Direct oxychlorination of ethylene to ethylene dichloride proceeds according to the stoichiometric equation C₂H₄ + 2 HCl + 0.5 O₂ → C₂H₄Cl₂ + H₂O; the widely reported standard enthalpy of reaction is -238 kJ/mol of EDC produced. The EDC is subsequently thermally cracked to vinyl chloride monomer in a separate furnace, with HCl recycled to the oxychlorinator. The VCM yield attributable to oxychlorination-derived EDC is therefore the product of the oxychlorination selectivity to EDC on an ethylene basis, the cracking per-pass conversion, the cracking selectivity to VCM, and the EDC purification recovery. In a balanced vinyl complex, direct chlorination and oxychlorination are coupled so that the HCl released from cracking is consumed by oxychlorination; this makes the oxychlorination HCl conversion a boundary condition for the recycle loop. Commercial fixed-bed oxychlorinators are typically multitubular reactors operating at 200–250 °C and 0.2–0.6 MPa with a HCl-to-ethylene molar ratio of 1.90–2.05 and an O₂-to-HCl molar ratio of 0.50–0.60. These ranges are derived from published licensor documentation and standard chemical engineering references rather than from a single plant dataset.

Thermal cracking of EDC in fired heaters uses coil outlet temperatures of 480–520 °C, residence times of 2–5 s, and outlet pressures of 1.5–2.5 MPa. Per-pass EDC conversion is intentionally limited to 50–60% to suppress coking and byproduct formation; VCM selectivity at these conditions is generally 96–99 mol%. Unconverted EDC is recovered by distillation and recycled, and the hot cracked gas is quenched to prevent secondary reactions. The overall VCM yield from oxychlorination-derived EDC is therefore approximately the product of ethylene selectivity to EDC in oxychlorination, EDC purification recovery, cracking selectivity, and VCM purification recovery. Published data for this specific configuration is limited, but balanced-plant engineering studies typically report an overall VCM yield of 93–96 mol% on consumed ethylene across the coupled direct chlorination–oxychlorination–cracking sequence.

In a balanced VCM complex, direct chlorination and oxychlorination operate in parallel to produce EDC. The direct chlorination unit consumes ethylene and chlorine at 50–70 °C in a liquid-phase EDC reactor with ferric chloride catalyst, while the oxychlorination unit consumes the HCl recycled from EDC cracking. The ratio of direct chlorination to oxychlorination EDC production is set by the plant chlorine balance and is typically 1:0.45–0.55 on a molar basis. The overall VCM yield from the balanced process can be expressed as moles of VCM produced per mole of fresh ethylene consumed. The theoretical stoichiometry gives 1.0 mol VCM per mol ethylene when chlorine and oxygen are included, but real plants achieve 0.93–0.96 mol/mol because of purge losses, byproduct formation, and incomplete recovery. This ratio is governed by the oxychlorination selectivity and the cracking furnace severity. The direct oxychlorination section must therefore achieve HCl conversion above 99% to avoid HCl breakthrough into the vent; otherwise the recycled HCl loop becomes depleted and the VCM yield decreases. The direct chlorination reactor is excluded from this analysis, but its EDC is combined with oxychlorination EDC before distillation.

What Limits HCl Conversion in a Single-Stage Fixed-Bed Oxychlorinator?

Fixed-bed direct oxychlorinators operate with a gas-phase feed distributed across several thousand tubes. The principal constraint on HCl conversion is the exotherm. The reaction releases 238 kJ/mol, and local hot spots above 230–250 °C accelerate copper chloride volatilization from the supported CuCl₂–KCl/γ-Al₂O₃ catalyst. Hot spots also promote ethylene oxidation to CO and CO₂, reducing EDC selectivity. Industrial fixed-bed units therefore use a coolant, often boiling water or a high-flux organic heat transfer fluid, to maintain the tube wall at 200–230 °C while the centreline gas temperature is allowed to reach 250–260 °C. HCl conversion in a single stage is typically 99.0–99.8%; pushing beyond this range requires either lower gas hourly space velocity or higher temperature, both of which increase byproduct formation and catalyst deactivation. Gas hourly space velocity is commonly held between 800 h^-1 and 1500 h^-1 on a dry total feed basis. The molar HCl-to-ethylene ratio is maintained at 1.95–2.05 to avoid ethylene-rich zones that can form ethyl chloride, while the O₂-to-HCl ratio is kept at 0.50–0.55 to avoid oxygen breakthrough to the downstream vent. Tube inner diameters in commercial units are generally in the 32–40 mm range, with tube lengths of 5–7 m, constrained by the need to limit pressure drop to 0.1–0.3 bar across the bed. The design pressure and temperature of these tubes fall under ASME B31.3-2022, Chapter II, Part 2, and manufacturers qualify the tube-to-tubesheet joints by reference to the same code. When hydrochloric acid is present in the feed, the metallurgy is usually a duplex stainless steel or a high-nickel alloy such as Alloy 200/201 for the reactor inlet sparger and effluent piping, whereas the reactor tubes themselves may be a low-alloy steel such as 13CrMo44 because the tube wall remains hot enough to avoid aqueous condensation. This configuration is limited by the onset of hot-spot-driven CuCl₂ sublimation; operation beyond 235 °C at the bed centre for long periods produces visible green copper chloride deposits in the effluent quench and shortens catalyst cycle length.

