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Impact of Alkyl Methacrylate-Maleic Anhydride Copolymers as Pour Point Depressant on Crystallization Behavior of Diesel Fuel Sheng Han,†,§ Yuping Song,‡,§ and Tianhui Ren*,§ Department of Chemical Engineering, Shanghai Institute of Technology, Shanghai, 200233, P. R. China, Research Institute of the Dushanzi Petrochemical Company, Dushanzi, 833600, P. R. China, and School of Chemistry and Chemical Engineering, Shanghai Jiao Tong UniVersity, Shanghai 200240, P. R. China ReceiVed October 27, 2008. ReVised Manuscript ReceiVed February 21, 2009
Alkyl methacrylate-maleic anhydride (RMC-MA) copolymers are among the most widely used pour point depressants (PPD). In order to develop more efficient RMC-MA copolymers, it is necessary to study the crystallization behavior of n-alkanes when adding RMC-MA into diesel fuel. In this paper, RMC-MA is prepared by the reaction of long-chain alkyl methacrylate and maleic anhydride. The diesel fuel before and after adding C14MC-MA is in situ filtered at its cold filter plugging point (CFPP) in a manual CFPP apparatus. Extensive measurements of composition variations of n-alkanes are conducted by gas chromatography and the results are compared. The experimental results show that after adding C14MC-MA, the concentration distribution of n-alkanes in the filtrate is wide and ranges from 8 to 28 carbon atoms, mainly centralizing from 10 to 19 carbon atoms. For the precipitate, the concentration distribution of n-alkanes gets richer in the lighter n-alkanes and poorer in the heavier n-alkanes. The concentration distribution of n-alkanes in the crystal solid shows a decreasing trend, especially with high carbon number n-alkanes (heavier than 20 carbon atoms). About 60% of the residual crystal solid is composed of nonparaffins such as isoparaffin, naphthene, and other components. Crystallinities of n-alkanes show a tiny decreasing trend from C8 to C20. When the carbon number of the n-alkanes is more than C20, the crystallinities of n-alkanes begin to sharply reduce with an increase of carbon number. The largest decline of crystallinity is C26 n-alkane from 38.4% to 3.4%.
1. Introduction Crystallization of n-alkanes is well-known to be responsible for the solid deposit formation of middle distillate fuels or petroleum cuts. Accumulation of these deposits constitutes a real problem for refiners and diesel fuel consumers in very cold regions.1,2 Adding pour point depressants (PPD) to lower the cold filter plugging point (CFPP) of diesel fuels has been proven to be an effective and economic way of improving the cold flow properties of the oils. Many kinds of polymers have been developed and used as PPD to influence the behavior of the paraffin crystallites formation.3-5 Among the most extensively used PPDs for waxy fuel oils are highly branched poly-Rolefins,6 alkyl esters of unsaturated carboxylic acid-R-olefin copolymers,7 ethylene-vinyl fatty acid ester copolymers,8 vinyl acetate-R-olefin copolymers,9 styrene-maleic anhydride co* Corresponding author. Tel: 86-21-64941192. Fax: 86-21-64941286. E-mail:
[email protected]. † Shanghai Institute of Technology. § Shanghai Jiao Tong University. ‡ Research Institute of the Dushanzi Petrochemical Co. (1) Mirante, F. I. C.; Coutinho, J. A. P. Fluid Phase Equilib. 2001, 180, 247–255. (2) Coutinho, J. A. P. Fuel 2002, 81, 963–967. (3) Chuanjie, W. Fuel 2005, 84, 2039–2047. (4) Wang, S. L.; Flamberg, A.; Kikabhai, T. Hydrocarbon Process. 1999, 78, 59–62. (5) Chanda, D.; et al. Fuel 1998, 77, 1163–1167. (6) Morduchowitz, A.; Bialy, J. J. US Patent No. 4022590, 1977. (7) Vander, M. P.; Bloembergen, R. H. US Patent No. 3726653, 1973. (8) Cole, E. W.; Bialy, J. J.; Sweeey, W. M. US Patent No. 3792984, 1974. (9) Miller, F. R.; Tex, H. US Patent No. 4419106, 1983.
