Municipal potable water systems drawing surface water with natural organic matter feed liquid chlorine through vacuum-operated gas chlorinators that reduce cylinder pressure from approximately 0.6 MPa at 20°C to subatmospheric levels before the gas is entrained into a booster water stream. The resulting chlorine-water mixture is injected into raw water, settled water, or filtered water at doses commonly between 0.2 mg/L and 2.0 mg/L free chlorine residual, depending on pH, temperature, contact time, and the CT inactivation table selected for the target pathogen. Upon dissolution, chlorine hydrolyses rapidly to hypochlorous acid and hydrochloric acid; hypochlorous acid then dissociates to hypochlorite ion with a pKa near 7.54 at 25°C. Process control holds the water pH between 6.5 and 7.5 because hypochlorous acid is the more rapid biocide, while lower pH increases corrosion and haloacetic acid formation, and higher pH shifts the equilibrium toward hypochlorite ion and increases trihalomethane formation. Fixed monitoring uses EPA Method 4500-Cl G for free chlorine residual and SM 5710 B or EPA Method 524.2 for trihalomethanes. Liquid chlorine quality for water utility feed is covered by AWWA B301-18; process safety thresholds are 680 kg for 29 CFR 1910.119 and 1,134 kg for 40 CFR Part 68.

When distribution residual is converted to chloramines, ammonia solution is added at a chlorine-to-ammonia-nitrogen weight ratio of 3.0:1 to 5.0:1, and pH is maintained at 8.0–9.0 to suppress dichloramine and trichloramine formation. The chloramination step is applied only after primary disinfection CT requirements have been met with free chlorine, because monochloramine is a weaker virucide and requires contact times often an order of magnitude longer than free chlorine under the same cold-water conditions.

Why Is Moisture Intrusion Below 1 ppm the Operational Fault Line in Direct Ethylene Chlorination?

The ethylene-to-ethylene dichloride step of a balanced vinyl chloride monomer complex is a liquid-phase exothermic reaction operated in a boiling EDC loop. Vaporized liquid chlorine is sparged into a circulating 1,2-dichloroethane stream containing ferric chloride catalyst at 50–70°C and 1.2–1.8 bar. Ethylene is maintained at a molar excess of 1.0:1.0 to 1.05:1.0 over chlorine; this prevents unreacted chlorine from reaching downstream distillation and forming chlorinated byproducts in the vent system. Moisture is held below 1 ppm because water hydrolyses ferric chloride to hydrochloric acid and iron oxides, causing catalyst loss, emulsion formation in the EDC wash train, and pitting of carbon steel. The reactor heat is removed by vaporization of EDC from the boiling loop, and the crude EDC is washed with caustic, dried, and distilled to a purity above 99.5 wt% before thermal cracking.

The purified EDC is cracked in alloy furnace tubes at 480–530°C, with residence time typically 5–30 s, conversion 55–60%, and selectivity to vinyl chloride monomer above 98%. Hydrogen chloride from this cracking step is routed to oxychlorination, where ethylene, HCl, and oxygen react over a copper(II) chloride/alumina catalyst at 220–240°C to produce additional EDC. Vinyl chloride monomer is then polymerised in suspension at 50–65°C to PVC with K-values from 55 to 68 for pipe and profile grades, measured according to ISO 1628-2 and classified under ASTM D1755. Occupational exposure to vinyl chloride monomer in the downstream polymerisation building is limited by 29 CFR 1910.1017, which sets an 1 ppm 8-hour time-weighted average and a 5 ppm 15-minute short-term exposure limit.

For sodium hypochlorite producers operating continuous vacuum chlorination into a caustic recirculation loop, the controlling variables are pH, temperature, and excess alkalinity. Membrane-grade caustic soda is diluted to 19–20 wt% NaOH before contacting vaporized chlorine in a falling-film absorber or packed tower, and the reaction follows the stoichiometry 2 NaOH + Cl₂ → NaOCl + NaCl + H₂O. Heat of reaction is removed by titanium plate heat exchangers to maintain product temperature below 30°C, and the finished liquor is trimmed to leave 0.5–1.0 wt% free caustic soda, preserving pH above 11 and slowing chlorate formation. Without that excess caustic, hypochlorite disproportionates to chlorate and chloride; the rate increases sharply above 30°C, below pH 10, and in the presence of nickel or copper ions. The commercial output is typically 12.5% available chlorine by weight with a specific gravity near 1.16; it is analysed for free alkali, iron, and sodium chlorate according to ANSI/AWWA B300 and must be listed under NSF/ANSI 60 when used in drinking water. Materials of construction downstream of the absorber are limited to CPVC, PVC, titanium, or fluoropolymer-lined steel because stainless steel and nickel-based alloys catalyse oxygen evolution and rapid bleach decomposition. The same liquor is subsequently diluted and packaged as household laundry bleach, swimming pool shock, or industrial biocide at concentrations ranging from 5.25% to 15.0% available chlorine depending on end-use specification and regulatory labelling.

