Flow Improvement of Waxy Oils by Modulating Long-Chain Paraffin

Nov 30, 2010 - UniVersity, Princeton, New Jersey 08544, United States ... and transportation of waxy distillate fuels and crude oils is a problem for ...
0 downloads 0 Views 346KB Size
Download PDF
316

Ind. Eng. Chem. Res. 2011, 50, 316–321

Flow Improvement of Waxy Oils by Modulating Long-Chain Paraffin Crystallization with Comb Polymers: An Observation by X-ray Diffraction Li Li,† Xuhong Guo,*,† Douglas H. Adamson,‡ Brian A. Pethica,§ John S. Huang,§ and Robert K. Prud’homme*,§ State Key Laboratory of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai 200237, China, Department of Chemistry, UniVersity of Connecticut, Storrs, Connecticut 06269-3060, United States, and Department of Chemical and Biological Engineering, Princeton UniVersity, Princeton, New Jersey 08544, United States

To understand the flow improvement of waxy oils by comb-type polymer additives, the crystallization of long-chain n-paraffins from solutions of decane was studied by X-ray diffraction. The effects of poly(ethylenebutene) (PEB) and poly(maleic anhydride amide-co-R-olefin) (MAC) polymers on the crystallization of C28, C32, and C36 n-paraffins and their mixtures were studied. In the absence of the polymer additives, paraffin platelet crystals give well-resolved low-angle spectra. PEB and MAC reduce the low-angle scattering from the layered structure, which is expected since the polymers cocrystallize with the paraffin and the noncrystallizable portion of the chain provides a steric barrier against platelet stacking. This is consistent with observations by optical microscopy and differential scanning calorimetry and can account for the significant reduction of yield stresses of model waxy oils upon addition of these polymers. The assembly of PEB and MAC with paraffins during crystallization results in improved flow of waxy oils on cooling. X-ray diffraction also reveals that the cooling rate has significant impact on the crystal structure of long-chain n-paraffins. Introduction Wax deposition in pipelines due to cooling during production and transportation of waxy distillate fuels and crude oils is a problem for the petroleum industry, resulting in reduced throughput or even blockage.1-3 Polymer additives with crystalline/amorphous diblock structure such as ethylene-vinyl acetate copolymer4 or with comb structures such as poly(maleic anhydride ester-co-R-olefin)5 are commercially used as wax modifiers to alleviate wax deposition. But the mechanism of flow improvement of waxy oils by polymer additives is still incompletely understood, and thus this topic is under intensive research.6-13 Deposition and gelation arise from the crystallization of longchain n-paraffins from waxy oils. Therefore, it is important to understand the modification of paraffin crystallization in the presence of polymer additives. In our previous work we explored the mechanism of flow improvement of model waxy oils upon addition of polymeric additives, such as ethylene-vinyl acetate copolymer,9 polyethylene-poly(ethylenepropylene),10 and poly(ethylene-butene) (PEB) (Figure 1a)11,12 by using rheology, optical microscopy, and differential scanning calorimetry (DSC).14 But none of these methods give direct insight into crystal structure of the long-chain n-paraffins with polymer additives. Since X-ray diffraction is a powerful method to analyze crystal structures including crystal-phase identification, lattice parameters, and dimensions of the unit cell,15-20 we use this method to further explore the modification mechanism of polymer additives on long-chain n-paraffin crystallization. The crystal structures of single paraffins and paraffin mixtures from the melt and from solution in decane with and without addition * Authors to whom correspondence should be addressed. E-mail: [email protected] (X.G.); [email protected] (R.K.P.). † East China University of Science and Technology. ‡ University of Connecticut. § Princeton University.

of PEB or poly(maleic anhydride amide-co-R-olefin) (MAC) (Figure 1b) are examined by X-ray diffraction. The motivation is to reveal the role and behavior of these comb polymer additives in the crystal structure of long-chain n-paraffins. Experimental Section Materials. Decane (anhydrous, 99+%), octacosane (C28, >99%, F.W. 394.77, mp 59 °C), dotriacontane (C32, >97%, F.W. 450.88, mp 69 °C), and hexatriacontane (C36, >98%, F.W. 506.99, mp 74-76 °C) were purchased from Aldrich and were used as obtained. Analysis by gas chromatography (GC) for C36 gave a purity of 97.6% C36, 2.1% C34, and 0.2% C38. For the C32 sample, it gave 98.8% C32, 0.4% C31 and C33, and 0.8% other paraffin isomers. The C28 sample gave a purity of 99.6% C28, with 0.38% being a combined total of C26 and C30. No fatty acids or alkanols could be observed in the samples by IR analysis.

