On the Optimal Reactor Type and Operation for Continuous Emulsion

6 On the Optimal Reactor Type and Operation for Continuous Emulsion Polymerization MAMORU NOMURA Department of Industrial Chemistry, Fukui University, Fukui, Japan

Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch006

MAKOTO HARADA Institute of Atomic Energy, Kyoto University, Uji, Japan

Continuous emulsion polymerization processes are presently employed for large scale production of synthetic rubber latexes. Owing to the recent growth of the market for polymers in latex form, this process is becoming more and more important also in the production of a number of other synthetic latexes, and hence, the necessity of the knowledge of continuous emulsion polymerization kinetics has recently increased. Nevertheless, the study of continuous emulsion polymerization kinetics has, to date, received comparatively scant attention in contrast to batch kinetics, and very little published work is available at present, especially as to the reactor optimization of continuous emulsion polymerization processes. For the theoretical optimization of continuous emulsion polymerization reactors, it is desirable to understand the kinetics of emulsion polymerization as deeply and quantitatively as possible. The present review paper, therefore, refers firstly to the particle formation mechanism in emulsion polymerization, the complete understanding of which is indispensable for establishing a correct kinetic model, and then, deals with the present subject, that is, what type of reactor and operating conditions are the most suitable for a continuous emulsion polymerization process from the standpoint of increasing the volume efficiency and the stability of the reactors. Although the early literature described the application of a tubular reactor for the production of SBR latexes(1), the standard continuous emulsion polymerization processes for SBR polymerization still consist of continuous stirred tank reactors(CSTR's) and all of the recipe ingredients are normally fed into the first reactor and a latex is removed from the last one, as shown in Figure 1. However, it is doubtful whether this conventional reactor combination and operation method is the most efficient in continuous emulsion polymerization. As is well known, the kinetic behavior of continuous emulsion polymerization differs very much according to the kind of monomers. In this paper, therefore, the discussion about the present subject will be advanced using the 0097-6156/81/0165-0121$05.75/0 © 1981 American Chemical Society

In Emulsion Polymers and Emulsion Polymerization; Bassett, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


122

EMULSION POLYMERS AND EMULSION POLYMERIZATION

s t y r e n e e m u l s i o n p o l y m e r i z a t i o n s y s t e m b e c a u s e t h e b a s i c mechanism and t h e m a t h e m a t i c a l r e a c t i o n model o f t h i s s y s t e m a r e f a i r l y w e l l established. In c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e i n a s e r i e s o f CSTR's, i t was c l a r i f i e d t h a t a l m o s t a l l t h e p a r t i c l e s formed i n the f i r s t reactor(2,3). Since the r a t e of p o l y m e r i z a t i o n i s , under n o r m a l r e a c t i o n c o n d i t i o n s , p r o p o r t i o n a l t o t h e number o f p o l y m e r p a r t i c l e s p r e s e n t , t h e number o f s u c c e e d i n g r e a c t o r s a f t e r t h e f i r s t c a n be d e c r e a s e d i f t h e number o f p o l y m e r p a r t i c l e s p r o duced i n t h e f i r s t s t a g e r e a c t o r i s i n c r e a s e d . T h i s c a n be r e a l i zed by i n c r e a s i n g e m u l s i f i e r and i n i t i a t o r c o n c e n t r a t i o n s i n t h e f e e d s t r e a m and by l o w e r i n g t h e t e m p e r a t u r e o f t h e f i r s t r e a c t o r where p a r t i c l e f o r m a t i o n i s t a k i n g p l a c e ( 3 ) . The f o r m e r c h o i c e i s n o t d e s i r a b l e b e c a u s e p r o d u c t i o n c o s t and i m p u r i t i e s w h i c h may be i n v o l v e d i n t h e p o l y m e r s w i l l i n c r e a s e . The l a t t e r p r a c t i c e c o u l d be employed i n p a r a l l e l w i t h t h e t e c h n i q u e g i v e n i n t h i s p a p e r . Our f i n a l g o a l i n t h e p r e s e n t p a p e r i s t o d e v i s e a n o p t i m a l t y p e o f t h e f i r s t s t a g e r e a c t o r and i t s o p e r a t i o n method w h i c h w i l l m a x i m i z e the number o f p o l y m e r p a r t i c l e s p r o d u c e d i n c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n . F o r t h i s p u r p o s e , we need a m a t h e m a t i c a l r e a c t i o n model w h i c h e x p l a i n s p a r t i c l e f o r m a t i o n and o t h e r k i n e t i c behavior o f continuous emulsion p o l y m e r i z a t i o n of s t y r e n e .

Water Monomer Initiator Emulsifier Modif ier — Stopper

Figure 1.

—

Flow diagram of typical continuous emulsion polymerization reactor system

Mathematical R e a c t i o n Model Basic Equations. M a t h e m a t i c a l r e a c t i o n models f o r c o n t i n u o u s emulsion p o l y m e r i z a t i o n of styrene proposed t o date are roughly c l a s s i f i e d i n t o three groups a c c o r d i n g t o the p a r t i c l e f o r m a t i o n mechanism w h i c h t h e y a d o p t e d . D i c k i n s o n (_4) , and M i n and Ray (5) took i n t o c o n s i d e r a t i o n t h e homogeneous p a r t i c l e f o r m a t i o n w h i c h F i t c h and T s a i ( 6 ) made q u a n t i t a t i v e . G e r s h b e r g and L o n g f i e l d ( 3 ) , D e g r a f f and P o e h l e i n ( 7 ) , and Omi and coworkers(Q) f o r m one g r o u p , where t h e y employed t h e S m i t h - E w a r t s e c o n d i d e a l i z e d s i t u a t i o n f o r p a r t i c l e formation(9), t h a t i s , the f r e e r a d i c a l s i n the water phase a r e c a p t u r e d i n p r o p o r t i o n t o t h e s u r f a c e a r e a o f m i c e l l e s and p o l y m e r p a r t i c l e s . On t h e o t h e r hand, Nomura and coworkers(10) presented a d i f f e r e n t expression which i n v o l v e s a concept of r a d i c a l capture e f f i c i e n c y of m i c e l l e s , although adopting the m i c e l l e hypothesis. The k i n e t i c b e h a v i o r s o f s t y r e n e e m u l s i o n p o l y m e r i z a t i o n c a n be p r e d i c t e d s u f f i c i e n t l y w e l l , a s a w h o l e , b y m i c e l l a r


6.

