Particle Size Distribution II - American Chemical Society

Chapter 4

On-Line Particle Size Distribution Measurements for Latex Reactors A. Brandolin and L. H. Garcia-Rubio

1

Downloaded by UNIV OF ARIZONA on August 2, 2012 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch004

Department of Chemical Engineering, University of South Florida, Tampa, F L 33620

The on-line measurement of the particle size distribution has been an outstanding problem in the design, analysis and control of latex reactors. Most available techniques have long time delays associated, primarily, with sample preparation and/or the actual measurements. Recent developments in the interpretation of turbidity spectra have resulted in a technique for the measurement of the complete particle size distribution with instrumentation that can be easily implemented in a plant environment. In this paper, the effects of process variables such as sampling time and dilution factors are analyzed within the context of continuous on-line turbidity measurements. Important results for the design of sampling/measurement strategies are presented and discussed. The o n - l i n e measurement o f t h e p a r t i c l e s i z e d i s t r i b u t i o n h a s b e e n an o u t s t a n d i n g p r o b l e m i n t h e d e s i g n , analysis and c o n t r o l o f l a t e x r e a c t o r s . Most a v a i l a b l e t e c h n i q u e s h a v e l o n g t i m e d e l a y s due t o t h e s a m p l e p r e p a r a t i o n a n d t o the time i t takes t o conduct the a c t u a l measurements. Recent developments i n t h e i n t e r p r e t a t i o n o f t u r b i d i t y spectra (1) have resulted in a spectrophotometric technique f o r t h e e s t i m a t i o n of the complete p a r t i c l e s i z e d i s t r i b u t i o n with instrumentation that can be easily implemented i n a p l a n t environment ( 1 , 2 ) . In t h i s paper, some a l t e r n a t i v e s f o r t h e d e s i g n o f a c o n t i n u o u s s a m p l i n g and measurement s y s t e m a r e e x p l o r e d . One o f t h e major problems with the design of c o n t i n u o u s measurement s y s t e m s u s i n g s p e c t r o p h o t o m e t r i c techniques i s the considerable d i l u t i o n required f o r the measurements. The d i l u t i o n s t e p s n o t o n l y i n c r e a s e t h e d e l a y s a s s o c i a t e d w i t h t h e measurements b u t a l s o may 1

Corresponding author 0097-6156/91/0472-0064$06.50/0 © 1991 American Chemical Society

In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


4.

BRANDOLIN & GARCIA-RUBIO

On-Line Distribution Measurements

65

increase the experimental errors and bias the i n t e r p r e t a t i o n of the data. E r r o r s i n the d i l u t i o n steps generally w i l l r e f l e c t i n the estimation of the p a r t i c l e concentration and, depending on the interpretation equation, the average p a r t i c l e d i a m e t e r may a l s o be a f f e c t e d . The d i l u t i o n s t e p s h a v e i n h e r e n t l y two e f f e c t s : 1) . S h i f t t h e d a t a i n t i m e r e l a t i v e t o t h e p o p u l a t i o n s e x i s t i n g i n t h e r e a c t o r a n d , 2) A v e r a g e t h e d a t a o v e r t h e d i l u t i o n i n t e r v a l with a concomitant l o s s i n r e s o l u t i o n . T h e s e two e f f e c t s c a n b i a s t h e i n t e r p r e t a t i o n o f t h e PSD data. Therefore, i t i s important to i n v e s t i g a t e , through realistic simulations of complete reactor-measurement s y s t e m s , t h e s a m p l i n g and d i l u t i o n s t r a t e g i e s ( i e . : batch versus continuous sampling). For this purpose, the 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 o f s t y r e n e has been investigated as a base case. The particle size distributions, g e n e r a t e d i n t h e r e a c t o r as f u n c t i o n o f t i m e , a r e s a m p l e d and d i l u t e d u s i n g b a t c h a n d c o n t i n u o u s strategies. Then, the turbidity spectra for the distribution at the exit of the dilution vessel is d e c o n v o l u t e d t o e s t i m a t e t h e PSD i n t h e r e a c t o r . Figure 1 shows a b l o c k d i a g r a m f o r t h e d i r e c t s i m u l a t i o n and t h e d e c o n v o l u t i o n s t e p s . A continuous d i l u t i o n system coupled w i t h t u r b i d i t y measurements i s d e m o n s t r a t e d . B e c a u s e o f t h e c o n t i n u o u s r e c o r d i n g o f t h e d a t a , and b e c a u s e t h e complete size distribution is obtained for every measurement p o i n t , t h e c o n t i n u o u s s y s t e m o f f e r s a s u i t a b l e alternative for the measurement and control of the particle size distributions. Reactor

