Crystal Types of Pure Hydrocarbons in the Paraffin Wax Range EDGAR W. CLARKE ,Ipplication Research Diwision, Research and Dewelopment Department, The Atlantir Refining Co., Philadelphia, Pa. T h i s study of the crystal types of pure hrdrocarbons was undertaken to secure data for application to wax processing, and to determine why a paraffin wax niay form three different types of crystals-needles, plates, and malcrystalline masses. Twenty-three pure hydrocarbons comprising paraffinic, naphthenic, and aromatic compounds in the molecular weight range of paraffin wax were obtained from il.P.1. Project 42. These pure hydrocarbons were crystallized from the melt a t different rates and from solutions of ethyl acetate and nitrobenzene a t different rates and over a wide range of temperatures. Crystals of each pure hydrocarbon except n-hexacosane were prepared in each of three predominating types: plates, needles, and nialcrystalline masses. Needle crystals could be obtained from n-hexacosane only by the additinn of small amounts of
resinous impurities. The t w o major factors in determining whether needles, plates, or malcrystalline masses were formed by each of the pure hydrocarbons were: the rate of crystallization of the solute or the melt, and the temperature difference between the melting point of the pure hydrocarbon and the cloud point (or crystallizing temperature of the solution). This work indicated that waxes which are predominantly naphthenic, or predominantly branched-chain paraffins, may be crystallized as needles, plates, or malcrystalline masses by varying the temperature and rate of crystallization. Waxes which are predominantly n-paraffins may be crystallized as plates or malcrystalline masses by varying the temperature and rate of crystallization, or as needles hy the addition of small amounts of petroleum resins.
T
cannot be obtained from the sweating operation, the slack wax is unsweatable. A much smaller type of crystal than the needle or plate crystal has been observed in unpressable wax distillates. Irregularly shaped masses of these microscopically small petroleum wax crystals are commonly termed malcrystalline masses. The term “malcrystalline” signifies that mames of this wax assume no definite shape and not that the individual crystals are misshaped or malformed. The exact shape or crystal habit of the very small crystals which compose a malcrystalline mass has not been established by microscopic observation because of the small size of the individual crystals, although x-ray diffraction patterns have indicated a uniform crystal latice. Typical examples of the three types of wax crystals are shown in Figure 1. These photomicrographs of wax crystals are from the work of Ferris and coworkers (4). Based on their crystal study of wax fractions from successive crystallizations of waxeB, Ferris and Cowles (8)have suggested that plates, needles, and malcrystalline waxes represented separate homologous series of normal paraffinic, naphthenic, and branched-chain paraffinic compounds. Padgett (11) and others have reported that a wax may be crystallized as needles, plates, or malcrystalline masses depending on the conditions of crystallization. Gray (6) suggested that this apparent difference in theory could be clarified by a crystal study of pure paraffinic,naphthenic, and aromatic compounds in the paraffin wax molecular weight range; the study, however, would have to be conducted under a wide range of temperatures. A crystal study of these three classes of hydrocarbons became feasible after the synthesis of pure paraffinic, naphthenic, and aromatic compounds in the paraffin wax molecular weight range by A.P.I. Project 42. The evidence presented in this paper indicates that the point of view of Ferris and coworkers is not necessarily opposed to the point of view of Padgett and others. Ferris and coworkers conducted their crystal studies over a relatively narrow range of crystallizing temperatures for each wax fraction. It would have been a gigantic task indeed to carry out their crystal studies for
HE crystallization of paraffin waxes as needles, plates, and malcrystalline masses has been the subject of many studies. Various theories have been advanced to explain why a paraffin wax may form these three different types of crystals. This paper presents data which reconcile differences in the theories. This study of the crystal types of pure hydrocarbons was undertaken in order to secure accurate fundamental data for application to wax processing. Twenty-three pure hydrocarbons comprising paraffinic, naphthenic, and aromatic compounds in the paraffin wax molecular weight range were obtained from A.P.I. Project 42. These pure hydrocarbons were crystallized from the melt and from solution under widely different conditions. The study of these pure hydrocarbon crystals included: the size and external shape of the crystals, the factors which