Optimization of ultrasonic-assisted oxidative desulfurization of gasoline and crude oil

Journal Pre-proof Optimization of Ultrasonic-Assisted Oxidative Desulfurization of Gasoline and Crude Oil Cuihong Zhou, Yanying Wang, Xintong Huang, Yupeng Wu, Jiarui Chen

PII:

S0255-2701(19)30479-9

DOI:

https://doi.org/10.1016/j.cep.2019.107789

Reference:

CEP 107789

To appear in:

Chemical Engineering and Processing - Process Intensification

Received Date:

21 April 2019

Revised Date:

22 October 2019

Accepted Date:

11 December 2019

Please cite this article as: Zhou C, Wang Y, Huang X, Wu Y, Chen J, Optimization of Ultrasonic-Assisted Oxidative Desulfurization of Gasoline and Crude Oil, Chemical Engineering and Processing - Process Intensification (2019), doi: https://doi.org/10.1016/j.cep.2019.107789

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Optimization of Ultrasonic-Assisted Oxidative Desulfurization of Gasoline and Crude Oil

Cuihong Zhoua,*, Yanying Wangb, Xintong Huanga,c, Yupeng Wub, Jiarui Chena

a

Department of Environmental Engineering, Beijing Institute of Petrochemical Technology,

b College

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Beijing 102617, China

of Environmental & Energy Engineering, Beijing University of Technology, Beijing

100124, China

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Sinopec Tenth Construction CO., Ltd, Shandong Province 266555, China

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c

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Graphical abstract

Highlights: 

The cavitation effect can generate radicals and strengthen the oxidation process.



Crude oil was desulfurized more than gasoline with ultrasonic treatment.



The significant factors for crude oil are the oxidant amount and ultrasonic power. 1

Abstract: As environmental protection standards have improved, the reduction of sulfur contained in crude oil and the realization of energy-efficient gasoline desulfurization have become popular

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research topics. In this study, the optimal conditions for ultrasonic-assisted oxidative desulfurization of gasoline and crude oil were investigated, the effects of ultrasonic and simple

oxidative desulfurization were compared, and the mechanism of the ultrasonic effect was verified.

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The desulfurization effects were analyzed according to the response surface methodology under different ultrasonic power levels, irradiation times, and oxidant amounts. The maximal

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desulfurization rate of gasoline was achieved with an ultrasonic power of 400 W, irradiation time

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of 7 min, and oxidant amount of 8 mL. Meanwhile, the optimal desulfurization effect for crude oil was achieved with an ultrasonic power of 700 W, irradiation time of 10 min, and oxidant amount

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of 10 mL. The results of the comparison experiments showed that ultrasonic treatment enhanced processing and that ultrasonic-assisted oxidative desulfurization was more effective for crude oil

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than for gasoline. The results of this study provide valuable reference information for the

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application of ultrasonic treatment in oil desulfurization. Keywords: Ultrasonic; Desulfurization; Response surface methodology; Gasoline; Crude oil

1. Introduction In recent years, automobile exhaust emissions have increased considerably, due to economic improvement and transportation development, leading to more serious air pollution. Sulfur 2

oxides, sulfureted hydrogen, and sulfur compounds are among the main pollutants in exhaust gas [1, 2]. Currently, hydrodesulfurization (HDS) and non-hydrodesulfurization (NHDS) are commonly used for the removal of sulfur compounds [3, 4]. However, HDS is unable to remove thiophene [5], benzothiophene [6], dibenzothiophene, the naphthothiophene compound, and the naphthylbenzothiophene compound [7] completely, because the structure and properties of these sulfur compounds are similar to those of benzene fused-ring compounds, which have high

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thermal stability and are inactive in chemical reactions [8, 9]. However, some NHDS methods are significantly effective for removal of such sulfur-containing compounds. With continuous developments in research technology, NHDS methods will become indispensable in the

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petrochemical industry.

