Performance-Based Designing of Wax Crystal Growth Inhibitors

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Energy & Fuels 2008, 22, 3930–3938

Performance-Based Designing of Wax Crystal Growth Inhibitors Hemant P. Soni,*,† Kiranbala,‡ and D. P. Bharambe‡ Department of Chemistry, Faculty of Science, The Maharaja Sayajirao UniVersity of Baroda, Vadodara-390 002, Gujarat, India, and Applied Chemistry Department, Faculty of Technology and Engineering, The Maharaja Sayajirao UniVersity of Baroda, Vadodara-390 001, Gujarat, India ReceiVed April 23, 2008. ReVised Manuscript ReceiVed July 30, 2008

Four new comb-shape maleic anhydride copolymeric diesters with aliphatic and/or aromatic pendant chains were synthesized by suspension polymerization and characterized by Fourier transform infrared spectroscopy (FTIR) and gel-permeation chromatography. These polymers were evaluated as pour-point depressant (PPD) and rheology modifiers on Kosamba (KS) crude oil (Gujarat, India). A complete rheological analysis of crude oil with and without additives has been carried out using an advanced rheometer AR 500. A correlation between pour-point depressing power and structure of the polymer has been established. The role of asphaltene content in the crude oil has been highlighted in connection with performance of PPD. The proposed correlation has been supported by optical microscopy.

1. Introduction Transportation of crude oil through pipelines is one of the major problems for oil industries all over the world. This is especially true while restarting the pipelines after a long shutdown period or during winter. The solubility of highmolecular-weight paraffins present in crude oil decreases with a decreasing temperature, which results in a formation of stable wax crystals. These crystals having platelets or orthorhombic structures overlap and interlock with each other and form threedimensional networks. The oil present around this network gets trapped in it, and below the cloud point, it results in a gel-like structure. This gel behaves as a porous medium into which wax molecules continue to diffuse, and therefore, the wax content of deposited gel increases with time.1 Upon further decrease in temperature, this gel becomes hardened and chokes the pipelines. Thus, the rheological behavior of crude shifts from Newtonian to non-Newtonian with the lowering of the temperature. A lot of energy is wasted to pump such crude oil from the reservoir to the refinery. Various remedies are suggested to overcome this problem, e.g., preheating of crude and pipelines, subjecting the crude initially to a special heating-cooling treatment cycle to modify the wax crystal structure, and controlled thermal conditioning to take advantage of the natural pour-point depressing effect of resin and asphaltene present in oil.2-4 There are also processes such as application of microwave and * To whom correspondence should be addressed. Telephone: +91-2652795552. E-mail: [email protected]. † Department of Chemistry. ‡ Applied Chemistry Department. (1) Singh, P.; Ramchandran, V.; Fogler, H. S.; Nagrajan, N. R. Morphological evolution of thick wax deposits during aging. AIChE J. 2001, 47, 6–18. (2) Ramchandran, V.; Ostlund, J. A.; Chawla, H.; Wattana, P.; Magnus, N.; Fogler, H. S. The effect of asphaltenes on the gelation of waxy oils. Energy Fuels 2003, 17, 1630–1640. (3) Thompson, D. G.; Taylor, A. S.; Graham, D. E. Emulsification and demulsification related to crude oil. Colloids Surf. 1985, 15, 175–189. (4) Chanda, D.; Sarmaha, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam, B.; Das, H. C. Combined effect of asphaltenes and flow improvers on the rheological behaviour of Indian waxy crude oil. Fuel 1998, 77, 1163–1167.

Figure 1. Chemical structure of the R-olefin maleic anhydride copolymer.

Figure 2. Chemical structures of four different newly synthesized combshaped polymers evaluated as PPDs for Indian crude oil.

