Supercritical fluid extraction of a high-ash Brazilian coal

Fuel Vol. 76, No. 7, pp. 585-591, 1997 0 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0016.2361/97 $17.00+0.00

PII: SOO16-2361(97)00060-4

Supercritical fluid extraction of a high-ash Brazilian coal Extraction with pure ethanol and isopropanol and their aqueous solutions

Clhudio Darivaa, Josh V. de Oliveiraaf*, Elina B. Caram50b

Maria G. R. Valeb and

Themical Engineering Programme, Federal University of Rio de Janeiro, 21945-970 Rio de Janeiro, RJ, Brazil blnstitute of Chemistry, Federal University of Rio Grande do Sul, 91507-970 RS, Brazil (Received 2 October 1996; revised 31 January 1997)

Port0 Alegre,

The effects of extraction temperature, pressure and addition of water on the liquid yield and on the characteristics of the extracts and residues from supercritical fluid extraction (SCFE) of a high-ash Brazilian mineral coal were experimentally investigated. The experiments were performed in a semi-batch laboratory-scale unit in the coal

pyrolysis region 598-698 K at pressures up to 12.5 MPa, using supercritical ethanol and isopropanol as primary solvents. An increase in either temperature or pressure caused an increase in the liquid yield for both the pure alcohols and alcohol-water mixtures, although at the same temperature and pressure, the latter led to a lower liquid yield compared with the pure alcohols. The extracts were characterized by a reliable, non-destructive preparative liquid chromatography method providing eight discrete fractions with well-defined chemical functionality. The amount of the lightest compounds in the extract decreased with increasing pressure, while the opposite trend was observed when the temperature increased. The coal after extraction maintained most of its heating value. Ethanol and isopropanol are compared with regard to both the liquid yield and the gasification reactivity of residual coals. 0 1997 Elsevier Science Ltd. (Keywords:

coal; supercritical

fluid extraction;

preparative

Coal reserves are important non-renewable energy sources in the world’. In Brazil, the coal reserves are even more important: they represent 60% of the non-renewable energy sources. The largest reserves are located in southern Brazil and, following the example of other countries with large coal reserves, SCFE is proposed as an upgrading process for this coal. Since the pioneering work of the National Coal Board in Britain2, several authors have studied the effects of primary extraction temperature3-5, solvent density5-‘, solvent5’6, addition of co-solvents’, coal properties’, use of catalysts”-12 and process design13 on SCFE of coals. From the vast literature available on this subject, it can be concluded that coal properties exert an influence on the liquid yield9,14 and also on the characteristics of the extracts15. Though toluene is the most common solvent used in SCFE of coals, it has been pointed out in the literature that the solvent should have a hydrogen-donor capability, to stabilize the free radicals generated and therefore reduce undesirable repolymerization reactions7’15. Supercritical alcohols have been used for this purpose, since they act as

* Author

to whom correspondence

should be addressed.

liquid chromatography)

hydrogen donors’1*‘2 and it seems that they attack the coal structure14, resulting in higher conversions compared with hydrocarbons under equivalent conditions5. Furthermore, the use of aqueous solutions of hydrogen-donor solvents appears to be promising with regard to the liquid yield for coals of low rank16. Owing to the chemical complexity of coal-derived liquids, it is difficult to assess the influence of process variables on the characteristics of the extracts without prior fractionation. Many fractionation procedures are based on the precipitation of heavier compounds (asphaltenes and asphaltols)‘7. Such methods suffer from drawbacks such as poor reproducibility, lack of selectivity in the classes of compounds obtained, problems with solvent evaporation, co-extraction and loss of volatile corn ounds”. I! As pointed out by Shishido et al. , another interesting aspect that should be taken into account is that the residual coal obtained from SCFE has a high gasification reactivity due to its high porosity, making coal in this way not only a chemical resource but also a promising energy resource. The aim of the present work was to evaluate the effect of extraction temperature, pressure and addition of water on the liquid yield and on the characteristics of the extracts and residues obtained from SCFE of a high-ash (33.4 wt%) Brazilian coal, using ethanol and isopropanol as primary

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SCFE of high-ash Brazilian coal: C. Dariva et al.

