A New Outlook on Coal Liquefaction Through Short-Contact-Time

In fact, Wiser showed that there exists a strong simi larity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that s...
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7 A New Outlook on Coal Liquefaction Through Short-Contact-Time Thermal Reactions: Factors Leading to High Reactivity D. DUAYNE WHITEHURST Mobil Research and Development Corporation, Princeton, ΝJ 08540 About 35 years ago German investigators observed that the initial phases of coal liquefaction in presence of hydrogen donors involved the conversion of the insoluble coal matrix into a form which is soluble in strong organic solvents such as pyridine ( 1 ) . Work in the United States by Gorin (2) and Hill (3) showed that such transformations are extremely rapid and require the consump­ tion of relatively little hydrogen from the solvent. These initial products are highly functional molecules having molecular weights of 300-1000 and become soluble in weaker solvents such as benzene or hexane only after the degree of functionality and molecular weight are reduced ( 4 ) . They have been referred to by a variety of names but in this paper they will subsequently be called asphaltols Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions ( 2 , 3 ) . The fast initial reactions have been pro­ posed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong simi­ larity between coal pyrolysis and liquefaction ( 5 ) . Recent studies by Petrakis have shown that suspensions of coals in various sol­ vents when heated to ~450°C produce large quantities of free radi­ cals (~.1 molar solutions!) even when subsequently measured at room temperature. The radical concentration was significantly lower in Η-donor solvents (Tetralin) then in non-donor solvents (naphthalene) ( 6 ) . The production of such high concentrations of radicals leads to a very unstable situation and if the radicals are not stabil­ ized via Η-donation, they undergo a variety of undesired reactions such as condensation, elimination or rearrangement ( 7 ) . Neavel has shown that at short times (~5 min) a vitrinite enriched bitu­ minous coal can be converted to~80%pyridine soluble form in even non-donor reaction solvents (naphthalene) ( 8 ) . But if reaction times are extended, the soluble products revert to an insoluble form via condensation reactions. Such condensation reactions were 0-8412-0587-6/80/47-139-133$08.00/0 © 1980 American Chemical Society

134

COAL LIQUEFACTION

FUNDAMENTALS

proposed to i n v o l v e hydrogen a b s t r a c t i o n from hydroaromatic c o a l s t r u c t u r e (7 8) . Indeed, c o a l s have been shown to contain l a r g e q u a n t i t i e s of l a b i l e hydrogen (9) and i n s o l v e n t s c o n t a i n i n g l i m i t ­ ed Η-donors, c o a l products are more aromatic than those i n H-donor r i c h s o l v e n t s . F i g u r e 1 shows the hydrogen consumption measured f o r a s e r i e s of conversions of a bituminous coal ( I l l i n o i s #6 Monterey Mine) i n which s o l v e n t s of v a r y i n g Η-donor content were used. I t can be seen that the hydrogen r e q u i r e d to produce 450""°C products and lower heteroatom contents were e s s e n t i a l l y the same f o r a l l s o l v e n t s but as the s o l v e n t Η-donor content was decreased, H2 gas and SRC provided the needed hydrogen. The s i g n i f i c a n c e of the above-described work i s that i n a l l of the p r e s e n t l y developing c o a l l i q u e f a c t i o n processes, the i n i t i a l step i n the conversion i s thermal fragmentation of the c o a l s t r u c t u r e to produce very f r a g i l e molecules which are h i g h l y func­ t i o n a l , low i n s o l u b i l i t y , and extremely r e a c t i v e toward dehydrogenation and char formation. A more d e t a i l e d d i s c u s s i o n of the chemical nature of these i n i t i a l products has been presented e l s e ­ where (4) . The formation of these thermal fragments i s necessary to c a t a ­ l y t i c l i q u e f a c t i o n processes b e f o r e the c a t a l y s t s can become e f f e c ­ t i v e f o r hydrogen i n t r o d u c t i o n , c r a c k i n g and/or heteroatom r e ­ moval (10) . In thermal processes the formation o f a s p h a l t o l s always p r e ­ cedes other r e a c t i o n s such as major heteroatom r e j e c t i o n and d i s ­ t i l l a t e formation. In f a c t , i n the SRC process bituminous c o a l s are a c t u a l l y d i s s o l v e d by the time the c o a l s l u r r y e x i t s the p r e heater (4,11). T h i s has r e c e n t l y been demonstrated at the SRC process development unit (PDU) i n W i l s o n v i l l e , Alabama (11) (see F i g u r e 2). These o b s e r v a t i o n s suggest that new coal l i q u e f a c t i o n t e c h ­ nology may be p o s s i b l e based on short contact time r e a c t i o n s . The purpose of t h i s and the r e l a t e d papers i n t h i s volume by R.H. Heck and W.C. R o v e s t i i s to show some p o t e n t i a l advantages f o r o p t i ­ mized o r i n t e g r a t e d s h o r t contact time l i q u e f a c t i o n processes over conventional technology. T h i s paper w i l l concentrate on f a c t o r s which l e a d to high conversion at short time. R.H. Heck, T.O. M i t c h e l l , T.R. S t e i n and M.J. Dabkowski d i s c u s s the r e l a t i v e ease o f conversion of short and long contact time SRCs to higher q u a l i t y products. C.J. K u l i k , W.C. R o v e s t i and H.E. Liebowitz d i s c u s s some new leads p r e s e n t l y being explored at the W i l s o n v i l l e PDU i n which s h o r t contact time l i q u e f a c t i o n i s being coupled w i t h r a p i d product i s o l a t i o n v i a the Kerr-McGee C r i t i a l Solvent Deashing Process. 9