influence the type of crystal formed, the optical properties and crystal systems of the crystals, and the similarity of the pure hydrocarbon crystals to paraffin wax crystals. The shape and size of wax crystals have long been studied. The microscopist refers t o the external shape of a crystal as its crystal habit. The two crystal habits which are most frequently observed for paraffin waxes are plates and needles. Plates are found in pressable wax distillates and in refined waxes (8). Needles are found in sweatable slack waxes. A wax distillate is the fraction of a crude petroleum which contains the paraffin wax. To separate the paraffin wax from the oil, the wax distillate may be chilled until the wax crystallizes and the resultant slurry may be filter pressed. If the wax forms a filter cake from which the oil drains easily the wax distillate is pressable. If the oil cannot be separated well from the wax crystals in the pressing operation, the wax distillate is unpressable. The wax left on the filter presses is called the slack wax. The slack wax is melted and charged into the sweat ovens where the wax is chilled to solidify it somewhat and then it is slowly heated. If oil and low melting waxes drain away from the slack wax during the sweating operation and leave a high yield of wax with low oil content, the slack wax is sweatable. If a wax of low oil content
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1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
aaoh wax fraction over 8 much wider range of crystallizing temperatures. The crygtal ryetents of norm1 pnwffins have been studied extensively. Crystal xy@,omR dssaify crystals an tire bapis of orientstion of pointr; on the eryntirl laticc. On thia ha& of cl-ifieation, 811 eryatule have beon grouped into .six differenl system (t). Both the x-ray epeetrometei atid thc petrographic micmecop b a r e been u a d l o dotennine the crystal ayetern of
2927
Each pure hydroosrbon was aubsequently orystelilfized over B wide range of temperatures and a t different rates of heat withdrawal to determine experimentally the 0tTwt of each oft h e e vsri~blwut, cryrstal size and habit. For each temperature of orystallization and rate of heat withdrawal, 10 separate slid- of tho hydrocarbon were prepared t o emure an sceurately measured rate of cryitallization and reproducible data Approximately 1.D nip. uf h\-atal of hydroosrbon 80 ~ s r b o nwere ~ comparable. that the solution remained indefinitely dear when held in t h e r m ] . Before carrying out the complete experimentat program, these vis! a t 1- F. above the cloud point temperature. After strrnng coi~eluaionswere tested on one psraffiinie, one naphthenio, and g?ntly for 10 to 30 minutes a t the eloud point temperature, R one aromatic compound. Each of t.hese repmentstive comsltght ham of solid orystals nopeared in the eolution. T h e nohtion in the vial wae then warmed s?vernl d c g F e and divided i d pounds was crystallized a t B eeiies of temperatures ranging fmm rtions, each of which contaaied approxmately 1.0 5s. o i the melting point to temperatures 50' to 1W' F. below the melt&%carbon. Each aliquot aolution \mas either. pi etted pt4 B ing point. Within this temperature range for each of the repretared hanging drop elide or onto the surface of B tarecfglass microsentative compounds, malerystalline mmaes, needles, and plates slide. The hanging dro slides w e n used, to obtain erystsls from saturated solutions. TKe plain glass mioroslidee were used to were OhaeNed with 6 petrographic miomscope. In addition t o obtain orystals hy total evaporation of the solvent. the temperature a t which crystallieation occurred in these prp The hanging drop slides had B flat bottom recess which wa8 16 liminary experiments, the rate of heat withdrawel was also a f s f mm. in diameter and 4 mm. in de th The reoeas of the han 'ng tor in determining the type of crystal. drop slide was olmast filled with t& &quat solution of the hy%
OT
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INDUSTRIAL AND ENGINEERING CHEMISTRY
carbon. A standard cover glass, approximately 0.18 mm. in thickness and 22 mm. in diameter, was moved over the hanging drop slide in such a way as to cause an air bubble about 10 mm. in diameter to form and to adhere to the bottom of the cover glass. Each hanging drop slide was placed in a desiccator which was located in a cold room. The cold room was held within l o or 2' C. of the predetermined cloud point temperature. To compensate for any slight differences between the temperature of the cold room and the predetermined cloud point temperature, a small piece of dry ice or a small quantity of warm water was introduced into the desiccator through a ground-glass joint in the lid. In this manner the interior of the desiccator was maintained within = k O . l " C. of the cloud point temperature. By looking throu h the glass of the desiccator, the