Polymerization Reactors and Processes - American Chemical Society

5 Continuous-Emulsion Polymerization of Styrene in a Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 25, 2018 | https://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch005

Tubular Reactor 1,

A. L. ROLLIN, W. IAN PATTERSON, J. ARCHAMBAULT and P. BATAILLE Chemical Engineering Department, Ecole Polytechnique de Montreal, C.P. 6079, Succursale "A", Montreal, Quebec, H3C 3A7, Canada

The advantages of continuous tubular r e a c t o r s are w e l l known. They i n c l u d e the e l i m i n a t i o n of batch t o batch v a r i a t i o n s , a l a r g e heat t r a n s f e r area and minimal handling o f chemical products. Despite these advantages there are no reported commerc i a l instances o f emulsion polymerizations done i n a tubular r e a c t o r ; i n s t e a d the continuous emulsion process has been r e a l i z e d i n series-connected s t i r r e d tank r e a c t o r s (1, 2^ 3 ) . A few workers have examined the continuous emulsion polymerizat i o n process i n a tubular r e a c t o r (4, _5, 6), the i n i t i a l work being done i n the turbulent regime. T h i s flow c o n d i t i o n was chosen to maximise heat t r a n s f e r and mixing, however i t was found that a pre-coagulum was formed and r e s u l t e d i n plugging of the r e a c t o r (4). Ghosh and F o r s y t h (6) examined the emulsion p o l y m e r i z a t i o n of styrene i n a continuous tubular r e a c t o r . They r e s t r i c t e d o p e r a t i o n to the laminar flow regime and a l s o encountered r e a c t o r plugging except when high soap concentrations were employed. They a t t r i b u t e d the plugging t o emulsion i n s t a b i l i t y and the flow r e s t r i c t i o n s caused by the thermocouple w e l l s and they a l s o concluded that the k i n e t i c s were e s s e n t i a l l y those of the Smith-Ewart model. Recently, R o l l i n e t a l examined the e f f e c t of the flow regime on the emulsion p o l y m e r i z a t i o n of styrene i n a tubular r e a c t o r (7). I t was found that the r a t e of p o l y m e r i z a t i o n was a maximum a t the laminar-turbulent t r a n s i t i o n when an "emulsion Reynolds number, ( % ) i s defined as f o r a c i r c u l a r tube, except that the f l u i d p r o p e r t i e s , p a r t i c u l a r l y the v i s c o s i t y , are those of the emulsion before any a p p r e c i a b l e r e a c t i o n has occurred. (Previous workers had c a l c u l a t e d t h e i r values of N u s i n g the v i s c o s i t y of the l a t e x product). This r e s u l t was i n t e r p r e t e d i n the l i g h t of work done on the e f f e c t of s t i r r i n g speed (degree of a g i t a t i o n ) f o r batch emulsion polymerizations (8, % 10, 11) where i t was found that an optimum s t i r r i n g speed e x i s t e d . T h i s optimum was f i r s t explained by Evans et a l (8). I t may be i n t e r p r e t e d f o r tubular r e a c t o r s a t low e

e

R e

'Current address:

Hydro-Quebec, Montreal

0-8412-0506-x/79/47-104-113$06.00/0 © 1979 American Chemical Society Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

POLYMERIZATION

114

REACTORS AND PROCESSES

Reynolds numbers as f o l l o w s : i n the laminar flow regime the r e a c t i o n i s d i f f u s i o n c o n t r o l l e d although the v e l o c i t y gradient a i d s the d i f f u s i o n . Very small values of N permit phase s e p a r a t i o n to occur thus g r e a t l y d i m i n i s h i n g the area a v a i l a b l e f o r monomer t r a n s f e r . Highly t u r b u l e n t flow ( l a r g e values of ( N ) ) promotes the break-up of monomer drops thus reducing the soap a v a i l a b l e to form m i c e l l e s and hence the number of polymer p a r t i c l e s i s reduced. Since the r a t e of p o l y m e r i z a t i o n (Smith-Ewart) i s given by: R e

