ccumulation of paraffin deposits in tubular goods and production equipment is an old and expensive problem in petroleum production. Many methods ha\-e been developed to treat the problem, but little investigation has been conducted on the crystal habit, structure, and the crystallization mechanism of waxes which, if better understood, might help in programs to prevent or inhibit deposition. A detailed study of the crystallization of petroleum waxes with x-ray and electron microscope techniques was undertaken to
A
-determine the structure, crvstallization mechanism, and orientation of paraffin wax crystals -determine the degree of modification of such crystals by certain chemical additives -stud) the nucleation and growth mechanisms of 11ax crvstals on a single cr) stal basis
PREVIOUS INVESTIGATIONS
M. CHlCHAKLl
86
F. W. JESSEN
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
The crystallization of paraffin waxes has been the subject of man); studies, yet little agreement is found on many aspects of crystallinit)-. The two crystal habits most frequently described are plates and needles. A much smaller type of crystal than the needle or plate cr)-stal has been observed by Clarke (9). Irregularly shaped forms of microscopically small petroleum wax crytals are commonly termed malcrystalline masses. Buchler and Graves (6) found that all purified petroleurn waxes Lvere straight-chain paraffin hydrocarbons and crystallized as plates. The); reported, however, an “impurity” termed “soft wax” which crystallized as needles and was capable of imparting its form to purified fractions. Padgett, Hefley, and Henriksen (2.5) reported the crystal form depended on the amount of oil present, as \vel1 as upon the handling; slow cooling tended to produce plates. Carpenter (8) observed needles when crystallization occurred from high-boiling solvents, and also from lowboiling solvents provided crystals were formed within 15’ to 20’ C. of the melting point of the wax. Rhodes, Mason, and Sutton (27) stated crystallization aliva)-s began by plate formation, and rapid cooling (iiiore than 0.1’ C. per minute) caused the plates to curl at the edges. Seedles, then, Tvere rolled plates. Tanaka, Kobayashi, and Ohno (37) supported the L-iew of Rhodes but considered viscosity to be the controlling factor, high viscosity fostering needles. Katz (7.9) deemed plate formation a necessary precursor to needle formation and considered needles to consist of plates characteristically formed in layers. He introduced the c2ncepts “concentration of change” and “temperature of change” ; both “changes” were from plates to needles, and the rel’erse change was impossible. Ferris and Coivles (75)presented evidence in support of the theory that petroleum \vases consist of mixtures of h)-drocarboiis belonging to various homolo5ous series. The members of each series crystallize similarly, as either plates, inalcr)-stals (nials), or needles. If the types are iiiixed, and if the solubility relations are such that more
than one type can crystallize simultaneously, either the needle or ma1cr)stal 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 and needles can then deposit upon them and thus take the form of plates. Those of highest melting points (cyclic hydrocarbons) form as needles; those of intermediate melting point (possibly branchedchain paraffins) form mal crystals. E. IV. Clarke (9) studied the crystal types of 23 pure hydrocarbons comprising paraffinic, naphthenic, and aromatic compounds crystallized at different rates both from the melt and from solutions of ethyl acetate and nitrobenzene over a wide range of temperatures. The two 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 melt; the temperature difference between the melting point of the pure hydrocarbon and the cloud point (or crystallizing temperature of the solution). Needles were produced from n-paraffins by the addition of small amounts of petroleum resins. Edwards (74) found refining properties of paraffin wax had no inherent tendency to crystallize in any particular form, though plates were most common. Other forms were produced by distortion or truncation. Birdwell (4) concluded from studies on the effects of various additives on crystal habit of petroleum waxes that when n-paraffin waxes crystallized from hydrocarbon solvents at relatively dilute concentrations and moderately slow rates of cooling, the crystal habit was generally thin plates and that compounds most likely to produce modifications appeared to be high molecular weight hydrocarbons which promoted a more isotropic growth through cocrystallization or lattice dislocation. Effective wax crystal modifiers were found to be present in crude oils. From studies on wax transitions, Barmby, Bostwick, and Huston ( 3 ) , using x-ray diffraction, differential thermal analysis, and dilatometric techniques showed that waxes melting below about 60' C. exhibit two solidsolid phase transitions, those melting between 60' and 75' C. only a single transition, and waxes melting above 75' C. no transitions. Holder and Winkler (78) stated that many needlelike wax crystals may be actually thin plates. They suggested also that the polymeric additive molecules by virtue of their great chain length and their structure, are able to incorporate themselves at the crystal growth step and stop the growth of the crystals.
The practical problems associated with the presence of waxes in tubular equipment have only occasionally been attacked systematically. The authors have renewed a systematic investigation of the structure, mechanism of crystallization, and orientation of
parafin wax crystals, and they present the first results. The results should aid in the elimination of fouling and other problems in petrochemical equipment
X-RAY STU D I ES The following materials were used : -A pure standard sample of hydrocarbon wax, ndocosane, C22H46, melting point 47' C., API Research Project 58B, obtained from T h e Carnegie Institute of Technology -A highly refined paraffin wax, Gulfwax (Gulf Oil Co.), melting point 132' to 133' F. vot.
59
NO. 5
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1967
a7
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0
2
4
I
I
I
6
a
IO
36 34 32 30
20.
