8 Monolithic Reactor-Heat Exchanger T. F . D E G N A N , JR. and J. W E I
Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch008
University of Delaware, Newark, DE 19711
A novel monolithic reactor-heat exchanger has been constructed and operated i n five d i f f e r e n t modes. Experiments were conducted with the oxidation of c a r bon monoxide over copper chromite p e l l e t e d c a t a l y s t s . The experimental temperatures of reactants and c o o l ants, and concentrations agree with computations with a cell model. V i r t u a l l y f l a t temperature p r o f i l e s can be obtained i n the co-current mode. Equipment A novel chemical reactor has been constructed of four crossflow monoliths arranged i n series to act as reactor-heat exchangers. Every second pass of each crossflow i s packed with a p e l l e t e d c a t a l y s t or has c a t a l y s t deposited on the monolith w a l l s . The remain ing passes are empty to f a c i l i t a t e the flow of heat exchange medium. A p e l l e t e d crossflow monolith i s shown i n F i g . 1, and its dimensions are shown i n Table I. For the coated monolith, the heights of the two passes are the same. Note the large heat transfer area to volume r a t i o achieved i n the cross flow design, which leads to a high number of trans ference units for heat exchange. Arranging the crossflows i n series approximates true cocurrent or countercurrent flow. Mathematical analyses of simple heat transfer were published on cocurrent or countercurrent behavior for a series of crossflows (1,2). This paper deals with the mathe matical modeling and experiments for simultaneous reaction and heat transfer. The oxidation of carbon monoxide over a base metal c a t a l y s t was selected because i t i s a highly exothermic reaction whose rate has an approximate f i r s t order dependence on the concentration of ©
0-8412-0401-2/78/47-065-083$05.00/0
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CHEMICAL REACTION ENGINEERING—HOUSTON
84
Table I P h y s i c a l Dimensions o f C r o s s f l o w M o n o l i t h s P e l l e t F i l l e d Monoliths R e a c t i o n Pass C o o l a n t Pass Cross S e c t i o n a l
Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch008
of
Flow, A
Area
x
2.326 i n
Heat T r a n s f e r A r e a , A
86.2
H e i g h t Between F l a t s , b Thickness of F l a t s ,
a
1
Volume o f Pass, V H y d r a u l i c Radius,
in
0.867 i n
2
6 7.82
in
0.315 i n
0.125 i n
0.0336in
0.0336in
4.652 i n r
2
3
1.735 i n
0.054 i n
2
2
3
0.026 i n
Heat T r a n s f e r A r e a t o Volume R a t i o , a 18.49 in" 39.10 i n " CO Q/i,,^) ; p e l l e t e d copper chrome "aero ban" c a t a l y s t p r o v i d e d by American Cyanamid Co. was c h a r a c t e r i z e d , u s i n g a 1.25 cm OD i n t e g r a l r e a c t o r i n a c o n s t a n t temperature sand b a t h . F i r s t o r d e r k i n e t i c s were o b t a i n e d i n the range o f 425 t o 1000°F. The f o l l o w i n g k i n e t i c parameters were o b t a i n e d : 1
I
= 6,300 °R1
1
1
1
k = 1.194x1ο * s e c " . oo
The p e l l e t e d copper chrome c a t a l y s t was metered i n t o each o f the l a r g e s i n u s o i d a l d u c t s o f the r e a c t i o n pass o f each o f the f o u r m o n o l i t h s c o m p r i s i n g the r e a c t o r - h e a t exchanger. A h" l a y e r o f q u a r t z c h i p s a t each o f t h e r e a c t i o n pass f a c e s o f t h e c r o s s f l o w s guaranteed t h a t the r e a c t i o n was c o n f i n e d t o the volume o f the m o n o l i t h where h e a t exchange c o u l d take p l a c e . In a s e p a r a t e s e t o f f o u r c r o s s f l o w m o n o l i t h s , a c a t a l y s t o f s i m i l a r c o m p o s i t i o n (1.442% Cu and 0.966% Cr) and d e n s i t y was c o a t e d e x c l u s i v e l y on the w a l l s o f the r e a c t i o n p a s s . No attempt was made t o keep the c a t a l y s t from a d h e r i n g t o the s e c t i o n o f w a l l a d j a c e n t t o which no h e a t t r a n s f e r medium f l o w e d . In b o t h c a s e s , the f o u r c o r d i e r i t e m o n o l i t h s were p o s i t i o n e d i n a s t e e l m a n i f o l d shown i n F i g . 2.
