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Article

Biodiesel as Dispersant to Improve the Stability of Asphaltene in Marine Very-Low-Sulfur Fuel Oil

by
Daping Zhou
1,
Haijun Wei
1,*,
Zhiwen Tan
2,
Shuye Xue
1,
Ye Qiu
1 and
Shen Wu
3
1
Merchant Marine College, Shanghai Maritime University, Shanghai 201306, China
2
Marine Design & Research Institute of China, Shanghai 200011, China
3
Institute of Power Plants and Automation, Shanghai Jiaotong University, Shanghai 201306, China
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(2), 315; https://doi.org/10.3390/jmse11020315
Submission received: 25 December 2022 / Revised: 13 January 2023 / Accepted: 18 January 2023 / Published: 2 February 2023
(This article belongs to the Special Issue Marine Fuels and Green Energy)
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Versions Notes

Abstract

:
Since the implementation of the sulfur cap legislation in 2020, marine very-low-sulfur fuel oil, often known as VLSFO, has become a crucial source of fuel for the contemporary shipping industry. However, both the production and utilization processes of VLSFO are plagued by the poor miscibility of the cutter fraction and the residual fraction, which can result in the precipitation of asphaltene. In this study, biodiesel was chosen as a cutter fraction to improve the stability and compatibility of asphaltene in VLSFO because of its environmental benefit and strong solubility. The average chemical structure of asphaltene derived from the marine low-sulfur fuel oil sample was analyzed using element analysis, FTIR, 1HNMR, and time-flight spectroscopy. The composition of biodiesel was analyzed using GC-MS. It was found that the asphaltene had a feature of a short side chain, low H/C ratio, high aromaticity, and a high proportion of heteroatoms. Both laboratory experiments and molecular dynamic simulations were applied to investigate the dispersion effect and mechanism compared with other dispersants. The dispersion effect of biodiesel was studied using measurements of the initial precipitation point (IPP), dispersion improvement rate, and morphology of asphaltene in the model oil. Experimental results revealed that biodiesel was fully compatible with heavy fuel oil and that it can postpone the IPP from 46% to 54% and increase the dispersion improvement rate to 35%. Molecular dynamics (MDs) simulation results show that biodiesel can form strong interactions with the fused aromatics structures and heteroatoms in the asphaltene; such interactions can increase the solubility of asphaltene and acts as a “connection bridge” to promote the dispersion effect of asphaltene molecules.

1. Introduction

On 1 January 2020, the regulations made by the International Maritime Organization (IMO) about sulfur oxide (SOx) emissions reduction from the shipping sector went into effect [1,2,3,4,5]. The sulfur limit for fuel oil used by vessels sailing in designated Emission Control Areas (ECAs) is limited to 0.10 wt.%, and for fuel oil used in non-Emission Control Areas, the sulfur content should be within 0.5% unless alternative mitigation methods are used, such as the exhaust scrubber or other similar systems [6,7]. Marine very-low-sulfur fuel oil is produced by blending a suitable amount of residual fractions with low-sulfur distillates. The primary purpose of mixing the low-sulfur cutter fraction is to reduce the sulfur content of the final products, and the ratio of cutter fraction can reach up to 30% [8,9,10]. However, asphaltene may precipitate from maritime low-sulfur fuel oil due to the inherent immiscibility with the low-sulfur distillates [11,12,13,14]. Asphaltene precipitation can cause various problems, such as blocked fuel filters and excessive wearing of engine moving parts [15,16]. Consequently, an effective cutter fraction that can inhibit the asphaltene precipitation is necessary to produce a stable low-sulfur marine fuel oil.
Many researchers endeavor to develop cost-effective and environment-friendly asphaltene inhibitors to solve the asphaltene deposition problem in fuel oil. Cheng et al. have studied the effect of maleic polymer to inhibit two different types of asphaltene extracted from heavy crude oil; they found that the initial precipitation point of both asphaltenes could be postponed with the copolymer containing PMO2-P, and the average molecular structure of asphaltenes is the one of the major factor affect the dispersion effect [17]. Saeedi et al. investigated the dispersion effect of commercial inhibitors, and they found that DBSA was the most effective inhibitor for its high inhibition efficiency and commercial availability [18]. However, most of the synthesized asphaltene dispersants can only be applied to crude oil during the storage stage at a very low percentage, and they are not suitable to be directly added into heavy fuel oil.
Biofuel is a typical kind of carbon-neutral fuel, and the development of biofuel in the marine section has drawn many researchers’ attention [19,20]. Because of its relatively high polarity and absorption capacity, biofuel can serve as an efficient asphaltene inhibitor for marine low-sulfur fuel oil production. Michael et al. studied the properties of low-sulfur fuel oil produced by biodiesel blended with heavy fuel oil. They found that the combustion properties of the fuel oil had improved significantly, and the compatibility between biodiesel and heavy fuel is satisfied [21]. Biodiesel is a kind of green diesel fuel obtained through the transesterification of vegetable oils or animal fats with methanol [22,23,24]. Waste cooking oil can also be used to produce biodiesel, and it is environmentally friendly because it recycles waste cooking oil and gives renewable energy with lower pollution. According to reports, the Bio-VLSFO currently provided in Singapore is biodiesel derived from waste cooking oil, which not only solves the problem of waste oil treatment but also avoids the consumption of vegetable oil. Fatty acid methyl esters, often known as FAMEs, are the primary components of biodiesel [25]. FAMEs are oxygen-containing polar compounds; they have both lipophilic and hydrophilic components, which enable them to function as an amphiphile and to form micelles that inhibit the asphaltene molecule from self-aggregating [26,27]. Furthermore, in comparison to fuel derived from crude oil, biodiesel demonstrates a higher cetane value and better combustion efficiency, as well as low sulfur content, toxicity, and particle emissions [28,29,30,31]. Therefore, biodiesel can help ship operators make a significant step toward meeting their carbon emissions reduction targets.
Molecular dynamic simulations can predict the macroscopic physical properties of asphaltene at the molecular level and offer a theoretical basis for researching the physicochemical properties of asphaltene inhibitors [32,33,34]. In addition, the molecular simulation can demonstrate the distribution state of asphaltene in model oil with different dispersants. Meanwhile, it can also provide different means of calculations to quantitatively characterize the strength of the interaction between biodiesel and asphaltene in real time.
In this paper, the effects of biodiesel as an additive in stabilizing marine very-low-sulfur fuel oil were researched both experimentally and theoretically. First, the average chemical structure of asphaltene was deduced. Then, UV-Vis spectrophotometry and optical microscopy were used to evaluate the effectiveness of rasphaltene inhibition. Finally, with the aid of molecular dynamics simulations, a theoretical investigation of the molecular interactions between asphaltene and biodiesel was carried out, and the interaction energies and RDF curves between biodiesel molecules and asphaltenes were also studied.

