Molecular Dynamics Simulation on the Process of Ultrasonic Viscosity Reduction

Abstract

In this work, through experiments and molecular dynamics simulations, it was found that the viscosity of heavy oil decreased significantly after ultrasonic treatment, and the viscosity reduction rate can be up to 60%. The simulation results show that under the action of ultrasound, the macromolecules in the heavy oil were broken into small molecular hydrocarbons accounting for 89.2% with fewer carbon numbers (<6) and simple structures, as well as small molecules containing heteroatoms. The fracture rate of different bonds in the macromolecule under the action of ultrasound was in the range of 25% to 43%. The simulation results provide a theoretical basis for the industrial application of ultrasonic viscosity reduction.

1. Introduction

Heavy oil is characterized by high viscosity, high density, and poor flowability at room temperature [1], which makes it difficult to mix in a reactor. Therefore, the key to enhancing the mixing efficiency of reactants is to reduce its viscosity. At the present time, a number of viscosity reduction methods for heavy oil have been practically applied, such as the heating method, mixed-with-thin-oil method, pour point reduction method, emulsification method, alkali addition method, microbial method, and composite viscosity reduction method [2,3,4,5,6,7,8]. These current viscosity reduction methods have some problems, such as unsatisfactory viscosity reduction effects, high energy consumption, high cost, secondary pollution, and other issues. The mechanical, cavitation, and thermal effects of ultrasonication can significantly reduce the viscosity of heavy oil. It has the advantages of simple equipment, convenient operation, mild conditions, high efficiency, low cost, and environmental friendliness [9]. Therefore, ultrasonic viscosity reduction has become a new and efficient method for heavy oil viscosity reduction.

At present, some studies on ultrasonic heavy oil viscosity reduction have been carried out. It was suggested through experiments that ultrasound can reduce the viscosity of heavy oil. Gopinath et al. [10] treated heavy gas oil with ultrasonic waves, and the viscosity was reduced by 5%. Ershov et al. [11] found that the viscosity of crude oil from the East Zhetybai and Ashchisai oilfields decreased by 35% and 42%, respectively, after ultrasonic treatment and adding chemical agents. Chakma and Berruti [12] observed that the oil viscosity decreased by 15% when ultrasound was applied, and this decrease might be due to the partial decomposition of asphaltene by ultrasonic waves. The effect of ultrasound on the viscosity reduction in heavy oil was studied. Pu et al. [13] conducted field experiments, and the results showed that an increase in ultrasonic frequency and power was conducive to the removal of asphaltene. Rahimi et al. [14] found that the viscosity of an ultraheavy oil sample diluted with light crude oil was the lowest after 10 min of ultrasonic irradiation.

Some researchers have studied the effect of ultrasound on the molecular structure of heavy oil from a microscopic point of view. Jaber et al. [15] found that the macromolecular chains of asphaltenes break under the action of microwaves and ultrasound, resulting in a light component that reduces viscosity. Daniel et al. [16] showed that the synergistic effect of ultrasonic cavitation and nanoparticle addition would destroy the viscoelastic microstructure of heavy oil. Cui et al. [17] found that under the effect of ultrasonic cavitation catalyzed by nickel oleate, the wax crystal structure in crude oil was destroyed, resulting in a decrease in viscosity.

In recent years, molecular dynamics has been used to study the crude oil properties to explore the micro composition and mechanism of crude oil. Chen et al. [18] used a molecular dynamics simulation method to prove that an electric field could reduce the viscosity of waxy crude oil by promoting the aggregation of paraffin. The reason was that aggregation caused an increase in the mean free path of molecules, such that the molecules moved more freely, causing the decrease in viscosity. Kong et al. [19] established molecular models of amino silicone oil with different structures and simulated the changes in macro-properties such as the density, viscosity, fluidity, adsorption, melting point, and boiling point of the amino silicone oil system through molecular dynamics simulation. Xu et al. [20] used molecular dynamics and atomic force microscopy to study the changes in asphalt morphology and adhesion during the relaxation process, revealing the microstructures affecting the adhesion of asphalt. Yuan et al. [21] showed, through molecular dynamics simulation, that in the oil–water interface, oil and water would affect the solubility of asphaltenes, and the structure and dynamic properties of asphaltenes in the oil–water interface and water–water interface were different.

