2 Inhibition of Coke Formation in Ethylene Steam Cracking D . E . B R O W N , J. T . K . C L A R K , A . I. F O S T E R , J. J. M c C A R R O L L and M . L . SIMS
Downloaded by MONASH UNIV on September 15, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch002
The British Petroleum Company p.l.c., Research Centre Sunbury, Chertsey Road, Sunbury-on-Thames, Middlesex, United Kingdom
This paper describes the d e r i v a t i o n and e v a l u a t i o n of a silica c o a t i n g process to reduce the r a t e of coke formation in the p y r o l y s i s s e c t i o n of ethylene steam crackers (ESC). The coating process utilises the chemical vapour d e p o s i t i o n of an a l k o x y s i l a n e in steam c a r r i e r gas to produce a t h i n , impervious, adherent and amorphous silica f i l m on the i n t e r n a l surfaces o f high a l l o y s t e e l tubing a t 700-800°C. E v a l u a t i o n in a laboratory s c a l e ethylene steam c r a c k i n g r e a c t o r employing a t y p i c a l ESC feedstock (LDF 160) has revealed that the coking r a t e can be reduced by up to x10 in short term t e s t s and by x3-4 in longer term t e s t s .
The steam c r a c k i n g o f h y d r o c a r b o n s ( e g n a p h t h a ) t o p r o d u c e t h e l o w e r o l e f i n s ( e t h y l e n e , p r o p y l e n e , butènes) a n d b u t a d i e n e i s the basic petroleum chemicals operation. Such steam c r a c k i n g p l a n t s u f f e r s coke d e p o s i t i o n problems t h a t r e s t r i c t performance. The c r a c k i n g o p e r a t i o n i n v o l v e s h e a t i n g n a p h t h a d i l u t e d w i t h steam in r a d i a n t l y h e a t e d c o i l s o f h i g h a l l o y s t e e l t u b i n g t o t e m p e r a t u r e s o f t y p i c a l l y 850°C. D u r i n g c r a c k i n g , coke d e p o s i t i o n o c c u r s in t h e c o i l s and a s s o c i a t e d downstream e x c h a n g e r s . The c o k e , w h i c h f o r m s t o d e p t h s o f s e v e r a l mm, reduces heat t r a n s f e r and n o r m a l l y l i m i t s r u n l e n g t h t o f o u r t o e i g h t weeks. The c r a c k i n g f u r n a c e must t h e n be t a k e n o f f - s t r e a m f o r d e c o k i n g , u s u a l l y by s t e a m - a i r o x i d a t i o n , w i t h c o n s e q u e n t i a l loss o f production. Also, frequent decoking operations present a d d i t i o n a l f u e l and u t i l i t i e s c o s t s , and p r o b a b l y a f f e c t coil life. A r e s e a r c h programme was u n d e r t a k e n t o i n v e s t i g a t e t h e c o k e d e p o s i t i o n phenomena a n d t o d e v e l o p a p o t e n t i a l method for i t s inhibition.
0097-6156/82/0202-0023$06.50/0 © 1982 American Chemical Society
In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Downloaded by MONASH UNIV on September 15, 2013 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch002
ESC
COKE FORMATION
Plant Materials Investigation
The r e s e a r c h programme was i n i t i a t e d t h r o u g h a n i n v e s t i g a t i o n of r a d i a n t c o i l a l l o y s and d e p o s i t e d coke. Radiant c o i l s are c o n s t r u c t e d in h i g h a l l o y s t e e l s ( e g I n c o l o y 8 0 0 , HK 4 0 , 34 C T ) , w i t h i r o n , n i c k e l a n d chromium a s m a j o r components a n d a r a n g e o f m i n o r c o n s t i t u e n t s s u c h a s s i l i c o n , t i t a n i u m a n d manganese. S o f t X - r a y a p p e a r a n c e p o t e n t i a l s p e c t r o s c o p y (SXAPS) s t u d i e s o f r a d i a n t c o i l a l l o y s were c a r r i e d o u t t o d e t e r m i n e s u r f a c e compositions r e s u l t i n g from exposure t o a range o f environments. I n p a r t i c u l a r , t h e o x i d e s u r f a c e f i l m s g e n e r a t e d in o x i d i s i n g c o n d i t i o n s a t h i g h t e m p e r a t u r e s (>800°C) r e v e a l e d t h e s e g r e g a t i o n t o t h e s u r f a c e o f m e t a l s f o r m i n g s t a b l e o x i d e s in p r e f e r e n c e to m e t a l s f o r m i n g l e s s s t a b l e o x i d e s . T h i s e f f e c t was a l s o f o u n d on t h e i n t e r n a l s u r f a c e s o f a n aged I n c o l o y 800 r a d i a n t c o i l f r o m a c o m m e r c i a l ESC, w h i c h h a d c o n t i n u a l l y b e e n e x p o s e d t o h i g h t e m p e r a t u r e steam c r a c k i n g / d e c o k i n g c y c l e s . F i g u r e 1 shows t h e SXAPS s p e c t r u m o f t h e s u r f a c e o x i d e , w h i c h ( t a k e n together w i t h the bulk a l l o y composition) demonstrates the g e n e r a l o b s e r v a t i o n o f a t r e n d in t h e r a t i o o f s u r f a c e o x i d e composition t o bulk metal composition f o r the m e t a l l i c a l l o y i n g e l e m e n t s o f T i > C r > Fe > N i , in l i n e w i t h d e c r e a s i n g o x i d e stability. Coke d e p o s i t s f r o m a c o m m e r c i a l ESC p l a n t were a l s o e x a m i n e d . B u l k m e t a l s c o n c e n t r a t i o n s were d e t e r m i n e d by a s h i n g a t 900°C p r i o r t o emission spectroscopy a n a l y s i s . High l e v e l s of metals were f o u n d a t t h e r a d i a n t c o i l - c o k e i n t e r f a c e d u e , p r e s u m a b l y , to o x i d e s p a l l a t i o n , b u t c o n c e n t r a t i o n s o f m e t a l s ( p r i m a r i l y chromium, i r o n a n d t i t a n i u m , t o t a l l i n g c a 2 0 0 ppm) were a l s o f o u n d i n t o t h e b u l k o f t h e coke and a t t h e c o k e - p r o c e s s stream i n t e r f a c e ( T a b l e I ) . SXAPS e x a m i n a t i o n o f c o k e h e a t e d u n d e r vacuum f o r s e v e r a l h o u r s a t 900°C a l s o r e v e a l e d t h e a p p e a r a n c e o f up t o 1% c o n c e n t r a t i o n o f m e t a l s a t t h e s u r f a c e o f some c o k e s a m p l e s , a n d s i g n i f i c a n t l y chromium, i r o n a n d t i t a n i u m were again the metals observed. TABLE I BULK ANALYSIS OF METALS IN COKE DEPOSITS FROM COMMERCIAL ETHYLENE STEAM CRACKER ( I N ppm) Sample
Ca
Ti
Cr
Mn
Fe
Ni
Coke f r o m c e n t r a l s e c t i o n between metal tube and gas p h a s e i n t e r f a c e s
82
27
105
5
53
6
Coke f r o m g a s phase interface
11
3
22
3
175
In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
