The Oxidation of o-Xylene in a Transported Bed Reactor - Advances in

51 The Oxidation of o-Xylene in a Transported Bed Reactor MARK

S. WAINWRIGHT

and TERRENCE

W.

HOFFMAN

Downloaded by FUDAN UNIV on February 22, 2017 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch051

Department of C h e m i c a l Engineering, M c M a s t e r University, Hamilton, Ontario, C a n a d a

o-Xylene was oxidized on a vanadia-on-silica catalyst in a bench-scale fixed bed reactor. Reaction products were analyzed at short on-stream times. These results indicated that reaction rates on freshly oxidized catalyst are much higher than those observed after steady state has been achieved; selectivity to organic oxidation products is also improved significantly. This reaction was also done in a pilot-scale transported bed reactor in which the catalyst was conveyed by the reactant/product gases. Stable operation was achieved at very high solid loadings (solid/gas ratios up to 250 and voidages from 0.60 to 0.86). The high reaction rates and selectivities can be exploited in such reactors, and suggestions for further research on such reactor systems are included.

O

- X y l e n e w a s o x i d i z e d i n a t r a n s p o r t e d b e d r e a c t o r w h e r e t h e finely d i v i d e d catalyst (vanadia on a silica support) was conveyed u p w a r d i n a t u b e b y t h e r e a c t i n g gas m i x t u r e . T h e a d v a n t a g e s o f a t r a n s p o r t e d b e d r e a c t o r a r e : ( a ) n e a r l y p l u g f l o w b e h a v i o r of t h e gas a n d s o l i d a n d h e n c e b e t t e r c o n t r o l of r e s i d e n c e t i m e of t h e r e a c t i n g a n d p r o d u c t gases, ( b ) e s s e n t i a l l y i s o t h e r m a l o p e r a t i o n e v e n a t h i g h r e a c t i o n rates b e c a u s e o f t h e g o o d h e a t t r a n s f e r rates f r o m s o l i d p a r t i c l e s t o gas a n d s o l i d / g a s s l u r r y to w a l l a n d t h e h e a t s i n k p r o v i d e d b y t h e m a s s of s o l i d e n t r a i n e d i n t h e gas, a n d ( c ) c o n t i n u o u s u s e of r e a c t i v a t e d c a t a l y s t s i n c e t h e s o l i d s a r e c o n t i n u a l l y a d d e d a n d withdrawn (I). T h e m a i n disadvantages are: (a) possibly the dilute con centrations of solids i n p n e u m a t i c a l l y c o n v e y e d systems a n d h e n c e the n e e d f o r h i g h s p e c i f i c r e a c t i v i t y of t h e c a t a l y s t , ( b ) p o s s i b l e h i g h a t t r i t i o n of t h e catalyst a n d erosion of reactor internals, a n d (c) c o m p l i c a t e d e q u i p m e n t , especially the catalyst recovery system. T h i s s t u d y w a s d o n e to e v a l u a t e t h e p e r f o r m a n c e of t h i s u n i q u e c o n t a c t i n g r e a c t o r f o r o x i d a t i o n r e a c t i o n s l i k e o - x y l e n e . I n t h i s r e a c t i o n t h e rates a r e h i g h e n o u g h a n d i n t e r m e d i a t e p r o d u c t s a r e d e s i r e d ; m o r e o v e r , i t is h i g h l y e x o t h e r m i c . W o r k w i t h transported b e d reactors has been l i m i t e d because the reactions m u s t b e v e r y fast. M o s t of t h e r e s e a r c h h a s b e e n d o n e b y i n d u s t r i a l g r o u p s a n d is d e s c r i b e d i n p a t e n t s . T h e i r u s e i n c l u d e s c a t a l y t i c c r a c k i n g , F i s h e r T r o p s c h h y d r o c a r b o n synthesis, coal gasification, acetylene generation, a n d 669

Hulburt; Chemical Reaction Engineering—II Advances in Chemistry; American Chemical Society: Washington, DC, 1974.

670

CHEMICAL

REACTION ENGINEERING

II

n u c l e a r r e a c t o r s . E c h i g o y a et al. (2) r e p o r t a b e n c h - s c a l e r e a c t o r f o r c u m e n e decomposition on an alumina-silicate catalyst, a n d recently d e L a s a a n d G a u (3) report the d e c o m p o s i t i o n of ozone i n a pilot-scale u n i t . E x c e p t for the l a t t e r , n o n e o f t h e s e s t u d i e s r e p o r t s o n t h e s e u n i t s as c h e m i c a l r e a c t o r s .


The

Oxidation

Reaction

R e a c t i o n Steps. Definitive studies on o-xylene oxidation on v a n a d i a cata lysts h a v e b e e n done b y N o v e l l a a n d B e n l l o c h ( 4 ) , H e r t e n a n d F r o m e n t ( 5 ) , a n d J u u s o l a ( 6 ) . A l t h o u g h these w o r k e r s u s e d different v a n a d i a catalysts, e a c h w i t h d i f f e r e n t a m o u n t s of v a n a d i u m p e n t o x i d e , w i t h p r o m o t e r s of d i f f e r e n t concentration a n d w i t h different catalyst supports, t h e y detected essentially the same p r o d u c t s ; catalyst performance differed only i n reaction rate a n d s e l e c t i v i t y . I n g e n e r a l , t h e r e a c t i o n s c h e m e c a n b e r e p r e s e n t e d b y F i g u r e 1. B o t h M a n n ( 7 ) a n d J u u s o l a ( 6 ) i n d i c a t e p - b e n z o q u i n o n e as a s i g n i f i c a n t r e a c t i o n p r o d u c t , b u t i t is d i f f i c u l t t o see h o w i t c a n b e p a r t o f t h e m a i n r e a c t i o n s e q u e n c e . M o r e o v e r , u s u a l l y o - t o l u i c a c i d is p r e s e n t o n l y i n s m a l l q u a n t i t i e s , i n d i c a t i n g t h a t i t s r e a c t i o n r a t e to p h t h a l i d e is q u i t e fast. O x i d a t i o n rate studies of p h t h a l i d e a n d p h t h a l i c a n h y d r i d e to the c a r b o n oxides a n d m a l e i c a n h y d r i d e suggest t h a t these reactions are n e g l i g i b l e b e l o w 4 0 0 ° C , i n d i c a t i n g t h a t these c o m p o u n d s are s t a b i l i z e d b y the a n h y d r i d e r i n g structure. M a l e i c a n h y d r i d e concentration is u s u a l l y s m a l l . O x i d a t i o n M e c h a n i s m . T h e o x i d a t i o n state o f v a n a d i u m s i g n i f i c a n t l y affects p r o d u c t d i s t r i b u t i o n s . S i m a r d et al. (8) s h o w e d t h a t t h e o x i d a t i o n state of v a n a d i u m i n these catalysts is s o m e w h e r e b e t w e e n 4 a n d 5 d u r i n g reaction. I n n a p h t h a l e n e o x i d a t i o n s t h e h i g h e r o x i d a t i o n state f a v o r s f o r m a t i o n o f t h e o r g a n i c o x i d e s w h e r e a s t h e l o w e r state f a v o r s t h e f o r m a t i o n o f c a r b o n o x i d e s ( 9 ) . T h e most significant systematic studies r e l a t i n g to catalyst properties h a v e b e e n d o n e b y t h e J a p a n e s e g r o u p (10, 11, 12, 13). T h e y were concerned w i t h t h e effect o f s u p p o r t m a t e r i a l , p o t a s s i u m s u l f a t e ( p r o m o t e r ) c o n c e n t r a t i o n , a n d sulfur trioxide a d d i t i o n o n the catalyst activity a n d selectivity. It was s h o w n that the o p t i m a l K S 0 - t o - V 0 mole ratio d e p e n d e d on support material. F r o m a phase d i a g r a m for this system, they c o n c l u d e d that w i t h a silicas u p p o r t e d c a t a l y s t , m o s t o f t h e s u l f a t e is a s s o c i a t e d w i t h t h e s u p p o r t , a n d t h i s , i n t u r n , is i m p o r t a n t i n d e t e r m i n i n g c a t a l y s t p e r f o r m a n c e . T h e i m p o r t a n c e o f s u l f u r t r i o x i d e as a c a t a l y s t a d d i t i o n w a s d e t e r m i n e d b y p r e p a r i n g c a t a l y s t s treated w i t h o l e u m ; p h t h a l i c a n h y d r i d e y i e l d w a s d r a m a t i c a l l y affected. They c o n c l u d e d that the active catalyst c o m p o n e n t has a c o m p o s i t i o n V 0 + 3 V O · 2 K S 0 + n S 0 w i t h p e r h a p s the sulfate i n the f o r m of p o t a s s i u m pyrosulfate. A U t h e s e s t u d i e s h a v e r e l a t e d m o r e to m e a s u r i n g t h e effects o f a d d i t i o n s r a t h e r t h a n d e f i n i n g t h e i r r o l e o r t h e m e c h a n i s m s b y w h i c h t h e s e c o m p o n e n t s affect catalyst behavior. 2

4

2

5

2

2

4

5

2

s

3

M a r s a n d v a n K r e v e l e n (14) p u b l i s h e d a c o m p r e h e n s i v e s t u d y of o x i d a t i o n reactions o n v a n a d i u m pentoxide catalysts. T h e results w e r e interpreted b y a m e c h a n i s m w h i c h h a s b e c o m e k n o w n as t h e r e d o x m e c h a n i s m . I n t h e i r m o d e l t h e o x y g e n f o r t h e o r g a n i c o x i d a t i o n is a s s u m e d to c o m e f r o m t h e v a n a d i a ; it is t h e n a s s u m e d to b e r e p l a c e d b y t h e c a t a l y s t r e a c t i o n w i t h g a s e o u s o x y g e n . Steady-state b e h a v i o r occurs w h e n these t w o o x i d a t i o n rates are e q u a l . T h e m o d e l explains the m u c h higher catalyst activity w i t h fresh catalyst a n d the v e r y fast c a t a l y s t d e c a y t h a t W a i n w r i g h t a n d H o f f m a n (15) o b s e r v e d just after the reactor is p u t o n stream.