Figure 1. Chemical structure of PEB and poly(maleic anhydride amideco-R-olefin).

10.1021/ie101575w  2011 American Chemical Society Published on Web 11/30/2010

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

317

Figure 2. X-ray diffractograms of crystal samples of C36, C32, and C28 with and without PEBs grown from decane solutions at a cooling rate of 1 °C/min. The curves from top to bottom denote crystals with 0.1% PEB 10, with 0.1% PEB7.5, and without PEB. Optical micrographs of each sample are shown as insets.

Model waxy oil samples were prepared by dissolving C28, C32, C36, or their binary mixtures in decane. Solid crystal samples were prepared by heating the model wax oils to 70 °C, holding at this temperature for 5 min and cooling from 70 to 0 °C at a cooling rate of 1, 5, or 10 °C/min. The precipitated crystals were separated by filtration with vacuum in an ice bath and drying under vacuum at room temperature for 3 days. Crystal samples were sealed and stored at ca. -4 °C before use. The synthesis of the microcrystalline random copolymers PEBs (Figure 1a) was described elsewhere.11 The final polymers had molecular weights of ca. 7000 g/mol and a distribution of molecular weight of ca. 1.03. Two PEBs (PEB7.5 and PEB10) were used, where the number denotes the ethyl side branches per 100 backbone carbons as determined by 1H NMR. Comb polymers MACs (Figure 1b) were prepared by amidation of poly(maleic anhydride-co-R-octadecene) with octadecylamine, tetradecylamine, or dodecylamine, defined as MAC18-18, MAC18-14, and MAC18-12 where the two numbers represent the carbon number of R-olefin and amine, respectively. Poly(maleic anhydride-co-R-octadecene) was copolymerized from 1-octadecene and maleic anhydride in pxylene by using benzoyl peroxide as initiator. The molecular weights and molecular weight distributions determined by gel permeation chromatography were 12300 g/mol and 1.4 for MAC18-18, 11500 g/mol and 1.4 for MAC18-14, and 11200 g/mol and 1.3 for MAC18-12.21 X-ray Scattering. X-ray diffraction patterns were collected at room temperature from samples of long-chain n-paraffin crystals from decane solution in the reflection mode using an evacuated Statton camera manufactured by Philips-Norelco. The crystals were held on glass slides with double-layer tape (Winmore, Frank W. Winne & Son, Inc., Pennsylvania). The slides were arranged with their planes orthogonal to the plane of the X-ray source, sample, and detector. Detector and sample were synchronously rotated. The scattering from the glass slides and tape was subtracted from the gross scattering. X-rays with a source wavelength of 0.154 nm were produced using a sealed tube generator with a Cu target and a Huber graphite monochromator.

The scattering patterns were recorded over the range of angles corresponding to 2θ from 1.5° to 50°. Subtraction of the diffraction of background (tape and slide) from raw spectra gave the X-ray diffractograms of paraffin crystals. Optical Microscopy. Wax crystal morphologies were observed using a Nikon TE200 inverted microscope with phase and DIC optics (Micron Optics, Cedar Knoll, NJ). Images were captured using a Kodak ES 310 CCD camera connected to a PC via a PIXCID imaging board (EPIX, Buffalo Grove, IL). A small quantity of waxy oil was transferred from storage at -4 °C directly to a glass slide inside a copper stage with a central window for observation. The temperature of the copper stage was controlled at 0 °C by a circulating bath. Images were taken at five sites on the slides. Results and Discussion Paraffin Crystallization with PEB. For Single-Component Paraffin. The crystal structures of C28, C32, and C36 are considered by most authors to be monoclinic as determined by X-ray powder diffraction.15,17,22,23 In their experiments, the paraffin crystals were generally prepared from melt, and the effect of cooling rates was not considered. At very slow cooling rates, the paraffin crystals should be orthorhombic.3 The observed monoclinic phase is probably metastable. In our experiments, the crystals were prepared by relatively rapid cooling and for this discussion we will describe them as monoclinic. In our previous studies,11,12 we found that 0.05 wt % PEB7.5 or PEB10 could reduce the yield stress of model waxy oils (4% C36, C32, or C28 solutions in decane) by more than 3 orders of magnitude, and the size of paraffin crystals was reduced and their shapes changed significantly. To find the correlation between the reduction of the yield stress and the change in crystal structure, X-ray diffraction was employed in this work to observe the paraffin crystal structures before and after the addition of PEB (Figure 2). Figure 2 shows the comparison of the scattering of C36, C32, and C28 crystal samples in the absence and presence of PEB7.5

