Sizing a Packed-Bed Scrubber: Airflow, Diameter and Bed Depth

Start With the Gas Flow You Really Need

In packed bed scrubber sizing, the first number to confirm is not the tower diameter. It is the actual gas flow rate entering the scrubber.

Many mistakes happen because drawings show normal flow or standard flow, while the scrubber must handle actual flow at operating temperature, pressure and moisture content.

Use:

`text Q_actual = Q_standard × (T_actual / T_standard) × (P_standard / P_actual) `

Where:

• Q_actual = gas flow at scrubber inlet conditions
• Q_standard = flow at standard or normal conditions
• T = absolute temperature, K
• P = absolute pressure

For example, if a process exhaust is 20,000 Nm³/h at 25°C and enters the scrubber at 60°C near atmospheric pressure:

`text Q_actual = 20,000 × (333 / 298) = 22,350 m³/h `

The scrubber diameter, packing volume, fan selection and duct velocity should all be based on approximately 22,350 m³/h, not 20,000 Nm³/h.

Also check whether the stated flow is:

• Maximum fan flow or normal process flow
• Dry gas flow or wet gas flow
• One exhaust source or several sources combined
• Continuous operation or batch peak operation
• Before or after dilution air

As a rule of thumb, for chemical exhaust systems, design the scrubber for the maximum expected actual flow, plus a reasonable margin. A common design margin is 10–15%, but too much margin can create low gas velocity and poor liquid distribution.

Select Tower Diameter From Superficial Gas Velocity

For vertical packed towers, the diameter is normally selected from the superficial gas velocity through the empty tower cross-section.

`text Vg = Q / A `

Where:

• Vg = superficial gas velocity, m/s
• Q = actual gas flow, m³/s
• A = tower cross-sectional area, m²

Then:

`text D = √(4Q / πVg) `

For many PP packed-bed scrubbers used for acid gas, alkali gas and water-soluble fumes, typical design velocities are:

Application	Typical superficial gas velocity	Notes
Acid mist, HCl, HF, NH₃, soluble gases	1.5–2.5 m/s	Common range for PP scrubbers
High removal efficiency requirement	1.2–1.8 m/s	Lower velocity improves contact time
Dirty gas with dust or sticky mist	1.0–1.8 m/s	Reduces plugging and entrainment
Compact design with low efficiency demand	2.5–3.0 m/s	Higher pressure drop and carryover risk
Odor polishing or low concentration gas	1.5–2.2 m/s	Depends strongly on chemistry

For PP towers, we often start preliminary sizing at 1.8–2.2 m/s for normal acid or alkaline gas absorption. Final selection depends on removal target, liquid rate, packing type and allowable pressure drop.

Example:

Gas flow = 22,350 m³/h

`text Q = 22,350 / 3,600 = 6.21 m³/s `

If selecting Vg = 2.0 m/s:

`text A = 6.21 / 2.0 = 3.105 m² D = √(4 × 3.105 / 3.1416) = 1.99 m `

So the practical scrubber shell diameter would be about DN2000.

If the same gas flow is designed at 1.5 m/s:

`text A = 6.21 / 1.5 = 4.14 m² D = 2.30 m `

This shows why velocity selection is important. A lower velocity gives better contact and lower entrainment risk, but increases equipment size.

Estimate Packed Bed Depth and Contact Time

Packed bed depth is selected according to the gas solubility, reaction speed, required removal efficiency and allowable pressure drop. It is not only a mechanical dimension.

For common vertical PP scrubbers, typical random packing depths are:

• 800–1,200 mm: simple acid mist washing, low concentration, moderate efficiency
• 1,200–1,800 mm: common acid or alkaline gas absorption, such as HCl, NH₃ or SO₂ with proper reagent
• 1,800–2,500 mm: higher removal target, lower solubility gas or variable inlet load
• Two packed sections: used when pH control, two-stage chemistry or high efficiency is required

A simple check is empty-bed contact time inside the packing:

`text EBCT = H / Vg `

Where:

• EBCT = empty-bed contact time, seconds
• H = packed bed depth, m
• Vg = superficial gas velocity, m/s

Example:

If bed depth is 1.5 m and gas velocity is 2.0 m/s:

`text EBCT = 1.5 / 2.0 = 0.75 s `

For many fast acid-base absorption duties, an EBCT of 0.5–1.0 s can be workable when liquid distribution and pH control are good. For less soluble gases or higher removal expectations, 1.0–1.5 s or more may be needed. Actual performance depends on mass transfer, not contact time alone.

