Technology Type

The thermal cracking (pyrolysis) of ethylene dichloride (EDC) to produce vinyl chloride monomer (VCM) is a critical and energy-intensive step in the integrated VCM production chain. It is chemically classified as a dehydrochlorination reaction (also called dehydrohalogenation), which proceeds through a homogeneous, first-order, free-radical chain mechanism.

Reaction Chemistry

Primary Reaction (endothermic, ΔH = +71 to +97 kJ/mol):

The elimination reaction converting conversion of EDC into VCM and hydrogen chloride proceeds through a homogeneous, first-order free-radical chain mechanism involving chlorine and hydrogen radical formation.

By-product Formation

Secondary reactions produce acetylene, benzene, chloroprene, and other chlorinated hydrocarbons through free-radical mechanisms. Heavy tars and coke can form if products are not rapidly quenched.

Process Configuration

Furnace Design

EDC cracking furnaces typically consist of four main sections:

• Convection Section: Preheats the liquid EDC feed using hot flue gases from combustion, raising the temperature to approximately 670K (397°C)
• Evaporation Zone: Completely vaporizes the preheated EDC feed
• Radiation Section (Firebox): Contains reaction tubes where thermal cracking occurs, heated by burners. This is where the endothermic pyrolysis reaction takes place
• Shock/Quench Section: Rapidly cools product gases to prevent coke and tar formation

The furnace uses a tubular reactor configuration with multiple parallel passes. Industrial units typically feature 31 or more passes in the radiation section.

Figure 1 - EDC Cracking - Simplified Process Flow Diagram^[1]

Operating Parameters and Conditions

Temperature and Pressure

• Reaction Temperature: 480–510°C (753–783K) in the radiation zone
• Feed Inlet Temperature: 280°C after preheating
• Operating Pressure: 15–30 atm (20–22 bar typical)
• Maximum Tube Wall Temperature: Up to 1080°C

Residence Time and Flow Conditions

• Residence Time: 10–30 seconds
• Gas Flow Velocity: 5–20 m/s in cracking tubes

Feed Purity Requirements

• EDC Purity: >99.5 wt% required to minimize coke formation and reactor fouling

Efficiency Metrics

Conversion and Selectivity

• Single-Pass EDC Conversion: 45–65% (typically 50–60%)
• VCM Selectivity: >90% (commonly 96–99%)
• Overall VCM Yield: 95–99% after EDC recycling

Higher conversion rates (>55%) can be achieved but result in increased coke formation, reducing furnace run length. Addition of carbon tetrachloride (CCl₄) at ~1200 ppm can boost conversion to 60% while maintaining 98.8% selectivity by promoting free chlorine radical formation.

Energy Efficiency

• Fuel Efficiency: Modern designs achieve >94% thermal efficiency
• Heat Recovery: Advanced designs use cracked gas to preheat and evaporate feed EDC, cooling the gas from 510°C to 180–350°C before final quenching
• Quench Temperature: Rapid cooling to ~80°C prevents undesired secondary reactions

Operational Considerations

• Furnace Run Length: Coke deposition on tube walls necessitates periodic decoking, with run cycles optimized through dynamic operating parameters
• Emissions Reduction: Modern designs achieve >95% reduction in emissions with emission control technology

Process Integration Features

Material Recycle Streams

• Unconverted EDC is separated in VCM distillation and recycled to the cracking furnace after purification
• HCl by-product is recycled to the oxychlorination unit, enabling the "balanced" process configuration

Heat Integration

• Exothermic heat from direct chlorination and oxychlorination can be used to generate steam for EDC preheatin?
• Cracked gas sensible heat is recovered through heat exchange with feed EDC

Advanced Process Control

Modern VCM plants implement Advanced Process Control (APC) systems with multivariable algorithms and mathematical models to optimize furnace operation, reduce human intervention, and maximize run length.

Key Performance Indicators Summary

Technology Providers and Licensors

Major Technology Providers

1. ThyssenKrupp
   • Exclusive engineering partner with Vinnolit for EDC/VCM technology
   • Has built plants with >10 million tonnes/year total EDC capacity and 6.5 million tonnes/year VCM capacity
   • Market share: ~40% of global EDC capacity
   • Offers complete technology package including direct chlorination, oxychlorination, EDC distillation, EDC cracking, VCM distillation, and HCl recovery
2. Westlake Vinnolit (formerly Vinnolit/Hoechst)
   • Technology heritage from Hoechst AG's vinyls division
   • Licenses integrated EDC/VCM process technology
   • Operates multiple commercial plants in Europe
3. Heurtey Petrochem Solutions (now part of Axens)
   • Claims "highest number of EDC references in the world"
   • Offers advanced cracking furnace designs with:
   • High fuel efficiency (94%+)
   • Fully modularized construction
   • Longer run length
   • Optimized plot plans
4. Linde Engineering
   • Provides both fired and electrified EDC furnace technologies
   • Furnace capacities exceeding 300 kilotonnes/annum
   • Features unique offset convection section design with dynamic tube technology
5. Shandong Qilu Petrochemical Engineering Co. (QPEC)
   • Part of SINOPEC Group, China's major state-owned oil and gas enterprise
   • Completed engineering design for multiple VCM plants:
   • Revamping of 230,000 T/Y VCM Plant
   • Additional 370,000 T/Y VCM Plant at Qilu Petrochemical
   • 400,000 T/Y VCM Plant (commissioned in 2004)
   • Developed nationalized oxychlorination reactor technology awarded SINOPEC Science and Technology Progress Prize
   • Established Qilu Petrochemical as China's largest chlorine and caustic soda production base with 630,000 T/Y processing capacity
   • Offers complete plant design and engineering services for VCM production

References

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