ultrasound irradiation,5 magnetic field,6 lining and coating pipelines with fiber-reinforced plastics to reduce the wettability of paraffins with walls,7 and covering inner wall surfaces with polypropylene to inhibit wax deposition.8 However, each one has its own drawback and thus is not widely accepted. The addition of the light distillate to the crude (before pumping) is another alternative, but it wastes the precious light distillates. The addition of the oil-soluble surfactants or treating the crude with chemical additives (before pumping) is the most viable (5) Bjø´rndalen, N.; Mustafiz, S.; Islam, M. R. Numerical modeling of petroleum fluids under microwave irradiation for improved horizontal well performance. Int. Commun. Heat Mass Transfer 2003, 30, 765–774. (6) Rocha, N. O.; Gonzalez, G.; Vaitsman, D. S. Magnetic field effect on paraffin deposition. Quim. NoVa 1998, 12, 11–17. (7) Slack, M. Polyethylene liners for internal rehabilitation of oil pipelines. Mater. Perform. 1992, 31, 49–52. (8) Quintella, C. M.; Musse, A. P. S.; Martha, T. P. O.; Castro, S.; Mikelsons, L.; Watanabe, Y. N. Polymeric surfaces for heavy oil pipelines to inhibit wax deposition: PP, EVA28, and HDPE. Energy Fuels 2006, 20, 620–624.

10.1021/ef8002763 CCC: $40.75  2008 American Chemical Society Published on Web 09/17/2008

Wax Crystal Growth Inhibitors

Energy & Fuels, Vol. 22, No. 6, 2008 3931

Table 1. Molecular Weights of Polymer Additives

Table 2. Physical Characteristics of KS and ND Crude Oils

molecular weight serial number

additive

Mw

Mn

1 2 3 4

18-OA18 BAUn-18 18-ABA 18-MBA

71 118 23 382 35 841 56 640

26 226 10 982 12 453 23 438

solution of this problem.9,10 Before selecting any treatment, the factors such as heat transfer, mass transfer, design of the pipelines, shutdown and restarting period, and overall cost of the treatment should also be considered. Any material acting as a pour-point depressant (PPD) should be soluble in oil and have an ability to co-crystalize with the growing wax crystals from the crude oil below its cloud point. In general, it has a wax-like paraffin part along with a polar component in the form of acrylate, methacrylate, acetate, etc. If such material is present in a comb-like shape, then generally the pendant chains act as a paraffin part and co-crystalize with wax, while the backbone and polar end groups limit such crystallization. This results into modifying the habit of wax crystals, which is generally orthorhombic, to a compact pyramidal form. This prevents the crystals from agglomerating and forming a gel-like structure.11-19 Theoretically, the interaction between ethylene-vinyl acetate copolymer (EVA)-type PPD molecules and wax crystals was studied using molecular mechanics, molecular dynamics, and quantum mechanical methods. It was proven from the molecular simulation that the addition of PPD molecules inhibit the growth of wax crystals along with the surfaces (010) and (110), which are perpendicular to the surface (001). Thus uni-directional growth of wax crystals along the surface (001) results in relatively small cubic wax particles, which prevent the formation of the wax gel network in oil upon cooling.20 Maleic anhydride copolymerizes readily with vinyl-type monomers to produce alternating 1:1 copolymers (Figure 1). (9) Dong, L.; Xie, H.; Zhang, F. A study on BEM series of pour point depressants and their application in China. Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, Feb 1316, 2001; SPE 65381-MS. (10) Tung, N. P.; Phong, N. T. P.; Long, B. Q. K.; Thuc, P. D.; Son, T. C. Studying the mechanisms of crude oil pour point and viscosity reductions when developing chemical additives with the use of advanced analytical tools. Proceedings of the SPE International Symposium on Oilfield Chemistry, Houston, TX, Feb 13-16, 2001; SPE 65024-MS. (11) Wang, S. L.; Flamberg, A.; Kikabhai, T. Select the optimum pour point depressant. Hydrocarbon Process. 1999, 59, 59–62. (12) Zhang, J.; Wu, C.; Li, W.; Wang, Y.; Han, Z. Study on performance mechanism of pour point depressants with differential scanning calorimeter and X-ray diffraction methods. Fuel 2003, 82, 1419–1426. (13) Pedersen, K. S.; Rø´nningsen, H. P. Influence of wax inhibitors on wax appearance temperature, pour point and viscosity of waxy crude oils. Energy Fuels 2003, 17, 321–328. (14) Holder, G. A.; Winkler, J. Wax crystallization from distillate fuels I: Cloud and pour phenomena exhibited by solutions of binary n-paraffins mixtures. J. Inst. Pet. 1965, 51 (499), 228–234. (15) Holder, G. A.; Winkler, J. Wax crystallization from distillate fuels II: Mechanism of pour depression. J. Inst. Pet. 1965, 51 (499), 235–252. (16) Fremel, T. V.; Zubova, M. A.; Yunovich, M. E.; Mitusova, T. N. Mechanism of action of pour point depressant. Chem. Technol. Fuels Oils 1993, 29, 8–15. (17) Srivastva, S. P.; Tandon, R. S.; Verma, P. S.; Pandey, D. C.; Goyal, S. K. Phase transition in middle distillate waxes: Effect of pour point depressant additive. Fuel 1995, 74, 928–931. (18) Srivastva, S. P.; Tandon, R. S.; Verma, P. S.; Saxena, A. K.; Joshi, G. C.; Pathak, S. D. Crystallization behaviour of n-paraffins in Bombay high middle distillate wax/gel. Fuel 1992, 71, 533–537. (19) Continho, J. A.; Danphin, C.; Daridon, J. L. Measurement and modeling of wax formation in disel fuels. Fuel 2000, 79, 607–616. (20) Zhang, J.; Zhang, M.; Wan, J.; Li, W. Theoretical study of the prohibited mechanism of ethylene/vinyl acetate co-polymers to the wax crystal growth. J. Phys. Chem. B 2008, 112, 36–43.

