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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch004
Technology of Styrenic Polymerization Reactors and Processes R. H. M.SIMONand D. C.CHAPPELEAR Monsanto Company, Springfield, MA 01151
1.
Introduction
In considering the broad commercial applications of both crystal polystyrene (PS) and rubber modified "high impact" polystyrene (HIPS), it bears reemphasis that the process and process conditions each have major effects on product properties and fabrication behavior as well as product costs. With crystal polystyrene, product molecular weight, molecular weight distribution, oligomer and residual monomer levels, color and c l a r i t y are closely process related. With HIPS, rubber phase particle size, size distribution and morphology, graft copolymer level and molecular weight are additionally affected. To the manufacturer, therefore, the selection of the optimum process and conditions w i l l underly the most relevant polymer "property": the cost of the product which meets performance requirements. A characteristic of styrene polymerization processes i s that different reactor types are frequently used i n varying series combinations. The goal of this review i s therefore twofold: f i r s t , to describe how and why different reactors have been employed in batch and continuous processes; and second, to outline some of the bridges between available theory and actual practice by highlighting some of the major design problems that are amenable to such an approach. Hopefully, this may encourage more pertinent research i n the area. Industrial practice i s reflected in the patent a r t , and a few general reviews such as Bishop (1). Much information remains proprietary. Answers to many practical problems have to be obtained by licensing or extensive development. 2.
Processes and Reactor Process Elements
2.1 Classification of Processes and Reactors. Most styrene polymers are produced by batch suspension or continuous mass processes. Some are produced by batch mass processes. "Mass" in this sense includes bulk polymerization of the polymer
0-8412-0506-x/79/47-104-071$10.50/0 © 1979 American Chemical Society In Polymerization Reactors and Processes; Henderson, J. Neil, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 4, 2016 | http://pubs.acs.org Publication Date: July 31, 1979 | doi: 10.1021/bk-1979-0104.ch004
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POLYMERIZATION REACTORS AND PROCESSES
d i s s o l v e d i n i t s monomer and, i n some cases, some amount of solvent. PS mass p o l y m e r i z a t i o n i s homogeneous ( s i n g l e phase viscous f l u i d ) . In mass HIPS p o l y m e r i z a t i o n s , the rubber forms an e m u l s i f i e d second phase. Table I provides an overview o f general r e a c t o r designs used with PS and HIPS processes on the b a s i s o f r e a c t o r f u n c t i o n . The polymer concentrations c h a r a c t e r i z i n g the mass p o l y m e r i z a t i o n s are approximate; there c o u l d be some o v e r l a p p i n g o f a g i t a t o r types with s o l i d s l e v e l beyond t h a t shown i n the t a b l e . Polymer c o n c e n t r a t i o n l i m i t s on HIPS w i l l be lower because o f increased viscosity. There are a l s o a d d i t i o n a l a p p l i c a t i o n s . Tubular r e a c t o r s , f o r example, i n e f f e c t , o f t e n e x i s t as the t r a n s f e r l i n e s between r e a c t o r s and i n e x t e r n a l c i r c u l a t i n g loops assoc i a t e d with continuous r e a c t o r s . Various r e a c t o r combinations are used. For example, the product from a r e l a t i v e l y low s o l i d s batch-mass r e a c t o r may be t r a n s f e r r e d to a suspension r e a c t o r ( f o r HIPS), press ( f o r PS), o r unagitated batch tower ( f o r PS) f o r f i n i s h i n g . In a s i m i l a r f a s h i o n , the e f f l u e n t from a continuous s t i r r e d tank r e a c t o r (CSTR) may be t r a n s f e r r e d to a t u b u l a r r e a c t o r or an unagitated or a g i t a t e d tower f o r f u r t h e r p o l y m e r i z a t i o n before d e v o l a t i l i z a tion. Greater d e t a i l w i l l be provided i n the s e c t i o n s f o l l o w i n g . TABLE I Styrene Polymer Reactors - C l a s s i f i c a t i o n Process Type Reactor
Batch
Continuous
Function
Mass Polymerization
Conventional k e t t l e with: turbine agitator
CSTR with:
large turbine, anchor or h e l i c a l agitator
turbine,anchor or h e l i c a l agitator
Polymer 30-80% concentration
anchor or h e l i c a l agitator p r o p r i e t a r y and patented s t i r r e d reactors
anchor,helical a g i t a t o r s or s p e c i a l designs
Polymer > 80% concentration
press, unagitated batch tower
Polymer
