Biogas Water Scrubber Packing: Plastic Pall Ring Selection for CO₂ Removal

Biogas upgrading systems remove carbon dioxide and other unwanted components from raw biogas so that the methane-rich product can be used as fuel, injected into a gas grid, or further processed.

One established upgrading method is water scrubbing.

In a pressurized water scrubber, raw biogas contacts downward-flowing water inside an absorption column. Carbon dioxide is significantly more soluble in water than methane, allowing a large portion of the CO₂ to transfer from the gas phase into the liquid.

Random packing provides the gas-liquid contact area required for this process.

Plastic Pall Ring can be a practical packing option where the system requires:

• corrosion-resistant construction
• relatively low packed-bed weight
• open gas-flow passages
• effective water redistribution
• economical random packing
• reliable operation under continuous gas-liquid contact

However, biogas upgrading introduces several selection issues that are different from an ordinary air scrubber.

The important engineering question is:

When does Plastic Pall Ring provide the right balance of CO₂ mass transfer, gas capacity, pressure drop, water circulation, and methane retention in a biogas water scrubber?

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How a Biogas Water Scrubber Works

Raw biogas commonly contains:

• methane
• carbon dioxide
• water vapor
• varying amounts of hydrogen sulfide
• traces of other contaminants

In a water scrubber, the gas normally flows upward through the absorption column while water flows downward.

CO₂ preferentially dissolves into the water.

The treated gas leaving the absorber contains a higher methane concentration.

The CO₂-rich water may then be:

• regenerated
• depressurized
• stripped
• recirculated

depending on the process design.

The packed absorber therefore operates as part of a complete upgrading system rather than as an isolated tower.

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1. What Does the Packing Actually Do?

Plastic Pall Ring does not selectively absorb CO₂ itself.

The separation is produced by differences in gas solubility and the operating conditions of the water-scrubbing process.

The packing provides:

• wetted surface
• repeated gas-liquid contact
• liquid redistribution
• open passages for upward gas flow
• renewal of the liquid film

Better use of the available packing surface generally increases the opportunity for CO₂ to transfer into the water.

But packing performance must always be considered together with:

• operating pressure
• water flow rate
• gas flow rate
• temperature
• packed height
• inlet CO₂ concentration
• required methane purity

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2. Why Pressure Is Important in Water Scrubbing

Biogas water scrubbers are often operated under elevated pressure.

Higher pressure increases the driving force for dissolving CO₂ into water.

This can improve absorption performance, but it also changes:

• gas density
• volumetric gas flow
• hydraulic loading
• compressor requirements
• dissolved gas behavior

Packing selection must therefore use the actual absorber operating pressure, not atmospheric-condition gas data.

A packing selected only from normal cubic meters per hour without considering actual operating pressure can lead to an incorrect hydraulic evaluation.

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3. Why Plastic Pall Ring Is a Practical Candidate

Pall Ring is an open cylindrical random packing with windows and internal structural elements.

Compared with a simple Raschig Ring, this geometry provides:

• more open wall area
• improved access to internal packing surfaces
• multiple gas-flow routes
• repeated liquid redistribution
• relatively high void fraction

For a pressurized biogas absorber, this combination can be useful because the column requires both:

effective water contact + sufficient gas capacity.

Plastic construction also keeps the packing bed substantially lighter than comparable ceramic packing.

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4. CO₂ Removal Requires More Than Maximum Surface Area

It is tempting to select the smallest packing simply because smaller random packing generally provides greater surface area per cubic meter.

But this creates a trade-off.

Smaller packing may provide:

• more contact area
• more frequent liquid redistribution
• potentially better mass transfer per unit bed height

while also creating:

• higher pressure drop
• smaller gas passages
• greater fouling sensitivity
• reduced hydraulic capacity

For biogas upgrading, the correct choice must consider whether the absorber is primarily limited by:

mass-transfer efficiency or hydraulic capacity.

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5. Packing Size Matters

Plastic Pall Rings are available in several nominal sizes.

Smaller Pall Ring

Usually offers:

• more elements per cubic meter
• greater contact opportunity
• potentially shorter required packed height

But can also produce:

• greater hydraulic resistance
• greater sensitivity to contamination
• lower maximum gas capacity

Larger Pall Ring

Generally offers:

• larger gas passages
• lower pressure drop
• greater capacity
• improved fouling tolerance

But normally provides lower surface area per unit volume.