properties

KS

ND

pour point (°C) cloud point (°C) density (g/cc) specific gravity API gravity (at 60 °F) wax content (wt %) saturates (wt %) aromatics (wt %) N, S, O asphaltene (wt %) IBP (°C)

36 57 0.8772 0.8776 29.7 30.55 80.78 10.22 6.23 0.98 57

33 54 0.8341 0.8345 38.06 31.26 84.97 12.99 4.81 0.54 52

Table 3. Distillation Characteristics of KS and ND Crude Oilsa KS

ND

initial boiling point (57 °C)

initial boiling point (52 °C)

serial number

temperature (°C)

volume of oil (mL)

temperature (°C)

volume of oil (mL)

1 2 3 4 5 6 7 8 9 10

57-75 75-100 100-125 125-150 150-175 175-200 200-225 225-250 250-275 275-300

02 06 11 19 25 31 39 45 53 62

57-75 75-100 100-125 125-150 150-175 175-200 200-225 225-250 250-275 275-300

04 08 13 21 27 33 42 49 57 68

a

Volume taken ) 100 mL.

Dependent upon the requirements, one can tune the length of pendant chains. Under catalytic conditions these maleic anhydride units could be broken up into comb-shaped polymer or polymeric brushes. In our previous studies, it has been reported that such polymers act as PPD for crude oils from different Indian oil fields.21-23 Khidr24 reported R-olifin maleic anhydride copolymers acting as PPD for Umbarka waxy crude oil. It has been concluded that alkyl chains of the prepared copolymers are an essential factor for intensive interaction with crude oil. It was demonstrated that the structure of maleic anhydride polymers affected the pour point and viscosity of the paraffinic crude oil and the more similar the polymer structure (backbone and pendant groups) to the wax component, the better its performance.25 In the present work, we have selected two different crude oils [Kosamba (KS) and Nada (ND)] from the Cambay basin of western India, having different physical properties and systematically developed polymer additives, which work both as PPD and as rheological modifiers for these crudes. We have shown that pendant chain length can affect pour depression. The chain made from C18 alcohol is more effective for oils from the Cambay basin region.23 Therefore, we are reporting here only the additives of a different series made from C18 alcohol (21) Deshmukh, S.; Bharambe, D. P. Synthesis of polymeric pour point depressants for Nada crude oil (Gujarat, India) and its impact on oil rheology. Fuel Process. Technol. 2008, 89, 227–233. (22) Soni, H. P.; Bharambe, D. P. Synthesis and evaluation of polymeric additives as flow improvers for Indian crude oil. Iran. Polym. J. 2006, 15 (12), 943–954. (23) Soni, H. P.; Bharambe, D. P.; Nagar, A.; Kiranbala. Synthesis of chemical additives and their effect on Akholjuni crude oil (Gujarat, India). Ind. J. Chem. Technol. 2005, 12, 55–61. (24) Khidr, T. Synthesis and evaluation of copolymers as pour point depressants. Pet. Sci. Technol. 2007, 25, 671–681. (25) Son, A. J.; Graugnard, R. B.; Chai, B. J. Proceedings of the SPE International Symposium on Oilfield Chemistry, New Orleans, LA, March 2-5, 1993; SPE 25186-MS.

3932 Energy & Fuels, Vol. 22, No. 6, 2008

Soni et al.

