Desorption Influence on Benzene Alkylation with Olefins over Y Zeolites

51 Desorption Influence on Benzene Alkylation with Olefins over Y Zeolites

Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch051

Alkylation over Zeolites 1

J. P. NOLLEY, JR. and J. R. KATZER Department of Chemical Engineering, University of Delaware, Newark, Del. 19711

Vapor-phase alkylation of benzene by ethene and propene over HY, LaY, and REHY has been studied in a tubularflowreactor. Transient data were obtained. The observed rate o reaction passes through a maximum with time, which resu from build-up of product concentration in the zeolite pores coupled with catalyst deactivation. The rate decay is related aromatic: olefin ratio, temperature, and olefin type. The observed ratefitsa model involving desorption of product from the zeolite crystallites into the gas phase as a rate-limiting step. The activation energy for the desorption term is 16.5 kcal/mole, approximately equivalent to the heat of adsorption of ethylbenzene. For low molecular weight alkylates intracrystalline diffusion limitations do not exist. 'he r a t e - l i m i t i n g processes i n c a t a l y t i c r e a c t i o n o v e r zeolites r e m a i n *·• l a r g e l y u n d e f i n e d , m a i n l y because of t h e l a c k of i n f o r m a t i o n o n c o u n t e r diffusion rates a t r e a c t i o n c o n d i t i o n s . T h o m a s a n d B a r m b y (1), C h e n et al. (2, S), a n d N a c e (4) speculate o n possible d i f f u s i o n a l l i m i t a t i o n s i n c a t a l y t i c c r a c k i n g o v e r zeolites, a n d K a t z e r (5) has s h o w n t h a t i n t r a c r y s t a l l i n e d i f f u s i o n a l l i m i t a t i o n s do n o t exist i n l i q u i d - p h a s e benzene a l k y l a t i o n w i t h p r o p e n e . T a n a n d F u l l e r (6) propose i n t e r n a l mass t r a n s f e r l i m i t a t i o n s a n d r a p i d f o u l i n g i n benzene a l k y l a t i o n w i t h c y c l o h e x e n e o v e r Y zeolite, b a s e d o n t h e o c c u r r e n c e of a m a x i m u m i n t h e r e a c t i o n r a t e a t a b o u t 100 m i n i n flow r e a c t i o n studies. V e n u t o et al. (7, 8, 9) r e p o r t s i m i l a r r a t e m a x i m a for v a p o r - a n d l i q u i d - p h a s e a l k y l a t i o n of benzene a n d d e h y d r o Present address: Universal Oil Products Co., Process Division, 20 UOP Plaza, Algonquin and Mt. Prospect Rds, Des Plaines, Ill. 60016. 1

563 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

564

MOLECULAR SIEVES

h a l o g e n a t i o n of e t h y l c h l o r i d e o v e r X a n d Y zeolites.

T h e exact l o c a t i o n s

of t h e m a x i m a were n o t d e t e r m i n e d because of t h e i n t e g r a l s a m p l i n g t e c h n i q u e , n o r was a n e x p l a n a t i o n p r o p o s e d .

R i e k e r t (10) f o u n d t h a t i n t r a

c r y s t a l l i n e a d s o r p t i v e d i f f u s i o n of ethene is m u c h m o r e r a p i d t h a n t h e r a t e of ethene p o l y m e r i z a t i o n o v e r N a N i Y a t 3 4 3 ° K , a n d he suggests t h a t d e s o r p t i o n c o n t r o l s t h e r a t e of f o r m a t i o n of gaseous p r o d u c t s b u t he d i d n o t q u a n t i f y h i s suggestion.

V e n u t o a n d L a n d i s (11, 12) h a v e s p e c u l a t e d o n

t h e i m p o r t a n c e of t h e d e s o r p t i o n step, p a r t i c u l a r l y w i t h respect t o

de

activation. B e n z e n e a l k y l a t i o n o v e r Y zeolites has been s t u d i e d as a f u n c t i o n of


olefin, olefin ."aromatic r a t i o , t e m p e r a t u r e , a n d z e o l i t e c a t i o n f o r m .

The

r a t e has been m o d e l e d , a n d t h e r a t e - l i m i t i n g process has been q u a n t i f i e d as p r o d u c t d e s o r p t i o n . Experimental