NOMURA AND HARADA

Optimal

Reactor

Type

and

Operation

123

p a r t i c l e f o r m a t i o n and w i t h o u t u s i n g a c o m p l i c a t e d r e a c t i o n model w i t h a number o f k i n e t i c p a r a m e t e r s . T h e r e f o r e , a s i m p l e r e a c t i o n model d e v e l o p e d by Nomura a n d c o w o r k e r s ( 1 1 ) ( r e f e r r e d t o a s t h e Nomura a n d H a r a d a model) w i l l be u s e d f o r t h e d i s c u s s i o n o f t h e p r e s e n t s u b j e c t because t h e i r r e a c t i o n model a l s o i n c l u d e s t h e Smith-Ewart second i d e a l i z e d s i t u a t i o n i n p a r t i c l e f o r m a t i o n ( 9 ) and t h e G e r s h b e r g m o d e l (3_) i s a s p e c i a l c a s e o f t h e Nomura a n d H a r a d a m o d e l , a s shown l a t e r . The Nomura a n d H a r a d a m o d e l p r o c e e d s a s f o l l o w s , u s i n g t h e e l e m e n t a r y r e a c t i o n s a n d t h e i r r a t e e x p r e s s i o n s shown i n T a b l e I (11) . Table I Elementary R e a c t i o n s and T h e i r r a t e s f


Reaction rate

Reaction type

Reaction Initiation of radicals Initiation of particle from micelle Initiation Termination Propagation in particle

/ — 2R* R* + m, A * R* + A* — A'* R* + A * — N P*i + M - PV*

r = 2kJI (A) k m,R* (B) k XR* (C) t

r

x

t

k*V*R* (D) k M N* (E) p

p

* P*j is a polymer radical containing j monomer units in an active particle N*J.

(I)

Particle

Formation: dN

N

m

5 T - V T where i s t h e r a t e o f p a r t i c l e f o r m a t i o n , N i s t h e number o f p o l y m e r p a r t i c l e s p e r u n i t volume o f w a t e r a n d 0 i s t h e mean r e s i dence t i m e i n t h e f i r s t r e a c t o r . S i n c e t h e number o f r a d i c a l s f l o w i n g o u t o f t h e f i r s t r e a c t o r i n t h e e x i t s t r e a m c a n be n e g l e c t e d under n o r m a l r e a c t i o n c o n d i t i o n s , we h a v e : k_m R* R. = r . [ ] 1

1

k

m

R

i s *

+

k

N

R

2 T * r

1

=

here,

r

e= ( k / k . ) M 2 1 m 0

where r ^ i s t h e r a t e o f r a d i c a l p r o d u c t i o n i n t h e w a t e r p h a s e , m i s t h e m i c e l l e c o n c e n t r a t i o n , M i s t h e a g g r e g a t i o n number p e r m i c e l l e , S i s t h e number o f e m u l s i f i e r m o l e c u l e s f o r m i n g m i c e l l e s i n u n i t voTume o f w a t e r a n d R* i s 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 water phase and g i v e n by; g

dR* R* ^ f - = r . - L m R * - k N R * - ^~ dt l i s 2 T 9 C o n s i d e r i n g t h a t t h e t e r m R*/0 c a n be n e g l e c t e d a n d s t e a d y h y p o t h e s i s i s a p p l i c a b l e t o R*, E q . (3) l e a d s t o : 0

r

R

*

=

state

i

k.m [1+ ( C N / S J ] Is T m


( 3 )

124

EMULSION

POLYMERS

AND

EMULSION

POLYMERIZATION

A c c o r d i n g t o t h e G e r s h b e r g model where i t was assumed t h a t t h e r a d i c a l s i n t h e w a t e r p h a s e e n t e r b o t h m i c e l l e s and p a r t i c l e s i n proportion to t h e i r t o t a l surface areas, i s expressed as: A R

= r

a S

S

[

(4)

i i —'^i'rr'^i'r'

m p s F F where A^ i s t h e s u r f a c e a r e a o f m i c e l l e s , A i s the s u r f a c e a r e a of polymer p a r t i c l e s , a i s t h e s u r f a c e a r e l o c c u p i e d by an e m u l s i f i e r m o l e c u l e and S i s the e m u l s i f i e r c o n c e n t r a t i o n i n the f e e d stream. C o n s i d e r i n g t h a t A = ird m and A = 7Td N where d and d a r e t h e a v e r a g e d i a m e t e r s o f a m i c e l l e aSd a p o l y m e r p a r ? i c l e , Fespectivel V r Eq. (4) can be a l s o e x p r e s s e d by t h e f o l l o w i n g e q u a t i o n : p

2

2

TTd m m


2

r l

i

l

r.

s ; ;—i 7Td m + TTd N 1 + m s p T

l

z

(d

p

;

/d

m

f

(5 ) M

m

(N/S

T

m

) 7 1

E q u a t i o n ( 2 ) can i n c l u d e t h i s e x p r e s s i o n by s u p p o s i n g t h a t k ^ " ^ and k = T T d . We h a v e , t h e r e f o r e , t h e f o l l o w i n g e x p r e s s i o n s f o r e a c c o r d i n g ? o t h e mechanism o f r a d i c a l e n t r y i n t o m i c e l l e s and p a r ticles. 2

C a s e A: I f k^ and k^ do n o t depend on t h e d i a m e t e r s o f m i c e l l e and p a r t i c l e , we h a v e : G= ( k . / k J M = c o n s t a n t = K (6) z l m o Case B: I f F i c k ' s d i f f u s i o n t h e o r y c a n be a p p l i e d t o t h e e n t r y o f r a d i c a l s i n t o m i c e l l e s and p a r t i c l e s , we h a v e : k

1

1 G=

=

2TTD d

and

k

w m 2 (d / d J M = K H p m m 1 P i s the d i f f u s i o n c o e f f i c i e n t

=

2TTD d

w

(7

p

)

(8 )

where D of r a d i c a l i n the water w phase. Case C: T h i s s i t u a t i o n corresponds t o the Smith-Ewart second i d e a l i z e d s i t u a t i o n f o r p a r t i c l e f o r m a t i o n and t o t h e G e r s h b e r g model , as shown by E q . ( 5 ) . I n t h i s c a s e , we h a v e : k

n

l

TTd

=

2

m

and

k

z

TTd

=

2

( 9

p

)

2

8=

(d /d f M = K d (10) p m m 2 p Case D : U g e l s t a d s t a t e s t h a t r a d i c a l e n t r y i n t o m i c e l l e s and part i c l e s i s , under some c o n d i t i o n s , p r o p o r t i o n a l t o t h e i r v o l u m e s . I f t h e s e c o n d i t i o n s a r e f u l f i l l e d , we h a v e : k

l

v

« d

e= ( I I ) The b y :

0

x

3

m

3

and

k * d 2 p

3

(11)

3

( d V d ) M = K_d p m m 3 p

(12)

number o f a c t i v e p a r t i c l e s c o n t a i n i n g a r a d i c a l i s g i v e n dN*/dt= k ^ R *

+ k N R * - k^N*R* 2

Q

N*/0


6. NOMURA AND HARADA

Optimal

Reactor

Type

and Operation

125

2N*/N - i ^ - I T T s T ^ ,

3

N

" *

/

e

( 1 3

>

(IH) Monomer c o n v e r s i o n i s e x p r e s s e d b y : k [M ]M


D

D T =

V

(P

9

N

M NF A> * - V

9

( 1 4 )

