Allyl chloride feedstock purity controls the dynamic stability of the entire epichlorohydrin chain through its effect on the hypochlorous acid partition ratio, the organic-phase density envelope, and the accumulation of aldehyde-derived oligomers in the chlorohydrin reactor. In a continuous bubble-column hypochlorinator fabricated from glass-lined carbon steel and fitted with a 7.5 kW agitator and a removable titanium chlorine sparger, allyl chloride, water, and chlorine are contacted at 45–55 °C and 1.5–2.5 bar g. The reaction mass is held at pH 1.8–2.5 because hypochlorous acid availability in the brine phase is maximized in this range; below 1.5 chlorine hydrolysis produces excessive chloride, and above 3.0 the per-pass conversion of allyl chloride falls by more than 15%. Feedstock acidity, reported as HCl by ASTM D1613, is relatively manageable in this section because the reactor already operates under hydrochloric acid conditions; however, free water above 300 mg/kg in the allyl chloride feed increases the continuous aqueous-phase volume fraction from 55–60 vol% to 68–74 vol%, dilutes hypochlorous acid, and alters gas hold-up in the downcomer. The resulting dichloropropanol-rich organic phase must remain within 1.25–1.32 g/cm³ at 50 °C to permit clean decanting. When the co-feed of 1,2-dichloropropane, density 1.156 g/cm³ at 20 °C, or 1,3-dichloropropene, density 1.217 g/cm³ at 20 °C, exceeds 0.5 wt% of the allyl chloride mass, the organic phase density drifts below 1.24 g/cm³, and the decanter rag layer grows from 3–5 cm to 12–20 cm on a 1.5 m-diameter disengagement vessel. Acrolein at 50–150 mg/kg in technical-grade feedstock accelerates the growth of a tacky aldehyde oligomer film on the polypropylene filter elements installed before the neutralizer. Pressure drop across those elements rises from 10 kPa to 25–40 kPa within 72 h, and filter life falls from 14 days to 3–5 days. Published data for this specific filter configuration is limited, but the observed pressure-drop trend is consistent with acrolein aldol condensation followed by acid-catalyzed Michael addition to form cross-linked polyacrolein residues. The chlorine-to-allyl chloride molar ratio is maintained at 0.95:1 to 1.05:1; excess chlorine is removed in the off-gas caustic scrubber whose oxidation-reduction potential is held at +600 mV to +700 mV versus silver/silver chloride. Allyl chloride with elevated 1,3-dichloropropene content, above 0.3 wt%, also increases the chlorine demand for allylic substitution and shifts the ratio of 1,3-dichloropropan-2-ol to 2,3-dichloropropan-1-ol by 0.04–0.08 mol/mol, which subsequently changes the viscosity of the crude chlorohydrin mixture and alters the saponifier feed pump discharge pressure from 2.2 bar g to 2.8–3.2 bar g.

