SO₂ Scrubber Packing: Plastic Pall Ring vs Tri-Pack for Caustic Absorption

Sulfur dioxide scrubbers are used in chemical plants, metal processing, combustion systems, sulfur-handling facilities, and other industrial processes where SO₂ must be removed from exhaust gas.

One common treatment method uses an alkaline scrubbing liquid such as sodium hydroxide or another suitable alkaline solution.

Inside the packed tower, SO₂-containing gas contacts downward-flowing scrubbing liquid. The packing provides the gas-liquid contact area required for SO₂ to transfer into the liquid and react.

Two plastic random packings often worth evaluating are:

• Plastic Pall Ring
• Plastic Tri-Pack

Both can provide effective gas-liquid contact, but their geometry creates different balances between:

• mass-transfer area
• gas capacity
• pressure drop
• liquid drainage
• fouling tolerance
• long-term maintainability

The practical engineering question is:

When is Plastic Pall Ring the better general-purpose choice, and when does the more open Tri-Pack geometry become worth considering for an SO₂ caustic scrubber?

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Why SO₂ Scrubber Packing Selection Is Not Only About Surface Area

SO₂ is transferred from the gas into the alkaline liquid and then converted into dissolved reaction products.

Depending on the chemistry and oxidation conditions, the circulating liquor may contain species such as:

• sulfite
• bisulfite
• sulfate
• sodium salts
• suspended solids
• other process contaminants

As the scrubber operates, the liquid can become very different from clean make-up caustic.

This matters because the packing must continue operating while exposed to:

• reactive chemistry
• increasing dissolved salt concentration
• possible precipitation
• solids
• high gas throughput

For this reason, the best packing is not necessarily the one with the highest nominal specific surface area.

The bed must also remain hydraulically open over time.

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1. What Does the Packing Actually Do?

The packing itself does not neutralize SO₂.

Its main function is to create a large and repeatedly renewed interface between:

SO₂-containing gasandalkaline scrubbing liquid.

A suitable packing helps provide:

• liquid spreading
• gas-flow passages
• wetted contact surface
• repeated gas-liquid redistribution
• stable drainage

The actual SO₂ removal efficiency also depends on:

• inlet SO₂ concentration
• outlet requirement
• scrubbing-liquid chemistry
• pH
• liquid circulation rate
• gas flow
• packed-bed height
• temperature

Therefore, packing selection and scrubber chemistry must be considered together.

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2. Why Plastic Packing Is Commonly Considered

Plastic random packing can be attractive for SO₂ scrubbers because it offers:

• relatively low weight
• corrosion resistance
• economical cost
• broad size availability
• easy installation
• low support loading

PP is commonly considered for many wet-scrubber applications, although final suitability should always be checked against:

• actual chemical composition
• operating temperature
• maximum temperature
• oxidizing environment
• cleaning chemicals

The choice between Pall Ring and Tri-Pack is mainly a geometry and hydraulic decision once a suitable polymer has been established.

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3. Plastic Pall Ring: The Conventional Starting Point

Plastic Pall Ring is one of the most established wet-scrubber random packings.

Its open cylindrical geometry provides:

• wall openings
• internal liquid-contact surfaces
• good gas-flow area
• repeated liquid redistribution
• relatively high void fraction

For many standard SO₂ scrubbers, Pall Ring provides a practical balance between:

mass transfer + hydraulic capacity + cost + availability.

If the gas is reasonably clean and the scrubber chemistry is well controlled, there may be little reason to move to a more specialized packing.

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4. What Makes Tri-Pack Different?

Tri-Pack uses a highly open three-dimensional cage-like geometry rather than a conventional ring wall.

This creates:

• large interconnected gas passages
• substantial void space
• multiple liquid-flow routes
• relatively little solid obstruction
• good drainage

The main attraction is therefore hydraulic openness.

Tri-Pack becomes especially interesting when the scrubber has problems related to:

• high gas flow
• excessive pressure drop
• solids
• salt deposition
• packing blockage

Its value is not simply that it looks more open.

The engineering value comes from maintaining gas and liquid pathways under demanding operating conditions.

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5. SO₂ Reaction Products Can Influence Long-Term Operation

A caustic SO₂ scrubber does not circulate fresh NaOH forever.