Methods

T h e a p p a r a t u s i n c l u d e d a s a t u r a t o r a n d a t u b u l a r flow m i c r o - r e a c t o r . T h e s a t u r a t o r c o n s i s t e d of a 1000 m l t h r e e - n e c k flask i m m e r s e d i n a n o i l filled c o n s t a n t t e m p e r a t u r e b a t h ( ± 0 . 1 ° K ) . O l e f i n was s p a r g e d i n t o t h e l i q u i d a r o m a t i c h y d r o c a r b o n i n t h e flask, a n d t h e v a p o r s t r e a m f r o m t h e flask w a s p a s s e d t h r o u g h a c o n d e n s o r m a i n t a i n e d 3 ° K b e l o w b a t h t e m p e r a t u r e t o p r o d u c e some b a c k - c o n d e n s a t i o n of a r o m a t i c . T h e r e a c t o r l o o p w a s V 4 i n c h stainless steel t u b i n g a n d c o n t a i n e d a b y p a s s t o a l l o w c a t a l y s t i s o l a t i o n f r o m t h e flow s y s t e m d u r i n g flow e q u i l i b r a t i o n . T h e r e a c t o r l o o p , except for b y p a s s , w a s i m m e r s e d i n a fluidized s a n d b a t h ( ± 0 . 5 ° K ) . T h e z e o l i t e c a t a l y s t , a b o u t 0.1 g r a m i n t h e p o w d e r f o r m , w a s a d d e d t o t h e reactor mixed w i t h quartz chips or placed between and somewhat dis p e r s e d i n t w o p l u g s of glass w o o l . T h e r e s u l t s were e s s e n t i a l l y t h e same for b o t h methods. R e a c t o r o p e r a t i o n was d i f f e r e n t i a l . A 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 h e a t e d gas s a m p l e v a l v e w a s u s e d f o r a n a l y s i s of r e a c t o r effluent. S a m p l e s w e r e i n j e c t e d as f r e q u e n t l y as p o s s i b l e ; t h u s p o i n t v a l u e s of r a t e w e r e o b t a i n e d . T h e z e o l i t e w a s p r e a c t i v a t e d b y h e a t i n g f r o m 323° t o 8 2 3 ° K at 1 ° K / m i n u n d e r flowing p r e d r i e d a i r a n d h o l d i n g a t 8 2 3 ° K for 3 h r . The c a t a l y s t w a s r e a c t i v a t e d i n t h e r e a c t o r u n d e r flowing a i r b y h e a t i n g t o 8 2 3 ° K a t 1 0 ° K / m i n j u s t p r i o r t o a r u n . A t t h e s t a r t of a r u n t h e z e o l i t e w a s first c o n t a c t e d w i t h benzene a n d t h e n w i t h t h e r e a c t a n t s t r e a m (7). D e t a i l s are g i v e n b y N o l l e y (13). M a t e r i a l s . A U Y zeolites w e r e u s e d i n t h e p o w d e r ( i n d i v i d u a l c r y s t a l lites) f o r m . T h e H Y (lot # 149499-00-007) h a d 8 7 % exchange of N a + b y N H , Si0 :Al 0 = 4.98, a n d a c r y s t a l l i n i t y g r e a t e r t h a n 9 5 % o n an a b s o l u t e scale (14)· T h e R E H Y , d e n o t e d S K - 5 0 0 b e l o w (lot # 12979-17), h a d the calculated unit cell formula ΚΕΜ(ΝΗ4)Η.ΙΝΒΜ[(Α10*)Ι5.7: (Si0 )i36.3]-ZH 0, where the rare earth is h i g h i n l a n t h a n u m a n d l o w i n c e r i u m . T h e L a Y was p r e p a r e d b y L a exchange of N a Y (lot # 3607-67; S i 0 : A 1 0 = 5.01), u s i n g s t a n d a r d t e c h n i q u e s (13,16) ; 8 5 % exchange was achieved. E t h e n e a n d propene w e r e M a t h e s o n C P g r a d e a n d c o n t a i n e d n o d e tectable impurities and 0.0003% impurities, respectively. Fisher A C S benzene w a s u s e d ; a l l r e a c t a n t s were d r i e d o v e r 4 A z e o l i t e before use. +

3

2

3

2

2

2

2

2

3

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

51.

Benzene Alkylation over Y Zeolites

NOLLEY AND KATZER

Theoretical

565

Considerations


T h r e e o b v i o u s models w h i c h c o u l d describe t h e o b s e r v e d r e a c t i o n r a t e a r e : (a) c o n c e n t r a t i o n e q u i l i b r i u m b e t w e e n a l l p a r t s of t h e i n t r a c r y s t a l l i n e pore s t r u c t u r e a n d t h e e x t e r i o r gas phase (reaction r a t e l i m i t i n g ) , (b) e q u i l i b r i u m between t h e gas phase a n d the surface of t h e zeolite c r y s t a l l i t e s b u t d i f f u s i o n a l l i m i t a t i o n s w i t h i n t h e i n t r a c r y s t a l l i n e pore s t r u c t u r e , a n d (c) c o n c e n t r a t i o n u n i f o r m i t y w i t h i n t h e i n t r a c r y s t a l l i n e pore s t r u c t u r e b u t a large difference f r o m e q u i l i b r i u m a t t h e i n t e r f a c e between t h e zeolite c r y s t a l (pore m o u t h ) a n d t h e gas phase ( p r o d u c t d e s o r p t i o n l i m i t a t i o n ) . C o m b i n a t i o n s of t h e a b o v e m a y occur, a n d a l l models m u s t i n c l u d e c a t a l y s t deactivation. F o r t h e m o d e l i n v o l v i n g a d e s o r p t i o n l i m i t a t i o n (model c) a c o m p o n e n t m o l e b a l a n c e is w r i t t e n o v e r t h e gas phase a n d c r y s t a l l i t e phase for p r o d uct D (A + Β D ) . T h e s e are, r e s p e c t i v e l y :

at and r~~

+

Of

KTJCOT

=

(2)

