What is the recommended L/G Ratio of a scrubber ?

The recommended L/G ratio (liquid-to-gas ratio) varies depending on several factors, including the type of scrubber, the specific airborne hazard or pollutant being removed, and the desired removal efficiency subjective to the permission emission limit values stipulated in Clean Air Regulation 2014. L/G ratio basically manipulates the absorption liquid to enhance the scrubbing efficiency of the scrubber while balancing the system static pressure to be optimal. Here are some general guidelines and recommended ranges:

General Recommended L/G Ratios by Scrubber Type

1. Packed Bed Scrubbers:

• Typical range: 0.5 to 5 gallons per 1,000 cubic feet of gas (gal/1,000 ft³) or 0.05 to 0.5 liters per cubic meter of gas (L/m³).

• For gases with moderate to high solubility, such as ammonia or sulfur dioxide (SO₂), lower L/G ratios (closer to 0.5–2 gal/1,000 ft³) are usually effective.

• For less soluble VOCs or gases, higher L/G ratios within this range may be necessary.

2. Spray Towers:

• Typical range: 0.5 to 5 gal/1,000 ft³.

• Spray towers generally operate with moderate L/G ratios since they rely on droplets rather than packing to achieve gas-liquid contact. For more soluble gases, lower L/G ratios suffice, while higher values are used for low-solubility gases.

3. Venturi Scrubbers:

• Typical range: 5 to 15 gal/1,000 ft³ (0.5 to 1.5 L/m³).

• Venturi scrubbers require higher L/G ratios because they depend on high-velocity gas-liquid interactions. The higher liquid flow helps increase pollutant capture efficiency, especially for particulate matter and certain VOCs.

Recommended L/G Ratios for Specific Air Borne Hazards or Pollutants

1. Acid Gases (e.g., SO₂, HCl):

• Range: 1 to 3 gal/1,000 ft³.

• Acid gases typically dissolve well in water, so moderate L/G ratios are usually effective.

2. Ammonia (NH₃):

• Range: 0.5 to 2 gal/1,000 ft³.

• Ammonia is highly soluble in water, allowing for lower L/G ratios.

3. Volatile Organic Compounds (VOCs):

• Range: 3 to 10 gal/1,000 ft³.

• VOCs like MEK and xylene often require higher L/G ratios, especially if a water-based scrubber is used. Using solvents or a secondary activated carbon stage can reduce the liquid requirements while improving VOC capture.

4. Particulate Matter:

• Range: 5 to 15 gal/1,000 ft³ for Venturi scrubbers.

• Particulate scrubbing in a Venturi scrubber generally requires a high L/G ratio to ensure efficient capture, especially for fine particles.

Additional Considerations

• Pollutant Solubility: Higher L/G ratios are needed for less soluble compounds (e.g., some VOCs), while lower L/G ratios suffice for soluble gases (e.g., NH₃ or SO₂).

• Scrubber Design and Efficiency: The type and design of the scrubber influence the L/G ratio. For instance, packed beds often allow lower L/G ratios than Venturi scrubbers, given their reliance on packed material for gas-liquid interaction.

• Desired Removal Efficiency: Higher L/G ratios generally improve removal efficiency but increase operational costs and can potentially cause flooding or high pressure drops.

Starting Point and Adjustments

A good starting point for most applications is to operate at the lower end of the recommended range and adjust upward if removal efficiency is insufficient. Pilot testing or system monitoring will help identify the optimal L/G ratio for specific applications. As for those who operates scrubber without knowing the need to MAINTAIN the Pump Volumetric Flow Rate and Gas Volumetric Flow Rate within the ALLOWABLE RANGE of SYSTEM DESIGN POINT, do immediate determine these values and gauge if your current operation is within this BASELINE POINT. Any deviation from this point is a CLEAR CUT indication of system deterioration in performance.