A simulation on the formation of organic sulfur compounds in petroleum from thermochemical sulfate reduction

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 36, Issue 1, February 2008 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2008, 36(1), 48−54

RESEARCH PAPER

A simulation on the formation of organic sulfur compounds in petroleum from thermochemical sulfate reduction DING Kang-le 1, *, LI Shu-yuan1, YUE Chang-tao1, ZHONG Ning-ning2 1

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China

2

Faculty of Natural Resource and Information Technology, China University of Petroleum (Beijing), Beijing 102249, China Abstract: Thermochemical reduction of magnesium sulphate with n-pentane was carried out in an autoclave in the presence of water at high temperature and pressure to simulate the origination of organic sulfur compounds in petroleum from thermochemical sulphate reduction (TSR). The products of 3 phases (gas, oil, and solid) were analyzed by some advanced analytical methods including gas chromatography, microcoulometry, FT-IR, and X-ray diffraction. Results show that the TSR reaction occurred at 425–525°C with MgO, C, H2 S, CO2 , and a series of organic sulfur compounds such as mercaptans, sulfoethers, and thiophenes as the main products. The reaction kinetics was studied and the calculated activation energy is 58.0 kJ/mol. Key Words:

thermochemical sulfate reduction; n-pentane; magnesium sulphate; simulation experiment; organic sulfur compounds;

kinetics

The present decrease in petroleum resources makes high sulfur crude oil an important choice for our energy demand [1] . There are a series of sulfur-containing compounds in crude petroleum, such as hydrogen sulfide, element sulfur, mercaptan, sulfoether, bisulphide, thiophene, etc [2]. Direct cracking or biodegradation of kerogen and oil is routinely assumed as the main natural source of these sulfur-containing compounds in crude petroleum[3–6]. Sulfates are usually found in carbonate rock reservoirs. Under certain temperature and pressure, hydrocarbons in oil and gas reservoirs can react with sulfates; this is called thermochemical sulfate reduction (TSR). Several organosulphur species such as sulphur-containing diamantane and mercaptan were detected in some deep buried carbonate reservoirs, which may be associated with TSR and may be the results of organic-inorganic interactions between crude petroleum and sulfates [7,8]. The origin and distribution of sulfur compounds are highly valuable for investigating the source of petroleum, the degree of oil-gas maturity, and the correlation of oil-gas sources ; moreover, they have great influence over oil quality, processing equipment maintenance, and environmental protection. Therefore, the study of the thermochemical origin of

sulfur-containing compounds in crude petroleum is scientifically and economically important for the exploration of oil reservoirs associated with TSR and the processing of high sulfur crude oil. Most of the previous simulation experiments were focused on the TSR origin of high H2S and its geological significance [9–13], while a simulation on the TSR origin of organic sulfur compounds (OSCs) has not yet been reported. Based on sulfur isotopic changes, Amrani et al[14] studied the possibilities of incorporating inorganic sulfur into oil and gas reservoirs. In this study, thermochemical reduction of magnesium sulphate with n-pentane was carried out in an autoclave in the presence of water at high temperature and pressure to simulate the origination of organic sulfur compounds in petroleum from the TSR. The products of 3 phases (gas, oil, and solid) were analyzed by some advanced analytical methods including gas chromatography (GC), microcoulometry, FT-IR, and X-ray diffraction.

