Chemical Reaction EngineeringâII

50 Copper-Substituted Zirconium Phosphate— a New Oxidation Catalyst THOMAS

J.

KALMAN

1

and MILORAD

DUDUKOVIĆ

Downloaded by CORNELL UNIV on September 15, 2016 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1974-0133.ch050

C h e m i c a l Engineering Department, O h i o University, Athens, O h i o 45701 ABRAHAM

CLEARFIELD

Department of Chemistry, O h i o University, Athens, O h i o 45701

A novel catalytically active compound is obtained by substitution of copper cations into crystalline α-zirconium phosphate. Cata lytic oxidation of carbon monoxide on crystalline copper-substi tuted zirconium phosphate was studied at different gas composi tions, flow rates, and temperatures up to 400°C. A 1-ft long, tubular reactor with a preheater section is used for analysis. The rate of reaction is proportional to the square root of the carbon monoxide concentration. Oxygen, when present in large excess over its stoichiometric ratio, has no influence on the reaction rate. The apparent activation energy of 12.5 kcal/mole and the rate constant for the reaction are determined. Results indicate that copper-substituted α-zirconium phosphate is comparable in ac tivity with a number of active catalysts for carbon monoxide oxidation.

T

he c a t a l y t i c o x i d a t i o n of c a r b o n m o n o x i d e ( C O ) has b e e n s t u d i e d e x t e n sively w i t h m a n y catalysts. A complete literature r e v i e w was c o m p i l e d b y K a t z ( J ) a n d D i x o n a n d L o n g f i e l d ( 2 ) . M o s t of this e a r l y w o r k dealt p r i m a r i l y w i t h predictions of catalytic activity of various m e t a l l i c oxides, especially the transition group m e t a l l i c oxides. R e a c t i o n m e c h a n i s m a n d possible surface intermediates were t h o r o u g h l y e x a m i n e d . L a i d l e r (3) s u m m a r i z e d a n u m b e r of s t u d i e s d o n e o n C O o x i d a t i o n w i t h d i f f e r e n t s u r f a c e s s u c h as q u a r t z glass, r o c k crystal, p l a t i n u m , a n d copper oxide a n d tabulated the empirically f o u n d reaction rate orders w h i c h v a r i e d f r o m — 1 to 1 w i t h respect to C O a n d f r o m 0 to 1 w i t h r e s p e c t to o x y g e n . S c h w a b a n d G o s s n e r (4) s t u d i e d t h e o x i d a t i o n o f C O o n silver, p a l l a d i u m , a n d s i l v e r - p a l l a d i u m alloys a n d f o u n d different rate expres s i o n s , w h i c h t h e y q u a l i t a t i v e l y e x p l a i n e d o n t h e basis o f t h e c a t a l y s t e l e c t r o n configuration a n d L a n g m u i r adsorption theory. Parravano (5) reported the a d d i t i o n o f f o r e i g n i o n s t o n i c k e l o x i d e a n d t h e s u b s e q u e n t use o f t h e o x i d e i n

1

State of Ohio Environmental Protection Agency, Logan, Ohio. 654

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

50.

KALMAN E T AL.

New

Oxidation

655

Catalyst

C O o x i d a t i o n . T a j b l et al. (6) s t u d i e d t h e k i n e t i c s o f C O o x i d a t i o n b y 0.5 w t % p a l l a d i u m o n α-alumina pellets a n d f o u n d t h e rate to b e p r o p o r t i o n a l to o x y g e n t o c a r b o n m o n o x i d e r a t i o . C o e k e l b e r g s et al. ( 7 ) u s e d x - r a y i r r a d i a t e d a l u m i n a as c a t a l y s t a n d d e t e r m i n e d t h e r e a c t i o n r a t e o r d e r t o b e 1 w i t h r e s p e c t t o C O a n d 0.5 w i t h r e s p e c t t o 0 .