—

ep

w i t h initial conditions CDP(< = 0 ) = 0,

CDQ(< = 0 ) = 0

(3)

ULD is t h e d e s o r p t i o n coefficient for p r o d u c t D . T h e first-order d e s o r p t i o n t e r m s h o u l d b e s t r i c t l y JK"D(CDP — H C D G ) , a l l o w i n g for a n e q u i l i b r i u m b a c k pressure, where Η is a n e q u i l i b r i u m a d s o r p t i o n c o n s t a n t r e l a t i n g m o l e f r a c t i o n s i n t h e gas a n d zeolite phases. Η COG was s h o w n e m p i r i c a l l y t o be small compared w i t h CDP under our conditions. T h e r a t e d e c a y is a s s u m e d t o be e x p o n e n t i a l i n t i m e r p = M exp (-t/M ) 1

D

(4)

2

w h e r e M — i n i t i a l r a t e a n d M = t i m e c o n s t a n t for r a t e decay. T a n and F u l l e r (6) j u s t i f y t h i s r a t e d e c a y b e h a v i o r f r o m a m e c h a n i s t i c v i e w p o i n t , a n d e x p o n e n t i a l d e c a y fit o u r d a t a w e l l . x

2

E q u a t i o n s 1-4 c a n t h e n be s o l v e d g i v i n g

where a =

-

1/M , 2

b =

-

Κ, Ό

c =

-

F/V . G


566

MOLECULAR SIEVES

A l l p a r a m e t e r s i n E q u a t i o n 5 except KO c a n be e s t i m a t e d f r o m t h e p h y s i c a l c o n d i t i o n s of t h e e x p e r i m e n t o r f r o m t h e m e a s u r e d r a t e d e c a y (Mi a n d M ) ; 2

ULD is t h u s t h e o n l y f i t t i n g p a r a m e t e r r e m a i n i n g i n t h e m o d e l . B y w r i t i n g m o l a r c o m p o n e n t balances for gas a n d c r y s t a l l i t e phases, a s s u m i n g H e n r y ' s l a w e q u i l i b r i u m b e t w e e n these phases a n d a p p l y i n g e x p o n e n t i a l r a t e d e c a y , t h e e q u i l i b r i u m m o d e l (model a) is o b t a i n e d (13).

All

p a r a m e t e r s i n t h e m o d e l c a n be m e a s u r e d or e s t i m a t e d , i n d e p e n d e n t of r e a c t i o n studies, w i t h t h e e x c e p t i o n of Mi a n d M ,

w h i c h are o b t a i n e d f r o m

2

the observed rate decay.

T h e H e n r y ' s l a w constant H was estimated f r o m

a liquid-phase equilibrium study.

A model i n v o l v i n g diffusional l i m i t a -

t i o n s w i t h i n t h e c r y s t a l l i t e s has been d e v e l o p e d b y T a n a n d F u l l e r (6) a n d Downloaded by UNIV OF AUCKLAND on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch051

is n o t discussed here. Results B e n z e n e a l k y l a t i o n w i t h ethene w a s s t u d i e d o v e r H Y , L a Y , a n d S K 500 b e t w e e n 4 8 8 ° a n d 5 9 9 ° K a n d for C : C f r o m 0.7 t o 10. 6

e t h y l a t i o n w a s also s t u d i e d .

2

F o r propene

Ethylbenzene

a l k y l a t i o n , conditions

were

s i m i l a r except t h a t t h e t e m p e r a t u r e range w a s 350° t o 4 9 3 ° K , a n d t h e s t u d y w a s less c o m p l e t e t h a n for t h e ethene s y s t e m .

T h e experimental rate data

t y p i c a l l y e x h i b i t e d a m a x i m u m w i t h respect t o t i m e a n d u n d e r w e n t ext e n d e d d e c a y ( F i g u r e 1).

T h e l o c a t i o n of t h e p e a k is a f u n c t i o n of r e -

action conditions, particularly temperature.

T h e propene

a c t i v a t e d m o r e r a p i d l y t h a n t h e ethene s y s t e m .

system

t e m were r e p r o d u c i b l e t o 1 0 % .

Figure

1.

Rate of benzene alkylatwn

ethene over HY 1.9.

Line

at 529°K

represents

by desorption

C :C

and

6

simulation

limitation

de-

D a t a f o r t h e ethene s y s -

of

by 2

=

data

model


51.

