5 Industrial Process Models—State of the Art V E R N W. W E E K M A N , JR.
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Mobil Research and Development Corp., Research Dept., Paulsboro, N.J. 08066
Kinetic used
process
models
increasingly
operation.
in
have
come
process
The current
of age and are
development,
design,
and
state of the art in the building
and
use of such reactor process models is reviewed. complete
process
various these
pieces,
problems. ing
models
incomplete
have
pieces
complex
Few, if any,
published;
appeared
to assess
attention
reaction
lems and benefits.
been
have
it is possible
Particular
however,
recently.
current
is given to kinetically
mixtures
along with
In addition,
recent
effects,
bed reactors are reviewed.
remaining
problem
T)rocess models
areas
fluid
in the
prob-
on
flow-packed
particle
bed comparisons,
development
and lump-
attendant
findings
heat and mass transfer
are
From
trends
two-phase models
being
and Finally,
of
process
addressed.
u s e d i n t h e d e s i g n , o p e r a t i o n , a n d o p t i m i z a t i o n of
·*· i n d u s t r i a l reactors a r e t h e raison
d'etre o f r e a c t i o n e n g i n e e r i n g .
They
represent its final p r o d u c t a n d t h e v e h i c l e b y w h i c h t h e b o d y of r e a c t i o n e n g i n e e r i n g t h e o r y is a p p l i e d .
I n this r e v i e w , t h e state of t h e a r t of
process m o d e l s is j u d g e d p r i m a r i l y f r o m r e c e n t l y p u b l i s h e d pieces o f i n d u s t r i a l models. T h e w o r d pieces is a p p r o p r i a t e since, to this r e v i e w e r s k n o w l e d g e , f e w i f a n y c o m p l e t e process m o d e l s h a v e b e e n p u b l i s h e d . Typically, a model
is p u b l i s h e d w i t h t h e rate constants
missing, or
w i t h o u t m e n t i o n o f the m i x i n g patterns t h a t o c c u r i n t h e reactor, o r w i t h the c h e m i s t r y n o t i d e n t i f i e d specifically. T h i s is u n d e r s t a n d a b l e i n v i e w of t h e usefulness operation.
of i n d u s t r i a l process m o d e l s
Complete
models
to reactor d e s i g n a n d
w i l l b e p u b l i s h e d e v e n t u a l l y , b u t , since
t h e y w e r e d e v e l o p e d o n l y d u r i n g t h e last 5 - 1 0 years, a p p a r e n t l y m o r e t i m e m u s t pass before a n y c o m p l e t e , t h o u g h obsolete, m o d e l s w i l l b e published.
B y l o o k i n g b e t w e e n t h e lines, h o w e v e r , w e c a n m a k e some
j u d g m e n t s as to t h e c u r r e n t state of t h e art. I t is also w e l l t o k e e p i n 98 In Chemical Reaction Engineering Reviews; Hulburt, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
5.
Industrial
W E E K M A N , JR.
mind
Process
99
Models
that those pieces a l r e a d y p u b l i s h e d are p r o b a b l y
from
earlier
versions of the c u r r e n t l y u s e d m o d e l a n d t h a t t h e y do n o t represent the latest m o d e l that is a c t u a l l y b e i n g u s e d i n t h e Selected a c a d e m i c
field.
c o n t r i b u t i o n s , w h i c h i n this r e v i e w e r ' s
opinion
bear d i r e c t l y o n the c u r r e n t state of the art, w e r e i n c l u d e d i n this r e v i e w . W i t h the emphasis o n the present state, m a n y v a l u a b l e a c a d e m i c tributions w h i c h m a y have
a large i m p a c t o n f u t u r e generations
conof
m o d e l s w e r e not i n c l u d e d . M o s t of the r e v i e w e d papers w e r e p u b l i s h e d Downloaded by UNIV OF MICHIGAN ANN ARBOR on June 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0148.ch005
w i t h i n the last three years, most since the last I n t e r n a t i o n a l R e a c t i o n E n g i n e e r i n g S y m p o s i u m i n A m s t e r d a m . F i n a l l y , at the e n d of t h e r e v i e w , research areas w h i c h c o u l d g r e a t l y i m p r o v e the c u r r e n t state of the art are discussed.
The Comptent Process Model It is i m p o r t a n t to r e m e m b e r
w h a t t h e i n d u s t r i a l l y u s e f u l process
m o d e l s h o u l d c o n t a i n . T h e c o m p l e t e process m o d e l ( a ) accounts for the effects of the f u l l range of process v a r i a b l e s (e.g.
pressure, flow rates,
and temperature) on product yields and properties; ( b )
allows predic-
t i o n of the effect of a w i d e r a n g e of charge stock c o m p o s i t i o n o n p r o d u c t yields and properties; (c)
p r e d i c t s t h e effects of catalyst a g i n g a n d of
changes i n catalyst properties o n a c t i v i t y a n d s e l e c t i v i t y ; ( d )
encom-
passes t h e effects of process v a r i a b l e s o n m i x i n g a n d o n h y d r o d y n a m i c phenomena; and (e)
has b e e n v e r i f i e d b y extensive p i l o t p l a n t or c o m -
m e r c i a l tests. P r o d u c t properties are u s u a l l y t h e most difficult to q u a n t i f y i n terms of basic c h e m i c a l or p h y s i c a l p h e n o m e n a .