Figure 1. N m d a h s a n e pocking mode in thc pure state
-Two highly refined par& waxes, Aristowax 143' to 150" F. and Aristowax 165' F., Petroleum Specialties Co. -A hard microcrystalline wax, White Be-Square Wax 180' to 185' F., Bareco Wax Co. Additives used were: -Stearyl methacrylate. A possible pour point depressant, obtained from Rohm and Haas Co. -Asphaltenes. Pentane-insoluble portion from crude oil fractionation, 700' F. residua; highmolecular weight hydrocarbons containing nitrogen, sulfur, and oxygen -Resins. Highly polar components of crude oil 700' F. residua. Elemental analysis shows the presence of oxygen, nitrogen, and sulfur -Aromatics. Compounds separated from high boiling fractions (650' to 700' F. and 700 F.) of several crude oils, and believed to be primarily single aromatic rings with alkyl side chains (73) The resin and aromatic fractions were obtained through adsorption chromatography separation of distillation fractions and residuals of five crude oils. The asphaltene fraction was obtained through precipitation with pentan-i.e., the pentane-insoluble fraction from the distillation residue. A detailed description of the method of separation is covered by Patton (26). T h e solvent used for all samples was n-heptane (Phillips Petroleum Co., Commercial Grade). A small amount of benzene was used for those samples which were not completely soluble in n-heptane.
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88
INDUSTRIAL AND ENGINEERING CHEMISTRY
(dqmrl
Figure 2. C h i n length of waxes used in thc invcstigntion
Experimental
An XRD-6 General Electric diffraction unit was used to obtain the diffraction patterns, utilizing copper K a radiation of wavelength equal to 1.54050 A. T h e unit operated under 35 kv.p. and 20 ma. The x-rays were filtered by nickel foil. The SPG spectrogoniometer scanned the sample at a speed of 2' per minute (2 S), with the SPG beam defining a slit of 1' with appropriate Soller slits, and a n SPG detector slit of 0.1 T h e accompanying chart speed was 2 e equal to 4' per inch. The diffraction patterns were obtained from 0' to 50' (2 e). The reflection peaks were checked for any shifting of positions due to improper placing of the slide or the marking pen on the charts. In a few charts, a very small shift was detected and corrected. The slides were placed vertically on their long side edge in the sample holder of the machine. The lirst peak of most of the patterns does not belong to the crystal structure of the wax, but was caused by the primary beam. Weighed amounts of each wax were added to a measured volume of n-heptane a t room temperature to yield 5, 10, and 20% solutions. The samples were heated to dissolve the wax. A weighed amount of additive then was added to a measured portion of the original waxheptane solution to give a concentration of 10% by weight of the wax present. Further dilution with the original solution gave samples with 5, 1, 0.1, and 0.01% concentration of additive. For x-ray analysis glass slides were used as sub-
'.
strata for the specimens. By use of a small dropper equal amounts of sample were spread on the glass surface to obtain a thin homogeneous layer. The slide was placed in a desiccator at room temperature and a few drops of heptane were placed in the bottom to control the evaporation rate. After all the heptane was evaporated, x-ray diffraction patterns were taken. I t was difficult to get a layer of the same thickness for some samples because as the crystals solidified some aggregation was noted. In preparing the specimens from solid wax, a small portion was put on the slide and the slide was placed in an oven maintained at a temperature a little higher than the melting point. As soon as the wax melted, a thin layer spread on the surface. The slide was removed and the wax solidified at room temperature.
TABLE I .
T H E DIMENSIONS OF THE U N I T CELL OF GULFWAX
ze
d
(‘/2d)s a
(L/2d)2
21.4
4.1486 3,7354 2.9857 2,4926
0.01450 0.01786 0.02793 0.0404 0,0450
0.01458 0.01776 0.02790 0,04056 0,04500
0.0507
0.05010 0,05832 0.07104 0.08062 0.09126 0.09570
23.8 29.9 36.0 38.1 40.6 43.7 48.7 52.3 55.2 56.8
2.3599 2.2202 2.0696 1.8682 1.7477 1.6625 1.6195
0.0583 0.0714 0.0820 0.0910 0.0954
Crystallographic Plane 110 020 120 200 210 130 220 040 140 300 310
Calculatrd from the diffraction pattern. Calculated from Smith’s 0.005 A . and bo = 7.478 i 0.005 A . (30)ialues of ad = 4.970
RESULTS AND DISCUSSION Crystallization from the Melt
The thickness of the diverse slide preparations was not subject to uniform control. For each case it was necessary to make the best extrapolation possible to reach the best estimate of the doolspacing. The quality of each measure is a function of the number of observed orders of diffraction. The diffraction pattern of normal docosane was found to follow Muller’s second series for long chain normal hydrocarbons (24). The specimen had side spacings of 4.6 and 3.81 A . By trial and error the dimensions of the unit cell and the arrangement of the crystal molecules could be determined from the side spacings. Muller and Lonsdale (23) reported a triclinic form for this crystal as it crystallized in a pure state with one molecule in the unit cell. This crystal form is of lower symmetry than the orthorhombic form. The cross section of the unit cell is not rectangular and the chain is tilted relative to the base. Figure 1 shows the packing arrangement of the C22 molecules projected parallel to their long axis onto the ab plane. The calculated side dimensions are reproduced from Segerrnan ( 2 9 ) . All the diffraction patterns of the commercial traxes used showed chain lengths similar to those of Sluller’s (24) first series of pure loiig chain normal hydrocarbons ivith molecular weight above 400. These hydrocarbons 3.6 A. rather have chain lengths of 1.25 -4.( n - 1) 1.8 A. as for the second series. than 1.25 A . (n - 1) Figure 2 gives the extrapolated first-order peaks for chain lengths of all the waxes used and is included only to substantiate the precision obtained and to support the coniparison with AIuller’s formula. The waxes showed a strong side spacing of 4.13 to 4.2 A. The structure, packing arrangements, and di-
