Continuous Emulsion Polymerization of Styrene in a Tubular Reactor

24 Continuous Emulsion Polymerization of Styrene i n a Tubular Reactor MAINAK GHOSH and T. H. FORSYTH†

Downloaded by MICHIGAN STATE UNIV on February 19, 2015 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0024.ch024

The University of Akron, Akron, Ohio 44325

P o l y s t y r e n e c a n be e a s i l y p r e p a r e d by e m u l s i o n o r suspension techniques. H a r k i n s ( 1_), S m i t h a n d E w a r t ( 2 j a n d G a r d o n (3) h a v e d e s c r i b e d t h e m e c h a n i s m s o f e m u l s T o n p o l y m e r i z a t i o n i n batch r e a c t o r s , a n d t h e r e s u l t s have been extended t o a s e r i e s o f c o n t i n u o u s s t i r r e d tank r e a c t o r s (CSTR)(4J Much i n f o r m a t i o n on c o n t i n u o u s emul sion r e a c t o r s i sdocumented i n t h epatent literature, w i t h s u c h i n n o v a t i o n s a s : u s e o f a s e e d l a t e x (5), u s e of p u l s a t i l e flow t o reduce plugging o f t h etube ( £ ) , a n d t u r b u l e n t f l o w t o r e d u c e p l u g g i n g (7_). Feldon (8) d i s c u s s e s t h e t u b u l a r p o l y m e r i z a t i o n o f SBR r u b b e r wTth l a m i n a r f l o w ( a tReynolds numbers o f 660). There have been r e c e n t s t u d i e s on c o n t i n u o u s s t i r r e d tank r e a c t o r s u t i l i z i n g S m i t h - E w a r t k i n e t i c s i n a s i n g l e C S T R (9) a s well as predictions o f p a r t i c l e size d i s t r i b u t i o n (10). C o n t i n u o u s t u b u l a r r e a c t o r s have been examined f o r n o n polymeric r e a c t i o n s (JjJ and p o l y m e r i c r e a c t i o n s (12,13V The o b j e c t i v e o f t h i s s t u d y w a s t o d e v e l o p a m o d e l for t h econtinuous emulsion polymerization o f styrene i n a t u b u l a r r e a c t o r , a n d t o v e r i f y t h emodel w i t h e x p e r i mental d a t a . 0

Experimental F i g u r e 1 shows t h e e q u i p m e n t used. The tubular r e a c t o r w a s 2 4 0 f t ( 7 3 m ) l o n g , 0.5 i n c h ( 1 . 2 7 c m ) 0 D , T y p e 316 s t a i n l e s s s t e e l . The r e a c t o r was p l a c e d i n an a g i tated, constant temperature water bath. Two g e a r pumps were used t o g i v e metered f l o w o f t h etwo feed streamsan e m u l s i o n o f s t y r e n e i n a n e q u a l v o l u m e o f w a t e r , a n d a s o l u t i o n o f potassium p e r s u l f a t e i nwater. Table 1 shows t h er e c i p e used f o r p o l y m e r i z a t i o n . t

A u t h o r t o whom c o r r e s p o n d e n c e may b e d i r e c t e d . 367

In Emulsion Polymerization; Piirma, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

EMULSION

368

Table

I

Polymerization


POLYMERIZATION

Recipe

Component

Standard

Uninhibited Styrene % De-Ionized Water % S o d i u m L a u r y l S u l f a t e ( T e c h n i c a l ) %* P o t a s s i u m P e r s u l f a t e ( R e a g e n t ) %* T e m p e r a t u r e °C Total Flow Rate, cc/min R e s i d e n c e T i m e , min V i s c o s i t y Average M o l e c u l a r Weight