318

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

Figure 3. X-ray diffractograms of polymer samples.

and PEB10. Crystals from solutions with or without addition of PEBs were separated from their decane solutions after cooling from 70 to 0 °C at a cooling rate of 1 °C/min. Samples were dusted on to the support glass slide with double-layer tape and studied at laboratory temperature. All the crystals of C36, C32, and C28 without PEBs from solution show good resolution of the low-angle diffraction, corresponding to the lamellar spacing of the extended long chains (Figure 2). The layer spacing decreases from 4.16 nm for C36 to 3.65 nm for C32. Interestingly, C28 shows doublets in the low-angle spacings (3.52 and 3.36 nm). The high-angle spacing for monoclinic paraffins in this range changes little between the several monoclinic forms, but the low-angle spacing does change significantly. We may therefore conclude that the pattern for C28 in Figure 2c shows the crystals to be polycrystalline monoclinics. But the high-angle scatterings around 22 and 24° (2θ) from the monoclinic unit cell are missing, as expected from the literature.19,20 Upon addition of PEBs, the high-angle diffraction is prominent and the low angles show diminished diffraction peaks, especially in the case of C32 (Figure 2).

The crystal morphologies of these long-chain n-paraffins are platelike (Figure 2). The near perfect platelike crystals tend accordingly to be in a preferred orientation parallel to the support. Since the support and detector are rotated in one plane with the stationary radiation source, well-oriented platelet samples should show the low-angle scattering well, but will not show contributions from off-plane high-angle scattering spots. The high-angle scattering will show up increasingly as the platelet sizes decrease. This coincides with our observation by optical microscopy: PEBs reduce the paraffin crystal size and modify the crystal shape significantly (Figure 2).11,12 The appearance of the 1D high-angle scatterings around 22 and 24° (2θ) from the monoclinic unit cell in the presence of PEB shown in Figure 2 is striking. It might be argued that these peaks come from PEBs, but as shown in Figure 3, both PEB7.5 and PEB10 are almost amorphous, although PEB7.5 seems the more crystalline of the two with two tiny peaks around 20° (2θ). The PEB7.5, with fewer steric defects and longer crystalline runs, produces higher levels of crystallinity (7.5 ethyl side branches per 100 backbone carbon atoms) compared to PEB10 (10 per 100 carbon atoms). Binary Paraffin Mixtures. As shown in Figure 4, the lowangle scattering of the binary mixtures of C36 + C32 and C36 + C28 is weaker than those for the pure components (Figure 2) while the high-angle peaks are enhanced. As revealed by DSC, the binary mixture of C36 + C32 can form solid solution while the mixture C36 + C28 crystallizes separately.12,14 Upon addition of PEB10 or PEB7.5, a decrease of low-angle scattering is observed, and the high-angle scattering from the monoclinic unit cell is enhanced in both mixtures (Figure 4). Obviously, the effect of PEBs on binary paraffin mixtures is weaker than that on single components as observed by X-ray diffraction. This is consistent with our previous observations by rheology, microscopy, and DSC.12,14 Paraffin Crystallization with MACs. Effect of MAC on Paraffin Crystallization. Figure 5 shows the diffractograms of C28, C32, and C36 crystal samples with MAC18-18. Compared with their pure crystal samples (Figure 2), MAC1818 reduces the low-angle scattering from the layered structure of the paraffin crystals and the 1D high-angle scattering from

Figure 4. X-ray diffractograms of crystal samples of C36 + C32 and C36 + C28 with PEBs grown from decane solutions at a cooling rate of 1 °C/min. The curves from top to bottom denote crystals with 0.1% PEB10, with 0.1% PEB7.5, and without PEB.

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

Figure 5. X-ray diffractograms of crystals of C36, C32, and C28 with MAC18-18 from 4 wt % solutions in decane.