Advantages f o r Short Contact Time Coal L i q u e f a c t i o n In order to understand the p o t e n t i a l advantages f o r short contact time l i q u e f a c t i o n processes, l e t us f i r s t consider some of the disadvantages f o r p r e s e n t l y developing long contact time processes. These are enumerated below.

WHiTEHURST

Figure 1.

Short-Contact-Time

Thermal

Reactions

The source of hydrogen is controlled by the solvent

136

COAL LIQUEFACTION

FUNDAMENTALS

Figure 2. Effect of temperature on coal conversion: (O), 600-650 Ib/hr; 300-350 Ib/hr; (A), 730-830 Ib/hr; (O), 400-450 Ib/hr. Gas rate = 0-10flOO scfh

7.

WHITEHURST

Short-Contact-Time

Thermal

Reactions

137

Long contact time thermal processes have the i n t r i n s i c disadvantage o f poor s e l e c t i v i t y f o r l i g h t hydrocarbon gas formation r e l a t i v e to heteroatom removal (see F i g u r e 3) . Some d e s u l f u r i z a t i o n occurs thermally but e s s e n t i a l l y no d e n i t r o g e n a t i o n occurs without the a i d o f c a t a lysts . In long contact time thermal processes, e s s e n t i a l l y no net hydrogen i s i n t r o d u c e d i n t o the heavy l i q u i d products and the major product (SRC) c o n t i n u a l l y dehydrogenates w i t h i n c r e a s i n g time (4,11). These l a s t two p o i n t s are i l l u s t r a t e d i n Table I and Figure 4. In c a t a l y t i c coal l i q u e f a c t i o n processes, r e a c t i o n temperatures must be h i g h i n order to insure that thermal r e a c t i o n s d i s r u p t the c o a l s t r u c t u r e to the point that the c a t a l y s t can a c t on the products. As a r e s u l t , s e l e c t i v i t y i s not optimal and excessive hydrocracking r e s u l t s (10). C a t a l y s t aging i s also e x c e s s i v e . Poor hydrogen u t i l i z a t i o n r e s u l t s i n l e s s than optimal thermal e f f i c i e n c i e s f o r a l l developing processes.

TABLE I HETEROATOM REMOVAL General Formula

Number Heteroatoms/100 C

Monterey Coal C

H

N

S

(maf) 100 88 1.6°13.2 1.74 Monterey SRC, short contact time (AC-59) W ^ N ^ O ^ S ^ Monterey SRC, long contact time (AC-58) C

^

H

^

^

S

^

1

4

'

9

12.0

7.5

To improve s e l e c t i v i t y and conservation of hydrogen over p r e sent l i q u e f a c t i o n technology i n the conversion o f c o a l to h i g h q u a l i t y l i q u i d s , we b e l i e v e that thermal r e a c t i o n s should be kept as short as p o s s i b l e . C a t a l y t i c processes must be used f o r upgrading but should be used i n a temperature regime which i s o p t i mal f o r such c a t a l y s t s .

138

COAL LIQUEFACTION FUNDAMENTALS

Moles of Oxygen Removed From Coal Products Figure 3.

Sensitivity of hydrogen consumption to oxygen removal

WHiTEHURST

Short-Contact-Time

Figure 4.

Thermal

Reactions

H/C mole ratio of coal products vs. time

140

COAL LIQUEFACTION

FUNDAMENTALS

Conversion l e v e l s should be l i m i t e d so as to be compatible w i t h the r e q u i r e d hydrogen manufacture from unconverted c o a l . To i l l u s t r a t e t h i s l a t t e r p o i n t , Figure 5 shows the c a l c u l a t e d amount of c o a l r e q u i r e d f o r hydrogen manufacture as a f u n c t i o n of the rank of the s t a r t i n g c o a l and the composition of the d e s i r e d products (10). In these c a l c u l a t i o n s a 12.5% methane byproduct was assumed and the thermal e f f i c i e n c y of the hydrogen generation was assumed to be 70%. The s i g n i f i c a n c e of these c a l c u l a t i o n s i s that lower rank coals w i l l r e q u i r e ^5% lower conversion than h i g h e r rank c o a l s f o r a given end product. A l s o , the more severe a coal i s to be upgraded, the lower i t s conversion has to be i n the i n i t i a l phases of l i q u e f a c t i o n . One very p e r t i n e n t question to be addressed i s whether or not c o a l s can be converted to the l e v e l s shown i n F i g ­ ure 5 i n a short contact time process. T h i s paper w i l l deal with that question as w e l l as what compositional f e a t u r e s of the coal and the solvent i n f l u e n c e short contact time conversions. A combination of t h i s and the r e l a t e d papers w i l l show the f o l l o w i n g p o t e n t i a l advantages f o r short contact time o p t i m i z e d or inte-grated processes. •

A g r e a t e r degree of f l e x i b i l i t y i s achieved by decoupling thermal and c a t a l y t i c processes.

