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SOLVPNT USED Ethylene diohloride Nitrobenrene
Figures 23 to 25. Crystallization of Pure Paraffins (from Their Own Melts) Frowarn NO. 23 24 26
PABAFPIN Dodeoane Tetradeoane 2,2,3,3-Tetramethylbutane
MELTINQ POINT.O C. -9.6 5.6 101.6
1058
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vd. 3l. No. 11
EQUAL VOLUMES
BLENDED AND CRYSTALLIZED
28 SOLUTION
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Figures 26 to 29. Influence of Certain Types on Crystal Form of Other Types (in Nitmbenzene Solution) Moltin. mint. I_ .iron in
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Brapbs of orystels forming io concentmted eolutions or in their own melts, beeauee changes normslly occur far too rapidly to be properly recorded by ordinmy %ti!". Since it wm "Denssly to extend tbe atudiea to concentrsted wax solutions and pure waxes, tbemoving picture camera w w utilized to obtain pictures while cryatalliastionwas proceeding st normal rates. Individual frmen have been enlarged and given B rectaogulsr form to distJnpuis6 them from the ordinmy stills or photomicrographs which are circular. Mngni6eationa are not reported bemuse they appear to have no bearing on the propoaed theory; &o, psrticularly with plate uul needle orystals, the size is dependent on the rate of cryatslli-
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estion. with slow rates favoring large crystals. This is much leer, pronounced with mal waaea. At no time, however, were the autbors sble to secure evidence that the rate of crystalli%ation exerted any influence on the type (plate, needle, or msl) of crystal appe&g. CRKSTALLIWTION OF FFiACXIONS -0 F€&4CllONS. figurea 6 to 11 P W n t th0 C I Y S b b fowed by plate-type waxes, of vsriaua melting pointa Experimenta were chosen to test other theories; thua wncentTatMn was vmied fmm very dilute eolutions (Figures 6 snd 7). thmW
wmmnbu, 1949
INDUSTRIAL A N D KNGINEERING CHEMISTRY
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wneentrated solutions (6 and 9). and finally to the absence of aolvent (Figure 11). Viseoaity varied from the very low vslues of the ethylene dichloride solution to that indicated for Figure 10. The aetual viscosity during crystallization w88 far higher than the value given, inksmuch as the crystals formed at about 77*F. I n Bpite of the%e drwtic vsriationa, the phte waxes rema‘ined true to type. The cinemicrograph (Figure 11) shows an ~~tatsnding ohmtesiatic of @ate wax in concentrated solution--nsmely, the tendsncy,for the crystals to pile one upon the other. Croeaing the field diagonally are crystals which might be taken for needles. The motion picture iteelf demonstrates mquivocally that they rn plstes on edge. Fkum 12 to IS offer similar evidenee for mal crystailine waxes, and Figurea I9 to 22, for needle waxes. Figures 23, 24, and 25 show the crystal form of three pure parstfin hydrocsrbons; 23 and 24 offer good confirmation of the suggestionsslready made, Since the substances studied are straightohin hydrocarbons and form plates. The suhjwt material of Figure 25, however, is eertsinly branched. Study of Figure 1 suggests that branching m y become rather pmnounoed before the plate hahit is destroyed. AIthough tetrmethylbutane (Figure 73) is highly branched, there are no long branches. A weas may be haasrded, therefore, that fample 6C or 5D (Figure 1) is made up of chaine carrying short aide chine, whein 5E or 5F the side chains are longer. k C n O N S CONTAINTNO MOBE %AN ONE TTPE. rimes 26 to 31 show the &st of one type of wax upon another. In the 6mt tent (28) a dilute solution of plate wax, which orystallieed 80 shown in the upper portion of the lefbhand circle, was mixed with a dilute solution of B mal crystalline wax, whose crystal is slsa shown. When crystals were obtained from the mixture, they were perfect PIE*, that is, the mal crystal wax b d no effeet. The fame msl cryah1 wax, however, completely destroyed the pbte clystsle in the next trial (27).
