Effect of poly(ethylene-vinyl acetate) pour point depressant on the cold flow properties and crystallization behavior of soybean biodiesel blends fuel

Although biodiesel-diesel blends are being widely used in diesel engines, investigations on its cold flow properties and crystallization behavior are still scarce. In this paper, poly(ethylene–vinyl acetate) (PEVA) pour point depressant and petroleum diesel were worked together to enhance the cold flow properties of soybean biodiesel. PEVA presented a better positive effect in reducing the cold filter plugging point (CFPP) of biodiesel blends. B40 treated with 1% PEVA exerted the best cold flow properties, and its CFPP was decreased by −14 °C. In addition, the crystallization behavior was changed variously. The sizes of crystals were decreased as well as the number of crystals was increased notably.


Introduction
Along with social and economic speediness development, the need for petroleum diesel fuels increases day by day.Biodiesel is an alternative diesel fuel derived from vegetable oils, animal fats, and other lipids [1,2].Interest in developing biodiesel

Experimental 2.1. Materials
Diesel fuel untreated with any other PPDs was obtained from Sinopec Group, Shanghai, China.PEVA pour-point depressant was obtained from Rohmax Corporation, Germany.Soybean oil was obtained from a supermarket (Shanghai, China), and the soybean oil biodiesel (SBD) was prepared in our laboratory by transesterification method according to previous literature [17].

Biodiesel compositions measurements
Agilent 7890A-5975c gas chromatography-mass spectrometer (GC-MS) was used to analyze the biodiesel composition [18].The GC operation conditions were as follows: HP-Innowax quartz capillary column (60 m × 0.25 mm × 0.25 μm); capillary-column temperature initially raised by 10 °C/min from 60 °C to 150 °C, and then raised by 5 °C/min from 150 °C to 230 °C; interface temperature of 250 °C; injector temperature of 250 °C; diffluent ratio of 100:1; high-purity helium carrier gas with a flow rate of 1 mL/min; and injection volume of 0.2 μL.The compositions of the prepared SBD are listed in Table 1.

CFPP measurements
CFPP corresponds to the temperature where wax crystals have agglomerated in sufficient quantity to cause a diesel fuel filter to plug.At present, CFPP is normally used locally to evaluate the cold flow property of diesel.It was determined using a SYP1022-2 multifunctional low-temperature tester (Shanghai Boli Instrument Co., Ltd.) according to ASTM D6371 standard [20].In addition, other fuel properties, such as cloud point, pour point, flash point, oxidation stability, kinematic viscosity, and acid value, were also determined, and the fuel properties of the diesel and SBD used in this work are shown in Table 2.

Crystal morphology and crystallization behavior measurements
A DSC27HP differential scanning calorimeter (Mettler Corporation, Switzerland) was used to determine the wax precipitation point.The operating conditions are as follows: 8-10 mg samples were put in standard crucibles.Transition temperatures and enthalpies were determined using a computer during the heating cycle at a scanning rate of 5 °C /min and a range of 30 °C -60 °C.A DM2500P polarizing optical microscope (Leica Microsystems, Wetzlar, Germany) equipped with a Leica DFC420C digital camera was used to determine the low-temperature phase/crystallization behavior of diesel fuel.A sample was dropped onto a slide and then observed at a cooling rate of 0.8 °C/min from 20 °C to -30 °C, and the micrographs were captured in 1°C increment under a magnification of 100× [21].
X'Pert PRD XRD system (PANalytical Corporation, Netherlands) was used to determine the lattice parameter and structure of wax crystals under the following operating conditions: tube voltage of 40 kV, tube current of 40 mA, graphite monochromator, and Cu Kα radiation (λ=1.542Å) [22].

Composition and fuel properties of SBD
The compositions and fuel properties of SBD are shown in Table 1 and Table 2.As shown in Table 1 and Table 2, SBD consists of various FAMEs with different carbon chains and saturated compositions, and the content of unsaturated FAMEs is 81.88% higher than saturated FAMEs (17.41%).All fuel properties of the prepared SBD satisfied the ASTM D6751 standards.However, due to the relatively higher contents and freezing point of saturated FAMEs, the biodiesel and its blends with petro-diesel used in a diesel engine in cold climates were always limited by their weaker cold flow properties.

