Flake caustic soda with a minimum sodium hydroxide assay of 98.5 wt% and a typical sodium chloride content of 0.1–0.5 wt% is received in 25 kg bags or supersacks and transferred through bag dump stations equipped with dry-air purges. The flake is hygroscopic; at ambient relative humidity above 60%, surface hydration produces a concentrated liquid film and causes caking, leading to screw feeder torque excursions and mass flow interruptions in loss-in-weight systems. The flake hopper is fitted with vibratory pads and a dry-air purge to prevent rat-holing; a twin-screw feeder with 10:1 length-to-diameter ratio meters flake to the dissolution tank. Dissolution into demineralized water with conductivity below 5 µS/cm is performed in a jacketed 316L stainless steel vessel with an agitator tip speed of 2.5–4.0 m/s. The dissolution enthalpy of sodium hydroxide in water is approximately 44.5 kJ/mol; preparing a 20 wt% NaOH solution from ambient flake raises the batch temperature by 30–45 K, requiring cooling water at ≤303 K before transfer to the dehydrochlorination dosing header. The 20 wt% solution has a viscosity of approximately 4.5 mPa·s at 293 K, which is suitable for metering by diaphragm pumps without elevated-temperature correction. The solution is filtered through a 50 µm duplex strainer to remove insoluble carbonates and iron oxides. Batches with sodium carbonate above 0.8 wt% as Na₂CO₃ produce visible haze and can blind the strainer within 4–8 h; incoming lot analysis per ISO 979:1975 or GB/T 209-2018 is used to route high-carbonate flake to non-critical neutralisation service. The dissolution tank and transfer lines are fabricated from welded 316L stainless steel with crevice-free internal welds, and the tank vent is routed to a caustic mist scrubber to meet workplace exposure limits for sodium hydroxide aerosol.
| Parameter | Required range or value | Test method | Process consequence if out of range |
|---|---|---|---|
| Sodium hydroxide assay | ≥98.5 wt% | ISO 979:1975 | Dosing variability and pH loop instability |
| Sodium carbonate as Na₂CO₃ | ≤0.6 wt% | ISO 3196:1975 | Buffering, CO₂ evolution, scaling |
| Sodium chloride | ≤0.5 wt% | Potentiometric titration with AgNO₃ | Higher brine purge load; salting in stripper |
| Iron | ≤10 ppm | ICP-OES | Oxidative epichlorohydrin byproducts; sensor fouling |
| Water-insoluble matter | ≤0.05 wt% | Filtration | Strainer blinding; pump wear |
At the core of caustic demand in dichloropropanol dehydrochlorination lies the stoichiometric requirement of 1 mol sodium hydroxide per 1 mol dichloropropanol, with the overall reaction C₃H₆Cl₂O + NaOH → C₃H₅ClO + NaCl + H₂O. Industrial feed mixtures contain both 1,3-dichloropropan-2-ol and 2,3-dichloropropan-1-ol; the former undergoes cyclization with NaOH at measurable rates even below 333 K, while the latter exhibits slower dehydrochlorination and a higher tendency to form chlorinated ethers when caustic availability is locally deficient. Caustic utilization efficiency is therefore governed by micromixing rather than simple bulk stoichiometry. In continuous stirred-tank reactors with a nominal residence time of 10–30 min, the molar NaOH-to-dichloropropanol ratio is typically maintained between 1.02 and 1.12, but local pH excursions above 12.5 at the caustic injection point hydrolyse epichlorohydrin to 3-chloro-1,2-propanediol and glycidol. This side pathway consumes up to 5–12% of the fed caustic in poorly baffled reactors. High-intensity mixing with a tip speed of 3.0–4.5 m/s and injection through a sparger ring with 1.5–3.0 mm orifices reduces alkali segregation. The presence of sodium chloride at 10–20 wt% in the reaction medium suppresses organic solubility and increases the apparent viscosity of the brine-organic dispersion. At temperatures above 363 K, the saponification of epichlorohydrin to glycerol becomes kinetically significant, and the caustic demand shifts from stoichiometric dehydrochlorination to hydrolysis. Published data for specific proprietary epichlorohydrin train configurations with flake caustic dosing is limited, but operating experience indicates that caustic conversion to epichlorohydrin above 85% requires feed nozzles maintained within 0.5 pH unit of the set point and chloride-controlled brine recycle.
