Organic Chemistry of Coal - American Chemical Society

13 Supercritical Solvents and the Dissolution of Coal and

Downloaded by UNIV OF MICHIGAN ANN ARBOR on May 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0071.ch013

Lignite JAMES E. BLESSING and DAVID S. ROSS SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025

The unique solvent properties of supercritical fluids suggest their use in coal extraction as a novel scheme for isolating syncrude-like materials. One advantage such a process might offer is the ease of separating solvent from extract. We have studied the degree of coal dissolution possible with a number of solvents in the supercritical state and examined the importance of system parameters, such as solvent type, density, and temperature, on the success of extractions. Shortly after we began our work, Bartle, Martin, and Williams of the National Coal Board of Britain reported a 17% yield of low -ash, high-H/C material from the extraction of coal with superc r i t i c a l toluene at 350°C (1). Since then, Maddocks and Gibson have reported greater yields, with up to one-third extraction of an I l l i n o i s No. 6 coal with toluene at 400°C (2). They estimated that their process would be economically competitive with the COED and SRC operations. This paper reviews the fundamentals of supercritical extraction, discusses our data in terms of theoretical expectations, and draws some conclusions regarding the role of extraction per se i n obtaining products from coal. Background A "supercritical" fluid i s one that i s above i t s c r i t i c a l temperature (T ), the point beyond which a phase boundary no longer exists between gas and liquid. In the supercritical region, the density of a f l u i d is a continuous function of i t s pressure, no distinction exists between gas and liquid, and the f l u i d has no surface tension. One hundred years ago, Hannay and Hogarth observed the dissolution of KI in supercritical ethanol (3). Yet, until now, little practical use has been made of supercritical extractants. Paul and Wise have described the theoretically based expectations of the use of supercritical fluids as solvents or extractants, both c

0-8412-0427-6/78/47-071-171$05.00/0 © 1978 American Chemical Society In Organic Chemistry of Coal; Larsen, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

172

ORGANIC CHEMISTRY OF COAL


g e n e r a l l y and with some emphasis on c o a l d i s s o l u t i o n (4). Given below are some of the p r o p e r t i e s of s u p e r c r i t i c a l f l u i d s , d i s cussed i n t h e i r monograph: •

At low d e n s i t i e s , gases have no solvent power, and the concentration of a compound s o l u t e i n the gas phase i s described by i t s p a r t i a l pressure. However, at a given temperature, the solvent power of any gas increases d r a m a t i c a l l y as i t s density approaches that of l i q u i d s .

•

For a given pressure, a gas i s at i t s highest d e n s i t y near i t s c r i t i c a l temperature, where i t i s l e a s t i d e a l .

•

For a given gas density, the concentration of a s o l u t e i n a gas increases with i n c r e a s i n g temperature due to increased s o l u t e v o l a t i l i t y , but solvent pressures r i s e r a p i d l y as the temperature exceeds T and the solvent gas becomes more i d e a l . Theory p r e d i c t s that the solvent power of such a gas i s p r i m a r i l y a f u n c t i o n of i t s p h y s i c a l p r o p e r t i e s and i s r e l a t i v e l y independent of i t s chemical s t r u c t u r e and f u n c t i o n a l i t y . c

•

Because a gas i s g e n e r a l l y l e s s viscous than a l i q u i d , i t can b e t t e r penetrate porous substrates, such as coal.

•

Though the solvent power of a dense gas may not be high compared with l i q u i d s , the gas i s more e a s i l y separated from m a t e r i a l s l i k e c o a l , and solvent r e c o v e r i e s can therefore be b e t t e r .

The conclusions of Paul and Wise thus suggest that superc r i t i c a l e x t r a c t i o n i s a promising procedure f o r c o a l conversion. The r e s u l t s of our research v e r i f y the a p p l i c a b i l i t y of these p r i n c i p l e s to c o a l e x t r a c t i o n , but a l s o point to the importance of processes other than simple e x t r a c t i o n s i n the production of c o a l products. Experimental Procedures A v a r i e t y of experiments were performed using s e v e r a l s o l vents over a range of c o n d i t i o n s to e x t r a c t samples of I l l i n o i s No. 6 c o a l and North Dakota l i g n i t e . The coals are c h a r a c t e r i z e d i n Table I. A l l experiments were done i n batch mode, i n a 300 cm , 316 SS AE MagneDrive autoclave. Most experiments were run f o r 90 min at 335°C. The run procedure i s summarized i n Figure 1. The workup procedure i s shown i n Figure 2. Each r e a c t i o n product i s separated i n t o a f i l t r a t e and a s o l i d s f r a c t i o n . In every case, the f i l t r a t e s were f u l l y p y r i d i n e s o l u b l e . The p y r i d i n e s o l u b i l i t i e s of the s o l i d s were determined by s t i r r i n g 0.5 g s o l i d i n 50 ml p y r i d i n e f o r 1 hr at room temperature, and 3

In Organic Chemistry of Coal; Larsen, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


COAL OR LIGNITE

Figure 1.

