Epichlorohydrin Alternatives & Bio-Based Glycerol Route Trade-offs are evaluated through feedstock chloride efficiency, chlorohydrin selectivity, and downstream epoxy resin equivalency. The conventional propylene-based route proceeds via allyl chloride generation at 480-510 °C, hypochlorous acid addition to dichloropropanols, and alkaline dehydrochlorination with Ca(OH)₂ or NaOH. The bio-based glycerol route replaces propylene with glycerol recovered from oleochemical transesterification or refined glycerin streams; the primary trade-off shifts from high-temperature propylene handling to aqueous chloride management, brine volume, and impurity-driven selectivity loss. Under Regulation (EC) No 1272/2008, ECH is classified as a carcinogen category 1B; this classification drives closed-loop handling in both propylene and glycerol routes rather than selective advantage for one feedstock.

What Process Variables Govern Selectivity Toward 1,3-Dichloropropanol in Glycerol Hydrochlorination?

In the bio-based glycerol route, glycerol is hydrochlorinated with gaseous HCl in the presence of a carboxylic acid catalyst to form a mixture of 1,3-dichloropropan-2-ol and 2,3-dichloropropan-1-ol, with the former preferred for ECH formation. Selectivity to 1,3-dichloropropanol is governed by HCl partial pressure, water activity, and catalyst acidity. Published laboratory kinetic studies report 1,3-isomer selectivity between 80% and 95% for refined glycerol at HCl-to-glycerol molar ratios of 2.5:1 to 4.0:1 and temperatures of 105-130 °C; selectivity falls toward the lower bound as water accumulates in the reactor because protonated glycerol intermediates shift toward mono-chloropropanediol derivatives and oligomeric ethers.

Water removal is central to achieving high conversion because the reaction generates 2 mol water per mol glycerol converted to dichloropropanol. Continuous trains use azeotropic distillation with a chlorinated solvent or vacuum-assisted stripping at 60-100 kPa to maintain reactor water below 5-8 wt%. Above this water concentration, the solubility of HCl in the liquid phase decreases and the equilibrium concentration of mono-chloropropanediol remains high; the result is a lower throughput in the dehydrochlorination section and higher steam demand in the ECH stripper.

Production-scale continuous trains typically feed gaseous HCl through a sparger into a stirred or bubble-column reactor with an external circulation loop and a graphite block cooler. The reaction is exothermic; heat removal capacity is often the bottleneck rather than intrinsic kinetics, particularly when crude glycerol contains methanol that forms methyl chloride as an additional vapour-phase component. At HCl partial pressures above 3 bar, vent recovery becomes necessary to avoid exceeding halogenated volatile organic compound limits; at below 1 bar, conversion per pass drops below 60%, requiring larger recycle volumes and additional stripping steam. At temperatures above 140 °C, glycerol dehydration to acrolein becomes significant, consuming glycerol and generating unsaturated aldehydes that condense into fouling precursors on heat transfer surfaces. This imposes a narrow processing window; the practical upper bound is 130 °C for bubble-column operation with 2-8 wt% acetic acid catalyst, while the lower bound is set by reaction rate and HCl solubility at 95-100 °C.

Comparative life cycle assessments conducted under ISO 14040:2006 and ISO 14044:2006 generally assign the bio-based glycerol route a lower fossil depletion and global warming impact than the propylene route when glycerol is credited as a by-product of biodiesel, but the result is sensitive to allocation method, crude glycerol purification, and HCl recovery energy. A parallel alternative, direct epoxidation of allyl chloride with hydrogen peroxide over titanium silicalite TS-1, bypasses the chlorohydrin saponification brine but introduces a peroxide cost boundary and safety constraints at H₂O₂ concentrations above 50 wt%; published data for industrial-scale ECH trains using this route is limited.

Corrosion, Chloride Management, and Equipment Boundaries in Continuous ECH Trains

Hot aqueous HCl service dictates equipment selection: graphite blocks, tantalum heat exchange surfaces, and acid-resistant fluoropolymer-lined vessels are used where temperatures exceed 80 °C and chloride concentrations exceed 15 wt%. Carbon steel is unacceptable in the hydrochlorination and dehydrochlorination sections because pitting rates exceed 0.5 mm/year in published failure analyses. Evaporators and stripping columns handling dichloropropanol-water mixtures are prone to fouling when crude glycerol contains residual sodium chloride or sodium sulfate; salt deposits nucleate on reboiler tube surfaces and reduce the overall heat transfer coefficient by 20-30% over 200-400 h, requiring caustic washing cycles.

