44 Multiple Steady States of a Moving Bed Reactor—Theory and Experiment
Downloaded by IOWA STATE UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch044
K L A U S T H O M A and D I E T E R V O R T M E Y E R Institut Β fürThermodynamik,Technische Universität München, 8000 München 2, Arcisstr. 21, West Germany
A moving bed chemical reactor is characterized by a c c - or countercurrent movement i n the presence of chemical reac t i o n s . Furthermore heat and mass is exchanged between the two phases. Due to the solid movement the reactors are difficult to model i n laboratory experiments. The only experiments known to us are reported by weekman and Nace (1). These authors were mainly i n t e r e s t e d i n the behaviour of an isothermal moving bed reactor with respect to catalytic cracking and used f o r t h e i r experiments more or less free falling p a r t i c l e s i n a cocurrent gas stream. In the present paper model experiments are reported under defined conditions f o r gas and solid flow by counter current o p e r a t i o n . In this s i t u a t i o n the energetic feedback by the e f f e c t i v e heat conduction is increased through the countercurrent movement. I n t e r e s t i n g stability problems a r i s e concerning ignition/extinction phenomena. In p r i n c i p l e these effects are rather s i m i l a r to those encountered i n f i x e d bed reactors ( 2 ) , ( 3 ) , (4) although the s i t u a t i o n gets a new dimension by the solid movement. A s i m p l i f i e d t h e o r e t i c a l i n v e s t i g a t i o n of a countercurrent moving bed reactor was presented by Schaefer, Vortmeyer and Watson (5). The work neglects heat conduction i n both phases, the only feedback mechanism being the movement of one phase against the o t h e r . Three s o l u t i o n s were obtained f o r c e r t a i n parameter ranges. The upper and lower s o l u t i o n s were stable and the middle one unstable. I t i s i n t e r e s t i n g to note that the governing equations for a moving bed are i d e n t i c a l with the two phase model of a l i q u i d / l i q u i d spray column which was i n v e s t i g a t e d t h e o r e t i c a l l y by Luss and Amundson ( 6 ) . The aim of our work was to design a laboratory model of a moving bed reactor and to compare experimental r e s u l t s with numerically predicted m u l t i p l e steady state s o l u t i o n s of the governing equations. © 0-8412-0401-2/78/47-065-539$05.00/0
In Chemical Reaction Engineering—Houston; Weekman, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
CHEMICAL REACTION ENGINEERING—HOUSTON
540
Downloaded by IOWA STATE UNIV on October 19, 2014 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0065.ch044
D e s i g n o f a moving bed r e a c t o r The most d i f f i c u l t problem was t o o b t a i n a d e f i n e d s t e a d y movement o f t h e s o l i d . A movement due t o g r a v i t a t i o n a l f o r c e o n l y r e s u l t s i n a s t i c k - s l i p f l o u ( Y o s h i d a e t a l (2)) w h i c h makes t h e d e s i r e d measurement u n c o r r e c t , i f n o t i m p o s s i b l e . T h e r e f o r e t h e c a t a l y s t p a r t i c l e s were s u r r o u n d e d by a f i n e w i r e c l o t h . T h i s t u b u l a r arrangement o f 2.5 m l e n g t h was p u l l e d m e c h a n i c a l l y t h r o u g h a s t a i n l e s s s t e e l tube ( l e n g t h H-.95 m, i n n e r d i a m e t e r 5 cm, w a l l t h i c k n e s s i n t h e r e a c t i o n s e c t i o n 0.2 mm). T h i s tube was c l o s e d a t b o t h ends. A 2k cm l o n g c e n t r a l p a r t o f t h i s tube was t h e r e a c t i o n s e c t i o n ( f i g . 1 ) . A vacuum and t h e r m a l i n s u l a t i o n t o g e t h e r w i t h a d d i t i o n a l h e a t e r s kept r a d i a l h e a t l o s s e s low. The s o l i d was moved t h r o u g h t h e r e a c t i o n s e c t i o n w i t h a speed o f 5 mm/min. The gas m i x t u r e e n t e r s and l e a v e s t h e r e a c t o r s h o r t l y b e f o r e and s h o r t l y a f t e r t h e r e a c t i o n s e c t i o n as i n d i c a t e d i n f i g . 1. S i n c e both ends o f t h e l o n g c a t a l y s t s t o r a g e p i p e were c l o s e d t h e gas c o u l d o n l y move t h r o u g h t h e r e a c t i o n s e c t i o n . Temperature p r o f i l e s w i t h i n t h e r e a c t i o n s e c t i o n were measured by a movable t h e r m o c o u p l e w i t h i n a t h i n a x i a l tube. In t h e r e a c t i o n s e c t i o n ethane was o x i d i z e d by a Pd-Al203 c a t a l y s t . O v e r a l l c h e m i c a l r e a c t i o n r a t e s were d e t e r m i n e d e x p e r i m e n t a l l y i n a d i f f e r e n t i a l r e a c t o r as a f u n c t i o n o f c o n c e n t r a t i o n and t e m p e r a t u r e by Simon ( 8 ) , ( 9 ) . Theory W i t h r e g a r d t o a f i x e d bed t h e moving bed r e a c t o r has two more o p e r a t i o n a l p a r a m e t e r s . They a r e t h e p r e h e a t tempera t u r e T s and t h e mass f l o w r a t e mg o f t h e s o l i d . Because o f the i n c r e a s e d number o f v a r i a b l e s c a l c u l a t i o n s had t o be made i n advance i n o r d e r t o f i n d t h e i n t e r e s t i n g e x p e r i m e n t a l range where m u l t i p l i c i t y c o u l d be e x p e c t e d . The f a l l o w i n g s t e a d y s t a t e model e q u a t i o n s were s a l v e d : 0
Energy b a l a n c e s solid:„ d Τ NT
Τ2Γ dx
-
U
d Τ
s ' s -ΈΓ c
~ hB(T -T )s
F
a i
( T )=-AH.r V
u
(1)
gas : C
F
dT" -hS