2

T h e interest i n catalytic C O oxidation was greatly r e n e w e d i n the late 1960's w h e n the increasing l e v e l of C O emissions forced the U . S . government t o i m p o s e r e s t r i c t i o n s o n t h e a m o u n t e m i t t e d f r o m m a j o r s o u r c e s (8). Auto m o b i l e c o m b u s t i o n of gasoline accounted for over 8 0 % of the total U . S . C O e m i s s i o n s ( 9 ) . A t p r e s e n t , e x h a u s t t r e a t m e n t d e v i c e s , s u c h as c a t a l y t i c c o n verters, s h o w p r o m i s e i n r e d u c i n g these emissions to the l o w levels r e q u i r e d (10,11). T h e catalysts to be u s e d b y auto manufacturers i n the next f e w years c o n t a i n n o b l e m e t a l s , s u c h as p l a t i n u m , a n d d o n o t r e p r e s e n t a n o p t i m a l c h o i c e f o r s e v e r a l r e a s o n s . P l a t i n u m is e x p e n s i v e a n d is e a s i l y p o i s o n e d b y a l l t y p e s o f i m p u r i t i e s ; i t is e v e n s e l f - p o i s o n e d b y excess C O , t h e o x i d a t i o n r a t e is h i g h at h i g h t e m p e r a t u r e s b u t i n t o l e r a b l y s l o w at l o w t e m p e r a t u r e s w h e n m o s t o f t h e C O e m i s s i o n s o c c u r . T h u s , t h e r e s t i l l is a n e e d f o r a c h e a p e r c a t a l y s t w i t h g o o d performance. T h i s w o r k evaluates the performance of an i o n exchange compound, copper-substituted α-zirconium phosphate, Z r C u ( P 0 ) , i n the catalytic o x i d a tion of C O . T h i s w i l l initiate further studies into the possible use of transition m e t a l - s u b s t i t u t e d z i r c o n i u m p h o s p h a t e s as o x i d a t i o n c a t a l y s t s w i t h p o t e n t i a l applications for catalytic treatment of automobile exhaust. T h e present study w a s d o n e w h e n t h e p r e l i m i n a r y tests p r e s e n t e d i n T a b l e I s h o w e d t h e c o m p o u n d to be c a t a l y t i c a l l y active. T h e s e results w e r e o b t a i n e d b y i n j e c t i n g a small a m o u n t of a i r - c a r b o n m o n o x i d e into a h e l i u m stream entering a pulse r e a c t o r f i l l e d w i t h c a t a l y s t . T h e e x i t gas c o m p o s i t i o n w a s m o n i t o r e d b y a gas chromatograph. 4

T a b l e I.

C a r b o n Monoxide Oxidation on Copper-Substituted Z i r c o n i u m Phosphate (Preliminary Results)

Reactor Temperature, 60 100 150 250 300 300

2

°C

Carbon Monoxide: Air 3:2 1:3 1:4 1:5 1:10 1:1

Reactor Flow Rate (ml/min) 60 60 8 8 8 12

Products CO CO Trace C O 75% C 0 , 2 5 % C O 100% C O 70% C 0 , 30% C O 2

2

2

2

Catalyst Selection and Preparation Selection of the Catalyst. T h e m a i n effect of a c a t a l y s t is t o l o w e r t h e p o t e n t i a l e n e r g y b a r r i e r w h i c h r e a c t a n t gas m o l e c u l e s h a v e t o o v e r c o m e i n order for a c h e m i c a l reaction to take place a n d thus a l l o w a larger n u m b e r of molecules to react per unit t i m e . T h e activity of a s o l i d surface i n a p a r t i c u l a r r e a c t i o n d e p e n d s i n g e n e r a l o n its e l e c t r o n c o n f i g u r a t i o n , c r y s t a l s t r u c t u r e , p u r i t y , a n d porosity. T h e studies reported i n the literature indicate that besides n o b l e m e t a l s s u c h as p a l l a d i u m , p l a t i n u m , a n d s i l v e r , m a n y c o m p o u n d s c o n t a i n i n g t r a n s i t i o n m e t a l s are a c t i v e c a t a l y s t s f o r C O o x i d a t i o n . O n t h e o t h e r h a n d , c e r t a i n c r y s t a l s t r u c t u r e s s u c h as t h o s e o f n a t u r a l l y o c c u r r i n g z e o l i t e s , are v e r y c o n d u c i v e to c a t a l y t i c a c t i v i t y .


656

CHEMICAL

REACTION

ENGINEERING

II

I n p r e p a r i n g n e w catalysts for C O o x i d a t i o n reaction t w o n o v e l approaches m a y be taken: ( a ) s u b s t i t u t i o n of t r a n s i t i o n m e t a l s i n t o t h e z e o l i t e f r a m e w o r k w i t h f u l l preservation of the zeolite crystal configuration ( b ) s u b s t i t u t i o n of t r a n s i t i o n m e t a l s i n t o a n i o n e x c h a n g e c o m p o u n d a d e f i n e d c r y s t a l structure s i m i l a r to zeolites.

