Decarbonator Tower Packing for CO₂ Removal: Plastic Pall Ring vs Raschig Ring

Decarbonator towers are commonly used in industrial water-treatment systems to remove dissolved carbon dioxide before water enters downstream demineralization equipment.

A typical application occurs after a hydrogen-form cation exchanger.

Bicarbonate alkalinity in the raw water is converted to carbonic acid, which then decomposes into water and dissolved CO₂. Instead of sending this carbon dioxide load directly to the anion exchanger, a packed decarbonator can remove a large portion of it by contacting the water with air.

The random packing inside the tower is therefore a critical part of the system.

Two packing types commonly encountered in new and legacy installations are:

• Plastic Pall Ring
• Plastic Raschig Ring

Both can provide gas-liquid contact, but their geometry and hydraulic behavior are different.

The practical engineering question is:

When should a decarbonator keep conventional Raschig Ring packing, and when is Plastic Pall Ring the more logical choice for CO₂ removal?

---

Why Remove CO₂ Before the Anion Exchanger?

In a demineralization system, carbon dioxide entering the anion exchanger becomes part of the ionic load that must be removed.

If a significant CO₂ load can be removed physically before the anion unit, the downstream treatment burden can be reduced.

A decarbonator tower therefore helps by stripping dissolved CO₂ from the water into an upward-flowing air stream.

The basic process is:

CO₂-rich water enters the top

↓

Water distributes over the packing

↓

Air enters from below

↓

Packing creates gas-liquid contact

↓

CO₂ transfers from water to air

↓

Treated water leaves the bottom

The packing is not chemically absorbing CO₂.

Its function is to create enough interfacial contact for mass transfer.

---

1. What the Packing Must Do

A good decarbonator packing must provide several functions simultaneously.

It should:

• spread water over a large effective area
• allow sufficient upward airflow
• keep packed-bed pressure drop reasonable
• provide enough gas-liquid contact
• drain water without excessive holdup
• resist corrosion from the water chemistry
• remain mechanically stable during long service

This means packing selection cannot be based on surface area alone.

A packing with high nominal area but excessive airflow resistance may increase fan demand or reduce the practical air-to-water ratio.

---

2. Why Plastic Packing Is Common in Decarbonators

Water-treatment decarbonators usually operate at temperatures where thermoplastic random packing can be practical.

Plastic packing offers several advantages:

• corrosion resistance
• low weight
• easy installation
• low support load
• broad size availability
• economical replacement

PP is frequently considered for many ordinary water-treatment applications, although the final polymer should always be checked against:

• water chemistry
• temperature
• cleaning chemicals
• oxidizing agents
• operating environment

The choice between Pall Ring and Raschig Ring is therefore mainly a geometry and hydraulic decision when the same suitable polymer is used.

---

3. What Is a Plastic Raschig Ring?

Raschig Ring is one of the simplest random packing geometries.

It consists essentially of a short cylindrical element.

Its advantages include:

• simple construction
• easy manufacturing
• low product complexity
• long history of industrial use

Many older water-treatment towers were designed around Raschig Ring packing.

For that reason, Raschig Rings remain important in:

• existing-plant maintenance
• like-for-like replacement
• legacy tower specifications
• systems where hydraulic performance is already satisfactory

However, the simple cylindrical wall also creates more flow obstruction than later open-ring designs.

---

4. What Is a Plastic Pall Ring?

Pall Ring was developed as an improvement over the traditional Raschig Ring.

Instead of a largely closed cylindrical wall, Pall Ring contains open windows and internal structural features.

This creates:

• more open gas-flow passages
• improved use of internal surface
• better liquid redistribution
• lower obstruction to upward airflow
• more three-dimensional gas-liquid contact

For a decarbonator, these characteristics can be attractive because the tower often needs substantial airflow to remove dissolved CO₂ efficiently.

---

5. Why Airflow Matters in CO₂ Removal

A decarbonator depends on maintaining a suitable driving force for CO₂ transfer.

Air entering the tower contains relatively little CO₂ compared with the CO₂-rich water.

