Epichlorohydrin for Synthetic Glycerol & Glycidol: Selection and Route Options

Epichlorohydrin (ECH; C₃H₅ClO; CAS 106-89-8; boiling point 116.1 °C at 101.3 kPa; density 1.180 g/cm³ at 20 °C; epoxide equivalent mass 92.52 g/mol) functions as a bifunctional alkylating intermediate for synthetic glycerol and glycidol. Selection between direct alkaline hydrolysis and the two-step glycidol route is governed by water content, hydrolyzable chlorine, iron concentration, and the monochloropropanediol precursor profile in the ECH feedstock. Industrial ECH produced via allyl chloride hypochlorination and dehydrochlorination typically carries 50–300 mg/kg hydrolyzable chlorine and 50–200 mg/kg water after fractional distillation. For pharmaceutical-grade synthetic glycerol, ECH purity is specified at not less than 99.5 wt%, water not more than 200 mg/kg, and iron not more than 1 mg/kg. These thresholds reduce downstream salt loading and Lewis-acid-catalysed polyglycerol formation. Feedstock ECH is verified by gas chromatography with flame ionization detection; retention time markers for allyl chloride, 1,3-dichloropropene, and 2,3-dichloropropene are used to reject lots exceeding the total volatile organic impurity limit.

Which ECH Impurity Thresholds Limit Direct Alkaline Hydrolysis to Synthetic Glycerol?

Direct alkaline hydrolysis of ECH to glycerol proceeds through glycidol as the transient intermediate at NaOH:ECH molar ratios from 1.00:1 to 1.20:1. In a continuous stirred-tank cascade with 15–25 m³ glass-lined reactors, the feed ECH is dispersed into an aqueous sodium hydroxide solution at 8–12 wt% NaOH. Reaction temperature is maintained at 85–105 °C and absolute pressure at 0.4–0.8 MPa to keep ECH in the liquid phase. Under these conditions ECH conversion exceeds 99.5%; residual glycidol is hydrolysed to glycerol by maintaining reactor pH between 9.2 and 10.8. Hydrolyzable chlorine in ECH above 100 mg/kg increases the formation of 3-chloro-1,2-propanediol and 2-chloro-1,3-propanediol, which later require wiped-film evaporation at 1.0–2.0 kPa and 150–170 °C to reduce total chlorinated diols below 10 mg/kg in finished glycerol. Iron above 1 mg/kg accelerates polyglycerol ether formation through Lewis-acid-catalysed ring-opening, producing diglycerol and triglycerol oligomers that foul the reboilers of the downstream distillation train.

Crude hydrolysis liquor typically contains 85–90 wt% glycerol, 6–10 wt% water, 1–3 wt% sodium chloride, and 0.5–1.5 wt% unreacted ECH/glycidol before neutralization. Neutralization with hydrochloric acid to pH 6.5–7.5 is followed by triple-effect evaporation. Color is controlled to <10 Pt-Co per ASTM D1209-15, and water content to <0.5 wt% per ASTM E203-16. Chloride in the final synthetic glycerol is held below 10 mg/kg by passing the 80 wt% crude glycerol through a weak-base anion exchange resin bed at 2–4 bed volumes/h.

For ECH-derived glycidol, the ring closure is operated as a low-water alkaline dehydrochlorination using aqueous sodium hydroxide or calcium hydroxide. In a 5–10 m³ baffled reactor with retreat-curve impeller, ECH is added below the liquid surface over 2–4 h at 40–60 °C. The NaOH/ECH molar ratio is maintained at 0.95:1 to 1.05:1, and absolute pressure is reduced to 8–15 kPa during the final reaction phase to remove glycidol from the salt slurry as it forms. The vapour stream is condensed and fractionated in a column with 20–30 theoretical stages; the glycidol cut is collected at 61–64 °C at 2.0 kPa. Residual water in the glycidol cut is 0.3–1.0 wt%, and residual ECH is controlled below 0.1 wt% by a final thin-film stripping step at 60–70 °C and 0.5–1.0 kPa. Sodium chloride content in the feed to the vacuum column is limited to <50 mg/kg to prevent salt crust build-up on structured packing surfaces; wash columns with demineralized water are required every 5–7 days in continuous operation.

