Study on the Desulfurization of High-Sulfur Crude Oil by the

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Study on the desulfurization of high sulfur crude oil by electrochemical method Dong Liu, Ming Li, Raja L. AL Otaibi, Linhua Song, Wen Li, Qingyin Li, Hamid O. Almigrin, and Zifeng Yan Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 27 Sep 2015 Downloaded from http://pubs.acs.org on September 28, 2015

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Study on the desulfurization of high sulfur crude oil by electrochemical method Dong Liu1’*, Ming Li1, Raja L. AL Otaibi2,*, Linhua Song3, Wen Li4, Qingyin Li3, Hamid O. Almigrin2, Zifeng Yan1’*

1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China 2. Petrochemical Research Institute, P.O. Box 6086 Riyadh 11442, King Abdulaziz City for Science and Technology, Kingdom of Saudi Arabia 3. College of Science, China University of Petroleum, Qingdao 266580, China 4. Heavy Oil Development Company, Xinjiang oilfield company, Petrochina, Karamay, Xinjiang 834000, China

Corresponding author: Dong Liu, E-mail: [email protected] Raja L. AL Otaibi, E-mail: [email protected] Zifeng Yan, E-mail: [email protected]

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ABSTRACT: SnSb intermetallic compound was synthesized by chemical precipitation with NaBH4 as reducer, and then it was applied in the electro-desulfurization of Saudi crude oil as functional desulfurization material. The prepared SnSb intermetallic compound was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). Additionally, the raw material and treated oil were analyzed through gas chromatography-pulsed flame photometric detector (GC-PFPD) and gas chromatograph-mass spectrometer (GC-MS). The results showed that SnSb intermetallic compound restored with NaBH4 with lager particle size, lower surface area, lower crystallinity and ‘brick-shaped’ structure. The desulfurization efficiency for sulfocompounds with small molecular weight is higher. Especially, it should be noted that benzonaphthothiophene was hardly removed by traditional hydrogenation desulfurization whereas could be subtracted partly via electrochemical-adsorption desulfurization. The proposed desulfurization mechanism would be attribute to the adsorption reacts on the surface of SnSb intermetallic compound. KEYWORDS: SnSb intermetallic compound; Crude oil; Desulfurization; Sulfur-containing compounds; Adsorption


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1. INTRODUCTION Crude oil supplement in worldwide is deteriorated greatly day by day. The increases of sulfur in crude oil will make a huge difficulty to obtain high quality oil products, and require the more complex oil refining because of the corroding of equipment.1 The quality of gasoline, diesel oil and automobile fuel could affect the produced harmful air pollutants in vehicle emissions directly. In China, the new gasoline standard has been launched which limited sulfur content in 10 µg·g-1. Nevertheless it was demanded to maintained in 15 µg·g-1 in Europe, Japan and the United States. Therefore, it is significant and urgent to decrease the sulfur content in crude oil.2 Desulfurization process is conducted under high temperature and pressure recently, requiring large energy consumption and processing equipment with high quality. Thus, more and more researchers have paid attention to exploit the low-power and low-cost method for desulfurization, especially under moderate pressure and temperature. Adsorption desulfurization is considered as one of promising deep desulfurization technologies. Akira have studied the effects of types of molecular sieve adsorbents including Cu-Y, Ag-Y, Na-ZSM-S, H-USY and other adsorbents on the adsorption of sulfur compounds.3 The results showed that the macromolecular sulphur compounds were unable to be removed completely. 13X molecular sieve and activated carbon were utilized to remove the sulfur compounds in naphtha.4-5 Consequently, it was pointed out that 13X molecular sieve should be suitable for the higher temperature and sulfide concentration. Ca-Y molecular sieve were studied to remove the sulfur compounds in kerosene in Nanjing Refinery.6 The desulfurization rate can reach 50%, however, quantities of aromatics were adsorbed during desulfurization process, resulting in its poor desulfurization selectivity. Intermetallic compound, a type of alloys, was attracted the increasing attentions, performing great potential in desulfurization.7-8 The compound can be


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formed by covalent bond in both metal, or metalloid submetallic and metal atoms. Its atom was arranged highly in order.9-10 As a functional material, it shows high strength, hardness and thermo stability. On the other hand, it has become one of the research hot-spot due to its unique characteristics in electromagnetism, catalysis and optics.11 Lu S H found firstly that the SnSb intermetallic compound could react with organic sulfide under magnetic field.12 Roger demonstrated that the sulfur content can be reduced clearly in crude oil with SnSb intermetallic compound through magnetic tubular reactor.13 The desulfurization mechanism with SnSb intermetallic compound as desulfurizing agent is different from the current widely used hydrogenation desulfurization mechanism.14-15 Compared with traditional desulfurization technologies, this method could be conducted under normal pressure and temperature, and the the sulfur and asphaltene content in petroleum was declined. Thus, it was beneficial to the refining process with the low desulfurization cost and weak corrosion of refinery equipment. The characteristics of different desulfurization methods are shown in Table 1. Table 1 Characteristics of different desulfurization methods