2. Experiments Section

2.1. Materials

Toluene, n-heptane, and oleic acid were supplied by Aladdin Reagent Shanghai with 99% purity; biodiesel was produced using the transesterification of wasted cooking oil; diesel oil and very-low-sulfur fuel oil were supplied by SINOPEC China. The basic properties of low-sulfur fuel oil are listed in Table 1.

2.2. Characterization of Biodiesel

Agilent 7890 gas chromatography coupled with Agilent 7000 A triple quadrupole mass spectrometer was used to analyze the composition of biodiesel. A total of 1.0 μL of biodiesel was injected under the split mode of 60:1, and the injector temperature was kept at 260 °C; nitrogen was used as carrier gas. The temperature of the oven was maintained at 70 °C for 2 min before being raised to 250 degrees Celsius at a rate of 10 °C/min for 20 min. The mass spectrometer was set to operate in the electron impact mode at 70 eV. The peaks of the biodiesel were identified using a comparison with the standard retention time and then comparing the mass spectra with the available libraries. Table 2 shows the basic properties of biodiesel.

2.3. Asphaltene Extraction and Characterization

Asphaltene was extracted from the low-sulfur fuel oil sample. First, n-heptane was mixed with fuel oil in a ratio of 40 mL per gram oil and kept at room temperature for 24 h. Next, the solid asphaltene was recovered using filtration and flushed with heptane to remove the resin. Finally, the purified asphaltene was kept in a vacuum oven at 60 °C for 1 h. The chemical structure of asphaltene was characterized using FTIR (Nicolet Nexus 470), elemental analysis (VARIO EL III), proton nuclear magnetic resonance (1HNMR) (Bruker 500), and time-of-flight mass spectrometry (TOFMS).

2.4. Model Oil Preparation

Analyzing the precipitation behaviors of asphaltene in heavy fuel oil is fairly difficult due to the complexity of the fuel oil. In this study, a series of model oils with different concentrations of asphaltene was prepared, in which toluene was selected as it is a typical solvent used in the definition of asphaltene [35]. A total of 0.25 g of the extracted asphaltene and 0.5 g of biodiesel were added to 1000 mL of toluene to prepare a stock solution in which the concentration of asphaltene and biodiesel were 250 mg/L and 500 mg/L, respectively. Ultrasonic mixing was utilized to improve the dispersion effect of asphaltene in the model oil. Then, a volume ratio of n-heptane ranging from 0 to 90% was added to the model oil. All samples were shaken for 10 min and left to settle for 24 h. Next, the asphaltene that had precipitated out of the samples was extracted using centrifugation at 4000 rpm for 30 min. Then, only the top part of each sample was taken to measure the absorbance using the UV-Vis spectrometer. In order to guarantee the experiment’s repeatability, all the tests were carried out three times.