(IV) E m u l s i f i e r b a l a n c e : S i n c e t h e d e p l e t i o n o f e m u l s i f i e r m i c e l l e s o c c u r s o n l y because t h e y b r e a k up and t h e i r m o l e c u l e s a r e a d s o r b e d o n t o t h e s u r f a c e o f growing p a r t i c l e s . The b a l a n c e o n t h e m i c e l l e s i n t h e f i r s t s t a g e r e a c t o r i s g i v e n by t h e f o l l o w i n g e q u a t i o n i f t h e r e a c t i o n i s s t a r t e d s o t h a t t h e e m u l s i f i e r c o n c e n t r a t i o n i n t h e f e e d stream does n o t change w i t h t i m e : S = S - A /a (15) m F P s where, A = (36Trr v N (16) P P 3

/

3

When monomer d r o p l e t s e x i s t , X

2 P m 2 1 0 3 k

n e


M /M w n

Case A 2.0

Case B 2.4

Case C

Case D

4.8

>4.8

Figure 2. Exyerimentaldata of average molecular weight of polystyrene formed in a CSTR: (%) M ; (O) M ; ( ) theory (1) ((Mt) 0 = 58.5 min; I = 0.8 g/kg H 0; reaction temp. T = 70°C; frightj 6 = 7.2 min; S = 27.9 g/kg H 0; reaction temp. T = 70°C) n

2

w

0

0

2


128


o s c i l l a t i o n s i n monomer c o n v e r s i o n and t h e number o f p o l y m e r p a r t i c l e s when a CSTR i s o p e r a t e d a t a h i g h t e m p e r a t u r e ( 3 ) o r extremely h i g h c o n v e r s i o n range(13). T h i s s o r t o f o s c i l l a t o r y response d i d n o t a p p e a r a t a m o d e r a t e t e m p e r a t u r e s u c h as 50°C, as shown i n F i g u r e 3, e v e n when a CSTR i s u s e d as t h e f i r s t s t a g e r e a c t o r where p a r t i c l e formation mainly takes place. Figure 4 shows a t y p i c a l example o f t h e c o u r s e o f c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e , w h i c h was s t a r t e d w i t h t h e f o l l o w i n g p r o c e d u r e . The r e a c t o r i s f i l l e d w i t h t h e d e s i r e d amount o f a l l r e c i p e i n g r e d i a n t s e x c e p t i n i t i a t o r , and t h e c o n t i n u o u s r u n i s s t a r t e d by s i m u l t a n e o u s l y p o u r i n g t h e g i v e n amount o f an a q u e o u s i n i t i a t o r s o l u t i o n and pumping monomer and an aqueous i n i t i a t o r and e m u l s i f i e r s o l u t i o n i n t o t h e f i r s t r e a c t o r . The s o l i d l i n e s show t h e m o d e l p r e d i c t i o n s c a l c u l a t e d u s i n g t h e Nomura and H a r a d a m o d e l w i t h e= 1.28 X10 t h e v a l u e o f w h i c h was o b t a i n e d i n b a t c h e x p e r i m e n t s ( 1 0 ) . The d o t t e d l i n e s a r e t h o s e c a l c u l a t e d u s i n g Eq.(4) i n p l a c e o f E q . (2) a s t h e e x p r e s s i o n f o r p a r t i c l e f o r m a t i o n , t h e o t h e r e q u a t i o n s and c a l c u l a t i o n c o n d i t i o n s b e i n g t h e same a s t h o s e i n t h e above calculations. T h i s s i t u a t i o n i s i d e n t i c a l t o t h e G e r s h b e r g and L o n g f i e l d m o d e l ( 3 ) ( o r t h e Nomura and H a r a d a m o d e l w i t h £ o f C a s e C ), e x c e p t t h a t E q . ( 1 6 ) was u s e d i n s t e a d o f E q . ( 2 3 ) . I t i s clear f r o m F i g u r e 4 t h a t t h e Nomura and H a r a d a model w i t h t h e c o n s t a n t v a l u e o f £ = 1.28x 1 0 p r e d i c t s w e l l t h e k i n e t i c b e h a v i o r o f c o n t i nuous e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e , i n c l u d i n g t h e b e h a v i o r i n t r a n s i e n t p e r i o d , and t h a t t h e G e r s h b e r g and L o n g f i e l d m o d e l which supposes t h a t r a d i c a l s i n the water phase e n t e r the m i c e l l e s and p a r t i c l e s i n p r o p o r t i o n t o t h e i r s u r f a c e a r e a s d o e s n o t e x p l a i n transient behavior. The r e a s o n why b o t h m o d e l s c o i n c i d e w i t h e a c h o t h e r a t s t e a d y s t a t e w i l l be shown i n t h e s u c c e e d i n g s e c t i o n .


5

5

I f Case B i s r a t h e r l i k e l y f o r £, a s _ t h e d i f f u s i o n t h e o r y and t h e e x a m i n a t i o n o f e x p e r i m e n t a l v a l u e o f M /M p r e d i c t , one must i n t r o d u c e the concept of r a d i c a l capture e f f i c i e n c y of a m i c e l l e r e l a t i v e t o a polymer p a r t i c l e , a i n the form o f ak where k i s the r a d i c a l c a p t u r e c o e f f i c i e n t f o r a polymer p a r t i c l e . The a p p r o x i m a t e v a l u e o f a i s e s t i m a t e d t o be 0.01 f o r e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e b e c a u s e t h e v a l u e o f £ i s 1.28x 10 , t h e v a l u e o f d /d i s a t t h e h i g h e s t 10 and t h e v a l u e o f M i s a b o u t 100. P m m 2

2

5

Pre-Reactor P r i n c i p l e L e t us c o n s i d e r t h e s t e a d y s t a t e c h a r a c t e r i s t i c s o f c o n t i n u ous e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e i n t h e f i r s t s t a g e r e a c t o r . The s t e a d y s t a t e v a l u e o f t h e number o f p o l y m e r p a r t i c l e s f o r m e d i n t h e f i r s t s t a g e r e a c t o r c a n be c a l c u l a t e d u s i n g t h e f o l l o w i n g equations. From E q s . (1) and ( 2 ) , we h a v e :

N

T

=

1

M N / TS

m The v a l u e o f S i s g i v e n by t h e f o l l o w i n g e q u a t i o n o b t a i n e d E q s . ( 1 5 ) and (?6) and v = uO: P


(

from

2

4

)

6.

