High Chlorine-to-Carbon Ratio in 1,1,2,2-Tetrachloroethane for Catalytic TCE Production

In continuous catalytic production of trichloroethylene (TCE), the selection of 1,1,2,2-tetrachloroethane (TeCA) as a chlorine-donating feedstock rests on its chlorine-to-carbon molar ratio of 2:1—a value significantly exceeding the 1.5:1 stoichiometry of the target olefin. This intrinsic excess enables a dehydrochlorination pathway that, when operated over a supported barium chloride catalyst at 280–350 °C, yields TCE with a per-pass conversion exceeding 85% at gas hourly space velocities (GHSV) of 80–150 h⁻¹. The reaction C₂H₂Cl₄ → C₂HCl₃ + HCl proceeds with an endothermic demand of approximately 72 kJ·mol⁻¹, requiring multitubular fixed-bed reactors—typically of 25.4 mm inner diameter tubes arranged in a 600-tube bundle—to maintain near-isothermal profiles through circulating molten salt heat transfer media. Product quality must align with ASTM D4080-21 for vapour degreasing grades, mandating a non-volatile residue below 10 ppm and an acid acceptance (as NaOH) exceeding 0.04 wt%, parameters directly influenced by the crude TCE stream’s residual HCl and unconverted TeCA fractions. The high chlorine load also imposes a corrosion management burden: reactor internals in contact with hot HCl typically employ UNS N10276 (Hastelloy C-276), and the pre-heating section upstream requires internal borosilicate glass lining when feed impurities include adventitious water above 50 ppm, a condition known to accelerate pitting rates from 0.02 mm/yr to over 0.15 mm/yr as measured per NACE TM0169-2020 in condensing zones. Additionally, the high chlorine-to-carbon ratio relaxes the need for external chlorine sources and reduces the formation of low-chlorinated by-products such as vinylidene chloride, which complicate distillation through azeotrope formation with TCE at 87.8 °C and 760 mmHg.

What Operating Temperature Window Maximises Selectivity Over BaCl₂/SiO₂ Catalysts?

The dehydrochlorination of TeCA over 8–12 wt% BaCl₂ supported on silica gel (surface area 250–350 m²/g as per ISO 9277:2022) is governed by a surface-mediated β-elimination mechanism whose apparent activation energy lies in the range 95–110 kJ·mol⁻¹. Laboratory-scale isothermal reactor data, obtained using a 10 mm internal diameter quartz reactor loaded with 15 g of catalyst crushed and sieved to 0.5–0.7 mm particles, indicate that the selectivity to TCE—defined as moles TCE per mole TeCA converted—plateaus above 96% only when the catalyst bed midpoint temperature is maintained at 310 ± 5 °C. Outside this window, the competing over-dehydrochlorination to perchloroethylene (PCE) accelerates; at 335 °C, selectivity to TCE drops to 88%, with PCE yield climbing to 7–9% and carbonaceous deposit formation rate doubling from 0.02 g·(g cat)⁻¹·h⁻¹ to 0.05 g·(g cat)⁻¹·h⁻¹, as determined by temperature-programmed oxidation of spent catalyst samples. The following data, obtained under a constant TeCA partial pressure of 0.3 bar in nitrogen carrier and a GHSV of 120 h⁻¹, illustrate the sensitivity of the product distribution to the temperature set point, highlighting the necessity of cascade PID control loops with thermocouple response times below 0.5 s on commercial shell-and-tube units.

Bed temperature (°C)	TeCA conversion (%)	TCE selectivity (%)	PCE selectivity (%)	Coke selectivity (g/100 g feed)
285	62.3	97.1	0.8	0.12
300	78.7	96.8	1.1	0.18
310	86.5	96.5	1.5	0.22
320	91.2	94.3	3.2	0.35
335	94.8	88.0	7.5	0.52

Pre-conditioning of the BaCl₂/SiO₂ catalyst bed by pre-treatment with anhydrous HCl at 250 °C for 4 h—a step that converts residual silanol groups to stable Si–Cl species—is essential to avoid HCl absorption during the initial hours of operation, which would otherwise cause a transient pH excursion in downstream scrubbing loops and alter the acid acceptance value of crude TCE beyond the 0.02 wt% NaOH threshold required for subsequent stabilisation. Regeneration of coked catalyst is performed in situ using an air-nitrogen mixture with oxygen concentration not exceeding 2 vol%, ramped from 300 °C to 450 °C at 20 °C/h, to prevent runaway exotherms that sinter the BaCl₂ crystallites and reduce the BET surface area below the minimum serviceable limit of 150 m²/g.