As SO₂ is absorbed, alkaline capacity is consumed and reaction products accumulate.

The circulating liquor may therefore experience increasing concentrations of dissolved salts.

Depending on chemistry, temperature, concentration and water balance, this can contribute to:

• deposits
• crystallization
• suspended solids
• blocked distributor openings
• packing fouling

Once solids begin occupying the void spaces in the bed, packed-bed pressure drop can rise significantly.

This is one of the main reasons that fouling tolerance can matter as much as clean-bed surface area.

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6. Pall Ring vs Tri-Pack Under Fouling Conditions

If the scrubber stream and circulating liquid remain clean, Pall Ring may provide excellent service.

But if the application has a history of deposits, Tri-Pack may deserve stronger consideration.

Pall Ring

Can provide:

• strong general-purpose mass transfer
• familiar operating behavior
• easy sourcing
• good hydraulic capacity

But smaller Pall Ring sizes can become increasingly sensitive to solids accumulation.

Tri-Pack

Can provide:

• larger open passages
• greater tolerance to partial deposits
• easier gas and liquid movement through a partially fouled bed
• lower risk of very tight blocked passages

However, Tri-Pack is not immune to scale.

Severe precipitation can eventually obstruct any packed bed.

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7. High Gas Flow Can Favor the More Open Geometry

SO₂ scrubbers may handle large exhaust-gas volumes.

As gas velocity increases:

• pressure drop rises
• liquid drainage becomes more difficult
• liquid holdup increases
• the tower approaches loading and flooding

A more open packing geometry may provide additional hydraulic margin.

This is where Tri-Pack can become attractive.

However, if the tower already operates comfortably below its hydraulic limit, the benefit may not justify changing from Pall Ring.

The packing change should solve a real problem.

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8. Pressure Drop Affects Fan Energy

The scrubber fan must overcome resistance from:

• inlet ductwork
• packing support
• packed bed
• mist eliminator
• outlet ductwork

If packed-bed pressure drop is unnecessarily high, the system may require:

• a larger fan
• greater motor power
• higher operating cost
• reduced maximum gas throughput

For continuously operating SO₂ scrubbers, lower hydraulic resistance can therefore provide lifecycle value.

Packing price alone does not represent the total operating cost.

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9. Smaller Packing Is Not Automatically Better

A common selection mistake is choosing the smallest available random packing to maximize nominal surface area.

Smaller packing generally provides:

• more elements per cubic meter
• greater surface area
• more frequent gas-liquid redistribution

But it can also create:

• higher pressure drop
• smaller passages
• greater sensitivity to deposits

Larger packing generally provides:

• lower resistance
• greater gas capacity
• larger flow passages
• greater fouling tolerance

but lower area per unit packed volume.

For SO₂ caustic service, packing size should therefore balance:

removal efficiency + pressure drop + fouling risk.

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10. Tri-Pack Does Not Automatically Mean Better SO₂ Removal

A more open structure may improve hydraulics, but the scrubber still needs enough effective wetted area to absorb SO₂.

If a packing is too open for the available bed height and liquid rate, mass-transfer performance may become limiting.

The correct optimization is therefore not:

maximum openness

but:

enough contact area at acceptable hydraulic resistance.

This is why Pall Ring can remain a stronger option for many conventional clean SO₂ scrubbers.

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11. Liquid Distribution Is Critical

No random packing can compensate for poor caustic distribution.

If liquid is unevenly distributed:

• some regions become dry
• SO₂ can channel through poorly wetted areas
• local flooding can occur
• effective packing area decreases

Large SO₂ scrubbers require particular attention to:

• distributor coverage
• irrigation-point density
• blocked holes
• nozzle condition
• distributor levelness
• wall flow

A fouled distributor can create poor removal even when the packing itself remains in good condition.

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12. Salt Deposits Can Begin at the Distributor

When solids or salts are present, distributor openings may foul before the random packing itself.

Blocked distributor holes create:

• uneven irrigation
• dry bed zones
• localized high liquid load
• reduced SO₂ removal

Therefore, if performance deteriorates, the operator should inspect:

distributor + packing + support

rather than assuming the packing alone is responsible.

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13. The Packing Support Can Become a Hidden Restriction

The support grid must retain the random packing while providing high open area.

