Nucleation mechanism of biodiesel derived from palm oil at low temperatures

Xiu Chen1,a, Xin Jin1, Yongbin Lai2,b, Jiamin Hu1, Junfeng Shu2, Yuqi Zhang1, Bo Wang2, Menghong Yuan1

1School of Chemical Engineering, Anhui University of Science & Technology, Huainan, Anhui, 232001, China

2School of Mechanical Engineering, Anhui University of Science & Technology, Huainan, Anhui, 232001, China

achenxiuhn@163.com, byblai@163.com

Keywords: Biodiesel, Crystallization, Nucleation

Abstract. The compositions and crystallization process at low temperature of palm-based biodiesel

(PME) are investigated. In this work, we show that PME is mainly composed of fatty acid methyl

esters of 14-24 even-numbered C atoms: C14:0-C24:0, C16:1-C22:1, C18:2, and C18:3. Palm-based biodiesel

crystallization comprises three steps, viz., forming supersaturated solution, nucleation and ester

crystal growth; the driving force for saturated fatty acid methyl esters nucleation is the degree of

supersaturation. The rate equation of nucleation is put forward. The objective of this research is to

provide theoretical support for hindering the ester crystal nucleation and growth, and improving the

cold flow properties of PME.

Introduction

Due to the fast fossil fuel depletion estimated for the next coming years, the necessity of

developing alternative fuels became an important issue. Biodiesel from vegetable oils was seen as a

solution due to the bio-renewable character. Biodiesel has several distinct advantages compared to

petrodiesel: derived from a renewable domestic resource, thus reducing dependence on and

preserving petroleum; biodegradability; reduces most regulated exhaust emissions (with the

exception of NOx); higher flash point leading to safer handling and storage; and excellent lubricity

etc. Nevertheless, its performance during cold weather may restrict its commercial viability during

cold weather in moderate climates. As ambient temperatures decrease, high-molecular weight

(C14:0–C24:0) long chain saturated fatty acid methyl esters (SFAMEs) in biodiesel nucleate form ester

crystals suspended in a liquid phase composed of shorter-chain saturated fatty acid methyl esters and

unsaturated fatty acid methyl esters (UFAMEs) [1-5]. If the fuel is left unattended in cold

temperatures for a long period of time (e.g., overnight) the solid ester crystals may lead to start-up and

operability problems.

Experimental

Materials. Palm methyl ester (PME) was prepared in laboratory. It is following the GB/T

20828-2007.

Instruments. PME was analyzed by gas chromatography-mass spectrometer (GC-MS) (Finnigan,

Trace MS, FID, USA), equipped with a capillary column (DB-WAX, 30 m × 0.25 mm × 0.25 µm).

The carrier gas was helium (0.8 mL/min). The sample injection volume was 1 µL. Temperature

program was started at 160 °C, staying at this temperature for 0.5 min, heated to 215 °C at 6 °C /min,

then heated to 230 °C at 3 °C /min, staying at this temperature for 13 min.

Ester crystal morphologies of PME at low temperatures was observed using a polarizing

microscope (Leica, DM2500P, Germany), equipped with a hot and cold stages (Linkam, LTS120,

U.K.). Temperature program was started at ambient temperature, staying at this temperature for 5 min,

the cooling rate is 0.01 °C /min, and the end temperature is 9 °C.

Advanced Materials Research Vols. 953-954 (2014) pp 183-186Online available since 2014/Jun/18 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.953-954.183

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Results and Discussion

Composition. The chromatogram of PME analyzed by GC-MS is showed in Fig. 1 and the

compositions are showed in Table 1.

Fig. 1 The gas chromatogram of palm methyl ester

Table 1 The main compositions of palm methyl ester PME C14:0 C16:0 C18:0 C20:0 C22:0 C24:0 C16:1 C18:1 C20:1 C18:2 C18:3

Content / w% 1.63 31.04 6.64 0.61 0.11 0.10 0.32 43.94 0.28 14.44 0.59

Note: Cm:n is the shorthand of fatty acid methyl ester; m means the carbon number of fatty acid; n

means the number of C=C.

From Table 1, we can see that dominate the compositions of PME is the fatty acid methyl ester

(FAME) composed by 14-24 even number carbon atoms: myristic methyl ester (C14:0), palmitic

methyl ester (C16:0), stearic methyl ester (C18:0), arachidic methyl ester (C20:0), behenic methyl ester

(C22:0), lignoceric methyl ester (C24:0), palmtoleic methyl ester (C16:1), oleic methyl ester (C18:1),

eicosenoic (C20:1), linoleic methyl ester (C18:2), and linolenic methyl ester (C18:3). The mass fraction of

SFAMEs (C14:0~C24:0) and UFAMEs (C16:1~C20:1, C18:2 and C18:3) is 40.13% and 59.57% respectively.