•

Higher y i e l d s of d e s i r e d l i q u i d products are possible.

•

C a t a l y t i c upgrading of short contact time pro­ ducts allows c a t a l y s t s to be used i n more optimal c o n d i t i o n s ; s e l e c t i v i t i e s are improved and aging r a t e s decreased.

•

Less hydrogen consumption may be r e q u i r e d f o r the production of h i g h q u a l i t y products because l e s s gaseous hydrocarbon byproducts are produced.

•

Low s u l f u r b o i l e r f u e l can p o t e n t i a l l y be pro­ duced from low s u l f u r western c o a l s w i t h reduced c a p i t a l investment.

The E f f e c t of Coal Composition

on Short Contact Time Conversion

The c l a s s i c work of Storch and co-workers showed that essen­ t i a l l y a l l c o a l s below ^89% C ^ f can be converted i n h i g h y i e l d s to acetone s o l u b l e m a t e r i a l s on extended r e a c t i o n (12). We have i n v e s t i g a t e d the behavior of c o a l s o f v a r y i n g rank toward short contact time l i q u e f a c t i o n . In one s e r i e s of experiments, c o a l s were admixed w i t h about 5 volumes of a s o l v e n t of l i m i t e d H-donor content (8.5% T e t r a l i n ) and heated to 425°C f o r e i t h e r 3 o r 90 minutes. The s o l v e n t a l s o contained 18% p - c r e s o l , 2% γ-picolene, and 71.5% 2-methylnaphthalene and represented a s y n t h e t i c SRC r e ­ c y c l e s o l v e n t . The conversions of a v a r i e t y of c o a l s w i t h t h i s

WHITEHURST

Short-Contact-Time

Figure 5.

Thermal

Reactions

Hydrogen requirement for conversion of coal

142

COAL LIQUEFACTION FUNDAMENTALS

s o l v e n t to p y r i d i n e s o l u b l e m a t e r i a l s are shown i n F i g u r e 6. As i n the work o f Storch (12) , we observed that c o a l conversion at long times was high f o r a l l c o a l s having l e s s than 88% C ^ f . At short contact time, however, both low and h i g h rank coals were not converted i n high y i e l d to p y r i d i n e s o l u b l e form. Only c o a l s from 77 to 87% Cmaf were converted to more than 70% i n 3 minutes. The l a c k of conversion f o r low rank coals l e d to an i n v e s t i g a t i o n of what compositional f e a t u r e s of the c o a l had l i m i t e d t h e i r conversion. T h i s i n v e s t i g a t i o n showed that although the low rank c o a l s d i d not produce as much p y r i d i n e s o l u b l e products, they had indeed undergone major compositional change. T h i s was evidenced by high hydrogen consumption and the production of l a r g e q u a n t i t i e s of C02 as shown i n F i g u r e 7. The r e f l e c t i v i t y of the unconverted s o l i d s was much higher than that of the o r i g i n a l coals as shown i n Table II. T h i s a l s o i n d i c a t e s a major compositional change. An oxygen balance c a l c u l a t i o n of the products of short cont a c t time conversion showed that the percent l o s s of oxygen from the c o a l was a l s o much more advanced f o r low rank coals than f o r high rank c o a l s . F i g u r e 8 i l l u s t r a t e s the percent oxygen convers i o n f o r a v a r i e t y of c o a l s at short times (2-5 minutes). If this f r a c t i o n o f the t o t a l oxygen i s compared to the f r a c t i o n of the t o t a l oxygen present as carboxyl (13) and carbonyl groups (14), an almost 1 to 1 c o r r e l a t i o n r e s u l t s . The average d i s t r i b u t i o n of the v a r i o u s oxygen-containing f u n c t i o n a l groups i n c o a l i s shown i n F i g u r e 9 ( 15) . It i s b e l i e v e d that a major reason f o r the i n s o l u b i l i t y of short contact time products o f low rank coals i s that the i n s o l uble m a t e r i a l s are s t i l l too h i g h l y f u n c t i o n a l (phenolic) to be s o l u b l e . The short contact time SRCs from low rank c o a l s do i n deed c o n t a i n l e s s p o l y f u n c t i o n a l m a t e r i a l s than the SRCs of, h i g h rank c o a l s (see F i g u r e 10). We have also shown that f o r low rank c o a l s , the i n i t i a l low conversion SRCs contain a lower p r o p o r t i o n of p h e n o l i c oxygen than long contact time, h i g h conversion SRCs (16) . A p o s s i b l e explanation i s that the p h e n o l i c content of low rank c o a l products are too high to allow s o l u b i l i t y i n even p y r i dine. This point has yet to be proven, however. In a d d i t i o n to f u n c t i o n a l i t y , s k e l e t a l s t r u c t u r e and the p h y s i c a l make-up o f the c o a l was found to be important i n achievi n g h i g h conversions at short time. Neavel has p r e v i o u s l y c a l l e d a t t e n t i o n to the importance of p l a s t i c i t y i n c o a l l i q u e f a c t i o n (17) . Mochida and co-workers have a l s o shown that the degree o f s o l u b i l i z a t i o n o f c o a l s i n polyaromatic s o l v e n t s r e l a t e s d i r e c t l y to t h e i r f l u i d i t y (18). We d i d not o b t a i n f l u i d i t y measurements d i r e c t l y on t h i s s e r i e s of c o a l s but data developed by Honda (19) i n d i c a t e that a maximum i n f l u i d i t y occurs at ^85% C f (see F i g u r e 11). T h i s i s i n the same region o f carbon content i n which we observed a maximum i n conversion at short time (see F i g u r e 6). m a

