Continuous Reaction Gas Chromatography: The Dehydrogenation of

25 Continuous Reaction Gas Chromatography: The Dehydrogenation of Cyclohexane over Pt/γ-Al O 2

1

Downloaded by CALIFORNIA INST OF TECHNOLOGY on April 3, 2017 | http://pubs.acs.org Publication Date: September 16, 1982 | doi: 10.1021/bk-1982-0196.ch025

A L A N W. WARDWELL ,

3

ROBERT W. CARR, JR., and RUTHERFORD ARIS

University of Minnesota, Department of Chemical Engineering and Materials Science, Minneapolis, MN 55455

The construction and operation of a continuous rotating annular chromatographic reactor are describ ed. Experimental data for the dehydration of cyclo hexane over a Pt/Al 03 catalyst are presented, and the performance of the apparatus as a combined reac tor-separator i s discussed. A mathematical model i s developed, and the results of numerical simulation of reactor performance are presented. 2

Continuous chromatography i n the packed annular space between the walls of two concentric cylinders can be done by rotating the assembly about i t s longitudinal axis (1_, _2, 3). Rotation trans forms the temporal separation that would be obtained under fixed, pulsed operation into a spatial separation that permits continuous operation. It has recently been shown that continuous reaction chromatography can be done i n similar apparatus (4, 5). This not only provides a means of carrying out chemical reaction and separa tion simultaneously i n one unit, but for A :£ Β + C the product se paration suppresses the rate of the back reaction and provides a means of enhancing the reaction y i e l d . Yield enhancement i n pulsed column chromatography has been demonstrated (6, 8). Yields of 100% were obtained from the acid catalyzed hydrolysis of methyl formate i n a continous annular chromatographic reactor (4, 5). This paper describes the development and operation of a con tinuous rotating annular chromatographic reactor (CRACR) for gassolid reaction systems at elevated temperatures. Experimental and numerical simulation results for the dehydrogenation of cyclohexane on a Pt/Al2Û3 catalyst are presented. Experimental Section Reactor. Figure 1 shows a cross-sectional view of the aluminum alloy reactor. The rotating cylinder, 7, i s 18 i n . long by 10 i n . diam., and the annular space, 6, (approx. 0.3 i n . wide) i s uniformly f i l l e d with a 0.75% Pt on AI2O3 reforming catalyst crushed and graded up to 40 to 60 mesh. Circular Teflon rings, 4, 1

Current address: Conoco, Ponca City, OK 74602. 0097-6156/82/0196-0297$06.00/0 © 1982 American Chemical Society Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


298

CHEMICAL REACTION ENGINEERING

Figure 1. Reactor cross-sectional view. Key: 1, carrier inlet; 2, top cover plate; 3, distributor (stationary); 4, seal (one of six); 5, upper endpiece; 6, packed bed; 7, bed support walls; 8, distributor support rods; 9, drive shaft; 10, lower endpiece; 11, mount (stationary); 12, collector tightening ring; 13, collector (manually movable); 14, bottom plate; and 15, main exit.

Wei and Georgakis; Chemical Reaction Engineering—Boston ACS Symposium Series; American Chemical Society: Washington, DC, 1982.


25.

WARDWELL E T A L .