As air moves upward, it carries stripped CO₂ out of the system.

If airflow is insufficient:

• the gas phase becomes richer in CO₂
• mass-transfer driving force decreases
• removal efficiency may fall

But increasing airflow also increases gas velocity through the packing.

Higher gas velocity generally means:

• greater pressure drop
• more interaction with descending water
• greater risk of hydraulic loading
• increased fan requirement

This makes packing openness an important design parameter.

---

6. Pall Ring Can Offer a Hydraulic Advantage

Compared with a conventional Raschig Ring of comparable scale, Pall Ring geometry generally provides a more open flow structure.

For a decarbonator this can be useful when:

• high airflow is required
• fan pressure is limited
• tower capacity is being increased
• packed-bed pressure drop is already significant

Lower hydraulic resistance can allow more of the fan's available pressure to be used efficiently across the overall system.

But this does not mean every Pall Ring automatically produces lower pressure drop than every Raschig Ring.

Actual performance still depends on:

• packing size
• gas rate
• liquid rate
• packing geometry
• bed height
• fouling condition

The real products must be compared under the intended operating conditions.

---

7. Mass Transfer Still Matters

Low pressure drop alone does not make a good decarbonator packing.

The objective remains CO₂ removal.

The descending water must repeatedly contact fresh packing surfaces and interact with the upward airflow.

Effective performance depends on:

• liquid distribution
• wetting
• packing surface
• air-to-water ratio
• water temperature
• inlet CO₂ concentration
• required outlet CO₂ level
• packed-bed height

Pall Ring's open internal geometry can provide more usable gas-liquid interaction than the simpler Raschig Ring geometry.

This is one reason Pall Ring is often a logical modernization option.

---

8. Why Raschig Ring May Still Be the Correct Replacement

Older does not automatically mean wrong.

If an existing decarbonator using Plastic Raschig Rings has:

• acceptable CO₂ removal
• acceptable fan load
• stable pressure drop
• no significant fouling
• sufficient treatment capacity

there may be little reason to change packing type during routine maintenance.

A like-for-like replacement can offer:

• low engineering risk
• no major change in tower hydraulics
• simpler procurement
• easier matching to the original design

This is especially relevant when the original design calculations are no longer available.

Changing packing geometry should have a clear objective.

---

9. When Pall Ring Is Worth Evaluating

Plastic Pall Ring becomes more interesting when the project has a specific performance problem.

Higher Water Treatment Capacity

If the plant must process more water through the existing tower, airflow and hydraulic capacity may become limiting.

Excessive Pressure Drop

A more open packing geometry may provide additional gas-flow area.

Fan Capacity Is Limited

If the existing fan cannot provide substantially more pressure, reducing unnecessary packing resistance may be valuable.

Old Packing Is Being Modernized

A major tower refurbishment creates an opportunity to evaluate whether the original Raschig Ring remains the best choice.

Existing Packing Has Poor Flow Distribution

A more open geometry may improve bed utilization, although the liquid distributor must also be checked.

These are real reasons to compare packing types.

---

10. Packing Size May Matter More Than the Product Name

One of the biggest selection mistakes is comparing:

Pall Ring vs Raschig Ring

without comparing their actual sizes.

Packing size affects:

• surface area
• pressure drop
• void space
• number of pieces per cubic meter
• water redistribution
• air capacity

Smaller packing generally provides more contact opportunity but increases hydraulic resistance.

Larger packing generally provides:

• more open flow passages
• lower pressure drop
• better tolerance to fouling

but less contact area per unit packed volume.

Therefore, a meaningful upgrade study should compare:

packing family + packing size + bed height

rather than product family alone.

---

11. The Liquid Distributor Is Critical

A decarbonator depends on uniform water distribution across the packing.

If water enters the bed unevenly, some areas become heavily irrigated while others remain partially dry.

This can create:

• gas channeling
• poor CO₂ removal
• reduced effective packing area
• localized hydraulic loading

Pall Ring cannot compensate for a seriously damaged or poorly designed distributor.

