Industrial production of monochloroacetic acid (MCA) through continuous liquid-phase chlorination of acetic acid is governed by a kinetic selectivity envelope that depends on the steady-state concentrations of acetyl chloride and sulfur chlorides. The reaction mass typically consists of glacial acetic acid, dissolved chlorine, acetic anhydride (Ac₂O), and elemental sulfur, with anhydrous hydrogen chloride removed as a gas. The molar ratio of acetic anhydride to sulfur controls the relative concentrations of acetyl chloride, chloroacetyl chloride, sulfur monochloride, and sulfur dichloride, all of which participate in the chlorination sequence. Selectivity to MCA, expressed as moles of MCA formed per mole of acetic acid converted, is sensitive to this ratio because the second chlorination of MCA to dichloroacetic acid (DCA) competes with the first chlorination. In continuous stirred-tank reactors, the ratio is not uniform throughout the vessel: acetic anhydride hydrolyses at the liquid-vapour interface, sulfur chlorides accumulate near the chlorine sparger, and the local stoichiometry can deviate from the bulk charge analysis. The resulting product distribution is therefore determined by both intrinsic kinetics and the local mass transfer rates of chlorine and HCl.

The selectivity coefficient S = k₁[CH₃COOH]/(k₂[MCA]) provides a convenient dimensionless parameter for process control. Because the concentration of unreacted acetic acid remains high in a once-through CSTR at 85–95% conversion, S remains above unity even when k₂/k₁ is as high as 0.3. Published industrial data for acetic acid chlorination indicate that MCA selectivity in the range of 90–95 mol% is achievable when the reaction temperature is held between 95°C and 110°C and the liquid residence time is maintained between 4 h and 8 h. The ratio of acetic anhydride to sulfur in commercial chlorinators is typically reported to lie between 2:1 and 10:1 on a molar basis, although published data for specific ratio optimization is limited and often proprietary. The lower boundary is set by sulfur chloride phase formation and chlorine absorption collapse, while the upper boundary is set by progressive DCA formation caused by insufficient inhibition of the second chlorination.

How Does the Ac₂O:S Ratio Alter the Steady-State Concentration of Acetyl Chloride?

Acetic anhydride reacts rapidly with the hydrogen chloride generated during chlorination to form acetyl chloride and acetic acid. This reaction is not a side reaction but an integral part of the catalytic cycle, because acetyl chloride enolizes more readily than acetic acid under the acidic conditions of the chlorinator. Chlorine then attacks the enol form of acetyl chloride to produce chloroacetyl chloride, which exchanges with acetic acid to yield MCA and regenerate acetyl chloride. The steady-state concentration of acetyl chloride therefore depends on the feed ratio of acetic anhydride to acetic acid, the rate of HCl removal, and the water concentration in the recycle stream. When the Ac₂O:S molar ratio is raised at fixed sulfur loading, the acetyl chloride inventory increases. This accelerates the first chlorination, but it also accelerates the chlorination of chloroacetyl chloride to dichloroacetyl chloride, because chloroacetyl chloride is an intermediate in the same catalytic cycle. Sulfur, by contrast, buffers the active chlorine concentration by forming sulfur chlorides. Elemental sulfur is converted to sulfur monochloride and sulfur dichloride in the presence of chlorine, and these species are weaker chlorinating agents than molecular chlorine toward MCA. The net effect is that sulfur suppresses the second chlorination while maintaining a sufficient chlorine transfer rate for the first chlorination.

At an Ac₂O:S molar ratio below about 2:1, the sulfur chloride concentration becomes sufficient to form a separate dense liquid phase. The resulting two-phase dispersion reduces gas-liquid mass transfer and creates stagnant regions in the CSTR. At a ratio above about 10:1, the inhibitory effect of sulfur is diluted, and the selectivity loss to DCA becomes measurable. The transition between these regimes is not sharp because the solubility of sulfur chlorides in acetic acid is a function of temperature, water content, and the presence of acetyl chloride. Plant-scale retention samples show that the sulfur chloride phase can persist for several hours after a ratio excursion, making the system slow to recover after a feed interruption.