with


I n a r e c e n t effort o n e of t h e a u t h o r s (12) m a n a g e d to i n c o r p o r a t e c h r o m i u m ( I I I ) , v a n a d i u m ( V ) , a n d m o l y b d e n u m ( V I ) into the zeolite framework. N o t h i n g is k n o w n y e t a b o u t t h e c a t a l y t i c p r o p e r t i e s o f t h e s e n e w z e o l i t e s . T h e second approach was taken i n this w o r k , a n d z i r c o n i u m phosphate was selected as a n i o n e x c h a n g e c o m p o u n d i n t o w h i c h d e s i r e d m e t a l c a t i o n s m a y b e s u b s t i tuted for hydrogen. T h e i o n exchange properties of z i r c o n i u m p h o s p h a t e h a v e b e e n k n o w n for s o m e t i m e , a n d its h i g h s e l e c t i v i t y h a s b e e n u s e d t o s e p a r a t e u r a n i u m a n d p l u t o n i u m fission p r o d u c t s (13) a n d to treat c o n t a m i n a t e d w a t e r i n nuclear r e a c t o r s (14). Practically a l l m e t a l l i c cations c a n be exchanged for h y d r o g e n in zirconium phosphate b y a h i g h temperature reaction between a metallic h a l i d e a n d z i r c o n i u m p h o s p h a t e (15). T h e s u b s t i t u t i o n of transition m e t a l ions w i l l render compounds catalytically active, thus presenting an enormous amount of p o t e n t i a l c a t a l y s t s . Z i r c o n i u m p h o s p h a t e h a s a l a y e r e d s t r u c t u r e (16). E a c h l a y e r consists of a p l a n e of z i r c o n i u m atoms a r r a n g e d i n a h e x a g o n a l array. T h e p h o s p h a t e g r o u p s are s i t u a t e d a l t e r n a t i v e l y a b o v e a n d b e l o w t h e p l a n e s a n d h a v e t h r e e of t h e i r o x y g e n atoms b o n d e d to three different z i r c o n i u m atoms. T h e f o u r t h o x y g e n bears the h y d r o g e n a t o m a n d points r o u g h l y p e r p e n d i c u l a r to the layers. T h e l a y e r s a r e s t a g g e r e d so t h a t t h e p h o s p h a t e g r o u p s i n o n e l a y e r are a b o v e z i r c o n i u m a t o m s i n t h e n e x t l a y e r . T h i s a r r a n g e m e n t f o r m s z e o l i t e t y p e cages b e t w e e n t h e l a y e r s . T h e i n t e r l a y e r d i s t a n c e is 7 . 5 6 A . T h e i n t r o d u c t i o n o f v a r i o u s c a t i o n s t e n d s t o m o v e t h e l a y e r s f a r t h e r a p a r t to a c c o m m o d a t e a l a r g e ion. C o p p e r - s u b s t i t u t e d z i r c o n i u m phosphate was selected for this w o r k since c o p p e r c o m p o u n d s are some of the most active catalysts for C O o x i d a t i o n . A s e x p e c t e d , t h e p r e l i m i n a r y tests s h o w e d c o p p e r - s u b s t i t u t e d z i r c o n i u m p h o s p h a t e to be a more active oxidation catalyst ( C O , m e t h a n o l , S 0 oxidation) than c o b a l t s u b s t i t u t e d z i r c o n i u m p h o s p h a t e (17). These preliminary qualitative r e s u l t s w e r e o b t a i n e d b y p a s s i n g t h e gas m i x t u r e t h r o u g h a U - t u b e filled w i t h catalyst. T h e copper-containing catalyst readily converted m e t h a n o l a n d C O t o t h e i r u l t i m a t e o x i d a t i o n p r o d u c t s , C 0 a n d w a t e r , at t e m p e r a t u r e s less t h a n 3 0 0 ° C . I t also s h o w e d p r o m i s e f o r t h e o x i d a t i o n o f h y d r o c a r b o n s a n d S 0 at h i g h e r t e m p e r a t u r e s . T h e c o b a l t - c o n t a i n i n g c a t a l y s t w a s less e f f e c t i v e f o r t o t a l oxidation, a n d it r e q u i r e d higher temperatures for comparable performance. H o w e v e r , i t s h o w e d p r o m i s e as a c a t a l y s t f o r s p e c i f i c o x i d a t i o n s s i n c e i t p r o d u c e d a 3 0 % y i e l d o f b u t y r a l d e h y d e at 9 4 % c o n v e r s i o n o f η - b u t y l a l c o h o l , thus, y i e l d i n g o n l y a s m a l l a m o u n t of side products. 2

2

2

C a t a l y s t P r e p a r a t i o n . T h e p r e p a r a t i o n of the catalyst i n v o l v e d four major steps: (1) p u r i f i c a t i o n of z i r c o n y l c h l o r i d e , (2) p r e p a r a t i o n of α-zirconium phosphate, (3) c u p r i c i o n exchange for h y d r o g e n i n the α-zirconium phosphate, a n d ( 4 ) d e p o s i t i o n o f t h e c a t a l y s t o n a s u p p o r t . T h e first t w o steps w e r e p r e s e n t e d i n d e t a i l b y K a l m a n (18) a n d to some extent b y C l e a r f i e l d a n d Stynes (19). T h e c u p r i c i o n e x c h a n g e r e a c t i o n c a n b e d o n e b y e i t h e r a s o l i d - s o l i d (17) o r w e t m e t h o d (18) using amorphous or crystalline z i r c o n i u m phosphate. T h e


50.

New

KALMAN E T AL.

Oxidation

Catalyst

D = 7.56

657

A

z ο Cl eo ce


Lu

ο ce ο ο

LU

ce

JL

_i_

0

6

1 2

D I F F R A C T I O N Figure

1.

1 8

A N G L E

2

4

( D E G R E E S )

X-ray diffraction pattern of α-zirconium phosphate

crystalline

starting material i n this w o r k was crystalline α - z i r c o n i u m phosphate. A n x-ray d i f f r a c t i o n p a t t e r n ( F i g u r e 1) r e v e a l e d t h a t t h e s t a r t i n g m a t e r i a l w a s c r y s t a l l i n e , as i n d i c a t e d b y t h e s h a r p p e a k at d s p a c i n g o f 7 . 5 6 A . T h e s o l i d - s o l i d m e t h o d , w h i c h c o n s i s t s i n h e a t i n g a finely g r o u n d m i x t u r e o f z i r c o n i u m p h o s p h a t e a n d c u p r i c chloride, was not convenient for m a k i n g large amounts of catalyst. T h e wet m e t h o d , developed i n this w o r k , i n v o l v e d refluxing α - z i r c o n i u m phosphate

Lu

(/)

D

Ο CL

== 9 . 5 1

Â

ω ce Lu

α ce ο ο

ι 7

ι 9

D I F F R A C T I O N Figure

2.