At present, there are few studies on ultrasonic viscosity reduction by molecular dynamics simulation. To study the mechanism of ultrasonic viscosity reduction, a molecular dynamics simulation method was used in this study to study the effect of ultrasound on the viscosity reduction in heavy oil. A molecular model of heavy oil composed of asphaltenes and resins molecules was established, and molecular dynamics simulations were carried out to explore the mechanism during the process of ultrasonic viscosity reduction. The molecular simulation results were compared with the experimental results to study the influence of different factors on ultrasonic viscosity reduction and analyze the changes in heavy oil molecules after ultrasonication. In addition, the activity of bonds at different positions in heavy oil molecules under the action of ultrasonication and the difficulty of fracture were discussed. The simulation results help explain the mechanism of reducing the viscosity of heavy oil by ultrasound from the microscopic point of view.

2. Experiment

2.1. Experimental Setup

The experimental setup used in this study is shown in Figure 1. The experimental device consisted of an ultrasonic generating part, viscosity measuring part, and ultraviolet light measuring part. The ultrasonic generating part was mainly composed of a THD-T1 ultrasonic generator (Taiheda Ultrasonic Technology Co., Ltd., Dongguan, China) and ultrasonic transducer (Kemeida Ultrasonic Equipment Co., Ltd., Shenzhen, China). Ultrasonic waves generated by an ultrasonic generator acted on the heavy oil through the ultrasonic transducer. The viscosity change in heavy oil before and after ultrasonic treatment was measured with a NDJ-5S digital viscometer (Changji Geological Instrument Co., Ltd., Shanghai, China).

2.2. Experimental Methods

At room temperature (300 K), the viscosity of heavy oil before ultrasonic treatment was 3573 mPa·s, as measured with a viscometer. A 200 mL oil sample was placed in a glass beaker and put in a temperature-controlled water bath to keep the oil temperature constant. Three groups of experiments were designed to study the effects of the ultrasonic sound intensity, frequency, and temperature on the viscosity of heavy oil. The specific experimental parameters can be found in

Table 1. Operating parameters of the experiment and molecular dynamics simulation.

	Experiment			Molecular Dynamics Simulation
	Power	Frequency	Temperature	Amplitude	Frequency	Temperature
1	60 W	20 kHz	300 K	1 Å	108 kHz	300 K
	180 W	20 kHz	300 K	1.5 Å	108 kHz	300 K
	300 W	20 kHz	300 K	2 Å	108 kHz	300 K
	420 W	20 kHz	300 K	2.5 Å	108 kHz	300 K
	540 W	20 kHz	300 K	3 Å	108 kHz	300 K
2	300 W	20 kHz	300 K	2 Å	0.125 × 108 kHz	300 K
	300 W	24 kHz	300 K	2 Å	0.25 × 108 kHz	300 K
	300 W	28 kHz	300 K	2 Å	0.5 × 108 kHz	300 K
	300 W	32 kHz	300 K	2 Å	108 kHz	300 K
	300 W	36 kHz	300 K	2 Å	2 × 108 kHz	300 K
3	300 W	20 kHz	303 K	2 Å	108 kHz	303 K
	300 W	20 kHz	313 K	2 Å	108 kHz	313 K
	300 W	20 kHz	323 K	2 Å	108 kHz	323 K
	300 W	20 kHz	333 K	2 Å	108 kHz	333 K
	300 W	20 kHz	343 K	2 Å	108 kHz	343 K

in comparison with the simulated parameters. After setting the ultrasonic power, frequency, and the water bath temperature, the heavy oil was treated by ultrasonication for a certain time. After each experiment, the viscosity was measured with a viscometer, and the viscosity reduction rate was calculated.

