Energy & Fuels 2000, 14, 1043-1048
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Crude Oil Wax Crystallization. The Effect of Heavy n-Paraffins and Flocculated Asphaltenes Marı´a del Carmen Garcı´a* Production Department, PDVSA-Intevep, P.O. Box 76343, Caracas 1070A, Venezuela
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Received February 23, 2000. Revised Manuscript Received June 2, 2000
The influence of heavy linear alkanes concentration on the wax crystallization tendency or wax appearance temperature (WAT) of a paraffinic crude oil and on a paraffin inhibitor effectiveness was evaluated by means of polarized light microscopy. A crude oil insensitive to the addition of wax inhibitors was fractionated into its hydrocarbon class fractions. Virgin crude oil distillation, deasphaltation of the 385 °C+ residue, HPLC separation of the heavy saturated hydrocarbons, and molecular sieves adduction enabled us to isolate the linear paraffins fraction with a carbon distribution of more than 24 atoms (nC24+ paraffins). A synthetic crude was prepared by mixing all separated fractions, except nC24+ paraffins, and the effect of this fraction concentration was evaluated by its controlled addition to the synthetic crude followed by cloud point measurements of all doped samples. The influence of this hydrocarbon class family concentration on a maleic anhydride copolymer derivative paraffin inhibitor (MAC) efficiency was also evaluated in all doped synthetic crudes. Abundance of these large linear alkanes increases the crude oil tendency to wax crystallization, a fact that was demonstrated by a linear correlation between the concentration of this hydrocarbon family and the crude oil cloud point. nC24+ paraffins proved to be deleterious for the efficiency of the selected paraffin inhibitor, affording an asymptotic curve which tends to a zero activity value at 30 wt % concentration of this fraction. Additionally, it was demonstrated that the flocculated asphaltenes provide wax crystallization sites whose presence increases the cloud point of the crude oil and also interferes with the crystal inhibition mechanism.
Introduction It has been clearly demonstrated that wax deposits formed during production and transport stages consist mainly of n-paraffins (linear alkanes) and small amounts of branched paraffins and aromatic compounds.1 Naphthenic (cyclic) and long chain paraffins also have a notorious contribution to microcrystalline wax and have a remarkable influence on the macrocrystalline growing pattern.2 The carbon number of paraffinic molecules present in wax deposits is known to be higher than 15 atoms and to reach values of more than 80 carbon atoms, as reported by several authors.3-5 Furthermore, advanced analytical techniques, including high-temperature capillary gas chromatography,6-12 supercritical fluid chro* E-mail:
[email protected]. (1) Jorda, R. M. J. Pet. Tech., Trans. AIME 1966, 237, 1605-1612. (2) Shock, D. A.; Sudbury, J. D.; Crockett, J. J. J. Pet. Technol. 1955, 7, 23-28. (3) Swetgoff, J. Oil Gas J. 1984, 82, 79-82. (4) O’Donell, G. Anal. Chem. 1951, 23 (6), 894. (5) Woo, F. T.; Garbis, S. J.; Gray, T. C. Paper SPE 13126, presented at the 59th Annual Fall Technical Conference and Exhibition, Houston, TX, 1984. (6) Del Rı´o, J. C.; Philp, R. P. TRAC Trends Anal. Chem. 1992, 11 (5), 187-193. (7) Del Rı´o, J. C.; Philp, R. P.; Allen, J. Org. Geochem. 1992, 18 (4), 541-553. (8) Philp, R. P. J. High Resolut. Chromatogr. Chromatogr. Commun. 1994, 17 (6), 398-406. (9) Aquino Neto, F. R.; Cadoso, J. N.; Dos Santos Pereira, A.; Zupo Ferna´ndez, M. C.; Caetano, C. A.; Castro Machado, A. L. J. High Resolut. Chromatogr. Chromatogr. Commun. 1994, 17 (4), 259-263.