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on February 25, 2018 | https://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch005

R e

r

p

= (k

p

M

p

e

N )/2

(1)

t

where = the number of polymer p a r t i c l e s per u n i t volume i t i s expected that the r a t e of p o l y m e r i z a t i o n w i l l d i m i n i s h due to the decreased value of N . Two a d d i t i o n a l r e s u l t s were noted d u r i n g t h i s s e r i e s of experiments. I t was found that plugging of the r e a c t o r occurred when the conversion reached about 60%. No s a t i s f a c t o r y explanat i o n or cause f o r the plugging was determined. I t was a l s o noted t h a t , r e g a r d l e s s of the r a t e of p o l y m e r i z a t i o n , no f u r t h e r r e a c t i o n occurred a f t e r a p e r i o d of about 60 to 75 minutes. T h i s i s i n c o n t r a s t to r e a c t i o n times of up to three hours f o r the same r e c i p e used i n a batch r e a c t o r . t

E f f e c t of Reactor Geometry The s t u d i e s described above have a l l been done i n e s s e n t i a l l y s t r a i g h t , tubular r e a c t o r s . The curved part (the elbows) c o n s t i tuted a very small p r o p o r t i o n of the t o t a l l e n g t h . A r e a c t o r of commercial i n t e r e s t operated at an ( N ) i n the v i c i n i t y of 2100 would be on the order of 60 metres to §o8 metres i n l e n g t h depend i n g on tube diameter. The advantages of good heat t r a n s f e r and temperature c o n t r o l of emulsion p o l y m e r i z a t i o n are g e n e r a l l y r e a l i z e d by the immersion of the r e a c t o r i n a heat t r a n s f e r medium of s u b s t a n t i a l thermal c a p a c i t y . T h i s i s d i f f i c u l t and c o s t l y to achieve when a l i n e a r tubular r e a c t o r i s used and a h e l i c a l c o n f i g u r a t i o n suggests i t s e l f as a v i a b l e a l t e r n a t i v e . The emulsion p o l y m e r i z a t i o n of styrene i n a s t r a i g h t t u b u l a r r e a c t o r has been shown to be s e n s i t i v e to the hydrodynamics of the flow. Thus i t i s reasonable to ask what the e f f e c t of a h e l i c a l r e a c t o r geomet r y would be. The flow through h e l i c a l l y configured, round tubes was f i r s t examined by E u s t i c e (12) i n 1910 and the f i r s t t h e o r e t i c a l analys i s was published by Dean i n 1927 and 1928 (13). He showed that the f l u i d flow could be c h a r a c t e r i z e d by the dimensionless group R

(

I T

where:

=

V

2

d i = the tube i n s i d e diameter. D = the diameter of the h e l i c a l bend. N « the Dean number

Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

5.

ROLLTN ET AL.

N

=

in a Tubular

115

Reactor

i e

R * conventional Reynolds number but was unable to account f o r the secondary flow observed by E u s t i c e . White (14) derived an expression f o r the laminar flow pressure drop i n a h e l i x and T a y l o r (15) l a t e r showed that the e f f e c t of tube curvature was t o damp flow p e r t u r b a t i o n s and i n c r e a s e the value of the Reynolds number a t which the laminart u r b u l e n t t r a n s i t i o n occurs. Since then a number of workers have d e r i v e d v a r i o u s expressions to p r e d i c t the t r a n s i t i o n and the flow c h a r a c t e r i s t i c s i n the d i f f e r e n t regimes. T h i s work i s summarized by S r i n i v a s a n et a l (16). The c o n f l i c t i n g p r e d i c t i o n s of the v a r i o u s equations f o r the t r a n s i t i o n p o i n t have l e d us to experimentally determine the laminar-turbulent t r a n s i t i o n f o r the p a r t i c u l a r c o n f i g u r a t i o n employed i n t h i s work. T h i s i s reported i n the s e c t i o n on results. e


