Gas Scrubber Unit Overview: Purpose, Off-Gas Control, Design, Chemistry, and Filters

Author: Anna Frank, Equipment Selection Expert

Gas scrubber unit is an industrial device used for cleaning exhaust polluted air from various impurities. It is considered one of the most effective systems for removing solid particles of any degree of dispersion. In addition to dust collection, it can also perform heat exchange and absorption processes.

An off-gas scrubber is a unit used to clean secondary (by-product) process streams before they are released into the atmosphere. These units ensure compliance with environmental regulations.

Such emissions often contain toxic, explosive, odorous, or corrosive substances, including CO, SO₂, H₂S, NOₓ, VOCs, NH₃, as well as acid and solvent vapors.

Before selecting a waste gas scrubber, the following parameters must be identified:

The specific type of gas scrubber equipment is selected based on the composition of the process stream.

If the exhaust stream contains acids, we recommend an alkaline gas scrubber using a NaOH solution. Packed towers are the most effective option in such cases. The packing significantly increases the contact time and surface area between phases. Gas slowly flows upward through the packed bed, while the liquid flows downward—creating a countercurrent flow that enhances absorption efficiency.

Depending on operating conditions, sour gas scrubbers in packed towers can utilize either structured packing or random packing.

Structured packing consists of regularly arranged blocks with corrugated or mesh surfaces, forming ordered channels for gas and liquid flow. It is especially effective when column height is limited, offering low pressure drop and very high mass transfer efficiency—critical for applications with strict purification requirements. However, structured packing is more expensive to manufacture and install, is sensitive to fouling.

Random packing consists of loose, irregularly shaped elements such as Raschig rings, Berl saddles, Pall rings, etc. It is ideal for medium-complexity systems or those with variable air composition, offering flexibility and reliability. It is easy to install, resistant to clogging, and well-suited to demanding or fluctuating conditions. However, in height-limited columns, it is less efficient, requiring greater column height to achieve the same level of cleaning. For highly variable stream composition or risk of fouling, random packing is preferred. For high temperatures and aggressive environments, metal or ceramic packing materials are employed.

The toxic gas scrubber is built using corrosion-resistant materials such as high-grade steel or industrial plastic for extended durability.

In the case of heavy dust loading, a Venturi equipment performs exceptionally well. Its operation is based on Bernoulli’s principle, where air velocity increases through a narrowing of the duct, promoting particle-liquid droplet collisions and particle coagulation.

To treat odors, organic compounds, H₂S, and ammonia, we use a noxious gas scrubber with acidic or alkaline solutions. Foul-smelling substances can also be captured using dry adsorption modules.

When the airflow contains high levels of vapor and sticky aerosols, a foam or barbotage tower is selected. The foam layer effectively dissolves such contaminants. Cross-flow trays are most commonly applied in these units. This includes bubble-cap, sieve, valve, S-type, and bubble-cap-sieve hybrid trays.

A compact gas scrubber is typically designed with a horizontal column layout. They are ideal for use in laboratories and small production areas where space is limited. Their lightweight and modular design also makes them easy to transport, install, and maintain.

Regeneration gas scrubber enables the reuse of a chemical or physical medium that captures pollutants.

During the purification process, the contaminated airflow passes through a layer of adsorbent or absorbent (such as activated carbon, zeolites, NaOH solutions, or amines), which traps the pollutants. As the medium becomes saturated, its efficiency decreases — instead of being disposed of, it is regenerated through washing, purging with hot air or inert gas, or by a chemical reaction. After regeneration, the sorbent is reused in the same cycle.

This type of equipment is applied in systems for the removal of H₂S, CO₂, NH₃, SO₂, and VOCs. It is commonly applied in petrochemical, chemical, pharmaceutical, biogas, and power generation facilities — especially where reducing operating costs and minimizing waste are critical.

In essence, this applies to any industrial process where air passes through a reaction or technological stage, is then purified, and reenters the same cycle—remaining within the system instead of being wasted. In such a scheme, the scrubber plays a vital role by removing impurities that would otherwise interfere with gas reuse. This is precisely what makes the unit a key component of the closed-loop system.

Such systems require the purification unit to:

• provide a high degree of purification,
• resist the buildup of contaminants,
• maintain pressure and temperature suitable for the process.

The chemistry of gas scrubbers revolves around the removal of undesirable components—such as acidic and basic compounds, particulates, or volatile organics—from a process stream through chemical or physical interaction with a scrubbing medium.
Here’s a breakdown of the fundamental chemistry involved in a typical scrubber for a gas stream.
1. Acid Removal (e.g., H₂S, SO₂, CO₂)
These are typically removed using alkaline solutions such as sodium hydroxide (NaOH), lime (Ca(OH)₂), or ammonia (NH₃).
Example reactions:
SO₂ + 2 NaOH → Na₂SO₃ + H₂O
CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O
H₂S + NaOH → NaHS + H₂O
This is acid–base neutralization, forming salts (like sulfites, carbonates) and water. Precipitates like CaCO₃ can be filtered out.

2. Basic Gas Removal (e.g., NH₃, amines)
These are captured using acidic solutions, such as sulfuric acid (H₂SO₄).
Example:
2 NH₃ + H₂SO₄ → (NH₄)₂SO₄
This forms a stable ammonium salt in the scrubbing liquid.

3. Particulate & Aerosol Capture
Though primarily physical, chemistry may assist: humidification and wetting agents increase droplet adhesion, and coagulants may be added to agglomerate fine particles. For example, acids or salts in water may neutralize electrostatic charges that keep particles suspended.

4. Oxidation-Reduction Scrubbing (e.g., H₂S → SO₄²⁻)
Some absorption towers use oxidizing agents to convert toxic compounds into less harmful forms.
Examples:
H₂S + 2 NaOH + ½ O₂ → Na₂SO₄ + H₂O
NO + KMnO₄ + H₂O → HNO₃ + MnO₂ (simplified)
This is more complex, redox-type scrubbing equipment, often employed in multi-stage wet scrubbers.

5. Solvent Absorption (e.g., VOCs)
Volatile organic compounds (VOCs) are absorbed into organic solvents or aqueous solutions that physically or chemically bind them. In some cases, reactive scrubbing is applied, such as:
Aldehydes + bisulfite → stable adducts
Amine absorption of CO₂:
CO₂ + RNH₂ → RNHCOOH (carbamate formation)

Scrubber chemistry is based on neutralization, absorption, precipitation, and redox reactions, adjusted to the composition of the process stream. The selection of appropriate chemical reactions and treatment media depends on stream composition, desired removal performance, and waste management strategy.

In wet gas scrubbers, the filter element most commonly consists of demisters, washable filters, or foam materials. In dry units, typical elements include sorbent cartridges, mechanical filters, and HEPA filters.

Universal or compact units often use combined filter elements that integrate multiple functions.

When selecting gas scrubber vendors, it’s important to compare product quality, customization options, and after-sales support to ensure reliable long-term performance. As a manufacturer, we provide full customer support and tailored solutions. Contact us — we're here to help.