Reactor Surface Effects During Propylene Pyrolysis - American

13 Reactor Surface Effects During Propylene Pyrolysis M. A. GHALY and B. L. CRYNES

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0032.ch013

School of Chemical Engineering, Oklahoma State University, Stillwater, Okla. 74074

The t h e r m a l d e c o m p o s i t i o n o f p r o p y l e n e i n v o l v e s a s e r i e s o f p r i m a r y and secondary r e a c t i o n s l e a d i n g t o a complex m i x t u r e o f products. S t u d i e s showed t h a t t h e d i s t r i b u t i o n o f p y r o l y s i s p r o d u c t s v a r i e s c o n s i d e r a b l y w i t h t h e p y r o l y s i s c o n d i t i o n s and t h e t y p e o f r e a c t o r u s e d . T h e r e i s a g r e e m e n t among t h e s t u d i e s on p r o p y l e n e p y r o l y s i s t h a t t h e t h r e e m a j o r p r o d u c t s o f p y r o l y s i s a r e m e t h a n e , e t h y l e n e , and h y d r o g e n . However, t h e r e i s d i s a g r e e ment on t h e t y p e s a n d amounts o f m i n o r o r s e c o n d a r y p r o d u c t species. Ethane, b u t è n e s , acetylene, methylacetylene, a l i è n e , and h e a v i e r a r o m a t i c components a r e r e p o r t e d i n d i f f e r e n t s t u d i e s , L a i d l e r and W o j c i e c h o w s k i ( 1 9 6 0 ) , K a l l e n d , e t a l . ( 1 9 6 7 ) , Amano and U c h i y a m a ( 1 9 6 3 ) , S a k a k i b a r a ( 1 9 6 4 ) , S i m s , e t a l . ( 1 9 7 1 ) , Kunugi, e t a l . (1970), M e l l o u t t e e , et a l . (1969), conducted a t d i f f e r e n t c o n v e r s i o n and t e m p e r a t u r e l e v e l s . C a r b o n was a l s o r e p o r t e d a s a p r o d u c t i n t h e e a r l y w o r k o f H u r d and E i l e r s (1943) and i n t h e more r e c e n t w o r k o f S i m s , e t a l . ( 1 9 7 1 ) . The o v e r a l l r a t e o f p r o p y l e n e p y r o l y s i s h a s b e e n r e p o r t e d t o be f i r s t o r d e r i n some s t u d i e s and as 3/2 o r d e r i n o t h e r s . Most s t u d i e s c o n d u c t e d a t l o w t e m p e r a t u r e s (up t o 650C) t e n d t o f a v o r 3/2 o r d e r , w h i l e s t u d i e s c o n d u c t e d a t h i g h e r t e m p e r a t u r e s i n d i cated mostly â f i r s t order r a t e . C l e a r l y , the o v e r a l l order of r e a c t i o n i s a t b e s t o n l y a pseudo o r an a p p a r e n t o r d e r r e p r e s e n t i n g a c o m b i n a t i o n o f many e l e m e n t a r y s t e p s . One f e a t u r e o f t h e p y r o l y s i s r e a c t i o n w h i c h i s n o t f u l l y understood i s the r o l e of r e a c t o r s u r f a c e . Although the thermal d e c o m p o s i t i o n o f p r o p y l e n e h a s b e e n a t l e a s t i m p l i c i t l y assumed i n some c a s e s t o b e a c o m p l e t e l y homogeneous g a s - p h a s e r e a c t i o n , the i n f l u e n c e o f r e a c t o r s u r f a c e s has been demonstrated f r e quently. F a c t o r s such as the m a t e r i a l of c o n s t r u c t i o n , s u r f a c e v o l u m e r a t i o o f t h e r e a c t o r , and c h e m i c a l t r e a t m e n t o f r e a c t o r s u r f a c e o f t e n a f f e c t t h e r a t e o f r e a c t i o n and/or t h e p r o d u c t d i s tribution. Many p r e v i o u s i n v e s t i g a t i o n s a r e o f l i m i t e d v a l u e i n d e t e r m i n i n g t h e r o l e *of s u r f a c e i n t h e p y r o l y s i s r e a c t i o n . The l i t e r a t u r e on s u r f a c e e f f e c t s c o n t a i n s p r e d o m i n a n t l y q u a l i t a t i v e i n f o r m a t i o n and o f t e n c o n t r a d i c t i o n s and a n o m a l i e s . Early studies 218

Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


13.

GHALY AND CRYNES

Reactor Surface Effects

219

t h a t r e p o r t e d heterogeneous s u r f a c e e f f e c t s were completed s e v e r a l y e a r s ago b e f o r e r e l i a b l e a n a l y t i c a l methods w e r e a v a i l a b l e t h u s r e s t r i c t i n g t h e a c c u r a c y o f p r o d u c t a n a l y s i s and d e t e c t i o n of minor products. M o s t r e c e n t s t u d i e s h a v e b e e n made u s i n g batch reactors or tubular glass or quartz reactors u s u a l l y at c o n d i t i o n s d i f f e r e n t from those o f commercial i n t e r e s t . Recent s t u d i e s o f t h e i n f l u e n c e o f r e a c t o r w a l l s on p y r o l y s i s r e a c t i o n s h a v e b e e n c o n d u c t e d by C r y n e s (1969) and H e r r i o t t ( 1 9 7 1 ) . Their s t u d i e s u n e q u i v o c a l l y demonstrated the i n f l u e n c e of r e a c t o r w a l l s on p r o p a n e p y r o l y s i s i n r e a c t o r s c o n s t r u c t e d o f v a r i o u s m e t a l s and w i t h c e r t a i n gas t r e a t m e n t s . The m a t e r i a l o f c o n s t r u c t i o n , r e a c t o r h i s t o r y , a n d t h e t y p e o f w a l l t r e a t m e n t w e r e shown t o b e important v a r i a b l e s . A n o t h e r r e c e n t s t u d y by T a y l o r , e t a l . (1972) on p r o p y l e n e p y r o l y s i s i n a w a l l - l e s s r e a c t o r , i . e . , a " c o m p l e t e l y " homogeneous s y s t e m , showed t h a t t h e p r o d u c t s p e c i e s , p r o d u c t d i s t r i b u t i o n , and r e a c t i o n r a t e i n t h e a b s e n c e o f a s u r f a c e were s i g n i f i c a n t l y d i f f e r e n t - f r o m those o b t a i n e d a f t e r a s t a i n l e s s s t e e l s u r f a c e has been i n t r o d u c e d i n t o the r e a c t i o n m i x ture. With r e s p e c t to t h e o r i e s of w a l l heterogeneous e f f e c t s i n hydrocarbon p y r o l y s i s r e a c t i o n s , the l i t e r a t u r e i s almost v o i d . R i c e and H e r z f e l d (1951) h a v e p r e s e n t e d some t h e o r e t i c a l a r g u ments b u t w i t h some s e v e r e l y s i m p l i f y i n g a s s u m p t i o n s . Polotrak, e t a l . (1959) p r o p o s e d mechanisms i n v o l v i n g b o t h c h a i n i n i t i a t i o n and t e r m i n a t i o n a s h e t e r o g e n e o u s p r o c e s s e s . More e l a b o r a t e t h e o r e t i c a l w o r k on t h e i n t e r a c t i o n b e t w e e n h y d r o c a r b o n s ( p a r a f f i n s and o l e f i n s ) and m e t a l o x i d e s u r f a c e s was done by Semenov (1958) and K a s a n s k y and P a r i i s k y ( 1 9 6 5 ) i n w h i c h t h e a u t h o r s t r i e d to e x p l a i n the heterogeneous e f f e c t s ( a c t i v i t y ) of the s u r f a c e s i n terms o f e l e c t r o n i c c o n d u c t i v i t y . A r e c e n t s t u d y by T s a i and A l b r i g h t (1975) c l e a r l y i n d i c a t e d some o f t h e i m p o r t a n t s u r f a c e r e a c t i o n s o c c u r i n g i n r e a c t o r s c o n s t r u c t e d of d i f f e r e n t metals d u r i n g the p y r o l y s i s of l i g h t p a r a f fins. These r e a c t i o n s i n c l u d e f o r m a t i o n o f c a r b o n , r e m o v a l o f carbon, o x i d a t i o n of metal s u r f a c e s , r e d u c t i o n of s u r f a c e o x i d e s , f o r m a t i o n o f m e t a l s u l f i d e s , and d e s t r u c t i o n o f m e t a l s u l f i d e s . The above a u t h o r s s t a t e d t h e i m p o r t a n t n e e d f o r i n f o r m a t i o n on t h e i n t e r a c t i o n o f e t h y l e n e , p r o p y l e n e , and o t h e r o l e f i n s w i t h the s u r f a c e oxides i n o r d e r to c l a r i f y the l e v e l of s u r f a c e oxides p r e s e n t on t h e r e a c t o r w a l l s when v a r i o u s h y d r o c a r b o n f e e d s t o c k s are used. T h e r e i s a n e e d t o d e t e r m i n e numerous f a c t o r s a b o u t t h e mechanisms o f h y d r o c a r b o n p y r o l y s i s , i n c l u d i n g t h e r o l e o f r e a c tor surfaces. The p r e s e n t i n v e s t i g a t i o n was made i n t u b u l a r f l o w r e a c t o r s c o n s t r u c t e d o f s e v e r a l m e t a l s , and d a t a h a v e b e e n o b t a i n e d on t h e i n f l u e n c e o f r e a c t o r s u r f a c e s on t h e p r o d u c t d i s t r i b u t i o n s , f e e d c o n v e r s i o n s and c a r b o n f o r m a t i o n w h i l e p y r o l y z i n g propylene. f l