7.

WHITEHURST

901

55

Figure 6.

Short-Contact-Time

1

1

60

65

1

Thermal

1

70 75 % MAF C IN COAL

143

Reactions

1

Γ

80

85

Conversion vs. percent MAF after 3 and 90 min: (A), 90-min ring.

90

3-min ring;

144

COAL LIQUEFACTION FUNDAMENTALS

Figure 7. Liquefaction behavior at 3 min as a function of rank

Short-Contact-Time

WHITEHURST

Thermal

Reactions

Figure 8. Percent oxygen converted vs. rank (2-5 min): (%), percent oxygen lost as CO or C0 ; (X), percent oxygen in coal as carboxyl or carboxyl groups. 2

COAL LIQUEFACTION FUNDAMENTALS

τ

Γ

PERCENT CARBON IN DMMF COAL Figure 9.

Distribution of oxygen functionality in coals

1

PERCENT MAF COAL Figure 10.

SRC composition at short time

WHITEHURST

Short-Contact-ΊIme

Thermal

Reactions

PERCENT CARBON (MAF) IN COAL Figure 11.

Maximum fluidity vs. rank of coal

American Cheminai Society Library 1155 16th St. N. W. Washington, 0. C. 20036

1.64

1.68

17

lvb

405

AC-148

+16 1.46

1.26

65

mvb

256

AC-149

+23 1.23

1.00

79

372

AC-150

+79

1.36

0.76

70

*Percentage o f i n c i d e n t l i g h t , i n o i l .

hvAb

hvBb

330

AC-151

+120

0.97

0.44

% Change i n Reflectance

51

hvCb

312

AC-152

Conversion

%

Reflectance* of V i t r o p l a s t i n Residue

Rank

PSOC

Mobil Run No.

Reflectance* of V i t r i n i t e i n Coal

COMPARISON AMONG RANK, CONVERSION AND REFLECTANCE OF HIGH VITRINITE COALS AND RESIDUES

TABLE I I

r

H >

m

C

δ

H

Sc w

r

ο

oo

7.

