Elsevier

Fuel

Volume 272, 15 July 2020, 117666
Fuel

Full Length Article
Effects of N-containing pour point depressants on the cold flow properties of diesel fuel

https://doi.org/10.1016/j.fuel.2020.117666Get rights and content

Highlights

  • A series of N-containing copolymers were synthesized as pour point depressants (PPDs) for diesel.

  • C14MC-NVIM copolymers in diesel presented better depressing effects compared to C14MC-NVP and C14MC-NVCL.

  • C14MC-NVIM (9:1) treated diesel exhibited the highest ΔSP (26°C) and ΔCFPP (13°C) at 2000ppm.

  • The interaction mechanisms of these N-containing PPDs in diesel were investigated.

Abstract

N-containing pour point depressants (PPDs) for diesel fuel were synthesized by polymerizing tetradecyl methacrylate (C14MC) with N-vinyl-2-pyrrolidinone (NVP), N-vinylimidazole (NVIM), and N-vinylcaprolactam (NVCL) at different with molar ratios (1:1, 3:1, 6:1, 9:1, 12:1, and 15:1) through free radical polymerization. The resulting C14MC-N-containing copolymers (C14MC-NVP, C14MC-NVIM and C14MC-NVCL, respectively) were characterized by Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance (1H NMR), and Thermogravimetric analysis (TGA), and the depressive effects of the PPDs added into diesel fuel were investigated. Results showed that C14MC-NVP (12:1), C14MC-NVCL (9:1), and C14MC-NVIM (9:1) at 2000 ppm exhibited improved depressive effects, and C14MC-NVIM (9:1) at 2000 ppm exhibited the best cold flowability. That is, the cold filter plugging (CFPP) and solid points (SP) decreased by 13 °C and 26 °C, respectively. The interaction mechanisms of these N-containing PPDs in diesel were investigated by differential scanning calorimetry and polarizing optical microscopy and by using viscosity–temperature curves.

Introduction

Diesel fuel is a complex hydrocarbon mixture composed of n-alkanes C8–C30 and isoparaffins [1], [2]. At low temperatures, the solubility of n-alkanes gradually decreases and precipitates into wax crystals in diesel fuel. With the further decrease of temperature, wax crystals accumulate under the action of van der Waals forces and finally form a three-dimensional network structure, which wrap the remaining liquid diesel and further affect the cold flow properties [3], [4], [5].

One of the many methods used to solve the poor fluidity of diesel at low temperatures, adding pour point depressant (PPDs) has become the most effective and cheapest method for improving fluidity at low temperatures [6], [7]. PPDs as cold flow improvers can effectively reduce the cold filter plugging point (CFPP) and solid point (SP) of diesel by altering the sizes and crystal morphology of wax crystals, thus affording the flow ability at cold climates [8], [9]. The molecular structures of PPDs generally consist of long-chain alkanes and polar group segments. Long-chain alkanes provide nucleation sites or co-crystallize with wax crystals, and the strongly polar groups can change the growth characteristics of wax crystals and interfere with wax crystal accumulation [10].

Many types of polymeric additives have been developed as PPDs to improve the cold flow properties of diesel. These polymeric additives include ethylene-vinyl acetate copolymer [11], [12], [13], [14], [15], [16], α-olefin copolymer [17], [18], maleic anhydride polymers [19], [20], alkyl methacrylate polymer [21], and polar nitrogenous compounds [22]. The improved depressive effect is influenced by several aspects. For example, polymer molecules interact with wax crystals and result in cold flow performance when the long alkyl side chain is similar to the number of n-alkane carbons in diesel fuel [23], [24]. PPD performance can be significantly improved by adjusting the ratio between long-chain alkyl and polar groups in the side chain [25]. Meanwhile, polar groups are necessary for investigating new and efficient PPDs.