Therefore, the most appropriate size must be selected from actual process data rather than simply choosing the smallest available Pall Ring.

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6. Pressure Drop Has an Energy Cost

Biogas upgrading already requires energy for gas compression, water circulation, regeneration, or other process equipment.

Additional unnecessary packed-bed pressure drop increases the burden on the system.

A high absorber pressure does not mean packed-bed pressure loss is unimportant.

Excessive differential pressure can:

• reduce available process pressure
• increase compression requirement
• limit gas capacity
• reduce operating margin
• indicate loading or fouling

For a continuously operating biogas plant, even relatively small energy penalties can become significant over time.

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7. Flooding Must Be Avoided

The absorber contains counter-current flow:

gas upwardwater downward

As gas velocity increases, the gas increasingly interferes with downward water drainage.

As the tower approaches flooding:

• liquid holdup increases
• pressure drop rises sharply
• flow becomes unstable
• gas-liquid distribution deteriorates

The packing must therefore provide sufficient capacity at the maximum expected gas and water rates.

Design should not target operation directly at the flooding point.

A practical safety margin is required.

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8. Water Circulation Rate Is a Key Variable

Increasing water flow can increase available absorption capacity, but it also raises the hydraulic load on the packed bed.

Higher liquid rate can increase:

• liquid holdup
• pressure drop
• pump duty
• flooding tendency

The correct water circulation rate is therefore a process optimization.

Too little water can reduce CO₂ removal.

Too much water may create unnecessary energy consumption and hydraulic loading.

The packing must be selected together with the intended liquid-to-gas ratio.

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9. Water Temperature Affects CO₂ Absorption

Gas solubility in water is temperature-dependent.

Cooler water generally favors physical absorption of CO₂ compared with warmer water.

This means absorber performance can change with:

• seasonal water temperature
• cooling-system performance
• recycled-water temperature
• compression heat
• ambient conditions

Packing selection cannot correct unfavorable thermodynamics.

If the water is too warm, adding more packing may not provide the same benefit as improving the process temperature.

Therefore, design should consider both:

normal water temperatureandmaximum expected water temperature.

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10. Methane Loss Must Also Be Considered

CO₂ is more soluble in water than methane, but methane is not completely insoluble.

Some methane can dissolve into the scrubbing water.

This creates an important process trade-off.

The system must maximize CO₂ removal while minimizing valuable methane loss.

Methane recovery may depend on:

• absorber pressure
• water circulation
• regeneration method
• flash pressure
• process configuration

Packing itself does not determine methane selectivity, but efficient mass transfer affects how closely the absorber approaches the available phase equilibrium.

Therefore, packing selection must be integrated with the overall upgrading process.

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11. H₂S Can Change the Material and Process Review

Raw biogas may also contain hydrogen sulfide.

Water can remove some H₂S, depending on operating conditions.

But H₂S introduces additional concerns such as:

• corrosion
• odor
• toxicity
• downstream water treatment
• material compatibility

The packing material should therefore be selected using the complete raw gas composition.

A specification that only states:

Biogas with 40% CO₂

may be incomplete if significant H₂S is also present.

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12. Liquid Distribution Is Critical

A good packing cannot compensate for poor water distribution.

If the absorber distributor sends too much water to one region and too little to another, the bed can develop:

• dry zones
• gas channeling
• localized hydraulic loading
• reduced effective contact area
• lower CO₂ removal

Large-diameter absorbers require particular attention to distribution quality.

Useful checks include:

• number of distribution points
• distributor levelness
• flow uniformity
• blocked holes or nozzles
• wall-flow tendency

Packing and distributor should therefore be treated as one hydraulic system.

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13. Wall Flow Reduces Packing Utilization

If significant water flows directly down the vessel wall, it bypasses much of the packing surface.

This reduces useful gas-liquid contact.

Wall flow may be influenced by:

• distributor design
• packing size
• tower diameter
• packing arrangement

A packing that is too large relative to the absorber diameter can increase wall-effect problems.

Therefore, packing size must always be checked against vessel diameter.