51.

W A I NW R I G H T

S h e l s t a d et al. (16)

therefore models.

a n d chemisorbed

are i d e n t i c a l to those

reaction

671

Reactor

H e r e t h e r e a c t i o n is a s s u m e d t o t a k e

the hydrocarbon molecule

rate expressions

Bed

p r o p o s e d a steady-state adsorption m o d e l to e x p l a i n

t h e b e h a v i o r o f these c a t a l y s t s . between

Transported

AND HOFFMAN

kinetic

oxygen.

derived from

experiments

cannot

place

T h e resultant

the redox

model, and

discriminate between

these

H o w e v e r , t h e f a s t c o l o r c h a n g e o f t h e c a t a l y s t a n d t h e o x i d a t i o n state

of t h e v a n a d i a suggest that t h e redox m e c h a n i s m is t h e m o r e l i k e l y , a l t h o u g h the chemisorbed oxygen m a y be a n important link i n the catalyst reoxidation. T h e essence of t h e m o d e l c a n b e d e m o n s t r a t e d b y d e r i v i n g t h e k i n e t i c rate expression for the o-xylene to t o l u a l d e h y d e reaction.

Consider the follow


ing: hydrocarbon - f oxidized catalyst —• reduced catalyst + product reduced catalyst -f- oxygen —* oxidized catalyst A s s u m e , o n t h e b a s i s o f e x p e r i m e n t a l e v i d e n c e , t h a t b o t h r e a c t i o n s a r e first order w i t h respect to h y d r o c a r b o n a n d oxygen, respectively. T h e rate expres sions f o r e a c h s t e p b e c o m e : r

= k C'ιιθ

t

(I)

r

and r

= k Co &

a

Θ)

(1 -

2

(2)

w h e r e θ i s t h e f r a c t i o n o f o x y g e n sites a v a i l a b l e f o r h y d r o c a r b o n o x i d a t i o n . I n the steady state, these rates m u s t b e

equal—viz.,

r

= nr

&

(3)

r

w h e r e η is a s t o i c h i o m e t r i c coefficient f o r the n u m b e r of o x y g e n m o l e c u l e s u s e d p e r m o l e o f h y d r o c a r b o n o x i d i z e d . T h u s t h e f r a c t i o n o f a c t i v e sites a v a i l a b l e becomes ka.Co + nk Cu 2

θ

~ k Co a

2

r

1

+

1 nkrCn

(4)

k&Co

2

o-TOOilC ACID rT^^YCOOH ο - TOLUALDE HYDE I ζ

^YCHO I I

HC

\ ^ C H PHTHALIC / .ANHYDRIDE 2

1

M A L E I C ANHYDRIDE

«=*C0 + H 0 + C O 2

Figure 1.

2

Reaction scheme


672

CHEMICAL

REACTION

ENGINEERING

II

a n d the rate of o v e r a l l h y d r o c a r b o n o x i d a t i o n becomes:


Γ γ

"

/c Co + nkrCn R

2

K

}

S i m i l a r b u t s o m e w h a t m o r e c o m p l i c a t e d expressions c a n be d e r i v e d for the r e a c t i o n s c h e m e s h o w n i n F i g u r e 1 i n v o l v i n g p a r a l l e l a n d series r e a c t i o n steps. I n i t i a l l y w i t h a f u l l y o x i d i z e d c a t a l y s t 0 = 1, a n d t h e r e a c t i o n r a t e w o u l d b e E q u a t i o n 6, w h i c h is i n d e p e n d e n t o f o x y g e n c o n c e n t r a t i o n . T h i s a s s u m e s t h a t t h e r e a c t i o n is n o t m a s s t r a n s f e r c o n t r o l l e d n o r c o n t r o l l e d b y t h e r a t e of a d s o r p t i o n o f t h e h y d r o c a r b o n m o l e c u l e s o n t o t h e r e a c t i o n sites. T h e a b i l i t y of t h i s m o d e l t o d e s c r i b e t h e s t e a d y - a n d u n s t e a d y - s t a t e b e h a v i o r o f t h i s r e a c t i o n s y s t e m is s h o w n e l s e w h e r e ( 1 5 ) . T h e m o d e l p r o v i d e s a reasonable d e s c r i p t i o n of a n u m b e r of catalysts u s i n g different supports; the i n d i v i d u a l pre-exponential factors a n d activation energies for each catalyst are, h o w e v e r , quite different. H e r e w e r e v i e w briefly some salient results for one catalyst u s e d i n the t r a n s p o r t e d b e d r e a c t o r so its p e r f o r m a n c e i n a f i x e d b e d a n d a t r a n s p o r t e d b e d can be compared. Packed

Bed

Studies

Apparatus. T h e p a c k e d b e d apparatus was designed for all o-xylene con v e r s i o n s i n c l u d i n g 1 0 0 % . T h e f o l l o w i n g effects w e r e a l s o t a k e n i n t o a c c o u n t : (1) T h e reactor must be isothermal, a n d the temperature range cover those e n c o u n t e r e d i n i n d u s t r i a l practice

must

( 2 ) C a t a l y s t d i l u t i o n s h o u l d b e a v o i d e d , i f p o s s i b l e , to a v o i d a n y c a t a l y t i c effect of t h e d i l u e n t s i n c e a n o m o l o u s effects of o t h e r m a t e r i a l s h a v e b e e n reported (6,7) (3) P a r t i c l e s i z e s h o u l d b e s m a l l to a v o i d c a t a l y s t e f f e c t i v e n e s s a n d k e e p t h e p a r t i c l e e s s e n t i a l l y a t t h e s a m e t e m p e r a t u r e as t h e gas (4) effects

effects

Reactor-to-catalyst d i a m e t e r s h o u l d be large to a v o i d s h o r t - c i r c u i t i n g

(5) R e a c t o r length-to-diameter ratio s h o u l d be large to a v o i d a p p r e c i a b l e a x i a l d i f f u s i o n effects ( 6 ) T h e r e a c t o r s h o u l d o p e r a t e at a n a p p r e c i a b l e v e l o c i t y t o e n s u r e g o o d h e a t a n d m a s s t r a n s f e r f r o m gas t o p a r t i c l e ( 7 ) T h e o p e r a t i n g c o n d i t i o n s r e l a t i v e to m a s s o f c a t a l y s t - t o - f l o w r a t e o f reactant s h o u l d be similar to those expected i n the transported b e d reactor. T h e a p p a r a t u s is s h o w n i n F i g u r e 2. O x y g e n a n d n i t r o g e n a r e f e d f r o m h i g h p r e s s u r e c y l i n d e r s a n d m e t e r e d b y c a p i l l a r y flowmeters. A b a c k p r e s s u r e v a l v e m a i n t a i n e d a c o n s t a n t p r e s s u r e o n t h e m e t e r i n g s y s t e m s . T h i s gas m i x ture was f e d t h r o u g h a n o-xylene saturator p l a c e d i n a constant temperature b a t h , a n d the pressure i n the saturator was measured. T h i s mixture was heated to r e a c t i o n t e m p e r a t u r e i n a c o i l i m m e r s e d i n t h e salt b a t h s u r r o u n d i n g t h e reactor a n d t h e n f e d to the top of the reactor. T h e reactor w a s a 1 4 - c m l e n g t h o f 0 . 4 7 5 - c m i d stainless t u b i n g , a n d a k n o w n a m o u n t o f c a t a l y s t w a s c h a r g e d to it ( 6 c m to 1 1 - c m d e p t h ) . T h e reactor w a s p l a c e d i n a 1 2 - i n c h i d b y 1 2 - i n c h h i g h m o l t e n salt b a t h ( p o t a s s i u m n i t r a t e - s o d i u m n i t r a t e - s o d i u m n i t r i t e e u t e c t i c ) . T h e e x i t gases p a s s e d t h r o u g h a C a r l e s a m p l i n g v a l v e . A l l l i n e s a n d t h e v a l v e w e r e h e a t e d t o ca. 2 0 0 ° C b y h o t a i r o r e l e c t r i c h e a t i n g t a p e t o p r e v e n t c o n d e n s a t i o n of p r o d u c t s . I n l e t gases flowed t h r o u g h t h e s a m p l i n g v a l v e a n d t h e n to the reactor or w e r e e x h a u s t e d to t h e atmosphere. T h i s a l l o w e d steadystate o p e r a t i o n o f t h e s a t u r a t o r t o b e a c h i e v e d b e f o r e t h e gases w e r e p a s s e d to the reactor.


