Epichlorohydrin (CAS 106-89-8) is a chlorinated C3 oxirane intermediate with a molecular weight of 92.52 g mol⁻¹, a normal boiling point of 116.1 °C, and a closed-cup flash point below 31 °C. More than 80% of global output is consumed in liquid epoxy resin synthesis, where ECH is condensed with bisphenol A under alkaline conditions to yield DGEBA resins having epoxy equivalent weights between 170 g eq⁻¹ and 190 g eq⁻¹. Secondary outlets include epichlorohydrin elastomers, polyamide wet-strength resins, and synthetic glycerol. Two production routes dominate: the propylene-based allyl chloride chlorohydrin sequence and the glycerol hydrochlorination sequence. The latter has become the focus of process decarbonisation because it replaces naphtha-derived propylene with biodiesel-derived glycerol, reduces elemental chlorine demand, and enables valorisation of sodium chloride brine and organic residues.

Where the Propylene Route Concentrates Carbon Burden

The propylene route is a three-stage sequence. Propylene is chlorinated in the vapour phase at 450–510 °C with a propylene-to-chlorine molar ratio above 3:1 to yield allyl chloride; the reaction is exothermic and requires rapid quench to prevent addition side products such as 1,2,3-trichloropropane. Allyl chloride then reacts with hypochlorous acid generated from chlorine and water, producing an isomeric mixture of dichloropropanols in which 1,3-dichloropropan-2-ol typically accounts for more than 90% of the chlorinated C3 cut. Dehydrochlorination with lime milk at pH 9–11 produces ECH and calcium chloride brine. Carbon burden is concentrated in naphtha-derived propylene from steam cracking, electrolytic chlorine from chlor-alkali cells, and calcium oxide from lime calcination; the final step also generates a high-volume inorganic salt by-product and chlorinated organic residues requiring AOX abatement. Existing allyl chloride units using shell-and-tube quench exchangers have shown that fouling from polymerised C3 residues is influenced by propylene feed grade and phosphorus stabiliser dosage, with inspection intervals shortened when low-grade propylene is processed.

ECH is classified under Regulation (EC) No 1272/2008 as H226, H301+H311+H331, H314, H317, and H350. The oxirane ring is sensitive to hydrolysis, and storage specifications typically require water contents below 100 mg kg⁻¹ to limit 3-chloro-1,2-propanediol formation. Carbon steel is unsuitable for wet ECH service; lined carbon steel or stainless steel is used for storage and transfer. Incompatibilities include ammonia, amines, strong acids, and iron chlorides, which catalyse oligomerisation or ring-opening addition.

Why Is Crude Glycerol Pretreatment a Governing Variable?

Crude glycerol from biodiesel transesterification is heterogeneous. It typically contains 40–80 wt% glycerol, 10–30 wt% methanol, 2–8 wt% inorganic ash consisting largely of sodium or potassium chloride, and variable water and soap content. Direct feeding of this material into a hydrochlorination reactor deactivates the carboxylic acid catalyst, increases fouling in the feed/effluent heat exchangers, and raises the organic chlorine load in the dehydrochlorination brine. Pretreatment therefore begins with acidulation with hydrochloric acid to split soaps at pH 3–5, followed by atmospheric methanol recovery, phase separation, and vacuum distillation in a wiped-film or short-path evaporator. The refined glycerol stream is normally held to 95.0–99.5 wt% glycerol before entering the hydrochlorination loop. Batch-to-batch ash variation is a primary control variable; without online density or conductivity monitoring, salt carryover into the wiped-film evaporator causes rotor blade fouling and unplanned cleaning cycles.

Catalyst selection is not inert. Monocarboxylic acids such as acetic acid are active but partially distilled from the reactor with water, requiring acid recovery in the overhead system. Dicarboxylic acids such as adipic acid are less volatile but can form esters with unreacted glycerol, which accumulate in the heavy ends. The hydrochlorination reaction is run in a continuous stirred-tank cascade with staged hydrogen chloride injection to keep water activity low; excess HCl is absorbed in a graphite block absorption column and recycled to the first reactor. The spent acid stream is neutralised before dehydrochlorination to avoid excessive caustic consumption. Dehydrochlorination control is critical: pH below 9 slows ring closure and leaves residual dichloropropanol in crude ECH, while pH above 12 accelerates ECH hydrolysis to 3-chloro-1,2-propanediol and raises the brine COD. Continuous stirred-tank reactors therefore maintain pH 9.5–10.5 by cascade control of sodium hydroxide dosing based on reactor effluent pH and conductivity.