Model;

For t h e s i m u l a t i o n o f t h e r e a c t o r , a g e n e r a l i z e d s t i r r e d t a n k r e a c t o r model h a s b e e n i m p l e m e n t e d ( t h e detailed d e s c r i p t i o n s o f e m u l s i o n p o l y m e r i z a t i o n p r o c e s s e s and t h e mathematical model for the continuous emulsion p o l y m e r i z a t i o n r e a c t o r a r e g i v e n i n ( 2 - 5 ) ) . The c o n t i n u o u s reactor model i s b a s e d on t h e b i r t h t i m e d i s t r i b u t i o n o f p a r t i c l e s (4) and on t h e p h y s i c a l and c h e m i c a l p r o c e s s e s known to take place during emulsion polymerization r e a c t i o n s (5). T h u s , t h e model c a n s i m u l a t e t h e e m u l s i o n p o l y m e r i z a t i o n s o f v i n y l a c e t a t e and m e t h y l m e t h a c r y l a t e w h i c h h a v e h i g h r a d i c a l d e s o r p t i o n r a t e s and may e x h i b i t s u s t a i n e d o s c i l l a t o r y b e h a v i o r , as w e l l a s , t h e 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 p r e s e n t s low d e s o r p t i o n rates with very attenuated or n o n - o s c i l l a t o r y behavior. The output from t h e r e a c t o r model c o n s i s t s o f latex properties such as particle size distribution, total p a r t i c l e s u r f a c e a r e a , t o t a l number o f p a r t i c l e s a n d f r e e emulsifier concentration.



P A R T I C L E S I Z E D I S T R I B U T I O N II

F i g u r e 1: B l o c k Diagram f o r t h e D i r e c t S i m u l a t i o n and Deconvolution steps of a Latex Reactor-Measurement System.


4.

BRANDOLIN & GARCIA-RUBIO


Sampling

and Measurement

On-Line Distribution Measurements

67

Systems:

The d e l a y s a s s o c i a t e d w i t h t h e t r a d i t i o n a l t e c h n i q u e s f o r s a m p l i n g and measurement o f p a r t i c l e s i z e distribution make them suitable only for off-line measurements. Examples o f such t e c h n i q u e s a r e : e l e c t r o n m i c r o s c o p y , d i s c centrifuge photosedimentometry and hydrodynamic chromatography. A good r e v i e w o f t h e t e c h n i q u e s a v a i l a b l e for determining p a r t i c l e size d i s t r i b u t i o n i s given in (6) . On-line measurements of the particle size d i s t r i b u t i o n h a v e o n l y r e c e n t l y become a r e a l p o s s i b i l i t y through the development of turbidimetry (1,2,7) and d y n a m i c l i g h t s c a t t e r i n g (8,9) t e c h n i q u e s . Zollars (7) , p r e s e n t e d an o n - l i n e s y s t e m b a s e d on t u r b i d i t y measurements f o r ; t h e e s t i m a t i o n o f t h e p a r t i c l e size distribution, the refractive index and the concentration of p a r t i c l e s . Z o l l a r s considered the e f f e c t o f t h e r e s i d e n c e t i m e i n t h e d i l u t i o n s t e p a n d assumed a two p a r a m e t e r l o g - n o r m a l d i s t r i b u t i o n t o i n t e r p r e t t h e turbidity data. This method will give reasonable a p p r o x i m a t i o n s whenever u n i m o d a l d i s t r i b u t i o n s a r e b e i n g g e n e r a t e d i n t h e r e a c t o r . However, b e c a u s e t h e s h a p e o f the PSD is fixed, the values f o r the p a r t i c l e size distribution will not be reliable for cases where s e c o n d a r y n u c l e a t i o n s o c c u r and m u l t i - m o d a l d i s t r i b u t i o n s are present. Dynamic l i g h t s c a t t e r i n g h a s a l s o b e e n p r o p o s e d a s a suitable alternative for o n - l i n e measurements of the p a r t i c l e s i z e d i s t r i b u t i o n i n l a t e x p o l y m e r i z a t i o n s (8,9) . With dynamic l i g h t s c a t t e r i n g , a s i n g l e average d i a m e t e r i s t y p i c a l l y m e a s u r e d . T h e d i l u t i o n s t e p p l u s measurement consuming 10-15 m i n u t e s . Dynamic l i g h t scattering was proven u s e f u l i n monitoring the average p a r t i c l e diameter during batch polymerizations of v i n y l acetate. Throughout t h e c o u r s e o f t h e r e a c t i o n , t h r e e t o f i v e s a m p l e s were taken, the d i f f e r e n c e i n t h e average d i a m e t e r between e a c h s a m p l e r a n g e d f r o m 50 t o 100%, i m p l i y i n g t h a t t h e d e l a y t i m e s a s s o c i a t e d w i t h t h e measurements c o u l d l e a d t o i n a c c u r a t e c o n t r o l a c t i o n s i f t h e p a r t i c l e d i a m e t e r was t o b e c o n t r o l l e d on t h e b a s i s o f t h e s e m e a s u r e m e n t s . Simulation