51.

WAINWRIGHT A N D H O F F M A N

Transported

Bed

Reactor

673


AIR

Figure 2. Β Ε F Gc

Flowsheet of packed bed apparatus

backpressure regulator electrical heating capillary flow meter gas sample valve to gas chromatograph heating elements manometer agitator needle valve

He M Ms Ν

Table I.

NR Ρ R S SB Τ TC W V

non-return valve preheating coil reactor xylene saturator molten salt bath three-way valve thermocouple from constant tempera ture water bath vent

Approximate Composition (wt %)

a n d Properties of C a t a l y s t s

Silica Supported (No. 902)

νο 2

2

so

9 29

δ

K S0 K 0

4

2

3

S i 0 or T i 0 Sb 0 Surface area, m /gram Particle size (screened from original) Bulk density, grams/cm Average pore size 2

2

2

3

2

3

12 50 40 50/70 mesh 0.6 30 A

Titania

Supported

2 2 84 6 5 50/70 mesh 1.2

T h e r a n g e of e x p e r i m e n t a l c o n d i t i o n s c o v e r e d w a s : reactor temperature catalyst o-xylene concentration o-xylene conversions oxygen concentration

330°-390°C 1.0 g r a m 1-3% 1-100% 10-30%

Initially the reactor was instrumented internally w i t h thermocouples i n serted at a b o u t 1-cm intervals over its l e n g t h . E v e n at 1 0 0 % c o n v e r s i o n t h e observed m a x i m u m variation i n temperature never exceeded 2°C. Blank reaction was negligible at a l l temperatures used. If silver solder was i n contact w i t h t h e r e a c t i o n gases, c o n s i d e r a b l e o r g a n i c m a t e r i a l o x i d i z e d so t h a t o n l y stainless s t e e l w a s a l l o w e d t o c o n t a c t t h e gases. F o r t h i s r e a s o n , t h e i n t e r n a l temperature of the reactor w a s not m e a s u r e d d u r i n g the studies; three t h e r m o couples o n the reactor w a l l w e r e used to m o n i t o r the reaction temperature.



674

CHEMICAL

REACTION

ENGINEERING

II

T o m a i n t a i n c o n s t a n t c a t a l y s t a c t i v i t y , a s m a l l a m o u n t of s u l f u r d i o x i d e gas w a s i n t r o d u c e d w i t h t h e i n c o m i n g gas ( 0 . 0 1 v o l % ) , c o n s i s t e n t w i t h i n d u s trial experience ( 1 7 ) . I n our experience, no appreciable l o n g term catalyst d e c a y c o u l d b e a t t r i b u t e d t h i s effect. T h e c a t a l y s t w a s a l w a y s o x i d i z e d o v e r n i g h t w i t h a i r a n d s u l f u r d i o x i d e b e f o r e a n y e x p e r i m e n t t o m a i n t a i n its f u l l y o x i d i z e d state. G a s s a m p l e s o f a b o u t 5 m l w e r e a n a l y z e d b y gas c h r o m a t o g r a p h y u s i n g a c o m b i n a t i o n of columns c o n t a i n i n g P o r o p a k Q , m o l e c u l a r sieves, a n d S E 5 2 silicone g u m r u b b e r , w i t h temperature p r o g r a m i n g a n d a complicated switching procedure described elsewhere ( 1 8 ) . P e a k areas were measured b y a H e w l e t t - P a c k a r d 3370 Β electronic integrator. N i t r o g e n w a s u s e d as t h e t i e c o m p o n e n t to a l l o w d i r e c t c o m p a r i s o n o f i n l e t a n d o u t l e t compositions. T h e a c c u r a c y of t h e a n a l y t i c a l p r o c e d u r e w a s t e s t e d o v e r o - x y l e n e c o n v e r s i o n s f r o m 1 t o 1 0 0 % a n d f o r s e l e c t i v i t i e s of a l l o t h e r c o m ponents f r o m 0 to 8 5 % . I n general, a l l c a r b o n balances w e r e w i t h i n 9 6 - 1 0 4 % . L o w e r carbon balances were observed w h e n considerable tar formation o c c u r r e d . P r o b l e m s associated w i t h a c h i e v i n g this a n a l y t i c a l a c c u r a c y are dis cussed elsewhere (18). T h e reactants used were highest p u r i t y oxygen a n d nitrogen f r o m C a n a d i a n L i q u i d A i r , h i g h p u r i t y o-xylene ( E a s t m a n ) , a n d a 0 . 5 0 2 % m i x t u r e of S 0 in nitrogen (Matheson). P a c k e d b e d studies w e r e done (19) w i t h v a n a d i a catalysts i n i n d u s t r i a l u s e . T h e i r a p p r o x i m a t e c o m p o s i t i o n a n d p r o p e r t i e s a r e s h o w n i n T a b l e I , as supplied by W . R. Grace Co. 2

100 T I M E O F Figure 3.

200 R E A C T I O N

300 ( S E C )

Approach to steady-state operation of packed bed

E f f l u e n t gases w e r e a n a l y z e d at d i f f e r e n t t i m e s f r o m t h e start o f r e a c t a n t flows to the reactor i n o r d e r to m o n i t o r catalyst d e c a y a n d p e r f o r m a n c e w i t h time. Because the analysis took approximately 35 minutes, the catalyst was g e n e r a l l y r e a c t i v a t e d f o r at l e a s t 1 h r a t r e a c t i o n t e m p e r a t u r e . T h i s p r o c e d u r e g a v e r e p r o d u c i b l e r e s u l t s . A t r e a c t i o n t i m e s less t h a n a b o u t 2 0 sec, a m a s s


51.

WAINWRIGHT

AND

Transported

H O F F M A N

ι Ti0

2

ι

Reactor

675

r

CATALYST 200

T O T A L GAS FLOW WEIGHT OF C A T A L Y S T

08

Bed

CC/MIN.

10

1 0 GM-

TEMPERATURE

3 7 0

ο ο

0-4

φ

υ ΰ to

ANHYDRIDE

PHTHALIDE 0-TOLUALDEHYDE CO CO

STEADY

t

STATE_

CONVERSION

"

0315

0-2

01

0 0 40

80 TIME

Figure

4.

120

(SEC)

Initial rates and selectivities of catalyst

TiOi-supported

b a l a n c e w a s o f t e a n o t a c h i e v e d ; t h e b a l a n c e w a s p o o r e r as r e a c t i o n t i m e d e c r e a s e d , p r o b a b l y b e c a u s e of a d s o r p t i o n a n d flow effects. Results. S i n c e t h e r e s u l t s o f t h i s p r o g r a m a l o n g w i t h t h a t w i t h o t h e r catalysts are r e p o r t e d i n great d e t a i l elsewhere ( 1 5 ) , o n l y those results salient to the t r a n s p o r t e d b e d are i n d i c a t e d here. A c o m p l e t e t w o - l e v e l f a c t o r i a l d e s i g n was conducted on the 902 catalyst. I n a d d i t i o n , eight center-point experiments w e r e d o n e b e t w e e n e a c h e x p e r i m e n t to m o n i t o r l o n g t e r m c a t a l y s t effects. R e p l i c a t i o n of f o u r o p e r a t i n g c o n d i t i o n s p r o v i d e d v a r i a n c e e s t i m a t e s o v e r a w i d e range of conversions. Unsteady-State Performance. A t y p i c a l p l o t of o - x y l e n e c o n v e r s i o n w i t h t i m e is s h o w n i n F i g u r e 3 for this catalyst. I n i t i a l conversions of o-xylene, a n d h e n c e r e a c t i o n rates, are a p p r o x i m a t e l y 3 0 to 4 0 times those o b t a i n e d after t h e c a t a l y s t a c h i e v e s s t e a d y - s t a t e a c t i v i t y . O x y g e n i n t h e f e e d gas d o e s affect t h e c o n v e r s i o n , b u t t h i s effect b e c o m e s less i m p o r t a n t at s h o r t r e a c t i o n t i m e s . T h e r e is a s u g g e s t i o n t h a t a t z e r o t i m e t h e effect o f o x y g e n c o n c e n t r a t i o n d i s a p p e a r s , t h u s s u g g e s t i n g t h a t a m o d e l w i t h θ = 1 is r e a s o n a b l e . I n a d d i t i o n , i t has b e e n s h o w n h e r e a n d b y H e r t e n a n d F r o m e n t ( 5 ) t h a t selectivities to p a r t i a l o x i d a t i o n p r o d u c t s i m p r o v e w i t h decreasing temperature. T h i s is i m p o r t a n t s i n c e a p p r e c i a b l e r e a c t i o n rates c a n b e a c h i e v e d a t t e m p e r a t u r e s m u c h l o w e r t h a n those u s u a l l y e m p l o y e d i n d u s t r i a l l y .


