Coke Formation on Metal Surfaces - American Chemical Society

12 Influence of Total Pressure and Hydrogen:Hydrocarbon Ratio on Coke Formation over Naphtha-Reforming Catalyst NORA S. FÍGOLI, JORGE N. BELTRAMINI, A M A D O F. BARRA, E L O Y E . MARTINELLI, MARIO R. SAD and JOSÉ M . PARERA

Downloaded by UNIV LAVAL on November 5, 2015 | http://pubs.acs.org Publication Date: October 25, 1983 | doi: 10.1021/bk-1983-0202.ch012

Instituto de Investigaciones en Catálisis y Petriquímica-INCAPE-Santiago del Estero 2654, 3000 Sante Fe, Argentina

The decreasing of the total pressure and of the hydrogen to naphtha ratio produces the increment of the coke formation over Pt/Al O . C r i t i c a l values below which there i s a great increment of the amount of coke and i t s degree of polymerization exist. For a given feed the catalyst deactivation i s a function of the deposited coke obtained either by diminution of the pressure or of the hydrogen to naphtha ratio. A great carbon deposition -mainly on the metal- i s observed during the f i r s t hours of operation, and later a slow deposition -mainly on the alumina-continues.When the soluble fraction of the coke i s extracted with solvents, a C to H ratio 0.79 i s obtained. 2

3

The o b j e c t i v e s of the c a t a l y t i c reforming of naphtha are to increase the naphtha octane number (petroleum r e f i n a t i o n ) or to produce aromatic hydrocarbons (petrochemistry). B i f u n c t i o n a l c a t a l y s t s that promote hydrocarbon dehydrogenation, i s o m e r i z a t i o n , cracking and d e h y d r o c y c l i z a t i o n are used t o accomplish such purposes. Together with these r e a c t i o n s , a carbon d e p o s i t i o n which d e a c t i v a t e s the c a t a l y s t takes p l a c e . T h i s d e a c t i v a t i o n l i m i t s the i n d u s t r i a l operation to a time which depends on the o p e r a t i o n a l c o n d i t i o n s . As t h i s time may be very long, to study c a t a l y s t s t a b i l i t y i n laboratory, accelerated deactivation tests are r e q u i r e d . The knowledge of the i n f l u e n c e of o p e r a t i o n a l c o n d i t i o n s on coke d e p o s i t i o n and on i t s nature, may help i n the e f f o r t s to avoid i t s formation. The i n f l u e n c e of the t o t a l pressure and hydrogen t o naphtha molar r a t i o on the amount and nature of the coke deposited on a monometallic b i f u n c t i o n a l c a t a l y s t was s t u d i e d by means of a standardized c a t a l y s t t e s t .

0097-6156/82/0202-0239$06.00/0 © 1982 American Chemical Society

In Coke Formation on Metal Surfaces; Albright, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

COKE FORMATION

240

Experimental Catalyst. A P t / A l 0 3 - C l c a t a l y s t c o n t a i n i n g 0.375% P t and 0.87% C I was u s e d . I t was p r e p a r e d i m p r e g n a t i n g w i t h H PtCl£ s a m p l e s o f CK 300 a l u m i n a f r o m K e t j e n f o l l o w i n g t h e method o f C a s t r o e t a l . ( 1 ) . The s u p p o r t s u r f a c e a r e a was 200 m /g and t h e c a t a l y s t had 90% m e t a l d i s p e r s i o n . 2

2


2

Catalyst Tests. They w e r e p e r f o r m e d in a b e n c h s c a l e equipment t h a t was d e s c r i b e d e l s e w h e r e ( 2 ) . O c t a n e number (RON) was t a k e n a s a m e a s u r e o f a c t i v i t y and l i q u i d y i e l d (r|C^) as a measure o f s e l e c t i v i t y . These p r o p e r t i e s , the p e r c e n t a g e o f a r o m a t i c s in t h e l i q u i d (%ArL) and t h e p e r c e n t a g e o f C 1 - C 3 w e r e c a l c u l a t e d from c h r o m a t o g r a p h i c a n a l y s i s d a t a , as p r e v i o u s l y described (2). The a d o p t e d s t a n d a r d t e s t (3) c o n s i s t e d in t h e o p e r a t i o n o f t h e c a t a l y s t a t n o r m a l c o n d i t i o n s (P = 30 atm; H : n a p h t h a m o l a r r a t i o = 8) d u r i n g s e v e n h o u r s ( p e r i o d I ) . Then p r e s s u r e and H : n a p h t h a r a t i o were d e c r e a s e d d u r i n g twenty h o u r s ( p e r i o d I I ) . F i n a l l y , p r e s s u r e and H : n a p h t h a r a t i o w e r e r e t u r n e d t o i n i t i a l v a l u e s f o r s e v e n h o u r s ( p e r i o d I I I ) . A l l t h e t e s t was c a r r i e d o u t a t 515°C and WHSV = 6 h ' . An example o f t h e t e s t i s shown in F i g u r e 1. Two i m p o r t a n t t y p e s o f v a l u e s c a n be o b t a i n e d t h a t a r e u s e f u l f o r i n v e s t i g a t i n g c a t a l y s t d e a c t i v a t i o n by c o k e : a) The d i f f e r e n c e s b e t w e e n t h e v a l u e s o b t a i n e d d u r i n g p e r i o d s I I I and I : m o d i f i c a t i o n in RON (AR0N), in T)C§ (ΔηΟ^), e t c . , due t o the a c c e l e r a t e d d e p o s i t i o n of coke. b) The s l o p e o f change o f RON, T)C$, e t c . d u r i n g p e r i o d I I p r o d u c e d by t h e c o n t i n u o u s d e p o s i t i o n o f c o k e . The c o k e c o n t e n t o f t h e c a t a l y s t s was m e a s u r e d by c o m b u s t i o n v o l u m e t r y a f t e r t h e r u n . The d i f f e r e n t i a l t h e r m a l a n a l y s i s (DTA) o f t h e u s e d c a t a l y s t s was p e r f o r m e d w i t h an Aminco T h e r m o a n a l y z e r u s i n g o x y g e n as dynamic g a s . The s o l u b l e f r a c t i o n o f c o k e f r o m u s e d c a t a l y s t s was e x t r a c t e d w i t h s o l v e n t s and t h e s o l u t i o n s w e r e a n a l y z e d by s e v e r a l t e c h n i q u e s in o r d e r t o d e t e r m i n e t h e i r n a t u r e . 2

