Kinetics of Catalytic Liquefaction of Big Horn Coal - ACS Publications

25 Kinetics of Catalytic Liquefaction of Big Horn Coal Y. T. S H A H , D . C. C R O N A U E R , H . G . M c I L V R I E D , and

Downloaded by IOWA STATE UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch025

Gulf Research and

Development Co.,

J. A.

PARASK0S

Pittsburgh, PA 15230

A number o f coal l i q u e f a c t i o n processes, i n c l u d i n g Gulf CCL, are being developed to help counter the coming l i q u i d f u e l shortages. In most coal l i q u e f a c t i o n processes, coal i s l i q u e f i e d i n the presence o f a solvent. In a d d i t i o n , the c o a l - o i l s l u r r y may be contacted by a hydrogen-rich gas, and a c a t a l y s t may also be present. The process of l i q u e f a c t i o n produces a wide b o i l i n g range l i q u i d , as well as l i g h t gases (C -C ) and by-products, such as water, ammonia, hydrogen s u l f i d e , e t c . Data f o r the k i n e t i c s of coal l i q u e f a c t i o n have been p u b l i s h ed i n the l i t e r a t u r e (1-11). A review o f the reported studies has r e c e n t l y been given by Oblad (12). The reported data were mostly obtained i n bench-scale r e a c t o r s . Guin et al. (7) studied the mechanism of coal p a r t i c l e d i s s o l u t i o n , whereas Neavel (7), Kang et al. (8), and Gleim (10) examined the r o l e of solvent on coal l i q u e f a c t i o n . T a r r e r et al. (9) examined the e f f e c t s of coal minerals on r e a c t i o n rates during coal l i q u e f a c t i o n , whereas Whitehurst and M i t c h e l l (11) studied the short contact time coal l i q u e f a c t i o n process. It i s b e l i e v e d that hydrogen donor solvent plays an important r o l e i n the coal l i q u e f a c t i o n process. The r e a c t i o n paths i n a donor solvent coal l i q u e f a c t i o n process have been reviewed by Squires (6). The reported studies examined both thermal and c a t a l y t i c l i q u e f a c t i o n processes. So f a r , however, very little e f f o r t has been made to present a d e t a i l e d k i n e t i c model f o r the intrinsic k i n e t i c s of coal l i q u e f a c t i o n . The primary purpose of t h i s paper i s to describe the k i n e t i c s of c o a l l i q u e f a c t i o n derived from a p i l o t - s c a l e u n i t . The s p e c i f i c coal studied was Big Horn subbituminous c o a l . The experimental data were obtained i n a p i l o t - s c a l e Gulf patented (13) r e a c t o r . The data i l l u s t r a t e the e f f e c t s of r e a c t o r space time and temperature on the product d i s t r i b u t i o n . Since the r e a c t o r behaves as a bubble column, the i n t r i n s i c k i n e t i c s of the l i q u e f a c t i o n process can be extracted by means of a kinematic model o f the r e a c t o r . The experimental data were found to be 1

©

4

0-8412-0401-2/78/47-065-303$05.00/0

Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

304

CHEMICAL

REACTION

ENGINEERING—HOUSTON

adequately c o r r e l a t e d by a simple r e a c t i o n mechanism.


Experimental A schematic o f the experimental u n i t i s shown i n Figure 1. This u n i t p r i m a r i l y c o n s i s t e d o f a feed tank, charge pump, p r e heater, r e a c t o r and product r e c e i v e r s . A separate hydroclone u n i t was used t o prepare r e c y c l e s o l v e n t . The r e a c t o r was 6 cm I.D. by 122 cm long. The u n i t was operated w i t h a cocurrent upflow c o a l s l u r r y r a t e o f 1.8 kg/hr, the feed s l u r r y c o n s i s t e d of mixture o f 40% c o a l and 60% hydroclone overflow. Big Horn (WY) subbituminous c o a l was used f o r t h e experiments. I t was p u l v e r i z e d and screened t o minus 20 mesh, but was not p r e d r i e d ; t h e r e f o r e , t h e moisture l e v e l o f t h e feed c o a l was t y p i c a l l y 22 wt%. The ash content was 3.4 wt%. On a m o i s t u r e - f r e e b a s i s , carbon, hydrogen and oxygen contents o f the c o a l were 69.34, 4.6 and 19.9 wt%, r e s p e c t i v e l y . A l l runs were made a t 24.1 MPa pressure and a gas r a t e o f 2575 dm /hr. A G u l f developed c a t a l y s t was used. S u f f i c i e n t analyses were made t o o b t a i n d e t a i l e d m a t e r i a l balances over the l i q u e f a c t i o n r e a c t o r . S o l v a t i o n o f the c o a l organic matter on a moisture and ash-free (maf) b a s i s was c a l c u l a t e d u s i n g t h e f o l l o w i n g equation: 3

