Plastic Tri-Pack Packing for Air Stripping Towers: Pressure Drop, Mass Transfer & Fouling Considerations

Air stripping towers remove volatile contaminants from water by bringing the liquid into contact with a large flow of air.

Typical applications may involve removal of volatile organic compounds, dissolved gases, odors, or other transferable contaminants from water and wastewater.

Because these towers often operate with a relatively high gas-to-liquid ratio, packing selection must provide more than surface area.

The packing must also support:

• high air throughput
• effective water spreading
• low packed-bed pressure drop
• sufficient mass-transfer area
• resistance to fouling
• stable long-term hydraulic operation

Plastic Tri-Pack is one random packing geometry worth evaluating for these duties because of its open three-dimensional structure and relatively large flow passages.

The important engineering question is:

When does Tri-Pack provide a useful balance of air capacity, water distribution, mass transfer, and fouling tolerance in an air stripping tower?

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Why Air Stripping Places Special Demands on Packing

An air stripper normally operates with water flowing downward through a packed bed while air flows upward.

The objective is to transfer volatile components from the liquid phase into the gas phase.

Packing increases the interfacial contact between the two phases.

However, the hydraulic conditions can differ significantly from many conventional absorption towers.

Air stripping often involves:

• large air volumetric flow
• relatively low-density gas
• substantial gas-to-liquid ratios
• water containing suspended solids or minerals
• outdoor or wastewater service
• long continuous operating periods

This makes gas capacity and fouling resistance particularly important.

A packing with excellent theoretical surface area but excessive resistance to airflow may increase blower requirements or limit tower throughput.

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What Is Different About Tri-Pack Geometry?

Tri-Pack is a plastic random packing with an open spherical or cage-like three-dimensional structure.

Instead of using a conventional cylindrical wall, the geometry consists of interconnected ribs and open spaces.

This provides:

• relatively large passages through each packing element
• multiple directions for air and water movement
• low obstruction to gas flow
• repeated liquid redistribution
• fewer enclosed regions where deposits can accumulate

Its value therefore comes primarily from the relationship between:

open flow area + wetted structure + three-dimensional redistribution.

This can be attractive in towers where hydraulic openness is as important as nominal surface area.

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1. Airflow Capacity Is Often a Major Selection Factor

Air stripping requires enough air to maintain the driving force for contaminant removal.

Increasing airflow can improve stripping performance in some systems, but it also increases gas velocity through the packed bed.

As gas velocity increases:

• pressure drop rises
• liquid drainage becomes more difficult
• liquid holdup may increase
• the tower moves closer to loading and flooding

For this reason, packing with large interconnected voids can be useful.

Tri-Pack's open geometry may allow a relatively large gas volume to pass through the bed without creating the same degree of obstruction as tighter packing geometries.

This does not mean the tower can operate at any airflow.

Actual allowable gas velocity still depends on:

• packing size
• tower diameter
• liquid rate
• fluid properties
• required flooding margin

But the packing geometry can materially influence the available hydraulic capacity.

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2. Pressure Drop Directly Affects Blower Duty

In an air stripping system, the blower must overcome pressure losses across the entire gas-flow path.

These losses may include:

• inlet ducting
• packing support
• packed bed
• mist eliminator
• outlet ducting
• other tower internals

The packed bed may represent an important portion of the total resistance.

If packing pressure drop is unnecessarily high, the system may require:

• a larger blower
• greater motor power
• higher operating cost
• reduced maximum airflow

For long-running water-treatment equipment, energy consumption can become a meaningful lifecycle cost.

Therefore, selecting packing only on purchase price can be misleading.

A more open random packing can sometimes provide economic value through lower hydraulic resistance over years of operation.

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3. Mass Transfer Still Requires Effective Wetting

Low pressure drop alone is not enough.

The purpose of the tower is contaminant removal.

Water must repeatedly spread over packing surfaces so that volatile components can transfer into the upward-flowing air.

Effective stripping depends on:

• wetted surface area
• liquid distribution
• air-to-water ratio
• contaminant volatility
• water temperature
• packed-bed height
• contact efficiency

Tri-Pack's open structure allows water to contact ribs and surfaces while repeatedly breaking and redistributing as it moves downward.

However, actual effective surface area depends on how well the liquid wets the packing.

This means the packing and liquid distributor must be evaluated together.

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4. Distributor Quality Can Determine Real Performance

Poor water distribution can reduce the performance of even a good packing.

If the distributor sends too much liquid to one part of the bed and too little to another, the tower may develop:

• dry zones
• channeling
• localized high liquid load
• uneven mass transfer
• poor contaminant removal

Air can preferentially move through regions with lower liquid resistance.

This reduces effective gas-liquid contact.

For this reason, large-diameter air stripping towers require particular attention to:

• distributor coverage
• number of liquid distribution points
• distributor levelness
• orifice blockage
• irrigation uniformity

Changing packing alone will not solve a severe liquid-distribution problem.

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5. Fouling Resistance Can Be More Important Than Maximum Surface Area

Water-treatment systems frequently operate with fluids that are not perfectly clean.

Potential contaminants include:

• suspended solids
• iron deposits
• mineral scale
• biological growth
• precipitated salts
• organic matter

Deposits can gradually reduce the available gas-flow area within the packed bed.

This leads to:

• increasing pressure drop
• reduced air capacity
• uneven water flow
• declining tower performance
• more frequent cleaning

Open packing geometries can be advantageous because they provide larger passages and fewer tight internal spaces.

Tri-Pack may therefore be worth evaluating where fouling tolerance is a major requirement.

