1997. catalytic process development_a process designer's point of view
TRANSCRIPT
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7/23/2019 1997. Catalytic Process Development_A Process Designer's Point of View
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E L S E V I E R Catalysis Today 36 (1997) 367-374
C T LYSIS
TOD Y
C a t a l y t ic pr o c e s s de v e l o pm e nt : A pr o c e s s de s i g ne r s
A . B i s i o *
Atro Associates PO. Box 1367 Mountainside NJ USA
p o i n t o f v i e w
P r oc e s s de s igne r s a r e no t m e m be r s o f t he c a t a ly s t
c o m m u n i t y ; f e w h a v e e v e r d e v e l o p e d o r e v e n t e st e d a
c a t a ly s t . H ow e ve r , in a n im por t a n t w a y , t he y a r e t he
f i r s t c u s tom e r s f o r ne w o r im pr ove d c a t a ly s t s . The i r
i n t e r e s t is l im i t e d t o da t a t ha t w i l l e na b l e t he m to m a k e
a s e r i es o f c om ple x de c i s ions a bou t c om m e r c i a l c a t -
a ly t i c r e a c to r sy s t e m s in a m a nuf a c tu r ing e nv i r on -
m e n t . U nde r s t a nd ing the i r ne e ds ( a nd p r e jud i c e s )
shou ld e na b le t he c a t a ly s t c om m uni ty t o de l i ve r
' ne w ' c a t a ly s ts m or e e f f e c t i ve ly .
M os t , i f no t a l l o f the c a t a ly s t s u se d c om m e r c i a l l y
toda y f o r m a nuf a c tu r ing c he m ic a l s a nd pe t r o l e um
r e f in ing w e r e l a r ge ly de ve lope d by t e d ious t r i a l a nd
e r ro r . H undr e ds o f m a n- hou r s ha ve b e e n spe n t i n t he
l a bo r a to r ie s o f u se rs a nd c a t a ly s t ve ndo r s t o de ve lop
a nd e vo lve a c om m e r c i a l l y v i a b l e c a t a ly s t . N o t su r -
pr is ingly , the re i s a growing in te res t in u t i l iz ing more
r a t i ona l a pp r oa c he s t ow a r d t he de ve lop m e n t o f c a t a -
ly s ts , pa r t i c u l a r ly i f one i s t r y ing t o a c h i e ve de s i r a b l e
e nv i r onm e n ta l goa l s .
A r e v i e w o f r e c e n t l i t e r a tu r e on c a t a ly s i s w i l l
unc o ve r tw o in t e r w o ve n s t r ands :
A be l i e f t ha t c a t a ly s ts c a n be de s igne d ; a nd
A s t rong fee l ing , a lmo st mess ian ic , tha t new ca t -
a ly s t s c a n l e a d t o r e duc t ion o f t he po l lu t ion bu r de n
on the e nv i r onm e n t .
I nde e d , a m o s t e l e m e n ta r y l i te r a tu re s e a r c h c ove r -
ing t he pa s t 10 ye a r s w i ll unc o ve r ove r 350 a r t ic l e s a nd
one pa t e n t u s ing t he ph r a se s , de s ign ing c a t a ly s t s o r
*Corresponding author.
des igner ca ta lys ts , in the t i t le and probably severa l
t housa nd tha t u se t he se w or ds ( o r s im i l ar one s ) i n t he
text.
'B e n ign by d e s ign ' i s a c onc e p t ( a lt hough i t w a s no t
o r ig ina t e d by h im ) t ha t ha s be e n pop u la r i z e d i n r e c e n t
ye a r s by P r o f e s so r M a r k D a v i s o f t he C a l i f o r n i a
I n s t it u t e o f T e c hno logy . Le t u s f i rs t c ons ide r b r i e f ly
f r om a p r oc e s s de s igne r ' s v i e w po in t , t he im p l i c a t ions
o f t he c onc e p t ' b e n ign by de s ign ' o r a ' c l e a n t e c hno l -
o g y ' .
To a r r i ve a t a l e ve l o f pe r f o r m a n c e im p l i e d by t he
c onc e p t o f a ' c l e a n t e c hno log y ' r e qu i r e s m or e t ha n j u s t
a n im pr ov e m e n t in t he pe r f o r m a n c e o f a c a t a ly s t , suc h
as a h igher y ie ld or improved se lec t iv i ty . At a mini -
m um , w e m us t a c c e p t t ha t the a r e a o f i n t er e s t is a lw a ys
a m a nuf a c tu r ing s i t e ( c r a d l e - to - g r a ve o r l i f e - c yc l e
th ink ing , w h i l e im por t a n t , a r e no t pa r t o f ou r c on -
s ide r at i ons a t t h i s junc tu r e ) . I f t he po l lu t i on bu r de n on
the e nv i r onm e n t i s t o be r e duc e d , t he n w e m us t
r e c ogn iz e t ha t e ve r y th ing t ha t ha ppe ns o r m a y ha ppe n
w i th in a p l a n t s i te i s li nke d . A c ha n ge in t he m o de o f
ope r a t i on o f a p r oc e s s un i t c a n qu i c k ly c a sc a de
th r ough the ope r a t i on o f t he e m is s ions a nd w a s t e -
m a na ge m e n t sy s t e m s , t he e ne r gy sys t e m s , a nd m a y
e ve n ha ve a n im pa c t on r a w m a te r ia l , p r oduc t s t o r a ge
a nd t r a nspo r t at i on . G a in ing a n u nde r s t a nd ing o f t he se
l i nka ge s a nd the i r c onse que nc e s i s o f p r im e c o nc e r n t o
a process des igner .
