Polyvinyl Chloride Production from VCM (Suspension Polymerization)

The process under analysis comprises three major sections: (1) Polymerization; (2) VCM Recovery; and (3) Drying.

Polymerization. Vinyl chloride monomer feed stream, hot demineralized water and chemicals - polymerization initiators, dispersing agents and additives – are supplied to a stirred reactor. The reaction occurs substantially in the liquid phase, in VCM droplets dispersed in water, used as diluent. After VCM polymerization ignition, the heat from the highly exothermic reaction is removed by means of a cooling system, for an isothermal temperature control. The molecular weight of the polymer produced is defined by the length of the resulting polymer, which depends on the polymerization temperature. As the polymer is formed, it precipitates out of the reaction medium, floating in the liquid monomer phase. The polymerization is interrupted when the conversion of VCM to PVC reaches about 90%.

VCM Recovery. The polymer slurry is fed to a blowdown vessel, operated at a lower pressure. Most of the unreacted VCM flashes-off. The polymer slurry is pumped to a pre-degassing vessel and a sieve-tray degassing column, where remaining VCM is removed from the polymer suspension. The VCM-rich off-gases are compressed and condensed in liquid ring compressors and fed to a vessel, in such a way that non-condensed light contaminants are separated from the liquid VCM and released as waste gas. The VCM obtained is recycled to the polymerization.

Drying. The slurry is passed through a solid-bowl decantation centrifuge and a wet PVC cake containing only residual moisture is obtained. At this point, the wet PVC cake is conveyed to a drying step, which comprises a combination of a flash dryer with a cyclone dryer, where it is dried to a moisture content ranging from 0.2 - 0.3 wt%. The PVC powder obtained is then packed and sent to storage facilities.