NOLLEY AND KATZER

567


σ "00 u Ε σ» ι

£ V. CO


φ ο Ε Ε «Γ 50 ο

0 0

5 MOLE RATIO, C Q / ^

10

Figure 2. Dependence of the extrapolated in itial rate of benzene ethylation on aromatw:olefin mole ratio at 578°K and one atmosphere total pressure F o r S K - 5 0 0 t h e r a t e a t 5 7 3 ° Κ a n d 4 0 0 sec after t h e i n i t i a t i o n of r e a c t a n t flow i s i n d e p e n d e n t of r e a c t a n t m o l e r a t i o f o r C e : C = 0.7 t o 10. U n d e r these c o n d i t i o n s t h e 400-sec p o i n t is j u s t b e y o n d t h e m a x i m u m i n t h e r a t e c u r v e . S i m i l a r b e h a v i o r w a s o b s e r v e d a t one o t h e r c o n d i t i o n . I n i t i a l r a t e of r e a c t i o n e s t i m a t e d b y e x t r a p o l a t i n g t h e d e c a y p o r t i o n of t h e r a t e c u r v e s f o r t h i s d a t a t o zero t i m e (see b e l o w ) i n d i c a t e s a m a x i m u m i n t h e r a t e a t C : C = 3.5 ( F i g u r e 2 ) . E r r o r b a r s represent e s t i m a t e d 9 5 % confidence l i m i t s . T h e o b s e r v e d a c t i v i t y f o r H Y is a b o u t t w i c e t h a t of S K - 5 0 0 , t h a t f o r L a Y i s a b o u t t w o - t h i r d s t h a t of S K - 5 0 0 ( F i g u r e 2 ) . T h i s is consistent w i t h t h e t r e n d e x p e c t e d (7) since a l l c a t a l y s t s were a c t i v a t e d t o t h e same t e m p e r a t u r e . T h e t e m p e r a t u r e dependence of t h e o b s e r v e d r a t e i s l a r g e f o r a l l s y s t e m s s t u d i e d i n d i c a t i n g t h e absence of e x t e r n a l mass transfer limitations. 2

6

2

T h e d e c a y p o r t i o n of t h e r a t e c u r v e w a s fitted t o a n e x p o n e n t i a l r a t e d e c a y expression (6), r a t e = M χ exp (—t/M ), w h e r e Μι is t h e i n i t i a l r a t e a n d M is a t i m e c o n s t a n t for r a t e d e c a y , a n d v a l u e s of Mi a n d M were d e t e r m i n e d . I n m o s t cases e x p o n e n t i a l d e c a y fit t h e d a t a w e l l as s h o w n i n F i g u r e 3. F i g u r e s 4 a n d 5 s h o w t h e effect of C : olefin m o l e r a t i o o n t h e d e c a y t i m e c o n s t a n t . T h e m a x i m u m possible e r r o r i n these p o i n t s is ± 10^o I n t h e p r o p e n e s y s t e m a t a C : C 3 r a t i o of 20 t h e d e c a y c o n s t a n t i s 48,000 sec. W i t h H Y a t 4 9 3 ° K a n d C : olefin = 2, t h e r a t i o of t h e t i m e c o n s t a n t for t h e ethene s y s t e m t o t h a t f o r t h e propene s y s t e m i s a b o u t 20. F i g u r e 4 shows t h a t t h e d e c a y t i m e c o n s t a n t is i n d e p e n d e n t of c a t i o n f o r m , a n d f o r 2

2

2

e

e

6


MOLECULAR SIEVES

568 100

ι-

1

1

1

|

1

Γ


8 soL

Ο

2000

4000

6000

time, sec

Figure 8. Fit of exponential rate decay to experimental data for benzene alkyla tion with propene over H Y at 492°Κ and C :C = 9 e

3

e t h y l b e n z e n e e t h y l a t i o n the v a l u e is h i g h e r t h a n t h a t for benzene e t h y l ation. Discussion K i n e t i c C o n s i d e r a t i o n s . T h e r e a c t i o n k i n e t i c s are m a s k e d b y a d e s o r p t i o n process as s h o w n b e l o w a n d are f u r t h e r c o m p l i c a t e d b y r a t e d e a c t i v a t i o n . T h e independence of t h e 400-sec r a t e o n r e a c t a n t m o l e r a t i o is n o t i n d i c a t i v e of zero-order k i n e t i c s b u t r e s u l t s because of t h e n a t u r e of t h e p a r t i c u l a r k i n e t i c , d e s o r p t i o n , a n d r a t e d e c a y r e l a t i o n s h i p s u n d e r these c o n d i t i o n s . I t w o u l d n o t be e x p e c t e d t o be m o r e g e n e r a l l y o b s e r v e d u n d e r w i d e l y v a r y i n g c o n d i t i o n s . T h e i n i t i a l r a t e b e h a v i o r is considered m o r e i n d i c a t i v e of t h e i n t r i n s i c k i n e t i c s of t h e s y s t e m a n d is consistent w i t h a m o d e l i n v o l v i n g c o m p e t i t i v e a d s o r p t i o n between t h e t w o r e a c t a n t s w i t h t h e olefin b e i n g m o r e s t r o n g l y adsorbed. S u c h k i n e t i c b e h a v i o r is c o n s i s t e n t w i t h t h a t r e p o r t e d b y V e n u t o (16). K i n e t i c a n a l y s i s depends o n t h e a s s u m p t i o n t h a t q u a s i - s t e a d y state b e h a v i o r holds for t h e r a t e d u r i n g r a t e d e c a y a n d t h a t t h e e x p o n e n t i a l d e c a y e x t r a p o l a t i o n is v a l i d as t i m e approaches zero. D e t a i l e d q u a n t i f i c a t i o n of t h e i n t r i n s i c k i n e t i c s was n o t attempted i n this work. RATE DECAY. c e n t r a t i o n increases.

