Epichlorohydrin (ECH; CAS 106-89-8; C₃H₅ClO; molar mass 92.52 g/mol; boiling point 116.1 °C at 101.3 kPa; flash point 33 °C closed cup) is produced by three industrial routes: propylene via allyl chloride, glycerol via dichloropropanol, and propylene via allyl acetate/allyl alcohol. The epoxide ring and chloromethyl substituent create a bifunctional intermediate used in liquid epoxy resins, epoxy novolacs, epichlorohydrin rubber, polyamide wet-strength resins, and quaternary ammonium water-treatment polymers. Epoxy-grade ECH is routinely controlled by capillary gas chromatography with internal standard; water is measured by Karl Fischer titration per ASTM E203, and color is reported against platinum-cobalt references per ASTM D1209. Under EU CLP Regulation 1272/2008, ECH is classified as Flam. Liq. 3, Acute Tox. 3, Skin Corr. 1B, Skin Sens. 1, and Carc. 1B; closed-loop handling and double mechanical seals are therefore standard on production lines.

The process selection problem is usually framed as a trade-off among chlorine intensity, feedstock flexibility, aqueous effluent volume, and purification burden. The matrix below—Propylene Route vs Glycerol Route vs Allyl Acetate Route: ECH Process Comparison Table—summarizes route-specific intermediates, operating conditions, waste profiles, and material demands. The table omits side-by-side yield claims where the separation train, catalyst age, and feedstock purity are not defined; published data for identical downstream purification across all three routes are limited. Values should be treated as engineering ranges from public process patents, safety dossiers, and plant technical summaries rather than guarantee levels.

Why Does Feedstock Selection Govern ECH Process Economics?

Feedstock cost contribution is not uniform across the three routes. Published cash-cost models for the propylene route consistently show that propylene and chlorine dominate variable cost when the plant is not credited for caustic soda co-product. In an integrated chlor-alkali site, chlorine is transferred at marginal cost, while exported 50% sodium hydroxide can reduce net operating cost; this creates a site-specific price floor that glycerol-route plants without chlor-alkali integration may not match. The glycerol route inserts crude glycerol, which is available from biodiesel transesterification. Its value is tied to local biodiesel capacity and logistics; crude glycerol containing 80–90 wt% glycerol can require acidulation, filtration, methanol stripping, and vacuum distillation before hydrochlorination. The allyl acetate route adds acetic acid and oxygen costs while retaining propylene consumption; acetic acid recovery becomes the main variable-cost bridge because the hydrolysis equilibrium is not fully shifted by water removal alone.

Process equipment count also follows feedstock chemistry. The propylene route includes a high-temperature chlorination furnace, a free-radical reactor, a hypochlorous acid generator, and a calcium hydroxide or sodium hydroxide dehydrochlorination train. The glycerol route eliminates the high-temperature propylene chlorination furnace but adds a crude glycerol pretreatment unit and a high-alloy HCl absorption-dehydration loop. The allyl acetate route adds an acetoxylation reactor, an acetic acid recovery column, an allyl acetate hydrolysis reactor, and an allyl alcohol chlorination unit. Each unit contributes to fixed capital intensity and maintenance cost; published total fixed capital comparisons vary widely by site because of differences in tankage, wastewater treatment, and incineration duty.

Thermal Chlorination Limits and Corrosion Boundaries in the Propylene Route

Propylene chlorination proceeds by free-radical substitution at 450–500 °C and near-atmospheric pressure. The furnace is designed for a short residence time, typically 5–15 s, to suppress secondary reactions that form 1,2-dichloropropane and 1,2,3-trichloropropane. Furnace tubes are exposed to hot HCl, unreacted chlorine, and carbon deposits; common tube alloys include Incoloy 800H and Inconel 600. Tube skin temperatures are maintained above the dew point of aqueous HCl to avoid wet chloride pitting, but below the alloy carburization limit. At the furnace outlet, cracked gas is quenched with recycled chlorinated organics or water to below 100 °C before entering rubber-lined or graphite equipment. Hypochlorous acid addition to allyl chloride is carried out at 30–60 °C in a stirred reactor because HOCl decomposes at elevated temperature and can liberate oxygen into the vapor space. The chlorine content of the off-gas is monitored continuously; excess chlorine is absorbed in caustic scrubbers.

Dehydrochlorination in the propylene route uses either calcium hydroxide slurry or sodium hydroxide solution. Calcium hydroxide produces a calcium chloride brine with lower alkalinity and a filterable sludge, while sodium hydroxide produces sodium chloride brine that can be returned to the chlor-alkali cell room if organic carryover is below cell-room limits. The dehydrochlorination reactor is typically glass-lined or PTFE-lined because the chlorohydrin mixture contains free chloride ions. Over-dehydrochlorination can produce glycidol or resinous oligomers; temperature is therefore limited to 50–100 °C, and pH is controlled by staged alkali addition. The crude ECH is steam-stripped from the brine and purified by extractive distillation or azeotropic drying.