Deposits can accumulate on or beneath the support.

This may result in:

• poor liquid drainage
• rising pressure drop
• localized flooding
• liquid backup

A very open Tri-Pack bed installed above a badly fouled support can still operate poorly.

For retrofit projects, support condition must therefore be checked.

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14. Mist Eliminator Pressure Drop Also Matters

SO₂ scrubbers typically operate wet and can entrain droplets.

A downstream mist eliminator reduces liquid carryover.

But as the demister fouls, its pressure drop may rise substantially.

If total scrubber differential pressure becomes excessive, the operator should determine whether the restriction comes from:

• packing
• support
• mist eliminator
• ductwork

before changing packing.

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15. Plastic Pall Ring vs Tri-Pack: Practical Comparison

The table is a selection starting point, not a universal performance ranking.

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16. When Pall Ring Is Usually the Better Starting Point

Plastic Pall Ring remains highly logical when:

• gas and liquid streams are relatively clean
• pressure drop is already acceptable
• the tower has a proven Pall Ring design
• standardization matters
• replacement availability matters
• fouling is manageable
• adequate packed height is available

For a reliable existing Pall Ring tower, there may be no benefit in changing packing simply because another geometry exists.

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17. When Tri-Pack Becomes Worth Evaluating

Tri-Pack becomes more attractive when:

Packed-Bed Pressure Drop Is Limiting Capacity

The system needs greater airflow without increasing tower diameter.

Salt or Solids Deposition Is Frequent

More open passages may provide better tolerance.

The Existing Packing Frequently Blocks

A geometry change may extend cleaning intervals.

Fan Pressure Is Limited

Lower hydraulic resistance can provide useful operating margin.

A Major Retrofit Is Already Planned

A shutdown provides an opportunity to compare a more open packing rather than simply repeating the old specification.

These are meaningful reasons to evaluate Tri-Pack.

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18. Chemical Control Can Be More Important Than Packing Geometry

If the recirculation chemistry is poorly controlled, no packing will remain clean indefinitely.

Important operating variables may include:

• pH
• alkalinity
• blowdown rate
• dissolved salt concentration
• oxidation state
• make-up water
• temperature

If the liquor becomes highly concentrated and precipitation begins, changing from Pall Ring to Tri-Pack may only delay the problem.

The chemistry causing deposition should be addressed as well.

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19. Retrofit Projects Should Start With the Failure Mode

Before selecting new packing, determine what is actually wrong.

Poor SO₂ Removal + Normal Pressure Drop

Check:

• pH
• caustic concentration
• distributor
• liquid circulation
• packed height

Increasing Pressure Drop Over Time

Check:

• packing deposits
• support blockage
• mist eliminator fouling
• crystallization

Insufficient Gas Capacity

Check:

• packing hydraulics
• gas rate
• distributor/support free area
• fan capability

Only then should the packing geometry be changed.

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20. Data Needed for an SO₂ Scrubber Packing RFQ

Useful information includes:

• tower internal diameter
• gas flow rate
• maximum gas flow
• inlet SO₂ concentration
• required outlet SO₂ concentration
• scrubbing chemical
• NaOH concentration if applicable
• operating pH
• liquid circulation rate
• operating temperature
• maximum temperature
• current packing type
• packing size
• packed-bed height
• current pressure drop
• salt or solids concentration
• fouling history
• cleaning frequency
• distributor design
• support details
• mist eliminator information

For retrofit projects, photographs of deposits and removed packing can be especially useful.

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Final Selection Principle

Plastic Pall Ring and Tri-Pack can both be effective random packings for SO₂ caustic scrubbers.

Pall Ring remains a strong general-purpose choice where:

mass transfer + availability + proven operation + economical replacement

are the main priorities.

Tri-Pack becomes more attractive where:

high gas throughput + lower hydraulic restriction + deposit tolerance

solve a real operating problem.

The correct question is therefore not:

“Which packing has the more advanced shape?”

It is:

“Is this SO₂ scrubber primarily limited by mass transfer, hydraulic capacity, or long-term fouling?”

If the existing Pall Ring system already performs reliably, keeping it may be the best decision.

If pressure drop and salt-related blockage repeatedly limit operation, a more open Tri-Pack geometry is worth evaluating.