At present, copolymers with polar N-containing groups are commonly used in PPDs. Xu et al. [26] synthesized a tetradecylamine modified C14MC-MA-a copolymer as a PPD to reduce the CFPP and SP of diesel. After 1000 ppm of this copolymer was added to diesel fuel, the SP and CFPP decreased by 13 °C and 5 °C, respectively. Zhou et al. [27] used N-containing morpholine groups to prepare methacrylate-maleic anhydride-methylamidemorpholine terpolymers. When the methacrylate-maleic anhydride (1:1) additive concentration was 500 ppm, the CFPP and SP of diesel decreased by 6 °C and 18 °C, respectively. Kuzmić et al. [28] introduced pyrrolidone in acrylate-styrene-1-vinyl-2-pyrrolidone polymers as cold flow improvers for crude oil. These results showed that N compounds in PPDs considerably improve the low-temperature flow performance of oils. However, N-containing PPDs that effectively decrease CFPP remain scarce, and the mechanisms have not been sufficiently investigated.

In this study, three N-containing compounds, namely, N-vinyl-2-pyrrolidinone (NVP), N-vinylimidazole (NVIM), and N-vinylcaprolactam (NVCL) were introduced as polar monomers and polymerized with tetradecyl methacrylate (C14MC). A series of N-containing copolymers (C14MC-NVP, C14MC-NVIM, and C14MC-NVCL) was synthesized and used in improving the cold flow properties of 0# diesel fuel. The depression mechanism was then explored by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and viscosity–temperature curves and used in optimizing the effect of PPDs.

Section snippets

Materials

1-Tetradecanol, methacrylic acid, p-toluenesulfonic acid, hydroquinone, toluene, sodium hydroxide, distilled water, benzoyl peroxide, N-vinyl-2-pyrrolidinone, N-vinylimidazole, N-vinylcaprolactam, and anhydrous ethanol were purchased from Titan Scientific Co., Ltd. (Shanghai, China). Diesel fuel (0#) untreated with any other additive was provided by Sinopec Group, Shanghai, China.

Synthesis of copolymers

The synthesis routines of tetradecyl methacrylate and copolymers are shown in Fig. 1. C14MC was prepared by using

1H NMR spectra

The C14MC-NVP, C14MC-NVIM, and C14MC-NVCL typical signals of the 1H NMR spectra are provided in Fig. 3. As shown in Fig. (a), (b), (c), and (d), the H of single bondCH2single bond that was connected with the single bondCdouble bondOsingle bondOsingle bond bonds of tetradecyl methacrylate and copolymers showed that chemical shifts occurred at 4.15 and 3.93 ppm, respectively. The H of the long-chain alkyl (single bond(CH2)nsingle bond and n > 4) exhibited a chemical shift of 1.27 or 1.28 ppm, and the chemical shifts for the single bondCH3 ranged from 0.88 ppm to 1.03 ppm. As shown in Fig. (b),

Conclusion

  • (1)

    A series of N-containing copolymers were synthesized and used as the PPDs, and all of them exhibited excellent depressing effect in diesel fuel.

  • (2)

    C14MC-NVIM copolymers in diesel presented better depressing effects than C14MC-NVP and C14MC-NVCL. C14MC-NVIM (9:1) exerted the best depressing effect, and the SP and CFPP decreased by 26 °C and 13 °C at 2000 ppm, respectively.

  • (3)

    The depressive effect of the N-containing PPDs was attributed to the combined action of the N-containing polar group and long

CRediT authorship contribution statement

Taishun Yang: Investigation, Data curation, Writing - review & editing. Suya Yin: Investigation, Validation, Formal analysis. Maiying Xie: Validation, Formal analysis, Methodology. Fengfei Chen: Validation, Formal analysis, Data curation. Baoting Su: Formal analysis, Validation, Investigation. Hualin Lin: Resources, Formal analysis, Funding acquisition, Supervision. Yuan Xue: Conceptualization, Methodology, Investigation, Supervision, Writing - review & editing. Sheng Han: Conceptualization,

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was is sponsored by the National Natural Science Foundation of China (Project Number 21878188 and 21975161), Shanghai Excellent Technology Leaders Program (Project Number 17XD1424900), Chenguang Program supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (Project Number 19CGA69), Science and Technology Commission of Shanghai Municipality Project (Project Number 18090503800), Shanghai Education Development Foundation and Shanghai Municipal

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