1 1.1

Experimental Apparatus and reagents

Received: 2007-07-11; Revised: 2007-10-09 * Corresponding author. E-mail: [email protected] Foundation item: Supported by the National Natural Science Foundation of China (40472076, 40702019) and the Item of Cooperative Fund of Beijing Educational Committee (XK114140479). Copyright  2008, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

Thermochemical reduction of magnesium sulphate with n-pentane was carried out in an autoclave made of stainless steel WYF-1 with a volume of 200 mL. The reaction temperature was controlled with the precision of ±1 °C. For each test, a sample basket of quartz glass containing 10.000 g MgSO4 powder was placed at the bottom of the autoclave, which was then vacuumed, and 20 mL n-pentane and 10 mL distilled water were loaded into the autoclave through a feed regulator. Analytical reagents n-pentane and magnesium sulphate were used for the TSR tests. For GC analysis, all the sulfur compounds are analytical reagents purchased from Fluka Chemi AG and Acros Organics: dimethyl sulfide, isopropylmercaptan, tert-butylmercaptan, n-propyl mercaptan, methyl ethyl sulfide, thiophene, diethyl sulfide, n-butyl mercaptan, dimethyl disulfide, 2-methyl thiophene, 3-methyl thiophene, tetrahydrothiophene, 2-methyl tetrahydrothiophene, n-amylmercaptan, 2-ethyl thiophene, 2,5-dimethylthiophene, 2,4-dimethylthiophene, 2,3-dimethylthiophene, dipropyl sulfide, diethyl disulfide, n-hexylmercaptan, 3,4-dimethylthiophene, n-heptyl mercaptan, n-butyl sulfide. The standard solutions of sulfur-containing compounds were prepared with n-octane. 1.2 Reaction conditions and analytical methods Since it is hard to detect any reaction product under a temperature lower than 300°C, the reactor was first heated to 340°C directly and then programmed to the final temperatures. The final temperatures were 425°C, 450°C, 475°C, 500°C, and 525°C, and the heating times were 48 h, 36 h, 24 h, 12 h, and 6 h, respectively. The final pressure of the reaction system ranged from 15.0 MPa to 20.0 MPa. When the desired reaction temperature or time was attained, the stainless steel autoclave was withdrawn from the oven, air cooled for 30 min, and then rapidly cooled to room temperature by quenching in water. After the sample basket (quartz glass cylinder) with solid products was taken out from the autoclave, the mixture of oil and water was drawn from the reactor with a pipette and separated with a micro extraction funnel. The autoclave wall was then rinsed with distilled water 3–4 times, each with about 10 mL water. The quartz glass cylinder with solid products, the separated water, and the rinse solution were all put together into a ceramic crucible. The crucible was kept at 120°C for 2 h in an oven to remove water, and then calcinated at 550°C for 2 h in a muffle oven. The weight change of the solid reactant and the solid products after calcinations owing to the TSR reaction was measured by an electronic balance. The products of 3 phases (gas, oil, and solid) were analyzed by microcoulometry, gas chromatography, capillary gas chromatography combined with a pulsed flame photometric

detector (GC -PFPD), FT-IR, and X-ray diffraction, respectively. Microcoulometry To determine the total sulfur of gaseous products in n-pentane and magnesium sulphate system, the microcoulometry (WK-2B) was used. The furnace entrance temperature was 500°C and the exit temperature was 850°C. The temperature of the gasification zone was 60°C. The flow rates of air, nitrogen, and sample gas were 40 mL/min, 160 mL/min, and 30 mL/min, respectively. GC analysis A gas chromatograph of Agilent 6890 was equipped with two detectors (TCD and FID) and five mixed columns (capillary columns and packed columns). The detector temperature was 250°C. The oven temperature was first kept at 50°C for 3 min, then heated to 100°C with a heating rate of 5°C/min, to 180°C with a heating rate of 10°C/min, and maintained at 180°C for 3 min. GC-PFPD An Agilent 6890 series gas chromatograph with a RIPP PONA column (50 m × 0.2 mm i.d. × 0.5 µm film thickness), an electronic pressure controller, and an automatic sample injector with a model of Agilent 7683 was used with ultra-high pure nitrogen as carrier gas. The injector temperature was kept at 250°C and its pressure is kept at 14 psi. A pulsed flame photometric detector (PFPD, O.I. Analytical 5380) was used for GC-PFPD analysis and its temperature was kept at 250°C. The injection volume was 1.0 µL and the split ratio was 30:1. For the oil products, the column temperature was set at 35°C for 1 min, from 35°C to 100°C at 1.5°C/min, and then to 250°C at 10°C/min, and maintained at 250°C for 15 min. FT-IR All spectra were recorded from 4000 cm-1 to 400 cm–1, using the Nicolet FT-IR spectrometer. Each spectrum provided the averaged value of 32 scans using a spectral resolution of 4 cm–1. XRD The products were mechanically crushed and ground to a size of < 200 mesh. X-ray diffraction (XRD) patterns were collected at room temperature with a RIGAKU D/ max-rB automatic X-ray diffractometer under a voltage of 50 kV, and a current of 150 mA at a scanning speed of 2°/min, using graphite monochromated Cu-Kα radiation. The experiment parameters were as follows: DS = 1°, SS = 1°, RS = 0.3 mm.