The viscosity reduction rate was used as the standard to evaluate the ultrasonic cracking effect of the heavy oil. The viscosity reduction rate of heavy oil was calculated as follows:

$$R={\displaystyle \frac{{\eta }_1-{\eta }_2}{{\eta }_1}}\times 100\%$$

(1)

where R, η₁, and η₂ are the viscosity reduction rate, viscosity before the reaction, and viscosity after the reaction, respectively.

3. Molecular Dynamics Simulation

3.1. Molecular Models

Although the main components of heavy oil are asphaltene, resin, paraffin, and other compounds, the complex macromolecules formed by the interaction between the heavy asphaltene and resin macromolecules are the main factors leading to the high viscosity of heavy oil [22]. Therefore, to simplify the molecular model of heavy oil, asphaltene and resin, which have the most significant impact on viscosity, were selected as the main components of heavy oil for molecular modeling.

As the most complex component with the largest molecular weight in heavy oil, asphaltene is a complex macromolecule composed of a large number of nonhydrocarbon compounds with various structures. Asphaltene’s main structures include an aromatic sheet, condensed aromatic rings, functional groups of alkyl side chains, and heteroatom substituents (containing sulfur, oxygen, and nitrogen). One of the most important parameters for characterizing the molecular structure is molecular weight, and the average molecular weight of an asphaltene polymerization molecule is generally between 3000 and 10,000. In heavy oil, the molecular weight of asphaltenes is the largest, and the molecular weight of resins is second only to asphaltenes. Resin is a complex macromolecular substance composed of 4~6 methylene groups that connect aromatic rings and contains sulfur, nitrogen, and other heteroatomic substituents. The average molecular weight of resins is between 600 and 3000 [23]. The approximate structural model of asphaltene and resin (Figure 2) was obtained by infrared spectroscopy, ultraviolet spectroscopy, and fluorescence spectroscopy analysis [24]. In order to make the molecular model as close as possible to the heavy oil and easy to model and calculate, the above molecular structure model was adopted in this study.

The molecular model was established with the commercial molecular simulation software Materials Studio 17.1. The heavy oil model was composed of asphaltene molecules and resin molecules. Various component models were constructed under the 3D atomic document module, and the benzene rings and sidechains were constructed and optimized. The molecular model was determined from the heavy oil composition used in the experiment. The molecular number ratio of asphaltene and resin was 1:3, and the relative molecular mass ratio was 1:2 [25,26]. The molecular model included 5 asphaltene molecules, 15 resin molecules, and 170 water molecules. Molecules were randomly generated in a cube box of 46 Å × 46 Å × 46 Å using the Amorphous Cell module, and the energy was optimized.

3.2. Simulation Method

Molecular dynamics simulations were performed with the open source software Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS 12Dec2018) [27]. The cutoff radius was set to 16 Å, and periodic boundary conditions were used in the X, Y, and Z directions. Long-range coulombic interactions were calculated using the particle–particle–particle–mesh (PPPM) method with an accuracy value of 10⁻⁶. The main simulated force field used was the CVFF force field. In addition, an NPT ensemble was used, and the pressure was set to 0.1 MPa. The energy of the system was minimized, where the maximum number of iterations was 10,000. The system underwent a relaxation process of 200 ps to ensure that an equilibrium state was reached. The time step was 1 fs, the molecular dynamics calculation of 400 ps was performed, and the calculation results were output every 200 steps. Viscosity was calculated using the Green–Kubo method, which is enough to calculate the viscosity of the system at low pressure. The viscosity of the system was calculated as follows [28]:

$$\eta ={\displaystyle \frac{V}{3k_{\mathrm{B}}T}}{\displaystyle {\int }_0^{\infty }\mathrm{d}t{\displaystyle \sum _{\alpha }{\displaystyle \sum _{\beta }〈P_{\alpha \beta }\left(0\right)P_{\alpha \beta }\left(t\right)〉}}}$$

(2)

where V is the volume of the system, k_B is Boltzmann’s constant, T is the temperature of the system, α and β denote the directions in the Cartesian coordinate system, the sharp brackets denote the average value of the autocorrelation function in the equilibrium system, and P_αβ(t) is the shear stress component of the system in the α and β directions at time t.