2.5. Determination of the Asphaltene Inhibition Effect

2.5.1. Asphaltene Initial Precipitation Point (IPP)

The IPP describes the proportion of n-heptane at which asphaltene flocculent begins to precipitate out of the solution. The UV-Vis spectrophotometer was used to measure the absorbance of the asphaltene in an n-heptane toluene solution. Furthermore, the onset point of precipitation appears when the aggregation formation overcomes the effect of dilution. According to previous research [36,37,38,39], the wavelength around 280 nm can distinguish the absorption of asphaltene and solvent, so the wavelength of 280 nm was selected. The 10 mm quartz cell was used.

2.5.2. Asphaltene Dispersion Efficiency

The biodiesel dispersion and inhibition effect on asphaltene were evaluated by measuring the change of asphaltene concentration in saturated asphaltene solution before and after the addition of inhibitors. The dispersion rate R was defined to evaluate the dispersion effect of each component on asphaltene as follows [18]:
R = C - C 0 C 0 × 100 %
where: C0 indicates the saturated concentration of asphaltene mg/L before adding the dispersant, and C indicates the saturated concentration of asphaltene after adding the dispersant.
A total of 500 mL of model oil at a concentration 500 mg/L was prepared for the adsorption capacity tests, with one sample serving as the reference (without an inhibitor) and the remaining samples serving as sample oil with varying concentrations of inhibitors.

2.5.3. Optical Microscope Observation

The OLYMPUS BX51 microscope with a digital camera was used for the microscopic observation. Model oil samples with inhibitors were transferred to the glass slide and observed after diluting with 70% n-heptane. All samples were prepared at room temperature and settled for 10 min before imaging.

3. Simulation Section

3.1. Molecular Model of Asphaltene and Biodiesel

The molecular structure of asphaltene is very complicated, and there is no one particular molecular structure suitable for all MD simulations [40]. It is commonly accepted that the number of polyaromatic rings and side chains can affect the inhibition efficiency [41]. In order to ensure the simulation results can reflect the actual behavior of the inhibitor, the average asphaltene structure obtained with the experimental analysis was used in the MD simulations. Figure 1 and Table 3 present detailed information about the molecular compositions of the biodiesel.

3.2. Simulation Procedure

The molecular dynamics method was used to investigate the interaction between asphaltene and biodiesel at the molecule level, and all the simulation calculations were performed using Materials Studio (version 8.0). The Condensed-phase Optimized Molecular Potentials for Atomistic Simulation Studies (COMPASS) force field is ideal for this study since the research subjects are organic molecules. COMPASS force field, which includes bonded potential and non-bonded potential, is one of the most frequently used force fields in the petroleum industry. It can forecast the structural, vibrational, and thermophysical characteristics of a wide variety of organic and inorganic compounds, such as asphaltene and biodiesel, with great accuracy [42].
The simulation procedure was as follows: ① both the asphaltene and biodiesel model were built using the Visualizer module, and all the models were optimized for energy minimization and geometric configuration; ② the asphaltene and biodiesel molecules were placed randomly in the 3 × 3 × 3 nm simulation box; ③ the established simulation box was equilibrated for 300 ps in the NVT ensemble; and ④ the simulation box was Kinetic annealed under a Nose constant temperature hot bath; the calculation time step was 1 fs; the temperature was 298 K, and the calculation time was 2000 ps. To acquire the average value for further study, the MD simulations were run three times separately for each project.

4. Results and Discussion

4.1. Experimental Results

4.1.1. GC/MS Analysis

GC-MS was used to characterize the biodiesel [43]. The spectrum is presented in Figure 2. Six significant peaks are identified, each of which relates to different kind of fatty acid methyl ester. Table 4 shows the percentages of each component in the biodiesel, and the test results are in accordance with the reported composition of the previous research.

