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Author:
Nikulin V, Head of Engineering
Nikulin V, Head of Engineering
A wet scrubber is an industrial device used for air cleaning with the help of a liquid. Proper calculation of the system’s efficiency makes it possible to determine the optimal operating parameters and ensure the required level of purification to meet environmental regulations.
Flow Scheme and Process Structure
- The main elements of a standard wet scrubber flow diagram:
- Incoming contaminated air enters the unit.
- The air then passes through the contact zone with the cleaning fluid.
- Captured particles are collected in the sump, while the cleaned air leaves the system through a demister or droplet separator.
- Liquid recirculation is maintained, along with monitoring of its quality and flow rate.
Flow Diagram of a Wet Scrubber
Efficiency Calculation and Particle Capture
Wet scrubber particulate efficiency depends both on the tower design and multiple operating parameters. Table of key factors:
Typical effectiveness in particle removal of the equipment:
- Up to 99% for dust >5 µm (Venturi, spray tower).
- 90–95% for fine particles in the range of 1–3 µm (with intensive interaction).
- For submicron solids, elevated pressure drops (>200 mm H₂O) are required to achieve effective removal.
where,
Cin — mass concentration of the particulate matter at the inlet, Cout — at the outlet.
Collection performance of wet scrubber particulate matter with sizes above 5 µm can reach 98–100%, depending on design and operating conditions.
It is critical to consider size: the removal of submicron fractions (<2 µm) is reduced due to the lower probability of collisions with fine mist.
Cin — mass concentration of the particulate matter at the inlet, Cout — at the outlet.
Collection performance of wet scrubber particulate matter with sizes above 5 µm can reach 98–100%, depending on design and operating conditions.
It is critical to consider size: the removal of submicron fractions (<2 µm) is reduced due to the lower probability of collisions with fine mist.
VOC Wet Scrubber: An Effective Air Pollution Control Solution
Let us examine two key parameters that govern absorption tower operation and must be considered during their design.
Wet scrubber air flow reflects the volumetric flow of gas passing through the system. It is usually measured in m³/h or CFM and indicates how much contaminated air can be treated per unit of time. Elevated liquid rates increase throughput, but excessively high rates can reduce pollutant removal due to shorter interaction time between phases.
Wet scrubber air velocity is a critical parameter, defined as the speed of the gas within the chamber, expressed in m/s.
Air velocity plays a critical role: low velocities reduce turbulence and the likelihood of particle–droplet collisions, while excessively elevated velocities sharply increase pressure drop and energy consumption for the fan. For most systems, the optimal velocity range is 1–5 m/s, although in Venturi units it can reach 10–15 m/s.
Wet scrubber air flow reflects the volumetric flow of gas passing through the system. It is usually measured in m³/h or CFM and indicates how much contaminated air can be treated per unit of time. Elevated liquid rates increase throughput, but excessively high rates can reduce pollutant removal due to shorter interaction time between phases.
Wet scrubber air velocity is a critical parameter, defined as the speed of the gas within the chamber, expressed in m/s.
Air velocity plays a critical role: low velocities reduce turbulence and the likelihood of particle–droplet collisions, while excessively elevated velocities sharply increase pressure drop and energy consumption for the fan. For most systems, the optimal velocity range is 1–5 m/s, although in Venturi units it can reach 10–15 m/s.
Calculation of the L/G Ratio
The L/G ratio in a wet scrubber is the volume of water (liters or gallons) per 1 m³ (or 1000 ft³) of air passing through the absorption tower.
For dust removal and highly soluble contaminants, typical standards are 0.5–3 liters per cubic meter (L/m³) or 4–20 gallons per 1000 ft³ of air.
Increasing the L/G ratio enhances the probability of contact between contaminants and the solution, resulting in improved performance. However, beyond a certain point, further increases provide only marginal gains in performance, while hydraulic resistance and power demand continue to rise.
Let’s review the typical ranges of wet scrubber water flow rate for different types of designs. These are approximate values; exact figures depend on the system size, composition of the gaseous stream, and contaminants.
For dust removal and highly soluble contaminants, typical standards are 0.5–3 liters per cubic meter (L/m³) or 4–20 gallons per 1000 ft³ of air.
Increasing the L/G ratio enhances the probability of contact between contaminants and the solution, resulting in improved performance. However, beyond a certain point, further increases provide only marginal gains in performance, while hydraulic resistance and power demand continue to rise.
Let’s review the typical ranges of wet scrubber water flow rate for different types of designs. These are approximate values; exact figures depend on the system size, composition of the gaseous stream, and contaminants.