Molecular configurations were visualized with Visual Molecular Dynamics (VMD 1.9.2) and Open Visualization Tool (OVITO 3.0.0) [29]. In this work, the equilibrium state of the simulation system was determined by examining the system energy. It is believed that the system approaches its equilibrium state as the energy converges. The system energy is shown in Figure 3. It was found that the total energy becomes stable and converges after 30 ps. Therefore, a simulation time of 400 ps for the system was enough.

3.3. Ultrasonic Field Imposition

Ultrasonic vibration was loaded according to the following formula. The loading speed and displacement are shown in Equations (3) and (4):

$$x=Asin{\displaystyle \frac{2\pi }{T}}t$$

(3)

$$v={\displaystyle \frac{2\pi A}{T}}\cos {\displaystyle \frac{2\pi }{T}}t$$

(4)

where A is the applied ultrasonic amplitude, and T is the applied ultrasonic vibration period.

4. Results and Discussion

4.1. Comparison of Experimental and Simulation Results

When the ultrasonic power was 300 W, the frequency was 20 kHz, and the temperature was 300 K; the viscosity change in the oil sample treated for 30 min is shown in Figure 4a. In the first 20 min, under the action of ultrasound, the viscosity decreased continuously. After 20 min, the viscosity of the heavy oil changed little. In the molecular dynamics software LAMMPS 12Dec2018, the temperature was set to 300 K, the ultrasonic amplitude was 2 Å, and the ultrasonic frequency was 10⁸ kHz. After 200 ps relaxation, the system reached equilibrium, and then ultrasound was applied to the heavy oil molecules for 300 ps. The viscosity change after ultrasonic application is shown in Figure 4b. The viscosity decreased rapidly in the first 50 ps, slowed down at 50~200 ps, and tended to be stable after 200 ps. The viscosity of the molecule was small, so only the decreasing trend of the viscosity was compared. The viscosity decreasing trend was consistent with the experimental data in Figure 4a.

To further confirm the rationality of the molecular dynamics method to simulate the ultrasonic viscosity reduction process, the experimental and molecular dynamics simulation results under different sound intensities, frequencies, and temperatures were compared. Since the simulation time and model size of molecular simulations are smaller than the actual situation, the sound intensity and frequency of molecular simulation cannot be the same as in the experiment. Therefore, to compare the results of the experiment and molecular simulation, the sound intensity and frequency of the best viscosity reduction effect in the experiment and molecular simulation and their nearby values were taken as independent variables to study the regular changes in the viscosity reduction effect under different sound intensities and frequencies. The experimental settings of the three groups are shown in Table 1. The ultrasonic interaction time of the experiment and molecular dynamics simulation were 20 min and 200 ps, respectively. The output power and amplitude of the ultrasound were directly proportional to the sound intensity so that the sound intensity could be characterized by the output power and amplitude. Since temperature has a significant influence on viscosity, the relative viscosity reduction in ultrasonic treatment and nonultrasonic treatment at the same temperature could better reflect the effect of ultrasonication on viscosity reduction.