+
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mensions of the unit cell of the Gulfwax were determined by trial and error using ( h , k , 0) data. The Q, and b, spacings were obtained from a pure sample of normal hydrocarbon in the same rnolecular weight range in the manner proposed by Smith (30). The results of these calculations are reported in Table I, along with the corresponding values calculated from Smith’s pure hydrocarbon. The correlation is excellent. Figure 3, a construction of the general form of the orthorhombic subcell, shows the perpendicular projection of the molecules to their long axes onto the ab.plane. The side dimensions of Gulfwax are in good agreement with Smith‘s values for pure n-Cnshydrocarbon. Crystallization from Solution
n-Docosane. \\‘hen stearyl methacrylate \cas uscd as an additive, the \vax gave diffraction patterns u i t h the same main and side spacings obtained on the pure material, lvhich means that the unit cell dimensions, crystal form, and packing arrangement of the molecules in the C22 crystal did not change. A s the per cent of stearyl methacrylate increased, the intensity of the peaks on the pattern declined, indicating a decrease in the number of crystals. Lt’ith resins, crystal formation decreased even more. Gulfwax. The crystallization mechanism of Gulfrvax from heptane solution Ljith and \\-ithout additives showed that iyith no additive, the extrapolated chain length \vas equal to 41 X., Jvhich is longer than the chain of the same wax crystallized from the melt. This mayresult from the longer molecules in the Gulfwax coming out of solution first, trapping between them the shorter molecules and thereby creating big gaps between some of the chains. VOL. 5 9
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With additives, some fractional crystallization was observed which gave rise to two crystal phases of different chain length. By increasing the percentage of additive, crystals with shorter chain length increased and became more abundant. The diffraction patterns of Gulfwax with different percentages of stearyl methacrylate (Figure 4) show fractional crystallization quite well. Ft’ith 5%;, additive, the two phases are of approximately equal amounts. Fractional crystallization may be explained by hypothesizing that the additive caused crystallization to slow down, allowing long molecules time to come out of solution, coalesce, and form independent crystals before shorter ones begin to crystallize. The increasing percentage of additive decreases the number of crystals with long molecules. The most effective additives were asphaltenes, resins, aromatics, and stearyl methacrylate in that order. The effect of asphaltenes (Sun-Hobbs crude oil 700+O F.) was very severe. FVith only O.Olx, the number of crystals formed diminished by more than two thirds. With low additive concentration, one crystal phase occurred with long molecules (30 carbon atoms) and tolerated shorter molecules, while with high additive concentration a second crystal phase with shorter inolecules (27 carbon atoms) was either equally distributed or perhaps was more abundant than the first. The side spacings gave the same Orthorhombic unit cell and packing arrangement as Gulfwax crystallized from the melt, indicating no change in crystal structure. Aristowax 143/150. The diffraction patterns showed this wax to be from a large cut containing several hydrocarbons. The resins lessened the number of crystals observed but no fractional crystallization was apparent. 7il’ith a low percentage of resins, a crystal with a 49 A. long molecule was found; but with 10% additive, the crystal formed contained molecules 43.8 A. in length. Aristowax 165 showed essentially the same behavior as Gulfwax. With the addition of small amounts of resins, fractional crystallization appeared and the crystals of the shorter molecule increased as the per cent of resins increased. Again, the formation of crystals, as a whole, decreased rapidly with increasing amount of additive. Be-Square Wax 180/’185. The (001) main spacings of this wax were very poor, and only one order, the (002) reflection, showed on the diffraction patterns. Failure to show the peaks was due to the greater length of the molecule, about 67 A., in this crystal. The effect of additives on this wax, as on the other waxes, was fewer crystals with some fractional crystallization. For example, with no additives, the intensity of (002) was 7, while the intensity of (002) with lOy0 resins was equal to 3. Wax Mixtures
The effect of additives on crystals with large differences in chain lengths was investigated by mixing various waxes in equal amounts by weight. CZZGulfwax. The C Z Gulfwax ~ mixture gave two 90
INDUSTRIAL A N D ENGINEERING CHEMISTRY
