Polymerization Reactors and Processes - American Chemical Societypubs.acs.org/doi/pdf/10.1021/bk-1979-0104.ch010between...
2 downloads
230 Views
2MB Size
10 Free-Radical Polymerization: Sensitivity of Conversion and Molecular Weights to Reactor Conditions KIU
H . LEE and J O H N P. M A R A N O , JR.1
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
Union Carbide Corporation, P. O . Box 8361, South Charleston, W V 25303
One o n - g o i n g objective in a c o m m e r c i a l polymerization reactor of a fixed size is to m a x i m i z e t h e reactor productivity at the desired p r o d u c t properties. Polymerization reactors a r e sensitive to c h a n g e s in operating p a r a m e t e r s b e c a u s e the reactors involve highly e x o t h e r m i c reactions. A relatively m i n o r fluctuation in t h e operating variables could c a u s e w i d e fluctuations in the reactor r e s p o n s e s . T h e r e f o r e , it is i m p o r t a n t to search o u t underlying relationships c o n c e r n i n g t h e reactor p e r f o r m a n c e and t h e modes of reactor operations. The conceptual t r e n d s obtained in this t y p e of investigation provide valuable information regarding t h e operating limits o f a given reactor a n d also aid in d e t e r m i n i n g t h e future actions a i m e d a t further i m p r o v i n g t h e limits o f the reactor p e r f o r m a n c e . The c o n c e p t u a l s t u d y may dictate c h a n g e s in the initiator s y s t e m , t h e solvent s y s t e m ( c h a i n transfer a g e n t s ) and the h e a t transfer s y s t e m for a reactor of fixed size to provide t h e maximum possible c o n v e r s i o n a t desired p r o d u c t properties. The s t u d y o f t h e peak t e m p e r a t u r e s e n s i t i v i t y t o t h e r e a c t o r o p e r a t i n g p a r a m e t e r s and t h e c o n s t r u c t i o n o f s e n s i t i v i t y b o u n d a r y c u r v e s f o r s t a b l e r e a c t o r o p e r a t i o n were p r e v i o u s l y r e p o r t e d ( l ) . T h i s p a p e r p r e s e n t s a computer s t u d y on c o n c e p t u a l r e l a t i o n s h i p s between t h e c o n v e r s i o n - p r o d u c t p r o p e r t i e s and t h e r e a c t o r o p e r a t i n g parameters i na p l u g f l o w t u b u l a r r e a c t o r of f r e e r a d i c a l polymerization. I n p a r t i c u l a r , a c o n t o u r map o f c o n v e r s i o n molecular weight r e l a t i o n s h i p s i n a r e a c t o r o f f i x e d s i z e i s presented and t h e s e n s i t i v i t y o f i t s r e l a t i o n s h i p t o t h e c h o i c e o f i n i t i a t o r system, s o l v e n t system and heat t r a n s f e r system a r e discussed. In t h e study, t h e k i n e t i c r a t e constants a p p l i c a b l e t o t h e p o l y m e r i z a t i o n o f e t h y l e n e (2_,3_) were u s e d w i t h a n assumed a c t i v a t i o n v o l u m e . These v a l u e s a p p e a r t o b e a r e a s o n a b l y c o n s i s t e n t s e t o f c o n s t a n t s f o r t h e p o l y m e r i z a t i o n o f e t h y l e n e and, a s shown Current
a d d r e s s : 'Now l o c a t e d a t M o b i l C h e m i c a l Company, E d i s o n , New J e r s e y .
0-8412-0506-x/79/47-104-221$07.75/0 © 1979 American Chemical Society In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
222
POLYMERIZATION REACTORS AND PROCESSES
i n F i g u r e 1 , g e n e r a t e a t e m p e r a t u r e p r o f i l e w h i c h a p p e a r s t o be r e a s o n a b l y t y p i c a l o f t h e one m e a s u r e d i n a h i g h - p r e s s u r e ethylene polymerization reactor. T h i s work p a r t i c u l a r l y emphasizes t h e importance o f s e l e c t i n g t h e i n i t i a t o r s y s t e m f o r optimum r e a c t o r o p e r a t i o n a n d r e v e a l s g e n e r a l concepts w h i c h s p e c i f y t h e d e s i r e d p r o p e r t i e s and o p e r a t i o n a l modes o f an optimum i n i t i a t o r s y s t e m . In addition,the e f f e c t s o f t h e s y s t e m h e a t t r a n s f e r a n d t h e CTA ( c h a i n t r a n s f e r a g e n t ) l e v e l on t h e c o n v e r s i o n - m o l e c u l a r w e i g h t s r e l a t i o n s h i p s are presented.
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
P o l y m e r i z a t i o n Tubular
Reactor
Model
The c o m p u t e r m o d e l u s e d f o r t h i s a n a l y s i s i s b a s e d on a p l u g f l o w t u b u l a r r e a c t o r o p e r a t i n g u n d e r r e s t r a i n t s o f t h e commonly a c c e p t e d k i n e t i c mechanism f o r p o l y m e r i z a t i o n r e a c t i o n s (3. h). The c o m p u t e r m o d e l c o n s i s t s o f t h e n u m e r i c a l i n t e g r a t i o n o f a s e t of d i f f e r e n t i a l equations which conceptualizes t h e high-pressure p o l y e t h y l e n e r e a c t o r . A Runge-Kutta technique i s used f o r i n t e g r a t i o n w i t h t h e use o f an a u t o m a t i c a l l y a d j u s t e d i n t e g r a t i o n step s i z e . The e q u a t i o n s u s e d f o r t h e c o m p u t e r m o d e l a r e shown i n A p p e n d i x A. The e l e m e n t s o f t h e m o d e l a r e t h e r e a c t i o n m e c h a n i s m , h e a t and mass b a l a n c e e q u a t i o n s , a n d t h e m o l e c u l a r w e i g h t moment e q u a t i o n s , w h i c h a r e n u m e r i c a l l y i n t e g r a t e d w i t h r e a c t o r l e n g t h . The m o l e c u l a r w e i g h t moment e q u a t i o n s w e r e d e r i v e d u s i n g moment g e n e r a t i n g f u n c t i o n s (5.). T h i s method o f d e r i v a t i o n a p p e a r s t o be t h e most r e l i a b l e t e c h n i q u e f o r d e r i v i n g m o l e c u l a r w e i g h t moments. The a s s u m p t i o n s u s e d i n t h e c o m p u t e r p r o g r a m a r e l i s t e d i n t h e Appendix. B u t u n l i k e many p o l y m e r i z a t i o n m o d e l s , no a s s u m p t i o n s a r e made c o n c e r n i n g t h e s t e a d y - s t a t e c o n c e n t r a t i o n o f r a d i c a l s s i n c e t h e r a d i c a l s w i l l n o t be a t s t e a d y - s t a t e under c o n d i t i o n s o f r a p i d l y changing temperature over t h e e n t i r e range o f r e a c t o r c o n d i t i o n s w h i c h must b e c o n s i d e r e d i n t h i s a n a l y s i s . The c o m p u t e r m o d e l u s e d i n t h i s a n a l y s i s was d i s c u s s e d p r e v i o u s l y (l 6) a n d a r e s i m i l a r , i n g e n e r a l c o n c e p t s , t o o t h e r models ( £ , 8 ) d i s c u s s e d i n t h e l i t e r a t u r e . The c o m p u t e r p r o g r a m was w r i t t e n f o r u s e on IBM 370/65 c o m p u t e r . 9
