Ethylene Dichloride High Purity Material: Advanced Purification Technologies And Industrial Applications

Molecular Structure And Physicochemical Properties Of Ethylene Dichloride High Purity Material

Ethylene dichloride (C₂H₄Cl₂, CAS 107-06-2) is a chlorinated hydrocarbon with a molecular weight of 98.96 g/mol. The compound exhibits a symmetric structure with two chlorine atoms bonded to adjacent carbon atoms, resulting in distinct physical and chemical characteristics critical for its industrial utility.

Key Physicochemical Parameters:

• Boiling Point: 83.5°C at 101.3 kPa, enabling efficient distillation-based separation 1
• Density: 1.253 g/cm³ at 20°C, facilitating liquid-phase handling and storage
• Vapor Pressure: 8.7 kPa at 20°C, requiring controlled storage conditions to minimize evaporative losses
• Solubility: Miscible with most organic solvents; limited water solubility (8.6 g/L at 20°C), which influences aqueous-phase separation strategies
• Refractive Index: 1.4448 at 20°C, used as a quality control parameter in analytical protocols
• Flash Point: 13°C (closed cup), classifying EDC as a flammable liquid requiring stringent safety protocols

High purity ethylene dichloride specifications typically mandate total impurity levels below 0.1–0.5%, with individual contaminants such as chloroform (<100 ppm) 2, carbon tetrachloride (<50 ppm), trichloroethylene (<30 ppm) 1, and water content (<50 ppm) strictly controlled to prevent catalyst poisoning and side reactions in downstream VCM synthesis. The presence of unsaturated organics, particularly acetylene derivatives and benzene (<10 ppm) 1, accelerates coking in pyrolysis furnaces, necessitating their rigorous removal during purification.

Feedstock Quality And Ethylene Purity Requirements For Ethylene Dichloride Production

Historically, ethylene dichloride manufacturing has relied on ethylene feedstocks exceeding 99.8% purity, obtained through energy-intensive cryogenic distillation of steam-cracked petroleum fractions 3 6 8. However, the high capital and operational costs associated with ultra-high-purity ethylene production have driven research into processes capable of utilizing lower-purity ethylene (95–99.5%) while maintaining EDC product quality.

Impurity Impact Analysis:

• Ethane: Inert diluent that reduces ethylene partial pressure, lowering chlorination reaction rates and requiring larger reactor volumes; can be separated and recycled via cryogenic or pressure-swing adsorption 7
• Propylene and C₃+ Olefins: Undergo competitive chlorination to form chlorinated propanes and higher homologs, which contaminate the EDC product and complicate downstream distillation 6
• Acetylene: Highly reactive species that polymerizes under chlorination conditions, leading to reactor fouling and catalyst deactivation; typically removed via selective hydrogenation or solvent extraction upstream 3
• Methane and Hydrogen: Light gases that dilute reactants and increase off-gas handling requirements but do not directly contaminate EDC

Recent process innovations 3 6 7 enable the use of ethylene with purity as low as 95%, achieved by integrating ethane separation units upstream of the chlorination reactor and employing selective oxychlorination processes that tolerate higher impurity levels. For instance, Solvay's patented process 7 separates ethane from the ethylene feed, optionally purifies it to >98% ethylene, and recycles it to the chlorination reactor, reducing feedstock costs by 15–25% compared to conventional routes while maintaining EDC purity above 99.5%.

Advanced Purification Technologies For Ethylene Dichloride High Purity Material

Extractive Distillation With High-Boiling Chloroalkene Solvents

Extractive distillation represents a cornerstone technology for removing close-boiling and azeotrope-forming impurities from crude EDC. The process employs high-boiling chlorinated solvents, most commonly perchloroethylene (C₂Cl₄, boiling point 121°C), to selectively enhance the relative volatility of target impurities 1.

Process Configuration:

1. Feed Preparation: Crude EDC from the chlorination reactor (typically 97–98.5% purity) is preheated to 60–80°C and fed to the middle section of an extractive distillation column operating at 1.5–2.0 bar absolute pressure
2. Solvent Introduction: Perchloroethylene solvent is introduced near the top of the column at a solvent-to-feed mass ratio of 2:1 to 4:1, creating a liquid phase enriched in high-boiling impurities (trichloroethylene, chlorobenzenes) and unsaturated organics (benzene, styrene) 1
3. Overhead Recovery: Purified EDC is recovered as overhead vapor (99.3–99.7% purity), condensed, and sent to final polishing distillation
4. Solvent Regeneration: The bottoms stream, containing solvent and concentrated impurities, is fed to a solvent recovery column where perchloroethylene is distilled overhead and recycled; impurities are removed as a small bottoms purge stream