2

2

1

Feed. P u r e L a O x i g e n a h y d r o g e n was u s e d . I t was p r e v i o u s l y p u r i f i e d by p a s s a g e t h r o u g h a d e o x y g e n a t i n g c o p p e r bed and dehydrated p a s s i n g t h r o u g h a M o l e c u l a r S i e v e s 4 A bed. The n a p h t h a had t h e f o l l o w i n g p r o p e r t i e s : D e n s i t y : 0.736 g/ml Mean M o l e c u l a r W e i g h t : 109 B o i l i n g p o i n t r a n g e : 65-147°C RON: 59 C o m p o s i t i o n ( w e i g h t % ) : P a r a f f i n s : 64.2 N a p h t h e n i c s : 24.2 A r o m a t i c s : 11.6



FiGOLi ET AL.

100

Coke Formation

30 atm H2:naphtha=8

over Naphtha-Reforming

Catalyst

241

30 atm Η2: naphtha =8

7.5 atm Η2: naphtha = 4

ο

x

χ

χ

^ Α

50

Δ

Δ

-κ

χ

χ

^

Ζ

_

[

"

, Δ

+

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Iι

0

5

χ—«

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I

II

I1

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I

15

20

25

TIME,

hour

10

Α

—Λ-

Λ

&

μ—+

ι I

I U_

30

35

Figure 1. Diagram of a standard test, showing modifications of RON (O), rjC , (X), % Ar (A), and CJd + C + C Χ 10 (+). Conditions: temperature, 515°C, andWHSV,6hK 5

L

2

3

2


COKE FORMATION

242

Results Influence of Pressure. In order t o study the i n f l u e n c e of t o t a l pressure on coke d e p o s i t i o n , the operating pressure was v a r i e d between 2 and 20 atm in p e r i o d I I , keeping the other c o n d i t i o n s p r e v i o u s l y d e s c r i b e d as standard. So, when r e f e r i n g to an experiment a t 2 atm pressure, i t i s meant beginning at 30 atm and H :naphtha molar r a t i o 8 f o r seven hours, then decreased and maintained the pressure a t 2 atm and H :naphtha molar r a t i o at 4 for twenty hours, and coming afterwards back t o the i n i t i a l c o n d i t i o n s . The r e s u l t s are presented in Table I . I t may be observed that decreasing the pressure during p e r i o d I I , increments in the amount of coke deposited on the c a t a l y s t and in the negative d i f f e r e n c e s ARON and A % A r between I and I I I are produced. Lower pressures produce lower increments in Ariclj. C o r r e l a t i o n s between the carbon percentage on the c a t a l y s t s , pressures and ARON were d e r i v e d using the l e a s t square method. L i n e a r , e x p o n e n t i a l , power law and semilog r e l a t i o n s h i p s were considered in a l l cases, t a k i n g that w i t h the best c o r r e l a t i o n c o e f f i c i e n t ( r ^ ) as the most accurate one. The best c o r r e l a t i o n obtained f o r the carbon percentage and pressure was: 2


2

L

0

%C = 7.71 Ρ " ·

9 6

r

2

= 0.95

(1)

The amount of carbon deposited on the c a t a l y s t m o d i f i e d ARON according t o : ARON = 2.50 e

0 , 3 9

(

%

C

)

r

2

= 0.97

(2)

The r e l a t i o n between pressure during p e r i o d I I and ARON was: ARON = 11.76 e "

0 , 9 8

P

r

2

= 0.99

(3)

The slope of decay of RON during I I was higher when a lower pressure was used and i t s r e l a t i o n s h i p was: Slope of RON = -1.52 e ~

0 , 1 2

P

r

2

= 0.99

(4)