Figure 6. X-ray diffractograms of crystals of C36 with MACs from 4 wt % solutions in decane. Optical micrographs of each sample are shown as inserts over the curves.

the monoclinic unit cell is recovered, especially in the case of C28 (Figure 5). The influence of MAC is similar to, but less effective than, the effect of PEBs. Increasing MAC18-18 concentration leads to a reduction of the intensities of low-angle scattering and a growth of the two high-angle peaks. The same tendency appears if the side-chain length is decreased from C18 (MAC18-18) to C14 (MAC1814) and C12 (MAC18-12) (Figure 6). Again, the contribution of MACs themselves to the X-ray scattering intensity should be negligible since all three MAC samples are mostly amorphous as shown in Figure 3. As observed by optical microscopy (Figure 6), the sizes of C36 crystals from 4 wt % solutions in decane at 0 °C were reduced by MAC18-18, MAC18-14, or MAC18-12, and the shapes changed significantly from large platelike crystals (Figure 2a) to spindle shapes or particles of indistinct geometry. MAC18-18 reduces the C36 crystals to smaller size than MAC18-14 or MAC18-12. These observations are consistent with the rheological results in that crystal size and platelet shape

319

Figure 7. Schematic structure of long-chain n-paraffin crystal assembled with comb polymers.

correlate qualitatively with the yield stress of cooled model waxy oils. The reduction of yield stress is in the order MAC18-18 > MAC18-14 > MAC18-12).21 The X-ray diffraction results allow us to conclude that the interactions of PEB or MAC with long-chain n-paraffins take place by a cocrystallization mechanism, which is a consequence of assembly between paraffin and the crystallizable sections of the polymer structure. The noncrystallizable parts of the polymer structure will form an amorphous steric layer that prevents the stacking of wax plates (Figure 7). Effect of Cooling Rate. It is known that the cooling rate can affect the morphology of paraffin crystals, the transition temperature, and the yield stress of gels formed from waxy oils.24 However, investigation of the influence of cooling rate on the behavior of waxy oils is incomplete, especially with polymer additives. In our previous work,25 crystallization of long-chain n-paraffins from both melts and from decane solutions was studied by DSC and the morphology of these paraffin crystals was observed by optical microscopy. We also investigated the effect of cooling rate on paraffin crystallization when PEBs were added to the decane solutions.25 Microstructural characterization of wax morphology using X-ray diffraction should help in understanding the effect of cooling rate on polymer-paraffin interactions. Figure 8 shows the scattering for C36, C32, and C28 samples crystallized from melt and from solution with various cooling rates. The diffraction of crystals from the melt shows the highangle scattering characteristic of the monoclinic paraffin unit cell at angles of 21.5° and 23.7° (2θ).15,19 For the crystals grown from solution, this high-angle scattering is not represented but strong scattering from the layered platelet structure is observed. As would be expected, the slower cooling rate produces more ordered structures and stronger diffraction from the layered structure. The lack of high-angle scattering from the monoclinic unit cell in these 1D diffractograms indicates less lateral correlation between the chains in the crystals from solution, as compared with the crystals from the melt. This may be the consequence of the (relatively) rapid crystallization or interference in lateral organization from the alkane solvent during crystallization. The 2D transmission scattering patterns (Figure 9) for samples of C32 from melt and solution confirmed that the absence of peaks from the monoclinic unit cell was not due to diffraction spots that might be missed in the 1D detector used in our experiments. The high-angle 2D diffraction rings were complete and sharp

320

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

Figure 8. X-ray diffractograms of crystal samples (a) C36, (b) C32, and (c) C28. The curves from top to bottom denote melt-grown crystals at a cooling rate of 20 °C/min and crystals from decane solutions at a cooling rate of 10 and 1 °C/min, respectively.

paraffins do not. In addition, the diffraction from the C28 layered structures shows two sets of peaks with slightly different layer repeat distances at the slowest cooling rate (Figure 8c). In our previous work,25 we observed that C28 was anomalous in the effect of cooling rate on yield stress. In addition, we studied the crystallization of paraffin mixtures from decane solutions.12 Mixtures that phase-separated into coexisting crystal phases showed lower yield stresses than gels formed from either of the two pure phases at the same overall solids concentration. These three observations indicate that the anomalous coolingrate dependence of the rheological behavior of C28 gels from solution may arise from the two phases that we observe at the lowest cooling rate. Clearly more detailed work is warranted. Figure 9. 2D X-ray diffractograms of C32 crystal samples (a) from 4 wt % in decane solution with a cooling rate of 1 °C/min and (b) from the melt with a cooling rate of 20 °C/min.

