Thermal behavior of coal-derived asphaltenes

Fuel Processing Technology, 9 (1984) 307--313

307

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

THERMAL BEHAVIOR OF COAL-DERIVED ASPHALTENES

RYOICHI YOSHIDA, SHOHEI TAKEDA

Government Industrial Development Laboratory, Hokkaido, Sapporo 061-01 (Japan) SHINICHI TERAMOTO

Pfizer--Quigley K.K., Kobe 653 (Japan) TORU MATSUSHITA

Department of Applied Chemistry, Faculty of Engineering, Hokkaido Univeraity, Sapporo 060-01 (Japan) and GEN TAKEYA

Hakodate Technical College, Hakodate 042 (Japan) (Received June 1, 1984; accepted June 25th, 1984)

ABSTRACT

Thermal behaviour of coal-derived asphaltenes was investigated under atmospheric pressure by thermogravimetry (TG). The weight loss is rapid from 300 to 500°C and is slow above 500°C. The asphaltene comparatively low in molecular weight shows the greater weight loss. The temperature at which the differential thermogravimetry peak appears, Try, correlates to the asphaltene aromaticity: the asphaltene comparatively high in aromaticity shows higher Tin. Of the residual asphaltene after heating up to 600°C, the obtained crystallite thickness through the c-axis direction (Lc00~) has a correlation with the molecular weight of the parent asphaltene: the parent asphaltene comparatively low in molecular weight produces residual asphaltene of larger Lcoo2. INTRO DUCTION

Coal-derived asphaltene, hexane insoluble and benzene soluble, is an intermediate product of coal hydrogenation. A series of fundamental studies of the utilization of coal-derived asphaltenes as industrial raw materials and energy resources is being made, particularly using Japanese Hokkaido coals, and reports have been made of the chemical structure [1,2] and the reactivity for hydrogenation [3--6] of asphaltenes derived through the hydrogenation of those coals. As a result, it was clarified that the rate constants of hydrogenation of asphaltenes are smaller by one order of magnitude than those in the initial step of hydrogenation of original coals [ 5,7]. Stannous sulfide-ammonium chloride [ 8], red-mud plus sulfur [ 3,5,9] and metal halides [ 10 ] have been used as the catalysts for the hydrogenation of asphaltene. However, these catalytic materials have very low activities. Therefore, it became necessary to search for a highly active catalyst for hydrogenation of asphal-

0378-3820/84/$03.00

© 1984 Elsevier Science Publishers B.V.

308

tenes and to investigate a process to treat asphaltenes by means other than hydrogenation. Carbonization and graphitization of coal liquids have been investigated [11,12] and recently the feasibility of producing olefins by pyrolysis of coal liquids was examined [13--16]. However, the thermal behavior of asphaltene has not been investigated from the point of view of designing a new and effective processing method. In the present paper, thermogravimetry (TG) was carried o u t to investigate the thermal behaviour of asphaltenes. Residual asphaltenes after thermal treatment were studied by X-ray diffractometry. The results of TG and Xray diffractometry are discussed in the light of the chemical structure of parent asphaltenes. EXPERIMENTAL

Asphaltenes were prepared from 5 Japanese Hokkaido coals varying in rank: Soya coal (C: 68.7%, H: 5.5%), Haboro coal (C: 75.2%, H: 6.2%), Taiheiyo coal (C: 76.9%, H: 6.4%), Horonai coal (C: 78.4%, H: 5.9%) and Oyubari coal (C: 85.6%, H: 6.2%). A shaking-type autoclave (500 ml) was used as the experimental apparatus for hydrogenation. The catalyst used was red-mud, which is a discharge product from bauxite when alumina is refined and consists mainly of Fe203 [3]. Sulfur was used as a promoter. Decrystallized anthracene oil was used as the vehicle. Reaction conditions were 400°C and 5--31 minutes reaction time. Initial hydrogen pressure was 9.8 MPa. For Taiheiyo and Oyubari coals a rotating-type autoclave (12 litres) was also used without vehicle under the conditions of 400°C, 19.0--25.4 MPa and about 120 min. After reaction, hexane and benzene extractions were carried o u t successively using a Soxhlet extractor. Finally, asphaltenes were obtained by distilling benzene out of the benzene solutions. The ultimate analysis, the 1H-NMR analysis, and the molecular-weight determination by means of vapour-pressure osmometry were carried out to analyze the asphaltenes by the Brown--Ladner method [17] and the method of Takeya et al. [ 18]. Under atmospheric pressure in nitrogen gas flow, a TG curve of the asphaltenes was measured up to 600°C with a Mitamura Riken model 2-50 TGA-1R. The quartz sample holder of 12.5 mm inside diameter and 13.0 mm height was charged with 100 mg of asphaltene. The system was degassed by using a vacuum pump and nitrogen gas was introduced to avoid oxidation of the sample. The sample was heated to 600°C at a rate of a b o u t 10°C per minute under atmospheric pressure in nitrogen gas flow at a rate of 100 cm 3 per minute. The residual asphaltenes, after heating to 600°C, were pulverized for X-ray powder diffractometry (Ni-filtered CuKa). The interlayer spacing d002 and crystallite thickness through the c-axis direction Lc002 were determined by referring to the internal standard of silicon.