matography,13 and size exclusion chromatography,14 have allowed researchers to detect up to 160 carbon atoms in paraffinic deposits, in accordance with Boduszynski results in 1981.15 It was demonstrated that light paraffinic fractions along with the absence of a well-defined maximum in carbon number distribution curve or a bimodal behavior in simulated distillation chromatograms are responsible for keeping the pour point low enough even at high concentrations of heavier paraffins.16 Other authors reported that doping nC16 solutions with nC13 paraffins reduces the melting point, only for cases in which the heavier paraffin concentrations are kept relatively low.17 Paraffin blends containing n-paraffins with small chain length difference respond more effectively to crystal(10) Aquino Neto, F. R.; Cadoso, J. N.; Dos Santos Pereira, A.; Zupo Ferna´ndez, M. C. Proc. ISCOP′95, First Intl. Symp. Colloid Chem. Oil Prod., 1995, Rı´o de Janeiro, Brazil, pp 63-67. (11) Hsu, J. J. C.; Santamarı´a, M. M.; Brubaker, J. P. Paper SPE 28480, 1994, presented at the 69th Annual Technical Conference and Exhibition, New Orleans, LA. (12) Wavrek, D. A.; Dahdah, N. F. Paper SPE 28965, 1995, presented at the SPE International Symposium on Oil Field Chemistry, San Antonio, TX. (13) Thompson, J. S.; Rynaski, A. F. J. High Resolut. Chromatogr. Chromatogr. Commun. 1992, 15 (4), 227-234. (14) Carbognani, L. J. Chromatogr. A 1997, 788, 63-73. (15) Boduszynski, M. M.; McKay, J. F.; Latham, D. R. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1981, 26 (4), 865-881. (16) Wang, B.; Dong, L. Paper SPE 29954, 1995, presented at the SPE International Meeting on Petroleum Engineering, Beijin, PR China, 1995, pp 33-48. (17) Affens, W. A.; Hall, J. M.; Hazlett, R. N. Fuel 1984, 63, 543547.
10.1021/ef0000330 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/26/2000
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lization inhibitors than those where this difference is more significant.18 A good correlation between oil composition and polymeric paraffin inhibitors activity was recently found.19,20 Several crude oil compositional aspects such as [n-/(cyclo + isoparaffins)] ratio, molecular weight distribution curve shape, and concentration of components with more than 24 carbon atoms (C24+) were demonstrated to have a remarkable influence on crude oil wax crystallization tendency and on paraffins crystallization inhibition mechanisms. In that work, paraffinic crude oils were classified into two structural categories in terms of their molecular weight distribution, C24+ paraffins concentration, and [n-/(cyclo + isoparaffins)] ratio. Also, different responses to commercial paraffin inhibitors were obtained by these two crude oil types, assessing the inefficiency of some paraffin inhibitors when added to crudes rich in C24+ alkanes. In a more recent paper,21 it was demonstrated that the crude oil (cyclo + isoparaffins) fraction, increases the cloud point at concentrations above 50 wt %, probably due to the higher average molecular weight introduced by these components. A crystal modifier effect enhancement was also observed within this concentration interval, as a consequence of structural effects such as loose packing of crystals due to the steric effect of naphthenic and branched structures.21-23 As can be seen, there is a complex interaction between crude oil fractions and the tendency of the crude oil to wax precipitation, especially in the case of paraffin class hydrocarbons. The main goal of this work is to study the effect of heavy linear paraffins concentration on wax appearance temperature (WAT) of a Type I paraffinic crude oil, and on the effectiveness of a maleic anhydride copolymer derivative paraffin inhibitor. The effect of flocculated asphaltenes on the paraffins crystal growing is also evaluated. Experimental Section Crude Oil and Paraffin Inhibitor. The oil sample used in this study (M-4) is a dead (tank) paraffinic crude from an Eastern Venezuelan reservoir. This crude corresponds to Type I oil, a class previously defined regarding its molecular weight distribution and heavy paraffins content. It also remains unaltered when treated with paraffin inhibitor additives, keeping its pour and cloud point depression within the technique’s experimental uncertainty.19,20 M-4 crude has an API gravity of 35.3°, which is typical for light crude oils, but its highly paraffinic character (49.35 wt % of wax) and elevated [n-/(cyclo + isoparaffins)] ratio (1.26) are (18) Kitaigorodski, A. I. Mixed Crystal; Springer: Berlin, 1984; p 219. (19) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Paper SPE 49200, presented at the SPE Annual Technical Conference and Exhibition, New Orleans, LA, 1998, pp 681-687. (20) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Pet. Sci. Technol. 1998, 16, 1001-1021. (21) Garcı´a, M. C.; Orea, M.; Urbina, A.; Carbognani, L. Paper No. 58g, presented at the Spring AIChE Meeting, Second International Symposium on Wax Thermodynamic and Deposition I, Houston, TX, March 14-18, 1999. (22) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. Asphaltenes and Wax Deposition. Proceedings of the 3rd International Symposium on Colloids and Oil Production, Huatulco, Oaxaca, Mexico, 1999. (23) Garcı´a, M. C.; Carbognani, L.; Orea, M.; Urbina, A. Pet. Sci. Eng. 2000, 25, 99-105.