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14. Fouling May Develop in Recycled-Water Systems

Biogas water scrubbers may recirculate water.

Depending on gas quality and water treatment, the circulating liquid may accumulate:

• suspended solids
• biological material
• dissolved contaminants
• corrosion products
• process deposits

Fouling can gradually reduce the open area of the bed.

Possible consequences include:

• increasing pressure drop
• poorer water distribution
• gas channeling
• reduced mass transfer

More open packing geometries and appropriate Pall Ring sizes can improve tolerance, but they cannot eliminate a severe water-quality problem.

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15. Plastic Pall Ring vs Plastic Raschig Ring

Older packed absorbers may use Plastic Raschig Rings.

Compared with the simple Raschig Ring geometry, Pall Ring generally provides:

• more open gas passages
• improved use of packing interior
• better liquid redistribution
• more favorable hydraulic behavior

For a retrofit, Pall Ring may therefore be worth evaluating if the existing system has:

• excessive pressure drop
• limited gas capacity
• old or damaged packing
• planned throughput increase

However, a Raschig-to-Pall conversion should be reviewed hydraulically.

Equal packing volume does not necessarily mean equal performance.

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16. Plastic Pall Ring vs Structured Packing

Structured packing may also be used in gas absorption systems.

A simplified comparison is:

Structured packing may be attractive for high-performance upgrading systems.

Pall Ring remains useful when the project prioritizes economical random packing, simple installation and robust operation.

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17. Packing Support Must Handle Both Load and Flow

The support grid must:

• retain the packing
• carry the wet bed load
• resist process chemistry
• provide high open area
• avoid excessive gas restriction

A support plate with insufficient free area can become the real hydraulic bottleneck.

This is especially important when an existing absorber is being upgraded for higher gas throughput.

Replacing the packing without reviewing the support may fail to deliver the expected capacity increase.

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18. When Plastic Pall Ring Is a Strong Candidate

Plastic Pall Ring is worth serious consideration when:

Water Scrubbing Is the Selected Process

The absorber uses physical CO₂ absorption into water.

Plastic Is Compatible With the Service

Temperature and gas/liquid chemistry remain within the selected polymer limits.

Random Packing Simplicity Is Valuable

The project prefers easy installation and economical replacement.

Moderate-to-High Gas Capacity Is Required

Open ring geometry provides useful gas passages.

Tower Weight Should Be Limited

Plastic significantly reduces packed-bed weight.

Existing Random Packing Is Being Replaced

A Pall Ring retrofit can be straightforward to evaluate.

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19. When Another Packing May Be Better

Another packing may be preferred when:

• extremely low pressure drop is required
• very high mass-transfer efficiency per unit height is needed
• tower diameter is very large
• the water is severely fouling
• operating temperature exceeds polymer limits
• structured packing offers better whole-system economics

The correct packing is therefore determined by the absorber constraints rather than by one isolated packing property.

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20. Data Needed for a Biogas Water Scrubber Packing RFQ

Useful information includes:

• absorber internal diameter
• operating pressure
• operating temperature
• raw biogas flow rate
• methane concentration
• CO₂ concentration
• H₂S concentration
• other contaminants
• water circulation rate
• water temperature
• required methane concentration
• required CO₂ removal
• packed-bed height
• allowable pressure drop
• existing packing type and size
• fouling history
• distributor design
• packing support details

For retrofit projects, current tower differential pressure and product-gas composition are particularly useful.

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Final Selection Principle

Plastic Pall Ring can be a practical random packing option for biogas water scrubbers because it provides a useful combination of:

gas-liquid contact + open gas passages + corrosion-resistant plastic construction + low bed weight + economical installation.

But CO₂ removal performance depends on the complete physical-absorption system.

The packing decision must consider:

• absorber pressure
• water temperature
• gas and liquid flow
• CO₂ concentration
• methane loss
• H₂S content
• packing size
• packed height
• distributor quality
• allowable pressure drop

The practical engineering question is:

Can the selected Pall Ring provide sufficient CO₂-water mass transfer while maintaining acceptable pressure drop, gas capacity and methane recovery under the actual biogas upgrading conditions?

That is the basis for a reliable water-scrubber packing specification.