F o r e x a m p l e , the
o m n i p o t e n t octane n u m b e r i n the p e t r o l e u m i n d u s t r y is difficult to c h a r acterize i n a basic sense. I d e a l l y , the process v a r i a b l e s s h o u l d b e l i n k e d to the y i e l d s a n d properties i n terms of f u n d a m e n t a l p h y s i o c h e m i c a l p h e n o m e n a .
In prac-
t i c a l m o d e l s it is not a l w a y s possible to d e s c r i b e e a c h effect i n its most f u n d a m e n t a l f o r m , a n d correlations i n v o l v i n g adjustable p a r a m e t e r s are u s u a l l y r e q u i r e d . B a s i c f u n d a m e n t a l r e l a t i o n s h i p s for a l l v a r i a b l e s m a y b e expensive to ascertain, a n d Prater's o p t i m u m sloppiness p r i n c i p l e ( J ) must be invoked.
T h i s p r i n c i p l e is i l l u s t r a t e d i n F i g u r e 1 w h e r e
fundamentalness of the m o d e l is p l o t t e d vs. its usefulness a n d cost.
the The
f u n d a m e n t a l n e s s p a r a m e t e r has b e e n r o u g h l y q u a n t i f i e d i n terms of the n u m b e r of p h e n o m e n o l o g i c a l l a w s d i v i d e d b y the n u m b e r of adjustable p a r a m e t e r s . W h e n this r a t i o is zero, w e h a v e a p u r e l y c o r r e l a t i v e m o d e l w h e r e a s , w h e n i t is infinite, w e h a v e a p u r e l y t h e o r e t i c a l m o d e l w h i c h contains no adjustable coefficients.
W h i l e the c o r r e l a t i v e m o d e l is c h e a p ,
y o u get w h a t y o u p a y for, a n d its e x t r a p o l a t i v e p r o p e r t i e s are p o o r .
In Chemical Reaction Engineering Reviews; Hulburt, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
On
100
CHEMICAL
REACTION
ENGINEERING
REVIEWS
USEFULNESS
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COST
NET VALUE
0
NUMBER OF PHENOMENOLOGICAL LAWS NUMBER OF ADJUSTABLE CONSTANTS INCREASINGLY FUNDAMENTAL
Figure 1.
Principle
of optimum
-
sloppiness
t h e other h a n d , a p u r e l y t h e o r e t i c a l m o d e l that explains a l l p h e n o m e n a m a y g i v e accurate extrapolations b u t at a n e x o r b i t a n t d e v e l o p m e n t
cost.
T h u s , w e see that t h e net v a l u e w i l l p r o b a b l y h a v e a n o p t i m u m somew h a t short of a l l p h e n o m e n a b e i n g q u a n t i f i e d . I t is u s u a l l y best to k e e p the m o d e l as s i m p l e as p o s s i b l e a n d to a d d o n l y p h e n o m e n a w h i c h c o n t r i b u t e s i g n i f i c a n t l y to a n u n d e r s t a n d i n g of the process v a r i a b l e b e h a v i o r . T o r e t u r n to o u r d e s c r i p t i o n of the c o m p l e t e process m o d e l , i t is c r i t i c a l that i t b e able to p r e d i c t the p r o d u c t d i s t r i b u t i o n f r o m t h e r e a c t i o n over t h e f u l l range of a n t i c i p a t e d charge stock c o m p o s i t i o n .
Few
i n d u s t r i a l processes h a v e s i n g l e - c o m p o n e n t
often
feedstocks,
and, more
t h a n n o t , a c o m p l e x m i x t u r e m u s t b e r e a c t e d . I t is v e r y difficult to treat the r e a c t i o n o f e a c h species, a n d b y necessity w e m u s t l u m p
species
together k i n e t i c a l l y . A s u b s t a n t i a l p a r t of t h e r e v i e w is d e v o t e d to t h e state of t h e a r t o f k i n e t i c l u m p i n g s i n c e i t is so v i t a l to most process models. T h e next c r i t i c a l a t t r i b u t e of o u r i n d u s t r i a l process m o d e l is t h e a b i l i t y to p r e d i c t t h e effects o f catalyst a g i n g a n d changes i n catalyst p r o p e r t i e s o n a c t i v i t y a n d selectivity. O n c e w e k n o w the rate constants for a g i v e n r e a c t i o n scheme, w e h a v e t h e f u l l p o w e r of the m a n y tools of r e a c t i o n e n g i n e e r i n g to a i d us i n d e s i g n a n d o p e r a t i o n . U n f o r t u n a t e l y , w e h a v e f e w g u i d e l i n e s f o r p r e d i c t i n g these rate constants f r o m t h e p r o p e r t i e s of t h e catalyst. A s a consequence, enormous sums of m o n e y are spent e a c h y e a r b y i n d u s t r y f o r r e d e t e r m i n i n g k i n e t i c parameters
In Chemical Reaction Engineering Reviews; Hulburt, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
5.