Continuous Reaction Gas Chromatography

299

having a r e c t a n g u l a r cross s e c t i o n serve as r o t a t i n g s e a l s between the r e a c t o r end p l a t e s , 5 and 10 , and the s t a t i o n a r y d i s t r i b u t o r , 3 , and c o l l e c t o r , 13 p l a t e s . At room temperature t h e s e a l s a r e t i g h t , but i n the 200 to 250°C o p e r a t i n g range, the s e a l s leaked, presumably due to thermal expansion e f f e c t s . The r e a c t o r i s r o t a t e d by a 1/4 HP motor, coupled through a Graham N29MW23 v a r i a b l e speed t r a n s m i s s i o n and a Boston Gears 315C speed reductor (50/1), t o the d r i v e s h a f t , 9 , p r o v i d i n g angular r o t a t i o n s o f 0.4 to 1.04 rpm. I t i s thermostatted i n a r e s i s t i v e l y heated, i n s u l a ted a i r oven equipped w i t h a s q u i r r e l cage blower f o r c i r c u l a t i o n . Temperature i s c o n t r o l l e d w i t h a Cole-Parmer 2155 p r o p o r t i o n a l controller. Oven temperatures were s p a t i a l l y uniform to w i t h i n a few degrees Centigrade, and were steady to w i t h i n 1°C. Cyclohexane i s fed by a s y r i n g e pump t o a heated v a p o r i z a t i o n b l o c k before e n t e r i n g the top of the r o t a t i n g annulus through a s t a t i o n a r y i n l e t p o r t , 1 . The He c a r r i e r gas f l o w r a t e i s measured w i t h a c a l i b r a ted rotameter b e f o r e n e t e r i n g u n i f o r m l y around the top o f the annul u s . The bottom i s f i t t e d w i t h a manually r o t a t a b l e bottom sampl i n g space, 13 , c o n t a i n i n g the e x i t p o r t , 15 . T h i s permits sampling the e f f l u e n t a t a r b i t r a r y p o s i t i o n s about the e n t i r e 360°C. A Gow-Mac 10-952 thermal c o n d u c t i v i t y c e l l , connected t o the e f f l u e n t sampling l i n e , i n d i c a t e s changes i n t o t a l e f f l u e n t composition v i a changes i n thermal c o n d u c t i v i t y . The e f f l u e n t l i n e i s connected to the gas sampling v a l v e o f a Barber-Colman 5000 s e r i e s FID-TC gas chromatograph. Chemical a n a l y s i s o f e f f l u e n t comp o s i t i o n as a f u n c t i o n o f angle was done on a 1/4 i n . OD packed alumina column a t 200°C. F u r t h e r d e t a i l s o f c o n s t r u c t i o n and opera t i o n may be found i n r e f . 9. Experimental R e s u l t s D i s p e r s i o n . Estimates o f d i s p e r s i o n i n the bed were obtained by room temperature experiments i n which N t r a c e r was fed through the s t a t i o n a r y i n l e t , and He c a r r i e r was i n t r o d u c e d everywhere e l s e . The Gow-Mac TC c e l l was used t o monitor N c o n c e n t r a t i o n s as a f u n c t i o n o f angular p o s i t i o n . The r e s u l t s a r e summarized i n Table I and i n F i g u r e s 2 and 3. Experiments done w i t h the bed h e l d s t a t i o n a r y show symmetrical N p r o f i l e s . The v a r i a n c e o f the N d i s t r i b u t i o n decreases with i n c r e a s i n g flow r a t e , as expected. F i gure 2 shows some N 2 d i s t r i b u t i o n s taken a t v a r i o u s r o t a t i o n r a t e s and approximately the same flow r a t e . The i n s e n s i t i v i t y o f the v a r i a n c e o f the N p r o f i l e s t o r o t a t i o n speed gives confidence that the bed packing i s approximately uniform. Some measurements were made w i t h the bed removed, that i s , w i t h the flow d i s t r i b u t i o n and sampling p l a t e s d i r e c t l y attached to each o t h e r . The r e s u l t s showed that s i g n i f i c a n t d i s p e r s i o n occurred i n the end spaces. 2

2

2

2


2


300


POSITION (DEGREES) Figure 2.

N tracer profiles. Key: O, bed stationary; • , 0.36 rpm; Δ , 0.70 rpm ; and 0,1.03 rpm. g

J 1

ι

I 3

ι

I 5

ι

I

ι L

7

9

FLOW RATE(L/MIN) Figure 3. Variance of N tracer profile vs. He carrierflowrate. Key: O, with bed; in absence of bed; and , difference between curves through Ο and • . t


25.

WARDWELL E T A L .


301

Table I Estimates o f d i s p e r s i o n c h a r a c t e r i s t i c s o f the packed bed


Q(Jl min-1)

σ2

P

U(cm

e,L

1

sec" )

E(cm

z

1

sec" )

2.0

15

300

65

18

4.0

9.4

470

130

24

6.0

6.5

680

200

25

8.0

5.0

900

260

26

D i s p e r s i o n c o e f f i c i e n t s were obtained by assuming that a l l d i s p e r s i o n occurs i n the azimuthal d i r e c t i o n , p e r p e n d i c u l a r t o the flow, and by assuming that the annulus i s a t h i n , i n f i n i t e l y wide s l a b . T h i s i s not unreasonable s i n c e the bed i s t h i n compared with i t s r a d i u s , and the t r a c e r i s t y p i c a l l y d i s t r i b u t e d over one quadrant. The steady s t a t e equation i s 3[N ] 2

2

3 [N J 2

(1) Comparing the s o l u t i o n o f t h i s equation to a Gaussian d i s t r i b u t i o n shows that an estimate o f the P e c l e t number can be obtained from the v a r i a n c e o f the N d i s t r i b u t i o n ; P ^ = 2 L / o . To o b t a i n e s timates o f P e c l e t numbers i n the bed alone the assumption was made that the mixing a t the ends o f the bed and the d i s p e r s i o n w i t h i n the bed a r e independent i n the sense that the v a r i a n c e s o f the N p r o f i l e s a r e a d d i t i v e . F i g u r e 3 shows data both w i t h and without the bed, and the r e s u l t o f s u b t r a c t i n g the v a r i a n c e due to end mix i n g from the t o t a l v a r i a n c e . 2