WHITEHURST

Short-Contact-Time

Thermal

Reactions

149

Another parameter i s the i n t r i n s i c e x t r a c t a b i l i t y o f the parent coals by p y r i d i n e . As can be seen i n Figure 12, the shape of the curve o f p y r i d i n e e x t r a c t y i e l d from the various c o a l s v s . t h e i r carbon content follows the same trend as the short contact time conversions o f these c o a l s . It has been proposed (17) that the p o r t i o n o f c o a l which i s mobile under l i q u e f a c t i o n c o n d i t i o n s , c o n t r i b u t e s to the s t a b i l i ­ z a t i o n o f thermally-generated r a d i c a l s . Thus, coals which are h i g h l y f l u i d o r contain l a r g e contents o f e x t r a c t a b l e m a t e r i a l might be expected to provide hydrogen and thus promote conversion. C o l l i n s has reported that v i t r i n i t e i s a b e t t e r donor o f hydrogen than i s T e t r a l i n (20). Our own measurements o f the aromatic con­ tent and elemental analyses o f the coals (16,21) ( o r coal products) b e f o r e and a f t e r conversion at short time are i n s u f f i c i e n t t o con­ f i r m o r deny the s u p p o s i t i o n that coal acts as i t s own H-donor even at short times. There i s a c l e a r trend, however, i n the content o f aromatic carbon i n a coal and i t s c o n v e r t i b i l i t y at short times. T h i s i s shown i n Figure 13. I t can be seen that high c o n v e r t i b i l i t y occurs f o r coals which are intermediate i n aromatic carbon content. This observation i s consistent w i t h the common b e l i e f that thermal f r a g ­ mentation occurs at a l i p h a t i c p o s i t i o n s α o r $ to aromatic r i n g s . If the aromatic content becomes too h i g h , the concentration o f such a l i p h a t i c linkages must become l i m i t e d . Working c o - o p e r a t i v e l y w i t h o t h e r s , we have found some i n d i ­ c a t i o n that c e r t a i n a l i l p h a t i c l i n k a g e s between aromatic n u c l e i i are i n v o l v e d i n the r a p i d d i s s o l u t i o n o f c o a l . The absolute a l i ­ p h a t i c hydrogen content as determined by P. Solomon using FTIR (22) shows a very good l i n e a r r e l a t i o n s h i p w i t h conversion o f coal i n 3 minutes to p y r i d i n e s o l u b l e m a t e r i a l s (Figure 14a). Deno has a l s o developed an a n a l y t i c a l procedure f o r determin­ ing the type and amount o f a l i p h a t i c c o n s t i t u e n t s i n coals and n a t u r a l products (23). This procedure s e l e c t i v e l y o x i d i z e s aro­ matic n u c l e i i and does not attack saturated a l i p h a t i c s t r u c t u r e s . Among the s t r u c t u r e s which can be i d e n t i f i e d are Ar-CH2~CH2-Ar and Ar-CH2-aliphatic. Such s t r u c t u r e s could c o n s t i t u t e some e a s i l y broken C-C bonds i n c o a l s . A l i m i t e d number o f coals were o x i d i z e d and t h e amont o f hydrogen o f the types i d e n t i f i e d above were deter­ mined. These r e s u l t s were compared w i t h the y i e l d o f SRC achieved i n short contact time conversions. Figure 14b shows that there i s a rough c o r r e l a t i o n between SRC y i e l d and c e r t a i n a l i p h a t i c s t r u c t u r e s . These encouraging i n i t i a l r e s u l t s do suggest that f u r t h e r work i n t h i s area could help i n understanding the nature of t h e r e a c t i v e a l i p a h t i c s t r u c t u r e s i n c o a l . These r e s u l t s i n d i c a t e that the a l i p h a t i c p o r t i o n o f the coal i s very important i n the i n i t i a l phases o f c o a l conversion. Weak l i n k a g e s must be a s s o c i a t e d w i t h the a l i p h a t i c s i n coal though they have not as yet been completely i d e n t i f i e d . Both o f the above methods show an i n c r e a s e i n the aromatic methyl content o f SRCs at short times which i n d i c a t e s that cleavage at a b e n z y l i c carbon i s important i n d i s s o l v i n g the c o a l .

150

COAL LIQUEFACTION

Figure 12.

FUNDAMENTALS

Yield of extract (percent weight recovered) vs. rank (percent MAF carbon)

7.

WHiTEHURST

201 40

I 45

Figure 13.

Short-Contact-ΊIme

Thermal

Reactions

I I I 1 —I— 50 55 60 65 70 Grams of Aromatic Carbon in 100 g MAF Coal

Relationship between conversion and aromatic carbon

151

COAL LIQUEFACTION

Figure 14a.

FUNDAMENTALS

Response of coal conversion to aliphatic hydrogen content

7.

WHITEHURST

Short-Contact-Time

CH

CH

( {& 2~ 2~©L

Thermal

a

Reactions

j^)-CH -Aliphatic) 2

% MAF REACTIVE H IN COAL Figure 14b.

Relationship between SRC yield and reactive aliphatic hydrogen

153

154

COAL LIQUEFACTION FUNDAMENTALS

There i s a d d i t i o n a l evidence f o r the importance of a l i p h a t i c hydrogen and i t s r e l a t i o n s h i p to c o a l rank and r e a c t i v i t y i n l i q u e ­ faction. Reggel, Wender, and Raymond (9) s t u d i e d the dehydrogenation of v i t r a i n s from a v a r i e t y of c o a l s w i t h 1% P d / C A ( C 0 3 ) 2 i n r e f l u x i n g phenanthridine. Coals i n the rank range which r a p i d l y give high SRC y i e l d s are r i c h i n hydrogen, which t h e i r technique can remove. Furthermore, t h e r e was a d i s t i n c t d i f f e r e n c e between bituminous c o a l s , subbituminous c o a l s and l i g n i t e s . The lower rank m a t e r i a l s y i e l d e d l e s s H2 i n t h e i r t e s t ; we f i n d these to be very r e a c t i v e but slow to y i e l d p y r i d i n e - s o l u b l e products. These work­ ers concluded from t h e i r work "that l i g n i t e s and subbituminous c o a l s contain some c y c l i c carbon s t r u c t u r e s which are n e i t h e r a r o ­ matic nor hydroaromatic; that low rank bituminous c o a l s c o n t a i n l a r g e amounts of hydroaromatic s t r u c t u r e s ; and that h i g h e r rank bituminous coals contain i n c r e a s i n g amounts o f aromatic s t r u c t u r e s " . The f o l l o w i n g summarizes the compositional f e a t u r e s o f c o a l s which have been i d e n t i f i e d as s i g n i f i c a n t to t h e i r p o t e n t i a l con­ v e r t i b i l i t y to SRC at short times: • • • • • •