676

CHEMICAL

REACTION ENGINEERING

II

-2


10

-4

10

Ο

1-4

1-6

o-i

3

Ι_ι_ 1-8

2 0

( T K).I0

Figure 5. Rates of xylene congersion for different catalysts at .01 atm o-xylene and 0.21 aim oxygen pressures A after 10 sec contact on 902 cat alyst in packed bed Β transported bed data for 902 catalyst C steady-state packed bed data of Herten and Froment (5) D steady-state packed bed data on 902 catalyst Ε steady-state packed bed data of Juusola (6)

A h i g h l y o x i d i z e d c a t a l y s t also p r o d u c e s g r e a t e r s e l e c t i v i t y t o p a r t i a l oxidation products. P h t h a l i c a n h y d r i d e is m o r e e a s i l y o x i d i z e d to m a l e i c a n h y d r i d e w h e n o x y g e n l e a n m i x t u r e s a r e u s e d (20). T h i s is t h o u g h t to b e t h e r e s u l t of t h e h i g h r a t i o o f V 0 / V 0 r ; e x i s t i n g i n t h e c a t a l y s t . T h u s , t h e i n i t i a l s e l e c t i v i t i e s s h o u l d b e h i g h e r t h a n t h o s e o b t a i n e d i n t h e s t e a d y s t a t e ; t h i s effect is s h o w n i n F i g u r e 4. 2

4

2

Steady-State Performance. W a i n w r i g h t a n d H o f f m a n (15) show that the redox reaction m o d e l , w i t h suitable parameter estimates, predicts the reaction p r o d u c t d i s t r i b u t i o n q u i t e w e l l o v e r t h e r a n g e of o p e r a t i n g c o n d i t i o n s u s e d here a n d thus provides support for this m e c h a n i s m . F i g u r e 5 summarizes the steady-state reaction rate d a t a for the 9 0 2 catalyst. T h e p a c k e d - b e d e x p e r i m e n t s are a n e x c e l l e n t b a s e f o r d i s c u s s i n g t h e p e r f o r m a n c e of t h e t r a n s p o r t e d b e d reactor ( b e l o w ) .

Transported Bed Studies I n a t r a n s p o r t e d b e d r e a c t o r , t h e r e a c t a n t a n d p r o d u c t gases p n e u m a t i c a l l y c o n v e y t h e p a r t i c l e s u s e d to c a t a l y z e t h e r e a c t i o n i n a v e r t i c a l p i p e . The o - x y l e n e o x i d a t i o n , c a t a l y z e d b y v a n a d i a , w a s u s e d to d e t e r m i n e t h e o p e r a t i n g c h a r a c t e r i s t i c s a n d to e v a l u a t e s u c h a r e a c t o r f o r o x i d a t i o n r e a c t i o n s . P r e l i m i n a r y design consideration based on meager literature information suggested the f o l l o w i n g criteria for a pilot plant u n i t : ( a ) T o a c h i e v e s m o o t h o p e r a t i o n , c h a r a c t e r i s t i c of n o n - c h o k i n g s o l i d s - g a s flow, s o l i d l o a d i n g s w o u l d h a v e to b e l o w — i . e . , d i l u t e p h a s e t r a n s p o r t w o u l d p r e v a i l a n d voidages w o u l d be greater t h a n 9 7 % . T h u s , v e r y little catalyst w o u l d b e i n c o n t a c t w i t h t h e r e a c t i n g gases. ( b ) S u p e r f i c i a l gas v e l o c i t i e s w o u l d h a v e t o b e g r e a t e r t h a n 2 0 f t / s e c to c o n v e y the particles. T h i s , c o u p l e d w i t h the l o w solids h o l d - u p , suggested that t h e v e r t i c a l p i p e w o u l d h a v e t o b e f a i r l y l o n g to a c h i e v e r e a s o n a b l e c o n t a c t times. (c) T o achieve near t u r b u l e n t conditions i n the c o n v e y i n g system to p r o v i d e s o m e m i x i n g of t h e gas, t h e c o n v e y i n g p i p e d i a m e t e r w o u l d h a v e to b e about % inch.


51.

WAINWRIGHT

AND

H O F F M A N

Transported

Bed

Reactor

677

( d ) T o a c h i v e f u l l c o n t r o l of t h e s o l i d s flow r a t e t o t h e s y s t e m , s o m e f o r m of f o r c e d s o l i d s f e e d i n g w o u l d b e n e e d e d . (e) Solids l o a d i n g s h o u l d be h i g h e n o u g h to p r o v i d e a g o o d sink for the heat generated b y the reaction a n d thus control the reaction temperature over the reactor length.


(f) Solids f e e d rate a n d solids h o l d - u p measurements evaluate reactor performance.

are necessary

to

T h e s e criteria w e r e u s e d to d e s i g n the a p p a r a t u s u s e d here. A p p a r a t u s . T h e a p p a r a t u s is s h o w n i n F i g u r e 6. T h e s o l i d s c i r c u i t is a solids hold-tank, a solids feed-control tank, the entry section, the v e r t i c a l reactor, a cyclone separator, a n d a solids receiver tank. T h e t w o u p p e r tanks a r e 1 5 i n c h e s i n d i a m e t e r , a n d t h e s o l i d s f e e d c o n t r o l t a n k is 6 i n c h e s i n d i a m e t e r ; a l l are h e a t e d to a n y d e s i r e d t e m p e r a t u r e b y l o w v o l t a g e , h i g h c u r r e n t K a n t h a l s t r i p h e a t e r s i m b e d d e d i n H i l o s e t c e m e n t a r o u n d t h e t a n k s . A i r is s u p p l i e d f r o m t h e 1 0 0 p s i g m a i n s , filtered, a n d p a s s e d t h r o u g h a s o n i c o r i f i c e w h o s e u p s t r e a m p r e s s u r e is c o n t r o l l e d b y a c o n t r o l l e r / p n e u m a t i c v a l v e . S o m e of t h e gas is s p l i t to p r o v i d e t h e d r i v i n g gas f o r t h e s o l i d s f e e d s y s t e m ; t h e r e m a i n d e r is h e a t e d i n a s t e a m h e a t e r a n d e l e c t r i c a l l y h e a t e d s y s t e m a n d t h e n p a s s e d t h r o u g h a s h o r t h o r i z o n t a l s e c t i o n of p i p e w h e r e t h e s o l i d s a r e d r o p p e d i n t o t h e gas. T h e y a r e c o n v e y e d a b o u t 8 i n c h e s a n d t h e n t r a n s p o r t e d a r o u n d a 9 0 ° b e n d t o a v e r t i c a l % - i n c h o d b y 0 . 6 8 - i n c h i d stainless s t e e l t u b e 2 7 f t l o n g . D u r i n g a n y r u n , t h e s o l i d s a n d gas t e m p e r a t u r e s w e r e c o n t r o l l e d t o b e at e s s e n t i a l l y t h e s a m e t e m p e r a t u r e at t h e i n l e t . E l e c t r i c a l h e a t e r s o n t h e reactor m a i n t a i n e d isothermal operation. A 90° b e n d a n d an 8-inch horizontal s e c t i o n of t u b i n g a n d e x p a n s i o n b e l l o w s c o n n e c t t h e r e a c t o r t o t h e c y c l o n e s e p a r a t o r . T h e gas l e a v i n g t h e c y c l o n e is c o n v e y e d t o a n o t h e r c y c l o n e a n d a s p r a y s c r u b b i n g s e c t i o n b e f o r e i t is d i s c h a r g e d . T h e p r i m a r y c y c l o n e s e p a r a t e d m o s t of t h e s o l i d s so t h a t s o l i d s loss w a s s m a l l . T h e s o l i d s r e c e i v e r h a d a 2 - i n c h b a l l v a l v e o n its b o t t o m t o c o l l e c t s o l i d s . T h e s e w e r e d i s c h a r g e d b a c k i n t o t h e s o l i d s f e e d t a n k at t h e e n d of a r u n ; t h u s , t h e s y s t e m w a s o p e r a t e d b a t c h w i s e . A s o l i d s i n v e n t o r y o f a p p r o x i m a t e l y 3 0 0 - 3 5 0 l b s a l l o w e d a t least a 1 0 - m i n r u n at t h e h i g h e s t s o l i d s f e e d r a t e . T h e s o l i d s r e c e i v e r w a s c o n n e c t e d t o t h e s o l i d s f e e d t a n k b y a 2 - i n c h s t a i n l e s s steel b e l l o w s . T h e r e c e i v e r is h e l d b y a y o k e a r r a n g e m e n t c o n n e c t e d t o a c a l i b r a t e d s t r a i n - g a g e w e i g h s y s t e m so t h e receiver c o u l d be w e i g h e d continuously. A % - i n c h d i a m e t e r orifice w a s inserted o n t h e b o t t o m of t h e s o l i d s f e e d c o n t r o l t a n k . T h e s o l i d s l e v e l h e r e w a s c o n trolled b y measuring it b y a capacitance probe (Drexelbrook E n g . C o . , G l e n s i d e , P a . ) w h i c h c o n t r o l l e d t h e o p e n - c l o s e o p e r a t i o n of a 2 - i n c h b u t t e r f l y v a l v e o n t h e b o t t o m of t h e t a n k . A i r p r e s s u r e i n b o t h t a n k s w a s c o n t r o l l e d b y p n e u m a t i c v a l v e s o n t h e a i r l i n e s . U p s t r e a m p r e s s u r e o n these v a l v e s w a s k e p t c o n s t a n t b y r e s t r i c t i n g t h e m a i n a i r flow b y a m a n u a l l y c o n t r o l l e d v a l v e . T h e s y s t e m p r o v i d e d e x c e l l e n t c o n t r o l as e v i d e n c e d b y t h e r a t e o f w e i g h t i n c r e a s e of t h e s o l i d s r e c e i v e r , a n d i t a l l o w e d a f a i r l y w i d e r a n g e o f s o l i d s f e e d rates to b e a c h i e v e d . o - X y l e n e w a s f e d as l i q u i d f r o m a n i t r o g e n p r e s s u r i z e d f e e d t a n k ; its flow rate w a s c o n t r o l l e d b y a needle v a l v e a n d m e t e r e d b y a R o t a m e t e r . A s t e a m h e a t e d e x c h a n g e r v a p o r i z e d t h e o - x y l e n e just p r i o r t o e n t e r i n g t h e r e a c t o r , a n d this v a p o r w a s f e d t h r o u g h a V s - i n c h t u b e to m i x w i t h the a i r - s o l i d m i x t u r e about 2 inches b e y o n d the elbow. O r i g i n a l l y , o-xylene was v a p o r i z e d i n a c a r b u r e t o r u p s t r e a m of t h e s o l i d s a d d i t i o n p o i n t , b u t s i n c e a p p r e c i a b l e r e a c t i o n o c c u r r e d i n t h e h o r i z o n t a l s e c t i o n p r i o r to t h e v e r t i c a l r e a c t o r , t h e f e e d p o i n t l o c a t i o n w a s c h a n g e d . I n one e x p e r i m e n t o-xylene w a s a d m i t t e d 7 ft a b o v e the elbow. T h r e e a i r - a c t i v a t e d , q u i c k s h u t - o f f b a l l v a l v e s w e r e i n s t a l l e d at 9, 1 5 8 , a n d 281 inches f r o m the reactor bottom. G a s samples were r e m o v e d , a n d pressure was m e a s u r e d b y m e r c u r y m a n o m e t e r s near these valves. G a s samples w e r e