Because the dehydrochlorination of mixed dichloropropanols is strongly exothermic, the reactor is configured as a vertical baffled vessel with an external circulation loop and a shell-and-tube heat exchanger. The circulation pump is typically a 316L stainless steel centrifugal unit with a flow rate selected to provide 20–30 turnovers per hour. Reaction temperature is controlled at 343–358 K by throttling cooling water on the exchanger. Overcooling below 338 K reduces the dehydrochlorination rate and leaves unconverted dichloropropanol in the stripper feed, while overheating above 363 K increases epichlorohydrin hydrolysis and can cause rapid pressure fluctuations in the overhead line due to volatile byproducts. The reactor is blanketed with nitrogen at 5–20 kPa(g) to exclude oxygen, which otherwise forms chlorinated organic peroxides and accelerates corrosion of carbon steel downstream. Continuous pH measurement in the reactor loop uses high-temperature glass electrodes mounted in retractable holders; the set point is maintained at 9.8–11.2 by cascading the caustic dosing stroke. The caustic feed line from the dissolution tank is heat-traced at 293–303 K to prevent sodium carbonate deposition. A loss-of-flow interlock on the circulation pump closes the caustic feed valve and opens a quench water valve within 5 seconds to terminate the exotherm. Batch-to-batch variance in flake caustic soda sodium carbonate content from 0.2 wt% to 1.2 wt% shifts the neutralisation curve by as much as 0.3 pH units at constant stroke length, so feed-forward correction based on carbonate assay is applied. The circulation loop is also fitted with a static mixer containing 6 helical elements to disperse the incoming caustic into the bulk brine-organic flow before the pH electrode, thereby reducing measurement dead time.
Steam stripping of the reactor effluent at 353–363 K and 15–30 kPa(g) separates epichlorohydrin from the brine-organic matrix. The epichlorohydrin–water overhead stream is condensed and decanted; the epichlorohydrin-rich organic phase is returned as reflux or withdrawn as crude product, while the aqueous phase is recycled to the stripping column. Caustic alkalinity in the stripper feed above 0.5 wt% free NaOH increases the hydrolysis rate of epichlorohydrin in the column sump, producing glycerol and glycidol that remain in the brine phase and raise the chemical oxygen demand of the effluent. Conversely, caustic alkalinity below 0.05 wt% leaves residual dichloropropanol in the stripped brine, causing a volatile halogenated organic load in the wastewater treatment feed. The liquid hourly space velocity in the stripping column is typically limited to 0.8–2.0 h⁻¹ for sieve trays with 5–8 m of packing height; exceeding this range entrains brine droplets into the overhead system and increases chloride content of the crude epichlorohydrin. The overhead receiver is operated at 303–308 K to reduce epichlorohydrin solubility in the aqueous phase and to maintain phase separation between the epichlorohydrin-rich phase with a density of 1.18 g/cm³ and the water phase. Elevated sodium carbonate in the caustic feed results in carbon dioxide evolution in the stripper, which can disrupt tray hydraulics and produce pressure oscillations of 5–15 kPa in the column. Published data for specific epichlorohydrin–water azeotropic composition at atmospheric pressure is limited in the open literature; industrial train control is therefore based on online density and refractive index rather than fixed overhead temperature alone.