T h e d e c a y t i m e c o n s t a n t decreases as t h e olefin c o n T h i s is consistent w i t h a d e a c t i v a t i o n m e c h a n i s m


51.


NOLLEY AND KATZER

i n v o l v i n g olefin p o l y m e r i z a t i o n .

569

T h a t t h e c o n s t a n t i s c o n s i d e r a b l y less

for propene t h a n ethene ( F i g u r e s 4 a n d 5) i s also consistent w i t h olefin p o l y m e r i z a t i o n b e i n g m a i n l y responsible f o r r a t e decay.

T h i s is further

substantiated b y the observation t h a t t h e rate decay for ethylbenzene e t h y l a t i o n i s less t h a n t h a t f o r benzene e t h y l a t i o n ( F i g u r e 4 ) . I f d e a c t i v a t i o n w e r e caused b y t h e f o r m a t i o n o f h i g h e r m o l e c u l a r w e i g h t a l k y l ates, r a t e d e c a y s h o u l d b e m o r e severe for e t h y l b e n z e n e e t h y l a t i o n t h a n for benzene e t h y l a t i o n .

T h a t d e a c t i v a t i o n i n zeolite a l k y l a t i o n i s m a i n l y

t h e r e s u l t of olefin r e a c t i o n s o t h e r t h a n a l k y l a t i o n has also been s h o w n b y V e n u t o etal (8,17).


3000

τ—ι—ι—ι

ι

ι

r

υ

9 C

σ Ι υ

2000

«

Ε Η >» σ υ

SK-500 Ο SK-500: Ethylbenzene · Ethylation • • ' I I I 1 1

Ο

-

MOLE RATIO, Ce/Cg

Figure 4. Dependence of rate decay time constant on reactant mole ratio for benzene ethylation over Y zeolites at 578°K T h e rate decay t i m e constant is independent of cation f o r m of t h e zeolite i n t h e ethene s y s t e m ( F i g u r e 4) a l t h o u g h t h e a l k y l a t i o n a c t i v i t y o f t h e three forms i s c o n s i d e r a b l y different ( F i g u r e 2). T h i s i n d i cates t h a t t h e a c t i v e site w i t h i n t h e zeolite (at least for d e a c t i v a t i o n ) is t h e same f o r a l l three c a t i o n f o r m s as expected f r o m o u r c u r r e n t p i c t u r e o f a c t i v e sites for a c i d - c a t a l y z e d reactions i n these zeolites (8, 18, 19). T h e t h r e e c a t a l y s t s s h o u l d h a v e different n u m b e r s o f a c t i v e sites because o f t h e i r i n d i v i d u a l response t o a c t i v a t i o n a t 8 2 3 ° K , b u t these sites s h o u l d b e s i m i l a r ; t h u s M s h o u l d b e i n d e p e n d e n t o f c a t i o n f o r m , Mi s h o u l d d e p e n d on it. 2

T h e a c t i v a t i o n energy for t h e r a t e d e c a y t i m e c o n s t a n t w i t h benzene e t h y l a t i o n o v e r S K - 5 0 0 a t C : C = 8 is 13.6 ± 1 k c a l / m o l e . T h a t for H Y i n t h e ethene s y s t e m a t C : C = 2 is 11 k c a l / m o l e . F o r propene a l k y l a t i o n o v e r H Y t h e a c t i v a t i o n energy for r a t e d e c a y i s 4 k c a l / m o l e a n d i s i n d e p e n d e n t of C : Ce m o l e r a t i o . 6

