Caustic saponification of propylene chlorohydrin is the final chemical step in the chlorohydrin route to propylene oxide. The feed liquor, generated in a chlorohydrination absorber where propylene, chlorine, and water are contacted at 35–50 °C, contains an aqueous mixture of 1-chloro-2-propanol and 2-chloro-1-propanol at combined concentrations of 3.5–5.5 wt%. When this liquor is mixed with sodium hydroxide solution, the chlorohydrin undergoes intramolecular dehydrochlorination to propylene oxide and sodium chloride according to C₃H₆ClOH + NaOH → C₃H₆O + NaCl + H₂O. The reaction is exothermic and must be carried out in equipment designed for simultaneous volatile product removal because propylene oxide hydrolyses to propylene glycol if allowed to remain in hot alkaline solution. The spent brine leaving the saponification system therefore contains unreacted chlorohydrin, propylene glycol, dichloropropane, and residual caustic, and its handling determines both environmental compliance and process economics.

Why Does the Isomer Ratio of Propylene Chlorohydrin Influence Saponification Selectivity?

The chlorohydrination step produces a distribution of isomers, typically 85–95 mol% 1-chloro-2-propanol and 5–15 mol% 2-chloro-1-propanol depending on reactor pH and the propylene-to-chlorine ratio. In the saponification reactor, hydroxide ion first abstracts the hydroxyl proton to form a chloroalkoxide intermediate. For 1-chloro-2-propanol, the resulting secondary alkoxide attacks the adjacent carbon bearing chlorine to form propylene oxide through a five-membered ring closure at the same carbon backbone. The primary isomer, 2-chloro-1-propanol, forms a primary alkoxide that cyclizes more slowly under identical pH and temperature conditions. This kinetic difference means that at fixed residence time, the primary isomer is more likely to undergo nucleophilic substitution by water to yield 1,2-propanediol or to remain unconverted and report to the brine. The rate constants published for chlorohydrin ring closure in dilute aqueous alkali indicate that the secondary isomer reacts approximately 2–4 times faster than the primary isomer at 60–80 °C. Consequently, control of chlorohydrination selectivity towards the secondary isomer directly reduces glycol formation and improves propylene oxide yield. Saponification reactors are therefore operated with excess caustic and staged pH profiling to keep the primary isomer conversion above 98% without excessive propylene oxide hydrolysis.

Continuous Caustic Addition and pH Control in Staged Saponification Reactors

Industrial saponification sections are configured as two or three continuous stirred-tank reactors in series, each with an external circulation rate of 5–15 turnovers per hour and an impeller power input of 0.5–1.0 kW/m³. The vessels are usually glass-lined carbon steel or nickel alloy-clad because hot alkaline chloride mixtures cause chloride-induced stress corrosion cracking in austenitic stainless steels. pH is maintained at 10.0–11.5 in the first stage and 12.0–13.0 in the final stage by ratio control of 10–20 wt% sodium hydroxide solution. Temperature is held at 60–85 °C in the first reactor and 90–105 °C in the final stripping reactor. The first-stage pH is deliberately lower to limit propylene oxide hydrolysis, while the final-stage higher caustic concentration forces conversion of residual chlorohydrin. Propylene oxide flashes from the reactor headspace and is condensed in a water-cooled exchanger at 10–15 °C. A fraction of the condensate is returned as reflux to the saponification tower to reduce propylene glycol formation. pH measurement uses high-temperature glass electrodes with temperature compensation per IEC 60746-2. Published data for specific reactor configurations is limited, but operating plants generally report a propylene oxide yield of 88–95 mol% from chlorohydrin when the final reactor pH is above 12.0 and the residence time does not exceed 10–20 minutes.