polymers,10,11 long-chain fatty acid amides,12 and poly(n-alkyl acrylates).13,14 Of these, alkyl methacrylate-maleic anhydride (RMC-MA) copolymers offer the best effect and adaptability to most diesel fuels in lowering the CFPP. In earlier works,15 we systematically studied the structure-activity relationship between molecule structure and cold flow property of diesel fuel. The performance mechanism, however, has not yet been studied. In order to investigate the mechanism and instruct the further development of this type of product, it is necessary to acquire a good understanding of the impact of the paraffin molecules on crystallization behavior when adding RMC-MA. In this paper, tetradecyl methacrylate-maleic anhydride (C14MC-MA) copolymer is selected as the representative PPD in this experiment (since long-chain alkyl, i.e. tetradecyl, of this copolymer is close to the average carbon number of diesel fuel, it has the best effect on improving flow property at low temperature). The diesel fuel before and after adding C14MC-MA is in situ filtered at its CFPP in a manual CFPP apparatus. Intermediate octane is used as a reference, and a mathematical correction based on the system investigated is employed just as described in previous works.16,17 Under these conditions, paraffins between octane (n-C8) and octacosane (10) Ei-Gamal M. I. Ph.D thesis, University of Cario, 1985. (11) Abou, E. H.; Wedad, M. E.; Ahmed, M. M. J. Chem. Technol. Biotechnol. A 1985, 35, 241–247. (12) Ei-Gamal, M. I.; et al. Faculty Sci. Mansoura UniV. 1992, 19, 1. (13) Sweeney, W. M.; Wappingers, N. Y. US Patent No. 3904385, 1975. (14) Ei-Gamal, I. M.; Atta, A. M.; Ai-sabbagh, A. M. Fuel 1997, 76, 1471–1478. (15) Yuping, S.; et al. Fuel Process. Technol. 2005, 86, 641–650. (16) Coutinho, J. A. P.; Dauphin, C.; Daridon, J. L. Fuel 2000, 79, 607– 616. (17) Dauphin, C.; et al. Fluid Phase Equilib. 1999, 161, 135–151.
10.1021/ef800936s CCC: $40.75 2009 American Chemical Society Published on Web 03/23/2009
RMC-MA as Pour Point Depressant of Diesel Fuel
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Table 1. Physical and Chemical Characteristic of Diesel Fuel test
method
fuel
specific gravity (kg/m3 at 20 °C) kinematic viscosity (mm2/s at 40 °C) flash point (°C) cold filter plugging point (°C) solid point (°C) total S, µg/mL saturated hydrocarbon (wt %) aromatic hydrocarbon (wt %) n-paraffins average carbon number boiling distillation (°C)
SH/T0604 GB/T265 GB/T261 SH/T0248 GB/T510 SH/T0253 MS MS GC GC GB/T6536
808.1 2.062 49 -2 -9 529 83.9 16.1 41.2 14.5 152-344
Figure 1. Composition of the diesel measured by GC.
(n-C28) could be quantified. Extensive measurements of composition variations of n-alkanes are conducted by gas chromatography and the results are compared.