9
Use o f R e a c t o r M o d e l . I n order t o begin t h e study o f t h e s e n s i t i v i t y o f t h e r e a c t o r responses, t h e s i m p l e s t r e a c t o r conf i g u r a t i o n p o s s i b l e was c h o s e n . T h i s paper considers t h e case o f c o n s t a n t p r e s s u r e , c o n s t a n t heat t r a n s f e r c o e f f i c i e n t and constant j a c k e t o r w a l l temperature, w i t h i n i t i a t i o n o c c u r r i n g by a f r e e r a d i c a l g e n e r a t o r w h i c h decomposes b y a f i r s t - o r d e r r a t e process. The e f f i c i e n c y o f t h e i n i t i a t o r s c o n s i d e r e d i s assumed c o n s t a n t (=0.5) and, as w i t h t h e i n i t i a t o r e f f i c i e n c y , a l l o t h e r r a t e c o n s t a n t s a r e assumed i n d e p e n d e n t o f v i s c o s i t y . Following t h i s i n i t i a l i n v e s t i g a t i o n , an o p t i m i z a t i o n s t u d y o f r e a c t o r c o n f i g u r a t i o n s i s p l a n n e d , l i f t i n g most o f t h e s e i n i t i a l r e s t r i c t i o n s .
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
LEE AND MARANO
Polymerization
223
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
Free-Radical
I
o5
0
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
1 90
I 100
P e r Cent R e a c t o r L e n g t h Figure 1. Typical reactor temperature profile for continuous addition polymerization: a plug-flow tubular reactor. Kinetic parameters for the initiator: I = 10 ppm; E = 32.921 kcal/mol; In k ' = 26.492 In sec' ; f = 0.5. Reactor parameter: [(4hTp)/ (DpC )] = 5148.2. [(C ) = heat capacity of the reaction mixture; (p) = density of the reaction mixture; (h) = overall heat-transfer coefficient; (Tj) = reactor jacket temperature; (t) = reactor residence time; (D) — reactor diameter]. o
d
d
p
1
p
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
224
The f i x e d v a r i a b l e s u s e d i n t h e c o m p u t e r s i m u l a t i o n a r e shown i n Table 1 along w i t h the k i n e t i c r a t e constants f o r the polymerization reactions. Since i t i s a conceptual study employing a t h e o r e t i c a l r e a c t o r model, i t i s a l s o important t o appreciate the l i m i t s o f t h i s type of i n v e s t i g a t i o n . The a d v a n t a g e o f t h e c o m p u t e r i n v e s t i g a t i o n over a p i l o t or p r o d u c t i o n r e a c t o r i n v e s t i g a t i o n i s the o b v i o u s c o s t and t i m e s a v i n g o v e r t h e r e a l r e a c t o r e x p e r i m e n t . The c o m p u t e r i n v e s t i g a t i o n c a n a l s o y i e l d a more d e f i n a b l e r e l a t i o n s h i p w i t h f e w e r p a r a m e t e r e x c u r s i o n s s i n c e t h e o u t p u t w i l l be free of s c a t t e r . I n a d d i t i o n , excursions i n r e a c t o r parameters c a n be t a k e n w h i c h m i g h t be c o n s i d e r e d u n s a f e on o r b e y o n d t h e e q u i p m e n t l i m i t a t i o n s o f an e x i s t i n g r e a l r e a c t o r . The p i t f a l l s o f a c o m p u t e r m o d e l a r e o b v i o u s i n t h a t i t i s o n l y a c o n c e p t u a l r e p r e s e n t a t i o n o f t h e r e a c t o r and i n c l u d e s o n l y as many a s p e c t s o f t h e r e a l r e a c t o r as p r e s e n t k n o w l e d g e permits. I n a d d i t i o n , e v e n t h e most p e r f e c t l y c o n c e i v e d d e s c r i p t i o n w i l l s t i l l depend u p o n t h e a c c u r a c y o f t h e p h y s i c a l l y measu r e d c o n s t a n t s used i n t h e model f o r t h e q u a l i t y o f t h e p r o c e s s representation. The g o a l o f t h i s r e p o r t i s , h o w e v e r , o n l y t o show c o n c e p t u a l t r e n d s and t h e t e c h n o l o g i c a l b a s e i s d e v e l o p e d t o t h e e x t e n t t h a t t h e c o n c e p t u a l t r e n d s w i l l be c o r r e c t . In some r e s p e c t s t h e c o m p u t e r m o d e l i s a b e t t e r p r o c e s s d e v e l o p m e n t t o o l t h a n t h e p i l o t p l a n t u s e d f o r t h e LDPE p r o c e s s s i n c e t h e p i l o t r e a c t o r does n o t y i e l d d i r e c t l y s c a l e a b l e i n f o r m a t i o n . The reader should take care t o d i r e c t h i s a t t e n t i o n t o the t r e n d i n f o r m a t i o n and c o n c e p t u a l d i f f e r e n c e s d e v e l o p e d i n t h i s w o r k ; v e r y l i t t l e a t t e n t i o n s h o u l d be p a i d t o t h e a b s o l u t e v a l u e s o f t h e parameters given. Theoretical
Considerations
The o v e r a l l r e a c t i o n s i n v o l v e d i n a f r e e r a d i c a l p o l y m e r i z a t i o n are d e s c r i b e d i n the Appendix. I t i s i n t e r e s t i n g however, t o l o o k i n t o s e v e r a l r e a c t i o n s t e p s w h i c h c o n t a i n t h e key r e a c t i o n p a r a m e t e r s and c o n t r o l t h e r a t e o f p r o d u c t i o n and t h e m o l e c u l a r weights o f the polymer. F o r i l l u s t r a t i o n o f some s i m p l e h i g h l i g h t s , t h e r a t e o f p o l y m e r i z a t i o n i s g i v e n by t h e r e l a t i o n s h i p Rp
= k [M][R*]
(1)
p
t h e r a t e o f r a d i c a l g e n e r a t i o n i s g i v e n by t h e R
R
= R.