t

Styrene

t

Experimental The emulsion p o l y m e r i z a t i o n of styrene was c a r r i e d out i n an open loop r e a c t o r shown s c h e m a t i c a l l y i n F i g u r e 1. The apparatus c o n s i s t e d o f an e m u l s i f i c a t i o n c i r c u i t connected to a t u b u l a r r e a c t o r of a 2.23 cm i . d . f l u o r i n a t e d polymer tube 155.7 m. long i n a h e l i c a l l y c o i l e d c o n f i g u r a t i o n . The diameter, the number of turns and the tube l e n g t h of each c o i l a r e presented i n Table I. I t i s observed that the tube l e n g t h , or number of loops, of each of the four c o i l s i s p r o g r e s s i v e l y longer. The connections between the c o i l s a r e s t r a i g h t lengths of s t a i n l e s s pipe w i t h taps t o permit sampling of the s o l u t i o n a t the p o i n t s S2 to S6« An a g i t a t e d v e s s e l c o n t a i n i n g the monomer, e m u l s i f i e r and water was used to supply the mixture t o a sonic e m u l s i f i e r (Sonolator A from Sonic Eng. L t d ) f e d by a gear pump. A r e t u r n l i n e was i n s t a l l e d t o r e g u l a t e the flow of the s t a b i l i z e d emuls i o n through the r e a c t o r . The aqueous s o l u t i o n of the i n i t i a t o r was c o n t i n o u s l y i n j e c t e d by a p e r i s t a l t i c pump (Masterflex model 7016) upstream of a r e s t r i c t i o n a t the entrance to the heating section. Instantaneous mixing of the i n i t i a t o r i n the emulsion was achieved by the energetic eddies created by the r e s t r i c t i o n (0.63 cm diameter). The s t r a i g h t s t a i n l e s s s t e e l heating s e c t i o n (2.54 cm i.d.) was jacketed by a 6.3 cm i . d . galvanized pipe and c o u n t e r - c u r r e n t l y flowing water at 62°C c i r c u l a t e d i n the j a c k e t t o maintain the d e s i r e d emulsion temperature o f 60°C a t the e x i t of t h i s s e c t i o n . The r e a c t o r was operated i n the f o l l o w i n g manner. First, the r e q u i r e d volumes o f the emulsion i n g r e d i e n t s were placed i n the a g i t a t e d r e s e r v o i r and the o p e r a t i n g temperatures were e s t a b l i s h e d i n the heat exchanger, the r e a c t o r c o i l tanks and the heating j a c k e t o f the a g i t a t e d v e s s e l . A n i t r o g e n blanket was i n j e c t e d a t the top o f the r e s e r v o i r t o avoid oxygen absorpt i o n and the a g i t a t o r was s t a r t e d (constant speed of 110 rpm). E m u l s i f i c a t i o n was achieved by c i r c u l a t i n g the r e s e r v o i r mixture



latex

pump-A

hot water-

COM 4

Figure

pump-B

1.

Schematic

a.

T3.

coiled tubular

a

initiator

coil 2

, S 4

'iHc orifice

LSI

of the helically

coil 3

sonolator

emulsion reservoir

reactor

coil l

steam injection

= u v * »

hot water

Tubular reactor


Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 2.23 2.23 2.23

21.70

32.55

37.98

54.26

Coil # 1

Coil # II

Coil # III

C o i l # IV 2.23

2.54

3.45

Heat Exchanger

0.63 2.54

(cm)

0.59 0.12

(m)

Length

Tube diameter

Mixing of initiator

Section

159.98

102.03

61.91

27.57

4.17

0.59 0.71

24 28 40

43.2 43.2 43.2

24,387 40,187 62,487

16

43.2 11,087

_

loops

Number of

-

Coil

-

_

(cm)

Diameter

2,027

107

(cm )

volume

length Cm)