NOMURA AND HARADA


~

Optimal

Reactor

Type and

129

Operation

80-

2

3

4

D i m e n s i o n l e s s time t / 9

[- ]

Figure 3. Polystyrene conversion transient at start-up in a CSTR (S = 12.5 g/L H 0; l = 1.25 g/L H 0; M = 0.5 g/cc H 0; T = 50°C; mean residence time 6: ((D) 155 min; (%) 67.5 min; (O) 38.6 min) F

2

F

2

F

2

T 0.3

8.O.0

' o >0.l

r 8.

ifbft\

40^

1

100 Reaction time

300

200 [min]

t (a)

b il. T^f

—

»

1

\ ^# \

- A 200

100

Reaction time (b)

t

300

[minj

Figure 4. Typical course of continuous emulsion polymerization of styrene: (a) (O) conversion; (%) particle number (calculated: ( ) Smith and Ewart; ( ) Nomura et al. (b) emulsifier balance S = 12.5 g/L H 0; l = 1.25 g/L H 0; M = 0.50 g/cc H 0; 50°C; experimental: 6 = 67 min) F

F

2

F

2


2

130

EMULSION POLYMERS AND EMULSION POLYMERIZATION V

S

F"

A

/ a

p s

=

( 3 6 T T j ( y e f N / a =S p -

V

L/3

/3

T

s

B^

(25)

B = (36TT) (ye? a" s of Eqs.(24) and (25) leads t o :

where,

L/3

Combination

/3

1

(s - r.eB) - r.eej - 4(B-e )r.es =—I 1 JL_I 1 1_£ 2

N

T

2(B-e)

(26)

At l i m i t i n g values of 0 , Eq.(26) g i v e s the f o l l o w i n g values of N^ which do not depend on e, when 0 + 0 , N^= r^0 (27)


when 0 - * ° ° ,

N = S /B= 0.21 ( y 9 ) ~ T F

2 / 3

#

(a S f ° s F

(28)

T h i s i s the reason why the steady s t a t e value of the number of polymer p a r t i c l e s c o i n c i d e with each other, as shown i n Figure 4, regardless of the form of e when the f i r s t stage reactor i s operated at comparatively longer residence time. On the other hand, i f Eq.(23) i s used instead of Eq.(16) f o r c a l c u l a t i n g Ap value, we h

a

V

e

:

N= T

-2/3 1 0 S / r (5/3)6= O.23(y0) (a S ) Z / J

p

g

F

(29)

This equation was derived by Gershberg and L o n g f i e l d (3) at 0 + ° ° . Figure 5 represents a t y p i c a l example of the v a r i a t i o n of the number of polymer p a r t i c l e s with mean residence time 0 . The s o l i d l i n e shows the t h e o r e t i c a l value p r e d i c t e d by the Nomura and Harada model with e= 1.28x 10 . The dotted l i n e i s that p r e d i c t e d by the Gershberg model(or the Nomura and Harada model with Case C for £) , where Eq.(23) was used instead of Eq.(16) f o r Ap. The value of N produced at longer mean residence time d i f f e r s , theref o r e , by a f a c t o r of T(5/3) between the s o l i d and dotted l i n e s i n Figure 5. From the comparison between the experimental and theor e t i c a l r e s u l t s shown i n Figure 5, i t i s confirmed that the steady state p a r t i c l e number can be maximized by operating the f i r s t stage reactor at a c e r t a i n low value of mean residence time 0 , which i s c o n s i d e r a b l y lower than that i n the succeeding r e a c t o r s . This i s s o - c a l l e d "pre-reactor p r i n c i p l e " . I t i s , t h e r e f o r e , d e s i r a b l e to operate the f i r s t reactor at such mean residence time as producing something l i k e a maximum number of polymer p a r t i c l e s i n order to increase the r a t e of p o l y m e r i z a t i o n i n the succeeding r e a c t o r s . This w i l l r e s u l t i n a decrease i n the number of necessary r e a c t o r s and hence, i n the c a p i t a l c o s t . 0 where maximum number of polymer p a r t i c l e s can be formed i s p r e d i c t e d t h e o r e t i c a l l y by d i f f e r e n t i a t i o n of Eq.(26) by 0 . Thus, we have: 0 / 7 9 0/7 5

T

m a x

W

0

'

2

5

*

(

V

E )

(

W

( 3 0 )

On the other hand, 0 has the f o l l o w i n g r e l a t i o n with #the disappearance time o ? m i c e l l e s i n batch operation conducted with the same r e c i p e as that i n continuous operation(11). a



NOMURA AND HARADA

Optimal

Reactor

Type and

Operation

Figure 5. Effect of mean residence time of the first reactor on the number c polymer particles formed (S = 12.5 g/L H 0; l = 1.25 g/L H 0; M = 0. g/cc H 0; experimental: (O) 1st reactor; (Φ) 2nd reactor; 50°C styrene) F

2

F

2

2


F

132

EMULSION

POLYMERS

AND

EMULSION

POLYMERIZATION

θ = 0.83t . max c l

(31)

I t i s apparent from Eq.(30) t h a t t h e h i g h e r t h e temperature o f t h e f i r s t s t a g e r e a c t o r and t h e v a l u e o f r ^ , t h e s m a l l e r t h e v a l u e o f %ax* iY r e a s o n why D e g r a f f and P o e h l e i n c o u l d n o t f i n d e m a x i n t h e i r e x p e r i m e n t s . E q u a t i o n ( 3 1 ) s u g g e s t s t h a t one can e s t i m a t e the v a l u e o f & s i m p l y by d e t e r m i n g t by measur i n g s u r f a c e t e n s i o n w i t h t h e use o f , f o r e x a m p l e , a du-Nouy t e n s i ometer. The maximum number o f p o l y m e r p a r t i c l e s p r o d u c e d i n t h e f i r s t stage reactor being operated a t 9 c a n be o b t a i n e d by i n t r o d u c i n g Eq.(30) i n t o E q . ( 2 6 ) . Thus, T n

S m a

b

e

t

n

e

m a x

c

l


m a x

On t h e o t h e r h a n d , t h e number o f p o l y m e r p a r t i c l e s f o r m e d i n b a t c h o p e r a t i o n w i t h t h e same r e c i p e a s i n c o n t i n u o u s o p e r a t i o n i s g i v e n N

7

T B

=0.56af (

2

r i

2

7

/ , /%- / S^

(33)

e

E q u a t i o n s ( 3 2 ) and (33) g i v e : Ν

Tmax

= 0.57 Ν

(34)

TB

T h i s means t h a t a s l o n g a s a CSTR i s u s e d a s t h e f i r s t s t a g e r e a c t o r and a l l t h e r e c i p e i n g r e d i a n t s a r e f e d i n t o t h e f i r s t s t a g e r e a c t o r , one c a n n o t have more t h a n 57% o f t h e number o f p a r t i c l e s p r o d u c e d i n a b a t c h r e a c t o r w i t h t h e same r e c i p e a s i n c o n t i n u o u s operation. The v a l i d i t y o f t h e s e e x p r e s s i o n i s c l e a r f r o m t h e c o m p a r i s o n between t h e e x p e r i m e n t a l and t h e o r e t i c a l v a l u e s shown i n F i g u r e 5. From F i g u r e 5, i t i s f o u n d t h a t t h e optimum mean r e s i dence t i m e o f t h e f i r s t s t a g e r e a c t o r i s a b o u t 10 m i n u t e s under these r e a c t i o n c o n d i t i o n s . E q u a t i o n ( 3 0 ) p r e d i c t s 10.0 m i n u t e s , w h i l e e x p e r i m e n t a l v a l u e i s 10.4 m i n u t e s where t h e number o f p o l y mer p a r t i c l e s i s a b o u t 60% o f t h a t p r o d u c e d i n a b a t c h r e a c t o r . The o b s e r v e d d i s a p p e a r a n c e t i m e o f m i c e l l e s t i n a batch reactor w i t h t h e same r e c i p e a s i n t h i s c o n t i n u o u s o p e r a t i o n was 12.8 m i n u t e s and 10.4 m i n u t e s i s a b o u t 8 0 % o f 12.8 m i n u t e s , a s E q . ( 3 1 ) predicts. T h u s , i t p r o v e s t h a t E q s . ( 3 1 ) , (32) and (34) a r e v a l i d . c