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch008
70.10 inches
Figure 2. Top view of MRHE
Weekman and Luss; Chemical Reaction Engineering—Houston ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF NEW ENGLAND on February 10, 2017 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch008
86
CHEMICAL REACTION ENGINEERING—HOUSTON
F i f t y - s e v e n chromel-alumel thermocouples measured the temperatures w i t h i n the r e a c t o r . A D o r i c m u l t i p o i n t d i g i t a l r e c o r d e r m o n i t o r e d the thermocouple r e a d i n g s and p r i n t e d these r e a d i n g s on paper tape. A c o n v e r s i o n p r o f i l e a l o n g the r e a c t i o n pass was o b t a i n e d by measuring the c o m p o s i t i o n i n each o f the t r i a n g u l a r shaped empty volumes between the m o n o l i t h s . F i f t e e n centimeters long pieces of s t a i n l e s s s t e e l were welded i n t o the c e n t e r o f each o f the t e n t r i a n g u l a r shaped volumes f o r use as gas sample p o r t s . Sample a n a l y s i s was performed u s i n g a gas chromatograph. Cocurrent,
Countercurrent
and A u t o t h e r m a l
Operation
By f e e d i n g the i n l e t r e a c t a n t and c o o l a n t streams i n t o s e p a r a t e p o r t s o f the r e a c t o r , i t was p o s s i b l e to a c h i e v e f i v e d i f f e r e n t flow schemes, shown i n F i g . 3. An a d i a b a t i c r e a c t o r was s i m u l a t e d by f e e d i n g a stream o f p r e h e a t e d a i r and CO to the c a t a l y s t c o n t a i n i n g r e a c t i o n pass and by s e a l i n g o f f the c o o l a n t p a s s . S i m i l a r l y , the c o c u r r e n t and c o u n t e r c u r r e n t schemes were approximated by f l o w i n g c o o l a n t and r e a c t a n t streams e i t h e r i n t o a d j a c e n t p o r t s o r i n t o p o r t s which l i e a t o p p o s i t e ends o f the r e a c t o r - h e a t exchanger r e s p e c t i v e l y . F i n a l l y , a u t o t h e r m a l c o c u r r e n t and c o u n t e r c u r r e n t schemes were approximated by f e e d i n g a c o o l stream o f CO i n a i r i n t o the c o o l a n t pass and f e e d i n g the h e a t e d stream l e a v i n g t h i s pass t o the c a t a l y s t c o n t a i n i n g r e a c t i o n pass. F o r the a u t o t h e r m a l c o u n t e r c u r r e n t r u n , the p a r t i t i o n between p o r t s G and H was removed, the p o r t s were s e a l e d , and the CO c o n t a i n i n g s t r e a m was f e d t o p o r t E. F o r the a u t o t h e r m a l c o c u r r e n t c a s e t h i s p a r t i t i o n was r e s t o r e d , and the m i x t u r e o f CO i n a i r was f e d i n t o P o r t G. An i n s u l a t e d s t a i n l e s s s t e e l f l e x hose was used t o c a r r y the gas stream from p o r t Ε t o p o r t H. The r e a c t e d stream e x i t e d a t p o r t F. M a t h e m a t i c a l Models The c e l l model (6., 7) i s p a r t i c u l a r l y w e l l s u i t e d to d e s c r i b i n g the two d i m e n s i o n a l temperature and con centration p r o f i l e s i n crossflow, including a x i a l d i s p e r s i o n i n these s h o r t p a s s e s (L/Dp