The experimental and molecular dynamics simulation results under different sound intensities are illustrated in Figure 5a. The viscosity reduction rate increased with increasing sound intensity and decreased when the sound intensity increased to a certain extent. The ultrasonic cracking of heavy oil mainly relies on instantaneous high temperature and high pressure generated by cavitation. These extreme physical and chemical conditions disturbed the heavy oil macromolecules and reduced the viscosity of heavy oil. The intensity of ultrasonic cavitation was related to the intensity of the sound. The greater the sound intensity, the more intense the cavitation phenomenon, and the viscosity reduction effect of heavy oil was more significant. When the sound intensity reached a certain value, the cavitation was suppressed by the sound intensity and tended to be saturated. At this time, with a further increase in sound intensity, the viscosity reduction deteriorated.

According to the comparison results of the viscosity reduction rate under different ultrasound frequencies (Figure 5b), the viscosity reduction rate of ultrasonic viscosity reduction decreased with increasing frequency in the experiment, and the viscosity reduction rate of ultrasonic viscosity reduction first increased and then decreased with the increase in frequency in molecular dynamics simulation. This was because the higher the frequency of the ultrasound, the greater the energy loss during propagation. The cavitation threshold was the minimum sound pressure or sound intensity amplitude required for the liquid to produce cavitation. The ultrasonic cavitation threshold would increase with increasing frequency, and an increase in frequency would make ultrasonic cavitation more difficult to occur. Moreover, as the frequency of ultrasonic waves increased, their period shortened, leaving insufficient time for the growth and collapse of cavitation bubbles, and the cavitation effect was not fully generated. Therefore, the increase in frequency was not conducive to the occurrence of cavitation.

For temperature, the relative viscosity reduction rate of ultrasonic viscosity reduction increased with increasing temperature in both experiments and molecular dynamics simulations and decreased when it increased to a certain extent (Figure 5c). When the temperature was low, the ultrasonic cavitation threshold was higher, and the cavitation phenomenon struggled to occur. It would not cause a decrease in the viscosity of heavy oil. With increasing temperature, the cavitation threshold decreased gradually, cavitation easily occurred, and the viscosity reduction effect of ultrasonic waves increased. When the temperature reached a certain extent, the relative viscosity reduction rate decreased gradually with increasing temperature. The reason was that the increase in temperature intensifies the motion of molecules; at this time, ultrasonic waves were no longer the main reason for the heavy oil viscosity decrease. The temperature had a more significant influence on the viscosity of heavy oil at high temperatures. For the process of viscosity reduction, both temperature and ultrasound play a role, but they have a competitive relationship. Too high or too low a temperature will inhibit the viscosity reduction effect of ultrasonic waves, in which case the temperature takes the dominant role. When the temperature is moderate and there is no significant change, ultrasound will play a dominant role, so that a significant viscosity reduction effect can be realized at a general level of temperature.

4.2. Mechanism of Ultrasonic Viscosity Reduction

Figure 6 shows the total energy change in the system during molecular simulation, and Figure 7 shows the temperature change in the system. After the system energy reached equilibrium through the relaxation process, the total energy and temperature of the system rapidly increased to a higher value at the moment of applying ultrasound. In order to more obviously observe and analyze the specific changes in total energy and temperature at the moment of ultrasonic application, the time step and output time were reduced at the moment of ultrasonic application, and the frequency and amplitude of ultrasonic were increased. The total energy and temperature of the system increased to a very high value in a short action time, and then changed into a similar sinusoidal curve with the vibration cycle of ultrasound. Therefore, heavy oil molecules would be periodically affected by ultrasound.

Figure 8 shows the pressure change in the system during molecular simulation, and the pressure fluctuates continuously in the positive and negative pressure range. Under the action of ultrasound, cavitation bubbles were formed between heavy oil molecules. With the change in pressure, the positive pressure spatio-temporal cavitation bubbles shrank and the negative pressure spatio-temporal cavitation bubbles increased. The change in cavitation bubbles with the action time of ultrasound is shown in Figure 9. The high-speed jet generated when the cavitation bubble collapses acted on the macromolecules in the heavy oil, breaking the chemical bonds of the macromolecules and breaking the macromolecules. At the same time, the cavitation bubble would produce mechanical shear force on the heavy oil during the collapse process, breaking it further. Through the action of ultrasonication, the vibration speed and amplitude of particles in heavy oil would be greatly improved. The relative motion between molecules became more intense, and the friction between molecules could break the C–C bond, so as to split the complex macromolecular groups of asphaltene and gum into small molecules with a simple structure and reduce the viscosity of heavy oil.