of

the Sampling-Dilution

System:

A v i a b l e o n - l i n e d i l u t i o n s y s t e m c o n s i s t s o f M>1 d i l u t i o n v e s s e l s i n s e r i e s o f e q u a l v o l u m e and i d e a l r e s i d e n c e t i m e d i s t r i b u t i o n . In t h i s system, a stream from t h e r e a c t o r i s c o n t i n u o u s l y d i l u t e d and p a s s e d t h r o u g h a d i o d e array s p e c t r o m e t e r where a c o m p l e t e u v - v i s s p e c t r u m ( 2 0 0 - 9 0 0 nm) is recorded at the desired measurement intervals. Depending upon t h e d e s i r e d w a v e l e n g t h r e s o l u t i o n , the measurement times c o u l d range b e t w e e n 0.1 second to s e v e r a l seconds. Therefore, f o r most a p p l i c a t i o n s to latex reactors, the measurements can be considered instantaneous r e l a t i v e to the d i l u t i o n times.


68

PARTICLE SIZE DISTRIBUTION II

The f o l l o w i n g mass b a l a n c e s must be s o l v e d i n o r d e r to p r e d i c t the p a r t i c l e s i z e d i s t r i b u t i o n at the e x i t of t h e "M" d i l u t i o n v e s s e l :

dn (t Dj) k

_ n -Ht,Dj)Xf

- n (t,Dj)

k

9

dt

~

k

6*

for Downloaded by UNIV OF ARIZONA on August 2, 2012 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch004

( 1 )

j = l , . . , N D and

R = (Cd/Cr) 0

f

k=l,..,M

(2)

5

where " k " and " j " r e p r e s e n t t h e d i l u t i o n v e s s e l s and d i s c r e t i z e d ^diameters r e s p e c t i v e l y , R i s the dilution f a c t o r and 9* i s t h e mean r e s i d e n c e t i m e i n e a c h d i l u t i o n vessel. C r i s t h e p o l y m e r mass 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 Cd i s t h e c o n c e n t r a t i o n e x p e c t e d a t t h e e x i t o f the d i l u t i o n system. I f t h e s y s t e m i s o p e r a t e d i n a b a t c h mode, E q . [1] becomes; f

n (t,D.) k

= n Ht,Dj)R k

(?)

f

f o r k = l , . . , M and

j=l,..,ND

F o r s i m u l a t i o n p u r p o s e s , t h e d i a m e t e r D was d i s c r e t i z e d w i t h t h e minimum and maximum p a r t i c l e d i a m e t e r s c o n s i d e r e d t o b e t h e m i c e l l e d i a m e t e r and 150 nm r e s p e c t i v e l y . These limits can be e a s i l y changed i n accordance with the p a r t i c l e d i a m e t e r s p r e d i c t e d by t h e r e a c t o r m o d e l . Because the polymer mass concentration and the particle size distribution in the reactor vary continuously during the course of the polymerization, i t i s n o t f e a s i b l e t o use a c o n s t a n t d i l u t i o n f a c t o r and a t the same time attain the linear range for the spectrophotometer

(ie;

Particle Size Distribution II - American Chemical Society

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