The d i f f e r e n t i a l thermal a n a l y s i s of the d i f f e r e n t c a t a l y s t s i s shown in F i g u r e 2. Two d i f f e r e n t combustion zones can be observed, which may be considered as produced by the combustion of substances of d i f f e r e n t nature: one zone from 123 to 369°C and the second one from 369 to 555°C. The f i r s t zone i s not very w e l l defined: d i f f e r e n t peaks can be seen and there i s an i n c r e a s e of the zone when the pressure i s decreased. The second zone appears as more s e n s i t i v e to the pressure change. The pressure decreasing produced a great increment of t h i s zone and p a r t i c u l a r l y of the peaks appearing a t h i g h combustion temperatures. I t i s important to observe that m o d i f i c a t i o n s of pressure above 7.5 atm d i d not produce a very s i g n i f i c a t i v e change in the amount and nature of the coke. But very important changes were produced when pressure was below 7.5 atm, being s p e c i a l l y m o d i f i e d


12. FiGOLi ET AL.

Coke Formation over Naphtha-Reforming Catalyst

vO r-H

vO CN

ΙΠ

Ο

m Ο

ο

ο

ο

Ο

I


α

cd

Φ ο Φ U

φ

o0|

G

•H

ο

u
CM

m

Ο

ι-Η

rH

Ph


243

COKE FORMATION


244


12.

FIGOLI ET AL.

Coke Formation


Catalyst

245

the amount of coke that burns at higher temperatures which i s more s t a b l e and d i f f i c u l t to e l i m i n a t e . The c r i t i c a l pressure value seems to be 7.5 atm, as i t can be seen in Figure 3. Influence of hydrogen-hydrocarbon r a t i o . Molar values from 8 to 2 and a pressure of 10 atm during p e r i o d I I were used, being the r e s u l t s presented in Table I I . More severe c o n d i t i o n s , as smaller hydrogen to naphtha r a t i o s , meant more carbon d e p o s i t i o n and higher slopes of decrease of RON and %ArL» Comparing the c a t a l y s t behavior before and a f t e r the severe p e r i o d , i t may be seen that when smaller the used r a t i o s , higher was ARON as w e l l as the A % A r and a smaller increment in Ar|C£ was n o t i c e d . As when pressure m o d i f i c a t i o n s were s t u d i e d , c o r r e l a t i o n s between %C, ARON and H :naphtha molar r a t i o , were d e r i v e d :


L

2

%C = 4.72

(H :naphtha)"

ARON = 3.05

1 , 2 8

r

2

e

0 , 4 4

( % C )

r 0 , 6 7

ARON = 11.88

(H :HC)"

Slope of RON

during I I = -1.18

2

r

2

e

2

= 0.97 = 0.97

2

= 0.97

(5) (6) (7)

"0.21(H :naphtha) 2

r

= 0.99

(8)

When the carbon nature was s t u d i e d by d i f f e r e n t i a l thermal a n a l y s i s , two zones appeared c l e a r l y d i s t i n c t i v e , as shown in Figure 4. In the f i r s t zone there were l i t t l e d i f f e r e n c e s between the c a t a l y s t s operated at d i f f e r e n t hydrogen to naphtha r a t i o s , being g r e a t e r the d i f f e r e n c e s in the second. Smaller r a t i o s increased the peaks of the second zone and a l s o produced a f u r t h e r i n c r e a s e on the ones of higher combustion temperatures. As when c o n s i d e r i n g the e f f e c t of pressure, a c r i t i c a l value f o r hydrogen to naphtha r a t i o -approximately 3- may be pointed out, as shown in F i g u r e 5. Discussion The amount and nature of coke deposited on the c a t a l y s t i s a f f e c t e d by s e v e r a l f a c t o r s , as the o p e r a t i o n a l c o n d i t i o n s (pressure, H :naphtha r a t i o , temperature and WHSV) and the c a t a l y s t nature, such as i t s c h l o r i n e content (4). Studying the i n f l u e n c e of t o t a l pressure and hydrogen to naphtha r a t i o , i t was found that there are c r i t i c a l values that must be avoided during o p e r a t i o n in order to prevent g r e a t e r and more polymerized coke d e p o s i t s , which make decrease the c a t a l y s t a c t i v i t y and modify i t s s e l e c t i v i t y , as had been shown in Figures 3 and 5. I f the data from DTA a n a l y s i s i s considered, i t may be a l s o n o t i c e d that when the carbon deposit i s higher than 1%, there i s a great increment of the peaks which corresponds to a more stable deposit. 2


COKE FORMATION


246

V \

I 0

ι 5

ι 10

ι 15

U 20

PRESSURE, a t m Figure 3. Modifications of ARON (-o-), A% Ar (-X-), and AyC (—Δ—) and percentage of carbon on catalysts (- · —|— · -) used under differ ent pressures during the standard test. Conditions: temperature, 515°C; and WHSV, 6 hr . L

1


+

5

12.

FiGOLi ET AL.

Coke Formation


CM

00 rH

+

Ο

ON CO

VO

Coke Formation on Metal Surfaces - American Chemical Society

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