The rate constant ratio k₂/k₁ in acetic acid chlorination is a function of the leaving-group ability of the α-substituent and the polarity of the transition state. MCA is less reactive than acetic acid because the electron-withdrawing chlorine atom destabilizes the enol intermediate and increases the activation energy for the second chlorination. However, the presence of acetyl chloride lowers the energy barrier for both chlorinations by providing a more readily enolizable substrate. The selectivity loss observed at high Ac₂O:S ratio is therefore attributed to the increased concentration of chloroacetyl chloride, which can undergo exchange with MCA to form DCA. Sulfur-containing species preferentially react with chloroacetyl chloride or with HCl to regenerate acetyl chloride, thereby reducing the steady-state concentration of the over-chlorination precursor. The exact speciation of sulfur in the reaction mixture is complex; sulfur monochloride, sulfur dichloride, and polysulfides have all been identified in plant samples. Published data for the rate constants of individual sulfur species in this system is limited, and the practical control of the Ac₂O:S ratio remains based on selectivity feedback from on-line gas chromatography or potentiometric titration.

Byproduct formation in the chlorination section includes DCA, trichloroacetic acid (TCA), acetyl chloride, chloroacetyl chloride, and minor amounts of sulfur-containing esters. The DCA concentration in the crude reaction product typically ranges from 3 wt% to 8 wt% depending on the Ac₂O:S ratio and the temperature profile. TCA formation is usually below 1 wt% in continuous processes because the reaction mass is withdrawn before the concentration of MCA exceeds the kinetic threshold for the third chlorination. The separation of these components is achieved by fractional crystallization of MCA or by distillation under reduced pressure. The crystallization route relies on the large melting point difference between MCA (61–63°C) and DCA (10–12°C), and the mother liquor is recycled to the chlorinator after removal of heavy ends. Distillation is less common because MCA is corrosive and thermally sensitive, but wiped-film evaporators with tantalum internals are used in some plants.

Continuous Stirred-Tank Chlorinator Mass Balance and Heat Transfer Constraints

The selectivity to MCA in a continuous stirred-tank chlorinator is governed by the Damköhler number for chlorine absorption relative to the liquid-phase reaction kinetics. Chlorine is sparged into the vessel through a dip pipe or a gas distributor, and the rate of absorption is proportional to the interfacial area, the partial pressure of chlorine, and the liquid-phase mass transfer coefficient. In a glass-lined CSTR with a radial turbine agitator operating at tip speeds of 2.5–4.0 m/s, gas hold-up is typically 5–12% by volume. The exothermic heat of chlorination is removed through external circulation loops and tantalum or glass-lined heat exchangers. Because the reaction enthalpy for the first chlorination is approximately −120 kJ/mol to −140 kJ/mol, the temperature rise across the heat exchanger must be limited to prevent local hot spots that promote DCA formation. The liquid residence time is maintained by the feed rate of acetic acid and the withdrawal rate of the crude reaction product. Short residence times reduce conversion but can improve selectivity by limiting the contact time of MCA with chlorine. Long residence times increase conversion but allow the second chlorination to proceed. In practice, CSTR cascades of two or three vessels are used to decouple conversion from selectivity. The Ac₂O:S ratio in the first vessel is set at the lower end of the operating window to maximize selectivity while chlorine demand is high, and the ratio in subsequent vessels is adjusted upward to maintain reaction rate as acetic acid concentration falls.

The sparger design and chlorine dispersion influence the local Ac₂O:S stoichiometry because sulfur chlorides have higher density and viscosity than the bulk liquid. If the agitator tip speed falls below 2.5 m/s, sulfur chloride-rich droplets segregate near the bottom and reduce effective sulfur concentration in the reaction zone. This condition mimics a high Ac₂O:S ratio locally, even though bulk analysis shows a ratio within the target range. Conversely, if the sparger is located close to the liquid surface, chlorine gas bypasses the reaction mixture and reacts with acetyl chloride in the vapour space, leading to chloroacetyl chloride accumulation and DCA formation in the downstream condenser. Plant-scale measurements of agitator power draw and gas hold-up are therefore used to infer the dispersion quality of sulfur chlorides. Published data for this specific configuration is limited, but the phenomenon is qualitatively well documented in industrial process safety reviews.