4.1.2. Chemical Structure of Asphaltene

Table 5 shows the element composition of asphaltene, and it is one of the critical factors determining the structural properties. It can be seen that asphaltene derived from marine heavy fuel oil is mainly composed of C and H, the total amount exceeds 80%, and it also contains different types of heroatoms such as sulfur, oxygen, nitrogen, etc. From the perspective of molecular composition, the hydrogen-carbon ratio (NH/NC) is 0.964, which means the asphaltene is a typical high aromatic condensed hydrocarbon. The sulfur content in asphaltene is much higher than the upper limit of marine fuel oil, which means that the sulfur content of the fuel oil is mainly affected by the asphaltene content. The molecular weight of asphaltene was measured using the TOF-MS method, and the result is 821. Combined with the element analysis result, the average molecular formula of the extracted asphaltene is C51.6 H49.6 O6.9 N0.7 S2.
The functional groups in the asphaltene sample were analyzed using FTIR spectroscopy. The asphaltene’s FTIR spectrum is displayed in Figure 3. The absorption peaks at 2923 cm−1 and 1614 cm−1 can be attributed to the C–H stretching vibration on the aromatic ring and the C=C skeleton stretching vibration, respectively, indicating the presence of aromatic hydrocarbon structures. The weak absorption peak at 3057 cm−1 means that asphaltene contains a highly condensed aromatic ring structure, which is consistent with the element analysis results showing a low H/C atomic ratio. Multiple absorption peaks between 650 and 910 cm−1 are linked to the out-of-plane bending C–H on the substituted aromatic ring, indicating that the aromatic ring has many substituents. The absorption peak at 813 cm−1 means that there are three adjacent aromatic hydrogens. What is more, no prominent peaks are found at 718 cm−1 and 727 cm−1, which means that there is no long-chain alkane structure in the asphaltene. The C–O ester linkage vibration is represented at the peaks at 1090 cm−1 and 1150 cm−1, which is consistent with the high oxygen concentration in the elemental analysis. Peaks presented at 2855 and 1376 cm−1 mean that there are multiple C–H bonds in-plane symmetric and asymmetric vibration in the methyl and methylene group. As asphaltene is insoluble in n-heptane, these alkane structures should exist in the form of side chains bonded to aromatics. Since the vibration peak for nitrogen-containing functional groups ranged from 1415 to 1660 cm−1, which overlaps with peaks of aromatic hydrocarbons (1430–1650 cm−1), it is difficult to identify the existence of nitrogen-containing heterocycles using FTIR analysis [44].
1HNMR spectra was used to interpret the chemical structures of asphaltene. Figure 4 shows the 1HNMR spectrum of asphaltene. The distribution of hydrogen is divided into four major integration intervals according to the chemical shift, the aromatic hydrogen HA, α hydrogen, β, and γ hydrogen on the substituents. The detailed results of each fraction are shown in Table 6. It can be seen that the β hydrogen takes the highest proportion, followed by the γ hydrogen, and the portion of α hydrogen and aromatic hydrogen are similar, indicating that the aliphatic side chains in the aromatic ring structure of the sample asphaltenes are relatively short, mainly α-methyl, α-methylene, α-methine, which agrees with the findings of the FTIR analysis.
By analyzing the 1H spectra, ratios of four kinds of hydrogen can be obtained using peak integration. Several structural parameters have been calculated according to the derivation formula in the literature; the results are shown in Table 7. The aromatic skeleton is analyzed using the carbon number and the aromatic ring number; the substitution position can be inferred by using the improved B-L method. Figure 5 shows the possible average chemical structure of asphaltene.

4.1.3. Asphaltene Initial Precipitation Point

From the previous research, we know that the concentration of the asphaltene in the solution is proportional to the absorption peak of the UV-Vis [45]. Figure 6 plots the asphaltene absorbance intensity at different n-heptane percentages in the presence and absence of various inhibitors at a concentration of 200 mg/L. Changes in absorbance are not apparent in systems with relatively low n-heptane contents. By increasing the amount of n-heptane, the absorbance decreases slowly due to the dilution effect. A noticeable decline in absorbance, which corresponds to a lower concentration of asphaltene dispersed in the solvent medium, indicates the onset of asphaltene precipitation (initial precipitation point). From the diagram, it can be seen that the IPP of the untreated asphaltene solution appears at 46% n-heptane, and it is postponed with the addition of oleic acid and biodiesel, but the IPP is shifted to a low n-heptane ratio after adding diesel oil. The IPP increases from 46% to 56% with oleic acid. In comparison, the IPP increases to 54% with biodiesel, so the dispersion and stabilization effect of oleic acid on asphaltene is slightly better than biodiesel, but diesel oil has a negative effect on asphaltene dispersion. It can significantly accelerate the precipitation of asphaltene. The dispersion effect increased with the increasing polarity of the inhibitor, and this trend can be attributed to the stronger interactions of polar functional groups between the inhibitor and asphaltene; in other words, the interaction of non-bond forces improves the stability of asphaltene in a mixed n-heptane solution.