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Oxidative desulfurization (ODS) is considered one of the most promising methods for deep

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desulfurization of fuels due to the mild operating conditions (<100°C, environmental pressure), high desulfurization efficiency, low equipment cost, and simplicity of operation [10, 11]. It is

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widely regarded as an innovative and green alternative process for performing deep HDS [12] and has received extensive attention from researchers at home and abroad [13-16]. ODS is often

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combined with adsorption desulfurization and extraction desulfurization technology. Compared to conventional adsorbents, clay materials have become promising low-cost

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adsorbents [17, 18]. However, the properties of organic sulfides are similar to those of oils, and the solubility in extractants is almost the same. It has been found that oxygen atoms can greatly increase the polarity of sulfides in the extractant, so the ODS method has begun to be researched and applied in practice [19]. To date, various oxidants have been studied, such as hydrogen peroxide (H2O2) [7, 20], peroxy organic acids [21-23], nitrogen oxides [24], peroxy salts [25, 3

26], ozone [27, 28], molecular oxygen (O2) [29], and tertiary butyl hydroperoxide [30], which can donate oxygen atoms to the sulfur in mercaptans (thiols), sulfides, disulfides, and thiophenes to form sulfoxides or sulfones [19, 31]. Extracting agents such as dimethylformamide [32, 33], dimethyl sulfoxide (DMSO) [34], ionic liquids (ILs) [35, 36], acetonitrile [37], methanol [38], and sulfolane [37] are used to extract oxidation products. Extractive desulfurization based on iron liquids (ILs) is also considered a promising

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desulfurization technique [39], but it is used less due to its high cost and the difficulty of

preparation. The rate of the conventional ODS reaction has been slow in most studies due to the

low degree of mixing between the different phases and the small mass transfer interface area [23].

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Ultrasonic-assisted oxidative desulfurization (UAOD) is an emerging ODS method that

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adequately utilizes the action mechanism of ultrasonic waves to form a fine emulsion [23],

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enhances the mixing intensity between two immiscible phases [40], and increases the rate of the ODS reaction [41]. Bolla et al. [1] reported that the beneficial effect of a UAOD system is its

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physical property that enhances the interfacial area by promoting fine emulsification of fuel and oxidants. It creates extreme conditions (high temperature and high pressure) for the oxidation

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process and reduces the amount of solvent used. Ultrasonic technology is environmentally friendly [42], and the combination of ultrasonic treatment and oxidation can not only achieve a certain

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desulfurization effect, but also is in line with the vigorous promotion of environmental protection technology. It can fundamentally solve the problem of deep desulfurization of petroleum and related products and is conducive to the production of clean fuel. Hosseini et al. [43] used a combination of ODS and ultrasonic assisted methods, which can effectively remove sulfur from crude oil. They demonstrated that the desulfurization efficiencies of formic acid, acetic 4

acid, and propionic acid can be ranked as follows: acetic acid > formic acid > propionic acid. In addition, they concluded that the desulfurization rate of UAOD is about 30% higher than that of mechanical stirring-assisted ODS. In this study, the UAOD of real gasoline using a phosphotungstic acid (PW)/H2O2/formic acid system and that of crude oil using an H2O2/formic acid/acetonitrile/methanol system were investigated. The main objective was to determine the oxidation performance of sulfide in gasoline

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and crude oil under different reaction conditions. Experiments were conducted on gasoline and

crude oil desulfurization, and the effects of the ultrasonic power, irradiation time, and oxidant amount on gasoline desulfurization were studied. Simultaneously, comparative analyses of the

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desulfurization mechanism, the selected extraction method and oxidation time, and ultrasonic

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versus non-ultrasonic treatment were performed experimentally. In addition the effects of UAOD

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on the desulfurization of gasoline and high-sulfur crude oil were compared. The ultrasonic mechanism was verified, and the main factors affecting the desulfurization of high-sulfur

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crude oil were analyzed.

2. Experiment

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2.1 Materials and instruments

The experimental materials were actual fluid catalytic cracking (FCC) gasoline with a

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sulfur content of 0.1207%, actual crude oil with a sulfur content of 2.862%, H2O2 (Analytical Reagent (AR), Beijing Chemical Works), formic acid (AR, Tianjin Fuchen Chemical Reagents Factory), PW (AR, Baomanbio), DMSO (AR, Beijing Chemical Works), octane (99.99%, Tianjin Fuchen Chemical Reagents Factory), acetonitrile (AR, Beijing Chemical Works), methanol (AR, >99.5%, Beijing Chemical Works), and potassium bromide (AR, 5

