PVC

Polyvinyl Chloride（PVC）

1. Polymerization Method
Suspension polymerization technology is adopted for the production of general-purpose PVC resin. This method uses water as the dispersion medium, with VCM droplets undergoing suspension polymerization, and employs an oil-soluble initiator and dispersant system. It is the mainstream industrial production method, accounting for over 80% of global production capacity. The process is mature, reliable, and suitable for large-scale production. It can produce both porous and compact resin grades with adjustable porosity, low raw material consumption, and low overall production cost. The products are mainly used in pipes, profiles, rigid sheets, injection molding, and general-purpose flexible cable compounds.

2. Reaction Mechanism
Reaction equation: 

nCH₂=CHCL→ ~CH₂-CHCL-CH₂-CHCL~

(1) Chain Initiation
The initiator decomposes upon heating to generate primary free radicals: 

I一^△→2R

The primary free radicals then attack the double bond of the VCM monomer to form monomer free radicals:

R·+ CH₂=CHCL→ R-CH₂-CHCL

(2) Chain Propagation
Monomer free radicals continuously add VCM monomers, and the molecular chain rapidly lengthens: 

R-CH₂-CHCL+ nCH₂=CHCL → R-(CH₂-CHCL)_n-CHCL

(3) Chain Termination
Two macromolecular free radicals collide and deactivate through two mechanisms: combination and disproportionation. The PVC system predominantly terminates via disproportionation.
(4) Chain Transfer
Macromolecular free radicals can undergo chain transfer to monomers, polymers, solvents, and additives.
Chain transfer to monomer: One macromolecular chain terminates while a new free radical is generated to continue propagation. This directly determines the molecular weight of PVC. In industrial practice, molecular weight is controlled by temperature: higher temperature promotes chain transfer, resulting in lower molecular weight.

~CH₂-CHCL+ CH₂=CHCL → ~CH=CHCL + CH₂=CHCL

Chain transfer to polymer: Branched chains are formed. More branches lead to poorer processing flowability and thermal stability.
The chain transfer constant of VCM radical polymerization is large, making it nearly impossible to control molecular weight by initiator dosage; it is primarily regulated by polymerization temperature.

3. Process Description
According to the raw material types and quantities specified in the PVC product formulation and the addition sequence programmed in the DCS, cold demineralized water, hot demineralized water, VCM, dispersant, buffer, and initiator are automatically added to the polymerization reactor under closed conditions. After the initiator is automatically charged, the polymerization reaction begins. The reaction temperature is maintained by automatically adjusting the circulating cooling water flow rate. The polymerization reaction proceeds according to a predetermined temperature profile. The temperature and pressure during the reaction are continuously measured, and the computer calculates the monomer conversion rate based on these measurements. When the set conversion rate is reached, the terminator addition system is activated, and the terminator is automatically added to the reactor to stop the polymerization. The PVC slurry in the reactor is then automatically discharged to the discharge tank. Meanwhile, unreacted VCM (gaseous) from the polymerization is recovered by venting in the post-reaction recovery system. After reactor discharge, the reactor is evacuated to reduce the residual pressure from approximately 0.5 MPa to below 0.1 MPa. Between discharge and feeding, the reactor wall is flushed with approximately 1.4 MPa pressure water, and an anti-fouling coating is applied to prevent polymer buildup in the next batch. During reactor maintenance, if a small amount of polymer deposits are found on the wall, a high-pressure water wash at 50–60 MPa is performed. This polymerization process achieves high production efficiency and stable, high-quality products.
After polymerization discharge, the slurry undergoes high-pressure recovery and low-pressure recovery. High-pressure recovery is carried out in the discharge tank, and low-pressure recovery is carried out in the stripping column. After stripping, the VCM content of the slurry is reduced to below 10 ppm, and the slurry then enters the drying system. The dried product is packaged, stored, and sold.
DCS automatic control system and SIS emergency shutdown system are adopted to produce SG3 to SG8 grade resins.