Quantification of MCA, DCA, and TCA in the crude reaction mixture is performed by gas chromatography after derivatization with methanol-boron trifluoride or by high-performance liquid chromatography with a reversed-phase C18 column and UV detection at 210 nm. The anhydride content is determined by titration with aniline in pyridine, while free water is measured by volumetric Karl Fischer titration according to ASTM E203-24. Total acidity is determined by potentiometric titration with ethanolic potassium hydroxide using ASTM D664-18e2. Chloride ion from hydrolysed samples is measured by ion chromatography with suppressed conductivity using EPA Method 300.1. These analytical methods are validated under ISO/IEC 17025:2017, and the measurement uncertainty for MCA selectivity is typically reported as ±0.5 mol% at the 95% confidence level. The Ac₂O:S molar ratio is calculated from the titrated acetic anhydride content and the gravimetric sulfur content, with elemental sulfur determined by extraction and gravimetric analysis or by X-ray fluorescence. The water content of the acetic acid feed must be maintained below 0.2 wt% because water hydrolyses acetic anhydride and changes the effective ratio. If the water content exceeds 0.3 wt%, the acetic anhydride feed rate must be increased by 1.5–2.0 times the stoichiometric hydrolysis demand, which shifts the ratio and complicates process control.

When Sulfur Loading Is Increased at Fixed Acetic Anhydride Concentration

Raising the sulfur loading at a fixed acetic anhydride concentration lowers the Ac₂O:S molar ratio and increases the steady-state concentration of sulfur chlorides. At moderate loadings, this shift improves MCA selectivity by reducing the chlorination activity toward MCA, but the benefit is not monotonic. Above a sulfur loading of approximately 1.5 wt% of the liquid charge, a separate sulfur chloride-rich phase can form, depending on the water content and the temperature of the reaction mass. This dense phase is corrosive to carbon steel and attacks the glass lining at the liquid-liquid interface. The phase separation also reduces the effective interfacial area for chlorine absorption, so the chlorine feed must be reduced or the agitator speed increased to maintain conversion. If the agitator speed is increased, mechanical energy input raises the bulk temperature and partially offsets the selectivity benefit. The operational boundary is therefore defined by a maximum allowable sulfur loading rather than a minimum, and the Ac₂O:S ratio is adjusted by changing acetic anhydride feed rather than by adding excess sulfur.

The thermal runaway risk is higher at low Ac₂O:S ratios because sulfur chloride-mediated chlorination has a lower apparent activation energy than molecular chlorine-mediated chlorination. Under these conditions, the reaction rate is less sensitive to temperature, and conventional jacket cooling may be insufficient to remove the heat released during a chlorine feed interruption. The process control system should include an automatic chlorine shut-off interlock triggered by high reactor pressure or high temperature, as specified in the site safety integrity level assessment according to IEC 61511-1:2016. Published data for this specific configuration is limited, but process hazard analyses for MCA chlorinators consistently identify low Ac₂O:S operation as a critical scenario requiring interlocks. The interlock setpoints are typically based on a maximum allowable temperature of 115°C and a maximum allowable pressure of 150 kPa gauge.

The materials of construction for the chlorinator, overhead condenser, and crude product storage must tolerate the combined effects of liquid acetic acid, HCl gas, sulfur chlorides, and trace water. Glass-lined carbon steel is preferred for the reactor because it resists the corrosive liquid phase, but the glass lining is susceptible to thermal shock if the temperature difference across the wall exceeds the manufacturer's specified limit, typically 100 K. Tantalum is used for the sparger and the thermowell because it resists both oxidizing and reducing acidic conditions. The overhead vapour line is constructed of PTFE-lined carbon steel or graphite to withstand condensation of acetic acid and HCl. The absorption column for HCl is fabricated from graphite or glass-reinforced plastic with a dilute acetic acid solution as the scrubbing medium. The selection of these materials is governed by the chloride ion concentration, the presence of elemental sulfur, and the operating temperature; published data for specific Ac₂O:S configurations in these materials is limited, but the industry practice is conservative due to the risk of stress corrosion cracking.

Elemental sulfur is charged into the chlorinator as prills or as a molten liquid through a heated line. The prill size distribution affects the dissolution rate; prills larger than 2 mm persist as solids and create local sulfur-rich zones, while sulfur powder finer than 75 μm can form dust and agglomerates. The acetic anhydride is fed through a separate corrosion-resistant line to prevent pre-reaction in the feed pipe. The two feeds are mixed in the reactor rather than in a static mixer upstream, because pre-mixing generates acetyl chloride and heat before the chlorine sparger, shifting the local stoichiometry away from the target Ac₂O:S ratio. The acetic acid recycle stream must be dried by azeotropic distillation to below 0.2 wt% water before returning to the chlorinator. If the water content exceeds 0.3 wt%, the acetic anhydride feed rate must be increased by 1.5–2.0 times the stoichiometric hydrolysis demand, which shifts the ratio and complicates process control. Published data for this specific configuration is limited, but the effect of water on acetyl chloride hydrolysis is well documented in industrial operating manuals.