Desulfurization types

Temperature and Pressure

Production cost and energy consumption

Desulfurization efficiency

Catalytic desulfurization

High temperature and high pressure

High-cost (large energy consumption; high-quality processing equipment)

High-efficiency desulfurization for light oils. But not used in heavy oils.

Biological desulfurization

Not fixed

High-cost (complex production; expensive catalyst)

High-efficiency in the desulfurization of light oils But not suitable for industrialization

Adsorptive desulfurization (molecular sieves)

Not fixed

High-cost (complex production; expensive adsorbents)

High-efficiency desulfurization for light oils. But not used in heavy oils.

Low-cost (low energy consumption; simple production process)

High-efficiency desulfurization for crude oil. (e.g. take SnSb intermetallic compound as functional desulfurization material)

Electro-desulfurization

Normal temperature normal pressure

and


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The study of desulfurization is mostly applied in gasoline and diesel fractions, but almost sulfur compounds with high molecular weight are present in the heavy fraction of crude oil. Thus it was investigated that the electro-desulfurization of crude oil was conducted using SnSb intermetallic compound as functional desulfurization material. Some achievements have been found in desulfurization with the SnSb intermetallic compound research recently16-20. However, few researches were focused on the determining of sulphur-containing compounds types and exploration of desulfurization mechanism in crude oil desulfurization. In our work, the SnSb intermetallic compound is synthesized by chemical co-precipitation with NaBH4 as a reducer. And then the electrochemical adsorption desulfurization for Saudi crude oil is carried out using SnSb intermetallic compound. Additionally, 21-24 the influences of reaction time and voltage on the desulfurization was discussed. The morphology and element composition of the SnSb intermetallic compound during the desulfurization were explored. The change in sulphur content from crude oil and sulphur types from different fractions were further studied to investigate the desulfurization reaction mechanism and SnSb intermetallic compound effect.25-26

2. EXPERIMENTAL SECTION 2.1 Materials The Saudi Arabian crude oil provided by China Petroleum and Chemical Corporation was used as raw materials and its characteristics were shown in Table 2. Table 2. Properties of the Saudi crude oil


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Element (wt %) ρ20 (g·cm-3)

0.8710

η50 (mm2·S-1)

14.12

Ash (wt %)

7.32

C

H

S

N

84.18

12.00

2.80

0.15

2.2. Preparation of SnSb Intermetallic Compound The SnSb intermetallic compound was synthesized by chemical co-precipitation method. The reducing agent (the mixture of 200 ml of NaOH (4.5 mol·L-1) and 200 ml of NaBH4 (5 mol·L-1)) was added to the oxidant (the mixture of 200 ml of SnCl2 • 2H2O (0.1 mol·L-1) and 200 ml of SbCl3 (0.1 mol·L-1) with stirring at the temperature controlled at 1-5℃ (ice bath). After the reaction, the products were centrifuged, washed by distilled water and ethanol for several times and dried for 2 hours at 120℃. The SnSb intermetallic compound particles were acquired through grinding for further analysis. Reaction equation: 4SnCl2 + 4SbCl3 + 5NaBH 4 aq→ 4SnSb + 5NaBCl4 + 10H 2 ↑

(1)

2.3. Desulfurization Experiment The Saudi crude oil was used as raw material to prepared water/oil emulsion27 (the proportion of oil and water was 3:1) with Span-80 as emulsifier. The experiment was carried out in the emulsion system using SnSb intermetallic compound under normal pressure and temperature and applied electric field. Experimental instrument was shown in Figure 1. The size of the reactor is 150mm×120mm×100mm (lengh×width×height). The external DC regulated power supply adopts


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copper rod as anode and carbon rod as cathode. The electromagnetic stirrer is used to mix the solution uniformly at the bottom of the reactor. SnSb intermetallic compound was dispersed in the emulsion system uniformly. NaCl was added to the emulsion to increase the electrical conductivity before adding voltage, and it was also added to the emulsion to help emulsification after the reaction. The oil was separated into two phases via centrifugal. The sulfur content was determined by tubular oven method, and the SnSb intermetallic compound was collected from the lower aqueous phase for further analysis.