S yn the t i c r ou t e s t ha t u se h igh ly t ox i c a nd v o l a t i l e
r a w m a te r i a l s , suc h a s phosge ne , m us t be a vo ide d i f
pos s ib le . W he n no t pos s ib l e a nd i f the r a w m a te ri a l s
c a nno t be ge n e r a t e d a s ne e de d , t he de s igne r w i l l ha ve
0920-5861/97/$32.00 Copyright 1997 ElsevierScienceB.V. All rights reserved.
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to have a sophis t ica ted unders tanding of wha t i s
involved in t ranspor t ing an d s tor ing them. Procedu res
tha t protec t pe rsonne l aga ins t exposure to them and
handle acc identa l re leases in a re spons ib le man ner a re
also absolutely vi ta l .
Whether used as a reac t ion media or to fac i l i t a te
separa t ions , the ma gni tud e of the so lvent inventor ies
and the c i rcula t ion ra tes a re impo rtant . Smal l re la t ive
losses (e .g . ,
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369
Commerc ia l Inpu ts
Screen ing S t ra tegy
I Prel imina r ' Tech nical
A s s e s s m e n t
Labora tory
Screen ing
Kinetic/Thermodynamic
Data
T
Deta i led Techn ica l
Ana lys is
Labora tory Reac tor
Eva lua t ion
Pi lot P lant
Eva lua t ion
I C o m m e r c i a l
Des ign
Fig. 1. M ajor tasks in a m arket-drivencatalytic process develop-
ment program . Adopted from Ref. [2].
Prel iminary
Techn ica l /Commerc ia l
Ana lys is
Technical
A s s e s s m e n t
R e s e a r c h C o n c e p t
Mater ia l Screen ing
Discovery
Labora tory Reac tor
Eva lua t ions
Opt im iza t ion
Se lec t ion o f Sys tem
G o o d S c i e n c e
Pi lot Scale
Opera t ion
I~ ta ikKI I
Techn ica l /Commerc ia l
Ana lys is
C o m m e r c i a l
Des ign
Fig. 2. M ajor tasks in a science-driven catalytic proc ess develop-
ment program . Adopted from Ref. [2].
v e r s i o n . T h e r e f o r e , h i s p r o j e c t i o n s w i l l b e f o r t h c o m -
i n g i f n o t g e n e r o u s a n d h i s c o n c e r n s m i n i m a l .
A s c i e n c e - d r i v e n d e v e l o p m e n t i n v o l v e s i n v e n t i n g a
n o v e l ( o r n e w ) c a t a l y s t t h a t m a y ( o r m a y n o t ) f o r m t h e
b a s i s f o r a p r o f i t a b l e c o m m e r c i a l p l a n t , u n l i k e a
m a r k e t - d r i v e n d e v e l o p m e n t i n w h i c h c a s e a s ig n i f i ca n t
p o r t i o n o f t h e w o r k w i l l b e e x p l o r a t o r y i n n a t u r e ,
b r o a d i n i t s s c o p e , a n d u n c e r t a i n i n o u t c o m e . O n l y
a l i m i t e d d a t a - b a s e m a y e x i s t t o g u i d e t h e s e l e c ti o n o f
i n i t i a l c a t a l y s t c o m p o s i t i o n s , a s s o c i a t e d p r e p a r a t i o n
p r o c e d u r e s , a n d ' t e s t ' r e a c t i o n c o n d i t i o n s ; t h i s c a n
r e s u l t i n t h e p r o c e s s d e s i g n e r b e i n g q u i t e r e l u c t a n t t o
m a k e p r o j e c t i o n s , e v e n c o n s e r v a t i v e o n e s .
T h e m a j o r t a s k s t h a t m u s t b e c o n d u c t e d b e f o r e a
c a t a l y s t a n d t h e a s s o c i a t e d p r o c e s s ( e . g . , r e a c t o r a n d
s u p p o r t s y s t e m s ) a r e p r o j e c t e d t o b e c o m m e r c i a l l y
v i a b l e , a r e s h o w n i n F i g . 2 . T h e k e y t e c h n i c a l h u r d l e s
w i l l i n c l u d e ( a t a m i n i m u m ) :
A n i n i t ia l d i s c o v e r y t h a t r e s u l ts f r o m a h i g h l y
e x p l o r a t o r y l a b o r a t o r y r e se a r c h p r o g r a m ;
E x t e n s i v e l a b o r a t o r y r e a c t o r t e s ti n g ; a n d
D e m o n s t r a t i o n s o n a p i l o t - p l a n t s c a le .