In an oxygen-based direct oxychlorination loop, the inert nitrogen load is reduced by substituting 99.5 mol% oxygen for air. The reactor feed is then a recycle gas consisting largely of unreacted ethylene, HCl, and saturated water vapour plus small amounts of CO, CO₂, and nitrogen from purge. Flammability control requires that oxygen concentration at the reactor inlet be maintained below the lower explosive limit boundary for ethylene–oxygen–water–HCl mixtures; typical oxygen-based units operate with oxygen concentrations of 6–8 vol% at the reactor inlet and a total pressure of 0.3–0.6 MPa to keep the mixture oxygen-limited. The oxygen-feed purity specification is set at 99.5 mol% minimum because argon and nitrogen accumulate in the recycle loop and force a larger purge. A purge stream of 0.5–2.0% of recycle flow is typically routed to a vent incinerator or catalytic oxidation unit, and the resulting HCl is recovered in a wet scrubber. Ethylene selectivity to EDC in oxygen-based fixed-bed operation is generally 96–98 mol%, with carbon oxides accounting for 0.5–1.5 mol% of consumed ethylene and ethyl chloride plus heavier chlorinated compounds accounting for the remainder. The lower oxygen partial pressure reduces hot-spot intensity relative to air-based operation, but it also makes the reaction rate sensitive to oxygen starvation at high HCl conversion. When oxygen is not uniformly distributed across the tube field, local oxygen-poor zones produce free carbon and increase copper chloride reduction, leading to ethylene slip and lower VCM yield. Published data for this specific configuration is limited, but the general behaviour is consistent with fixed-bed oxychlorination studies in the open literature.

Fluidised-Bed Catalyst Attrition and Temperature Homogeneity in Direct Oxychlorination

Fluidised-bed direct oxychlorinators use a fluidised catalyst of CuCl₂–KCl on a microspheroidal γ-Al₂O₃ or silica-alumina support with a mean particle diameter of 30–80 µm. The bed is fluidised at superficial gas velocities of 0.3–0.6 m/s, and the resulting dense-phase bed operates at 210–235 °C with a high degree of lateral temperature uniformity. Because the solid phase undergoes continuous back-mixing, the bed temperature can be controlled within ±2 °C of the setpoint, which is a significant advantage over fixed-bed systems where hot spots may exceed 20 °C above the coolant temperature. Heat removal is accomplished by vertical or U-tube cooling coils immersed in the bed, using boiler feed water or a high-flux heat transfer fluid. The predominant long-term operational risk is catalyst attrition, not copper volatilization. Attrition generates fines smaller than 20 µm, which are carried into the reactor cyclone and, if the cyclone is undersized or the gas velocity is too high, into the quench system. The attrition rate is typically expressed as a loss of 0.1–0.5 wt% catalyst inventory per day in commercial units, although published data for this specific configuration is limited. Catalyst makeup is added continuously or every shift to maintain the bed inventory and the required CuCl₂ loading, typically 4–12 wt% CuCl₂ and 1–4 wt% KCl. The fluidised-bed reactor outlet gas passes through two or three stages of cyclones followed by a hot gas filter in some modern designs. The use of a hot gas filter reduces downstream fouling in the quench column and protects the HCl recovery absorber from fine particle contamination. The main process conflict in fluidised-bed operation is that higher gas velocity improves heat transfer and reduces bed height, but increases attrition and cyclone loading. Acceptable HCl conversion is 99.0–99.9%, and ethylene selectivity to EDC is generally 96–98 mol% in oxygen-based operation, but the selectivity depends strongly on the oxygen-to-HCl ratio.