Garcı´a
Figure 1. Main structure of MAC inhibitor actives.
Figure 2. nC24+ isolation from crude M-4 scheme. responsible for its high pour point (45 °C).24 Although the paraffinicity qualified this crude as potential raw material for wax derivatives manufacturing, it drastically affects production operations due to paraffin deposition during production, transportation, and oil storage,19,20,24 an effect previously reported for similar crude oils.25 The paraffin inhibitor used for this study was a commercial crystal modifier. This product’s active ingredient (62 wt % of the formulation) is a maleic anhydride R-olefin alkyl ester copolymer, (MAC). The R-olefin fragment of the copolymer contains a maximum of 26 carbon atoms, and the ester substitution ranges from C20 to C24 (Figure 1). nC24+ Paraffins Isolation. Linear paraffins with more than 24 carbon atoms on their hydrocarbon chains were isolated from M-4 crude oil following the scheme illustrated on Figure 2. A simple method, that involves insignificant losses due to evaporation or irreversible adsorption on chromatographic sorbents, as well as low overlapping with shorter n-paraffins, was used. It consists of a preliminary hydrocarbon light ends remotion (bp below 385 °C). Once the 385 °C+ residue is obtained, a straightforward physical and chromatographic separation sequence allows us to assess the nC24+ paraffin fraction isolation. Distillation of 515 g of M-4 crude oil, performed by a standard method (ASTM D-2892), allowed us to obtain several distillation fractions that were put aside for later experiments. Fifteen grams of the 385 °C+ distillation residue were deasphalted with 600 mL of a precipitating solvent (n-heptane), following previously reported procedures.19,20 The 385 °C maltenes fraction, obtained from the n-heptane/toluene soluble portion of the deasphaltation procedure evaporation, was fractionated into its saturate, aromatic, and resin hydrocarbon (24) Garcı´a, M. C.; Carbognani, L.; Urbina, A.; Orea, M. PDVSAIntevep, Report INT-3687, 1997; p 18. (25) Misra, S.; Baruah, S.; Singh, K. SPE Prod. Facil. 1995, 5054.
Crude Oil Wax Crystallization
Energy & Fuels, Vol. 14, No. 5, 2000 1045 Table 1. M-4 Crude Oil Distillation Fractions bp (°C)
distributiona
IP-200 200-300 300-350 350-385 385+
C6-C16 C9-C18 C12-C22 C15-C24 C24+
cumulated distributiona C6-C16 C6-C18 C6-C22 C6-C24 (C24-)
% (wt)
cumulated % (wt)
6.43 9.07 7.83 9.13 67.54
6.43 15.50 23.33 32.46 100
a Carbon number distribution by simulated distillation using paraffins standards.