4. Main Equipment Selection
(1) Polymerization Reactors
Commonly used mainstream polymerization reactors in China include sizes of 70 m³, 105 m³, 108 m³, 127 m³, and 135 m³:
70 m³ reactor: bottom-entry agitator, 4 sets of internal cooling coils, half-pipe jacket, single-train capacity up to 25,000 t/a.
105m³reactor: bottom-entry agitator, external jacket and large overhead condenser for heat removal, single-train capacity 35,000–40,000 t/a.
108m³ reactor: bottom-entry agitator, 4-blade straight impeller, 4 sets of internal cooling baffles, internal jacket and overhead condenser, single-train capacity up to 40,000 t/a.
127 m³ reactor: bottom-entry agitator, jacket and overhead condenser for heat removal, single-train capacity up to 50,000 t/a.
135 m³ reactor: bottom-entry agitator, jacket and overhead condenser for heat removal, single-train capacity up to 50,000 t/a.
(2) Stripping tower
Through-flow sieve tray stripping column: No overflow weirs, no independent downcomers. The slurry and steam pass through the sieve holes in countercurrent flow—slurry downward, steam upward, sharing the same sieve holes. Sieve hole size: φ10–15 mm, open area ratio: 8%–11%. Number of trays: 20–40. Tray spacing: 300–600 mm. Operates under slight positive pressure.
Advantages: Simple structure, low fabrication and maintenance cost, not prone to plugging，and good operational flexibility.
Disadvantages: Relatively high steam consumption, high column temperature (112–114℃) which has some effect on resin whiteness, limited stripping efficiency, and prone to channeling at low loads. This is an earlier classic general-purpose design.
Overflow weir sieve tray stripping column: Equipped with overflow weirs and dedicated downcomers. Slurry flows horizontally across the tray, over the weir, and down through the downcomer to the next tray. Steam rises only through the sieve holes, with gas-liquid separation at the passages.
Advantages: Baffle plates are installed on the trays to force the slurry to flow in a serpentine pattern, eliminating dead zones. Each tray section is independently equipped with spray rinsing and sight glasses. Fewer trays (typically 6–10) are used; generally operates under vacuum. Sieve holes are smaller (φ1.1–2.0 mm). Advantages include uniform slurry distribution, high mass transfer efficiency, low-temperature vacuum operation (<100℃) resulting in good resin whiteness, significantly lower steam consumption than through-flow towers, and deep stripping with residual VCM consistently below 10 ppm.
Lowest energy consumption: internal tube bundle for indirect heating greatly reduces hot air demand, with significant reduction in fan capacity and system resistance. Thermal efficiency: 75%–85%.
(3) Drying 
1) Fluidized Bed Drying
Advantages:
Lowest energy consumption: Internal tube bundle for indirect heating greatly reduces hot air demand, with significant reduction in fan capacity and system resistance. Thermal efficiency: 75%–85%.
Stable product quality: Adjustable residence time, uniform bed temperature, low risk of localized overheating resulting in low levels of black/yellow specks and impurities, and reduced particle breakage and fines generation.
High operational flexibility: Strong adaptability to fluctuations in feed moisture content and load. Suitable for grade switching production across the full range of SG3 to SG8 general-purpose PVC.
Integrated cooling section allows low discharge temperature, reducing thermal agglomeration in downstream conveying.
Small exhaust gas volume, smaller cyclone and baghouse dust collection scale, lower capital and operating costs for dust removal.
Disadvantages:
High initial capital investment: costs for large pressure vessels, air distributor plates, internal tube bundles, steel structures, and civil works.
Risks of dead zones and channeling; severe filter cake agglomeration can cause flow maldistribution, requiring stable centrifuge feed.
Significant manual labor for bed cleaning during shutdown; large material holdup in the bed; prolonged shutdowns can lead to degradation.
Larger equipment footprint, requiring more space for plant layout.
2) Cyclone Drying
Advantages:
Simple main equipment structure, no internal moving parts, lower equipment cost than fluidized bed; compact footprint.
Fewer dead zones than fluidized bed, much less material holdup, shorter cleaning time during shutdown, less degraded material.
Quick start-up, fast response to load changes; short construction period for small to medium-scale plants (≤100,000 t/a).
Self-classification of particles in the swirling flow; wet particles have longer residence time, offering some tolerance to centrifuge feed fluctuations.
Disadvantages:
Higher energy consumption: Relies entirely on hot air for convective heat transfer, with huge hot air demand. The installed power of the blower and induced draft fan is large, and steam consumption is higher than that of the fluidized bed.
Limited residence time for materials, reducing the ability to remove internal bound moisture from particles. If the moisture content of the centrifuge discharge is high, the moisture content of the finished product can easily exceed specifications.
High particle collision velocity, resulting in more breakage and increased fines generation, which affects bulk density and fish-eye index.
High system resistance, with fans running under high load year-round, leading to high maintenance workload. Large exhaust gas volume results in high dust collection load.
Inlet elbows and the bottom of the dryer are prone to periodic material accumulation; if not properly controlled, this can lead to the formation of off-spec products (red specks, black particles).
3) Swirl Drying
Advantages:
Optimized flow guide structure based on cyclone drying; more uniform internal flow field; lower risk of material accumulation and wall sticking than traditional cyclone drying.
Slightly lower system resistance than conventional cyclone drying at same throughput; improved impurity particle performance.
Retains the moderate investment, flexible start-up and shutdown, and easy cleaning characteristics of cyclone drying.
Disadvantages:
No internal heat source; steam and power consumption remain close to cyclone drying, with only minor improvement; energy consumption still significantly higher than fluidized bed.
Narrow operating window; stringent control of air pressure and temperature; parameter fluctuations can easily cause moisture variations and bottom plugging.
Fewer large-scale commercial examples in China than cyclone drying; less experience in scale-up design.
Fines generation issue not fundamentally improved; particle attrition still present due to high-speed swirling flow.

5. Performance Indicators
Product quality:qualified product rate: 100%，Premium-grade product rate: ≥95%.
VCM monomer consumption: 1.003 t/t PVC.
Demineralized water consumption: 2.2 t/t PVC.
Steam consumption: 1.1 t/t PVC.
Anti-fouling capability: ≥300 consecutive batches before cleaning (SG-5).

6. Process Block Diagram