tri- and polycyclic aromatics), one of intermediate polarity (resins), and two high-molecular-weight polar fractions (asphaltenes and asphaltols). This method has already been applied to Brazilian coals2”22. The gasification reactivity of the coal residues obtained from SCAB under the best extraction conditions with respect to liquid yield for ethanol and isopropanol was also examined. The liquid yields under the same operating conditions with ethanol and isopropanol are compared. EXPERIMENTAL Apparatus and procedure

Figure 1 Schematic diagram of the SCFE apparatus. A, solvent reservoir; B, high-pressure pump; C, preheating coil; D, extraction vessel; E, absolute-pressure transducer; F, glass collector; G, trap

Table 1

Proximate and ultimate analysis of Butia coal

Proximate analysis (wt% db) Moisture’ Ash Volatile matter Fixed carbon Ultimate analysis (wt% daf) C H N 0 + Sb

1.1 33.4 27.3 39.3 78.1 5.1 1.5 15.3

a Air-dried bBy difference

solvents. These solvents were chosen on the basis that Brazil is one of the world’s leading ethanol producers and also to investigate the influence of solvent chain length. For this purpose Butia subbituminous coal was used under different

experimental conditions in a semi-batch laboratory-scale unit, over the temperature range 598-698 K and pressures up to 12.5 MPa. The extracts were analysed by the preparative liquid chromatography-eight fractions (PLC-8) method*‘, specially developed for coal-derived liquids. The PLC-8 method furnishes eight distinct chemical classes: five nonpolar fractions (saturated hydrocarbons (HC) and mono-, di-,

Table 2

n (wt%) = 100 x

wt extract wt daf coal >

Coal characterization High-ash subbituminous coal from Buti& Brazil, was used in the experiments. Its proximate and ultimate analyses are given in Table 1.

Preparative liquid chromatography (PLC-8) elution characteristics

Fraction Fl F2 F3 F4 F5 F6 F7 F8 “Bz, benzene; MeCl, dichloromethane;

586

The experimental apparatus is shown in Figure 1. It consisted of a 97.5 mL extraction vessel with an electric heater provided with a PID temperature controller with a precision of + 0.1 K. The pressure was monitored by an absolute-pressure transducer with a precision of + 0.012 MPa and the data were acquired by a portable programmer. After the desired extraction temperature was reached, -40 g of coal with a particle size of 0.85-1.18 mm, dried overnight at 383 K, was charged into the extraction vessel, supported by two 260 mesh wire disks. The system was immediately closed and the solvent continuously fed into the vessel by a high-pressure pump. The product recovery system consisted of a glass collector at atmospheric pressure, which collected the solvent and the extract after depressurization by a micrometering valve and a cold trap to avoid solvent losses. The experiments were accomplished in 90 min isothermally at constant pressure and using a solvent:coal ratio of 6.3 + 0.1. After the extraction was completed, the solvent delivery was turned off and the system was brought to atmospheric pressure. The liquid product collected was evaporated under vacuum and the extract analysed. The residue was removed from the extraction vessel and also analysed. The liquid yield is here defined as:

Fuel 1997 Volume

Mobile phase’

Volume (rnL)

Chemical identity

hexane (C 6) hexane 11.5 vol.% Bz in Cg 32 vol.% Bz in C6 32 vol.% Bz in C6 Bz-Ac-MeCl (3:4:3 v/v) AC-THF (28 v/v) methanol

40 27 36 24

saturated HC monoaromatics diaromatics triaromatics polyaromatics resins

AC, acetone

76 Number

7

25 65 60 65

asphaltenes asphaltols

of high-ash Brazilian coal: C. Dariva et al.

SCFE

Extract and residue analysis

Gasification reactivity tests

Samples (300 mg) were dissolved in a small amount of tetrahydrofuran (THF) and the solution was stirred with silica gel (2 g). After evaporation of the solvent, the sample-coated silica gel was placed on a 70 cm X 11 mm i.d. glass column with a Teflon stopcock, previously wet-packed with silica gel (18 g) in hexane. Elution was performed with the mobile phases listed in Table 2, with manual collection of each fraction. The solvent was removed from each fraction under vacuum by rotary evaporation (323 K). The extracts were further analysed for C, H and N. The proximate and ultimate analyses and calorific value of the residues were determined.