Cumulative

REACTOR DIMENSIONS

Cumulative

TABLE I :


POLYMERIZATION REACTORS AND PROCESSES


118

through the gear pump which was monitored f o r safe o p e r a t i n g conditions. The c l o s e d loop r e c i r c u l a t i o n v i a the pump and Sonolator f o r a p e r i o d of approximately 75 minutes provided a very stable enulsion. When the e m u l s i f i c a t i o n was complete, the gear pump speed was reduced to the d e s i r e d l e v e l and the flow r a t e s of the emulsion and of the i n i t i a t o r were e s t a b l i s h e d . Temperature p r o f i l e s and pressure drops along the r e a c t o r were recorded and the flow r a t e was measured g r a v i m e t r i c a l l y . F i f t e e n experimental runs were c a r r i e d out i n a range of emulsion Reynolds numbers from 1350 to 10600 at a constant temperature of 60°C. The d u r a t i o n of each run was approximately three (3) residence times. A l l runs used the e m u l s i f i c a t i o n formulation given i n Table I I . The f i v e thermistors placed along the r e a c t o r v e r i f i e d that no temperature gradient e x i s t e d d u r i n g a run and the range of flow r a t e s v a r i e d from 0.64 to 5.17 Kg/min. Samples were c o l l e c t e d a t f i x e d times from the sample p o i n t s and placed i n c l o s e d t e s t tubes c o n t a i n i n g a small amount of hydroquinone i n h i b i t o r . The styrene conversion was g r a v i m e t r i c a l l y determined.

TABLE I I :

EMULSION FORMULATION

Potassium Persulfate

[s]x 10~

Workers

3

(9)

Rollin

(7)

B-3 & B-4 ( t h i s work)

Note:

10-

3

Styrene

(mol/1)

(mol/1)

5.88*

2.42

1.69

6.52**

2.55

1.74

12.60**

3.07

1.74

(mol/1)

Omi

[l] x

c o n c e n t r a t i o n s r e f e r r e d to u n i t volume of emulsion

*

Oleate Sodium

**

Dodecyl Sodium S u l f a t e


5.

ROLLIN ET AL.

119

Styrene in a Tubular Reactor

Molecular weights were measured u s i n g a Waters g e l permeation chromatograph (model 200). Complete d e t a i l s of the equipment and procedures can be found i n r e f e r e n c e s (17, 18, 19).