Tianjin Fuchen Chemical Reagents Factory). The instruments used in the experiment included an XO-1000D-type ultrasonic system, Fourier transform infrared (FTIR) spectrometer, and TCS2000S-type ultraviolet fluorescent sulfur analyzer. FTIR was used to analyze the changes in the functional groups of the materials after the experiment. Before the experiment, potassium bromide was placed in an oven at a temperature below 100°C for 4 h. The dried potassium bromide powder was ground and pressed into a transparent tablet. Then the oil was applied

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to the potassium bromide transparent tablet to analyze the infrared spectrum. An ultraviolet fluorescent sulfur analyzer was used to determine the sulfur content. First, the apparatus

was heated to 1000°C under an argon gas (Ar) atmosphere. Then oxygen was introduced and

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the O2:O2:Ar flow rate ratio was adjusted to 200:50:50 (mL/min). Finally, using computer

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software, the standard curve was selected, and the sample was analyzed, where each

2.2 Experimental procedures

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injection volume was 8 μL.

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2.2.1 Ultrasonic assisted oxidation treatment of gasoline experiment To begin, 0.2 g of PW as well as H2O2 and formic acid (1:1 by volume) were added to 50 mL

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of gasoline. The ultrasonic probe was immersed in the center of the beaker at about 3/4 of the depth of the oil sample, and the action mode of 2 s action time, 2 s intermittent time, was selected.

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A single-factor experiment was conducted using the control variable method. The ultrasonic power was controlled to 100 W, 250 W, 400 W, 550 W, and 700 W; the irradiation time was set to 1 min, 3 min, 5 min, 7 min, and 9 min; and the oxidant amount was 4 mL, 6 mL, 8 mL, 10 mL, and 12 mL. After oxidation, 10 mL supernatant was taken, 10 mL of DMSO was added for extraction, extraction was performed three times, and water washing was performed four times. The mass 6

fraction of sulfur was measured using an ultraviolet fluorescence sulfur analyzer. Each group of experiments was repeated twice, and the results were averaged. The factor-level values were determined according to the optimal range determined by the single-factor experiments. The response surface methodology (RSM) experimental design scheme is shown in Table 1. The specific version of the Design Expert software used in this study is Design Expert.V8.0.6.

Value level Independent variable −1

0

Ultrasonic power (W)

100

400

Irradiation time (min)

3

7

Oxidant amount (mL)

6

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Table 1 Factors and levels of experiment by response surface methodology

1

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700

10

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8

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2.2.2 Ultrasonic-assisted oxidation treatment of crude oil experiment

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Different volumes of H2O2 and formic acid (1:1 by volume) were added to 100 mL of crude oil. The ultrasonic probe was immersed in the center of the beaker at about 3/4 of the depth of the

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oil sample, and the action mode of 2 s action time and 2 s intermittent time was selected. Subsequently, the oil sample was subjected to magnetic stirring for 1 h after ultrasonic treatment.

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A single-factor experiment was conducted using the control variable method. The ultrasonic power was controlled to 100 W, 300 W, 500 W, 700 W, and 900 W; the irradiation time was set to 4 min, 6 min, 8 min, 10 min, and 12 min; and the oxidant amount was 8 mL, 10 mL, 12 mL, 14 mL, and 16 mL. A mixture of 20 mL of acetonitrile, 20 mL of methanol, and 20 mL of water was added for extraction. The extracted mixture was slowly transferred into a separator funnel and allowed to 7

sit for 10 min, until the extracted mixture no longer increased. The mass fraction of sulfur was measured using the ultraviolet fluorescent sulfur analyzer. The experiment was conducted according to the previous procedure, with no change in any conditions, except that no ultrasonic was added. The sulfur contents in the samples after UAOD and ODS treatment were compared. Each group of experiments was repeated three times, and the results were averaged. The factor-level values were determined according to the optimal range identified from the

Table 2.

Value level

Irradiation time (min)

500

1

700

900

6

10

14

8

10

12

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Oxidant amount (mL)

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Ultrasonic power (W)

−1

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Independent variable

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Table 2 Factors and levels of experiment by response surface methodology

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single-factor experiments. The details of the RSM experimental design scheme are summarized in

2.3 Comparison experiment

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The byproduct of the use of H2O2 in this reaction is water, which does not pollute the environment; furthermore, it does not produce organic acids and does not corrode the reaction

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equipment. Therefore, H2O2 was selected as the oxidant. Four main aspects were considered in the comparative experiments: the desulfurization mechanism, the selected extraction method and oxidation time, and ultrasonic versus non-ultrasonic treatment. To study the changes of the substances in crude oil, the infrared spectra of the oil samples before and after desulfurization were analyzed. 8

The desulfurization mechanism verification experiment was performed under optimized RSM conditions: an oxidant amount of 8.1 mL and an oxidation time of 12 min. The experiments were conducted by mechanical stirring and heating and by stirring in a constant temperature water bath; then, comparisons were performed with the RSM verification experiment. The room temperature was 15°C, and the heating condition was a water bath with a constant temperature of 62°C (the gasoline temperature was 50°C at the end of oxidation).

