Chloroform (trichloromethane) technical grade, when qualified as refrigerant R-22 feedstock, is a chlorinated methanes intermediate with CAS 67-66-3 and EC 200-663-8. The compound has molecular formula CHCl₃, molecular weight 119.38 g/mol, normal boiling point 61.2 °C, freezing point −63.5 °C, density 1.483 g/cm³ at 20 °C, vapour pressure 21.2 kPa at 20 °C, and water solubility 8.09 g/L at 20 °C. In anhydrous fluorination service, the material is distinguished from general technical grade chloroform by tighter control of moisture, oxygenated stabiliser, and heavy chlorocarbon impurities. The primary industrial use of this qualification is as liquid feed to antimony pentachloride-catalysed hydrofluorination to chlorodifluoromethane (R-22), CAS 75-45-6, according to the stoichiometry CHCl₃ + 2 HF → CHClF₂ + 2 HCl. Production-scale specifications for this feedstock typically require assay of ≥99.9% area by GC-FID, moisture ≤50 mg/kg by ASTM E203-16, acidity as HCl ≤10 mg/kg, and Pt-Co colour ≤10 by ASTM D1209-15. Ethanol-stabilised technical grades are excluded from many fluorination units because the oxygenated stabiliser consumes anhydrous HF and increases water load.

Transfer of fluorination-grade chloroform into the R-22 unit is typically performed with sealless diaphragm or canned-motor pumps because the combination of low viscosity 0.56 mPa·s at 20 °C and high vapour pressure 21.2 kPa at 20 °C creates a narrow available NPSH margin at elevated ambient temperatures. The feed line is traced with tempered water, not steam, and the preheater skin temperature is controlled to avoid localised degradation of the amylene stabiliser. Coriolis mass flow meters with Hastelloy C-276 wetted parts are used where custody transfer or continuous mass balance is required.

What Limits Direct Fluorination Selectivity in Commercial R-22 Reactors?

The liquid-phase reaction of CHCl₃ with anhydrous HF proceeds through sequential chlorine-fluorine exchange at a Lewis-acid catalytic surface. Industrial reactors operate with SbCl₅ or partially fluorinated SbCl_xF_5−x as the active catalyst, maintained in a mixed chlorofluorocarbon-HF liquid phase. The chlorination selectivity is governed by the local HF-to-CHCl₃ ratio, reactor temperature, and the rate of HCl removal from the catalyst layer. Under-fluorination produces dichlorofluoromethane (R-21), CHCl₂F, while over-fluorination produces trifluoromethane (R-23), CHF₃.

Field analysers on the reactor exit monitor R-21/R-22/R-23 weight ratios. A rising R-21 concentration indicates insufficient HF activity or low catalyst inventory; a rising R-23 concentration indicates excessive bed temperature or residence time. Producers typically treat R-21 as the key low-HF constraint, not the raw HF/CHCl₃ feed ratio, because catalyst preconditioning and organic recycle alter the effective ratio at the active site. Plant control bands commonly hold R-21 below 5 wt% and R-23 below 2 wt% in the crude reactor effluent, though published data for exact limits in specific reactor configurations is limited.

The reaction exotherm requires heat removal through a recirculation loop or internal cooling surface fabricated for mixed acid service. Graphite and silicon carbide exchangers are used where hot HF-HCl condensate is present. The liquid-phase reaction is operated at sufficient pressure to maintain HF and chloroform in the liquid state; supplier process descriptions place the practical upper temperature below 150 °C to limit catalyst degradation and over-fluorination to R-23.

Specification Thresholds and Impurity Interactions in Anhydrous Fluorination

The distinction between general technical grade chloroform and fluorination-grade chloroform is defined less by the main assay than by the oxygenated and halogenated impurities that interact with HF or the catalyst. Representative values from producer certificates of analysis are shown below.

Representative technical-grade trichloromethane specification for R-22 feedstock

Parameter	Limit	Test method
Trichloromethane assay	≥99.9% area	Capillary GC-FID with internal standard
Moisture	≤50 mg/kg	ASTM E203-16
Acidity as HCl	≤10 mg/kg	ASTM D1613-06(2021)
Non-volatile residue	≤10 mg/kg	ASTM D1353-13
Colour, Pt-Co	≤10	ASTM D1209-15
Amylene stabiliser	25–50 mg/kg	GC-MS or hydrolysis screening

Ethanol is the most common stabiliser in general technical grades. In fluorination service, ethanol at concentrations above 100 mg/kg reacts with HF to form water and ethyl fluoride; each mole of ethanol consumes at least one mole of HF and generates one mole of water. Carbon tetrachloride, if present above 500 mg/kg, is co-fluorinated and increases the load on downstream distillation. Brominated methanes, even below 10 mg/kg, can generate corrosive bromide salts and discolour the catalyst phase. Fluorination-grade suppliers therefore often specify amylene stabilisation at 25–50 mg/kg or an inhibitor-free condition under inert gas.