Table 4. Pour Points (°C) of Additive-Treated (500 ppm) Crude Oils KS

ND

additive

pour-point blank

pour point of additive-treated crude oil

extent of pour depression

pour-point blank

pour point of additive-treated crude oil

extent of pour depression

18-OA18 BAUn-18 18-ABA 18-MBA

36 36 36 36

27 30 15 18

09 06 21 18

33 33 33 33

30 30 30 30

03 03 03 03

Table 5. Viscosities and Yield Stresses of KS and ND (Blank) Crudes at Different Temperatures KS

ND

temperature (°C)

viscosity (CPs)

yield stress (Pa)

viscosity (CPs)

yield stress (Pa)

20 25 30 33 35 36 40 50

995.5 957.4 907.8

132.8 130.2 69.95

616 560.8 444.8 392

47.46 38.66 47.13 14.33

955.7 753.2 231.3 282.9 207.8 207.6 122.9 33

82.45 63.99 22.73 21.97 21.19 20.99 14.96 2.91

Table 6. Rheological Measurements of KS and ND Crude Oils KS

ND

temperature viscosity yield temperature viscosity yield additive (°C) (CPs) stress (Pa) (°C) (CPs) stress (Pa) blank 18-OA18 BAUn-18 18-ABA 18-MBA

36 27 30 15 18

560.8 304.1 306.7 59.89 59.07

38.66 14.76 239.8 1.264 1.477

30 30 30 30 30

231.3 31.32 23.75 21.91 22.80

22.73 1.53 1.15 1.14 1.15

Table 7. Wax Deposit Analysis of KS and ND Crude Oils serial number

parameter

KS

ND

1 2 3 4 5 6 7 8

water (vol %) wax (wt %) asphaltene (wt %) oil (wt %) inorganics (wt %) resin (wt %) mp of wax (°C) mp of deposit (°C)

05 84.5 0.1 6.4 2.11 1.89 91 96

03 86.3 0.13 5.6 3.9 1.01 94 95

for the purpose of comparison. In previous studies, it has also been shown that polymer units containing a short-chain backbone particularly of four carbon atoms are more effective as PPD.26 Therefore, we purposely selected all of the four series of PPDs (containing a backbone of four carbon atoms) and synthesized them from easily available raw materials using a simple methodology. 2. Experimental Methods 2.1. Materials. Maleic anhydride, benzyl alcohol, undecylenic acid, and solvents were purchased from S. D. Fine Chemicals Ltd., India. n-Stearyl alcohol was received as a gift sample from Godrej Ltd., India. Oleic acid was received as a gift sample from Jayant Oil Mill Ltd., India. 2.2. Synthesis of n-Alkyl Esters. n-Steryl acrylate, n-stearyl oleate, and benzyl undecylenate were synthesized by a direct acidcatalyzed esterification method in the presence of sulfuric acid and p-toluene sulfonic acid as a catalyst using a Dean-Stark apparatus for water separation. After the calculated amount of water separation (12-15 h) and usual workup, the unreacted alcohol was removed by passing the reaction mixture through an activated alumina column. The yields were 80-85%. (26) Bharambe, D. P. Ph.D. Thesis, The Maharaja Sayajirao University of Baroda, Vadodara, India, Sept 24, 1987.