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3. Results and discussion 3.1 Oxidation and ultrasonic mechanisms 3.1.1 Oxidation mechanism

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Peroxyformic acid is a powerful oxidizing agent in this process, which forms by the

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reaction of formic acid and H2O2. The reaction mechanisms can be expressed as follows. In

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the first step, H2O2 reacts with formic acid, leading to the generation of an unstable intermediate compound that is quickly dehydrated to form peroxyformic acid [formula (1)]

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[44, 45]:

.

(1)

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Then, the sulfide, such as 4, 6-Dimethyl dibenzothiophene, in oil is oxidized by peroxyformic

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acid to sulfoxide and sulfone [formula (2)] [22, 46]:

.

(2)

3.1.2 Ultrasonic mechanism The ultrasonic power intensity is a critical factor in the UAOD effect. After the ultrasonic intensity exceeds a certain value, the cavitation tends to be saturated and generates a large amount 9

of bubbles, which coalesce and form clouds at high power. Cavitation shielding [47] causes attenuation of the ultrasonic scattering due to the presence of bubble clouds, which tend to reduce the cavitation activity [2, 48]. Fan et al. [49] determined that the optimum power intensity for diesel desulfurization is 8 W/cm2. The desulfurization rate decreases when the power intensity exceeds 8 W/cm2, due to cavitation shielding. Liu et al. [50] analyzed the relation between the relative intensity of the cavitation field in the

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ultrasonic vessel and the desulfurization rate. In general, from the center to the sides of the vessel, the cavitation intensity gradually weakens. The rate at which the desulfurization efficiency

ultrasonic power intensity on the desulfurization.

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changed was well matched with the cavitation field measurement results, proving the effect of the

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Related research indicated that the preferable frequency for UAOD is about 20–50 kHz [51].

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Mello et al. [52] also clarified that ultrasonic treatment provides better performance during UAOD at low frequencies, usually following the order 20 kHz > 35 kHz > 130 kHz > 582 kHz. It is likely

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that continuous operation with high frequency irradiation leads to erosion of the transducer surface and requires a higher power to generate cavitation events [53]. In this study, the ultrasonic

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frequency was 19–21 kHz. The specific operating frequency of the system will change as the power changes.

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3.2 Single-factor experiment results The desulfurization rates of gasoline and crude oil are shown in Fig. 1. It can be clearly seen

that the ultrasonic power, irradiation time, and oxidant amount required for the treatment of crude oil are significantly higher than those required for gasoline desulfurization. This difference could be due to the complexity of the crude oil composition, the more complex components in 10

crude oil, or the higher sulfur content in crude oil. As shown in Fig. 1(a), the optimal power levels for ultrasonic treatment of gasoline and crude oil are 400 W and 700 W, respectively, and the best results cannot be achieved with power levels that are too low or too high. During the reaction, the temperature rose very rapidly, and the atomization phenomenon caused by the accelerated decomposition of the oxidant can be seen. As the ultrasonic power increases, the sound power per unit area increases, which is beneficial for the

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occurrence of ultrasonic cavitation effects and accelerates the completion of the oxidation reaction until equilibrium is reached. However, this characteristic does not mean that the higher the

ultrasonic power, the higher the desulfurization efficiency, because excessive ultrasonic power

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causes oxidant gasification loss and the reaction becomes too severe, as well as unsafe.