Under conditions of local caustic excess and temperatures above 358 K, epichlorohydrin hydrolysis produces 3-chloro-1,2-propanediol, glycidol, and glycerol. These oxygenated species partition into the aqueous phase and increase the total organic carbon of the brine purge. Chlorinated ethers such as bis(3-chloro-2-hydroxypropyl) ether and 1,3-dichloropropan-2-ol-derived oligomers are formed when the dichloropropanol concentration is high relative to caustic availability; these compounds have boiling points above 473 K and are not removed by steam stripping. They accumulate in the bottoms loop and can form a viscous organic layer that reduces heat-transfer coefficients by 20–40% over a 30-day run. The selectivity to epichlorohydrin is therefore monitored by gas chromatography with flame ionisation detection using a 30 m capillary column with 0.25 mm internal diameter and a 0.25 µm film thickness. The analytical method quantifies epichlorohydrin, 1,3-dichloropropan-2-ol, 2,3-dichloropropan-1-ol, glycidol, and glycerol against an internal standard; a typical target is residual dichloropropanol below 0.1 wt% in the stripped brine. Because flake caustic soda contains trace transition metals such as iron at 5–25 ppm, soluble iron can catalyse oxidative coupling of epichlorohydrin in the presence of dissolved oxygen. The nitrogen blanketing described in reactor control is therefore extended to the stripping column and the decanter. Iron precipitation as ferric hydroxide at pH above 10 can also coat pH sensor bulbs and cause measurement drift of 0.2–0.5 pH units within 24 h; sensors are therefore equipped with automatic KCl electrolyte flush and ultrasonic cleaning.
When sodium carbonate in the flake caustic exceeds 0.8 wt% as Na₂CO₃, the stripper bottoms develop a buffered alkalinity that cannot be reduced by simple acid dosing without generating carbon dioxide gas. The carbonate ion reacts with calcium and magnesium in make-up water and forms scale on reboiler tubes; this scale, predominantly calcium carbonate, has a thermal conductivity of 2.2 W/(m·K) compared with 15 W/(m·K) for 316L stainless steel, leading to a reduction in heat-transfer efficiency of 30–50% after 10–14 days of operation. In one production-scale train using demineralized water with hardness below 5 mg/L as CaCO₃, carbonate-related scaling was confined to the caustic dissolution tank and pH electrode housings. In a second train using municipal water with hardness up to 120 mg/L as CaCO₃, reboiler cleaning frequency increased from every 90 days to every 21 days. The pressure drop across the stripping column increased from 8 kPa to 25 kPa as carbonate-derived precipitates accumulated on the distributor trays. The operational boundary for flake caustic soda used in epichlorohydrin trains is therefore a maximum sodium carbonate content of 0.6 wt% and a maximum sodium chloride content of 0.5 wt%, with the chloride limit set by chloride accumulation in the brine purge rather than by reaction stoichiometry. The brine purge from the train typically contains 15–22 wt% sodium chloride and must meet the receiving site’s total organic carbon and adsorbable organic halogen limits. Adsorbable organic halogen values above 10 mg/L are often subject to discharge restrictions under local permits; published data for specific site permit limits is limited.