Impact of PEVA on soybean biodiesel blends
The effect of various dosages of PEVA on the CFPP of soybean biodiesel blends is shown in Figure 1.As it can be seen from Figure 1, the CFPP of biodiesel blends without PEVA was stable between 0 °C and -2 °C, dosing different PEVA into various biodiesel blends present distinct-different depression effects.PEVA had essentially slight or reactive effects on the CFPP of B100 (pure biodiesel), and the CFPP of B100 was increased from -2 °C to 0 °C after treated with 1% PEVA, indicating that the sensitivity of PEVA to pure SBD was poor.After the biodiesel blends were treated with PEVA, the reduction of CFPP increased firstly and it remained almost unchanged or slightly decreased as the dosage of PEVA above 1 wt.%.The CFPP of B0 (pure diesel), B20 (20vol.%biodiesel + 80vol.%diesel), and B40 (40vol.%biodiesel + 60vol.%diesel) decreased obviously with the PEVA increased from 0.4wt.% to 1.2wt.%than that of B60 (60vol.%biodiesel + 40vol.%diesel), B80 (80vol.%biodiesel + 20vol.%diesel) and B100, and adding 1% PEVA produced the lowest CFPP.Meanwhile, the effects of PEVA on B20 and B40 were similar to those on B0.B0 treated with 1 wt.%PEVA exhibited a relatively low CFPP of -13 °C, while adding 1wt.%PEVA to B20 and B40 produced the lowest CFPP at -14 °C.It indicates that 1wt.%PEVA exhibited a better sensitivity to the biodiesel-diesel blends with a low percentage of biodiesel (B40 and B20).Therefore, given its environmental friendliness and to make full use of more biodiesel, formulated B40 with 1wt.% was considered to be the ideal biodiesel blends with the best cold flow properties.

DSC analysis
DSC can quantitatively analyze energetic changes in the phase-change process in biodiesel blends with PEVA.The starting temperature of peak (onset) in a curve reflects the starting temperature of precipitation of crystals, the slope of peak reflects the rate of precipitation in diesel, and the solid-liquid phase-change energy (ΔH) reflects the stability of the dispersion.The DSC curves and analyses of B0, B40, and B100 with 1% PEVA are shown in Figure 2 and Table 3, respectively.
Table 3 shows that the onset of the peak of B40 was -12.05 °C between B0 (-11.67 °C) and B100 (-3.98 °C).Two crystals precipitated at a low temperature, and peak temperature was consistent with that of onset temperature, indicating the time of three samples from onset to the peak were not very different.The absolute value of ΔH of B40 was the smallest (0.4320J.g -1 ), indicating that the solid-liquid phase-change energy of biodiesel blends was smaller and dispersion was more stable.The crystallization-peak area of B40 was the smallest (3.348), indicating that crystal content was the least.The crystallization-peak slope further revealed that the crystallization rate of B100 was fast, the crystallization rate of B0 was slow, and the crystallization rate of B40 was the slowest.Thus, the CFPP of PEVA-supplemented B40 was lower than that of other biodiesel blends.

POM analysis
POM has proven to be an effective method that can be used to observe changes in wax crystals in the biodiesel blends with and without PEVA [23,24].The crystal morphology and crystallization behavior of untreated and 1% PEVA treated B0, B40, and B100 were observed by POM at -5 °C and -15 °C, and are shown in Figure 3.
Figure 3 shows dense net-like wax crystals in B100 (Figure 3a 1 ), needle-like wax crystals in B0 (Figure 3b 1 ), and grain shaped crystals in smaller sizes at -5 °C (Figure 3c 1 ).In the presence of PEVA, the crystallization behavior was changed variously.The sizes of crystals were decreased as well as the number of crystals was increased notably (Figure 3d 1, e 1 and f 1 ).The crystals in treated B100 became more regular but intensive-huddled, resulting in the deviation of CFPP (Figure 3d 1 ).The shapes of crystals in treated B0 (Figure 3e 1 ) and B40 (Figure 3f 1 ) changed from needle-and grain-like to fine granules, whereas the size of crystals in treated B40 was far less and the quantity was much more than those in treated B0.Following the temperature drops to -15 °C , larger and more crystals were appeared and aggregated in three-dimensional network structures in both untreated and treated B100, B0, and B40, thus losing their flowability at low temperature (Figure 3 a 2~f2 ).However, two kinds of crystals from biodiesel and diesel precipitated together at low temperatures, one crystal scattered around another crystal and produced some exclusion, thereby inhibiting formation and growth of large and stripe-like crystals by cocrystallization (Figure 3f 1 and f 2 ).Ultimately, the PEVA treated B40 presented a lower CFPP.