Hydrogen-Chlorine Burner Stoichiometry with Excess Hydrogen Control

Anhydrous HCl furnaces operate with a hydrogen-rich diffusion flame in a graphite combustion chamber. Vaporized liquid chlorine and dried hydrogen are metered separately to the burner at a molar ratio of 1.05:1.0 to 1.10:1.0 H₂:Cl₂, so that free chlorine is suppressed in the product gas and the hydrogen slip remains below the lower explosive limit in downstream vent streams. The flame can exceed 2300°C, and the combustion gas is immediately cooled in a water-tube waste-heat boiler before entering isothermal absorbers fed with deionised water to produce 32–37 wt% hydrochloric acid. Chlorine flow is delivered through a self-contained vaporizer equipped with a pressure-reducing valve and a knock-out pot; moisture ingestion must be prevented because wet chlorine forms hydrochloric acid and corrodes stainless steel vaporizer tubing.

The hydrochloric acid product for steel pickling is typically sold at 18–20 wt% HCl and heated to 60–80°C in acid regeneration baths, while food-grade and pharmaceutical-grade acid is assayed for heavy metals and free chlorine according to compendial methods. Published data for specific semiconductor-grade HCl impurity budgets is limited, but electronics-grade material normally requires moisture below 1.0 ppm and total metals below 1.0 µg/L after further distillation and filtration. The main operational boundary is the prevention of chlorine backflow into the hydrogen header; this is controlled by differential pressure interlocks, flame arrestors, and shutoff valves tied to burner flame intensity.

Within titanium dioxide pigment facilities that operate the chloride route, liquid chlorine is vaporized and metered to a fluidised-bed chlorinator together with fine petroleum coke and rutile ore at 850–1000°C. The principal reaction converts titanium(IV) oxide to titanium tetrachloride while the carbon source consumes liberated oxygen; the chlorine-to-titanium molar ratio is maintained slightly above 2.0:1 to hold excess chlorine in the off-gas and limit unreacted rutile carryover. Residual moisture in the ore or coke is controlled to avoid hydrolysis of TiCl₄ to TiO₂ scale and HCl, which plug condenser tubes and erode refractory linings. Crude TiCl₄ contains metal chloride impurities such as FeCl₃, AlCl₃, VCl₄, and SiCl₄, and is purified by fractional distillation before oxidation at 900–1400°C with preheated oxygen. Aluminium chloride is added as a rutile promoter during oxidation to control crystal structure and pigment durability. The terminal titanium dioxide pigment is classified under ISO 591-1 and used in architectural paints, automotive coatings, and outdoor-grade plastics requiring UV resistance.

When Free-Radical Chlorination of Methane Requires Tight Chlorine-to-Methane Molar Control

Free-radical chlorination of methane produces methyl chloride, methylene chloride, chloroform, and carbon tetrachloride through consecutive substitution reactions, each generating hydrogen chloride as a co-product. Liquid chlorine is vaporized, mixed with dry natural gas methane, and fed to gas-phase chlorination reactors at 400–500°C, where the target product split is controlled by the staged Cl₂:CH₄ molar ratio, residence time, and recycle of underchlorinated intermediates. The mixed chloromethanes are quenched, neutralised, dried, and separated by distillation; the hydrogen chloride off-gas is absorbed in water for sale or captive use. When methylene chloride is the target, the chlorine-to-methane ratio and reactor temperature are set to suppress over-chlorination to chloroform and carbon tetrachloride, but a recycle stream of methyl chloride is often returned to the reactor to improve selectivity. Methyl chloride is sold as a methylating agent and as the precursor to organosilane and silicone production; methylene chloride is used in extraction and, where still permitted, paint stripping; chloroform is routed to fluorochemical intermediates. Solvent-grade material is tested for acid acceptance using ASTM D2106-07, and methylene chloride use in paint strippers in the EU is restricted under REACH Annex XVII. Carbon tetrachloride is controlled by the Montreal Protocol, so modern methyl chlorination trains are configured to minimise its formation. Process equipment downstream of the free-radical reactor is fabricated from acid-resistant alloys or graphite because wet HCl forms at the quench stage, and any free chlorine breakthrough into the absorber can generate hypochlorous acid and impose severe pitting loads.

Propylene Chlorohydrin Route: Caustic Saponification and Waste Brine Handling

Hypochlorous acid addition to propylene proceeds through a two-step sequence: chlorine gas is dissolved in water to generate hypochlorous acid, which then reacts with propylene to a mixture of 1-chloro-2-propanol and 2-chloro-1-propanol. The propylene-to-chlorine molar ratio is held above 1.0:1.0 to minimise 1,2-dichloropropane formation, and the aqueous chlorohydrin solution is then treated with milk of lime or sodium hydroxide at 60–90°C and pH 10–12 to close the epoxide ring. The propylene oxide is flashed off, purified, and used as a raw material for polyether polyols that are characterised by hydroxyl number and acid number under ISO 14900 and related methods. Published process yields for the chlorohydrin route vary with feed purity and saponification conditions; the open literature typically reports propylene oxide yields in the range 85–90% based on propylene. The main operational burden is the calcium chloride-rich waste brine, which carries residual chlorinated byproducts and COD and must be stripped, neutralised, and biologically treated before discharge. The route remains in service where low-cost chlorine and lime are available, but the effluent load and energy consumption of saponification create a narrower compliance envelope under EU industrial emissions directives.