4.1.4. Asphaltene Dispersion Efficiency

The dispersion improvement rate, R, after adding different concentrations of inhibitors to the model oil is shown in Figure 7. It can be seen that oleic acid has the highest dispersion improvement rate, biodiesel is less efficient than oleic acid, and the negative value of the dispersion rate of diesel oil indicates that it not only fails to inhibit asphaltene but also accelerates the aggregation and precipitation of asphaltene. In general, the improvement rate of asphaltene shows an increasing trend with the increase of inhibitor concentration. The change in the improvement rate in the model oil is not obvious when the biodiesel concentration is at a relatively low concentration. As the biodiesel concentration continuously increased, the asphaltene improvement rate began to increase significantly, and when the biodiesel content reached 60%, the dispersion rate reached more than 35%. This means that only when the biodiesel concentration has reached a certain level, it has a stabilizing effect on the asphaltene in the model oil. However, the amount of biodiesel added also has an upper limit, and when the content exceeds 60%, the increase rate of the improvement effect decreases significantly. The improvement rate of oleic acid can reach 85% at 30% concentration, but when the concentration exceeds 30%, the improvement effect tends to be stable and will not increase with the increase in concentration.

4.1.5. Morphology

An optical microscope is used to examine the morphology of asphaltene aggregates in the presence and absence of biodiesel and other inhibitors. From Figure 8 we can find that the size and quantity of asphaltene precipitates (black particles in Figure 8a) are quite evident in the absence of any inhibitor, and the diameter of most particles is larger than 3 μm. When diesel oil is added, a high number of asphaltene tend to grow into larger size particles by interacting with each other, which means that diesel oil cannot disperse asphaltene; instead, it accelerates the asphaltene precipitation. After adding biodiesel, the size of asphaltene precipitates reduces significantly, and particles larger than 2 μm are rarely found. The size of asphaltene particles precipitate in the presence of oleic acid is similar to biodiesel, and most of the particles are less than 2 μm. These observations from morphology are consistent with the result of the UV-Vis measurements, which confirm that the functions of ester and carboxy function groups can interact with asphaltene and inhibit its self-aggregation.

4.2. Simulation Results

4.2.1. Interaction Energies

The parameter defined as “interaction energy” (Einter) measures the intensity of the interaction between various molecules; in this study, it can be used to predict the inhibition rate of the inhibitor on the asphaltene molecules [46]. If the calculated interaction energy is a negative value, it means the inhibitor can form a stable interaction asphaltene, a larger absolute value represents a stronger mutual interaction, and the asphaltene molecules are less likely to self-aggregate in the presence of an inhibitor. It can be expressed as follows [47]:
Einter = Etotal − EA−A − EB−B
where Einter is the interaction energy between inhibitor and asphaltene, Etotal is the total energy of the inhibitor and asphaltene molecules, and EA−A and EB−B are the combined energy of the same kind of molecules, respectively.
Van der Waals and Coulombic interactions are the primary contributors to inter-molecular interactions [48]. The strength of van der Waals force is determined by the surface area and its electronic polarizability. The Coulombic interaction is affected by the polarity and the presence of heteroatoms (O, N, S) [49].
As demonstrated in Table 8, the interaction energy between asphaltene and each component in biodiesel is greater than the mutual interaction of the asphaltene–asphaltene pairs, which means that asphaltene is more likely to combine with biodiesel molecules. In addition, the interaction energy between methyl ester derived from palmitic acid, stearic acid, oleic acid, linoleic acid, oleic acid, and asphaltene decreases, respectively. This can be explained as the higher the degree of unsaturation, the more double bonds it contains and the stronger the polarity, so it can form a more stable combination with asphaltene molecules [50]. From a microscopic perspective, the primary reason that asphaltene tends to aggregate is because of its large number of aromatic rings, which can form a strong aromatic ring stacking effect, and the density of asphaltene aggregation is proportional to the contact force [51,52]. What is more, the presence of heteroatoms in the asphaltene molecules can form strong polar groups that will mutually interact, causing the asphaltene molecules to aggregate [53]. When the fuel oil solution has a high concentration of alkane fraction, it may dilute the ratio of inhibitor molecules around the asphaltene, resulting in aggregation or precipitation of the asphaltene. From previous research, we know that only physical adsorption occurred between biodiesel and asphaltene, which does not change the chemical structure of asphaltene [54]. However, it breaks the original colloidal balance of the asphaltene solution, reducing the mutual between asphaltene molecules.
In order to further analyze the effect of biodiesel and oleic acid on the colloidal structure of asphaltene, the structural state of asphaltene molecules before and after adding dispersants were analyzed, and the results are shown in Figure 9. It can be found that in the pure asphaltene solution, asphaltene is more uniformly dispersed in the whole cubic space. When biodiesel is added, asphaltene tends to move around, which means that the addition of biodiesel has inhibited the intermolecular interaction of asphaltene. After adding the oleic acid, the distribution of asphaltene molecules is more extensive, and the intermolecular distance is slightly higher than that after adding biodiesel but significantly higher than that of the asphaltene model oil without any dispersant. According to the calculation results of binding energy, the binding energy of methyl linolenate and asphaltene is the highest, and it is the component with the best dispersion effect among various components of biodiesel.