- 2k R*
=
(2)
2
t
and t h e r a t e o f i n i t i a t i o n i s g i v e n toy t h e %
relationship
relationship
2fk [l] d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
(3)
10.
LEE AND MARANO
Free-Radical
Polymerization
225
TABLE I
REACTION AND REACTOR PARAMETERS USED I N THE COMPUTER SIMULATION
KINETIC RATE CONSTANTS
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
Log Base e Frequency F a c t o r ( l , mole, s e c ) Pll TC T CFM CFP CFS BETA*
IT.891 20.796 1.198 -U.616 -U.963 -0.577 27.0
A c t i v a t i o n Energy/R (°C) 3573.0 150.0 150.0 1988.k 30U.3 1U76.2 139^0.0
A c t i v a t i o n Volume/R (°C/atmo) -0.2800 -0.1710 -0.1100 0.0366 0.0366 0.0366 0.21+U
PHYSICAL PARAMETERS BMO = 1 7 . 1 7 6 PP = 33,000 CP = 16.2399 BM = 28.0 HEATRO = 22,300.00 TF = 5 8 . 0 DMO = 1 7 . 1 7 6 P l l = propagation rate TC = t e r m i n a t i o n b y c o m b i n a t i o n r a t e c o n s t a n t T = r a t i o o f t h e t e r m i n a t i o n r a t e constant f o r combination to t h e r a t e constant f o r d i s p r o p o r t i o n a t i o n CFM = r a t i o o f t h e r a t e c o n s t a n t f o r monomer t r a n s f e r t o t h e constant f o r propagation CFP = r a t i o o f t h e r a t e c o n s t a n t f o r p o l y m e r t r a n s f e r ( l o n g chain branching) t o t h e constant f o r propagation CFS = r a t i o o f t h e r a t e c o n s t a n t f o r s o l v e n t t r a n s f e r t o t h e constant f o r propagation BMO = i n l e t monomer ( e t h y l e n e ) c o n c e n t r a t i o n , m o l e / l PP = r e a c t o r p r e s s u r e , p s i a BM = monomer ( e t h y l e n e ) m o l e c u l a r -weight CP = h e a t c a p a c i t y a t c o n s t a n t p r e s s u r e o f t h e r e a c t i o n f l u i d , cal/mole-°C HEATRO = h e a t o f r e a c t i o n f o r t h e p o l y m e r i z a t i o n , c a l / m o l e TF = r e a c t o r i n l e t t e m p e r a t u r e , °C DMO = r e a c t o r f l u i d d e n s i t y , m o l e / 1 BETA = 3 - s c i s s i o n r e a c t i o n r a t e c o n s t a n t *Though t h i s r e a c t i o n i s i m p o r t a n t i n LDPE r e a c t o r s , i t was i g n o r e d i n t h e present s i m u l a t i o n because o f t h e u n c e r t a i n t y o f t h e r a t e c o n s t a n t v a l u e and f o r s i m p l i f i c a t i o n aimed a t r e p r e s e n t i n g t r e n d s .
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
226
F u r t h e r , the major m o l e c u l a r weight c o n t r o l s are accomplished i n t h e r a t e o f c h a i n t r a n s f e r "by s o l v e n t ( R t s ^ * " "ke o f r a d i c a l t e r m i n a t i o n s (R-t ) b y c o m b i n a t i o n and d i s p r o p o r t i o n a t i o n . a n c
tlie
ra
r
*ts R
t r
= k = k
t s
t r
W
[s][R*] [R*]
2
(5)
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
where kp = p r o p a g a t i o n r a t e "constant" k^ = t e r m i n a t i o n r a t e "constant" k^ = f i r s t o r d e r r a t e " c o n s t a n t " f o r t h e i n i t i a t o r breakdown [M] = monomer c o n c e n t r a t i o n [i] = initiator concentration [R*] = r a d i c a l c o n c e n t r a t i o n f = initiator efficiency •^ts chain t r a n s f e r rate "constant" ktr t e r m i n a t i o n (combination or d i s p r o p o r t i o n a t i o n ) rate constant [s] = solvent (chain t r a n s f e r agent) c o n c e n t r a t i o n =
=
E q u a t i o n ( l ) shows t h e r a t e o f p o l y m e r i z a t i o n i s c o n t r o l l e d by t h e r a d i c a l c o n c e n t r a t i o n and as d e s c r i b e d by E q u a t i o n (2) t h e r a t e o f g e n e r a t i o n o f f r e e r a d i c a l s i s c o n t r o l l e d by t h e i n i t i a tion rate. I n a d d i t i o n , E q u a t i o n (3) shows t h i s r a t e o f g e n e r a t i o n i s c o n t r o l l e d b y t h e i n i t i a t o r and i n i t i a t o r concentration. F u r t h e r , the r a t e of i n i t i a t i o n c o n t r o l s the r a t e of propagation which c o n t r o l s the rate of generation of heat. T h i s combined w i t h t h e h e a t t r a n s f e r c o n t r o l s t h e r e a c t i o n t e m p e r a t u r e and t h e v a l u e o f t h e v a r i o u s r e a c t i o n r a t e c o n s t a n t s o f t h e k i n e t i c mechanism. T h r o u g h t h e s e e v e n t s i t becomes o b v i o u s t h a t t h e i n i t i a t o r i s a prime c o n t r o l v a r i a b l e i n the t u b u l a r p o l y m e r i z a t i o n r e a c t i o n system. F u r t h e r , t h e r a t e c o n s t a n t s may be w r i t t e n i n i t s u s e f u l f o r m as k
= k"e-
E / R T
e"
A
V
P
/
R
T
(6)
where E R T P AV k"
= = = = = =
a c t i v a t i o n energy i d e a l gas c o n s t a n t absolute temperature pressure a c t i v a t i o n volume frequency f a c t o r
The v a l u e s o f k" and E a r e h i g h l y d e p e n d e n t on t h e i n i t i a t o r t y p e s and t h e i r e f f e c t s on t h e s o l v e n t t y p e s a r e l e s s o v e r w h e l m i n g . The t y p e s o f s o l v e n t u s e d as c h a i n t r a n s f e r a g e n t a r e u s u a l l y f i x e d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
LEE AND MARANO
Free-Radical
227
Polymerization
f o r a g i v e n r e a c t o r and o n l y t h e c o n c e n t r a t i o n o f t h e s o l v e n t s are v a r i e d t o c o n t r o l t h e molecular weights. The i n i t i a t o r t y p e s , h o w e v e r , a r e c h a r a c t e r i z e d b y t h e s e p a r a m e t e r s , a n d s i n c e t h e e f f e c t o f p r e s s u r e i s s m a l l (l>9) a n d the t u b u l a r polymerization o f ethylene i sundertaken w i t h i n a n a r r o w r a n g e o f p r e s s u r e , t h e d e s c r i p t i v e c o n s t a n t becomes -E, /RT (T)
d
k' e
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
d
Any i n i t i a t o r w h i c h decomposes b y a f i r s t - o r d e r r a t e p r o c e s s c a n , t h e r e f o r e , b e c h a r a c t e r i z e d b y t h e t w o p a r a m e t e r s k*£ a n d E^. S u c h i m p o r t a n t m a t e r i a l s a s o r g a n i c p e r o x i d e s , a z o compounds, a s w e l l a s many o t h e r t y p e s o f m a t e r i a l s , a r e d e s c r i b e d b y t h e f i r s t order p r o c e s s and as such f o l l o w t h e g e n e r a l development g i v e n i n t h i s work. The e f f i c i e n c y w i l l b e assumed c o n s t a n t a n d t h e same f o r a l l i n i t i a t o r s w i t h 0.5. I n a case o f where t h e r a d i c a l s t e a d y - s t a t e a s s u m p t i o n c a n be made, t h e r e a c t o r h e a t b a l a n c e c a n b e w r i t t e n i n d i m e n s i o n l e s s f o r m a t c o n s t a n t p r e s s u r e (l_) T
?