kinds of crystals, one consisting of molecules with a length of 41 A . , the other with molecules of chain length 32 A . The first crystal was identical with that obtained from the Gulfwax-heptane solution. The shorter chain length crystal was the impure form of CZZhydrocarbon crystal. This form appears if the wax contains more than about 2.2YG of the neighboring hydrocarbons. By mixing pure CZZwax with Gulfwax it became contaminated and changed form from triclinic to orthorhombic, with a long spacing of about 31 A . , about 3 A. longer than the projected length of the first crystal form. The addition of resins to this system gave results similar to those obtained previously. With 10% resins, very few long chain crystals were present, while the C Z Zcrystals remained the abundant phase. Czz-Aristowax 1431150. The diffraction patterns of this system showed three different types of crystals, all orthorhombic in form. Chain lengths of 30 A., the impure form of CZZ;45 A., the typical crystal from Aristowax 143/150; and one of 35-A. length were found present. This was the only system which gave the latter crystal phase. Possibly the 45-4. hydrocarbon chains of Aristowax were oriented at a large angle, giving the perpendicular projection of the chain length. Such an arrangement was observed in studies made by Segerman (29), where the chains were tilted so much they crossed each other. All three types of crystals persisted as additives were introduced into the system; but the intensities of the peaks of all three crystals declined with increasing per cent of additive. However, the rate of decrease was different; the two crystals with the long chains diminished at a much faster rate than the C Z Z crystals. CzZ-Aristowax 165. This wax mixture also produced three kinds of crystals. One had a chain length of 27.8 A, and the same side spacing as the pure wax, indicative of a pure crystal of triclinic structure. The second had a 30-A. chain length, the orthorhombic form of the C Z Zwax, while the third phase had a very long molecule (53.5 A.), the orthorhombic crystal of Aristowax 165. The resin additive acted as before. With a high percentage of additive, the Aristowax crystals decreased rapidly, almost disappearing with 10% additive. &-Be-Square Wax i80/185. This wax system reacted in very much the same way as the CzTAristowax 165 mixture. I t showed three kinds of crystals: 70 A. for Be-Square Wax, 30 A. for the impure C Z wax, ~ and 27.8 A. for the pure C??material. Again, the resin additive gave the same results with this combination of waxes. Gulfwax-Aristowax 143/150. il’ith no additive this Combination showed onIy one crystal phase with a chain length of 47 A. of the Aristowax. KO reflection peaks were shown for the Gulfwax crystals indicating the shorter molecules may- have been incorporated between the longer molecules of the -4ristowax. With a low percentage of additive, the resulting crystals had a chain length of 45 to 46 A., which still showed Aristowax crystals to be predominant. The small reduction in
r
-
i
I
l
l
l
l
l
I
1
I
I
-i
7.471 A
-
74.985 A
PIIUMDER OF (uIo(( AIW
Figure 3. The packing mode of Culfwu
Figure 5. Tha range of n-hydrocarbow in the waxes used
I 4443632
~
1612 8 I
c 20
Figure 4.
The effect of stcmyl methaqlofe on
Culfwax q s t d s 01. 5 9
NO. 5
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91
I
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1
I
I
I 11 I I
I
I
I
I
IO
I
i
I
I 1
I,
I
/ I
I A
I
0.01
0.05
’
I
I
0
I
J
2
3
4
1
0.05 0
I
I
L
I
0.4
0.8
1.2
1.6
ll~l4Wa
14~~~1JII
FigMa 7. The efecf of d&mt ciys:als
additives on ths m’mtafionof Gulfwax
chain length was the result of packing the molecules closer to each other by eliminating some of the gaps between them. This closer packing is evidence of slower crystallization. With 10% additive only crystals with chain lengths of 39 A,, typical of Gulfwax, were obtained. The Aristowax crystals did not show in the diffraction pattern. Gulfwax-histowax 165. Two types of crystals, one with a chain length of 53.7 A. and the other with a chain length of 39.7 A., were present. Both crystals were distinctly those from Aristowax and Gulfwax. The introduction of resin additive to this system, as before, decreased the amount of both crystals. The side spacing showed all crystals to be orthorhombic. Gulfwax-BhSquare Wax 180/185. This mixture acted the same way as the Gulfwax-Aristowax 165 combination. The long chain molecules of the BeSquare Wax 180/185 did not show on the diffraction patterns, because their reflections occurred at very small values of 2 e. The range of carbon atoms present in each wax was ascertained by using Muller’s constant slope line, and the diffraction patterns of the waxes used are indicated in Figure 5.
92
I
2.0
(1) Gulfwax 132/133
cts to cso
(2) Aristowax 143/150
CSr to C86
(3) Aristowax 165
CSS
(4) Be-Square Wax 180/185
Cso to Csr
to
c 4 0
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
Figure 8. The efec: of resin on fhs orientattm of molecular of &@nf wax ctys:afs Cryslol Oilemlation
The crystals produced showed an abundance of platy structure. By introducing additives, a change i‘n orientation was observed. With increasing amounts of additives the side spacing intensities increased, indicative of a high percentage of the hydrocarbon chains lying horizontally. This change in orientation of the molecules resulted in turning the crystals vertically on their edges, giving them a needlelie appearance. The diffraction patterns (Figure 6) show change of orientation Well. An attempt was made to determine the per cent of the crystals orientated. The ratio of the intensity of the (110) plane to the intensity of the (002) plane was calculated for each wax additive system and these values plotted against the per cent additive on log paper. Figures 7,8,9,and 10 show the rate of change of orientation with increasing amount of additive. The waxes do not show constant slopes or similar patterns of change of orientation; it appears, therefore, that the change of orientation is not a direct function of the concentration of additive.
