2026-03-25

Gas Monitoring in the Chlor-alkali Industry

There are often legislation and emission limits to protect health and the environment, and there are usually requirements on monitoring that the limits are not exceeded. An example is the monitoring of aggressive gases in the chlor-alkali industry, producing chlorine and other bulk chemicals.

The Chlor-alkali Process

The chlor-alkali process is a method to convert sodium chlorine (NaCl) into chloride gas (Cl₂) and caustic soda (sodium hydroxide, NaOH). The process also generates hydrogen gas (H₂).

In the process, an aqueous solution of NaCl is fed into one side of an electrolysis chamber where the anode splits the NaCl bond. Cl₂ gas and Na⁺ ions are formed.
In the other side of the chamber, fed by water only, the cathode splits the water whereby gaseous hydrogen and OH^- ions are formed.
The two sides are separated by a membrane which allows the Na⁺ ions to travel to the cathode side. There, they react with the OH^- ions and form caustic soda.

The Cl₂ and H₂ gases are caught at the electrodes on their respective side of the membrane, while the NaOH solution is tapped on the cathode side.

Focus on Chlorine

Let us focus on the chlorine being produced. The gas is cooled whereby gas-phase water residue condenses and is removed from the gas mixture. Compression and further cooling follow, producing liquid Cl₂ which can be stored and distributed.

There are also a variety of options to manage and use the hydrogen being generated, and the caustic soda is of course also utilized.

Emissions Monitoring

In practice, the chlorine is not completely liquified in the cooling stages, and there may also occur gaseous impurities in the chlorine gas which are not condensed. This tail gas is usually lead to a series of scrubbers catching and sometimes utilizing the remaining gas.

At the end of the process, there is waste gas emitted to the ambient air. The gas might contain a variety of pollutants such as gaseous Cl₂ and HCl. These emissions may cause health and environmental concerns. There are therefore often requirements to monitor the gas concentrations continuously.

Practical Challenges

There are several ways to monitor gases like Cl₂ and HCl, but when it comes to practical aspects, the options narrow down.

The gases themselves are aggressive also in low concentrations which excludes most sampling instrumentation based on probes and thin hoses.
The ambient environment at a chlor-alkali industry can be harsh. It calls for very ruggedized equipment to achieve a reasonable lifetime with acceptable maintenance efforts.
At the same time, the emission limits can be low, calling for very precise monitoring solutions with low detection limits.

This leaves few options.

The Fast-loop Approach

A solution that has proven to be very good is the OPSIS fast-loop monitoring system. It operates with non-contact optical detection along a light path. To gain low-enough detection limits, a part of the flue gas is led with a relatively high flow via durable tubing to a monitoring cell.
The cell can be several metres long, enabling detection of the gases at very low concentrations. The gas is then returned to the duct, hence the name “fast loop”. To withstand the aggressive gases, the tubing and the cell is often manufactured in stainless steel or durable plastics.

Protecting the Delicate Parts of the System

It is only the measurement cell with its light emitter and receiver that is exposed to the tough ambient conditions. The light from the cell is led via an optical fibre to a shelter where the core instrumentation is located. That is an analyser where the spectrum of the light is evaluated and the sought-after gas concentrations are calculated.

Other Monitoring Applications

The fast-loop gas concentration measurement principle can also be used for process control. The concentration levels might be much higher but both fast-loop monitoring systems and direct monitoring in gas ducts (“in-situ”) can still be applied.

With multi-gas analysers, the systems also become very cost-effective. Proven and durable monitoring systems keep maintenance efforts and operational disturbances at a minimum.

Ambient Air Monitoring

The open-path technology can also be applied to ambient air quality monitoring. In the context of chemical industries, so called fence-line monitoring is of particular interest. Then there are monitoring light beams in the open air around the industry (“along the fence”, hence the name), allowing quick detection of unintentional emissions irrespective of wind direction.

Alarms can be raised for immediate attention to the situation, and data from the long-term monitoring can be used to assess the air quality in general and for studies on how to reduce the impact of the industry on the environment.

Do you want to learn more about the OPSIS fast-loop or ambient air quality monitoring systems? Don’t hesitate to contact an OPSIS representative!

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