The Effects of Ultrasonic Treatment on the Molecular Structure of Residual Oil

Pelayo Envo Esono Maye; Yang Jingyi; Yan Taoyan; Xu Xinru

(State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237)

1 Introduction

The oil industry today faces tremendous challenges due to the increasing demand for gasoline and diesel fuel along with more stringent environmental regulations.The gradual decline of conventional petroleum resources has forced refiners to process residual oil[1]. However,the use of residual oil causes other serious problems such as the deactivation of catalysts caused by the formation of coke.

To provide optimal processing conditions for the effective conversion of residual oil, it is essential to elucidate their structural characteristics. Residual oil is an extremely complex mixture of various compounds[2-4].Thus, ultrasonic treatment is considered an effective way to transform residual oil to facilitate secondary processing in oil refineries. It is also an energy-efficient method of intensifying the effects of physicochemical processing[5]. Ultrasonic treatment produces the thermal scission of the bonds of heavy oil and the generation of hydrogen atoms, which are significant for upgrading heavy molecules. Moreover, carbon residue is correlated with unstable hydrocarbons, condensed aromatic hydrocarbons, non-hydrocarbons in oils, and especially asphaltenes and resins, reflecting the tendency of coke formation during residual oil processing. In catalytic cracking, carbon residue, as an important control index for feedstock, can affect the distribution of products: the more the carbon residue, the more the coke deposition along with lower liquid yields. Fortunately, ultrasonic treatment can reduce asphaltene and resin content, so that the carbon residue is reduced[6]. This measure can change the resin and asphaltene content and the structure of residual oil after ultrasonic treatment and affect the content of carbon residue and processing performance of oil, which are mainly derived from compounds with condensed rings and heterocyclic structure in resins and asphaltenes[7]. Ultrasonic treatment can also improve the stability of the colloidal system of asphaltenes to a certain extent[8].

The effects of ultrasonic treatment in upgrading residual oil are gaining growing attention[9-10]. The studies of Sun,et al.[11]showed that ultrasonic treatment has a significant impact on changes in residual oil properties. Gopinath,et al.[12]adopted fl ow tests and displacement experiments in order to treat heavy gas oil with ultrasonic energy for producing lighter gaseous hydrocarbons. Wang, et al.[13]studied the effects of ultrasonic treatment on the properties of petroleum coke oil slurry and discovered that the apparent viscosity was reduced as the duration of ultrasonic treatment increased.

The investigation performed by Mousavi, et al.[14]regarding the effect of ultrasonic treatment on the rheological properties of residual oil showed that the viscosity of residual oil was reduced by 10.98%. It also showed that the frequency and power of ultrasonic radiation at different controlled temperatures decreased oil viscosity and reduced the asphaltene content through dehydrogenation and cracking reactions[6,15]. The significance of frequency, power, and temperature is unclear. Although the applications of ultrasonic treatment for heavy oil have been widely studied, their effects on the physicochemical properties and structural parameters of residual oil remain unclear.

Therefore, the goal of this paper is to study the effects of ultrasonic radiation on the structural changes and physical properties of vacuum residue by1H-NMR, FT-IR, with the changes in SARA fraction and structural parameters calculated. The upgrading of residual oil was conducted at different duration of ultrasonic radiation with an ultrasonic horn reactor, while the pyrolysis kinetics was calculated by means of the activation energy, and the pyrolysis kinetics of heavy oils was also observed.

2 Experimental

2.1 Feedstock

The residual oil used in this study was obtained from the PetroChina’s Urumqi Petrochemical Company. Some important properties of the residual oil are shown in Table 1. Neutral aluminum oxide (FCP) was purchased from the Sinopharm Chemical Reagent Co., Ltd. Petroleum ether(AR), xylene (AR), toluene (AR), normal heptane (AR)and ethanol (AR) were purchased from the Shanghai Lingfeng Reagent Co., Ltd.

Table 1 Properties of the feedstock

2.2 Experimental procedure

The upgrading of residual oil was done with an ultrasonic horn reactor over different time intervals (0, 5, 7, 9, and 11 minutes, respectively). The reactor consisted of an ultrasonic generator with a frequency of 40 kHz and a total supplied power input of 800 W at 70 °C. 150 mL of vacuum residue were placed in a 250-mL breaker and directly exposed to ultrasonic waves. After ultrasonic treatment, the oil sample was cooled down to ambient temperature (25 °C) and prepared for the following experiments.

2.3 Characterization of the residual oil sample

Residual oil analysis was performed according to the American Society for Testing and Materials (ASTM)Standards. The density, viscosity, and carbon residue were determined according the ASTM D5002-99, ASTM D445, and ASTM D189-06e1 methods, respectively.The contents of SARA fractions for the residual oil were determined by column chromatographic separation technology based on the ASTM D4124-01 method.

The presence of functional groups in the residual oil was characterized by a Nicolet Magna 550 FT-IR instrument according to the manufacturer's instruction. The detection wave numbers ranged from 500—4 000 cm-1with an accuracy of 2 cm-1.