The difference is in the soluhilities. Figure 3 indicate that solubility of B wm in a given solvent is largely a function of the melting paint of the wax. The mal ahawn in Figure 26 was. therefore, much more soluble than the plate; it could not cry+ talliee while the high-melting plate were forming. In Figure 27,however, ita salubility was much the same 88 that of the plate wax, snd it therefore impressed its own form on the plate. A similar ability on the part of needle wax to csst plate wax into ita own mold when solubility relations are favorable is demonstrated by Figures 28. When waxes of all tbree types (23) were present, the crystal WBB mal. Pwrly formed needles from “abnormal” shmk wax were ohanged to clean needles by adding n pure needle wax (30). The impure frsction shown in Figure 31 exhibita the typical mal crystal form, but that needle crystals had lost their normal form is demonstrsted b$ the resuita of fractional wystalliae.tiotion. Coedeting crptnls of more than one type are shost never encountered. Whichever type is able h t y to establish itaelf nppem to dominate. Thus, while i t has not bean rigorowly proved, the mal cryebltine wgx shown in Figure 26 appears to have deposited on the preformed plstea and thus taken up their habit. In other words, either needles or mal cry&& cao impress their fom on p i a h during simultaneous crystsllization; platee require conditions where they can cry&di.se alone and appear to take up the other types. On this basis the wax in p a r a h distillate sheds ita chameleon behavior. The highest-mdting (least-soluble) wax= in the distillate are plates, but the total wax concentration is relatively low. When, therefore, the distillate is chilled prior to pressing, needles and mals are held in solution while the plstea form. In deck wax, OD the other hand, there is insufficient oil to hold the needles snd msls in solution; hence one or the other will prevail. If however, the boiling range of the distillate be extended (Figun?s 1 and Z), the proportion of pletes tends to decrease while the
1060
INDUSTRIAL A N D ENGINEERING CHEMISTRY
proportion of high-melting msls increases. Ae a result the plate crystals do not become established; mal crystals predominate, snd the distillate fails to press. SLAM WAXES
Figures 32 to 45 (obteined by own-melt orystalliaation) ill”+
trate the application of the plate-needle-mal theories to the study of the behavior of two slack waxes; one was “normal” in that it meated well. the other “abnormal”, in thet it failed to sweat properly. Figure 32 shows the appearance of the normal slsek wax, the orystals being typioal of the interlacing needles most suitable for sweating. Figure 33 demonstrates that the crystallmation is not altered by removal of oil
Vol. 37, No. 11
The norms1 slack wax was separated into fraotions by vscuum distillation in B fractionating tower. The 12.5-25% cut (Figure 34) exhibited unmistakable plate crystals, as did the three frsctions which followed (35,36, and 37). The approximate boiling range of saoh cut is indioated in the upper portion of Figure 2, which reveals that the 6=stlour fraotions of the normal slack WBX Bhould oootain little except plate wmes; the microscope verifies this. In the next cut, however, the needle waxes are increasing in proportion and impress their farm on the plates (Figure 38). The next cut also gives needles (39). The still residue, however, contains enough of the mal crystaltine type to destroy both plates although not enough to influenee the crystals and needles (e), of the entire slsck wax,
November, 1945
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1061
Figure 41. Abnoirmal Slack Wax Figun 42. 1540% Ovarhead fmm Abnormal Slack Wax Figure 44. 70-R5% Overhead from Abnormal Slack War Figure 43. 3 l b i O % Overbead fmm Abnormal Slack W a x Figure 45. 15% Reaidusl from Abnormal Slaok Wax
Figure 41 shows why the slack wax is abnormal in operation; such a structure is better adapted to holding oil than to releasina. it. When, however, this material is cut into fraotions by simple v~cuumdistillation, the law-boiling 15-30% cut (Figure 34) again exhibits plates ZE it should, since neither needle nor mal crystal wax- oecw within this boiling range (Figuce 2). The next long (30-70%) cut includes enough needle wax (Figure 43) 90 that the plates are suppressed, and the same is true of tho 7 0 4 5 % h e tion (44). The residue, however (45), exhibits especially poor
VoL 37. No. I1
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1062
46
48
41
FigUM Mi to 48
Figures 42, 33, and 41 show, respeetively, the typical manner of growth of plates, needles, and mals. P l s t e ~are very thin and freguentty pile one upon the other; needles "shoot" aoroee the field, and in either o a ~ ethe size of individual crystals may be varied between wide limits by control of the cooling rate. Msls 8eem alwsys to be relatively small, and differences in cooling rate have little &e&. Inetead of single cry&&, growing gradually, amsll crystals appesr suddenly, and more crystals appear until the field is obscured. Indeed, they aot as if the lattice is too eomplieated to permit large crystals to form. That either needle or mal crystal can impress its form on plates neems clear; it is not so definite as to whether needle OT mal cryatat is more powerful. Evidence indicates, however, that in the middle ranges of para& distillate there is little difference, the needles being possibly s bit more powerful. As the boiling range climbs, however, the power of the mals increases rapidly and completely controls all the heavier wax distillates. These findings are rather consistent with the suggestions made earlier in the paper for the molecular oontiguratian of the three crystal types of WBX: that plates are either straight chain or chaina with short branches, that compounds crystallizing as needles contain cyclic etructures, and that the msls have long branches. One would expect the crystal lattice of straight-chain substances to be relatively simple, and able to accommodate the more irregular compound8 only with some di5culty or after the plate crystal has atready attained considerable size. On the other hand, the straight or slightly branched molecule could probably adapt itself rather r e d l y to the lattice of a crystal more complex than its own.
