Effects of Ultrasonic Treatment on Residue Properties

Sun Yudong; Zhang Qiang; Shi Honghong; Wang Xue; Liu Bo

(State Key Laboratory of Heaνy Oil Processing, China Uniνersity of Petroleum, Qingdao 266580)

Effects of Ultrasonic Treatment on Residue Properties

Sun Yudong; Zhang Qiang; Shi Honghong; Wang Xue; Liu Bo

(State Key Laboratory of Heaνy Oil Processing, China Uniνersity of Petroleum, Qingdao 266580)

The changes in properties and structural parameters of four vacuum residue samples before and after ultrasonic treatment were analyzed. Ultrasonic treatment could increase the carbon residue value, decrease the average molecular weight and viscosity, which can barely influence the density of vacuum residue. Meanwhile the constitution of residue can be varied including the decrease in the content of saturates, aromatics and asphaltenes, while the increase in the content of resins can lead to an increase in the total content of asphaltenes and resins. Among the four kinds of residue samples, there is a common trend that the more the content of asphaltenes in feedstock is, the more the increase in the content of resins, the more significant decrease in the aromatic content and the less decrease in the saturates content after ultrasonic treatment of residue would be. Changes in the structure and content of asphaltenes caused by ultrasonic treatment have a significant impact on the changes in residue properties. Ultrasonic treatment has changed the structural parameters of residue such as decrease in the total carbon number of average molecule (CTotal), the total number of rings (RT), the aromatic carbon number (CA),the aromatic rings number (RA) and the naphthenic rings number (RN) , and increase of characterization factor (KH). The study has indicated that ultrasonic treatment of vacuum residue can change the average structure of residue, and the changes in the content and structure of asphaltenes are the main cause leading to property changes. The results of residue hydrotreating revealed that coke yield decreased, whereas the gas and light oil yield and conversion increased after ultrasonic treatment of vacuum residue.

ultrasonic treatment; residue; asphaltene; SARA fractions; structure parameters

1 Introduction

Nowadays, the contradiction between the surging demand for light oil and the situation of increasingly heavier feedstocks to be processed has become a crucial problem. Conventionally, there are only two ways to meet the demand and tackle the current challenges of heavy oil processing, namely: one is the decarbonization and another is the hydrogen addition[1]. Novel technique of residue hydrotreating is one of the hotspots of recent research. The advantage of the process is that it not only can remove heteroatoms effectively but also can contribute to the maximum utilization of residue[2]. In-depth research on hydrotreating to improve the depth of heavy oil processing can minimize the grim situation associated with supply and demand of crudes in China[3]. As regards the causes leading to this serious situation, it has been found out that asphaltene plays a major role. Researches in the area of asphaltene study have shown that asphaltene is the most complicated component of residue and it is also the key to improve the conversion of residue during hydrotreating process[4]. Dickie, et al.[5]pointed out that reducing the association number of asphaltene can be regarded as an effective way to upgrade the performance of asphaltene hydrotreating. Heating, dilution with polar solvent, applying shear stress and other measures, are all efficient conventional means of dissociation. Zhang, et al.[6]mentioned that supersonic wave as a type of power, can affect the chemical and physical properties of asphaltene simultaneously. Researchers in ultrasonic technique pointed out that the mechanical effect of supersonic wave can release powerful shear stress, and cavitation can produce extremely physical condition such as high transient pressure and temperature and luminescence and highspeed jet at local site resulting in mass energy release[7-8]. The interaction of both effects makes it possible to break the bond between asphaltene units, destroy the sheet layerstructure and change the distribution state of asphaltene[9-10]. T. F. Yen, et al.[11-12]applied ultrasonic wave to the study of asphaltene in the 1990s. After decades of development, the ultrasonic wave has been applied in numerous fields. However, the previous papers paid few attention to using ultrasonic technology in treating the residual oil, especially in the details of series changes in basic properties after ultrasonic treatment of residue under special conditions. So lots of data are still needed to support the obtained conclusion of preliminary research. In this paper, the influence of ultrasonic treatment on residue have been investigated to delve in the changes of basic properties and structural parameters of residue, as well as the changes in the content and structure of asphaltene.