In the calcium hydroxide saponification loop, the neutralized dichloropropanol stream is preheated in a plate-and-frame exchanger to 55–65 °C before entering the stirred saponifier where effluent pH is controlled at 9.0–9.5 by lime slurry addition. Water-soluble impurities inherited from impure allyl chloride, particularly acrolein-derived 3-chloropropionaldehyde and its aldol dimers, consume calcium hydroxide at 0.03–0.05 mol per mol of impurity and increase the saponifier solids loading. A technical-grade allyl chloride campaign with acrolein at 100 mg/kg can increase Ca(OH)₂ consumption from 0.35 kg/kg dichloropropanol to 0.42–0.48 kg/kg dichloropropanol while simultaneously shifting the calcium chloride brine product concentration from 42 wt% to 46–48 wt%. The higher salt concentration raises the boiling point of the saponifier bottoms and requires the steam stripping column reboiler temperature to be raised from 112 °C to 118–122 °C, which approaches the thermal degradation threshold for epichlorohydrin hydrolysis to glycerol chlorohydrin. The coarse fraction of the saponifier underflow, composed primarily of excess calcium hydroxide and calcium carbonate, exhibits a settling velocity below 0.4 m/h when the feed contains unreacted 1,3-dichloropropene above 0.2 wt%, causing periodic carry-over of calcium chloride fines into the stripped crude epichlorohydrin. Washing of the crude organic phase with demineralized water at 40 °C reduces calcium ion carry-over to <5 mg/kg, but only when the organic phase density remains above 1.16 g/cm³; lower density due to light chloropropanes impairs the water-wash phase split and raises sodium ion content in the final epichlorohydrin to ≥2 mg/kg, which is outside the specification of <1 mg/kg for epoxy-grade product. The vacuum stripping column, operated at 180–220 mbar top pressure, is particularly sensitive to 1,2-dichloropropane because its boiling point of 96.4 °C is close enough to the water-ECH azeotrope at 88–89 °C at atmospheric pressure to create an overhead profile disturbance when feed concentration exceeds 0.4 wt%. Operators compensate by increasing the reflux ratio from 0.8 to 1.2–1.5, which raises steam consumption per tonne of ECH by 0.3–0.5 t/t and reduces column capacity by 10–15%.

What Limits the Recovery of Epichlorohydrin in the Steam Stripping Column?

Steam stripping of the saponifier effluent is conducted in a packed column with structured packing and a liquid distributor designed for 25 m³/h feed at 0.5 wt% epichlorohydrin. The column is operated at 180–220 mbar top pressure and 65–80 °C top temperature. Volatile chloropropane impurities from allyl chloride feedstock, primarily 1,2-dichloropropane and 1,3-dichloropropene, accumulate in the overhead decanter and create a separate light phase that is recycled to the chlorohydrin reactor. When the light phase recycle exceeds 8 vol% of the total reflux flow, the liquid-liquid interface in the overhead decanter moves downward, and aqueous phase carry-over into the vacuum pump increases. The liquid ring vacuum pump, specified with 250 m³/h capacity at 150 mbar, then experiences cavitation and seal-water temperature rises from 25 °C to 40 °C over 90 min. Recovery of epichlorohydrin in the stripped crude drops from 98.5% to 96.0–97.2% when the feed contains 0.4–0.8 wt% total light halocarbons. The overhead condenser, a shell-and-tube unit with 18 m² surface area, exhibits a rapid fouling rate when acrolein-derived polymers deposit on the cooling water side outlet weir; outlet temperature rises from 35 °C to 48 °C and the cooling water control valve opens from 45% to 85% position. Under these conditions, the condenser duty falls from 1.8 MW to 1.3–1.5 MW, and the column pressure controller must increase the vacuum pump speed to maintain top pressure. The resulting pressure fluctuations of ±10 mbar disturb the bottom level control and produce epichlorohydrin product with fluctuating water content between 0.05 wt% and 0.25 wt%, which fails the ≤0.10 wt% specification for direct sale. A side-draw phase boot is installed on the overhead decanter to remove the light phase continuously; its level setpoint is 35% of boot height, and its discharge rate is limited to 2.0 L/min to avoid emulsions. When 1,2-dichloropropane exceeds 0.4 wt% in the crude feed, the boot discharge rate must be increased to 3.5 L/min, and the light phase contains 12–18 wt% epichlorohydrin, which is recovered by recycle but at the cost of additional steam and reflux.