Run procedure

CAREFULLY COLLECT REACTOR CONTENTS

HEAT-UP (50 MIN) HOLD COOL-DOWN (60 MIN)

FILL AND V E N T 1000 psig OF NITROGEN, TWICE

3

A E MAGNEDRIVE 300 c m , 316 SS AUTOCLAVE STIRRING 1500 rpm

DRY A T 120°C, < 1 TORR

REACTION SOLVENT

Figure 2.

Low melting solids. Typical MW_ « 450

Wt% Filt. based on MAF starting coal.

"SOLIDS"

Workup procedure

Typical MW

n

* 1100

Wt% solids pyr. sol based on MAF 0.5 g sample

EVAPORATE SOLVENT AND DRY, 120°C, < 1 TORR

MED. POROSITY FILTER

STIR 0.5 g IN 50 ml PYRIDINE FOR 1 HOUR A T ROOM T E M P E R A T U R E

DRY, 120°C, < 1 TORR

MEDIUM POROSITY FRITTED FILTER (10-15 Mm)

EVAPORATE SOLVENT AND DRY, 120°C, < 1 TORR

"FILT"

REACTION MIXTURE


174


Table I CHARACTERISTICS OF COAL AND LIGNITE SAMPLES


Analyses

Beneficiated I l l i n o i s No. 6 Coal Dried Over night at 120°C and < 1 t o r r . ASTM HVC (PSOC-26?

%C %H Molar H/C %N %S org %S. morg %0 Δ %Ash % Pyridine solubility % Solubility i n a l l reac tion solvents

North Dakota L i g n i t e Dried Overnight a t 120°C and < 1 t o r r (PSOC-246)'

77.2 5.1 0.79 1.7 2.1

62.0 4.5 0.87 1.0 0.7

b

% 0 11.9

14.8

2.0 13

17 2

Λ

* £ the duced d e n s i t y of l i q u i d s , taken to be about 2.66. Thus δ i s a l i n e a r f u n c t i o n of the experimental d e n s i t y , a l l other v a r i a b l e s i n the equation being constant f o r any given s o l v e n t . Product p y r i d i n e s o l u b i l i t y versus δ i s p l o t t e d i n F i g u r e 4. The data f o r a l l the s o l v e n t s appear to f a l l about a l i n e . * Regardless of t h e i r s t r u c t u r a l d i f f e r e n c e s , a l l these compounds l a r g e l y perform i n accordance with t h e i r solvent c a p a b i l i t i e s . Thus, the media are a c t i n g simply as solvents and are apparently not chemically a c t i v e . Since the p y r i d i n e s o l u b i l i t y of the s t a r t i n g c o a l i s 13%, the use of these solvents at lower den s i t i e s i s a c t u a l l y counterproductive. c

a n c

r

p

i s

r e

C

A comparable run with T e t r a l i n and c o a l y i e l d s a product that i s about 50% p y r i d i n e s o l u b l e .



BLESSING AND Ross

Figure 4.

Supercritical

Solvents

Product pyridine solubilities vs. 8 (reaction conditions: 90 min, 335°C, approx. 500-5000 psig)


182



Conclusion The conclusions presented here are based on the r e s u l t s of our experiments with c o a l . On the b a s i s of some l i m i t e d research with l i g n i t e , we suggest that these conclusions a l s o apply to lignite. The y i e l d s f o r l i g n i t e , however, were g e n e r a l l y lower than those f o r c o a l under comparable c o n d i t i o n s . The c o r r e l a t i o n of p y r i d i n e s o l u b i l i t y of the c o a l products with the Hildebrand s o l u b i l i t y parameter coupled with the tempera ture dependence of the product y i e l d s leads us to suggest that the s o l u b l e product m a t e r i a l s r e s u l t from an i n i t i a l , thermally induced fragmentation of the c o a l i n v o l v i n g the a c t i o n of a dense solvent. Thermochemical c o n s i d e r a t i o n s suggest that the r a t e of thermal fragmentation i s too slow to account f o r the r e s u l t s . As s t a t e d , however, these thermochemical c a l c u l a t i o n s are f o r a gas phase system, a t d e n s i t i e s s e v e r a l orders of magnitude lower than those used i n our experiments. Wiser has suggested that c o a l thermolysis r a t e s may be s i g n i f i c a n t l y enhanced i n the presence of a s o l v e n t (10). Although n e i t h e r t h e o r e t i c a l nor independent experimental j u s t i f i c a t i o n e x i s t s f o r t h i s suggestion,* our data, and p a r t i c u l a r l y , our f i n d i n g of a correspondence between the d e n s i t y of the medium and the conversion to p y r i d i n e - s o l u b l e products, are best explained that way. Thus, a model c o n s i s t e n t with the data i s

c

_

c

solvent participation 1

c

m

2

C-0 bonds can be considered s i m i l a r l y .