l

N

θ , and Ν w h i c h a r e p r e d i c t e d by t h e G e r s h b e r g and L o n g f i e l d model w i t h o u t t a k i n g i n t o a c c o u n t t h e r a d i c a l c a p t u r e e f f i c i e n c y o f m i c e l l e s ( o r t h e Nomura and H a r a d a model w i t h C a s e C f o r ε) a r e g i v e n , f o r c o m p a r i s o n , a s f o l l o w s : T

m

a

x

V x = ° - < ^ > N

Tmax

N

T B

= 0

-

2 1 ( r

3

i/^

= 0.37 (r . / μ J

/

/ 5

2

V a

/

S

5

< s F

2 7 5

^^)

) 3 / 5

3 7 5


( 3 6 )

(37)

6.

NOMURA A N D HARADA

Optimal

Reactor

Type

and

Operation

133

On the other hand, Eqs. (36) and (37) g i v e the same r e l a t i o n s h i p as Eq.(34). Equation(35) underestimates G , while Eq.(36) overes timates the value of N , as shown i n Figure 5. Tma χ Another method t o increase the number o f polymer p a r t i c l e s produced i n the f i r s t stage r e a c t o r with i n i t i a t o r and e m u l s i f i e r c o n c e n t r a t i o n s f i x e d i s t o employ a plug flow type r e a c t o r such as a tubular r e a c t o r f o r the f i r s t stage. The minimum residence time of a plug flow reactor θ necessary t o produce the same number of polymer p a r t i c l e s as i n 1 batch r e a c t o r i s t . Thus, from Eq.(31) We have: θ = t =1.2Θ (38) ρ c l max T h i s means that one can increase the number o f polymer p a r t i c l e s about 75% higher than that formed when a CSTR o f θ i s used, by employing a plug flow type r e a c t o r which i s only 20% bigger i n volume than a CSTR o f θ When the f i r s t stage reactor i s operated a t such a low mean residence time as θ„,_ or θ^, monomer conversion i n the f i r s t stage r e a c t o r i s c o n s i d e r a b l y lower, say s e v e r a l %, and hence, a l most a l l the monomer f e d i n t o the f i r s t stage r e a c t o r only passes through i t without p l a y i n g any important r o l e . T h i s suggests that one can f u r t h e r decrease the r e a c t o r volume o f the f i r s t stage r e actor by supplying only a small p o r t i o n of the t o t a l monomer feed i n t o the f i r s t stage and the r e s t o f the monomer i n t o the second stage. But how f a r can we decrease the monomer feed r a t e i n t o the f i r s t stage r e a c t o r , and what happens, then, t o p a r t i c l e formation behavior ? m a x

m


c

l

ν

P r o p o s i t i o n o f Operation with Divided Monomer Feed Nomura and Harada already reported an experimental and theor e t i c a l study on the e f f e c t o f lowering the amount o f monomer i n i t i a l l y charged on the number o f polymer p a r t i c l e s formed i n a batch r e a c t o r ( L 4 ) . Under usual c o n d i t i o n s i n batch o p e r a t i o n , mi c e l l e s disappear and the formation o f p a r t i c l e s terminates before the disappearance o f monomer d r o p l e t s i n the water phase. However, i f the i n i t i a l monomer c o n c e n t r a t i o n i s extremely low, m i c e l l e s would e x i s t even a f t e r the disappearance of monomer d r o p l e t s and hence, p a r t i c l e formation w i l l continue u n t i l a l l e m u l s i f i e r mole c u l e s are adsorbed on the s u r f a c e s o f polymer p a r t i c l e s . This c o n d i t i o n i s q u a n t i t a t i v e l y expressed by the f o l l o w i n g e m u l s i f i e r balance equation. S =S - (36^ ; ) N / (39) / 3

m

o

(

p

T

a s

A f t e r the time when monomer d r o p l e t s have disappeared, ν i s app roximately given by ( / P ) · Then, Eq.(39) i s r e w r i t t e n as: M

N

0

T

p

1/3

1

S = S - (36π) (Μ /ρ f ^ N y V m o ο *p T s

When r e a c t i o n time becomes i n f i n i t e , m i c e l l e s w i l l disappear mately, and then, Equation(39') becomes:


(39') ulti

134

EMULSION POLYMERS AND

N = T m

EMULSION POLYMERIZATION

3 2 3 -2 0 (a p /367T)S M I s ρ ο ο ο κ

(40)

/

where S , M and I a r e t h e e m u l s i f i e r , monomer and i n i t i a t o r c o n centrations, respectively. F i g u r e s 6, 7 and 8 show e x p e r i m e n t a l v e r i f i c a t i o n o f E q . ( 4 0 ) i n b a t c h e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e (1£) . The number o f p o l y mer p a r t i c l e s was m e a s u r e d by e l e c t r o n m i c r s c o p y , n o t a t f i n i t e but a t 1 h o u r a f t e r t h e s t a r t o f p o l y m e r i z a t i o n . F i g u r e 6 r e p r e s e n t s t h e e f f e c t o f l o w e r i n g t h e i n i t i a l monomer c o n c e n t r a t i o n , M . ο on t h e number o f p o l y m e r p a r t i c l e s f o r m e d a t f i x e d i n i t i a l i n i t i a t o r and e m u l s i f i e r c o n c e n t r a t i o n s . The number o f p o l y m e r p a r t i c l e s formed i s c o n s t a n t even i f M i s lowered t o the c r i t i c a l value M . T h i s i s because normal c o n d i t i o n t h a t m i c e l l e s d i s a p p e ar b e f o r e t h e d i s a p p e a r a n c e o f monomer d r o p l e t s i s s a t i s f i e d i n t h e r a n g e o f monomer c o n c e n t r a t i o n above M . The v a l u e o f M can be c a l c u l a t e d by t h e f o l l o w i n g e q u a t i o n o b t a i n e d by e q u a t i n g X ^ c l ' t h e monomer c o n v e r s i o n where m i c e l l e s d i s a p p e a r , t o Xj4c2' " omer c o n v e r s i o n where monomer d r o p l e t s d i s a p p e a r .


c

c

c

t î r i e

M

= 0.126 a

c

9 / 7

(p

s

p /

(1

+

γ) Κμε/r . )