678

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ENGINEERING

II

c o l l e c t e d t h r o u g h p o r o u s stainless s t e e l filters i n t o p r e - e v a c u a t e d b o t t l e s (ca. 1 0 0 0 c c ) . T h e l i n e s to these b o t t l e s w e r e s t e a m t r a c e d ( 1 0 0 - p s i g ) t o p r e v e n t c o n d e n s a t i o n . A l l l i n e s a n d filters w e r e flushed w i t h h e l i u m j u s t b e f o r e s a m p l i n g . G a s samples w e r e d r a w n t h r o u g h the s a m p l i n g v a l v e a n d a n a l y z e d b y the chromatographic technique described. S u l f u r d i o x i d e w a s f e d t o m a i n t a i n t h e s a m e c o n c e n t r a t i o n l e v e l as i n t h e p a c k e d b e d experiments. Before a r u n , the solids w e r e c i r c u l a t e d t h r o u g h the system w i t h air a n d S 0 to ensure that the catalyst w a s f u l l y regenerated. A n o r i f i c e o n t h e e x i t l i n e i n d i c a t e d c o n s t a n t gas flow r a t e a n d p r o v i d e d a m a t e r i a l b a l a n c e c h e c k o n t h e a i r flow r a t e . T h e c a t a l y s t u s e d w a s N o . 9 0 2 ( W . R . G r a c e ) , t h e s a m e as i n t h e p a c k e d b e d e x p e r i m e n t s . T h e m e a n p a r t i c l e s i z e w a s 125μ a n d its d e n s i t y w a s d e t e r m i n e d as 0 . 9 5 g r a m / c c . T h e e x p e r i m e n t a l c o n d i t i o n s are s h o w n i n T a b l e I I . N o s i g n i f i c a n t h o m o g e n e o u s r e a c t i o n o c c u r r e d i n t h e r e a c t o r at t e m p e r a t u r e s a b o v e 4 0 0 ° C o r i n t h e s a m p l e b o t t l e s at t h e i r o p e r a t i n g t e m p e r a t u r e o f ca. 2 0 0 ° C .


2

Figure 6. Flowsheet of transported bed apparatus AB AN AV Β C CA CP D F FC FT G GC H HT LC Ν Ο OX Ρ PC R SB SG SO ST Τ TV V WT W

air-operated butter fly valve air-operated needle valve air-operated ball valve stainless steel bellows cyclone carburetor capacitance probe water overflow filter catalyst fines feed tank sight ghss filtered sample to gas chromatograph preheater catalyst hold tank on-off level con troller manually-operated needle valve orifice xylene feed tank pressure tap proportional pres sure controller Rotameter support beam strain gage weigh ing device sonic orifice scrubbing tower thermocouple three-way valve manually-operated ball valve high pressure water weighing tank


51.

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T a b l e II.

AND

H O F F M A N

Bed

Reactor

679

Experimental Conditions for the Transported B e d Reactor

Temperature Solids flow rate Air flow rate o-Xylene concentration W/F ao


Transported

233°-342 °C 35 lb/min. 2.0-3.5 S C F M 1-3 mole % 63-350 grams hr/gram mole

T h e p r o b l e m of i n t e r p r e t i n g r e s u l t s f r o m a c h e m i c a l r e a c t o r r e s o l v e s i t s e l f i n t o t w o s e p a r a t e p r o b l e m s . F i r s t , t h e fluid m e c h a n i c a l b e h a v i o r o f t h e r e a c tants a n d catalyst m u s t be established a n d (perhaps) d e s c r i b e d m a t h e m a t i c a l l y . S e c o n d , s o m e u n d e r s t a n d i n g of t h e c h e m i c a l b e h a v i o r a n d d e s c r i p t i o n of t h e chemical kinetics should be obtained. T h e p a c k e d b e d study p r o v i d e d the c h e m i c a l u n d e r s t a n d i n g a n d s o m e i n d i c a t i o n of t h e m o d e l i n g t o d e s c r i b e t h e k i n e t i c s . S i n c e t h e r e is l i t t l e i n f o r m a t i o n o n s o l i d s - g a s flow, p a r t i c u l a r l y a t h i g h solids l o a d i n g , it was i m p o r t a n t to h a v e e x p e r i m e n t a l i n f o r m a t i o n relating to t h e i r fluid m e c h a n i c a l b e h a v i o r u n d e r r e a c t o r c o n d i t i o n s (see b e l o w ) .

