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Articles
Continuous tetrachloroethylene chlorination to hexachloroethane is a radical-chain addition process in which molecular chlorine is consumed across the double bond of tetrachloroethylene according to C₂Cl₄ + Cl₂ → C₂Cl₆. In continuous production, liquid tetrachloroethylene is circulated through a reactor train while chlorine gas is sparged into the liquid under ultraviolet radiation, and the chlorine conversion efficiency is defined as the molar percentage of chlorine feed that is irreversibly incorporated into chlorinated organic products, excluding vent-gas chlorine recovered in downstream absorption. The overall process objective is not simply to maximize single-pass chlorine conversion, but to balance conversion against the cost of excess chlorine, the thermal load from the exothermic addition, and the formation of byproducts that arise from thermal reversal or radical side reactions. Industrial reactors for this chemistry are typically glass-lined or fluoropolymer-lined vessels fitted with internal quartz lamp wells, external heat exchangers, and multi-stage gas-liquid separation, because the combination of wet chlorine, hydrogen chloride, and chlorinated organics is aggressively corrosive to common metals. The efficiency of chlorine utilization is therefore an equipment-specific result rather than a fixed property of the reaction, and it is influenced by bubble size distribution, photon penetration depth, liquid backmixing, moisture ingress, and the concentration of dissolved radical scavengers.
In a continuous stirred-tank cascade, the chlorine feed is often split across three to five reaction stages to overcome the inherent backmixing of a single well-mixed vessel. Each stage operates at a liquid hourly space velocity between 0.05 h⁻¹ and 0.15 h⁻¹, with chlorine supplied through a sparging ring beneath the lower impeller. The gas-liquid mass transfer coefficient kLa in such mechanically agitated reactors generally falls between 0.03 s⁻¹ and 0.20 s⁻¹ when the superficial gas velocity is maintained between 0.01 m/s and 0.05 m/s and the volumetric gas holdup is kept in the range of 4% to 12%. Below that range, bubble coalescence in the liquid produces large gas voids that reduce interfacial area; above that range, liquid entrainment in the vent gas increases chlorine losses and destabilizes downstream caustic scrubbing. The local energy dissipation rate in the impeller discharge zone controls the equilibrium bubble size, and the sparger orifices are typically 1.0 mm to 2.0 mm in diameter to generate bubbles that remain in the dispersion long enough for chlorine dissolution. Because chlorine solubility in tetrachloroethylene decreases with rising temperature and increases with rising pressure, the reactor is usually held at 60°C to 85°C and 150 kPa(g) to 250 kPa(g) to keep the equilibrium dissolved chlorine concentration high while avoiding the reverse dissociation of hexachloroethane.
Liquid-phase photochlorination is preferred over vapor-phase chlorination because the liquid inventory absorbs the exothermic reaction heat and allows the use of external circulation loops with plate-and-frame or shell-and-tube heat exchangers. The radical chain is initiated by photodissociation of molecular chlorine under 330 nm to 400 nm radiation, and the liquid phase provides a high concentration of tetrachloroethylene for propagation. In commercial reactors, medium-pressure mercury vapor lamps are housed inside quartz immersion wells, and the effective photon flux at the sleeve face is typically 100 W/m² to 300 W/m². Quartz sleeve fouling is a major field failure mode; when dissolved iron exceeds 2 mg/kg or suspended solids exceed 5 mg/kg, the penetration depth of ultraviolet light is reduced and chlorine conversion falls by as much as 10% to 15% before the lamp is cleaned. The fouling layer is often a mixture of hydrolyzed chlorides and oligomeric radical-termination products, and it requires shutdown, mechanical cleaning, and passivation with dilute hydrochloric acid. Published data for specific lamp replacement intervals is limited; equipment manufacturers commonly recommend cleaning and recalibration when the 365 nm output measured through a clean quartz reference falls by 20% to 25%.
In a single continuous stirred-tank reactor, the bulk liquid chlorine concentration approaches the exit concentration, reducing the concentration difference between the gas-liquid interface and the liquid phase. This lowers the absorption rate and leaves unconverted chlorine in the headspace. By arranging three to five reactors in series with interstage gas disengagement, the cascade approximates plug-flow behavior and raises chlorine conversion from a single-stage range of 70% to 85% to an overall range of 94% to 98% under optimized conditions. The liquid hourly space velocity is a critical control variable; above 0.15 h⁻¹, the residence time becomes insufficient for the radical chain to reach completion, and chlorine breakthrough in the vent gas increases rapidly. The Peclet number of the liquid phase in an agitated vessel with a draft tube is generally in the range of 0.5 to 5, indicating significant backmixing, whereas a multi-stage cascade with submerged overflow piping can achieve a vessel dispersion number below 0.05. Interstage transfer lines are designed to avoid gas carryover, because bubbles entrained from one stage to the next create localized zones of high chlorine concentration and increase the risk of hot spots at the impeller hub.