f l


220

INDUSTRIAL AND LABORATORY PYROLYSES TO V E N T

MANOMETER

TO CHROMATOGRAPH

OC

LU 5

Ο

ζ

oc

CL Ο

PREHEATER GEN

OGEN

Lu

*LEN

oc

|PR


S

'AIR S U P P L Y FOR FLUIDIZATION

Χ

ο

Figure 1. Experimentalflowsystem

• HIGH-^ INTENSITY LAMP

1 0 " —

KQTC

TC

TO

CONTROLLER

*^L^VIEWING WINDOW

AIR*

^DISENGAGING SECTION

14

-HEATING WIRE 8 0 0 WATT

-MAIN HEATING WIRE I500WATT TEMPERATURE CONTROLLER

18

- H E A T I N G WIRE 4 0 0 WATT

100-MESH METAL SCREEN

Figure 2.

Reactor details


13.

GHALY AND CRYNES


Experimental


221

Details

The f l o w s y s t e m u s e d i n t h i s i n v e s t i g a t i o n i s shown i n F i g u r e 1. The r e a c t o r s w e r e c o n s t r u c t e d f r o m 0 . 6 3 5 cm O.D. ( 1 / 4 - i n c h O.D., 20-gauge) t u b i n g . The t u b i n g was c o i l e d t o a d i a m e t e r o f 1 2 . 7 cm, and a b o u t 4 . 5 2 t o 4 . 6 2 m o f t u b e w e r e immersed i n a f l u i d i z e d sand b a t h to o b t a i n the d e s i r e d temperatures. A total of seven r e a c t o r s were used i n t h i s s t u d y . R e a c t o r s were c o n s t r u c t e d o f 304 s t a i n l e s s s t e e l , l o w - c a r b o n s t e e l , n i c k e l , i n c o n e l , and i n c o l o y , a l l c o m m e r c i a l l y a v a i l a b l e . The t h e r m o c o u p l e s immersed i n t h e s a n d b a t h ( F i g u r e 2) w e r e a c c u r a t e t o w i t h i n 1 C ; and a t s t e a d y - s t a t e , t h e t e m p e r a t u r e s i n s i d e t h e f l u i d i z e d s a n d b a t h a t v a r i o u s r a d i a l and a x i a l p o s i t i o n s d i d n o t d i f f e r by more t h a n 3C i n a l l r u n s . Thermocouples were a l s o p l a c e d i n t h e r e a c t o r i n l e t and e x i t t o measure gas h e a t i n g and c o o l i n g p r o f i l e s , b u t t h e s e t h e r m o c o u p l e s w e r e removed d u r i n g actual runs. The r e a c t i n g gas was e s s e n t i a l l y i s o t h e r m a l s i n c e l e s s t h a n 8 . 9 cm w e r e r e q u i r e d f o r h e a t i n g and c o o l i n g , r e s p e c t i v e l y , b u t t h e gas t e m p e r a t u r e was a l w a y s l e s s t h a n t h e b a t h t e m p e r a t u r e by a b o u t 4 t o 8C. Sand was f l u i d i z e d by a i r t h a t was p r e h e a t e d t o a b o u t 300C b e f o r e e n t e r i n g t h e r e a c t o r s y s t e m . Det a i l s o f t h e r e a c t o r u s e d i n t h i s s t u d y a r e shown i n F i g u r e 2 . P r o d u c t s a m p l e s w e r e a n a l y z e d u s i n g a M o d e l 1200 A e r o g r a p h gas c h r o m a t o g r a p h e q u i p p e d w i t h f l a m e i o n i z a t i o n d e t e c t o r . The c h r o m a t o g r a p h columns w e r e a s e r i e s a r r a n g e m e n t o f 1 . 8 3 c m , 0 . 3 2 c m O.D. t u b i n g p a c k e d w i t h s i l i c a g e l f o l l o w e d b y 1 . 8 3 c m , 0 . 3 2 c m O.D. t u b i n g packed w i t h a c t i v a t e d alumina. These c o l u m n s p r o v i d e d f o r r e s o l u t i o n of a l l hydrocarbons through C 4 s . Hydrogen c o u l d not b e d e t e r m i n e d b y t h e c h r o m a t o g r a p h and was c a l c u l a t e d f r o m a hydrogen b a l a n c e . f

R e a c t i o n P r o d u c t s and K i n e t i c s , 7 0 0 - 8 5 0 C A s e r i e s o f r u n s was made i n a r e a c t o r c o n s t r u c t e d f r o m 304 s t a i n l e s s s t e e l a t t e m p e r a t u r e s r a n g i n g f r o m 700 t o 850C and f o r space times r a n g i n g from 0 . 1 to 3 seconds. S p a c e t i m e was o b t a i n e d by d i v i d i n g t h e r e a c t i o n z o n e v o l u m e by t h e f e e d v o l u m e t r i c r a t e m e a s u r e d a t r e a c t i o n t e m p e r a t u r e and p r e s s u r e . Hydrogen, m e t h a n e , e t h y l e n e , and c a r b o n w e r e t h e m a j o r p r o d u c t s f o r m e d a t t h e s e t e m p e r a t u r e s , w h i l e b u t è n e s , 1 - 3 b u t a d i e n e , and s o m e t i m e s ethane were the minor p r o d u c t s . C a r b o n y i e l d was d e t e r m i n e d f r o m a c a r b o n b a l a n c e . H o w e v e r , c a r b o n was d e t e r m i n e d l a t e r q u a n t i t a t i v e l y i n an i d e n t i c a l r e a c t o r b y b u r n i n g t h e r e a c t o r w a l l s w i t h o x y g e n and a d s o r b i n g t h e r e s u l t i n g CO2 on a s c a r i t e . Results showed a good a g r e e m e n t b e t w e e n t h e amount o f c a r b o n d e t e r m i n e d q u a n t i t a t i v e l y and t h e v a l u e d e t e r m i n e d f r o m a n i n d e p e n d e n t c a r bon b a l a n c e . P r o p y l e n e c o n v e r s i o n s r a n g e d f r o m 4 t o 48% a t a l l t e m p e r a t u r e s and s p a c e t i m e s . At low c o n v e r s i o n , e t h y l e n e , m e t h a n e , h y d r o g e n , b u t è n e s , and b u t a d i e n e formed i n t h e


INDUSTRIAL AND LABORATORY PYROLYSES

222 TABLE

I

KINETIC PARAMETERS OF PROPYLENE PYROLYSIS UNTREATED 304 STAINLESS STEEL REACTOR

IN


. Temperature (C)

R e a c t i o n Order

700

1.4 + 0 . 0 3

17.60

750

1.2 + 0 . 0 4

2.49

800

1.08

1.31

850

1.0 + 0.05

Rate Constant [

(n)

+ 0.03

1.80


sec

1-n ]


13.

