Chlorine Compressor Selection in Chlor-Alkali Plants: Dry Chlorine, Seal Liquid and Materials

Chlorine leaves the electrolysis cells hot, wet, and near atmospheric pressure. Before it can be liquefied, stored, or sent to a downstream unit, it must be cooled, dried, and compressed. The chlorine compressor sits at the centre of that chain, and few compressor duties are less forgiving.

Chlorine is toxic, corrosive in the presence of water and, under the wrong conditions, able to burn steel. A chlorine gas compressor must therefore keep the gas dry, cool, and contained. Liquid ring technology meets all three demands, which is why it has long been a standard choice for dry chlorine compression.

Where the Chlorine Compressor Sits in a Chlor-Alkali Plant

In a chlor-alkali plant, brine electrolysis produces chlorine at the anode and hydrogen at the cathode. The chlorine stream then passes through a fixed sequence of steps:

1. Electrolysis. Chlorine leaves the cells saturated with water vapour, typically carrying about 0.5–2.0% water by volume, along with traces of brine.
2. Cooling. The gas is cooled from around 95 °C to around 15 °C. Most of the water vapour and brine spray condenses out at this stage.
3. Drying. The cooled gas contacts concentrated sulfuric acid, usually in drying towers, which removes the remaining moisture down to the ppm level.
4. Compression. The dry chlorine is compressed to the pressure needed for liquefaction or direct downstream use.
5. Liquefaction. The compressed gas is cooled until most of the chlorine condenses into liquid for storage and transport.

The choice of liquefaction pressure shapes the whole compression duty. Higher pressure lets the chlorine condense at a warmer temperature and requires less refrigeration. Lower pressure eases the compression duty but requires colder condensing. Each plant strikes its own balance between compressor power and refrigeration.

Why Chlorine Is Hard on Compressors

Three chlorine properties make compressor selection critical.

Moisture turns chlorine corrosive. Wet chlorine hydrolyses to form hydrochloric and hypochlorous acids, which are highly corrosive to most common metals. Even trace moisture that slips past the drying tower can corrode the chlorine compressor and force a shutdown.

Steel can burn in chlorine. Steel is the most common material in dry chlorine service, but above about 149 °C (300 °F) a chlorine-steel fire can start. Heat from compression, friction or an external source must never push metal surfaces towards that limit.

Titanium burns in dry chlorine. Titanium resists wet chlorine well, but it is flammable in dry chlorine. A material that suits one stream can be dangerous in the other, so wet and dry duties need different construction.

Together, these properties set the requirements for a chlorine compressor: keep the gas dry, keep its temperature low, and keep it sealed inside the machine.

Why Liquid Ring Compressors Suit Dry Chlorine

Large plants sometimes use multi-stage centrifugal machines with intercooling, and reciprocating machines also appear in chlorine service. The liquid ring compressor, however, offers advantages that directly meet chlorine’s demands.

• The seal liquid keeps the gas dry. With concentrated sulfuric acid as the service liquid, the gas stays in contact with a strong drying agent throughout compression. Any moisture that reaches the compressor is absorbed rather than corroding the casing.
• Compression stays cool. The service liquid absorbs the heat of compression, and its temperature rises by only about 6–8 °C as it passes through the machine. Metal surfaces stay far below the temperature at which chlorine attacks steel.
• Carryover is tolerated. Occasional traces of salt or acid mist carried in with the gas are washed through to the separator, rather than damaging valves or rotors.
• No rubbing parts in the gas path. The impeller assembly is the only rotating part, and no metal-to-metal contact occurs in the compression chamber. With no friction inside the casing, there is no local hot spot to start a chlorine-steel fire.
• No lubricating oil. The gas touches only the service liquid, so there is no oil to react with chlorine or contaminate the product.

Compared with other types of process gas compressors, the liquid ring handles chlorine service as part of normal operation, rather than through added protection systems.

Seal Liquid: Sulfuric Acid for Dry Chlorine, Water for Wet Chlorine

The service liquid is the defining choice in a chlorine compressor package.

Dry chlorine: concentrated sulfuric acid. Acid of about 93–98% H₂SO₄ is widely used as the seal liquid. It completes gas drying, protects the compressor from moisture-related corrosion, and carries away the heat of compression. The circulating acid is cooled, typically to between 0 °C and 30 °C, before it returns to the compressor. Operators gradually replace acid that has picked up water with fresh acid to maintain its strength.

Wet chlorine: water. Where chlorine is compressed before drying, water serves as the seal liquid and the gas stays saturated throughout. Wet chlorine is aggressively corrosive, so these systems are built in titanium.

In both cases, the package around the compressor matters as much as the machine itself. A separator splits the gas from the seal liquid, a cooler controls the liquid temperature, and level control keeps the liquid ring stable.

Material Selection for Chlorine Service

Material choice follows the seal liquid, the pressure, and above all whether the chlorine is wet or dry.

In low-pressure dry service, the strong acid and low temperature leave little risk of rust, so cast iron is a cost-effective choice. Higher-pressure duties call for austenitic stainless steel. Titanium is reserved for wet chlorine and must never be used in dry service.

Dry Hydrogen: The Second Gas Stream

Every tonne of chlorine from brine electrolysis comes with hydrogen from the cathode side. This hydrogen is cooled, dried and often compressed for use as a fuel or as a feedstock for other units. Hydrogen is flammable across a very wide range of concentrations, so the same qualities that suit chlorine service apply here: near-isothermal compression, no internal friction and a liquid seal around the gas. Kenflo lists dry hydrogen compression in the chlor-alkali industry among the standard duties of its CBK compressor.

Kenflo CBK for Chlorine and Hydrogen Service

The Kenflo CBK liquid ring compressor is listed for compressing dry chlorine and dry hydrogen in the chlor-alkali industry. Its design features match the demands of a chlor-alkali compressor:

• Discharge pressure up to 0.7 MPa(G) at a capacity of 1 to 150 m³/min, suited to high discharge pressure at medium and low capacity.
• Wetted parts in SS304, SS304L, SS316L, or duplex stainless steel, suitable for dry chlorine service.
• Shaft sealing for toxic gas: single or double mechanical seals, or a dry gas seal, to keep chlorine inside the machine.
• Rubber O-ring casing seals rated to withstand high pressure.
• A symmetrical double-suction impeller that brings the theoretical axial force to zero, with SKF bearings supporting the shaft at both ends.
• Open end shields and impeller for straightforward inspection and maintenance.

Where a higher pressure is needed, Kenflo’s 2BWX tandem package reaches an ultimate discharge pressure of 1.3 MPa(G). Kenflo also supplies complete compression packages, including auxiliary equipment, instrumentation, and controls, with CE- and ATEX-certified options available.

Conclusion

Dry chlorine compression asks a compressor to stay dry, stay cool, and stay sealed. A liquid ring compressor sealed with concentrated sulfuric acid does all three as part of its normal operation, which is why it remains a trusted chlorine compressor in chlor-alkali plants.

Guangdong Kenflo Pump Co., Ltd. has built pumps since 1954 and supplies liquid ring compressors and complete packages for chemical and petrochemical plants. To discuss a dry chlorine or hydrogen compression duty, contact the Kenflo engineering team.