309 RESULTS AND DISCUSSION Table 1 shows yield and analytical data of the asphaltenes prepared from the Hokkaido coals hydrogenated at 400°C. The yield ranges from 20 to 33%, the carbon content ranges from 83.0 to 88.6%, the hydrogen content ranges from 6.5 to 6.9%, the aromaticity, fa, ranges from 0.62 to 0.69, and the molecular weight ranges from 420 to 830. Thermogravimetry curves of the asphaltenes are shown in Fig. 1. For all the asphaltenes, weight loss at temperatures from 300 to 500°C is rapid, but above 500°C is slow. The TG curves up to 500°C show that Hokkaido coal asphaltenes can be classified into two groups: (1) Soya coal asphaltene and Taiheiyo coal asphaltene; and (2) Haboro coal asphaltene, Horonai coal asphaltene and Oyubari coal asphaltene. This is because Taiheiyo coal asphaltene shows larger weight loss than that expected from the rank of Taiheiyo coal. The uniqueness of Taiheiyo coal can also be seen in the yield of asphaltene [1] and also in the molecular weight of asphaltene (Table 1). Group (1) shows more weight loss than group (2). This trend appears clearly above 500°C.

10

2O v u)

o

3o

4J r"

~

4O

Horonai Haboro 50 Taiheiyo Soya 60

1

~

l

q

I

[

I00

200

300

400

500

600

Temperature of heatino ('C)

Fig. 1. Thermogravimetry curves of asphaltenes. The relationship between weight loss up to 600°C and molecular weight of asphaltenes is shown in Fig. 2. The asphaltenes comparatively low in molecular weight show more weight loss. In both cases of Taiheiyo and

310 0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 ~ 0 0

0

% 0 0 0 0 0 o 0 0

~ 0 0 0 0 0 0

0

o 0 o 0

o

t% 0

oo

o .~

.°

o

~2

311

Oyubari coals, the asphaltenes showed lower molecular weight and more weight loss when the reaction time was extended up to ca. 120 min. These results indicate that the TG curves for asphaltenes correlated well with their molecular weights. Molecular weights for asphaltenes correlate with the peak positions for the components as shown by GPC elution curves [1]. Therefore, the TG curve shows the distillation behavior of asphaltenes. The peak temperature of differential thermogravimetry (DTG), Tin, in Table 2 may be an index of the boiling-point range for the main components in as70 v

60 0

50 0 c-

40

30

300

i

J

I

i

I

400

500

600

700

800

900

Molecular wei0ht Fig. 2. Relation between weight loss up to 600°C and molecular weight. Filled symbols: 400°C, 5--31 rain; unfilled symbols: 400°C, 123--124 min. o, Soya coal asphaltene; o, Haboro coal asphaltene; o, Horonai coal asphaltene; A, Taiheiyo coal asphaltene; m, Oyubari coal asphaltene; a, Taiheiyo coal asphaltene; o, Oyubari coal asphaltene. TABLE 2 Weight loss up to 600°C, Tm a of Hokkaido coals and asphaltenes, and crystallite parameters of residual Hokkaido coals and residual asphaltenes Sample Soya coal asphaltene Haboro coal asphaltene Taiheiyo original coal coal asphaltene coal asphalteneb Horonai coal asphaltene Oyubari original coal coal asphaltene coal asphaltene b

Weight loss up to 600°C (% d.a.f.)

Tm a (°C)

doo ~

Lcoo 2

(A)

(A)

56.3 43.0 31.6 52.3 57.9 44.5 21.2 41.3 51.8

467 462 450 476 467 465 469 469 492

3.46 3.44 3.49 3.45 3.44 3.46 3.41 3.44 3.42

17.5 16.2 10.6 15.3 18.5 16.4 10.8 15.4 19.1

aTemperature of appearance of a peak in differential TG curve. bHydrogenation was carried out by using a 12 liter rotating-type autoclave.