To evaluate the effects of SCFE on gasification performance, the reactivities to carbon dioxide of the parent coal and residues from the extraction were investigated. Three samples were tested: the original coal and two residues from SCFE under the best extraction conditions with respect to liquid yield for both solvents. All data were compared with those for a medium-reactivity standard coal tested under the same conditions. The reactivity measurements were carried out in a thermobalance by Cientec method 701 .Ol .032”. The reactivities of the coal and residues were characterized by

Table 3 ethanol

Effects of temperature, pressure and addition of water on liquid yield and characteristics of residues from SCFE of Buti coal with Residue

Coal T (K)

598

P (MPa)

635

7.0

Cosolvent (mol%)

12.5

-

10

673

9.8

-

10

7.0

5

12.5

-

10

-

10

-

2.7

2.2

5.7

4.6

3.6

3.8

3.7

8.5

7.7

C

78.1

80.6

76.7

79.1

78.6

80.2

79.1

80.9

79.8

83.0

H N

5.1 1.5

5.2

4.9

5.1

5.1

5.1

5.1

5.1

5.2

5.3

1.5

1.4

1.4

1.4

1.5

1.4

1.4

1.4

1.5

o+ S” 15.3 Proximate analysis (wt% db)

12.7

16.9

14.4

14.9

13.2

14.4

12.6

13.6

10.2

33.4

36.2

33.7

36.0

36.1

35.5

34.9

36.6

36.3

37.2

23.9

24.9

24.0

25.3

21.8

22.0

24.9

22.6 40.2

7

(wt% daf)

Ultimate analysis (wt% daf)

Ash VM

27.3

25.9

FC

39.3

37.9

42.4

39.1

39.9

39.2

43.3

41.4

38.8

19.50

17.29

20.25

17.82

19.79

17.40

16.40

17.84

18.02

18.99

89

84

91

92

97

CV (MJ kg-‘) CVMh (%)

89

100

C/H atomic ratio

1.27 ____ a By difference bCalorific value maintenance

Table 4

104

1.29

91

1.31

102 1.29

1.30

1.30

1.30

Effects of temperature, pressure and addition of water on liquid yield and characteristics

1.33

1.27

1.30

of residues from SCFE of Butii coal with

isopropanol Residue

Coal 598

T (K) 5.5

P (MPa) Cosolvent

648 9.5

-

(mol%)

10

7.5

10

-

698 5.5

5

9.5

-

10

-

10

-

1.9

1.7

4.3

4.1

4.0

4.8

4.5

7.3

6.5

C

78.1

79.9

79.1

79.3

77.9

75.9

80.5

81.7

80.8

76.0

H

5.1

5.1

5.1

5.2

5.1

4.9

4.9

4.9

4.9

4.8

N

1.5

1.4

1.4

1.3

1.4

1.4

1.4

1.5

1.4

1.4

15.3

13.6

14.4

14.2

15.6

17.8

13.2

11.9

12.9

17.8 33.9

p (wt% daf) Ultimate analysis (wt% daf)

0 + S”

Proximate analysis (wt% db) Ash

33.4

33.2

33.3

37.0

36.5

32.9

35.6

34.0

34.5

VM

27.3

26.4

25.6

24.6

23.7

24.0

19.1

21.2

21.9

22.8

FC

39.3

40.4

41.1

38.4

39.8

43.1

45.3

44.8

43.6

43.3

19.60

16.75

16.49

86

85

CV (MJ kg-‘) CVM (%) C/H atomic ratio

19.50 100 1.27

16.32

15.93

17.89

16.22

16.94

18.36

84

82

92

83

87

94

1.29

1.30

1.28

1.28

1.29

1.36

100 1.38

1.38

1.33

“By difference

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SCFE of high-ash

coal: C. Dariva et al.