During a retrofit, inspect:

• distributor holes
• spray nozzles if used
• distributor levelness
• blocked openings
• water coverage
• tower wall flow

Sometimes the distributor is the real performance limitation.

---

12. Wall Flow Can Reduce Effective Mass Transfer

Water flowing directly down the tower wall receives less useful contact with the random packing.

This can reduce overall decarbonation efficiency.

Wall flow becomes especially relevant when:

• the distributor does not cover the tower cross-section well
• packing arrangement near the wall is poor
• tower diameter is small relative to packing size

Packing size should therefore be compatible with the actual vessel diameter.

Selecting an excessively large packing simply to reduce pressure drop can create undesirable wall effects.

---

13. Fouling and Water Quality Still Matter

Water-treatment streams can contain:

• suspended solids
• iron compounds
• mineral scale
• biological deposits
• upstream resin fines
• treatment-chemical residues

Deposits can accumulate on random packing and gradually reduce its open area.

The result may be:

• higher pressure drop
• reduced airflow
• poorer water distribution
• lower CO₂ removal

Where fouling is a known issue, a larger or more open packing may provide better long-term operation.

But packing selection should also be combined with improvements in:

• upstream filtration
• cleaning
• water chemistry control
• distributor maintenance

---

14. Fan Energy Is Part of Lifecycle Cost

Random packing price is only one part of the total operating economics.

The fan may operate continuously for many years.

If one packing design requires significantly greater pressure than another at the same treatment rate, the difference in electricity consumption can become much larger than the original packing price difference.

Therefore, a decarbonator packing decision should consider:

purchase cost + pressure drop + fan power + maintenance

rather than simply comparing unit prices per cubic meter.

---

15. Pall Ring vs Raschig Ring: Practical Comparison

The final choice should still be based on actual hydraulic and process requirements.

---

16. Should an Old Raschig Ring Tower Be Converted?

Not automatically.

Before converting, ask:

Is CO₂ Removal Currently Poor?

If yes, determine whether the cause is actually the packing.

Is Pressure Drop Too High?

If yes, more open packing may help.

Is More Throughput Required?

If yes, hydraulic capacity becomes more important.

Is the Existing Distributor in Good Condition?

If no, repair it before assuming packing is the problem.

Is the Fan Already at Its Limit?

If yes, reducing packed-bed resistance may provide useful operating margin.

Only after answering these questions does a packing change become an engineering decision rather than a product substitution.

---

17. Support Grid Compatibility Must Be Checked

Changing packing size can affect the existing support.

The support grid must:

• retain the selected packing
• carry the wet packing load
• provide high open area
• resist the water chemistry

If a new packing is significantly smaller than the old one, it may require a different support opening or retaining layer.

If the support itself is blocked or restrictive, replacing the packing may not solve the tower's pressure-drop problem.

---

18. Data Needed for a Decarbonator Packing RFQ

For preliminary selection or retrofit evaluation, useful data include:

• tower internal diameter
• water flow rate
• air flow rate
• water temperature
• inlet CO₂ concentration
• required outlet CO₂ concentration
• existing packing type
• existing packing size
• packed-bed height
• current pressure drop
• fan capacity
• tower operating pressure
• water chemistry
• fouling history
• distributor type
• packing support details

For legacy towers, photos of the existing packing can also help confirm the installed product.

---

Final Selection Principle

Plastic Raschig Ring and Plastic Pall Ring can both function as random packing in water-treatment decarbonators.

Raschig Ring remains relevant where an existing tower already operates reliably and the objective is simple maintenance replacement.

Plastic Pall Ring becomes more attractive where the project requires:

better gas-flow openness + improved liquid redistribution + lower hydraulic resistance + greater capacity potential.

The correct decision is therefore not simply:

“Which packing is newer?”

It is:

“Does changing the packing geometry solve a real CO₂ removal, pressure-drop, airflow, or capacity problem in this decarbonator?”

If the answer is yes, Pall Ring may provide a meaningful modernization path.

If the existing Raschig Ring tower already performs well, a like-for-like replacement may still be the safest solution.