2. Experimental Section 2.1. The Composition of Diesel Fuel. A diesel fuel blend is derived from distilled diesel fuel and hydrotreated diesel fuel in Lanzhou, China, to evaluate the crystallization behavior of fuel and the response of the C14MC-MA copolymers. The physicochemical characteristics are given in Table 1. 2.2. Syntheses of C14MC-MA Copolymers and Evaluation Tests of Cold Flow Performance. C14MC-MA was prepared by the authors, and the synthesis method is the same as that referred to in the literature.15 The tested fuel with additive sample was prepared by mixing the above-prepared C14MC-MA with diesel fuel at a concentration of 500 ppm at 50 °C and stirred for 40 min to guarantee the complete dissolution of the PPD. The cold flow performance, solidifying point (SP), and CFPP of diesel fuel were determined according to GB/T510-83 and ASTM6371-99 standard methods, respectively. SP was determined on the BSN(S)-4 solid point instruments, and CFPP was determined on the BCL(Y)-2 cold filter plugging point instrument. The SP and CFPP of diesel fuel are -9 and -2 °C, respectively. The SP and CFPP of the diesel fuel with additive are -21 and -4 °C, respectively. The C14MC-MA copolymer obviously improves the SP and CFPP performance of diesel fuel. The change of CFPP is much less than that of SP after adding the PPD. It is considered that PPD might cocrystallize with n-alkane to reduce the size of wax crystallization and allow the smaller sized crystal through the filter, thus lowering the CFPP. If these smaller crystals lose fluidity (i.e., reached their SP), a lower temperature is needed; thus, PPD generally have a more obvious effect on SP than it does on CFPP. 2.3. Phase Separation. The phase separation, which was obtained by compression of the liquid-solid system through a filter, is carried out in the CFPP determining instrument, which is described in ASTM6371-99. A specimen of the sample is cooled under specified conditions and, at intervals of 1 °C, is drawn into a pipet under a controlled vacuum through a standardized wire 363 mesh filter. The procedure is repeated, as the specimen continues to cool to the CFPP of diesel fuel, and the two recovery phases are collected in a beaker and then weighed. The instrument was provided by Shanghai BoLi Instrument Co., Ltd. 2.4. Gas Chromatography Analysis. The GC-2010 gas chromatograph, which was purchased from the Shimadzu Corp., was used to analyze the carbon number distribution of n-alkanes in the diesel fuel, the fuel with additive, and their separation fractions (i.e., filtration and precipitate). The conditions of GC were as follows: the temperature of the capillary was procedurally raised by 5 °C/min from 120 to 280 °C. The interface temperature and injector temperature were 290 and 300 °C, respectively, and the
Figure 2. Composition of the diesel with additive measured by GC. Table 2. Mass Recovery Ratio of Phase Separation of Diesel Fuel sample
diesel fuel
diesel fuel with additive
% filtrate % precipitate % recovery ratio
97.4 2.1 99.5
97.3 0.8 98.1
diffluent ratio was 150:1. The compositions of the diesel and the diesel with additives by GC are given in Figures 1 and 2, respectively.
3. Results and Discussion 3.1. Mass Recovery Ratio of Phase Separation in Diesel Fuel. The mass percentage of the filtrate (F) and precipitate (P) in the diesel fuel and recovery ratio might be evaluated as follows: F ) WF /Wo
(1)
P ) WP /Wo
(2)
recovery ratio ) F + P
(3)
where WF, WP, and Wo correspond to the mass of filtrate, precipitate, and diesel fuel, respectively. The results are listed in Table 2. It can be seen from Table 2 that the recovery ratio of diesel fuel and fuel with additive are 99.5% and 98.1%, respectively. The recovery effect is satisfactory. 3.2. Concentration Distribution of n-Alkanes in the Fuel, Precipitate, and Filtrate before and after Adding C14MC-MA. The precipitate and filtrate, separated from diesel fuel and the fuel with C14MC-MA, were obtained through the filter. The respective concentration distributions of n-alkanes in the fuel as well as its separation fractions (i.e., filtrate and precipitate) before and after adding C14MC-MA are given in the Tables 3 and 4. It can be noted from Table 3 that the concentration distribution of n-alkanes in the filtrate is basically similar to that of the original fuel. The concentration distributions of n-alkanes are wide, ranging from 8 to 28 carbon atoms, and are mainly centralized from 10 to 19 carbon atoms. When the carbon