2 2.1

Results and discussion Gas products

The gas products at 425–525°C had a foul odor (rotten egg smell). The wetted lead acetate indicator paper turned black when exposed to the gas products, which indicated that hydrogen sulfur was produced and TSR had taken place. The total sulfur contents in gaseous products determined by

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

microcoulometry are shown in Fig. 1, which suggests that the total sulfur in the gaseous products increases with temperature and that high temperature is favorable for TSR.

Fig. 1

Sulfur contents in gaseous products of TSR at different reaction temperatures

Fig. 2

CH4 /SCn H2n+2 and content of CO2 in gaseous products at different reaction temperatures

The compositions in the gaseous products at different temperatures are listed in Table 1. The gas products are complex, including C1–C5 saturated hydrocarbons, C3–C5 unsaturated hydrocarbons, H2, and CO2. C1–C4 saturated hydrocarbons, C3–C5 unsaturated hydrocarbons, and H2 are possibly derived from the oxidation of n-pentane by magnesium sulphate or from the cracking of n-pentane. It should be noted that CO2 is also formed as a gaseous product besides H2S. Table 1

and the content of CO2 increase with temperature, which indicates that carbon chains of hydrocarbons are reduced and the oxidation of hydrocarbons by magnesium sulphate tends to be intensified, as the temperature increases.

Compositions of hydrocarbons in gaseous products at different temperatures Gas composition ϕ / %

Species 425°C

450°C

475°C

500°C

525°C

CH4

13.4255

27.3384

28.9309

33.5728

34.9377

C 2 H6

38.9646

46.3822

43.3221

38.0693

36.0636

C 3 H8

13.8487

8.3369

9.9720

8.8917

8.4671

C 3 H6

6.7999

2.7800

0.0000

0.0000

0.0000

n-C 4 H10

2.5137

0.8108

1.2847

0.7548

0.8178

n-C 4 H8

0.6763

0.1338

0.2047

0.0787

0.1022

i-C 4 H8

0.4886

0.2162

0.1771

0.0907

0.1097

i-C 5 H12

0.4246

0.0944

0.2326

0.2238

0.0762

n-C 5 H12

0.6176

0.3004

4.9349

5.2204

3.5125

1,3-butadiene

17.6573

8.2162

0.0000

0.0000

0.0000

n-C 5 H10

0.4998

0.0687

0.0994

0.0354

0.0564

H2

2.3451

2.6028

5.4532

6.3093

6.8230

CO2

1.7383

2.7191

5.3885

6.7530

9.0338

It is seen in Fig. 2 that both CH4/SCnH2n+2 (gas dryness)