33 67 0.8 1.0 60. 3.0 60 2.0·10

Range S t u d i e d

6

33 67 0.4-1.2 0.21-1.5 40 - 9 0 0.5-5.1 10 - 102 0.7-8.Π0

* wt % based on water Unsteady s t a r t - u p b e h a v i o r h a s been r e p o r t e d i n some c o n t i n u o u s r e a c t o r s ( ϋ ) . I n t h i s work, the re a c t o r was i n i t i a l l y f i l l e d w i t h water o r p a r t i a l l y polymerized l a t e x , and the % conversion a s y m p t o t i c a l l y a p p r o a c h e d a c o n s t a n t s t e a d y - s t a t e v a l u e , w h i c h required l e s s than twice the residence time i nthe r e a c t o r . In e a r l y t e s t s , t h e t e m p e r a t u r e o f t h e emulsion was m e a s u r e d b y t h e r m o c o u p l e s . The thermocouples were r e m o v e d f o r l a t e r t e s t s , b e c a u s e no s i g n i f i c a n t exotherm was o b s e r v e d a n d b e c a u s e t u b e p l u g g i n g s o m e t i m e s o c c u r red near the thermocouple probe. P l u g g i n g o f t h e r e a c t o r o c c u r r e d when soap c o n c e n t r a t i o n was l e s s than 0.4%a n d a t high temperatures. P l u g g i n g was o f t e n p r e c e d e d b y a p u l s a t i n g f l o w o f twophase l i q u i d . I n t e n s e l y a g i t a t i n g the emulsion feed i n the storage tank helped i npreventing plugging a l s o . The p e r c e n t c o n v e r s i o n w a s m e a s u r e d a f t e r s h o r t s t o p p i n g w i t h 2 0 0 ppm h y d r o q u i n o n e , b y c o a g u l a t i n g i n i s o p r o p a n o l , and then d r y i n g t o constant weight. The m o l e c u l a r w e i g h t was o b t a i n e d from the v i s c o s i t y o f a 10% s o l u t i o n o f c o a g u l a t e d p o l y m e r i n t o l u e n e , a s meas u r e d i n a n O s t w a l d C a p i l l a r y V i s c o m e t e r ( s i z e 5 0 ) . The per cent c o n v e r s i o n was r e p r o d u c i b l e t o w i t h i n 15%, w h i l e the m o l e c u l a r w e i g h t was r e p r o d u c i b l e t o w i t h i n 50%. Mathematical

Model

The f l o w i n g e m u l s i o n w a s a s s u m e d h o m o g e n e o u s , s o t h a t the c o n t i n u i t y e q u a t i o n s c o u l d be used. Additional assumptions were: the f l u i d i s an i n c o m p r e s s i b l e Newton ian w i t h c o n s t a n t p r o p e r t i e s ; the f l o w i s l a m i n a r a t the maximum e x p e r i m e n t a l R e y n o l d s number o f 2 1 0 a n d l e s s ; there i s n e g l i g i b l e viscous heating; flow i s a t steady


Emukion

GHOSH A N D FORSYTH

24.

Polymerization

of Styrene

369

s t a t e w i t h n o e n t r a n c e e f f e c t s o r r a d i a l v e l o c i t y com ponents; body f o r c e s a r e n e g l e c t e d ; a x i a l heat conduc t i o n i s small compared t or a d i a l c o n d u c t i o n ; Region I o f S m i t h - E w a r t k i n e t i c s ( i . e . , when m i c e l l e s a r e f i r s t forming) i sneglected and t h ei n i t i a t o r c o n c e n t r a t i o n i s constant. T h e model may be summarized a s : V = V [ l - ( r / R ] = - 9 . [l-(£)*]= f

|1 [ 1 - φ ]

2

z

2

z

(1)


TTR

pC

p

Vl-(r/R)^]|I=^[r|I]

2V

Z

[l-(r/R*] § - ^ [ - r f Ê l - R p H 0

Ν = C (C ) -

6

p

=

J

?N7

A e x

(2)

p

(3)

0

(Cj) '*

s

R

AHR M

+

(

P -

E / R

(4)

g

T )

( 5 )

where [ M ] i s t h e c o n c e n t r a t i o n o f monomer i n t h e swollen d r o p l e t s a n d i sc o n s t a n t up t o 5 0 % c o n v e r s i o n . Between 50 and 1 0 0 % c o n v e r s i o n [ M ] i s t h e b u l k m o n o m e r c o n c e n tration. Further combinations o f terms s i m p l i f y t h e above e q u a t i o n s a n d make i t p o s s i b l e t o a n a l y z e i n t e r m s o f dimensionless numbers. The energy equation i s :

where:

y = r/R k 27 pC R

G

z

G 2

3

p

1 (Reynolds)(Prandtl)

(AH)(N)(A)(R)

=

2

G

_

=

[

c

]

V pX aH w) -E C

N

T

g w The

equation u

y

where:

' 3z G*

f o r monomer c o n c e n t r a t i o n L

R

l

D 2V R z

J

9 3 r y y3 y3 y _

J

R

e x p

Τ

becomes: κ

η

1 (Reynolds)(Schmidt)


370

EMULSION

G

.