XRD analysis
Low-temperature XRD can be used to analyze the lattice parameters and structure of wax crystals in biodiesel [25].The low-temperature XRD of 1% PEVA treated B0, B40, and B100 at -10 °C are shown in Figure 4.
As it can be seen from Figure 4, the diffusion peak at 10°-30° was quite obvious in B0/PEVA and B100/PEVA.Sharp orthorhombic diffraction peaks were observed at 21.5° and 23.8°, and sharp monoclinic diffraction peaks were observed at 41.3°, 42.9°, 44.0°, 46.2°, 50.0°, 51.3°, and 52.3°, indicating that amorphous and crystalline wax precipitated at low temperatures.The broad of diffusion peaks of B0/PEVA was the first, next was that of B100/PEVA followed by B40/PEVA, so the content of amorphous-wax crystals in pure diesel was more than that in pure biodiesel.
As shown in Table 4 and Table 5, the crystallization-peak areas of three diesels were in the order B0 (136082) > B100 (100045) > B40 (85001), indicating that crystal content in treated B40 decreased at the same temperature.The ratio of orthorhombic to monoclinic peak area in B40/PEVA was 0.14, which is lower than that of 0.15 in B0/PEVA and 0.43 in B100/PEVA.In Table 5, A 0 refers to the total area of crystallization peak (10.0°−60.0°),A 1 refers to the total area of sharp monoclinic diffraction peaks ranges from 40.0° to 60.0°, and S refers to the relative area of orthorhombic area to monoclinic area.This finding indicates that PEVA changed the ratio of two types of crystals by cocrystallization.For another, the ratio of monoclinic peak area (A 1 ) to A 0 in B40/PEVA (0.87) was higher than that of B0/PEVA (0.86) B100/ PEVA (0.69).As the aggregation and growth of monoclinic crystals are known to be difficult the crystal size in the PEVA treated B40 was smaller and more easily passed through the filter, resulting in a lower CFPP.

Effect of PEVA on the fuel properties of biodiesel blends
In summary, B40 treated with 1% PEVA presents the best cold flow properties.Table 6 shows the fuel properties of B40, and 1% PEVA treated B40.As it can be seen from Table 6 and Table 2, the fuel properties of B40 are different from that   of B0 and B100 due to their huge differences in compositions.After being treated with 1% PEVA, the CFPP of PEVAsupplemented B40 is rand the CP and PP are reduced from 0 °C to -14 °C, and the CP and PP are reduced from 1 °C and -4 °C to -3 °C and -17 °C , respectively.For another, the presence of PEVA in B40 appeared to have no significant effect on other fuel properties of B40, and the flash point, oxidation stability, kinematic viscosity, acid value of B40 just occur a slight increase.

Conclusions
(1) The effects of PEVA on the cold flow properties of soybean biodiesel blends were studied; the CFPP of PEVAsupplemented B40 was lower than that of other blends.B40 treated with 1% PEVA presents the best cold flow properties.
(2) Performance mechanisms of PEVA in soybean biodiesel blends are described as follows: PEVA in B40 effectively lowered the crystallization rate of wax crystals, changed the process of crystal growth by cocrystallization, and many small spherical wax crystals existed in B40 that inhibited the formation and growth of the larger crystal, therefore, the CFPP of PEVA-supplemented B40 was lower than that of untreated B40.The crystal size in the PEVA treated B40 was smaller and more easily passed through the filter, resulting in a lower CFPP.

Figure 1 .
Figure 1.Effect of PEVA on the CFPP of soybean biodiesel blends.

Table 1 .
The compositions of SBD.