2.2 Oil products On the basis of the qualitative analysis of sulfur compounds in the gasoline[15,16] and the change of boiling points of sulfur compounds, the organosulphur species of the oil phase at 425–525°C were qualitatively and quantitatively analyzed with GC -PFPD based on the present twenty-four standard sulfur-containing compounds. The retention times of sulfur compounds in oil phase were qualitatively determined according to the laws: the less the number of the substituting group, the shorter was the retention time; the nearer the location of the substituting group around sulfur atom, the shorter was the retention time; thiophene compounds with adjacent substituting groups have longer retention times than others. Since PFPD is an equimolar detector irrelevant to the structures or types of sulfur compounds, the given concentrations of standard sulfur compounds were adopted to create a calibration curve to measure the contents of each sulfur compound in oil phase. 10 standard samples containing 3-methyl thiophene as sulfur compounds in n-octane were prepared and used for determining the standard curve of sulfur response. The standard curve is given by: y = 0.0014x3 – 0.0609x2 + 79.215x –213.10, R2 = 0.9999, where, y is the peak area and x is the sulfur content (µg· g–1). Fig. 3 depicts the GC-PFPD chromatogram of sulfur compounds in oil products (425– 525°C). The qualitative and quantitative results of sulfur compounds in oil phase are listed in Table 2. Besides hydrogen sulfur, mercaptans, sulfoethers, and thiophenes were also detected as the products of TSR between n-pentane and magnesium sulphate. The contents of hydrogen sulphide, mercaptans, sulfoethers, thio phenes, and total sulfides at

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

different reaction temperatures are given in Figs. 4–8, Table 2

respectively.

Sulfur-containing compounds in the reaction of n-pentane with magnesium sulfate Contents of sulfur compounds in oil phase w /µg⋅g–1

Retention time Compound t / min

425°C

450°C

475°C

500°C

525°C

Hydrogen sulphide

6.597

24.4308

22.0844

17.5714

18.0220

14.9954

Methyl mercaptan

7.683

13.3624

10.0796

48.7617

30.8954

16.8535

Ethyl mercaptan

9.266

88.2910

78. 5402

17.6711

223.1614

134.3174

Propyl mercaptan

11.002

151.1812

55.1551

62.5910

246.9821

146.4300

Ethylmethyl sulfide

12.651

11.8998

12.1132

7.9709

51.6071

22.8915

Isopropyl mercaptan

13.320

112.5921

109.5960

33.1579

183.3950

112.7457

Thiophene

17.103

311.9236

644.6603

671.6448

662.0842

650.0378

Butyl mercaptan

20.657

14.4773

11.0458

6.6170

17.2461

23.9059

C 5 -Mercaptan

23.490

38.1943

20.1963

4.7528

43.7786

54.0432

2-Methylthiophene

25.308

421.7116

342.2635

623.5988

613.4564

619.7543

3-Methylthiophene

25.931

66.2396

73.0979

351.0963

652.5428

665.6889

THF

28.055

0

3.8568

13.6027

11.7781

C 6 -Mercaptan

32.501

11.0337

0

5.9726

19.9307

13.0018

2-Ethylthiophene

34.473

45.1502

61.8758

2,5-Dimethylthiophene

35.060

34.8584

27.2286

68.0690

C 6 -Sulfide

35.664

8.4710

11.3732

133.0175

2,4-Dimethylthiophene

35.808

44.3662

47.8910

2,3-Dimethylthiophene

36.969

17.1093

20.3372

61.9586

3,4-Dimethylthiophene

38.570

6.9567

8.9035

27.4382

0

0

2-Propylthiophene

44.604

9.1796

9.2804

35.1977

0

0

3-Propylthiophene

45.297

11.9293

15.9137

44.9491

0

0

C 7 -Sulfide

46.300

0

13.3148

23.7213

2-Methyl-4-ethylthiophene

46.313

0

0

23.6480

3-Methyl-4-ethylthiophene

46.816

0

0

14.3247

2-Methyl-5-ethylthiophene

46.309

0

0

2,3,5-Trimethylthiophene

48.099

8.0830

7.8017

2,3,4-Trimethylthiophene

52.187

0

0

C 4 -Thiophene

57.340

C 5 -Thiophene

62.166

0

0

0

6.2917

C 6 -Thiophene

69.887

0

0

0

7.0128

16.3144

Benzothiophene

67.950

381.2778

479.0958

0

20.0094

45.0365

With increasing temperature, the contents of hydrogen sulphide in oil phase decrease (Fig. 4), which can be explained by the further reaction between hydrogen sulphide and remained hydrocarbons in autoclave. High temperatures induce the secondary reaction and then cause the negative correlation between the H2S content and temperature.