AOX loading to the biotreater increases when allyl chloride feedstocks contain elevated unsaturated chlorinated hydrocarbons, because their saponification products are more polar chlorinated ethers and aldehydes that partition into the wastewater phase rather than into the crude epichlorohydrin oil. The activated sludge basin, designed for 250 m³/h combined wastewater at 8 g/L mixed liquor volatile suspended solids, can maintain a dissolved organic carbon removal efficiency of 95% only when the influent adsorbable organic halogen remains below 2.0 mg/L, measured by EN ISO 9562:2004. A rise in 1,3-dichloropropene feed content from 0.2 wt% to 0.8 wt% increases the AOX concentration in the saponifier wastewater from 120 mg/L to 340–450 mg/L, and after dilution the biotreater influent may exceed 3.5 mg/L. The corresponding sludge retention time must be increased from 12 days to 18–20 days to maintain nitrification, but this is not always possible within the existing basin volume of 4500 m³. Foaming events occur when the influent AOX exceeds 3.0 mg/L, and the surface foam thickness reaches 0.5–1.0 m within 6 h if the anti-foam dosing pump is limited to 5 L/h of a 30 wt% silicone emulsion. The final effluent chemical oxygen demand then increases from 60 mg/L to 120–180 mg/L, exceeding the local discharge limit of 100 mg/L. Published data for this specific wastewater configuration is limited, but the trend is consistent with AOX-mediated inhibition of heterotrophic bacteria.

Feedstock Quality Assurance and In-Line Process Analytical Technology

Crude allyl chloride offloading is directed to a raw material settling tank, and the feed to the chlorohydrin reactor is sampled through a fast-loop analyzer skid that includes a gas chromatograph with a flame ionization detector, a Karl Fischer titrator, and an inductively coupled plasma optical emission spectrometer. The gas chromatograph uses a 60 m capillary column with a 0.32 mm internal diameter and a 1.0 µm bonded polyethylene glycol stationary phase, and the method quantifies acrolein, 1,2-dichloropropane, and 1,3-dichloropropene down to 0.1 mg/kg. Water is determined by ASTM E203-16 and acidity by ASTM D1613; halide content in the nonaqueous phase is cross-checked by ASTM D2988-96. The analyzer loop is designed for a sample update time of 8 min, but the fast loop flush period is 15 min to avoid cross-contamination between batches. When the measured acrolein concentration exceeds 30 mg/kg, the distributed control system automatically reduces the allyl chloride feed rate from 8.0 m³/h to 6.0 m³/h and increases the decanter skimming cycle from every 4 h to every 2 h. This action keeps the chlorohydrin reactor space velocity within 0.4–0.6 h⁻¹, below the knockout threshold at which the rag layer becomes uncontrollable. The cost of such analyser-based feed rate reduction is a drop in ECH production from 12.5 t/h to 9.0–9.5 t/h, but it prevents a forced shutdown that typically lasts 48–72 h for filter replacement and decanter cleaning. Calibration of the GC with traceable reference mixtures is performed every 7 days, and the Karl Fischer titrator is validated against ISO 760:1978 using standard water-in-methanol mixtures. The sampling system uses Hastelloy C-276 tubing and a 0.5 µm sintered filter; iron and nickel shedding from corrosion is monitored by inductively coupled plasma optical emission spectrometry at 5 mg/kg detection limits.

Unloading and tank-farm temperature control are often overlooked but directly affect feedstock purity in the epichlorohydrin train because allyl chloride undergoes slow hydrolysis in the presence of free water and iron chloride. The storage tank is maintained at 5–15 °C to reduce the hydrolysis rate, and the tank is padded with nitrogen containing ≤5 ppm oxygen to minimize aldehyde formation. Water content in the tank is monitored by a capacitance probe calibrated for 0–500 mg/kg water; the lower detection limit is 50 mg/kg, and the alarm is set at 300 mg/kg. A layer of bromine-treated activated alumina, 4–8 mesh, installed in the transfer line removes residual water, but its capacity is limited to 0.1 kg water per kg adsorbent, and spent alumina must be replaced after 30 days of continuous service. The pressure drop across the adsorbent bed increases from 0.2 bar to 1.0 bar as the bed saturates, and the bed is equipped with a differential pressure transmitter with a high-high alarm at 1.2 bar. Iron chloride, which may be present at 5–20 mg/kg in technical-grade allyl chloride, accelerates the formation of a brown residue that settles in the tank boot and is drained every 48 h. The use of technical-grade material with iron above 15 mg/kg raises the saponifier Fe content to 2–5 mg/kg in the crude epichlorohydrin, which can increase the rate of oxidative colour formation in the final product and push the platinum-cobalt colour above 10 APHA, the typical epoxy-grade limit. Pre-drying of allyl chloride is required when the relative humidity during tank breathing exceeds 60%, and the use of a chilled water condenser on the tank vent is recommended to reduce moisture ingress. When technical-grade material is held for more than 30 days, a weekly purge of the tank boot is insufficient to maintain the water specification, and a distillation or adsorption step may be required to restore polymer-grade purity.