Brazilian

0 Without co-colvent 0 5 mol % of water * 10 mol % of water

0 Without co-colvent 0 5 mol % of water * 10 mol % of water

Figure 2 Effect of temperature, pressure and addition of water on liquid yield from SCFE with ethanol

Figure 3 Effect of temperature, pressure and addition of water on liquid yield from SCFE with isopropanol

loss in weight during oxidation 1223 K.

the central point, giving a reproducibility 3.2%.

with carbon

dioxide

at

better than

Effect of temperature

RESULTS AND DISCUSSION

It can be seen from Tables 3 and 4 and Figures 2 and 3 that the liquid yield increases with increasing temperature even when water is added. This effect can be associated with a higher pyrolysis yield at the higher temperatures used. It can be observed that very low liquid yields were obtained compared with the results reported in the literature for similar conditions5’14. As pointed out by Cahill et al.15,

A full factorial 23 experimental design was performed to evaluate the effects of temperature, pressure and addition of water to both ethanol and isopropanol on the liquid yield and also on the characteristics of the extracts and residues obtained from SCFE of the highash Brazilian coal. Triplicate runs were performed for

Table 5 Effects of temperature, pressure and addition of water on distribution of fractions and ultimate analysis of extracts from SCFE of Butii coal with ethanol

-

Cosolvent (mol%) Fraction Fl

12.5

7.0

P (MPa) .

10

673

635

598

T (W

-

-

9.8 5

10

12.5

7.0 -

10

-

0.9 1.1

0.5 0.1

1.2

2.1

1.9

0.3

2.4

0.3

0.7

0.6

1.4

0.1

0.6 1.7 4.4

0.2 1.5 4.5 73.6 9.6

F2

0.8

2.4 0.4

F3 F4

1.1 2.9

2.9 2.7

0.6 1.5

0.1 1.0

0.2 2.2

1.2 3.3

0.3 5.1

F5

6.5

6.4

3.2

F6

61.4

5.3 70.1

7.9 71.0

F7

17.1

58.4 11.9

3.3 63.0

11.2

5.6

8.1 65.4 7.9

69.7 15.3

F8 Fl-F5 F6 F7-F8

7.8 13.7 61.4 24.9

15.0 14.7 58.4 26.9

12.7 7.5 63.0 29.5

78.0 7.6 1.1 13.3 0.85

78.2 7.2 1.2 13.4 0.91

2.3

Ultimate analysis (wt% daf) C 78.6 H 7.7 N 1.1 ots 12.6 0.85 C/H atomic ratio

588

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16.8

7

10

65.3 12.5 17.5

9.6

8.1

10.7

6.6

8.4

4.7 65.3 30.0

9.2 70.1 20.7

15.3 71.0 13.7

16.0 65.4 18.6

8.4 69.7 21.9

8.4 73.6 18.0

76.0

78.2

80.0

79.1

77.5

78.0

8.6 1.0 11.3 0.77

7.8 1.0 13.7 0.83

8.0 1.1 12.9 0.81

7.3 1.2 15.5 0.87

7.9 1.1 12.8 0.83

8.5 1.0 10.5 0.78

SCFE

60

of high-ash Brazilian coal: C. Dariva et al.

r

70

59% 5S5K 873K

7.OMPa lZ.SMPa 7.WPa

EZI 673K

12.5MPa

0 I G3

60

j: d,

20

10 0 Fl-F5

F6

F7-F8

FB PLCB Qmup fraction

Fl-F5

PLC-E group fraction

Figure 4 Effect of temperature and pressure on extracts from SCFE with pure ethanol characterized by PLC-8 with grouped fractions

Figure 5 Effect of temperature and pressure on extracts from SCFE with addition of 10 mol% water to ethanol characterized by PLC-8 with grouped fractions

low liquid yields might be related to both the high ash and high oxygen content of the coal: see Table I. To evaluate the effect of temperature on the distribution of the fractions of the extracts, the PLC-8 method was used. The results are presented in Table 5 and Figures 4 and 5 for ethanol. It can be seen from these data that an increase in temperature, leading to more severe thermolysis of the coal structure, results in an increase in the amount of lighter and intermediate compounds (Fl-F5 and F6) at the expense of the higher-molecular-weight compounds (M-F8). Also, comparison of Figures 4 and 5 shows that the addition of water to ethanol does not significantly change the distribution of the group fractions. For isopropanol, however, from Table 6 and Figures 6 and 7, is is seen that there is no specific trend for the heavier group fractions (F6 and F7-F8), indicating that there was

