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SUSPENSION POLYMERIZATION

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The main redox system for acrylonitrile polymerization is the base of the persulfate, ferric iron and bisulfite ions at pH 2 to 3.5 where bisulfite ions are predominant and the ferric ions generated by the oxidation are sufficiently soluble. When necessary, a solution of sulfuric acid is added to maintain the pH within that range. The polymerization initiation stage occurs with the formation of the free radical through two reactions: the oxidation of the ferrous cations by the persulfate and the reduction of the ferric cations by the bisulfite ion.





These reactions produce the sulfate and sulfonate free radicals that react with the monomer by rapidly initiating chain growth. In addition to acting as a reducing agent the bisulfite ion functions as a chain transfer agent that reacts with the growing polymer radical, thereby controlling the molar mass of the polymer without practically changing the rate of polymerization.

As shown in the process flow chart of PAN polymerization via redox, the aqueous suspension copolymerization of acrylonitrile (AN) with a neutral comonomer (such as methyl acrylate or vinyl acetate)

(1) begins by dosing these products continuously in a premix tank

(2) by means of pumps specifically programmed in computer, the mixture of AN with the comonomers and demineralized water in this tank is delivered by means of a centrifugal pump

(3) to the polymerization reactor

(4) consisting of a vessel constructed of stainless steel or aluminum alloy, jacketed, and closed at atmospheric pressure.
The reactor (preheated with steam from a boiler) is about 2/3 of its volume filled with the demineralized water mixture with the catalyst and activator

(5) Thus, the solution of the premix tank meets the necessary conditions to initiate the copolymerization during its launch in the reactor.
The copolymerization reaction of the AN is highly exothermic and therefore the reactor must be cooled by the circulation of water from a chilled water unit (chiller) which generally maintains the reaction temperature constant between 55 and 60 ° C. The aqueous slurry that forms inside the reactor is continuously agitated by a shaft with propellers at a rotation between 15 and 20 rpm

(6) which maintains the viscosity of the suspension at about 50 cP.
The polymerization continues until the reactor is completely filled and its overflow through a tubular duct

(7) coupled between the reactor cap and the depletion column

(8) In this duct, the redox reaction forming the free radicals is interrupted by the addition of ethylenediaminetetraacetic acid (EDTA)

(9) which acts as a chelating agent forming stable complexes with the iron ions.
The EDTA is constantly injected into the duct in the direction of the depletion column.
Upon falling into the depletion column, the aqueous sludge encounters an upward current of superheated steam that acts on the volatilization of unreacted monomers in the reactor, then initiating a recovery process of these unpolymerized monomers. Water vapor entrains the monomers to a heat exchanger

(10) cooled by the circulation of ice water recovering the monomers by condensation

(11) thus returning to the premix tank.
Aqueous sludge passing through the depletion column can be collected in a reservoir where it is pumped

(12) for later steps

(13) which will culminate in the dry powdered polymer of PAN.

A vacuum cleaning and filtration system removes water, dissolved salts and residual EDTA.
Thereafter, the still wet polymer mass is continuously pressed into the pellets which provides greater drying efficiency and better storage of the finished polymer in several ton silos.


After drying the PAN is a white powder, insoluble in water and only soluble in highly polar solvents such as dimethylformamide (DMF), dimethylacetamide (DMA)






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