5

POLYMERIZATION

(N)(A)(R)[C]


S o l u t i o n o f t h e d i f f e r e n t i a l e q u a t i o n s w a s b y GaussSeidel iteration (with the f l u i d property values given in Table I I ) on an IBM 370 d i g i t a l computer u s i n g i m p l i c i t d i f f e r e n c e e q u a t i o n s o f t h e C r a n k - N i c h o l s o n type. The p r o g r a m was c o n v e r g e n t a n d s t a b l e f o r a l l conditions tested.

Constants A [C] C D Ε ΔΗ k M a R Rg μ P D P

N

Used

Table I I i nComputer Program f o r Styrene

3

cm /mo1e-sec wt f r a c t i o n cal/gm-°C cm /sec cal/gmole cal/gm cal/sec-cm-°C gm/gmole molecules/mole cm cal/gmole-°K gm/cm-sec gm/cm 2

2.24*10 0.572 1.0 10" 17,570 160.6 0.0015 104 6.02-10 0.546 1.987 0.01 1.0 6

Using Gardon's ( 3 ) e x p r e s s i o n f o r N, t h e number o f p a r t i c l e s p e r u n i t voTume, gave c o n v e r s i o n r a t e s t w o o r d e r s o f magnitude l a r g e r than were e x p e r i m e n t a l l y o b served. To o b t a i n a c c u r a t e e s t i m a t e s o f t h e c o n v e r s i o n , Ν was e m p i r i c a l l y e v a l u a t e d ( a t about 10 particles/cm ) from one data s e t a t each temperature. l l f

Results

3

and Discussion

Experimental. Figure 2 compares molecular weight d a t a r e p o r t e d b y G a r d o n Q_0) f o r b a t c h r e a c t o r s a n d b y P o e h l e i n f o r C S T R r e a c t o r s (J5), w i t h t h e d a t a o b t a i n e d in this study f o r a tubular reactor. The solid lines are p r e d i c t e d by Gardon's theory ( 1 0 ) . The molecular weights o b t a i n e d i nt h i s e x p e r i m e n t a l study were pre d i c t e d w i t h i n a f a c t o r o f 3 by Gardon's theory. No d i r e c t c o m p a r i s o n c a n b e made w i t h t h e d a t a o f o t h e r workers, y e t this molecular weight data i s c o n s i s t e n t ( a t l e a s t w i t h i n e x p e r i m e n t a l e r r o r ) w i t h d a t a obtained in other types o f r e a c t o r s . F i g u r e 3 compares t h e e x p e r i m e n t a l r a t e o f monomer loss i nGardon's batch r e a c t o r , the t u b u l a r reactor,and


Emuhion

GHOSH A N D FORSYTH

Polymerization

of Styrene


ι

Figure 1.

STEAM

Sketch of equipment used for the continuous emulsion polymerization of styrene in a tubular reactor

2.0

I

I

I

MONOMER • GARDON POEHLEIN

40 % 35 %

585MIN-

• THISMoSTUDY /

33

6 0 MIN

1.6 - A

% .

!.2

A 0.65

l o

0.8

•

3.8/

0.4

1 TIME

"

#

l.8^>

A 0.30

-

"•1.4

- /

MOL. WT. IN 10

0

1

ι

0.4

08

1.2

1.6

2.0

INITIATOR CONCENTRATION(%) Figure 2. Comparison of molecular weights obtained in different types of reactors, with Gardons theory for batch reactors shown as a solid line