| Variable | Design window | Control method | Observed impact on chlorine conversion |
|---|---|---|---|
| Chlorine to tetrachloroethylene molar feed ratio | 1.03 to 1.10 | mass flow ratio control | drives forward reaction; excess ratio raises vent-gas chlorine load |
| Reactor bulk temperature | 60°C to 85°C | external circulation cooler | higher temperature improves kinetics but lowers chlorine solubility and favors reverse dissociation |
| Operating pressure | 150 kPa(g) to 250 kPa(g) | backpressure control valve | higher pressure increases dissolved chlorine and interfacial driving force |
| Liquid hourly space velocity | 0.05 h⁻¹ to 0.15 h⁻¹ | feed pump stroke and level control | values above 0.15 h⁻¹ produce chlorine breakthrough |
| Moisture content in liquid feed | ≤ 20 mg/kg | azeotropic drying or molecular sieve | free water hydrolyzes chlorine to HCl and corrodes quartz wells |
| Dissolved oxygen | ≤ 1 mg/kg | vacuum degassing of feed | oxygen acts as radical scavenger and reduces quantum efficiency |
The radical chain consists of initiation, propagation, and termination reactions. Photodissociation of molecular chlorine produces two chlorine atoms. A chlorine atom adds to tetrachloroethylene to form the pentachloroethyl radical, and that radical abstracts a chlorine atom from molecular chlorine to produce hexachloroethane and regenerate a chlorine atom. The propagation steps are fast, but the overall quantum yield falls when dissolved oxygen or unsaturated impurities compete for chlorine atoms. Barium chloride and ferric chloride have been described as thermal catalysts for chlorination of tetrachloroethylene, but published data for continuously operated BaCl₂-catalyzed systems is limited, and the presence of free water converts these chlorides into acidic hydrolysis products that precipitate on heat-transfer surfaces. If a metal-chloride catalyst is used, the salt is typically pre-dried and blended into the tetrachloroethylene feed at a concentration below 1 wt%, and the reactor is operated with a moisture content below 20 mg/kg to prevent hydrolysis. The catalyst is removed from the crude product by water washing before hexachloroethane is isolated by melt crystallization or vacuum distillation. The incompatibility of hexachloroethane with strong alkalis is an important operational boundary; contact with sodium hydroxide or potassium hydroxide can generate chlorinated acetylenes and accelerate decomposition even at moderate temperatures.
Hexachloroethane undergoes reverse dissociation to tetrachloroethylene and molecular chlorine when heated to elevated temperatures, and this equilibrium imposes an upper temperature limit on continuous reactors. The bulk liquid is normally held at 60°C to 85°C, but local surface temperatures at the quartz lamp wells and impeller hubs can exceed the bulk temperature by 10°C to 20°C. In those zones, thermal cracking of hexachloroethane produces lower chlorinated intermediates and reduces net chlorine conversion, even when the measured bulk temperature is within specification. Reactors using BaCl₂ or FeCl₃ thermal catalysts are particularly sensitive because Lewis acid sites can promote dechlorination and rearrangement at elevated temperatures. Continuous units therefore require external cooling loops with sufficient heat-transfer surface to remove the exothermic reaction enthalpy without relying on boiling of the reaction mass. The off-gas from the reactor headspace is routed to a vent condenser and then to a caustic scrubbing system; if the condenser is undersized, hexachloroethane vapor pressure in the vent gas increases and the apparent chlorine conversion efficiency improves falsely because chlorine is consumed while product is lost as vapor. Moisture ingress into the vent system forms hydrochloric acid at the dew point, leading to corrosion of stainless steel vent piping.
Liquid hourly space velocity is among the most sensitive control parameters in continuous chlorination. When the value rises above 0.15 h⁻¹, the mean residence time of liquid in the reactor falls below the time required for the slower radical termination and chain-transfer steps to approach completion, and unreacted chlorine exits the vent gas. The resulting chlorine conversion efficiency may decline from 94%–98% to 82%–88% over a relatively narrow feed-rate increase, depending on the number of stages and the photon flux available. Because the reaction is mass-transfer-limited rather than purely kinetically limited, increasing agitation alone is not sufficient to recover conversion; the higher shear can reduce bubble diameter and increase interfacial area, but it also increases backmixing and can entrain gas bubbles through the interstage overflow. The optimum agitation power input in a 4 m³ to 10 m³ production reactor is usually specified in the range of 0.8 kW/m³ to 2.5 kW/m³, and exceeding this range tends to raise the liquid temperature and reduce the dissolved chlorine concentration. Variable-frequency drives on the agitators, coupled with online vent-gas chlorine analyzers, allow the control system to reduce the chlorine feed rate when the vent-gas chlorine concentration exceeds 2 vol% to 5 vol%. The vent-gas analyzer itself is typically an ultraviolet absorption or electrochemical cell calibrated in accordance with the equipment manufacturer’s procedure, and the sample line is heated above the dew point to prevent condensation artifacts.
