Chemical Scrubber Systems: Principles, Design, Reactions, and Cost Analysis

Author: Nikulin V, Head of Engineering

Customized chemical scrubber design solutions can be tailored to address unique process conditions, space limitations, and regulatory demands.

Step 1 — Initial Data
The process of gathering initial data and analyzing contaminants is a key part of the design workflow. Based on the information received, the designer incorporates certain coefficients into the calculations to achieve the best result.

• Flow rate, temperature, and pressure.
• Nature of contaminants and special conditions — presence of dust, noise reduction needs, energy consumption limits, and criteria for effluent quality and volume.
• Contaminant concentration at the inlet and desired levels following treatment.
• Maximum allowable height, diameter, weight, and installation footprint, as dictated by the site or building.
• Permissible pressure drop — the maximum pressure differential that the ventilation equipment can handle; the design must not exceed this value.
• Corrosive or temperature conditions that determine the choice of material: plastic, composite, stainless steel, etc.

Step 2 — Selection of Solution

The concentration and flow rate of the liquid are selected to ensure at least 120% stoichiometric excess.
In some advanced units, chemical scrubber injection is integrated with automated monitoring to adjust dosing rates in real time.

Step 3 — Selection of Contact Apparatus

Explanations:
Δp, kPa denotes the hydraulic resistance across the working section of the apparatus — that is, how much the pressure decreases as the stream passes through this type of chemical scrubber system. Δp is the pressure difference between the inlet and outlet through the column (or through the packing, trays, foam layer, etc.), expressed in kilopascals (kPa). The higher the Δp, the more energy (fan or compressor work) is needed to "push" the stream through.

In equipment design, this value is always considered: it must be manageable for the ventilation setup and not result in excessive energy consumption.

Δp serves as a guideline for how much pressure is "lost" in an apparatus of this type and how energy-intensive its use will be for the ventilation system.

HETP, m — Height Equivalent to a Theoretical Plate — is the notional height of a contact element (packing, foam, one tray, etc.) at which the same mass transfer (cleaning, absorption) is achieved as on one ideal (theoretical) stage. For instance, with HETP = 0.6 m, every 0.6 meters of the fill inside the column delivers the equivalent performance of a single ideal tray.

In practice, a smaller HETP means the apparatus is more efficient, achieving the target degree of treatment at a shorter height.

Selection guidelines:

• First, analyze the particle size distribution and flow rate.
• Then, evaluate the permissible pressure drop.
• Finally, check whether it is possible to ensure uniform distribution of liquid/air dispersion for the selected type of apparatus.

Step 4 — Hydrodynamics and Mass Transfer
Degree of purification → necessary number of mass transfer stages
The greater the reduction in contaminant concentration required, the more “theoretical stages” (NTU — Number of Transfer Units) are needed. In practice, this means either a taller packed bed, more contact trays, or more intense foam/sparging.

Height of the active (contact) zone
Selected to ensure the calculated number of stages, considering the efficiency of the specific packing or trays. For structured elements, 0.4–1 m per stage is standard; spray towers generally use 2–3 m per stage.

Apparatus diameter
Determined by the maximum allowable gas velocity. The flow must not “flood” the packing or trays, but should not be too slow either (an excessive diameter means an expensive vessel). In practice, the selected velocity is compared to the “flooding limit” tables for each type of fill or tray, with a safety margin of approximately 60% applied.

L/G ratio
Must ensure:

• sufficient stoichiometric excess of neutralizing agent;
• uniform wetting of the entire cross-sectional area;
• acceptable temperature (the fluid absorbs heat produced during treatment).

For packed towers, 2–10 liters per normal cubic meter are typical; for Venturi unit, values are much higher—tens of liters.

Pressure and hydraulic resistance
The pressure drop across the contact zone and the demister is summed. If the total pressure drop exceeds the fan’s ∆pₘₐₓ, a “softer” scheme is adopted: increasing diameter, switching to a tray or spray tower, or reducing the L/G ratio.

Auxiliary circuits
Recirculation pump — sized for 3–6 column volumes per hour to prevent salt accumulation.
pH / ORP control — acts as a trigger for dosing fresh reagent.
Blowdown — removes accumulated salts; typically, 3–8 wt% solids are maintained.

The designer selects a combination of height, diameter, and liquid flow rate that achieves the necessary degree of mass transfer, prevents flooding or foam carryover, and keeps the total resistance within the capabilities of the ventilation equipment.

Step 5 — Materials
Many chemical scrubber manufacturers now focus on corrosion-resistant materials and modular designs to improve equipment reliability and ease of maintenance, and we integrate these strategies into our own equipment.

• Casing: FRP, CPVC, PP; for temperatures above 80 °C — alloys such as 904L, C-276, or duplex stainless steels.
• Fittings: HDPE or PVDF in the wet zone; carbon steel above the liquid is permissible at temperatures below 60 °C and in the absence of condensate.