Equipment Examples by Torch Air
Pressure Drop
Wet scrubber pressure drop is the difference in gas pressure between the inlet and outlet of the unit. It arises from resistance to gas movement caused by the liquid (droplets, foam, films) and the structural components of the equipment.
Pressure drop is usually measured in mm H₂O or Pa. It indicates the energy required to operate the system: the greater the value, the more power is needed for the fan. It is also directly connected to cleaning effectiveness: higher resistance enhances interfacial interaction, generates smaller spray, and improves solid and gas capture.
Typical values:
Pressure drop is usually measured in mm H₂O or Pa. It indicates the energy required to operate the system: the greater the value, the more power is needed for the fan. It is also directly connected to cleaning effectiveness: higher resistance enhances interfacial interaction, generates smaller spray, and improves solid and gas capture.
Typical values:
- Spray towers: low ΔP, ~25–100 mm H₂O (~250–1000 Pa). Low energy consumption, but less efficient for fine dust.
- Venturi systems: high ΔP, 500–2500 mm H₂O (~5–25 kPa). Very energy-intensive, but excellent for fine solids (down to 1 µm).
- Foam apparatus: medium ΔP, 100–400 mm H₂O (~1–4 kPa). Good balance between energy consumption and operational results.
- Packed bed columns: 100–500 mm H₂O (~1–5 kPa).
Mass Transfer Processes
When a contaminated gas comes into contact with an absorbent solution, impurities are transferred from the gas phase into the absorbing medium. This process is described as mass transfer across the gas–liquid interface.
Main stages:
Main stages:
- Transport of the contaminant in the gas phase to the interfacial area (diffusion).
- Overcoming interfacial resistance.
- Dissolution and diffusion in the fluid phase.
- Mass transfer coefficient (KGa, KLa): depends on gas turbulence, droplet size, and velocity of the absorbing phase. KGa (gas-phase coefficient) is especially important for poorly soluble gaseous species (e.g., SO₂).
- Interfacial area (a): formed through spray dispersion, packing surface, or foam layer. Smaller droplets and enhanced dispersion increase the area.
- Equilibrium (Henry’s law): the solubility of the contaminant in the fluid determines the absorption limit.
- Contact time: determined by the height of the spray zone, gas velocity, and liquid flow rate.
TORNADO Packed Bed Towers
Calculation using the mass transfer equation
The overall material balance in the unit can be expressed as:
where,
N — mass flux of the substance transferred to the fluid (mol/m²·s),
KG — overall gas-phase transfer rate (mol/m²·s·Pa),
a — interfacial area per unit volume (m²/m³),
p — partial pressure of the contaminant in the gas,
p* — equilibrium pressure corresponding to the concentration of the contaminant in the fluid.
Integrating this equation over the height of the tower allows estimation of the degree of removal. Packed and foam towers have high interfacial area and are often used for absorption. Spray towers have lower transfer rates (limited by contact area). Venturi setups provide very elevated mass transfer intensity due to the atomization of the liquid into microdroplets.
The mass transfer project wet scrubber focuses on factors such as mass transfer coefficients, interfacial contact area, contact time, and gas–liquid equilibrium.
N — mass flux of the substance transferred to the fluid (mol/m²·s),
KG — overall gas-phase transfer rate (mol/m²·s·Pa),
a — interfacial area per unit volume (m²/m³),
p — partial pressure of the contaminant in the gas,
p* — equilibrium pressure corresponding to the concentration of the contaminant in the fluid.
Integrating this equation over the height of the tower allows estimation of the degree of removal. Packed and foam towers have high interfacial area and are often used for absorption. Spray towers have lower transfer rates (limited by contact area). Venturi setups provide very elevated mass transfer intensity due to the atomization of the liquid into microdroplets.
The mass transfer project wet scrubber focuses on factors such as mass transfer coefficients, interfacial contact area, contact time, and gas–liquid equilibrium.
Technical Specifications
A cut sheet for a wet particulate scrubber may include:
- Diameter and length of the unit.
- Maximum air and solvent flow rates.
- L/G ratio at nominal load.
- Pressure rating, construction material, type of nozzles/packing.
- Maximum solid size to be removed.
- Performance and datasheet information of the demister.
How does a TORNADO-ST work
Example: Wet scrubber specification — TORNADO ST
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We always perform precise calculations and offer expert assistance in selecting the optimal dust collection or gas cleaning systems, typically completing this process within 1 to 2 days
Head of Engineering,
Vladimir Nikulin
Vladimir Nikulin
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