2.3. Synthesis of Copolymers. Vinylic esters (or benzylic ester) and maleic anhydride dissolved in dry benzene in a 1:1 mol ratio were taken in a three-necked flask. The contents were heated up to 70-80 °C with constant stirring and in a nitrogen atmosphere. Then, benzoyl peroxide (1 mol wt %) dissolved in 20 mL of benzene was added drop by drop to it. The whole content was refluxed at the same temperature for 18 h. Finally, the thick slurry was added into excess petroleum ether (40-60) for the precipitation of the white powdered copolymer. 2.4. Synthesis of Comb-Shape Polymer Additives. The above synthesized copolymers and stearyl alcohol were dissolved in xylene in a 1:2 mol ratio. p-Toluene sulfonic acid (1 wt %) along with 1-2 drops of sulfuric acid were used as a catalyst. The apparatus used for the synthesis of diesters consists of a three-necked flask fitted in a controlled heating mantle, a mercury seal stirrer, a Dean-Stark apparatus (for water separation), and inlet for nitrogen gas. For a better reflux and azeotrope, the proportion of xylene was kept between 20 and 25 mL/0.01 mol of the copolymer unit. At the beginning, the reaction was carried out at 145-150 °C for 15 h followed by raising the temperature slowly to 160-165 °C (removal of xylene by distillation). 2.5. Characterization. The four different polymeric diesters, namely, poly(n-stearyl oleate-co-maleicanhydride)distearate (18OA18, Figure 2a), poly(benzyl undecylenate-co-maleicanhydride)distearate (BAUn-18, Figure 2b), poly(n-stearyl acrylate-co-maleicanhydride)dibenzylate (18-ABA, Figure 2c), and poly(n-stearyl methacrylate-co-maleicanhydride)dibenzylate (18-MBA, Figure 2d), were characterized for their molecular weight (in terms of Mw and Mn) and polydispersity index using Shimadzu’s gel-permeation chromatograph equipped with a refractive index detector and polydivinylbenzene mix gel-D column. Tetrahydrofuran (THF) with a flow rate of 1 mL/min was used as a mobile phase. Polystyrene was used as the standard. The results are given in Table 1. The structure of the copolymers and diesters was confirmed by Fourier transform infrared (FTIR) spectroscopy using a Perkin-Elmer FTIR spectrophotometer IR1420. In a representative spectrum (see the Supporting Information), the presence of characteristic bands at 2918 and 2849 cm-1 (CH stretch), 1736 cm-1 (CdO stretching in aliphatic esters), 1470 cm-1 (CH3 bending), 1173 cm-1 (C-C stretching), and 722 cm-1, (-(CH2)n-) indicate the presence of hydrophobic long pendant side chains,27 confirming the comb-shape structure of the polymer. Bands at 1455, 1498, and 1587 cm-1 are because of the aromatic parts of the polymer. The structures of the basic polymer unit are given in Figure 2 2.6. Physical Testing and Analysis of Crude Oils. Wax content of the crude oils was determined using the UOP method 46-85 and other variations.28-30 The IP-143 method was used to find out the asphaltene present in the crude. The percentage of resin was (27) Suryanarayana, I.; Rao, K. V.; Duttachaudhury, S. R.; Subrahmanyam, S.; Saikia, B. K. Infrared spectroscopic studies on the interaction of pour point depressants with asphaltene, resin and wax fractions of Bombay high crude. Fuel 1990, 69, 1546–1551. (28) Burger, E. D.; Perkins, T. K.; Striegler, J. H. Studies of wax deposition in the trans Alaska pipeline. J. Pet. Technol. 1981, 33, 1075– 1086. (29) Rø´nningsen, H. P.; Bjø´rndal, B.; Hansen, A. B.; Pedersen, W. B. Wax precipitation from North Sea crude oils. 1. Crystallization and dissolution temperatures, and Newtonian and non-Newtonian flow properties. Energy Fuels 1991, 5, 895–907. (30) Sjoblom, J.; Mingyuan, L.; Hoiland, H.; Johansen, E. J. Water-incrude oil emulsions from the Norwegian continental shelf part III. A comparative destabilization of model systems. Colloids Surf. 1990, 46, 127– 139.



Figure 3. Laboratory setup for paraffin deposition measurement (coldfinger test). Table 8. Coldfinger Data for KS and ND Crude Oils KS

ND

temperature gradient (65-35 °C)

temperature gradient (60-30 °C)

additive

weight of deposit (g)

percent inhibition

mp of wax (°C)

wax content (%)

weight of deposit (g)

percent inhibition

wax content (%)

mp of wax (°C)

