Wet Gas Scrubber: Design, Operation, L/G Ratio, and Efficiency

Author: Michael Klepik, Chief Executive Officer

Wet dust collectors have several advantages over devices of other types:

• They are relatively inexpensive and demonstrate superior performance in particle collection compared to dry mechanical systems.
• They can be used to remove particles as small as 0.1 µm from process streams;
• They are suitable for high-temperature and high-humidity streams, especially where there is a risk of ignition or explosion, and can also function as mixing heat exchangers.

These advantages allow for wide application of wet gas scrubber in dust collection systems of drying plants, especially as a second stage of purification.

However, this method also has some disadvantages:

• The collected product is discharged as sludge, requiring wastewater treatment and thereby increasing overall process costs;
• When the purified flow is cooled to near the dew point, or when droplets are entrained by the stream, dust may settle in the ductwork, ventilation systems, and exhaust equipment. Additionally, droplet carryover leads to irreversible loss of working fluid;
• When handling aggressive compounds, equipment and pipelines must be protected with corrosion-resistant materials.

Before deciding on a method, it is essential to thoroughly analyze the properties of the emissions to be treated.
It is necessary to consider:

• The solubility, reactivity (with potential for forming explosive, corrosive, or secondary contaminants), and corrosiveness of pollutant components should be taken into account.
• For solid contaminants: wettability, adhesion, and tendency to agglomerate.
• For liquids: wettability, density, and phase transition parameters.

Comparison of wet and dry purification methods shows that wet cleaning (not considering sludge handling) is less expensive and, as a rule, more efficient than dry methods. The majority of such systems are capable of capturing ultra-fine particles, even those smaller than 1 micron.

In any type of scrubber, particles are removed through one or more of the following main capture mechanisms: gravitational sedimentation, centrifugal separation, inertia and impaction, Brownian diffusion, thermophoresis, diffusiophoresis, and electrostatic precipitation. The rate of particle removal can be increased by the enlargement of particles due to agglomeration and condensational growth.

Depending on the method of organizing the contact surface between phases and the operating principle, absorption towers can be classified into the following groups:

• Spray Tower or Spray Chamber;
• Packed Bed Towers;
• Tray Tower (Plate Column)
• Rotoclones (Impaction-inertia type);
• Centrifugal units;
• Venturi.

Sometimes, equipment can be classified by energy consumption:

• Low-pressure devices: hydraulic resistance does not exceed 1500 Pa (spray chambers and centrifugal devices).
• Medium-pressure devices: hydraulic resistance from 1500 to 3000 Pa (packed bed, tray towers, and those with moving packing).
• High-pressure devices: hydraulic resistance above 3000 Pa (impaction-inertia devices, mechanical, and high-velocity units).

Typical liquid-gas scrubber components include a main vessel, spray nozzles or trays for solution distribution, inlet and outlet ports, a demister, a sump or reservoir, and recirculation pumps.

Packed bed towers additionally contain packing material to increase the contact area between the phases.

Wet gas scrubber companies typically use water as the washing solution in systems designed for dust and ash removal. Its consumption can vary for different types of units from 0.1 to 10 m³ per 1,000 m³ of processed flow.
When addressing both particulate control and chemical air purification, the choice of absorbent is determined by the conditions of the absorption process.

The liquid-to-gas ratio (L/G) is a key parameter in wet gas scrubber operation, indicating how much liquid is used to treat a given volume of air. It is usually expressed in liters per normal cubic meter (L/Nm³) or gallons per 1,000 cubic feet.
To eliminate volatile contaminants (e.g., SO₂):
Typically, a liquid-to-gas ratio in a scrubber from 1:1 to 10:1 (L/Nm³) is used.
For the removal of solid particles:
The liquid-to-gas ratio in a wet scrubber may be lower—from 0.5:1 to 3:1 (L/Nm³)—depending on the configuration and particle concentration.

L/G values generally depend on the type of pollutant: for example, highly soluble compounds require more working fluid. The higher the concentration of harmful substances and the required degree of purification, the higher the L/G. The optimal value is always a compromise between the efficiency of the wet gas scrubber and the overall cost-effectiveness of the process.

