ARTICLE pubs.acs.org/EF
Exploration of the Function of Diesel Fuel Additives Influencing Flow Ya L€u,* Xiao Zhang, and Guchao Yao Research Centre of Petroleum Processing, East China University of Science and Technology, 200237 Shanghai, People’s Republic of China ABSTRACT: Precipitation of n-alkanes in diesel fuel affects their cold-fluidity properties. Diesel fluidity improvers (DFIs) decrease the cold filter plugging point (CFPP) of diesel fuels by interacting with n-alkane crystals. The relationship between fuel response to DFI treatment and some factors such as n-alkane contents and distributions was investigated. The effects of DFIs are excellent when they is used in those diesel fuels with moderate n-alkane contents. The melting point of the most efficient DFI for a diesel is similar to that of the n-alkane, which has about two carbons longer than the average carbon number in this diesel. From the X-ray diffraction (XRD) spectra, we can find that effective DFIs have the same crystal plane as n-alkane crystals in diesels and improve wax crystallinity. From above results, we deduce that the function of DFIs is that they influence flow by nucleation and adsorption and modify the growth habit of the waxes.
1. INTRODUCTION In a diesel fuel vehicle, the presence of wax crystals can cause operating difficulties by impeding flow through fuel lines and filters. The problems usually occur when starting the engine after overnight shutdown in temperatures below the cloud point of the diesel fuel. Before diesel fluidity improvers (DFIs) became available, avoidance of waxing problems required a cloud point at or below the minimum temperature expected during the winter period to minimize the likelihood of wax forming in the diesel fuel. The purpose of fluidity improvers, as the name implies, is to increase the fluidity of the fuel at low temperature when waxes are present. Diesel fuel is a complex mixture of n-alkanes, isoalkanes, aromatics, olefins, other hydrocarbons, and non-hydrocarbons. It is known that crystallization in diesel fuels is mainly due to the presence of normal or slightly branched alkanes. The mechanism of action of DFIs indicates that they improve the cold-fluidity properties of diesel by changing the crystallization behaviors of n-alkanes.1,2 The crystallization behaviors of n-alkanes were deeply researched by differential scanning calorimetry, a low-temperature optical microscope, a near-infrared (NIR) scattering technique, etc.3 7 X-ray diffraction (XRD) is a method by which the crystal structures may be determined. Few studies have been carried out to research the crystallization of wax in diesel by the XRD spectra. In this paper, the relationship between fuel response to flow-improver treatment and some factors such as n-alkane contents and distribution was investigated. In addition, the XRD spectra of DFIs, wax, and the mixture of wax and DFI was also analyzed. On the basis of these findings, the function of DFI was deduced.
DFI MJ was prepared by the co-polymerization of vinyl acetate with maleate esters in our lab. The structure of MJ is shown below.
2. MATERIALS AND METHODS
In MJ2 MJ4, the alcohols employed to obtain the maleate esters are 1-tetradecanol, 1-hexadecanol, and 1-octadecanol, respectively. The molecular weights (Mw) of MJ2 MJ4 are 12 000 15 000. Maleate ester of MJ1 is synthesized by 1-dodecanol. The Mw of MJ1 is less than 10 000. 2.2. Methods. 2.2.1. Separation of n-Alkanes from Diesels. n-Alkanes in diesel fuels were separated at room temperature by urea adduct formation with methanol as an activator. The urea adduct was collected by filtration and subsequently disintegrated by treatment with hot water.8 The supernatant of the decomposition product consisted of n-alkanes from the diesel fuels. 2.2.2. Analysis of n-Alkanes. The carbon distribution of n-alkanes is tested by a temperature-programmed gas chromatograph GC-14A and computed by the software for simulating distillation. Analysis conditions of GC-14A are as follows: j 4 1000 mm column with supporter Chromosorb W (80 100 meshes) and fixed phase 2% Dexsil-300; column temperature, 35 330 °C; programming rate, 10 °C/min; detector, flame ionization detector (FID); detector temperature, 320 °C; flow rate of N2, 50 mL/min; flow rate of H2, 50 mL/min; flow rate of air, 500 mL/min; and injecting amount, 0.2 μL. 2.2.3. XRD Spectrum. A graphite monochromator was used to select Cu KR radiation (λ = 1.542 Å) operated at 35 kV and 20 mA. The samples were scanned with a scanning rate of 4θ/min in the range of 3 60°. The scanning step of 2θ was 0.02θ. Sample preparation process: put the sample on the holder, heat the holder to 50 °C, wait for the sample to melt, cool the holder to 0 °C, and take the XRD graphite at once.
2.1. Materials. Seven indigenous diesel fuels from different crude oils were used for evaluating the performance of the synthesized polymeric additives.
Received: January 24, 2011 Revised: April 7, 2011 Published: April 07, 2011
r 2011 American Chemical Society
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Table 1. Effects of DFI MJ4 sample
TCFPP (°C)
TCFPP (°C, ωMJ4 = 0.0005)
phenomena in the sample at CFPP blocky wax sink at the bottom with a clear upper fluid
A
10
10
B
8
6
C
1
3
granular wax crystal suspending in the fluid
D
2
9
wax microcrystal suspending in the fluid
blocky wax sink at the bottom with microcrystal in the upper fluid
E
4
9
granular wax crystal suspending in the fluid
F
6
10
granular wax crystal suspending in the fluid
G
18
20
blocky wax sink at the bottom with microcrystal in the upper fluid
2.2.4. Determination of the Cold Filter Plugging Point (CFPP). The CFPP is evaluated using American Society for Testing and Materials (ASTM) D6371-2005: Standard Test Method for Cold Filter Plugging Point of Diesel and Heating Fuels.