T h e s c o p e a n d i n t e n s i t y o f t h e p i l o t - p l a n t d e m o n -
s t r a t i o n s w i l l b e g r e a t l y i n f l u e n c e d b y t h e p e r c e i v e d
u n c e r t a i n t ie s o f t h e p r o c e s s d e s i g n e r . I f a n s w e r s a r e
n o t f o r t h c o m i n g ( o r b e t t e r y e t , a n t i c i p a t e d a n d v o l u n -
t e e r e d ) , t h e d e v e l o p m e n t w i l l s t a ll .
R e g a r d l e s s o f w h e t h e r a p r o g r a m i s p e r c e i v e d t o b e
m a r k e t d r i v e n o r s c i e n c e d r i v e n , s o m e e x p e r i m e n t a l
d a t a a t e a c h s t a g e o f a p r o g r a m w i l l b e v i e w e d b y a
p r o c e s s d e s i g n e r a s p a r t i c u l a r l y c r it i ca l . I n m y e x p e r i -
e n c e , u n d e r s t a n d i n g t h e i m p l i c a t i o n s o f th e :
cha rac te r i s t i c s of the ca ta lys t ,
p r o t o c o l f o r m a n u f a c t u r e o f t h e c a t a l y s t, a n d
c o n t e m p l a t e d ( p r o j e c t e d ) c o m m e r c i a l r e a c t o r , a r e
c r i ti c a l t o t h e s u c c e s s o f t h e p r o g r a m .
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Table 1
Characteristics of industrial heterogeneous catalysts a
Activity
Selectivity
Stability
Morphology
Mechanical
Thermal
Regenerability
Manufacturability
Reactor productivity/operatingconditions
Undesirable/competitive eactions
Poisoning, fouling, sintering, phase transfor-
mation
Form, g rain size, filling density
Strength/crushing/attrition abrasion/erosion
Therm al conductivity, specific heat
Activity, selectivity,mech anical strength
Reproducibility
Adopted from Ref. [9].
T h e a p p a r e n t s i m i l a r it y o f F i g s . 1 a n d 2 m a y b e
m i s l e a d i n g t o s o m e . T h e e n g i n e e r i n g a n a l y s e s d o n e
b e t w e e n e a c h o f t h e in d i c a t e d s t a g e s a n d t h e c o m -
m e r c i a l d e s i g n a r e , in m y e x p e r i e n c e , q u i t e d i f f e r e n t,
f o r m a r k e t - d r i v e n a n d s c i e n c e - d r i v e n p r o g r a m s . A
c o m m e r c i a l d e s i g n n o t o n l y a d d r e s s e s e q u i p m e n t ,
s y s t e m s , a n d p h y s i c a l l a y o u t , b u t a l s o c a t a l y s t p r e -
p a r a t i o n o n a c o m m e r c i a l s c a l e , s a f e o p e r a t i n g p r o -
c e d u r e s , a n a l y t i c a l m e t h o d s , p r o d u c t s p e c i f i c a t i o n s ,
e t c . I n a s c i e n c e - d r i v e n p r o g r a m , m a n y o f t h e s e w i l l b e
d o n e f o r t h e ' f i rs t t i m e ' o n a c o m m e r c i a l s c a l e .
W h e n i n a s e r i e s o f s t u d i e s o n e i g n o r e s o r m i n i -
m i z e s th e i m p o r t a n c e o f s t u d y i n g s o m e c h a r a c t e r i st i c s
o f t h e c a t a l y s t ( T a b l e 1 ) b y e i t h e r a s s u m i n g t h e a n s w e r
o r r e a s o n i n g b y a n a l o g y , o n e i s a s k i n g f o r f u t u r e
d i f fi c u lt ie s . F o r e x a m p l e , t h e m o r p h o l o g y o f a c a t a l y s t
i s o f t e n c r i t i c a l t o t h e s e l e c t i o n o f t h e c o m m e r c i a l
r e a c t o r s y s t e m . T h e p h y s i c a l f o r m o f a c a t a ly s t a n d i t s
g r a i n s i z e m u s t b e s u i t e d t o t h e p r o j e c t e d r e a c t o r .
U n f o r t u n a t e l y , t h e f o r m , g r a i n s i z e , a n d f i l li n g d e n s i t y
o f a c a t a l y s t a r e a l l r e l a t e d t o i t s p o r e v o l u m e ; t h e r e -
f o r e , c h a n g e s i n w h a t a p p e a r t o b e u n i m p o r t a n t p h y -
s i c a l p r o p e r t i e s m a y h a v e a c o m p l e x i n t e r a c t i o n o n
ca ta lys t ac t iv i ty and , pa r t i cu la r ly , s e l ec t iv i ty .
