Ethylene Dichloride (EDC /1,2-Dichloroethane): Primary Feedstock for VCM is the technical-grade chlorinated hydrocarbon that integrated vinyl-production sites procure, dry, and crack into vinyl chloride monomer. The product, identified by CAS number 107-06-2 and EC number 203-444-5, has molecular formula C₂H₄Cl₂, molar mass 98.96 g/mol, normal boiling point 83.5 °C at 101.3 kPa, and density 1.253 g/cm³ at 20 °C. Commercial cracker-feed specifications generally require EDC assay not less than 99.5 wt%, water below 150 mg/kg, and acidity below 5 mg/kg as HCl; release testing is commonly anchored to ASTM D5960 for technical-grade ethylene dichloride, with water determined by Karl Fischer titration per ASTM E203.

Representative EDC cracker-feed release limits and test methods

Parameter	Representative limit	Test method
EDC assay	99.5 wt% min	ASTM D5960
Water	150 mg/kg max	ASTM E203
Acidity as HCl	5 mg/kg max	ASTM D1613
Distillation range	82.5–84.5 °C	ASTM D1078
Colour	10 Pt-Co max	ASTM D1209

The dominance of EDC in the vinyl chain is not based on solvent versatility but on chlorine stoichiometry. In the balanced vinyl loop, direct chlorination of ethylene with chlorine and oxychlorination of ethylene with hydrogen chloride and oxygen both generate EDC; subsequent thermal cracking liberates one mole of hydrogen chloride per mole of EDC converted, and that hydrogen chloride returns to the oxychlorination reactor. This closed-chlorine route to VCM makes EDC the singular high-volume intermediate and primary feedstock for vinyl chloride monomer. Secondary uses include synthesis of ethylenediamine and vinylidene chloride, but those are minor relative to VCM.

Why Does the Vinyl Chain Center on Ethylene Dichloride?

Because EDC pyrolysis delivers VCM with per-pass molar selectivity generally reported between 98% and 99%, while unconverted EDC is recovered and recycled. Direct chlorination typically operates in the liquid phase at 50–70 °C with dissolved FeCl₃ as catalyst; ethylene is sparged into a recirculating EDC loop, and chlorine is introduced through a gas-liquid contactor. Oxychlorination operates in either fixed-bed or fluid-bed reactors at 200–250 °C over a CuCl₂/Al₂O₃ catalyst, consuming HCl recycled from the cracking furnace. The parallel EDC-generating routes allow a site to balance chlorine, because direct chlorination consumes purchased chlorine while oxychlorination consumes hydrogen chloride liberated in pyrolysis.

Direct chlorination is strongly exothermic and requires external recirculation coolers and tight ethylene-to-chlorine ratio control to avoid chlorine breakthrough and subsequent safety excursions. In a commercial agitated gas-liquid reactor, the high heat of reaction is removed by external coolers or by operating at the EDC boiling point; metallic impurities and water must be controlled because they catalyse polar byproduct formation. Oxychlorination heat management is more demanding: fluid-bed reactors permit continuous catalyst withdrawal and replenishment, while fixed-bed configurations rely on multiple reactor tubes and careful hot-spot control to prevent CuCl₂ sintering. Crude EDC from direct chlorination is relatively clean after water wash and drying; oxychlorination crude carries chloral, ethylene chlorohydrin, and oxygenated byproducts that must be removed by caustic washing and distillation before the material is acceptable for cracking.

Because direct chlorination yields EDC with fewer oxygenated impurities, it is often preferred for the cracker feed blend when high-purity chlorine is available. Oxychlorination is indispensable, however, because it consumes the HCl generated in cracking and reduces the need for external chlorine purchases. Oxygen-based oxychlorination reduces inert gas load, while air-based operation has lower oxygen concentration but larger vent-gas flow; both require flammability control and catalyst hot-spot management. The oxychlorination effluent is quenched and condensed, and the resulting water/HCl/EDC mixture is separated in a quench tower and decanter before caustic washing.

At the radiant-coil pyrolysis section, the actual processing window is narrower than the bulk stoichiometry suggests. Industrial EDC cracking furnaces operate with coil outlet temperatures between 500 °C and 530 °C, coil outlet pressures of 2.0–2.8 MPa, and vapour residence times between 8 s and 20 s. Under these conditions, per-pass EDC conversion is deliberately held at 50% to 60%. The reason is a competing pair of process penalties: the endothermic dehydrochlorination requires high heat flux through the coil wall, while the free-radical reaction network accelerates coke deposition as temperature and residence time increase. Radiant-section coils are specified in centrifugally cast HP-grade or similar high-nickel-chromium alloys, and furnace control relies on tube-skin thermocouples because wall temperature, not process outlet temperature, limits run length.

The EDC pyrolysis reaction is endothermic, with a heat of reaction near 70 kJ/mol. The apparent activation energy for dehydrochlorination is commonly reported near 200–250 kJ/mol, so a modest increase in coil outlet temperature produces a larger increase in required tube-wall temperature. Modern furnace designs preheat EDC in the convection section, vaporize the feed, and distribute the vapour to parallel radiant coils. Distribution must be balanced; uneven flow across parallel coils creates local overheating, accelerated coke formation, and risk of tube failure. The pressure range is a compromise: higher pressure improves downstream separation and quench efficiency, but it also increases residence time and fouling tendency.