ELECTRON MICROSCOPE STUDIES Although the resolving power for structure studies by x-ray diffraction is greatly superior to that of the electron microscope, there are a number of problems in the study of order crystalline structures that best can be answered by microscopy rather than by diffraction. In diffrac-
30 20 IO
ri
!i 1.0
i
IO
0. I
E
0.05
a
2
1.0
blAlm Figurc 9. Tha cfcct of rcsin on tlu &ation
of
WM
ntixturc nyrrOrr
tion studies, the results obtained are a statistical mean of the packing arrangement prevailing throughout the whole volume of the crystal and represent the mean values 0f.pome millions of molecules. It is therefore not possible by this method to get information on the precik. arrangement of molecules at faces and edges of the crystal, a t which an important role is p l a y 4 in both growth and strength.
0.1
0.05 0
0.4
0.8
1.2
1.6
2.0
WLl F’ti
70. Tha cfect of rcsin on !ha aimtdim of wax mixture
VStdS
Materials
I n addition to the waxes examined by x-ray diffraction, an ‘extra-hard microcrystalline wax, White BeSquare Wax, wag also used. This high melting point wax (19Oo/195O F.) is made by Bareco Wax Co. Resin and asphaltene fractions obtained from the 700+ F. residue of East Texas &de oil, a commercial asphaliic material, “Atlanite 5,” of known composition, made by Atlantic Relining Co., and all additives described previously were employed. O d y one solvent was used, Commercial Grade nheptane obtained from Phillips Petroleum Co.
’
Exp.rimontd
A JEM-150 Electran Microscope, produced by Japan Electron Optics Laboratory Co. Ltd., was used. This instrument has an operating range from 80 to 150 kv., but only the 80 and 100 kv. were used because the higher power generated such a n intense beam that the wax melted and broke through the replica. VOL 59
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After many unsuccessful trials using concentrations of 1% or more of wax, samples with 0.2% concentration produced the best crystals. Hence, most observations were made from this dilution of wax in heptane. One gram of wax was dissolved in 500 ml. of nheptane solution to form the reference sample. A weighed amount of the different additives was then mixed with a measured portion from the reference sample to form a concentration of 10% by weight of the wax-Le., 0.02y0 of additive in the heptane-wax-additive system. All the samples were then labeled for future use. Preparation of Electron Microscopy Specimens
Methods of preparation of crystal samples for electron microscope studies, replica techniques, and shadowcasting are discussed by Dawson ( I I ) , Bradley (5) Williams and Wyckoff (33), and others. In this investigation, three techniques were tried. The first method involved the use of collodion film. A few drops of 10-to-1 solution of methyl ethyl ketone (MEK) and collodion were placed on a clean glass slide and when evaporated formed a thin film. Specimen mounts (copper grids) then were placed on the slide, and another film of collodion formed on the top of the grids. The wax crystals then were grown on the collodion film by evaporation of a small drop of the dilute wax-heptane solution. These specimens did not give a good image when examined owing to the low surface contrast of the crystals examined. A more serious problem was the exposure of the crystals themselves to the electron beam, which instantly melted most of the crystals of low melting point (less than 150' F.). This method was deemed unsuitable. T o avoid burning of the collodion film by the electron beam and to give the specimen a strong base, a carbon film was used. This film was prepared by shadowcasting the copper grids (covered by collodion film) mni. Hg, with carbon at a pressure of the order of then placing a drop of the sample on the carbon surface and letting it evaporate. This replica also did not give satisfactory results, again because of the low contrast of the crystal surface, so this method also was discontinued. Finally, the grids from the first technique mentioned were placed in the shadowcasting machine, and after the pressure reached 10-5 mm. Hg the specimens were shadowcast first with platinum and then with carbon. Platinum was used to give surface contrast, and carbon served as a supporting film. The collodion film was dissolved by placing the grids in a MEK vapor bath for 1 5 to 20 minutes, followed by immersion in MEK solution for 10 minutes. The direct examination of these shadowcast specimens in the electron microscope gave excellent results. Results and Discussion
Inasmuch as the x-ray studies had shown the basic crystal form to be the same for all the waxes studied, it was felt that nucleation and crystal growth phenomena 94
INDUSTRIAL A N D ENGINEERING CHEMISTRY
could be evaluated from investigations using the higher melting point (130°/1350 F.) paraffin wax. Nucleation. Crystal growth is controlled by nucleation. The nucleus acts as a growth center for the attachment of other molecules. Very little is known about the nature of the primary nucleus in crystal formation from solution. Walton (32) gives a n explanation which states that prior to the nucleation there is continuous formation and dissolution of ionic or molecular clusters in equilibrium with other clusters. If the concentration of solute ions or molecules is large enough the clusters become sufficiently large to become consolidated into small crystallites, whereupon the crystal growth is ensured. By adding one molecule to the largest cluster (the critical cluster) nucleation occurs. A current crystallization theory for polymers, referred to by Geil (77) states that the nucleus should be thicker than the remainder of the crystal. H e showed a small bump in