2

2

e

2

Bed voidage. The v o i d f r a c t i o n was determined by weighing a known volume o f c a t a l y s t o f known p a r t i c l e d e n s i t y , and by timing N2 t r a c e r breakthrough f o r known f l o w r a t e s . Both methods gave ε = 0.54. Cyclohexane dehydrogenation. F i g u r e s 4 and 5 show data f o r the dehydrogenation o f cyclohexane a t 204°C and 227°C, r e s p e c t i v e l y . The products are benzene, hydrogen, and a s m a l l amount (~1%) of an u n i d e n t i f i e d compound which, j u d g i n g from i t s e l u t i o n p o s i t i o n , may be the p r e v i o u s l y r e p o r t e d (10) methylcyclopentadiene. Hydrogen could not be r e l i a b l y determined from the chromatograms because low s e n s i t i v i t y o f H i n the He c a r r i e r gave r i s e t o l a r g e measurement e r r o r s . Thus, data f o r H are not presented here. The H peak maxima were approximately a t 330° (Figure 4) and 340° ( F i gure 5 ) . R e l a t i v e y i e l d s o f benzene and cyclohexane were obtained by i n t e g r a t i n g the areas under t h e i r "peaks" i n the chromatograms o f Figures 4 and 5. Conversions o f 0.87 and 0.88, r e s p e c t i v e l y , were obtained by t h i s procedure. 2

2

2


302


τ—ι—ι—ι—*r


Δ

hexane; and

, simulation.

Figure 5. Reactor effluent profiles at 227°C. Conditions: Q = 2.9 L/min; \£ cyclohexane = 8.3 mL/min. Key: Δ , ben zene; • , unknown compound; O, cyclo hexane; and , simulation. He

ENTRANCE

30 0 330 300 270 240 210 ANGLE FROM FEED ENTRANCE


25.

WARDWELL ET AL.


303


Reactor S i m u l a t i o n Model. A d i f f e r e n c e equation f o r the m a t e r i a l balance was ob t a i n e d from a d i s c r e t e r e a c t o r model which was devised by d i v i d i n g the annulus i n t o a two dimensional a r r a y o f c e l l s , each taken t o be a w e l l s t i r r e d batch r e a c t o r . The model supposes t h a t a x i a l mo t i o n o f the mobile phase and bed r o t a t i o n occur by instantaneous discontinuous jumps, between c e l l s . Reaction occurs o n l y on the s o l i d s u r f a c e , and f o r the r e a c t i o n type Α Φ Β + C used i n t h i s work, - d n / d t = K^n^ - K 2 n n . L i n e a r isotherms, n^ = 3iC^, were used, and w h i l e d i s p e r s i o n was not e x p l i c i t l y i n c l u d e d , i t could be simulated by a d j u s t i n g the number o f c e l l s . The balance i s g i v e n by Eq. 2, where s u b s c r i p t η i s the c e l l index i n the a x i a l d i r e c t i o n , and s u b s c r i p t m i s the index i n the c i r c u m f e r e n t i a l d i r e c tion. A