1

/

7

S ^

/

7

^

m o n

(41)

2

In c a s e o f s t y r e n e e m u l s i o n p o l y m e r i z a t i o n , i s 0.43. If M i s f u r t h e r d e c r e a s e d b e l o w M , t h e number o f p o l y m e r p a r t i c l e s f o r m e d i n c r e a s e s w i t h d e c r e a s i n g M , as shown i n F i g u r e 6. The s o l i d l i n e s i n the f i g u r e s represent t h e o r e t i c a l v a l u e s p r e d i c t e d by E q . ( 4 0 ) u s i n g t h e f o l l o w i n g n u m e r i c a l c o n s t a n t s : Q

C

Q

Ζ/mol.sec,

k =

212

a =

35x 10~

p

s

1 6

[M ]=5.48 mol/Z, P

2

cm /molecule, X

molecule/cm .sec at I = 3

q

1.25

M c 2

= 0.43,

p^=

1.0

r./e=

g/cm 2.9x

3

10

7

g/Z-water

The r e a s o n why t h e e x p e r i m e n t a l v a l u e s o f p a r t i c l e number a r e somewhat l o w e r t h a n t h e t h e o r e t i c a l v a l u e s seems t o be t h a t t h e t i m e where t h e number o f p o l y m e r p a r t i c l e s was m e a s u r e d i s n o t a t i n f i n i t e but a t o n l y 1 hour a f t e r the s t a r t o f p o l y m e r i z a t i o n . F i g u r e 9 shows t h a t t h e number o f p o l y m e r p a r t i c l e s i n c r e a s e s w i t h r e a c t i o n time. The s o l i d l i n e s r e p r e s e n t t h e t h e o r e t i c a l v a l u e s p r e d i c t e d by t h e Nomura and H a r a d a m o d e l . However, s i n c e N = 0 when M = 0, t h e r e w o u l d be an optimum v a l u e o f M where t h e number o f p o l y m e r p a r t i c l e s f o r m e d becomes maximum. U n f o r t u n a t e l y , i t i s d i f f i c u l t a t p r e s e n t t o p r e d i c t t h e optimum v a l u e o f M theoreti c a l l y b e c a u s e any r e a c t i o n m o d e l c a n n o t y e t e x p l a i n p e r f e c t l y t h e k i n e t i c behavior a t h i g h monomer-conversion range. T h e r e f o r e , one c a n n o t h e l p d e t e r m i n i n g , a t p r e s e n t , t h e optimum v a l u e o f M expe rimentally. F i g u r e s 7 and 8 a l s o show t h a t E q . ( 4 0 ) r o u g h l y s a t i s f i e s the e x p e r i m e n t a l r e s u l t s . T

Q

Q

Q

Q

A p p l y i n g t h e above m e n t i o n e d k n o w l e d g e t o c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n o f s t y r e n e , we c a n p r o p o s e a v e r y e f f e c t i v e o p e r a t i o n method ( 1 4 ) . A s c h e m a t i c d i a g r a m o f p o r p o s e d p r o c e s s i s shown


NOMURA AND HARADA

Optimal

xio

Τ

Reactor

Type

and

Operation

I

ΓΊΓ

iI

20h

Cale.

Exp. Ο •

Eq.(33)

θ—

o " 'Ό—σ

4h Downloaded by TUFTS UNIV on October 14, 2014 | http://pubs.acs.org Publication Date: October 7, 1981 | doi: 10.1021/bk-1981-0165.ch006

2 υ

0.01

j L JL 0.04 0.06 0.1

0.02

0.4 0.6

0.2 M

Monomer c o n c e n t r a t i o n

q

[g/cc-water]

Figure 6. Effect of lowering initial monomer concentration on particle formation in batch operation (S = 6.25 g/L H 0; l„ = 1.25 g/L H 0; Τ = 50°C) 0

Figure 7.

2

2

Effect of initial emulsified concentration when initial monomer concen tration is very low (l = 12.5 g/L H 0; M g/cc H 0) 0

2

0

2


136


14 -xlO 8 20

Eq. (40)

10

μ

— - e r -

Expl.


• Ο

0.6

1

I n i t i a t o r Concentration

Figure 8.

Γ

τ—I

M

Cale.

Ο 0.03 0.5 I

1

1

4

6

8

I

q

[g/7--water]

Effect of initiator concentration on particle formation when monomer concentration is very low (S = 6.25 g/L H O 50°C) 0

2

f

Figure 9. Variation of particle number with reaction time in batch operation (cal culation conditions: S = 6.25 g/L H 0; I = 1.25 g/L H 0; S = 0.50 g/L H 0; η = 0.5) 0

2

0

2

CMC

2


6.

NOMURA AND HARADA

Optimal

Reactor Type and

Operation

137

i n F i g u r e 10. In t h i s o p e r a t i o n , a very small p o r t i o n of the t o t a l monomer feed and a l l other r e c i p e i n g r e d i a n t s are b a s i c a l l y fed i n t o the f i r s t stage r e a c t o r and the r e s t of the monomer i n t o the second stage. T h i s o p e r a t i o n method i s named "an operation with d i v i d e d monomer feed". (1) When a s t i r r e d tank r e a c t o r i s used as the f i r s t stage. Figure 11 shows a t y p i c a l example o f the t h e o r e t i c a l r e l a t i o n s h i p between the steady s t a t e p a r t i c l e number and mean residence time i n the f i r s t stage r e a c t o r . The s o l i d l i n e i n d i c a t e s the t h e o r e t i c a l values c a l c u l a t e d by Eq. (26) using the value of ε = 1 . 2 8 χ 1 0 , where monomer d r o p l e t s e x i s t i n the water phase. On the other hand, the broken l i n e s represent the t h e o r e t i c a l values p r e d i c t e d by Eqs.(24) and (39')r where monomer d r o p l e t s disappear. When , the r a t e o f monomer feed i n t o the f i r s t stage r e a c t o r decreases to zero, the number of polymer p a r t i c l e s formed approaches the value c a l c u l a t e d by the f o l l o w i n g equation:


5

N = (r.Sp/e) T

1 7 2

θ

1

/

2

(42)