4.2.2. RDF Analysis

The radial distribution function g(r) is a curve that describes the variation in the molecular density distribution of a specific molecule with the distance from the reference particle by specifying some atoms as reference particles, which can indicate the dispersing effect of the inhibitor on asphaltenes [55,56]. Since the research object is the aggregation phenomenon of asphaltenes before and after adding dispersant, the asphaltene aggregates are the centroid of the reference particle [57]. Therefore, the formula for the calculation can be refined as the following equation [58]:
g ( r ) = d N ρ 4 π r 2 d r
In this equation, r represents the distance between the particles; ρ is the average density of the entire system; N is the total number of target atoms; T is the total simulation time; r is the radial distance of the reference particle; δr is the set distance; ΔN is the number of atoms from r to δr.
Since the asphaltene mode oil is a kind of amorphous mixture, changes in the RDF curves can be utilized to explain how asphaltenes aggregate. Agglomeration of asphaltene happens when the RDF curve peaks are noticeably higher than others. The first peak location and peak height, which are related to the agglomeration mode and aggregation probability of each component, respectively, are the most crucial characteristics of the RDF curves in this research. Figure 10 displays RDF curves for asphaltenes–asphaltene pairs in the model oil before and after adding biodiesel. In this study, we focused on the trajectories from the last 50 ps of the MD simulations.
In accordance with the real scenario of RDF curve convergence, the RDF curve of asphaltene in model oil without inhibitor exhibits apparent fluctuations in the range of 1 to 10 Å and converges to 1 when the distance r is sufficiently large. The image shows that there are three asphaltene aggregation peaks, demonstrating a high asphaltene aggregation density in this area. The assertive aggregation behavior is closely related to the existence of aromatic rings and heteroatoms in asphaltene molecules [59,60].
The illustration clearly shows that there are three obvious aggregation peaks in the asphaltene model oil; peaks within 5 Å are quite sharp, suggesting that the asphaltene aggregation tendency is high. According to previous research, the distribution peak at 5 Å corresponds to the edge-to-face stacking of asphaltenes, and this is because the aromatic ring in asphaltene molecules tends to develop a strong “π−π” stacking [61,62].
After adding biodiesel, the RDF first peak position of asphaltene has postponed from 2.3 to 3.5, and the peak value g (r) decreased from 39 to 26, which means that the molecular density has decreased. What is more, the presence of biodiesel does not change the shape of the overall RDF curve, the peak intensity of the probability density of asphaltene molecules decreased to a certain extent, and the number of peaks also increased, indicating that the aggregation behavior of asphaltene was weakened, proving that the addition of biodiesel can slow down asphaltene aggregation.

5. Conclusions

In this study, the average structure of asphaltene extracted from very-low-sulfur fuel oil was analyzed, and the effect of biodiesel on improving the stability of asphaltene was evaluated using experiments and MD simulations. Finally, we came to the following conclusions:
  • The average chemical structure of asphaltene was characterized using elemental, 1HNMR, and FTIR analysis, and the results show that asphaltene was derived from marine low-sulfur fuel oil feature with a low H/C ratio, high aromaticity, and strong polarity, which made it easy to aggregate between asphaltene molecules.
  • The composition of biodiesel produced with waste cooking oil is quite complex, and oleic acid and linoleic acid are the major fatty acids in WCO biodiesel, with 44.2 wt.% and 29.4 wt.%, respectively, followed by palmitic acid and linoleic acid.
  • In the UV-Vis analysis, the IPP value shifted from 46% to 56% after adding biodiesel, while IPP was shifted to a low n-heptane ratio with the addition of diesel oil. Through the comparison of the IPP and improvement rate of asphaltene with oleic acid, biodiesel, and diesel oil using UV-Vis, it was found that both oleic acid and biodiesel can reduce the IPP of asphaltene and improve the asphaltene effectively. The solubility of oleic acid has a certain effect, and the analysis effect of oleic acid with the strongest polarity is better than that of biodiesel, but diesel has no dispersing effect on asphaltenes and accelerates the precipitation of asphaltenes.
  • Molecular dynamics simulation results show that the interaction energy between each component in biodiesel and asphaltene is negative, and the absolute value is higher than the mutual-interaction energy of asphaltene, indicating that biodiesel molecules can be effectively adsorbed on asphaltene molecule, inhibiting the asphaltene from aggregation and precipitation.
  • Both oleic acid and biodiesel can improve the stability of asphaltene in marine fuel oil, but due to the acid limit and combustion quality, biodiesel is a better choice for VLSFO production.