When Technical-Grade Allyl Chloride Is Substituted for Polymer Grade During a Campaign Transition

A campaign transition from polymer-grade to technical-grade allyl chloride is typically planned over 24–48 h to avoid shocking the pH control loop of the chlorohydrin reactor and the saponifier. During the transition, the feed blend is adjusted from 0% technical-grade material to 100% technical-grade material in 25% increments every 4–6 h. The saponifier alkali-to-dichloropropanol ratio is increased from 0.35 kg/kg to 0.42–0.48 kg/kg as the technical-grade portion rises, and the steam stripping column reflux ratio is raised from 0.8 to 1.2–1.5. The chlorohydrin reactor effluent is analysed every 30 min for density and for the mass ratio of 1,3-dichloropropan-2-ol to 2,3-dichloropropan-1-ol; this ratio shifts from 1.10:1 to 0.95:1 when 1,3-dichloropropene in the feed exceeds 0.3 wt%, and the saponifier residence time must be increased from 45 min to 60 min to maintain a dichloropropanol conversion above 99.0%. The vacuum column overhead light phase boot level controller is placed in manual mode during the transition because the interface can move abruptly; the level target is held at 35% and the discharge rate is manually set at 3.0 L/min. Operators monitor the liquid ring vacuum pump seal-water temperature, which should remain below 35 °C; a rise to 42 °C indicates excessive light-phase carry-over and requires immediate reduction of the technical-grade feed fraction to 50%. The transition is aborted if the decanter rag layer thickness exceeds 15 cm on the 1.5 m-diameter vessel or if the saponifier filtrate turbidity exceeds 50 NTU. These limits define the operational envelope for safe substitution without a full feedstock changeover shutdown.

Acrolein Accumulation Shifts the Chlorohydrin Decanter Rag-Layer Cycle

Acrolein-derived oligomers are the most refractory contributors to decanter instability because their surface activity stabilizes a persistent rag layer at the chlorohydrin reactor phase boundary. The decanter is maintained at 50±2 °C; above 55 °C, oligomerization accelerates rapidly, and below 45 °C the viscosity of the rag layer rises above 150 mPa·s, preventing adequate skimming. When the acrolein concentration in the feed reaches 100 mg/kg, the rag layer becomes a structured emulsion with a solids loading of 3–5 wt% and a shear-thinning flow index of 0.45–0.55, measured on a rotational rheometer at 50 °C. The decanter skimming pump, a progressive cavity unit rated for 2.0 m³/h at 5 bar, is then unable to remove the rag layer without entraining more than 20 vol% organic phase. The operator response is to reduce the decanter feed temperature setpoint to 48 °C and to inject demineralized water at 0.5 wt% of the feed rate to increase the aqueous-phase gravity separation driving force. This action restores the rag layer to 8–10 cm within 4 h, but it also raises the water load to the downstream neutralizer and reduces the saponifier feed dichloropropanol concentration by 0.5–1.0 wt%. The maximum tolerable acrolein concentration in the chlorohydrin feed is therefore 30 mg/kg for continuous operation without modifying the decanter geometry or installing a dedicated rag-layer centrifuge. This constraint defines the upper limit for acrolein in allyl chloride when the existing decanter is designed for a liquid-liquid dispersion separation time of 12 min.