Equation(42) i s the s o l u t i o n of simultaneous equations(24) and (39') at Mpi"*- 0. L e t us consider the v a r i a t i o n of N with θ i n F i g u r e 11 when M ^ i s , f o r example, 0.03 g/cc-water. The stady s t a t e p a r t i c l e number produced i n the f i r s t stage r e a c t o r f i r s t l y i n c r e ases along the s o l i d l i n e with i n c r e a s i n g the mean residence time of the f i r s t stage r e a c t o r Θ, begins t o decrease, passing through Ν at θ , and then, s h i f t s t o the broken l i n e a t θ„ where nu a χ max , , , , , c monomer d r o p l e t s j u s t disappear. With an increase i n the value of θ above p a r t i c l e number again increases g r a d u a l l y along the broken l i n e . As i s c l e a r from F i g u r e 11, a CSTR i s l e s s e f f i c i e n t i n the a b i l i t y o f p a r t i c l e formation i n the range of shorter mean residence time even i f the o p e r a t i o n method with d i v i d e d monomer feed i s employed, comparing a plug flow type r e a c t o r with the r e s idence time o f θρ which produces the same number of polymer p a r t i c l e s as i n a batch r e a c t o r . However, the e f f i c i e n c y of a CSTR i n p a r t i c l e formation i s g r e a t l y improved when the o p e r a t i o n method with d i v i d e d monomer feed i s employed i n the range of longer mean residence time, as shown i n Figure 11. (2) When a plug flow type r e a c t o r i s used as the f i r s t stage. The t h e o r e t i c a l r e l a t i o n s h i p between N and θ i s shown i n Figure 12 when a plug flow type r e a c t o r i s used f o r the f i r s t stage. Figure 12 can be obtained by r e p l a c i n g t by θ i n Figure 9, because the k i n e t i c s o f continuous emulsion p o l y m e r i z a t i o n of styrene i n a plug flow type r e a c t o r i s the same as i n a batch r e a c t o r . The s o l i d l i n e corresponding t o the c o n d i t i o n Μ + 0 i s obtained by Eq.(44) which can be d e r i v e d as f o l l o w s . In a plug flow type r e a c t o r , the r a t e o f p a r t i c l e formation i s ex pressed by(10) : dN_ r. r.S_ T

F

T

ρ

1


138


*F1 F2 —Monôme r— * -Water

2

Reactor


Initiator Emulsifier Figure 10.

η

3

Train

Schematic of proposed process and its operation method

u

φ

4

6 8 10

2

2

4

6 810

3

2

4

6

10

4

Mean R e s i d e n c e time β [ s e c ]

Figure 11. Effect of mean residence time and monomer concentration on steadystate particle number when a CSTR is used for the first stage (S = 6.25 g/L H 0; l =1.25 g/L H O;50°C) F

F

2


2

NOMURA AND HARADA

6.

Optimal

Reactor

Type and

Operation

139

S i n c e monomer d r o p l e t s do n o t e x i s t i n t h e r e a c t o r i n t h i s c a s e , E q u a t i o n ( 3 9 ' ) c a n be u s e d f o r S . T h u s , c o m b i n i n g E q s . ( 3 9 ' ) a n d

(43) and integrating at t h e conSition M ^-> 0, we have: F

N=


T

/2~(r.S /e) F

1 / 2

e

1 / 2

(44)

T h e o r e t i c a l v a l u e s c a l c u l a t e d by E q . ( 4 4 ) a r e a l s o shown i n F i g u r e 11 by a d o t t e d l i n e , f o r c o m p a r i s o n . I t i s f o u n d f r o m t h e compa r i s o n between F i g u r e s 11 and 12 t h a t a p l u g f l o w t y p e r e a c t o r w i t h a d i v i d e d monomer f e e d i s much b e t t e r i n t h e e f f i c i e n c y o f p a r t i c l e f o r m a t i o n t h a n a CSTR w i t h a d i v i d e d monomer f e e d a n d a p l u g f l o w type r e a c t o r w i t h a l l r e c i p e i n g r e d i a n t s f e d together i n t o t h e f i r s t s t a g e r e a c t o r . The v a l i d i t y o f t h e t h e o r e t i c a l p r e d i c t i o n s r e p r e s e n t e d i n F i g u r e 12 was e x p e r i m e n t a l l y v e r i f i e d u s i n g a p l u g f l o w t y p e r e a c t o r w i t h r e s i d e n c e t i m e o f θ = 20 m i n u t e s , w h i c h was made o f a c o i l e d g l a s s t u b e , and i s shown i n F i g u r e 13 where t h e s o l i d l i n e i s t h e t h e o r e t i c a l v a l u e s c a l c u l a t e d by E q s . ( 3 9 ) a n d (43). I t i s s e e n f r o m F i g u r e 13 t h a t t h e s t e a d y s t a t e p a r t i c l e number p r o d u c e d a t = 0.01 g / c c - w a t e r i s a b o u t 1.5 t i m e s t h a t a t M ^= 0.2 g / c c - w a t e r . I t i s c l e a r t h a t t h e volume o f t h e r e a c t o r operated a t = 0.2 g / c c - w a t e r i s b i g g e r t h a n t h a t o p e r a t e d a t F1 g/cc-water because t h e r e s i d e n c e times o f both r e a c t o r s a r e f i x e d a t θ = 20 m i n u t e s . From t h i s t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s , t h e r e f o r e , a p l u g f l o w type r e a c t o r w i t h a d i v i d e d monomer f e e d i s recommended f o r t h e f i r s t s t a g e r e a c t o r ( p r e - r e a c t o r ) , b e c a u s e t h e volume o f t h e r e a c t o r c a n be d e c r e a s e d by d e c r e a s i n g monomer c o n c e n t r a t i o n i n a f e e d s t r e a m . N e v e r t h e l e s s , t h e s t e a d y s t a t e p a r t i c l e number a t t a i n e d i n t h i s r e a c t o r c a n be i n c r eased. F

M

=

0

,

0

1

Further, continuous emulsion polymerization i n a plug flow type r e a c t o r has another advantage. I n continuous emulsion poly m e r i z a t i o n i n a CSTR, s u s t a i n e d o s c i l l a t i o n s o f monomer c o n v e r s i o n , p a r t i c l e number and a v e r a g e m o l e c u l a r w e i g h t o f p o l y m e r s f o r m e d a r e sometimes o b s e r v e d ( 1 ^ ) . T h i s phenomenon i s r e g a r d e d a s d i s a d vantage from t h e s t a n d p o i n t o f t h e s t a b i l i t y o f continuous r e a c t o rs and t h e q u a l i t y o f p r o d u c t s . However, i t was e x p e r i m e n t a l l y f o u n d t h a t s u s t a i n e d o s c i l l a t i o n c a n be a v o i d e d when p a r t i c l e f o r m a t i o n i s c o n d u c t e d i n a p l u g f l o w t y p e r e a c t o r , a s shown i n F i g u r e 14. F i g u r e 14b d i s p l a y s a c o u r s e o f c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n o f v i n y l a c e t a t e o b t a i n e d when a p l u g f l o w t y p e r e a c t o r w i t h t h e r e s i d e n c e t i m e o f 8 m i n u t e s i s u s e d a s a seeder i n f r o n t o f a CSTR w i t h t h e mean r e s i d e n c e t i m e o f 12 m i n u t e s , a n d a l l t h e r e c i p e i n g r e d i e n t s a r e f e d i n t o t h e p l u g f l o w t y p e seeder. E x p e r i m e n t a l d a t a p o i n t s r e p r e s e n t t h e monomer c o n v e r s i o n i n t h e CSTR and r e a c h a s t e a d y s t a t e v a l u e v e r y s m o o t h l y . On t h e o t h e r h a n d , F i g u r e 14a shows t h e c o u r s e o f c o n t i n u o u s e m u l s i o n p o l y m e r i z a t i o n o f v i n y l a c e t a t e c o n d u c t e d i n a CSTR w i t h t h e mean r e s i d e n ce t i m e o f 20 m i n u t e s , o t h e r r e a c t i o n c o n d i t i o n s b e i n g i d e n t i c a l to t h o s e i n F i g u r e 14b(15 - 1 6 ) .