Dehydrochlorination of dichloropropanols with NaOH produces sodium chloride brine at 18-22 wt% NaCl, while lime-based saponification produces calcium chloride brine at 10-12 wt% CaCl₂. Brine concentration by evaporation or electrodialysis adds approximately 1.5-2.5 GJ thermal-equivalent energy per tonne ECH, depending on the required salt concentration and plant steam integration. Homogeneous carboxylic acids exit the reactor as sodium salts after dehydrochlorination; recovering them from brine by acidulation with HCl returns organic acids to the hydrochlorination loop but adds a liquid-liquid separation step using a coalescer with fluoropolymer internals. In campaigns where brine sale or disposal is permitted, unrecovered carboxylic acids increase chemical oxygen demand of the brine above 5000 mg/L, which may require oxidation or biological treatment before discharge under local trade effluent limits.

Liquid epoxy resin producers convert ECH with bisphenol A under alkaline conditions. Epoxy equivalent weight of the resulting diglycidyl ether of bisphenol A is measured by ASTM D1652-11e1 or ISO 3001:1999; elevated hydrolyzable chloride from incomplete ECH dehydrochlorination above 0.05 wt% shifts the epoxide equivalent weight and raises resin viscosity measured by ASTM D445. Bio-based ECH with residual 2,3-dichloropropanol above 0.1 wt% contributes to resin colour values above 50 Pt-Co as determined by ASTM D1209-05(2019). Vacuum distillation of ECH at 20-40 mbar overhead pressure is therefore retained in most production trains, even when crude glycerol feedstock economics favour a single-pass distillation.

When Crude Glycerol Replaces Refined Glycerol in Hydrochlorination

Crude glycerol from biodiesel transesterification contains methanol, sodium methylate catalyst residues, fatty acid methyl esters, soaps, and non-glycerol organic matter. Before hydrochlorination, methanol must be stripped to below 0.05 wt% because methanol reacts with HCl to form methyl chloride; methyl chloride increases volatile chlorinated organic loading in the vent system and consumes chloride without producing ECH. Sodium salts must be reduced to below 0.1 wt% because they deposit on reactor internals and poison acidic catalysts by ion exchange; sulfated ash above 2 wt% contributes to reboiler fouling and requires periodic alkaline cleaning cycles.

Batch-fed pilot campaigns comparing refined glycerol and crude glycerol at the same HCl-to-glycerol molar ratio recorded a 1,3-DCP selectivity loss of 8-15 percentage points for crude glycerol and increased tar formation in the distillation column. The reduction is attributed to soap residues that form stable emulsions in the phase separator and to non-glycerol organic matter that undergoes chlorination and condensation reactions. Published data for continuous crude glycerol campaigns longer than 1000 h is limited; available technical bulletins suggest that a wiped-film evaporator ahead of the dehydrochlorination reactor extends run length and removes high-boiling chlorinated ethers before saponification.

Chlorinated by-products and aqueous acid streams from the bio-based glycerol route should not be neutralized with amine-based additives before the ECH stripping column, because amines react with ECH via ring-opening addition and form chlorohydroxy amines that precipitate in overhead condensers. Pre-drying of crude glycerol to <0.5 wt% water is recommended at storage relative humidity above 60%; otherwise HCl absorption in free water reduces the effective HCl-to-glycerol ratio. Commercial ECH entering epoxy resin synthesis is normally specified at ≥99.5 wt% purity by gas chromatography, water content below 0.05 wt% (ASTM E203), and Pt-Co colour below 15 (ASTM D1209). The process boundary for economic feasibility is generally set by the combined cost of brine concentration, HCl recovery as azeotropic hydrochloric acid, and crude glycerol purification; these trade-offs determine whether a bio-based ECH train reaches parity with the propylene route within a given regulatory and feedstock-pricing environment.