blank 18-OA18 BAUn-18 18-ABA 18-MBA

6.50 3.5 7.2 2.0 4.0

46.15 -15.20 66.66 38.46

60 40 50 40 46

56 47 60 42 51

6.0 4.0 4.5 4.0 5.0

33.33 25.00 33.33 16.66

45 43 44 43 45

48 47 46 47 48

determined by the method developed by Hubberd et al.31 The physical characteristics of both the crude oils are given in Table 2. Distillation characteristics of the crude oils were determined using the IP/24/84 method as shown in Table 3. Before doing any rheological measurement, the crude oil sample was conditioned at 70 °C for 30 min to erase the previous shear and thermal history of the crude oil.32,33 Pour points of the crude in the absence and presence of polymer additive were obtained using the ASTM-D/ 97 method. The results are given in Table 4. The rheological studies of these crude oils were performed on an advanced rheometer AR500 of the TA Instrument Company with a smooth stainless-steel truncated cone plate geometry (4 cm in diameter, cone angle of 2°, and truncation of 55 µm). The temperature was controlled within 0.1 °C by a peltier element inside the plate. The above conditioned crude sample was cooled below its pour point to precipitate wax, and then, a static stress was applied to it and, simultaneously, the viscosity and shear rate were measured. The same system was used to produce the shear rate versus shear stress curves at any particular temperature. The results are given in Tables 5 and 6. Analysis of the wax deposit was carried out using methods reported in the literature,34,35 and results are given in Table 7. The performance of the polymer was also tested by the coldfinger test. In this test, the existing field pipeline conditions were simulated in the laboratory using copper coil (5.5 ft length and 5 mm in diameter). A total of 800 mL of oil (with or without additive) was kept in a water bath, having the temperature near the cloud point, while the other bath was kept at oil well head temperature (35 °C for KS and 30 °C for ND). The schematic diagram of the coldfinger test assembly is shown in Figure 3. The oil was circulated at a constant flow rate (31) Hubbard, R. L.; Stanfield, K. E. Determination of asphaltenes, oils, and resins in asphalt. Anal. Chem. 1948, 20, 460–465. (32) Singh, P.; Fogler, H. S.; Nagarajan, N. R. Prediction of the wax content of the incipient wax-oil gel in a pipeline: An application of the controlled-stress rheometer. J. Rheol. 1999, 43, 1437–1459. (33) Ashbaugh, H. S.; Fetters, L. J.; Adamson, D. H.; Prud’homme, R. K. Flow improvement of waxy oils mediated by self aggregating partially crystallizable diblock copolymers. J. Rheol. 2002, 46, 763–776. (34) Musser, B. J.; Kilpatrick, P. K. Molecular characterization of wax isolated from a variety of crude oils. Energy Fuels 1998, 12, 715–725. (35) Venkatesan, R.; Singh, P.; Fogler, H. S. Delineating the pour point and gelation temperature of waxy crude oils. Soc. Pet. Eng. J. 2002, 7, 349–352.

(200 mL/min) for 3 h using a peristaltic pump then flushed with diesel oil and dried overnight. The paraffin deposit in the coil was weighed after flushing it with chilled acetone to remove loose oil if any. The percentage of wax inhibition was calculated using the formula: percentage of wax inhibition ) (B - C)/(B - A) × 100, where A is the weight of empty coil, B is the weight of copper coil plus wax deposited with untreated crude oil, and C is the weight of copper coil plus wax deposited with treated crude oil. The percentage of wax inhibition is reported in Table 8. The microscopic studies of the wax crystals have been carried out using Leitz, Laborlux 12 Pol D with a Cannon 12× zoom camera with Leica Qwil software. The neat and additive-treated crude samples were cooled near their pour points and taken on a glass slide covered by a coverslip. The change in size and shapes of wax crystals at a magnification of 25× were photographed.

3. Results and Discussion Table 2 describes the physical characteristics of both KS and ND crude oils. Figures 4 and 5 indicate that both crude oils exhibit non-Newtonian behavior with a yield stress satisfying the Bingham model. Figures 6 and 7 show the effect of the temperature on viscosities of both the crude at a constant shear rate of 10 s-1. The efficacy of the additive largely depends upon the rheological properties of the crude oil. However, crude containing asphaltenes in a comparable amount may not react with the additives in the same manner as the waxy ones. Commonly, at or below the cloud point, the wax-like part (crystalline part) of pendant chains of the chemical additive co-crystallizes with wax crystals, while the polar part (amorphous part) of the same creates a barrier to the formation of interlocking wax network as shown in Figure 8. Because of this, the shape and size of the wax crystals reduce and avoid the formation of interlocking networks, which results in the lowering of the pour point. Out of four different additives selected from four different series for this study, the first one to be discussed here is 18OA18 (Figure 2a). It has the structure similar to the ideal


Soni et al.

Figure 4. Flow behavior of KS crude oil at different temperatures.

Figure 5. Flow behavior of ND crude oil at different temperatures.

Figure 6. KS temperature ramp from 65 to 30 °C at a 10 s-1 shear rate.