F o r a p r o c e s s d e s i g n e r t h e r e p r o d u c i b i l i t y o f t h e
c a t a l y s t m a n u f a c t u r i n g p r o t o c o l i s c r i t ic a l i n h i s e v a -
l u a t i o n o f t h e l a b o r a t o r y a n d p i l o t - p l a n t d a t a . W e a r e
a l l a w a r e t h a t t h e p e r f o r m a n c e o f a c a t a l y s t d e p e n d s o n
h o w i t h a s b e e n m a n u f a c t u r e d ; s e e m i n g l y , s m a l l d i f -
f e r e n c e s in t h e p r o c e d u r e c a n h a v e a m a r k e d i m p a c t o n
t h e p e r f o r m a n c e o f a c a t a l y s t . H o w e v e r , I h a v e b e e n
i n v o l v e d i n s e v e r a l d e v e l o p m e n t p r o g r a m s w h e r e
s i g n i f ic a n t e f f o r t a n d t i m e h a d t o b e d e v o t e d t o r e p r o -
d u c i n g ' t h e b a t c h ' o f t he c a t a l y s t t h at p e r f o r m e d v e r y
w e l l m o n t h s b e f o r e . W h e n t h i s h a p p e n s , a p r o g r a m
c a n b e s e r i o u s l y d i sr u p t e d ; i f i t o c c u r s m o r e t h a n o n c e ,
p a r t i c u l a r l y w h e n s t u d i e s ar e b e i n g c o n d u c t e d i n p i l o t
u n i ts , t h e p r o g r a m i s i n j e o p a r d y o f b e i n g t e r m i n a t e d .
T h e l i f e o f a c a t a l y s t ( n u m b e r o f r e g e n e r a t i o n s a n d
h o u r s o n - l i n e ) w i l l d e t e r m i n e m a n y i m p o r t a n t f a c t o r s
i n t h e p r o c e s s d e s i g n , m a n u f a c t u r i n g o p e r a b i l i t y , a n d
p r o j e c t e c o n o m i c s . U n f o r t u n a t e l y , l a b o r a t o r y p r o -
g r a m s a n d p i l o t - p l a n t t e s t s w i l l n o t ( a t l e a s t f o r a
p r o c e s s d e s i g n e r ) y i e ld u n e q u i v o c a l i n f o r m a t i o n a b o u t
t h e c a t a l y s t l i fe , w h i c h o n l y a c o m m e r c i a l e x p e r i e n c e
w i l l . T h e r e f o r e , t h e d e v e l o p m e n t p r o g r a m s h o u l d
g e n e r a t e i n f o r m a t i o n a b o u t e a c h o f t h e p a t h w a y s ,
e .g . , po i soning , foul ing , s in te r ing , e t c . t ha t can spec i -
f i c a ll y i m p a c t t h e l i f e o f th e c a t a l y s t u n d e r s t u d y a n d
h o w o n e g o e s a b o u t t o r e c o v e r a c t i v i t y [ 3 , 4 ] . P r o j e c -
t i o n s a b o u t t h e c a t a l y s t l i f e a r e m o r e r e l i a b l e i n a
m a r k e t - d r i v e n p r o g r a m .
A l a r g e n u m b e r o f d i v e r s e r e a c t o r t y p e s a r e u s e d
c o m m e r c i a l l y . I n t e x t b o o k s , t h e r e i s a t e n d e n c y t o
d i s c u s s t h e s e i n i d e a l i z e d t e r m s s u c h a s P F R A a n d
C S T R S . A f e e l i n g f o r t h e r i c h n e s s a n d v a r i e t y o f
Table 2
There is a comp lex nteraction between the catalyst and the reactor a
Reac tor type Residence time distribution Catalyst
Fluid(s) Solid Activity Tem perature
Fixed bed PF Batch f(t) f(Z,R)
Fluid bed PF CST, STR f(t) 'Uniform'
Slurry CST or PF CST, STR f(t) 'Uniform'
Gauze 'PF/CST' Batch f(t) 'Uniform'
Trickle bed 'PF' Batch f(0 f(ZvR)
Mo ving bed PF PF Constant f(Z,R)
Transport line PF PF Constant f(Z)
a Adopted from Ref. [1].
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c o m m e r c i a l r e a c t o r s i s e s s e n t ia l ; t h i s c a n b e o b t a i n e d
f r o m a s t u d y o f t h e p a t e n t l i t e r a t u r e o r a c a r e f u l
r e a d i n g o f R e f . [ 5] .