312 phaltenes. The relationship between Tm and chemical structure of asphaltenes shows that Tm decreases with increasing H/C atomic ratio and with an increase in the measure of the substitution of the aromatic system, a, and increases with increasing aromatic hydrogen, Ha, and carbon aromaticity, fa; that is, asphaltenes comparatively high in aromaticity show higher Tin. Crystallite parameters for the residual Hokkaido coals and residual asphaltenes after heating up to 600°C are shown in Table 2. The interlayer spacing, do02, obtained from the residual asphaltenes is within the range of 3.42 to 3.46 A, which has no clear correlation with the chemical structure of parent asphaltenes. On the other hand, the crystallite thickness, Lc00:, has a correlation with the molecular weight of the parent asphaltene: the parent asphaltenes comparatively low in molecular weight produce residual asphaltenes of the larger Lcoo2. This result suggests that the lower the molecular weight of asphaltene, the more easily it passes through its plastic state and the more its stacking is promoted, because smaller molecules have larger degrees of freedom for migration. That Lcoo2 of residue from original coal is smaller than that of residual asphaltene appears to result from the difference in the mobility of molecules. In Fig. 3 the TG curves of the original coal and asphaltene are compared. Although it was proved that the hydrogenation of asphaltene is more difficult than that of the original coal [5,7], the weight loss of asphaltene during the TG measurement is more rapid than that of the original coal. For the secondary processing of asphaltene, a thermal process may be more effective than hydrogenation.

-.

" "

coal

10

2O _o "~

"~,

Taiheiyo coal

c o a ~

Oyubari

\

30

\ i

; asphaltene

40

asphaltene""

50 60 0

I

I

I

I

I

I

I

I

I

I

I

I

100

200

300

400

500

600

100

200

300

400

500

600

T e m p e r a t u r e o f heating (X])

Fig. 3. Comparisonof TG curvesof originalcoal and asphaltene. CONCLUSION The weight loss of coal-derived asphaltenes in the temperature range of 300 to 500°C is rapid, and above 500°C is slow. The weight loss up to 600°C

313

correlates with the molecular weight of the parent asphaltene. The temperature at which the differential thermogravimetry peak appears, Tin, correlates to the aromaticity. Of the residual asphaltenes after heating up to 600°C, the obtained crystallite thickness through the c-axis direction, Lcoo2, has a correlation with the molecular weight of parent asphaltene.

REFERENCES 1 Yoshida, R., Maekawa, Y. and Takeya, G., 1974. J. Fuel Soc. Jpn., 53: 1011. 2 Yoshida, R., Maekawa, Y., Yokoyama, S. and Takeya, G., 1975. J. Fuel Soc. Jpn., 54: 332. 3 Yoshida, R., Maekawa, Y., Ishii, T. and Takeya, G., 1976. Fuel, 55: 337. 4 Yoshida, R., Maekawa, Y., Ishii, T. and Takeya, G., 1976. Fuel, 55: 341. 5 Yoshida, R., Maekawa, Y., Makabe, M. and Takeya, G., 1976. J. Fuel Soc. Jpn., 55: 322. 6 Yoshida, R., Yoshida, Y., Bodily, D.M. and Takeya, G., 1982. Fuel Processing Technology, 6: 225. 7 Yoshida, R., Maekawa, Y., Ishii, T. and Takeya, G., 1972. Nippon Kagaku Kaishi (J. Chem. Soc. Jpn., Chem. Ind. Chem.), 1885. 8 Weller, S., Pelipetz, M.G. and Friedman, S., 1951. Ind. Eng. Chem., 43: 1572, 1575. 9 Ishii, T., Maekawa, Y. and Takeya, G., 1965. Kagaku Kogaku (Chem. Eng. Japan), 29 : 988. 10 Kawa, W., Feldmann, H.F. and Hiteshue, R.W., 1970. Amer. Chem. Soc., Div. Fuel Chem., Prepr., 14(4): 19. 11 Mochida, I., Amamoto, K., Maeda, K. and Takeshita, K., 1977. Fuel, 56: 49. 12 Mochida, I., Ando, T. and Takeshita, K., 1979. J. Fuel Soc. Jpn., 58: 321. 13 Korosi, A., Woebcke, H.N. and Virk, P.S., 1976. Amer. Chem. Soc., Div. Fuel Chem., Prepr., 21(6): 190. 14 Krishnamurthy, S., Shah, Y.T. and Stiegel, G.J., 1979. Ind. Eng. Chem., Process Des. Dev., 18: 254. 15 Krishnamurthy, S., Shah, Y.T. and Stiegel, G.J., 1979. Ind. Eng. Chem., Process Des. Dev., 18: 466. 16 Krishnamurthy, S., Shah, Y.T. and Stiegel, G.J., 1980. Fuel, 59: 738. 17 Brown, J.K. and Ladner, W.R., 1960. Fuel, 39: 87. 18 Takeya, G., Itoh, M., Suzuki, A. and Yokoyama, S., 1964. J. Fuel Soc. Jpn., 43: 837.