ο ο , ^. maf c o a l feed - maf u n d i s s o l v e d c o a l % Solvation = τ—ζ—ι x 100 maf c o a l feed Ί Λ η

Έ

Reactor. The r e a c t o r used i n t h i s study was a p i l o t p l a n t v e r s i o n o f the G u l f patented (13) segmented bed r e a c t o r . The c a t a l y s t was h e l d i n tubes o f 17 mm I.D. by 114 cm length con s t r u c t e d o f 10-mesh (U.S.) s t a i n l e s s s t e e l screen. In many o f the runs, the c a t a l y s t charge was placed i n f o u r o f the above tubes which t y p i c a l l y h e l d 750 g o f the c a t a l y s t . Some runs were made u s i n g only one o r two c a t a l y s t tubes by b l o c k i n g o f f a p o r t i o n o f the r e a c t o r cross s e c t i o n . Glass model s t u d i e s (14) w i t h an upflow a i r - w a t e r system have shown t h a t gas flow p l a y s a s i g n i f i c a n t r o l e i n a c h i e v i n g a x i a l mixing i n t h e open spaces between the c a t a l y s t tubes and r a d i a l mixing w i t h i n t h e c a t a l y s t tubes; both a x i a l and r a d i a l mixing i n c r e a s e d w i t h an i n c r e a s e i n the hydrogen flow r a t e . I t i s b e l i e v e d t h a t the r e a c t i o n process i n v o l v e s depolymeri z a t i o n and s o l v a t i o n o f t h e c o a l , w i t h these r e a c t i o n s o c c u r r i n g p r i m a r i l y i n t h e open spaces between the c a t a l y s t tubes. Once the c o a l fragments a r e l i b e r a t e d , they a r e e i t h e r s t a b i l i z e d by hydrogen o r repolymerized. Thus, the molecular weight o f the l i q u e f a c t i o n products i s determined by a combination o f c o a l p r o p e r t i e s and hydrogénation a c t i v i t y w i t h i n the r e a c t o r . The process o f s o l v a t i o n i s improved as t h e hydrogen donor c a p a c i t y of the l i q u i d phase i s increased. I n order t o improve the a v a i l a b i l i t y o f the c a t a l y s t surface t o the r e a c t a n t s , entrapment


SHAH

ET AL.

Catalytic Liquefaction of Coal

Simplified Flow Unit Bench Scale Liquefaction Unit SOLVENT HYDROGEN


COAL

VENT

GAS METER COLD SEPARATOR

LIGHT E N D S RECEIVER

CATALYST TUBES.114cm

LONG

SLURRY RECEIVER

Hvdrocyclone Unit (Batch Operation) NITROGEN

VENT

Figure I.