W h a t i s i m p o r t a n t f o r t h e p r o c e s s d e s i g n e r a n d
u l t i m a t e l y t h e o p e r a t o r o f t h e p l a n t i s t h e c o m p l e x
i n t e r a c t i o n b e t w e e n t h e c a t a l y s t , t h e r e a c t a n t s a n d
p r o d u c t s , a n d t h e r e a c t o r ( T a b l e 2 ). T h e p h a s e r e l a -
t ions in the reac t ion space , e .g . , so l id- l iquid , a re
c r i t i c a l t o b o t h t h e s e l e c t i o n a n d t h e d e s i g n o f a
r e a c t o r . T h e y a r e p a r t i c u l a r l y i m p o r t a n t w h e n , t o
o p t i m i z e c o n v e r s io n a n d y i e ld , a n u m b e r o f r e a c t o rs
a r e c o m b i n e d ( i n t e r c o n n e c t e d ) i n t o a r e a c t o r s y s t e m .
W h i l e m a n y t e x t b o o k s d i s c u s s g i v e n s p e c if i c f l o w a n d
t h e r m a l c o n d i t i o n s , c e r t a i n r e a c t o r s a s ' i d e a l ' , n e v e r -
t h e l e s s a r o b u s t r e a c t o r d e s i g n m u s t , a t a m i n i m u m ,
addres s :
F o r m a t i o n o f f l o w p a t t e r n s i n t h e r e a c t o r , e . g ., d e a d
z o n e s , s h o r t - c i r c u i t f l o w s , a n d c h a n n e l i n g ;
T r a n s p o r t p r o c e s s e s i n t h e i n d i v i d u a l p h a s e s i n t h e
r e a c t o r , e . g . , a x i a l b a c k m i x i n g ;
T e m p e r a t u r e a n d c o n c e n t r a t i o n p r o f i l e s t h a t a r e a
r e s u l t o f t r a n s p o r t r e s i s t a n c e i n , a n d b e t w e e n
phases ;
S e g r e g a t i o n p r o c e s s e s ;
I n c o m p l e t e m i x i n g o f r e a ct a n ts ;
A c c o m m o d a t i o n o f c a t a ly s t a c t iv i ty ; a n d
D e a c t i v a t i o n .
I n m y j u d g m e n t , t h e b e s t d i s c u ss i o n o f t h e c o m p l e x
i n t e r a c ti o n s t h a t a p r o c e s s d e s i g n e r m u s t c o p e w i t h c a n
b e f o u n d i n [ 1 ] . T h o s e w h o c a n o b t a i n a r e a s o n a b l e
m a s t e r y o v e r t h e m a t e r i a l i n t h i s t e x t ( p a r t i c u l a r l y ,
s i n c e i t w a s n o t w e l l p r o o f r e a d ) a r e p r e p a r e d t o
a d d r e s s t h e c h a l l e n g e o f t h e c u r r e n t l i t e r a t u r e .
I n t h i n k i n g a b o u t c a t a l y s t p r o c e s s d e v e l o p m e n t , i t i s
u s e f u l t o c o n s id e r w h a t m e a n i n g f u l i n f o r m a t i o n c a n b e
d e r i v e d f r o m e x p e r i m e n t s i n l a b o r a t o r y - s c a l e a n d
p i l o t - p l a n t r e a c t o r s ( T a b l e 3 ) . I n p r a c t i c e , t h e s c o p e
o f t h e l i s t e d s t u d i es o f t e n o v e r l a p , s o m e t i m e s s i g n if i -
c a n t l y ; t h e d e g r e e o f o v e r l a p a n d t h e t i m e s p e n t f o r
e a c h s t u d y w i l l d e p e n d s i g n i f i c a n t l y o n t h e l e v e l o f
e x p e r i e n c e a n o r g a n i z a t i o n h a s w i t h t h e n a t u r e o f t h e
c a t a l y s t - r e a c t o r c o m b i n a t i o n u n d e r s tu d y . F o r e x a m -
p l e , L u r g i ( a w e l l - e s t a b l i s h e d l i c e n s o r o f m e t h a n o l
c a t a l y s t s a n d p r o c e s s t e c h n o l o g y ) w o u l d , i n m y j u d g -
m e n t , a p p r o a c h t h e d e v e l o p m e n t o f a n F C C c a t a l y s t
v e r y d i f f e r e n t l y f r o m h o w E x x o n ( a p i o n e e r i n F C C
t e c h n o l o g y ) w o u l d . T h e c o n v e r s e w o u l d b e t ru e , in m y
36 1997) 367-3 74 371
Table 3
Applicability of laboratory-scale reactors to the development of
catalytic processes
Objective of st ud y Understanding nformation sought
Catalyst screening
Param eter screening
Reaction network
Kinetic parameters
Catalyst life
Process 'optimization'
Scaleup"
Developm ent of catalyst candidates
Identification of reaction variables
Characterization of reactio n pathways
(in a lumped sense)
Development of a 'kinetic model'
Impa ct of operating conditions
Def'me best combination' of catalyst and
conditions
Validation o f critical reactor design issues
a Scaleup studies can be conducted in a variety of units, e.g., pilot
plants, comm ercialunits and hydraulic mockups; the issues being
studied will determine what unit is used. Adopted from Ref. [6].
j u d g e m e n t , i f E x x o n w e r e t o u n d e r ta k e t h e d e v e l o p -
m e n t o f a m e t h a n o l p r o c e s s a n d c a t a l y s t .