GHALY AND CRYNES

223


approximate r a t i o of 5 : 5 : 2 : 1 : 1 . As p r o p y l e n e c o n v e r s i o n i n c r e a s e d , methane and h y d r o g e n y i e l d s i n c r e a s e d r a p i d l y w h i l e e t h y l e n e , b u t è n e s , and b u t a d i e n e y i e l d s d e c r e a s e d s t e a d i l y . At t h e s e h i g h c o n v e r s i o n s , t h e p r o d u c t s r a t i o became a p p r o x i m a t e l y 10:13:8:1:1. C a r b o n y i e l d m a r k e d l y i n c r e a s e d f r o m 34% a t l o w c o n v e r s i o n t o 95% a t h i g h c o n v e r s i o n . T o t a l molar y i e l d of p r o d u c t s p e r m o l e o f p r o p y l e n e r e a c t e d seemed t o i n c r e a s e w i t h c o n v e r s i o n a n d / o r t e m p e r a t u r e l e v e l s r a n g i n g f r o m 1 . 1 5 ( 7 0 0 C , 4% c o n v e r s i o n ) t o 1 . 4 1 ( 8 5 0 C , 48% c o n v e r s i o n ) . F i g u r e 3 shows t h e e f f e c t o f c o n v e r s i o n on major p r o d u c t d i s t r i b u t i o n . Note t h a t the a b s c i s s a o f F i g u r e 3 i s l o g a r i t h m i c to emphasize lower c o n v e r sion data. H y d r o g e n , e t h y l e n e , and methane a p p e a r t o b e p r i m a r y p r o d u c t s , and c a r b o n i s p o s s i b l y a p r i m a r y p r o d u c t , a l t h o u g h n o t c o n c l u s i v e l y shown h e r e . C a r b o n d a t a a r e somewhat l i m i t e d r e l a t i v e t o t h a t o f o t h e r s p e c i e s . Few r e s e a r c h e r s h a v e o b s e r v e d carbon as a p r i m a r y r e a c t i o n p r o d u c t . The h y d r o g e n and methane y i e l d s a r e more s e n s i t i v e t o t e m p e r a t u r e t h a n was e t h y l e n e . Both a r e p r o b a b l y p r o d u c e d by p r i m a r y a s w e l l a s s e c o n d a r y r e a c t i o n s ( d e c o m p o s i t i o n o f b u t è n e s and b u t a d i e n e and a l s o e t h y l e n e , t h e l a t t e r a t h i g h t e m p e r a t u r e and c o n v e r s i o n ) . The c o n v e r s i o n - s p a c e t i m e d a t a w e r e u s e d t o c a l c u l a t e r a t e c o n s t a n t s and o v e r a l l r e a c t i o n o r d e r s f o r p r o p y l e n e d e c o m p o s i t i o n . R e a c t i o n o r d e r was o b t a i n e d b y p l o t t i n g t h e l o g a r i t h m o f r a t e versus the l o g a r i t h m of propylene c o n c e n t r a t i o n a c c o r d i n g to the equation : -r = k C η

n

R e a c t i o n r a t e s were o b t a i n e d by n u m e r i c a l d i f f e r e n t i a t i o n o f conversion-space time curves. The r e a c t i o n r a t e c o n s t a n t s w e r e c a l c u l a t e d a t e a c h t e m p e r a t u r e by n u m e r i c a l l y i n t e g r a t i n g t h e expression k

η

-

X

t C

t

1 + (ε - 1 ) X ( 1 - X)

]

n

dX

w h i c h t a k e s i n t o account the volume e x p a n s i o n as r e a c t i o n proceeds. The r e a c t i o n o r d e r s and r a t e c o n s t a n t s o b t a i n e d a t t h e v a r i o u s t e m p e r a t u r e s a r e shown i n T a b l e I. I n o r d e r t o compare the r e s u l t s from t h i s work to those g i v e n i n the l i t e r a t u r e , f i r s t and 3/2 o r d e r r a t e e x p r e s s i o n s w e r e u s e d t o o b t a i n r a t e c o n s t a n t s from the experimental d a t a . A c o m p a r i s o n o f f i r s t and 3 / 2 - o r d e r r a t e constants i s given i n Table I I . The v a l u e s g i v e n i n t h e t a b l e f o r t h i s work w e r e . t h o s e c o r r e s p o n d i n g to zero c o n v e r s i o n , and t h e l i t e r a t u r e v a l u e s a r e f r o m a v a r i e t y o f c o n d i t i o n s i n c l u d i n g b a t c h , f l o w , and a t b o t h l o w and h i g h t e m p e r a t u r e s . The values of the f i r s t order r a t e constant are c l o s e to the v a l u e s



on r a t e c o n s t a n t a r e

+ Wall-less

reactor.

3.3

0.40

conversion.

(g mole) ^ ' ^ (sec)

are seconds

*** Rate c o n s t a n t s a r e e s t i m a t e d a t z e r o

** U n i t s

on r a t e c o n s t a n t

2.5

0.10

2.0

850

* Units

0.48

0.03

0.38

800

0.07

0.10

750

S z w a r c Amano M e l l o u t t e ( 1 9 4 9 ) (1963) (1969)

0.035

T h i s Work***

lst-order*

700

Temperature (C)

II

460 602

1.1

35.0

5.8

Kunugi (1970)

125

0.10

Melloutte (1969)

order**

112

32.5

12.3

T h i s Work***

3/2

0.26

Taylor"*" (1972)

COMPARISON OF REACTION RATE CONSTANTS

TABLE


13.

GHALY AND CRYNES


225


r e p o r t e d b y Amano and U c h i y a m a ( 1 9 6 3 ) , a t l e a s t o v e r t h e temperature range of 750-850C. The 3 / 2 - o r d e r d a t a a r e g e n e r a l l y i n agreement w i t h t h o s e p r e s e n t e d by K u n u g i , e t a l . ( 1 9 7 0 ) , a g a i n over the range o f 750-850C. M e l l o u t t e e , e t a l . (1969) r e p o r t e d a change o f r e a c t i o n o r d e r f r o m 3/2 ( 6 0 0 - 6 9 8 C ) t o f i r s t o r d e r ( 7 0 0 900C) and s u g g e s t e d a change i n d e c o m p o s i t i o n mechanism a t 700C as a p o s s i b l e r e a s o n . The r a t e c o n s t a n t s o b t a i n e d b y M e l l o u t t e e a r e , however, q u i t e d i f f e r e n t from those o b t a i n e d i n t h i s work a t t h e same t e m p e r a t u r e s . The p y r o l y s i s o f p r o p y l e n e i s o b v i o u s l y a c o m p l e x r e a c t i o n s y s t e m , a n d any a t t e m p t t o r e p r e s e n t t h e r a t e o f the r e a c t i o n by a s i m p l e o v e r a l l n t h o r d e r e x p r e s s i o n o v e r a n y t h i n g but a very narrow range of c o n d i t i o n s i s not j u s t i f i e d . Results i n Untreated Reactors A t o t a l o f t w e n t y r u n s w e r e made i n 5 r e a c t o r s c o n s t r u c t e d o f d i f f e r e n t m a t e r i a l s a t a t e m p e r a t u r e o f 750C and d i f f e r e n t s p a c e t i m e s r a n g i n g b e t w e e n 0 . 5 and 2 s e c o n d s . The r e a c t o r s u s e d i n t h e s e r u n s a r e shown i n T a b l e I I I . S t a i n l e s s s t e e l R e a c t o r 2 was c h o s e n a s a r e f e r e n c e r e a c t o r , and t h e r e s u l t s ( c o n v e r s i o n s and p r o d u c t d i s t r i b u t i o n ) o b t a i n e d i n t h i s r e a c t o r w e r e t h e b a s i s by which the comparative b e h a v i o r of o t h e r r e a c t o r s were judged. R e s u l t s o b t a i n e d u n d e r t h e same e x p e r i m e n t a l c o n d i t i o n s b u t i n d i f f e r e n t r e a c t o r s w e r e sometimes s i g n i f i c a n t l y d i f f e r e n t a s w i l l b e shown b e l o w . The i n f l u e n c e o f w a l l m a t e r i a l was j u d g e d by t h e change i n p r o p y l e n e c o n v e r s i o n ( r e a c t i o n r a t e ) and/or a s h i f t i n p r o d u c t d i s t r i b u t i o n . When c o m p a r i n g t h e d i f f e r e n t w a l l m a t e r i a l s b e h a v i o r t o t h a t o f t h e r e f e r e n c e w a l l m a t e r i a l , i . e . , 304 s t a i n l e s s s t e e l , c a r e was t a k e n t o d i s t i n g u i s h b e t w e e n t h e t r a n s i e n t and s t e a d y y i e l d s and c o n v e r s i o n s ; t h e l a t t e r was u s u a l l y o b t a i n e d i n reactors with carbon-conditioned w a l l s . S t a i n l e s s S t e e l (304) R e a c t o r ( R e f e r e n c e R e a c t o r ) . The c o n v e r s i o n s and p r o d u c t y i e l d s o b t a i n e d a t t h e d i f f e r e n t s p a c e t i m e s and a t 750C w e r e a l m o s t i d e n t i c a l t o t h o s e o b t a i n e d p r e v i o u s l y i n 304 s t a i n l e s s s t e e l R e a c t o r 1 ( u s e d m a i n l y t o g e n e r a t e k i n e t i c d a t a ) , thus i n d i c a t i n g the h i g h r e p r o d u c i b i l i t y of r e sults. No i n i t i a l t r a n s i e n t b e h a v i o r o f t h e r e a c t o r was n o t i c e d s i n c e t h e p r o d u c t s c o n c e n t r a t i o n s and p r o p y l e n e c o n v e r s i o n d i d n o t change f o r s a m p l e s drawn o v e r a p e r i o d o f 30 m i n u t e s i n t h e f i r s t r u n ( 7 5 0 C , 1 . 5 s e c o n d s ) made i n t h i s r e a c t o r . No u n s t e a d y s t a t e b e h a v i o r was o b s e r v e d i n s u b s e q u e n t r u n s as w e l l . Typical results a r e shown i n T a b l e I V . P r o p y l e n e c o n v e r s i o n r a n g e d f r o m 5 t o 18%. H y d r o g e n y i e l d i n c r e a s e d s t e a d i l y f r o m 12 t o 20%, methane y i e l d i n c r e a s e d f r o m 31 t o 43% and e t h y l e n e y i e l d r e m a i n e d p r a c t i c a l l y c o n s t a n t a t 45% a s c o n v e r s i o n i n c r e a s e d w i t h s p a c e t i m e . Carbon y i e l d i n c r e a s e d s u b s t a n t i a l l y f r o m 38 t o 60% w i t h i n c r e a s e i n c o n v e r s i o n , w h i l e b u t è n e s y i e l d d e c r e a s e d f r o m 15 t o 11% and b u t a d i e n e y i e l d d e c r e a s e d f r o m 11 t o 8% w i t h i n c r e a s e i n c o n v e r s i o n . The r e s u l t s s u g g e s t t h a t s e c o n d a r y d e c o m p o s i t i o n o f p r o d u c t s