• Too low an L/G will lead to insufficient cleaning efficiency: some of the pollutant will not have time to dissolve or react with the liquid.
• Too high an L/G will improve efficiency only up to a certain limit; further increases in liquid consumption will have little effect on cleaning quality, but will significantly raise operational costs (consumption of water or reagent, energy for pumping, wastewater disposal costs, etc.), as well as hydraulic resistance.

The engineer's task is to select such an L/G ratio that:

• the required purification efficiency according to regulations is achieved,
• water and reagents are not excessively consumed,
• stable and reliable equipment operation is ensured without overloads.

Typically, the optimal value is selected experimentally (through laboratory or pilot-scale tests), or recommended figures from similar processes are used, with adjustments made for the specifics of the particular facility.

To maximize process efficiency, wet gas scrubber factories employ automated solutions for monitoring and adjusting liquid and air flows. Regulation of the L/G ratio is carried out using frequency-controlled pumps, control valves, and flow meters.

For specific gaseous contaminants (such as SO₂, HCl, NH₃, Cl₂, etc.), the solution must not only capture the pollutant physically, but also chemically react with it or actively absorb it. For this purpose, special solutions are used: alkalis (NaOH, KOH), acids (H₂SO₄), oxidizers (sodium hypochlorite, hydrogen peroxide), and others.

The absorbent should effectively bind or neutralize the pollutant. For example, lime or alkali solutions are often applied to treat sulfur compounds, while acids serve to remove ammonia.

Some solutions may promote the formation of hard-to-remove deposits, foaming, or secondary pollutants.

In imperial units: 10–100 gallons per 1,000 cubic feet (gal/1,000 ft³) is common.

The chemical composition of the absorbent affects the choice of materials, pumps, piping, and fittings. For example, applying acid or oxidizing solutions calls for corrosion-resistant materials like stainless steel, plastics (PVC, PP), composites, or special coatings.

The gas velocity in a wet scrubber has a significant impact on its performance. Increased flow velocity may improve particle collection efficiency in systems like Venturi units, but can reduce interphase contact time in others. Additionally, increasing the velocity generally results in a higher pressure drop. If the flow becomes too rapid, it can entrain droplets, leading to carryover and potential problems in downstream equipment.

The optimal scrubber gas velocity is chosen based on:

• Configuration
• Efficiency requirement
• Acceptable pressure drop
• Risk of droplet carryover

The key to an effective system is proper design. Mistakes at this stage are costly, so it is crucial to pay close attention to all information requested by the equipment manufacturer, as the calculations will be based on it.
The main stages include:
1. Collection of Initial Data

• Air volume and composition: flow rate, temperature, pressure, humidity.
• Type and concentration of contaminants: particulate matter, SO₂, HCl, NH₃, etc.
• Required degree of purification: emission standards, target values.

2. Selection of wet gas scrubber design

• Venturi: for fine dust and aerosol removal.
• Spray tower: for airborne pollutants requiring moderate treatment efficiency.
• Packed column: suitable for absorbing soluble contaminants.
• Combined designs: for all-in-one emission control.

3. Calculation and Selection of L/G

• Determine the optimal scrubber liquid-to-gas ratio, considering pollutant type, required efficiency, and process economy.
• L/G values are usually taken from literature or determined via laboratory/pilot testing.

4. Selection of Absorbent

• Water: for particulate removal and soluble contaminants.
• Chemical solutions: for specific contaminants (alkalis, acids, etc.).

5. Calculation of Contact Surface and Time

• Determine the required contact area and/or volume for interaction (e.g., packing, spray nozzles, trays, etc.).
• Calculate the residence time of both phases in the apparatus.

6. Hydraulic Calculations

• Determine pressure drop (hydraulic resistance) across the system.
• Calculate the required fan power.
• Calculate solution consumption and pump power requirements.

7. Equipment Materials

• Select materials considering corrosion resistance to the absorbent and contaminants (stainless steel, plastics, rubbers).

8. Control System

• Design a system to monitor air and fluid flow rates, automate reagent dosing, and install pressure drop sensors, etc.

9. Wastewater Treatment

• Organize systems for collection, treatment, or neutralization of wastewater.
• Provide measures to prevent clogging and deposition of solids.

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