Hexachloroethane produced in continuous chlorination is used in aluminum melt degassing and grain refining, in smoke-generating pyrotechnic compositions, and in certain rubber processing and chemical synthesis applications. The downstream application imposes specific purity and handling requirements that feed back into reactor design. For aluminum degassing, the hexachloroethane is often compacted into tablets with a minimum assay of 99.0 wt% and a moisture content below 0.1 wt%; residual tetrachloroethylene above 0.5 wt% can volatilize during melt treatment and create worker exposure issues. For pyrotechnic smoke formulations, the particle size distribution and the absence of free moisture are critical because the condensed-phase reaction with zinc oxide or zinc dust is sensitive to particle surface area. Continuous chlorination units that feed these markets typically include a multistage purification sequence: water washing, neutralization, drying, and vacuum melt crystallization. The melt crystallization step operates just above the melting temperature of hexachloroethane, normally 186°C to 187°C, but prolonged holding at that temperature in the presence of dissolved chlorine causes darkening and the formation of chlorinated oligomers. Analytical surveillance of the purified product is performed by gas chromatography according to ASTM D6806-17, and semivolatile impurity screening is carried out by US EPA SW-846 Method 8270E. Batch-to-batch variance in continuous production is most commonly introduced by fluctuations in moisture content of the tetrachloroethylene feed and by drift in the vent-gas analyzer calibration.
The selection of materials of construction determines the operational availability of a continuous chlorination train. Glass-lined carbon steel is used for reactor bodies and disengagement vessels because it resists wet chlorine and hydrochloric acid at the operating temperatures; however, the glass lining is vulnerable to thermal shock if cold tetrachloroethylene is charged into a hot vessel. PTFE-lined piping and gaskets are used for transfer lines carrying liquid containing dissolved chlorine, while Hastelloy C-276 or tantalum is specified for thermowells and rupture-disc holders exposed to wet chlorine at elevated temperature. Carbon steel is unacceptable for any wetted surface, and 316L stainless steel is generally inadequate in the presence of aqueous hydrochloric acid at temperatures above 50°C. The vent gas from the final disengagement vessel contains unreacted chlorine, saturated tetrachloroethylene vapor, and traces of hydrogen chloride from moisture ingress. The chlorine recovery system typically consists of a vent condenser operating at −10°C to 5°C to return chlorinated organics to the reactor, followed by a packed-bed caustic scrubber using 10% to 20% sodium hydroxide solution. The scrubber is designed for a maximum chlorine concentration of 5 vol% in the inlet gas, and the spent hypochlorite solution is monitored for active chlorine before discharge to the plant wastewater treatment system. Hydrogen chloride formation is controlled by maintaining the moisture content of the chlorine feed and the tetrachloroethylene feed below 20 mg/kg, and by drying the reactor system with dry nitrogen before startup after any maintenance opening.
| Parameter | Method or standard | Purpose |
|---|---|---|
| Hexachloroethane assay | ASTM D6806-17 | Gas chromatographic purity |
| Semivolatile chlorinated impurities | US EPA SW-846 Method 8270E | GC/MS screening |
| Color of halogenated solvents | ASTM D2108-10 | Detection of oxidative darkening |
| pH of water extraction | ASTM D2110-00 | Free acidity after washing |
| Reagent water quality | ASTM D1193-06 | Wash and neutralization water |
| REACH registration | Regulation (EC) No 1907/2006 | Manufacture/import compliance above 1 t/a |
Field data from production-scale continuous units indicate that the most common process conflict is the interaction between moisture, lamp fouling, and vent-gas chlorine breakthrough. A rise in feed moisture from 10 mg/kg to 30 mg/kg can produce sufficient hydrogen chloride to attack quartz well seals and generate a haze on the inner reactor wall, reducing ultraviolet transmission and lowering chlorine conversion by several percentage points within 24 h. The failure is frequently misdiagnosed as lamp aging because the vent-gas analyzer shows simultaneous chlorine breakthrough and the product assay remains within specification; the distinguishing symptom is a rise in the pH of the water wash below the reactor and an increase in chloride in the closed cooling-water circuit. Continuous trains that operate without an online moisture analyzer on the tetrachloroethylene feed are therefore constrained to more frequent lamp inspection and to conservative space velocities below 0.10 h⁻¹. Published data for the effect of dissolved moisture on quantum yield in this specific configuration is limited, but the operational boundary is well understood from corrosion and fouling records. The process also exhibits incompatibility with amine-based radical inhibitors that are sometimes present in recovered chlorinated solvents; even low concentrations of amines can react with chlorine and form nitrogen-chlorine compounds that alter the radical chain and deposit sticky residues on the heat exchanger surfaces.