Albright and Crynes; Industrial and Laboratory Pyrolyses ACS Symposium Series; American Chemical Society: Washington, DC, 1976. 455 460

I n c o l o y 801

I n c o l o y 801

6*

7*

18 g a g e )

53.0

452

I n c o n e l 600

5*

(1/4 i n c h O . D . ,

8.7

75.8

462

Nickel

4

* 0 . 6 3 5 cm O.D.

8.7

73.0

445

Low-Carbon S t e e l ( A I S I 1015)

32.5

20.5 20.5

45.5 45.5

10.4 10.4

53.3 54.0

tubing

76.4

15.8 7.2

10.4

32.5

99.5

—

0.15

Bal.

19

8.7

75.0

457

Stainless Steel ( A I S I 304)

70

19

8.7

75.0

457

0.04 0.04 0.04

0.80 0.80

0.06 0.20

0.25

0.13-0.18

0.3-0.6

max.

0.08

2 . 0 max.

max.

0.08

% Μη

2 . 0 max.

Composition, Cr Ni

Stainless Steel ( A I S I 304)

Fe

S/V, cm

Submerged V o l u m e , cm

Submerged L e n g t h , cm

Material

Reactor

III

REACTOR DIMENSIONS AND PROPERTIES

TABLE


GHALY AND CRYNES

13.

227

Reactor Surface Effects TABLE

IV

RESULTS FROM 304 STAINLESS STEEL REACTOR AT 7 5 0 C , 1 ATM

Space Time

(min.)

0.5

1.0

1.5

2.0

0.66

1.52

2.52

3.25

1.61

3.71

5.21

7.10

2.48

4.35

6.24

7.84

0.05

0.04

0.61

0.95


Mole P e r c e n t

CH

4

C

2 4

C

2 4

C

3 6

C

4 8

C

4 6

H

H

H

H

H

93.9

87.5

82.4

77.4

0.77

1.35

1.71

1.93

0.57

1.03

1.31

1.56

C o n v e r s i o n (%)

5.3

Total Yield*

1.16

10.1 1.25

14.0 1.30

17.8 1.33

12.5

15.3

18.8

19.4

Y i e l d (%)

30.6

37.4

39.0

42.4

C H Y i e l d (%)

47.1

44.1

46.7

46.8

C Y i e l d (%)

38.0

50.0

55.1

60.0

H

Y i e l d (%)

2

CH 2

4

4

* T o t a l y i e l d o f gaseous p r o d u c t s



228


b u t è n e and b u t a d i e n e t a k e s p l a c e as c o n v e r s i o n i n c r e a s e s , and m e t h a n e , h y d r o g e n , and p r o b a b l y c a r b o n a r e p r o d u c t s o f s u c h decomposition. H i g h c a r b o n y i e l d a t c o n v e r s i o n s as l o w a s 5% s u g g e s t s s t r o n g l y t h a t c a r b o n may be formed d i r e c t l y b y t h e i n t e r a c t i o n o f p r o p y l e n e m o l e c u l e w i t h the m e t a l s u r f a c e r a t h e r t h a n by a s e c o n d a r y c o k i n g o f some h i g h e r m o l e c u l a r w e i g h t p r o d u c t s . Direct d e c o m p o s i t i o n o f p a r a f f i n s and o l e f i n s on m e t a l s u r f a c e s t o p r o duce c a r b o n was s u g g e s t e d e a r l y by Thomas, e t a l . (1939) and was l a t e r c o n f i r m e d by T e s n e r (1959) and T a m a i , e t a l . ( 1 9 6 8 ) . The a b s e n c e o f a n o t i c e a b l e i n i t i a l s u r f a c e a c t i v i t y p e r i o d does n o t c o n c l u s i v e l y p r o v e t h a t s t a i n l e s s s t e e l s u r f a c e h a s no h e t e r o g e n eous e f f e c t on p r o p y l e n e d e c o m p o s i t i o n . The f o r m a t i o n o f c a r b o n i n much h i g h e r q u a n t i t i e s ( a t l o w c o n v e r s i o n ) t h a n t h o s e r e p o r t e d i n r e a c t o r s made o f g l a s s o r q u a r t z u n d e r s i m i l a r c o n d i t i o n s s t r o n g l y i n d i c a t e s a s u r f a c e a c t i v i t y a s s o c i a t e d w i t h the metal reactor. Carbon formed d u r i n g p y r o l y s i s o f s e v e r a l hydrocarbons h a s b e e n shown t o p o s s e s s some c a t a l y t i c a c t i v i t y a s shown b y T e s n e r (1959) and H o l b r o o k , e t a l . ( 1 9 6 8 ) , and i t i s p o s s i b l e t h a t t h e c a r b o n f o r m e d by i n t e r a c t i o n o f p r o p y l e n e w i t h s t a i n l e s s s t e e l s u f a c e i n d u c e s t h e same a c t i v i t y t o w a r d s p r o p y l e n e d e c o m p o s i t i o n a s t h a t o f t h e p a r e n t s u r f a c e ; h e n c e , no change i n s u r f a c e a c t i v i t y w o u l d b e n o t i c e d w i t h t i m e as more c a r b o n i s f o r m e d . Low C a r b o n S t e e l R e a c t o r . The l o w c a r b o n s t e e l (LCS) r e a c t o r e x h i b i t e d a p e c u l i a r behavior i n the e a r l y p e r i o d of the f i r s t run (750C, 1.5 s e c o n d s ) . An i n c r e a s e i n a c t i v i t y o c c u r r e d d u r i n g t h e 20 m i n u t e p e r i o d , marked by i n c r e a s e d h y d r o g e n and c a r b o n f o r m a t i o n , and t h e n t h e a c t i v i t y d r o p p e d s t e a d i l y u n t i l a s t e a d y - s t a t e was r e a c h e d a f t e r 60 m i n u t e s . No i n i t i a l t r a n s i e n t a c t i v i t y p e r i o d was o b s e r v e d i n s u b s e q u e n t r u n s c a r r i e d o u t i n t h e c a r b o n conditioned reactor. At the p o i n t of highest a c t i v i t y , propylene c o n v e r s i o n was a h i g h 2 7 % , w h i l e h y d r o g e n , m e t h a n e , e t h y l e n e , and c a r b o n y i e l d s w e r e 9 0 , 5 6 , 3 5 , and 146% r e s p e c t i v e l y . Using h y d r o g e n y i e l d as a measure o f a c t i v i t y , F i g u r e 4 p r e s e n t s t h e a c t i v i t y p r o f i l e f o r t h e LCS r e a c t o r . The r e s u l t s a l s o i n d i c a t e d t h a t e v e n t h e s t e a d y - s t a t e c a r b o n - c o n d i t i o n e d LCS r e a c t o r was more a c t i v e t h a n t h e 304 r e f erence r e a c t o r . F i g u r e 5 shows t h e h i g h e r h y d r o g e n and methane c o n c e n t r a t i o n s o b t a i n e d i n t h e LCS r e a c t o r a s compared t o t h e reference reactor. C a r b o n y i e l d was much h i g h e r i n l o w c a r b o n s t e e l t h a n i n t h e r e f e r e n c e r e a c t o r , w h i l e e t h y l e n e y i e l d was s l i g h t l y lower than that i n the r e f e r e n c e r e a c t o r . Conversions ( r e a c t i o n r a t e s ) i n low carbon s t e e l were always 25-35% h i g h e r than those obtained i n the r e f e r e n c e r e a c t o r . The same s h a p e d i n i t i a l a c t i v i t y p r o f i l e was o b s e r v e d b y C r y n e s (1968) i n a LCS r e a c t o r d u r i n g t h e p y r o l y s i s o f p r o p a n e r a t h e r than p r o p y l e n e the h i g h e r i n i t i a l a c t i v i t y of low carbon s t e e l a s compared t o s t a i n l e s s s t e e l c o u l d b e a t t r i b u t e d t o t h e much higher i r o n content of the former. Thomas, e t a l . (1939) s t u d y i n d i c a t e d i r o n t o b e a s t r o n g c a t a l y s t i n p r o m o t i n g c a r b o n and


GHALY AND CRYNES

13.