X-ray stituted

ι Η A N G L E

ι 1 3 ( D E G R E E S )

diffraction pattern of α-zirconium phosphate

fully-sub


658

CHEMICAL

w i t h a c u p r i c acetate

REACTION

ENGINEERING

solution u n t i l the supernatant b e c a m e colorless.

II

The

reaction is: Zr(HP0 )


4

2

+ Cu(CH COO) = ZrCu(P0 ) + 2CH COOH 3

2

4

2

(1)

3

X - r a y p o w d e r patterns w e r e t a k e n d u r i n g the reaction to determine the extent of t h e c o p p e r s u b s t i t u t i o n . C o p p e r - s u b s t i t u t e d α - z i r c o n i u m p h o s p h a t e has a c h a r a c t e r i s t i c d s p a c i n g o f 9 . 5 1 A , as s h o w n i n F i g u r e 2 , a n d t h u s c a n b e d i s t i n g u i s h e d f r o m α - z i r c o n i u m p h o s p h a t e (d = 7 . 5 6 A ) . T h e s o l i d c a t a l y s t w a s filtered a n d d r i e d , a n d the percentage of c o p p e r was d e t e r m i n e d a n a l y t i c a l l y b y an iodometric titration w i t h sodium thiosulfate using the modified F r i t z a n d S c h e n k (20) m e t h o d . T h e p e r c e n t a g e o f c o p p e r s u b s t i t u t e d i n e v e r y b a t c h of catalyst m a d e was greater t h a n 9 0 % . T h e r m a l g r a v i m e t r i c analysis ( T G A ) g a v e t h e w e i g h t loss o f c a t a l y s t w i t h i n c r e a s i n g t e m p e r a t u r e . A T G A o f a 0 . 4 0 1 5 - g r a m s a m p l e o f 9 3 % c o p p e r - s u b s t i t u t e d z i r c o n i u m p h o s p h a t e is s h o w n i n F i g u r e 3 w i t h a n e x p l a n a t i o n o f t h e w e i g h t loss at c e r t a i n t e m p e r a t u r e r a n g e s . T h e catalyst has a density of 2.78 g r a m s / c m , a n d a B E T area of 7.85 m / g r a m was determined. T h i s relatively l o w surface area c a n r e a d i l y be i m p r o v e d b y modifying the preparation procedure. 3

2

T h e last s t e p i n t h e p r e p a r a t i o n c o n s i s t e d o f d e p o s i t i n g t h e c a t a l y s t o n a suitable support. A s u p p o r t w a s necessary since the catalyst consisted of micron-sized crystals. A l t h o u g h the catalyst adhered w e l l to t h e m , conventional s u p p o r t s s u c h as a l u m i n a a n d c l a y w e r e n o t u s e d i n t h i s w o r k b e c a u s e t h e y

0

40

80

120 160 TIME (MINI)

200

240

280

Figure 3. Thermogravimetric analysis of 93% copper-substituted a-zirconium phosphate. A , assumed to be surface moisture; Β and C , weight loss from two moles of water; D , from incomplete substitution which results in condensation of phosphate groups. m a s k e d the catalyst activity b y b e i n g catalytically active themselves. Asbestos w a s u s e d s i n c e i t d o e s n o t affect c a t a l y s t a c t i v i t y a n d a d h e r e t o i t v e r y w e l l . T o d e p o s i t t h e c a t a l y s t o n asbestos a s l u r r y w a s first p r e p a r e d w i t h desired a m o u n t of copper-substituted α - z i r c o n i u m phosphate (4 grams) b e n z e n e . T h e n , 8.0 g r a m s o f a c i d - w a s h e d , m e d i u m fiber p u r i f i e d asbestos m i x e d i n w i t h the slurry. T h e benzene was next a l l o w e d to evaporate, a n d


the and was the


50.

KALMAN ET

AL.

New

Oxidation

659

Catalyst

Figure 4. Apparatus for carbon monoxide oxidation. A—needle valve, Β—flowmeter, C—manometer, D—feed sampling port, Ε—temperature controller, F—preheater, G— thermocouple, H—catalytic bed, I—heating tape, J—insulation, Κ—condenser, L— effluent sampling port, M—wet test meter, Ν—junction board, Ο—temperature re corder, Ρ—gas chromâtο graph oven, Q—gas chromatograph control module, R—oven injection ports, S—strip chart recorder, Τ—digital integrator. r e s i d u a l vapors w e r e r e m o v e d b y h e a t i n g the catalyst to 1 2 0 ° C for 12 hrs. T h e s u r f a c e a r e a of t h e c a t a l y s t o n s u p p o r t w a s 1 0 . 9 1 m / g r a m s as d e t e r m i n e d b y the B E T m e t h o d . 2