Figure 7. Fressure drop per unit length of reactor as a function of gas velocity at con stant solids flow rate. V„ superficial gas ve locity. ΔΡ total pressure drop. ΔΡ, pressure drop caused by solids. &P„ frictional loss. Γ

S o l i d - G a s F l o w Experiments. Z e n z a n d O t h m e r (21) suggest that the flow c h a r a c t e r i s t i c s of a g a s - s o l i d s c o n v e y i n g s y s t e m c a n b e i n t e r p r e t e d f r o m p r e s s u r e d r o p o b s e r v a t i o n s as a f u n c t i o n o f s o l i d s a n d gas flow r a t e . We o b t a i n e d t h i s i n f o r m a t i o n at r e a c t i o n t e m p e r a t u r e s , a n d a t y p i c a l p l o t is s h o w n i n F i g u r e 7 f o r t h e u p p e r s e c t i o n o f t h e r e a c t o r . H e r e , t h e s o l i d s flow r a t e w a s


680

CHEMICAL

REACTION

ENGINEERING

II

0 4 r

0 3 U - € ) -


0 2 -

0 1 -

• MEASURED [ ο CALCULATED | W

Q 0 10

S

= 3 2 L B / M I N

Τ = 2 2 0 I

2 0

3 0

U

g

C ι

4 0

FT/SEC

Figure 8. Solids loading as a function of super ficial gas velocity at constant solids flow rate h e l d constant b y j u d i c i o u s c h o i c e of a i r pressure i n t h e f e e d t a n k , a n d t h e gas flow w a s v a r i e d . S i n c e t h e s o l i d s h o l d - u p i n t h e 1 0 - f t s e c t i o n o f t u b i n g w a s m e a s u r e d directly, the static pressure caused b y the solids-gas m i x t u r e c o u l d be evaluated. T h e frictional pressure d r o p w a s obtained b y difference. T h e p r e s s u r e d r o p b e h a v i o r i n F i g u r e 7 is s i m i l a r t o t h a t r e p o r t e d b y Z e n z a n d O t h m e r ( 2 1 ) , b u t t h e p r e s s u r e d r o p fluctuations r e c o r d e d o v e r t h e r a n g e o f gas flows a r e n o t . A t g a s flow rates b e l o w t h a t w h e r e t h e m i n i m u m p r e s s u r e d r o p is r e c o r d e d , t h e s y s t e m is e x p e c t e d t o b e i n c h o k e d flow a n d e x h i b i t characteristic large pressure fluctuations. These were observed i n the region s h o w n o n F i g u r e 7—i.e., n e a r t h e m i n i m u m p r e s s u r e d r o p ; h o w e v e r , a t l o w e r gas flow r a t e s , t h e s e fluctuations b e c a m e s m a l l , e v e n less t h a n t h o s e a t t h e h i g h g a s flow r a t e s , a n d t h e s y s t e m w a s r e m a r k a b l y s t a b l e . F i g u r e 8 shows the fraction of solids i n t h e tube c o r r e s p o n d i n g to t h e e x p e r i m e n t s i n d i c a t e d i n F i g u r e 7. T h o s e c a l c u l a t e d v o i d a g e s c o r r e s p o n d i n g t o h o m o g e n e o u s t w o - p h a s e flow a r e also s h o w n f o r c o m p a r i s o n . T h e r e g i o n o f l a r g e p r e s s u r e fluctuations is i n d i c a t e d . I f w e a s s u m e t h a t t h e p r e s s u r e fluctua t i o n s i n d i c a t e s l u g flow, t h e n f o r W = 3 2 l b / m i n a n d b e l o w a s u p e r f i c i a l g a s v e l o c i t y o f a b o u t 2 8 f t / s e c , t h e s o l i d - g a s s u s p e n s i o n flows as a h o m o g e n e o u s mass similar to particulate fluidization b u t w i t h a s o l i d s v e l o c i t y m u c h less t h a n t h a t o f t h e gas. T h i s flow p h e n o m e n o n is d i f f i c u l t t o e x p l a i n ; i f t h e p a r t i c l e s are f r e e l y s u s p e n d e d a n d i n p a r t i c u l a t e flow, t h e d r a g coefficient o n e a c h p a r ticle w o u l d have to b e r e d u c e d b y orders of m a g n i t u d e to achieve s u c h h i g h p a r t i c l e - g a s s l i p v e l o c i t i e s . A t t h e s a m e t i m e , t h e v o i d a g e s a r e so h i g h t h a t particles m u s t b e freely s u s p e n d e d a n d n o t m o v i n g t h r o u g h t h e b e d (as a m o v i n g fixed b e d ) as m i g h t b e e x p e c t e d i f t h e e x i t l i n e w e r e r e s t r i c t i n g t h e flow of p a r t i c l e s b u t n o t t h e gas. I f , h o w e v e r , t h e p a r t i c l e s f o r m e d c o n c e n t r a t e d c l o u d s o r a g g l o m e r a t e s w h i c h c a n b r e a k a n d r e f o r m i n t h e r e a c t o r via t h e w a k e s


51.

WAINWRIGHT

mechanism

(21),

Transported

AND HOFFMAN

Bed

681

Reactor

t h e n larger s l i p velocities are possible since this

agglomerate

h a s a t e r m i n a l v e l o c i t y d e t e r m i n e d b y its d i a m e t e r a n d a p p a r e n t s o l i d s d e n s i t y . Another which

explanation

a slow

might be

moving

the f o r m a t i o n

concentrated

particle

of

an annular

mass

flowed

as

flow an

regime outer

a r o u n d a c e n t r a l c o r e of g a s - s o M d s u s p e n s i o n i n d i l u t e p h a s e t r a n s p o r t . knowledge

t h i s is t h e first r e p o r t i n g of t h i s s o l i d s - g a s

h i g h solids-to-gas

flow

in ring

To

our

behavior with such

loadings.

O p e r a t i o n of t h e t r a n s p o r t e d b e d r e a c t o r w a s r e s t r i c t e d t o t h e h i g h s o l i d s f r a c t i o n r e g i o n b e l o w t h e o n s e t of l a r g e p r e s s u r e

fluctuations.

F o r lack of

better

i n f o r m a t i o n , w e a s s u m e d t h a t t h e gas a n d s o l i d s a r e i n i n t i m a t e c o n t a c t t h r o u g h out

the

reactor

length;

assumed negligible.


mechanics

thus,

by-passing

is n o w u n d e r s t u d y .

ditions were

not

of

gas

by

bubble

formation

T h i s is a w e a k n e s s i n o u r a n a l y s i s , a n d t h e s o l i d - g a s

excessive.

c a t a l y s t as s u p p l i e d .

C a t a l y s t losses f r o m a t t r i t i o n u n d e r t h e s e

Some

fine

material was

lost

initially

from

was fluid con the

L i k e w i s e , erosion of the reactor t u b e d i d not o c c u r .

9 0 2

C A T A L Y S T

T R A N S P O R T E D

0- TOLL)ALDEHYDE

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R E A C T O R

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ο

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0 8

φ φ

0 4

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Figure 9.

φ

^•j.or^oooro-cpo-o-oo

0 0 1

0

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2 3

I 0Ι _ 4 O - X Y L E N E

ι

0I -ι6 _ 0 - 8 C O N V E R S I O N

Selectivity data from transported bed reactor

C h e m i c a l R e a c t i o n . F i g u r e 9 s h o w s s e l e c t i v i t i e s at ca. 3 0 0 ° a n d 3 4 0 ° C . S e l e c t i v i t y t o t h e m a i n p a r t i a l o x i d a t i o n p r o d u c t o t o l u a l d e h y d e is v e r y h i g h . I n d u s t r i a l r e a c t o r s r a r e l y e x c e e d 7 0 % . T h e l o w y i e l d s of p h t h a l i c a n h y d r i d e are c a u s e d b y t h e c a t a l y s t u s e d . W a i n w r i g h t a n d H o f f m a n (15) found that silica gel-supported catalysts have p o o r selectivity for p h a t h a l i c a n h y d r i d e production. T h i s accounts for the lack of success i n o x i d i z i n g o-xylene i n fluidized beds. A s i l i c a - s u p p o r t e d catalyst w a s u s e d since n o catalyst w i t h the desired selectivity a n d fluidization c h a r a c t e r i s t i c s is a v a i l a b l e i n q u a n t i t i e s sufficient for a p i l o t - p l a n t s t u d y . F i g u r e 9 also shows t h a t the amounts of over-oxidation products—carbon dioxide and carbon monoxide—are doubled


682

CHEMICAL

b y a 4 0 ° C temperature change.

REACTION

ENGINEERING

II

Therefore, the reactions s h o u l d b e done at l o w

temperature for h i g h yields of partial oxidation products.

Selectivities for other

r e a c t i o n p r o d u c t s — p h t h a l i c a n h y d r i d e , p h t h a l i d e , m a l e i c a n h y d r i d e , a n d ot o l u i c a c i d — a r e n o t i n c l u d e d i n F i g u r e 9 since these w e r e present i n s m a l l amounts. A n a l y s i s o f R e s u l t s . Because of t h e gas analysis a n d solids h o l d - u p meas u r e m e n t s , t h e reactor w a s a n a l y z e d i n t w o sections.

G a s a n a l y s i s a t t h e first

sample port gave the feed composition for the lower section.

T h e composition

of t h e f e e d f o r t h e u p p e r s e c t i o n w a s t h a t o b t a i n e d f r o m m i d - r e a c t o r s a m p l e s . T h e d a t a w e r e a n a l y z e d as i f t h e y c a m e f r o m t w o s e p a r a t e r e a c t o r s .

Since no

significant difference c o u l d b e detected b e t w e e n the p e r f o r m a n c e of these t w o reactors, t h e results are presented f o r t h e o v e r a l l reactor.


T h e data were

analyzed according to a simple parallel m o d e l i n w h i c h

o-xylene reacts to o - t o l u a l d e h y d e

a n d c a r b o n oxides.

T h i s is r e a s o n a b l e

since

f e w other products were detected, consistent w i t h the p a c k e d b e d study w i t h this catalyst. 100

χ ω 10 LU

cr

•

O-XYLENE FED 7 FT-FROM ENTRANCE

16

_L_

IT

1-8

(T Figure

.

