Coke Formation on Metal Surfaces - American Chemical Society

3 Surface Effects on the Steam Cracking of Propane A. H O L M E N and O. A. LINDVAAG Norwegian Institute of Technology, Department of Industrial Chemistry, N-7034 Trondheim-NTH Norway D. L. TRIMM

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University of New South Wales, School of Chemical Engineering and Industrial Chemistry, P.O. Box 1, Kensington, 2033 Australia

The e f f e c t o f s u r f a c e s on the gaseous and solid products o f the steam c r a c k i n g o f propane has been s t u d i e d . The chemical nature o f the surface near the r e a c t o r inlet has a s i g n i f i c a n t e f f ect on the r e a c t i o n products while the surface near the e x i t does not. The m a t e r i a l o f the r e a c t o r tube appears t o c a t a l y z e gas phase r e a c t i o n s as w e l l as coke formation and gasification. P r e treatment o f the r e a c t o r tube a l t e r s the chemical nature o f the surface and, as a r e s u l t , a l t e r s the e f f e c t o f the m a t e r i a l on the r e a c t i o n products. The steam c r a c k i n g o f hydrocarbons i s one o f the most important sources o f light o l e f i n s f o r the chemical i n d u s t r y(1).The o v e r a l l r e a c t i o n is a high temperature p y r o l y s i s process in which a range o f products are formed in the gas phase as a r e s u l t o f a f r e e r a d i c a l chain r e a c t i o n . Tars and coke are a l s o formed as s i d e products to the main r e a c t i o n . Steam is present mainly as an i n e r t d i l u e n t (2), although it may help t o g a s i f y t a r s and carbon produced during the r e a c t i o n (1, 3). Because o f the formation o f coke deposits on industrial r e a c t o r s u r f a c e s , the r e a c t o r must be r e moved from o p e r a t i o n from time t o time and the coke l a y e r burned out with oxygen/steam mixtures. Although the major r e a c t i o n s occur in the gas phase, surface r e a c t i o n s can be very important in these systems. The absence or presence o f a surface p l a y s an important r o l e in the formation o f s o l i d products such as coke (4 - 8 ) . There i s a l s o good evidence that the w a l l can have a s i g n i f i c a n t e f f e c t on gas phase r e a c t i ons, a t l e a s t in part as a r e s u l t o f the i n i t i a t i o n or t e r m i n a t i o n of f r e e r a d i c a l chains thereon (9., JTO). Given the nature o f the f r e e r a d i c a l chains, both the chemical nature and the p h y s i c a l form (1J_ - 16.) o f the surface can i n f l u e n c e the r e a c t i o n and i t s products.

0097-6156/82/0202-0045$06.00/0 © 1982 American Chemical Society

Albright and Baker; Coke Formation on Metal Surfaces ACS Symposium Series; American Chemical Society: Washington, DC, 1983.