Experimental Apparatus. T h e experimental apparatus for C O oxidation on coppers u b s t i t u t e d α - z i r c o n i u m p h o s p h a t e o n asbestos is s h o w n i n F i g u r e 4. T h e gas flow r a t e f r o m e a c h t a n k w a s c o n t r o l l e d b y a n e e d l e v a l v e ( A ) a n d m e a s u r e d b y a flowmeter ( B ) . T h e flowmeters w e r e c a l i b r a t e d w i t h a w e t test m e t e r ( M ) a n d a 1 0 - c c s o a p b u b b l e m e t e r . T h e gas p r e s s u r e u p s t r e a m f r o m t h e r e a c t o r was measured b y a m e r c u r y manometer ( C ) , a n d the downstream line was v e n t e d t o t h e a t m o s p h e r e . T h e r e a c t o r c o n s i s t e d of 2 . 5 ft o f 2 5 m m d i a m e t e r b o r o s i l i c a t e glass t u b i n g , 1 ft of w h i c h w a s u s e d f o r t h e c a t a l y t i c b e d ( H ) a n d 1.5 ft f o r t h e p r e h e a t s e c t i o n ( F ) . B o t h sections w e r e w r a p p e d w i t h h e a t i n g t a p e ( I ) , p o w e r e d b y v a r i a c s ( E ) , a n d e n c l o s e d w i t h fiberglass i n s u l a t i o n ( J ) . T h e c a t a l y t i c b e d w a s filled w i t h 4 g r a m s of c a t a l y s t ( 9 3 % c o p p e r - s u b s t i t u t e d z i r c o n i u m p h o s p h a t e ) o n 8 g r a m s o f asbestos fibers as s u p p o r t . T h e t e m p e r a t u r e i n t h e r e a c t o r w a s m e a s u r e d b y t w o t h e r m o c o u p l e s ( G ) , o n e at t h e e n t r a n c e a n d t h e o t h e r at t h e e x i t o f t h e c a t a l y t i c b e d . T h e t e m p e r a t u r e w a s continuously controlled w i t h i n ± 3 ° C a n d monitored b y a strip chart potentiom e t r i c r e c o r d e d ( O ) . T h e b l i n d p r o b e w i t h asbestos fibers i n d i c a t e d n o C O c o n v e r s i o n . T h e effluent gases f r o m t h e r e a c t o r w e r e c o o l e d i n a c o n d e n s e r ( K ) a n d expelled t h r o u g h an exhaust hood. T w o s a m p l i n g ports ( D , L ) were l o c a t e d b e f o r e a n d after t h e r e a c t o r t o w i t h d r a w f e e d a n d effluent s a m p l e s f o r analysis. T h e y contained a r u b b e r s e p t u m t h r o u g h w h i c h a syringe needle c o u l d be inserted for sample r e m o v a l . T h e gas s a m p l e s w e r e a n a l y z e d b y a G o w M a c t h e r m a l c o n d u c t i v i t y gas chromatograph ( P , Q , R ) . T h e analysis r e q u i r e d that N , 0 , C O a n d C 0 b e s e p a r a t e d b y t h e c o l u m n s i n t h e gas c h r o m a t o g r a p h i n o r d e r t o c a l c u l a t e t h e c o m p o s i t i o n of t h e f e e d a n d effluent s t r e a m s . A c o l u m n c o n t a i n i n g m o l e c u l a r sieve 5 A c o u l d s e p a r a t e N , 0 , a n d C O b u t a d s o r b e d C 0 (21, 22). A 2