d T / d Z = 6X ( l - Y ) " a Q ( T - 1 / T
f
!
) - y(T -l)
(8)
r
where
AHk
f
fk
2
M I e^T p o o L
Q a i T
and
Z T T
Mo f p Cp Tn
h D
!
t
a/T, Ep + ( E - E / 2 ) / R T/T, L/L temperature residence time monomer i n l e t c o n c e n t r a t i o n i n i t i a t o r i n l e t concentration initiator efficiency density heat c a p a c i t y j a c k e t o r w a l l t e m p e r a t u r e (The j a c k e t t e m p e r a t u r e when h i s d e f i n e d a s a n o v e r a l l heat-transfer coefficient; the inside wall t e m p e r a t u r e when h i s d e f i n e d a s a n h e a t transfer coefficient.) heat-transfer coefficient reactor diameter d
t
J
0
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
228
and
X Y AH R L L k p ,Ep
monomer c o n v e r s i o n i n i t i a t o r conversion heat of r e a c t i o n i d e a l gas c o n s t a n t reactor length t o t a l reactor length f r e q u e n c y f a c t o r and a c t i v a t i o n for chain propagation k , E = f r e q u e n c y f a c t o r and a c t i v a t i o n f o r i n i t i a t o r breakdown k ^.,E^. = f r e q u e n c y f a c t o r and a c t i v a t i o n for radical termination Q
T
f
d
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
f
d
= = = = = = =
energy energy energy
E q u a t i o n (8) p r o v i d e s a g e n e r a l r e l a t i o n s h i p b e t w e e n t h e r e a c t o r t e m p e r a t u r e p r o f i l e and t h e o p e r a t i n g p a r a m e t e r s . In r e l a t i n g the system heat t r a n s f e r t o t h e c o n v e r s i o n - m o l e c u l a r weights r e l a t i o n s h i p f o r a r e a c t o r o f f i x e d s i z e , t h e heat t r a n s f e r c o e f f i c i e n t emerges as t h e c o r r e l a t i n g p a r a m e t e r . E f f e c t s o f I n i t i a t o r C o n c e n t r a t i o n and J a c k e t T e m p e r a t u r e . The a b i l i t y t o m a n i p u l a t e r e a c t o r t e m p e r a t u r e p r o f i l e i n t h e polymerization tubular reactor i s very important since i t d i r e c t l y r e l a t e s t o c o n v e r s i o n and r e s i n p r o d u c t p r o p e r t i e s . This i s often done b y u s i n g d i f f e r e n t i n i t i a t o r s a t v a r i o u s c o n c e n t r a t i o n s and at d i f f e r e n t r e a c t o r jacket temperature. The r e a c t o r t e m p e r a t u r e response i n terms o f t h e d i f f e r e n c e between t h e j a c k e t temperature and t h e p e a k t e m p e r a t u r e (6=Tp-Tj) i s p l o t t e d i n F i g u r e 2 as a f u n c t i o n of the j a c k e t temperature f o r v a r i o u s i n l e t i n i t i a t o r concentrations. The t e m p e r a t u r e r e s p o n s e n o t o n l y depends on t h e j a c k e t temperature but a l s o , f o r c e r t a i n combinations o f the v a r i a b l e s , i t i s very s e n s i t i v e t o the jacket temperature. The c o n v e r s i o n r e f l e c t s t h e t e m p e r a t u r e r e s p o n s e r e a l i z e d i n t h e r e a c t o r and t h e t e m p e r a t u r e r e s p o n s e shown i n F i g u r e 2 c a n be r e p l o t t e d i n t e r m s o f c o n v e r s i o n r e s p o n s e s and t h e y a r e shown i n F i g u r e 3. The f i g u r e c l e a r l y shows t h a t t h e c o n v e r s i o n i n a r e a c t o r o f f i x e d s i z e depends on b o t h t h e i n l e t i n i t i a t o r c o n c e n t r a t i o n and t h e j a c k e t t e m p e r a t u r e . T h e r e e x i s t optimum o p e r a t i n g c o n d i t i o n s t o maximize the c o n v e r s i o n i n a r e a c t o r o f f i x e d s i z e . The d a s h e d l i n e s i n F i g u r e s 2 and 3 n o t o n l y i n d i c a t e t h e optimum o p e r a t i n g c o n d i t i o n b u t a l s o show t h e l i m i t s o f s t a b l e r e a c t o r o p e r a t i o n f o r a g i v e n i n i t i a t o r system i n a f i x e d r e a c t o r . In a d d i t i o n , the average polymer m o l e c u l a r weights t h a t are produced i n t h e r e a c t o r depend on t h e t e m p e r a t u r e r e s p o n s e and t h u s , a r e r e l a t e d t o t h e c o n v e r s i o n . An example o f t h i s r e l a t i o n s h i p i s shown i n F i g u r e h. Optimum O p e r a t i n g L i n e . The r e l a t i o n s h i p s b e t w e e n t h e c o n v e r s i o n and t h e a v e r a g e m o l e c u l a r w e i g h t c a n be p l o t t e d as a f u n c t i o n o f i n i t i a t o r c o n c e n t r a t i o n w h i l e v a r y i n g t h e j a c k e t temperat u r e t o o p t i m i z e t h e c o n v e r s i o n . The r e l a t i o n s h i p s a r e shown i n
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
LEE AND MARANO
Free-Radical
Polymerization
229
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10.