Figure 3. M-4 synthetic crude II preparation scheme. families (SAR) by means of an HPLC technique under previously reported conditions.22,23 The 385 °C+ saturate fraction was treated with 5 Å molecular sieves, under adduction conditions with isooctane previously described,19,20 obtaining the nC24+ paraffin fraction. All the remaining fractions of the M-4 crude oil fractionation steps (IP-385 °C, asphaltenes, 385 °C+ aromatics, 385 °C resins, and 385 °C+ (cyclo + isoparaffins)) were set aside for further experimental work in this study. The purity of the saturate, aromatic, resin, and asphaltene fractions was verified by thin-layer chromatography with flame ionization detection (TLC/FID), as described before.26 All distillation, n-, and (cyclo + isoparaffins) fractions were characterized by simulated distillation (SD), in terms of molecular weight (carbon number) distribution.14,20 “Synthetic Crudes” Preparation. Reconstituted or “synthetic” crude oils attempt to reproduce the original M-4 crude oil composition, containing all its components except the nC24+ paraffin fraction, which will be used for later controlled doping experiments. Three different synthetic crudes were prepared in order to study the effect of this hydrocarbon fraction on the wax crystallization and to evaluate the influence of flocculated asphaltenes in this process. For “synthetic crude I” preparation, the IP-385 °C, asphaltene, 385 °C+ aromatic, 385 °C resin, and 385 °C+ (cyclo + isoparaffins) M-4 fractions (see Figure 2) were combined in adequate proportions, keeping the original hydrocarbon families concentrations in the crude oil. Sample homogenization was carried out by means of warming and ultrasonication of the mixture at 60 °C for 1 h, inside a closed vial, avoiding the evaporation of light components. “Synthetic crude I” (6 g) was obtained. “Synthetic crude II” had the same composition as “synthetic crude I”, but it was prepared under different conditions trying to avoid asphaltenes flocculation. This synthetic crude (6 g) was obtained following the scheme illustrated in Figure 3. All the heavy fractions, except the nC24+ paraffins (asphaltenes, 385 °C+ aromatics, 385 °C+ resins, and 385 °C+ (cyclo + isoparaffins)) were placed in a 100 mL round-bottom flask along with 20 mL of a carbon disulfide/methylene chloride 1:1 mixture to assist the total dissolution of the asphaltenes in the crude bulk. After complete dissolution of the heavy components, solvents were removed at 60 °C in a rotary evaporator, followed by vacuum-drying until a constant weight was reached. Finally, the distilled fraction (IP-385 °C) was added to the evaporation residue and the resulting mixture (26) Sol, B.; Romero, E.; Carbognani, L.; Sa´nchez, V.; Sucre, L. Rev. Te´ cn. Intevep 1985, 5, 39-43.
was carefully homogenized, avoiding light-ends losses due to evaporation and ultrasonication. “Synthetic crude III” was prepared in order to evaluate the flocculated asphaltenes effect on the wax crystallization tendency of the crude oil. This synthetic crude contains the same components as “synthetic crudes I and II” except for the asphaltene fraction, which was not added to the mixture. This synthetic crude (5 g) was obtained under the same experimental conditions used in the preparation of “synthetic crude I”. Doping with nC24+ Paraffin Fraction. All the “synthetic crudes” were doped with the nC24+ isolated fraction, spanning the concentration range from 0 wt % to a value above the nC24+ paraffins concentration in the virgin M-4 crude oil. Enriched mixtures (500 mg) were prepared by placing the synthetic crude oil in 10 mL vials and adding a specific amount of the dopant fraction in order to reach the desired concentration of nC24+ paraffins. The total system homogenization was accomplished under the warming and ultrasonication conditions mentioned above. Crude Oil Wax Appearance Temperature and Paraffin Inhibitor Evaluation. Cloud points were measured by means of polarized light microscopy (PLM), following the already reported conditions.19,20 The uncertainty obtained in these measurements was (0.2 °C. Cloud point values were used to evaluate the wax appearance temperature (WAT) of the crude oils and the paraffin inhibitor activity in each case. For this last set of experiments, 4000 ppm of the commercial product (MAC) were added to the particular sample under study (virgin crude oil, and synthetic and doped synthetic crudes). After mild heating and thorough homogenization, the cloud points were measured. Crude samples with no additives were used as controls for each experiment.
Results and Discussion M-4 Crude Oil Fractionation. Distillation of M-4 crude oil afforded 5 fractions (4 distillation cuts and 385 °C+ residue), whose yields and characteristics are summarized in Table 1. Each distillation fraction was characterized by simulated distillation, affording the carbon number distribution with respect to paraffin references (Figure 4). According to these results, M-4 crude oil was easily separated into two distillation fractions: IP-385 °C (all distillates re-combination) and 385 °C+ (distillation residue), keeping the original distribution of M-4 crude oil (Table 1). The 385 °C+ residue fractionation results are summarized in Table 2. TLC/FID analysis of the saturate, aromatic, resin, and asphaltene fractions confirmed their purity in terms of overlapping with neighbor hydrocarbon families. Simulated distillation of the two paraffinic fractions (nC24+ and (cyclo + isoparaffins)) confirmed a clean separation. Concentration Effect of nC24+ Paraffins on the Cloud Point and MAC Activity. The influence that nC24+ paraffins concentration has on the crude oil wax crystallization tendency, measured by its cloud point,
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Garcı´a
Figure 5. MAC inhibitor activity evaluation with nC24+sdoped M-4 synthetic crudes. Table 3. Synthetic Crudes Formulationa concentration (%, wt)
Figure 4. M-4 crude oil distillation fractions chromatograms: (a) IP-200 °C, (b) 350-385 °C, (c) 385 °C+. Table 2. M-4 Crude Oil Fractionation Yields fractionation stage distillation (ASTM D 2892) residue 385 °C+ deasphaltation maltenes 385 °C+ HPLC 385 °C+ saturates adduction
fraction IP-385 °C 385 °C+ asphaltenes maltenes 385 °C+ 385 °C+ saturates 385 °C+ aromatics 385 °C+ resins nC24+ paraffins 385 °C+ cyclo/ isoparaffins
yield (%, wt)a 32.46 (32.46)b 67.54 (67.54) 0.26 (0.18)b 99.70 (67.33) 81.9 (55.14) 17.39 (11.71)b 1.33 (0.90)b 24.8 (13.28)b 75.2 (41.47)b
a Values in parentheses indicate wt % with respect to the original crude oil. b wt % used for mass balance.