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As shown in Fig. 1(b), when the amount of oxidant is 8 mL, the desulfurization rate of

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gasoline is the highest, while the desulfurization rate of crude oil increases with increasing oxidant amounts. However, the desulfurization effect on crude oil approaches saturation with the addition

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of 10 mL of oxidant. This occurs because many more molecules participate in the reaction, increasing the probability of molecular collisions. When the amount of oxidant is increased, the

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mass fraction of sulfur is decreased. However, this does not mean that the more oxidant added, the better the desulfurization effect. The excessive addition of oxidant will cause excessive oxidation

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of the oil and destroy the oil components. Considering the desulfurization effect, environmental safety, and economic benefits, the optimal oxidant amounts are 8 mL and 10 mL for gasoline and crude oil, respectively. Bhasarkar et al. [54] found that H2O2 is the crucial component of the oxidant system which balances between the different competing reaction pathways of the process. It was demonstrated that the addition of an excess of H2O2 was unfavorable to the 11

reaction process. This is consistent with the conclusion that excess oxidant is not conducive to desulfurization. As shown in Fig. 1(c), the desulfurization rates of gasoline and crude oil are optimal when the irradiation time is 7 min and 10 min, respectively. If irradiation time is too long or too short, the decomposition of the oxidant is terminated. The oxidation effect is optimized after the reaction reaches a certain time and degree. Excessive irradiation makes the oxidant decomposition invalid,

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which is not conducive to the progress of the oxidation reaction. Considering the desulfurization effect and economic benefits, the optimal irradiation times for gasoline and crude oil are 7 min and 10 min, respectively.

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3.3 Response surface optimization and result analysis

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The RSM was designed using Design Expert software based on the previous experimental

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results and methods, and the experimental design and results are summarized in Table 3.

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Table 3 Response surface methodology results

Ultrasonic power Experiment (W)

Oxidant amount (mL)

(min)

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number

Desulfurization rate

Irradiation time

(%)

Gasoline

Crude oil

Gasoline

Crude oil

Gasoline

Crude oil

Gasoline

Crude oil

100.00

500.00

3.00

10.00

8.00

8.00

74.04

54.36

2

700.00

500.00

3.00

10.00

8.00

12.00

78.67

61.25

3

100.00

900.00

11.00

10.00

8.00

8.00

73.35

56.24

4

700.00

900.00

11.00

10.00

8.00

12.00

73.98

68.89

5

100.00

700.00

7.00

6.00

6.00

8.00

71.31

57.89

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1

12

700.00

700.00

7.00

6.00

6.00

12.00

75.94

72.12

7

100.00

700.00

7.00

14.00

10.00

8.00

72.47

56.49

8

700.00

700.00

7.00

14.00

10.00

12.00

71.69

70.08

9

400.00

500.00

3.00

6.00

6.00

10.00

73.23

58.94

10

400.00

900.00

11.00

6.00

6.00

10.00

74.14

69.85

11

400.00

500.00

3.00

14.00

10.00

10.00

77.68

63.85

12

400.00

900.00

11.00

14.00

10.00

10.00

72.00

68.54

13

400.00

700.00

7.00

10.00

8.00

10.00

80.35

72.54

14

400.00

700.00

7.00

10.00

8.00

10.00

79.94

72.53

15

400.00

700.00

7.00

10.00

8.00

10.00

72.41

16

400.00

700.00

7.00

10.00

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81.41

10.00

80.42

72.32

17

400.00

700.00

7.00

8.00

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10.00

80.18

72.39

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10.00

8.00

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In this study, the RSM was used to design the high-sulfur crude oil desulfurization

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experiment. It was found that the desulfurization effect was optimal when the oxidant amount was 10 mL, the ultrasonic power was 700 W, and the irradiation time was 10 min, which yielded a

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desulfurization rate of 72.54%. In addition, greater desulfurization of crude oil than of gasoline was achieved under these conditions. Thus, UAOD has a greater effect on high-sulfur oil products.

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The rationality and predictability of the model design can be measured using methods such as

variance analysis and significance testing. The main variance analysis results for gasoline and crude oil are shown in Tables 4 and 5. Table 4 Variance analysis results for gasoline

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P-value Source

Sum of squares

df

Mean square

F-value Prob(P) > F

200.50

9

22.28

29.96

<0.0001

A-Ultrasonic power

10.37

1

10.37

13.95

0.0073

B-Irradiation time

12.88

1

12.88

17.32

0.0042

C-Oxidant amount

0.076

1

0.076

0.106

0.7584

AB

4.00

1

4.00

5.38

0.0534

AC

7.32

1

7.32

9.84

0.0164

BC

10.86

1

10.86

14.60

0.0065

Lack of Fit

3.94

3

1.31

4.15

0.1013

Pure error

1.26

4

Cor Tota

205.70

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Model

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0.32

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Table 5 Variance analysis results for crude oil