372

EMULSION

POLYMERIZATION

two CSTR. A l s o shown i s a p r e d i c t e d c u r v e b a s e d o n S m i t h E w a r t t h e o r y f o r R e g i o n II f o r s t a n d a r d c o n d i t i o n s o f the t u b u l a r r e a c t o r s t u d i e s . The experimental r e s u l t s indicate that there i s a constant rate period i n t h e t u b u l a r r e a c t o r , but c o n v e r s i o n r a t e s are lower than p r e d i c t e d by Smith and Ewart theory. A t long residence times, there i s a sharp drop i nc o n v e r s i o n r a t e which c o u l d be due t o Region I I I k i n e t i c s (where r e a c t i o n rate i s l i m i t e d by d e c r e a s i n g amounts o f a v a i l a b l e monomer) or t o i n i t i a t o r decay. F i g u r e 4 shows the e f f e c t o f i n i t i a t o r on t h e average c o n v e r s i o n r a t e a f t e r a r e s i d e n c e time o f60 minutes. A t h i g h s o a p a n d i n i t i a t o r l e v e l s , t h e number o f p a r t i c l e s , N, a n d r a t e o f p o l y m e r i z a t i o n a r e h i g h . E q u a t i o n ( 4 ) i n d i c a t e s a 0 . 4 p o w e r d e p e n d e n c y o f number of p a r t i c l e s t o i n i t i a t o r c o n c e n t r a t i o n , and a l e a s t s q u a r e f i t o f t h e d a t a i n F i g u r e 4 g a v e t h i s same d e pendency f o r r a t e o f p o l y m e r i z a t i o n . Model F i g u r e 5 shows the e f f e c t o f the dimensionl e s s v a r i a b l e , G i , on c o n v e r s i o n a t d i f f e r e n t p o s i t i o n s down t h e t u b e . The curve marked X i s f o r the standard c o n d i t i o n s o f T a b l e 1. I t c a n b e s e e n t h a t t h e r e i s l i t t l e c h a n g e i n c o n v e r s i o n a s a f u n c t i o n o f Gi i f heat c o n d u c t i o n i n the f l u i d i shigh. F i g u r e 5 shows t h a t the temperature f o r curves Β and X reach an asymptote s l i g h t l y higher than the w a l l temperature, such t h a t heat g e n e r a t i o n i s n u m e r i c a l l y equal t o heat t r a n s f e r . When t h e r a t e o f r e a c t i o n d e c r e a s e s a t h i g h e r c o n v e r s i o n , t h e a v e r a g e f l u i d t e m p e r a t u r e w o u l d d e c r e a s e t o 60°C. With high convection (or high flow r a t e s ) the f l u i d i s not heated, s o t h a t c o n v e r s i o n i slow a t l o w v a l u e s o f Gi.

Radial temperature p r o f i l e s were almost constant as e x p e r i m e n t a l l y o b s e r v e d a n d l a t e r p r e d i c t e d b y t h e model. This i n d i c a t e s that the thermal c o n d u c t i v i t y f o r the styrene/water systems s t u d i e d i s s u f f i c i e n t l y high t o q u i c k l y heat the l i q u i d as i t enters the tube, and e f f i c i e n t l y remove t h e heat r e l e a s e d b y t h e r e a c t i o n . In F i g u r e 6 i t i s seen t h a t a s G i n c r e a s e s , a h i g h r a t i o o f heat r e l e a s e d by r e a c t i o n t o heat removed by convection causes a temperature overshoot. Such an overshoot was not observed f o r s t y r e n e , e i t h e r e x p e r i mentally o r with the model. Figure 7 presents predicted a x i a l t e m p e r a t u r e p r o f i l e s f o r s e v e r a l monomers a t s t a n d a r d c o n d i t i o n s , w i t h c o n s t a n t s a s i n T a b l e III. W h i l e e q u a t i o n s (4) a n d (5) r e q u i r e t h e monomer t obe o n l y p a r t i a l l y s o l u b l e , t h e r e s u l t s shown i n F i g u r e 7 indicate a strong exotherm f o r vinyl c h l o r i d e , a c r y l o n i t r i l e and vinyl acetate. 2


24.

Emulsion

GHOSH A N D FORSYTH

Polymerization

of

Styrene

373

6 ^CURVE BASED 01 OÎK^^POEHLEIN SMITH-EWART THEORr^V. OF THIS STUDY DY

3

Κ

! THIS S T U D Y ^ A Jr

06


GARDON

?

0.3-

0.1 0.061

Continuous Emulsion Polymerization of Styrene in a Tubular Reactor

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