1H-NMR analysis of the residual oil was conducted with an Avance III 500 superconducting Fourier transform NMR spectrometer (Bruker, Switzerland) to determine the various H ratios in the components.

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The thermogravimetric analysis of the residual oil was performed with a thermogravimetry analyzer (TG/DTGSTD Q 600, TA Instruments, USA) at a heating rate of 40°C/min under a N2fl ow of 150 mL/min.

3 Results and Discussion

3.1 Effects of ultrasound cavitation on the properties of residual oil

The experiment was carried out to investigate the effects of ultrasonic radiation on the physical properties of residual oil. As shown in Figure 1, in the time interval ranging from 0 to 7 minutes, as the time increased, the oil viscosity decreased at a slightly faster pace over 3 to 5 minutes of radiation. After 7 minutes of ultrasonic treatment the oil viscosity had decreased by 14.1%.Longer interval of radiation had no apparent effect on the viscosity. The experiment thereby demonstrated that the ultrasonic treatment could reduce the viscosity of the studied residual oil sample[14,16-17].

Carbon residue could condense to become coke[16]. Residual oil contains not only a large number of aromatics but also many resins and asphaltenes, among which there are many polycyclic aromatic hydrocarbons and heterocyclic aromatic hydrocarbons. The reduction of carbon residue content after different ultrasonic exposure time as displayed in Figure 2 shows that when the ultrasonic treatment time increased, the carbon residue content gradually decreased to reach a reduction of 7.4% after 7 minutes. Beyond 7 minutes of ultrasonic treatment no further big change in the carbon residue content was observed. It was obvious that ultrasonic treatment could effectively reduce the carbon residue content in the residual oil.

Figure 2 Effect of ultrasonic exposure time on carbon residue

3.2 Effects of different ultrasonic exposure time on SARA fractions of residual oil

The changes in the SARA fractions of the oil after different ultrasonic exposure times are displayed in Figure 3.

Figure 3 Effect of ultrasonic exposure time on SARA fractions

It was observed that as the ultrasonic exposure time increased, the saturate content of the residual oil also increased, while the aromatic, resins, and asphaltenes content decreased, which was in agreement with the literature description[6,18]. In detail, when the ultrasonic exposure time varied from 0 to 7 minutes, the saturate content increased by up to 28.25%, while the aromatics,resins, and asphaltenes content decreases by up to 21.50%, 5.80%, and 82.23%, respectively. Beyond 7 minutes of exposure, the content of SARA fractions was stabilized. The decrease in carbon residue content indicates that the process of ultrasonic treatment not only can decrease viscosity and carbon residue, but also changes the contents of SARA fractions through weakening the bonds between the molecular structural units in residual oil, and reducing the number of bonds between structural units and the degree of aromaticity.

3.3 Effects of ultrasound cavitation on the average molecular structure parameters for residual oil

The average molecular structure parameters for the residual oil were calculated based on the analytical data from FT-IR and1H-NMR, while the average relative molecular mass was obtained by using the improved Brown-Ladner method (B-L method)[12].

FT-IR spectrometry was applied to characterize the residual oil to further confirm the effects of ultrasonic radiation on the chemical structure of the residual oil sample. According to Figure 4, the absorption peak at 1 600 cm-1, 1 454 cm-1and 1 370 cm-1, and the absorption peak intensity at 690 cm-1to 870 cm-1were significantly changed after ultrasonic treatment, and the intensity of strong absorption peak at 2 850 cm-1was not obvious in 6 min and 7 min of ultrasonic treatment, and the intensity of adsorption peaks at 1 600 cm-1, 1 454 cm-1,1 370 cm-1, 690 cm-1to 870 cm-1was gradually decreased,while after being subjected to ultrasonic treatment in 9 min and 11 min, the peak area of residue samples remained essentially unchanged. The results show that the ultrasonic wave has some influences on the functional groups of the residual oil in a certain treatment time,which can destroy the weak bonds such as hydrogen bonds and C-C bonds to some extent, resulting in the decomposition of fused aromatic hydrocarbons and the cleavage of alkyl side chains. The role of ultrasound on the C = C bonds can also have a strong impact.HA,Hα,Hβ, andHγgenerally represent the number of hydrogen atoms on naphthenic and aromatic rings and alkyl side chains of the dense aromatic nuclei in the unit structures of residual oil. The changes in the different categories of hydrogen atoms can, to some extent,indicate the changes in the aromatic ring structures in the residue.

Table 2 Relative contents of hydrogen atoms in different positions of molecules in residual oil

Figure 5 1H-NMR spectra of residual oil

As shown in Table 2 and Figure 5, after the ultrasonic treatment, the proportion ofHα(hydrogen which is connected to α carbons connected to aromatic rings in the residual oil) in the total amount of hydrogen atoms decreased, while the proportion ofHA(hydrogen directly connected to an aromatic nucleus) in the total amount of hydrogen atoms,Hβ(hydrogen attached to carbon atoms in the β position relative to aromatic rings), andHγ(hydrogen attached to carbon atoms in the γ position) in the molecular structure increased.