The spent brine from the saponification section leaves the final reactor at 85–100 °C and contains 12–20 wt% sodium chloride, 0.05–0.5 wt% residual sodium hydroxide, 0.1–1.0 wt% sodium carbonate, 2,000–8,000 mg/L chemical oxygen demand, and suspended solids at 50–300 mg/L. The organic fraction comprises propylene glycol, unconverted chlorohydrins, 1,2-dichloropropane, bis(2-chloropropyl) ether, and traces of propylene oxide. Because the brine is hot, alkaline, and high in dissolved solids, it is first cooled by heat exchange with incoming dilute chlorohydrin or cooling water to 40–50 °C before atmospheric storage. Storage tanks for this brine are typically fabricated from rubber-lined carbon steel or fibre-reinforced plastic with a corrosion allowance of 3–6 mm. Vent systems on storage tanks are connected to a caustic scrubber to capture residual propylene oxide and chlorinated volatiles. Without neutralization, this brine would exceed most municipal discharge limits for pH and chloride; therefore, it is routed either to a waste brine treatment unit or to chlor-alkali recycle after polishing.

Brine Polishing Across Dissolved Air Flotation and Ultrafiltration

Primary treatment of saponification brine begins with neutralization using 32% hydrochloric acid to pH 7.0–8.0 in a well-mixed neutralization tank with a residence time of 15–30 minutes. The neutralized brine is then processed in a dissolved air flotation unit where air is dissolved under pressure of 4–6 bar and released at a recycle ratio of 20–40%, producing a hydraulic rise rate of 5–8 m/h. Suspended solids, coagulated organic droplets, and metal hydroxides are skimmed from the surface, and the clarified brine is sent to ultrafiltration. Ultrafiltration membranes made from polyvinylidene fluoride with a nominal molecular weight cut-off of 100–150 kDa are operated at a transmembrane pressure of 1.5–3.0 bar and a crossflow velocity of 3–5 m/s. Permeate turbidity is maintained below 1 NTU, and backwashing with permeate is performed every 30–60 min to restore flux. Filtrate quality is verified by turbidity measurement according to ISO 7027-1:2016 and suspended solids determination according to ASTM D5907-18. The solids-laden backwash stream is thickened and dewatered, while the polished brine advances to ion exchange, carbon adsorption, biological treatment, or thermal recovery.

When Waste Brine Is Recycled to Chlor-Alkali Electrolysis, What Impurity Thresholds Apply?

Recovery of saponification brine as feed to membrane chlor-alkali electrolysis offers a chloride sink, but the impurity specifications for ion-exchange membrane cells are severe. Sodium chloride brine entering a membrane electrolyser must contain less than 20 µg/L combined calcium and magnesium, less than 0.5 mg/L aluminium, and total organic carbon below 1 mg/L to prevent fouling of the carboxylate polymer layer on the membrane. Sulfate must be controlled below 5 g/L and chlorate below 10 g/L to avoid current efficiency loss. The saponification brine contains calcium from raw water and magnesium from process additives, and these must be removed by primary softening with sodium carbonate and subsequent chelating ion-exchange resin polishing. Organics are destroyed by sodium hypochlorite oxidation, activated carbon adsorption, and ultraviolet/hydrogen peroxide advanced oxidation before final ion exchange. If the total organic carbon exceeds 2 mg/L, membrane cell voltage rises due to organic film formation. Therefore, most propylene oxide producers do not directly recycle saponification brine without a dedicated brine purification plant. Alternatively, the brine is sold for de-icing or dust control when local regulations permit.

Steam Stripping of Volatile Chlorinated By-Products from Saponification Liquor

Steam stripping is applied to saponification brine to remove volatile chlorinated hydrocarbons before discharge or biological treatment. A packed column with structured packing height of 8–15 m operates at 101–110 °C and a liquid loading of 15–30 m³/m²·h. The brine is preheated by economizers and enters the top of the column; live steam injected at the bottom at a mass ratio of 0.1–0.3 kg steam/kg brine strips 1,2-dichloropropane and residual chlorohydrins. Overhead vapours are condensed and separated; the organic phase, rich in dichloropropane, is sent to incineration or waste solvent recovery. The stripped brine leaves the column with a dichloropropane concentration below 1 mg/L, while propylene glycol is essentially unchanged because propylene glycol is not volatile under these conditions. The column is fabricated of duplex stainless steel or lined carbon steel, and the reboiler is designed for a fouling factor of 0.00035 m²·K/W. Steam stripping alone does not remove non-volatile ethers such as bis(2-chloropropyl) ether, which remain in the brine and contribute to adsorbable organic halogen.