229


25

750 ° C , 1 A T M t = 1.5 S E C O N D S

LOW-CARBON

STEEL


_D

0

20

40 SAMPLE

Figure 4.

60

80

100

120

TIME , MINUTES

Transient behavior of metal reactors

16 7 5 0 ° C , 1 ATMOSPHERE

14

ο 304 STAINLESS-STEEL REACTOR - • LOW-CARBON STEEL π REACTOR A NICKEL REACTOR

12 HYDROGEN-. tu

ο

ι

iO|

4

/VMETHANE

8

Ο Q_

Έ

Ο u

f

J*

/

^-HYDROGEN.

-

1

1

0.5 1.0 SPACE TIME,

1—

1.5 2.0 SECONDS

2.5

Figure 5. Effect of reactor mate rial on product distribution


230



hydrogen f o r m a t i o n d u r i n g hydrocarbon p y r o l y s i s , w h i l e a l l o y s of i r o n c o n t a i n i n g a l a r g e amount o f chromium w e r e much l e s s a c t i v e due t o t h e C r a b i l i t y t o d e c r e a s e o r d e s t r o y t h e a c t i v i t y o f i r o n . The a c t i v i t y o f c a r b o n - c o n d i t i o n e d l o w c a r b o n s t e e l was h i g h e r t h a n t h a t o f s t a i n l e s s s t e e l p e r h a p s b e c a u s e o f a more c a t a l y t i c a l l y a c t i v e carbon l a y e r . S t u d i e s by Thomas, e t a l . ( 1 9 3 9 ) and T a m a i , e t a l . (1968) on c a r b o n f o r m a t i o n d u r i n g t h e p y r o l y s i s o f o l e f i n s and p a r a f f i n s on d i f f e r e n t m e t a l s i n d i c a t e t h e p o s s i b i l i t y o f c a r b o n s h o w i n g a c a t a l y t i c a c t i v i t y due t o c a r r y i n g i r o n atoms i n the carbon l a y e r perhaps i n the form of minute p a r t i c l e s . Nickel Reactor. A n i n i t i a l u n s t e a d y s t a t e p e r i o d marked by h i g h a c t i v i t y l a s t e d f o r 50 m i n u t e s i n a r e a c t o r t u b e c o n s t r u c t e d of n i c k e l . The h i g h e s t a c t i v i t y was o b t a i n e d a t t h e b e g i n n i n g o f t h e r u n ( 7 5 0 C , 1.5 s e c o n d s ) and d e c r e a s e d w i t h t i m e a s more c a r b o n was d e p o s i t e d on t h e r e a c t o r w a l l s . The t r a n s i e n t b e h a v i o r o f t h e n i c k e l r e a c t o r i s shown i n F i g u r e 4 a l o n g w i t h t h o s e o f other reactors. Highest w a l l a c t i v i t y corresponded to a hydrogen y i e l d o f 74%, methane y i e l d o f 4 7 % , e t h y l e n e y i e l d o f 4 1 % , and c a r b o n y i e l d o f 1 2 2 % ; c o n v e r s i o n was s l i g h t l y h i g h e r t h a n t h a t obtained i n the reference r e a c t o r . At s t e a d y - s t a t e , c o n v e r s i o n s were i d e n t i c a l to those o b t a i n e d i n t h e 304 r e a c t o r , b u t p r o d u c t s d i s t r i b u t i o n was d i f f e r e n t i n d i c a t i n g a r e a c t o r w a l l more a c t i v e ( s e l e c t i v e ) t h a n s t a i n l e s s s t e e l b u t l e s s a c t i v e t h a n l o w c a r b o n s t e e l as shown i n F i g u r e 5 by t h e l e v e l s o f h y d r o g e n and methane o b t a i n e d i n t h e carbon-conditioning nickel reactor. The i n i t i a l w a l l a c t i v i t y d i m i n i s h e d l e s s a c t i v e carbon b u i l d s upon t h e w a l l s d e c r e a s i n g a v a i l a b l e a c t i v e s i t e s . The i n i t i a l a c t i v i t y o f n i c k e l was c l e a r l y l o w e r t h a n t h a t o f l o w c a r bon s t e e l , b u t h i g h e r than t h a t of s t a i n l e s s s t e e l . The r e s u l t s a r e i n g e n e r a l agreement w i t h t h e c o n c l u s i o n s o f T a m a i , e t a l . (1968) and B u e l l and Weber ( 1 9 5 0 ) . The f o r m e r i n d i c a t e d t h a t n i c k e l had a l o w e r " a f f i n i t y " to o l e f i n s t h a n i r o n , w h i l e t h e l a t t e r concluded t h a t the n i c k e l content i n a u s t e n i t i c s t e e l a l l o y s i s p r i m a r i l y r e s p o n s i b l e f o r t h e i r a c t i v i t y (carbon format i o n ) when compared t o t h e l e s s a c t i v e chrome s t e e l a l l o y s . The c a r b o n - c o n d i t i o n e d n i c k e l w a l l s were l e s s a c t i v e than those o f low carbon s t e e l r e a c t o r probably because the c a t a l y t i c a c t i v i t y of the base m e t a l d i d not p e n e t r a t e through the carbon l a y e r as e f f e c t i v e l y as i t d i d w i t h low carbon s t e e l . a

s

I n c o n e l and I n c o l o y R e a c t o r s . I n c o n e l showed a n i n i t i a l a c t i v i t y p e r i o d t h a t l a s t e d f o r 30 m i n t u e s . During t h i s period, propylene conversion remained c o n s t a n t , but the product d i s t r i b u t i o n changed m a r k e d l y . The h i g h e s t w a l l a c t i v i t y was o b t a i n e d i n t h e f i r s t two m i n u t e s o f t h e f i r s t r u n ( 7 5 0 C , 1 . 5 s e c o n d s ) and then decreased s t e a d i l y w i t h time. Highest a c t i v i t y corresponded t o a h y d r o g e n y i e l d o f 4 3 % , methane y i e l d o f 5 1 % , and c a r b o n y i e l d o f 9 5 % . S t e a d y - s t a t e c o n v e r s i o n s and p r o d u c t d i s t r i b u t i o n i n t h e


13.

GHALY AND CRYNES


231


i n c o n e l r e a c t o r were s i m i l a r t o those o f the r e f e r e n c e r e a c t o r . F i g u r e 4 shows I n c o n e l t r a n s i e n t b e h a v i o r as m e a s u r e d b y h y d r o g e n yield. The l o w e r i n i t i a l a c t i v i t y o f I n c o n e l as compared t o t h a t o f low carbon s t e e l o r n i c k e l c l e a r l y i n d i c a t e s t h a t a l l o y s c o n t a i n i n g i r o n and n i c k e l a r e i n g e n e r a l l e s s e f f e c t i v e t h a n t h e p u r e metals i n i n t e r a c t i n g w i t h propylene. I n c o l o y r e a c t o r showed a n i n i t i a l a c t i v i t y p r o f i l e s i m i l a r to that of Inconel r e a c t o r . S t e a d y - s t a t e a c t i v i t y was r e a c h e d a f t e r 40 m i n u t e s , and t h e r e s u l t s w e r e s i m i l a r t o t h o s e o b t a i n e d i n the r e f e r e n c e r e a c t o r . The i n i t i a l a c t i v i t y p r o f i l e o f I n c o l o g y i s shown i n F i g u r e 4 . Results i n Surface-Treated