Performance Metrics:

• Trichloroethylene Removal Efficiency: >99.5%, reducing concentration from 2000–5000 ppm in crude EDC to <30 ppm in purified product 1
• Benzene Removal: >98%, achieving final concentrations <10 ppm 1
• Energy Consumption: 1.2–1.8 GJ per metric ton of purified EDC, primarily for reboiler duty
• Solvent Losses: <0.5 kg perchloroethylene per metric ton EDC, managed through closed-loop recovery systems

This technology is particularly effective for removing impurities that form minimum-boiling azeotropes with EDC or have boiling points within ±5°C of EDC's normal boiling point, which are difficult to separate via conventional distillation.

Reflux-Controlled Distillation For Light Fraction Separation

Separation of light chlorinated impurities—primarily carbon tetrachloride (CCl₄, boiling point 76.7°C) and chloroform (CHCl₃, boiling point 61.2°C)—from EDC presents a significant challenge due to the formation of azeotropes and close boiling points. A patented process by PPG Industries 2 addresses this through precise reflux control to maintain specific composition profiles within the distillation column.

Technical Approach:

• Reflux Composition Control: The distillation column is operated under total reflux conditions initially, then gradually transitioned to partial reflux while maintaining chloroform concentration in the reflux liquid above 51.5 mole percent 2
• Operating Pressure: 1.0–1.3 bar absolute, optimized to maximize relative volatility between chloroform and EDC
• Temperature Profile: Column top temperature maintained at 58–62°C; bottom temperature at 82–85°C
• Reflux Ratio: 5:1 to 8:1 during steady-state operation, adjusted based on real-time composition monitoring via online gas chromatography

Separation Performance:

• Carbon Tetrachloride Removal: Reduced from 1500–3000 ppm in feed to <50 ppm in EDC product 2
• Chloroform Removal: Reduced from 3000–8000 ppm to <100 ppm 2
• EDC Recovery: >99.2% in the bottoms product, minimizing valuable product loss to the light fraction
• Light Fraction Composition: Typically 60–75% chloroform, 20–30% carbon tetrachloride, 5–10% EDC, suitable for solvent recovery or incineration

This method significantly reduces EDC losses compared to conventional distillation approaches, which often sacrifice 2–5% of EDC to the light fraction to achieve comparable impurity removal. The economic benefit is substantial: for a 500,000 metric ton per year EDC plant, reducing EDC loss from 3% to 0.8% saves approximately $7.5 million annually at current EDC market prices ($500–600 per metric ton).

Multi-Stage Rectification With Reverse Heat Integration

A recent innovation disclosed in Chinese patent CN116655409A 4 describes a multi-stage purification system specifically designed for EDC recovered as a by-product from calcium carbide-based vinyl chloride production. This process is particularly relevant for facilities seeking to upgrade low-purity EDC (95–97%) to high-purity material (>99%) suitable for direct use or sale.

System Architecture:

1. First Purification Unit: Operates at 0.8–1.2 bar with a distillation tower containing 40–60 theoretical stages; removes light impurities (chloroform, carbon tetrachloride) as overhead product
2. Second Purification Unit: Operates at 1.2–1.6 bar with 50–70 theoretical stages; separates high-purity EDC as overhead while rejecting heavy impurities (trichloroethylene, chlorobenzenes) to bottoms
3. Third Purification Unit: Final polishing column operating at 1.0–1.3 bar with 30–50 stages; achieves >99.5% EDC purity with trace impurity control
4. Reverse Contact Heat Exchange: Overhead vapor from the second unit provides reboiler duty for the first unit; overhead vapor from the third unit heats the feed to the second unit, reducing external energy input by 30–40% 4

Process Performance:

• Final EDC Purity: 99.0–99.5%, meeting specifications for short-distance transportation and direct use in VCM synthesis 4
• Energy Efficiency: 0.9–1.3 GJ per metric ton of purified EDC, representing a 25–35% reduction compared to conventional multi-column systems without heat integration
• Capital Cost: Approximately 15% higher than single-stage purification but justified by superior product quality and reduced operating costs
• Throughput: Demonstrated at 50,000–100,000 metric tons per year scale with linear scalability to larger capacities

This approach addresses a critical market need in regions with significant calcium carbide-based PVC production (primarily China, which accounts for ~70% of global carbide-based PVC capacity), where by-product EDC quality has historically been insufficient for commercial sale, resulting in resource waste and safety concerns during handling of impure, flammable material 4.