Figure 2. Effect of jacket temperature on the maximum temperature in a plugflow reactor (f = 0.5; In k ' = 43.2261 In sec ; E,/R = 23481.06 °C; r/ C 0.67898 sec-ft?-°C/BTU; T = 58°C) d
1
P
0
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
p
=
POLYMERIZATION REACTORS AND PROCESSES
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
230
A i r = 40 ppm
1.7
1.8
1.9
2.0
2.1
2.2
2.3
Jacket Temp/Ref., Temp., Dimensionless Figure
3.
Conversion-jacket temperature relation transfer coefficient: 75 cal/m 2
(computer sec °C
simulation).
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Heat
10.
LEE AND MARANO
Free-Radical
231
Polymerization
50,000
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
40
1 •H
—
30
40,000 0
o •H
W CD >
o o
20
30,000
10
20,000
Jacket
2.8
2.7
2.6
2.5
I
Temp/T f, re
Dimensionless Figure 4. Operation of a plug-flow tubular addition polymerization reactor fixed size using a specified free-radical initiator (initiator kinetic parameters: E 32.921 Kcal/mol; In k ' = 26.492 In sec' ; f = 0.5; 10 ppm initiation, 1.0 mol solvent)
d
d
1
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
of = %
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
232
POLYMERIZATION REACTORS AND PROCESSES
F i g u r e 5 and i t p r o v i d e s a b a s i s f o r c o n s t r u c t i n g t h e optimum operating l i n e . The f i g u r e shows t h a t t h e r e i s a maximum c o n v e r s i o n t h a t c a n be a c h i e v e d a t a g i v e n i n i t i a t o r c o n c e n t r a t i o n . In t e r m s o f m o l e c u l a r w e i g h t , no o p t i m i z a t i o n e x i s t s . H o w e v e r , an o p e r a t i o n a l l i m i t a t i o n f o r c e s an o p t i m i z a t i o n s i n c e t h e o p e r a t i o n o f t h e r e a c t o r a t m o l e c u l a r w e i g h t s w h i c h a r e h i g h e r t h a n c a n be o b t a i n e d a t optimum c o n v e r s i o n c o u l d r e s u l t i n an e x c e s s i v e i n i t i a t o r c o n c e n t r a t i o n a t t h e r e a c t o r e x i t and an e x c e s s i v e number o f f r e e r a d i c a l s i n the polymer r e c o v e r y system. T h i s o p e r a t i o n a l l i m i t a t i o n c o u p l e d w i t h t h e maximum c o n v e r s i o n s p e c i f i c a t i o n r e s u l t s i n a d e f i n e d o p t i m a l o p e r a t i o n a l mode which i s unique t o a given i n i t i a t o r type. The optimum o p e r a t i n g l i n e i s i l l u s t r a t e d i n F i g u r e 5 as t h e d a s h e d l i n e . T h i s optimum o p e r a t i n g l i n e c a n now be p l o t t e d f o r d i f f e r e n t i n i t i a t o r t y p e s a t v a r i o u s s o l v e n t c o n c e n t r a t i o n s and h e a t t r a n s f e r c o n d i t i o n s t o compare i n i t i a t o r t y p e s and t o s t u d y t h e r e a c t o r r e s p o n s e s t o t h e operating parameters. The o p e r a t i n g l i n e shows t h a t f o r a g i v e n i n i t i a t o r t y p e t h e r e i s a maximum m o l e c u l a r w e i g h t and maximum c o n v e r s i o n w h i c h c a n be p r o d u c e d i n a r e a c t o r o f f i x e d s i z e . The o p e r a t i n g l i n e s e r v e s as a s o u n d b a s i s f o r c o m p a r i n g t h e p e r f o r mance o f t h e r e a c t o r as t h e v a r i o u s i n i t i a t o r s a r e u s e d and f u r t h e r p r o v i d e s t h e d i r e c t i o n o f s e a r c h f o r optimum i n i t i a t o r system f o r a given product i n a r e a c t o r of f i x e d s i z e . E f f e c t of Solvent Concentration. The optimum c o n v e r s i o n m o l e c u l a r w e i g h t c u r v e c a n be d i v i d e d i n t o two z o n e s : one c h a i n t r a n s f e r c o n t r o l l e d and t h e o t h e r i n i t i a t o r c o n t r o l l e d . I n t h e upper m o l e c u l a r w e i g h t or c h a i n t r a n s f e r c o n t r o l l e d r e g i o n changes i n t h e i n i t i a t o r c o n c e n t r a t i o n s i g n i f i c a n t l y change t h e r e a c t o r c o n v e r s i o n b u t h a v e l i t t l e e f f e c t on t h e m o l e c u l a r w e i g h t . As s e e n i n F i g u r e 6, h o w e v e r , i t i s i n t h i s r e g i o n t h a t t h e c h a i n t r a n s f e r a g e n t has i t s l a r g e s t e f f e c t . I n the low molecular w e i g h t or i n i t i a t o r c o n t r o l l e d r e g i o n changes i n t h e i n i t i a t o r c o n c e n t r a t i o n a l t e r t h e m o l e c u l a r weight but have l i t t l e e f f e c t on t h e r e a c t o r c o n v e r s i o n . A f u r t h e r examination of a s i n g l e operating l i n e i n d i c a t e s t h a t t h e r e a c t o r i s m o l e c u l a r w e i g h t l i m i t e d b e c a u s e o f an i n v e r s e r e l a t i o n s h i p o f t h e i n i t i a t o r c o n c e n t r a t i o n and t h e j a c k e t t e m p e r ature. The l i m i t i n g m o l e c u l a r w e i g h t i s a p p r o a c h e d as t h e i n l e t i n i t i a t o r c o n c e n t r a t i o n a p p r o a c h e s z e r o and t h e j a c k e t t e m p e r a t u r e approaches a l i m i t i n g v a l u e d i c t a t e d by the i n i t i a t o r t y p e . The m o l e c u l a r w e i g h t at t h a t p o i n t i s g i v e n s i m p l y by t h e r a t i o o f t h e p r o p a g a t i o n t o monomer and s o l v e n t c h a i n - t r a n s f e r r a t e c o n s t a n t s evaluated at the l i m i t i n g r e a c t i o n temperature. On t h e o t h e r h a n d , t h e l i m i t i n g c o n v e r s i o n i n a r e a c t o r o f f i x e d s i z e i s d e p e n d e n t on t h e t e m p e r a t u r e and t h e r a d i c a l c o n c e n t r a t i o n i n t h e r e a c t o r and r e s u l t s f r o m a p r e d o m i n a t i n g radicalr a d i c a l i n t e r a c t i o n p r e c i p i t a t e d b y an i n c r e a s e d i n i t i a t o r c o n c e n t r a t i o n and t h e a c c o m p a n y i n g t e m p e r a t u r e e x c u r s i o n . At t h i s p o i n t t h e s o l v e n t c o n c e n t r a t i o n s h a v e l i t t l e e f f e c t on t h e m o l e c u l a r