Reactors

R e a c t o r s u r f a c e s w e r e t r e a t e d w i t h o x y g e n and h y d r o g e n s u l f i d e s i n c e t h e s e gases a r e e i t h e r used o r have been suggested i n i n d u s t r i a l p y r o l y s i s a p p l i c a t i o n s . Oxygen t r e a t m e n t s c o n s i s t e d o f p a s s i n g 550-600 cc/min o f oxygen through the r e a c t o r f o r t i m e s up t o 60 m i n u t e s w i t h t h e r e a c t o r b a t h a t t e m p e r a t u r e s r a n g i n g f r o m 600 t o 800C. I n h y d r o g e n s u l f i d e t r e a t m e n t , 250 c c / m i n o f t h e gas was p a s s e d t h r o u g h t h e r e a c t o r f o r 2 0 - 6 0 m i n u t e s , w h i l e t h e r e a c t o r b a t h was a t 750C. The r e a c t o r s w e r e thoroughly purged w i t h h e l i u m a f t e r each treatment. A l l propyl e n e r u n s f o l l o w i n g a s u r f a c e t r e a t m e n t w e r e c a r r i e d o u t a t 750C and 1 . 5 s e c o n d s s p a c e t i m e a t e s s e n t i a l l y a t m o s p h e r i c p r e s s u r e . S t a i n l e s s S t e e l (304) R e a c t o r a. Oxygen T r e a t m e n t : The c o u r s e o f t h e r e a c t i o n was f o u n d to be r a t h e r d i f f e r e n t a f t e r the s t a i n l e s s s t e e l r e a c t o r (Reactor 2) was t r e a t e d w i t h o x y g e n . Figure 6 i n d i c a t e s the r e s u l t s of a r u n made a t 750C a f t e r t h e r e a c t o r w a l l s h a d b e e n t r e a t e d f o r 30 m i n u t e s w i t h o x y g e n a t a t e m p e r a t u r e o f 600C. An i n i t i a l a c t i v i t y p e r i o d was o b s e r v e d d u r i n g w h i c h p r o p y l e n e c o n v e r s i o n and p r o d u c t c o m p o s i t i o n c h a n g e d s t e a d i l y w i t h t i m e u n t i l a s t e a d y - s t a t e was r e a c h e d a f t e r 80 m i n u t e s . The change i n w a l l a c t i v i t y was more d r a s t i c i n t h e f i r s t 15 m i n u t e s o f t h e r u n . Hydrogen y i e l d d e c r e a s e d f r o m 40 t o 2 9 % , methane and e t h y l e n e y i e l d s r e m a i n e d u n c h a n g e d , b u t è n e s and b u t a d i e n e s y i e l d s i n c r e a s e d m o d e r a t e l y , and c a r b o n y i e l d d e c r e a s e d f r o m 73 t o 6 3 % . The r e s u l t s s u g g e s t t h a t the oxygen-treated s u r f a c e d i r e c t l y c a t a l y z e d the secondary d e c o m p o s i t i o n o f b u t è n e s and b u t a d i e n e s . A f t e r 80 m i n u t e s , v a l u e s o f c o n v e r s i o n and p r o d u c t c o m p o s i t i o n a p p r o x i m a t e d t h o s e o b t a i n e d i n the untreated reference r e a c t o r . The i n i t i a l r e a c t o r a c t i v i t y was f o u n d t o change w i t h o x y g e n t r e a t m e n t t e m p e r a t u r e r a t h e r t h a n treatment time. The h i g h e s t s u r f a c e a c t i v i t y was o b t a i n e d f o r a t r e a t m e n t t e m p e r a t u r e o f 600C w h i l e t h e l o w e s t s u r f a c e a c t i v i t y was o b t a i n e d f o r a t r e a t m e n t t e m p e r a t u r e o f 800C. T h i s change i n i n i t i a l a c t i v i t y seems t o i n d i c a t e a change i n t h e s t r u c t u r e o f a



ι 1

OXYGEN TREATED UNTREATED

1

1

1

1

1

1

304 STAINLESS STEEL 750 °C, 1 ATM t = 1.5 SECONDS



13.

GHALY AND CRYNES


233

c a t a l y t i c s u r f a c e s p e c i e s ( p r o b a b l y a n o x i d e ) when t h e s u r f a c e i s c o n t a c t e d w i t h oxygen a t d i f f e r e n t t e m p e r a t u r e s . b. Hydrogen S u l f i d e Treatment: T r e a t i n g the r e a c t o r w i t h H2S f o r 20 m i n u t e s e l i m i n a t e d t h e s u r f a c e a c t i v i t y e f f e c t s o f p r e v i o u s oxygen t r e a t m e n t s . I n t h e e a r l y p e r i o d o f one r u n ( 2 - 2 0 m i n u t e s ) t h e s u r f a c e was a c t u a l l y l e s s a c t i v e t h a n t h a t o f t h e u n t r e a t e d r e f e r e n c e r e a c t o r as i n d i c a t e d by s l i g h t l y l o w e r h y d r o g e n , m e t h a n e , and c a r b o n y i e l d s . H o w e v e r , as t h e r e a c t i o n p r o c e e d e d and c a r b o n was b u i l t u p , t h e a c t i v i t y i n c r e a s e d u n t i l a s t e a d y - s t a t e c o n v e r s i o n and p r o d u c t c o m p o s i t i o n s i m i l a r t o t h a t i n the r e f e r e n c e r e a c t o r were o b t a i n e d . Subsequent t r e a t m e n t s w i t h o x y g e n f o r a s l o n g as 60 m i n u t e s c o u l d n o t r e s t o r e w a l l a c t i v i t y . T h i s b e h a v i o r was s i m i l a r t o t h a t i n d i c a t e d by C r y n e s and A l b r i g h t (1969) d u r i n g propane p y r o l y s i s i n s u r f a c e - t r e a t e d r e a c tors. During s u l f i d i n g w i t h H2S, the s u r f a c e oxide l a y e r i s probably converted to s u l f i d e e i t h e r e n t i r e l y or at l e a s t to a s u f f i c i e n t d e p t h ( F a r b e r and E h r e n b e r g , 1 9 5 2 ) , and a d u r a b l e p r o t e c t i v e s u l f i d e l a y e r i s formed. T h i s s u l f i d e l a y e r i s more p a s s i v e t h a n e i t h e r t h e u n t r e a t e d 304 s t a i n l e s s s t e e l s u r f a c e o r the c a r b o n l a y e r formed d u r i n g p y r o l y s i s . Thus t h e r e i s an o b s e r v e d i n i t i a l i n c r e a s e i n a c t i v i t y w i t h c a r b o n b u i l d - u p as t h e s u r f a c e t y p e and i t s a c t i v i t y s t a b i l i z e . Low C a r b o n S t e e l R e a c t o r . a. Oxygen T r e a t m e n t : A f t e r t r e a t i n g t h e LCS r e a c t o r w i t h o x y g e n , an a c t i v e r e a c t o r s u r f a c e i s p r o d u c e d , and a t r a n s i e n t a c t i v i t y p r o f i l e , s i m i l a r t o t h a t o b t a i n e d w i t h 304 s t a i n l e s s steel, is noticed. A comparison of the t r a n s i e n t a c t i v i t y o b t a i n e d i n b o t h t h e u n t r e a t e d and o x y g e n - t r e a t e d LCS r e a c t o r i s shown i n T a b l e V . The t a b l e shows t h a t b o t h t h e a c t i v i t y p r o f i l e and a c t i v i t y l e v e l p r o d u c e d b y t h e two s u r f a c e s w e r e c o n s i d e r a b l y different. The r i s e - f a l l a c t i v i t y p r o f i l e was a b s e n t i n t h e o x y g e n - t r e a t e d r e a c t o r , and t h e a c t i v i t y p r o d u c e d by t h e o x y g e n t r e a t e d s u r f a c e was g e n e r a l l y l o w e r t h a n t h a t o f t h e u n t r e a t e d surface. V a l u e s o f i n i t i a l c o n v e r s i o n s a n d p r o d u c t y i e l d s shown f o r t h e o x y g e n - t r e a t e d LCS r e a c t o r w e r e o b t a i n e d a f t e r t h e r e a c t o r was t r e a t e d w i t h o x y g e n f o r 20 m i n t u e s a t 750C. A steady-state a c t i v i t y was e v e n t u a l l y r e a c h e d f o r t h e o x y g e n - t r e a t e d s u r f a c e a f t e r p e r i o d s o f 50 t o 70 m i n u t e s . This carbon-conditioned r e a c t o r a c t i v i t y was a p p r o x i m a t e l y s i m i l a r t o t h a t o f t h e c a r b o n c o n d i t i o n e d u n t r e a t e d LCS s u r f a c e . The r e s u l t s s u g g e s t t h a t t h e d e c r e a s e i n i n i t i a l s u r f a c e a c t i v i t y o f l o w c a r b o n s t e e l when t r e a t e d w i t h o x y g e n i s p r o b a b l y due t o t h e f o r m a t i o n o f a p r o t e c t i v e o x i d e f i l m w h i c h c a t a l y t i c a l l y i s l e s s a c t i v e than the base m e t a l ; i . e . , i r o n . Many l i t e r a t u r e s o u r c e s i n d e e d i n d i c a t e the f o r m a t i o n o f an o x i d e l a y e r c o n s i s t i n g m a i n l y o f Fe304 u n d e r c o n d i t i o n s s i m i l a r t o t h o s e u s e d i n this study. T h i s b l a c k i r o n o x i d e u s u a l l y forms a c o h e r e n t l a y e r which i s not e a s i l y reduced.



4

4

**

*

H y d r o g e n s u l f i d e a t 750C f o r 20 m i n u t e s

54

52 46 43

54

62

128

103

102

45 46 46 46

46

47

38

44

41

37

34 33

37

37

48

50

43

39

18 16 15

19

20

27

52

78

62

14.0

13.0

12.5

14.0

14.4

17.2

24.6

22.7

20.0

1.5

1.5

1.5

1.5

1.5

1.5

1.5

1.5

750

750

750

750

750

750

750

750

750

1.5

60

20

2

60

40

2

70

2

H S--Treated**

35

Oxygen-Treated*

2

Oxygen a t 750C f o r 20 m i n u t e s

(%)

(%)

(%)

(%)

Yield

Yield

C Yield

2

C H

CH

Yield

(%)

Conversion

2

(Sec)

Space Time

H

(C)

Temperature

Sample Time ( M i n )

Untreated

COMPARISON OF I N I T I A L SURFACE ACTIVITY I N UNTREATED AND TREATED LOW CARBON STEEL REACTOR

TABLE V


13.