D e s i g n o f a r e a c t o r i s f a c i l i ta t e d b y t h e a v a i l a b i l i t y
o f a m a t h e m a t i c a l m o d e l t h a t c o m b i n e s i n f o r m a t i o n
o n r e a c t i o n t h e r m o d y n a m i c s , k i n e t i c s , a n d t r a n s p o r t -
k i n e t i c i n t e r a c t i o n s , e . g . , h e a t a n d m a s s t r a n s f e r
e f f e c t s . T h e e x p e r i m e n t a l i n p u t s n e e d e d t o s u p p o r t
t h e d e v e l o p m e n t o f t h e m o d e l c a n b e o b t a i n e d f r o m
s t u d i e s i n l a b o r a t o r y - s c a l e r e a c t o r s y s t e m s . T h e t r a d i -
t i o n a l a p p r o a c h ( w h i c h i s t h e o n l y a p p r o a c h u n d e r -
s t o o d b y a n o v e r w h e l m i n g m a j o r i t y o f p r o c e s s
d e s i g n e r s , i n c l u d i n g t h e w r i t e r) i n v o l v e s i n t e r p r e t a ti o n
o f s t e a d y - s t a t e o r q u a s i - s t e a d y - s t a t e r e a c t o r p e r f o r -
m a n c e d a t a f r o m w h i c h k i n e t i c r a t e e q u a t i o n c o n s t a n t s
a r e e v e n t u a l ly e x t r a c t e d b y p a r a m e t e r e s t im a t i o n t e c h -
n i q u e s . W h i l e i t i s k n o w n t h a t s u c h d a t a c a n b e
i n t e r p r e t e d b y m o r e t h a n o n e r e a c t i o n m e c h a n i s m , a
p r o c e s s d e s i g n e r w i l l b e s a t is f ie d i f th e k i n e t i c e x p r e s -
s i o n s a r e b a s e d o n a n u n d e r s t a n d i n g o f t h e p r i n c i p a l
c h e m i c a l p a t h w a y s ( i n a l u m p e d s e n s e ) . F o r t u n a t e l y ,
t h e e x t e n s i v e s tu d i e s o f M o b i l a n d r e l a t e d a c a d e m i c
s t u d ie s p u b l i s h e d i n [ 7] , s u m m a r i z e i n a u s e a b l e f o r m ,
h o w t o g o a b o u t o b t a i n i n g a r e a c t i o n n e t w o r k f o r
m i x t u r e s o f r e a c t a n t s.
C o n t i n u o u s -f l o w l a b o r a t o r y r e a c to r s c a n b e b r o a d l y
c l a s si f i e d b y t h e f l o w p a t t e r n o f t h e g a s , l i q u i d , a n d
sol id phases (F ig . 3) ; quant i t a t ive in t e rpre ta t ion of
e x p e r i m e n t a l d a t a f r o m r e a c t o r s t h a t h a v e a n o n - i d e a l
f lo w , is m u c h m o r e c o m p l e x a n d n o t a s s tr a i g h t f o r w a r d
c o m p a r e d t o t h a t o b t a in e d f r o m r e a c t o rs w h o s e f lo w
p a t t e r n i s e i t h e r p e r f e c t l y b a c k m i x e d o r i n p l u g f l o w
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i N o n - s t ~ s t m I
I
I
Fig. 3. Classification of multiphase laboratory reactors. Adopted from Ref. [6].
[ 8] . M a k i n g a p r o j e c t io n o f t h e p e r f o r m a n c e o f a
c o m m e r c i a l r e a c t o r f r o m s t u d i e s i n a l a b o r a t o r y
r e q u i r e s , a t a m i n i m u m , c a l c u l a t i o n o f
m a s s b a l a n c e s ,
o n e e n e r g y b a l a n c e , a n d
f o r c e b a l a n c e s f o r e a c h f l u i d i n t h e r e a c t o r.
I n a d d i t io n , o n e m u s t d e c o u p l e t h e k i n e t i c s f o r t h e
r e a c t i o n n e t w o r k f r o m t h e d e a c t i v a t i o n o f c a t a l y s t
ac t iv i ty .
M e a n i n g f u l a n d u s e f u l d a t a ( u s e f u l t o a p r o c e s s
d e s i g n e r ) c a n b e s t b e o b t a i n e d i n c o n t i n u o u s i s o t h e r -
m a l r e a c t o r s t h a t h a v e a w e l l - d e f i n e d f l o w p a t t e r n a n d
l o w p r e s s ur e d r o p . C o n t i n u o u s m i c r o r e a c t o r s m e e t i n g
t h e s e c r i t e r i a a r e a v a i l a b l e t o d a y a t a r e a s o n a b l e c o s t ;
m o d e r n m i c r o r e a c t o r s y s t e m s i n c o r p o r a t e :
R e l i a b le m a s s f l o w m e t e r s w i t h a 1 / 2 - 1 % a c c u r a c y
u p t o 5 0 a t m ;
P C - b a s e d c o n t r o l s y s t e m s ; a n d
C o n t i n u o u s a n a l y t i c a l c a p a b i l i t y f o r s t r e a m s c o n -
t a i n i n g g a s e s , l i q u i d s , a n d s e m i s o l i d s , e . g . , w a x e s .