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
LEE AND MARANO
Free-Radical
Polymerization
233
icr
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
—
1
ppm
^~
2
io S^" *^^^^^ \
\ \
10
q
1
I . I . I 10
20
30
40
Conversion, % Figure 5. Molecular weight-conversion contour map for various concentrations of a free-radical initiator operating in a tubular-addition polymerization reactor of fixed size. Curves were constructed using varying jacket temperatures (kinetic parameters for the initiator: E = 32.921 Kcal/mol; In k ' = 26.494 In sec' ; f = 0.5; ( ) optimum operating line) d
d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
1
POLYMERIZATION REACTORS AND PROCESSES
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
234
Conversion, % Figure 6. Effect of solvent concentration on the molecular weight-conversion relationships of a tubular-addition polymerization reactor of fixed size using a specified initiator type. Each point along the curves represents an optimum initiator feed concentration—reactor jacket temperature combination, (kinetic parameters of the initiator: E = 24.948 Kcal/mol; In k ' = 26.494 In sec' ; f = 0.5) d
d
1
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
LEE AND MARANO
Free-Radical
Polymerization
235
weight. The l i m i t i n g c o n d i t i o n i s r e a c h e d when t h e c h a i n r a d i c a l s which a r e formed i m m e d i a t e l y t e r m i n a t e . T h i s c o n d i t i o n i s depend e n t on t h e t y p e o f i n i t i a t o r u s e d i n t h e r e a c t o r . E f f e c t s o f I n i t i a t o r Parameters. I n i t i a t o r t y p e s can b e s t be c h a r a c t e r i z e d b y t h e f r e q u e n c y f a c t o r ( k ^ ) and t h e a c t i v a t i o n e n e r g y ( E ^ ) , a n d t h e e f f e c t o f t h e s e p a r a m e t e r s on t h e m o l e c u l a r w e i g h t - c o n v e r s i o n r e l a t i o n s h i p i s shown i n F i g u r e s 7 a n d 8 . The c u r v e s shown a r e t h e r e s u l t o f c h o o s i n g t h e j a c k e t t e m p e r a t u r e i n l e t i n i t i a t o r c o n c e n t r a t i o n combination which maximizes t h e r e a c t o r c o n v e r s i o n f o r each i n i t i a t o r t y p e i n v e s t i g a t e d . F i g u r e 7 shows t h e l i m i t i n g maximum m o l e c u l a r w e i g h t o f p r o ducts from a r e a c t o r o f f i x e d s i z e v a r i e s d i r e c t l y w i t h t h e f r e quency f a c t o r o f t h e i n i t i a t o r a t a f i x e d a c t i v a t i o n energy, w h i l e the l i m i t i n g conversion v a r i e s i n v e r s e l y w i t h t h e frequency f a c t o r . I n a d d i t i o n , t h e l e n g t h o f t h e c h a i n - t r a n s f e r c o n t r o l l e d zone i s increased i n v e r s e l y w i t h t h e frequency f a c t o r . F i g u r e 8 shows t h e l i m i t i n g maximum m o l e c u l a r w e i g h t o f p r o ducts produced i n a r e a c t o r o f f i x e d s i z e v a r i e s i n v e r s e l y w i t h t h e a c t i v a t i o n energy o f t h e i n i t i a t o r a t a f i x e d frequency f a c t o r , while the l i m i t i n g conversion varies d i r e c t l y with the activation energy. In addition, the length of the chain-transfer controlled zone i n c r e a s e s d i r e c t l y w i t h t h e a c t i v a t i o n e n e r g y . T h e o r e t i c a l l y , as t h e i n i t i a t o r a c t i v a t i o n energy approaches z e r o , a v e r y h i g h m o l e c u l a r weight m a t e r i a l w i l l be produced a t a v e r y s m a l l c o n v e r s i o n and as t h e i n i t i a t o r a c t i v a t i o n approaches i n f i n i t y , a v e r y l o w m o l e c u l a r weight m a t e r i a l w i l l be produced a t v e r y h i g h c o n v e r s i o n . T h i s i m p l i e s t h a t an optimum c o m b i n a t i o n o f Ea a n d k ^ w h i c h p r o d u c e s a n i n f i n i t e r a n g e o f m o l e c u l a r w e i g h t s does n o t e x i s t . T h e r e i s , h o w e v e r , a n optimum c o m b i n a t i o n o f E and k*a f o r a g i v e n p r o d u c t ( g i v e n m o l e c u l a r w e i g h t ) p r o d u c e d i n a given reactor. I n a d d i t i o n t o t h e number a v e r a g e m o l e c u l a r w e i g h t o f t h e produced polymer, t h e breadth o f t h e molecular weight d i s t r i b u t i o n has i m p o r t a n t e f f e c t s on t h e p r o d u c t p r o p e r t i e s . F i g u r e s 9 a n d 10 show t h e e f f e c t o f i n i t i a t o r t y p e on t h e m o l e c u l a r w e i g h t d i s t r i b u t i o n o f t h e r e s i n as d e f i n e d by t h e r a t i o o f t h e weight t o number a v e r a g e m o l e c u l a r w e i g h t . The f i g u r e s show t h a t t h e b r e a d t h of molecular weight d i s t r i b u t i o n v a r i e s i n v e r s e l y w i t h t h e a c t i v a t i o n e n e r g y o f t h e i n i t i a t o r a t a n y g i v e n c o n v e r s i o n f o r an i n i t i a t o r o f s p e c i f i e d f r e q u e n c y f a c t o r and v a r i e s d i r e c t l y w i t h t h e f r e q u e n c y f a c t o r f o r an i n i t i a t o r o f s p e c i f i e d a c t i v a t i o n e n e r g y . This suggests t h a t t h e molecular weight d i s t r i b u t i o n o f a r e s i n c a n b e made t o assume a n y d e s i r e d v a l u e b y a p r o p e r c h o i c e o f t h e initiator. The i n i t i a t o r u s a g e c a n p l a y a r o l e i n t h e e c o n o m i c s o f r e s i n p r o d u c t i o n . The c o m p u t e r s i m u l a t i o n s show t h e u s a g e t o b e d e p e n d e n t on t h e i n i t i a t o r t y p e . The e f f e c t o f t h e i n i t i a t o r t y p e on t h e amount o f i n i t i a t o r r e q u i r e d t o p r o d u c e a g i v e n q u a n t i t y o f r e s i n a t optimum r e a c t o r c o n d i t i o n s i s shown i n F i g u r e s 11 a n d 1 2 .