R e s u l t s and

Discussion

A summary of the nine batch r e a c t o r emulsion p o l y m e r i z a t i o n s and f i f t e e n t u b u l a r r e a c t o r emulsion p o l y m e r i z a t i o n s are presented i n Tables I I I & IV. A l s o , many tubular r e a c t o r pressure drop measurements were performed at d i f f e r e n t Reynolds numbers using d i s t i l l e d water to determined the laminar-turbulent t r a n s i t i o n a l flow regime. Pressure drop measurements: The pressure drop i n a c o i l e d tube c o n f i g u r a t i o n i s p r e d i c t e d to be higher than i n a s t r a i g h t tubular r e a c t o r and the l a m i n a r - t u r b u l e n t t r a n s i t i o n i s a l s o p r e d i c t e d to be s h i f t e d to higher Reynolds numbers because of the presence of the c e n t r i f u g a l f o r c e a c t i n g on the f l u i d elements. Shown i n F i g u r e 2 are curves r e p r e s e n t i n g the hydraul i c behavior i n s t r a i g h t tube and i n a c o i l of dimensions i d e n t i c a l to that used i n the experimental r e a c t o r . Curves 1 and 2 were c a l c u l a t e d using I t o ' s equation (20)and the curves D are the w e l l known Moody curves f o r s t r a i g h t smooth c i r c u l a r tubes. The experimental data, represented by the p o i n t s i n F i g u r e 2, f a l l on the upper curves (1 & 2) confirming that the measured pressure drops correspond to a c o i l e d c o n f i g u r a t i o n r a t h e r than a s t r a i g h t tube. White (14) proposed a g r a p h i c a l method of determining the c r i t i c a l Reynolds number N at which the f u l l y t u r b u l e n t flow e x i s t s i n a c o i l e d tube. As shown i n F i g u r e 3, the curve r e p r e s e n t i n g White's equation f o r the r a t i o of the l o g a r i t h m of the f r i c t i o n f a c t o r i n a c o i l ( f c ) to the f r i c t i o n f a c t o r i n a s t r a i g h t tube ( f ) versus the logarithm of the Dean number, N , fits the experimental data w e l l f o r Reynolds numbers smaller than approximately 5500. The i n t e r s e c t i o n of the s t r a i g h t l i n e passing through the experimental p o i n t s at h i g h Reynolds numbers w i t h White's curve represents the flow c o n d i t i o n at which f u l l y t u r bulent flow occurs. I t can be observed from F i g u r e 3 that the i n t e r s e c t i o n occurs i n the r e g i o n 5500 £ Nre 6. 7100 compared to a value of N = 7830 c a l c u l a t e d using Holland's equation (16). The d i f f e r e n c e between the c a l c u l a t e d and experimentally d e t e r mined c r i t i c a l Reynolds number can be explained from the r e a c t o r c o n f i g u r a t i o n , which c o n s i s t e d of four c o i l s connected by s t r a i g h t tubing s e c t i o n . The s t r a i g h t s e c t i o n s would lower the N relat i v e to the value f o r a s i n g l e c o i l . I t i s j u s t i f i e d to note that higher p o l y m e r i z a t i o n r a t e s are f o r e c a s t e d f o r Reynolds numbers i n the v i c i n i t y of 5000 to 7000. R e c

D n

R e c

R e c

Batch P o l y m e r i z a t i o n s : Nine batch p o l y m e r i z a t i o n s were performed to v e r i f y that our f o r m u l a t i o n behaviour was unchanged from


Henderson and Bouton; Polymerization Reactors and Processes ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 1.74 1.74

3.07

3.07

3.07

3.07

3.07

3.00

2.96

3.06

12.60

12.60

12.60

12.60

12.60

12.50

11.96

12.20

3-3

3-4

B-5

B-6

B-7

B-8

B-9

30

20 45

1.0

3.0 1.0

I

1.74

195.0

200.0

D = d i s t i l l e d Styrene and I = i n h i b i t e d styrene

D

75

60

» Apparatus C-2 0.73

670

670

670 Apparatus E-2

30

200.0

D

930 670 + 20 rpm

45

1.0

670

670 + 20 rpm

550

D

670

650

670 + 20 rpm

Apparatus C-1 1.0

Agitation speed (rpm)

650

20 rpm

Volume

Apparatus E - l

650

A g i t a t i o n speed

Reaction

670 + 20 rpm

45

1.0

D

45

go

Time (min)

Volume

1.0

D

D

D

1.74

1.74

1.74

1.74

1.74

1.33

3-2

1.77

D

Styr

c C

(N ^) R

e

dimensionless

v a l u e o f t h e R e y n o l d s number a t t h e l a m i n a r t r a n s i t i o n , dimensionless R e y n o l d s number b a s e d o n e m u l s i o n p r o p e r t i e s polymerization, dimensionless

N^ Dn

Dean number,

r^

rate o f polymerization, mol 1 ^ h ^

Note:

turbulent

prior to

dimensionless

A p p a r a t u s C - l i s a l a b o r a t o r y r o u n d - b o t t o m f l a s k and laboratory s t i r r e r C - 2 i s a l a b o r a t o r y r o u n d - b o t t o m f l a s k and laboratory s t i r r e r E - l i s a stainless, cylindrical reservoir, piston pump a n d s o n o l a t o r E-2 i s t h e r e s e r v o i r o f t h e h e l i c a l r e a c t o r , gear Dump a n d s o n o l a t o r