Direct Chlorination Process Optimization For High-Purity Ethylene Dichloride Production

Direct chlorination of ethylene with molecular chlorine represents the primary industrial route to EDC, typically conducted in liquid-phase reactors using ferric chloride (FeCl₃) as catalyst. Achieving high-purity EDC directly from the chlorination reactor minimizes downstream purification requirements and reduces overall production costs.

Catalyst System Optimization

A patented process by Hoechst AG 14 demonstrates that maintaining a specific molar ratio of sodium chloride (NaCl) to ferric chloride below 0.5 during the chlorination reaction enables production of high-purity EDC (>99.5%) without the need for catalyst removal via distillation, which is conventionally required and energy-intensive.

Mechanistic Basis:

• Catalyst Solubility Control: Excess NaCl forms a separate solid phase that acts as a heterogeneous catalyst support, reducing FeCl₃ solubility in the EDC product phase from 500–1000 ppm to <50 ppm 14
• Selectivity Enhancement: Lower FeCl₃ concentration in the liquid phase reduces side reactions (e.g., formation of 1,1,2-trichloroethane, tetrachloroethane) by 60–80%, improving EDC selectivity from 96–97% to 98.5–99.2% 14
• Coking Suppression: Reduced catalyst concentration minimizes polymerization of trace acetylene and diolefins, decreasing reactor fouling rates by 50–70% and extending run lengths from 6–9 months to 12–18 months 14

Operating Parameters:

• Reactor Type: Bubble column or stirred tank reactor with liquid EDC as reaction medium
• Temperature: 40–60°C, controlled via external cooling to remove the 218 kJ/mol reaction enthalpy
• Pressure: 1.5–3.0 bar, using gaseous chlorine feed to maintain intimate gas-liquid contact
• Ethylene Utilization: >99.5% per pass, with unreacted ethylene recovered from the reactor off-gas, compressed, and recycled 14
• Chlorine Utilization: >99.8% per pass, with trace unreacted chlorine absorbed in caustic scrubbers

Product Quality:

• EDC Purity: 99.5–99.8% directly from the reactor, requiring only light-ends removal via a single distillation column 14
• Catalyst Content: <50 ppm FeCl₃, eliminating the need for caustic washing or distillative catalyst removal
• Water Content: <30 ppm, achieved by using dry chlorine and ethylene feeds and maintaining anhydrous reaction conditions
• Heavy Chlorinated Impurities: <0.2%, primarily 1,1,2-trichloroethane and tetrachloroethanes, easily removed in downstream distillation

This process innovation reduces capital costs by eliminating one or two distillation columns from the purification train and decreases energy consumption by 0.3–0.5 GJ per metric ton EDC. For a world-scale EDC plant (1 million metric tons per year), this translates to annual energy savings of $15–25 million and capital cost reduction of $30–50 million.

Integrated Reaction And Rectification

An alternative approach disclosed by Stauffer Chemical Company 15 integrates the chlorination reaction with in-situ product vaporization and rectification, utilizing the exothermic heat of reaction (218 kJ/mol) to drive the separation process.

Process Description:

1. Reaction Zone: Liquid-phase chlorination conducted in a reactor maintained below the EDC boiling point (75–80°C at operating pressure) with continuous circulation of liquid EDC medium
2. Vaporization Zone: A portion of the circulating EDC is continuously withdrawn and fed to a vaporization/rectification column where the reaction heat (transferred via an internal heat exchanger) vaporizes and partially rectifies the EDC
3. Product Recovery: Purified EDC vapor (98.5–99.2% purity) is condensed and collected; a portion is returned as reflux to the rectification section
4. Impurity Rejection: Light impurities are vented from the column overhead; heavy impurities accumulate in the circulating liquid and are periodically purged

Advantages:

• Energy Integration: Eliminates the need for external reboiler duty in the first distillation column, reducing energy consumption by 0.4–0.6 GJ per metric ton EDC 15
• Simplified Equipment: Combines reaction and primary separation in a single integrated unit, reducing plot space requirements by 20–30%
• Improved Heat Management: Direct utilization of reaction heat prevents the need for large external cooling systems and subsequent reheating for distillation
• Reduced Residence Time: Continuous product removal minimizes EDC exposure to high temperatures and catalyst, reducing formation of heavy by-products by 30–50%

Operational Considerations:

• Pressure Control: Critical to maintain liquid-phase reaction while enabling efficient vaporization; typically operated at 2.0–3.5 bar
• Circulation Rate: Liquid EDC circulation rate must be 5–10 times the production rate to maintain stable reaction