initial formation of cr stals in t i e hangi'ng drop slides was observed with the aid of a i a n d lens and a flash light. When crystals began to form, the han in drop slide was removed from the desiccator. The liquid in taeaanging drop slide was gently splashed over the bottom of the cover glass in order to cause a drop of solution to adhere to the bottom of the cover glass. Since the major portion of the liquid in the recess of the hangin drop slide was separated by the air bubble from the drop of so%tion on the cover glaas, the crystals on the under side of the cover glass were immobilized. By means of the petrographic microscope, these crystals on the cover glass were observed in the cold room under stationary conditions which could not exist in the main body of the liquid. When no further increase in the number or size of crystals in the hanging drop slide was detected by microscopic observation, it was considered that equilibrium had been attained in the saturated solution. The length of time recorded for crystallization from a saturated solution was the time interval between the appearance of the first crystal and the attainment of equilibrium. A glass microslide WM used to obtain crystals from the aliquot solution of the hydrocarbon by total evaporation of the solvent. The glass microslide with the aliquot solution on the surface, was placed inside a smafl glass container which was equipped with a transparent glass cover. The tightness of the cover on the glass container controlled the rate of evaporation of solvent from the aliquot solution. The entire glass assembly was placed in an insulated desiccator in which the crystallization took place. The rate a t which heat was withdrawn from the solution was determined by the type of insulation surroundin the desiccator and by the magnitude of the temperature difkrence between the inside of the desiccator and the cold room. The temperature of the cold room was always maintained constant within 1 or 2' C. If the temperature inside the desiccator was initially higher than the constant temperature of the cold room, the temperature inside the desiccator was gradually lowered. The more completely the desiccator waa insulated from the surroundings, however, the less rapid was the rate of cooling of the crystalliein solution. By using a hand magnifying glass and a flash light, t i e crystallization was observed without altering the conditions of crystallization or disturbing the glass assembly in the desiccator. The length of time recorded for crystallization of solutions from microslides was the time interval between the appearance of the first crystal and the disappearance of the aolvent. Glass microslides were used to obtain crystals of the pure hydrocarbons from the melt. The glass microslide with 1.0 m . of pure hydrocarbon was heated a t least 10' C. above the meyting point of the pure hydrocarbon in order to obtain a uniform distribution of the liquid melt over the surface of the microslide. Crystals were obtained by cooling the liquid melt to temperatures well below the melting point of the pure hydrocarbon. A very high rate of cooling was attained by direct contact of the microslide with a moisture-free surface of solid carbon dioxide. Less rapid degrees of cooling were effected in the manner which has been described in detail for the glass microslides with aliquot solutions. The time required for crystallization was recorded as the time interval between the appearance of the first crystal and the complete solidification of the melt. A petrographic microscope with accessory plates was used for observations of the crystals. The light source was a 108-watt, &volt, ribbon filament lamp. A liquid cell served as a heat filter to prevent warming of the slide and consequent fraying of the crystal edges. Silhouette angles of the plate crystals were determined by use of a graduated stage and vernier, and by triangulation. Forty different crystals of n-hexacosane were obtained under identical conditions on a series of hanging drop slides. Their acute silhouette angles (4 angles) were measured and it was found that the d, angles of the 40 different crystals of n-hexacosane varied over a range of 2' to 3". The majority of the @ angles, however, lay within 1 0 . l O o of the average value. These observations confirmed the observations of Hubbard ('7) that the values of angles for crystals of a normal paraffin might vary over a range of Several degrees. This small variation in magnitude of the @
Vol. 43, No. 11