Figure 7. ND temperature ramp from 65 to 30 °C at a 10 s-1 shear rate.

additive as shown in Figure 8. Its repeating unit has four pendant chains, out of which two are C18 and the remaining two are C8 chains. A 500 ppm dose of this additive can depress the pour point of KS crude up to 9 °C, while the same dose of additive reduces yield stress and viscosity to 304.1 CPs and 14.76 Pa from 560.8 CPs and 38.66 Pa, respectively, at the modified pourpoint temperature (27 °C). The 500-1000 ppm dose of any additive is permissible, while the higher doses are not economically advisable. Thus, 18-OA18 additive did not perform satisfactorily. This is also clear from shear stress Vs shear rate and viscosity curve shown in Figure 9. Therefore, to improve the performance, we decided to introduce an aromatic unit in the pendant chains of the additive.

Thus, the new additive synthesized was BAUn18 (Figure 2b) containing the benzyl group attached to the polar -COOgroup, while the remaining two aliphatic pendant C18 chains remained as they are. However, from Table 4, we can see that 500 ppm of this additive can reduce the pour point of KS crude to 6 °C only. Instead, this additive increases the yield stress drastically from 38.66 to 239.8 Pa (Figure 10 and Table 6). Richter et al.36 and Monkenbusch et al.37 explained this effect. At 39 °C, this polymer additive becomes insoluble in crude oil and forms aggregates, which precipitate out from the crude along with wax crystals without being properly adsorbed on the surface.



Figure 8. Characteristic structure of the general PPD polymer additive.

Figure 9. Rheological behavior of KS crude in the presence of 500 ppm of 18-OA18. The black continuous lines show the ideal behavior.

Figure 10. Rheological behavior of KS crude in the presence of 500 ppm of BAUn18. The black continuous lines show the ideal behavior.

Figure 11. Rheological behavior of KS crude in the presence of 500 ppm of 18-ABA. The black continuous lines show the ideal behavior.

On our further attempt to modify the structure of the additive, we introduced two aromatic units in a pendant chain and one aliphatic unit of the C18 chain. This is the case of additive 18(36) Richter, D.; Scneiders, M.; Mokenbusch, L.; Willner, L. J.; Fetters, J. S.; Huang, M. L.; Mortensen, K.; Farago, B. Polymer aggregates with crystalline cores: The system polyethylene-poly(ethylenepropylene). Macromolecules 1997, 30, 1053–1068.

ABA (Figure 2c). To our surprise, this additive worked better than the previous two. Its 500 ppm dose reduced the pour point of KS crude to 21 from 36 °C. Its same dose reduced the viscosity and yield stress of the same crude to 59.89 CPs and (37) Monkenbusch, M.; Schneiders, D.; Richter, D.; Farago, B.; Fetters, L.; Huang, J. Aggregating block copolymers as model systems to study polymer block dynamics. NuoVo Cimento 1994, 16, 747–755.


Soni et al.

Figure 12. Rheological behavior of KS crude in the presence of 500 ppm of 18-MBA. The black continuous lines show the ideal behavior.

Figure 13. Prevention of interlocking of wax crystals by polymer additives by (a) providing nucleating sites (brown) to asphaltene molecules (blue) as well as wax molecules (blue). (b) Polar parts (green) hinder the co-crystallization of both wax as well as asphaltenes.

1.264 Pa, respectively (Table 6), at 15 °C. This can even be seen from the rheogram shown in Figure 11. For the sake of comparison, we had taken methacrylic acid instead of acrylic acid to synthesize the same type of additive. This was the case of a new series of additive MBA-18 (Figure 2d). This additives differs from the Figure 2c series by methyl group branching in the backbone of the polymer. This additive behaves almost similar to 18-ABA, which can be seen from Tables 4 and 6 and also from the rheogram of Figure 12.