1-9 —I

3

10. Arrhenius plot for k values ob tained in transported bed reactor r

T h e disappearance of o-xylene w a s m o d e l e d b y a r that is, assuming θ =

20

K ) . 10

1 i n a l l cases.

r

first-order

reaction:

= hCn

(6)

V a l u e s o f k a r e p l o t t e d vs. l/T r

in Figure

1 0 . A n A r r h e n i u s - t y p e t e m p e r a t u r e d e p e n d e n c e is o b e y e d b e l o w 3 0 0 ° C , w i t h a n activation produce

energy

of

16,700

cal/gram

mole.

o n l y s m a l l increases i n reaction rate

t h i r d of that at lower

H i g h e r reaction

temperatures

(activation energies

at least a

temperatures), except for one experiment where the

solids l o a d i n g i n the reactor w a s quite h i g h .

T h e f o l l o w i n g analysis attempts


51.

WAINWRIGHT

AND

H O F F M A N

Transported

Bed

Reactor

683

to e x p l a i n these o b s e r v a t i o n s a n d t o i n d i c a t e t h e v a r i o u s effects w h i c h m u s t be considered i n transported b e d reactors. MASS TRANSFER LIMITED REACTION. T h e mass transfer rate m a y be l i m i t i n g t h e r e a c t i o n r a t e at t h e h i g h e r t e m p e r a t u r e s . T h e c a l c u l a t e d m a s s t r a n s f e r r a t e is at least t w o o r d e r s of m a g n i t u d e l a r g e r t h a n t h e r e a c t i o n r a t e ; m o r e o v e r , t h e m e a s u r e d c o n v e r s i o n s are a l m o s t t h e s a m e f o r s o m e of t h e l o w e r t e m p e r a t u r e r u n s as t h o s e at t h e h i g h t e m p e r a t u r e , t h u s i n d i c a t i n g a l m o s t t h e s a m e r a t e of r e a c t i o n . Loss O F C A T A L Y T I C A C T I V I T Y A T T H E H I G H E R T E M P E R A T U R E S . Perhaps c a t a l y s t a c t i v i t y is r e d u c e d at h i g h e r t e m p e r a t u r e s . T h e l o n g t e r m a c t i v i t y d i d n o t c h a n g e s i n c e t h e e x p e r i m e n t s ca. 3 0 0 ° C ( 1 / T = 1.75 Χ 1 0 " ) w e r e d o n e a f t e r e a c h e x p e r i m e n t at o t h e r c o n d i t i o n s t o m o n i t o r c a t a l y s t a c t i v i t y . N o s i g n i f i c a n t o r s y s t e m a t i c v a r i a t i o n is o b s e r v e d . S h o r t - t e r m a c t i v i t y r e l a t e s to t h e a s s u m p t i o n t h a t 0 = 1. I f t h e o x y g e n c o n s u m e d p e r g r a m of c a t a l y s t w a s g r e a t e r at h i g h e r t e m p e r a t u r e s t h a n at l o w e r o n e s , a d e c r e a s e i n a c t i v i t y m i g h t be s u g g e s t e d . A g a i n t h e r e is n o c o r r e l a t i o n of t h e d e v i a t i o n w i t h t h i s r a t i o . F u r t h e r , t h e r e a c t i o n r a t e c o n s t a n t s are e s s e n t i a l l y t h e s a m e f o r b o t h r e a c t o r s e c t i o n s . I t m i g h t b e a r g u e d t h a t less o f t h e c a t a l y s t c o n t a c t s t h e r e a c t i o n gases i n t h e l o w e r s e c t i o n b e c a u s e of t h e r e a c t o r l e n g t h r e q u i r e d t o disperse the o-xylene over the entire reactor cross-section. It seems i m p r o b a b l e t h a t t h i s effect w o u l d p r o d u c e t h e s a m e r e s u l t f o r a l l t h e e x p e r i m e n t s c o n s i d e r i n g t h e r a n g e of o p e r a t i n g v a r i a b l e s .


3

T h e e x t r e m e l y h i g h r e a c t i o n rates i n t h e t r a n s p o r t e d a n d p a c k e d b e d r e a c t o r s at s h o r t o n - s t r e a m t i m e s s u g g e s t p o s s i b l e d i f f e r e n t t y p e s of a c t i v e o x y g e n — i . e . , o x y g e n i n t h e s u r f a c e l a y e r s of t h e c a t a l y s t l a t t i c e a n d o x y g e n w h i c h is c h e m i s o r b e d t o t h e c a t a l y s t s u r f a c e . K a k i n o k i et al. (11) h a v e s h o w n , b y e x p e r i m e n t s i n a B E T a p p a r a t u s a t 4 0 0 ° C , t h a t 2 c c of o x y g e n c a n b e a d s o r b e d o n 1 g r a m of a s i m i l a r c a t a l y s t . E v e n m o r e o x y g e n is e x p e c t e d t o b e a d s o r b e d at l o w e r t e m p e r a t u r e s . T h e r e g e n e r a t i o n stage i n t h e t r a n s p o r t e d b e d reactor not only reoxidizes the catalyst b u t replenishes the chemisorbed o x y g e n . T h e a m o u n t of c h e m i s o r b e d o x y g e n w i l l b e r e d u c e d at t h e h i g h e r temperatures, a n d this c o u l d explain the r e d u c e d activity. T h i s m e c h a n i s m , h o w e v e r , s h o u l d p r o d u c e a g r a d u a l c h a n g e of a c t i v i t y w i t h t e m p e r a t u r e u n l e s s t h e r e are d i f f e r e n t t y p e s of c h e m i s o r b e d o x y g e n a b o v e a n d b e l o w 3 0 0 ° C . S u c h differences have b e e n detected i n other catalyst systems. ADSORPTION OF HYDROCARBON ON T H E CATALYST. T h e m o d e l assumes that t h e r a t e o f a d s o r p t i o n is i n s t a n t a n e o u s — i . e . , i t r e a c h e s its e q u i l i b r i u m c o n c e n tration o n the catalyst surface w i t h i n a short reactor l e n g t h a n d hence does not i n f l u e n c e t h e r e a c t i o n r a t e . I t is also a s s u m e d t h a t t h e a m o u n t o f r e a c t a n t a d s o r b e d is s m a l l r e l a t i v e t o t h a t i n t h e gas p h a s e . I f t h i s w e r e a n i m p o r t a n t effect, p e r f o r m a n c e w o u l d c o r r e l a t e s t r o n g l y w i t h s o l i d s h o l d - u p ; t h i s w a s n o t o b s e r v e d . A l s o , a d s o r p t i o n is a s s u m e d to b e n o n - s e l e c t i v e — i . e . , t h e r a t i o of c o n c e n t r a t i o n s of a d s o r b e d species is t h e s a m e as i n t h e gas p h a s e .

T h e s e a d s o r p t i o n effects c o u l d b e i m p o r t a n t i n t r a n s p o r t e d b e d r e a c t o r s s i n c e t h e y d i f f e r i n t h i s r e s p e c t f r o m n o r m a l r e a c t o r s w h i c h c o n t a i n a fixed c a t a l y s t c h a r g e w h i c h h a s t i m e to e q u i l i b r a t e w i t h t h e s u r r o u n d i n g gas. H o w ever, this adsorption p h e n o m e n a cannot e x p l a i n the observed v a r i a t i o n w i t h temperature. SOLIDS-GAS CONTACTING. H e r e t h e gas a n d s o l i d s are a s s u m e d t o b e i n t i m a t e l y m i x e d . T h e fluid m e c h a n i c a l b e h a v i o r is a s s u m e d to b e t h e s a m e u n d e r a l l o p e r a t i n g c o n d i t i o n s . O n the o t h e r h a n d , as t h e r e a c t i o n t e m p e r a t u r e