the center of a single crystal. Figure 11 illustrates single crystal plates of Be-Square Wax crystallized from heptane solution. The appearance of a little bump in the center of each plate strongly supports the theory referred to by Geil. The nucleus may serve for other plates on each side of the primary plate. From a n examination of the photographs in Figures 12, 13, 14, and 15, clear evidence is presented that this position may well be the nucleation point i n t h e formation of the crystal. If one may rely on measurements made from the photographs, there are a large number of molecules present in the initial cluster. Plate Thickness. The x-ray diffraction studies established that the paraffin and microcrystalline waxes used all had orthorhombic crystal structure, composed of successive molecular layers in which the molecules ivere arranged along their long axis (c-axis) normal to the ab plane. From the measurements of the shadows of the crystal plates, and with a shadowing angle of 9", the hright of the individual layers could be calculated. The thickness of about 25 crystal plates was calculated, and values between 70 and 120 A. were obtained. The average length from these measurements agreed with the length of one molecule obtained from the x-ray diffraction pattern, indicating most of the crystal plates were monomolecular. They consisted of vertically oriented molecules, their ends packed in a single crystallographic plane, the (001) plane, the plates terminating at the edge of the (110) plane. By knowing the plate thickness and observing the face of these single crystal plates, we concluded that no folding of the molecules in the crystal plane occurred. This folding of molecules in the lamella was reported by IVunderlich (35) and Lindenmeyer (20) in the crys-
M . Chickakli is on the staJ of Bellaire Research Laboratories, Texaco, Inc., Bellaire, T e x . F. W. Jessen is Professor of Pelroleurn Engineering, T h e University of Texas, Austin. T h e authors acknowledge the help and suggestions of Dr. W. F. Bradley andfinancial supfiort from the ACS Petroleum Research Fund. AUTHORS
Figure 77. Electron micrograph of a single crystal showing the nucleus i n the center. T h e crystal &)asgrown from 0.7 gram of Be-Square w a x (790'/795' F.) i n 50 m l . of n-heptane Magnification = 24,000
Figure 14. Single crystals showing fractional crystallization and nucleation sites. T h e crystals were grown from 0.7 gram of BeSquare w a x (190'/795' F.) in 50 m l . of n-heptane with 0.07 gram of resin Magniycation = 27,000
Figure 72. Electron micrograph showing three diamond-shaped crystals w i t h irregular edges and several nucleation stites. T h e crystals grown f r o m 0.7 gram of Be-Square wax (790'/795' F.) i n 50 m i . of n-heptane LCith 0 . 0 7 gram of aromatic fraction MagniJication = 20,000
Figure 75. Single lajer crystal with ta'o nucleation sites in the center. T h e crystal zeNasgrown from 0.7 gram of Be-Square uiax ( 7 9 0 ° / 7 9 5 0 F.) in 50 m l . of n-heptane w i t h 0.07 gram of asphaltene MagniJication = 47,000
Figure 73. Electron micrograph of three single crystals showing dtyerent kinds of crystal grorc'th. T h e dtyerence i n thickness of the crystal i n the upper left is evidence of fractional crystallization. T h e crystal w a s grown f r o m 0.7 gram of Be-Square Le'ax (790'/795' F.) i n 50 ml. of n-heptane with 0.07 gram of resin
Figure 7 6 . A single crystal showing a monomolecular spiral growth f r o m the screw dislocation in the center of the crystal. T h e crystal w a s grown f r o m 0.7 gram of Be-Square w a x (79Oo/7Q5' F.) in ,50 m l . of n-heptane w i t h 0.07 gram of asphaltene Magnijication = 47,000
MagniS;cation = 25,000 VOL. 5 9
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tallization of polymers. There was also no evidence of migration of molecules from the surface to the growing edges as suggested by Cabrera and Burton (7). The crystal plate was formed by adding the condensing molecules to the edges of the nucleus. The number of the condensed molecules was more concentrated at the edges of the plates. This high concentration may be due to the fact that the molecules at the edgeswere attracted toward each other because of the discontinuity between the crystal-noncrystal phase. Practically all of the crystals, whether of one or more layers, were diamond shaped. From a few well formed crystals, the acute angle between the (110) edges was measured to be between 6 6 O and 71". This is in good agreement with the value of 67" calculated by Muller (22) for the unit cell dimensions of pure normal hydrocarbons, and also with the value obtained by Birdwell (4) for waxes in the same melting point range of BeSquare Wax. Such agreement further confirms the crystal plates of Be-Square Wax to be orthorhombic. Screw Dislocation. The theory of growth of single crystals from solution states that after the formation of the nucleus, a dislocation in the lattice of the growing crystal greatly helps the growth. Figures 16 and 17 show a spiral structure running from the apex of the crystal to its base. The apex of the crystal is the growth center where the screw disloc,ation occurs. Figure 16 shows a simple spiral growth with one screw dislocation, while Figure 17 shows two screw dislocations with the same sign, resulting in the growth of two spirals. An explanation of the formation of screw dislocation given by Dawson (70) indicates that the stacking fault responsible for the lattice dislocation occurs in the original crystal when adjacent molecules fail to align themselves in exact juxtaposition but condense in a position such that the ends of the molecules project above or below the plane of the normal