interaction among the process variables. This is clearly confirmed by considering the effect of addition of water with regard to the F6 and F7-F8 fractions; this subject is under investigation. The PLC-8 characterization presented in Tables 5 and 6 is of great value, since it allows the analysis of the individual fractions. For example, the addition of water to ethanol causes a reduction in the F2 fraction under all experimental conditions, whereas such behaviour is not verified for the whole Fl-F.5 group fraction. The residues and extracts were further analysed for C, H and N. Tables 3 and 5 for ethanol and Tables 4 and 6 for isopropanol show that the C/H ratio of the residues is greater than that of the parent coal, while the C/I-l ratio of the extracts is lower than that of the original coal. It should be noted from these tables that a rise in temperature decreases

these

Table 6 Effects of temperature, Butia coal with isopropanol

pressure and addition of water on distribution of fractions and ultimate analysis of extracts from SCFE of

T 6)

598

Cosolvent

648

5.5

P (MPa) (mol%)

-

9.5 10

-

698 5.5

7.5 10

5

-

9.5 10

-

10

Fraction Fl

2.5

2.9

1.3

F2

0.8

3.1

F3

1.5

1.5

F4

5.2

3.6

2.9

2.9

0.3

1.3

1.1

2.7

3.7

6.2

0.9

1.9

0.5

0.5

1.1

0.7

3.1

1.5

2.6

1.9

2.0

1.1

2.1

2.0

0.9

0.9

4.6

3.5

3.5

F5

8.8

8.4

4.9

4.8

7.1

9.0

9.6

8.3

5.9

F6

54.0

61.6

71.6

65.4

63.7

65.4

59.0

63.4

69.9

4.5

13.6

5.5 10.3

F7

7.3

8.0

11.2

9.9

7.7

4.9

F8

19.9

10.9

6.3

16.3

14.5

7.6

15.4

6.6

Fl -F5

18.8

19.5

10.9

8.4

14.1

22.1

21.1

16.4

14.3

F6

54.0

61.6

71.6

65.4

63.7

65.4

59.0

63.4

69.9

F7-F8

27.2

18.9

17.5

26.2

22.2

12.5

19.9

20.2

15.8

Ultimate analysis (wt% daf) C

79.9

80.9

79.8

77.9

76.5

80.3

80.6

79.0

77.0

H

7.9

8.1

7.4

7.9

8.5

8.7

8.5

7.9

8.0

N

1.0

0.9

0.9

1.1

0.8

0.9

1.1

1.2

1.0

11.2

10.1

11.9

13.1

14.2

10.1

9.8

11.9

14.0

0.82 __

0.75 __-

o+s C/H atomic ratio

0.84

0.83

0.90

0.77

0.79

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SCFE of high-ash

Table 7

coal: C. Dariva et

Brazilian

al.

Gasification reactivity data Conversion (wt%)

Sample

t= 1000s

Gasification rate per unit surface area, r, (wt% min-‘)

t=3OOos

Relative reactivity Standard

Butii coal

Butid coal Ethanol SCFE residue

71.0 68.9

100 100

5.22

1.22

1.oo

4.94

1.11

0.95

Isopropanol SCFE residue Standard

59.6 70.9

100 100

3.90 4.45

0.89 -

0.75 -

and increases that of the residues, indicating a progressive extraction of the hydrogen-rich fraction of the coal matrix due to the higher depolymerization levels. These results are confirmed by the PLC-8 fractionation of the extracts. the C/H ratio of the extracts

Effect of pressure Tables 3 and 4 summarize the data on the liquid yield as a function of pressure. From Figures 2 and 3 it can be noted that a rise in pressure causes an increase in the liquid yield as a result of an increase in solvent density (solvent power). Again the addition of water has a negative effect on the liquid yields. As expected, an increase in pressure results in a decrease