F o r t u na t e l y , t h e c a p a b i l it y o f t h e e q u i p m e n t n e e d e d
f o r c o n t i n u o u s s t u d i e s i n s m a l l - s c a l e r e a c t o r s c o n -
t i n u e s t o i m p r o v e .
T h e n a t u r e o f t h e s m a l l - s c a l e r e a c t o r ( T a b l e 4 ) t o b e
u s e d d e p e n d s b o t h o n t h e o b j e c t iv e s o f t h e s t u d y
( T a b l e 3 ) a n d t h e n u m b e r o f p h a s e s i n v o l v e d i n t h e
r e a c t i o n s y s t e m . T h e r e c o m m e n d a t i o n s g i v e n i n
T a b l e 4 w e r e d e v e l o p e d f r o m p r a c t i c a l a n d t h e o r e ti c a l
r e a s o n i n g s t h a t a r e t o o l e n g t h y t o b e g i v e n h e r e
( u n f o r t u n a t e l y , t h e o n l y t h o r o u g h d i s c u s s i o n , k n o w n
t o t h e w r i t e r, o f t h e r e a s o n i n g s u p p o r t i n g t h e s e r e c o m -
m e n d a t i o n s i s i n a p r o p r i e t a r y r e p o r t [ 1 0 ] ) . T h e g u i -
d a n c e g i v e n i n T a b l e 4 s h o u l d o n l y b e r e g a r d e d a s a
' m a j o r i t y o p i n i o n ' ; s i t u a t i o n s o f t e n e x i s t w h e r e
l a b o r a t o r y r e a c t o r s o t h e r t h a n t h o s e r e c o m m e n d e d
i n T a b l e 4 a r e a b e t t e r a l t e r n at i v e . F o r e x a m p l e , m a n y
f in e c h e m i c a l m a n u f a c t u r i n g p r o c e s s e s u t i l iz e b a t c h o r
Table 4
A laboratory reactor is not a comm ercial reactor
Objective of study Solid-gas Solid-liquid Solid-gas-liquid
Catalyst screening PFR PFR STR (batch) STR (semi-batch)
Parameter screening CSTR CST R STR (batch) STR (semi-batch)
Reaction network CSTR CSTR Trickle bed CST R
Kinetic parameters CSTR CSTR Trickle bed CST R
Catalyst life CSTR PFR CSTR PFK Trickle bed CSTR
Process 'optimization' CSTR PFR CST R PFR Trickle bed CST R
Scaleup PFR fluidized bed PFR slurry Trickle bed slurry
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sem i-ba tch ag i ta ted reac tors . In these cases , the objec -
t i ve s o f a s t udy m a y be a c h i e ve d m or e e f f i c i e n t ly
th r ough the u se o f s im i l a r l a bo r a to r y r e a c to r s .
O ne m us t a lw a ys r e m e m be r t ha t t he sm a l l - s c a l e
e xpe r im e n ta l s t ud i es e nv i s ion e d in Ta b le 4 ge ne r a t e
da t a ; t he y do no t de m ons t r a t e a c om m e r c i a l r e a c to r .
F o r e xa m ple , a C S TR i s r a r e ly u se d i n a c om m e r c i a l
p r oc e s s , s im p ly be c a use f o r a g ive n p r oduc t ion r a t e a
C S T R w i l l ha ve l a r ge r vo lum e tha n a p lug - f low re a c -
to r ( o f t e n c os t i ng m or e ) . H ow e ve r , f o r m a ny sm a l l -
sca le s tudies , CSTRs a re opt ima l s ince :
R a t e s c a n be de r ive d d i r e c t l y f r om s im p le m a te ri a l
ba lances ;
The t e m pe r a tu r e a nd p r e s su r e a r e un i f o r m ;
P a r a m e te r s f o r r e a c t i ve k ine t i c s a nd c a t a ly s t de c a y
can be separa ted; and
C ond i t i ons i n t he r e a c to r do no t va r y by pos i ti on ;
the re fore , the tr anspor t r e s is tances a t phase boun d-
a r ies and wi th in a ca ta lys t can be es t ima ted .
The m e r i t s a nd de m e r i t s o f t he C a r be r r y S p inn ing
B a ske t , B e r ty i n t e r na l r e c yc l e r e a c to r o r e x t e r na l
r e c yc l e C S TR ha ve be e n e x t e ns ive ly d i s c us se d i n
the l i t e r atu r e ; e xpe r i e nc e a nd pe r sona l p r e jud i c e o f t e n
are the bas is for makin g a ch oice in a spec i f ic s itua t ion .