2 Experimental

2.1 Feedstocks

The experiments were carried out using Shengli vacuum residue (SLVR), Karamay vacuum residue (KVR), Arabian light vacuum residue (ALVR) and Fushun vacuum residue (FSVR) as raw materials. The SARA fractions of residue are shown in Table 1. Due to the difference in the content of asphaltenes, the selected raw materials are suitable for make an in-depth study on the role which asphaltene plays in the experiment.

Table 1 Influence of ultrasonic treatment on SARA fractions of vacuum residuew, %

2.2 Ultrasonic treatment

A high-frequency ultrasonic reactor with a frequency of 80 kHz and an output power of 45 W was used to carry out the experiments. A definite amount of vacuum residue (50 g) was put in a sealed container prior to undergoing ultrasonic treatment for 1.5 h at 50 ℃. After that treatment the samples were put in the dark place to be stored for 12 h. After all programs were completed, the properties of vacuum residue were measured.

2.3 Analytical methods

The main methods for analysis of residue properties are shown in Table 2.

2.4 Code names

The asphaltenes of Shengli residue before and after ultrasonic treatment are denoted as SLAs and USLAs, respectively. ALAs and UALAs represent the asphaltenes of Arabian light vacuum residue before and after ultrasonic treatment, respectively. “BP” means before the process of ultrasonic treatment, “AP” means after the process of ultrasonic treatment.

Table 2 Main basic properties and relevant methods for analysis of vacuum residue

3 Results and Discussion

3.1 Effects of ultrasonic treatment on SARA fractions of residue

Table 1 shows the changes of SARA fractions before and after ultrasonic treatment of vacuum residue samples. Tests showed that after the ultrasonic treatment of VR samples, the content of saturates, aromatics and as-phaltenes decreased, however, the content of resins and the total content of resins and asphaltenes increased. Among the four kinds of residue samples that had undergone ultrasonic treatment, there was a common trend that the more the content of asphaltenes in feedstock was, the more the increase of resins content, and the more the decrease of aromatics content along with less decrease of saturates content would be.

Firstly, as shown in Table 3, the association state of asphaltene units has been destroyed after ultrasonic treatment of VR samples. The structure of asphaltene has been changed, as evidenced by the decrease of the association number of asphaltene units, which can boost the transformation of asphaltenes to resins. The content of resins has increased. Secondly, there are lots of aromatic rings and saturated side chains in the resin molecules, so, according to the theory of “Likes Dissolνe Likes”, more saturates and aromatics will be adsorbed or dissolved inevitably in resins with the increases of resin molecules. The increase of resin content is caused by the interaction of these two aspects, but the extent of the trend is limited as we can see from the results. Above all, these are the reasons of the changing trend of SARA fractions caused by ultrasonic treatment of vacuum residue samples.

The quality and content of asphaltenes play an important role in the changes of residue property. The aromaticity of resins after ultrasonic treatment is determined by the content of asphaltenes. The more the content of asphaltenes in feedstock is, the more the increase in the aromaticity of resins after ultrasonic treatment of residue samples would be.This trend will enhance the resin ability in adsorbing aromatics, and lower its ability in adsorbing saturates.

Table 3 Structural parameters changes of asphaltenes before and after ultrasonic treatment of VR

3.2 Effects of ultrasonic treatment on molecular weight

It can be seen from Table 4 that the average molecular weight of vacuum residue was reduced through ultrasonic treatment, which depended on the type of raw materials. The largest decrease in the molecular weight occurred to SLVR, followed by ALVR. By combining the statistical data with those depicted in Table 1, a conclusion can be drawn that the increase of asphaltenes in amount could result in a remarkable decrease of average molecular weight of vacuum residue during the process of ultrasonic treatment. The ultrasonic irradiation could break the association state of macromolecules in the vacuum residue, lower the association degree of asphaltene units (Table 3) and turn a part of the macromolecules into smaller molecules. Meanwhile, aromatics absorbed in the asphaltene micellae were released into resins. The content, structure and distribution state of all fractions would be changed, and the average molecular weight of vacuum residue was reduced.