e

2

2

6



570

MOLECULAR SIEVES

MOLE RATIO,

C /C 6

3

Figure 5. Dependence of rate decay time constant on reactant mole ratio for benzene isopropylation over H Y Catalysis S y s t e m M o d e l . T h e e q u i l i b r i u m m o d e l ( m o d e l a) d i d n o t p r o p e r l y represent t h e o b s e r v e d r a t e c u r v e because t h e p r e d i c t e d p e a k m a x i m u m , u s i n g t h i s m o d e l , a l w a y s o c c u r r e d at least a n order of m a g n i t u d e earlier i n t i m e t h a n was a c t u a l l y o b s e r v e d w h e n m e a s u r e d v a l u e s for a l l p a r a m e t e r s were s u b s t i t u t e d i n t o t h e e q u i l i b r i u m m o d e l . T h u s a mass t r a n s f e r i n fluence—e.g., i n t r a c r y s t a l l i n e diffusional l i m i t a t i o n s or p r o d u c t d e s o r p t i o n l i m i t a t i o n s — m u s t be i n v o k e d to e x p l a i n the d a t a . T h e d i f f u s i o n a l l i m i t a t i o n s m o d e l m i g h t fit t h e d a t a q u a l i t a t i v e l y as T a n a n d F u l l e r (6) s h o w for t h e i r s y s t e m . H o w e v e r , t h i s m o d e l c o n t a i n s three fitting constants a n d s h o u l d be a p p l i e d o n l y w h e n there is sufficient evidence of diffusional l i m i tations. T h e p r o b a b l e absence of i n t r a c r y s t a l l i n e diffusional l i m i t a t i o n s i n benzene e t h y l a t i o n a n d i s o p r o p y l a t i o n c a n be s h o w n b y e x t r a p o l a t i o n of a v a i l a b l e l i q u i d - p h a s e counterdiffusion d a t a (20, 21) t o r e a c t i o n c o n d i t i o n s w i t h the a i d of reasonable a s s u m p t i o n s . T h e effective c o u n t e r d i f f u s i o n coefficient for benzene-cumene counterdiffusion i n S K - 5 0 0 a t 2 9 8 ° Κ (the v a l u e for b e n z e n e - e t h y l b e n z e n e counterdiffusion s h o u l d be s i m i l a r or higher) was e x t r a p o l a t e d t o several t e m p e r a t u r e s i n t h e r e a c t i o n


51.

571


NOLLEY AND KATZER

range i n c l u d i n g 5 7 3 ° K , u s i n g a c t i v a t i o n energies of 10, 13, 15, a n d 20 k c a l / mole. T h e a c t i v a t i o n energy p r o b a b l y f a l l s near t h e center of t h i s range


(21) , a n d t h e presence of t h e s m a l l olefin molecule p r o b a b l y does n o t h a v e a large effect o n t h e c o u n t e r d i f f u s i o n r a t e . T h e m o d i f i e d T h i e l e m o d u l u s (22) was t h e n c a l c u l a t e d u s i n g a m e a s u r e d r a t e of r e a c t i o n n e a r t h e m a x i m u m observed a n d o t h e r p a r a m e t e r s of t h e s y s t e m (18); t h e results are s u m m a r i z e d i n T a b l e I . I n a l l cases t h e effectiveness f a c t o r is greater t h a n 0.9, i n d i c a t i n g t h a t i n t r a c r y s t a l l i n e diffusional l i m i t a t i o n s are n o t present. T h e s e c o n d i t i o n s are considered t h e severest test because t h e a c t i v a t i o n energy for r e a c t i o n is greater t h a n t h a t for diffusion a n d l o w e r t e m p e r a t u r e s s h o u l d be less l i k e l y t o i n v o l v e diffusional l i m i t a t i o n s . C a l c u l a t i o n s v e r i f y t h i s for b o t h systems. A l t h o u g h e x t r a p o l a t i o n of l i q u i d - p h a s e m e a s u r e d diffusivities t o 5 7 3 ° Κ is questionable, i t is q u i t e l i k e l y t h a t a t temperatures considerably above the reactant boiling point a relatively dense " l i q u i d - l i k e p h a s e " r e m a i n s w i t h i n t h e i n t r a c r y s t a l l i n e pores (12), a n d u n d e r these c o n d i t i o n s t h e e x t r a p o l a t i o n s h o u l d h o l d . I f t h i s is w i t h i n t h e t e m p e r a t u r e range of t h e r e a c t i o n studies, w h i c h is t r u e for benzene i s o p r o p y l a t i o n , or i f Z) ff a n d t h e associated a c t i v a t i o n energy do n o t d e crease g r e a t l y w i t h a decrease i n t h e d e n s i t y of t h e i n t r a c r y s t a l l i n e organic " p h a s e , " l i m i t a t i o n s seem u n l i k e l y for t h e ethene a n d propene systems. e

Table I.