llnd appnrcntly grown wrttcally, snd the microscope, focused at B plane slong the crystal sxis, accomplished optical sectioning. Fi~res46,4i,snd48bndaomeeredeocetotheideaofadrilled needle. They could a190 I* used to nuppon the contention of Khdrsend eo-workers (IO, that crystals are rolled plates. This ia partirularly true of Fimrc 47. If, Iiowever, we m u m e that needles m u i t from the r o l t i n ~of thin plnces. the vowing process of 3 needle should be Isryrly tlmf of mrrraaing ils diameter without morknlly changing iE leriah. and the aides should be e p proximately pnrsllel C'ount1e.w ubservations have eonvineed I I ~ P prru.nr nutliorn ttmt nwdlcs do "01 grow in this manner; they m w d y lengtht~n.tloeken, and taper toward the growing tip. Thus they &haw exactly 8s one would expect them u, if the deposiwd mnrerisl were all cominp; fmm the m u o d i n g ~oluticn (Figure 33). Ir ia riot nccesylly to w u m e that any of the cry* tal surface is netunlly r u n d The ems mtioo of the needle mny he polygunnl, nith t11e mmber of aides too large to be re solved by the microwope. rnerpectvd support fur the unlikely hypothesis of a " c r y d with a hole" was offend by A . 11. Thornpsoo of the Unim.lsity of Pennsylvania, d u n g the discuuwn which followed the oral pe n t ~ t i wof 11u. paper. The authors appreciate his prmidon to q w t c lam 8 8 fv1lon.s: "Io the course of 8 study of the Bolubilitiw (of ~ t ~ r g aSn~i L~in S ~lqwcrusnlruhol wlutwns. it waa found that if such a solution of ammonium nitrate were cooled alowly, the 6811 crpsrsllircd i n thc form uf oeedlcs with 8 hole rbmugh tha reiiwr *' Tlw r.uthuw do not b e l w e that they have o E c d de& c wax nmdlrs, but have no other nite proof of thr c x ~ ~ t c n cofdnlltvl e ~ ~ i l n i . n t m iw n t L c vv+:r-pn..wet median line, and present the cviilcncc nirh the holn. i l i i l t o t h m can propove B lwtter solution.
STRUCTURE O F NEEDLES
An outstanding peculiarity of needles is what appears 8 8 8 line or "rib" extending along the crystal and always equidistant from the aides; it is apparent in some of the crystals of Figures I9 to 22, 29,30,a3,43, and 44, and can slways be seen hy adjustinq the focus. It is quite'dSerent from the lines appearing at the edges of plate crystals, as in Figure 5 or 9; the latter is the result of optical phenomena depending on the focus and the refrraetive indices of the solvent and orystal, respectively. This needle with a central rib has been B puzzling phenomenon. It manifestly cannot be the kind of rib which would result from s rectangular or triangular c m ~ section, s for then it would not always he Been at the center of the crystal. The only u p l a n h o n which seemed to fit the facts (orappearances) was the existence of 8 hole through the center of each needle. This seemed so improbable, however, that the authors wwe seoustomed to offering it BB a joke. Then on one ooossion (when the idiosyncracies of needles was not the subject under investigation), B slack wax was crystallised very slowly on the atage of a micromope (Figures 48, 47, and 48). This slaok wax was known (as were all "normal" slsek waxas) to crystslliee in needles; in this instance the cryatals
ACILYOWLF.KMEXT
The authors seknoalLdKu their indebtedness to John Campbell, w w r d many of the phota-phs used herewith.
who
LlTERlTURE C I T W (1) Uuel,lsrsnd Graver. IEU. E m . CREY.. 19, 718(1927)). W Carpmtrr. 1. I w l . Pplrolnm Tech., 12. 2h8 (lU2G). (3) Ferrrs. Curules. and Henderson. 1.~0.Ero. Ctcru.. 21. loo0 (1929).
(4) Ibid.. 23. 881 (1931). (6) Ivanovasky. P&oleum (London), 4, 100 (1941). (6) Kats. J . Inat. Pdroleum !red.,16.870 (1930). Ibid., 18, 37 (1932). Mair and Sohioktanz. IND. E m . Casu.. 28. 1056 (1936). Padgatt. HeBey. and Henrichen. Ibid.. 18. 832 (1926). Rhodea. Msson. and Sutton. Ixn. EN*.Casu.. 19,935 (1927). Sechanen.Pdrolelrm Z.,21,735 (1925). Sulbvan, MoGilI, and Frenoh, IND. ENO.Caex., 19,1042 (1927). Tanaka. Kobayashi, and Ohm, J. FanJtv Ew. Tokw Zmp. Unio., 17, 275 (1928): abstracted in J . Imt. Pdrdnrtn Ted.. 15. 74A (19291.