1

V

A

3

2

4

Figure 1. Experiment of electro-suspension desulfurization 1- Power; 2-Copper electrode; 3-Graphite electrode; 4-SnSb intermetallic compound The light fraction were separated from the crude oil by distillation below 360℃, and the saturates and aromatics were separated and enriched through column chromatography method from the atmospheric residue. The sulfur contents of saturates and aromatics were determined by tubular oven method.


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2.4. Characterization The sulfur content was analyzed according to GB/T 887-90. Firstly, the samples were burned in the tubular in air flow. The generated sulfurous acid anhydride was then absorbed by hydrogen peroxide and sulfuric acid solution. Lastly, the sodium hydroxide standard solution was utilized as titrated medium to measure the content of sulfuric acid formed in the adsorption process above. The sulfur content was calculated according to the content of sulfuric acid. Crystallographic information of the products was investigated with X-ray powder diffraction (XRD, X’Pert PRO MPD, Cu KR) at a scanning rate of 1℃ min-1. XRD data were collected in the 2θ ranges from 10 to 90˚. The morphology of the SnSb intermetallic was examined with field-emission scanning electron microscopy (SEM, Hitachi, S-4800). The element of the SnSb intermetallic compound was characterized by energy dispersive spectrometer (EDS, Germany, 1791-N-016-000). Gas chromatography-pulsed flame photometric detector (GC-PFPD, USA, GC3800) was used to analyze sulfocompounds in light fractions. In addition, gas chromatograph-mass spectrometer (GC-MS, Agilent Technologies Co., Ltd, GC6890) was used to analyze sulfocompounds in saturates and aromatics. The GC was fitted with a RX1-5HT column (30m (×) 0.25uM df) and temperature was initially 60℃ and ramped 10℃/min to a final temp of 225℃ and hold for 10 min at 225℃. Helium was used as the carrier gas at a flow rate of 1ml/min. The Mass Spectrometer was set to record ranges of spectra from 35 to 400 m/v at a scan speed of 6000 scans/sec.

3. RESULTS AND DISCUSSION 3.1. Characterization of SnSb Intermetallic Compound 3.1.1. XRD and SEM analysis


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8000

*: 101

*:SnSb 7000 6000 5000 4000 3000

*: 012 2000

*: 003 1000

*: 202 *: 211

0 -1000 10

20

30

40

50

60

70

2θ/(°)

Figure 2. XRD spectrum of SnSb intermetallic compound

The XRD spectrum of SnSb intermetallic compound is shown in Figure 2. Due to the standard card of SnSb intermetallic compound, the characteristic diffraction peaks positions and intensities in the XRD spectra of SnSb intermetallic compound (29.092°, 41.745°, 51.662°, 60.272° and 68.522°) were consistent with those appeared in the standard XRD spectra of β-SnSb hexagonal crystal. It was indicated that the synthesized SnSb alloy was with high purity by this method. The sharp peaks (101, 012, 003, 202 and 211) in the spectrum indicated that the SnSb intermetallic compound with high crystallinity was produced by chemical precipitation method.


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Figure 3. SEM spectra of SnSb intermetallic compound (a) before desulfurization; (b) after desulfurization

SnSb intermetallic compound was washed by toluene thoroughly after desulfurization experiment. The morphology of the SnSb intermetallic compound is characterized by SEM. As shown in Figure 3, the SnSb intermetallic compound particles were stacked together with “brick-like” structure, and the particulates are distributed uniformly with little aggregation. The particles size is ranged from tens nanometers to hundreds nanometers. Besides, the morphology of SnSb intermetallic compound is changed little after desulphurization experiment. 3.1.2. EDS analysis The changes of elements adsorbed on the surface of SnSb intermetallic compounds are shown in Figure 4, and the analysis of element contents is shown in Table 3.


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Figure 4. EDS spectra of SnSb intermetallic compounds (a) and (b): before desulfurization; (c) and (d): after desulfurization

Table 3. Element analysis of SnSb intermetallic compounds

S

Sn

Sb

Element weight%

atomic%

Before desulfurization After desulfurization

1.15

4.19

weight%

atomic%

weight%

atomic%

39.94

40.55

60.06

59.45

39.48

38.85

59.37

56.96

From the EDS before desulfurization, it can be found that Sb is enriched on the surface of SnSb intermetallic compound and the atom ratio of Sn and Sb is 40.55:59.45. According to the standard reduction potential of Sn and Sb, Sb would be reverted to "nuclear" firstly, and thus the