The distance between atoms of heavy oil before and after ultrasonic treatment was obtained with molecular dynamics software LAMMPS 12Dec2018. When the distance between atoms was greater than 1.2 times the original bond’s length, the chemical bond was judged to be broken. According to the molecular simulation results, the heavy oil cracked under the action of ultrasound. Heavy components such as asphaltenes and resins in the heavy oil were converted into small molecular hydrocarbons and small molecules containing heteroatoms. Figure 10 shows the model of some asphaltenes and resins after ultrasonic treatment. The figure shows the molecular model with broken bonds removed without considering the fracture of hydrogen bonds. The maximum molecular mass of asphaltene changed from 2442 to 658 after ultrasonic treatment, and that of resin changed from 1634 to 436. The dispersion force was the force between nonpolar molecules attracted by dipole fluctuations. Among the nonpolar molecules in heavy oil, the intermolecular force was mainly the dispersion force, and the dispersion force depended on the polarizability. The molecules with large molecular mass were more polarizable as they had a more polarizable electron cloud. In addition to molecular mass, the dispersion force was also affected by the contact area, and a greater contact area led to more dipole fluctuations. With the decrease in molecular mass and contact area, the dispersion force and viscosity of heavy oil decreased. The decrease in molecular mass and contact area was due to the change in molecular structure. In total, 4 asphaltene macromolecules were decomposed into 5 aromatic hydrocarbons, 56 unsaturated hydrocarbons, 162 saturated hydrocarbons, 10 molecules containing sulfur, 2 molecules containing sulfur and nitrogen, 6 molecules containing nitrogen, and 4 molecules containing oxygen. A total of 12 resin macromolecules were decomposed into 3 aromatic hydrocarbons, 148 unsaturated hydrocarbons, 446 saturated hydrocarbons, 22 molecules containing sulfur, 2 molecule containing sulfur and nitrogen, and 10 molecules containing nitrogen. The complex macromolecules were decomposed into small molecular hydrocarbons such as aromatic hydrocarbons, unsaturated hydrocarbons, and saturated hydrocarbons with relatively simple structures, as well as small molecules containing sulfur, nitrogen, and oxygen heteroatoms, which significantly reduced the viscosity of heavy oil.

There are three sulfur atoms, two nitrogen atoms, and one oxygen atom in each initial molecular model of asphaltene and two sulfur atoms and one nitrogen atom in each initial molecular model of resin. Heteroatoms are the association nodes of some heavy oil macromolecules, and the main body of the association reaction is asphaltene, resin, and other heavy components. Heteroatoms are an important factor affecting the viscosity of heavy oil [30]. The molecular simulation results show that the fracture reaction of carbon–sulfur, carbon–oxygen, carbon–nitrogen, and other heteroatomic bonds occurred in heavy oil under the action of ultrasonication (Figure 10).

The main reason for the cracking of large molecules into small molecules in heavy oil is the carbon–carbon bond-breaking reaction of asphaltene and resin (such as the chain-breaking reaction of long-chain alkanes and side-chain reaction of aromatic ring dealkylation) caused by ultrasonic treatment, which makes large-carbon-number hydrocarbons crack into small-carbon-number hydrocarbons. After ultrasonic treatment, the molecules with a carbon number less than 6 account for 89.2%, the molecules with carbon number between 6 and 10 account for 7.36%, the molecules with carbon number between 11 and 15 account for 2.3%, and the molecules with carbon number greater than 15 account for 1.15%, as shown in

Table 2. Distribution of carbon numbers in asphaltene and resin after ultrasonic treatment.