References (1) (2) (3)

Ghosh, M. and Forsyth, T.H., ACS Symp. Series, No. 24, paper 24 (1976) R o l l i n , A.L., Patterson, I . , Huneault, R. & Bataille, P . , Can. J . Chem. E n g . , 55, 565 (1977) White, C . M . , Proc. Royal Soc., A-123, 645 (1929)

Literature cited (1) (2) (3) (4) (5) (6)

Harada, Μ., Nomura, M . , Kojima, H., Eguchi, W. and Nagata, S., J . Appl. Poly. S c i . , 16, 811 (1972) U.S. Patent No. 2, 831, 842, Dupont de Nemours & Co. De Graff, A.W. and Poehlein, G.W., J. Poly. S c i . , A-2, 9, 1955 (1971) Feldon, M . , McCann, R . F . and L a u n d r i e , R.W., India Rubber World 128, 1 (1953) Canadian Patent No. 907795, G u l f O i l Canada Ltd (1972) Ghosh, M. and Forsyth, T . H . , ACS Symposium Series, No. 24, paper No. 24 (1976)


136

POLYMERIZATION REACTORS AND PROCESSES


(7)

R o l l i n , A.L., P a t t e r s o n , I . , Huneault, R., R a t a i l l e , P., "The E f f e c t of Flow Regime on the Continuous Emulsion P o l y m e r i s a t i o n of Styrene i n a Tubular Reactor", Can. J. of Chem. 55, 565 (1977) (8) Evans, C.P., L i g h t , J.D., Marker, L., S a n t o a i a l a , A.T. and Swetting, O.J., J. Appl. P o l y . S c i . , 5, 31 (1961) (9) Omi, S., S h i r a i s h i , Y., Sato, H. and Kubota, H., J . Chem. Eng. Japan, 2, 1. 64 (1969) (10) Nomura, M., Harada, M., Eguchi, W. and Nagata, S., J . A p p l i e d Poly. S c i . , 16, 835 (1972)

(11) Omi, S., Kuwabara, L. and Kubota, H., J . Chem. Eng. Japan, 6, 343 (1973) (12) E u s t i c e , J . , Proc. Royal Soc. A 84, 107 (1910) (13) Dean, W.R., P h i l . Mag., 4, 208 (1927) and Phil. Mag., 5, 673 (1928) (14) White, C.M., "Streamline Flow through Curved P i p e s " , Proc. Royal Soc., A 123, 645 (1929) (15) T a y l o r , G.I., "The C r i t e r i o n f o r Turbulence i n Curved Pipes", Proc. Royal Soc., A 124, 243 (1930) (16) S r i n i v a s a n , P.S., Nandapurkar, S.S. & Holland, F.A., "Pressure Drop and Heat T r a n s f e r in C o i l s " , The Chem. Engr. 218, CE 113 (1968) (17) Blanc, M., M.Sc.A. Thesis Chem. Eng. Dept., E c o l e P o l y technique, Montreal, (1977) (18) Huneault, R., M.Sc.A. T h e s i s Chem. Eng. Dept., E c o l e Polytechnique, Montreal (1976) (19) Archambault, J . , M.Sc.A. T h e s i s Chem. Eng. Dept., E c o l e Polytechnique, Montreal (1977) (20) I t o , H., " F r i c t i o n F a c t o r s f o r Turbulent Flow in curved P i p e s " , J . B a s i c , Eng. Trans. A.S.M.E., D, 81, 123 (1959) (21) Omi, S., Sato, H. and Kubota, H., J . Chem. Eng. Japan, 2, 1, 55 (1969) (22) Gardon, J.L., "Mechanism o f Emulsion P o l y m e r i s a t i o n " , AIChE Symp., May 1969 (23) B l a c k l e y , D . C , "Emulsion P o l y m e r i z a t i o n " , Wiley, New York (1975) RECEIVED February 6, 1979.


Polymerization Reactors and Processes - American Chemical Society

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