Author Contributions

Conceptualization, D.Z. and H.W.; methodology, Z.T.; software, S.W.; validation, D.Z. and Z.T.; formal analysis, Y.Q.; investigation, S.X.; resources, S.X.; data curation, S.X.; writing—original draft preparation, D.Z.; writing—review and editing, H.W.; visualization, S.W.; supervision, H.W., project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Shanghai Commission of Science and Technology (STSC), grant number: 20DZ2252300.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Informed consent was obtained from all subjects involved in the study.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 11 00315 g001 550
Figure 1. Molecular structure of each component in biodiesel (carbon: grey, hydrogen: white, oxygen: red).
Figure 1. Molecular structure of each component in biodiesel (carbon: grey, hydrogen: white, oxygen: red).
Jmse 11 00315 g001
Jmse 11 00315 g002 550
Figure 2. The GC-MS chromatogram of waste cooking oil biodiesel.
Figure 2. The GC-MS chromatogram of waste cooking oil biodiesel.
Jmse 11 00315 g002
Jmse 11 00315 g003 550
Figure 3. FTIR spectrum of the asphaltene.
Figure 3. FTIR spectrum of the asphaltene.
Jmse 11 00315 g003
Jmse 11 00315 g004 550
Figure 4. 1H-NMR spectrum of VLSFO asphaltene.
Figure 4. 1H-NMR spectrum of VLSFO asphaltene.
Jmse 11 00315 g004
Jmse 11 00315 g005 550
Figure 5. The average structure of asphaltene derived from marine very-low-sulfur fuel oil (carbon: grey, hydrogen: white, oxygen: red, sulfur: yellow, nitrogen: blue).
Figure 5. The average structure of asphaltene derived from marine very-low-sulfur fuel oil (carbon: grey, hydrogen: white, oxygen: red, sulfur: yellow, nitrogen: blue).
Jmse 11 00315 g005
Jmse 11 00315 g006 550
Figure 6. Asphaltene IPP in the existence and absence of an inhibitor (inhibitor concentration is 10% wt./v).
Figure 6. Asphaltene IPP in the existence and absence of an inhibitor (inhibitor concentration is 10% wt./v).
Jmse 11 00315 g006
Jmse 11 00315 g007 550
Figure 7. The improvement rate of each dispersant at different concentrations.
Figure 7. The improvement rate of each dispersant at different concentrations.
Jmse 11 00315 g007
Jmse 11 00315 g008 550
Figure 8. The morphology of asphaltene precipitates in the absence (a) and presence of biodiesel (b), oleic acid (c), and diesel oil (d).
Figure 8. The morphology of asphaltene precipitates in the absence (a) and presence of biodiesel (b), oleic acid (c), and diesel oil (d).
Jmse 11 00315 g008
Jmse 11 00315 g009 550
Figure 9. Equilibrium structure of (a) asphaltene in model oil (b) asphaltene in model oil in the presence of biodiesel (c) asphaltene in model oil in the presence of oleic acid (Yellow is asphaltene).
Figure 9. Equilibrium structure of (a) asphaltene in model oil (b) asphaltene in model oil in the presence of biodiesel (c) asphaltene in model oil in the presence of oleic acid (Yellow is asphaltene).
Jmse 11 00315 g009
Jmse 11 00315 g010 550
Figure 10. RDF curves of asphaltene pairs (a) asphaltene model oil (b) asphaltene with biodiesel oil.
Figure 10. RDF curves of asphaltene pairs (a) asphaltene model oil (b) asphaltene with biodiesel oil.
Jmse 11 00315 g010
Table
Table 1. Basic Characterization of VLSFO.
Table 1. Basic Characterization of VLSFO.
PropertyVLSFOISO8217
Density at 15 °C/(g·mL−1)0.972Max. 1010
Flash point/°C96Min. 60
Viscosity at 50 °C/(mm2·s −1)275.2Max. 700
Sulfur content, %0.46Max. 3.5
TSA (wt.%)0.03Max. 0.1
Total acid number (mg KOH−1)0.08Max. 2.5
Table
Table 2. Basic Characterization of biodiesel.
Table 2. Basic Characterization of biodiesel.
PropertyBiodieselTest Method
Density at 15 °C/(g·mL−1)0.876ASTM D4052
Viscosity at 40 °C/(mm2·s −1)4.004ASTM D445
Sulfur content, %NAASTM D129
Flash point (°C)173ASTM D93
Pour point (°C)−1ASTM D97
Acid number (mg KOH−1)0.31ASTM D664
Free glycerine (wt.%)0.01ASTM D6584
Cetane number53ASTM D613
Table
Table 3. Diagrammatic sketches and properties of each component in the biodiesel oil.
Table 3. Diagrammatic sketches and properties of each component in the biodiesel oil.
NameMolecule ModelFormulaMolecular WeightAtom NumberH/C Ratio
Palmitic acid methyl ester Jmse 11 00315 i001C17H34O2270.457532
Stearic acid methyl ester Jmse 11 00315 i002C19H38O2298.511592
Oleic acid methyl ester Jmse 11 00315 i003C19H36O2296.495571.895
Linoleic acid methyl ester Jmse 11 00315 i004C19H34O2294.479551.789
Linolenic acid methyl ester Jmse 11 00315 i005C19H32O2292.463531.684
Table
Table 4. The chemical composition of biodiesel.
Table 4. The chemical composition of biodiesel.
PeakRetention TimeNamewt.% in Biodiesel
114.95Palmitic acid methyl ester15.9
215.71Stearic acid methyl ester1.2
317.08Oleic acid methyl ester44.2
418.61Linoleic acid methyl ester29.4
520.03Linolenic acid methyl ester3.1
Others6.2
Table
Table 5. Elemental composition of the asphaltene.
Table 5. Elemental composition of the asphaltene.
Elemental Composition (%)Molecular Weight
CHONSNH/NC
75.466.0513.411.223.810.962821
Table
Table 6. Chemical shift of asphaltene.
Table 6. Chemical shift of asphaltene.
Hydrogen TypeHydrogen DescriptionChemical ShiftRelative Amount
HADirectly linked to aromatic carbon9.0–6.013.2%
HαLinked to α carbon of aromatic ring4.5–1.914.3%
HβLinked to β carbon of aromatic rings and H on CH2 CH farther than β carbon1.9–1.055.1%
HγLinked to γ carbon of aromatic rings and H on CH2 CH farther than β carbon1.0–0.517.4%
Table
Table 7. Parameters and values of average asphaltene molecular structure.
Table 7. Parameters and values of average asphaltene molecular structure.
SymbolDefinitionResult
CTTotal carbon51.6
HTTotal hydrogen49.6
fAAromatic carbon weight ratio0.58
HAU/CAAromatic rings condensation degree0.38
σAromatic rings substitution degree0.31
CAProton aromatic carbon number30
CSAromatic carbon number of lateral branches12
CPPeripheral Aromatic carbons9
RAAromatic ring number8.5
RTTotal rings12.5
RNNaphthenic rings4
CNNaphthenic carbon12
LThe average length of substituted chain1.5
Table
Table 8. Interaction energies of different pairs at 298 K.
Table 8. Interaction energies of different pairs at 298 K.
Molecular PairsInteraction Energy/(KJ/·mol−1)
asphaltene–asphaltene−1203.16
asphaltene–palmitic acid methyl ester−1338.31
asphaltene–stearic acid methyl ester−1397.61
asphaltene–oleic acid methyl ester−1559.24
asphaltene–linoleic acid methyl ester−1736.19
asphaltene–oleic acid−1902.28
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MDPI and ACS Style