Carbon Number	Asphaltene	Resin	Total
<C6	194	582	776
C6–C10	24	40	64
C11–C15	8	12	20
>C15	6	4	10

.

With the shortening of the carbon chain length, the relative molecular mass decreases, the molecular structure becomes simpler, the intermolecular force decreases, and the viscosity decreases. After ultrasonic treatment, the carbon number of most molecules is less than 10, and the molecular structure is relatively simple. Therefore, ultrasonication greatly reduces the viscosity of heavy oil.

In this study, the bond-breaking rate is introduced to express the difficulty of breaking bonds in macromolecules’ different positions under ultrasound. The fracture rates of bonds at different positions in asphaltene and resin macromolecules are shown in

Table 3. Fracture rates of bonds at different positions in macromolecules (bond-breaking rate = number of broken bonds/total number of bonds in this position × 100%).

Bond Location	Color	Fracture Rate/%
Common bond between benzene rings	Yellow	36.9
A bond directly connected to the common bond of the benzene rings	Red	40.4
A bond not directly connected to the common bond of the benzene rings	Blue	35.3
α position of side chain of benzene ring	Red	40.8
β position of side chain of benzene ring	Yellow	42.2
γ position of side chain of benzene ring	Blue	39.6
Methyl group	Red	42.8
Between methylene groups	Yellow	38.6
Between methyne and methylene	Blue	41.1
Quaternary carbon atom	Green	35.2
Carbon-sulfur		38.9
Carbon-nitrogen		35
Carbon-oxygen		25

.

Among the connecting bonds between benzene rings in Figure 11a, the bonds directly connected to the common bond of the benzene ring have a higher fracture rate. In contrast, the shared bonds between the benzene rings and bonds that share bonds between benzene rings that are not directly connected have a lower breaking rate. Among the side chains of the benzene ring in Figure 11b, the β position has the highest breaking rate, followed by the γ position and α position. Among the bonds at different positions on the side chain of the benzene ring in Figure 11c, the fracture rate of the methyl group and between methyne and methylene is higher, that between methylene is second, and that of quaternary carbon atom is low. Among the three kinds of heteroatoms, carbon–sulfur bonds have the highest breaking rate, followed by carbon–nitrogen bonds, and the carbon–oxygen bond fracture rate is low. The bond energy of the carbon–sulfur bond is 272 kJ/mol, that of the carbon–oxygen bond is 326 kJ/mol, and that of the carbon–nitrogen bond is 305 kJ/mol. Because the carbon–sulfur bond energy is relatively low, the carbon–sulfur bond is more likely to be broken under the action of ultrasound, which indicates that ultrasonication has a special effect on the hydrodesulfurization reaction of heavy oil. According to the fracture rate, the bond’s activity at different positions and the difficulty of fracture under the action of ultrasound can be obtained, and it helps to understand the specific effect of ultrasound on the structure of macromolecules.

5. Conclusions

In this work, ultrasonic viscosity reduction experiments and molecular dynamics simulations were carried out. Based on the results of the study, the following conclusions can be drawn:

(1)

The total energy and temperature of the system would increase rapidly after applying ultrasound to heavy oil macromolecules. The viscosity of heavy oil decreased significantly after ultrasonic treatment, and the viscosity reduction rate can be up to 60%.

(2)

Temperature and ultrasound are both effective in viscosity reduction in heavy oil, but there is a competitive relationship. A significant viscosity reduction effect can be realized at a general level of temperature.

(3)

Under the action of ultrasound, the macromolecules in the heavy oil will be broken into small molecular hydrocarbons accounting for 89.2% with fewer carbon numbers (<6) and simple structures, which significantly reduce the heavy oil’s viscosity. In addition, the fracture rate of different bonds in the macromolecule under the action of ultrasound is in the range of 25% to 43%.