6.5850

75.7119

0

0

0

0 0 193.1903 0 115.2648

42.2883 0 28.1758 49.5706

777.5385 0 60.1948 0 0

20.3637 0 0 15.4529

15.0328

0

0

8.3099

0

0

11.2090

0

83.3147

21.6780 0

Figure 5 shows that there is no evident correlation between the content of mercaptans and the temperature at 425–525°C. This can be attributed to the low thermal stability of mercaptans at high temperatures; the content of mercaptans in oil phase is controlled simultaneously by the generation rate and the thermal decomposition rate that depend on the

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

reaction temperature. As seen in Fig. 6, the content of sulfoethers increases with temperature from 425°C to 500°C, but then declines with further increase of the reaction temperature. The reason may be that the thermal decomposition rates of sulfoethers exceed their generation rates at 500–525°C. The positive correlation between the content of thiophenes and the reaction temperature is shown in Fig. 7.

300

Sulfoethers content w / 10

-6

250 200 150 100 50 0 420

Fig. 6

440

460 480 o Temperature t / C

500

520

540

Content of sulfoethers in oil phase at different reaction temperatures

3500

Thiophenes content w / 10

-6

3000

Fig. 3 GC-PFPD Chromatogram of sulfur compounds in oil

2500

2000

1500

1000

products (425–525°C) 420

440

26

H2S content w / 10

-6

Fig. 7

460 480 500 o Temperature t / C

520

540

Content of thiophenes in oil phase at different reaction

24

temperatures

22

Thiophenes are thermally more stable than mercaptans or sulfoethers, therefore, the conversion of inorganic sulfur to thiophene sulfur in the TSR system increases with temperature. The total content of sulfur-containing compounds in oil phase correlates positively with the reaction temperature (Fig. 8), which coincides with the results of total sulfur in the gaseous products. All these suggest that the conversion of inorganic sulfur to organic sulfur in the whole TSR process, especially to thiophene sulfur, increases with the reaction temperature.

20

18

16

14 420

440

460

480

500

520

540

o

Temperature t / C

Fig. 4 Content of H 2 S in oil phase at different reaction temperatures

4000

800 3500 -6

Sulfides content w / 10

Mercaptans content w / 10-6

700 600 500 400

3000 2500 2000 1500

300

1000

200

420

100 420

Fig. 5

440

460 480 Temperature t / oC

500

520

540

Content of mercaptans in oil phase at different reaction temperatures

Fig. 8

440

460 480 o Temperature t / C

500

520

540

Total content of sulfur -containing compounds in oil phase at different reaction temperat ures

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

2.3

Solid products

430

1166

3423

1638

1018 896

Some black material was observed as the solid product for TSR; after calcination at 550°C, the black material turns white. Therefore, the black deposit is probably coke. The FT-IR spectrum of the solid product after calcination at 475°C is shown in Fig. 9. The peaks at 3423 cm–1 and 1638 cm–1 are the vibrational bands of water. The peak at 1166 cm–1 is the vibrational band of sulfate radical. Especially, the appearance of peak at 430 cm–1 can be explained by the formation of MgO[17]. It should be noted that the decomposition of magnesium sulphate may only take place at a temperature over 1124°C [18]. The temperatures for TSR simulation here are far lower than the decomposition temperature of magnesium sulphate; therefore, MgO produced should be attributed to the reduction of MgSO4 by hydrocarbons.