2

2

2

2

2


660

CHEMICAL

REACTION

ENGINEERING

II

s e c o n d c o l u m n c o n t a i n i n g s i l i c a g e l , w h i c h s e p a r a t e d C O f r o m t h e o t h e r gases, completed the analysis. T h e best separation w a s obtained w i t h 16 ft of m o l e c u lar sieve 5 A a n d 4.5 ft of silica g e l , u s i n g 0.25-inch c o p p e r t u b i n g f o r c o l u m n s at a n o v e n t e m p e r a t u r e o f 1 0 0 ° C a n d a c a r r i e r gas flow r a t e o f 5 0 c c / m i n o f helium. A digital integrator ( T ) was used d u r i n g a l l experimental runs to e v a l u a t e c o m p o s i t i o n . S i n c e t h e v o l u m e p e r c e n t o f e a c h gas f o u n d f r o m t h e i n t e g r a t o r d i d n o t e x a c t l y e q u a l t h e a c t u a l v o l u m e p e r c e n t o f t h e gases, a calibration curve w a s d e t e r m i n e d for a l l ranges of composition. T h e results o b t a i n e d w i t h t w o f e e d mixtures consisting of 0 a n d C O are reported i n this study. O n e contained a C O : 0 ratio of 1:4, the other h a d a C O : Q r a t i o o f 1 : 9 . T h e flow r a t e t h r o u g h t h e r e a c t o r w a s v a r i e d f r o m 2 7 . 5 to 3 1 2 c c / m i n , c o r r e s p o n d i n g to t h e range of m e a n contact times of 0.214 to 1.77 g r a m h r / l i t e r . I n t h i s flow r a n g e t h e p r e s s u r e d r o p across t h e r e a c t o r w a s negligible. H o w e v e r , the runs performed w i t h N / 0 mixture, corresponding to t h e c o m p o s i t i o n of a i r w i t h t h e a d d i t i o n of a s m a l l percentage of C O , resulted i n complete conversion of C O since large contact times h a d to b e u s e d to a v o i d large pressure drops. P r o c e d u r e . E a c h b a t c h o f c a t a l y s t o n asbestos s u p p o r t w a s i n i t i a l l y h e a t e d at 4 0 0 ° C i n t h e r e a c t o r f o r 3 h r s , u n d e r N p u r g e t o r e m o v e a n y r e s i d u a l moisture. T h e catalyst w a s t h e n heated at t h e desired temperature w i t h the required 0 flow r a t e f o r 1 h r . T h e C O w a s n e x t i n t r o d u c e d a n d w a s a l l o w e d t o flow f o r 1 5 m i n u n t i l a s t e a d y state w a s e s t a b l i s h e d . T h e t e m p e r a t u r e w a s kept constant to w i t h i n 3 ° C d u r i n g this time a n d throughout the r u n . A feed sample (0.5 cc) w a s taken w i t h a syringe from the entrance port to t h e reactor a n d injected i n t o t h e m o l e c u l a r sieve c o l u m n , w h i c h separated 0 and C O . T w o effluent s a m p l e s w e r e t a k e n f r o m t h e e x i t p o r t o f t h e r e a c t o r . T h e first was injected i n t h e m o l e c u l a r sieve c o l u m n , w h i c h a g a i n separated C O a n d 0 , but adsorbed C 0 . T h e second sample was injected into the silica g e l column, w h i c h separated C 0 from the mixture. T h i s procedure was repeated several t i m e s f o r e v e r y flow r a t e t o assure t h a t s t e a d y - s t a t e c o m p o s i t i o n w a s b e i n g obtained. T h e data for the compositions were then averaged a n d corrected using the calibration curve. s

2

2


2

2

2

2

2

2

2

2

2

T a b l e II.

K i n e t i c D a t a f o r C a r b o n M o n o x i d e O x i d a t i o n ( C 0 : 0 r a t i o i s 1:4) Contact Time Feed Rate Temperature τ, (gram hr/liter) Run No. (cc/min) Conversion xco (°C) 1.77 1 325 37.5 1.00 2 1.54 43.3 325 1.00 1.18 3 56.5 325 1.00 4 0.889 75.1 325 0.866 0.592 5 325 112.5 0.608 6 0.427 156.0 325 0.471 1.77 7 37.5 300 0.921 1.54 8 300 0.842 43.3 1.81 9 300 56.5 0.729 0.889 10 300 75.1 0.649 0.592 11 300 112.5 0.405 12 0.427 156.0 300 0.294 1.77 13 37.5 275 0.689 1.54 14 0.574 43.3 275 1.18 15 56.5 275 0.469 0.889 16 75.1 275 0.413 0.592 17 275 112.5 0.276 0.427 18 156.0 275 0.152 19 1.77 250 37.5 0.433 20 1.54 250 43.3 0.376 21 1.18 250 56.5 0.289 0.889 22 250 0.232 75.1 0.592 0.0984 23 250 112.5 24 0.427 156.0 250 0.0919 2


50.

KALMAN

Table III.

661

Catalyst

Kinetic D a t a for Carbon Monoxide Oxidation ( C 0 : 0 Temperature (°C)

Run No.


New Oxidation

ET AL.

25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

r a t i o i s 1:9)

2

Conversion xco

Feed Rate (cc/min)

Space Time τ, (gram hr/liter)

0.972 0.878 0.817 0.623 0.497 0.340 0.826 0.694 0.555 0.465 0.299 0.204 0.552 0.484 0.368 0.256 0.170 0.143 0.339 0.252 0.220 0.133 0.0530 0.0349

75.1 86.6 113.3 150.1 225.2 312.0 75.1 86.6 113.3 150.1 225.2 312.0 75.1 86.6 113.3 150.1 225.2 312.0 75.1 86.6 113.3 150.1 225.2 312.0

0.889 0.770 0.588 0.444 0.296 0.214 0.889 0.770 0.588 0.444 0.296 0.214 0.889 0.770 0.588 0.444 0.296 0.214 0.889 0.770 0.588 0.444 0.296 0.214

325 325 325 325 325 325 300 300 300 300 300 300 275 275 275 275 275 275 250 250 250 250 250 250

Discussion I n this study 4 8 experimental runs under v a r y i n g conditions of tempera t u r e , flow r a t e , a n d c o m p o s i t i o n w e r e m a d e . T h e w e i g h t o f c a t a l y s t u s e d i n all of t h e runs w a s 4 grams ( 9 3 % copper-substituted α-zirconium phosphate) o n 8 g r a m s asbestos s u p p o r t . C O o x i d a t i o n p r o c e e d s a c c o r d i n g t o t h e f o l l o w i n g stoichiometry: CO +