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enrichment of Sn on the surface was observed. However, the nucleation rate of Sn is faster than that of Sb, so Sb is enriched on the surface of SnSb intermetallic compound. Therefore, it was suggested that the chemical co-precipitation method to synthesize SnSb intermetallic compound was able to be dynamically controlled.19 As shown in Figure 4 and Table 3, the difference in the two spectra illustrates that S peaks appear on the surface of SnSb intermetallic compound after desulfurization, and the elements on the surface of SnSb intermetallic compound are mainly composed of Sn(wt, 1.15%), Sb(wt, 39.48%), S(wt, 59.37%). The sulfide atoms was adsorbed on the surface of SnSb intermetallic compound, and other part of molecules were removed during the attachment by the cleavage of α bond by vibrational or rotational force. Therefore, the removal of sulfur on the surface of SnSb intermetallic compound depended on the physical adsorption and chemical adsorption. The element analysis shows that atomic ratio of Sn and Sb is change little, whereas the content of sulfur increases from zero to 4.19%. It was conclude that a part of sulfide was adsorbed on the surface of SnSb intermetallic compound. 3.2. Effect of Reaction Parameters on The Desulfurization Performance 3.2.1 Effect of time The effects of reaction time on desulfurization were shown in Figure 5. The process was carried out at 20V voltage, oil to water ratio of 3:1. The desulfurization efficiency increased with reaction time, which might be due to more interaction between sulfur compound and SnSb active sites, however, it seemed to level off since the reaction time furthered extended from 18 to 30 h, which could be attributed to limited active sites in the sulfur compound. Thus, the optimal


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reaction time was chose to 18 h, under which the desulfurization process was conducted completely.

Figure 5. Effect of reaction time on desulfurization rate of crude oil

3.2.2 Effect of voltage The effects of voltage on desulfurization were shown in Figure 6. The process was carried out at reaction time of 18 h, oil to water ratio of 3:1. The desulfurization efficiency increased with the increasing of voltage and it reached a maximum value and then decreased quickly. Due to the properties and characteristic structure of SbSn, the charge carriers transferred between the n-type Sn and p-type Sb, which led positive charged Sb on the surface. This interior electrical field with the presence of outer field promoted connection between the polar sulfur and the intermetallic surface. Thus, the polar sulfur in the emulsion could be attached with Sb atoms on the alloy surface under the induced polarized field. The electrochemical reaction during the attachment resulted in the removal of sulfur compounds.


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Figure 6. Effect of loading voltage on desulfurization of crude oil

3.3. Desulfurization Performance 3.3.1. Desulfurization of crude oil The changes of sulfur content in crude oil, light fraction (the fraction distilled below 360℃), saturates and aromatics after electrochemical desulfurization are shown in Table 4. The data indicated that 22.43% of the sulfide was removed from the crude oil by electrochemical desulfurization. The desulfurization rates of light fraction, saturates and aromatics were 54.11%, 1.16% and 27.85%. Table 4. Change of sulfur content in crude oil

Raw oil

Crude oil

Light fraction(＜360℃)

Saturate

Aromatic

Sulfur content (w %)

2.80

2.00

0.09

0.09


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Desulfurization rate (w %)

22.43

54.11

1.16

27.85

The changes of basic properties in crude oil are shown in Table 5. The analysis showed that the density, viscosity, flash point, pour point, and carbon residue of the crude oil decreased after desulfurization process, which may be attributed to the removal of sulfur compounds with lager molecular weight. Table 5. Changes of basic properties in crude oil

Crude oil

Density（20 ℃）/g·cm-3

Viscosity（50 ℃）/mm2·s-1

Flash point/ ℃

Pour point/℃

Carbon residue/m%

Before desulfurization

0.87

14.12

25

-30

5.84

After desulfurization

0.86

14.07

23

-33

5.76

3.3.2. Desulfurization of light fraction GC-PFPD is a kind of effective method to analyze the contents and distributions of sulfocompounds for light fraction of crude oil (the fraction distilled below 360℃). Most of the qualitative and quantitative analysis of sulfocompounds can be identified by temperature programmed sulfidation (TPS).28 The changes of sulfur contents in light fraction and desulfurization rates are shown in Table 6. Table 6 Sulfur types and desulfurization rates of sulfide