(i)

Hydrogen-transfer coal, —C-

(ii)

conversion to s t a b l e products

Step (2) may i n v o l v e e i t h e r

from a hydrogen-rich p o r t i o n of the

+ RH—^R- +

—CH

D i s p r o p o r t i o n a t i o n of the r a d i c a l s p e c i e s , 2

—CH-CH

3

—*~

—CH=CH

2

+ —CH -CH 2

3

or ( i i i ) β-scission, s p l i t t i n g o f f a s m a l l , r e l a t i v e l y f r e e r a d i c a l (R=H, a l k y l , benzyl) —C-C-R — — C = C

stable

+ R-

* For the simple, thermal homolysis, R-R 2R«, no evidence e x i s t s that the r a t e i s s i g n i f i c a n t l y enhanced by the presence o f s o l vent. Moreover, i f the process i s s t r i c t l y one i n which no charge s e p a r a t i o n occurs i n the t r a n s i t i o n s t a t e , there i s no t h e o r e t i c a l expectation of a s i g n i f i c a n t solvent e f f e c t .



13.

BLESSING AND Ross

Supercritical

183

Solvents

(An a l t e r n a t i v e mechanism i s discussed i n the Appendix.) S u p e r c r i t i c a l s o l v e n t s , t h e r e f o r e , c l e a r l y can provide moderate y i e l d s of s y n c r u d e - l i k e m a t e r i a l s from c o a l , and these y i e l d s are not p r i m a r i l y due to any unique c h a r a c t e r i s t i c s of s u p e r c r i t i c a l c o n d i t i o n s . Since high solvent d e n s i t i e s are d e s i r a b l e , solvents that are l i q u i d at l i q u e f a c t i o n temperatures could prove at l e a s t as e f f e c t i v e as those that are s u p e r c r i t i c a l , and at lower pressures. L i q u i d s , however, are subject to the l i m i t a t i o n s of surface t e n s i o n and higher v i s c o s i t i e s , which dimi n i s h t h e i r usefulness i n t h i s scheme. A d d i t i o n a l l y , s o l v e n t s that are l i q u i d at l i q u e f a c t i o n temperatures are very d i f f i c u l t to separate from c o a l products. Thus, s u p e r c r i t i c a l s o l v e n t s o f f e r solvent f l u i d i t y , a r e l a t i v e l y wide range of usable types of compounds, and e a s i l y o b t a i n a b l e high solvent r e c o v e r i e s i n the e x t r a c t i o n of low molecular weight m a t e r i a l s from c o a l . An understanding of the importance of the thermal processes i n v o l v e d i n the treatment of c o a l with hot, dense solvents and the p r i n c i p l e s of s u p e r c r i t i c a l e x t r a c t i o n , as enumerated at the outset of t h i s paper, could l e a d to an e f f e c t i v e use of s u p e r c r i t i c a l solvents i n c o a l and l i g n i t e processing.

Appendix An a l t e r n a t i v e scheme i n v o l v i n g charge s e p a r a t i o n i s suggested f o r c o a l t h e r m o l y s i s : coal + coal ^ ± r w

+· coal

/

.+· (coal ....coal

solvent )

solvent ^ separated

pair

+·

> coalH coal or H-donor

2

As u n l i k e l y as t h i s suggestion may seem at the o u t s e t , the propos i t i o n i s c o n s i s t e n t with the f o l l o w i n g : •

As discussed i n the t e x t , an i n c r e a s e i n d e n s i t y of the media increases the degrees of c o a l conversion. The conversion process, i n turn, e n t a i l s i r r e v e r s i b l e changes i n the c o a l , and thus i s not j u s t a simple s o l v e n t - s o l u t e interaction.

•

Wiser s t a t e s that c o a l p y r o l y s i s i n the absence of T e t r a l i n i s second order i n c o a l , and when T e t r a l i n i s present the process i s f i r s t order i n both T e t r a l i n and c o a l (10).



184

ORGANIC CHEMISTRY OF

COAL

•

T e t r a l i n r e a d i l y donates hydrogen to electron-poor systems at 50 to 160°C. T y p i c a l Η-acceptors are quinones. The r e a c t i o n i s a c c e l e r a t e d by electron-withdrawing subs t i t u e n t s on the quinone, i s a c c e l e r a t e d by polor s o l v e n t s , and i s unaffected by f r e e r a d i c a l i n i t i a t o r s (11) .