46

COKE FORMATION

The present s t u d i e s were i n i t i a t e d in order to i n v e s t i g a t e the e f f e c t o f the r e a c t o r s u r f a c e on the product d i s t r i b u t i o n and on the tendency f o r coke formation during the steam c r a c k i n g o f propane in a t u b u l a r r e a c t o r . A t t e n t i o n has been focused on c o r r e l a t i n g v a r i o u s e f f e c t s which can a r i s e in the system. Previous s t u d i e s o f the p y r o l y s i s o f propane has been reviewed r e c e n t l y (17 > 18), and the f i n d i n g s o f the present work are r e l a t e d to these s t u d i e s l a t e r in t h i s paper. Coke formation w i l l always be a p o t e n t i a l problem in any high temperature process i n v o l v i n g hydrocarbons. Coke and t a r s formed during r e a c t i o n can deposit on r e a c t o r s u r f a c e s or in quench heat exchangers, and s e v e r a l e f f e c t s have been observed in a steam c r a cker tube (£, 9_)· As the r e a c t i o n proceeds the tube w i l l accumu l a t e coke towards the r e a c t o r e x i t , while the i n l e t zone remains r e l a t i v e l y coke f r e e , as a r e s u l t o f the i n d u c t i o n time a s s o c i a t e d with the formation o f t a r s and coke (JjSO. Gas phase r e a c t a n t s may adsorb and react on t h i s bare metal, and the components o f the gas phase in the r e a c t o r should and do r e f l e c t t h i s i n t e r a c t i o n (4, 9^, JO). Since carbon formation depends on the nature o f the gas phase s p e c i e s , these i n t e r a c t i o n s should a l s o i n f l u e n c e coke formation towards the e x i t o f the r a c t o r . Again, t h i s i s found to be the case ( 4, 9_ - JJ_) · As a r e s u l t o f these c o n s i d e r a t i o n s , i t would be expected that surface e f f e c t s during steam c r a c k i n g would be apparent in the formation o f gaseous, l i q u i d and s o l i d products. The present s t u d i e s show that t h i s i s indeed the case. EXPERIMENTAL Coke formation has been studied in a conventional flow appa r a t u s . A schematic diagram o f the experimental apparatus i s shown in Figure 1. Coke formation i s measured on f o i l s o f d i f f e r e n t ma t e r i a l s hanging from one arm o f the microbalance ( C . I . Mark 2B) in such a way that the f o i l i s in the constant temperature zone of the r e a c t o r . The amount o f coke deposited on the f o i l s i n c l u des the d e p o s i t i o n o f coke formed in the gas phase as w e l l as the coke formed on the s u r f a c e . The tubular r e a c t o r i s made o f s t a i n l e s s s t e e l (Uddeholm S t a i n l e s s 24, 18-2689) with an i n s i d e diameter o f 22mm. The l e n g t h of the r e a c t o r i s 37 cm, with a constant temperature zone o f c a . 16 cm ( + 3 C). Experiments have a l s o been c a r r i e d out in a t u bular r e a c t o r made o f quartz ( i . e . quartz as a l i n i n g in the s t e e l r e a c t o r ) . The i n s i d e diameter o f the quartz r e a c t o r i s 14.8 mm. The dimensions o f the f o i l hanging in e i t h e r r e a c t o r were u s u a l l y ca. 8 mm χ 28 mm and the weight about 0,3 gram i f a s t e e l f o i l was used. A l l the experiments were c a r r i e d out with steam and/or n i t r o gen as d i l u e n t s . Steam was f e d to the r e a c t o r by evaporating water i n t o a stream o f n i t r o g e n . A stream o f n i t r o g e n i s always main-


Steam Cracking

of

Propane


HOLMEN ET A L .

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American Chemical Society Library 1155 16th St. N. w. Albright and Baker; Coke Formation on Metal Surfaces D. C. 20036 ACS Symposium Series;WHMngton, American Chemical Society: Washington, DC, 1983.

COKE FORMATION

48

tained through the microbalance in order to p r o t e c t the balance against c o r r o s i v e atmospheres. The e x i t gas from the r e a c t o r was quenched and the gas volume measured before complete gas a n a l y s i s using two o n - l i n e gas chromatographs. The permanent gases ( H , N^, CO, C0 ) were separated on a 2 m column (1/4") packed with Porapak Ν (80/100 mesh), followed by a 3 m column (1/4") packed with 13X molecular sieve (30/50 mesh). Hydrocarbons were separated on a 3 m column (1/8") packed with squalane on Alumina H (100/120 mesh). Two d i f f e r e n t pre-treatments of the f o i l and o f the r e a c t o r surface have been used: e i t h e r p r e r e d u c t i o n or p r e o x i d a t i o n (with oxygen). During p r e r e d u c t i o n each f o i l was reduced f o r 18 h in flowing hydrogen at 770°C and 1 h at the a c t u a l experimental temperature p r i o r to the run. With s t e e l f o i l s i t was impossible to o x i d i z e the f o i l to constant weight and p r e o x i d a t i o n i n v o l v e d heating in oxygen (850°C) f o r v a r i o u s times as d e t a i l e d in the text. 2