Subsequently, the reactor effluent at 200–250 °C is quenched in a water-cooled graphite or rubber-lined quench column to 80–100 °C to condense water, HCl, and heavy chlorinated byproducts while leaving most EDC and light gases in the vapour phase. The quench medium is a circulating stream of dilute hydrochloric acid at 1–5 wt% HCl. The aqueous HCl concentration is controlled by water balance and the addition of demineralised water; the overhead EDC vapour is then condensed and sent to a decanter. The quench column is a significant corrosion zone because aqueous HCl condenses on the walls and internals. Materials of construction include graphite, fluoropolymer-lined steel, or high-nickel alloys such as Hastelloy C-276 for the upper section, while the lower section may use fibre-reinforced plastic with a sacrificial corrosion allowance. The pH of the quench bottoms is typically maintained below 1.0 to keep ferric chloride and copper chloride species in solution and to avoid precipitation of hydroxides that promote fouling. Chloral and other oxygenated byproducts form in trace amounts in the presence of residual oxygen; their concentrations are normally below 100 mg/kg in the circulating quench liquid, but can rise if the O₂-to-HCl ratio exceeds 0.60 or if the quench temperature is too high. Amine-based neutralisers are generally avoided because they form amine hydrochlorides, which accumulate in the quench loop and block the pH control instrumentation. The use of antifoam agents is likewise constrained to polydimethylsiloxane emulsions at low dosage because the aqueous phase has a tendency to emulsify with chlorinated hydrocarbons. The quench overhead is routed to a series of distillation columns where EDC is separated from water, light ends, and heavy ends. The EDC purity after purification is typically 99.5–99.9 wt%, as measured by ASTM D1078-11 distillation range and ASTM D1209-05 colour.

When Tetrachloroethane Replaces Methylene Chloride in Heavy Ends Extraction

Downstream purification of oxychlorination-derived EDC produces a heavy ends stream containing 1,1,2-trichloroethane, trichloroethylene, tetrachloroethane, and other polychlorinated C₂ compounds. In older units, methylene chloride was used as a solvent to extract or dilute heavy ends before thermal or catalytic incineration. When tetrachloroethane replaces methylene chloride, the extraction duty is changed because tetrachloroethane has a higher boiling point and higher chlorine content. The heavy ends column then requires a higher reboiler temperature, typically 150–175 °C compared with 100–125 °C for a methylene chloride system, and the column metallurgy must be upgraded to resist decomposition-generated HCl. The advantage is that tetrachloroethane does not introduce additional methylene chloride into the product train, and it can be co-fed to the EDC cracking furnace after recovery; however, tetrachloroethane itself can form trichloroethylene and perchloroethylene in the cracking furnace, reducing VCM selectivity. Published data for this specific configuration is limited. Process simulations based on chlorinated hydrocarbon vapour-liquid equilibrium data indicate that the heavy ends viscosity at 40 °C increases from 0.8–1.0 mPa·s to 1.2–1.5 mPa·s when methylene chloride is replaced by tetrachloroethane, and the heat exchanger fouling factor should be increased accordingly. The extraction system is often constructed from Alloy 200 or fluoropolymer-lined carbon steel, and the API 650 storage tank must be inerted with nitrogen to avoid oxygen ingress. The overall VCM yield effect is usually neutral to slightly negative, with a loss of 0.1–0.3% VCM yield due to additional heavy ends recycle, but the substitution eliminates a regulated solvent from the purification area. The replacement is limited to units that already have a tetrachloroethane recovery column; retrofitting a methylene chloride unit requires additional piping rated under ASME B31.3-2022 and a dedicated vent scrubber.

Catalyst deactivation in direct oxychlorination is governed by copper chloride migration, hot-spot-induced volatilisation, and accumulation of carbonaceous deposits. In a fixed-bed unit, the axial temperature profile shows a hot band located 30–50% of the bed length from the inlet, and this is where the copper chloride loss is most pronounced. The potassium chloride promoter is present at 1–4 wt% to suppress the formation of copper chloride hydrates and to lower the melting point of the active phase; excess KCl above 5 wt% can produce a low-melting eutectic that accelerates agglomeration at temperatures above 220 °C. The CuCl₂–KCl/γ-Al₂O₃ catalyst is typically dried to a moisture content below 0.5 wt% before loading, and the feed gas must be preheated to at least 10 °C above the water dew point to avoid liquid water condensation in the bed. When a fixed-bed reactor is shut down, the catalyst is purged with nitrogen for 4–8 h to remove HCl and oxygen before the reactor is opened; failure to purge leads to severe corrosion of the tube sheet and support grids. Regeneration is sometimes accomplished by air treatment at 200–220 °C with stepwise oxygen introduction, but repeated regeneration causes irreversible loss of surface area. Catalyst cycle life in fixed-bed oxygen-based units is typically 12–24 months, while fluidised-bed catalyst may be maintained by continuous additions for 3–5 years of unit operation. These cycle lives are affected by feedstock purity, especially acetylene, ethylene oxide, and sulfur compounds in the ethylene feed; acetylene should be below 5 ppmv to prevent formation of explosive copper acetylide compounds. This is an operational boundary anchored to the lower explosive limit of copper acetylide and standard process safety information under 29 CFR 1910.119(d).