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
f
T
d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
236
Conversion, % Figure 7. Tubular plug-flow addition polymer reactor: effect of the frequency factor (k ) of the initiator on the molecular weight-conversion relationship at constant activation energy (E ). Each point along the curves represents an optimum initiator feed concentration-reactor jacket temperature combination and their values are all different. (E = 32.921 Kcal/mol; In k = 35.000 In sec' ; 0.0 mol % solvent) d
d
d
d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
1
LEE AND MARANO
Free-Radical
237
Polymerization
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10.
10
20
30
40
50
Conversion, % Figure 8. Tubular plug-flow addition polymer reactor: effect of the activation energy (E) of the initiator on the molecular weight-conversion relationship at constant frequency factor (k ). Each point along the curves represents an optimum initiator feed concentration-reactor jacket temperature combination and their values are all different. (In k' = 26.494 In sec' ; 0.0 mol % solvent) f
1
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 1
Conversion,
%
d
Figure 9. Effect of the initiator activation energy on the molecular weight distribution of an addition polymer produced in a tubular reactor: constant frequency factor and at widely different values of initiator-jacket temperature combination (the conversion is optimized: In k ' = 26.492 In sec' ; 0.0 mol % solvent)
20
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979. %
d
Figure 10. Effect of the initiator frequency factor on the molecular weight distribution addition polymer produced in a tubular reactor: constant activation energy and at widely ent values of initiator-jacket temperature combination (the conversion is optimized: E = kcal/mol; 0.0 mol % solvent)
Conversion,
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
of an differ32.921
CO
POLYMERIZATION REACTORS AND PROCESSES
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
240
Figure 11. Effect of the initiator frequency factor on the initiator usage in an addition polymerization reactor: constant activation energy (the conversion is optimized; E = 32.921 kcal/mol) d
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
LEE AND MARANO
241
Free-Radical Polymerization
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
60
Conversion, %
10
20
30
40
Square Root of the I n l e t I n i t i a t o r Concentration Figure 12. Effect of the initiator activation energy on the initiator usage in a tubular-addition polymerization reactor: constant frequency factor (the conversion is optimized; In k' = 26.492 In sec' ) 1
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
POLYMERIZATION REACTORS AND PROCESSES
242
As s e e n i n t h e f i g u r e s , t h e q u a n t i t y o f i n i t i a t o r r e q u i r e d t o y i e l d a given conversion v a r i e s d i r e c t l y w i t h the frequency f a c t o r f o r an i n i t i a t o r w i t h a s p e c i f i e d a c t i v a t i o n e n e r g y and i n v e r s e l y w i t h t h e a c t i v a t i o n e n e r g y f o r an i n i t i a t o r o f s p e c i f i e d f r e q u e n c y factor. The r e l a t i o n s h i p s do n o t show a l i n e a r p r o p o r t i o n a l i t y b e t w e e n t h e r e a c t o r c o n v e r s i o n and s q u a r e r o o t o f t h e i n l e t i n i t i a t o r concentration. E f f e c t o f I n i t i a t o r Change on C o n v e r s i o n Improvement. Based on t h e s e d i s c u s s i o n s , i t i s a p p a r e n t t h a t a s e l e c t e d i n i t i a t o r c a n a l l o w c o n v e r s i o n improvements f o r a s p e c i f i e d m o l e c u l a r w e i g h t . T h i s c a n be i l l u s t r a t e d i n F i g u r e 13. I f product A i s using i n i t i a t o r A, c o n v e r s i o n A w o u l d r e s u l t . A switch to i n i t i a t o r B w o u l d c a u s e t h e p r o d u c t D t o be p r o d u c e d . The i n i t i a t o r c c n c e n t r a t i o n c o u l d t h e n be i n c r e a s e d a l o n g c u r v e B t o p r o d u c t A at c o n v e r s i o n A". This simple increasing of the i n i t i a t o r concentrat i o n a n d , t h e r e f o r e , t h e c o n v e r s i o n c o u l d n o t h a v e b e e n done w i t h the o r i g i n a l i n i t i a t o r s i n c e a decrease i n t h e m o l e c u l a r weight w o u l d have o c c u r r e d . I n o t h e r w o r d s a 0.1 m e l t i n d e x m a t e r i a l c a n be p r o d u c e d a t t h e same r a t e as a 10 m e l t i n d e x m a t e r i a l b y u s i n g an i n i t i a t o r o f t h e p r o p e r d e s i g n . The c o n v e r s i o n i m p r o v e ment f o r C p r o d u c t f r o m C c o n v e r s i o n t o C" c o n v e r s i o n i s now done b y t h e r e v e r s i n g o f t h e i n i t i a t o r t y p e s and shows t h e s e n s i t i v i t y o f t h e p r o d u c t p r o p e r t i e s on c o n v e r s i o n improvement w i t h i n i t i a t o r changes. The LDPE r e a c t o r i s sometimes t e r m e d h e a t t r a n s f e r l i m i t e d i n conversion. While t h i s i s t r u e , the molecular weight (or melt i n d e x ) — c o n v e r s i o n r e l a t i o n s h i p i s n o t s i n c e t h i s w o r k shows t h a t a s e l e c t e d i n i t i a t o r c a n a l l o w c o n v e r s i o n i m p r o v e m e n t s t o be made under a d i a b a t i c c o n d i t i o n s f o r a s p e c i f i e d m o l e c u l a r weight. The a c t u a l l i m i t a t i o n t o conversion i s the decomposition temperature o f t h e e t h y l e n e and g i v e n t h a t t e m p e r a t u r e as a maximum l i m i t a t i o n , an i n i t i a t o r ( n o t n e c e s s a r i l y c o m m e r c i a l o r even known w i t h p r e s e n t i n i t i a t o r t e c h n o l o g y ) c a n be f o u n d w h i c h w i l l a l l o w any p r o d u c t t o be made a t t h e r a t e d i c t a t e d b y t h i s t e m p e r a t u r e . Conc e p t u a l l y , t h i s i s a c o n s t a n t (maximum) c o n v e r s i o n r e a c t o r , r u n n i n g a t c o n s t a n t o p e r a t i n g c o n d i t i o n s where t h e p r o d u c t p r o d u c e d d i c t a t e s t h e i n i t i a t o r t o be u s e d . 1
1
f
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
f
f
1
E f f e c t o f Heat T r a n s f e r . Because t h e r e a c t o r i s heat t r a n s f e r l i m i t e d , e f f o r t s a r e o f t e n made t o i m p r o v e t h e h e a t t r a n s f e r and c o n v e r s i o n . However, f o r a g i v e n i n i t i a t o r s y s t e m i n a s p e c i f i e d r e a c t o r , t h e r e are a l s o unique conversion-molecular weightheat t r a n s f e r r e l a t i o n s h i p s . F i g u r e ih shows a r e l a t i o n s h i p b e t w e e n t h e a v e r a g e m o l e c u l a r w e i g h t and t h e c o n v e r s i o n w i t h h e a t t r a n s f e r c o e f f i c i e n t as a parameter. The c u r v e i s b a s e d on o p t i mized conversion-jacket temperature r e l a t i o n s h i p s f o r d i f f e r e n t number a v e r a g e m o l e c u l a r w e i g h t s . The shape o f t h e c u r v e i m p l i e s a g i v e n i n i t i a t o r s y s t e m and r e a c t o r c o n f i g u r a t i o n . The shape o f c u r v e may change w i t h d i f f e r e n t r e a c t o r s y s t e m s , b u t i t does show
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
LEE AND MARANO
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10.