orthorhombic molecular layer. In a few instances the molecules adopted a spiral structure having hexagonal symmetry. Dawson (72) reported on studies of the single crystal of n-pentacontanol1 that at higher temperatures a slight change occurred in the intermolecular distances of the lattice, and that if the molecules adopted a coiled structure having hexagonal symmetry, or were free to rotate, a hexagonal crystalline form occurred. Twist Boundaries. Figures 18 and 19 illustrate spiral crystals with an interlayered twist between the successive layers. This twist between the layers of hydrocarbon crystals is referred to as polytypism. Amelinckx (7), Wilman ( 3 4 , and Frank (76) reported polytypism in long chain acids, alcohols, and in a few cases in n-paraffins. This twist between layers could arise through mechanical shear or through layer displacement as in the case illustrated. As crystals grow larger, the likelihood of twist boundary formation, either through shear or through noncrystallographic coalescence of layers, will increase with increasing size. Spiral Cancellation. An examination of Figure 20 shows a layer structure, not a spiral growth step. This 96
INDUSTRIAL AND ENGINEERING CHEMISTRY
suggests that dislocation did not occur in the nucleus and growth took place by the successive deposition of layers. This is unlikely because of the high order and degree of symmetry of each single layer with respect to the other. If growth did happen by successive deposition of layers, nucleation would have been at random for each plate and the resulting crystal would exhibit low order of symmetrv. Instead, a more likely explanation is the movement of two screw dislocations in opposite directions giving two spirals with destructive interference. As they grew they canceled each other, resulting in a successive layer structure. Anderson and Dawson (2) and Mott (27) suggested that the two dislocations of opposite sense in the paraffin crystals were canceled by moving together through mutual attraction. Effect of Additives. The size of the crystal plates obtained from the wax-solvent solution containing a small per cent of additive was a great deal smaller than
Figure 77. Electron micrograph of the center portion of a x a x crystal showing tzeo screLe8 dislocations with tzeo spirals originating from them. T h e taso growth spirals are related to a 120' rotation betaeen the layers, rcihich i s an example of polytypism. T h e crystal &as grown from 0.7 gram of &-Square wax (790°/195' F.) i n 50 mi. of nheptane Magn$cacation = 53,000
the size of the crystals obtained from the original wax-heptane sample. Figure 1 2 shows the crystal plates obtained by adding 0.025y6 of aromatics, and Figures 13, 14, and 21 show crystal plates obtained by adding 0.02557, of resins to the wax-heptane solutions. The width of the (001) plane for these single crystals varied from 2 to 4 p , the most frequently occurring width being close to 2 p . The width of the (001) plane in the original sample with no additive varied between 5 and 3 p , the most frequently occurring width being close to 7 p . Another effect of the additives was the reduction of the number of layers in the spiral crystals from an average of 15 layers to an average of 5 layers. This reduction of size and number of steps in the crystal indicated the number of molecules forming the crystals from the samples containing additives was 4 or 5 times less than that of the original sample. The effectiveness of the additives may be attributed to dispersing action in preventing molecules from coalescing or condensing
Figure 78. Poljtypic crystal consisting of tzoo spirals related by 720' rotation zcith their lajers interleaved with each other. T h e crystal was groien from 0.7 gram of Be-Square ze'ax (790'/795' F.) in 50 ml. of n-heptane Magnijication = 38,000
Figure 27. Several single crystals showing the relative size and the irregularity of the crystal edges to the presence of the additive. T h e crystals were grown from 0.7 gram of Be-Square w a x in 50 ml. of n-heptane with 0.07 gram nf resin MagniJcation = 7500
Figure 79. A large single crystal nf Be-Square Zc'ax shoceing a twist between the indic'idual layers of the crystal. T h e crystal was grown from 0.7 gram of Be-Square zcax (790'/795' F.) in 50 rnl. of nheptane Magn$cation = 30,000
Figure 22. Electron micrograph shoming dendritic cry c'tals of BeSquare wax. T h e crystals were grown from 0.7 gram of Be-Square w a x in 7 0 0 ml. of n-heptane with 0.07 gram nj' asphaltene ikfagnification = 60,000
Figure 20. Single-wax crystal showing a growth p y a m i d consisting of closed diamond-shaped plates. Careful examination of the apex of t h crystal shoas tzco screw dislocations nf opposite sign forming the crystal, and canceling themselves as the crystal grew. The crystal icas grown from 0.1 gram of Be-Square w a x ( 1 9 0 ° / 7 9 5 0 F.) in 50 m l . of n-heptane Magnijcation = 40,000
Figure 23. Electron micrograph showing several irregular plates of Be-Square Leiax crystals and dendritic crystals beside them. T h e crystals were grown from 0.7 gram of Be-Square w a x (79O0/795' F.) i n 50 ml. of n-heptane with 0.01 gram of Atlanite-5 .Magnification = 60,000