Typical published ranges for industrial EDC pyrolysis furnace operation

Parameter	Range	Process consequence
Coil outlet temperature	500–530 °C	Higher outlet temperature raises per-pass conversion but accelerates coke deposition
Coil outlet pressure	2.0–2.8 MPa	Maintains residence time and quench driving force; high pressure increases fouling tendency
Residence time	8–20 s	Short residence time preserves VCM selectivity but reduces conversion
Per-pass EDC conversion	50–60%	Limited by selectivity and coke penalties; unreacted EDC is recycled
VCM selectivity	98–99%	Requires rapid quench and low feed moisture; byproducts include light chlorinated hydrocarbons and tars

Rapid quench after the coil is essential. Furnace effluent is cooled to below 150 °C within milliseconds to freeze dehydrochlorination chemistry; otherwise secondary condensation reactions form high-boiling chlorinated byproducts and increase fouling in the downstream quench and distillation train. EDC-furnace run lengths are governed less by catalyst deactivation than by pressure drop across the coil and the maximum allowable tube metal temperature. When coke thickness increases, the same fired duty produces a higher wall temperature; operators reduce coil outlet temperature or schedule decoking to avoid tube carburization and metal dusting. Decoking is performed by steam-air oxidation or mechanical pigging, depending on furnace design, and frequent decoking cycles accelerate tube aging.

Feed water is controlled below 150 mg/kg because free water increases the acid load on the quench tower and accelerates corrosion. In addition, the presence of free water in EDC storage can create acid-rich bottom phases that corrode carbon steel; tank-bottom water draws and Karl Fischer monitoring per ASTM E203 are used to maintain cracker-feed quality. The EDC distillation train upstream of the furnace typically includes drying and heavy-ends columns; molecular sieve or azeotropic drying may be applied when water limits are tight. Quench-tower metallurgy is another boundary condition because the quench water contains HCl and traces of chlorinated hydrocarbons; tower linings or corrosion-resistant alloys are specified for wet acid service.

When Per-Pass Conversion Approaches 65%, What Fails First?

Published process studies indicate that raising per-pass EDC conversion beyond 60% reduces VCM selectivity through secondary chlorination and condensation pathways. Because the apparent activation energy for EDC dehydrochlorination is in the 200–250 kJ/mol range, higher conversion requires an outlet-temperature increase that drives tube-wall temperature upward disproportionately. Operations that attempt to recover more VCM per pass therefore observe rising levels of acetylene, ethylene, chloroprene, and heavy tars. These byproducts complicate downstream distillation and increase coke precursors in the recycle EDC stream.

At coil outlet temperatures above 530 °C, coke deposition can become autocatalytic: the roughened coke surface lowers heat-transfer coefficient and provides sites for further dehydrochlorination. Pressure-drop increase across a fouled coil commonly becomes the limiting run-length factor before tube metal temperature reaches the specified maximum. Site-dependent decoking or furnace turnaround thresholds are often set when coil pressure drop rises 20–30% above clean-tube baseline or when tube-skin temperature approaches the alloy limit. Published data for a given furnace geometry and feedstock impurity profile remain site-specific; however, the trend of higher temperature, higher conversion, lower selectivity, and shorter run length is consistent across technical literature.

Downstream of the furnace, the quench and distillation sequence separates hydrogen chloride, unreacted EDC, VCM, and heavies. Recovered HCl is routed to the oxychlorination reactor at a concentration controlled to maintain catalyst activity. The VCM purification train typically includes a light-ends column for HCl and light gases, a VCM product column, and a heavies removal system; polymer-grade VCM specifications require 99.9 wt% purity and tight acetaldehyde, iron, and moisture limits for suspension polymerization. Unreacted EDC recycle is distilled to remove heavies, preventing accumulation of unsaturated chlorinated species that would otherwise increase cracking-furnace fouling.

The HCl recovery section is designed for wet HCl or anhydrous HCl depending on the downstream oxychlorination pressure and reactor metallurgy. Column overhead streams containing HCl and light gases are processed through condensers and, where necessary, a vent-gas recovery system. Liquid EDC recycle is returned to the cracking furnaces after heavies rejection.

Storage, Handling, and Stability Boundaries

Carbon steel storage tanks are standard for dry EDC service, provided free water and acid are excluded. The compound is classified as flammable liquid category 2 under CLP with a closed-cup flash point near 13 °C; storage vessels are nitrogen-blanketed and grounded, and transfer systems use pumps rated for low-viscosity chlorinated hydrocarbon service. EDC vapour is heavier than air and may accumulate in pits or sumps; continuous low-level detection is applied at unloading and tank-farm areas. Workplace exposure is controlled under jurisdiction-specific occupational exposure limits, with common 8-hour time-weighted average values in the range 10–50 ppm where nationally adopted.

Under EU CLP, EDC also carries carcinogenicity category 1B and acute toxicity category 3 classifications. The product is incompatible with strong oxidizers, alkali metals, powdered aluminum, magnesium, zinc, and other reactive metals. Hot concentrated alkali hydrolyzes EDC to ethylene glycol and sodium chloride; caustic wash systems must therefore operate at temperature and concentration conditions that prevent meaningful yield loss. In ambient storage, moisture ingress is managed by nitrogen blanketing and periodic water draw from tank bottoms, with Karl Fischer monitoring per ASTM E203.

Vapour pressure at 20 °C is near 87 hPa; this means tank breathing and filling losses must be routed to vapour recovery or thermal oxidation. Fixed-roof tanks with nitrogen blanketing and conservation vents are commonly used, and loading/unloading is closed-loop with vapour return. Where EDC is stored in humid coastal sites, tank-bottom water draws are sampled weekly; free water is drained before transfer to cracking furnaces to keep the feed water below the cracker specification.