Conclusions

with crystals from the melt, but diffuse and incomplete with crystals from solution, again indicating loss of lateral order. From the diffraction patterns of Figure 8, the long spacings for the long-chain n-paraffin layers are 4.22 and 3.77 nm for C36 and C32 crystals from solution and 3.49 and 3.36 nm for the two spacings observed with C28 at the slowest solution cooling rate. These spacings are less than those reported for these paraffins with crystals from the melt and with crystals formed very slowly (1.5 °C/h) from solution, namely, 4.84, 4.31, and 3.81 nm for C36, C32, and C28, respectively.15,19 The 2D diffraction patterns confirmed the difference in layer spacing for C32 crystals from the melt cooled at 20 °C/min (4.27 nm) and from solutions cooled at 1 °C/min (3.76 nm). Reynhardt et al. report19 identical long spacings for C36 crystals from the melt and from solutions cooled at 1.5 °C/h, but give no long spacing results for their crystals prepared by rapid cooling. We can conclude that paraffin crystals from rapidly cooled solutions show less short-range lateral order and correspondingly smaller layer spacings due to altered packing angles and possibly some coiling of the chains. A noteworthy observation from this set of data is the anomalous or distinct behavior of C28 compared to that of C32 or C36. The C28 displays strong scattering from layered structures when crystallized from the melt, while the other two

As observed by X-ray diffraction, poly(ethylene-butene) (PEB) copolymers and the comb-type poly(maleic anhydride amide-co-R-olefin) (MAC) can affect the crystal structures of long-chain n-paraffins C36, C32, C28, and their binary mixtures. In the absence of the polymer additives, paraffin platelet crystals give well-resolved low-angle spectra. Upon addition of PEB or MAC, the low-angle scatterings from the layered structures were reduced and even disappeared. It implies that the polymers anchor into crystals by cocrystallization with the paraffins by their polyethylene blocks (for PEB) or alkyl grafts (for MAC) and the noncrystallizable parts of the polymer chain form a polymer layer to provide a steric barrier against platelet stacking. This is consistent with the observation by optical microscopy and DSC, which can account for the significant reduction of yield stresses of model waxy oils upon addition of these polymers. X-ray scattering confirms greater ordering of lamellar crystals upon slower cooling, and greater lamellar ordering for C32 and C36 from solution than from the melt. The C28 diffraction spectra show doublets in both low- and high-angle spacings with decreasing cooling rate, which imply the formation of two crystal phases. Also, the C28 shows low-angle diffraction from lamellar ordering from the melt but the C32 and C36 do not.

Ind. Eng. Chem. Res., Vol. 50, No. 1, 2011

Acknowledgment We thank Dr. Qiang Qiu (Unilever Research, Trumbull, CT) and Dr. Katsuyuki Wakabayashi and Prof. Richard Register (Princeton University) for assistance with the diffraction experiments and much appreciated discussion. This work was supported by Halliburton Energy Services Inc., NSFC Grant 20774030, Shanghai Pujiang Talent Project 08PJ14036, 111 Project Grant B08021, and the Fundamental Research Funds for the Central Universities. Literature Cited (1) Singh, P.; Venkatesan, R.; Fogler, H. S.; Nagarajan, N. R. Morphological evolution of thick wax deposits during aging. AIChE J. 2001, 47, 6. (2) Denis, J.; Durand, J. P. Modification of wax crystallization in petroleum products. ReV. Inst. Fr. Pet. 1991, 46, 637. (3) Misra, S.; Baruah, S.; Singh, K. Paraffin problems in crude oil production and transportation: A review. SPE Prod. Facilities 1995, 10, 50. (4) Owen, K. Gasoline and diesel fuel additiVes; Wiley: New York, 1989. (5) Chang, C. L.; Fogler, H. S. Peptization and coagulation of asphaltenes in apolar media using oil-soluble polymers. Fuel Sci. Technol. Int. 1996, 14, 75. (6) Price, R. C. Flow Improvers for Waxy Crudes. J. Inst. Pet. 1971, 57, 106. (7) Kern, R.; Dassonville, R. Growth inhibitors and promoters exemplified on solution growth of paraffin crystals. J. Cryst. Growth 1992, 116, 191. (8) Pederson, K. S.; Ronningsen, H. P. Influence of wax inhibitors on wax appearance temperature, pour point, and viscosity of waxy crude oils. Energy Fuels 2003, 17, 321. (9) Ashbaugh, H. S.; Guo, X.; Schwahn, D.; Prud’homme, R. K.; Richter, D.; Fetters, L. J. Interaction of paraffin wax gels with ethylene/vinyl acetate co-polymers. Energy Fuels 2005, 19, 138. (10) Ashbaugh, H. S.; Fetters, L. J.; Adamson, D. H.; Prud’homme, R. K. Flow improvement of waxy oils mediated by self-aggregation partially crystallizable diblock copolymers. J. Rheol. 2002, 46, 763. (11) Ashbaugh, H. S.; Radulescu, A.; Prud’homme, R. K.; Schwahn, D.; Richter, D.; Fetters, L. J. Interaction of paraffin wax gels with random crystalline/amorphous hydrocarbon copolymers. Macromolecules 2002, 35, 7044. (12) Guo, X.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K.; Adamson, D. H.; Fetters, L. J. Crystallization of mixed paraffin from model waxy oils and the influence of micro-crystalline poly(ethylene-butene) random copolymers. Energy Fuels 2004, 18, 930.

