Industrial Process ModelsState of the Art

of the usefulness of industrial process models to reactor design and operation. ...... cloud and wake model does not appear to be appreciably more acc...
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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