When TeCA Feed Purity Falls Below 99.5 wt% – Effects on Reactor Corrosion Rates and Uncondensable Gas Formation

Procurement specifications for TeCA destined for catalytic TCE trains typically mandate a minimum purity of 99.5 wt% with total chlorinated ethane isomers including 1,1,1,2-tetrachloroethane and pentachloroethane held below 0.2 wt% combined, moisture below 30 ppm, and non-volatile residue below 5 ppm. When the combined impurity load exceeds 0.5 wt%—a scenario observed during periods of upstream oxychlorination reactor upset in ethylene-based vinyl chloride monomer plants that supply crude TeCA as a by-product—the reactor effluent hydrogen chloride stream carries trace amounts of 1,1-dichloroethylene and vinyl chloride formed by dehydrochlorination of the isomeric impurities at lower temperatures. These unsaturates undergo polymerisation in the HCl recovery column overheads, forming fouling deposits in the graphite block heat exchangers that typically operate with a cooling water ΔT of 8–12 °C and require quarterly mechanical cleaning cycles instead of the baseline annual turnaround. More critically, moisture ingress above 50 ppm combines with the high-chlorine environment to produce a mixed hydrochloric acid condensate film on the tube side of the feed preheater, accelerating general corrosion of the UNS N10276 alloy from 0.08 mm/yr to 0.45 mm/yr at condensation zone temperatures of 110–130 °C, as determined by electrical resistance probes logged in accordance with NACE SP0113-2022. Stray oxygen entrained during tank farm transfers—even at concentrations as low as 0.1 vol% in the vapour space—synergises with HCl to induce stress corrosion cracking in the sensitized heat-affected zones of weldments, making mandatory the use of solution-annealed and quenched fabrication for all pressure boundary components per ASME BPVC Section VIII Division 1 with post-weld heat treatment at 1120 °C followed by rapid water quenching.

Bulk distillation of the reactor condensate, consisting of unreacted TeCA, crude TCE, and dissolved HCl, is executed in a two-column train. The first column, a 40-tray valve-tray unit with a reboiler duty of 2.8 MW, strips anhydrous HCl overhead at 2.5 bar gauge to an adiabatic falling-film absorber producing 32 wt% hydrochloric acid suitable for in-plant oxychlorination recycle, provided the acid iron content remains below 5 ppm as measured per ISO 6685:2021. The bottoms stream, carrying TCE, heavy ends, and inhibitor precursors, feeds a second vacuum distillation column operating at a head pressure of 200 mbar absolute and a reflux ratio of 3:1, where a heart cut of TCE is withdrawn with a boiling range of 86.7–87.3 °C conforming to the narrow-range requirement of ASTM D1078-11 for vapour degreasing. The high chlorine-to-carbon ratio of the original TeCA feedstock maintains a favourable relative volatility of 1.8 between TCE and unreacted TeCA, enabling a 95% recovery of TCE in the heart cut while recycling TeCA back to the reactor feed surge drum, which is blanketed with nitrogen containing less than 10 ppm oxygen to prevent peroxide formation.

Fixed-Bed Pressure Drop and Catalyst Pellet Mechanical Integrity

Catalyst pellets for TeCA dehydrochlorination are produced by extrusion of a BaCl₂-impregnated silica-alumina powder (80:20 SiO₂:Al₂O₃) with a methylcellulose binder, yielding cylindrical extrudates of diameter 3.2 mm and length-to-diameter ratio 1.5–2.5. Single-pellet radial crush strength, tested following ASTM D4179-22 on a 50 N load cell apparatus, must exceed 38 N to withstand the bed weight and thermal cycling in a commercial reactor of 3 m bed height. Bulk crush strength of the packed bed, characterised by a pressure drop not exceeding 0.45 bar at the design mass flux of 4.5 kg·m⁻²·s⁻¹, is maintained through a particle size distribution with fines (<0.5 mm) limited to <1 wt% as determined by dry sieving on a 200 mm diameter sieve stack conforming to ISO 3310-1:2016. Quarterly in-situ pressure drop monitoring, using differential pressure transmitters with 0.1 mbar resolution across the catalyst bed, serves as a leading indicator of pellet attrition; an increase exceeding 25% over baseline triggers a scheduled catalyst screening shutdown involving vacuum extraction of the top 300 mm of the bed to remove dislodged fines that would otherwise migrate into the quench zone and foul the tube-side surfaces of the product cooler, a shell-and-tube exchanger with 19.05 mm OD tubes on a 25.4 mm triangular pitch.

Application of this TCE in aerospace vapour degreasing per MIL-PRF-680C necessitates a tailored inhibitor package. A standard blend contains 0.03 wt% of 1,2-butylene oxide and 0.01 wt% of N-methylmorpholine, which together buffer the solvent pH to 8.5–9.5 as measured by a 50 vol% water extraction method derived from ASTM D2119-19. The acid acceptance value—a critical parameter for preventing metal attack on aluminium-zinc alloys—must remain above 0.04 wt% NaOH as per ASTM D2942-21 after 24 h of reflux exposure in the presence of aluminium shavings, a test that simulates the recycling conditions of a vapour degreaser where stabilised TCE can accumulate hydrolysis-derived chloride ions if the inhibitor level drops below the depletion threshold of 0.005 wt% per eight-hour shift. The relationship between inhibitor concentration and stabilised solvent service lifetime in a continuous degreasing unit operating at sump temperatures of 83–86 °C with 0.5 vol% water contamination is summarised below, highlighting the operational boundary where acid acceptance falls below the acceptable floor of 0.02 wt% NaOH, at which point copper mirror corrosion per ASTM D1617-21 exhibits a rating of 3b or worse.

1,2-butylene oxide (wt%)	N-methylmorpholine (wt%)	Initial acid acceptance (wt% NaOH)	Acid acceptance after 72 h reflux (wt% NaOH)	Copper mirror rating (ASTM D1617)
0.02	0.01	0.035	0.018	3b
0.03	0.01	0.048	0.032	2a
0.04	0.015	0.052	0.041	1b
0.05	0.02	0.060	0.053	1a