4000

3500

3000

2500

2000

1500

1000

500

Wavenumbers σ / cm

-1

Fig. 9

FT -IR spectrum of solid products after calcination

The XRD patterns of solid products of TSR at 475°C are shown in Fig. 10.

results of FT-IR. Based on the analysis of the 3-phase products (gas -oil-solid), it is concluded that the main products for TSR reaction between n-pentane and magnesium sulphate are MgO, C, H2S, CO2 , and a series of organic sulfur compounds such as mercaptans, sulfoethers, and thiophenes .

3

Kinetics

The reaction of n-pentane and magnesium sulphate is assumed as n order reaction, and its kinetic equation is written as follows [19]:

dx = k (1- x )n = Ae- E / RT (1- x ) n dt

(1)

For n ≠ 1, the approximate integration of equation (1) will give 1 − (1 − x)1- n AR 2 RT E (2) ln = ln (1 − )− (1 − n )T 2 βE E RT For n = 1, equation (1) after taking the approximate integration changes into: AR 2 RT E  − ln(1 − x )  (3) ln  = ln (1 − )− 2  T βE E RT   Where, x is the reaction conversion; t is the reaction time in s; A is the frequency factor in s –1; E is the activation energy in kJ· mol–1; T is the absolute temperature in K; n is the reaction order; R is the gas constant (8.314 J·mol–1· K–1); and β is the –1 heating rate in K· s . Based on the conversions at different temperatures, the 1-n linear regression coefficients of lines for - ln 1- (1- x) vs. (1- n) T 2 103/T (0 ≤ n <1) and - ln -ln(1 - x ) vs. 103/T (n = 1) can be T2 drawn. For the reaction of n-pentane and magnesium sulphate, the conversion was calculated from the weight change of the solid products after calcinations and the solid reactant; the conversions at 425°C, 450°C, 475°C, 500°C, and 525°C are 5.23%, 7.77%, 10.94%, 16.56%, and 24.01%, respectively. The linear regression coefficients are listed in Table 3, which suggest that the reaction of n-pentane and magnesium sulphate may be best described in zero order. Table 3 Linear regression coefficients for the reaction of n-pentane and magnesium sulphate

Fig. 10

X-ray patterns of the solid products after calcinations

Two distinct crystallographic phases, MgO and MgSO4 are found in the products; these suggest that MgO is produced during the TSR process, which coincides with the

Reaction order

Regression coefficient

0.0

0.9983

0.2

0.9981

0.4

0.9979

0.6

0.9977

0.8

0.9975

1.0

0.9872

DING Kang-le et al. / Journal of Fuel Chemistry and Technology, 2008, 36(1): 48−54

This indicates that the reaction rate is dependent of neither the volume of n-pentane nor the weight of MgSO4; it is a function of temperature. Figure 11 depicts the regression line for the reaction between n-pentane and magnesium sulfate. The apparent activation energy E is 58.0 kJ/mol and the apparent frequency factor A is 6.22×10–2 s –1, as determined by the slope and intercept of regressed line. Tang et al[20] reported that MgSO4 may be more effective than other sulfates in the oxidation of hydrocarbons. This study demonstrates that n-pentane can react with magnesium sulphate at 425°C in the presence of water. The activation energy is relatively low and the reaction mechanism requires further investigation.

T — absolute temperature, K; n — reaction order; R — gas constant, 8.314 J· mol–1· K–1; –1 β— heating rate, K· s

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Thermochemical reduction of magnesium sulphate with n-pentane can take place at 425–525°C in the presence of water at high temperature and pressure , with MgO, C, H2S, CO2, and a series of organic sulfur compounds such as mercaptans, sulfoethers, and thiophenes as the main products. It was observed that with increasing temperature, the oxidation of hydrocarbons by magnesium sulphate tends to be intensified, the carbon chains of hydrocarbons decrease, and the conversion of inorganic sulfur to organic sulfur, especially to thiophene sulfur, increases in the whole process of TSR. The TSR of n-pentane and magnesium sulphate may follow a zero order kinetic with the apparent activation energy of 58.0 kJ/mol.

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