High v i t r i n i t e content High f l u i d i t y High e x t r a c t a b i l i t y Carbon contents (maf) near 85% Intermediate aromatic carbon content Presence of c e r t a i n a l i p h a t i c s t r u c t u r e s

E f f e c t o f Process Parameters on Short Contact Time Conversions The data on Figure 6 i n d i c a t e that some coals are d i f f i c u l t to convert to s o l u b l e form at short times. In f a c t , the degree of conversion at 425°C with the solvent chosen would not be h i g h enough to balance the hydrogen manufacture/conversion s t o i c h i o metry, shown i n Figure 5. Several a l t e r n a t i v e s are a v a i l a b l e t o i n c r e a s e t h i s conversion. Among these are to i n c r e a s e the temper­ ature and/or pressure of the r e a c t i o n . At present, our data are not d e f i n i t i v e on the e f f e c t o f i n ­ c r e a s i n g H2 p r e s s u r e . However, i n c r e a s i n g the temperature has a profound e f f e c t . T h i s was c l e a r l y demonstrated by e a r l y workers i n the f i e l d 02,3). More r e c e n t l y i t has been shown by M o r i t a and Hirosawa that f o r a given c o a l there i s a temperature above which conversion to s o l u b l e form no longer increases even at short time (24). T h e i r data f o r one coal i s shown i n F i g u r e 15. Kleinpeter and Burke have reported a s i m i l a r r e s u l t f o r a bituminous c o a l o f the U n i t e d States (25). S e n s i t i v i t y to temperature i s most prob­ ably dependent on the nature of the solvent used f o r the conver­ s i o n . T h i s p r e l i m i n a r y c o n c l u s i o n i s based on the work o f Neavel (8) and our own work with s o l v e n t s having high Η-donor contents (40% T e t r a l i n ) . F i g u r e 16 shows that r a i s i n g the temperature to ^450°C has no d e t r i m e n t a l e f f e c t f o r 3 U.S. bituminous c o a l s w i t h this solvent.

LA

S'

I

S ft

ri ·*•.

I

1

H

00

*>

H W

s

156

COAL LIQUEFACTION

Figure 16.

Conversions of various coals

FUNDAMENTALS

7.

WHITEHURST

Short'Contact-Time

Thermal

157

Reactions

S i m i l a r l y , a subbituminous coal (Wyodak-Anderson) which gave only 60% conversion at 427°C i n 2 minutes could be converted to >70% at 460°C w i t h no i l l e f f e c t s (see Figure 17). At ^470°C there was an i n d i c a t i o n that conversion had begun to d e c l i n e at ^2 minutes; however, t h i s data i s extremely l i m i t e d . The i m p l i c a ­ t i o n s of these r e s u l t s w i t h a western subbituminous c o a l i s that a low s u l f u r b o i l e r f u e l may p o t e n t i a l l y be produced i n a s i n g l e stage short contact time process. E f f e c t of Solvent Composition Time Convers ions

of Short

Contact

As discussed above, the composition of the solvent used i n short contact time conversions can be important. The concentra­ t i o n of Η-donors i s one f a c t o r to be considered. I t i s known that i n long contact time conversions, s o l v e n t s having high H-donor contents have a b e t t e r a b i l i t y to prevent char formation as s u l f u r i s removed from the SRC. Thus, higher y i e l d s of upgraded l i q u i d s are observed when s o l v e n t s c o n t a i n i n g high concentrations of Hdonors are used. If the i n i t i a l r e a c t i o n s o f c o a l are p u r e l y thermal, one might expect that the Η-donor l e v e l w i l l be of minor importance i f times are kept short. In f a c t , a l l coals c o n t a i n a c e r t a i n p o r t i o n of m a t e r i a l that i s e x t r a c t a b l e by p y r i d i n e . On heating coals to l i q u e f a c t i o n temperatures, some a d d i t i o n a l m a t e r i a l also becomes s o l u b l e i n even non-donor s o l v e n t s . Thus, there i s a por­ t i o n of a l l c o a l s which can be s o l u b i l i z e d w i t h l i t t l e dependence on the nature of the s o l v e n t . Table I I I shows that hydrogenated and unhydrogenated SRC r e ­ c y c l e s o l v e n t s were e q u a l l y e f f e c t i v e f o r the conversion of a western subbituminous coal at low r e a c t i o n s e v e r i t y . At higher s e v e r i t y but at times s h o r t e r than 10 minutes, s i g n i f i c a n t l y h i g h ­ er conversions were achieved only w i t h the hydrogenated s o l v e n t s which could donate more hydrogen. TABLE I I I SOLVENT EFFECTS ON SHORT TIME CONVERSION OF BELLE AYR SUBBITUMINOUS COAL (800°F, ^3 min., 1500 p s i H2) Solvent 400-800°F

Recycle Solvent

(Wilsonville)

400-800°F

Hydrogenated Recycle Solvent

%H

Conversion

8.15

59

9.67

58

We have observed that at short contact times the conversion of bituminous c o a l s i s a l s o responsive to the l e v e l of Η-donor i n the s o l v e n t . Table IV shows the conversions of an I l l i n o i s #6

158

COAL LIQUEFACTION F U N D A M E N T A L S

Figure 17.