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Table 3. Carbon Number Distribution of n-Alkanes in Diesel Fuel, Its Precipitate, and Its Filtrate n-alkanes
diesel fuel
filtrate
precipitate
% C8 % C9 % C10 % C11 % C12 % C13 % C14 % C15 % C16 % C17 % C18 % C19 % C20 % C21 % C22 % C23 % C24 % C25 % C26
0.11 0.96 2.85 2 2.42 2.34 2.33 2.41 2.49 2.12 2.18 2.25 1.99 1.82 1.3 0.81 0.54 0.31 0.06
0.16 0.92 2.78 1.99 2.42 2.36 2.02 2.44 2.53 2.13 2.15 2.4 2.04 1.83 1.25 0.78 0.47 0.26 0.02
0.05 0.51 1.98 1.61 2.09 2.14 2.11 2.18 2.28 1.93 2.05 2.32 2.48 3.06 3.38 3.52 3.06 2.1 1.09
Table 4. Carbon Number Distribution of N-Alkanes in Diesel Fuel with Additive, Its Precipitate, and Its Filtrate n-alkanes
diesel fuel
filtrate
precipitate
% C8 % C9 % C10 % C11 % C12 % C13 % C14 % C15 % C16 % C17 % C18 % C19 % C20 % C21 % C22 % C23 % C24 % C25 % C26
0.17 0.98 2.96 2.1 2.58 2.48 2.15 2.54 2.64 2.16 2.2 2.39 2.02 1.83 1.1 0.85 0.58 0.33 0.16
0.15 0.81 2.53 1.84 2.24 2.2 1.93 2.31 2.4 2.04 2.2 2.32 2.04 1.97 1.41 0.98 0.5 0.29 0.13
0.06 0.37 1.76 1.56 2.07 2.17 2.36 2.28 2.33 1.97 2.28 2.35 2.27 2.65 2.85 2.65 2.07 1.37 0.71
number exceeds 19 carbon atoms, the concentration distributions obviously begin to decline. For the precipitate fraction of fuel, at the beginning the concentration proportion of n-alkanes increases with an increase of carbon number; when it goes through 23 carbon atoms, its concentration percentage reaches maximum and then it sharply declines with an increase of carbon number. Compared with the original fuel and its filtrate, the concentration distribution of n-alkanes in the precipitate exhibits a slight decrease in the concentration of low carbon number n-alkanes (lighter than n-C19) concomitant with an increase in the concentration of high carbon number n-alkanes (heavier than n-C19). This is consistent with general facts. Similarly, it can be seen from Table 4 that the concentration distribution of n-alkanes in the filtrate after adding additive is almost similar to that of diesel fuel. The concentration distributions of n-alkanes are also wide, ranging from 8 to 28 carbon atoms, and are mainly centralized from 10 to 19 carbon atoms. When the carbon number exceeds 19 carbon atoms, it begins to decline. For the precipitate fraction of fuel with additive, at the beginning the concentration proportion of n-alkanes increases with an increase of carbon number; when it goes through 22 carbon atoms, its percentage reaches a maximum and then it declines with an increase of carbon number. Compared with fuel and its filtrate, the concentration distribution of n-alkanes in the precipitate shows a slight decrease in the concentration of low carbon number n-alkanes (lighter than n-C19) concomitant
with an increase in the concentration of high carbon number n-alkanes (heavier than n-C19). Comparing Tables 3 and 4, the trends are similar before and after adding C14MC-MA, as both fuel and its filtrate show the same variation trend of n-alkanes; it indicates that PPD have little effect on the crystallization behavior of low carbon n-alkanes. However, there still two differences: (1) precipitate after adding additive gets richer in the lighter n-alkanes and poorer in the heavier n-alkanes than the original fuel and (2) the carbon number of n-alkanes with the largest concentration in the precipitate before and after adding additive is 23 and 22 carbon atoms, respectively. It shows a slightly decreasing trend after adding additive. This can be explained by the fact that C14MC-MA contains an oil-soluble long-chain alkyl group and a polar structure moiety in the molecular structure. The longchain alkyl group can be inserted into the wax crystal in the fuels, and the polar moiety exists on the surface of the wax crystal, thereby inhibiting the crystal lattice formation and reducing the wax’s crystal