Chlorine Gas Scrubber Solutions: Neutralization Techniques, Custom Technologies, and Emergency Preparedness

Author: Michael Klepik, Chief Executive Officer

The main principle of a chlorine gas scrubber's operation lies in the interaction between the pollutant and absorbing liquids or solid materials, which allows for its conversion into less harmful compounds. The most effective type of equipment is the packed chlorine gas wet scrubber.

A circulation pump delivers water or a special solution to the nozzles inside the unit. Dirty air enters through the duct into the column, where it comes into contact with the liquid. The packing is also located there. The cleaned stream passes through a droplet separator and exits to the atmosphere. The contaminated liquid is drained into a recirculation water tank, where it settles and can then be reused.

The cleaning process in a caustic scrubber for chlorine gas is often carried out in several stages to enhance efficiency. This includes the sequential passage of the air stream through the following stages:

1. Initial Absorption in an Alkaline Solution (e.g., sodium hydroxide), where the majority of chlorine is removed through a chemical reaction:
Cl₂ + 2NaOH → NaCl + NaClO + H₂O
In this reaction, a chlorine molecule (Cl₂) reacts with a sodium hydroxide (NaOH) solution. One chlorine atom is reduced to sodium chloride (NaCl), while the other is oxidized to sodium hypochlorite (NaClO).

2. Use of Lime Milk:
Cl₂ + 2Ca(OH)₂ → CaCl₂ + Ca(ClO)₂ + 2H₂O
Depending on the operating conditions (temperature, concentration, pH of the mixture), other absorbents may also be used. For example:

3. Sodium or Potassium Carbonates are used in alkaline environments, where they react with chlorine to form chlorides and carbon dioxide:
Cl₂ + Na₂CO₃ + H₂O → NaCl + NaClO + CO₂ + H₂O

4. Sodium Thiosulfate is applied in conditions where chlorine needs to be reduced, leading to the formation of safe compounds such as sodium chloride and tetrathionate:
Cl₂ + 2Na₂S₂O₃ → 2NaCl + Na₂S₄O₆
These reactions are effective for low-concentration chlorine gas and help minimize residual concentrations in emissions.

After the main absorption, by-products such as hydrogen chloride (HCl) may form, requiring additional removal. This can involve a stage using water or solid absorbents. A multi-stage system allows for a more complete removal of pollutants, as each stage specifically reduces the concentration of Cl2. This is especially important in cases of high Cl₂ concentrations in the feed gases or for meeting strict emission regulations.

HCl can be absorbed by water or alkaline mixtures to lower its concentration in emissions:
HCl + H₂O → HCl (hydrochloric acid)
HCl + Ca(OH)₂ → CaCl₂ + H₂O
This is necessary for the final removal of toxic components and to prevent corrosion of the equipment.

Wet scrubbers are essential for ensuring compliance with environmental regulations and improving air quality in industrial settings. Each model offers specific advantages depending on the application, allowing for tailored solutions to meet varying operational needs.

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Absorption apparatus with packed columns uses a liquid alkaline absorbent (e.g., NaOH or Na₂CO₃ solutions). The design includes a packed column where counter-current interaction between gas and liquid occurs on the surface of the packing. Advantages of this method include:

• High degree of purification due to intense phase contact.
• The ability to use various alkaline solutions that react well with the pollutant and effectively neutralize it.

Disadvantages include:

• The time required to fill the column with absorbent liquid, which delays startup and may worsen the consequences of an accident.
• The possibility of "breakthrough" of the pollutant during the initial operating period due to the unstable flow regime of the liquid over the packing.
• Bulkiness of the design and complexity of pipeline connections.

The Adsorption Emergency Chlorine Scrubber uses solid adsorbents, such as activated carbon, to absorb chlorine.
Advantages include:

• High degree of absorption due to the porous structure of the adsorbent.
• The ability to reuse adsorbents after regeneration, reducing operational costs.

Despite their effectiveness, they have disadvantages:

• Require large volumes of adsorbent, making the equipment bulky.
• After regeneration of the adsorbent, usually conducted with steam, Cl2 is released again, creating a recurring neutralization problem.

The Mass Transfer Apparatus is an absorption system with an ejector that ensures effective mixing of phases. The device uses NaOH and sodium thiosulfate solutions for absorption. Advantages of the setup include increased process efficiency, simplified design, and the presence of a spent fluid disposal system, making it more compact.
Disadvantages of this apparatus:

• Requires strict control of air flow rate (0.3–4 m/s) to ensure effective neutralization; deviations lead to reduced efficiency or secondary liquid carryover.
• Complexity of the design due to the presence of a gas distribution contact device and the need to maintain certain process parameters.

The tray emergency chlorine scrubber system consists of two mesh trays that create boiling foam layers for intense mass transfer. The apparatus operates on the principle of countercurrent flow: the contaminated stream passes upwards through the trays, which are sprayed from above with a calcined soda solution. Droplets are collected by a droplet separator, and the cleaned flow is discharged into the atmosphere. The advantage is the inertialess startup, which eliminates Cl2 breakthrough, although the low gas flow rate limits the device's efficiency.

The design of the chlorine air scrubber and the type of absorber are selected by a specialist based on the technological process and analysis of all factors.