Conversion of Wyodak coal with time: (O), 800°C; (Ah 840°-850°C; ( V A > 860 C; (Y), 878 C. e

e

820°C;

7.

WHITEHURST

Short-Contact-Time

Thermal

Reactions

159

bituminous coal which was heated f o r 2-3 minutes at 425°C i n s o l ­ vents o f v a r y i n g Η-donor contents. The conversions increased from 50 to 85% conversion as the t e t r a l i n l e v e l was r a i s e d from 0% to 43% o f the s o l v e n t . TABLE IV EFFECT OF SOLVENT* COMPOSITION OF CONVERSION OF ILLINOIS #6 (BURNING STAR) COAL AT SHORT TIME (2-3 minutes, 425°C) % T e t r a l i n i n Solvent 0

% Conversion 50

8.5

68

43

85

*A11 solvents contained 2-methylnaphthalene as the major component. In some cases ^18% p - c r e s o l was also present. Hydrogen donors a r e , however, not the only important compo­ nents of s o l v e n t s i n short contact time r e a c t i o n s . We have shown (4,7,16) that condensed aromatic hydrocarbons a l s o promote coal conversion. Figure 18 shows the r e s u l t s o f a s e r i e s of conversions of West Kentucky 9,14 coal i n a v a r i e t y o f process-derived s o l ­ vents, a l l of which contained only small amounts of hydroaromatic hydrocarbons. The concentration o f d i - and polyaromatic r i n g s t r u c t u r e s were obtained by a l i q u i d chromatographic technique (4c). I t i s i n t e r e s t i n g to note that a number o f these processderived s o l v e n t s were as e f f e c t i v e o r were more e f f e c t i v e than a s y n t h e t i c solvent which contained 40% t e t r a l i n . The balance be­ tween the concentration of Η-donors and condensed aromatic hydro­ carbons may be an important c r i t e r i o n i n a d j u s t i n g solvent e f f e c ­ t i v e n e s s at short times. K l e i n p e t e r and Burke have r e c e n t l y reported (24) that s o l v e n t s can a l s o be over hydrogenated and thus become l e s s e f f e c t i v e i n short time processes. Figure 19 shows some of t h e i r work i n which a p r o c e s s - d e r i v e d SRC r e c y c l e solvent was hydrogenated to v a r i o u s s e v e r i t i e s and used f o r the conversion of an Indian V bituminous c o a l . The r e s u l t s c l e a r l y show a maximum at intermediate hydro­ génation s e v e r i t i e s . Our assessment o f t h i s o b s e r v a t i o n i s that the l o s s i n conversion was due p r i m a r i l y to the l o s s i n condensed aromatic n u c l e i i rather than conversion o f hydrogen donors to saturates. Summary To summarize, we have i d e n t i f i e d a number o f f e a t u r e s unique to short contact time c o a l l i q u e f a c t i o n . The important f a c t o r s

160

COAL LIQUEFACTION

Figure 18.

FUNDAMENTALS

Conversion of West Kentucky coal in various solvents

Figure 19.

Microautoclave

tests—Indiana V coal. SRT conversion vs. percent hydrogen content. Batch I solvent.

to

δ*

1

ι

?

Η

CO

Η m

162

COAL LIQUEFACTION FUNDAMENTALS

which affect the application of short contact time conversions are tabulated below: •

Very high conversion i s not necessary because of hydrogen manufacture requirements.

•

Practically a l l coals can be converted to the desired stoichiometry at short time.

•

High reactivity is associated with coals having high f l u i d i t y , high extractability, intermediate aromaticity, and the presence of weak aliphatic linkages.

•

For each coal an optimal temperature for conver­ sion exists.

•

For a given coal a certain portion can be con­ verted to soluble form very easily and is inde­ pendent of solvent composition.

•

Beyond this easily converted portion of the coal even at short times, the composition of the solvent is important - high concentrations of Η-donors and polyaromatics are beneficial. Over hydrogénation i s detrimental.

Acknowledgement I would l i k e to acknowledge EPRI and MRDC who funded the work and my co-workers F. J. Derbyshire, J. J. Dickert, M. Farcasiu, B. Heady, T. 0. Mitchell, and G. A. Odoerfer. Literature Cited 1.

Rank, Report of Ludwigshafen, Germany (1942), U.S. Bureau of Mines T-346.

2.

Curran, G.P.; Struck, R . T . ; Gorin, E. I&EC Process Design and Development, 1967, 6, 166.

3.

H i l l , G.R. Fuel, 1966, 45, 326.

4.