Crude glycerol entering the hydrochlorination section is acidulated with HCl or sulfuric acid to hydrolyze soaps, then filtered through a plate-and-frame or rotary drum filter to remove fatty acid methyl ester waxes and ash. Excess methanol and water are stripped under vacuum; residual water above 5 wt% can shift the reaction toward monochlorohydrin instead of dichloropropanol and raise sodium hydroxide consumption downstream. The hydrochlorination reactor operates at 90–130 °C under 30–150 kPa absolute. Anhydrous HCl gas is sparged through the liquid phase, while water is removed as an HCl-water overhead stream; entrained HCl is recovered in a falling-film absorber. Carboxylic acid catalysts such as acetic acid or adipic acid are used at 0.5–5.0 wt% of the glycerol feed; catalyst losses occur through ester formation with residual methanol and through water-soluble carryover into the brine. The glass-lined or fluoropolymer-lined reactor is coupled to graphite block heat exchangers because hot wet HCl rapidly corrodes carbon steel and many stainless steels. Agitator mechanical seals are double-pressurized and nitrogen-purged to prevent HCl fugitive emissions.

Glycerol-route dehydrochlorination produces a brine that contains residual glycerol and chlorinated diglycerol ethers. These compounds can be biologically refractory, so the wastewater treatment unit is usually designed with an equalization basin and an acclimated activated sludge population. Published data comparing chemical oxygen demand removal for propylene-route and glycerol-route brines under identical OECD 301F conditions are limited. In practice, some producers route the brine to a chloride recovery unit rather than biological treatment because chlorinated ethers can pass through standard secondary treatment. The glycerol route has lower chlorinated hydrocarbon byproduct formation than the propylene route, but it is not chlorine-free; hydrochloric acid is still consumed in the hydrochlorination step, and sodium hydroxide demand remains approximately stoichiometric to produce ECH.

When Allyl Acetate Is Hydrolyzed Before Chlorination, Where Does the Cost Penalty Concentrate?

Allyl acetate is produced by the acetoxylation of propylene in the presence of a supported palladium-based catalyst. The reaction is run at 160–200 °C and 0.5–1.0 MPa with oxygen, and the hot reactor effluent contains acetic acid, water, propylene, oxygen, and allyl acetate. The acetic acid/water separation is the main energy sink. Simple distillation cannot produce dry acetic acid because water and acetic acid form a low relative volatility mixture; the separation is completed by azeotropic or extractive distillation. The recovered acetic acid is recycled to the acetoxylation reactor after concentration to at least 95 wt%. Hot acetic acid combined with trace chloride species is highly corrosive; UNS S31603 stainless steel may be acceptable in dry acetic acid service, but titanium or higher alloys are specified where acid and chloride coexist. The hydrolysis step converts allyl acetate to allyl alcohol and acetic acid over an acid catalyst, and the allyl alcohol is then chlorinated at 0–20 °C to limit exothermic ring-opening and chlorinated ether formation. The chlorination reactor is glass-lined or PTFE-lined, and the cooling system is designed for an instantaneous heat-release scenario because the chlorination of allyl alcohol is strongly exothermic. The cost penalty concentrates in acetic acid recovery, oxygen purity, and allyl acetate/allyl alcohol purification, rather than in the dehydrochlorination section.

The allyl acetate route can achieve high allyl alcohol purity, which reduces the formation of propane-derived chlorinated byproducts. However, the route requires a more complex liquid-phase separation train with multiple distillation columns and a dedicated acetic acid recycle loop. Published side-by-side purification data for allyl acetate-route ECH and glycerol-route ECH at equivalent epoxy-grade product specifications are limited. In practice, allyl acetate-route plants are integrated with acetic acid recovery capacity and often operate at smaller scale than propylene-route plants because the process is more sensitive to energy cost and oxygen handling.

Downstream epoxy resin producers set ECH water below 0.05 wt% and hydrolyzable chloride below 50 mg/kg to avoid side reactions with bisphenol A sodium salt. Liquid epoxy resin produced from propylene-route ECH and glycerol-route ECH with identical purity can meet the epoxy equivalent weight limits of ASTM D1652-11 if low-boiling chlorinated impurities are removed to below 0.1 wt%. Published comparative data for resin color and hydrolyzable chloride at equal formulation levels using allyl acetate-route ECH are limited.