Modified Thiele Modulus for Benzene Ethylation at 5 7 3 ° K

Activation Energy for Diffusion, kcal/mole

10

0

Modified Thiele M o d u l u s , (22) a

D

e{{

13

0.76

6

at 298°K (21) assumed to be 140>: 10 m /sec 17

2

0.065

15 0.013

20 0.00023

2

where R = 0.5 μ —

= 75 mmole/hr-gram catalyst

CAS = concentration of A at surface of catalyst. T h e influence of d e a c t i v a t i o n o n t h e c o u n t e r d i f f u s i o n r a t e c a n n o t b e q u a n t i f i e d here, b u t for early t i m e s ( < 1 h r ) i t was p r o b a b l y n o t u n d u l y i m p o r t a n t . C a t a l y s t w h i c h was r e m o v e d f r o m t h e r e a c t o r 1 h r a n d several h o u r s after t h e s t a r t of a r u n w a s s t i l l very w h i t e , i n d i c a t i n g a l o w degree of c o k i n g i n those t i m e periods. A f t e r r e m a i n i n g i n t h e r e a c t o r o v e r n i g h t t h e c a t a l y s t d i d t u r n d a r k b r o w n . A l t h o u g h we w o u l d expect d i f f u s i o n a l l i m i t a t i o n s t o exist i n t h e presence of m u c h coke, we a t t r i b u t e t h e r a t e d e c a y o v e r m o s t of t h e r a t e c u r v e m e a s u r e d i n t h i s w o r k ( i n c l u d i n g t h e m a x i m u m ) m o r e t o a s i t e - t y p e d e a c t i v a t i o n t h a n t o m a s s i v e c o k i n g (see a b o v e ) , w h i c h does e v e n t u a l l y occur, a n d we propose t h a t for t h i s i n i t i a l p o r t i o n t h e c o u n t e r d i f f u s i o n rates are n o t g r e a t l y r e d u c e d .


572

MOLECULAR SIEVES

T h e p r o d u c t d e s o r p t i o n l i m i t a t i o n m o d e l appears t o represent p r o p e r l y a n u m b e r of s y s t e m d e t a i l s a n d was f i t t e d t o t h e d a t a b y a d j u s t i n g t h e d e s o r p t i o n coefficient Κτ>. Κτ> was i n s e n s i t i v e t o t h e v a l u e of M . In all cases t h e p r o d u c t d e s o r p t i o n l i m i t a t i o n m o d e l a c c u r a t e l y s i m u l a t e d t h e d a t a . F i g u r e s 1 a n d 6 show t y p i c a l fits of the m o d e l t o observed r a t e d a t a for benzene e t h y l a t i o n o v e r H Y a t 529° Κ a n d for e t h y l b e n z e n e e t h y l a t i o n


2

o v e r S K - 5 0 0 a t 577° Κ r e s p e c t i v e l y . V a l u e s of KT> for benzene e t h y l a t i o n o v e r H Y are s h o w n i n F i g u r e 7 a n d e x h i b i t a n a c t i v a t i o n energy of 16.5 ± 2.5 k c a l / m o l e . T h i s is too h i g h for a s t a n d a r d mass t r a n s f e r coefficient, a n d since i t is e s s e n t i a l l y e q u i v a l e n t t o t h e h e a t of a d s o r p t i o n of e t h y l benzene o n H Y (23, 24), KO m u s t be a coefficient for d e s o r p t i o n . The observed a c t i v a t i o n energy represents t h e heat of a d s o r p t i o n , t h e energy b a r r i e r w h i c h a d e s o r b i n g molecule m u s t cross. T h e p r e d i c t e d e x t e r i o r mass t r a n s f e r coefficient c a l c u l a t e d u s i n g k n o w n correlations is m a n y orders of m a g n i t u d e l a r g e r , f u r t h e r i n d i c a t i n g t h a t a n exterior mass t r a n s f e r p r o cess is n o t l i m i t i n g . Κτ> for e t h y l b e n z e n e e t h y l a t i o n is a l m o s t a n o r d e r of m a g n i t u d e less t h a n t h a t for benzene e t h y l a t i o n . T h e d i a l k y l a t e w o u l d be expected t o desorb m o r e s l o w l y t h a n t h e m o n o a l k y l a t e . T h u s t h e p r o p e r t i e s of Kjy o b t a i n e d f r o m fitting t h e d a t a are those w h i c h w o u l d be expected i f Κτ> were a p r o d u c t d e s o r p t i o n coefficient. A l t h o u g h other p o s s i b i l i t i e s c a n n o t as y e t be a b s o l u t e l y r u l e d o u t , t h e evidence s t r o n g l y i n d i c a t e s t h a t i n t h i s s t u d y t h e d e s o r p t i o n of p r o d u c t molecules f r o m t h e surface (pore m o u t h s ) of t h e zeolite c r y s t a l l i t e s is a r a t e - l i m i t i n g step. F u r t h e r , p r o d u c t d e s o r p t i o n l i m i t a t i o n s are p r o b a b l y also responsible for t h e m a x i m a i n rates p r e v i o u s l y r e p o r t e d (7, 8, 9) a n d m a y be a m o r e general p h e n o m e n o n for zeolite systems. S u c h l i m i t a t i o n s

Figure 6. Simulation of ethylbenzene ethylation over SK-500 at 577°Κ and Cs:C = 0.2 by product desorption limitation model 2


51.

NOLLEY AND KATZER


573


τ

ο I 1.6

I

I

1.8

2.0

I0 /T, 3

Figure 7.