Table 4 Change in molecular weight of vacuum residue after ultrasonic treatment

3.3 Effects of ultrasonic treatment on viscosity

Viscosity reflects the mobility and diffusivity of oil which can affect the mass transfer speed of residue molecule in catalyst’s micropores. Changes of four residue samples at 100 ℃ before and after ultrasonic treatment are shown in Table 5.

The viscosity decreased at different degrees, which was in agreement with the work reported by Li[13]. The maximum reduction of viscosity was found in ALVR and was equalto about 12%. As it had been reported, several factors could affect the changes in viscosity of vacuum residue samples, such as the average molecular weight of vacuum residue, the number and size of branched chains, the number of rings and the quality and content of asphaltenes[14]. The content and structure of asphaltenes remarkably differed from each other between different vacuum residue samples[15], which mighty have a significant effect on viscosity. Table 5 also reveals that the change in viscosity of vacuum residue was not coincident with the phenomenon denoting that the more the content of asphaltenes in feedstock was, the more the decrease of viscosity after ultrasonic treatment would be. There is no obvious relationship between asphaltene content in feedstock and its viscosity change. The result may indicate that different types and contents of asphaltene varied in their effect on the viscosity of vacuum residue. Changes in asphaltene structural parameters caused by ultrasonic treatment such as the decrease in association degree and total number of rings not only could reduce the complexity of asphaltene, but might also contribute to the change of viscosity.

Table 5 Influence of ultrasonic treatment on the viscosity of vacuum residue at 100℃

It was found that the viscosity reduction of asphaltenerich SLVR after ultrasonic treatment was only 4.84%, and the change was relatively small. In fact, as it has been reported, a lot of structures with long chains can be found in asphaltene fraction of SLVR[16]. The layered structure of asphaltene has been destroyed by the ultrasonic treatment, while the branches are still strong enough to keep the viscosity unchanged. But as for KVR which has low content of asphaltenes, the change of viscosity is relatively considerable. Because asphaltene fraction of KVR is provided with short alkyl side chains, and as it has been reported, the quality of asphaltene fraction is not the only determining factor that can influence its viscosity[17]. However, upon considering the experimental results it is realized that the ultrasonic treatment process has decreased the viscosity of all vacuum residue samples studied.

3.4 Effect of ultrasonic treatment on the density

Table 6 shows the influence of ultrasonic treatment on the density of vacuum residue at 20 ℃. It is realized that the density of four kinds of vacuum residue samples has not been changed after ultrasonic treatment. The test results reveal that after ultrasonic treatment the changes of all fractions in vacuum residue may maintain a counterbalance with each other, which makes the density of four kinds of VR samples basically unchanged.

Table 6 Influence of ultrasonic treatment on the density of vacuum residue at 20℃

3.5 Effect of ultrasonic treatment on Conradson carbon residue

Table 7 shows the changes of CCR value before and after ultrasonic treatment of VR samples. It can be seen from Table 7 that CCR values of four residue samples did increase. The increase in CCR values of VR samples after ultrasonic treatment decreased successively in the following order: ALVR＞SLVR＞FSVR＞KVR, denoting that with the increase of asphaltene content in VR, the Conradson carbon residue value was increased more remarkably after ultrasonic treatment of the relevant VR sample.