2

RESULTS Experiments were c a r r i e d out (a) in which the m a t e r i a l o f the r e a c t o r tube remained constant and the formation o f coke was mea sured on f o i l s of d i f f e r e n t m a t e r i a l s and (b) in which the mater i a l of the r e a c t o r tube was v a r i e d and coke formation was measured on a f o i l of the same m a t e r i a l . Both p r e r e d u c t i o n and p r e o x i d a t i o n of the r e a c t o r system have been used as standard pretreatment in d i f f e r e n t cases. Not s u r p r i s i n g l y , the f i r s t geometry gave very s i m i l a r r e s u l t s in a l l cases. Using the quartz r e a c t o r , coke formation was measured on f o i l s f a b r i c a t e d from s t a i n l e s s s t e e l , c o b a l t , molyb denum, copper and q u a r t z . T y p i c a l e x i t gas compositions are shown in Figure 2 and r a t e s of coke formation on the f o i l s a f t e r p r e r e duction are shown in F i g u r e 3· The nature of the f o i l does not have any s i g n i f i c a n t i n f l u e n c e on the e x i t gas composition. On the other hand i t i s c l e a r that the coke formation on f r e s h f o i l s i s dependent of the f o i l m a t e r i a l . However, t h i s i n i t i a l v a r i a t i o n in coke d e p o s i t i o n on the f o i l s i s s u b s t a n t i a l l y decreased as the f o i l s become more covered in coke. T h i s coke o r i g i n a t e s at l e a s t in part in the gas phase and encapsulates the f o i l and breakaway metal p a r t i c l e s (20). I t i s i n t e r e s t i n g to note that the higher i n i t i a l r a t e s were not observed on f o i l s made of quartz or molybdenum: in f a c t , the r a t e f o r coke formation seems to increase somewhat as the r e a c t i o n proceeds on these f o i l s . The e f f e c t of r e a c t o r m a t e r i a l was s t u d i e d u s i n g three d i f f e r e n t s u r f a c e s . These were quartz and o x i d i z e d and reduced s t a i n l e s s s t e e l (Uddeholm 24 c o n t a i n i n g 17,2% Cr, 11,5% N i , 2,7% Mo, 1,7% Μη, 0,5% S i and 66,4% F e ) . Previous s t u d i e s have revealed that the surface of the o x i d i z e d s t e e l c o n t a i n s Fe and Ni oxides together with the Cr oxide, while the s u r f a c e e n r i c h ment by Mn and Cr oxides i s very pronounced during r e d u c t i o n o f


3.

HOLMEN E T A L .

Steam Cracking

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of

CH

— ν

A

C

-


ο

49

Propane

2 4 H

-

C

< ο 70

3 6 H

1

1

80

90

CONVERSION (%) Figure 2. Exit gas composition from steam cracking of propane in quartz reactor with steel (Sandvik 15RelO) as the foil material after 10 min on stream. Conditions: temperature range, 800-870°C; feed gas composition, 29 mol% C H , 32% H 0, and 39% N ; and total feed rate, 0.42 L gas/min. 3

8

2

2

10

20

30

U0

50

60

70

80

90

100

110

TIME ( m i n )

Figure 3. Coke formation during steam cracking of propane in the quartz reactor at 850°C on foils made from different materials. Key: O, steel; V , Co; A , Mo; •, Cu; and ·, quartz. Conversion of C H — 98%. Feed gas as in Figure 2. S