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on the crystal faces, or to adsorption on the surface of the crystal, thereby inhibiting the normal growth pattern. The additives also reduced the number of crystals. Examination of a large number of samples containing additives showed the number of plate crystals per unit area to be far less than in samples containing no additive. This decrease occurred because the crystals formed in the presence of an additive did not solidify completely, probably a result of an adsorbed resin or aromatic or inclusion of such material in the crystal structure. Sometimes the temperature developed during shadowcasting was sufficient to melt the semisolid crystals making them appear on the electron scope as “globs” of wax. Figure 13 shows crystal plates with two different regions on their surfaces. I n the center region a fine texture and relatively thicker section is developed while the second region, which formed the outer boundaries of the crystal, was granular. By observing the shadows of each region, it was concluded that the molecules forming the inside of the crystal plates were longer than those forming the outside region. These two distinct regions are evidence of fractional crystallization caused by introduction of the resin additive to the system. The crystals produced from wax-additives solutions show low degree of symmetry, irregular plates with rounded corners, and truncated front. This may be explained by the accumulation of the additives around the faces of the crystals which limit the growth of the crystal and lower the temperature of crystallization. A large number of crystal plates were obtained from samples containing an additive that did not show a common center of growth. While most of the samples containing additives produced some dendritic crystals, the degree of dendritic growth varied substantially. The resins and aromatics, for example, produced very few dendritic crystals and mostly small plates, while the introduction of the asphaltene fraction from East Texas crude oil caused almost complete dendritic growth (Figure 22). The Atlanite 5 additive, consisting of a mixture of about equal proportions of asphaltenes and resins showed approximately equal amounts of dendritic growth crystals and platy crystals (Figure 23). Dendritic growth can be explained in this manner: As any crystal face grows, “poisons” present in the solution will accumulate around the faces. As a result, the crystal will tend to grow as rapidly as possible where the impurity concentration is a t a minimum. Accordingly, a more complicated, treelike dendrite will form (28). Geil (77) refers to the major determining factor leading to spherulite and dendritic growth as the presence of impurities in the system. The crystal habit develops under given circumstances, depending in large part upon the magnitude of the ratio D / G where D is the diffusion coefficient of impurities (additives) in the sample, and G is the rate of advance of a growing crystal phase. When the ratio is relatively large, crystallization is severely restricted by diffusion processes, and dendritic growth takes place. 98
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CONCLUSIONS X-ray diffraction studies on paraffin waxes of wide melting point range showed these to exhibit orthorhombic crystal form, with main and side spacings similar to those of long chain n-hydrocarbons. Modification of crystal by inhibitors was occasioned through in size and number of crystals, ( b ) fractional crystallization, and (c) change of crystal orientation. ( 0 )decrease
Dendritic growth appears to be the principal form of crystal development when asphaltenes are present. Electron micrographs of single crystals show crystal growth to be by spiral growth-steps ending on screw dislocations. Nucleation centers, appearing as small bumps on electron micrographs of single plate crystals, represent the nuclei of the crystals and are taken to be the growth centers. O n e or more such nuclei may b e present.
The predominant, if not practically exclusive, form of the nparaffinsis a platy, diamond-shaped crystal.
REFERENCES i\melinckx, S. “Growth Features o n Crystals of Long-chain Compounds,” (‘!4cta Cryst. 9, 21; (1956b). (2) Anderson, N. G., Dawson, I. M., “ T h e Study of C r rstal Growth with the Electron hlicroscope,” Part 111, Proc. Roy.Soc. A218, 255 4953). (3) Barmby, D . S., Bostwick, L. G., Huston, J. A,, Jr., “Some Studies o n the Physics of Paraffin Wax a n d U a x / P o l y m r r Systems,” Sec. VI, Paper 21, Sun Oil Co., 1956. (4) Birdwell, B. F., Effects of Various Additives o n Crystal H a b i t a n d O t h e r Properties of Petroleum Wax Solutions, Ph.D. dissertation, T h e University of Texas, .4ustin, September 1963. ( 5 ) Bradley, D . E., “A High-Resolution Evaporated-Carbon Replica Technique for the Electron Microscope,” J . Inst. Mctoir 83,35 (1954). (6) Buchler, C. C., Graves, G. D., “ T h e Petroleum Waxes,” IND. END.CHEM.19, 718 (1927). (7) Cabrera, N., Burton, M’. K., “Crystal Growth a n d Surface Structure,” Discursions Foroday Sot. (5), 40 (1949). ( 8 ) Carpenter, J. A., “ T h e Physical a n d Chemical Properties of Paraffin Wax, Particularly in the Solid State,” Inst. Pet. Tech. 12, 288 (1926). ( 9 ) Clark;, E. W., “Crystal Types of Pure Hydrocarbons in the Paraffin W a x Range, IXD. E N G .CHEM.43,2526 (1951). (10) Dawson, I. M. “ T h e Study of Crystal Growth with the Electron Microscope,” Parr 11, Prof. Roy.’Soc. A214, 72 (1952). (11) Dawson, I. M., “Techniques for the Electron Microscopy of Crystals,” Bnt. J . A,@[. Phis. 4, 177 (1953). (12) Dawson, I. M., \ F t s o n , D. H., “ T h e Study of Crystal Growth with the Electron hlicroscope, Part V,Pror. Roy. Sac. A239, 349 (1957). (13) Cby, L. T., “Properties of High Boiling Petroleum Products,” And. Chcm. 25, 1057 (1953). (14) Edwards, R. T., “Crystal Habit of Paraffin IVax,” IND. €NO. CHEM.49, 750 (1957). (15) Ferris, S. W.,Cowles, H . C., “Crystal Behavior of Paraffin Wax,” Ibid., 37, 1054 (1745). (16) Frank, F. C., “ T h e Growth of C a r b o r u n d u m : Dislocations and Polytypism,” Phi/..Mng. 42, 1014 (1951). (17) Geil, P . H., “Polymer Sforphology,” Chem. En