Nucleation. According to compositions and theirs melting points (m.p.) in Table2 [6], PME may be

considered a pseudobinary mixture consisting of high-melting-point long chain SFAMEs (C14:0-C24:0)

and low-melting-point UFAMEs (C16:1-C22:1, C18:2, and C18:3) [7].

Table 2 Melting point of fatty acid methyl ester FAME C14:0 C16:0 C18:0 C20:0 C22:0 C24:0 C18:1 C18:2 C18:3

m.p. /°C 18.5 30.5 39.1 54.5 55.0 57.0 -20.0 -35.0 -55.0

Palm biodiesel crystallization comprises three steps, viz., forming supersaturated solution,

nucleation and ester crystal growth. As ambient temperatures decrease, PME becomes supersaturated

solution. Once a sufficiently large thermodynamic driving force (the degree of supersaturation) has

been attained, nucleation can occur, whereby ester crystals are generated as a result of bringing

growth units together so that a crystal lattice can be formed. From then on, proper crystal growth can

proceed predominately in X-dimension and Y-dimension, and forming plate crystals. Ester crystal

growth of PME shows in Fig. 2.

0 2 4 6 8 10 12 14 16 18 20 22

Time (min)

0

10

20

30

40

50

60

70

80

90

100

Rela

tive A

bundance

8.85

6.11

4.16

11.242.90 15.18 21.557.20 17.96 20.0013.405.02

NL:

1.52E8

TIC MS

2011-675

184 Advanced Energy Technology

(a) 15.0 °C (b) 13.0°C

Fig. 1 Microscopic images of PME (×100)

The crystallization is the formation of a new phase in the body of the mother phase (PME). The

potential barrier which a system must overcome in order to create a (crystalline) nucleus in the mother

phase and which determines the rate of nucleation is defined by the interface energy. The nucleation

rate R[s-1

m-3

]:

= exp −∆ ∗ k⁄ exp −∆ ∗ k⁄ (1)

Where A represents the global kinetic coefficient[s-1

m-3

], A = , with N the number of

molecules per m3 [m

-3], kB the Boltzmann constant [1.380 10-

23 J K

-1], T the absolute temperature[K]

and h Planck’s constant [6.626 10-34

J s]. The ∆G∗ denotes the critical free energy of activation for

nucleation [J] and ∆G∗ activation for diffusion [J].

Conclusions

The above discussion shows that:

• Palm-based biodiesel is mainly composed of fatty acid methyl esters of 14-24 even-numbered

C atoms: C14:0-C24:0, C16:1-C22:1, C18:2, and C18:3. The mass fraction of saturated fatty acid

methyl esters (C14:0-C24:0) and unsaturated fatty acid methyl esters (C16:1-C22:1, C18:2, and C18:3)

is 40.13% and 59.57% respectively.

• Palm-based biodiesel may be considered a pseudobinary mixture consisting of

high-melting-point long chain saturated fatty acid methyl esters (C14:0-C24:0) and

low-melting-point unsaturated fatty acid methyl esters (C16:1-C22:1, C18:2, and C18:3).

Palm-based biodiesel crystallization comprises three steps, viz., forming supersaturated

solution, nucleation and ester crystal growth; the driving force for saturated fatty acid methyl

esters nucleation is the degree of supersaturation. The rate equation of nucleation is put

forward.

Acknowledgments

This research was supported by Anhui Provincial Natural Science Foundation (1408085ME109).

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Advanced Materials Research Vols. 953-954 185

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186 Advanced Energy Technology

Advanced Energy Technology 10.4028/www.scientific.net/AMR.953-954 Nucleation Mechanism of Biodiesel Derived from Palm Oil at Low Temperatures 10.4028/www.scientific.net/AMR.953-954.183

DOI References

[2] K. Gerhard: Fuel Processing Technology Vol. 86 (2005), P. 1059.

http://dx.doi.org/10.1016/j.fuproc.2004.11.002 [5] B. S. Chen, Y. Q. Sun: Biomass and Bioenergy Vol. 34 (2010), P. 1309.

http://dx.doi.org/10.1016/j.biombioe.2010.04.001