8



50

COKE

FORMATION

the s t e e l s u r f a c e . (The content of Ni and Fe in the surface o f the reduced s t e e l i s l e s s than 0.3 wt-%) (20). Coke formation was measured from d e p o s i t i o n on f o i l s made from a steam cracker tube m a t e r i a l (Sandvik 15Re10 c o n t a i n i n g 24,5% Cr, 20,5% N i , 1,8% Μη and 52,5% F e ) . Some experiments have a l s o been performed with f o i l s made of n i c k e l . T y p i c a l e x i t gas compositions are shown in F i g u r e s 4 and 5. The amount of coke deposited on the f o i l s are summarised in Figures 6, 7, 8 and 9· I t i s seen that coke formation on the sample of f o i l i s de pendent on the m a t e r i a l of the t u b u l a r r e a c t o r as i s the product gas composition. With the s t e e l r e a c t o r , p r e o x i d a t i o n leads to a s u b s t a n t i a l increase in the coke formation on the s t e e l f o i l (Figures 6 and 7 ) . The e f f e c t of p r e o x i d a t i o n i s even more pron ounced with Ni as the f o i l m a t e r i a l (Figure 8). On the other hand, p r e o x i d a t i o n does not lead to a high rate of coke forma t i o n on the s t e e l f o i l i f a quartz l i n e r i s used in the t u b u l a r r e a c t o r (Figure 9). G a s i f i c a t i o n of the f i r s t coke deposit l a i d down on a new f o i l always r e s u l t e d in higher r a t e s f o r coke formation during subsequent runs (Figure 9)· However, i t has so f a r been impossi ble to e s t a b l i s h a r e l a t i o n s h i p between repeated coke formation/ g a s i f i c a t i o n and the r a t e s f o r coke formation during the runs f o l l o w i n g the f i r s t g a s i f i c a t i o n . I t was found that carbon oxides do not form in the quartz r e actor whereas a large amount of carbon oxides was formed in the preoxidized s t e e l reactor (Table I ) . The content of other products in the e x i t gas i s a l s o dependent of the r e a c t o r surface ( F i g u r e s 2, 4 and 5 ) . Increasing content of hydrogen was a l s o observed when the same s t e e l r e a c t o r was used f o r a l a r g e number of experimental runs with repeated coke f o r m a t i o n / g a s i f i c a t i o n . S i m i l a r e f f e c t s have a l s o been observed in a t u b u l a r r e a c t o r f i l l e d with pieces o f the r e a c t o r m a t e r i a l (Table II) during p y r o l y s i s of ethane and ethylene. The o x i d i z e d surface d i d not e f f e c t the conversion o f ethane very much, but the s e l e c t i v i t y of ethylene formation was reduced from about 80% in the untreated r e a c t o r to about 15% when the surface was pretreated with oxygen. DISCUSSION The present s t u d i e s have been concerned with the o v e r a l l e f f e c t of s u r f a c e s on r e a c t i o n s o c c u r i n g during steam c r a c k i n g . The formation of gaseous and s o l i d products has been r e l a t e d to the nature of the r e a c t o r s u r f a c e . Steam c r a c k i n g i s , however, a high temperature p y r o l y s i s r e a c t i o n in which free r a d i c a l intermedi ates play an important r o l e . No attempt has been made to r e l a t e the experimental r e s u l t s to the nature and amount of free r a d i c a l s present in the system. Coke formation on the f o i l s i s found to be dependent not only on the nature of the f o i l but a l s o on the m a t e r i a l of the t u b u l a r r e a c t o r . E s s e n t i a l l y , the s t u d i e s involved three r e a c t o r surface



2

70

CONVERSION (%)

80

2

90

3

8

Figure 4. Exit gas composition from steam cracking of propane in the prereduced steel reactor with steel (Sandvik 15RelO) as the foil material after 10 min on stream. Conditions: temperature range, 800S70°C; feed gas composition, 29 mol% C H , 32% H 0, and 39% N ; and total feed rate, 0.92 L/min.

35 h

Figure 5. Exit gas composition from steam cracking of propane. Comparisons between prereduced and preoxidized systems. Conditions as in Figure 4.

CONVERSION (%)




2!

δ

> H

ο

8

to

3.

HOLMEN E T AL.

Steam Cracking

of

53

Propane


1.00

10

20

30

TIME (min) Figure 7. Coke formation during steam cracking of propane at 840°C on a steel foil (Sandvik 15Re 10) in a preoxidized steel reactor. The reactor surface and the foil were preoxidized for 95 min using 46% 0 in N at 840°C. Conversion of C H :89%. Feed gas as in Figure 4. 2

3

2

8

4.0

E

pre-reduced a l l o y surface (Cr and Mn on the surface) > quartz (no metals on the surface of the r e a c t o r ) . As seen in Figures 2 and 5 t h i s e f f e c t has been observed e x p e r i mentally. The r e s u l t s show c l e a r l y the importance of the chemical n a ture of the a c t u a l surface in determining the product spectrum observed during steam c r a c k i n g . The surface has an important bea r i n g on both the s o l i d and gaseous products of the r e a c t i o n . I t would be very i n t e r e s t i n g to e s t a b l i s h the exact mechanism by which t h i s occurs, but the f r e e r a d i c a l intermediates i n v o l v e d are not amenable to study using the present system. Acknowledgments The support of t h i s work by the Royal Norwegian Council f o r S c i e n t i f i c and I n d u s t r i a l Research i s g r a t e f u l l y acknowledged. Literature Cited 1. 2. 3·

4. 5. 6.