Catalyst Selection and pH Boundaries in Glycidol Ring Closure

Ring closure of ECH to glycidol is catalysed by both homogeneous alkali and heterogeneous base resins. Sodium hydroxide at 1.0 molar equivalent yields glycidol with 92–96% selectivity at 50 °C; excess NaOH above 1.05 equivalents promotes glycidol hydrolysis to glycerol and reduces glycidol recovery in the vacuum distillation step. When calcium hydroxide is used, the reaction slurry has a pH near 12.2 and produces calcium chloride as a filterable solid; glycidol selectivity in the organic phase is 88–93%, but the lower alkalinity reduces diglycidol ether formation. Heterogeneous tertiary amine resins operated at 35–45 °C and liquid hourly space velocity 0.5–1.0 h⁻¹ provide glycidol selectivity up to 97%, but published data for long-term resin fouling in ECH ring closure is limited. The operational pH window is narrow: below pH 8.5, ECH conversion falls below 85% within 2 h; above pH 12.5, glycidol polymerization to polyglycerol is accelerated. Dissolved oxygen in stored glycidol is maintained below 0.5 mg/kg with inert gas blanketing to suppress peroxide formation.

When downstream applications require USP/EP-grade glycerol with chloride below 10 mg/kg, sulfate below 20 mg/kg, and diethylene glycol content below 0.1 wt%, direct ECH hydrolysis is selected. The two-step ECH-to-glycidol-to-glycerol route is retained when a glycidol intermediate is needed for epoxy reactive diluents, glycidyl ethers, or pharmaceutical intermediates. Both routes share the same distillation and ion-exchange finishing sequence, but the two-step option requires additional vacuum rectification and has a 3–5% lower overall glycerol yield.

When Direct Hydrolysis Replaces Bio-Glycerol Refining for High-Purity Polyol Supply

At production capacities of 20–50 kt/year, direct ECH hydrolysis trains are constructed with a reactor cascade, neutralization vessel, triple-effect evaporator, salt centrifuge, and vacuum distillation system. The dominant energy load is evaporation of water and salt removal: 7–9 tonnes of 32 wt% NaCl brine are produced per tonne of glycerol from ECH. The salt brine must be purified by membrane cell chlor-alkali electrolysis or disposed under site permits; this is the largest site-selection constraint. In contrast, the glycidol route generates 1.0–1.2 tonnes NaCl per tonne of glycidol, but the salt is removed as a dry filter cake, lowering liquid waste handling. When ECH is selected for synthetic glycerol rather than refined bio-glycerol, the process advantage is the absence of fermentation-derived odorants, fatty acid methyl ester residues, and methanol; the cost penalty is chlorine handling and chloride corrosion. Equipment in contact with ECH and aqueous chloride is specified in glass-lined carbon steel, nickel alloy C-276, or fluoropolymer-lined steel; 316L stainless steel is unsuitable above 50 °C due to chloride pitting. Gaskets are limited to polytetrafluoroethylene or flexible graphite with oxidizing-acid service restrictions.

Vapour-Liquid Equilibrium Boundaries in Crude Glycerol Distillation

Crude glycerol from ECH hydrolysis is distilled in a two-stage high-vacuum column. The first column removes water and residual ECH at 5–10 kPa and overhead 60–80 °C; the second column rectifies glycerol at 0.5–1.0 kPa and bottoms 160–170 °C. Decomposition of glycerol accelerates above 175 °C, so reboiler residence time is limited to less than 10 min using a falling-film or short-tube reboiler. Polyglycerol heavies collect in the reboiler and are purged at 2–5 wt% of the feed. Glycerol purity after this step is 99.5–99.7 wt%. Final pharmaceutical grade requires deodorization with activated carbon and deionization with mixed-bed resins to meet the Ph. Eur. 0496 glycerol monograph and the USP Glycerin monograph. The vacuum system is designed for chloride-containing distillates; carbon steel ejector internals are replaced with titanium or fluoropolymer-coated parts where wet chloride vapours condense. The condensed water phase from the first column is sent to the ECH recovery stripper; recovered ECH is recycled to the hydrolysis cascade unless the hydrolyzable chlorine exceeds 150 mg/kg, in which case it is rerouted to the ECH distillation column.

Closed-loop transfer and automated dosing are required for ECH and glycidol operations. ECH exposure is monitored by colorimetric detector tubes and real-time photoionization detection with alarm setpoints at 0.5 ppm and 2.0 ppm work-area excursion. Glycidol is classified as Carc. 1B under EU CLP and requires enclosed sampling, nitrogen blanketing, and peroxide testing at 7-day intervals during prolonged storage. Bromine-index or epoxide titration data are used to terminate storage when residual epoxide polymerization increases viscosity above 5 mPa·s at 20 °C or when peroxide exceeds 50 mg/kg. Wastewater from ECH hydrolysis contains absorbable organic halides and must be stripped with steam at 0.2 MPa to reduce AOX below 1 mg/L before discharge under site permits.