F o r e xa m ple , I do no t l i ke da t a f r om a B e t ty r e a c to r
s inc e one c a nno t be c e r t a in t ha t ga s a nd l i qu id a r e
r e c yc l e d i n kn ow n ( c a l c u la t e d ) p r opo r t ions .
The de ve lopm e n t o f c a t a ly s t s a nd p r oc e s s sy s t e m s
ba se d on t h e m invo lve s m a n ipu la t i on o f a l a r ge num -
be r o f va r ia b l e s ; t he m os t im p or t a n t o f t he se be ing t he
f o l low ing :
C om pos i t i on o f t he c a t a ly s t ;
P r oc e du r e u se d to m a nu f a c tu r e t he c a t a ly s t ; a nd
N a tu r e o f t he e nv i r onm e n t t he c a t a ly s t i s e xpose d
to (e .g . , r eac tants , poison, opera t ing condi t ions) .
S im i l a r ly , c a t a ly s t pe r f o r m a nc e i s a c om pos i t e o f
reac t ion ra te , y ie lds and se lec t iv i ty , ca ta lys t l i f e , and
the pu r i t y o f t he d e s i r e d p r oduc t ( s ).
O ne m igh t b e l i e ve , i n v i e w o f t h is c om ple x i ty , t ha t
c a t a ly s t de ve lopm e n t p r og r a m s w ou ld be ne f i t f r om the
use o f c a r e f u l l y p l a nne d e xpe r im e n ta l de s igns t ha t
pe r m i t c onc lu s ions t o be d r a w n th r ough a r i go r ous
s ta t i s t ica l ana lys is o f the da ta . In recent yea rs , doze ns
o f ' c o o k b o o k s ' o n h o w t o u s e e x p e r im e n t a l de s ig n s
ha ve be e n pub l i she d ; ' c ook so f tw a r e ' t ha t f a c il i t at e s
the des ign and ana lys is of exper iments i s a lso ava i l -
a b l e. H o w e ve r , f a r too f e w s tud ie s , i n m y e xpe r i e nc e ,
u t i l i z e e ve n a s e m b la nc e o f a n e xpe r im e n ta l de s ign .
F o r e xa m ple , I r e c e n t ly s tud i e d ove r 2000 p a t e n t s on a
c om m e r c i a l r e a c t i on o f i n t e r e s t t o a c l i e n t . I n on ly
a bou t 50 o f t he pa t e n t s do t he r e po r t e d c onc lu s ions
a bou t t he r e sponse s e xpe c t e d f r om c ha nge s i n t he
process fac tor s have any s ta t i s t ica l bas is .
W hi l e he t e r oge ne ous c a t a ly s t s c a nno t ye t be
de s igne d , a n unde r s t a nd ing o f t he i r be ha v io r su f f i c ie n t
f o r t he de s ign a nd ope r a t i on o f c om m e r c i a l p r oc e s se s
tha t a r e e nv i r onm e n ta l l y ' a c c e p t a b l e ' , i s i n ou r g r a sp
toda y . H ow e ve r , de ve lop ing a nd c onduc t ing e xpe r i -
menta l s tudies tha t genera te use fu l da ta requi res
answer ing the ques t ion (be fore s ta r t ing a s tudy) :
w hy i s t he s tudy be ing done ? The na tu r e o f t he
phe nom e na to be s tud i e d a nd the p r o j e c t i on o f c om -
m e r c i a l pe r f o r m a nc e t ha t w i l l be m a de f r om the
exper imenta l da ta , i s of c r i t ica l impor tance . Only
w he n th i s ha s be e n c l e a r ly e s t a b l i she d c a n t he be s t
m e thod f o r a n e xpe r im e n ta l s t udy be c hose n . O the r -
wise , the s tudy may resul t in da ta which i s of l i t t le
va lue a nd , un f o r tuna t e ly , m a y be m i s l e ad ing ; a s m a ny
studies do
A r e a d ing l i s t m a y be m or e a pp r op r i a t e f o r m a ny
tha n a s e t o f r e f e re nc e s ; in m y judg m e n t t he f o l l ow ing
are pa r t icu la r ly re levant to the i ssues revi ewe d in th is
ar t ic le .
A c k n o w l e d g e m e n t s
I w ou ld l i ke t o e xp r e s s m y g r a t i tude t o m a n y e xpe r t s
a nd f r i e nds ( p r oc es s de s igne r s) w h o he lpe d m e in t he
p r e pa r a t i on o f t h i s a r t ic l e by p r ov id ing ke y in f o r m a -
t i on o r r e v i e w ing the t e s t . I w ou ld e spe c i a l l y l i ke t o
tha nk G ia nn i A s t a r i t a , M or de c ha i H e r skow i t z a nd
R e ue l S h inna r ; how e ve r , t he pa s s ion w i th w h ic h som e
of t he v i e w s a r e e xp r e s se d i s so l e ly t he r e spons ib i li t y
of the author .
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