Source

Mean square

F-value

P-value

735.47

9

81.72

29.45

<0.0001 Prob(P)>F

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Model

Sum of squares df

280.37

1

280.37

101.05

<0.0001

B- Ultrasonic power

78.88

1

78.88

28.43

0.0011

C- Irradiation time

0.0032

1

0.0032

0.001153

0.9739

Pure error

0.036

4

0.00897

Cor Total

754.89

16

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A- Oxidant amount

As can be seen from Table 4, the F-value of 29.96 indicates statistical significance. There is only a 0.01% chance that a “model F-Value” this large could occur due to noise. Values of “Prob > 14

F” less than 0.05 indicate model terms that are significant. In this case, A, B, AC, and BC are significant. Among them, the irradiation time and ultrasonic power are significant influencing factors, the irradiation time is more significant than the ultrasonic power, and the oxidant amount is not a significant affecting factor. Thus, the model established in this experiment is feasible and the influencing factors are significant. The “Lack of Fit” is used to indicate the degree to which the model used fits the experimental results; specifically, it is the degree of difference

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between the results of the model and experiment. The determination coefficient (R2) of this

model is 0.9747, which is close to 1, indicating a high correlation between the predicted and measured values. A correction decision coefficient (Adj. R2) value of 0.9422 means that this

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model can explain approximately 94.22% of the change in the response value, so the

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regression equation fits well. The difference between the model R2 and Adj. R2 is small,

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indicating that the equation fits the actual situation well. In this model, P = 0.1013 > 0.05, which is beneficial, and there is no missing factor. Therefore, the following regression equation

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(Equation 3), could be used instead of the actual experimental points to analyze the experimental results. The regression equation for this model is as follows:

(3)

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Y=80.46 + 1.14A − 1.27B − 0.098C − 1.00AB − 1.35AC − 1.65BC − 3.43A2 − 2.02B2 − 4.18C2

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where Y is the desulfurization rate, %; A is the ultrasonic power, W; B is the irradiation time, min; and C is the oxidant amount, mL. The response surface figures are shown in Fig. 2. Fig. 2(a) shows the significant interaction of ultrasonic power and oxidant amount on sulfur removal from gasoline. The edge of the surface representing the effect of the ultrasonic power on the removal rate is clearly steeper than the edge 15

of oxidant amount, indicating that it has a significant influence on the removal rate. As the ultrasonic power increases, the sulfur removal rate first increases and then decreases. Oxidant addition can convert sulfides that are not easily removed from gasoline into more polar sulfones and sulfoxides that can be removed easily. However, if the oxidant content is too high, it will cause excessive oxidation of the oil sample, destroy the oil sample components, and reduce the desulfurization effect.

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Fig. 2(b) shows that the irradiation time and oxidant amount significantly impacts the rate of sulfur removal from gasoline. It can be seen clearly from Fig. 2 that the oxidant amount has a higher influence on the sulfur removal rate when the two factors interact.

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The optimal processing conditions predicted by response surface optimization are an

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ultrasonic power of 464.7 W, irradiation time of 5.5 min, and oxidant amount of 8.1 mL. The

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experimental conditions were revised through repeated rounding: the ultrasonic power was 460 W, irradiation time was 6 min, and oxidant amount was 8.1 mL. The desulfurization rate of the

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model is 79.93–81.72%, which is within the 95% confidence interval, and the optimal desulfurization rate of surface optimization is 80.82%, which is also within the 95%

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confidence interval. The desulfurization rates obtained from the two sets of verification experiments were 80.53% and 81.20%, with an average of 80.87%. The error between the

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experimental value and the response surface optimization result was 0.06%. From the variance analysis results in Table 5, the model established this time is feasible and

the influencing factors are significant. Among them, the oxidant amount and ultrasonic power are significant factors. The oxidant amount is more significant than the ultrasonic power, and the irradiation time is not a significant factor. The R2 and Adj. R2 are 0.9743 and 0.9412, 16

respectively. The response surface for the UAOD of crude oil is shown in Fig. 4. The regression equation for this model is as follows: Y = 72.44 + 5.92A + 3.14B + 0.020C + 1.44AB − 0.16AC − 1.55BC − 6.70A2 − 5.55B2 − 1.59C2

(4)

where Y is the desulfurization rate, %; A is the oxidant amount, mL; B is the ultrasonic power, W; and C is the irradiation time, min.