140


Figure 12. Effect of mean residence time and monomer concentration on steadystate particle number when a plug flow-type prereactor is used for the first stage (calculation conditions: S = 6.25 g/L H 0; l = 1.25 g/L H 0; S MC = 0.50 g/L H O;n = 0.5) F

2

F

2

C

2

Figure 13. Effect of monomer concentration fed into a piston flow prereactor on steady-state particle number produced (reaction conditions: S = 6.25 g/L H 0; h •= 1.25 g/L H 0; residence time 6 = 20 min) F

2

t


2

6.

NOMURA AND HARADA

Optimal

Reactor

Type and

141

Operation

In some cases, the f o l l o w i n g operation method i s more e f f i c i ent than that s t a t e d above(14). F i g u r e 15 shows a schematic d i a gram o f the proposed process and operation method which i s the same as that represented i n Figure 10, except the method o f water feed. In t h i s operation method, water feed i s a l s o d i v i d e d . A small amount o f water which j u s t d i s s o l v e s i n i t i a t o r and emulsi f i e r i s fed i n t o the f i r s t stage reactor along with a l l the i n i t i ator and e m u l s i f i e r , and the r e s t o f water i n t o the second stage reactor. The number o f polymer p a r t i c l e s formed i n the f i r s t stage r e actor can be g e n e r a l l y expressed by the f o l l o w i n g form: a b c . »-» N =KS I 9 (45)


T

F

F

When 1/f o f the t o t a l water feed i s s u p p l i e d i n t o the f i r s t stage reactor and i t s mean residence time i s kept constant by a d j u s t i n g i t s reactor volume, the number of polymer p a r t i c l e s formed i n the f i r s t stage reactor i s : N

T1

= K ( fS

F

f

S

θ

F 4

°

(

4

6

)

However, the c o n c e n t r a t i o n of polymer p a r t i c l e s w i l l be d i l u t e d by f times with the r e s t of water fed i n t o the second stage. There f o r e , the number o f polymer p a r t i c l e s i n the second stage reactor N i s given by: T 2

N

T

2

=f

a

+

b

1

- KS I^

C

F

Since the value o f f i s greater than u n i t y , i t i s concluded

(47) that:

(1) when a + b> 1, one can increase the number of polymer par t i c l e s formed and furthermore, decrease the volume of the reactor by employing a d i v i d e d water feed along with a monomer d i v i d e d feed, (2) when a+ b = 1, the number o f polymer p a r t i c l e s does not change. However, the volume of the f i r s t stage reactor can be de creased i n p r o p o r t i o n t o a decrease i n the volume of the water fed i n t o the f i r s t stage, and (3) when a+ b< 1, one can decrease the volume of the f i r s t stage r e a c t o r . However, the number of polymer p a r t i c l e s a l s o dec reases with a decrease i n the volume of the f i r s t stage r e a c t o r . The o p e r a t i o n method with a d i v i d e d water feed i s not n e c e s a r i l y useful. In case of continuous emulsion polymerization of styrene, i t seems that a + b = 1 holds i n a wide range of operation c o n d i t i o n s , c o n s i d e r i n g Eqs. (27),(28),(42) and (44). The operation method with a d i v i d e d water feed would, t h e r e f o r e , be u s e f u l , although the v a l i d i t y of above d i s c u s s i o n i s not yet proved experimentally. To Conclude Optimal reactor type and i t s operation method f o r the f i r s t stage i n continuous emulsion polymerization was discussed i n t h i s paper. I t was c l a r i f i e d t h e o r e t i c a l l y and experimentally uaing a



142


Figure 14. Effect of prereactor type on vinyl acetate conversion transient at start up (15, 16; (S = 2.0 g/L H 0; l = 1.25 g/L H 0; M = 0.20 g/cc H 0; θ = 20 min, 50°C) F

2

Λ

F

F

2

M Monôme r — * Water H

-Initiator •Emulsifier

Figure 15.

2

Reactor

Train

Schematic of proposed process and its operation method


6. NOMURA AND HARADA

Optimal Reactor Type and Operation 143


styrene emulsion polymerization system that the most suitable rea ctor was a plug flow type reactor with a divided monomer feed and in some cases, with a divided water feed, additionally. The basic principles developed in this paper would be useful in designing the first stage reactor(pre-reactor) and its operation method for continuous emulsion polymerization of other monomers. Literature Cited 1. Feldon, M.; McCann, R. F.; Laundrie, R. W. India Rubber World 1953, 128, 51. 2. Poehlein, G. W.; Dpughherty, D. J. Rubber Chem. Technol. 1977, 50, 601. 3. Gershberg, D. B.; Longfield, J. E. Simp. Polym. Kinetics and Catalyst System Preprints 10, 45th Α. I. Ch. Ε. Meeting, New York 1961. 4. Dickinson, R. F. Ph.D. dissertation, Dept. of Chem. Eng., Waterloo Ontario, Canada, 1976. 5. Min, K. W.; Ray, W. H. J. Macromol. Chem. 1974, C11(2) 177. 6. Fitch, R. M.; Tsai, C. Η. "Polymer Colloids" Fitch, R. M. Ed., Plenum Press: New York, 1971; Ch.6. 7. Degraff, A. W.; Poehlein, G. W. J. Polym. Sci. 1971 A-2, 9, 1955. 8. Omi. S.; Ueda, T.; Kubota, H. J. Chem. Eng. Jpn. 1969, 2, 123. 9. Smith, W. V.; Ewart, R. H. J. Chem. Phys. 1948 16, 592. 10. Harada, M.; Nomura. M.; Kojima, H.; Eguchi, W.; Nagata, S. J. Appl. Polym. Sci. 1972, 16, 811. 11. Nomura, M.; Harada, M.; Kojima, H.; Eguchi, W.; Nagata, S. J. Appl. Polym. Sci. 1971, 15, 675. 12. Vanderhoff, J. W.; Vitkuske, J . F.; Bradford, E. B.; Alfrey, T, J. Polym. Sci. 1956, 20, 225. 13. Ley, G.; Gerrens, H. Makromol. Chem. 1974, 175, 563. 14. Harada, M.; Nomura, M.; Eguchi, W.; Nagata, S. Kobunshi Kagaku 1972, 29, 844. 15. Nomura, M.; Sasaki, S.; Fujita, K.; Harada, M.; Eguchi,W.ACS Preprints of Organic Coatings and Plastics Chemistry 1980, 43, 834. 16. Nomura, M. Unpublished data. RECEIVED

April 6, 1981.


On the Optimal Reactor Type and Operation for Continuous Emulsion

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