GHALY AND CRYNES


235


b. Hydrogen S u l f i d e Treatment: T r e a t i n g t h e LCS r e a c t o r s u r f a c e w i t h H2S e l i m i n a t e d t h e s u r f a c e a c t i v i t y e f f e c t s o f p r e v i o u s o x y g e n t r e a t m e n t s and p r o d u c e d a more p a s s i v e s u r f a c e w h i c h e x h i b i t e d e s s e n t i a l l y t h e same c o n v e r s i o n and p r o d u c t d i s t r i b u t i o n o b t a i n e d i n t h e 304 r e f e r e n c e r e a c t o r . Results obtained i n the LCS r e a c t o r a f t e r t r e a t i n g t h e w a l l s w i t h H£S a r e shown i n T a b l e V. Subsequent t r e a t m e n t w i t h oxygen f o r a r a t h e r l o n g p e r i o d did not r e s t o r e the o r i g i n a l surface a c t i v i t y . Nickel Reactor. The n i c k e l r e a c t o r e x h i b i t e d a p e c u l i a r i n i t i a l a c t i v i t y p r o f i l e a f t e r the w a l l s had been t r e a t e d w i t h oxygen. The same p e c u l i a r p r o f i l e was a l w a y s o b t a i n e d r e g a r d l e s s o f t h e oxygen t r e a t m e n t temperature o r d u r a t i o n . F i g u r e 7 shows t h a t t h e a c t i v i t y o f w a l l s d r o p p e d s t e a d i l y i n t h e f i r s t 15 m i n utes of the r u n , then suddenly the w a l l a c t i v i t y i n c r e a s e d s h a r p l y o v e r t h e n e x t 25 m i n u t e s as i n d i c a t e d b y t h e s h a r p i n c r e a s e s i n h y d r o g e n and methane y i e l d s . Propylene conversion a l s o i n c r e a s e d b y 30% d u r i n g t h a t 25 m i n u t e p e r i o d . A sharp decrease i n a c t i v i t y t h e n f o l l o w e d and a s t e a d y s t a t e a c t i v i t y l e v e l was f i n a l l y r e a c h e d a f t e r 90 m i n u t e s o f r u n . The i n i t i a l a c t i v i t y o f t h e r e a c t o r w i t h i n t h e f i r s t 10 m i n u t e s o r s o was a l w a y s l e s s t h a n t h e c o m p a r a b l e a c t i v i t y o f t h e untreated n i c k e l surface. However, t h e p r o d u c t c o m p o s i t i o n s o b t a i n e d d u r i n g the l a t e r p e r i o d of decreased a c t i v i t y were a p p r o x i m a t e l y s i m i l a r to those obtained d u r i n g t r a n s i e n t a c t i v i t y behavior of untreated n i c k e l surface. One p r o b a b l e e x p l a n a t i o n o f t h e above p e c u l i a r a c t i v i t y b e h a v i o r i s t h a t t h e c a r b o n f o r m e d on t h e r e a c t o r s u r f a c e , t o g e t h e r w i t h h y d r o g e n , i n t h e e a r l y e x p e r i m e n t a l p e r i o d may h a v e c a u s e d , a t l e a s t , a p a r t i a l r e d u c t i o n o f t h e n i c k e l o x i d e s u r f a c e l a y e r t o f o r m t h e more c a t a l y t i c a l l y a c t i v e n i c k e l , and a s more r e d u c t i o n t o o k p l a c e more n i c k e l was formed l e a d i n g t o h i g h e r a c t i v i t y . A p o i n t must t h e n be r e a c h e d when no more n i c k e l f o r m a t i o n t a k e s p l a c e , and t h e a c t i v i t y e v e n t u a l l y d e c l i n e s i n a manner s i m i l a r t o t h a t o b t a i n e d i n a n u n treated nickel reactor. The r e d u c t i o n o f n i c k e l o x i d e t o n i c k e l under the i n f l u e n c e o f hydrogen and/or c a r b o n a t h i g h t e m p e r a t u r e s i s sometimes r e f e r r e d t o a s n i c k e l w i l d n e s s . S i n c e n i c k e l i s s e v e r e l y a t t a c k e d by H2S a t h i g h t e m p e r a t u r e s a t e v e n s h o r t p e r i o d s o f t i m e , no a t t e m p t was made t o c o n d i t i o n t h e n i c k e l r e a c t o r w i t h H^S. I n c o n e l and I n c o l o y R e a c t o r s . The a c t i v i t y o f i n c o n e l and i n c o l o y r e a c t o r s i n c r e a s e d s i g n i f i c a n t l y a f t e r the s u r f a c e s had been t r e a t e d w i t h oxygen. R e s u l t s i n t h e s e two r e a c t o r s w e r e p r a c t i c a l l y s i m i l a r t o t h o s e o b t a i n e d i n o x y g e n - t r e a t e d 304 s t a i n less steel. A g a i n because of the h i g h n i c k e l content of these r e a c t o r s , no H S t r e a t m e n t was a t t e m p t e d . ?



V

0

Ο

10 "

20

30

40

50

60

1

10

1

Figure 7.

20

NICKEL REACTOR 750°C, 1 ATM t = 1.5 SECONDS

—**/

1

40

1

50

1

60

\.

•

70

1 :

80

1

^-METHANE

TO

^-ETHYLENE

Transit behavior of oxygen-treated nickel reactor

S A M P L E TIME (MINUTES)

1

30

^ o — ^ _

/\^~HYDROGEN


•

90

13.

GHALY AND CRYNES


237


Summary Simple n t h o r d e r k i n e t i c s are not adequate f o r r e p r e s e n t i n g p r o p y l e n e p y r o l y s i s o v e r any b u t a r e l a t i v e l y s h o r t t e m p e r a t u r e and/or c o n v e r s i o n r a n g e . A t r a n s i t i o n does o c c u r f r o m a n o v e r a l l p r o p y l e n e p y r o l y s i s o r d e r o f 1 . 5 t o 1.0 a s t h e t e m p e r a t u r e o f r e a c t i o n i n c r e a s e s f r o m 700 t o 850C. S i g n i f i c a n t w a l l a c t i v i t y was o b s e r v e d i n r e a c t o r s c o n s t r u c t e d o f l o w c a r b o n s t e e l , n i c k e l , and o t h e r a l l o y s . Such a c t i v i t y may r e s u l t i n e x c e s s i v e c a r b o n and h y d r o g e n f o r m a t i o n a s w e l l as i n c r e a s e d c o n v e r s i o n . Low c a r b o n s t e e l i s a n e s p e c i a l l y a c t i v e r e a c t o r m a t e r i a l w i t h a r a t h e r u n u s u a l and l o n g t r a n s i e n t profile. C a r b o n a p p e a r s t o b e an i n i t i a l r e a c t i o n p r o d u c t a l o n g w i t h h y d r o g e n , m e t h a n e , and e t h y l e n e . Not a l l c a r b o n - c o n d i t i o n e d r e a c t o r s produced i d e n t i c a l results. The c a r b o n l a y e r seems t o h a v e a c a t a l y t i c a c t i v i t y whose l e v e l depended somewhat on t h e t y p e o f s u r f a c e p r e s e n t underneath. B o t h t h e t y p e o f c a r b o n f o r m e d and m i g r a t i o n and m i x i n g o f m e t a l forms w i t h i n t h e c a r b o n p r o b a b l y c o n t r i b u t e t o t h e increased w a l l a c t i v i t y . R e a c t o r s u r f a c e s w h i c h a r e t r e a t e d w i t h oxygen produced strong surface effects. However, the l e v e l o f a c t i v i t y o f t h e o x i d i z e d s u r f a c e as compared t o t h a t o f t h e u n t r e a t e d s u r f a c e depends l a r g e l y on t h e t y p e o f m e t a l o r a l l o y . The i n i t i a l a c t i v i t y o f 304 s t a i n l e s s s t e e l , i n c o l o y , and i n c o n e l i n c r e a s e d a f t e r oxygen-treatment of the s u r f a c e . The n i c k e l r e a c t o r p r o d u c e d a r a t h e r u n u s u a l a c t i v i t y p r o f i l e a f t e r i t was t r e a t e d w i t h o x y g e n . Reduction of n i c k e l oxide to n i c k e l f o l l o w e d by c a r b o n b u i l d - u p i s a p r o b a b l e e x p l a n a t i o n o f the unusual a c t i v i t y p r o f i l e shape. Hydrogen s u l f i d e treatment of r e a c t o r s u r f a c e s produced a r e l a t i v e l y durable, passive, metal s u l f i d e layer. The o x i d e l a y e r i s probably converted to s u l f i d e s e i t h e r e n t i r e l y or at l e a s t to a s u f f i c i e n t depth t h a t very l i t t l e c a t a l y t i c oxides are exposed. C e r t a i n r e a c t o r w a l l s ( t r e a t e d and u n t r e a t e d ) a r e r a t h e r e f f e c t i v e i n p r o m o t i n g t h e f o r m a t i o n o f c a r b o n and h y d r o g e n and to a l e s s e r e x t e n t methane, p r o b a b l y t h r o u g h b o t h p r i m a r y and secondary r e a c t i o n s . T r a n s i e n t b e h a v i o r i s always a s s o c i a t e d w i t h a c t i v e w a l l s a n d t h e t r a n s i e n t a c t i v i t y p r o f i l e i s d e p e n d e n t on the type of w a l l . Complex c h e m i c a l and p h y s i c a l c h a n g e s a r e u n d o u b t e d l y o c c u r r i n g on t h e r e a c t i o n w a l l s r e s u l t i n g i n s u c h t r a n sient behavior. A l l o f t h e a c t i v e w a l l s p e c i e s c a n n o t now be c h a r a c t e r i z e d , b u t g e n e r a l r e a c t i o n s s u c h a s r e d u c t i o n and c a r b u r i z a t i o n from r e a c t a n t and p r o d u c t s c o u l d c o n c e i v a b l y cause an i n i t i a l buildup of c a t a l y t i c a l l y a c t i v e w a l l s p e c i e s . Activity p a s s e s t h r o u g h a maximum and t h e n d i m i n i s h e s as c a r b o n o r c a r b o n a c e o u s p r o d u c t s b u i l d upon t h e w a l l s a n d / o r a d d i t i o n a l r e a c t i o n products decrease the a v a i l a b l e a c t i v e s p e c i e s .



