Technology Type

Process Overview

Suspension polymerization is the dominant method for commercial PVC production, accounting for approximately 85% of global capacity. This process involves polymerizing vinyl chloride monomer (VCM) droplets suspended in water, forming polymer particles that precipitate as they grow. The attached reactor diagram shows a typical suspension polymerization vessel with its associated components including cooling systems, inlets for raw materials, and safety features.

Key Chemical Reactions

Initiation

The polymerization begins when an initiator (typically peroxydicarbonate compounds) decomposes thermally to generate free radicals:

These radicals then react with VCM molecules:

Propagation

The activated monomer radical continues reacting with additional monomer molecules:

The propagation rate constant is given by:

Termination

Termination occurs through disproportionation or combination reactions:

Dispropornation

Combination

Chain Transfer

Chain transfer to monomer significantly affects molecular weight distribution:

Process Parameters

Reactor Conditions

• Temperature: 45-70°C (controls molecular weight; lower temperatures yield higher MW)
• Pressure: Initially ~10 bar, dropping to 3-4 bar at ~90% conversion 
• Reactor Size: 60-200 m³, typically stainless steel or glass-lined
• Conversion: 85-90% (higher conversion risks "fish eyes")
• Cycle Time: Approximately 6.5 hours 

Formulation Components

• VCM: 100 parts
• Demineralized water: 120-137 parts
• Suspending agents: 0.05-0.1 parts (typically polyvinyl alcohol with 70-75% hydrolysis)
• Initiators: 0.03-0.16 parts (e.g., peroxydicarbonate compounds)

Equipment and Processing Steps

Process Flow Diagram

Illustration shows the process flowsheet of the PVC suspension process that consists of a VCM polymerization reaction stage, PVC purification stage, unreacted monomer recovery stage, and PVC drying stage (water removal):

• R-101: Reactor
• V-101: Flash Separator
• T-101: Stripping Column
• B-101: High-Pressure Boiler
• E-101: Heat Exchanger
• C-101 / C-102: Compressor
• E-102 / E-103: Condenser
• C-102: Compressor
• S-101: Centriguge
• D-101: Rotary Dryer
• S-102: Cyclone
• B-101: Water Boiler
• H-101: Air Heater

Polymerization Reactor

A typical suspension polymerization reactor includes:

• Bottom-drive agitators to maintain droplet suspension
• External jacket cooling system and internal coils
• Various instrumentation (pressure, temperature controls)
• Feed inlets for VCM, water, initiators, and dispersing agents
• Safety features including pressure relief valves

Process Sequence

1. Reactor Charging: VCM, demineralized water, and suspending agents are fed into the reactor
2. Polymerization: Initiator addition starts the reaction; heat removal maintains isothermal conditions
3. VCM Recovery: After reaching target conversion, slurry is sent to blowdown vessels and strippers
4. Dewatering: The slurry is passed through centrifuges to remove water 
5. Drying: Wet PVC cake is dried to 0.2-0.3% moisture in fluidized bed or rotary dryers
6. Screening: Final product is screened to remove oversized particles

Particle Formation Mechanism

1. Initial Stage: VCM droplets (50-150 µm) are stabilized by suspending agents
2. Polymerization: As polymerization progresses, PVC precipitates as primary particles within droplets 
3. Agglomeration: Primary particles aggregate and form a porous membrane structure 
4. Final Structure: Completed particles have 10-30% porosity, which affects plasticizer absorption 

Process Yields and Economics*

• VCM Conversion: 85-90% per batch
• Production Cost: Approximately \$650/ton, representing the lowest among PVC processes
• Steam Consumption: 0.25 kg/kg PVC during stripping 
• Reactor Productivity: Modern plants achieve 450 kta capacity 
  
  *2015 figures

Key Process Considerations

• Suspending Agent Selection: Affects particle size distribution, bulk density, and porosity
• Polymerization Temperature: Crucial for molecular weight control via chain transfer reactions
• Reactor Cooling: The exothermic reaction (106 kJ/mol) requires efficient cooling
• Stripping Efficiency: Residual VCM must be reduced to <10 ppm
• Reactor Cleaning: Anti-fouling coatings reduce cleaning frequency and prevent "fish-eyes"

References

Perplexity A.I. Research assisted description based upon publicly retrievable sources and documents.