Table 7 Influence of ultrasonic treatment on the carbon residue value of vacuum residue

The Conradson carbon residue is caused by gathering and build-up of macromolecules with low H/C ratio, and ultrasonic treatment has enhanced this trend. As it is known to all, ultrasonic treatment can break the association struc-ture of vacuum residue. Meanwhile, it can also accelerate the collision of residue particles that will boost the process of gathering. Thus, the CCR value will be increased. In another aspect, the polarity value of asphaltene unit (X/n) has been increased after ultrasonic treatment of VR. It means that this process has promoted the gathering of a part of asphaltene fraction which could hardly be dispersed. The increase of CCR value indicates that the process of ultrasonic treatment not only can improve the structure and distribution state of asphaltene, but can also promote the gathering process of asphaltene fraction to some extent.

So, it can lead to a conclusion that asphaltene plays an important part in the changing of the properties of vacuum residue during the ultrasonic treatment.

3.6 Effect of ultrasonic treatment on structural parameters of residue

The calculated structural parameters of vacuum residue according to the theory of densimetry proposed by Van Krevelen and William are shown in Table 8[18]. It can be seen from Table 8 that the total carbon number of average molecule (CTotal),RT,CA,RAandRNdecreased to different extent, while the heavy oil characterization factor (KH) which can reflect the chemical property and secondary processing performance of vacuum residue increased. This fact indicates that the size of vacuum residue samples decreased after ultrasonic treatment which could facilitate their secondary processing performance. TheCTotaldecreased by about 2 to 5, andCAdecreased by about 0.4 to 1 after ultrasonic treatment of VR samples. Upon taking into account the content of asphaltene fraction in VR samples, it can be found out thatCTotalandRTdecreased more, whileKHincreased more obviously when there was more asphaltene fraction in feedstock as shown in Table 1. The average structure of four kinds of vacuum residue samples had been changed after ultrasonic treatment of these samples.

Table 8 Influence of ultrasonic treatment on the structural parameters of vacuum residue

3.7 Changes in reaction performance of vacuum residue

The property changes may have an effect on the secondary processing performance of vacuum residue. The vacuum residue hydrotreating has been applied to verify the reaction performance of the vacuum residue after ultrasonic treatment. The distribution of main products obtained from residue hydrotreating process is shown in Table 9 before and after ultrasonic treatment of VR samples. The test results have revealed that the coke yield decreased, whereas the yield of gas and light oil as well as the residue conversion increased. It is confirmed that the ultrasonic treatment can change not only the properties, but also the reaction performance of vacuum residue. Therefore, the ultrasonic treatment has improved the secondary processing performance of vacuum residue.

Table 9 Comparison on hydrotreating product distribution before and after ultrasonic treatment of VR samples

4 Conclusions

The VR samples with different properties showed similar trend of changing after ultrasonic treatment. The results showed that the average molecular weight and viscosity of four kinds of vacuum residue samples decreased after ultrasonic treatment, while Conradson carbon residue value increased, with the density basically unchanged. The contents of saturates, aromatics and asphaltenes also decreased. However, the total content of resins and asphaltenes increased because of significant increase in resins content. The more the content of asphaltenes in feedstock, the more the increase in the ratio of resins, and the more remarkable decrease of aromatics and less decrease of saturates after ultrasonic treatment of VR samples.

Ultrasonic treatment has changed the structural parameters of four kinds of vacuum residue samples, withCTotal,CA,RT,RAandRNdecreased, andKHincreased. This fact indicates that the structure of vacuum residue has changed, and the quality and performance of vacuum residue during secondary processing have improved. Changes in the structure and content of asphaltenes caused by ultrasonic treatment of VR have a significant influence on the changes in basic properties of vacuum residue.

Acknowledgment:The authors acknowledge the financial support provided by the Fundamental Research Funds for the Central Universities (Grant No. 11CX05008A), the PetroChina Innovation Foundation (Grant No. 2011D-5006-0405) and the UPC Innovation Project of Postgraduate (Grant No. CX201304).

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Recieved date: 2013-06-27; Accepted date: 2013-10-15.

Sun Yudong, Telephone: +86-532-86984702; E-mail: ydsun@upc.edu.cn; ydsun1970@126.com.