angles may have been caused by the crystals being actually imperceptibly tipped in one direction or another although they appeared to lie in a plane normal to the axis of the microsco e. The d, angle which corresponded to the average value is probagly the value which would be obtained by measuring the acute angle of a crystal which was perfectly oriented for study. It appeared necessary, then, to measure the d, angles for approximately 10 crystals of each hydrocarbon. The probable error of a single reading was about f0.12' and the mean about =k0.05'. Pleochroism was determined by the usual method. Plane polarized light was transmitted through the cr stal; the microscope stage was rotated; and any chan e in co3br of the crystal, pleochroism, during the revolution of t f e microscopic stage was noted. The crystals were also observed with crossed Nicol prisms and parallel light-orthoscopic observation. Orthoscopic observation was used to determine anisotropism, type of extinction, birefringence, and the direction of the slow ray in a crystal. These optical properties are qualitative aids in identifying a crystal. Because of the ease with which the significance of these optical properties escape the individual who is not constantly using them, these optical properties and the method of measuring them are briefly described. If a crystal under orthoscopic observation remained uniformly dark on rotating the microscope stage, that particular view of the crystal was isotropic or singly refracting. On the other hand, if the crystal became alternately light and dark during the rotation of the stage, that particular principal view of the crystal was anisotropic or doubly refracting. The darkening of a crystal which is being observed orthoscopically is called extinction. Isotropic views of crystals showed total extinction. Anisotropic views of crystals showed parallel, oblique, or symmetrical extinction. At extinction, if the principal edges of the crystal were parallel to the cross hairs of the microscope, the extinction was parallel. If the principal edges of the crystal were at an angle to the cross hairs of the microscope a t extinction, the extinction was oblique. If the cross hairs of the microscope bisected a principal angle of the crystal a t extinction, the extinction was symmetrical. Provided the extinctions of enough principal views of a crystal were known, the system to which a crystal belonged could be identified from extinction data alone (16). The direction of the slow ray in an anisotropic crystal was determined by means of a selenite retardation plate. The crystal was turned 45' from a position of extinction to a position of maximum brightness. If the anisotropic crystal became yellow in color when the selenite plate was inserted into the slot through the body of the microscope, the slow ray in the crystal was perpendicular to the slow ray in the selenite plate. On the other hand, if the anisotropic crystal appeared blue when the selenite plate was inserted, the slow ray in the crystal was parallel to the slow ray in the selenite plate. If the slow ray of the crystal was parallel to the length of the crystal, the crystal was length slow and the sign of elongation was positive. On the other hand, if the slow ray of the crystal was perpendicular to the length of the crystal, the crystal was length fast and the sign of elongation was negative. The birefringence of an anisotropic crystal is numerically equal to the difference between the highest and lowest refractive indexes of the crystal. A qualitative measure of this difference in refractive indexes of an anisotropic crystal may be obtained by use of a quartz-wedge retardation plate. The crystal was adjusted so that the slow ray of the crystal was perpendicular to the slow ray of the quartz wedge, and the quartz wedge was slowly inserted. The number of times that a red band moved across the field before the crystal became gray determined the order of birefringence. If the anisotropic crystal was initially gray when the quartz wedge was inserted, the crystal was weakly birefringent. If a red band crossed the field a number of times before the crystal became gray, the crystal was highly birefringent. The crystals were also observed with high aperture objectives, a full condenser, and Bertrand lens (conoscopic observation). The
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
interferenee figures obtained by mnoscopic ohservstion of s eryatal aaeist in determining the system W which a crystal belongs
(6). Caneiderable msehsnical difficulties were encountered in preparing slides witahle for Bertrand interferencefigures. For 8 well defined interference figure, the crystal s h d d be perfectly farmed sod pmperly entered on the stage. A crystal which is slowly gmwing does not give sn clear sod distinct an interference figure sn B crystal which is not inorwing in siec. Although crystale which were prepared fmm the melt or from coneentrsted ~olutiooswere large, they were somewhat deformed and imperfect because of the rapidity of growth and the interference of contiguous crystals. Cwatals whioh were very slowly oryatsllhed fmm dilute solutions st sufficiently low temperatures were perfeotly formed hut were very small, thereby providing grcat difficulty in obtaining oonelusive interference figures.