For packed bed scrubber sizing, do not increase bed depth without checking pressure drop and liquid distribution. A very deep single bed may perform worse than expected if liquid channeling occurs. For tall beds, it may be better to use two packing layers with redistributors.

Choose Packing, Liquid Rate and Pressure Drop Together

Packing type affects mass transfer, fouling resistance and pressure drop. In PP scrubbers, common choices include PP Pall rings, PP Tellerette packing and structured or special plastic packing.

Typical random packing sizes:

• 25 mm: high surface area, higher pressure drop, more plugging risk
• 38 mm: balanced choice for many chemical scrubbers
• 50 mm: lower pressure drop, better for higher gas flow or dirtier gas
• 76 mm: used for large towers or low pressure drop, lower surface area

For many industrial exhaust scrubbers, 38 mm or 50 mm PP packing is a practical starting point.

Liquid-to-gas ratio is normally expressed as:

`text L/G = liquid flow rate / gas flow rate `

Common design ranges:

• Acid/alkali gas absorption: 2–8 L/m³ of gas
• Mist and soluble fume washing: 1–4 L/m³
• High heat load or high concentration gas: may require higher circulation and cooling
• Dusty gas: liquid rate must also keep the bed washed, not only absorb gas

For example, with 22,350 m³/h gas and L/G = 3 L/m³:

`text Liquid flow = 22,350 × 3 = 67,050 L/h = 67 m³/h `

This is the circulation pump flow, not the blowdown flow. Chemical dosing and wastewater discharge are calculated separately from material balance and water quality limits.

Pressure drop across a clean packed bed often falls in this broad range:

• 500–1,000 Pa: low velocity, large packing, shallow bed
• 1,000–2,000 Pa: common industrial scrubber range
• 2,000–3,000 Pa or higher: high velocity, small packing, deep bed or fouling

A preliminary fan static pressure allowance should include:

`text Total static pressure = duct loss + scrubber body loss + packed bed loss + mist eliminator loss + stack loss + margin `

For a typical PP scrubber system, total fan pressure may be in the range of 2,000–4,000 Pa, but this depends strongly on duct length, elbows, dampers, packing depth and mist eliminator condition.

Check Mechanical and Operating Details Before Finalizing

After the basic diameter and bed depth are selected, several details can decide whether the scrubber works reliably.

Important checks include:

• Mist eliminator face velocity: often designed around 2.0–3.5 m/s, depending on type and droplet loading. Too high causes water carryover.
• Spray nozzle coverage: nozzles must wet the full tower area. Poor distribution causes dry zones and low efficiency.
• Packing support strength: design for wet packing weight, liquid holdup and fouling load.
• Freeboard above packing: allow space for spray, droplet disengagement and mist eliminator access.
• Sump volume: large enough to prevent pump vortexing and handle pH control stability. A practical minimum circulation residence time is often 1–3 minutes, but process variation may require more.
• Access doors: provide access to nozzles, packing, support grid and mist eliminator.
• Drain and overflow: avoid dead zones where solids collect.
• Material selection: PP is suitable for many acid and alkali duties, but temperature and oxidizing chemicals must be checked.

For PP construction, continuous operating temperature is commonly kept below about 70–80°C, depending on sheet grade, wall thickness, reinforcement and chemical exposure. Higher temperatures require special review or different materials.

The scrubber diameter also affects fabrication. Very small towers may have poor internal access. Very large PP towers need stronger shell reinforcement, external ribs, wind load checks and careful transport planning.

Practical Next Step

For a first packed bed scrubber sizing, prepare these data before contacting a supplier:

1. Actual gas flow, temperature and pressure at scrubber inlet
2. Gas composition and inlet concentration, including peaks
3. Required outlet concentration or removal efficiency
4. Dust, oil mist, sticky components or crystals present
5. Available layout height and footprint
6. Preferred chemical reagent and wastewater limits
7. Duct route and fan location

With these values, a supplier can calculate tower diameter, packing depth, circulation flow, pressure drop, pump size and fan duty with fewer assumptions. If any data are uncertain, state the expected range; this is better than using one inaccurate design point.