B

α (η-1,ιη-1) - C ( n , m ) + ±

+

±

ί

c

3^(λ(η,ιη-1) - 0 (η,ιη)] ±

[ 3 C ( n , m ) - I^B Β C ( n , m ) C ( n , m ) ] d t Kl

A

A

c

•Ό A f t e r c a s t i n g the equations i n dimensionless form, they were s o l v e d by an E u l e r ' s method i n t e g r a t i o n on the U n i v e r s i t y o f Minnesota Cy ber 7000 computer system. The accuracy o f t h i s method was checked by a f o u r t h order Runge-Kutta i n t e g r a t i o n , which gave agreement t o w i t h i n 0.5%. Simulation. Input data f o r s i m u l a t i o n o f r e a c t o r performance were obtained as f o l l o w s . A d s o r p t i o n constants f o r the l i n e a r isotherms were determined by c o n v e n t i o n a l column gas chromato graphy. However, r e a c t i o n r a t e s c o u l d not be measured by column r e a c t i o n chromatography a t temperatures g r e a t e r than 152°C, s i n c e a t that temperature the r e a c t i o n was a l r e a d y e q u i l i b r i u m l i m i t e d . Rate constants were t h e r e f o r e estimated by comparing computer c a l c u l a t i o n s o f conversion as a f u n c t i o n o f with experimentally observed conversions. D i s p e r s i o n was simulated by changing M, the number o f c'ells i n the c i r c u m f e r e n t i a l d i r e c t i o n u n t i l simulated peak widths matched experiment. Simulations done w i t h the above a d s o r p t i o n constants d i d not g i v e enough s e p a r a t i o n o f benzene and cyclohexane t o show the p a r t i a l l y r e s o l v e d "peaks" observed e x p e r i m e n t a l l y . Furthermore, conversions were underestimated a t 204°C, and overestimated a t 227°C. To o b t a i n b e t t e r agreement i t was necessary to decrease the values o f the cyclohexane a d s o r p t i o n constants, and to a d j u s t the r e a c t i o n r a t e c o n s t a n t s . Table I I summarizes the measured and adjusted parameter v a l u e s , and the s o l i d l i n e s i n F i g u r e s 4 and 5 g i v e the s i m u l a t i o n r e s u l t s f o r t h e adjusted parameters.


304

CHEMICAL REACTION ENGINEERING Table I I Comparison o f experimental r e s u l t s with reactor simulations

T,°C

Q,£

min-

1

X,%

n,%

d

R,%

d

β(Η ) 2

87

a

3.2

a

93

a

72

b

19

b

39

b

86

c

22°

e(c H > 6

1 2

e ( c H ) K^.mlh ^ 6

6

__

««._


204 2.3

88a 227

2.9

98

b

88°

93°

1.9

a

65

a

7.6

b

88

b

6.7

C

95°

0.18

b

7.5

b

c

4.0

C

0.18

10.5°

0.14

b

6.7

b

7.5

b

10. o

0.14

c

2.3

C

7.0

C

3.4

—

—

10. o

b

—

7.5 9.4

b

C

— b

C

a, Experiment; b, S i m u l a t i o n w i t h independently determined para meters; c, Simulation w i t h f i t t e d parameters; d, see Appendix. Discussion The benzene y i e l d s given by the data o f F i g u r e s 4 and 5, 87% at 204°C and 88% a t 227°C, may be compared w i t h computed e q u i l i brium y i e l d s of 13% and 19%, based on i n l e t c o n d i t i o n s . T h i s c l e a r l y shows the advantage of the continuous annular chromato g r a p h i c r e a c t o r over, say, a t u b u l a r r e a c t o r . The comparison i s not e n t i r e l y s t r a i g h t f o r w a r d , because d i l u t i o n o f the cyclohexane by He c a r r i e r as i t d i s p e r s e s c i r c u m f e r e n t i a l l y s h i f t s the e q u i l i brium toward products; t h i s would have to be taken i n t o account i n any q u a n t i t a t i v e comparison. The data show o n l y p a r t i a l s e p a r a t i o n of benzene and cyclohexane. T h i s p a r t i a l s e p a r a t i o n must r e s u l t i n p a r t i a l suppression of the back r e a c t i o n , and must a l s o c o n t r i b u t e to the observed y i e l d enhancement ( i n a d d i t i o n to the d i l u t i o n e f fect). Comparison o f product peak widths i n F i g u r e s 4 and 5 w i t h peak widths of weakly adsorbed N i n F i g u r e 2 i n d i c a t e s that spreading due to d i s p e r s i v e flows dominates peak broadening due to adsorp t i o n . D i s p e r s i o n causes s u f f i c i e n t peak o v e r l a p that i t seems r e a sonable to a t t r i b u t e f a i l u r e to observe l a r g e r r e a c t i o n y i e l d s to t h i s f a i l u r e of s e p a r a t i o n . For example, the l i q u i d - s o l i d c o n t i n u ous annular chromatographic r e a c t o r , where d i s p e r s i o n was s i g n i f i c a n t l y l e s s , gave 100% y i e l d s f o r the h y d r o l y s i s o f methyl formate (4^, 5 ) . L i m i t a t i o n of y i e l d enhancement due to d i s p e r s i o n w i l l l i k e l y occur i n any g a s - s o l i d r e a c t o r o f t h i s c o n f i g u r a t i o n . I t may be p o s s i b l e t o overcome t h i s d i f f i c u l t y by p l a c i n g t h i n p a r t i t i o n s i n the annulus as a b a r r i e r to t r a n s p o r t i n the c i r c u m f e r e n t i a l d i r e c t i o n . However, w i t h t h i s arrangement, a x i a l d i s p e r s i o n i n the r e s u l t i n g tubes would be manifested as output peak broaden2


25.

WARDWELL E T A L .