High-salinity wastewater from propylene oxide saponification is treated biologically only after equalization and careful dilution because the chloride concentration often exceeds the tolerance range of conventional activated sludge. Halotolerant bacterial consortia acclimated to 3–6 wt% sodium chloride are used in a moving-bed biofilm reactor with a carrier fill fraction of 40–60% and dissolved oxygen maintained at 2–4 mg/L. The hydraulic retention time typically ranges from 18–36 h for a chemical oxygen demand removal efficiency of 75–90% at organic loading rates of 0.5–1.5 kg COD/m³·d. After biological polishing, residual recalcitrant organics are removed by granular activated carbon contactors designed for an empty bed contact time of 15–30 minutes. The treated effluent can meet a discharge limit of 100 mg/L chemical oxygen demand and 0.5 mg/L adsorbable organic halogen when the influent is properly pretreated. However, salt loading in the receiving water body often controls the final discharge permit more than the organic parameters.

Caustic Saponification Versus Lime Saponification: Brine Volume and Disposal Implications

The choice of caustic or lime saponification changes the waste brine composition and the available handling routes. With caustic, the stoichiometric chloride salt is sodium chloride, and the brine can be recycled to chlor-alkali production after extensive purification. With lime, the reaction produces calcium chloride and water, and the resulting brine is not compatible with chlor-alkali cells but can be marketed as a liquid calcium chloride product for de-icing, dust suppression, or oilfield completion fluids. Lime saponification generates a larger mass of dissolved solids per tonne of propylene oxide because calcium chloride has a molar mass of 110.98 g/mol compared with 58.44 g/mol for sodium chloride, and the reject brine volume is roughly 1.5–1.8 times that of caustic saponification when compared at equivalent propylene oxide output. The lime route also introduces calcium hydroxide impurities that can precipitate as calcium carbonate in downstream equipment. Caustic saponification is therefore preferred in integrated plants with access to membrane chlor-alkali capacity or where sodium chloride brine can be deep-well injected; lime saponification is more common when the calcium chloride brine has a stable local market.

What Are the Scaling and Fouling Mechanisms in Forced-Circulation Brine Evaporators?

Zero liquid discharge for saponification brine typically uses forced-circulation evaporators followed by a crystallizer. The dominant scaling species are calcium carbonate, magnesium hydroxide, calcium sulfate, and silica. Calcium carbonate scaling is controlled by acidification with hydrochloric acid to pH 4.5–5.5 and degassing of carbon dioxide before evaporation. Magnesium hydroxide and calcium sulfate scale are managed by maintaining a high sulfate-to-calcium ratio and by seeding the evaporator with gypsum slurry at 10–20 g/L suspended solids. Silica scaling becomes limiting when the silica concentration exceeds 150–180 mg/L in the circulating brine; therefore, silica is removed by alum or sodium aluminate coagulation before evaporation. The forced-circulation heater typically operates with a tube-side velocity of 1.8–2.5 m/s and a heat flux of 30–50 kW/m² to minimize boiling on the heat transfer surface. Heat transfer coefficients decline from initial values of 1,200–1,800 W/m²·K to as low as 600–900 W/m²·K when scale layers reach 0.2–0.5 mm. Cleaning uses inhibited hydrochloric acid at 5–10 wt% and 50–60 °C for 4–8 h, followed by alkaline detergent circulation.

The crystallized sodium chloride discharged from the centrifuge contains 3–8 wt% residual moisture and 0.5–2.0 wt% organic impurities, making it unsuitable for direct reuse in chlor-alkali brine saturation without thermal reoxidation. A rotary kiln or fluidized-bed dryer operating at 250–400 °C removes moisture and partially oxidizes organic contaminants. The dried salt is then dissolved in process water and polished through the same chelating ion-exchange system used for recovered brine. The purge stream from the crystallizer, containing concentrated organic residues and non-volatile salts, is disposed of by controlled incineration at 1,100–1,200 °C with flue-gas scrubbing to meet ISO 14001 environmental management requirements. Partitioning of organics between salt cake and purge liquor varies with crystallizer temperature and sodium chloride solubility; lower crystallizer temperatures increase organic inclusion in the salt lattice and require higher wash water rates.