7.

8. 9.

Zdonik, S.B.; Green, E.J.; H a l l e e , C.P. "Manufacturing E t y l e n e " . Petroleum P u b l i s h i n g Corp. 1970. T u l s a , Oklahoma. Goossens, A.G.; Dente, M . ; R a n z i , E . H y d r . P r o c . 1970. 57 (9), 227. Bennett, M.J.; P r i c e , J.B. "Behaviour In Simulated Cracker Environments of HK40 S t e e l , Silica and an ESC-Tube Coke". 14th B i e n n i a l American Carbon Conference E x t . Abstract Progr. 1979. 137 - 8 . Trimm, D . L . C a t a l . Rev. - Sci. Eng. 1977. 15 (2), 155. Sundaram, K.M.; Froment, G . F . Chem. Eng. Sci. 1979. 34, 635. Newsome, D.S.; Leftin, H . P . "Coking Rates in a Laboratory P y r o l y s i s Furnace". 72nd Annual AIChE Meeting. San F r a n c i s c o . Nov. 25-29. 1979. Paper no 21d. A l b r i g h t , L.F.; Yu, Y.C.; Welther, K. "Coke Formation During P y r o l y s i s O p e r a t i o n " . 85th N a t i o n a l AIChE Meeting. June 4-8, P h i l a d e l p h i a . Paper no 15E. Lahaye, J.; Badie, P . ; Ducret, J. Carbon 1977. 15, 87. T s a i , C.H.; A l b r i g h t , L . F . "Surface Reactions Occurring


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10. 11. 12.

13.


14. 15. 16. 17. 18. 19. 20. 21. 22.

COKE

FORMATION

During P y r o l y s i s o f L i g h t P a r a f f i n s " . ACS Symp. Ser. 1976. 32, 274. Crynes, B.L.; A l b r i g h t , L.F. Ind. Eng. Chem. Process. Des. Dev. 1969. 8 ( 1 ) , 25. Leathard, D.A.; P u r n e l l , J.A. " P a r a f f i n P y r o l y s i s " . Annual Review Phys. Chem. 1970. 21, 197. Dunkleman, J.J.; A l b r i g h t , L.F. "Surface E f f e c t s During Pyro lysis of Ethane in Tubular Flow Reactors". ACS Symp. Ser. 1976. 32, 241. Trimm, D.L.; Holmen, Α.; Lindvåg, O.A. J. Chem. Tech. B i o t e c h n o l . 1981. 32, 241. Shah, Y.T.; S t u a r t , E.B.; Sheth, K.D. Ind. Eng. Chem., Process Des. Dev. 1976. 15 ( 4 ) , 518. L a i d l e r , K.J.; Sagert, N.H.; Wojciechowski, B.W. Proc. Roy. Soc. (London). 1962. 270A, 242. Tamai, Y.; Nishiyama, Y. Bull. J. P e t r . I n s t i t u t e 1970. 12, 16. Volkan, A.R.; April, G.C. Ind. Eng. Chem., Process Des. Dev. 1977. 16 (4), 429. A l b r i g h t , L.F. I b i d . 1978. 17 (3), 377. Trimm, D.L.; Turner, C.J. J. Chem. Tech. B i o t e c h n o l 1981. 31, 195. Holmen, Α.; Trimm, D.L. "Coke Formation during Cracking o f Hydrocarbons. Part I I . In press. Lobo, L.S.; Trimm, D.L.; F i g u e i r e d o , J.L. Proc. 5th I n t . Congr. C a t a l . 1973. 1125. C a t a l y s t Handbook. Wolfe Scientific Texts (London). 1970.

RECEIVED

June 28,

1982.


Coke Formation on Metal Surfaces - American Chemical Society

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