60

(Fl -F5) and, in a general sense, in an increase in the heavier fractions. It should also be observed that the pressure has a more pronounced effect than the extraction temperature on the Fl-F5 group fraction. Another interesting effect is that for pure ethanol (Figure 4) the Fl-F5 group fraction decreases with increasing pressure at the expense of the heavier group fraction (F7-FS), while with the addition of water (Figure 5) this decrease is more pronounced and is due mainly to an increase in the intermediate fraction (F6). This might be explained in terms of the physicochemical properties of water at high temperatures and pressures and also on the basis of the intermediate polarity of the F6 fraction (resins). As pointed out by Cane1 and Missalz4, water behaves as a moderately polar organic liquid in these conditions. Concerning the C/H ratio of the residues and extracts it can be observed from Tables 3 and 5 for ethanol and Tables 4 and 6 for isopropanol that pressure has an opposite effect to extraction temperature. To compare the performance of isopropanol and ethanol with regard to the liquid yield under the same operating conditions, another experiment was conducted at 673 K and 12.5 MPa with pure solvent, resulting in 6.6 wt% for isopropanol and 8.5 wt% for ethanol. in the lighter compounds

Gasification reactivity 10 0 Fl-F5

F7.FB

F6

Figure 6 Effect of temperature and pressure on extracts from SCPE with pure isopropanol characterized by PLC-8 with grouped fractions

Average values of the reaction rate over the range 2080 wt% conversion were used. The reactivity index was defined as the ratio of the mean reaction rate of the sample to the standard within this conversion range. Figure 8 shows conversion versus time for the three samples; at 1223 K these curves are very similar in shape. Table 7 summarizes all the experimental gasification data. Like most Brazilian coals, Butii is classified as

60

90

-

0

Fl-F5

Fuel 1997 Volume

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isopmpnol

.......

ethanol SCFE residue

SCFE residue

F7-FB

F6

Figure 7 Effect of temperature and pressure on extracts from SCPE with addition of 10 mol% water to isopropanol characterized by PLC-8 with grouped fractions

590

originalcoal

-.-.-

7

Figure 8 Gasification tests on residues from SCPE using ethanol and isopropanol and on the original coal

SCFE of high-ash

medium-reactive even though it is 20% more reactive than the standard used. The results show that the two residues are marginally less reactive than the coal from which they were obtained. Finally, despite a slight reduction in their reactivities-5 and 25% for ethanol and isopropanol respectively-both residues can still be considered as medium-reactivity coals, suggesting that they might be used subsequently in a gasification process. ACKNOWLEDGEMENTS This work was supported by FUNCITEC (Santa Catarina State, Brazil), Grant No. 004194. The authors would like to thank CNPq (Conselho National de Desenvolvimento Cientifico e Tecnol6gico, Brazil) and FAPERGS (Rio Grande do Sul State, Brazil) for financial support.

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2 3 4 5

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Engineering Chemistry Process Design & Development, 1985, 24, 121. Kershaw, J. R. and Jezko, J. S., Separation Science and Technology, 1982, 17, 151. Amestica, L. A. and Wolf, E. E., Fuel, 1984, 63, 227. Kershaw, J. R. and Bagnell, L. J., Fuel, 1987, 66, 1739. Stolarski, M. and Szczygiel, J., Fuel, 1991, 70, 1421. Canel, M., Hedden, K. and Wilhelm, A., Fuel, 1990, 69, 471. Ross, D. S. and Blessing, J. E., Fuel, 1979, 58, 433. Ross, D. S. and Blessing, J. E., Fuel, 1979, 58, 438. Maddocks, R. R., Gibson, J. and Williams, D., Chemical and Engineering Progress, 1979, 75(6), 49. Shishido, M., Mashiko, T. and Arai, K., Fuel, 1991,70,545. Cahill, P., Harrison, G. and Lawson, G., Fuel, 1989, 68, 1152. Kershaw, J. R., Journal of Supercritical Fluids, 1989,2,35. Patrakov, Y. F. and Denisov, S. V., Fuel, 1991, 70, 267. Ellington, R. T., Liquid Fuels from Coal. Academic Press, New York, 1977. Shishido, M., Mashiko, T., Adschiri, T. and Arai, K., Fuel, 1991, 70, 539. Karam, H. S., McNair, H. M. and LanGas, F. M., Lc-Cc, 1987, 5, 41. Rocha, S. R. P., Oliveira, J. V., d’Avila, S. G., Pereira, D. M. and Lanqas, F. M., Fuel, 1997, 76, 93. LanGas, F. M. and CaramLo, E. B., Fuel Science and Technology International (in press). Funda@o de Ci&ncia e Tecnologia, Technical Report 1396, Brazil, 1996. Cane], M. and Missal, P., Fuel, 1994, 73, 1776.

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