Condensed from an OPSIS AB article by Bengt Löfstedt, first published 2026-03-30.
Chemical plants handle dangerous and aggressive substances, where leaks and spills can be catastrophic for production, people and the environment, and long exposure to low concentrations is harmful too. Regulations therefore set emission limits and require monitoring to prove they are met, and the chlor-alkali industry, which produces chlorine and other basic chemicals, is a clear example. The process converts sodium chloride into chlorine and sodium hydroxide, with hydrogen as a by-product. Brine is electrolysed: chlorine forms at the anode together with Na⁺ ions, water is split at the cathode into hydrogen and OH⁻ ions, and the sodium ions cross a diaphragm to meet the hydroxide and form caustic soda. Chlorine is cooled so that residual water condenses out, then compressed and cooled further to liquid chlorine for storage and transport. Not all of it liquefies, and the tail gas still carries gaseous impurities; it passes through a series of scrubbers that capture and sometimes recover the remaining gas before the exhaust reaches the atmosphere, where it may still contain Cl₂ and HCl. Continuous monitoring is usually required, yet the practical difficulty is severe: these gases attack probes and fine tubing even at low concentrations, the plant environment is harsh, and the limits can be very low.
Chinese chlor-alkali capacity is concentrated in a few provinces, and the duty is set by the emission standard for the industry together with the CEMS rules in HJ 75 and HJ 76. The engineering problem is corrosion: chlorine and hydrogen chloride attack stainless steel, and any moisture turns a sample line into acid. Extractive systems need expensive materials and frequent replacement, while an in-situ or open-path optical measurement keeps the instrument out of the gas stream — which is why it is chosen for tail gas and for fence-line duty around chlorine storage and loading. Three practical points. The measurement has to survive a scrubber outlet where the gas is saturated, so the optical path and its windows need heating or dry purge. The range has to cover normal operation and a release, which are orders of magnitude apart, so specify two ranges rather than one compromise. And the alarm logic belongs with the plant safety system, because a chlorine reading is a safety input, not only an environmental one.
Gas Monitoring in the Chlor-alkali Industry — read the original at opsis.se ↗
Written by Bengt Löfstedt, OPSIS AB, first published 2026-03-30. The summary and the A.Johnson note above are our own text; the OPSIS article remains the authoritative version. Chinese reader? See the full Chinese article.
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