238


The s t r o n g s u r f a c e e f f e c t s o b s e r v e d a f t e r o x y g e n t r e a t m e n t o f t h e r e a c t o r s a r e p r o b a b l y c a u s e d by o x y g e n a d s o r p t i o n a n d / o r r e a c t i o n o f the oxygen w i t h t h e r e a c t o r w a l l s t o form complex metal oxides. Transformations i n the s t r u c t u r e of these oxides c a n o c c u r due t o change i n t e m p e r a t u r e , e x p o s u r e t o a r e d u c i n g a t m o s p h e r e , o r a t t a c k by a n o t h e r c h e m i c a l s u c h as h y d r o g e n s u l fide. A l l o f t h i s i s , o f c o u r s e , s p e c u l a t i v e a t p r e s e n t and w i l l r e m a i n s o u n t i l more d e t a i l s become a v a i l a b l e a b o u t t h e w a l l a c t i v e s p e c i e s , t h e e l e m e n t a r y r e a c t i o n s t e p s , and t h e p h y s i c a l transformations occurring within a pyrolysis reactor. However, p r e c i s e c o n t r o l o f r e a c t o r t r e a t m e n t t i m e s and c o n d i t i o n s s t i l l provides e x c e l l e n t c o n d i t i o n s f o r i n v e s t i g a t i n g the e f f e c t of s u c h t r e a t m e n t s d u r i n g p y r o l y s i s r e a c t i o n s . As r e s e a r c h w o r k e r s c o n t i n u e t o i d e n t i f y t h e e f f e c t s a n d r e a c t i o n s o f b o t h t h e homogeneous and h e t e r o g e n e o u s p y r o l y s i s o f h y d r o c a r b o n s , and as t h e s e r e a c t i o n s mechanisms become q u a n t i t a t i v e t h e n s u c h e f f o r t s w i l l l e a d t o b e t t e r c o n t r o l o f c o n v e r s i o n , p r o d u c t y i e l d s and i n carbon laydown i n commercial t u b e s .

Abstract Propylene pyrolysis was studied in tubular flow reactors of various materials of construction. The influence of the reactor wall material on both conversion and product yield distribution was assessed over a temperature range of 700 to 850C and for conversions up to 48%. Neither a f i r s t order nor three-halves order overall reaction model adequately represent the data over this temperature range. Low carbon steel, nickel, and other alloy reactor materials exhibited a catalytic wall effect as evidenced by product yield changes and/or increased conversions. In a 304 stainless steel reference reactor, carbon appears to be a primary or initial reaction product. Reactor surfaces treated with oxygen showed strong initial surface activity that diminished as carbon was b u i l t on reactor walls. Hydrogen sulfide "passivated" the reactor surfaces by forming a protective metal sulfide film which prevented excessive carbon and hydrogen formation by secondary reactions. Acknowledgement A p o r t i o n o f t h i s w o r k was s u p p o r t e d b y t h e P e t r o l e u m R e s e a r c h Fund o f t h e A m e r i c a n C h e m i c a l S o c i e t y , and p a r t was s u p p o r t e d b y i n s t i t u t i o n a l r e s e a r c h f u n d s o f t h e Oklahoma S t a t e University.


239

13. GHALY AND CRYNES Reactor Surface Effects Nomenclature C

reactant concentration, compatible units

C

i n i t i a l concentration of propylene

k

s p e c i f i c r a t e c o n s t a n t , c o m p a t i b l e u n i t s , d e p e n d i n g on o r d e r ,

η

(gmole/cc)"** sec reaction order

R

gas c o n s t a n t , c o m p a t i b l e

r

rate of formation,

Τ

temperature,

t

space t i m e ,

X

conversion of propylene,

e

expansion f a c t o r , moles o f products mole o f p r o p y l e n e r e a c t e d .

Q

i n feed, compatible units


n

units

gmoles/cc s e c

Κ seconds moles r e a c t e d p e r mole o f

feed

( e x c l u d i n g carbon) per

Literature Cited 1. 2. 3.

4. 5. 6.

7. 8. 9.

Amano, Α . , and Uchiyama, Μ., J. Phy. Chem., (1963), 67, 1241. Buell, C., and Weber, L., Petroleum Processing, (April, 1950), 387. Crynes, B. L., Ph.D. Thesis, "Surface Effects During Propane Pyrolysis in Tubular Flow Reactors," Purdue University (1968). Crynes, B. L., and Albright, L. F., Ind. Eng. Chem. Proc. Des. and Dev., (1969), 8, 25. Faber, Μ., and Ehrenburg, D., J. Electrochem. Soc., (1952), 99, 427. Herriott, G . , Eckert, R., and Albright, L., "Kinetics of Propane Pyrolysis," 68th National Meeting AIChE, Houston, Texas (February, 1971). Holbrook, Κ., Walker, R., and Watson, W., J. Chem. Soc. (B) Phy. O r g . , (1969), 1089. Hurd, C. D., and E i l e r s , Κ . , Ind. Eng. Chem., (1943), 26, 776. Kallend, Α . , Prunell, J., and Shurlock, Β . , Proc. Roy. Soc. London, (1967), 300A, 120.


240

10.

11. 12. 13. 14.


15. 16. 17.

18. 19. 20. 21. 22. 23. 24.

INDUSTRIAL AND LABORATORY PYROLYSES Kasansky, V., and Pariisky, G., "Proceedings Third International Congress on Catalysis," Vol. 1, 368, Amsterdam (1965). Kunugi, T., et al., Ind. Eng. Chem. Fundam., (1970), 9, 314. Laidler, Κ . , and Wojciechowski, Proc. Roy. Soc. London, (1960), 259A, 257. Mellouttee, H . , Maleissye, J., and Delbourgo, R., B u l l . Soc. Chemi. Fr., (1969), 8, 2652. Polotrak, V., L e i t i s , L., and Voevodskii, V., Russ. J. Phy. Chem., (1959), 33, 379. Rice, F., and Herzfeld, K., J. Phy. C o l l . Chem., (1951), 55, 975. Sakakibara, Υ . , B u l l . Chem. Soc. Japan, (1964), 37, 1262. Semenov, Ν . , "Some Problems in Chemical Kinetics and Reactivity," V o l . 1, Princeton University Press, Princeton (1958). Sims, J., Kershenbaum, L., and Shroff, J., Ind. Eng. Chem. Proc. Pes. Dev., (1971), 10, 265. Szwarc, M . , J. Chem. Phys., (1949), 17, 284. Tamai, Y., et al., Carbon, (1968), 6, 593. Taylor, J., et al., Proc. Am. Chem. Soc., Div. Pet. Chem., B47, Boston (1972). Tesner, P . , "Seventh International Symposium on Combustion," Butterworth, London (1959). Thomas, C., Egloff, G., and Morrell, J., Ind. Eng. Chem., (1939), 31, 1090. Tsai, C., and Albright L., "Surface Reactions Occurring During Pyrolysis of Light Paraffins," ACS Meeting, Philadel phia (April, 1975).


Reactor Surface Effects During Propylene Pyrolysis - American

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