684

CHEMICAL

REACTION

ENGINEERING

II


is i n c r e a s e d , f o r t h e s a m e m a s s flow r a t e o f gases, t h e v e l o c i t y m u s t i n c r e a s e . M o s t o f t h e r u n s at t h e h i g h e r t e m p e r a t u r e s w e r e at a b o u t t h e s a m e m a s s flow rates as those a t t h e l o w e r t e m p e r a t u r e s . I f t h i s i n c r e a s e d gas v e l o c i t y c a u s e d a c h a n g e i n flow b e h a v i o r of t h e s o l i d s - g a s m i x t u r e at j u s t a b o v e 3 0 0 ° C , a n d t h i s i n t u r n c a u s e d p a r t of t h e gas t o b y p a s s t h e c a t a l y s t , a n a p p a r e n t loss i n r e a c t i o n r a t e w o u l d b e o b s e r v e d . S i n c e o p e r a t i o n is so c l o s e to t h e a p p a r e n t s l u g g i n g r e g i m e , this seems to be a l o g i c a l e x p l a n a t i o n for the l o w e r reaction rates. T h e o n e p o i n t w h i c h is m u c h h i g h e r is at a m u c h h i g h e r s o l i d s l o a d i n g a n d l o w e r gas v e l o c i t y , a n d t h e flow r e g i m e u n d e r these c o n d i t i o n s m a y b e s i m i l a r t o t h a t at t h e l o w e r t e m p e r a t u r e s , t h u s s o m e w h a t s u b s t a n t i a t i n g t h i s h y p o t h e s i s . T h u s , o n l y w h e n t h e flow r e g i m e s are p r o p e r l y m a p p e d f o r t h i s reactor, can reactor performance be a n a l y z e d adequately. O v e r a l l R e s u l t s . T h e t r a n s p o r t e d b e d is o n e w a y t o o b t a i n e x t r e m e l y h i g h r e a c t i o n rates f o r o x i d a t i o n s i n v o l v i n g t h e c a t a l y t i c m e c h a n i s m s a s s o c i a t e d w i t h o - x y l e n e o x i d a t i o n . F i g u r e 5 s h o w s t h a t t h e r e a c t i o n rates o b s e r v e d i n t h i s s y s t e m are m u c h h i g h e r t h a n those o b s e r v e d b y o t h e r s f o r s i m i l a r c a t a l y s t s i n fixed b e d s ; t h e y a r e e v e n h i g h e r t h a n t h o s e i n t h e p a c k e d b e d a f t e r i t w a s o n - s t r e a m f o r o n l y 1 0 sec. T h e s e h i g h rates c a n b e e x p l o i t e d b y d o i n g h y d r o c a r b o n oxidations at temperatures l o w e r t h a n n o r m a l l y used. I n this w a y , h i g h e r selectivities to p a r t i a l o x i d a t i o n p r o d u c t s c a n be a c h i e v e d . T h e i n d u s t r i a l potential becomes a n e c o n o m i c tradeoff b e t w e e n the savings i n feedstocks a n d t h e a d d i t i o n a l costs of m o r e c o m p l e x e q u i p m e n t a n d p o s s i b l e c a t a l y s t losses. T h e u n s t e a d y - s t a t e p e r f o r m a n c e of a n o t h e r i n d u s t r i a l c a t a l y s t u s e d t o p r o d u c e p h t h a l i c a n h y d r i d e f r o m o - x y l e n e is s h o w n i n F i g u r e 4. I n i t i a l rates are o n l y s e v e r a l t i m e s g r e a t e r t h a n s t e a d y - s t a t e rates. H o w e v e r , e v e n t h i s i n c r e a s e c o u l d r e s u l t i n t h e s a m e y i e l d as f r o m a fixed b e d at a 4 0 ° C l o w e r r e a c t i o n temperature, thereby i m p r o v i n g selectivity to p h t h a l i c a n h y d r i d e . A c o m p a r i s o n b e t w e e n t h e t w o c a t a l y s t t y p e s is n o t n e c e s s a r y . B o t h s h o w c o n s i d e r a b l y i n c r e a s e d a c t i v i t y i n a h i g h l y o x i d i z e d state. Conclusions T h e a d v a n t a g e s of c o n d u c t i n g o - x y l e n e o x i d a t i o n o n a h i g h l y o x i d i z e d v a n a d i a c a t a l y s t h a v e b e e n d e s c r i b e d , p a r t i c u l a r l y w i t h r e s p e c t to r e a c t i o n rates a n d s e l e c t i v i t i e s . T h e d e n s e - p h a s e c o n v e y i n g of a c o m m e r c i a l fluidization c a t a l y s t is e x t r e m e l y s t a b l e w h e r e s l u g g i n g w o u l d b e e x p e c t e d . T h e o x i d a t i o n of o - x y l e n e i n a t r a n s p o r t e d b e d is w e l l d e s c r i b e d b y a s s u m i n g i t to b e first o r d e r w i t h r e s p e c t to o - x y l e n e c o n c e n t r a t i o n a n d a f u l l y o x i d i z e d c a t a l y s t (Θ = 1). T h i s provides additional support for the redox a n d S S A M models. Acknowledgments T h i s research p r o g r a m has b e e n s u p p o r t e d b y grants f r o m the N a t i o n a l R e s e a r c h C o u n c i l of C a n a d a . C y a n a m i d of C a n a d a p r o v i d e d c a t a l y s t f o r t h e initial studies, a n d W . R . G r a c e s u p p l i e d catalyst samples. C . M . C r o w e pro v i d e d m a n y u s e f u l s u g g e s t i o n s i n t h e e a r l y stages of t h e p r o g r a m . Nomenclature CR> C02

c o n c e n t r a t i o n of h y d r o c a r b o n a n d o x y g e n , r e s p e c t i v e l y , gram-moles/liter


51. F k

A

WAINWRIGHT

o

r

k

&

η ΔΡ , ΔΡ , A P Τ

r

8

F

r

r Si


a

Γ U W W

g

s

Greek Θ ε

A N D H O F F M A N

Transported

Bed Reactor

685

f e e d rate of o-xylene, g r a m - m o l e s / h r r e a c t i o n rate c o n s t a n t f o r t h e h y d r o c a r b o n r e a c t i o n , l i t e r s / ( g r a m catalyst ) ( h r ) reaction rate constant f o r catalyst o x i d a t i o n reaction, liters / ( g r a m catalyst ) ( h r ) stoichiometric coefficient, gram-moles of o x y g e n c o n s u m e d / gram-mole of hydrocarbon reacted total pressure d r o p , hydrostatic pressure of solids, frictional pressure d r o p p e r ft of reactor, respectively, p s i / f t rate of reaction of h y d r o c a r b o n reactant, g r a m - m o l e s / ( g r a m catalyst) ( h r ) rate o f oxidation of catalyst, g r a m m o l e s / ( g r a m catalyst) ( h r ) selectivity of component i , g r a m moles of component i pro d u c e d p e r g r a m moles of o-xylene reacted temperature, °K. superficial velocity of gas, f t / s e c mass of catalyst i n the reactor, g r a m mass of catalyst f e d to the reactor, l b / m i n

Letters f r a c t i o n o f c a t a l y s t s u r f a c e sites i n f u l l y o x i d i z e d state v o i d fraction i n the reactor

Literature Cited 1. Weekman, V. W., Jr., Ind. Eng. Chem., Process Design Develop. (1968 ) 7, 90. 2. Echigoya, E., Yen, S., Morikawa, K., Kagaku Kogaku (1969) 32, 1002-7. 3. de Losa, H., Gau, G., Chem. Eng. Sci. (1973) 28, 1875-84. 4. Novella, E. C., Benllock, A. E., An. Real Soc. Espan. Fis. Quint. Ser. (1962) 783-802. 5. Herten,J.,Froment, G. F., Ind. Eng. Chem., Process Design Develop. (1968) 7, 516-26. 6. Juusola, J. Α., Ph.D. Thesis, Queen's University, Kingston, Canada (1971). 7. Mann, R. F., Ph.D. Thesis, Queen's University, Kingston, Canada (1966). 8. Simard, G. L., Steger, J. F., Arnott, R.J.,Siegel, L. Α., Ind. Eng. Chem. (1955) 47, 1424-30. 9. Ushakova, V. P., Korneichuk, G. P., Zhigailo, Ya. V., Ukrain, Khim. Zh. (1957) 23, 191. 10. Kakinoki, H., Sahara, N., Kamata, I., Aigami, Y., Shokubar. (1962 ) 4, 113. 11. Kakinoki, H., Mizushina, F., Tanaka, T., Aigami, Y., Suzuki, H., Sekiyu, Gakkai, Shi. (1964) 7 (3), 164. 12. Mizushina, F., Tanaka, T., Aigami, Y., Kakinoki, H., Suzuki, H., Sekiyu, Gakkai, Shi. (1964) 7 (11), 30. 13. Suzuki, H., Kakinoki, H., Mizushina, F., Kamata, I., Sekiyu, Gakkai, Shi. (1964) 7 (1), 15. 14. Mars, P., Van Krevelen, D. W., Chem. Eng. Sci. (Spec. Suppl.) (1954) 3, 41-59. 15. Wainwright, M. S., Hoffman, T. W., unpublished data. 16. Shelstad, Κ. Α., Downie,J.,Graydon, W. F., Can. J. Chem. Eng. (1960) 38, 102. 17. Froment, G. F., personal communication (1971). 18. Wainwright, M. S., Hoffman, T. W., unpublished data. 19. Wainwright, M. S., Ph.D. Thesis, McMaster University, Hamilton, Canada (1974). 20. Hughes, M. F., Adams, R. T., J. Phys. Chem. (1960) 64, 781-784. 21. Zenz, F. Α., Othmer, D. F., "Fluidization and Fluid-Particle Systems," Reinhold, New York, 1960. RECEIVED January 2, 1974.


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The Oxidation of o-Xylene in a Transported Bed Reactor - Advances in

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