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Fig. 3(a) shows that as the ultrasonic power and oxidant amount increase to certain levels, the desulfurization rate tends toward the optimum value. As shown in Fig. 3(b), the software predicts that the desulfurization rate is optimal, reaching 73.37%, and the expected value is up to 1 when

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the irradiation time is 6.2 min, the oxidant amount is 11.4 mL, and the ultrasonic power is 785.1

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W.

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3.4 Analysis of comparison experiment results

3.4.1 Desulfurization mechanism experiment on gasoline

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In the UAOD process, the sample temperature increased significantly. To explore the ultrasonic mechanism, the desulfurization rates of gasoline under three conditions were compared.

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The desulfurization rates obtained by mechanical stirring at room temperature (15°C; case No. 1), heating while stirring (case No. 2), and ultrasonic treatment (case No. 3) were 56.57%, 71.05%,

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and 80.87%, respectively.

Comparing cases No. 1 and No. 2, the gasoline desulfurization rate can be increased by

14.48% through heating, which proves that the thermal effect is significant in the UAOD process. Comparing cases No. 2 and No. 3, the gasoline desulfurization rate after ultrasonic application was increased by 9.82% compared with that obtained by heating. The experimental results show that in 17

addition to the thermal effect, the mechanical and cavitation effects of ultrasonic treatment also play significant roles in oil desulfurization. The extraction methods are compared in Fig. 4. The addition of deionized water can reduce the DMSO extraction effect by about 20%, because DMSO is highly polar and soluble in water. The higher the DMSO concentration, the better the extraction. Therefore, when DMSO is used as the extractant, it should not be combined with water.

reaction rate constant can be calculated using equation (5):

c  ln  t   kt  c0 

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Assuming that the reaction conforms to the apparent first-order reaction kinetics, the

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(5)

where ct is the sulfide concentration after irradiation t time, mg/L; c0 is the initial sulfur

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content, mg/L; t is the irradiation time, min; and k is the reaction rate constants.

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By fitting the first-order reaction kinetic equation of UAOD at the same power for different times, the k value is 0.0304 and the R2 is 0.6811 within 1–7 min. The k value is

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0.0558 and the R2 is 0.9950 within 3–7 min. Therefore, the reaction conforms to the apparent first-order reaction kinetics within 3–7 min. This shows that ultrasonic treatment

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significantly promotes gasoline desulfurization.

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3.4.2 Desulfurization mechanism experiment on crude oil The oxidant amount is the most important factor affecting the desulfurization of crude oil

according to the RSM. Therefore, the desulfurization data for crude oil with and without ultrasonic treatment under different oxidant amounts are shown in Fig. 5. The desulfurization rate of crude oil that is simply subjected to ODS is significantly lower than that of crude oil after ultrasonicassisted oxidation treatment. 18

As is clear from Fig. 5, the desulfurization rate increases as the oxidant amount increases. The desulfurization rate of crude oil with ultrasonic treatment is 30% higher than that without ultrasonic treatment. Based on these results, ultrasonic treatment is effective in the ODS process, because the physicochemical effect of ultrasonic treatment enhances the oxidizing conditions and forms cavitation bubbles, when acting on crude oil containing a small amount of oxidant. The cavitation bubbles rupture during repeated oscillation under the positive and negative pressure

ro of

phases, instantaneously creating a local high temperature and high pressure and causing intense agitation. The generated radicals, such as hydroxyl and hydrogen radicals, and the stimulated

reactive oxygen species can not only rapidly oxidize sulfides, but also create an ideal environment

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for the desulfurization process, leading the oxidability of the oxidant to increase. Ultrasonic

re

treatment causes intense emulsification, with the generation of high interfacial area, and

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helps to overcome the mass transfer limitations, enhances mass transfer efficiency, and improves the desulfurization effect. This is also consistent with the study by Bhasarkar et al.

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[55].

