Bulk and Solution Polymerizations Reactors

Bulk and Solution Polymerizations Reactors

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Vol. 5 BULK AND SOLUTION POLYMERIZATIONS REACTORS 307

Introduction

This article discusses polymerization reactors where the continuous phase is a solution of a polymer in its own monomer or in a solvent. When the low molecular weight species is primarily monomer, the reaction is a bulk polymerization; and when it is a solvent, the reaction is a solution polymerization. This distinction has little practical importance. The important consideration is that a high viscosity polymer solution is the continuous phase and is in contact with the reactor walls andtheagitator.Incontrast,suspended-phasepolymerizations(suchasemulsion, dispersion, and suspension) and gas-phase polymerizations have a low viscosity continuous phase (see HETEROPHASE POLYMERIZATION).

Table 1 lists representative bulk and solution polymerizations. All these polymerizations give a polymer-rich continuous phase that, at the end of the reaction, will have a viscosity 104 –1 07 times higher than the feed solution. Laminar flow is the norm. Heat and mass transfer coefficients are much lower than is common in the chemical industry, and specialized equipment is often necessary.

The manner in which viscosity increases with conversion has a strong influence on reactor design. Polymerizations with long chain lives, such as condensation and anionic polymerizations increase chain length and viscosity slowly. When half the monomer has reacted, the number-average chain length will be about 2 and the viscosity of the mixture will be about twice that of the pure monomer. Polymerizations with short chain lives, as is typical of free-radical and transition-metal catalyses, yield high molecular weight polymers from the onset. A mixture containing 50% monomer by weight might contain a polymer with an

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Tab le 1. Representative Bulk and Solution Pol ymerizations

Heat of Reaction

Type of reaction, exotherm, By-product

Polymer polymerization Process Reaction medium kJ/mol a K to be removed

Polyethylene Vinyl addition High pressure Solution of polyethylene in ethylene 95.0 1610 Heat High density

Polyethylene

Vinyl addition Solution Solution of polyethylene in hexane 95.0 1610 Heat

Poly(vinyl chloride)

Vinyl addition Bulk Polymer-ric h phase in contact with a (suspended) vinyl chloride phase 95.8 730 Heat

Polystyrene Vinyl addition Bulk Solution of polystyrene in styrene and ethyl benzene 69.9 320 Heat

PMMA Vinyl addition Bulk Solution of polymer in methyl methacrylate 56.5 270 Heat Nylon-6 Ring opening Bulk Solution of polymer in caprolactam 15.9 68 None Polysulfone Condensation Solution Solution of polymers and monomers in monoc hlorobenzene with a suspended NaCl phase

Polycarbonate Condensation Interfacial Polymer solution in methylene chloride in contact with an aqueous NaOH phase 0 0 HCl (as NaCl)

Poly(butyleneterephthalate) Condensation Bulk Polymer solution in diglycol terephthalate 0 0 Ethylene glycol a To convert kJ/mol to kcal/mol, divide by 4.184.

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Conversion

Long chain lives

Short chain lives Natural log of viscosity in mPa s(=cP)

Fig. 1. Representative curves of viscosity as a function of conversion for short and long chain lives. The end point in both cases is a number-average chain length of 1000 and 0.1% residual monomer.

average chain length of 1000 and a viscosity that is 104 times that of the monomer. Figure 1 illustrates the difference between short and long chain lives for a polymerization that starts and ends at the same points but proceeds by different mechanisms. There are three key design issues in bulk and solution polymerizations:

(1) Removing the heat of polymerization. This is applicable mainly to vinyl addition polymerizations.

(2) Achieving adequate stoichiometry and by-product removal to obtain high molecular weight polymers. This is applicable mainly to condensation polymers.

(3) Achieving desired copolymer composition distributions.

Managing the Reaction Exotherm

For vinyl addition and diene polymerizations, heat removal is a primary concern. ThereactionexothermlistedinTable1istheheatofpolymerizationdividedbythe specific heat of the polymer. It is a fictitious quantity for energetic polymerizations since bonds break and the polymer reaches a ceiling temperature well before

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Table 2. Methods for Temperature Control

Method for controlling temperature Example system

React to low conversion High pressure polyethylene Dilute with solvent Solution polyethylene Dilute with dead polymer PMMA casting Adiabatic reactors Nylon-6 casting Boiling CSTRs Polystyrene bulk continuous Cold feed to a CSTR PMMA bulk continuous CSTRs with heat transfer to jackets Specialty polymers in small reactors or internal coils

Flow inside a single tube High pressure polyethylene Shell-and-tube reactors Solution polyethylene Stirred-tube reactors Polystyrene bulk continuous the full exotherm is realized, but it provides a rough measure of the difficulty of controlling the reaction temperature.