I0 2

• C0

2

ZrCu(P0 ) 4

(2)

2

2

R u n s 1 t h r o u g h 2 4 w e r e d o n e w i t h a v o l u m e r a t i o o f C O : 0 o f 1:4 a n d runs 2 5 t h r o u g h 4 8 w i t h a ratio of 1:9. N o signs of catalyst d e a c t i v a t i o n w e r e detected d u r i n g several months of experimentation. T h e results are g i v e n i n Tables II and III. A material balance was done for each r u n b y stoichiometrically balancing t h e n u m b e r o f m o l e s o f a n y t w o o f t h e t h r e e gas s p e c i e s p a r t i c i p a t i n g i n E q u a t i o n 2. T h e calculation i n v o l v e d the determination of the total n u m b e r of m o l e s o f effluent p e r m o l e o f f e e d b y e i t h e r o f t h e t h r e e b a l a n c e s ( C O a n d 0 , C 0 a n d 0 , C O a n d C 0 ) w h i c h i d e a l l y s h o u l d give the same value i f Reaction 2 w e r e t h e o n l y o n e t a k i n g p l a c e as a s s u m e d (18). T h e m a x i m u m d i f f e r e n c e between the three component balances w a s b e l o w 4 % for a l l the runs p r e s e n t e d h e r e , w h i c h is a r e a s o n a b l y g o o d a g r e e m e n t c o n s i d e r i n g t h e t y p e o f apparatus used a n d possible e x p e r i m e n t a l errors. T o reduce t h e influence of this e r r o r i n e v a l u a t i n g k i n e t i c d a t a , a n a v e r a g e o f t h e t h r e e v a l u e s o f t h e t o t a l m o l e s o f effluent p e r m o l e o f f e e d w a s t a k e n . 2

2

2

2

2

T h e p o r o s i t y o f t h e p a c k e d b e d o f asbestos fibers u s e d w a s ca. 0 . 7 5 , t h e a p p a r e n t d e n s i t y o f t h e fibers i m p r e g n a t e d w i t h t h e c a t a l y s t w a s less t h a n t h e d e n s i t y o f w a t e r . T h e fibers u s e d w e r e o f d i f f e r e n t l e n g t h s , a n d r a d i i a n d


662

CHEMICAL

REACTION

ENGINEERING

II

f o r m e d i r r e g u l a r n e t w o r k s o f h i g h t o r t u o s i t y f a c t o r . T h e m e a n fiber l e n g t h a n d d i a m e t e r w e r e a p p r o x i m a t e l y 1 c m a n d 0.1 c m , r e s p e c t i v e l y . E v e n f o r l o w R e y n o l d s n u m b e r s , sufficient m i x i n g o c c u r r e d i n t h e b e d so t h a t m a s s t r a n s f e r w a s n o t r a t e c o n t r o l l i n g . F o r t h e flow r a t e s a n d t e m p e r a t u r e s u s e d , t h e R e y n o l d s n u m b e r w a s b e t w e e n 2 a n d 10. I t is d i f f i c u l t t o a p p l y a n e m p i r i c a l c o r r e l a t i o n f o r m a s s t r a n s f e r coefficient p r e d i c t i o n s , s u c h as t h e o n e o b t a i n e d b y P e t r o v i c a n d T h o d o s (23) f o r b e d s p a c k e d w i t h s p h e r e s t o t h i s i r r e g u l a r p a c k e d b e d o f fine fibers. H o w e v e r , i t c a n b e s h o w n , f o l l o w i n g S a t t e r f i e l d (24), t h a t i f m a s s t r a n s f e r w e r e r a t e c o n t r o l l i n g , a 99% conversion of CO w o u l d b e r e a c h e d at a L/d r a t i o o f 3.2 a n d 7.4 at R e y n o l d s n u m b e r s o f 1 a n d 10, r e s p e c t i v e l y . T h e e q u i v a l e n t a v e r a g e p a r t i c l e d i a m e t e r , d , is 0.38 c m at t h e m o s t . A b e d o f o v e r 30 c m i n l e n g t h , L, w a s u s e d , a n d m u c h l o w e r c o n v e r s i o n s w e r e o b t a i n e d at t h e e x i t , w h i c h s e e m s t o i n d i c a t e t h a t t h e c h e m i c a l r e a c t i o n is r a t e c o n t r o l l i n g . T h i s a s s u m p t i o n w a s j u s t i f i e d a posteriori b y t h e m a g n i t u d e of the e v a l u a t e d activation energy. W e assumed that axial dispersion m a y be n e g l e c t e d b e c a u s e o f t h e l a r g e L/d ratio. T h i s should be verified b y a tracer experiment. I n t u i t i v e l y , the assumptions of g o o d r a d i a l m i x i n g a n d negligible a x i a l d i s p e r s i o n are j u s t i f i e d s i n c e i t s e e m s t h a t t h e n e t w o r k s o f asbestos fibers act l i k e m u l t i p l e screens to b r e a k r e p e a t e d l y the v e l o c i t y profile a n d create mixing. p