But it is not fouling-proof.

Severe scaling or biological growth can still block the bed.

Long-term operation also depends on water pretreatment, chemistry control, cleaning procedures, and packing size.

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6. Packing Size Creates a Trade-Off

Tri-Pack is available in different nominal sizes depending on the manufacturer.

Size has a strong influence on tower performance.

Smaller Packing

Smaller elements generally provide:

• more packing pieces per unit volume
• greater potential contact area
• more frequent water redistribution

But they may also produce:

• greater pressure drop
• smaller flow passages
• greater sensitivity to fouling

Larger Packing

Larger elements generally provide:

• larger open passages
• lower hydraulic resistance
• higher tolerance to solids and deposits
• potentially higher gas capacity

But they may provide less effective surface area per unit packed volume.

For air stripping towers, packing size should therefore be selected according to both:

required contaminant removal and hydraulic operating margin.

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7. High Surface Area Is Not Always the Best Choice

A common mistake in packed-tower selection is assuming:

Higher surface area always means better treatment performance.

This is incomplete.

Surface area is valuable only when the liquid actually wets it and the gas can move through the bed without excessive resistance.

A very tight packing may theoretically provide more area but may also:

• increase pressure drop
• increase fouling sensitivity
• reduce air capacity
• require more blower power

For air stripping, a lower-area but more hydraulically open packing may sometimes provide the better overall design.

The correct decision is based on the complete mass-transfer and hydraulic system.

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8. Plastic Material Is Usually Well Suited to Water Treatment

Plastic random packing is widely considered in water and wastewater equipment because it can provide corrosion resistance at relatively low weight.

Depending on process requirements, materials may include PP or other suitable polymers.

Material selection should consider:

• water chemistry
• pH
• operating temperature
• oxidizing chemicals
• cleaning chemicals
• organic contaminants
• UV exposure if relevant
• long-term mechanical stability

For ordinary aqueous service, PP is frequently considered, but chemical compatibility should still be confirmed for the actual treatment system.

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9. Tower Diameter and Packing Size Must Be Compatible

A packing element should not be excessively large relative to the tower diameter.

In smaller columns, large random packing can create strong wall effects.

This may cause:

• non-uniform packing arrangement
• preferential flow near the wall
• poorer liquid spreading
• reduced effective contact

Therefore, selecting the largest possible Tri-Pack simply to minimize pressure drop is not always appropriate.

Packing size should be matched to the actual tower diameter.

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10. When Tri-Pack Is Particularly Worth Evaluating

Plastic Tri-Pack can be a strong candidate when an air stripping tower requires several of the following:

High Airflow

The process needs a large gas volume and packed-bed pressure drop is important.

Fouling Tolerance

Water contains minerals, suspended matter, or contaminants that may gradually deposit on the packing.

Low Blower Energy

The system operates continuously and energy consumption is important.

Corrosion Resistance

Metal packing would create unnecessary corrosion concerns.

Stable Hydraulic Operation

The tower needs sufficient operating margin below loading and flooding.

In these situations, the open geometry has a clear engineering purpose.

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11. When Another Packing May Be More Appropriate

Tri-Pack should not automatically be selected for every air stripper.

Another packing may be preferable when:

• maximum mass-transfer efficiency per unit packed height is required
• tower diameter is very small
• packing fouling is negligible
• the existing system already performs well with Pall Ring or another packing
• detailed design data favor a different geometry
• temperature or chemistry exceeds the plastic material limits

Product selection should follow process requirements rather than packing appearance.

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12. Tri-Pack vs Pall Ring in Air Stripping

Plastic Pall Ring is another common random packing that may be considered in air stripping systems.

A simplified selection perspective is:

This does not mean Tri-Pack is universally superior.

Actual performance depends on packing size and operating conditions.

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13. Retrofit Projects Should Start With the Existing Problem

For an existing air stripping tower, changing packing should have a clear objective.

Useful reasons may include:

• pressure drop has increased
• blower cannot achieve required airflow
• packing repeatedly plugs
• tower floods at higher flow rates
• contaminant removal has declined
• packing is physically damaged

Before changing the packing, determine whether the real cause is:

• packing fouling
• poor liquid distribution
• blocked support plate
• insufficient blower capacity
• incorrect operating flow
• inadequate packed height

This avoids replacing packing when the problem lies elsewhere.

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Data Needed for Tri-Pack Selection

For preliminary selection of Plastic Tri-Pack in an air stripping tower, useful information includes:

• tower internal diameter
• water flow rate
• air flow rate
• operating temperature
• operating pressure
• contaminant type
• inlet contaminant concentration
• target outlet concentration
• water density
• water viscosity where relevant
• solids or scaling tendency
• available packed height
• allowable pressure drop
• current packing type for retrofit projects
• packing support and distributor information

For an existing tower, current pressure drop and operating problems are especially valuable.

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Final Selection Principle

Plastic Tri-Pack can be an attractive random packing for air stripping towers because its open three-dimensional geometry can provide a useful balance between:

air capacity + low hydraulic resistance + water redistribution + fouling tolerance.

But air stripping performance cannot be judged from packing geometry alone.

The final selection must also consider:

• contaminant mass transfer
• air-to-water ratio
• packing size
• tower diameter
• packed height
• liquid distribution
• fouling conditions
• material compatibility

The practical engineering question is:

Can the packing provide enough gas-liquid contact to reach the required contaminant removal while allowing the necessary air volume to pass through the tower at an acceptable pressure drop?

For air stripping towers, that balance is more important than maximizing any single packing parameter.