Table 2. Relationship between n-Alkane Contents and Effects of DFIs n-alkane content added to 45 mL of dewaxed
3. RESULTS AND DISCUSSION 3.1. Effects of DFIs. In the study of the improvement in CFPP provided by DFI, it was found that the additive response for different fuel types was significantly different. The effects of DFI MJ4 to seven diesel fuels in the same dosage were presented in Table 1. We found from Table 1 that the effects of DFIs (ΔTCFPP) on diesel fuels increased from 0 to 7 °C with a decrease of the CFPP of diesel fuels from 10 to 2 °C, indicating that the effects on those diesel fuels became better. Next, with a further decrease of the CFPP of diesel fuels from 4 to 18 °C, the effects of DFIs became weaker as ΔTCFPP from 5 to 2 °C. The phenomenon above demonstrated that the effects of DFIs were related to the CFPPs of diesel fuels. When wax crystals were observed in diesel samples without DFIs at CFPP, it was known that blocky wax sank at the bottom with a clear upper fluid, demonstrated that crystals grow in the plate-like pattern, adhere, gelled large crystals, stick together, and prevent the fuel from flowing. There are some relationships between the effects of DFIs and the crystallization shape at CFPP. When DFI has no effect, such as in sample A, the crystal shape is the same as that in the diesel without DFIs, illustrating that there was no reaction between DFI and wax crystals. When DFI has a weak effect, such as in samples B and G, there is blocky wax sink at the bottom with microcrystal in the upper fluid, illustrating that DFI interacted with a fraction of the wax crystals. When DFI has effective results, such as in sample D, wax microcrystals suspending in the fluid showed that DFIs interact with the wax crystals and modify their growth habit. DFIs promote the formation of a large number of very small crystals rather than relatively few large crystals. These smaller wax crystals are more likely to pass through filter screens or form a permeable coating on the surface of finer filters. 3.2. Relationship between n-Alkane Contents and Effects of DFIs. n-Alkane content was obtained by isolating the waxes in diesel fuel content by urea dewaxing. Then, n-alkane content was added in a certain proportion to dewaxed diesel sample C. Their CFPP with or without DFI were exhibited in Table 2. When n-alkane contents decreased from 4 to 0 g, the CFPP decreased from 1 to 12 °C. When the content was 1.5 g, DFI brought the best effectiveness of the decrease of CFPP at 8 °C. However, when the wax content was as little as 0 g, the effects of DFIs tended to become weaker at only 2 °C, because DFIs have few wax crystals to interact. When there was too many waxes in
diesel (g)
TCFPP
TCFPP
ΔTCFPP
(°C, ωMJ4 = 0) (°C, ωMJ4 = 0.0005) (°C, ωMJ4 = 0.0005)
0 0.5
12 10
14 15
2 5
1.0
7
14
7
1.5
6
14
8
4.0
1
5
4
the sample, the effect of DFIs was weak too. As the fuel cloud point temperature is reached, DFIs creates a large number of nuclei to which the first separating wax molecules attach themselves and form crystals. Because there are many crystals in diesel with 4 g of n-alkane at CFPP, a DFI cannot effectively perform adsorption with so many n-alkanes. The n-alkanes, which were not adsorbed with DFIs, form larger dimension crystals and then clog the filter screen. This is one of the reasons that the CFPP of diesel with 4 g of n-alkane is difficult to be reduced by adding DFI.9 According to the discussion of sections 3.1 and 3.2, there is a relationship between n-alkane contents and the effects of DFIs. The effects of DFIs are excellent when they are used in those diesel fuels with moderate n-alkane content. The effects of DFIs used in diesels with low or high wax contents become worse. Hereby, it can be supposed that there was adsorption between DFIs and n-alkanes. 3.3. Relationship between the Melting Point and Effects of DFI. The polymers MJ2, MJ3, and MJ4 had the same structure of the main chain but different carbon number lengths in the side chain, which have different melting point ranges (MP) (Table 3). Meanwhile, Table 3 also listed their effects of decreasing CFPPs on diesels B, D, and G. The distribution of n-alkane in three diesels is in Table 4. We found from Tables 3 and 4 that the most favorable DFIs to three diesel fuels were different. The higher the average carbon number of n-alkane, the higher the MP of the most favorable DFI. The MP of MJ4 is close to that of docosane (MP is 44.4 °C). It played a good role in diesel D, whose average carbon number of n-alkane was 19.8. The MP of MJ3 is between that of nonadecane (MP is 32 °C) and eicosane (MP is 36.8 °C). It rose to an efficient effect in diesel B, whose carbon number was 17.5. The MP of the most efficient DFI for a diesel is similar to that of the n-alkane, which has about two carbons longer than the average carbon number. It is illuminated that DFIs started to come out of solution as nuclei before the main n-alkane settling temperature is reached. It is the nucleation, one of two ways in which DFI work. 2116
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Table 3. Relationship between the Melting Point and Effects of DFI ΔTCFPP in
ΔTCFPP in
ΔTCFPP in
diesel D
diesel B
diesel G
melting
DFI point (°C) (°C, ω = 0.0005) (°C, ω = 0.0005) (°C, ω = 0.0005) MJ2