305

ing, and the remedy would not be completely e f f e c t i v e . Better s e p a r a t i o n , hence b e t t e r performance, would be obtained i n s y s tems having l a r g e r d i f f e r e n c e s i n a d s o r p t i o n c o e f f i c i e n t s than those found f o r the s p e c i e s i n t h i s i n v e s t i g a t i o n . I n connection with t h i s i t should be noted that s e p a r a t i o n i s poorer a t higher temperatures, s i n c e the a d s o r p t i o n c o e f f i c i e n t s decrease with i n c r e a s i n g temperature.


Legend of Symbols c^

f l u i d phase c o n c e n t r a t i o n s

Ε

d i s p e r s i o n constant

F

f r a c t i o n o f bed fed

Κ

r e a c t i o n r a t e constant

L

reactor length

M

number o f c e l l s about r e a c t o r circumference

m

c i r c u m f e r e n t i a l c e l l counting

η

a x i a l c e l l counting

n^

surface concentrations

Ρ

P e c l e t number based on r e a c t o r l e n g t h

_ e, L

index

index

Q

flow r a t e

t

time

U

linear velocity

X

conversion

^ef

e q u i l i b r i u m c o n v e r s i o n based on feed c o n d i t i o n s

y

circumferential direction

ζ

axial direction

3^

a d s o r p t i o n constant

ε

void fraction

η

efficiency

σ

variance

Acknowledgment T h i s work was supported by the U.S. Department o f Energy un der Contract No. DE-AC02-76ER02945. We are g r a t e f u l to Amoco O i l Co., N a p e r v i l l e , I l l n o i s , f o r f u r n i s h i n g the c a t a l y s t used i n t h i s work.


306


Literature Cited


1. 2.

Giddings, J. C. Anal. Chem. 1962, 34, 37. Fox, J. B . ; Calhoun, R. C . ; Eglinton, W. J. J . Chromatog. 1969, 43, 48. 3. Scott, C. D . ; Spence, R. D . ; Sisson, W. G. J . Chromatog. 1976, 126, 381. 4. Cho, Β. K . ; Carr, Jr., R. W.; A r i s , R. Chem. Engr. S c i . 1980, 35, 74. 5. Cho, Β. K . ; Carr, R. W.; A r i s , R. Sep. S c i . and Tech. 1980, 15, 679. 6. Roginskii, S. Z . ; Yanovskii, M. I.; Gaziev, G. A. Dokl. Akad. Nauk. S.S.R. 1961, 140, 1125. 7. Matsen, J. M . ; Harding, J. W.; Magee, E . M. J . Phys. Chem. 1965, 69, 522. 8. Wetherold, R. G . ; Wissler, E . J.; Bischoff, Κ. B. Adv. Chem. 1974, 133, 181. 9. Wardwell, A. W. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 1981. 10. P o l l i t z e r , E . L.; Hayes, J. C . ; Haensel, V. Adv. Chem. 1970, 97, 20. Appendix A r e a c t o r e f f i c i e n c y , η, defined by eq. 1A, i s used i n t h i s work. ^. ». « * « conversion o f chromatographic r e a c t o r n=fraction o f bed fed χ ττττ—: : — °„ . — e q u i l i b r i u m conversion a t i n l e t c o n d i t i o n s (1A) r

I t provides a comparison o f the p r o d u c t i v i t y o f the chromatogra phic r e a c t o r w i t h the p r o d u c t i v i t y t h a t would be obtained i f the annulus were f e d uniformly, ( f r a c t i o n o f bed f e d = 1 ) and reacted to a s p e c i f i e d conversion (conversion o f r e a c t o r / e q u i l i b r i u m con v e r s i o n < 1 ) . This e f f i c i e n c y i s thus a measure o f the penalty p a i d f o r using only a p o r t i o n o f the bed t o c a r r y out the r e a c tion. I t i s a c o n s e r v a t i v e f i g u r e , however, s i n c e i t ignores the b e n e f i t o f s e p a r a t i n g r e a c t a n t and products. A measure o f the chromatographic r e a c t o r as a separator as w e l l as a r e a c t o r i s the recovery, R. conversion

v

In eq. 2A the y i e l d i s a y i e l d a t p u r i t y , the amount o f d e s i r e d product that can be removed from the r e a c t o r a t s p e c i f i e d p u r i t y . In t h i s work a p u r i t y o f 99% was s p e c i f i e d . Thus the recovery i s the f r a c t i o n of the product that can be removed a t the s p e c i f i e d purity. R E C E I V E D April 2 7 , 1982.


Continuous Reaction Gas Chromatography: The Dehydrogenation of

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