The infrared spectroscopy results obtained before and after ultrasonic treatment are shown in

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Fig. 6. The peak around 570–715 cm−1 can be assigned to the stretching vibrations of C-S, while the peak appearing at 1310–1335 cm−1 can be assigned to the deformation vibrations of C-S. The

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stretching vibration peak disappears, and the deformation vibration peak becomes smaller after UAOD. The peak around 655–695 cm−1 can be identified as the C-S-C vibration absorption peak, which disappears after UAOD. These results demonstrate the remarkable effects of UAOD. The peaks that appear at 1280 cm−1, 1156 cm−1, and 1060 cm−1 can be identified as the S=O characteristic absorption peaks of thiophene sulfoxide and sulfone after UAOD, indicating that 19

sulfoxide and sulfone were formed after thiophene oxidation. These findings illustrate the feasibility of UAOD. The core of the ultrasonic cavitation effect is the thermal decomposition reaction inside the microbubbles. This cavitation effect can also promote the liquid to generate hydroxyl radicals and other chemically active groups, strengthening the oxidation process.

4. Conclusion In this study, UAOD experiments on gasoline and crude oil were performed and optimized,

ro of

using single factor and response surface methodology. The effects of UAOD and the simple

oxidation method on the desulfurization of crude oil were compared. The following conclusions were drawn:

-p

(1) Comparison of the desulfurization of real gasoline and crude oil samples revealed that the

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UAOD technique has a better treatment effect on high-sulfur oil than on low-sulfur oil.

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(2) Ultrasonic power and irradiation time are significant factors affecting the UAOD of gasoline. The order of influence of the different factors is as follows: irradiation time > ultrasonic

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power > oxidant amount. The desulfurization efficiency is optimal with an ultrasonic power of 464.7 W, irradiation time of 5.5 min, and oxidant amount of 8.1 mL, as predicted using the RSM.

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The rate of sulfur removal from gasoline was predicted to reach 80.82%, and the average desulfurization rate obtained experimentally was close to this value, at 80.87%.

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(3) The oxidant amount and ultrasonic power are significant factors affecting the UAOD of

crude oil. The order of influence of the different factors is as follows: oxidant amount > ultrasonic power > irradiation time. The desulfurization rate of crude oil is the highest when the oxidant amount is 11.4 mL, ultrasonic power is 785.1 W, and irradiation time is 6.2 min, reaching up to 73.37%. 20

(4) Through comparison experiments, the mechanism of ultrasonic action was verified: the thermal, mechanical, and cavitation effects of ultrasonic treatment all had significant effects on the UAOD experiment. Simultaneously, an appropriate extraction method and oxidation time for gasoline desulfurization were selected. The addition of ultrasonic treatment can increase the efficiency of ODS by 30% for crude oil.

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Conflict of interest: We declare that we do not have any commercial or associative interest that represents a

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conflict of interest in connection with the work submitted.

Acknowledgements

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Funding: This work was supported by a National Natural Science Fund project (51104022); and

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teacher team construction Top-notch Youth Project (municipal) (PXM2016 014222 000043).

21

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28

80 75 70 65 Gasoline Crude oil

60 55

4

Ultrasonic power/W (a) Ultrasonic power

6

8

10

12

14

Desulfurization rate/%

Desulfurization rate/%

Gasoline Crude oil

0 10 0 20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 10 00

Desulfurization rate/%

80 75 70 65 60 55 50 45 40 35

16

74 72 70 68 66 64 Gasoline Crude oil

62 60

0 1 2 3 4 5 6 7 8 9 10 11 12 13

Oxidant amount/mL

Irradiation time/min

(b) Oxidant amount

(c) Irradiation time

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Fig. 1 Results of single factor experiments

(b) Oxidant amount and irradiation time.

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(a) Ultrasonic power and oxidant amount.

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Fig. 2 Response surfaces of gasoline.

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(a) Ultrasonic power and oxidant amount.

(b) Oxidant amount and irradiation time.

Fig. 3 Response surfaces for crude oil.

29

Fig. 4 Extraction method selection. 75

1.0

65

0.8

With ultrasonic treatment Without ultrasonic treatment

55 50 45

0.6 0.4 0.2

40

0.0

35 10

8

12

14

After treatment Crude oil

-0.2 4500 4000 3500 3000 2500 2000 1500 1000 500

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Oxidant addition amount/mL

16

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30

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60

Absorbance

Desulfurization rate/%

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1.2

70

Fig. 5 Comparison of desulfurization data obtained

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for crude oil with and without ultrasonic treatment.

30

Wave number/cm-1

Fig. 6 Infrared spectroscopy results obtained

before and after ultrasonic treatment.