Chemical scrubber media is the material (or blend) that provides the required conversion and/or offers a contact surface for removing gas-phase pollutants.

There are two main types:

Cleaning solution: An aqueous solution of an alkali, acid, oxidizer, or other reactive substance that neutralizes or oxidizes contaminants.

Solid media: Granules, pellets, activated carbon, zeolites, or engineered polymers infused with functional agents, installed in cartridges or as bed material.

Examples:
In a dry chemical air scrubber, the media might be granulated sodium hydroxide or a mixture of activated carbon with specific reagents. In a wet chemical scrubber, the media is typically the liquid (e.g., NaOH, H₂SO₄, NaOCl, etc.), but hybrid systems may also use granular or fibrous elements.

Scrubber media is the “working reagent” or “active medium” where mass transfer and/or reactions with pollutants occurs.

Chemical scrubber packing refers to internal structured elements or bulk fill that enhance the contact area between phases within the column or reactor.

Random packing: Small parts of regular or irregular shapes—Raschig rings, Super Intalox, saddles, sticks, plastic balls, etc. It is usually selected if there is a risk of dust, precipitate formation, or frequent maintenance, as it is cheaper and easier to service.

Structured packing is chosen when maximum efficiency at minimal height and energy consumption is needed, provided the stream is clean and free from particulates.

Main functions:

• Increase the contact surface between phases.
• Ensure even liquid distribution throughout the column.
• Minimize flow resistance while maximizing mass transfer.

A chemical reaction in a scrubber is a specific transformation occurring between a pollutant present in the stream and a reagent supplied to the apparatus.
The aim is to transform harmful substances into safe, non-volatile, or readily separable forms, usually either dissolved in solution or precipitated as solids.
Examples:
SO₂​ + 2NaOH → Na₂SO₃​ + H₂​O
H₂S + 4NaOCl + 2H₂​O → Na₂​SO₄+ 4NaCl + 4H⁺
NH₃​ + H₂​SO₄​ → (NH₄​)₂SO₄​
HCl + NaOH → NaCl + H₂​O
2 HF + Ca(OH)₂ → CaF₂ + 2H₂​O
2NO₂ + 2NaOH → NaNO₂ + NaNO₃ + H₂​O
3H₂S + 2KMnO₄ → 3S + 2MnO₂ + 2KO + 2H₂​O
2 NH₃ + NaOCl → N₂ + 3NaCl + 3 H₂​O(oxidation of ammonia to nitrogen)
3HCHO + 2KMnO₄ + 2H₂​O → 3HCOOH + 2MnO₂ + 2KOH (removal of volatile organic compounds, VOCs)
Cl₂ + 2NaOH → NaCl + NaOCl + H₂​O
The selection of treatment agents is determined by the specific chemical reactions in scrubber systems required to neutralize or oxidize the target contaminants.

2. Cost Ranges (based on Western sources, 2024):

• Typical pricing from a chemical scrubber supplier for a small unit (1000–5000 Nm³/h, plastic materials, standard package) ranges from $20000 to $60000 USD turnkey.
• Medium units (10000–30000 Nm³/h, FRP, automation, 1–2 stages):
• $70000 – $200000 USD
• Industrial complexes (50,000–200,000 Nm³/h, highly resistant materials, multi-step process, complex integration): $300000 – $1000,000+ USD (depending on requirements and region)

3. Example Specific Costs

• Approximate reference: $50–$200 USD per 1,000 m³/h of capacity (equipment only, excluding installation).
• Wet chemical scrubber systems are generally cheaper per cubic meter than dry cassette units (but require higher OPEX for reagents and effluent disposal).

4. Factors That Can Significantly Increase Cost

• Handling aggressive substances (HCl, HF, Cl₂) — requires expensive materials (PVDF, PTFE, Hastelloy).
• For elevated air temperatures, reinforced casings and additional cooling are typically used.
• Attaining purification below 1 ppm calls for more complex process schemes, extra stages, and improved automation.
• Certification for food/pharmaceutical industry.
• Requirements for noise, explosion-proofing, ATEX, etc.

5. Typical Operating Costs (approximate):

• Reagents: $500–$2000 USD/year per 10000 Nm³/h (depends on loading and type)
• Maintenance: 2–5% of CAPEX per year
• Electricity: 0.5–2 kWh per 1000 m³ of gas (for pumps and fans)

The Equipment cost varies greatly—from $20000–30000 for small units to several hundred thousand or millions of dollars for complex industrial systems. The final price is determined by factors including air flow rate, purification goals, selected active substances and materials, automation, safety demands, site, and installation specifics.

For those wondering what is a chemical scrubber, it is a type of air pollution control equipment that neutralizes harmful substances before they are released into the atmosphere. We are a chemical scrubber factory and know every aspect of their operation. If you need assistance with selection or design, please contact us — we are here to help.