Free-Radical
Polymerization
243
si •H
Q)
A'
A" B' C
C"
Reactor Conversion Figure
13.
Effect of an initiator change on the conversion improvement tubular-addition polymerization reactor
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
in
the
POLYMERIZATION REACTORS AND PROCESSES
244
5
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
LEE AND MARANO
Free-Radical Polymerization
245
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
a general trend of what can be expected of molecular weightconversion-heat transfer coefficient relationship. The figure shows that the conversion does increase with increasing heat transfer but the degree of increase depends on the average molecular weights of polymer being produced. At high average molecular weights (lower melt index), the lines representing different heat transfer levels converge. This implies that the conversion improvement due to heat transfer is small at high molecular weight. At lower molecular weights (higher melt index), the degree of conversion improvement is much larger. In order to utilize the improvement in heat transfer for a given product, therefore, an initiator system must be selected to provide a maximum conversion spread with increasing heat transfer.
Concluding Remarks The computer simulation study of the operation of the tubular free radical polymerization reactor has shown that the conversion and the product properties are sensitive to the operating parameters such as initiator type, jacket temperature, and heat transfer for a reactor of fixed size. The molecular weight-conversion contour map is particularly significant and it is used in this paper as a basis for a comparison of the reactor performances. The type of initiator used affects the molecular weight and conversion limits in a reactor of fixed size and the molecular weight distribution of the material produced at a given conversion level. The initiator type also dictates the amount of initiator which is necessary to yield a given conversion to polymer, the operating temperature range of the reactor and the sensitivity of the reactor to an unstable condition. Clearly, the initiator is the most important reaction parameter in the polymer process. The full utilization of improved heat transfer in a given reactor can only be made when the molecular weight-conversion relationships are carefully studied with various initiator types at different heat transfer levels. Then a particular initiator system must be selected for a maximum conversion improvement for a specified product. A study of this kind can be further extended to develop optimum reactor configurations which are needed to produce given products at the highest possible conversion.
Abstract A theoretical polymerization tubular reactor model was used to study the effects of reactor operating parameters on conversion
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
246
POLYMERIZATION REACTORS AND PROCESSES
APPENDIX A
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
EQUATIONS FOR A PLUG FLOW POLYMER TUBULAR REACTOR WITH BRANCHING KINETICS
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
LEE AND MARANO
Free-Radical
Polymerization
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10.
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
247
POLYMERIZATION REACTORS AND
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
248
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
PROCESSES
L E E AND MARANO
Free-Radical Polymerization
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
10.
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
249
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
250
POLYMERIZATION REACTORS A N D PROCESSES
and average molecular weight. In particular, the kinetic rate constants specific to high pressure ethylene polymerization were used in the computer-study. There exists an optimum jacket temperature for maximizing conversion at a given average molecular weight product. The study further suggests that an unstable operating region exists where wide conversion fluctuations result from attempts to increase the reactor conversion by minor adjustments in initiator amount or jacket temperature. The initiator is the most important reactor parameter in the polymer process. The initiator type affects the molecular weight and conversion limits in a reactor of fixed size and the molecular weight distribution of the material at a given conversion level. The initiator type dictates the initiator amount for a given conversion, the operating temperature range and sensitivity of the reactor to an unstable condition. Optimized molecular weight-conversion relationship is related to the system heat transfer coefficient. The degree of conversion improvement from improved heat transfer depends on the average molecular weights of polymer being produced for a given initiator system. Acknowledgements The authors gratefully acknowledge Union Carbide Corporation for permission to publish this paper.
Literature Cited 1
Lee, K. H. and Marano, J . P., J r . , Paper No. 67d, AIChE Annual Meeting, November, 1977, New York.
2
Szabo, J., Luft, G. and Steiner, R., Chemie-Ing.-Techn., 41, 1007 (1969).
3
Ehrlich, P. and Mortimer, G. A . , Adv. Polymer S c i . , 1, 386 (1970).
4
Graessley, W. W., AIChE-I. Chem. E. Symposium Series, No. 3, London, 1965.
5
Ray, W. H., Canadian Journal of Chemical Engineering, 45, 356 (1967).
6
Marano, J . P., J r . , A Seminar Presented at Northwestern University, Evanston, Illinois (February, 1974).
7
Gilles, E. D. and Schuchmann, H . , Chemie-Ing.-Techn., 38, 1278 (1966).
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
10.
8
Downloaded by TROY UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch010
9
LEE AND MARANO
Free-Radical
Ehrlich, P., et al, A I C h E
251
Polymerization
Journal, 2 2 , 4 6 3 , May, 1 9 7 6 .
W e a l e , K. E., C h e m i c a l R e a c t i o n s a t High Pressure, W i l l m e r Brothers, Ltd., B e r k e n h e a d , G r e a t Britain, 217 ( 1 9 6 7 ) .
10
B e v i n g t o n , C., Jr., et al., J. of P o l y m e r Science, 14, 463, May, 1 9 7 6 .
11
B a m f o r d , C. H., et al., The Kinetics o f Vinyl Polymerization "by Radical M e c h a n i s m s , B u t t e r w o r t h s Scientific P u b . , London (1958).
12
Hammond, G. S., et al., J. Am. Chem. S o c . , 77,
3244,
RECEIVED January 29, 1979.
In Polymerization Reactors and Processes; Henderson, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
(1955).