Experimental set-up


CHEMICAL

306

REACTION

ENGINEERING—HOUSTON


of c o a l p a r t i c l e s w i t h i n the c a t a l y s t tube must be minimized, and a high degree o f r a d i a l mixing w i t h i n the c a t a l y s t tubes must be achieved. Both o f these can be improved by increased hydrogen and o i l flow r a t e s . Upgrading o f the product o i l occurs p r i m a r i l y w i t h i n the c a t a l y s t tubes. The d i s s o l v e d hydrogen r e q u i r e d f o r both s o l v a t i o n and upgrading i s continuously s u p p l i e d from the gas phase. Both the l i t e r a t u r e and a v a i l a b l e experimental data on hydrogen s o l u b i l i t y i n c o a l l i q u i d s at r e a c t i o n temperature and pressure i n d i c a t e that mass t r a n s f e r of hydrogen from the gas i n t o the l i q u i d phase i s not a l i m i t i n g f a c t o r i n the r e a c t i o n process. Reactor Model. The i n t r i n s i c k i n e t i c i n f o r m a t i o n f o r the l i q u e f a c t i o n process can be evaluated from the data by accounting f o r mass t r a n s f e r e f f e c t s . The o p e r a t i o n of the present r e a c t o r system was modeled by making the f o l l o w i n g assumptions: (1) Coal and l i q u i d flow as a uniform s l u r r y w i t h i n the r e a c t o r . Based on the study o f Kato et al», (15) i t i s reasonable to assume that the backmixing c h a r a c t e r i s t i c s of c o a l and o i l are about the same. (2) There are no r a d i a l c o n c e n t r a t i o n or temperature gradients w i t h i n the r e a c t o r . (3) The c o n c e n t r a t i o n of hydrogen i n the s l u r r y i s always i n excess o f that r e q u i r e d f o r r e a c t i o n , and (4) The a x i a l d i s p e r s i o n e f f e c t w i t h i n the r e a c t o r i s s i g n i f i c a n t . In the present study, we have assumed a simple r e a c t i o n mechanism f o r c o a l s o l v a t i o n i l l u s t r a t e d by Equation (1). Gaseous Products (H 0, 9

k Coal

1->

l i g h t gases, N H

Oils (

V

H S, ?

naphtha, furnace and heavy f u e l o i l s , e t c .

etc.)

CD

This mechanism i s c o n s i s t e n t w i t h our understanding of the low s e v e r i t y c a t a l y t i c l i q u e f a c t i o n process. More complex r e a c t i o n mechanisms which i n c l u d e hydrocracking ( i . e . , degeneration of high er b o i l i n g hydrocarbons i n t o lower b o i l i n g components) and hydroger donor r e a c t i o n s may be important under high s e v e r i t y thermal process. Gas and s l u r r y flow c o c u r r e n t l y upwards through the open spaces between the c a t a l y s t tubes. The gas flow i s the primary cause o f backmixing i n the s l u r r y phase. L i t e r a t u r e on hydroprocessing operations i n d i c a t e s t h a t the mass t r a n s f e r r e s i s t a n c e f o r the t r a n s f e r o f hydrogen from the gas to the l i q u i d phase can be neglected. Furthermore, very high s o l u b i l i t y of hydrogen i n c o a l l i q u i d s d i c t a t e s t h a t , f o r a l l p r a c t i c a l purposes, the mass t r a n s f e r r e s i s t a n c e of hydrogen at the g a s - l i q u i d i n t e r f a c e can be neglected. This does not mean, however, that the r a t e of d i f f u s i o n o f hydrogen through the l i q u i d i s n e g l i g i b l e compared with the r a t e at which hydrogen i s consumed at the c a t a l y s t surface. However, because of the reasons mentioned l a t e r , i t i s



25.


SHAH E T A L .