WEEKMAN, JR.
Industrial
after o n l y s m a l l changes
101
Process Models
i n catalyst properties that r e s u l t f r o m e i t h e r
a g i n g or changes i n c o m p o s i t i o n . A n o t h e r k e y p a r t of o u r process m o d e l is the a b i l i t y to p r e d i c t the effect of process v a r i a b l e s o n m i x i n g a n d fluid d y n a m i c p h e n o m e n a .
In
this reviewer's experience, a significant p o r t i o n of the difficulties e n c o u n t e r e d i n a p p l y i n g i n d u s t r i a l m o d e l s lies i n not f u l l y u n d e r s t a n d i n g these phenomena.
T h u s changes
i n t e m p e r a t u r e or
flow
rate can, i n turn,
change m i x i n g patterns so as to alter the r e a c t i o n greatly. F i n a l l y , o u r Downloaded by UNIV OF MICHIGAN ANN ARBOR on June 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/ba-1975-0148.ch005
m o d e l s h o u l d be v e r i f i e d , at the v e r y least i n extensive p i l o t p l a n t w o r k or, m o r e d e s i r a b l y , i n l a r g e scale c o m m e r c i a l tests. O n l y b y s u c h l a r g e scale testing c a n w e b e sure t h a t t h e m o d e l successfully p r e d i c t s the scale-up of a l l the k e y p h e n o m e n a .
M a n y times s u c h large scale testing
leads to the d i s c o v e r y of p r e v i o u s l y u n a c c o u n t e d for p h e n o m e n a .
Indeed,
i m p o r t a n t discoveries of c r i t i c a l p h e n o m e n a w e r e m a d e b y the f a i l u r e of process models i n s u c h tests. P r a t e r (2)
c a l l e d this t h e strategy of f a i l -
u r e ; w h e n a p p l i e d a l e r t l y , it c a n b e h i g h l y u s e f u l i n s o r t i n g o u t
the
c r i t i c a l b e h a v i o r of the process reactor. Lumped Kinetics in Recent Process Models One
of the k e y p r o b l e m s
i n d e s c r i b i n g the k i n e t i c s of
systems is h o w to l u m p the m a n y components
complex
so t h a t t h e r e s u l t a n t
l u m p e d kinetics describe the system b e h a v i o r a d e q u a t e l y .
Some of the
earliest t h e o r e t i c a l w o r k d e s c r i b i n g the n a t u r e of l u m p e d systems b y A r i s a n d G a v a l a s ( 3 ) a n d A r i s (4). W e i a n d K u o ( 5 , 6) m o l e c u l a r systems.
I n a comprehensive
was
treatment,
d e s c r i b e d the errors i n v o l v e d i n l u m p i n g m o n o -
A s a matter of p r a c t i c a l i t y , most i n d u s t r i a l systems
are s t r o n g l y c o n s t r a i n e d to l u m p i n g those species w h i c h c a n b e r e a d i l y identified.
T h e r e is also strong i n c e n t i v e to l u m p species i n terms of
those t h a t are the final p r o d u c t s of the process. Catalytic Reforming.
I n the area of the c a t a l y t i c r e f o r m i n g
petroleum naphthas, Smith (7) kinetics.
His
reaction
scheme
paraffins as single components. compounds
of
w a s one of the first to present l u m p e d treated
aromatics,
naphthenes,
E a c h l u m p e d species c o n t a i n e d
and many
w h i c h w e r e v e r y l i k e l y to h a v e different r e a c t i o n rates.
In
spite of this, the k i n e t i c s w e r e a d e q u a t e to d e s c r i b e the o v e r a l l b e h a v i o r of the reformers.
R e c e n t l y D o r o k h o v et al. ( 8 )
also successfully
s c r i b e d r e f o r m e r b e h a v i o r u s i n g a m o d i f i c a t i o n of this s c h e m e 2).
de-
(Figure
U n f o r t u n a t e l y , t h i s l u m p i n g is so coarse t h a t i t is sometimes difficult
to d e s c r i b e the i m p o r t a n t properties of the system (e.g.
octane).
Here
the d i s t r i b u t i o n of c o m p o u n d s a m o n g the paraffins, n a p h t h e n e s , a n d a r o matics becomes i m p o r t a n t . T h i s s h o r t c o m i n g was
rectified b y
K m a k (9)
who
described
a
l u m p e d system for r e f o r m i n g t h a t c o n t a i n e d 22 l u m p e d species. H i s b a s i c
In Chemical Reaction Engineering Reviews; Hulburt, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1975.
102
CHEMICAL
Ρ
t
5 0
0.1
j-υ·
/Y 0.2 0.3 0.5 0.7 1.0 2.0 3.0 5.0 AROMATIC TO NAPHTHENE WT RATIO
10.0
Industrial and Engineering Chemistry, Process Design and Development
Figure 16. Relationship between catalyst de cay constant and aromatic-to-naphthene ratio (16) 900 °F
ο
/
0
°/ ° °/o •β 30