The vinyl addition polymers undergo polymerization by opening a double bond in a substituted ethylene molecule. The energy release is 50–100 kJ/mol. The reaction exotherm varies over a broader range since it depends on the mass of the substituted groups. Thus ethylene and vinyl chloride polymerizations have the same heat of reaction per mole but very different exotherms due to the relative mass of the monomers. The vinyl addition polymerizations are too energetic to allow adiabatic polymerizations, except in very special cases. Heat removal is a dominant consideration in reactor design. In sharp contrast, a condensation polymer such as poly(ethylene terephthalate) is formed by a series of ester exchange reactions where the bonds being made and broken have nearly the same energy. Adiabatic polymerizations are possible for such polymers, and heat may even be added to evaporate the condensation by-product. Conversion is typically limited by by-product removal.

Table 2 lists various methods that have been used to control the temperature in bulk and solution polymerizations. All these processes continued to be practiced industrially but some would no longer be chosen for new construction.

Reaction to low conversion and the use of large amounts of solvent are conceptually similar. The high pressure process for low density polyethylene limits conversion to about 15%. This reduces the exotherm to about 250◦C. A similar result is achieved in the solution process for high density polyethylene, where the conversion of monomer is high but the reactor contains about 85% solvent. Both processes rely on supplemental cooling by sensible heat transfer to tube walls. The high pressure uses a single tube and is “once through” with respect to polymer. The solvent process employs a shell-and-tube heat exchanger in a recycle loop.

Figure 2 shows the reaction exotherm for the semi adiabatic batch polymerization of a methyl methacrylate casting system. The methyl methacrylate monomer has been diluted with dead polymer to limit the exotherm and to increase the viscosity of the casting syrup. The polymerizing mass undergoes a glass transition and polymerization stops before the temperature reaches the atmospheric boiling point of methyl methacrylate, thus avoiding bubbles in the cast

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Time, min

Temperature,

Fig. 2. Reaction exotherm for a methyl methacrylate casting system.

product. A less energetic polymerization such as that for polycaprolactam can be cast in large parts without need for dilution with dead polymer.

Continuous-flow stirred-tank reactors (CSTRs) can be cooled in three ways.

The most elegant method is to allow boiling of the monomer or solvent so that the heat of reaction is removed in an overhead condenser. The pressure in the vessel is set to give the desired temperature. The condensate can be returned to the vessel or recycled back to the feed. This process is commonly used for polystyrene. ChillingthefeedisanothermeansformanagingtheexotherminaCSTR.Refrigeration to −40◦C has been used for the bulk, continuous polymerization of PMMA. Laboratory reactors and small-scale industrial reactors can be cooled using jackets or internal coils, but this method scales up poorly.

Removing Chemical By-products

Most condensation polymerizations are reversible and become limited by equilibrium unless the condensation by-product is removed. Spontaneous removal occurs when the by-product is insoluble in the reaction medium. The low solubility of NaCl in organic solvents means reactions like those for polysulfone and phenoxy (ie a high molecular epoxy) proceed irreversibly with the chain length depending only on the reaction time and initial stoichiometry. Phenol–formaldehyde condensations are limited both by the initial stoichiometry and by the formation of by-product water. Dual limitations are difficult to satisfy when a high molecular weight polymer is desired, and it is common to change the chemistry so that the stoichiometric limitation is eliminated. If poly(ethylene terephthalate) were made by the direct condensation of its two monomers, terephthalic acid and ethylene glycol, exact stoichiometry and removal of by-product water would both be needed to achieve high molecular weights. The standard industrial process uses

312 BULK AND SOLUTION POLYMERIZATIONS REACTORS Vol. 5 a preliminary esterification and separation to obtain a self-condensing monomer, diglycol terephthalate. The monomer has inherently perfect stoichiometry, but its polymerization is strictly limited by the presence of by-product ethylene gycol. By-product removal is easy when the viscosity is low. A typical process has one or two stirred tanks in series that remove ethylene glycol in overhead condensers. The effluent from the last stirred tank is fed to a specially designed reactor that approximates piston flow (also known as plug flow). It is typically a twin-screw device machine that exposes the now viscous polymer solution to vacuum while moving it forward.

Copolymerization

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