307

assumed t h a t the hydrogen c o n c e n t r a t i o n i s i n excess and constant for a l l pertinent reactions. The c o a l depolymerizes, d i s s o l v e s i n the l i q u i d , and forms v a r i o u s gaseous and l i q u i d products. The s i m p l i f i e d mechanism assumes t h a t both gaseous and l i q u i d products are d i r e c t l y formed from c o a l . The c a t a l y s t w i t h i n the c a t a l y s t tubes con t i n u o u s l y s u p p l i e s hydrogen-rich solvent f o r hydrogen t r a n s f e r r e a c t i o n s . The s o l v a t i o n o f c o a l i s undoubtedly accompanied by some hydrocracking r e a c t i o n s (degeneration o f higher b o i l i n g hydrocarbons i n t o lower b o i l i n g components), but these appear t o be r e l a t i v e l y unimportant, and i n the present study, they were not i n c l u d e d i n the mechanism. The r e a c t o r feed i n c l u d e s a moisture- and ash-free coal component designated "C." and a c a r r i e r s o l v e n t . Feed hydrogen and f l u s h o i l are incluàed i n the s o l v e n t f o r s i m p l i c i t y . The product stream i s composed o f a p f r a c t i o n ( l i g h t gases H 0 , CO, C 0 , H S, NH ), a "C" f r a c t i o n (unconverted moisture- and ash-free c o a l ) , and an f r a c t i o n (coal l i q u i d s p l u s s o l v e n t , i . e . , C^+). The concentrations o f the above-defined feed and product components are expressed i n terms o f dimensionless weight f r a c t i o n s ; m a t e r i a l balance feed and product q u a n t i t i e s have been normalized w i t h respect t o feed moisture- and ash-free c o a l ; i . e . , ρ = Ρ / C , c = C /C. , and £ = L /C. , where Ρ , e t c . , * o ο 1* O 0 1* Ο 0 1* ο . ' are the c o n c e n t r a t i o n s by weight o f components p, e t c . , i n the product, and C. i s the c o n c e n t r a t i o n o f the maf c o a l a t the reactor i n l e t . D i f f e r e n t i a l mass balances based on the standard a x i a l d i s p e r s i o n model can be expressed a s : f!

2

2

( R dx

• R )

2

ff

3

r

- ± Pe dx

_i_£i-dR Pe dx dx

Pe dx

+

R

2

C =

0

c = 0

dx

The independent v a r i a b l e "x" i s the dimensionless r e a c t o r l e n g t h , i . e . , χ = ζ/ξ, where ζ i s the d i s t a n c e from the r e a c t o r i n l e t and ξ i s the t o t a l r e a c t o r l e n g t h . and R2 are the dimensionless r a t e constants. They can be expressed as: R^

—

k^f,

(3) R2 — ^2^*


CHEMICAL REACTION ENGINEERING—HOUSTON

308

where k and k a r e i n t r i n s i c r a t e constants which i n c l u d e c a t a l y s t v o i d t r a c t i o n and d i l u t i o n e f f e c t s . The q u a n t i t y Γ i s defined as the r e c i p r o c a l o f s l u r r y space v e l o c i t y (g s l u r r y / g c a t / h r ) , i . e . , s l u r r y space time. Pe i s the P e c l e t number d e f i n e d as ϋξ/ϋ , where U i s the s u p e r f i c i a l v e l o c i t y o f l i q u i d s l u r r y and D the a x i a l d i s p e r s i o n c o e f f i c i e n t measured i n a g l a s s model under the present r e a c t o r c o n f i g u r a t i o n (14). Equation (2) assumes t h a t the P e c l e t numbers f o r a l l species are equal. The magnitude o f the P e c l e t number c h a r a c t e r i z e s the extent o f backmixing w i t h i n the r e a c t o r . At the l i m i t i n g condi t i o n s , an i n f i n i t e P e c l e t number means plug flow, while a P e c l e t number o f zero means completely backmixed flow, i . e . , the r e a c t o r operates j u s t l i k e a continuous s t i r r e d tank r e a c t o r . Equation (2) i s subjected t o t h e standard boundary condi tions: ?


α

C

1 dc " P e dx -±J&

0 = ρ . P

Pe dx

ι

(4)

Pe dx

and dc dp dil dx = dx = c E =

^

Λ 0

ι χ = 1-

, (5) r c

Equation (2) assumes that t h e feed contains only c o a l and s o l v e n t . An a n a l y t i c a l s o l u t i o n t o Equations (2)-(5) can be obtained i n a s t r a i g h t f o r w a r d manner. The s o l u t i o n f o r the coal concen tration i s

c =

V

T-

^

x

- ae

Ψ

2

^

x

6

where

q = / 4 (R R ) / 1 + k: — Pe +

α

' α

2

= +

U q)

- (1-q) e

F e q

il^I -Peq (i+q) ι e

2

S i m i l a r s o l u t i o n s f o r "p" and

can be obtained.