Epichlorohydrin Process Decarbonisation: Glycerol Route, By-Product Valorisation

In the glycerol-to-ECH sequence, hydrogen chloride adds to glycerol in the liquid phase at 80–120 °C over a sub-stoichiometric carboxylic acid catalyst, producing 1,3-dichloropropan-2-ol and 2,3-dichloropropan-1-ol. Selectivity is sensitive to water concentration and temperature: operation above 120 °C increases acrolein and polyglycerol side products, while excessive water shifts equilibrium toward monochloropropanediols and reduces dichloropropanol yield. Dehydrochlorination is carried out with aqueous sodium hydroxide in a continuous stirred-tank reactor with live steam stripping to remove ECH as it forms, suppressing ring-opening hydrolysis. Overhead ECH-water vapour is condensed, decanted, and distilled; the water azeotrope and vacuum distillation are used to reach polymer-grade purity above 99.9 wt%.

Comparative carbon accounting under ISO 14040:2006 and ISO 14044:2006 indicates that the glycerol route lowers cradle-to-gate global warming potential by approximately 40–60% relative to propylene-based ECH when glycerol is treated as a biodiesel by-product with mass allocation. The reduction narrows when energy allocation, system expansion, or oilseed cultivation burdens are included; published data for specific regional configurations is limited and varies with the indirect land-use change factor assigned to the biodiesel supply chain. The route also reduces elemental chlorine demand by using hydrogen chloride, which can be sourced from waste HCl streams in integrated chemical parks, and displaces naphtha-based propylene with a renewable C3 backbone. Commercially, the glycerol-to-ECH route has been operated as the Solvay Epicerol process and comparable licensed configurations.

By-product valorisation is not limited to brine. The crude glycerol feed itself is a valorised biodiesel by-product; its use as a C3 feedstock avoids the energy-intensive refinery separation of glycerol into waste or combustion. In integrated plants, waste hydrogen chloride from vinyl chloride monomer or chlorinated solvent facilities can be purified and used as the hydrochlorination agent, further reducing the demand for fresh chlorine and the associated chlor-alkali power consumption.

Some glycerol-to-ECH plants retain lime dehydrochlorination to avoid sodium ion input, but this shifts the by-product from NaCl brine to CaCl₂ brine. The calcium chloride stream is more difficult to valorise, and its disposal generally requires evaporation or landfilling. Sodium hydroxide dehydrochlorination is therefore preferred where a membrane cell chlor-alkali unit is available, because the NaCl brine can be polished and recycled. The choice is a site-specific mass-balance decision that determines the net process decarbonisation outcome.

When Sodium Chloride Brine Recycles to Chlor-Alkali

Dehydrochlorination with sodium hydroxide generates a sodium chloride brine containing residual ECH, dichloropropanols, and carboxylic acid catalyst fragments. The brine is not directly compatible with membrane cell chlor-alkali electrolysis. Published membrane supplier specifications typically require total hardness below 50 ppb as CaCO₃, sulfate below 5 g L⁻¹, and total organic carbon below 10 mg L⁻¹; therefore the brine is purified by acid stripping, peroxide or hypochlorite oxidation, activated carbon adsorption, and selective ion exchange. The recovered brine can be returned to a chlor-alkali unit, closing the chlorine loop and improving the mass balance of the glycerol-to-ECH sequence. Where brine recycle is not economically feasible, the NaCl stream is routed to wastewater treatment after organic destruction. Published data for brine recycle rates in glycerol-based ECH plants is limited; the benefit depends on the local electrolysis configuration and the transport distance between the ECH plant and the chlor-alkali consumer.

Under vacuum, the polymer-grade ECH distillation section is operated to keep reboiler metal skin temperatures below 150 °C; the ECH-water heterogeneous azeotrope is condensed and decanted before final drying. Reflux ratios in the light-ends column are often between 1.5 and 3.0, and the bottom product is transferred with 20–50 mg kg⁻¹ of a stabiliser when storage beyond 30 days is required. The reboiler tube-side velocity is maintained above 2 m s⁻¹ to reduce polymer film deposition on heat-transfer surfaces. Quantitative fouling rates for specific ECH distillation stabilisers are not widely published; plant data are generally proprietary.