To evaluate the performance of the additive in the field, the field pipeline conditions were simulated in the laboratory and the coldfinger test was performed. It is clear from the Table 8 that neat KS crude oil has given a wax deposit of 6.5 g, showing a very high wax deposition tendency of the same crude oil. When crude was treated with 500 ppm dose of 18-ABA, the deposition decreased to 2 g. These results indicate that the additive 18-ABA is very effective to KS crude oil. The additive BAUn-18 shows an adverse effect because it increases the wax



Figure 14. Photomicrographs of waxes of KS and ND crude oils. (A) Neat KS crude, (B) KS crude with 500 ppm of 18-ABA, (C) neat ND crude, and (D) ND crude with 500 ppm of 18-ABA.

deposits of KS crude instead of decreasing it. This can be understood by negative percent inhibition values in Table 8. To explain the behavior of the above four synthesized additives on KS crude, we proposed that the asphaltene present in the crude serves as a natural PPD. As a result, it coprecipitates along with waxes (or preventing the aggregation of the wax crystal network by working as a surfactant).2,29 Therefore, it enhances the performance of the polymer additives containing aromatic parts as the pendant chain. Hence, it is clear that this aromatic part of the pendant chains serve as nucleating sites for asphaltenes. The planar aromatic parts of asphaltene may stabilize with planar aromatic rings of the pendant chains of the additive with attractive forces, such as π-π stacking. Two aromatic units can stabilize the asphaltene better than a single one. The aliphatic pendant chain present in the same polymer unit can serve similarly as nucleating sites for waxes present in the crude. It can be easily envisaged that the wax crystals cannot interlock with each other and form a house-of-card-type structure because of the presence of asphaltene and remain in the crude as suspended particles. A schematic diagram of the proposed mechanism is shown in Figure 13. To support our viewpoint, we selected crude oil (ND crude), which was almost similar in physical properties and rheological parameters (Tables 2 and 4) to that of KS crude, except the asphaltene content. KS crude contains 1% asphaltene, while ND crude contains 0.5%. Generally, the behavior (rheological properties and even color also) of the crude is very sensitive to

their asphaltene content. A minute difference in the percentage of asphaltenes can result in a major change of the behavior of the crude, but in case of waxes, the same does not affect very much. We can see from Table 6 that all four additives depress the pour point of ND crude from 33 to 30 °C only; i.e., they almost respond similarly. Thus, the excess asphaltene present in the KS crude definitely enhances the activity of the additives, particularly 18-ABA, and improves its performance. The microscopic images of both KS and ND wax crystals at the pour point in the absence and presence of additive are shown in Figure 14. Parts A and C of Figure 14 show the wax crystals formed in the absence of polymer (18-ABA), whereas parts B and D of Figure 14 show the wax crystals formed when 500 ppm of polymer additive was added for KS and ND crude, respectively. The wax crystals formed without polymer additive in the system are thin and feather-shaped, indicating their growth at nucleating sites. However, in the presence of polymer additive, their shapes are modified to a globular one. This observation is in agreement with that of Venkatesan et al.,2 which allowed us to conclude that the asphaltene provides local nucleating sites for the waxes to crystallize. Additive 18-ABA provides nucleating sites for both asphaltene as well as wax present in the crude simultaneously, which limits the wax crystallization process. Thus, wax cannot form strong interpenetrating networks and remains in the dispersed state, which results in reduced yield stress and improved flow of the crude oil. For ND crude, a less amount of asphaltene causes a weaker


interaction with the additive (18-ABA). Hence, the wax crystallization process could not be hindered much, and wax crystals disperse less (Figure 14d), which results in a poor performance of the additive. 4. Conclusion In search for universal flow improvers, four new comb-shape polymers were synthesized, having both aromatic and aliphatic units as pendant chains with polar functional groups. All four additives satisfy most of the requirements to act as PPD and flow improvers. The presence of asphaltenes in crude definitely affects the performance of the additive. They act as natural PPDs, collect at the surfaces of wax crystals, and hinder their growth. The additive (18-ABA) that attracts asphaltene on the surface of wax crystals performs better than any other additives. Thus, the study allowed us to conclude that the flow behavior of crude oil in the pipeline can be improved with the help of

Soni et al.

suitable polymer additives. The chemical structure of the pendant chains of polymer additives has a distinct role to play in boosting the flow. Probably, this study is one of the few that directly relates the performance to the structure of PPD. The data open a new avenue in which the structure-performance relationship finds an important role to develop an effective and economical additive. Acknowledgment. The authors are grateful to the Regional Laboratory, ONGC, Vadodara, India, for providing an advanced rheometer for rheological measurements and other facilities and to Dr. Sanjeev Kumar and Dr. Amar Ballabh for constructive and fruitful discussions. Supporting Information Available: IR spectrum of the representative PPD polymer additive. This material is available free of charge via the Internet at http://pubs.acs.org. EF8002763

Performance-Based Designing of Wax Crystal Growth Inhibitors

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