The decrease inHαand the increase inHAafter ultrasonic treatment indicate that the number of side chains on aromatic rings reduced. A possible explanation for the increase inHβandHγis that the aromatic side chain length increased, or the degree of branching of side chains increased[19-20]. In short, after ultrasonic treatment, the aromatic ring structures in the residual oil were altered.

Upon calculating the parameters of the average molecular structure in residual oil, it is assumed that the residue is associated with several structural units with a fused aromatic ring system serving as their core. A simple average structure model is used to approximately express the structural characteristics of the residual oil in order to obtain the changes in the molecular structure of the residue after exposure to ultrasonic treatment.

In this work, the B-L method was used to calculate the average molecular structural parameters based on

1H-NMR analysis spectrograms[12]. By using the FTIR spectroscopy, the relative amount of methyl and methylene groups and the parameters of the average molecular structure of the residue samples can be calculated, combined with a proposed formula. The average molecular structure parameters for the residue after different ultrasonic exposure times are given in Table 3.

Table 3 Average molecular structure parameters obtained after different ultrasonic treatment time

According to Table 3, the average molecular structure parameters, including the aromatic-carbon ratio (fA),naphthenic-carbon ratio (fN), alkyl-carbon ratio (fP),total number of rings (RT), number of aromatic rings(RA), and number of naphthenic rings (RN) in the residue,presented regular changes, with ultrasonic exposure realized between 0 and 7 minutes. Beyond 7 minutes,the ultrasonic treatment time had almost no effect on the molecular parameters. This means that after ultrasonic treatment, some condensed aromatics in the residual oil experienced scission, and some aromatic rings underwent ring-opening reactions[21]. Over time, with an increase in ultrasonic radiation time, the number of molecular structural units (n) decreased, and the average relative molecular mass of the structural units (usw) increased.Moreover, the aromatic-carbon ratio (fA) and the naphthenic-carbon ratio (fN) gradually decreased, while the alkyl-carbon ratio (fP) increased slightly.

Meanwhile, the total number of rings (RT) and number of aromatic rings (RA) slightly decreased. The naphthenic rings (RN) did not change significantly. The degree of condensation (HAU/CA) in the aromatic ring systems and the rate of replacement (σ) of the hydrogen surrounding the aromatic ring systems decreased, whereas the average length of side chains (L) increased. The structural units in some condensed aromatics underwent separation, and some aromatic and cycloalkane rings underwent ringopening reactions, resulting in longer alkyl side chains,which caused decrease infA,RAandRT, and increase inLandfP. The rate of replacement (σ) of the hydrogen surrounding the aromatic ring systems was associated with the ratiosHAandHα, which can be expressed asσ=Hα/ (2HA+Hα). When Hα sharply decreased, the rate of replacement (σ) of the hydrogen surrounding the aromatic ring systems decreased as well.

3.4 Effects of different ultrasonic exposure time on thermal reaction kinetics of residual oil

As shown in the measurements above, after the ultrasonic treatment of the residual oil, the average molecular structure of the residual oil changed, which is shown by the decrease in aromatic properties (fA,RA) and the increase in alkyl properties (fP,L). This kind of change in molecular structures indicates the relative change in thermal reaction properties.

This section used thermogravimetry analysis to investigate the effects of ultrasonic treatment on the properties of residual oil.

As can be observed in Figure 6 and Figure 7, the pyrolysis of the residual oil mainly occurred at temperatures ranging from 350 °C to 510 °C. After ultrasonic treatment, the initial weight-loss temperature, the final weight-loss temperature, and the derivative thermogravimetry peak temperature all showed a declining trend, among which the derivative temperature and the pyrolysis kinetics of residual oil can be described by in finite parallel reaction models, as shown in Table 4. When the pyrolysis reaction order was 1,the pyrolysis activation energy of the residual oil decreased as the ultrasonic exposure time increased. Thus, it can be seen that the residual oil can more easily undergo pyrolysis reactions after ultrasonic treatment, as evidenced by the changes in the colloid structure of vacuum residue and the average molecular structure after the treatment.

Figure 6 TG curve of residual oil

Figure 7 DTG curve of residual oil

Table 4 Thermal reaction kinetics parameters of residual oil

4 Conclusions

Throughout this study, the consistent data have been gathered concerning the effect of ultrasonic treatment on the different properties and structural parameters of residual oil. Insights have been gained into the mechanisms by which ultrasound cavitation acts on such samples. The results show that the carbon residue content, viscosity, and average molecular weight decrease after ultrasonic treatment.

The content of asphaltenes, resins, and aromatics decreased, whereas that of saturates increased.

As for the structural parameters relating to the number of structural units (n), the fraction of carbon atoms in aromatic structures (fA) and naphthenic structures (fN),and the number of naphthenic rings (RN) decreased, while the fraction of carbon atoms in alkyl structures (fP) and unit structure weights (usw) increased.

By applying the thermogravimetry method to analyze the changes in thermal performance, the results showed that with an increase in ultrasonic processing time, the apparent activation energy and the temperature needed for pyrolysis reactions decreased gradually.

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