| Operational parameter | Lower boundary | Upper boundary | Consequence outside boundary |
|---|---|---|---|
| Reactor temperature | 338 K | 358 K | Low conversion below; epichlorohydrin hydrolysis above |
| Reactor pH | 9.8 | 11.2 | Residual dichloropropanol below; epichlorohydrin hydration above |
| NaOH/dichloropropanol molar ratio | 1.02 | 1.12 | Low conversion below; excess alkaline brine above |
| Stripper sump free NaOH | 0.05 wt% | 0.5 wt% | Dichloropropanol carryover below; glycerol formation above |
| Stripping column LHSV | 0.8 h⁻¹ | 2.0 h⁻¹ | Low throughput below; brine entrainment above |
| Flake storage relative humidity | — | 60% | Caking and feeder bridging above |
| Nitrogen-blanketed headspace oxygen | — | 4 vol% | Oxidative polymerisation and flammability risk above |
Materials of construction for flake caustic dehydrochlorination service are selected primarily for chloride stress-corrosion cracking resistance and hot caustic corrosion. Carbon steel is avoided in reactor and stripper internals because the chloride concentration in the brine phase exceeds 10 wt% and the operating temperature exceeds 333 K, conditions under which chloride stress-corrosion cracking can initiate in austenitic stainless steels. 316L stainless steel is generally acceptable for the caustic dissolution tank at temperatures below 333 K, but the reactor circulation piping and heat exchanger tubes are specified as duplex 2205 or Alloy 904L where the pH exceeds 12 and the temperature exceeds 353 K. The selection is consistent with the chloride stress-corrosion cracking guidance of ISO 15156-3:2020. Flake caustic handling equipment—screw conveyors, bucket elevators, and bag dump hoppers—is fabricated from 304L stainless steel with crevice-free welds to avoid chloride attack. Experience with production-scale trains shows that 316L pH electrode bodies fail by crevice corrosion after 6–12 months in the reactor loop; titanium-bodied electrodes with PTFE junctions are used where continuous service exceeds 350 K. Gaskets are specified as PTFE or flexible graphite with stainless steel reinforcement; EPDM is excluded because it swells in the presence of epichlorohydrin and chlorinated organics. Pumps handling brine with suspended solids are specified with silicon carbide mechanical seals and closed impellers with 2–4 mm wear clearances to manage salt crystal abrasion. The nitrogen purge system maintains a dew point below 233 K in the flake storage hopper to prevent caking and to reduce the risk of electrical grounding failures caused by conductive surface moisture.
Because the chloride load in the brine purge from the dehydrochlorination train is directly coupled to the sodium chloride generated by the reaction and to the sodium chloride introduced with flake caustic, caustic-borne salt must be included in purge volume calculations. At a caustic soda feed assay of 98.5 wt% with 0.3 wt% NaCl, the incremental chloride load from caustic-borne salt is small; however, at 1.0 wt% NaCl, the purge volume required to maintain a target brine density increases by 3–7%, depending on the dichloropropanol isomer ratio. The brine is analysed for total organic carbon according to EN 1484:1997 and for adsorbable organic halogen according to EN ISO 9562:2004. Production-scale experience indicates that a brine purge with TOC above 5 mg/L and AOX above 10 mg/L requires additional oxidative treatment before discharge or reuse. Sodium chloride recovery to membrane chlor-alkali electrolysis imposes tighter limits: calcium and magnesium below 20 ppb, suspended solids below 1 mg/L, and total organic carbon below 5 mg/L. Epichlorohydrin itself is classified under the CLP Regulation as Carc. 1B and is subject to REACH exposure scenario restrictions; closed-loop handling and continuous monitoring of the bag dump station ventilation at a capture velocity of 0.5–1.0 m/s are required to maintain the workplace air concentration below the ACGIH TLV-TWA of 0.5 ppm. The process headspace is kept below 4 vol% oxygen, well below the lower explosive limit of epichlorohydrin at 3.8 vol%, to prevent flammable vapour formation in the decanter and storage tanks.
At the back end of the epichlorohydrin train, the stripped brine is cooled to 318–323 K and sent to a clarifier or hydrocyclone for removal of precipitated salts and organic solids. The clarified brine is either discharged under permit or further concentrated by evaporation; evaporative concentrate can reach 25–30 wt% sodium chloride but carries residual glycidol and glycerol that lower the quality of recovered salt. Wet air oxidation or Fenton-like oxidation is required when AOX exceeds 10 mg/L; the selection depends on the site’s discharge permit and the sodium chloride recovery route. Published data for specific flake caustic-based epichlorohydrin train configurations is limited, particularly for long-term accumulation of chlorinated oligomers in the brine recycle loop. The operational boundary for continuous operation is therefore established by weekly monitoring of AOX, TOC, and free caustic in the stripper bottoms, with automatic diversion of the brine purge to emergency hold-up when AOX exceeds 10 mg/L or free NaOH exceeds 0.5 wt% for more than 15 minutes.
Committed to quality, innovation, and reliable chemical solutions for a sustainable future.