size. C14MC-MA is more prone to cocrystallization with high carbon number alkanes and reduces the concentration of heavier paraffins in precipitate so that more wax crystal can get through the filter and improve the cold flow property of diesel fuel. 3.3. Carbon Number Distribution of n-Alkanes in the Crystal Solid before and after Adding C14MC-MA. At the stage of separation, only the carbon number distribution of n-alkanes in the filtrate might be determined directly. The precipitate, which is made up of a myriad of microcrystals, tends to trap part of the liquid during the compression filtering, so the mass of the precipitate is heavier than the actual solid crystals and cannot be directly analyzed or calculated by gas chromatography. In this experiment, intermediate octane is used as a reference, and a mathematical correction based on the system investigated is employed, just as described in previous works.16-19 Under these conditions, paraffins between octane (n-C8) and octacosane (n-C28) could be quantified. Extensive measurements of composition variation are calculated by distribution coefficient Z to correct for the entrapped liquid in the precipitate, and the results are compared. The proportion Z of trapped liquid in the precipitate can be deduced from the equilibrium for the paraffin octane, which cannot freeze in the experimental temperature range due to the fact that its freezing point is -56.8 °C, Z ) XP /XF
(4)
where XP and XF are the mass fraction of octane in the precipitate and filtrate. So the mass of liquid (WL) and solid (WS) in the precipitate can be calculated by WL ) ZWF
(5)
WS ) (1 - Z)WP
(6)
in which WP corresponds to the mass of precipitate. Finally, the mass fraction of any one n-alkane XS in the solid can be obtained by using XS ) (XP - ZXF)/(1 - Z)
(7)
The solid (the solid part calculated after removing trapped liquid in the solid residue by calculations) and the entrapped (18) Moynihan, C. T.; Mossadegh, R.; Bruce, A. J. Fuel 1984, 63, 378– 384. (19) Winkle, T. L.; et al. Fuel 1987, 66, 890–896.
RMC-MA as Pour Point Depressant of Diesel Fuel
Figure 3. Distribution of concentrations of n-alkanes in crystallized solid in diesel fuel before and after adding C14MC-MA.
Figure 4. Change of the concentration of n-alkanes in diesel fuel before and after adding C14MC-MA.
liquid separated from precipitate is weighted exactly, and the proportion Z of entrapped liquid calculated by eq 4 in the precipitate was 0.31 and 0.4, respectively. It can be seen that the mass of crystal solid reduces largely in macroscopic analysis after adding C14MC-MA. When fuel lowers to its CFPP, 1.5% wax crystallizes to precipitate in the original fuel, but only 0.5% wax crystallizes in the fuel after adding C14MC-MA. This indicates that C14MC-MA copolymers could reduce the size of wax crystallization and reduce the size of the crystal through the filter. The increasing Z value implies that the smaller crystal solid could coat more liquid tightly during the compression filtration. The carbon number distributions of n-alkanes in the crystal solid separated from the original fuel and the fuel with C14MC-MA copolymers are reported in Figure 3. The result shows that the two curves are very similar. Concentrations of high carbon number n-alkanes account for the majority of total crystal solid, especially those heavier than n-C20. At the beginning, the concentration proportion of n-alkanes increased with an increase of carbon number; when it goes through 23 carbon atoms, its percentage reaches a maximum and then it declines with an increase of carbon number. In contrast with the two curves in the Figure 3, the concentration distribution of n-alkanes in the crystal solid shows a decreasing trend after adding C14MC-MA, especially high
Energy & Fuels, Vol. 23, 2009 2579