I

•K'

1

Arrhenius temperature dependence of KD for benzene ethylation over HY

c o m p l i c a t e t h e i n t e r p r e t a t i o n of k i n e t i c studies w i t h zeolites a n d c o u l d w e l l be i m p o r t a n t t o zeolite c a t a l y s t b e h a v i o r i n c y c l i c systems, e.g., fluid-bed catalytic cracking. Acknowledgments T h e a u t h o r s a c k n o w l e d g e t h e U n i v e r s i t y of D e l a w a r e R e s e a r c h F o u n d a t i o n a n d t h e c o n t r i b u t o r s of I n d u s t r i a l F e l l o w s h i p s t o t h e D e p a r t m e n t for t h e i r s u p p o r t of J . P . N o l l e y , J r . a n d of t h e research. U n i o n C a r b i d e C o r p . s u p p l i e d t h e zeolites. Nomenclature COG CDP

C o n c e n t r a t i o n of D i n gas phase, m o l e s / v o l u m e C o n c e n t r a t i o n of D i n c r y s t a l l i t e phase, m o l e s / z e o l i t e pore v o l u m e

Z>eff F H

E f f e c t i v e diffusion coefficient, (length) / t i m e V o l u m e t r i c feed r a t e t o r e a c t o r , v o l u m e / t i m e H e n r y ' s l a w - t y p e c o n s t a n t r e l a t i n g gas phase

KB Μχ M r p

c r y s t a l l i t e phase mole f r a c t i o n Coefficient of d e s o r p t i o n for species D , ( m o l e s ) / ( l e n g t h ) - t i m e I n i t i a l r a t e of r e a c t i o n , m o l e s / t i m e - w t c a t a l y s t T i m e c o n s t a n t for r a t e decay, t i m e R a t e of p r o d u c t i o n of p r o d u c t D , m o l e s / t i m e - w t c a t a l y s t

2

D

2

mole

fraction 2


to

574 t VG Vc e € P

MOLECULAR SIEVES

Time Volume of interparticle gas space in bed, volume Volume of catalyst in bed, volume Interparticle void fraction of bed Intracrystalline void fraction of zeolite


Literature Cited 1. 2. 3. 4. 5.

Thomas, C. L., Barmby, D. S., J. Catalysis (1968), 12, 341. Chen, Ν. Y., Preprint 5th Int. Congr. Catalysis (Palm Beach, Fla.), 1972. Miale, J. N., Chen, N. Y., Weisz, P. B., J. Catalysis (1966), 6, 278. Nace, D. M., Ind. Eng. Chem. Prod. Res. Develop. (1970), 9, 203. Katzer, J. R., Ph.D. Thesis, Massachusetts Institute of Technology, Cam bridge, Mass., 1969. 6. Tan, C. H., Fuller, Ο. M., Canad. J. Chem. Eng. (1970) 48, 174. 7. Venuto, P. B., Hamilton, L. Α., Landis, P. S., Wise, J. J., J. Catalysis (1966), 4, 81. 8. Venuto, P. B., Hamilton, L. Α., Landis, P. S., J. Catalysis (1966), 5, 484. 9. Venuto, P. B., Givens, Ε. N., Hamilton, L. Α., Landis, P. S., J. Catalysis (1966) 6, 253. 10. Riekert, L., J. Catalysis (1970) 19, 8. 11. Venuto, P. B., Landis, P. S., Advan. Catalysis (1968) 18, 259. 12. Venuto, P. B., Chem. Tech. (1971) 215. 13. Nolley, J. P., Jr., M.Ch.E. Thesis, University of Delaware, Newark, Del., 1972. 14. Sherman, J. D., private communication, Union Carbide Corp., Tarrytown, Ν. Y., Sept. 1970. 15. Sherry, H. S., ADVAN. CHEM. SER. (1971) 101, 350. 16. Venuto, P. B., ADVAN. CHEM. SER. (1971) 102, 260.

17. Venuto, P. B., Hamilton, L. Α., Ind. Eng. Chem. Prod. Res. Develop. (1967) 6, 190. 18. Ward, J. W., J. Catalysis (1972) 26, 451. 19. Eberly, P. E., Jr., Kimberlin, C. N., Jr., ADVAN. CHEM. SER. (1971) 102, 347. 20. Moore, R. M., Katzer, J. R., AIChE J. (1972) 18, 816. 21. Satterfield, C. N., Katzer, J. R., ADVAN. CHEM. SER. (1971) 102, 193.

22. Satterfield, C. N., "Mass Transfer in Heterogeneous Catalysis," MIT Press, Cambridge, Mass., 1970, pp. 129-151. 23. Khudiev, A. T., Klyachko-Gurvich, A. L., Brueva, T. R., Isakov, Y . I., Rubinstein, A. M., Bull. Acad. Sci. U.S.S.R. Div. Chem. Sci. (1968) No. 4, 694. 24. Romanovskii, Β. Α., Hoshi, T., Topchieva, K. U., Piguzova, L. I., Kinet. Katal. (1966) 7, 841. RECEIVED

December 4, 1972.


Desorption Influence on Benzene Alkylation with Olefins over Y Zeolites

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