Zhou, D.; Wei, H.; Tan, Z.; Xue, S.; Qiu, Y.; Wu, S. Biodiesel as Dispersant to Improve the Stability of Asphaltene in Marine Very-Low-Sulfur Fuel Oil. J. Mar. Sci. Eng. 2023, 11, 315. https://doi.org/10.3390/jmse11020315

AMA Style

Zhou D, Wei H, Tan Z, Xue S, Qiu Y, Wu S. Biodiesel as Dispersant to Improve the Stability of Asphaltene in Marine Very-Low-Sulfur Fuel Oil. Journal of Marine Science and Engineering. 2023; 11(2):315. https://doi.org/10.3390/jmse11020315

Chicago/Turabian Style

Zhou, Daping, Haijun Wei, Zhiwen Tan, Shuye Xue, Ye Qiu, and Shen Wu. 2023. "Biodiesel as Dispersant to Improve the Stability of Asphaltene in Marine Very-Low-Sulfur Fuel Oil" Journal of Marine Science and Engineering 11, no. 2: 315. https://doi.org/10.3390/jmse11020315

APA Style

Zhou, D., Wei, H., Tan, Z., Xue, S., Qiu, Y., & Wu, S. (2023). Biodiesel as Dispersant to Improve the Stability of Asphaltene in Marine Very-Low-Sulfur Fuel Oil. Journal of Marine Science and Engineering, 11(2), 315. https://doi.org/10.3390/jmse11020315

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