(7)

25.

SHAH E T A L .


309


Results and D i s c u s s i o n The best values o f the r a t e constants were obtained by non l i n e a r l e a s t square f i t t i n g o f the data t o the a n a l y t i c a l equations (16). Experimental c o a l s o l v a t i o n s and the y i e l d s o f gases and o i l s are compared w i t h model p r e d i c t i o n s i n Figure 2. As shown by t h i s f i g u r e , the model c o r r e l a t e s the experimental data q u i t e w e l l . From the experimental data, r a t e constants f o r two r e a c t i o n s were obtained a t three temperature l e v e l s . The Arrehenius p l o t s f o r the r a t e constants are shown i n Figure 3. I n t e r e s t i n g l y , the a c t i v a t i o n energies f o r a l l the r e a c t i o n s were found t o be c o n s i d e r a b l y higher than normally encountered i n f i r s t order c a t a l y t i c r e a c t i o n s . This o f f e r s f u r t h e r evidence that the c a t a l y s t i s not d i r e c t l y i n v o l v e d i n the s o l v a t i o n reaction. The major d i f f e r e n c e between the k i n e t i c model presented i n t h i s study and those presented i n the l i t e r a t u r e i s t h a t , here, c o a l l i q u e f a c t i o n i s assumed t o occur i n a p a r a l l e l r e a c t i o n mechanism. A l l the products, l i g h t o r heavy, are assumed t o be formed d i r e c t l y from c o a l . The models proposed i n the l i t e r a t u r e assume a s e r i e s r e a c t i o n mechanism, wherein only heavy components ( i . e . , high b o i l i n g o i l f r a c t i o n s ) are formed d i r e c t l y from c o a l , and the l i g h t e r components ( i . e . , low b o i l i n g o i l f r a c t i o n s ) and the gases are produced by the c r a c k i n g o f the heavy compon ents. The present model a l s o assumes that the c a t a l y s t provides an excess o f hydrogen donor solvent r e q u i r e d f o r the c o a l l i q u e f a c t i o n . A more r i g o r o u s model (which would a l s o take c a t a l y s t aging e f f e c t s i n t o account) should i n c l u d e the r o l e o f hydrogen donor s o l v e n t . Separate measurements o f water, l i g h t gases (C^-C^) and by products as f u n c t i o n s o f space time and temperature were a l s o carried out. These data were c o r r e l a t e d by a k i n e t i c model which assumes t h a t water, l i g h t gas and by-products a l l are produced d i r e c t l y from c o a l by f i r s t - o r d e r i r r e v e r s i b l e r e a c t i o n s . This type of'shooting star" mechanism c o r r e l a t e d the separate data f o r water, l i g h t gases and by-products as f u n c t i o n s o f space time as w e l l as the data f o r the t o t a l gas shown i n Figure 2. Further more, the a c t i v a t i o n energies f o r the r e a c t i o n s c o a l --> water, c o a l --> by-product and c o a l --> l i g h t gases were found to be 53,500, 63,500 and 85,200 c a l / g mole. In a backmixed r e a c t o r , the gas flow r a t e should have a s i g n i f i c a n t e f f e c t on the product d i s t r i b u t i o n . High gas flow i s important f o r e l i m i n a t i n g p o s s i b l e r e s i s t a n c e s t o the t r a n s f e r o f hydrogen from the gas phase t o the c a t a l y s t surface. Two important r e s i s t a n c e s i n the present case are the g a s - l i q u i d i n t e r f a c e r e s i s t a n c e and the i n t e r - p a r t i c l e d i f f u s i o n a l r e s i s t a n c e w i t h i n the c a t a l y s t tube. A l a r g e gas flow would, however, a l s o give s i g n i f i c a n t backmixing i n the open p o r t i o n o f the r e a c t o r , 1


CHEMICAL REACTION ENGINEERING—HOUSTON

310

1.0 α

CD

Ζ

ional

«-* Φ

duct/

>

0.8

Kinetics of Catalytic Liquefaction of Big Horn Coal - ACS Publications

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