and on MAC paraffin inhibitor activity was evaluated through M-4 crude oil doping experiments. Controlled aliquots of nC24+ fraction were added, following the sequence illustrated in Figure 5. “Synthetic crude I”, prepared with the formulation shown in Table 3, was used for the first experiment of this series. The resulting mixture had a homogeneous appearance and a solid consistence at room temperature, with a cloud point of 45 °C, which is lower than the value obtained for M-4 original crude oil (47 °C),24 due to the remotion of the heavy linear (nC24+) paraffin fraction. This sample was doped with specific proportions of the nC24+ paraffin
fraction
original crude oil
IP-385 °Cb nC24+ paraffins 385 °C+ (cyclo + isoparaffins) 385 °C+ aromatics 385 °C+ resins asphaltenes total
32.46 13.28 41.47 11.71 0.90 0.18 100.00
synthetic crude oils I and II III 37.43
37.51
47.82 13.50 1.04 0.21 100.00
47.93 13.53 1.04 100.00
a
From M-4 crude oil fractions. b Combination of all M-4 crude oil distillates. Table 4. nC24+ Paraffins Fraction Dosages Used in the “Synthetic Crudes” Doping synthetic crudea (mg) 500 490 480 470 460 450 440 400 375 350
added C24+ paraffins fraction mass (mg) % (wt) 0 10 20 30 40 50 60 100 125 150
0 2 4 6 8 10 12 20 25 30
a Synthetic crudes I, II, and III: from the combination of M-4 crude oil fractions except nC24+ paraffins (I and II); nC24+ paraffins and asphaltenes (III).
fraction, as shown in Table 4. An increment of “synthetic crude I” cloud point is observed upon the addition of nC24+ paraffin fraction, as shown in Figure 6a. This tendency confirms previous findings that suggested that the greater the concentration of the high-melting nalkanes in the wax, the higher is their dissolution temperature (WDT).27 A positive deviation of the curve, compared to the original crude oil sample (47 °C), is observed. This effect can be explained by the presence
Crude Oil Wax Crystallization
Figure 6. nC24+ paraffins effect on M-4 synthetic crude oils cloud point: (a) synthetic crude I), (b) synthetic crude II, and (c) synthetic crude III (asphaltenes removed).
of flocculated asphaltene particles in “synthetic crude I”, visualized under PLM as well-defined black particles (see Figure 7a), as a consequence of the irreversibility of the oil reconstitution, previously reported by Ackroyd and Cawley in 1953.28 These particles probably behave as paraffin nucleation sites for the wax crystal growth (Figure 7b,c), shifting the cloud point to a value above that one of the original oil. “Synthetic crude II”, prepared with a polar solvent asphaltene dissolution aid, was used in the second experimental set, trying to overcome the crystal nucleation sites problem due to the asphaltene flocculation during “synthetic crude I” preparation. The new sample (27) Srivastava, S. P.; Saxena, A. K.; Tandon, R. S.; Shekher, V. Fuel 1997, 76 (7), 625-630. (28) Ackroyd, G. C.; Cawley, C. M. J. Inst. Pet. 1953, 39, 82-111.