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of m o t h s c t m of hydrocarbons. Perfect hexagonal plates with 120- anglea were obtained for two ammstic compounds-1phenyleioossne snd 7-phenyleimaane-and for two psrsffinie compound-11-~ecylheneioosane and 6 , 1 1 d I ~ y l h e x a d e c ane. Hexsgoaally shaped plates with angles which differedby ~everaldegrees from 1%' angles of B perfect hexagonal plate were obtained for two naphthenic o o m p o u o d s - 1 , 5 d i c y c l ~ h ~ y l ~ ( ~ cyclohexylethyl>pentane and I -cyolopentylheneimsanae. Square m d rectangular plates with perfeot 90" mgles were obtained for
DISCUSSION
Although pure hydrocarbon crystals at times consisted of oomplea mixtures of p l a h , needles, and rnderyatsls, it VBBusus1 for one type of ory&.I to predominate under m y given set of conditione. The crystals were in d l CRBBR classified according to the predominant type.
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Figure 2. Crystal Types
Table I aod Figure 2 show that by varying the rate of heat withdmwd aod cloud point temperatureof thesolution armelt, crystale of each pure hydmcarbon except n-hexseanane were pr+ pared in eseh of three predominstiig types: tshulsr crystals having uniform thickness, designated as plates; acieular crystals of oval em8~1section, deeigosted aa needles; irregularly ahaped maam of tiny crystale (generdly less than 1 X IO-' om. in overall length), designsted 88 malcryetalline mwam. Needle crystals muld not he obtained from n-hexscasane except l of reeioous impurities. By adding by addition of ~ m damounts to the n-hexsoosane leas then 1 % of B resio which had been ~epsratedhy adsorption from micmcrydallioe wax, needle c w tala were obtained from the melt. This observation agrees with the observation of Hubhard (7) that normal parsffins crystallized m needles fmm the melt or from solvente only when impurities were p-nt. Although Carothers and coworkers ( 1 ) claimed that the normal paraffin, n-heptscontane, orystallised in small needles fmm hot butyl acetate and from benzene, their method of prepsring n-heptaeontane did not exolude the presence of impurities. PL~TE-TYPE CRYBTALS.Plate crystals aith regular outlines were nearly dwsys produced by slow crystallization of n pure hydrocarbon from a dilute aolution which WBB maintained at eonatant temperature. Details of the eonditiona under which these plate cryatsls were obtained for eaob pure hydroesrbon are given in Table I. The hapa of 1-e plate ciystals which W ~ Bobtained far each pure hydrocarbon is given in Tshle I1 dong with the optical properties aod the acute silhouette angle of the crystal. Typical plate crystals are &own in Figure 3. For the three classes of hydrocarbons, no one shape of plate crystal wsn more characteristic of one olase of hydrocarbons than
A
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C
Examples of Plate Crystals
the naphthenie compound 1-cyolohexyleioasane. Far the other pure hydrocarbons, the dnts of Table I1 indicate that the majority of plate crystals were shaped like B psrallefogram or a rhombus with acute angles varying from 51"30' for the paraffioic mmpound, 11-neopentylheneimsane, to 8O"lO' for the naphthenie compound, 1,ldicyolohexyItetrad~n~~. The temperature of crystslliw.tion snd the rate of moling inkbeneed the shape of the plate crystale. This frequently observed phenomenon is illustrated by the deta of several pure hydroosrhons for which plstes of quite different shapes were ohtained. Plate-type crystals which formed very slowly fmm the melt tended to w u m e an hexagonal shape with 1 W Bugles (Table I, PSC No. 90). Phte-type crystala which crystallieed fmm eolvent tended toward p~r~rallelogrsm plates (Table I, PSC N08. 