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Sulfur types

Before desulfurization /ppm

After desulfurization /ppm

Desulfurization rate /%

H2S

0.04

0

100

thiols

0.30

0.01

97.85

thioethers

1.09

0.24

78.01

disulfides

0.95

0.37

60.92

thiophene

0.24

0.14

42.4

C2- hiophene

0.20

0.14

30.56

≥C3-thiophene

6.76

4.97

26.21

BT

2.71

2.27

16.17

C1-BT

8.77

7.85

10.47

C2-BT

31.26

28.00

10.43

≥C3-BT

43.19

39.81

7.52

DBT

0.86

0.83

3.44

C1-DBT

0.79

0.78

1.14

C2-DBT

0.25

0.25

0.35

≥C3-DBT

0.07

0.06

0

As seen in Table 6, the desulfurization rates of the reactive sulfocompounds such as H2S, thiols, thioethers and disulfides are 100%, 97.85%, 78.01% and 60.93% respectively, the desulfurization rates of thiophene, C2-thiophene and other thiophene (which were deprived secondly) are 42.4%, 30.56% and 26.21% respectively, the desulfurization rates of BT, C1-BT, C2-BT and other BT were 16.17%, 10.47%, 10.43% and 7.52% respectively, but the desulfurization experiments have no influence on dibenzothiophene. Through the results of calculation in Table 3, it can be seen that micromolecule sulfocompounds were taken off from


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the system firstly, and the desulfurization efficiency of the micromolecule sulfocompounds is higher than that of macromolecular sulfocompounds. It is difficult for macromolecular sulfocompounds like C4DBT to be adsorbed on the surface of the SnSb intermetallic compound because of the sulfocompounds’ steric hindrance. 3.3.3. Desulfurization of saturates and aromatics The possible sulfur-containing compounds and desulfurization rates of saturates and aromatics were shown in Table 7 and 8. Table 7. Possible sulfur structures and desulfurization rates of saturates Molecular formula

Structural formula

Desulfurization degree/%

C12H14N2S

6.51

C13H17N2S

3.45

C13H16N2O2S

1.32

The possible structures of sulfocompounds and the desulfurization rates were calculated from GC-MS spectra, as shown Table 7. A few of sulfocompounds could be identified by GC-MS, because saturates fraction contain many hydrocarbons with lower sulfur content. Some spectral peaks of sulfocompounds are overlapped by the spectral peaks of hydrocarbons, which is


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difficult to be identified. Although the structures of sulfur types in saturate is complex, SnSb intermetallic compound still has some effect on desulfurization under the condition of NPT (normal pressure and temperature) and appropriate voltage. It shows that sulfide on the interface of water/oil emulsion is polarized under the electric field. The removal of sulfocompounds with complex structures owes to adsorption of sulfide on the surface of porous SnSb intermetallic compound. Table 8. Possible sulfur structures and removal rates of aromatics Molecular formula

Structural formula

Desulfurization degree/%

C16H10S

17.4

C17H12S

8.59

C18H14S

1.61

As shown in Table 8, the desulfurization rate of benzo-naphtho-thiophene reaches up to 17.4% after electrochemistry desulfurization experiment, and SnSb intermetallic compound still have some effect on benzo-bis-benzothiophenes. It is difficult for hydrogen desulfurization to remove benzonaphthothiophene because of its high stability, but electrochemistry desulfurization can remove it partly in some degree under normal pressure and temperature, which proves that SnSb intermetallic compound is superior to hydrogen desulfurization.


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3.4. Desulfurization Mechanism SnSb intermetallic compound is semiconductor with narrower band gap. Sb is enriched on the surface of SnSb intermetallic compound as donor and Sn as acceptor.26 Under the function of electricity, electrons in valence band are excited to a higher energy state, and then the free holes are formed due to the valence electrons of Sb transfer. Furthermore, the SnSb intermetallic compound exhibited the characteristic of electron deficiency. Additionally, sulfur atoms in sulfocompounds have single pair electrons and high electronic cloud density. Therefore, polar sulfur with rich electron is adsorbed to electron-deficient SnSb intermetallic compound. The pole-pole interation led to the absorption of sulfur on the surface of Sb elements, and the polar sulfide could be removed by the cleavage of α or β bond with vibrational or rotational force .25，27

4. CONCLUSION In the study, the Saudi crude oil was electro-desulfurizatied using SnSb intermetallic compound as desulfurizeris. The optimal desulfurization could be achieved at 18 h and 20 V voltage. Clearly, the morphology of SnSb intermetallic compound is not changed obviously after desulphurization experiment and S spectrum peaks appear on the surface of SnSb intermetallic compound. The sulfur-containing compounds adsorbed on the surface of SnSb intermetallic compound are removed by the cleavage of sulfocompounds at α or β bond by vibrational or rotational force. The desulfurization efficiency of the micromolecule sulfocompounds is higher than that of macromolecular sulfocompounds. It is difficult for macromolecular sulfocompounds to be adsorbed on the surface of the SnSb intermetallic compound because of the steric hindrance.


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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21176259) and the Fundamental Research Funds for the Central Universities (15CX05009A).

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Study on the Desulfurization of High-Sulfur Crude Oil by the

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