carbon number n-alkanes (heavier than C20). This implies that the C14MC-MA copolymers could reduce the high carbon number n-alkanes concentration of crystal solid in the diesel fuel, thus leading to low CFPP. A very interesting phenomenon in the experiment is observed in that the accumulative concentration proportion of n-alkanes in the solid separated from the original fuel and the fuel with additive is 40%. The data indicates that n-alkanes only account for 40% of total crystal solid. The other 60% residual crystal solid is composed of nonparaffins such as isoparaffin, naphthene, and other components. Prior studies generally focus on crystallization behavior of n-alkanes at low temperature. Little work has been done to address the response performance mechanism of nonparaffins. To better understand the performance mechanism of diesel fuel under low temperatures, it is necessary to strengthen related research work on the nonparaffins crystallization mechanism at low temperature. 3.4. Crystallinity Change of n-Alkanes in the Fuel before and after Adding C14MC-MA. The crystallinities of n-alkanes in the diesel fuel, which can help to further explain the crystallization behavior of diesel fuel at CFPP and the effect of pour point depressant C14MC-MA copolymers on the diesel fuel, are calculated by XS2 ) (XS /Xo)S
(8)
XS2 ) [XP - ZXF /(1 - Z)](1/Xo)P(1 - Z)
(9)
XS2 ) P(XP - ZXF)/Xo
(10)
in which XS2 corresponds to the crystallinity of any one n-alkane in the diesel fuel and Xo corresponds to the mass fraction of any one n-alkane in the diesel fuel samples. The results are reported in Figure 4. It can be seen from Figure 4 that the crystallinities of n-alkanes almost show no change when the n-alkanes number is less than 20, but when the n-alkanes number is more than 20 carbon atoms, it increases sharply. This indicates that the high carbon number of n-alkanes is more prone to crystallize than the low carbon number of n-alkanes. By comparing the crystallinities of n-alkanes in the diesel fuel before and after C14MC-MA, it can be observed that crystallinities of n-alkanes show a slightly decreasing trend from C8 to C20 after adding C14MC-MA. When the carbon number of the n-alkanes is more than C20, the crystallinities of n-alkanes begin to sharply reduce with the increase of carbon number. The largest decline of crystallinity is C26 n-alkanes from 38.4% to 3.4%. The reason for this is that C14MC-MA is more prone to cocrystallize with high carbon number n-alkanes, decreasing their crystallinities at low temperature and diminishing the wax’s crystal size so that more wax crystal can get through the filter. 4. Conclusions In this paper, the impact of RMC-MA on the crystallization behavior of diesel fuel is studied in detail. The following conclusions can be drawn. (1) After adding C14MC-MA, the concentration distribution of n-alkanes in the filtrate is almost similar to that of the original fuel. The concentration distributions of n-alkanes are wide and range from 8 to 28 carbon atoms, mainly centralizing from 10 to 19 carbon atoms. For the concentration distribution of n-alkanes in the precipitate, it gets richer
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in the lighter n-alkanes and poorer in the heavier n-alkanes than the original fuel. (2) After adding C14MC-MA, the concentration distribution of n-alkanes in the crystal solid shows a decreasing trend, especially with the high carbon number n-alkanes (heavier than 20 carbon atoms). The accumulative concentration proportion of n-alkanes accounts for about 40% of the total crystal solid, while the other 60% residual crystal solid is composed of nonparaffins such as isoparaffin, naphthene, and other components. (3) After adding C14MC-MA, crystallinities of n-alkanes show a slightly decreasing trend from C8 to C20. When the carbon number of the n-alkanes is more than C20, the crystallinities of n-alkanes begin to sharply reduce with an
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increase of carbon number; the largest decline of crystallinity is C26 n-alkane from 38.4% to 3.4%. Acknowledgment. This project was supported by Innovation Program of Shanghai Municipal Education Commission; Project Number 09YZ387. The authors thank the R&D Centre of PetroChina Lubricating Oil Co., Lanzhou, China, for the financial support. Note Added after ASAP Publication. After eq 10, XS2 was defined. This paper was originally posted to the Web on March 23, 2009. The paper was reposted to the Web on March 25, 2009. EF800936S