Assessing Catalyst Volatilisation and Ethyl Chloride Accumulation by On-Line Gas Chromatography

Continuous on-line gas chromatography is used to monitor the oxychlorinator outlet for HCl, oxygen, ethylene, EDC, CO, CO₂, and light chlorinated byproducts. The analyser sample system is steam-traced and constructed of tantalum or fluoropolymer to avoid corrosion from wet HCl. The analyser cycle time is typically 2–5 min, and the results are used in closed-loop control of the oxygen feed and HCl-to-ethylene ratio. The calibration gas mixture is traceable to a certified reference material and includes 0–100 ppmv CO, 0–5 vol% O₂, and 0–10 vol% ethylene in nitrogen. The flame ionisation detector quantifies EDC and ethyl chloride; the thermal conductivity detector quantifies fixed gases and HCl. The accuracy of the on-line system is verified monthly against manual grab samples analysed by ASTM D2505-88(2015) for ethylene feed impurities and by ASTM D1078-11 for distilled EDC purity. Oxygen analysers in the recycle loop are typically zirconia or paramagnetic types with a response time of less than 10 s; this is required because an oxygen excursion above the flammable envelope triggers an automatic trip under IEC 61511-1:2016. The trip logic closes the oxygen control valve and opens the inert nitrogen purge within 1–2 s, and the safety instrumented function is tested at an interval not exceeding 6 months. The reactor pressure is recorded and restrained by a relief valve sized according to ASME B31.3-2022, Chapter II, Part 2, and the relief discharge is routed to a caustic scrubber. The effluent monitoring system also includes pH probes in the quench bottoms and conductivity cells in the demineralised water feed. When the on-line gas chromatograph detects an increase in ethyl chloride from 200 mg/kg to above 500 mg/kg, the control system reduces ethylene feed by 0.5–1.0% and increases oxygen flow by 0.2–0.5% until the ratio returns to target. This closed-loop action prevents the accumulation of ethyl chloride in the recycle loop and preserves overall VCM yield.

During start-up transients in an air-based oxychlorination loop, the reactor is first heated with nitrogen and steam to 150–180 °C before HCl is introduced. The introduction sequence is critical: HCl is admitted at a low flow of 5–10% of design while monitoring the reactor outlet for oxygen and water. Ethylene is not introduced until the oxygen feed is stable and the catalyst bed has been exposed to HCl for 2–4 h; this preconditioning chlorinates copper oxide species and avoids a rapid exotherm when ethylene is initially fed. The air flow is then brought up stepwise over 4–6 h while maintaining an oxygen concentration below 6 vol% at the reactor inlet. Failure sequences in industrial units commonly involve premature ethylene introduction, which produces a temperature excursion of 20–40 °C within 10 min and can permanently deactivate the top section of a fixed bed by copper chloride migration. In a fluidised-bed unit, the same excursion is moderated by the high solids inventory, but cyclone plugging can occur if liquid EDC condenses during heat-up. If the start-up is performed after a maintenance shutdown, the reactor pressure boundary is first leak-tested with nitrogen at 1.1 times the design pressure according to ASME B31.3-2022, and the relief valve setpoints are verified. The quench system is filled with demineralised water and acidified to 1–2 wt% HCl before reactor effluent is admitted. During the initial hours of operation, the on-line gas chromatograph is set to a 1 min cycle time instead of the normal 2–5 min to detect HCl breakthrough early. HCl breakthrough above 500 ppmv in the vent stream indicates that the oxychlorination reaction is not yet stable, and the oxygen flow is reduced by 0.5% until the measured HCl conversion exceeds 99%. After 12–24 h of stable operation, the unit is gradually raised to design throughput at a rate not exceeding 5% of design load per hour to avoid disturbing the catalyst bed and the EDC purification train. Published data for this specific configuration is limited, but the described sequence is consistent with standard air-based oxychlorination start-up procedures documented in process licensor safety reviews and in OSHA PSM compliance audits.