(a) Farcasiu, M . ; Mitchell, T . O . ; Whitehurst, D.D. Chem. Tech., 1977, 7, 680. (b) Whitehurst, D.D.; Farcasiu, M . ; Mitchell, T . O . , "The Nature and Origin of Asphaltenes in Processed Coals," EPRI Report AF-252, First Annual Report Under Project RP-410, February 1976. (c) Whitehurst, D.D.; Farcasiu, M . ; M i t c h e l l , T . O . ; Dickert, J . J . , "The Nature and Origin of Asphaltenes in Processed Coals," EPRI Report AF-480, Second Annual Report Under Project RP-410-1, July 1977.

7.

WHITEHURST

Short-Contact-Time Thermal Reactions

163

5.

Wiser, W., Fuel, 1968, 47, 475.

6.

Petrakis, L.; Grandy, D.W., Fuel Div. Preprints, 146th ACS Meeting, Miami, Florida, November 1978, p. 147.

7.

Whitehurst, D.D., "Relationships Between Recycle Solvent Composition and Coal Liquefaction Behavior", Proceedings of the EPRI Coal Liquefaction Contractors Meeting, Palo A l t o , C a l i f . , May 1978.

8.

Neavel, R. C . , Fuel, 1976, 55, 237.

9.

Reggel, L . ; Wender, I.; Raymond, R., (a) Science, 1962, 41, 67. (b) Fuel, 1964, 43, 229. (c) Fuel, 1968, 47, 373. (d) Fuel, 1970, 49, 281. (e) Fuel, 1970, 49, 287. (f) Fuel, 1971, 50, 152. (g) Fuel, 1973, 52, 163.

10.

Whitehurst, D.D.; M i t c h e l l , T . O . ; Farcasiu, M . ; Dickert, J . J . , J r . , "Exploratory Studies in Catalytic Coal Liquefaction", Final Report from Mobil Research and Development Corporation to EPRI under Project RP-779-18, 1979.

11.

Lewis, H . E . ; Weber, W.H.; Usnick, G.B.; Hollenack, W.R.; Hooks, H.W.; Boykin, R.G., Solvent Refined Coal Process Quarterly Technical Progress Report for the period January­ -March 1978, from Catalytic Inc. to EPRI and DOE, July 1978.

12.

(a) Fisher, C . H . ; Sprunk, G . C . ; Eisner, Α . ; O'Donnell, H . J . ; Clarke, L . ; Storch, N . H . , U.S. Bureau of Mines Technical Paper 642 (1942). (b) Storch, H . H . ; Fisher, C . H . ; Hawk, C.O.; Eisner, Α . , U.S. Bureau of Mines Technical Paper 654 (1943).

13.

Schafer, H.N.S., Fuel, 1970, 49, 197.

14.

Blom, L . ; Edelhausen, L . ; VanKrevlen, D.W., Fuel, 1957, 36, 135.

15.

Whitehurst, D.D. The development of the correlations shown shown in Figure 9 w i l l be published i n the future.

16.

Whitehurst, D.D.; Farcasiu, M . ; M i t c h e l l , T . O . ; Dickert, Jr., "The Nature and Origin of Asphaltenes in Processed Coals," EPRI Report AF-1298, Final Report Under Project RP-410-1, December 1979.

J.J.,

164

COAL LIQUEFACTION

FUNDAMENTALS

17.

Neavel, R . C . , "Coal P l a s t i c i t y Mechanism: Inferences from Liquefaction Studies", Proceedings of the Coal Agglomeration and Conversion Symposium, Morgantown, W.Va., May 1975 (pub­ lished April 1976). 18.

Mochida, I.; Takarabe, Α . ; Takeshita, Κ . , Fuel, 1979, 58, 17.

19.

Sanada, Y . ; Honda, Η . , Fuel, 1966, 45, 295.

20.

C o l l i n s , C. J ; Raaen, V. F.; Benjamin, B. M . ; Kabalka, G. W., Fuel, 1977, 56, 107,

21.

Pines, Α . , "Aromatic Carbon Contents of Coals, SRCs, and Residues Were Determined by CP- 13 C-NMR", Under a Subcontract to EPRI, RP-410-1.

22.

(a) Solomon, P.R., "Relation Between Coal Structure and Thermal Decomposition Products", Preprints Fuel Division, ACS/CJS Chemical Congress, Honolulu, Hawaii, March 1979, p. 184. (b) Solomon, P.R.; Whitehurst, D . D . , to be published.

23.

Deno, N . C . ; Greigger, Β. Α . ; Jones, A . D . ; Rakitsky, W.G.; Stroud, S . G . , "Coal Structure and Coal Liquefaction", Final Report to EPRI, Project RP-779-16 (1979).

24.

Morita, M . ; Hirosawa, Κ . , Nenryo Kyokai-Shi, 1974, 53, 263.

25.

Kleinpeter, J . A . ; Burke, F . P . , Proceedings of the EPRI Contractors Conference on Coal Liquefaction, Palo A l t o , CA, May 1979.

RECEIVED May

21,

1980.