•

R a d i c a l cations r e a d i l y accept H (12) .

•

The e s t a b l i s h e d acid-base character of c o a l - d e r i v e d asphaltenes (13) suggests that charge separation, i . e . , donor-acceptor complexes, e i t h e r are present i n c o a l i t s e l f or form thermally.

•

Poly-condensed aromatic s t r u c t u r e s , l i k e those i n c o a l , are known to form r e a d i l y both r a d i c a l c a t i o n s and r a d i c a l anions (15) .

2

from hydrocarbon donors

This suggested model could provide p r a c t i c a l i n s i g h t i n t o the a c t i o n of c a t a l y s t s i n conversion processes. One might consider, thus, the use of c a t a l y s t s that promote C-C s c i s s i o n by r a d i c a l c a t i o n intermediates.* The i m p l i c a t i o n s of t h i s scheme await the r e s u l t s of f u r t h e r research i n t o the Η-donor process and c o a l con v e r s i o n chemistry. Acknowledgment We acknowledge the support of the Department o f Energy f o r t h i s work on Contract EF-76-C-01-2202. Literature Cited 1. 2. 3. 4.

B a r t l e , K e i t h D., Martin, Terence G., and Williams, Dereck F., F u e l (1975), 54, 226. Maddocks, R. R., and Gibson, J . , Chem. Eng. Prog. (June 1977), 73, 6, 59-63. Hannay, J . B., and Hogarth, J . , Proc. Roy. Soc. (London), Ser. A (1879), 29, 324-26. Paul, P.F.M., and Wise, W.S., "The P r i n c i p l e s of Gas E x t r a c t i o n , " M i l l s and Boon Limited, London, 1971.

*Trahanovsky and B r i x i u s have shown that at temperatures below 100°C, Ce(IV) promotes the cleavage of PhCH -CH Ph, y i e l d i n g o x i d a t i o n products by way of a r a d i c a l c a t i o n intermediate (16). I t would be of i n t e r e s t to c a r r y out the r e a c t i o n with H-donor solvents present. 2

2


BLESSING AND Ross 5.

6.

7.


8. 9. 10. 11. 12.

13. 14.

15.

16.

Supercritical

Solvents

185

Ross, D. S., and B l e s s i n g , J . Ε., " I s o r p o p y l A l c o h o l as Coal L i q u e f a c t i o n Agent," F u e l D i v i s i o n P r e p r i n t s f o r the 173rd N a t i o n a l Meeting of the Amer. Chem. Soc., New Orleans, LA, (March 1977). Manuscript i n p r e p a r a t i o n . Benson, S. W., and O'Neal, Η., N a t i o n a l Standard Refer ence Data S e r i e s — NBS 21, U.S. Government P r i n t i n g O f f i c e , Washington, D.C., 1970. Blanks, R., and Peausnitz, J . , Ind. Eng. Chem. Funda mentals (1964), 3, 1. Angelovich, J . , Pastor, G., and S i l v e r , Η., Ind. Eng. Chem. Process Des. Dev. (1970), 9, 160. Giddings, J . , Myers, Μ., McLaren, L., and K e l l e r , R., Science (4 October 1968), 162, 67. Wiser, N., F u e l (1968), 47, 475. Braude, Ε. Α., Jackman, L., and L i n s t e a d , R., J . Chem. Soc. (1954), 3548, 3564, 3569. Doepker, R., and Ausloos, P., J . Chem. Phys. (1960), 44 (5), 1951; Kramer, G., and Pancirov, R., J . Org. Chem. (1973), 38, 349. Sternberg, Η., Raymond, R., and Schweighardt, F., Science (4 April 1975), 188, 49. F r a n k l i n , J . , Dillard, S., Rosenstock, Η., Herron, J . , and D r a x l , Κ., " I o n i z a t i o n P o t e n t i a l s , Appearance P o t e n t i a l s , and Heats of Formation of Gaseous P o s i t i v e Ions," N a t i o n a l Standard Reference Data S e r i e s — NBS 26, U.S. Government P r i n t i n g O f f i c e , Washington, D.C., 1969. Compton, R., and Huebner, R., in "Advances in R a d i a t i o n Chemistry," Burton, Μ., and Magee, J . , Ed., V o l . 2., 1970 p. 281; Christophorou, L., and Compton, R., Heath P h y s i c s (1967), 13, 1277. Trahanovsky, W., and B r i x i u s , W., J . Amer. Chem. Soc. (1973), 95 (20), 6778.

RECEIVED February 10,

1978


Organic Chemistry of Coal - American Chemical Society

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