p

p

T h e i n t e g r a l m e t h o d o f a n a l y s i s w a s a p p l i e d t o a l l t h e d a t a (24). A r e a c t i o n r a t e e x p r e s s i o n w a s h y p o t h e s i z e d , i n t r o d u c e d i n t o t h e p l u g flow r e a c t o r mass balance, a n d integrated. T h e integrated expression was p l o t t e d against c o n t a c t t i m e u s i n g t h e e x p e r i m e n t a l l y d e t e r m i n e d v a l u e s . A p l o t o f I n (1/1 — x) vs. c o n t a c t t i m e , W/q, c o r r e s p o n d i n g t o t h e first-order r e a c t i o n w i t h r e s p e c t to CO, f a i l e d to y i e l d a straight line, e x h i b i t i n g a definite convex curvature. A p l o t o f c o n v e r s i o n , x, vs. c o n t a c t t i m e , W/q, c o r r e s p o n d i n g to a zero-order

2-4

CONTACT

TIME,. W/2

V

IHR-G/LITER)

Figure 5. Carbon monoxide oxidation on copper-substituted zirconium phosphate at a CO:O ratio of 1:4. Least-squares fit for: —rco = kCco" . t

5

reaction, resulted i n a concave curve. T h i s i n d i c a t e d that the order of reaction w i t h r e s p e c t t o C O w a s b e t w e e n z e r o a n d o n e . T h e r e a c t i o n r a t e of o r d e r 0.5 fitted t h e d a t a v e r y w e l l .



50.

New

KALMAN E T AL.

0.1

0.2

Oxidation

0.3

Catalyst

0.4

CONTACT

TIME

0.5 W/£

v

N

663

0.6

0.7

0.8

0.9

(HR-G/LITEFU

Figure 6. Carbon monoxide oxidation on copper-substituted zirconium phosphate at a CO:O ratio of 1:9. Least-squares fit for: —r o = k C o ° . t

C

The

rco =

5

C

fc(C o) C

integrated expression, taking into account

( 3 )

0 5

the slight contraction of

the

reaction m i x t u r e caused b y reaction, has the f o l l o w i n g f o r m :

k W

W^ç

"

1

, (1 + ε)

,

Λ

~ ^

" *>

+ > ε

+

7 =

Γ V l + sx -

**[

V-

ε (1 -

ζ)Ί,

l-V^e

J

B y p l o t t i n g t h e r i g h t s i d e o f E q u a t i o n 4 vs. c o n t a c t t i m e , W/q, lines w e r e o b t a i n e d b y a least-squares m e t h o d for b o t h C O : 0 r e s u l t s a r e p r e s e n t e d i n F i g u r e s 5 a n d 6.

2

4

)

straight

ratios.

These

T h e reaction rate constant for

temperature was e v a l u a t e d f r o m the slope of the straight line

(

Ν

fitted

each

by

the

least-squares m e t h o d . T h e rate of C O o x i d a t i o n p r o b a b l y d e p e n d s i n some m a n n e r o n the c o n c e n t r a t i o n also.

presented here since 0

2

2

f e e d ratios.

Therefore, the 0

2

of its entrance v a l u e for

concentration m a y be

2

concentration b y

12.5%

f r o m t h e 1:4

to the

1:9

both

a s s u m e d to

c o n s t a n t a n d e q u a l t o its i n i t i a l v a l u e at t h e r e a c t o r e n t r a n c e . initial 0

2

w a s u s e d i n l a r g e excess o v e r t h e s t o i c h i o m e t r i c r a t i o

a n d its c o n c e n t r a t i o n s t a y e d w e l l w i t h i n 3 % CO:0

0

T h i s d e p e n d e n c e is d i f f i c u l t t o e s t a b l i s h f r o m t h e r e s u l t s

be

Increasing the

mixture d i d

not

h a v e a n y effect, w i t h i n e x p e r i m e n t a l e r r o r , o n t h e r e a c t i o n r a t e c o n s t a n t ; t h i s i n d i c a t e s t h a t t h e r e a c t i o n r a t e is i n d e p e n d e n t o f 0 excess o f i t are p r e s e n t .

A t ratios of 0

2

2

concentration w h e n large

t o C O l a r g e r t h a n 8, t h e s t o i c h i o m e t r i c

r a t i o t h e c a t a l y s t s u r f a c e is a l w a y s s a t u r a t e d w i t h 0 , 2

reaction mechanisms involving 0 The

2

a n d t h e steps i n t h e

are n o t r a t e l i m i t i n g .

reaction rate constants, e v a l u a t e d i n F i g u r e s 5

a n d 6 at

different

t e m p e r a t u r e s w e r e u s e d t o p r e p a r e a n A r r h e n i u s p l o t o f l o g k vs. 1/T i n F i g u r e 7.

as s h o w n

A single straight line was o b t a i n e d for b o t h v o l u m e ratios C O : 0 .

F r o m the slope of the line a n apparent activation energy of

2

12.5

kcal/mole



664

CHEMICAL

O.oi I

1.65

1

1

1.7

1.75

1

1

1.8 1.85 I/Tx I0

Chemical Reaction EngineeringâII

Recommend Documents

Chemical Reaction EngineeringâII