Stirred Tanks with a Continuous Polymer Phase

1 Stirred Tanks with a Continuous Polymer Phase 

Continuous-flow stirred tank reactors are widely used for free-radical polymerizations. They have two main advantages the solvent or monomer can be boiled to remove the heat of polymerization, and fairly narrow molecular weight and copolymer composition distributions can be achieved. Stirred tanks or 

Continuous phase Dispersed phase Type of polymerization Example systems 

Polymer dissolved in monomer None Homogeneous bulk polymerization Poly(methyl methacrylate) in methyl methacrylate monomer 

Polymer dissolved in solvent None Homogeneous solution polymerization Polymerization of high-density polyethylene in hexane 

Polymer dissolved None Homogeneous Poly(methyl methacrylate) 

Molecular Weight Distributions. The CSTRs produce the narrowest possible molecular weight distributions for fast chain growth, short chain lifetime 

Example 13.8 Apply the method of moments to an anionic polymerization in a CSTR. 

CSTRs produce the narrowest possible MWDs for fast-chain-growth, short-chain lifetime polymerizations like free-radical and coordination metal catalysis. The mean residence time in the CSTR will be minutes to hours, and the chain lifetimes are fractions of a second. Any chain that initiates in the CSTR will finish its growth there. All the polymer molecules are under identical, well-mixed conditions and will have as narrow an MWD (typically PD 2) as is possible for the given kinetic scheme. 

SOLUTION Assume that initiation occnrs instantly as fresh monomer enters the reactor. A monomer balance for the CSTR gives 

A balance on polymer of length I gives an infinite set of algebraic equations  

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Continuous stirred-tank reactors (CSTRs) are used for large productions of a reduced number of polymer grades. Coordination catalysts are used in the production of LLDPE by solution polymerization (Dowlex, DSM Compact process [29]), of HDPE in slurry (Mitsui CX-process [30]) and of polypropylene in stirred bed gas phase reactors (BP process [22], Novolen process [31]). LDPE and ethylene-vinyl acetate copolymers (EVA) are produced by free-radical polymerization in bulk in a continuous autoclave reactor [30]. A substantial fraction of the SBR used for tires is produced by coagulating the SBR latex produced by emulsion polymerization in a battery of about 10 CSTRs in series [32]. The CSTRs are characterized by a broad residence time distribution, which affects to product properties. For example, latexes with narrow particle size distribution cannot be produced in CSTRs. 

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