321

(13) Chen, W.; Zhao, Z.; Yin, C. The interaction of waxes with pour point depressants. Fuel 2010, 89, 1127. (14) Guo, X.; Pethica, B. A.; Huang, J. S.; Prud’homme, R. K. Crystallization of long-chain n-paraffins from solutions and melts as observed by differential scanning calorimetry. Macromolecules 2004, 37, 5638. (15) Heyding, R. D.; Russell, K. E.; Varty, T. L. The Normal Paraffins Revisited. Powder Diffr. 1990, 5, 93. (16) Chatterjee, R. K.; Phatak, S. D.; Murthy, P. S.; Joshi, G. C. Combtype polymers and their interaction with wax crystals in waxy hydrocarbon fluids: Wide-angle X-ray diffraction studies. J. Appl. Polym. Sci. 1994, 52, 887. (17) Craig, S. R.; Hastie, G. P.; Roberts, K. J.; Sherwood, J. N. Investigation into the structures of some normal alkanes within the homologous series C13H28 to C60H122 using high-resolution synchrotron X-ray powder diffraction. J. Mater. Chem. 1994, 4, 977. (18) Craig, S. R.; Hastie, G. P.; Roberts, K. J.; Gerson, A. R.; Tack, R. D. Investigation into the structures of binary-, tertiary- and quinternarymixtures of n-alkanes and real diesel waxes using high-resolution synchrotron X-ray powder diffraction. J. Mater. Chem. 1998, 8, 859. (19) Reynhardt, E. C.; Fenrych, J.; Basson, I. Structures and molecular dynamics of solution-grown and melt-grown samples of n-hexatriacontane. J. Phys.: Condens. Matter 1994, 6, 7605. (20) Abdallah, D. J.; Sirchio, S. A.; Weiss, R. G. Hexatriacontane Organogels. The First Determination of the Conformation and Molecular Packing of a Low-Molecular-Mass Organogelator in Its Gelled State. Langmuir 2000, 16, 7558. (21) Guo, X.; Prud’homme, R. K. Synthesis of comb polymers based on poly(maleic anhydride-co-R-olefin) and their influence on paraffin crystallization. Polym. Prepr. 2005, 46, 1051. (22) Shearer, H. M. M.; Vand, V. The crystal structure of the monoclinic form of n-hexatriacontant. Acta Crystallogr. 1956, 9, 379. (23) Imai, T.; Nakamura, K.; Shibata, M. Relationship between the hardness of an oil-wax gel and the surface structure of the wax crystals. Colloids Surf., A 2001, 194, 233. (24) Singh, P.; Fogler, H. S.; Nagarajan, N. Predicting of the wax content of the incipient wax-oil gel in a flowloop: An application of the controlledstress rheometer. J. Rheol. 1999, 43, 1437. (25) Guo, X.; Pethica, B. A.; Huang, J. S.; Douglas, H. A.; Prud’homme, R. K. Effect of cooling rate on crystallization of model waxy oils with microcrystalline poly(ethylene butane). Energy Fuels 2006, 20, 250.

ReceiVed for reView July 23, 2010 ReVised manuscript receiVed October 27, 2010 Accepted November 16, 2010 IE101575W