Receipt of technical-grade trichloromethane for R-22 service requires a closed-loop nitrogen pressure transfer system. Moisture ingress through temporary hose connections or opened manways has been observed to raise feedstock water from below 50 mg/kg to above 150 mg/kg within a single shift at ambient relative humidity above 60%. Because the downstream fluorination unit uses anhydrous HF, water in the feed hydrolyses SbCl₅ to antimony oxychlorides and reduces HF activity through dilution; this increases acid load and can shorten catalyst life. The material is stored in lined carbon steel or stainless steel tanks fitted with desiccant breathers; copper and aluminium alloys are avoided for wetted components because chloroform can slowly hydrolyse and generate HCl. Stabiliser depletion under heat and ultraviolet exposure is monitored by periodic amylene concentration measurement. Once amylene concentration falls below 10 mg/kg, phosgene formation becomes possible if oxygen is present, so the storage headspace is maintained under nitrogen blanketing with oxygen below 5%.

When Recycled HCl from R-22 Synthesis Is Returned to the Chloromethanes Train

Hydrogen chloride generated in the R-22 reactor is often recovered from the vent gas in a falling-film absorber or adiabatic absorber using water to produce 30–35 wt% hydrochloric acid. Where an integrated chloromethanes plant is co-located, this recovered acid can be returned to the methanol hydrochlorination or methyl chloride chlorination step, but only after treatment to remove residual fluorides. Fluoride contamination in recycled HCl can poison oxychlorination catalysts or corrode downstream equipment; a fluoride concentration below 10 mg/L in circulating acid is often used as the control limit. The absorption train is therefore equipped with a caustic scrubber or acid clay guard bed before the acid is returned.

If the recovered HCl is not returned, it is neutralised or sold as technical-grade acid; in either case the mass balance from the R-22 reactor is 2 mol HCl per 1 mol CHCl₃ converted. This HCl balance becomes a plant-wide acid integration constraint when the chloromethanes unit operates at high throughput, because the fluorination section may produce more aqueous acid than the chlorination section can absorb.

Antimony pentachloride in the R-22 fluorination unit is not a true catalyst in the sense of being unchanged; it undergoes continuous ligand exchange with HF to form mixed chlorofluoride species. The active liquid catalyst phase is sensitive to water and to neutralising species. Organic impurities that enter with chloroform can form high-boiling oligomers that accumulate in the catalyst phase and raise viscosity; the removal of a slipstream of spent catalyst and its replacement with fresh SbCl₅ is a standard operational practice. The replacement interval is dictated by the concentration of non-volatile residue in the feedstock and by the efficiency of the catalyst recovery system. Published data for specific deactivation rates in technical-grade chloroform service is limited, but plant records commonly link high non-volatile residue in the feedstock to shortened catalyst runs and increased operator intervention.

Vapour-Liquid Separation and Catalyst Inventory Management

The R-22 reactor effluent is a pressurised mixture of CHClF₂, HCl, unreacted HF, CHCl₃, CHCl₂F, and CHF₃. Separation is conducted in a series of columns: the first column strips HCl by distillation; the second removes HF by acid absorption or phase separation; and the final column separates R-22 from under- and over-fluorinated byproducts. The feed quality of chloroform directly affects the accumulation of high-boiling residues in the reboiler sections. Reboiler fouling from chlorinated oligomers has been observed when the non-volatile residue of the chloroform feedstock remains at or above 20 mg/kg. Plant units that switch from ethanol-stabilised technical grade to fluorination-grade material typically observe reduced reboiler cleaning frequency and lower pressure drop across the distillation train. Vapour-liquid separation efficiency is also influenced by the presence of non-condensable lights formed from stabiliser breakdown; these lights raise condenser pressure and can shift the R-22 overhead composition unless vented continuously.

Process Alarm Bands for Moisture and Stabiliser Breakthrough

Continuous moisture analysers on the chloroform feed line are typically set to warn at 30 mg/kg and to initiate feed diversion at 50 mg/kg. This alarm band is derived from the observed correlation between feed moisture and corrosion rates in the HF feed preheater. Stabiliser breakthrough is monitored indirectly by the appearance of ethyl fluoride or C5 fluorinated fragments in the reactor vent gas. The presence of ethyl fluoride in the crude R-22 stream is a specific indicator of ethanol contamination, because ethyl fluoride is not a significant product from chloroform fluorination alone. The process control response is to switch to the reserve tank of qualified feedstock and to isolate the contaminated delivery line. In units that receive multiple suppliers, the online alarm threshold is cross-checked against the certificate of analysis for each batch, and operator response is documented under ISO 9001 batch traceability procedures.

Compliance under the Montreal Protocol requires that R-22 produced from trichloromethane feedstock be recorded separately as a feedstock-grade product if it is destined for further chemical transformation. UNEP reporting definitions exclude feedstock from calculated consumption, but the burden of documenting downstream conversion remains with the operator. Under REACH, chloroform is indexed with harmonised classification for acute toxicity, carcinogenicity, and reproductive toxicity; the technical-grade R-22 feedstock stream is managed as a closed-system intermediate with exposure monitoring according to local occupational exposure limits, which for chloroform are often set at 2 ppm as an 8-hour TWA or lower depending on jurisdiction. Anhydrous fluorination of technical-grade trichloromethane is not suitable for ethanol-stabilised material, for feed with water above 50 mg/kg, or for reactors that lack continuous acid-gas scrubbing and catalyst slipstream management.