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Figure 7. Crude M-4 photomicrographs showing the effect of flocculated asphaltenes on WAT: (a) above cloud point, (b) at cloud point, and (c) below cloud point.
was doped with the same dosages of nC24+ paraffin fraction as in the first experiment (Table 3), under the same homogenization conditions. The effect of these paraffins concentration in the synthetic crude oil cloud point is shown in Figure 6b. Again, an increase in the cloud point is observed and the synthetic crude oil shows a higher cloud point than the original crude oil. This deviation is a consequence of the presence of flocculated asphaltene particles in the synthetic crude oil (observed under the PLM in similar amount as in “synthetic crude I”) despite the careful dissolution technique used. I is evidenced in Figure 6a,b, that “synthetic crude” I and II afforded similar tendencies. Finally, a third set of experiments was performed using “synthetic crude III”, prepared in a total absence of asphaltenes, with the formulation shown in Table 3 in order to avoid the presence of asphaltenic crystal wax nucleation sites. Doping experiments with nC24+
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paraffin fraction controlled aliquots (Table 4), under the same homogenization conditions used for the first two experiments, allowed us to observe again an incremental cloud point response to the addition of this paraffinic fraction. As in “synthetic crude II” a linear correlation was obtained, but in this case a perfect correspondence with the original crude oil was observed (Figure 6c). Figure 6b,c shows that, although both curves have the same slope (almost parallel), there is an appreciable shift between them (ca. 3 °C). These results confirm the hypothesis which states that the flocculated asphaltenes enhance the wax crystallization process due to the presence of nucleation sites in the crude oil matrix. A qualitative correspondence is established at this moment. This evidence must be taken into consideration, when paraffin crystallization inhibition treatments are to be designed, in terms of including an asphaltene flocculation inhibitor in the formulation. Quantitative correlations between the amount of flocculated asphaltenes and the cloud point deviations are under study in our laboratories. It is interesting to note that at nC24+ concentrations above 20 wt %, the effect of this fraction on the asphaltenes flocculation, and hence on the crude oil cloud point, is rather unpredictable. This phenomenon can be observed in Figure 6a-c, where “synthetic crude I” tends to a plateau for the last three points, becoming insensitive to the addition of nC24+ paraffins, and “synthetic crudes I and II” afford a higher cloud point for such a high concentration. This behavior remains unexplained, especially when taking into consideration that the uncertainty of the technique employed for these measurements was (0.2 °C. The effect of nC24+ paraffin fraction concentration on the MAC inhibitor activity was assessed with all the “synthetic crudes”. The doped samples were additivated with 4000 ppm of the product. No correlation was observed with “synthetic crudes I and II”. All the inhibitor activity values were found to be around zero point, probably due to the flocculated asphaltenes interference effect described above. However, a good correlation was obtained between the inhibitor activity and the concentration of nC24+ paraffins in “synthetic crude III”. This correlation (asymptotic curve) tends to an activity value of zero at 30 wt % concentration of the nC24+ paraffins (Figure 8). Further research on the matter, which includes the effect of colloidal asphaltenes
Garcı´a
Figure 8. nC24+ paraffins effect on MAC inhibitor activity over M-4 synthetic crude oil III nC24+ paraffins (asphaltenes removed).
in the wax crystallization tendency, is being made in our laboratories. Conclusions The abundance of large linear alkanes (nC24+) in paraffinic crude oils increases their tendency to the wax crystallization, which can be demonstrated by a linear correlation between the concentration of this hydrocarbon family and the crude oil cloud point. nC24+ paraffins proved to be deleterious for the efficiency of a maleic anhydride paraffin inhibitor or crystal modifier, when added to a Type I crude oil, affording an asymptotic curve, which tends to an activity value of zero at a 30 wt % concentration of this fraction. The presence of flocculated asphaltenes in the crude oil bulk generates wax crystallization sites, which increases the cloud point of the crude oil and interferes with the crystal inhibition mechanism of a maleic anhydride crystal modifier. This aspect should be taken into consideration in the design of paraffin inhibition treatments, by the addition of asphaltene flocculation inhibitors to the formulation, when necessary. Acknowledgment. The author thanks Petro´leos de Venezuela, S.A. for providing the funding. Helpful discussions with Lante Carbognani and microphotographs provided by Argelia Urbina are also acknowledged. The detailed review of the manuscript by Estrella Rogel and Laura Carvajal is appreciated. Finally, the author thanks PDVSA-Intevep for allowing the publication of this work. EF0000330