106, 4, and 51). Clystals which formed more slowly from solvent or at lower tempersturea tended toward rhombic plates (Table I, PSC Nos. 106, 4, 51, 90, and 99). The crystallieation of these pure hydrocarbons from the melt ea hexegonal plates wm investigated more fully. Micmscopic observstions were made of slides of pure hydrocarbone in e, thermally oontmlled stage. All of the hydrocarbons exwpt I q c b hexyleimsme exhibited some tendency toward cry&llising BB hexagonal plates aith 120' angles, provided the pure hydrocarbons were crystallized over s period of several hours at a temperature differentially below the melting point. The readioess with whioh bexagond plates formed under other conditions, however, varied greatly with molecular configuration. Thus, 1,5dicyclohexyl312oyclahexylethyl)-pentsne fonned hevsgonsl platea on rapid and slow oooling of the melt and on slow evaporation of a aolution of the hydrocarbon in ethyl acetate at 111' F. below the melting point. On the other hand, I-oyclopentylheneicossne formed mslcry~tslsupon repid cooling of the melt, but formed hexsgonal plates upon rapid or slow evsporstion of I) ~olutionof the hydrocarbon in ethyl ncetate a t 103" F. below the melting point. The plate-type crystsls of the paraffinic, naphthenio, and aromstie hydmosrhons were sll very thin. All plate crystals were weakly birefringent and showed no evidence of plwehroism 88 shown in Table 11. The direction of the slow my in the plate eiystale wsn uniform only for the pamffinio compounds. For the psraffiaa studied, regardlesa of the degree of branching, the slow rsy waa perpendicular to the length of the plate crystal and the Zign of elongationwas uniformly negstive. For the plate crystals of naphthenea in which oyclohexyl groups were substituted on the m e carbon, such sn l,l-dioyelohexyl tetradwane aod 1,5dicyclohexyl-3-(2
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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November 1951 cyclohexylethyl)-pentane, the sign of elongation WBB positive. The plate crystals of all other naphthenea which were atudied had negative signa of elongation. The rhombic plates of the aromatic CamDound. I-Dhenvkieosme. had 8 oositive e i r n of eloneation
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
7.534
tained by very rapid cooling of the melt. Regardless uf the pure h y d m r b o n which w m crystallized, thew rnslcrretalliie inasses showed many conmoo physical rind optical properties. Figure 5 shore typical crystals of this type. The individual niirrwrystals which composed the maleryathlline mwes were generally lesa than 1 X IO-' cm. i n lenah. Thc thicknew of an individual microcryetal WBO generally le" than 5 % of the leuah. Extino-
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Figure 5.
Examples of Maler,-atalline Masses A . PSC No. 11 E . PSCNo.55
tion WM parsllel to the longest dimension of the microcrystals and the sign of elcngatiotion w m negative. No deteotsble Bertrsnd interference figures oould be obtained for any of these mioroorystals. Manses of these miemerystals were malformed and exhibited no uniform oatlines and no straight edges.
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unless tho difference between the ~ e l t i n point g of the crystal and the cloud point ternwrature of the solution ereeeded 130° F. If the temperature of the er).stallirstian w m within 50' UI Bo' F.of the melting point of the pure hydruearbou. lor slow 01 malemtclg slow races of crystallizstion, the seeJle-type crystal xhs the stnble type (Table 1. P3C Xm. 27. 23, 45. 22, C,: 8, 6.1, 09, and 81). On the other hand, if the dilfcrriiee lierurrir the temperature of crsitalliration a d the melting ,mint oi tlra pure hydrocarbon "as yreater than 60' to (io" F..fw low 01 m d e r ately blow rat.+ of er).stailuatioi,, the plate-typs of c r y i t a l R.". the rurble type ( T a l k 1. PSC So* 4 %51, 11. and Yu,. If the tempernrum of erynullirutioi, i n s iwre than :30° 4 b d o w t k melting poinl of the pure hydroc.irlmzi far w r y SIUU r a t e of rr)*lalltzarion, plate-type rryatala w ~ r ethe eihl,ie t y y r (Table 1. PSC So.. 106, 4,3,2, 5l,27. 23, 55.22,C7. 100, ;ti, I17 0.1, 11, "J, 125, 9!l, la?,81, and 12,. FordiRcrcnren I,rtuwii tire t e n i p c r u t u r ~of rrystnllizatim anJ the meltirip. Imiiit of t h e p u n hgJrornrbon which wrre Icw thao BO' F., nnd for very OW r t t v * of c o h > g , t I w rcmllctype cryetal WFRS the stable type (Ta1.l.. I PSC s*4. s anti loo,. The value of the temperaiurr dilfcietire RI which n plni
